Epoxy Peeling Problem

Q. I work for a surgical instrument repair company that uses epoxy powder coat to insulate instruments that use electricity during surgeries. I’m currently having a problem with the bond of the epoxy, however. After running the parts through a sterilization cycle, I lose the bond and the epoxy—it can be picked off with your fingers.

We are prepping the parts by sandblasting with 100-grit aluminum oxide, washing them in a hot ultrasonic soap solution, rinsing in clean, hot water, rinsing again in clean, cold water, and finally pre-heating for 10 min at cure temperature. The epoxy requires curing of 10 min at 375°F. We are using an electrostatic gun to apply the powder.

After the powder is cured, the bond is very strong and we are unable to remove it with a razor, yet when we sterilize for 20 min at 250°F, the epoxy is removable in large pieces. All of the parts we coat are stainless steel, and because they are for medical use, we’re limited to FDA-approved epoxies only. We have tried different manufacturers but the results are the same. I’m hoping you can give me a little direction as to what is causing this. The manufactures have no idea what to do to fix it. J. C.

A. There could be several reasons for the adhesion failures you’re experiencing. In our consultation practice we have always found failures to fall into one, or more, of the following categories: Design, Process, Consumable Materials. If the product is designed poorly, you can never make up for this deficiency in process or with consumable materials. The design can be the greatest, but if the coating process is poorly executed, you will never realize the inherent benefits of the design. If the design is appropriate and the process is properly executed but the incoming consumable materials are “bad,” then you will still have problems. I will provide my advice to you using these conventions.

Design: This category describes the use of substrates, pretreatments, coating powders, etc. required to attain a desired set of design goals or requirements. In your case the substrate is given as stainless steel, which seems appropriate. The pretreatment using grit blasting and ultrasonic cleaning makes sense for providing a good clean surface with sufficient roughness (tooth) for good powder adhesion. Lastly, the use of FDA-approved epoxy is also appropriate, given the end-use of the product.

However, there are some things that can be modified in the formulation without affecting the FDA approval. Furthermore, there are many FDA approved epoxies that are used for many applications (i.e. hospital furniture, food preparation, etc.), so selecting the one that has the properties you need is very important. One issue is that the wetting factor of the powder material will affect adhesion to the substrate. Second, thermal expansion of the powder material can affect adhesion if it isn’t matched to the expansion of the substrate. Third, the powder must have high humidity resistance to survive the sterilization process. All of these design requirements must be inherent in the powder coating selected.

The only way to know for sure that the design is appropriate is to manufacture products in “ideal laboratory” environments and test the results for meeting all your stated requirements. If the design works, you will know right away. If it doesn’t, you have to go back to the “drawing board” and select other combinations to achieve your stated goals. If you cannot prove it works under ideal circumstances, you can forget about proper implementation in production.

Process: This category describes the effectiveness of the process employed to achieve the desired goals. There are several areas where your process can go awry. For instance, handling of the product with bare hands between the cleaning and coating steps and under curing the powder coating are two that immediately come to mind. Not changing the cleaning solution frequently in the ultrasonic cleaner is another area where this methodology can be deficient. If you have proved your design as stated above, then you just have to perform several design or experiments (DOE) to determine what area of your process needs improvement.

Consumable Materials: This category describes the batch-to-batch consistency of the consumed materials (powder, chemicals, etc.) used in the process. There is always potential for things to go wrong here, but since you have tried other suppliers with similar failures, I doubt that this is an area in which we should spend much time.

There is a lot of good stuff in this answer to assist you in solving your problem, but no definitive corrective action plan. For that, I apologize. However, this information shows you how to go about developing your own answers. You know the old saying: “Give a man a fish and you solve his hunger pains; teach him to fish and he will never go hungry again.” I just hope you like fish.