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 of these materials have been studied for decades, there are key gaps in knowledge on the phototransformations that occur under aqueous conditions. Therefore, it is the goal of this project to characterize the phototransformations of PP and PE in simulated natural water 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) (10 mgC/L Suwanee River natural organic matter) to simulate natural systems. Irradiated plastics were then evaluated for a variety of chemical transformations. It was observed using Fourier transform infrared spectroscopy (FTIR) that oxidation occurredboth in air and aqueous environments, with oxidation in aqueous environments happening at a slower rate. However, in the presence of DOM, indirect photochemistry accelerated oxidation compared to ultrapure water, providing the first evidence of indirect photo-transformations of plastics. The vinyl characteristic and crystallinity, both measured with FTIR, was also monitored as an evaluation of polymer scission. An increase in vinyl groups and crystallinity was observed for all samples indicating that the polymer matrix was rearranged during photodegradation, with an indication that these photoproducts are more likely to occur when oxidation is less favorable. Compared to naturally weathered sample, lab observed transformations were in line with natural sampling supporting the importance of the lab based measurements. Through this work, we have gained a clearer perspective on the chemical weathering of materials found in aquatic plastic debris, which will allow us to predict the behavior of these materials, including the breakdown into microplastics.

Organic matter accelerates photodegradation of polyolefins

ACS Spring 2022

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