Patent Application
20080269441
This invention relates to the polymerization of ethylene to produce ethylene homopolymers and copolymers with a chromium-based polymerization catalyst in the presence of triethylboron co-catalyst under conditions to provide a polymer product of good optical properties while retaining good mechanical or physical properties.
Polyethylene as a homopolymer or an ethylene alpha olefin copolymer can be employed in a number of commercial applications in which good visual or optical properties are important. For example, polyethylene may be employed in the production of various products such as bottles or other containers and the like which can be produced by blow molding or extrusion molding operations. In such applications, it is desirable to arrive at a product having good optical characteristics in which a desired color is maintained without extensive yellowing of the bottle or other container with time. The resistance of a polymer product to yellowing with time can be measured by the Yellowness Index (YI) as determined in accordance with American Society for Testing Material Standard ASTM-D1925. As understood by those skilled in the art, an increase in the Yellowness Index with time is a measure of the undesirable discoloration of the polymer product.
Other significant physical characteristics of polyethylene polymers include the molecular weight distribution, MWD (a ratio of the weight average molecular weight, Mw, to the number average molecular weight, Mn), and shear response as determined by the ratio of melt indices as determined in accordance with standard ASTM D1238. Thus, the shear response, SR2, is characterized as a ratio of the high load melt index (HLMI) to the melt index MI2 and the shear response, SR5, is the ratio of the high load melt index to the melt index MI5. The various melt indices are conventionally reported in terms of melt flows in grams/10 minutes (g/10 min.) or the equivalent measure as expressed in terms of decigrams/minute (dg/min.).
The polymer fluff withdrawn from the polymerization reactor is typically separated from the diluent in which the polymerization reaction proceeds, and then melted and extruded to produce particles of the polymer product, typically in the nature of pellets having dimensions of about ⅛″-¼″ which then are ultimately used to produce the polyethylene containers or other commercial products. During the extrusion process, stabilizing agents may be incorporated into the polymer. Such stabilizing agents typically include phenolic antioxidants, such as sterically-hindered phenols and phosphite antioxidants. Other polymer characteristics which are significant in terms of suitability of the polymer for the end product include resistance to mechanical failure as measured by notched constant ligament stress (NCLS) and environmental stress crack resistance (ESCR) as determined in accordance with American Society Testing Standard ASTM D1693.
In accordance with the present invention, there is provided a process for the polymerization of ethylene to provide an ethylene homopolymer or copolymer of a reduced Yellowness Index. In carrying out the invention, a feed stream, comprising an inert hydrocarbon diluent containing ethylene, and optionally a higher alpha olefin comonomer, is supplied to a polymerization reaction zone. The feed stream is composed primarily of the inert hydrocarbon diluent, such as a normally liquid alkane or an aromatic compound, with the ethylene being present in a minor amount, usually no more than 10 wt. % of the diluent. The higher molecular weight alpha olefin comonomer, if present, will be employed in an amount that is less than the amount of the ethylene in the feed stream. Hydrogen may also be supplied to the polymerization reaction zone.
A chromium-based polymerization catalyst and a triethylboron co-catalyst are incorporated into the feed stream within the polymerization reactor. The polymerization catalyst will normally be used in an amount within the range of 0.008-0.1 wt. % of the diluent in the feed stream and the triethylboron co-catalyst will be incorporated in an amount within the range of 0.1-50 parts per million (ppm) of the diluent. The catalyst and the co-catalyst may be supplied separately or mixed and supplied either continuously or intermittently to the feed stream as it is fed into the polymerization reactor. The polymerization reaction zone is operated under polymerization conditions to produce an ethylene polymer fluff by the polymerization or co-polymerization of the ethylene monomer. The polymer fluff is withdrawn from the polymerization reaction zone and then heated to a temperature sufficient to melt the fluff for extrusion. The melted fluff is then extruded to produce particles of the ethylene homopolymer or copolymer. In accordance with the invention, the reaction zone is operated under conditions effective to produce a polymer product, which has a reduced Yellowness Index (YI) than would be the case where the chromium-based polymerization catalyst is employed without the addition of the triethylboron co-catalyst. Specifically, the polymer product resulting from the extrusion of the fluff has a Yellowness Index after aging at a temperature of 175° F. for 60 hours, which is at least 5% less than the corresponding Yellowness Index of the polymer product produced without the use of the triethylboron co-catalyst.
In a further aspect of the invention, the polymer product is a copolymer of ethylene and a C3-C8 olefin, more specifically, hexene. The hexene, or other higher molecular weight olefin, may be employed in a concentration that is less than 50 wt. % of the concentration of the ethylene in the feed stream. In one embodiment of the invention, the triethylboron co-catalyst is incorporated into the feed stream in an amount effective to increase the activity of the polymerization catalyst by an amount which is at least 10% greater than the activity of the catalyst without the addition of the triethylboron co-catalyst. In yet a further aspect of the invention, the triethylboron co-catalyst is employed in the feed stream in an amount to produce a polymer product having a broader molecular weight distribution than the molecular weight distribution of the corresponding polymer product produced without the addition of the triethylboron co-catalyst.
The invention will be described with reference to a loop-type reactor used in the production of ethylene homopolymers or copolymers. Referring to
The catalyst and co-catalyst may be introduced into the polymerization reactor by any suitable technique. In one mode of operation, the catalyst system may be introduced into the reactor employing a catalyst injection system of a type often employed for Phillips-type silica supported chromium catalysts. In this mode of application a catalyst system, comprising a chromium-based polymerization catalyst as described previously and a triethylboron (TEB) co-catalyst, is incorporated into the polymerization reactor through catalyst feed line 14. In the catalyst injection system, a diluent, such as isobutane, is supplied to a mixing line 18 via a supply line 19. The TEB co-catalyst is supplied through line 21 and the chromium-based catalyst is introduced through line 22, and the catalyst system is then introduced into the reactor 10 via line 14. Alternatively or in addition to introduction through line 14, the catalyst system may be passed through line 16 to line 12 for introduction to reactor 10. The catalyst may be supplied either continuously or intermittently to the carrier stream for introduction into the reactor. The catalyst may be prepolymerized prior to introduction into the polymerization reactor 10. For example, the chromium based catalyst and the TEB cocatalyst may be polymerized in a tubular reactor prior to introduction into the reactor, as described in U.S. Pat. No. 4,767,735 to Ewen et al. For a further description of suitable prepolymerization procedures which may be employed in carrying out the invention, reference is made to the aforementioned patent U.S. Pat. No. 4,767,735, the entire disclosure of which is incorporated herein by reference. In another mode of operation, the chromium-based catalyst and the TEB co-catalyst may be introduced into the polymerization reactor through separate feed lines. For example, referring to
At the product side of the reactor, the ethylene homopolymer or copolymer is withdrawn via line 26. Typically, a deactivator is incorporated into the product stream in order to terminate the polymerization reaction in the solvent stream containing the polyethylene. The product is supplied through line 26 to a concentration and recovery system 28 in which polyethylene fluff is extracted. Diluent and unreacted ethylene are recovered through a suitable purification and recovery system (not shown) and recycled to the reactor 10. The product stream containing the polyethylene fluff, which is now free of gaseous ethylene, is withdrawn from the recovery system via line 30.
The polyethylene fluff is supplied to the input hopper 32 of an extruder-die system 34. Stabilization additives are supplied to the hopper 32 through line 31. In the extruder-die system, the polymer is heated to a molten state, and the molten polymer is extruded and then cut into appropriate particles. Typically, the polyethylene product may be extruded and die cut into pellets which are discharged from the product end 36 of the extruder-die system 34. These pellets may then be heated and extruded and molded in various applications, such as in the production of bottles or other polyethylene products.
The chromium-based catalyst employed in carrying out the present invention may be of any suitable type that is effective in the polymerization or copolymerization of ethylene. Typically the chromium-based catalyst will incorporate a silica support and have a chromium content of ranging to ½ weight % to 5 weight % chromium. The chromium-based catalyst may also include titanium which normally will be present in the amount of 1-5 weight %. Suitable chromium-based catalysts which may be employed in carrying out the present invention are disclosed in U.S. Pat. No. 6,423,663 to Debras and U.S. Pat. No. 6,489,428 to Debras, et al, the entire disclosures of which are incorporated herein by reference.
In experimental work respecting the present invention, ethylene homopolymers and ethylene-hexene copolymers were produced in standard laboratory polymerization runs to produce the corresponding polymer fluff. In each case, the polymer fluff was stabilized by the addition to the fluff during extrusion to form pellets of a stabilized package having 400 ppm of a phenolic antioxidant identified as Irgonox 1010 and 1,600 ppm of a phosphite antioxidant identified as Irgafos 168. After extrusion to form the polymer pellets, the pellets were heat aged under standard conditions for 60 hours with the Yellowness Index numbers determined at approximately 12, 36 and 60 hours.
The catalysts employed in the experimental work were commercially available chromium-based catalysts and are identified herein as Catalysts A, B, and C, characterized by a chromium content of about 1.0 wt. % for each catalyst. Catalysts A, B and C also contained titanium in respective amounts of 2.4, 2.3 and 3.7 weight % titanium. In the laboratory polymerization runs, polymerization was carried out without a co-catalyst and with triethylboron as a co-catalyst in amounts ranging from 4-12 ppm of the diluent. The diluent used was isobutane. The ethylene was used in the polymerization runs in a concentration of 8 wt. % of the isobutane diluent and for the copolymers, the comonomer 1-hexene was used in a concentration of up to 72 wt. %. The polymerization or copolymerization runs were carried out in a bench reactor at temperatures ranging from 94 to 104° C. The catalysts were activated at an activation temperature of about 1,100° F.
The homopolymer or copolymer fluff recovered from the polymerization reactor was blended with the antioxidant additive package identified above and for the color studies then extruded into pellets to produce polymer products identified herein as products PA, PB and PC, corresponding respectively to the catalyst used as identified above as catalysts A, B an C in the polymerization runs.
In one set of experiments, ethylene homopolymer was produced without the TEB co-catalyst and with the TEB co-catalyst at concentrations of 4, 8 and 12 ppm to produce homopolymer polymers PA, PB and PC. The activities of the catalyst in grams of polymer per grams of catalyst per hour for runs varying from 0 ppm TEB up to 12 ppm TEB are set forth in Table I.
The melt flow values of MI2, MI5 and HLMI as a function of the various triethylboron concentrations for the polymer products PA, PB, and PC are set forth in Tables II-IV.
The shear ratios SR2 (HLMI/MI2) and SR5 (HLMI/MI5) for the polymer products are set forth in Tables V and VI.
As can be seen from an examination of the data in Tables I-VI, the low levels of the TEB used have a significant effect on polymerization kinetics. For Catalyst A, the catalyst showed a maximum or a near maximum activity at 4 ppm of TEB with roughly the same activity shown at 8 ppm TEB with a slightly increased activity at 12 ppm TEB. For Catalysts B and C, the greatest activities occurred in the 4-8 ppm TEB range and then decreased somewhat at the highest level tested, 12 ppm TEB. As indicated in Tables V and VI, the shear ratios SR2 and SR5 were generally increased by the addition of the TEB co-catalyst throughout the 4-12 ppm range tested.
In further experimental work, copolymers were produced employing hexene as the comonomer in concentrations of 0.18 wt. % and 0.36 wt. % in the diluent. In this experimental work, the TEB concentration was held constant at 4 ppm. The same antioxidant additive package as described above was added to the polymer fluff during the extrusion procedure. The values of MI2, MI5 and the high load melt index, HLMI, corresponding to the various hexene concentrations are set forth in Tables VII, VIII and IX, respectively.
The resulting shear ratio values of SR2 and SR5 for the polymer products A, B and C are set forth in Tables X and XI.
In further experimental work to determine the color integrity of polymer products polymerized employing triethylboron as a co-catalyst, color integrity studies were carried out on ethylene-hexene co-polymers polymerized with the chromium-based catalysts identified above as Catalysts A and B without the addition of triethylboron and with the triethylboron added to the isobutene diluent in an amount of 4 ppm. The ethylene monomer was added to the diluent in the polymerization system in an amount of 4-8 wt. %. The hexene comonomer was added to the diluent in an amount of up to 0.72 wt. %. The fluff recovered from the laboratory polymerization reactor was extruded after stabilization of the fluff with the additive package described above, 400 ppm of the phenolic antioxidant Irgonox 1010 and 1,600 ppm of the phosphite antioxidant Irgafos 168. After recovery of the pelletized polymer products from the extrusion system, they were aged at a temperature of 175° F. for a period of 60 hours. In the course of the aging studies, Yellowness Index values of the polymer products were measured at times of approximately 12 hours, 36 hours and 60 hours. The Yellowness Index values were determined in accordance with American Society for Testing Materials Standards ASTM-D1925. The experimental work uniformly showed a reduction in the Yellowness Index of the polymer product through the use of the triethylboron as a co-catalyst. The results of this experimental work are illustrated in
As can be seen from an examination of the data presented in
As indicated by the foregoing experimental work, the use of a triethylboron co-catalyst in accordance with the present invention enables the production of polymers of reduced Yellowness Index and improved aging characteristics in terms of Yellowness Index, while at the same time providing for enhanced catalyst activity and improved polymer characteristics.
Having described specific embodiments of the present invention, it will be understood that modifications thereof may be suggested to those skilled in the art, and it is intended to cover all such modifications as fall within the scope of the appended claims.
