Heat resistant silicone rubber
Heat resistance is one of the remarkable characteristics of silicone rubber. Its high temperature resistance depends on the silicone rubber rubber molecular framework is composed of si-O-Si bonds with high bond energy (bond energy 451 kJ/mol), and si-C bonds (bond energy 214 kJ/mol) linked to the Si atomic side group are also very strong. Therefore, silicone rubber has high stability under high and low temperature conditions, and some silicone rubber can be used for a long time in -100 ~ 300℃ environment. However, silicone rubber cannot meet the requirements of higher temperature. In high temperature environment, silicone rubber rubber molecules lose physical and mechanical properties because of oxidation of side organic groups and thermal rearrangement degradation of main chain.
There are mainly the following methods to improve the heat resistance of silicone rubber:
(1) Introducing special groups (such as phenyl) into the side chain of silicone rubber molecules to prevent oxidative degradation of side groups and thermal rearrangement degradation of the main chain;
(2) To improve the thermal stability of the crosslinked bond of the vulcanized rubber, large chain segments (such as phenyl, cyclodisilazane, cardecaboryl) were introduced into the main chain of the silicone rubber molecule.
(3) Heat resistant additives (such as SnO2, Fe2O3, etc.) are introduced into the glue formulation system to prevent the cyclic degradation of the main chain and oxidative cross-linking of the side chain. Among them, the introduction of heat-resistant additives into the formula system is the most commonly used and effective method.
Flame retardant silicone rubber
Silicon rubber flash point up to 750℃, 450℃ ignition point, its oxygen index is higher than other carbon - based rubber chain. Combustion, heat release rate is low, flame propagation is slow, no dripping, combustion products are almost non-toxic, non-corrosive, on the surface of the formation is still flame retardant ceramic carbon silicon layer. Despite its excellent flame retardancy, silicone rubber still has defects, especially its tendency to smoulder and potential combustion hazard. In some special occasions of high temperature, heat and high voltage (such as aerospace, electronic and electrical transmission lines, etc.), the flame retardant of silicone rubber is highly required.
Improving the flame retardancy of silicone rubber is generally considered from the following aspects:
(1) promote the formation of the barrier of the ceramic carbon silicon layer;
(2) Promote the cross-linking of silicone rubber at high temperature to form a stable structure
(3) add inorganic filler, acting as flame retardant to reduce the surface temperature of combustion, improve thermal conductivity or form an air barrier layer to play a flame retardant role;
(4) Capture free radicals generated during combustion, and inhibit and slow down combustion;
(5) To prevent the unlocking degradation, reduce the release of combustible small molecules.
Commonly used flame retardants are generally divided into additive flame retardants and reactive flame retardants. The former is only physically dispersed in the matrix without chemical reaction. The latter, as a monomer or auxiliary reagent, participates in the jurisprudence reaction and becomes a part of the silicone rubber structure.
The oxygen index of MMT/ silicone rubber composites prepared by melt blending method increased with the increase of MMT content and then increased slowly. The mixed flame retardant system of magnesium hydroxide/zinc borate can also improve the flame retardant of silicone rubber and enhance the ceramic layer formed after combustion.
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