mechanical properties comparison of autochthonous natural … · 2020. 5. 6. · mechanical...

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).

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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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