Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2001-04-03
2004-03-16
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S495000
Reexamination Certificate
active
06706791
ABSTRACT:
TECHNICAL FIELD
This invention relates to a power cable having a semiconductive shield and moisture cured insulation.
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 (internal) semiconductive shield layer (conductor or strand shield), an insulating layer, a second semiconductive shield layer (insulation shield or external semiconductive layer), a metallic tape or wire shield, and a protective jacket. The external semiconductive 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 semiconductive 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 semiconductive materials is typically in the range of 10
−1
to 10
8
ohm-centimeters 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 internal or external 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. The electrical conductor can be, for example, made of annealed copper, semihard drawn copper, hard drawn copper, or aluminum.
Polyethylenes, which are typically used as the polymeric component in the insulation layer, can be made moisture curable by making the resin hydrolyzable, which is accomplished by adding hydrolyzable groups such as —Si(OR)
3
wherein R is a hydrocarbyl radical to the resin structure through conventional copolymerization or grafting techniques. Grafting can be effected at 210 to 250 degrees C. Suitable crosslinking agents are organic peroxides such as dicumyl peroxide; 2,5-dimethyl-2,5-di(t-butylperoxy)hexane; t-butyl cumyl peroxide; and 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3. Dicumyl peroxide is preferred. The amount of organic peroxide used in the grafting process can be in the range of 0.01 to 4 parts by weight per 100 parts by weight of the base resin.
Suitable alkoxysilane compounds, which can be used to provide the hydrolyzable group can be represented by the following formula: RR′SiY
2
wherein R is an aliphatic unsaturated hydrocarbon group or a hydrocarbonoxy group, R′ is a hydrogen atom or a saturated monovalent hydrocarbon group, and Y is an alkoxy group. Examples of R are vinyl, allyl, butenyl, cyclohexenyl, and cyclopentadienyl. The vinyl group is preferred. Examples of Y are ethoxy, methoxy, and butoxy.
Examples of ethylenically unsaturated alkoxysilanes are vinyl triethoxysilane, vinyl trimethoxysilane, and gamma-methacryloxypropyltrimethoxy-silane. The amount of alkoxysilane compound that can be used is preferably about 0.5 to about 20 parts by weight per 100 parts by weight of base resin.
Hydrolyzable groups can be added, for example, by copolymerizing ethylene with an ethylenically unsaturated compound having one or more —Si(OR)
3
groups or grafting these silane compounds to the resin in the presence of the aforementioned organic peroxides. The hydrolyzable resins are then crosslinked by moisture, e.g., steam or hot water, in the presence of a silanol condensation catalvst such as dibutyltin dilaurate, dioctyltin maleate, dibutyltin diacetate, stannous acetate, lead naphthenate, and zinc caprylate. Dibutyltin dilaurate is preferred. The amount of silanol condensation catalyst can be in the range of about 0.001 to about 20 parts by weight per 100 parts by weight of base resin, and is preferably about 0.005 to about 5 parts by weight.
Examples of hydrolyzable copolymers and hydrolyzable grafted copolymers are ethylene/vinyltrimethoxy silane copolymer, ethylene/gamma-methacryloxypropyltrimethoxy silane copolymer, vinyltrimethoxy silane grafted ethylene/ethyl acrylate copolymer, vinyltrimethoxy silane grafted linear low density ethylene/1-butene copolymer, and vinyltrimethoxy silane grafted low density polyethylene.
In applications where moisture cured insulation is used, it is desirable to provide a moisture cured strippable semiconductive shield to protect the insulation. The shield composition would then be prepared in the same manner as the moisture cured insulation as outlined above. Unfortunately, shield compositions, which could be moisture cured, were found to have a tendency to scorch, i.e., to prematurely crosslink during extrusion. In addition to solving the scorch problem, the shield had to be easily strippable by hand or with the aid of an appropriate tool.
Further, the use of a peroxide crosslinkable insulation shield over a moisture curable insulation is not considered viable because of the incompatibility of the processing requirements for each. Typically, the peroxide system utilizes higher operating temperatures during the cure cycle, and these high temperatures interfere with the dimensional stability of the “uncured” moisture curable insulation. The upshot is that the peroxide system requires a pressurized curing tube, which is an integral part of the extrusion process, while the moisture curable insulation is cured in a post extrusion stage. It was also found that while crosslinking via a peroxide did improve scorch, it did not enhance strippability.
It is apparent, then, that both the peroxide system and the moisture cure system for the insulation shield each have their drawbacks. Further, it is found especially desirable that the insulation shield have the following characteristics:
(1) a volume specific resistance of 100 ohm-centimeters or less to prevent corona degradation caused by partial delamination and gap formation;
(2) an elongation of 100 percent or more to maintain elasticity, and prevent partial delamination and gap formation when the power cable is bent or is exposed in the heat cycle;
(3) a smooth interface between the moisture cured insulation layer and the insulation shield with an absence of micro-protrusions;
(4) capable of being extruded high temperatures similar to the temperatures used for the moisture cured insulation layer, i.e., 210 to 250 degrees C.;
(5) a tensile strength of 10 Mpa or more so that the insulation shield will not be cut during stripping;
(6) a cold temperature resistance;
(7) a peel strength of 4 kilograms per 0.5 inch or less to provide for easy stripping from the moisture cured insulation layer; and
(8) a heat deformation at 120 degrees C. of 40 percent or less.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a composition useful for an insulation shield for a moisture cured insulation layer, which has the above characteristics and avoids the drawbacks of peroxide and moisture cured shields. Other objects and advantages will become apparent hereinafter.
According to the invention, such a composition has been discovered. The composition comprises:
(a) one or
Ishihara Koji
Ohki Ariyoshi
Tsukada Kiroku
Cain Edward J.
Nippon Unicar Company Limited
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