Chemically Recyclable Supramolecular Thermosets toward Strong and Reusable Hot-Melt Adhesives

Chemically recyclable thermosets are an ideal substitute for traditional thermosets in the development of a circular economy and sustainable environment. However, the development of efficient and easy-to-achieve chemical recycling strategies remains challenging. Herein, a series of supramolecular thermosets that can be chemically recycled under mild acid conditions at room temperature are fabricated by cross-linking the polyimine polymers with dynamic boroxines (PIx-Boroxine). By tailoring the molar content of boroxines, the PI1.2-boroxine can exhibit a tensile strength of ∼30.6 MPa, a tensile yield strength of 33.0 MPa, an elongation at break of ∼111.6%, and a Young's modulus of ∼679.6 MPa. Because of the dynamic nature of boroxines and imine bonds, the PIx-boroxine supramolecular thermoset exhibits fast stress relaxation behavior, which enables them to have good reprocessing ability. These unique features can also guarantee the PIx-boroxine supramolecular thermosets to be a high-performance reusable hot-melt adhesive. The maximum lap shear strength of the PIx-Boroxine-based hot-melt adhesives in stainless steel bonding adhesive can reach ∼18.6 MPa, which is comparable to that of commercial hot-melt adhesives. Meanwhile, the PIx-Boroxine-based hot-melt adhesives can be reused at least 10 times with only a small amount of reduction in lap shear strength. More importantly, the PIx-boroxine supramolecular thermosets can be easily depolymerized in a 0.1 M HCl/H2O solution at room temperature. Further, the monomers can be easily and efficiently separated by a simple separation procedure. The recovered monomers can also be reused to fabricate new PIx-boroxine supramolecular thermosets without losing their mechanical properties. This work provides a new design strategy to develop high-performance thermosets with easy-to-achieve chemical recyclability, which will contribute to the sustainable development of modern society.