Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Reexamination Certificate
1999-04-22
2001-07-24
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
C526S096000, C526S106000, C526S348000, C526S352000
Reexamination Certificate
active
06265500
ABSTRACT:
BACKGROUND TO THE INVENTION
The present invention relates to a process for producing polyethylene, in particular high density polyethylene having improved mechanical properties.
DESCRIPTION OF THE PRIOR ART
Polyethylene is known for use in the manufacture of a wide variety of articles. The polyethylene polymerisation process can be varied in a number of respects to product a wide variety of resultant polyethylene resins having different physical properties which render the various resins suitable for use in different applications. In particular, it is known to use polyethylene for use in applications where the polyethylene is required to have crack resistance, both resistance to rapid and to slow crack growth. For example, polyethylene is known for use in the manufacture of pipes where it is required that the material of the pipe has sufficient crack resistance so as to avoid inadvertent fracture in service. Polyethylene is also known for use in the manufacture of blow moulded articles where a high environmental stress cracking resistance (ESCR) is required.
Chromium-based catalysts used for the production of polyethylene have been known for some time. It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene product can vary depending on what catalytic system was employed to make the polyethylene. This is because different catalyst systems tend to yield different molecular weight distributions in the polyethylene produced.
It is further known in the art that the mechanical properties of a polyethylene resin, for example the ESCR for blow moulding resins, the impact resistance for film forming resins, and slow crack growth resistance for pipe forming resins, tend to be better when the high molecular weight fraction of the polyethylene is copolymerised.
U.S. Pat. No. 5,208,309 discloses the manufacture of linear very low density polyethylene in which a copolymer of ethylene and a higher alpha-olefin is produced using an activated and subsequently carbon monoxide reduced chromium containing catalyst system and an alkyl aluminium or alkyl boron co-catalyst. The specification states that in the process disclosed therein, it is believed that additional comonomers, i.e. in addition to those added to the polymerisation reactor, can be generated in-situ in the polymerisation reactor or in the reaction zone. Such comonomer is incorporated into the copolymer.
EP-A-0307907 discloses a process for olefin polymerisation in which in-situ comonomer production is employed using a carbon monoxide reduced polymerisation catalyst system. The addition of hydrogen to the reactor enables the regulation and control of the characteristics of the resultant polymer.
The processes disclosed in those two patent specifications suffer from the disadvantage that apart from the density of the resultant polyethylene being low, around 0.890 to 0.915 g/cc for U.S. Pat. No. 5,208,309 and around 0.93 to 0.94 g/cc for EP-A-0307907, the resultant polymer does not have particular broad molecular weight distribution or a particularly high shear response (SR), the shear response being the ratio of the high load melt index (HLMI) and the melt index (MI
2
).
EP-A-0832905 discloses a process for preparing polyethylene having a large molecular weight distribution employing two reactors in series and in which a polyethylene homopolymer is produced in the first reactor and a polyethylene copolymer with 1-hexene is produced in the second reactor by the addition of hexene into the second reactor.
EP-A-0739909 discloses the production of ethylene polymers using, for example, a single reactor having two separated stages simulating two reactors in series. The first polymer is a copolymer of ethylene with hexene and the second polymer is produced by additionally adding hydrogen into the reactor.
SUMMARY OF THE INVENTION
The present invention aims in one aspect to provide a process for producing polyethylene having improved mechanical properties.
Accordingly, the present invention provides a process for producing a polyethylene resin having improved mechanical properties, the process comprising polymerising ethylene in the presence of a chromium-based catalyst to make polyethylene homopolymer in a first polymerisation reactor and in a second polymerisation reactor downstream of the first polymerisation reactor copolymerising ethylene in the presence of the chromium-based catalyst and a co-catalyst from ethylene monomer and comonomer generated in-situ in the second polymerisation reactor to make polyethylene copolymer.
As a result of the in-situ generation of comonomer, no comonomer is introduced into the second reactor. This avoids the need for a comonomer feed to the reactor system.
Preferably, the chromium-based catalyst has been chemically reduced, for example by carbon monoxide, prior to the introduction thereof into the first polymerisation reactor. More preferably, the chromium-based catalyst additionally contains titanium.
The present invention is predicated on the surprising discovery by the present inventors that the use of a two-stage polymerisation process using a chromium-based catalyst, wherein only ethylene is introduced as a monomer and the catalyst systems present in the first and second stages are controlled so as to produce in the first stage a homopolymer and the in the second stage a copolymer in which comonomer generated in-situ in the second stage is incorporated into the copolymer, can in turn yield a broader molecular weight distribution and a higher shear response for the resultant polyethylene resin, in turn yielding improved mechanical properties for the resin.
The chromium-based catalyst preferably comprises a supported chromium oxide catalyst having a titania-containing support, for example a composite silica and titania support. A particularly preferred chromium-based catalyst may comprise from 0.5 to 5 wt % chromium, preferably around 1 wt % chromium, such as 0.9 wt % chromium based on the weight of the chromium-based catalyst. The support comprises from 1 to 5 wt % titanium, preferably at least 2 wt % titanium, more preferably around 2 to 3 wt % titanium, yet more preferably around 2.3 wt % titanium based on the weight of the chromium-based catalyst. The chromium-based catalyst may have a specific surface area of from 200 to 700 m
2
/g, preferably from 400 to 550 m
2
/g and a volume porosity of greater than 2 cc/g preferably from 2 to 3 cc/g.
A particularly preferred chromium-based catalyst (“catalyst 1”) for use in the present invention has an average pore radius of 190 A, a pore volume of around 2.1 cc/g, a specific surface area of around 510 m
2
/g and a chromium content of around 0.9 wt % based on the weight of the chromium-containing catalyst. The support comprises a composite silica and titania support. The amount of titania in the support provides that the catalyst as a whole comprises around 2.3 wt % titanium.
The catalyst may be subjected to an initial activation step in air at an elevated activation temperature. The activation temperature preferably ranges from 500 to 850° C., more preferably around 700 to 800° C.
The chromium-based catalyst is preferably subjected to a chemical reduction process in which at least a portion of the chromium is reduced to a low valence state. Preferably, the chromium-based catalyst is reduced in an atmosphere of dry carbon monoxide in nitrogen gas, typically 8% CO in N
2
at a temperature of from 250 to 500° C., more preferably around 340° C., for a period typically around 30 minutes.
In the preferred polymerisation process of the present invention, the homopolymerisation and copolymerisation processes are carried out in the liquid phase, the liquid comprising ethylene in an inert diluent. The inert diluent is preferably isobutane. The homopolymerisation or copolymerisation process is typically carried out at a temperature of from 80 to 110° C., more preferably from 90 to 100° C., and at a pressure of from 20 to 42 bars, more preferably at a minimum pressure of around 24 bars. The temperature is selected
Cheung William K
Fina Research S.A.
Wheelington Jim D.
Wu David W.
LandOfFree
Production of polyethylene having improved mechanical... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Production of polyethylene having improved mechanical..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Production of polyethylene having improved mechanical... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2531394