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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

Ndiripo, Anthony (2015-04)

Thesis (MSc)--Stellenbosch University, 2015.


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.

AFRIKAANSE OPSOMMING: Lineêre lae digtheid poliëtileen (LLDPE), een van die vinnigste groeiende poliëtileen tipes, word produseer deur die ko-polimerisasie van etileen en ‘n hoër 1-olefien ko-monomeer. 1-okteen is die ko-monomeer wat die meeste gebruik word aangesien dit LLDPE met die beste meganiese eienskappe produseer. Daar is egter ‘n tekort aan 1-okteen in die globale mark. Aangesien etileen/1-hepteen (EH) kopolimere moontlik soortgelyke eienskappe het as etileen/1-okteen (EO), kan 1-okteen moontlik vervang word deur 1-hepteen as ‘n komonomeer in die produksie van LLDPE. Om dit te doen is die meganiese en mikrostrukturele eienskappe van beide polimere geëvalueer. Verskeie LLDPE polimere is gesintetiseer met behulp van Ziegler-Natta (ZN) en metalloseen kataliste. Die komonomeer inhoud is gevarieer om LLDPE polimere te produseer met verskillende komposisie. Tien van die gesintetiseerde ZN-LLDPE polimere is gekies en is die kernfokus van die huidige studie. 13-Koolstof kern magnetiese resonans spektroskopie (13C KMR) het die variasie in ko-monomeer inhoud bevestig van beide die EH en EO polimere. Kristallisasie analise fraksioneering (CRYSTAF), differensiële skandeer kalorimetrie (DSC) en 'n hoë temperatuur hoë verrigting vloeistof chromatografie (HT-HPLC) het die teenwoordigheid van ten minste twee fraksies binne die EH en EO ko-polimeer bevestig wat ‘n variasie in hoeveelheid en chemise samestelling getoon het met ‘n toename van die ko-monomeer inhoud in die ko-polimeer. CRYSTAF en HT-HPLC data het getoon dat hierdie fraksies in EH en EO ooreenkomstige mikrostrukturele gedrag getoon het. Preparatiewe temperatuur styging elueering fraksioneering (prep-TREF) is gebruik om die polimere te fraksioneer om sodoende kwantitief die poliëtileen fraksies te verky en te analiseer. Verdere analise van die fraksies deur DSC, HT-HPLC, CRYSTAF en 13C KMR het getoon dat die fraksies, bekom van die EH en EO kopolimere met vergelykbare ko-monomeer inhoud, baie dieselfde eienskappe toon. Die analises het ook getoon dat die TREF fraksionering beinvloed word deur die ko-monomeer inhoud van die oorspronklike ko-polimeer.

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