Hardener Isomerism and Content of Dynamic Disulfide Bond Effect on Chemical Recycling of Epoxy Networks

Abstract

Nowadays, there is an important concern in the scientific community related to the end-of-life products derived from polymeric matrix composites. In this regard, covalent adaptable networks and, more specifically, the disulfide bond-based ones are a promising approach to develop composite parts able to be dissolved in a specific solvent, thus regaining the continuous fiber reinforcement. In this work, the effect of hardener isomerism, using 2-aminophenyl disulfide (2-AFD) and 4-aminophenyl disulfide (4-AFD), and amine/epoxy ratio (1.0–1.2) was studied to optimize the chemical recycling capabilities at different temperatures. Results confirmed the need for using hardener excesses for dissolving these vitrimers. Networks based on 2-AFD were dissolved in considerably lower times than the 4-AFD ones, which is interesting since the latter one is quite more used for this purpose and currently way more expensive. In this context, a composite laminate, reinforced with six layers of carbon fiber fabric, was manufactured as the proof-of-concept.

Epoxy Thermosets Designed for Chemical Recycling

“…Recently, a crucial and detailed study about the importance of the composition ratio of the reactants and selection of the monomers for disulfide‐based epoxy thermosets was reported by Prolongo et al [136] . In this study, DGEBA was cured at 160 °C for 6 h by employing two hardeners, such as h19 and h20 , and the effects of these regioisomers on the structural properties and recycling of the network were investigated.…”

“…These reductions were attributed to a less compact nature of the networks consisting of ortho isomers compared to the para equivalents due to the lower phenyl ring interactions. In addition, they incorporated carbon fiber fabric layers into the epoxy system and recovered them after immersing the thermosetting material in a 2ME solution in DMF at 100 °C for 24 h. The structure of the chemicals regained after depolymerization has not been analyzed [136] …”

“…Also, they showed that it is possible to depolymerize the epoxy network by immersing it into a dithiothreitol (DTT) solution in DMF. [135] Recently, a crucial and detailed study about the importance of the composition ratio of the reactants and selection of the monomers for disulfide-based epoxy thermosets was reported by Prolongo et al [136] In this study, DGEBA was cured at 160 °C for 6 h by employing two hardeners, such as h19 and h20, and the effects of these regioisomers on the structural properties and recycling of the network were investigated. Also, the influence of the NH 2 /epoxy ratio on the system was explored.…”

“…In addition, they incorporated carbon fiber fabric layers into the epoxy system and recovered them after immersing the thermosetting material in a 2ME solution in DMF at 100 °C for 24 h. The structure of the chemicals regained after depolymerization has not been analyzed. [136] Kessler et al incorporated disulfide bonds into the epoxy systems to synthesize recyclable liquid crystals. [137] The epoxy monomer was synthesized by reacting 4,4'-dihydroxybiphenyl with epichlorohydrin to yield 4,4'-diglycidyloxybiphenyl (BP).…”

Regioselective photocycloaddition for light-stabilised dynamic materials design

“…[42][43][44] Specifically for covalent dynamic polymer materials, the exact nature of the molecular network connectivity before and after (re)processing can be significantly altered. 45,46 For instance, networks based on the bench-mark thermoreversible furan/maleimide system have been observed to undergo chemical aging when subjected to repeated material (re)processing, resulting from differences in network rearrangement caused by endo/exo isomerisation of the dynamic crosslinks (Fig. 1b, left).…”