Silica gel (Silica), also known as silica gel, is a highly active adsorbent material, which belongs to amorphous material. Its chemical formula is mSiO2*nH2O. Insoluble in water and any solvent, non-toxic and tasteless, stable chemical properties, no reaction with any substance except strong base and hydrofluoric acid.

Silica gel can be divided into organic silica gel and inorganic silica gel according to its properties and components.

Organic silica gel is a kind of organic silicon compound, which contains Si-C bond and at least one organic group is directly connected with silicon atom. It is also used to treat those compounds that connect organic group with silicon atom through oxygen, sulfur, nitrogen, etc. as organic silicon compounds. Among them, polysiloxane with silicon oxygen bond (- Si-O-Si -) as the skeleton is the most widely studied and widely used one among the organosilicon compounds. The basic structural unit of organic silica gel products is composed of silicon oxygen links, and the side chain is connected with other organic groups through silicon atoms. Therefore, the structure of silicone products contains both "organic group" and "inorganic structure". This special composition and molecular structure make it integrate the characteristics of organic matter and the functions of inorganic matter.

Preparation and properties of silicone resin

Silicone is mainly divided into silicone rubber, silicone resin, silicone oil and silicone coupling agent.

Silicone rubber is mainly divided into room temperature vulcanized silicone rubber, high temperature vulcanized silicone rubber, and liquid silicone rubber. The main chain of silicone rubber is formed by silicon and oxygen atoms, and the side chain is carbon containing group. The largest amount of silicone rubber is ethylene side chain. It is heat-resistant and cold resistant. The service temperature is between 100 ℃ and 300 ℃. It has excellent weather resistance and ozone resistance for good insulation. The disadvantages are low strength, poor tear resistance and poor wear resistance.

Silicone resin is a kind of semi inorganic polymer with silicon oxygen silicon as main chain and organic groups connected on silicon atoms. Polysiloxane with highly crosslinked network structure is a highly branched polymer (compared with linear silicone oil), which can be solidified into solid. It has the dual characteristics of organic resin and inorganic materials, and has unique physical and chemical properties. The properties of the final products depend on the amount of organic groups (i.e. the ratio of R to SI).

The monomer for preparing silicone resin is chlorosilane, which can be alcoholized to obtain corresponding alkoxysilane. They are widely used as monomers due to their non corrosiveness, higher hydrolytic stability, easier preservation and separation than corresponding chlorosilanes. By changing the number of functional groups in monomers and choosing different substituents, polymers with different degrees of polymerization, branching and crosslinking can be prepared, and products with different properties can be obtained to adapt to different uses.

The number of functional groups in monomers can be expressed by the ratio R / Si of the mixture of monomers (R is the number of substituents and Si is the number of silicon atoms). Generally, the ratio of R to Si in the molecular composition of practical silicone resin ranges from 1.2 to 1.6. The general rule is that the lower the R: Si value is, the more the silicone resin can be cured at lower temperature; the higher the R: Si value is, the silicone resin needs to be cured at 200 ℃ ~ 250 ℃ for a long time, and the hardness of the film is poor, but the thermal elasticity is much better than the former. Silicone resin also has excellent properties of moisture resistance, water resistance, rust prevention, cold resistance, ozone resistance and weather resistance. It has good corrosion resistance to most water containing chemical reagents such as dilute mineral acids, but poor solvent resistance.

Take the hydrolysis condensation of methylchlorosilane as an example: when R / Si > 2, the mixture of (CH3) 2sicl2 and (CH3) 3sicl is used to hydrolyze to form a low molecular weight oily polymer, namely silicone oil. When R / Si = 2, pure (CH3) 2sicl2 is used to hydrolyze and condense to form linear polymer with high molecular weight, which is the base material of silicone rubber adhesive, also known as raw silicone rubber. When R / Si is less than 2, (CH3) 2sicl2, CH3SiCl3, or CH3SiCl3, (CH3) 2sicl2 and SiCl4 are used to hydrolyze and polycondensate to form polymer with network structure, i.e. silicone resin. Different properties of silicon containing polymer can be obtained by changing the R / Si value appropriately. When the R / Si ratio is small, that is, when the ratio of three or four functional silicon oxygen units is high, the crosslinking degree is high after curing, and the silicone resin is hard and brittle; while when the R / Si ratio is large, that is, the ratio of two functional silicon oxygen units is high, the flexibility of the cured silicone resin is good, and the R / Si ratio is usually between 1.2 and 1.5.