Effect of Humidity and Water Content on the Cure Behavior of a Natural-Rubber Accelerated Sulfur Compound

Abstract

Abstract Certain accelerators are susceptible to chemical attack by water. Consequently, the cure behavior of compounds containing these accelerators is likely to be affected by the water content of the compound and the humidity conditions of processing and storage. A quantitative investigation into the influence of humidity and water content on cure behavior was carried out using a natural-rubber compound with MBS (N-morpholylbenzothiozole-2-sulfenamide) as the principal accelerator. Samples of the compound were treated at different humidities, their water contents determined, and their cure curves measured. Scorch time was inversely related to treatment humidity and water content of the compound, whereas cure rate was directly related to humidity and water content. A study of the mechanism by which water affects the cure behavior of the compound revealed that hydrolysis of the accelerator, MBS, would inhibit its scorch-delay action and could produce MBT (2-mercaptobenzothiazole), which would increase the cure rate. It is concluded that compounds containing other benzothiazole sulfenamide accelerators are likely to exhibit similar behavior. The adverse effect of moisture on the cure behavior of compounds can be minimized by carrying out storage and processing at as low a humidity and temperature and for as short a time as possible.