The substances that can vulcanize rubber under certain conditions are collectively referred to as vulcanizing agents. The so-called vulcanization is to make the linear molecular structure of rubber become a three-dimensional network structure through the "bridging" of the vulcanizing agent, so that the mechanical and physical properties of the rubber are obvious. improve.
1. Sulfur: yellow solid substance, widely used in natural rubber and some synthetic rubber. Commonly used sulfur includes sulfur powder, sublimated sulfur (also known as sulfur bloom) and precipitated sulfur. Sulfur is insoluble in water, slightly soluble in ethanol and ether, and soluble in carbon disulfide and carbon tetrachloride. It is characterized by low heat resistance, high strength, and corrosive effect on copper wires. It is suitable for natural rubber and certain synthetic rubbers. In the wire and cable rubber formula, the amount of sulfur is about 0.2 to 5 parts, but due to the addition of accelerators, the amount of sulfur can be reduced accordingly.
2. Metal oxides: Metal oxides are mainly used as vulcanizing agents for chloroprene rubber and chlorosulfonated polyethylene. Commonly used are zinc oxide, magnesium oxide, lead oxide, and lead tetroxide.
Zinc oxide has a specific gravity of 5.6 and is a white powder, non-toxic and tasteless. Zinc oxide is widely used in rubber, and is often used as the main vulcanizing agent with magnesium oxide in general-purpose chloroprene rubber. It can be used as an activator of accelerator in natural rubber and other olefin rubbers. In addition, it also has a reinforcing effect. Plays the role of shielding ultraviolet rays in the rubber that is resistant to sun aging. The amount of zinc oxide used in natural rubber and butyl rubber is 5-10 parts, and the general amount used in combination with magnesium oxide in chloroprene rubber is 5 parts.
Magnesium oxide is used as a secondary vulcanizing agent in chloroprene rubber to prevent early vulcanization of chloroprene rubber during mixing. This product can improve the tensile strength, fixed elongation strength and hardness of chloroprene rubber. It can neutralize a small amount of hydrogen sulfide generated during vulcanization of halogenated rubber or under other oxidizing conditions. The chlorosulfonated polyethylene rubber can be given good physical and mechanical properties, especially the permanent deformation is relatively small. But the water resistance is poor. The general dosage is 3-7 parts. Magnesium oxide is a white loose powder with a specific gravity of 3.2. It can gradually absorb moisture and carbon dioxide in the air to become alkali or magnesium carbonate and reduce its activity. Therefore, it should be kept strictly sealed.
3. Resin vulcanizing agent: The resin vulcanizing agent is mainly some thermosetting alkyl phenolic resin and epoxy resin. The use of alkyl phenolic resin to vulcanize unsaturated carbon chain rubber and butyl rubber can significantly improve the heat resistance of the vulcanized rubber. The main varieties commonly used are phenol formaldehyde resins, such as tert-butyl phenol formaldehyde resin and tert-octyl phenol formaldehyde resin. Epoxy resin has a good vulcanization effect on carboxyl rubber and neoprene, and its vulcanized rubber has good flex resistance.
4. Thiuram: The full name is tetramethylthiuram disulfide, and the trade name is TMTD. It is a widely used vulcanizing agent in wire and cable rubber, and can also be used as a vulcanization accelerator. The pure product has a melting point of 147℃～148℃ and a specific gravity of 1.29. It is an off-white powder. It is an overspeed accelerator for natural rubber, which can be decomposed to generate free radicals at 100°C, so it can be cross-linked. Using thiuram as a vulcanizing agent can improve the heat resistance and aging resistance of the rubber. The vulcanization curve is flat and not easy to burn. It is suitable for natural rubber, styrene butadiene rubber, nitrile butadiene rubber and all unsaturated rubbers containing double bonds. In general heat-resistant rubber, the amount of thiuram is 2 to 3 parts, and in the continuous vulcanization rubber formulation, the amount is 2 to 5 parts, and when used as an accelerator, the amount is 0.3 to 0.5 parts.