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Polymer structure characteristics, properties and applications of silicone resin part Ⅲ

Nov 26, 2019

3.2 Highly cross-linked methyl silicone resin


Typical silicone resins are bulky silicone polymers with reactive groups. In order to improve the hardness of the cured silicone resin, the following two methods are commonly used: ① increase the functionality of the silicone polymer, that is, reduce the R / Si value of the silicone resin component; ② all the side chain groups of the silicone resin polymer adopt the shortest chain organic Group-methyl, and avoid the use of side chain groups such as phenyl.


The synthetic process route of methyl silicone resin has already been mentioned. It is necessary to prepare high hardness silicone resin, that is, to synthesize methyl silicone resin with high cross-linking degree or ultra-high cross-linking degree. In order to avoid cross-linked gel during hydrolysis, it is not suitable. Using methylchlorosilane as the raw material, only starting from alkoxysilane. In order to achieve the synthesis of high-crosslinking silicone resins that can be stored stably, in addition to the raw material monomers, in addition to acid treatment, the ratio of the 3 and 4 functional components in the raw material components of the low R / Si methyl silicone resin is relatively large. Even without bifunctional components, the reaction activity is relatively higher, so a mild reaction catalyst should be used for the hydrolysis and condensation reaction.


It is difficult to synthesize methyl silicone resin with high cross-linking degree, especially ultra-high cross-linking degree. The main problem is that the gel component is easily formed in the silicone resin solution obtained by the synthesis reaction, that is, the silicone resin solution. Jelly-like gel particles are often found in these gel particles. These gel particles are often active centers that cause the silicone resin solution to continue to condense and crosslink during the storage period. The gel particles generated during the synthesis process not only damage the storage of the silicone resin solution. Stability. In coating applications, these gel particles may also form "fish eyes" or "crystal points" in the coating, causing defects such as optical distortion of the silicone coating film and reduction of light transmittance, which seriously damage the silicone resin. Use performance.


To obtain storage-stable, ultra-high-crosslinking methyl silicone resins, such gel centers must be avoided. The key to its technology is to effectively control the smooth and balanced progress of the hydrolysis and condensation reaction. The feasible technical means is to use a mild catalyst and a mildly controllable synthesis process to regulate the hydrolysis and condensation reaction process of the silicone monomer, so as to avoid excessive "preemption" of the 4-functional monomer "Hydrolyzed and" condensed on its own ", thereby achieving a stable polymer that prevents the formation of a gel center and obtains a predetermined composition structure. For example, according to the composition of [CH3] / [Si] = 0.85, using methyltriethoxysilane and ethyl orthosilicate as the main raw materials, using a mild acid of a suitable concentration as a catalyst, and using a mild stepwise hydrolysis process, A silicone resin prepolymer having good storage stability is obtained. The methyl silicone resin prepared by the above process is added with a compound solvent, a suitable curing agent, and a cured silicone resin coating. The coating film is uniform and highly transparent. The coating film is firmly adhered to the substrate. After curing, the pencil hardness of the coating film is measured. 6H.


3.3 Ultra-low carbon content silicone resin soluble in non-polar solvents


In aerospace engineering, the electromagnetic wave communication components of space-operating devices need to be protected by high temperature and high transmission silicon dioxide materials. Silicon dioxide can easily damage the wave transmission performance due to moisture absorption. Therefore, it is necessary to apply hydrophobic and moisture-proof materials. Silicone parts are subjected to moisture-proof treatment. Silicone resin is not only resistant to high temperatures, water and moisture, but also has excellent electromagnetic wave transmission properties. It is an ideal high-wave transmission protection material. For the return spacecraft, when the spacecraft returns to the atmosphere, the high temperature caused by the friction of high-speed airflow will ablate and carbonize the organic matter, and the generated carbon will seriously hinder the transmission of electromagnetic waves. Therefore, the transparent window ’s electromagnetic wave is protected from moisture. , Not only requires high temperature resistance and hydrophobicity and moisture, but also requires that the carbon content of the protective material is as low as possible.


The hydrophobic and moisture-proof functions of silicone resins benefit from the hydrophobic effect of organic groups in silicone polymers, but organic groups are the source of ablation into carbon. In order to obtain highly permeable silicone resins, On the premise of ensuring sufficient hydrophobic and moisture-proof properties of the material, the content of organic groups should be reduced as much as possible, that is, a relatively low R / Si chemical component is selected. In addition, it also includes the use of carbon-free or low-carbon-containing hydrophobic silicone links. As a component of the silicone resin, it achieves the excellent performance of taking into account heat resistance, hydrophobicity, moisture resistance, and maintaining high wave transmission before and after ablation.


The synthesis of ultra-low R / Si methyl silicone resin requires methyldiethoxysilane, methyltriethoxysilane and ethyl orthosilicate as starting materials, and the methyl alcohol is obtained by hydrolysis and condensation. Solution. Ethanol is a polar solvent. Whether it is the ethanol produced by the hydrolysis of the alkoxysilane or the additional solvent ethanol, it must contain a trace amount of water. The trace amount of water that cannot be ruled out in polar solvents will seriously damage the second wave as a transparent material. For the dielectric properties of silicon oxide, non-polar solvents must be used for high-wave-transmitting silicone resins.


As mentioned earlier, the hydrolysis and condensation reaction of ultra-low R / Si methyl silicone resin is extremely sensitive and fast. If ethanol is removed by distillation and then converted to a non-polar solvent, the silicone resin will be crosslinked and scrapped in advance. Moreover, the ultra-low R / Si methyl silicone resin prepared by conventional processes is not soluble in non-polar solvents. In order to solve this key problem, special technical measures need to be taken. In the initial stage of alkoxysilane monomer hydrolysis, it is necessary to try to create "a specific reaction environment". Solubility in the reaction, and in the subsequent condensation reaction process, it is necessary to continuously control the solubility of the reaction product in a non-polar solvent. Under this prerequisite, the reaction by-product ethanol is gradually removed, and The polar solvent is replaced, and through such a complicated and special synthetic process, an ultra-low carbon content silicone resin that can be dissolved in a non-polar solvent is prepared.


RH-SH305-High-Reactivity-Transparent-Liquid-Phenyl



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