Reverse Engineering of Chemically Similar Bimodal High Density Polyethylenes: A Comprehensive Study Using Advanced Chromatographic Techniques

Abstract

Bimodal high-density polyethylene (bHDPE) is a complex, multicomponent polyethylene (PE) material whose synthesis in a multistage process can be challenging. Three bHDPEs with good and bad end-use properties are reverse engineered using advanced analytical techniques. Average chemical composition is determined using 13C NMR and 1-butene is identified as the comonomer for the good resin (bHDPE 1) while 1-hexene is the comonomer in bHDPE 2 and 3. The presence of comonomer in the high molar mass fractions of the samples is shown using high-temperature triple-detection size exclusion chromatography (HT-SEC-d3). Chemical composition separation using high-temperature interaction chromatography (HT-IC) is achieved using porous graphitic carbon (PGC) and silica stationary phases. Some problems in temperature gradient interaction chromatography (TGIC) on PGC are overcome by using a non-adsorptive stationary phase, enabling better separation and visualization of homopolymer and copolymer components. Coupling HT-SEC in 2D liquid chromatography (2D-LC) analyses at high temperatures reveals the presence of a larger copolymer component in bHDPE 1 at high elution volume. In contrast, bHDPE 2 and bHDPE 3 have copolymer components at low elution volumes, indicating poor comonomer distribution in the copolymer component which ultimately explains the poor mechanical properties at similar comonomer contents.