Thermomechanical Characterization of Recyclable Epoxies By the Diels-Alder Reaction

Epoxy products are typically non-recyclable and difficult to remove. Synthesis and characterization of recyclable epoxies crosslinked by a Diels-Alder (DA) reaction were investigated. The recyclable epoxies were synthesized between maleimide and furan functional group to form cyclo-adducts (e.g., polyetheramines and epoxides precursors). Different architecture (e.g., 4-arm or 6-arm) and stoichiometric ratios of furan to maleimide groups were examined to determine the optimum functionality of the recyclable epoxy. The epoxies demonstrated calorimetric glass transition temperatures (T_g) in the range 0 to 30°C depending on molecular composition, and a high-temperature (~120°C) endotherm corresponding to the retro-DA (rDA) reaction was observed. In addition to the low-temperature glassy phase, rheometry detected a rubbery plateau at moderate temperature followed by a rapid decrease in modulus indicative of liquid behavior. This high temperature (above ~140°C) decrease is a consequence of the rDA reaction depolymerizing the network. Plasmonic titanium nitride (TiN) nanoparticles have significant visible light absorption and were incorporated into the recyclable epoxies for targeted and instantaneous triggering of the rDA reaction by a high-intensity light source. The TiN filler (<1.0wt%) slightly increased the modulus of the epoxies and permitted localized heating. The epoxy was further tested by looking at its reusability. Two glass slides were attached using the reversible epoxy; and the sample was then heated using a high-intensity light source. This light was converted to heat in the epoxy by the TiN nanoparticles, causing the epoxy to soften and the slides to separate under stress. The rDA reaction was evident after separation, as epoxy remained on both glass slides. This showed that the epoxy did not simply succumb to adhesive failure. After the slides separated, they were capable of reattachment by exposing the sample to the light source and pressing the slides together. These healed slides were then able to undergo the same stress as before the photothermal test without failing.