Surface Treatment is Straightforward – Right? Wrong!

Published: January 12, 2026

On the face of it, there is no reason to think that a liquid, like ink or varnish, will not adhere to any surface — at least in the minds of those not involved in the package printing industry. But for those to whom this is an everyday commercial operation, the truth is very different.

It was back in the 1950s, when plastic materials first entered the package printing sector (think early supermarket carrier bags) that the issue of poor ink adherence first came to light. And a chance conversation among a group of friends that included a converter, and an electronics engineer created a process that is now universal and known as corona treatment. The electronics engineer was Verner Eisby, and his patented invention has gone on to become a global standard that, over the years, has been developed and refined to accommodate new technology in inks, substrates, lacquers and adhesives.

In essence, the principle has remained the same — it is a question of matching the surface tension (or energy) of the liquid (ink) to that of the solid (substrate). And the process of using an electric discharge at close quarters remains unchanged too. But there, the similarity ends. To understand the process requires some basic knowledge of science. If the surface energy of the liquid is higher than that of the substrate, the ink will not ‘wet out’ but bead-up on the surface. Remember, we’re talking about plastic material that is non-absorbent.

To ‘persuade’ the ink to adhere requires a change to the molecular composition of the surface of the substrate. The electrical discharge breaks up the long molecular chains on the surface to allow more oxygen molecules to attach and in doing so allows the ink to ‘wet out’ and adhere securely.

The Vetaphone Test Lab which opened in 2020 performs more than 150 substrate tests each year. Image courtesy of Vetaphone A/S

Complex Substrates

And in the early days that was all there was to it — simple process on basic material. But that was more than 70 years ago, and just about every aspect of package printing has moved on since then. There is now a wide variety of plastic substrates, with new ones being developed all the time to cater for international legislation on foodstuff and environmental protection. Lightweighting and a reduction in the number of layers required to achieve conformity are strong drivers in today’s package print market as brand owners come under increasing pressure to improve their carbon footprint and ‘go green.’

And the problem is not confined to new substrates. With the ongoing growth in demand for plastic packaging, a vast supply chain has developed globally that even with JIT delivery requires storage and transportation, and both can have significant adverse effects on the performance of the substrate when the times comes for it to be printed and converted.

This is known as ‘dyne decay’ and is a measure of what happens to a substrate between being corona treated at extrusion and the moment it is processed. The problem is not insoluble — boost (or bump) treatment prior to using the roll of material will restore its dyne level. But to know if it is required and how much is needed can only be determined by testing — and this is where todays’ scientific approach to surface treatment comes to the fore.

Laboratory Analysis

Simple testing, using dyne pens, has been available for many years, but in more recent times, as substrates have become more complex, the need for deeper investigation into behavioural characteristics has been highlighted. For this reason, we established our own unique Test Lab facility in 2020. Equipped with both corona and plasma treaters the test production line can provide detailed analysis of substrates in roll or sheet form and typically carries out around 150 different tests each year.

It offers a unique opportunity for converters to run tests under controlled laboratory conditions prior to committing to the expense of commercial production. The same facilities are available to ink, lacquer, and substrate manufacturers too, each of which is under close market scrutiny to ensure that their new products meet or exceed the current international standards.

Since opening, the facility has been used to test a wide variety of material including BOPP, OPP, PET and numerous foils with corona technology, as well as ETFE and FEP with the plasma treater. Interestingly, one of the most challenging materials is TPO (thermoplastic olefin). This polymer/filler blend is often used for outdoor applications, like roofing, because it does not degrade under solar UV radiation, which is a common problem with nylons. TPO is also used extensively in the automobile industry and is just one example of the ‘new’ substrates being developed for specific applications.

This new high-powered corona treater has 8 ceramic electrodes and a web width of 520mm. Image courtesy of Vetaphone A/S

Surface Treatment

Returning to the surface treatment process, we see a continuing increase in production line speeds across all industry sectors, and this requires a higher dosage of corona. A common misconception is that higher dosage equates to higher power, when what is needed is fine control of the corona discharge.

The key to this is in Eisby’s original patent that allows the generator to monitor the treatment process and optimize the correct frequency to ensure the most efficient discharge. This gives the best treatment to the substrate. The generator will automatically monitor the output and self-match to any material, altering the frequency to ensure an efficient discharge. One of the key benefits of optimizing this process is that it prevents excessive heat, which when you’re dealing with sensitive substrates is vital.

So far, I have spoken only of corona surface treatment, but another and more sophisticated method is plasma. Where corona works in ambient conditions, plasma requires a controlled environment to allow dosage of inert gases that are fine-tuned to each substrate and its intended use. Plasma also produces higher dyne levels that are easier to maintain — this is particularly useful with a substrate like BOPP that is notoriously difficult to treat.

Although both methods offer effective surface treatment, the two are not directly comparable. Apart from the process described above, plasma requires far more investment in the technology and consumables (gas) that are required to make it work. This is why most plasma installations are with major international groups that use it for developing new packaging. Its value is both as an R&D tool and its specialist commercial applications. However, in 95% of cases, corona treatment still offers the perfect solution.

As market demand continues to change and new substrates and processes are developed to meet international legislation, so surface treatment technology must adapt and meet these more complex requirements. If the need and basic process has changed little since the 1950s, almost everything else has!

About the Author

Kevin McKell is Chief Sales Officer (CSO) at Vetaphone A/S. Visit: www.vetaphone.com