Corona-discharge Treatment of Polypropylene films-Effects of Process Parameters

Abstract

Corona treatment of films, mainly polypropylene (PP)-copolymers, was studied at commercial levels in a 2.7 kVA treater. The films were produced on a flat-film extruder with chill rolls. Degree of treatment was characterized by power of the generator divided by web speed and width of film (m Ws/cm²). The effectiveness of the treatment was measured in terms of the polar and dispersion components of surface-energy, the peel adhesion of pressure sensitive tape (similar to ASTM Adhesion Ratio) and the peel adhesion of polyurethane adhesives. The polar component of surface energy is a measure of the effectiveness of corona pretreatment. For a given degree of treatment, the polar surface energy component becomes greater as the film cooling rate increases (and the degree of crystallization falls). A comparison of homopolymers and copolymers does, however, reveal that even where these have the same density or the same degree of crystallization one cannot count on them having equally-sized polar components. Peel strengths of pressure-sensitive tapes and polyurethane-bonded patches confirm the influence of cooling conditions on wetting properties. Contrary to the case for tape adhesion, the polyurethane adhesive strengths reach their maximum value at much lower treatment intensities, i.e. with much lower polar surface energy components, and thus question the validity of the ASTM tests for adhesion properties.