Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2002-09-03
2003-12-02
Choi, Ling-Siu (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S125300, C526S124300, C526S158000, C526S351000, C502S103000, C502S115000, C502S116000, C502S133000, C502S158000, C502S126000, C502S125000
Reexamination Certificate
active
06657024
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to polymerization catalyst systems and processes for the preparation of polypropylene, and more particularly relates, in one embodiment, to polymerization catalyst systems for and controlled polymerization processes for the preparation of polypropylene of specified microtacticity that give improvement in physical properties and processability of polypropylene film.
BACKGROUND OF THE INVENTION
Thermoplastic olefin polymers, such as linear polyethylene, polypropylene, and olefin copolymers, are formed in polymerization reactions where a monomer is introduced into a reactor with an appropriate catalyst to produce the olefin homopolymer or copolymer. The polymer is withdrawn from the catalyst reactor and may be subjected to appropriate processing steps and then extruded as a thermoplastic mass through an extruder and die mechanism to produce the polymer as a raw material in particulate form, usually as pellets or granules. The polymer particles are ultimately heated and processed in the formation of the desired end products.
Polypropylene manufacturing processes typically involve the polymerization of propylene monomer with an organometallic catalyst of the Ziegler-Natta type. The Ziegler-Natta type catalyst polymerizes the propylene monomer to produce predominantly solid crystalline polypropylene. Polypropylene is most often produced as a stereospecific polymer. Many desirable product properties, such as strength and durability, depend on the crystallinity of the polypropylene that in turn is dependent on the stereospecific arrangement of methyl groups on the polymer backbone.
Stereospecific polymers are polymers that have a defined arrangement of molecules in space. Both isotactic and syndiotactic propylene polymers, for example, are stereospecific. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or all below the plane. Isotactic polypropylene can be illustrated by the following chemical formula:
This structure provides a highly crystalline polymer molecule. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene may be shown as follows:
Another way of describing the structure is through the use of NMR spectroscopy. Bovey's NMR nomenclature for an isotactic pentad is mmmm with each “m” representing a “meso” dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
This crystallinity distinguishes isotactic polymers from an amorphous or atactic polymer, which is soluble in an aromatic solvent such as xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. That is, the methyl groups in atactic polypropylene are randomly positioned. While it is possible for a catalyst to produce both amorphous and crystalline fractions, it is generally desirable for a catalyst to produce predominantly crystalline polymer with very little atactic polymer.
Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst; a co-catalyst, usually an organoaluminum compound; and an external electron donor compound or selectivity control agent, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos.: 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference. These are just a few of the scores of issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene.
Ziegler-Natta catalysts for the polymerization of isotactic polyolefins are well known in the art. The Ziegler-Natta catalysts are stereospecific complexes derived from a halide of a transition metal, such as titanium, chromium or vanadium with a metal hydride and/or metal alkyl, typically an organoaluminum compound as a co-catalyst. The catalyst is usually comprised of a titanium halide supported on a magnesium compound. Ziegler-Natta catalysts, such as titanium tetrachloride (TiCl
4
) supported on an active magnesium dihalide, such as magnesium dichloride or magnesium dibromide, as disclosed, for example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Mayr, et al. are supported catalysts. Silica may also be used as a support. The supported catalyst may be employed in conjunction with a co-catalyst such as an alkylaluminum compound, for example, triethyl aluminum (TEAL), trimethyl aluminum (TMA) and triisobutyl aluminum (TIBAL).
The development of these polymerization catalysts has advanced in generations of catalysts. The catalysts disclosed in the patents referenced above are considered by most to be third or fourth generation catalysts. With each new generation of catalysts, the catalyst properties have improved, particularly the efficiencies of the catalysts, as expressed in kilograms of polymer product per gram of catalyst over a particular time.
In the utilization of a Ziegler-Natta catalyst for the polymerization of propylene, it is generally desirable to add an external donor. External donors act as stereoselective control agents to control the amount of atactic or non-stereo-regular polymer produced during the reaction, thus reducing the amount of xylene solubles. Examples of external donors include the organosilicon compounds such as cyclohexylmethyidimethoxysilane (CMDS), dicyclopentyidimethoxysilane (CPDS) and diisopropyldimethoxysilane (DIDS). External donors, however, tend to reduce catalyst activity and tend to reduce the melt flow of the resulting polymer.
In addition to the improved catalysts, improved activation methods have also lead to increases in the catalyst efficiency. For example, one discovery involved a process for pre-polymerizing the catalyst just prior to introducing the catalyst into the reaction zone.
It is generally possible to control catalyst productivity (i.e., lbs. of polypropylene/lb. catalyst or other weight ratios) and product isotacticity within limits by adjusting the molar feed ratio of co-catalyst to external electron donor (and their corresponding ratios to the active metal content, e.g., titanium, in the Ziegler-Natta catalyst). Increasing the amount of external electron donor decreases the xylene solubles but may reduce activity and hence catalyst productivity. The xylene solubles (XS) content of the polypropylene product is a measure of the degree of stereoselectivity. Further, the polymer stereoregularity may be obtained by directly measuring the microtacticity of the product via
13
C Nuclear Magnetic Resonance spectroscopy.
Selectivity to isotactic polypropylene is typically determined under the XS test by measuring the amount of polypropylene materials that are xylene soluble. The xylene-solubles were measured by dissolving polymer in hot xylene, cooling the solution to 0° C. and precipitating out the crystalline material. The xylene solubles are the wt. % of the polymer that was soluble in the cold xylene.
In particular with respect to film grade polyolefin resins for biaxially oriented polypropylene (BOPP) applications, there is continuing interest in identifying catalyst systems that offer potential improvements in polymer physical properties and processability. Some previous studies have focused on efforts to enhance resin processability/extrusion characteristics via broadening of polymer molecular weight distribution through utilization of particular donor types (e.g., bis(perhydroisoquinolino)dimethoxysilane (BPIQ)). Other, more recent studies have focused on the use of fluoroalkylsilane compounds (e.g., 3,3,3-trifluoro-propylmethyldimethoxysilane (“E” don
Blackmon Kenneth Paul
Rauscher David J.
Wallace Michael Ray
Choi Ling-Siu
Fina Technology, Inc.
Madan Mossman & Sriram PC
Misley Bradley A.
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