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Prevention and control of common defects in application of water-based paint for steel drums
Water-based paint is an indispensable paint in paint coatings, which protects steel drums, prolongs the service life of steel drums, and beautifies packaging. Water-based paint is the most commonly used one at present, but water-based paint has some defects. Let's take a look at these defects and the causes and solutions.
Common defects of water-based coatings
The advantages or defects of a batch of water-based paints that have just been made and packaged are unclear. Only when the cans and barrels are opened can the appearance and stability be seen. The defects that generally appear are: There is floating water on the top, agglomeration on the bottom, bubbly on the surface, deterioration and mildew, etc. Also, the effect is not visible until after the coating is applied. May appear: paint sagging, poor covering power, film surface shrinkage, pinholes, surface unevenness, lack of fullness and gloss of the outer paint, etc. After a period of time, if the water-based paint or coating quality is not good, there will be defects such as cracking, peeling, blistering, rust and so on.
Causes of Common Defects in Waterborne Coatings
1. The main reasons for the foam control of the coating film are: improper selection and matching of defoamer; the defoamer is not dispersed and evenly mixed in the beating and paint mixing stages; the viscosity of the coating is too low during roller coating, and the spray gun is not adjusted to the best when spraying. There are bubbles, and there is no material to cover the cavity, water and oxygen can easily invade, and corrosion occurs.
2. The main reasons for the shrinkage of the coating film are: the components in the coating system cannot be stirred evenly, substances with high surface tension such as emulsions, etc., have poor spreading fluidity; the amount of defoamer and film-forming agent is too large. There are many shrinkage holes, the primer is easy to rust, and the topcoat has poor appearance.
3. The main reasons for the hardness problem are: related to the Tg and dosage of the base material, related to the oil absorption and dosage of the powder, and related to the environmental conditions during the film forming process, that is, the temperature and humidity. If the coating film is too hard, the flexibility, flexibility and impact resistance will not pass, and if the coating film is too soft, the effect of water penetration and plasticization will be obvious.
4. The main reasons for the poor water resistance (salt spray) are: the film-forming property and water whitening resistance of the base material are poor, and the solid particles are coarse; the amount of the base material is insufficient; the anti-rust emulsion does not actually prevent rust; Due to the surface tension of the material, the adhesion between the water paint and the substrate is poor; there are also fatal defects such as blistering, shrinkage, pinholes, etc.; Salt spray resistance can be said.
5. Appearance defects of the coating film As far as the exterior paint is concerned, the main reasons are: uneven paint film and lack of fullness, because the surface tension of the coating is large, the interior does not flow and spread, the leveling of the film surface is poor; the gloss is not high , The visual effect is poor, which is related to the higher pigment base or Pvc, the quality of the base material and the pigment, and the flatness of the bottom layer.
Prevention and treatment of common problems in water-based coatings
According to the experience of research and development practice, we take the following measures to prevent and solve the above defects: to prevent and control from the source of the formula, that is, to try to reduce the surface tension of water-based coatings. The main difference between water-based coatings and solvent-based coatings is that the dispersion and dilution medium used is different, resulting in different surface tensions of the two coatings. Water-based coatings have more problems than solvent-based coatings. Therefore, for water-based coatings, whether it is high-grade or low-grade, it is necessary to reduce the surface tension of the coating and improve the adhesion of the coating in the first place.
1. The first step of reducing the surface tension of the coating is completed by using a substrate wetting agent with a small amount of high performance and a high-efficiency dispersing agent.
2. Add adhesion promoter for water-based paint. Silane coupling agents have strong fluidity and can migrate freely to the organic/inorganic interface.
3. Supplement a certain amount of co-solvents of alcohol ethers with different volatilization rates. On the one hand, it can better help the base material to form a film, and on the other hand, it can also reduce the surface tension of the coating, but the relationship between the dosage and drying and construction must be taken into account. The primary and secondary positioning of these additives, synergistic action, the surface tension of the coating will be reduced, the adhesion will be improved.
The above measures are taken reasonably, those defects such as peeling, blistering, and poor adhesion of the self-drying primer or baking paint, the surface of the outer paint is uneven, not smooth, not full and other defects, and the service life of the paint layer is not long, etc. Defects can be effectively prevented and solved. For the softness, sag, and uneven surface of the water-based baking paint of the packaging barrels, we screened the appropriate resin and adjusted the ratio, and replaced it with additives that reduce surface tension, prevent sagging and increase leveling. For general water-based paints such as roughness, foaming, shrinkage, poor appearance, post-thickening or post-thinning, etc., we use the "four two" additive method to overcome. "Four twos", that is, two or more wetting and dispersing agents, two or more anti-foaming and defoaming agents, two or more film-forming cosolvents, and two or more rheological additives.