Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1998-02-09
2001-02-27
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S107000, C526S130000, C502S242000, C502S256000
Reexamination Certificate
active
06194528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing polyethylene having impact resistance and to a catalyst for use in such a process.
2. Description of the Prior Art
Polyethylene is known for use in the manufacture of a wide variety of articles. The polyethylene polymerization process can be varied in a number of respects to produce 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 impact resistance. For example, polyethylene is known for use in the manufacture of pipes where it is obviously required that the material of the pipe has sufficient impact resistance so as to avoid inadvertent fracture in service. Furthermore, polyethylene is known for use as a film where the property of impact resistance of the film can be of importance.
Chromium-based catalysts used for the production of polyethylene have been known for some time. The catalysts may have been subjected to pre-treatment processes prior to the polymerization reaction. The pre-treatment processes may include chemical reduction of the chromium to a lower valence state, e.g. by carbon monoxide. Such a process is disclosed in, for example, EP-A-0591968.
U.S. Pat. No. 5,208,309 discloses a linear, very low density polyethylene polymerization process. In the polymerization process, a cocatalyst such as a boron alkyl, most particularly triethyl borane (TEB), is employed which tends to generate an in situ comonomer and thus depresses the density of the resultant polyethylene resin. The catalyst support is a silica-titania support and for producing a polymer with the most desirable characteristics it is preferred for the support to contain 5 to 8 wt % titanium. The specification does not address the problem of the production of polyethylene having impact resistance.
U.S. Pat. No. 4,151,122 discloses the reduction and reoxidation of a cogel or self-reduced catalyst in which oxidation of a catalyst is employed after reduction of a catalyst, such as by carbon monoxide, for boosting the melt index of the resultant polyethylene resin. There is no disclosure of a process for producing polyethylene having impact resistance.
U.S. Pat. No. 4,405,768 discloses the addition of titanium to a catalyst for boosting the melt index of the polyethylene resin. Again, there is no disclosure of a process for producing polyethylene having impact resistance.
EP-A-0000751 and its equivalent U.S. Pat. No. 4,180,481 disclose a process for the manufacture of a supported chromium oxide catalyst for olefin polymerization, the catalyst having been treated with carbon monoxide. There is no disclosure of a process for producing polyethylene having impact resistance.
SUMMARY OF THE INVENTION
The present invention aims in one aspect to provide a process for producing polyethylene having improved impact resistance.
It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene product 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. Thus, for example, the properties of a polyethylene product produced using a chromium-based catalyst (i.e. a catalyst known in the art as an “Phillips catalyst”) tend to be different from the properties of a polyethylene produced using a different catalyst, for example a Ziegler-Natta catalyst. The production of polyethylene using a chromium-based catalyst is desirable to enable the particular polyethylene product obtainable thereby to be manufactured.
In a second aspect the present invention aims to provide a process for producing polyethylene having impact resistance using a chromium-based catalyst.
Accordingly, the present invention provides a process for producing polyethylene having impact resistance, the process comprising polymerizing ethylene, or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms, in the presence of a chemically reduced chromium-based catalyst containing in a support thereof from 2 to 3 wt % of titanium, based on the weight of the catalyst.
DETAILED DESCRIPTION OF THE INVENTION
The present invention further provides a chromium-based catalyst for the production of polyethylene by polymerizing ethylene or copolymerizing ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms, the catalyst being chemically reduced and containing in a support from 2 to 3 wt % of titanium, based on the weight of the catalyst.
The present invention is predicated on the surprising discovery by the present inventor that the chemical reduction, preferably by carbon monoxide, of a chromium-based catalyst containing from 2 to 3 wt % titanium in its support can unexpectedly yield a polyethylene product having improved impact resistance.
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-containing catalyst. The support comprises 2 to 3 wt % titanium, more preferably around 2.3 wt % titanium, based on the weight of the chromium containing 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 for use in the present invention comprises a catalyst, (“catalyst 1”) having an average pore radius of 190 Å, 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 600 to 750° C., and is most particularly around 635° C.
The chromium-based catalyst is subjected to a chemical reduction process in which at least a portion of the chromium is reduced to a low valance state. Preferably, the chromium-based catalyst is reduced in an atmosphere of dry carbon monoxide at a temperature of from 250 to 500° C., more preferably 350 to 450° C., and most preferably at a temperature of around 370° C.
In the preferred polymerization process of the present invention, the polymerization or copolymerization process is carried out in the liquid phase, the liquid comprising ethylene, and where required an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms, in an inert diluent. The comonomer may be selected from 1-butene, 1-pentene, 1-hexene, 4-methyl 1-pentene, 1-heptene and 1-octene. The inert diluent is preferably isobutane. The polymerization or copolymerization process is typically carried out at a temperature of from 90 to 100° C., more preferably from 95 to 100° C., and at a pressure of 20 to 42 bars, more preferably at a minimum pressure of around 24 bars.
Typically, in the polymerization process, the ethylene monomer comprises from 0.5 to 8% by weight, typically around 6% by weight, of the total weight of the ethylene in the inert diluent. Typically, in the copolymerization process, the ethylene monomer comprises from 0.5 to 6% by weight and the comonomer comprises from 0.5 to 3% by weight, each based on the total weight of the ethylene monomer and comonomer in the inert diluent.
The carbon monoxide-reduced catalyst for u
Fina Research S.A.
Rabago R.
Wheelington Jim
Wu David W.
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