Q: I’m working on a marketing research project and have a question regarding powder coatings used on medium-density fiberboard (MDF). Do they always have to be UV cured?
A: No, they do not. There are essentially two options: UV-curable and low-temperature-thermosetting powder coatings. The UV curables require an additional process step but cure in a shorter process footprint. They also provide better coating durability (mainly hardness and chemical resistance).
UV cure requires line-of-sight curing that is also critical to the distance of the lamps to the coating surface. In addition, film thickness must be carefully controlled. If the coating is too thick (or too opaque), it will not cure completely. And as you can guess, the material cost is higher than more conventional products
Thermosetting powders can be used as well. They require longer curing times (typically 20-30 minutes), although infrared heat can mitigate some of this. They are highly catalyzed and therefore must be stored in a controlled environment and can advance chemically, which affects coating smoothness. The thermosets are generally less smooth than the UV-curable types and typically less durable.
As for specifics regarding cure requirements, this mainly depends on the chemistry. Polyester TGIC systems work with cure cycles of about 25 minutes at 265°F. Acrylics can cure as low as 25 minutes at 275°F. Epoxies have been formulated to cure at 25 minutes at 230°F. Epoxy-polyesters (hybrids) can be processed at 25 minutes at 250°F. Polyurethanes and polyester-HAA require cure temperatures over 300°F. More conventional powders cure with less dwell time at much higher temperatures and therefore cannot be used for MDF.
UV-curable powder technology requires a melt stage, typically around 60-90 seconds at a peak temperature of 200-240°F. The additional UV-cure process usually takes only a few seconds, depending on the complexity of the part.
Lots of film performance and process issues are very specific to the formulation, so you need to be careful with generalizations. Several powder manufacturers make and sell both types of chemistries, as one shoe doesn’t fit all.