Surface Treatment for the Extrusion Market

Published: August 25, 2025

By Giuseppe Rossi, Area Sales Manager, Vetaphone

Surface treatment in the form of Corona is a well-known and acknowledged part of the printing, converting and laminating sectors of the industry, but it has an even more fundamental part to play in the extrusion process. In this article, we explain the need for the process and how technology is responding to changes in market demand.

Why is surface treatment so important in the extrusion process? Well, it's not so much important in the process itself, as afterwards. To ensure good adhesion of the inks and lacquers during downstream converting processes, the molecular structure of the film surface needs to be modified, and it needs to be done immediately after the cooling phase of the melt before the polymer is completely post-crystallized.

By applying the Corona charge at this point to the top layer (to 1-micron deep), we can break the molecular chains and add more oxygen. This alters the surface tension and improves adhesion. The longer you leave it before treatment, the more difficult the molecular chains are to break - in fact, it's often impossible, so timing is critical.

There are also different requirements for blown and cast film extrusion. Blown film is the more common use for Corona treatment. Because of the high incidence of LDPE in this process and the relatively slow production speed, compared with cast extrusion or any of the converting processes, the Corona system requires only low energy to achieve a good result. The technology in this sector is well consolidated and mature, and with good control allows consistently high-quality film to be produced and treated.

Cast film is a far more demanding process because the PP material and higher line speeds require a more complex Corona system layout. Even a single-sided treater (unlike the double-sided in blown extrusion) will typically need higher power, a cooled backing roller, direct drive and a nip roller - effectively a proper pull set up.

There is a third type of extrusion that applies to Bioriented Cast Films like BOPP, BOPET, BOPA, where the width of the line and the high output demands that the Corona unit is contained within the extruder.

Surface treatment requirements also differ depending on the material being extruded and its intended use. How it will be converted after manufacture brings a number of variables into the equation. For a start, every polymer has its own initial dyne level - that's its ability to adhere inks and lacquers. Some materials, like PVC or PA, require very little power to surface treat to the correct dyne level - PE requires a little more power, and PP is notoriously the most difficult to treat. You also need to allow for the additives mixed in with the polymers, because these can significantly affect the level of Corona treatment needed, and the power consumed.

With all these differing requirements, control of the treatment process is critical, and two principles stand out here: simplicity and high efficiency. The unit itself needs to be user-friendly, so typically needing a slide-in, slide-out cartridge system to facilitate cleaning and setting. This must be backed up with high-efficiency generators capable of delivering maximum input power direct to the electrodes. This reduces the heat level, which is an obvious advantage when you are handling lightweight extruded films.

Like most markets, extrusion has changed over the years because the markets it's supplying have changed. If you go back 20 years, the majority of films being extruded were up to five layers for use in the technical and industrial packaging sectors. Nowadays, with the emphasis more on meeting the ever-growing demands of the food, pharmaceutical, and hygiene markets, packaging with up to 13 multi-layers are far more common, and the product is required to meet a variety of demands. These include freshness, protection and recycling, and pose complex problems for manufacturers. The COVID pandemic, for example, highlighted the vital role that packaging plays in our daily lives and welfare. Situations like this drive demand for new technology and future applications that enhance the value of so-called "clever" packaging. And, it all starts with extrusion!

Of course, we are speaking of extrusion for plastic packaging here, which seems to be "Public Enemy No. 1" right now, so it's important that the extrusion market takes and is seen to be taking steps to mitigate environmental impact. But the blame is often directed at the wrong target. Despite public opinion to the contrary, plastic packaging has a very low carbon footprint as far as manufacturing is concerned. But, mindful of its image, and the need to take its responsibilities seriously, there are two ways in which extrusion is helping to go greener.

The first is in downgauging the packaging by using new resins that allow for less volume of plastic to be used. This helps with problems like shelf-life and hygiene where the extrusion of a special material removes the need for several laminated and  heterogenous substrates. The second is the way the industry is working to simplify the structure of packaging by using compatible resins to improve recyclability as part of the circular economy. It's not the manufacture of plastic that is the problem - it's how it's disposed of after use that is the real issue.

This is the next big challenge for the industry because very ambitious international targets have been set for reducing the production of plastic and increasing the level of recycling. But it's very difficult to equate the oxygen barrier demand with the sole use of Polyolefin resins. One solution is to extrude bioriented blown films, so-called triple bubble technology, where the orientation of the molecules dramatically improves the properties of the standard resins used in general purpose packaging. The oxygen barrier will not be as good as that offered by EVOH or PA, but good enough to replace much of the packaging where they are currently being used. There are no simple solutions, but demand drives R&D and gives birth to new technology - extrusion is no different in that respect.

About the Author

Giuseppe Rossi is Area Sales Manager for Vetaphone in Italy, representing the company's corona and plasma surface treatment technology across industry sectors from extrusion to printing and laminating. He has extensive experience in the manufacture and conversion of plastic film substrates.