Synthesis and characterisation of new sulphur-containing epoxy networks

Abstract

The position of sulphur within a cured epoxy amine network and its impact upon the thermal, mechanical, chemical and physical properties has been investigated in this study. Sulphur-containing epoxy resins, diglycidyl thioether of bisphenol A (DGTEBA) and diglycidyl ether of dithio diphenyl (DGTED) have been synthesised, cured and characterised and then compared with the benchmark epoxy resin, diglycidyl ether of bisphenol A (DGEBA). DGTEBA has two sulphur atoms located terminally to the epoxide groups, whilst DGTED has a sulphur atom located centrally between two phenyl groups. Evaluation of the properties and cure mechanism using 4,4′-diamino diphenyl sulphone (4,4′-DDS) as the hardener showed that when the sulphur was terminally located, the glass transition temperature, cure conversion, thermal stability, yield strain and stress decrease substantially, whilst the rate of cure and methyl ethyl ketone (MEK) ingress was comparatively higher. Near-infrared spectroscopy revealed that a competing hydrogen abstraction reaction mechanism deactivated the epoxide ring to nucleophilic attack to produce a heterogeneous network and poorer properties. In contrast, the properties and network structure of the DGTED-cured network, which had sulphur placed centrally between the diphenyl groups were similar to DGEBA except that MEK fluid ingress was reduced. The DGTEBA epoxy was also cured with other amines of varying reactivity to reveal that the hydrogen abstraction mechanism was dependent upon the reactivity of the hardener. Less reactive hardeners were dominated by the hydrogen abstraction mechanism, whilst the more reactive amines displayed a step-growth mechanism more typically associated with epoxy amine cure.