The significant difference between powder coatings and solvent-based coatings is the different dispersion media. 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 when sprayed, and the granularity of the coating cannot be adjusted. Therefore, the fineness of the powder particles suitable for electrostatic spraying is important.
For powder electrostatic spraying process, the key point to consider is the powder coating particles to accept the charge, maintain the charge and charge distribution, which directly affects the powder adsorption to the workpiece and deposition efficiency, in addition, it is important that the uncured powder coating must withstand the mechanical vibration of the transfer mechanism without dropping the powder. In fact, the main factor affecting the powder particles to accept the charge and maintain the charge is the dielectric constant of the powder coating, the lower the dielectric constant of the powder, the easier the particles are charged, but the loss of charge is also easier, which reflects the powder on the workpiece suction force is not firm, slightly by vibration on the powder, for electrostatic spraying powder coating, should be as high as possible with high dielectric constant, it will make the powder adsorption force greatly improved. The coating film is more uniform. But the high dielectric constant of powder coating, charged more difficult, which needs to be improved in the structure of electrostatic powder spraying gun, the use of multiple electrodes of the forced charged structure.
Powder coating itself moisture absorption (water content) directly affects the powder's own resistance dielectric constant. If the powder is seriously hygroscopic, it will agglomerate. This is not possible for electrostatic spraying. General moisture absorption, in addition to affecting its charged performance, will also reduce the fluidity and film-forming properties of the powder, so that the coating film is not smooth or even difficult to adsorb on the workpiece and get the coating film will produce bubbles and pinholes.
Powder coating moisture absorption in addition to preservation and storage is not caused by attention, but also with the spraying of compressed air purification degree. Because the compressed air is easy to produce condensation, there must be a filtering and moisture absorption device in the air purification system, so that the water content in the air leading to the powder supplier can be reduced to a minimum level.
The stability of powder coating refers to whether the powder will be caked in storage or use, the leveling characteristics become worse, the charging effect becomes worse, the orange pattern of the coating film is obvious, the gloss is weakened, the occurrence of pinhole bubbles, etc.
In the trial powder coating, must pay attention to its storage stability, only with a certain stability of the powder coating, in order to allow users to use. Foreign countries often add some additives in powder coatings to enhance the stability of powder coatings, so this type of powder coatings in the general humid air or temperature up to 70 ~ 75 ℃ will not produce caking.
The stability of powder coating is determined by measuring the change of leveling of powder coating at a certain temperature and after a certain time. Because the stability of powder coating illustrates the degree of cross-linking reaction of powder coating molecules under storage conditions; the more intense the cross-linking reaction of powder occurs, the larger the molecular weight of powder becomes, which is reflected in the increased viscosity of powder under curing temperature, and the leveling characteristics become worse.
The test confirms that the nature of powder coating as one of the two components of the powder electrostatic spraying process is not to be ignored; in the development, exploration of electrostatic spraying equipment at the same time, should also pay attention to the technical requirements of powder coating. This two aspects of the joint research, in order to achieve the purpose of obtaining high-quality powder coating film.
After several tests, analysis, comparison, based on the conclusion that the powder coating to achieve the requirements of the electrostatic spraying process, should pay attention to several technical parameters: powder particle size distribution, powder resistivity and dielectric constant, powder moisture absorption and stability, etc.. These parameters directly affect the powder charging efficiency and adsorption force, as well as the quality of the coating film. If the design of the electrostatic powder gun has a good charging mechanism and diffusion mechanism. And do not pay attention to the quality of the powder coating, is not to get a high quality powder coating film.