E. Rahman, S. BinAhmed, P. Keyes, C. Alberg, S. Godfreey-Igwe, G. Haugstad, B. Xiong
University of Minnesota, Twin Cities,
United States
Keywords: wear, nanoplastic, UV-aging, lateral force microscopy
Summary:
Despite the detrimental effects of nanoplastics (NPs) on human health and the environment, the mechanisms underlying NP release via abrasive nanoscale wear remain unclear. Moreover, the changes in plastics resulting from prolonged sunlight exposure in terrestrial environments and their subsequent effects on nanoscale NP release are unknown. To address these research gaps, we developed a novel protocol based on a Lateral Force Microscope (LFM) to mechanistically understand the abrasion process of sand particles (replicated using a 300nm Si probe) on pristine and UV-aged Low Density Polyethylene (LDPE) films under environmentally relevant load and length scales. Prolonged sunlight exposure of LDPE films was simulated using a QUV accelerated weathering chamber following ASTM G154 protocol, where the LDPE films were subjected to a total irradiation of 229, 448, 596, 860, 1124, and 1860 Wh/m2 (@304 nm). Preliminary results indicated the wear mechanism transition occurred abruptly between 448 and 860 W h/m2. The wear mechanism before 448 W h/m2 consistently resembled that of pristine LDPE, and after 860 W h/m2, it gradually transitioned to cutting wear mechanism. Inspired by the research on metals, we proposed a new parameter (β’), which is the ratio of the piled-up materials at the end of the scratch to the total pile-up volume, to express the wear mechanism quantitatively. The transitions in wear mechanisms could be correlated with transitions from ductile to brittle phases (i.e., embrittlement) in semicrystalline plastic films under UV-aging, as confirmed by surface chemical, molecular, morphological, and nanomechanical characterization. Results from Attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR), contact angle measurements, and gel permeation chromatography (GPC) indicated earlier surface oxidation and chain scission, rather than the wear transition. In comparison, the sudden increase in crystallinity measured by differential scanning calorimetry (DSC) occurred between 596 and 860 W h/m2, during which surface nanomechanical properties (elastic modulus and hardness) also began to increase. Combined characterization results suggest that chemi-crystallization and nanomechanical properties are the dominant contributors to the transition in the wear mechanism, rather than surface chemistry or chain scission. Following the trend of the wear mechanism transition, wear and NP release rates also showed a rapid jump around 596 – 860 Wh/m2, likely due to the transition. Thus, we divided the evolution of NP release rates due to UV-aging into two steps: before 596 Wh/m2 (ductile regime) and after 596 Wh/m2 (brittle regime). Wear and NP release rates in each regime were primarily controlled by surface oxidation and chain scission, but the distinction of these two regimes was determined by crystallinity. These fundamental insights inform the chemical-structure-degradability relationships for LDPE under combined stressors, improving our understanding of nanoplastic release rate as a function of UV-aging in the environment.