(1 ) Low solid content during construction
When spraying, radiation coating or curtain coating is used, the solid content is about 20% to 30% (NVV). Low solids means that to achieve the same dry film thickness, the wet film thickness needs to be thicker. In automotive metal glitter coatings, low solid content is an advantage, which can make the aluminum powder pigment achieve better orientation in the paint film and obtain a good glitter effect.
( 2) Influence of humidity
The viscosity of water-thinning resin coatings is strongly dependent on the ratio of water to solvent. The humidity of the air affects the ratio of water to solvent remaining in the paint . The critical relative humidity is the humidity at which water and solvent evaporate in the same proportion as in the paint composition, that is, neither water nor solvent is enriched during the evaporation process. When the ambient humidity is greater than the critical relative humidity, water will be enriched due to slow volatilization, causing the viscosity of the coating film to be too low and sag. The resin with a relative molecular weight of 82000 does not sag at a humidity lower than 60%, while the resin with a relative molecular weight of 42000 does not sag at a relative humidity lower than 50% . Use in environments with high humidity.
If the RH (relative humidity) exceeds 70% during construction, the volatilization rate of water is extremely low, and when it is 100%, the water will not volatilize. If the humidity is extremely high and construction is required, the air needs to be cooled to condense some of the water in the air, and then the air should be reheated before construction. Factory-applied coatings generally require a higher critical RH of the coating, such as 60%.
(3) Blistering and blasting
The formation of bubbles near the surface of the paint film is called blistering. The surface viscosity of the wet film is very high, and the bubbles of the solvent rise to the surface without breaking, which is foaming. Bubbles burst on the surface of the paint film but are not leveled, which is called popping. The surface viscosity is high enough, and the bubbles of the solvent can burst but cannot be leveled, which is called popping. Very small popping holes are called pinholes.
① Surface layer viscosity
Bursts and blisters occur when the wet paint film starts to dry, and the solvent on the surface volatilizes quickly, making the viscosity higher than that of the bottom layer. After entering the drying tunnel, the bubbles formed by the solvent escape from the bottom layer cannot easily pass through the high viscous surface . As the temperature rises, the air bubbles expand, and eventually break through the surface to form a burst hole. At this time, the wet film viscosity is too high to flow to level the burst hole. The wet paint film takes longer to dry before entering the drying tunnel; the heating of the drying tunnel is divided into zones, that is, the first part of the drying tunnel is a lower temperature zone, and the water can diffuse out through the paint film before the surface viscosity increases too much; Blowouts can be reduced by using an infrared-heated tunnel to expel most of the water before baking .
The faster the solvent volatilizes, the easier it is to burst holes. Adding a good solvent with slow volatilization to the mixed solvent can reduce the hole burst. It can keep the viscosity of the surface layer low enough to allow air bubbles to pass through before the viscosity becomes too high. Leveling, but slow volatile solvents, such as alcohol ethers , often cause sagging, while fast volatile solvents such as sec-butanol can prevent sagging. When working in a humid environment, water volatilizes slowly, and the viscosity of the wet paint film is too low to sag, so it is especially necessary to add a fast-volatile solvent. Therefore, the coating place usually controls the relative humidity at 30% to 70%, and then adjusts the ratio between the co-solvent and water to prevent sagging.
②The heat of vaporization of water is 2260J/g, which is higher than that of organic solvents, such as 373J/g for ethylene glycol butyl ether. The high heat of vaporization slows down the temperature rise of the wet film, makes the surface of the paint film dry and has solvent inside, which increases the possibility of cracking. Similarly, water will form a strong hydrogen bond with the polar group in the resin molecule at room temperature and is not easy to volatilize. Before the amine evaporates, the polar carboxylammonium salt group is easy to retain water. At higher temperatures, the hydrogen bond The water is released when it is damaged, which also increases the possibility of a hole explosion.
The lower the Tg of the emulsion polymer , the more susceptible it is to popping. Because before the water is completely volatilized, the surface of the low T g latex paint film coalesces better, preventing the volatilization of water.
③ Prepare the sample under standard conditions, dry and bake, and then measure the maximum film thickness without bursting, which is called the critical film thickness of bursting. The critical film thickness of the water-based baking paint is lower, and the burst hole is more serious than that of the corresponding solvent-based paint. Resins with high T g are far more likely to burst holes than resins with low T g , and resins diluted with water are far more likely to burst holes than resins diluted with solvents.
The probability of popping increases with film thickness because the gradient of solvent content increases with film thickness. Popping can be reduced by spraying more coats, ie thinner wet coats each time.
④ Popping holes can also be caused by air bubbles trapped in the wet film. The surface layer of the wet film is highly viscous, and the air bubbles can stay in the wet film and enter the drying tunnel , then heat up and expand, and break through the surface layer. Bubbles generated by mechanical stirring are also factors that cause bursting. When spraying water-based paint, it is easy to trap air bubbles. Thin coatings applied by radiation or curtain coating are less prone to popping problems because the film does not have the problem of air bubble entrapment compared to spray coating . The drying time is longer, the curtain coating is thicker, and the paint film does not appear burst holes.
In order to spray a uniform film thickness, ensure that there is enough paint in all application areas. The actual film thickness will be thicker than the average film thickness. When spraying with high-pressure airless spray, air bubbles are more likely to occur because more air dissolves in the paint at high pressure. Spraying with supercritical carbon dioxide can reduce blow holes because the carbon dioxide evaporates when the paint mist particles are between the Spray Gun and the surface to be painted.
⑤ The solvent remaining in the primer will also cause blistering and bursting when the topcoat dries. When coating plastic, the solvent will dissolve in the plastic, causing blistering or popping during baking.
