As a kind of silicone resin, what are the modified silicone resins? Many people do not know it well, and the application of modified silicone resin is also very wide. Follow this article to learn more about modified silicone resin.
The basic structural unit (the main chain) of traditional silicone polymers is composed of si-0 links, and the side chains are connected to other various organic groups through silicon atoms. Therefore, the structure of silicone products contains both "organic groups" and "inorganic structures". This special composition and molecular structure make it combine the characteristics of organic substances with the functions of inorganic substances, and have high and low temperature resistance , Weathering resistance, electrical insulation, ozone resistance, flame resistance, non-toxic non-corrosion and physiological inertia and many other excellent properties, and some varieties also have oil resistance, solvent resistance, radiation resistance. Compared with other polymer materials, the si-o bond energy (450KJ / MOL) in the silicone resin is much larger than the C-C bond energy (345KJ / mol) and C-O bond energy (351KJ-mol).
Therefore, the outstanding properties of silicone products are excellent temperature resistance, dielectric properties, weather resistance, physiological inertia, and low surface ability. They have been widely used in coatings, adhesives and high temperature resins. However, pure silicone resin generally requires high temperature drying (250-300 degrees), long curing time, inconvenient construction on large areas, poor adhesion and resistance to organic solvents, and high mechanical strength of the paint film at higher temperatures, so there are often Resin was renamed to make up for its shortcomings. In recent years, domestic and foreign modified silicone resins include: epoxy resin, polyester resin, polyurethane resin, acrylic resin, alkyd resin, etc.
Silicone resins are highly crosslinked, network-structured polyorganosiloxanes, typically methyltrichlorosilane, dimethyldichlorosilane, phenyltrichlorosilane, diphenyldichlorosilane, or methylbenzene. Various dichlorosilane mixtures are hydrolyzed at lower temperatures in the presence of organic solvents such as toluene to obtain acidic hydrolysates. The initial product of the hydrolysis is a mixture of cyclic, linear, and crosslinked polymers, and usually also contains a considerable number of hydroxyl groups. The hydrolysate is washed with water to remove the acid, the neutral primary polycondensate is thermally oxidized in the air or is further polycondensed in the presence of a catalyst, and then forms a highly cross-linked three-dimensional network structure.
The requirements for the mechanical properties of silicone resins mainly depend on the application. Silicone resins used as electrical insulating paints, coatings and adhesives are more concerned about their hardness, elasticity, thermoplasticity and adhesion. The hardness and elasticity of the silicone resin paint film can be changed within a wide range by adjusting the molecular structure of the resin. When the tri- or tetra-functional chain content is higher, that is, the higher the crosslinking density, the higher the hardness and the lower the elasticity of the paint film can be obtained; the introduction of a large steric hindrance group can improve flexibility and thermoelasticity. It is the reason why methylphenyl silicone resin is more flexible and thermoplastic than methyl silicone resin. Therefore, silicone resins do not need to use special plasticizers, but rely on the appropriate combination of soft and hard silicone resins to meet the plasticity requirements. When used as some coatings, the hardness of the pure silicone resin coating film is insufficient, while the thermoplasticity is more than enough; if an organically modified silicone resin is used, this contradiction can be easily resolved. Pigments and catalysts can also affect the hardness and elasticity of silicone resins. Pigments have the effect of accelerating the oxidation of silicone paint films and transforming them into harder silica glass. With low-activity catalysts, only soft coatings can be obtained due to the incomplete condensation reaction. Conversely, with high-activity catalysts (such as compounds of Pb, Al, etc.), hard and brittle coatings can be obtained. Acid ester) can effectively improve the hardness of the coating without seriously reducing the elasticity. Silicone resin has good adhesion to iron, aluminum, silver, tin, glass, and ceramics, but it has poor adhesion to copper, especially after high temperature and long-term thermal aging, which may cause oxide films on other surfaces of copper. There is a reason to accelerate the thermal cracking reaction of silicone resin. The adhesion of silicone grease to organic materials such as plastics and silicone rubber depends mainly on the surface energy of the latter and its compatibility with silicone resins. The lower the surface energy and the less compatible the material, the more difficult it is to adhere. By treating the surface of the substrate (including sanding and priming), especially by introducing a tackifier into the silicone resin, the adhesion of the silicone resin to difficult-to-stick substrates can be improved to a certain extent.
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