Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Reexamination Certificate
2000-03-29
2001-06-12
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
C526S064000, C526S073000, C526S106000, C526S348500
Reexamination Certificate
active
06245867
ABSTRACT:
BACKGROUND TO THE INVENTION
The present invention relates to the polymerisation of ethylene using at least two independent catalysts.
DESCRIPTION OF THE PRIOR ART
Various techniques have been employed in the past for the polymerisation of polymers and copolymers of olefins. One of the approaches has involved employing catalysts based upon transition metal compounds such as titanium. Another approach has involved the employment of catalysts containing chromium. As a general rule, these two types of catalysts produce polyolefins having somewhat different physical characteristics. For some applications, it is desirable to have polyolefins which have a blend of the properties that are produced by the titanium and the chromium catalysts. For example, it may be desirable to obtain polyolefins with a broad or bimodal molecular weight distribution in order to combine the advantages of the low molecular weight polyolefins such as good processability and high melt index, and those of the high molecular weight polyolefins, such as good physical properties.
The molecular weight distribution can be completely defined by means of a curve obtained by gel permeation chromatography. Generally, the molecular weight distribution (MWD) is more simply defined by a parameter, known as the dispersion index D, which is the ratio between the average molecular weight by weight (Mw) and the average molecular weight by number (Mn). The dispersion index constitutes a measure of the width of the molecular weight distribution. For most applications, the dispersion index varies between 10 and 30.
Some techniques for preparing broad molecular weight polymers have involved the use of multiple reactor arrangements, sometimes loops, sometimes stirred tanks in which different polymerisation conditions such as temperature, hydrogen, or comonomer are employed in the different reaction zones. Such multiple reactor schemes, while offering versatility in resin characteristics, are not particularly efficient. In addition, the control of the multiple reactor schemes is difficult.
Another technique which has been used to broaden the molecular weight distribution involves the physical blend, in an extruder, of polyolefin resins having different properties.
In addition, some attempts have been made to combine titanium and chromium either on a single catalyst, as disclosed for example in U.S. Pat. No. 3,622,521, U.S. Pat. No. 4,041,224, and EP-A-0,480,376, or as two different supported catalysts such as disclosed for example in U.S. Pat. No. 4,285,834, U.S. Pat. No. 5,237,025 and U.S. Pat. No. 5,330,950 which discloses a mixture of Ziegler-Natta and chromium catalysts for making bimodal or broad molecular weight distribution high density polyethylenes.
However, the polymers obtained with the above mentioned processes do not exhibit the desired good processing and mechanical properties together with a high catalyst activity.
WO-A-97/32905 discloses a method for transitions between two different catalysts in olefin polymerisations. First and second catalysts are successively introduced into a polymerisation reactor and a transition is performed between a chromium-based catalyst and a metallocene catalyst. This method suffers from the problem of the existence of a transition period between the two catalysts which, although shortened as compared to the background art discussed in the specification, nevertheless is still undesirable. The specification discloses that a direct transition between chromium oxide-based catalysts and metallocene catalysts in olefin polymerisation normally is difficult.
EP-A-0832905 discloses a process for the preparation of polyethylene having a large molecular weight distribution in which a single chromium-based catalyst is employed in two reactors in series, ethylene and the chromium catalyst being introduced into the first reactor at a temperature from 95 to 110° C. and the ethylene homopolymer thereby obtained is transferred into the second reactor with added ethylene and optionally an alpha-olefinic comonomer at a temperature of 80 to 90° C.
EP-A-0739909 discloses a number of processes for the production of ethylene polymer. In a first process ethylene is polymerised optionally with one or more comonomers in two reactors in series in the presence of a solid catalyst comprising titanium and zirconium in a molar ratio of at least 2 together with a cocatalyst. No chromium-based catalyst is employed. In a second process the ethylene homopolymerisation or copolymerisation is carried out in two reactors in series in the presence of a first catalyst consisting essentially of from 10 to 30 wt % titanium, from 20 to 60 wt % halogen, from 0.5 to 20 wt % magnesium and from 0.1 to 10 wt % aluminium and a second catalyst consisting essentially of from 0.5 to 10 wt % titanium, from 5 to 40 wt % zirconium, from 20 to 80 wt % halogen, from 1 to 30 wt % magnesium and from 0.5 to 10 wt % aluminium. Again, no chromium catalyst is employed. A third process polymerises ethylene optionally with one or more comonomers in a single reactor in the presence of a chromium-based catalyst and a support consisting of at least 2 constituents chosen from silica, alumina and aluminium phosphate. A fourth process is disclosed in which ethylene is polymerised optionally with one or more comonomers in two reactors in series in the presence of a single chromium-based catalyst on a support containing at least 2 constituents chosen from silica, alumina and aluminium phosphate.
DE-A-19723003 discloses the production of polymer mixtures in which two polymers have been physically blended. One polymer may comprise an ethylene copolymer produced using a metallocene catalyst and one polymer may comprise an ethylene copolymer produced using a chromium-based or Ziegler-Natta catalyst.
SUMMARY OF THE INVENTION
It is an aim of the present invention at least partially to overcome these problems in the prior art.
The present invention provides a process for producing polyethylene in a two step polymerisation process that comprises the steps of:
(a) homopolymerising ethylene in the presence of a first catalyst comprising an activated chromium catalyst in a first reactor to produce a first polyethylene fraction;
(b) transferring the first polyethylene fraction produced in the first reactor and at least a portion of the first catalyst to a second reactor;
(c) in the second reactor homopolymerising ethylene or copolymerising ethylene and an alpha-olefinic comonomer having from 3 to 10 carbon atoms in the presence of a second catalyst under conditions which suppress residual activity in the first catalyst to produce a second polyethylene fraction; and
(d) retrieving from the second reactor polyethylene comprising a blend of the first and second polyethylene fractions.
The second polyethylene fraction preferably has a narrow molecular weight distribution and higher molecular weight than the first polyethylene fraction.
The second catalyst may be a Ziegler-Natta catalyst or metallocene catalyst, and may have been precontacted with a cocatalyst, such as an organoaluminium compound.
The first and second reactors may be liquid full loop reactors, and may be connected in series by a transfer line. The homopolymer fraction is produced in the first reactor at a temperature preferably ranging from 100 to 110° C. and may have an ethylene off-gas concentration in the range of from 3 to 4% by weight, and optionally hydrogen.
The second reactor is preferably operated at a temperature of less than 85° C. so as to suppress the activity of the first chromium-based catalyst. More preferably, in the second reactor the polymerisation temperature is controlled so as to be from 65 to 80° C. and comonomer and hydrogen are added, as needed, together with any additional ethylene which may be required in order to obtain a final polyethylene of the desired density.
At the low temperature at which the second reactor is operated, the first chromium catalyst has a limited activity. At such temperatures the first catalyst produces a polyethylene exhibiting the required low melt in
Cheung William
Fina Research S.A.
Wheelington Jim D.
Wu David W.
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