Gaining a comprehensive understanding about the (impact of) degradation of substances such as additives in plastic recycling is crucial for producing high-quality recycled plastics, ensuring regulatory compliance, mitigating environmental impact, and maintaining the long term sustainability of recycling operations. The overall objective of this project is to generate systematic insights into the impact of additives and their degradation in mechanical and chemical plastic recycling processes and to provide a method to predict potential problems caused by degradation products depending on input composition. In first instance a selection of representative additives, based on functionality and chemical classes, typically used in polyolefin packaging, will be made. Next, analytical methods to quantify their degradation products are developed by means of py-GC-MS and TGA-FTIR. Next a large range of experiments is performed to study the thermal degradation behavior of a broad range of individual additives and functional groups. Next, a mixture design is made to include synergistic effects. Based on this dataset, a model to predict this degradation based on additive type and functional groups is developed, relying on statistical approaches. This delivers insights in potential risks such as toxicity, corrosion potential, and electrophilicity, which will be used to formulate guidelines towards more responsible use of additives and the creation of more robust recycling processes.