Plasma-modified interfaces between polypropylene films and vacuum roll-to-roll coated thin barrier layers

Vacuum roll-to-roll coating is an important method to improve the functionality of polymer films, especially in flexible packaging applications. The most common coating material is Al, and oxides like SiO_x or Al₂O₃ are used less frequently. The final functionality - as the adhesion and permeation barrier - depends very much on the character or chemical composition of the interface between the substrate film and the coated barrier layer. For polypropylene films, this interface can be modified by different oxygen plasma pretreatments. In order to obtain the systematic correlations between the roll-to-roll pretreatment and the functionality of the roll-to-roll coated films: plasma conditions were characterized by a quadrupole-type plasma monitor and a Langmuir probe, untreated and pretreated films were analyzed by atomic force microscopy (AFM), contact angle measurements and X-ray photoelectron spectroscopy (XPS), films were coated with Al, Al₂O₃ and SiO_x, and finally, the functionality of the coated films was assessed by peel strength and O₂ permeability measurements.The results can be summarized as follows: Smoothing of the polymer surface by plasma pretreatment before vacuum coating increases the oxygen barrier of the investigated types of vacuum coated films. Oxygen plasmas improve the unpolar polypropylene surface by producing oxygen groups. These groups form chemical Al-O-C bonds to vacuum coated aluminum and improve the adhesion of aluminum barrier layers. The oxygen pretreatment does not increase the adhesion of layers from Al₂O₃ and SiO_x to polypropylene any further. The electrons, ions and UV radiation also present in the oxygen plasma weaken the cohesive strength of the polymer by inner chain scissions. Therefore, the atomic oxygen of the plasma should dominate the pretreatment process.