Method and catalyst system for producing polyolefins with...

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...

Reexamination Certificate

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C526S114000, C526S119000, C526S160000, C526S943000, C526S153000, C502S103000, C502S107000, C502S110000, C502S117000

Reexamination Certificate

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06384161

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a supported metallocene catalyst system having a modified support useful for the polymerization of olefins. More particularly, but not by way of limitation, the present invention relates to a supported metallocene system having supports modified by more than one drying condition. In another aspect the present invention relates to a method for polymerization of olefins with a broad molecular weight distribution using the inventive supported metallocene catalyst system.
BACKGROUND OF THE INVENTION
Metallocene catalyst systems are extensively used in a variety of polymerization systems, including the polymerization of olefins. The term “metallocene” as used herein refers to a compound containing at least one cyclopentadienyl-type group bonded to a transition metal. The transition metal is selected from Groups IVB, VB, and VIB, preferably IVB and VIB. Examples include titanium, zirconium, hafnium, chromium and vanadium. Generally, the more preferred catalysts in the polymerization of olefins are metallocenes of Zr, Hf, or Ti.
Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane cocatalyst, such as methylaluminoxane. The resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerization media. These homogenous catalysts systems have the disadvantage that when they are used under slurry polymerization conditions, they produce polymer which sticks to reactor walls during the polymerization process (generally referred to as “fouling”) and/or polymer having small particle size and low bulk density which limits the commercial utility.
Various methods have been proposed in an effort to overcome the disadvantages of the homogenous metallocene catalyst systems. One such method involves the prepolymerization of the metallocene aluminoxane catalyst system and/or supporting the catalyst system components on a porous carrier (also known as a “particulate solid” or “support”).
Another important consideration in development of metallocene catalysts is the molecular weight distribution of the solid polymer generated using such catalysts. Broad molecular weight distributions have been linked to improved processability and improved performance of the resulting polymer. Although there have been numerous approaches to broadening the molecular weight distribution of polymers generated from metallocene catalyst systems, there continues to be a need for improved techniques and catalysts.
One aim of the present invention is to provide a new method for preparing a supported metallocene catalyst system capable of producing polyolefins having a broad molecular weight distribution. In accordance with another aspect of the present invention, a method is provided for polymerizing olefins using the new type of supported metallocene catalyst system.
SUMMARY OF THE INVENTION
The present invention provides a process for preparing a supported metallocene catalyst system that can be used to catalyze formation of polyolefins having a wide molecular weight range. In accordance with the process of the present invention, a first portion of an inorganic oxide is dried under a first set of conditions, wherein upon treatment with an organoaluminoxide and a metallocene, a first catalyst system is formed usable to produce a polyolefin product having a first molecular weight average distribution. A second portion of an inorganic oxide under a second set of conditions, wherein upon treatment with an organoaluminoxane and a metallocene, a second catalyst system is formed usable to produce polyolefin product having a second molecular weight distribution. An amount of the first portion is mixed with an amount of the second portion to produce a final inorganic oxide support. The final inorganic oxide support is combined with an organoaluminoxane and a metallocene to produce a supported metallocene catalyst system usable to catalyze formation of polyolefins with a broadened molecular weight average distribution.
In an alternative embodiment, the inventive metallocene catalyst system is prepared by drying a first portion of an inorganic oxide under a first set of conditions and combining the first portion with an organoaluminoxane and a metallocene to create a first catalyst system usable to produce a polyolefin product having a first molecular weight average distribution. A second portion of an inorganic oxide is dried under a second set of conditions and combined with an organoaluminoxane and a metallocene to create a second catalyst system usable to produce a polyolefin product having a second molecular weight average distribution. An amount of the first catalyst system can be mixed with an amount of the second catalyst system to form a combined catalyst system usable to catalyze the formation of polyolefins having a broadened molecular weight average distribution.
In another aspect, the present invention provides a process for polymerizing polyolefins using the inventive supported metallocene catalyst system. The process comprises preparing the final catalyst as mentioned above and then contacting an olefin with the final metallocene catalyst system under conditions supporting polymerization.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a particulate solid or “support,” useful in forming a liquid catalyst system for the polymerization of olefins. Any number of supports can be employed as the particulate solid to be modified. Typically the support can be any inorganic oxide, including metal oxides such as silica, alumina, silica-alumina, and mixtures thereof. Other examples of inorganic oxides are magnesia, titania, zirconia, and the like. It is within the scope of the present invention to use a mixture of one or more of the particulate solids.
The present inventors have observed that the extent and type of drying influences the molecular weight distribution of polyolefin product. Combining metallocene catalysts with inorganic oxide dried under varying conditions allows a proportional modification of the molecular weight distribution of the catalyzed polyolefin. Thus, partially dried and more extensively dried inorganic oxide-metallocene catalyst preparations can be mixed in amounts effective to catalyze formation of desired molecular weight distributions of polyolefin, e.g. polyethylene, products. The polyolefin catalyzed by the mixed catalyst preparations will have a broader molecular weight distribution than polyolefin produced by either catalyst system alone. Catalysts prepared under varying drying conditions may be combined before or after TMA treatment in order to achieve a hybrid polyolefin product.
In accordance with a preferred embodiment of the present invention, the solid support is carefully dehydrated to a desired extent prior to use. Preferably for substantially complete dehydration, the solid is dehydrated so as to contain less than 7% weight loss on ignition. Complete or partial dehydration treatment may be carried out in substantial vacuum or while purging with a dry inert gas such as nitrogen or dry air at a temperature of about 20° C. to about 1200° C., and preferably, from about 300° C. to about 800° C. for substantially complete dehydration. Pressure considerations are usually not critical. The duration of thermal treatment can be from about 0.1 to about 24 hours, depending on the extent of thermal treatment desired.
In a preferred embodiment, the inorganic oxide support is placed in a ceramic tray which is then placed in a muffle furnace with ambient air. The temperature of the muffle furnace is increased at a rate of 10° C. per minute until the desired temperature is reached. The silica is left at the indicated temperatures for the desired time period, depending on the extent of dehydration desired. After removal from the muffle furnace the heated silicon dioxide is placed in a sealed container for further use.
Dehydration is pr

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