Aminosilanes and hyperbranched polymers for adhesion tailoring between metallic oxides and polyethylene

Bonding between polyolefins and steel is widely found in applications as diverse as steel wire/cord reinforced polymers, and automotive components where steel and thermoplastics are commonly used. As polyolefins are highly unreactive, because of their low polarity, and present poor adhesion to polar, hydrophilic substrates, it is necessary to optimise the interface for adhesion promotion. An optimised interface also affects corrosion resistance which can play an important role in practical applications, where a long in-service life has to be guaranteed. The present work focused on the tailoring of adhesion between steel or galvanised steel and high density polyethylene. A maleic anhydride grafted polyethylene was selected as a base polymer, and flat substrates, wires, and cords as the metallic substrates. Particular attention was paid to metal treatments based on silane. The first stage of the work concentrated on gaining a thorough understanding of the interactions and interphase formation mechanisms taking place between a liquid aminosilane oligomer (γ-APS) and steel surfaces, as a function of the pH of the liquid aminosilane. When liquid γ-APS controlled at pH 8 was applied onto steel substrates and oligomerised, properties were the same as the bulk ones. In contrast, when the liquid γ-APS (pH 11.6) was applied onto steel substrates and cured, an interphase was created between the substrate and the oligomer, with chemical, physical and mechanical properties quite different from those of the oligomer. Using various analytical techniques (DSC, FTIR, ICP, OM, SEM, AFM, nano-indentation and XPS) it was shown that the amino-silane chemically reacted with and dissolves the oxide or hydrated layers. Then metallic ions diffused through the organic layer to form a complex, assumed to be of a coordination type, with the amine function of the oligomer molecule. These organo-metallic complexes were insoluble at room temperature and crystallized into the form of sharp needles. The Young's modulus of the resulting crystal was equal to approximately 5 GPa, i.e. seventy times higher than that of the silane. In other words, these organo-metallic complexes may act as a short fibre in a matrix. As a result a new schematic description of the mechanisms of interface formation was proposed. The practical adhesion of maleic anhydride grafted polyethylene to galvanised steel was then studied by various testing methods, including 3-point flexure tests and fibre pull-out tests. Compared to non-treated steel substrate, which initially performed well, silane coatings revealed their potential with ageing. Indeed, it was shown that silane coatings applied at basic pH performed better than those applied at quasi-neutral pH. This observation was contradictory to most results reported in the literature so far, but can be explained by the creation of a tailored interphase, as proposed by the model developed for silane coatings applied at basic pH. Besides the influence of sila