Terahertz Identification of Black Pigment Concentration in Plastics

Automated sorting of polymers commonly relies on sensors operating in the near-infrared (NIR) and short-wave infrared (SWIR) spectral ranges, but black pigmented plastics remain difficult to sort and identify because carbon black (CB) strongly absorbs light and suppresses distinctive spectral features. This preliminary work investigates terahertz time-domain spectroscopy (THz-TDS) for the characterization of black high-density polyethylene (HDPE) with different carbon black concentrations. Black HDPE samples were fabricated by melt compounding and compression molding, then measured in transmission using a THz-TDS system in a nitrogen-purged chamber. The measured spectra show increasing attenuation and phase delay with increasing carbon black concentration, with the strongest amplitude separation observed below 1 THz. In addition, the transmission at 0.803 THz exhibits an approximately linear dependence on carbon black concentration, indicating that a single-frequency feature can be used for CB concentration estimation. These results show that THz-TDS can distinguish pigment loading differences in black HDPE and may support improved characterization approaches for better product circularity in plastic recycling.