Non-invasive optical property-based strategy to distinguish poly(ethylene terephthalate) containers of different recycled content

Abstract

For Poly(ethylene terephthalate) (PET), one of the most widely used and re-used plastics, no definitive analytical method has been established to distinguish between the virgin form and its recycled counterpart. Numerous efforts have been made to achieve this purpose by mainly measuring thermal or mechanical properties and in this sense, the challenge arises from the fact that several factors can affect the characteristics of PET, recycled or not. In this study, the optical properties and their correlations for 28 PET containers with different mass, thickness and content of recycled material have been studied, normalized and parameterized to ultimately differentiate between both forms. Diffuse reflectance UV–Vis (DRS) spectra were unique for each sample and, while near-infrared (NIR) and Fourier-transform infrared-attenuated total reflectance (FT-IR-ATR) provided constant overall parameters regardless of the recycled %, fluorescence and Raman results indicated some differences between non-recycled and recycled PET. Furthermore, the Raman identification accuracy of the two groups was 96.4% and the fluorescence intensity could be related to the content of recycled PET. Moreover, water permeability tests were carried out on some samples, concluding that this property was correlated with the thickness of the plastic and not the recycled %. The study was completed by performing a multivariate analysis (PCA, box plot, K-means and hierarchical clustering analysis) to better understand the correlations within the variables studied and finally confirm the selection of those that allowed for a correct and sequential discrimination of the groups, resulting in a fast and non-invasive strategy that allows for an almost complete vPET-rPET distinction.