Color-coated board production technology was first created in the United States in 1927. Since the 1960s, the production of color-coated plates has developed rapidly. Among them, the United States and Japan have more color plate production lines with larger production capacity and complete varieties and specifications. In foreign countries, color panels have been widely used in construction, home appliances, automobile manufacturing and other industries. With the growth of color-coated board consumption and the continuous expansion of application fields, as well as the continuous improvement of users' requirements on the quality and performance of their products, various new technologies for color board production have been continuously developed and applied, and the varieties of color boards have gradually become diversified. Development in the direction of modernization and advancedization.
In China, the consumption and production of colored plates began to rise in the late 1990s, and its growth momentum was extremely rapid. By the end of 2003, 124 companies had built 169 coating units with a production capacity of 8.74 million tons, of which 7 were member units of the China Iron and Steel Industry Association. The production capacity is 1.04 million tons; there are 162 non-member units with a production capacity of 7.7 million tons. However, because most of the production lines are affected by factors such as low level of equipment, weak technical force, and unstable supply of raw materials, the unit operation rate is low. At present, the domestic color plate market is still in a state of half self-produced, half imported, and supply exceeds demand. In 2003, the total domestic output was 1.4546 million tons, the import was 1.1016 million tons, the export was 16,200 tons, and the total apparent consumption was 2.5399 million tons.
The low-level repeated construction in China has caused vicious competition in the color coating market, and has caused many units with an investment of tens of millions to stop production. China Iron and Steel Association predicts that the domestic demand for color plates will reach 4.3 million tons in 2005 and will reach 8 million tons in 2010, and the proportion of non-construction plates will be greatly increased. According to data from the ECCA 2003 annual meeting, the proportions of color-coated steel sheets and color-coated aluminum sheets used for construction in North America were 66% and 53.3%, respectively. At present, my country’s consumer market is mainly in the building decoration industry. The proportion of boards is as high as 95%. The application in the home appliance industry is still in the trial stage of small batches, and the application in the transportation industry has basically not started. Therefore, color-coated sheets still have broad market prospects in China.
Color-coated board production process and coating
color-coated sheets mainly include ordinary coated sheets and special color-coated sheets. At present, domestic ordinary coated steel sheets are mainly used in the construction industry. The process flow is as follows:
The basic production process of coated board: unwinding→double-cutting→suture machine→(pre-degreasing)→looping→degreasing cleaning→chemical coating treatment→drying→primary coating→curing and drying→cooling (water, wind)→fine coating Finishing paint→curing and drying→cooling (water, wind)→looping→coiling.
color coating substrates include cold-rolled annealed sheet, ordinary spangle smooth hot-dip galvanized sheet, small spangle smooth hot-dip galvanized sheet, zinc-free hot-dip galvanized sheet and electro-galvanized sheet. Among them, the number of cold-rolled sheets used for color-coated substrates is gradually decreasing, and electro-galvanized sheets are gradually being replaced by hot-dip galvanized sheets. The use of Galfan, Galvalume, Gal-vanneal and electroplated Zn-Ni steel coils as substrates is increasing. In particular, as the Galvalume production lines of Baosteel and Wuhan Iron and Steel Co., Ltd. are put into production one after another in the next two years, the color-coated plates using GL as the substrate will replace imports. In addition, color-coated substrates for construction are developing toward thin specifications (<5mm), large widths (>1500mm) and high strength (UTS>600MPa). Color-coated substrates for home appliances have increasingly higher requirements for surface quality. At present, the larger width of color plates that can be produced in China is 1600mm (Wugang No. 2 Line).
Epoxy primer is the most classic type of primer. Before 1990, it was used for almost all types of colored boards. It has the best anti-corrosion performance under the film, and has strong adhesion to the pretreatment layer and most topcoats. However, its PMT is high, and the curing energy consumption is high, and it cannot be matched with the top coat to reach 0T or 1T. Polyester primer is a new type of primer developed by European and North American paint companies in recent years. Its corrosion resistance is comparable to epoxy primer, low PMT (<200℃), and its high flexibility is particularly suitable for the production of color panels for home appliances that require high T-bends. Water-based acrylic primer is another new type of primer, which is an environmentally friendly product. Its PMT is lower (160℃), and other properties are similar to polyester primer. However, it easily leads to a decrease in the chemical stability of the coating, so it is still not universally accepted.
Polyester's comprehensive advantages in corrosion resistance, decoration and cost make it occupy a dominant position in color panel topcoats, and its consumption accounts for more than 50% of all topcoats. In addition, in Europe, PVC plastisols are mostly used as topcoats for outdoor construction, accounting for 31% of the total consumption of topcoats; while the United States uses modified polyesters, accounting for 39%.
The roller coating method is the most widely used in the production of color-coated plates, accounting for about 85% to 90%. The process section of the process involves the dip roller of the roller coater with the liquid paint, the paint is applied to the upper and lower surfaces of the pre-treated strip through the application roller, and then it is heated and baked in the curing furnace to completely volatilize the solvent in the paint. The exhaust gas containing solvent is discharged and sent to the incinerator for treatment. The coating is baked and solidified, and then quenched by water and dried by hot air. The roll coating process is usually limited by the thickness of the one-time coating film of the roll coater and the capacity of the baking curing oven. If the product requires a large coating film thickness, multiple coatings and baking curing are required. Therefore, in order to meet the requirements of different coating thicknesses, the rolling process has one coating and one baking, two coating and two baking, or even three coating and three baking. At present, the widely used roller coating process is two-coating and two-baking. At present, the maximum process speed of the domestic color coating unit is 160m/min, and many units in the United States can be driven to a speed of more than 180m/min, which greatly increases the production capacity.
The powder coating method is a new process: the surface of the pretreated substrate is coated with a powder adhesive, which is cured by induction heating, and then the substrate is cooled by an internal water-cooled roller and then enters the powder coating device. The powder coating device is in a strong electrostatic Under the field, the powder rotating brush generates a coating powder cloud. The solid coating particles of the powder cloud are highly charged and fly to the high-speed running substrate, generating sufficient boundary penetration force, and the powder particles are evenly deposited on the strip. On the surface. At present, there are 5 powder color coating lines in the world, the width of the coated steel strip is 1300mm, and the process speed of the unit has reached 100m/min.
Color plate companies should cooperate with upstream and downstream manufacturers for a win-win situation
For domestic color-coated steel sheets, developing new varieties of color-coated steel sheets and improving the technical content and added value of products is a problem that we are facing urgently to be studied and tackled. In the next 10 years, my country's color coated steel sheet market will get better and better, and there is a lot of room for development. However, in the face of increasing integration in the international market, competition will become increasingly fierce. Therefore, color plate manufacturers should always maintain close cooperation with paint manufacturers in the upstream of the industry and household appliances and other users in the downstream of the industry to face the market together and achieve a win-win situation for all parties.