Extrusion International 6-2022

59 Extrusion International 6/2022 Surface Treatment and Extrusion It all begins with the simple fact that plastic film is non-absorbent. Also irrefutable, is that plastic film has become an integral part of our daily life, especially in packaging where it is printed, laminated, and converted. And this is where surface treatment plays a vital part. By changing the chemical construction of the surface layer, ink, lacquer, or any other liquid will adhere to the plastic. “A dherence” and “adhesion” are terms that fre - quently appear in any text relating to filmic sub - strates and their ability to be printed, coated, and laminated. The issue relates to striking a balance between the surface energy of the liq- uid and the solid to which it is being applied. A mismatch results in poor adhesion, which is the reason that controllable corona treatment is essential for consistent performance. 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 adhere. 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 process of sur- face treatment starts at the very beginning of plastic film manufacture, when it is extruded. But, like most industrial processes, it is not straightfor- ward. Different plastic film formulations have different dyne values and different processes demand different dyne levels to be successful. For example, materials like PP, PE- LD, PE-HD, and BOPP have a native dyne value from 29 to 32. Equally, dyne levels required for printing with solvent- based or water-based inks, or coating or laminating vary from 40 to 42 at the low end to 46 to 56 at the high end. And it all starts with the extrusion process, which can raise dyne levels from 32 to 52 or more, whereas after ex- trusion, 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 before 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 co- rona treatment during extrusion was poor. The Blown film process There are two separate types of film extrusion: Blown film and Cast film. In the Blown process the film is extrud - ed vertically and wound into one flat tubular or two reels that are then converted by printing/coating/sealing into the final packaging product. The corona station is normal - ly located at the top where the flattened tubular film is treated on the outside only. It is then split in two and can be fully treated again, or only on the inside, before being wound onto reels. On some extruders, the winder oper- ates in both directions, so just one double-sided corona treater can be used. The reels are then passed through a converting process where the welding takes place, and as corona treatment is notoriously the enemy of welding, because the oxida- tion generated by the corona process weakens the weld- ing area, it is important to evaluate the likely effect in each case. Blown extrusion is typically used for PE based materials. Product applications include pallet hoods, collation shrink film, stretch hoods, lamination film, deep freeze film, heavy duty film, protective film, and food packaging film. These are single layer up to normally 3 layers but can be up to 11 layers in the case of food packaging film. The Cast film process In the Cast film process, the substrate is produced from a flat die and wound as a single film after being side trimmed. Because the edges of the Cast film are thicker than the rest of the web they need to be cut off before en- tering the corona station and winder to prevent any dam- age to the rubberised rollers. The Vetaphone C modelrange is designed for modern blown film lines running at high speed or producing more complex substrates. Its construction allows more power to be added to the corona treatment process