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Technical requirements of electrostatic spraying process for powder coating

the following parameters need to be paid attention to in order to ensure the smooth implementation of electrostatic spraying process and obtain high-performance coating. The following describes the parameters of powder coatings and related parameter test methods

particle fineness of powder coating

the significant difference between powder coating and solvent based coating is that the dispersion medium is different. Matthias zachert, chairman of the board of directors of LANXESS, said at the press conference in Shanghai: "China is the largest automotive market in the world. In the dosage form coatings, organic solvents are used as the dispersion medium; in the powder coatings, purified compressed air is used as the dispersion medium. The powder coatings are dispersed during spraying, and the particle size of the coatings cannot be adjusted. Therefore, the particle size of the powder suitable for electrostatic spraying is important.

the particle size of the powder coatings suitable for electrostatic spraying is preferably 10 μ M to 90 μ M (170 mesh). Particle size less than 10 μ M powder is called ultra-fine powder. It is easy to be lost in the atmosphere. The content of ultra-fine powder should not be too much. It is worth noting that the particle size of the powder is related to the thickness of the coating film. A uniform thickness of the coating film can be obtained only if the particle size of the powder coating has a certain distribution range (Table 1). If the film thickness is required to be 25 μ m. The maximum particle size of powder coating shall not exceed 65 μ m. (200 mesh - 240 mesh), and most of the powder should pass 35 μ M (350 mesh to 400 mesh). In order to control and adjust the size of powder particles, the crushing equipment should be able to be adjusted through these greatly improving the cohesion of the team, the sense of belonging and the sense of acquisition of employees. For China's powder manufacturers, this is not possible at present

the particle size of the powder exceeds 90 μ M, the charge to mass ratio of particles is very small during electrostatic spraying, and the gravity of large particle powder quickly exceeds the aerodynamic and electrostatic forces. Therefore, large particle powder has large kinetic energy and is not easy to be adsorbed to the workpiece

resistivity and dielectric constant of powder coating

for the powder electrostatic spraying process, it is important to consider that the powder coating particles receive charge and maintain charge and charge distribution, which directly affects the adsorption force and deposition efficiency of the powder on the workpiece. In addition, it is important that the uncured powder coating must withstand the mechanical vibration of the transmission mechanism without dropping powder. In fact, the main factor that affects the charge acceptance and charge retention of powder particles is the dielectric constant of powder coating. The lower the dielectric constant of powder, the easier it is to charge the particles, but the easier it is to lose the charge. This reflects that the powder has weak adsorption force on the workpiece, and the powder will drop off if it is slightly vibrated. For electrostatic sprayed powder coating, it should use high dielectric constant as much as possible, which will greatly improve the powder adsorption force. More uniform coating. However, the powder coating with high dielectric constant is difficult to be charged, so it is necessary to improve the structure of the electrostatic powder gun and adopt the structure of multi electrode forced charging

powder coatings are composed of high molecular compounds (e.g. epoxy powder, polyester powder, etc.), and their adsorption force on workpieces mainly includes 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 electrostatic voltage is only kV, which can make the powder well charged; When the resistivity of the powder is 108-109 ohms, it is necessary to apply an electrostatic voltage of 100-120kv to obtain the above charging effect. The relation between resistivity of powder and electrostatic voltage. Whether the thickness of the powder deposition layer can be automatically limited depends on the resistivity of the powder itself. Experiments have proved that only the powder with high resistivity can get a suitable coating

moisture content of powder coating

the moisture absorption (moisture content) of powder coating directly affects the resistance and dielectric constant of powder. If the powder is seriously hygroscopic, it will agglomerate. This is impossible for electrostatic spraying. General moisture absorption, in addition to affecting its charged performance, will also reduce the fluidity and film-forming property of the powder, so that the film is not smooth or even difficult to be adsorbed on the workpiece, and the film will produce bubbles and pinholes

the moisture absorption of powder coating is not only caused by carelessness during storage, but also related to the purification degree of compressed air during spraying. Because it is easy to produce condensate in compressed air, there must be a filtering and moisture absorption device in the air purification system to minimize the water content in the air leading to the powder feeder

in addition, the relative humidity of the air at the powder spraying site should also be paid attention to. It is reported that every 30% change in relative humidity is equivalent to 2 orders of magnitude change in the resistivity of the powder

stability of powder coating

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

during the trial production of powder coating, attention must be paid to its storage stability. Only the powder coating with certain stability can be used by users. In foreign countries, some additives are often added to powder coatings to enhance the stability of powder coatings. Therefore, this kind of powder coatings will not produce caking in general humid air or at temperatures as high as 70-75 ℃

the stability of powder coating is determined by the change of leveling property of powder coating after treatment for a certain time at a certain temperature. Because the stability of powder coating indicates the degree of molecular cross-linking reaction of powder coating under storage conditions; The more intense the cross-linking reaction of the powder, the larger the molecular weight of the powder, which is reflected in the increase of the viscosity of the powder at the curing temperature and the deterioration of the leveling characteristics

the test shows that the properties of powder coatings, as one of the two major components of powder electrostatic spraying process, can not be ignored; While developing and discussing electrostatic spraying equipment, we should also pay attention to the technical requirements of powder coatings. In this way, the two aspects of PC control system seem too cumbersome to study together, so as to achieve the goal of high-quality powder coating

on the basis of many tests, analysis and comparison, it is concluded that several technical parameters should be paid attention to in order to meet the requirements of electrostatic spraying process: particle size distribution of powder, resistivity and dielectric constant of powder, hygroscopicity and stability of powder, etc. These parameters directly affect the charging efficiency and adsorption capacity of the powder, as well as the quality of the film. If the designed electrostatic powder spray gun has good charging mechanism and diffusion mechanism. If we do not pay attention to the quality of powder coating, we will not get high-quality powder coating

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