A comparative analysis of the chemical composition of linear low density polyethylene polymers synthesised with 1- hexene comonomer under different catalytic conditions

Naidoo, Preloshni (2013-03)

Thesis (MSc)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: A comparative study of the chemical composition of linear low density polyethylene polymers, synthesised with 1 - hexene as comonomer was conducted. Catalyst trials were conducted on the linear low density 1 - hexene polymer grade material to evaluate alternative catalysts. A comparative analysis was performed in order to investigate if the samples synthesised under catalyst trial conditions showed any significant differences in terms of crystallinity and mechanical properties with the reference sample that was synthesised using the reference catalyst. The results showed that the macro product properties, namely melt flow Index, density, and level of hexene extractables are different for the trial samples in comparison with the reference sample. The differences observed implied that the trial samples were synthesised with differences on a molecular level. The differences in the chemical composition between the reference sample and the comparative samples were fully explored using a wide range of analytical techniques, namely crystallisation analysis by fractionation (CRYSTAF), temperature rising elution fractionation (TREF), differential scanning calorimetry (DSC), Carbon 13 nuclear magnetic resonance (13C NMR), Size exclusion chromatography (SEC), Positron analysis lifetime spectroscopy (PALS) and micro hardness analysis. The results of the characterisation studies indicated the following: - Crystallinity and hardness analysis of the reference sample, catalyst trial sample 1 and catalyst trial sample 2 indicate that the catalyst trial sample 2 having a low cocatalyst concentration is the most crystalline of all the samples. - The reference sample, catalyst trial sample 1 and catalyst trial sample 2 were further fractionated using TREF at fractionation temperature intervals of 10°C. TREF analysis indicates that the bulk of the material is observed to elute between 70°C - 10°C. - 13C NMR analyses of the TREF fractions identified four populations of fractions that could be selectively removed, allowing the bulk of the material to be recombined. As these highly crystalline fractions were removed, there was an observed decrease in the total crystallinity of the bulk recombined material. This trend was further verified by the free volume analysis. - Free volume analysis indicated of the bulk recombined material indicated a general increase in the T3 lifetime and T4 lifetime intervals. Free volume analysis further confirmed a decrease in crystallinity of the bulk recombined material as highly crystalline material was removed. - Micro hardness analysis of the polymers further verified the crystallinity trends observed. As the molecular composition of the polymer changed due to removal of highly crystalline fractions, the total mechanical strength of the material indicated by the hardness value decreased. The study showed that by changing the chemical composition of the polymer by removing highly crystalline fractions, there was an observed change in the mechanical properties of the polymer. It can be concluded that the samples synthesised under catalyst trial conditions show significant differences in terms of crystallinity and mechanical properties in comparison with the sample that was synthesised using the standard reference catalyst.

AFRIKAANSE OPSOMMING: ‘n Vergelykende analise studie is onderneem van die chemiese samestellings van lineêre lae digtheid poliëtileen polimere, gesintetiseer met 1-hekseen as ko-monomeer. Alternatiewe kataliste is ge-evavuleer ten opsigte van lineêre lae digtheid 1-hekseen Sasol polimeer graad materiaal. Die vergelykende analise is uitgevoer om die monsters onder katalis proef kondisies te evalueer en te merk of enige beduidende verskille in terme van kristalliniteit en meganiese eienskappe met die verwysings monster voorkom. Die resultate toon dat die makro-produk eienskappe, naamlik smelt vloei indeks, digtheid en vlak van hekseen onttrekking, verskillend is vir die proef monsters in vergelyking met die verwysings monster. Die waargenome verskille impliseer dat die proef monsters op molekulêre vlak verskil. Die verskille in chemiese samestelling tussen die verwysings monster en die vergelykende monsters is ten volle ondersoek deur gebruik te maak van 'n wye verskeidenheid van analitiese tegnieke, naamlik kristallisasie analise fraksionering (CRYSTAF), temperatuur stygende eluering fraksionering (TREF), differensiële skandeer kalorimetrie (DSC), koolstof 13 kernmagnetiese resonansie (13C KMR), gelpermeasie chromatografie (SEC), positron analise leeftyd spektroskopie (PALS) en mikro-hardheid analise. Die resultate van die karakterisering studies het die volgende aangedui:- Kristalliniteit en hardheid analises van die verwysings monster en katalis proef monsters 1 en 2 het getoon dat katalis proef monster 2, wat ‘n lae ko-katalis konsentrasie bevat, die mees kristallyn is. - Die verwysings monster en katalis proef monster 1 en 2 is gefraksioneer met behulp van ‘n TREF met temperatuur tussenposes van 10°C. TREF analise toon dat oormaat materiaal ge-elueer word tussen 70°C en 100°C. - 13C KMR analise van die TREF fraksies het 4 verskillende fraksies geidentifiseer wat selektief verwyder kan word. Dit laat ook toe dat die grootste deel van die materiaal weer geherkombineer kan word. Soos die hoogs kristallyne fraksies verwyder is, is ‘n afname in die totale kristalliniteit van die geherkombineerde materiaal waargeneem. Hierdie tendens is bevestig deur vrye volume analises. - Vrye volume analises van die geherkombineerde materiaal toon ‘n algemene toename in die T3en T4 leeftyd aan. Vrye volume analises toon verder dat ‘n afname in die kristalliniteit van die geherkombineerde materiaal plaasvind soos meer kristallyne fraksies verwyder word. - Verdere mikro-hardheid analises van die polimere bevestig die waargenome kristalliniteit tendense. Soos die molekulêre samestelling van die polimere verander as gevolg van die verwydering van die hoogs kristallyne fraksies, so neem die totale meganiese sterkte van die materiaal af; soos aangedui deur die afname in hardheid waarde. Die studie toon dat die verandering van die chemiese samestelling van die polimeer, deur die verwydering van hoogs kristallyne fraksies, 'n waargenome verandering in die meganiese eienskappe van die polimeer laat plaasvind. Daar kan afgelei word dat die monsters, vervaardig onder die katalis proef voorwaardes, beduidende verskille toon in terme van kristalliniteit en meganiese eienskappe in vergelyking met die monster vervaardig deur die huidige verwysings katalis.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/80272
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