These ingredients are used to reduce the accelerator activity during processing and storage i.e. to prevent scorch during processing and prevulcanization during storage. They should either decompose or not interfere with the accelerator during normal curing at elevated temperature. In general, these materials are organic acids such as salicylic acid, phthalic anhydride (PA) which function by lowering the pH of the mixture thus retarding vulcanization. Presently, though N - (cyclohexylthio) phthalimide (CTP) is the largest tonnage retarder used in rubber industry, N-nitrosodiphenylamine (NDPA) and thiosulfonamides also constitute special class of retarders. The use of retarders should be avoided, if possible, by the proper selection of accelerator – sulfur combination and careful control of processing conditions.
