Silicone resin adhesive

Synthesis of silicone resins

The monomer for the preparation of silicone resin is chlorosilane, which can be alcoholyzed to obtain the corresponding alkoxy silane. They are widely used monomer components because they are not corrosive, and have greater hydrolysis stability, easy preservation and easy separation than the corresponding chlorosilane. By changing the number of functional groups in monomer and choosing different substituents, polymers with different degrees of polymerization, branching and cross-linking can be prepared, and products with different properties can be obtained to meet different applications.

The number of functional groups in a monomer can be expressed by the ratio of R to Si of the monomer mixture, R/Si (R is the substitutional cardinal order, Si is the number of silicon atoms). Take the hydrolysis condensation of methyl chlorosilane as an example:

When R/Si>2, namely (CH3) 2SiCl2 and (CH3) 3SiCl mixed co-hydrolysis, the formation of low molecular weight oily polymer, namely silicon oil. When R/Si=2, that is, with pure (CH3) 2SiCl2 hydrolysis condensation, the generation of high molecular weight linear polymer that is said behind the base material of silicone rubber adhesive, also known as silicon rubber.

When R/Si<2, that is, (CH3) 2SiCl2,CH3SiCl3 co-hydrolysis. Or CH3SiCl3,(CH3)2SiCl2 and SiCl4 co-hydrolytic polycondensation, the formation of a network structure of the polymer, namely silicone resin, appropriate change in the value of R/Si, can get different properties of silicon-containing polymer. When R/Si is small, that is, the ratio of tri-functional or four-functional silicon-oxygen unit is high, after curing, the cross-linking degree is high, and the silicone resin is hard and brittle; The R/Si is large, that is, the ratio of bifunctional silicon-oxane unit is high, and the flexibility of the cured silicone resin is good. The R/Si usually used is between 1.2-1.5.

Silicone resins are different from silicone oil and silicone rubber. After curing, the glass transition temperature (Tg) of silicone resins is >200℃, while the Tg of typical silicone rubber is <-60℃. Methyl silicone resin has the lowest carbon content, it has a very high heat resistance, silicon atoms connected to the methyl group the least steric resistance, resin crosslinking degree is high, high hardness, small thermoplasticity, as a waterproof, moisture-proof adhesive is very ideal. However, pure methyl silicone resin has poor compatibility with pigments and small thermal elasticity. The introduction of phenyl in polymethyl siloxane can improve the thermal elasticity of the product and the miscibility of pigments, as well as the adhesive property of various substances, and can also improve its thermal stability. The properties of methylphenyl silicone resins depend mainly on two factors: one is the R/Si mentioned above, and the ratio of methyl to phenyl group (CH3/C6H5) is closely related