Basic Information of Silicone Resin

Oct 28, 2020

Silicone resin is a highly cross-linked polyorganosiloxane with a network structure, usually methyltrichlorosilane, dimethyldichlorosilane, phenyltrichlorosilane, diphenyldichlorosilane or methylbenzene Various mixtures of dichlorosilanes, in the presence of organic solvents such as toluene, are hydrolyzed at lower temperatures to obtain acid hydrolysates. The initial product of the hydrolysis is a mixture of cyclic, linear and cross-linked polymers, usually containing a considerable number of hydroxyl groups. The hydrolyzate is washed with water to remove the acid, and 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.
Silicone resin and modified silicone resin products are widely used in the national defense, military, and electrical industries due to their excellent thermal oxidation stability, electrical insulation properties, weather resistance, waterproof, salt spray resistance, mold resistance, and biocompatibility. , Leather industry, light industrial products, rubber and plastics, food hygiene and other industries, playing an irreplaceable role. my country's organic silicon industry has developed from the early 1950s to the present, and has made great progress in material properties, mechanisms and applications. Compared with advanced countries, my country's technological gap is relatively small, but the gap in application is relatively large. With the continuous increase in the demand for high temperature resistant materials, as a class of materials with outstanding characteristics, silicone polymers can be modified and matched with organic resins and inorganic materials to realize the integration of structure and function, and have application prospects in high-tech industries and cutting-edge fields. Very broad
Composition structure
Curing is usually achieved by condensation of silanol to form siloxy chain links. When the condensation reaction is proceeding, the silanol concentration gradually decreases, increasing steric hindrance and poor fluidity, resulting in a decrease in the reaction rate. Therefore, to make the resin fully cured, it must be heated and a catalyst added to accelerate the reaction. Many substances can play a catalytic role in the silanol condensation reaction, including acids and bases, soluble organic salts of lead, cobalt, tin, iron and other metals, and organic compounds such as dibutyltin dilaurate.
The performance of the final processed silicone resin product depends on the number of organic groups contained (that is, 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 value of the obtained silicone resin needs to be 200~250. Bake for a long time at a high temperature of ℃, the resulting paint film has poor hardness, but the thermal elasticity is much better than the former.
In addition, the ratio of methyl to phenyl groups in the organic groups 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, the faster the condensation, the higher the phenyl content, the harder the resulting paint film, the more thermoplastic. The phenyl content is between 20 to 60%, and the paint film has the best bending resistance and heat resistance. In addition, the introduction of phenyl groups 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.
Featuresedit
(1) One of the most outstanding properties of silicone resin is excellent thermal oxidation stability. After heating at 250°C for 24 hours, the weight loss of silicone is only 2%-8%, polycarbonate is 55.5%, polystyrene is 65.6%, and epoxy resin is 22.7%; heated at 350°C for 24 hours Later, the weight loss of organic resins is generally 70% to 99%, while the weight loss of silicone resins is less than 20% [2].
(2) Another outstanding performance of silicone resin is excellent electrical insulation performance, which can maintain good insulation performance in a wide temperature and frequency range. Generally, the electrical breakdown strength of silicone resin is 50kV mm, the volume resistivity is 1013～1015Ψ·cm, the dielectric constant is 3, and the dielectric loss tangent is about 10-3.
(3) Silicone resin also has outstanding weather resistance, and is resistant to yellowing even under strong ultraviolet radiation, which is unmatched by any organic resin. In addition, silicone resin also has the characteristics of waterproof, salt spray, and mildew resistance.
Type and performanceedit
According to the different organic substituents on the silicon atoms in the silicon-oxygen chain, silicone resins can be basically divided into three categories: polyalkyl silicone resins, polyaryl silicone resins and polyalkylaryl silicone resins [3 ].
Polyalkyl silicone resin
Polymethyl silicone resin
Polymethyl silicone resin is generally a copolymer composed of siloxane chains such as SiO3/2, CH3SiO3/2, (CH3)3SiO2/2, (CH)SiO1/2, etc. Using raw materials with only two or less methyl groups attached to each silicon atom (such as methyltrichlorosilane, dimethyldichlorosilane), a polymethyl silicone resin with a network structure can be prepared, which can be transformed into Insoluble insoluble product. Polymethyl silicone resin has high heat resistance and strong oxidation resistance. The methyl silicone resin is made into a sheet sample, heated to 550°C in a vacuum or heated to 500°C in a hydrogen stream, it will not be destroyed, and will remain infusible for a long time; the made mica tablet is heated to There is almost no smoke at 600～700℃. The resin plastic sheet was heated at 200°C for one year without damage. When the temperature exceeds 300°C, its surface is slowly oxidized by oxygen in the air. When the maximum working temperature is exceeded, the resin will not crack into carbon, and its surface will be oxidized to silicic anhydride. The resin also has very high electrical properties, and it is not sensitive to water, and its properties change little after being boiled in water at 100°C for 30 minutes.
Polyethyl silicone resin
Polyethyl silicone resin is a copolymer containing ethyl groups in the siloxane chain. The polymerization speed of polyethyl silicone resin is slower than that of polymethyl silicone resin, but the ethyl group connected with silicon atoms in the siloxane chain can increase the solubility of the resin and reduce its hardness. In order to prepare insoluble and infusible polyethyl silicone resin, the ratio of the number of ethyl groups to the number of silicon atoms in the polymer is preferably 0.5 to 1.5. When this value is less than 0.5, the resin polymerizes too fast, and a larger amount of water is produced during the condensation process, which makes the resin brittle and not strong, and it is easy to crack at high temperature; when this value is 0.5 to 1, the condensation product The flexibility and elasticity of the polymer are increased. When this value is about 1, the product formed has good elasticity and can form a paint film with adhesion ability; after this value continues to increase to 1.5, the content of low molecular products in the polymer It is more difficult to condense into a solid; the condensation product with this value of 2 is already a typical elastomer. Polyethyl silicone resin is easier to intermix and copolymerize with polyester, polyacetal and other organic polymers than polymethyl silicone resin.
Polyaryl silicone resin
Polyaryl silicone resin is a copolymer containing only phenyl groups in the siloxane chain, and has excellent properties such as high heat resistance and strong oxidation resistance. Heat the polyaryl silicone resin plastic sheet in the air to 400℃ or 500℃, and the phenyl group will not fall off the silicon after several hours; heat it at 400℃ for a longer time or place the polymer in the sealing The phenyl group can only fall off by heating with dilute acid or bromine water in the sealed tube. Using trifunctional organosilicon monomer (phenyltrichlorosilane), after hydrolysis and rearrangement, a ladder-shaped polymer is formed-all phenyl silicone resin has higher heat resistance than general resin.
Polyalkylaryl silicone resin
The properties of the two types of polyalkyl silicone resins and polyaryl silicone resins can be changed by adding another type of resin to one type of resin to form a polyalkylaryl silicone resin. In fact, it is not a simple mixing, but the alkyl group and the aryl group are directly connected to the same silicon atom during synthesis, or the alkyl group and the aryl chlorosilane are hydrolyzed and co-condensed to form a copolymer. Polyalkylaryl silicone resins have better mechanical properties and hardness than pure alkyl or aryl silicone resins.