Terahertz Detection of Halogen Additive-Containing Plastics

Terahertz (THz) waves are transparent with respect to non-polar substances, such as plastics. The refractive index at THz frequencies and the specific spectral features of plastic materials, such as polypropene (PP), polyethylene (PE), polyvinyl alcohol (PVC) and polyethylene terephthalate (PET) can be used to discriminate waste plastics into each material regardless of color. This is important since colored plastic cannot be identified by the present method of near-infrared (NIR) reflection spectroscopy due to absorption of NIR light by the coloring. In addition, the THz refractive index of acrylonitrile butadiene styrene (ABS) increases in proportion to the content of bromine (Br), which can be used to quantitatively evaluate the presence of halogen additives in waste plastic. We show that the non-contact THz identification of materials and additives in plastics can be an effective method for sorting plastic waste. Safe & efficient waste management is one of the most urgent social requirements for smart city innovation.