Kinetic insights into high-density polyethylene, polyethylene terephthalate, and their blend using thermogravimetric analysis and model-free methods

The accumulation of plastic waste, particularly from high-density polyethylene (HDPE) and polyethylene terephthalate (PET), poses significant environmental challenges due to their persistence and the complexity of recycling mixed polymer. Accordingly, this study was conducted to investigate the thermal degradation behavior and kinetic parameters of virgin HDPE, PET, and their binary mixture to support waste-to-energy applications. Thermogravimetric analysis (TGA) and Differential thermogravimetry (DTG) were performed under pyrolytic conditions using nitrogen as the carrier gas at multiple heating rates, and degradation kinetics were evaluated using five isoconversional methods: Friedman (FR), Kissinger–Akahira–Sunose (KAS), Flynn–Wall–Ozawa (FWO), Starink (STK), and Vyazovkin (Vy). Results showed that both HDPE and PET undergo single-step degradation, with HDPE decomposing at higher temperatures in a narrower range (449–497 °C) than PET (394–471 °C) at 15 °C/min. The HDPE–PET blend showed a broader decomposition range (417–495 °C) with an onset temperature between PET and HDPE. Comparatively, the Friedman (FR) method provided reliable activation energies for HDPE and PET (259.55 ± 7.3 and 193.16 ± 17.07 kJ/mol), as it effectively captures the single-step degradation of individual polymers with minimal variation across conversion levels. For the HDPE–PET binary blend, the Vyazovkin (Vy) method yielded the most consistent activation energy profile (173.51–217.45 kJ/mol; average 210.47 ± 7.2 kJ/mol), demonstrating its robustness in handling the complex, overlapping decomposition behaviors of mixed polymer systems. Model-fitting via y(α)/y(0.5) analysis identified the autocatalytic model 1 − α n α m + α ∗ as the most appropriate for all samples, with simulated curves showing excellent agreement with experimental data (R 2 > 0.92). These findings demonstrate the feasibility of predicting pyrolysis behavior for both individual and mixed plastics, contributing to improved strategies for managing mixed plastic waste streams. • At β = 15 °C/min, the HDPE–PET blend showed a broader decomposition range (417–495 °C). • Eα by Fr averaged 259.55 kJ/mol for HDPE, 193.16 kJ/mol for PET, and Vyazovkin 210.47 kJ/mol for the blend. • The autocatalytic model obtained the f(α) for degradation kinetics for HDPE, PET, and their blend. • Simulated autocatalytic model curves matched experimental data (R 2 > 0.92),