Polyethylene Pyrolysis:  Theory and Experiments for Molecular-Weight-Distribution Kinetics

A novel reactor for pyrolysis of a polyethylene melt stirred by bubbles of flowing nitrogen gas at atmospheric pressure permits uniform-temperature depolymerization. Sweep-gas experiments at temperatures 370−410 °C allowed pyrolysis products to be collected separately as reactor residue (solidified polyethylene melt), condensed vapor, and uncondensed gas products. Molecular-weight distributions (MWDs) determined by gel permeation chromatography indicated that random scission and repolymerization (cross-linking) broadened the polymer-melt MWD. Observing the repolymerization reaction is difficult if the condensate and melt are not recovered separately as in the described experiments. The C1−C5 uncondensed gas products were formed by chain-end scission, according to the continuous-distribution theory proposed to interpret the experimental data. The mathematical model accounts for the mass transfer of vaporized products from the polymer melt to gas bubbles. The driving force for mass transfer is the interphase difference of MWDs based on equilibrium at the vapor−liquid interface. The chain-end-scission activation energy and preexponential were determined to be 35 kcal/mol and 1.14 × 108 s-1, respectively. The data did not permit determination of the random scission and repolymerization rate coefficients.