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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/51747 |
| Date | 12 1900 |
| Creators | Joubert, D. J. (Dawid Johannes) |
| Contributors | Van Reenen, A. J., Stellenbosch University. Faculty of Science . Dept. of Chemistry & Polymer Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 288 p. |
| Rights | Stellenbosch University |
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