PC Summitry:Polyester Vs. Polyurethane
Posted on Friday, March 1, 2013
By Kevin Biller
One of the neatest parts of being
in this industry is the fact that
you sometimes get to meet really
cool people. Cool people are pretty
easy to identify, but for me they always
engender an inherent curiosity,
a thirst for knowledge and quest to
continuously innovate and improve
their operations. One of these “cool”
people posed a great question to me
recently. Here’s what she asked:
Polyester vs. polyurethane: I have
a question concerning these two
chemistries. Which one is a better
choice for outdoor furniture? I hear
great things about polyurethane
being great for outdoor applications,
and yet I hear the same for polyester.
I am not sure what the difference in
the two would be. I thought maybe it
had something to do with the gloss
level. Maybe the polyurethanes need
high gloss to fulfill their corrosion
protection quality? Or maybe
polyester has better impact and mar
resistance. Anything that you could
offer on this would be helpful.
From a fundamental standpoint,
polyester and polyurethane powder
coatings have many similarities. The
base polymer used in both chemistries
is very similar. The reason for
this pertains to the limited amount
of monomers available to the
polymer scientist to create a solid
resin. Hence, the major portion of
the polymers used in polyester and
polyurethane powders is comprised
of the same building blocks. Consequently,
the same building blocks
yield the same performance that is
to a point.
The main differences in these two
chemistries emanate more from the
crosslinkers than the base polymers.
Polyester powder coatings are formulated
with a polyester resin cured
with triglycidyl isocyanurate (TGIC)
or alternately hydroxyl-alkyl-amide
(HAA). Primid® is a common
trade name for HAA. Polyurethanes,
on the other hand, are cured with
a multifunctional isocyanate and
derive much of its performance from
this crosslinker.
The binder or resinous portion
of polyester powders is comprised
of a carboxyl polyester cured with
a relatively low level of crosslinker
(typically 5 to 10 percent). Polyurethane
powder binders contain a
hydroxyl functional polyester cured
with an isocyanate crosslinker. The
isocyanate crosslinker level can
range from 15 to 60 percent and
therefore strongly influences the
ultimate coating properties.
Here is a rundown of the top
properties and how they compare:
Exterior Durability
Excellent exterior durability
is possible with either chemistry.
Standard grade and “Superdurable”
polymers are available in both
polyester and polyurethane varieties.
The superdurable products have the
potential to maintain appearance
and film performance for up to 6
or 7 years in south Florida environment.
Hardness and Abrasion
Resistance
Polyesters are typically somewhat
softer and less abrasion resistant
than their polyurethane counterparts.
The additional crosslinker
inherent in a polyurethane powder
formulation is responsible for the
added performance. Using the somewhat
archaic pencil hardness test,
polyesters clock in at H hardness
whereas polyurethanes can achieve
H to 5H depending upon the specific
formulation. The higher hardness
products typically sacrifice mechanical
flexibility.
Low Gloss and Matte Finishes
Polyurethanes can be formulated
from matte to high gloss with very
good to excellent coating performance.
Polyesters are more difficult
to formulate in lower gloss and film
performance can be reduced with
some of these formulation schemes.
Chemical Resistance
Both chemistries provide fairly
good chemical resistance however
polyurethanes significantly outshine
polyesters if formulated appropriately.
In fact a highly crosslinked polyurethane
powder coating provides
chemical resistance that rivals epoxy
powder performance.
Cure Response
Low temperature cure is much
easier to develop in a polyester than
a polyurethane powder coating. A
couple caveats need to be noted.
HAA polyesters are more difficult to
catalyze than TGIC based polyesters.
HAA polyesters can cure as low as
325°F whereas TGIC polyesters can
be formulated to cure at temperatures
as low as 250°F. Polyurethanes
bottom-out around 300°F.
Film Thickness
One peculiarity of polyurethanes
is the need for a chemical blocking
agent in the crosslinker. These
blocking agents volatilize out of the
coating during the bake cycle. Blocking
agents make the chemistry work
best (balance of reactivity and cure)
but also can create pinhole defects in
thick film coatings. Hence polyurethanes
are difficult to apply successfully
at film builds greater than 5.0
mils, whereas TGIC based polyester
powders can be applied at upwards
of 10 mils. On the other hand polyurethanes
inherently provide very
good film smoothness at thin films
(>1.5 mils).
Cost
In general, you will pay more
for a polyurethane powder coating
as compared to a polyester based
powder. That said, the powder
formulator can cheapen a polyurethane
powder product by taking a
substantial amount of quality out of
the coating.
In the end, both chemistries have
their place in the powder coating
industry. Polyesters and polyurethanes
can provide good to excellent
outdoor durability. More often than
not, polyurethanes have polyesters
beat on chemical resistance, hardness
and thin film appearance; however,
this added performance almost
always comes at a premium.
Thanks for the cool question,
Bonnie.
Kevin Biller is technical editor for
Powder Coated Tough magazine. He can
be reached at kevinbiller@yahoo.com.
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