Aluminum coils are not produced in the form of coils at the beginning. In fact, aluminum coils have some work to do before they are rolled into coils. Manufacturers need to do some pretreatment sections to ensure the cleanliness of the surface of aluminum coils. In order to avoid the residual grease and lubricant on the surface, it will affect the painting and use. Therefore, the aluminum coil should be pretreated in the following aspects before forming.
First, the aluminum coil can be cleaned with hot water. Hot water cleaning is mainly to clean the residual degreaser on the surface of the substrate to ensure that these residues can be dissolved in it, so as to prevent these degreasers from causing secondary pollution to the substrate. The use methods are mostly dipping and spray brushing. The water used is good and the hardness should not be too high, otherwise, the minerals in the water will form mineral spots on the surface of the substrate.
Second, because of the fast line speed of precoating line, the degreaser used by hot alkali degreasing generally has a high concentration. Typical degreasers contain sodium hydroxide (NaOH), sodium carbonate (Na2Co3), water glass (Na2SiO3), phosphate and other components. Generally, the hot degreasing process is divided into two steps to ensure that the surface of aluminum coil substrate is clean, and the spray brushing method is often used.
Third, passivate the aluminum coil. Passivation treatment is to make the passivation agent form a conversion film on the surface of the substrate by means of pressure spray, dip coating or roll coating. Generally, the method of pressurized spraying is used, and the sludge generated during the use of the passivation solution will often block the spray holes, thus affecting the spraying effect. Although dip coating solves this problem, the consumption of passivation solution is relatively large.
All the above methods need to wash the excess passivation solution with water in actual operation, which will lead to the problem of wastewater recovery and purification. Roller coating is a good passivation construction method, and it has the advantages of uniform coating, economy and practicality, and no need of rinsing. Generally, chromate/oxide treatment agents are used in aluminum coil, which contain chromate, chromic acid, phosphoric acid and accelerator fluoride and molybdate. This kind of treatment agent must be added with phosphoric acid, otherwise it can't be used to treat aluminum rolls for food and beverage.
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