This article compares three different surface treatment methods for building aluminum profiles from the perspectives of coating performance and application performance, and summarizes and analyzes the research progress of weather resistance and low-temperature curing as important development directions of powder coatings, combined with the characteristics of powder coating.

The advantages of powder coating in surface treatment of building aluminum profiles

At present, the surface treatment of aluminum profiles mainly includes three types: anodizing, electrophoretic coating, and powder coating. The complete anodizing process usually involves five steps: mechanical pretreatment, chemical pretreatment, anodizing, coloring, and sealing. The electrophoretic coating process is generally the same as the anodic oxidation process, with the difference being that electrophoretic coating replaces the sealing process with electrophoretic coating after the anodic oxidation coloring process. Therefore, the surface of aluminum profiles coated by electrophoretic coating is actually a composite film of anodic oxidation film and electrophoretic coating, also known as anodic oxidation composite film. Powder spraying also requires chemical pretreatment, followed by electrostatic spraying of powder coating.

The coating properties obtained from the three surface treatments of aluminum profiles each have their own characteristics. Anodizing was the main method for surface treatment of aluminum profiles in China in the early days. Anodizing film has high wear resistance, good insulation and corrosion resistance, and is still one of the main methods for surface treatment of aluminum profiles today. Electrophoretic coating was mature in Japan, a country with a maritime climate surrounded by the sea. The corrosion problem of aluminum profiles caused by sea salt particles or mortar mixed with sea sand is particularly prominent, and the anodizing treatment process is difficult to achieve effective protection in highly corrosive environments. Electrophoretic coating has excellent weather resistance and corrosion resistance, as well as a bright appearance and easy cleaning, which has led to rapid development. According to the exposure test data in Florida, USA, the anodic oxidation composite film obtained by electrophoretic coating (with a light retention rate of 5a) is comparable to fluorocarbon coating, and the color difference is smaller than that of fluorocarbon coating. However, electrophoretic coating also has the defect of easy scratching of the paint film. In addition, the anodic oxide film as the base layer has poor toughness and is prone to cracking under mechanical or thermal stress. Reports show that the anodic oxide film with cold sealed holes can only withstand baking at 66 ℃, and only half of the samples are qualified after baking at 82 ℃.

In the early 1990s, powder coating began to be widely applied in the surface treatment of aluminum profiles in China, and has developed rapidly in the past decade. The performance advantages of powder coating are not obvious. In terms of appearance flatness and coating uniformity, it is inferior to anodizing and electrophoretic coating, and its weather resistance is between anodizing and electrophoretic coating. However, its wear resistance, acid resistance, and flexibility are significantly better than anodizing and electrophoretic coating. As a semi permanent structure, the durability of architectural aluminum profiles is very important, so it is particularly important to resist mechanical action and aging to maintain the integrity and functionality of the coating film. The commonly used electrophoretic paint is acrylic coating, which has excellent weather resistance. The minimum level of accelerated weather resistance in GB 5237-2008 also requires a xenon lamp aging gloss retention rate of over 80% after 1000 hours, and the highest level even requires a xenon lamp aging gloss retention rate of over 80% after 4000 hours; The main structure of the universal powder coating for building aluminum profiles is polyester resin, which has slightly worse weather resistance than acrylic acid. The highest level of accelerated weather resistance in GB 5237-2008 only requires a xenon lamp aging retention rate of>90% after 1000 hours.

This indicates that the average weather resistance of electrophoretic coating is significantly higher than that of powder coating, and the weather resistance of powder coating on building aluminum profiles has fallen behind the actual demand. Powder spraying has significant advantages in application. Powder coating can achieve up to thousands of colors and various texture decorative effects, which is difficult to achieve with anodizing and electrophoretic coating. In addition, powder coating has obvious environmental advantages. In the anodic oxidation and electrophoretic coating processes, the consumption of water and electricity is quite large. In the oxidation process, the output current of the rectifier can reach 8-11kA, the voltage is between 15-17.5V (the oxidation voltage of sulfuric acid DC anodizing process is generally 12-18V), and the power consumption per ton can reach about 1000 kWh. In addition, anodizing, coloring, and sealing processes require a large amount of acid, alkali, and nickel salts, and the post-treatment pressure of wastewater and exhaust gas is high. The pre-treatment process of powder spraying is simpler than that of anodizing, mainly involving degreasing and chroming, without the need for anodizing and electrophoresis processes, and with lower energy consumption. Powder coating does not contain solvents, with almost zero VOC emissions and low environmental pressure. Compared with anodizing and electrophoretic coating, powder coating of aluminum profiles consumes much less electricity. However, the curing temperature of mainstream powder coatings can reach up to 180-200 ℃, and its energy consumption cannot be ignored. Reducing the curing conditions of powder coatings is a long-term development trend.

Research progress on powder coatings for building aluminum profiles

In recent years, the country and society have increasingly high requirements for environmental protection, and the trend of policy guidance gradually restricting and reducing the use of high-energy consumption and high pollution production processes is very obvious. Powder coating has ushered in a good opportunity for development. However, in order to expand the application of powder coating in the surface treatment of building aluminum profiles, it is necessary to improve the weather resistance and compensate for the shortcomings in performance while reducing the curing temperature and energy consumption of powder coating while maintaining its own application advantages.

2.1 Improvement of Weather Resistance of Powder Coatings

There have been many studies on the weather resistance of powder coatings both domestically and internationally. In the synthesis of polyester resin for powder coatings, increasing the proportion of terephthalic acid appropriately and reducing the amount of terephthalic acid, as well as using neopentyl glycol as much as possible and reducing or not using ethylene glycol to ensure weather resistance, have been widely recognized in the industry. However, the conventional method of replacing phthalic acid has the problem of deteriorating mechanical properties. Currently, the vast majority of commercially available super weather resistant polyester resins in China use the full phthalic acid solution, and the powder coating prepared from this type of super weather resistant polyester resin usually only has a recoil of 20cm. Poor mechanical properties are a common problem faced by these super weather resistant resins.

Among various types of powder coatings, fluorocarbon powder coatings have the best weather resistance and can meet the requirements of super weather resistance. Gong Yongzhong and others have conducted long-term research on fluorocarbon powder coatings and their key raw materials fluorocarbon resins. Currently, the processing performance of PEVE fluorocarbon powder has been greatly improved. The FEVE fluorocarbon powder coating prepared using the same equipment and process as conventional powder coatings has passed QUALICOAT-2009 III and AAMA 2605-2005 certifications, and the curing temperature has been reduced to 180-200 ℃. There are no application problems with mechanical properties and adhesion. However, the complex processing technology and high price of FEVE fluorocarbon resin limit its application. In order to reduce costs, domestic powder coating manufacturers introduce some fluorocarbon resins into conventional powder coatings, and produce weather resistant powder coatings through blending or layer separation techniques. This not only reduces costs but also improves the wetting and mechanical properties of fluorocarbon resins. Wei Yufu et al. introduced 6%~17% FEVE fluorocarbon resin into TGIC cured powder coatings, and the resulting powder coatings still have excellent weather resistance, with a xenon lamp aging retention rate of over 90% after 1000 hours. Zhang Yunwei achieved heavy anti-corrosion and super weather resistance by dry mixing epoxy powder coating and fluorocarbon powder coating, and achieving delamination after one coating through the difference in surface energy between epoxy resin and fluorocarbon resin. The prepared coating still maintained a gloss retention rate of over 90% after 2000 hours of xenon lamp accelerated aging. Gao Qingfu et al. combined TGIC cured polyester resin with isocyanate cured fluorocarbon resin to produce a composite super weather resistant powder coating. Research has shown that when the mass ratio of polyester resin to fluorocarbon resin is 1:1, the QUV-B 1000h artificial accelerated aging light retention rate is still over 60%, which can achieve a good balance between weather resistance and cost. However, under the same experimental conditions, the light retention rate of polyester resin powder coating is only 19.1%.

Introducing new weather resistant monomers to improve the weather resistance of the main structure of polyester resin is also a feasible solution. Chang et al. found that polyester resins synthesized using monomers 1,2-cyclohexanedicarboxylic acid or 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol as the main components without benzene rings have excellent weather resistance when cured with hydroxyalkylamides at approximately 177 ℃/20 min.

The aging time of QUV-B with 50% light retention rate is above 1500 hours; The aging time of QUV-B with 50% light retention rate using 1,2-cyclohexanedicarboxylic acid as a dicarboxylic acid can even reach 5000 hours, while the aging time of QUV-B with 50% light retention rate for coatings prepared with conventional polyester resin is below 300 hours. Yang Xiaoqing and others also found that polyester resins prepared using monomers without benzene rings have excellent weather resistance. Zheng Ronghui et al. introduced fluorinated monomers 1H, 1H, 10H, 10H-perfluoro-1,10-decanediol, tetrafluoroisophthalic acid, and hexafluoroglutaric acid into the synthesis process of polyester resin, and compared the prepared fluorinated polyester resin with beta- Hydroxyalkylamide curing can produce excellent weather resistant coatings. However, the price of these weather resistant monomers is much higher than that of conventional monomers, and the coatings prepared from the above-mentioned benzene ring free monomers still have the defect of low Tg.

In addition to improving the weather resistance of film-forming materials, the use of modified fillers and additives to enhance the weather resistance of powder coatings has also been reported. Guo Gang and Shi Qiwu found that adding surface modified rutile (R) - type nano TiO2 as a UV absorber to powder coatings at a dosage of 2% can significantly improve the weather resistance of the coating. Research by Tu Qinghua and others has shown that powder coatings are prone to white spots on the surface of the coating in high temperature and high humidity environments. These white spots are caused by the coating absorbing water. By using 10% to 40% surface treated BaSO4 and Al2O3 hydrophobic fillers, the white spots are basically eliminated, and the weather resistance of the coating is improved by enhancing hydrophobicity.

2.2 Research on Low Temperature Curing Powder Coatings

At present, powder coatings with curing conditions below 160 ℃ are referred to as low-temperature curing powder coatings in the industry. To achieve low-temperature solidification, the film-forming material needs to have high reactivity and low melt viscosity. At the same time, to ensure the necessary mechanical properties and powder storage stability of the coating, the relative molecular weight of the film-forming material before solidification cannot be too low.

Among different types of powder coatings, TGIC curing system, hydroxyalkylamide curing system, closed isocyanate curing system, and acrylic powder coating can meet the weather resistance requirements of building aluminum profiles. The blocked isocyanate curing system is difficult to meet the requirements of low-temperature curing due to the high unsealing temperature of commonly used caprolactam blocking curing agents, which can reach up to 160 ℃. Acrylic resin has high activity and excellent weather resistance, and is widely used in low-temperature curing. L· Mornes has prepared a powder coating that can cure at temperatures below 150 ℃ to achieve excellent coating properties. The powder coating consists of amorphous carboxyl end polyester resin A, amorphous or semi crystalline carboxyl end hydroxyl bifunctional polyester resin B1 and/or crystalline polyacid B2, glycidyl acrylic acid copolymer C, and other compounds D that can react with carboxyl groups. The mechanical properties of the coating obtained by curing the powder coating at 140 ℃/15 min are comparable to those of the room temperature cured powder coating. The QUVA artificial accelerated aging 50% light retention time is between 2200 and 2500 hours, and it has excellent weather resistance. Bin Wu has disclosed a semi crystalline polyester resin and its preparation method. The powder coating prepared by co extruding semi crystalline resin with conventional amorphous resin and glycidyl acrylic resin can be fully cured at 130 ℃/25 minutes, and has good mechanical properties and appearance leveling. Li Guang et al. prepared low-temperature curing acrylic powder coating by selecting high epoxy equivalent acrylic resin, low epoxy equivalent acrylic resin, dodecanedioic acid, and other additives. The coating was fully cured by baking at 150 ℃ for 20 minutes. After artificial accelerated aging for 1400 hours with QUV-A, the gloss retention rate of the coating was over 90%, and it was applied to the gloss paint of aluminum wheel hub covers. Zhang Jian et al. prepared a powder coating for outdoor MDF by blending polyester resin and acrylic resin, and using a low-temperature curing agent for polyester resin. The coating can achieve rapid curing at 130-150 ℃ under medium wave infrared pulse radiation heating.

At present, the TGIC curing system and hydroxyalkylamide curing system have the largest usage of weather resistant powder coatings. In terms of low-temperature curing, the hydroxyalkylamide system has more advantages. Due to the significant impact of TGIC on the Tg of powder coatings, TGIC curing resins require a high Tg, usually above 60 ℃. TGIC has high reactivity and usually requires the addition of curing accelerators to ensure sufficient curing at 200 ℃/10min. The lowest curing temperature that can be achieved through curing accelerators is also above 160 ℃, making the development of TGIC low-temperature cured polyester very difficult. Zheng Ronghui et al. prepared a polyester resin capable of curing TGIC systems at 140-160 ℃ by increasing the types and amounts of highly branched polyols, as well as increasing the amount of isophthalic acid in the polyacid component and introducing maleic anhydride and adipic acid. The resin was capped with highly active pyromellitic dianhydride, but the Tg of the polyester resin was only 53-57 ℃. The commonly used hydroxyalkylamide T-105 has four functional groups and is used in small quantities. Its effect on the Tg of powder coatings is much smaller than TGIC, and it has high reactivity. It can usually be completely cured at 180 ℃/10 min. Ma Hongying optimized the formula by selecting a combination of trimethylolpropane, neopentyl glycol, and 2-ethyl-2-butyl-1,3propanediol. The proportions of terephthalic acid, isophthalic acid, and adipic acid in the formula were adjusted, and trimellitic anhydride was used as the end capping agent to synthesize a polyester resin with an acid value of 50mg KOH/g and a Tg of 57 ℃. This polyester resin uses hydroxyalkylamide as a curing agent and can achieve complete curing at 120 ℃/40 min, 130 ℃/30 min, 140 ℃/20 min, and 150 ℃/15 min. Under the above curing conditions, the coating achieved a 50cm positive and negative impact, and the QUV-B aging light retention rate was above 80% after 240 hours. Deng Muqiang et al. prepared a hydroxyalkylamide cured polyester resin that can be cured at 130-140 ℃ by introducing aliphatic 1,6-hexanediol, cycloaliphatic polyol 1,4-cyclohexanedimethanol, and methyl acrylic acid, with a Tg above 55 ℃. Ma Zhiping et al. introduced hydrogenated dimeric fatty acids to achieve a balance between the flexibility and Tg of polyester resin. By adding 1,4-cyclohexanedimethanol afterwards, the viscosity of the polyester resin was reduced. The resulting hydroxyalkylamide cured resin had an acid value of 50-55mgKOH/g and could be fully cured at 140 ℃. Zhang Jian et al. selected high acid value ultra weather resistant polyester resin with an acid value of 42-56mgKOH/g, used hydroxyalkylamide as a curing agent, and achieved rapid curing at 150-160 ℃ on the surface of fiberglass under the action of curing accelerator. The prepared coating has excellent weather resistance and good adhesion.

Summary

The three coating processes for building aluminum profiles in China each have their own characteristics in terms of performance. In terms of application performance, powder coating has significant advantages in selecting diversity and personalization. However, China's powder coatings have not yet made breakthrough progress in improving weather resistance and reducing curing temperature and energy consumption. At present, fluorocarbon powder coatings are expensive and limited in application, and there are other performance shortcomings in the weather resistance improvement solutions that are acceptable in cost; There are very few commercial products of low-temperature curing powder coatings, and there are many difficulties to be solved in both upstream raw material supply and downstream application markets. With the increasing attention of the Chinese people to environmental issues, policy guidance is conducive to expanding the application proportion of powder coating, but there is still a need to strengthen technology research and development within the industry to solve various problems faced.