Properties of Organic Silicon Coatings
Silicone resin boasts exceptional properties, including heat resistance, cold resistance, moisture resistance, hydrophobicity, and weather resistance. As a result, it is widely utilized in the special coatings industry, particularly as a base for silicone coatings. Organic silicon coatings come in various forms, including high-temperature resistant coatings, weather-resistant coatings, wear-resistant hard coatings, anti-adhesive release coatings, moisture-proof and hydrophobic coatings, and radiation-resistant coatings.
Among heat-resistant coatings, organic silicon and inorganic heat-resistant coatings are the most commonly used. Pure silicone varnish, for example, can withstand temperatures between 200-250°C. This high-temperature resistant coating is formulated with silicone resin as the base material, combined with heat-resistant pigments (such as coloring pigments, additive pigments, metal powder, heat-resistant fillers, glass powder, black iron oxide powder, and oxide labels), solvents, curing agents, and additives. It can endure temperatures of 300-700°C and can be used continuously at 250-400°C while retaining its color and gloss. Organic silicon heat-resistant coatings are known for their excellent heat resistance (typically effective within the 250-400°C range), water resistance, electrical insulation, and good mechanical properties. However, they have drawbacks, such as lower hardness, poor flame resistance, and relatively high cost. Cross-linked silicone resin begins to decompose around 300°C, with decomposition accelerating at higher temperatures (depending on the phenyl/methyl ratio). The final decomposition product is silica, which, although brittle, can still function as a binder for high-temperature resistant pigments.
Inorganic heat-resistant coatings, on the other hand, can withstand even higher temperatures, ranging from 400°C to 1000°C or more. These coatings offer excellent flame resistance and high hardness, though the paint film is brittle and exhibits poor water resistance before full curing. Additionally, inorganic coatings require strict substrate preparation.
The definition of heat resistance in coatings is not universally agreed upon, but it generally refers to coatings that maintain their physical and mechanical properties and protective effects at temperatures above 200°C, with minimal changes in color and gloss, an intact coating, and no cracking. Heat-resistant coatings are widely used in high-temperature environments, such as steel pipes, high-temperature steam pipelines, heat exchangers, high-temperature furnaces, petroleum cracking equipment, high-temperature reaction equipment, engine parts and exhaust pipes, and military equipment. These coatings prevent thermal oxidation and corrosion of steel and other metals at high temperatures, ensuring the long-term durability of equipment. As the aviation and aerospace industries expand, the national economy grows, and awareness of corrosion protection increases, the demand for these coatings is rising, along with their usage.
Heat-resistant coatings are generally classified into two categories: organic heat-resistant coatings and inorganic heat-resistant coatings. By combining the strengths and mitigating the weaknesses of both, organic resin and inorganic coatings are often used together or chemically modified to create hybrid heat-resistant coatings. Silicone resin, when used as a base material for heat-resistant coatings, is not limited to pure silicone resin. Modified silicone resins, combined with organic resins such as phenolic, epoxy, alkyd, polyurethane, and acrylic resins, have also gained widespread use. The organic silicon content in these modified organic resins typically exceeds 50%, significantly enhancing their heat resistance. Consequently, these coatings play a vital role in heat-resistant applications.
In general, although modified resins may not be as heat-resistant as pure silicone resin, they outperform it in terms of solvent resistance, chemical resistance, and adhesion. Moreover, under specific conditions, using silicone resin containing organic resin as the base material can yield superior performance. Attention must be paid to the methyl/phenyl ratio in the organosilicon oligomers used for modified resins. Phenyl groups offer better heat resistance but poorer physical and mechanical properties compared to methyl groups. In modified resins with the same organic silicon content, increasing the methyl/phenyl ratio of the organic silicon oligomers may reduce the coating's thermoplasticity, photopolymerization, color retention, and heat-resistant lubricating properties. However, it will improve the coating's physical and mechanical properties, as well as its resistance to microcracking over time. Therefore, the modified resin and the organosilicon oligomer formulation should be tailored to meet the specific application requirements.
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