Valorisation of the urban plastic waste in high-quality recyclates is an imperative challenge in the new paradigm of the circular economy. In this scenario, a key role in the improvement of the recycling process is exerted by the optimization of waste sorting. In spite of the enormous developments achieved in the field of automated sorting systems, the quest for the reduction of cross-contamination of incompatible polymers as well as a rapid and punctual sorting of the unmatched polymers has not been sufficiently developed. In this paper, we demonstrate that a miniaturized handheld near-infrared (NIR) spectrometer can be used to successfully fingerprint and classify different plastic polymers. The investigated urban plastic waste comprised polyethylene (PE), polypropylene (PP), poly(vinyl chloride) (PVC), poly(ethylene terephthalate) (PET), and poly(styrene) (PS), collected directly in a recycling plastic waste plant, without any kind of sample washing or treatment. The application of unsupervised and supervised chemometric tools such as principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) on the NIR dataset resulted in a complete classification of the polymer classes. In addition, several kinds of PET (clear, blue, coloured, opaque, and boxes) were correctly classified as PET class, and PE samples with different branching degrees were properly separated.

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The abundance of microplastics in the environment poses a constant threat to all parts of the ecosystem, and the scientific community is called upon to help solve the problem. Numerous studies have been published for microplastic analysis, especially in the last decade, with vibrational spectroscopy being the preferred method. According to recent literature, portable spectrometers operating in the near-infrared (NIR) range are being used for the analysis of different types of polymers, and this technique has recently found its way into the analysis of microplastics as a good alternative to expensive and complicated benchtop instruments, such as Fourier-transform infrared (FTIR) spectrometers. The aim of this study is to investigate and evaluate research trends, leading publications, authors, countries, and limitations of the use of NIR spectroscopy in microplastics research, with a comparison to the established FTIR technique.