Lu Ying1, Bu Qing Peng2，Wang Xiaoqiang2, Pan Jianliang2
(1、Changzhou Coatings Association; 2、Jiangsu Huaguang Powder Co., Ltd.)
Summary:This study focuses on developing a high-temperature powder coating suitable for automotive exhaust systems. The coating, based on organic silicone resin and epoxy resin, achieves a temperature resistance of 600°C. The curing agent for the organic silicone resin is VN-302, while the curing agent for the epoxy resin is a phenolic curing agent. Adding boron-containing glass powder significantly improves the high-temperature resistance, while talc powder improves the integrity of the coating after baking. Silicon microspheres enhance the thermal stability of the coating, and silane coupling agents improve the adhesion of the coating. The organic silicone/epoxy high-temperature powder coating was successfully prepared through raw material pre-mixing, melting extrusion, and grinding. The performance of the organic silicone/epoxy high-temperature powder coating was characterized and tested using impact resistance tests, high-temperature resistance tests, and color difference analysis. The effects of the ratio of organic silicone resin to epoxy resin, as well as the amounts of boron-containing glass powder, talc powder, leveling agent, and silane coupling agent, on the high-temperature performance of the coating were also investigated. The results show that the optimized coating exhibits excellent high-temperature resistance and good mechanical properties, making it suitable for use in automotive exhaust systems.
Keywords:Automotive exhaust pipes, silicone/epoxy resin, high-temperature resistance, thermal stability.
0 Introduction
Due to increasingly stringent environmental regulations, more and more paint manufacturers are switching to powder coatings that are free from pollution and have high utilization rates. The development of functional powder coatings for specific applications, such as high-temperature powder coatings, is becoming a trend. High-temperature powder coatings have a wide range of applications, including ovens, heaters, furnaces, vehicle exhaust systems, and special heat pipes. High-temperature powder coatings not only have the properties of ordinary powder coatings but also have excellent heat resistance and corrosion resistance. Given the harsh conditions for using powder coatings in vehicle exhaust systems, where exhaust temperatures exceed 500°C and operating times are long, traditional solvent-based coatings are gradually being restricted due to stricter environmental regulations. The development of high-temperature powder coatings specifically for vehicle exhaust systems is of great significance.
This research utilized organic epoxy resin as the film-forming material, VN-203 as the organic silicone resin curing agent, phenolic curing agent for the epoxy resin, and added fillers such as glass powder, talc powder, silicon powder, manganese iron black, dispersants, and silane coupling agents to significantly improve the coating's high-temperature resistance and good mechanical properties.
1. Experimental Part
1.1 Experimental Materials
Organosilicon resin, VN-203, silane coupling agent: Changzhou Jiano Organosilicon Co., Ltd.; Silicon powder: Industrial grade, Suzhou Jin Yi Inorganic New Material Technology Co., Ltd.; Epoxy resin E-12: Industrial grade, Huangshan Jinfeng Industrial Co., Ltd.; Manganese iron black: Kunshan Qichao Chemical Co., Ltd.; Talc powder: Industrial grade, Changzhou Fengxu Chemical Co., Ltd.: Retarder: Ningbo Nan Hai Chemical Co., Ltd.: Phenolic curing agent: Industrial grade, Luan'an Jietongda Chemical Co., Ltd.
1.2 Experimental Equipment
- Spectrophotometer (SP60): X-Rite (USA); Thickness meter (QNIX4500): Nix (Germany); Maofu Furnace: Changzhou Xingguang Kiln Co., Ltd.; Curing Oven: Nanjing Boyuntong Instrument Technology Co., Ltd.; Static Powder Coating Spray Gun: Golden Horse Co., Ltd. (Switzerland); Electronic Scale: Mettler Toledo (Shanghai) Co., Ltd.; Impact Test Instrument (QCJ type): Kunshan Guojing Electronic Co., Ltd.; Twin-Screw Extruder (SLJ30A), Grinding System (ACM02): Yantai Donghui Powder Equipment Co., Ltd.; Particle Size Analyzer (Topsizer Plus): Zhuhai Omeck Instrument Co., Ltd.
1.3 Experimental Procedure
1.3.1 Formulation Design for High-Temperature Powder Coatings
The conditions for using high-temperature powder coatings are quite harsh, with prolonged high-temperature operation, therefore, the coating performance requirements are very high. Firstly, the raw materials should have low heat loss under high-temperature conditions. Secondly, the coating must maintain its color effectively under high temperatures to avoid affecting aesthetics, meaning that the colorants must maintain their color stability under high-temperature conditions. Thirdly, the mechanical properties of the coating after use under high-temperature conditions should be high. If the coating experiences powdering, detachment, or cracking after high-temperature use, which significantly reduces its mechanical properties, it will not meet the usage requirements.
According to the literature, silicone organic silicone resin exhibits the best high-temperature resistance, therefore, this experiment selected silicone organic silicone resin as the film-forming material for high-temperature powder coatings. To reduce costs, a small amount of epoxy resin was also added. Table 1 shows a typical formulation for high-temperature powder coatings.
1.3.2 Screening of Pigments
Place fillers from different manufacturers under identical quality standards in a muffle furnace and bake at 600°C for 1 hour. Then, remove the samples and observe the volatilization and color changes. Color changes should primarily be measured using a colorimeter (60°).
1.3.3 Manufacturing Process of Powder Coatings
Combine the selected pigments, organic silicone resin, epoxy resin, phenolic curing agent, and other raw materials according to the formula in Table 1 in the mixing tank. Mix at a speed of 400 r/min for 4 minutes. After thorough mixing, pour into a twin-screw extruder. Heat the extruder according to the following conditions: Zone I temperature: 110℃, Zone II temperature: 125℃. After extrusion, cool to room temperature and feed into a grinder. Grind with a main grinding speed of 8500～9500 r/min and a secondary grinding speed of 6500～7500 r/min, with a mesh size of 180, to obtain a powder coating with a particle size of 32～37 μm.
1.3.4 Preparation of Coatings
The coating preparation process includes degreasing, rust removal, and electrostatic spraying of the coating on the substrate.
The cold-rolled sheet is immersed in hot acid and hot alkali to remove rust and protective oil, with an immersion time of 7-10 minutes. After treatment, allow to air dry naturally. Using electrostatic spraying, the powder is sprayed onto the cold-rolled sheet. At 230°C, it is cured for 30 minutes, resulting in a high-temperature powder coating with a coating thickness of approximately 60-80μm.
1.4 Analysis and Testing
1.4.1 High-Temperature Testing
Place the aforementioned high-temperature powder-coated steel drums in a muffle furnace and heat them to 600°C for 1 hour. After the baking process, remove the drums and perform performance and coating color difference analysis.
1.4.2 Other Performance Tests
Color difference measurements were performed on the coating using a colorimeter, and all testing procedures were conducted at room temperature (23±2℃). The coating's performance was tested using a QCJ type coating impact tester, following the HG/T 2006-2006 standard. The coating adhesion test was performed using the cross-hatch method.
2. Results and Discussion
2.1 Selection of Pigments
Pigments are the second most important component, besides resin, and directly affect the decorative and heat resistance of the coating. According to the literature, silicon materials, barium sulfate, talc, mica, glass powder, and manganese iron black are the most commonly used pigments in high-temperature coatings. This article conducts high-temperature tests on the above pigments available on the market, primarily testing for volatility and color difference. The color difference test method involves weighing the pigments according to the formula in Table 2 before baking, then preparing powder coatings according to 1.3.3 and coatings according to 1.3.4, and finally testing the color difference according to 1.4. The results of the volatility and color difference tests are shown in Table 3.
As shown in Table 3, glass powder has the lowest volatility, at 0.06%; manganese iron black is second, at 0.12%; silica powder is third, at 0.13%; barium sulfate is fourth, at 1.52%; and C311 has the highest volatility, at 36.4%, which is not suitable for high-temperature powder coatings. Additionally, talc has a high linear expansion coefficient but a low volumetric expansion coefficient, which can alter its resistance to cracking at high temperatures.
As shown in Table 3, the lowest color difference was observed for the silicon micro powder, followed by precipitated barium sulfate, and then nano-sized barium sulfate. Even though the color difference between precipitated and nano-sized barium sulfate was relatively good, it was still greater than 2, which is visually perceptible, so these were discarded. In this experiment, silicon micro powder was selected as the main filler. The main colors in the current market for high-temperature powder coatings are black, so the next black pigment will be manganese iron black.
2.2 The impact of different ratios of organic silicone resin and epoxy resin on the performance of high-temperature powder coatings.
Prepare the powder coating according to step 1.3.3. Under the conditions of a curing temperature of 230℃, a curing time of 30 minutes, and constant quality of other components, change the ratio of organic silicone resin to epoxy resin. After baking the coating at 600℃ for 1 hour, test the results as shown in Table 4.
The bond energy of Si-O (425 kJ/mol) is greater than that of C-O (351 kJ/mol) and C-C (345 kJ/mol). Therefore, organic silicon resins are more suitable for producing high-temperature powder coatings. Since the market for high-temperature powder coatings is dominated by sand-textured coatings, the compatibility between epoxy resins and organic silicon resins is poor, which can lead to sand-textured coatings and reduce production costs. Therefore, epoxy resin is added in this article.
As shown in Table 4, all coatings exhibit good mechanical properties and meet the required impact resistance and adhesion before high-temperature baking. However, after high-temperature baking, with the increase in epoxy resin content, the coating's heat loss also increases, and this increase is only 1-2 percentage points higher than the relative formulation. Therefore, it can be confidently concluded that the epoxy resin is completely lost under high temperatures. When the amount of silicone resin exceeds 45%, the coating exhibits good high-temperature resistance and good mechanical properties, and the color difference ΔE before and after baking is<2，完全可以满足市场需求；当有机硅树脂的用量低于45％（质量分数，后同）时，涂层烘烤之后，热损失太大，导致涂层的机械性能太差，涂层触碰之下就会掉落，且涂层颜色由黑色变成灰红色。所以接下来的实验有机硅树脂和环氧树脂的用量比例固定在45:15。
2.3 The effect of the amount of boron glass powder on the high-temperature resistance of the coating
After comparison, it was found that boron-containing glass powder offers better value and performance for high-temperature powder coatings, therefore, boron-containing glass powder is used in this article. The impact of different amounts of boron-containing glass powder on coating performance is shown in Table 5.
According to Table 5, as the amount of borosilicate glass powder increases, the coating's heat loss decreases, while its mechanical properties increase and then decrease. When the amount of borosilicate glass powder is 5%, the heat loss is 17.38%, but the coating performance is poor. As the amount increases, both the coating's mechanical properties and heat loss gradually decrease. When the amount exceeds 13%, the heat loss decreases, but the coating's mechanical properties are poor. Low-melting borosilicate glass powder melts at high temperatures and has a certain bonding effect, but when used in small amounts, the bonding effect is not obvious. If used in excess, the coating's toughness and mechanical properties are reduced. Considering the use of 10% borosilicate glass powder (by weight).
2.4 The effect of the amount of talc on coating performance
The flake-like structure of talc powder exhibits good coating crack resistance at high temperatures, and the flake structure can provide the coating with certain impact resistance. Additionally, the Van der Waals forces between the layers are relatively weak, so adding an appropriate amount of talc powder can improve the mechanical properties of the coating. The effects of different amounts of talc powder on coating performance are shown in Table 6.
As shown in Table 6, with the increase in talc powder content, the heat loss also gradually increases. This is because talc powder contains bound water, and the more talc powder used, the greater the heat loss of the coating. The apparent performance of the coating initially increases with the increase in talc powder content, but then decreases. Therefore, the following experiments will use a talc powder content of 3% (by weight).
2. 5 The Impact of Silane Coupling Agents on Coating Performance
Silane coupling agents can enhance the adhesion between the coating and the substrate, as well as improve the bonding strength between different materials, thereby enhancing the overall performance of the coating. The impact of different amounts of silane coupling agents on the coating performance is shown in Table 7.
As shown in Table 7, the coating adhesion increases with increasing dosage. However, the adhesion no longer increases when the dosage exceeds 0.9%, so the subsequent experiments selected a dosage of 0.9% (by weight).
2.6 The amount of leveling agent used affects the coating performance.
Due to the low compatibility between the epoxy resin in the powder coating system and organic silicone resins, the resulting textured surface exhibits highlights, which increases the coating's gloss and reduces its decorative effect. Therefore, a small amount of leveling agent is required to reduce the highlights and improve the coating's decorative effect. However, the leveling agent is susceptible to loss at high temperatures, so it should not be used in excess to avoid reducing the coating's mechanical properties. The effect of different amounts of leveling agent on the coating's gloss is shown in Table 8.
As shown in Table 8, with the increase in the amount of leveling agent, the gloss gradually decreases. When the amount exceeds 0.7%, the gloss no longer decreases, and the specular highlights disappear. Therefore, the amount of leveling agent should be selected as 0.7% (by weight).
3. Conclusion
(1) By matching organic silicone resin with epoxy resin and incorporating high-temperature pigment, a high-temperature powder coating with a service temperature of 600°C for 1 hour was successfully developed, exhibiting good mechanical properties and suitability for use in automotive exhaust systems.
(2) based on the above experiments, it can be concluded that silicon powder, when used as a filler, exhibits the best high-temperature resistance, and has the least impact on color variations and heat loss in the coating. Manganese iron black should be selected as the pigment. The amounts of organic silicone resin and epoxy resin are fixed at 45% (by weight), and 15% (by weight), respectively. The amount of boron glass powder is 10% (by weight). The amount of talc powder is 3% (by weight). The amount of silane coupling agent is 0.9% (by weight). The amount of superplasticizer is 0.7% (by weight).