I. Introduction

In recent years, the application scope of powder coatings in China has been continuously expanding, and people's requirements for powder coatings and coating equipment are also increasing. Currently, in addition to epoxy powders, polyester-modified epoxy powders and polyester powders have emerged, which greatly increase the resistance to outdoor performance of powder coatings; in color, current powder coatings have more than 40 colors. Can meet different needs. As a result, powder coatings have evolved from the past with high-performance anti-corrosion coating applications to the current thin coating of gorgeous decorative applications. This is a leap in China's powder coating category.

The majority of powder coatings are currently electrostatically sprayed—the powder's cold-coating technology forms a film after heat curing. The electrostatic spraying process of powder coating essentially consists of two major parts: the performance of powder coating and electrostatic spraying equipment (including spraying process). The two major parts are interdependent and mutually reinforcing. Only with the ideal (for electrostatic spraying) powder coating, but also with a good electrostatic spraying equipment, in order to obtain high-quality powder coating film, on the contrary lack one of them. For good performance, no high-quality coating is obtained, which has been confirmed by powder coating application practice for many years. For this reason, the in-depth study of powder coatings and electrostatic spraying equipment is the current direction of efforts. In the electrostatic spraying experiment, in addition to the exploration and improvement of the spraying equipment (electrostatic powder gun, powder feeder, etc.), the powder coating suitable for electrostatic spraying was also analyzed. The conclusion is obvious, with the same spraying equipment and The same process conditions, using different powder coatings, the obtained powder coating deposition rate (adsorption rate) and the adsorption force is not the same, reflecting the quality of the coating film.

Second, electrostatic spraying technology requirements for powder coatings

In the experimental powder electrostatic spraying process, the characteristics of the powder coating are often not taken into account, so that when the coating film has some rickets (such as bubbles, pits, depressions, shrinkage holes, spotted spots, and even no powder, etc.) ), no way to start, it is difficult to analyze. In essence, for the powder coating for electrostatic spraying, the following parameters should be noted to ensure the smooth implementation of the electrostatic spraying process and obtain high-performance coating film.

1, powder coating particle fineness

The significant difference between powder coatings and solvent-based coatings is the dispersion medium. In solvent-based coatings, organic solvents are used as the dispersion medium; in powder coatings, purified compressed air is used as the dispersion medium. The powder coating is in a dispersed state when sprayed and cannot adjust the particle size of the coating. Therefore, the powder particle size suitable for electrostatic spraying is important.

Powder coatings suitable for electrostatic spraying preferably have a particle size of between 10 microns and 90 microns (i.e., >170 mesh). Powders with a particle size of less than 10 μm are called ultra-fine powders, which are easily lost to the atmosphere, and the content of ultra-fine powders cannot be too high. It is worth noting here that the particle size of the powder is related to the thickness of the coating film, and the particle size of the powder coating must have a certain distribution range (Table 1) to obtain a uniform coating film thickness. If the thickness of the coating is required to be 25 μm, the maximum particle size of the powder coating should not exceed 65 μm (200 mesh - 240 mesh) and most of the powder should pass 35 μm (350 mesh - 400 mesh). In order to control and adjust the size of the powder particles should be able to adjust the smashing equipment. For China's powder manufacturers, this is not yet possible.

Powder coating particle distribution can be tested with KCY automatic record particle analyzer

When the particle size of the powder exceeds 90 micrometers, the ratio of charge to mass of the particles is small during electrostatic spraying, and the gravity of the large particle powder quickly exceeds the aerodynamic and electrostatic forces. Therefore, the large particle powder has a large kinetic energy. , not easy to adsorb to the workpiece.

2. The resistivity and dielectric constant of powder coatings

For the powder electrostatic spraying process, the key point to consider is that the powder coating particles receive electric charge, maintain the charge and charge distribution, which directly affects the power of the powder on the workpiece adsorption and deposition efficiency, in addition, it is important that the uncured powder coating The layer must withstand the mechanical vibration of the conveying mechanism without falling out of powder. In fact, the main factor affecting the charge and charge retention of the powder particles is the dielectric constant of the powder coating. The lower the dielectric constant of the powder, the easier it is to charge the particles, but the easier it is to lose the charge, which reflects the absorption of the powder on the workpiece. The enthalpy is not strong, and it is powdered off by vibration. For the electrostatic spraying powder coating, high dielectric constant should be used as much as possible, which will greatly increase the adsorption force of the powder. The coating is more uniform. However, powder coatings with high dielectric constants are more difficult to charge. This requires improvements in the structure of the electrostatic powder guns and the use of a multi-electrode forced-charged structure.

For the powder coating, it is composed of high molecular compounds (such as: epoxy powder, polyester powder, etc.), and there are two main adsorption forces for the workpiece: Coulomb force (electrostatic force) and Van de Waals force ( Molecular force). Polymer compounds have high resistivity, so the Coulomb force (electrostatic force) is large and reliable. The resistivity of the powder itself will determine the charged state of the powder at a certain electrostatic field strength; for example, when the resistivity of the powder is 1013 ohms, the static voltage as long as 30-50 kV can make the powder charged well; and the powder When the resistivity is 108-109 ohms, an electrostatic voltage of 100-120 kV is applied to obtain the above charging effect. The relationship between the resistivity of the powder and the electrostatic voltage (figure omitted). Whether it can automatically limit the thickness of the powder deposited yttrium, which is related to the resistance of the powder itself, experiments confirmed that only high-resistivity powder can get the appropriate coating.

3, the moisture content of the powder moisture

The hygroscopicity (moisture content) of the powder coating itself directly affects the dielectric constant of the powder itself. If the powder is heavily hygroscopic it will agglomerate. This cannot be electrostatically sprayed. In general, moisture absorption, in addition to affecting its charging performance, will also reduce the powder's fluidity and film formation, so that the coating film is not smooth or even difficult to adsorb on the workpiece and the resulting coating will produce bubbles and pinholes.

The moisture absorption of powder coatings is not related to the preservation and storage, but also related to the degree of compressed air purification during spraying. Because condensed water is easily generated in the compressed air, a filter and moisture absorption device must be provided in the air purification system to minimize the moisture content in the air leading to the powder feeder.

In addition, the relative humidity of the air at the powder coating site is also important. The data reported that every 30% change in relative humidity corresponds to two orders of magnitude change in the resistivity of the powder (figure omitted).

4, the stability of powder coating

The stability of powder coating refers to whether the powder will agglomerate during storage or use, the leveling characteristics will be worse, the charging effect will be worse, the orange pattern of the coating film will be obvious, the gloss will be weakened, and the pinhole bubbles will occur.

In the trial production of powder coatings, we must pay attention to the stability of its storage, only a certain degree of stability of the powder coating, to allow users to use. In foreign countries, some auxiliaries are often added to powder coatings to enhance the stability of powder coatings. Therefore, these powder coatings will not produce agglomerates in normal moist air or temperatures up to 70-75°C.

The stability of a powder coating is determined by measuring the leveling behavior of the powder coating after a certain period of time at a certain temperature. Because the stability of the powder coating indicates the degree of cross-linking reaction of the powder coating under the storage conditions; the more intense the cross-linking reaction of the powder, the higher the molecular weight of the powder, which is reflected in the increased viscosity of the powder at the curing temperature. Leveling characteristics deteriorate.

Third, the conclusion

1. The test confirms that the properties of the powder coating, which is one of the two major components of the powder electrostatic spraying process, cannot be ignored; while developing and exploring the electrostatic spraying equipment, it should also pay attention to the technical requirements of the powder coating. The joint research of these two aspects can achieve the purpose of obtaining high-quality powder coatings.

2, after many experiments, analysis, comparison, obtained, powder coatings to achieve the electrostatic spray process requirements, attention should be paid to these technical parameters: the fineness of the powder, the powder resistivity and permittivity, powder Hygroscopicity and powder stability. These technical parameters directly affect the charging efficiency and adsorption power of the powder, as well as the coating quality. When the electrostatic spray gun designed has a good charging mechanism and a diffusion mechanism, and does not attach importance to the quality of the powder coating, a high-quality powder coating film cannot be obtained.

3. When understanding the characteristics of the powder, the cause of the formation of the powder coating film may also be found and solved. At the same time, the design of the electrostatic powder gun and the powder feeder is provided. Realistic parameters.