Bamboo fibre filled natural rubber composites: the effects of filler loading and bonding agent

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1 Polymer Testing 21 (2002) Material Properties Bamboo fibre filled natural rubber composites: the effects of filler loading and bonding agent Hanafi Ismail a,*, M.R. Edyham a, B. Wirjosentono b a School of Industrial Technology, Universiti Sains Malaysia, Minden, Penang, Malaysia b Department of Chemistry, Universitas Sumatera Utara, Medan 20155, Indonesia Received 19 March 2001; accepted 25 May 2001 Abstract Bamboo fibre reinforced natural rubber composites were prepared by incorporation of different loadings of bamboo fibre. Two series of composites were studied i.e. composites with and without the presence of a bonding The curing characteristics of the composites were determined and the composites were vulcanized at 150 C using a hot press. The properties of the composites such as tensile strength, tensile modulus, tear strength, elongation at break and hardness were studied. The adhesion between the bamboo fibre and the natural rubber were enhanced by the addition of bonding agent as exhibited by the tensile fracture surfaces of the composites using scanning electron microscopy (SEM). The presence of bonding agent also gave shorter curing time and enhanced mechanical properties Elsevier Science Ltd. All rights reserved. Keywords: Bamboo fibre; Mechanical properties; Curing characteristics; Natural rubber; Bonding agent 1. Introduction Short fibre reinforcement of rubbers is a subject that has interested a large number of researchers [1 8]. Among the advantages obtained for short fibre reinforced rubber composites are, design flexibility, high low strain moduli, stiffness, damping and processing economy. In addition, natural fibres have advantages due to their renewable nature, low cost, easy availability, and ease of chemical and mechanical modification. Many researchers have reported the processing advantages and improvements in the mechanical properties of short fibre reinforced rubber composites [9 11]. The objective of this study is to develop a new type of composite material bamboo fibre reinforced rubber (BFRR) composite. Bamboo fibre was selected as the reinforcement because bamboo is an abundant natural * Corresponding author. Tel.: x2214; fax: address: ihanafi@usm.my (H. Ismail). resource in Asia and its overall mechanical properties are comparable to those of wood [12]. One of the most important aspects of composite manufacture is to achieve adequate adhesion between fibres and the rubber matrix. In this work we report studies on the effects of filler loading on the curing characteristics and mechanical properties of bamboo fibre reinforced natural rubber composites. The comparison was carried out in order to examine the effect of a bonding agent for composites. Scanning electron microscopy (SEM) studies were made to determine the failure mechanism of the composites. 2. Experimental 2.1. Compounding ingredients and formulation The formulation used in this study is shown in Table 1. Natural rubber (SMR L) was obtained from the Rubber Research Institute of Malaysia (RRIM). The bamboo used in this work belongs to the species of Bambusa Paravariabilis and was supplied by Forest Research /02/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S (01)

2 140 H. Ismail et al. / Polymer Testing 21 (2002) Table 1 Formulations for bamboo fibre reinforced natural rubber composites Compound composition (phr) A B C D E F G H I SMR L Zinc oxide Stearic acid CBS Sulphur Flectol H Phenol formaldehyde (PF) Silica (Sil) Hexamethylene tetramine (Hexa) Bamboo fibre Table 2 Chemical compositions of bamboo Chemical composition (%) Moisture 11.7 Ash 2.3 Hot Water solubility 7.7 Alcohol-benzene solubility 2.2 Lignin 21.4 Pentosan 20.3 Holocellulose 73.3 Alpha-cellulose 48.2 Institute of Malaysia (FRIM). Bamboo chips were produced by means of a wood planer which were then ground to µm and used in rubber compounding without further treatment (other than drying). Table 2 shows the chemical composition of the bamboo fibre used in this study. Other chemicals such as sulphur, zinc oxide, stearic acid n-cyclohexylbenthiazyl sulphenamide (CBS) and silica were all purchased from Bayer (M) Ltd whereas Flectol H (poly-1,2-dihydro-2,2,4- trimethylquinoline) was supplied by Monsanto (M) Company. Bonding agents used in this study were phenol formaldehyde [Borden Chemical (M) Ltd] and hexamethylenetetramine [Fluka Chemical (M) Ltd]. Fig. 1. Variation of scorch time and filler loading of bamboo 2.2. Compounding and testing Bamboo fibres were dried in an oven at 80 C for 24 h to expel moisture. Mixing was carried out on a conventional laboratory two roll mill size ( mm) according to ASTM designation D Nip gap, mill roll speed ratio, time of mixing and the sequence of addition of the ingredients were kept the same for all the composites. The sheeted rubber compound was conditioned at a temperature of 23±2 C for 24 h in a closed container before cure assessment using a Monsanto Mov- Fig. 2. Variation of cure time and filler loading of bamboo

3 H. Ismail et al. / Polymer Testing 21 (2002) Fig. 3. Mooney scorch time versus filler loading of bamboo Fig. 5. The effect of filler loading on tensile strength of bamboo fibre filled natural rubber composites with and without bonding Fig. 4. Mooney viscosity versus filler loading of bamboo fibre filled natural rubber composites with and without bonding ing Die rheometer (MDR 2000). The respective scorch time, t 2 and cure time, t 90 were obtained from the MDR 2000 at 150 C. The Mooney scorch time, t 5 and Mooney viscosity were also determined by using a Monsanto automatic Mooney Viscometer (MV 2000) at 120 C. The testing procedure was conducted according to the method described in ASTM D The Mooney scorch time (t 5 )isdefined as the time required for an increase of five units above the minimum viscosity as determined from a plot of the Mooney viscosity versus time. The stress strain properties and tear strength of the composites were measured on an Instron Universal Testing Machine, model 1114 according to ASTM D412 and ASTM D624 respectively at 500 mm/min cross-head speed. The hardness of the composites was measured by Fig. 6. The effect of filler loading on tear strength of bamboo the shore type A Durometer according to ASTM All tests were conducted at room temperature (25 C) Scanning electron microscopy Examination of the tensile fracture surface was carried out using a scanning electron microscope (SEM) model Leica Cambridge S-360. All the surfaces were examined after first sputter coating with gold to avoid electrostatic charging and poor image resolution. 3. Results and discussion Figs. 1 and 2 show the results for scorch time, t 2 and cure time, t 90 for bamboo fibre reinforced natural rubber composites with and without bonding agents. It can be

4 142 H. Ismail et al. / Polymer Testing 21 (2002) Fig. 7. Scanning electron microscopy of bamboo fibres (Magnification 20 ). Fig. 9. Scanning electron microscopy tensile fracture surface of bamboo fibre filled natural rubber composites at 10 phr of filler and with bonding agent at magnification 350. Fig. 8. Scanning electron microscopy tensile fracture surface of bamboo fibre filled natural rubber composites at 10 phr of filler and with bonding agent at magnification 150. seen that both properties decrease with increasing filler loading. The trend might be due to the longer time the rubber composites remain on the mill during mixing. According to Geethamma et al. [13], as the filler loading increases, the incorporation time of filler into rubber matrix also increases and consequently generates more heat due to additional friction. The Mooney scorch time, t 5 obtained from the Mooney viscometer at 120 C also exhibit a similar trend (Fig. 3). However, at similar filler loading, the scorch and cure times of composites with bonding agent are shorter than for the composites without bonding agent, which is attributed to adhesion between the fibre and the rubber matrix being enhanced and more energy needed to incorporate the fibres due to increased viscosity. The effect of filler loading on Mooney viscosity of the composites with and without bonding agent is shown in Fig. 10. Scanning electron microscopy tensile fracture surface of bamboo fibre filled natural rubber composites at 30 phr of filler and with bonding agent at magnification 150. Fig. 4. It can be seen that the Mooney viscosity increases with increasing filler loading and the addition of bonding This indicates that the addition of both filler and bonding agent has increased the stiffness of the composites. Similar observation was also reported by Kumar et al. [14] for short sisal fibre reinforced styrene butadiene rubber composites. Figs. 5 and 6 show the effect of filler loading on tensile strength and tear strength of the composites with and without bonding It can be seen that both properties decrease with increasing filler loading. This result is in agreement with Arumugam et al. [15] who worked on coconut fibre reinforced rubber composites. However, other researchers [16 18] reported increased tensile strength with increased fibre loading. For irregularly shape fillers, the strength of the composites decreases

5 H. Ismail et al. / Polymer Testing 21 (2002) Fig. 11. Scanning electron microscopy tensile fracture surface of bamboo fibre filled natural rubber composites at 10 phr of filler and without bonding agent at magnification 150. Fig. 13. Variation of tensile modulus and filler loading of bamboo fibre filled natural rubber composites with and without bonding Fig. 12. Scanning electron microscopy tensile fracture surface of bamboo fibre filled natural rubber composites at 30 phr of filler and without bonding agent at magnification 150. Fig. 14. Variation of hardness and filler loading of bamboo due to the inability of the filler to support stresses transferred from the polymer matrix. Fig. 7 shows the SEM micrograph of bamboo fibres used in this study which have various shapes, size and lengths. The irregular shape of bamboo fibre together with poor adhesion (due to hydrophilic nature) to the natural rubber matrix (hydrophobic nature) are main factors for the deterioration of tensile strength and tear strength with increasing filler loading. However, at a similar filler loading both properties improved with the addition of bonding This is due to the increase in adhesion between the rubber and bamboo fibre. Figs show the SEM of tensile failure surfaces of natural rubber composites with 10 phr (Fig. 8/Mag: 150 and Fig. 9/Mag: 350 ) and 30 phr (Fig. 10/Mag: 150 ) of filler loading with the presence of bonding The failure surfaces show feature of well-developed interfacial interaction. It can be seen there is very low pull-out of fibres on the fracture surface. The surfaces of composites show that failure occurred at the fibre due to strong adhesion between fibre and rubber matrix (see Figs. 8 and 9). However, the failure surface of similar composites but without the presence of bonding agent in Fig. 11 (10 phr of bamboo fibre/mag: 150 ) and Fig. 12 (30 phr of bamboo fibre/mag: 150 ) exhibit the weak interfacial adhesion between the fibre and rubber matrix. Both figures show many holes left after the fibres were pulled out from the matrix when the stress was applied and the failure occurred at the weak fibre and rubber matrix interface. Figs. 13 and 14 show the effect of filler loading on tensile modulus and hardness of bamboo fibre reinforced

6 144 H. Ismail et al. / Polymer Testing 21 (2002) The curing characteristics and mechanical properties of bamboo fibre reinforced natural rubber composites were examined as a function of fibre loading and bonding The scorch time, t 2 and cure time, t 90 decrease with increasing filler loading and the presence of bonding Tensile modulus and hardness of composites increase with increasing filler loading and the presence of bonding agents. The adhesion between the bamboo fibre and natural rubber can be enhanced by use of a bonding References Fig. 15. Elongation at break versus filler loading of bamboo natural rubber composites with and without bonding It can be seen that both properties increase with increasing filler loading. These results indicate the fact that the incorporation of bamboo fibre into the rubber matrix enhanced the stiffness of the composites. However, with the presence of bonding agent, these properties were further increased as a result of better interaction between fibre and rubber matrix. Fig. 15 shows that the elongation at break, E b of the composites decreases with increasing filler loading and the presence of bonding As discussed before with the increase in filler loading and the presence of bonding agent, the stiffness and brittleness of the composite increased with an associated decrease in E b. 4. Conclusions [1] R.L. Ibarra, A.C. Chamorro, R.M.C. Tabernero, Polymer Composites 9 (3) (1988) 198. [2] S. Varghese, B. Kuriakose, S. Thomas, A.T. Koshy, Industrial Journal of Natural Rubber Research 4 (1) (1991) 55. [3] L.A. Goettler, K.S. Shen, Rubber Chemistry and Technology 56 (1983) 620. [4] G.C. Derringer, Rubber World 45 (1975) 165. [5] H. Ismail, I.N. Hasliza, Polymer and Plastics Technology and Engineering 38 (1) (1998) 137. [6] H. Ismail, N. Rosnah, H.D. Rozman, Polymer 38 (1997) [7] V.M. Murty, S.K. De, Rubber Chemistry and Technology 55 (1982) 287. [8] V.M. Murty, S.K. De, Journal of Applied Polymer Science 29 (1984) [9] K. Boustany, P. Hamed, Rubber World 171 (1974) 39. [10] A.Y. Coran, Rubber Chemistry and Technology 47 (1974) 396. [11] K. Boustany, R.L. Arnold, Journal of Elastoplasticity 8 (1976) 160. [12] X. Lakkad, J.M. Patel, Fibre Science and Technology 14 (1980) 319. [13] V.G. Geethamma, R. Joseph, S. Thomas, Journal of Applied Polymer Science 55 (1995) 583. [14] R.P. Kumar, M.L. Geethakumari Amma, S. Thomas, Journal of Applied Polymer Science 58 (1995) 597. [15] N. Arumugan, K. Tamare Selvy, K.V. Rao, Journal of Applied Polymer Science 25 (1980) 795. [16] G.C. Derringer, Journal of Elastoplasticity 3 (1971) 230. [17] J.E. O Connor, Rubber Chemistry and Technology 50 (1977) 945. [18] D.K. Setua, S.K. De, Rubber Chemistry and Technology 56 (1983) 808.