The change of each performance index in rubber vulcanization process.

The so-called vulcanization means that under the conditions of heating or irradiation, the raw rubber and the vulcanization agent in the rubber have a chemical reaction, and the linear structure of the polymer cross-linked into the three-dimensional network structure of the macromolecule, and the physical mechanical properties and other properties of the rubber have fundamental changes.

In industrial production, the vulcanization crosslinking reaction is generally completed under certain conditions of temperature, time and pressure. These conditions are commonly referred to as the three elements of vulcanization. In the production of vulcanization conditions, the correct selection of vulcanization equipment and the selection of heating media are important technical content in the vulcanization process.

In the vulcanization process, a series of properties of the rubber material has undergone significant changes, taking different vulcanization time of the test pieces for various physical and mechanical properties, it can be seen that after the tensile strength, fixed elongation stress, elasticity and other properties reach the peak, the vulcanization time is extended, its value decreases, while the hardness remains unchanged. The properties of elongation and permanent deformation decrease gradually with the vulcanization time. After reaching the lowest value, the vulcanization increases slowly. Other properties such as heat resistance, wear resistance and swelling resistance are improved with the increase of curing time.

The general rule of several main physical and mechanical properties of rubber (taking natural rubber as an example) is as follows:

1, fixed elongation stress

When the rubber is not vulcanized, the linear molecules can flow freely among each other, showing non-Newtonian flow characteristics in the plastic range. However, with the deepening of the degree of vulcanization, the flow freedom becomes smaller and smaller, and the deformation force required for fixed-length stretching becomes larger and larger. This is called "constant elongation stress".

2. Tensile strength

The tensile strength of soft rubber is gradually increased with the increase of the degree of crosslinking until the highest value appears. When further vulcanized, the tensile strength decreases sharply over a flat area. In ebonite with a large amount of sulfur, the tensile strength decreases and then rises until it reaches the level of ebonite.

3, elongation

The elongation of rubber decreases gradually with the increase of crosslinking degree.

4, compression permanent deformation

The compressive permanent deformation of rubber also decreases gradually with the increase of crosslinking degree. For the rubber with vulcanization reduction, after positive vulcanization, the compression permanent deformation gradually increases.

5. Flexibility

The elasticity of rubber comes from the reversible change of the micro-Brownian motion position of the flexible chain segment of macromolecules. Due to the existence of this property, a small external force will cause a large deformation of rubber.

In the plastic state, the displacement of rubber molecules is irreversible, but after the rubber molecules are cross-linked, there is a relative positioning between each other, resulting in a strong tendency to reset. However, when the degree of cross-linking continues to increase, the resetting tendency of the macromolecules after deformation is reduced due to the excessive relative stability. Therefore, when the vulcanized rubber is seriously over-sulfur, the elasticity is weakened, and the elasticity is changed from elastomer to rigid body elasticity.

6. Hardness

The hardness of vulcanized rubber increases rapidly after the beginning of vulcanization, reaches a maximum value at the positive vulcanization point, and then remains basically constant.

7, anti-swelling

Unvulcanized adhesives, like other polymers, swell in some solvents and absorb them until they lose cohesion. Rubber molecules enter the solution, and swelling occurs only when the osmotic pressure of the solvent on the rubber is greater than the cohesion of the rubber molecules.

8. Breathability

After the degree of rubber crosslinking increases, the void in the network structure gradually decreases, and the ability of gas to pass and diffuse in the rubber is weakened due to the increase in resistance, so the permeability resistance of the former is better than that of the fully vulcanized elastomer and the insufficient vulcanization.

9. Heat resistance

The heat resistance is best under positive vulcanization.

10. Wear resistance

After the beginning of vulcanization, the wear resistance gradually increases and reaches the highest level in positive vulcanization. Both under-sulfur and over-sulfur are detrimental to wear resistance, but over-sulfur is less affected.