loading

Cure Dynamics

Written by: Nick Liberto P.E., Powder Coating Consultants, Division of Ninan, Inc.

Bruno Fawer RESOTEX Associates, LLC and Powder Coating Consultants

Commercial application line operators, laboratory and customer service personnel working for powder coatings manufacturers, and even field engineers and application line designers have often relied on online trial and error methods and procedures to establish viable time and temperature data to assure that powder coatings are fully cured. This article offers a relatively simple formula to calculate the time temperature dynamics for powder coating cure.

There is still a lot of confusion in the industry regarding oven dwell time (the total time a part remains in the cure oven) and actual cure time (the time it takes for powder finishes to cure up completely at a set and stable substrate or part temperature). Many people, especially those in a laboratory environment, still believe that oven dwell time and cure time are essentially the same. The formula featured here is for calculating cure time only, but when oven bring-up times (the time it takes for parts to reach cure temperature) are known or can be estimated with some accuracy, the cure-time formula can be modified to calculate oven dwell time.

Technical data sheets issued by powder coatings manufacturers generally show only one single bake schedule, for example 10 minutes at 400°F. How- ever, engineers designing application lines and powder coating applicators need to consider cure-time schedules under different time-temperature conditions to optimize space, equipment, and process, to minimize costs, and to assure that the finished products meet the specifications set down by internal quality assurance and by the end
user of the coated products.

Understanding reaction mechanisms

Following are scientific explanations of the complexity of chemical reactions and kinetics of organic binder systems used in powder coatings. The information is from the Web site [http://ibchem.com/IB/ibnotes/brief/kin-hl.htm]. It has been modified to explain some details about powder coating chemistries and cure mechanisms. For a summary, see Table 1.

• Rate determining step: The slowest step in a reaction. It determines the rate of the overall reaction. This may apply for the cure mechanism in emissive poly- urethane powder coatings—the blocking agent (usually E-caprolactam) has to dissociate (split off) from the curative before the resulting isocyanate groups can react with the hydroxyl groups of the polyester.

• Rate of reaction: Rate of reaction is concerned with how quickly a reaction reaches a certain point. Applied to powder coatings, this would be equivalent to cure time.

• Collision theory: Reactions take place as a result of particles (atoms or molecules) colliding and then undergoing a reaction. Not all collisions cause reactions, even in a system where the reaction is spontaneous. The particles must have sufficient kinetic energy and the correct orientation with respect to each other for them to react.

• Activation energy: This is the minimum energy that colliding particles must have to produce successful reaction. The energy of particles is expressed by their speed. The kinetic energy of molecules is related to heat energy; higher heat means higher collision energy. Higher heat also means lower viscosities for liquefied powder coatings, resulting in higher mobility of the resin and curative molecules to create favourable conditions for collisions. (See Collision theory.)

• Changing the conditions: Increasing the temperature of a substance increases the average speed (energy) of the particles and consequently the number of particles colliding with sufficient energy (see Activation energy) to react; it also reduces the viscosity of liquefied powder.

— More successful collisions, and therefore a faster reaction , occur at higher temperatures, which means shorter cure times for powder coatings.

— Catalysts lower the activation energy by providing an alternative mechanism for the reaction and greater probability of proper orientation. This results in a faster reaction.

Establishing a cure dynamics curve for powder coatings

A rule of thumb in chemistry declares that the reactivity doubles (or is cut in half) whenever the temperature is increased (decreased) by 18°F (10°C). Given the facts that we have the reactions taking place in a relatively thin film on (mostly metal) substrates with different specific heat characteristics, and taking changing bake schedules into consideration, establishing a formula to eliminate guesswork and calculate cure time or total oven dwell time at various temperatures is at least a very difficult undertaking.

Oven dwell time, the total time (in minutes) parts spend inside a cure oven, consists of bring-up time (the time in minutes it takes for the parts to reach cure temperature) and cure time (the time in minutes it takes for a powder finish to reach full cure):

Oven dwell time = Bring-up Time + Cure Time

Depending on the chemical nature of the powder coating, a slow cure reaction may start during bring-up time after the powder liquefies and reaches temperatures of about 230°F-302°F (110°C-150°C). However, oven air and substrate temperatures must reach a certain and predetermined point for the cure reaction to proceed at a speed suitable to meet the equipment design specifications of an application line. (See Activation energy in

Explanations of the complexity of chemical reactions and kinetics of organic binder systems used in powder coatings

Condition Effect on rate Explanation
Temperature Increasing the temperature, increases the rate of a reaction. Two reasons
1. There are more particles with sufficient energy to react—more successful collisions.
2. Lower viscosity results in higher molecular mobility—more collisions
Concentration Increasing the concentration of a reactant increases the rate of the reaction (usually). There are more collisions as there are more particles in closer proximity. Inert products (pigments, fillers, additives) ʻdiluteʼ the binder (resins + curatives) systems and act as heat sinks, resulting in lower reactivity.
Particle size The smaller the particles, the faster the reaction. (Note: particles in solutions have the smallest particle size, and so react fastest). Collisions occur at the surface of particles. The larger the particle size, the smaller the surface area and the fewer collisions can occur. This explains the relatively leisurely cure reaction speed of the higher molecular binder systems in paints and powder coatings.
Catalysts The presence of a catalyst increases the rate of a reaction. Catalysts provide an alternative mechanism with a lower activation energy.

the previous section.) Cure time begins when oven air and substrate temperatures reach this point. Depending on oven designs (convection ovens only), the nature of the parts to be coated, and cure temperature settings, the bring-up time can be a substantial percentage (more than 50 percent for heavier parts) of the total oven dwell time.

All this makes it virtually impossible to establish a purely scientific formula that can be used to calculate cure time-temperature curves from one single time temperature cure schedule set.

Using an empirical formula

Approximately 18-20 years ago, I established an empirical formula to calculate multiple time-temperature cure schedules for commercial application line convection ovens. At that time, I was involved in a series of new product line trials for appliance, under-the-hood auto- motive, functional epoxy, and general metals applications. This formula is based on years of laboratory and field observations, applicator feedback, and on-line trials and errors:

t0
t1 = ———————
1.024 [±AT (°F)]

t1 = New cure time after temperature change t0 = Cure time before temperature change
AT = New temperature minus initial (base) temperature = Temperature change in degrees Fahrenheit (positive: temperature increase; negative: temperature decrease)

For Celsius, use the same formula; how-ever, use 1.0436, instead of 1.024.

Example. Calculate the cure times in 10°F intervals between 350°F (177°C) and 400°F (204°C) for a polyurethane and a TGIC (triglycidyl isocyanurate) powder coating based on the following technical data sheet information and using the (non- metric) formula for degrees Fahrenheit:

Polyurethane: 15 min at 375°F (191°C) TGIC: 10 min at 400°F (204°C

See the cure times in Table 2.

Discussing the accuracy of an empirical formula

In my experience, this formula is quite widely applicable for commercial application lines containing convection ovens, but results should be verified. Whenever time-temperature changes based on calculations from this formula are made on an application line, the resulting finishes should be tested for full cure, for example by using the MEK cure test. (See Table 3.)

Calculating cure temperatures below the activation energy, the energy needed to start the chemical reaction, is obviously nonsensical. Unless powder coatings were clearly labelled and designated as low temperature cure, commercial application line oven temperatures below 340°F-350°F (171°C-177°C) shouldn’t be considered for standard epoxy-polyester hybrid, (emissive) polyester- polyurethane, polyester-TGIC, polyester-HAA (beta-

Cure times in 10°F intervals for a polyurethane and a TGIC powder coating

Note: Data-sheet time-temperature: Polyurethane: 15 min at 375°F (191°C); TGIC: 10 min at 400°F (204°C)

Temperature ((°F)) Polyurethane cure time (minutes) TGIC cure time (minutes)
350 27 32-33
360 21-22 26
370 17 20-21
380 13-14 16
390 10-11 12-13
400 8-9 10

hydroxyl alkyl amide), and polyester-TMMGU (tetra methoxy methyl glycoluril) chemistries. Powder coatings manufacturers should be consulted for low- (and possibly high-) temperature limits when applicators are using specially formulated powder coatings designed for commercial cure below 300°F (149°C).

On the other end, cure-oven conditions resulting in substrate temperatures above 410°F-420°F (210°C-216°C) can be detrimental to the formation and performance of quality standard powder finishes (except for silicone resin based products or special higher functionality epoxies) because of heat yellowing of films, volatile inclusions (micro-bubbles) and micro-pinholing in the film as a result of rapid cure, or heat degradation of phosphate pre-treatments.

Based on my experience over the years, this formula produces usable results within a ±50°F (±28°C) range from cure schedules featured by powder coatings manufacturers in technical data sheets and calculated within the upper and lower temperature limits discussed in the previous paragraph. Any results from calculations beyond this range, especially on the very low or very high bake temperature side, for example for powder coatings commercially curable between 280°F-390°F (138°C-199°C), must be regarded as raw approximations only and subject to verification through line trials.

This cure time-temperature formula is a useful tool for engineers and technicians running oven temperature profilers to estimate the percentage of cure. However, line operators are primarily interested in determining oven dwell times over a given temperature range. Therefore, once oven bring-up times are known over a preferred temperature range (for example, by using oven temperature profilers), or can be estimated with a degree of certainty, the cure time-temperature formula can be modified to determine oven dwell time:

DT = Oven Dwell Time

tBUT = Oven Bring-Up Time (known)

For Celsius, use the same formula; however, use 1.0436, instead of 1.024. -PC

References

IB Chemistry Web site [http://ibchem.com/index.htm]

Powder Coating-The Complete Finisher’s Handbook (Alexandria, Va.: The Powder Coating Institute), Chapter 12.

Need Help?

The services PCC provides are of the highest quality, with your results in mind. Browse through our powder coating services and let us help you achieve your business goals.

Editor’s note

For further reading on the topics discussed in this article, see Powder Coating magazine’s Web site at [www. pcoating.com]. Click on Article Index and search by subject category. Have a question? Click on Problem solving to submit one.

Bruno Fawer is the president of RESOTEX Associates, LLC, a business development and improvement company, as well as a part-time associate consultant for Powder Coating Consultants. He has about 40 years of operation, formulation, application, technical sales/marketing, and customer service experience in industrial and powder coatings, and has held various management positions before becoming a consultant. He has a bachelor of science degree in chemical engineering and organic chemistry from the College of Technology, Zurich-Winterthur (Switzerland), and a master’s degree in marketing and operations management (MBA) from Case Western Reserve University, Cleveland, Ohio. He has published a number of major papers and technical articles in trade journals, and presented several technical papers to trade organizations in the US, Canada, and South Korea. In addition, he participated in the initial creation of Powder Coating-The Complete Finisher’s Handbook published by the Powder Coating Institute (PCI), and he was a major contributor to the Powder Coating Formulator’s Desktop Reference published in May 2003 by Powder Coating magazine. He is a member of the magazine’s editorial advisory board and several trade organizations. Contact Fawer