1, The phenomenon of rain marks
After the exterior wall paint is dry on the surface, it is not completely dried, and it is exposed to light rain or dew. Water droplets flow down on the surface, and the flow marks are uneven. After the surface is dried, the surface of the paint film forms bright vertical stripes.
2, The cause
The water-based paint contains a certain amount of hydrophilic substances. The exterior wall latex paint is not completely dry after construction, transparent wax for coating, and the surface of the paint film is wetted by rainwater. The hydrophilic substances in the paint film are dissolved by water and brought to the surface. A bright spot was formed. Hydrophilic substances are usually emulsifiers and protective glues in emulsions, dispersants, wetting agents and thickeners in latex paint formulations.
The paint will be dry to the surface for about 24 hours after application. When the exterior wall is exposed to condensation or light rain caused by low temperature and high humidity, this kind of paint film sickness is most likely to occur. Because the precipitated hydrophilic substances are irregularly aggregated on the surface, the shape is a flowing line.
Rain marks are usually not produced when encountering heavy rain. At this time, the hydrophilic substances will also be dissolved and dissolved, but they will be washed away by the rain and will not accumulate on the surface of the paint film, so it is not easy to produce rain marks; even if there is residual distribution, it is evenly distributed. No obvious abnormality.
But why are dark paints more prone to rain marks? Color and gloss will affect the visual sensitivity of the human eye; where the rain marks appear, the reflection of light is a little more, and the human eye feels that there are traces of translucent objects; white and light colors, because of the high brightness value, the bright spots of the rain marks It is not obvious, and no or less color paste is used. The hydrophilic substances are relatively small and the degree is lighter, so it is not obvious to the naked eye. The dark color is usually darker, and the human eye is very sensitive to it. A slight change in color can be observed. The bright spots such as rain marks become more conspicuous. In addition, the dark color paint uses a large amount of color paste, and there are relatively more hydrophilic substances. .
3. Preventive measures
1. Optimize product formula
1) Choose a good water-resistant emulsion. The type of emulsion has the greatest impact, and the less emulsifier in the same type of emulsion, the better the water resistance and the better the rain mark phenomenon. The elastic emulsion is relatively the worst, styrene-acrylic and pure acrylic are better, and silicone-acrylic, silicone resin, and fluorocarbon emulsions have excellent water resistance.
2) Selection of dispersant: Homopolymer ammonium salt dispersant is better than homopolymer sodium salt, copolymer dispersant is better than homopolymer, larger molecular weight is better than smaller molecular weight, so hydrophobically modified copolymer ammonium salt can be used powder.
3) Wetting agent: use medium and low HLB value.
4) Thickener: mainly use associative thickener.
5) Selection of fillers: fibrous wollastonite and flaky mica powder can be used; calcium carbonate can be selected with relatively large particle size. Through the compounding of fillers, the PVC of the product is reduced as much as possible, and the proportion of the emulsion is increased, thereby improving the compactness of the paint film.
6) Try to improve the gloss of the product, the mercerized will be better than the matte, and the place where the rain marks appear will reflect the light. If the gloss of the paint film is slightly higher, the rain marks will not look too obvious.
7) Color paste selection: choose products with high pigment content and excellent water resistance.
2. Coating process
The hydrophilic substances in the exterior paint formulation cannot be completely removed, so the construction environment should be paid full attention to avoid or reduce the phenomenon of rain marks. In the rainy season, pay attention to the weather forecast and make a construction plan. In low temperature, high humidity, and rainy weather, do not apply exterior paint; or apply a primer, a topcoat, and the last topcoat after the weather turns clear.
4, corrective measures
Shortly after the rain marks appeared, another heavy rain happened to completely wet the walls, and the rain marks could be eliminated. If the customer complains that they want to rush to eliminate the rain marks, they can manually rinse the wall with tap water after the coating film is dry. Repainting a top coat can also cover the rain marks, but the material and labor budget will increase.
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