Silicone rubber, referred to as silica gel for short, refers to a polymer material whose main chain is composed of silicon atoms and oxygen atoms alternately, and whose molecular side chain is connected with methyl, phenyl and other organic groups. The whole molecular chain of silica gel material has the special structure of organic and inorganic materials. The combination of the two makes it have the dual advantages of organic and inorganic materials. It has excellent characteristics of high and low temperature resistance, weather resistance, arc resistance, electrical insulation and physiological inertia.

According to its curing temperature, silica gel can be divided into high-temperature curing silica gel (HTV) and room-temperature curing silica gel (RTV); According to the product form and mixing method, it can be divided into mixed silica gel and liquid silica gel; According to its crosslinking reaction mechanism, it can be divided into condensation reaction type, addition reaction type and organic peroxide type. Next, according to the reaction type, we will briefly introduce the silica gel material.

1. Condensation reaction silica gel
Condensation reaction type silica gel is generally based on hydroxy-terminated polysiloxane, polyfunctional silane or siloxane as cross-linking agent. Under the action of catalyst, condensation reaction can occur when it meets water vapor or is mixed evenly at room temperature, forming three-dimensional network elastomer. Condensed silica gel is generally used as building sealant, sealing of lamp devices such as automobile lamps, and antifouling materials for electrical insulators.
2. Addition silicone
Addition silica gel is based on polysiloxane containing vinyl group as the basic polymer, polysiloxane containing Si-H chemical bond as the cross-linking agent, and carries out addition reaction under the action of platinum catalyst to obtain silicone rubber with three-dimensional cross-linking structure. Addition silica gel is mainly composed of basic polymer, cross-linking agent, catalyst, reinforcing filler and functional additives. Because addition silicone has the advantages of low shrinkage, no release of small molecules during crosslinking, and excellent resistance to high and low temperatures, most of the current LED packaging adhesives are addition silicone.
3. Peroxide-induced silica gel
Peroxide-induced silica gel is a three-dimensional network structure formed by crosslinking reaction between peroxide and vinyl base polymer with peroxide as curing agent. Peroxide-induced silica gel has two obvious disadvantages: first, it needs to use expensive high-purity silane monomers and intermediates as raw materials; second, due to the presence of peroxides in silica gel, air will seriously affect curing, and inert gas protection is required during curing, so the production and use costs are high, which also greatly limits its application scope.