Evaluation of plastic packaging waste degradation in seawater and simulated solar radiation by spectroscopic techniques

Abstract

The goal of this study was to evaluate the degradation of plastic packaging waste in seawater and under simulated solar radiation by different spectroscopic techniques in view of their potential recycling. Selected packaging products made of polypropylene (PP), high-density polyethylene (HDPE) and polystyrene (PS) were weathered in a Solar Box for 4 cycles of 504 h each (21 days), for a total of 2016 h (84 days). At each aging cycle, digital images of the polymer samples were acquired to investigate surface changes in terms of cracking and fragmentation. Furthermore, color measurements were carried out by reflectance spectrophotometry in the visible range (VIS: 400–700 nm). Finally, hyperspectral imaging (HSI) in the shortwave infrared region (SWIR: 1000–2500 nm) coupled with chemometric techniques was applied to monitor the aging process in terms of reflectance spectral variations and to evaluate the possibility of predicting the polymer aging time. The results showed that plastic packaging samples are characterized by different degradation behaviors, such as surface alteration (i.e. fractures, grooves and deep cracks), color variation (i.e. yellowing and discoloration) and fragmentation and microplastic formation, corresponding to changes in reflectance and chemical bonds vibrations. The degradation phenomena are influenced by both thickness and polymer type, being more evident for PP and PS samples rather than for HDPE ones. Concerning the results based on SWIR-HSI, principal component analysis (PCA) highlighted the spectral variation of polymers occurring during the artificial aging process through the different clustering in the score plot. Furthermore, good correlations were obtained between polymer spectral signature and aging time applying Support Vector Machines (SVM) prediction model. The use of the proposed spectroscopic techniques, as well as the development of predictive models based on HSI, contributes to understand the behavior of polymers when exposed to marine environment. This approach represents a promising tool for the definition of the degradation status of polymers and for the selection of marine plastics characterized by a level of degradation compatible with their recycling, fully in line with the principles of circular and sustainable economy.