ã€Abstractã€‘The principle of electrostatic coating of powder coating was introduced. The factors affecting the powder rate on powder were analyzed, and the process conditions for increasing the powdering rate were proposed.
Key words: powder coating; powder rate; process parameters; electrostatic spraying; factors
Powder coatings have been widely recognized in the world for their high production efficiency, excellent film properties, good eco-friendliness and outstanding economics. Powder coatings are growing at a rate of more than 10% per year. Especially in China in the late 1990s, many powder coating manufacturers have emerged. Under the premise of ensuring the appearance and mechanical properties of the powder coating film, the manufacturer of the powder coating material is most concerned about the amount of powder coating material consumed by the unit profile, that is, the production cost problem, and thus the powder coating rate of the powder coating becomes the manufacturer and Use issues that the manufacturer cares about.
1, powder coating electrostatic spraying principle
The electrostatic coating of powder coating is based on the principle of high-voltage electrostatic corona electric field. The metal spray cup and the pole needle of the spray gun head are connected with a high-voltage negative electrode (generally 60~100kV), and the grounded workpiece is grounded to form a positive pole, which forms between the spray gun and the workpiece. A strong electrostatic field. As the compressed air of the carrier gas, when the powder coating is sent from the powder supply barrel to the spray cup and the pole of the spray gun through the powder tube, since it is connected to the high-voltage negative electrode to generate a corona discharge, a dense negative charge is generated in the vicinity thereof, so that The negative charge of the powder enters the electrostatic field with high electric field strength. Under the dual action of electrostatic force and driving force of the carrier gas, the powder evenly flies to the surface of the grounded workpiece to form a thick and uniform powder layer, which is then heated and solidified into a coating film. .
The powdering rate of powder coating generally refers to the efficiency of powder coating coverage. It is known from the spraying mechanism that the main adsorption force of electrostatic spraying is electrostatic force, and the powder rate on the powder depends mainly on how much powder particles are charged. According to Coulomb's law, the charge amount of powder coating particles has the following relationship for a certain period of time:
Therefore, by increasing the charging ability of the powder particles, the powdering rate of the powder coating can be improved.
2, affect the powder rate on the powder coating factor
2.1 Powder particle size
It can be seen from the formula (1) that the charge amount of the powder is proportional to the square of the particle size of the powder, and the charge amount of the powder of the powder is increased, and the powder loading rate is increased. Conversely, the powder particle size is reduced, and the charge amount of the powder is reduced. The rate of powdering is reduced. Therefore, in the powder coating production process, the small particle size powder coating (10Î¼m) should be reduced as much as possible; however, the particle size of the powder should not be too large, the particle size is too large, the appearance quality of the coating film is degraded, and the orange peel is serious. It is possible that because the gravity of the large particle powder exceeds the aerodynamic force and the electrostatic force, the powder coating material has fallen due to the effect of gravity during the flight, but the powdering rate is lowered. Therefore, controlling powder particle size is one of the key issues in controlling the quality of powder coatings. Fig. 1 is a particle size distribution diagram of an epoxy-polyester hybrid CH-08 powder coating, which is detected by a laser particle size distribution meter, and has a good spraying effect and a high powdering rate.
It can be seen from Fig. 1 that the powder coating suitable for electrostatic spraying is preferably controlled to have a particle size of 20 to 90 Î¼m.
2.2 Dielectric constant
It can be seen from the formula (1) that the charge amount of the powder is proportional to the dielectric constant of the powder. Selecting a coating component with a higher dielectric constant can increase the charge amount of the powder particles, so that the adsorption power of the powder is greatly improved, and the powder loading rate is increased.
2.2.1 Coating formula
The powder coating is mainly composed of a polymer compound such as epoxy powder or polyester powder. These polymer compounds have a high dielectric constant, so that the Coulomb force (electrostatic force) is large and the powder loading rate is high. However, due to its relatively high price, some powder manufacturers reduce the cost of raw materials in order to compete in the market, add too much filler, reduce the ratio of pigment to base, and greatly reduce the powdering rate of the powder. Table 1 shows the formulation of CH-08 powder coating. After several years of production inspection, the effect is good and the powdering rate is high.
2.2.2 Use and storage conditions of powder coatings
Compared with solvent-based coatings, powder coatings tend to have a slight orange peel. Therefore, in order to pursue the smooth and smooth surface of the workpiece, many powder coating manufacturers minimize the particle size of the powder coating within the allowable particle size range, and increase the leveling property to achieve a better coating effect. However, the finer the powder, the easier it is to absorb moisture. The hygroscopicity (water content) of the powder coating directly affects the dielectric constant of the powder. Slight moisture absorption will affect its charging performance, reduce the powder loading rate, affect the flowability of the powder, film forming properties, etc., so that the coating film is not smooth, even difficult to adsorb on the workpiece, the film will produce bubbles and pinholes and spray gun Blocking guns and other ills; when the powder is heavily hydrated, it will agglomerate and cannot be electrostatically sprayed. Therefore, in order to increase the powder loading rate, the following measures can be taken: (1) The compressed air purification system, through the secondary filtration moisture absorption device, the water content of the compressed air can be reduced as much as possible; (2) pay attention to the powder coating site. The relative humidity of the air and the relative humidity change of 30% are equivalent to two orders of magnitude change in the resistivity of the powder. Especially in areas with relatively high relative humidity in the south, conditional manufacturers recommend installing dehumidifying air conditioners in the dusting room; (3) the warehouse keeps air circulation, and the powder coating packaging box should be placed on the wooden board with the ground on the ground, and the stacking height should not exceed 4 Floor.
2.3 spraying process conditions
2.3.1 Static voltage
It can be known from the formula (1) that the charge amount of the powder coating is proportional to the electric field strength E. Therefore, increasing the static voltage and increasing the charge amount of the powder are advantageous for improving the powder rate on the powder. The relationship between static voltage and powder adhesion is shown in Figure 2 through production testing.
It can be seen from Fig. 2 that the adhesion of the powder increases rapidly with the increase of static voltage. When the viscosity reaches 90kV, the change of adhesion is not obvious. Therefore, it is more suitable to control the powder voltage during production to be 60~90kV.
2.3.2 Distance between the spray gun and the workpiece
In the case where the static voltage is determined, the electric field strength is inversely proportional to the distance between the poles. When the distance between the spray gun and the workpiece is too short, a spark discharge will occur; if the distance is too far, the powder loading rate will decrease. The relationship between the distance between the spray gun and the breakdown voltage is shown in Fig. 3. It can be seen from the figure that when the static voltage is controlled at 60~90kV, the electrostatic spray distance is preferably between 250mm and 350mm.
3, the conclusion
The powder powder rate is a technical indicator determined by various factors. The powder coating process is not only a fixed parameter, but each manufacturer has its own enterprise standard. In addition to the main factors discussed above, there are other factors, such as the shape of the surface of the sprayed workpiece, the type of spray gun, the performance of the recovery system, etc., which will affect the powder loading rate of the powder, and need to be continuously explored and researched in production.
Liu Hong Liu Zhengyu
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