Classification of rubber vulcanization system

The vulcanization of rubber is the process of converting essentially plastic rubber into elastic and dimensional stable products through chemical cross-linking between rubber molecules. The physical properties of the vulcanized rubber are stable, and the temperature range of use is expanded. The term 'curing' is widely used throughout the rubber industry and holds an important position in rubber chemistry. 

The vulcanization (crosslinking) reaction ability between rubber molecular chains depends on their structure:

1. Unsaturated diene rubber (such as natural rubber, styrene butadiene rubber, nitrile butadiene rubber, etc.) contains unsaturated double bonds in its molecular chain, which can form intermolecular crosslinking with sulfur,phenolic resin, organic peroxide, etc. through substitution or addition reaction；

2. Saturated rubber is generally cross-linked with free radicals with a certain amount of energy (such as organic peroxides) and high-energy radiation.

3. Rubber containing special functional groups (such as chlorosulfonated polyethylene) forms cross-linking through specific reactions between various functional groups and established substances, such as the sulfinyl amide group in rubber cross-linking by reacting with metal oxides and amines.

Different types of rubber react with various crosslinking agents to generate different cross-linking bond structures, and the properties of vulcanizates also vary.

The first type is when sulfur or sulfur donors are used as crosslinking agents, which can generate single sulfur bonds (x=1), double sulfur bonds (x=2), and polysulfide bonds (x=3-8);

The second type is the use of resin crosslinking and oxime crosslinking;

The third type is the case of peroxide vulcanization using peroxide crosslinking and radiation vulcanization using radiation crosslinking, generating carbon carbon bonds.

Most universal rubbers are vulcanized using sulfur or sulfur donors, which include adding sulfur or sulfur donors, accelerators to shorten vulcanization time, and active agents composed of zinc oxide and stearic acid to ensure sulfur crosslinking efficiency. In practice, it is usually divided into the following typical vulcanization systems based on the amount of sulfur and its ratio to the accelerator:

① The ordinary sulfur sulfide system is composed of a commonly used sulfur content (>1.5 parts) and a commonly used promoting dose. Using this vulcanization system can cause the vulcanizate to form more polysulfide bonds and a small amount of low sulfur bonds (single sulfur bonds and double sulfur bonds). Vulcanized rubber has high tensile strength and good fatigue resistance. The disadvantage is poor heat and aging resistance.

② The semi effective vulcanization system consists of a sulfur content of 0.8-1.5 parts (or partial sulfur donors) in combination with commonly used promoting doses. The use of this vulcanization system can form an appropriate proportion of low sulfur and polysulfide bonds in the vulcanized rubber. The tensile strength and fatigue resistance of the vulcanized rubber are moderate, and the heat and aging resistance are good.

③ The effective vulcanization system consists of a low sulfur yellow content (0.3-0.5 parts) or a partial sulfur donor combined with a high promotion dose (generally 2-4 parts). The use of this curing system can make the vulcanizate form low sulfur bonds (single sulfur bond and double sulfur bond) that occupy absolute advantage. The vulcanizate has good heat resistance and aging resistance, but its disadvantage is low tensile strength and fatigue resistance.

④ The sulfur free vulcanization system is composed entirely of sulfur donors and accelerators without the use of sulfur. The performance of this vulcanization system is similar to that of an effective vulcanization system.

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