Technical requirements of electrostatic spraying process for epoxy powder coating

In recent years, the application scope of powder coating in China has been expanding, and people's requirements for powder coating and coating equipment are also increasing At present, in addition to epoxy powder, polyester modified epoxy powder and polyester powder appear, which greatly improves the outdoor resistance of powder coating; in terms of color, the current powder coating has more than 40 colors, which can meet different needs Therefore, the powder coating has developed from the past anticorrosive application of thick film with high performance to the present decorative application of thin film This is a leap in the variety of powder coatings in China At present, most of the powder coating adopts the electrostatic spraying technology - the cold coating technology of the powder after the heat curing The electrostatic spraying process of powder coating essentially includes two parts: the performance of powder coating and electrostatic spraying equipment (including spraying process) These two parts are interdependent and mutually promoting Only with ideal (suitable for electrostatic spraying) powder coating and good electrostatic spraying equipment, can we get high-quality powder coating Otherwise, without one of the good performance, we can not get high-quality coating, which has been proved by powder coating spraying practice for many years Therefore, in-depth study of powder coating and electrostatic spraying equipment is the current direction of efforts In the electrostatic spraying experiment, in addition to the exploration and improvement of the spraying equipment (electrostatic powder gun, powder feeder, etc.), we also analyzed the powder coating suitable for electrostatic spraying The conclusion is obvious that the deposition rate (adsorption rate) and adsorption force of the powder coating are different with the same spraying equipment and the same process conditions and different powder coating, The difference is reflected in the quality of the coating 2 Technical requirements of electrostatic spraying process for powder coating During the construction of experimental electrostatic spraying process for powder coating, the characteristics of powder coating are often ignored, so that when there are some defects (such as bubbles, pockmarks, depressions, shrinkage holes, speckles, or even coating powder, etc.) on the coating film, there is no way to start, and it is difficult to analyze In fact, for the powder coating for electrostatic spraying, the following parameters should be paid attention to in order to ensure the smooth implementation of electrostatic spraying process and obtain high-performance coating 1 The particle size of powder coating is different from that of solvent coating In solvent coating, organic solvent is used as dispersion medium, while in powder coating, purified compressed air is used as dispersion medium The powder coating is dispersive during spraying, so the particle size of the coating cannot be adjusted Therefore, the powder particle size suitable for electrostatic spraying is important The particle size of powder coating suitable for electrostatic spraying is preferably between 10 μ m and 90 μ m (i.e > 170 mesh) Powder with particle size less than 10 microns is called ultrafine powder It is easy to lose in the atmosphere, and the content of ultrafine powder cannot be too much It is worth noting that the particle size of the powder is related to the thickness of the coating film For the particle size of the powder coating, a certain distribution range (Table 1) is required to obtain a uniform thickness of the coating film If the thickness of the coating is required to be 25 microns, the maximum particle size of the powder coating shall not exceed 65 microns (200 mesh-240 mesh), and most of the powder shall pass 35 microns (350 mesh-400 mesh) In order to control and adjust the size of powder particles, it should be able to be adjusted on the crushing equipment As far as the powder factories in China are concerned, we can't do this at present Table 1 particle size distribution of 180 mesh powder coating with nominal mesh number of 400 mesh, 240 mesh, 180 mesh and 170 mesh passing percentage of 40-50%, 70-85%, 95-99% and 100% powder coating can be tested by kcy type automatic recording particle tester When the particle size of the powder is more than 90 μ m, the ratio of the charge to the mass of the particles is very small during electrostatic spraying, and the gravity of the large particle powder quickly exceeds the aerodynamic force and electrostatic force Therefore, the large particle powder has great kinetic energy and is not easy to be adsorbed on the workpiece 2 For electrostatic powder spraying process, the resistivity and dielectric constant of powder coating should be taken into account It is important to keep the charge and distribution of the powder coating particles, which directly affects the adsorption force and deposition efficiency of the powder on the workpiece In addition, it is important that the uncured powder coating can withstand the mechanical vibration of the conveying mechanism without dropping the powder In fact, the main factor that affects the charge acceptance and retention of powder particles is the dielectric constant of the powder coating The lower the dielectric constant of the powder, the easier it is for the particles to be charged, but the easier it is to lose the charge This reflects that the adsorption force of the powder on the workpiece is not strong, and the powder will fall off if it is slightly vibrated For the electrostatic spraying powder coating, the high dielectric constant should be used as much as possible, which will make the powder powder The final adsorption capacity is greatly improved More uniform coating But the powder coating with high dielectric constant is difficult to be charged, so it needs to improve the structure of electrostatic powder gun and adopt the structure of multi electrode forced charging For powder coatings, they are all composed of high molecular compounds (such as epoxy powder, polyester powder, etc.), and they have two main adsorption forces on workpieces: Coulomb force (electrostatic force) and Van de Waals force (molecular force) Polymer compounds have high resistivity, so the 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 field strength; for example, when the resistivity of the powder is 1013 ohm, the electrostatic voltage is only 30-50kv, which can make the powder well charged; and when the resistivity of the powder is 108-109 ohm, the electrostatic voltage of 100-120kv must be applied to get the above charged effect The relationship between powder resistivity and electrostatic voltage (Figure omitted) Whether the thickness of the powder deposition manifold can be automatically limited depends on the resistivity of the powder itself Experiments show that only the powder with high resistivity can obtain the appropriate coating 3 Moisture content of powder moisture content of powder coating itself moisture absorption (moisture content) directly affects the resistance and dielectric constant of powder itself If the powder is seriously hygroscopic, it will agglomerate It is impossible to carry out electrostatic spraying General moisture absorption, in addition to affecting its charged performance, will also reduce the fluidity and film-forming of the powder, so that the film is not smooth or even difficult to adsorb on the workpiece and the film will produce bubbles and pinholes The moisture absorption of powder coating is not only caused by careless storage, but also related to the degree of compressed air purification during spraying Because condensed water is easy to be produced in compressed air, filter and moisture absorption device must be provided in 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 spraying site should also be paid attention to It is reported that every 30% change in relative humidity is equivalent to two orders of magnitude change in powder resistivity (Figure omitted) 4 The stability of powder coating the stability of powder coating refers to whether the powder will agglomerate in storage or use, whether the leveling characteristics will be poor, whether the electrified effect will be poor, whether the orange lines of the coating film are obvious, the luster will be weakened, and whether the pinhole bubbles will occur, etc In the trial production of powder coating, we must pay attention to its storage stability Only the powder coating with certain stability can be used by users In foreign countries, some additives are often added to the powder coating to enhance the stability of the powder coating Therefore, this kind of powder coating will not produce caking in the general humid air or the temperature as high as 70-75 ℃ The stability of powder coating is determined by measuring the change of leveling property of powder coating at a certain temperature and after a certain time Because the stability of the powder coating indicates the degree of the crosslinking reaction of the molecules under the storage condition; the more intense the crosslinking reaction of the powder, the larger the molecular weight of the powder, the higher the viscosity of the powder under the curing temperature, and the worse the leveling property 3、 Conclusion 1 As one of the two parts of electrostatic powder spraying process, the properties of powder coating can not be ignored In the development of electrostatic spraying equipment, attention should be paid to the technical requirements of powder coating 2 In this way, we can achieve the goal of high quality powder coating 2 On the basis of many experiments, analysis and comparison, it is concluded that these technical parameters, such as fineness of powder, resistivity and dielectric constant of powder, hygroscopicity of powder and stability of powder, should be paid attention to in order to meet the requirements of electrostatic spraying process These technical parameters have a direct impact on the charge efficiency and adsorption capacity of the powder, as well as the quality of the coating When the designed electrostatic powder gun has a good charged mechanism and diffusion mechanism, but does not pay attention to the quality of powder coating, it can not get high-quality powder coating 3 When we understand the characteristics of powder, we can find out the causes of some defects of powder coating and solve them At the same time, we can provide practical parameters for the design of electrostatic powder gun and powder feeder