Tree resistant cable

Details Tree resistant cable Tree resistant cable

2000-09-26

2002-08-27

Reichard, Dean A. (Department: 2831)

Electricity: conductors and insulators

Conduits, cables or conductors

Insulated

C174S1100PM

Reexamination Certificate

active

06441309

ABSTRACT:

TECHNICAL FIELD
This invention relates to electric power cable insulated with a polyethylene composition having an improved resistance to water trees.
BACKGROUND INFORMATION
A typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric material including a first semiconducting shield layer, an insulating layer, a second semiconducting shield layer, a metallic tape or wire shield, and a jacket.
These insulated cables are known to suffer from shortened life when installed in an environment where the insulation is exposed to water, e.g., underground or locations of high humidity. The shortened life has been attributed to the formation of water trees, which occur when an organic polymeric material is subjected to an electrical field over a long period of time in the presence of water in liquid or vapor form. The formation of water trees is believed to be caused by a complex interaction of the AC electrical field, moisture, time, and the presence of ions. The net result is a reduction in the dielectric strength of the insulation.
Many solutions have been proposed for increasing the resistance of organic insulating materials to degradation by water treeing. One solution involves the addition of polyethylene glycol, as a water tree growth inhibitor, to a low density polyethylene made by a high pressure process. This solution has been applied for many years; however, there is a continuous industrial demand for improvement with respect to four features, i.e., tree retardancy, processability, peroxide response, and flexibility.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a cable based on a polyethylene composition, which does provide improvement in the four features mentioned above. Other objects and advantages will become apparent hereinafter.
According to the invention, a cable has been discovered which meets the above object.
The cable comprises one or more electrical conductors or a core of electrical conductors, each conductor or core being surrounded by a layer of a composition comprising at least about 95 percent by weight of a very low density polyethylene (VLDPE) having a density in the range of 0.860 to 0.915 gram per cubic centimeter, said VLDPE having a number average molecular weight in the range of about 10,000 to about 20,000 and a CHMS equal to or greater than about 4.5 percent by weight as determined by SEC.
CHMS=Concentration of High Molecular Weight Species. The High Molecular Weight Species (HMS) of the CHMS has a number average molecular weight equal to or greater than about 500,000.
SEC=Size Exclusion Chromatography.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The very low density polyethylene (VLDPE) is a linear copolymer of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms, and preferably 4 to 8 carbon atoms, and, optionally, a diene. Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene.
The VLDPE can be homogeneous or heterogeneous. The homogeneous VLDPEs have an essentially uniform comonomer distribution, and are characterized by single and relatively low DSC melting points. The heterogeneous VLDPEs, on the other hand, do not have a uniform comonomer distribution. The VLDPEs can have a density in the range of 0.860 to 0.915 gram per cubic centimeter, and preferably have a density in the range of 0.880 to 0.910 gram per cubic centimeter.
The VLDPEs are generally produced by low pressure processes. They are preferably produced in the gas phase, but they can also be produced in the liquid phase in solutions or slurries by conventional techniques. Low pressure processes are typically run at pressures below 1000 psi.
Catalyst systems which can be used to prepare these VLDPE resins can be magnesium/titanium or vanadium-based systems; chrome-based systems; or metallocene systems. The chief requirement for these catalysts is that they can produce resins having the required molecular architecture, molecular weight, and density. These resins can be produced in either two or more reactors featuring the required process conditions to generate the main body of the resin in one reactor, and the high molecular weight tail in another reactor. In the case of this multistage polymerization system, a wide range of catalysts can be used. Magnesium/titanium based catalyst systems can be exemplified by the catalyst system described in U.S. Pat. No. 4,302,565 (heterogeneous polyethylenes); vanadium based catalyst systems by those described in U.S. Pat. No. 4,508,842 (heterogeneous polyethylenes) and U.S. Pat. Nos. 5,332,793; 5,342,907; and 5,410,003 (homogeneous polyethylenes); a chromium based catalyst system by that described in U.S. Pat. No. 4,101,445; a metallocene catalyst system by those described in U.S. Pat. Nos. 4,937,299 and 5,317,036 (homogeneous polyethylenes); or other transition metal catalyst systems. Many of these catalyst systems are often referred to as Ziegler-Natta catalyst systems or Phillips catalyst systems. Catalyst systems, which use chromium or molybdenum oxides on silica-alumina supports, can be included here. Typical processes for preparing the VLDPEs are also described in the aforementioned patents.
In the case of polymerization in a single reactor, catalysts may be used giving rise to two intimately mixed populations of resins whose sum produces the resin of the current invention. One suitable catalyst system is a silica-supported magnesium/titanium catalyst available from Grace Davison under the designation of Sylopol™ 5950, which produces suitable resins when polymerized in the presence of a mild aluminum alkyl cocatalyst such as tri-n-hexyl aluminum or tri-isobutyl aluminum at about a 30:1 Al/Ti weight ratio.
The melt index of the VLDPE can be in the range of about 0.1 to about 20 grams per 10 minutes and is preferably in the range of about 0.3 to about 5 grams per 10 minutes. The portion of the VLDPE attributed to the comonomer(s), other than ethylene, can be in the range of about 1 to about 49 percent by weight based on the weight of the copolymer and is preferably in the range of about 15 to about 40 percent by weight. A third comonomer can be included, e.g., another alpha-olefin or a diene such as ethylidene norbornene, butadiene, 1,4-hexadiene, or a dicyclopentadiene. The third comonomer can be present in an amount of about 1 to 15 percent by weight based on the weight of the copolymer and is preferably present in an amount of about 1 to about 10 percent by weight. It is preferred that the copolymer contain two or three comonomers inclusive of ethylene.
It will be understood that, if one or more additional resins are introduced into the composition, the amount of the additional resins will either make up the about 5 percent by weight balance or will be based on 100 parts by weight of the VLDPE. These resins can be various polyethylenes (low, medium, or high density) or polypropylenes, or other polymer additives conventionally used in wire and cable applications.
As noted, the polyethylene composition, which is used in the cable of the invention, comprises at least about 95 percent by weight of VLDPE having a number average molecular weight in the range of about 10,000 to about 20,000 and a CHMS equal to or greater than about 4.5 percent by weight as determined by SEC. HMS has a number average molecular weight equal to or greater than about 500,000, preferably in the range of about 500,000 to about 2,000,000. In order to provide the conventional molecular weight together with the high molecular weight tail in fully commingled form, the VLDPE can be prepared, as noted above, with a silica supported magnesium/titanium catalyst preactivated with an aluminum alkyl using the following steps and conditions:
An 8-inch gas phase fluid bed reactor of reaction volume 50 liters, which is capable of polymerizing olefins at a rate of 5 to 7 pounds per hour at 300 psi (pounds per square inch) pressure, is used. Reaction conditions for the VLDPE are: reaction temperature 60

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