PC Summitry: Options for Low Temperature Cure Powder Coatings

Posted on Monday, October 1, 2012

By Kevin Biller

Why low temperature cure? The two main drivers for low temperature cure are: energy savings and the ability to coat heat-sensitive substrates. Reducing energy consumption is a rather clear and measurable objective. Tackling heat sensitive substrates is a more complex proposition. Standard cure powder coatings typically require a bake of 15 to 20 minutes at a temperature ranging from 350°F to 400°F. Depending on your oven type, production volumes and energy costs, significant savings can be realized by reducing the oven temperature by 25°F or more.

Low Cure vs. Ultra-Low Cure

It is important to differentiate between coatings technologies that are suitable for energy reduction purposes vs. a more complicated approach that allow heat-sensitive substrates to be powder coated. Typically a low cure powder will provide a reduction in oven operating temperature with a minimal change to your application process. Ultra Low Cure technology usually involves spe- 44 Powder Coated Tough PCSUMMITRY cialized transport, handling, storage, application and curing equipment.

Low cure powder coatings are available from most powder coating suppliers and will carry a modest premium. The value of making the transition from a standard curing product to a low cure alternative will have to be carefully calculated. I have provided an example on how to determine potential savings of running the same ware through an oven on a conventional conveyorized line. It is important to input the proper data to develop a meaningful estimate of savings. The worksheet below requires input of a number of data relating to your product and oven engineering. It also takes into account the type(s) of metal you are coating, the density of your conveyor and trolleys and the insulation of your oven walls.

As you can see, reducing your curing temperature can provide measurable savings in operating costs. It is important to compare these potential savings to any added costs associated with procuring a low temperature cure product.

What type of performance can you expect of the lower temperature cure product? It is always wise to thoroughly review the technical data sheet associated with every product that you buy, however you shouldn’t expect a reduction in film performance by switching to a lower curing product. Exterior durability, chemical resistance and mechanical performance should be comparable to the analogous higher curing product. Appearance properties may be a little different. Gloss and smoothness are appearance properties that can be affected by a lower curing temperature. It is strongly recommended to run the new product through your oven with typical parts and oven loading before committing to a wholesale change in your product. You may have to tweak your oven setting and line speed a little to accommodate the new product.

Ultra-Low Cure Technology

Coating heat sensitive substrates such as engineered boards (e.g., medium density fiberboard—MDF), plastics and light alloys may require the use of ultra-low cure powder coatings. These are products that can cure at temperatures from 200°F to 275°F. Your options for these applications are either highly catalyzed thermosetting chemistries or UV curable powder coatings.

With thermosetting types the same or very similar chemistry is used as higher baking types; however, the curing mechanism has been greatly accelerated with the addition of a specialized catalyst. This formulation modification allows the powder to cure at sub-300°F temperatures. Ultra-low cure thermosets typically require a bake time of 20 to 30 minutes. This rather long duration can be mitigated with the use of infrared curing technology which can deliver relatively intense energy for a short period of time without degrading the substrate.

UV curing powder technology relies on first melting the coating then exposing it to high doses of intense ultraviolet energy to affect cure. The advantage of this approach is twofold; the curing process can be significantly shortened to around 1 to 5 minutes at quite low temperatures and the cure chemistry is stable at ambient and elevated temperatures. It is very important to note the criticality of having all coated surfaces exposed to the required dosage of UV energy. If any coated areas do not receive the necessary dosage, the coating will undoubtedly suffer catastrophic failure due to absence of cure.

If you are contemplating applying powder coating to non-conductive substrates then you will have to identify a novel technique to apply the powder. This can be achieved by preheating the substrate as commonly implemented with MDF. If the substrate is plastic you have a couple options. The substrate material can be made conductive by the polymer supplier by imbedding the plastic with a conductive agent. This can be expensive. Otherwise a conductive agent can be applied to the plastic to make the surface conductive and the powder can then be applied electrostatically.

Another very important caveat if you are considering venturing into ultra-low cure powder technology – regardless of cure type (thermoset or UV cure), you will have to be very careful with the transport, handling, storing and application of these products. To achieve ultra-low temperature film formation, the powder coating will possess a low melt point which can cause the powder to agglomerate at ambient conditions. Most ultra-low temp powders will remain stable up to 75 to 80°F. Exposure to higher temperatures can cause serious agglomeration. Hence you may have to use refrigerated transportation for delivery of the product and you will definitely require air conditioned storage and application environments. The application process will have to be carefully managed as well. Ultra-low temp powders may require careful fluidization conditions and high velocity powder transport air must be avoided lest you invite impact fusion in the transport hoses and interior gun parts.

In conclusion, low temperature cure powder coating technology can reduce your overall operating costs by providing lower oven temperatures while providing film performance equal to a higher baking product. Furthermore, ultra-low cure powder coatings can provide new avenues allowing powder coatings to penetrate unattainable markets that utilize heat sensitive substrates. Your best path forward is to thoroughly research the possibilities and conduct careful evaluations before making the transition.

Kevin Biller is a contributing editor of Powder Coated Tough magazine. He can be reached via email at kevinbiller@yahoo.com