Ethylene and propylene copolymers utilizing Fishcer-Tropsch 1-olefins

Joubert, D. J. (Dawid Johannes) (2000-12)

Thesis (PhD)--Stellenbosch University, 2000.


ENGLISH ABSTRACT: From the Sasol Fischer- Tropsch process, a variety of different a-olefins are produced. Sasol recently started presenting these a-olefins to polymer producers. To demonstrate the application possibilities of these a-olefins as comonomers for ethylene and propylene polymerization, it was necessary to first synthesize catalysts having a combination of high activity and good comonomer incorporation, and in the case of propylene copolymers, also sufficient stereospecificities. Different methods to produce catalysts conforming to these requirements were investigated and it was found that catalysts produced from a MgCb-support activated by a combination of chemical and mechanical means produced suitable catalysts. The amount of alcohol used during the support activation step and the time allowed for alkylation of the active centers were important. No clear correlation between total titanium content and activity was observed. The degree to which active sites are protected was evaluated from the amount of comonomer present in the final copolymer based on the amount added to the reaction. Cyclopentadiene was used to selectively deactivate the unprotected active sites to determine the ratio between protected and "open" active sites. High activity catalysts are not suitable for gas-phase copolymerization and were consequently "diluted" by dispersion in a pre-formed polymer powder and by prepolymerization. Catalyst activity based on titanium content was substantially decreased, but comonomer incorporation was not. Catalysts for producing crystalline polypropylene require the presence of both an internal and external electron donor. It was shown that isotacticity increased linearly with an increase in external modifier at the expense of catalyst activity and that a double treatment of the support or catalyst before the final TiCl4 fixation was more effective at increasing stereospecificity. The less stereospecific sites are more capable of accepting bulky comonomers in the coordination complex and thus by decreasing the amount of less-stereospecific active sites, the overall capability of the catalyst to incorporate comonomer was decreased. Comonomer sequence distributions and average lamellar thicknesses of different ethylene / a-olefin copolymers were calculated from CH2 dyad concentrations determined by I3C NMR spectroscopy. Ethylene sequences in the I-butene containing copolymers are generally longer than those where a higher a-olefin was used as comonomer which indicates that a more random comonomer distribution is obtained when the higher u-olefins are used. It was shown that an inverse relationship exists between branch size and density. For density, no effect resulting from the comonomer type was observed. This same inverse relationship was also observed for tensile strength. Modulus, hardness and impact strength, on the other hand, did show an effect resulting from the comonomer type. Modulus and hardness were not depressed as much and impact strength improved more than what was expected from calculations based on branch size. Homogeneous copolymers have broad melting peaks. It was shown that at sufficiently high comonomer content, peak broadening occurs when the higher cc-olefins are used as comonomer, which also indicates that more random comonomer distributions are obtained with the higher aolefins. From the chain propagation probabilities calculated it was observed that two types of active sites are present. Those responsible for producing mainly polyethylene have an alternating character while the sites capable of incorporating comonomer have a blocky character. It was expected that the additional introduction of a third a-olefin during ethylene / 1- pentene copolymerization will produce a terpolymer with density and related properties similar to the mathematical average between those of the relevant copolymers. This was only observed for the terpolymers containing l-heptene, 1- octene and l-nonene. The I-butene containing terpolymers have densities well below the expected values while the I-hexene containing terpolymers have values very similar to that of the ethylene / l-hexene copolymer densities, but still below the expected values. Properties related to density, such as tensile strength and modulus, follow this same trend. It is believed that the presence of l-pentene breaks up the tendency of the lower c-olefins to cluster which results in improved randomness. Compared to the copolymers, I-butene and l-hexene containing terpolymers seem to reach the impact strength maximum at a lower total comonomer content than that of the I-pentene copolymers which also indicates an enhanced effect from the combined use of I-pentene with these o-olefins. No substantial difference between impact strengths of co- and terpolymers prepared with higher a-olefins was observed. In general, the melting temperatures of the terpolymers are slightly lower and spread over a wider temperature range than those of the copolymers which can be realized if the comonomer units are less clustered and thus more randomly distributed. Decrease in melting temperature was, however, not as much as for the metallocene catalyzed terpolymers. From sequence length calculations from l3C NMR spectroscopy it was found that the crystallizable ethylene sequences of l-butene containing terpolymers were shorter than those of the corresponding copolymers, which confirms the notion that the introduction of a third comonomer resulted in an increase in randomness. Crystallizable sequence lengths became gradually shorter when higher cc-olefins were used in co- and terpolymers and those of the terpolymers are generally shorter. From the different types of active centers present on a Ziegler-Natta catalyst, it was reasoned that three main types of polymer chains can be present in the terpolymers: (a) ethylene homopolymer, (b) ethylene / lower a-olefin copolymer and (c) ethylene / lower a-olefin / higher a-olefin terpolymer. The ratio between these components in the final terpolymer depends primarily on the size of the higher a-olefin. The larger the third a-olefin becomes, the more active sites will reject it, resulting in a higher amount of ethylene / lower a-olefin copolymer. It was thus suggested that the large decrease in density and the associated change in related properties observed for ethylene / l-pentene / l-butene terpolymers can be related to the combined result of improved random comonomer incorporation together with the decrease in the amount of ethylene homopolymer. The possibilities of using the higher a-olefins having uneven carbon numbers were investigated in random propylene copolymers. Similar to that observed for the ethylene copolymers, less of the higher a-olefins was necessary to achieve a certain level of crystallinity. A good agreement was observed between tensile properties and comonomer type and content and the size of the branch and the resulting defect it causes in the crystal structure is the primary factor affecting tensile strength. For impact strength a close correlation between the size of the comonomer side chain and comonomer content was observed. It was shown that the effect of the heptyl branch derived from a l-nonene unit was 2.3 times that of the propyl group derived from the I-pentene unit. Properties of block copolymers can not be related directly to l-pentene content as is the case with random copolymers, mainly due to the heterogeneity of the block copolymers. The activating effect of hydrogen on catalyst activity was observed. It was also observed that the amount of l-pentene incorporated in the copolymer as well as the copolymer yield were higher in the presence of hydrogen than when the reaction was carried out in the absence of hydrogen. By using DSC it was possible to identify different crystalline phases in the propylene / I-pentene block copolymers due to the differences in their crystallization kinetics. A connection between the low-temperature peak and impact strength was observed. It was found that the presence of the low-temperature peak resulting from thin lamellae formed by chain containing many defects was undesirable when high impact strength is required. It was not possible to quantify the extent to which the intensity of this peak affected mechanical properties of the block copolymers. However, from sequence length calculations it was found that the ratio between the propylene and 1- pentene sequence lengths could be related quantitatively to impact strength, modulus, hardness and tensile strength of the polymers investigated.

AFRIKAANSE OPSOMMING: 'n Verskeidenheid van verskillende «-olefiene word in die Sasol Fischer- Tropsch proses vervaardig. Sasol het onlangs begin om hierdie a-olefiene aan polimeervervaardigers te bied. Om die toepassingsmoontlikhede van hierdie aolefiene as komonomere vir etileen en propileen polimerisasie te demonstreer was dit nodig om eerstens 'n katalis met 'n kombinasie van hoë aktiwiteit en goeie komonomeer invoeging te sintetiseer. In die geval van propileen word voldoende stereospesifisiteit ook vereis. Verskillende metodes om kataliste wat aan hierdie vereistes voldoen te vervaardig is ondersoek. Daar is gevind dat kataliste waarvan die MgCb basis deur 'n kombinasie van chemiese- en meganiese metodes geaktiveer is, die vereiste eienskappe besit. Die hoeveelheid alkohol gebruik tydens die basis aktivering stap en die tyd toegelaat vir die alkilering van die aktiewe spesies was belangrik. Geen duidelike verband tussen totale titaan inhoud en aktiwiteit is waargeneem nie. Die mate waartoe aktiewe spesies beskerm is, is bepaal vanaf die hoeveelheid komonomeer teenwoordig in die finale kopolimeer in verhouding met die hoeveelheid bygevoeg tydens die reaksie. Siklopentadieen is gebruik om onbeskermde aktiewe spesies selektief te deaktiveer om die verhouding tussen beskermde en oop aktiewe spesies te bepaal. Hoëaktiwiteit kataliste is nie geskik vir gasfase kopolimerisasie nie en is gevolglik verdun deur dit te versprei in 'n voorafgevormde polimeer poeier en deur prepolimerisasie. Katalis aktiwiteit gebaseer op titaan inhoud was aansienlik laer maar komonomeer invoeging was nie merkbaar beinvloed nie. Kataliste VIr die vervaardiging van kristallyne polipropileen vereis die teenwoordigheid van beide interne- en eksterne elektron donors. Daar is gewys dat isotaktisiteit liniêr verhoog met 'n toename in eksterne modifiseerder ten koste van katalis aktiwiteit en dat 'n dubbele behandeling van die basis of katalis, voor die finale titaan fiksering, meer effektief was om stereospesifisiteit te verhoog. Die spesies met laer stereospesifisiteit is meer bevoeg om bonkige komonomere in die koërdinasie kompleks toe te laat en deur dus die konsentrasie van hierdie spesies te verlaag is die bevoegdheid van die katalis om bonkige komonomeer te inkorporeer, verlaag. Komonomeer reeksverspreiding en gemiddelde lamellêre dikte van verskillende etileen / o-olefien kopolimere is bereken vanaf CH2 diad konsentrasie bepaal deur KMR spektroskopie. Etileen reekse in die kopolimere wat l-buteen bevat is oor die algemeen langer as dié waarin 'n hoër a-olefien as komonomeer gebruik was, wat aandui dat 'n meer egalige komonomeer verspreiding verkry word as hoër «-olefiene gebruik word. Daar is getoon dat 'n inverse verhouding tussen die grootte van die vertakking en digtheid bestaan. Geen effek komende van die komonomeer tipe kon waargeneem word nie. Hierdie soortgelyke inverse verhouding was ook waargeneem vir treksterkte. Modulus, hardheid en impaksterkte aan die ander kant, hét 'n effek komende van die komonomeer tipe getoon. Modulus en hardheid was nie soveel verlaag, en impak sterkte soveel verhoog as wat verwag is vanaf berekeninge gebaseer op vertakking grootte nie. Homogene kopolimere toon breë smeltpieke. Daar is gewys dat by voldoende komonomeer inhoud 'n verbreding van die pieke voorgekom het wanneer hoër a-olefiene as komonomere gebruik is, wat ook aandui dat 'n meer egalige komonomeer verspreiding met hierdie c-olefiene verkry kan word. Vanaf berekening van die ketting voortplantingsmoontlikhede is waargeneem dat twee tipes aktiewe spesies teenwoordig is. Die verantwoordelik vir die vorming van polietileen het 'n alternerende karakter terwyl die sentra wat komonomeer kan inkorporeer 'n blokagtige karakter het. Daar is verwag dat die addisionele byvoeging van 'n derde o.-olefien tydens etileen / I-penteen kopolimerisasie, 'n terpolimeer met digtheid en verwante eienskappe soortgelyk aan die wiskundige gemiddelde tussen dié van die relevante kopolimere tot gevolg sal hê. Dit was egter slegs waargeneem vir terpolimere wat I-hepteen 1- okteen en l-noneen bevat. Die I-buteen bevattende terpolimere het digthede ver onder die verwagte waardes terwyl die I-hekseen bevattende terpolimere waardes soortgelyk aan die etileen / l-hekseen kopolimeer digthede het' wat steeds onder die verwagte waardes is. Eienskappe verwant aan digtheid, soos treksterkte en modulus, volg dieselfde neiging. Dit word geglo dat die teenwoordigheid van l-penteen die neiging van die laer a-olefiene om saam te bondelopbreek wat 'n verbeterde egaligheid in komonomeerverspreiding tot gevolg het. Vergeleke by die kopolimere blyk dit dat die terpolimere wat l-buteen en I-hekseen bevat, die impaksterkte maksimum by 'n laer totale komonomer inhoud bereik as dié van die 1-penteen polimere. Dit dui ook op 'n verbeterde effek as gevolg van die gekombineerde gebruik van l-penteen met ander a-olefiene. Geen duidelike verskil tussen die impaksterktes van ko- en terpolimere, wat met die hoër a-olefiene berei was, is waargeneem me. In die algemeen is die smeltingstemperature van die terpolimere effens laer, en versprei oor 'n wyer temperatuurgebied as dié van die kopolimere wat verklaar kan word as komonomere minder saamgebondel is en dus meer homogeen versprei is. Die afname in smelt temperatuur was egter nie soveel as dié van die metalloseengekataliseerde terpolimere nie. Vanaf reekslengte berekeninge met behulp van KMR spektroskopie is daar gevind dat die kristalliseerbare etileen reekse van die l-buteen bevattende terpolimere korter was as dié van die ooreenkomstige kopolimere, wat die gevoel dat die byvoeging van 'n derde komonomeer 'n verbeterde komonomeerverspreiding tot gevolg het, bevestig. Vanaf die verskillende aktiewe spesies teenwoordig in 'n Ziegler-Natta katalis is daar geredeneer dat drie hooftipes polimeerkettings in die terpolimere teenwoordig kan wees: (a) etileen hompolimeer, (b) etileen / laer a-olefien kopolimeer en (c) etileen / laer a-olefien / hoër a-olefien terpolimeer. Die verhouding tussen hierdie komponente in die finale terpolimeer hang primêr van die grootte van die hoër aolefien af. Hoe groter die derde a-olefien is, deur hoe meer van die aktiewe spesies sal dit verwerp word wat 'n groter hoeveelheid etileen / laer a-olefien kopolimeer tot gevolg sal hê. Daar word dus voorgestel dat die groot afname in digtheid en die geassosieerde veranderings in die toepaslike eiensappe waargeneem vir etileen / 1- penteen / I-buteen terpolimeere, herlei kan word na die gekombineerde effek van verbeterde komonomeerverspreiding tesame met die afname lil die hoeveelheid etileen homopolimeer. Die moontlikheid om hoër cc-olefiene met onewe koolstofgetalle te gebruik in homogene propileen kopolimere is ondersoek. Soortgelyk aan dit wat waargeneem is vir die etileen kopolimere, was minder van die hoër cc-olefiene nodig om 'n spesifieke vlak van kristalliniteit te bereik. 'n Goeie ooreenkoms tussen trek-eienskappe en . komonomeer tipe- en inhoud is waargeneem en die grootte van die vertakking en die gevolglike defek wat dit veroorsaak in die kristal struktuur is die primêre faktor wat treksterkte beinvloed. Vir impaksterkte is 'n noue verband tusssen die grootte van die vertakking en komonomeer inhoud waargeneem. Daar is aangetoon dat die effek van die heptiel vertakking vanf die l-noneen eenheid 2.3 keer dié van die propiel groep van die l-penteen eenheid is. Eienskappe van blok kopolimere kan nie direk na l-penteen inhoud herlei word soos die geval met die homogene kopolimere was nie, hoofsaaklik as gevolg van die heterogeniteit van die blok kopolimere. Die aktiverende effek van waterstof op katalis aktiwiteit is waargeneem. Daar is ook gesien dat die hoeveelheid l-penteen geïnkorporeer in die kopolimeer, sowel as die kopolimeer opbrengs, hoër was in die teenwoordigheid van waterstof as wanneer die reaksie sonder waterstof uitgevoer is. Deur DSC te gebruik was dit moontlik om verskillende kristallyne fases in die propileen / l-penteen blok kopolimere vanaf die verskille in hulle krisallisasie kinetika, te identifiseer. 'n Verbintenis tussen die lae-temperatuur piek en impaksterkte is waargeneem. Daar is gevind dat die teenwoordigheid van die laetemperatuur piek, komende van die dun lamellas gevorm, deur kettings wat baie defekte bevat, ongewens is wanneer hoë impaksterkte vereis word. Dit was nie moontlik om die bereik waartoe die intensiteit van hierdie piek die meganiese eienskappe van die blok kopolimere affekteer, te kwantifiseer nie. Vanaf reekslengte bepalings is daar gevind dat die verhouding tussen die propileen en I-penteen reekslengtes kwantitiatief herlei kan word na impaksterkte, modulus, hardheid en treksterkte van die ondersoekte polimere.

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