Common problems in thermosetting powder coating construction (2)

2018-12-24 14:20

Coating loss

For high-gloss powder coatings, it means that after curing into a film, the film layer gives people a light feeling of dancing or dust. There are many reasons for the loss of gloss:

1. Workshop environmental sanitation: The dust in the workshop is too large, and it is often infusible or difficult to melt with the powder coating, which will cause the coating film to lose gloss. In view of this situation, in addition to improving the hygiene of the workshop, each process should be isolated in space, that is, the ingredients are isolated from the extrusion, and the extrusion is isolated from the pulverizer.

2. Low-molecular-weight materials in raw materials: Some resins remain low-molecular-weight due to their high viscosity and low vacuum during synthesis. Some fillers are treated with low-molecular-weight surfactants and volatilize when the powder coating is cured at a high temperature. Many tiny pores appear in the coating, causing the film to be microscopically uneven and lose its luster. Although the low-molecular-weight polymers contained in some materials cannot be volatilized, their curing speed is very different from the main components, resulting in immiscibility and loss of gloss.

3. The influence of moisture: the filler has high moisture content or the material absorbs moisture; local water seepage in the extruder; water seepage from the roller of the tablet press or the frost on the roller enters the tablet material; the wet air enters the ACM mill without treatment Air inlet; the compressed air for cleaning the cloth bag is not filtered or the compressor has not been discharged for a long time; the finished powder coating is not good for moisture absorption due to packaging. The effect of moisture on gloss is similar to that of low molecular volatiles.

4. Uneven composition: The powder coating is composed of a variety of materials. These materials have different densities and shapes. The process from the batching pot to the extrusion port requires multiple process flows. It is easy to separate during the transmission process, resulting in the composition Non-uniformity, segregation phenomenon occurs, leading to long gelation time for some coatings and short gelation time for other coatings. In continuous production, these two groups of materials may enter the pulverizer at the same time to form a mechanical mixture. The final product contains powders with different curing speeds. This is similar to the matting principle of matting agents: that is, different curing speeds form microscopic unevenness. Therefore, the reflectivity is low and the light is lost.

5. Extrusion temperature is too low: the temperature of the extruder is too low, the components cannot be fully mixed, and the filler cannot be well-wrapped by various organic polymers, and they are not mutually soluble, so they lose gloss.

6. Failure to strictly abide by the process regulations: In actual production, in order to meet the requirements of market competition, an ACM mill cannot be used to produce a single product, or a product made of the same resin, which is often made of different varieties or different materials. Products are sometimes produced on the same shredder. If the ACM mill and sieve are not thoroughly cleaned, it is easy to cause dry mixing of different powders. Due to the different molecular structure and relative molecular weight distribution of the film-forming resin, the miscibility and curing mechanism of the film-forming resin are different, and the coating loses its gloss. When turning on the pulverizer, if the induced draft fan is not turned on, but the pulse is turned on first, it is easy to invert the ultrafine powder into the cyclone separator. Although the ultrafine powder is very small, the impact is great and it is easy to make a difference. Variety powder loses its luster in dry mixing.