By: Mike Knoblauch and Robert VanAmburgh
Cure is the measure of the crosslinked
oligomer chains or fully reacted double
bonds residing in the coating matrix following
exposure to the curing system, which can be
thermal energy or Ultra Violet light exposure.
Here, let’s measure, describe and evaluate it.
Speed changes everything. The
demand for coatings that cure faster has
changed how coatings are made. To cure
faster, the polymeric chemical structure
of coatings had to change. Historically,
liquid coatings were thermally dried to
reach the state of being cured. Or, in the
case of thermal cured powder coatings,
put in an oven and baked at 400°F for
30 or more minutes. The advent of
ultraviolet (UV) light curing of liquid
and powder coatings and the use of
catalysts and additives such as peroxides
to initiate faster cure has fundamentally
changed the nature of coatings.
Reducing the time to cure a coating,
increases the number of chemical
reactions that have to occur in a smaller
process time. As speed and complexity
increases, the need for more robust
process controls and scientifically based
process analysis becomes necessary.
The use of proxy methods to test for
cure—thumb nail, scratch, swell and
MEK double rub—is very common.
These can be done quickly, but each
of these is unreliable, subject to
interpretation by the evaluator and can
give a false negative or a false positive
result. These are mechanical methods
that only evaluate by observation
the surface condition of the coating.
None of these techniques are able
to analytically measure cure and
scientifically describe the chemical state
of the entire coating.
Cure is the measure of the
crosslinked oligomer chains or fully
reacted double bonds residing in the
coating matrix following exposure to
the curing system, which can be thermal
energy or Ultra Violet light exposure.
Differential Scanning Calorimetry
(DSC) is a scientifically recognized,
reliable and repeatable methodology to
measure the cure of a coating system.
This paper looks at a number of powder
coated finishes, thermal cured and UV
cured, and uses DSC and photo-DSC
to measure, describe and evaluate their
respective cure profiles.
Thermal cured powder coatings
and UV-cured powder coatings are
fundamentally different chemistry
systems. Thermal coatings contain
functional groups that respond when
thermal (heat) energy is introduced
to the coating. The initial heat
reduces the viscosity of the coating
until a thermal threshold is achieved,
initiating the cross-linking of the
oligomers. The addition of a catalyst is
sometimes required to initiate or aid in
crosslinking. The application of heat
to cure creates a network of oligomer
chains in the coating. Adding additional
heat to the coating “fixes” or cures the
system. Thermal powder coatings cure
on a time and temperature continuum.
Insufficient time, temperature, or a
combination of both will result in a
failure to completely cure the coating.
Resin and powder manufacturers have
made investments in chemistries that
shorten time and reduce temperature
needed to cure. These are marketed as
“low-bake” and “ultra low bake” (ULB)
Ultra-Violet (UV) powder coatings
are photopolymerized coatings
containing a chemical photoinitiator
that instantly responds to UV light
energy initiating a chain reaction and
curing the coating. The melt stage
is separate from the cure stage of
the coating. Melting is typically 1-2
minutes and is followed by UV-curing.
As long as the proper amount of UV
energy is present, the UV-cured powder
coating will cure in a matter of seconds.
(See Illustration 1 above.)
Distinguishing a Cured Coating from an Uncured Coating
How is it possible to distinguish
a cured coating from an uncured
coating when there are no outward or
observable indicators? For the purpose
of this experiment, we purchased five
products advertised as powder coated
MDF products. The product literature
described two products as red and
yellow epoxy powder coatings, one
white polyester powder coating, one
black epoxy/polyester powder coating
and the other as an unspecified white
powder coating. Two products were
manufactured in China and three in
Europe. Knowing the retailer, all are
thermoset powder coatings.
Four standard UV-cured powder
coatings were prepared in the Keyland
Polymer laboratory and applied using
standard conditions to MDF samples.
These were a white epoxy, yellow
epoxy, red polyester/epoxy and black
epoxy. The samples were cured with
300W and 600W UV microwave
lamps. The data reported in this paper
are the samples cured on the 600W
lamp. After cure, the lab technician
performed the 50 double rub MEK
solvent resistant test. The results were
evaluated and are reported in Table 1.
The cure profile of the four samples
was measured and evaluated using
DSC. The white and black UV-cured
samples were also analyzed using
photocalorimetric DSC (photo-DSC).
DSC is an analytical tool that measures
and analyzes the thermal activity and
chemical behavior of a powder coating.
The graphs below are examples of
two thermal cured powder coatings and
illustrate two different cure profiles.
Using DSC to measure the cure profile
of each, the NKSW1 sample is cured
and the NKSW2 sample is uncured.
The standard method to measure
the cure profile using DSC utilizes a
heat-cool-heat cycle. The first heat
cycle erases the thermal history of the
coating and will cure any remaining
uncured or unreacted material resident
in the coating. The second heat
cycle measures the glass transition
temperature (Tg) and also shows if
there is any further chemical activity
in the form of a curing reaction. Glass
transition temperature Tg is a range of
temperatures that a material transitions
from a hard or “glass” state to a molten
or rubber like state, as the temperature
of the material is increased. A constant
slope on a DSC graph indicates there
are no physical or chemical changes
occurring, as more heat is applied. If
a thermoset powder has a constant
slope above its cure temperature it is
described as being completely cured.
Sample NKSW1 is an example
of a “cured” coating with a Tg of
~94°C. The first heat (red line) is
fairly comparable to the second heat
(black line) indicating no unreacted or
uncured material was cured in the first
heat cycle. The sample NKSW2 is an
example of an “uncured” coating. In the
first heat (red line) a sharp and rapid
exothermic (release of heat) reaction is
seen starting ~140°C. The second heat
shows a constant slope, or no thermal
or chemical events indicating it is a
completely “cured” coating. The first
heating cycle completely cured the
residual uncured material.
It is important to bear in mind that
sample NKSW2 scored higher in the
MEK test than NKSW1. Based upon
the MEK test, the observed result
suggests that NKSW2 is the “more”
The DSC graphs 3,4 and 5 are the other thermal powders
tested; NKSWB1 black, NKSR1 red and NKSY1 yellow.
Each of these appeared to have a primer material under
the powder coating. The DSC is measuring the cure of
the powder coating and it is not possible to speculate on
the influence of the primer coating on cure of the powder
coating. Based upon DSC the NKSWB1 black and NKSR1
red are cured. The DSC for the NKSY1 yellow shows an
exothermic reaction starting at ~150°C; first heat (red
line), again an indication of unreacted or uncured material
present in the coating. This is an indication that the coating
is not fully cured. The slope of the second heat black line is
constant, indicating that the first heat was able to cure the
UV-cured powder coatings are fundamentally different
chemistries than thermoset powder coatings. Thermoset
powder coatings cure on a time and temperature continuum.
The powder coating is heated and the powder enters a melt
& flow phase that continues until the chemical crosslinker is
activated. The material must be maintained at the activation
temperature until the crosslinking is
complete. Typically, the temperature the
material must reach is 350 to 400°F for
15 or more minutes. “Ultra Low Bake”
(ULB) powder manufacturers claim
cure can be achieved in 5 minutes at
300°F. These materials use peroxides as
thermal initiators for crosslinking.
UV-cured powder coating separates
the melt & flow phase and cure phase.
Following powder application the
powder is heated in the gel phase
with IR, convection, or a combination
of both. The UV powder will melt at
230 to 300°F in 60 to 120 seconds,
depending upon the mass of the part.
The melt phase is immediately followed
by nearly instant UV light cure.
Graph 6 on the next page, the
ZT16C28R4 UV/White/epoxy is an
example of a fully cured UV-cured
powder coating as measured using
DSC. The green line is the first heat and
the blue line is the second heat. The
Onset Tg is measured at 85°C, Offset
Tg is 98°C and final Tg is 92°C. Graph
7 on the next page, the ZR16D15R4
UV/Yellow/epoxy is an example of an
uncured UV-cured powder coating.
Typically, UV-cured failure is the result
of UV absorption interference between
the photo initiator and the pigment.
The black line is the first heat and red
line is the second heat. The rising slope
of the red line shows the presence of
unreacted photoinitiator in the coating not consumed in
the initial UV-curing process. As the MEK test result of the
NKSW2 Thermal/White/polyester showed a high MEK score
so too does ZR16D15R4 UV/Yellow/epoxy. Each of these is a
false positive if the MEK test result is used as an evaluation
The ZR16D15R4 UV/Yellow/epoxy Onset Tg is
measured at 62°C, Offset Tg is 73°C and final Tg is 68°C.
These low Tg’s are further indication that the coating was
not cured. The higher the Tg the higher the crosslink
density of the coating and the more certain that the
cure state has been reached. 70° to 75°C is a reasonable
minimum Tg range for a cure determination, anything
below that Tg range is uncertain for cure.
The data shown in Graphs 8 and 9 indicate ZR16C31R1
UV/Red polyester/epoxy and ZR16C15R4 UV/Black/epoxy
coatings are cured.
Photo-DSC measures unreacted photoinitiating material
in the coating. Photo-DSC replicates the two steps of the
UV-curing process. The first step is heating of the sample
powder to the melted or gelled state. This erases the thermal
history residing in the coating. The second step is heating
and UV irradiation; heating the coating to 130°C and
exposing it to UV light energy for a period of two minutes.
This will determine if the photoinitiator has crosslinked and
reacted (cured) the double bonds in the coating material.
Graph 10 shows the result of a series of photo-DSC trials
on the ZT16C28R4 UV/White/epoxy and ZR16C15R4 UV/
Black/epoxy samples. Each graph line is smooth indicating
that no exothermic reactions occurred. This demonstrates
each material is fully cured.
Thermal powder coatings and UV-cured powder coatings
are used throughout the world and are a growing segment
of the global coatings market. This rate of growth will
accelerate as firms move away from solvent-borne and solvent
containing liquid coatings. In many instances waterborne
coatings are failing as replacement for solvent coatings. These
market conditions are increasing the market opportunity
for powder coatings. Cure determines the performance of a
coating. Chemistry complexity requires more accurate process
and analytic tools to measure, describe and ultimately validate
cure. Powder coating manufacturers and applicators cannot
continue to rely on proxy methods to measure and evaluate
cure. To do so exposes firms to unnecessary product liability,
business risks and ultimately compromises the market
acceptance of UV-cured and thermally cured powder coatings.
DSC and photo-DSC are scientifically recognized and reliable
methods that measure and describe the cure characteristics of
Michael Knoblauch is president and Robert VanAmburgh is lead R&D chemist at Keyland Polymer, LLC. They can be reached at 216-741-7915.
Keyland Polymer, LLC. Cleveland, Ohio
Thermodynamics Laboratory Department of the ETSEIB,
Universitat Politecnica de Catalunya (UPC, Barcelona, Spain)
Xavier Ramis Juan, Professor and Director Laboratori de
Termodinàmica ETSEIB Universitat Politècnica de Catalunya
Xavier Fernández Francos, Laboratori de Termodinàmica
ETSEIB Universitat Politècnica de Catalunya