Adhesion to Powder Coatings

Q. Our company uses UV-curable industrial strength adhesives to hold typically metal parts during machining. We have had problems achieving a strong adhesive joint when adhering to black powder coatings, but we have normal strength bonds with white powder coatings. We also achieve lower-strength bonds to glossy coatings than to flat or matte coatings. We theorize now that the UV light is being absorbed by the black, hindering the cure process of our adhesive. Are the different powder coatings, such as TGIC-polyester or epoxy, known to have different properties as concerns adhesive compatibility? Is there a component in glossy powder coating formulations that could adversely affect adhesion? D. C.

A. There are many things in powder coatings that can affect adhesive bonding. First is surface smoothness. The smoother the surface (i.e. glossy colors), the more difficult it is for adhesives to bond. The second problem is compatibility. Adhesives often use some form of solvent to etch the surfaces for good bonding. Some powder coatings have very strong solvent resistance that can be defeating this etching mechanism. Typically, epoxies are more solvent-resistant than TGIC polyesters.
Lastly, there are numerous additives used in powder coating formulations that can affect your adhesive bond. Waxes, slip agents, talc and other additives are often used in powder coating formulas to control gloss or improve durability or UV stability. These same additives can have a direct affect on the bond that a particular adhesive may have on a powder coated surface. As I have said in the past, always check with the powder coating formulator and adhesive manufacture for advice on compatibility issues with powder coatings and adhesives. Finally, verify good adhesive bond with product testing.