Molybdate doping of networks in epoxy–silica hybrids: Domain structuring and corrosion inhibition

Epoxy–silica hybrids were prepared from a silane-functionalized resin mixture of two diglycidyl ether bis-phenol A oligomers with different molecular weights, using molybdate anions as dopants for the siloxane domains.The functionalization of the epoxy resin was carried out using suitable trimethoxysilane compounds, respectively, containing amine and mercaptan reactive groups.The molybdate anions were added as “bound” species in the form of salts produced from pre-reactions with a fractional amount of the cycloaliphatic amine hardener, and as “free” molybdic acid. The co-continuous silica domains within the epoxy networks were produced by the incorporation of pre-hydrolysed mixtures of tetraethoxy silane and γ-glycidoxyl trimethoxysilane into the precursor solution used for the production of epoxy–silica hybrids.The addition of a source of molybdate anions to the hybrid precursor solution gave rise to the formation of a denser siloxane network and to a substantial increase in the Tg of the organic phase. This resulted also in a large increase in the rubbery-plateau modulus, and in a substantial reduction in both the rate of solvent uptake and the equilibrium amount of solvent absorption.It was shown that the presence of organic–inorganic hybrid networks allows the “entrapped” molybdate anions to be released at a slow and controlled rate. Furthermore, when the precursor solution mixtures for the formation of such doped networks are used as coatings for ferrous metal substrates, the molybdate anions slowly migrate to the interface and provide an enhanced mechanism for the passivation of the metal substrate. The enhancement of the corrosion protection capability of the coatings was found to be greater when the molybdate anions were added in “bound” form, as amine molybdate, than as “free” molybdic acid.