Influence of zinc oxide during different stages of sulfur vulcanization. Elucidated by model compound studies

The addition of zinc oxide (ZnO) as an activator for the sulfur vulcanization of rubbers enhances the vulcanization efficiency and vulcanizate properties and reduces the vulcanization time. The first part of this article deals with the reduction and optimization of the amount of ZnO. Two different rubbers, solution-styrene-butadiene rubber and ethylene-propylene-diene rubber, have been selected for this study. The results demonstrate that the curing and physical properties can be retained when the level of ZnO (Red Seal) is reduced to 1 or 2 phr, respectively. Of particular interest is nano-ZnO, characterized by a nanoscale particle distribution. The cure characteristics indicate that with nano-ZnO, a reduction of zinc by a factor of 10 can be obtained. In the second part, model compound vulcanization is introduced to investigate the effects of ZnO during the different stages of vulcanization. Experiments are described with two models, squalene and 2,3-dimethyl-2-butene, both with benzothiazolesulfenamide-accelerated vulcanization systems. The results demonstrate the influence of ZnO during the different stages of the vulcanization. With ZnO present, a marked decrease can be observed in the sulfur concentration during an early stage of vulcanization, along with a slight delay in the disappearance of the crosslink precursor. The crosslinked product distribution is influenced as well.