Understanding enhanced char formation in the thermal decomposition of PVC resin: Role of intermolecular chlorine loss

Abstract

Owing to the importance of controlling the decomposition products of plastic pyrolysis, in this study the mechanisms of char formation in the thermal decomposition of PVC were investigated quantitatively using a kinetic approach. Thermal decomposition behavior of pure PVC, PE, PP, and mixtures of PVC + NiO and PVC + TiO2 powders was probed using a thermogravimetry-differential scanning calorimetry analyzer under an Ar atmosphere from room temperature to 1073 K. Morphological and structural analyses of solid residues were conducted using X-ray diffraction, scanning electron microscopy-energy dispersive spectrometry and Raman spectroscopy. Chlorine was removed from PVC up to 673 K with an activation energy of 104.7 kJ/mol, resulting in 38 % mass of solid residue polyene. When temperature was further increased, the decomposition of polyene occurred almost in the same temperature range of PE and PP decomposition, leading to the final solid residue at 1073 K of 11 %, nil and 1 % for PVC, PE and PP respectively. Central to our examination, two competing routes of chlorine loss in the pyrolysis of PVC (involving intramolecular and intermolecular intermediates respectively) were compared, with respect to their contributions towards char formation. By juxtaposing the pyrolysis behavior of PVC against those of PE and PP, it was deduced that chlorine loss through intermolecular intermediates contributed overwhelmingly to char precursor formation. Theoretical calculations revealed around 35 % of PVC subject to pyrolysis underwent chlorine loss via intermolecular intermediates. This theoretically derived proportionality is in good agreement with experimental results, which indicated a char yield of 28.6 % in the thermal decomposition of PVC. This char formation mechanism was further confirmed by the co-pyrolysis results of PVC with each of two transition metal oxides, i.e. chemically reactive NiO and inactive TiO2, showing that the intermolecular chlorine loss was enhanced in the presence of NiO due to the formation of NiCl2, whereas no obvious change in char yield was observed in case of TiO2 because no chlorination of TiO2 was detected. This combination of experimental and theoretical considerations unanimously verifies that intermolecular mechanisms of chlorine loss in the pyrolysis of PVC play a leading role to the formation of net structures giving rise to char. These findings allude to the possibility of manipulating the decomposition products such as to synthesize net structured carbon based materials through the pyrolysis of waste plastics.