Mechanical properties comparison of autochthonous natural fibers reinforced polyester composites: flax and hemp
J. Rocha1, J.E. Ribeiro1, L. Queijo1
1Polytechnic Institute of Bragança, ESTIG, C. Sta. Apolónia, 5301-857 Bragança, PT.
ABSTRACT
Natural fibre composites have some mechanical and environmental advantages when compared
with synthetic ones and the environmental advantage can, even, be improved if the base materials
are autochthonous. In this work are analysed two factors concerning natural fibre composites
characteristics: fibre type influence and fibre surface alkali-silane treatment. For this purpose, it
was defined an orthogonal array of experiments where the levels of sodium hydroxide (NaOH)
concentration were changed and used over flax and hemp fibres. The matrix of composite was,
always, polyester resin and six plates were manufactured with different combinations among
alkali-silane treatment and fibre types. To evaluate the composite mechanical characteristics
eighteen tensile tests were performed and it was calculated the average tensile strength for each
combination. The combination that brings the highest value of tensile strength was the flax
composite associated with the alkali-silane fibre surface treatment with 5% of concentration of
NaOH, which resulted in 113 MPa. The most influent factor to maximize the tensile strength was
the alkali-silane fibre surface treatment, with a contribution of 53.0%.
Key words: green composite, flax, hemp, natural fibre composites, polyester resin, tensile
strength
1. INTRODUCTION
Nowadays, the climate change caused by pollution increasing and, therefore, the carbon footprint
need of reduction brings the growing use of autochthonous natural materials. On the other hand,
there are a new world population demand that require the increased use of recyclable materials,
role for which the natural fibres are excellent candidates. However, only in last few years this
subject has been studied for technical applications [1]. The natural fibres are very interesting
materials when they are associated with a matrix, forming a composite material. The natural fibre
composites (NFCs) have some advantages compared with synthetic ones which can be
emphasized in its lower density, its higher specific strength and stiffness and in the fact that the
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fibres are a renewable resource which production requires little energy and involves CO2
absorption. However, NFCs have, also, disadvantages like a lower durability than synthetic fibre
composites, which can be improved, significantly, with specific treatment, a greater variability of
properties and they suffer a higher moisture absorption, which results in swelling [2].
There are many factors that can influence natural fibre reinforced composites performance, from
which the most important is the selected fibre [3]. The properties of the natural fibre reinforced
composites can, also, be influenced by fibre dispersion or fibre volume fraction as well as fibre
orientation. In general, an high fibre volume fraction is essential to accomplish high performance
of composites [4] while fibres orientation yields composites with very different properties [5]. It
is, frequently, observed that the increase in fibre loading leads to a growth of tensile properties
[6]. An additional factor which has an important influence in NFCs mechanical properties is the
composites interfacial strength that can be changed using chemical treatments [7]. Hence,
appropriate processing techniques and parameters must be cautiously selected in order to yield
optimum composite materials [8]. Despite all these factors, matrix selection is also a very
important factor which must be analysed to characterize the NFCs mechanical properties [9]. In
this work, the studied factors that affect NFCs mechanical performance are the fibre type and the
interfacial strength which was changed by chemical treatments.
Some of the most used fibres used in Portugal northern region are hemp and flax. These fibres
have origin in plants that have been grown in fields in country’s northeast. In the territory that is
now Portugal, growing of flax and its derived fabrics manufacturing date from prehistory. There
are traces of linen dated form 2500 BC in the Algarve region. More recently, in the 19th century,
flax cultivation had a great economic and social importance in the North of Portugal, having
suffered a decline with the emergence of simpler and cheaper fibres as was the case of cotton. In
several historical periods it was tried to relaunch the linen industry [10]. In Portugal’s northeast,
more specifically in Vilariça valley, hemp was grown for cables manufacturing in the National
Cordage, which equipped caravels and other ships during Portuguese discoveries period, across
15th and 16th centuries. In the 20th century there were several hemp producers in Vilariça valley.
Industrial hemp growth has its complexity once it is a variant of said cannabis (Cannabis sativa),
and these two variants differ only in terms of THC content (TetraHidroCannabiol), thus the
production of this plant is regulated by law [11].
The interfacial adhesion between fibres and matrix plays an important role over mechanical
properties of composites. As the stress is transferred from fibres to fibres across the matrix
interface, a good interfacial adhesion is required to reach good reinforcement, nevertheless, if the
interface is too strong, fissures are enable to propagate which can reduce toughness and strength.
Though, for fibre composites based on plant, the interaction between the fibres, usually
hydrophilic, and matrices that are, generally, hydrophobic is very limited which leads to a low
interfacial adhesion affecting the mechanical properties. In other hand, a weak humidity
resistance decreases the properties at long time period. To guarantee a good adhesion, matrix and
fibre must be very closed and, as is usual in any adhesion phenomenon, the property of
wettability is fundamental to assure it between the adhesive and adherent. In this particular case,
insufficient fibre wetting origins interfacial flaws that can act as stress concentrators [12] and
affect mechanical properties [13]. There are different types of fibre surface treatment divided by
type: physical and chemical, that can improve the wettability of the fibre and therefore improve
the interfacial strength [14] [15] [16].
Interfacial adhesion may happen by means of mechanisms of chemical bonding, inter-diffusion
bonding, mechanical interlocking, and electrostatic bonding [17]. To improve interfacial adhesion
in NFCs it has been used a chemical approach. Chemical approach can be divided in many
different techniques that use chemical products like zirconate, peroxide, benzyl, acryl, titanate,
permanganate, acetyl, alkali and silane, among others [18]. These products can be used in a single
way or combined among them [7].The most widespread used products are the alkali, acetyl and
silane [19] and, for this reason, it was chosen the alkali-silane treatment [20] to implement in this
work. Alkaline treatment consists in immerging the fibres in an alkaline solution, normally
NaOH, for a period of time. This treatment removes fibre constituents including lignin,
hemicellulose, pectin and wax which exposes cellulose and increases surface roughness per area
providing an improved interfacial adhesion [21]. Silane treatment, generally, involves moisten the
fibres in a solution of diluted silane in a water/alcohol mixture that will be broke down into
silanol and alcohol by the water presence. Silanol reacts with the cellulose OH groups in the
natural fibres, forming stable covalent bonds in the cell walls [22]. Silane treatment improves the
amount of reticulation in the interface region and increases the fibre surface area, implementing a
stronger adhesion between the matrix and the fibre [23].
To evaluate the influence of fibre type and chemical treatment it is need to develop experimental
work in which there are used different factors combination and, for that reason, it is important to
organize experiments, systematically, using experimental techniques design. These techniques
allow to conveniently organize the experiments and, then, make results statistical processing. The
first published works using experimental design were done by Fisher [24], who used, initially,
these techniques in agriculture field. His technique was based in factorial design in which it was
created an orthogonal array with multi factors and different levels for each factor.
The factorial design has different approaches that depend on the number of experiments that the
researcher wants to do, general or fractional. In general factorial design, all the possible
combination must be done, so, this approach is only viable if the number of factors and levels are
few. However, if the number of factor and levels is high it is unpractical to implement all
experiments for economical and time costs. There are some methods which use the fractional
approach, but, the more popular method was developed by Genichi Taguchi (Tokamachi, Japan,
1924-2012) [25]. In the present work, where are only used two-factor - two types of fibre and its
surface treatment, it is appropriate to implement an experiment design model based in the general
factorial approach.
2. EXPERIMENTAL PROCEDURE
2.1 Materials and properties
The natural fibre composites are constituted by the matrix and fibres. For this case, the matrix
used in this work was the polyester with 1% of catalyst to activate the polymerization. The used
natural fibres were flax and hemp which were produced in northeast of Portugal. The typical
mechanical properties of these materials are represented in Table 1.
Table 1. Mechanical Properties of matrix and natural fibres.
Density [g/cm3] Tensile strength
[MPa] Young’s modulus
[GPa]
Matrix Polyester [26] 1.2 70-103 2.1-4.4
Natural Fibre Flax [1] 1.5 345-1830 27-80
Hemp [1] 1.5 550-1100 58-70
For the alkali-silane treatment were prepared different chemical solutions which are specific for
each treatment. Thus, the solution of sodium hydroxide (NaOH) with concentrations of 2% and
5% by mass were used for the alkali treatment. On the other hand, the silane treatment was
performed using of 5% by weight of three-aminopropyltriethoxysilane, which was diluted in a
50% aqueous solution of methanol.
2.2 Plan of experiments
The experiments were conducted in agreement with a standard orthogonal array where each
column defines the factor control. So, the first column is correspondent to the type of fibre (A)
and the second one is used for the fibre surface alkali-silane treatment (B), as can be seen in table
2.
Table 1. Orthogonal array of experiments. Experiment A B
1 1 1
2 2 1
3 1 2
4 2 2
5 1 3
6 2 3
As said before, in this work were used two types of natural fibres which are the hemp and flax
from which some of them were used untreated and others suffered a fiber surface alkali-silane
treatment with 2% and 5% of NaOH concentration. The reason that led to choose these
concentrations was based in the paper published by Asumani et al. [20]. According with their
work, the maximum improvement in tensile strength occurred for the 5% of NaOH concentration
and 2% corresponds to an intermediate value between the untreated situation and the 5% of
concentration. In table 2 are presented the levels for each factor.
Table 2. Factors and respectively levels.
Levels
1 2 3
Factors A – Type of fibre Hemp Flax -
B - Surface alkali-silane treatment Untreated 2% of NaOH 5% of NaOH
2.3 Specimen manufacturing and tensile teste
The specimens used in tensile test to determine the tensile strength of each factors and levels
combination were cut from a composite plate (200x200x1mm) manufactured previously. The
fibres of these composite plates were aligned in one direction which is correspondent to the
direction of tensile load application. Firstly, to prepare the composite plate, the fibres, untreated
and treated, were curl around a steel plate with 2 mm of thickness using a lathe. To perform this
operation, the metallic plate was previously greased with release agent and, after that, fixed in the
chuck and set the spindle in a very slow rotation while the fibres were wrapped around the plate.
During the process it was guarantee that the fibres were very close and aligned in one direction.
Subsequently to this operation, the metallic plate with the fibres were immersed in the polyester
resin (99% of resin with 1% of catalyst) during proximally to 30 seconds and while un-
polymerized, it was placed into a two parts mould. This mould was placed on a press table and it
was applied a pressure of 1 MPa [27] over the composite plate. After 12 hours of polymerization
in the press, the composite was removed from the mould and the metallic plate was taken away
from the middle of composite. By cutting the composite set by same plane of fibres direction was
possible to obtain two composite plates as seen in figure 1.
Figure 1. Two composite plates obtained after the manufacturing process.
The specimens were cut from the composite plates using a laser system (X252 from GCC). The
dimensions and geometry of specimens were chosen according with ASTM D 3039M standard
[28]. In figure 2, it is possible to observe some specimen examples used for tensile tests. For
each, corresponding a combination of different factors and levels, were cut three specimens in a
total of 18.
Figure 2. Composite specimens for tensile tests.
The alkali-silane treatment of fibres was done in two steps, the first step was the alkali treatment
using sodium hydroxide (NaOH) solutions with concentrations 2% and 5% by mass. Hemp and
flax fibre mats were immersed in the NaOH solution for 24 h at a temperature of 45 ºC. After this
period, the fibres were washed with tap water and submerged in distilled water which contained
1% acetic acid to neutralise the residual NaOH. These fibres were, then, dried in an oven at 45 ºC
during 12 h. The second step was the silane treatment. For this treatment it was used a solution of
5% three-aminopropyltriethoxysilane by weight (weight of silane relative to the weight of hemp
or flax mat) diluted in a 50% aqueous solution of methanol. The pH of the solution was preserved
between 4 and 5 using acetic acid. The mats were submerged in the solution for 4 h at a
temperature of 28 ºC. After that period, they were washed with distilled water and, finally, dried
in the oven at 45 ºC for a period of 12 h.
After manufacturing the specimens, the tensile tests were performed using a universal test
machine (Instron 4485). The specimens were fixed in the machine grips and the tensile test was
performed with the test speed of 1 mm/s. For each experiment (table 1) were executed three
tensile tests, corresponding to a total of 18 trials.
In figure 3 it is possible to see the graphical representation for the extracted tensile tests data
considering the percentage variation on surface treatment. First row indicates Hemp data for 0%,
2% and 5% (from left to right) while in the second row, the same date is shown for Flax.
Figure 3. Tensile tests with different surface treatment percentages (Hemp in the top, Flax in the bottom
and 0%, 2% and 5% from left to right).
3. RESULTS AND DISCUSSION
In table 3 and figure 4 it is possible to observe the average of obtained results. Analysing the table
3 and figure 4 it is seen that the experiments 1, 5 and 6 give the higher values of tensile strength,
especially, the experiment 6 in which a flax composite was tested using an alkali-silane surface
treatment with 5% of concentration of NaOH.
Table 3. Experimental results: average tensile strength and standard deviation. Experiment A B Average Tensile Strength [MPa] Standard Deviation [MPa]
1 1 1 112.8 11.8
2 2 1 49.7 4.6
3 1 2 70.8 12.7
4 2 2 32.2 4.7
5 1 3 102.0 25.4
6 2 3 113.4 7.8
In table 3, is also possible to verify that the experiment 5 has the highest value of standard
deviation, 25.4 MPa, which means that the data is widely spread (less reliable), while the
experiment 2, 4 and 6 have values of standard deviations are relatively low for NFCs which are
4.6 MPa, 4.7 MPa and 7.8 MPa, respectively, what, in other words, means that the experimental
data are closely clustered around the mean (more reliable).
Sticky NoteImpossible to read this figure. Enlarge and make it clearer.
Figure 4. The average tensile strength for each tensile test.
Using the data presented in table 3 is possible to obtain an analysis of variance (ANOVA). The
ANOVA is a group of statistical models used to analyse the changes among group averages and
their associated procedures developed by Ronald Fisher [24]. In table 4 is presented the ANOVA
for the performed tensile tests.
Table 4. ANOVA results. Source DF Adj SS Adj MS F-Value % contribution
A – Type o fibre 1 1358 1358.1 1.89 22.8
B – Surface treatment 2 3157 1578.6 2.19 53.0
RESIDUAL 2 1441 720.3 24.2
TOTAL 5 5956
In this table, DF are the degrees of freedom, Adj. SS are the sum of squares, Adj. MS are the
mean squares. The F test is a statistical tool to determine which are the parameters that more
significantly affect the quality. Observing the table 4, it turns out that the chemical surface
treatment of the fibre has the largest influence to tensile strength 53.0%.
4. CONCLUSIONS
The influence of two fibres type and alkali-silane surface treatment on the tensile strength of
NFCs was studied in this work. It was implement an array of experiments using three levels for
the surface treatment and two levels for type of fibres.
0
20
40
60
80
100
120
1 2 3 4 5 6
Ave
rage
Ten
sile
Ste
ngth
[M
Pa]
Test Number
Sticky NoteInclude error bars.
To determine the tensile strength of NFCs went performed a total of 18 tensile tests. From these
tests it was verified that the combination that brings the higher value of tensile strength was the
flax composite associated an alkali-silane surface treatment with 5% of concentration of NaOH
and, for this case, the average tensile strength had the value of 113.4 MPa. Based on the
experimental results and using the ANOVA approach it was determined that the most influent
factor to maximize the tensile strength was the alkali-silane surface treatment, with a contribution
of 53.0%.
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Mechanical properties comparison of autochthonous natural fibers reinforced polyester composites: flax and hemp
J. Rocha1, J.E. Ribeiro1, L. Queijo1
1Polytechnic Institute of Bragança, ESTIG, C. Sta. Apolónia, 5301-857 Bragança, PT.
ABSTRACT
Natural fibre composites have some mechanical and environmental advantages when compared
with synthetic ones and the environmental advantage can, even, be improved if the base materials
are autochthonous. In this work are analysed two factors concerning natural fibre composites
characteristics: fibre type influence and fibre surface alkali-silane treatment. For this purpose, it
was defined an orthogonal array of experiments where the levels of sodium hydroxide (NaOH)
concentration were changed and used over flax and hemp fibres. The matrix of composite was,
always, polyester resin and six plates were manufactured with different combinations among
alkali-silane treatment and fibre types. To evaluate the composite mechanical characteristics
eighteen tensile tests were performed and it was calculated the average tensile strength for each
combination. The combination that brings the highest value of tensile strength was the flax
composite associated with the alkali-silane fibre surface treatment with 5% of concentration of
NaOH, which resulted in 113 MPa. The most influent factor to maximize the tensile strength was
the alkali-silane fibre surface treatment, with a contribution of 53.0%.
Key words: green composite, flax, hemp, natural fibre composites, polyester resin, tensile
strength
1. INTRODUCTION
Nowadays, the climate change caused by pollution increasing and, therefore, the carbon footprint
need of reduction brings the growing use of autochthonous natural materials. On the other hand,
there are a new world population demand that require the increased use of recyclable materials,
role for which the natural fibres are excellent candidates. However, only in last few years this
subject has been studied for technical applications [1]. The natural fibres are very interesting
materials when they are associated with a matrix, forming a composite material. The natural fibre
composites (NFCs) have some advantages compared with synthetic ones which can be
emphasized in its lower density, its higher specific strength and stiffness and in the fact that the
Sticky NoteIndicate corresponding author.
Give email of all authors.
Cross-Out
Inserted Textto
Cross-Out
Inserted Text,
Inserted Text are analysed
Cross-Out
Inserted Textwas defined
Inserted Textthe
Inserted Text,
Cross-Out
Inserted Text was calculated
Sticky NoteProofread English by native speaker.
fibres are a renewable resource which production requires little energy and involves CO2
absorption. However, NFCs have, also, disadvantages like a lower durability than synthetic fibre
composites, which can be improved, significantly, with specific treatment, a greater variability of
properties and they suffer a higher moisture absorption, which results in swelling [2].
There are many factors that can influence natural fibre reinforced composites performance, from
which the most important is the selected fibre [3]. The properties of the natural fibre reinforced
composites can, also, be influenced by fibre dispersion or fibre volume fraction as well as fibre
orientation. In general, an high fibre volume fraction is essential to accomplish high performance
of composites [4] while fibres orientation yields composites with very different properties [5]. It
is, frequently, observed that the increase in fibre loading leads to a growth of tensile properties
[6]. An additional factor which has an important influence in NFCs mechanical properties is the
composites interfacial strength that can be changed using chemical treatments [7]. Hence,
appropriate processing techniques and parameters must be cautiously selected in order to yield
optimum composite materials [8]. Despite all these factors, matrix selection is also a very
important factor which must be analysed to characterize the NFCs mechanical properties [9]. In
this work, the studied factors that affect NFCs mechanical performance are the fibre type and the
interfacial strength which was changed by chemical treatments.
Some of the most used fibres used in Portugal northern region are hemp and flax. These fibres
have origin in plants that have been grown in fields in country’s northeast. In the territory that is
now Portugal, growing of flax and its derived fabrics manufacturing date from prehistory. There
are traces of linen dated form 2500 BC in the Algarve region. More recently, in the 19th century,
flax cultivation had a great economic and social importance in the North of Portugal, having
suffered a decline with the emergence of simpler and cheaper fibres as was the case of cotton. In
several historical periods it was tried to relaunch the linen industry [10]. In Portugal’s northeast,
more specifically in Vilariça valley, hemp was grown for cables manufacturing in the National
Cordage, which equipped caravels and other ships during Portuguese discoveries period, across
15th and 16th centuries. In the 20th century there were several hemp producers in Vilariça valley.
Industrial hemp growth has its complexity once it is a variant of said cannabis (Cannabis sativa),
and these two variants differ only in terms of THC content (TetraHidroCannabiol), thus the
production of this plant is regulated by law [11].
The interfacial adhesion between fibres and matrix plays an important role over mechanical
properties of composites. As the stress is transferred from fibres to fibres across the matrix
interface, a good interfacial adhesion is required to reach good reinforcement, nevertheless, if the
interface is too strong, fissures are enable to propagate which can reduce toughness and strength.
Though, for fibre composites based on plant, the interaction between the fibres, usually
hydrophilic, and matrices that are, generally, hydrophobic is very limited which leads to a low
interfacial adhesion affecting the mechanical properties. In other hand, a weak humidity
resistance decreases the properties at long time period. To guarantee a good adhesion, matrix and
fibre must be very closed and, as is usual in any adhesion phenomenon, the property of
wettability is fundamental to assure it between the adhesive and adherent. In this particular case,
insufficient fibre wetting origins interfacial flaws that can act as stress concentrators [12] and
affect mechanical properties [13]. There are different types of fibre surface treatment divided by
type: physical and chemical, that can improve the wettability of the fibre and therefore improve
the interfacial strength [14] [15] [16].
Interfacial adhesion may happen by means of mechanisms of chemical bonding, inter-diffusion
bonding, mechanical interlocking, and electrostatic bonding [17]. To improve interfacial adhesion
in NFCs it has been used a chemical approach. Chemical approach can be divided in many
different techniques that use chemical products like zirconate, peroxide, benzyl, acryl, titanate,
permanganate, acetyl, alkali and silane, among others [18]. These products can be used in a single
way or combined among them [7].The most widespread used products are the alkali, acetyl and
silane [19] and, for this reason, it was chosen the alkali-silane treatment [20] to implement in this
work. Alkaline treatment consists in immerging the fibres in an alkaline solution, normally
NaOH, for a period of time. This treatment removes fibre constituents including lignin,
hemicellulose, pectin and wax which exposes cellulose and increases surface roughness per area
providing an improved interfacial adhesion [21]. Silane treatment, generally, involves moisten the
fibres in a solution of diluted silane in a water/alcohol mixture that will be broke down into
silanol and alcohol by the water presence. Silanol reacts with the cellulose OH groups in the
natural fibres, forming stable covalent bonds in the cell walls [22]. Silane treatment improves the
amount of reticulation in the interface region and increases the fibre surface area, implementing a
stronger adhesion between the matrix and the fibre [23].
To evaluate the influence of fibre type and chemical treatment it is need to develop experimental
work in which there are used different factors combination and, for that reason, it is important to
organize experiments, systematically, using experimental techniques design. These techniques
allow to conveniently organize the experiments and, then, make results statistical processing. The
first published works using experimental design were done by Fisher [24], who used, initially,
these techniques in agriculture field. His technique was based in factorial design in which it was
created an orthogonal array with multi factors and different levels for each factor.
The factorial design has different approaches that depend on the number of experiments that the
researcher wants to do, general or fractional. In general factorial design, all the possible
combination must be done, so, this approach is only viable if the number of factors and levels are
few. However, if the number of factor and levels is high it is unpractical to implement all
experiments for economical and time costs. There are some methods which use the fractional
approach, but, the more popular method was developed by Genichi Taguchi (Tokamachi, Japan,
1924-2012) [25]. In the present work, where are only used two-factor - two types of fibre and its
surface treatment, it is appropriate to implement an experiment design model based in the general
factorial approach.
2. EXPERIMENTAL PROCEDURE
2.1 Materials and properties
The natural fibre composites are constituted by the matrix and fibres. For this case, the matrix
used in this work was the polyester with 1% of catalyst to activate the polymerization. The used
natural fibres were flax and hemp which were produced in northeast of Portugal. The typical
mechanical properties of these materials are represented in Table 1.
Table 1. Mechanical Properties of matrix and natural fibres.
Density [g/cm3] Tensile strength
[MPa] Young’s modulus
[GPa]
Matrix Polyester [26] 1.2 70-103 2.1-4.4
Natural Fibre Flax [1] 1.5 345-1830 27-80
Hemp [1] 1.5 550-1100 58-70
For the alkali-silane treatment were prepared different chemical solutions which are specific for
each treatment. Thus, the solution of sodium hydroxide (NaOH) with concentrations of 2% and
5% by mass were used for the alkali treatment. On the other hand, the silane treatment was
performed using of 5% by weight of three-aminopropyltriethoxysilane, which was diluted in a
50% aqueous solution of methanol.
2.2 Plan of experiments
The experiments were conducted in agreement with a standard orthogonal array where each
column defines the factor control. So, the first column is correspondent to the type of fibre (A)
and the second one is used for the fibre surface alkali-silane treatment (B), as can be seen in table
2.
Table 1. Orthogonal array of experiments. Experiment A B
1 1 1
2 2 1
3 1 2
4 2 2
5 1 3
6 2 3
As said before, in this work were used two types of natural fibres which are the hemp and flax
from which some of them were used untreated and others suffered a fiber surface alkali-silane
treatment with 2% and 5% of NaOH concentration. The reason that led to choose these
concentrations was based in the paper published by Asumani et al. [20]. According with their
work, the maximum improvement in tensile strength occurred for the 5% of NaOH concentration
and 2% corresponds to an intermediate value between the untreated situation and the 5% of
concentration. In table 2 are presented the levels for each factor.
Table 2. Factors and respectively levels.
Levels
1 2 3
Factors A – Type of fibre Hemp Flax -
B - Surface alkali-silane treatment Untreated 2% of NaOH 5% of NaOH
2.3 Specimen manufacturing and tensile teste
The specimens used in tensile test to determine the tensile strength of each factors and levels
combination were cut from a composite plate (200x200x1mm) manufactured previously. The
fibres of these composite plates were aligned in one direction which is correspondent to the
direction of tensile load application. Firstly, to prepare the composite plate, the fibres, untreated
and treated, were curl around a steel plate with 2 mm of thickness using a lathe. To perform this
operation, the metallic plate was previously greased with release agent and, after that, fixed in the
chuck and set the spindle in a very slow rotation while the fibres were wrapped around the plate.
During the process it was guarantee that the fibres were very close and aligned in one direction.
Subsequently to this operation, the metallic plate with the fibres were immersed in the polyester
resin (99% of resin with 1% of catalyst) during proximally to 30 seconds and while un-
polymerized, it was placed into a two parts mould. This mould was placed on a press table and it
was applied a pressure of 1 MPa [27] over the composite plate. After 12 hours of polymerization
in the press, the composite was removed from the mould and the metallic plate was taken away
from the middle of composite. By cutting the composite set by same plane of fibres direction was
possible to obtain two composite plates as seen in figure 1.
Figure 1. Two composite plates obtained after the manufacturing process.
The specimens were cut from the composite plates using a laser system (X252 from GCC). The
dimensions and geometry of specimens were chosen according with ASTM D 3039M standard
[28]. In figure 2, it is possible to observe some specimen examples used for tensile tests. For
each, corresponding a combination of different factors and levels, were cut three specimens in a
total of 18.
Figure 2. Composite specimens for tensile tests.
The alkali-silane treatment of fibres was done in two steps, the first step was the alkali treatment
using sodium hydroxide (NaOH) solutions with concentrations 2% and 5% by mass. Hemp and
flax fibre mats were immersed in the NaOH solution for 24 h at a temperature of 45 ºC. After this
period, the fibres were washed with tap water and submerged in distilled water which contained
1% acetic acid to neutralise the residual NaOH. These fibres were, then, dried in an oven at 45 ºC
during 12 h. The second step was the silane treatment. For this treatment it was used a solution of
5% three-aminopropyltriethoxysilane by weight (weight of silane relative to the weight of hemp
or flax mat) diluted in a 50% aqueous solution of methanol. The pH of the solution was preserved
between 4 and 5 using acetic acid. The mats were submerged in the solution for 4 h at a
temperature of 28 ºC. After that period, they were washed with distilled water and, finally, dried
in the oven at 45 ºC for a period of 12 h.
After manufacturing the specimens, the tensile tests were performed using a universal test
machine (Instron 4485). The specimens were fixed in the machine grips and the tensile test was
performed with the test speed of 1 mm/s. For each experiment (table 1) were executed three
tensile tests, corresponding to a total of 18 trials.
In figure 3 it is possible to see the graphical representation for the extracted tensile tests data
considering the percentage variation on surface treatment. First row indicates Hemp data for 0%,
2% and 5% (from left to right) while in the second row, the same date is shown for Flax.
Figure 3. Tensile tests with different surface treatment percentages (Hemp in the top, Flax in the bottom
and 0%, 2% and 5% from left to right).
3. RESULTS AND DISCUSSION
In table 3 and figure 4 it is possible to observe the average of obtained results. Analysing the table
3 and figure 4 it is seen that the experiments 1, 5 and 6 give the higher values of tensile strength,
especially, the experiment 6 in which a flax composite was tested using an alkali-silane surface
treatment with 5% of concentration of NaOH.
Table 3. Experimental results: average tensile strength and standard deviation. Experiment A B Average Tensile Strength [MPa] Standard Deviation [MPa]
1 1 1 112.8 11.8
2 2 1 49.7 4.6
3 1 2 70.8 12.7
4 2 2 32.2 4.7
5 1 3 102.0 25.4
6 2 3 113.4 7.8
In table 3, is also possible to verify that the experiment 5 has the highest value of standard
deviation, 25.4 MPa, which means that the data is widely spread (less reliable), while the
experiment 2, 4 and 6 have values of standard deviations are relatively low for NFCs which are
4.6 MPa, 4.7 MPa and 7.8 MPa, respectively, what, in other words, means that the experimental
data are closely clustered around the mean (more reliable).
Sticky NoteImpossible to read this figure. Enlarge and make it clearer.
Figure 4. The average tensile strength for each tensile test.
Using the data presented in table 3 is possible to obtain an analysis of variance (ANOVA). The
ANOVA is a group of statistical models used to analyse the changes among group averages and
their associated procedures developed by Ronald Fisher [24]. In table 4 is presented the ANOVA
for the performed tensile tests.
Table 4. ANOVA results. Source DF Adj SS Adj MS F-Value % contribution
A – Type o fibre 1 1358 1358.1 1.89 22.8
B – Surface treatment 2 3157 1578.6 2.19 53.0
RESIDUAL 2 1441 720.3 24.2
TOTAL 5 5956
In this table, DF are the degrees of freedom, Adj. SS are the sum of squares, Adj. MS are the
mean squares. The F test is a statistical tool to determine which are the parameters that more
significantly affect the quality. Observing the table 4, it turns out that the chemical surface
treatment of the fibre has the largest influence to tensile strength 53.0%.
4. CONCLUSIONS
The influence of two fibres type and alkali-silane surface treatment on the tensile strength of
NFCs was studied in this work. It was implement an array of experiments using three levels for
the surface treatment and two levels for type of fibres.
0
20
40
60
80
100
120
1 2 3 4 5 6
Ave
rage
Ten
sile
Ste
ngth
[M
Pa]
Test Number
Sticky NoteInclude error bars.
To determine the tensile strength of NFCs went performed a total of 18 tensile tests. From these
tests it was verified that the combination that brings the higher value of tensile strength was the
flax composite associated an alkali-silane surface treatment with 5% of concentration of NaOH
and, for this case, the average tensile strength had the value of 113.4 MPa. Based on the
experimental results and using the ANOVA approach it was determined that the most influent
factor to maximize the tensile strength was the alkali-silane surface treatment, with a contribution
of 53.0%.
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