At present, most pre-coated coiled color plates are coated with liquid coil coatings, but the large amount of VOCs emitted during the production and use of the coatings has attracted great attention from relevant national departments and the public. In 2016. the Ministry of Industry and Information Technology issued two policy documents, "The Ministry of Industry and Information Technology's Notice on Printing and Distributing the Development Plan of the Building Materials Industry (2016-2020)" and "The Action Plan for Reducing Volatile Organic Compounds in Key Industries", both of which clearly focus on promoting powder coating products, making the development of environmentally friendly powder coatings in the coil field imperative.
Coil coating is an important sub-industry in the field of industrial coatings. The State Council's "Notice on Printing and Distributing the Three-Year Action Plan for Winning the Blue Sky Defense War" (Guofa [2018] No. 22) requires "before the end of 2019. complete the formulation of mandatory national standards for VOCs content limits for coatings, inks, adhesives, cleaning agents and other products", "promote the upgrading and transformation of pollution control in key industries, and fully implement special emission limits for volatile organic compounds (VOCs) in key areas".
Background of powder coating on coils
Currently, the annual production capacity of color-coated coils in China has reached 52.39 million tons, of which Shandong has a production capacity of 22.93 million tons, accounting for 63.03% of the production capacity in East China and 43.76% of the country. In addition, Shandong is also the largest color-coated plate production base in China, with more than 300 production lines in China, of which Shandong accounts for more than 100.
From the perspective of subdivision, coatings mainly include architectural coatings, general industrial paints, protective coatings, powder coatings, wood coatings, etc., accounting for 52%, 13%, 10%, 8% and 5% of the national coatings industry respectively. In 2021. the output of powder coatings reached 2.35 million tons, accounting for 8% of the coatings field. Among them, coil coatings accounted for 3%, with an annual output of more than 800.000 tons.
Color-coated plates are currently produced using roller coating technology, and solvent-based coatings are used. The advantages are mature technology, thin coating, good leveling, good covering, fast color change, and high production efficiency. However, they also emit a large amount of VOC, requiring a VOC recovery system. Although solvent incineration can utilize heat energy, solvent incineration wastes resources on the one hand and pollutes the atmosphere on the other. Small coil factories that are not closed for coating have a bad production environment and a bad smell.
Currently, VOC is one of the important factors that hinder the city's air quality from reaching the standard. After significantly reducing the particle concentration, the annual increase in ozone concentration has become the primary problem that plagues cities' non-compliance with standards. Therefore, reducing VOC emissions is currently a national priority for improving the quality of urban and regional atmospheric environments.
Therefore, the state and local governments have formulated the "Three-Year Action Plan to Win the Blue Sky Defense", focusing on reducing VOC emissions in paints and coatings; the 2013 "Air Pollution Prevention and Control Action Plan" and the "2020 Volatile Organic Compounds Governance Tough Plan" for atmospheric governance, vigorously promoting the replacement of raw and auxiliary materials with low (no) VOC content, and the collection of consumption tax on coatings.
Development of domestic powder coating technology for coils
At present, the technical direction of domestic powder coating for coils is:
Powder cloud chamber spraying method is in the experimental stage.
High-speed rotary cup spraying, horizontal powder spraying, has been mass-produced, the rotary cup needs to be replaced regularly, and powder nodules and particles are prone to occur.
High-efficiency electrostatic spraying, the first generation horizontal, can be produced stably, it is easy to have powder nodules and particles, and is not suitable for plates with high surface requirements.
High-efficiency electrostatic spraying, the second generation vertical, has a high powder coating rate, good surface effect, good technical maturity, and has been mass-produced.
Foreign coil powder coating technology
Powder block technology
Australia has developed the "Solid Block" powder coating technology, which uses a pre-fused powder coating block (similar to chalk) pressed on a moving preheated coil plate, and the powder coating is transferred to the hot coil plate through a physical mechanical process.
However, this technology has not achieved commercial success due to speed limitations and inconsistent coil surface conditions. Any irregular surface defects or edges will cause leaks or streaks.
Powder cloud chamber
A company has developed the "Powder cloud" technology. It uses a series of brushes to atomize powder coatings, and uses electrostatic areas to make powder coating particles reach the surface of the moving coil. The cloud of powder coatings can be applied to the metal surface at high speed.
However, due to the high cost and difficulty in controlling in practical applications, it is currently in the semi-commercial stage. At the same time, the coating thickness is difficult to reduce, which limits the widespread application of powder cloud chamber technology.
MB technology
A Dutch company has developed the "Electro-magneticbrush (EMB)" powder coating technology, which uses a principle similar to that of a copier and laser printer. The charged powder coating particles are applied through a mixing wheel along the action of a strong magnetic field, called an "electromagnetic brush", which can be transferred to charged metals.
However, this technology has not achieved great success so far. It can only apply very thin coatings, and it is impossible to obtain coatings of different thicknesses through a single construction. At the same time, because the coating equipment is complex, the production requirements for powder are high, and the investment cost is high, the production equipment has not been widely used.
