2020 年 93 巻 11 号 p. 345-351
Insoluble sulfur (IS), which has linear long chain of sulfur and is an allotrope of sulfur, is prepared by thermal ring-opening polymerization of cyclooctasulfur (S8). IS is widely used as a vulcanizing agent for rubber. Enhancing thermal stability of IS and its yield from S8 are important for usage of IS in the tire manufacturing process and its industrial production. Typical procedure for IS preparation has three steps, melting sulfur (S8), cooling the molten sulfur, and post-heating the solidified sulfur that is mixture (Sx) of IS and S8. Our previous investigation demonstrated that the long term post-heat treatments (at 30 °C for 960 h) enhanced thermal stability of the resulted IS without yield loss. Such long term process is not acceptable for industrial preparation of IS. In this paper, we report that the acceptable post-heat treatment condition of the Sx is at 60 °C for 18 hours through the investigation under various post heat treatment conditions. We estimate the apparent reaction rates, elongation reaction of sulfur chains, their scission reaction and formation of S8 by using ESR technique. In addition, the multistep post-heat treatments also enhance thermal stability of the resulted IS with small amount of yield loss.
