Controlled depolymerization of polypropylene via selective partial oxidation in a supercritical water medium

Waste polypropylene has become a major environmental concern due to its vast amount of usage and non-biodegradable nature. Based on the ability of the high molecular weight material to be broken down into useful low molecular weight hydrocarbons, waste polypropylene can be viewed as a recyclable feedstock. This processing step can be accomplished by using supercritical water partial oxidation (SCWPO) technology. This technology uses the unique properties of supercritical water in an oxygen-deficient environment to partially oxidize the polymer. This study is designed to give insight into the optimization of this process by examining the reaction time and the percent of stoichiometric oxygen needed for complete combustion of polypropylene.; Experimental investigation was conducted under well-controlled process environments following a statistical design of experiments. The experiments were carried out using a one-liter Hastelloy C-276 reactor that is equipped with complete process peripherals including on-line effluent composition analysis and data acquisition. This investigation indicated that the largest amount of propylene monomer was recovered while operating at a reaction time of 15 minutes and oxygen feed amount of 5.86% with a conversion of polypropylene to propylene monomer of approximately 11%. Analysis of the residual polypropylene structure determined that the H/C ratio was consistent with the original structure. High-temperature GPC analysis identified a decrease in the molecular weight from 155,000 to an average of 14,000 and the polydispersity index was reduced from 5.2 to an average of 2.0. This process offers an exciting opportunity for conversion of spent polypropylene into propylene monomer and useful product of low molecular weight polypropylene wax of a narrow molecular weight distribution.