Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Reexamination Certificate
1999-03-02
2001-12-11
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in two or more physically distinct zones
C526S066000, C526S067000, C526S068000, C526S158000, C526S127000, C526S130000, C526S160000, C526S170000, C525S053000, C525S240000, C525S270000, C525S323000
Reexamination Certificate
active
06329477
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of making polymer compositions, e.g. EPDM compositions, in a multistage reactor with monomers chosen from ethylene, C
3
-C
8
alpha olefins, and a non-conjugated diene. In particular, the invention relates to a process for improving diene conversion during the manufacture of the aforementioned polymer compositions by use of a multistage reactor and a metallocene catalyst system.
EPDM is a terpolymer or a polymer composed of three different types of monomers. The monomers which make up EPDM are: 1) ethylene, 2) propylene or some other higher alpha-olefin, and 3) a non-conjugated diene. Ethylene and propylene monomers provide a saturated backbone interrupted by the incorporation of non-conjugated diene monomers which provide for unsaturated groups in the EPDM chain. The presence of unsaturated groups is essential for curing or crosslinking of EPDM to produce a final rubber product because unsaturated groups provide readily available crosslinking sites. Improving the conversion of diene monomer in the polymerization reactor more efficiently incorporates unsaturated groups into polymer chains to make an EPDM with desirable curing or crosslinking properties.
Improved diene conversion also represents cost saving in the production of EPDM. Ethylene and propylene monomers are relatively cheap raw materials for EPDM when compared to significantly more expensive diene monomers such as dicyclopentadiene (DCPD), ethylidene norbornene (ENB) or 1,4 hexadiene. Improved diene conversion reduces the amount of unreacted expensive diene and reduces the necessity of recovering unreacted diene from the polymerization mixture for recycle back to the polymerization reactor. Recovery procedures are necessary not only to conserve diene monomers but also to remove unreacted diene which, if left unremoved, would lead to EPDM with undesirable cure properties. However, recovery procedures are costly and often lead to diene losses. Thus, it is desirable to obtain a diene conversion high enough to reduce or eliminate the need for diene recycling thereby lowering manufacturing costs.
EPDM has been produced in multistage reactors with Ziegler catalysts. An example is U.S. Pat. No. 3,629,212 which discloses that certain Ziegler catalysts can be reactivated by using a series of at least three reactors where ethylene, propylene and non-conjugated diene are added to each reactor. Catalyst is added to the first reactor with addition of reactivator to subsequent stages. This method has various disadvantages such as contamination of the polymer with reactivator residues and equipment corrosion due to the nature of these residues.
U.S. Pat. No. 4,016,342 discloses another method using multi-staged reaction operations to make EPDM. The method comprises adding ethylene, higher alpha-olefin, diene, Ziegler catalyst, co-catalyst and solvent in the first stage of the reaction. The same components are also added to the second stage reactor with the exception of catalyst. The method produces EPDM elastomers with relatively broad molecular weight distributions and very narrow compositional distributions. A disadvantage of this method is a relatively high amount of unreacted diene resulting from the addition of diene in the second reaction stage where catalyst concentration is low. The relatively high amount of diene leads to increased manufacturing costs and problems in curing the polymer.
U.S. Pat. No. 4,306,041 discloses a method for obtaining improved diene conversion in the manufacture of EPDM type terpolymers. Ethylene, higher alpha olefin, and diene and a Ziegler vanadium catalyst, co-catalyst and solvent are added to a reactor where polymerization occurs to make a polymer cement (polymer dissolved in solvent). The reactor contents including the polymer cement are passed to a second reactor where additional monomers are added and further polymerization occurs.
International Application WO 97/36942 discloses a non-adiabatic solution polymerization process using a metallocene catalyst for making ethylene, propylene, and styrene polymers such as polypropylene, styrene block copolymers, ethylene-propylene-diene (EPDM) elastomers, ethylene-propylene (EP) elastomers, ethylene-styrene copolymers, ethylene/alpha-olefin interpolymers, and polyethylene. This polymerization system and process includes the use of two flow loop reactors in series wherein catalyst is added to both reactors.
SUMMARY OF THE INVENTION
The present invention departs from the prior art by the use of a metallocene catalyst rather than prior art Zeigler vanadium catalysts (e.g. vanadium alkyl halide) in a multi-stage system to increase diene conversion. In general, during single reactor operation, metallocene catalysts of this invention have diene conversion of typically 15-40% as compared to the conversions of 60-85% with the vanadium catalysts of the prior art. Consequently, it is important to find economical methods to increase diene conversion with metallocenes. As a result of the high conversion obtained in a single reactor with vanadium catalysts, use of series reactors gives only a small additional conversion benefit. However, we have surprisingly found that moving from single to multiple reactors with metallocene catalysts causes a very significant improvement in diene conversion, much greater than one would anticipate in comparison to moving from single to multiple reactors with vanadium catalysts. In addition, by use of series reactors with metallocenes it is possible to obtain EPDM compositions that are not economical to produce with the prior vanadium systems. Note that the terms “multi-stage reactor” and “series reactor” are used interchangeably herein.
The use of a metallocene catalyst in this invention leads to the following advantages over prior art vanadium catalysts: 1) capability of producing a wider range of EPDM compositions, 2) capability of producing high propylene content EPDM compositions while maintaining good diene conversion, 3) capability of producing EPDM with propylene crystallinity, 4) capability of reactor operation at higher temperatures.
In contrast to multi-stage processes using vanadium catalysts, the process of this invention using metallocenes catalysts is capable of making more than 35 wt % of total polymer in a second reactor without any added amount of catalyst to the second reactor. Ziegler catalysts generally do not have such capability because their catalyst lifetimes are relatively short and leave low levels of active catalyst in a second or successive reactor. Because more of the total polymer product may be made in a second or successive reactor, a wider range of EPDM compositions is possible by varying the monomer makeup in each reactor while only adding catalyst to a first reactor. Thus the method of this invention conserves catalyst and reduces manufacturing costs.
While traditional Zeigler catalysts for EPDM such as vanadium alkyl halide catalysts show good diene conversion in general and particularly at high ethylene compositions, these catalysts are unsuitable for making EPDM polymers at low ethylene content because of an inability to efficiently polymerize propylene and other higher alpha olefins. Catalyst activity and polymer molecular weight decrease rapidly at ethylene contents less than about 40 wt %. Even when these catalysts are used in multi-staged reactor operation, propylene and diene conversion are uneconomically low for compositions with less than about 40 wt % ethylene. Metallocene catalysts maintain reasonable propylene and higher alpha olefin conversion at lower ethylene compositions (less than about 40 wt %). However, diene conversion in a single reactor is lower than desired. When these catalysts are used in multi-stage reactor operation, dramatic improvements in diene conversion result for low ethylene compositions as well as higher ethylene compositions. Therefore in addition to providing for high diene conversion, the process of this invention is capable of producing EPDM in the composition ranges of 5-20 40 wt % ethylene
Cozewith Charles C.
Crowther Donna J.
Datta Sudhin
Folie Bernard J.
Harrington Bruce A.
Cheung William
Chi Anthony
ExxonMobil Chemical Patents Inc.
Prodnuk Stephen D.
Wu David W.
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