Many materials that are printed flexographically have nonporous surfaces. The most common of these substrates are polyethylene and polypropylene. These nonporous surfaces have low inherent surface energies, causing them to be non-receptive for bonding with printing inks, coatings and adhesives.

Other materials, such as foils and some papers, also exhibit low surface energies that can cause concerns with bonding as stated above. It is for these reasons that an altering of surface energy is required in order to print these materials using flexo.

Corona treating is a method used to alter the surface energy of many of these substrates to make them conducive to accept, or bond with, printing inks and adhesives. All of these substrates will have better bonding characteristics when they are treated at the same time they are manufactured. This process is referred to as post-treatment.

It needs to be stated that corona treatment isn't the only way surface energy can be altered at the time of manufacture. Other methods used include flame treating and priming. Selection is based mainly on the construction of the various materials.

Most people think that corona treating somewhat roughens the surface of the substrate to make it more accepting of printing inks and adhesives, but scanning electron microscopy has disproven this. A popular theory is that corona treating causes a realignment of the molecular structure at the surface making more polar sites available for bonding.

A measurement unit called a dyne is used to define surface energy. All liquids and most substrates (except porous) can be assigned dyne values. The rule of thumb is that for an ink to have success in adhering and wetting when placed on the substrate, that substrate needs to have a number in dynes that is 10 higher than the surface energy in dynes of the ink being applied.

Water-based inks have higher surface energies than solvent inks, therefore their substrates must also have higher surface energies. Everything in nature will try to return to its most relaxed state, so that the higher the dyne level the converter is trying to achieve, the faster that treat level will degrade. It is for this reason that when printing water inks on films, foils, and some papers, it is necessary to secondary treat at the press before printing.

Use of a corona treater on the flexo press, when sized correctly, will boost the treat level of the film back to its original level or slightly higher.

As I mentioned, treat levels will degrade with time. Films that have slip packages associated with them could also benefit from secondary treatment. Slip will "bloom" to the surface of the film over time and this could cause various unacceptable print results. There are also other contaminants that could be on the film or foil surfaces. Corona treating will burn off this surface slip and other contaminants and help to achieve a better print not only now for adhesion but for visual quality. It is for this reason that experts recommend that secondary treatment be used for all applications for film, foil and some papers when using either solvent, water-based or UV ink.

Packaging materials and constructions continue to evolve technologically with various sealing methods, laminations, and tighter printing and ink property specifications. The substrates being developed for these applications are also being modified and becoming more challenging. Through all of this, corona treating plays a major role in helping to address the surface energy requirements within the process to better meet the needs of the end user.

Steve Utschig

Consulting Technical Editor

920/735-4882

utschig@foxvalleytech.com