The effect of in-process ethylene incorporation on the evolution of particle morphology and molecular characteristics of commercial heterophasic ethylene propylene copolymers (HEPCs)

by Linda Botha

Institution: Stellenbosch University
Department: Chemistry and Polymer Science
Degree: PhD
Year: 2014
Keywords: Polymer science
Record ID: 1475583
Full text PDF: http://hdl.handle.net/10019.1/86532


ENGLISH ABSTRACT: Impact copolymers or heterophasic polypropylene-ethylene-co-propylene copolymers (HEPCs) commonly produced in industry are valued for their good mechanical properties, combining the rigidity of the polypropylene matrix with the toughness of the dispersed ethylene-propylene copolymer. The potential for further optimisation and tailoring of product properties can be realised through an improved understanding of how the copolymer phase produced in the second reactor develops with increasing ethylene incorporation, providing an intermediate link between predicted physical behaviour and the process parameters required to achieve this. To this end, the morphological development of heterophasic or impact copolymers, has been a topic of interest of many studies to date, yet due to the complexity of these polymers, there is still some uncertainty with regards to the mechanism of copolymer growth as well as the structure-function relationships that exist. These studies were limited either due to the use of autoclave products or final impact copolymer products obtained from industry. The work presented in this study was aimed at understanding how the nascent copolymer phase develops during a transition from homopolymer to the final copolymer. This was done by selecting samples at certain intervals from two different commercial gas-phase processes, yielding two sets of four samples, each with a range of increasing ethylene contents. These samples provided the unique opportunity to study the early development of copolymer in a sequential manner (as each sample builds on the morphology of the previous one). The morphological development of copolymer in these samples was investigated by high resolution FE-SEM and it was observed that the copolymers showed different degrees of internal and external distribution as well as porosity for the different sets, determined by the initial porosity of the homopolymer. It was also found that the copolymer was radially distributed throughout the particle in all instances, suggesting that ethylene monomer diffusion limitations did not play a significant role in the copolymerization process. A further aim of the study was to determine the effect of ethylene incorporation on bulk sample crystallinity, microstructure and chemical composition. It was observed by SCALLS and TREF that increasing ethylene incorporation attenuated the crystallinity of the homopolymer, resulting in a distribution of components with different crystallinities within the samples, suggesting some interaction between the developing copolymer and existing homopolymer. During the microstructural development of these samples, longer or more blocky ethylene sequences seemed to be favoured above isolated ethylene sequences with increasing ethylene incorporation and it was shown by solid-state NMR that ethylene partitioning between both amorphous and rigid environments occurred. Detailed characterization (solution and solid-state 13C NMR, HT-SEC and HT-HPLC) of the semi-crystalline copolymer fractions provided…