Overcoming the Black Plastics Sorting Barrier with a Portable Non-Destructive Specular-Reflection MEMS-FTIR Spectrometer (Preprint)

Abstract

Efficient sorting and recycling of black plastics is challenging because of their strong absorption in the visible and near-infrared (NIR) regions, which makes conventional optical sorting methods ineffective. Classification of black plastics in the NIR range is particularly difficult due to low reflectance and weak spectral contrast among different polymer types. This study presents an approach based on MEMS Fourier Transform Infrared (FTIR) spectroscopy with a specular reflection setup to classify and measure six common black plastic types: polypropylene (PP), polystyrene (PS), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). Nondestructive measurements were performed using developed low-cost, portable and compact MEMS FTIR spectrometers covering the mid-infrared (2130 : 7000 cm-1) and wide infrared ranges (5000 : 1000 cm-1). Specular reflection spectra were collected in the mid-infrared fingerprint region, where absorption bands arise from molecular vibrational modes and are described by the imaginary component of the polymer’s complex refractive index. The core of the system is a MEMS-based FTIR chip that contains a Michelson interferometer with one mirror moved by a micro comb-drive actuator. A very wide spectral range is achieved because the chip is made entirely of silicon and uses free-space light propagation. The design also uses micro-reflective mirrors to couple light into the interferometer without passing it through any material, which avoids absorption losses. Spectral preprocessing involved standard normal variate (SNV) correction followed by mean centering to minimize scattering effects and standardize spectral intensity across samples. These steps enhanced signal consistency and emphasized relevant spectral features, enabling effective multivariate statistical classification with accuracies exceeding 90%. These results demonstrate that MEMS-FTIR spectroscopy in specular reflection mode provides a reliable, non-destructive, and contactless method for identifying black plastics, overcoming the limitations of NIR-based detection, and offering potential for integration into automated recycling and optical sensing systems.