Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-05-03
2001-06-05
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S160000, C526S243000, C526S943000, C526S129000, C526S905000, C526S095000, C526S104000, C502S117000, C502S152000
Reexamination Certificate
active
06242543
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for the polymerization of ethylene, optionally with &agr;-olefins, to obtain polyethylenes having a broad molecular weight distribution, and a catalyst system for use in said process. More particularly, the present invention relates to a process for the production of ethylene homopolymers and copolymers that have an especially broad molecular weight distribution; polyethylenes produced thereby; and a novel and improved catalyst system which includes two independent catalysts, comprising chromium oxide on an inorganic support and the other comprising bis-cyclopentadienyl chromium compound reacted with an inorganic support. Articles made from the obtained polyethylene compositions exhibit particularly good environmental stress cracking resistance (ESCR) performance.
PRIOR ART
Polyethylenes, homo- and copolymers, that are linear and have few and short branches on its backbone, such as high density polyethylene (PEHD) and linear low density polyethylene (PELLD), are produced commercially by catalyzed polymerizations. To obtain linear polyethylenes with a particularly broad or bimodal molecular weight distribution, two reactors are usually used in a series. Ethylene and the total amount of catalysts are in general fed into the first reactor, from where the obtained polymer is fed to the second reactor and mixed with an additional amount of ethylene which is then polymerized. Comonomers, usually &agr;-olefins, may be added either in the first or second reactor. Morphology, molecular weight and molecular weight distribution of the final polyethylene depends on the type of catalyst used and the polymerization conditions. To obtain polyethylenes with specific properties, certain combined catalyst systems may be used.
Catalysts often used in the polymerizations of &agr;-olefins are chromium oxide based catalysts, such as those disclosed in Kirk-Othmer, “Encyclopedia of Chemical Technology”, 1981, Vol. 16, p. 402. Such catalysts give high molecular weight ethylene homopolymers and copolymers, but the molecular weight can be difficult to control. Another well known catalyst is bis-cyclopentadienyl chromium disposed on a support of calcined silica. In the absence of hydrogen this catalyst gives high molecular weight polyethylenes, and in the presence of hydrogen low molecular weight polyethylenes.
U.S. Pat. No. 3,378,536 discloses a process for polymerizing ethylene by the use of a two-component catalyst system consisting of (a) chromium deposited on a support, e.g. silica, where the chromium is activated in dry air or an oxygen-containing gas at a temperature of 400 to 815° C., and then reduced with CO before use; and (b) chromium or vanadium arene where the arene is an aromatic C
6
ring, which optionally is substituted with C
1-20
alkyl, cycloalkyl or aryl. The catalyst: components are preferably fed to the polymerization reactor separately.
U.S. Pat. No. 3,709,853 states that bis-cyclopentadienyl chromium(II) on an inorganic oxide support is a very efficient catalyst for the polymerization of ethylene over a wide range of reaction conditions, in particular in the presence of hydrogen. Produced polyethylenes have a high molecular weight and a narrow molecular weight distribution.
U.S. Pat. No. 4,530,914 relates to polyolefins having a broad molecular weight distribution obtained by the use of a catalyst system comprising two or more metal cyclopentadienyl compounds, each having different polymerization rates. The metal component is preferably Ti or Zr, and also an alumoxan is used. The working examples reveal that the molecular weight distributions are not particularly broad, with a highest polydispersity, M
w
/M
n
, of only 7.8, which today is common incommercial polymers. Comonomers are not used.
U.S. Pat. No. 4,918,038 discloses a process for the production of polyethylenes having a broad and/or bimodal molecular weight distribution. It is used a complex catalyst system comprising a plurality of metal halide compounds. The catalyst system results in an inclusion of comonomers in the polymer backbone.
U.S. Pat. No. 4,015,059 discloses the use of a catalyst prepared by reacting an inorganic oxide with bis-cyclopentadienyl, bis-indenyl or bis-fluorenyl chromium in the polymerization of ethylene, optionally with other &agr;-olefins. The chromium atoms in the cyclopentadienyl ligands are believed to be connected through oxygen to the metal atoms in the support. However, the obtained polymers have narrow molecular weight distributions.
U.S. Pat. No. 4,424,139 teaches the use of phosphate-containing chromocene or chromoxide catalysts.
EP 088 562 discloses a modified polymerization catalyst comprising a silica support and chromium deposited thereon. After oxidation in dry air the chromium is modified by being contacted with a transition metal compound bonded to an inorganic rest. The transition metal is Ti, V or Cr, preferably Ti. Preferably, the ligand is an unsaturated carbocyclic or heterocyclic ring system containing 6 delocalized &pgr;-electrons, for example cyclic carbanions, such as cyclopentadienyl anion, and derivatives thereof. Only the use of bis-toluene titan is exemplified, and the obtained polyethylenes have a substantial degree of branching and a medium broad molecular weight distribution.
U.S. Pat. No. 5,330,950 and U.S. Pat. No. 5,408,015 relate to ethylene polymers having a broad molecular weight distribution. The polyethylenes are produced by the use of a mixture of a MgO-supported Ziegler catalyst and a chromium oxide catalyst.
U.S. Pat. No. 5,399,622 discloses a process for the polymerization of ethylene by starting with a chromium oxide catalyst to obtain granules of low density polyethylene, and then the polymerization is continued by adding a cocatalyst and an yttrium-containing catalyst of formula (Cp
2
YX
x
)
y
M
z
L
n
, wherein Cp is cyclopentadienyl optionally substituted with alkyl or alkylsilyl radicals, X is a halide, M is an alkali metal and L is an electron donor ligand, to obtain an outer shell of high density polyethylene on said granules.
Instead of using two reactors in series operating at different conditions, it would be desirable to use one single reactor and two different types of catalysts to obtain a composition consisting of two different ethylene polymers. If the catalysts are used simultanously in the presence of each other a precondition is that neither of them get their activity substantially reduced. Through a suitable choice of catalysts and polymerization conditions, the obtained polyethylene composition should have a broad molecular weight distribution. Such polyethylenes are notably well suited for use in extrusion processes, and provided they have an adequate comonomer distribution, blow moulded articles having especially good environmental stress cracking resistances can be obtained.
SUMMARY OF THE INVENTION
It has now surprisingly been found that ethylene polymers having a broad molecular weight distribution can be obtained by a process performed in one single reactor by the use of a catalyst system consisting of two independent catalysts, which comprises (A) divalent chromium oxide on an inorganic support and (B) chromium bonded to a cyclopentaclienyl and reacted with an inorganic support.
The present invention thus provides a process for the polymerization of ethylene, optionally with &agr;-olefins, to ethylene homopolymers or copolymers having a broad molecular weight distribution, comprising the steps of polymerizing 100 to 80% by weight of ethylene and 0 to 20% by weight of comonomer in the presence of two independent, simultaneously present catalysts A and B, and optionally also a cocatalyst. The catalysts and the optional cocatalyst are fed into the reactor concurrently, either separately or in the form of a blend. Catalyst A comprises chromium oxide supported on an inorganic support, and the chromium has an oxidation number of predominantly two. Catalyst B comprises a bis-cyclopentadienyl chromium compound reacted with an inorganic support. The bis-cyclopentadienyl chr
Blom Richard
Dahl Ivar Martin
Eggen Svein Staal
Follestad Arild
Jens Klaus-Joachim
Borealis A/S
Choi Ling Siu
Scully Scott Murphy & Presser
Wu David W.
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