Improving Adhesion of Thermoplastic Powder

Q. We have used your users guide, many times, with post-it notes and highlighted areas. It’s a big help. However, in the guide, you address PVC coating, which we are sampling with an outside supplier. The issue is that the supplier’s black PVC coating, applied at 8–10 mils, over a 304 stainless plate, welded onto a 1008 low carbon steel grid will not adhere to the stainless portion. The coating adheres well to the grid but peels away from the stainless plate.

The supplier has tried both solvent and water-based primers, and is confident that the water-based primer is better [no solvent pops]. He has tried elevated cure temps and cure times, but the coating will start to sag, if he exceeds the fluidized powder mfr. specs. (Though higher temps will increase the adhesive properties, runs and sags occur on the grid portion.)

Since we can’t substitute the stainless for another material, is there a chemical treatment or sub-primer that could be sprayed on the steel, prior to the fluidized powder, to improve adhesion? Or can you make suggested changes to the pre-heat, time, temps, etc.? N. G.

 

A. The User’s Guide to which you refer is entitled “User’s Guide to Powder Coating” as published by the Society of Manufacturing Engineers. I did not write the entire thing myself, I just wrote part of it and edited the rest as Editor. Many Industry experts provided the information contained in this manuscript and it has been around for a long time.

There is another great book called “Powder Coating, The Complete Finisher’s Handbook” as published by the Powder Coating Institute and edited by yours truly, as well. Now that I plugged both books, let’s get back to your question.

There are several things you can do to improve adhesion of a thermoplastic powder (in this case PVC). First, always use the liquid primer in accordance with the supplier’s recommendations. Follow the thinning, application and flash-off instructions to the “T” if you want to avoid solvent pops and other problems.

Next you can improve adhesion to stainless steel by roughing-up the surface. Sand or media blast the surface to provide “tooth” for the PVC coating to bond. You can even try an adhesion promoter like Silane after blasting in place of the primer. It is an adhesion promoter that has had great success over stainless steel by improving the wet-out properties of the powder coating.

Lastly, make sure the oven used to flow the material has a high ramp-up rate. This means the heat is delivered to the part fast and heat-up occurs quickly. This will melt the powder fast to ensure proper wet-out of the surface and will improve adhesion. Follow these simple hints and I am sure you will see a dramatic improvement.