Evidence of ionic intermediates in rubber vulcanization detected by on-line electric current measurements in natural rubber/sulfur/tetramethylthiurammonosulfide/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 oxid e (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 curre nt, e.g. in the range of 10(-9) A, was recorded during vulcanization. The c omplicated 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 crosslink ing and also earlier. The final reaction product zinc dimethyldithiocarbama te (ZnDMTC) also provided some conductivity. For comparison, a reaction inv olving radical intermediates, the cross-linking of NR with dicumyl peroxide (DCP), was considered. (C) 2000 Elsevier Science Ltd. All rights reserved.