Chemical Recycling of Poly(ethylene terephthalate): Effects of Mechanical Stress and Radiation Damage on Hydrolysis

Zhang, Tongjie

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Chemical Recycling of Poly(ethylene terephthalate): Effects of Mechanical Stress and Radiation Damage on Hydrolysis

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Finding an effective recycling process for oceanic plastic waste is increasingly important to address environmental pollution. Plastic waste is a bountiful and sustainable resource for energy production and chemical recycling. Polyethylene terephthalate (PET), one of the most common commercialized polymers, is promising to be recycled by hydrolysis. The objective of this work was to study the effects of mechanical stress and radiation damage on PET structure and hydrolysis reactivity. Ball-milling and photo-damaging pretreatments were carried out to imitate the environmental degradation in the ocean environment. An evident decrease in crystallinity with increasing ball-milling time was observed. This decrease in crystallinity caused by fractures of chemical bonds induced by photoaging and ball-milling treatments were investigated. PET was hydrolyzed into ethylene glycol (EG) and terephthalic acid (TPA) at 200°C in tube hydrolysis reactors without catalysts. EG and TPA were recovered after PET depolymerization. Thermogravimetric analysis and Fourier-transform infrared spectroscopy indicated that the recovered TPA was purified. Qualitative and quantitative analysis of TPA and EG were performed by using UV-Visible spectrophotometer and High-Performance Liquid Chromatograph (HPLC) separately. At 200 ℃, the conversion rate of fresh and ball-milled PET samples was improved from 16-18% with a one-hour reaction time compared to 87-91% with a two-hour reaction time. This depolymerization behavior supported that the conversion rate of PET was increased with increasing reaction time at the same temperature. The experiment results, however, showed PET conversion, TPA yield, and EG yield did not improve after ball-milling and photo-damaging treatment. After retention time of 1 hour, PET conversions, yields of TPA, yields of EG of various PET samples were 16.5±1.5%, 11.5±1.5% and 0.70 ± 0.20%, respectively. Mechanical treatment and radiation damage did not affect PET reactivity significantly in this experiment. Two main reasons were discussed to explain this result. The effect of radiation damage and mechanical stress was obscured by the more dominant reaction condition, temperature. The pretreatments in this experiment were not strong enough to affect PET reactivity. Based on the current results, recommendations for the hydrolysis temperature, potential catalysts, and more robust pretreatment methods were provided for further outlook of studying environmental effects on PET hydrolysis.

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