Extrusion International 2-2023

34 Extrusion International 2/2023 CORONA TREATMENT The Importance of Surface Treatment at the Extrusion Stage of Film Production According to leading surface treatment authority, Vetaphone, understanding the extrusion process and the effect it has on the production chain of printing and converting film is an essential key to success. P lastic film has become so much a part of our daily life that we often don’t even notice it’s there – hidden in plain sight you might say. But its importance is undeni- able, certainly in the form of flexible packaging, and this is where its natural ability to repel liquids creates issues in the printing, laminating, and converting processes that are inherent in its production. The fundamental issue is that of adhesion. Plastic poses a problem when it comes to the adherence of a liquid. Use a ballpoint pen to write on a plastic bag and you can smudge the ink by rubbing your thumb over it. The problem is a mismatch of surface energy between the two, and this is resolved by modifying the molecular structure of the plastic surface. This process is known as corona treatment and starts at the very beginning of plastic film manufacture, when it is extruded. Controlled corona By applying a carefully controlled electronic discharge at close range to the plastic film, the chemical make-up of its surface layer is changed by breaking down the long molecule chains which then allows the liquid to ad- here. By increasing the surface energy of the plastic film, which is measured in dynes, it is possible to perform a range of different printing and converting processes that would otherwise prove impossible. And, this pro- cess of surface treatment starts at the very beginning of plastic film manufacture, when it is extruded. But, like most industrial processes, it is not straight- forward. Different plastic film formulations have dif- ferent levels of surface energy, which is measured in Dynes, and different processes demand different Dyne levels to be successful. Basic materials like PP, PE-LD, PE-HD, and BOPP have a range of native Dyne values from 29 to 32. However, the Dyne level requirement, for example, of printing with solvent-based inks, or water-based inks, or for coating or laminating vary from 40 - 42 at the low end to 46 - 56 at the high end. To achieve these higher Dyne levels, the corona system will require more power. And it all starts with the extrusion process, which can raise dyne levels from 32 to 52 or more, whereas after extrusion, the polymer chains take 48 to 72 hours to post-crystallise, with additives like slip agents and those for anti-fogging migrating back to the surface and in turn affecting the adhesion level. Securing correct treatment at extrusion is vital for two reasons: first, because dyne levels decline over time – typically 4 to 6 dynes over a period of 2 to 3 weeks be - fore stabilising; and second, because subsequent ‘boost’ or ‘bump’ treatment may be required later, depending on the intended process, and this may not be possible if corona treatment during extrusion was poor. Increas- ing Dyne levels at extrusion is easy, but after a period of crystallisation, additives like slip agents and those for anti-fogging migrate back to the surface and reduce the Dyne level and lower adhesion. Correct treatment at extrusion is therefore vital for two reasons: first, be- cause Dyne levels decline over a period, and second, be- cause subsequent ‘boost’ or ‘bump’ treatment may be required depending on the intended process, and this may not be possible if corona treatment during extru- sion was poor. The Vetaphone C model is a high-power corona treater for blown film applications