Technical requirements of electrostatic spraying process for epoxy powder coatings

Qiu Henian, foreword
In recent years, the application of powder coatings in China has been expanding, people's requirements for powder coatings and coating equipment are getting higher and higherAt present, in addition to epoxy powder, there is a polyester modified epoxy powder and polyester powder, which greatly improves the outdoor performance of powder coating; As a result, powder coatings have evolved from the high-performance anti-corrosion use of thick coating films in the past to the current use of thin coating spand spouts ornate decorative usesThis is a leap in the variety of powder coatings in Chinapowder coating softer powder coating is currently the vast majority of the use of electrostatic spraying process - powder cold coating technology after thermal curing filmThe electrostatic spray process of powder coating essentially consists of two parts: the performance of powder coating and the electrostatic spray equipment (including the spray process)These two are interdependent and mutually reinforcing, only with the ideal (suitable for electrostatic spray) powder coating, and with good equipment of electrostatic spray equipment, in order to get high-quality powder coating, on the contrary, the lack of one of the good performance, will not get high-quality coating, which has been confirmed by the practice of powder coating over the yearsTo this end, the in-depth study of powder coatings and electrostatic spray equipment is the current direction of effortsIn our electrostatic spraying experiment, in addition to the spraying equipment (electrostatic powder gun, powder dispenser, etc.) to explore and improve, but also suitable for electrostatic spraying powder coating analysis, the conclusion is obvious, with the same spray equipment and the same process conditions, the use of different powder coatings, the resulting powder coating deposition rate (absorption rate) and adsorption force is not the same, reflected in the quality of the coating there is a differencesecond, electrostatic spraying process on the technical requirements of powder coating
in the experimental powder electrostatic spray process construction, often do not pay attention to the characteristics of powder coating, so that when the coating film appears some defects (such as bubbles, numb spots, depressions, shrinkholes, plaques, and even not coated powder, etc.), it is difficult to analyzeIn essence, the powder coating for electrostatic spraying should pay attention to the following parameters, in order to ensure the smooth implementation of electrostatic spraying process and get high-performance film coating1, powder coating particle fineness
powder coating and solvent-based coatings are differentIn solvent-based coatings, organic solvents are used as dispersive media, while in powder coatings, purified compressed air is used as dispersive mediaPowder coatings are dispersed when sprayed and cannot regulate the particle size of the coating Therefore, powder particle fineness suitable for electrostatic spray is important powder coating suitable for electrostatic spray, the particle size is best between 10 microns and 90 microns (i.e 170 eyes) Powders with a particle size of less than 10 microns are called ultrafine powders, which can easily be lost in the atmosphere and cannot be too fine It is worth noting here that the particle size of the powder and the thickness of the coating film are related, the particle size of the powder coating must have a certain distribution range (Table 1) in order to obtain uniform thickness of the coating film If the thickness of the coating is required to be 25 microns, the maximum particle of the powder coating can not exceed 65 microns (200 mesh - 240 eyes), and most powders should pass 35 microns (350 eyes - 400 mesh) In order to control and adjust the size of powder particles, they should be adjusted on the crushing equipment For China's powder manufacturers, it is not possible to do so at present Table 1 to 180-eye powder coating particle size distribution
the nominal number of 400 eyes 240 eye 180 eye 170 eye
through the percentage 40-50% 70-85% 95-99% 100% powder coating particle distribution can be used KCY type automatic recording particle meter test powder particle size of more than 90 microns, in electrostatic spraying, the particle with a small ratio of charge and mass, the gravity of large particle powder quickly exceeds aerodynamics and static power, therefore, large particle powder has a larger kinetic energy, not easy to adsorbed to the workpiece 2, powder coating resistivity and dielectric constant
for the powder electrostatic coating process, the focus is to consider the powder coating particles to accept the charge, to maintain the charge and charge distribution, which directly affects the powder to the workpiece adsorption and deposition efficiency, in addition, it is important that the uncured powder coating must withstand the mechanical vibration of the transmission mechanism without powder In fact, the main factor affecting 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 particle is charged, but the easier the loss of charge, which reflects the powder in the workpiece of the suction force is not fast, slightly affected by vibration on the powder, for electrostatic spray powder coating, should be as high as possible with the dielectric constant, it will make the powder adsorpation force greatly improved The coating is more uniform However, the dielectric constant high powder coating, charge is more difficult, which needs to be improved in the structure of electrostatic powder gun, the use of multi-electrode forced charged structure for powder coatings, it is composed of polymer compounds (e.g epoxy powder, polyester powder, etc.), they have two main adsorption forces on the workpiece: coulomb (static power) and Vande Waals force (molecular force) Polymer compounds have high resistivity, so coulomb (static power) is large and reliable The resistivity of the powder itself will determine the live state of the powder at a certain electrostatic electric field strength, for example: when the resistivity of the powder is at 1013 ohm, the electrostatic voltage is as long as 30-50KV, the powder can be well charged, and the resistance of the powder at 108-109 ohm, it is necessary to apply 100-120 KV to obtain the above-mentioned live effect The relationship between the resistivity of the powder and the electrostatic voltage (Chart) Whether the thickness of powder deposition can be automatically limited is very much related to the resistivity of the powder itself, and the experiment confirms that only powderwithe with high resistivity can get the appropriate coating 3, the moisture content of the powder humidity the moisture absorption (water content) of the powder coating itself directly affects the powder's own resistance dielectric constant If the powder is heavily absorbed, it will be clumped This is not possible for electrostatic spraying General moisture absorption, in addition to affecting its live performance, will also reduce the fluidity and film formation of the powder, so that the coating film is not smooth or even difficult to adsorption on the workpiece and get the coating film will produce bubbles and pinholes the moisture absorption of powder coatings is not caused by the absorption of water, but also related to the degree of compressed air purification during spraying Because compressed air is prone to condensate, in the air purification system must have a filter moisture absorption device, so that the air to the powder supply water content to a minimum in addition, the relative humidity of the air at the powder spray site is also to be paid attention to Data reported that for every 30% change in relative humidity, it is equivalent to two orders of magnitude of change in the resistivity of the powder (Chart) 4, the stability of powder coating
powder coating stability refers to whether the powder in storage or use will occur powder clumps, leveling characteristics vary, the effect of charge disparate difference, coating orange pattern is obvious, gloss weakened, pinhole bubbles and so on in the trial powder coating, we must pay attention to the stability of its storage, only with a certain stability of the powder coating, in order to allow users to use Foreign powder coatings often add some additives to enhance the stability of powder coatings, so this powder coating in general humid air or temperature up to 70-75 degrees C will not produce clumps the stability of powder coating is determined by the powder coating at a certain temperature, after processing a certain period of time to determine its leveling changes Because the stability of the powder coating indicates the degree of molecular cross-linking reaction under storage conditions, the more intense the cross-linking reaction of the powder occurs, the greater the molecular weight of the powder, which is reflected in the increase dissonance of the powder at the curing temperature, and the leveling characteristics become worse III, conclusion
1, test confirmed that as one of the two components of powder electrostatic spray process, the nature of powder coating can not be ignored; In this way, the two aspects of joint research, in order to achieve high-quality powder coating purposes 2, after many experiments, analysis, comparison, the basis, to obtain, powder coating to meet the requirements of electrostatic spraying process, should pay attention to these technical parameters: powder fineness, powder resistance and dielectric constant, powder moisture absorption and powder stability These technical parameters directly affect the live efficiency and adsorption of the powder, as well as the coating quality When the design of electrostatic powder gun has a good live mechanism and diffusion mechanism, without paying attention to the quality of powder coating, is not able to get high-quality powder coating 3, the characteristics of the powder have some understanding, the powder coating film can also find the cause of its formation, and solve it, at the same time, the design of electrostatic powder spray gun and powder feeder provides realistic parameters