The demand for special silicone rubber has exploded. How to improve the heat resistance and flame retardancy of silicone rubber?
Heat-resistant silicone rubber
Heat resistance is one of the remarkable characteristics of silicone rubber. Its high temperature resistance depends on the molecular skeleton of the raw silicone rubber is composed of Si-O-Si bonds with high bond energy (bond energy of 451 kJ/mol), and Si-C bonds connected to the side groups of Si atoms ( The bond energy is 214 kJ/mol) is also very strong. Therefore, silicone rubber has high stability under high and low temperature conditions, and some silicone rubbers are used for a long time in an environment of -100 to 300°C. However, silicone rubber cannot meet the requirements of higher temperatures. Under high temperature environments, the raw silicone rubber molecules lose their physical and mechanical properties due to oxidation of side organic groups and thermal rearrangement degradation of the main chain.
The main methods to improve the heat resistance of silicone rubber are as follows:
(1)Introduce special Groups (such as phenyl) to prevent the oxidative degradation of side groups and thermal rearrangement degradation of the main chain;
(2)(2) In the silicone rubber molecule Introduce larger segments (such as phenylene, cyclodisilazane, carbodecaboryl) into the main chain to improve the thermal stability of the cross-linked bond of the vulcanizate;
(3)(3) in the glue Heat-resistant additives (such as SnO2, Fe2O3, etc.) are introduced into the formulation system to prevent the main chain cyclization degradation and side chain oxidative cross-linking. Among them, the introduction of heat-resistant additives into the rubber formulation system is currently the most commonly used and effective method.
Flame retardant silicone rubber
Silicon The flash point of rubber is as high as 750°C and the ignition point is 450°C. Its oxygen index is higher than other rubbers with carbon as the main chain. When burning, the heat release rate is low, the flame spreads slowly, and there is no dripping. The combustion products are almost non-toxic and non-corrosive, and a ceramic carbon silicon layer that is still flame-retardant is formed on the surface. Despite its excellent flame retardancy, silicone rubber still has defects, in particular it is easy to smolder and has a potential burning hazard. In some special occasions with high temperature, heat, and high voltage (such as aerospace, electronic and electrical and power transmission lines, etc.), the flame retardancy of silicone rubber is extremely high.
Improving the flame retardancy of silicone rubber is generally considered from the following aspects:
(1)to promote the formation of ceramic carbon with a barrier effect Silicon layer;
(2) promote the crosslinking of silicone rubber at high temperature to form a stable structure;
(3) Add inorganic filler , Acts as a flame retardant to reduce the surface temperature during combustion, improve thermal conductivity or form an air barrier layer to play a flame retardant effect;
(4) Capture free radicals generated during combustion, Suppress and slow down combustion;
(5) Prevent tripping degradation and reduce the release of flammable small molecules.
Commonly used flame retardants are generally divided into additive flame retardant materials and reactive flame retardants. The former is only physically dispersed in the matrix and does not undergo chemical reactions; the latter acts as a monomer or auxiliary reagent to participate in legal reactions and become a part of the structure of silicone rubber. The oxygen index of MMT/silicone rubber composites prepared by melt blending method increases first and then slowly with the increase of the amount of MMT added. The mixed flame retardant system of magnesium hydroxide/zinc borate can also improve the flame retardancy of silicone rubber and enhance the ceramic layer formed after combustion.
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