Heat resistant silicone rubber:

Heat resistance is one of the remarkable characteristics of silicone rubber. Its high temperature resistance depends on that the molecular skeleton of the raw silicone rubber is composed of Si-O-Si bonds with high bond energy (451 kJ/mol), and the Si-C bond (214 kJ/mol) attached to the side group of the Si atom is also strong. Therefore, silicone rubber has high stability at high and low temperatures, and some silicone rubber can be used for a long time at - 100~300 ℃. However, silicone rubber cannot meet the requirements of higher temperature. Under high temperature environment, silicone rubber molecules lose physical and mechanical properties due to oxidation of side organic groups and thermal rearrangement degradation of the main chain.

The methods to improve the heat resistance of silicone rubber are as follows:
(1) Special groups (such as phenyl) are introduced into the side chains of silicone rubber molecules to prevent the oxidative degradation of the side groups and the thermal rearrangement degradation of the main chain;
(2) The thermal stability of crosslink bond of silicone rubber was improved by introducing large segments (such as phenylene, cyclodisilazane, carbodecane) into the main chain of silicone rubber molecule;
(3) Heat resistant additives (such as SnO2, Fe2O3, etc.) are introduced into the compound formulation to prevent the cyclization degradation of the main chain and oxidative crosslinking of the side chain. Among them, the most commonly used and effective method is to introduce heat-resistant additives into the compound formulation system.

Flame retardant silicone rubber:
The flash point of silicone rubber is as high as 750 ℃, the ignition point is 450 ℃, and its oxygen index is higher than other rubber with carbon as the main chain. During combustion, the heat release rate is low, the flame spread is slow, and there is no drop. The combustion products are almost non-toxic and non corrosive, forming a ceramic carbon silicon layer that is still flame retardant on the surface. Despite its excellent flame retardancy, silicone rubber still has defects, especially its smoldering and potential combustion hazards. In some special occasions with high temperature, heating and high voltage (such as aerospace, electronic and electrical and transmission lines), the flame retardancy of silicone rubber is highly required.

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

(1) It can promote the formation of ceramic carbon silicon layer with barrier effect;
(2) Promote the crosslinking of silicone rubber at high temperature to form a stable structure;
(3) The inorganic filler is added to serve as a flame retardant to reduce the surface temperature during combustion, improve the thermal conductivity or form an air barrier layer to play a flame retardant role;
(4) Capture free radicals generated during combustion to inhibit and slow down combustion;
(5) Prevent release degradation and reduce release of flammable 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 legal reaction and becomes a part of the structure of silicone rubber.

The oxygen index of MMT/silicone rubber composites prepared by melt blending method increased sharply at first and then slowly 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.