silicone refers to a polymer whose organic group is directly connected to silicon, and mainly includes three categories: silicone oil, silicone rubber, and silicone resin. As a derivative product of silicone, what is the structure of silicone resin different from silicone?
Organosilicon, that is, organosilicon compound, refers to a compound containing Si-C bond, and at least one organic group is directly connected to the silicon atom, and it is customary to often use oxygen, sulfur, nitrogen, etc. to make the organic group and silicon atom The connected compounds are also regarded as organosilicon compounds. Among them, the polysiloxane composed of silicon-oxygen bond (-Si-0-Si-) as the skeleton is the most widely studied, most widely used and most widely used organosilicon compound, accounting for more than 90% of the total dosage .
Silicone materials have a unique structure:
(1) Sufficient methyl groups on the Si atom shield the high-energy polysiloxane backbone;
(2) C-H has no polarity, which makes the interaction between molecules very weak;
(3) The length of the Si-O bond is long, and the bond angle of the Si-O-Si bond is large.
(4) Si-O bond is a covalent bond with 50% ionic bond characteristics (covalent bond has directionality, ionic bond has no directionality).
2. Silicone resin
Silicone resin is a highly cross-linked network structure of polyorganosiloxane, usually with methyltrichlorosilane, dimethyldichlorosilane, phenyltrichlorosilane, diphenyldichlorosilane or
Various mixtures of methylphenyl dichlorosilane are decomposed by adding water at a relatively low temperature in the presence of an organic solvent such as toluene to obtain an acid hydrolysate. The initial hydrolysis product is a mixture of cyclic, linear and cross-linked polymers,
Usually also contains quite a lot of hydroxyl groups. The hydrolysate is washed with water to remove the acid, the neutral primary polycondensate is thermally oxidized in the air or further polycondensed in the presence of a catalyst, and finally forms a highly cross-linked three-dimensional network structure.
Curing is usually achieved by condensation of silanol to form silicone links. When the condensation reaction is in progress, the silanol concentration gradually decreases, which increases the steric hindrance and the fluidity is poor, resulting in a decrease in the reaction rate. Therefore, to fully cure the resin, the reaction must be accelerated by heating and adding a catalyst. Many substances can catalyze the condensation reaction of silanol. They include acids and bases, soluble organic salts of lead, cobalt, tin, iron and other metals. Organic compounds such as dibutyltin dilaurate or N, N, N ', N'-tetramethylguanidine salt, etc.
The performance of the final processed product of silicone resin depends on the number of organic groups contained (that is, the ratio of R to Si). Generally useful silicone resin, the ratio of R to Si in the molecular composition is between 1.2 ~ 1.6. The general rule is that the smaller the value of R: Si, the more silicone resin can be cured at a lower temperature; the larger the value of R: Si, the silicone resin needs to be 200 ~ 250 to cure it Baking at a high temperature of ℃ for a long time, the resulting film hardness is poor, but the thermal elasticity is much better than the former.
In addition, the ratio of the methyl group to the phenyl group in the organic group also has a great influence on the performance of the silicone resin. The lower the phenyl content in the organic group, the softer the resulting paint film and the faster the condensation, and the higher the phenyl content, the harder the resulting paint film and the more thermoplastic it is. The phenyl content is between 20 and 60%, and the paint film has the best bending resistance and heat resistance. In addition, the introduction of phenyl can improve the compatibility of silicone resins with pigments, as well as the compatibility of silicone resins with other silicone resins and the adhesion of silicone resins to various substrates.
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