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

Botha, Linda (2014-04)

Thesis (PhD)--Stellenbosch University, 2014.

Thesis

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 some information on the development of microstructure and chemical composition in these fractions that are responsible for compatibilization between the homopolymer matrix and dispersed rubber phase. Based on the different observations from the investigations outlined above, a model for copolymer development in each set was proposed and related to the physical property development observed for these samples.

AFRIKAANSE OPSOMMING: Die impak-kopolimere – ook bekend as heterofase polipropileen-etileen-kopropileen kopolimere (HEPC’s) – wat tans in die bedryf vervaardig word, is bekend vir hul goeie meganiese eienskappe, naamlik om die styfheid van ’n polipropileenmatriks met die sterkte van ’n etileen-propileen kopolimeer, wat in die matriks versprei is, te kombineer. Die potensiaal vir die optimisering en pasmaak van produkeienskappe kan bewerkstellig word deur beter begrip ten opsigte van hoe die kopolimeerfase wat in die tweede reaktor vervaardig word, ontwikkel as gevolg van toenemende inkorporasie van etileen, en hoe dit ’n skakel skep tussen voorspelbare fisiese gedrag en die prosesparameters wat nodig is hiervoor. Tot datum het heelparty studies gefokus op die morfologiese ontwikkeling van heterofase of impak-kopolimere, maar as gevolg van die komplekse aard van hierdie polimere is daar nog steeds onsekerheid oor die meganisme van kopolimeerontwikkeling, asook die verwantskappe tussen die polimeerstruktuur en -funksie. Sodanige studies was beperk omdat óf outoklaafprodukte óf finale produkte van industriële prosesse gebruik is. Die doel van hierdie studie was om begrip te kry vir hoe die kopolimeerfase ontwikkel tydens ’n oorgang van homopolimeer tot die finale produk. Hiervoor is twee stelle van vier monsters met toenemende etileeninhoude tydens die oorgang in twee verskillende gasfaseprosesse verkry. Hierdie monsters het die unieke geleentheid gebied vir die opvolgende bestudering van die vroeë ontwikkeling van die kopolimeer, deurdat elke monster voortgebou het op die morfologie van die vorige monster. Die morfologiese ontwikkeling van die kopolimeer is ondersoek deur van hoëresolusie FE-SEM gebruik te maak. Verskillende wyses van interne en eksterne verspreiding, sowel as porositeit van die onderskeie stelle (soos bepaal deur die aanvanklike porositeit van die homopolimeer), is vir die verskillende prosesse waargeneem. Daar is ook waargeneem dat die kopolimeer in alle gevalle op verskeie straalposisies binne die partikel versprei is, waarvan afgelei kan word dat monomeerdiffusiebeperking nie ’n beduidende rol in die kopolimerisasieproses speel nie. ’n Verdere doel van hierdie studie was om die uitwerking van etileen-inkorporasie op die kristalliniteit, mikrostruktuur en chemiese samestelling van die polimeer te bepaal. Deur middel van SCALLS en TREF is bevind dat toenemende etileen-inkorporasie die kristalliniteit van die homopolimeer verswak het. Die gevolg was die vorming van ’n verskeidenheid komponente met verskillende kristalliniteite, wat dui op ’n interaksie tussen die groeiende kopolimeer en die bestaande homopolimeer. Tydens die ontwikkeling van die mikrostruktuur van die monsters het dit geblyk dat lang, opeenvolgende etileeneenhede tydens toenemende etileen-inkorporasie vinniger ontwikkel as afgesonderde etileen- en propileeneenhede. Deur middel van soliedefase-KMR is daar bewys dat die etileen in beide amorfe en kristalagtige areas versprei is. Die semi-kristallyne kopolimere wat deur TREF verkry is, is verder gekarakteriseer met behulp van KMR in oplossing sowel as die soliede fase, HT-SEC en HT-HPLC, wat meer inligting verskaf oor die ontwikkeling van die mikrostruktuur en chemiese samestelling van hierdie fraksies wat normaalweg verantwoordelik is vir die interaksies tussen die homopolimeermatriks en die verspreide rubberfase. Op grond van die waarnemings soos hierbo vermeld, word ’n model vir die kopolimeerfase-ontwikkeling van elke stel monsters in hierdie studie voorgestel en verbind met die ontwikkeling van die waargenome fisiese eienskappe.

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