Pyrolysis of end-of-life photovoltaic laminates: Kinetic study and pyrolysis mechanism

This study examines the thermal decomposition kinetics and mechanisms of ethylene-vinyl acetate (EVA) copolymer and Tedlar-polyethylene terephthalate-Tedlar (TPT) laminates. Kinetic parameters were evaluated using the Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) methods. Pyrolysis experiments for EVA (350-450 °C) and TPT (375-450 °C) were conducted in a fixed-bed reactor and using pyrolysis gas chromatography-mass spectrometry (Py-GC/MS), respectively. In the fixed-bed experiments, the resulting oil products were characterized by gas chromatography-mass spectrometry (GC/MS). EVA exhibited a two-stage thermal decomposition process involving initial deacetylation followed by main chain scission, with an activation energy of 289.12 kJ mol −1 . In the Py-GC/MS experiments, the combination of high temperature (550 °C) and rapid heating caused deacetylation and thermal cracking to occur simultaneously. The resulting unsaturated segments were immediately subjected to thermal cracking, cyclization, and dehydrogenation, yielding primarily 1-olefins, 1,n-dienes, and cycloolefins. In contrast, low temperature slow pyrolysis in the fixed bed reactor favored random scission pathways leading to alkane formation. The pyrolysis of TPT proceeded as a continuous single stage reaction (173.92 kJ mol −1 ), initiated by the dehydrofluorination of polyvinyl fluoride (PVF) with the release of hydrogen fluoride (HF). This process was accompanied by synergistic bond cleavage in polyethylene terephthalate (PET) via a six membered cyclic transition state, yielding benzoic acid, terephthalic acid, and related derivatives. Under high temperature, fast pyrolysis conditions, aromatic compounds such as benzene and naphthalene were increasingly formed through two dominant pathways: decarboxylation of benzoic acid derivatives and cyclization/aromatization of defluorinated conjugated olefins produced during PVF dehydrofluorination. Combustion, driven by oxidative reactions, shifted the thermal decomposition of both EVA (172.29 kJ mol −1 ) and TPT (138.45 kJ mol −1 ) to lower temperatures and introduced an additional char combustion stage.