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POWDER COATING, April 2016 29 When science meets practical powder applications Thermal and chemical properties of powders are critical as they synergisti- cally influence film formation and end properties. Consider a powder resin that has a glass transition temperature (T gr ) relatively close to its ambient conditions. At an ambient tempera- ture of about 25°C (77°F), the powder coating was formulated to have the T gr around 80°C. Should this T gr allow us to use lower curing temperatures? What happens when the T gr is too close to the storage temperature (T s )? Segmental motion of macromolecules resulting from relatively small differ- ences between the storage temperature and the T gr is high, thus resulting in a slow, but pronounced, crosslinking of the resin before it has been applied. As a result, powder will slowly sinter in a bag. Similar attributes will occur when the reactivity of a crosslinker is too high at lower temperatures. It will not take much energy to initiate even par- tial chemical reactions, and crosslink- ing will occur. Therefore, the T gr of the resin should be such that it is usually about 40°C to 50°C above a maximum storage temperature, thus giving roughly the T gr of about 80°C to 90°C. The next question is: What is the rela- tionship between the temperature of crosslinking (or curing, T c ) and the temperature at which powder particles may flow (T f )? The best scenario would be when the T f is lower than the T c so the deposited powder particles can smoothly flow before crosslinking reactions take place. In reality, how- ever, when energy is applied, the size of the particles and their thermal proper- ties will determine how quickly they can melt and flow. At the same time, the same energy supplied to the system will cause crosslinking. Thus, the rela- tionship between these two tempera- tures is very important and this is why the effect of time-temperature-particle size transformation (TTPST) is essen- tial. It should also be understood that dur- ing crosslinking reactions, the T gr in- creases as higher molecular weight is being built to reach the final value of the T g of the crosslinked film (T gf ). If the buildup of molecular weight is too fast with respect to crosslinking reac- tions, it may inhibit chemical reactions due to limited mobility of reactive g r o u p s , t h u s s l o w i n g d o w n t h e crosslinking process and ultimately leading to poor leveling and/or film and other appearance issues. Figure 1 illustrates the relationship be- tween the discussed temperatures be- fore and after crosslinking and their ef- fect on coatings properties. One im- portant component in these consider- ations is time, specifically the time re- quired to obtain desirable flow at a g i v e n T f a n d t o o b t a i n s u i t a b l e crosslinking at T c , which brings up again the concept of the cure window, where TTPST becomes crucial. PC Marek W. Urban is J.E. Sirrine Found- ation e ndow e d chair and professor in the Department of Materials Science and Engine er ing at C lemson Univ er sity, 299 A S irrine H all, Clemson, SC 29634; 864/656-1155; mareku@clemson.edu. Until 2012, he was a professor of polymer science and engineering in the School of Polymers and High Performance Materials at the University of Southern Mississippi- Hattiesburg (USM). After 13 years at North Dakota State University, he joined USM where he established and dir e cte d the National Sc ienc e Foundation M ate r ials R esearch Powder Coatings Clinic Marek W. Urban, Ph.D. Clemson University Figure 1