Photoaging of Polyvinyl Chloride and Polystyrene Under UVA Radiation in Diverse Environmental Conditions

Plastic pollution has become a major environmental issue due to extensive use, durability, and poor waste management practices. Once plastics are released into the environment, they undergo aging processes that include mechanical abrasion, temperature changes, and exposure to oxygen and UV radiation, causing them to break down into microplastics. These microparticles can accumulate in the environment and present unknown health risks to humans and animals. Plastics aging studies have been conducted using techniques including thermal, chemical and photodegradation (frequently with UVB or UVC radiation). Physical changes have been identified, such as brittleness, discoloration, surface roughness, and cracks. Additionally, chemical oxidation and the formation of new functional groups such as carbonyl and hydroxyl moieties have been reported. However, UVA radiation is not as well studied, even though it is the primary type of UV radiation reaching the Earth’s surface. This study investigates the photoaging of polyvinyl chloride (PVC) and polystyrene (PS) under UVA radiation (365 nm), after 108 days of exposure using a custom-built setup delivering high irradiance conditions at 78 W/ m2, corresponding to the equivalent of over two years of natural sunlight at mid-latitudes and around 5 years in polar conditions. Four different simulated environments were studied: air, seawater (SW, 0.56 M NaCl), freshwater (FW, 0.01 M NaCl), and ultrapure water (UW). Physical changes were monitored through scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), while chemical changes were evaluated by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Changes in surface reactivity were assessed through potentiometric titrations and modeling to quantify newly formed acidic groups and their pKa values. Results indicate that PVC exhibited significant photostability in air, FW, and UW, with no detectable physical or chemical changes. However, SW-aged PVC showed a temporary increase in the vinyl index (1620 cm⁻¹) and slight alterations in surface protonation behavior at high pH. However, this did not support the formation of distinct proton-active sites. In contrast, PS underwent noticeable photo-oxidation across all conditions, evidenced by yellowing, and spectroscopic analyses that showed meaningful changes, especially increases in the carbonyl (1718 cm-1) and the hydroxyl (3458 cm-1) indices and decreases in aliphatic C–H (2919 cm⁻¹) and aromatic ring deformation (752 cm-1) band intensities. Potentiometric titration data fit well by invoking three acidic surface sites with pKa values of ≈ 5.49, 7.76, and 9.98, corresponding to carboxylic acid groups, phenolic groups, and hydroperoxides, respectively. No morphological changes were observed in SEM-EDS. These findings demonstrate the influence of polymer composition and ionic medium in UVA photoaging. While PS becomes chemically reactive and may act as a pollutant carrier, PVC persists with minimal surface modification, representing primarily a physical hazard in aquatic environments.