The effect of controlled rheology on the evolution of chemical composition distribution of commercial Heterophasic Ethylene Propylene Copolymers (HEPCs)
Thesis (PhD)--Stellenbosch University, 2020.
ENGLISH ABSTRACT: The vis-breaking of heterophasic ethylene-propylene copolymers (HEPCs) is an important industrial process. While the changes in molar mass properties are easy to track using hightemperature size exclusion chromatography (HT-SEC) before and after vis-breaking, changes in the chemical composition are much more challenging due to the complex chemical composition resulting from different ethylene contents and ethylene block lengths. The present work tracks the microstructural changes of three samples with 0 – 10.8 mol.% ethylene content before and after vis-breaking with the aid of molar mass and chemical composition sensitive chromatographic techniques. Bulk sample analyses indicated that the rubber phase, which is ethylene rich was essential in aiding peroxide mobility during vis-breaking as indicated by the narrower dispersities and lower peak molar masses with increasing ethylene content. Preparative temperature rising elution fractionation (p-TREF) and solvent gradient elution fractionation (SGEF) were used to fractionate the non vis-broken and vis-broken HEPC bulk samples into several fractions for further analyses. Upon vis-breaking, the SGEF fractions with high molar mass were observed to diminish after vis-breaking. Furthermore, the increase in ethylene content was observed to reduce the impact of the peroxide on the fraction quantities before and after vis-breaking implying that vis-breaking affects more the polyolefin chains with more PP segments and less those with more ethylene co-monomer. However, SGEF may not be suitable for the fractionation of mixtures of homopolymer and copolymers due to the different solubilities in the solvent/non-solvent mixtures. Furthermore, in-situ solid state NMR experiments revealed the greater sensitivity of copolymers containing short ethylene sequences to peroxide degradation. Long chain ethylene sequences were found to be resistant to radical degradation.
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