Energy-saving and environmentally friendly building aluminum profile powder coating technology and application

Abstract:
In view of the problems in the development of powder coating technology in the direction of energy saving and environmental protection, the development trend of coating technology and application is expounded from the three aspects of green new powder, chromium-free pre-treatment and waste heat utilization technology and application.
aluminum profile has the characteristics of low density, light quality, processing and plasticity, widely used in architecture, home, decoration and other fields. China has overtaken the United States since 2006 to become the world's largest aluminum profile production and sales of countries, according to the official website of the China Nonferrous Metals Industry Association statistics: 2017 the world's electrolytic aluminum production of about 63 million t, while China's aluminum profile production reached 19.5 million t, of which construction aluminum profiles accounted for about 70%
China
. Although aluminum profiles can form a dense al2O3 oxide film in the air and have good self-healing properties, but the oxide film is very thin, the protection of aluminum profiles is limited, affecting the life and aesthetics of the profile. Therefore, aluminum profiles need to go through a certain amount of surface protection treatment: on the one hand, improve the wear, corrosion resistance of the profile, extend its service life, on the other hand, improve the aluminum profile in the process of processing caused by a small number of surface defects, and provide a variety of colorful surface decoration effect, improve the aesthetics of the profile.
, aluminum profile surface treatment methods are mainly anode oxidation, electrophoresis coating and powder coating. Each of these three treatment methods has advantages and characteristics, and has a considerable market share in the market. Anode oxidation is a method of forming a layer of oxide film on the surface of aluminum profile by using the principle of electrolytic oxidation. The oxidizing film prepared by this legal system has high wear resistance, good insulation and corrosion resistance, and can meet the service requirements in the continental climate. Typical anode sulfate oxidation processes include: mechanical pre-treatment, chemical pre-treatment, anode oxidation, coloring and closure processes. Electrophoresis coating process and anode oxidation process is basically the same, the difference is that electrophoresis coating in the anode oxidation coloring process after the electrophoresis coating process to replace the closed process, further improve the film layer weather resistance and corrosion resistance, to meet the coastal or offshore and other harsh climate requirements. Powder coating is a coating method that uses the principle of corona discharge to make the atomized coating load negative electricity under the action of high voltage DC electric field and absorb discharge on the surface of the load positive substrate.
in order to improve the binding force and corrosion resistance of the coating, aluminum profiles are usually pre-treated before coating, such as chromate conversion or chromate-free conversion. Compared with anode oxidation and electrophoresis, the performance of coating prepared with powder coating process, such as appearance uniformity, weather resistance and so on, has no obvious advantages, but its wear resistance, acid resistance, flexibility and decorative properties are significantly better than those prepared with anode oxidation and electrophoresis coating. In addition, due to the simple powder coating process, there are obvious advantages in energy conservation and environmental protection. Anode oxidation tons of electricity consumption can reach about 1000 degrees, anode oxidation, coloring and closure, electrophoresis and other processes need to use a large number of acids, alkalis and nickel salts, waste water and exhaust gas after treatment pressure. The pre-treatment of powder coating is mainly degreasing and passivation, energy consumption is mainly reflected in coating curing, energy consumption is greatly reduced, and powder coating is solvent-free, VOC emissions are almost 0, environmental pressure is small, has become a widely used aluminum profile coating technology.
with the increasingly strict requirements of energy conservation and environmental protection, powder spraying gradually to a more energy-saving and environmental protection direction. The main performance in the following three aspects: First, the emergence of new coating powders, such as environmentally friendly low toxic curing agent, high reflectivity energy-saving powder and self-cleaning functional powder, etc. ; This paper mainly expounds the energy-saving and environmental protection technology and its application in the powder coating process from the above three aspects.
1 New powder coating
1.1 environmentally friendly low toxic curing agent
Currently, the powder coating commonly used in aluminum profiles is weather-resistant polyester powder coating, the polyester powder coating is mostly saturated, according to its end-base structure can be divided into end-to-end niobium and end hydroxyl two categories. The commonly used powder coating curing agent is isocyanuric acid triglycerides, or TGIC (Tri-glycidyl iso-cyanurate). Due to its good heat resistance (yellowing) and weather resistance, the gloss and texture of the cured products are one, stable and adjustable, making TGIC the most important curing agent for weather-resistant polyester powder coatings. However, with the understanding of its toxicity, especially environmental pollution, such as oral will produce moderate acute toxicity, direct contact with the eyes and skin after serious eye irritation and allergic skin reactions, in addition, there may be affecting male fertility, serious harm to the physical and mental health of operators, especially first-line workers, for this reason, countries have developed a large number of TGIC alternatives. The curing agents used for powder coating of aluminum alloy profiles are mainly hydroxyamide systems (HAA, Hydroxyl alky amide). The process characteristics of the curing agent TGIC and HAA are listed in Table 1. In terms of energy saving, HAA curing agent has low temperature curing characteristics (starting curing at 130 degrees C), which is about 20% energy saving compared to TGIC curing agent; By comparing the process and cost of the two types of curing agents, it is found that HAA has obvious advantages. However, HAA curing agent also has obvious disadvantages: First, the coating film is relatively difficult to eliminate light, less than 20% gloss conditions, it is difficult to ensure the aesthetic properties of the coating. Second, the coating film is prone to pinholes, especially when thick coating, which is caused by the escape of small molecules. Third, over-baking coating film is prone to yellowing. Fourth, hydrolysolysing is slightly worse than TGIC, but the problem can be solved with matching polyester.HaA powder curing agent is slow to be popularized in China. Check out the network report of powder coatings on curing agents, for the domestic market, TGIC curing agent powder accounted for 70%, HAA curing agent powder accounted for 30%, for the global outdoor market, TGIC curing agent powder accounted for 44%, HAA curing agent powder accounted for 46%, the other share is mainly aminomethyl acid, acrylic and propylene triacid. HAA curing agents are mainly used for the import of large brands of powders, such as Akzo, Tiger, DuPont and Jotun, domestic brands such as Time, Huajiang and Jingaoli are rarely used, HAA curing agent powder manufacturers and their dosage listed in Table 2., HAA curing agent environmental protection powder has the advantages of energy saving and environmental protection, but its weather resistance, gloss, yellow resistance, hydrolysis resistance and membrane thickness easy pinholes still need to be improved.
1.2 high reflectivity energy-saving powder
a coating that allows the coating to produce temperature-regulating effects in sunlight is called a reflective coating. High reflectivity energy-saving powder has excellent weather resistance and good near-infrared reflection characteristics, suitable for outdoor building applications, especially in the need to strengthen the reflection of sunlight or near-infrared, such as the building's exterior walls, roofs, doors and windows and large curtain wall single board. It can effectively reduce the temperature in the building, reduce the energy consumption of air conditioning refrigeration, while the low surface temperature will reduce the coating film thermal expansion and contraction caused by film bending, deformation and partial damage, so the coating can delay the coating color, gloss, elasticity and some other physical properties of thermal aging. In addition, the coating has an energy-saving effect, especially in colder winter areas, which further impedes the rapid exchange of heat inside and outside windows. In addition, with the bridge insulation can make the door wall system to achieve a good insulation effect. Figures 1 and 2 are the series of insulation windows and insulation curtain walls independently developed by Guangdong Haomei New Materials Co., Ltd.1.3 self-cleaning functional powder
self-cleaning powder has good weather resistance, adhesion resistance and self-cleaning. Self-cleaning refers to dust and water and powder surface binding force is low, rolling water droplets can take away some dust sludge particles, to achieve the effect of self-cleaning, to achieve a spotless effect, as shown in Figure 3. The self-cleaning function of the powder is to apply the principle of ultra-hydrophobic and self-cleaning effect of the lotus leaf surface, as shown in Figure 4. The introduction of macromolecule perfluoropolyther and active nanoAl2O3 particles in the coating structure, the F-C bond of perfluoropolyte has a very low surface energy and the small size effect of active nanoparticles, showing abnormal dual dredging on the coating surface. Inorgesty dust in the air generally has a certain polarity, in the super-hydrophobic surface can not form a strong adsorption, easy to be washed away by rain. Even if the coating surface is adsorbed by organic matter, the ultra-strong photocatalytic activity of nanoparticles can be used to eventually break down the organic matter into CO2, H2O, H2SO4, HNO3 and other easily washable substances. Therefore, this powder ensures that the coating surface is as clean as new. The powder coating is generally used in curtain wall products, and when combined with clean glass, the overall cleaning-free function can be achieved.2 Chromium-free pre-treatment
the usual chrome (hexavalent chromium) pre-treatment in the spray industry produces passivation films to improve the product's salt atom resistance and binding force between substrates and coatings. However, hexavalent chromium is easily absorbed by the human body, it can invade the body through the digestive tract, respiratory tract, skin and mucous membranes, long-term or short-term contact or inhalation of the risk of cancer. The industry is phasing out pre-chromium processing and replacing chrome treatment with chromium-free passivation technology. At present, there are more chromium-free passivation processes, such as titanium passivation, rare earth salt passivation, permanganate passivation, palladium passivation, lithium acid passivation, tannin passivation, phosphate passivation and organic passivation.
the test, Guangdong Haomei New Materials Co., Ltd. finally selected the tantalum titanium system process (titanium system is currently the only EuropeanQualicoat certification system) for the pre-treatment of aluminum profiles. The treatment solution of the process is mainly composed of titanium, zirconium metal salts, fluoride, nitrates and organic additives, which form a transformation film by impregnation and spraying. The film layer is mainly composed of niobium titanium salt, aluminum oxide, aluminum fluoride and niobium titanium compound, and the binding force of organic polymers is strong. There are some differences in operation between chromium passivation and chromium-free passivation: first, the chromium-free passivation process requires high water quality, especially before passivation and pure water washing after passivation. If the de-oiling dose brought in by the water wash before passivation is too high, it is easy to lead to the instability of chromium-free passivation, and if the pure water wash after passivation is mixed with chromium ions, adhesion will also be unstable. Secondly, aluminum profiles should be kept clean after pre-treatment, avoid bare-handed touch, especially not to touch strong oxidizing substances or items with chromium ions, which will affect their adhesion. Third, the heat resistance of chromium-free pre-treatment film is good, and the temperature can be appropriately increased when drying, which can improve production efficiency.
, there are still some problems when chromium-free pre-treatment technology moves towards large-scale application. First of all, in terms of adhesion, due to the chromium-free passivation product system more, the technical characteristics are uneven, some of the system itself has the defect of poor adhesion, but also due to harsh process conditions, improper operation resulting in a serious decline in adhesion after the membrane weight exceeds the standard.
, product color changes are not obvious, field operators in this process is difficult to use color changes to judge the final product quality. Finally, the non-chromium pre-treatment products and structural glue incompatible is more obvious. The test results of a well-known brand of structural glue in the United States show that a number of chromium-free passivation products in the anti-peeling adhesion test, there is a peeling phenomenon, so the compatible between the two has become a bottleneck in the promotion of chromium-free pre-treatment technology, is one of the key to break through this process at this stage.
3 Waste heat utilization technology
the heating equipment in the powder spraying process has a pre-treatment drying furnace, pre-curing furnace and curing furnace, the pre-treatment liquid is also equipped with a constant temperature heating equipment. The temperature of the pre-treatment drying furnace is generally 95 degrees C, the temperature of the curing furnace is generally 180 degrees C, there is a large temperature difference between the two processes, Guangdong Haomei New Materials Co., Ltd. set up a waste heat recovery system, that is, under the premise of controlling the temperature of the curing furnace, so that the exhaust heat of the curing furnace into the pre-treatment dryer, to achieve the residual heat recovery and energy-saving consumption purposes of the curing furnace, the effect is more obvious. Figure 5 shows a brief diagram of the waste heat recovery process of Guangdong Haomei New Materials Co., Ltd. using vertical spraying production line, and the company's tons of natural gas consumption data before and after the use of residual heat are listed in Table 3. Table 3 shows that the consumption of natural gas tons decreased by 26.82% after the use of residual heat.in the aluminum profile production process, a lot of residual heat can be used, such as cast furnaces, homogeneity furnaces and cast rod heating furnaces. The range of residual heat utilization is also very wide, if the requirements for flue gas is not high can be used directly to heat flue gas, such as pre-treatment drying furnace, but also the use of heat exchange to anode oxidation hot water tank or dormitory with hot water heating. In addition, now large aluminum plants are using natural gas, flue gas quality is good, can be more convenient to achieve residual heat utilization. In general, the rational use of residual heat, enterprises can achieve energy conservation and consumption reduction.4 Conclusion
Energy-saving and environmentally friendly powder coatings, such as HAA curing agents, high reflectivity energy-saving powders and self-cleaning functional powders, these products are environmentally friendly, and can significantly reduce energy consumption, is one of the future trends of powder coatings. Chromium-free pre-treatment alternative chromate treatment is an inevitable requirement for the development of aluminum profile pre-treatment, but there are problems such as insufficient adhesion, harsh process conditions and poor compatibility with structural glue, which need to be further studied. In the aluminum profile processing industry, energy saving and consumption reduction can be achieved through the use of residual heat.
Wen/Zhu Shi'an, Wu Shichen, Xu Lianjun, Wang Zhiyi,
Guangdong Haomei New Materials Co.,