Study of the surface, rheological and morphological characteristics of isotactic polypropylene microplastics aged in a photothermal oxidation chamber

Abstract

The aim of this study was to investigate, in addition to degradation processes, the rheological, crystalline, and molecular dynamics changes in isotactic polypropylene microplastics with average sizes of 0.5 and 2 mm, aged in a photothermal oxidation chamber for 166 days. In addition to commonly used techniques in microplastics research, time-domain nuclear magnetic resonance (TD-NMR) in fast field cycling and fixed field modes, was employed for the first time in the study of these materials, providing non-destructive information that supports other results. Concurrently with X-ray diffractometry (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), rheometry, Fourier-transform infrared spectroscopy (FTIR), surface wettability, and scanning electron microscopy (SEM) techniques, two antagonistic phenomena were observed over the aging period. One associated with the surface degradation of microplastics and increased molecular mobility, and the other related to morphological changes in the internal layers, leading to increased rigidity. These phenomena occur slightly differently depending on the average particle size. Through multivariate data analysis with principal component analysis (PCA) and hierarchical cluster analysis (HCA), it was possible to determine that the degradation process intensifies after 62 days of simultaneous exposure to ultraviolet radiation, temperature, and oxidizing atmosphere. The obtained data highlight that even synthetic polymers with long degradation periods can undergo, in the form of microplastics, initial morphological changes in just 15 days and degradation processes within a few months of exposure to weathering conditions.