Thermal Stability Change of Insoluble Sulfur by a Heat Treatment and Its Mechanism Study

Insoluble sulfur (IS), used as a vulcanizing reagent of rubber, is prepared by the thermal ring-opening polymerization of sulfur (S₈). Enhancing its thermal stability and content ratio (yield) is important for the industrial production of IS. The post-heating process at a high temperature of 70 or 90°C of the mixture of IS and S₈ enhanced the thermal stability of IS and reduced the yield of IS. Further, the process at 30°C enhanced its thermal stability and maintained its yield. Since the thermal stability of IS is considered to be closely related to the chain length of polymer sulfur, a method for determining the chain length of IS was investigated by quantifying the amount of electron spin of radicals from sulfur, estimated from electron spin resonance (ESR) measurements. We confirmed that the long-period post-heating process at 30°C induced high thermal stability without reducing the yield of IS due to growth of the sulfur polymer chains.