ADHESION MECHANISM OF ALUMINUM, ALUMINUM-OXIDE, AND SILICON-OXIDE ON BIAXIALLY ORIENTED POLYPROPYLENE (BOPP), POLY(ETHYLENETEREPHTHALATE) (PET), AND POLY(VINYL CHLORIDE) (PVC)

In a roil-to-roll vacuum coater, homopolymer and copolymer biaxially o riented polypropylene films (BOPP HOMO and BOPP P/E COPO) were pretrea ted by a microwave-powered oxygen plasma. A plasma monitor and a Langm uir probe were used to analyze the plasma parameters, i.e. the composi tion of neutral and positive ions, and floating and plasma potential. With this set-up, surface overtreatment, i.e. destruction of the polym er surface, was also detected in situ. Atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), and polar surface energy measu rements characterized the surface before and after the pretreatment. T he BOPP samples, poly(vinyl chloride) (PVC), and poly(ethylenelerephth alate) (PET) were coated with thin aluminum, aluminum oxide, and silic on monoxide layers by an electron beam evaporator. It is shown that th e commonly used rule of thumb - a higher polar surface energy of a pol ymer results in better adhesion of a thin layer coated onto this surfa ce - fails in some cases. High adhesion of thin aluminum, aluminum oxi de, and silicon monoxide layers on a polymer substrate is obtained if there is oxygen at the interface between the polymer and the overlayer . It is not hydrogen bonds - being responsible for the polar surface e nergy - but covalent oxygen bonds (C-O-Al) that are responsible for th e adhesion of aluminum and aluminum oxide. Silicon oxide is covalently bonded to polymers by C-O-Si and/or C-Si bonds.