Powder Coatings Industry Concerns

Optimal particle size distribution

Controlling the particle size distribution of powders is vital in order to deliver a final coating with the required properties. Particle size influences the rate of melting of polymer particles during processing, when the coating is heated to allow the powder particles to fuse and form a smooth film. Fines may melt too rapidly, causing specks and imperfections, and giving a poor surface finish known as ‘orange peeling’ whereas coarse particles may not melt at all leading to inclusions which affect both surface finish and mechanical strength. In electrostatic application systems, the presence of fines can lead to "electrostatic orange peel". This is caused by charge build-up on the surface which causes a "back-spray" effect. This is more likely to occur when fines are deposited on the surface as they have a higher charge density.

Control of polydispersity is also important as this defines the powder packing density. Fine particles will fill the voids between coarser particles, excluding air and increasing the overall thermal conductivity of the powder and the rate of melt formation. If the melt is formed rapidly then the degree of cross-linking will be high (there is more time for cross-link reactions to occur). This has a positive impact on the strength and flexibility of the final coating . Conversely if the distribution width is too narrow then inclusions can be formed unless the part is held at high temperatures for a long enough time to allow the particles to melt together, excluding any air. Moving to a finer overall particle size improves packing and the rate at which melting occurs, leading to the formation of a coating free of inclusions. With larger particles, higher processing temperatures are needed to ensure adequate melting. The final properties of the film are essentially controlled using the parameters of particle size, and time and temperature of processing.

Particle size also affects processing. Fine powders tend to exhibit poor flowability, partly because of their packing behavior (the particles interlock) and partly because they tend to absorb more moisture as a result of their relatively high specific surface area. In spray applications, this can lead to inconsistent powder delivery, unless high-energy compressed air conveying systems are used. This is not always an attractive option however as high pressures can lead to particle attrition and an increase in the fines content.

During spray application, fine particles are more likely to follow air streams which may mean they miss the target, resulting in a high degree of over-spray and waste. Electrostatic application systems (where the powder is electrostatically charged in order to improve deposition on an earthed metal part) can, however, reduce over-spray where fine particle sizes are used. In this case, fines can be useful as they are more likely to wrap around the object being sprayed and therefore coat both sides. In contrast, larger particles, as a result of their straight-line inertia, are more likely to penetrate into the corner of a part, rather than be attracted to the surfaces on each side.

To produce a powder coating that can be used easily and efficiently, and which will result in a surface coating with the required properties, it is essential to control particle size distribution, often within a tightly a defined specification.

Management of multiple lines

Most powder coatings facilities have many milling lines, each producing several batches per day of different powders. Mill start-up and product changeover therefore occur frequently throughout the day. To optimize facility productivity it is imperative to minimize changeover time and yet at many facilities mill adjustments are made manually using off-line particle size measurement. With off-line analysis, achieving a new specification is an iterative time-consuming process.

More on how controlling milling particle size can dramatically improve start-up times

Product consistency

Client demand for improved product consistency is becoming a major threat to existing powder coatings suppliers and in addition is a barrier to the penetration of new markets. Users desire more consistent end results when applying the powder coatings to their products, and less powder recycle. As discussed above particle size distribution impacts directly on product performance and therefore tight control of particle size is critical for the manufacture of consistent product.

See how a binder manufacturer has significantly improved product consistency using a Malvern Insitec on-line particle sizer

Contamination

Given that each product line produces many batches of powder paint per day, each of which can be of a different color, contamination between batches is a real issue. Even one speck of red in a batch of green can ruin a production run.

See the features of the Insitec which are designed to prevent contamination.

Mill Optimization

See the reduction in overgrinding that on-line milling particle size analysis delivers.

Environmental and safety risk

As well as impacting on product quality, fines can represent a dust hazard. Powder coatings dust poses both a respiratory hazard and an explosion risk. Dusting is a huge environmental concern. As such, the level of fines needs to be consistently controlled, with the amount of material below 40 microns being particularly important.