Polarity of Corona-Treated Polymer Film

Question: We have had UV-cured ink adhesion problems on some print jobs, and our ink supplier was concerned with the low polarity of our substrate. How is polarity related to the dyne level of the material that we are printing, and how is it affected by corona treatment?

Answer: Most polymers are relatively non-polar, as shown in our polymer surface energy table at http://www.accudynetest.com/polytable_02.html. The polar component of the surface energy is shown in column 5 (8.0 dynes/cm for ABS, for example). Untreated polyethylene has a polarity of only about 1.4 dynes/cm. Corona, plasma, and flame treatments all increase the polar component, which is what increases the total surface energy, providing the improvement in wetting and adhesion.

The polar component is basically a measure of free electrons (or free radicals) available on the material’s surface for bonding, whereas the dispersive component of surface energy (also known as dispersion forces, London forces, or van der Waals forces) is based on general atomic-level forces involving the entire structure of the polymer molecules on the surface of the substrate.

Dispersion forces tend to be greater for larger molecules, and the surface energy of most untreated polymer surfaces is primarily determined by them. Unless you are a physical or surface chemist or have similar training, they are not easy to visualize. The polar component is more intuitive: an extra (or missing) electron is available for bonding at an atomic level with an oppositely charged surface.

Fortunately (from the point of view of understanding, at least), the dispersion forces are relatively unimportant when it comes to the adhesion of a printing ink. Inks are designed to find polar sites which will anchor and attract them until they are fully cured or dried. Surface treatment provides such polarity, and increases the overall dyne level (which is comprised of both polar and dispersion forces) by doing so. Thus, the change in dyne level between untreated and treated polymer surfaces is essentially a measure of the increase in polarity on that surface.

Published by

Russ Smith

Russ Smith formed Diversified Enterprises - the first business to focus specifically on applications of the dyne test - in 1986, and has served as President of the company ever since. He has over 30 years of experience in the fields of surface treatment and analysis, and deals with technical inquiries from customers worldwide on a daily basis. Russ is a member of ASTM, the Society of Plastics Engineers, the American Chemical Society, the American Society for Quality, the American Association for the Advancement of Science, and TAPPI.

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