In general, our answer is that the main factor affecting the charge and retention of the powder particles is the dielectric constant of the powder. 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 is reflected in the powder on the workpiece. The suction force is not strong, and the powder is dropped slightly by vibration. For electrostatically sprayed powder coatings, high dielectric constants should be used as much as possible, which will greatly increase the adsorption capacity of the powder.
It is known from electrostatics that the surface charge distribution of a charged isolated conductor is related to the radius of curvature of the surface. The maximum curvature (ie, the sharpest part of the surface) has the highest charge density, and the electric field strength of the nearby space is also the largest. When the gas is ionized, the tip of the conductor is discharged. In the case of a negative high-voltage discharge, the electrons leaving the conductor are accelerated by a strong electric field, causing them to collide with air molecules, causing the air molecules to ionize to generate positive ions and electrons. The new electrons are accelerated to collide, making the air molecules form an “electronic avalanche” process. The mass of the electron is very small. When it rushes out of the ionization zone, it is quickly attracted by a much heavier gas molecule, and the gas molecule becomes a negative ion in the free state. The negative ions rush to the positive electrode under the action of the electric field force, and a layer of halation is generated at the ionosphere, so-called halo discharge. When the powder passes through the periphery of the corona, it is charged by the negative ions running toward the positive electrode.
Most industrial powder coatings are polymer insulators with complex structures. Only when there is a position on the surface of the powder that is suitable for accepting electric charges, negative ions can be adsorbed to this part of the surface of the powder. For negative ions, this site may be a positive charge impurity in the powder composition or a potential energy pit in the composition, or it may be purely mechanical. However, regardless of the mechanism of adsorption, deposition on each particle is not easy for ions. The surface resistance of the particles is very high, and the charge is not redistributed by conduction, so the surface charge distribution is not uniform.
Powder coating particles carry a negative charge near the electrodes due to corona discharge. When the powder particles just leave the muzzle and are blown close to the workpiece (positive electrode) by the compressed air delivery force, the coating is firmly adhered to the workpiece by the electric field force. It usually takes only a few seconds for the coating to reach a thickness of 50 to 100 μm. While the powder layer reaches a certain thickness, the surface stores a thick negative-charge shield, so that the subsequent negative particles are repelled and the coating is no longer thickened. This completes the coating process.
A thicker paint film has been applied to the surface of the reflowing member. According to the resistivity and the applied voltage curve, the higher resistivity is favorable for the charge, but the negative effect is not easy to release the charge. According to the knowledge, the reduction can reduce the transfer speed and charge of the particles, so that the powder particles are not rebounded by strong rejection, and the powdering efficiency is further improved; if E is large, the coating will establish an “inductive electric field”. The workpiece has not been coated with a lot of powder and the negative charge density region is very high, thereby repelling the later negatively charged powder particles and being difficult to adsorb, but the powder layer is very thin.
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 film of the coating film is obvious, the gloss is weakened, the pinhole bubble occurs, and the like.
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.
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.
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.
For powder coatings, it is 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 molecular force. Polymer compounds have high electrical resistivity, so 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 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 108-109 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 the thickness of the powder deposition can be automatically limited depends on the electrical resistivity of the powder itself. Experiments have confirmed that only a high resistivity powder can obtain a suitable coating film.
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