Advances in Powder Coating Research and Analysis of Coating for Architectural Aluminum Profiles

Advantages of Powder Coating in Surface Treatment of Architectural Aluminum Profiles

Currently, surface treatment of aluminum profiles mainly includes three methods: anodizing, electrophoretic coating, and powder coating. A complete anodizing process typically requires five steps: mechanical pretreatment, chemical pretreatment, anodizing, coloring, and sealing. Electrophoretic coating is largely the same as anodizing, the difference being that after the anodizing coloring step, the sealing step is replaced by an electrophoretic coating step. Therefore, the surface of an aluminum profile after electrophoretic coating is actually a composite film of anodized film and electrophoretic coating, also known as anodized composite film. Powder coating also requires chemical pretreatment, followed by electrostatic spraying of the powder coating.

The coatings obtained from the three surface treatments of aluminum profiles each have their own characteristics. Anodizing was the primary method for surface treatment of architectural aluminum profiles in my country in the early days. Anodized films have high wear resistance, good thermal insulation properties, and corrosion resistance, and it remains one of the main methods for surface treatment of aluminum profiles. Electrophoretic coating matured in Japan, a maritime country surrounded by the sea. Corrosion of aluminum profiles caused by sea salt particles or sludge mixed with sea sand is a significant problem, making anodizing difficult to effectively protect against such high corrosion. Electrophoretic coating offers excellent weather resistance and corrosion resistance, along with a bright appearance and ease of cleaning, leading to its rapid development. Data from exposure tests in Florida, USA, shows that the gloss retention of anodized composite films obtained through electrophoretic coating (5-year exposure) is comparable to that of fluorocarbon coatings, with even less color difference. However, electrophoretic coating also suffers from drawbacks such as susceptibility to scratches. Furthermore, the anodized film as the base layer has poor toughness and is prone to cracking under mechanical or thermal stress. Reports indicate that cold-sealed anodized films can only withstand baking at 66℃, and only half of the samples passed baking at 82℃.

In the early 1990s, powder coating began large-scale application in the surface treatment of aluminum profiles in my country, developing rapidly over the past decade. However, the performance advantages of powder coating are not yet significant. While its appearance smoothness and coating uniformity are inferior to anodizing and electrophoretic coating, and its weather resistance falls between anodizing and electrophoretic coating, its abrasion resistance, acid resistance, and flexibility are significantly superior to both. As a semi-permanent structure, the durability of architectural aluminum profiles is paramount; therefore, resistance to mechanical action and anti-aging to maintain the integrity and functionality of the coating film are particularly important. Commonly used electrophoretic paints are acrylic coatings, possessing excellent weather resistance. GB 5237-2008's accelerated weathering standard requires a gloss retention rate of >80% after 1000 hours of xenon lamp aging at the lowest level, with the highest level requiring >80% after 4000 hours. General-purpose powder coatings for architectural aluminum profiles are primarily composed of polyester resin, whose weather resistance is slightly inferior to acrylic. GB 5237-2008's highest accelerated weathering standard only requires a gloss retention rate of >90% after 1000 hours of xenon lamp aging.

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 architectural aluminum profiles has fallen behind actual needs. Powder coating has significant advantages in application. It can achieve thousands of colors and a wide variety of textures, which is difficult to achieve with anodizing and electrophoretic coating. Furthermore, powder coating has significant environmental advantages. Anodizing and electrophoretic coating processes consume considerable amounts of water and electricity. In the oxidation process, the rectifier output current can reach 8-11 kA, and the voltage is 15-17.5 V (the voltage for sulfuric acid DC anodizing is generally 12-18 V), with electricity consumption reaching approximately 1000 kWh per ton. In addition, anodizing, coloring, and sealing processes require large amounts of acids, alkalis, and nickel salts, resulting in significant wastewater and exhaust gas post-treatment pressure. The pretreatment process for powder coating is simpler than that for anodizing, mainly involving degreasing and chromating, eliminating the need for anodizing and electrophoretic processes, thus reducing energy consumption. Powder coatings do not contain solvents, and VOC emissions are almost zero, reducing environmental pressure. Compared to anodizing and electrophoretic coating, powder coating of aluminum profiles consumes significantly less electricity. However, the curing temperature of mainstream powder coatings is currently as high as 180-200℃, and its energy consumption remains significant. Reducing the curing conditions of powder coatings is a long-term development trend.

Research Progress of Powder Coatings for Architectural Aluminum Profiles

In recent years, national and societal requirements for environmental protection have become increasingly stringent. Policy guidance has clearly shown a trend of restricting and reducing the use of high-energy-consuming and high-polluting production processes, presenting a significant opportunity for powder coating development. However, to expand the application of powder coating in the surface treatment of architectural aluminum profiles, it is essential to improve weather resistance to compensate for performance deficiencies while maintaining its own application advantages, and simultaneously reduce the curing temperature to decrease energy consumption.

2.1 Improvement of Weather Resistance of Powder Coatings

There has been considerable research on the weather resistance of powder coatings both domestically and internationally. In the synthesis of polyester resins for powder coatings, appropriately increasing the proportion of isophthalic acid and reducing the amount of terephthalic acid, as well as using neopentyl glycol as much as possible and reducing or eliminating the use of ethylene glycol to ensure weather resistance, has gained widespread acceptance within the industry. However, conventional isophthalic acid substitution methods suffer from deteriorated mechanical properties. Currently, most commercially available ultra-weather-resistant polyester resins in China use a full isophthalic acid solution. Powder coatings prepared from this type of ultra-weather-resistant polyester resin typically only achieve a backwash of 20cm, and poor mechanical properties are a common problem faced by these ultra-weather-resistant resins.

Among various types of powder coatings, fluorocarbon powder coatings exhibit the best weather resistance, meeting ultra-weather-resistant requirements. Gong Yongzhong et al. have conducted long-term research on fluorocarbon powder coatings and their key raw material, fluorocarbon resin. Currently, the processing performance of PEVE fluorocarbon powder has been greatly improved. FEVE fluorocarbon powder coatings prepared using the same equipment and processes as conventional powder coatings have passed QUALICOAT-2009Ⅲ and AAMA 2605-2005 certifications, and the curing temperature has been reduced to 180~200℃. Mechanical properties and adhesion are not problematic for application. However, the complex processing technology and high price of FEVE fluorocarbon resin limit its application. To reduce costs, domestic powder coating manufacturers have introduced fluorocarbon resins into conventional powder coatings, using blending or layer separation techniques to produce powder coatings with excellent weather resistance. This approach improves the wetting and mechanical properties of the fluorocarbon resin while reducing costs. Wei Yufu et al. introduced 6%–17% FEVE fluorocarbon resin into TGIC-cured powder coatings, resulting in powder coatings that still exhibited excellent weather resistance, with a gloss retention rate exceeding 90% after 1000 hours of xenon lamp aging. Zhang Yunwei achieved heavy-duty corrosion protection and ultra-weather resistance by dry-mixing epoxy and fluorocarbon powder coatings, utilizing the surface energy difference between the two resins to achieve layering after a single coating application. The resulting coating maintained a gloss retention rate of over 90% after 2000 hours of accelerated xenon lamp aging. Gao Qingfu et al. prepared a composite ultra-weather-resistant powder coating by blending TGIC-cured polyester resin and isocyanate-cured fluorocarbon resin. Studies showed that when the mass ratio of polyester resin to fluorocarbon resin was 1:1, its gloss retention rate after 1000 hours of artificial accelerated aging with QUV-B was still over 60%, achieving a good balance between weather resistance and cost. Under the same experimental conditions, the gloss retention rate of polyester resin powder coating was only 19.1%.

Improving the weather resistance of the polyester resin matrix by introducing new weather-resistant monomers is also a feasible approach. Chang et al. found that a polyester resin synthesized using benzene-free monomers such as 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, cured with hydroxyalkylamide at approximately 177℃/20 min, produced a coating with excellent weather resistance.

The aging time of QUV-B with 50% gloss retention was over 1500 hours; the aging time of QUV-B with 50% gloss retention using 1,2-cyclohexanedicarboxylic acid as the dicarboxylic acid even reached 5000 hours, while the aging time of QUV-B with 50% gloss retention of coatings prepared from conventional polyester resins was less than 300 hours. Yang Xiaoqing et al. also found that polyester resins prepared using monomers without benzene rings had 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 polyester resin synthesis process, and cured the prepared fluorinated polyester resin with β-hydroxyalkylamide to obtain a coating with excellent weather resistance. However, the price of these weather-resistant monomers is much higher than that of conventional monomers, and the coatings prepared from the above-mentioned monomers without benzene rings also have the defect of low Tg.

Besides improving the weather resistance of the film-forming material, the use of modified fillers and additives to enhance the weather resistance of powder coatings has also been reported. Guo Gang and Shi Qiwu independently discovered 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. Tu Qinghua et al.'s research shows that white spots easily appear on the surface of powder coatings in high-temperature and high-humidity environments. These white spots are caused by water absorption by the coating. By using 10%~40% of surface-treated BaSO4 and Al2O3 hydrophobic fillers, the white spots basically disappear, thus improving the weather resistance of the coating by increasing hydrophobicity.

2.2 Research on Low-Temperature Curing Powder Coatings

Currently, the industry refers to powder coatings with curing conditions <160℃ as low-temperature curing powder coatings. Achieving low-temperature curing requires the film-forming material to have high reactivity and low melt viscosity. Simultaneously, to ensure the necessary mechanical properties of the coating and the powder's storage stability, the relative molecular mass of the film-forming material before curing cannot be too low.

Among different types of powder coatings, those that can meet the weather resistance requirements of architectural aluminum profiles include TGIC curing systems, hydroxyalkylamide curing systems, blocked isocyanate curing systems, and acrylic powder coatings. Among these, blocked isocyanate curing systems, due to the high deblocking temperature (up to 160℃) of commonly used caprolactam curing agents, are difficult to meet the requirements for low-temperature curing. Acrylic resins, with their high activity and excellent weather resistance, are widely used in low-temperature curing. L. Moens prepared a powder coating that can be cured below 150℃ to achieve excellent film performance. This powder coating consists of an amorphous carboxyl-terminated polyester resin A, an amorphous or semi-crystalline carboxyl-terminated hydroxyl-terminated bifunctional polyester resin B1 and/or a crystalline polyacid B2, a glycidyl acrylate copolymer C, and other compounds D that can react with carboxyl groups. The mechanical properties of the film obtained after curing at 140℃ for 15 min are comparable to those of room-temperature cured powder coatings, and the gloss retention time under QUVA accelerated aging (50%) is 2200~2500 h, demonstrating excellent weather resistance. Bin Wu disclosed a semi-crystalline polyester resin and its preparation method. The powder coating prepared by co-extruding the semi-crystalline resin with conventional amorphous resin and glycidyl acrylate resin can be fully cured at 130℃/25min, exhibiting excellent mechanical properties and surface leveling. Li Guang et al. prepared a low-temperature curing acrylic powder coating by selecting high epoxy equivalent acrylic resin, low epoxy equivalent acrylic resin, dodecanoic acid, and other additives. Full curing was achieved by baking at 150℃ for 20min. After 1400h of accelerated aging under QUV-A, the gloss retention rate was over 90%, and it was applied to aluminum wheel clear coats. Zhang Jian et al. prepared an outdoor MDF powder coating by blending polyester resin and acrylic resin, with the aid of a low-temperature curing agent. This coating can achieve rapid curing at 130~150℃ under mid-wave infrared pulse radiation heating.

Currently, the TGIC curing system and the hydroxyalkylamide curing system are the most widely used weather-resistant powder coatings. In terms of low-temperature curing, the hydroxyalkylamide system has a greater advantage. Because the addition of TGIC has a significant impact on the Tg of powder coatings, TGIC-cured resins require a high Tg, typically above 60℃. TGIC is highly reactive, usually requiring the addition of curing accelerators to ensure full curing at 200℃/10min. However, the lowest curing temperature achievable with curing accelerators is above 160℃, making the development of low-temperature curable polyesters for TGIC extremely difficult. Zheng Ronghui et al. prepared a polyester resin capable of curing the TGIC system at 140-160℃ by increasing the types and amounts of highly branched triols, increasing the amount of isophthalic acid in the polybasic acid component, and introducing maleic anhydride and adipic acid, and end-capping with highly reactive pyromellitic dianhydride. However, the Tg of this polyester resin was only 53-57℃. The commonly used hydroxyalkylamide T-105 has four functionalities, requires a small amount, has a much smaller impact on the Tg of powder coatings than TGIC, and is highly reactive, typically curing completely at 180℃/10min. Ma Hongying optimized the formulation, selecting the optimal combination of trimethylolpropane, neopentyl glycol, and 2-ethyl,2-butyl-1,3-propanediol. She also adjusted the proportions of terephthalic acid, isophthalic acid, and adipic acid in the formulation, and used trimellitic anhydride as the end-capping agent to synthesize a polyester resin with an acid value of approximately 50 mg KOH/g and a Tg of 57 °C. This polyester resin, using hydroxyalkylamide as a curing agent, can achieve complete curing at 120 °C/40 min, 130 °C/30 min, 140 °C/20 min, and 150 °C/15 min. Under these curing conditions, the coatings achieved a 50 cm forward and reverse impact, and the gloss retention rate after 240 h of QUV-B aging was above 80%. Deng Muqiang et al. prepared a hydroxyalkylamide-cured polyester resin that could cure at 130-140℃, with a Tg above 55℃, by introducing aliphatic 1,6-hexanediol, alicyclic polyol 1,4-cyclohexanediethanol, and methacrylic acid. Ma Zhiping et al. achieved a balance between the flexibility and Tg of the polyester resin by introducing hydrogenated dimer fatty acids and reduced the viscosity of the polyester resin by adding 1,4-cyclohexanediethanol later. The resulting hydroxyalkylamide-cured resin had an acid value of 50-55 mgKOH/g and could be fully cured at 140℃. Zhang Jian et al. selected a high-acid-value, ultra-weather-resistant polyester resin with an acid value of 42-56 mgKOH/g, used hydroxyalkylamide as a curing agent, and achieved rapid curing at 150-160℃ when coated on fiberglass surfaces with the aid of a curing accelerator. The resulting coating exhibited excellent weather resistance and good adhesion.

Summary

The three coating processes for architectural aluminum profiles in my country each have their own performance characteristics. In terms of application performance, powder coating has a significant advantage in terms of versatility and personalization. However, my country's powder coating industry has not yet made breakthrough progress in improving weather resistance and reducing curing temperature to decrease energy consumption. Currently, fluorocarbon powder coatings are expensive and their applications are limited, while cost-effective weather resistance improvement solutions have other performance shortcomings. There are very few commercially available low-temperature curing powder coatings, and many difficulties need to be addressed in both upstream raw material supply and downstream application markets. With the increasing public awareness of environmental protection in my country, policy guidance is favorable for expanding the application of powder coating, but the industry still needs to strengthen technological research and development to solve the various problems it faces.