Fractionation of high density polyethylene in propane by isothermal pressure profiling and isobaric temperature profiling

Abstract

High density polyethylene was fractionated with respect to molecular weight and dissolution temperature in supercritical and near-critical propane. Isothermal pressure profiling in the liquid (polymer)-super-critical fluid regime resulted in 14 fractions with narrow polydispersity. The liquid-crystal phase separation technique of Pennings was extended from organic solvents to compressed propane for fractionation with respect to dissolution temperature and therefore crystallizability in the semicrystalline solid-supercritical fluid regime by isobaric temperature profiling above the second critical endpoint pressure. The influence of molecular weight, crystalline content, and mass transfer limitations is discussed. The authors believe this is the first time a crystal phase fractionation has been reported in supercritical fluids. The process is suggested as an alternative to temperature rising elution fractionation (TREF) developed to fractionate linear low density polyethylene on the basis of short chain branching.