Composites of natural rubber and polyaramid short fibres
The present work was carried out to study the properties of both natural rubber composites reinforced by short polyaramid fibres and natural rubber laminates reinforced by filament polyaramid. Initial work was concentrated to obtain a suitable direct bonding agent and curing system for high modulus nylon 6,6 which was used as the control fibre. Two different types of bonding and curing systems were established as useful for bonding polyaramid to rubbers namely resorcinol-hexamethylene tetraminesilica (HRH) and sulphur, and a blocked diisocyanate in conjunction with a diurethane-crosslinker. Each system was found to offer a different type of adhesion, the HRH formed physical bonds and the NCO/urethane chemical links between fibre and rubber. When the proportions of these bonding agents were optimised in the NR rubber compound vulcanizate properties were significantly improved. Investigation established that for polyaramid fibres the optimum aspect ratio of approximately 1250 was found to give maximum composite reinforcement as judged by the tensile properties of the composite. The experimentally found optimum of 4 parts of blocked diisocyanate was used for extensive investigations of composites properties with respect to fibre orientation and content. Further studies on composites with polyaramid short fibres demonstrated the necessity to optimize the proportions of blocked diisocyanate in the composite to achieve maximum strength properties, e. g. unoptimised with 30% fibre gave a tensile strength of 26.7 MPa whereas optimised, with 30% fibre, gave a tensile strength of 34.4 MPa. Post treatment of composites and laminates by heating or with high energy radiation, using a Co60 source, improved the quality of adhesion still further by the formation of more chemical links between polyaramid and rubber though some degradation of rubber strength simultaneously took place. The bonding agent, diisocyanate, was found unsuitable for use in a sulphur curing system, and was considered to form complexes with the accelerator DCBS and sulphur, to react with zinc salts, and further to act as a crosslinker for rubber. It was therefore found necessary to vulcanize the rubber with a diurethane crosslinker which did not show cure interference with the diisocyanate and the latter then functions primarily as a bonding agent though a minor proportion is shown to simultaneous crosslink the rubber. The Resorcinol-hexamethylene tetramine-silica system. was found to contribute a type of physical adhesion, possibly that of hydrogen bonding, between polyaramid and rubber since this resin formed insitu during vulcanization of rubber was found to crosslink the rubber; reaction mechanisms for both these bonding systems are postulated.