Zinc chloride promoted the inimitable dissolution and degradation of polyethylene in a deep eutectic solvent under white light†

Abstract

Environmental pollution caused by the excessive use of polyethylene (PE) and its non-biodegradable nature are major concerns in many countries. Most of the processes available for the degradation of PE are cumbersome and involve chemically harsh conditions at elevated temperatures. Therefore, new methods for the dissolution and degradation of PE should be devised. The major challenge in the development of such processes is to adopt environmentally benign conditions. Herein, we have prepared and utilized ZnCl₂ and lactic acid (LA) based deep eutectic solvents (DESs) (LA : ZnCl₂) for the dissolution and degradation of PE under white light at 60 °C. The dissolved PE has been regenerated using water as an antisolvent. The regenerated material has been characterized and compared with virgin PE for alteration in the inherent structure by employing scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) investigations. The composition and induction of functional groups into the regenerated material have been further probed by Fourier-transform infrared (FTIR), ¹H and ¹³C nuclear magnetic resonance (NMR), and X-ray photoelectron (XPS) spectroscopy. The mechanism governing the dissolution of PE in DES has been observed to be free-radical mediated under controlled reaction conditions. It is proposed that ZnCl₂ activates the CO of LA, resulting in lowering of the bond energy of the π-bond of CO, which is compensated by the energy provided by white light and temperature, simultaneously. The recyclability of the DES and no alterations in the properties of the regenerated material obtained from recycled DES in comparison to that obtained from the native DES further add to the sustainable nature of the process. It is believed that the present work would not only provide basic understanding about the dissolution and degradation of PE but also prompt other researchers to develop new DESs for the dissolution of hard to dissolve materials in a sustainable manner.