Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Effecting a change in a polymerization process in response...
Reexamination Certificate
2000-09-22
2002-10-08
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Effecting a change in a polymerization process in response...
C526S059000, C526S113000, C526S114000, C526S118000, C526S119000, C526S124300, C526S160000, C526S352000, C526S943000, C526S116000, C525S240000
Reexamination Certificate
active
06462149
ABSTRACT:
TECHNICAL FIELD
This invention relates to the manufacture of polyolefin resins, and in particular to the manufacture in a single reactor of polyolefin resins having polymodal, especially bimodal, molecular weight, density, or other characteristics. For bimodal effects, it employs two separate bimetallic catalyst compositions fed in ratios which may be varied to control the bimodal properties of the product more closely than has been possible in the past. Additional catalyst species may be employed in more complex processes.
BACKGROUND OF THE INVENTION
The term “bimodal” as applied to polyolefin resins usually means that the resin has two distinct ranges of molecular weight or density, which can impart desired properties to the product in great variety. Originally, bimodal resins were made in two separate reactors or reaction chambers—that is, a product having a first molecular weight was moved directly from the reaction zone in which it was made and introduced to a reaction zone having conditions for making a resin of a different molecular weight, where more resin was made. The two resins are thus mixed or, in some cases, even present in the same particles. Various 2-stage and bimodal processes are reviewed by Cozewith et al in U.S. Pat. No. 4,786,697. Two-stage processes are difficult to control and, perhaps more important, have a capital disadvantage in that two reactors, or at least two reaction zones, are required to make them. Moreover, frequently the products are not homogeneously mixed, in that at least some particles will be entirely of one mode or the other. It is therefore desirable to find ways of making homogeneous bimodal polyolefins in a single reactor.
One approach to making bimodal polyolefins in a single reactor has been to employ a mixed catalyst system, in which one catalyst component (because of specific termination and/or chain transfer kinetics) makes a primarily low molecular weight (LMW) product, and the other catalyst component produces a primarily high molecular weight (HMW) product, because of different termination and/or chain transfer kinetics. By including both of these catalyst components in the same catalyst composition, a bimodal product may be produced; the molecular weight modes of the product will be intimately mixed, providing a resin product that is relatively free of gels compared to similar products made in staged-reactor processes or by the blending of two distinct unimodal resins.
In addition to tailoring the molecular weight distribution of the polymer, the different comonomer incorporation kinetics of specific catalysts can be applied in making products that are bimodal in density. A catalyst with favorable kinetics can incorporate alpha-olefins into polyethylene very effectively. A mixed catalyst system that uses two catalysts having different comonomer incorporation efficiencies can be used to produce such bimodal density products. Producing bimodal products in a single reactor relieves the necessity of a separate blending step, and allows them to be produced more quickly and efficiently.
Controlling the ratio of the components in the bimodal product is a significant manufacturing concern. Product properties of bimodal resins are often extremely sensitive to component split. For instance, in the manufacture of high-density, high-molecular-weight film, to achieve the desired specification requires control of component split within ~2% of the setpoint.
The weight percentage, or “split,” of HMW or high density (“HD”) in the total product including low density (“LD”) components in a single-reactor manufactured bimodal resin is primarily a function of the relative amount of each type of catalyst in the catalyst system. While, theoretically, a catalyst system containing proper amounts of each catalyst could be generated and used to produce the desired split in a particular case, in practice using such a system would be difficult, as the relative productivities of the catalyst components may change with variations in reactor conditions or poison levels.
There have been attempts in the past to control product component split by controlling catalyst split. Disclosed in WO 96/07478 is a method for determining the molecular weight distribution of a bimodal product made in a single reactor, which uses a supported bimetallic catalyst system with the addition of a make-up feed consisting of one of the metallic components. While this scheme can be used to control split, it has a major disadvantage in that the resin produced may contain particles consisting of only one component. The resulting heterogeneity of the resin is known to degrade product film appearance and performance.
Another proposed method for controlling product component split is the use of separate feeds for the HMW and LMW components. This method is most practical in liquid catalyst systems, where the components will become intimately mixed before polymerization begins. This method has a major disadvantage in that the catalyst split is very sensitive to fluctuations in the relative feed rates of each catalyst.
Each of the above described approaches to the problem of making bimodal polyolefins in a single reactor has difficulties and shortcomings. The art is in need of a method of making bimodal products with improved control and convenience.
SUMMARY OF THE INVENTION
One of the most challenging aspects of bimodal (in molecular weight distribution or density) polyolefin production in one reactor is the control of component split. This invention includes the simultaneous use of two multi-component catalyst blends to achieve a desired split control. Preferably, the first blend contains the same catalytic species as the second blend, but in different relative amounts. By simultaneously feeding the two catalyst blends to the reactor and varying the relative feed rates, the resin component split can be controlled at the desired setpoint.
In our invention, we feed two complex catalyst compositions or mixtures (blends), each capable of making both HMW and LMW components, or HD and LD components, or bimodal in some other aspect, such as productivities or reaction rates with respect to a comonomer, but (where the components are the same) having different fixed ratios of catalysts. By controlling the ratio of the two multi-component fixed-ratio catalyst blends, we can modulate or otherwise control the ratio of HMW product to LMW product (or other bimodal feature) rather precisely within a desired range.
By using only mixed catalyst compositions having fixed ratios of catalyst species, we avoid the possible manufacture of particles of only high or low molecular weight. Each particle will be a product of the mixed system. Further, the system is substantially less sensitive to perturbations in catalyst feed rates or feed ratios.
By feeding two distinct catalyst compositions, each having LMW and HMW producing components, the possibility of making particles of only high or only low molecular weight is avoided—all resin particles in this invention will contain both HMW and LMW components. Furthermore, carefully choosing the composition of each catalyst mixed composition will ensure that each resin particle has a HMW-to-LMW ratio that lies in a range known or believed to produce acceptable film or other properties. Our method is effective in controlling the product split (the weight percent of HMW component in the overall product) with supported catalyst systems, spray-dried catalyst systems, or liquid phase catalyst systems.
The wide variety of specific catalysts we can use is illustrated in the following review of catalyst compositions useful in olefin polymerization.
Bimetallic catalysts are described by Kissin et al in U.S. Pat. No. 6,001,766. At least one of the two transition metal compounds they use is a cyclopentadienyl compound, and the resulting catalyst composition is said to produce polymer of broad molecular weight distribution. The ratio of the cyclopentadienyl compound (which preferably includes zirconium) to the other transition metal may vary.
Various other patents owned by Mobil
Davis Mark Bradley
Maheshwari Vinayak
Tilston Michael W.
Rabago R.
Union Carbide Chemicals & Plastics Technology Corporation
Wu David W.
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