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Effect of water absorption on the mechanical properties of hybrid interwoven cellulosic-cellulosic fibre reinforced epoxy composites
Composite Structures 167 (2017) 227–237
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Effect of water absorption on the mechanical properties of hybrid interwoven cellulosic-cellulosic fibre reinforced epoxy composites A.B. Maslinda a,b, M.S. Abdul Majid a,⇑, M.J.M. Ridzuan a, M. Afendi a, A.G. Gibson c a
School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, 02600 Arau, Perlis, Malaysia Faculty of Engineering Technology, Universiti Malaysia Perlis, Uniciti Alam Campus, 02100 Padang Besar, Perlis, Malaysia c School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK b
a r t i c l e
i n f o
Article history: Received 5 December 2016 Revised 16 December 2016 Accepted 3 February 2017 Available online 6 February 2017 Keywords: Natural fibre Hybrid Mechanical properties Strength Fibre/matrix bond
a b s t r a c t The absorption behaviour of water and its effect on the tensile and flexural properties of interwoven cellulosic fibres were investigated. Hybrid composites consisting of interwoven kenaf/jute and kenaf/hemp yarns were prepared by an infusion process that used epoxy as the polymer matrix. The water absorption characteristics of the fibres were obtained by immersing the composite samples in tap water at room temperature, until reaching their water content saturation point. The dry and water-immersed woven and interwoven hybrid composite samples were subjected to tensile and flexural tests. To study the effect of water penetration in the fibre/matrix interface, fractured samples were examined using field emission scanning electron microscopy (FESEM). The study shows that the mechanical and water-resistant properties of the kenaf, jute, and hemp fibres were improved through hybridization. However, as a result of water penetrating the fibre/matrix interface, longer water-immersion times reduced the tensile and flexural strength of the composites. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Due to environmental concerns, the potential use of natural fibres as replacements of synthetic fibres has been investigated. In comparison with glass fibres, one of the advantages of using natural fibres is their low density, which endows them with excellent specific mechanical properties, easier handling and processing, recyclability, and good thermal and acoustic insulation [1]. Despite these advantages, natural fibre reinforced composites have several drawbacks that limit their application, such as a low strength, variability in quality, high moisture absorption, limited processing temperature, and a lesser durability and incompatibility between fibres and polymer matrices [1,2]. However, through continuous studies, researchers have come out with a number of methods and treatments to improve the performance of natural fibre composites. Nowadays, natural fibre composites have been widely implemented in load bearing and outdoor applications, such as in the exterior underfloor panelling of cars, sports equipment, and marine structures [3]. Faruk et al. [4] summarised the development ⇑ Corresponding author. E-mail addresses: [email protected] (A.B. Maslinda), [email protected] (M.S. Abdul Majid), [email protected] (M.J.M. Ridzuan), afendirojan@ gmail.com (M. Afendi), [email protected] (A.G. Gibson). http://dx.doi.org/10.1016/j.compstruct.2017.02.023 0263-8223/Ó 2017 Elsevier Ltd. All rights reserved.
of biocomposites from the years 2000 to 2010, and reported that flax, jute, hemp, sisal, ramie, and kenaf fibres are among the most widely used and studied fibre materials. Hybrid composites are commonly termed as the mixture of two or more reinforcing fibres in a single matrix system. Through proper fibre selection and design, the balance between cost and performance of hybrid composites could be achieved through hybridization [5]. For example, the incorporation of glass with different cellulosic fibres such as abaca, jute, banana, hemp, and napier have been previously reported in the literature [6–9], with results that showed the mechanical properties of the hybrid composites being superior to those of the single-fibre reinforced composites. There are also published works about synthetic/synthetic [10], and cellulosic/cellulosic [11–13] fibres based on reinforced hybrid composites. Furthermore, the performance of a hybrid kenaf/kevlar composite was investigated by Yahaya et al. [14,15], who found that the composite had the potential to be used in impact applications. In the industry, FlexForm Technologies mixed hemp with kenaf fibre and used it in Chrysler’s Sebring door panels, and the company is currently in the process of using the natural composite in other automotive parts [5]. A review by Swolfs et al. [16] indicates that hybridization is a method that can be used to increase the toughness of fibre reinforced composites, and that
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there are three main hybrid configurations that can be used to combine two fibres: the (1) interlayer or layer-by-layer, (2) intra-layer or yarn-by-yarn, and (3) intra-yarn or fibre-by-fibre configurations. One important factor that contributes to the excellent properties of hybrid composites is the fibre orientation. For example, a continuous and long fibre aligned parallel to the load direction exhibits a higher strength, in comparison with a randomly oriented fibre [3]. To attain a greater degree of fibre alignment, different textile processing methods such as weaving, knitting, and braiding can be applied [17]. Specifically, weaving is the interlacing of two sets of yarns to construct a woven fabric. In a plain weave, for example, the warp is defined as the yarn that runs along the length of the fabric, and the weft is defined as the yarn that has been weaved over and underneath the warp yarn [18]. As reported in the literature [19,20], the tensile, flexural and impact properties of woven fabric composites are higher than those of unidirectional and randomly oriented composites. In woven structures, the stress is uniformly transferred, and the interlocking between the reinforced fibres further increases the strength of the composite [21]. Khan et al. [19] investigated the influence of the fibre direction and found that the mechanical properties of a woven jute reinforced poly(l-lactic acid) composite are greater when tested along the warp direction, than along the weft direction. A woven banana reinforced epoxy composite was used by Sapuan et al. [22] in the fabrication of household telephone stands, and they proved that the natural based composite was able to replace the conventional material in the furniture industry. In regards to natural based composites, the presence of polar groups (e.g. hydroxyl, among others) in natural fibres allows them to absorb high amounts of moisture, and also makes them incompatible with the polymer matrix. Together, these two factors lead to a reduced fibre/matrix bonding strength, which results in a weaker mechanical performance of fibre/matrix hybrid composites [23,24]. Akil et al. [25] found that when pultruded jute/glass fibre reinforced polyester hybrid composites were exposed to humidity or aqueous environments, water molecules penetrated the composites through three routes, as follows: through the flaws at the composites interphase attributed to the poor wettability between the fibre and matrix, through micro-gaps between the polymer chains, and through the cracks in the polymer matrix induced by the fibre swelling. In this work, water absorption and its effect on the mechanical properties of interwoven kenaf/jute and kenaf/hemp hybrid composites were experimentally investigated as an extension of the research regarding hybrid- and woven-structured composites. Up to date, only few works have been conducted involving interwoven cellulosic fibres, and this study will provide useful information to the research community. 2. Experimental procedure 2.1. Materials The kenaf, jute, and hemp yarns that were to produce the woven fabrics were obtained from a local supplier. The EpoxAmite 100 series resin was selected as the polymer matrix, and it was mixed with a hardener in a ratio of 3:1 to form the binder for the composite preparation. The chemical compositions of the
reinforcement fibres are listed in Table 1, while Table 2 details their mechanical properties, alongside those of the matrix. The kenaf, jute and hemp fibres were chosen as reinforcements because they are easily available in Malaysia, and they are also among the most widely used materials in the natural fibre reinforced polymer composite industry [3]. 2.2. Woven fabric production Woven fabrics were produced by weaving the fibre yarns using a wooden frame. The wooden frame (400 mm 400 mm) was manually constructed, and it contained nails that acted as the warp yarn guider on both of its sides. The weaving process was done by passing the weft yarn over and underneath the warp yarn, which had been previously arranged on the frame with the help of the warp yarn guider. The wooden frame, weaving process, and completed woven fabric are shown in Fig. 1. Fig. 2 illustrates the construction of the interwoven kenaf/jute and kenaf/hemp hybrid composites, where the interlacing of the kenaf yarns followed the warp fibre direction, while the jute and hemp yarns were arranged in the weft fibre direction. 2.3. Composite fabrication A vacuum infusion manufacturing technique was used to prepare the composite specimens, which had a fibre weight content of 30% ± 2%. Five types of composites were prepared, and the symbols that represent each type of composite are listed in Table 3. The fabrication process began by polishing the glass surface with acetone to remove any dirt and balanced resin from the previous infusion process. A thin layer of wax, or catalyst, was used for the easy removal of the composites after infusion. The woven fabrics were positioned on the glass surface followed by a peel ply, netting, and an enka channel, as shown in Fig. 3. The resin inlet and outlet, which were made from a PVC hose, were placed over the mould area before it was wrapped with a plastic sheet. The vacuum pump was switched on, and the in-mould pressure was controlled at below 2000 Pa, to make sure that the air was fully evacuated. The resin mixture, which was prepared according to the manufacturer’s specification, was infused into the mould, where it flowed evenly until it reached the end. Excess resin flowed into a resin trap. The infused composite was removed from the mould and cured for 24 h at room temperature (25 °C). 2.4. Moisture absorption test The water absorption tests were carried out as elucidated in the ASTM D570 standard [31]. First, the specimens were submerged in a container filled with tap water at room temperature for up to 1400 h. Thereafter, to monitor the mass during the ageing process, the specimens were withdrawn from the water, wiped dry to remove any surface moisture, and then weighted using a high accuracy 4-digit analytical balance. For each type of composite, five specimens were tested, and the average result was recorded. The moisture content percentage, DM(t), was calculated using the following equation:
DMðtÞ ¼
Mt M0 100 M0
ð1Þ
Table 1 Chemical composition of the reinforcement [4]. Constituent/Fibre
Cellulose (wt%)
Hemicellulose (wt%)
Lignin (wt%)
Waxes (wt%)
Kenaf Jute Hemp
72 61–71 68
20.3 14–20 15
9 12–13 10
– 0.5 0.8
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Tensile strength (MPa)
Elastic modulus (GPa)
Strain at failure (%)
Moisture absorption rate (%)
References
Kenaf Jute Hemp Epoxy
930 393–773 690 55
53 26.5 70 1.75
1.6 1.5–1.8 2–4 6
17–20 12 8 –
[4,20,26–29] [25,26,28,29] [25,26,28] [30]
Fig. 1. Woven fabric fabrication including the a) wooden frame, b) weaving process, and c) completed woven fabric.
Kenaf Yarns (warp) Jute/Hemp Yarns (weft)
Warp Weft
Fig. 2. Warp and weft fibre directions of the interwoven hybrid composites.
Table 3 Symbols used to represent the different types of composites prepared. Type of composite
Symbols
Woven kenaf/kenaf Woven jute/jute Woven hemp/hemp Interwoven kenaf/jute Interwoven kenaf/hemp
KK JJ HH KJ KH
where M0 and Mt represent the mass of the dry and immersed sample, respectively, at a specific time. The percentage of the moisture absorption was plotted against the square root of time (hours). The effects of ageing on the tensile and flexural properties of the woven composites were investigated after 2, 5, 10, 15, 24, 48, and 1400 h (at saturation). The diffusion coefficient, D is defined as the ability of water molecules to penetrate through laminate composites. It is computed from the slope of moisture content versus the square root of time by:
D¼p
h 4M1
2
M2 M1 pffiffiffiffi p t2 t1
2 ð2Þ
where M1 is the percent moisture absorbed at saturation, h is the specimen thickness, M2 M1 is the slope of the plot of the moisture pffiffiffiffi pffiffiffiffi absorption rate during the initial ageing time and t2 t 1 is the linear portion of the curve. Assuming the absorption process is linear at an early stage of immersion; times are taken at the beginning of absorption process, so that the weight change is expected to vary linearly with the square root of time. 2.5. Tensile testing
Resin Outlet Hose Plastic Sheet Woven Fabric
Netting
Enka Channel Peel Ply Resin Inlet Hose
Resin Trap Vacuum Pump
For this test, ‘‘dog-bone” shaped specimens with dimensions of 165 19 (3.2 ± 0.4) mm3 were prepared using a Dremel 4000 tool. The tensile strength and elastic modulus of the plain weave kenaf/jute, kenaf/hemp, and their individually woven composites were determined according to the ASTM D638 [32] standard, which was performed using a 100 kN universal testing machine with a crosshead speed of 1 mm/min. These tests were performed for samples at dry and wet conditions. The strain value was obtained using a clip-on extensometer attached to the gauge length of the specimen. Five specimens were tested for each type of woven composite, and the average value was recorded. The specimens before and after the tensile tests are shown in Fig. 4. 2.6. Flexural testing
Fig. 3. Vacuum infusion system.
The flexural tests were performed using an Instron 5848 universal testing machine, with a crosshead speed of 2.5 mm/min, in
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(b)
(a)
Fig. 4. Tensile test specimens (a) before and (b) after the test.
accordance with the ASTM D790-10 standard [33]. The rectangular shaped three-point bending specimens with dimensions of 125 mm 13 mm (3.2 ± 0.4 mm) were prepared using a Dremel 4000 tool. The distance between the supports was kept at 52 mm as per the standard, with a ratio of 16:1. The specimens before and after the flexural test are shown in Fig. 5. For each case, five specimens were tested, and the average values were recorded. The flexural strength of the composites was calculated using the following equation:
rf ¼
3PL 2bd
ð3Þ
2
where P, L, b, and d represent the bending load, support span length, width, and depth of the samples, respectively. 2.7. Field emission scanning electron microscopy In order to study the morphological features of the fibre-matrix interface after the tensile and flexural tests, fractured portions of the samples were examined using a field emission electron microscope (FESEM). Prior to their examination, the surfaces of the samples were coated with platinum. Images of the fractured specimens were taken by subjecting them to a voltage of 3–5 kV. 3. Results and discussion 3.1. Moisture absorption behaviour The moisture absorption by the samples was determined by the weight gain relative to the dry weight of the samples. The moisture content of each sample was computed using Eq. 1. It was assumed that the samples reached equilibrium moisture absorption when the daily weight gain of the samples was less than 0.01% [34]. Fig. 6 shows the moisture absorption percentage as a function of
(a)
the square root of time (hours) for various woven composites, which had been immersed in tap water at room temperature. From the water absorption curves, the absorbed water content increased with increasing immersion time. A similar finding was reported in a previous study where the water absorption effects of natural fibre reinforced polymer composites were investigated [24,35]. The water uptake process of the woven composites was linear at the beginning, especially in the woven kenaf (KK), woven jute (JJ) and woven hemp (HH) composites, demonstrating the rapid water penetration into the composite materials. The water uptake then slowed down and approached saturation after a prolonged time period. The moisture uptake is following non-Fickian behaviour and seems more sigmoidal in nature. This is due to material lost, most likely by leaching out of resin particles leading to a loss of weight of the composites. As far as individual fibres are concerned, woven kenaf (KK) composites absorbed a higher amount of water, compared with the woven jute (JJ) and hemp (HH) composites. This was expected due to the higher cellulose content in the kenaf fibre, as was presented in Table 1. Cellulose, which is the main constituent of plant fibre, is hygroscopic, and thus able to absorb moisture in comparatively large quantities [26]. The resin matrix, besides serving as the binder of the reinforcements and helping to transfer the load to the fibres, also protects the fibres from environmental attacks [21]. However, when the composites are continuously exposed to moisture, the brittle thermosetting resin will experience microcracking due to the swelling behaviour of the fibres. The high cellulose content in the kenaf fibres further contributes to more water penetrating into the fibre-matrix interphase creating stress concentration leading to failure of the composite. As more microcracking take place, water transport via these cracks become active [36]. The water molecules flow within the capillary cracks and continuously attack the interface, resulting in debonding of the fibre and the matrix. This explanation is supported by the micrograph
(b)
Fig. 5. Flexural test specimens (a) before and (b) after the test.
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hybrid composites, compared with their individual woven composites. The diffusion coefficient of the woven composites are shown in Table 4 and from the result, the amount of water uptake are correspond well with the value of diffusion coefficient. Woven kenaf composite (KK) had the highest diffusion coefficient, D and maximum of the water content, M1. As explained earlier, high lignin content hemp fibre hybridised with kenaf fibre; resulted in lower diffusion coefficient and present good behaviour against water absorption. Higher porosity or the existence of voids, formed during processing could be other reasons that accelerate the diffusion process into the matrix of the composite materials and reflect their water uptake [37]. 3.2. Tensile strength
Fig. 6. Water absorption curves of various woven composites.
in Fig. 7, where matrix cracking is observed in water immersed sample. When compared to the individually woven composites, a reduction of the water uptake was observed for the interwoven kenaf/ jute (KJ) and kenaf/hemp (KH) hybrid composite. The interwoven kenaf/jute (KJ) hybrid composite absorbed 46% less water than the woven kenaf (KK) and woven jute (JJ) composites. Meanwhile, the amount of water absorption of the interwoven kenaf/hemp (KH) hybrid composite was reduced by 64% and 58%, in comparison with the woven kenaf (KK) and hemp (HH) composites, respectively. This reduction shows that the hybridisation enhanced the water resistance properties of the kenaf, jute, and hemp fibres. A similar behaviour was also reported by Venkateshwaran et al. where a decrease in the water absorption properties of composites was also found when sisal fibre was hybridised into a banana/ epoxy composite [13]. Lignin protect the fibres from hydrothermal degradation due to its hydrophobic features and theoretically, composites with fibre containing higher lignin content as a filler should present lower values of water uptake [13,26]. Referring to Table 1, incorporation of high lignin content fibres such as jute and hemp with kenaf fibre reduced the absorbed water of the
The woven and interwoven hybrid composites were subjected to a tensile test, under dry and wet conditions, to determine their strength, elastic modulus, and strain to failure. For the wet samples, the test was carried out at different immersion times to observe their degradation response. A summary of the tensile properties is listed in Table 5. The effect that the hybridization had on the tensile properties of the woven composites can be seen in Fig. 8, where it is observed that the interwoven hybrid composites had the highest tensile strength, elastic modulus, and failure strain in both the dry and saturated conditions. In the dry condition, the tensile strength, elastic modulus and failure strain of the interwoven kenaf/jute hybrid composite (KJ) was found to be 11, 23, and 41% higher than that of the woven kenaf composite (KK), respectively; and 16, 30, and 29% greater than that of the woven jute composite (JJ), respectively. For the interwoven kenaf/hemp hybrid composite (KH), increment of 4, 22 and 59% over the woven kenaf composite (KK); and 9, 38 and 21% over the woven hemp composite (HH) were recorded for the tensile strength, elastic modulus and failure strain, respectively. A similar trend was observed for the saturated samples, where the tensile strength, elastic modulus, and failure strain increased due to the hybridization effect. The tensile strength, elastic modulus, and failure strain of the interwoven kenaf/jute hybrid composite (KJ) at the saturation state increased by 25, 40, and 14%, respectively, in comparison with those of the woven kenaf composite (KK); and by 4, 8, and 9%, respectively, in comparison with those of the woven jute composite (JJ). For the saturated samples of the interwoven kenaf/hemp hybrid composite (KH), an increment of 30, 50, and 18% over the woven kenaf composite (KK); and 4, 7, and 9% over woven hemp composite (HH), were observed for the strength, modulus, and failure strain, respectively. These results indicate that the incorporation of a high strength kenaf fibre with the jute and hemp fibres enhanced the ability of the interwoven hybrid composites to resist breaks and deformation under a tensile load, in comparison with the individually woven composites. To achieve the desired properties, it is important to properly select the reinforcement fibres that will be used in the hybrid composites [21]. In this case, the kenaf fibre was chosen to be hybridised in a woven structure with the jute and hemp fibres due to its outstanding mechanical properties,
Table 4 Diffusion coefficients of the composite materials.
Fig. 7. Matrix cracking of an immersed sample due to attacks by the water molecules.
Specimens
Diffusion coefficients, D (m2/s)
KK JJ HH KJ KH
6.32 108 6.29 108 4.36 108 3.57 109 1.47 109
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Table 5 Tensile properties data for the dry and saturated woven composite samples. Specimens Properties
KK
JJ
HH
KJ
KH
Tensile strength (MPa)
Dry Saturated
80 ± 4 20 ± 4
77 ± 3 24 ± 1
76 ± 5 25 ± 3
89 ± 3 25 ± 2
83 ± 1 26 ± 1
Tensile Modulus (GPa)
Dry Saturated
6 ± 0.5 1 ± 0.2
5.7 ± 0.3 1.3 ± 0.1
5.3 ± 0.5 1.4 ± 0.1
7.4 ± 0.5 1.4 ± 0.2
7.3 ± 0.3 1.5 ± 0.1
Tensile strain (%)
Dry Saturated
2.2 ± 0.3 5.1 ± 0.2
2.4 ± 0.2 5.3 ± 0.3
2.9 ± 0.6 5.5 ± 0.1
3.1 ± 0.5 5.8 ± 0.6
3.5 ± 0.3 6.0 ± 0.2
Fig. 8. Tensile properties of various woven composites in dry and saturated condition.
as shown in Table 2. Furthermore, the two fibres with difference properties created a complex load-sharing property, between the longitudinal and transverse directions, when they were weaved together. This resulted in a greater stress uptake by the interwoven hybrid composites, which were observed to break at higher failure strains, in comparison with the individually woven composites. The proper stress transfer from the matrix to the fibres contributed to a lower crack propagation rate, which resulted in the higher ductility of the interwoven hybrid composites [20]. Comparing the results between the dry and saturated samples in Fig. 8, the tensile properties of the woven composites were reduced by the effects of water absorption. The tensile strength of the woven kenaf (KK), woven jute (JJ), woven hemp (HH), interwoven kenaf/jute (KJ), and interwoven kenaf/hemp (KH) hybrid composites at the saturation state were reduced by 75, 69, 67, 72, and 69% of their strength under the dry condition, respectively. Similarly, the tensile modulus of the saturated samples (following the aforementioned sequence) was also reduced by 83, 77, 74, 81, and 79%, respectively, from the dry condition samples. The strength and tensile modulus of the composites were found to be dependent on the amount of water absorbed. The interwoven kenaf/hemp (KH) hybrid composite, which was the composite with the lowest water absorption, had the highest strength and modulus; whereas the woven kenaf composite (KK) had the lowest strength and modulus due to its greater water uptake. The mechanical performance of the composites was reduced as the water immersion time was prolonged [9,38]. Referring to Table 6, the tensile strength was observed to significantly drop during the first 10 h of the water immersion time, and it slowly decreased as the immersion time increased, following the water
absorption curve in Fig. 6. During the water absorption test, the reinforced fibres absorbed water and swelled. The swelling of the fibre changed the dimensions of the composites, and micro cracks started to appear on the matrix. With longer immersion times, larger water molecules penetrated the interphase of the composites through the micro cracks, and this resulted in the detachment of the fibres and the matrix. Poor fibre/matrix adhesion is one of the main factors that affect the final properties of a composite, as it results in lower mechanical properties [39]. An increase in the immersion time also reduced the tensile modulus of the composites, as is shown in Table 7. When a composite is continuously exposed to an aqueous environment, hydrogen bonds between the macromolecules of the matrix and reinforcement fibre are formed. The hydrogen bonds then react with the hydroxyl groups (–OH) in the fibre structure forming a large number of hydrogen bonds between the macromolecules of the polymer and cellulose. This formation tends to be less susceptible to moisture exposures which lead to poor fibre-matrix interfacial bonding, and a reduction of the tensile modulus is observed [38]. The ductility of the woven composites was expressed by the strain to failure presented in Table 8. Different from the tensile strength and modulus, the failure strain increased with increasing immersion times due to a plasticization effect. As a result of water absorption, the cellulose content of the reinforcement fibre was reduced when the water molecules embedded themselves between the polymer chains. This made the structure of the fibre more flexible [36]. FESEM images of the woven composites after their fracture under tensile loading are shown in Figs. 9 and 10. The fractured surfaces of the highest tensile strength composite under dry
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A.B. Maslinda et al. / Composite Structures 167 (2017) 227–237 Table 6 Tensile strength of the woven composites when tested at difference immersion times. Specimens
Dry (MPa)
KK JJ KJ
HH KH
Wet (MPa)
0h
2h
5h
15 h
24 h
80 ± 4 77 ± 3 89 ± 3
67 ± 2 71 ± 1 74 ± 3
42 ± 3 44 ± 1 48 ± 1
38 ± 1 39 ± 4 40 ± 2
31 ± 4 33 ± 5 37 ± 1
0h
2h
10 h
24 h
48 h
76 ± 5 83 ± 1
74 ± 3 75 ± 2
40 ± 2 42 ± 3
35 ± 4 39 ± 3
28 ± 2 30 ± 2
Table 7 Tensile modulus of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (GPa)
Wet (GPa)
0h
2h
5h
15 h
24 h
6 ± 0.5 5.7 ± 0.3 7.4 ± 0.5
3 ± 0.3 3.7 ± 0.4 4.3 ± 0.3
3 ± 0.1 3.6 ± 0.2 4.2 ± 0.2
2.9 ± 0.2 3 ± 0.2 3.3 ± 0.2
1.9 ± 0.1 2 ± 0.4 2.1 ± 0.3
0h
2h
10 h
24 h
48 h
5.3 ± 0.5 7.3 ± 0.3
3.8 ± 0.2 5 ± 0.2
2.7 ± 0.4 3.1 ± 0.3
2.1 ± 0.3 2.7 ± 0.1
1.8 ± 0.2 2.1 ± 0.2
Table 8 Tensile strain of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
(a)
Dry (%)
Wet (%)
0h
2h
5h
15 h
24 h
5.1 ± 0.3 5.3 ± 0.2 5.8 ± 0.5
5.5 ± 0.2 5.6 ± 0.5 6.3 ± 0.4
5.8 ± 0.5 6.1 ± 0.4 6.5 ± 0.6
6.2 ± 0.5 6.6 ± 0.7 6.8 ± 0.5
6.5 ± 0.6 6.7 ± 0.4 7.1 ± 0.3
0h
2h
10 h
24 h
48 h
5.5 ± 0.6 6 ± 0.3
6.1 ± 0.6 6.4 ± 0.5
6.6 ± 0.5 6.8 ± 0.5
6.9 ± 0.2 7.3 ± 0.5
7 ± 0.3 7.7 ± 0.4
(b)
Pull out
Pull out Fibre breakage
Fibre/matrix debonding Fibre bending
Fibre breakage Fibre/matrix debonding
Fig. 9. FESEM images of tensile fractured specimens under dry conditions where (a) good fibre/matrix bonding, and (b) the less ‘pull out’ phenomena can be observed.
conditions, and vice versa for the wet condition, were selected as representative samples in order to fully understand the water absorption effect towards the fibre/matrix bond. Closely packed interfacial bonding between the fibre and the matrix, as shown in Fig. 9a, contributed to the higher tensile performance of the composites in dry conditions. In Fig. 9b, a less hollowed surface was observed in the dry sample, compared with the water-
immersed sample. This hollowed portion indicated the ‘pull out’ phenomenon, which largely occurred in the water-immersed samples, as shown in Fig 10a. Matrix cracking and delamination as observed in Fig. 10b are among the physical damage contributed by the water absorption that further increase the fibre/matrix detachment and reduced the tensile properties of the water immersed samples [5].
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Delamination
Pull out Fibre/matrix debonding Fibre bending Matrix cracking
Fig. 10. FESEM images of saturated samples after the tensile test. In (a) the hollowed portion indicates the ‘pull out’ phenomena; and in (d) the presence of matrix cracking and delamination, which result from water adsorption, can be observed.
work, where the presence of the kenaf fibre also increased the flexural strength of a woven kenaf/banana hybrid composite [20]. The interlocking structure of the reinforcement fibres further contributed to the higher bending capacity of the interwoven hybrid composites, which resulted in a higher load required to break the tight structure. The combination of compression, shear, and tension modes determined the failure mode of the flexural samples [36]. In comparison with the individually woven composites, the interwoven hybrid composites experienced a higher flexural strain before reaching their breaking point, as a result of hybridization. Different types of fibres in the warp and weft directions of the interwoven hybrid composites resulted in a higher extension of the fibre yarns along the transverse and longitudinal directions [20]. When the samples were subjected to a bending load, the stress was initially absorbed by the fibres along the longitudinal direction, and then it was transferred to the fibres in the transverse direction, before final failure of the composite occurred through the complete removal of the fibre bundle. From Fig. 11, it is observed that the flexural strength was reduced by 73, 64, 57, 69, and 67% for the woven kenaf (KK), woven jute (JJ), woven hemp (HH), interwoven kenaf/jute (KJ), and interwoven kenaf/hemp (KH) hybrid composites, respectively, when they were immersed in water up until their saturation state. Similar to the trend observed for the flexural strength, a reduction of the flexural modulus was also observed. The flexural modulus of the saturated samples of woven kenaf (KK), woven jute (JJ), woven hemp (HH), interwoven kenaf/jute (KJ) and interwoven kenaf/ hemp (KH) hybrid composites was 78, 70, 68, 76 and 73% lower than that of the dry samples. Among the hybrid composites, the interwoven kenaf/jute hybrid composite (KJ) had the highest flexural strength and modulus under dry conditions, but it was unable to surpass the strength and modulus of the interwoven kenaf/ hemp hybrid composite (KH) in the wet (saturated) condition. This
3.3. Flexural strength The flexural strength and modulus describe ability of composites to withstand a bending load and deformation before reaching their breaking point [14,15]. Table 9 summarises the results obtained from the flexural test. As a result of hybridization, excellent flexural properties were achieved. As presented in Fig. 11, a higher flexural performance was recorded for the interwoven hybrid composites, at the dry and saturation states, in comparison with the individually woven composites. In the dry condition, the flexural strength, modulus, and failure strain of the interwoven kenaf/jute hybrid composite (KJ) were found to be 22, 12, and 16% higher than those of the woven kenaf composite (KK), respectively; and 39, 26 and 16% greater than those of woven jute composite (JJ), respectively. For the interwoven kenaf/hemp hybrid composite (KH), increments of 17, 4 and 20% over the woven kenaf composite (KK); and 33, 23, and 5% over the woven hemp composite (HH) were recorded for the flexural strength, modulus, and failure strain, respectively. At the saturation state, the flexural strength, modulus, and failure strain of the interwoven kenaf/jute hybrid composite (KJ) were 41, 27, and 13% higher than those of the woven kenaf composite (KK), respectively; and 18, 3, and 10% greater than those of the woven jute composite (JJ), respectively. Meanwhile, an increment of 44, 32, and 13% over the woven kenaf composite (KK); and of 2, 6, and 10% over the woven hemp composite (HH), were experienced by the saturated interwoven kenaf/hemp hybrid composite (KH) for the flexural strength, modulus, and failure strain, respectively. The increments of the flexural strength and modulus clearly show that the interwoven hybrid composites were stronger and more rigid, in comparison with the individually woven composites. This indicates that hybridization with high strength fibres, such as kenaf, yields a material with a better flexural performance. This is in accordance with a previous
Table 9 Flexural properties of dry and saturated woven composite samples. Specimens Properties Flexural Strength (MPa) Flexural Modulus (GPa) Flexural Strain (%)
Dry Saturated Dry Saturated Dry Saturated
KK
JJ
HH
KJ
KH
77.6 ± 2.9 20.7 ± 3.6 2.6 ± 0.1 0.56 ± 0.2 5 ± 0.5 9.7 ± 0.4
68.5 ± 5.3 24.8 ± 3.8 2.3 ± 0.2 0.69 ± 0.1 5 ± 0.6 10 ± 0.7
68.2 ± 4.9 29.2 ± 2.3 2.2 ± 0.2 0.7 ± 0.2 5.7 ± 0.6 10 ± 0.2
95 ± 3.4 29.2 ± 5.1 2.9 ± 0.1 0.71 ± 0.2 5.8 ± 0.2 11 ± 1
90.8 ± 6.3 29.8 ± 2.7 2.7 ± 0.2 0.74 ± 0.1 6 ± 0.7 11 ± 1
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Fig. 11. Flexural properties of various woven composites in dry and saturated conditions.
is due to the higher water resistance properties of the hemp fibre, which led to less water being absorbed by the saturated samples of the interwoven kenaf/hemp hybrid composite (KH). The mechanical properties of the composites were greatly influenced by the water absorption [11,23]. As presented in Tables 10 and 11, the flexural strength and modulus of the woven composites decreased with increasing immersion times. The poor wettability and micro-gaps between the polymer chains, and the cracking induced by the fibre swelling, increased the water uptake and weakened the fibre/matrix bonding, which resulted in a reduction of the flexural strength and modulus of the composites when they were continuously immersed in water [25,40]. However, the flexural strain behaved differently from the flexural strength and modulus. From Table 12, it can be observed that a longer immersion time increased the flexural strain of the woven composites. The flexural strain percentage of the woven kenaf (KK), woven jute
(JJ), woven hemp (HH), interwoven kenaf/jute (KJ), and interwoven kenaf/hemp (KH) hybrid composites were 94,100, 75, 90, and 83% higher than their strain in dry conditions. It is believed that, the cellulose contents in the fibres reduced during the water ingress making these fibres more flexible due to plasticisation effect. The molecules of the reinforcing fibres were freer to move when the water molecules filled the cracks and cavities of the composites and act as a plasticiser to the composite and render the structure more flexible, and thus the strain value was significantly increased [35]. FESEM images of the woven composites after their fracture under the flexural test are shown in Fig. 12. The surface morphology of the water-immersed samples was different from that of the dry samples in terms of voids, porosity swelling, and sorption through the microcracks [11]. A micrograph of the dry sample, shown in Fig. 12a, displays a good interface between the matrix
Table 10 Flexural strength of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (MPa)
Wet (MPa)
0h
2h
5h
15 h
24 h
77.6 ± 2.9 68.5 ± 5.3 95 ± 3.4
52.1 ± 2.5 54 ± 3.6 62.6 ± 1.7
49.9 ± 1.3 50.2 ± 4.7 51.5 ± 1.6
42.9 ± 1.3 44 ± 5.9 46.3 ± 2.2
35.8 ± 1.2 38.3 ± 2.6 39 ± 2.7
0h
2h
10 h
24 h
48 h
68.2 ± 4.9 90.8 ± 6.3
55.5 ± 1.2 63.3 ± 5.5
48.3 ± 1.4 48.4 ± 1.1
39 ± 2.4 42.6 ± 4.4
29.8 ± 1.4 30.6 ± 4.3
Table 11 Flexural modulus of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (GPa)
Wet (GPa)
0h
2h
5h
15 h
24 h
2.6 ± 0.1 2.3 ± 0.2 2.9 ± 0.1
1.7 ± 0.2 1.8 ± 0.2 2 ± 0.3
1.6 ± 0.1 1.6 ± 0.3 1.7 ± 0.2
1.4 ± 0.2 1.4 ± 0.1 1.4 ± 0.1
1 ± 0.1 1.1 ± 0.1 1.3 ± 0.1
0h
2h
10 h
24 h
48 h
2.2 ± 0.2 2.7 ± 0.2
1.9 ± 0.2 2.1 ± 0.2
1.5 ± 0.2 1.6 ± 0.1
1.3 ± 0.2 1.3 ± 0.1
0.8 ± 0.1 1 ± 0.1
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Table 12 Flexural strain of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (%)
Wet (%)
0h
2h
5h
15 h
24 h
5 ± 0.5 5 ± 0.6 5.8 ± 0.2
6 ± 0.4 6 ± 0.9 6.4 ± 0.3
7 ± 0.8 7±1 7 ± 0.8
8.5 ± 0.8 8.7 ± 0.6 9 ± 0.9
9±1 9 ± 0.8 9 ± 1.3
0h
2h
10 h
24 h
48 h
5.7 ± 0.6 6 ± 0.7
6±1 6.5 ± 0.8
7.3 ± 0.9 7.4 ± 0.5
9±1 9.3 ± 0.3
9.3 ± 1 9.5 ± 0.4
(a)
Void
(b)
Pull out
Fiber/matrix debonds
Fibre bending
Fiber/matrix debonding
Pull out
Fig. 12. FESEM images of the fractured flexural specimen at the (a) dry and (b) saturated conditions.
and the reinforcement fibre, which contributed to its high strength when the bending load was applied. As for the water-immersed sample, its flexural strength was reduced due to a poor fibre/matrix adhesion, as shown in Fig. 12b. When the water molecules penetrate the macrovoids and free space of the polymer, new cavities and cracks are formed; acts as the water transport pathway within the composites and this gradually reduced the interface bonding [23]. Furthermore, the voids seen in Fig. 12b act as stress concentrators [36], and lead to the failure of the water-immersed samples. Observation of the fracture surface from the flexural test sample further underlines the importance of fibre/matrix adhesion on flexural strength. 4. Conclusions The water absorption of interwoven kenaf/jute and kenaf/hemp hybrid composites, and its effect on their mechanical properties, was investigated following their immersion in tap water at room temperature. The conclusions from this experiment are as follow: The water absorption pattern of these composites is found to follow a non-Fickian behaviour. Furthermore, through their hybridisation, the water-resistant properties of woven kenaf, jute, and hemp fibre were improved. The tensile and flexural strength of the interwoven hybrid composites were superior to those of the individual woven composites due to the different load sharing properties between the longitudinal and transverse directional fibres in the woven structure. A greater stress uptake of the interwoven hybrid composites was obtained by the interlocking structure between the fibre yarns; therefore, a larger load was required to break the structure. Longer immersion times reduced the strength and modulus of the composites as a result of water absorption. However, the failure strain increased with increasing immersion time due to a breakdown of the cellulose structure.
Matrix cracking, delamination, and voids (as shown by the FESEM images) increased the penetration of water, and reduced the strength and modulus of the water immersed samples, by weakening the interface between the fibre and matrix.
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Contents lists available at ScienceDirect
Composite Structures journal homepage: www.elsevier.com/locate/compstruct
Effect of water absorption on the mechanical properties of hybrid interwoven cellulosic-cellulosic fibre reinforced epoxy composites A.B. Maslinda a,b, M.S. Abdul Majid a,⇑, M.J.M. Ridzuan a, M. Afendi a, A.G. Gibson c a
School of Mechatronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, 02600 Arau, Perlis, Malaysia Faculty of Engineering Technology, Universiti Malaysia Perlis, Uniciti Alam Campus, 02100 Padang Besar, Perlis, Malaysia c School of Mechanical and Systems Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK b
a r t i c l e
i n f o
Article history: Received 5 December 2016 Revised 16 December 2016 Accepted 3 February 2017 Available online 6 February 2017 Keywords: Natural fibre Hybrid Mechanical properties Strength Fibre/matrix bond
a b s t r a c t The absorption behaviour of water and its effect on the tensile and flexural properties of interwoven cellulosic fibres were investigated. Hybrid composites consisting of interwoven kenaf/jute and kenaf/hemp yarns were prepared by an infusion process that used epoxy as the polymer matrix. The water absorption characteristics of the fibres were obtained by immersing the composite samples in tap water at room temperature, until reaching their water content saturation point. The dry and water-immersed woven and interwoven hybrid composite samples were subjected to tensile and flexural tests. To study the effect of water penetration in the fibre/matrix interface, fractured samples were examined using field emission scanning electron microscopy (FESEM). The study shows that the mechanical and water-resistant properties of the kenaf, jute, and hemp fibres were improved through hybridization. However, as a result of water penetrating the fibre/matrix interface, longer water-immersion times reduced the tensile and flexural strength of the composites. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Due to environmental concerns, the potential use of natural fibres as replacements of synthetic fibres has been investigated. In comparison with glass fibres, one of the advantages of using natural fibres is their low density, which endows them with excellent specific mechanical properties, easier handling and processing, recyclability, and good thermal and acoustic insulation [1]. Despite these advantages, natural fibre reinforced composites have several drawbacks that limit their application, such as a low strength, variability in quality, high moisture absorption, limited processing temperature, and a lesser durability and incompatibility between fibres and polymer matrices [1,2]. However, through continuous studies, researchers have come out with a number of methods and treatments to improve the performance of natural fibre composites. Nowadays, natural fibre composites have been widely implemented in load bearing and outdoor applications, such as in the exterior underfloor panelling of cars, sports equipment, and marine structures [3]. Faruk et al. [4] summarised the development ⇑ Corresponding author. E-mail addresses: [email protected] (A.B. Maslinda), [email protected] (M.S. Abdul Majid), [email protected] (M.J.M. Ridzuan), afendirojan@ gmail.com (M. Afendi), [email protected] (A.G. Gibson). http://dx.doi.org/10.1016/j.compstruct.2017.02.023 0263-8223/Ó 2017 Elsevier Ltd. All rights reserved.
of biocomposites from the years 2000 to 2010, and reported that flax, jute, hemp, sisal, ramie, and kenaf fibres are among the most widely used and studied fibre materials. Hybrid composites are commonly termed as the mixture of two or more reinforcing fibres in a single matrix system. Through proper fibre selection and design, the balance between cost and performance of hybrid composites could be achieved through hybridization [5]. For example, the incorporation of glass with different cellulosic fibres such as abaca, jute, banana, hemp, and napier have been previously reported in the literature [6–9], with results that showed the mechanical properties of the hybrid composites being superior to those of the single-fibre reinforced composites. There are also published works about synthetic/synthetic [10], and cellulosic/cellulosic [11–13] fibres based on reinforced hybrid composites. Furthermore, the performance of a hybrid kenaf/kevlar composite was investigated by Yahaya et al. [14,15], who found that the composite had the potential to be used in impact applications. In the industry, FlexForm Technologies mixed hemp with kenaf fibre and used it in Chrysler’s Sebring door panels, and the company is currently in the process of using the natural composite in other automotive parts [5]. A review by Swolfs et al. [16] indicates that hybridization is a method that can be used to increase the toughness of fibre reinforced composites, and that
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there are three main hybrid configurations that can be used to combine two fibres: the (1) interlayer or layer-by-layer, (2) intra-layer or yarn-by-yarn, and (3) intra-yarn or fibre-by-fibre configurations. One important factor that contributes to the excellent properties of hybrid composites is the fibre orientation. For example, a continuous and long fibre aligned parallel to the load direction exhibits a higher strength, in comparison with a randomly oriented fibre [3]. To attain a greater degree of fibre alignment, different textile processing methods such as weaving, knitting, and braiding can be applied [17]. Specifically, weaving is the interlacing of two sets of yarns to construct a woven fabric. In a plain weave, for example, the warp is defined as the yarn that runs along the length of the fabric, and the weft is defined as the yarn that has been weaved over and underneath the warp yarn [18]. As reported in the literature [19,20], the tensile, flexural and impact properties of woven fabric composites are higher than those of unidirectional and randomly oriented composites. In woven structures, the stress is uniformly transferred, and the interlocking between the reinforced fibres further increases the strength of the composite [21]. Khan et al. [19] investigated the influence of the fibre direction and found that the mechanical properties of a woven jute reinforced poly(l-lactic acid) composite are greater when tested along the warp direction, than along the weft direction. A woven banana reinforced epoxy composite was used by Sapuan et al. [22] in the fabrication of household telephone stands, and they proved that the natural based composite was able to replace the conventional material in the furniture industry. In regards to natural based composites, the presence of polar groups (e.g. hydroxyl, among others) in natural fibres allows them to absorb high amounts of moisture, and also makes them incompatible with the polymer matrix. Together, these two factors lead to a reduced fibre/matrix bonding strength, which results in a weaker mechanical performance of fibre/matrix hybrid composites [23,24]. Akil et al. [25] found that when pultruded jute/glass fibre reinforced polyester hybrid composites were exposed to humidity or aqueous environments, water molecules penetrated the composites through three routes, as follows: through the flaws at the composites interphase attributed to the poor wettability between the fibre and matrix, through micro-gaps between the polymer chains, and through the cracks in the polymer matrix induced by the fibre swelling. In this work, water absorption and its effect on the mechanical properties of interwoven kenaf/jute and kenaf/hemp hybrid composites were experimentally investigated as an extension of the research regarding hybrid- and woven-structured composites. Up to date, only few works have been conducted involving interwoven cellulosic fibres, and this study will provide useful information to the research community. 2. Experimental procedure 2.1. Materials The kenaf, jute, and hemp yarns that were to produce the woven fabrics were obtained from a local supplier. The EpoxAmite 100 series resin was selected as the polymer matrix, and it was mixed with a hardener in a ratio of 3:1 to form the binder for the composite preparation. The chemical compositions of the
reinforcement fibres are listed in Table 1, while Table 2 details their mechanical properties, alongside those of the matrix. The kenaf, jute and hemp fibres were chosen as reinforcements because they are easily available in Malaysia, and they are also among the most widely used materials in the natural fibre reinforced polymer composite industry [3]. 2.2. Woven fabric production Woven fabrics were produced by weaving the fibre yarns using a wooden frame. The wooden frame (400 mm 400 mm) was manually constructed, and it contained nails that acted as the warp yarn guider on both of its sides. The weaving process was done by passing the weft yarn over and underneath the warp yarn, which had been previously arranged on the frame with the help of the warp yarn guider. The wooden frame, weaving process, and completed woven fabric are shown in Fig. 1. Fig. 2 illustrates the construction of the interwoven kenaf/jute and kenaf/hemp hybrid composites, where the interlacing of the kenaf yarns followed the warp fibre direction, while the jute and hemp yarns were arranged in the weft fibre direction. 2.3. Composite fabrication A vacuum infusion manufacturing technique was used to prepare the composite specimens, which had a fibre weight content of 30% ± 2%. Five types of composites were prepared, and the symbols that represent each type of composite are listed in Table 3. The fabrication process began by polishing the glass surface with acetone to remove any dirt and balanced resin from the previous infusion process. A thin layer of wax, or catalyst, was used for the easy removal of the composites after infusion. The woven fabrics were positioned on the glass surface followed by a peel ply, netting, and an enka channel, as shown in Fig. 3. The resin inlet and outlet, which were made from a PVC hose, were placed over the mould area before it was wrapped with a plastic sheet. The vacuum pump was switched on, and the in-mould pressure was controlled at below 2000 Pa, to make sure that the air was fully evacuated. The resin mixture, which was prepared according to the manufacturer’s specification, was infused into the mould, where it flowed evenly until it reached the end. Excess resin flowed into a resin trap. The infused composite was removed from the mould and cured for 24 h at room temperature (25 °C). 2.4. Moisture absorption test The water absorption tests were carried out as elucidated in the ASTM D570 standard [31]. First, the specimens were submerged in a container filled with tap water at room temperature for up to 1400 h. Thereafter, to monitor the mass during the ageing process, the specimens were withdrawn from the water, wiped dry to remove any surface moisture, and then weighted using a high accuracy 4-digit analytical balance. For each type of composite, five specimens were tested, and the average result was recorded. The moisture content percentage, DM(t), was calculated using the following equation:
DMðtÞ ¼
Mt M0 100 M0
ð1Þ
Table 1 Chemical composition of the reinforcement [4]. Constituent/Fibre
Cellulose (wt%)
Hemicellulose (wt%)
Lignin (wt%)
Waxes (wt%)
Kenaf Jute Hemp
72 61–71 68
20.3 14–20 15
9 12–13 10
– 0.5 0.8
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A.B. Maslinda et al. / Composite Structures 167 (2017) 227–237 Table 2 Mechanical properties of fibres and epoxy resin. Properties/Fibre
Tensile strength (MPa)
Elastic modulus (GPa)
Strain at failure (%)
Moisture absorption rate (%)
References
Kenaf Jute Hemp Epoxy
930 393–773 690 55
53 26.5 70 1.75
1.6 1.5–1.8 2–4 6
17–20 12 8 –
[4,20,26–29] [25,26,28,29] [25,26,28] [30]
Fig. 1. Woven fabric fabrication including the a) wooden frame, b) weaving process, and c) completed woven fabric.
Kenaf Yarns (warp) Jute/Hemp Yarns (weft)
Warp Weft
Fig. 2. Warp and weft fibre directions of the interwoven hybrid composites.
Table 3 Symbols used to represent the different types of composites prepared. Type of composite
Symbols
Woven kenaf/kenaf Woven jute/jute Woven hemp/hemp Interwoven kenaf/jute Interwoven kenaf/hemp
KK JJ HH KJ KH
where M0 and Mt represent the mass of the dry and immersed sample, respectively, at a specific time. The percentage of the moisture absorption was plotted against the square root of time (hours). The effects of ageing on the tensile and flexural properties of the woven composites were investigated after 2, 5, 10, 15, 24, 48, and 1400 h (at saturation). The diffusion coefficient, D is defined as the ability of water molecules to penetrate through laminate composites. It is computed from the slope of moisture content versus the square root of time by:
D¼p
h 4M1
2
M2 M1 pffiffiffiffi p t2 t1
2 ð2Þ
where M1 is the percent moisture absorbed at saturation, h is the specimen thickness, M2 M1 is the slope of the plot of the moisture pffiffiffiffi pffiffiffiffi absorption rate during the initial ageing time and t2 t 1 is the linear portion of the curve. Assuming the absorption process is linear at an early stage of immersion; times are taken at the beginning of absorption process, so that the weight change is expected to vary linearly with the square root of time. 2.5. Tensile testing
Resin Outlet Hose Plastic Sheet Woven Fabric
Netting
Enka Channel Peel Ply Resin Inlet Hose
Resin Trap Vacuum Pump
For this test, ‘‘dog-bone” shaped specimens with dimensions of 165 19 (3.2 ± 0.4) mm3 were prepared using a Dremel 4000 tool. The tensile strength and elastic modulus of the plain weave kenaf/jute, kenaf/hemp, and their individually woven composites were determined according to the ASTM D638 [32] standard, which was performed using a 100 kN universal testing machine with a crosshead speed of 1 mm/min. These tests were performed for samples at dry and wet conditions. The strain value was obtained using a clip-on extensometer attached to the gauge length of the specimen. Five specimens were tested for each type of woven composite, and the average value was recorded. The specimens before and after the tensile tests are shown in Fig. 4. 2.6. Flexural testing
Fig. 3. Vacuum infusion system.
The flexural tests were performed using an Instron 5848 universal testing machine, with a crosshead speed of 2.5 mm/min, in
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(b)
(a)
Fig. 4. Tensile test specimens (a) before and (b) after the test.
accordance with the ASTM D790-10 standard [33]. The rectangular shaped three-point bending specimens with dimensions of 125 mm 13 mm (3.2 ± 0.4 mm) were prepared using a Dremel 4000 tool. The distance between the supports was kept at 52 mm as per the standard, with a ratio of 16:1. The specimens before and after the flexural test are shown in Fig. 5. For each case, five specimens were tested, and the average values were recorded. The flexural strength of the composites was calculated using the following equation:
rf ¼
3PL 2bd
ð3Þ
2
where P, L, b, and d represent the bending load, support span length, width, and depth of the samples, respectively. 2.7. Field emission scanning electron microscopy In order to study the morphological features of the fibre-matrix interface after the tensile and flexural tests, fractured portions of the samples were examined using a field emission electron microscope (FESEM). Prior to their examination, the surfaces of the samples were coated with platinum. Images of the fractured specimens were taken by subjecting them to a voltage of 3–5 kV. 3. Results and discussion 3.1. Moisture absorption behaviour The moisture absorption by the samples was determined by the weight gain relative to the dry weight of the samples. The moisture content of each sample was computed using Eq. 1. It was assumed that the samples reached equilibrium moisture absorption when the daily weight gain of the samples was less than 0.01% [34]. Fig. 6 shows the moisture absorption percentage as a function of
(a)
the square root of time (hours) for various woven composites, which had been immersed in tap water at room temperature. From the water absorption curves, the absorbed water content increased with increasing immersion time. A similar finding was reported in a previous study where the water absorption effects of natural fibre reinforced polymer composites were investigated [24,35]. The water uptake process of the woven composites was linear at the beginning, especially in the woven kenaf (KK), woven jute (JJ) and woven hemp (HH) composites, demonstrating the rapid water penetration into the composite materials. The water uptake then slowed down and approached saturation after a prolonged time period. The moisture uptake is following non-Fickian behaviour and seems more sigmoidal in nature. This is due to material lost, most likely by leaching out of resin particles leading to a loss of weight of the composites. As far as individual fibres are concerned, woven kenaf (KK) composites absorbed a higher amount of water, compared with the woven jute (JJ) and hemp (HH) composites. This was expected due to the higher cellulose content in the kenaf fibre, as was presented in Table 1. Cellulose, which is the main constituent of plant fibre, is hygroscopic, and thus able to absorb moisture in comparatively large quantities [26]. The resin matrix, besides serving as the binder of the reinforcements and helping to transfer the load to the fibres, also protects the fibres from environmental attacks [21]. However, when the composites are continuously exposed to moisture, the brittle thermosetting resin will experience microcracking due to the swelling behaviour of the fibres. The high cellulose content in the kenaf fibres further contributes to more water penetrating into the fibre-matrix interphase creating stress concentration leading to failure of the composite. As more microcracking take place, water transport via these cracks become active [36]. The water molecules flow within the capillary cracks and continuously attack the interface, resulting in debonding of the fibre and the matrix. This explanation is supported by the micrograph
(b)
Fig. 5. Flexural test specimens (a) before and (b) after the test.
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hybrid composites, compared with their individual woven composites. The diffusion coefficient of the woven composites are shown in Table 4 and from the result, the amount of water uptake are correspond well with the value of diffusion coefficient. Woven kenaf composite (KK) had the highest diffusion coefficient, D and maximum of the water content, M1. As explained earlier, high lignin content hemp fibre hybridised with kenaf fibre; resulted in lower diffusion coefficient and present good behaviour against water absorption. Higher porosity or the existence of voids, formed during processing could be other reasons that accelerate the diffusion process into the matrix of the composite materials and reflect their water uptake [37]. 3.2. Tensile strength
Fig. 6. Water absorption curves of various woven composites.
in Fig. 7, where matrix cracking is observed in water immersed sample. When compared to the individually woven composites, a reduction of the water uptake was observed for the interwoven kenaf/ jute (KJ) and kenaf/hemp (KH) hybrid composite. The interwoven kenaf/jute (KJ) hybrid composite absorbed 46% less water than the woven kenaf (KK) and woven jute (JJ) composites. Meanwhile, the amount of water absorption of the interwoven kenaf/hemp (KH) hybrid composite was reduced by 64% and 58%, in comparison with the woven kenaf (KK) and hemp (HH) composites, respectively. This reduction shows that the hybridisation enhanced the water resistance properties of the kenaf, jute, and hemp fibres. A similar behaviour was also reported by Venkateshwaran et al. where a decrease in the water absorption properties of composites was also found when sisal fibre was hybridised into a banana/ epoxy composite [13]. Lignin protect the fibres from hydrothermal degradation due to its hydrophobic features and theoretically, composites with fibre containing higher lignin content as a filler should present lower values of water uptake [13,26]. Referring to Table 1, incorporation of high lignin content fibres such as jute and hemp with kenaf fibre reduced the absorbed water of the
The woven and interwoven hybrid composites were subjected to a tensile test, under dry and wet conditions, to determine their strength, elastic modulus, and strain to failure. For the wet samples, the test was carried out at different immersion times to observe their degradation response. A summary of the tensile properties is listed in Table 5. The effect that the hybridization had on the tensile properties of the woven composites can be seen in Fig. 8, where it is observed that the interwoven hybrid composites had the highest tensile strength, elastic modulus, and failure strain in both the dry and saturated conditions. In the dry condition, the tensile strength, elastic modulus and failure strain of the interwoven kenaf/jute hybrid composite (KJ) was found to be 11, 23, and 41% higher than that of the woven kenaf composite (KK), respectively; and 16, 30, and 29% greater than that of the woven jute composite (JJ), respectively. For the interwoven kenaf/hemp hybrid composite (KH), increment of 4, 22 and 59% over the woven kenaf composite (KK); and 9, 38 and 21% over the woven hemp composite (HH) were recorded for the tensile strength, elastic modulus and failure strain, respectively. A similar trend was observed for the saturated samples, where the tensile strength, elastic modulus, and failure strain increased due to the hybridization effect. The tensile strength, elastic modulus, and failure strain of the interwoven kenaf/jute hybrid composite (KJ) at the saturation state increased by 25, 40, and 14%, respectively, in comparison with those of the woven kenaf composite (KK); and by 4, 8, and 9%, respectively, in comparison with those of the woven jute composite (JJ). For the saturated samples of the interwoven kenaf/hemp hybrid composite (KH), an increment of 30, 50, and 18% over the woven kenaf composite (KK); and 4, 7, and 9% over woven hemp composite (HH), were observed for the strength, modulus, and failure strain, respectively. These results indicate that the incorporation of a high strength kenaf fibre with the jute and hemp fibres enhanced the ability of the interwoven hybrid composites to resist breaks and deformation under a tensile load, in comparison with the individually woven composites. To achieve the desired properties, it is important to properly select the reinforcement fibres that will be used in the hybrid composites [21]. In this case, the kenaf fibre was chosen to be hybridised in a woven structure with the jute and hemp fibres due to its outstanding mechanical properties,
Table 4 Diffusion coefficients of the composite materials.
Fig. 7. Matrix cracking of an immersed sample due to attacks by the water molecules.
Specimens
Diffusion coefficients, D (m2/s)
KK JJ HH KJ KH
6.32 108 6.29 108 4.36 108 3.57 109 1.47 109
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Table 5 Tensile properties data for the dry and saturated woven composite samples. Specimens Properties
KK
JJ
HH
KJ
KH
Tensile strength (MPa)
Dry Saturated
80 ± 4 20 ± 4
77 ± 3 24 ± 1
76 ± 5 25 ± 3
89 ± 3 25 ± 2
83 ± 1 26 ± 1
Tensile Modulus (GPa)
Dry Saturated
6 ± 0.5 1 ± 0.2
5.7 ± 0.3 1.3 ± 0.1
5.3 ± 0.5 1.4 ± 0.1
7.4 ± 0.5 1.4 ± 0.2
7.3 ± 0.3 1.5 ± 0.1
Tensile strain (%)
Dry Saturated
2.2 ± 0.3 5.1 ± 0.2
2.4 ± 0.2 5.3 ± 0.3
2.9 ± 0.6 5.5 ± 0.1
3.1 ± 0.5 5.8 ± 0.6
3.5 ± 0.3 6.0 ± 0.2
Fig. 8. Tensile properties of various woven composites in dry and saturated condition.
as shown in Table 2. Furthermore, the two fibres with difference properties created a complex load-sharing property, between the longitudinal and transverse directions, when they were weaved together. This resulted in a greater stress uptake by the interwoven hybrid composites, which were observed to break at higher failure strains, in comparison with the individually woven composites. The proper stress transfer from the matrix to the fibres contributed to a lower crack propagation rate, which resulted in the higher ductility of the interwoven hybrid composites [20]. Comparing the results between the dry and saturated samples in Fig. 8, the tensile properties of the woven composites were reduced by the effects of water absorption. The tensile strength of the woven kenaf (KK), woven jute (JJ), woven hemp (HH), interwoven kenaf/jute (KJ), and interwoven kenaf/hemp (KH) hybrid composites at the saturation state were reduced by 75, 69, 67, 72, and 69% of their strength under the dry condition, respectively. Similarly, the tensile modulus of the saturated samples (following the aforementioned sequence) was also reduced by 83, 77, 74, 81, and 79%, respectively, from the dry condition samples. The strength and tensile modulus of the composites were found to be dependent on the amount of water absorbed. The interwoven kenaf/hemp (KH) hybrid composite, which was the composite with the lowest water absorption, had the highest strength and modulus; whereas the woven kenaf composite (KK) had the lowest strength and modulus due to its greater water uptake. The mechanical performance of the composites was reduced as the water immersion time was prolonged [9,38]. Referring to Table 6, the tensile strength was observed to significantly drop during the first 10 h of the water immersion time, and it slowly decreased as the immersion time increased, following the water
absorption curve in Fig. 6. During the water absorption test, the reinforced fibres absorbed water and swelled. The swelling of the fibre changed the dimensions of the composites, and micro cracks started to appear on the matrix. With longer immersion times, larger water molecules penetrated the interphase of the composites through the micro cracks, and this resulted in the detachment of the fibres and the matrix. Poor fibre/matrix adhesion is one of the main factors that affect the final properties of a composite, as it results in lower mechanical properties [39]. An increase in the immersion time also reduced the tensile modulus of the composites, as is shown in Table 7. When a composite is continuously exposed to an aqueous environment, hydrogen bonds between the macromolecules of the matrix and reinforcement fibre are formed. The hydrogen bonds then react with the hydroxyl groups (–OH) in the fibre structure forming a large number of hydrogen bonds between the macromolecules of the polymer and cellulose. This formation tends to be less susceptible to moisture exposures which lead to poor fibre-matrix interfacial bonding, and a reduction of the tensile modulus is observed [38]. The ductility of the woven composites was expressed by the strain to failure presented in Table 8. Different from the tensile strength and modulus, the failure strain increased with increasing immersion times due to a plasticization effect. As a result of water absorption, the cellulose content of the reinforcement fibre was reduced when the water molecules embedded themselves between the polymer chains. This made the structure of the fibre more flexible [36]. FESEM images of the woven composites after their fracture under tensile loading are shown in Figs. 9 and 10. The fractured surfaces of the highest tensile strength composite under dry
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Dry (MPa)
KK JJ KJ
HH KH
Wet (MPa)
0h
2h
5h
15 h
24 h
80 ± 4 77 ± 3 89 ± 3
67 ± 2 71 ± 1 74 ± 3
42 ± 3 44 ± 1 48 ± 1
38 ± 1 39 ± 4 40 ± 2
31 ± 4 33 ± 5 37 ± 1
0h
2h
10 h
24 h
48 h
76 ± 5 83 ± 1
74 ± 3 75 ± 2
40 ± 2 42 ± 3
35 ± 4 39 ± 3
28 ± 2 30 ± 2
Table 7 Tensile modulus of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (GPa)
Wet (GPa)
0h
2h
5h
15 h
24 h
6 ± 0.5 5.7 ± 0.3 7.4 ± 0.5
3 ± 0.3 3.7 ± 0.4 4.3 ± 0.3
3 ± 0.1 3.6 ± 0.2 4.2 ± 0.2
2.9 ± 0.2 3 ± 0.2 3.3 ± 0.2
1.9 ± 0.1 2 ± 0.4 2.1 ± 0.3
0h
2h
10 h
24 h
48 h
5.3 ± 0.5 7.3 ± 0.3
3.8 ± 0.2 5 ± 0.2
2.7 ± 0.4 3.1 ± 0.3
2.1 ± 0.3 2.7 ± 0.1
1.8 ± 0.2 2.1 ± 0.2
Table 8 Tensile strain of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
(a)
Dry (%)
Wet (%)
0h
2h
5h
15 h
24 h
5.1 ± 0.3 5.3 ± 0.2 5.8 ± 0.5
5.5 ± 0.2 5.6 ± 0.5 6.3 ± 0.4
5.8 ± 0.5 6.1 ± 0.4 6.5 ± 0.6
6.2 ± 0.5 6.6 ± 0.7 6.8 ± 0.5
6.5 ± 0.6 6.7 ± 0.4 7.1 ± 0.3
0h
2h
10 h
24 h
48 h
5.5 ± 0.6 6 ± 0.3
6.1 ± 0.6 6.4 ± 0.5
6.6 ± 0.5 6.8 ± 0.5
6.9 ± 0.2 7.3 ± 0.5
7 ± 0.3 7.7 ± 0.4
(b)
Pull out
Pull out Fibre breakage
Fibre/matrix debonding Fibre bending
Fibre breakage Fibre/matrix debonding
Fig. 9. FESEM images of tensile fractured specimens under dry conditions where (a) good fibre/matrix bonding, and (b) the less ‘pull out’ phenomena can be observed.
conditions, and vice versa for the wet condition, were selected as representative samples in order to fully understand the water absorption effect towards the fibre/matrix bond. Closely packed interfacial bonding between the fibre and the matrix, as shown in Fig. 9a, contributed to the higher tensile performance of the composites in dry conditions. In Fig. 9b, a less hollowed surface was observed in the dry sample, compared with the water-
immersed sample. This hollowed portion indicated the ‘pull out’ phenomenon, which largely occurred in the water-immersed samples, as shown in Fig 10a. Matrix cracking and delamination as observed in Fig. 10b are among the physical damage contributed by the water absorption that further increase the fibre/matrix detachment and reduced the tensile properties of the water immersed samples [5].
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Delamination
Pull out Fibre/matrix debonding Fibre bending Matrix cracking
Fig. 10. FESEM images of saturated samples after the tensile test. In (a) the hollowed portion indicates the ‘pull out’ phenomena; and in (d) the presence of matrix cracking and delamination, which result from water adsorption, can be observed.
work, where the presence of the kenaf fibre also increased the flexural strength of a woven kenaf/banana hybrid composite [20]. The interlocking structure of the reinforcement fibres further contributed to the higher bending capacity of the interwoven hybrid composites, which resulted in a higher load required to break the tight structure. The combination of compression, shear, and tension modes determined the failure mode of the flexural samples [36]. In comparison with the individually woven composites, the interwoven hybrid composites experienced a higher flexural strain before reaching their breaking point, as a result of hybridization. Different types of fibres in the warp and weft directions of the interwoven hybrid composites resulted in a higher extension of the fibre yarns along the transverse and longitudinal directions [20]. When the samples were subjected to a bending load, the stress was initially absorbed by the fibres along the longitudinal direction, and then it was transferred to the fibres in the transverse direction, before final failure of the composite occurred through the complete removal of the fibre bundle. From Fig. 11, it is observed that the flexural strength was reduced by 73, 64, 57, 69, and 67% for the woven kenaf (KK), woven jute (JJ), woven hemp (HH), interwoven kenaf/jute (KJ), and interwoven kenaf/hemp (KH) hybrid composites, respectively, when they were immersed in water up until their saturation state. Similar to the trend observed for the flexural strength, a reduction of the flexural modulus was also observed. The flexural modulus of the saturated samples of woven kenaf (KK), woven jute (JJ), woven hemp (HH), interwoven kenaf/jute (KJ) and interwoven kenaf/ hemp (KH) hybrid composites was 78, 70, 68, 76 and 73% lower than that of the dry samples. Among the hybrid composites, the interwoven kenaf/jute hybrid composite (KJ) had the highest flexural strength and modulus under dry conditions, but it was unable to surpass the strength and modulus of the interwoven kenaf/ hemp hybrid composite (KH) in the wet (saturated) condition. This
3.3. Flexural strength The flexural strength and modulus describe ability of composites to withstand a bending load and deformation before reaching their breaking point [14,15]. Table 9 summarises the results obtained from the flexural test. As a result of hybridization, excellent flexural properties were achieved. As presented in Fig. 11, a higher flexural performance was recorded for the interwoven hybrid composites, at the dry and saturation states, in comparison with the individually woven composites. In the dry condition, the flexural strength, modulus, and failure strain of the interwoven kenaf/jute hybrid composite (KJ) were found to be 22, 12, and 16% higher than those of the woven kenaf composite (KK), respectively; and 39, 26 and 16% greater than those of woven jute composite (JJ), respectively. For the interwoven kenaf/hemp hybrid composite (KH), increments of 17, 4 and 20% over the woven kenaf composite (KK); and 33, 23, and 5% over the woven hemp composite (HH) were recorded for the flexural strength, modulus, and failure strain, respectively. At the saturation state, the flexural strength, modulus, and failure strain of the interwoven kenaf/jute hybrid composite (KJ) were 41, 27, and 13% higher than those of the woven kenaf composite (KK), respectively; and 18, 3, and 10% greater than those of the woven jute composite (JJ), respectively. Meanwhile, an increment of 44, 32, and 13% over the woven kenaf composite (KK); and of 2, 6, and 10% over the woven hemp composite (HH), were experienced by the saturated interwoven kenaf/hemp hybrid composite (KH) for the flexural strength, modulus, and failure strain, respectively. The increments of the flexural strength and modulus clearly show that the interwoven hybrid composites were stronger and more rigid, in comparison with the individually woven composites. This indicates that hybridization with high strength fibres, such as kenaf, yields a material with a better flexural performance. This is in accordance with a previous
Table 9 Flexural properties of dry and saturated woven composite samples. Specimens Properties Flexural Strength (MPa) Flexural Modulus (GPa) Flexural Strain (%)
Dry Saturated Dry Saturated Dry Saturated
KK
JJ
HH
KJ
KH
77.6 ± 2.9 20.7 ± 3.6 2.6 ± 0.1 0.56 ± 0.2 5 ± 0.5 9.7 ± 0.4
68.5 ± 5.3 24.8 ± 3.8 2.3 ± 0.2 0.69 ± 0.1 5 ± 0.6 10 ± 0.7
68.2 ± 4.9 29.2 ± 2.3 2.2 ± 0.2 0.7 ± 0.2 5.7 ± 0.6 10 ± 0.2
95 ± 3.4 29.2 ± 5.1 2.9 ± 0.1 0.71 ± 0.2 5.8 ± 0.2 11 ± 1
90.8 ± 6.3 29.8 ± 2.7 2.7 ± 0.2 0.74 ± 0.1 6 ± 0.7 11 ± 1
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Fig. 11. Flexural properties of various woven composites in dry and saturated conditions.
is due to the higher water resistance properties of the hemp fibre, which led to less water being absorbed by the saturated samples of the interwoven kenaf/hemp hybrid composite (KH). The mechanical properties of the composites were greatly influenced by the water absorption [11,23]. As presented in Tables 10 and 11, the flexural strength and modulus of the woven composites decreased with increasing immersion times. The poor wettability and micro-gaps between the polymer chains, and the cracking induced by the fibre swelling, increased the water uptake and weakened the fibre/matrix bonding, which resulted in a reduction of the flexural strength and modulus of the composites when they were continuously immersed in water [25,40]. However, the flexural strain behaved differently from the flexural strength and modulus. From Table 12, it can be observed that a longer immersion time increased the flexural strain of the woven composites. The flexural strain percentage of the woven kenaf (KK), woven jute
(JJ), woven hemp (HH), interwoven kenaf/jute (KJ), and interwoven kenaf/hemp (KH) hybrid composites were 94,100, 75, 90, and 83% higher than their strain in dry conditions. It is believed that, the cellulose contents in the fibres reduced during the water ingress making these fibres more flexible due to plasticisation effect. The molecules of the reinforcing fibres were freer to move when the water molecules filled the cracks and cavities of the composites and act as a plasticiser to the composite and render the structure more flexible, and thus the strain value was significantly increased [35]. FESEM images of the woven composites after their fracture under the flexural test are shown in Fig. 12. The surface morphology of the water-immersed samples was different from that of the dry samples in terms of voids, porosity swelling, and sorption through the microcracks [11]. A micrograph of the dry sample, shown in Fig. 12a, displays a good interface between the matrix
Table 10 Flexural strength of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (MPa)
Wet (MPa)
0h
2h
5h
15 h
24 h
77.6 ± 2.9 68.5 ± 5.3 95 ± 3.4
52.1 ± 2.5 54 ± 3.6 62.6 ± 1.7
49.9 ± 1.3 50.2 ± 4.7 51.5 ± 1.6
42.9 ± 1.3 44 ± 5.9 46.3 ± 2.2
35.8 ± 1.2 38.3 ± 2.6 39 ± 2.7
0h
2h
10 h
24 h
48 h
68.2 ± 4.9 90.8 ± 6.3
55.5 ± 1.2 63.3 ± 5.5
48.3 ± 1.4 48.4 ± 1.1
39 ± 2.4 42.6 ± 4.4
29.8 ± 1.4 30.6 ± 4.3
Table 11 Flexural modulus of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (GPa)
Wet (GPa)
0h
2h
5h
15 h
24 h
2.6 ± 0.1 2.3 ± 0.2 2.9 ± 0.1
1.7 ± 0.2 1.8 ± 0.2 2 ± 0.3
1.6 ± 0.1 1.6 ± 0.3 1.7 ± 0.2
1.4 ± 0.2 1.4 ± 0.1 1.4 ± 0.1
1 ± 0.1 1.1 ± 0.1 1.3 ± 0.1
0h
2h
10 h
24 h
48 h
2.2 ± 0.2 2.7 ± 0.2
1.9 ± 0.2 2.1 ± 0.2
1.5 ± 0.2 1.6 ± 0.1
1.3 ± 0.2 1.3 ± 0.1
0.8 ± 0.1 1 ± 0.1
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Table 12 Flexural strain of the woven composites when tested at difference immersion times. Specimens
KK JJ KJ
HH KH
Dry (%)
Wet (%)
0h
2h
5h
15 h
24 h
5 ± 0.5 5 ± 0.6 5.8 ± 0.2
6 ± 0.4 6 ± 0.9 6.4 ± 0.3
7 ± 0.8 7±1 7 ± 0.8
8.5 ± 0.8 8.7 ± 0.6 9 ± 0.9
9±1 9 ± 0.8 9 ± 1.3
0h
2h
10 h
24 h
48 h
5.7 ± 0.6 6 ± 0.7
6±1 6.5 ± 0.8
7.3 ± 0.9 7.4 ± 0.5
9±1 9.3 ± 0.3
9.3 ± 1 9.5 ± 0.4
(a)
Void
(b)
Pull out
Fiber/matrix debonds
Fibre bending
Fiber/matrix debonding
Pull out
Fig. 12. FESEM images of the fractured flexural specimen at the (a) dry and (b) saturated conditions.
and the reinforcement fibre, which contributed to its high strength when the bending load was applied. As for the water-immersed sample, its flexural strength was reduced due to a poor fibre/matrix adhesion, as shown in Fig. 12b. When the water molecules penetrate the macrovoids and free space of the polymer, new cavities and cracks are formed; acts as the water transport pathway within the composites and this gradually reduced the interface bonding [23]. Furthermore, the voids seen in Fig. 12b act as stress concentrators [36], and lead to the failure of the water-immersed samples. Observation of the fracture surface from the flexural test sample further underlines the importance of fibre/matrix adhesion on flexural strength. 4. Conclusions The water absorption of interwoven kenaf/jute and kenaf/hemp hybrid composites, and its effect on their mechanical properties, was investigated following their immersion in tap water at room temperature. The conclusions from this experiment are as follow: The water absorption pattern of these composites is found to follow a non-Fickian behaviour. Furthermore, through their hybridisation, the water-resistant properties of woven kenaf, jute, and hemp fibre were improved. The tensile and flexural strength of the interwoven hybrid composites were superior to those of the individual woven composites due to the different load sharing properties between the longitudinal and transverse directional fibres in the woven structure. A greater stress uptake of the interwoven hybrid composites was obtained by the interlocking structure between the fibre yarns; therefore, a larger load was required to break the structure. Longer immersion times reduced the strength and modulus of the composites as a result of water absorption. However, the failure strain increased with increasing immersion time due to a breakdown of the cellulose structure.
Matrix cracking, delamination, and voids (as shown by the FESEM images) increased the penetration of water, and reduced the strength and modulus of the water immersed samples, by weakening the interface between the fibre and matrix.
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