Application of Silicone Resin in Heat-resistant Powder Coatings

Song Linyong, Xu Wanxuan, Yan Lachun
Department of Polymer Science and Engineering, China University of Science and Technology
Guangzhou Dynamite Powder Coatings Co., Ltd.
Abstract: the use of silicone resin to mix and change epoxy resins, and the use of inorganic pigments in the coating to select, significantly improve the heat resistance of powder coatings, so that the coating can be used in environments above 250 degrees C for a long time.
introduction isWith the development of the coating industry, powder coating has been widely used in the coating of metal substrates, in recent years, antibacterial, high wear-resistant, heat-resistant and other functional products development and application has become the direction of the development of powder coatings.
-resistant powder coating refers to a powder coating that can withstand temperatures above 200 degrees C for a long time, has a good coating, and enables the protected object to function normally in high temperature

.
the thermal stability of polymers, the heat resistance of polymers depends mainly on their molecular structure. The thermal stability of polymers can be improved by introducing larger or more polar side foundations on the main chain and increasing the interaction force between molecules.
way to improve the heat resistance of powder coatings is to add heat-resistant pigments and fillers to polymers. Commonly used pigments are aluminum powder, mica powder, stainless steel powder, cadmium powder, silicon dioxide and so on.
1, test part
1.1 coating film preparation
according to the formula preparation powder coating, mixed crushing, twin screw extruder extrusion, extrusion, extrusion, sliver, crushing, screening (180 eyes), electrostrical spray to the blasted steel plate substrate, 200 degrees C /20min solidification.
1.2 Performance Test
Heat resistance: 300C oven; Impact resistance: GB 1732-79; Gloss: GB 1743-79.
2, results and discussion
2.1 resin heat resistance
resin as the main film-forming substance of the coating is the most basic factor to determine the heat resistance of the coating, the usual resin product heat resistance index can be seen in Table 1.
powder coating is generally in 180 to 200 degrees C, 20min conditions cured into a film, belongs to the thermo-curing coating, its coating forms a mesh cross-linking structure, so the heat resistance of the thermoplastic coating has been improved.
Heat-resistant tests of powder coatings such as epoxy, epoxy polyester blend, polyester/TGIC (Table 2) show that these coatings are basically capable of long-term use at temperatures below 150 degrees C.
But in environments above 250 degrees C, the coating shows damage such as loss of light, decreased adhesion, brittle coating, reduced flexibility and powder, causing the coating to lose its protective effect on the substrate.Table 2 of
shows that existing generic products are defective in the following areas: severe loss of light on the surface of the
(1) coating, significantly poor mechanical performance of the
(2) coating, significantly reduced flexibility of the
(3) coating, and the continuous use time of the
(4) coating is basically less than 30h.
most widely used heat-resistant resins are silicone and fluorine resins. Silicone resin with silicone oxygen bond (-Si-O-) as the main chain.
Because of its high bond energy, it has high oxidation stability, and silicone resin can create a stable chain - Si-O-Si - protective layer on the coating surface, reducing the impact on the polymer interior;
silicone resin in heat-resistant coatings has a wide range of applications, but the use of silicone resin alone due to its small molecular force, poor adhesion, and high price.
the preliminary test results, the method of adding the appropriate amount of silicone resin modified epoxy resin is used to ensure a certain degree of heat resistance and meet market demand.
silicone resins selected for the test were all resins containing hydroxyl hydroxyl hydrosynin groups, si/Epoxy tested at 0.1 and 0.3 2 ratios, and the results were found in Table 3.
the application of silicone resins in different systems, it can be seen that the addition of silicone resins significantly improves the heat resistance of the coating.
Through continuous baking damage test proved that the heat resistance of the coating from the original 10h or more extended to more than 100h, on the other hand, the flexibility of the coating has also been significantly improved, in the continuous use of 9h can maintain considerable flexibility.
2 different silicone resins have different properties. When silicone resins account for 0.1 of the total, the flexible properties of the coating are significantly improved, but their heat resistance time is still low, about 50h.
the coating can last longer than 100h when its ratio increases to 0.3. This shows that the increase in silicone content makes it more in proportion to epoxy, so that the coating can maintain good performance in high temperature environment.
Silicone resin 1 and 2 performance is not very different, silicone resin 1 in improving the flexibility of the coating than resin 2 has advantages, but it has a negative impact on the surface hardness after coating high temperature roasting, and resin 2 is better than resin 1 in terms of coating surface hardness.
3, the selection of heat-resistant pigment test
3.1 heat-resistant body filler
heat-resistant filler selection will also directly affect the life of the coating, the selection of the market more commonly used body filler, a series of experiments, the results can be seen in Table 4.
of all the tried fillers, the impact resistance of the pre-baking coating was from good to bad: silica ash, mica powder, kaolin >silica wire stone> quartz. After baking is: mica powder is the best, the rest is similar.
From Table 4, it can be seen that mica powder as a heat-resistant filler is better, the choice of diameter thickness ratio >80 fine scale-like structure, in the coating can form a good inter-layer structure, thus effectively preventing oxygen infiltration, slowing the aging of the coating resin substate, to extend the protection of coating life.
other fillers, such as molybric acid, kaolin, etc. because of their sperical shape, so in high temperature environment oxygen easy to penetrate, so that the coating internal resin is oxidized damage, thereby reducing the adhesion of the coating.
3.2 Heat-resistant pigments
Common organic pigments can change color or even decompose in environments above 200 degrees C, so inorganic pigments can only be used in heat-resistant powder coatings.
e.g. iron oxide, graphite, carbon black, etc., through experiments, comprehensive antichromic resistance, coating mechanical properties and other factors, iron oxide and graphite are the best black pigments, not only have good anti-high temperature color change, but also do not affect the mechanical properties of the coating, its dosing is large (can account for 5% to 20% of the total amount of resin).
The total amount of pigment in heat-resistant powder coating has a more obvious effect on the heat resistance of the coating, generally the more fillers the coating heat resistance performance is better, but the mechanical performance of the coating is worse, through experiments to determine its optimal dosing range between 60% to 100% (compared to the total amount of resin).
3.3 Antioxidants
Because the coating is used in high temperature environments, organic polymers degrade, especially in the presence of oxygen, which accelerates the aging process of the coating, so add a certain amount of antioxidants to slow down the aging process of the coating.
antioxidants used in powder coatings are mainly ephedrophosates and blocked phenols. The main function of phosphate is to prevent the coating from yellowing during baking, and the main function of
blocked phenols is to prevent the oxidation of polymers in the long term. Phosphate and blocked phenol, antioxidant-like compound users can achieve the best anti-aging effect. The results of the experiment can be found in Table 5.
From Table 5, it can be seen that the antioxidant alone has no significant effect on the anti-yellowing performance of the coating, but when the two are mixed and the ratio is 4/1, the coating shows obvious anti-yellowing properties.
But when the temperature of use is raised to 300 degrees C, it is found that the role of antioxidants is not obvious, this may be due to the antioxidant heat volatility caused by its content decreased, the resin in the coating decomposition speed is too fast, so that antioxidants can not be timely elimination of resin due to heat-produced free fundamentals.
4, conclusion
by adding 10% to 30% of the total amount of resin silicone resin in epoxy resin, and the choice of heat-resistant fillers - mica powder and the appropriate amount of composite antioxidants, so that the powder coating heat resistance significantly improved (life from the original 300 degrees C1h, increased to more than 70h).
can meet the long-term use in environments below 350 degrees C, and the coating has good adhesion and impact resistance, a certain degree of flexibility, while minimizing the color change of the coating.