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Ask Joe Powder

Posted on Tuesday, April 1, 2014

The Cure for Board-om

Dear Joe,

I’m working on a marketing research project and have a question regarding powder coatings used on MDF (medium density fiberboard). Do they always have to be UV-cured?

Regards, Pamela S.

Dear Pamela,

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-ofsight curing that also is 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 to 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 upon 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.

The UV curables require an additional process step but cure in a shorter process footprint. They also provide better coating durability...

UV-curable powder technology requires a melt stage, typically around 60 to 90 seconds at a peak temperature of 200 to 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. A number of powder manufacturers make and sell both types of chemistries as one shoe doesn’t fit all.

--Joe Powder

Performance Anxiety

Dear Mr. Powder,

We are manufacturer of polyester resins for powder coatings. Our customers are having difficulty in achieving consistent high gloss and impact resistant in their powder coatings. Can you help us determine the root cause of this problem.

Kind regards, Vijay R.

Dear Vijay,

I understand that you are a polyester resin manufacturer and supplier and that your customer (a powder coating formulator/producer) is getting low gloss and/or poor impact. One basic question: How is the solvent resistance of these coatings? Chemical resistance is a good indication of cure and it will be poor if the stoichiometry of the binder is incorrect. Poor cure as evidenced by poor solvent resistance will produce poor impact resistance. If the stoichiometry is theoretically correct and the impact resistance remains poor then I would investigate the molecular weight distribution of the polyester resin. If it is too broad then there are too many polymeric species which can account for poor cure due to variable functionality. Broad molecular weight distribution can also account for poor gloss.

--Joe Powder