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Adhesion to Powder Coated PaintsWhy Pressure Sensitive Materials Fail to Adhere to Powder Coated Surfaces?
Adhesion failures of pressure sensitive materials, such as labels, decorative films, foam tapes and graphic panels, to powder coated substrates are quite common. Just as there are several reasons for these failures, there are several steps that the fabricator can take to avoid these problems. Most importantly, before manufacturing a pressure sensitive product, such as a label, you must have a thorough understanding of physical properties of the substrate. Equally important is selecting the right adhesive product for the application.
Powder Coating Basics
I like to describe powder coating as dried paint in a powdered state. Unlike liquid paint, it has no solvent and consequently none of those nasty VOCs.
If you were wondering how the powder was applied, the process is really quite simple. The powder, which consists of very fine plastic particles, is sprayed on the substrate using a spray gun similar to conventional spray guns used for paint.
In conventional applications, the powder sticks electrostatically. That’s the trick. A metal part, for example, is negatively charged with electrical current. As the powder is sprayed, it is positively charged. The tiny plastic particles are attracted to the oppositely charged part.
When these plastic powders are baked at high temperatures (in the range of 400⁰F or 204⁰C), they flow out and fuse together form a tough, solid, chemically resistant coating. To top it off some of these coatings are absolutely beautiful. It’s no wonder that powder coating is growing in popularity.
Low Energy Surfaces
What’s important to remember is that powder coating is plastic. Some of these coating materials are thermoplastic and some are thermoset. Typically, these coatings are low energy surfaces.
Because surface energy affects how an adhesive wets out or spreads over the substrate, it is critical to achieving a good adhesion. If the adhesive wets out well, it will flow into the microscopic pores of the surface and create a strong mechanical bond. For a pressure sensitive adhesive to wet out and adhere adequately, the surface energy of the adhesive should be lower than the surface energy of the substrate.
Standard adhesives used on pressure sensitive films typically do not wet out well on low surface energy materials, such as polyethylene and polypropylene. The metaphor generally used to describe the disparity of surface energies is a beaded up droplet of rainwater on a newly waxed hood of a car. By comparison, on a high energy surface the adhesive will readily wet out. The point is that low energy surfaces, such as powder coated paints, can be very difficult to adhere to and require special adhesives.
Other factors which can inhibit adhesion of a pressure sensitive material to a powder coated surface are additives in the powder. Waxes and slip agents, in the powder coated paints can inhibit adhesion, resulting in problems, such as edge lift and tunneling.
The texture of the substrate can also affect adhesion. Some powder coated jobs have a very smooth and high gloss finish. Other surfaces have a rough “orange peel” finish. Both high gloss finishes and rough surfaces can be problematic.
Highly textured surfaces are especially troublesome. On rough surfaces, a thin coating of adhesive, in the range of 1 to 2 mils, may not be adequate. Here’s why. Thinner coats of adhesive may not flow into the crevices or valleys of the surface. Instead, the adhesive merely makes contact with peaks of surface. Insufficient surface contact generally results in adhesion failure.
High gloss coatings are also prone to problems. These smooth, glossy surfaces are extremely slick, and have less tooth for a good bond.
As a fabricator, prior to manufacturing, thoroughly investigate the type of surface that you are adhering to. Then select the right product for the application. Powder coat paints typically have a low surface energy and require a low surface energy adhesive. Rough textures will require adhesives with heavier coat weights. Products with 1 and 2 mils of adhesive are generally not adequate. For rough surfaces, an adhesive coat weight of 5 mils is typically required.
Finally, whenever you are trying something new always "Test, Don't Guess" before going into production. For example, before attempting to laminate a 5 mil LSE adhesive to a pressure sensitive label stock, test the two adhesives for chemical compatibility.