Towards Understanding Drivers of Plastic Embrittlement and Fragmentation in Coastal Environments

The fragmentation of plastic in the environment directly influences particle size, buoyancy, transport, and sedimentation dynamics, shaping their fate and interactions within aquatic systems. Notably fragmentation of plastic particles in coastal environments has been observed to be faster than in the open ocean due to high temperature (heat-build up in the sand), high oxygen levels, high exposure to solar radiation (due to little vegetation coverage) and mechanical forces from waves and sediment movements which accelerates the cracking and fragmentation of plastic debris. However, understanding the drivers of fragmentation based on intrinsic properties of plastic—such as brittleness, a key precursor to fragmentation—remains challenging due to the irregular shapes, weathered conditions, and small sizes of environmental samples, which often do not meet the criteria for standardized mechanical testing. Here we present our study where we investigated the fragmentation and brittleness of field-collected plastic particles using a simple laboratory fragmentation test. Our fragmentation test assessed whether beach-sampled plastic particles would break (brittle) or remain intact (ductile) under fixed pressure, enabling the collection of a large dataset (16,322 plastic particles) for statistical analysis of the number of brittle particles on the beach. To further investigate what drives the brittleness of the sampled polypropylene (PP) and polyethylene (PE) particles, we investigated a subsample of PE and PP particles, including both brittle and ductile particles, focusing on their physicochemical traits that might explain their extent of weathering and links to their observed brittleness. We found that the brittleness of sampled plastics strongly correlates with advanced degradation, characterized by very low average molecular weights (as low as 7 kg mol⁻¹) and the presence of oxidation products. Furthermore, we observed that brittle particles were significantly smaller than their ductile counterparts, underscoring the role of fragmentation in shaping the size distribution of plastics on beaches. Additionally, through this fragmentation test, we established, to our knowledge, the first embrittlement criterion for plastics collected from the environment. To further explore fragmentation of plastics in coastal environments, we use a beach swash-zone laboratory simulator to investigate the fragmentation of polymers aged under controlled laboratory conditions. This allows us to correlate the extent of degradation with fragmentation behavior, providing additional insights into the fragmentation processes of brittle plastics in dynamic coastal environments. By highlighting the brittleness of plastic samples on beaches, our study underscores the significant risks of secondary microplastic generation from degraded, brittle plastics in dynamic beach environments. This study also aims to contribute to the overall understanding of the chemical and physical processes that drive fragmentation in aquatic systems, in our case coastal environments, which can aid to inform targeted intervention strategies to prevent microplastics from entering the environment.