What are the application areas of silicone resins?

Application of silicone resin in heat-resistant powder coating The use of silicone resin to blend and modify epoxy resin, and the selection of inorganic pigments and fillers used in coatings, significantly improve the heat resistance of powder coatings, so that the coatings can be used for a long time in an environment above 250 ℃ . introduction With the development of the coatings industry, powder coatings have been widely used in the coating of metal substrates. Direction of development. Heat-resistant powder coatings refer to powder coatings that can withstand temperatures above 200 °C for a long time, have a good coating film, and enable the protected object to function normally in a high-temperature environment. In terms of the thermal stability mechanism of polymers, the thermal resistance of polymers mainly depends on their molecular structure. The thermal stability of the polymer can be improved by introducing larger or more polar side groups on the main chain and increasing the intermolecular interaction force. Another way to improve the heat resistance of powder coatings is to add heat-resistant pigments and fillers to the polymer. Commonly used pigments and fillers are aluminum powder, mica powder, stainless steel powder, cadmium powder, silica and so on. 1. Test part 1.1 Preparation of coating film The powder coating was prepared according to the formula, mixed and pulverized, extruded by twin-screw extruder, tableted, pulverized, sieved (180 mesh), electrostatically sprayed onto the sandblasted steel plate substrate, and cured at 200°C/20min. 1.2 Performance test Heat resistance: 300℃ oven; Impact resistance: GB 1732-79; Gloss: GB 1743-79. 2. Results and discussion 2.1 Heat resistance of resin Resin as the main film-forming substance of coatings is the most basic factor determining the heat resistance of coatings. Powder coatings are generally cured at 180~200 ° C for 20 minutes to form a film. They belong to thermosetting coatings. The coating forms a network cross-linked structure, so the heat resistance of thermoplastic coatings is improved to a certain extent. The heat resistance test of epoxy type, epoxy polyester mixed type, polyester/TGIC type powder coatings (Table 2) shows that these coatings can basically be used for a long time under the condition of lower than 150℃. However, in an environment higher than 250 °C, the coating exhibits damage such as loss of gloss, decreased adhesion, coating embrittlement, decreased flexibility, and chalking, which makes the coating lose its protective effect on the substrate.   The results in Table 2 show that the existing general-purpose products have defects in the following aspects: ①The surface of the coating is seriously tarnished; ②The mechanical properties of the coating are obviously deteriorated; ③ The flexibility of the coating decreases significantly; ④The continuous use time of the coating is basically less than 30h. At present, the most widely used heat-resistant resins are mainly silicone resins and fluororesins. Silicone resins use a silicon-oxygen bond (—Si—O—) as the main chain. Due to its high bond energy, it has high oxidative stability, and the silicone resin can generate a protective layer of stable chains—Si—O—Si— on the surface of the coating, reducing the impact on the interior of the polymer; Silicone resin has a wide range of applications in heat-resistant coatings, but the use of silicone resin alone is due to its small intermolecular force, poor adhesion, and high price. According to the preliminary test results, the epoxy resin was modified by adding an appropriate amount of silicone resin to achieve the purpose of ensuring a certain heat resistance and meeting the market demand. The two selected silicone resins are resins containing hydroxyl functional groups, and Si/Epoxy is tested with two ratios of 0.1 and 0.3. The experimental results are shown in Table 3.  It can be seen from the application of silicone resin in different systems that the addition of silicone resin significantly improves the heat resistance of the coating. The continuous baking failure test proves that the heat resistance time of the coating has been extended from more than 10h to more than 100h. On the other hand, the flexibility of the coating has also been significantly improved, and it can maintain considerable flexibility during the continuous use of 9h. sex. The properties of 2 different silicone resins are different with different dosages. When the silicone resin accounts for 0.1 of the total, the flexibility of the coating can be significantly improved, but the heat resistance time is still low, about 50h. But when its ratio is increased to 0.3, the service life of the coating can exceed 100h. This shows that the increase of silicone content increases the graft ratio to epoxy resin, so that the coating can maintain good performance in high temperature environment. Silicone resin 1 and 2 have little difference in performance. Silicone resin 1 has an advantage over resin 2 in improving the flexibility of the coating, but it has a negative impact on the surface hardness of the coating after high temperature baking; Coating surface hardness is better than resin 1. 3. Selection of heat-resistant pigments and fillers 3.1 Heat-resistant body filler The selection of heat-resistant fillers will also directly affect the service life of the coating. A series of experiments were carried out with the more commonly used physique fillers in the market. The results are shown in Table 4.  Among all tested fillers, the order of impact resistance of the coating before baking is from good to poor: wollastonite, mica powder, kaolin > sillimanite > quartz. After baking: mica powder is the best, and the rest are almost the same. It can be seen from Table 4 that mica powder is better as a heat-resistant filler, and a fine scale structure with a diameter-to-thickness ratio > 80 is selected, which can form a good interlayer structure in the coating, thereby effectively preventing the infiltration of oxygen and slowing down the coating. The aging of the resin base material can achieve the protective effect of prolonging the life of the coating. Other fillers such as barium sulfate, kaolin, etc. are spherical in shape, so oxygen is easy to penetrate in a high temperature environment, so that the resin inside the coating is oxidized and damaged, thereby reducing the adhesion of the coating. 3.2 Heat Resistant Pigments Ordinary organic pigments will change color or even decompose in an environment higher than 200 ℃, so only inorganic pigments can be used in heat-resistant powder coatings. Such as iron oxide, graphite, carbon black, etc., through experiments, comprehensive factors such as discoloration resistance, coating mechanical properties, iron oxides and graphite are the best black pigments, not only have good high temperature discoloration resistance, but also It does not affect the mechanical properties of the coating, and its dosage is large (can account for 5% to 20% of the total resin). The total amount of pigments and fillers in heat-resistant powder coatings has a significant impact on the heat resistance of the coating. Generally, the more fillers, the better the heat resistance of the coating, but the mechanical properties of the coating become worse, which is finally determined by experiments. The optimal dosage range is between 60% and 100% (ratio to the total resin). 3.3 Antioxidants Since the coating is used in a high temperature environment, the organic polymer will degrade, especially in the presence of oxygen, which will accelerate the aging process of the coating. Therefore, a certain amount of antioxidants should be added to slow down the aging process of the coating. The antioxidants used in powder coatings are mainly phosphites and hindered phenols. The main function of phosphites is to prevent the coating from yellowing during the baking process; The main function of hindered phenols is to prevent the oxidation of polymers in the long term. The best anti-aging effect can be achieved by the combined use of phosphite and hindered phenolic antioxidants. The experimental results are shown in Table 5.  It can be seen from Table 5 that the antioxidant has no obvious effect on the anti-yellowing performance of the coating when used alone, but when the two are used in combination and the ratio is 4/1, the coating shows obvious anti-yellowing performance variable performance. However, when the operating temperature was increased to 300 °C, it was found that the effect of antioxidants was not obvious. This may be because the content of antioxidants was reduced due to the volatilization of heat, and the decomposition rate of the resin in the coating was too fast, making the antioxidants The free radicals generated by the resin due to heat cannot be eliminated in time. 4. Conclusion By adding 10% to 30% of the total amount of silicone resin to the epoxy resin, and selecting the heat-resistant filler - mica powder and an appropriate amount of composite antioxidant, the heat-resistant performance of the powder coating is significantly improved (service life). From the original 300 ℃ 1h to more than 70h). It can be used for a long time in the environment below 350 °C, and the coating has good adhesion and impact resistance, a certain flexibility, and at the same time minimizes the discoloration of the coating