Electrostatic spraying coating principle

In 1962, the French Sames Company successfully researched powder electrostatic spraying equipment, which laid the foundation for the rapid development of powder coating technology. Powder electrostatic spraying method is the most widely used process in electrostatic coating construction. The electrostatic spray gun adsorbs the powder on the surface of the object to be coated by the electrostatic Coulomb force. The most important feature of powder electrostatic spraying technology is that the workpiece is coated at room temperature. Coating utilization rate of 95% or more, thin film (50-100μm) and uniform, no hanging phenomenon, in the workpiece sharp edges and rough surfaces can form a continuous smooth coating film, easy to achieve industrial production line.

2.3.1 Working principle of electrostatic spraying

       High-voltage electrostatic spraying in the high-voltage electrostatic generator to provide. The working principle of the spray gun is based on corona discharge theory. As shown in Figure 9 electrostatic spray gun muzzle of the high voltage discharge needle and high voltage generator output of negative high voltage connected, air atomized powder coating from the muzzle of the gun spray. As the end of the discharge needle produces a corona discharge so that there is a large number of free electrons in the space around it. When the powder passes through this area, it absorbs electrons and becomes negatively charged powder particles, which run to the positively charged grounded workpiece and adsorb to its surface under the action of air thrust and electric field force. This powder can be permanently adsorbed on the surface of the workpiece and does not fall off. However, the powder can be removed by brush or compressed air. This section focuses on corona charging theory for discussion.

(1) corona

       Electrostatic theory tells us that the distribution of charge on the surface of the charged isolated conductor is related to the surface curvature half by, the curvature of the largest place (i.e., the sharpest place) charge density is the largest, the electric field strength of the space near it is also the largest. When the electric field strength reaches enough to ionize the surrounding gas, the tip of the conductor produces a discharge, if it is a negative high-voltage discharge, then the electrons leaving the conductor will be accelerated by the strong electric field, it collides with air molecules, so that the air molecules ionize and produce positive ions and electrons, the newborn electrons are accelerated to collide with air molecules, thus forming an electron avalanche process. Positive ions run to the negative polarity of the discharge needle, accepting electron reduction into neutral molecules. This ionization phenomenon occurs only around the electrode needle. The electron mass is very light, when it hits the ionization region, it is quickly absorbed by the gas molecules which are much heavier than it, and the gas molecules become free state of negative ions, this negative ions run to the positive electrode under the action of electric field force, and produce a layer of corona light at the ionization layer, which is called corona discharge. When the powder passes through the outer area of the corona, it will collide with the negative ions running to the positive pole and charge.

      Theoretically, positive and negative corona can be used for powder charging. But in practice, electrostatic spraying mostly use negative corona, because the positive corona produces an occasional spark breakdown voltage is lower than the voltage of the negative corona, it can get the corona current is relatively small, and thus the charging efficiency to be lower.

(2) Charging of powder

       Most industrial powder coatings are polymer insulating materials with complex structure. Negative ions can be adsorbed to the surface of the powder only when there exists a location on the surface of the powder particle that can accept the charge. For negative ions, the acceptance point on the powder surface can be a positively charged impurity or a potential energy pit in the powder composition. The absorption of ions can also be purely mechanical. But regardless of the mechanism resulting in adsorption, effective deposition on each powder particle is not easy for ions, and the high resistivity of the powder particles is itself a limitation to effective charging.

Analyze the powder particle charging process shown in Figure 11. Assuming that each colliding ion is strictly "locked" at the point of collision on the surface of the powder, the charge is not redistributed due to electrical conductivity, as it is on the surface of a conducting particle, because of the high surface resistance of the powder, so that the charge density is the same everywhere on the surface. Therefore the pattern of charging of insulating powder particles in Figure 11 is representative. That is, the surface of the charged powder particles adsorbed to the workpiece has an island state of charge, and the distribution of the surface charge is not uniform.

       The ion charging pattern of the insulating powder particles envisioned above, together with three qualifications, allows the calculation of the powder particle charging amount.

(1) An insulating powder particle in an ion cloud, when its potential is not equal to that of its surroundings it will adsorb ions until the two potentials are exactly equal. (2) The powder is spherical, and the ions have an equal chance of colliding with the powder in all directions, so the charge distribution on the surface of the powder will be uniform (this assumption is only possible if the powder is completely motionless. (The actual spraying is unlikely to happen). (3) For insulating powder, there exists a maximum value of charging surface charge.

        Assume that all areas of the powder particle surface are charged. Then the motion of ions colliding on the surface of the powder particle will terminate immediately when the potential of the powder particle is equal to the ambient potential. This is because the electric field E in the figure is generated by the surface charge of the powder particle, which is the interfacial electric field between the powder particle and the surrounding environment. As the accumulated charge increases, the value of E will also increase simultaneously. When E reaches a certain value, the ions can no longer attach to the surface of the powder particle, at which time the accumulated surface charge of the powder particle is the maximum surface charge, and the limit value of the surface charge can be calculated by the Pauthenier formula as

ε

q = 12πε0----Ea2

ε + 2

Where: ε0 - free space dielectric coefficient

ε - powder relative permittivity

E - electric field strength

α - spherical powder particle radius

       The above analysis is the charging of powder particles under negative corona, if it is positive corona charging, its charging characteristics and the maximum surface charge obtained by the Pauthenier formula is still valid. It is only the way of corona charging of powder particles in the ionized region that is different from negative corona charging. Due to the positive high voltage applied to the electrode, electrons will be stripped from the neutral air molecules to produce positive ions, while the electrons are quickly collected by the electrode. The positive ions move toward the grounded workpiece and collide with the powder particles for charging, making the powder become positively charged particles. Scholars at home and abroad have made a lot of research on the two corona charging mechanisms of the above powder particles. But for each powder particle surface absorption mechanism and ion attachment mechanism (electro-adsorption, mechanical attachment or a combination of both) has not been very clear.

(3) Powder adsorption

Powder electrostatic adsorption can be roughly divided into three stages 

       Negatively charged powder in the electrostatic field along the power line flying to the workpiece, powder uniformly adsorbed on the surface of the workpiece; B for the second stage, the attraction of the workpiece to the powder is greater than the repulsion of the powder accumulated on the surface of the workpiece to the subsequent deposition of powder, the surface of the workpiece continues to accumulate powder; C for the third stage, with the continuous thickening of the powder deposition layer, the powder layer to the flying powder repulsion increases, when the workpiece to the powder Attractive force and powder layer on the powder repulsion equal, the workpiece will no longer adsorption of the flying charged powder.

Powder adsorbed on the surface of the workpiece is heated, so that the original "loose" accumulation of solid particles on the surface melt leveling curing (plasticization) into a uniform, continuous, flat, smooth coating film.

2.3.2 electrostatic spraying construction process; construction process is essential to the impact of powder film; powder electrostatic coating construction affects the quality of the coating film many factors; (1) electrostatic spraying process parameters spraying voltage; within a certain range, the spraying voltage increases, the amount of powder adhesion increases, but when; Table 12 different powder coatings electrostatic spraying voltage and current parameters; supply powder air pressure; in the spray powder volume remains unchanged, other spraying conditions are the same, the powder supply; ﹡ calculated with 0.05Mpa deposition efficiency of 100%; the amount of powder sprayed; the initial growth of the powder layer thickness.