High-Density Polyethylene Fractionation with Supercritical Propane L. J. D. BRITTO, 1 J. B. P. SOARES, 1 A. PENLIDIS, 1 V. KRUKONIS 2 1 University of Waterloo, Department of Chemical Engineering, Waterloo, Ontario N2L 3G1, Canada 2 Phasex Corporation, 360 Merrimack Street, Lawrence, Massachusetts 01843 Received 24 April 1998; revised 21 October 1998; accepted 27 October 1998 ABSTRACT: During the development of column extraction techniques, two methods of separation were identified. The first method is based on altering polymer solubility by varying the ratio of solvent in a solvent/nonsolvent mixture at a constant temperature above the polymer melting point (gradient solvent elution fractionation). This method fractionates polymers according to molecular weight. The second method is based on altering polymer solubility by varying solvent temperature (temperature rising elution fractionation—TREF). TREF fractionates semicrystalline polymers with respect to their crystallizability, independently of molecular weight effects. In the present article, supercritical propane will be used to fractionate a high-density polyethylene sample by molecular weight and short chain branching. The main advantage of supercritical fluid fractionation is that large polymer fractions with narrow molecular weight distribu- tions (isothermal fractionation) or narrow short chain branching distributions (isobaric fractionation) can be obtained without using hazardous organic chlorinated solvents. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 553–560, 1999 Keywords: supercritical fluid fractionation; supercritical isothermal fractionation; supercritical isobaric fractionation; CITREF; TREF; temperature rising elution frac- tionation; chemical composition distribution; molecular weight distribution; polyolefins; high density polyethylene INTRODUCTION The wide range of physical properties of polyole- fins results from the variety of microstructural distributions obtained with different catalyst types and polymerization processes. The need to understand the relations between these distribu- tions and end-use properties has led to the devel- opment of sophisticated polymer analysis tech- niques such as column extraction fractionation methods. The development of column extraction tech- niques began in the fifties with the replacement of the classical fractional solubility methods by col- umn extraction methods. Classical fractional sol- ubility methods (fractional precipitation and frac- tional solution/extraction) were mainly concerned with fractionation of polymers by molecular weight. However, both molecular weight and short chain branching (chemical composition), in- fluenced these techniques. No method was known to fractionate polymers exclusively on a chemical composition basis. 1 These techniques were based on the fact that the solubility of polymers in a given solvent decreases with increasing molecu- lar weight and that solvent power depends on solvent temperature and on the ratio of solvent to nonsolvent in a binary mixture. The use of these methods was not successful in obtaining a precise molecular weight distribution Correspondence to: J. B. P. Soares Journal of Polymer Science: Part B: Polymer Physics, Vol. 37, 553–560 (1999) © 1999 John Wiley & Sons, Inc. CCC 0887-6266/99/060553-08 553