Microwave-Induced Construction of Flame-Retardant TPU Foams with Adjustable Sensitivity for Piezoresistive Sensing

CNTs feature ultrahigh electrical conductivity and multiwall structure, contributing to the favorable interaction with microwaves, and accordingly evoke drastic energy conversion to the form of heat upon microwave irradiation. Herein, CNTs were ball milling assembled at the surface of expandable thermoplastic polyurethane/black phosphorus (e-TPU/BP) beads, where BP was used as a flame retardant, and served as the "interfacial sensitizer", resulting in exceptional selective heating to inducing bubble formation in a continuous conductive network of TBC composite foams under the microwave field setting. The sinter-molded TBC composite foam possesses a high sensitivity of 550 kPa–1 over 0–0.4 MPa of compressive stress with an ultralow CNT content of 0.5 wt %, stability of over 1000 cycles, and durability of 5000 s. Moreover, BP also provided excellent flame retardant and antidripping properties at 2 wt % loading. Therefore, by utilizing the intense response of CNTs to microwaves, a universal approach was developed not only for foam molding but also for the functionalization of thermoplastic materials.