Plastics have a wide range of applications in the medical device, textile, construction, and automotive industries by virtue of low cost and high performance. Information on the radiation intensity of samples can be obtained by conventional detection methods, while information on other parameters such as phase and refractive index are not available. Terahertz time-domain spectroscopy is a low destructive, non-contact, high penetration and high sensitivity technique with a wide range of applications in material property analysis. In this paper, terahertz time-domain spectroscopy is used to characterize particle size, type and content of blends as well as absorption properties of high-density polyethylene (HDPE), polypropylene (PP), and polyvinyl chloride (PVC). The variations of optical parameters (refractive index and absorption coefficient) of PVC-PP blends in the range of 0.2-1.6 THz with content and frequency are compared and analyzed, and the variations of optical parameters of HDPE, PP, and PVC blended with calcium carbonate (CaCO₃) and Kaolin fillers in the range of 0.2-1.4 THz with content and frequency are characterized and the correlation curves are established. A database is provided for the effective use of the optical properties of plastics in the terahertz range, which can be used as a supplement to the near-infrared spectroscopy plastic detection method and provides a technical means to monitor the filler content in industrial production. In addition, the terahertz time-domain spectroscopy technique can distinguish the absorption of common seawater pollutant methyl orange by different micro-plastics. These phenomena have important implications for the analysis of the environmental pollution of micro-plastics.

PP, PVC and HDPE plastic particles with different particle sizes (less than 74 μm and 178 μm) are prepared. First, PVC and PP with particle size of less than 74 μm are mixed in a mortar and pressed into PVC-PP mixtures with different proportions. CaCO₃ and Kaolin powder with the mass fraction of 0%-50% is added to PVC, PP, and HDPE, respectively. In the above experiments, 0.2 g mixed powder samples are mixed in a mortar for 20 min and pressed with a hydraulic press at 10 MPa for 2 min. The diameter of the sample is 13 mm and the thickness is 1-2 mm. The samples are tested by terahertz time-domain spectroscopy (THz-TDS) at 22 ℃ and 2% humidity to avoid moisture entering the samples to influence the experimental results. The liquid samples are placed in a liquid bath with a thickness of 0.1 mm. The THz-TDS system is used in the transmission mode without considering the influence of multiple reflection of terahertz pulses.

The terahertz spectral lines of PP, HDPE and PVC with particle size of less than 74 μm are different in the range of 0.2-1.6 THz. The scattering effect becomes stronger when the particle size is close to the wavelength, and the absorbance of the samples at the same frequency is higher (Fig. 2). To avoid the influence of scattering effect of plastics, we mix the PVC and PP with particle size of less than 74 μm, and we find that the absorbance and refractive index increase with the increase of the PVC mass ratio (Fig. 3). The utility of terahertz elements made from the blends in the terahertz range can be improved by the curve fitting of the mixing results. Blends of three plastics (PVC, PP and HDPE) with two fillers (Kaolin and calcium carbonate) are characterized separately (Fig. 4). The dependence of optical parameters on filler concentration is characterized by establishing the correlation curves (Fig. 5), where CaCO₃ has a greater effect on the samples than Kaolin. The high absorbance of PVC could be clearly distinguished from those of the other two plastics, and the effects of filler on HDPE and PP are closer but distinguishable by observing the fitting parameters (Tables 1 and 2). Finally, the absorption effect of plastics is explored. The adsorption phenomenon of micro-plastics could be distinguished by THz-TDS (Fig. 6), and the absorption effect of PVC is stronger than those of the other two plastics. The absorption reaction within 24 h could be distinguished by THz-TDS (Fig. 7), and the smaller the particle size of the plastic, the stronger its absorption capacity (Fig. 8).

In this paper, HDPE, PP, PVC and their filler-containing compounds are characterized by the terahertz time-domain spectroscopy. The characterizations of refractive indices and absorbance of PVC-PP blends compared to those of single polymers are useful for the fabrication of terahertz optical components. Blends made with different proportions of fillers can be identified by the terahertz time-domain spectroscopy technique. In addition, the time-domain spectra and absorbance of three plastics are related to the particle sizes of the constituent samples. The terahertz time-domain spectroscopy technique can detect different absorption capacities of three plastics and the differences in the absorption capacity to methyl orange of plastics at different time or with different particle sizes. Our study highlights the feasibility and promise of the terahertz time-domain spectroscopy for the identification and detection of plastics and their mixtures, and provides a new characterization method for the absorption behaviors of micro-plastics in the environment.