Investigation of factors affecting the breakdown of plastics in the environment

Plastics are durable, lightweight, cheap to manufacture and can be made with diverse properties, shapes and strengths. As such they are used widely across an array of sectors from construction, medical care through to packaging. However, owing to their ubiquity and abundance, coupled with poor end-of-life practices, plastics are now polluting almost every environment on Earth. The consequences of this contamination are well recorded across a range of settings, but most notably in marine environments. Yet, most negative effects focus on a short-term perspective, for example, on wildlife entanglements or ingestion of plastics causing death; much less is known about plastic contamination over longer timescales. In part this is because plastic was first manufactured and used in the 1950s but its widespread occurrence in the environment has been a more recent phenomenon – simply stated, insufficient time has elapsed for us have directly observed the fate of plastic on timescales longer than a couple of decades. It is, therefore, important to consider plastic as a geological material, given its durability and recalcitrance in the environment, and to predict what the fate of plastic materials will be as they are released and transported through many different environments, over decades, hundreds and probably thousands of years. This is a significant knowledge gap that needs to be filled as at present there is an estimated 6.3 billion tonnes of plastic in the environment which is exposed to a diverse range of environmental conditions. Without an understanding of the degradation rates and processes that will affect such plastic we cannot begin to attempt mitigation operations or know where plastic will do the most significant harm.

This thesis addresses this knowledge gap by firstly evaluating the effects of historical degradation on plastic litter, beginning to join the dots between where a plastic has been, for how long, and what physical and chemical degradation it has experienced. Secondly, case studies of how plastics degrade in certain types of conditions are presented to show how greatly degradation can vary within a single environmental context. Finally, the non- polymer content within plastics, in particular the colour, is examined to determine the role this plays in degradation variability.

The results of the research presented herein demonstrate that age is not necessarily an indication of the level of degradation a plastic item has experienced, and that the historical conditions the material has been exposed to, as well as the type of plastic, have a greater influence on the breakdown. Volatile degradation products are observed at environmental temperatures for some plastic types, even in the absence of sunlight. Interactions between degrading polymers in proximity to one another were observed, especially those containing stabilising additives, where the degradation of one polymer can hinder or promote the degradation of another. This may have consequences for not only littered plastics, but also in landfills and waste management. Differences between thermal and photo-degradation were shown, demonstrating the importance of sunlight in degradation of plastics with C-C backbones, and the importance of water exposure in plastics with C- O  backbones.  Finally,  experimental  analyses demonstrated  that  environmental degradation is highly dependent on colour, from red plastics extensively and rapidly forming microplastics, to black plastics displaying minimal signs of degradation. Overall, these are small but important steps to understanding how plastic litter will affect our planet in the long-term and are the first stages in considering the material in a more holistic way as part of a non-linear and ever-changing process from their initial source to final sink.