Sulfur vulcanization of polyisoprene accelerated by benzothiazole derivatives. IV. The reaction of polyisoprene with N-cyclohexylbenzothiazole sulfenamide,sulfur, and zinc oxide

Polyisoprene was vulcanized with N-cyclohexylbenzothiazole sulfenamide (CBS), sulfur, and zinc oxide by heating in a Differential Scanning Calorimeter (DSC) at a programmed rate to given temperatures. The reaction was quenched and the product analyzed. Soluble curatives and reaction intermediates were analyzed by high-performance liquid chomatography (HPLC) and the crosslink density of the network determined by swelling. The delayed action of the CBS accelerator is explained in terms of an exchange reaction between benzothiazole terminated polysulfidic pendent groups on the polymer chain and CBS to yield unreactive amine terminated pendent groups and 2-bisbenzothiazole-2,2′-disulfide (MBTS). MBTS reacts with sulfur to form 2-bisbenzothiazole-2,2′-polysulfides (MBTPs), which also form pendent groups. Crosslinking does not commence until all of the CBS has been consumed and pendent groups are no longer deactivated. 2-Mercaptobenzothiazole (MBT) is released only on crosslinking. When MBT is present in the formulation at the outset of the reaction it traps cyclohexylamine released when CBS adds to the chain as a pendent group. The MBT-amine salt participates in a reaction that regenerates MBTS, which is, thus, not consumed in the vulcanization process. ZnO does not react with CBS, and its role in increasing the crosslink density is attributed to its promoting crosslinking reactions between pendent groups and neighboring chains rather than intramolecular reactions, which lead to cyclization. © 1996 John Wiley & Sons, Inc.