Pinholes and shrinkage cavities are common defects in the use of powder coatings. Their presence not only affects the appearance quality of the products, but also seriously affects the mechanical properties and protective properties of the products. Most of the corrosion of the materials to be protected extends from this point on. Loss of the use value of powder coatings. Therefore, it is of practical significance to minimize the occurrence of pinholes and shrinkage cavities during the use of powder coatings.
Pinhole refers to the defect that the gas in the powder coating passes from the bottom layer through the high-viscosity, nearly closed elastic resin layer to the surface of the coating, and does not escape, from the time of melting to solidification. It is formed on the surface of the paint film because the solvent volatilizes too quickly during the construction or the air bubbles in the coating film cannot overflow and the needle-like small holes are left after drying.
The surface defects of the shrinkage film are mainly depressions, pinholes, and the edges or thick edges of the corners. There are two kinds of depressions on the surface of the coating film, one is a circular depression and the other is a hexagonal polygonal depression. The depression on the surface of the coating film is caused by the surface tension gradient. The surface tension is uneven due to changes in the composition of the coating and temperature changes. The fluid flows from the low surface tension to the high surface tension, resulting in a depression on the surface of the fluid. For the Maragoni effect, there is a marginal bulge, a circular crater at the center, or a Benaldo vortex with a ridged ridge and a central sag into a hexagonal groove. The material with low surface tension in the center of the shrinkage cavity exists, and the surface tension of the surrounding paint is poor. This difference is the power of the shrinkage hole formation, which causes the surrounding liquid fluid to flow away from it (the shrinkage source) and flow away. Depression.
Several characteristics of shrinkage holes
1) There is a core - there is a core particle in the middle of the shrinkage hole, which is fluffy or granular, and the shrinkage hole is large and deep, exposing the substrate;
2) No core - there is no mass point in the middle of the shrinkage hole, and it is mostly on the surface of the coating.
1) Uniform distribution - the shrinkage holes are evenly distributed throughout the painted workpiece;
2) There are characteristic distributions - the parts are very concentrated, and even appear as a comet-like shape;
3) Anisotropy - When the sprayed workpiece has an upward plane, the upper surface has more shrinkage cavities.
1) When the temperature changes greatly in spring and autumn, the shrinkage cavity problem is easy to occur;
2) It is easy to appear when there is abnormality in the compressed air drying equipment;
3) The spraying equipment has a long downtime and is easy to appear when restarting;
4) After the shrinkage hole problem occurs, the raw materials in the equipment are cleaned up, and the problem will be alleviated or solved.
Cause of formation
The coating forms shrinkage cavities during the construction drying process, which has problems with the coating itself and the problem of cleaning the substrate. Due to the presence of low surface tension droplets in the coating itself or the low surface tension zone due to contamination of the coated surface, the surface tension is not uniform, and the coating flows from a low surface tension to a high surface tension due to the difference in surface tension. At the same time, the result is a hole in the center of the depression - shrinkage hole. We classify the unevenness of surface tension as the internal cause of shrinkage formation. In fact, some properties of the coating itself, such as paint viscosity, thixotropy, paint drying speed and film thickness, can aggravate or reduce the flow ability of the coating fluid, which will aggravate or weaken the degree of shrinkage. We classify these factors as shrinking. The external cause of the hole. The internal cause is an inevitable condition for shrinkage, and the external cause can appropriately control or aggravate the degree of shrinkage.
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 bulk and form depressions. In general, thicker coatings (> The liquid coating of 4mm), mainly the density gradient drives the flow, and the thinner liquid coating is the surface tension gradient control flow (Fig. 1), forming a shrinkage hole.
Pinholes are produced by the discharge of gas (air or reaction-generated gas, etc.), but the shrinkage holes are caused by poor wetting, such as the difference in surface tension between the substrate and the upper coating, or the substrate is contaminated, and some are The paint is contaminated by a matting agent or a crepe agent. This type is generally evenly distributed in the film, and there is no regularity when and where it appears, and the shape is a small point-like shrinkage hole.
Cause of formation
1. The formulation of the coating and the reason for the manufacturer. The varnish is poorly refined, the solvent is selected and the mixing ratio is improper, the dispersion of the pigment and filler is poor, and air bubbles and water vapor are entrained in the paint production.
2. The storage temperature is too low, which makes the miscibility of the resin worse, the viscosity increases or partially precipitates, and it is easy to cause granules or pinhole defects (special asphalt coating)
3. Stirring for a long time, mixing air into the paint to generate numerous bubbles.
4. The humidity of the construction environment is too high, the oil-water separator of the spraying equipment fails, the air is not filtered, and the moisture is sprayed into the air tube when spraying, causing pinholes and even blisters on the surface of the coating film. The pressure is too high when spraying, and the distance is too far, which destroys the solvent balance of the wet film. When the brush is applied, the force is too large, and the rotation speed of the roller is too fast, so that the generated bubbles cannot escape.
5. After the coating, the solvent is volatilized to the initial film-forming stage. Because the solvent evaporates too fast, or it is applied at a higher temperature, especially when it is baked at a high temperature, the coating film itself is too late to make up the gap, forming a series of small holes, ie pinholes. .
6. The surface of the coated object is improperly treated and painted on the oily surface. The steel has a high moisture content, and the putty and the seventh are not dry. The coating film is applied too thick at one time, the solvent cannot be volatilized in time to be wrapped in the coating, and after a period of time, the pinholes are formed when the volatilization escapes.
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