3.2 High degree of crosslinking methyl silicone resin
Typical silicone resins are bulk silicone polymers with reactive reactive groups. In order to improve the hardness of the cured silicone resin, the following two methods are generally used: 1 to improve the functionality of the silicone resin, that is, to reduce the R/Si value of the silicone component; 2 the side chain groups of the silicone polymer are all the shortest chain organic The group - methyl group, avoids the use of side chain groups such as phenyl.
The synthetic route of methyl silicone resin has been mentioned before, and high-hardness silicone resin is prepared, that is, methyl silicone resin with high crosslinking degree or super high crosslinking degree is synthesized, and it is not suitable to avoid cross-linking gel in the hydrolysis process. The use of methylchlorosilane as a starting material can only be started from alkoxysilanes. In order to achieve high-crosslinking degree silicone resin which can be stably stored, in addition to the raw material monomer, it is necessary to pre-acid removal treatment. In the raw material component of low R/Si methyl silicone resin, the ratio of 3 and 4 functional components is larger. Even if the bifunctional component is not used at all, the reactivity is relatively higher, so a mild reaction catalyst should be selected for the hydrolysis condensation reaction.
The synthesis of methyl silicone resin with high degree of crosslinking, especially ultra-high crosslinking degree, is difficult to control the synthesis reaction process. 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. It is often found that there are jelly-like gel points, which are often the active centers that cause the silicone resin solution to continue to condense and crosslink during storage; the gel particles generated during the synthesis process not only damage the storage of the silicone resin solution. Stability, when applied in coating, these gel particles may also form "fish eyes" or "crystal spots" in the coating, resulting in optical distortion of the silicone coating film and reducing light transmittance, etc., which seriously damage the silicone resin. Use performance.
In order to produce a storage-stable ultra-high cross-linking degree methyl silicone resin, such a gel center must be avoided. The key technology lies in the effective control of the smooth and balanced hydrolysis and condensation reaction. The feasible technical means is to use a mild catalyst and a mildly controllable synthesis process to regulate the progress of the hydrolysis and condensation reaction of the organosilicon monomer, so as to avoid excessive pre-emption of the tetrafunctional monomer. "Hydrolysis and "alone condensation" to achieve a stable polymer that prevents the formation of a gel center and gives a predetermined composition. For example, according to the composition of [CH3] / [Si] = 0.85, using methyl triethoxysilane and ethyl orthosilicate as the main raw materials, using a weak acid of appropriate concentration as a catalyst, using a gentle stepwise hydrolysis process, A silicone prepolymer having good storage stability is obtained. The methyl silicone resin obtained by the above process is added with a composite solvent, combined with a suitable curing agent, and the cured silicone resin coating is uniform and highly transparent, and the coating film adheres firmly to the substrate, and the hardness of the coated pencil after curing is reached. 6H.
3.3 Ultra-low carbon content silicone resin soluble in non-polar solvents
In aerospace engineering, the electromagnetic wave communication components of space-running devices need to be protected by high-temperature and high-transmission silica materials, and silica is very easy to damage the wave-transparent performance due to moisture absorption. The silica component is moisture-proof, and the silicone resin is resistant to high temperature, moisture and moisture, and has excellent electromagnetic wave transmission performance, and is an ideal high-wave protection material. For the return spacecraft, when the spacecraft re-enters the atmosphere, the high temperature generated by the high-speed airflow friction will cause the organic matter to ablate and carbonize, and the generated carbon will seriously hinder the electromagnetic wave transmission. Therefore, the moisture-proof protection of the electromagnetic wave transparent window of the return spacecraft It requires high temperature resistance and moisture resistance, and requires the carbon content of the protective material to be as low as possible.
The hydrophobic and moisture-proof function of the silicone resin benefits from the hydrophobic action of the organic groups in the silicone polymer, but the organic group is the source of ablation into carbon. In order to obtain a high-transparent silicone resin, it is necessary to Under the premise of ensuring that the material is sufficiently hydrophobic and moisture-proof, the content of organic groups is minimized, that is, the chemical composition of relatively low R/Si is selected, and the hydrophobic silicone chain without carbon or low carbon is also included. As a component of the silicone resin, it achieves excellent performance in maintaining high wave transmission both before and after heat resistance, moisture resistance, and ablation.
The synthesis of ultra-low R/Si methyl silicone resin is based on methyldiethoxysilane, methyltriethoxysilane and ethyl orthosilicate, and the methyl silicone resin is obtained by hydrolysis condensation. Solution. Ethanol is a polar solvent. Either the ethanol produced by the hydrolysis of the alkoxysilane of the reaction raw material or the added solvent ethanol must contain a trace amount of water, and the trace amount of water which cannot be excluded in the polar solvent will seriously damage the two as the wave-transparent material. The dielectric properties of silicon oxide, for which high-wavelength silicone resins must use non-polar solvents.
As mentioned above, the hydrolysis condensation reaction of the ultra-low R/Si methyl silicone resin is extremely sensitive and rapid. For example, the operation of removing ethanol by distillation and then converting into a non-polar solvent will inevitably lead to the early crosslinking of the silicone resin. Moreover, the ultra-low R/Si methyl silicone resin prepared by a conventional process is not dissolved in a non-polar solvent. In order to solve this critical problem, special technical measures are needed. In the initial stage of hydrolysis of the alkoxysilane monomer, it is necessary to try to create a "specific reaction environment", and the hydrolyzate "in situ" has a non-polar solvent. Solubility in the process, and in the subsequent condensation reaction process, the solubility of the reaction product in a non-polar solvent is continuously controlled. Under this premise, the by-product ethanol is gradually removed, and at the same time The polar solvent is replaced by an ultra-low carbon content silicone resin which is soluble in a non-polar solvent by such a complicated special synthesis process.
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Zhejiang Runhe Organosilicone New Material Co.,Ltd(Factory)
Zhejiang Runhe Chemical New Material Co.,Ltd(Exporter)
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