Why the rubber mixing material placed for a long time will be "self-sulfurization"

Mixing rubber material placed during the "self-sulfurization" is mainly due to: (1) the use of vulcanizing agent, accelerator is too much; (2) loading rubber capacity is large, the temperature of the refining machine is too high, the film cooling is not enough; (3) or too early to add sulfur, the dispersion of the material is not uniform, resulting in the local concentration of accelerant and sulfur; (4) improper parking, i.e., the parking place is too high a temperature, the air does not circulate.