High-Permittivity Carbonized Kapok Fiber/Waterborne Polyurethane Composite Films as Frequency-Selective Filtering Layers for Thin-Profile Multiband Microwave Absorption

Thin-profile broadband microwave absorbers must reconcile impedance matching and strong attenuation, two requirements that usually pull in opposite directions. Here, hollow carbonized kapok fibers (CKFs) obtained at 1000 °C were compounded with waterborne polyurethane (WPU) at a mass ratio of 8:2 and consolidated by hot pressing. The film reached a dielectric real part of about 325 and behaved as a frequency-selective layer: it transmitted low-frequency microwaves while reflecting high-frequency ones, governed by the frequency-dependent permittivity, the skin effect, a narrow resonance window, mesopore scattering, and the impedance mismatch with free space. Inserting a 0.62 mm CKF/WPU interlayer between a high-frequency surface layer and a low-frequency bottom layer broadened the effective absorption bandwidth from 4.71 to 9.27 GHz (windows at 3.33–9.30 and 11.6–14.9 GHz), with a minimum reflection loss of −54.8 dB at a total thickness of only 1.53–1.62 mm, covering the whole C band and parts of the S and X bands. This provides a low-cost route to thin, flexible multiband microwave absorbers through a biomass-derived filtering interlayer.