Silicone rubber is the largest category of silicone products. It is a kind of polymer material with both inorganic and organic properties, whose main chain is Si-O-Si bond inorganic structure and side chain is organic group. Compared with ordinary organic rubber, silicone rubber has many excellent characteristics, such as heat resistance, weather resistance, electrical insulation, chemical stability and physiological inertia. It plays an important role in all industrial fields represented by electrical and electronic industries, including office equipment, automobile, food, household products, leisure products and so on.

1. High temperature vulcanized silicone rubber

Silicone rubber mainly includes high temperature vulcanized silica gel (HTV) and room temperature vulcanized (RTV). The room temperature vulcanized type is divided into polycondensation reaction type and addition reaction type. High temperature silicone rubber is mainly used to manufacture various silicone rubber products, while room temperature silicone rubber is mainly used as adhesive, potting material or mold. The consumption of high-temperature vulcanization type is the largest, which is divided into methyl silicone rubber (MQ), methyl vinyl silicone rubber (VMQ, with the largest consumption and product brand), methyl vinyl phenyl silicone rubber pvmq (low temperature and radiation resistance), etc.

Methyl vinyl silicone rubber VMQ is copolymerized by dimethylsiloxane and a small amount of vinyl siloxane. Because this silicone rubber contains a small amount of vinyl side chain, it is easier to vulcanize than dimethyl silicone rubber, and more kinds of peroxides can be used as vulcanizing agent, which can greatly reduce the amount of peroxides. Compared with dimethyl silicone rubber, the use of silicone rubber containing a small amount of vinyl can effectively improve the compression set resistance of products, which is one of the necessary properties of O-rings and gaskets. Methyl vinyl silicone rubber has good process performance, convenient operation, can be made into thick products, and the surface of extruded and calendered semi-finished products is smooth. It is a kind of commonly used silicone rubber at present.

1.1 reasons for yellowing of silicone rubber

For high temperature vulcanized silicone rubber, two-stage vulcanization is usually required to remove the decomposition residue of peroxide, and further crosslink the silicone rubber to obtain the best physical properties. However, silicone rubber has yellowing in the process of practical application. Secondary vulcanization will reduce the transparency, directly affect the appearance of products, and limit its application in food, medical treatment, solar photovoltaic and other industries. Researchers have also done a lot of research in this area and used many anti yellowing additives.

Some researchers believe that the trace metal elements and NO-3 contained in the raw materials and additives for the production of silicone rubber lead to the serious yellowing of silicone rubber after secondary vulcanization. It is also considered that acetone is produced by the decomposition of dialkyl peroxide α Hydroxybutyraldehyde condenses and dehydrates to form conjugated compounds, resulting in yellowing [1]. Some people believe that the pH value of silica has an impact on controlling the yellowing of silicone rubber [2]. Therefore, the measures to solve the yellowing of silicone rubber in the market are mainly from raw rubber, silica and additives.

1.2 measures to improve yellowing of silicone rubber

Chen Chaohui of the school of materials, South China University of technology, etc. used self-made modified silicone oil [3]: on the one hand, it can inhibit the yellowing of silicone rubber after secondary vulcanization; On the other hand, it can improve the vulcanization speed and crosslinking density of silicone rubber, shorten the vulcanization time and play the role of crosslinking active assistant. Some experts have invented an anti yellowing paste double 2,5 vulcanizing agent. Because of the addition of high hydrogen containing silicone oil, the anti yellowing effect of vulcanizing agent is greatly enhanced, the appearance of silica gel products is more transparent, the splashing of vulcanizing agent is prevented during processing, and the potential safety hazards are reduced. Another expert added organic surfactant to control the size of secondary particles during the treatment of white carbon black. After being treated with composite anti yellowing agent, the product is applied to silicone rubber products, which has good anti yellowing effect. Ge Jianfang of Zhongkai College of agricultural engineering [4] enhanced the yellowing resistance of the compound and expanded its application environment by adding an anti yellowing additive pyrophosphoric acid without reducing the physical and mechanical properties of the compound.