The pre-coated galvanized steel coil is a steel sheet which is applied to the drum factory for direct production in rolls or sheets after being coated with a roll of steel, without further coating. The base steel plate is generally made of 0.2~2.0mm thick steel plate coil with a maximum width of 2m. Mainly cold rolled steel, hot-dip galvanized or electro-galvanized steel.
Development and application of pre-coated galvanized steel coils
Pre-coated coil technology first appeared in the United States in the 1930s. The earliest is an alkyd paint coated with a steel strip with a thickness of 0.2 mm and a width of 50 mm, which is used as a window panel for louvers instead of wood. After the Second World War, in order to restore the trauma of the war, it contributed to the prosperity of the construction industry. Taking this as an opportunity, pre-coated coils have been greatly developed as lightweight and inexpensive building materials. At the same time, with the economic recovery after the war, advances in science and technology have provided material and technical foundations for the development of pre-coated coils. At present, there are more than 300 pre-coated coil production lines in the world, with an annual production capacity of 15 million tons. The field of use has expanded from building materials to automotive manufacturing, ship interiors, household appliances, containers, drinking utensils and more. At present, it has been applied in the field of packaging at home and abroad, especially in the metal container manufacturing industry.
The main reason for the rapid development of pre-coated coils is that they have good economic benefits and meet the needs of social and economic development. First of all, it transforms the traditional finished product into a continuous coating of raw steel sheets, which not only facilitates the quality control of surface treatment and coating, but also does not easily produce edge defects of coating defects, and obtains the best coating quality. . Secondly, the traveling speed of the thin steel plate on the coating line is generally 40~100m/min, and the fastest has reached 240m/min, and the production efficiency is far greater than that of the finished product. Thirdly, when the paint film is cured, the coated flat plate passes through the oven, and the utilization rate of the furnace capacity is higher than that of the finished product. The solvent volatilized during baking can be collected and introduced into the burner for incineration, and the heat energy can be reused. It consumes only one-fifth to one-sixth of the finished product. Finally, the effluent in the production is convenient for centralized treatment. For example, after the oven exhaust gas is treated by the incinerator, the organic solvent content is less than 50 ppm. It is difficult to achieve this level of ovens with relatively small barrels, and it is not economical to do.
Pre-coated steel coil production process
The production process of pre-coated steel coils is mainly divided into three parts：
√ The introduction section
√ The pretreatment section
√ The coating section and the extraction section.
The introduction section includes equipment such as unwinding, trimming, seaming, and stocking loops, and the raw material coils are loosened and joined to continuously and uniformly supply the steel sheets to the unit.
The pretreatment section serves to clean the coated steel sheet and perform surface treatment to improve corrosion resistance and adhesion of the paint film.
The coating section is the core part of the unit. Usually, the two-coating and two-bake process of applying both the front and the back sides, that is, the primer-baking-coating-coating, is used. Depending on the product requirements, you can also apply one coat or only one side. The reverse roller coating is often used for painting. The advantage of this method is that various thicknesses of the paint film can be obtained by adjusting the ratio of the traveling speed of the steel sheet to the speed of the paint roller and adjusting the gap between the paint roller and the paint roller. In order to obtain a well-cured, fully cured paint film, the furnace temperature is controlled in stages, and the temperature of each section is determined by the type of coating used, the thickness of the steel sheet, and the time of passing the oven. Because it is double-sidedly coated, the coated steel plate cannot be supported in the furnace, so a hanging or air-floating oven is generally used.
Some materials require a laminating device in addition to the liquid coating, and a plastic film can be laminated on the bottom plate. At this time, the top coat roller and the oven can be used to coat and activate the adhesive. After attaching other auxiliary devices, the coating can be printed or embossed. In order to prevent the paint film from being damaged during transportation, processing and installation, a peelable protective film or a layer of wax is added to the prepared precoated plate.
Pretreatment method for substrate in pre-coated galvanized steel coil production
The coiled steel plate has a lubricant on the surface during the rolling process, and some products are also coated with an anti-rust grease before leaving the factory. In the pretreatment, these greases and other adhering substances are first removed, and the surface of the clean steel plate is chemically treated to form a stable conversion film to improve the corrosion resistance of the bottom plate and the bonding strength of the paint film. The speed of the bottom plate on the production line is very fast, and the pretreatment time is much shorter than the pretreatment time of the general steel drum. To ensure the quality of the pretreatment, it is required that the degreasing and chemical treatment are efficient, so the pretreatment liquid Process indicators also require more stringent control. The formed conversion film has good toughness and adhesion so as to be able to withstand the barrel forming process of the pre-rolled sheet.
Generally, the oil is sprayed and degreased with a hot alkaline liquid, for example, by spraying with a 0.5% to 1.5% aqueous sodium hydroxide solution containing a small amount of sodium polyphosphate at 60 ° C at a pressure of 3 x 105 Pa. There are also methods of removing oil by electrolysis. Brushing is typically done with a nylon brush to remove surface adherence and provide a clean surface for chemical processing.
The chemical treatment is to form a conversion film on the surface of the substrate by a pressure spray method or a dipping method. When the spray method is adopted, the sludge generated during the use of the conversion fluid tends to block the spray hole and affect the spray effect. The impregnation rule avoids this problem. In order to obtain the most ideal conversion film, different conversion solutions should be used for different steel coils.
Cold rolled sheets generally use an iron oxide/phosphate type conversion liquid. It is an acidic solution containing phosphoric acid, an alkali metal phosphate, and an oxidizing agent such as a chlorate, a nitrate, or a molybdate. The acid has a certain acid etching on the surface of the steel sheet to form an amorphous conversion film of iron oxide/phosphate. The ratio of iron oxide to phosphoric acid in the conversion film is determined by the pH of the conversion solution and the operating temperature. The conversion film weight is generally controlled to be about 0.3 g/m2.
In order to improve the corrosion resistance, it is necessary to pass the chromic acid solution containing 0.1% to 0.5% of chromium to passivate, and the chromic acid in the passivation solution is partially reduced to trivalent chromium. After adding a small amount of phosphoric acid and fluoride, the passivated conversion film has the best performance. This partially reduced system produces “chromium chromate” which fills the pores of the amorphous conversion film and improves its barrier properties. The contained hexavalent chromium ions can inhibit the corrosion of the steel sheet.
The pretreatment process must strictly control the various indicators of the treatment fluid, and must be continuously tested and adjusted during use. Chromium or other harmful metals in the rinsed water after treatment shall be treated with purification before discharge.
Properties and properties of pre-coated galvanized steel coil coatings
Pre-coated coil coatings must meet the characteristics of the pre-coated coil production process and processing.
The production process is fast roll coating construction, in order to ensure the film thickness and leveling, it is required to have a certain construction viscosity. Solvent-based coatings are suitable for 40~150s (coating -4 cups), and waterborne coatings are suitable for 28~35s. When the roller is coated, the traveling speed of the paint roller and the bottom plate is different, and the speed between the rollers is also different. For example, the full reverse three-roll coating machine, for example, if the running speed of the bottom plate is +100%, the painting roller is -120%, The paint roller is -140% and the control roller is -10%. Due to the large speed difference, the coating between them is subjected to a large shear force. This requires that the coating has no or only a small amount of pseudoplasticity. That is, the viscosity of the coating is not or rarely affected by the shear force. Otherwise, the viscosity of the coating will decrease significantly as the shearing force is applied, and the amount of coating required will not be adhered to the roller.
In the production line, the speed of the bottom plate is very fast. Because there is no support, the oven can’t be too long (about 50m). The baking time of the paint in the furnace is very short, and the coating is required to be within 30~60 seconds below the temperature of the bottom plate of 260 °C. Fully cured. In addition, the drying time after painting is very short. The roller coating machine is only a dozen meters away from the entrance of the oven. The bottom plate speed is 40m/min and the distance is 15 meters. The drying time is only 22 seconds. Therefore, it is necessary to use a solvent with a suitable volatilization speed to avoid blistering, pinhole formation and poor sea cucumber flatness. Commonly used are solvents such as alcohol ethers, ketone alcohols and high-boiling aromatic hydrocarbons.
From the requirements of the performance of the paint film, the coating has only one bottom and one coat. The primer should have good corrosion resistance and adhesion to the substrate and topcoat. The topcoat should have good hiding power and decorative properties. It is also required that the paint film does not crack or fall off during processing, and is resistant to collision and scratching during assembly, transportation and use. That is, the paint film should have good flexibility and hardness at the same time, and also have excellent weather resistance and corrosion resistance.
Development direction of pre-coated steel coil
1.Develop high durability pre-coated coils
(1) Improve the weather resistance of oil-free polyester coatings (coatings excellent in discoloration, fading, and powdering), and make them suitable for exterior coatings of steel drums.
(2) Vigorously develop polyfluoroolefin coatings. By blending with the rust-preventing substrate, pre-coated coils that are more suitable for steel drum processing can be obtained.
(3) The use of galvanized steel as the substrate, it not only has good processability, but also has good corrosion resistance.
2.Develop pre-coated coils with good hardness, elasticity and stain resistance.
(1) Use high-quality coiled substrate with good processing properties.
(2) Based on the polyester resin, the performance is improved by the adjustment of the molecular weight, the selection of the crosslinking agent, and the like.
3.Reduce the baking temperature, shorten the baking time
(1) Development of new high-efficiency catalysts and high-activity curing agents.
(2) Increasing the molecular weight of the resin, polyfunctionalizing the resin, and increasing the degree of branching of the resin.
(3) Development of electron beam curing coatings. A large amount of research work has been carried out on electron beam curing coatings for precoated coils, and industrialization has begun. However, compared with the ordinary baking type, the history is not long, no matter the variety or the output is small, and needs further development.
4.There will be a larger application in the packaging collar
(1) It can replace ordinary wood and cardboard to make ordinary packaging box, which is firm and reliable, beautiful, waterproof, fireproof and anti-corrosive. It has the advantages that wooden boxes and cartons are incomparable.
(2) Replacing ordinary thin steel plates to make metal containers, such as steel drums, steel tanks and small barrels, has the advantages of simple processing, energy saving and pollution reduction.
(3) It is a beautiful and elegant product with a low cost for the manufacture of commodity packaging boxes for tinplate.
(4) It can replace the glass bottles and the like for the packaging of food, beverages, cans, etc., and improve the safety of the packaging.
(5) A large number of large-scale packaging products, such as containers and headers, which reduce costs, save energy, reduce pollution, and simplify the processing.
Hello, if you have any intention to purchase the product or have any questions, please contact us！