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Home > Industry Information > Technical requirements for powder coating by electrostatic spraying

Technical requirements for powder coating by electrostatic spraying

2021-05-02

【China Aluminum Industry Network】 electrostatic powder coating powder coating need to pay attention to the following parameters in order to ensure the smooth implementation of electrostatic spraying process and get a high-performance coating. The following describes the powder coating parameters and related parameters test methods.

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 μm and 90 μm (i.e. >170 mesh). Powders with a particle size of less than 10 μm are called ultra-fine powders, which are easily lost in 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 film is required to be 25 μm, the larger particles 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.

静电喷涂工艺对粉末涂料的技术要求

When the particle size of the powder exceeds 90 μm, the ratio of charge to mass of the particles is small at the time of 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. It is not easily adsorbed on the workpiece.

Powder coating resistivity and dielectric constant

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. This reflects the absorption of the powder on the workpiece. The force is not strong, and it is powdered by vibrating slightly. 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.

Powder coatings consist of high molecular compounds (eg, epoxy powders, polyester powders, etc.) and have two main adsorption forces on the workpiece: Coulomb force (electrostatic force) and Van de Waals force (intermolecular 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 under 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 between 108 and 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. Whether or not the thickness of the powder deposit can be automatically limited depends on the resistivity of the powder itself. Experiments have confirmed that only a high-resistivity powder can obtain a suitable coating film.

Water content of powder coating

The hygroscopicity (moisture content) of the powder coating itself directly affects the self-resistance dielectric constant of the powder. 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 apt to be generated in the compressed air, a filtration and moisture absorption device must be provided in the air purification system so that the water content in the air leading to the powder feeder is reduced to a small extent.

In addition, the relative humidity of the air at the powder coating site is also important. Data reported that every 30% change in relative humidity corresponds to a change in the resistivity of the powder by 2 orders of magnitude.

Powder coating stability

The stability of powder coating refers to whether the powder will agglomerate during storage or use, the leveling characteristics will be deteriorated, 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, it must be noted that 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 cause agglomeration in normal moist air or at temperatures as high as 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.

in conclusion

The test confirmed that the nature 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.

After several tests, analysis and comparison, it was concluded that the powder coatings should meet the requirements of electrostatic spraying process, and several technical parameters should be noted: the particle size distribution of the powder, the resistivity and dielectric constant of the powder, and the powder Hygroscopicity and stability. These parameters directly affect the charging efficiency and the adsorption force of the powder, as well as the coating quality. If the designed electrostatic powder spray gun has a good charging mechanism and diffusion mechanism. Without paying attention to the quality of powder coatings, it is impossible to obtain high-quality powder coatings.


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