Research on high-temperature resistant powder coatings suitable for automotive exhaust pipes in terms of their usage environment. A powder coating with high temperature resistance of 600 ℃ was prepared using organic silicon resin and epoxy resin as base materials, with VN-302 as the silicon curing agent and phenolic curing agent as the epoxy resin curing agent. The addition of boron containing glass powder can significantly improve the high temperature resistance performance, talc powder can significantly improve the coating integrity after baking, silicon micro powder can improve the thermal weight loss stability of the coating, and silane coupling agent can improve the adhesion of the coating. After pre mixing the raw materials, melting extrusion, and grinding, a high-temperature resistant powder coating of organic silicon/epoxy resin was successfully prepared. The performance of high temperature resistant powder coatings of organic silicon/epoxy resin was characterized and tested through impact resistance testing, high temperature resistance, color difference analysis, and other methods. The influence of the ratio of organic silicon resin to epoxy resin on the high temperature resistance of the coating was investigated. At the same time, the influence of the dosage of boron containing glass powder, sliding stone powder, leveling agent, and silane coupling agent on the high temperature resistance of the coating was also investigated. The results indicate that the optimized coating has excellent high temperature resistance and good mechanical properties, making it fully suitable for use in automotive exhaust pipes.
The increasingly strict environmental policies nowadays have led more and more paint manufacturers to switch to pollution-free and highly efficient powder coatings. Developing functional powder coatings for special occasions has become a trend, such as high-temperature resistant powder coatings. High temperature resistant powder coatings are widely used, such as ovens, warm air blowers, high-temperature furnaces, vehicle exhaust pipes, special heat pipes, etc. High temperature resistant powder coatings not only have the performance of ordinary powder coatings, but also have excellent heat resistance and corrosion resistance. Due to the harsh usage conditions of powder coatings for automotive exhaust pipes, with temperatures above 500 ℃ and long working hours, traditional solvent based coatings will gradually be restricted under increasingly strict environmental policies. Therefore, the development of high-temperature resistant powder coatings for automotive exhaust pipes is of great significance.
This study used organic epoxy resin as the film-forming material, VN-203 as the organic silicon resin curing agent, phenolic curing agent as the epoxy resin curing agent, and added pigments and fillers such as glass powder, talc powder, silicon micro powder, manganese iron black, leveling agent, and silane coupling agent to significantly improve the high temperature resistance and good mechanical properties of the coating.
one
Experimental section
1.1 Experimental raw materials
Organic silicon resin VN-203、 Silane coupling agent: Changzhou Jianuo Organic Silicon Co., Ltd; Silicon micro powder: industrial grade, Suzhou Jinyi Inorganic New Material Technology Co., Ltd; Epoxy resin E-12: industrial grade, Mount Huangshan Jinfeng Industrial Co., Ltd; Manganese Iron Black: Kunshan Qichuang Chemical Co., Ltd; Talc powder: industrial grade, Changzhou Fengshuo Chemical Co., Ltd.: leveling agent: Ningbo Nanhai Chemical Co., Ltd.: phenolic curing agent: industrial grade, Lu'an Jietongda Chemical Co., Ltd.
1.2 Experimental Equipment
Color difference meter (SP60): X-rite from the United States; Thickness meter (QNIX4500): Nix from Germany; MaFu Furnace: Changzhou Xingguang Kiln Co., Ltd; Curing electric furnace: Nanjing Boyuntang Instrument Technology Co., Ltd; Electrostatic powder coating spray gun: Swiss Golden Horse Co., Ltd.: Electronic scale: Mettler Toledo Instruments (Shanghai) Co., Ltd; Impact resistant instrument (QCJ type): Kunshan Guojing Electronics Co., Ltd.: Double screw extruder (SLJ30A), milling system (ACM02): Yantai Donghui Powder Equipment Co., Ltd; Particle size analyzer (Topsizer Plus): Zhuhai Ouaike Instrument Co., Ltd.
1.3 Experimental Process
1.3.1 Formula design of high-temperature resistant powder coating
The use conditions of high-temperature resistant powder coatings are relatively harsh, and the high-temperature working time is long, so the requirements for coating performance are very high. Firstly, the raw materials have low heat loss under high temperature conditions. Secondly, the coating needs to have high color retention at high temperatures, otherwise it will affect the appearance. That is, the color retention requirements for pigments and fillers are high under high temperature conditions. Thirdly, the mechanical properties of the coating after use under high temperature conditions are high. If the coating undergoes powdering, peeling or cracking after high temperature use, the mechanical properties of the coating will be greatly reduced, which cannot meet the needs of use.
According to literature, silicone organic silicon resin has the best high temperature resistance. Therefore, in this experiment, silicone organic silicon resin was chosen as the film-forming material for high temperature resistant powder coatings, and a small amount of epoxy resin was added to reduce costs. Table 1 shows the formula composition of typical high-temperature resistant powder coatings.
1.3.2 Screening of pigments and fillers
Place pigments and fillers from different manufacturers in a muffle furnace of equal quality, bake at 600 ℃ for 1 hour, and then remove them. Observe the volatile matter and color changes, with the color change mainly measured by a colorimeter (60 ° C).
1.3.3 Preparation process of powder coating
Add the selected pigments, organic silicon resin, epoxy resin, phenolic curing agent, and other raw materials according to the formula in Table 1 into a mixing tank, stir at a speed of 400r/min for 4 minutes, stir evenly, and pour into a twin-screw extruder for melting and extrusion. The heating conditions of the extruder are 110 ℃ in Zone I and 125 ℃ in Zone II. After extrusion is completed, cool to room temperature and send them to a grinding machine for grinding. The grinding conditions are main grinding speed of 8500~9500r/min, secondary grinding speed of 6500~7500r/min, and mesh size of 180 mesh. When the particle size of the powder coating is adjusted to 32~37 μ m, a large amount of powder can be ground to obtain high-temperature resistant powder coating.
1.3.4 Preparation of Coatings
The preparation of coatings includes degreasing and rust removal of the board, as well as electrostatic spraying of the coating.
Immerse the cold-rolled sheet in hot acid and alkali to remove rust and protective oil, with an immersion time of 7-10 minutes. After processing, let it air dry naturally. Using the electrostatic spraying method, the powder is sprayed onto a cold-rolled plate and cured for 30 minutes at 230 ℃ to obtain a high-temperature resistant powder coating with a thickness of approximately 60-80 μ m.
1.4 Analysis and Testing
1.4.1 High temperature test
Place the solidified high-temperature resistant powder coated tinplate in a muffle furnace and bake it at 600 ℃ for 1 hour. After baking, take it out for performance and coating color difference analysis.
1.4.2 Other performance tests
Perform color difference analysis on the coating using a colorimeter, and all testing processes are conducted at room temperature (23 ± 2 ℃); QCJ type impact tester is used to test the coating performance: the coating performance is tested according to HG/T 2006-2007, and the coating adhesion is tested using the cross grid method.
two
results and discussion
2.1 Selection of pigments and fillers
Color fillers are the most important part besides resins, which directly affect the decorative and heat resistance of coatings. According to literature, silicon materials, barium sulfate, talc powder, mica powder, glass powder, and manganese iron black are the most commonly used pigments and fillers in high-temperature resistant coatings. This article conducts high temperature resistance tests on the above-mentioned pigments and fillers on the market, mainly testing volatile matter and color difference. The method of color difference testing is to uniformly weigh the fillers before and after baking according to the formula in Table 2, prepare powder coatings according to 1.3.3, then prepare coatings according to 1.3.4, and then test color difference according to 1.4. The results of volatile matter and color difference tests are shown in Table 3.
According to Table 3, the volatile matter of glass powder is the lowest, at 0.06%, followed by manganese iron black at 0.12%; Next is silicon micro powder, with a volatile content of 0.13%; Next is extinction barium, with a volatile content of 1.52%; C311 has the highest volatile content of 36.4% and cannot be used for high-temperature resistant powder coatings; Next is talc powder, with a volatile content of 10.8%, but its coefficient of linear expansion is large and its coefficient of volume expansion is small, which can change its crack resistance at high temperatures.
From Table 3, it can be seen that silicon micro powder has the lowest color difference, followed by precipitated barium sulfate, and then nano barium sulfate. Even if the color difference between precipitated barium sulfate and nano barium sulfate is good, the color difference is still greater than 2, which is visible to the naked eye. Therefore, silicon micro powder was chosen as the main filler in this experiment. The high-temperature resistant powder coatings in the current market are mainly black, so the next black pigment is manganese black.
2.2 Effects of Different Dosage of Organic Silicon Resin and Epoxy Resin on the Performance of High Temperature Resistant Powder Coatings
Prepare powder coating according to the steps in 1.3.3. Under the conditions of curing temperature of 230 ℃, curing time of 30 minutes, and constant mass of other components, change the dosage of organic silicon resin and epoxy resin. The coating is baked at high temperature of 600 ℃ for 1 hour. The test results are shown in Table 4.
The bond energy of silicon oxygen bond (Si-O) is 425 kJ/mol, while the bond energy of carbon oxygen bond (C-O) is 351 kJ/mol, and the bond energy of carbon carbon bond (C-C) is 345 kJ/mol. Therefore, silicone resin is more suitable for manufacturing high-temperature resistant powder coatings. Due to the fact that high-temperature resistant powder coatings on the market are mainly composed of sand patterns, the compatibility between epoxy resin and silicone resin is poor, resulting in the formation of sand patterns and reducing production costs. Therefore, epoxy resin is added in this article.
According to Table 4, all coatings have good mechanical properties, impact resistance, and adhesion before high-temperature baking, which meet the application requirements; But after high-temperature baking, as the amount of epoxy resin increases, the heat loss of the coating also increases, and the heat loss is only 1-2 percentage points higher than the epoxy resin content in the relative formula. Therefore, it can be confirmed that the epoxy resin is completely lost under high-temperature conditions: when the amount of silicone resin exceeds 45%, the high temperature resistance and mechanical properties of the coating are good, and the color difference Δ E before and after baking is less than 2, which can fully meet market demand; When the amount of silicone resin used is less than 45% (mass fraction, the same applies later), the heat loss after coating baking is too large, resulting in poor mechanical properties of the coating. The coating will fall off when touched, and the coating color will change from black to gray red. So the dosage ratio of silicone resin and epoxy resin in the following experiment is fixed at 45:15.
2.3 Effect of boron containing glass powder dosage on the high temperature resistance of coatings
After comparison, it was found that the cost-effectiveness and performance of boron containing glass powder are more suitable for high-temperature resistant powder coatings, so this article uses boron containing glass powder. The influence of different amounts of boron containing glass powder on the coating performance is shown in Table 5.
According to Table 5, as the amount of boron containing glass powder increases, the heat loss of the coating decreases, while the mechanical properties of the coating increase and then decrease. When the dosage of boron containing glass powder is 5%, although the heat loss is 17.38%, the coating performance is poor: with the increase of dosage, the mechanical properties and heat loss of the coating gradually decrease; When the dosage exceeds 13%, although the heat loss of the coating decreases, the mechanical properties of the coating are poor. Low melting point boron containing glass powder melts at high temperatures and has a certain bonding effect, but when used in small amounts, the bonding effect is not significant. If used in too much amount, the coating toughness is insufficient and the mechanical properties are reduced. Considering comprehensively, the amount of boron containing glass powder is 10% (mass fraction).
2.4 Effect of talc powder dosage on coating performance
Talc powder has a layered structure and good anti cracking performance of coatings at high temperatures. The layered structure can provide the coating with certain impact resistance, and the van der Waals force between layers is weak. Therefore, adding an appropriate amount of talc powder is beneficial for improving the mechanical properties of the coating. The influence of different amounts of talc powder on the coating performance is shown in Table 6.
From Table 6, it can be seen that as the amount of talc powder increases, the heat loss also gradually increases. This is because talc powder contains bound water, so the more it is used, the greater the heat loss of the coating. The apparent performance of the coating shows a trend of first increasing and then decreasing with the increase of dosage. Therefore, the following experiment chooses a dosage of 3% (mass fraction).
The Effect of Silane Coupling Agent on Coating Performance
Silane coupling agents can increase the adhesion between coatings and substrates, as well as the bonding strength between materials, thereby improving the overall performance of coatings. The effects of different amounts of silane coupling agents on coating performance are shown in Table 7.
According to Table 7, the adhesion of the coating increases with the increase of dosage. When the dosage is greater than 0.9%, the adhesion of the coating no longer increases. Therefore, the following experiment chooses a dosage of 0.9% (mass fraction).
2.6 Effect of leveling agent dosage on coating performance
Due to the addition of epoxy resin in the powder coating system, its compatibility with organic silicon resin is low, resulting in bright spots in the sand pattern and an increase in the gloss of the coating, which reduces the decorative value of the coating. So a small amount of leveling agent needs to be added to reduce bright spots and improve the decorative properties of the coating. But the leveling agent is completely lost at high temperatures, so it should not be too much to avoid reducing the mechanical properties of the coating. The effect of different leveling agent dosages on coating gloss is shown in Table 8.
According to Table 8, as the amount of leveling agent increases, the gloss gradually decreases; When the dosage exceeds 0.7%, the gloss of the coating no longer decreases and the bright spots disappear. So the selected dosage of leveling agent is 0.7% (mass fraction).
three
conclusion
(1) A high-temperature resistant powder coating that can withstand a high temperature of 600 ℃ for 1 hour was successfully prepared by matching organic silicon resin with epoxy resin and adding high-temperature resistant pigments and fillers. The coating has good mechanical properties and is completely suitable for use in automotive exhaust pipes.
(2) Through the above experiments, it can be concluded that silicon micro powder as a filler has the best high temperature resistance and has the least impact on the color difference and heat loss of the coating. Manganese black is selected as the black pigment; The dosage of silicone resin and epoxy resin is fixed at 45% (mass fraction) and 15% (mass fraction); The dosage of boron containing glass powder is 10% (mass fraction); The dosage of talc powder is 3% (mass fraction), the dosage of silane coupling agent is 0.9% (mass fraction), and the selected dosage of leveling agent is 0.7% (mass fraction).
