Relationship between Xylene Solubles and Crystallization Elution Fractionation of Polypropylene

Abstract

The Xylene Soluble (XS) is one of the most important parameters closely related to the performance of polypropylene (PP) materials. In order to obtain precise and accurate XS data, ASTM D 5492-17, based on the wet chemistry separation mechanism, strictly specifies each and every step of the procedure, such as a glassware setup, heating and cooling rates, etc. Meanwhile, crystallization elution fractionation (CEF), a newly developed technique, is capable of quantifying polyolefin structural regularity and comonomer distribution in a fast and automated manner. There have been a few pieces of preliminary work aiming to provide XS value from the CEF test with a limited number of samples. It is important to gain deep insights into the relationship between XS and the purge fraction of CEF by studying a large number of diverse polypropylene materials. In this paper, close to 300 commercial polypropylene and/or homemade polypropylene, both homopolypropylene (hPP) and propylene–ethylene copolymers (P/E) made with various catalysts under different conditions, were employed. XS values were obtained via CRYSTEX QC calibration. It was found that XS and the soluble fraction (SF) from CEF yielded a linear relationship with R² = 0.9942. This linearity is applicable regardless of polymer types, which can differ significantly in terms of molecular weight distribution, tacticity, chemical composition, and melt flow rate. One of the prominent advantages of using the CEF test instead of ASTM D 5492-17 is the elimination of the time-consuming procedure for much-improved precision and high productivity. In addition, the catalytic system was found to have an influence on SF and crystalline fractions of copolymers.