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by Ron Joseph
July, 2009
Paint Coating Peeling from EPDM
Q. Currently down here in South America we are facing a paint peel issue in a part injected in EPDM. One of the root cause pointed for that is the low surface energy. I'd like to understand how could it contribute to the paint peel-off from the EPDM, also how to measure the surface energy and where I can find more details about low surface energy causing peeling issue or affecting paint coating adherence.
Thanks in advance.
A. When you place a drop of water onto a substrate that has high surface energy, the drop will spread out and "wet" the surface. Water has a high surface tension and will only wet surfaces that have high surface energy. On the other hand, when you place a drop of water (high surface tension) onto a surface such as a piece of metal that has been contaminated with oil or grease (both of which have low surface tension) the water beads up. That is why it is so necessary to thoroughly clean metal surfaces that are contaminated. Similarly, if you place a drop of water onto a substrate of low surface energy, such as some plastics, it will bead up. EPDM obviously has a low surface energy, although I don't have data for the surface energy of EPDM.
The greater the difference between the surface tension of the liquid and the surface energy of the substrate, the more difficult it is for the liquid to wet the surface.
On the other hand, if you place a drop of oil (low surface tension) on a surface with high surface energy, the drop spreads out and easily wets the surface.
For good adhesion it is essential that the liquid must properly wet the surface. Therefore, for good wetting the surface tension of the liquid should be lower than or similar to the surface energy of the surface. Solvent based paints have relatively low surface tension, but waterborne paints have higher surface tension on account of the presence of water.
In your question you didn't tell me whether your paints are waterborne are solvent borne. Regardless, to improve adhesion you will need to do one of two things; lower the surface tension of the paint, or raise the surface energy of the EPDM. In the first case, you would need to speak to your paint supplier to see if he can reformulate the paint to provide a lower surface tension. In the second case, you must pretreat the EPDM to increase its surface energy. This can be done by various heat or plasma treatments.
The following article provides an additional explanation of the problem and solution. http://www.pcn.org/Technical%20Notes%20-%20Corona.html
The Federation of Societies of Coatings Technology (FSCT) www.coatingstech.org has published a monologue that discusses the painting of plastics. You can purchase a copy from the FSCT bookstore.
To get more information on how to measure the ability for a liquid to wet a substrate, you can search for articles on "measurement of contact angle."
Now that I have given you a basic understanding of the problem, you can search the Internet for more articles on surface energy, surface tension and wetting.
Regards,
Ron Joseph
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