Plasma surface modification of polymers using atmospheric pressure discharges

Abstract

Summary form only given. Atmospheric pressure plasmas, and corona discharges in particularly, are used to treat polymer films to improve their wetting and adhesion properties. Production of O, OH and HO/sub 2/ radicals in humid air discharges have been found to result in surface oxidation of the polymer and produce Low Molecular Weight Oxidized Material (LMWOM). Although a widely used industrial process, the fundamental plasma surface interactions which produce LMWOM and modify surface properties are not well understood. Results from a computational investigation of corona treatment of polypropylene will be discussed with the goal of determining the reaction mechanism which produce LMWOM and improve wetting properties. The gas phase plasma chemistry is modeled using a global kinetics model, GLOBAL-KIN, coupled to a surface kinetics model which addresses the plasma surface interactions. The region above the polymer surface is divided into two zones: a) a homogeneous bulk plasma; and b) a diffusion layer. Radicals produced in the bulk plasma diffuse to the polymer surface where they may adsorbed or directly react with surface sites. The products desorb from the polymer surface, transporting back through the diffusion layer to the bulk plasmas. The surface processes are simulated using a site-balance model with a variable area density of surface sites to account for modification of the surface morphology by the plasma. The surface reaction mechanism distinguishes between processes which produce hydrophilic and hydrophobic groups. Comparisons can then be made with experimental data for corona treated polypropylene based on wettability (contact angle). Results will be discussed for corona treatment of polypropylene in web-type reactors while varying energy density, humidity and web-speed. As probabilities of polymer surface reactions are in general uncertain, sensitivity studies will be made on the reaction mechanism. Comparison to experiments by industrial collaborators at 3M will aid in refining the mechanism.