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How to cleaning and coating of Aluminum

Alkaline etching involves a bigger proportion of cost in the anodic oxidation of aluminum alloys, especially for workpieces which are anodized after alkaline etching only, and its cost of alkali corrosion will account for 1/4 or even higher. The consumption of the alkaline etching process primarily made of two sides: the first one is the consumption of aluminum corrosion during the alkaline etching process; the other one is the consumption of the workpieces by themselves. The alkaline etching process is very intense. When the workpiece leaves the solution, the reaction is still there. The reactant which is generated at this moment is easily attached to the surface of the workpiece, it is hard to clean it out by acid cleaning instead; and it is very much likely to leave a mark. When the workpieces are taken out from the alkaline etching solution, it is quickly removed from the alkaline etching solution and placed in the clean water, so the carryover of the workpieces and the consumption of alkaline etching account for a huge proportion. If the workpiece is complicated or the alkaline etching time is shorter, and the amount of carryover will be even more than the etching consumption. Multiple methods for reducing the cost of alkali corrosion are introduced as followed.

One of which is to put a accumulator tank next to the alkaline etching tank. Since the alkaline etching implements at 50-60℃, the evaporation of the solution is fast, and the alkaline etching solution needs to be refilled every day. If there is an accumulator tank, it can be directly refilled with the solution in the tank, meanwhile the amount of carryover is also reduced, and the wastewater treatment pressure is mitigated.

The second one is to perform degreasing and grinding work well in advance. In case fail degreasing or secondary grinding, which will cause to increase the cost of alkaline corrosion due to multiple alkaline corrosion. At the same time, there will be an increase in etching volume on workpieces, cause an increase in the dimensional change of the workpieces and even cause the workpiece to be scrapped.

Calculation based on the chemical reaction formula of aluminum in alkaline etching solution, to dissolve 1g aluminum theoretically takes 1.482g sodium hydroxide, but it actually needs 2.5g sodium hydroxide as least. In production process, according to the shape and actual area of the workpieces, combine with the selected alkaline etching process formula and operating conditions, the sodium hydroxide consumption of the workpieces can be estimated. The consumption include the portion that contains solution during the etching process and the part when brought out.

In addition to the above main components, the degreaser also needs to add an appropriate amount of calcium and magnesium ion complexing agent to improve its degreasing performance. Commonly used complexing agents are trisodium citrate, EDTA-2Na, etc. For the aluminum alloy degreaser, an appropriate amount of sodium borate can be added during the making process to improve the corrosion resistance of the alkaline to the aluminum alloy.

As far as the current technology is concerned, a decent alkaline degreaser should meet the following requirements:

①Each component of the degreaser can be quickly and completely dissolved, and should have stable and good washing function, and no by-products which may have an influence on the washing effect during the washing process;

②The oil removal liquid has moderate alkalinity and can not cause obvious corrosion behavior to the metal. For aluminum alloy, the pH value is preferred to set between 9 and 12, and also has a strong buffering capacity to maintain the stable activity of the degreaser;

③Must have excellent wetting ability and high emulsifying ability to remove or disperse surface residue such as grease;

④Degreasing liquid must have good anti-soil and re-precipitation ability;

⑤Degreaser must be able to inhibit the erosion of the base metal to a minimum;

⑥Must be non-irritating to the skin and completely non-toxic;

⑦Degreaser should have a softener to prevent the formation of insoluble hard water salt deposits on the metal surface;

⑧Must be economically applicable.

The material of the working cylinder which are used in the alkaline degreasing treatment depends on the situation. The degreaser used at room temperature can be made of PP or hard PVC. For degreasers that need to be warmed, they should be made of high-quality steel or stainless steel, as well as insulation measures. The heating of the alkaline degreaser can be heated by a stainless steel heater.

After degreasing, the surface of the aluminum foil reaches the first level of brush water, and no matter what substance is adhered later, the composite property has always to be guaranteed. Be cautious to make the paint after cleaning.

Coating auxiliaries are very less likely to be used in coatings, but can significantly improve coating performance and have become an indispensable component of coatings. The thickener is a rheological additive that not only thickens the coating, prevents sagging during construction, but also do impart excellent mechanical properties and storage stability to the coating. It is a very important additive for water-based coatings with low viscosity.

I. The type of water-based paint thickener

There are many types of thickeners currently available on the market, mainly including inorganic thickeners, celluloses, polyacrylates and associative polyurethane thickeners. Inorganic thickeners are a sort of gel minerals that swell and form thixotropic properties. They are mainly bentonite, attapulgite, aluminum silicate, etc., of which bentonite is more commonly used. Cellulose thickeners have a long history of use and many varieties, such as methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, etc., which used to be the mainstream of thickeners. Among them, hydroxyethyl cellulose is more commonly used. Polyacrylate thickeners can be basically divided into two types: the first one is water-soluble polyacrylate; the other one is acrylic or methacrylic acid homopolymer or copolymer emulsion thickener. This kind of thickener is acidic in nature , and it must be neutralized with alkaline or ammonia to pH 8-9 to achieve thickening effect. It's also known as alkaline swell thickener. Polyurethane thickeners are newly developed associative thickeners in recent years.

II. The characteristics of various thickeners

1)cellulose thickener

Celluloid thickeners have high thickening efficiency, especially for thickening of the aqueous phase; less restrictions on coating formulations, wide application; large applicable pH range. However, there are disadvantages such as poor leveling, frequent splashing during roll coating, poor stability, and vulnerable to microbial degradation. Due to its low viscosity under high shear and high viscosity in static and low shear, the viscosity increases rapidly after coating to prevent sagging, but on the other hand, it causes poor leveling. Studies have shown that the relative molecular mass of the thickener positively correlated the spatter of the latex paint. Cellulose thickeners are prone to splashing because of their relatively large molecular mass. Since cellulose is hydrophilic, it will reduce the water resistance of the paint film.

2)Polyacrylic thickener

The polyacrylic thickener has strong thickening property and good leveling property, as good as good biological stability, but it is sensitive to pH value and poor in water resistance.

3)Associative polyurethane thickener

The associative structure of the associative polyurethane thickener is damaged by the shearing force, and the viscosity is lowered. When the shearing force disappears, the viscosity can be restored, and the sagging phenomenon during the construction process can be prevented. And its viscosity recovery has a certain hysteresis, which is conducive to the film leveling. The relative molecular mass (thousands to tens of thousands) of polyurethane thickeners is much lower than the relative molecular mass (hundreds of thousands to millions) of the first two types of thickeners and does not promote splashing. The polyurethane thickener has both hydrophilic and hydrophobic groups on the molecule, and the hydrophobic group has strong affinity with the substrate of the coating film, and can enhance the water resistance of the coating film.

Since the latex particles participate in the association, flocculation does not occur, so that the coating film can be made smooth and has a high gloss. Associative polyurethane thickeners have many properties superior to other thickeners, but due to their unique micellar thickening mechanism, those components of the coating formulation that affect the micelles necessarily have an effect on thickening. When using such thickeners, the effects of various factors on the thickening properties should be fully considered. Do not easily change the emulsion, defoamer, dispersant, filming aid, etc. used in the coating.

4)Inorganic thickener

Inorganic thickener has the advantages of strong thickening, good thixotropy, wide pH range and good stability. However, since bentonite is an inorganic powder, it has good light absorption and can significantly reduce the surface gloss of the coating film and acts like a matting agent.

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