Semiconducting shield compositions

Details Semiconducting shield compositions Semiconducting shield compositions

2001-03-08

2002-09-24

Reichard, Dean A. (Department: 2831)

Electricity: conductors and insulators

Anti-inductive structures

Conductor transposition

C174S126200, C174S1060SC, C525S050000

Reexamination Certificate

active

06455771

ABSTRACT:

TECHNICAL FIELD
This invention relates to semiconducting shield compositions for use in power cable, particularly medium and high voltage power cable.
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 materials including a first semiconducting shield layer (conductor or strand shield), an insulating layer, a second semiconducting shield layer (insulation shield), a metallic tape or wire shield, and a protective jacket. The outer semiconducting shield can be either bonded to the insulation or strippable, with most applications using strippable shields. Additional layers within this construction such as moisture impervious materials are often incorporated.
Polymeric semiconducting shields have been utilized in multilayered power cable construction for many decades. Generally, they are used to fabricate solid dielectric power cables rated for voltages greater than 1 kilo Volt (kV). These shields are used to provide layers of intermediate conductivity between the high potential conductor and the primary insulation, and between the primary insulation and the ground or neutral potential. The volume resistivity of these semiconducting materials is typically in the range of 10
−1
to 10
8
ohm-cm when measured on a completed power cable construction using the methods described in ICEA S-66-524, section 6.12, or IEC 60502-2 (1997), Annex C. Typical strippable shield compositions contain a polyolefin, such as ethylene/vinyl acetate copolymer with a high vinyl acetate content, conductive carbon black, an organic peroxide crosslinking agent, and other conventional additives such as a nitrile rubber, which functions as a strip force reduction aid, processing aids, and antioxidants. These compositions are usually prepared in granular or pellet form. Polyolefin formulations such as these are disclosed in U.S. Pat. No. 4,286,023 and European Patent Application 420 271. The shield composition is, typically, introduced into an extruder where it is co-extruded around an electrical conductor at a temperature lower than the decomposition temperature of the organic peroxide to form a cable. The cable is then exposed to higher temperatures at which the organic peroxide decomposes to provide free radicals, which crosslink the polymer.
In order to provide a semiconducting shield it is necessary, as noted above, to incorporate conductive particles (conductive filler) into the composition. Industry is constantly attempting to reduce the conductive filler loading and thus reduce formulation cost while maintaining a sufficient level of electrical conductivity and improve processability through reduced viscosity.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a multiphase semiconducting shield composition with reduced conductive filler loading while maintaining a high level of conductivity and improving processability. Other objects and advantages will become apparent hereinafter.
According to the invention, such a composition has been discovered. The semiconducting shield composition is a conducting polymer composite comprising:
(i) a phase I material consisting essentially of a polar copolymer of ethylene and an unsaturated ester having 4 to 20 carbon atoms, said copolymer having a crystallinity of 0 to about 30 percent as determined by differential scanning calorimetry analysis and having a melt viscosity &eegr;
I
;
(ii) a phase II material having a crystallinity of 0 to about 30 percent as determined by differential scanning calorimetry analysis and having a melt viscosity &eegr;
II
, said phase II material consisting essentially of (A) a non-polar copolymer of ethylene, an alpha-olefin having 3 to 12 carbon atoms, and, optionally, a diene, or (B) a non-polar elastomer, either of which, when mixed with the phase I material, will not enter into a completely homogeneous state, but is compatible with the phase I material; and
(iii) a conducting filler material dispersed in the phase I material and/or the phase II in an amount sufficient to be equal to or greater than the amount required to generate a continuous conducting network in the phase I and phase II materials,
with the proviso that the phase I and phase II materials, in the molten state, have the following relationship: (&eegr;
I
÷&eegr;
II
)×(V
II
÷V
I
)=about 0.5 to about 2.0 wherein V
I
and V
II
are the volume fractions of the phase I and phase II materials, respectively, and V
I
+V
II
=1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS(S)
The phase I material consists essentially of a polar copolymer of ethylene and an unsaturated ester. The copolymers of ethylene and unsaturated esters are generally made by high pressure processes. A conventional high pressure process is described in Introduction to Polymer Chemistry, Stille, Wiley and Sons, New York, 1962, pages 149 to 151. The high pressure processes are typically free radical initiated polymerizations conducted in a tubular reactor or a stirred autoclave. In the stirred autoclave, the pressure is in the range of about 10,000 to 30,000 psi and the temperature is in the range of about 175 to about 250 degrees C., and in the tubular reactor, the pressure is in the range of about 25,000 to about 45,000 psi and the temperature is in the range of about 200 to about 350 degrees C. The unsaturated esters can be alkyl acrylates, alkyl methacrylates, and vinyl carboxylates. The alkyl group can have 1 to 8 carbon atoms and preferably has 1 to 4 carbon atoms. The carboxylate group can have 2 to 8 carbon atoms and preferably has 2 to 5 carbon atoms. In semiconducting shields, the portion of the copolymer attributed to the ester comonomer can be in the range of about 20 to about 55 percent by weight based on the weight of the copolymer, and is preferably in the range of about 35 to about 55 percent by weight. In terms of mole percent, the ester comonomer can be present in an amount of about 5 to about 30 mole percent. The ester can have about 4 to about 20 carbon atoms, and preferably has about 4 to about 7 carbon atoms. Examples of vinyl esters (or carboxylates) are vinyl acetate, vinyl butyrate, vinyl pivalate, vinyl neononanoate, vinyl neodecanoate, and vinyl 2-ethylhexanoate. Vinyl acetate is preferred. Examples of acrylic and methacrylic acid esters are lauryl methacrylate; myristyl methacrylate; palmityl methacrylate; stearyl methacrylate; 3-methacryloxy-propyltrimethoxysilane; 3-methacryloxypropyltriethoxysilane; cyclohexyl methacrylate; n-hexylmethacrylate; isodecyl methacrylate; 2-methoxyethyl methacrylate; tetrahydrofurfuryl methacrylate; octyl methacrylate; 2-phenoxyethyl methacrylate; isobornyl methacrylate; isooctylmethacrylate; octyl methacrylate; isooctyl methacrylate; oleyl methacrylate; ethyl acrylate; methyl acrylate; t-butyl acrylate; n-butyl acrylate; and 2-ethylhexyl acrylate. Methyl acrylate, ethyl acrylate, and n- or t-butyl acrylate are preferred. The alkyl group can be substituted with an oxyalkyltrialkoxysilane, for example. The copolymers can have a density in the range of 0.900 to 0.990 gram per cubic centimeter, and preferably have a density in the range of 0.920 to 0.970 gram per cubic centimeter. The copolymers can also have a melt index in the range of about 0.1 to about 100 grams per 10 minutes, and preferably have a melt index in the range of about 1 to about 50 grams per 10 minutes. A typical process for the preparation of a copolymer of ethylene and an unsaturated ester is described in U.S. Pat. No. 3,334,081.
The phase II material consists essentially of a non-polar copolymer 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. Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The alpha-olefin comonomers can be present in the copolymer in amounts of about 5 to about 30 mole percent.
The phase II material can also be a non-polar elastome

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