Optimizing Atmospheric Pressure Plasma Surface Treatment of Polymer Melts to Achieve Adhesion‐Promoter Free Multilayer Films

Multilayer polymer films require strong interlayer adhesion, which is conventionally achieved with adhesion promoters that negatively affect recyclability and environmental impact. In this work, an adhesion‐promoter‐free approach is investigated using atmospheric‐pressure plasma treatment of polymer melts. A film with low surface energy is treated at the die exit and directly joined to another film, both still in the molten state. While feasibility has been demonstrated previously, the polyethylene/polyamide laminates exhibited rather low peel strength. Therefore, the process was optimized by varying plasma gasses, plasma parameters and jet‐surface distances. Plasma treatment increased the peel strength from 0.03 N/mm for the untreated laminate to up to 2.92 N/mm. Although the treatment also increased the surface energy of polyethylene, a higher surface energy did not necessarily result in a higher peel strength. ATR‐FTIR spectroscopy and optical microscopy indicate that gas‐dependent polar groups and pressure‐dependent changes in surface morphology additionally govern adhesion. The identified operating point provides a basis for further investigations of the adhesion mechanisms.