Epoxy/Polyester Compatibility

Question:

Mr. Liberto, we manufacture over-center brake lever components and assemblies, which we powder coat. We currently have two powders that we use. One is a brown epoxy and the other is a black polyester. Both are supplied from the same Formulator.

We normally don’t have a problem with either powder, except with one application. One customer (military) requires that we coat their product with the brown epoxy, (which was specified by our engineers, some years ago as a primer), then over coat the brown with the black polyester. In the past, we have had problems with some of the product coming back because of chipping or peeling black paint. The stock was verified; the problem parts touched up (with a lacquer touch up paint) and returned to the customer. In the past few months, we have had an increase of returns and our quality department has taken a hard look at our application.

The following are facts that I have put together, and I would like to know if I am heading in the right direction or not.

I have discussed the peeling problem with the powder supplier. Since the brown is an epoxy, the black polyester may not adhere. They suggested that we might get better adhesion if the brown isn’t completely cured before coating with the polyester. That will not work very well in our application. Handling and adjustment of cure temperatures might become a tracking nightmare.

We have asked our customer to allow a thicker black coating and eliminate the brown epoxy. Our tests have proven that the black will meet the corrosion requirements. This would have eliminated our problems, but since they require the brown epoxy for chemical, the black for UV protection, and since they have been getting the same product for several years (10+), they will not allow the change.

I have examined our components in the past but could not find a direct cause as to why sometimes the paint adheres and will not peel and other times a slight chip will allow a fingernail to peel large sections of the black polyester. The brown always adheres well to the substrate. I think I have found out why our returns have gone up lately. Before February of 2002, we did what we call touch-up paint of the product before it went out the door. The requirements were to make sure there was 100% coverage on some of our products. The products were touched up with the spray lacquer at a final assembly department before shipment. In February, with the economy staying slow, we began an aggressive cost savings push. One area was the large amount of spray paint we were using. After examination, we found that our operators were touching up almost all products to make them look good, and the operators were actually almost repainting the entire lever with black lacquer, over the black polyester, over the brown epoxy. We eliminated most of our “touch-up” painting and began to just cover inside spots that powder coating will not cover completely (Faraday areas). I have noticed that if a peeling component is touched up with the lacquer it will not peel as easily. It will still peel if damaged enough but the lacquer seems to seal and/ or strengthen the polyester somewhat. Our product actually was benefiting from the lacquer application. This didn’t eliminate the peeling but seemed to help.

Lost yet? I told you earlier that I need your advice. I think I have found out why, (after all the above), that our product only peels sometimes. I would like to have you tell me if I am heading in the right direction, or I am still chasing ghosts.

We were running the above product last week. The start of the production run went well, no peeling observed and the paint would not chip easily or peel (either epoxy or polyester). But at the end of the run, components were peeling easily. Nothing had apparently changed. Operators indicated that they had not changed or added powder, had no problems with equipment and had run the components back to back. Only one thing was different— the operators shut down the conveyor for 30 min for lunch. The last of the run during the brown epoxy cure was left in the cure oven to bake. It seems that over-curing the epoxy affects the adhesion of the black polyester. Am I on the right track? Have you any information that would help me confirm or deny what I think is happening? Your help is greatly appreciated. R. B.

Answer:

I am familiar with your products from my previous engineering life in DoD vehicle design. I had often specified these hand brake devices on vehicles I designed for the Military while working for a company that is no longer in business. Honestly, I had nothing to do with them closing their doors, since it happened several years after I left.

Because you described your problem in such great detail it becomes a lot easier to offer some suggestions to correct your problem. The first point is to understand why your powder supplier recommends that you slightly under-cure the epoxy primer before applying the polyester topcoat. Epoxies are very solvent and corrosion resistant materials, making them perfect for use as primers in outdoor products. This same attribute makes applying topcoats more difficult, since the solvent resistance of the epoxy prevents the polyester “biting” into the primer and reduces inter-coat adhesion. By under-curing the primer, the full solvent resistance properties of the epoxy are not yet developed, giving the polyester a better chance to bond to the primer. This would be the easiest solution to continue to use the two-coat process that your customer demands. Otherwise, scuff sanding the epoxy primer can help the inter-coat adhesion between the primer and polyester topcoat, obviously a more labor-intensive process than under-curing the epoxy. A change in gloss and/or texture (surface smoothness) of the epoxy primer can also help in inter-coat adhesion, as long as the gloss is not dulled using waxy additives.

Now to address why using liquid lacquer touch-up paint provides a part that does not readily chip. Since the polyester topcoat does not have very good solvent resistance, the lacquer paint can easily “bite” into the topcoat and provide good adhesion. Since the lacquer encapsulates and seals the polyester and has pretty good hardness, it would very difficult to chip the topcoat after it was applied. Now while I don’t recommend that you continue to perform this touch-up activity (especially in a open plant environment), explaining why this approach masked your underlying problem makes sense as to finding the best solution.

Your description of over-baking the parts during lunch break was very enlightening. Most powder coatings have 100% over-bake resistance. This means that if a particular powder coating is fully cured in 15 min at 325F metal temperature, you can over-bake the powder by allowing it to be in the oven for 30 min at the same temperature without loss of coating properties. After the over-bake resistance has been exceeded, the coating properties begin to suffer and the coating will eventually breakdown. Now try to relate this to your situation. You apply an epoxy powder primer and fully cure that material before applying a polyester topcoat and fully cure that coating. The epoxy was cured for twice as long as the polyester, which, if your oven is set-up correctly and you have not yet exceeded the over-bake resistance of the epoxy, will not cause any noticeable problems. However, when your production people go to lunch they stop the conveyor line allowing for the parts to sit in the oven for an additional 30 min. This will ensure that the epoxy has exceeded its over-bake limitations and epoxy primer coating will breakdown. When the epoxy breaks down the result will be lack of adhesion of the polyester topcoat.

My advice to you is to run an oven profile using a temperature recorder that will plot part metal temperature and oven air temperature during the complete normal oven cycle. Your powder supplier can perform this test for you. You should review the data with your powder supplier to determine if the epoxy coating is fully cured under normal operation and over cured during lunch breaks. If, as I suspect, the epoxy is more than 100% over-baked during lunch, then I recommend that you have substitute workers run the line during lunch or go to a staggered shift to ensure that the conveyor never stops for more than short (less than 5 min) periods of time.