What are the processes and advantages of pre-painted steel coils?

In the fabrication of a pre-painted steel coil plate, the substrate is pretreated as follows:

During the rolling process, the steel coil plate is lubricated on the surface, and some goods are also coated with anti-rust grease before leaving the plant. In the pretreatment, first, remove the grease and other adherence, and then chemically treat the clean surface of the steel plate to form a stable conversion coating, improving the corrosion resistance of the substrate and the combined strength of the paint film.

Because the bottom plate moves through the production line so quickly and the pretreatment time is shorter than it is for standard steel drums, it is necessary to control the pretreatment fluid process indicators more strictly to ensure that the pretreatment meets the high standards for grease removal and chemical treatment. To be able to endure the pre-coil barrel processing molding, the conversion film needs to be formed with good toughness and adhesion. Click here for Ral 1050 Galvanized Prepainted Steel Coil.

Generally use hot lye pressurized spray degreasing, such as with 60 ℃ containing a small amount of sodium polyphosphate 0.5 ~ 1.5% sodium hydroxide aqueous solution to 3 × 105Pa pressure spray. Electrolysis is also used to remove oil. Brushing is generally done with nylon brushes to remove surface adhesions and provide a clean surface for chemical treatment.

Chemical treatment is done by spraying under pressure or by impregnation so that the conversion solution forms a conversion film on the surface of the substrate. When spraying is used, the sludge produced during the use of the conversion fluid tends to clog the spray holes and affect the spraying effect. The impregnation method can avoid this problem. To get the most ideal conversion film, for different steel coils to choose a different conversion fluid.

Cold rolled plates generally use iron oxide/phosphate type conversion fluid. It is an acidic solution containing phosphoric acid, alkali metal phosphates, and oxidizing agents (such as chlorate, nitrate, and molybdate). Acid on the surface of the steel plate has a certain acid corrosion, the formation of iron oxide/phosphate amorphous transformation film. The ratio of iron oxide to phosphate in the conversion film is determined by the pH of the conversion solution and the operating temperature. General control of the conversion film weight in 0.3g/m2 or so.

To improve corrosion resistance also need to use chromium content of 0.1 ~ 0.5% of chromic acid solution drenching for passivation, the passivation solution in the chromic acid is partially reduced to trivalent chromium. And add a small amount of phosphoric acid and fluoride after the passivation of the conversion film has the best performance. This partially reduced system generates "chromium chromate" which fills the pores of the amorphous conversion film and improves its shielding properties. The hexavalent chromium ions contained can inhibit the corrosion of steel plates.

The pretreatment process strictly controls the indicators of the treatment solution and the use of continuous testing and adjustment. The water drenched after treatment contains chromium or other harmful metals, and must be treated with purification before discharge.

The nature and performance of the pre-painted steel coil coating:

Pre-coated coil coatings must meet the characteristics of both the production process and the processing and use of pre-coated coils.

The production process is a fast roller coating construction, to ensure the film thickness and leveling, so it requires a certain construction viscosity. Solvent-based paint to 40 ~ 150s (paint - 4 cups) is appropriate, and water-based paint to 28 ~ 35s is appropriate. In roller coating when the paint roller and the base plate traveling speed are different, the speed between the rollers is also different, to the full reversible three-roller coating machine, for example, such as the base plate traveling speed for +100%, then the paint roller for -120%, the paint roller for -140%, the control roller for -10%. Due to the large speed difference, the paint between them is subjected to a large shear force. This requires that the paint has no or very little pseudo plasticity. That is, the viscosity of the coating is not or very little affected by the shear force. Otherwise, the viscosity of the coating decreases significantly with the increase in shear force applied, which prevents the required amount of coating from adhering to the roll.

In the production line, the bottom plate traveling speed is very fast, because there can not be support, the oven can not be too long (50m or so), the paint in the oven baking time is very short, it is required that the paint in the bottom plate temperature of 260 ℃ below the 30 ~ 60 seconds to fully cured. In addition to painting the drying time is very short, roller coating machine at the entrance of the oven is only a dozen meters away, with a base plate speed of 40m/min, a distance of 15 meters, and the drying time is only 22 seconds and a half. Therefore, it is necessary to choose a solvent with suitable volatilization speed to avoid blistering, pinholes, and bad sea cucumber flatness. Commonly used are solvents such as alcohol ether, ketone alcohol, and high boiling point aromatic hydrocarbons.

Only use priming for each application and topcoat it according to the demands of the paint film performance. The top coat should have high hiding power and decoration, and the primer should have good corrosion resistance and adherence to the substrate. Assembling, transporting, and using collision and scratch resistance as well as molding paint coating that does not crack and does not fall off are also necessary. That is, the paint film should be both flexible and tough, and it should also have exceptional resistance to weather and corrosion.