Toughening of epoxy resin by in-situ generated silica reinforced rubbery inclusions

The performance of perfluoroether oligomers as toughness modifiers for a diglycidyl ether of bisphenol-A was enhanced with the incorporation nano-structured silica.

This was achieved by generating the inorganic oxide in-situ within the toughening perfluoroether-rich phase, separated from the precursor perfluoroether/epoxy mixture.

The ability to achieve such thermodynamically unfavourable repartition of the silica was realised by controlling the hydrolysis, aggregation and growth of the inorganic oxide in the sol-gel process and by appropriate telechelic modifications of the perfluoroether oligomers. The control of the production of silica was achieved by catalysing the hydrolysis reaction of the silicon alkoxide with p-toluene sulphonic acid, which was also found to inhibit the condensation reaction at room temperature. This, therefore, provided the opportunity for the two reactions to occur separately, enabling the very advanced stage of hydrolysis to take place before the condensation reaction is induced by neutralising the acid with a basic salt. A combination of two different telechelic modified perfluoroether oligomers was used to produce the final toughened epoxy resin, containing the silica reinforced perfluoroether inclusions. The first oligomer was alkoxysilane functionalised in the presence of the silicon alkoxide, which acts as a reactive diluent. In the curing of the epoxy resin, the oligomer and the evolving inorganic-oxide clusters were precipitated at room temperature so that the two components could be generated within the same particles. The second oligomer was allowed to be chain extended with epoxy resin in the presence of triphenylphosphine as a catalyst. This oligomer would then phase separate later at an elevated temperature and would encapsulate the initial particles formed by the first oligomer/silica mixture. The objective of the second oligomer was to prevent the formation of a silica-rich phase at the interfacial boundaries between the particles and the epoxy matrix, which was found to have very detrimental effects on mechanical properties.

The final epoxy-perfluoroether-silica hybrid system was found to displayed improvements in properties, which included Tg, flexural strength, flexural modulus, and toughness.