Browsing by Author "Lamola, Hellen"

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    Structure-property relationship of bimodal high density polyethylene: Effect on rheology
    (Stellenbosch : Stellenbosch University, 2021-12) Lamola, Hellen; Van Reenen, Albert; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Bimodal polyethylene is a version of HDPE that is tailor-made to possess better processability and mechanical strength. Polymerisation is carried out in a low-pressure dual reactor system in the presence of a catalyst. Hydrogen and ethylene are fed to the first reactor to produce a polyethylene homopolymer of very low molar mass. In the second reactor, comonomer of 1- alpha olefins is introduced to create short chain branching. A homogenous distribution of comonomer along the polyolefin chain is important in determining final resin performance. The present work looks at three bHDPE resins with almost similar microstructural properties but different rheological behaviour during the film blowing process. A benchmark industrial resin (Reference) was compared to two similar resins (Resin 1 and Resin 2) where Resin 2 exhibited poor processability regarding bubble instability and melt strength. In the first part of this work, bulk resins are analysed using various analytical techniques. Carbon-thirteen nuclear magnetic resonance (13C NMR) was used to determine the type and quantity of the comonomer in all samples. Other techniques used included high-temperature size exclusion chromatography (HT-SEC), differential scanning calorimetry (DSC), crystallisation analysis fractionation (CRYSTAF), high-temperature interaction chromatography (HT-IC), high-temperature liquid chromatography (2D-LC), dynamic mechanical analysis (DMA) and tensile strength. Results showed that the Reference had a different comonomer to Resin 1 and Resin 2. Also, Resin 2 had a slightly higher comonomer content but overall, comonomer content was very low (<0.8 %) for all samples. Fractionation was performed on a preparative scale and collected fractions were further analysed using 13C NMR, HT-SEC, and DSC. Firstly, preparative temperature rising elution fractionation (pTREF) was performed followed by preparative molar mass fractionation (pMMF). Analyses of the fractions showed that the comonomer in Resin 2 was concentrated in certain fractions and not broadly distributed along the polyolefin chain. As a result, Resin 2 had certain fractions of significantly higher crystallinity to those of the Reference and Resin 1. It was also shown that Resin 2 had high molar mass in the lower TREF temperature fractions where it is expected to be of low molar mass. The Reference conversely had an even distribution of very low and very high molar mass chains as well as an evenly distributed comonomer throughout the polymer chains. Overall observed differences in chemical composition distribution (CCD) and molar mass distribution (MMD) do have an effect on the rheology and processability of bHDPE resins. More specifically, the narrow distribution of iv comonomer in Resin 2 could justify the observed differences in rheological behaviour during processing.