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Method for improving heat resistance and flame retardancy of silicone rubber

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Heat-resistant silicone rubber

Heat resistance is one of the remarkable properties of silicone rubber. Its high temperature resistance depends on the molecular skeleton of silicone rubber raw rubber is composed of Si-O-Si bonds with high bond energy (bond energy is 451 kJ/mol), and the Si-C bonds on the side groups of Si atoms ( bond energy of 214 kJ/mol) is also 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 temperature. Under high temperature environment, silicone rubber raw rubber molecules lose physical and mechanical properties due to oxidation of side organic groups and thermal rearrangement and degradation of the main chain.

There are mainly the following methods to improve the heat resistance of silicone rubber:

(1) Introduce a special group (such as phenyl) into the side chain of the silicone rubber molecule to prevent the oxidative degradation of the side group and the thermal rearrangement degradation of the main chain;
(2) Introduce larger segments (such as phenylene, cyclodisilazane, carbodecaboryl) into the main chain of the silicone rubber molecule to improve the thermal stability of the crosslinking bond of the vulcanizate;
(3) Introduce heat-resistant additives (such as SnO2, Fe2O3, etc.) into the compound formulation system to prevent the cyclization degradation of the main chain and the oxidative crosslinking of the side chain. Among them, the introduction of heat-resistant additives into the compound formulation system is currently the most common and effective method.

Flame retardant silicone rubber

The flash point of silicone rubber is as high as 750°C and the ignition point is 450°C, and its oxygen index is higher than that of other carbon-based rubbers. When burning, the heat release rate is low, the flame spreads slowly, 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 excellent flame retardancy, silicone rubber still has drawbacks, especially its easy smoldering and potential burning hazard. In some special occasions with high temperature, heat and high voltage (such as aerospace, electrical and electronic and transmission lines, etc.), the flame retardancy of silicone rubber is extremely required.

Improving the flame retardancy of silicone rubber is generally considered from the following aspects:

(1) A ceramicized silicon carbon layer that promotes the formation of a barrier;
(2) Promote the cross-linking of silicone rubber at high temperature to form a stable structure;
(3) Add inorganic fillers to act as flame retardants to reduce the surface temperature during combustion, improve thermal conductivity or form an air barrier layer to play a flame retardant role;
(4) Capture free radicals generated during combustion, inhibit and slow down combustion;
(5) Prevent unbuttoned 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 without chemical reaction; the latter as a monomer or auxiliary reagent participates in the legal reaction and becomes a part of the silicone rubber structure.

The oxygen index of MMT/silicone rubber composites prepared by melt blending method firstly increased sharply and then slowly increased with the increase of MMT content. 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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