Ask Joe Powder
Posted on Tuesday, April 1, 2014
The Cure for Board-om
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.
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.
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.
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.