By E.J. (Ted) Lightfoot, Ph.D.
Coating is defined as replacing air with a layer of liquid at a solid interface. Often that liquid is either a solution, dispersion, or emulsion containing water or an organic solvent that needs to be removed before use. There are many ways to dry coatings—convective hot air and IR drying being the most common. IR drying is sometimes confused with IR heating. You certainly need heating in any dryer, but you also need a way to get the evaporated water or solvent out of the oven.
The best mental model for drying is the “drying curve”: when a product enters a dryer, it goes through (at least) three distinct drying periods. First, the product needs to come up to temperature in a “heat-up period.” The faster we heat the product, the sooner it gets through the this stage. The end of the heat-up period comes when evaporative cooling balances the rate of heating. When the evaporative cooling just balances the rate of heating, we reach a temporary steady state called the “constant (drying) rate period,” where the product acts like a wet bulb thermometer. For waterborne coatings, we can adjust the temperature of the product by varying the humidity in the dryer; however, for solvent-borne coatings, flammability constraints limit that option. For hot air drying, the rates of heating and the drying are linked by the Chilton-Colburn analogy: if you increase the airflow, heating and drying rates go up proportionally. But with IR drying, you can control heating independently, so you can dry a product at a much wider range of temperatures. But does product temperature matter? In some cases, yes; in other cases, no. When does drying temperature matter? Here are three common examples.
If your coating contains semi-crystalline polymers, the crystallinity can be very sensitive to drying temperature, and the end use properties can be very sensitive to crystallinity. In the photographic industry, it would be virtually impossible to sell X-ray film dried at typical temperatures for graphic arts films, which dry 10–20 degrees Fahrenheit colder than X-ray. Could you dry graphic arts films hotter? Sure—if you reformulate at a cost of at least a penny per square foot. Architectural paint is the biggest application that requires crystallinity control, but many advanced technologies use crystallinity to control mechanical properties, as well.
Does drying temperature matter for non-crystalline polymers, such as pressure-sensitive adhesives (PSAs)? For aqueous emulsion PSAs it does. If emulsion PSAs dry too fast, you can form a skin that traps water in the coating. But the limiting drying rate goes down as the drying temperature goes up.
Finally, film temperature in the dryer is a key control for residual stresses in the coating that cause curl (and some defects).
At what temperature should you dry, and what kind of dryer should you buy? That depends. But one thing you should definitely do is run pilot trials using different drying configurations and temperatures to see what works best for your product before you buy. █
E.J. (Ted) Lightfoot, Ph.D., is the principal Consultant at Ted Lightfoot LLC. He can be reached at 716-449-4455 or ejl@tedlightfoot.com.
