Four stages of vulcanization process and their characteristics

Four stages of vulcanization process and their characteristics

The whole vulcanization process can be divided into four stages: vulcanization induction, pre-sulfur, positive vulcanization and over-sulfur (for natural rubber vulcanization recovery).

During the vulcanization induction period (scorching time), the crosslinking has not yet started, and the compound has good fluidity. This stage determines the scorchability and processing safety of the compound. At the end of this stage, the compound begins to cross-link and lose its fluidity. The length of vulcanization induction period is related to the properties of raw rubber itself, mainly depending on the additives used. For example, a longer scorching time can be obtained with a delay accelerator, and a higher processing safety can be obtained.

After the induction period of vulcanization, the pre-vulcanization stage of cross-linking is carried out at a certain speed. The crosslinking degree of vulcanizate during pre-vulcanization period is low, and the elasticity of vulcanizate can not reach the expected level even at the later stage of vulcanization, but the tearing and dynamic cracking properties are better than the corresponding positive vulcanization.

After reaching the positive vulcanization stage, the physical properties of vulcanizate reach or approach the optimum point, or reach the comprehensive balance of properties.

After the positive vulcanization stage (flat vulcanization zone), that is, the over-sulphur stage, there are two situations: the natural rubber has the phenomenon of "returning to the original" (the elongation strength decreases), and the elongation strength of most synthetic rubber (except butyl rubber) continues to increase.

For any rubber, vulcanization not only produces crosslinking, but also produces breakage of chain of production and molecular chain due to heat and other factors. This phenomenon runs through the whole vulcanization process. In the sulfur excess stage, if the crosslinking is still dominant, the rubber will harden and the elongation strength will continue to rise. On the contrary, the rubber will soften, which means that the rubber will return to its original state.