Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – Clay
Reexamination Certificate
2002-04-10
2004-05-11
Choi, Ling-Siu (Department: 1713)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
Clay
C502S254000, C502S086000, C502S103000, C502S084000, C526S104000, C526S348000, C526S129000
Reexamination Certificate
active
06734131
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to novel chromium-based catalysts and to an improved process for producing polyolefins, in particular polyethylene, by contacting an olefin monomer with the catalysts of the subject invention in a reaction zone, typically a single reaction zone, to provide a resultant polyolefin that exhibits bimodal molecular weight distribution.
BACKGROUND OF THE INVENTION
In the field of polyolefin manufacture, much attention has been devoted to finding new and improved catalysts capable of producing polyolefins having unique and/or improved properties or capable of providing such polyolefins in a more economical manner.
The best known industrially used catalyst systems for the polymerization of olefins are those of the “Ziegler-Natta catalyst” type and the “Phillips catalyst” type. The former comprises the reaction product of a metal alkyl or hydride of elements of the first three main groups of the Periodic Table and a reducible compound of a transition metal element of Groups 4 to 7. The combination used most frequently comprising an aluminum alkyl, such as diethylaluminum chloride, and titanium (IV) chloride. More recently, highly active Ziegler-Natta catalyst systems have been formed in which the titanium compound is fixed chemically to the surface of magnesium compounds, such as, in particular, magnesium chloride.
The Phillips Process for ethylene polymerization developed around Phillips catalyst that is composed of chromium oxide on silica as the support. This catalyst was developed by Hogan and Banks and described in U.S. Pat. No. 2,825,721, as well as A. Clark et al. in Ind. Eng. Chem. 48, 1152 (1956). Commercialization of this process provided the first linear polyalkenes and accounts for a large amount of the high-density polyethylene (HDPE) produced today.
More recent developments have focused on single-site catalyst systems. Such systems are characterized by the fact that their metal centers behave alike during polymerization to make very uniform polymers. Catalysts are judged to behave in a single-site manner when the polymer they make meets some basic criteria (e.g., narrow molecular weight distributions, or uniform comonomer distribution). Thus, the metal can have any ligand set around it and be classified as “single-site” as long as the polymer that it produces has certain properties. Includable within single-site catalyst systems are metallocene catalysts, and constrained geometry catalysts.
A “metallocene” is conventionally understood to mean a metal (e.g., Zr, Ti, Hf, Sc, Y, Vi or La) complex that is bound to two cyclopentadienyl (Cp) rings, or derivatives thereof, such as indenyl, tetrahydroindenyl, fluorenyl and mixtures. In addition to the two Cp ligands, other groups can be attached to the metal center, most commonly halides and alkyls. The Cp rings can be linked together (so-called “bridged metallocene” structure), as in most polypropylene catalysts, or they can be independent and freely rotating, as in most (but not all) metallocene-based polyethylene catalysts. The defining feature is the presence of two Cp ligands or derivatives thereof.
Metallocene catalysts can be employed either as so-called “neutral metallocenes” in which case an alumoxane, such as methylalumonxane, is used as an activator or they can be employed as so-called “cationic metallocenes” which incorporate a stable and loosely bound non-coordinating anion as a counter ion to a cationic metal metallocene center. Cationic metallocenes are disclosed in U.S. Pat. Nos. 5,064,802; 5,225,500; 5,243,002; 5,321,106; 5,427,991; and 5,643,847; and EP 426 637 and EP 426 638.
“Constrained geometry” is a term that refers to a particular class of organometallic complexes in which the metal center is bound by only one modified Cp ring or derivative. The Cp ring is modified by bridging to a heteroatom such as nitrogen, phosphorus, oxygen, or sulfur, and this heteroatom also binds to the metal site. The bridged structure forms a fairly rigid system, thus the term “constrained geometry.” By virtue of its open structure, the constrained geometry catalyst can produce resins (long chain branching) that are not possible with normal metallocene catalysts.
The above-described single site catalyst systems are primarily based on early transition metal d
0
complexes useful in coordination polymerization processes. However, these catalysts are known to be oxophilic and, therefore, have low tolerance with respect to even small amounts of oxygenated impurities, such as oxygen, water and oxygenated hydrocarbons. Thus, these materials are difficult to handle and use.
More recently, late transition metal (e.g., Fe, Co, Ni, or Pd) bidentate and tridentate catalyst systems have been developed. Representative disclosures of such late transition metal catalysts are found in U.S. Pat. No. 5,880,241 and its divisional counterparts, U.S. Pat. Nos. 5,880,323; 5,866,663; 5,886,224; 5,891,963; 6,184,171; 6,174,976; 6,133,138; and PCT International Application Nos. PCT/US98/00316; PCT/IJS97/23556; PCT/GB99/00714; PCT/GB99/00715; and PCT/GB99/00716.
For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distribution (MWD) is a fundamental property which determines the properties of the polymer, and, thus, its applications. It is generally recognized in the art that the molecular weight distribution of a polyethylene resin can principally determine the physical, and in particular, the mechanical properties of the resin. Further, the provision of different molecular weight polyethylene molecules can significantly affect the Theological properties of the polyethylene as a whole.
Since an increase in the molecular weight normally improves the physical properties of polyethylene resins, there is a strong demand for polyethylene having high molecular weight. However, it is the high molecular weight molecules that render the polymers more difficult to process. On the other hand, a broadening or preferably a bimodal molecular weight distribution tends to improve the flow of the polymer when it is being processed at high rates of shear. Accordingly, in applications requiring a rapid transformation resulting in high expansion of the material through a die, for example in blowing and extrusion techniques, having a bimodal molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight (this being equivalent to a low melt index, as is known in the art). It is known that when the polyethylene has a high molecular weight and also a bimodal molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion and also the high molecular weight portion contributes to a good impact resistance for the polyethylene film. A polyethylene of this type may be processed utilizing less energy with higher processing yields.
It is known in the art that it is not practical to prepare a polyethylene having a bimodal molecular weight distribution and the required properties simply by mixing polyethylenes having different molecular weights.
As discussed above, high-density polyethylene consists of high and low molecular weight fractions. The high molecular weight fraction provides good mechanical properties to the high density polyethylene and the low molecular weight fraction is required to give good processability to the high density polyethylene, the high molecular weight fraction having relatively high viscosity which can lead to difficulties in processing such a high molecular weight fraction.
On the other hand, a bimodal molecular weight distribution provides a composite of high and low glass transition temperature weight fractions. Such bimodal weight distribution polymer composition provides desired material that exhibits good processability while providing a tough, resilient material which, as an example, exhibits high environmental stress cracking resistance (ESCR). The ESCR of polymers desirably will be greater than 200 hours, such as 500 hours or more and more
Denton Dean Alexander
Glemza Rimantas
Shih Keng-Yu
Choi Ling-Siu
Maggio Robert
Troffkin Howard J.
W. R. Grace & Co.,-Conn.
LandOfFree
Heterogeneous chromium catalysts and processes of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Heterogeneous chromium catalysts and processes of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Heterogeneous chromium catalysts and processes of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3208222