The lower the dielectric constant of powder, the easier it is to charge the particles, but the easier it is to lose the charge, which is reflected in the powder adsorption force on the workpiece, and the powder will fall off when it is slightly vibrated. For electrostatic spraying powder coating, should be as high as possible with the high dielectric constant, it will make the powder adsorption force greatly improved.
From electrostatics, it is known that the distribution of charge on the surface of the charged isolated conductor is related to the radius of curvature of the surface, the maximum curvature (i.e., the sharpest place on the surface) has the largest charge density, and the electric field strength of the nearby space is also the largest, when the electric field strength reaches enough to make the surrounding gas ionization, the tip of the conductor will discharge. If it is a negative high voltage discharge, the electrons leaving the conductor will be accelerated by the strong electric field to collide with the air molecules, causing the air molecules to ionize to produce positive ions and electrons. The newly generated electrons are accelerated by the collision, so that the air molecules form an "electron avalanche" process. Electron mass is very small, when it rushes out of the ionization region, it will soon be much heavier than it is attracted to the gas molecules, gas molecules become free state of negative ions. This negative ion in the electric field force under the action of running to the positive pole, at the ionization layer to produce a layer of halo, that is, the so-called halo discharge, when the powder through the corona periphery, it will be charged by the negative ion running to the positive pole collision.
Most industrial powder coatings are polymer insulators with complex structures, and negative ions can be adsorbed to this part of the powder particle surface only when a suitable location exists on the powder surface to receive the charge. For negative ions, this site can be a positively charged impurity in the composition of the powder or a potential energy pit in the composition, or it can be purely mechanical. But regardless of the mechanism causing the adsorption, the deposition on each powder particle is not easy for the ions. The surface resistance of the powder particles is high and the charge is not redistributed due to conductivity, so the surface charge distribution is not uniform.
Powder coating particles take on a negative charge near the electrode due to corona discharge. When the powder particles just leave the muzzle, they are blown out close to the workpiece (positive electrode) by the compressed air conveying force, and are guided by the electric field force, so that the paint is firmly adsorbed on the workpiece. Generally only a few seconds to make the coating thickness of 50 ~ 100μm. powder layer to a certain thickness at the same time, the surface storage of a very thick layer of negative charge shielding layer, resulting in the subsequent negative particles are repelled back, the coating is no longer thickened. So far the coating process is completed.
For the return of the sprayed surface has been coated with a thicker layer of paint film, according to the resistivity and applied voltage curve, higher resistivity is conducive to charge, but the negative effect is not easy to release the charge. According to know, reduce, can reduce the transfer rate of particles and charge, so that the powder particles are not strongly repelled and rebound, while further improving the efficiency of the powder on; if E is very large, the coating will establish a "sensible electric field", the workpiece has not been coated with a lot of powder and negative charge density area is very high, thus repelling the subsequent charge negative charge The powder particles are difficult to adsorb, but the powder layer is very thin.
Stability of powder coating
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 and so on.
When trying powder coating, we must pay attention to its storage stability, only powder coating with certain stability can be used by users.
The stability of powder coating is determined by measuring its leveling change after processing for a certain time under a certain temperature. Because the stability of powder coating explains 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 higher viscosity of powder under curing temperature, and the leveling characteristics become worse.
For powder electrostatic spraying process, the focus to consider is the powder coating particles to accept the charge, maintain the charge and charge distribution, which directly affects the adsorption of powder on the workpiece and deposition efficiency, in addition, it is important that the uncured powder coating must withstand the mechanical vibration of the transmission 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 requires the electrostatic powder spray gun structure to be improved, the use of multi-electrode forced charged structure.
For powder coatings, it is composed of polymer compounds (such as: epoxy powder, polyester powder, etc.), they have two main adsorption force on the workpiece: Coulomb force (electrostatic force) and molecular force. Polymer compounds have a high resistivity, so the Coulomb force (electrostatic force) is large and reliable. The resistivity of the powder itself, will determine the powder in a certain electrostatic electric field strength charged state; such as: when the resistivity of the powder in 1013 ohms, electrostatic voltage as long as 30-50KV, it can make the powder good charged; and the resistivity of the powder in 108-109 ohms, it is necessary to apply 100-120KV electrostatic voltage to get the above charged effect. The relationship between the resistivity of the powder and the electrostatic voltage. Can automatically limit the thickness of the powder deposited, which has a great relationship with the resistivity of the powder itself, the experiment confirms that only high resistivity of the powder to get a suitable coating film.