Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2002-04-10
2003-06-17
Lu, Caixia (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C525S319000, C525S324000
Reexamination Certificate
active
06579922
ABSTRACT:
BACKGROUND TO THE INVENTION
The present invention is concerned with high density multimodal polyethylenes, methods for their production and uses thereof. In particular the invention relates to high density multimodal polyethylenes having especially superior stress-crack resistance. These polyethylenes can be used in polyethylene pipes to increase their stress-crack resistance making them suitable for use in no-sand pipe installation.
DESCRIPTION OF THE PRIOR ART
Polyolefins such as polyethylenes which have high molecular weight generally have improved mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefins having a multimodal molecular weight distribution (MWD) are desirable because they can combine the advantageous mechanical properties of high molecular weight fraction with the improved processing properties of one or more lower molecular weight fractions.
For many high density polyethylene (HDPE) applications, polyethylene with enhanced toughness, strength and environmental stress cracking resistance (ESCR) is important. These enhanced properties are more readily attainable with high molecular weight (HMW) polyethylene. However, as the molecular weight of the polymer increases, the processibility of the resin decreases. By providing a polymer with a broad or bimodal MWD, the desired properties that are characteristic of high molecular weight resin are retained while processibility, particularly extrudibility, is improved.
There are several methods for the production of multimodal or broad molecular weight distribution resins: melt blending, reactor in series configuration, or single reactor with dual site catalysts. Use of a dual site catalyst for the production of a bimodal resin in a single reactor is also known.
Chromium catalysts for use in polyolefin production tend to broaden the molecular weight distribution and can in some cases produce bimodal molecular weight distribution but usually the low molecular part of these resins contains a substantial amount of the co-monomer. Whilst a broadened molecular weight distribution provides acceptable processing properties, a bimodal molecular weight distribution can provide excellent properties.
Ziegler-Natta catalysts are known to be capable of producing bimodal polyethylene using two reactors in series. Typically, in a first reactor, a low molecular weight homopolymer is formed by reaction between hydrogen and ethylene in the presence of the Ziegler-Natta catalyst. It is essential that excess hydrogen be used in this process and, as a result, it is necessary to remove all the hydrogen from the first reactor before the products are passed to the second reactor. In the second reactor, a copolymer of ethylene and hexene is made so as to produce a high molecular weight polyethylene.
Metallocene catalysts are also known in the production of polyolefins. For example, EP-A-0619325 describes a process for preparing polyolefins such as polyethylenes having a multimodal or at least bimodal molecular weight distribution. In this process, a catalyst system which includes at least two metallocenes is employed. The metallocenes used are, for example, a bis(cyclopentadienyl) zirconium dichloride and an ethylene-bis(indenyl) zirconium dichloride. By using the two different metallocene catalysts in the same reactor, a molecular weight distribution is obtained, which is at least bimodal.
Polyethylene resins are known for the production of pipes. Pipe resins require high resistance against slow crack growth as well as resistance to rapid crack propagation yielding impact toughness. However, there is a need to improve in the performance of currently available pipe resins.
Methods are known for producing an improved pipe resin by employing specific catalysts belonging to the general types discussed above, to produce a high molecular weight linear low density polyethylene fraction having a narrow MWD.
However, it is still necessary to produce a polyethylene that can be used in a pipe to increase the stress-crack resistance of the pipe sufficiently to render it suitable for use in no-sand pipe installation. Pipes for no-sand pipe installation require especially high stress-crack resistance, since their surfaces are not shielded from abrasive rock and earth surfaces by a layer of sand, but instead are directly in contact with the rock and/or earth.
It is known to use conventional cross-linked polyethylene for no-sand installable pipes. However, cross-linked polyethylene is very expensive (considerably more expensive than non-crosslinked PE). In addition, large diameter pipes formed from cross-linked PE are not available and it is not possible to join such pipes by butt-fusion. These characteristics of cross-linked polyethylene are discussed in “The creep behaviour of polyethylene under the influence of local stress concentrations”, 3
R international
34, volume 10-11, 1995, pages 573-579. In this document, cross-linked polyethylene (PE-X) having a full notch creep test value of 5100 hrs is disclosed (FIG. 12). Problems in welding PE-X are discussed in “Long term durability of welds involving cross-linked and non cross-linked polyethylene” 3
R international
37, volume 10-11, 1995, pages 694-699.
It is also known to provide a protective layer around conventional non-crosslinked PE to render it suitable for use in no-sand installation pipes. This also increases the cost considerably and has the disadvantage that the outer layer needs to be peeled off for welding, making installation laborious and expensive. The characteristics and problems associated with these materials are discussed in detail in the published newsletter of Werner Strumann GmbH & Co., October 1997, pages 1-3.
U.S. Pat. No. 5,405,901 discloses the production of polyethylene blends in gas phase using two reactors for the production of films. A low density resin is produced in the first reactor and a high density resin is produced in the second reactor. There is no disclosure of the production of a polyethylene blend having properties required by pipes, in particular pipes having stress crack resistance.
U.S. Pat. No. 5,284,613 discloses the production of bimodal molecular weight polyethylene resins containing two fractions of different molecular weight for the production of blown films exhibiting improved machine direction/transverse direction tear balance. Again, there is no disclosure of the production of polyethylene pipe resins, in particular having improved stress-crack resistance.
EP-A-0533154 discloses the production of ethylene polymer blends of a virgin or recycled low molecular weight ethylene polymer produced from a chromium-based catalyst and a high molecular weight ethylene polymer produced from a titanium-based catalyst. It is disclosed that the blends may be used for bottle, film, pipe and/or drum applications. However, the resins disclosed therein do not exhibit superior stress-crack resistance required by the art.
U.S. Pat. No. 4,547,551 discloses the production of ethylene polymer blends of high molecular weight and low molecular weight ethylene polymer, with the resins being useful for the manufacture of film or in blow moulding techniques, the production of pipes, and wire coating. There is no disclosure of the provision of pipe resins having enhanced stress crack resistance.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the problems of the above prior art and to provide a polyethylene suitable for use in a pipe that can be laid in a no-sand installation method. Accordingly, the present invention provides a high density multimodal polyethylene, having a shear ratio (SR) of 18 or more and comprising at least 20% by weight of a high molecular weight fraction, which high molecular weight fraction has:
(a) a density (&rgr;) of 0.930 g/cm
3
or less; and
(b) a high load melt index (HLMI) of 0.30 g/10 mins or less.
The present invention further provides a method for the production of a high density multimodal poly
Bergen Grady K.
Fina Research S.A.
Lu Caixia
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