The factors affecting the powder rate on powder coatings are discussed

China Coatings Online News Information
Abstract introduces the principle of static spraying of powder coatings, analyzes the factors that affect the powder rate on powders, and puts forward the process conditions for improving the powder rate.
"Keywords" powder coating; Powder rate; Process parameters; static spraying; Factor
powder coating has the characteristics of high production efficiency, excellent coating performance, good ecological environmental protection and outstanding economy, which has been widely paid attention to by the world. Powder coatings are growing at a rate of more than 10% per year, especially in the late 1990s, many powder coating manufacturers have emerged. Powder coating manufacturers in ensuring the appearance of powder coating, mechanical performance and other premises, the most concerned about the unit profile consumption of powder coating, that is, production costs, so powder coating powder rate has become a manufacturer and the use of manufacturers of common concern.
1, powder coating electrostitive spraying principle
powder coating electrostectrecturization is the use of high-voltage electrostation corona electric field principle, in the gun head metal spray cup and pole on the high-voltage negative (generally 60 to 100kV), sprayed work parts ground to form a positive pole, so that the spray gun and work piece between the formation of a strong electrostation field. As a compressed air carrying gas, the powder coating from the powder barrel through the powder tube to the spray cup and pole needle of the spray gun, because it is connected to the high-pressure negative pole to produce corona discharge, in its vicinity produced a dense negative charge, so that the powder with negative charge into the electrostation field with high intensity of the electric field, in the static electricity and carrying gas driving force of the dual action, the powder evenly flew to the surface of the grounding parts to form a thick and uniform powder layer, and then solidified into a coating film.
Powder coating powder rate, generally refers to the powder coating coverage efficiency, from the spraying system can be known, static spraying the main adsorption force is static electricity, powder rate on the powder, mainly depends on how much powder particles charged. According to Coulomb's law, at a certain time, the charge of powder coating particles has the following relationship:
Therefore, to improve the charge capacity of powder particles, can improve the powder rate of powder coating.
2, affecting the powder rate factor on the powder coating
2.1 powder particle size
from (1) type can be known powder charge and powder particle size squared to increase the powder particle size powder with increased charge, powder rate increased, on the other way, reduce powder particle size, reduce powder charge, powder rate decreased. Therefore, in the production process of powder coatings, small particle size powder coatings (10 m) should be minimized; However, the particle size of the powder can not be too large, the particle size is too large, the appearance of the film quality decline, orange peel serious, and possibly because the gravity of large particle powder more than aerodynamic and static electricity, powder coating in the course of flight due to gravity did not reach the surface of the work piece has fallen, but make the powder rate decreased. Therefore, controlling powder particle size is one of the key problems in controlling the quality of powder coating. Figure 1 shows the particle size distribution map of epoxy-polyester hybrid CH-08 powder coating, which is detected by laser particle size spreader, and the spray effect is very good and the powder rate is high.
can be seen from Figure 1: suitable for electrostature spray powder coating, its particle size control in 20 to 90 m is more suitable.
2.2 dielem constant
as can be seen from the (1) type: the charge of the powder is directly related to the dielem constant of the powder. Selecting the coating part with a higher dielecyte constant can increase the charge of the powder particles, make the powder adsorption force greatly increase, and increase the powder rate.
2.2.1 coating formula
powder coating is mainly composed of epoxy powder, polyester powder and other polymer compounds, these polymer compounds have a higher dielectic constant, so its coulomb force (static power) is larger, the powder rate is higher. However, due to its more expensive prices, some powder manufacturers in order to market competition needs to reduce the cost of raw materials, too many fillers, reduce the pigment ratio, so that the powder rate greatly reduced. Table 1 is ch-08 powder coating formulation, after several years of production and inspection, the use of good results, high powder rate.
2.2.2 powder coating use, storage conditions
powder coating compared with solvent-based paint, coating film is prone to slight orange peel. Therefore, many powder coating manufacturers in order to pursue the smooth and flat surface of the work parts, in the allowable powder particle size range, as far as possible to reduce the particle size of the powder coating, so that its leveling increased, to achieve a better coating effect. But the finer the powder, the easier it is to absorb moisture. The moisture absorption (water content) of the powder coating directly affects the dielescent constant of the powder. Slight moisture absorption will affect its charged performance, reduce the powder rate, affect the flow of powder, film performance, so that the coating film is not smooth, even difficult to adsorption on the work piece, the coating film will produce bubbles and pinholes and spray gun blocking gun and other ills; If the powder is severely moisture-absorbing, it will condense and cannot be electrostecturizing. Therefore, in order to improve the powder rate of powder, the following measures can be taken: (1) compressed air purification system, through the secondary filter moisture absorption device, so that the water content of compressed air as low as possible to use; Especially in the south of the relative humidity of large areas, conditional manufacturers recommend in the powder room installed dehumidification air conditioning;
2.3 Spraying process conditions
2.3.1 static voltage
by type (1) can be known: powder coating charge and electric field strength E directly related, so increase static voltage, increase the powder with charge, is conducive to the increase of powder rate. The relationship between static voltage and powder adhesion is shown in Figure 2.
as can be seen from Figure 2, the adhesion of the powder increases rapidly with the static voltage, and when 90kV is reached, the adhesion change is not obvious, so the powder voltage control in production is more suitable at 60 to 90 kV.
distance between the 2.3.2 spray gun and the work
in the case of static voltage determination, the intensity of the electric field is inversely inversely related to the distance between the poles. Spark discharge occurs when the distance between the spray gun and the work piece is too short; If the distance is too far, the powder rate of the powder will be reduced. The relationship between the gun distance and the knock-through voltage is shown in Figure 3, and it can be seen from the figure that when the static voltage is controlled at 60 to 90kV, the electrostitive spray distance is more suitable between 250 and 350mm.
3, conclusion
powder powder rate is a technical indicator determined by a variety of factors, powder coating process is not only a fixed parameter, each manufacturer has its own enterprise standards. In addition to the main factors discussed above, there are other factors, such as the surface shape of the spray workman, the type of spray gun, the performance of the recovery system, etc., will affect the powder rate, need to be constantly explored and studied in production.