With the increase of RTV compound and the emergence of high temperature vulcanization, the vulcanization temperature tends to two extremes. In terms of improving vulcanization efficiency, it should be considered that the higher the vulcanization temperature is, the better, but in fact, the vulcanization temperature cannot be increased indefinitely. Rubber is a high molecular polymer, and high temperature will cause the cracking reaction of rubber molecular chain, leading to the fracture of cross-linking bond, that is, the phenomenon of "vulcanization returning to the original" will appear, which will reduce the physical and mechanical properties of vulcanizate. For example, when vulcanizing natural rubber at high temperature, the oxygen dissolved in the rubber increases its activity with the increase of temperature, causing strong oxidation, destroying the structure of the rubber and reducing the physical and mechanical properties of the vulcanizate.
Considering the heat resistance and "vulcanization Reversion" of each rubber, the recommended vulcanization temperature of each rubber is as follows:
The best NR is 140-150 ºC, and the maximum is not more than 160 ºC;
CIS polybutadiene rubber, isoprene rubber and neoprene rubber are preferably 150-160 ºC, and the maximum temperature is not more than 170 ºC
Styrene butadiene rubber and butadiene rubber can be used above 150 ºC, but the maximum temperature is not more than 190 ºC;
Butyl rubber and EPDM are generally 160-180 ºC, and the maximum temperature is not more than 200 ºC;
Silicone rubber and fluororubber generally adopt two-stage sulfur addition. The first stage temperature can be 170-180 ºC, and the second stage vulcanization can be 200-230 ºC, which can be selected within 4-24h according to the process requirements.
For effective and semi effective vulcanization systems, the vulcanization temperature is generally controlled between 160-165 ºC. For non sulfur vulcanization systems such as peroxides and resins, the vulcanization temperature is suitable for 170-180 ºC In particular, it should be pointed out that for EPDM, NBR sulfur vulcanization formula, such as the design requires secondary sulfur addition, and the temperature and time of primary sulfur addition and secondary sulfur addition affect the mechanical properties such as compression set and hardness of the final product greatly; For peroxide vulcanization formula, the temperature of primary sulfur addition is particularly important, and the best is above 180 ºC. If the temperature of primary sulfur addition is insufficient, the effect of secondary sulfur addition is very low. That is, the formulation of peroxide vulcanization, and the secondary sulfur addition has little effect on the final physical properties.
Rubber is a poor conductor of heat, and its temperature rise is slow. For thick products, it is difficult to make the inner and outer rubber materials reach a flat range at the same time by using high-temperature vulcanization; The outer part of the product is just positive vulcanization, while the inner part is under vulcanization. Or when positive vulcanization happens to occur inside, while the outside has been vulcanized. In order to ensure uniform vulcanization of thick products, in addition to fully considering the vulcanization flatness of the compound when designing the formula, when selecting the vulcanization temperature, it is also necessary to consider the operation method of lower vulcanization temperature or gradual heating up.
For thin products, the vulcanization temperature can be appropriately high.
For rubber products with fabric, the vulcanization temperature is usually not higher than 140 ºC For foamed rubber, the appropriate vulcanization temperature should be selected according to the decomposition temperature of foaming agent and foaming agent.
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