Causes and control measures of particle formation in powder coatings and coatings

2021-09-15 11:00

Particles are a common and complex problem in powder coatings and coatings. This article categorizes and analyzes the causes of particle formation in powder coatings and coatings, and then provides targeted control measures to reduce the occurrence of particle problems in coatings.


Mechanical impurities

The mechanical impurities in powder coatings mainly include the following types:

1. From the pigment or filler itself: The pigment or filler itself has a certain particle size range due to the requirement of oil absorption, and coarser pigment or filler particles themselves can easily cause particles to appear in the coating.

Controlling such particles generally requires pigments or fillers to pass through a 325 mesh sieve, making it extremely difficult to cause particle formation due to this reason;

2. Insoluble substances from resin: Mechanical impurities or particularly large molecular weight particles present in the resin due to production process issues, which cannot be dissolved in organic solvents and will not melt at high temperatures.

Controlling such particles generally requires the use of acetone to test the raw material to determine whether its insoluble content meets the control range negotiated with the raw material supplier;

3. Mechanical wear from the powder production process: Metal chips formed by the mechanical parts of the extruder, grinder main mill, or cyclone separator due to friction with the powder coating.

Generally, the likelihood of such problems occurring is relatively low, and the particles generated have a serious scratch sensation when touched by hand, making it easier to distinguish. Controlling such particles mainly requires regular maintenance and upkeep of the equipment;

4. Fabric fibers or dust from the production process: the packaging mouth of raw materials, gloves and clothing of workers, and similar fibers or powders that exist in the environment can cause the generation of particles.

The control of such particles generally requires the use of gloves, clothing, and tools by workers and other on-site maintenance personnel that do not cause similar pollutants. The powder manufacturing environment should maintain a certain humidity and good ventilation.


Gelatinized particle

There are two main possibilities for the generation of colloidal particles in powder coatings:

1. Equipment reason: There is a certain dead angle between the extruder screw and the screw, and there is a certain gap between the screw and the screw barrel. During the extrusion process, there may be some materials remaining inside the extruder, and the resin undergoes a chemical reaction at a long-term extrusion temperature (usually 90-110 ℃), causing gelation.

The occurrence of gelatinized particles in this problem is objective and almost unavoidable. It is mainly necessary to adjust the arrangement of the screw blades to minimize the occurrence of gelatinized particles in the coating.

2. Process reason: If the extrusion temperature is too high, it will cause some of the resin to undergo chemical reactions, or if some materials cannot be extruded in the extruder due to material breakage and stay for too long, the resin will undergo chemical reactions, resulting in the generation of gelatinized particles;

The key to controlling such particles lies in strictly controlling the extrusion temperature according to the technical process, and controlling the actual feeding and discharging speeds.


Powder particle size too coarse

If the powder particle size is too coarse, there are generally the following possibilities:

1. The particle size of the powder itself does indeed exceed the agreed range: the coarse powder content of the powder itself is too large, and the average particle size far exceeds the normal requirements, resulting in the inability of the powder to melt completely during normal spraying, leading to the generation of particles.

Solution: Controlling such particles is mainly achieved by real-time detection of particle size or by detecting the particle size distribution after mixing the same batch of powder into a uniform state, ensuring that it meets the detection requirements;

2. Powder flakes appear in the powder: When the powder is sieved through a rotary screen, the screen breaks, causing small powder flakes to be used as finished powder and flow into the customer, resulting in the generation of particles.

Solution: Controlling such particles mainly involves conducting hand sieving (100-140 mesh sieve) tests on the powder at regular intervals when producing powder, to ensure that such abnormalities do not occur;

3. Spraying film thickness is too thin: The normal spraying film thickness of the powder is required to be about 2.0 times the average particle size of the powder. If the film thickness is too thin, the powder will appear in a melting and leveling state similar to the first possible situation.

Solution: Controlling such particles requires spraying to the agreed film thickness of the powder during coating construction.

Particles that appear during the coating process


Substrate defects

The substrate for powder electrostatic spraying of workpieces is mainly aluminum or iron. Generally, defects in the substrate result in particles, which mainly refer to the presence of point like protrusions or small pits on the surface of the substrate. The protrusion of the substrate exceeds the thickness of the coating, and the coating cannot cover it up, resulting in particles. Small pits are caused by the inability to completely discharge air from the pits, resulting in the coating being lifted and forming particles.

The main method of controlling such particles is to treat the surface of the substrate to make it flat. Generally speaking, aluminum substrate is suitable for alkaline corrosion, while iron substrate is suitable for polishing (polishing with oil stains on the surface needs to be carried out together with degreasing solution, otherwise black oil stains will form on the surface of the substrate, and degreasing cannot be completely removed);


Pre processing defects

Pre treatment defects mainly refer to the residual metal chips on the surface of various pre treatment agents or some substrates, resulting in the appearance of particles after spraying.

The control of such particles is mainly achieved by strengthening the washing effect of pre-treatment, ensuring that the surface of the substrate is treated clean and free of similar residues (air blowing can also be added before and after pre-treatment drying).

3. Spray system contamination

The pollution of the spraying system mainly includes the following situations:

1. Fluidization bucket pollution: There are other powder residues or agglomerates of powder at the bottom of the bucket that have not been cleaned up, resulting in the appearance of particles after powder spraying.

Solution: The main way to control such particles is to clean the powder fluidized barrel in a timely manner. If the fluidized barrel is not used for a long time (not used for more than 4 hours), all the powder in the fluidized barrel should be removed and stored;

2. Spray gun pollution: When the spray gun produces uneven powder and has excessive high-voltage discharge during spraying, the powder inside or at the nozzle of the spray gun may melt or undergo chemical reactions due to electric shock, resulting in the appearance of particles after powder spraying.

Solution: The main way to control such particles is to adjust the parameters of the spray gun to ensure uniform and good atomization of the powder, and the spray gun voltage should not be too high.

3. Chain feeding and hanging device pollution: Dust on the conveyor chain or coating on the hanging device falls onto the surface of the workpiece coating, as it is already solidified particles that will not melt and level again, resulting in particles appearing on the coating.

Solution: The main way to control such particles is to regularly clean the conveyor chain and fixtures, and keep them clean and tidy.


environmental pollution

Environmental pollution mainly includes the following situations:

1. Environmental pollution in the powder room: The clothing and tools used by the personnel in the powder room (such as cardboard boxes and dust removal cloth) have fibers or are themselves unclean; The dust on the ground of the powder room and the floating dust inside the powder room are severe; The powder room is not sealed, and external dust or particles caused by grinding entering the powder room can cause particles to be generated during the spraying process of the coating (in the case of electrostatic spraying, the workpiece will automatically adsorb solid small particles in the air due to static electricity).

Solution: Controlling such particles is mainly to keep the powder room clean and tidy, and maintaining a certain humidity in the powder room is also beneficial for reducing the generation of particles.

2. Environmental pollution in the drying tunnel: There are particulate waste formed by combustion materials in the drying tunnel (solid particles are generated by coal, biological particles, and natural gas, which are more obvious in direct heating curing furnaces). Dust on the inner wall and bottom of the drying tunnel, which adheres to the surface of the workpiece due to thermal cycling, can cause particles to appear on the coating surface.

Solution: The main way to control such particles is to use indirect heating to heat the curing furnace as much as possible, ensuring that the air entering the curing furnace is clean, and maintaining regular cleaning of the curing furnace.

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