What's in a Cure?

Posted on Tuesday, January 10, 2017

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) powder coatings.

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” cured coating.

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 unreacted material.

Different Chemistries

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 of cure.

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 powder coatings.

Michael Knoblauch is president and Robert VanAmburgh is lead R&D chemist at Keyland Polymer, LLC. They can be reached at 216-741-7915.

Testing Locations:
Keyland Polymer, LLC. Cleveland, Ohio
Thermodynamics Laboratory Department of the ETSEIB,
Universitat Politecnica de Catalunya (UPC, Barcelona, Spain)

Technical assistance:
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