Polyethylene, a low-cost and easy-to-process plastic, accounts for almost one third of the world’s plastic trash. The University of Bayreuth’s multidisciplinary team has examined the environment’s progressive degradation of polyethylene for the first time. Although the degradation process causes fragmentation into smaller particles, isolated particles of nanoplastic are rare in the environment. This is because such decay products don’t like to be left alone and instead attach rapidly to larger colloidal system that naturally occur in the environment. The journal now contains the researchers’ findings. Science of the Total Environment.
Polyethylene can be found in many molecular forms. Because of its high demand, low-density plasticethylene (LDPE), is a widely used material for packaging everyday consumer goods such as food. There have been no estimates of how this widely-used plastic will degrade after it is released into the environment as waste. This question was first investigated by the Collaborative Research Centre Microplastics, University of Bayreuth. This was possible thanks to a new, technically advanced experimental setup. This allows scientists to simulate in the laboratory two well-known, but also environmentally related processes of plastic degrading: 1. photo-oxidation is when long polyethylene chains are gradually broken down into smaller, more water-soluble molecules by exposure to light. Mechanical stress is increasing the likelihood of fragmentation. This provided detailed insight into the complex chemical and physical processes of LDPE decay.
Studies examining the environmental impact of polyethylene are particularly interested in the final stage of LDPE degrading. Researchers discovered that this process does not end with the packaging material being released into the environment. Instead, it results in many micro- or nanoplastic particles with a high level of crystallinity. These tiny particles are prone to aggregate and attach quickly to larger colloidal systems made of organic or inorganic compounds. They form part of the environment’s material cycle. Clay minerals, polysaccharides and humic acid are all examples of such colloidal systems. “The process of aggregation prevents individual polyethylene degradation nanoparticles from being freely available in nature and interfacing with animals and plants. However, this is not a clear signal. Living organisms are more likely to ingest larger aggregates which participate in the material cycle of the environment and contain nanoplastics. This is how nanoplastics can eventually get into the food chain,” Teresa Menzel, one the lead authors of the new study, and a doctoral researcher working in the field polymer materials, says.
To identify the degradation products formed when polyethylene decomposes, the researchers employed a method that has not been widely used in microplastics research: multi-cross-polarization in solid-state NMR spectroscopy. “This method even permits us to quantify degradation products yielded from photooxidation,” co-author Anika Mauel (a doctoral researcher on inorganic chemistry).
Bayreuth’s scientists also discovered that polyethylene can also be degraded and decomposed, resulting in the formation of peroxides. “Peroxides have been known to be cytotoxic, meaning that they can cause damage to living cells. Another way in which LDPE degrading poses a threat is through the use of cytotoxic substances. These interrelationships should be studied in greater detail in the future,” says Nora Meides (a doctoral researcher in macromolecular Chemistry).
Without the University of Bayreuth’s coordinated use of state of-the-art research technology and interdisciplinarity, detailed analysis of the chemical- and physical processes involved with the degradation of polyethylene wouldn’t have been possible. These include scanning electron microscopy, energy dispersive X ray spectroscopy and NMR spectroscopy.