Evidence of ionic intermediates in rubber vulcanization detected by on-line electric current measurements in natural rubber/sulfur/tetramethylthiuram monosulfide/zinc oxide and comparison mixtures

The vulcanization of a mixture of natural rubber (NR, cis-1,4-polyisoprene), elemental sulfur (S), tetramethylthiuram monosulfide (TMTM) and zinc oxide (ZnO), in addition to simpler systems containing some of these components, was investigated by means of an electrochemical method. A direct-current voltage was applied to the mixtures with two platinum electrodes. The current, e.g. in the range of 10−9A, was recorded during vulcanization. The complicated curves obtained from the mixture containing S, TMTM and ZnO were most likely caused by the superposition of the contributions of different transitory ionic species. Current maxima occurred at the onset of cross-linking and also earlier. The final reaction product zinc dimethyldithiocarbamate (ZnDMTC) also provided some conductivity. For comparison, a reaction involving radical intermediates, the cross-linking of NR with dicumyl peroxide (DCP), was considered.