Adhesion Improvement by Nitrogen Functionalization of Polymers Using DBD-Based Plasma Sources at Ambient Pressure

Nitrogen-bearing chemical functional groups have been generated on the surfaces of various polymers by plasma treatment using dielectric barrier discharges (DBDs) or AC corona discharges stabilized by a dielectric barrier. Investigations on the role of these functional groups for adhesion control are reported in this paper. The experiments were carried out under an almost oxygen-free atmosphere with nitrogen, forming gas and ammonia as process gases. It was possible to establish more than 10 primary amino groups per nm2 on polyethylene (PE) or biaxially oriented polypropylene (BOPP) with forming gas and optimized hydrogen content depending on the energy density. Adhesion experiments carried out on BOPP with natural rubber adhesives show a close relationship between the number of primary amines and adhesion improvement. Poly(ethylene terephthalate) (PET) functionalized by DBD treatments with forming gas and ammonia shows a strong improvement in adhesion with natural rubb er adhesives up to values typical of wet-chemically treated reference samples. Overtreatment effects become obvious for energy densities above 100 W-min/m2, resulting in a marked decrease of adhesion. In the case of PET other functional nitrogen groups may also play an important role in adhesion and this will need to be clarified in further investigations. In addition, treatment of the high-performance polymers poly(vinylidine fluoride) (PVDF) and poly(ether ether ketone) (PEEK) was investigated. It was found that energy densities of more than 3000 W-min/m2 are necessary to obtain a strong adhesion. For PVDF a strong improvement in adhesion to steel was achieved using hot-melt adhesives. The maximum value was 1.7 N/mm. It was possible to functionalize the PEEK in particular with forming gas in an AC corona discharge in such a way that there would be a cohesive failure in the PEEK after bonding with different epoxy adhesives.