It’s All in the PSD

Often a powder coating supplier will describe their product’s particulate nature by quoting a singular value usually “average” particle size or sometimes “median” particle size. These values only paint a minor picture of the story. What is more critical is an understanding of the entire particle size distribution (PSD).

By Kevin Biller

Ever wonder why one supplier’s powder coating applies much better than another manufacturer’s? Some powders fluidize well, never clump and always apply evenly and consistently while others are clumpy, surge and spit, and miss tight corners and recesses. It’s all in the particle size distribution (PSD). And I said distribution, not just particle size.

Powder coatings are made by fracturing corn-flake sized chips of extrudate. The milling technique typically employs a rotating hammer mechanism that impacts the flakes and flings them into a corrugated inner liner resulting in a relatively broad distribution of particle size. The finished product has a distribution ranging from under 1.0 micron up to over 100 microns. The relative distribution is the key as to how a powder coating will behave in an application system. (See Figure 1.)

Often a powder coating supplier will describe their product’s particulate nature by quoting a singular value usually “average” particle size or sometimes “median” particle size. These values only paint a minor picture of the story. What is more critical is an understanding of the entire particle size distribution. This will more comprehensibly predict how powder will handle in a finishing system.

Most telling is the concentration of “fines.” Fines can be defined as the portion of a PSD measuring 10 microns or smaller in diameter. An arbitrary threshold of fines concentration is 7 percent to 8 percent. Higher than this and you will experience myriad handling woes, including clumping, poor fluidization, gun spitting, poor film build and difficulty in penetrating Faraday Cage areas.

The origin of all these problems emanates from how small particles behave. Small particles possess an exponentially higher amount of surface compared to larger particles. This greater surface area can adsorb a significantly higher amount of ambient moisture which in turn, will cause a higher degree of clumping. In addition smaller particles have a greater tendency to agglomerate due to surface energies such as van der Waal forces, hydrogen bonding and electrostatic attractive forces.

Agglomeration causes powders to fluidize poorly and makes it more difficult to transport the product in tubing, hoses, venturi pumps and application gun cavities. This difficulty requires the powder coating user to employ higher fluidization air pressure in her hopper and higher pressure in the transport air. This causes a higher incidence of impact fusion in hoses and other interior surfaces of the application system. Surges and spitting are also phenomena related to agglomeration.

Not only do fine particles cause these problems but they wreak electrostatic mayhem as well. The high surface area of fine powders picks up a higher concentration of electrostatic charge (aka: charge to mass ratio) than larger particles. This higher charge to mass ratio introduces repulsive interparticle forces that reduce the transfer efficiency of a powder coating. Consequently film build is reduced and back ionization occurs at a lower film thickness. (Back ionization refers to the phenomenon of when a deposited layer of powder builds up an abundance of localized electrostatic charge. This excess charge causes eruptions of particles resulting in micro volcanic appearing defects in a powder coating film.)

Examples of Particle Size Distribution

Particle size can be measured by a number of means. The most common techniques include sieve analysis and laser diffraction. Sieve analysis is rather crude relying on vibrating, shaking or vacuuming a weight sample of powder through a screen of calibrated openings. The analysis is quoted as percentage of material retained on a prescribed screen opening size. Multiple sieves can be used to determine percent of material between certain particle sizes. (See Table 1.)

Laser diffraction methods analyze the diffraction of a laser light source caused by a particle passing through the beam. The amount of light diffraction correlates with the particle diameter. This technique provides a much higher resolution of particle size distribution and is the most common method used by powder coating manufacturers. Because of the sophistication of this technique particle size distribution curves can be created exhibiting PSD ranges from around 0.4 microns to beyond 200 microns. In addition, analysis of the data can be computed including percentile values, mean (x-bar), median (d50), mode and other distribution analysis.

With laser diffraction the quality technician can analyze important aspects of particle distribution such as concentration of fines (i.e., < 10 microns) and coarse particle fraction (> 100 microns). The analysis generated is based on volume measurements which correlate directly to weight. The data includes median and average particle size. (See Figure 2.)

Two production powders are depicted in Figures 2 and 3. As you can see “Powder T” exhibits a rather broad particle size distribution with a high concentration of fines. “Powder A” on the other hand has a reasonably tight PSD with only a modest amount of fines. (See Figure 3.)

Accordingly, “Powder A” will handle and apply significantly better than “Powder T”. “Powder A” shows only a 4.7 percent concentration of particles less than 10 microns whereas “Powder T” has a whopping 18.7 percent concentration of fines. It is important to note that it is questionable that this red flag would have been raised merely based on mean or median particle size values. (See Table 2.)

In summary, it is wise to understand the overall particle size distribution of the powder coatings you are using on your application line. Knowledge of a singular piece of data such as particle mean or median is helpful but it is not enough to qualify whether a powder will apply flawlessly. Just keep in mind, “It’s all in the PSD.” Know your powder’s particle size distribution and you will avoid costly and time consuming application issues.

Kevin Biller is technical editor of Powcomder Coated Tough and the president of The Powder Coating Research Group. He can be reached at kevinbiller@yahoo.