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epoxy composites
Materials and Design 40 (2012) 378–385
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Flexural properties of treated and untreated kenaf/epoxy composites B.F. Yousif a,⇑, A. Shalwan a, C.W. Chin a, K.C. Ming b a b
Faculty of Engineering and Surveying, University Southern Queensland, Toowoomba, 4350 Qld, Australia Faculty of Engineering, University of Nottingham-Malaysia Campus, Semenyih, 45300 Selangor, Malaysia
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Article history: Received 26 December 2011 Accepted 9 April 2012 Available online 16 April 2012 Keywords: Polymer matrix Natural materials Mechanical properties Kenaf fibres Composites Flexural properties
a b s t r a c t In the current work, flexural properties of unidirectional long kenaf fibre reinforced epoxy (KFRE) composites are studied. The kenaf fibres were prepared into two types as untreated and treated (with 6% NaOH). The failure mechanism and damage features of the materials were categorized with the surface observation by scanning electron microscope (SEM). The results revealed that reinforcement of epoxy with treated kenaf fibres increased the flexural strength of the composite by about 36%, while, untreated fibres introduced 20% improvement. This was mainly due to the high improvement of the chemical treatment (NaOH) on the interfacial adhesion of the fibres and the porosity of the composites which prevented the debonding, detachments or pull out of fibres. For untreated KFRE, the fracture mechanisms were debonding, tearing, detachments and pull out of fibres. The developed composite exhibited superior properties compared to the previous composites based on natural and synthetic fibres. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, natural fibres are drawing considerable attention as substitute candidate for synthetic fibres. The applications of natural fibres are growing in many sectors such as automobiles, furniture, packing and construction. This is mainly due to their advantages compared to synthetic fibres, i.e. low cost, low weight, less damage to processing equipment, improved surface finish of moulded parts composite, good relative mechanical properties, abundant and renewable resources [1,2]. Nowadays, plant fibres are the most commonly used natural fibres such as sisal, jute, coir and flax fibres which are used as reinforcement and filler for polymer composites. The main concept of reinforcing the polymer with such fibres is to enhance the mechanical properties of the polymers, i.e. tensile, impact and flexural properties [3,4]. The mechanical properties of natural fibre reinforced composites highly depend on the interface adhesion property between the fibres and the polymer matrix as have been reported by many researchers [5–8]. Natural fibres contain cellulose, hemicelluloses, pectins and lignin and are rich in hydroxy1 groups, natural fibres tend to be strong polar and hydrophilic materials whilst polymer materials are a polar and exhibit significant hydrophobicity. In other words, there are significant problems of compatibility between the fibre and the matrix due to weakness in the interfacial adhesion of the natural fibres with the synthetic matrices. Therefore, surface modification of natural fibres by means treatment is one of the largest areas of recent researches to improve compatibility and interfacial ⇑ Corresponding author. Tel.: +61 746315331. E-mail address: [email protected] (B.F. Yousif). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.04.017
bond strength [9,10]. Chemical treatments as bleaching, acetylation and alkali treatment found to be a technique to enhance the matrix– fibre adhesion by increasing roughness through of clean the fibre surface from impurities and by disrupting the moisture absorption process through of coat of OH groups in fibre [11,12]. Many investigations have focused on the treatment of fibres to improve the bonding with resin matrix. From reported works on mechanical properties of polymeric composites based on natural fibres, the flexural strength of high density polyethylene (HDPE) composite increased by about 22% when treated henequen fibres were used as reinforcement in the composite [5]. In [13], Vilay et al. investigated the effect of fibre surface treatment (NaOH) and fibre loading (0– 20 vol.%) on the flexural properties of bagasse fibre reinforced unsaturated polyester composites (BFRUSP). NaOH treated fibre composites showed better flexural strength and modulus (increase by about 11% and 20% respectively) compared to untreated fibre composites. These experiential results were resulted of the surface modification by treatment that improves the fibre–matrix interaction. In another work, the effect of concentration (1–10%) and period (24, 48 h) of alkalization treatment on the flexural properties of Alfa/polyester composites (40 wt.% Randomly orientated fibres) have been studied [14]. The flexural test results of that work showed that alkali treatment of fibres Alfa improves the quality of the fibre/matrix interface. Moreover, both NaOH concentration and time treatment have a significant effect on the flexural properties of Alfa fibres reinforced composites. For fibres treated with 10% NaOH for 24 h, the flexural strength and flexural modulus were improved by 60% and 62%, respectively, compared to the untreated fibre composites. In polymeric composites based on natural fibres, the shapes of composite and its surface appearance were awarded by matrix
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while fibres act as carriers of load and stress (stiffness and strength) when composite is subjected to load. Therefore, the orientation of natural fibres has significant effects and plays an important role to enhancement the mechanical properties in polymeric composites based on natural fibres [15–17]. Brahim and Cheikh [15] studied the influence of fibres orientation on the mechanical properties of the Alfa/polyester composites with a volume fraction in fibres of 45%. All specimens made of unidirectional Alfa fibres and tested at different orientation angles (a) 0°, 10°, 30°, 45° and 90°. The reduction percentage of tensile strength (r) with change angle from 0° (longitudinal specimens) 0 45° was 78% and it was 88% when change it to 90° (transverse direction). This is highly with the agreements of many published articles [16,17]. Kenaf fibres have very high characteristics compared to other natural fibres, i.e. long fibre, small diameter, and high interfacial adhesion to matrix [18]. However, there is less work has been done on the kenaf fibres to comprehensively understand the possibility of using such fibres for polymeric composites especially in flexural loading conditions. This paper is an attempt has been made to study the effect of untreated and alkali treated kenaf fibre on flexural properties of epoxy composites.
2. Materials and experimental details Raw kenaf fibres were supplied by Malaysian Agricultural Research and Development Institute (MARDI). The supplied fibres were hand-washed, and dried at ambient temperature for 48 h. The density of kenaf fibres is approximately 620.26 kg/m3. Portion of the cleaned fibres were treated with 6% NaOH solution. In the NaOH treatment process, the fibres were socked in 6% NaOH solution for 24 h, then washed and dried in an oven at 40 °C for 24 h. The matrix used is a combination of D.E.R.™ 331™ Liquid Epoxy Resin and Jointmine 950-3S as Hardener/Curing Agent/Accelerator. In the fabrication process, a metal mould (100 100 10 mm3) was coated with a layer of release agent (WD-40). Epoxy/hardener (2:1) mixture was stirred, and poured into the mould. Mechanical properties of such materials is highly dependent on fibre alignment and the location of resin-rich areas, i.e. unidirectional long fibres tend to give better strength compared to the short and other orientations of fibres [19]. In the current work, the untreated kenaf fibres were prepared in unidirectional alignment, cut into lengths of 80 mm and placed in the mould. It is important to ensure that bubbles are not trapped in between the fibres; thus, a steel roller was moved on the composite to remove trapped air. Finally, the blocks of the composite pressed, and covered with mould cover and left to cure for 24 h. The same procedure was done for treated kenaf fibres as well. The volume fraction of the fibre was determined to be about 38–41%. For the neat epoxy (NE), the material was fabricated in the same procedure to the composites procedure without adding the fibres. The prepared blocks were machined into specimens with the size of 80 mm 10 mm 4 mm according to ASTM D790-07 flexural testing standard, i.e. 3-point flexural technique was adopted in the current experiments [20]. Lloyd LR50 KPlus 50 kN Universal Testing Machine was used to perform the experiments. The cross-head speed was set to 2 mm/min which is recommended by [21,22]. Schematic drawing of the test technique is shown in Fig. 1. Scanning electron microscopy was used to observe the surface modification resulting from the aging effect.
3. Results and discussions The effect of NaOH treatment on fibre surface morphology, the composite microstructure, and flexural properties will be addressed in the following sections.
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Fig. 1. Drawings and dimensions of bending equipment.
3.1. Influence of NaOH treatment on the surface morphology of the fibre and the composites To clarify the effect of alkali treatment on kenaf fibre and the microstructure of the composite, surface examinations were carried out on the untreated and alkalized fibres and composites. The scanning electron microscope (SEM) photographs of morphology in diameter direction and in the cross-section of both untreated and alkalized surfaces are exhibited in Figs. 2 and 3. The alkali treatment leads to significant differences in the fibre surface morphology. As shown in Fig. 2a, the surface of untreated kenaf fibre was found to be considerably covered with waxy substances and impurities. Relatively, micrograph of the treated fibre (Fig. 2b) shows an improvement in the surface morphology after 6% NaOH treatment, i.e. using 6% NaOH treatment removed the waxy layer and impurities from surface and the treated surface of fibre becomes rather rougher and fibrillation as compared to that of untreated fibre. Moreover, it can be seen that the fibres have been spitted into finer fibres. This could lead to high interlock and adhesion between the fibres and the matrix. Fig. 3 shows the crosssection of kenaf/epoxy composites based on untreated (Fig. 3a) and treated fibres (Fig. 3b) and a penetration of epoxy into plant fibre cell walls. Both micrographs of the treated and untreated composite surfaces are taken at the same conditions where the composite surface were polished against smooth surface (AISI 304, 50 BH hardness and 0.09 lm Ra roughness) with an applied load of 50 N. Fig. 3b shows higher epoxy penetration into plant fibre cell walls in treated fibre than untreated fibre, Fig. 3a. The high epoxy penetration may be due to the leaching out of the wax substances and impurities and leading to fibre bundle fibrillation. Alvarez and Vazquez [23] reported that the remotion of the cementing material in the sisal fibres by alkaline treatment led to produce fibrillation and collapse the cellular structure, which leads to a better packing of cellulose chains. 3.2. Flexural properties The mechanical properties of the pure epoxy, untreated kenaf fibre reinforced epoxy (UT-KFRE), and treated kenaf fibre reinforced epoxy (T-KFRE) composites are displayed in Fig. 4a–c. The figure shows the results of set of experiments conducted on each material. One can see that the variation in the results is remarkable. This indicates the accuracy of the machine and the homogeneity of the composites. Fig. 4a shows a ductile behaviour for the neat epoxy and the maximum strength is noticed at about 180–200 MPa when the deflection reached about 12.5 mm. Fig. 4b indicates that the addition of untreated kenaf fibres to the epoxy enhances the flexural strength and reduces the deflection, i.e. the maximum strength of the UT-KFRE is about 225–250 MPa. On the other hand, the treated kenaf fibres highly improved the flexural strength of the epoxy
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Fig. 2. SEM micrographs of: (a) untreated kenaf fibres, (b) 6% sodium hydroxide, NaOH treated kenaf fibres.
Fig. 3. SEM micrographs on the cross-section of kenaf fibre reinforced epoxy composite: (a) untreated kenaf fibre, (b) treated kenaf fibres.
composite which reached about 300–350 MPa as shown in Fig. 4c. However, it is obvious that the addition of either treated or untreated kenaf fibres to epoxy composite reduces the deflection. Furthermore, the results presented in Fig. 4 is summarized in Figs. 5 and 6 showing the average of the flexural strength, deformation, and flexural modulus associated with maximum and minimum values of each set of test. As shown in Fig. 5, it is observed that the flexural strength of UT-KFRE and T-KFRE composites is high by about 20% and 36%, respectively, compared to neat epoxy. In addition, the flexural modulus of epoxy composites enhanced by 67% and 74% with the addition of untreated and treated kenaf fibres to the composites, Fig. 6. Furthermore, the 6% NaOH treated to the kenaf fibres contribute to the better performance of T-KFRE compared to the UT-KFRE. This could be due to two reasons as the treatment enhances the interfacial adhesion of the fibre with the matrix and allowing the resin to enter the fibres’ bundle during the manufacturing process. From the reported works on the effect of fibre treatment on the mechanical properties of polymers, treating jute [11], Agave [19], sisal [24], oil palm [25] and coir [26] fibres with 5–6% NaOH improved the tensile strength of thermoset polymers (polyester and epoxy). This has been explained with the fact that the waxy layers on the fibre surface are removed with the assistance of NaOH solution which in turn enhances the interfacial adhesion of the fibre with the matrix. This leads to the high carrying loading by the fibres in the matrix during loading conditions (tensile), i.e. the load is transferred to the fibres and the fibres are able to carry some of the load. The participant authors have another thought which the high resin penetration in the treated kenaf fibre which decreases the possibility of debonding, detachment or pull out of fibre from matrix during loading condition. To explain this thought, Fig. 7 shows schematically the cross-section of the untreated and treated kenaf fibres embedded in
the matrix and the failure behaviour of both composites. In the case of untreated kenaf fibres, the epoxy resin was not able to enter the fibres which lead to two phenomenons as the high porosity of the composite (empty fibres) and less interaction of the core of the fibres with the surrounding matrix. The density of T-KFRE and UT-KFRE were determined and found to be that the density of treated kenaf fibres is about 999.44 kg/m3, while the density of UT-KFRE is about 985.45 kg/m3 which indicates the higher porosity of the UT-KFRE compared to T-KFRE. Wucherer et al. [27] reported that high porosity materials have less mechanical performance compared to the ones with low porosity. In that work [27], the high porosity of barium titanate (BaTiO3) foams exhibited lower tensile strength compared to the low porosity barium titanate (BaTiO3). This is highly in agreement with the current results, i.e. it is evidence that the higher porosity fUN-KFRE composite is one of the reason of low flexural strength of the composite compared to the T-KFRE. Further evidence to that will be given in the SEM observation section. 3.3. Surface morphology of flexural test composites Scanning electron microscopy (SEM) analysis was used to observe the failure mechanism which occurred on the composites. The SEM micrographs for UT-KFRE composite fracture is shown in Fig. 8a–e. In general, there is clear pull out, detachment and debonding of some of the kenaf fibres can be seen which indicates the poor interfacial adhesion of the untreated kenaf fibres with the epoxy matrix. Tearing in the inner fine fibres can be seen in Fig. 8c and e which could be due to the lack of epoxy in the inner space of the fibres. Furthermore, Fig. 8b and e shows empty fibres where the resin was not able to penetrate in the bundle of the fibres. This confirms the thought given before. However, the bending
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Specimen 3
Stress, MPa
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Specimen 54
250 200 150 100 50 0
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Specimen 2
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Specimen 1
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Deflection, mm
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250 200 150 100 50 0
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2
3
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Deflection, mm
Flexural Modulus, GPa
Stress, MPa
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Fig. 5. Bar chart with flexural strength and flexural strain of pure epoxy, untreated KFRE and treated KFRE.
Specimen 2 Specimen 4 Specimen 6 Specimen 8
9 8 7 6 5 4 3 2 1 0
c) T-KFRE
Pure Epoxy
400
Untreated
Treated
Fig. 6. Bar chart with flexural modulus of pure epoxy, untreated KFRE and treated KFRE.
350
Stress, MPa
300 250
Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Specimen 6 Specimen 7 Specimen 8
200 150 100 50 0 0
1
2
3
4
5
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Deflection, mm Fig. 4. Stress-deformation diagram of the neat epoxy and both treated and untreated kenaf fibre reinforced epoxy composites.
area indicates good adhesion of the some of the fibres with the matrix. In contrast, Fig. 9a–c shows good bonding areas between fibres and epoxy and almost no signs of fibres debonding, detachment or pull out mechanisms at the treated KFRE composite fracture surface after flexural loading. This result suggests that interfacial adhesion between the kenaf fibres and epoxy matrix has become much more favourable and strong upon treated of fibres with NaOH. The remotion of the cementing material (waxy layer and impurities) in the kenaf fibres by alkaline treatment led to increase the roughness of fibre surface and then create a good interlocking mechanism with the surface of matrix. Furthermore, the clean the fibre surface of a large amount of waxy layer and impurities cause fibrillation of fibres. On the other word, the fibrillation increases the effective surface area available for contact with the matrix. This phenomenon could be explain the reason of bending of fibres, as
shown in Fig. 9a, due to improve the fibre surface adhesives characteristics and therefore improve the interfacial bonding between the fibre and resin matrix. Furthermore, during the fibrillation process, the spaces between fibres increase which enable epoxy to enter and fill the spaces and therefore increase the area contact between the fibre and resin and enhance the interfacial bonding, as shown in Fig. 9b. The epoxy debris in the fracture surface of TKFRE, Fig. 9c, indicates the degree of distribution of epoxy through the fibre bundle due to the fibrillation of fibres. The fibrillation of fibres could be led to increase the space areas between the fibres’ bundle and lead to smooth flowing for resin into these space areas. Furthermore, the defragment of fibre shows in Fig. 9c could be due to two reasons as the high degree of distribution of resin and high degree of penetration to fibres. These reasons could be attributed to enhance the interfacial bonding and increase the incorporation between the fibres and resin as a bulk material and reduce the defragment of fibre from matrix. 3.4. Comparison to other published works It is interesting to compare the current results with the previous works and examine the possibility of replacing glass fibres with kenaf fibres for flexural applications. Table 1 summarizes the available flexural properties of polymeric composites based on natural and synthetic fibres. In the previous attempted works on natural fibre/polymer composites, treated sisal fibres reinforced epoxy composites exhibited a flexural strength of 225 MPa [28] which is higher than others. In the current work, treated kenaf fibres/epoxy composite shows higher flexural strength (301.64 MPa) and modulus (6.74 GPa) values compared to the sisal/epoxy. This is due to two main reasons
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(a)
(b)
(a) Illustration on epoxy penetration for: (a) Untreated KFRE, (b) Treated KFRE
(b) Illustrated failure behaviour of untreated KFRE
(c) Illustrated failure behaviour of treated KFRE Fig. 7. Schematic drawing of untreated and treated kenaf fibre composite showing the flexural behaviour of both composites.
which are the high strength and the better interfacial adhesion of the kenaf fibres compared to the sisal fibre. Moreover, the single sisal fibre has more than 100 irregular hexagonal hollow ultimate cells, spongy tissue, vessel, sieve tube etc., [28]. Meanwhile, kenaf fibres are a bundle of fine fibres with same characteristics and structure as seen in the previous micrographs. For the other types of natural fibres with polymers, the low flexural properties is either due to the orientations of the fibres in the matrix [4,7,12], the usage of thermoplastic as matrix [5,7], or poor interfacial adhesion of the untreated fibres [6,7,12,18]. In [6], the resin used was the epoxy thermoset. It is well known that epoxy has very high mechanical properties compared to other types of resin. However, the flexural properties of the banana/epoxy composite were poor. This is mainly due to the poor interfacial adhesion of the untreated banana fibres with the epoxy. On the other hand, kenaf fibres have been used with the PLA in [8] and showed high flexural properties. In that work [8], the kenaf fibres assisted to increase the flexural strength by about 62% when the volume fraction of the kenaf fibres was about 45%. In [18], kenaf fibre/polyester composite showed a flexural strength of 123 MPa when the volume fraction was 64% and long fibres used. In the fabrication of the kenaf/polyester
composite, it has been mentioned that ‘‘the aim is to maximize the volume fraction of fibres in the composites to maximize the mechanical properties’’. However, it is well known that there is a critical volume fraction of fibres in the matrix which introduces the optimum mechanical properties, i.e. transfer the load from the matrix to the fibres. In fabricating natural fibre/polymer composites, it is recommended to consider few elements: the volume fraction (to be below 50%), the interfacial adhesion of the fibre with the matrix, orientation of the fibre and length of the fibre. On the other hand, glass/polyester composites, [29], exhibited better flexural strength compared to all the natural fibres including the kenaf/epoxy composites. In fact, glass fibres have much high tensile strength compared to the kenaf fibres. In addition, woven glass mats have been used to reinforce the polyester composite. Those two reasons attributes to the high flexural properties of the glass/polyester composites compared to the kenaf/ epoxy composites. However, for less than 250 MPa flexural load of 250 MPa (considering safety factor of 1.2 or below), kenaf/ epoxy composite most appropriate materials offering light weight, biodegradable, less cost, less damage to the manufacturing equipments.
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(b)
(a)
Debonding
Pull out fibres
bonding
Empty Bundle Debonding
Fibres breakage
Tearing Macro-cracks Macro-cracks
Brittle failure
(c)
(d)
Brittle failure
Tearing of fibres
(e) Detachment Empty Bundle
Debonding
Fig. 8. SEM micrographs of untreated KFRE after flexural failure.
bonding
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(a)
Bonding
(b) Filled fibre with Epoxy
Bending of fibres
Bending of fibre
(c) Epoxy debris Bonding
Defragmentation of fibre
Fig. 9. SEM micrographs of treated KFRE after bending showing: (a) bent fibre, (b) epoxy coverage, (c) short fibre pull out length.
Table 1 Comparisons between various existing fibre reinforced composites. Materials Pure epoxy Untreated KFRE Treated KFRE Palmyra/polyester [4] Palmyra–glass/polyester [4] Bamboo/corn starch [12] Kenaf/corn starch [12] Kenaf/polyester [18] Hemp/polyester [18] Woven banana/epoxy [6] Glass/polyester [29] Glass mat/polypropylene [7] Kenaf/polypropylene [7] Coir/polypropylene [7] Sisal/polypropylene [7] Hemp/polypropylene [7] Jute/polypropylene [7] Untreated sisal/epoxy [28] Treated sisal/epoxy [28] Kenaf/PLA [8] Henequen/HDPE [5]
Fibre orientation Unidirectional Unidirectional Randomly distributed 50 mm, 55 vol.% Randomly distributed 50 mm, 55 wt.% and 12 wt.% palmyra fibres Randomly distributed 10 mm, 72 vol.% fibre Randomly distributed short (10 mm) 60 vol.% fibre Unidirectional 64 vol.% Unidirectional 56 vol.% 60 vol.% 22 vol.% Random mat 40 vol.% Random mat 40 vol.% Random mat 40 wt.% Random mat 40 wt.% Random mat 40 wt.% Unidirectional 50 vol.% Unidirectional 50 vol.% Unidirectional 45 vol.% 46 vol.%
4. Conclusion The following points are drawn from the experimental results: 1. The flexural properties of the KFRE composite are highly influenced by the kenaf fibre surface characteristics. NaOH treatment highly enhanced the interfacial adhesion of the fibre with the matrix leading to better flexural properties compared to the untreated fibres. Thirty six percentage increment in the flexural strength of the epoxy composite achieved when treated kenaf fibres were used as reinforcement compared to the untreated fibres, which showed only 20%.
Flexural st (MPa)
Flexural mod (GPa)
192.73 ± 12 235.13 ± 27 301.64 ± 72 59 MPa 175 N/A N/A 123 101 28.181 590–720 60 27 25 23 54 34 200 225 210 95.9
1.726 ± 0.8 5.572 ± 0.8 6.743 ± 1.8 8.45 9.35 5.4 4.8 13 10 2.685 31–38 4.48 2.3 0.5 1.6 4.9 2.6 15 18 18 2.61 ± 102
2. In the case of untreated KFRE, the fracture mechanisms were debonding, tearing, detachments and pull out of fibres due to the low interfacial adhesion of fibres with the matrix. On the other hand, treated KFRE showed only breakage at the end of the fibres, i.e. the treated fibres were able to carry the load during the test. This was the main reason of the high flexural properties of treated KFRE compared to the untreated KFRE. 3. Treating the kenaf fibres with 6% NaOH split the kenaf fibre bundles into fine fibres which in turn allowed the epoxy resin to penetrate in the fibre bundles leading to high interlocking of the fibres in the matrix (high interfacial adhesion). However, the porosity of the treated kenaf fibres/epoxy composite is
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lower than the untreated fibre/epoxy composites which was another reason of the better flexural properties of the treated fibre/epoxy composite compared to the untreated one. 4. Kenaf fibres exhibited superiors properties as reinforcement for polymeric composites under flexural loading conditions compared to other types of natural fibres. Moreover, there is a possibility of replacing the synthetic fibres (glass) with kenaf fibres for flexural applications. References [1] Chin CW, Yousif BF. Potential of kenaf fibres as reinforcement for tribological applications. Wear 2009;267(9–10):1550–7. [2] Yousif BF, Ku H. Suitability of using coir fiber/polymeric composite for the design of liquid storage tanks. Mater amp Des 2012;36(0):847–53. [3] Joshi SV et al. Are natural fiber composites environmentally superior to glass fiber reinforced composites? Compos A Appl Sci Manuf 2004;35(3):371–6. [4] Velmurugan R, Manikandan V. Mechanical properties of palmyra/glass fiber hybrid composites. Compos A Appl Sci Manuf 2007;38(10):2216–26. [5] Herrera-Franco PJ, Valadez-González A. Mechanical properties of continuous natural fibre-reinforced polymer composites. Compos A Appl Sci Manuf 2004;35(3):339–45. [6] Sapuan SM et al. Mechanical properties of woven banana fibre reinforced epoxy composites. Mater amp Des 2006;27(8):689–93. [7] Wambua P, Ivens J, Verpoest I. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63(9):1259–64. [8] Shinji O. Mechanical properties of kenaf fibers and kenaf/PLA composites. Mech Mater 2008;40(4–5):446–52. [9] Alawar A, Hamed AM, Al-Kaabi K. Characterization of treated date palm tree fiber as composite reinforcement. Compos B Eng 2009;40(7):601–6. [10] Cantero G et al. Effects of fibre treatment on wettability and mechanical behaviour of flax/polypropylene composites. Compos Sci Technol 2003;63(9):1247–54. [11] Saha P et al. Enhancement of tensile strength of lignocellulosic jute fibers by alkali-steam treatment. Bioresour Technol 2010;101(9):3182–7. [12] Shibata S, Cao Y, Fukumoto I. Flexural modulus of the unidirectional and random composites made from biodegradable resin and bamboo and kenaf fibres. Compos A Appl Sci Manuf 2008;39(4):640–6. [13] Vilay V et al. Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber-reinforced unsaturated polyester composites. Compos Sci Technol 2008;68(3–4):631–8.
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Flexural properties of treated and untreated kenaf/epoxy composites B.F. Yousif a,⇑, A. Shalwan a, C.W. Chin a, K.C. Ming b a b
Faculty of Engineering and Surveying, University Southern Queensland, Toowoomba, 4350 Qld, Australia Faculty of Engineering, University of Nottingham-Malaysia Campus, Semenyih, 45300 Selangor, Malaysia
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Article history: Received 26 December 2011 Accepted 9 April 2012 Available online 16 April 2012 Keywords: Polymer matrix Natural materials Mechanical properties Kenaf fibres Composites Flexural properties
a b s t r a c t In the current work, flexural properties of unidirectional long kenaf fibre reinforced epoxy (KFRE) composites are studied. The kenaf fibres were prepared into two types as untreated and treated (with 6% NaOH). The failure mechanism and damage features of the materials were categorized with the surface observation by scanning electron microscope (SEM). The results revealed that reinforcement of epoxy with treated kenaf fibres increased the flexural strength of the composite by about 36%, while, untreated fibres introduced 20% improvement. This was mainly due to the high improvement of the chemical treatment (NaOH) on the interfacial adhesion of the fibres and the porosity of the composites which prevented the debonding, detachments or pull out of fibres. For untreated KFRE, the fracture mechanisms were debonding, tearing, detachments and pull out of fibres. The developed composite exhibited superior properties compared to the previous composites based on natural and synthetic fibres. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, natural fibres are drawing considerable attention as substitute candidate for synthetic fibres. The applications of natural fibres are growing in many sectors such as automobiles, furniture, packing and construction. This is mainly due to their advantages compared to synthetic fibres, i.e. low cost, low weight, less damage to processing equipment, improved surface finish of moulded parts composite, good relative mechanical properties, abundant and renewable resources [1,2]. Nowadays, plant fibres are the most commonly used natural fibres such as sisal, jute, coir and flax fibres which are used as reinforcement and filler for polymer composites. The main concept of reinforcing the polymer with such fibres is to enhance the mechanical properties of the polymers, i.e. tensile, impact and flexural properties [3,4]. The mechanical properties of natural fibre reinforced composites highly depend on the interface adhesion property between the fibres and the polymer matrix as have been reported by many researchers [5–8]. Natural fibres contain cellulose, hemicelluloses, pectins and lignin and are rich in hydroxy1 groups, natural fibres tend to be strong polar and hydrophilic materials whilst polymer materials are a polar and exhibit significant hydrophobicity. In other words, there are significant problems of compatibility between the fibre and the matrix due to weakness in the interfacial adhesion of the natural fibres with the synthetic matrices. Therefore, surface modification of natural fibres by means treatment is one of the largest areas of recent researches to improve compatibility and interfacial ⇑ Corresponding author. Tel.: +61 746315331. E-mail address: [email protected] (B.F. Yousif). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.04.017
bond strength [9,10]. Chemical treatments as bleaching, acetylation and alkali treatment found to be a technique to enhance the matrix– fibre adhesion by increasing roughness through of clean the fibre surface from impurities and by disrupting the moisture absorption process through of coat of OH groups in fibre [11,12]. Many investigations have focused on the treatment of fibres to improve the bonding with resin matrix. From reported works on mechanical properties of polymeric composites based on natural fibres, the flexural strength of high density polyethylene (HDPE) composite increased by about 22% when treated henequen fibres were used as reinforcement in the composite [5]. In [13], Vilay et al. investigated the effect of fibre surface treatment (NaOH) and fibre loading (0– 20 vol.%) on the flexural properties of bagasse fibre reinforced unsaturated polyester composites (BFRUSP). NaOH treated fibre composites showed better flexural strength and modulus (increase by about 11% and 20% respectively) compared to untreated fibre composites. These experiential results were resulted of the surface modification by treatment that improves the fibre–matrix interaction. In another work, the effect of concentration (1–10%) and period (24, 48 h) of alkalization treatment on the flexural properties of Alfa/polyester composites (40 wt.% Randomly orientated fibres) have been studied [14]. The flexural test results of that work showed that alkali treatment of fibres Alfa improves the quality of the fibre/matrix interface. Moreover, both NaOH concentration and time treatment have a significant effect on the flexural properties of Alfa fibres reinforced composites. For fibres treated with 10% NaOH for 24 h, the flexural strength and flexural modulus were improved by 60% and 62%, respectively, compared to the untreated fibre composites. In polymeric composites based on natural fibres, the shapes of composite and its surface appearance were awarded by matrix
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while fibres act as carriers of load and stress (stiffness and strength) when composite is subjected to load. Therefore, the orientation of natural fibres has significant effects and plays an important role to enhancement the mechanical properties in polymeric composites based on natural fibres [15–17]. Brahim and Cheikh [15] studied the influence of fibres orientation on the mechanical properties of the Alfa/polyester composites with a volume fraction in fibres of 45%. All specimens made of unidirectional Alfa fibres and tested at different orientation angles (a) 0°, 10°, 30°, 45° and 90°. The reduction percentage of tensile strength (r) with change angle from 0° (longitudinal specimens) 0 45° was 78% and it was 88% when change it to 90° (transverse direction). This is highly with the agreements of many published articles [16,17]. Kenaf fibres have very high characteristics compared to other natural fibres, i.e. long fibre, small diameter, and high interfacial adhesion to matrix [18]. However, there is less work has been done on the kenaf fibres to comprehensively understand the possibility of using such fibres for polymeric composites especially in flexural loading conditions. This paper is an attempt has been made to study the effect of untreated and alkali treated kenaf fibre on flexural properties of epoxy composites.
2. Materials and experimental details Raw kenaf fibres were supplied by Malaysian Agricultural Research and Development Institute (MARDI). The supplied fibres were hand-washed, and dried at ambient temperature for 48 h. The density of kenaf fibres is approximately 620.26 kg/m3. Portion of the cleaned fibres were treated with 6% NaOH solution. In the NaOH treatment process, the fibres were socked in 6% NaOH solution for 24 h, then washed and dried in an oven at 40 °C for 24 h. The matrix used is a combination of D.E.R.™ 331™ Liquid Epoxy Resin and Jointmine 950-3S as Hardener/Curing Agent/Accelerator. In the fabrication process, a metal mould (100 100 10 mm3) was coated with a layer of release agent (WD-40). Epoxy/hardener (2:1) mixture was stirred, and poured into the mould. Mechanical properties of such materials is highly dependent on fibre alignment and the location of resin-rich areas, i.e. unidirectional long fibres tend to give better strength compared to the short and other orientations of fibres [19]. In the current work, the untreated kenaf fibres were prepared in unidirectional alignment, cut into lengths of 80 mm and placed in the mould. It is important to ensure that bubbles are not trapped in between the fibres; thus, a steel roller was moved on the composite to remove trapped air. Finally, the blocks of the composite pressed, and covered with mould cover and left to cure for 24 h. The same procedure was done for treated kenaf fibres as well. The volume fraction of the fibre was determined to be about 38–41%. For the neat epoxy (NE), the material was fabricated in the same procedure to the composites procedure without adding the fibres. The prepared blocks were machined into specimens with the size of 80 mm 10 mm 4 mm according to ASTM D790-07 flexural testing standard, i.e. 3-point flexural technique was adopted in the current experiments [20]. Lloyd LR50 KPlus 50 kN Universal Testing Machine was used to perform the experiments. The cross-head speed was set to 2 mm/min which is recommended by [21,22]. Schematic drawing of the test technique is shown in Fig. 1. Scanning electron microscopy was used to observe the surface modification resulting from the aging effect.
3. Results and discussions The effect of NaOH treatment on fibre surface morphology, the composite microstructure, and flexural properties will be addressed in the following sections.
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Fig. 1. Drawings and dimensions of bending equipment.
3.1. Influence of NaOH treatment on the surface morphology of the fibre and the composites To clarify the effect of alkali treatment on kenaf fibre and the microstructure of the composite, surface examinations were carried out on the untreated and alkalized fibres and composites. The scanning electron microscope (SEM) photographs of morphology in diameter direction and in the cross-section of both untreated and alkalized surfaces are exhibited in Figs. 2 and 3. The alkali treatment leads to significant differences in the fibre surface morphology. As shown in Fig. 2a, the surface of untreated kenaf fibre was found to be considerably covered with waxy substances and impurities. Relatively, micrograph of the treated fibre (Fig. 2b) shows an improvement in the surface morphology after 6% NaOH treatment, i.e. using 6% NaOH treatment removed the waxy layer and impurities from surface and the treated surface of fibre becomes rather rougher and fibrillation as compared to that of untreated fibre. Moreover, it can be seen that the fibres have been spitted into finer fibres. This could lead to high interlock and adhesion between the fibres and the matrix. Fig. 3 shows the crosssection of kenaf/epoxy composites based on untreated (Fig. 3a) and treated fibres (Fig. 3b) and a penetration of epoxy into plant fibre cell walls. Both micrographs of the treated and untreated composite surfaces are taken at the same conditions where the composite surface were polished against smooth surface (AISI 304, 50 BH hardness and 0.09 lm Ra roughness) with an applied load of 50 N. Fig. 3b shows higher epoxy penetration into plant fibre cell walls in treated fibre than untreated fibre, Fig. 3a. The high epoxy penetration may be due to the leaching out of the wax substances and impurities and leading to fibre bundle fibrillation. Alvarez and Vazquez [23] reported that the remotion of the cementing material in the sisal fibres by alkaline treatment led to produce fibrillation and collapse the cellular structure, which leads to a better packing of cellulose chains. 3.2. Flexural properties The mechanical properties of the pure epoxy, untreated kenaf fibre reinforced epoxy (UT-KFRE), and treated kenaf fibre reinforced epoxy (T-KFRE) composites are displayed in Fig. 4a–c. The figure shows the results of set of experiments conducted on each material. One can see that the variation in the results is remarkable. This indicates the accuracy of the machine and the homogeneity of the composites. Fig. 4a shows a ductile behaviour for the neat epoxy and the maximum strength is noticed at about 180–200 MPa when the deflection reached about 12.5 mm. Fig. 4b indicates that the addition of untreated kenaf fibres to the epoxy enhances the flexural strength and reduces the deflection, i.e. the maximum strength of the UT-KFRE is about 225–250 MPa. On the other hand, the treated kenaf fibres highly improved the flexural strength of the epoxy
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Fig. 2. SEM micrographs of: (a) untreated kenaf fibres, (b) 6% sodium hydroxide, NaOH treated kenaf fibres.
Fig. 3. SEM micrographs on the cross-section of kenaf fibre reinforced epoxy composite: (a) untreated kenaf fibre, (b) treated kenaf fibres.
composite which reached about 300–350 MPa as shown in Fig. 4c. However, it is obvious that the addition of either treated or untreated kenaf fibres to epoxy composite reduces the deflection. Furthermore, the results presented in Fig. 4 is summarized in Figs. 5 and 6 showing the average of the flexural strength, deformation, and flexural modulus associated with maximum and minimum values of each set of test. As shown in Fig. 5, it is observed that the flexural strength of UT-KFRE and T-KFRE composites is high by about 20% and 36%, respectively, compared to neat epoxy. In addition, the flexural modulus of epoxy composites enhanced by 67% and 74% with the addition of untreated and treated kenaf fibres to the composites, Fig. 6. Furthermore, the 6% NaOH treated to the kenaf fibres contribute to the better performance of T-KFRE compared to the UT-KFRE. This could be due to two reasons as the treatment enhances the interfacial adhesion of the fibre with the matrix and allowing the resin to enter the fibres’ bundle during the manufacturing process. From the reported works on the effect of fibre treatment on the mechanical properties of polymers, treating jute [11], Agave [19], sisal [24], oil palm [25] and coir [26] fibres with 5–6% NaOH improved the tensile strength of thermoset polymers (polyester and epoxy). This has been explained with the fact that the waxy layers on the fibre surface are removed with the assistance of NaOH solution which in turn enhances the interfacial adhesion of the fibre with the matrix. This leads to the high carrying loading by the fibres in the matrix during loading conditions (tensile), i.e. the load is transferred to the fibres and the fibres are able to carry some of the load. The participant authors have another thought which the high resin penetration in the treated kenaf fibre which decreases the possibility of debonding, detachment or pull out of fibre from matrix during loading condition. To explain this thought, Fig. 7 shows schematically the cross-section of the untreated and treated kenaf fibres embedded in
the matrix and the failure behaviour of both composites. In the case of untreated kenaf fibres, the epoxy resin was not able to enter the fibres which lead to two phenomenons as the high porosity of the composite (empty fibres) and less interaction of the core of the fibres with the surrounding matrix. The density of T-KFRE and UT-KFRE were determined and found to be that the density of treated kenaf fibres is about 999.44 kg/m3, while the density of UT-KFRE is about 985.45 kg/m3 which indicates the higher porosity of the UT-KFRE compared to T-KFRE. Wucherer et al. [27] reported that high porosity materials have less mechanical performance compared to the ones with low porosity. In that work [27], the high porosity of barium titanate (BaTiO3) foams exhibited lower tensile strength compared to the low porosity barium titanate (BaTiO3). This is highly in agreement with the current results, i.e. it is evidence that the higher porosity fUN-KFRE composite is one of the reason of low flexural strength of the composite compared to the T-KFRE. Further evidence to that will be given in the SEM observation section. 3.3. Surface morphology of flexural test composites Scanning electron microscopy (SEM) analysis was used to observe the failure mechanism which occurred on the composites. The SEM micrographs for UT-KFRE composite fracture is shown in Fig. 8a–e. In general, there is clear pull out, detachment and debonding of some of the kenaf fibres can be seen which indicates the poor interfacial adhesion of the untreated kenaf fibres with the epoxy matrix. Tearing in the inner fine fibres can be seen in Fig. 8c and e which could be due to the lack of epoxy in the inner space of the fibres. Furthermore, Fig. 8b and e shows empty fibres where the resin was not able to penetrate in the bundle of the fibres. This confirms the thought given before. However, the bending
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Specimen 3
Stress, MPa
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Specimen 1 Specimen 3 Specimen 5 Specimen 7
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Flexural Modulus, GPa
Stress, MPa
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Fig. 5. Bar chart with flexural strength and flexural strain of pure epoxy, untreated KFRE and treated KFRE.
Specimen 2 Specimen 4 Specimen 6 Specimen 8
9 8 7 6 5 4 3 2 1 0
c) T-KFRE
Pure Epoxy
400
Untreated
Treated
Fig. 6. Bar chart with flexural modulus of pure epoxy, untreated KFRE and treated KFRE.
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Stress, MPa
300 250
Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5 Specimen 6 Specimen 7 Specimen 8
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1
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Deflection, mm Fig. 4. Stress-deformation diagram of the neat epoxy and both treated and untreated kenaf fibre reinforced epoxy composites.
area indicates good adhesion of the some of the fibres with the matrix. In contrast, Fig. 9a–c shows good bonding areas between fibres and epoxy and almost no signs of fibres debonding, detachment or pull out mechanisms at the treated KFRE composite fracture surface after flexural loading. This result suggests that interfacial adhesion between the kenaf fibres and epoxy matrix has become much more favourable and strong upon treated of fibres with NaOH. The remotion of the cementing material (waxy layer and impurities) in the kenaf fibres by alkaline treatment led to increase the roughness of fibre surface and then create a good interlocking mechanism with the surface of matrix. Furthermore, the clean the fibre surface of a large amount of waxy layer and impurities cause fibrillation of fibres. On the other word, the fibrillation increases the effective surface area available for contact with the matrix. This phenomenon could be explain the reason of bending of fibres, as
shown in Fig. 9a, due to improve the fibre surface adhesives characteristics and therefore improve the interfacial bonding between the fibre and resin matrix. Furthermore, during the fibrillation process, the spaces between fibres increase which enable epoxy to enter and fill the spaces and therefore increase the area contact between the fibre and resin and enhance the interfacial bonding, as shown in Fig. 9b. The epoxy debris in the fracture surface of TKFRE, Fig. 9c, indicates the degree of distribution of epoxy through the fibre bundle due to the fibrillation of fibres. The fibrillation of fibres could be led to increase the space areas between the fibres’ bundle and lead to smooth flowing for resin into these space areas. Furthermore, the defragment of fibre shows in Fig. 9c could be due to two reasons as the high degree of distribution of resin and high degree of penetration to fibres. These reasons could be attributed to enhance the interfacial bonding and increase the incorporation between the fibres and resin as a bulk material and reduce the defragment of fibre from matrix. 3.4. Comparison to other published works It is interesting to compare the current results with the previous works and examine the possibility of replacing glass fibres with kenaf fibres for flexural applications. Table 1 summarizes the available flexural properties of polymeric composites based on natural and synthetic fibres. In the previous attempted works on natural fibre/polymer composites, treated sisal fibres reinforced epoxy composites exhibited a flexural strength of 225 MPa [28] which is higher than others. In the current work, treated kenaf fibres/epoxy composite shows higher flexural strength (301.64 MPa) and modulus (6.74 GPa) values compared to the sisal/epoxy. This is due to two main reasons
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(a)
(b)
(a) Illustration on epoxy penetration for: (a) Untreated KFRE, (b) Treated KFRE
(b) Illustrated failure behaviour of untreated KFRE
(c) Illustrated failure behaviour of treated KFRE Fig. 7. Schematic drawing of untreated and treated kenaf fibre composite showing the flexural behaviour of both composites.
which are the high strength and the better interfacial adhesion of the kenaf fibres compared to the sisal fibre. Moreover, the single sisal fibre has more than 100 irregular hexagonal hollow ultimate cells, spongy tissue, vessel, sieve tube etc., [28]. Meanwhile, kenaf fibres are a bundle of fine fibres with same characteristics and structure as seen in the previous micrographs. For the other types of natural fibres with polymers, the low flexural properties is either due to the orientations of the fibres in the matrix [4,7,12], the usage of thermoplastic as matrix [5,7], or poor interfacial adhesion of the untreated fibres [6,7,12,18]. In [6], the resin used was the epoxy thermoset. It is well known that epoxy has very high mechanical properties compared to other types of resin. However, the flexural properties of the banana/epoxy composite were poor. This is mainly due to the poor interfacial adhesion of the untreated banana fibres with the epoxy. On the other hand, kenaf fibres have been used with the PLA in [8] and showed high flexural properties. In that work [8], the kenaf fibres assisted to increase the flexural strength by about 62% when the volume fraction of the kenaf fibres was about 45%. In [18], kenaf fibre/polyester composite showed a flexural strength of 123 MPa when the volume fraction was 64% and long fibres used. In the fabrication of the kenaf/polyester
composite, it has been mentioned that ‘‘the aim is to maximize the volume fraction of fibres in the composites to maximize the mechanical properties’’. However, it is well known that there is a critical volume fraction of fibres in the matrix which introduces the optimum mechanical properties, i.e. transfer the load from the matrix to the fibres. In fabricating natural fibre/polymer composites, it is recommended to consider few elements: the volume fraction (to be below 50%), the interfacial adhesion of the fibre with the matrix, orientation of the fibre and length of the fibre. On the other hand, glass/polyester composites, [29], exhibited better flexural strength compared to all the natural fibres including the kenaf/epoxy composites. In fact, glass fibres have much high tensile strength compared to the kenaf fibres. In addition, woven glass mats have been used to reinforce the polyester composite. Those two reasons attributes to the high flexural properties of the glass/polyester composites compared to the kenaf/ epoxy composites. However, for less than 250 MPa flexural load of 250 MPa (considering safety factor of 1.2 or below), kenaf/ epoxy composite most appropriate materials offering light weight, biodegradable, less cost, less damage to the manufacturing equipments.
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(b)
(a)
Debonding
Pull out fibres
bonding
Empty Bundle Debonding
Fibres breakage
Tearing Macro-cracks Macro-cracks
Brittle failure
(c)
(d)
Brittle failure
Tearing of fibres
(e) Detachment Empty Bundle
Debonding
Fig. 8. SEM micrographs of untreated KFRE after flexural failure.
bonding
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(a)
Bonding
(b) Filled fibre with Epoxy
Bending of fibres
Bending of fibre
(c) Epoxy debris Bonding
Defragmentation of fibre
Fig. 9. SEM micrographs of treated KFRE after bending showing: (a) bent fibre, (b) epoxy coverage, (c) short fibre pull out length.
Table 1 Comparisons between various existing fibre reinforced composites. Materials Pure epoxy Untreated KFRE Treated KFRE Palmyra/polyester [4] Palmyra–glass/polyester [4] Bamboo/corn starch [12] Kenaf/corn starch [12] Kenaf/polyester [18] Hemp/polyester [18] Woven banana/epoxy [6] Glass/polyester [29] Glass mat/polypropylene [7] Kenaf/polypropylene [7] Coir/polypropylene [7] Sisal/polypropylene [7] Hemp/polypropylene [7] Jute/polypropylene [7] Untreated sisal/epoxy [28] Treated sisal/epoxy [28] Kenaf/PLA [8] Henequen/HDPE [5]
Fibre orientation Unidirectional Unidirectional Randomly distributed 50 mm, 55 vol.% Randomly distributed 50 mm, 55 wt.% and 12 wt.% palmyra fibres Randomly distributed 10 mm, 72 vol.% fibre Randomly distributed short (10 mm) 60 vol.% fibre Unidirectional 64 vol.% Unidirectional 56 vol.% 60 vol.% 22 vol.% Random mat 40 vol.% Random mat 40 vol.% Random mat 40 wt.% Random mat 40 wt.% Random mat 40 wt.% Unidirectional 50 vol.% Unidirectional 50 vol.% Unidirectional 45 vol.% 46 vol.%
4. Conclusion The following points are drawn from the experimental results: 1. The flexural properties of the KFRE composite are highly influenced by the kenaf fibre surface characteristics. NaOH treatment highly enhanced the interfacial adhesion of the fibre with the matrix leading to better flexural properties compared to the untreated fibres. Thirty six percentage increment in the flexural strength of the epoxy composite achieved when treated kenaf fibres were used as reinforcement compared to the untreated fibres, which showed only 20%.
Flexural st (MPa)
Flexural mod (GPa)
192.73 ± 12 235.13 ± 27 301.64 ± 72 59 MPa 175 N/A N/A 123 101 28.181 590–720 60 27 25 23 54 34 200 225 210 95.9
1.726 ± 0.8 5.572 ± 0.8 6.743 ± 1.8 8.45 9.35 5.4 4.8 13 10 2.685 31–38 4.48 2.3 0.5 1.6 4.9 2.6 15 18 18 2.61 ± 102
2. In the case of untreated KFRE, the fracture mechanisms were debonding, tearing, detachments and pull out of fibres due to the low interfacial adhesion of fibres with the matrix. On the other hand, treated KFRE showed only breakage at the end of the fibres, i.e. the treated fibres were able to carry the load during the test. This was the main reason of the high flexural properties of treated KFRE compared to the untreated KFRE. 3. Treating the kenaf fibres with 6% NaOH split the kenaf fibre bundles into fine fibres which in turn allowed the epoxy resin to penetrate in the fibre bundles leading to high interlocking of the fibres in the matrix (high interfacial adhesion). However, the porosity of the treated kenaf fibres/epoxy composite is
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lower than the untreated fibre/epoxy composites which was another reason of the better flexural properties of the treated fibre/epoxy composite compared to the untreated one. 4. Kenaf fibres exhibited superiors properties as reinforcement for polymeric composites under flexural loading conditions compared to other types of natural fibres. Moreover, there is a possibility of replacing the synthetic fibres (glass) with kenaf fibres for flexural applications. References [1] Chin CW, Yousif BF. Potential of kenaf fibres as reinforcement for tribological applications. Wear 2009;267(9–10):1550–7. [2] Yousif BF, Ku H. Suitability of using coir fiber/polymeric composite for the design of liquid storage tanks. Mater amp Des 2012;36(0):847–53. [3] Joshi SV et al. Are natural fiber composites environmentally superior to glass fiber reinforced composites? Compos A Appl Sci Manuf 2004;35(3):371–6. [4] Velmurugan R, Manikandan V. Mechanical properties of palmyra/glass fiber hybrid composites. Compos A Appl Sci Manuf 2007;38(10):2216–26. [5] Herrera-Franco PJ, Valadez-González A. Mechanical properties of continuous natural fibre-reinforced polymer composites. Compos A Appl Sci Manuf 2004;35(3):339–45. [6] Sapuan SM et al. Mechanical properties of woven banana fibre reinforced epoxy composites. Mater amp Des 2006;27(8):689–93. [7] Wambua P, Ivens J, Verpoest I. Natural fibres: can they replace glass in fibre reinforced plastics? Compos Sci Technol 2003;63(9):1259–64. [8] Shinji O. Mechanical properties of kenaf fibers and kenaf/PLA composites. Mech Mater 2008;40(4–5):446–52. [9] Alawar A, Hamed AM, Al-Kaabi K. Characterization of treated date palm tree fiber as composite reinforcement. Compos B Eng 2009;40(7):601–6. [10] Cantero G et al. Effects of fibre treatment on wettability and mechanical behaviour of flax/polypropylene composites. Compos Sci Technol 2003;63(9):1247–54. [11] Saha P et al. Enhancement of tensile strength of lignocellulosic jute fibers by alkali-steam treatment. Bioresour Technol 2010;101(9):3182–7. [12] Shibata S, Cao Y, Fukumoto I. Flexural modulus of the unidirectional and random composites made from biodegradable resin and bamboo and kenaf fibres. Compos A Appl Sci Manuf 2008;39(4):640–6. [13] Vilay V et al. Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber-reinforced unsaturated polyester composites. Compos Sci Technol 2008;68(3–4):631–8.
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