Thermal degradation kinetics and conversion pathways of bioplastics: A comprehensive analysis using model-fitting methods

Conventional biological treatments (e.g., composting, anaerobic digestion) face limitations in achieving complete biodegradation of bioplastics under current policy and operational frameworks, driving interest in pyrolysis for energy/chemical recovery. This study systematically investigated the pyrolysis kinetics of three typical biodegradable plastics— poly(butylene adipate-co-terephthalate) (PBAT), poly(butylene succinate) (PBS), and polycaprolactone (PCL)— through thermogravimetric analysis and pyrolysis-gas chromatography-mass spectrometry. Kinetic analysis combining model-fitting and isoconversional methods revealed structure-dependent degradation patterns: PBAT exhibited progressively increasing activation energy (from 126 to 346 kJ mol−1 across α values of 0.05 and 0.95, respectively) due to phase transitions in its semi-crystalline structure, whereas PBS and PCL showed stable activation energies (114 ± 5 and 146 ± 12 kJ mol−1, respectively, with stability indices <30 %) from ordered crystalline breakdown. Mechanistic modeling identified distinct reaction pathways—PBAT followed a Bn (n≠1) nucleation branching model initiated from semi-molten regions, generating complex volatiles, while PBS/PCL adhered to An or B1 models with stage-specific products. These findings establish a critical structure-thermostability relationship: crystalline regularity enables predictable multi-stage degradation (PBS/PCL), whereas semi-crystalline systems (PBAT) require dynamic process control due to evolving kinetic barriers. By correlating polymer crystallinity with kinetic parameters and product profiles, this work advances the precision of pyrolysis reactor design and waste management strategies, particularly for optimizing temperature programs and residence times to achieve targeted conversion of heterogeneous bioplastic waste streams.