1. Chemical structure

(1) Main chain and side group
The main chain is the most basic skeleton of silicone rubber, which has a decisive influence on the thermal aging of silicone rubber. The main chain is composed of silicon-oxygen bond connection and arranged in a helical structure, which has large polarity and flexibility, so it is easy to degrade the main chain and react with side group oxidation at high temperature, resulting in gradual softening of silicone rubber and loss of use value. The main chain of silicone rubber is composed of different structures, and its heat resistance is also very different. The introduction of phenylene, cyclodisilazane, phenylene, phenylene, decaboryl and other groups in the main chain can greatly inhibit the possibility of cyclization degradation reaction and improve the overall heat resistance of the material. For the side groups, when the side groups of vinyl silicone rubber are phenyl, methyl, ethyl, propyl, butyl, in high temperature environment, oxidation will occur in sequence, resulting in a decline in the heat resistance of the material. In addition, according to the study, when the side chain alkyl group is increased, the thermal stability of silicone rubber will also decline. For example, the heat resistance of dimethyl silicone rubber is better than that of butyl silicone rubber.

(2). End group
When the end group of methyl silicone rubber is hydroxyl, intermolecular condensation reaction will occur at low temperature, which will promote the break of the main chain, and then affect the heat resistance of the material. At higher temperatures, the rebite reaction will occur, resulting in irregular degradation, and finally affect the heat resistance of silicone rubber through two aspects of silicon-benzene bond and silicon-oxygen bond fracture.

2. Semi-inorganic polymer composite materials
Silicone rubber as a semi-inorganic polymer composite material, when adding different fillers will have different effects on the heat resistance of silicone rubber. For example, some amphoteric metal compound fillers can absorb acidic or alkaline substances in silicone rubber, which can effectively reduce the catalytic degradation of these substances for silicone rubber, this measure can effectively reduce the possibility of silicone rubber thermal aging, commonly used materials are generally alumina. In general, the filler is not only an excellent filler, properly used to better improve the heat resistance of silicone rubber.

3. Environmental factors
The use of water in the environment, including water vapor, can cause the silicon-oxygen bond in silicone rubber to break, and finally the formation of silicon hydroxyl, and it is known that the presence of silicon hydroxyl has an adverse effect on the heat resistance of silicone rubber. In addition, it is interesting to note that when filling silicone rubber, the hydrolysis and rearrangement of siloxane have the same effect on mechanical properties, so it is difficult to effectively determine whether there is hydrolysis through mechanical properties analysis, but the difference in heat resistance can provide a basis for its determination. The presence of acid and alkali in the silicone rubber system will play a catalytic role in the degradation of silicone rubber. In addition, oxygen and ozone in the environment will also have a certain adverse effect on the thermal degradation of silicone rubber. At the same time, if there is cyclic stress of load in the application environment, it will accelerate the degradation of dimethylsiloxane and diphenyl siloxane copolymer, resulting in reduced heat resistance of the material, and the composition of the two components of the copolymer is different, and the degradation mechanism is also different.