Eco-Friendly Photoreversible Adhesives Derived from Coumarin-Functionalized Epoxy Soybean Oil

The increasing demand for high-performance thermosetting adhesive materials is causing the continuous depletion of fossil reserves and has led to environmental pollution by accumulating recalcitrant plastic waste as thermosetting polymer networks are hard to recycle or reuse. This is particularly true in the case of their use as adhesives, where not only can a crosslinked adhesive not be recovered but multi-material components and devices can be difficult to readily separate, making reuse and recycling complex. Bio-based feedstocks and reusable polymeric materials can start to address these issues and reduce their environmental impact. In this work, we developed sustainable green adhesives which are reversible in architecture (changing from thermoset to non-crosslinked material) by combining bio-renewable epoxidized soybean oil (ESO) and non-toxic coumarins, which can readily undergo reversible [2 + 2] cycloaddition reaction under the appropriate wavelength of UV light. A series of coumarin-modified ESO materials were synthesized and chemically characterized by 1H NMR, 13C NMR, Fourier-transform infrared (FTIR) spectroscopy, and gel permeation chromatography. The extent and kinetics of the polymerization of all systems were monitored and calculated using UV–visible spectroscopy. The photoreversibility of these synthesized polymer networks was also chemically investigated and quantified with UV–visible spectroscopy and FTIR techniques. The physical properties of the materials were tested as well as their ability to heal defects such as scratches on the surface when stimulated by the appropriate UV light. Systems with a more flexible coumarin arm showed a superior adhesion strength with a measurable lap shear strength of 3.1 MPa with a minimum of 0.045 J cm–2 dose of 365 nm UV irradiation and with an excellent reuse efficiency of 94%.