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Comparing outdoor and accelerated weathering of polystyrene, polyethylene and polypropylene – predicting the degradation of microplastics in the environment
Original title: Comparing outdoor and accelerated weathering of polystyrene, polyethylene and polypropylene – predicting the degradation of microplastics in the environment
Summary
Scientists tested how three common plastics (used in things like packaging, bottles, and containers) break down into microplastics when exposed to sunlight, using both real outdoor conditions and sped-up lab simulations. They found a way to accurately predict how long it actually takes plastics to degrade outdoors based on faster lab tests—and discovered that different plastic types break down at very different rates depending on temperature and sunlight exposure. This matters because it helps researchers better estimate how long microplastics linger in our environment (and potentially in our food and water), which is a key step toward understanding our real-world exposure to them.
Accelerated weathering becomes increasingly popular in microplastic research to produce defined, environmentally relevant reference particles and determine their degradation kinetics. To predict the persistence of environmental microplastics, models must be developed that correlate real-life with accelerated weathering. Therefore, we compare long-term accelerated and outdoor weathering studies using amorphous polystyrene, semi-crystalline low-density polyethylene and polypropylene particles. While mechanical stress was similar in all experiments, temperature and irradiance varied outdoors. In all cases, the particles weather due to photooxidation inducing chain scissions, polar functional groups and crosslinks. Macroscopically, particles degrade in three stages: surface abrasion, fragmentation and particle aggregation. By introducing a time-dependent acceleration factor composed of the Schwarzschild and Arrhenius laws, we derived a model with two parameters - the material-specific activation energy E A and the Schwarzschild coefficient p. While p was around 0.5 suggesting decreasing photon efficiency with increasing flux for all probed polymers, E A varied between 10 kJ/mol (PS) and 26 kJ/mol (LDPE, PP). Due to the E A differences, identical accelerated weathering durations correspond to markedly different outdoor timescales depending on polymer type and formulation. Using publicly available weather data, the model captures seasonal and regional variations more accurately than common models using average temperatures or constant acceleration factors. Synopsis: The characteristics of commodity plastics like PS, PE and PP weathered in natural environments can be predicted by parameterizing the irradiance and temperature from accelerated weathering studies.