The exterior wall coating has the characteristics of rich color, light weight, simple maintenance and renovation, high efficiency in construction and low cost. It has been widely used in the decoration of the exterior wall. However, in the process of application, there will be peeling, blistering, blooming, fading, rainmarks and other film diseases, which will affect the beauty of the building. These quality defects of coating are related to the quality of coating products, construction technology and drying conditions. It is of great significance to take corresponding preventive measures in advance.
The following analyzes the rain mark phenomenon of dark paint on the exterior wall, and explains the preventive and corrective measures taken.
1、 Rainmark phenomenon
After the exterior wall coating is dried, it is not completely dry, drenched or dewed by light rain, and there are water drops on the surface, which flow unevenly. After the surface water is dried, the surface of the paint film forms vertical bright traces.
2、 Causes
Waterborne coatings contain certain hydrophilic substances. The exterior wall latex paint is not completely dried after construction. The surface of the paint film is wetted by the rain. The hydrophilic material in the paint film is dissolved by water and then brought to the surface. After the water trail is dried, a bright spot is formed. The hydrophilic material is usually the emulsifier and protective glue in the emulsion, the dispersant, wetting agent and thickening agent in the latex paint formula.
When the surface of the coating dries to about 24 hours after construction, it is most likely to appear this kind of film disease when the surface of the external wall is dewy or drizzled due to low temperature and high humidity. Because the precipitated hydrophilic material shows irregular aggregation on the surface, and the shape is a downward flowing line.
When encountering heavy rain, it usually will not produce rainmarks. At this time, hydrophilic substances will also be dissolved and precipitated, but will be washed away by rain and will not gather on the surface of the paint film, so it is not easy to produce rainmarks; even if there is residual distribution, it is uniform and no obvious abnormality can be seen.
But why is dark paint more prone to rain marks? Color and luster will affect the visual sensitivity of the human eye; where rainmarks appear, there is a little more reflection of light, and the human eye feels that there are traces of translucency; white and light color, because of the high lightness value, the kind of bright spots of rainmarks is not obvious, and the color paste is not used or used less, and the hydrophilic substances are relatively less, and the degree is lighter, so it is not obvious to the naked eye. However, 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 like rainmarks become more prominent. In addition, the amount of dark color paint color paste is large, and the hydrophilic substances are relatively more.
3、 Preventive measures
1. Optimize product formulation
1) choose water resistant lotion. The emulsion type has the greatest influence, while the less emulsifier of the same kind of emulsion, the better water resistance and the rain proof phenomenon. The elastic emulsion is the worst, and styrene acrylate and acrylic acid are better. Silicone acrylate, silicone resin and fluorocarbon emulsion have excellent water resistance.
2) Selection of dispersant: the dispersant of homopolymer ammonium salt is better than that of homopolymer sodium salt, the dispersant of copolymer is better than homopolymer, and the dispersant with large molecular weight is better than that with small molecular weight. Therefore, hydrophobic modified copolymer ammonium salt dispersant can be selected.
3) Wetting agent: low HLB value.
4) Thickener: mainly associated thickener.
5) Selection of filler: fiber like wollastonite and mica powder with sheet structure can be used; calcium carbonate with large relative particle size can be selected. By mixing the filler, the PVC of the product can be reduced as far as possible, and the proportion of the emulsion can be increased, so as to improve the compactness of the film.
6) Try to improve the luster of the product. Mercerization is better than matte. The place with rainmarks reflects the light. If the luster of the paint film is slightly higher, rainmarks are not obvious.
7) Color paste selection: select products with high pigment content and excellent water resistance.
2. Coating process
It is impossible to remove the hydrophilic substances in the formula of exterior wall coating completely, so the construction environment should be paid enough attention to to avoid or reduce the phenomenon of rainmarks. In rainy season, pay attention to weather forecast and make construction plan. In low temperature, high humidity and rainy weather, do not apply exterior wall coating; or apply one primer and one finishing coat first, and then apply the last finishing coat after the weather turns sunny.
4、 Corrective actions
Shortly after the rain mark appeared, it happened that another heavy rain completely wetted the wall, and the rain mark could be eliminated. If the customer complains that they are eager to eliminate the rain mark, they can wash and drench the wall manually with tap water after the coating is dry. Reapplying a top coat can also cover rain marks, but material and labor budgets will increase.
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