Photodegradation of polyolefin thin films in simulated freshwater conditions

Abstract

Polypropylene (PP) and polyethylene (PE) are commonly used polyolefins in a variety of applications, which have resulted in their accumulation in the environment. Once in the environment, these polymers undergo various chemical and physical transformations as the result of environmental stressors such as sunlight. While photodegradation has been studied for decades, there are key gaps in knowledge on the phototransformations of polyolefins that occur under aqueous conditions. Therefore, the goal of this study is to characterize the phototransformations of PP and PE in simulated freshwater conditions. Polymer thin films were irradiated with 254 nm and 350 nm UV light in air, ultra-pure water, and solutions of dissolved organic matter (DOM) to simulate natural systems. Irradiated plastics were evaluated for oxidation and chain scission. It was observed using Fourier transform infrared spectroscopy (FTIR) that oxidation in aqueous environments happened at a slower rate compared to oxidations in air. However, photo-oxidation was accelerated in the presence of DOM compared to ultrapure water, with singlet oxygen and hydroxyl radical causing varied amounts of degradation depending on the polymer. The vinyl characteristic, a chain scission product, revealed an increased yield but the reaction rate showed that these photoproducts were more likely to occur when oxidation is less favorable. Compared to naturally weathered samples, lab observed transformations were on par with naturally degraded samples and support the importance of the in-lab measurements. This work quantifies the extent and rate of photodegradation pathways in PP and PE to demonstrate the importance of photodegradation in aquatic systems.