Application of the General Linear Model to Determine TiO2 Product Differences at Various Powder Coatings Film Thicknesses
The environmentally friendly nature of powder coatings over conventional liquid coatings continues to fuel its growth within various coatings segments.1 Although environmentally friendly, powder coatings still have challenges, especially those related to product manufacture. Powder coatings go through a series of process steps that impact the final product. First, the raw materials are dry blended in a pre-mixing stage. Then, the pre-mixed materials are passed through an extruder to incorporate pigments and additives within the binder system. The extrudate is then broken down using a grinding mechanism. This step can be further broken into two steps: large grinding, which reduces product to less than a few centimeters; followed by fine grinding, which brings the powder down to micron size. Ground extrudate is now sieved (scalped) until the desired particle size distribution is achieved. Normally, the final dry powder coating will have a distribution between 20 and 100 microns depending upon the final application and product specification. Note that this range is somewhat arbitrary and is dependent on the coatings manufacturer's needs. The sieved powder is now ready to be applied to the substrate surface. Spray application is used to apply the powder coating to the desired surface. After the coating has been spray applied to the substrate, the substrate undergoes thermal processing.
Powder coating film thickness differences become difficult to control, due to the nature of spray application methods and lack of free flow during the thermal curing process. While a conventional liquid coating will flow and self-level after application to the substrate surface and during curing, a powder coating is limited to flow only during the thermal curing process. Even then, the flow is much more restricted than a conventional liquid coating. Many coatings manufacturers have developed procedures to reduce the effects of this issue. However, film thickness differences in powder coatings still play a role in the final coated product performance. Film thickness differences within a powder coating panel are also larger than those encountered in conventional coatings. Variations in film thickness make comparisons between powder coatings at a constant film thickness difficult and sometimes impossible. Some methods applied to powder coatings, although suited to the needs of the industry, may not give the coatings manufacturer the same level of quantification that can be found in other areas of coatings manufacture.2