Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1998-04-20
2001-03-20
Krynski, William (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S383000, C174S1100PM, C174S1100SR, C174S1130AS, C525S240000
Reexamination Certificate
active
06203907
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, which form 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 net result is a reduction in the dielectric properties of the insulation.
Many solutions have been proposed for increasing the resistance of organic insulating materials to degradation by water treeing. The most recent solution involves the addition of polyethylene glycol (PEG), as a water tree growth inhibitor, to a heterogeneous low density polyethylene such as described in U.S. Pat. Nos. 4,305,849; 4,612,139; and 4,812,505. The addition of PEG to polyethylene, however, presents certain problems, particularly in the areas of process and long term heat stability and in compatibility with the host polymer. The latter is addressed by selecting a PEG of a particular molecular weight (weight average molecular weight); however, compounding conditions may still produce a low molecular weight fraction. The former requires the addition of a high level of certain heat stabilizers, which cause staining of the composition. The color (or stain) produced and the reduction in crosslinking, both due to the high level of heat stabilizer, lead to some commercial difficulties. Thus, there is an industrial demand for water tree retardant additives that are as effective as PEG, but do not present stability and compatibility concerns.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a polyethylene composition, which exhibits a much improved resistance to water trees. Other objects and advantages will become apparent hereinafter.
According to the invention, a composition has been discovered which meets the above object.
Such a composition comprises polyethylene and, for each 100 parts by weight of polyethylene, about 0.1 to about 3 parts by weight of the reaction product of (i) an aliphatic diacid anhydride or a polymer or copolymer thereof wherein the anhydride has 4 to 20 carbon atoms; and (ii) a polymer selected from the group consisting of a polycaprolactone, a polyalkylene glycol, a monoalkyl ether of a polyalkylene glycol, and a mixture of two or more of said polymers, the weight ratio of component (ii) to component (i) being in the range of about 0.05:1 to about 1:1.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Polyethylene, as that term is used herein, is a homopolymer of ethylene or a copolymer of ethylene and a minor proportion of one or more alpha-olefins having 3 to 12 carbon atoms, and preferably 4 to 8 carbon atoms, and, optionally, a diene, or a mixture of such homopolymers and copolymers. The mixture can be a mechanical blend or an in situ blend. Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene.
The polyethylene can be homogeneous or heterogeneous. The homogeneous polyethylenes usually have a polydispersity (Mw/Mn) in the range of about 1.5 to about 3.5 and an essentially uniform comonomer distribution, and are characterized by single and relatively low DSC melting points. The heterogeneous polyethylenes, on the other hand, have a polydispersity (Mw/Mn) greater than 3.5 and do not have a uniform comonomer distribution. Mw is defined as weight average molecular weight and Mn is defined as number average molecular weight. The polyethylenes of interest here can have a density in the range of 0.860 to 0.950 gram per cubic centimeter, and preferably have a density in the range of 0.870 to 0.925 gram per cubic centimeter. They also can have a melt index in the range of about 0.1 to about 20 grams per 10 minutes, and preferably have a melt index in the range of about 0.5 to about 5 grams per 10 minutes. The polyethylenes can be produced by low or high pressure processes. Under low pressure, 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 whereas high pressure processes are typically run at pressures above 15,000 psi. Typical catalyst systems, which can be used to prepare these polyethylenes, are organic peroxides for high pressure processes and, for low pressure processes, magnesium/titanium based catalyst systems, which can be exemplified by the catalyst system described in U.S. Pat. No. 4,302,565 (heterogeneous polyethylenes); vanadium based catalyst systems such as those described in U.S. Pat. Nos. 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 such as that described in U.S. Pat. No. 4,101,445; a metallocene catalyst system such as that 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. Catalyst systems, which use chromium or molybdenum oxides on silica-alumina supports, are also useful. Typical processes for preparing the polyethylenes are also described in the aforementioned patents. Typical in situ polyethylene blends and processes and catalyst systems for providing same are described in U.S. Pat. No. Nos. 5,371,145 and 5,405,901. The various polyethylenes can include low density homopolymers of ethylene (made by high pressure processes), linear low density polyethylenes, very low density polyethylenes, and medium density polyethylenes. The latter three polyethylenes are generally made by low pressure processes. A conventional high pressure process is described in Introduction to Polymer Chemistry, Stille, Wiley and Sons, N.Y., 1962, pages 149 to 151.
The reaction product of component (ii) and component (i), which is a water tree growth inhibitor, is present in the composition in an amount of about 0.1 to about 3 parts by weight for each 100 parts by weight of the primary polymer, which is polyethylene. [The acronym pph may be used. This stands for parts per hundred.] The reaction product is formed by a condensation reaction between the two components. The condensation reaction occurs under normal compounding process conditions for polyethylene hence no additional manufacturing step is introduced. The reaction product is preferably present in an amount of about 0.5 to about 2.5 parts by weight. It is understood that other polymers can be present in addition to the primary polymer, e.g., polypropylene, polybutylene, ethylene/propylene copolymer rubber, and ethylene/propylenen/diene terpolymer rubber, but the amount of these additional polymers will be based on the primary polymer.
The weight ratio of component (ii) to component (i) is in the range of about 0.05:1 to about 1:1, and is preferably in the range of about 0.1:1 to about 0.5:1.
Component (i) is a an aliphatic diacid anhydride or a polymer or copolymer thereof wherein the anhydride has 4 to 20 carbon atoms. The copolymer is considered to be a polymer of two or more monomers. Particularly included are terpolymers and graft copolymers of the anhydride. The copolymers can be formed by addition or grafting. Examples of monomers which can be used in the copolymer or graft copolymer are ethylene, propylene, 1-butene, 1-hexene, 1-octene, 1-decene, and 1-octadecene. One example is a poly (1-alkyl-unsaturated aliphatic d
Bresch Saul R.
Gray J. M.
Krynski William
Union Carbide Chemicals & Plastics Technology Corporation
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