The monomers for the preparation of silicone resins are chlorosilanes, which can be alcoholysed to give the corresponding alkoxysilanes. Because they are not corrosive, and have greater hydrolytic stability than the corresponding chlorosilanes, easy to preserve, easy to separate, are widely used monomer components. Changing the number of functional groups in the monomer and selecting different substituents can produce polymers with different degrees of polymerization, branching and crosslinking, and obtain products with different properties to adapt to different uses. The number of functional groups in the monomer can be expressed by the ratio R/Si of R to Si of the monomer mixture (R is the number of substituents and Si is the number of silicon atoms). Take the hydrolysis condensation of methyl chlorosilane as an example: when R/Si>2, (CH3)2SiCl2 and (CH3)3SiCl are mixed and hydrolyzed to produce oily polymers with lower molecular weight, namely silicone oil. When R/Si = 2, pure (CH3)2SiCl2 is used for hydrolysis and condensation to generate high molecular weight linear polymer, which is the base material of silicone rubber adhesive, also known as silicone rubber. When R/Si<2, (CH3)2SiCl2 is used to CH3SiCl3 co-hydrolysis. Or CH3SiCl3,(CH3)2SiCl2 and SiCl4 co-hydrolysis polycondensation, the formation of the network structure of the polymer, that is, silicone resin, appropriate change the value of R/Si, you can get different properties of silicon-containing polymers. R/Si is small, that is, when the proportion of trifunctional or tetrafunctional silicon-oxygen units is high, after curing, the crosslinking degree is high, and the silicone resin is hard and brittle; while R/Si is large, that is, the proportion of bifunctional silicon-oxygen units is high, and the flexibility of silicone resin after curing is good, and the R/Si usually used is between 1.2 and 1.5.
Silicone resins differ from silicone oils and silicone rubbers in that cured silicone resins have a glass transition temperature (Tg)>200°C, while typical silicone rubbers have a Tg<-60°C. 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 high crosslinking degree, high hardness and small thermoplasticity, and is ideal as a waterproof and moisture-proof adhesive. However, the compatibility of pure methyl silicone resin with pigments is poor, and the thermal elasticity is small. The introduction of phenyl in polymethyl siloxane can improve the thermal elasticity of the product and the miscibility of pigments, as well as the adhesion of various substances, and can also improve its thermal stability. The performance of methyl phenyl silicone resin mainly depends on two factors: one is the R/Si mentioned above, and it is also closely related to the ratio of methyl and phenyl (CH3/C6H5). The influence of phenyl content on the performance of silicone resin is shown in the following table:
Effect of Phenyl Content on Properties of Silicone Resin
Phenyl content (%)
condensation speed
film hardness
curing performance
　　0 20 40 60 80 100
Fast ---------- Slow
Hard ---------- soft
Thermosetting ------- Thermoplastic
The methyl groups in methyl silicone resin are all substituted by phenyl groups to give phenyl silicone resin, which is made by hydrolysis and polycondensation of phenyl trichloro (or trialkoxy) silane. The polymer with phenyl silsesquioxane as the structural unit, which is obtained by hydrolysis of phenyl trichlorosilane under specific conditions and then by equilibrium rearrangement reaction, is referred to as silicon ladder polymer, which is a ladder polymer with good comprehensive properties.
Silicon ladder polymer can be dissolved in benzene, tetrahydrofuran, dichloromethane and other solvents, and can be cast into colorless, transparent and tough films. The polymer has better wet and heat resistance than ordinary silicone resin. The thermal aging performance in steam is almost the same as that in air. The tensile strength is about twice that of ordinary silicone resin, reaching 27.5-41.2MPa. The most outstanding performance of silicon ladder polymer is its heat resistance, when the air is heated to 525 ℃, it begins to lose weight. The chemical structural defects of this ladder polymer, such as uncondensed hydroxyl groups, branching, crosslinking, etc., have a great influence on the performance.
It can be seen from the above three silicone resins that the type of organic group connected by silicon atoms has a great influence on the performance of the resin. When it is methyl, it can endow the silicone resin with thermal stability, mold release, hydrophobicity and arc resistance; When it is phenyl, it can endow the resin with oxidative stability, which can destroy the crystallinity of the polymer within a certain range; when it is vinyl, it can improve the curing characteristics of the silicone resin and bring coupling; when it is tetrachlorophenyl, it can improve the lubricity of the polymer; when it is phenylethyl, it can improve the miscibility of silicone resin and organic matter; when it is aminopropyl, it can improve the water solubility of the polymer and bring coupling at the same time; when it is pentyl, it can improve the hydrophobicity of silicone resin. Thus, 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 may be copolymerized with polyester, epoxy, phenolic, or the like, and the silicone-polyester copolymer may be prepared by condensation reaction of a hydroxyl-containing polyester with an alkoxy-containing silane (or siloxane), or with a hydroxyl-containing silane (or siloxane).
There are many ways to synthesize silicone-epoxy copolymer, but the industrial use of more commercial epoxy resin as raw material, according to different requirements, select the appropriate varieties and containing alkoxy or hydroxyl group of low molecular weight silicone resin co-condensation reaction to produce copolymers.
Silicone-phenolic copolymers can also be prepared by copolycondensation of soluble silicone resins with phenolic resins, or by copolycondensation of organoacetyl silanes with low molecular weight phenolic resins.