Pinholes and shrinkage holes are relatively common deficiencies in the use of powder coatings. Their presence not only affects the appearance quality of the product, but also seriously affects the mechanical properties and protection performance of the product. Most of the corrosion of the protected materials extends from this point, thus Lose the use value of powder coating. Therefore, in the use of powder coatings, it is of practically important to reduce the occurrence of pinholes and shrinkage holes as much as possible.
Pinhole refers to the time when the powder coating melts to solidify, the gas in the powder coating passes from the bottom layer through the high-viscosity, nearly closed elastic resin layer to reach the surface of the coating, which fails to escape and form the defect. It is formed on the surface of the paint film, because the solvent volatilizes too fast during construction or the air bubbles in the paint film can not be leveled, resulting in pinholes after drying.
The surface defects of the shrinkage coating film are mainly the phenomenon of depressions, pinholes, and shrinkage 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 depressions on the surface of the coating film are caused by the surface tension gradient. Due to the change in the composition of the coating and the temperature changes, the surface tension is uneven. The fluid flows from the low surface tension to the high surface tension. As a result, a depression is formed on the surface of the fluid, also known as In the Maragoni effect, the rim edging and the center sinking into a circular shrinkage hole, or the Bernaldo vortex with an edge bulging and a hexagonal groove in the center. There is a substance with low surface tension in the center of the shrinkage cavity, which has a surface tension difference with the surrounding paint. This difference is the motive force for the formation of the shrinkage cavity, which causes the surrounding liquid to flow away from it (shrinkage pollution source) Sunken.
Several features of shrinkage cavity
Shape feature
1) There is a core-there is a core particle in the middle of the shrinkage hole, which is in the form of fluff or granules, and the shrinkage hole is large and deep, exposing the substrate;
2) No core-there is no particle in the middle of the shrinkage cavity, which is mostly located on the surface of the coating.
Distribution characteristics
1) Uniform distribution-shrinkage holes are evenly distributed throughout the painted workpiece;
2) There is a characteristic distribution-the locality is very concentrated, and even appears as a comet;
3) Anisotropy-When the sprayed workpiece has an upward plane, the upper surface has more shrinkage holes.
Time feature
1) When the temperature changes greatly in spring and autumn, the shrinkage problem is easy to occur;
2) It is easy to appear when the compressed air drying equipment is abnormal;
3) When the spraying equipment has a long downtime, it is easy to appear when restarting;
4) After the shrinkage problem occurs, clean up the raw materials in the equipment, and the problem will be alleviated or solved.
Cause of formation
The paint forms shrinkage holes during the construction drying process, which has the problems of the paint itself and the problem of substrate cleaning. Due to the presence of low surface tension droplets in the paint itself, or the presence of low surface tension areas due to contamination on the coated surface, resulting in uneven surface tension, the paint flows from the low surface tension to the high surface tension under the effect of the difference in surface tension At the end, a hole in the center is formed-the shrinkage hole. We attribute the uneven surface tension to the internal cause of shrinkage cavity formation. In fact, some properties of the paint itself, such as paint viscosity, thixotropy, paint drying speed, and film thickness, can aggravate or weaken the flow ability of the coating fluid, which will aggravate or weaken the degree of shrinkage. We will classify these factors as shrinkage. 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 and film formation process, the surface solvent volatilizes, the surface polymer concentration increases, and the coating viscosity increases, which will cause the surface tension and surface density to exceed the body, forming pits. In general, a thicker coating film (>4mm ) The liquid coating is mainly driven by the density gradient, and the thinner liquid coating is controlled by the surface tension gradient (.1), forming one shrinkage cavity.
Characteristics of pinholes
Pinholes are caused by the discharge of gas (air or gas generated by the reaction), and shrinkage holes are caused by poor wetting. For example, the gap between the surface tension of the substrate and the coating is too small, or the substrate is contaminated. The paint is contaminated by matting agent or refining agent and so on-this type is generally evenly distributed in the coating film, there is no regularity when and where it appears, and the shape is a small point-shaped shrinkage cavity.
Cause of formation
1. The formula of the paint and the reason of the manufacturer. Poor refining of varnish, improper choice of solvent and mixing ratio, poor dispersion of pigments and fillers, entrapment of air bubbles and moisture in the production of paint.
2. If the storage temperature is too low, the miscibility of the resin will be poor, the viscosity will rise or be partially precipitated, which will easily cause the defects of particles or pinholes (special asphalt coating)
3. Stir vigorously for a long time, mix air into the paint, and generate countless bubbles.
4. The humidity of the construction environment is too high, the oil-water separator of the spraying equipment fails, and the air is not filtered. During spraying, the water is brought into the spray with the air pipe, causing pinholes or even blisters on the surface of the coating film. When spraying, the pressure is too high and the distance is too far, which destroys the solvent balance of the wet coating. Excessive force is applied during brushing, and the rotational speed is too fast during roller coating, etc., so that the generated bubbles cannot escape.
5. After the solvent volatilizes to the initial film formation stage after coating, due to the solvent volatilizing too fast, or construction at a higher temperature, especially under high temperature baking, the coating film itself is too late to fill the gap, forming a series of small holes, that is, pinholes .
6. Improper surface treatment of the object to be coated, paint on the surface with oil stains. The steel has a high moisture content, and the putty and the seventh are not completely dry. The coating film is too thick at one time, the solvent cannot be evaporated in time, and it is wrapped in the coating. When the volatilization escapes after a period of time, a pinhole is formed.
