Q: How can I increase the fluidity of my powder coatings?
A: Increasing the fluidity of a powder coating (and really any powder) can be a tricky business. For those unfamiliar with fluidity, we’re talking about the behavior in which a powder can allow air to evenly space the powder particles from each other in air. This is a critical property in the application of a powder coating.
In a powder application system, the bulk powder is introduced into a fluidized hopper. The fluidized hopper is either a cylindrical or box-shaped container that has a porous Teflon® plate at its bottom. Compressed air is passed through this porous plate to fluidize or prepare the powder to be transported through a powder pump, then hoses, and finally through a spray gun. Fluidity is crucial in getting the powder to feed evenly through the system.
A few factors are important in optimizing fluidity. First, particle size distribution is critical. Note that I said “distribution” and not just particle size. This means that you have to manage the entire size distribution of particles. Most important is to minimize “fines,” which can be defined as particles below 10 microns in diameter.
Less is better, and you should strive to have less than 6-7% fines. Too many fines will cause clumping, agglomeration, and poor fluidity. Ditto for large particles (about 100 microns and above). These are more difficult to physically move with compressed air and therefore should be minimized. I would target less than 1% over 100 microns for the best fluidization behavior.
After you nail your particle size distribution, it’s time to incorporate a fluid flow or dry-blend additive. These are fumed silicas or a fumed alumina and act as ball bearings between the powder particles. Keep in mind that a little goes a long way.
The addition should be made during the powder coating manufacturing process, after the extrusion process. Typically, people add the fumed material with the extrudate chips (flakes) just prior to the pulverization process. This ensures that the agglomerated fumed silica or alumina is evenly dispersed. Otherwise, it can cause seeds and bits in the finished powder film.
One problem is that the fumed material can get pulled out of the grinding system and into the waste stream that deposits into the baghouse. Some manufacturers have devised ways to introduce the silica after the grinding process with a deagglomeration technique. I mentioned a little goes a long way — I would shoot for a concentration well under 1%, say closer to 0.2%.