Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Plural component system comprising a - group i to iv metal...
Reexamination Certificate
1998-06-05
2001-03-13
Wu, David W. (Department: 1713)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Plural component system comprising a - group i to iv metal...
C502S232000, C502S254000, C502S256000, C502S240000, C502S242000, C526S113000, C526S096000, C526S106000, C526S348000
Reexamination Certificate
active
06200920
ABSTRACT:
The present invention relates to a catalyst for producing a high density polyethylene having a broad molecular weight distribution, in order to obtain good processability and good physical and chemical properties. In particular, the good physical properties may be improved tear properties when the polyethylene is made into films and/or improved environmental stress crack resistance. The present invention further relates to a process for producing said catalyst and to the use of such a catalyst.
For polyethylene, and for high density polyethylene (HDPE) in particular, the molecular weight distridution (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.
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 molecular dispersion index varies between 10 and 30.
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. These high molecular weight molecules, however render the polymer 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 shear rates. Accordingly, in applications requiring a rapid transformation 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 (high molecular weight polyethylenes have 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 broad molecular weight distribution, the processing of the polyethylene is made easier as a result of the low molecular weight portion while the high molecular weight portion contributes to a good impact resistance for the polyethylene resin. A polyethylene of this type may be processed using less energy with higher processing yields.
As a general rule, a polyethylene having a high density tends to have a high degree of stiffness. In general, however, the environment stress crack resistance (ESCR) of polyethylene has an inverse relationship with stiffness. In other words, as the stiffness of polyethylene is increased, the environment stress crack resistance decreases, and vice versa. This inverse relationship is known in the art as the ESCR-rigidity balance. It is required, for certain applications, to achieve a compromise between the environmental stress crack resistance and the rigidity of the polyethylene.
Polyethylene is well known in the art for use in making films. Typically, polyethylene films are blown or extruded through a die. The blowing and extrusion of the thin film defines for the film a machine direction in the direction of blowing or extrusion through the die and an orthogonal transverse direction. For many applications, the polyethylene film is required to have a high tear strength, and in particular a high isotropy in the tear strength between the machine and transverse directions. As a result of the blowing or extrusion technique, the polyethylene polymer chains can become substantially aligned in the machine direction of the blowing or extrusion process. This can yield a significantly higher tearing strength in the transverse direction of the film as compared to the tearing strength in the machine direction. There is generally a need for polyethylene films having good tear properties for use in the manufacture of films, and in particular a good isotropy in the tear properties between the machine and transverse directions.
A variety of catalyst systems are known for the manufacture of polyethylene. It is known in the art that the physical properties, in particular the mechanical properties, of a polyethylene resin can vary depending on what catalyst 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 resin produced using a chromium-based catalyst (i.e. a catalyst known in the art as a “Phillips catalyst”) tend to be different from the properties of a product employed using a Ziegler-Natta catalyst.
For the manufacture of polyethylene films, it is known that HDPE resins made using Ziegler-Natta catalysts have a good balance in their tear properties between the machine and transverse directions. In particular, such resins made using Ziegler-Natta catalysts, and having what is known in the art as a bimodal molecular weight distribution, have good isotropic tear properties. Such a bimodal HDPE resin has a bimodal distribution of the molecular weight of the high density polyethylene which is represented in a graph of the molecular weight distribution as determined for example by gel phase chromatography. The graph includes in the curve a “shoulder” on the high molecular weight side of the peak of the molecular weight distribution. Such a bimodal high density polyethylene consists of high and low molecular weight fractions in which the mixture of those fractions is adjusted as compared to a monomodal distribution so as to increase the proportion of high molecular weight species in the polymer.
The production of high density polyethylene 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 Ullman'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.
It is known in the art that in order to obtain the advantages of a broad molecular weight distribution, it is necessary to polymerise an intimate mixture of polyethylene molecules prepared in a common manufacturing process. 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 blending polyethylenes having different molecular weights.
It has thus been proposed to carry out the polymerisation by a two step process, using two reactors connected in series (GB-A-1233599; EP-A-057352; U.S. Pat. Nos. 4,414,369 and 4,338,424). In a first step and in the first reactor a fraction of the high density polyethylene is produced under specified conditions and in the following second step in the second reactor a second fraction of the high density polyethylene is produced using a different set of polymerisation conditions. 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. 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 has further been proposed to produce polyethylene with a broad molecular weight distribution with a two-catalyst mixture of one supported chromium catalyst and one Ziegler-Natta type catalyst
Dath Jean-Pierre
Debras Guy
Choi Ling-Siu
Fina Research S.A.
Wheelington Jim D.
Wu David W.
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