Heat resistance is one of the remarkable properties of silicone rubber. Its high temperature resistance depends on the fact that the molecular framework of silicone rubber is composed of Si-O-Si bonds with high bond energy (451 kJ / mol), and the Si-C bond (214 kJ / mol) on the side group of Si atom is also very strong. Therefore, silicone rubber has high stability at high and low temperature. Some silicone rubber can be used for a long time at - 100 ~ 300 ℃. However, silicone rubber can not meet the requirements of higher temperature. At high temperature, the raw silicone rubber molecules lose their physical and mechanical properties because of the oxidation of side organic groups and the thermal rearrangement of the main chain.
There are several ways to improve the heat resistance of silicone rubber
(1) A special group (such as phenyl) was introduced into the side chain of silicone rubber to prevent the oxidative degradation of the side group and the thermal rearrangement of the main chain;
(2) Large segments (such as phenylene, CYCLODISILAZANE and carbodecaborane) were introduced into the main chain of silicone rubber to improve the thermal stability of crosslinking bond;
(3) Heat resistant additives (such as SnO2, Fe2O3, etc.) were introduced into the compound formulation system to prevent cyclization and degradation of the main chain and oxidative crosslinking of the side chain. Among them, the most common and effective method is to introduce heat-resistant additives into the compound formulation system.
The flash point of silicone rubber is 750 ℃ and the ignition point is 450 ℃. The oxygen index of silicone rubber is higher than that of other rubber with carbon as main chain. During combustion, 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 the ceramic carbon silicon layer is still flame retardant on the surface. Despite its excellent flame retardancy, silicone rubber still has some defects, especially its smoldering and potential combustion hazard. In some high temperature, heating, high voltage special occasions (such as aerospace, electronic and electrical and transmission lines, etc.), the flame retardancy of silicone rubber is very high.
To improve the flame retardancy of silicone rubber, the following aspects should be considered:
(1) The ceramic carbon silicon layer with barrier effect is formed;
(2) The crosslinking of silicone rubber at high temperature was promoted to form stable structure;
(3) Inorganic fillers are added to act as flame retardants to reduce the surface temperature, improve the thermal conductivity or form an air barrier layer;
(4) Capture free radicals generated during combustion, inhibit and slow down combustion;
(5) It can prevent the uncoupling degradation and reduce the release of combustible small molecules.
The commonly used flame retardants are generally divided into additive flame retardants and reactive flame retardants. The former is physically dispersed in the matrix without chemical reaction; The latter, as a monomer or auxiliary reagent, participates in the reaction and becomes a part of the structure of silicone rubber.
The oxygen index of MMT / silicone rubber composites prepared by melt blending method increases sharply at first and then slowly with the increase of MMT content. The mixture of magnesium hydroxide and zinc borate can also improve the flame retardancy of silicone rubber and enhance the ceramic layer formed after combustion.