Production of polyethylene for blow moulding

Details Production of polyethylene for blow moulding Production of polyethylene for blow moulding

1998-06-05

2001-04-10

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S106000, C526S113000, C526S154000, C502S171000, C502S309000

Reexamination Certificate

active

06214947

ABSTRACT:

The present invention relates to a process for the production of a chromium-based catalyst for producing polyethylene suitable for blow moulding, a process for producing polyethylene using such a catalyst, and the use of such a catalyst.
Polyethylene is well known for use in the manufacture of blow moulded articles, for example bottles. It is known in the art that polyethylene resin produced for the manufacture of blow moulded articles must achieve a balance of (a) physical properties of the resin so that the resultant blow moulded article has the required physical characteristics and (b) processing properties of the resin so that the polyethylene melt may readily be processed into the blow moulded article. In order to achieve good processability of the polyethylene resins, it is desired that the flow properties and the shear response of the polyethylene are improved by broadening the molecular weight distribution of the polyethylene. Moreover, the physical properties of the solid resin when employed for blow moulding bottles require the resin to have a high density and a high environmental stress cracking resistance (ESCR).
As a general rule, a polyethylene having a higher density tends to have a higher degree of stiffness, thereby making it more suitable for blow moulding into bottles. A higher stiffness in the known polyethylene increases bottle strength and enables thinner walls to be employed. However, in general, the environment stress cracking resistance of polyethylene has an inverse relationship with stiffness. In other words, as the stiffness of polyethylene is increased, the environment stress cracking resistance decreases, and vice versa. This inverse relationship is known in the art as the ESCR-rigidity balance. It is required, for any given bottle grade polyethylene, to achieve a compromise between the environmental stress cracking resistance of the polyethylene and the rigidity of the polyethylene employed in the blown bottle.
A number of different catalyst systems have been disclosed for the manufacture of polyethylene, in particular high density polyethylene (HDPE) suitable for blow moulding. 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. It is known to employ a chromium-based catalyst (i.e. a catalyst known in the art as a “Phillips catalyst)”. Such a chromium-based catalyst enables the production of polyethylene having desirable physical and Theological properties.
It is known in the art to use chromium-based catalysts to polymerise HDPE and in particular to product high density polyethylene having high resistance to environmental stress cracking. For example, EP-A-0291824, EP-A-0591968 and U.S. Pat. No. 5,310,834 each disclose mixed catalyst compositions, incorporating chromium-based catalysts, for the polymerisation of polyethylene. Each of those prior proposals suffers from the disadvantage that mixed catalysts are required which can increase the complexity and cost of the process.
It is known in the art to provide titanium in a chromium-based catalyst. Titanium can be incorporated either into the support for the chromium catalyst or into the catalytic composition deposited on the support.
Titanium can be incorporated into the support by coprecipitation or terprecipitation as is the case for cogel and tergel type catalysts developed by Phillips Petroleum. Cogel and tergel catalysts respectively have binary and ternary supports.
Alternatively, titanium can be incorporated into the support by impregnation of the support as described for example in U.S. Pat. No. 4,402,864 or by chemisorption of a titanium compound into the support as described for example in U.S. Pat. No. 4,016,343.
Titanation of the catalytic composition has been disclosed in earlier patent specifications.
U.S. Pat. No. 5,006,506 discloses a titanated chromium-based catalyst having a silica or silica/alumina support by drying the catalyst and then treating the dried catalyst with tetraisopropyltitanate prior to activation of the catalyst.
U.S. Pat. No. 4,718,703 discloses that titanium can be incorporated into the catalytic composition by adding to a composite liquid suspension, of a carrier material (i.e. a support) and chromium trioxide, a titanium compound of the formula Ti(OR)
4
.
U.S. Pat. No. 4,224,428 discloses the titanation of a chromium-based catalyst having a silica support by treating the catalyst, after a drying step, with liquid tetraisopropoxide, followed by activation.
U.S. Pat. No. 4,184,979 discloses that titanium can be incorporated into the catalytic composition by adding at elevated temperature a titanium compound such as titanium tetraisopropoxide to a chromium-based catalyst which has been heated in a dry inert gas The titanated catalyst is then activated at elevated temperature.
U.S. Pat. No. 3,798,202 discloses the production of a titanated chromium-based catalyst for the production of low density polyethylene.
It is an aim of the present invention to provide a process for producing polyethylene suitable for blow moulding which employs a single chromium-based catalyst.
It is a further aim of the present invention to provide such a process for producing blow moulding grade polyethylene having a good balance of rheological and mechanical properties.
It is a yet further aim of the present invention to provide a process for producing blow moulding grade polyethylene having a good balance between the environmental stress cracking resistance and rigidity.
Accordingly, the present invention provides a process for preparing a supported chromium-based-catalyst for the production of high density polyethylene, by polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising 3 to 10 carbon atoms, which comprises the steps of: a) providing an alumina-containing support; b) depositing a chromium compound on the support to form a chromium-based catalyst; c) dehydrating the chromium-based catalyst to remove physically adsorbed water by heating the catalyst at a temperature of at least 300° C. in an atmosphere of dry, inert gas; d) titanating the chromium-based catalyst at a temperature of at least 300° C. in an atmosphere of dry, inert gas containing a titanium compound of the general formula selected from R
n
Ti(OR′)
m
and (RO)
n
Ti (OR′)
m
wherein R and R′ are the same or different and are hydrocarbyl groups containing from 1 to 12 carbon atoms, n is 0 to 3, m is 1 to 4 and m+n equals 4, to form a titanated chromium-based catalyst having a titanium content of from 1 to 5% by weight, based on the weight of the titanated catalyst; and e) activating the titanated catalyst at a temperature of from 500 to 900° C.
The present invention further provides a process for producing high density polyethylene suitable for blow moulding by polymerisation of ethylene or copolymerisation of ethylene with an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms in the present of a catalyst produced in accordance with the invention.
The present invention also provides the use, for improving the environmental stress crack resistance of polyethylene resins, of a catalyst produced in accordance with the invention in a process for producing high density polyethylene suitable for blow moulding, the process comprising polymerising ethylene, or copolymerising ethylene and an alpha-olefinic comonomer comprising from 3 to 10 carbon atoms.
The present invention is predicated on the surprising discovery by the present inventor that the use of the combination of particular treatment steps for producing a particular catalyst system comprising a chromium-based catalyst, having an alumina-containing support, and a titanated surface, can produce a blow moulding grade polyethylene exhibiting improved mechanical properties, in particular an improved ESCR-rigidity balance, than

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