Adhesion to Powder Coating Part II

I answered a question in the November Powder Coating Clinic that got some additional response from our readership. I will share their comments here and then add some more of my comments.

Response #1: I read your commentary on adhesion to powder coatings in the November 2004 magazine, and I take exception to the assertion that is made that the problem is solely one of the powder coating. Adhesion is a function of two things interacting and, while it is true that there are things used in powder coatings that reduce the surface energy of the coating and thereby make it more difficult for either decals or inks to develop good adhesion, it is just as possible that modifications are made to the adhesive in the decal to decrease surface energy as well, to counteract that. Similarly the use of more aggressive solvents in the ink may help “bite” into a powder coating that contains waxes and such and again overcome these adhesion issues. The key is good communication with the supplier of the ink or decals. B. M.

Response #2: I love reading your articles and just wanted to comment on an answer you gave in the November 2004 issue. A reader had a question regarding adhesion of silk-screening to powder coating. Your answer was that adhesion was based on the formulation of the powder, i.e. wax. This statement is incorrect. Definitely the use of wax as an additive makes stickers or labels a problem, however not silk-screening. We have been silk-screening here for 26 years and on powder for the last 15. I can tell you honestly and without hesitation that I have silk-screened on every chemistry of powder, with every possible formulation from textures to full gloss. Now without giving away some of my industry “secrets,” I will tell you that the chemistry of the ink is extremely important as well as the sequence that it is put on. M. R.

Answer:

Both these comments are great and prove that when I give a shorten answer, I sometimes leave out some important details. We all agree that waxy additives will affect the application of labels and decals. Let’s all remember that this is the original problem that the questioner asked about. He stated that he was going to look towards silk-screening as a “less desirable alternative” to using these decals. Therefore, my original answer about the waxy additives is absolutely on target.

However, I did not give the questioner much in the way of details on silk-screening. I just commented that the waxy additives would affect the silk-screen adhesion, as well. This part of my answer is way to simplistic and my readers called me on it. Good for them! I need a kick in the behind once in a while to stop me from getting too complacent and sloppy. I sometimes forget that even though I have answered this question many times in the past, new readers may not have seen these answers. That’s the problem with writing a column for more than 14 years in two different publications.

Now let’s discuss silk-screening over powder coating in detail. The key to silk-screening over a powder-coated surface is to have the silk-screen ink “bite into” the powder-coated surface to ensure good adhesion. This can present a problem if the particular powder coating used on the part is inherently resistant to solvents. It is a well-known fact that several powder coatings have excellent solvent resistance, specifically epoxy formulations. While this may be good for the end-use of the product it can be problematic for applying a silk-screen to the surface. This does not mean that you can’t apply silk-screening to epoxy and other solvent resistant powder coatings. It just means that you must take some precautions. The first precaution you must take is to discuss your silk-screen intentions with your powder coating formulator. They may adjust your formulation to better accommodate the ink you intend to use. Bringing in the silk-screen ink supplier during these discussions will help, since they may also have more aggressive inks and solvents that can help with adhesion over powder coated surfaces. These discussions need to be completely open and frank to yield the desired results.

Furthermore, these discussions should include product design and use to ensure that reducing the solvent resistance of a powder coating to accommodate silk-screening does not adversely impact the use of the product in the field. Next you can improve both product performance and silk-screening success by changing your powder coating and silk-screening process. Since solvent resistance is the last property that is developed during the powder coating cure cycle, slightly under-curing a powder coating may help the silk-screen ink penetrate the powder surface. You can always continue the powder cure after applying the silk-screen, if necessary. Just be sure that the silk-screen ink has been completely flashed-off to avoid solvent popping.

Lastly, you may need to apply two coats of powder coating to obtain the total design package for your product. This may be the best solution if your product is fielded in conditions that require high solvent and corrosion resistance. In this case, you may want to apply a polyester powder coating, with less solvent resistance, to allow you to get a good silk-screen adhesion, and then apply a clear epoxy over the top. This will yield a product that will have significantly improved solvent resistance and yet be relatively easy to silk-screen. It won’t, however, have very good UV resistance, because the epoxy powder coating will chalk. I hope that this answer better describes the issues with silk-screening over powder coating. I am sure that sometime in the next 14 years I will answer this question again. Also, please keep those cards, letters and e-mails coming!