Rubber vulcanization reaction process

（1） Vulcanization process

According to the analysis of vulcanization process, it can be divided into four stages: scorch stage, hot vulcanization stage, flat vulcanization stage and over vulcanization stage.

The ideal rubber vulcanization curve shall meet the following conditions:

(1) The vulcanization induction period should be long enough to fully ensure the safety of production and processing (2) vulcanization speed should be fast, improve production efficiency and reduce energy consumption; (3) The vulcanization flattening period is long.

Changes in properties of rubber during vulcanization

（1） Physical property change

During vulcanization, the physical and mechanical properties of rubber change significantly, so in the production process, the vulcanization degree is often measured by the change of physical properties. The physical properties of rubber generally refer to strength (tensile strength, tensile strength and tear strength), elongation at break, hardness, elasticity, permanent deformation, swelling degree, etc. Although the changes of physical and mechanical properties of rubber with different structures have different trends in the vulcanization process, most of the changes of properties are basically the same. During vulcanization, the plasticity of natural rubber decreased significantly, the strength and hardness increased significantly, while the elongation and swelling decreased accordingly. These phenomena are characterized by the transformation of linear macromolecules into network structures. For rubber with side vinyl structure, such as styrene butadiene rubber and nitrile butadiene rubber, there are similar changes in the vulcanization process, but in a long time, the changes of various properties are relatively flat, and the maximum or minimum of the curve is not obvious.

1. Soluble vulcanization process will gradually reduce the ability of rubber to dissolve in solvent, but only swell; After vulcanization for a certain period of time, the swelling appeared to be the minimum, and continued vulcanization tended to increase the swelling gradually.

2. Thermal stability vulcanization improves the thermal stability of rubber, that is, the change degree of physical and mechanical properties of rubber with temperature decreases. For example, when unvulcanized natural rubber is lower than 10 ℃, crystallization hardening will occur after long-term storage; When the temperature exceeds 70 ℃, the plasticity increases significantly; If it exceeds 100 ℃, it is in viscous flow state; Decomposition begins at 200 ℃. However, after vulcanization, the temperature range of high elasticity is expanded, the brittleness temperature can be reduced to below - 20 ~ - 40 ℃, and the plastic flow state of raw rubber does not appear. Therefore, vulcanization greatly improves the service temperature range of natural rubber.

3. Within a certain curing time range, the density and air permeability of rubber increased with the increase of crosslinking density, while the air permeability decreased with the increase of crosslinking density. This is because the thermal motion of macromolecular segments is limited.

（2） Changes in chemical properties

During vulcanization, due to crosslinking, the active functional groups or double bonds in the rubber macromolecular structure gradually decrease, thus increasing the chemical stability. On the other hand, due to the formation of network structure, the movement of rubber macromolecular chain segments is weakened, and the diffusion of low molecular substances is seriously hindered. As a result, the stability of rubber to chemical substances is improved.

Rubber vulcanizing agent

Vulcanizing agent is a substance that connects adjacent rubber macromolecular chains in the form of chemical bonds under certain conditions.

Vulcanizing agents are mainly divided into seven categories: sulfur, sulfur donor, organic peroxide, metal oxide, organic quinone, resin vulcanizing agent and amine vulcanizing agent.