Thermosetting polymers such as epoxies exhibit excellent mechanical properties and have wide applications ranging from adhesives to structural materials. However, recycling of epoxy is difficult because of their irreversible cross-linked network structure. We have developed a reversible epoxy which can be recycled using visible light. We employed Diels-Alder chemistry based on furan/maleimide reactions to develop epoxy with reversible cross-linking. At higher temperatures (above~ 120°C), the Diels-Alder reaction reverses, resulting in depolymerization of the epoxy and can polymerize again as it cools down. Most importantly, we incorporated photothermal refractory plasmonic titanium nitride (TiN) nanoparticles in the reversible epoxy. These nanoparticles can efficiently and rapidly convert solar light to nanoscale heat to initiate the retro Diels-Alder reaction. Interestingly, we could use four times of solar intensity to increase the temperature of epoxy/nanoparticles composite to above 120<sup>0</sup>C within 1 minute time frame and efficiently depolymerize epoxy showing flow like behavior. The polymer composite can cool down to room tempeature and resolidify within one minute of turning off light. However, increasing the temperature further (~above 150<sup>0</sup>C), the effect of side reactions such as homo-polymerization of maleimide becomes pronounced that causes loss of reversibility of epoxy. We are performing a detailed studies of the side reactions and comparing their kinetics with Diels-Alders reactions to develop a reversible epoxy which can be recycled multiple times without compromising any of its desirable mechanical properties.

Solar light enhanced recycling of thermoset polymers

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ACS Fall 2022

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