Rapid characterization of high-impact ethylene–propylene copolymer composition by crystallization extraction separation: comparability to standard separation methods

Abstract

High-impact copolymers of propylene and ethylene are complex materials with several components and at least two different phases, a crystalline polypropylene (PP) matrix and a predominantly amorphous ethylene–propylene copolymer (EPC) normally present as disperse particles. Next to overall composition analysis and morphology studies, fractionation into crystalline and amorphous components followed by further analysis of those components is the most important characterization technique for this technically very relevant class of copolymers. This process, involving separation into xylene cold-soluble (XCS) and insoluble fractions and follow-up determination of intrinsic viscosities (IV) as well as ethylene (C2) content is, however, tedious and time-consuming. A far more rapid approach is presented here through the separation of the amorphous and crystalline fractions of PP in 1,2,4-trichlorobenzene (TCB) using the crystallization extraction technique and the Crystex QC instrument. As the standard methods and Crystex technique are using different solvents (xylene and decalin versus TCB), correlations need to be established in order to be able to compare the results. Based on nine structurally very different polymers, respective correlation curves were developed and predictions for 95% confidence intervals were calculated statistically. In addition, these correlation curves were validated by testing over 100 different PP samples covering a broad range of all tested characteristics. The results show agreement and comparability between the traditional test methods techniques and the Crystex technique inside statistically calculated prediction intervals at 95% over a broad range of all tested characteristics (XCS, C2 and IV).