Particle fineness of powder coating
A significant difference between powder coatings and solventborne 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. Powder coatings are dispersed during spraying and do not adjust the particle size of the coating. Therefore, the fineness of the powder particles suitable for electrostatic spraying is important.
Powder coatings suitable for electrostatic spraying preferably have a particle size between 10 μm and 90 μm (ie >170 mesh). A powder having a particle size of less than 10 μm is called an ultrafine powder, and it is easily lost in the atmosphere, and the content of the ultrafine powder is not too much. 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 should have a certain distribution range (Table 1) in order to obtain a uniform thickness coating film. If the thickness of the coating film is required to be 25 μm, the maximum particle size of the powder coating should not exceed 65 μm (200 mesh to 240 mesh), and most of the powder should pass 35 μm (350 mesh to 400 mesh). In order to control and adjust the size of the powder particles, it should be adjustable on the pulverizing equipment. For China's powder manufacturers, this is not currently possible.
When the particle size of the powder exceeds 90 μm, the ratio of the charge to the mass of the particles during electrostatic spraying is small, and the gravity of the large particle powder quickly exceeds the aerodynamic force and the electrostatic force. Therefore, the large particle powder has a large kinetic energy. Not easy to adsorb onto the workpiece.
Resistivity and dielectric constant of powder coatings
For the powder electrostatic spraying process, the key consideration is that the powder coating particles receive the charge and maintain the charge and charge distribution, which directly affects the adsorption and deposition efficiency of the powder on the workpiece. In addition, it is important that the uncured powder is coated. The layer must withstand the mechanical shock of the conveyor without falling off the powder. In fact, the main factor affecting the charge and 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. If the force is not strong, the powder will be dropped slightly by vibration. For the powder coating of electrostatic spraying, the high dielectric constant should be used as much as possible, which will greatly increase the adsorption force of the powder. The coating film is more uniform. However, powder coatings with a high dielectric constant are more difficult to charge, which requires an improvement in the structure of the electrostatic powder gun, and a multi-electrode forced charging structure.
Powder coatings are composed of polymer compounds (such as epoxy powder, polyester powder, etc.), and their adsorption to the workpiece is mainly two: Coulomb force (electrostatic force) and VandeWaals force (intermolecular force). Polymer compounds have high electrical resistivity, so Coulomb force (electrostatic force) is large and reliable. The resistivity of the powder itself determines the charged state of the powder under a certain electrostatic electric field strength; for example, when the resistivity of the powder is 1013 ohms, the electrostatic voltage is 30-50 kV, which can make the powder well charged; When the resistivity is between 108 and 109 ohms, an electrostatic voltage of 100 to 120 kV is applied to obtain the above charging effect. The relationship between the resistivity of the powder and the electrostatic voltage. Whether the thickness of the powder deposit layer 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 (water content) of the powder coating itself directly affects the powder's own dielectric constant. If the powder is heavily hygroscopic, it will agglomerate. This is impossible to electrostatically spray. The general moisture absorption, in addition to affecting its charging performance, will also reduce the fluidity and film forming property of the powder, so that the coating film is not smooth or even difficult to adsorb on the workpiece and the coating film will generate bubbles and pinholes.
The moisture absorption of the powder coating is not related to the storage and storage, but also related to the degree of purification of the compressed air during spraying. Because condensed water is easily generated in compressed air, there must be a filter absorbing 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 coating site is also important. According to the data, every 30% change in relative humidity is equivalent to a change in the resistivity of the powder by two orders of magnitude.
Powder coating stability
The stability of the powder coating refers to whether the powder will agglomerate during storage or use, the leveling property is deteriorated, the charging effect is deteriorated, the orange grain of the coating film is obvious, the gloss is weakened, and pinhole bubbles are generated.
When testing powder coatings, it is necessary to pay attention to the stability of storage. Only powder coatings with certain stability can be used by users. Foreign countries often add some additives in powder coatings to enhance the stability of powder coatings. Therefore, such powder coatings will not cause agglomeration in normal humid air or at temperatures as high as 70-75 °C.
The stability of the powder coating is determined by measuring the leveling change of the powder coating at a certain temperature after a certain period of time. Because the stability of the powder coating indicates the degree of cross-linking reaction of the powder under the 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 viscosity increase of the powder at the curing temperature. The leveling characteristics deteriorate.
The test confirmed that the properties of the powder coating as one of the two major components of the powder electrostatic spraying process can not be ignored; while developing and discussing the electrostatic spraying equipment, the technical requirements of the powder coating should also be taken seriously. The joint research of these two aspects can achieve the purpose of obtaining high quality powder coating.
On the basis of many tests, analysis and comparison, it is concluded that the powder coating should meet the requirements of the electrostatic spraying process. 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 adsorption of the powder, as well as the quality of the coating. If the electrostatic powder spray gun is designed to have a good charging mechanism and a diffusion mechanism. Without paying attention to the quality of powder coatings, high quality powder coatings are not available.