Porous solvent-free polyurethane coating with 3D conductive network for efficient electrothermal/photothermal conversion and intelligent temperature sensing

Achieving lightweight, high electrical conductivity remains a central challenge for flexible conductive coatings, especially within environmentally friendly solvent-free processing systems. Herein, a porous solvent-free polyurethane (SFPU) coating with a three-dimensional (3D) conductive network was successfully constructed employing thermally expandable microspheres (TEMs) as foaming agents and carbon nanotubes (CNTs) as conductive fillers. Owing to the volumetric expansion of TEMs during curing, the CNTs were squeezed into the gaps between the microspheres. As a result, the electrical conductivity of the coating with 4 wt% TEMs increased by 16.3 times and the density decreased by over 61% compared with non-foamed references, showing light-weight, flexibility (Strain of 125%) and sensitive conductivity (0.88 S/m) characteristics. At a low voltage of 5 V or a weak light intensity of 150 mW·cm −2 , the temperature of the coating surface reached 160 °C and 68 °C, respectively, and maintained for at least 30 min, showing efficient electrothermal and photothermal conversion performance. Moreover, this coating also exhibited a linear and reproducible resistance–temperature relationship (TCR ≈ -0.95%·°C −1 ), enabling real-time temperature monitoring and showing potential for fire hazard warning applications. Owing to the rough surface and porous conductive structure, the coating also exhibited multiple protective functionalities, including waterproofness, effective UV-blocking performance and electromagnetic interference shielding. This work presents a scalable route to flexible multifunctional conductive coatings and introduces a general porous structural design strategy for environmentally friendly coating systems.