Heat resistant silicone rubber


Heat resistance is one of the most important properties of silicone rubber. The high temperature resistance of the silicone rubber rubber is determined by the high bond energy Si-O-Si bond (bond energy is 451 kj/mol), and the Si-C bond (bond energy is 214 kj/mol) on the Si side base is also strong. Therefore, silicone rubber has high stability at high and low temperature, and some silicone rubber is used for a long time in the environment of -100-300 ℃. However, silicone rubber can not meet the requirements of higher temperature. Under high temperature environment, the silicone rubber molecules lose physical and mechanical properties due to the oxidation of organic groups and the thermal rearrangement of main chain.


The methods to improve the heat resistance of silicone rubber are as follows:


(1) Special groups (such as phenyl) were introduced into the molecular side chain of silicone rubber to prevent the oxidation and thermal rearrangement of the main chain;


(2) The large chain segments (such as phenylene, cyclodisilane, carbodecarboxyl) were introduced into the main chain of silicone rubber to improve the thermal stability of crosslinking bond;


(3) Heat resistant additives were introduced into the formulation system to prevent the degradation of main chain cyclization and side chain oxidation crosslinking. Among them, the introduction of heat-resistant additives into the formulation system of rubber is the most commonly used and effective method.


Flame retardant silicone rubber


The flash point of silicone rubber is 750 ℃, and the ignition point is 450 ℃, and its oxygen index is higher than that of other rubber with carbon as the main chain. When burning, the heat release rate is low, the flame propagation is slow, and the combustion products are almost non-toxic and non corrosive. The ceramic carbon silicon layer is still flame retardant on the surface. Although the flame retardant is excellent, there are still defects in silicone rubber, especially its easy to ignite, and there is potential combustion risk. In some special occasions with high temperature, heat and high voltage (such as aerospace, electronic and electrical and transmission lines), the flame retardant of silicone rubber is required to be very high.


The following aspects are generally considered to improve the flame resistance of silicone rubber:


(1) The ceramic carbon silicon layer which promotes the formation of barrier effect;


(2) The crosslinking of silicone rubber at high temperature is promoted to form a stable structure;


(3) The inorganic filler is added as flame retardant to reduce the surface temperature, improve the heat conductivity or form air barrier layer;


(4) Capture the free radicals generated during combustion, and restrain and slow down combustion;


(5) Prevent the degradation of the buckle type and reduce the release of small combustible molecules.


The common flame retardant is generally divided into additive flame retardant and reactive flame retardant. The former is only dispersed in the matrix in physical way, and no chemical reaction occurs; The latter is a monomer or auxiliary reagent which participates in the legal reaction and becomes a part of the structure of silicone rubber.