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The raised particles on the surface of powder coating are a thorny issue for powder coating spray manufacturers and manufacturers. Particles can be said to be a fatal blow to the decorative properties of surface coatings, ultimately affecting the grade of coated products. The generation of particles in powder coatings is inseparable from the characteristics, production processes, and coating status of powder coatings. The non filterability of powder coatings determines the existence of particle problems in powder coatings. Below are specific reasons for the generation of powder coating particles and how to reduce the generation of particles.

Particles in Powder Coatings

The particles of powder coating are basically divided into mechanical impurity particles and adhesive particles.

1.1 Generation and prevention of mechanical impurity particles

Mechanical impurity particles mainly come from the material itself and its production process, equipment wear, and impurities brought in by the environment during powder coating production.

(1) The particles formed by mechanical impurities in resin may have sharp shapes due to the presence of various mechanical impurities in the raw materials or during the production process. This requires resin manufacturers to use qualified raw materials, maintain a clean production environment, strictly control production processes, and take corresponding filtering measures.

(2) 

(2) Impurities in pigments and fillers. For some low-quality products such as titanium dioxide, barium, calcium, etc., they all contain a considerable amount of coarse gravel and black impurities to varying degrees. Using this pigment filler will definitely form particles in the coating. For particles, finer pigments and fillers with no residue after passing 325 mesh are less prone to particle formation; The coarser the pigment filler, the more likely particles are to appear. However, if the pigment filler is too fine, it will increase its oil absorption, which may affect the leveling of the powder coating. This requires the pigment filler to have a reasonable particle size and strictly control the presence of coarse particles.

(3) During the production process of powder coatings, metal shavings may also be generated due to wear and component detachment in the grinding chamber, and the particles formed may have a scratching sensation when touched by hand.

(4) Fabric fibers can also cause particle formation. Some powder coating manufacturers use fabric tied to the ingredient discharge port or product packaging port to receive materials. After a certain period of wear, the fibers fall off and enter the material or product, producing string like particles. Mechanical impurities particles, including dust and other impurities that may be carried by the outer packaging, can also be brought into the pre mixing position, and packaging materials (paper, plastic fibers, packaging lines, etc.) can fall into the mixer. Therefore, it is necessary to conduct inspections at all times in these sections to reduce the generation of particles.

1.2 Colloidal particles

The shape and quantity of gelatinous particles are relatively round, mainly from the extrusion process in the production of resins and powder coatings used.

(1) Colloidal particles in resin. During the resin production process, the reaction kettle may overheat locally or the stirring condition may be poor, resulting in the appearance of high molecular weight and melting point particles in the entire batch of materials. These adhesive particles cannot melt during extrusion in the production process of powder coatings, and are difficult to be crushed into too fine particles in the subsequent crushing process. If these adhesive particles are mixed in normal powder coatings, they may form particles in the coating after spraying and curing.

(2) 

(3) The extrusion process in the production of powder coatings is a major factor in particle generation. On the one hand, it may be caused by the device itself; On the one hand, it may be caused by process control, either due to high temperature or extrusion material breakage and idling.

(4) 

The temperature of the extruder is generally set at around 110 ℃, and thermosetting powder coatings such as hybrid, polyester, epoxy, etc. generally do not undergo chemical reactions at this temperature, which can cause material gelation. However, there is always a certain gap between the extruder screw and barrel, and the gap becomes larger over time, which may result in residual material; In addition, there may be certain dead corners between the screw and the screw stem, which may result in the accumulation of residual materials. These materials will undergo chemical reactions under long-term exposure at a temperature of 110 ℃, leading to gelation and becoming substances that cannot be melted again. Regardless of the type of extruder, the production of gelatinous substances during the extrusion process cannot be avoided, but extruders with small gaps, appropriate aspect ratios, and strong self-cleaning capabilities produce much less gelatinous substances.

Particle problems and solutions in powder coating application

2.1 Material aspect of painted workpiece

In metal workpieces coated with powder coating, some materials such as castings (iron, aluminum), hot-rolled steel plates, galvanized, etc. have rough surfaces and micro pores, which makes the coating prone to particle formation. The formation mechanism is that during the baking and curing process of powder coatings, the surface of the coated material is sealed during the melting flow, causing the melted coating to arch. If the internal pressure is not sufficient to break through the coating, particles will form.

From the perspective of powder coating manufacturing, solving the above problems is to develop a breathable powder coating specifically for application. When designing the formula, two aspects can be considered: first, before the bubbles in the micropores reach the surface of the coating, the powder has already completed leveling and curing, leaving the pores below the coating; The second is to reduce the surface tension of powder coatings during melting, allowing gases in the micropores to easily escape. At the same time, the powder will flow flat in a timely manner and then solidify, achieving a smooth and even coating effect. In terms of coating, it is recommended to polish the coated workpiece as smooth and even as possible, with no oxide scale or micro pores on the surface. If conditions permit, it can also be pre baked, which can effectively reduce the formation of such particles.

2.2 Surface treatment of workpieces

Generally, powder coating workpieces need to undergo surface treatment processes such as degreasing, rust removal, degreasing, water washing, phosphating, water washing (passivation), deionized water washing, and drying. Impurities such as rust removal and phosphating residue may adhere to the workpiece, resulting in the presence of particles after film formation.

In the pre-treatment process, before phosphating, one or a combination of methods should be used to remove scale, rust, grease, fingerprints, and foreign substances on the surface to be phosphated, mainly to ensure that no coarse grains are produced. To avoid the formation of coarse crystalline phosphating films or films with excessive mass per unit area, strong acid or alkali cleaning should be minimized or avoided as much as possible. Processed workpieces should be thoroughly cleaned in cold or hot water to remove residue. Otherwise, if the residue is brought into the phosphating tank, it will damage the quality of the tank solution and directly affect the phosphating effect.

During the phosphating process, attention should be paid to the acidity of the phosphating solution, including total acidity and free acidity. Total acidity refers to the total concentration of various ions such as Zn2+, Fe2+, H+, H2PO4-, HPO42-, etc. in phosphating (such as zinc based). The total acidity is too low, the phosphating reaction rate is slow, and the phosphating film layer is thick and rough. Free acidity refers to the concentration of free H+in the phosphating tank solution. When the free acidity is too high, the phosphating reaction rate is slow, the film formation time is prolonged, and the phosphating film crystals are coarse and porous, resulting in more precipitation in the solution.

2.3 Working environment for painting

The production environment around the powder spraying room will directly affect the surface quality of the coating. For the coating of electrostatic spray guns with corona discharge, some dust and particles (such as flocculent fibers) in the environment are brought into the powder coating room, and charged impurities are adsorbed on the surface of the coating through electrostatic action. When the powder melts and flows, they become particles on the surface of the coating.

A clean production environment has a positive impact on product quality. Conditional manufacturers can effectively reduce surface coating particles and impurities by isolating the powder spraying room from the production workshop. The production workshop and surrounding environment must be clean.

2.4 Thickness control of coating film

If the coating is too thin, in addition to particles, there will also be leveling differences, severe orange peel, poor coverage of light colored varieties, and poor protective ability of the coating (such as acid, alkali, salt, and water resistance). Appropriately increasing the thickness of the coating can effectively reduce the formation of particles. But the coating should not be too thick, as it can affect the size of the coated workpiece and reduce its impact resistance, flexibility, adhesion, and other properties. It is generally controlled between 60-80 μ m; M is appropriate.

For painting companies, the thickness of the coating can be controlled through the following methods: ① the number and arrangement of spray guns in the powder room; ② Powder output and air pressure (powder output, atomization, fluidization parameters) of the spray gun; ③ The operating speed of the conveyor chain, also known as the chain speed; ④ The suspension method and arrangement of the coated workpiece.

2.5 Processing of Recycled Powder

For recycled powder, if it has not been sieved or the mesh size is too small, some non melting particles (including lifting equipment, foreign impurities, etc.) will be produced, which will become particles of the coating film.

For most coating companies, there are generally several ways to recycle powder: ① In the fully automatic powder coating system with conveyor belt recycling, the sprayed powder is automatically recycled and screened, and then mixed with new powder coating to continue supplying to the powder spray gun. The quality of this recycled powder is relatively stable and not easy to produce particles; ② In the two-stage recovery system of cyclone separator and bag filter, when the powder recovered in the first stage of cyclone separator is sieved by vibrating screen and circulated with the powder supply system, it has little effect on the coating and is not easy to produce particles For the powder recovered by the manual spraying coating recycling system, dust and impurities in the environment are easily carried into the recycled powder, and after recycling, it must pass through a vibrating screen with a mesh size of 120 or more.

In the three commonly used recycling systems mentioned above, due to the influence of the environment and the attachment of substances on the hanging chain, impurities, especially solid small particles, are easily carried into the recycled powder, which can gradually accumulate in the powder supply tank. A considerable part of them are solidified powder particles that fall from the hanging chain or lifting device.

3 Conclusion

Both powder coating manufacturing and coating companies are eager to have an effective method to control the occurrence of powder particles. Because once painted, the particles on the painted surface are obvious, which will inevitably cause waste of material and human resources. So treating particle problems correctly is of great practical significance.