Browsing by Author "Ndiripo, Anthony"

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    Comparative study on the molecular structure of ethylene/1-octene, ethylene/1-heptene and ethylene/1-pentene copolymers using advanced analytical methods
    (Stellenbosch : Stellenbosch University, 2015-04) Ndiripo, Anthony; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Linear low density polyethylene (LLDPE), one of the fastest growing types of polyethylene, is made from the copolymerisation of ethylene and higher 1-olefin comonomers. 1-octene is the comonomer of choice as it gives mechanically better LLDPEs as compared to other 1-olefins. Recently, a shortage of 1-octene has been observed in the global market. Considering the fact that ethylene/1-heptene (EH) copolymers may have properties that are very similar to those of ethylene/1-octene (EO), replacing 1-octene with 1-heptene as the comonomer in the manufacture of commercial linear low density polyethylene (LLDPE) is a viable option. In order to do so, evaluation of microstructural and mechanical properties of both types of resins and their comparison were carried out first. Several LLDPE resins were synthesised using Ziegler-Natta (ZN) and metallocene type catalysts. The LLDPE resins were made using varying amounts of the comonomer to obtain copolymers of different compositions. Ten of the ZN-LLDPE resins became the core focus of the present study. Carbon-13 nuclear magnetic resonance spectroscopy (13C NMR) showed the differences in the compositions of both the EH and EOresins. Crystallisation analysis fractionation (CRYSTAF), differential scanning calorimetry (DSC) and high temperature high performance liquid chromatography (HT-HPLC) revealed the presence of at least two fractions within the EH and EO copolymers which varied in quantity and chemical composition as the comonomer content was increased. The fractions were identified as being the copolymer (of ethylene and the comonomer) and polyethylene. Comparisons of the EH and EO CRYSTAF and HPLC data showed similarities in the microstructures of the resins. Preparative-temperature rising elution fractionation (prep-TREF) was used to obtain several fractions from each resin for quantification and analyses. DSC, HT-HPLC, CRYSTAF, and 13CNMR revealed close similarities in the fractions of EH and EO copolymers with comparable comonomer contents. It also was revealed that TREF fractionations are influenced by the bulk resin comonomer content. EH and EO copolymers demonstrated high similarities in tensile strength and Young’s modulus at comonomer contents of < 3 mol %. Minor differences in the mentioned properties at comonomer content of > 3 mol % were attributed to the slightly better ability of 1-octene at reducing crystallinity as compared to 1-heptene as well as small differences in the comonomer contents of the test samples. The results of the study suggest that 1-heptene can be used in the place of 1-octene in the commercial manufacture of LLDPE.
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    High temperature multidimensional chromatography of complex and functionalized polyolefins
    (Stellenbosch : Stellenbosch University, 2018., 2018-12) Ndiripo, Anthony; Pasch, Harald; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: Polyolefins like all other synthetic polymers have complex microstructures that must be understood in order to predict product performance and adjust catalyst and reactor technologies. Between synthesis and polymer application, characterization techniques act as the visual aid for polymer microstructure. In further conversion processes, products can be modified e.g. via oxidation or grafting which introduces functionality and functionality type distributions (FTD), in addition to the molar mass and chemical composition distributions (MMD and CCD, respectively). Furthermore, the use of multiple reactor and catalyst technologies to produce a given product results in complex mixtures or alloys of polymers with superior properties, but very complex microstructures. These microstructural properties must be defined and if possible, libraries of microstructural data and suitable comprehensive techniques should be established for each type of material. High-temperature chromatographic techniques have emerged as fast and reliable techniques for polyolefin characterization. High-temperature liquid chromatography (HT-LC) which can be operated using solvent gradient and thermal gradient interaction modes (SGIC and TGIC, respectively) provides interesting alternatives to gas chromatography (GC) for the analysis of low molar mass oxidized waxes. In addition, non-crystallizing elastomers that cannot be characterized using crystallization-based methods such as temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF) can be analysed using the mentioned modes of HT-LC. Polyolefin characterization is enhanced by the hyphenation of multiple techniques as no one method is able to fully define the multiple microstructural distributions. Preparative fractionation has emerged as an indispensable tool that, when coupled to other separation and fractionation techniques, yields a wealth of information on polyolefin microstructure. The aim of the thesis is to utilize comprehensive HT-LC methods for the separation of complex polyolefin materials such low molar mass oxidized waxes, elastomers, and bimodal high density polyethylene (HDPE). Each type of selected polyolefin material presents a set of specific characterization challenges that must be addressed by HT-LC and/or hyphenation to a suitable preparative fraction technique. Work presented in this study demonstrates the separation of oligomers on porous graphitic carbon (PGC) of oxidized waxes under tailored HT-LC conditions irrespective of oxidation level. However, oxidation levels are shown to affect detector response. As a preparative fractionation technique, solution crystallization fractionation (SCF) is also shown to enhance the separation and identification of smaller oligomers. For the first time, the separation according to polarity of oxidized low molar mass waxes using HT-SGIC and HT-TGIC on silica as the stationary phase is demonstrated under tailored conditions. In addition, the oxidation level is shown to influence the quantities of retained fractions. Using a set of propylene-ethylene copolymers with increasing comonomer contents, the separation ranges of HT-SGIC and HT-TGIC are compared. In HT-SGIC a linear dependency of elution volume on the ethylene content is obtained for the entire average chemical composition (ethylene content) range. However, with HT-TGIC a linear dependency only is obtained within certain ethylene content limits. From this set of samples, it is also shown that HT-SGIC has a greater separation range (26.8 – 100 mol% ethylene) as compared to that of HT-TGIC (50 – 100 mol% ethylene). Lastly, preparative TREF is coupled offline to HT-SGIC for the characterization of bimodal ethylene-1-hexene HDPEs. Two model bimodal HDPE samples with similar comonomer contents are compared to a HDPE homopolymer produced with the same catalyst technology. The presence of copolymer fractions in the two samples introduces complexity in the microstructure of the bimodal HDPE. For a detailed analysis of the resins, the use of multiple techniques is shown to provide more information on the bimodal HDPE microstructure. It is also shown that the separation and selectivity of HT-SGIC for the analysis of bimodal HDPEs improves when coupled to a preparative fractionation technique such p-TREF as using this approach the complexity of the materials is reduced. However, the co-elution of low molar mass PE with the copolymer remains an obstacle, which can be overcome.