Evidence of ionic components in vulcanization mixtures by means of electric current measurements: Investigations with natural rubber/sulfur/zinc bis(dimethyldithiocarbamate) and comparison mixtures

Continuous low-level current (CLLC) measurements for detecting ionic specie s in the course of vulcanization reactions were applied to investigate the vulcanization of a mixture of natural rubber (NR),sulfur (S), and zinc bis( dimetlhyldithiocarbamate) (ZnDMTC). A de voltage was applied to the reactio n mixture in a special vulcanization mold and the current (e.g., in the ran ge of 10(-9) A) was measured. Temperature-dependent current maxima were fou nd after reaction times t(max). The simplest explanation is that transitory ionic species occur during vulcanization. An activation energy (E-alpha) = 116.4 kJ/mol, similar to that obtained in previous chemical investigations , was determined from the decrease of t(max) with increasing temperature. T he maxima corresponded to reaction times where a strong increase of polymer crosslinking was observed, as measured using vulcametry. For comparison, d e measurements were carried out with the corresponding mixture without elem ental sulfur (NR/ZnDMTC) and mixtures containing zinc stearate (ZnST) inste ad of zinc bis(dimethyldithiocarbamate) (NR/S/ZnST and NR/ZnST). (C) 2000 J ohn Wiley & Sons, Inc.