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 functiona lity of polymer films, especially in flexible packaging applications. The m ost common coating material is Al, and oxides like SiOx or Al2O3 are used l ess frequently. The final functionality - as the adhesion and permeation ba rrier - depends very much on the character or chemical composition of the i nterface between the substrate film and the coated barrier layer. For polyp ropylene films, this interface can be modified by different oxygen plasma p retreatments. In order to obtain the systematic correlations between the ro ll-to-roll pretreatment and the functionality of the roll-to-roll coated fi lms: plasma conditions were characterized by a quadrupole-type plasma monitor an d a Langmuir probe, untreated and pretreated films were analyzed by atomic force microscopy (AF M), contact angle measurements and X-ray photoelectron spectroscopy (XPS), films were coated with Al, Al2O3 and SiOx, and finally, the functionality of the coated films was assessed by peel strength and O-2 permeability measurements. The results can be summarized as follows: Smoothing of the polymer surface by plasma pretreatment before vacuum coati ng increases the oxygen barrier of the investigated types of vacuum coated films. Oxygen plasmas improve the unpolar polypropylene surface by producing oxyge n groups. These groups form chemical Al-O-C bonds to vacuum coated aluminum and improve the adhesion of aluminum barrier layers. The oxygen pretreatme nt does not increase the adhesion of layers from Al2O3 and SiOx to polyprop ylene any further. The electrons, ions and UV radiation also present in the oxygen plasma weak en the cohesive strength of the polymer by inner chain scissions. Therefore , the atomic oxygen of the plasma should dominate the pretreatment process. (C) 1999 Elsevier Science S.A. All rights reserved.