What Are The Vulcanization Process Conditions?

Vulcanization temperature is one of the basic conditions for the vulcanization reaction of rubber. It directly affects the rate of vulcanization reaction and the physical and mechanical properties of vulcanized rubber, thus affecting the quality of the product.
Like general chemical reactions, the vulcanization reaction of rubber depends on temperature. As the temperature rises, the vulcanization reaction rate accelerates, the production efficiency is high and it is easy to generate more low-sulfur cross-links; on the contrary, the vulcanization temperature is low, the vulcanization rate is slow, the production efficiency is low, and it is easy to generate more polysulfide cross-links.
Obviously, to obtain high production efficiency, a higher vulcanization temperature should be used as much as possible. In fact, the vulcanization temperature cannot be increased indefinitely. Generally, the higher the vulcanization temperature, the lower the physical and mechanical properties of the rubber. Excessive temperature will cause the cracking of the rubber molecular chain and the occurrence of vulcanization reversion (especially natural rubber), which will reduce the performance and make process control difficult.
Therefore, the selection of vulcanization temperature should be comprehensively considered according to the type of product, rubber type and vulcanization system. Generally, the vulcanization temperature of rubber is in the range of 120~190℃.
Vulcanization pressure can be divided into normal pressure and high pressure. Usually, atmospheric pressure vulcanization is used for thin-walled products such as rubber sheets, while molded products require higher pressure.
There are two ways of vulcanization pressurization: using a hydraulic pump to transfer pressure to the model through a flat vulcanizer, and then transferring it to the rubber from the model, which is called flat plate pressurization; direct pressurization by the vulcanization medium, such as steam pressurization; pressurization by compressed air, that is, hot air pressurization; pressurization by individual vulcanizers and injection presses, etc.
The main effects of vulcanization pressure are as follows.
a. Prevent the formation of bubbles in the product during the vulcanization process and improve the density of the rubber. During vulcanization, the moisture and other volatile substances contained in the rubber, as well as the hydrogen sulfide gas that may be formed during the vulcanization reaction, will evaporate and escape at high temperature, causing bubbles in the rubber. If a vulcanization pressure greater than the internal pressure that may escape from the rubber is applied during vulcanization, the formation of bubbles can be prevented and the density of the rubber can be improved. Applying a higher vulcanization pressure can change the density of the rubber. If only bubbles are to be prevented, water absorbents such as gypsum and calcium oxide can also be added to the rubber compound to achieve normal pressure vulcanization.
b. Make the rubber compound easy to flow and fill the mold cavity. In order to obtain products with clear and full patterns, the rubber compound must be able to flow and fill the mold cavity. Especially during the vulcanization induction period when the rubber compound is in an uncrosslinked state, the effect of vulcanization pressure is more obvious. Experiments show that for molded rubber products, if the vulcanization temperature is 100-140℃, the vulcanization pressure should be 2~5MPa. If the vulcanization temperature is 40~50℃ (such as injection molding), the vulcanization pressure should be 50~80MPa, so as to ensure that the rubber compound flows well and fills the mold cavity.
c. Improve the density of the rubber compound and improve the adhesion between the rubber compound and the skeleton material. During vulcanization, as the vulcanization pressure increases, the depth of the rubber penetrating into the cloth layer increases, thereby improving the adhesion between the rubber and the cloth layer and the flexural resistance of the product. Experiments show that when natural rubber automobile outer tires are vulcanized, as the vulcanization pressure (water tire internal pressure) increases, the flexural resistance of the inner layer of the outer tire cord also increases.
d. It helps to improve the physical and mechanical properties of the vulcanized rubber. Experiments show that as the vulcanization pressure increases, some physical and mechanical properties of the vulcanized rubber, such as strength, dynamic modulus, fatigue resistance, and wear resistance, increase accordingly. Experiments show that the wear resistance of tires vulcanized with a pressure of 5.0MPa is 10% to 20% higher than that of tires vulcanized with a pressure of 2.0MPa. The wear resistance of the annular pre-vulcanized tread rubber produced with a pressure of 6 to 7MPa is almost the same as that of new tires.
However, excessive vulcanization pressure is also detrimental to the performance of rubber. This is because high pressure, like high temperature, will accelerate the thermal degradation of rubber molecules, which will reduce the performance of the rubber compound. In addition, under high pressure, the structure of textile materials will also be damaged, resulting in a decrease in flexural resistance. Excessive pressure also places high demands on equipment, causes greater damage to equipment, and has high power costs.
Vulcanization time is like many other chemical reactions. The progress of vulcanization reaction also depends on time. Under certain vulcanization temperature and pressure, only after a certain vulcanization time can the vulcanization degree that meets the design requirements be achieved.