Effect of Hydroxypropyl Methyl Cellulose on the Quality of Zinc-Chromium Coatings

Abstract: Two kinds of coatings were prepared by using hydroxypropyl methylcellulose and hydroxyethyl cellulose as thickeners in zinc-chrome coatings, and compared them through experiments. The test results show that under the condition of the same coating quality, using hydroxypropyl methylcellulose as a thickener can reduce the amount of thickener by half, and can greatly reduce the decomposition of organic matter during the sintering process of the coating and improve the coating quality. Layer cohesion.

Key words:zinc-chromium film; coating; thickener; hydroxypropylmethylcellulose; hydroxyethylcellulose

The preparation of zinc-chromium film is to first mix zinc powder, aluminum powder, chromic acid and thickener and other raw materials to form a viscous slurry, and then use methods such as immersion, brushing or spraying to adhere the coating solution to the steel workpiece. The surface is uniformly coated by centrifugal method and sintered to form a metal anti-corrosion coating treatment technology rich in zinc and aluminum coating.

During the paint formulation process, thickeners need to be added to stabilize the performance of the paint. Since the thickener is an organic substance, the volatilization and decomposition of the organic substance will affect the quality of the coating during the sintering process of the coating. Therefore, reducing the content of organic matter in the coating, especially reducing the decomposition of organic matter in the sintering stage, is of great significance to improving the quality of the coating.

1. Experimental method

1.1 Composition and ratio of paint

The basic formula of paint: Take 50mL water as an example, first add a specified amount of thickener, then add 5 g flake zinc powder (diameter of 15-20 μm), 1 g aluminum powder (diameter of flake is about 20 μm), 1 mL of OP-10 (10%) wetting agent, 5 g of polyethylene glycol-400, 5 mL of ethylene glycol, 1 g of chromic anhydride, stirring continuously during the addition process until the mixture is uniform.

1.2 Process steps of Zn-Cr film preparation

The laboratory coating film preparation process is: pickling and derusting → washing with water → dipping coating → 80°C, 30min drying → 300°C, 30min → sintering → recoating → inspecting the quality of the coating film. The coating is completed by one coating and one sintering. Since the test piece basically has no oily substances, the degreasing step is omitted.

1.3 Coating Appearance and Adhesion Test

The zinc-chromium coating prepared by one-time coating and one-time sintering has a silvery-white appearance, and the surface of the coating is lightly rubbed with fingers, and there is basically no metal powder falling off and fingerprints. It shows that the bonding force between the metal powders is very good, and the quality of the metal powders in the coating is also very good.

Adhesion test was carried out according to the 1.4 standard of GB/T 5270-2005. After the adhesion strength of the coating was tested by the tape test method, the coating did not peel off from the substrate or exposed, and the tape only adhered to a small amount of metal powder.

1.4 Effect of hydroxypropyl methylcellulose on coating quality

Add 5 g of zinc powder, 1 g of aluminum powder, 5 mL of ethylene glycol, 5 g of polyethylene glycol 400, 1 mL of OP-10, 1 g of chromic anhydride, and 0.25 to 0.75 g of thickener hydroxypropyl in 50 mL of water Base methyl cellulose (relative molecular weight 200,000). Under the condition of stirring for 8 hours, other components except the thickener were fixed, and zinc-chromium film layer coating was carried out.

The quality of hydroxypropyl methylcellulose per 50mL of water is controlled at 0.50g, and a zinc-chromium coating with uniform thickness can be obtained.

1.5 Comparative optimization selection of hydroxypropyl methylcellulose thickener

At present, hydroxyethyl cellulose and hydroxypropyl methyl cellulose are commonly used thickeners, and these two thickeners are selected for comparative experiments. Prepare 50 mL of coating solution for each thickener according to the proportion and prepare the coating (the quality of the remaining components is consistent with 1.4).

After choosing two kinds of thickeners, the dispersibility of the zinc-chromium coating solution is very good, the coating is uniformly dispersed, the coating quality is good, the coating appearance quality and the stabilization effect of the coating solution are equivalent to the current general thickeners, and the coating is stable. The average 8 is around 20pan. After analysis, the quality of the zinc-chromium coating obtained by the two thickeners is equivalent, the amount of hydroxypropyl methylcellulose is less than that of hydroxyethyl cellulose, and the thickening effect of using hydroxypropyl methylcellulose as a thickener is better. for the ideal.

1.6 Surface morphology of the coating

The surface morphology of zinc-chromium coating was analyzed by JCxA733 scanning electron microscope (El Ben Electronics Company), with an accelerating voltage of 15kV and a current of 0.001μA.

From the SEM photos, it can be seen that the metal powder in the coating is densely packed on the surface of the substrate layer by layer, and is closely combined with the substrate, indicating that both hydroxypropyl methylcellulose and hydroxyethyl cellulose can be used as zinc oxide. Thickener for chrome paints and gives good quality coatings. The amount of hydroxypropyl methylcellulose is half that of hydroxyethylcellulose, which is more guaranteed to obtain a qualified coating.

2. Conclusion

The metal powder density of zinc powder and aluminum powder in zinc-chromium film paint is relatively high, and it is easy to make the metal powder sink to the bottom of the paint during the production of liquid paint, so that the paint is layered, and the quality of the paint is difficult to guarantee. For this reason, a thickener must be added to increase the viscosity of the coating, so that the dispersed metal powder can exist stably in the coating solution. However, in the sintering step of coating preparation, the volatilization and decomposition process of organic matter will affect the bonding force between the coating and the substrate. Therefore, reducing the content of organic matter in the coating solution has a good guarantee for improving the quality of the coating. Using hydroxypropyl methylcellulose as a thickener in zinc-chrome film coatings has produced qualified coatings, and the amount of thickener is only half of that of hydroxyethylcellulose, which is currently used with better effects, making it The amount of organic matter decomposition is greatly reduced during the sintering process of the coating, and the quality of the coating is better improved and guaranteed, which has good application value.