Thesis for the Degree of Doctor of Philosophy
Development of thermoplastic biocomposites
based on aligned hybrid yarns for fast composite
manufacturing
Behnaz Baghaei
Copyright©Behnaz Baghaei
Swedish Centre for Resource Recovery
University of Borås
SE-501 90 Borås, Sweden
Digital version: http://urn.kb.se/resolve?urn=urn:nbn:se:hb:diva-764
ISBN 978-91-87525-79-7 (printed)
ISBN 978-91-87525-80-3 (pdf)
ISSN 0280-381X Skrifter från Högskolan i Borås, nr. 74
Cover Photo: Masoud Salehi
Printed in Sweden by Responstryck AB
Borås 2015
I
Abstract
The interest in natural fibres as reinforcement for composite materials has been steadily increasing
due to their attractive mechanical properties and the possibility of making more eco-friendly
materials. Currently, various alternatives are being introduced for commercial applications, as fibres
such as hemp, jute and flax exhibit properties, which make them appropriate for structural
composite components. Biocomposites offer reductions in weight and cost and have less reliance on
foreign oil resources, making them attractive. Several investigations have revealed that the full
utilisation of fibre mechanical properties in the final composites can be exploited, provided an
aligned fibre orientation is chosen. In fact, a major challenge for natural fibre reinforced composites
is to achieve high mechanical performance at competitive prices. The use of commingled/hybrid
yarns is one of the more promising methods for manufacturing structural thermoplastic composites.
Commingled yarns of thermoplastic and reinforcing fibres offer a potential for cost-effective
production of composite parts, thanks to reduced applied pressures and impregnation times during
processing. Besides economic advantages, there is also direct control over fibre placements and ease
of handling of fibres in yarn process. The yarn technologies provide homogenous distribution of
reinforcing fibre and matrix. Variation in natural fibre properties has been a major problem facing
composite manufacturers, compared to carbon and glass fibres that have well-defined production
processes. This issue can be addressed by regenerated cellulose fibres. These fibres can be
reproduced easily with high surface evenness and even quality, making it possible to get consistent
results, which is not possible with natural fibres. Combination of natural and regenerated cellulose
fibre brings together the best of both materials. The end result is a product with superior properties,
which could not be obtained by the individual components.
This thesis describes the development of aligned hybrid yarns with low fibre twist, for high
performance natural (hemp) and man-made (Lyocell) cellulose fibre-reinforced biocomposites,
suitable for use in structural or semi-structural applications. The properties of composites in terms
of fibre orientation, off-axis angle and alkali treatment were investigated, focusing on determining
void%, water absorption, mechanical and thermo-mechanical properties. The results show that
combining hemp and Lyocell in PLA composite leads to the reduction of moisture absorption and
can improve the mechanical properties. The mechanical properties of the composites were highly
affected by the fibre direction. The alkali treatment on hemp fibre improved the mechanical
properties of the composites.
Keywords: Hybrid yarns, Mechanical properties, Porosity, Weaving, Alkali treatment, Compression
moulding
II
III
List of Publications
This thesis is based on the results presented in the following publications:
I. Baghaei B, Skrifvars M and Berglin L. (2013) Manufacture and characterization of
thermoplastic composites made from PLA/hemp co-wrapped hybrid yarn prepregs. Composites
Part A: Applied Science and Manufacturing 50:93-101.
II. Baghaei B, Skrifvars M, Salehi M, Bashir T, Rissanen M and Nousiainen P. (2014) Novel
aligned hemp fibre reinforcement for structural biocomposites: Porosity, water absorption,
mechanical performances and viscoelastic behaviour. Composites Part A: Applied Science and
Manufacturing 61:1-12.
III. Baghaei B, Skrifvars M, Berglin L. (2015) Characterization of thermoplastic natural fibre
composites made from woven hybrid yarn prepregs with different weave pattern. Composites
Part A: Applied Science and Manufacturing 76:154-161.
IV. Baghaei B, M. Skrifvars, Rissanen M and Ramamoorthy SK. (2014): Mechanical and thermal
characterization of compression moulded polylactic acid natural fiber composites reinforced by
hemp and Lyocell fibers. Journal of Applied Polymer Science, 131(15) DOI:10.1002/app.40534.
V. Baghaei B, M. Skrifvars. Characterisation of polylactic acid biocomposites made from prepregs
composed of woven polylactic acid/hemp-Lyocell hybrid yarn fabrics. Submitted to Composites
Part A: Applied Science and Manufacturing
Statement of Contribution
The author of this thesis, Behnaz Baghaei, was main author of all the publications and performed
most of the experimental work, data analysis and manuscript writing in all publications. Dr Lena
Berglin prepared the textile fabrics for Publications III and V. Dr Marja Rissanen performed the
tensile testing of the fibres for Publications II and IV. Other co-authors assisted with data analysis
and manuscript writing.
SEM analysis was performed at Swerea IVF (Sweden) for Publication I and Chalmers University of
Technology (Sweden) for Publications II, III, IV and V.
Permission from publishers
Permission was obtained from:
- ELSEVIER to include Publications I, II and III in the printed version of this thesis
- John Wiley and Sons to include Publication IV in the printed version of this thesis
IV
Conference Contributions
1. Baghaei B and Skrifvars M. Investigation of pattern style of woven fabrics produced from
hybrid wrap spun yarns on fabricated composite, 20th International Conference on
Composite Materials in Copenhagen, Denmark, July 19-24, 2015 (oral presentation)
2. Baghaei B, Skrifvars M and Salehi M. Aligned hemp yarn reinforced biocomposites:
porosity, water absorption, thermal and mechanical properties, 16th European conference on
composite materials, Seville, Spain, June 22-26, 2014 (oral presentation)
3. Baghaei B, Skrifvars M and Berglin L. Tailoring of the mechanical and thermal properties
of hemp/PLA hybrid yarn composites, 3rd Avancell conference, Göteborg, Sweden, October
8-9, 2013 (Poster)
4. Baghaei B, Skrifvars M and Berglin L. Tailoring of the mechanical and thermal properties
of hemp/PLA hybrid yarn composites, IPLA Global Forum on Sustainable Waste
Management for the 21st Century Cities, Borås, Sweden, September 9-11, 2013 (Poster)
5. Baghaei B, Skrifvars M and Berglin L and Ramamoorthy SK. Hemp/PLA co-wrapped
hybrid yarns for structured thermoplastic composites, 50th
Anniversary Nordic Polymer
Days, Helsinki, Finland, May 28-31, 2013 (oral presentation)
V
Acknowledgements
I would like to thank everyone who has contributed immensely to my life in many remarkable ways.
First and foremost, I would like to express my deepest gratitude to my supervisor, Professor Mikael
Skrifvars, for his support during these past four years. I appreciate all his contributions of time, idea,
scientific advice and immense knowledge.
Besides my supervisor, I would like to thank my co-supervisors, Dr Lena Berglin and Dr Dan
Åkesson, for their insightful comments and encouragement.
I also wish to extend my gratitude to my examiner, Professor Tobias Richards, for making it
possible for me to present this dissertation.
Professor Mohammad Taherzadeh, I am so grateful for your valuable guidance.
The members of polymer group have contributed immensely to my personal and professional time
at Borås. The group has been a source of friendship as well as good advice and collaboration. To my
friends: Tariq, Fatimat, Haike, Sunil, Adib, I say thank you.
I would like to thank Dr Peter Therning, Tomas Wahnström, Sari Sarhamo, Susanne Borg, Louise
Holmgren and Thomas Södergren for your support throughout my studies.
I am also very grateful to my teachers, Dr.Magnus Lundin, Dr Kim Bolton and Dr Anita Pettersson.
I extend my gratitude to all PhD students and colleagues at the Swedish Centre of Resource
Recovery for being such wonderful friends and creating a pleasant working environment.
In addition, I thank my wonderful friends Khatereh, Abas, Narges, Ehsan, Noushin, Mahdi, Mehran,
Zohreh, Khosrow, Parvin, Nima, Shahram, Mehrsa, Johan, Kamran, Solmaz, Maryam, Gergely,
Marjan, Farhang, Samira, Farzad, Negar, Arman, Davood, Mina, Masoud, Faranak, Amin, Sima,
Raj, Rouzbeh and Sahar.
To my husband, Masoud, I express my deepest gratitude. You have always been there cheering me
up and standing by my side through the good times and bad. I would also like to thank my dear
parents, brother (Amirali), sister (Bahareh) and brother-in-law (Majid). They have always supported
me and encouraged me with their best wishes. I am thankful for all support from my parents-in-law.
I express my gratitude to Patrik Johansson and Joachim Almvång from Engtex Company for
making the fabrics for this project.
Finally, I would like to thank other institutions that were part of my research, either helping me with
instruments or providing support: Swerea AB, Sweden; Chalmers University, Sweden and Tampere
University of Technology, Finland.
This work was financed by Ångpanneföreningens (Åforsk) and Smart Textiles (University of Borås)
and their contributions are greatly acknowledged.
VI
VII
Abbreviations
NMMO N-Methylmorpholine N-oxide
NFC Natural fibre composites
NaOH Sodium hydroxide
SEM Scanning electron microscopy
WG Weight gain
Wt% Weight %
Vol % Volume %
l/d Aspect ratio
DMTA Dynamic mechanical thermal analysis
DSC Differential scanning calorimetry
Tg Glass transition temperature
TC Crystallisation temperature
Tm Melting temperature
ΔHcc Cold crystallisation enthalpy
ΔHm Melting enthalpy
Tex Mass of yarn in grams per 1,000 metres length
VIII
IX
TABLE OF CONTENTS
Abstract I
List of Publications III
Conference Contributions IV
Acknowledgements V
Abbreviations VII
1. INTRODUCTION 1
1.1 Justification 2
1.2 Aim of this study 3
1.3. Outline of the thesis 4
2. LITERATURE REVIEW 5
2.1 Natural fibres 5
2.1.1 Hemp 5
2.1.2 Regenerated cellulose fibre 6
2.2 Fibre surface modifications 7
2.2.1 Alkali treatment 7
2.3 Biodegradable polymers 8
2.3.1 Polylactic acid (PLA) 8
2.4 Biocomposites 8
2.4.1 Hybrid composites 9
2.4.2 Thermoplastic composites 10
2.4.3 Engineering applications of biocomposites 13
3. EXPERIMENTAL 15
3.1 Materials 15
3.1.1 Reinforcements 15
3.1.2 Matrices 15
3.1.3 Chemicals 15
3.2 Methods 15
3.2.1 Fibre alkali treatment 15
3.2.2 Manufacture of hybrid yarns 15
3.2.2.1 Co-wrapped hybrid yarn 15
3.2.2.2 Wrap spinning of hybrid yarns 16
3.2.3 Preparation of prepregs 17
3.2.4 Composite preparation 19
3.3 Characterisation 19
3.3.1 Single fibre tensile test 19
3.3.2 Composite density and porosity 19
3.3.3 Water absorption test 19
3.3.4 Tensile testing 20
3.3.5 Flexural testing 20
3.3.6 Impact testing 20
3.3.7 Dynamic mechanical thermal analysis 20
X
3.3.8 Differential scanning calorimetry 20
3.3.9 Scanning electron microscopy (SEM) 21
4. SUMMARY OF RESULTS 23
4.1 Effect of heat and alkali treatments on fibre properties (Publications II and IV) 23
4.2 Composites properties (Publications I, II, III, IV and V) 24
4.2.1 Composite porosity 24
4.2.2 Water absorption 25
4.2.3 Tensile properties 28
4.2.4 Flexural properties 30
4.2.5 Impact resistance 32
4.2.6 Dynamic mechanical thermal testing 34
4.2.7 Differential scanning calorimetry 37
5. DISCUSSION AND CONCLUSIONS 41
6. FUTURE STUDIES 43
REFERENCES 45
PUBLICATION I
PUBLICATION II
PUBLICATION III
PUBLICATION IV
PUBLICATION V
INTRODUCTION
1
1. INTRODUCTION
The idea of making composite materials is not a new one; nature has many examples wherein the
idea of composite materials is applied. For example, the coconut palm leaf is nothing but a basis
using the concept of fibre reinforcement. Wood is a fibrous composite; it is cellulose fibres in lignin
matrix. In addition to these naturally occurring composites, there are several other engineering
materials that are composites and that have been applied for a very long time [1]. In general the
composite materials consist of a matrix, reinforced with particles or fibres to create a material with
specific optimal properties. The two components control different aspects of the composite;
reinforcing fibres influence the strength and stiffness properties while the matrix acts to transfer
loads between fibres and protects against environmental conditions such as chemicals, heat and
moisture. The matrix binds the fibres together and is usually composed of metals, polymers or
ceramics [2].
Natural fibres have been applied as reinforcement in composites from very early on till the mid-
twentieth century; however, the interest in them had decreased with the development of synthetic
fibres such as carbon and glass fibres. Since mid-20th
century, there was an increased demand for
stronger and stiffer, still light weight, composites, in fields such as aerospace, transportation and
construction. This led to the incorporation of high performance fibres for reinforcement.
Consequently, the success of new synthetic materials almost ceased the development of natural
fibres. Nonetheless, there is a revivification of interest in natural fibres lately, due to environmental
reasons. They are coming back in high-tech world. The application of modern technologies,
knowledge and scientific tools for these natural materials has given rise to new developments in this
field [3, 4].
Currently, the use of thermoplastic resins in some of the composite application is clearly of higher
potential than the use of thermoset resins because of: better impact strength, easier recycling, faster
processing conditions (no time for curing is required), possibility of production in longer series,
lower cost, absence of toxic solvents and higher fracture toughness and elongation on fracture [5-7].
However, the high viscosity of molten thermoplastics, compared to thermosets makes the process
more problematic. Different impregnation methods have been proposed and applied.
A current trend in composites fabrication is to reduce the number of processes required to produce a
component rapidly and cost-effectively. Therefore, a series of required processes including
impregnation of reinforcement preforms, consolidation, forming into shape and solidification to be
done sequentially are now often combined into a single process, one that is highly integrated [8].
Among the possible methods for fabricating continuous fibre reinforced thermoplastic composites,
consolidation of preforms based on commingled/hybrid yarns, created on the principle of combining
reinforcing fibres and matrix fibres homogeneously during spinning, is an economical method for
the manufacture of complex shapes. The hybrid yarns can be further converted into a drapable
woven fabric preforms for forming the complex shaped-parts. The good distribution of
reinforcement and matrix in a non-molten state in commingled yarn leads to very short polymer
flow distance for impregnation, short cycle times and reduction in production cost [9, 10]. The
INTRODUCTION
2
quality of the component distribution in yarn is known to affect the mechanical properties of the
manufactured composites.
The fibre orientation (i.e. alignment of the fibres) must be controlled to ensure that the fibre
mechanical properties are efficiently utilised in order to attract industrial interest as an alternative to
the traditionally applied synthetic fibres (e.g. glass fibres). It is evident that PLA/hemp fibre
composites can compete with glass fibre composites regarding stiffness, whereas for tensile and
impact strength, the properties are still not on a satisfactory level [11-16]. Previous studies have
demonstrated that the full reinforcement potential of natural fibres can be exploited in
biocomposites if an aligned fibre orientation is used [16, 17]. Natural fibres are inherently
discontinuous; therefore, natural fibre reinforcements reported so far are based on twisted spun
staple yarns, which are produced by mechanical spinning methods, mainly ring spinning. These
spun yarns tend to be highly twisted, which leads to fibre misalignment due to their helical paths
around the yarn axis. Specifically, this misalignment contributes negatively to the mechanical
properties of the resultant composites. Another negative impact of yarn twist is that it tightens the
yarn structure, rendering resin impregnation difficult [18]. Therefore, in the textile industry, a broad
range of techniques for the alignment of natural fibres have been developed and optimised to
produce yarns with controlled fibre orientations by reducing or replacing twist in yarns.
To fully benefit from these advantages, the process parameters governing consolidation must be
identified and optimised. Consolidation of commingled yarns has been studied and modelled by
several authors [1-6] and has recently been reviewed by Svensson et al. [7]. In all cases, the
consolidation quality was characterised by the amount of residual porosity in the part. In general,
the polymer fibres melt rapidly and form liquid pools surrounding the reinforcing fibres. The
problem thus results in infiltration of dry fibre bundles surrounded by molten polymer pools. As
reinforcing fibres and polymer fibres are in general not distributed homogeneously within a yarn,
commingling quality becomes one of the most critical parameters [9].
1.1 Justification
There is growing interest in the use of natural fibre composites (NFCs) either in terms of industrial
applications or fundamental research studies. They are renewable, completely or partially recyclable
and relatively cheap. Plant fibres, such as hemp, sisal, flax, kenaf, ramie, etc., are often used as
reinforcement for composites due to their renewability, availability, low density, price and good
mechanical properties that make them an attractive ecological alternative to conventional fibres
such as carbon, glass and aramid fibres. Moreover, the European Union has set environmental
sustainability policies and regulations [19] aiming to reduce the land filling and apply renewable,
sustainable and biodegradable materials. Currently, NFCs are applied in transportation, military
applications, building and construction industries and packaging, etc. [20-23]. Despite the extensive
studies and development activities in the recent years aiming to optimise the properties and
performance of NFCs, current NFCs still cannot compete with glass fibre composites in terms of
mechanical performance [19]. In common with all engineering applications for NFCs, fibre
alignment should be optimised [22, 24] to suit the application. Unlike conventional continuous
reinforcement fibres, such as carbon and glass rovings, the natural fibres are of short length, being
INTRODUCTION
3
known as staple fibres in the textile industry. Usually, these natural fibres need to be processed into
continuous yarns using conventional spinning method before being made into directional
reinforcements for structural composite application, although research on unidirectional nonwoven
and woven preforms has been undertaken [17, 19]. Polylactic acid (PLA) is the most important bio-
thermoplastic. It shows also quite good properties, appropriate for applications that do not require
long-term durability or high mechanical performance at higher temperatures. The mechanical
properties of the PLA can be improved by using reinforcements like natural fibres in order to
increase its potential use in many industrial applications [25, 26]. The major attraction of these
biocomposites is that they are environmentally-friendly. These composites possess a high
biocontent and upon disposal can biodegrade with minimal harm to the environment, or at the end
of their life they can be recycled by energy recovery. Given the evident potential of the application
of thermoplastic biocomposites in high volume production, appropriate processing techniques are
needed. Due to the high melt viscosity of the thermoplastic matrix, the fibre impregnation is more
complex than for thermosets. Combining the reinforcement fibres and polymer at the fibre level
offers an improved distribution.
The idea of this research project was to modify the existing methods of commingled yarn
production from relatively short cellulose fibres and PLA (biopolymer) in order to improve
distribution and alignment of the fibres. The quality of the component distribution in the
commingled yarn affects the mechanical properties of the fabricated composites. The fibre
orientation (i.e. alignment of the fibres) must be controlled to ensure that the fibre mechanical
properties are efficiently utilised in order to attract industrial interest, as an alternative to the
traditionally applied synthetic fibres.
1.2 Aim of this study
The specific purpose of this thesis was to develop hybrid yarns with low fibre twist, optimised and
controlled fibre orientation for the fast processing, cost-effective and high performance
biocomposites suitable for use in load-bearing or semi-structural applications. I discussed the
development of two types of hybrid yarns: co-wrapped and aligned commingled yarns. The
influences of fibre content and wrap density on the properties of composites were reported. The
properties of composites, in terms of fibre orientation (aligned and random), off-axis angle and
alkali treatment were also investigated. The hybrid yarns were further processed into preforms in
the form of woven fabrics, which are consolidated and moulded in a single processing step. We also
studied the effect of two types of structures, 8-harness satin and basket, for the woven fabrics from
hybrid yarns on amount of porosity, moisture absorption, mechanical and thermo-mechanical
properties. In the context of this study, the mechanical characteristics of composites, in terms of the
reinforcing fibre characteristics, were examined. The natural (hemp) and man-made (Lyocell)
cellulose fibres were used. Different constituents can be used to tailor the composite characteristics
for diverse requirements, for instance, the high stiffness of the hemp fibres with the high elongation
at break and resin wetting and tenacity of the Lyocell fibres. Composites of PLA/hemp fibres,
PLA/hemp-Lyocell fibre mixtures, and PLA/Lyocell fibres were investigated.
INTRODUCTION
4
1.3. Outline of the thesis
The thesis is divided into six main chapters:
1. Chapter 1 introduces the motivation, justification and objectives of the research.
2. Chapter 2 reviews existing literatures related to the experimental work.
3. Chapter 3 explains the experimental work of the thesis.
4. Chapter 4 provides the summary of the obtained results.
5. Chapter 5 is the concluding chapter and discusses the results.
6. Chapter 6 suggests the avenues for future research.
LITERATURE REVIEW
5
2. LITERATURE REVIEW
2.1 Natural fibres
Textile fibre is a material mainly made from natural or synthetic sources. A general classification
for fibres is provided in Fig. 1.
Natural fibres generally can be classified as to their origin, that is, coming from plants, animals or
minerals.
Fig. 1 Classifications for Textile fibres
(Adapted and developed from Müeller et al. (2003) and Beldzki et al. (2002) [27, 28])
The plant-based fibres can be classified according to the part of the plant they are recovered from.
The precise composition of plant fibres, cellulose, hemicellulose, lignin, wax, etc. depends on
several factors such as the method of analysing, the growth environment and geographical location
of the plant and the level of maturity of the plant, etc.
2.1.1 Hemp
Hemp (Cannabis sativa L.) is a sustainable, multiuse, multifunctional, and high yielding industrial
crop, which can provide valuable raw material to meet the high-global demand for fibres. Hemp is
well known for requiring little or no pesticide use, and it needs low to medium quantities of
fertiliser [29]. The environmental friendly cultivation of hemp crop and the sustainability of its
products are the key drivers for future expansion of the hemp [30]. Currently, China, Europe and
Canada are the main hemp producing regions in the world [31]. Among natural fibres, bast fibres
have good mechanical properties, which make them an ideal candidate as the reinforcing media.
Hemp as a bast fibre has specific mechanical properties similar to the glass fibres, while it is
flexible. Furthermore, hemp fibres can be easily processed on the traditional opening and carding
LITERATURE REVIEW
6
machines. Many studies have been carried out in automotive market on olefin polymers reinforced
with hemp fibres. The above-mentioned reasons, along with industry wide acceptance, led to
selection of hemp fibres as reinforcement media in my research. Currently, hemp is the subject of a
European Union subsidy for non-food agriculture, and considerable strategies have been underway
for further development in Europe. Lear Corp., Johnson Controls and Ford are using hemp fibre for
reinforcing plastics in a number of automotive components [28].
2.1.2 Regenerated cellulose fibre
Polymers from renewable resources have gained attention due to their biodegradability and potential
for substituting petrochemicals in many applications. Cellulose, as a linear polysaccharide and the
most plentiful renewable polymeric material, has great properties and extensive applications.
Nonetheless, generally, processing of cellulose, natural polymer, is troublesome since it does not
melt or dissolve in common solvents because of its intramolecular or intermolecular hydrogen
bonds and crystalline structure. Furthermore, the traditional Viscose processes to produce cellulose-
regenerated materials result in critical environmental problems such as generating several
environmentally hazardous by-products, including CS2 and H2S. Thereby, additional facilities to
deal with the gaseous and aqueous waste emissions are required [32]. Accordingly, new
technologies have been investigated to avoid complex procedures and dangerous by-products [33].
Cellulose can be dissolved in solvents such as N-methylmorpholine-N-oxide (NMMO) [33],
dimethyl sulfoxide (DMSO)/paraformaldehyde (PF), N,N-dimethylacetamide (DMAc)/lithium
chloride (LiCl) and alkali solutions [34] as well as ionic liquids (ILs) [35]. Of these solvents, only
the use of NMMO in the process of commercialisation has resulted in technical success, namely, a
new man-made cellulose fibre: Lyocell [33, 36].
Lyocell is the first in a new generation of cellulosic fibres [37]. Lyocell is a 100% cellulosic fibre
made by a solvent spinning process. Demand for a process that is environmentally responsible and
utilises renewable resources as the raw materials is a major driving force for the development of
Lyocell fibre. Lyocell is derived from wood pulp, which is produced from sustainable managed
forests. The pulp is dissolved in NMMO, which is not classified as corrosive or toxic and is
biodegradable. Then the solution is spun to fibres and the solvent extracted as fibres pass through a
washing process [38].
After the fibre formation, the solvent is recovered from the fibre via multiple washing steps. Then it
is purified and returned to the concentration needed to dissolve cellulose, for reusing at the
beginning of the process. This is so called closed-loop technology. By paying careful attention at
each stage of recycling, the solvent recovery rates in excess of 99.5% can be achieved [38].
Lyocell is fully biodegradable. It has a relatively high strength, which facilitates its use in various
mechanical and chemical finishing treatments. The physical characteristics of Lyocell fibres aid in
its great blending characteristics with fibres such as silk, wool, linen and cashmere [37]. Lyocell
fibres are known to have good mechanical properties, wettability, high tenacity and good
drapability. Therefore, they have potential for applications such as reinforcements for composites
and can improve mechanical and physical properties [39].
LITERATURE REVIEW
7
2.2 Fibre surface modifications
The quality of the fibre–matrix interface is one of the most important factors, which control the
performance of the composite materials. In order to achieve optimum performance of the
biocomposites, an adequate degree of adhesion between the surface of hydrophilic cellulosic natural
fibres and the matrix is preferred. Since the interface determines the reinforcement efficiency of a
composite, strong bonding between matrix and fibre is necessary [22]. Because of the presence of
hydroxyl and other polar groups in several constituents of natural fibres, especially in the
amorphous region, the moisture uptake of bio-based composites is high, which causes weak
interfacial bonding between matrix and fibres, swelling and a plasticising effect, resulting in
dimensional instability and poor mechanical properties. Moreover, the presence of a natural waxy
substance on the natural fibre surface contributes to poor fibre-matrix bonding and ineffective
surface wetting.
Dewaxing, alkali treatment, isocyanate treatment, peroxide treatment, vinyl grafting,
cyanoethylation, bleaching, peroxide treatment, acetylation and treatment by coupling various
agents have achieved various levels of success in improving fibre–matrix adhesion in natural fibre
reinforced composites [40].
2.2.1 Alkali treatment
The optimum alkali treatment is an effective and low cost surface modification method for natural
fibres. The reaction of sodium hydroxide (NaOH) with natural fibre (Cell-OH) occurs as shown
below:
Cell − OH + NaOH → Cell − O−Na+ + H2O + surface impurities
During alkali treatment, a certain amount of lignin, impurities such as wax and natural oil covering
the external surface of the natural fibre cell wall may be removed; the cellulose structure will be
depolymerised and the short length crystallites will be exposed. A high alkali concentration may
depolymerise the cellulose, which can result in unfavourable effect on the fibre strength. Therefore,
the concentration of NaOH solution, treatment temperature and time play a vital role in obtaining
the optimum efficiency of the fibre.
Changes in performance of natural fibre composites as a result of alkali treatment have been
reported by many researchers [41-45]. The alkali treatment results in better fibre–matrix adhesion
because of the increase in the rough surface topography and fibrillar formation providing additional
sites of mechanical interlocking and more resin-fibre interpenetration at the surface. Furthermore,
this treatment leads to a decrease in the microfibril angle and an increase in degree of molecular
orientation. With removal of hemicellulose as a result of alkali treatment, the inter-fibrillar region
will be less dense and rigid, which makes the fibrils more capable of reorganising themselves along
the direction of tensile deformation [42].
The influence of alkali treatment on crystallisation of natural fibres has been studied by several
researchers [46, 47]. Alkali treatment results in better and closer packing of the cellulose chains or
rather increased crystallinity.
LITERATURE REVIEW
8
2.3 Biodegradable polymers
2.3.1 Polylactic acid (PLA)
There are plenty of thermoplastic polymers derived from renewable raw agricultural materials,
which are available to meet the ecological purpose of biodegradability. Polylactic acid, known as
PLA, is one of the fully biodegradable polymers that is commercially available and has received
particular attention for biocomposites manufacturing [48]. PLA is probably the most promising
matrix becuase its properties are close to the most widespread thermoplastic matrix of
polypropylene (PP) and polyethylene terephtalate (PET). PLA has many favourable markets due to
its biodegradability, making it one of the best candidates for substitution of PP and PET in some
applications. Despite the numerous advantages, however, there are considerable drawbacks which
still need to be overcome, such as its relatively poor impact properties arising from its inherent
brittleness, but also the moisture sensitivity, limited supply and higher cost of PLA, compared with
commodity polymers such as PET and PP [49]. PLA is commercially interesting because of its film
transparency, biocompatibility and good mechanical properties. Apart from several areas of
application in the packaging industry [1,2], currently PLA is applied as matrix in fibre reinforced
composites.
2.4 Biocomposites
Biocomposites are composite materials made from natural/bio fibre and petroleum based plastic or
renewable resource based plastic. Composites produced from synthetic fibres and biopolymers
could also be called biocomposites. The biocomposites derived by combining plant derived fibres
with bio-derived plastic are likely to be more eco-friendly [50, 51], thus, reducing the environmental
challenges. Being environmentally-friendly and sustainable are the major attractions for 100%
biocomposites. These biocomposites are suited for simply disposing or composting at the end of
their life without damaging the environment or can be recycled by energy recovery. These
composites are defined as composites that are designed with the lowest possible environmental
footprint [22].
Some of the biodegradable matrices (natural and synthetic) that can be applied for making
biocomposites are listed in Table 1 [52, 53].
LITERATURE REVIEW
9
Table 1 Biodegradable polymer matrices (Adapted and developed from (Stevens, 2002) [53])
Biodegradable polymers
Natural Polysaccharides: starch, cellulose, Chitin
Proteins: Collagene/gelatin, Casein, albumin, fibrogen, Silks
Polyesters: Polyhydroxyalkanoates
Other polymers: Lignin, Lipids, Shellac, Natural rubber
Synthetic
Poly(amides)
Poly(anhydrides)
Poly(amide-enamines)
Poly(vinyl alcohol)
Poly(vinyl acetate)
Polyesters: Poly(glycolic acid), Poly(lactic acid), Poly(caprolactone), Poly(orthoesters)
Poly(ethylene oxides)
Poly(phosphazenes)
2.4.1 Hybrid composites
One way of deliberately regulating the properties and performance of composite materials is to use
hybrid fibrous fillers (synthetic or natural fibre) into a single matrix. It will give these composite
materials specific properties, and allow for the expansion of the area of their application. Its
functionality depends on the balance between the properties of each individual component. The
properties of a hybrid composite depend directly on the fibre content, length of fibres, arrangement
and orientation of both the fibres and also fibre-matrix bonding [52]. Many interesting studies have
been conducted to explore different hybrid composites with different applications [54-56]. The
impact and tensile behaviour of epoxy resin reinforced by oil palm/glass fibre was investigated by
Bakar et al. (2005) [57]. Mishra et al. (2003) investigated the effect of the hybridisation of
sisal/glass and pineapple/glass-polyester composites on the moisture uptake characteristics of
composites. The authors observed that water absorption of the hybrid composites were less than that
of unhybridised composites [58]. In an interesting research, the hybrid biocomposites were made by
pineapple and sisal fibre reinforced thermosets and thermoplastics resins. The authors dealt with the
structure, composition and properties of hybrid biocomposites [51]. The mechanical performance of
short, randomly-oriented, sisal-banana hybrid fibre reinforced polyester composites was
investigated. A positive hybrid effect was observed in the flexural and tensile strength and modulus
of the hybrid composites [59]. The tensile properties of kenaf fibre/wood flour-polypropylene
hybrid composites were investigated by Mirbagheri et al. (2007) [60].
The possibility of improving the outdoor properties of NFCs needs to be investigated. Therefore,
studies on improving the mechanical properties and specifically reducing the water absorption in
NFCs need to be done. Making a composite system consisting of two different fibres with different
morphologies may be a good solution. The poor mechanical properties of bast fibre reinforced
composites can be improved by admixing cellulose fibres to bast fibres. Mieck et al. [61] studied the
effects of the admixture of Lyocell fibres with flax fibre reinforced polypropylene composites on
the mechanical properties. Further investigation was carried out by Graupner (2009). Composites of
the fibre mixtures of hemp/Lyocell were investigated [62].
LITERATURE REVIEW
10
2.4.2 Thermoplastic composites
Several fabrication methods have been developed for manufacturing thermoplastic biocomposites.
Film stacking [63, 64], injection moulding [14, 65] and compression moulding [14, 48, 66] are the
most widely applied production methods for fibre reinforced composite materials.
Many thermoplastic composites frequently suffer from inadequate fibre–matrix adhesion.
Furthermore, the use of thermoplastics introduces the problem of lack of adequate resin penetration
into the fibre tow. Due to the high viscosity of thermoplastic, it is very challenging to inject the
resin into a tightly woven textile structure and to fill its pores generated by the fibre interlacement.
This problem increases the porosity content in the composites and can be overcome by using high-
pressure injection pressure and heavier moulds. De-alignment of reinforcing fibres is another
problem caused by high matrix viscosity during consolidation. Reducing the melt flow distance
during the consolidation process can alleviate these problems. Mixing the fibres with the matrix
even before the preforming operation is a way to prepare composites with satisfactory matrix
dispersion within the fibre tows. There are several methods such as hot melt, solution, slurry,
emulsion, film, surface polymerisation, powder coating and commingling. Out of these techniques,
powder coating and commingling have the potential for producing prepregs with substantial
flexibility, being a critical necessity for textile processing [67].
Thermoplastic commingled/hybrid yarns are one of the possible intermediate products, so-called
“dry prepregs”, applied for continuous fibre reinforced thermoplastic composites. The commingled
yarns have emerged as a cost-effective method for manufacturing the different parts, especially
complex-shaped ones. The hybrid yarn has reinforcing and matrix-forming fibres combined together
in order to reduce the problems associated with high melt viscosity of thermoplastic matrices;
therefore, polymer flow distance for impregnation is reduced and the time needed for impregnation
or high-applied pressure are limited, leading to a reduction in production cost. Moreover, the yarns
have enough flexibility to go through textile preforming without getting damaged considerably, and
they can easily be converted into drapable woven fabrics being suitable for forming complex
shapes.
In this hybrid yarn, by applying heating the thermoplastic component melts and impregnates the
reinforcing component (fibre). After cooling, the system is transformed into a rigid composite [68].
To benefit from the mentioned advantages of hybrid yarn, the process parameters for yarn
production and consolidation should be optimised. Homogeneous distribution of reinforcing fibre
and polymer fibres or commingling quality is one of the most critical parameters since it governs
the consolidation quality, leads to a fast and complete impregnation of reinforcing fibres, and also
affects the amount of void content in the manufactured composites [9].
2.4.2.1 Types of hybrid yarns
Hybrid yarns can be prepared by different methods including co-wrapping, core spinning and
commingling.
LITERATURE REVIEW
11
In co-wrapping method, thermoplastic fibres are wrapped around a core of twistless reinforcing
fibres and thermoplastic filaments (Fig. 2), which brings individual filaments together and
consequently increases the structural integrity of the prepregs and also provides better protection for
the reinforcing fibres during further processing such as weaving or braiding [69]. However, the yarn
has poor impregnation due to inhomogeneous distribution of the reinforcing and matrix yarns and
requires higher pressure and processing temperature [70].
Fig. 2 Structure of wrap spun hybrid yarn
In core spinning (friction spinning), short staple fibres of thermoplastic matrix material are spun
around a core of high-performance continuous reinforcing fibres, as represented by Fig. 3. The
properties of these yarns are similar with those of the co-wrapped yarns. These yarns are very
flexible and make further processing easier [71].
Fig. 3 Structure of friction spun hybrid yarn
In commingling, the matrix and reinforcing fibres are mixed in a nozzle with the help of
compressed air. Fig. 4 shows the schematic of the structure of the commingled yarn. Among these
hybrid yarns, commingled yarn gives a good blending of matrix and reinforcing fibres; however,
this yarn tends to de-mingle under load and during preforming operations [72]. This process
provides soft, flexible and drapable yarn [73].
Fig. 4 Structure of commingled hybrid yarn
LITERATURE REVIEW
12
Apart from the above methods, thermoplastic hybrid yarns can be produced by the yarn
manufacturing processes such as rotor and ring spinning. However, the existing spinning techniques
cannot be applied to produce hybrid yarns without modifications at the process level [68].
The main application of natural fibres is today mainly in non-structural composites as they are
mostly available as randomly-oriented nonwovens [74-76]. The fibre orientation (i.e. alignment of
the fibres) must be controlled to ensure that the fibre mechanical properties are efficiently utilised in
order to attract industrial interest as an alternative to the traditionally applied synthetic fibres (e.g.
glass fibres). Previous studies have demonstrated that the full reinforcement potential of natural
fibres can be exploited in bio-composites if an aligned fibre orientation is used [17]. Natural fibres
are naturally discontinuous; therefore, natural fibre reinforcements reported so far are based on
twisted spun staple yarns, which are produced by spinning methods, mainly ring spinning. These
spun yarns tend to be highly twisted, which leads to fibre misalignment due to their helical paths
around the yarn axis. This misalignment contributes negatively to the mechanical properties of the
resultant composites. Another negative impact of yarn twist is that it tightens the yarn structure,
rendering resin impregnation difficult [77]. Therefore, in the textile industry, a broad range of
techniques for the alignment of natural fibres have been developed and optimised to produce yarns
with controlled fibre orientations by reducing or replacing twist in yarns. Goutianos et al. [77] tried
to produce flax yarns with the minimal level of twist for manufacturing aligned composites. Shah et
al. [78] used a sizing agent to substitute the use of twist in roving and yarn. Zhan et al. [17] studied
the effect of wrapped spun yarn with low twist for reinforcement purpose.
2.4.2.2 Textile processing of hybrid yarn
Textile production technology makes it possible to use hybrid yarns as the basis for obtaining an
intermediate prepreg, such as in the form of woven, weft knit or warp knit fabrics with the needed
properties. With subsequent processing under pressure and at elevated temperature, the melting
thermoplastic fibre component is converted into a polymer matrix, filling the mould and
impregnating the reinforcing fibres. This technology offers new possibilities for a shortened
production process for thermoplastic composites and articles made from them [79].
Fabric weaves are used in several applications in fibre reinforced polymers due to their good
mechanical properties, such as stiffness, strength and dimensional stability. With the weaving
technology, it is possible to fabricate high density woven structures with load-oriented fibre
positioning [80, 81]. The strength and stiffness of fabric reinforced composites are influenced not
only by the matrix and yarns properties, but also the structural parameters of materials such as
fabric count and weave pattern. The fabric count determines the number of weft and warp yarns per
cm, whereas the weave pattern specifies how the warp and the weft yarns are interlaced. Typical
weave patterns are plain, twill, basket and satin [82]. Fabrics that are produced by weaving a large
number of thick yarns such as warp and a few numbers of thin yarns as weft are called
unidirectional and are used in unidirectional composites. This weave type provides materials with
good processability, high stiffness and strength in the warp direction, which is specific for
unidirectional composites [83].
LITERATURE REVIEW
13
The structure and properties of the hybrid yarn in textile preforms has significant effect on preform
properties such as drape, flexibility, permeability and thickness. Ultimately, these properties are
important when making the composites from textile preforms.
2.4.3 Engineering applications of biocomposites
Over the last five decades, considerable research has been carried out in the field of bio-based
plastics and biocomposites, which demonstrates their significance. Nevertheless, part of product life
cycle is research and development. The actual engineering process begins when the developed
science is applied to a specific application. Bio-based composites offer great opportunities for an
increasing role as alternate material. The use of biocomposites has extended to almost all fields
including packaging, biomedical, building, civil, construction and automotive industry, etc. [84].
Some of the building and construction applications where bio-based composites are potentially used
include window frames, framing, fencing, walls, doors, flooring and ceiling panels. Roof structures
form biocomposites were manufactured from soy oil-based resin and cellulose fibres in the paper
sheets form, being made from recycled cardboard boxes [85]. Pulp from residual sisal and coir
fibres and eucalyptus waste has also been used as a replacement for asbestos in roofing components
[86]. The use of bamboo fibre as reinforcement in structural concrete elements has been studied
[87]. Building temporary housing can be another potential application of biocomposites. Usually,
temporary housings from wood plastic composites are set-up whenever a catastrophe occurs. When
the situation recovers to normalcy, the temporary housings are disassembled and the waste is put in
landfills. Thereby, to reduce this type of landfill waste, such applications necessitate applying the
biocomposites and more definitely green composites, which can possibly be composted or recycled
by energy recovery after their service life [84]. In the automotive industry, polyester matrix
reinforced by cotton fibres was used in the body of the East German ‘‘Trabant’’ car [88]. Another
example is the use of flax fibres in car disk brakes as a replacement for asbestos fibres. Mercedes
used jute-base reinforced door panels for their vehicle production in 1996. The bast fibres are
mainly used in automotive industry since they exhibit greatest strength and weight savings of
between 10% and 30% and corresponding cost savings. Many recent studies have shown that
thermoplastics reinforced by hemp fibre mat are promising candidates in automotive industries
where high specific stiffness is required [89]. Almost all the major car companies (Audi Group,
BMW, Mercedes, Volkswagen, Ford and Opel) now use bio-based composites in several
applications such as interior trim components like dashboards and door panels. For example, in
2000, Audi launched the A2 midrange car in which door trim panels were manufactured from
flax/sisal mixture mat reinforcing polyurethane.
LITERATURE REVIEW
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EXPERIMENTAL
15
3. EXPERIMENTAL
3.1 Materials
3.1.1 Reinforcements
Hemp in the form of baled loose staple fibres (genus species Cannabis Sativa L) was supplied by
Hempage AG (Adelsdorf, Germany). According to the manufacturer’s information, the average
diameter of the hemp fibre was 20–40 µm, and the mean fibre length was 30 mm (Publications I, II,
III, IV and V).
The Lyocell staple fibres were supplied by Lenzing AG (Lenzing, Austria). The average length and
diameter of the fibres were 38 mm and 13.4 µm, respectively (Publications IV and V).
3.1.2 Matrices
Two different types of PLA were used as the matrix in the hybrid reinforcements:
A PLA staple fibre, provided by Trevira GmbH (Hattersheim, Germany), had a fineness of 1.7 dtex
and a mean fibre length of 38 mm. Based on the manufacturer’s information, the PLA fibres were
made from Nature Works®
PLA 6202D from, Cargill Dow LLC (Minnetonka, USA). This
thermoplastic polymer has a density of 1.24 g/cm3, a melt temperature of 160–170 °C, and a glass
transition temperature of 60–65 °C (Publications II, III, IV and V).
An air-textured PLA continuous yarn with 72 filaments was provided by Torcitura Lei-Tsu S.r.l
(Bergamo, Italy). According to the supplier, the yarn was made from the PLA grade 6201D from
Nature Works®
, Cargill Dow LLC (Minnetonka, USA). This thermoplastic has a glass transition
temperature of 55–60 °C (Publications I, II, III and V).
3.1.3 Chemicals
Reagent grade NaOH was obtained from Scharlau and Sigma Aldrich and used for fibre alkali
treatment (Publication II). Absolute ethanol (> 99.8%) was supplied by Sigma Aldrich (Publications
II, III, IV and V) and used for porosity measurement test.
3.2 Methods
3.2.1 Fibre alkali treatment
The hemp fibres were first treated by 4 wt% sodium hydroxide (NaOH) solution for 1 h, then rinsed
with distilled water until it was neutral and finally dried at room temperature for 48 h (Publication
II).
3.2.2 Manufacture of hybrid yarns
3.2.2.1 Co-wrapped hybrid yarn
The PLA/hemp hybrid yarns were produced by using a laboratory yarn twist machine from
DirecTwist, AGTEKS Co. Ltd. (Istanbul, Turkey). In the co-wrapping, the PLA yarn filaments were
wrapped around low twist and very fine hemp yarns. These PLA filaments will melt during the
consolidation process and will be incorporated in the matrix. Thus, they need to be very fine yarns
so that the interfacial adhesion between the reinforcing hemp yarns and the PLA matrix is not
EXPERIMENTAL
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affected to any significant extent. However, inhomogeneous distribution of the reinforcing hemp
and the PLA matrix yarns may lead to poor impregnation in the consolidated composite; therefore,
it was decided to put PLA filaments in the core part of the hybrid yarn together with the hemp yarn.
The core yarn was assembled by putting aligned hemp yarns and PLA filaments into a bundle
without increasing the hemp yarn twist. The yarn structure obtained from the co-wrap spinning used
is shown in Fig. 5 (a, b and c). The wrap density, or number of wraps per unit length (1 m), ranged
from 150 (Fig. 5 b) to 250 (Fig. 5 c) in order to study the effect of wrap density on the properties of
the composite. The name of the hybrid yarn indicates the composition. For example, in the hybrid
yarn PLA/45 Hemp-150, one PLA filament is wrapped 150 times around 1 m of the core part
(composed of four hemp yarns and two PLA filaments), thus, giving a nominal ratio of 45 mass%
hemp fibre content in the hybrid yarn. The linear densities of the hybrid yarns were determined by
weighing a 10 m long yarn sample that was dried in a vacuum chamber (0.9 mbar; 20 °C) for at least
18 h, and calculating the tex value.
Fig. 5. a) Structure of co-wrapped hybrid yarn, b) yarn with 150 wrapping density and c) yarn with 250 wrapping
density (Publication I)
3.2.2.2 Wrap spinning of hybrid yarns
Previous studies have demonstrated that the full reinforcement potential of reinforcing fibres can be
exploited in composites if an aligned fibre orientation is applied [16, 17]. Therefore, the yarn
structure, which has the reinforcing fibres straight and parallel to the yarn axis, is preferred. Wrap
spinning can be used to produce such a yarn [17]. PLA/hemp hybrid wrap spun yarns were
produced by using a laboratory spinning machine from Mesdan S.p.A. (Brescia, Italy) and a
laboratory yarn twist machine from DirecTwist, AGTEKS Co., Ltd. (Istanbul, Turkey). The hemp
and PLA fibres arrived at our laboratory in baled loose fibre form. The PLA fibre and the hemp
fibre were weighed to the desired proportion (30 mass%), and the fibre mixture was then fed into
the carding machine. The blended PLA/hemp web was carded three times to parallelise the fibres
a)
b)
c)
EXPERIMENTAL
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and achieve sliver uniformity. Then the sliver was fed through a roving frame, where the strands of
fibre were further elongated. The sliver was drawn twice after carding to achieve the required
roving linear density. Although it was possible to create hybrid yarns with low twist, the cohesion of
the fibres was very low because PLA/hemp roving had a false twist, which means that they could
not form a roving of sufficient integrity. Moreover, the roving is too weak to be able to be collected
alone in the roving machine. In order to collect the roving without causing breakage in the roving
machine, the processable PLA filaments were used as a processing carrier for the PLA/hemp roving
in the final step. After the spinning, the roving was wrapped by PLA filaments in the twisting
machine. The wrap yarn was spun to the nominal count of 550 tex, and it had a wrapping intensity
of 200 turns/m. These wrappings provide better protection for the reinforcing fibres during further
processing, such as weaving [68] or making a prepreg. The yarn structure obtained from the wrap
spinning used is shown in Fig. 6.
Fig. 6. Structure of wrap spun hybrid yarn (Publications II, III and V)
3.2.3 Preparation of prepregs
3.2.3.1 Nonwoven fabric manufacturing process
The fibres in their loose form were manually mixed and fed to a carding machine from Mesdan
S.P.A (Brescian, Italy). The blended fibre web was carded three times to parallelise the fibres and to
achieve mat uniformity (Publication IV).
3.2.3.2 Bidirectional hybrid yarn prepreg
Before compression moulding, multilayer 0/90 bidirectional hybrid yarn prepregs were prepared by
winding the hybrid yarn around a 19×19-cm steel rectangular frame (Fig. 7). A 0/90 bidirectional
layup of the prepreg was chosen, due to the ease of the winding technique. Each prepreg had 12
layers of wound yarns, to achieve a composite thickness of about 1–3 mm (Publication I).
EXPERIMENTAL
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Fig. 7 Prepreg frame with the PLA/hemp hybrid yarns in a 0/90 configuration (Publication I)
3.2.3.3 Unidirectional woven fabrics from hybrid yarns
In order to manufacture samples of thermoplastic composites, we made woven unidirectional (UD)
fabrics from hybrid yarns and PLA filaments on a handloom weaving machine. The obtained woven
fabrics had the PLA/hemp hybrid yarn in the transverse direction of manufacture of the fabric (weft
direction) and were supported by the 18 tex PLA filaments as the warp yarns in the longitudinal
direction (8 threads per cm). The fabrics were woven in two different weave patterns: 8-harness
satin (4 threads per cm) and basket weave (3 threads per cm) as seen in Fig. 8 a and b. These fabrics
have similar characteristics as unidirectional fabric in the weft direction, as the thin warp yarn,
reduces the crimp considerably. The woven fabrics obtained can be considered as prepregs for
subsequent processing to make composites by compression moulding.
Fig. 8. Schematic diagram of a) 8-harness satin and b) basket weave patterns (Publications III and V)
a)
b)
EXPERIMENTAL
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3.2.4 Composite preparation
The prepreg mats were first put in a vacuum chamber (0.9 mbar; 70 °C) for at least 18 h, to dry
before the compression moulding. The prepreg was covered with a Teflon sheet to prevent sticking
of the matrix to the surface of the mould before being placed into a pre-heated steel mould with a
20×20-cm square cavity, and 10 mm depth. The steel mould was of our own design and machined
by a local machine shop. The composites were then prepared by pressing the prepreg at a
temperature of 175 °C and a pressure of 1.04 MPa for 15 min (Publication I), 195
°C and a pressure
of 1.7 MPa for 15 min (Publications II, III and V) and 195 °C and a pressure of 1.7 MPa for 20 min
(Publication IV) in a hydraulic compression moulding machine from Rondol Technology Ltd.
(Staffordshire, UK). Neat PLA sheets to be used as reference material were made by compression
moulding under the same processing conditions of the same PLA filaments formed to a similar
prepreg.
3.3 Characterisation
Before performing the testing, the specimens were conditioned for at least 24 hours at 23 °C and
50% relative humidity, according to DIN EN ISO. Specimens for different tests were cut by GCC
LaserPro Spirit laser cutting machine, according to the ISO test standards.
3.3.1 Single fibre tensile test
All staple fibres were tested on a Favigraph single fibre tensile tester from Textechno GmbH
(Mönchengladbach, Germany) equipped with a 20 cN load cell. The test speed was 20 mm/min, and
the gauge length was 20 mm and 20 replicated tests were done for each fibre type. The single fibres
were treated in a hot press at 190 °C and 1.7 MPa for 15 min in order to simulate the effect of
composite processing conditions (presented in Section 3.2.4) on the Lyocell and hemp
characteristics and then tested for tensile properties.
3.3.2 Composite density and porosity
Composite density was evaluated in order to assess the volumetric composition. Composite
densities were determined by the buoyancy method (Archimedes’ principle) using ethanol as the
displacement medium. To avoid absorption of liquid during immersion, the specimens were covered
by a varnish containing paraffin. Based on the data obtained regarding densities and mass fractions,
the volume fractions of fibre, matrix and porosity were calculated by using the proposed method by
Madsen et al. [5].
3.3.3 Water absorption test
Water absorption analysis was done on the composite specimens according to ASTM D570-98. The
specimens were dried in an oven for 24 h at 60°C, and then cooled to room temperature in a
desiccator. The initial weight of these specimens was denoted as W0. The specimens were then
immersed in two different baths: one with distilled water at room temperature and one with distilled
water bath at 80°C. The amount of water absorbed was measured every 24 hours for 10 days. At
each measurement, the specimen was taken out of the water and the surface wiped dry before the
EXPERIMENTAL
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weight was recorded as W. For each composite, five replicate samples were measured. The
percentage of apparent weight gain (WG) was then calculated using the following equation.
WG = [(W-Wo)/Wo] × 100 % (1)
3.3.4 Tensile testing
The tensile testing was performed according to the ISO 527 standard test method, using a universal
H10KT testing machine equipped with a model 100R mechanical extensometer, both supplied by
Tinius Olsen Ltd. (Salford, UK). The loading rate was 10 mm/min and the load range was 1 kN. Six
specimens were analysed for each composite laminate. The specimens were cut in 20 mm wide
dumbbell shape with an overall length of 150 mm and a 60 mm length of the narrow, 10 mm wide
parallel-sided portion. The gauge length was 50 mm.
3.3.5 Flexural testing
The flexural testing was performed according to ISO 14125, with the same testing machine as for
tensile testing. At least five specimens were tested for every composite composition. The loading
rate was 10 mm/min and the load range was 1 kN. The sample dimensions were: length 60 mm,
width 15 mm, while the thickness varied depending on the sample.
3.3.6 Impact testing
Charpy impact testing was done on un-notched specimens in accordance with ISO 179 using a
pendulum type Zwick test instrument from Zwick GmbH (Ulm, Germany). A total of 10 specimens
were tested edgewise to determine the mean impact resistance.
3.3.7 Dynamic mechanical thermal analysis
DMTA was done using a Q-series TA instrument supplied by Waters LLC (Newcastle, DE, USA).
The analysis was run in the dual cantilever bending mode and the sample dimensions were: length
50 mm, width 8 mm, while the thickness varied depending on the sample. The temperature interval
was 15–150 °C with a heating rate of 3 °C /min using a frequency of 1 Hz and amplitude of 15 lm.
3.3.8 Differential scanning calorimetry
DSC analysis was done on a DSC Q2000 supplied by TA Instruments (New Castle, DE, USA).
Samples of 8–10 mg were heated in a nitrogen-purge stream at a rate of 10 °C/min from 20 to 200
°C, cooled to 0 °C, and then heated again from 20 to 200°C. The data from the first scan were used
in order to see the effect of processing conditions on the thermal properties. For each PLA sample,
three replicates were scanned. The final results were the averages of three DSC runs. The
percentage crystallinity (XDSC) of PLA was calculated using the following equation [90]:
XDSC %= ∆𝐻𝑓−∆𝐻𝑐𝑐
∆𝐻𝑓𝑜 ×
100
𝑤 (2)
Where, ∆Hfo = 93 J/g for 100 % crystalline PLA, ∆Hf is the enthalpy of melting, ∆Hcc is the cold
crystallisation enthalpy and w is the weight fraction of PLA in the composite.
EXPERIMENTAL
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3.3.9 Scanning electron microscopy (SEM)
The fibre surface and the composite fracture surface obtained from the tensile testing were studied
using low-vacuum scanning electron microscopy with a Quanta 200 ESEM FEG instrument from
FEI, (Oregon, USA) with an operating voltage of 5–15 kV. For low vacuum imaging, no specific
preparation was required.
EXPERIMENTAL
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SUMMARY OF RESULTS
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4. SUMMARY OF RESULTS
4.1 Effect of heat and alkali treatments on fibre properties (Publications II and IV)
In order to evaluate the effect of moulding conditions, the mechanical properties of heat and alkali
treated single hemp fibres were determined, see Table 2.
The alkali treatment improves fibre tensile strength and modulus, which helps to improve the
properties of the composites [91, 92]. This treatment possibly orients fibrils along the direction of
tensile forces by removing hemicelluloses from fibre (Fig. 9), resulting in better load sharing
between the fibrils [93].
Heat treatment greatly reduces mechanical properties. Actually, cellulosic fibres are mixtures of
organic materials, and heat treatment at elevated temperatures can cause physical and chemical
changes. The physical changes are associated with enthalpy, weight, colour, strength, crystallinity,
and orientation of microfibril angle [94]. The chemical changes are related to the decomposition of
several chemical elements. Heat treatment results in the weight loss of moisture plus weight loss
due to thermal degradation. The thermal degradation of cellulosic fibres results in change in colour
and deterioration in mechanical properties of the fibres [94-96].
Table 2 Tenacity, modulus, elongation and linear density for hemp and Lyocell fibres before and after heat and alkali
treatments (Publications II and IV).
Treatment Fibre Tenacity
cN/dtex
E-Modulus
cN/dtex
Elongation
%
Linear
density
dtex
As received Hemp 4.66 (1.43) 97.71 (24.92) 3.93 (0.66) 4.15
Lyocell 3.27 (0.63) 55.87 (23.06) 8.70 (2.01) 1.36
After hot pressing Hemp 3.75 (1.10) 84.62 (26.60) 3.37 (0.88) 4.27
Lyocell 2.12 (0.92) 48.46 (20.29) 4.75 (1.84) 1.38
After alkali treatment Hemp 5.18 (1.30) 99.15 (32.64) 4.05 (0.86) 3.75
SUMMARY OF RESULTS
24
Fig. 9 SEM micrographs of the surface morphology of: (a) untreated and (b) alkali treated hemp fibres (Publication II)
4.2 Composites properties (Publications I, II, III, IV and V)
4.2.1 Composite porosity
Porosity is one of the most critical issues during the manufacturing process of composites. The
presence of porosities can significantly reduce the tensile, compressive, inter laminar shear and
structural strengths of a composite [97].
In Publication I, co-wrapped hybrid yarns were produced from hemp and PLA yarns in four
nominal mass ratios: 10/90, 20/80, 35/65 and 45/55 with two wrapping densities: 150 and 250
wraps/metre. In total, eight composites were manufactured with fibre volume fractions in the range
8–38 vol.% and with porosity in the range 6–9 vol.%. The physical composition of the composites
made are summarised in Table 3. From the results, it is possible to conclude that the higher the fibre
volume fraction, the higher the amount of porosity, which is obviously due to the physical structure
of the prepreg. Similar trends have also been seen in other studies [5, 98]. Furthermore, it appears
that the use of lower wrapping density causes a slight increase in the porosity. In the prepared
composite, the porosity fraction was still high even though the fibre volume fraction was low, which
could be due to several factors. Porosity is difficult to avoid in natural fibre composites and
influences on the composite properties. Thus, research on how to decrease the amount of porosity is
warranted.
In Publication II, we describe the developed natural fibre hybrid yarns with low twist, for use in
high performance composites with lower amount of porosity. The composites were produced from
PLA/hemp yarn prepregs with three off-axis fibre orientations (0°, 45° and 90°) and PLA/alkali
treated hemp yarn prepregs in a nominal mass ratio of 30 mass% reinforcement. Because the fibre
angle does not have any effect on the composite composition, only the composite of 0° off-axis
fibre was tested. The formation of a stronger interface by improved hemp fibre and PLA adhesion
could be the result of the reduced porosity of the composite from alkali treated hemp composites,
compared with the untreated hemp/PLA composites. This can be explained by the fact that alkali
treatment increases the available hydroxyl groups [91] due to removal of the noncellulosic materials
a) b)
SUMMARY OF RESULTS
25
covering the cellulose hydroxyl groups and cleans the surface impurities that lead to the formation
of the rough fibre surface, which would largely increase the surface area of the fibre [99].
In Publication III, composites were produced from satin PLA/hemp hybrid fabrics and basket
PLA/hemp hybrid fabrics in a nominal mass ratio of 30 mass% hemp reinforcement in order to
investigate the effect of different weave patterns on the properties of the composites. From the
results, it is possible to conclude that the composite made by unidirectional satin fabric gave
significantly lower porosities, compared to the winded hybrid yarn laminates (composite described
in Publication II). The porosity content for the composite made from satin fabric (0.96 %) was
clearly lower, compared to the composite made from basket fabric (4.55 %). It can probably be
attributed to the fact that satin weaves allow individual fibres to be woven in the closest proximity
and can therefore produce fabrics with a close ‘tight’ weave, compared to basket weave. Moreover,
basket weave has higher quantity of interlace points per unit area, compared to satin. Perhaps voids
might be present between the interlace points that are higher in basket systems than those of satin
[100].
In Publications IV and V, are composites produced from unidirectional satin hemp/PLA,
Lyocell/PLA and Lyocell-hemp/PLA fabric and the nonwovens discussed. All the composites had a
nominal mass ratio of 30 mass% reinforcement in order to investigate the effect of mixing different
fibres and their alignment on the composites properties. From the results, it is clear that the
composite made by unidirectional satin fabric gave significantly lower porosities, compared to the
nonwoven laminates [101, 102]. The tortuosity of the resin flow path is an important factor in the
process of void movements and their elimination. The nonwoven mat fibre has a more tortuous flow
path, compared to the composite made from hybrid yarn prepregs;, hence, it is more difficult for the
entrapped air to flow out of the system. Moreover, it can be observed that the Lyocell-based
composite had a lower void content, which is obviously due to the higher interfacial adhesion.
4.2.2 Water absorption
The apparent weight gain (WG) as a function of time at different temperatures for the manufactured
composites are shown in Fig. 10 a, b and c. It can be observed that the fibre-based composites had
significantly higher water absorption than neat PLA due to the hydrophilic nature of hemp and
Lyocell with polar groups such as hydroxyl and carboxyl groups on the fibre surfaces. Among all
composites, the composite from PLA/hemp nonwoven showed the highest water absorptions. It can
probably be attributed to the fact that it has less close and compact packing of hemp fibres in the
PLA matrix, compared to the composite made from PLA/hemp yarn. The compact and close
packing of PLA/hemp yarns reduces the porosity inside the composites, which in turn contributes to
the reduction in water absorption [103]. It is evident that the effect of weave pattern is significant
for the water absorption for the composites. Although the composite made from the winded hybrid
yarn prepreg has larger moisture absorption than that of the pure PLA, it exhibits lower moisture
absorption than the composites with woven fabrics (satin and basket). Actually, the matrix pockets
and yarn undulation within the weaves provide an easier diffusion path for the moisture to travel
within the weave, compared to a composite composed of unidirectional yarns (Publications II, III,
IV and V).
SUMMARY OF RESULTS
26
Table 3. Composition of the fabricated composites calculated from the density measurements.
Sample Density in
g/cm3
Fibre mass
fraction in %
Fibre
volume
fraction
in %
Matrix
volume
fraction in
%
Porosity
volume
fraction in
%
PLA 1.24 0.0 0.0 100.0 -
PLA/10 Hemp-150 1.192 10.1 9.1 85.3 5.60
PLA/10 Hemp-250 1.193 11.3 8.1 86.5 5.30
PLA/20 Hemp-150 1.196 22.3 18.8 74.0 7.20
PLA/20 Hemp-250 1.197 23.3 18.0 75.0 6.90
PLA/35 Hemp-150 1.210 34.9 29.0 62.9 8.00
PLA/35 Hemp-250 1.219 35.5 28.7 64.0 7.30
PLA/45 Hemp-150 1.224 45.5 38.1 53.2 8.70
PLA/45 Hemp-250 1.226 46.1 37.7 53.9 8.40
Alkali hemp/PLA yarn
(off-axis angle 0°)
1.294 30.0 26.23 72.53 1.23
Hemp/PLA yarn
(off-axis angle 0°)
1.271 30.0 25.77 71.25 2.97
Satin hemp/PLA fabric 1.298 30.0 26.31 72.74 0.96
Basket hemp/PLA fabric 1.251 30.0 25.35 70.10. 4.55
Satin Lyocell/PLA fabric 1.273 27.02 71.32 1.66
Satin Lyocell-hemp/PLA fabric 1.296 15.0 hemp
15.0 Lyocell
26.27 72.63 1.09
Nonwoven PLA/hemp 1.0914 30.0 23.0 61.2 10.8
Nonwoven PLA/Lyocell 1.2765 30.0 25.2 71.5 3.3
Nonwoven PLA/hemp-Lyocell 1.1222 15.0 hemp
15.0 Lyocell
22.4 76.4 2.1
Among all composites in this study, composites made from satin PLA/Lyocell fabric showed the
lowest water absorptions, which may be due to their better interfacial adhesion that decreases the
thickness of the interphase area between the fibres and the matrix, thus, decreasing the water
absorption through the interphase and further into inner parts of the structure [104, 105].
As a result of alkali treatment, the interfacial adhesion in PLA/hemp fibres improved, which is
necessary for the reduction of interfacial wicking of the water molecules. Thus, the moisture
absorption of the composites can be reduced by chemical treatments [103, 105, 106] (Publication
II).
Furthermore, as shown in Fig. 10 c, it can be seen that for all the composites investigated, WG
increases monotonically by time, initially before reaching a maximum. It can be observed that the
water immersion temperature does have an influence on the water absorption curves. Increasing the
immersion temperature from room temperature to 80 ˚C led to increased water absorption of the
neat PLA and composites as well as decreased the saturation time. The weight gain was found to
decrease after passing through a maximum. For the composites, this could be due to the neat PLA
layer peeling on the surface as a result of biodegradation and dissolving with time, together with the
removal of some substances from the hemp fibre surface during the immersion [107, 108]
(Publications II and IV).
SUMMARY OF RESULTS
27
a)
b)
SUMMARY OF RESULTS
28
Fig. 10 Apparent weight gain against time for the manufactured composites at: a) room temperature, and b) 80 ˚C
4.2.3 Tensile properties
An overview of the tensile strength, modulus and elongation at break of the composites, compared
to the values of the neat PLA matrix are given in Fig. 11 a and b and Table 4. It can be seen that an
increase in fibre content improves the tensile strength and modulus as expected. It is also obvious
from the results that composites made from the hybrid yarn with a wrapping density of 250 showed
a consistent improvement in tensile modulus over the composites with 150 wrapping density, across
the whole range of fibre fraction ratios investigated. Increasing wrapping density improved the
tensile modulus of the composites because the tortuosity of the hybrid yarns decreases as wrap twist
increases, and consequently the reinforcement alignment would be improved [109] (Publication I).
As can be seen in Table 4, the composites fabricated from PLA/hemp yarns (both untreated and
treated) had considerably higher tensile strength and modulus in the principal fibre direction (i.e.
longitudinal direction) than in the corresponding nonwoven mats. As expected, the composite
strength and stiffness decreased with increasing fibre orientation angle. The composite made from
the nonwoven demonstrated higher tensile properties in the principal fibre direction, compared with
hemp/PLA yarn composite with fibre orientation angles 45˚ and 90˚. Furthermore, the results
showed that tensile strength of composites made with fibre orientation angles 45˚ and 90˚ were
lower than pure PLA. The highest strength values were reached by treated hemp/PLA yarn with
fibre orientation angle 0˚. The decrease in elongation at break in the case of the composites is
principally due to the structural integrity of the matrix (PLA), which is destroyed by the loading of
the natural fibre and leads to faster fracture than pure matrix [110] (Publication II).
The composites from the woven fabric show better tensile properties, compared to the winded yarn
laminate composite. In fact, tightly woven fabrics are usually the right choice to maintain fibre
orientation during the fabrication process, which was not possible in the winded yarn laminate. The
c)
SUMMARY OF RESULTS
29
tensile modulus and strength of composite for basket 2/2 is lower, compared to satin due to its fabric
structure. The basket weave has a higher crimp, compared to the satin weave due to its more
interlacement (Publication III).
Specimens made from nonwoven reinforcement show lower values than specimens made from
fabric. The strength values of hemp-PLA, Lyocell-PLA and Lyocell-hemp PLA specimens differ
noticeably. The improvement of the tensile properties of hemp-reinforced PLA composites by
addition of Lyocell fibres could be attributed to the much higher fineness of the Lyocell fibres,
which cause a larger specific bonding surface and a better matrix–fibre adhesion [99] (Publications
IV and V).
Fig. 11. Tensile properties of PLA/hemp composites: a) tensile modulus, and b) tensile strength (Publication I)
SUMMARY OF RESULTS
30
Table 4 Tensile properties of PLA-based composites (Publications II, III, IV and V)
Sample Tensile
strength
(MPa)
Young’s
modulus
(GPa)
Elongation at
break (%)
Publication
PLA 41.21 (2.25) 2.91 (0.39) 1.92 (0.26) II, III, IV and
V
Hemp/PLA yarn (off-axis angle 0°) 72.75 (6.26) 8.77 (1.44) 1.24 (0.36) II
II
II
Hemp/PLA yarn (off-axis angle 45°) 34.75 (4.63) 4.62 (1.04) 0.66 (0.18)
Hemp/PLA yarn (off-axis angle 90°) 22.01 (3.38) 3.70 (0.65) 0.53 (0.11)
Satin hemp/PLA fabric 88.06 (7.70) 10.23 (2.67) 1.35 (0.30) III
III Basket hemp/PLA fabric 81.09 (5.62) 9.20 (1.81) 1.58 (0.22)
Alkali hemp/PLA yarn (off-axis angle 0°) 77.08 (2.60) 10.27 (1.36) 1.58 (0.26) II
Hemp/PLA nonwoven 53.63 (1.22) 5.60 (0.95) 1.10 (0.30) II
Hemp/Lyocell/PLA nonwoven 60.63 (8.36) 6.31 (0.51) - IV
IV Lyocell/PLA nonwoven 80.87 (2.94) 6.86 (0.40) -
Satin hemp/Lyocell/PLA fabric 96.01 (2.05) 11.15 (0.61) - V
V Satin Lyocell/PLA fabric 101.23 (1.27) 11.42 (0.84) -
Note: Data in table are mean (SD).
4.2.4 Flexural properties
The results of the flexural test are shown in Fig. 12 (a and b) and Fig. 13 (a and b). The composite
made from yarn with 250 wrapping density gave slightly higher values than the composite made
from yarn with 150 wrapping density, which is probably due to the lower porosity and more aligned
fibres (Fig. 12) (Publication I).
From Fig. 13 a and b, it can be seen that the composites made from PLA/hemp yarn (0˚ direction)
showed an evident improvement in flexural modulus over the composites produced from PLA/hemp
nonwoven. The flexural properties of unidirectional composites followed the reduction trend in their
value when the fibre orientation changed from 0˚ to the off-axial direction. Alkali treatments
enhanced the flexural strength and modulus of the composites, which can be attributed to the greater
interfacial bonding between treated hemp fibres and PLA [111-113] (Publication II).
Different weave pattern has significant influence on the flexural strength of the composites. Satin
fabric composite exhibited the highest flexural strength due to the presence of more aligned fibres in
the axial direction and lower number of interlacing points, which has less chance for stress
concentration [114] (Publication III).
From the results, it can be seen that the highest values were achieved by the composite reinforced
SUMMARY OF RESULTS
31
by satin Lyocell/PLA fabric. Fibre reinforcement resulted in a clear and significant increase of the
modulus values, compared to pure PLA. Between satin hemp-Lyocell/PLA and satin hemp/PLA
composites, no significant differences in modulus were determined, but their strength values differ
clearly (Publications IV and V).
Fig. 12. Flexural properties of PLA/hemp composites
a) b)
a)
SUMMARY OF RESULTS
32
Fig. 13 Flexural properties of PLA-based composites
4.2.5 Impact resistance
The results of the Charpy impact test are shown in Fig. 14 (a, b and c). It is apparent from Fig. 14
that the impact strength of the PLA/hemp composites increases with increasing mass proportion of
fibre. However, the value at 10 mass% fibre content is still lower than the value for neat PLA.
When longer fibres are introduced into the PLA matrix, the impact strength greatly increases due to
the frequent occurrence of long fibre pull-outs [113]. In our work, the higher impact strength of
composites, compared to neat PLA can be attributed to the fact that the reinforcing staple fibres are
in the form of yarn and can act as long fibres. The composites made from yarn with wrapping
density of 250 showed relatively lower impact strengths, compared to the composite from yarn with
wrapping density of 150 (Publication I).
A significant reinforcement effect was determined for the treated PLA/hemp yarn composite. In this
case, debonding occurred, followed by fibre breakage rather than by interfacial debonding, usually
associated with high energy absorption [115, 116]. The increased PLA crystallinity of the alkali
treated hemp fibre composites, compared with untreated hemp fibre composites could be another
factor leading to increased impact strengths [117, 118]. The value of the PLA/hemp nonwoven
composites was lower than that of the neat PLA matrix. Further increments of fibre angle caused a
decrease in impact strength of the composites, which is likely to be due to decreased dissipation of
energy by less fibre pull-out. The impact strength of the nonwoven composites was higher than that
of the yarn composites with 45˚ and 90˚ off-axis (Publication II).
It can be seen from the results (Fig. 14 b) that the variation in impact strength of the composites was
not significantly influenced by the weaving architecture, as the variation is meagre. The higher
impact energies for fabric laminate, compared with winded yarn laminate are mainly because of the
presence of a high number of axially oriented fibres [119] (Publication III).
b)
SUMMARY OF RESULTS
33
The improvement of the impact strength of hemp fibre-reinforced PLA was caused by the addition
of 50% Lyocell to the hemp fibres (Fig. 14 c). In fact, the higher impact energies for satin fabric,
compared with nonwoven laminate are mainly because of the presence of a greater number of
axially oriented fibres, since woven fabric composites provide more balanced properties in the
fabric plane than randomly oriented composites [32]. The significant increase in impact strength of
Lyocell/PLA composites can be referenced to higher elongation at break [120, 121] (Publications
IV and V).
a)
b)
SUMMARY OF RESULTS
34
Fig. 14 Impact strength of the manufactured composites
4.2.6 Dynamic mechanical thermal testing
Fig. 15 shows how fibre treatment and orientation influenced the storage modulus and damping
factor (tan δ) of PLA and its respective composites. It is evident from Fig. 15 a and b that
incorporation of hemp fibres results in an increase in the storage modulus of the biocomposites,
which reveals effective stress transfer from the fibre to the matrix at the interface and good adhesion
between them. The storage modulus of the composites showed remarkable dependence on fibre
orientation. The storage modulus was highest for the hemp/PLA (0) yarn composite, followed by
the hemp/PLA (45) yarn composite and the hemp/PLA (90) yarn composite. Composite from satin
and basket weaves had a better storage modulus, compared to the winded yarn laminate composite,
which is in line with the results from tensile, flexural and impact tests showing that composites from
satin and basket weave are superior to winded yarn laminate composite.
Fig. 15 b and c show the tan δ value of the composites as the function of temperature and the glass
transition temperature corresponding to the maximum tan δ peak. The glass transition temperatures
of composites are higher, compared to pure PLA. This indicates that with the presence of hemp
fibres, the polymer relaxation is delayed and segmental motion of the PLA polymer chains is
restricted due to increased crystallinity.
The area under this peak for PLA composites seems to be smaller especially for alkali treated hemp
fibre composite. A possible explanation is that there is a strong interaction between the fibre and the
matrix. The magnitude of tan δ values is also seen to increase in the case of composite from
nonwoven, compared to the composites from yarn and it could be because of its higher porosity. In
reality, the storage modulus favours long fibres (large l/d), and damping favours short fibres (small
l/d). Our experimental investigations indicated that composites with fibre off-axis 90° showed better
damping than composites with 45° and 0.
c)
SUMMARY OF RESULTS
35
a)
b)
SUMMARY OF RESULTS
36
Fig. 15. DMTA analysis of PLA reinforced with hemp and Lyocell fibres. a-b) storage modulus vs temperature; c-d)
tanδ curves (Publications II and III)
c)
d)
SUMMARY OF RESULTS
37
4.2.7 Differential scanning calorimetry
The DSC heating thermograms of neat PLA and produced composites are shown in Fig. 16, and the
corresponding transition temperatures are tabulated in Table 5. The data indicate that the Tg and Tm
of the PLA composites decreased less than 2 ˚C with the addition of hemp fibre to the PLA matrix;
however, the ΔHm and Tc of the PLA composites decreased significantly in the presence of hemp.
These results suggest that hemp does not significantly affect the crystallisation properties of the
PLA matrix. In the first article, the crystallisation temperature of the PLA/hemp composite
decreased by up to about 13–15°C (for pure PLA compared to composites with 45 mass% fibre
content), which signifies that the hemp fibres have a negative effect on crystallisation (Publication I).
However, the data from Publication II indicate that the crystallinity and Tc of PLA was found to
increase as a result of the presence of hemp, since the fillers acted as nucleating agents for the
crystallisation of the polymers [99, 122]. Also, it is apparent that the crystallinity of PLA in the
treated hemp/PLA composites increased, compared with that of the untreated hemp/PLA
composites. This could be due to the fact that after alkali treatment, the impurities including wax
and pectin were removed from the fibres, which in turn increased the number of nucleating sites of
the fibres [14, 122] (Publication II).
The data indicate that the Tg of PLA in its composites did not show much difference. However, Tm
was slightly affected by the introduction of hemp and Lyocell fibres with a few decreases.
Furthermore, the Tc decreased from pure PLA to composite containing hemp-Lyocell fibres. It can
be clearly seen that the degree of crystallisation increases from 4.6% for neat PLA up to 8.3% for
PLA/hemp, 16.9% for PLA/Lyocell and 11.8% for PLA/hemp-Lyocell. The double melting peaks
suggest the occurrence of the crystal’s reorganisation during the heating run (Publication IV).
a)
SUMMARY OF RESULTS
38
Fig. 16. DSC curves of neat PLA and PLA composites
b)
c)
SUMMARY OF RESULTS
39
Table 5. Calorimetric data for PLA-based composites for the first heating run (10˚C/min)
Sample ∆Hcc
(J/g)
∆Hm
(J/g)
XDSC
(%)
Tg
(°C)
TC
(°C)
Tm
(°C)
Publication
PLA (6201D, processed at 175 ºC) - 52.29 55.71 54.03 112 168.82 I
I
I
I
I
I
PLA/10 Hemp-250 3.92 34.94 37.06 57.13 100.34 166.76
PLA/20 Hemp-250 4.28 28.84 33.02 57.17 98.50 167.13
PLA/35Hemp-250 4.50 26.21 31.67 56.37 98.54 166.43
PLA/45 Hemp-250 5.02 21.39 28.18 57.18 97.58 166.52
PLA (6202D, processed at 195 ºC) 28.9 35.9 7.5 59.9 109.9 167.4 II
II
II
Untreated hemp/PLA 13.17 22.73 14.7 61.3 110.5 167.5
Treated hemp/PLA 19.3 30.3 16.9 61.8 111.8 167.9
PLA (6202D, processed at 190 ºC) - - 4.6 59.2 112.1 167.4 IV
IV
IV
IV
PLA/hemp - - 8.3 59.4 108.6 166.8
PLA/Lyocell - - 16.9 57.6 108.7 166.7
PLA/hemp-Lyocell - - 11.8 57.5 106.4 166.3
SUMMARY OF RESULTS
40
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DISCUSSION AND CONCLUSIONS
41
5. DISCUSSION AND CONCLUSIONS
Recently, the great research efforts into the development of materials derived from renewable
resources, such as biocomposites have been initiated thanks to concerns over environmental and
economic problems. However, most of the studies and researches to date in the field of
biocomposites have been directed towards non-structural applications. This is mainly attributed to
earlier research efforts showing that these composites do not possess the properties essential to
make them suitable for applications involving structural elements. Through the development and
characterisation of a novel biocomposite system, this project aims to overcome some of the current
challenges for these materials application under different mechanical conditions.
Unidirectional-aligned hemp fibre reinforced PLA composites were produced for this research,
applying selected processing procedures to allow for efficient fibre reinforcement and an
improvement in overall composite performance and properties. The aligned hemp/PLA yarns were
produced through two developed types of hybrid yarn process: co-wrapping and commingling. In
Publication I, the properties of 0/90 bi-directional composites made from PLA/hemp co-wrapped
hybrid yarn preforms as reinforcement, instead of using only hemp yarns impregnated with a melted
PLA matrix, were assessed. Composites were fabricated by compression moulding of 0/90 bi-
directional prepregs and characterised regarding porosity, mechanical strength and thermal
properties. Here, we used continuous PLA filaments, which were used to wrap a low twist hemp
yarn. The composites made from the hybrid yarn with higher wrapping density showed
improvements of mechanical properties due to lower porosity. However, the porosities of
composites were between 6–9 vol.%, and the porosity fraction was still high even though the fibre
volume fraction was low. Mechanical tests showed that the tensile and flexural strengths of the
composites markedly increased with the fibre content, reaching 59.3 and 124.2 MPa when
reinforced with 45 mass% fibre, which is approximately 2 and 3.3 times higher, compared to neat
PLA. Impact strength of the composites decreased, initially up to 10 mass% fibre; however, higher
fibre loading (up to 45 mass%) caused an increase in impact strength up to 26.3 KJ/m2, an
improvement of about 2 times higher than neat PLA.
In Publication II, we investigated the development of new hybrid yarns with low twist for high
performance natural fibre-reinforced composites suitable for use in structural or semi-structural
applications and with lower amount of porosity. We investigated the influences of different
orientation especially off-axial direction of hemp fibre as well as alkali treatment. The superior
mechanical properties of the treated fibre composites were attributed to the greater interfacial
bonding between treated hemp fibres and PLA. The results showed that the mechanical properties
of the composites were influenced by the fibre direction. Tensile, flexural and impact values of the
composites showed the decreasing trend for off-axial composites, compared to 0˚ axial-oriented
composite. The damping properties were highly affected by the testing direction; it was increased at
an off-axis angle of 45°and 90°. From water absorption test results, it was found that higher
temperature generally increased the WG% of the neat PLA and all of the composites, as well as
shortening the saturation time.
DISCUSSION AND CONCLUSIONS
42
Porosity is difficult to avoid in natural fibre composites and influences on the composite properties,
yet how to control the porosity has so far only received limited attention. Thus, research on how to
decrease the amount of porosity is warranted. Therefore, the effects of the choice of reinforcing
filler on the structure and properties of PLA-based composites have been studied, focusing on the
porosity, water absorption, mechanical and thermo-mechanical properties. The obtained results
showed that combining hemp and Lyocell in a PLA-based composite can significantly improve the
impact strength at ambient temperature, flexural and tensile strength and modulus, compared with
hemp fibre reinforced PLA since Lyocell-based composites had lower void content due to the
higher interfacial adhesion. The complexity of the surface chemistry and the irregularity of the
morphology of plant fibres is one of the most important considerations; in addition, the presence of
luminal cavities could be another factor. Although Lyocell is expensive, it is reproducible by
artificial production, and the admixture of Lyocell fibres to hemp fibres leads to less variation in
quality of the fabricated composites. Publications III and IV in this thesis confirmed that a
composite system consisting of two different fibres (Lyocell and hemp) with different morphologies
results in composites with better mechanical properties and lower moisture absorption, and their use
in outdoor applications can then be established.
In Publications III and V, we also studied the manufacture of hybrid textile yarns containing
reinforcing and thermoplastic components within their structure, the technology for processing them
into woven fabrics (prepregs), and the manufacture of thermoplastic composites based on them. The
effect of two types of structures for the fabrics, namely 8-harness satin and basket, was studied. The
results showed that the unidirectional hemp/PLA composite made by satin-weave architecture fabric
possessed the highest mechanical strength, compared to the composites manufactured with basket
weave architecture fabric and winded hybrid yarn laminate. This improvement in mechanical
strength was correlated to that of decrease in void content and fibre misalignments. The best overall
properties in this research were achieved with satin fabric from aligned Lyocell/PLA yarn, leading
to a tensile strength of 101.2 MPa, Young’s modulus of 11.4 GPa, flexural strength of 158.2 MPa,
flexural modulus of 9.7 GPa, and impact strength of 47.4 kJ/m2, followed by satin fabric from
Lyocell/hemp mixture.
This study, therefore, indicated that hemp fibres can be a successful solution for many load-bearing
and structural applications such as construction and automotive.
FUTURE STUDIES
43
6. FUTURE STUDIES
Although the research completed in the current study has successfully demonstrated that
unidirectional hemp/PLA and hemp-Lyocell/PLA biocomposites can be manufactured with high
levels of performance characteristics, additional studies on the durability, biodegradation, the
possibility of improving the outdoor properties, and long term performance of the composites are
necessary if they are to be used in load-bearing and structural applications. Hence, understanding
the degradation mechanisms under the lifetime of NFCs is of great importance.
Failure in structural composites occurs often thanks to mechanical fatigue. Certain facts such as the
behaviour of these composite materials under fatigue loading and their failure mechanisms should
be investigated. Indeed, fatigue analysing is very time consuming; therefore, modelling may help to
save time, energy and resources. This can be done by adopting the model being developed for
synthetic composite materials and applied for NFCs.
FUTURE STUDIES
44
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REFERENCES
45
REFERENCES
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Paper I
Composites: Part A 50 (2013) 93–101
Contents lists available at SciVerse ScienceDirect
Composites: Part A
journal homepage: www.elsevier .com/locate /composi tesa
Manufacture and characterisation of thermoplastic composites madefrom PLA/hemp co-wrapped hybrid yarn prepregs
1359-835X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compositesa.2013.03.012
⇑ Corresponding author. Tel.: +46 33 435 4497; fax: +46 33 435 4008.E-mail address: [email protected] (M. Skrifvars).
Behnaz Baghaei a, Mikael Skrifvars a,⇑, Lena Berglin b
a School of Engineering, University of Borås, SE-501 90 Borås, Swedenb The Swedish School of Textiles, University of Borås, SE-501 90 Borås, Sweden
a r t i c l e i n f o
Article history:Received 10 October 2012Received in revised form 10 March 2013Accepted 15 March 2013Available online 26 March 2013
Keywords:A. Natural fibre compositesB. Mechanical propertiesC. Thermal analysisD. Compression moulding
a b s t r a c t
PLA/hemp co-wrapped hybrid yarns were produced by wrapping PLA filaments around a core composedof a 400 twists/m and 25 tex hemp yarn (Cannabis sativa L) and 18 tex PLA filaments. The hemp contentvaried between 10 and 45 mass%, and the PLA wrapping density around the core was 150 and 250 turns/m. Composites were fabricated by compression moulding of 0/90 bidirectional prepregs, and character-ised regarding porosity, mechanical strength and thermal properties by dynamic mechanical thermalanalysis (DMTA) and differential scanning calorimetry (DSC). Mechanical tests showed that the tensileand flexural strengths of the composites markedly increased with the fibre content, reaching 59.3 and124.2 MPa when reinforced with 45 mass% fibre, which is approximately 2 and 3.3 times higher com-pared to neat PLA. Impact strength of the composites decreased initially up to 10 mass% fibre; whilehigher fibre loading (up to 45 mass%) caused an increase in impact strength up to 26.3 kJ/m2, an improve-ment of about 2 times higher compared to neat PLA. The composites made from the hybrid yarn with awrapping density of 250 turns/m showed improvements in mechanical properties, due to the lowerporosity. The fractured surfaces were investigated by scanning electron microscopy to study the fibre/matrix interface.
� 2013 Elsevier Ltd. All rights reserved.
1. Introduction
In recent years, natural fibre-reinforced polymer compositeshave been attracting attention from the viewpoint of reducingthe impact on the natural environment. Due to the increase inenvironmental consciousness, manufacturers are looking for moreecologically friendly bio-based materials for interior and outdoorproducts, and in these applications natural fibre-reinforcedcomposites could have great potential. Currently, the use ofthermoplastic resins in composites is clearly of higher potentialthan the use of thermoset resins because of their easier recycling,faster processing conditions (no time for curing is required), thepossibility of production in longer series, their lower cost, andthe absence of toxic solvents [1,2].
There are many thermoplastic polymers derived from renew-able raw materials, which are also biodegradable. Polylactic acid(PLA) is one such candidate, and it shows rather good propertiesthat are suitable for applications that do not require long-termdurability or elevated mechanical performance at higher tempera-tures. In order to make their possible use in many technical appli-cations more attractive, the mechanical properties of the PLA canbe enhanced by using reinforcements [2,3]. The reinforcement of
PLA with lignocellulosic fibres seems to be a logical alternative inorder to increase their mechanical performance and to preservethe environmentally friendly character of the final material [4].Hemp fibres can be considered to be a good choice for reinforcingpolymer composites, due to their high stiffness, strength, and as-pect ratio [5]. They also have an extremely high fibre yield per unitdensity and they are disease- and pest-resistant, enabling produc-tion methods with a low impact on the environment [6]. When thelife cycle for natural fibres such as hemp are evaluated they tend tohave a neutral CO2 balance, i.e. they release at decomposition asmuch CO2 as they have taken up during growth [7]. This has beenconfirmed by life cycle assessment of hemp fibres used in automo-tive applications [8,9]. Wötzel et al. [8] compared in a LCA hempreinforced fibre composite side panels and acrylonitrile butadienestyrene (ABS) copolymer composite side panels. The study showedthat the hemp fibre reduced the energy consumption by 45%compared to the ABS composite. Additionally the hemp fibre usedonly 5.3% of the cumulative energy demand, and same was foundfor the CO2.
Highly ordered textile reinforcements, such as interlaced wovenfabrics and unidirectional fabrics made from natural-fibre yarns,perform considerably better than random non-woven mats innatural-fibre composites [10]. A number of companies have intro-duced natural fibre composites, of these Libeco Lagae (Belgium),Lineo (Belgium) and NPSP (Netherlands) can be mentioned.
94 B. Baghaei et al. / Composites: Part A 50 (2013) 93–101
Furthermore, in a recently published technical report the mechan-ical and specific properties of flax and hemp used in polymer rein-forcement were discussed, and the suitability of flax and hempreinforcement was assessed, especially regarding the use in unidi-rectional reinforcements, fabrics, non-crimp fabrics, mats,prepregs, and compounds [11].
At present, the commercially available plant-fibre yarns are notintended for structural composites, but mainly for textiles, whichhave entirely different demands on the yarns. Thus, work is neededto tailor-make the best plant-fibre yarn for reinforcement ofcomposites. This also includes investigation of the possibility ofcombining plant-fibre yarns with the matrix polymer in fibre forminto one hybrid yarn (a composite preform), and how to do it(twisting or blending [12,13]). Previous studies [2,14,15] havedemonstrated that reinforcing hemp fibre increases the tensilestrength, Young’s modulus, impact strength, and flexural modulusof PLA biocomposites, which is a good indication of the compatibil-ity of hemp fibre with PLA. It is well known that fibres provide thehighest strength and stiffness when they are continuous andaligned in the direction of the applied load. Natural fibres arenaturally discontinuous and conventional spun staple yarns tendto be highly twisted, which leads to fibre misalignment and poorresin wet-out. The structured natural-fibre composites reportedso far are based on twisted yarns produced by long-establishedconventional spinning methods, mainly ring spinning [12,13,16].
In this paper, we report our work on improving the orientationof hemp fibres in composites by using our recent development ofco-wrapped yarn structures. This novel co-wrapped yarn consistsof low twist and very fine hemp yarns next to PLA filaments inthe core part, which are wrapped by PLA filaments. By varyingthe composition of hybrid yarn, it is possible to vary the hemp fibrecontent from 10 to 45 mass%. An exciting recent advancement hasbeen a new family of aligned natural-fibre reinforcements, whichhas overcome these issues by using ‘low twist’ yarns. We alsoreport the influence of fibre content and wrap density (numberof wraps per unit length) on the properties of composites.
2. Materials and methods
2.1. Materials
An air-textured PLA yarn with 72 filaments, provided byTorcitura Lei-Tsu S.r.l (Bergamo, Italy) was used as a the matrixmaterial. According to the supplier, the yarn was made from thePLA grade 6201D from NatureWorks�, Cargill Dow LLC (Minne-tonka, USA). This thermoplastic has a density of 1.24 g/cm3, a melttemperature of 160–170 �C, and a glass transition temperature of55–60 �C. The bleached hemp yarn (Cannabis sativa L), suppliedby Hempage AG, (Adelsdorf, Germany), was used in the core partof hybrid co-wrapped yarns. This staple fibre yarn have an averagefibre length in the range of 25–35 mm. The mechanical propertiesof the yarns used are shown in Table 1.
2.2. Methods
2.2.1. Manufacture of hybrid yarnsThe PLA/hemp hybrid yarns were produced by using a
laboratory yarn twist machine from DirecTwist, AGTEKS CO. Ltd.,(Istanbul, Turkey). In the co-wrapping, the PLA yarn filaments were
Table 1Mechanical properties of hemp and PLA yarns.
Raw yarn Linear density in tex Breaking tenacity in N/tex El
Hemp 25 0.35 ± 0.06 1.PLA 18 0.24 ± 0.01 7.
wrapped around low twist and very fine hemp yarns. These PLA fil-aments will melt during the consolidation process and will beincorporated in the matrix. Thus, they need to be very fine yarnsso that the interfacial adhesion between the reinforcing hempyarns and the PLA matrix is not affected to any significant extent.However, inhomogeneous distribution of the reinforcing hempand the PLA matrix yarns may lead to poor impregnation in theconsolidated composite [17]; it was therefore decided to put PLAfilaments in the core part of the hybrid yarn together with thehemp yarn. The core yarn was assembled by putting aligned hempyarns and PLA filaments into a bundle without increase the hempyarn twist. The yarn structure obtained from the co-wrap spinningused is shown in Fig. 1. The wrap density, or number of wraps perunit length (1 m), ranged from 150 to 250 in order to study theeffect of wrap density on the properties of the composite. The com-position of the hybrid yarns obtained is given in Table 2. The nameof the hybrid yarn indicates the composition. For example, in thehybrid yarn PLA/45 Hemp-150 one PLA filament is wrapped 150times around 1 m of the core part (composed of four hemp yarnsand two PLA filaments), thus giving a nominal ratio of 45 mass%hemp fibre content in the hybrid yarn. The linear densities of thehybrid yarns were determined by weighing a 10-m long yarn sam-ple which was dried in a vacuum chamber (0.9 mbar; 20 �C) for atleast 18 h, and calculating the tex value (1 tex = 1 g/1000 m).
2.2.2. Preparation of prepreg and compositesBefore compression moulding, multilayer 0/90 bidirectional
hybrid yarn prepregs were prepared by winding the hybrid yarnaround a 19 � 19-cm steel rectangular frame (Fig. 2). A 0/90 bidi-rectional layup of the prepreg was chosen, due to the ease of thewinding technique. Each prepreg had 12 layers of wound yarns,to achieve a composite thickness of about 1–3 mm. The prepregmats were first put in a vacuum chamber (0.9 mbar; 20 �C) for atleast 18 h, to dry before the compression moulding. The prepregwas covered with a Teflon sheet to prevent sticking of the matrixto the surface of the mould before being placed into a pre-heatedsteel mould with a 20 � 20-cm square cavity, and 10 mm depth.The steel mould was of own design, and machined by a localmachine shop. The composites were then prepared by pressingthe prepreg at a temperature of 175 �C and a pressure of1.04 MPa for 15 min in a hydraulic compression moulding machinefrom Rondol Technology Ltd. (Staffordshire, UK). Neat PLA sheetsto be used as reference material, were made by compressionmoulding under the same processing conditions of the same PLAfilaments formed to a similar prepreg.
2.2.3. Characterisation of compositesTo compare the mechanical and thermal properties of compos-
ites, tensile testing, dynamic mechanical thermal analysis (DMTA),differential scanning calorimetry (DSC), and scanning electronmicroscopy (SEM) were carried out. The specimens were storedat ambient conditions after processing, and then before testing,the specimens were conditioned for at least 24 h at 23 �C and50% relative humidity according to DIN EN ISO 291.
Statistical comparisons, based on linear regression analysis atthe 95% confidence level, were performed on the experimental datato test the effects of the various wrapping density and fibre loadingon the mechanical properties. Analysis of data was done usingMinitab (version 15).
ongation at break in% Initial modulus in N/tex Twists per metre
04 ± 0.19 33.14 ± 4.49 40020 ± 0.31 4.49 ± 0.83 –
Fig. 1. Structure of co-wrapped hybrid yarn.
Fig. 2. Prepreg frame with the PLA/hemp hybrid yarns in a 0/90 configuration.
B. Baghaei et al. / Composites: Part A 50 (2013) 93–101 95
2.2.3.1. Composite density and porosity. Composite density wasevaluated in order to assess the volumetric composition. Compos-ite and hemp fibre densities were determined by the buoyancymethod (Archimedes’ principle) using ethanol as the displacementmedium. Before the specimens were immersed in ethanol, theyhad been covered by a varnish containing paraffin to avoid absorp-tion of liquid during immersion. Based on the data obtainedregarding densities and mass fractions, the volume fractions offibre, matrix, and porosity were calculated by using the methodproposed by Madsen et al. [18]. The composite porosity contentwas estimated as the volume fraction not taken up by the fibreand matrix components.
2.2.3.2. Tensile test. The tensile testing was performed according tothe ISO 527 standard test method for the fibre-reinforced plasticcomposites, using a universal H10KT testing machine equippedwith a model 100R mechanical extensometer, both supplied byTinius Olsen Ltd. (Salford, UK). The loading rate was 10 mm/minand the load range was 1 kN. Six specimens were analysed for eachcomposite laminate. Cutting of the specimens was done with alaser machine. The specimens were cut in 20-mm-wide dumbell-shape with an overall length of 150 mm and a 60-mm length ofthe narrow, 10-mm-wide parallel-sided portion. The gauge lengthwas 50 mm and the initial distance between the grips was115 mm. The extensometer was attached to the central portionof the test specimen with clips.
2.2.3.3. Dynamic mechanical thermal analysis. DMTA on thecomposites was done using a Q-series TA instrument supplied byWaters LLC, Newcastle, DE, USA. The DMTA was run in the dual-cantilever bending mode and the sample dimensions were: thick-ness 1–2 mm, length 50 mm, and width 8 mm. The temperatureinterval was 15–150 �C with a heating rate of 3 �C/min using afrequency of 1 Hz and amplitude of 15 lm.
2.2.3.4. Flexural testing. The flexural testing was performedaccording to ISO 14125, with the same testing machine as for ten-sile testing. At least 5 specimens were tested for every compositecomposition. The loading rate was 10 mm/min and the load rangewas 1 kN. The specimen dimension was 60 mm � 15 mm(length �width), while the thickness varied depending on thesample. The outer span was taken to be 40 mm and the range ofdisplacement was 20 mm.
Table 2Composition of hybrid yarns and their properties.
Sample no. Wrap part Core part
PLA/45 hemp-150 1PLA 4H, 2PLAPLA/45 hemp-250PLA/35 hemp-150 2PLA 3H, 2PLAPLA/35 hemp-250PLA/20 hemp-150 3PLA 2H, 2PLAPLA/20 hemp-250PLA/10 hemp-150 4PLA 1H, 2PLAPLA/10 hemp-250
2.2.3.5. Impact testing. Charpy impact testing was done onun-notched specimens in accordance with ISO 179 using a pendu-lum type Zwick test instrument from Zwick GmbH (Ulm,Germany). A total of 10 specimens were tested edgewise to deter-mine the mean impact resistance.
2.2.3.6. Differential scanning calorimetry. The DSC analysis was doneon a DSC Q2000 supplied by TA Instruments, New Castle, DE, USA.Samples of 8–10 mg were heated in a nitrogen-purge stream at arate of 10 �C/min from 20 �C to 200 �C, cooled to 0 �C, and thenheated again from 20 �C to 200 �C. The data from the first scanwere used in order to see the effect of processing conditions onthe thermal properties. For each PLA sample, three replicates werescanned. The final results were the averages of three DSC runs. Thepercentage crystallinity (XDSC) of PLA was calculated using thefollowing equation [19]:
XDSC% ¼ DHf � DHcc
DH�f� 100
w
where DH�f = 93 J/g for 100% crystalline PLA, DHf is the enthalpy ofmelting, DHcc is the cold crystallisation enthalpy, and w is the massfraction of PLA in the composite.
2.2.3.7. Scanning electron microscopy (SEM). Composite fracturesurface morphologies were studied using low-vacuum scanningelectron microscopy using a JEOL JSM 6610LV instrument, JEOLLtd. (Tokyo, Japan) with an operating voltage of 5–10 kV. As lowvacuum SEM imaging was done, no specific preparation wasrequired.
3. Results and discussion
3.1. Influence of yarn structure
In this study, co-wrapped hybrid yarns were produced fromhemp and PLA yarns in four nominal mass ratios: 10/90, 20/80,35/65, and 45/50 with two wrapping densities: 150 and250 wraps/m. The mass fraction of the hemp fibre obtained in
Wrap density Linear densityin tex
Mass-percentageof hemp
150 191.5 46.2250 192.5 45.5150 190.0 35.5250 193.0 34.9150 185.8 23.3250 191.6 22.3150 182.7 11.3250 187.3 10.1
96 B. Baghaei et al. / Composites: Part A 50 (2013) 93–101
the hybrid yarn with a wrapping density of 250 was slightly lowerthan for the hybrid yarn with wrapping density of 150 because ofthe denser PLA filament wrapping (Table 2). The same natural-fibremass fractions were assumed for the compression-moulded com-posites since there was no loss of matrix during preparation ofcomposites. In total, 8 composites were manufactured with fibrevolume fractions in the range 8–38 vol-% and with porosity inthe range 6–9 vol-%. In the calculations, we used a density of1.48 ± 0.01 g/cm3 for the hemp yarn. The physical composition ofthe composites made are summarised in Table 3. From the results,it is possible to conclude that the higher the fibre volume fraction,the higher the amount of porosity—which is obviously due to thephysical structure of the prepreg. Similar trends have also beenseen in other studies [18,20]. Furthermore, it appears that theuse of lower wrapping density causes a slight increase in the poros-ity. However, it must be considered that the fibre volume fractionof the composite with a wrapping density of 150 is slightly higherthan for the composite with a wrapping density of 250, so thismight explain the increased degree of porosity. In the preparedcomposite, the porosity fraction is still high even if the fibrevolume fraction is low, which could be due to several factors.The complexity of the surface chemistry and the irregularity ofthe morphology of plant fibres is one of the most important consid-erations, in addition to the presence of luminal cavities [18]. Inaddition, in this case we also used PLA fibres as the matrix, andPLA used for melt spun fibres usually has a high molecular weight,and therefore lower melt viscosity. Composite porosity is known tobe linearly related to the logarithm of matrix viscosity [21]. Thishinders the penetration of PLA resin formed by melting of thePLA fibres between and around the hemp fibres to form a uniformmatrix, and it causes some of the porosity. Furthermore, porosity isalso affected by the processing techniques used. The effects of theprocessing factors such as preheating temperature on the mechan-ical properties of composites have been studied. Preheating wasfound to be the most important factor for good-quality moulding.However, shrinkage problems may be encountered when preheat-ing the prepreg yarns. The high draw ratio used in the meltspinning of the PLA fibres causes contraction of the matrix fibresand hence distortion of the fibre architecture when these fibresrelax on heating; therefore, preheating of the hybrid yarns at lowtemperature can also contribute to development of porosity [22].Svensson et al. [22] examined laminates made from co-wrappedyarns using aramid, carbon, and glass fibres as reinforcementsand spun polyamide and polyimide as matrix. They found thatlaminates manufactured from the co-wrapped yarns containedvoids and resin-rich pockets.
3.2. Tensile properties
An overview of the tensile modulus and strength of thecomposites compared to the values of the neat PLA matrix is given
Table 3Composition of the fabricated composites calculated from the density measurements.
Sample no. Density in g/cm3 Fibre mass fraction in% Fibre volum
PLA 1.24 0.0 0.0PLA/10 hemp-150 1.192 (0.007) 10.1 9.1 (0.1)PLA/10 hemp-250 1.193 (0.025) 11.3 8.1 (0.1)PLA/20 hemp-150 1.196 (0.004) 22.3 18.8 (0.1)PLA/20 hemp-250 1.197 (0.004) 23.3 18.0 (0.1)PLA/35 hemp-150 1.210 (0.009) 34.9 29.0 (0.2)PLA/35 hemp-250 1.219 (0.002) 35.5 28.7 (0.1)PLA/45 hemp-150 1.224 (0.006) 45.5 38.1 (0.2)PLA/45 hemp-250 1.226 (0.004) 46.1 37.7 (0.1)
Note: Data in table are mean (SD).
in Fig. 3a and b. It can be seen that an increase in fibre contentimproves the tensile strength, although the standard deviationfor each group of specimens was high, as shown by the error barsin Fig. 3. The statistical analysis of the data confirmed however asignificant increase in the modulus and the strength with theaddition of hemp fibre to the PLA-matrix (p-value < 0.001). Froma statistical viewpoint, the addition of hemp fibre to the PLA matrixwas found to have statistically significant contributions (at 5%significance level) on the response modulus and strength, and thisrelation is effectively modelled by a linear regression model. Inaddition, the statistical analysis demonstrated normal distributionof the values. The hemp yarn was bleached, which obviouslyenhances the quality of the fibre surface. This promotes betterinterfacial fibre-matrix adhesion and leads to better compositeproperties. It is also obvious from the results that composites madefrom the hybrid yarn with a wrapping density of 250 showed aconsistent improvement in tensile modulus over the compositeswith 150 wrapping density, across the whole range of fibre fractionratios investigated. At a low fibre fraction ratio (<25% by mass), theimprovement was about 4.6%; at a higher fibre fraction ratio (>40%by mass), the improvement increased to about 14.7%. This increasecould be attributed to improved fibre alignment. Increasing wrap-ping density improved the tensile modulus of the compositesbecause the tortuosity of the hybrid yarns decreases as wrap twistincreases, and consequently the reinforcement alignment would beimproved. At a low wrap twist, the buckling moment generated bythe wrapping filament is the main cause of yarn tortuosity. As wraptwist increases, the wrapping pitch (1/T) becomes smaller so thatthe bending span is shortened and the yarn core responds less tothe buckling action of the wrapping filament [23].
3.3. Flexural properties
The results of the flexural testing are shown in Fig. 4a and b. Boththe flexural strength and the modulus of hemp-reinforced PLA com-posites were all higher than for neat PLA, which is especially evi-dent for the 45 mass% fibre content with wrapping density of 250,with a flexural strength (124.2 MPa) 3.3 times higher than that ofneat PLA (37.7 MPa). The statistical analysis of the data confirmedsignificant increase in the flexural modulus and strength with theaddition of hemp fibre to the PLA matrix (p-value < 0.001). In addi-tion, the analysis demonstrated normal distribution of the results. Itis noteworthy that the above increase in flexural strength withrespect to the fibre content, which can be assigned to a good adhe-sion between the fibre and the matrix, is consistent with previouslypublished results on natural fibre/PLA composites. Hu and Lim [24]reported that the flexural strength of PLA/hemp composite in-creased with increase in hemp fibre content. Cao et al. [25] foundthat flexural strength and modulus increased with increase in fibrecontent for PLA/bagasse fibre composites. Okubo et al. [26] investi-gated PLA/bamboo and micro-fibrillated cellulose composites and
e fraction in% Matrix volume fraction in% Porosity volume fraction in%
100.0 –85.3 (0.5) 5.6 (0.6)86.5 (0.2) 5.3 (0.9)74.0 (0.2) 7.2 (0.3)75.0 (0.2) 6.9 (0.3)62.9 (0.5) 8.0 (0.7)64.0 (0.1) 7.3 (0.1)53.2 (0.3) 8.7 (0.4)53.9 (0.2) 8.4 (0.3)
Fig. 3. Tensile properties of PLA/hemp composites: (a) tensile modulus, and (b)tensile strength.
Fig. 4. Flexural properties of PLA/hemp composites: (a) flexural modulus, and (b)flexural strength.
B. Baghaei et al. / Composites: Part A 50 (2013) 93–101 97
Shih and Huang [27] conducted research on PLA/banana fibre com-posites, and they found improved flexural strength compared topure PLA. Ma and Joo [28] investigated PLA/jute composites andfound that the flexural strength and modulus of jute/PLA compos-ites increased with increase in the content of jute fibre. However,Shibata et al. [29] studied PLA/abaca composite and observed thatflexural strength decreased with increase in abaca fibre content.Similar trends have also been reported by Sawpan et al. [30] forPLA/hemp composite and by Serizawa et al. [31] for PLA/kenaf com-posite. In addition, the above finding is consistent with the tensilestrength results with PLA/hemp composites regarding fibre con-tent. The results show that there were differences in tensile andbending Young’s modulus values. It can be explained by the differ-ent oading modes in tensile and bending, and the fact that themanufactured composites have local defects which act as stressconcentration points, which causes local weakness. Furthermore,qualitatively, this can be explained from statistically consideringflaws and fractures and the fracture energy available in flexuralsamples under a constant rate of deflection as compared to tensilesamples under the same load conditions [32]. In the bending modeonly the inner (concave face) and outer (convex face) edges of thesample are at the largest stress of compressive and tensile respec-tively, and if these fibres are free from defects, the flexural strengthwill be controlled by the strength of these intact ‘fibres’. However, ifthe identical specimen is subjected to tensile forces then all the fi-bres in the composite are under the same stress and the failure willoccur when the weakest fibre reaches its limiting tensile stress.Therefore it is common for flexural strengths and modulus to behigher than tensile strengths and modulus for the same material[32]. A small difference in composite flexural strength and moduluswas found for the hybrid yarns with the two different wrapping
densities. At a fibre ratio of about 45% by mass, composites madefrom yarn with wrapping density of 250 had 10% higher flexuralstrength and 9% higher flexural modulus than composite fabricatedfrom yarn with 150 wrapping density. The composite made fromyarn with 250 wrapping density gave slightly higher values thanthe composite made from yarn with 150 wrapping density, whichis probably due to the lower porosity and more aligned fibres.
3.4. Impact resistance
The results of the Charpy impact test are shown in Fig. 5. It isapparent that the impact strength of the PLA/hemp compositesincreases with increasing mass proportion of fibre. But the valueat 10 mass% fibre content is still 8.6% lower than the value forpure PLA. Fibre pull-out is a major energy-absorbing mechanismsince the matrix deformation in the composites with 45 mass%fibre content will be lower than in the composites with10 mass% fibre content due to the lower composite failure strain[33]. The effect of fibre reinforcement on the impact strength ofcomposites is more complicated than bending and tensilestrengths, since the impact strength is attributed to the energyconsumption during failure. Higher interfacial strength doesnot always give higher impact strength. Medium or lower inter-facial strength is sometimes appropriate to increase the dissipa-tion of energy during fracture due to fibre pull-out. In this case,longer fibres are preferable. For fibre-reinforced composites, thelonger the fibre length, the more the energy consumption thatoccurs during fibre pull-out, as long as no breakage of fibre oc-curs. When longer fibres are introduced into the PLA matrix,the impact strength greatly increases due to the frequent occur-rence of long fibre pull-outs [34]. In our work, the higher impactstrength of composites compared to pure PLA can be attributedto the fact that the reinforcing staple fibres (which are between
Fig. 5. Impact strength of the composites relative to their proportion of fibre mass.
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25 and 35 mm long) are in the form of yarn and can act as longfibres. It is notable that the above increase in impact strengthwith respect to the fibre content is consistent with some otherwork on natural fibre/PLA composites [35,36]. The compositesmade from yarn with wrapping density of 250 showed relativelylower impact strengths compared to the composite from yarnwith wrapping density of 150. This can be attributed to betterfibre–matrix adhesion, which was also obvious in the flexuraland tensile testing. Higher fibre–matrix adhesion resulted inshorter average pull-out lengths, and therefore caused lowerimpact resistance or strength [33].
Fig. 6. DMTA analysis of thermoplastic PLA reinforced with various proportions ofhemp fibre. (a) Storage modulus vs. temperature; and (b) tand curves.
3.5. Dynamic mechanical thermal testing
DMTA is a technique for applying stress to a material andmeasuring its response. Since polymers are viscoelastic by nature,the analysis will give information about the storage modulus andthe loss modulus. As the storage modulus is conceptually equiva-lent to the modulus of traditional mechanical testing, it gives ameasurement of the stiffness of the material. In this study, onlythe composites made from the hybrid yarns with a wrapping den-sity of 250 were analysed. The results are shown Fig. 6. The storagemodulus (at 20 �C) increased from 3.07 GPa for pure PLA up to6.2 GPa for the composite with 45 mass% fibre (Fig. 6a). It isevident that incorporation of hemp fibres results in an increasein the storage modulus of the biocomposite which reveals effectivestress transfer from the fibre to the matrix at the interface andgood adhesion between the fibre and the matrix. These resultsare consistent with previously published results on natural fibre/PLA composites [37–39].
The dampening, or tand, is the ratio between the loss modulusand the storage modulus and gives information about the internalfriction of the material. For a composite, the molecular motion inthe interface will contribute to the dampening. The dampening willconsequently give information about the adhesion of the interface.There is a general trend of the data in Fig. 6b, with reduced tandvalues with higher fibre ratio. The same trend has also beenobserved by other authors for various reinforcements [37–39]. Apossible explanation is that there is a strong interaction betweenthe fibre and the matrix. For a composite with a low fibre ratio,the polymer chains are relatively free to move. Increasing the fibre
Fig. 7. DSC heating thermograms for fabricated PLA/hemp composites from co-wrapped hybrid yarn with (a) wrapping density of 250, and (b) wrapping density of150.
B. Baghaei et al. / Composites: Part A 50 (2013) 93–101 99
ratio will decrease the mobility of the polymer chains and conse-quently reduce the dampening. This was discussed by Pothanet al. [40]. The peak in dampening takes place in the region ofthe glass transition where the material changes from a glass to arubbery state. The glass transition temperature is often recorded
Table 4Calorimetric data for PLA/hemp composites for the first heating run (10 �C/min).
Sample DHcc in J/g DHm in J/g
PLA – 52.29PLA/10 hemp-150 4.78 29.88PLA/10 hemp-250 3.92 34.94PLA/20 hemp-150 5.45 29.23PLA/20 hemp-250 4.28 28.84PLA/35 hemp-150 6.09 25.40PLA/35 hemp-250 4.50 26.21PLA/45 hemp-150 6.25 21.93PLA/45 hemp-250 5.02 21.39
Fig. 8. (a–e) SEM images (at different magnifications) of tensile fracture surfaces of PLA
at the maximum of the tand. Using this method of determiningthe glass transition temperature, the recorded Tg of the samplesranged from 66 �C for pure PLA up to 73 �C for almost the highestfibre ratio (45 mass%). It is obvious that incorporating fibres en-hances the Tg. The slight shift in Tg to higher temperature (by a
XDSC in% Tg in �C Tc in �C Tm in �C
55.71 54.03 112 168.8235.54 56.78 100.24 166.4837.06 57.13 100.34 166.7632.05 56.96 99.50 167.4033.02 57.17 98.50 167.1331.95 56.73 99.09 166.9431.67 56.37 98.54 166.4330.65 56.37 99.12 166.1128.18 57.18 97.58 166.52
/hemp composite with 45 mass% hemp/fibre content and wrapping density of 150.
100 B. Baghaei et al. / Composites: Part A 50 (2013) 93–101
few degrees) indicates that the mobility of polymer chains is af-fected. Mathew et al. [41] discussed that the shift to higher tem-perature usually indicates restricted movement of moleculesbecause of better interaction between the fibre and the polymermatrix.
3.6. Differential scanning calorimetry
The DSC heating thermograms of composites produced areshown in Fig. 7a and b. The glass transition temperature (Tg),crystallisation temperature (Tc), melting temperature (Tm), coldcrystallisation enthalpy (DHcc), and melting enthalpy (DHm)obtained from the DSC studies are summarised in Table 4. The dataindicate that the Tg and Tm of the PLA composites decreased less than2 �C with the addition of hemp fibre to the PLA matrix, however theDHm and Tc of the PLA composites decreased significantly in thepresence of hemp. These results suggest that hemp does not signif-icantly affect the crystallisation properties of the PLA matrix. Twomain factors control the crystallisation of polymeric compositesystems [42]. Firstly, the additives have a nucleating effect that re-sults in an increase in the crystallisation temperature, which has apositive effect on the degree of crystallisation. Secondly, additiveshinder the migration and diffusion of polymer molecular chains tothe surface of the growing polymer crystal in the composites, result-ing in a reduction in the crystallisation temperature, which has anegative effect on crystallisation. In this study, the crystallisationtemperature of the PLA/hemp composite decreased by up to about13–15 �C (for pure PLA compared to composites with 45 mass% fibrecontent), which signifies that the hemp fibres have a negative effecton crystallisation. Almost similar results were obtained in the case offabricated composites with wrapping density of 150 and 250. It isnoteworthy that the above decrease in crystallinity of PLA withrespect to fibre content is consistent with some other work onnatural fibre/PLA composites [27,37,39,43,44]. However, crystallisa-tion properties of PLA/hemp composites have been reportedelsewhere. For example, Masirek et al. [45], Pickering et al. [2], andSawpan et al. [14] studied hemp-reinforced PLA composites andthe crystallinity of PLA was found to increase as a result of thepresence of hemp, since the fillers acted as nucleating agents forthe crystallisation of the polymers.
3.7. Scanning electron microscopy analysis
SEM micrographs of the tensile fractured surface of the PLA/hemp biocomposites with a hemp fibre content of 45 mass% anda wrapping density of 150 are shown at different magnificationsin Fig. 8. It was observed from fracture surfaces that in some areasthe fibres and matrix did not adhere to each other adequatelyenough, showing little imprints of fibres on the matrix, as can beseen in Fig. 8a and b. Pulled-out fibres and the corresponding holesare visible in the composite (Fig. 8c). Furthermore, it can be seenthat the surfaces of the pulled-out fibres are partially clean(Fig. 8d and e). There are gaps between the fibres and the PLA,which could either occur because of debonding during mechanicaltesting or because of poor wetting during production of composite,which both indicates poor matrix/fibre adhesion. These observa-tions suggest that the adhesion between the matrix and the fibreis not optimal. It should be noted that unmodified hemp fibreswere used in the composites, and modification of fibres woulddefinitely lead to better adhesion between the matrix and naturalfibres. Heinemann and Fritz [46] predicted a good adhesionbetween natural fibres and PLA because of the hydrophilic natureof PLA. PLA has slightly polar oxygen atoms, which could formhydrogen bonds with the hydroxyl groups of the natural fibres.However, because of the results of this work and the work by otherauthors, it can be assumed that these hydrogen bonds only have a
small influence on fibre/matrix adhesion which leads to not opti-mal adhesion between the matrix and natural fibres.
4. Conclusions
The main objective of the present study was assessment of theproperties of 0/90 bidirectional composite made from PLA/hempco-wrapped hybrid yarn preforms as reinforcement, instead ofusing only hemp yarns impregnated with a melted PLA matrix.We investigated the mechanical and thermo-mechanical proper-ties of hemp-reinforced PLA composites. Compared to neat PLA,the tensile and flexural modulus and the strength of the PLA–hempcomposites were significantly higher as a result of the increased fi-bre content. Impact strength of the composites decreased initiallyup to 10 mass% fibre loading, but even higher fibre loading causedan improvement in impact strength. From the DMTA results, it isevident that incorporation of the fibres gives a considerableincrease in storage modulus and a decrease in tand values. Theseresults show the reinforcing effect of hemp on PLA matrix. Fromthe general trend in the results obtained, it can be affirmed thatco-wrapped hybrid yarn with lower wrapping density leads tolower mechanical properties in the composite. The studyperformed with DSC revealed that the glass transition temperatureand the melting point of PLA were not affected significantly afterreinforcement with hemp. The crystallisation temperature of thehemp-reinforced PLA composites decreased compared to purePLA, which indicates that the hemp fibres hinder the migrationand diffusion of PLA molecular chains to the surface of the nucleusin the composites. No noteworthy differences in calorimetric datafrom DSC for composites were observed between the hybrid yarnpreforms with different wrapping density. Future work willconcentrate on efforts to evaluate the biodegradability of thesedeveloping and promising composites.
Acknowledgments
Financial support from ÅForsk, Sweden, for this work is grate-fully acknowledged. The authors thank Jan Johansson, SwereaIVF, Mölndal, Sweden, for his assistance in performing the SEManalysis. Facius Nielsen of JN Textile Trading ApS, Denmark, isgratefully acknowledged for supplying the PLA filaments.
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Paper II
Composites: Part A 61 (2014) 1–12
Contents lists available at ScienceDirect
Composites: Part A
journal homepage: www.elsevier .com/locate /composi tesa
Novel aligned hemp fibre reinforcement for structural biocomposites:Porosity, water absorption, mechanical performances and viscoelasticbehaviour
http://dx.doi.org/10.1016/j.compositesa.2014.01.0171359-835X/� 2014 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel.: +46 33 435 4497; fax: +46 33 435 4008.E-mail address: [email protected] (M. Skrifvars).
Behnaz Baghaei a, Mikael Skrifvars a,⇑, Masoud Salehi a, Tariq Bashir a, Marja Rissanen b, Pertti Nousiainen b
a School of Engineering, University of Borås, SE-501 90 Borås, Swedenb Department of Materials Science, Tampere University of Technology, P.O. Box 589, FI-33101 Tampere, Finland
a r t i c l e i n f o
Article history:Received 28 August 2013Received in revised form 29 January 2014Accepted 31 January 2014Available online 8 February 2014
Keywords:A. Fabrics/textilesB. Mechanical propertiesB. PorosityE. Compression moulding
a b s t r a c t
This paper examines the thermal and mechanical behaviour as well as moisture absorption of alignedhemp composites using hemp/PLA wrap spun yarns. Uniaxial composites were fabricated with 30 mass%hemp using compression moulding. The properties of composites in terms of hemp fibre orientation(aligned and random), off-axis angle and alkali treatment were investigated. It was found that the testingdirection influenced the mechanical properties of the composites. Compared with all the fabricated com-posites, the aligned alkali hemp/PLA yarn composite possessed the best mechanical properties, includingtensile, flexural and impact strengths, lower porosity and water absorption. The water absorption for allcomposites was higher than for neat PLA, both at room temperature and 80 �C. The PLA in its treated com-posites had higher crystallinity, which was attributed to effective heterogeneous nucleation induced byhemp. Based on SEM observation and theoretical analysis of DMTA data, there was a favourable interfa-cial adhesion in all composites.
� 2014 Elsevier Ltd. All rights reserved.
1. Introduction
The potential of natural fibres as reinforcement in compositematerials is well recognized due to their attractive mechanicalproperties which enhance the possibility of producing eco-friendlymaterials. Natural fibres such as hemp and flax are already used inthe automotive industry to reduce weight, cost and environmentalimpact. Hemp is an upcoming European industrial crop [1], withgood mechanical fibre properties which can be cultivated with alow consumption of fertilizers and almost no pesticides [2]. Con-cerning the matrix in the composites, the industrial trend for nat-ural fibre composites is giving more importance to a thermoplasticmatrix, rather than a thermosetting matrix [1,3].
Polylactic acid (PLA) is the most important biothermoplastic forapplications requiring biodegradability. It shows also quite goodproperties appropriate for applications that do not require long-term durability or high mechanical performance at higher temper-atures. The mechanical properties of the PLA can be improved byusing reinforcements like natural fibres in order to increase its po-tential use in many industrial applications [4,5]. Therefore, because
of the attractive properties of hemp fibre, it was used as reinforce-ment for PLA composite in the presented study.
The main application of natural fibres is today mainly in non-structural composites as they are mostly available as randomly ori-ented nonwovens [1,3,6]. The fibre orientation (i.e. alignment ofthe fibres) must be controlled to ensure that the fibre mechanicalproperties are efficiently utilized in order to attract industrialinterest as an alternative to the traditionally applied synthetic fi-bres (e.g. glass fibres). It is evident that PLA/hemp fibre compositescan compete with glass fibre composite regarding stiffness,whereas for tensile and impact strength, the properties are stillnot on a satisfactory level [7–12]. Previous studies have demon-strated that the full reinforcement potential of natural fibres canbe exploited in bio-composites if an aligned fibre orientation isused [12,13]. Natural fibres are naturally discontinuous; thereforenatural fibre reinforcements reported so far are based on twistedspun staple yarns, which are produced by spinning methods,mainly ring spinning. These spun yarns tend to be highly twisted,which leads to fibre misalignment due to their helical paths aroundthe yarn axis. This misalignment contributes negatively to themechanical properties of the resultant composites. Another nega-tive impact of yarn twist is that it tightens the yarn structure, ren-dering resin impregnation difficult [14]. Therefore, in the textileindustry a broad range of techniques for the alignment of natural
2 B. Baghaei et al. / Composites: Part A 61 (2014) 1–12
fibres have been developed and optimised to produce yarns withcontrolled fibre orientations by reducing or replacing twist inyarns. Goutianos and Peijs [14] tried to produce flax yarns withthe minimal level of twist for manufacturing aligned composites.Shah et al. [15] used a sizing agent to substitute the use of twistin roving and yarn. Zhan and Miao [13] studied the effect ofwrapped spun yarn with low twist for reinforcement purpose.
Our previous study [12] investigated the mechanical propertiesof composites manufactured from PLA/hemp co-wrapped hybridyarn prepregs. Here we used continuous PLA filaments, which wereused to wrap a low twist hemp yarn. The composites made fromthe hybrid yarn with higher wrapping density showed improve-ments of mechanical properties due to lower porosity. However,the porosities of composites were between 6 and 9 vol.%, and theporosity fraction was still high even if the fibre volume fractionwas low, which could be due to several factors. Porosity is difficultto avoid in natural fibre composites and influences on the compos-ite properties, yet how to control the porosity has so far only re-ceived limited attention. Thus, research on how to decrease theamount of porosity is warranted.
In the current paper, we discuss the development new hybridyarns with low twist for high performance natural fibre-reinforcedcomposites suitable for use in structural or semi-structural appli-cations and with lower amount of porosity. The overall was tostudy the mechanical properties of these novel aligned hemp fibreyarn composites and investigate the effect of a range of relevantparameters such as prepreg type such as nonwovens and hybridyarn prepregs with different off-axis angles (0�, 45� and 90�) andfibre treatment.
2. Materials and methods
2.1. Materials
Two types of staple fibres were used in this study: hemp andPLA fibres. The PLA staple fibre, provided by Trevira GmbH (Hat-tersheim, Germany), had a fineness of 1.7 dtex and a mean fibrelength of 38 mm. Based on the manufacturer’s information; thePLA fibres were made from PLA Polymer 6202D from Nature-Works�, Cargill Dow LLC (Minnetonka, USA). This thermoplastichas a density of 1.24 g/cm3, a melt temperature of 160–170 �C,and a glass transition temperature of 60–65 �C. The hemp in theform of baled loose staple fibres (genus species Cannabis SativaL) was supplied by Hempage AG (Adelsdorf, Germany). Accordingto the manufacturer’s information, the average diameter of thehemp fibre was 20–40 lm and had a mean fibre length of30 mm. The hemp fibres were treated by 4 wt% NaOH solutionfor 1 hr, rinsed with distilled water until it was neutral and finallydried at room temperature for 48 h. In addition, a 18-tex PLA mul-tifilament yarn, provided by Trevira GmbH (Hattersheim, Ger-many), was used as wrapping yarn in the wrap spun yarns.
Fig. 1. Structure of wrap spun hybrid yarn.
2.2. Methods
2.2.1. Wrap spinning of hybrid yarnsThe preferred yarn structure has the reinforcing fibres straight
and parallel to the yarn axis. Wrap spinning can be used to producesuch a yarn [13]. PLA/hemp hybrid wrap spun yarns were producedby using a laboratory spinning machine from Mesdan S.p.A., (Bre-scia–Italy) and a laboratory yarn twist machine from DirecTwist,AGTEKS Co., Ltd., (Istanbul, Turkey). The hemp and PLA fibres ar-rived at our laboratory in baled loose fibre form. The PLA fibreand the hemp fibre were weighed to the desired proportion(30 mass%) and the fibre mixture was then fed into the carding ma-chine. During carding, the longer PLA fibre supported the shorter
hemp fibre and provided the necessary fibre-to-fibre cohesion toget a web suitable for further processing. The blended PLA/hempweb was carded three times to parallelize the fibres and achievesliver uniformity. Then the sliver was fed through a roving frame,where the strands of fibre were further elongated. The sliver wasdrawn twice after carding to achieve the required roving lineardensity. Although it was possible to create hybrid yarns with lowtwist, the cohesion of the fibres was very low because PLA/hemproving had a false twist, which means that they could not form aroving of sufficient integrity. Moreover, the rowing is too weak tobe able to be collected alone in the roving machine. In order to col-lect the roving without causing breakage in the roving machine,the processable PLA filaments were used as a processing carrierfor the PLA/hemp roving in the final step. Then the roving waswrapped by PLA filaments in the twisting machine. The wrap yarnwas spun to the nominal count of 550 tex, and it had a wrappingintensity of 200 turns/m. These wrappings provide better protec-tion for the reinforcing fibres during further processing, such asweaving [16] or making a prepreg. The yarn structure obtainedfrom the wrap spinning used is shown in Fig. 1.
2.2.2. Preparation of prepreg and compositesBefore compression moulding, multilayer unidirectional pre-
pregs were prepared by winding the hybrid yarn around a19 � 19 cm2 rectangular steel frame. The off-axis fibres were ori-ented at different angles including 0�, 45� and 90�. In order toinvestigate the effect of random fibre orientation on composites,non-woven PLA/hemp prepreg was produced through cardingand the blended PLA/hemp web was carded three times. The pre-preg mats were first dried in a vacuum chamber (0.9 mbar;70 �C) for at least 18 hours before compression moulding. The pre-preg was then covered by a Teflon sheet to prevent sticking of thematrix to the surface of the mould, and then it was placed into apre-heated steel mould with a 20 � 20-cm2 square cavity, and10 mm depth. The composites were formed by pressing the pre-preg at 195 �C and at 1.7 MPa for 15 min. The thickness of pro-duced composite samples was between 2 and 3 mm. Neat PLAsheets to be used as reference material were made by meltingPLA fibres under the same processing conditions. Specimens forthe mechanical testing were cut by GCC LaserPro Spirit laser cut-ting machine according to the standards given below. Before per-forming the testing, the specimens were conditioned for at least
B. Baghaei et al. / Composites: Part A 61 (2014) 1–12 3
24 hours at 23 �C and 50% relative humidity according to DIN ENISO.
2.2.3. Single fibre tensile testThe hemp fibres were tested on a Favigraph single fibre tensile
tester from Textechno GmbH (Mönchengladbach, Germany)equipped with a 20 cN load cell. The gauge length was 20 mm,the test speed was 20 mm/min and 20 replicated tests were done.To simulate the effect of composite processing conditions on thehemp characteristics, single hemp fibres were treated in a hotpress at 190 �C and 1.7 MPa for 15 min, and then again tested fortensile properties. These processing conditions are identical tothose used for the fabrication of the composites presented in Sec-tion 2.2.2.
2.2.4. Composite density and porosityThe densities of the composites were determined by the buoy-
ancy method using ethanol as the displacement medium. In orderto avoid absorption of liquid during immersion in ethanol, thespecimens were covered by a varnish containing paraffin. Usingthe method proposed by Madsen et al. [17], the fibre volume frac-tion was calculated from the fibre weight fraction take in porosityinto account.
2.2.5. Water absorption testWater absorption analysis was done on the composite speci-
mens according to ASTM D570-98. The specimens were dried inan oven for 24 h at 60 �C, and then cooled them down to room tem-perature in a desiccator. The weight of these specimens was de-noted as W0. The specimens were immersed in two differentbaths: one of distilled water at room temperature and one distilledwater bath at 80 �C. The amount of water absorbed was measuredevery 24 hours for 10 days. At each measurement, the specimenwas taken out of the water and the surface wiped dry before theweight was recorded as W. For each composite, five replicatedsamples were measured. The percentage of apparent weight gain(WG) was then calculated using Eq. (1).
WG ¼ ½ðW �W0Þ=W0� � 100% ð1Þ
2.2.6. Mechanical testingTensile testing was done according to the ISO 527, using a uni-
versal H10KT testing machine equipped with a mechanical exten-someter (model 100R long travel extensometer) supplied by TiniusOlsen Ltd. (Salford, UK). The testing parameters were: 10 mm/minfor loading rate, and 1 kN for loading range. The extensometer wasattached to the central portion of the test specimen with clips.
Flexural testing was performed on the same testing machineaccording to ISO 14125. At least five specimens were tested foreach batch of samples. The loading rate was 10 mm/min and theload range was 1 kN. The outer span was taken to be 64 mm fordiscontinuous-fibre-reinforced composites and 40 mm for unidi-rectional composite, and the range of displacement was 20 mm.
To investigate the toughness of the composites, an un-notchedCharpy impact strength test was carried out according to ISO179 179, using a pendulum type Zwick test instrument from ZwickGmbH (Ulm, Germany). A total of 10 specimens were tested edge-wise to determine the mean impact resistance.
2.2.7. Dynamic mechanical thermal analysisThe time-temperature dependency of the mechanical proper-
ties was determined by DMTA, using a Q-series instrument sup-plied by TA Instruments, Newcastle, DE, USA. The DMTA was runin the dual-cantilever bending mode, whereas the temperaturerange was from 30 to 150 �C with a heating rate of 3 �C/min, using
a frequency of 1 Hz and amplitude of 15 lm. At least 3 specimenswere tested for every composite composition.
2.2.8. Differential scanning calorimetryThe DSC analysis was done on a DSC Q2000 supplied by TA
Instruments, Newcastle, DE, USA. Samples of �10 mg were heatedat 10 �C/min under nitrogen between 20 �C to 200 �C, cooled to0 �C, and then heated again from 20 �C to 200 �C. The data fromthe first scan were used. The percentage crystallinity (XDSC) ofPLA was calculated using Eq. (2) [18].
XDSC% ¼ DHf � DHcc
DH0f
� 100w
ð2Þ
where DH0f ¼ 93 J=g for 100% crystalline PLA, DHf is the enthalpy of
melting, DHcc is the cold crystallisation enthalpy, and w is theweight fraction of PLA in the composite.
2.2.9. Scanning electron microscopy (SEM)Fibres and composite fracture surface obtained from the tensile
testing and upper surface morphologies were studied using low-vacuum scanning electron microscopy with a Quanta 200 ESEMFEG instrument from FEI, (Oregon, USA) with an operating voltageof 10–15 kV. For low vacuum imaging, no specific preparation wasrequired.
3. Results and discussion
3.1. Single fibre tensile test
In order to evaluate the effect of moulding conditions, themechanical properties of heat and alkali treated single hemp fibreswere determined, see Table 1. The alkali treatment improves fibretensile strength and modulus which helps to improve the proper-ties of the composites [19,20]. This treatment possibly orients fi-brils along the direction of tensile forces by removinghemicelluloses from fibre, resulting in better load sharing betweenthe fibrils [21]. Heat treatments greatly reduce mechanical proper-ties. This could be due to thermal degradation of cellulose chains[22].
3.2. Composite porosity
Composite fabrication by compression moulding requires thatthe fibre and the thermoplastic matrix are well mixed before beingprocessed into composites. Such fibre/matrix mixing can be doneby fibre opening and web formation which is a technique well sui-ted for non-woven fibre mats. However, in the case of hybrid yarnsfor preparing unidirectional prepregs, fibre/matrix mixing can alsoachieved by fibre opening and web formation, which is followed byspinning. A spinning method known as wrap spinning can be usedto produce such a yarn, consisting of low twist staple fibreswrapped by a matrix filament [13]. In the current study, compos-ites were produced from PLA/hemp nonwoven fibre mats, PLA/hemp yarn prepregs with 3 off-axis fibre orientations (0�, 45�and 90�) and PLA/alkali treated hemp yarn prepregs in a nominalmass ratio of 30% reinforcement. When 70% or more PLA fibrewas used, the staple fibre mixture could be successfully spun intoa hybrid yarn with uniform distribution of the fibres in composite.Because the fibre angle does not have any effect on the compositecomposition, only the composite of 0� off-axis fibre was tested. Intotal, the manufactured composites had porosity in the range 1.23–16.7 vol.%. In the calculations, we used a density of 1.48 ± 0.01 g/cm3 for the hemp yarn. The obtained physical composition of thecomposites produced are summarised in Table 2. From the results,it is possible to conclude that the composite made from PLA/hemp
Table 1Hemp fibre properties before and after pressing and treatment.
Treatment Tenacity in cN/dtex E-Modulus in cN/dtex Elongation in % Linear density dtex
As received 4.66 (1.43) 97.71 (24.92) 3.93 (0.66) 4.15After hot pressing 3.75 (1.10) 84.62 (26.60) 3.37 (0.88) 4.27After alkali treatment 5.18 (1.30) 99.15 (32.64) 4.05 (0.86) 3.75
Table 2Composition of the fabricated composites calculated from the density measurements.
Sample Density (g/cm3)
Fibre mass fraction(%)
Fibre volume fraction(%)
Matrix volume fraction(%)
Porosity volume fraction(%)
PLA 1.2490(0.0002)
0.0 0.0 100.0 –
Alkali hemp/PLA yarn (off-axis angle0�)
1.2943(0.0019)
30.0 26.23 (0.03) 72.53 (0.10) 1.23 (0.15)
Hemp/PLA yarn (off-axis angle 0�) 1.2713(0.0268)
30.0 25.77 (0.54) 71.25 (1.50) 2.97 (2.04)
Hemp/PLA nonwoven 1.0914(0.0154)
30.0 21.12 (0.31) 61.16 (0.9) 16.71 (1.17)
Note: Data in table are mean (SD).
4 B. Baghaei et al. / Composites: Part A 61 (2014) 1–12
nonwoven had the higher porosity. The formation of voids can beattributed to many factors, the main one being entrapped air inthe prepreg, which is not released during compression moulding.The size of voids is affected by the available pore size and their po-sition along the melted resin flow path [23]. The tortuosity of theflow path is an important factor in the process of void movementsand their elimination. The nonwoven mat fibre has a more tortuousflow path compared with the composite made from hybrid yarnprepregs and, hence, it is more difficult for the entrapped air toflow out of the system. The formation of a stronger interface by im-proved hemp fibre and PLA adhesion could be the result of the re-duced porosity of the composite from alkali treated hempcomposites compared with the untreated hemp/PLA composites.This could be explained as an effect of the alkali treatment. Thiswill increase the amount of available hydroxyl groups [19] dueto removal of the noncellulosic materials covering the cellulose hy-droxyl groups. The fibre surface is also cleaned from impurities,which leads to the exposure of the neat and rough fibre surfacewith many pits (Fig. 2), which will largely increase the surface areaof the fibre. In addition to increasing the number of hydroxylgroups for hydrogen bonding, increased surface roughness wouldalso enable better mechanical interlocking with PLA [24]. The lackof interfacial interaction leads to porosity and increases theamount of moisture absorption [21,25].
Fig. 2. SEM micrographs of the surface morphology of
3.3. Water absorption characteristics
The apparent weight gain (WG) as a function of the square rootof immersion time (
ffiffi
tp
) at different temperatures for the manufac-tured composites are shown in Fig. 3a and b. In Fig. 3a, it can beseen that for all the composites investigated, WG increases mono-tonically by
ffiffi
tp
. This increase in WG is consistent with some otherstudies on natural fibre composites [3,6,12]. The absence of theinduction period characterized by zero weight gain at the initialstage of water immersion treatment, which is explained in termsof the different material packing between the skin (which is in con-tact with the hot metal mould surface) and core regions, indicatesthat the hemp fibres were uniformly dispersed in the PLA matrix inthe current study [26]. It can also be seen that after reaching themaximum value, WG decreases when immersion time is increased.Before reaching the maximum WG, the slopes of the WG vs.
ffiffi
tp
plotswere higher for composites compared with neat PLA. It can be ob-served that the fibre-based composites showed significantly higherwater absorption than did neat PLA due to the hydrophilic natureof hemp with polar groups such as hydroxyl and carboxyl groupson the fibre surfaces. These results are in agreement with pub-lished data, showing that the lignocellulosic fibres displayed ahigher tendency to absorb water than does the hydrophobic PLA[6,12]. SEM images of the fracture surface morphology of compos-
: (a) untreated and (b) alkali treated hemp fibres.
0
5
10
15
20
25
0 1 2 3 4 5 6 7 8 9 10111213141516
App
aren
t w
eigh
t ga
in (
wt%
)
Square root of immersion time (hr1/2)
Hemp/PLA nonwoven
Hemp/PLA (0) yarn
Alkali hemp/PLA yarn
Neat PLA
0
5
10
15
20
25
0 1 2 3 4 5 6 7 8 9 10111213141516
App
aren
t w
eigh
t ga
in (
wt%
)
Square root of immersion time (hr1/2)
Hemp/PLA nonwoven
Hemp/PLA (0) yarn
Alkali hemp/PLA yarn
Neat PLA
(a)
(b)
Fig. 3. Apparent weight gain against square root of immersion time for the differentPLA/hemp composites at: (a) room temperature and (b) 80 �C. For neat PLA no datawas recorded after 5 days, due to severe softening of the specimens.
B. Baghaei et al. / Composites: Part A 61 (2014) 1–12 5
ites made of the nonwoven hemp–PLA and the alkali treated hemp/PLA are shown in Fig. 4. Compared with all composites, compositefrom PLA/hemp nonwoven showed the highest water absorptions.It can probably attributed to the fact that it has less close and com-pact packing of hemp fibres in the PLA matrix compared with thecomposite made from PLA/hemp yarn (Fig. 4a). The compact andclose packing of PLA/hemp yarns reduces the porosity inside thecomposites, which in turn contributes to the reduction in waterabsorption [27]. As a result of alkali treatment, the interfacial adhe-sion in PLA/hemp fibres improved (Fig. 4b), which is necessary forthe reduction of interfacial wicking of the water molecules. Thus,the moisture absorption of the composites can be reduced bychemical treatments [14,27,28]. Furthermore, as shown in Fig. 3b, it can be seen that for all the composites investigated, WG in-
Fig. 4. SEM images of the fracture surface morphology of composites m
creases monotonically byffiffi
tp
initially before reaching a maximum.It can be observed that the water immersion temperature doeshave an influence on the water absorption curves. Increasing theimmersion temperature from room temperature to 80 �C led to in-creased water absorption of the neat PLA and composites as well asa shortening of the saturation time. After 5 days at 80 �C, a massloss of almost 20% was observed for neat PLA. At this point, theneat PLA sample was very soft, and it was difficult to decant waterand rinse the sample without losing some material; therefore, thewater immersion for neat PLA was discontinued after 5 days. Theeffect of immersion temperature was sharper on PLA/hemp non-woven than on PLA/hemp yarn composites. The weight gain wasfound to decrease after passing through a maximum. For the com-posites, this could be due to the neat PLA layer peeling on the sur-face as a result of biodegradation and dissolving with time togetherwith the removal of some substances from the hemp fibres duringthe immersion as can be seen in Fig. 5a and b, SEM images of theupper surface of the alkali hemp/PLA specimens before and afterbeing subjected to water immersion treatment [29,30].
3.4. Tensile properties
The tensile strength, modulus and elongation at break of thecomposites are compared with the values of the neat PLA matrixin Table 3. It was found that the composites fabricated from PLA/hemp yarns (both untreated and treated) had considerably highertensile strength and modulus in the principal fibre direction (i.e.,longitudinal direction) than in the corresponding nonwoven mats.Compared to the tensile modulus of the pure PLA matrix, there isan improvement of 255% for treated hemp/PLA yarn, 203% for un-treated hemp/PLA with fibre orientation angle 0�, and 86% for thePLA/hemp nonwoven composites. As expected, the compositestrength and stiffness decreased with increasing fibre orientationangle. When the external load is directed at a significant angle tothe primary fibre axes, large reductions in key composite proper-ties such as ultimate tensile strength can occur [31]. The tensileproperties in the perpendicular direction (90�) for the PLA/hempyarn composites were in almost the exact opposite order to thosein the principal fibre direction (0�). This was entirely within expec-tation because the prepreg with the highest fibre orientation alongthe principal fibre direction would have the lowest fibre orienta-tion in the perpendicular direction [32,33]. The composite madefrom the nonwoven demonstrated higher tensile properties inthe principal fibre direction compared with hemp/PLA yarn com-posite with fibre orientation angles 45� and 90�. Also the resultsshowed that tensile strength of composites made with fibre orien-tation angles 45� and 90� were lower than pure PLA. The reason for
ade of: (a) nonwoven hemp–PLA and (b) alkali treated hemp/PLA.
Fig. 5. SEM images of the upper surface of the alkali hemp/PLA specimens: (a) before and (b) after being subjected to water immersion treatment.
Table 3Tensile properties of PLA/hemp composites.
Sample Tensile strength (MPa) Young’s modulus (GPa) Elongation at break (%)
PLA 41.21 (2.25) 2.91 (0.39) 1.92 (0.26)Hemp/PLA yarn (off-axis angle 0�) 72.75 (6.26) 8.77 (1.44) 1.24 (0.36)Hemp/PLA yarn (off-axis angle 45�) 34.75 (4.63) 4.62 (1.04) 0.66 (0.18)Hemp/PLA yarn (off-axis angle 90�) 22.01 (3.38) 3.70 (0.65) 0.53 (0.11)Hemp/PLA nonwoven 53.63 (1.22) 5.60 (0.95) 1.10 (0.30)Alkali hemp/PLA yarn (off-axis angle 0�) 77.08 (2.60) 10.27 (1.36) 1.58 (0.26)
Note: Data in table are mean (SD).
Fig. 6. Flexural properties of PLA/hemp composites.
6 B. Baghaei et al. / Composites: Part A 61 (2014) 1–12
this may be attributed to the fact that the on-axis properties arestrongly dependent on fibre properties, and that off-axis propertiesare strongly dependent on matrix properties, particularly on theirstiffness and load-carrying ability, which are typically related totheir porosity content. For example, when a composite with porousmatrices were tested off-axis, relatively weak strengths were ob-served because the porous matrix could not carry significant loadcompared to the pure matrix which has low porosity [34]. In thecase of treated hemp/PLA yarn composites, the stronger interfaceformed due to increased potential hydrogen bonding [7]. The high-est strength values were reached by treated hemp/PLA yarn withfibre orientation angle 0� with 77.1 MPa, followed by untreatedhemp/PLA yarn 0�, PLA/hemp nonwoven, PLA/hemp with fibre ori-entation angle and hemp/PLA yarn with fibre orientation angle 90�.As can be seen in Table 3, the hemp fibres evidently improve thetensile modulus, strength and reduce the elongation at break ofthe PLA. The decrease in elongation at break for the compositesis due to the destruction of the structural integrity of the PLA ma-trix, due to the loading of the natural fibre, and which leads to fas-ter fracture than for a pure matrix [35].
3.5. Flexural properties
The results of the flexural test are shown in Fig. 6. As can beseen, the flexural modulus of the composites is higher than thatof neat PLA, which is especially evident for the treated hemp/PLAyarn with a flexural modulus of 7.1 GPa. It is also obvious that com-posites made from PLA/hemp yarn (0� direction) showed an evi-dent improvement in flexural modulus over the compositesproduced from PLA/hemp nonwoven. However, the compositesfrom nonwoven exhibited the optimum tensile and flexuralstrength of 52.3 and 70.9 MPa, respectively. The reason for thereduction of mechanical properties could be the fact that all fibreswith off-axis orientation failed at the particular point, and cannot
contribute furtherer in the composite failure. So, the remainingnumber of on-axis (0� direction) fibres almost exhibited more than50% of the mechanical properties of uniaxial composites [36,37].The flexural properties of unidirectional composites followed thereduction trend in their value when the fibre orientation changedfrom 0� to the off-axial direction. The lowest value of flexuralstrength was observed for the composites with fibre orientationangle 90�. Flexural strength and modulus value were 36.1 MPaand 4.5 GPa, respectively. Alkali treatments enhanced the flexuralstrength and modulus of the composites, which is consistent withother studies [38–40]. The superior flexural properties of the trea-ted fibre composites can be attributed to the greater interfacialbonding between treated hemp fibres and PLA [41,42]. These re-sults are consistent with the tensile properties described previ-ously. Moreover, our research work found the highest flexural
B. Baghaei et al. / Composites: Part A 61 (2014) 1–12 7
strength compared with the tensile strength for different fibre ori-entation. The main reason could be that the volume of materialbeing subjected to the maximum stress is smaller for flexural testthan for the tensile. So, the presence of the critical defects is muchlower than for tensile test [37,43,44].
Fig. 7. Impact strength of the composites relative to their proportion of fibre mass.
Fig. 8. DMTA analysis of PLA reinforced with hemp fibres of various orienta
3.6. Impact resistance
The results of the Charpy impact test are shown in Fig. 7. For theneat PLA matrix, impact strength of 11.5 kJ/m2 was measured. Asignificant reinforcement effect was determined for the treatedPLA/hemp yarn composite with an impact value of 18.8 kJ/m2
and for untreated PLA/hemp yarn composite (0�) with the valueof 16.0 kJ/m2, while the value of the PLA/hemp nonwoven compos-ites (9.7 kJ/m2) was lower than that of the neat PLA matrix which issimilar to flexural strength analysis. Further increment of fibres an-gle caused a decrease in impact strength of the composites, whichis likely to be due to decreased dissipation of energy by less fibrepull-out. For an equivalent amount of hemp fibres (30 mass%),the impact strength of the nonwoven composites was 128% and237% higher than that of the yarn composites with 45� and 90�off-axis. Similar to the tensile properties, it can be observed thatthe alkali treatments enhanced the impact strength. This findingis in agreement with other studies [38–40,45]. In the case of alkalitreated fibres, the dissipation of energy by fibre pull-out is muchless, and debonding occurred followed by fibre breakage ratherthan by interfacial debonding usually associated with high energyabsorption [37,46]. Moreover, the increased PLA crystallinity of the
ion and type. (a) storage modulus vs. temperature and (b) tand curves.
8 B. Baghaei et al. / Composites: Part A 61 (2014) 1–12
alkali treated hemp fibre composites compared with untreatedhemp fibre composites could be another factor leading to increasedimpact strengths [47,48]. The impact resistance of the nonwovencomposites was lower than that of neat PLA. The effect of fibrereinforcement on the impact strength of composites is more com-plicated than bending and tensile strength. The impact strength isattributed to the energy consumption during failure. The additionof fibres probably creates regions of stress concentration, requiringless energy to initiate a crack. In the case of yarn composites, be-cause the reinforcing fibres are in the form of yarn and can act aslong fibres, more energy consumption occurred during fibre pull-out, as long as no breakage of fibre occurred. When longer fibresare introduced into the PLA matrix, the impact strength greatly in-creases due to the frequent occurrence of long fibre pull-outs [40].
3.7. Dynamic mechanical thermal testing
Fig. 8a and b shows how fibre treatment and orientation influ-enced the storage modulus and damping factor (tand) of PLA andits respective composites. The storage modulus (at 30 �C) increasedfrom 2.5 GPa for neat PLA up to 6.5 GPa for treated hemp/PLA, fol-lowed by 4.9 GPa for untreated hemp/PLA and 3.9 GPa PLA/hempnonwoven composite (Fig. 8a). As expected the storage modulusof the composites showed remarkable dependence on fibre orien-tation. The storage modulus was highest for the hemp/PLA (0) yarncomposite, followed by the hemp/PLA (45) yarn composite and thehemp/PLA (90) yarn composite. The increase in the stiffness of fi-bre-containing samples, revealing effective stress transfer fromthe fibre to the matrix at the interface, can be interpreted as goodadhesion between the fibres and the matrices [7,30,49,50]. The in-crease in storage modulus is more prominent in the glassy region(below Tg). The sharp decrease in the storage modulus around
Table 4Thermo-mechanical and crystallization parameters used for the estimation of the constra
Sample XDSC % based on DSC Tand at Tg based
PLA 7.50 1.69Hemp/PLA nonwoven 14.70 0.49Hemp/PLA (90) yarn 14.70 0.46Hemp/PLA (45) yarn 14.70 0.18Hemp/PLA (0) yarn 14.70 0.13Treated hemp/PLA yarn 16.90 0.10
Fig. 9. DSC curves for neat PLA
57–61 �C corresponds to the a-relaxation of the amorphous re-gions in PLA [24]. This fall in modulus at higher temperature espe-cially for the composites can be associated with the easyinterlaminar failure of composites at higher temperature, the chainmobility of matrix and thermal expansion taking place in the ma-trix resulting in reduced intermolecular forces [51,52]. The storagemodulus started to increase again around 100 �C, which is the re-sult of the cold crystallization typical of PLA [53], and which wasalso observed in the DSC results. The decrease in modulus around140 �C indicates the softening of the sample before the onset ofmelting. The dampening, or tand, is the ratio between the lossmodulus and the storage modulus and gives information aboutthe internal friction of the material. The peak of tand curve inthe glass transition region is the most dominant feature, corre-sponding to high damping due to initiation of motion in long seg-ments of the main polymer chain [54]. For a composite, themolecular motion in the interface will contribute to the damping.The dampening will consequently give information about theadhesion of the interface. A larger area under the a-relaxation peakin the tand curves of a polymer indicates that the molecular chainsexhibit a higher degree of mobility i.e. better damping properties[55]. The area under this peak for PLA composites (Fig. 8b) seemsto be smaller especially for alkali treated hemp fibre composite.A possible explanation is that there is a strong interaction betweenthe fibre and the matrix. The fibre/matrix interfacial adhesion canbe indirectly quantified by estimating the damping term as it is atrue indicator of the molecular motions in a material. When a com-posite material is subjected to deformation, the deformation en-ergy is dissipated mainly in the matrix and at the interface. Ifmatrix, fibre volume fraction and fibre orientation are identical,then the damping term can be used to assess the interfacial prop-
ined region.
on DMA Wc according to Eq. (4) C % according to Eq. (3)
0.84 7.500.60 33.440.59 34.980.36 60.500.30 67.370.24 73.39
and PLA/hemp composites.
B. Baghaei et al. / Composites: Part A 61 (2014) 1–12 9
erties between fibre and matrix. For a weak interface, more energyis dissipated during testing [56,57].
The peak point values of tand curves of different samples werecompared and ordered as neat PLA, composites made by hemp/PLAnonwoven, hemp/PLA (90) yarn, hemp/PLA (45) yarn, hemp/PLA(0) yarn and alkali treated hemp/PLA yarn. The height reduction
Table 5Calorimetric data for PLA/hemp composites for the first heating run (10 �C/min).
Sample DHcc (J/g) DHm (J/g)
PLA 28.9 35.9Untreated hemp/PLA 13.17 22.73Treated hemp/PLA 19.3 30.3
IIImmpprriinnttss ooffffiibbrreess
PPuulllleedd--oouuttffiibbrreess
(a) (
(c) (
(e)
Fig. 10. SEM images (at different magnifications) of tensile fracture surfaces of PLA/hemPLA (90), (d) nonwoven hemp/PLA and (e) alkali hemp/PLA.
for the tand peak for the composites shows that there is a reduc-tion in the amount of the mobile polymer chains during the glasstransition. The magnitude of tand values is also seen to increasein the case of composite from nonwoven compared to the compos-ites from yarn. Actually, the storage modulus favours long fibres(large l/d), and damping favours short fibres (small l/d). It is also
XDSC (%) Tg (�C) TC (�C) Tm (�C)
7.5 59.9 109.9 167.414.7 61.3 110.5 167.516.9 61.8 111.8 167.9
IImmpprriinnttss ooffffiibbrreess
VVooiiddss
b)
d)
p composite made from: (a) hemp/PLA (0) yarn, (b) hemp/PLA (45) yarn, (c) hemp/
10 B. Baghaei et al. / Composites: Part A 61 (2014) 1–12
interesting to note that, for high aspect ratios l/d, damping for ran-domly oriented short fibre composites is higher than that of eitheraligned short fibre or continuous fibre composites [58–60]. In addi-tion, material and structural integrity determined by defects, suchas microstructure (voids, impurities and imperfections in resin/fi-bre bonding or debonding) are the points of stress concentrationwhich increase the damping [61]. Higher damping of compositefrom nonwoven compared to composite from yarn could be be-cause of its higher porosity. Damping properties of compositesfrom yarn were improved by increasing fibre off-axis angle whichis consistent with Gibson et al. study [58]. They found that the the-oretical optimum damping property of discontinuous aligned shortfibre composites when the axial load is parallel to the fibre direc-tion. Our experimental investigations indicated that compositewith fibre off-axis 90� showed better damping than compositewith 45� and 0. The glass transition temperature is often recordedat the maximum of the tand. Using this method of determining theglass transition temperature, the Tg seems to increase (by a few de-grees) as hemp fibres are added to PLA. This shows that the poly-mer relaxation is delayed in the presence of fibres which restrictthe segmental motion of the PLA polymer chains due to increasedcrystallinity [30].
DMA measurements can be used to detect changes in themolecular mobility of the polymer segments in the vicinity ofhemp fibres. The volume of the constrained polymer regions inthe PLA-based composites has been estimated from the dynamicmechanical measurements. The volume fraction of the constrainedregion, C is given by Eqs. (3) and (4) [62,63],
C ¼ 1�Wc
W0ð1� C0Þ ð3Þ
W ¼ p tan dp tan dþ 1
ð4Þ
where Wc and W0 denote energy loss ratio of the composite and thepure polymer at Tg, and C0 the degree of crystallinity for the purePLA (Eq. (2)). The results are presented in Table 4. The addition ofhemp fibres brings changes in the mobility of polymer matrix andit has been observed that the maximum restriction in segmentalmobility is obtained with treated hemp fibres. This may be due tothe strong intercalation of the polymer matrix into the treated fi-bres [64].
3.8. Differential scanning calorimetry
The DSC thermograms of neat PLA, untreated and alkali treatedhemp fibre reinforced composites are given in Fig. 9 and the corre-sponding transition temperatures are tabulated in Table 5. The dataindicate that the crystallinity and Tc of PLA was found to increase as aresult of the presence of hemp, since the fillers acted as nucleatingagents for the crystallisation of the polymers [24,65]. Also, it isapparent that the crystallinity of PLA in the treated hemp/PLA com-posites increased compared with that of the untreated hemp/PLAcomposites. This could be due to the fact that after alkali treatment,the impurities including wax and pectin were removed from the fi-bres, which in turn increased the number of nucleating sites (i.e.crystalline portion of cellulose) of the fibres [10,65]. The crystalliza-tion peak of composite is found to become large with addition oftreated fibre which that peak was small in the case of neat PLA. Itis shown that the surface treatment increases the degree of regular-ity of the molecular chain [38]. The composites exhibit an increase inTg compared with that of neat PLA. The results obtained are in agree-ment with those of DMA. The melting behaviour of PLA is also influ-enced by the presence of untreated and treated hemp fibres. Theneat PLA has a single melting point around 167 �C divided into twosmall peaks (around 161 and 167 �C) in the case of untreated and
treated hemp, indicating the formation of two different crystal types[66].
3.9. Fractography analysis of damaged specimens
It was seen on the tensile specimens that the off-axis samplestend to break in the inter-fibre cleavage region and extend towardthe fibre direction. Thus, most of fibre breakages happened alongthe fibre orientation direction.
The morphologies of the fractured surfaces of tensile specimenswere investigated using SEM (Fig. 10). Debonding between fibreand matrix as well as fibre imprinting was found for compositesamples with 45� and 90� orientation arrangements (Fig. 10b andc). Furthermore, it can be seen from Fig. 10d that the compositesfabricated from nonwoven have higher porosity content insidethe composites because of less close and compact packing of hempfibres in the PLA matrix compared with the composite made fromPLA/hemp. From the surface images, there is a difference in theadhesion between PLA matrix and hemp fibre. Fig. 10a and e showsthat untreated hemp reinforced composites had more fibres pulledout with relatively clean fibre surface which is an indication of lowadhesion between them; however, the treated hemp fibres werecoated by layers of the PLA matrix that significantly reduce thegaps between them.
4. Conclusions
Unidirectional hemp reinforced PLA composites were producedfrom PLA/hemp yarn using compression moulding. We investigatedthe influences of different orientation especially off-axial directionof hemp fibre as well as alkali treatment; focusing on the determin-ing void%, water absorption, mechanical and thermo-mechanicalproperties. The properties of PLA composites were significantly im-proved compared to the neat PLA matrix. The alkali treated hemp/PLA yarn gave maximum improvement in mechanical propertiescompared to untreated hemp/PLA yarn. The best overall propertieswere achieved with aligned alkali treated hemp/PLA yarn leadingto a tensile strength of 77.1 MPa, Young’s modulus of 10.3 GPa, flex-ural strength of 100.9 MPa, flexural modulus of 7.1 GPa, and impactstrength of 18.8 kJ/m2. The results showed that the mechanicalproperties of the composites were highly affected by the fibre direc-tion. Tensile, flexural, and impact values of the composites showedthe decreasing trend for off-axial composites compared to 0� axial-oriented composite. Furthermore, the nonwoven PLA/hemp com-posites had an evident improvement in mechanical properties overthe composites produced from yarn with off-axis angle of 45�and90�. The thermo-mechanical tests showed that composites contain-ing alkali treated hemp fibres had improved storage modulus due toenhanced interfacial bonding. The damping properties were highlyaffected by the testing direction; it was increased at an off-axis angleof 45�and 90�. Composites made from PLA/hemp nonwoven showedan evident improvement in viscoelactic properties over the compos-ites produced from yarn with off-axis angle of 45�and 90�. The PLA inthe composites had high orientation degree and crystallinity whichwas attributed to effective heterogeneous nucleation induced byhemp fibres, however, the degree of crystallinity of alkali treatedhemp/PLA composite was higher. From water absorption test re-sults, it was found that higher temperature generally increased theWG% of the neat PLA and all of the composites, as well as shorteningthe saturation time. At room temperature, neat PLA had the lowestWG% value followed by PLA/hemp nonwoven, untreated hemp/PLAyarn and alkali treated hemp/PLA yarn composites. Future work willconcentrate on efforts to evaluate the biodegradability of thesedeveloping and promising composites.
B. Baghaei et al. / Composites: Part A 61 (2014) 1–12 11
Acknowledgment
The authors thank Tommy Martinsson, Swedish School of Tex-tiles, University of Borås (Sweden) for his assistance in preparinghybrid yarns.
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Paper III
Composites: Part A 76 (2015) 154–161
Contents lists available at ScienceDirect
Composites: Part A
journal homepage: www.elsevier .com/locate /composi tesa
Characterization of thermoplastic natural fibre composites made fromwoven hybrid yarn prepregs with different weave pattern
http://dx.doi.org/10.1016/j.compositesa.2015.05.0291359-835X/� 2015 Elsevier Ltd. All rights reserved.
⇑ Corresponding author. Tel.: +46 33 435 4497; fax: +46 33 435 4008.E-mail address: [email protected] (M. Skrifvars).
Behnaz Baghaei, Mikael Skrifvars ⇑, Lena BerglinSwedish Centre for Resource Recovery, Academy for Textile, Engineering and Business, University of Borås, SE-501 90 Borås, Sweden
a r t i c l e i n f o a b s t r a c t
Article history:Received 25 February 2015Received in revised form 13 May 2015Accepted 29 May 2015Available online 6 June 2015
Keywords:A. Fabrics/textilesB. Mechanical propertiesE. WeavingE. Compression moulding
This paper focuses on the effect of weave structure on mechanical behaviour and moisture absorption ofthe PLA/hemp woven fabric composites made by compression moulding. The unidirectional woven fabricprepregs were made from PLA (warp) and PLA/hemp wrapped-spun hybrid yarn (weft) with two differentweave patterns; 8-harness satin and basket. Unidirectional composites with 30 mass% hemp contentwere fabricated from these prepregs, and compared to winded PLA/hemp hybrid yarn laminates withsame composition. The composite from the satin fabric had significantly lowest porosities and bestmechanical properties compared to the composite made from the winded hybrid yarn and basket fabric.The tensile, flexural, and impact strength were 88 MPa, 113.64 MPa, and 24.24 kJ/m2, respectively. Theeffect of weave pattern on water absorption is significant. Although the composite from hybrid yarn lam-inate has larger water absorption than that of the pure PLA, it exhibits lower moisture absorption thanboth weaves.
� 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Currently, the increasing global environmental awareness hascaused a paradigm shift toward producing environmental friendlymaterials. Therefore, in recent years, annually renewable naturalfibres such as hemp, flax, jute, kenaf and sisal have attracted moreinterest as reinforcements for both thermosetting and thermoplas-tic polymer composites [1,2].
One of the advantages of using thermoplastic composite is itsshort cycle production process which makes it possible for exten-sive application in several industries including automobile manu-facture, construction of aircraft and ship components and otherhigh volume products. In contrast to thermosetting composites,thermoplastic composites have better impact strength, easier recy-clability and in the case of PLA, even biodegradability. On the otherhand, due to the high melt viscosity of the thermoplastic matrix,the fibre impregnation is more complex than for thermosets.Different impregnation methods have been proposed and appliedin order to solve this problem. One promising technique is the fab-rication of hybrid yarns being based on the principle of mixingreinforcing fibres and thermoplastic matrix fibres homogeneouslyduring yarn spinning. Hybrid yarns are non-uniform yarns, whichcontain components with different properties: a reinforcing
component and a thermoplastic component [3–6]. Recently, hybridyarn manufacturing has been developed for rapid and cost effectivemethod of continuous fibre reinforced thermoplastic composites.Hybrid yarns offer an ideal opportunity to achieve short cycletimes due to the very short flow paths of the viscous thermoplasticmelt [7]. To achieve high mechanical performance of thermoplasticcomposites the homogeneous fibre/matrix distribution is neces-sary, which can be achieved with hybrid yarns.
Handling of hybrid yarn for making unidirectional composite isproblematic. Hybrid yarns can be woven into a wide variety of con-formable and drapable fabrics. Textile production technologymakes it possible to use hybrid yarns as the basis for obtainingan intermediate prepreg. With subsequent processing under pres-sure and at elevated temperature, the melting thermoplastic fibrecomponent is converted to a polymer matrix filling the mouldand impregnating the reinforcing fibres. This technology offersnew possibilities for a shortened production process for thermo-plastic composites and articles made from them [8–10].
Fabric weaves are using in several applications in fibre rein-forced polymers due to their good mechanical properties, such asstiffness, strength and dimensional stability. With the weavingtechnology, it is possible to fabricate high density woven struc-tures with load-oriented fibre positioning [11–13].
The strength and stiffness of fabric reinforced composites areinfluenced by not only the matrix and yarns properties, but alsostructural parameters of materials such as fabric count and weave
B. Baghaei et al. / Composites: Part A 76 (2015) 154–161 155
pattern. The fabric count determines the number of weft and warpyarns per cm, whereas the weave pattern specifies how the warpand the weft yarns are interlaced. Typical weave patterns are plain,twill, basket and satin [14].
Fabrics which are produced by weaving a large number of thickyarns as warp and a few numbers of thin yarns as weft are calledunidirectional and are used in unidirectional composites. Thisweave type provides materials with good processability, high stiff-ness and strength in the warp direction, which is specific for unidi-rectional composites [15].
Recently different studies have reported limited improvementin the mechanical properties of the composites mostly madedirectly from hybrid yarns. There are limited studies on the com-posites manufactured from fabric using hybrid yarn especially bio-composite [5,7,16–18].
The main focus of this study was to develop and characterizereinforcement fabrics made of novel aligned hybrid yarn and usethis fabric as a prepreg for manufacturing composite. In this study,unidirectional hemp-fibre/PLA composites were made by compres-sion moulding of fabrics composed of hybrid yarns [19], and fur-ther subjected to different tests in order to investigate the effectsof the pattern style of the woven fabrics (satin and basket patterns)on the mechanical properties.
2. Materials and methods
2.1. Materials
The studied composites consisted of a polylactide matrix rein-forced with hemp fibres. The components were prepared at aweight ratio of 70/30 (polymer/fibre).
NatureWorks™ PLA Polymer 6202D by the company CargillDow LLC (Minnetonka, USA) was used as the matrix material.This thermoplastic has a glass transition temperature (Tg) of60–65 �C, a melting point (Tm) of 160–170 �C and a density of1.24 g/cm3. The used PLA was in filament and fibre form. A 18tex PLA multifilament yarn and Ingeo™ staple fibre were providedby Trevira GmbH (Hattersheim, Germany). The staple fibres had afineness of 1.7 dtex and a mean fibre length of 38 mm. Hemp staplefibres (genus species Cannabis sativa L.), supplied by Hempage AG(Adelsdorf, Germany), with an average diameter of 20–40 lmand a mean fibre length of 30 mm were used as reinforcement.
Fig. 1. Structure of wrap spun hybrid yarn.
2.2. Methods
2.2.1. Hybrid yarn manufactureA wrap spinning method was used to make a hybrid yarn struc-
ture with the reinforcing hemp fibres straight and parallel to theyarn axis [5]. Within the framework of this study, the hybrid yarn(nominal count of 550 tex) consists of hemp staple fibres used as areinforcement and PLA staple fibre used as the thermoplastic com-ponent. This hybrid yarn was manufactured with wrap spinningtechnology using a laboratory spinning machine from MesdanS.p.A. (Brescia – Italy) and a laboratory yarn twist machine fromDirecTwist, AGTEKS CO. Ltd. (Istanbul, Turkey). The PLA-hemp fibreweight ratio was 30:70 [19]. A very uniform and aligned rovingwas produced in a carding and roving frame. No twist was addedto the roving during the drawing, this is important, as a twist willhave a negative influence on composite strength. The obtained rov-ing had however poor integrity, it was too weak to be able to becollected alone without causing yarn breakage in the rovingmachine. Therefore, PLA filaments were used as a processing car-rier for the PLA/hemp staple fibres in the roving in the final man-ufacturing step. The roving was further stabilised by using acontinuous PLA filament thread, which was wounded around the
roving strand (wrapping intensity of 200 turns/m) in the twistingmachine. This imparted strength to the strand to prevent it fallingapart and gave a better protection for the reinforcing staple fibresduring further textile processing, such as weaving [20] or making aprepreg, The hybrid wrap yarn thus consists of two components,one twist-free PLA/hemp staple fibres component in the yarn core,and a filament wound around the core. The yarn structure is shownin Fig. 1.
2.2.2. UD woven fabrics from hemp/PLA hybrid yarnsIn order to manufacture samples of thermoplastic composites,
we made woven unidirectional (UD) fabrics from hybrid yarnsand PLA filaments on a hand loom weaving machine. The obtainedwoven fabrics had the PLA/hemp hybrid yarn in the transversedirection of manufacture of the fabric (weft direction) and weresupported by the 18 tex PLA filaments as the warp yarns in the lon-gitudinal direction (8 threads per cm). The fabrics were woven intwo different weave patterns; 8-harness satin (4 threads per cm)and basket weave (3 threads per cm) as seen in Fig. 2. The con-structional particulars of fabric used in this study are presentedin Table 1. These fabrics have similar characteristics as an unidirec-tional fabric in the weft direction, as the thin warp yarn reducesthe crimp considerably. The woven fabrics obtained can be consid-ered as prepregs for subsequent processing to make composites bycompression moulding.
2.2.3. Composite manufacturing and testingMechanical tests for tensile strength, flexural strength and
impact behaviour were conducted on UD composite laminates asa preliminary characterization of the fabrics made from hempand PLA in various architecture patterns; satin (8HS) and basket(2/2). Each laminate had 6 layers of 20 � 20 cm2 prepreg to achievea composite thickness of about 3–4 mm. The laid up prepregs werefirst put in a vacuum chamber (0.9 mbar; 70 �C) for at least 18 h, todry before the compression moulding. The UD composite lami-nates were consolidated at a temperature of 195 �C and a pressureof 1.7 MPa for 15 min in a laboratory hydraulic compressionmoulding machine from Rondol Technology Ltd. (Staffordshire,UK). These parameters result in optimal impregnation and verylow voids in the composite. Neat unreinforced PLA sheets to be
Fig. 2. Schematic diagram of (a) 8-harness satin and (b) basket weave patterns. (For interpretation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)
Table 1Details of constructional particulars of fabrics.
Sample Weave style Weight (GSM) Weft yarn Warp yarn Wrap count (ends/cm) Weft count (ends/cm)
1 Satin 1⁄8 344.0 (22.9) Wrapped spun hemp/PLA hybrid yarn (550 tex) 18 tex PLA 8 42 Basket 482.0 (48.9) Wrapped spun hemp/PLA hybrid yarn (550 tex) 18 tex LA 8 3
156 B. Baghaei et al. / Composites: Part A 76 (2015) 154–161
used as reference material were made by compression mouldingunder the same processing conditions of the same PLA fibres andfilaments formed to a similar prepreg. Before performing themechanical testing, the specimens were conditioned for at least24 h at 23 �C and 50% relative humidity according to DIN EN ISO291. The tensile and flexural strength tests on the UD compositesheets were done using an universal H10KT testing machineequipped with a mechanical extensometer (model 100R long travelextensometer) supplied by Tinius Olsen Ltd. (Salford, UK) accord-ing to ISO 527 and ISO 14125. The impact test was carried out withthe help of a pendulum arm-type impact tester, which works onthe principle of the Charpy Impact Test. Standard method of testingwas applied as stated in the ISO 179. The specimens used in themechanical testing were cut from the laminates using laser cutting,and they were tested in the weft direction.
2.2.4. Composite density and porosityFibre volume fraction was measured using the density buoy-
ancy method in which the mass of sample (coated with paraffin)is recorded in air and ethanol. Therefore, having obtained compos-ite densities and knowing the densities of the constituent parts i.e.the hemp fibre and PLA resin, the percentage content of the fibresand porosity were calculated [3].
2.2.5. Water absorption testThe effect of water absorption on hemp fibre reinforced com-
posites was calculated using ASTM D570-98. The percentageweight gain of the samples (WG) was calculated by the weight dif-ference between the sample immersed in distilled water (W1) anddry sample (W0), using Eq. (1):
WG ¼ ½ðW1 �W0Þ=W0� � 100% ð1Þ
B. Baghaei et al. / Composites: Part A 76 (2015) 154–161 157
2.2.6. Determination of dynamic mechanical propertiesThe time–temperature dependency of the mechanical proper-
ties was determined by dynamic mechanical thermal analysis,with a DMA Q800 (dual cantilever) supplied by TA Instruments,Newcastle, DE, USA. The temperature range was from 30 to150 �C with a heating rate of 3 �C/min and at frequency of 1 Hzand the sample dimensions were: thickness 2–3 mm, length50 mm, and width 8 mm.
2.2.7. Scanning electron microscopy (SEM)Fractured surfaces of the composites obtained from the tensile
testing were examined using a low-vacuum scanning electronmicroscopy with a Quanta 200 ESEM FEG instrument from FEI,(Oregon, USA) with an operating voltage of 10–12.5 kV.
3. Results and discussion
3.1. Composite porosity
Porosity is one of the most critical issues during the manufac-turing process of composites. The presence of porosities can signif-icantly reduce the tensile, compressive, inter laminar shear andstructural strengths of a composite [21]. In the current study, com-posites were produced from satin PLA/hemp hybrid fabrics andbasket PLA/hemp hybrid fabrics in a nominal mass ratio of 30%hemp reinforcement in order to investigate the effect of differentweave patterns on the properties of the composites.Unidirectional composites previously made from identicalPLA/hemp hybrid yarns, produced by winding the yarn around ametal frame, were used as reference for comparison to the madewoven hybrid fabric based composites [19]. Data for compositesmade from PLA/hemp nonwoven mats from a previous study arealso used as reference in this study in order to show the effect offibre alignment on the porosity content [22]. Table 2 demonstratesthe quality of the produced composites from the current study andcompared to results from the previous studies [19,22]. Both thecomposite density and constituents of the composites were evalu-ated. From the results, it is possible to conclude that the compositemade by unidirectional satin fabric gave significantly lower porosi-ties compared to the winded hybrid yarn laminates, and the valueis in the acceptable range [19,22]. Generally, less than 2% porosityis considered as high quality composite [23].
Creation of voids during composite processing is a function ofseveral construction factors including volume fractions of fibres,fibre distribution, fibre diameter and laminate tightness. Theporosity content for the composite made from satin fabric(0.96%) was clearly lower compared to the composite made frombasket fabric (4.55%) and much lower compare to the compositemade from a nonwoven mat (16.71%) [19,22]. It can probably beattributed to the fact that satin weaves allow individual fibres tobe woven in the closest proximity and can therefore produce fab-rics with a close ‘tight’ weave compared to basket weave. Thetightly woven fabrics are typically the choice to maintaining fibreorientation during the manufacturing process and minimizingresin void size due to the shorter impregnation distance.
Table 2Composition of the fabricated composites calculated from the density measurements.
Sample Density (g/cm3) Fibre mass fraction (%) Fibre vol
PLA 1.2490 (0.0002) 0.0 0.0Satin hemp/PLA fabric 1.2978 (0.0032) 30.0 26.31 (0.Basket hemp/PLA fabric 1.2508 (0.0090) 30.0 25.35 (0.Hemp/PLA yarn laminate 1.2713 (0.0268) 30.0 25.77 (0.Hemp/PLA nonwoven 1.0914 (0.0154) 30.0 21.12 (0.
Note: Data in table are mean (SD).
Therefore, the laminar and crosswise flow of resin along andbetween the fibres, in between the yarns and fabric layers is easier[24,25]. Moreover, basket weave has higher quantity of interlacepoints per unit area compared to satin. Perhaps voids might be pre-sent between the interlace points which are higher in basket sys-tems than those of satin [26]. Therefore, the composite made ofthe satin fabric is of high quality, as shown in Fig. 3a and b, becauseof its lower porosity.
3.2. Water absorption characteristics
Fig. 4 shows apparent weight gain (WG) as a function of time forneat PLA and PLA/hemp reinforced composite samples immersedin water at room temperature (23 �C). All the composites havethe same overall fibre weight fraction (30%). As expected the neatPLA shows the lowest water absorption, which is due to the lack ofhydrophilic fibres. It is seen that the effect of weave pattern is sig-nificant for the composites. Although the composite made from thewinded hybrid yarn prepreg has larger moisture absorption thanthat of the pure PLA, it exhibits lower moisture absorption thanboth weaves (satin and basket). Actually the matrix pockets andyarn undulation within the weaves provides an easier diffusionpath for the moisture to travel within the weave, compared to acomposite composed of unidirectional yarns. If the volume of thewavy region is larger, the WG of a composite would be higher[27,28]. It is observed that basket weave has higher WG than satin.
3.3. Tensile properties
Table 3 shows the tensile properties of unidirectional (hempfibres in the longitudinal direction) hemp/PLA composites. It canbe seen that the measured values in the case of the winded yarnlaminate composite is significantly lower compared to the wovenfabric composites. Actually tightly woven fabrics are usually thechoice to maintain fibre orientation during the fabrication process,which was not possible in the winded yarn laminate. The tensilemodulus and strength of composite for basket 2/2 is lower com-pared to satin due to its fabric structure, which makes some partsof the composite weaker. The basket weave has a higher crimpcompared to the satin weave due to its more interlacement. (seeFig. 2) Less crimp will result in greater strength on the compositesas its free inter-fibres contribute toward the force. After the weav-ing process of the fabric, contraction occurs which causes floatingof adjacent yarns to join together making a jammed structurewhereas the end point of interlacement creates a gap withoutany reinforcement. This gap will give in the composite a resin richarea, which is the weakest point because of no reinforcementmaterials. The basket 2/2 weave has more number of interlacingsper surface area, which increase the number of stress concentra-tion points and resin rich areas which significantly affect the ten-sile properties. The jammed fabric structure has influence on theproperties of the composites since in this area the resin penetra-tion between the fibres is low. In the case of satin weave each adja-cent floating yarn has a different interlacing point which makes it
ume fraction (%) Matrix volume fraction (%) Porosity volume fraction (%)
100.0 –07) 72.74 (0.18) 0.96 (0.25)18) 70.10 (0.51) 4.55 (0.71)54) 71.25 (1.50) 2.97 (2.04)31) 61.16 (0.9) 16.71 (1.17)
Voids
(b)(a)
Fig. 3. SEM images of the fracture surface of the hemp/PLA specimens from: (a) satin-weave architecture fabric and (b) basket-weave architecture fabric.
0 1 2 3 4 5 6 7 8 9
10
0 1 2 3 4 5 6 7 8 9 10
App
aren
t wei
ght g
ain
(wt%
)
Day
Hemp/PLA yarn laminate
Basket hemp/PLA fabric
Satin hemp/PLA fabric
Neat PLA
Fig. 4. Apparent weight gain against time for the different PLA/hemp composites atroom temperature.
Table 3Tensile properties of PLA-based composites.
Sample Tensile strength(MPa)
Young’s modulus(GPa)
Elongation atbreak (%)
PLA 41.21 (2.25) 2.91 (0.39) 1.92 (0.26)Satin hemp/PLA
fabric88.06 (7.70) 10.23 (2.67) 1.35 (0.30)
Basket hemp/PLAfabric
81.09 (5.62) 9.20 (1.81) 1.58 (0.22)
Hemp/PLA yarnlaminate
72.75 (6.26) 8.77 (1.44) 1.24 (0.36)
Note: Data in table are mean (SD).
0 2 4 6 8 10 12
0 20 40 60 80
100 120 140
Flex
mod
ulus
in G
pa
Flex
stre
ngth
in M
Pa Flex StrengthFlex modulus
Fig. 5. Flexural properties of PLA/hemp composites.
15
20
25
30
35
tren
gth
in k
J/m
2
158 B. Baghaei et al. / Composites: Part A 76 (2015) 154–161
evenly distributed and the even yarns floating distribution withsmall gap makes it free for the resin to distribute evenly [29].
The highest strength values (88.06 ± 7.70 MPa) were registeredfor composites reinforced with hemp/PLA satin fabric, followed byhemp/PLA basket fabric and winded hemp/PLA yarn laminate. Ascan be seen from the results, the tensile strength and modulus ofthe PLA were clearly improved by adding hemp fibres, however;the elongation at break was reduced. The reduction in elongationat break in the composites is mainly due to the destruction ofthe PLA structural integrity by the loading of the natural fibrewhich leads to faster fracture of the PLA matrix [30].
0
5
10
PLA Satin hemp/PLA
fabric
Basket hemp/PLA
fabric
Hemp/PLA yarn laminate
Impa
ct s
Fig. 6. Impact strength of the composites relative to their proportion of fibre mass.
3.4. Flexural properties
Fig. 5 shows the flexural strength of hemp/PLA composites.One-Way ANOVA test showed that mean flexural strength andflexural modulus values of materials investigated were signifi-cantly different (P-value < 0.05). The flexural strength for all com-posites was significantly higher than for the unreinforced
specimens (neat PLA). Flexural modulus was lowest for the neatPLA (4.41 GPa) and highest for the satin composite sample(7.63 GPa), and had the same trend as for the flexural strength.
Different weave pattern has significant influence on the flexuralstrength of the composites. Satin fabric composite exhibited thehighest flexural strength. It is obviously due to the presence ofmore aligned fibres in the axial direction, which enabled it to holdthe flexural stress developed over the surface of the material.Moreover, the satin fabric composite had a less chance for stressconcentration due to lower number of interlacing points [29,31].
3.5. Impact resistance
Fig. 6 shows the impact energy of hemp/PLA yarn reinforcedcomposites for different laminates. The impact properties of the
B. Baghaei et al. / Composites: Part A 76 (2015) 154–161 159
composites followed the same trend as obtained in the flexural andtensile testing. The impact energy was around 24.2, 23.8 and16 kJ/m2 for satin fabric, basket and winded yarn laminatecomposite, respectively. It can be seen from the results that thevariation in impact strength of the composites was not signifi-cantly influenced by the weaving architecture, as the variation ismeagre. The higher impact energies for satin and basket laminatecompared with winded yarn laminate are mainly because of thepresence of a high number of axially oriented fibres since wovenfabric composites provide more balanced properties in the fabricplane than unidirectional composites [32].
3.6. Dynamic mechanical thermal testing
Storage modulus is related to the Young’s Modulus under ten-sile loading. The incorporation of fibres plays significant role onincreasing the stiffness of a composite. Fig. 7a shows the effect of
Fig. 7. DMTA analysis of thermoplastic PLA reinforced with of hemp
weave patterns on the storage modulus values with temperature.From the results, it is clearly seen that addition of fibres hastremendously increased the storage modulus value of the compos-ite when compared with pure PLA. The increase in storage moduluswas attributed by the incorporation of hemp fibres and it is clearlyseen for both types of weave pattern laminates and winded yarnlaminate. This increase is due to good fibre/matrix adhesion andgreat degree of stress transfer at the interface. Composite fromsatin and basket weaves had a better storage modulus of 8.5 and7 GPa compared to the winded yarn laminate composite which isin line with the results from tensile, flexural, and impact tests, thatcomposites from satin and basket weave are superior to windedyarn laminate composite.
Fig. 7b shows the tand value of the composites as the functionof temperature and the glass transition temperature correspondingto the maximum tand peak. The results indicate that the incorpo-ration of fibres in the matrix has considerably reduced the
fibre. (a) Storage modulus vs. temperature; and (b) tand curves.
Fig. 8. SEM images of the fracture surface morphology of composites made of: (a)satin-weave architecture fabric and (b) basket-weave architecture fabric.
160 B. Baghaei et al. / Composites: Part A 76 (2015) 154–161
damping property of the PLA matrix. The tand value for the satinand basket weave types of composites are found to be a little lowerand wider compared to winded yarn laminate which indicates thestress transfer from hemp fibres to PLA matrix occurs easily with-out the failure of matrix [33]. Lower damping property shows thecomposite absorb more amount of the energy when loaded.
The glass transition temperature for satin and basket type ofwoven and yarn laminate composite are found to be higher(71.5–74 �C) compared to pure PLA (68 �C). This indicates that withthe presence of hemp fibres, the polymer relaxation is delayed andsegmental motion of the PLA polymer chains is restricted due toincreased crystallinity and good interaction between the fibreand the polymer matrix [34,35]. According to the previous study[22], the PLA in the composites had high degree of orientationand crystallinity which was attributed to effective heterogeneousnucleation induced by hemp fibres.
3.7. Fractography analysis of damaged specimens
SEM analysis is carried out to investigate the fracture behaviourof the composites (Fig. 8a and b). Figures reveal that the interaction
of polymer matrix (PLA) with hemp fibres is strong since the fibrepull-out is too low. Moreover, there is uniform distribution of uni-directional hemp fibres in the PLA matrix.
4. Conclusions
We report the manufacture of hybrid textile yarns containingreinforcing (hemp) and thermoplastic (PLA) components withintheir structure, the technology for processing them into woven fab-rics (prepregs), and the manufacture of thermoplastic compositesbased on them. The effect of two types of structures for the fabrics,8-harness satin and basket, on amount of porosity, moistureabsorption, mechanical and thermo-mechanical properties of com-posites were studied. The results showed that the unidirectionalhemp/PLA composite made by satin-weave architecture fabric pos-sessed the highest tensile, flexural and impact strength comparedto the composites manufactured with basket weave fabric andwinded hybrid yarn laminate. This improvement in mechanicalstrength was correlated to the decrease in void content and fibremisalignments. This means that satin composite is the strongest,stiffest, and toughest. Mechanical properties of the compositesshowed the decreasing trend for winded hybrid yarn laminatecomposites compared to woven fabric composite. However, mois-ture absorption test did not show a similar trend. Thethermo-mechanical tests showed that composites manufacturedfrom satin and basket weave types had lower damping propertiescompared to yarn laminate which indicates the stress transfer fromthe hemp fibres to PLA matrix occurs easily without the failure ofmatrix. Composite from satin and basket weaves had a better stor-age modulus compared to yarn laminate composite which sup-ported all the results from tensile, flexural, and impact tests.
Acknowledgments
The authors would like to acknowledge Jan Johansson, SwereaIVF, Mölndal, Sweden, for his assistance with the impact testing.
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[9] Thanomsilp C, Hogg PJ. Interlaminar fracture toughness of hybrid compositesbased on commingled yarn fabrics. Compos Sci Technol 2005;65(10):1547–63.
[10] Stolyarov ON, Stolyarov IN, Kryachkova TA, Kravaev PG. Hybrid textile yarnsand thermoplastic composites based on them. Fibre Chem 2013;45(4):217–20.
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[12] Großmann K, Mühl A, Löser M, Cherif C, Hoffmann G, Torun AR. New solutionsfor the manufacturing of spacer preforms for thermoplastic textile-reinforcedlightweight structures. Prod Eng 2010;4(6):589–97.
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[14] Vasiliev V, Morozov E. Mechanics and analysis of composite materials. Oxford(UK): Elsevier Science; 2001.
[15] Morozov E. Mechanics and analysis of fabric composites and structures.AUTEX Res J 2004;4(2):60–71.
[16] Schafer J, Stolyarov O, Ali R, Greb C, Seide G, Gries T. Process-structurerelationship of carbon/polyphenylene sulfide commingled hybrid yarns usedfor thermoplastic composites. J Ind Text 2015.
[17] Kravaev P, Stolyarov O, Seide G, Gries T. Influence of process parameters onfilament distribution and blending quality in commingled yarns used forthermoplastic composites. J Thermoplast Compos Mater 2014;27(3):350–63.
[18] Prabhakaran RTD, Toftegaard H. Environmental effect on the mechanicalproperties of commingled-yarn-based carbon fibre/polyamide 6 composites. JCompos Mater 2014;48(21):2551–65.
[19] Baghaei B, Skrifvars M, Salehi M, Bashir T, Rissanen M, Nousiainen P. Novelaligned hemp fibre reinforcement for structural biocomposites: porosity,water absorption, mechanical performances and viscoelastic behaviour.Compos Part A: Appl Sci Manuf 2014;61:1–12.
[20] Alagirusamy R, Fangueiro R, Ogale V, Padaki N. Hybrid yarns and textilepreforming for thermoplastic composites. Text Prog 2006;38(4):1–71.
[21] Mallick PK. Fiber reinforced composites materials, manufacturing and design.3rd ed. CRC Press; 2007.
[22] Baghaei B, Skrifvars M, Rissanen M, Ramamoorthy SK. Mechanical and thermalcharacterization of compression moulded polylactic acid natural fibercomposites reinforced with hemp and lyocell fibers. J Appl Polym Sci2014;131(15). n/a–n/a.
[23] Prabhakaran RT, Andersen T, Markussen CM, Madsen B, Lilholt H. Tensile andcompression properties of hybrid composites – a comparative study. In: The19th international conference on composite materials; 2013. p. 1029–35.
[24] Indu Shekar R, Kotresh TM, Krishna Prasad AS, Damodhara Rao PM,Ananthakrishnan T, Satheesh Kumar MN, et al. Hybrid fabrics for structuralcomposites. J Ind Text 2011;41(1):70–103.
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[26] Ismail H, Nasir M, Mariatti M. Influence of different woven geometry and plyeffect in woven thermoplastic composite behaviour-Part 2. Int J Polym Mater2000;47(2):499–512.
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[29] Saiman MP, Wahab MS, Wahit MU. The effect of fabric weave on the tensilestrength of woven Kenaf reinforced unsaturated polyester composite. In:Ahmad MR, Yahya MF, editors. Proceedings of the International Colloquium inTextile Engineering, Fashion, Apparel and Design 2014 (ICTEFAD2014). Singapore: Springer; 2014. p. 25–9.
[30] Nam TH, Ogihara S, Tung NH, Kobayashi S. Effect of alkali treatment oninterfacial and mechanical properties of coir fiber reinforced poly(butylenesuccinate) biodegradable composites. Composites Part B 2011;42(6):1648–56.
[31] Kannan TG, Wu CM, Cheng KB. Open hole flexural and izod impact strength ofunidirectional flax yarn reinforced polypropylene composites as a function oflaminate lay-up. Polym Compos 2013;34(11):1912–20.
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Paper IV
Mechanical and Thermal Characterization of Compression MouldedPolylactic Acid Natural Fiber Composites Reinforced with Hemp andLyocell Fibers
Behnaz Baghaei,1 Mikael Skrifvars,1 Marja Rissanen,2 Sunil Kumar Ramamoorthy1
1School of Engineering, University of Borås, SE-501 90 Borås, Sweden2Department of Materials Science, Tampere University of Technology, FI-33101 Tampere, FinlandCorrespondence to: M. Skrifvars (E - mail: [email protected])
ABSTRACT: This research evaluates the effects of PLA/PP blend ratio and Lyocell/hemp mixture ratio on the morphology, water
absorption, mechanical and thermal properties of PLA-based composites. The composites were fabricated with 30 mass % hemp
using compression moulding. As a reference composites made from PP were also studied. Combining of hemp and Lyocell in PLA
composite leads to the reduction of moisture absorption and can improve the impact, tensile, flexural properties when compared
with PLA/hemp. Composite based on the PLA/PP blend-matrix could not improve the tensile and flexural properties compared with
PLA/hemp, however; the lighter composite with better impact properties was obtained. The crystallization temperature of the PLA-
PP/hemp increased compared with pure PLA. This result was also confirmed by the SEM micrographs. The moisture absorption of
PLA-PP/hemp was higher than PLA/hemp. Based on theoretical analysis of DMTA data, there was favorable adhesion in all compo-
sites. VC 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40534.
KEYWORDS: biopolymers and renewable polymers; composites; fibers; mechanical properties; molding
Received 18 December 2013; accepted 31 January 2014DOI: 10.1002/app.40534
INTRODUCTION
Poly(lactic acid) (PLA) is a biodegradable polymer produced
from lactic acid, which is made by fermentation of carbohydrate
sources, such as corn. It is considered as one of the most prom-
ising biomedical and packing materials with a broad market
prospect.1 Many of the PLA properties are compared to those
polyethylene (PE), polypropylene (PP), polystyrene (PS), and
polyethylene terephthalate (PET) (such as stiffness, tensile
strength, and gas permeability), turning PLA into a potential
substitutes to petroleum-based products.2 However, to date the
use of PLA in the engineering field is limited due to hydropho-
bicity, brittleness, low impact resistance, high cost as well as to
sensitivity to the temperature. To enhance the thermal stability
and mechanical properties of PLA, chemical modification, phys-
ical blending with some polymers such as poly(glycolic acid),
poly(hydroxyalkanoates), and poly(caprolactone), and using
natural fiber reinforcements can be done.1,3,4
Several researches on PLA blends with other polymers have
been carried out in order to modify the properties for PLA5–10
or to reduce the cost.11,12 Because PLA is biodegradable, blend-
ing of PLA with nonbiodegradable polymers, such as PP, HDPE,
LDPE, PS, and PET can improve the resistance of PLA to
hydrolysis, also the degradability of conventional polymers can
be improved by blending with PLA.13,14 PLA and PP are two
quite different candidate matrix materials with different advan-
tages, where PLA is a renewable biopolymer, and PP is a more
hydrophobic low-cost commodity thermoplastic and has high
toughness, and low density.15,16 In addition, PLA and PP have
similar processing temperatures (200–230�C).17 A brief attempt
has been made to blend and produce products such as fibers
from PLA and PP.14,18 However, there are limited numbers of
studies on the mechanical properties of the composites pro-
duced from PLA/PP bend.19
Low weight, low cost, recyclability, and biodegradability are
advantages of natural fibers compared to synthetic com-
pounds.20,21 These are also renewable and have relatively high
strength and stiffness. The reinforcement of PLA with lignocel-
lulosic fibers seems to be a viable alternative to increase their
mechanical performance and to preserve the environmentally
friendly character of the outcome.22
Hemp fibers can be considered an appropriate choice for rein-
forcing polymer composites due to their high stiffness, strength,
and aspect ratio.23 They also have an extremely high fiber yield
per unit density, and they are disease and pest-resistant,
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enabling production methods with a low impact on the envi-
ronment.24 There are already some studies aimed at optimizing
hemp-fibers-reinforced PLA composites. Researchers have inves-
tigated the effects of different manufacturing processes, fiber
pre-treatments, fiber loading and fiber orientation on PLA/
hemp composites.25,26 It was found that PLA/hemp composites
mostly have good stiffness and tensile characteristics. However,
the impact properties are often worse than those of the pure
matrix, which is typical for a natural fiber-reinforced PLA com-
posite. Bax et al.27 investigated PLA/flax composites and
observed that the impact strength increased with increased flax
fiber content, but the impact strength of the composites was
clearly lower than that of the pure PLA sample. Oksman et al.28
studied the Charpy impact behavior of PLA/flax composites (40
wt % fiber), it was found that the composite impact strength
decreased by >25% compared to pure PLA. This effect was also
observed by M€ussig.29 He investigated the mechanical properties
of hemp fiber (40 wt %) reinforced PLA. The impact strength
was halved compared with pure PLA.
In contrast with bast fibers (like hemp fibers) with an elonga-
tion at break of approximately 5%,17 man-made cellulose
fibers have a higher elongation at break (>8–10%).30 Because
of the high elongation at break for the regenerated cellulose
fibers such as Rayon, Cupro or Lyocell, compared to the elon-
gation at break for the matrix, composites with high impact
strength values can be created. Lyocell fibers are known to
have good mechanical properties, wettability, high tenacity,
and good drapability as well as being environmentally friendli-
ness. Therefore, they have potential applications as reinforce-
ments for composites and can improve mechanical and
physical properties.31
In the context of this study, the mechanical characteristics of
composites in terms of the reinforcing fiber and matrix charac-
teristics were examined. Composites of PLA and hemp fibers,
PLA and hemp-Lyocell fiber mixtures, a PLA/PP blend and
hemp fibers, PLA and Lyocell fibers as well as PP and hemp
were investigated. The aim was to manufacture several compo-
sites based on two different kinds of reinforcing fiber and two
different matrices to investigate the characteristics of the com-
posites related to the fiber and matrix used. Different constitu-
ents can be used to tailor the composite characteristics for
diverse requirements, for instance the high stiffness of the hemp
fibers with the high elongation at break and resin wetting and
tenacity of the Lyocell fibers also, by combining the good
mechanical properties of PLA, including stiffness and strength
with the positive characteristics of polypropylene, including
high toughness and being cheap polymer.
MATERIALS AND METHODS
Materials
The PLA staple fibers, provided by Trevira GmbH (Hattersheim,
Germany), had a fineness of 1.7 dtex and a mean fiber length of
38 mm. Based on the manufacture’s information, the PLA fibers
were made from PLA Polymer 6202D from NatureWorksVR , Car-
gill Dow LLC (Minnetonka, USA). It had a density of 1.24 g
cm23, a melt temperature of 160–170�C, a glass transition tem-
perature of 60–65�C, 98% L-lactide, and a molecular weight of
97,000. The hemp, in the form of baled loose staple fibers
(genus species Cannabis Sativa L) was supplied by Hempage AG
(Adelsdorf, Germany). According to the manufacture’s info, the
average diameter of the hemp fiber was 20–40 lm and had a
mean fiber length of 30 mm. The Lyocell staple fibers were sup-
plied by Lenzing AG (Lenzing, Austria). The average length and
diameter of the fibers were 38 mm and 13.4 lm, respectively.
The PP fibers were supplied by FiberVisions (Varde, Denmark)
and had a fineness of 3.3 dtex and a mean fiber length of
66 mm.
Single Fiber Tensile Test. The hemp and Lyocell fibers were
tested on a Favigraph single-fiber tensile tester from Textechno
GmbH (M€onchengladbach, Germany) equipped with a 20-cN
load cell and with gauge length of 20 mm. The test speed was
20 mm min21 and the averages from 20 tests are reported in
Table I. To evaluate the effect of the composite processing con-
ditions on the hemp and Lyocell characteristics, single fibers
were treated in a hot press machine at 190�C and 1.7 MPa for
15 min, identical to the conditions used for the fabrication of
the composites, and then tested for tensile properties.
Table I summarizes the measured properties of the hemp and
Lyocell fibers used in this investigation.
Methods
Preparation of Prepreg and Composites. Five prepreg mats
with different fiber compositions were prepared:
� 70 mass % PLA 1 30 mass % hemp
� 70 mass % PLA 1 30 mass % Lyocell
� 70 mass % PLA 1 15 mass % hemp 1 15 mass % Lyocell
� 35 mass % PLA 1 35 mass % PP 1 30 mass % hemp
� 70 mass % PP1 30 mass % hemp
The needed amount of PLA, PP, Lyocell and the hemp staple
fibers were first weighed in their loose form and then they were
manually mixed, and fed to a carding machine from Mesdan
S.P.A (Brescian, Italy). During carding, the longer PLA and PP
fibers supported the shorter hemp and Lyocell fibers and
Table I. Tenacity, Modulus, Elongation, and Linear Density for Hemp and Lyocell Fibers Before and After Heat Treatment
Treatment Fibre Tenacity (cN/dtex) E-modulus (cN/dtex) Elongation (%) Linear density (dtex)
As received Hemp 4.66 (1.43) 97.71 (24.92) 3.93 (0.66) 4.15
Lyocell 3.27 (0.63) 55.87 (23.06) 8.70 (2.01) 1.36
After hot pressing Hemp 3.75 (1.10) 84.62 (26.60) 3.37 (0.88) 4.27
Lyocell 2.12 (0.92) 48.46 (20.29) 4.75 (1.84) 1.38
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provided the fiber-to-fiber cohesion which resulted in a web
suitable for further processing. The blended fiber web was
carded three times to parallelize the fibers and to achieve mat
uniformity.
The obtained prepreg mats were dried in a vacuum chamber
(0.9 mbar; 70�C) for at least 18 h before compression molding.
After drying the mats were stored under dry conditions. The
prepreg was covered by a Teflon sheet to prevent sticking of the
matrix to the surface of the mould before it was placed into a
preheated steel mould with a 20 3 20 cm2 cavity and 10-mm
depth. The steel mould was of own design, and machined by a
local machine shop. The mould was then placed in a hydraulic
compression-molding machine from Rondol Technology (Staf-
fordshire, UK). Compression molding was done at a tempera-
ture of 195�C and a pressure of 1.7 MPa for 20 min. Neat PLA
sheets, to be used as reference material, were made by melting
PLA fibers under the same processing conditions.
Test Methods. The specimens were stored at ambient conditions
after processing. Then, before testing, the specimens were condi-
tioned for at least 24 h at 23�C and 50% relative humidity
according to DIN EN ISO 291. The test specimens were cut by
laser according to the standard dimensions, given below.
Composite density and porosity. The densities of the compo-
sites were determined by the buoyancy method (Archimedes’
principle), using ethanol as the displacement medium. Before
the specimens were immersed in ethanol, they had been covered
by a varnish containing paraffin to avoid absorption during
immersion. The fiber volume fraction was calculated from the
fiber weight fraction with allowance for porosity using the
method proposed by Madsen et al.32
Water absorption test. Water absorption analysis was done on
composite specimens according to ASTM D570-98. The specimen
dimension was �36 3 10 mm2. The specimens were first dried in
an oven for 24 h at 60�C, and then put in a desiccator in order
to cool down to room temperature. The measured weights of
these specimens were denoted as W0. The specimens were then
immersed in two different baths, one of distilled water at room
temperature and one of hot water at 80�C. The amount of water
absorbed was measured every 24 h for 10 days. At each measuring
point the specimen was removed from the bath, and the surface
was wiped dry and the weight was recorded as W. The percentage
of apparent weight gain (WG) was calculated using eq. (1).
WG5 W 2W0ð Þ=W0½ � 3 100% (1)
Tensile test. The tensile testing was done according to ISO 527
in a universal H10KT testing machine equipped with a mechan-
ical extensometer (model 100R long travel extensometer),
attached to the central part of the specimen by clips. The testing
parameters were a loading rate of 10 mm min21 and a loading
range of 1 kN. The average tensile values were collected from
six separate measurements. The testing machine and the exten-
someter were supplied by Tinius Olsen (Salford, UK).
Dynamic mechanical thermal analysis. The viscoelastic proper-
ties of the specimens were measured using a dynamic mechani-
cal analyser (DMTA Q-series TA instrument supplied by Waters
LLC, Newcastle, DE). Rectangular specimens with the dimen-
sions 50 mm 3 8 mm and 1–2 mm thickness were used using
the dual cantilever method. The measurements were performed
at a frequency of 1 Hz and amplitude of 15 lm. The tempera-
ture range was from 30 to 150�C at a scanning rate of 3�Cmin21. The storage modulus (E0) and loss factor (tan d) of the
specimens were measured as a function of temperature.
Flexural testing. The flexural test was performed using the same
testing machine as for tensile testing according to ISO 14125
standard test method for fiber–reinforced plastic composites. At
least five specimens were assessed for each batch of samples. The
loading rate was 10 mm min21 and the load range was 1 kN.
The specimen dimension was 80 3 15 mm2 (length 3 width),
and the thickness varied depending on the sample. The outer
span was 64 mm and the range of displacement was 20 mm. At
least five specimens were assessed for each sample reported the
average values.
Impact testing. The Charpy impact strength of the composites
was tested according to ISO 179. A pendulum type Zwick test
instrument from Zwick GmbH (Ulm, Germany) was used to
measure the unnotched, rectangular specimens (80 mm 3 10
mm 3 1–2 mm). For each material, ten specimens were tested
edgewise.
Differential scanning calorimetry. The DSC analysis was done
on a DSC Q2000 supplied by TA Instruments, New Castle, DE.
Samples of �10 mg were heated at a rate of 10�C min21 in a
nitrogen-purge stream from 20 to 200�C, then cooled to 0�C,
and heated again from 20 to 200�C. The data from the first
scan were used. For each sample, three replicates were scanned
in order to get average values. The percentage crystallinity
(XDSC) of PLA and PP was calculated using eq. (2)33:
XDSC %5DHf 2DHcc
DHof
3100
w(2)
where DHof 5 93 J g21 for 100% crystalline PLA and 209 J g21
for 100% crystalline PP DHf is the enthalpy of melting, DHcc is
the cold crystallization enthalpy, and w is the weight fraction of
PLA in the composite.
Scanning electron microscopy (SEM). Composite fracture sur-
face morphologies were studied using low-vacuum scanning-
electron microscopy, a JEOL JSM 6610LV instrument, JEOL
(Tokyo, Japan) with an operating voltage of 5–10 kV. For low
vacuum imaging, no specific preparation was required.
RESULTS AND DISCUSSION
Single Fiber Tensile Test
To evaluate the effect of the high temperature compression
molding conditions, the tensile properties of heat treated hemp
and Lyocell staple single fibers were determined and compared
to untreated single fibers, see Table I. Heat treatments greatly
reduce the mechanical properties. Cellulosic fibers are mixtures
of organic materials (cellulose, hemicellulose, and lignin) and
heat treatment at elevated temperatures can cause physical and
chemical changes. The physical changes are associated with
enthalpy, weight, color, strength, crystallinity, and orientation of
microfibril angle.34 The chemical changes are related to the
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decomposition of the molecular structure. Heat treatment
results in the weight loss of moisture plus weight loss due to
thermal degradation. The thermal degradation of cellulosic
fibers results in change in color and deterioration in mechanical
properties of the fibers.35,36
Influence of Different Natural Fibers
In this study, five different composites were produced by com-
pression molding of prepregs containing 30 mass % reinforce-
ment (hemp respective Lyocell) and 70 mass % matrix, (PP
respective PLA). The physical characteristics of the composites
made are summarized in Table II. The porosity ranged from 2.1
to 14.9 vol %, and the higher porosity was seen for the PP/
hemp and PLA-PP/hemp composites. The porosity fraction is
rather high even if the fiber volume fraction was as low as 30
mass %, and this could be due to several factors. The complex-
ity of the surface chemistry and the irregularity of the morphol-
ogy of plant fibers is one of the most important considerations
as well as to the presence of luminal cavities.32 It can be
observed that the Lyocell-based composite had the lower void
content which is obviously due to the higher interfacial
adhesion.
Water Absorption Characteristics
Figure 1 shows the apparent weight gain (WG) as a function of
the immersion time at different temperatures for the manufac-
tured composites. In Figure 1(a), it can be seen that for all the
composites investigated, WG increases monotonically with time.
This increase in WG is consistent with other studies of natural
fiber composites.37–39 The absence of the induction period char-
acterized by zero weight gain at the initial stage of water
immersion treatment, could be explained by the different mate-
rial packing between the skin and core regions, which indicates
that the hemp and Lyocell fibres were uniformly dispersed in
the PLA and PP matrix.16
It can also be seen that the slopes of the WG vs. time plots were
steeper for all composites compared with the slopes for neat PP
and PLA. It can be observed that the composites exhibited sig-
nificantly higher water absorption than for neat PLA and PP
due to the hydrophilic nature of Lyocell and hemp because of
the presence of polar groups such as AOH and ACOOH in the
fibers. These results are in accordance with published data
showing that the lignocellulosic fibres displayed a higher tend-
ency to absorb water than does the PLA and PP.39–41 Among all
composites in this study, PLA/Lyocell and PLA/hemp-Lyocell
showed the lowest water absorptions (up to 6.7 and 10.3 wt %,
respectively), which may be due to their lower porosity and also
better interfacial adhesion which decreases the thickness of the
interphase area between the fibers and the matrix, thus decreas-
ing the water absorption through the interphase and further
into inner parts of the structure.42,43 From the results, it can be
Table II. Composition of the Fabricated Composites Calculated from the Density Measurements
Sample Density (g cm23)
Fibre massfraction (%)
Fibre volumefraction (%)
Matrix volumefraction (%)
Porosity volumefraction (%)Hemp Lyocell
PLA 1.2490 (0.0002) – – 0.0 100.0 –
PLA/hemp 1.0914 (0.0154) 30.0 – 23.0 (0.6) 61.2 (0.9) 10.8 (0.2)
PLA/Lyocell 1.2765 (0.0112) – 30 25.2 (0.2) 71.5 (0.6) 3.3 (0.9)
PLA/hemp-Lyocell 1.1222 (0.0169) 15.0 15.0 22.4 (0.3) 76.4 (1.2) 2.1 (0.1)
PLA-PP/hemp 1.0171 (0.0402) 30 – 20.6 (0.8) 65.9 (12.6) 11.9 (3.5)
PP/hemp 0.8758 (0.0154) 30 – 17.8 (0.4) 67.4 (1.4) 14.9 (1.8)
PP 0.9103 (0.0019) – – 0.0 100.0 –
Note: Data in the brackets are mean (SD).
Figure 1. Apparent weight gain against time for the manufactured com-
posites at: (a) room temperature and (b) 80�C.
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concluded that the PP/hemp had higher moisture absorption
compared with PLA/hemp. It could be explained by the lower
interfacial compatibility between hemp fiber and PP matrix,
which can result in incomplete wetting resulting in microcracks,
offering channels for better moisture transport.44 The PLA-PP/
hemp composite showed higher moisture absorption than PLA-
hemp composite.
As shown in Figure 1(b), after 5 days at 80�C, the neat PLA
sample was very soft and it was difficult to decant water and
rinse the sample without losing some material. The experiment
for neat PLA was therefore discontinued after 5 days. Further-
more, it can be seen that the water immersion temperature
influences the water absorption curves. Increasing the immer-
sion temperature from room temperature to 80�C increased the
water absorption of the neat PLA and composites, as well as
shortened the saturation time. The immersion temperature
affected PLA/hemp-Lyocell more than it did for PLA/Lyocell
composites. The weight gain was found to decrease after passing
through a maximum. For PLA composites, this was due to the
formation of a neat resin peel on the surface resulting from
degradation and dissolving with time together with the removal
of some substances from the hemp and Lyocell fiber during the
immersion.45–47 In contrast, even though the apparent weight
gain for the PP composite also decreased after passing through
a maximum, the PP dissolution did not take place. In fact, it
has been reported that the PP is very resistant to moisture
attack within even at elevated temperature.37,39,46 It is of interest
to understand the mechanisms that cause the decrease in weight
gain. The hemp fibers are responsible for the weight loss
observed, which is due to the removal of certain fractions from
the hemp fibers during the water immersion.
Tensile Properties
An overview of the tensile strength and modulus of the compo-
sites compared with the values of the neat PLA and PP matrix is
given in Figure 2(a,b). Compared with the tensile modulus of the
pure PLA matrix [Figure 2(a)], there is an improvement of 138%
for PLA/Lyocell, 117% for PLA/hemp-Lyocell, 106% for PLA/
hemp and 59% for the PLA-PP/hemp composites. The tensile
modulus have been decreased by the admixture of the PP fibers
in the PLA/hemp composite compared with the PLA/hemp com-
posite since the tensile modulus of PP is lower than that of PLA.
As can be seen in Figure 2(b), the tensile strength of PLA com-
posites was significantly higher than the tensile strength of the
neat PLA and neat PP matrix, which was 39.7 and 24.6 MPa,
respectively. Neat PLA has better mechanical properties than neat
PP.28 In the comprehensive outline, the highest tensile strength
values were reached by PLA/Lyocell with 80.9 MPa, PLA/hemp-
Lyocell with 60.6 MPa, followed by PLA/hemp with 45.7 MPa,
PLA-PP/hemp with 29.8 MPa and PP/hemp with 26.9 MPa.
The improvement of the tensile properties of hemp-reinforced
PLA composites by addition of Lyocell fibers could be attributed
to the lower amount of porosity in the composite and the
much higher fineness of the Lyocell fibers, which promote a
better fiber–matrix adhesion.
Flexural Properties
The results of the flexural testing are shown in Figure 3(a,b).
The flexural modulus of the composites were all higher than for
neat PLA and neat PP, which is especially evident for the PLA/
Lyocell which had a flexural modulus (5.8 GPa) three times
higher than that of neat PLA (1.9 GPa) [Figure 3(a)]. Moreover,
the superior flexural properties of the PLA composites with
hemp-Lyocell compared with the PLA composites with hemp
can be attributed to the better adhesion between Lyocell fibers
with PLA matrix as well as lower porosity. It is also obvious
from the results that composites made from PLA-PP/hemp
showed an evident improvement in flexural modulus across the
PP/hemp composite, but lower than that of PLA/hemp compos-
ite. As shown in Figure 3(b), the flexural strength showed an
increase of 43.5% for the PLA/Lyocell composite and 23.5% for
the PLA/hemp-Lyocell composite compared with neat PLA.
However, the flexural strengths of the other composites were all
lower than of neat PLA.
Impact Resistance
The results of the Charpy impact test are shown in Figure 4. For
the neat PLA matrix, an impact strength value of 11.5 kJ m22
Figure 2. Tensile properties of the manufactured composites: (a) tensile modulus, and (b) tensile strength.
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was measured. Similar values have also been seen in other stud-
ies.27,28 A significant reinforcement effect was determined for the
PLA/Lyocell composites with an impact value of 26 kJ m22 and
for the PLA/hemp-Lyocell composite with an impact value of
21.7 kJ m22, while the value of the PLA/hemp composites
(9.7 kJ m22) was lower than for the neat PLA matrix. This can
be explained by a higher elongation at break of Lyocell compared
with hemp fibers resulting in an improvement of the impact
strength in the composites.48,49 Hence, the highest impact value
was reached by PLA/hemp-Lyocell due to the higher elongation
characteristics of the Lyocell fibers, while using hemp fibers
resulted in a brittle character of the PLA composites. The behav-
ior of the hemp fiber is in accordance with the known behavior
of natural fibers under an impact load.
The impact strength of pure PP was around 18.9 kJ m22. Simi-
lar values have also been seen in other studies.50,51 However, the
impact strength of PP-based composites was lower than that of
neat PP. It has been reported that the un-notched impact resist-
ance of thermoplastic natural fibers composites generally show a
decreasing trend. The behavior of fiber reinforcement under
impact load is more complicated than under bending and ten-
sile load since the impact strength is attributed to the energy
consumption during failure, and also attributed to the addition
of fibers which probably creates regions of stress concentration
which require less energy to initiate a crack.
A mixture of PLA-PP/hemp resulted in a further improvement
of the impact strength up to 9.8 kJ m22 compared with PLA/
hemp with 8.8 kJ m22.
Dynamic Mechanical Thermal Testing
Figure 5(a,b) shows the dynamic viscoelastic curves for neat
PLA and composites made from PLA. The storage modulus (at
30�C) increased from 2.5 GPa for pure PLA, up to 3.7 GPa for
PLA/Lyocell followed by 3.2 GPa for PLA/hemp-Lyocell, 3.1
GPa for PLA/hemp and 2.6 GPa for PLA-PP/hemp composite
[Figure 5(a)]. The storage modulus increased when hemp and
Lyocell fibers were introduced into PLA. The increase in the
stiffness of fiber-containing samples, which reveals effective
stress transfer from the fiber to the matrix at the interface, can
be interpreted as good adhesion between the fibers and the mat-
rices.1,52–54 The sharp decrease in the storage modulus (around
57–61�C for most of the samples) corresponds to the
a-relaxation of the amorphous regions in PLA.55
The storage modulus started to increase again at temperatures
of around 90–100�C, which is a result of the cold crystallization
of PLA and this peak shifted to a lower temperatures with the
addition of PP. The cold crystallization was also observed in the
DSC curves. This result suggested that the incorporation of PP
increased the cold-crystallization ability of PLA.56
The dampening, or tan d, is the ratio between the loss modulus
and the storage modulus and provides information about the
internal friction of the material and the adhesion of the inter-
face. For a composite, the molecular motion in the interphase
will contribute to the dampening. A larger area under the
a-relaxation peak in the tan d curves of a polymer indicates
that the molecular chains exhibit a higher degree of mobility
thus better damping properties.57 The area under this peak for
PLA-based composites [Figure 5(b)] seems to be smaller com-
pared with neat PLA. A possible explanation is that there is a
good interaction between the fiber and the matrix. Adding
Figure 3. Flexural properties of the composites: (a) flexural modulus, and (b) flexural strength.
Figure 4. Impact strength of the manufactured composites.
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fibers to the matrix will decrease the mobility of the polymer
chains, and consequently reduce the dampening, as reported by
Pothan et al.58 The glass-transition temperature is often
recorded at the maximum of the tan d. The recorded Tg of the
samples were 65.6�C for pure PLA, 67.5�C for PLA/Lyocell,
68�C for PLA/hemp-Lyocell, 68.8�C for PLA-PP/hemp and
69.7�C for PLA/hemp. It is obvious that incorporating fibers
increases the Tg for both PLA and PP. The slight shift in Tg to a
higher temperature (by a few degrees) indicates that the mobil-
ity of polymer chains is affected. Mathew et al.59 discussed that
the shift to higher temperature usually indicates restricted
movement of molecules because of better interaction between
the fiber and the polymer matrix.
The broad transition (between 90 and 110�C) for all the fiber-
reinforced PLA relates to the cold crystallization of PLA in this
temperature region.
Differential Scanning Calorimetry
To examine the effect of the added fibers on the crystallinity of
PLA, DSC analysis was performed. The DSC heating and cooling
thermograms for all composites produced are shown in Figure
6(a,b). The glass transition temperature (Tg), crystallization tem-
perature (Tc), and melting temperature (Tm) obtained from the
DSC studies are summarized in Table III. The data indicates that
Tg and Tm was slightly affected by the introduction of hemp and
Lyocell fibers with a few decrease. Furthermore the Tc decreased
from 112.1�C of pure PLA to 106.4�C of composite containing
hemp-Lyocell fibers. It can be clearly seen that the degree of crys-
tallization increases from 4.6% for neat PLA up to 8.3% for
PLA/hemp, 16.9% for PLA/Lyocell and 11.8% for PLA/hemp-
Lyocell. It is noteworthy to see that the above increase in the
crystallinity of PLA with respect to fiber content is consistent
with other studies of natural fiber/PLA composites.3,60,61 The
double melting peaks suggest that occurrence of the crystal’s reor-
ganization during the heating run.62–64
In comparison with PP, the incorporation of hemp fibers shifts
the crystallization temperature to the higher temperature. This
Figure 6. DSC thermograms from the cooling and the second heating run
for the fabricated composites.
Figure 5. DMTA analysis of PLA and PP reinforced with hemp and Lyo-
cell fibers. (a) Storage modulus vs. temperature; (b) tan d curves.
Table III. Differential Scanning Calorimetric Data for the Composites
from the Cooling and Second Heating Run (10�C min21)
Sample XDSC (%) Tg (�C) TC (�C) Tm (�C)
PLA 4.6 59.2 112.1 167.4
PLA/hemp 8.3 59.4 108.6 166.8
PLA/Lyocell 16.9 57.6 108.7 166.7
PLA/hemp-Lyocell 11.8 57.5 106.4 166.3
PLA-PP/hemp 5.3 58.8 117.9 166.5
PP/hemp 1.4 – 117.3 164.4
PP 0.6 – 112.5 166.6
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increase suggests that hemp fibers possibly act as heterogeneous
nucleating agents causing the PP to crystalize at higher temper-
ature.65 As evident from the crystallization thermogram Tc [Fig-
ure 6(b)], the incorporation of PP within the PLA matrix
increased Tc of the PLA from 112 to 117.9�C. Moreover, the
crystallinity of the PLA in its blend with PP slightly increased.
Scanning Electron Microscopy Analysis
Figure 7 shows the SEM micrographs of the fracture surface for
t tensile specimens for different composites. Compared to PLA/
hemp composites [Figure 7(a)], less pulled-out fibers and corre-
sponding holes are visible for the PLA/Lyocell-hemp and PLA/
Lyocell composites [Figure 7(b,c)]. These observations suggest
that the adhesion between the PLA matrix and the Lyocell fiber
is quite good. It should be noted that the finer Lyocell fibers
were used which lead to the bigger specific surface between fiber
and matrix and better adhesion between them.
As shown in Figure 7(d) the PP and PLA polymers do not have
the well-defined spherical shapes and the separation of the two
components or pores.
CONCLUSIONS
The effects of the choice of matrix and reinforcing filler on the
structure and properties of PLA-based composites have been
studied, focusing on the water absorption, mechanical and
thermo-mechanical properties. Based on the mechanical tests,
the obtained results showed that combining hemp and Lyocell
in a PLA-based composite can improve significantly the impact
strength at ambient temperature, flexural and tensile strength
and modulus, compared with hemp fiber reinforced PLA. More-
over, the moisture absorption was reduced by up to 47.4%.
From the DMTA results, it is evident that incorporation of the
hemp and Lyocell fiber gives a considerable increase in storage
modulus and a decrease in tan d values. The study performed
using DSC revealed that the melting point of PLA was not
affected significantly after reinforcement with the hemp-Lyocell
mixture but the glass transition temperature increased a few
degrees. The PLA in the composites had high orientation degree
and crystallinity which was attributed to effective heterogeneous
nucleation induced by hemp and Lyocell fibers, however, the
degree of crystallinity of PLA/hemp composite was higher.
Figure 7. SEM images of tensile fracture surfaces of (a) PLA/hemp, (b) PLA/Lyocell, (c) PLA/hemp-Lyocell, and (d) PLA-PP/hemp composites.
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Although Lyocell is expensive, it is reproducible by artificial
production and the admixture of Lyocell fibers to hemp fibers
leads to less quality variation in the fabricated composites.48
On the contrary, the combination of PP and PLA had a destruc-
tive effect on the mechanical properties. Composites based on
the PLA/PP blend-matrix containing hemp had only enhanced
impact strength of the composite, compared with the PLA/
hemp composite but not the flexural, tensile and damping
properties. The crystallization temperature of the hemp-
reinforced PLA-PP composites decreased compared with pure
PLA indicating that the incorporation of PP improved the
cold-crystallization ability of PLA. The changes in the crystal
structure and melting points PLA-PP/hemp composite indicate
partial compatibility between the two polymers. This result was
also confirmed by the SEM micrographs, in which no well-
defined spherical shapes and separation of the two polymers
were observed. The PLA-PP/hemp composite showed higher
water absorption than the PLA/hemp did. In addition to
improving the impact properties of PLA, the PLA-PP/composite
are lighter, with relatively lower price and can be engineered to
have controlled degradability for different applications.
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Paper V
1
Characterisation of polylactic acid biocomposites made
from prepregs composed of woven polylactic acid/hemp-
Lyocell hybrid yarn fabrics
Behnaz Baghaei
1 and Mikael Skrifvars
*,1
1 Swedish Centre for Resource Recovery, Academy for Textile, Engineering and Business
University of Borås, SE-501 90 Borås, Sweden
* Correspondence to Mikael Skrifvars: Tel: +46 33 435 4497; Fax: + 46 33 435 4008
E-mail address: [email protected]
2
Abstract
This paper describes the mechanical properties and water absorption characteristics for
biocomposites made from woven PLA/hemp/Lyocell prepregs. The aim was to improve
the properties with the addition of Lyocell fibre into a hybrid yarn. Well-aligned hybrid
yarns composed of hemp/PLA, hemp-Lyocell/PLA, respective, Lyocell/PLA were made
by wrap spinning. Unidirectional satin fabrics were made by weaving with PLA (warp)
and the hybrid yarns (weft). Uniaxial composites were fabricated with 30 fibre mass-%
using compression moulding. The composites were investigated for tensile, flexural and
impact properties. Combining hemp with Lyocell in a PLA matrix improves the
mechanical properties, compared to hemp/PLA composites. The composite made from
the satin Lyocell/PLA fabric gave the best mechanical properties. The type of fibre
reinforcement compositions did not significantly affect the water absorption of the
biocomposites. Scanning electron microscopy showed that fibre pull-outs appear more
often in hemp/PLA composites than in composites also including Lyocell fibre.
Keywords:
A. Fabrics/textiles
B. Mechanical properties
E. Weaving
E. Compression moulding
3
1. INTRODUCTION
Natural fibre reinforced polymers are an emerging material that has great potential to be
applied in lightweight engineering applications. Polymer matrices can be divided into
two categories: thermoplastic and thermosetting polymers. The thermosets have
traditionally been used as reinforced, but today thermoplastics find more applications as
reinforced composites. Since the early 1990s, hybrid composite materials made from
natural fibres and thermoplastic matrix polymers have been commonly processed in
compression moulding [1]. Thermoplastic composites have several advantages over
thermoset composites in terms of processing and mechanical properties. Thermoplastic
polymers can be reshaped upon the application of heat, and this can be done repeatedly.
On the other hand, thermosetting polymers undergo chemical reactions during curing,
which cross-link the polymer molecules, causing the resins to become permanently hard
and rigid. However, thermoplastic molecules are associated with physical
intermolecular forces, and they can move under stress. Thereby, thermoplastic
composites usually have higher strains at failure than thermoset composites; thus, they
possess better fracture toughness and fatigue endurance. Thermoplastic composites
usually need a shorter and simpler processing cycle which involves no chemical
reactions, and they are therefore used in composite applications requiring short process
time [2].
Bio-based polymers are increasingly replacing the petroleum-based polymers in
many industrial applications. The most important thermoplastic biopolymer is polylactic
acid (PLA), which is produced from lactic acid obtained from carbohydrate feedstock.
PLA is today commercially used in biodegradable packing materials and other
applications with short lifespan. Nevertheless, various studies have shown that PLA is
also suitable as matrix for embedding of fibres in composites [3, 4].
4
Several research studies have been carried out on cellulose fibre-reinforced PLA
composites, but there is still a need for more optimisation of these materials to meet the
demands required in automotive components and construction materials. Beside annual
plant natural fibres, man-made fibres based on regenerated cellulose such as Lyocell or
viscose can be used to reinforce PLA. Lyocell is obtained by dissolving native cellulose
under specific conditions in N-methyl-morpholine-N-oxide, known as the NMMO
process. The dissolved cellulose can be regenerated as a fibre out of this solution [5, 6].
Many studies have been carried out on cellulose fibre-reinforced PLA composites to
improve the mechanical properties [7-9].
Apart from the choice of reinforcing fibres in composites to improve the composite’s
mechanical properties, methods that combine the thermoplastic resins with reinforcing
fibres is also important since thermoplastic resins have very high melt viscosities, which
makes in-process melt impregnation of the reinforcement difficult. Various techniques
for combining natural fibres with thermoplastic resins have been developed to overcome
these problems. Generally, the technique giving the shortest average matrix flow
distance through the layup is preferable. Matrix sheets can be placed between layers of
fibre mats or woven cloths. Thermoplastic filaments can be filament wound together
with the reinforcing yarns by filament winding, or they can be mechanically blended
into the reinforcement yarns, to a commingled fibre/matrix yarn, also named as
commingled hybrid yarns [10-12].
Recently there has been much interest in developing unidirectional continuous fibre-
reinforced (UD) composites since they provide the best performance when the direction
of applied load is parallel to the orientation of fibre. In UD composites, it is necessary to
handle the orientation and the position of the reinforcing fibres embedded in the
composite. Hybrid yarns with oriented reinforcing fibres can be positioned parallel in
5
the fabric to form a unidirectional preform. This will result in a composite with optimal
mechanical properties in the yarn direction, which is desirable for many applications.
For such applications, low crimp unidirectional woven yarn fabrics [13] or very thin
tape-shaped UD composite preforms can be used [14].
Our previous study [13] investigated the effect of fabric weave structure on
mechanical behaviour and moisture absorption of a PLA/hemp woven fabric composite
made by compression moulding. The unidirectional woven fabric prepregs were made
from PLA and PLA/hemp wrapped-spun hybrid yarn with two different weave patterns:
8-harness satin and basket. The results showed that the composite made by satin-weave
architecture fabric possessed the highest tensile, flexural and impact strength, compared
to the composites manufactured with basket weave fabric. This improvement was
correlated to the decrease in void content and fibre misalignments.
In the present study, we have continued the development of hybrid thermoplastic
reinforcements, by including Lyocell fibres in the yarn. The main objective is to
compare the mechanical characteristics of uniaxial composites fabricated from
reinforcement made from hemp/PLA, hemp-Lyocell/PLA and Lyocell/PLA wrap spun
hybrid yarns. The hybrid yarns have been made from parallel non-twisted commingled
fibres in the yarn core, which is mechanically stabilised by a wrap-filament wound
around the core. Based on the results from our previous study, we selected a satin-
weave architecture prepreg fabric, which was processed by press heating and
consolidation into a UD composite laminate.
6
2. MATERIALS AND METHOS
2.1. Materials
Hemp fibres (genus species Cannabis Sativa L) with a medium fibre length of 30 mm
were provided by Hempage AG (Adelsdorf, Germany). Lyocell fibres with a length of
38 mm and a fineness of 1.3 dtex were supplied by Lenzing (Lenzing AG, Austria).
A thermoplastic biopolymer NatureWorks™ PLA polymer 6202D from Cargill Dow
LLC (Minnetonka, USA) was selected as the matrix component. The biopolymer was
supplied both in staple fibre and continuous filament forms by the company Trevira
GmbH (Hattersheim, Germany). The staple fibres had a fineness of 1.7 dtex and a mean
fibre length of 38 mm. The filament had a fineness of 18 tex.
2.2. Fibre processing and production of composites
The hybrid yarns were produced according to the wrap spinning process reported
earlier in our previous studies [13, 15]. The wrap spun yarn consists of parallel staple
fibres being wrapped helically by a continuous PLA filament [11]. The core is a well-
mixed strand of hemp/PLA, hemp-Lyocell/PLA or Lyocell/PLA staple.
Within the framework of this study, the hybrid yarns (nominal count of 550 tex)
consist of the following mixtures: hemp 30% + PLA 70%, Lyocell 30 mass% + PLA
70% and hemp 15% + Lyocell 15% + PLA 70%. Data from the composite from hemp
30% + PLA 70% were used from previous study [13]. In order to compare the effect of
random fibre orientation on composites on mechanical properties, data from the
nonwovens with the same compositions from previous study [16] are reported in this
article as well. As reference, webs of pure PLA fibres were made.
To produce the thermoplastic prepregs, unidirectional woven fabrics were made from
hybrid yarns (weft yarn) and PLA filaments (warp yarn) on a hand loom weaving
machine. The fabrics were woven with 8-Harness satin pattern. Fig. 1 shows the
7
unidirectional hemp/PLA fabric. The fabric specifications are presented in Table 1.
These fabrics are used as prepregs for subsequent processing to composites by
compression moulding. Prior to compression moulding, the fabrics were dried for 24 h
at 70°C and 0.9 mbar in a vacuum oven.
Compression moulding was carried out with a manual hydraulic press from Rondol
Technology Ltd., Staffordshire, UK. The pressure was set to 1.7 MPa, which was
maintained for 15 min at 195°C. Before demoulding, the composites were cooled down
to approximately room temperature. After demoulding, the composites were cut into
specimens with GCC LaserPro Spirit laser cutting machine for the different mechanical
testing experiments.
2.3. Mechanical testing
Tensile and flexural tests of the composites were performed by using a universal
H10KT testing machine equipped with a mechanical extensometer (model 100R long
travel extensometer) supplied by Tinius Olsen Ltd. (Salford, UK) based on ISO 527 and
ISO 14125. Charpy impact tests were carried out at ambient conditions on a QC-639D
mechanical impact tester (Cometech testing machines, Taichung Hsien, Taiwan)
according to the ISO 179. The morphology of fractured surfaces of the composites
obtained from the tensile testing was observed by scanning electron microscopy (SEM),
using a low-vacuum scanning electron microscopy with a Quanta 200 ESEM FEG
instrument from FEI, (Oregon, USA) with an operating voltage of 10-12.5 kV.
The density of the composites was experimentally determined by the buoyancy-
flotation method (Archimedes) in ethanol; fibre volume fraction and porosity were
calculated using density of hemp and PLA fibres [17].
The effect of water absorption on hemp, Lyocell and hemp/Lyocell fibres reinforced
hybrid composites were investigated in accordance with ASTM D570-98. Water
8
absorption tests were conducted by immersing the composite specimens in distilled
water at room temperature. The specimens were weighed regularly from 24 hours to 240
hours exposure, at an interval of 24 hours. The moisture absorption was calculated by
the weight difference. The percentage weight gain of the samples was measured at
different time intervals.
Statistical comparisons, based on a one-way analysis of variance (ANOVA) at the
95% confidence level, were performed to test the effects of different fibres on the
mechanical properties.
3. RESULTS AND DISCUSSION
3.1. Composite porosity
One of the most common manufacturing induced defects in composites is voids [18,
19]. In general, the voids have detrimental effects on the mechanical strength of
composite laminates [20], and they make them more susceptible to moisture penetration
and environmental conditions. The effect is more pronounced in compressive, bending
and inter-laminar shear loading, which are related to the matrix dominated mechanical
properties [21, 22]. In the current study, we studied the composites produced from
unidirectional satin hemp/PLA, Lyocell/PLA and Lyocell-hemp/PLA fabrics and from
nonwovens. All composites had the same reinforcement loading (30 mass-%) in order to
investigate the effect of combining different fibre types and their alignment on the
composites properties. The obtained data were compared to corresponding data for
composites made from PLA/hemp nonwoven mat and hemp/PLA fabric from a previous
study [13, 16]. The density and constituents of the composites were evaluated and are
shown in . According to the data, it is clear that the composite made by unidirectional
satin fabric gave significantly lower porosities, compared to the composites made from
nonwoven mats, and the values are in the acceptable range [15, 16]. Generally, the voids
9
are closely related to the used processing conditions and the laminate tightness. They
can also be formed due to the discontinuous resin matrix in the composites, which is
caused by the PLA fibre’s failure to form a continuous phase of matrix in the
composites or by the uneven distribution of the PLA staple fibres [23, 24]. This fact is
more evident in the case of the composites from nonwoven mats.
3.2. Water absorption characteristics
Fig. 2 illustrates the measured gravimetric water absorption for the PLA
biocomposites. Generally, the three types of fibre reinforcement compositions (hemp,
Lyocell and mixture of hemp and Lyocell) studied do not significantly affect the weight
percentage of water absorbed by the PLA biocomposite. However, it was noted that the
water uptake of the biocomposite made from nonwoven mats was significantly higher
than for the composites made from satin fabrics. This higher water absorption for
nonwoven mat reinforced PLA was expected, due to the higher void content. As shown
in , there are more voids and gaps present in the nonwoven reinforced composite. This
condition worsened the moisture uptake behaviour since voids favour moisture
absorption. The voids result in a larger amount of poorly bonded surface area between
the fibre and the matrix [19, 25].
3.3. Tensile properties
presents tensile properties of PLA composites reinforced with hemp, Lyocell and
glass fibres, as typically reported in the literature. This demonstrates that glass fibre
composites are superior to hemp fibre composites irrespective of fibre orientation, and
the tensile strength in particular is larger for glass fibre composites [26, 27]. Hence, the
table reveals the current technical performance for PLA/hemp and PLA/Lyocell
composites where the E-modulus is acceptable, but the tensile strength needs to be
somewhat improved. It has also been observed that for all composite characteristics,
10
there are clear differences between the values measured for the polymer composites
from fabric reinforcement and nonwoven mat due to the fibre orientation. Specimens
taken from nonwoven reinforcement show lower values than specimens taken from
fabric reinforcements. The strength values of hemp-PLA, Lyocell-PLA and Lyocell-
hemp PLA specimens differ noticeably. The improvement of the tensile properties of
hemp-reinforced PLA composites by addition of Lyocell fibres could be attributed to
the much higher fineness of the Lyocell fibres, which cause a larger specific bonding
surface and a better matrix-fibre adhesion [16, 28].
3.4. Flexural properties
Fig. 3 shows the flexural properties of PLA-based composites. In all diagrams, the
values of the pure PLA are presented next to the composite characteristics. The results
of the flexural strength revealed similar trends as the results of the tensile tests.
However, flexural strength values were higher than tensile strength values. This effect
has also been noticed by Mieck et al. [8] for 35 mass-% kenaf/PLA composites and by
Graupner et al. [9] for 30 mass-% Lyocell/PLA composites. Improved bending strength
values were achieved with both hemp and Lyocell fibre reinforcements. From the
results, it is evident that the highest values were achieved by the composite reinforced
by satin Lyocell/PLA fabric with 158.2 MPa followed by the satin hemp-Lyocell/PLA
fabric with 145.1 MPa and satin hemp/PLA fabric with 119 MPa.
With regard to the flexural modulus values, our measurements show a modulus of
2.91 GPa for neat PLA. Fibre reinforcement resulted in a clear and significant increase
of the modulus values. For satin Lyocell/PLA, the highest flexural modulus, a value of
9.72 GPa was found. One-Way ANOVA test showed that mean flexural strength and
flexural modulus values of materials investigated were significantly different (P-
value˂0.05); however, between satin hemp-Lyocell/PLA and satin hemp/PLA
11
composites, no significant differences in modulus were determined, but their strength
values differ clearly.
3.5. Impact resistance
Fig. 4 represents results of Charpy impact tests. It can be seen that for satin
hemp/PLA, hemp-Lyocell/PLA and Lyocell/PLA composites, the impact strength
increased by factor 1.1, 2.7 and 3.1, respectively, compared to pure PLA. Graupner et
al. [29] tested composites of nonwoven PLA/Lyocell and PLA/hemp-Lyocell and found
a similar tendency. In our test, the Charpy impact resistance of Lyocell/PLA composite
increased by factor 3.1, compared to factor 0.6 of Graupner. In fact, the higher impact
energies for satin fabric, compared with nonwoven laminate are mainly due to the
presence of a higher number of axially oriented fibres, as woven fabric composites
provide more balanced properties in the fabric plane than randomly oriented composites
[30]. The significant increase in impact strength of Lyocell/PLA composites can be
related to higher elongation at break [29]. The improvement of the impact strength of
hemp fibre-reinforced PLA was caused by the addition of 50 mass-% Lyocell to the
hemp fibres. The impact strength increased from 24.3 kJ/m2 for satin hemp/PLA fabric
composite up to 42.3 kJ/m2 for the satin hemp-Lyocell/PLA fabric composite.
3.6. Scanning electron microscopy
Fig. 5 shows the SEM micrographs of the fracture surface for the tensile specimens
for different composites. The SEM micrographs show that fibre pull-outs appear more
often with hemp than with Lyocell fibre. It can be interpreted as that the interfacial
adhesion between the Lyocell fibres and PLA is quite good. Actually, Lyocell fibres
are finer than hemp fibres being led to the larger specific surface between matrix and
fibres and consequently better adhesion between them. Thereby, composite from hemp
and Lyocell blending is a much stronger composite than the hemp/PLA composite.
12
4. CONCLUSIONS
The purpose of this study was the optimisation of the mechanical characteristics and
water absorption resistance of hemp fibre reinforced composites by mixing with Lyocell
fibres. According to the findings from our previous research, combining hemp and
Lyocell in nonwoven PLA composite can improve the mechanical properties when
compared with PLA/hemp reinforcements [16]. In the present study, we discuss
production of aligned hybrid yarns containing reinforcing (hemp and Lyocell and
hemp/Lyocell mixture) and thermoplastic (PLA) material, the technology for processing
them into unidirectional-satin woven fabrics (intermediates), and the manufacture of
UD composites by compression moulding. Satin-weave architecture was chosen
because of lower void content and fibre misalignments [13]. The obtained results
showed that the mechanical properties of the composites were significantly improved,
compared to the pure PLA matrix. Combining hemp and Lyocell in a hybrid yarn
(hemp-Lyocell/PLA hybrid yarn) can significantly improve the impact strength, flexural
and tensile strength and modulus, compared to the composite from hemp/PLA hybrid
yarn, which was manufactured in our previous study [13, 15]. In the general outline, the
best overall properties were achieved with satin Lyocell/PLA fabric composites leading
to a tensile strength of 101.2 MPa, Young’s modulus of 11.4 GPa, flexural strength of
158.2 MPa, flexural modulus of 9.7 GPa and impact strength of 47.4 kJ/m2. It has also
been observed that there are clear differences between the values measured for the
polymer composites from woven fabric reinforcement (woven from hybrid yarn) and
nonwoven mat, produced previously [16], due to the fibre orientation. Specimens taken
from nonwoven reinforcement show lower mechanical strength than specimens taken
from woven fabric.
13
Water absorption test indicated that moisture absorption decreased for composites
being manufactured from woven fabrics, compared to the nonwoven composites.
However, the studied woven fibre reinforcement compositions (hemp, Lyocell and
mixture of hemp and Lyocell) did not significantly affect the water absorption into the
composite.
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15
Fig. 1 Surface view of woven satin hemp-Lyocell/PLA prepreg fabric.
Fig. 2. Apparent weight gain with time for the different PLA/hemp composites at room temperature.
0
5
10
15
20
25
0 1 2 3 4 5 6 7 8 9 10
Ap
pa
ren
t w
eigh
t g
ain
(w
t%)
Day
Neat PLA
Lyocell-hemp/PLA satin
fabrics
Lyocell/PLA satin fabrics
Hemp/PLA satin fabric
Lyocell-hemp/PLA nonwoven
Hemp/PLA nonwoven
Lyocell/PLA nonwoven
16
Fig. 3. Flexural properties of PLA/hemp composites.
Fig. 4. Impact strength of PLA/hemp composites.
0
2
4
6
8
10
12
0
20
40
60
80
100
120
140
160
E-M
od
ulu
s in
GP
a
Fle
xu
ral
stre
ngth
in
MP
a Strength
E-Modulus
05
101520253035404550
Imp
act
str
eng
th in
kJ
/m2
17
Fig. 5. SEM images of tensile fracture surfaces of (a) hemp/PLA, b) hemp-Lyocell/PLA and c) Lyocell/PLA
composites.
Hemp
Lyocell
b)
Fibre pull-out
a)
18
Fig. 5 Continued.
c)
19
he
gv
ghh
PL Table 1. Specifications of PLA/hemp woven fabric used for making composite.
Sample Weft yarn Warp
yarn
Weave
type
Yarn linear
density (tex)
Density (per
cm)
Weight
(g/m2)
warp weft warp Weft
1 Hemp/PLA hybrid
yarn
PLA
Filament
8-harness
satin
18 550 8 4 344.0
(22.9)
2 Hemp-Lyocell/PLA
hybrid yarn
PLA
Filament
8-harness
satin
18 550 8 4 496.0
(18.2)
3 Lyocell/PLA hybrid
yarn
PLA
Filament
8-harness
satin
18 550 8 4 519.0
(25.3)
Note: Data in table are mean (SD).
Table 2. Composition of the fabricated composites calculated from the density measurements.
Sample Density
(g/cm3)
Fibre mass
fraction (%)
Fibre
volume
fraction (%)
Matrix
volume
fraction (%)
Porosity
volume
fraction (%)
Hemp Lyocell
PLA 1.2490
(0.0002)
0.0 0.0 0.0 100.0 -
Satin hemp/PLA fabric 1.2978
(0.0032)
30.0 0.0 26.31 (0.07) 72.74 (0.18) 0.96 (0.25) [13]
Satin Lyocell/PLA fabric 1.2725
(0.0360)
0.0 30.0 27.02 (0.03) 71.32 (2.02) 1.66 (0.57)
Satin Lyocell-hemp/PLA
fabric
1.2960
(0.0031)
15.0 15.0 26.27 (0.062) 72.63 (0.17) 1.09 (0.23)
Hemp/PLA nonwoven 1.0914
(0.0154)
30.0 0.0 23.0 (0.6) 61.2 (0.9) 10.8 (0.2) [16]
Lyocell/PLA nonwoven 1.2765
(0.0112)
0.0 30.0 25.2 (0.2) 71.5 (0.6) 3.3 (0.9) [16]
Lyocell-hemp/PLA
nonwoven
1.1222
(0.0169)
15.0 15.0 22.4 (0.3) 76.4 (1.2) 2.1 (0.1) [16]
Note: Data in table are mean (SD).
20
Table 3. Tensile properties of hemp and Lyocell/PLA-based composites, as typically reported in
literature. For comparison, reported properties of glass fibre composites are included.
Fibre
orientation
Fibre type Fibre
loading
E-modulus
(GPa)
Tensile
strength (MPa)
Reference
Random Glass reinforced PP
composite
Vf = 0.20 5.4 77 [27]
Aligned Vf = 0.60 45.0 1020 [31]
Random Glass reinforced PLA
composites
Wf = 30 - 114 [32]
Aligned Hemp-alkali treated Wf = 30
11 85 [33]
Hemp-untreated Wf = 30 8 63 [33]
Random
(*MD)
Hemp-untreated Vf = 0.21
(Wf = 30)
5.60 53.63
[16] Random (MD) Lyocell Wf = 30 6.86 80.87
Random Lyocell-hemp mixture Wf = 30 6.31 60.63
Aligned Off-axis
angle 0˚
Hemp-untreated
Vf = 0.25 8.77 72.75
[15] 45˚ 4.62 34.75
90˚ 3.70 22.01
Aligned Hemp-alkali treated Vf = 0.26 10.27 77.08
Aligned
Hemp-untreated Wf = 30 7.5 68
[34] Hemp-alkali treated Wf = 30 8.2 71
Hemp-silane treated Wf = 30 7.9 73
Hemp-alkali/silane
treated
Wf = 30 8 75
Random Hemp-untreated Wf = 30 5.6 41
[35]
Hemp-alkali treated Wf = 30 7.6 39
Aligned Hemp-untreated Wf = 35 5.8 50 [17]
Aligned Hemp-untreated Wf = 30 10.23 88.06 This study
Aligned Lyocell Wf = 30 11.42 101.23 This study
Aligned Lyocell/hemp mixture Wf = 30 11.15 96.01 This study
*MD = fibre orientation predominantly in length direction