Defining the degree of degradation in plastics: Quantification of accumulated degradation products and leached stabilizers for predicting end-of-life

• The degradation degree is defined as the weight fraction of "complete degraded polymer." • This definition enables visualization of plastic degradation in compositional diagrams. • Polymer degradation and stabilizer leaching are understood to occur concurrently. • A threshold for degradation triggering mechanical property changes is identified. Plastics degrade through various mechanisms, complicating the comprehensive chemical definition of degradation. As the accumulation of degraded products leads to sudden declines in material properties and triggers microplastic dispersion, a chemical degradation metric is essential for predicting plastics’ end-of-life. Here, we propose defining degradation by the weight ratio of “completely degraded polymer (CDP)” to pristine polymer, determined via pyrolysis mass spectrometry (MS). By simplifying the complex degradation evaluation into a straightforward framework of compositional analysis, a comprehensive degradation metric can be obtained without tracking individual degradation pathways. Moreover, by including stabilizers as system components, stabilizer leaching and polymer degradation can be concurrently visualized in the same framework. We demonstrate this approach in polyurethane films subjected to accelerated degradation tests and, by correlating the CDP-defined degradation metric with mechanical property evaluations, successfully identify the threshold degradation level that marks a material's end-of-life. Furthermore, this method reveals deeper insights into degradation mechanisms: stabilizer leaching and polymer degradation are complementary processes that induce each other. This comprehensive degradation metric will also be crucial for grading recycled materials, promoting the circular economy in plastics.