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Nov 27, 2020
Silica gel (Silica gel; Silica) Alias: Silicone rubber is a kind of highly active adsorption material, which is an amorphous substance, and its chemical formula is mSiO2.nH2O. It is insoluble in water and any solvents, non-toxic, tasteless, chemically stable, and does not react with any substances except strong alkalis and hydrofluoric acid.
According to its properties and components, silica gel can be divided into organic silica gel and inorganic silica gel.Organic silica gel is an organosilicon compound, which refers to a compound that contains Si-C bonds and at least one organic group is directly connected to a silicon atom. It is customary to combine those organic groups with oxygen, sulfur, nitrogen, etc. Compounds in which silicon atoms are connected are also regarded as organosilicon compounds. Among them, polysiloxanes with silicon-oxygen bonds (-Si-0O-Si-) as the skeleton are the most numerous, the most researched and the most widely used type of organosilicon compounds, accounting for about 90% of the total amount. the above. One type used for packaging.The basic structural unit of silicone products is composed of silicon-oxygen chain links, and the side chains are connected to various other organic groups through silicon atoms. Therefore, the structure of organic silicon products contains both "organic groups" and "inorganic structures". This special composition and molecular structure make it integrate the characteristics of organic substances and the functions of inorganic substances.
Silicones are mainly divided into three categories: silicone rubber, silicone resin, and silicone oil. Silicone rubber is mainly divided into room temperature vulcanized silicone rubber, high temperature vulcanized silicone rubber,
Silicone rubber is composed of silicon and oxygen atoms to form the main chain, the side chain is a carbon-containing group, and the largest amount is the side chain of ethylene. It is heat-resistant and cold-resistant. The use temperature is between 100-300"C. It has excellent weather resistance and ozone resistance for good insulation. The disadvantages are low strength, poor tear resistance, and wear resistance. Performance is also poor.Silicone resin is a cross-linked semi-inorganic polymer with silicon-oxygen-silicon as the main chain and organic groups connected to the silicon atoms. The polyorganosiloxane with a highly cross-linked network structure is a highly branched polymer (compared to linear silicone oil) that can be cured into a solid. It has the dual characteristics of organic resin and inorganic materials, and has unique physical and chemical properties.
The performance of the final processed silicone resin product depends on the number of organic groups contained (ie the ratio of R to Si). Generally, silicone resins with practical value have a ratio of R to Si in the molecular composition between 1.2 and 1.6. The general rule is that the smaller the value of R: Si, the more the obtained silicone resin can be cured at a lower temperature; the larger the value of R: Si, the greater the value of R: Si, the higher the temperature of the obtained silicone resin is to cure it at 200 material 250C If the paint film is baked for a long time at a high temperature, the hardness of the resulting paint film is poor, but the thermal elasticity is much better than the former. Silicone resin also has excellent moisture resistance, water resistance, rust resistance, cold resistance, ozone resistance and weather resistance. It has good corrosion resistance to most water-containing chemical reagents such as dilute acid, but has poor solvent resistance.
The monomer for preparing the silicone resin is chlorosilane, and these chlorosilanes can all be alcoholyzed to obtain the corresponding alkoxysilane. Because they are not corrosive, and have greater hydrolytic stability, easy to store, and easy to separate than the corresponding chlorosilanes, they are widely used monomer components. By changing the number of functional groups in the monomer 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 suit different uses.
Oily polymer with low molecular weight, namely silicone oil. When R/Si=2, pure (CH3) 2SiCI2 is used to hydrolyze and condense to produce high molecular weight linear polymer, which is the base material of silicone rubber adhesive mentioned later, also known as raw silicone rubber. When R/Si<2, use (CH3)2SiCl2, CH3SiCI3 for co-hydrolysis. Or CH3SiCI3, (CH3)2SiCI2 and siCl4 are co-hydrolyzed and polycondensed to form a polymer with a network structure, that is, silicone resin. By appropriately changing the value of R/Si, silicon-containing polymers with different properties can be obtained. R/Si is small, that is, when the ratio of trifunctional or tetrafunctional silicon-oxygen units is high, after curing, the degree of crosslinking is high, and the silicone resin is hard and brittle; while R/Si is large, that is, the ratio of bifunctional silicon-oxane units is high After curing, the flexibility of silicone resin is good, and R/Si is usually used between 1.2-1.5.
Silicone resin is different from silicone oil and silicone rubber, the glass transition temperature of the cured silicone resin (Tg) >200C, while typical silicone rubber Tg<-60C. Methyl silicone resin has the lowest carbon content, it has high heat resistance, the methyl group connected to the silicon atom has the smallest steric hindrance, the resin has a high degree of crosslinking, high hardness, and low thermoplasticity. It is used as a waterproof and moisture-proof The cement is ideal. However, pure methyl silicone resin has poor compatibility with pigments, etc., and has low thermal elasticity. The introduction of phenyl groups into polymethylsiloxane can improve the thermal elasticity of the product, the miscibility of the pigment, and the adhesion to various substances. , Can also improve its thermal stability. The performance of methyl phenyl silicone resin mainly depends on two factors:-is the R/Si mentioned above, and is also closely related to the ratio of methyl and phenyl (CH3/C6H5).
It can be seen from the above three silicone resins: the type of organic groups connected by silicon atoms have a great influence on the performance of the resin. When it is a methyl group, it can give the silicone resin thermal stability, mold release, hydrophobicity, and arc resistance; When it is phenyl, it endows the resin with oxidation stability, which can destroy the crystallinity of polymer within a certain range; when it is vinyl, it can improve the curing characteristics of silicone resin and bring coupling; it is tetrachloro When it is phenyl, it can improve the lubricity of the polymer; when it is phenylethyl, it can improve the miscibility of silicone resin with organics; when it is aminopropyl, it can improve the water solubility of the polymer and bring about coupling properties. ; When it is pentyl, it can improve the hydrophobicity of silicone resin. Therefore, different organic groups can be introduced during the preparation of the silicone resin. In order to improve the adhesiveness of the silicone resin, the silicone resin can be copolymerized with polyester, epoxy, phenolic aldehyde, etc. The silicone-polyester copolymer can be combined with a hydroxyl-containing polyester and an alkoxy-containing silane (or siloxane). ), or by condensation reaction with silane (or siloxane) containing silicon hydroxyl group.
There are many ways to synthesize organosilicon-epoxy copolymers, but commercial epoxy resins are mostly used in the industry. According to different requirements, select appropriate varieties and low molecular weights containing alkoxy or hydroxyl The silicone resin undergoes a co-condensation reaction to obtain a copolymer. :
The organosilicon-phenolic copolymer can also be prepared by co-condensation of soluble silicone resin and phenolic resin, or can be prepared by co-condensation of organic acetyl silane and low molecular weight phenolic resin.
The difference:
Silicone rubber is a linear structure of molecular reinforcement after vulcanization and cross-linking to form a solid structure. When it contains a small amount of terminal hydroxyl groups, the anti-aging performance is not good enough. The light transmittance of white carbon black reinforcement is a little worse. The light transmittance of silicone resin reinforcement Better; mechanical properties-generally better than silicone resin; large elasticity, soft, good heat and weather resistance, small internal stress, the disadvantage is that the gap between molecular chains is large, air permeability, oxygen permeability, and moisture permeability. Bonding strength-general temperature resistance -50- -200 range.Silicone resin is a three-dimensional structure molecule cross-linked into a larger three-dimensional structure. When it contains a small amount of terminal hydroxyl groups, it is much better against aging than silicone rubber. It has high hardness, good light permeability, air permeability and moisture permeability, and low oxygen permeability. Its mechanical properties are generally worse than silicone rubber.Silicone resin has outstanding weather resistance, which is unmatched by any organic resin. Even under strong ultraviolet radiation, silicone resin is resistant to yellowing.