Tough, high-strength, flame-retardant and recyclable polyurethane elastomers based on dynamic borate acid esters

With the wide application of polyurethane elastomers, it is necessary to develop recyclable, fire-retardant polyurethane elastomers with great mechanical properties to comply with industrial requirements. Herein, we fabricated a flame-retardant, recyclable, strong yet tough polyurethane elastomer (PIDB-1) based on dynamic borate acid esters. The introduction of phosphaphenanthrene and boron-containing groups endow PIDB-1 with great flame retardancy, as reflected by it achieving the vertical burning (UL-94) V-0 rating. The PIDB-1 film shows high visible light transmittance, and its transmittance reaches 90 % at the wavelength of 800 to 900 nm. The tensile strength of PIDB-1 is 54.9 MPa, and its toughness reaches 207.8 kJ/m³, indicative of superior mechanical properties. Meanwhile, the dynamic borate acid esters allow the PIDB-1 elastomer to possess physical and chemical recyclability. When using PIDB-1 as a polymer matrix for carbon fiber-reinforced polymer composites, the carbon fibers can be fully recycled. This work provides an integrated design strategy for creating transparent, flame-retardant, recyclable polyurethane elastomers combining high strength and toughness based on dynamic borate ester bonds, which is expected to find wide applications in different industries.