Formulators Forum—Ultrafine, Ultra-Smooth Powder Coatings

Ultrafine powder coatings offer some unique properties that make them one of the best choices to achieve thin films that compete with liquid coatings in many instances. Producing them involves careful formulation, special milling processes, and attentive application methods. This, coupled with powder’s benefits like ease of use, reclaimable overspray, high material usage, low waste, and low environmental impact, makes them a superior choice over alternatives like waterborne coatings.

What Are Ultrafine Powders?
Ultrafine powders have very low and narrow particle size distributions with a size range of 8-40 microns, allowing the formation of thinner films with very smooth surfaces, high gloss, great clarity, and low porosity. This is due to the ability of finer particles to pack denser during film deposition with fewer voids to minimize outgassing. When the powder particles melt and flow, the resulting overall thickness and levelling capability are directly related to the particle size distribution. The goal is to decrease the overall large particles while controlling the fines volume to prevent handling and application problems that might arise in a typical industrial spray application system.
This allows for a thinner film to be deposited on the substrate, reducing the overall amount of product required, resulting in material savings for the applicator.

Keys To Formulating and Producing Ultrafine Powders
Most powder coatings can be processed to be ultrafine, but selecting resins and chemistry optimized for more flow for the desired end use will give you the thinnest, ultra-smooth result. You could choose a slow curing system with longer flow times or a low viscosity, faster curing resin system. Keep the pigment volume concentration as low as possible to maintain color and hiding, to avoid unnecessarily restricting flow caused by increased resin demand from the pigments. Superior dispersion of all the ingredients is required for the highest possible finish quality. This is done by tightly controlling extrusion temperatures and speeds and using progressive screw configurations. The resulting flaked extrudate is ground using special milling processes that create a small and narrow particle size distribution (PSD). If the fines volume contains too many nanosized particles, it can be difficult to handle, fluidize, or apply.

An air classifier mill coupled with a cyclone classifier is best for controlling PSD with good product yields. A second cyclone should be used to capture the fines, which can be recycled back into the premix, up to 10%, to avoid unnecessary waste and achieve an even finer overall PSD.

Particle size and distribution in powder have a direct effect on fluidization, application transfer efficiency, film thickness capability, static charging, film deposition defects, trapped gases, and voids. The finer the PSD, the more particles there are to control and interact with each other. Powder additives like aluminum oxide are added during the milling step to enhance the powder’s charging ability and reduce particle- particle interactions for good fluidization and powder flowability, which is more critical for ultrafine powders.

Achieving An Ultra-Smooth Finish
Powder spray application influences the resulting film finish. Small particles are lighter and are more sensitive to electrostatic charges, introduce less air volume to the powder delivery mixture, and reduce particle velocity to improve application transfer efficiency (TE). If the airflow in the booth is too high, it will lower your film thickness and increase your overspray and reclaim amounts by carrying the particles past the target. Control electrostatic charging using lower voltage and microamp settings to reduce electrical disruptions on the coated part and provide a smoother surface with good TE and particle density.

Reclaim management is key to high product utilization and application stability and is important to all powders. It is not advisable to spray 100% reclaimed powder. Reclaim powder must be blended with virgin powder to avoid a death spiral of ever-increasing fines, which will lead to many application and feeding problems. A good rule of thumb is a mix of 40% reclaim to 60% virgin powder. Substrate roughness and cure temperatures also play a role in the smoothness of the final finish.

Ultrafine Powders—A Great Option!
Powder is 100% solids, very low VOC, with the ability to reach greater than 96% paint utilization on a coating line. There are no booth sludge or wastewater treatment chemicals to deal with, as the overspray is captured and reused and needs no special disposal. Combined with some of the newer, low-temperature systems, this makes ultrafine powders a smart choice.

Walter Zahn is powder technology manager for Axalta Coating Systems.