Thesis for the Degree of Doctor of Philosophy

Material development of a textile bioreactor

All-polyamide composite for the construction of bioreactors

Mostafa Jabbari

Material development of a textile bioreactor All-polyamide composite for the construction of bioreactors Copyright 2020 ©Mostafa Jabbari Swedish Centre for Resource Recovery Faculty of Textiles, Engineering, and Business University of Borås SE-501 90 Borås, Sweden Digital version: https://urn:nbn:se:hb:diva-15939 ISBN 978-91-88838-28-5 (printed) ISBN 978-91-88838-29-2 (pdf) ISSN 0280-381X, Skrifter från Högskolan i Borås, nr. 82 Cover photo: Depicts a cross-sectional microscopic picture of the composite introduced and prepared in this thesis (all-polyamide composite coated textile) Printed in Sweden by Stema Specialtryck AB Borås, January 2020

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Abstract Bioreactors are manufactured from stainless/carbon steel, concrete, glass, etc., which are

costly and time-consuming to install. Recently, several research studies have been

initiated to find cost-efficient materials for constructing bioreactors, one of which is

coated textiles. Polyvinyl chloride (PVC)-coated polyester textile (PVCT) has been used

for this purpose to make bioreactors more cost-effective and easier to install. In this thesis,

the thermal insulation property of PVCT was improved, that enhances the energy

efficiency of the process carried out within the bioreactor. However, recycling PVCT is

challenging, as it is a mixture of PVC, polyester fabric, a plasticizer for the PVC, chemical

linkers, and other processing-aid additives.

A possible solution to address these issues is to use a coated textile composed of a single

material. The polyester fabric can be replaced with a better performing fabric, such as

polyamide, that generally has a longer lifetime as well as higher mechanical stability and is

light-weight. A facile method was introduced to make a same-polymer coated textiles

composite out of polyamide through the partial dissolution of the fabric’s surface followed

by coagulation. The all-polyamide composite coated textiles (APCT) is mechanically

stronger and more thermally stable than the PVCT as well as having less weight.

Additionally, the APCT is fully recyclable as it contains only a single component. This

property can be beneficial for the recyclability of the material. The APCT can be used in the

construction of textile bioreactors as well as other applications that require gas-/water-

tightness and flexibility at the same time. In addition, a new solvent for polyamide was

proposed which can be used for the preparation of the APCT. A computer-assisted

theoretical solvent selection method based on the Hansen solubility parameters was also

introduced. The findings of this research can increase the economic efficiency of the biofuel

production process by decreasing the initial investment. From a technical perspective, the

methods introduced in this thesis can encourage researchers in related fields to produce

same-polymer composites and find/replace solvent(s) in a more efficient way.

Keywords: textile bioreactor; biofuel, coated fabric; all-polyamide composite, polyvinyl chloride (PVC), solvent replacement, Hansen solubility parameters (HSPs)

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Anyone who has never made a mistake has never tried anything new.

Albert Einstein

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To my family, friends, and teachers,

for their constant support and unconditional love,

and,

in loving memory of my mother

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List of publications

This thesis is based on the results presented in the following research articles:

I. Jabbari, M., Åkesson, D., Skrifvars. M., Taherzadeh, M.J., Novel lightweight and highly thermally insulative silica aerogel-doped poly(vinyl chloride)-coated fabric composite. J. Reinf. Plast. Compos., 2015. 34(19): 1581-1592.

II. Jabbari, M., Skrifvars. M., Åkesson, D., Taherzadeh, M.J., Introducing all-polyamide composite coated fabrics: a method to produce fully recyclable single-polymer composite coated fabrics. J. Appl. Polym. Sci., 2015. 133(7): 42829.

III. Jabbari, M., Osadolor, O. A., Nair, R.B., Taherzadeh, M.J., All-polyamide composite coated-fabric as an alternative material of construction for textile-bioreactors (TBRs). Energies, 2017. 10(11): 1928.

IV. Jabbari, M., Skrifvars. M., Åkesson, D., Taherzadeh, M.J., New solvent for polyamide 66 and its use for preparing a single-polymer composite-coated fabric. Int. J. Pol. Sci., 2018. 28(1): 6235165.

V. Jabbari, M., Lundin, M, Hatamvand, M, Skrifvars. M, Taherzadeh, M.J., Computer-aided theoretical solvent selection using the simplex method based on Hansen solubility parameters (HSPs). J. Inf. Tech. Software Eng., 2018. 1(1): 6235165.

VI. Jabbari, M., Lundin, M, Bahadorikhalili, S, Skrifvars. M, Taherzadeh, M.J., Finding solvent for polyamide 11 using a computer software. Zeitschrift für Physikalische Chemie, 2019. doi:10.1515/zpch-2018-1299.

Additional publications not included in this thesis:

VII. Bátori, V., Jabbari, M., Åkesson, D., Lennartsson, P.R., Taherzadeh, M.J., Zamani, A., Production of pectin-cellulose biofilms: a new approach for citrus waste recycling. Int. J. Polym. Sci., 2017(1): 9732329.

VIII. Osadolor, O. A., Jabbari, M., Nair, R.B., Taherzadeh, M.J., Effect of media rheology and bioreactor hydrodynamics on filamentous fungi fermentation of lignocellulosic and starch-based substrates under pseudoplastic flow conditions. Bioresour. Technol., 2018. 263(1): 250-257.

IX. Bátori, V., Jabbari, M., Åkesson, D., Lennartsson, P.R., Zamani, A., Taherzadeh, M.J., Synthesis and characterisation of maleic anhydride-grafted orange waste for potential use in biocomposites. Carbohyd. Polym., 2018. 13(3): 4986-4997.

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Statement of contribution

My contributions to the above publications included:

I. Responsible for the concept, performing all of the experimental work, analyzing the

data, and writing the manuscript.

II. Responsible for the concept, performing all of the experimental work, analyzing the

data, and writing the manuscript.

III. Responsible for a portion of the concept, performing the experimental work (except the

fermentation part), analyzing the data, and a major portion of the writing.

IV. Responsible for the concept, analyzing the data, and writing the manuscript.

V. Responsible for the concept, performing all of the theoretical work, writing the codes

and developing the software, and writing the manuscript.

VI. Responsible for the concept, performing all of the theoretical work, and writing the

manuscript.

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TABLE OF CONTENTS

Abstract .......................................................................................................... III

List of publications ....................................................................................... VII

Statement of contribution............................................................................. VIII

Research Journey ............................................................................................XI

Chapter 1 ...........................................................................................................

1 Introduction ................................................................................................. 1

1.1 The incentive to perform this research ........................................................ 1

1.2 Thesis development ..................................................................................... 2

1.2.1 The aim of this study ................................................................. 2

1.3 Novelties of this thesis ................................................................................ 2

1.4 Terminology ................................................................................................ 3

Chapter 2 ...........................................................................................................

2 Bioreactors: materials of construction and requirements............................. 5

2.1 Bioreactors .................................................................................................. 5

2.1.1 Types of bioreactors .................................................................. 5

2.2 Requirements of bioreactors........................................................................ 7

2.2.1 Chemically inertness ................................................................. 7

2.2.2 Waterproofness and/or gas-tightness ......................................... 7

2.2.3 Temperature regulation and thermal stability ............................ 8

2.2.4 Heat and mass transfers ............................................................. 8

2.2.5 Resistance to external conditions ............................................... 9

2.2.6 Materials for the construction of bioreactors ............................. 9

Chapter 3 ...........................................................................................................

3 Collapsible tanks and textile bioreactors ................................................... 13

3.1 Collapsible tanks: portable/flexible containers.......................................... 13

3.1.1 Applications and design of collapsible tanks ........................... 14

3.2 Coated textiles ........................................................................................... 15

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3.2.1 Emerging alternatives: textile bioreactors ............................... 18

Chapter 4 ...........................................................................................................

4 Improvement in the thermal insulation of textile bioreactors .................... 21

4.1 Challenges associated with conventional bioreactors ................................ 21

4.2 Temperature fluctuations .......................................................................... 21

4.3 Insulation and thermal conductivity .......................................................... 23

4.3.1 Aerogel .................................................................................... 24

4.4 Improving the existing textile bioreactor .................................................. 25

Chapter 5 ...........................................................................................................

5 All-polyamide composite coated textile bioreactors ................................. 31

5.1 Challenges associated with existing textile bioreactors ............................. 31

5.2 Other challenges of the existing coated textiles ........................................ 32

5.2.1 Adhesion.................................................................................. 32

5.2.2 Recyclability and lifetime ........................................................ 32

5.2.3 Strength and weight ................................................................. 33

5.2.4 An alternative polymer: why polyamide? ................................ 33

5.3 A solution to the challenges of existing coated textiles ............................. 34

5.4 Strong adhesion: a possible theory ............................................................ 36

5.5 Mechanical properties ............................................................................... 41

5.6 The fermentation process in textile bioreactors ......................................... 43

Chapter 6 ...........................................................................................................

6 Solvent replacement .................................................................................. 47

6.1 Experimental ............................................................................................. 47

6.2 Theoretical: Hansen solubility parameters ................................................ 50

Chapter 7 ...........................................................................................................

7 Summary of the key findings and future directions ................................... 55

7.1 Future recommendations ........................................................................... 57

Acknowledgements ......................................................................................... 58

References ...................................................................................................... 61

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Research Journey

My first “scientific” experiment took place when I was around 10 years old. I

made a glue using Styrofoam and gasoline, for which I had obtained the recipe from a

DIY (do-it-yourself) expert who was my friend. It was a simple experiment, but it

made me more curious about materials and their characteristics. That curiosity drove

me to focus more on chemistry and physics at high school as well as choosing

chemistry as my subject of study at university. I was so fascinated with the chemistry

that I was dealing with as a part-time teacher. In my postgraduate studies, at the

University of Tehran, the same university that I received my undergraduate degree

from, I pursued my passion for chemistry in the field of polymers, working on the

cellulose membranes used in batteries. After finishing my master's thesis, I moved to

Sweden to work on material development for textile bioreactors, which was in its early

phase.

Like any other development in materials, the first and easiest step is the

modification of the existing material. This approach has plenty of merits, including the

lack of necessity to change the production line drastically. With that said, I started by

modifying the existing materials in polyvinyl chloride(PVC)-coated textile (PVCT),

trying to enhance the properties to make PVCT more suitable for its intended purpose.

In that step, I decreased the weight of the composite and its thermal conductivity (Paper

I). Being lightweight makes transportation easier, which leads to a decrease in costs.

Having a better thermal insulation property diminishes the need for an energy supply

to maintain the temperature of the bioreactor (composed of the PVCT), as the

microorganism responsible for producing biofuel within the bioreactor leads to the

outcome of the best efficiency at an optimum temperature (approximately 35 °C).

Moreover, since the bioreactors are mostly used outdoors, the temperature fluctuations

could negatively affect the microorganism, as they are susceptible to an abrupt

temperature drop/rise.

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Although the results were favorable for the modification of the existing coated

textiles used in making textile bioreactors, there were (and still there are) several

challenges involving PVCT including the low mechanical properties, energy-

consuming production process (due to the need for curing the coating), complex

formulation (due to having various constituents), recycling issues (PVC and polyester),

and last but not least, challenges associated with the adhesion between the coating

material (PVC) and the textile (polyester).

While I was trying to figure out how to mitigate the aforementioned issues, I started

with the question of ‘when will I have a better adhesion between the coating and the

fabric?’ My answer to this question was inspired from the old-fashioned chemist’s rule of

thumb for solubility: ‘like dissolves like’, meaning a solute will be dissolved in a solvent

that shares enough similarity to establish a high interaction. I applied the idea in this way:

if I choose a coating and a textile from the same kind of polymer, then either the effort to

establish the interaction between them (i.e., adhesion) will be less, or in the best scenario,

I will need to have only a simple process or use few chemicals. As I had to stick with the

company’s material selection, I only had two choices for the textile: polyester and

polyamide. The polyester textile did not work with the idea as it was not readily soluble,

which is a key factor for making same-polymer composites using partial dissolution.

Interestingly, polyamide 66 exhibited a good trade-off among the relative solubility

easiness, strength, and durability.

The early tests showed promising results. Then, I decided to develop a new

material that eventually led to a novel concept of preparing coated textiles from only

a single component (Paper II). I called the new material APCT, which stands for all-

polyamide composite coated textile. The simplest way was to make a film (as the

coating) out of the polymer and place it onto the textile and press it. It turned out that

using this method established somewhat weak interactions and led to a poor adhesion.

Then, I decided to dissolve the polymer and then applying it onto the textile. The

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results were even better than expected. The details of the method will be discussed in

the following chapters.

The APCT uses a ‘binder-free’ method to combine the coating and the fabric.

Being binder-free makes the material more industrially viable, i.e., there is no need for

extra chemicals for adhesion. As the process is carried out at room temperature, the

production of this material does not need complex machinery for heating/curing or

temperature control. Hence, the machinery can be simple compared to similar of its

kinds among conventional counterparts.

I made two textile bioreactors out of PVC-coated textile and APCT as well as

performing the biological process (fermentation) in them. Then, I analyzed the

performance and characterized the different properties of the two textile bioreactors.

The results showed that both textile bioreactors performed similarly in terms of

fermentation performance; however, the material characteristics were different. The

APCT showed superior properties over the PVC-coated textile in terms of mechanical

stability, chemical resistance towards the actual biological media, etc. (paper III).

The progress did not stop there. I tried to make the process of producing the

APCT more industrially-friendly. One of the challenges with the APCT was the

solvent being used. Formic acid, the sole solvent for the APCT production, is a strong

acid, which poses several challenges. It is odorous, corrosive, expensive, etc., which

hinders the scale-up procedure for the mass production of the APCT. In the next step,

I tried to decrease the usage of formic acid by replacing a significant portion of the

solvent with some industrially friendly chemicals, including CaCl2, urea, and water.

Therefore, I introduced a new solvent for polyamides (paper IV). The efforts regarding

the solvent search led to the introduction of a computer-aided theoretical solvent

selection model based on the Hansen solubility parameters for finding a solvent

replacement, which reduces the time and effort of the process significantly (paper V

and VI).

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Chapter 1

1 Introduction

1.1 The incentive to perform this research

The world population is increasing, and this explosion has led to a rapid

consumption of various resources and a tremendous increase in the volume of wastes

generated. Globally, approximately 17 billion tonnes of total solid wastes are generated

per year [1], and the amount is estimated to reach 27 billion tonnes in 2050 [2].

Continuous emissions of CO2, CH4, and other greenhouse gases from these waste streams

and the utilization of fossil fuels have led to a global environmental crisis. Furthermore,

intensive agriculture practices for producing food also damages the environment through

the use of chemical fertilizers.

Additionally, near 16% of the world population do not have access to electricity,

and approximately 38% use solid waste (forest residue, animal manure, crop, and other

wastes residues) for residential heating and cooking in poorly ventilated areas [3], which

results in environmental and health hazards. Concerns over these environmental pressures

and energy insecurity have increased the need for research about energy generation from

renewable sources, for example, biofuels. It is worth to mention, fossil fuels are not green

but remain inexpensive to replace.

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The cost of alternative energy sources must be reduced to compete with fossil fuels.

One way is to reduce the initial investment of biofuel production plants. The main cost of

a biofuel production plant is the primary investment in the land and infrastructure,

including utility, design, etc., among which the investment in the heart of the plant –the

bioreactor– is a significant figure [3]. The primary goal of this thesis was to develop a

new material of construction for producing a biofuel reactor, which can serve as an

alternative to the current stainless-steel-/concrete-based bioreactors. One approach to

reach this goal is the replacement of costly conventional bioreactors with cost-effective

textile bioreactors, and this is the topic of the thesis.

1.2 Thesis development

1.2.1 The aim of this study

The primary goal of the current thesis was to prepare a textile-based flexible

bioreactor. From a materials perspective, this thesis elaborates on the developments that

have been made to the textile-based bioreactor for aerobic and anaerobic reactions, using

fungi-based bioethanol fermentation within the textile bioreactor as a case study.

1.3 Novelties of this thesis

This thesis’s contribution is incorporating aerogel into an existing PVCT bioreactor

to enhance the insulation properties, as well as introducing a method to prepare a single-

polymer coated textiles composite through partially dissolving the fabric’s surface using

the solvent employed for the polymer solution. Additionally, a new solvent for dissolving

polyamide 66 and preparing the composite was reported in this thesis. Furthermore, a

computer-aided theoretical solvent selection was also introduced.

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1.4 Terminology

In our publications, we used ‘all polyamide-composite coated-fabric’ for the

composite developed during my PhD studies. The term was used interchangeably with

‘same-polymer composite’ as well. In this thesis, I used APCT, abbreviated from ‘all-

polyamide composite coated textile’.

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Chapter 2

2 Bioreactors: materials of construction and requirements

2.1 Bioreactors

A reactor, as a general term, is any container or vessel whereby one/multiple

reactant(s) is converted into product(s) in a controlled way. A bioreactor, a type of reactor,

is a vessel in which a chemical process is carried out that involves organisms or

biochemically active substances derived from such organisms [3]. This process can be

either aerobic (in the presence of air/oxygen) or anaerobic (in the absence of air/oxygen).

2.1.1 Types of bioreactors

Various types of bioreactors have been developed and are being utilized. The

continuous stirred-tank bioreactor design was introduced to achieve good mass transfer

and mixing, with the latter being carried out utilizing mechanical stirrers [3]. Airlift, bubble

column and fixed bed bioreactors were introduced for fermentation applications with

shear-sensitive microorganisms such as filamentous fungi [4]. For the bubble column

bioreactor design, mixing is carried out solely by aeration occurring in one direction, while

the airlift design uses aeration with mixing occurring in both the riser and down-comer

sections of the bioreactor [3]. Wave or single-use bioreactors were introduced for batch-

based fermentation applications, such as plant-based fermentation or shear-sensitive

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mammalian cell fermentation, in which the growth rate or productivity is too low to be

economic for continuous operation [5]. The wave design uses the rocking motion of the

chamber to create a wavelike motion in the fermentation medium inside the bioreactor.

Schematic representations of certain bioreactor designs are shown in Figure 2.1.

Figure 2.1 Common types of bioreactors: a) continuous stirred-tank reactor, b) bubble

column bioreactor, c) internal loop airlift bioreactor, e) external loop airlift bioreactor, (adapted from [6, 7]).

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2.2 Requirements of bioreactors

2.2.1 Chemically inertness

Although bioreactors are a specific type of chemical reactor, they have a key

feature that distinguishes them from other chemical reactors; bioreactors work with living

microorganisms, cells or enzymes [3]. A bioreactor should be inert to the underlying

biological and chemical process conditions [8]. As the fermentation process is a series of

chemical reactions, there are abundant chemicals involved in the process. According to

Glittenberg [9], seven sets of chemicals are added to or produced during the fermentation

process, including ethanol, amino acids, antibiotics, organic acids, polysaccharides,

vitamins, and enzymes.

Moreover, microorganisms are added to the medium to achieve the actual

fermentation. Some salts (e.g., nitrate salts as a nitrogen source) together with pH-

adjusting chemicals (such as sodium hydroxide) are also added to the process [8]. As a

result, the fermentation medium becomes a potentially harsh environment for the material

of construction of the bioreactor. The bioreactor’s material of construction should not

react with any of the constituents making up the cultivation media.

2.2.2 Waterproofness and/or gas-tightness

The presence of oxygen could be lethal for strictly anaerobic microorganisms.

Also, for an anaerobic process, possible bad odors leakage is an issue to consider [10].

Therefore, the bioreactor must be made out of gas-proof materials to make the chamber

gas-tight so that the air/gas(s) cannot pass through. In some cases, including the

production of biogas, the product(s) of the process is in gas form, and the gas-tightness of

the bioreactor becomes even more important.

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2.2.3 Temperature regulation and thermal stability

The bioreactor should provide adequate temperature control and be stable under

sterilization conditions [11]. Comparatively speaking, a system having adequate

insulation properties will decrease the heat loss and provide a more constant temperature

for the process. Controlling the temperature in a bioreactor, based on its heat transfer

requirement, can be achieved by using either an external heat exchange loop, an external

heating jacket or an internal vessel surface in the bioreactor. However, the design of such

a heat exchange element must be done in a way to avoid exposing the microorganisms to

heat shock or oxygen deprivation (for aerobic bioreactors) [12]. On the small scale, such

as with laboratory bioreactors, the challenge of sterilization is the limiting factor in the

material selection of the bioreactor, as the sterilization occurs at high temperature and

pressure. Other methods of bioreactor sterilization, including CIP (clean-in-place), WIP

(wash-in-place), SIP (sterilization-in-place), and high-pressure sterilization, are used for

large-scale bioreactors.

2.2.4 Heat and mass transfers

Heat transfer describes the flow of thermal energy due to temperature differences,

from one entity to another. When bioreactors are in operation, the overall goal in terms of

heat transfer is that the temperature should be maintained uniformly throughout the

bioreactor within the range needed for growing the microorganisms.

Mass transfer is the movement of mass from one position to another, as evidenced

by a concentration change. Good mass transfer in a bioreactor is important to ensure that

the transformation of the substrate to the product in the bioreactor proceeds as planned

[13]. The mass transfer can be enhanced by using mechanical stirrers, in the case where

the microorganisms are not sensitive to shear stresses.

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2.2.5 Resistance to external conditions

Bioreactors are usually placed outdoors and exposed to ultraviolet (UV) light from

the sun. Furthermore, the wind has the potential to exert a force on the bioreactor, which

eventually might rupture as a result. Temperature fluctuation between day and night as

well as among different seasons is another risk that threatens the lifetime of the bioreactor.

External microorganisms, as well as the roots of surrounding plants, are other sources

capable of decreasing the bioreactor’s lifetime. For large-scale bioreactors made out of

concrete, the risk of concrete corrosion is a key factor that must be taken into account. In

short, a bioreactor has to tolerate external factors, such as sunlight, rain, physical damage,

etc., up to a certain extent.

2.2.6 Materials for the construction of bioreactors

Bioreactors are usually constructed with materials such as stainless/carbon steel,

carbon steel and borosilicate glass, which are generally suitable for the growth of

fermenting microorganisms, inert and corrosion-proof. The materials used for

constructing bioreactors must be able to withstand the physiochemical conditions

encountered while running the bioreactor, as well as during clean-up and sterilization [14].

Table 2.1 lists some advantages and disadvantages of common materials used for the

construction of bioreactors.

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Table 2.1 Advantages and disadvantages of common materials of construction for bioreactors [14-17].

Material Advantage Disadvantage

Stainless steel 304

Cheapest of all stainless steels.

Can withstand high temperatures

and pressures.

Corrosion-resistant.

Long lifespan.

Quite expensive.

Carbon steel Cheaper than 304 stainless steel.

Corrosion and

contamination.

Time-consuming installation

process.

Concrete

bioreactors

More cost-efficient than steel-based

bioreactors.

Time-consuming installation

process.

Borosilicate glass

Transparent (perfect see-through

property).

Extremely inert to chemicals.

Fragile.

Limited scale.

Plastic Cheap.

Light-weight.

Leaks and short lifespan

Difficult transportation (as

small/mid-scale bioreactor)

Ceramic Chemically stable.

Wear-resistant.

Brittle.

Prone to thermal shock.

Coated textiles

Foldable. Portable. Flexible

geometry. Ease of setup.

Better corrosion resistance.

Relatively low cost. Good sterility.

More cost-effective than stainless steel.

Can be designed to have transparent

regions, for easy process monitoring.

Currently, a horizontal

vessel.

More susceptible to physical

damage (piercing cutting,

etc.) compared to stainless

steel.

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Concrete-based bioreactors (from concrete reinforced with steel rods) are common

in most plants (Figure 2.2a). They are generally more cost-effective than steel-based

bioreactors; however, the time-consuming installation process hinders concrete-based

systems usage in some plants. Stainless-steel bioreactors are fast to install and available

in many different sizes (Figure 2.2b). They mainly consist of carbon steel or stainless

steel. However, steel-based bioreactors are costly, especially when the annual

maintenance costs for material crack inspection are included. The high cost of

conventional bioreactors increases the capital investment of fermentation-based facilities.

For example, in ethanol production facilities, the bioreactor contributes 25–35% of the

fermentation-based capital investment cost [18].

Borosilicate glass is the most common material of construction for glass-based

bioreactors. The main perk of these bioreactors is their ‘see-through’ property (Figure

2.2c). Glass-based bioreactors are mostly used for low-/mid-scale fermentations, as the

building and reinforcing of large-sized shaped borosilicate glass is challenging. Other

materials that can be employed for making bioreactors include carbon steel, borosilicate

glass, polytetrafluoroethylene (PTFE) plastic, and ceramics [14].

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Figure 2.2 a) Concrete bioreactor under installation. b) A 5000 L stainless-steel

bioreactor. c) A pilot-scale glass bioreactor from Büchiglas.

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Chapter 3

3 Collapsible tanks and textile bioreactors

3.1 Collapsible tanks: portable/flexible containers

Bioreactors are basically tanks (or containers) with some additional features. Over

time, the need for storage, transportation and containment have led to the use and

development of tanks. Conventionally, tanks are rigid and possess a definite shape once

installed. However, certain applications that require easy installation, portability,

flexibility and short-term use have led to the introduction of collapsible tanks [19].

Collapsible tanks, also called pillow tanks, inflatable tanks, and bladder tanks, are

vessels made with light, easily deformable and flexible materials, thus, giving them a

naturally deformable shape defined by the nature of the material contained within them

[20]. The benefits of collapsible tanks over rigid tanks include ease of transportation, ease

of setup, portability, relatively low cost, ease of utilization, and multiplicity of application

[3, 19]. In recent decades, collapsible tanks have always been available as an option for

storing liquids. They are economically cheaper than conventional tanks [21]. Unlike rigid

tanks, collapsible tanks can be transported while folded, which makes them a logical

choice for applications in remote sites (e.g., mining exploration camps) and temporary

installations (e.g., military operations).

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Figure 3.1 Collapsible tanks containing different fluids: a) 100 m3 pillow-shaped filled

with gas (FOV Biogas AB), b) pillow-shaped filled with liquid, c) round-shaped with a self-reinforcement at the top and d) rectangular-shaped with structural support.

3.1.1 Applications and design of collapsible tanks

Collapsible tanks are used for several purposes such as waste storage, fuel storage,

water storage, chemical storage, and more recently, as bioreactors [8, 16, 22].

Additionally, an important distinguishing feature of collapsible tanks is that they can be

designed for use in nonconventional or customized applications in unusual geometries.

Once the collapsible tank is formed, its natural morphology when filled with a fluid

becomes similar to that of a pillow. Sometimes, the fluid or material to be contained exerts

a high tension on the joints of the collapsible tank [3]. To overcome this challenge,

particularly for medium-scale tanks, the joints are usually framed, or extra self-support is

added to the collapsible unit (Figure 3.1). Additionally, the natural pillow morphology

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can be changed by using different methods such as having metallic reinforcement at the

joints of the construction material, putting the collapsible tank inside a rigid material to

define the shape or using adhesives to join the material of construction to the desired

shape.

Considering the properties of the collapsible tanks, they are potential candidates as

the material of construction of bioreactors. However, their structure needs to be adjusted

to conform to this application. Collapsible tanks comprise coated textiles which consist

of a fabric coated with other polymer(s) to make it water-/gas-proof, chemical resistant,

decorative, etc.

3.2 Coated textiles

Coated textiles are flexible composite materials comprising two main components:

a textile and one or more polymer coatings on the textile, which makes the material

impenetrable to liquids, gases, solvents, etc., that protect the textile as well. This

configuration provides additional properties and functionalities to the system. The textile

substrate contributes to the mechanical strength of the composite, whereas the polymer

coating(s) helps to introduce penetration resistance and impermeability (to liquids, gases

and dust particles), as well as improve the fabric abrasion strength.

A typical structure of coated textiles is shown in Figure 3.2. It usually consists of

a base fabric as the backbone of the composite in the center (when the textile is coated on

both sides) or in the bottom/top (when the textile is coated on one side) together with (an)

adhesion layer(s) to increase the bonding between the fabric and the next layer. A topcoat

with the intended color or special properties, for example, weathering-resistance, is

usually included. Synthetic leather is an everyday-use example of a coated textiles. Coated

textiles are popular mainly due to their affordable price, high strength, durability,

16

resistance to wear and tear, good toughness, various colors, and soft texture [23]. The

types of fiber commonly used for this purpose are polyester, cotton, and rayon, depending

on the end-use requirements [24]. For applications where high strength is required,

polyamide and polyester are used, as they possess considerably high strength-to-weight

ratios [25].

Figure 3.2 A schematic structure of coated textiles (adapted from mehler-

texnologies.com).

Coated woven fabrics are exploited in a wide range of structural applications to

provide lightweight, architecturally striking solutions [26]. They are often used for wide-

span surfaces, membrane-cable structures, hanging roofs (such as roofs of sports

structures and stadiums) and pneumatic constructions [27]. Coated textiles are widely

employed for permanent works in various applications, such as transportation and

commercial constructions [28]. Polymer-coated textiles are extensively used in clothing,

agriculture, construction, sports and leisure, inflatable structures, and medical

applications. Some examples of these applications are illustrated in Figure 3.3.

17

Figure 3.3 Examples of different applications of coated textiles: a stadium hanging roof,

a military (camping) tent, an airbag, ventilation vanes, and inflatable structures at an amusement center (Cape Cod Inflatable Park).

The combination possibilities between the textile and coating are almost infinite

and are dependent upon the final use. The polymer coating can be on one (or both) side(s)

of the substrate (with identical or different types of polymers on each side) [24], whereas

the substrate can be a woven, knitted or even nonwoven textile material. For structural

18

applications where a substantial load is exerted onto the composite, a woven fabric is

used, while for nonstructural applications, a knitted fabric can be utilized.

Polyvinyl chloride-coated textile (PVCT) is the material of construction for the

existing textile bioreactor, produced by FOV Biogas AB, Borås, Sweden. The PVCT,

comprising a PVC coating on polyester fabric, has been used and tested starting from the

1950s [29]. Polyester fabric is one of the most common fabrics employed in making

coated textiles. A commercially available polyester fiber is polyethylene terephthalate

(PET) [24].

3.2.1 Emerging alternatives: textile bioreactors

Because of the aforementioned reasons, alternatives to conventional bioreactors

have emerged over time. To address some of these challenges, in recent decades, several

biopharmaceutical and protein production facilities have started employing single-use or

disposable bioreactors made from polymeric materials [30]. These bioreactors are entirely

made of flexible materials, yielding a natural pillow shape when the reactors are filled

with liquid [3] while they can also be kept inside rigid vessels to provide the desired

geometry and support [31]. As a non-single-use flexible bioreactor, in 2013, a collapsible

tank constructed with a textile as its backbone material of construction was introduced for

biogas production [22]. Replacing the steel/concrete/glass with textiles imparts the

bioreactor with a greater movability, cost-effectiveness and a fast installation. Figure 3.4

presents a picture of one of the textile-based bioreactor vessels. The product was

manufactured by a local company, FOV Fabrics AB, Borås, Sweden.

19

Figure 3.4 Actual and schematic picture of a dome-shaped textile bioreactor (top) [22],

and a laboratory-scale prototype of a pillow-shaped textile-based bioreactor (bottom) equipped with a heat exchanger underneath. Both are made of a PVC-coated polyester fabric produced by FOV Biogas AB, Sweden.

20

Textile bioreactors are foldable (Figure 3.5); hence, they possess a high portability

prior to installation at the biofuel production plant.

Figure 3.5 A 100 m3 pillow-shaped textile bioreactor prior to installation (FOV Biogas AB).

Although single-use and textile bioreactors are both made from polymeric

materials, textile bioreactors must not be regarded as similar to single-use bioreactors.

The single-use bioreactor presents limited applications for several reasons such as the

limited scale of its application (below 2 m3 on its own), unsuitability for continuous

production and the high potential for incurring damage [30]. In single-use bioreactors,

there is no structural reinforcement, while in textile bioreactors the material of

construction is reinforced with textiles. As their name implies, such bioreactors are used

only once and then discarded, and this restriction makes them unsuitable for facilities with

high production rates or for long-term continuous production applications [3]. Due to their

structure, single-use reactors have limited scalability [32].

21

Chapter 4

4 Improvement in the thermal insulation of textile bioreactors

4.1 Challenges associated with conventional bioreactors

Similar to any other chemical process, the fermentation process within a bioreactor

also has an optimum temperature.

4.2 Temperature fluctuations

Microorganisms require an optimum temperature for growth, and their active

performance during the biogas process can be divided into psychrophilic (optimum

temperature approximately 10 °C), mesophilic (optimum temperature ca. 30-40 °C),

thermophilic (optimum temperature approximately 55 °C) and hyperthermophilic

(optimum temperature approximately 85 °C).

Temperature fluctuations adversely affect the performance of, for instance, a

biogas process. A decrease in temperature may result in the reduced volatile fatty acid

production rate, substrate decomposition rate, and metabolic rate of the microorganism

[33]. For a given operational temperature type, the fluctuation of a few degrees of

temperature can have a severe impact on methane yield, as microorganisms adjust to one

22

certain temperature and re-adaptation corresponding to a different temperature requires

an alternated microbial structure [34]. Noticeably, a variation in mesophilic temperature

of ±4 °C and thermophilic temperature of ±1 °C was found to result in a sharp decrease

in biogas yield [35, 36]. Therefore, it is necessary to keep the temperature fluctuations

within the bioreactor to a minimum for a stable and consistent process. In conventional

bioreactors (e.g., stainless-steel bioreactors) this is achieved using external insulation

materials (Figure 4.1). In the case of flexible materials (collapsible tanks) using external

insulation materials is very difficult, if not undoable. However, this goal can be achieved

through an improvement in the insulation properties of the material of construction of the

textile bioreactor (coated textile) to decrease the thermal conductivity of the material.

Figure 4.1 A 304 Stainless-steel bioreactor enclosed in an insulation jacket to

avoid/decrease temperature fluctuations (adapted from www.pitt.edu).

23

4.3 Insulation and thermal conductivity

The passage of thermal energy through an insulating material occurs through three

mechanisms: solid conductivity (conduction), fluid conductivity (convection), and

radiative transmission (also known as radiation). The sum of these three components gives

the total thermal conductivity of the material [37] (measured in W/m.K, Watt per meter

per Kelvin). The less thermal conductivity a material has, the lower its value in W/m.K

will be. Table 4.1 summarizes the thermal conductivity values of some common porous

insulation materials. To increase the thermal insulative property of the PVCT, aerogel,

the most insulative material known, could be incorporated into the material (Paper I).

Table 4.1 Thermal conductivity values of some commonly used insulation materials (Paper I)

Insulation material Thermal conductivity

(W/m.K)

Mineral wool 33–40

Expanded/extruded

polystyrene 30–40

Loose-fill cellulose fiber 39–42

Foam glass 39–45

Aerogel 5–100

Enova aerogel IC3120 12

24

4.3.1 Aerogel

Aerogel is a synthetic ultralightweight material that has an excellent insulation

performance because of its nanoporous structure (several to tens of nanometer) [38-40].

Silica aerogels are a three-dimensional network of silica particles, which are obtained by

extracting the liquid phase of silica gels [41] and replacing it with a gas; therefore, they

are highly porous solids that hold gas (usually air) within pores or network of solid

substances [42]. Silica-based aerogel is an ideal thermal insulator with a composition of

up to 99% associated air [43]. It exhibits many intriguing and unique properties, which

include an extremely low thermal conductivity (5–100 mW/m-K), very low density

(0.003–0.5 g/cm3), high porosity (80–99.8%), high inner surface area (500–1200 m2/g),

ultralow dielectric constant (k = 1.0–2.0) and low index of refraction (∼ 1.05) [41, 44-

48].

Figure 4.2 Demonstration of the superinsulation property and lightness of silica aerogel.

Aerogels have been used in applications, such as windows and glasses, to save

energy [49]. Because of the aforementioned fascinating and extraordinary properties,

silica aerogels have found an excellent potential for applications in many fields such as

adsorption, window insulating systems, drug delivery systems [50-54], and thermal

superinsulators [55, 56]. They also are employed in thermal insulation systems for

25

aerospace, systems for environmental cleanup and protection, heat storage devices,

transparent insulative windows systems, thickening agents in paints, etc. [41]. Silica

aerogel also has many commercial applications, such as acoustic barriers and

supercapacitors [57]. As a result of their lightness, low heat conductivity, and large inner

surface area, aerogels are useful in many ways such as particle filters, particle trappers,

and catalyst supports [42, 58].

4.4 Improving the existing textile bioreactor

To improve the insulation property of the PVC-coated textile and to decrease the

weight of the composite, an aerogel was incorporated into the PVC plastisol before the

coating process (Paper I). The aerogel was selected as an insulating constituent in the

coated textile due to its particular characteristics. This additive had not been incorporated

in coated textiles before our experimental work. According to the results of the density

measurement and the transient plane source method of thermal conductivity measurement

(Figure 4.3), significant improvements in both reducing the thermal conductivity and

decreasing the density by 26% (from 205 to 152 mW/m-K) and 17% (from 1.132 to 0.941

g/cm3), respectively, were achieved. As mentioned earlier, improving these properties can

enhance the features of the textile reactor made out of PVCT.

26

Figure 4.3 (a) Density values for 0–4% aerogel-containing PVC-coated textile

composites; (b) thermal conductivity values for PVC/aerogel composite-coated textiles (adapted from Paper I).

In composite preparations, a good dispersion of the particles is crucial to

maintaining the uniformity of the properties. Scanning electron microscopy (SEM) did

not show any agglomeration of the silica aerogel particles on the surface of the PVCT

composite (Figure 4.4).

27

Figure 4.4 Scanning electron microscopy (SEM) micrograph of the aerogel-doped PVC-

coated textile composites with different aerogel percentages: (a) 0% (neat composite); (b) 2%, no agglomeration; (c) and (d) 3%, very slight agglomeration; and (e) and (f) 4%, more agglomeration (adapted from Paper I).

28

Introducing silica aerogel, which is extremely hydrophobic, into the PVC-coated

textile, makes the ultimate composite more hydrophobic. The untreated PVC-coated

textile exhibits a water contact angle of 76.02°, while with the addition of 3% silica

aerogel, the value increases by approximately 17% to 88.67° (Figure 4.5)

.

Figure 4.5 Water contact angle values (above) and pictures of the 0–4% aerogel-

containing PVC-coated textile composites (below) (adapted from Paper I).

29

Therefore, silica aerogel causes the composite to become more hydrophobic and

hence less prone to environmental failure due to humidity (Paper I), which is in addition

to the improvement in thermal insulation property and weight decrease. The existing

textile bioreactor made out of PVC-coated textile was improved in terms of thermal

insulation, weight and surface properties. However, this material continued to present

certain challenges that will be addressed as follows.

30

31

Chapter 5

5 All-polyamide composite coated textile bioreactors

5.1 Challenges associated with existing textile bioreactors

The non-single-use flexible bioreactor uses PVC-coated textile as the material of

construction; hence, it is mechanically strong and has a good scalability [16, 22]. PVC is

dimensionally stable, largely non-flammable, and resistant to weathering, but it possesses

a limited thermal stability and it is prone to be attacked by many solvents [59]. The coating

(PVC) and the textile (polyester) exhibit a high density (compared to that with other

common polymers). The low thermal stability of PVCT makes the thermal sterilization of

the bioreactor difficult. The fermentation medium contains various solvents and

chemicals that can chemically attack the PVC in the long term. Furthermore, the thermal

insulation of the composite can also be improved. The first part of our experimental work

involved improvements in the thermal insulation, density, thermal stability, and to some

extent, surface hydrophobicity by the incorporation of a highly porous material into the

plastisol formulation (paper I).

32

5.2 Other challenges of the existing coated textiles

This section elaborates the challenges of the existing coated textiles, mostly the

difficulties associated with adhesion, weight and strength, and recyclability after the

lifetime of the material is reached. In the following sections, a solution to address these

issues is proposed. One drawback of the materials of construction of collapsible tanks, in

comparison with those of rigid tanks, is that they are prone to failure due to the low

material strength of the employed polymers (compared to, e.g., stainless steel) or to the

separation of the materials of construction where they are welded or joined together. This

failure might affect the sealing properties of the joints.

5.2.1 Adhesion

Adhesion is the state in which two surfaces are held together at an interface by

forces, an interlocking action or both [60]. Adhesion between different components in a

composite is an important parameter to achieve good composite properties. Furthermore,

although the components usually have good adhesion between them, they sometimes

exhibit a poor matrix–fiber adhesion due to the chemical incompatibility of the

components [61, 62]. Poor adhesion results in failure caused by the insufficient stress

transfer between the binder and the reinforcing fabric. This is also the case when

producing coated textiles. To obtain good adhesion between a coating and fabric, it is also

necessary to select the right coating binder that can adhere the coating to the fabric. There

are several ways to increase the adhesion (e.g., using linker chemicals, surface treatments,

etc.), but they are often associated with drawbacks, including high cost, complicating the

formulation, degrading the fabric (to be bonded to the binder), etc [24].

5.2.2 Recyclability and lifetime

Heterogeneous composites pose a recycling challenge [61] as they consist of at

least two different components. The growing interest in the recycling of materials is

33

brought about by the desire to preserve the environment, as there is limited landfill space

due to the abundance of waste that is being dumped [25]. Global warming is also of

concern due to incinerator emissions [63]. These negative impacts can be reduced by

recycling the products that would otherwise go into landfills [64, 65]. In addition, a strong

need to reduce the energy requirements of the recycling process also exists [61]. This

impetus has stimulated interest in the development of environmentally friendly materials

[66, 67].

A textile bioreactor has a lifetime that depends on the materials used to produce it.

From the resource recovery point of view, when the lifetime is over, the textile bioreactor

needs to go through a recycling process. If the textile bioreactor is made of only one

component, there is no need for the separation of components (when possible), and it can

be used again as a single-component material.

5.2.3 Strength and weight

The weight of the final composite is also another challenge. PVC has a density of

approximately 1.4 g/cm3, almost the same as that of the polyester fabric (the other

constituent in the PVC-coated textile). A number of polymer candidates for the

replacement of PVC and polyester exist. However, I decided to use polyamide 66, as it

has a low density (1.14 g/ cm3) and strong mechanical properties (Paper II).

5.2.4 An alternative polymer: why polyamide?

Polyamide is a well-known high-performance engineering plastic with a high

strength and good fatigue resistance [68], as well as excellent mechanical and physical

properties [69]. Generally, polyamides have a high thermal stability and excellent

properties such as a high impact strength, tear resistance, low coefficient of friction and

34

excellent tensile strength [70]. Such good properties are due to strong intermolecular

hydrogen-bond interactions [70]. Infrared spectroscopy has shown that almost all of the

amide groups in polyamide 66 are linked together via H-bonding at room temperature

[71]. The high strength, elasticity, and abrasion resistance enable polyamide to be used

for a variety of industrial end uses, including filter fabrics, nets, webbings, cordages,

parachutes, ropes, and ballistic fabrics [24]. Polyamides have a wide range of applications,

from an aroma-proof, transparent packaging film to hydraulic lines, metal coatings, and

onto cable pulleys, bearing boxes and fuel oil storage tanks [72]. Due to their mechanical

properties and their barrier behavior against gases and aromatics, they are widely used in

films for flexible food packaging [73]. In the family of polyamide fibers, polyamide 66

fiber possesses relatively better mechanical properties, and the polymer is applied in the

production of tires, airbags, bullet-proof vests [74], and more [68]. Polyamides’

properties, more importantly, their tensile and impact strength, relatively low density,

good fatigue resistance, and gas barrier characteristics, make the polymer a good

candidate to coat textiles for use in bioreactors.

5.3 A solution to the challenges of existing coated textiles

To address some of the challenges of the existing coated textiles, I decided to use

polyamides to make a coated textile as the material of construction for the bioreactor.

Polyamides possess better mechanical properties and lower densities than do polyester

and PVC.

One promising approach to enhance composite recyclability is choosing the

composite components out of one sort of polymer, aka single-polymer composite (SPC)

[63], which identifies an emerging class of materials that has specific economic and

ecological advantages [61]. These materials are often described as one-polymer

composites, homocomposites, all-(the same-)polymer composites, self-reinforced, or

35

homogeneous composites [61, 63]. Such composites represent the correct alternative to

traditional composites because both the reinforcement and matrix are from the same

polymer; therefore, recyclability is enhanced [75]. Apart from recyclability, the interest

in the concept of SPCs is based upon the premise that interfacial bonding should improve

when matrix and reinforcement are made from the same polymer [67, 76, 77]. The

employed method used the noticeable difference in melting temperatures between the

high-density polyethylene (HDPE) matrix and the HDPE reinforcement to fabricate an

HDPE homocomposite. SPCs were first introduced by Capiati and Porter [78]

approximately four decades ago. Among these composites, all-cellulose and all-

polypropylene composites are probably the most well-known ones. All-polypropylene

composites have a broad processing window, and their exceptionally high fiber volume

fraction of approximately 90% makes them extremely competitive with conventional

glass-fiber-reinforced composites of substantially lower fiber loadings [79].

One possible method to increase the adhesion (i.e., the holding forces, interlocking

action or both between two surfaces together at the interface) involves using the same

material in the coating of the polyamide fabric and preparing an SPC-coated textile. By

using this method, the surface of the fabric is partially dissolved by the solvent from the

coating solution to entangle the polymer chains of the coating with the dissolved polymer

chains of the fabric’ surface. As the two components (fabric fibers and the coating) have

the same nature and as there is the same solvent between them, the adhesion (which in

this case is a nonreactive adhesion) could be strong. In addition, there is no need to use a

linker material between the coating and the solvent. By this method, a novel all-polyamide

composite coated textiles (APCT) can be obtained.

36

5.4 Strong adhesion: a possible theory

The theory/mechanism behind the excellent adhesion (Paper II) is proposed in this

way: the solvent in the solution penetrates the fabric and swells to the surface of the fabric

because it is capable of dissolving the fabric as well (Figure 5.1, b). Once the chains at

the surface of the fabric are solvated, the chains in the solution interpenetrate the chains

of the solvated surface of the fabric, becoming entangled (Figure 5.1, c). After a certain

time (ca. 30 s), the sample is immersed in water (or any other nonsolvent) which extracts

the solvent from the solvated polymers, fixing the chains in position. As a result, the

coating and the fabric are fused together without using any chemicals or bonding agent.

Figure 5.1 Mechanism proposed in this thesis for adhering the coating to the textile in an

all-polyamide composite coated textiles.

37

Figure 5.2 shows the adhesion between the coating (the non-filamentous part) and

the fabric (the yarns). It is clear that the adhesion in APCT (b, c, and d) is stronger the

adhesion in the PVC-coated textile (a). In the case of APCT, the adhesion is not only

between the fabric and the coating, as some of the yarns closer to the surface appear fused

together as well ("merged filaments" in Figure 5.2). This feature can enhance the coating

properties, meaning that a less thick coating would be needed to make the composite gas-

proof.

38

Figure 5.2 Cross-sectional SEM pictures of PVCT (a) and APCT (b, c). The fused parts

are shown with the ovals (d). In APCT, the first row of the fiber-filaments of the polyamide fabric

39

is merged and adhered to the coating. This arrangement will decrease the chance of delamination over a long span of time (adapted from Paper IV).

All-polyamide composite is a composite in which both the reinforcement and the

matrix are made from one kind of polymer. The reports regarding all-polyamide

composites in the literature describe the use of two different polyamide grades with

different melting points, which have been used to produce laminates [68, 80, 81]. All-

polymer composites, with both constituents made from the same polymer, are easily

recycled by mechanical methods, as they are composed of the same polymer.

The all-polyamide composite coated textile (APCT) that I have introduced during

my PhD is a specific form of single polymer composite intended to replace traditional

coated textiles with a good interfacial adhesion and enhanced recyclability. The composite

is fully recyclable since it contains no other materials except polyamide 66. The assembly

has a wide range of semi-structural applications to provide lightweight, architecturally

striking solutions as well as wide-span surfaces, membrane-cable structures, hanging

roofs, pneumatic constructions, water-/gas-proof textile reactors, temporary houses and

tents, facade coverings, container linings, and tarpaulins.

40

Figure 5.3 Cross-sectional scanning electron microscopy images of all-polyamide

composite coated textile samples. From top to bottom, 17%, 23%, 29%, 35%, 41%, 47%, and 53% polyamide dope concentrations, and from left to right, 15 s, 30 s, and 45 s gelling times.

41

The micrographs of the specimens were taken after quenching them in liquid nitrogen and breaking by hand. The magnification is 500X for all of the micrographs (adapted from Paper II).

Full recyclability and low recycling cost are additional advantages of the all-

polyamide composite coated textiles. Possibility of using waste polyamides to make the

coating solution and prepare the APCT is also beneficial (finding an application for a

waste: fabric waste). As the entire composite is made of a single polymer, separation is

not required for recycling the material, after its life span is met.

5.5 Mechanical properties

Partial surface dissolution of the fabric in the composite might decrease the overall

strength of the composite. However, the results (Figure 5.4) show that the decrease in

mechanical properties is negligible for the optimum solution concentration and waiting

time (Paper II).

42

Figure 5.4 Max force, elongation at max load and thickness from the tensile test of the

APCTs, as well as the fabric, for different solution concentrations at 15 s, 30 s, and 45 s gelling times. The variations can be attributed to several factors, yet mainly involve the dissolution power of the dopes. “Fab” stands for fabric, and it has been added to the graph for comparison purposes (adapted from Paper II).

43

The thickness of the composite is related to the amount of added coating (the

polymer solution), the waiting time before the phase change (immersion in water) and the

concentration of the polymer solution. The final thickness varies from approximately 1.2

times to twice the initial thickness of the fabric (Figure 5.4).

5.6 The fermentation process in textile bioreactors

Ultimately, the coated textiles discussed in this thesis were used as the material of

construction for textile bioreactors to run a fermentation process (Paper IV). The two

textile bioreactors applied showed equally the same performance in the process (Figure

5.5). Therefore, we can conclude that APCT is inert towards the microorganisms in the

fermentation process and is capable of maintaining the process.

Figure 5.5 Fermentation performed in the lab-scale prototype of a textile bioreactor at 30 °C

showing sucrose (circle) and ethanol (square) as the primary axis and glycerol (triangle) as the secondary axis (adapted from Paper III).

Having the same performance in the fermentation process for both bioreactors

might seem to suggest there is no considerable influence of the material type (APCT or

44

PVCT) as the material of construction for bioreactors. However, apart from the

aforementioned prominence of APCT in terms of environmental effects, material-wise,

the lifetimes of the two reactors differ significantly. According to Table 5.1 (Paper IV),

compared with the PVC-coated textile, the APCT performed better in the presence of

chemicals common to the fermentation process during the ageing test.

Table 5.1 Tensile strength comparison of PVC-coated polyester fabric and all-polyamide composite coated textiles (APCT). Un-aged samples represent the material before ageing (adapted from Paper III).

This characteristic can be beneficial in terms of a lifetime extension for the textile

bioreactor made by using APCT.

45

Figure 5.6 The comparative density values of PVCT and APCT. PVCT is composed of

two main constituents: PVC and polyester. These polymers are both heavier (in mass/volume unit) than polyamide, the sole component of APCT, resulting in a lighter final material when composed of the latter (adapted from Paper III).

Additionally, APCT is approximately 16% lighter than the PVC-coated textile,

which is logistically preferable as the material of construction of textile bioreactors

(Figure 5.6). Furthermore, less material per cubic meter of the textile bioreactor is

required to build, making it more cost-efficient.

PVC coated textile, 1.42

All-polyamide composite

coated textile, 1.19

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

Dens

ity (g

/cm

3)

46

47

Chapter 6

6 Solvent replacement

Although all-polyamide composite coated textile (APCT) is environmentally

friendly in terms of recyclability, it uses formic acid (methanoic acid) in its preparation.

I tried to replace this solvent with some less-harsh solvents. For this purpose, one

experimental and one theoretical study were carried out.

6.1 Experimental

Solvent selection is a crucial step in all solvent-involved processes. The solution

processing of aliphatic polyamides is quite challenging because only a few solvents, such

as formic acid and cresol [82] or fluoric solvents [83], can dissolve them, all of which

have environmental challenges. A few attempts have been made to replace the existing

solvents or to propose new solvents for aliphatic polyamides. Papadopoulou et al. [82]

mixed formic acid with trifluoroacetic acid and acetone. Charlet et al., [84], studied the

dissolution behavior of polyamide 6-water systems under pressure. Nirmala et al., [85],

used formic acid (85 wt%), acetic acid, dichloromethane, 1,1,1,3,3,3-hexafluoro-2-

propanol, trifluoroacetic acid, and chlorophenol in their study.

48

Basically, dissolving polyamides is difficult due to two reasons: (a) polyamides are

highly crystalline, and (b) solvents for polyamides are believed to act by virtue of strong,

highly specific polar forces, especially hydrogen bonds [86]. Aside from these limitations,

a fast dissolution in the production process of APCT is required, and the only solvent that

can provide favorable results is FA. Hence, I tried to replace a substantial portion of

formic acid in the ‘solvent’ mixture. Finally, a mixture of FA/CaCl2/urea (FAUCa) was

made to dissolve the polyamide 66. Urea imparts the hydrogen bonding, and calcium ions

from the calcium chloride, a Lewis acid, was added to the system to compensate for the

pH decrease due to the presence of urea. The results show that the proposed solvent,

FAUCa, can readily dissolve polyamides, resulting in a negligible decrease in the

mechanical properties during the dissolution (Paper IV). The adhesion between the fabric

and the coating is also good (Figure 6.1), as the new solvent is sufficient at dissolving the

fabric and penetrating into it.

49

Figure 6.1 Selected cross-sectional SEM micrographs of the APCT composite prepared

using FAUCa. Strong adhesion is seen between the coating (the top film) and the fibers from the fabric. The fibrils of the fibers closer to the coating are fused to each other (adapted from Paper IV).

The composite prepared using the FAUCa exhibits almost the same properties as

the one prepared using the formic acid solution. The solution was applied to a polyamide

66 fabric to make an all-polyamide composite coated textiles, which was then

characterized. The FAUCa solution has a higher viscosity than that of the one prepared

using the neat formic acid solvent; this property can be an advantage in applications that

need a high viscosity. A more viscous solution produces a denser coating, which will

increase the water/gas impermeability. In conclusion, experimentally, a mixture of

calcium chloride, formic acid, urea and water, which can dissolve polyamide with ease

and has the potential to be used in mass production of APCT, was proposed in this thesis

50

(Paper IV). Using the FAUCa solvent presents a significant merit: the replacement of 40%

formic acid with less harmful and environmentally friendly chemicals.

6.2 Theoretical: Hansen solubility parameters

Solvents, defined as substances able to dissolve or solvate a separate species, are

commonly used in many industries and applications [87]. For any solvent-based process,

the best-suited solvent or solvent-mixture must be selected [88]. On the other hand,

solvent selection and design is a complex process, which requires decision making on

several levels to identify the best candidates, and is dependent on different multiobjective

criteria, namely, environment, health, safety, process feasibility and economics [89].

Currently, solvent selection relies significantly on previous experience, i.e., trial and error

with different solvent candidates. The use of experimental thermophysical properties

stored in a reference database for the selection has an advantage in that the results are very

reliable; however, the solvent selection is limited to the experimental data pool [88]. Such

a heuristic approach, while valuable on its own, is arguably not fit to deal with a complex

multicriteria optimization and search problem, which is the case for solvent selection [89].

On the other hand, actual (physical) trials of mixing different solvents and checking

the solvation in the laboratory, is a tough and time-consuming job. A number of modern

tools are increasingly becoming available to reduce the efforts needed to select the right

solvent [90]. The use of prediction models has the advantage that for the selection

procedure, any solvent can be considered for which the required group interaction

parameters are available, and by using predictive methods, an extended variety of solvents

can be taken into account for selection [88].

Several graphing and modelling techniques have been developed to aid in the

prediction of polymer solubility [91]. The basic principle has been “like dissolves like”,

51

meaning that polymers will dissolve in solvents whose solubility parameters are not too

different from their own [92]. Solubility parameters help attach quantitative values to

qualitative textual data [93]. By 1950, Hildebrand had defined the solubility parameter as

the sum of all the attractive intermolecular forces, which he found to be empirically related

to the extent of the mutual solubility of many chemical species [94]. However, solubility

behavior cannot be accurately predicted by only the Hildebrand solubility parameter [91].

In 1967, Charles Hansen improved the concept and introduced his three-

dimensional solubility parameters [95]. The Hansen approach provides an empirical yet

effective [96] method for determining the dissolution possibility of a solute. The Hansen

solubility parameters (HSPs) have been used for many years to select solvents for coating

materials [92].

Some authors have proposed methods to find a proper solvent mixture for very

specific applications such as electrospinning [97]; however, a more general method that

is applicable to a broader range of processes appears necessary to propose. Certain

publications used HSPs to predict solvent systems that are likely to dissolve, e.g.,

Aghanouri et al. [98], but these approaches are empirically based and not computer-aided,

meaning they cannot consider a broad range of solvents. Nelson et al. [99] developed a

computer-based formulating technique that allows for the selection of minimum-cost

solvent blends, but the approach is not capable of suggesting a solvent substitution or

solvent mixture for a solute with known HSPs. Moreover, the authors used the Hildebrand

solubility parameters, which have been updated and replaced by the more reliable and

more accurate values of HSPs. As there has been no report regarding a general computer-

aided method of finding a solvent-mixture for a solute with known HSPs, I proposed a

computer-aided selection of solvents out of a vast number of solvents’ HSPs values stored

in a database (Paper V).

52

The Hansen model is usually considered as a sphere. The center of the sphere

consists of the 𝛿𝛿𝛿𝛿, 𝛿𝛿𝛿𝛿, and 𝛿𝛿ℎ values of the polymer (solute) [91]. is square root of the

cohesion energy density; 𝛿𝛿𝛿𝛿, 𝛿𝛿𝛿𝛿, and 𝛿𝛿ℎ represent the dispersive forces, polar inteactions,

and hydrogen bonding, respectively. The radius of the sphere, RO, is termed the interaction

radius [91]. The values of RO have been reported for some polymers in the literature. RA

is the distance in HSP space between the solute/polymer and the solvent [95]. The

boundary of the spherical characterization is based on the requirement that ‘good’ solvents

have a distance from the center of the sphere, RA (also termed the solubility parameter

distance), that is less than RO [91], where 𝛿𝛿𝛿𝛿𝑓𝑓 , 𝛿𝛿𝛿𝛿𝑓𝑓 , and 𝛿𝛿ℎ𝑓𝑓 are the Hansen solubility

components for the polymer/solute (our favorite values) and 𝛿𝛿𝛿𝛿𝑠𝑠 , 𝛿𝛿𝛿𝛿𝑠𝑠, and 𝛿𝛿ℎ𝑠𝑠 are the

Hansen solubility components for the solvent [91]. Equation 1 was developed from plots

of experimental data, where the constant ‘4’ was found convenient and correctly

represented the solubility data as a sphere encompassing the good solvent [91].

𝑅𝑅𝐴𝐴 = √4 × (𝛿𝛿𝛿𝛿𝑠𝑠 − 𝛿𝛿𝛿𝛿𝑓𝑓)2 + (𝛿𝛿𝛿𝛿𝑠𝑠 − 𝛿𝛿𝛿𝛿𝑓𝑓)2 + (𝛿𝛿ℎ𝑠𝑠 − 𝛿𝛿ℎ𝑓𝑓)2 (1)

Although it is possible to find a solvent mixture based on HSPs, the question

becomes: how can one screen the vast number of solvents’ combinations to find the

correct solvent mixture? Moreover, what is the amount of each solvent (volume fraction)

in the mixture?

To address the above questions, a computer-aided method was proposed (Paper V)

for selecting solvents and finding the adequate amount of each solvent to form a mixture

of solvents to dissolve a solute with known HSPs or to replace a solvent. To achieve this,

a sophisticated computer software package was developed to find the optimized mixture

using the mathematical Simplex algorithm based on HSPs values from a database of 234

solvents.

53

Polyamide 11 (or Nylon 11) is a biobased aliphatic polyamide produced by the

polymerization of 11-aminoundecanoic acid. It is also produced from castor beans by

Arkema under the trade name Rilsan [100]. Polyamide 11 is applied in the fields of oil

and gas, aerospace, automotive, textiles, electronics, and sports equipment, frequently in

the tubing, wire sheathing, and metal coatings. Bio-based polyamides, like polyamide 11,

generally have lower melting temperatures than conventional polyamide 6 and 66 [101].

Polyamide 11 is also more hydrophobic than polyamide 66, which makes it more

resistance towards humid conditions. This property makes polyamide 11 a possible

candidate to make an all-polyamide composite coated textile in order to be used as the

material of construction of textile bioreactors. There has been a number of research

articles in the literature related to Polyamide 11 solvents [102-106].

To obtain a list of solvent-mixtures, polyamide 66 (Paper V) and polyamide 11

(Paper VI) were tested using their HSPs as case studies. This technique reduces the

laboratory effort required in selecting and screening solvent blends while allowing a

multitude of candidate solvents to be considered for inclusion in a blend. The outcome of

this model significantly diminishes the time required for solvent development by

experimentation via decreasing the possible/necessary trials. Thus, the most suitable

solvent/solvent-substitution can be found by the least possible effort; hence, this approach

will save time and reduce the cost of all solvent-involved processes in the fields of

chemistry, industrial polymers and coatings, chemical engineering, etc.

54

55

Chapter 7

7 Summary of the key findings and future directions

This thesis proposes a new method of making coated textiles intended for the

construction of textile bioreactors. In this method, a film-forming solution of polyamide

66 is smeared over the polyamide 66 fabric and then immersed in water to initiate the

phase inversion (from solution state to solid-state) process.

In the course of the experiments, firstly, the conventional textile bioreactor material

(out of PVC and polyester) was improved by increasing the thermal insulation properties.

Then, a novel method of making a same-polymer composite coated textile was developed

and introduced to address adhesion and recyclability challenges, as well as enhancing

other properties. Furthermore, the new material production method was improved through

solvent replacement. Finally, two textile bioreactors, one constructed from the new

material and the other from the existing material, were utilized to run a fermentation

process, and their performances were evaluated.

Using the textile-based bioreactor is a cost-effective technology that is simple to

operate. This environmental and energy solution can be easily accessed by developing

countries, where the required expertise may not be available, or in remote villages, where

local ‘small-scale’ biofuel production units are going to be installed.

56

By insulating the existing coated textiles, the thermal conductivity of the whole

composite decreases, which eventually reduces temperature fluctuations (favorable for

the microorganisms inside the bioreactor). In this thesis, a method to increase the

insulation properties of the PVC-coated textile was described. This enhancement was

achieved by incorporating silica aerogel into the PVC plastisol. The incorporation of

aerogel into the PVC plastisol yielded a final composite that is approximately 1/3 lighter

than the unmodified one (Paper I).

As heterogeneous composites suffer from adhesion problems, a new material that

exhibits a better adhesion between the textile and coating was developed and introduced

(Paper II). The introduced composite, all-polyamide composite coated textile, uses waste

polyamide fibers to make a strong composite. The composite is almost ¼ lighter than

PVC, which will reduce transportation costs (based on the density). According to

Rajendran et al., [22], for a PVC-coated textile-based bioreactor, a 15-year biogas

production lifetime was assumed. Making a bioreactor out of APCT is expected to provide

a lifetime of more than 15 years (Paper III). For all-polyamide composite coated textile,

a replacement solvent was introduced (Paper IV) that is more environmentally and

industrially friendly due to the replacing 40 percent of the formic acid. Finally, I

introduced a computer-aided method to find replacement solvents for different solutes or

polymers and was used to find dolvents for polyamide 66 (Paper V) and polyamide 11

(Paper VI).

All the aforementioned discussions are general assessments and can be taken as the

preliminary steps of upscaling of constructing the textile bioreactors for biofuel

production. For further industrial development, more elaborative studies and trials are

required.

57

7.1 Future recommendations

Due to the limitation of time, several interesting areas that could affect the safety,

economics, application and productivity of the textile-based bioreactors were not covered

in this thesis. Consequently, they are recommended for future research or applications of

the textile-based bioreactor as follows:

Supported textile bioreactors with steel beams/frames

Adding carbon nanotubes to the composite of APCT

Giving an antifungal property to the outer side of the textile bioreactor

Loading PCMs (phase change materials) to make the material thermostatic,

provides the material with the ability to maintain its desired temperature.

Determining and improving the flame-retardancy of the APCT

Replacing the oil-based polymer (polyamide 66) with a bio-based polymer

(polyamide11) to make the APCT more environmentally friendly and water-

resistant.

To endow the APCT with a longer lifetime, a post-treatment for stress-releasing

can be carried out by annealing.

58

Acknowledgements

I take the opportunity to thank all the teachers, advisors, and supervisors, who

transferred their knowledge to me over the past 26 years, starting from primary school to

the end of my PhD studies.

I deeply express my gratitude to my supervisors, Professor Mohammad Taherzadeh

and Professor Mikael Skrifvars, whom I had insightful discussions with, and received

helpful advice from. I have learnt a lot from both.

I am thankful to Professor Parviz Rashidi, Dr. Dan Åkesson, and Professor Hossein

Mahdavi for their guidance, support, and insights.

I am thankful to my thesis examiner Professor Tobias Richards, and Tomas

Wahnström –the director of studies– for their valuable comments through the ISP

meetings and guidance on my PhD studies. I also thank Peter Therning and Peter

Axelberg, the current and former head of the department, for their time and support over

these years.

I would like to thank my previous teachers at the University of Borås; Kim Bolton,

Kenneth Tingsvik, Magnus Lundin, Dan Åkesson, Ilona Sárvári Horváth, Peter Ahlström,

Anita Pettersson, and my supervisors.

I am sincerely grateful for the efforts made and time dedicated by our

administrative colleagues, Jonas Edberg and Jonas Anderson, Susanne Borg, Sari

Sarhamo, Irene Lammassari, and Lolo Lebedinski.

59

I want to express my gratitude to my officemates: Kostas, Jorge, Swarnima, Adib,

Babak, Kehinde, Regina, Azam, and Ugwu for all the memorable moments we had

together.

I would like to further express my gratitude to the current and past fellow PhD,

visiting PhD students, other colleagues in Resource Recovery and Swedish School of

Textiles, and especially to those, I became friends with.

Thanks Ram, Veronika, Rebecca and Caspar, Kamran, Patrik, Supansa, Sunil,

Gülru, Abas, Johan, Mohsen, Dan, Razieh, Anette, Farzad, Sabina, Akram, Eboh, Hamed,

Pedro, Amir, Andreas, Mohsen, Martin, Amir, Maryam, Madu, Masud, Mohammad,

Mohsen, Mojtaba, Sajad, Tariq, Veronica, Mohammadhossain, Sina, Shahram, Supri,

Päivi, Pedro, Ruben, Jhosané, Alex, Azadeh, Behnaz, David, Fatimat, Foluke, Johan,

Julius, and Vitek for your presence and smiles

I am grateful to the lab technicians and administrators: Kristina Laurila, Marlén

Kilberg, Haike Hilke, Faranak Bazooyar, Thomas Södergren, Sofie Svensson, and Jonas

Hansson.

I am grateful to the FOV Fabrics AB staff, especially Fredrik Johansson and Jesper

Carlsson, for their support. I would like to thank other institutions who were assisting me

with instruments and providing support: Chalmers University of Technology, the

University of Tehran, KTH Royal Institute of Technology, and Amirkabir University of

Technology. This work was financed by the University of Borås, FOV Fabrics AB, and

Swedbank Sjuhärad; their contributions are greatly appreciated.

60

I am thankful to my family for their unconditional love and support, and I wish I

could share this moment with my mother, who passed away while I was working on my

PhD project.

Last, but not least, a special thanks goes to Ghazaleh, my dear wife. This journey

would have never been possible without her patience, love, and support. I am very happy

and blessed to have you in my life.

61

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Original Article

Novel lightweight and highly thermallyinsulative silica aerogel-doped poly(vinylchloride)-coated fabric composite

Mostafa Jabbari, Dan Akesson, Mikael Skrifvars andMohammad J Taherzadeh

Abstract

Novel lightweight and highly thermal insulative aerogel-doped poly(vinyl chloride)-coated fabric composites were

prepared on woven fabrics made of polyester fibres using knife coating method, and their performances were compared

with neat composite. The composites were prepared by incorporating a commercial aerogel to a ‘green’ poly(vinyl

chloride) (PVC) plastisol. The effect of aerogel-content, thermal insulating property, thermal degradation, surface char-

acteristics, tensile and physical properties of the composites were investigated. Results revealed that aerogel could

reduce thermal conductivity, density and hydrophilicity of the composites dramatically without significant decrease in

other properties. Experimental results showed that thermal insulation properties were enhanced by �26% (from 205 to

152 mW/m-K), density decreased by �17% (from 1.132 to 0.941 g/cm3) and hydrophobicity increased by 16.4% (from

76.02 to 88.67� 1.48�) with respect to the unmodified coated fabric. Analyses proved that composite with 3% aerogel is

the lightest by weight, while 4% showed the highest thermal insulation. The results showed that 4% is the critical

percentage, and preparation of composites with aerogel content higher than 4% has limitations with the given formu-

lation due to high viscosity of plastisol. The prepared composite has potential applications in many fields such as

development of textile bioreactors for ethanol/biogas production from waste materials, temporary houses and tents,

facade coverings, container linings and tarpaulins. The prepared composite can be considered ‘green’ due to usage of a

non-phthalate environment-friendly plasticiser.

Keywords

Poly(vinyl chloride)-coated fabric, silica aerogel composite, thermal insulation, lightweight PVC, thermal conductivity

coefficient, Knudsen effect, transient plane source, environment-friendly (green) poly(vinyl chloride)

Introduction

Coated textiles are flexible composites, consisting of atextile substrate and a polymeric coating. The coatingcould be on one side or on both sides, either with thesame or a different polymeric coating per side.1 Coatedwoven fabrics are used in a wide range of structuralapplications to provide lightweight, architecturallystriking solutions.2 The physical properties of acoated fabric depend on properties of the substrate,coating formulation, coating technique and processingconditions during coating.3 There are two principaltypes of coated woven fabric: glass fibre fabric with apolytetrafluoroethylene (PTFE) coating and polyesterfabric with poly(vinyl chloride) (PVC) coating.2 BothPTFE- and PVC-coated fabrics are employed today indifferent types of tents and architectural membrane

structures all over the world.4 Fabric structures resistenvironmental loads, as tensile stresses in the plane ofthe fabric.2 PVC-coated polyester fabrics are the mostcommonly used material in construction of structuralfabrics.5 These fabrics are popular mainly due to theiraffordable prices, high strength, durability, resistance to

Swedish Centre for Resource Recovery, University of Boras,

Boras, Sweden

This article was submitted as part of the Multi-Functional Materials and

Structures Special Issue.

Corresponding author:

Mostafa Jabbari, Swedish Centre for Resource Recovery, University of

Boras, Boras 501 90, Sweden.

Email: Mostafa.Jabbari@hb.se

Journal of Reinforced Plastics

and Composites

2015, Vol. 34(19) 1581–1592

! The Author(s) 2015

Reprints and permissions:

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DOI: 10.1177/0731684415578306

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wear and tear, good toughness, various colours and softtexture.6 They are often used for wide-span sur-faces, membrane-cable structures and pneumaticconstructions.7

Thermal insulation is the reduction of heat transferbetween objects with differing temperatures. The insu-lating capability of a material is measured by thermalconductivity (often denoted �), which is the property ofa material to conduct heat. Low � is equivalent to highinsulating capability (R-value). The � values of somecommon insulation materials are listed in Table 1. Thefunction of insulation materials is to minimise thetransport of heat through the construction. Insulationmaterials are highly porous with small amount of solidstructure. Out of all known solid porous materials,aerogels are particularly known for their high specificsurface area, high porosity, low density and high ther-mal insulation value,10 properties that are required fora high-performance insulator.

Aerogel is a synthetic, ultra-lightweight, highlynano-porous, super insulation material derived from agel, made by sol-gel process and supercritical dryingtechnology, which has excellent insulation performancefor its nano-porous structure (several to tens-of-nano-metre).11–13 Aerogels are the highest efficient thermalinsulator with a � value even lower than for air andwith a density as low as only four times that of air,which makes them the lightest solids known12 so far,up to 1000 times lighter than glass. Aerogels are goodconductive insulators because they are composedalmost entirely of gas. Since air is a low-� material,aerogels are super insulating materials, and they haveexcellent heat insulation properties.13 Aerogels mayhave a � which is smaller than the gas they contain.This is caused by the Knudsen effect. Some types ofaerogel provide 39 times more insulation than fibre-glass.12 Silica aerogel is a kind of aerogel, with athree-dimensional network of silica particles, whichare obtained by extracting the liquid phase of silicagels9 and replacing it with a gas; therefore, they arehighly porous solids that hold gas (usually air) within

pores or networks of solid substances.14 Silica-basedaerogel is an ideal thermal insulator with a makeup ofup to 99% air associated with the highly porous natureof this material.15 Silica aerogel is especially valuablebecause silica is also a poor conductor of heat. They aregood convective inhibitors because air cannot circulatethrough the lattice. The pores in silica aerogel andhence in its composites are open and allow the diffusionof gas; but as they are of micrometre or nanometresize, natural convection of the gas can be ignored.16

Silica aerogels exhibit many intriguing and uniqueproperties, which include extremely low thermalconductivity (5–100mW/m-K), very low density(0.003–0.5 g/cm3), high porosity (80–99.8%), highinner surface area (500–1200m2/g), ultra-low dielectricconstant (k¼ 1.0–2.0) and low index of refraction(�1.05).9,17

PVC is dimensionally stable, largely non-flammableand of low cost,1 but it has limited thermal stability andhigh density.18 Although PVC-based materials (and ingeneral, polymers) are insulators, better insulatingproperties might be needed for some applications,such as preparing a biogas digester from PVC-coatedfabric19 in which, having a high insulation property inthe wall of the digester can enhance the process.Although some researchers8 have been interested inpreparing insulated polymers or in enhancing the insu-lation properties (such as polyurethane and polyisocya-nurate), no previous publications have focused onenhancing thermal insulation of PVC-coated fabricsand making this material lightweight.

Thus, the objective of this work was to enhance thethermal insulation property and decrease the density ofPVC-coated fabric made from PVC green plastisol,incorporated by aerogel to form a novel composite ofPVC/aerogel on technical woven fabrics made of poly-ester fibres.

Materials and methods

Materials

Enova� aerogel IC3120 (Cabot Corporation, Boston,MA, USA) was used in this work. It is a fine particleadditive suited for insulative coatings with a thermalconductivity half that of still air (12mW/m-K at25�C) and particle density of 0.12–0.15 g/cm3. The plas-tisols prepared with the given composition are pre-sented in Table 2. PevikonTM P1412 is a fine particlepaste to prepare vinyl chloride homopolymer. Eastman168TM plasticiser is an excellent environment-friendlynon-phthalate plasticiser for PVC, without health con-cerns of phthalate plasticisers. The polyester wovenfabric was provided by FOV Fabrics AB (Boras,Sweden).

Table 1. Thermal conductivity (�) values of a number of

common porous insulation materials.8,9

Insulation material � [mW/(m-K)]

Mineral wool 33–40

Expanded/extruded polystyrene 30–40

Loose-fill cellulose fibre 39–42

Foam glass 39–45

Aerogel 5–100

Enova� aerogel IC3120 12

1582 Journal of Reinforced Plastics and Composites 34(19)

Methods

Composite preparation. Four composites of PVC/aerogelwere prepared using 0, 2, 3 and 4% aerogel (all of thepercentages in this report is mass percentage, w/w). Forthis purpose, aerogel was weighed and poured into a250-mL beaker; then the coating mixture with theabove-mentioned composition was poured on it andmixed with a mechanical mixer at ambient temperatureand pressure with a three-blade propeller (5 cm diam-eter), first at the speed of 400 r/min for 2 h, then 900 r/min for 4 h and finally 1700 r/min for 6 h to obtain ahomogeny and smooth plastisol without any agglomer-ates. Then, the as-prepared plastisol (PVC coating ingre-dients, including aerogel) was applied on both sides of apolyester fabric with a knife coating method bymeans ofa lab coating instrument (the thickness of each layer ofcoating was 0.4mm and thickness of fabric was 0.2mm,giving a total thickness of 1mm for each composite). Thecuring temperature and time were 180�C and 2.5min,respectively, for the first side and 190�C and 1.5minfor the other side. The neat composite was named‘0%-aerogel’ and the 2, 3 and 4% aerogel-containingcomposites were named as ‘2%-aerogel’, ‘3%-aerogel’and ‘4%-aerogel’, respectively.

Characterisation methods. The densities of the compositeswere obtained by dividing the weight of the compositesby their volume. Measurements of the composites’weights were performed using a balance to determinethe most possible accurate weight, and the volume wasmeasured by a graduated cylinder containing distilledwater. Three specimens were tested for each composite.

Thermal conductivities of the composites were deter-mined by a Hot Disk 2500s apparatus (Hot Disk AB,Gothenburg, Sweden), which is based on the transientplane source (TPS) method at room temperature ofapproximately 25�C in air. The sensor supplied a heatpulse of 20 mW for 20 s to the composite and associatedchange in temperature was recorded. The thermal con-ductivity of the composites was obtained by fitting the

data of 200 points for transient, according toGustavsson et al.20 First, six pieces of compositeswith the dimensions of 20� 20� 1mm were prepared,and a stack from the PVC mats was made for eachcomposite. Then, the sensor was placed symmetricallybetween two identical composite pieces (three layers ofcomposites stacking together on each side of the sensor)and was totally covered. A sample holder (located in airat room temperature) was used to clamp the piecesfirmly together. The height of the stacks, under pressurein the cell, was 5.6mm. The 5501 Kapton� sensor with6.4mm radius is an insulated nickel double-spiral,which was utilised for both transient heating (as a heat-ing source) and precise temperature readings.

The neat and aerogel-loaded composites as well asaerogel powder were analysed using Fourier transforminfrared (FT-IR) spectroscopy in attenuated totalreflection (ATR) mode within a wave number rangeof 600–4000 cm–1 on a Nicolet 6700 spectrometer(Thermo Fisher Scientific, Nicolet Instrument, USA,MA). The spectrum data were controlled by NicoletOMNIC 4.1 (Nicolet Instrument Corp.) software.

Contact angle experiments were performed using theAttension Theta optical tensiometer (Biolin Scientific,Finland). OneAttension� software was used for datacollection and analysis. The sessile drop method wasused to apply the droplet on the substrate surface, inwhich a 4 -mL drop was dispensed on the surface, andimages were captured. Three individual measurementswere obtained for each set of data, and the mean valuescalculated. Thermogravimetric analysis (TGA) was per-formed on the composites using Q500 machine (TAInstruments, MA, USA). About 10mg of the materialwas heated from room temperature to 750�C at a heat-ing rate of 10�C/min in a nitrogen purge stream.

The tensile properties were evaluated in accordancewith the standard method ISO 527. The dumbbell-shaped test bodies were cut by a laser cutter. The testbodies were tested on a MTS 20/M (MTS SystemsCorporation, Eden Prairie, MN, USA), fitted with a

Table 2. Composition materials of the plastisol for 0–4% aerogel-containing poly(vinyl chloride) (PVC)-coated fabric composites.

Ingredient Mass (g) Percentage (%) phr Supplier

PevikonTM P1412 (a fine particle paste making PVC) 53.72 41.18 100 Ineos (Switzerland)

Eastman 168 (a non-phthalate plasticiser) 40.28 30.88 74.98 IMCD (Sweden)

Lankroflex E2307 (epoxidised soyabean oil, a

low odour epoxy plasticiser)

16.20 12.42 30.16 IMCD (Sweden)

Akcrostab LZB6148 (liquid Barium Zinc stabiliser) 16.20 12.42 30.16 IMCD (Sweden)

Titaniumdioxid + Eastman 168 (54.55%

Titaniumdioxid + 45.45% Eastman 168)

3.87 2.96 7.2 Kronos Titan AS

(Leverkusen - Germany)

Pigment (orange colour) 0.19 0.15 0.35

Total 130.46 100 242.86

Jabbari et al. 1583

10 kN load cell and a special grip for films, using acrosshead speed of 5mm/min. The gauge length, pre-load force and first approach speed were 0.5N, 2mm/min and 33mm, respectively. A minimum of five testbodies was tested for each material. The specimens wereall cut in the warp direction. As the composites containwoven fabric, to obtain high-quality test specimens fortensile strength measurement, cutting of all specimenswere done with a laser cutting machine (GCC LaserProSpirit GLS, GCC, Taiwan) with the speed of 7.8mm/min, power of 84 milliwatt (mW) and pulses per inch(PPI) of 1486 with compressed air purging as coolerstream.

Scanning electron microscopy (SEM) was used tomonitor the fracture surface (morphology of thecross-section) of the composites after quenching inliquid nitrogen and sputtering with a layer of goldbefore the measurements. SEM analysis was performedusing AIS2100 (Seron Technology, Korea) operated atan acceleration voltage of 18 kV.

Results and discussion

A novel composite of aerogel/PVC on polyester fabricwas developed and characterised. During preparationof the plastisol for aerogel-doped PVC-coated fabriccomposites, it was observed that by increasing the aero-gel-content in plastisol, the viscosity increases, where5% aerogel was too viscous and not possible to mix.Therefore, up to 4% aerogel was developed and studiedin this work.

Density

The results of density measurement showed that aerogelhad reduced the density of the composites (Figure 1a). Itis most likely because aerogel particles carry plenty of airby themselves (inside the nanometric scale pores of silicaaerogel); thus, by introducing aerogel to PVC plastisol,the entrapped air inside the composite increases, whicheventually makes it lighter. By increasing the aerogelpercentage, the weight of the composite decreased.However, the 4%-aerogel composite deviates from thistrend, showing a higher density than 3%-aerogel, butstill lower than 2%-aerogel. The fact that densityincreases at 4% aerogel loading can be an indicationof aerogel particle damage. The increase in viscosity isdue to extra dissipation of the particles and, at higherconcentrations, due to particle–particle interactions.According to Einstein’s viscosity law for solid particlesin a slurry at low concentrations the relative viscosity isequal to 1+2.5�, where � is the particle volume frac-tion in the dispersion.21 As the aerogel is a very low-density material, its volume is ultimately high (for only4wt%of aerogel in the composite, the volume fraction is

around 26%, see Table 3); therefore by increasing theloading of aerogel from 3% to 4%, the viscosity of thesol (the plastisol) increases hugely.

For the 3% aerogel loading the following calculationcan be made:

�4%�aerogel

�3%�aerogel¼ 1þ ð2:5��4%�aerogel Þ

1þ ð2:5��3%�aerogel Þ

¼ 1þ ð2:5� 0:259Þ1þ ð2:5� 0:206Þ ~¼1:087

It means that by only 1% increasing aerogel contentin composite (from 3%-aerogel to 4%-aerogel), the vis-cosity increases around 9%. In other words, if theweight load of aerogel in plastisol increases a bit, thenthere is a high increase in viscosity. Relative viscositywith respect to the 0%-aerogel sample calculated viaEinstein’s viscosity law for solid particles in a slurryversus aerogel loading in composites are shown inFigure 7.

Figure 1. (a) Density values for 0–4% aerogel-containing PVC-

coated fabric composites; (b) thermal conductivity curve for

PVC/aerogel composites from TPS analysis.

1584 Journal of Reinforced Plastics and Composites 34(19)

The densities of the composite can be calculatedthrough the role of mixtures (ROMs)22

Dc ¼ Dpvc ��pvc

� �þ Daerogel ��

aerogel

� �

where the Dc, Dpvc and Daerogel are the densities of com-posite, matrix (PVC together with the fabric) and aero-gel, respectively. Here it is assumed that PVC is thecomposite’s matrix and aerogel is the reinforcement(filler). By inserting the corresponding values fromTable 3 into the ROM formula, it is clear that the the-oretical densities calculated from the ROM formula for2% and 3% aerogel loading composite are consistentwith the experimental values, with only 0.01 g/cm3 devi-ation (�). But in 4% loading, the difference is 0.10 whichis due to aerogel particle damages. The density of aerogelused in ROM formula is the average of 0.12–0.15 g/cm3

and as the fabric is present in all of the composites, it wasnot taken into account in ROM calculations.

Thermal conductivity

From the TPS analysis values (Figure 1b), it is obviousthat the thermal insulation properties enhanced by 26%

for the most-concentrated composite (in terms of aero-gel content). This is due to extreme insulative propertiesof silica aerogel, which in turn originates from its ultim-ate low density as well as the Knudsen effect,23 which isthe reduction of thermal conductivity in gases when thesize of the cavity encompassing the gas becomes com-parable to the mean free path (MFP).8 The MFP is theaverage distance a molecule travels before collidingwith another molecule. For nitrogen and oxygen, themain components of air, the molecular cross-sectionalarea is around 0.4 nm2.24 The MFP becomes approxi-mately 70 nm at standard temperature and pressure(STP). Effectively, the cavity restricts the movementof the gas particles, decreasing the thermal conductiv-ity, in addition to eliminating air convection. For exam-ple, thermal conductivity of air is about 25 mW/m-K atSTP and in a large container, but decreases to about5mW/m-K in a pore 30 nm in diameter.8 In general, thethermal conductivity of the gas decreases by decreasingits container volume. By using calculated gas conduct-ivity for air, as a function of characteristic system size(pore size, cavity encompassing the air), Berge andJohansson8 showed that by increasing the pore size of

Figure 2. FT-IR spectra of 0–4% aerogel-containing PVC-coated fabric composites and aerogel powder; the characteristic peaks of

aerogel are not seen in 0%-aerogel, 2%-aerogel, 3%-aerogel and 4%-aerogel composites, indicating that there is no aerogel particle

near the surface.

Jabbari et al. 1585

the material that contains air, the thermal conductivityincreases. Thermal conductivities of the composite canbe calculated via ROM:

�c ¼ �pvc ��pvc

� �þ �aerogel ��aerogel

� �

where the �c, �pvc and �aerogel are the � of composite,matrix and aerogel, respectively. The values are calcu-lated and tabulated in Table 3. In this case, the absolutedeviation (j�j) for 4% aerogel loading is more than3%. It is probably because of particle damage whichwas discussed. Interestingly, even with particle damage,the experimental � values are lower than the theoreticalones, indicating better insulation properties. It might beconfusing why damaged or aggregated aerogel particlesstill can impart their insulation properties to the com-posite. It can be because of this fact that silica by itselfis a poor conductor of heat.16 So even by crushing,although the crushed ones are not perfect insulator(due to not having nanometric pores to showKnudsen effect), they are still good heat insulatorbecause of silica’s nature. Therefore, by incorporatingaerogel in a composite, it allows it to become a betterthermal insulator. The results showed a regular trend indecreasing thermal conductivity in aerogel-doped PVC-coated fabric composites.

FT-IR spectroscopic analysis

The FT-IR spectra was used to verify if there were anychanges in surface bonds or if a new bond had beenformed between aerogel particles and PVC coating mix-ture ingredients, by comparing the aerogel-loaded com-posites to the spectrum of neat one (0%-aerogel). Here,the aerogel powder was analysed as well. The FT-IRresults (Figure 2) indicated that there were no differ-ences in FT-IR spectrum of 0%-aerogel and other aero-gel containing composites’ spectra.

Siloxanes show one or more strong infrared bands inthe region 1130–1000 cm–1. Disiloxanes and small-ringcyclosiloxanes show a single Si—O—Si band. As thesiloxane chains become larger and more complex (likesilica aerogel, which has a three-dimensional siloxanenetwork), the Si—O—Si absorption becomesbroader.25 These peaks are characteristic, showing atypical silica aerogel network structure. Therefore, thebroad and strong peaks in the aerogel powder’s FT-IRspectra belong to Si—O—Si bond in the aerogel.26,27

The strong absorption peaks near 1100 and 1220 cm–1

(vibration band28) and the weak peak around 800 cm–1

were assigned to the asymmetric and symmetric bend-ing modes of Si—O—Si, respectively. The peaks forSi—CH3 occur at near 2950 and 850 cm–1,29 as shownin the spectra. Aerogels by themselves are hydrophilic,but chemical treatment can make them hydrophobic.

Hydrophobic aerogels have higher stability.30 If aero-gels absorb moisture, they usually suffer a structuralchange, such as contraction and deterioration, but deg-radation can be prevented by making them hydropho-bic. Aerogels with hydrophobic interiors are lesssusceptible to degradation than aerogels with only anouter hydrophobic layer, because maybe a crack pene-trates into the surface. The peaks at 1260, 1150 andnear 830 cm–1 indicate the presence of Si—Cbond31–33 (and also the peak around 750 cm–1 corres-ponds to Si—C bond34). The presence of Si—C bondpeaks in the aerogel powder’s spectrum originates fromthe fact that the aerogel powder had been hydropho-bised to some extent by replacing some end–OH groups(in Si—OH) with methyl (—CH3) groups (Si—CH3).These peaks show that the aerogels are modified into ahydrophobic form.31

The FT-IR spectra of the four composites do notshow much significant difference, indicating that thereare no changes in the bonds existing in the surface,because in the ATR mode, the infrared ray cannotpenetrate into the composite too much. The penetrationdepth into the material under analysis is typicallybetween 0.5 and 2 micrometres, with the exact valuebeing determined by the wavelength of light, the angle

76.02 77.20

88.67

80.92

60

65

70

75

80

85

90

org 202 203 204

wat

er c

onta

ct a

ngle

(d

egre

e)

Figure 3. Water contact angle values (above) and pictures of

the 0–4% aerogel-containing PVC-coated fabric composites

(below).

1586 Journal of Reinforced Plastics and Composites 34(19)

of incidence, the indices of refraction for the ATR crys-tal and the medium being probed.35 Therefore, thespectrum shows the bonds existing in the surface ofthe composite. None of the aerogel characteristicpeaks is shown in the aerogel-doped composites’ FT-IR spectrum. Even if we consider that they may beoverlaid by the other peaks of PVC-coated fabric,there is no difference between 0%-aerogel composite’sspectra and the other aerogel-containing composites.From this result, we can conclude that the aerogel par-ticles are mostly in the inner part of the surface (nearthe middle fabric) rather than near the surface. In otherwords, until the depth of two micrometres, no aerogelparticles exist. The aerogel particles are not in or nearthe surface, so the surface properties have not changed.Silica aerogels are fragile and sensitive at relatively lowstresses, which limits their application;9 therefore, not

existing the fragile and less-impact material (in com-parison to high impact properties of plasticised PVC)in/near the surface is beneficial, because the surfaceremains in a high-impact state.

Contact angle measurement

The water contact angle measurement results (Figure 3)show that by increasing the aerogel-content (a hydro-phobic particle), the surface of the PVC-coated fabricbecomes more hydrophobic, which is expected. Thetrend is true up to 3%-aerogel, but the hydrophobicityof 4%-aerogel is between 2%-aerogel and 3%-aerogel.This trend is similar to the trend in density, and it is dueto aggregation of aerogel particles and also maybebecause the brittle aerogel particles are crushed at 4%loading and the particles interior is hydrophilic

Figure 4. Cross-sectional scanning electron microscope (SEM) micrograph of the aerogel-doped PVC-coated fabric composites with

different aerogel percentages: (a) 0% (neat composite); (b) 2%, no agglomeration; (c) and (d) 3%, very slightly agglomeration; (e) and (f)

4%, more agglomeration.

Jabbari et al. 1587

(because aerogel particles have been surface-treated tohave hydrophobic surface, as mentioned before). Thedecrease in contact angle at 4% loading could be due toexposing some crushed particles’ hydrophilic interior.Therefore, aerogels may not impart and show its hydro-phobicity completely.

Although based on FT-IR results there is no aerogelnear the surface, aerogel particles can affect the orien-tation and reorganisation of the PVC chains near thesurface in a way that the more hydrophobic partbecomes exposed to the outside of the surface.

Morphology

The microscopic photographs from cross section of theaerogel-doped PVC-coated fabrics are shown inFigure 4. Figure 4(b) represents the 2%-aerogel compos-ite, indicating the good dispersion of aerogel particlesinside the composite. This is also valid for 3%-aerogelcontaining composite (Figure 4c and d), but some ‘veryslight’ agglomerations are seen as well, indicating a verygood dispersion of aerogel particles on the surface. It canbe related to the results from TGA, because more partsof the composite can be affected from aerogel particles toincrease the onset decomposition temperature (ODT).The agglomeration is much higher in 4%-aerogel com-posite (Figure 4e and f). By increasing the size of theagglomerates of aerogel particles, they might play therole of plasticiser in a more stronger way; this interpret-ation is consistent with the results from tensile testing,that is, as the size of agglomerates increases, the plasti-cising effect becomes more noticeable (composite 4%-aerogel has a higher elongation than for composites 2%-aerogel and 3%-aerogel). It is also consistent with resultsfrom the density measurements. It can be interpretedthat by increasing the aerogel percentage in the aero-gel-doped PVC-coated fabric, agglomeration increasesand the effect of aerogel particles on the composite(like decreasing density and increasing thermal decom-position temperature) decreases because of agglomer-ation. In other words, by agglomeration, the surfacearea of the aerogel particles decreases (a larger collectionof particles are closely together); therefore, they cannotimpart their properties to PVC.

Thermal properties

From TGA results (Figure 5), it was observed that theeffect of aerogel percentage on thermal stability ofPVC-coated fabrics has no obvious effect on thermalstability. The ODT values of the three composites arejust a little lower than that of the neat composite, whichperhaps originates from hindrance in partially cross-linking and catalysing the decomposition reaction bymeans of oxygen atoms in the aerogel structure.

TGA curves (Figure 5a) revealed that the thermal deg-radation of the analysed materials comprises twostages. During the thermolysis of PVC, both HCl andbenzene evolution reach a maximum at 340�C, suggest-ing that the dehydrochlorination process and the cyc-lisation of triene segments from polyene segments toform benzene are simultaneous,36 which is obvious inthe neat and aerogel-doped PVC-coated fabric TGAcurves, with only a slight deviation from the mentionedtemperature. The experimental phenomena of the

Figure 5. (a) A typical TGA thermogram of 0%-aerogel and

2%-aerogel composites; (b) and (c) Onset decomposition tem-

perature and residue in TGA analysis for neat and aerogel-doped

PVC-coated fabric composites; the black bars correspond ‘with-

fabric’ composites (PVC layer on the fabric along with the fabric

between two layers of PVC) and the gray bars are for ‘without-

fabric’ composites (taking only PVC part of the PVC-coated

fabric by means of a blade).

1588 Journal of Reinforced Plastics and Composites 34(19)

composite pyrolysis would verify the above-mentionedprocedure. Table 4 lists the key temperature points ofthe TGA curves of every composite, containing initialpyrolysis temperature (ODT) for with-/without-fabriccomposites, residue at 750�C (%) and their correspond-ing differences (between with-/without-fabric). Thevalues extracted from the TGA curves are shown inthe charts in Figure 5(b). Fundamental informationregarding the thermal stability of the composites mater-ials can be obtained fromTGA analysis. Themechanismof PVC thermal degradation has been a controversialand extensively studied topic.36 The weight loss proceedsby two different steps. In general, it is established in lit-erature that the thermal decomposition for PVC takesplace in two processes.37 The first step, arising from thedechlorination from the side group, forms HCl, leavinga conjugated polyene structure and increasing the weakpoint on the main chains.37 Secondary processes in PVCthermal degradation includes crosslinking and gel for-mation, cyclisation, chain scission and benzene forma-tion, Diels-Alder reaction with additives as well as

oxidation of polyenes.38 During the second stage, usu-ally only one reaction is expected by the majority ofpeople, that is, with the occurrence of crosslinking anddepolymerisation of polyene chain, the evolution oftoluene together with a small quantity of other alkylaromatics takes place, yielding a residual char.37

The characteristic difference of the composites maylead to the fluctuation of the kinetic parameters; in par-ticular, the additives could hinder or improve the

Table 4. TGA extracted values for 0–4wt% aerogel-containing poly(vinyl chloride) (PVC)-coated fabrics.

Composite ODT (�C) � ODT (�C) Residue at 750�C (%) � residue (%)

Aerogel powder 469.80 – 88.590 –

Polyester fabric 405.72 – 4.259 –

0%-aerogel without-fabric 237.88 2.81 5.625 0.634

with-fabric 240.69 6.259

2%-aerogel without-fabric 232.34 6.67 8.651 �0.815

with-fabric 239.01 7.836

3%-aerogel without-fabric 228.42 12.48 8.356 �1.092

with-fabric 240.90 7.264

4%-aerogel without-fabric 232.34 �4.32 8.617 �1.049

with-fabric 228.02 7.568

ODT: onset decomposition temperature; � ODT: Difference in ODT between with- and without-fabric samples; � residue: difference

in residue at 750�C between with- and without-fabric samples.

Table 3. Theoretical values derived from the rule of mixtures and comparison with the experimental values for 2–4wt%

aerogel-containing poly(vinyl chloride) (PVC)-coated fabric composites.

Component W (g) D (g/cm3) V (cm3) f

Density Thermal conductivity

Theor Exp � Theor Exp �

Aerogel 2 0.135 14.815 0.146 0.98 0.99 0.01 176.82 – –

PVC 98 1.130 86.726 0.854

Aerogel 3 0.135 22.222 0.206 0.93 0.94 0.01 165.24 163 �2.24

PVC 97 1.130 85.841 0.794

Aerogel 4 0.135 29.630 0.259 0.87 0.97 0.10 155.01 152 �3.01

PVC 96 1.130 84.956 0.741

Theor: Theoretical; Exp: Experimental; W: weight; D: density; V: volume; f: volume fraction; �: the difference between theoretical and experimental

values. Here we considered ‘‘PVC+Fab’’ as the composite’s matrix and aerogel as the reinforcement (filler).

Table 5. Tensile strength test data for 0–4wt% aerogel-

containing poly(vinyl chloride) (PVC)-coated fabrics.

Composite

Tensile

modulus

(MPa)

Elongation

at max (%)

Stress at

break (MPa)

0%-aerogel 56.89� 7.35 33.54� 3.15 56.88� 7.37

2%-aerogel 54.48� 2.56 36.27� 5.02 52.40� 7.08

3%-aerogel 53.50� 2.64 37.92� 2.20 51.75� 5.09

4%-aerogel 52.07� 4.86 41.40� 10.17 50.88� 4.25

Jabbari et al. 1589

pyrolysis kinetics of PVC product significantly, espe-cially during the first degradation reaction stage.37,39

If we consider the without-fabric composites, theODT goes lower by increasing the aerogel-contentwithout a regular pattern. As the aerogel is an inorganicmaterial (a 3-dimensional network with Si—O bonds),logically, the material should be more stable thermallyand decompose at higher temperatures, in comparisonwith 0%-aerogel composite. It is probably due to thedecline in the partially crosslinking because of sterichindrance generated by aerogel particles. Accordingto Woolley,40 oxygen plays a catalytic role during thedegradation by assisting the dehydrochlorination andproduct elimination, possibly by increasing the numberof initiation sites.36 On the other hand, aerogels haveoxygen atoms in their structure. One of the reasons forthe slight decrease in ODT might originate from thepossibility that aerogel’s oxygen catalyses the decom-position reactions. The with-fabric composites havehigher ODT, which is reasonable because fabric startsto decompose at very high temperatures (&406�C). The4%-aerogel composite has higher decomposition

temperature than 3%-aerogel (without fabric), prob-ably due to agglomeration of aerogels, which makes ithard for oxygen atoms in the aerogel particles to playtheir catalytic decomposition role. With regard to thedifference between ODT of with- and without-fabriccomposites, it may be that the coating has closelyadhered to the fabric, more so than the others, whichis why the ODT of with-fabric composite has more of adifference than without-fabric of 3%-aerogel compos-ite, with respect to the others. It is concluded that thethermogravimetric behaviour for aerogel-doped com-posites are similar to that of PVC, which reveals thatthe pyrolysis mechanism of the aerogel-doped compos-ites are mainly dominated by PVC, its main ingredient.Therefore, results obtained through thermal analysescan be regarded as good, as the addition of the additive(aerogel) did not prejudice the thermal properties of thecomposites. It is worth mentioning that at normal uses,the material will never experience temperatures above200�C, for example the application of this material inpreparing biogas textile reactor.

Tensile strength measurement test

The results of tensile strength test are tabulated inTable 5 and also shown as drawn charts (Figure 6).A typical stress–strain curve for 0%-aerogel compositeis shown in Figure 6. Tensile strength tests showedthat the tensile modulus and stress upon breakage ofthe aerogel-doped composites are slightly lower thanfor the neat composite. The decrease in tensilestrength is probably due to the increase of interfacial

Figure 6. (a) Tensile at max and elongation at max versus

aerogel percentage; (b) A typical example of stress–strain curve

for aerogel-doped PVC-coated fabric composite (0%-aerogel).

Figure 7. Relative viscosity with respect to the 0%-aerogel

sample, calculated via Einstein’s viscosity law for solid particles in

a slurry, versus aeogel loading in aerogel-doped PVC-coated

fabric composites.

1590 Journal of Reinforced Plastics and Composites 34(19)

defects between the fabric and coating, as one mightalready expect. The increase in the elongation uponbreakage is probably due to the placement of aerogelparticles between the chains of PVC, allowing them tomove more easily, especially in the higher percentageof aerogel-containing composite in which we haveagglomeration (from SEM results). It can be proposedthat aerogel particles (especially in higher percentageand size, i.e. more agglomeration) play a role, likebeing a plasticiser of PVC at least in the way thatincreases the elongation to the maximum (elongationat max). As a result, the values of the three compositesare just a little higher/lower than that of neat compos-ite, demonstrating that incorporating aerogel withPVC does not decrease mechanical propertiesdramatically.

Conclusion

In this work, a novel PVC/aerogel composite wasobtained by mixing commercial aerogel powder withgreen PVC plastisol. It was found that silica aerogelcould enhance thermal insulation, density and surfaceproperties of PVC-coated fabrics without significantlychanging the other properties. Analyses proved thatcomposite with 3% aerogel is the lightest by weight,while 4% showed the highest thermal insulation. Thenovel lightweight and highly thermal insulative PVCfabric-reinforced composite has potential applicationsin some fields such as development of biogas textilereactor (which needs to be water/gas-tight, thermalinsulative and light-weight), temporary houses andtents, facade coverings, container linings and otheradvanced tarpaulins, which need high thermal insula-tion properties and has to be low in weight.

Acknowledgements

The authors are grateful to FOV Fabrics AB (Sweden) forproviding polyester fabric and the lab coating machine. Theauthors would also extend their gratitude to Dr. Magnus

Bratt for helping in laser cutting and Kristina Laurila forher efforts with laboratory work.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest withrespect to the research, authorship, and/or publication of this

article.

Funding

The author(s) disclosed receipt of the following financial sup-port for the research, authorship, and/or publication of this

article: This research was funded by FOV Fabrics AB(Sweden) and University of Boras.

Nomenclature

� thermal conductivity

Z viscosity� volume fraction

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1592 Journal of Reinforced Plastics and Composites 34(19) Paper II

Introducing all-polyamide composite coated fabrics: A method toproduce fully recyclable single-polymer composite coated fabrics

Mostafa Jabbari, Mikael Skrifvars, Dan Åkesson, Mohammad J. TaherzadehSwedish Centre for Resource Recovery, University of Borås, Borås, SwedenCorrespondence to: M. Jabbari (E-mail: mostafa.jabbari@hb.se)

ABSTRACT: Novel all-polyamide composite (APC) has been developed to replace traditional coated fabrics with good interfacial adhe-

sion and enhanced recyclability. The composite is fully recyclable since it contains no other materials except polyamide. APC was pre-

pared by partially dissolving a polyamide fabric by treatment with a film-forming polyamide solution. The effect of the polyamide

solution concentration and gelling time on tensile and viscoelastic properties of APCs was investigated to explore the optimum proc-

essing parameters for balancing the good interfacial adhesion. The composite properties were studied by dynamic mechanical thermal

analysis (DMTA), tensile testing and scanning electron microscopy (SEM). The results showed a good adhesion between the coating

and the fabric. A new method was introduced to convert a low value added textile waste to a high value-added product. The compos-

ite is tunable, in terms of having a dense or a porous top-layer depending on the end-use requirements. VC 2015 Wiley Periodicals, Inc. J.

Appl. Polym. Sci. 2015, 132, 42829.

KEYWORDS: coatings; films; polyamides; recycling; textiles; coated fabrics; single-polymer composites

Received 31 May 2015; accepted 9 August 2015DOI: 10.1002/app.42829

INTRODUCTION

Heterogeneous composites pose a recycling challenge1 because

of their composition. Furthermore, they often suffer from poor

matrix–fiber adhesion due to the chemical incompatibility of

the components.2–4 The growing interest in the recycling of

materials is brought about by the desire to preserve the environ-

ment, as there is limited landfill space due to the large amount

of wastes that is being dumped.5 Global warming is also of con-

cern due to emissions from waste incineration.4 These chal-

lenges have stimulated an interest in the development of

environmentally friendly materials.6,7 One promising approach

to deal with the composites recycling challenge is the introduc-

tion of single-polymer composite (SPC).6 In SPCs both the

reinforcing and the continuous phase are polymers with the

same chemical composition5; therefore, recyclability is

enhanced8,9 as no material separation is necessary. These mate-

rials are often described as one-polymer composites, homocom-

posites, all-(the same)polymer composites, self-reinforced, or

homogeneous composites.4,5 Besides recyclability, the interest in

the concept of SPCs is based upon the premise that interfacial

bonding should improve if matrix and reinforcement are made

from the same polymer.2,5

Polymer-coated textiles are flexible composite materials com-

prising a coating (the polymer) and a substrate (textile/fabric

layer(s)) adhered together through a specific coating process.10

The coating may be on one side or both sides either with the

same or a different polymeric coating per side.11 Coated woven

fabrics are used in a broad range of semi-structural applications

to provide lightweight, architecturally striking solutions.12 They

are often used for wide-span surfaces, membrane-cable struc-

tures, hanging roofs (e.g., roofs of sports structures like stadi-

ums), pneumatic constructions,13 transportation, and

commercial constructions.14 The combination of the different

properties of each layer determines the overall properties of the

system.10 Coated fabrics can be tailored to be used as a textile

reactor, temporary houses and tents, facade coverings, container

linings, and tarpaulins.15 Fabric structures resist environmental

loads, as the tensile stresses are dispersed in the plane of the

fabric.12 This means that the textile substrate contributes to the

mechanical strength (tear, tensile), elongational, and dimen-

sional properties of the system in general; whereas, the polymer

coating helps to introduce penetration resistance and imperme-

ability (to liquids, gases, and dust particles), as well as improve

fabric abrasion strength.10 They can also help to modify the

appearance for decorative purposes.16 Coated fabrics are popu-

lar mainly due to their affordable prices, high strength, durabil-

ity, resistance to wear and tear, good toughness, various colors,

and soft texture.6

A wide range of textile materials are used as substrates for

coated fabrics such as cotton, rayon, polyamide (PA), and

VC 2015 Wiley Periodicals, Inc.

WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2015, DOI: 10.1002/APP.4282942829 (1 of 9)

polyester, depending on the end-use requirements.11 A military

tent can be made of a material that combines a PA fabric with

a neoprene coating. For applications where high strength is

required, PA and polyester are used, as they have considerably

higher strength-to-weight ratios.11 PA or Nylon is a well-known

high-performance engineering thermoplastic with excellent

properties, including high tensile and impact strength, tear and

fatigue resistance, thermal stability, and low coefficient of fric-

tion.17–19 Because of their mechanical properties and their

barrier-like behavior toward gasses and flavors, they are widely

used in film products for flexible food packaging.20 Because of

its excellent wearing durability,17 the most important field of

application for PAs is in the textile industry.21 In the family of

PA fibers, PA66 possesses relatively better mechanical properties

and is applied in the production of tires, airbags, bullet-proof

vests, and so on.17

Adhesion is the state in which two surfaces are held together

at an interface by forces or interlocking action or both.22

Adhesion between different components in composites is an

important parameter, to achieve good composite properties.

Poor adhesion results in failure caused by the insufficient

stress transfer between the matrix and the reinforcing fabric

(as the reinforcement). This is also the case when producing

coated fabrics, to get a good adhesion between the coating and

the fabric; moreover, it is necessary to select the right coating

binder that can bind the coating to the fabric. There are sev-

eral ways to increase the adhesion (or hold the surfaces

together at interface by forces or interlocking action or both);

however, they are often associated with drawbacks like high

cost, formulation complexicity, or fabric degradation (in order

to be bonded to the binder), etc.

One possible method to increase the adhesion in PA-coated fab-

rics could be using the same material in the coating of the PA

fabric and preparing a SPC-coated fabric. By using this method,

the surface of the fabric will be partially dissolved in order to

entangle the polymer chains of the coating (as a continuous

phase of the composite) with the dissolved polymer chains of

the fabric’s surface. Since the two components (fibers of the fab-

ric and the coating) are the same, and the same solvent (formic

acid) exists between them, the adhesion (which in this case is a

non-reactive adhesive) would be very strong. In addition, there

is no need to use any other material except PA and the solvent.

By this method, novel All-Polyamide Composites (APC) coated

fabric, a special form of SPC, can be obtained. APCs are com-

posites in which both constituents are made from one kind of

polymer. Furthermore, they are easily recycled, as they are com-

posed of one and the same polymer. A number of techniques

have been used or developed to prepare SPCs, including film

stacking, powder or solution impregnation, hot compaction, co-

extrusion, and selective surface dissolution.23 The reports about

APC in the literature describe the use of two different PA grades

with different melting points that are used to produce a lami-

nate,17,24,25 or show the preparation of all-aramid composites by

partial fiber dissolution.26 However, there is no report about

producing SPC-based PA-coated fabrics via a non-solvent

induced phase separation (phase inversion) method.

The aim of this paper was to introduce a novel, facile, low cost,

and environmentally-friendly method to produce an APC

coated fabric from a PA by solvent-casting of only one compo-

nent, specifically, PA66 onto the PA66 fabric in order to have

strong adhesion between the coating and the fabric as well as

enhanced recyclability.

MATERIAL AND METHODS

Materials

The formic acid used in this work was supplied by Sigma-

Aldrich (ACS reagent grade, >98%). The PA66 plain woven fab-

ric was provided by FOV Fabrics AB (Boras, Sweden). As a

polymer source to produce a solution, PA fiber production

waste from the weaving process at FOV Fabrics was used.

Composite Preparation

All-polyamide composites were prepared in the form of a flat

laminate on the substrate fabric by an isothermal immersion-

precipitation method. Seven solutions of PA production waste

in the formic acid were made by dissolving 17, 23, 29, 35, 41,

47, and 53 g of PA in 100 g (82 ml) formic acid at room tem-

perature. In low concentrations (less than �30% w/w), PA66

readily dissolves in the formic acid at room temperature, but

for higher concentrations, the solution should be agitated for a

longer time. In order to assure the completion of dissolution

and have the same agitation condition for all the solutions, the

sealed solution flasks were put in a shaker at the speed of

100 rpm for 20 h at 558C to obtain a homogeneous solution

(hereinafter, called dope). The dopes were cooled to room tem-

perature, and after centrifugation for 10 minutes at the speed of

16,0003g to remove the bubbles, the dopes were casted on a

PA fabric with the size of 18 3 24 cm fixed on a glass with

adhesive tape, using a universal film applicator with a thickness

of 175 lm. Once the casting process was done, after waiting 15,

30, and 45 s (three different times), the glass plate (carrying the

fabric and a layer of the PA solution on top of it) was immersed

in a distilled water coagulation bath at room temperature to

induce polymer precipitation (phase separation). From then on,

the waiting time, which is the time between applying the solu-

tion to the fabric and immersion in the coagulant, is called gel-

ling time. After 1 h coagulation (in the water bath), the

composites obtained were first washed three times with distilled

water and then they were held under light press between two

sheets of filter papers and dried at 558C in a vacuum oven

(�0.1 bar) for 2 h. The composites were named according to

their corresponding solution concentrations and gelling time

(Table I).

Characterization Methods

The tensile properties were evaluated in accordance with the

standard method ISO 527.15 Dumbbell shaped test bodies,

75 mm long (with the width of 4 mm), were tested on a MTS

20/M tensile tester (MTS Systems Corporation, Eden Prairie,

MN), fitted with a 5 kN load cell and a special grip for films,

using a crosshead speed of 5 mm/min. The gauge length, pre-

load force, and first approach speed were 0.5 N, 2 mm/min and

33 mm, respectively. The thickness of the composites was meas-

ured by Elastocon thickness meter (Elastocon AB, Sweden). A

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minimum of five test bodies was tested for each material. The

specimens were all cut in the warp direction of the fabric.

To investigate the viscoelastic properties of the composites,

dynamic mechanical thermal analysis (DMA Q800, TA Instru-

ments, Waters LLC) was performed on the prepared composites.

The specimens were run with a film tension clamp using the

temperature ramp procedure with a sample dimension of �15

3 9 mm. The temperature ranged from room temperature to

1808C with a heating rate of 38C/min; the frequency and the

amplitude were 2 Hz and 15% strain, respectively.

Scanning electron microscopy (SEM) was used to monitor the

fracture surface morphology of the cross-sections of the compo-

sites. The specimens were obtained by quenching in liquid

nitrogen and breaking by hand. The studied surface was sput-

tered with a layer of gold before the measurements. SEM analy-

sis was performed using AIS2100 (Seron Technology, Korea)

operated at an acceleration voltage of 18 kV.

RESULTS AND DISCUSSION

To overcome the recyclability problem of the PA-coated fabric

and to increase the adhesion between the coating and the fabric,

a PA-coated PA fabric single-polymer composite out of PA66

was prepared. It was done by applying the PA66 solution to the

PA fabric by means of a universal film applicator and consecu-

tively, coagulation in a water bath as a non-solvent to induce

phase separation (phase inversion). As a result, a composite

composed of a thin continuous PA layer (the coating) and a PA

textile fabric out of the most common type of aliphatic PA

(PA66) was obtained. The hypothesis in this work was that since

the coating and the fabric were the same, and there was a com-

mon solvent in between them, the adhesion would be very

strong. Different properties were analyzed, and the process

parameters were optimized.

Mechanical Properties

The maximum force needed to break the composite in the ten-

sile mode, as well as elongation at max, and the thickness of the

APCs are shown in Figure 1. As the thickness of the composites

was not the same, the maximum force was chosen as the crite-

rion to compare the mechanical properties of the composites.

This was decided since the mechanical characteristics of the fab-

ric (a spun fiber, high crystallinity), and the formed film (an

amorphous film, low crystallinity) differs greatly with each

other. Moreover, in different composites, different amounts of

PA film exists; thus, in this case, the stress, that is, force divided

by cross-sectional area, is not a good criterion to compare the

strength of the APCs.

The results from the tensile testing giving the maximum force

(Figure 1) showed a slightly good trend. The results indicated

the increase of mechanical strength with the concentration of

the dope for each gelling time group individually. If only the

15 s gelling time composites were considered, the maximum

force needed to break the composites increased as the corre-

sponding dopes contained more PA (higher concentrations).

This trend was valid for 30 s and 45 s gelling times. However, if

the three different gelling times of one single group of the dope

concentration were considered, the trends were not the same for

all the 7 groups. There is no clear trend in increasing the gelling

time. Some dope concentration (for example 17%) possibly

show an increasing trend of the maximum force when the gel-

ling time is increased. However, the trend is not so significant

considering the standard deviations.

Comparing the fabric maximum force (Figure 1) with the other

APCs, showed that by applying PA dope on PA fabric, the fabric

became weak in terms of mechanical strength that can be attrib-

uted to the dissolution of the surface of the fabric by the sol-

vent existing in the dope. However, the 53% APCs had very

close maximum force values to those of the fabric. This decreas-

ing in mechanical properties can be regarded acceptable in com-

pensation of giving a new property to the fabric (making the

fabric gas-/water-proof) and resulting a coated fabric. For elon-

gation at max, more or less the same trend was valid with the

exception that the APCs made out of concentrated PA dopes

(especially 47% and 53%) had higher elongation at max values.

This was due to the difference between elastic properties of

amorphous and crystalline polymers. The fabric is made from a

spun polymer fiber that has a higher crystallinity than the

formed-film that is an amorphous polymer. Therefore, by

increasing the amorphous (more elastic) polymer in the com-

posite, it is reasonable to have a higher elasticity, elongation at

max value. It is in accordance with the rule of mixtures mecha-

nistic model,27 which says the resulting mechanical properties

of a composite, is the individual property of the components

based on their volume fraction.

Table I. Solution Concentrations, Gelling Times, and Codes for APCs

Code Solution concentration (%) Gelling time (s)

APC1715 17 15

APC1730 17 30

APC1745 17 45

APC2315 23 15

APC2330 23 30

APC2345 23 45

APC2915 29 15

APC2930 29 30

APC2945 29 45

APC3515 35 15

APC3530 35 30

APC3545 35 45

APC4115 41 15

APC4130 41 30

APC4145 41 45

APC4715 47 15

APC4730 47 30

APC4745 47 45

APC5315 53 15

APC5330 53 30

APC5345 53 45

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By increasing the PA concentration of the dope solution, the

content of formic acid in the solution with respect to the poly-

mer, that is, the ability of the dope to dissolve the surface of the

fabric decreases while the amount of the PA film on the fabric

increases. The more the fabric surface dissolves, the more the

strength of the fabric diminishes. On the other hand, in order

to obtain a good adhesion of the PA layer to the fabric sub-

strate, it is necessary to have a certain amount of surface disso-

lution. It is obvious that for higher concentrations of the dope,

the composite has a higher maximum force and is thus

mechanically stronger. It is due to the higher concentration of

the film formed (higher loading of PA) on the fabric and better

opportunities to form entanglements between the formed film’s

polymer chains and the polymer chains of the fibers in the fab-

ric. The mentioned trend is slightly valid when increasing the

gelling time for each dope concentration. By increasing the pen-

etration time for the solvent into the fabric surface (and dissolv-

ing the surface of it), logically the amount of penetrated solvent

(and hence the number of polymer chains that can entangle

with the chains of the polymers in the film) increases; while the

concentration of the solvent is the same for the three different

gelling times.

The thickness of the composites (Figure 1) varied with differ-

ent concentrations of the dopes and even with various gelling

times. It can be explained as follows: by increasing either the

dissolution power (lowering concentration) of the solvent or

the dissolution time of the solvent in the dope to dissolve the

fabric (gelling time), a greater amount of the fabric is dis-

solved; hence, the thickness decreases. The trend is more or

less valid for all the composites except for APC2945, which

can be related to a critical concentration or experimental

errors. Although the same universal film applicator (with the

thickness of 175 lm) was used for all of the composites, the

thickness varied. The thickness decreasing with the gelling

time indicated that the time dedicated to the solvent to dis-

solve and go through to the fabric, had an inverse proportion

with the thickness due to the dissolution power, as mentioned

before. The increasing of the thickness by increasing the dope

concentration may very well be attributed to increased poly-

mer content in the formed-film. A film with more polymer

content would more than likely have an increased thickness.

However, the decreasing the thickness in each group by

increasing the gelling time, supported the “dissolution power”

proposal for the thickness variation.

We propose that entanglement effects may explain the trend in

the maximum force and elongation data shown in Figure 1. At

low polymer solution concentrations, there are fewer entangle-

ments so that the polymer can more easily order its chains.28

These ordered regions can serve as nucleation sites, promoting

more crystallization in the precipitated sample.28 If the polymer

entanglement proposal is correct, the increasing elongation at

max by increasing the dope concentration is due to having a

higher amount of amorphous regions at the higher concentra-

tions of the dope. Therefore, they are more elastic than those at

the lower concentrations. At higher dope concentrations, how-

ever, polymer entanglements can interfere with the ordering

(making the ordering difficult) and thus reduce the number of

ordered formed regions.28

If the polymer entanglement proposal is correct, then there is a

critical concentration, C*, at which entanglements begin to

Figure 1. Max force, elongation at max and thickness in the tensile test of

the APCs as well as the fabric, versus different solution concentrations for

15 s, 30 s, and 45 s gelling times. The variations can be attributed to

many factors, mainly the dissolution power of the dopes. “Fab” stands for

fabric, and it has been added to the graph for comparison purposes.

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form, and the solution becomes semi-diluted.29,30 This critical

concentration can be approximated as follows30:

C � 5 l= g½ �

where [g] is the intrinsic viscosity. Intrinsic viscosity [g] is

related to the molecular weight (M) by the Mark-Houwink

equation, defined as:

g½ � 5KMa

where K and a are constants, that depend on the polymer, sol-

vent, and temperature.28 K depends on the width of the molec-

ular weight distribution, and a is a measure of the polymer-

solvent interactions.31 Thus, by increasing the molecular weight

of PA, C* will decrease, and entanglements will occur at lower

concentrations. This means that every time that PA scraps (as

PA source in this work) with a different molecular weight is

going to be used, the parameters need to be optimized again.

Another reason for improved mechanical properties by increas-

ing the dope concentrations is probably due to increased mass

(surface weight). Higher dope concentrations contain a higher

amount of polymer and form a denser layer on the fabric. At

very low dope concentrations (less than 35%), the concentration

of the solvent is so high that the dissolution of the fabric is too

much and more or less destroys the fabric according to maxi-

mum force results. But at higher concentrations, especially at

47% and 53%, the amount of the solvent seems to be optimized

so it does not dissolve the fabric more than what is needed; i.e.,

dissolves the fabric’s surface only to an extent that is required

to establish the adhesion. Therefore, the higher concentrations

(41%, 47%, and 53%) are more optimized concentrations to

prepare the APC in terms of the mechanical properties. With

the same argument, the longer gelling times in each of the

aforementioned three optimized concentrations give better

mechanical properties to the corresponding composites.

Viscoelastic Properties

The three composites APC1715, APC1730, and APC 1745 were

yielded during the analysis in the DMTA machine at 568C,

438C, and 328C, respectively; therefore, no results were reported

for them. An exemplary DMTA graph for the APC4745 com-

posite with appointed maximums, onset-, mid- and end-points,

as well as step transition, are shown in Figure 2.

Although there is no regular trend for the peak of the loss mod-

ulus (� glass transition temperature, Tg) of the composites, as

for the peak of the loss modulus versus temperature curve (Fig-

ure 3), it is clear that APC4130 has the highest value. For the

two more concentrated dopes, i.e., 47 and 53%, loss modulus

Figure 2. An exemplary DMTA graph for APC4745 composite; maxi-

mums, onset-, mid-, and end-points, as well as step transition, are shown.

The straight lines are drawn by the DMTA machine’s software to find the

points shown by arrows.

Figure 3. Loss modulus, tan delta, and stiffness of the APCs versus differ-

ent solution concentrations for 15 s, 30 s, and 45 s gelling times.

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for 30 s gelling time is greater than for 15 s and 45 s. However,

for the rest dopes, it is smaller than the other two gelling time.

The physical and mechanical properties of the coating, along

with its glass transition temperature and its thickness are key

parameters affecting properties, such as the interfacial strength

and the fracture toughness.32 Loss modulus describes the vis-

cous properties, meaning that composites with longer gelling

time have better elastic properties than those with shorter gel-

ling time. The variations in the viscoelastic results (Figure 3)

are probably related to a larger part of the PA fabric being dis-

solved and changing it from a higher crystalline form (spun

fiber) to a more amorphous polymer, from lower to higher

dope concentrations as well as from shorter to longer gelling

times. Since amorphous polymers are disorganized, they can

align in the direction of the force imposed on them, and by

this, they can dampen the energy, at least more than the

amount of dampening energy in more crystalline regions.

By increasing the temperature, the stiffness of the polymer

decreases. The temperatures at which the drop in stiffness

occurs are shown in Figure 3. Comparing the stiffness step tran-

sition onset point and gelling time of the APCs at various dope

concentration, it is difficult to infer the effect of the gelling

time/dope concentration on the stiffness. Again there is no clear

and regular trend, but longer gelling time composites have

higher storage moduli (see Figure 4). When the gelling time is

longer, the solvent has more time to penetrate into the fabric,

resulting in better adhesion. This indicates that the polymer

chains in the formed film and the polymer chains in the dis-

solved parts of the fabric can form more entanglements with

each other. Therefore, the longer gelling time gives not only bet-

ter adhesion, but also improved damping properties that can

dissipate more energy, which, as a result, gives higher tan delta

(loss tangent) (Figure 4). In this system, there might be a criti-

cal entanglement degree in which the polymer chains can entan-

gle in each other enough to make the resulting composite stiff,

but not to the extent that make it rigid. The variations, espe-

cially in 35% and 41% dope concentrations, can be due to the

critical degree of entanglements. As Varelidis et al.33 discuss, the

glass transition temperature and the thickness of the flexible

interlayer have been identified as the major parameters affecting

the end properties of the composite. Therefore, in this regard,

APC4130 is the optimum APC.

Morphological Properties

According to the scanning electron microscopic images of the

cross-section (Figure 5), in almost all the APC specimens there

is a good adhesion between the formed film and the fabric. In

other words, the boundaries between the fiber of the fabric and

the formed film are not clear because they have faded due to

the adhesion of the two components.

Figure 4. Tan delta, storage modulus, and loss modulus for some selected

All-Polyamide Composite (APC) coated fabrics. Although there is no clear

and regular trend, longer gelling time composites have higher storage

moduli. Longer gelling time gives more opportunity for the solvent to

penetrate into the fabric, resulting in better adhesion and more entangle-

ments, which eventually improved damping properties that can dissipate

more energy, which as a consequence gives higher tan delta. [Color figure

can be viewed in the online issue, which is available at wileyonlinelibrary.

com.]

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At higher concentrations of the solvent (lower concentrations of

the polymer solution), the dissolving power of the dope is

higher; hence, it can penetrate more into the fabric and dissolve

a larger part of the fabric, e.g., shown in APC2930 and

APC2945 SEM micrographs. It is obvious that a larger part of

the cross-sectional area of the fabric is dissolved, and the

Figure 5. Cross-sectional scanning electron microscopic images of the APCs. From top to bottom, 17%, 23%, 29%, 35%, 41%, 47%, and 53% polyamide

dope concentrations, and from left to right 15 s, 30 s, and 45 s gelling times, respectively. The micrographs of the specimens were taken after quenching

them in liquid nitrogen and breaking by hand. The magnification is 500 times for all of the micrographs.

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polymer chains are inter-diffused into each other, compared to

the SEM micrographs for APC5330 and APC5345. Although

higher surface dissolution helps to create a better adhesion

between the fabric and the formed film, it also degrades the fab-

ric structure and changes the fabric from a fibrous form to a

film form. Fibers are spun polymers and pose a high crystallin-

ity and thus a good strength while films are amorphous. There-

fore, converting PA from a high crystalline form to a less

crystalline (high amorphous) form is not favorable from the

mechanical point of view. The amount of the surface dissolu-

tion should be as low as possible to impart a good adhesion

between the fabric and the formed film, but higher dissolution

is not favorable.

In the APC5345 composite, good adhesion is seen, but there are

some voids in the formed film (Figure 6). The voids make the

film porous, which can be due to the difficulty of the solvent

inside the dope to evaporate from the gel during the coagula-

tion process due to the high viscosity of the dope. When the

solvent migrates from the gel, as the dope concentration/viscos-

ity and polymer chains entanglements are great while leaving,

Figure 6. Illustration of the porosity in APC5345 with different magnifications.

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the solvent leaves the channel and creates a void. Because of the

entanglement, the chains next to the migrating solvent mole-

cules are not able to rearrange themselves to fill the channel

formed by the solvent while leaving. In other words, the solvent

creates a route for itself to be able to come out and by this,

leaves behind a channel. This property can be useful for some

applications, in which the porosity of the composite is required,

like supported membranes.

CONCLUSIONS

The preparation of a novel all-polyamide composite (APC)

coated fabric, a specific form of single-polymer composites, made

through a phase inversion method is described in this report.

The prepared composite has a strong adhesion between the two

constituents due to the use of the same polymer (PA66) in the

constituents and a common solvent. The composite is fully recy-

clable since it contains no other materials except PA, which can

be melted or dissolved and reused like a virgin PA source. The

optimum concentration is 47% if a dense film is needed, but if it

is necessary to present a porous film on the fabric, then 53%

solution should be used to prepare the APC membrane, that can

be regarded as a fiber-supported membrane. The composite can

be tailored in various ways with different properties depending

on the end-use requirements. The prepared composite has a wide

range of semi-structural applications to provide lightweight,

architecturally striking solutions as well as wide-span surfaces,

membrane-cable structures, hanging roofs, pneumatic construc-

tions, water-/gas-proof textile reactors, temporary houses and

tents, facade coverings, container linings, and tarpaulins. Apart

from the novelty of the method to prepare a single-polymer com-

posite coated fabric out of PA, in this work, a new method was

introduced to convert a low value-added waste (PA scraps) to a

high value-added product (APC coated fabric).

ACKNOWLEDGMENTS

The authors are grateful to FOV Fabrics AB (Sweden) for provid-

ing PA fabric and PA residue.

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Paper III

energies

Article

All-Polyamide Composite Coated-Fabric asan Alternative Material of Construction forTextile-Bioreactors (TBRs)

Mostafa Jabbari * ID , Osagie A. Osadolor ID , Ramkumar B. Nair and Mohammad J. Taherzadeh ID

Swedish Centre for Resource Recovery, University of Borås, SE-50190 Borås, Sweden;alex.osagie@hb.se (O.A.O.); Ramkumar.nair@hb.se (R.B.N.); mohammad.taherzadeh@hb.se (M.J.T.)* Correspondence: mostafa.jabbari@hb.se; Tel.: +46-33-4354-636; Fax: +46-33-4354-003

Received: 15 October 2017; Accepted: 14 November 2017; Published: 21 November 2017

Abstract: All-polyamide composite coated-fabric (APCCF) was used as an alternative material forthe construction of textile-bioreactors (TBRs), which are prepared as a replacement of the traditionalstainless steel bioreactors (SSBRs) or concrete-based bioreactors. The material characteristics, as wellas the fermentation process performance of the APCCF-TBR, was compared with a TBR made usingthe polyvinyl chloride (PVC)-coated polyester fabric (PVCCF). The TBRs were used for the anaerobicfermentation process using baker’s yeast; and, for aerobic fermentation process using filamentousfungi, primarily by using waste streams from ethanol industries as the substrates. The results from thefermentation experiments were similar with those that were obtained from the cultivations that werecarried out in conventional bioreactors. The techno-economic analysis conducted using a 5000 m3

APCCF-TBR for a typical fermentation facility would lead to a reduction of the annual productioncost of the plant by $128,000,000 when compared to similar processes in SSBR. The comparativeanalyses (including mechanical and morphological studies, density measurements, thermal stability,ageing, and techno-economic analyses) revealed that the APCCF is a better candidate for the materialof construction of the TBR. As the APCCF is a 100% recyclable single-polymer composite, which wasprepared from Nylon 66 textile production-line waste, it could be considered as an environmentallysustainable product.

Keywords: single-polymer composite; textile bioreactor; all-polyamide coated-fabric; polyvinylchloride (PVC) coated-textile; waste management; Nylon 66; yeast fermentation; edible filamentousfungi cultivation; techno-economic analysis

1. Introduction

‘Energy’—its production and its use in contemporary society—is a formidable topic [1]. Renewableenergies are being increasingly adopted across Europe, partly due to the European Union’s (EU’s) energypolicy based on its 20-2020 commitments, i.e., 20% renewables by 2020 [2]. As biofuels must compete withfossil fuels, any attempt to reduce their investment and operational costs will contribute to stimulate theirconsumption [3]. During the last few years, several research projects have been conducted to minimisebiofuel production costs [4], regarding the process technologies, as well as the equipment or infrastructurefacilities, such as the bioreactors or distillation columns. A bioreactor is a vessel that provides an environmentsuitable for fermentation reactions where the controlled growth of microorganisms result in the production ofbiofuels [5]. Bioreactors are made out of stainless steel or concrete, which are expensive and time-consumingto install [5]. To be used as a bioreactor, the construction material has to meet several prerequisites, such asthe ability to provide a suitable environment for the microbial proliferation, withstand high pressure [6],be inert to the underlying biological and chemical process conditions [3], corrosion proof, and water-proofand/or gas-proof, depending on the fermentation process. Recently, research studies have come-up with

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Energies 2017, 10, 1928 2 of 14

alternative material for making microbial bioreactors [3]. One such example is using polyvinyl chloride(PVC)-coated polyester fabric (hereafter referred as PVCCF). Polyester being one of the most conventionalbut less advanced fabric in the textile industry, could be replaced with a better performing textile, such aspolyamide (PA) that possess a longer lifetime, has a higher mechanical and dimensional stability, and is lightweighted [7]. However, for economic reasons polyester fabric has to be used as the base material in manycases. Nevertheless, the coating of the PVC onto the polyester fabric involves chemical formulations thatmight be harmful to the microorganisms. Additionally, the recycling of the materials used will also posesevere challenges, as it consists of the mixture of PVC, polyester fabric, a plasticizer for the PVC, chemicallinkers, and some other processing-aid additives [8].

A possible solution to effectively address these issues is to use a coated-fabric composite made ofa single material—single-polymer composite—called all-polyamide composite coated-fabric (APCCF),which is mechanically stronger and thermally stable and weighs less than PVCCF [9]. Additionally, APCCFis fully recyclable as it contains only a single material,l (PA) that is prepared by adhesion of the PA fabricand the PA solution made out of waste, making them a cost-effective and eco-friendly material [9]. In theperspective of environmental sustainability, the recycling of fabric and textiles decreases the use of naturalresources, such as water or petroleum, that are being used for generating new fabric or textiles [10]. It alsolowers the extent of chemical usage and the associated pollution that is encountered during the textilemanufacturing process [11]. Currently, the most common method of recycling textiles is to use them ascomposite filler [12]. This method is however not effective as they do not maintain the quality or propertiesof the composites [13]. Hence, the development of a fabric-based bioreactor using recycled textile opens upthe possibility of resource recovery and energy balance for an economically sustainable biofuel process.

This study hence introduces a novel and first-of-its-kind ‘single-polymer composite’-based bioreactor,which is made from recycled textile or fabric waste. The potential application of APCCF bioreactorin a conventional ethanol industry was achieved by using it as a bioreactor for biofuel productionusing yeast and fungi to convert sugar and organic waste streams (thin stillage or vinasse) into valuableproducts (ethanol and protein-rich fungal biomass). A series of material performance analysis togetherwith an economic analysis were also carried out to compare the performance and cost-effectiveness ofAPCCF-based bioreactor with the conventional bioreactors.

2. Materials and Methods

2.1. Material

Formic acid that was used in this study was supplied by Sigma-Aldrich (Saint Louis, MO, USA,ACS reagent grade, >98%). Acetic acid, ethanol, DL-lactic acid, glycerol, butyric acid, and acetonewere purchased from Sigma-Aldrich and were used without further modifications. The PA66 plainwoven fabric was provided by a local supplier. The PA66 fibre production waste from the weavingprocess at a local Swedish textile company (FOV Fabrics AB, Borås, Sweden) was used as a polymersource. Sugar-to-ethanol industry waste stream, vinasse, was provided by Sepahan Bio-product Company(Isfahan, Iran). Thin stillage, a residual product from the wheat-based first-generation ethanol facility,was provided by Lantmännen Agroetanol (Norrköping, Sweden). Both of the substrates were useddirectly without any further laboratory treatment and were stored at 4 ◦C cold room prior to its use.Detailed chemical characteristics of vinasse and thin stillage are described in previous studies [14].

2.2. Composite Material and Reactor Design

All-polyamide composites were prepared in the form of a flat laminate on the substrate fabricfollowing an isothermal immersion-precipitation non-solvent induced phase separation (NIPS) method,according to our previous report [9]. The glass plate (carrying the fabric and a layer of the PA solution)was immersed in a distilled water bath at room temperature to induce polymer precipitation (phaseseparation) at the end of the casting process. The coagulation process (in the water bath to obtain thecomposites) was prolonged for 1 h, following which the composites were washed with distilled

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water (consecutively for at least 3–4 times) and were subsequently held under the filter-paperpress to remove the moisture. The composites (hereafter referred to as APC sheet) were furtherdried at 55 ◦C in a vacuum oven (≈0.1 bar) for 2 h. For the lab scale APCCF bioreactor design,the APC sheets were attached at their sides and were glued using a commercial adhesive—UniversalPower Epoxy (Loctite, Düsseldorf, Germany) to attain the shape of the bioreactor. To assure theadhesion, the adhesive-containing edges were placed under a hot press (60 ◦C and 220 kN) for 15 min.The detailed design of the fabric reactor is depicted in Figure 1. As the APCCF is made of PA, which isknown as a thermally-resistant polymer, sterilization using heat (autoclave condition) will not have anydetrimental effect on the properties of the composite. The thermal stability properties of the APCCFare tabulated in Figure 2. The reactor did not have any frame; hence, made as a stand-alone bioreactor.The cap (inlet/outlet) was a polyethylene bottle cap glued to the bioreactor using epoxy adhesive.

Figure 1. (a) Schematic of the laboratory scale prototype of the all-polyamide composite-coated-fabric(APCCF) bioreactor; (b) General schematic representation of APCCF bioreactor.

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Figure 2. Comparative onset decomposition temperatures for polyvinyl chloride (PVC) coated-fabric(PVCCF) and APCCF samples in different conditions (un-aged: before the ageing test, the rest arecorresponding to ageing the samples in various mediums). The dark-black corresponds to ‘beforeageing’ and the grey columns correspond to the ‘aged samples’.

Characterization of the Reactor Material

The water impermeability of the APCCFs was analysed using a dead-end diffusion cell. APC sheetwith the diameter of 35 mm was placed in the cell, filled with 25 mL ultrapure water, with subsequentpressure (in the range 0.5, 1, 1.5, 2, and 2.5 bar) being applied to the cell using nitrogen gas. The APCCFsheet was shown to be highly water-proof, with no permeation even at a pressure of 2.5 bar. The tensilestrength properties of the fabric fermenter material were evaluated in accordance with the ISO527standard method [8]. The dumbbell-shaped test bodies (75 mm long, 4 mm width) were tested onan MTS 20/M tensile strength tester (MTS Systems Corporation, Eden Prairie, MN, USA), fittedwith a 5 kN load cell and a special grip for films, using a crosshead speed of 5 mm/min. The gaugelength, preload force, and first approach speed were 0.5 N, 2 mm/min and 33 mm, respectively.The thickness of the composites was measured by Elastocon thickness meter (Elastocon AB, Brämhult,Sweden). A minimum of five test bodies was tested for each material. The specimens were allcut in the warp direction of the fabric. To conduct the tensile strength comparison between theAPCCF and PVCCF, the PVCCF was prepared according to the method previously reported [8].The densities of the composites were obtained by dividing the weight of the composites by theirvolume. Measurements of the composites’ weights were performed by using a balance to determinethe most possible accurate weight, and the volume was measured by a graduated cylinder containingdistilled water. Five specimens were tested for each composite.

The ageing test was performed in the following way: a piece of sample from either PVCCFand APCCF (70 mm × 20 mm) were cut with normal scissor and were placed in six different media,

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including acetic acid, ethanol, DL-lactic acid, glycerol, butyric acid, and acetone, all used in 100%purity. The beakers containing the samples and the media were kept at the room temperature for14 days. Then, the samples were taken out, washed three times with distilled water subsequently threetimes with acetone. Then, the samples were dried in a ventilated oven at 50 ◦C for 24 h. The sampleswere analysed using the tensile strength testing machine and thermogravimetric analysis (TGA)Q500 machine (TA Instruments, MA, USA). About 10 mg of the material was heated from roomtemperature to 600 ◦C at a heating rate of 10 ◦C/min in a nitrogen purge stream.

2.3. Yeast Fermentation

Dry ethanol red yeast (Saccharomyces cerevisiae) from Fermentis (Strasbourg, France) was usedto carry out the yeast fermentation experiments. A starting concentration of 10 g/L of the dry yeastwas used for the process. Fermentation was performed in a 2 L laboratory scale prototype of thebioreactor with a working volume of 1 L at 30 ◦C. The dimensions of the bioreactor were 25 cmlength, 20 cm breadth, and 4 cm width. A schematic of the lab scale prototype is shown in Figure 1a.Sucrose (280 g/L) was used as energy and carbon source, while 7.5 g/L (NH4)2SO4, 3.5 g/L KH2PO4,0.75 g/L MgSO4·7H2O, and 1.0 g/L yeast extract were also added to supply additionally needednutrients. Sucrose concentration dropped to 259.3 ± 1.5 g/L after the feed was autoclaved. Temperaturecontrol was carried out using a GD 120 grant thermostatic circulator (GD Grant Instrument Ltd.,Cambridge, UK). The thermostatic circulator was connected to a 4 m PVC tubing, which was woundeight times and was placed at the bottom of the bioreactor to maintain the desired temperature.

2.4. Fungi Cultivation

A filamentous fungus Neurospora intermedia CBS 131.92 (Centraalbureau voor Schimmelcultures,Utrecht, The Netherlands) was maintained on potato dextrose agar (PDA) slants containing (in g/L):potato extract 4, D-glucose 20, agar 15, and the plates were renewed every six months. For the regularexperimental purpose, the fungus was transferred to fresh PDA plates. The fungal plates were thenincubated aerobically for three to five days at 30 ◦C. For preparing spore suspension, fungal plates wereflooded with 20 mL sterile distilled water and the spores were released by gently agitating the myceliumwith a disposable cell spreader. An inoculum of 50 mL spore suspension per L medium with a sporeconcentration of 6.3 ± 0.8 × 105 spores/mL was used for the cultivations. The fungal cultivations werecarried out aerobically in thin stillage, and vinasse using 3 L capacity APCCF bioreactor, with a totalworking volume of 2 L. Aeration at the rate of 1.0 vvm (volumeair/volumemedia/min) was maintainedthroughout the cultivation, using a perforated sparger with a pore size of 100 µm. Filtration of the inletair was achieved by using a membrane filter (0.1 µm pore size, Whatman, Florham Park, NJ, USA).Samples were collected every 24 h and were stored at 4 ◦C until analyses (unless otherwise specified).Temperature control was carried out using a GD 120 grant thermostatic circulator (GD Grant InstrumentLtd., Cambridge, UK). The pH was adjusted with either 2 M HCl or 2 M NaOH.

All of the experiments and analyses were carried out in duplicate and the results were reportedwith error bars and intervals representing two standard deviations.

2.5. Fermentation Analyses

High-Performance Liquid Chromatography-HPLC system (Waters 2695, Waters Corporation,USA) was used to analyze all liquid fractions from the fermentation experiment. A hydrogen-basedion-exchange column (Aminex HPX-87H, Bio-Rad Hercules, CA, USA) at 60 ◦C with a Micro-Guardcation-H guard column (Bio-Rad) and 0.6 mL/min 5 mM H2SO4 (eluent), was used with a refractiveindex detector (Waters2414, Waters Corporation, Milford, MA, USA) and a UV detector (Waters 2487).The samples that were used for the HPLC analysis were centrifuged for 10 min at 10,000 × g and theliquid portion stored for analysis. The samples were stored at −20 ◦C prior to HPLC analysis. All ofthe experiments and analyses were carried out in duplicate and the results are reported with error bars

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and intervals representing two standard deviations. The yield from the fermentation experiment wascalculated using concentration values after autoclaving, as reported by the HPLC.

2.6. Economic Analysis

The investment cost that was needed for procurement of stainless steel, the predominatingmaterial of construction for the conventional bioreactors used by the industries [3], was estimatedusing Equation (1):

C = Fmexp[11.662 − 0.6104(lnV) + 0.04536(lnV)2] (1)

where C is cost ($) Fm is 2.4 for 304 stainless steel and V is volume (gallons) [15]. The procuring cost ofthe stainless steel reactors was updated to 2017 using the projected Chemical Engineering Plant CostIndex (CEPCI) for 2017, which was 574.1, based on the current low oil price [16]:

Cupdated = C (Iupdated/I) (2)

The purchasing cost of the fabric material for reactor construction was obtained from the estimatesat a local textile company in Borås, Sweden. The investment cost in the bioreactor is a key part of thecapital expenditure on the fermentation or waste-to-product transformation process. It influences thecost of handling waste or production of the desired products, as shown in Equation (3) [17], where FC isthe cost of feedstock ($/tonnes), Y is product yield (m3/tonne), ACE is the annual capital expenditure($/m3), OC is the operation cost ($/m3), Ye is the electricity yield (kWh/m3), and EC is electricitycredit ($/kWh):

Annual production cost (APC) = FC/Y + (ACE + OC) − Ye.EC (3)

3. Results and Discussion

To overcome the challenges that are associated with the conventional microbial bioreactors,a robust polymer composite (APCCF) was used to prepare the textile bioreactor in this study.A PA-coated PA fabric single-polymer composite (based out of PA66) was prepared in the study toaddress the issues surrounding the current coated-fabrics (material of construction of textile-bioreactors(TBRs)), such as the recyclability, the adhesion between the coating, and the fabrics. This was achievedby applying the PA66 solution to the PA fabric using a universal film applicator with subsequentcoagulation in a water bath, to induce phase separation (phase inversion). Hence, a compositecomposed of a thin continuous PA layer (the coating) and a PA textile fabric encompassing themost common type of aliphatic polyamide (PA66) was obtained, which forms the base material for theAPCCF bioreactor.

3.1. Material Development

Mechanical stability of materials that are used for constructing bioreactors is generally of highimportance [5]. Both stainless steel and concrete, which are conventionally being used for bioreactorconstruction, have got higher levesl of tensile strength than any available commercial polymer.The fabric-based bioreactor possesses several merits, such as being cost-effective, less time consumingto install, easiness with transportation, and foldability. However, the bioreactors made from PVCCF willhave several challenges, such as being susceptible to shear stresses, and in some cases, the delamination(detaching of the coating from the fabric). The results from this study suggest that in the all-polyamidecomposite-coated-fabric (APCCF), the mechanical properties have been improved (Table 1). Hence, it wasclear that the APPCF has superior mechanical properties (increase by around 20%) than the PVCCF.This could be attributed to the nature of the polymers. PA generally contains amide groups that areprominent acceptor/donator in the hydrogen-bonding [18,19], which establishes strong intermolecularinteractions. However, PVC by nature does not have strong intermolecular interaction; therefore, it is more

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susceptible to fast-breaking. Hence, the APCCF could be more robust, resulting in an extended lifetime forthe APCCF bioreactor material.

Table 1. Comparison between the tensile strength properties of PVC-coated polyester fabric (PVCCF)and all-polyamide composite-coated-fabric (APCCF). Un-aged samples are the ones before ageing.

Medium PVCCF ∆ (%) APCCF ∆ (%)

un-aged 57.2 ± 2.19 n.d. 68.6 ± 1.77 * n.d.acetic acid 49.8 ± 1.71 12.9 63.2 ± 1.21 7.9

acetone 52.5 ± 2.3 8.2 67.3 ± 1.9 1.9butyric acid 53.1 ± 1.8 7.2 65.7 ± 1.61 4.2

ethanol 55.3 ± 1.6 3.3 67.4 ± 1.2 1.7lactic acid 51.2 ± 1.4 10.5 63.1 ± 2.1 8.0glycerol 55.1 ± 1.7 3.7 68.1 ± 1.4 0.7

* The numbers after ‘±’ represent the standard deviations.

In the ageing test, the APCCF as well as the PVCCF, were kept in six different organic solvents,which are the most commonly produced metabolites in the microbial processes [20]. According to theresults (Table 1), in all of the test solvents, both APCCF and PVCCF were affected with a decline in itsmechanical properties; however, in all of the cases, the decrease in tensile strength value for the APCCFwere lower than the one for the PVCCF. For acetic acid, which is the second strongest unmodifiedorganic acid (after formic acid), the PA chain was found interacting with acetic acid molecules.

According to Chen et. al. [21], there is an interaction between acids and the PA polymer atthe surface. The aforementioned interaction can weaken the intramolecular interactions, as eachof the amide groups can only have two hydrogen bonding interactions with other amide groups.While there is better hydrogen bonding, donor/acceptor exists in the vicinity of the surface PA chain,the amide groups prefer them and loosen the previous interaction with the inner PA chains. In thiscase, acetic acid is a better hydrogen bonding acceptor (due to having a partially negative chargeon the oxygen in its carbonyl) and a better hydrogen bonding donator from the acidic hydrogen.Apart from this interaction, as PA can behave like a base, there could be another interaction in the formof acid/base interaction. Both of these two interactions decrease the internal chains (from the surfacetowards the bulk of the polymer), which leads to the decrease in crystallinity of the polymer, which inturn decreases the tensile strength value. This is the case with other acids (lactic/butyric acid) with thedifference in the intensity of the effect.

Butyric acid has the same structure similar to acetic acid, with a longer hydrophobic chain,which in this case, decreases the hydrogen bonding ability. The lower hydrogen bonding ability is dueto having a longer electron-donor alkyl group that decreases the density of partially positive chargeon the carbon atom in the carbonyl group, which in turn decreases the difference in charge densitybetween carbon and oxygen in the carbonyl group. Furthermore, the longer aliphatic chain decreasesthe mobility of the molecule and creates a bigger repellence between the butyric acid molecule andthe PA chain, which leads to less interaction between them [22]. Less interaction between the PA andbutyric acid means that more intramolecular interactions will remain. Hence, the crystallinity will bechanged less. It can be confirmed with the tensile strength values in Table 1.

For the case of lactic acid, although it has a longer aliphatic chain than acetic acid, it hasone more hydrogen bonding site —hydroxyl group— which will increase the interactions betweenlactic acid and PA. That might be the reason of a lower measured value for lactic acid than aceticacid in tensile strength testing. The values for ethanol and glycerol are reasonable with the aboveproposal; however, for acetone, which has less hydrogen bonding ability when compared to all of theother five solvents, the decrease in the tensile strength value should not be more than the one forglycerol, if the above proposal is correct. We assume that there might be other interactions involvedbetween acetone and PA, which needs to be elaborated in a separate study. Although the actionof the chemical substances towards the materials of construction of the bioreactor might be weak

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(as cultivation media mainly consists of water), we used the pure chemicals to accelerate the ageingprocess. Since the ageing study was a comparative-based study between APCCF and PVCCF, the effectof the chemicals’ concentration would be the same for both composites. Hence, the results could beassumed as meaningful findings.

PVC is also affected by protic and polar solvents [23]. The reason for more decrease in tensilestrength value for PVCCF when compared to APCCF in different mediums could be related to thereason that PVC contains a soft chlorine ion in its structure, which increases the tendency of establishinghydrogen binding between the surface PVC chains and the medium surrounding it. Also, PVC innature is a more amorphous polymer when compared to PA [24,25], which makes it more susceptibletowards surrounding chemicals/medium.

Figure 2 shows the TGA results for the un-aged samples and the samples that are aged in sixdifferent solvents. Similar to the above proposal for the tensile strength testing, in all of the cases,there is a decrease in the onset decomposition temperature (ODT) value after ageing, which couldbe related to the hydrogen bonding (discussed above). Although the differences are not significant,the decrease in ODT values for APCCF samples are less than the ones for APCCF, meaning thatAPCCF has superior thermal stability. Though the textile bioreactor will never experience those hightemperatures (e.g., 300–400 ◦C), having a higher ODT value (both in un-aged and aged samples)will give a better long-term stability to the APCCF [26]. From the tensile strength testing and TGAresults, both in the un-aged sample and in the aged samples in different mediums, we can concludethat APCCF is a better candidate for making textile bioreactor than PVCCF. As the APCCF hasthe ODT (371 ◦C) above the autoclave temperature (121 ◦C), one can assume that the APCCF asan ‘autoclave-proof’ material.

PA is a polymer containing monomers of amides joined by peptide bonds. They can either occurnaturally (for instance, proteins such as wool and silk) or can be made artificially, for example, nylons,aramids, and sodium poly (aspartate). According to McCrum and Buckley [27], in general, polyamidespresents a proper conciliation between toughness and strength with a low coefficient of friction andhigh thermal resistance (melting temperatures above 200 ◦C and thermal deflection—under low loadsuperior to 160 ◦C). Using this polymer can hence impart superior properties to the fabric bioreactorrather than using other commercially available polymers. Polyamide 66, with its high abrasionproperties together with the high strength, can be considered as the most suitable candidate polymerfor the development of fabric bioreactor. Polyamide 66, hence imparts high strength (withstanding thehigh pressure of fermentation media inside the reactor), as well as enough chemical resistance towardsthe chemical or biological process occurring within the reactor.

Figure 3 demonstrates the cross-sectional morphology of the two coated-fabrics. One can observethat only a few of the fibre-filaments in the PVCCF are attached to the coating, while in the APCCF,almost all of the fibre-filaments in the first row of the side of the fabric facing to the coating are adheredto the coating as well as adhering to each other. Adhesion on coating industries plays a crucial role [28].If the adhesion is not good enough, then the coating will be detached from the fabric after a certain time,called delamination [29]. In our case, the adhesion is much enough for the first-row fibre-filaments tobe fused to each other. The fibre-filaments not only are adhered to the coating but are also fused toeach other making the coating stronger.

Figure 4 shows the comparative density of the PVCCF and APCCF. PVCCF is composed oftwo main ingredients, PVC and polyester, which are heavy in comparison with other conventionalpolymers [30]. PVC and polyester are both heavier (in volume/mass unit) than PA—the sole ingredientof the APCCF, making APCCF a lighter material. The decrease in density (by around 16%) shows thatthe transportation cost of the bioreactor would be lower and also it would be easier. On the other hand,the total weight of the reactor in case of using APCCF as the material of construction of the bioreactorwould be less than the cost of the PVCCF one.

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Figure 3. Cross-sectional SEM picture of PVCCF (a) and APCCF (b,c). The fused parts are shown withthe yellow ovals (d). In PVCCF, the adhesion is not homogeneous and strong while in APCCF, the firstrow of the fibre-filaments of the PA fabric are merged together and adhered to the coating. It willdecrease the chance of delamination in a long span of time.

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Figure 4. The comparative density of the PVCCF and APCCF. PVCCF is composed of two main ingredients:PVC and polyester which both are heavier in volume/mass unit than that of polyamide—the solecomponent of the APCCF, resulting in a lighter material.

PA chains contain amide groups (a weak organic base), so they are not susceptible to mildacidic conditions (up to pH > 3), that represents the pH conditions of most fermentation media(pH 5–7). In this regard, polyolefins (polyethylene and polypropylene) are also comparable withpolyamides; however, they are not readily soluble, unlike polyamides, hence posing potentialchallenges during the production process. Another excellent property of PA that makes it the solematerial for the single-polymer composite is its thermal stability. Due to the presence of nitrogenalong with the hydrogen atoms on each polymer chain, they pose strong hydrogen bonding thathinders the disentanglements of the chains, which in-turn limits the thermal decomposition ofthe polymer. However, the argument that the fabric-based bioreactor will never experience hightemperatures (more than 40–50 ◦C), which also eliminates the need to consider the onset thermaldecomposition temperature for this material, could remain valid. Nevertheless, the long-term exposureof the industrial scale APCCF bioreactor to the atmospheric temperature (around 35 ◦C) could beconsidered since the polymers will thermally decompose both at high temperatures (for the relativelyshort time) and also at moderate/low temperatures for a longer exposure time. In this regard, the useof material such as PA66, with a higher thermal resistance will guarantee a longer shelf-life for thebioreactor. Additionally, in an environmental perspective, the use of single-polymer composite material(i.e., PA66) that is recycled from the textile industry presents the opportunity for making the fabricbased reactor an environmentally sustainable product.

3.2. APCCF Bioreactor for Valorization of Waste-Stream from Conventional Ethanol Industries

Introduction of the APCCF bioreactor to the conventional ethanol industries that follows eitherstarch or sugar-based processes was achieved by using edible filamentous fungi. The use of filamentousfungi for generating value-added products from ethanol industry waste streams, such as thin stillage(from the starch-based process) or vinasse has been previously studied in conventional bioreactors [14].Comparable results were obtained from the present study using the fungus neurospora intermedia,proving the potential application of fabric bioreactor (APCCF bioreactor) for filamentous fungicultivation. The results from the fermentation of thin stillage and vinasse in the fabric are depicted inFigure 5. Fermentation of thin stillage resulted in the formation of 3.5 ± 0.3 g/L of dry weight fungalbiomass corresponding to a biomass yield 24.6% from the total fermentable sugar, which is comparableto 4 g/L of dry weight biomass obtained from bubble column bioreactor that used thin stillage fora continuous cultivation process [14]. An ethanol maximum of 4.9 ± 0.6 g/L was observed at the

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cultivation time of 36 h with a high rate of fermentable sugar assimilation within the first 24 h of fungalgrowth (93.3% reduction). Similarly, the fermentation of vinasse at a dilution rate of 10% resulted inthe formation of 8.5 ± 0.7 g/L of dry weight fungal biomass. The higher fungal biomass productionin vinasse could be attributed to the presence of essential mineral components that are present init, which support prolific fungal growth. As opposed to the thin stillage cultivation, a much slowersugar assimilation rate was observed with vinasse, with only 22.2% reduction within the first 24 h.However, the complete utilization of the fermentable sugar was observed within the next 12 h, withno sugar being left after 36 h of the cultivation time (Figure 5). Cultivations at the pilot scale APCCFbioreactor resulted in the growth of fungal biomass that attributes for a total crude protein contentof 51%.

Figure 5. Fermentation of thin stillage (a) and vinase (b) in the textile bioreactor showing totalfermentable sugar ( ), ethanol (�), and glycerol ( ).

3.3. APCCF Bioreactor for Conventional Ethanol Production

Fermentation was carried out in the lab scale prototype of the APCCF bioreactor at 30 ◦C and pHof 6.0 ± 0.2 using the ethanol-producing yeast as explained in Section 2.3. The result of the fermentationis shown in Figure 6. Ethanol yield 96.3% of the theoretical maximum was attained using averageethanol concentration during the stationary phase. Ethanol specific productivity and substrate-specificconsumption rate of 4.0 g/L/h and 0.7 g/g/h were attained in the bioreactor, which are comparable tothe reported values from sugarcane-based ethanol production facilities [31,32].

Figure 6. Fermentation performed in the lab scale prototype of textile bioreactor at 30 ◦C showingsucrose ( ), ethanol (�) in the primary axis and glycerol (�) in the secondary axis.

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3.4. Economic Evaluation and Cost Comparisons

A cost-competitive bioreactor installation is one of the many opportunities that are currentlyevaluated to create an economically sustainable biofuel process. The procuring cost of conventionalstainless steel bioreactor vessels at different volumes was estimated using Equations (1) and (2),as depicted in Table 2. Similarly, the procurement cost of varying textile bioreactor volumes is alsoshown in Table 2. It can be observed that the textile bioreactor capital investment cost is at least thriceless expensive than the cheapest of the stainless steel reactor that meets the requirements of a bioreactor,which is 304-stainless steel.

Table 2. Comparative procurement cost for APCCF bioreactor and 304-SSBR vessels.

Reactor Size (m3) Purchase Cost of Developed TBR ($) Purchase Cost of 304 SSBR ($)

100 25,000 108,000200 35,000 137,000300 45,000 160,000400 58,000 181,000500 66,000 200,000

The estimated operation and investment cost of a stainless steel reactor is 1.7 times of itsprocurement cost after it has been installed [15], while that for a fabric bioreactor is 1.5 times ofits procurement cost for a 15 year period [33]. This contributes to the annual production cost, as shownin Equation (3). Assuming that the production facility requires a 500-m3 bioreactor for producing thedesired product, and the capital expenditure is depreciated using straight-line depreciation for 15 years.The developed bioreactor would contribute $7700/m3/year to the annual production cost, while thestainless steel bioreactor would contribute $33,333/m3/year to the annual production cost. If 5000 m3

of feedstock is processed in a year, using the currently developed bioreactor would potentially lead toa reduction of the annual production cost by $128,000,000.

4. Conclusions

A textile-bioreactor, prepared using the industrial waste polymer (all-polyamide compositecoated-fabric (APCCF)), was introduced in this study. The tensile strength testing, density measurements,ageing test, and thermal stability analyses showed that the APCCF poses superior characteristics thanthe PVCCF—the material that is currently being used to prepare the conventional textile-bioreactors.Hence, APCCF could be considered as a better candidate for the material of construction of thetextile-bioreactors. Introduction of the APCCF bioreactor to the ethanol industry was achieved usingyeast for ethanol production and filamentous fungi for generating value-added products, such as fungalbiomass protein, using the waste streams from the ethanol industry. The fungal fermentation and theeconomic analysis of the textile bioreactor showed comparative results with the conventional bioreactors.In an environmental perspective, the use of single-polymer composite material (i.e., polyamide 66) thatis recycled from the textile industry, presents the opportunity for making the fabric-based bioreactoran environmentally sustainable product.

Acknowledgments: The authors would like to acknowledge FOV Fabrics AB (Borås, Sweden) for providing thepolyamide fabric, the PVC-coated polyester fabric and the polyamide 66 scraps. This project was financiallysupported by University of Borås, Borås, Sweden and FOV Fabrics AB.

Author Contributions: M.J. developed and designed the bioreactor; M.J., R.B.N., and O.A.O. conceived, designed,and performed the experiments; M.J., O.A.O., and R.B.N. contributed to analyzing the data and writing the paper;M.J.T. supervised the study and revised the manuscript.

Conflicts of Interest: The authors declare no conflict of interest.

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tanks and bioreactors. Biochem. Eng. J. 2016, 114, 62–69. [CrossRef]7. Chernukhina, A.I.; Gabrielyan, G.A. Thermostabilization of aliphatic polyamides and fibres on their base

(review). Khimicheskie Volokna 1993, 30–34.8. Jabbari, M.; Åkesson, D.; Skrifvars, M.; Taherzadeh, M.J. Novel lightweight and highly thermally insulative

silica aerogel-doped poly(vinyl chloride)-coated-fabric composite. J. Reinf. Plast. Compos. 2015, 34. [CrossRef]9. Jabbari, M.; Skrifvars, M.; Åkesson, D.; Taherzadeh, M.J. Introducing all-polyamide composite coated-fabrics:

A method to produce fully recyclable single-polymer composite coated-fabrics. J. Appl. Polym. Sci. 2016, 133,42829. [CrossRef]

10. Angelov, R.R.; Georgieva, B.C.; Karashanova, D.B. Films of recycled polyethylene terephthalate, obtained byelectrospraying, for paper and textile impregnation. Bulg. Chem. Commun. 2016, 48, 156–160.

11. Riley, K.; Williams, J.; Waldron, D. End of life opportunities for textiles in the UK healthcare sector.J. Fiber Bioeng. Inform. 2009, 709–717.

12. Sommers, J.; Kho, H.S.; Al-Ghamedi, R.; Low, I.M.; Davies, I.J.; Latella, B.A. Mechanical and physicalproperties of recycled cellulose fibre-reinforced epoxy eco-composites. Adv. Mater. Res. 2008, 41–42, 317–322.[CrossRef]

13. Low, I.M.; Somers, J.; Kho, H.S.; Davies, I.J.; Latella, B.A. Fabrication and properties of recycled cellulosefibre-reinforced epoxy composites. Compos. Interfaces 2009, 16, 659–669. [CrossRef]

14. Ferreira, J.A.; Lennartsson, P.R.; Taherzadeh, M.J. Production of ethanol and biomass from thin stillage byneurospora intermedia: A pilot study for process diversification. Eng. Life Sci. 2015, 15, 751–759. [CrossRef]

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16. Mignard, D. Correlating the chemical engineering plant cost index with macro-economic indicators.Chem. Eng. Res. Des. 2014, 92, 285–294. [CrossRef]

17. Bergeron, P. Bioethanol market forces. In Handbook on Bioethanol: Production and Utilization; Wyman, C., Ed.;CRC Press: Boca Raton, FL, USA, 1996; pp. 61–88.

18. Vinken, E.; Terry, A.E.; Spoelstra, A.B.; Koning, C.E.; Rastogi, S. Influence of superheated water on thehydrogen bonding and crystallography of piperazine-based (co)polyamides. Langmuir 2009, 25, 5294–5303.[CrossRef] [PubMed]

19. Behler, K.; Havel, M.; Mattia, D.; Gogotsi, Y. Self-assembled multi-walled carbon nanotube coatings.In Proceedings of the Materials Research Society (MRS) Fall Meeting, Boston, MA, USA, 1–5 December 2008;pp. 133–138.

20. Sathitsuksanoh, N.; George, A.; Zhang, Y.H.P. New lignocellulose pretreatments using cellulose solvents:A review. J. Chem. Technol. Biotechnol. 2013, 88, 169–180. [CrossRef]

21. Chen, F.; Chang, W.V. Applicability study of a new acid-base interaction-model in polypeptides andpolyamides. Langmuir 1991, 7, 2401–2404. [CrossRef]

22. Carey, F.A.; Sundberg, R.J. Advanced Organic Chemistry; Springer Science & Business Media: Berlin, Germany, 2000.23. Herrero, M.; Tiemblo, P.; Reyes-Labarta, J.; Mijangos, C.; Reinecke, H. PVC modification with new functional

groups. Influence of hydrogen bonds on reactivity, stiffness and specific volume. Polymer 2002, 43, 2631–2636.[CrossRef]

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24. Bao, Y.; Weng, Z.; Huang, Z.; Pan, Z. The crystallinity of pvc and its effect on physical properties.Int. Polym. Process. 1996, 11, 369–372. [CrossRef]

25. Gilbert, M. Importance of crystallinity in pvc. Prog. Rubber Plast. Technol. 1993, 9, 143–158.26. Rathi, S.; Dahiya, J.B. Effect on thermal behaviour of polyamide 66/clay nanocomposites with inorganic

flame retardant additives. Indian J. Chem. A 2012, 51, 1677–1685.27. Buckley, R.W. Polymer Enhancement of Technical Textiles; iSmithers Rapra Publishing: Akron, OH, USA, 2003.28. Sargent, J.G.; Lee, J.S.; Reynaud, E.; Gilbert, M.D.; Sloan, J.M. Study of selectively permeable coatings to

textiles. In Proceedings of the 2010 Materials Research Society (MRS) Fall Meeting, Boston, MA, USA,29 November–2 December 2011; pp. 497–502.

29. Rohwerder, M.; Stratmann, M. From molecular aspects of delamination to new polymeric coating. Macromol. Symp.2002, 187, 35–42. [CrossRef]

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© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Paper IV

Research ArticleNew Solvent for Polyamide 66 and Its Use for Preparing a Single-Polymer Composite-Coated Fabric

Mostafa Jabbari , Mikael Skrifvars, Dan Åkesson, and Mohammad J. Taherzadeh

Swedish Centre for Resource Recovery, University of Borås, Borås, Sweden

Correspondence should be addressed to Mostafa Jabbari; mostafa.jabbari@hb.se

Received 1 July 2018; Accepted 13 September 2018; Published 17 October 2018

Academic Editor: Andrea Camposeo

Copyright © 2018 Mostafa Jabbari et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Polyamides (PAs) are one of the most important engineering polymers; however, the difficulty in dissolving them hinders theirapplications. Formic acid (FA) is the most common solvent for PAs, but it has industrial limitations. In this contribution, weproposed a new solvent system for PAs by replacing a portion of the FA with urea and calcium chloride (FAUCa). Urea impartsthe hydrogen bonding and calcium ion from the calcium chloride, as a Lewis acid was added to the system to compensate forthe pH decrease due to the addition of urea. The results showed that the proposed solvent (FAUCa) could readily dissolve PAs,resulting in a less decrease in the mechanical properties during the dissolution. The composite prepared using the FAUCa hasalmost the same properties as the one prepared using the FA solution. The solution was applied on a polyamide 66 fabric tomake an all-polyamide composite-coated fabric, which then was characterized. The FAUCa solution had a higher viscosity thanthe one prepared using the neat FA solvent, which can be an advantage in the applications which need higher viscosity likepreparing the all-polyamide composite-coated fabric. A more viscous solution makes a denser coating which will increase thewater /gas tightness. In conclusion, using the FAUCa solvent has two merits: (1) replacement of 40% of the FA with lessharmful and environmentally friendly chemicals and (2) enabling for the preparation of more viscous solutions, which makes adenser coating.

1. Introduction

Aliphatic polyamides (PAs), also called nylons, are a classof semicrystalline polymers that contain amide groupswhich are intercalated along linear alkane chains [1]. Solu-tion processing of aliphatic PAs is quite challenging due tothe fact that only a few solvents, such as formic acid (FA)and cresol [2] or fluoric solvents [3], can dissolve them.All of the solvents being used for the dissolution of PAshave severe environmental challenges. A few attempts havebeen made [2] to replace the existing solvents or proposenew solvents. Papadopoulou et al. [2] mixed FA with tri-fluoroacetic acid and acetone. Charlet et al. [4] studiedthe crystallization and dissolution behavior of polyamide6-water systems under pressure. Nirmala et al. [5] usedFA (85wt%), acetic acid, dichloromethane, 1,1,1,3,3,3-hex-afluoro-2-propanol, trifluoroacetic acid, and chlorophenolin their study. Basically, dissolving PAs is difficult due totwo reasons: (a) polyamides are highly crystalline, whereas

the above treatment holds for amorphous polymers, and(b) solvents for polyamides are believed to act by virtueof strong, highly specific polar forces [6].

Polyamide (PA) is a well-known high-performance engi-neering plastic (technical thermoplastic [7]) with highstrength and good fatigue resistance [8] and excellentmechanical and physical properties, which is why it isincreasingly used in industrial machinery [7]. Nylon is thecommon name of linear aliphatic PAs. Nylons are importantcommercial polymers, with uses ranging from fibers to cook-ing bags to coatings [9], carpets, upholstery, and apparel [10].

Coated fabrics are flexible composites, consisting of afabric substrate and a polymeric coating [10]. The coatingcould be on one side or on both sides, with either thesame or different polymeric coating per side [10]. Coatedwoven fabrics are used in a wide range of structural appli-cations to provide lightweight, architecturally striking solu-tions [11]. The physical properties of a coated fabricdepend on the specific properties of the substrate, coating

International Journal of Polymer Science Volume 2018, Article ID 6235165, 9 pages https://doi.org/10.1155/2018/6235165

formulation, coating technique, and processing conditionsduring coating [10]. There are two principal types ofcoated woven fabric: a glass fiber fabric with a PTFE(polytetrafluoroethylene) coating and a polyester fabricwith PVC (polyvinyl chloride) coating [12]. Both PTFE-and PVC-coated fabrics are employed today in differenttypes of tents and architectural membrane structures allover the world [13]. As the fabric is prepared from ahigh-crystalline polymer (spun fiber), it can resist environ-mental loads, as tensile stresses in the plane of the fabric[12]. These fabrics are popular mainly due to their afford-able prices, high strength, durability, resistance to wearand tear, various colors, and soft texture [11]. They areoften used for wide-span surfaces, membrane-cable struc-tures, and pneumatic constructions [13]. One recent appli-cation of coated fabrics is their application as a textilebioreactor which is a vessel to conduct the biological pro-cesses (fermentations) [14]. The PVC-coated fabric wasthe first material proposed for this purpose. The recentpromising candidate was the all-polyamide composite-coated fabric (APCCF) which showed superior propertiesover the former one [14]. However, using a high amountof formic acid as the solvent is still one industrial chal-lenge for scaling-up the production.

The aim of this paper was to introduce a new solventbased on the replacement of FA with urea, calcium chloride(industrially abundant chemicals), and water to produce anall-polyamide composite-coated fabric from a PA by solventcasting of only one component to have strong adhesionbetween the coating and the fabric as well as enhancedrecyclability.

2. Material and Methods

2.1. Materials. The formic acid, urea, and calcium chlorideused in this work were supplied by Sigma-Aldrich (ACSreagent grade, >98%). The PA66 plain woven fabric (70grams per square meter (gsm)) was provided by FOV FabricsAB (Borås, Sweden). As a polymer source to produce a solu-tion, PA fiber production waste from the weaving process atFOV Fabrics was used.

2.2. Solvent Preparation. The solutions were prepared byadding 35 g of PA waste scraps into 100 g solvent mix-tures, which, in turn, were obtained by mixing differentamounts of formic acid (FA), urea (U), calcium chloride(Ca), and distilled water (W) at room temperature,according to the values tabulated in Table 1. For the casesthat had more urea and calcium chloride, it took a while(1–3 minutes) to get a clear solution.

2.3. Composite Preparation. All-polyamide composites wereprepared in the form of a flat laminate on the substrate fabricusing an isothermal immersion-precipitation method. Thesolutions of the PA production waste in the formic acid weremade by dissolving 35 g of PA in 100 g solvent at room tem-perature. In low concentrations (less than ≈30% w/w), PA66readily dissolves in the formic acid containing the solventmixture at room temperature, but for higher concentrations,

the solution should be agitated for a longer time. In order toassure the completion of dissolution and have the same agi-tation condition for all the solutions, the sealed solutionflasks were put in a shaker at a speed of 100 rpm for 20 hoursat 55°C to obtain a homogeneous solution [11]. The solu-tions were cooled to room temperature, and after centrifuga-tion for 10 minutes at a speed of 16,000×g (meaning 16,000times Earth’s gravitational force) to remove the bubbles, thedopes were cast on a PA fabric with a size of 18× 24 cm fixedon a glass with adhesive tape, using a ZUA 2000 universalfilm applicator (Zehntner GmbH Testing Instruments, Sis-sach, Switzerland) with a gap of 175μm. Once the castingprocess was done, after waiting 30 seconds, the glass plate(carrying the fabric and a layer of the PA solution on topof it) was immersed in a distilled water coagulation bath(10 L) at room temperature to induce polymer precipitation(phase separation). After one hour of coagulation (in thewater bath), the composites obtained were first washed threetimes with distilled water and then held under light pressbetween two sheets of filter papers and dried at 55°C in avacuum oven (≈0.1 bar) for 2 hours. The samples were keptin a vacuum zipper storage bag for further analysis. Thecomposites were named according to their correspondingsolution ingredients (Table 1). The thickness of the compos-ites varied between 150 and 200μm. The reason of variationin the thickness could be the strength of the solvent. In otherwords, if the solvent is stronger, it will dissolve the surface ofthe fabric in a greater extent which leads to a more reductionof the composite thickness. The thickness of the fabric was100μm initially.

Table 1: Ingredients for the solutions (in some cases, the sum is not100%—due to the mathematical rounding of the numbers—thenames are only for labeling.)

Sample name (FA-U-Ca-W, %) FA (%) U (%) Ca (%) W (%)

100-0-0-0 100.00 0.00 0.00 0.00

60-7-20-13 60.00 6.67 20.00 13.33

64-7-14-14 64.29 7.14 14.29 14.29

90-0-0-10 90.00 0.00 0.00 10.00

90-10-0-0 90.00 10.00 0.00 0.00

82-9-9-0 81.82 9.09 9.09 0.00

69-8-15-8 69.23 7.69 15.38 7.69

67-4-30-0 66.67 3.70 29.63 0.00

82-0-0-18 81.82 0.00 0.00 18.18

75-8-17-0 75.00 8.33 16.67 0.00

75-0-0-25 75.00 0.00 0.00 25.00

55-6-39-0 54.55 6.06 39.39 0.00

50-8-42-0 50.00 8.33 41.67 0.00

47-8-39-5 47.37 7.89 39.47 5.26

56-6-25-13 56.25 6.25 25.00 12.50

50-6-28-17 50.00 5.56 27.78 16.67

43-7-36-14 42.86 7.14 35.71 14.29

38-6-43-13 38.30 6.38 42.55 12.77

35-6-48-12 34.62 5.77 48.08 11.54

2 International Journal of Polymer Science

2.4. Characterization Methods. The tensile strength proper-ties were evaluated in accordance with the standard methodISO 527 [15]. Dumbbell-shaped test bodies, 75mm long(with a width of 4mm), were tested on an MTS 20/M tensilestrength tester (MTS Systems Corporation, Eden Prairie,MN, USA), fitted with a 5 kN load cell and a special grip forfilms, using a crosshead speed of 5mm/min. The gaugelength, the preload force, and the first approach speed were33mm, 0.5N, and 2mm/min, respectively. The thickness ofthe composites was measured using an Elastocon thicknessmeter (Elastocon, Sweden). A minimum of five test bodieswas tested for each material. The specimens were all cut alongthe warp direction of the fabric.

To investigate the viscoelastic properties of the compos-ites, dynamic mechanical thermal analysis (DMA Q800, TAInstruments, Waters LLC, USA) was performed on the pre-pared composites. The specimens were run with a film ten-sion clamp using the temperature ramp procedure with asample dimension of approximately 15× 9mm. The temper-ature ranged from room temperature to 180°C with a heatingrate of 3°C/min; the frequency and the amplitude were 2Hzand 15% elongation at max, respectively.

Scanning electron microscopy (SEM) was used to moni-tor the fracture surface morphology of the cross sections ofthe composites and to check the adhesion between the coat-ing (the PA film) and the fabric. The specimens wereobtained by quenching in liquid nitrogen and breaking byhand. As the samples contained fabric, using only quenchingin liquid nitrogen did not take apart the sample completely;therefore, after breaking, the unbroken parts were cut by asharp blade. The studied surface was sputtered with a layerof gold before the measurements. SEM analysis was per-formed using AIS2100 (Seron Technology, Korea) operatedat an acceleration voltage of 18 kV.

The viscosity measurements were carried out with aBrookfield viscometer (MA, USA) at 40°C. The tests weredone within 30 seconds for each sample.

Thermogravimetric analysis (TGA) was performed onthe composites using the Q500 machine (TA Instruments,MA, USA). About 8mg of the material was heated fromroom temperature to 700°C at a heating rate of 10°C/min ina nitrogen purge stream.

The pH measurements were carried out using a Jenway3505 pH meter (Staffordshire, UK) at room temperature.The pH sensor was placed in the samples for 30 secondswhile being stirred at 200 rpm to reach to an equilibrium.

3. Results and Discussion

The most common solvent for polyamides is formic acid(FA), which is neither safe nor easy to handle. To replace aportion of the FA to make it more appropriate, more eco-nomical, and safer to handle solvent mixtures, differentamounts of urea, calcium chloride, and water were mixedand added to the FA. Thereafter, the polymer was added tothe solvent mixtures to make homogeneous solutions. Thesolutions were used to make all-polyamide composite-coated fabrics by means of a universal film applicator andconsecutive coagulation in a water bath as a nonsolvent in

order to induce phase separation (phase inversion). As aresult, a composite composed of a thin continuous PAlayer (the coating) and a PA fabric out of the most com-mon type of aliphatic PA (PA66) was obtained. Thehypothesis in this work was that since each urea moleculecontains four hydrogens and can establish strong intermo-lecular interactions (H-bonding) with the amide groups inthe PA, it can disrupt the intermolecular interactions(hydrogen bonding) among the polymer chains, thereforeresulting in an easier dissolution. In general, the dissolu-tion of semicrystalline polymers such as polyamide com-prises several steps including solvent penetration [16],decrystallization of crystalline domains, amorphous poly-mer swelling, and chain untangling [17]. Formic acid (asthe main solvent) has enough penetration power to pene-trate into the PA chains [18]. However, the main goal inthis paper was to replace a portion of the FA, meaningthat the replacement molecule needed to have enoughinteractions with the PA chains to start the penetration.As discussed earlier, urea has four hydrogen bonding perunit of molecule, and even if all the four hydrogen cannotengage in hydrogen bonding, urea still has high capabilityof hydrogen bonding interactions. On the other hand, PAchains contain amide groups which are the strong hydro-gen bonding donor/acceptor groups [19], while FA hasonly one hydrogen bonding per unit of molecule. There-fore, the mixture of FA/urea will have a higher tendencyto penetrate into the PA chains. The other steps of disso-lution (decrystallization of crystalline domains, amorphouspolymer swelling, and chain untangling) would be donefaster and easier if the first step (solvent penetration)occurs faster/easier [20, 21]. This process will be enhancedby using calcium chloride as a Lewis acid (to nullify theincreased pH due to the addition of urea—an organicbase) as well as localizing the paired electrons on theamide groups of PA chains. Different properties were ana-lyzed, and the process parameters were optimized. Theselection of different percentages of the solvent’s compo-nents (FA, urea, calcium chloride, and water) was doneby trying to keep the percentage of the FA as low as pos-sible. Water was added to the solvent mixture, both forimparting hydrogen bonding [22] and also for its well-known effect on the dissolution of polyamides [4, 23]when it is used in lower percentages.

3.1. Mechanical Properties. The maximum force divided bythe cross-sectional surface area of the samples needed tobreak the composite in the tensile strength mode; also, theelongation at max of the composites is shown in Figure 1.The thickness of the composites varied between 150 and200μm.

The results from the tensile strength testing (Figure 1)implied an irregular trend. However, a slight trend is evidentin terms of the FA content. Four composites had the tensilestrength values higher than the one obtained from a solventout of pure FA. It could be related to the fact that FA partiallydecomposes the PA chains during the dissolution process[11]. In other composites, which have an FA content that iseven less than the FA content of those four good composites,

3International Journal of Polymer Science

the tensile strength values are not higher than those of thecomposite 100-0-0-0 (with pure FA). Although it might seemcontradictory with the above proposal for increasing the ten-sile strength value, it could be the effect of the urea/calciumchloride content (and even water content) on the crystalliza-tion of the PA chains in the phase inversion process. Regard-ing the elongation at max, all the composites prepared fromthe new proposed solvent mixtures in this work have lowervalues compared to the 100-0-0-0 composite. This might berelated to the effect of the localization of the paired electronon the amide groups of PA chains, which plays a role like aspacer [24] during the phase inversion process. We can con-clude that the optimized FA content for dissolving PA withthe least possible decomposition/degradation is 60% (thesample 60-7-20-13).

3.2. Viscoelastic Properties. The loss modulus is a measure ofenergy dissipation. In the region of the glass transition,molecular segmental motions are activated; however,motions occur with difficulty, described as the molecular fric-tion that dissipates much of the force. Therefore, though thematerial is less stiff, more force is dissipated as heat, increas-ing the loss modulus. However, after that region, as thechains are free to move (at the temperature higher than theglass transition temperature), much less energy is storedsince the molecules can move with the force, resulting in arapid decline in the storage modulus. As evident inFigure 2, in the sample prepared from pure FA, the maxi-mum in the curve of the loss modulus decreases with thereduction in the amount of FA in the solution. The solvent60-7-20-13 contains water, and there might be some watermolecules left. As the water molecules can act as a plasticizerfor polyamide [25], it could plasticise the polymer. More so, itmight be related to the effect of higher density of hydrogenbonding on a urea-containing solvent (in 60-7-20-13) as wellas the localization of the paired electrons of the amide groupsfrom the PA chains. One of the main characteristics of PAs isthe strong interchain interaction that arises from the hydro-gen bonding between the amide groups on adjacent chains

[26]. In other words, in the solvation process, the amidegroups from the PA chains meet a higher density of hydrogenbonding (offering by urea), leading to weaker intrainterac-tions among the PA chains, which, in return, decreases theinteraction among them. Therefore, they have much morefreedom to move in the solvent. This freedom increases theBrownian motion of the chains, which increases their dis-tances from each other [27]. Finally, when they are about tochange their phase from solution to solid (coagulation pro-cess in the coagulant), they come together in a relativelyless-packed structure. Moreover, when they are less-packedin the solid form (having lower crystallinity), their Tg isslightly lower (Figure 2 for tan delta), and the peak of the lossmodulus is seen at a lower temperature for a solution with ahigher amount of urea. This proposed reason is intensifiedwith the presence of calcium ion as a Lewis acid in a differentway. Calcium ion has free orbital to localize the paired elec-trons of the nitrogen from the amide group of the PA chains;hence, the hydrogen bonding ability of the amide groups ofthe PA chains also decreases, and finally the same effect hap-pens. The same trend is seen in the storage modulus as well asin the tan delta.

3.3. Viscosity. The viscosity values are different for varioussolutions (Figure 3). However, in all solutions, the viscos-ity value is higher than the one for the solution using onlyformic acid as a solvent (100-0-0-0). Regarding the meritof the increase in viscosity, during the process of makingthe APCCF, it was observed several times that when the vis-cosity of the solution is low, the formed film (coating) doesnot have enough coherency. It makes sense as it might bedue to the less aggregation of polymer chains on top of thefabric and also letting the solution go through more intothe fabric which is not favorable. Because, for the applicationof making the APCCF, only the surface of the fabric must bedissolved partially, not the whole fabric which will decreasethe whole tensile strength of the composite, a solution whichhas higher viscosity is more favorable for making APCCF asit makes a well-formed coating on top of the PA fabric. When

05

101520253035404550

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10.0020.0030.0040.0050.0060.0070.0080.00

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(b)

Figure 1: An exemplary stress-elongation at max curve for the sample 60-7-20-13 (a), and the tensile strength and the elongation at maxvalues for the composites (b).

4 International Journal of Polymer Science

the viscosity of the solution is low, the formed film does nothave enough coherency. Comparatively, when the same per-centage of PA solutions in different solvents gives a differentviscosity and, on the other hand, the viscosity of around4000 cP (for the solution 100-0-0-0) is enough to make aproper coating, the solution 75-8-17-0 (which has the highest

viscosity value, around 10,500 cP) will use less polymer.Additionally, as the APCCF must be waterproof and be asgas tight as possible, the coating being formed from a higherviscosity solution will be denser and will increase theAPCCF’s gas tightness.

The reason for the difference in the viscosity values in dif-ferent solutions could be related to the difference in the inter-molecular interactions between the solvent’s molecules andthe PA’s molecules. The ones that have a higher amount ofurea have higher values in viscosity. This is due to the factthat urea can establish hydrogen bonding in large numbers(up to four units) from two sides, which can make a bridgebetween two PA chains (two bonds from the one side andtwo bonds from the other side). By establishing these fourbidirectional hydrogen bonds, the fluidity of the PA solutiondecreases, which, in turn, increases the viscosity.

3.4. Morphological Properties. According to the scanningelectron microscopic images of the cross section (Figure 4),in almost all the APCCF specimens, there is an excellentadhesion between the coating (formed film) and the fabric.In other words, the boundaries between the fiber of the fabricand the coating are not clear because they have faded due tothe adhesion of the two components.

It is obvious that a large part of the cross-sectional area ofthe fabric is dissolved, and the polymer chains are interdif-fused with each other, enabling the APCCF to have a goodadhesion between the coating and the fabric, both of whichare made out of PA. Although higher surface dissolutionhelps to create a better adhesion between the fabric and thecoating, it also disintegrates the fabric structure and changesthe fabric, from being a fibrous form to an amorphous film.Fibers are spun and pose a high crystallinity and thus a goodstrength while the films are amorphous as they do not haveenough time to rearrange their chains in a nice ordered way(fast coagulation in water). Therefore, converting PA froma highly crystalline form to a less crystalline (highly amor-phous) form is not favorable from a mechanical point ofview. The amount of the surface dissolution should be aslow as possible to impart a good adhesion between the fabric

50

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40 60 80 100Temperature (°C)

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Figure 2: Loss modulus, storage modulus, and tan delta for thecomposites prepared from the new solvents.

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-9-0

69-8

-15-

8

67-4

-30-

0

82-0

-0-1

8

75-8

-17-

0

75-0

-0-2

5

55-6

-39-

0

50-8

-42-

0

47-8

-39-

5

Figure 3: The viscosity values for the composites. The solutionmade from the solvent mixture 75-8-17-0 is the most viscoussolution due to the intermolecular interactions between theingredients of the solvent mixture and the polymer.

5International Journal of Polymer Science

and the coating, but a higher dissolution is not favorable [11].Using this new solvent helps the fabric’s surface to be par-tially dissolved more quickly, due to the higher hydrogen

bond intensity (due to the presence of urea); hence, the adhe-sion between the fabric and the coating—the coating (formedfilm) on top of the fabric—would be stronger.

Amirkabir University AIS2300C SEI WD = 10.8 15.0 kV X 500 100 �휇m

Amirkabir University AIS2300C SEI WD = 10.4 15.0 kV X 500 100 �휇m

Amirkabir University AIS2300C SEI WD = 10.8 15.0 kV X 1.5K 30 �휇m

Figure 4: Selected cross-sectional SEM micrographs of the composite 60-7-20-13. Strong adhesion is seen between the coating (the top film)and the fibers from the fabric. The fibrils of the fiber closer to the coating are fused to each other.

6 International Journal of Polymer Science

3.5. Thermogravimetric Analysis (TGA). The TGA curvesshowed almost the same values and shape (Figure 5). Theonly composite that deviates more from the rest is the 75-8-18-0, which tends to decompose slightly earlier than theothers, though the difference is not significant. However, ithas higher residue percentage compared to the rest of thecomposites (except the 64-7-14-14). Although PA66 is par-tially crystalline, all good solvents penetrate the crystallites,disrupting them completely [6]. Calcium ion (which is theacidic ion) is smaller than the urea molecule, resulting in aneasier entrance to the crystallites. The final swollen gel is thusamorphous, even though the initial polymer is crystalline [6].In the composite 75-8-17-0, the ratio of urea to calcium chlo-ride (the source of calcium ion) is 0.5 (Table 2) and the ratioof U/FA is 0.11. These ratios are the same as those of thecomposites 69-8-15-8 and 64-7-14-14; however, their Ca/(FA+U+W) and U/(FA+Ca+W) are different. From thelast two ratios and the TGA curves, it could be concluded thatout of the four ingredients of the new solvent mixture (U, FA,Ca, and W), the ratio of U and Ca to the other ingredientsshould be kept as low as possible to have the least disruptionof PA crystallites in order not to sacrifice the thermal stabil-ity. It is noteworthy that the composite 82-9-9-0, which hasthe same Ca/(FA+U+W) and U/(FA+Ca+W) ratios(equal to 10), has the lowest residue in the TGA curve, mean-ing that the closer the two ratios, the less the thermal stabilityof the composite.

3.6. pH Values and Observations. From the pH measure-ments, it is obvious and reasonable that the solutions thathave a higher amount of FA have lower pH. The amine group(-NH2) in urea can accept a hydrogen ion, making it a basicsubstance. However, the carbonyl group (-C=O) offers a sig-nificant opportunity for resonance, which will stabilise theamine group, meaning that the paired electrons on N is inresonance. This means that urea is very slightly basic (andvery close to neutral). However, in the presence of an acid,the basicity of urea increases (as there is plenty of H+ in the

solution and the –NH2 is converted to –NH3+). Hence, inthe solution, urea is a base, so the reason for adding cal-cium chloride (which acts as a Lewis base) was to decreasethe pH to make the solution as similar as possible to thepure FA solution. The highest pH is attributed to the solu-tion 75-0-0-25 in which 25% of the water has increasedthe pH from −1.74 to −0.30. Basically, polyamides arepolymers with a relatively high density of hydrogen bonds[1]. As PA is a weak acid, a solvent with acidic propertiescan protonate the amide group; consequently, the amidegroup will have less possibility to establish hydrogenbonding with other chains. Therefore, it will be easier forthe solvent to swell into the polymer and dissolve it. Inthe first six solutions of the solutions tabulated inTable 2, it was observed that the solution 90-0-0-10 wasexcellent in terms of homogeneity and dissolution power.This might be related to the lower pH they have comparedto the others, which have higher pH. However, the pH byitself is not a good criterion to judge the dissolution powerof these solutions. For example, in the solution 35-6-48-12,which has a pH value very close to the pH of FA (−1.66vs. −1.74), the observation showed that the solution doesnot have a high power of dissolution. This might berelated to the calcium chloride content (100∗Ca/(FA+urea +W)), which is 92.59%, while the value is less than25% in those showing “excellent” dissolution. The reasonfor this might be due to the effect of the counterion, chlo-ride. Chloride is a relatively big ion. In lower numbers,they can help the dissolution; however, when they areincreased, they most probably promote some interferenceswith the interactions of other ingredients (FA, urea, andwater) with polyamide chains. Therefore, it could be con-cluded that the calcium chloride content should be lessthan 25%.

4. Conclusions

The preparation of all-polyamide composite-coated fabric(APCCF), a specific form of single-polymer composites,made through a phase inversion method using a new solventis described. The solvent was obtained by replacing 40% ofthe FA with less harmful, cost-effective, more environmen-tally friendly, safer-to-handle, and industrially available che-micals: urea, calcium chloride, and water. The APCCFprepared in this work using the new solvent mixture doesnot have inferior properties over the one prepared throughFA, such as mechanical and thermal properties. The pre-pared composite has a strong adhesion between the two con-stituents due to the use of the same polymer (PA66) in theconstituents. The composite is fully recyclable since it con-tains no other materials except PA, which can be melted ordissolved and reused as a PA source. The prepared compos-ite has a broad range of semistructural applications for pro-viding lightweight, architecturally striking solutions as wellas wide-span surfaces, membrane-cable structures, hangingroofs, pneumatic constructions, water-/gas-proof fabricreactors, temporary houses and tents, facade coverings, con-tainer linings, and tarpaulins and as material of constructionof textile bioreactors.

100

80

60

Wei

ght (

%)

40

20

00 100 200 300 400

Temperature (ºC)500 600 700

64-7-14-1475-0-0-2560-7-20-1382-9-9-0

75-8-17-082-0-0-18100-0-0-0

Figure 5: TGA curves of the composites prepared via the newsolvent.

7International Journal of Polymer Science

Data Availability

The data used to support the findings of this study are avail-able from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors are grateful to FOV Fabrics AB (Sweden) forproviding the PA fabric and PA residue and Prof. Dr. ParvizRashidi Ranjbar for his help.

References

[1] M. Laurati, A. Arbe, A. Rios de Anda, L. A. Fillot, and P. Sotta,“Effect of polar solvents on the crystalline phase of polyam-ides,” Polymer, vol. 55, no. 12, pp. 2867–2881, 2014.

[2] E. L. Papadopoulou, F. Pignatelli, S. Marras et al., “Nylon 6, 6/graphene nanoplatelet composite films obtained from a newsolvent,” RSC Advances, vol. 6, no. 8, pp. 6823–6831, 2016.

[3] K. Behler, M. Havel, and Y. Gogotsi, “New solvent for polyam-ides and its application to the electrospinning of polyamides 11and 12,” Polymer, vol. 48, no. 22, pp. 6617–6621, 2007.

[4] K. Charlet, V. Mathot, and J. Devaux, “Crystallization and dis-solution behaviour of polyamide 6-water systems under pres-sure,” Polymer International, vol. 60, no. 1, pp. 119–125, 2011.

[5] R. Nirmala, H. R. Panth, C. Yi et al., “Effect of solvents on highaspect ratio polyamide-6 nanofibers via electrospinning,”Macromolecular Research, vol. 18, no. 8, pp. 759–765, 2010.

[6] L. Valentine, “Interaction of polyamides with solvents. I. Apreliminary survey of the swelling of crosslinked nylon 66 invarious types of solvents,” Journal of Polymer Science, vol. 23,no. 103, pp. 297–314, 1957.

[7] G. Vasile, C. Fetecau, and A. Serban, “Experimental researchon the roughness of surfaces processed through milling poly-amide composites,” Materiale Plastice, vol. 51, pp. 205–212,2014.

[8] Y. Gong and G. Yang, “Manufacturing and physical propertiesof all-polyamide composites,” Journal of Materials Science,vol. 44, no. 17, pp. 4639–4644, 2009.

[9] C. G. Johnson and L. J. Mathias, “In synthesis and solid stateNMR of 15n-labeled nylon 12,” in 1989 Boston, MassachusettsMeeting of ACS, pp. 523-524, Boston, MA, USA, 1990.

[10] A. K. Sen, Coated Textiles: Principles and Applications, CRCPress, Boca Raton, FL, USA, 2nd edition, 2007.

[11] M. Jabbari, M. Skrifvars, D. Åkesson, and M. J. Taherzadeh,“Introducing all-polyamide composite coated fabrics: amethod to produce fully recyclable single-polymer compositecoated fabrics,” Journal of Applied Polymer Science, vol. 133,no. 7, 2016.

[12] B. N. Bridgens and P. D. Gosling, “Direct stress–strain repre-sentation for coated woven fabrics,” Computers & Structures,vol. 82, no. 23-26, pp. 1913–1927, 2004.

[13] A. Ambroziak and P. Kłosowski, “Mechanical properties forpreliminary design of structures made from PVC coated fab-ric,” Construction and Building Materials, vol. 50, pp. 74–81,2014.

[14] M. Jabbari, O. Osadolor, R. Nair, and M. Taherzadeh, “All-polyamide composite coated-fabric as an alternative materialof construction for textile-bioreactors (TBRs),” Energies,vol. 10, no. 11, p. 1928, 2017.

Table 2: Values for the pH and the relative components and also the observations.

Sample name pH U/Ca U/FA 100∗U/(FA+Ca +W) 100∗Ca/(FA+U+W) Observation∗

100-0-0-0 −1.74 — 0.00 0.00 0.00 Excellent

60-7-20-13 −0.74 0.33 0.11 7.14 25.00 Excellent+

64-7-14-14 −0.54 0.50 0.11 7.69 16.67 Excellent

90-0-0-10 −1.10 — 0.00 0.00 0.00 Excellent

90-10-0-0 −0.18 — 0.11 11.11 0.00 Excellent

82-9-9-0 −0.75 1.00 0.11 10.00 10.00 Excellent

69-8-15-8 −0.74 0.50 0.11 8.33 18.18 Good

67-4-30-0 −1.64 0.13 0.06 3.85 42.11 Good

82-0-0-18 −0.56 — 0.00 0.00 0.00 Good

75-8-17-0 −0.95 0.50 0.11 9.09 20.00 Good

75-0-0-25 −0.30 — 0.00 0.00 0.00 Fairly good

55-6-39-0 −1.69 0.15 0.11 6.45 65.00 Fairly poor

50-8-42-0 −1.61 0.20 0.17 9.09 71.43 Fairly poor

47-8-39-5 −1.50 0.20 0.17 8.57 65.22 Fairly poor

56-6-25-13 −0.91 0.25 0.11 6.67 33.33 Poor

50-6-28-17 −0.90 0.20 0.11 5.88 38.46 Poor

43-7-36-14 −1.08 0.20 0.17 7.69 55.56 Bad

38-6-43-13 −1.42 0.15 0.17 6.82 74.07 Bad

35-6-48-12 −1.66 0.12 0.17 6.12 92.59 Bad∗The markings were assigned based on the homogeneity of the solutions and the uniformity of the cast film on the glass.

8 International Journal of Polymer Science

[15] M. Jabbari, D. Åkesson, M. Skrifvars, and M. J. Taherzadeh,“Novel lightweight and highly thermally insulative silicaaerogel-doped poly (vinyl chloride)-coated fabric composite,”Journal of Reinforced Plastics and Composites, vol. 34, no. 19,pp. 1581–1592, 2015.

[16] B. A. Miller-Chou and J. L. Koenig, “A review of polymer dis-solution,” Progress in Polymer Science, vol. 28, no. 8, pp. 1223–1270, 2003.

[17] M. Ghasemi, M. Tsianou, and P. Alexandridis, “Dissolution ofsemicrystalline polymers: solvent-induced decrystallizationand chain untangling,” in 2015 AIChE Annual Meeting,p. 29, Salt Lake City, UT,USA, 2015.

[18] K. V. S. N. Raju and M. Yaseen, “Influence of nonsolvents ondissolution characteristics of nylon-6,” Journal of Applied Poly-mer Science, vol. 43, no. 8, pp. 1533–1538, 1991.

[19] E. Vinken, A. E. Terry, O. van Asselen, A. B. Spoelstra, R. Graf,and S. Rastogi, “Role of superheated water in the dissolutionand perturbation of hydrogen bonding in the crystalline latticeof polyamide 4, 6,” Langmuir, vol. 24, no. 12, pp. 6313–6326,2008.

[20] C. H. R. M. Wilsens, Y. S. Deshmukh, B. A. J. Noordover, andS. Rastogi, “Influence of the 2, 5-furandicarboxamide moietyon hydrogen bonding in aliphatic-aromatic poly (esteramide)s,” Macromolecules, vol. 47, no. 18, pp. 6196–6206,2014.

[21] A. N. Derbyshire, E. D. Harvey, and D. Parr, “Solvent-assisteddyeing of nylon 6.6 and polyester fibres,” Journal of the Societyof Dyers and Colourists, vol. 91, no. 4, pp. 106–111, 1975.

[22] M. G. M. Wevers, V. B. F. Mathot, T. F. J. Pijpers, B. Goderis,and G. Groeninckx, “Full dissolution and crystallization ofpolyamide 6 and polyamide 4.6 in water and ethanol,” in Lec-ture Notes in Physics, G. Reiter and G. R. Strobl, Eds., vol. 714,pp. 151–168, 2007.

[23] M. Pegoraro, A. Penati, M. Zocchi, and G. Albertini, “Influ-ence of nylon-6 film solvent treatments on water and watersolutions permeability,” Annali di Chimica, vol. 74, pp. 589–605, 1984.

[24] Z. L. Wang, J. L. Xu, L. J. Wu et al., “Dissolution, hydrolysisand crystallization behavior of polyamide 6 in superheatedwater,” Chinese Journal of Polymer Science, vol. 33, no. 9,pp. 1334–1343, 2015.

[25] P. Y. Le Gac, M. Arhant, M. Le Gall, and P. Davies, “Yieldstress changes induced by water in polyamide 6: characteriza-tion and modeling,” Polymer Degradation and Stability,vol. 137, pp. 272–280, 2017.

[26] Q. Zhou, J. Zhang, J. Fang, and W. Li, “The influence of nano-fillers migration on the mechanical property of pa 6/chitosannanocomposites,” RSC Advances, vol. 5, no. 22, pp. 16631–16639, 2015.

[27] S.-i. Manabe and R. Fujioka, “Solvent concentration in a spec-ified region of regenerated cellulose solid evaluated fromdynamic viscoelasticity in a hydrophilic solvent,” Carbohy-drate Polymers, vol. 41, no. 1, pp. 75–82, 2000.

9International Journal of Polymer Science

Paper V

Volume 8 • Issue 4 • 1000242J Inform Tech Softw Eng, an open access journalISSN: 2165-7866

Jabbari et al., J Inform Tech Softw Eng 2018, 8:4DOI: 10.4172/2165-7866.1000242

Research Article

Journal of

Info

rmat

ion

Technology & Softw

are Engineering

ISSN: 2165-7866

Journal ofInformation Technology & Software Engineering

Keywords: Hansen solubility parameters; Solvent mixture; Solventsubstitution; Simplex method; Linear programming; Quadratic minimization; Solvent screening

Abbreviations: 11dCE:1,1-Dichloroethane; 12dCE: 1,2-diChloroEthane;B2CiPE: Bis(2-chloroisopropyl)ether; bCEE: Bis(chloroethyl)ether; BCM: Bromochloromethane; cHC: Cyclohexyl chloride; dCEthylene: 1,1-Dichloroethylene; DCM: Dichloromethane; dCtFE: 1,2-DichloroTetraFluoroEthane; dEAE: 2-(DiEthylAmino)Ethanol; dEGmBE: Diethylene glycol monoButyl ether; dEGmME: Diethylene glycol monoMethyl ether; EGmMEA: Ethylene glycol monoMethyl ether acetate; MiAK: Methyl isoAmyl ketone; MMA: Methyl methacrylate; odCB: o-diChloroBenzene; oMePh: o-MethoxyPhenol; PG: Propylene glycol; prC: Propyl chloride; THF: Tetrahydrofuran; tMPD: 2,2,4-triMethyl-1,3-pentanediol

IntroductionSolvents, defined as substances able to dissolve or solvate other

substances, are commonly used in many industries and applications [1]. For any solvent-based process, the best-suited solvent or solvent-mixture must be selected [2]. On the other hand, solvent selection and design is a complex problem, which requires decision making in several levels for identifying the best candidates depending on different multi-objective criteria namely environment, health, safety, process feasibility and economics [3]. Currently, solvent selection relies very much on previous experiences, trial and error with different solvent candidates. Use of experimental thermo-physical properties stored in a factual database for the selection has the advantage that the results are very reliable; however, solvent selection is limited to the experimental data pool [2]. Such heuristic approach while valuable on their own, however arguably are not fit to deal with a complex multi-criteria optimization and search problem, which is the case for solvent selection [3].

On the other hand, actual (physical) trials in the laboratory of mixing different solvents and checking the solvation, is a tough and time-consuming job. A number of modern tools are increasingly becoming available to reduce the efforts needed to select the right solvent [4]. The use of prediction models has the advantage that for the selection procedure, any solvent can be considered for which the required group

Computer-Aided Theoretical Solvent Selection using the Simplex Method Based on Hansen Solubility Parameters (HSPs)Mostafa Jabbari1*, Magnus Lundin1, Mohammad Hatamvand1,2, Mikael Skrifvars1 and Mohammad Taherzadeh1

1Swedish Centre for Resource Recovery, University of Borås, Borås, Sweden2Computer Science and Mathematics Faculty, Bielefeld University of Applied Sciences, Bielefeld, Germany

AbstractSolvent selection is a crucial step in all solvent-involved processes. Using the Hansen solubility parameters (HSPs)

could provide a solvent/solvent-mixture, but there are two main challenges: 1) What solvents should be selected? 2) From each solvent, how much should be added to the mixture? There is no straightforward way to answer the two challenging questions. This contribution proposes a computer-aided method for selecting solvents (answer to the question 1) and finding the adequate amount of each solvent (answer to the question 2) to form a mixture of 2, 3 or 4 solvents to dissolve a solute with known HSPs or to replace a solvent. To achieve this, a sophisticated computer software package was developed to find the optimized mixture using the mathematical Simplex algorithm based on HSPs values from a database of 234 solvents. To get a list of solvent-mixtures, polyamide 66 was tested using its HSPs. This technique reduces the laboratory effort required in selecting and screening solvent blends while allowing a large number of candidate solvents to be considered for inclusion in a blend. The outcome of this paper significantly diminished the time of solvent development experimentation by decreasing the possible/necessary trials. Thus, the most suitable solvent/solvent-substitution can be found by the least possible effort; hence, it will save time and cost of all solvent-involved processes in the fields of chemistry, polymer and coating industries, chemical engineering, etc.

interaction parameters are available and by using predictive methods, an extended variety of solvents can be taken into account for selection [2]. Solubility parameters have found their greatest use in the selection of solvents [5].

Although it is possible to find a solvent mixture based on Hansen solubility parameters (HSPs), the question is: how one can screen the vast number of solvents to find the desired ones? Moreover, which solvents should be selected? Also, how much is the amount of each solvent (volume fraction) in the mixture?

Selection of the appropriate solvents and finding the best volume fractions could be made by computer programming through minimization of RA formula (discussed in ‘background’ section); however, finding the minimum of RA takes much time for each set of solvents by normal linear programming. Because it has to sweep all the decimal values of the volume fraction from 0 to 1 for all the solvents; while by using the Simplex algorithm (discussed in ‘background’ section), it can be done within a few milliseconds; hence, more combinations of solvents would be taken into account for solvent selection.

Some authors proposed methods to find a proper solvent mixture for very specific applications like electro-spinning [6]; however, a more general method applicable to a broader range of processes seems to be necessary to propose. Some publications use HSPs to predict solvent systems that are likely to dissolve, like Aghanouri and Sun [7], but they are empirically based and not computer-assisted, meaning that they

*Corresponding author: Mostafa Jabbari, Swedish Centre for Resource Recovery, University of Boras, Boras 501 90, Sweden, Tel: 46 33 435 4636; E-mail:Mostafa.Jabbari@hb.se

Received February 15, 2018; Accepted October 01, 2018; Published October 08, 2018

Citation: Jabbari M, Lundin M, Hatamvand M, Skrifvars M, Taherzadeh M (2018) Computer-Aided Theoretical Solvent Selection using the Simplex Method Based on Hansen Solubility Parameters (HSPs). J Inform Tech Softw Eng 8: 242. doi: 10.4172/2165-7866.1000242

Copyright: © 2018 Jabbari M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Volume 8 • Issue 4 • 1000242J Inform Tech Softw Eng, an open access journalISSN: 2165-7866

Citation: Jabbari M, Lundin M, Hatamvand M, Skrifvars M, Taherzadeh M (2018) Computer-Aided Theoretical Solvent Selection using the Simplex Method Based on Hansen Solubility Parameters (HSPs). J Inform Tech Softw Eng 8: 242. doi: 10.4172/2165-7866.1000242

Page 2 of 6

cannot consider a large range of solvents. Nelson developed a computer-based formulating technique that allows selection of minimum cost solvent blends, but it was not capable of suggesting a solvent-substitution or solvent mixture for a solute with known HSPs [8]. Moreover, they used Hildebrand solubility parameters which have been updated and replaced by a more reliable and more accurate value, HSPs. To our knowledge, there is no report about a general computer-assisted method of finding a solvent-mixture for a solute with known HSPs. This contribution proposes a computer-assisted selection of solvents out of a vast number of solvents’ HSPs values stored in a database.

The purpose of this paper is to illustrate how to develop a systematic framework using HSPs to aid in finding a solvent-mixture/-substitution for many applications including organic synthesis, complex reaction systems, and solvent-based separations that decreases the laboratory labour and saves the experimentation time. The approach taken is the traditional one used in HSPs theory: matching the solvent’s HSPs to the polymer’s. HSPs values of the solvents will be those matched to the polymer.

Our contributions are:

• We found a way to decrease the experimental effort in solventscreening.

• We decreased the computational process time in minimization of RA formula, by uing the Simplex method.

To set the scene for this paper, we bring a brief description of the Hansen approach for solving the problem of finding a solvent-mixture. Then the linear programming and the Simplex algorithm are briefly introduced. The following overview is to facilitate reading for those who are not familiar with these concepts; therefore, experts can skip it.

BackgroundSolubility parameters help put numbers into this simple qualitative

idea [5]. Liquids with similar solubility parameters will be miscible, and polymers will dissolve in solvents whose solubility parameters are not too different from their own [5]. Several graphing and modelling techniques have been developed to aid in the prediction of polymer solubility [9]. The basic principle has been “like dissolves like” [5]. By 1950, Hildebrand had defined the solubility parameter as the sum of all the intermolecular attractive forces, which he found to be empirically related to the extent of mutual solubility of many chemical species [10,11]. Solubility behavior cannot be accurately predicted by only the Hildebrand solubility parameter [9]. In 1967, Charles Hansen improved the concept and introduced his three-dimensional solubility parameters. The Hansen approach provides an empirical, yet effective method for determining the dissolution possibility of solutes [9]. The solubility parameter has been used for many years to select solvents for coatings materials [5].

The Hansen model is usually considered as a sphere. Th e center of the sphere has the δd, δp, and δh values of the polymer in question (solute) [9]. δ is the square root of cohesion energy density, δd, δp, and δh represent the dispersive forces, polar interactions, and hydrogen bonding, respectively. The radius of the sphere, RO, is termed the interaction radius [9]. The values of RO have been reported for some polymers in the literature. RA is the distance in HSPs space between the solute/polymer and the solvent [12]. The boundary of the spherical characterization is based on the requirement that ‘good’ solvents have a distance from the center of the sphere, RA (also termed the solubility parameter distance) less than RO [9]. RA is given by the following relation:

( ) ( ) ( )2 2 2R Solution = 4× d - d + p - p + h - hs s sA f f fδ δ δ δ δ δ

(1)

where δdf, δpf, and δhf are the Hansen solubility components for the polymer/solute (our favourite values), and δds, δps, and δhs are the Hansen solubility components for the solvent [9]. Eq. 1 was developed from plots of experimental data where the constant ‘4’ was found convenient and correctly represented the solubility data as a sphere encompassing the good solvent [9].

Solubility parameters of mixtures are linear [13]. That is, each of the three HSPs of a solvent mixture is a linear function of composition. In this case, the composition value to be used in calculating solubility parameters for solvent mixtures is the volume fraction (φ) for each component [13]. For a binary (two-solvent) mixture, the equation for all three solubility parameters is Eq. 2 [13].

º × + × comp1 comp1 comp2 comp2blendδ ϕ σ ϕ σ (2)

This equation is correct for more than two components where the HSPs values are known [13]. Traditionally, without specific data, it is usually assumed that there is no volume change upon mixing of solvents. That is:

( )

Wt. Fraction

Density 1vol. Fraction = (3)1 Wt. Fraction Wt. Fraction+

Density Density1 2

(3)

In Eq. 2, φ is the volume fraction of component 1, and δ is any solubility parameter. It is understood that φcomp1 + φcomp 2 = 1. The volume fraction is easy to compute because solvents are stored in pails or drums and used by volume, although they are sold by weight.

Linear programming (LP) is a technique for the optimization of a linear objective function, subject to linear equality and linear inequality constraints. Although the RA formula is not linear (it is quadratic), its constraint is linear. Its feasible region is a convex poly-tope, which is a set defined as the intersection of finitely many half spaces, each of which is defined by a linear inequality [14]. Its objective function is a real-valued affine (linear) function defined on this polyhedron. A linear programming algorithm finds a point in the polyhedron where this function has the smallest (or largest) value if such a point exists. There are a few LP methods such as ellipsoid and interior-point, but for more than 35 years now, George B. Dantzig's Simplex-Method has been the most efficient mathematical tool for solving linear programming problems [15]. It is probably that mathematical algorithm for which the most computation time on computers is spent. This fact explains the great interest of experts and of the public to understand the method and its efficiency [15].

A convenient single parameter to describe solvent quality is the relative energy difference, RED, number: RED=RA/RO [9]. According to the basic principle in dissolution, “like dissolves like” [5], the more similarity, the less RA and hence, the higher the probability of dissolution. In other words, the distance in HSPs space between the solute/polymer, the Hansen space, should be as small as possible (RA≈0) [12]. By taking square root from both sides of Eq. 4, we have:

( ) ( ) ( )2 2 22R Solution = 4× d - d + p - p + h - hs s sA f f fδ δ δ δ δ δ

(4)

2A

2A

Therefore, we need to minimize R , as much as possible. In this paper, we minimize the R , with the Simplex method by development of a computer program.

Volume 8 • Issue 4 • 1000242J Inform Tech Softw Eng, an open access journalISSN: 2165-7866

Citation: Jabbari M, Lundin M, Hatamvand M, Skrifvars M, Taherzadeh M (2018) Computer-Aided Theoretical Solvent Selection using the Simplex Method Based on Hansen Solubility Parameters (HSPs). J Inform Tech Softw Eng 8: 242. doi: 10.4172/2165-7866.1000242

Page 3 of 6

MethodologyThe Microsoft Visual Studio software package was used to develop

the program. The Microsoft.SolverFoundation.Services.dll library that contains the Simplex algorithm was imported at the beginning of the program’s codes. The codes were written in the Visual Basic programming language that operates within the Microsoft. Net framework. All the HSPs values of 234 solvents together with their names and CAS number, health NFPA index were imported into a database. The database was connected to the program in a way that each time the program initiates, it loads the solvents’ δd, δp and δh values to predefined arrays–D(), P(), and H(), respectively. Out of 234 solvents, 81085 different combinations were examined for polyamide66 (PA66) as the case study. It was counted by a counter variable in the program’s code (this will be mentioned later, in “the code” section). The HSPs values of the solvents obtained from ASTM STP1133, Hansen [5], Mark [16], and Barton [17]. The values for NFPA health index were obtained from Sigma-Aldrich material safety data sheets, sciencelab.com, cameochemicals.noaa.gov, synquestlabs.com, and mathesongas.com.

The program’s codes

To count the different combinations in an integer variable, called “Processes”:

Processes=0

For i=0 To NumberOfSolventsInDB - 4

For j=i+1 To NumberOfSolventsInDB - 3

Processes=Processes+1

Next

Next

For i=0 To NumberOfSolventsInDB - 3

For j=i+1 To NumberOfSolventsInDB - 2

Processes=Processes+1

Next

Next

For i=0 To NumberOfSolventsInDB - 2

For j=i+1 To NumberOfSolventsInDB - 1

Processes=Processes+1

Next

Next.

The core part of the program

Dim solver=SolverContext.GetContext()

Dim model=solver.CreateModel()

Dim x1=New Decision(Domain.RealNonnegative, "QuantA" & i & j)

Dim x2=New Decision(Domain.RealNonnegative, "QuantB" & i & j)

Dim x3=New Decision(Domain.RealNonnegative, "QuantC" & i & j)

Dim x4=New Decision(Domain.RealNonnegative, "QuantD" & i & j)

model.AddDecisions(x1, x2, x3, x4)

model.AddGoal("Goal" & i & j, GoalKind.Minimize, 4 * (D(i) *

x1+D(j) * x2+D(j+1) * x3+D(j+2) * x4 - FD) * (D(i) * x1+D(j) * x2+D(j+1) * x3+D(j+2) * x4 - FD)+(P(i) * x1+P(j) * x2+P(j+1) * x3+P(j+2) * x4 -FP) * (P(i) * x1+P(j) * x2+P(j+1) * x3+P(j+2) * x4 - FP)+(H(i) * x1+H(j) * x2+H(j+1) * x3+H(j+2) * x4 - FH) * (H(i) * x1+H(j) * x2+H(j+1) *x3+H(j+2) * x4 - FH))

model.AddConstraint("sigmaN" & i & j, x1+x2+x3+x4=1)

model.AddConstraint("rangeX1" & i & j, x1 <= Maximum PercentageOfComponent)

model.AddConstraint("rangeX2" & i & j, x2 <= Maximum PercentageOfComponent)

model.AddConstraint("rangeX3" & i & j, x3 <= Maximum PercentageOfComponent)

model.AddConstraint("rangeX4" & i & j, x4 <= Maximum PercentageOfComponent)

solver.Solve()

n1=x1.GetDouble()

n2=x2.GetDouble()

n3=x3.GetDouble()

n4=x4.GetDouble()

R=4 * (D(i) * n1+D(j) * n2+D(j+1) * n3+D(j+2) * n4 - FD) * (D(i) * n1+D(j) * n2+D(j+1) * n3+D(j+2) * n4 - FD)+(P(i) * n1+P(j) * n2+P(j+1) * n3+P(j+2) * n4 - FP) * (P(i) * n1+P(j) * n2+P(j+1) * n3+P(j+2) * n4- FP)+(H(i) * n1+H(j) * n2+H(j+1) * n3+H(j+2) * n4 - FH) * (H(i) * n1+H(j) * n2+H(j+1) * n3+H(j+2) * n4 - FH)

And once the RA is calculated, the program checks if the calculated RA (based on the found n1, n2, n3 and n4 to the minimized valued with respect to Hansen space) is less than the maximmum accepted RA (Rlimite) or not:

If R<= Rlimite Then

“copy the results to listview”

End if

This code is for a four-solvent mixture system. For two- and three-solvent mixture system the code is the same, with the difference that only there are 2 and 3 parameters, respectively. As the calculation of taking squre root is far slower than linear calculations, to increase the speed of the program, we calculated the squre root of R (RA) and compared with the squre root of Rlimite which gives the same result but it is faster. Rlimite is the highest accepted RA which the user of the program inputs to the program.

The sweeping loops

The aforementioned calculation is only for one single combination while we need to check all the possible combination. To do this, we have to take the values of the all the solvents in the database:

For i=0 To NumberOfSolventsInDB - 4

For j=i+1 To NumberOfSolventsInDB - 3

“the core part of the program”

Next

Next

For i=0 To NumberOfSolventsInDB - 3

Volume 8 • Issue 4 • 1000242J Inform Tech Softw Eng, an open access journalISSN: 2165-7866

Citation: Jabbari M, Lundin M, Hatamvand M, Skrifvars M, Taherzadeh M (2018) Computer-Aided Theoretical Solvent Selection using the Simplex Method Based on Hansen Solubility Parameters (HSPs). J Inform Tech Softw Eng 8: 242. doi: 10.4172/2165-7866.1000242

Page 4 of 6

For j=i+1 To NumberOfSolventsInDB - 2

“the core part of the program”

Next

Next

For i=0 To NumberOfSolventsInDB - 2

For j=Me.i+1 To NumberOfSolventsInDB - 1

“the core part of the program”

Next

Next

in which, the first, second and third ‘for loop’ sweeps δd, δp and δh values for the four-solvent mixture, three-solvent mixture and two-solvent mixture, respectively. In a non-technical word, the ‘for loop’ means the variable i is increasing by 1 each time the process inside the loop is done (in this case: “the core part of the program”) and this will be continued until the i value is less than “NumberOfSolventsInDB - 4” which is 4 subtracted from the number of the solvents in the stored database that contains the HSPs of the 234 solvents. This is true for the other 'for loops’ as well as NumberOfSolventsInDB - 3 and NumberOfSolventsInDB - 2.

All the δd, δp and δh values are stored in D(), P() and H() arrays, respectively; and by changing the i and j variables’ vales in the three ‘for loops’, the δd, δp and δh values of the corresponding solvent are taken and considered for the core part of the program.

Results and DiscussionThe subset of solvent-mixtures results was obtained from running

the software with the HSPs of the selected polymer (PA66) out of 234 solvents. Using five main descriptors, namely, the dispersive (δd), polar (δp), and hydrogen (δh) Hansen solubility parameters, CAS registry number, and the health indexes according to the NFPR classifications. The Rlimite was assigned equal to “0.2”.

Miscibility challenge

Solvents used in this study are of different sorts–polar, non-polar, protic, aprotic, etc. The closeness of HSPs values of solvent-mixture and the solute only guarantees the likeness of the interactions between the solvent-mixture and the solute; however, there is no guarantee for miscibility of all the components of a solvent-mixture.

As the “polar” interactions of molecules are enumerated in δp values (in Hansen theory, see the ‘background’ section), we assumed that the closeness in δp values of the components could increase the possibility of miscibility, i.e., the difference in δp values (called ΔP, hereinafter) will decrease the chance of immiscibility of each solvent in the other one.

Results for PA66

The Hansen solubility parameters of PA66 have been reported as 18.5, 5.1 and 12.2 for δD, δP and δH, respectively [18]. The results were sorted first by ΔP, and then R and then by health index, descendingly. The used solvents are listed in Table 1.

As shown in Table 1, 30 records of 50 records of the results have R=0, meaning that they have the same HSPs with PA66’s HSPs. In other words, theoretically, they are exactly the same with PA66 regarding dispersion forces (δd), polar interactions (δp) and hydrogen bonding (δh). However, some results with a large ΔP are not good candidates

for being a solvent for PA66. For instance, the record #50 has the ΔP=8.4 (ethylene glycol δP: 9.4 — o-Xylene δP: 1), that they are not miscible in each other. Even without knowing the δP values of ethylene glycol (a polar solvent) and o-Xylene (a non-polar organic solvent), it is obvious that preparation of one-phase liquid from those two solvents is not possible. However, for example, the record #2 and #3 have the ΔP=1.8, which shows a higher possibility of giving a one-phase liquid. Proposed solvents using the developed software for PA66: only the first 50 are shown in Table 1.

Therefore, according to our above-mentioned proposal about miscibility, the record #=1 is the best candidate regarding miscibility. On the other hand, the record #2 and #3 have the higher possibility of dissolving PA66 (the closer R than record #1). Thus, the best candidates in terms of having the lowest chance of immiscibility (lowest ΔP) and at the same time having the highest possibility of dissolving PA66 (the closest possible R) is the record #2 and #3.

Some of the results may seem weird, as one might notice that how a solvent that is considered a non-solvent for a polymer could be part of the solvent-mixture for that polymer? For example in the result number #50 for polyamide 66, biphenyl is a part of the four-solvent mixture system, while it is non-solvent for polyamide 66. As Hansen discussed [5], a solvent can dissolve a given polymer in a mixture of two solvents, neither of which can dissolve the polymer by itself. Although Hansen discussed it for a binary system, but, as Durkee [13] discussed later, the relationship between HSPs are correct for more than two components where the HSPs values for a given polymer/solute are known.

As Hansen approach is a theoretical approach, all the suggested solvents are “theoretically proposed” solvents. As Hansen argued [9], solubility can be affected by any specific interactions, especially H-bonds, polymer morphology (crystallinity) and cross-linking, temperature, and changes in temperature [9]. Also, of importance is the size and shape of the solvent molecules. Therefore, a ccording t o Gmehling, by using predictive methods (such as using HSPs), a vast number of solvents can be considered for selection, but, of course, the quality of the predicted separation factors is less accurate than the use of highly reliable experimental data [2]. More specifically about PA66, as Anda argues, due to the presence of polar amide groups along polyamide chains, strong interactions between the polyamide and polar solvents like water can be formed [19]. Although there have been some co-solvent studies for polyamide [20], they were obtained by practical trial and errors, and one cannot generalize that method to other polymers/solutes. While, using the method proposed in this paper, as mentioned before, has the advantage of being applicable to all polymers/solutes and at the same time, is less time-consuming than the practical method. Therefore, a greater number of applicants can be taken into account to be the candidates (Table 1).

ConclusionA sophisticated software package for the selection of the most

suitable solvent-mixture for a solute/polymer with known HSPs or solvent-substitution for any solvent-involved process, e.g., liquid-liquid extraction was developed. The technique introduced in this paper decreases the time of solvent selection process tremendously by screening the vast number of different combination of an enormous number of solvents and narrow it down to a smaller set of solvents. This technique reduces the laboratory effort required in screening solvent blends while allowing a large number of candidate solvents to be considered for inclusion in a blend. Using Hansen solubility parameters (HSPs) to find solvent mixture for a polymer/solute with known HSPs is a very good tool but still is not the perfect method;

Volume 8 • Issue 4 • 1000242J Inform Tech Softw Eng, an open access journalISSN: 2165-7866

Citation: Jabbari M, Lundin M, Hatamvand M, Skrifvars M, Taherzadeh M (2018) Computer-Aided Theoretical Solvent Selection using the Simplex Method Based on Hansen Solubility Parameters (HSPs). J Inform Tech Softw Eng 8: 242. doi: 10.4172/2165-7866.1000242

Page 5 of 6

# R n1 n2 n3 n4 Health d, p, h ΔP max & min δP1 0.093 2-Phenoxyethano : 35.21% Cyclohexanol: 19.05% Aniline: 45.74% - 2.619 18.46, 5.12, 12.18 1.6 2-Phenoxyethanol {P: 5.7} - Cyclohexanol

{P: 4.1}2 0 2-Phenoxyethanol: 13.98% Cyclohexanol: 21.95% Aniline: 47.12% Phenol: 16.95% 2.561 18.5, 5.1, 12.2 1.8 Phenol{P: 5.9}-Cyclohexanol {P: 4.1}3 0 m-Cresol: 15.87% Cyclohexanol: 18.08% Aniline: 43.46% Phenol: 22.6% 2.638 18.5, 5.1, 12.2 1.8 Phenol{P: 5.9}- Cyclohexanol {P: 4.1}4 0.144 m-Cresol: 59.5% Aniline: 31.41% Phenol: 9.09% - 3 18.44, 5.17, 12.17 1.8 Benzoic acid {P: 6.9} - m-Cresol {P: 5.1}5 0 Benzyl alcohol: 13.31% Cyclohexanol: 27.9% Aniline: 43.87% Phenol: 14.92% 2.176 18.5, 5.1, 12.2 2.2 Benzyl alcohol {P: 6.3} - Cyclohexanol {P: 4.1}6 0.069 Benzyl alcohol: 28.39% Cyclohexanol: 32.45% Aniline: 39.17% - 1.783 18.47, 5.12, 12.19 2.2 Benzyl alcohol {P: 6.3} - Cyclohexanol

{P: 4.1}7 0 Cyclohexanol: 24.92% Aniline: 46.14% Phenol: 27.17% Benzoic acid: 1.77% 2.484 18.5, 5.1, 12.2 2.8 Benzoic acid {P: 6.9} - Cyclohexanol {P: 4.1}8 0.114 3-Chloro-1-propanol: 70.62% Amyl acetate: 2.88% Bromoform: 26.5% - 1.53 18.48, 5.21, 12.17 3.5 dCEthylene (1,1-Dichloroethylene) {P: 6.8}

- Amyl acetate {P: 3.3}9 0.156 2-Phenoxyethanol: 73.93% Amyl acetate: 4.63% Bromoform: 21.44% - 2.907 18.48, 5.25, 12.16 3.5 dCEthylene (1,1-Dichloroethylene) {P: 6.8}

- Amyl acetate {P: 3.3}10 0.168 Phenol: 68.86% Amyl acetate: 10.38% Bromoform: 20.76% - 2.792 18.48, 5.26, 12.16 3.5 dCEthylene (1,1-Dichloroethylene) {P: 6.8}

- Amyl acetate {P: 3.3}11 0.195 Benzyl alcohol: 60.7% 1-Octanol: 13.67% Ethylene diBromide: 25.63% - 1.513 18.41, 5.17, 12.15 3.6 Benzyl alcohol {P: 6.3} - 1-Decanol {P: 2.7}12 0.107 3-Chloro-1-propanol: 70.31% Bromoform: 26.78% IsoAmyl acetate: 2.91% - 1.536 18.48, 5.2, 12.17 4.3 12dCE {P: 7.4} - IsoAmyl acetate {P: 3.1}13 0.135 Phenol: 67.89% Bromoform: 22% IsoAmyl acetate: 10.11% - 2.798 18.48, 5.22, 12.17 4.3 12dCE {P: 7.4} - IsoAmyl acetate {P: 3.1}14 0.144 2-Phenoxyethanol: 73.43% Bromoform: 21.93% IsoAmyl acetate: 4.64% - 2.907 18.47, 5.23, 12.16 4.3 12dCE {P: 7.4} - IsoAmyl acetate {P: 3.1}15 0.143 Benzyl alcohol: 75.27% 1,4-Dioxane: 24.73% - - 1.247 18.55, 5.19, 12.14 4.5 Benzyl alcohol {P: 6.3} - 1,4-Dioxane {P: 1.8}16 0.138 Benzyl alcohol: 75.16% 1,4-Dioxane: 24.7% PG monoMethyl ether:

0.14%- 1.247 18.54, 5.19, 12.14 4.7 butyl lactate {P: 6.5} - 1,4-Dioxane {P: 1.8}

17 0.007 Phenol: 74.99% Styrene: 4.62% 1-bromonaphthalene: 20.39%

- 2.75 18.5, 5.1, 12.2 4.9 Phenol {P: 5.9} - Styrene {P: 1}

18 0.117 Phenol: 43.75% Ethylene diBromide: 43.32%

Furfuryl alcohol: 12.93% - 3 18.44, 5.08, 12.2 4.9 Furfuryl alcohol {P: 7.6} - 1-Decanol {P: 2.7}

19 0 Ethylene diBromide: 59.85% isoButyl isoButyrate: 4.61%

Methyl isoAmyl ketone: 4.23%

Resorcinol: 31.32%

2.464 18.5, 5.1, 12.2 5.5 Resorcinol {P: 8.4} - isoButyl isoButyrate {P: 2.9}

20 0.095 Benzyl alcohol: 72.09% 2-Butanol: 2.99% 1,4-Dioxane: 24.92% - 1.249 18.47, 5.16, 12.15 6.2 Methyl salicylate {P: 8} - 1,4-Dioxane {P: 1.8}

21 0.176 Napthalene: 48.95% Methyl isoAmyl ketone: 8.47%

Resorcinol: 42.58% - 1.915 18.42, 5.04, 12.22 6.4 Resorcinol {P: 8.4} - Napthalene {P: 2}

22 0.008 Bromoform: 51.68% Lactic acid: 31.86% Dichlorodifluoromethane: 16.45%

- 2.835 18.5, 5.09, 12.2 6.5 Lactic acid {P: 8.3} - dCtFE {P: 1.8}

23 0.182 Phenol: 74.92% 1-bromonaphthalene: 25.08%

- - 2.749 18.58, 5.2, 12.19 6.5 DEP (diEthyl phthalate) {P: 9.6} - 1-bromonaphthalene {P: 3.1}

24 0.192 Ethylene diBromide: 72.77% 1,3-Butanediol: 27.23% - - 2.455 18.49, 5.27, 12.11 6.5 1,3-Butanediol {P: 10} - Ethylene diBromide {P: 3.5}

25 0.192 Ethylene diBromide: 72.76% 1,3-Butanediol: 27.23% tEGmOE: 0.01% - 2.455 18.49, 5.27, 12.11 6.9 1,3-Butanediol {P: 10} - tEGmOE {P: 3.1}26 0 Resorcinol: 46.22% DEK (diEthyl ketone):

5.85%1-MethylNaphthalene: 34.51%

di(isoButyl) ketone: 13.42%

1.807 18.5, 5.1, 12.2 7.6 Resorcinol {P: 8.4} - 1-MethylNaphthalene {P: 0.8}

27 0 isoButyl isoButyrate: 10.95% dibutyl sebacate: 1.42% PG: 45.74% Biphenyl: 41.89% 0.433 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}28 0 Butyric acid: 9.11% dibutyl sebacate: 3.89% PG: 43.83% Biphenyl: 43.17% 0.471 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}29 0 Methylal: 5.07% dibutyl sebacate: 6.32% PG: 45.69% Biphenyl: 42.92% 0.543 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}30 0 tEGmOE: 6.29% dibutyl sebacate: 3.9% PG: 45.61% Biphenyl: 44.2% 0.544 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}31 0 Oleyl alcohol: 6.3% dibutyl sebacate: 4.71% PG: 45.65% Biphenyl: 43.35% 0.544 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}32 0 MethylAmyl acetate: 10.67% dibutyl sebacate: 2.07% PG: 45.28% Biphenyl: 41.98% 0.547 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}33 0 1-Tridecanol: 6.72% dibutyl sebacate: 4.56% PG: 45.23% Biphenyl: 43.49% 0.548 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}34 0 IsoAmyl acetate: 10.53% dibutyl sebacate: 2.39% PG: 45.18% Biphenyl: 41.9% 0.548 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}35 0 DiisoButyl carbinol: 6% dibutyl sebacate: 5.98% PG: 44.82% Biphenyl: 43.2% 0.552 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}36 0 sec-Butyl acetate: 12.66% dibutyl sebacate: 0.57% PG: 44.5% Biphenyl: 42.27% 0.555 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}37 0 1-Decanol: 8.34% dibutyl sebacate: 7.44% PG: 44.02% Biphenyl: 40.2% 0.56 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}38 0 1-Octanol: 7.59% dibutyl sebacate: 7.46% PG: 43.62% Biphenyl: 41.33% 0.564 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}39 0 1-Pentanol: 8.11% dibutyl sebacate: 6.52% PG: 42.71% Biphenyl: 42.65% 0.573 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}40 0 Cyclohexanol: 9.3% dibutyl sebacate: 7.89% PG: 42.09% Biphenyl: 40.72% 0.579 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}41 0 isobutanol: 9.06% dibutyl sebacate: 5.68% PG: 41.37% Biphenyl: 43.89% 0.586 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}42 0 2-Ethoxyethyl acetate: 17.42% dibutyl sebacate: 0.89% PG: 40.77% Biphenyl: 40.92% 0.592 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}43 0 1-Butanol: 11.26% dibutyl sebacate: 6.03% PG: 40% Biphenyl: 42.71% 0.6 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}44 0 2-Octanol: 20.27% dibutyl sebacate: 0.24% PG: 39.3% Biphenyl: 40.19% 0.607 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}45 0 2-Butanol: 14.06% dibutyl sebacate: 4.11% PG: 39.23% Biphenyl: 42.6% 0.608 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}46 0 2-Propanol: 12.27% dibutyl sebacate: 5.56% PG: 39.04% Biphenyl: 43.13% 0.61 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}47 0 4-Methyl-2-pentanol: 5.85% dibutyl sebacate: 6.76% PG: 44.39% Biphenyl: 43% 0.615 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}48 0 2-Ethyl-1-hexanol: 6.56% dibutyl sebacate: 6.74% PG: 44.2% Biphenyl: 42.49% 0.624 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Biphenyl {P: 1}49 0 Xylene (Xylol): 10.47% dibutyl sebacate: 4.6% PG: 46.89% Biphenyl: 38.04% 0.636 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - Xylene (Xylol) {P: 1}50 0 o-Xylene: 10.94% dibutyl sebacate: 4.68% PG: 46.86% Biphenyl: 37.53% 0.641 18.5, 5.1, 12.2 8.4 PG {P: 9.4} - o-Xylene {P: 1}

Table 1: Proposed solvents using the developed software for PA66.

Volume 8 • Issue 4 • 1000242J Inform Tech Softw Eng, an open access journalISSN: 2165-7866

Citation: Jabbari M, Lundin M, Hatamvand M, Skrifvars M, Taherzadeh M (2018) Computer-Aided Theoretical Solvent Selection using the Simplex Method Based on Hansen Solubility Parameters (HSPs). J Inform Tech Softw Eng 8: 242. doi: 10.4172/2165-7866.1000242

Page 6 of 6

therefore, more research is needed in this field to increase the quality of the proposed set of solvent-mixtures. As Jiaa studied [21], there is a relationship between HSPs and acidity of the polymers/solutes. Therefore, in the future studies in this regard, pKa/pKb values could be taken into account in order to get better results.

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Paper VI

Z. Phys. Chem. 2019; aop

Mostafa Jabbari*, Magnus Lundin, Saeed Bahadorikhalili,Mikael Skrifvars and Mohammad J. TaherzadehFinding Solvent for Polyamide 11 Using aComputer Softwarehttps://doi.org/10.1515/zpch-2018-1299Received September 8, 2018; accepted July 15, 2019

Abstract: The solvent finding step has always been a time-consuming job inchemical-involved processes. The source of difficultymainly comes from the trial-and-errors, as a repetitive process of chosing solvents and mixing them in dif-ferent proportions. Computers are good at doing repetitive processes; however,they can only deal with numerical values, rather than qulitative scales. Numer-ification of qualitative parameters (like solubility) has already been introduced.The most recent one is the Hansen solubility parameters (HSPs). Using the HSPscould provide a solvent or solvent-mixture. In our previous study, we introduced acomputer-aided model and a software to find a solvent mixture. In this study, wehave used the computer-aided solvent selection model to find some solvent mix-tures for polyamide 11, a biobased polymer which has attracted enormous atten-tion recently. Using this numerical model significantly diminished the time of sol-vent development experimentation by decreasing the possible/necessary trials.

Keywords: Hansen solubility parameters; polyamide 11; nylon; solvent mixture;solvent screening; solvent substitution; the Simplex method.

1 IntroductionSolvents are inseparable elements in chemical-involved processes, from solarcells [1] to the production of biofilms [2]. Solvent selection and design is a com-plex problem, which requires decisionmaking in several levels for identifying thebest candidates depending on different multi-objective criteria namely environ-ment, health, safety, process feasibility and economics [3, 4]. The conventionalmethod of solvent selection is based on previous experiences, trial, and error with

*Corresponding author: Mostafa Jabbari, Swedish Centre for Resource Recovery, University ofBorås, Borås 501 90, Sweden, e-mail: Mostafa.Jabbari@hb.seMagnus Lundin, Mikael Skrifvars and Mohammad J. Taherzadeh: Swedish Centre for ResourceRecovery, University of Borås, Borås, SwedenSaeed Bahadorikhalili: Department of Chemistry, Ångström Laboratory, Uppsala University,Box 538, Uppsala, Sweden

2 | M. Jabbari et al.

different solvent candidates. This limits the candidates to the already existingexperimental data pool [4, 5]. A number of modern tools are increasingly becom-ing available to reduce the efforts needed to select the right solvent [6]. In ourprevious study [4] we introduced a computer-aided model and software to finda solvent mixture which uses the Simplex numerical model for minimizing the‘unsimilarity’ of the solvent and the polymer.

Solubility is essentially a thermodynamic property related to the cohesiveenergy between the polymer in question and the solvent, which could be in theform of another solid, liquid, or gas. Solubility theory is based on the notion that“like” dissolves “like”, where a molecule can be considered “like” another one, ifit has similar cohesive energy [7].

The means for predicting the extent of solubility between a polymer and sol-vent is to determine the difference in solubility parameters for each component.The total (or Hildebrand) solubility parameter (δ) is a measure of the cohesiveenergy for a given substance [8]. Materials having similar values of δ are likelyto interact with each other and exhibit high mutual solubility (or swelling) [7]. InHansen solubility parameters (HSPs), the total solubility parameter is separatedinto the following three subparameters [2, 4]:1. δD, the dispersive energy2. δP, the polarity intermolecular force, and3. δH, the hydrogen bonding.

These three parameters serve as coordinates for a site in three dimensions alsoknown as the Hansen space. The distance between two molecules in this spacedetermines how likely they are to dissolve into each other; the closer they are, themore likely they are to mutually dissolve each other (become miscible). For solidmaterials (solutes), their HSPs define the coordinates (location) of the center ofa sphere, the radius of which is known as the interaction radius. The interactionradius represents a region of high solubility, and solvents having HSP values thatare inside this sphere are considered highly soluble with the polymer solid, whilethose outside the sphere are not [8].

The use of prediction models has the advantage that for the selection pro-cedure, any solvent can be considered for which the required group interactionparameters are available and by using predictive methods, an extended varietyof solvents can be taken into account for selection [5]. Solubility parameters havefound their greatest use in the selection of solvents [9]. Selection of the appro-priate solvents and finding the best volume fractions could be done by computerprogramming throughminimization of RA formula (discussed in ‘Section 3’); how-ever, finding the minimum of RA takes much time for each set of solvents bynormal linear programming. Because it has to sweep all the decimal values of the

Finding Solvent for Polyamide 11 Using a Computer Software | 3

volume fraction from 0 to 1 for all the solvents; while by using the Simplex algo-rithm (discussed in ‘Section 3’), it can be done within a few milliseconds; hence,more combinations of solvents would be taken into account for solvent selection[4].

2 Polyamide 11Polyamide 11 (or Nylon 11) is a biobased polyamide, a member of the nylon familyof polymers produced by the polymerization of 11-aminoundecanoic acid. Bio-based polyamides, like polyamide 11 (PA 11), generally have lower melting tem-peratures than conventional PA 6 and PA 66 [10]. This enables themelt processingof wood fiber reinforced composites before the start of thermal degradation ofthe wood fibers, which has been reported to be at around 220 °C [11]. Nylon 11is applied in the fields of oil and gas, aerospace, automotive, textiles, electronics,and sports equipment, frequently in the tubing, wire sheathing, and metal coat-ings. It is produced from castor beans by Arkema under the trade name Rilsan[12]. Researchers Joseph Zeltner, Michel Genas, and Marcel Kastner, perfected themonomer process of polyamide 11 (Figure 1) in 1944 [13]. There has been a numberof research articles in the literature related to polyamide 11 solvents [14–18].

In this contribution, we used computer software for finding a solvent-mixturefor polyamide 11. To set the scene for this paper, we bring a brief description ofthe Hansen approach to solving the problem of finding a solvent-mixture. The fol-lowing overview is to facilitate reading for those who are not familiar with theseconcepts; therefore, experts can skip it.

3 BackgroundSolubility parameters help put numbers into this simple qualitative idea [9].Liquids with similar solubility parameters will be miscible, and polymers willdissolve in solvents whose solubility parameters are not too different from theirown [9]. Several graphing and modeling techniques have been developed to aid

Fig. 1: The structure of polyamide 11.

4 | M. Jabbari et al.

in the prediction of polymer solubility [8]. The basic principle has been “likedissolves like” [9]. By 1950, Hildebrand had defined the solubility parameter asthe sum of all the attractive intermolecular forces, which he found to be empir-ically related to the extent of mutual solubility of many chemical species [19].Solubility behavior cannot be accurately predictedbyonly theHildebrand solubil-ity parameter [8]. In 1967, Charles Hansen improved the concept and introducedhis three-dimensional solubility parameters. The Hansen approach provides anempirical, yet effective [20] method for determining the dissolution possibility ofsolutes. The solubility parameter has been used for many years to select solventsfor coatings materials [9].

The Hansenmodel is usually considered as a sphere. The center of the spherehas the δd, δp, and δh values of the polymer in question (solute) [8]. δ is squarthee root of cohesion energy density δd, δp, and δh represent the dispersive forces,polar inteactions, and hydrogen bonding, respectively. The radius of the sphere,RO, is termed the interaction radius [8]. The values of RO have been reported forsome polymers in the literature. RA is the distance in HSPs space between thesolute/polymer and the solvent [21]. The boundary of the spherical characteriza-tion is based on the requirement that ‘good’ solvents have a distance from thecenter of the sphere, RA (also termed the solubility parameter distance) less thanRO [8]. RA is given by the following relation:

RA Solution =√[

4 ×(δds − δdf

)2] +(δps − δpf

)2 +(δhs − δhf

)2 (1)

where δdf , δpf , and δhf are the Hansen solubility components for the poly-mer/solute (our favorite values), and δds, δps, and δhs are the Hansen solu-bility components for the solvent [8]. Equation 1 was developed from plots ofexperimental data where the constant ‘4’ was found convenient and correctlyrepresented the solubility data as a sphere encompassing the good solvent [8].

Solubility parameters of mixtures are linear [22]. That is, each of the threeHSPs of a solvent mixture is a linear function of composition. In this case, thecomposition value to be used in calculating solubility parameters for solvent mix-tures is the volume fraction (φ) for each component [22]. For a binary (two-solvent)mixture, the equation for all three solubility parameters is Equation 2 [22].

δblend ≡[φcomp1 × σcomp1

]+

[φcomp2 × σcomp2

](2)

This equation is correct for more than two components where the HSPs val-ues are known [22]. Traditionally, without specific data, it is usually assumed that

Finding Solvent for Polyamide 11 Using a Computer Software | 5

there is no volume change upon mixing of solvents. That is:

(vol. Fraction)1 =

(Wt. FractionDensity

)1(

Wt. FractionDensity

)1

+(Wt. FractionDensity

)2

(3)

In Eq. 2, φ is the volume fraction of component 1, and δ is any solubilityparameter. It is understood that φcomp1 + φcomp 2 = 1. The volume fraction iseasy to computebecause solvents are stored inpails or drumsandusedbyvolume,although they are sold by weight.

Linear programming (LP) is a technique for the optimization of a linearobjective function, subject to linear equality and linear inequality constraints.Although the R formula is not linear (it is quadratic), its constraint is linear. Itsfeasible region is a convex polytope, which is a set defined as the intersection offinitely many half spaces, each of which is defined by a linear inequality [23]. Itsobjective function is a real-valued affine (linear) function defined on this polyhe-dron. A linear programming algorithm finds a point in the polyhedron where thisfunction has the smallest (or largest) value if such a point exists. There are a fewLP methods such as ellipsoid and interior-point, but for more than 35 years now,George B. Dantzig’s Simplex-Method has been the most efficient mathematicaltool for solving linear programming problems [24]. It is probably that mathemati-cal algorithm for which the most computation time on computers is spent. Thisfact explains the great interest of experts and of the public to understand themethod and its efficiency [24].

A convenient single parameter to describe solvent quality is the relativeenergy difference, RED, number: RED = RA/RO [8]. According to the basic prin-ciple in dissolution, “like dissolves like” [9], the more similarity, the less RA andhence, the higher the probability of dissolution. In other words, the distance inHSPs space between the solute/polymer, the Hansen space, should be as small aspossible (RA ≈ 0) [21]. By taking square root from both sides of Eq. 1, we have:

R2A Solution =[4 ×

(δds − δdf

)2] +(δps − δpf

)2 +(δhs − δhf

)2 (4)

Therefore, we need to minimize R2A, as much as possible. In this paper, weminimize the R2A with the Simplex method by devethe lopment of a computerprogram.

4 MethodologyThe Microsoft Visual Studio software package was used to develop the program.The Microsoft.SolverFoundation.Services.dll library that contains the Simplex

6 | M. Jabbari et al.

Tab.1:So

lventvaluesused

inthesoftw

areforp

olyamide11.

#So

lventn

ame

CAS

registry

number

δT( √

(MJ/m

3 ))

δD( √

(MJ/m

3 ))

δP( √

(MJ/m

3 ))

δH( √

(MJ/m

3 ))

MV

(mL/mol)

MW

(g/m

ol)

Health

index

(NFPA)

11-Brom

onaphthalene

90-11-9

20.9

20.3

3.1

4.1

139.06

620

7.07

22

1-Bu

tanol

71-36-3

23.2

165.7

15.8

91.5

74.12

13

1-Oc

tanol

111-87

-521

173.3

11.9

157.7

8787

14

1-Pentanol

71-41-0

21.6

15.9

4.5

13.9

108.6

8787

15

2-Bu

tanol

78-92-2

22.2

15.8

5.7

14.5

9274

.12

16

4-Methyl-2

-pentanol

108-11

-220

15.4

3.3

12.3

127.2

8787

27

Brom

obenzene

108-86

-121

.620

.55.5

4.1

105.02

315

7.01

28

Brom

ochlorom

ethane

74-97-5

18.5

17.3

5.7

3.5

64.982

129.38

29

Butylchloride

109-69

-317

.216

.25.5

210

4.01

192

.57

110

Butyric

acid

107-92

-618

.714

.94.1

10.6

92.475

88.11

011

Chloroform

67-66-3

18.9

17.8

3.1

5.7

80.175

119.38

212

Chloromethane

74-87-3

16.9

15.3

6.1

3.9

55.4

8787

213

Cycloh

exanol

108-93

-022

.417

.44.1

13.5

106

100.15

81

14Cycloh

exylchlorid

e54

2-18

-718

.317

.35.5

211

8.6

8787

215

Dibenzylether

103-50

-419

.117

.33.7

7.3

192.7

33,333

116

diEthylcarbo

nate

105-58

-818

16.6

3.1

6.1

121

8787

117

DiisoB

utylcarbinol

108-82

-718

.714

.93.1

10.8

177.8

8787

118

DPGM

E(diPropylene

glycol

methylether)

34,590

-94-8

2015

.55.7

11.2

157.4

8787

2

19IsoA

mylacetate

123-92

-217

.115

.33.1

714

8.8

33,333

120

isob

utanol

78-83-1

22.7

15.1

5.7

1692

.421

74.122

121

Methylbutylketone

591-78

-617

15.3

6.1

4.1

123.6

8787

2

Finding Solvent for Polyamide 11 Using a Computer Software | 7

Tab.1(continued)

#So

lventn

ame

CAS

registry

number

δT( √

(MJ/m

3 ))

δD( √

(MJ/m

3 ))

δP( √

(MJ/m

3 ))

δH( √

(MJ/m

3 ))

MV

(mL/mol)

MW

(g/m

ol)

Health

index

(NFPA)

22Methyloleate

112-62

-915

.514

.53.9

3.7

340

296.48

794

123

MethylAmylacetate

108-84

-916

.915

.23.1

6.8

167.4

8787

124

n-Bu

tylamine

109-73

-918

.616

.24.5

898

.838

73.14

225

o-diCh

loroBe

nzene

95-50-1

20.5

19.2

6.3

3.3

112.8

8787

226

Propyleneglycol

monoM

ethylether

107-98

-220

.415

.66.3

11.6

93.8

8787

1

27sec-Bu

tylacetate

105-46

-417

.215

3.7

7.6

133.51

711

6.16

128

Tetra

ChloroEthylene

127-18

-420

.319

6.5

2.9

101.1

8787

229

triEthylene

glycol

monoO

leylether

9004

-98-2

1613

.33.1

8.4

418.5

8787

1

8 | M. Jabbari et al.

algorithm was imported at the beginning of the program’s codes. The codeswere written in VisualBasic programming language that operates within theMicrosoft.Net framework. All the HSPs values of 234 solvents together with theirnames and CAS number, health NFPA index were imported into a database(Table 1). The database was connected to the program in a way that each timethe program initiates, it loads the solvents’ δd, δp and δh values to predefinedarrays–D(), P(), and H(), respectively. Out of 234 solvents, 81,085 different com-binations were examined for polyamide 66 (PA 66) as the case study. It wascounted by a counter variable in the program’s code. The HSPs values of the sol-vents obtained from ASTM STP1133, Hansen [9], Mark [25], and Barton [26]. Thevalues for NFPA health index were obtained from Sigma-Aldrich material safetydata sheets, sciencelab.com, cameochemicals.noaa.gov, synquestlabs.com, andmathesongas.com. A detailed description of the software code is available in ourprevious study [4].

5 Results and discussionThe subset of solvent-mixtures results was obtained from running the softwarewith the HSPs of the selected polymer (PA 11) out of 234 organic solvents. Usingfive main descriptors, namely, the dispersive (δd), polar (δp), and hydrogen(δh) Hansen solubility parameters, CAS registry number, and the health indexesaccording to the NFPR classifications. The Rlimite was assigned equal to “0.2”.

5.1 Miscibility challenge

Solvents used in this study are of different sorts–polar, non-polar, protic, apro-tic, etc. The closeness of HSPs values of solvent-mixture and the solute onlyguarantees the likeness of the interactions between the solvent-mixture and thesolute; however, there is no guarantee for miscibility of all the components of asolvent-mixture.

As the “polar” interactions of molecules are numerated in δp values (inHansen theory, see the ‘Section 3’), we assumed that the closeness in δp valuesof the components could increase the possibility of miscibility, i.e. the differencein δp values (called ∆P, hereinafter) will decrease the chance of immiscibility ofeach solvent in the other one.

5.2 Results for polyamide 11

The solubility of polyamide 11 was reported as 17.0, 4.4, and 10.6 for δD, δP, andδH, respectively [7, 27]. The results were sorted first by ∆P, and then R and then

Finding Solvent for Polyamide 11 Using a Computer Software | 9

Tab.2:Prop

osed

solventsusingthedevelopedsoftw

areforp

olyamide11.

#R

ϕ1a

ϕ2

ϕ3

Health

δd,δ

p,δh

( √(M

J/m

3 ))

∆Pb

10

Butyric

acid:1

5.14

2%Cycloh

exanol:6

3.45

9%Cycloh

exylchlorid

e:21

.399

%1.06

317

,4.4,10.6

1.4

20.00

1Cycloh

exanol:6

6.66

1%Dichloromethane:

15.306

%Methyloleate:18

.033

%1.15

316

.999

,4.401

,10.6

2.4

30.00

21-brom

onaphthalene:

30.572

%2-Bu

tanol:49

.967

%DiisoB

utylcarbinol:19.46

1%1.30

617

.001

,4.399

,10.6

2.6

40.00

3Brom

ochlorom

ethane:

24.341

%Cycloh

exanol:66.51

8%triEthylene

glycolmonoO

leyl

ether:9.14

1%1.24

317

.001

,4.398

,10.6

2.6

50.00

33-Ch

loro-1-propanol:

50.101

%Ch

loroform

:35.45

1%triEthylene

glycolmonoO

leyl

ether:14

.448

%1.35

517

,4.403

,10.59

92.6

60.00

44-Methyl-2

-pentanol:5.27

9%Bu

tylchloride:24

.662

%Cycloh

exanol:7

0.05

9%1.05

316

.998

,4.403

,10.60

12.2

70.00

41-brom

onaphthalene:

27.183

%1-Pentanol:49.27

9%DP

GME(diPropylene

glycol

methylether):23

.539

%1.50

717

.002

,4.402

,10.60

12.6

80.00

41-Oc

tanol:63

.3%

o-diCh

loroBe

nzene:

14.326

%PropyleneglycolmonoM

ethyl

ether:22

.374

%1.14

317

.002

,4.401

,10.60

13

90.00

5Cycloh

exanol:5

2.23

7%n-Bu

tylamine:41

.941

%o-diCh

loroBe

nzene:5.82

2%1.47

817

.002

,4.396

,10.59

92.2

100.00

51-Tridecanol:3

.631

%Bu

tylchloride:23

.801

%Cycloh

exanol:72.56

7%1

17.002

,4.397

,10.59

92.4

110.00

51-brom

onaphthalene:

27.038

%1-Bu

tanol:49

.833

%IsoA

mylacetate:23

.129

%1.27

17.001

,4.396

,10.60

12.6

120.00

6Bu

tylchloride:24

.218

%Cycloh

exanol:71.49

8%DiisoB

utylcarbinol:4.285

%1

17.002

,4.396

,10.59

92.4

130.00

71-brom

onaphthalene:

38.394

%1-Tridecanol:11.84

4%isobutanol:4

9.76

3%1.38

417

.002

,4.394

,10.60

22.6

140.00

81-brom

onaphthalene:

18.935

%1-Bu

tanol:50

.275

%diEthylcarbo

nate:30.79

%1.18

916

.999

,4.407

,10.59

82.6

150.00

9Brom

ochlorom

ethane:

11.025

%Cycloh

exanol:56.31

4%n-Bu

tylamine:32

.661

%1.43

716

.997

,4.407

,10.60

11.6

10 | M. Jabbari et al.

Tab.2(contin

ued)

#R

ϕ1a

ϕ2

ϕ3

Health

δd,δ

p,δh

( √(M

J/m

3 ))

∆Pb

160.00

9Bu

tylchloride:22

.53%

Cycloh

exanol:72.21

4%sec-Bu

tylacetate:5.255

%1

17.003

,4.394

,10.59

91.8

170.00

9Cycloh

exanol:5

2.73

8%n-Bu

tylamine:41

.389

%Tetra

ChloroEthylene:5.873

%1.47

316

.997

,4.407

,10.60

12.4

180.00

91-Bu

tanol:19

.369

%4-Methyl-2

-pentanol:

51.614

%Brom

obenzene:29.01

8%1.80

616

.996

,4.403

,10.59

92.4

190.00

91-brom

onaphthalene:

22.636

%3-Ch

loro-1-propanol:

50.331

%triEthylene

glycolmonoO

leyl

ether:27

.033

%1.22

616

.998

,4.409

,10.59

82.6

200.01

Butylchloride:23

.955

%Cycloh

exanol:73.22

1%triEthylene

glycolmonoO

leyl

ether:2.82

4%1

16.997

,4.407

,10.60

12.4

210.01

1-brom

onaphthalene:

30.984

%1-Bu

tanol:38

.682

%Bu

tyric

acid:30.33

3%1.00

616

.998

,4.409

,10.59

72.6

220.01

Chloroform

:11.78

4%Cycloh

exanol:67.14

1%Methylbutylketone:21.07

5%1.32

917

.005

,4.404

,10.6

323

0.01

11-brom

onaphthalene:

27.359

%1-Bu

tanol:50

.398

%MethylAmylacetate:22

.243

%1.27

416

.998

,4.41,10

.597

2.6

240.01

2Bu

tylchloride:17

.451

%Cycloh

exanol:66.33

8%n-Bu

tylamine:16

.211

%1.16

216

.996

,4.409

,10.60

21.4

250.01

2Ch

loromethane:1

8.44

%Cycloh

exanol:63.34

4%Dibenzylether:18

.216

%1.18

416

.995

,4.396

,10.6

2.4

a ϕrepresentsvolumefra

ction;therefore:ϕ1

+ϕ2

+ϕ3

=1.

b∆P

referstothediffe

renceintheδp

values

ofthesolventsineach

mixture,greatest

δp–sm

allestδp

inthesetofthree

solvents.

Finding Solvent for Polyamide 11 Using a Computer Software | 11

Fig. 2: The structure of the three solvents for polyamide 11 solvent-mixture.

by health index, descendingly (Table 2). The used solvents are listed in Table 1.The first record (row), a mixture of butyric acid: 15.142%, cyclohexanol: 63.459%,and cyclohexyl chloride: 21.399% is the best candidate for dissolving polyamide11. The percentages are the products of the volume fraction by 100. For the firstrecord, the NFPA health index is 1.063, representing a safe cumulative healthindex. Theoretically, the chance of immiscibility of the three solvents (butyricacid, cyclohexanol, and cyclohexyl chloride) is low enough (1.4). According to theHSPs of each solvents (Table 1), the δp for cyclohexyl chloride, butyric acid, andcyclohexanol are 5.5, 4.1, and 4.1, respectively, giving a difference of 1.4 whichis less than the limit (8) we set in the parameters in the software. The structureof the three solvents is depicted in Figure 2. While, using the method proposedin this paper, as mentioned before, has the advantage of being applicable to allpolymers/solutes and at the same time, is less time-consuming than the practicalmethod. Therefore, a greater number of applicants can be taken into account tobe the candidates.

6 ConclusionA sophisticated software package for the selection of the most suitable solvent-mixture for a solute/polymer with known HSPs or solvent-substitution for anysolvent-involved process, e.g. liquid-liquid extraction was used to find a solventfor polyamide 11. The technique introduced in this paper decreases the time ofsolvent selection process tremendously by screening the vast number of differ-ent combination of an enormous number of solvents and narrow it down to asmaller set of solvents. This technique reduces the laboratory effort required inscreening solvent blends while allowing a large number of candidate solvents tobe considered for inclusion in a blend.

12 | M. Jabbari et al.

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