The common curing systems of rubber are sulfur curing system and peroxide curing system. Sulfur vulcanization is mainly used in diene rubber to obtain vulcanizate with sulfur crosslinking bond. Peroxide vulcanization is a kind of vulcanization system that has developed rapidly and been widely used in recent years. Peroxide curing system can vulcanize diene rubber and non diene rubber. The crosslinking bond generated by peroxide curing system is C-C bond, and the bond energy is larger than that of single sulfur, double sulfur and multi sulfur bonds generated by sulfur curing system. Therefore, the vulcanizate vulcanized by peroxide has excellent heat resistance, small compression permanent deformation, simple combination, and is not easy to spray frost, so it will not produce vulcanization reversion like sulfur curing system; However, the tensile stress-strain properties and fatigue resistance of vulcanizates are poor.
Except butyl rubber and halogenated butyl rubber, almost all other rubbers can be vulcanized with peroxide. The most commonly used peroxide vulcanizates are silicone rubber (VMQ), ethylene propylene diene monomer (EPDM), nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), chlorinated polyethylene (CM), thermoplastic elastomer (TPV), neoprene (CR), etc. Although the peroxide curing system is simple to mix, in the formula components, the type of rubber, the type and amount of peroxide and crosslinking aid, and the interaction of other compounding agents have a greater impact on the crosslinking of rubber compound and the properties of vulcanizate; The process conditions of rubber processing also need to adapt to the curing system. Some scholars improved the peroxide curing system by adding multifunctional active crosslinking aids and composite curing system to meet the overall requirements of product performance, processing technology and cost.
1 Peroxide curing system
The mechanism of peroxide crosslinking is mainly as follows: when peroxide is heated with rubber, it splits evenly to produce free radicals, and then through free radical addition reaction or seizing ɑ- The methylene active hydrogen undergoes the crosslinking reaction, so that the C-C cross-linking bond is continuously formed in the reaction process. The vulcanization process of the elastomer by peroxide is mainly divided into three steps: (1) the peroxide splits evenly to form two alkoxy radicals; (2) Alkoxy radical snatches hydrogen atom from polymer chain; (3) The free radicals of two adjacent polymer chains combine to form a carbon carbon bond.
Peroxide used for vulcanization of rubber is a kind of material containing O-O bond. At present, the five types of peroxides mainly produced in industry are diacyl peroxides, tertiary alkyl peroxides, alkyl hydroperoxides, dialkyl peroxides, and dialkyl peroxyketals. The most commonly used ones are dicumyl peroxide (DCP), benzoyl peroxide (BPO), 1,1-di-tert-butylperoxy-3,3,5-trimethylcyclohexane (BPMC), 2,5-dimethyl-2,5-bis (t-butylperoxy) hexane (DBPMH), 1,3-bis (t-butylperoxy isopropyl) benzene (BIPB), etc.
In peroxide curing system, the decomposition temperature of peroxide is very important, which is mainly related to the stability of peroxide bond. The stability of the peroxide bond depends on the groups around it. The degree of peroxide decomposition depends on the time and temperature of heating. When peroxide decomposes, the lower the heating temperature, the longer the decomposition time. One method of characterizing the stability of peroxides is called half-life temperature. The half-life temperature varies greatly with the peroxide. At any temperature, the decomposition rate of peroxides with lower half life temperature is faster than that of peroxides with higher half life temperature. Peroxides with a higher half-life temperature will have a better scorch resistance. Slow curing speed usually requires a slower processing speed. Peroxides with a lower half-life temperature have a faster curing speed, allowing faster processing speed, but may cause scorch. This equilibrium relationship is often considered when choosing peroxides.
The crosslinking time and temperature are related to the half-life of the peroxide. For typical peroxides used in the rubber industry, the half-life is reduced to about 1/3 of its original value for every 10 ℃ rise in temperature.