Journal of the Rubber Research Institute of Sri Lanka (2015) 95, 1-13 Effect of latex coated coconut fibre on cure characteristics and physico-mechanical properties of natural rubber-coir composites D G Edirisinghe*, W D M Sampath* and P C Wettasinghe* *Rubber Research Institute of Sri Lanka, Telewala Road, Ratmalana, Sri Lanka ___________________________________________________________________________ Abstract A wide variety of natural fibres such as coconut fibre (coir), bagasse, banana fibre, sisal, etc. can be used to reinforce polymers. Natural fibre reinforced, biodegradable composites are eco-friendly. However, the interfacial adhesion between natural fibre and most of the biodegradable polymers is not adequate. The objective of this study was to modify readily available coir by coating with natural rubber (NR) as well as synthetic rubber latices with the aim of improving adhesion between coir and rubber in order to develop a coir filled NR based composite suitable in manufacture of tyre treads. Coir was coated with different compounded latices namely, neoprene, nitrile and NR and with uncompounded NR latex. Thereafter, composites were produced by mixing 15 phr of the latex coated coir fibres with virgin NR according to a tyre tread formulation and properties of the composites were evaluated and compared. Results revealed that processing safety of the compounded NR latex coated coir filled rubber composite is higher than that of the other three composites, whereas uncompounded NR latex coated coir filled rubber composite showed a similar processing safety to that of the two synthetic latex coated coir filled rubber composites. However, the two NR latex coated coir filled rubber composites were faster curing than the synthetic latex coated coir filled rubber composites. Minimum torque results indicated that compounded NR latex coated coir filled rubber composite has a higher processability when compared to the other composites. However, the latter composites indicated a higher state of cure and cross-link density in comparison to the former composite. Tensile strength, elongation at break, resilience and abrasion volume loss of the compounded NR latex coated coir filled rubber vulcanisate are superior to those of the other three vulcanisates. In overall cure characteristics and mechanical properties, especially abrasion resistance of the composite based on compounded NR latex coated coir indicates that it could be used in manufacture of tyre treads. Keywords: coir, natural rubber composites, natural rubber latex, neoprene latex, nitrile latex 1 Natural rubber composites with surface treated coir Introduction Coir is a versatile lignocellulose fibre Natural fibres are generally obtained from the coconut tree (Cocos lignocellulosic in nature, consisting of nucifera), which grows extensively in helically wound cellulose microfibrils in tropical countries. Because of its hard- a matrix of lignin and hemicellulose. wearing quality, durability and other Natural fibres have been used to advantages, it is used for making a wide reinforce materials for over 3,000 years. variety of floor furnishing materials, More recently they have been employed yarn and rope. However, these in combination with rubber and plastics traditional coir products consume only a (Sakdapipanich et al., 2007). Many small percentage of the potential total types of natural fibers have been world production of coconut husk. In investigated for use in rubber and the past, composites of coconut plastic including coconut fibre (coir), fibre/natural rubber (NR) latex were bamboo fibre, oil palm fibre, isora fibre, extensively used by the automotive banana fibre, sisal, etc. Natural fibres industry (Geethamma et al, 1998) and have the advantage that they are presently mattresses, pots, etc. are renewable resources and have widely produced using this marketing appeal. They are increasingly combination. Apart from the used in automotive and packaging conventional uses of coir as mentioned materials. above, research and development work Natural fibres degrade faster than has been conducted to find new synthetic fibres in natural environments, applications for coir, including minimizing environmental pollution. utilization of coir as a reinforcing However, biodegradation of natural material for plastics (Owolabi et al., fibre can decrease the life time of fibre 1985). reinforced polymer composites. To Coir is an inexpensive fibre among the prolong their performance in natural various natural fibres available in the environments, natural fibres are treated world. Furthermore, it possesses the to protect from any circumferential advantages of a lignocellulose fibre. It is agents. Hydroxyl groups from cellulose not toxic and possesses no waste and lignin are natural substrates for disposal problem. Unfortunately, the modification. In addition, coating of performance of coir as a reinforcement fibres often reduces water absorption, in polymer composites is unsatisfactory protection from bacteria and fungi and not comparable even with other attack. The treatment methods which natural fibres. This inferior performance have been used for fibre reinforced of coir is due to various factors such as polymer composites are not suitable for its low cellulose content, high lignin and protection of the fibres in contact with hemicellulose content, high soils (Farshid et al., 2011). microfibrillar angle and large and 2 D G Edirisinghe et al. variable diameter (Geethamma et al., al., 1993). Prasad et al. (1983) reported 1998). that the use of alkali treatment on coir The hydrophobic NR matrix and fibres improves the mechanical hydrophilic cellulose fibre can be made properties of coir-polyester composites. compatible through modification of Jacob et al. investigated the effect of polymer or fibre surface. The extent of chemical modification of banana fibre adhesion is usually increased by the use in NR (Communicated). Modification of of bonding agents and chemical banana fibre resulted in superior modification of fibres. The effects of a mechanical properties. Chemical silane coupling agent (Si69) on curing modification of both sisal and oil palm characteristics and mechanical fibres was imperative for increased properties of bamboo fibre filled NR interfacial adhesion and resulted in composites were studied (Ismail et al., enhanced properties (Jacob et al., 2004). 2002). The mechanical properties of The influence of alkali treatment on oil composites such as hardness, modulus, palm fiber reinforced NR composites tensile and tear strengths improved with was investigated by Joseph et al. the addition of Si69. Chemical (2006). They observed superior treatment of cellulose fibres usually mechanical properties for the changes the physical and chemical composites after chemical modification structure of the surface. The use of due to better adhesion. Effects of alkali, maleic anhydride grafted copolymer and silane coupling agent and acetylation on alkaline treatment of the natural fibre oil palm fibres have been studied by are the most used techniques with the Mahato et al. (1993). De et al. (2006) aim to improve the fibre-matrix were able to improve adhesion between interfacial bonding (Ku et al., 2011). grass fiber and NR matrix by alkali Arumugam et al. (1989) evaluated treatment. mechanical properties of NR Chemical modification of pineapple leaf composites prepared with pristine and fiber in NR was investigated by sodium hydroxide treated coconut Lopattananon et al. (2006). Sodium fibers. They added the same coupling hydroxide and benzoyl peroxide were agents as Ismail et al. (2001) in order to used to treat the surfaces of fiber. It was improve rubber-filler interactions. found that all surface modifications Results of the compounds prepared with enhanced adhesion and tensile pristine and treated fibres were properties. The effects of different compared with those of untreated fibres chemical treatments, including and an enhancement in the mechanical mercerisation, acetylation, benzoylation properties was observed. It was reported and treatment with toluene diisocyanate that alkali treatment on coir fibre (TDI) and silane coupling agents, on enhances the thermal stability and isora fibre properties and mechanical maximum moisture retention (Mahato et properties were analyzed. Isora fibre 3 Natural rubber composites with surface treated coir was seen to have immense potential as Experimental reinforcement in NR. Acetylation and Materials TDI treatments gave higher tensile Neoprene and nitrile latex compounds strength values compared to other were supplied by Dipped products Plc., treatments (Mathew et al., 2004). Maize Sri Lanka. Compounded NR latex, stalk fibres were also chemically treated uncompounded NR latex and virgin NR with acetic anhydride (acetylation) to (RSS 2) were purchased from a local enhance their compatibility with the supplier. Zinc oxide (ZnO), stearic acid, hydrophobic rubber matrix (Chigondo carbon black (N 330-high abrasion et al., 2013). The NR-maize stalk fibre furnace black), rubber processing oil filled composites showed good (Dutrex R), IPPD (N-isopropyl, N’- processing safety and when compared phenyl paraphenylene diamine), TMQ with untreated NR-maize stalk fibre (2,2,4 trimethyl 1,2-dihydroquinoline), composites, the acetylated composites TBBS (N-t-butyl-2 benzothiazole exhibited higher mechanical properties, sulphenamide) and sulphur were reduced moisture absorption and higher purchased from the local market. resistance to hydrothermal aging. Coconut