Adhesion improvement in thermoplastic composites by polypropylene-glass grafting pp.933-952
Authors: (Mariana Etcheverry, María Luján Ferreira, Silvia Barbosa, Numa Capiati, Plapiqui (Uns-Conicet), Argentina)
Abstract: Final properties of thermoplastic matrix composites are mainly determined by the strength and stability of polymer-fiber interphase. When the adhesion between fiber and matrix is poor, the fibers do not act as an effective reinforcing material. Also, the adhesion between phases can be easily degraded in aggressive environmental conditions such as high temperature and/or elevated moisture, and by the stress fields to which the material may be exposed. This is of particular importance in reinforced materials since they are mostly used in industrial applications involving an extended exposure to water, as for components used in marine or transportation fields). Many efforts have been done to improve polymer-glass adhesion. The most commonly used technique includes glass surface modifications via silicates or titanates, combined with proper polymer modification. Particularly, in the case of polypropylene (PP) filled with glass fibers (GF), an inclusion of small amount of acrylates-PP copolymers have shown to improve the adhesion. A new approach to increase PP-GF adhesion, based on propylene polymerization directly onto the fibers surface, is explored in the present work. The chemical anchoring of the matrix polymer on glass fibers was improved by direct metallocenic copolymerization of propylene onto the fibers. The experimental route involves an initial contact with methylaluminoxane and a hydroxy-α-olefin to generate anchorage points on the fiber surface, followed by a propylene polymerization catalyzed by EtInd2ZrCl2 (metallocene)/methylaluminoxane. As a result of this reaction, PP chains grow by copolymerization of propylene with the olefin anchored to the GF surface. The reaction occurrence was verified by scanning electron microscopy (SEM) with X-ray disperse energy microanalysis (EDX). Part of the tested samples was subjected to solvent extraction to eliminate the PP physically adhered and then compared to non-extracted samples to determine if PP is chemically bonded to glass. Different morphologies of grafted PP, cluster or layer type, result as the hydroxy-α-olefin concentration increases. In order to characterize the PP-GF adhesion, the interfacial shear strength (ISS) was determined by single-fiber fragmentation tests on model composites for different hydroxy-α-olefin concentrations. The surface treatment induced increases of ISS ranging from 1.7 to 2.1 times as compared to the untreated fibers. The improved interfacial adhesion level was confirmed by SEM observation of the morphology at the fiber-matrix region of cryogenic-fractured samples. This route for polymer grafting onto a glass surface can be suitable for technological applications to improve the fiber-matrix adhesion in glass fiber thermoplastic composites.