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 silane pH, the thickness of the silane coatings is also important in the performance of steel/silane/polymer bonds. When considering issues of corrosion resistance, a thick, homogeneous silane layer may be of benefit as a water barrier. Silane coatings, however, are brittle compared to the steel substrate, in particular if they are applied as a thick coating. The coating may thus crack during further material forming or in use if high strains are expected. As a consequence, and in order to optimise adhesion together with the mechanical behaviour of the silane interphase, a new hybrid coating was proposed, based on γ-APS blended with epoxy-functional hyperbranched polymers. The toughening effect of the hybrid coatings was shown to be promising. On the other hand, adhesion tests revealed that hyperbranched polymers had a negative effect on the initial adhesion build-up, but showed a positive effect when ageing is considered. The results were then scaled up to evaluate the potential use of steel cords in polyethylene matrices. Pull-out tests performed on cords showed results consistent with previous findings in wire reinforced polyethylene, with the increased interlocking effect of the twisted cords. In parallel, impregnation of the cord by the polymer was investigated in order to fulfil industrial requirements for the final application product. Due to the high viscosity of polyethylene, full impregnation of the compact cords was very difficult to achieve. Low viscosity copolymers showed promising results by easily impregnating compact cords, but the low shear strength of the used copolymers limits their use in bonding to steel and to the HDPE. Other cord fillers, such as epoxies could be used, but their material bending ability may be reduced, and the mechanical role of the cords be compromised. Solutions should be selected according to the application requirements of these materials.
Section de sciences et génie des matériaux
Faculté des sciences et techniques de l'ingénieur
Institut des matériaux
Jury: Heinrich Hofmann, Hans Jörg Mathieu, Daniel Mauer, Véronique Michaud, John Nairn
Public defense: 2005-10-28
Record created on 2005-08-31, modified on 2016-08-08