Surface Functionalized Polypropylene: Synthesis, Characterization, and Adhesion Properties

Modification of polypropylene by hyperbranched grafting with a poly(acrylic acid) graft was carried out using techniques previously used with gold, aluminum, silicon, and polyethylene surfaces. An initial etching oxidation produced a modified polypropylene that was presumed to contain carboxylic acid functional groups (though none were detected by IR spectroscopy). Then, a series of repetitive grafting experiments using an α,ω-diamine derivative of poly(tert-butyl acrylate) were used to produce surfaces containing significant amounts of poly(acrylic acid). The resulting surfaces were characterized by ATR-IR spectroscopy, contact angle measurements, and XPS spectroscopy. Treatment of the surfaces with alkali produced a more hydrophilic carboxylate surface. Treatment of these surfaces first with ethyl chloroformate followed by pentadecylfluorooctylamine produced a hydrophobic fluorinated surface. Mechanical tests show that such surface modification not only serves as a route to modify polypropylene's hydrophilicity/hydrophobicitysuch modification substantially affects the adhesive strength between this modified polypropylene and an epoxy adhesive. Double cantilever beam tests show that adhesion increases from 2 J/m2 for unmodified polypropylene to up to 29 J/m2 with the modified polypropylene.