Tensile and flexural properties of polylactic acid-based hybrid green composites reinforced by kenaf, bamboo and coir fibers
Introduction
Plant fibers are favourable materials for industrial use, as they have high specific strength, renewability, sustainability, and eco-efficiency. Their capability of absorbing carbon dioxide shows promising value in mitigating environmental pollution (La Mantia and Morreale, 2011). Unlike plant fiber composites, the prolonged lifetime of synthetic fiber composites has detrimental environmental impact through over-accumulation of landfill sites (Assamoi and Lawryshyn, 2012).
In order to pursue the biodegradable composites, matrix resin of polylactic acid (PLA) is frequently used to reinforce with plant fibers. Produced via fermentation of corn starch to lactic acid, PLA can be reconverted and naturally decomposed to satisfy the environmental impact. Compared to polypropylene (PP), PLA not only exhibits higher modulus but also higher storage modulus and flexural properties (Han et al., 2012b) as well as high mechanical and thermal properties (Huda et al., 2008, Nishino et al., 2003) which is comparable to polystyrene (Suryanegara et al., 2009). Single plant fiber type-reinforced PLA composites have been reported extensively, though providing much variability in strength properties (Akil et al., 2011, Dong et al., 2014, Han et al., 2012a, Han et al., 2012b, Saba et al., 2015, Sukmawan et al., 2016, Takagi and Ichihara, 2004). The variability of properties was due to the size of testing samples, fiber orientation, fiber content, fiber treatment, fiber length and fabrication method.
Research on single plant fiber reinforcement in biodegradable green composites mainly focuses on improving the physical and chemical qualities (such as poor wettability, insufficient adhesion, high moisture absorption, and thermal conductivity) (Aziz and Ansell, 2004, Bledzki et al., 1996, Gustavsson et al., 2005, Li and Pickering, 2008, Liu et al., 2012, Xie et al., 2010) to replace non-degradable synthetic fiber-reinforced composites. Hybridization in material design has been a popular method for plant/synthetic fiber-reinforced polymer composites to obtain high mechanical properties and to overcome the disadvantages of moisture absorption and poor adhesion with matrices of the composites (Alex and Retnam, 2014, Almeida et al., 2013, Satyanarayana et al., 2009). Nevertheless, there are few studies based on the combination of different kinds of plant fibers-reinforced biodegradable polymer in hybrid green composites (Saba et al., 2014, Sathishkumar et al., 2014).
Hybrid composites are defined as composites produced by incorporation of two or more fibers in a matrix (John and Thomas, 2008). The combination of two or more plant fibers in a matrix is known as a hybrid green composite. The high mechanical properties of these composites have made them popular in material design, as they overcome traditional disadvantages. To obtain the desired properties of hybrid green composites, the compatibility between constituent fibers and the matrix is of prime concern. Commonly, the selection of plant fiber determines the end properties of composites. Fibers such as kenaf, coir, bamboo, sisal, flax, hemp, and jute have been used in consumer goods, low cost housing and structural parts due to their high specific properties, comparable to glass fiber (Ramesh et al., 2013, Sudhir et al., 2014, Zhang et al., 2013) and conventional materials (Gosline et al., 1999). The fiber-matrix interfacial bonding strength, fiber’s alignment and dispersion and different fiber contents are likely to produce high strength properties of composites (Bourmaud et al., 2016, Pickering and Aruan Efendy, 2016) comparable to PLA/flax (Duhovic et al., 2009, McGregor et al., 2010). On the other hand, preparing long continuous and well aligned fibers are time consuming and limited to certain fibers. Therefore, hybrid composites are simply cost effective to develop high performance composites based on natural properties of plant fibers (Venkateshwaran et al., 2012). For optimal hybrid green composites, the plant fibers should be highly strain compatible (Sreekala et al., 2002), determined by microfibrillar angle (MFA) (Barnett and Bonham, 2004, Bledzki et al., 1996). Additionally, high strength and stiffness, a small diameter (Monteiro et al., 2011), a volume fraction between 60 and 70% (Campbell, 2010) and a continuous unidirectional configuration (Fei et al., 2014, Okubo et al., 2004, Salleh et al., 2012, Shah, 2013) are also desirable.
We used biodegradable PLA polymer and three different plant fibers (kenaf, bamboo, and coir fibers) in three different stacking designs to investigate the mechanical properties of hybrid green composites. Hereafter, the stacking patterns are referred to the fibers’ name (K = kenaf, B = bamboo and C = coir). The three kinds of composite were: kenaf and coir fibers (KCCK/PLA), bamboo and coir (BCCB/PLA), and kenaf, bamboo and coir (KBCCBK/PLA). Each type of fiber-reinforced PLA composites had similar weight percentages (60:40) fiber to matrix content. These composites were fabricated by a hot-press molding method. We carried out tensile and flexural tests on hybrid green composites and observed the effects on the composites’ microstructures through optical and scanning electron microscopy. Reinforcing bamboo and kenaf fibers improved tensile and flexural strengths as well as flexural modulus. KBCCBK/PLA outperformed the other two hybrid green composites on tensile and elastic modulus and its mechanical properties are comparable to plant/glass fiber reinforced polymer and conventional materials.
Section snippets
Fibers and polymeric matrix
Coir fibers were obtained from Kamenoko-Tawashi Nishio-Shoten Co., Ltd. Japan with an average cross-sectional area of 0.11 mm2 and density of 1.20 g/cm3. The bamboo culms were freshly harvested in Anan City, Tokushima, Japan, and bamboo fibers were extracted by a steam-explosion method (Takagi et al., 2003). The average cross-sectional area and density of bamboo fibers are 0.15 mm2 and 1.23 g/cm3, respectively. Kenaf cultivated in Kelantan, Malaysia was supplied by Lembaga Kenaf dan Tembakau Negara
Tensile properties of hybrid green composites
As shown in Table 2, the averages of Young’s modulus of fiber bundles after tensile testing are high; both bamboo and kenaf fiber bundles were much higher in Young’s modulus than coir fiber bundles, which were 20 GPa and 22 GPa, respectively. It is well known that the fiber’s elasticity decreases with the increase of the microfibrillar angle (MFA), however, it increases linearly with cellulose content (Baley, 2002). Table 1 shows the chemical composition and physical properties of the related
Conclusions
Tensile and flexural properties of plant fiber-reinforced hybrid green composites containing kenaf, bamboo and coir fibers with bio-based polymer (PLA) were assessed. Overall, hybrid green composites of kenaf-coir/PLA (KCCK/PLA), bamboo-coir/PLA (BCCB/PLA) and kenaf-bamboo-coir/PLA (KBCCBK/PLA) produced a diverse range of results – the hybrid combinations (hybrid) compensated for the inherent disadvantages of the individual materials. Higher tensile strength was obtained from the combination of
Acknowledgements
The authors wish to thank the Malaysian Ministry of Higher Education for financial support of the first author’s PhD study, Tokushima University for supporting research activity, Lembaga Kenaf dan Tembakau Negara (LKTN) for supplying kenaf fibers, and Kamenoko-Tawashi Nishio-Shoten Co., Ltd. Japan for supplying coir fibers.
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