Ethylene and propylene copolymers utilizing Fishcer-Tropsch 1-olefins

[摘要] 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. Todemonstrate the application possibilities of these a-olefins as comonomers forethylene and propylene polymerization, it was necessary to first synthesize catalystshaving a combination of high activity and good comonomer incorporation, and in thecase of propylene copolymers, also sufficient stereospecificities.Different methods to produce catalysts conforming to these requirements wereinvestigated and it was found that catalysts produced from a MgCb-support activatedby a combination of chemical and mechanical means produced suitable catalysts. Theamount of alcohol used during the support activation step and the time allowed foralkylation of the active centers were important. No clear correlation between totaltitanium content and activity was observed. The degree to which active sites areprotected was evaluated from the amount of comonomer present in the final copolymerbased on the amount added to the reaction. Cyclopentadiene was used to selectivelydeactivate the unprotected active sites to determine the ratio between protected andopen active sites. High activity catalysts are not suitable for gas-phasecopolymerization and were consequently diluted by dispersion in a pre-formedpolymer powder and by prepolymerization. Catalyst activity based on titaniumcontent was substantially decreased, but comonomer incorporation was not.Catalysts for producing crystalline polypropylene require the presence of both aninternal and external electron donor. It was shown that isotacticity increased linearlywith an increase in external modifier at the expense of catalyst activity and that adouble treatment of the support or catalyst before the final TiCl4 fixation was moreeffective at increasing stereospecificity. The less stereospecific sites are more capableof accepting bulky comonomers in the coordination complex and thus by decreasingthe amount of less-stereospecific active sites, the overall capability of the catalyst toincorporate comonomer was decreased.Comonomer sequence distributions and average lamellar thicknesses of differentethylene / a-olefin copolymers were calculated from CH2 dyad concentrationsdetermined by I3C NMR spectroscopy. Ethylene sequences in the I-butenecontaining copolymers are generally longer than those where a higher a-olefin wasused as comonomer which indicates that a more random comonomer distribution isobtained when the higher u-olefins are used. It was shown that an inverserelationship exists between branch size and density. For density, no effect resultingfrom the comonomer type was observed. This same inverse relationship was alsoobserved for tensile strength. Modulus, hardness and impact strength, on the otherhand, did show an effect resulting from the comonomer type. Modulus and hardnesswere not depressed as much and impact strength improved more than what wasexpected from calculations based on branch size. Homogeneous copolymers havebroad melting peaks. It was shown that at sufficiently high comonomer content, peakbroadening occurs when the higher cc-olefins are used as comonomer, which alsoindicates that more random comonomer distributions are obtained with the higher aolefins.From the chain propagation probabilities calculated it was observed that twotypes of active sites are present. Those responsible for producing mainlypolyethylene have an alternating character while the sites capable of incorporatingcomonomer 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 relatedproperties similar to the mathematical average between those of the relevantcopolymers. This was only observed for the terpolymers containing l-heptene, 1-octene and l-nonene. The I-butene containing terpolymers have densities well belowthe expected values while the I-hexene containing terpolymers have values verysimilar to that of the ethylene / l-hexene copolymer densities, but still below theexpected 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 thetendency of the lower c-olefins to cluster which results in improved randomness.Compared to the copolymers, I-butene and l-hexene containing terpolymers seem toreach the impact strength maximum at a lower total comonomer content than that ofthe I-pentene copolymers which also indicates an enhanced effect from the combineduse of I-pentene with these o-olefins. No substantial difference between impactstrengths of co- and terpolymers prepared with higher a-olefins was observed.In general, the melting temperatures of the terpolymers are slightly lower and spreadover a wider temperature range than those of the copolymers which can be realized ifthe comonomer units are less clustered and thus more randomly distributed. Decreasein melting temperature was, however, not as much as for the metallocene catalyzedterpolymers.From sequence length calculations from l3C NMR spectroscopy it was found that thecrystallizable ethylene sequences of l-butene containing terpolymers were shorterthan those of the corresponding copolymers, which confirms the notion that theintroduction of a third comonomer resulted in an increase in randomness.Crystallizable sequence lengths became gradually shorter when higher cc-olefins wereused 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 wasreasoned 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 inthe final terpolymer depends primarily on the size of the higher a-olefin. The largerthe third a-olefin becomes, the more active sites will reject it, resulting in a higheramount of ethylene / lower a-olefin copolymer. It was thus suggested that the largedecrease in density and the associated change in related properties observed forethylene / l-pentene / l-butene terpolymers can be related to the combined result ofimproved random comonomer incorporation together with the decrease in the amountof ethylene homopolymer.The possibilities of using the higher a-olefins having uneven carbon numbers wereinvestigated in random propylene copolymers. Similar to that observed for theethylene copolymers, less of the higher a-olefins was necessary to achieve a certainlevel of crystallinity. A good agreement was observed between tensile properties andcomonomer type and content and the size of the branch and the resulting defect itcauses in the crystal structure is the primary factor affecting tensile strength. Forimpact strength a close correlation between the size of the comonomer side chain andcomonomer content was observed. It was shown that the effect of the heptyl branchderived from a l-nonene unit was 2.3 times that of the propyl group derived from theI-pentene unit.Properties of block copolymers can not be related directly to l-pentene content as isthe case with random copolymers, mainly due to the heterogeneity of the blockcopolymers. The activating effect of hydrogen on catalyst activity was observed. Itwas also observed that the amount of l-pentene incorporated in the copolymer as wellas the copolymer yield were higher in the presence of hydrogen than when thereaction 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. Aconnection between the low-temperature peak and impact strength was observed. Itwas found that the presence of the low-temperature peak resulting from thin lamellaeformed by chain containing many defects was undesirable when high impact strengthis required. It was not possible to quantify the extent to which the intensity of thispeak affected mechanical properties of the block copolymers. However, fromsequence 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.

[发布机构] Stellenbosch University