The cause of the formation of keyholes
The coating forms shrinkage porosity during the construction drying process, which is mainly a problem of base cleaning. Due to the existence of low surface tension area on the surface of the coated finish due to pollution, resulting in uneven surface tension, the coating flows from low surface tension to high surface tension under the action of poor surface tension, resulting in the formation of holes with concave centers - shrinkage porosity, which is the internal cause. In fact, some properties of the coating itself, such as coating viscosity, thixotropy, coating drying speed and coating film thickness, etc., can intensify or weaken the flow ability of the coating fluid, thereby aggravating or weakening the degree of shrinkage porosity, and we attribute these factors to external factors. Internal factors are the inevitable conditions for porosity, and external factors can appropriately control or aggravate the degree of porosity.
After the coating is applied, during the drying process, the surface solvent volatilizes, the concentration of the surface polymer increases, and the viscosity of the coating increases, which will cause the surface tension and surface density to exceed the body and form depressions. In general, thicker liquid coatings (>4 mm) are primarily flow-driven by density gradients, while thinner liquid coatings are surface tension gradient-controlled flows, creating a shrinkage porosity.
Hazards of shrinkage porosity in coating films
The existence of pinholes and shrinkage holes not only affects the appearance of the coating film, it is difficult to pass the coating acceptance, but more seriously, it affects the waterproof penetration function of the coating film, the existence of shrinkage holes, reduces the tightness of the coating film, and makes it easier for water to enter and exit the coating film. The permeability of the coating film is enhanced, and the penetration of moisture will eventually lead to an increase in ground humidity, indoor humidity, mold breeding, discoloration of the coating film, blistering, falling off, panalkali, and accelerated aging of the coating film.
Prevention and resolution of shrinkage porosity
If the craters have been caused, it is usually difficult to completely solve them, and it is necessary to control the degree of shrinkage porosity by combining internal and external factors.
1. Fill the closed floor sand hole in the middle coat or top coat;
2. The primer coating should be thin and uniform, and wait for the hand to dry before coating the surface layer;
3. The material should be coated immediately after stirring and mixing (the stirring equipment that produces less mechanical bubbles should be selected), and the trowel plane or plastic broom should be used to eliminate the bubbles when coating.
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