Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2000-11-07
2002-09-24
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S065000, C526S124200, C526S348100, C526S901000, C526S905000, C525S240000
Reexamination Certificate
active
06455642
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for polymerising ethylene in two or more stages to produce a homogeneous polyethylene material that is advantageously used in film-making. More particularly, the invention relates to a process comprising a loop and a gas phase reactor, where the material has a good homogeneity and the fines level of the polymer powder is low.
BACKGROUND OF THE INVENTION
A number of processes designed to produce bimodal polyethylene are known in the art. Processes comprising two or more cascaded slurry reactors are known to produce homogeneous polyethylene materials having a good processability in the end use applications. However, these processes have a limitation of only being able to produce bimodal polyethylenes having a relatively high density, higher than about 935 kg/m
3
.
On the other hand, processes disclosing the use of two or more cascaded gas phase reactors are also known in the art. These processes have the advantage of being able to produce polyethylenes over a wide density range. However, the homogeneity and processability of the materials produced in these processes and which are available on the market have not been on such a level that they could seriously compete with the materials produced in the processes comprising cascaded slurry reactors.
A process comprising a cascade of a loop and a gas phase reactor is also known in the art. While this kind of a process can successfully be used to produce polyethylenes with a fairly good balance between processability and homogeneity over a fairly broad density range, they have occasionally shown problems to produce very demanding materials having a good homogeneity. Typical examples of such materials are bimodal film materials, especially bimodal high density film material. Also high density pipe materials can be included into this product category.
DESCRIPTION OF RELATED ART
Processes to produce bimodal materials for high density PE film are known from e.g. EP-B-517868, EP-A-691353 and WO-A-9618662.
EP-B-517868
The patent discloses a process for producing bimodal polyethylene in a loop and a gas phase reactor. The publication teaches the use of different inert hydrocarbons as a diluent in the loop reactor, but it states that propane, especially in supercritical conditions, is preferred. The publication does not refer to the homogeneity of the film material nor does it discuss the possibilities to reduce the level of the fine polymer particles. Silica based catalyst has been used in all examples.
WO-A-9618662
The patent application discloses a process comprising at least two loop reactors and at least one gas phase reactor. It also teaches the preparation of material to be used in HD film applications. Again, the publication mentions that different inert hydrocarbons can be used as a diluent in the loop reactor, but that specifically propane especially in supercritical state is preferred. The document discusses both the homogeneity of the film material and the level of fine polymer, and teaches that the homogeneity can be improved and the fines level can be reduced by installing a prepolymeriser in the process. Also this document discloses only the use of silica-supported catalysts.
EP-A-691353
The patent application discloses a process for producing an in situ blend of ethylene polymers giving a low gel film. The process comprises two gas phase reactors. A low MFR copolymer is made in the first reactor and a high MFR copolymer is made in the second reactor.
EP-A-754708
The patent application discloses a process for producing an in situ polyethylene blend. The modality of the polymer is increased by adding into the first reactor a saturated alicyclic hydrocarbon, which is liquid at process conditions. The addition of the saturated alicyclic hydrocarbon reduced the gel level of the film made of the polymer.
The document also discloses that the gas phase processes have problems with the resulting material having a too high level of gels compared with slurry or solution processes. Further, it reveals that the gas phase resins exhibit significant compositional, molecular and rheological heterogeneities. The use of a non-supported catalyst is disclosed in the document.
U.S. Pat. No. 4,859,749
The patent discloses a two stage polymerization process of ethylene, which uses a catalyst which consists of (a) a transition metal component which is the reaction product of magnesium alcoholate with a chlorine containing titanium compound and a chlorine containing organoaluminum compound and (b) organoaluminum cocatalyst. The examples disclose that a homogeneous material in a two stage slurry process has been obtained.
SUMMARY OF THE INVENTION
The objective of the present invention is to provide a process for producing polyethylene materials over a wide density range with good processability in the end use applications and an excellent homogeneity. In particular, it is an aim to provide a process for producing homogeneous polyethylene film and pipe materials having a good processability.
It is a further objective of the invention to provide a film-making process.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from the specification which follows, are accomplished with the invention as hereinafter described and claimed.
According to the present invention, bimodal polyethylene materials having a very broad molecular weight distribution are prepared in at least two of the stages in a process comprising a cascade of one or more loop reactor(s) and one or more gas phase reactor(s). The invention is based on the surprising finding that the homogeneity of the material can be improved by performing the polymerization in the presence of a specific catalyst. The catalyst is unsupported and comprises titanium and magnesium as active constituents.
The homogeneous material is produced by polymerizing or copolymerizing ethylene in the presence of a ethylene-polymerizing catalyst system in a reactor cascade formed by at least two reactors, one of which is a gas phase reactor and one of which is a loop reactor, said loop reactor being operated with an inert hydrocarbon, namely linear or branched aliphatic C
3
-C
6
-hydrocarbon. The reactors are operated with different amounts of hydrogen and comonomers to produce a high molecular weight portion in one of the reactors and a low molecular weight portion in the other, so as to provide a bimodal polyethylene composition comprising a relatively low molecular weight part and relatively high molecular weight part.
In particular, the present invention comprises a process for polymerising ethylene and comonomer(s) in at least two stages, in a process comprising a loop and a gas phase reactor, of which
(i) in the first stage, a low molecular weight, relatively high density polymer fraction having a melt flow rate MFR
2
of at least 250 g/10 min is prepared in one or more loop reactor(s) in the presence of an unsupported ethylene-polymerizing catalyst system which comprises titanium and magnesium as active components, and
(ii) in the second stage, a high molecular weight, relatively low density copolymer is produced in one or more gas phase reactor(s) using an alpha-olefin, like 1-butene, 1-hexene or 1-octene, as a comonomer. The polymerization conditions are selected so that the final polymer has a predetermined melt flow rate, preferably so that MFR
5
is 0.7 g/10 min or less.
More specifically, the present process is a process for producing polyethylene compositions comprising bimodal ethylene homo- and copolymers in a multistage reaction sequence of successive polymerization stages in the presence of an ethylene-polymerizing catalyst system characterized by using an unsupported catalyst comprising magnesium and titanium as active constituents, and carrying out the process in at least one loop polymerization stage and at least one gas phase polymerization stage, operated with different amounts of hydrogen and comonomers to produce a high molecular weight portion in one of the pol
Garoff Thomas
Lindroos Jarmo
Myhre Ole Jan
Nummila-Pakarinen Auli
Waldvogel Päivi
Birch & Stewart Kolasch & Birch, LLP
Borealis Technology Oy
Rabago R.
Wu David W.
LandOfFree
Process for producing a homogeneous polyethylene material in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for producing a homogeneous polyethylene material in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for producing a homogeneous polyethylene material in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2887152