Surface physicochemistry of corona-discharge-treated polypropylene film

Corona discharge treatment (CDT) is a surface modification technique commonly used to treat plastic films prior to adhesive bonding, printing with inks, lamination to other films and other coating applications. In this study, the treatment conditions are, in energy terms, representative of those used in industrial and laboratory coating applications. The physicochemistry of the surface of untreated and corona-discharge-treated biaxially oriented polypropylene (BOPP) film was investigated using a number of complementary surface analytical techniques: contact angle analysis; x-ray photoelectron spectroscopy (XPS); atomic force microscopy (AFM). This report describes the surface energetics, chemical functionality and morphology of polypropylene film before and after CDT. Both AFM and XPS were utilized, along with washing experiments, to investigate the presence of a weak boundary layer. The surface energy was found, as expected, to increase with increasing energy of the corona. The functional groups incorporated onto the surface have been identified as hydroxyl [C-OH], peroxy [C-O-O], carbonyl [C=O], ester [C-O-C=O], carboxylic acid [HOC=O] and carbonate [OC(O)O]. These groups are present in varying relative concentrations, depending on the energy of the corona utilized. The morphology of the film changed after CDT. Initially, a fibrillar crystalline structure was observed, whereas after CDT a globular morphology became apparent. These globular features were attributed to low-molecular-weight oxidized material (LMWOM) created by CDT. The roughness of the film was not found to increase under the corona conditions employed. Formation of LMWOM was found to be independent of treatment energy. However, two mechanisms have been suggested for its formation, dependent on the energy of treatment: below a threshold energy of similar to4 kJ m(-2), oxidation and scission of the inherent low-molecular-weight boundary layer present on polyolefin films is the dominant means for the formation of LMWOM; above 4 kJ m(-2), oxidation and scission of the polymer backbone is the main process. This work provides a comprehensive reference around CDT of polypropylene film for industrial applications, while also informing how the optimal level of treatment can be determined. In the case of adhesion of silicones, it would be expected that optimal adhesion would be obtained where the maximum amount of oxygen incorporated was in a water-insoluble form. Copyright (C) 2002 John Wiley Sons, Ltd.