Understanding poly(ethylene terephthalate) degradation using gas-mediated simulated recycling

•PET degradation during mechanical recycling limits its recyclability.•PET degradation mechanism can be controlled by tuning the ratio of air:N2 during melt processing.•Recycling under as little as 5% O2 causes chain scission, N2 can be used to improve molecular weights by 20%.•CO2 has plasticising effects during melt processing, promoting more sustainable recycling.•The use of gas can replace migration-prone additives to promote circularity of PET and other packaging plastics. While mechanical recycling is a key enabler of a plastics circular economy, poly(ethylene terephthalate) (PET) is highly susceptible to degradation during melt processing. Through rheology-simulated and extrusion recycling experiments we demonstrate that the degradation of PET during high temperature processing can be understood and mitigated by control of gaseous environment. Processing under nitrogen induces up-shifts in melt viscosity and molecular weights by up to 20%. By contrast, the presence of oxygen initiates chain scission reactions, decreasing molecular weight and viscosity. Carbon dioxide plasticises the PET melt, offering the potential for improved processing and property retention. Expanding experiments to nitrogen-oxygen mixed-gas systems, we identify the presence of two distinct regimes associated with chain growth and scission which are subsequently tuned by controlling the oxygen concentration of the environment. This research facilitates a simple, additive-free approach to improving PET recyclability, thus enabling improved circularity of an often-challenging plastic material to recycle. [Display omitted]