Three-dimensional transient thermal barcode for waste plastic identification

Low global recycling rates of plastics are constrained by the lack of rapid, accurate, and scalable technologies for waste sorting. Here we report a standoff Transient Thermal Barcode (TTB) technique that identifies plastics using their mid-infrared (IR) absorption-induced thermal signatures. When the IR irradiation wavenumber matches a polymer absorption band in the molecular fingerprint regime, the non-radiative decay produces localized heating, yielding a transient hotspot that can be recorded with a thermal camera. Scanning with a tunable mid-IR source produces thermal spectra that closely track conventional FTIR spectrum. We show that a set of six mid-IR wavelengths can generate distinct thermal barcodes that enable the unique identification of all six major commercially relevant plastic classes in waste plastics. These results show the potential of mid-IR based non-contact plastic identification according to their resin codes for automated sorting in recycling.