Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
2000-11-08
2003-11-04
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, C526S201000, C526S348200, C526S348500, C526S348600, C526S901000
Reexamination Certificate
active
06642323
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing polyethylene compositions suitable for film-making. More particularly, the invention relates to polyethylene compositions with a good homogeneity and a low fines level in the polymer powder.
2. Description of Related Art
A number of processes designed to produce bimodal polyethylene are known in the art. Processes comprising two or more cascaded loop 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 involving 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 this kind of processes 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. It is possible to produce polyethylenes over a fairly broad density range with this kind of a process. In addition, the polyethylenes produced with a loop-gas phase-process have a fairly good balance between processability and homogeneity. It has, however, occasionally been slightly problematic to produce very demanding materials having a good homogeneity. Typical examples of such materials are bimodal film materials, especially bimodal high density film or pipe material.
One problem typical for the production of bimodal polyethylene, especially in a case where the low molecular weight fraction is produced in the first polymerization stage, is the formation of fines. In the subsequent discussion, the term “fines” is used to designate the weight fraction of those polymer particles which pass a 105 &mgr;m sieve. If the fines are present in a high amount, they reduce the flowability of the powder. This may cause problems in some process stages, like in powder conveying and gas phase reactor. It has been found that the fine polymer is found already in the samples taken after the first polymerization stage, i.e., from the flash tank of the loop reactor. It also has been found that the level of fine polymer strongly depends on the MFR and density of the fraction produced in the loop reactor. The level of fines has been found to some extent to be influenced by prepolymerization and conditions employed in the prepolymerization reactor. The following table shows how the fines level is influenced by the MFR produced in the loop reactor and the prepolymerization conditions in a process where propane has been used as a diluent.
TABLE 1
Fines level after a pilot scale loop reactor at different MFR
of loop material and different prepolymerization conditions
Fines,
Fines, no
comonomer
Fines, no
comonomer used
used in
Loop MFR
2
prepolymerization
in prepolymerization
prepolymerization
g/10 min
%
%
%
150
15-25
10-12
5-10
500
24-30
16-20
9-12
1000
17-23
10-15
Thus, when the MFR
2
of the polymer produced in the loop reactor is increased, the level of fine polymer increases also. The problem becomes apparent in the production of bimodal polyethylene; the fraction produced in the loop reactor of the polymer usually has an MFR
2
of about 300-800 g/10 min. As can be seen in Table 1, it is possible to reduce the level of fines by employing a prepolymerization stage and feeding a sufficient amount of comonomer into the prepolymerization reactor.
Processes for producing 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 to produce bimodal polyethylene comprising 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 discuss the possibilities to reduce the level of the fine polymer particles.
Example 6 of EP-B-517868 shows a process where isobutane has been used as a diluent and hexene as a comonomer. In the example, the material was further processed by extruding to pipes. It shows further that the MFR
2
of the material produced in the loop reactor was 140 g/10 min and the MFR
5
of the final product was 0.8 g/10 min. As already mentioned, the publication did not refer to the homogeneity of the product, but in general it can be stated that a material having such a high final MFR
5
and quite a low MFR
2
after the first polymerization stage, i.e., in the loop reactor, should not be difficult to homogenize. Thus, the example gives no indication whether a more difficult material having a lower final MFR
5
and a higher MFR
2
of the material produced in the loop reactor would be easier to homogenize than a corresponding material produced with a process where propane is used as a diluent.
WO-A-9618662
The patent application discloses a process comprising at least two loop reactors and at least one gas phase reactor. 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. In the examples, only propane is used, and the obtained material is in some cases blown to film. The document does discuss both the homogeneity of the film material and the level of fine polymer, but teaches that the homogeneity can be improved and the fines level can be reduced by installing a prepolymeriser in the process.
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 copolymer with low MFR is made in the first reactor and a copolymer with high MFR 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.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the problems of the prior art and to provide a process for producing polyethylene materials over a wide density range. In particular, the invention aims at producing polyethylene materials having a good processability in the end use applications and an excellent homogeneity. Furthermore, it is an aim of the present invention to provide a process in which only minute amounts of fine polymer particles are produced.
It is a further object of the present invention to provide homogeneous films and pipe materials.
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 by the invention as hereinafter described and claimed.
According to the invention bimodal polyethylene materials having a very broad molecular weight distribution are prepared in at least two stages in a process comprising a cascade of one or more loop reactor(s) and one or more gas phase reactor(s). The present invention is based on the surprising finding that the homogeneity of the material can be improved by a proper selection of diluent used in the loop reactor. Equally surprisingly, also the level of fine polymer can be reduced at the same time.
The homogeneous material is preferably produced by polymerizing or copolymerizing ethylene 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 o
Äärilä Jari
Jansen Bjarne
Myhre Ole Jan
Nummila-Pakarinen Auli
Birch & Stewart Kolasch & Birch, LLP
Borealis Technology Oy
Cheung William
Wu David W.
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