I am working on a project aimed at improving the paintability of TPE (Thermo Plastic Elastomer) compounds using powder coating techniques. The first task was to improve electrical conductivity of TPE to ensure that the paint/powder particles are attracted to the part. This was achieved using an additive and the conductivity is now high enough for this purpose. But, once the paint is cured, it forms a coat that can be easily peeled off, revealing the lack of bonding between the TPE and the paint. TPEs are based on SEBS (Styrene Ethylene Butylene Styrene copolymers) and therefore are non-polar material; the paint was Epoxy/Polyester.
What kind of paint should I use to ensure a good level of adhesion between the coating and the TPE part? Is pretreatment of the part necessary to achieve adhesion? S.S.
Since powder coatings have no solvents inherent in their formulations, they cannot “bite” into a surface as a solvent-borne liquid paint might. This is the primary cause of your problem. You have a smooth thermoplastic surface and you are applying a powder coating onto it. The powder coating lies onto the surface and even after fully curing the material, there is no bond to the substrate. The result is that the coating just peels off.
This leaves you several choices. First, make sure that there are no soils on the substrate that are impeding good bond to the powder coating. Some soils that come to mind are mold release agents used in making the thermoplastic part and hand oils that may be added during part handling. Select a cleaning agent that will remove these impediments for adhesion and not destroy the substrate.
Next, after the part is completely cleaned, you must provide a surface profile for the powder coating to “bite” onto. This is often referred to as providing “tooth.” (It almost seems as though we are discussing dentistry doesn’t it?) This surface profile can be achieved using mechanical methods (sanding, grinding, blasting, etc.) or chemically, using liquid primers. If you choose a liquid primer, make sure the solvent is compatible to your substrate and is completely dried before you apply the powder coating. In either case, you should end up with a surface that is rough enough to allow the powder to “bite” onto or bond with the substrate. If you achieve this goal, then you will have surface bond between your thermoplastic substrate and the powder coating. Oh, by the way, don’t forget to floss afterwards. Just kidding.