Compounding Variables Influencing the Reversion Process in Accelerated Curing of Natural Rubber

Abstract

Abstract Reversion of natural rubber is dependent on the nature of the curing system, choice of accelerator, and the temperature and time of cure. Conventional cure systems which contain a high ratio of sulfur to accelerator have higher moduli at optimum cure, faster rates of cure, but are more vulnerable to reversion. The EV systems, which contain a low ratio of sulfur to accelerator, demonstrate good resistance to reversion but lower moduli at optimum cure and slower rates of cure. The sulfurless cure systems, which represent an extreme case, show greater reversion resistance than EV systems. Of the three accelerators tetramethyl thiuramdisulfide (TMTD), diphenylguanidine (DPG), and N-oxydiethylene benzothiazole-2-sulfenamide (MOR) studied, MOR gives the best performance, followed by DPG, then TMTD in terms of reversion resistance for the cure times studied. In the injection molding of rubber, MOR offers better performance with respect to the requirements of safe scorch times, fast rates of cure, and good reversion resistance. Whenever the conventional cure systems cannot give satisfactory performance, the EV cure system is strongly recommended. If further improvement is needed, such as higher injection and mold temperatures, compensation for the loss of modulus can be made by addition of accelerator and sulfur or simply accelerator alone. Other alternatives are consideration of blends of accelerators as well as blends of natural rubber with other rubbers.