Improves the ease of spraying of powder coatings

'Improve Spraying Performance of Powder Powder SCoatings Zhu Wei (Ningbo Chengxiang Plastic Powder Co., Ltd.) Abstract: This article discusses the factors that affect the efficiency of powder coating spraying, as well as measures to improve the fluidity of powder coatings, starting with the mechanism of electrostatic spraying of powder coatings1 Powder coating machine 1.1 commonly used coating method introduction 1.1.1.1 fluidized bed immersion bed immersion method is to preheat the workpiece immersed in a fluidized powder container, so that the powder melt attached to the workpiece, and then put the workpiece in the oven or oven to bakeThe preheating temperature of the workpiece should be higher than the melting temperature of the powder, but should be lower than the decomposition temperature of the powder, and can be coated at a time up to 200 s micro;m or moreThe equipment is simple, no recycling device, powder utilization is high, can be 100% to use, but the coating is not uniform, can not be thin lying, difficult to coat the complex structure of large-scale workpieces, workpieces must be warmed up, heat lossThe coating method is suitable for small workpieces, such as motor stators, special tubes, valves, refrigerator shelves, birdcage, bicycle basket coatingSome of the larger workpieces such as metal guardrails and protective nets are also coated with this method1.1.2 Flame spraying powder coating through high temperature flame gas, after melting sprayon ontheta, by the fluid leveling of curing into coatingThis method does not require an oven or oven, can be coated on-site, can be thick coating to 500 micro;m or moreThe applicable powder varieties are few eras, mainly used to spray thermoplastic powders such as ethylene-acetate copolymers (EVA), polyethylene, polyamide, etc1.1.3 Static powder coating method uses high-voltage electric field effect, so that the powder and workpieceare are induced and respectively with the opposite charge, the powder adsorption to the workpieceThis method is the most important powder coating processing method, which can adapt to the coating of almost all varieties of powderThere are two ways to be charged: high-voltage static electricity and friction altogether1.1.4 Static current bed immersion coating This is a coating process that combines the fluidized bed immersion coating method with the electrostatic powder spray methodWorkpieces do not need to warm up, powder utilization is high, easy to achieve automated production1.1.5 Vacuum attraction coating method is suitable for spraying small diameter pipes1.1.6 Electric field cloud coating method This is the latest development of a coating process, also is electrostatic spraying, it uses fixed nozzles and parallel arrangement of electrodes instead of the usual active spray gunPowder blown out by compressed air is fed into the electrode space and charged by ions produced by the corona needle, which is an electric field cloudThe grounding workpiece enters the electric field cloud, forming an electric field between the corona needle and the workpiece, so that the charged powder is adsorbed to the workpiece and then coated by heating and curingThe biggest advantage of this method is that the powder rate can reach 95%, which is 1/3 to 1/2 powder saving than the general electrostatic spraying method, the coating is dense and flat, and the production line for PCM (precoated steel plate) has been developed2 Electrostatic spraying principle 2.1 high-voltage electrostatic spraying mainly uses corona discharge electrostatic powder spray gun system to complete powder spraying, including: 1) high-voltage electrostatic generator (consisting of high-frequency transformer and boost circuit); With the continuous development and innovation of spraying technology, it is now possible to install the high-voltage static generator inside the gun, turn on the low voltage power supply, by a multi-stage booster into high-voltage electricity, applied to the end of the spray gun corona discharge electrode or ring electrode, so that the air around the electrode ionization, and with the grounded workpiece (generally anode, positively charged) between the formation of an electric fieldThe powder coating is transported from compressed air to the front of the gun, atomized, and the powder passes through the electric field with a negative charge, flying towards the surface of the workpiece under the action of electric field force (electrostatic gravity) and compressed air, and adsorbed on the workpiece2.2 The main principle of frictional electrostatic spraying friction electrostatic spraying is that two different non-conductors create a positive and negative charge when rubbing against each other, so that the conductor sons a static chargeIn the powder coating process mainly using teflon (PTFE) as friction material, placed in the inside wall of the spray gun, when the powder with compressed air into the nozzle, due to rapid movement and severe friction with PTFE, powder and PTFE performance is very different, so that the powder particles with positive charge, in the thrust of the air flow to the workpiece (due to groundas, as a cathode), adsorporised on the surface of the workpiece, and then through baking3 Factors affecting the efficiency of powder spraying Powder spraying efficiency depends on the sound spraying equipment, "work to do good, must be the first to be a good factor"; 3.1 The factors that affect the powder spray efficiency can be referred to table 1The author has examined dozens of small and medium-sized powder production plants in China, which supply uneven powder quality, but have a prominent problem, namely, poor powder flow, low powder rate, high recovery rate when spraying powder, large recovery rate, spray efficiency is not easy to improveIn contrast, some large companies such as DuPont Huajia, Akzo Nobel, PPG, etc., they provide good powder fluidity, high powder rate, has a clear advantageAccording to the powder production process to analyze, the domestic small powder factory mainly in three aspects and these large enterprises there is a gap: 1) equipment; 3.2 What is a liquid powder? Good-fluid powder coating in the process of use is easy to fluidize, loose, has a flowing effect, from the powder bucket to the spray gun powder transmission light, and the powder atomization effect is good, there is no group or spitting powder phenomenonWe have used laser particle size analyzer and electron microscope for many manufacturers of powder detection and observation, from which found some regular phenomena, simple generalization such as Table 2According to our experience, the production of a fluid powder must focus on three points: 1) low temperature crushing; 2) after-mixed flow aids, 3) control powder particle size distribution, and will be analyzed one by one below4 Measures to improve powder fluidity 4.1 The main component of cryogenic crushing powder coating is resinThe molecular weight of the resin with powder coating is low, generally only a few thousand, the softening point is less than 120 degrees C on average, the epoxy resin is only about 95 degrees C, and the glass temperature (Tg) is lower, generally around 60 degrees CNow the vast majority of crushing powder coatings use ACM grinding, powder crushing and grading at the same time, because there is a large amount of air through the grinding cavity, so the material and grinding body heating is small However, in the actual use process, we found that the equipment for a long time or hot summer, the production efficiency of the powder significantly decreased, the powder out of the screen temperature is high, immediately after packaging there will be loose clumps, affecting normal use Such powders are observed under an electron microscope, and there will be phenomena described in Table 2, such as irregular lysesofic powder shapes, many particles are jagged and so on Domestic ACM grinding air inlet is very short, air has not been processed directly into the grinding body, and foreign ACM grinding air inlet is very long, and air into the grinding body before cooling treatment, generally below 15 degrees C, can ensure that the grinding out of the powder temperature does not exceed 25 degrees C, far below the powder Tg point It is suggested that the domestic powder ingenuity plant should improve the air duct and install the refrigeration device Powder plants can add cooling air conditioners or special cooling fans at the ACM grinding air intakes Many powder factories have adopted this method and are working well Powder plant in the conditions permit, the extruder out of the semi-finished product should be as cold as possible, do not immediately crush 4.2 After mixed flow aid powder coating itself is very fine, usually particle size in microns (micro;m) measured The powders with normal distribution of granularity are mostly fluid However, the liquidity of the powder itself is very weak, to improve its mobility should be extruded and crushed at the same time added gas phase silica Powder coatings with a certain amount of gas-phase silica are observed under an electron microscope, and the powder particles are not adhesion between them and have a strong particle sense The reason is that powder particles float between or flow with finer particle sizes, less specific colloidal silica particles Commonly used gas phase silica are the American CABOT M-5, EH-5 and the AeroSILSIL200 and AEROSILSILSIL272 of DEGUSSA, Germany Specific technical indicators for these products are shown in tables 3 and 4 The gas phase silica of smh.com.au is a very thin, non-stereotyped silicon dioxide obtained by flame hydrolysis, a spherical particle with an average particle size of 7 to 40 nm Gas-phase silica can be divided into hydrophilic and hydrophobic two categories according to polarity, in which hydrophobic silica is beneficial to improve the process application of powder coating, usually adding a total powder of 0.1% to 0.2% of the total powder when crushing is the ideal effect Such a small number of gas-phase silica has little adverse effect on the fluidity of powder coatingfilm filming and curing Sometimes it also increases the ability to coat the edges However, the substance is very counterproductive, too much addition of gas phase silica will have a negative effect From some of the physical indicators of gas-phase silica (see Table 3 and Table 4), colloidal gas-phase silica is very fine, measured in nanometers its diameter, has a very strong dispersion, than the surface area is particularly large, easy to float So how to accurately and quantitatively add gas-phase silica to the shredder and crush the sieve with the powder, which is a difficult task For many of the actual situation of domestic powder factory, here are three methods of use for your reference (1) If only a small amount of powder (e.g dozens or one or two hundred kg), the pre-calculated and called gas-phase silica and powder semi-finished mechanical mixing - manual or machine stirring, and then crushed (2) Add the input port in the feed section of the shredder, add the silicon dioxide quantitatively with a special gas-phase silica feeder, and complete the common crushing with the extrusion of semi-finished products (3) Add an inlet on the air intake mask of ACM grinding, using negative pressure, the silicon dioxide output from the gas-phasesilid silica feeder is sucked into the grinding chamber and crushed together with the extruded semi-finished product 4.3 The thickness of the powder particle size distribution powder should be a statistical concept from a macro perspective, because the finished powder must be made up of coarse powder, fine powder, and powder between coarse and fine powder The powder particle size is measured in microns (micro;m) and was earlier measured by screening That is, a certain quality of the powder, after a combination of different particle size of the screen, statistics through each layer of the screen powder quality, to express the particle size distribution of the powder Screening method is time-consuming, each test takes at least a quarter of an hour, is poor lyson, data fluctuations, and is no longer recommended in the powder industry With the progress of technology, laser diffraction particle size analysis method has matured day by day, and has been used to test the particle size distribution of powder coatings abroad very early Laser diffraction particle size analysis method is fast and convenient, can be detected online, usually a complete test of a sample powder only 2 to 3 minutes, and in a chart can be described in a set or set of important parameters and particle size distribution graph, very intuitive The particle size distribution of the powder should theoretically be normally distributed (i.e., Gauscurve curve), which is reflected in the graph that the particle size of the powder on both sides of a central point is evenly reduced or increased, the peak is obvious at the center point, and must be a single peak distribution In layman's terms, the particle size of the powder should be concentrated as much as possible around the average particle size, too coarse and too fine powder particles should be very small The particle size distribution of the powder should be analyzed from the test chart (1) The average particle size is generally expressed in Dv50, which is a statistical concept, that is, the powder is treated as a ball of equal diameter, the value of which is measured in microns (micro;m) (2) Fine powder content is also a statistic, meaning that the cumulative volume of a powder less than a particle size as a percentage of the entire powder particle volume, mainly considering the content of less than 10 micro;m particles (3) The coarse powder content is also a statistic, corresponding to the fine powder content, mainly considering the powder content of more than 90 s micro;m particles (4) graphic distribution, to observe whether it is a single peak or twin peaks, peak-shaped towering or flat, etc Based on practical experience, the particle size distribution parameters for conventional powders are provided here (1) Powder for high-pressure electrostatic spray gun: Dv50- 35 to 38-micro;m Fine powders less than 10 s micro;m are 8%, greater than 3% of the rough powder of 90?micro;m (2) Friction gun powder: Dv50 s 40 to 45 s micro; m, 10 s micro; m fine powder content of 6%, 70 s micro; m coarse powder content, 90% (3) Fluidized bed powder: Dv50 is generally in 50 to 60 s micro;m The fine powder content of 10 s micro; 170 s micro;m is 3% thick powder content The spray properties of the powder coating have a lot to do with the fine powder content, and the charged properties of fine powders (especially the ultrafine powders of 5 and micro m) are so poor that they can't be charged at all So this part of the powder is not charged at all So this part of the powder is not charged at all It is usually a direct loss that goes into the recycling system Powders that have been tested in many small powder factories, the vast majority of which have a thin particle size, with an average particle size of 25 to 32?micro;m, and 10-micro;m, are mostly about 15 percent, and the rough powder of 75-micro;m is less Too fine powder fluidity is very poor, ultrafine powder content is high powder charge weak, high recovery powder, powder utilization rate is reduced, spraying speed is not easy to improve Compared with high-pressure electrostatic powder, friction gun powder should pay more attention to particle size control, because in the friction gun, the powder to be carried with sufficient positive charge, powder particles must be at a certain speed with the barrel of the inner wall of the Poly4fluoroethylene material friction If the powder particles are too fine, the area of friction with the inner wall of the tube is too small, not enough to produce and maintain sufficient positive charge, friction charged spray system, there is no electrostatic field between the spray gun and the workpiece, the powder should be deposited on the workpiece, mainly rely on the amount of charge carried by powder friction, the powder with small charge, low electrostatic attraction, powder is not easy to adsorb to the workpiece It has been observed that powder particles of 5 and micro;m are not normally used in friction gun spray systems The powder used for friction guns must be as dry as possible, and humidity affects the friction allive of the powder When producing powder coatings for friction guns, the flow additive should be made with gas phase alumina It increases the charge of friction.