It's All About Adhesion A Simple Guide to Corona Treatment

Published: March 03, 2025

By Jan Eisby, Chief Business Officer, Vetaphone A/S, Denmark

Corona treatment has come a long way since Verner Eisby, the founder of Vetaphone, invented it in 1950. Today, his pioneering technology is used all over the world wherever liquids need to adhere to a non-absorbent surface and adhering ink or lacquer to film or foil in the printing industry is one of its major uses. But many converters don't understand the technology.

So, what is it that makes it stick? The answer is control of surface tension between the two elements, the liquid and the substrate. Adhesion relies on the tendency for dissimilar surfaces to cling to one another. The adhesive force between a liquid and a solid causes droplets to spread across the surface — cohesive forces within the liquid cause the drop to "bead-up" and avoid actual contact with the surface. Wetting is the ability of a liquid to maintain contact with a solid surface resulting from intermolecular interactions when the two are brought together. Regardless of the amount of wetting, the shape of the liquid drop on a rigid surface always resembles a truncated sphere.

The substrate has long molecular chains that pose a problem for liquid adherence because of the infrequency of oxygen molecules. It means that the liquid beads-up because it is not wetting out properly. Eisby set about creating a method of breaking up the long chains to create more open-ended bonding points for the liquid. Breaking the molecule chains creates more open ends and free valances are formed. This is done by directing a high frequency electrical discharge at the plastic surface from close-range.

The electrons that are accelerated into the surface disrupt the molecules by oxidizing it.

The discharge splits the carbon molecules and breaks the oxygen into iodes, some of which enter the surface layer of the substrate and improve the bonding, while others form into ozone that needs to be extracted. The new carbonyl groups created have a higher surface energy, and this improves the chemical connectivity (dyne/cm) between the liquid and the plastic. Corona treatment affects only the surface layer of the plastic, to a depth of 0.00001mm (or 10m/0.01 Micron) and does not change its appearance or strength.

Key to the success of the discharge is the maintenance of an air gap between the electrodes and the substrate that is passing underneath. Too large, and the discharge won't jump the gap; too small and it will burn the material. An optimum gap is approximately 1.5mm and this should be checked regularly as part of routine maintenance of the Corona system.

But the really clever part is the way that it monitors and measures the discharge. The principle used today is exactly as it was developed in the 1950s and remains unique among all Corona manufacturers. When the power is supplied by the generator to the transformer and subsequently on to the treating unit, every cycle is measured to ensure that the system is performing at maximum efficiency.

Each transformer is matched to the electrodes supplying the discharge. It's complex and a more expensive means of achieving Corona treatment, but consistently gives a very high efficiency rating.

It's not enough to know you need it - you need to understand how it works and what it does to get the full benefit of surface treatment because its effect will vary from substrate to substrate and even from roll to roll of the same substrate.

Corona or Plasma?

But the problems facing converters now require an even greater degree of understanding if the surface treatment applied is to be effective in today's increasingly sophisticated market. Far from "buy, install, set and forget," surface treatment has become as integral to quality production as any other part of the printing process, and to ensure you are using it correctly, you need to understand its operational parameters.

The market can be broadly split into three clearly definable requirements. First, there is the need for simple Cora treatment, and this applies to virtually all narrow web press applications. It will typically require a Corona unit to deliver a power rating of 1.5-2.0 kW and provide excellent performance on standard substrates run off at low to medium speeds.

The second section has been created by the development of new and more difficult substrates. These are run off on the latest generation of presses that have been optimized by manufacturers for high-speed production. These factors place an increased demand on the Corona unit to deliver the correct dyne level, which typically needs to be rated at 3.0-5.0 kW. To understand this concept, think of the similar situation with UV curing, where dwell time under the lamp, defined by power rating and running speed, are critical to achieving a complete cure.

The third and final section relates to those substrates that have been developed to produce "unique" products, typically using special inks and lacquers, lamination, multi web applications and other techniques to produce sophisticated products. The chemical constituents of these substrates require a chemical treatment process as well as a physical one — and in these instances Plasma treatment is necessary to ensure bonding.

Plasma should not be considered as a replacement for Corona, more as a logical development of technology to keep pace with the requirements of more sophisticated materials and processes. The fundamental difference between Corona and Plasma is that the former is a physical process carried out in ambient conditions; the latter is a physical and chemical process carried out in controlled atmospheric conditions.

About the Author

As Chief Business Officer at Vetaphone A/S, Denmark, Jan Eisby runs the Vetaphone Academy that offers a unique knowledge of corona and plasma technology for perfect surface adhesion in the film and foil packaging industry. Learn more at www.vetaphone.com.