Scission and recombination efficiency of hybrid crosslinks in natural rubber

Adding a post-vulcanisation stabiliser (PVS) to a natural rubber mix forms hybrid crosslinks. One PVS is a hexamethylene-1,6-bisthiosulphate disodium salt dihydrate (HTS) that promotes the formation of flexible hybrid crosslinks. It is said that the fatigue life of natural rubber vulcanisates containing hybrid crosslinks is enhanced because of the flexible nature of such crosslinks. However, based on current literature, it is claimed that improvement in fatigue life is due to an energy dissipation process within the vulcanisate engendered by the scission and recombination of network chains. Thus, the aim of the investigation was to ascertain whether hybrid crosslinks break and recombine in the same way, as do crosslinks in natural rubber vulcanisates introduced by conventional curing systems. Results indicate that non-affine or finite extensibility effects cause scission and recombination of crosslinks. Such mechanisms manifest themselves as permanent set. At normal levels of crosslink density these effects appear to be very pronounced in vulcanisates containing HTS compared with vulcanisates containing conventional curing systems. Enhanced non-affine behaviour in vulcanisates containing HTS appears to be associated with poor dispersion. Inadequate dispersion can lead to localised areas of high crosslink density. Such areas may be regarded as being the major source contributing to finite extensibility effects. The present work has lead to an interesting paradox: inadequate dispersion in compounding is undesirable; however, inadvertently poor dispersion seems to be responsible for enhanced fatigue life, a feature that is clearly highly desirable.