Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Halogen or compound containing same
Reexamination Certificate
1999-11-02
2001-09-25
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Halogen or compound containing same
C502S224000, C502S227000, C502S231000, C502S240000, C502S305000, C502S309000, C502S319000, C502S320000
Reexamination Certificate
active
06294500
ABSTRACT:
The present invention relates to a process for the production of high density polyethylene (HDPE), in particular such a polyethylene having a bimodal molecular weight distribution. The present invention further relates to a catalyst system for producing such HDPE and to the use of such a system.
For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distribution (MWD) is a fundamental property which determines the properties of the polymer, and thus its applications. It is generally recognised in the art that the molecular weight distribution of a polyethylene resin can principally determine the physical, and in particular the mechanical, properties of the resin and that the provision of different molecular weight polyethylene molecules can significantly affect the rheological properties of the polyethylene as a whole.
Since an increase in the molecular weight normally improves the physical properties of polyethylene resins, there is a strong demand for polyethylene having high molecular weight. However, it is the high molecular weight molecules which render the polymers more difficult to process. On the other hand, a broadening in the molecular weight distribution tends to improve the flow of the polymer when it is being processed at high rates of shear. Accordingly, in applications requiring a rapid transformation employing quite high inflation of the material through a die, for example in blowing and extrusion techniques, the broadening of the molecular weight distribution permits an improvement in the processing of polyethylene at high molecular weight (this being equivalent to a low melt index, as is known in the art). It is known that when the polyethylene has a high molecular weight and also a wide molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion and also the high molecular weight portion contributes to a good impact resistance for the polyethylene film. A polyethylene of this type may be processed utilising less energy with higher processing yields.
The molecular weight distribution can be completely defined by means of a curve obtained by gel permeation chromatography. Generally, the molecular weight distribution is defined by a parameter, known as the dispersion index D, which is the ratio between the average molecular 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.
It is known in the art that it is not possible to prepare a polyethylene having a broad molecular weight distribution and the required properties simply by mixing polyethylenes having different molecular weights.
As discussed above, high density polyethylene consists of high and low molecular weight fractions. The high molecular weight fraction provides good mechanical properties to the high density polyethylene and the low molecular weight fraction is required to give good processability to the high density polyethylene, the high molecular weight fraction having relatively high viscosity which can lead to difficulties in processing such a high molecular weight fraction. In a bimodal high density polyethylene, the mixture of the high and low melting weight fractions is adjusted as compared to a monomodal distribution so as to increase the proportion of high molecular weight species in the polymer. This can provide improved mechanical properties.
It is accordingly recognised in the art that it is desirable to have a bimodal distribution of molecular weight in the high density polyethylene. For a bimodal distribution a graph of the molecular weight distribution as determined for example by gel phase chromatography, may for example include in the curve a “shoulder” on the high molecular weight side of the peak of the molecular weight distribution.
The manufacture of bimodal polyethylene is known in the art. It is known in the art that in order to achieve a bimodal distribution, which reflects the production of two polymer fractions, having different molecular weights, two catalysts are required which provide two different catalytic properties and establish two different active sites. Those two sites in turn catalyse two reactions for the production of the two polymers to enable the bimodal distribution to be achieved. Currently, as has been known for many years, as exemplified by EP-A-0057420, the commercial production of bimodal high density polyethylene is carried out by a two step process, using two reactors in series. In the two step process, the process conditions and the catalyst can be optimised in order to provide a high efficiency and yield for each step in the overall process. However, the currently commercially employed two step processes suffer from the disadvantage that because two separate serial processes are employed, the overall process has a low throughput.
It would be desirable to provide a one step process for manufacturing bimodal high density polyethylene. EP-A-480376 discloses the production of bimodal polyethylene using a two catalyst mixture of a supported chromium catalyst and a Ziegler-Natta type catalyst. This process suffers from the disadvantage that the Ziegler-Natta catalyst requires a co-catalyst to give an active catalytic system but the co-catalyst can influence the supported chromium catalyst and in particular can detrimentally affect its activity. The applicant believes that the process disclosed in that prior patent specification has not been used commercially.
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 polyethylen product produced using a chromium-based catalyst tend to be different from the properties of a product employed using a Ziegler-Natta catalyst. The production of HDPE using just a chromium-based catalyst is thus desirable to enable the particular polyethylene product to be manufactured. The Encyclopedia of Polymer Science and Engineering, Volume 6, pages 431-432 and 466-470 (John Wiley & Sons, Inc., 1986, ISBN 0-471-80050-3) and Ullmann's Encyclopedia of Industrial Chemistry, Fifth Edition, Volume A21, pages 501-502 (VCH Verlagsgesellschaft mbH, 1992, ISBN 3-527-20121-1) each discuss Phillips and Ziegler-Natta catalysts and the production of HDPE.
The prior art relating to the use of chromium-based catalysts to polymerise HDPE can be classified into two broad groups, the first group teaching a catalyst system having two or more catalytic sites, at least one being a Cr site, on a single catalyst support and the second group teaching mixed catalysts, each on a respective support, the mixture comprising chromium-based catalysts, or one such chromium-based catalyst with a catalyst containing active sites of a non-Cr nature.
The first group includes as a representative prior publication U.S. Pat. No. 5,034,364. This document discloses two chromium species supported on a single refractory oxide. One species comprises CrO
3
or a Cr compound calcinable to CrO
3
and the other species comprises a silylchromate-type compound. Such a catalyst does not produce a bimodal HDPE.
The second group includes as representative prior publications EP-A-480376 discussed above, EP-A-291824, U.S. Pat. No. 4,025,707, U.S. Pat. No. 5,310,834 and EP-A-591968.
EP-A-291824 discloses the production of HDPE using a mixture of two chromium-containing catalyst components having silica supports of different pore volumes and one of the components also containing Al. U.S. Pat. No. 4,025,707 discloses a mixed catalyst comprising several portions of the same or different supported components and metal promoted variations thereof, each portion being activated at a different temperature.
Dath Jean-Pierre
Debras Guy
Fina Research S.A.
Wheelington Jim D.
Wood Elizabeth D.
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