J. Bouchet

Formation of aminosilane-oxide interphases

The aim of this work is to understand the interactions and interphase formation mechanisms between a liquid aminosilane oligomer (γ-APS) and glass, steel or gold surfaces as a function of the pH of the liquid aminosilane. When basic liquid γ-APS (pH 11.4) is applied onto a gold coated substrate, no interphase is detected. Similarly, when liquid γ-APS controlled at pH 8 is applied onto steel or glass substrates and cured, properties are the same as the bulk ones. In contrast, when the liquid γ-APS (pH 11.4) is applied onto steel or glass substrates and cured, an interphase, with chemical, physical and mechanical properties quite different from those of the bulk oligomer, is created between the substrate and the oligomer. Using various analytical techniques (DSC, FTIR, ICP, SEM, AFM, nano-indentation and XPS) it was shown that the amino-silane chemically reacts with and dissolves the oxide or hydroxide layers. Then metallic ions diffuse through the organic layer to form a complex, assumed to be of coordination type with the amine function of the oligomer molecule. These organometallic complexes are insoluble at room temperature and crystallize in the form of sharp needles. The Youngs modulus of the resulting crystal is equal to approximately 5 GPa, i.e.over two orders of magnitude higher than that of the silane. In other words, these organometallic complexes act as a short fiber in a matrix.

Tailoring of the practical adhesion between polyethylene and galvanised steel

The practical adhesion of maleic anhydride grafted polyethylene (MAH-PE) to galvanised steel was studied using 3-point flexure tests, before and after hydrothermal ageing. Before bonding, the electro-galvanised steel was treated with γ-aminopropyltriethoxysilane (γ-APS). The influence of the silane coating thickness and deposition pH on the practical adhesion of MAH-PE to steel was investigated. FT-IR spectroscopy and microscopy enabled to gain understanding of the interphase formation between the silane and the metal substrate. It was found that, at the natural pH of the γ-APS, Zn ions dissolved in the silane coating with subsequent formation of crystals. This interphase could be held responsible for the better durability of the bonds than for silane coatings applied at quasi-neutral pH, for which dissolution of Zn ions was not observed.