Short fibre woodcellulose as a filler for natural rubber
This research work was carried out to investigate the effect of unregenerated short cellulose fibres from woodpulp on the properties of rubber vulcanisates. It was also an objective of this work to find the type of woodpulp which was able to produce short fibre-rubber composites of good physical properties at a reduced cost. A screening procedure was carried out to identify one particular type of woodcellulose short fibre for detailed investigation. Mechanical woodpulps containing short cellulose fibres were selected based on their dispersibility properties in rubber, low cost, and universatility of application. Among various types of bonding agents studied, a resorcinolhexamethylenetetramine (RH) system was found to be most effective to bond woodcellulose fibre (mechanical type) to natural rubber -matrix. This was judged by certain properties namely, restricted equilibrium swelling, yield stress and tear strength. Restricted equilibrium swelling in particular was established to be a very useful technique by which the degree of adhesion can be predicted. In-this technique, the short fibre-rubber composites were swollen in toluene. for one week, after which, the volume restriction value, Vr was calculated using the following formula: Vr= (VI - VF)/VI, where VI and VF are the volume fraction of rubber in dry and swollen samples respectively. Based on Vr value, it has been observed that, with the exception of silane, all bonding systems studied improved woodcellulose fibre-natural rubber adhesion. Optimisation work to determine the optimum level of resorcinol-hexamethylenetetramine combination was also carried out based on a statistical factorial experimental design, in particular the 'central composite design'. Contour graphs of different properties of compounds for various levels of bonding agent combination were obtained by means of computer programmes. From these contour graphs it has been established that the 3 parts of hexamethylenetetramine (HMT) and 4 parts of resorcinol combination gives the best overall properties. Mechanical properties of composites containing optimum level of resorcinol-HMT combination measured in the direction of fibre orientation have indicated their dependence on fibre concentration. Young's modulus in particular increases exponentially with increasing fibre concentration. Experimental and theoretical curves of Young's modulus versus volume concentration of fibres show good correlation up to fibre volume concentration of 30%; beyond which increasing divergence occurred. Besides physical and mechanical properties, the cost factor of woodcellulose fibre-rubber composite is also considered. Cost advantage is clearly evident as a result of incorporation of woodcellulose fibre in natural rubber compound.