Effect of Polymer Properties on the Biodegradation of Polyurethane Microplastics

The release of fragments from plastic products, that is, secondary microplastics, is a major concern in the context of the global plastic pollution. Currently available (thermoplastic) polyurethanes [(T)­PU] are not biodegradable and therefore should be recycled. However, the ester bond in (T)­PUs might be sufficiently hydrolysable to enable at least partial biodegradation of polyurethane particles. Here, we investigated biodegradation in compost of different types of (T)­PU to gain insights into their fragmentation and biodegradation mechanisms. The studied (T)­PUs varied regarding the chemistry of their polymer backbone (aromatic/aliphatic), hard phase content, cross-linking degree, and presence of a hydrolysis-stabilizing additive. We developed and validated an efficient and non-destructive polymer particle extraction process for partially biodegraded (T)­PUs based on ultrasonication and density separation. Our results showed that biodegradation rates and extents decreased with increasing cross-linking density and hard-segment content. We found that the presence of a hydrolysis stabilizer reduced (T)­PU fragmentation while not affecting the conversion of (T)­PU carbon into CO2. We propose a biodegradation mechanism for (T)­PUs that includes both mother particle shrinkage by surface erosion and fragmentation. The presented results help to understand structure–degradation relationships of (T)­PUs and support recycling strategies.