Based on the analysis of the vulcanization process, it can be divided into four stages, namely the scorching stage, the hot vulcanization stage, the flat vulcanization stage and the over-vulcanization stage. An ideal rubber vulcanization curve should meet the following conditions: (1) The vulcanization induction period should be long enough to fully ensure the safety of production and processing; (2) The vulcanization speed should be fast to improve production efficiency and reduce energy consumption. (3) The vulcanization flattening period should be long.

Performance Changes of Rubber during vulcanization

(1) Physical Property Changes

During the vulcanization process, the physical and mechanical properties of rubber change significantly. Therefore, in the production process, the degree of vulcanization is often measured by the changes in physical properties. The physical properties of rubber generally refer to strength (tensile strength, elongation at a given length, and tear strength, etc.), elongation at break, hardness, elasticity, permanent deformation, and degree of swelling, etc. Although the changes in physical and mechanical properties of rubbers with different structures during the vulcanization process have different trends, the changes in most properties are basically the same. During the vulcanization process of natural rubber, its plasticity decreases significantly, while its strength and hardness increase markedly, while the elongation and swelling degree decrease accordingly. These phenomena are all characteristics of the transformation of linear macromolecules into network structures. For rubbers with a side vinyl structure, such as styrene-butadiene rubber and nitrile rubber, similar changes occur during the vulcanization process. However, over a relatively long period of time, the changes in various properties are relatively flat, and the maximum or minimum values of the curves are not very obvious.

1.The soluble vulcanization process will gradually reduce the rubber's ability to dissolve in solvents, and it can only swell. After vulcanization for a certain period of time, the swelling property reaches its minimum value. If vulcanization continues, there is a tendency to gradually increase the swelling property.
 2. Thermal stability Vulcanization enhances the thermal stability of rubber, meaning that the degree to which the physical and mechanical properties of rubber change with temperature decreases. For instance, unvulcanized natural rubber will undergo crystallization hardening during long-term storage below 10℃. When the temperature exceeds 70℃, plasticity increases significantly. When the temperature exceeds 100℃, it is in a viscous flow state. Decomposition begins at 200℃. However, after vulcanization, the temperature range of high elasticity is expanded. The brittle temperature can be reduced to below -20 to -40℃, and the plastic flow state of raw rubber does not occur. Therefore, vulcanization greatly increases the operating temperature range of natural rubber.
 3. Density and air permeability: Within a certain vulcanization time range, as the crosslinking density increases, the rubber density increases, while the air permeability decreases with the increase of the crosslinking density. This is caused by the thermal motion of the macromolecular chain segments being restricted to a certain extent.

(2) Changes in chemical properties

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