Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Patent
1998-02-10
1999-11-16
Wu, David W.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
526 73, 526160, 526943, 526904, C08F 200, C08G 8500
Patent
active
059860219
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process for manufacturing ethylene copolymers, especially linear low density polyethylene polymers having a narrow molecular weight distribution and a narrow composition distribution.
The prior art includes several various processes for manufacturing polyethylene. Thus typical processes are gas-phase and slurry processes. In the latter process the polymerization is typically performed in a reaction medium or diluent, which is formed by isobutane, pentane, hexane or some other saturated aliphatic hydrocarbon. The polymerization is often performed in the presence of a Ziegler-Natta catalyst at an elevated temperature. In addition of monomers, hydrogen is often used as a modifier in the polymerization for affecting the molecular weight of the polymers achieved.
The production of LLDPE materials with traditional Ziegler-Natta catalysts in slurry process is difficult, because the solubility of the polymer is too high due to broad molecular weight distribution and comonomer distribution achieved by traditional Z-N catalysts. This limits strongly the reactor operating temperatures and hence also the productivity of the catalyst. Due to solubility limitations LLDPE materials are normally produced commercially in gas-phase reactors and solution process.
Recent developments in the field of olefin polymerization catalysts include metallocene catalysts, which comprise metallocene compounds of transition metals together with alumoxane compounds. These catalysts have been suggested to be used as a homogenous system or deposited onto carrier, for example inorganic oxide carriers. Thus these catalysts typically comprise as a procatalyst component a metallocene compound, for example bis(cyclopentadienyl)titanium dialkyl or bis(cyclopentadienyl)zirconium alkonyl or chlorides thereof, and an activator component, which typically is alumoxane or an ionic activator.
WO94/21691 describes a process, where ethylene and C.sub.3 -C.sub.8 alpha-olefin are polymerized in a slurry in the presence of a metallocene catalyst. The polymerization is carried out in a stirred tank reactor in an alkane solvent selected from isobutane, pentane or heavier alkanes. Although the publication states that a polymerization temperature of 70-100.degree. C. can be used, the examples of the publication are all carried out in the temperature of 70.degree. C.
Also this process describes only a batch process. There is no indication that a process could be carried out as a continuous process.
It has been found that metallocene catalysts have a property of producing polyethylene having much more narrow molecular weight distribution and much more homogenous comonomer incorporation, which means that all polymer chains will have an equal amount of comonomer and it will be distributed homogenously. Therefore it would be expected that they could be applied for manufacturing in slurry process such LLDPE polymers which would have a relatively narrow molecular weight distribution and narrow composition distribution.
It has also been observed that in a standard slurry process polymer solubility into the diluent in order to produce low density products requires temperatures which are rather low and this results in low production capacity and long and expensive transition periods. Also the operation in standard slurry process has indicated that the productivity of the metallocene catalysts decreases when the reactor temperature is increased. According to the invention it has surprisingly been found out that when the temperature is further increased the productivity of the catalyst can be improved.
Thus the object of the invention is a process for manufacturing ethylene copolymers such as medium weight polymers and linear low density polyethylene by polymerizing ethylene and a minor amount of C.sub.3 -C.sub.8 alphaolefins in particle form in slurry reactor in the presence of an ethylene polymerizing catalyst. The process of the invention is characterized in that the polymerization is carried out in propane diluent in a temperature above 80.degr
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Hokkanen Harri
Kallio Kalle
Knuuttila Hilkka
Palmroos Ari
Salminen Hannu
Borealis A/S
Rabago Roberto
Wu David W.
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