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 oriented polypropylene films (BOPP HOMO and BOPP P/E COPO) were pretreated by a microwave-powered oxygen plasma. A plasma monitor and a Langmuir probe were used to analyze the plasma parameters, i.e. the composition of neutral and positive ions, and floating and plasma potential. With this set-up, surface overtreatment, i.e. destruction of the polymer surface, was also detected in situ. Atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), and polar surface energy measurements characterized the surface before and after the pretreatment. The BOPP samples, poly(vinyl chloride) (PVC), and poly(ethylenelerephthalate) (PET) were coated with thin aluminum, aluminum oxide, and silicon monoxide layers by an electron beam evaporator. It is shown that the commonly used rule of thumb - a higher polar surface energy of a polymer results in better adhesion of a thin layer coated onto this surface - fails in some cases. High adhesion of thin aluminum, aluminum oxide, 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 energy - but covalent oxygen bonds (C-O-Al) that are responsible for the adhesion of aluminum and aluminum oxide. Silicon oxide is covalently bonded to polymers by C-O-Si and/or C-Si bonds.