M. Finale, J. Logan, J. McCoy, Y. Lee, S. Chowdhury
New Mexico Institute of Mining and Technology,
United States
Keywords: photothermal nanoparticles, epoxy composites, solar energy, Diels−Alder reaction, dynamic covalent bonds, recycling
Summary:
Due to the irreversible network structure, the recycling of thermosets such as epoxies is considered challenging. Recycling of epoxy is achieved by modifying it with Diels–Alder chemistry which enables depolymerizing the polymer at higher temperature and then repolymerizing upon cooling. However, driving the recycling process using conventional heating can cause long processing time and large temperature gradient in the sample due to their low conductivity and high energy cost compromising the benefits of recycling. Herein, we develop a solar energy driven complete recycling of epoxy by incorporating photothermal nanoparticles such as carbon black and refractory plasmonic titanium nitride nanoparticles into Diels-Alder modified thermosets. Both above nanoparticles can strongly absorb solar light to generate heat inside the polymer allowing precise control of the temperature of the matrix. An optimally designed epoxy/photothermal nanomaterials composites can generate sufficient heat (~140°C) to completely liquefy, reprocess and reshape the epoxy multiple times under concentrated (10 times) sunlight. Using an integrated experimental and theoretical approach we provide a valuable insight into how the choice of nanoparticles, and their dispersion, loading in the matrix, and the sample thickness can influence the sunlight absorption and resulting temperature in the matrix. Our studies also revealed the importance of considering the temperature dependent thermal properties of the matrix which transitions from solid to liquid for accurate prediction of the temperature profile of the matrix. Our study provides an insight into the design of epoxy/photothermal composites which can be sustainably recycled using abundant solar energy.