Water Impervious Rubber or Plastic Insulated Power Cable

The present invention relates to an improved water impervious rubber or plastic insulated power cable.

A typical water impervious rubber or plastic insulated power cable has a structure wherein a water impervious layer comprising a metal-plastic laminated tape, a hold tape layer, and a metal shield layer are sequentially formed on a cable core. The cable core has a conductor shield, a rubber or plastic insulation layer, and an insulation shield which are formed on a conductor. Crosslinked polyethylene, an ethylene-propylene rubber, and the like are mainly used as the insulation layer in water impervious rubber or plastic insulated power cables having such a structure. Laminated tapes comprising lead or a lead alloy and polyolefin are mainly used as plastic laminated tapes constituting water impervious layers since they have excellent flexibility and chemical resistance. In addition, semiconductive cloth tapes are generally used as hold tapes applied on water impervious layers. A semiconductive cloth tape is obtained by friction-treating a woven fabric with electrically conductive rubber. The warps and wefts of the woven fabric are, in general, cotton yarn, viscose rayon yarn, acetate yarn, vinylon yarn, nylon yarn, or polyester yarn.

In the manufacture of water impervious rubber or plastic insulated power cables having the above conventional structure, a laminated tape comprising a metal foil and a plastic film is longitudinally applied on the cable core. After a hold tape is wound on the laminated tape, heating is performed to bond the laminated tape to the cable core, thereby forming a water impervious layer. In this case, the adhesive strength between the laminated tape and the cable core is largely influenced by the temperature and time during heating. That is, the higher the heating temperature and heating time, the higher the adhesive strength obtained.

A water impervious rubber or plastic insulated power cable manufactured in the manner described above, however, has the following problems.

A semiconductive cloth tape normally used as a hold tape on a conventional water impervious layer has a memory rate of expansion and contraction of 0 to 15%. Therefore, when a water impervious rubber or plastic insulated power cable having a hold tape constituted by such a tape is subjected to repeated bending, the water impervious tape cracks along the edge of the hold tape layer.

When the cable is subjected to a heat cycle of 130.degree. C..revreaction.R.T. (Room Temperature) with power applied to the conductor, as the cable core expands or contracts, the water impervious layer comprising a metal-plastic laminated tape bonded on the core also expands or contracts. However, the hold tape on the water impervious layer, once expanded, cannot contract. Therefore, when other layers expand and then contract, the hold tape layer forms wrinkles. Such wrinkles in the hold tape layer cause damage to the water impervious layer formed within the hold tape layer and/or the metal shield layer formed around the hold tape layer, thereby significantly degrading the characteristics of the cable.

The memory rate of expansion and contraction, which represents the degree of contraction of a yarn is an index for stretchability of a fabric of the yarn, and is calculated in accordance with JIS (Japanese Industrial Standard) L 1090 by the following equation:

Memory rate of expansion and contraction A(%)=(a-b)/a.times.100

where,

a=hank (mm) in the case of applying a load of 2/1000 gf.times.20 number per 1d of the indicated denier and another load of 1/10 gf.times.20 number per 1d of the indicated denier; and

b=hank (mm) in the case of applying a load of 2/1000 gf.times.20 number per 1d of the indicated denier.

In view of the fact as described above for a water impervious rubber or plastic insulated power cable, extensive studies have been made and it has been found that the above problems can be effectively solved by using, as a hold tape layer, a tape having an excellent expansion property, capable of following the heat cycle of the cable. Based on this finding, an improved water impervious rubber or plastic insulated power cable has been obtained by using, as a tape having such a characteristic, an electrically conductive tape obtained by forming an electrically conductive rubber layer on at least one side of a woven fabric in which the warps or wefts or both have an excellent memory rate of expansion and contraction, preferably 30% or more, and which, therefore, has a memory rate of expansion and contraction of at least 30% in the transverse or longitudinal or both directions.

FIG. 1 is a sectional view showing an example of a cable construction according to the present invention;

FIG. 1A shows a fragmentary cross-sectional view of a water impervious layer of FIG. 1;

FIG. 1B shows a fragmentary cross-sectional view of a hold tape of FIG. 1; and

FIG. 2 is a sectional view showing another example of a cable construction according to the present invention.

According to the present invention, an electrically conductive tape using as a base cloth a woven fabric obtained by weaving a yarn having a memory rate of expansion and contraction of 30% or more is used as a hold tape formed on a water impervious layer. Therefore, the hold tape can follow the movement of the cable core, and therefore does not cause twisting or shifting or forming of wrinkles.

The effect of this hold tape layer is further enhanced when a lubricant such as silicone resin, graphite powder, talc, paraffin, hydrocarbon resin, fatty amide, fatty ester, carbon powder, or a stearate is applied on the surface of this electrically conductive tape.

In order to impart good characteristics to a cable, a yarn having a memory rate of expansion and contraction of 40% or more is used for the woven fabric, and wefts have a larger elongation than warps. The weaving method can be plain weaving, diagonal weaving, or twill weaving.

As the material for the yarn, a synthetic resin material having a low hygroscopicity is preferable. Preferred examples of such resin materials may include nylon, polyester, polybutylene terephthalate, polyurethane, and polypropylene.

As a yarn having a memory rate of expansion and contraction of 30% or more by crimping, wooly tetron, wooly nylon, and P.B.T. fiber ("Samora" available from Toray Industries, Inc.) are commercially available. As an electrically conductive rubber to be formed on at least one side of a woven fabric as a base material for a hold tape, an electrically conductive compound obtained by drying a solvent solution of a mixture of butyl rubber as a base and an electrically conductive carbon black such as "Ketjenblack EC" (trade name of AKZO Co.) or acetylene black and by vulcanizing the mixture by heating is used. The electrically conductive tape forming the hold tape has a volume resistivity of 5.times.10.sup.6 .OMEGA..multidot.cm or less.

When at least one side of the electrically conductive rubber layer formed on the semiconductive woven fabric is subjected to a smoothening treatment by topping or the like, better results are obtained in respect to stress relaxation in and prevention of external damage to the water impervious layer.

In the water impervious rubber or plastic insulated power cable according to the present invention, the water impervious layer formed under the hold tape layer compries a laminated tape comprising a metal foil of lead, copper, aluminum or the like, 20 to 200 .mu.m in thick, and an electrically conductive plastic film having a thickness of 20 to 200 .mu.m and a volume resistivity of 10.sup.6 .OMEGA..multidot.cm or less. A copper wire or copper tape is used for a metal shield layer to be formed around the hold tape layer. A laminated tape using a lead or lead alloy foil is most preferred since it is flexible to provide excellent formability and can impart excellent chemical resistance to the cable.

Crosslinked polyethylene, polyethylene, ethylenepropylene rubber or the like is used for the insulation layer. Although a stranded conductor is normally used as a conductor, in applications where water infiltration from the conductor side is feared and must be prevented, a watertight conductor obtained by filling the gaps between the strands with an electrically conductive watertight compound is used. Although the electrically conductive watertight compound is not limited to a particular type, a mixture (developed by the present inventors) comprising 100 parts of butyl rubber (trade name: polysar-butyl), 10 to 30 parts of conductive carbon black (trade name: Ketjenblack EC), and 100 to 200 parts of calcium carbonate, and having a volume resistivity of 10.sup.6 .OMEGA..multidot.cm or less is preferable due to its excellent filling and other properties.

In a water impervious rubber or plastic insulated power cable according to the present invention, since no wrinkles are formed in the hold tape layer, the inner water impervious layer and the outer metal shield layer are not adversely affected and the cable can exhibit excellent characteristics over a long period of time, thereby providing excellent industrial effects.

In each of Examples 1 to 4 and Comparative Examples 1 to 4, as shown in FIG. 1, each watertight conductor had stranded conductors 1 of 500 mm.sup.2 area which were filled with an electrically conductive watertight compound (volume resistivity: 5.times.10.sup.2 .OMEGA..multidot.cm) consisting of 100 parts of butyl rubber (polysar-butyl), 25 parts of conductive carbon black (Ketjenblack EC), and 100 parts of calcium carbonate. Conductor shield 2 consisting of ethylene-ethyl acrylate copolymer (EEA) and electrically conductive carbon, insulation layer 3 of crosslinked polyethylene, and insulation shield 4 consisting of EEA and electrically conductive carbon were formed around the watertight conductor to provide a cable core. After the cable core was dried at 60.degree. C. to make the amount of methane gas 0.2 cc or less per cc of the crosslinked polyethylene, a water impervious tape was longitudinally applied around the cable core to form water impervious layer 5. The water impervious tape forming layer 5 was as shown in FIG. 1A, a 50 .mu.m-thick lead foil 11 laminated on both sides with 100 .mu.m-thick adhesive EEA films 12, 13. Hold tape 6 of each material shown in Table 1 below was wound around the resultant structure with a 1/2 lap. Metal shield layer 7 comprising a copper wire having a diameter of 1.2 mm were formed. Jacket 9 of polyvinyl chloride was formed as an outermost layer to provide 66 kV water impervious crosslinked polyethylene insulated power cable. FIG. 1B shows a typical hold tape 6 comprising a woven fabric layer 14 with an electrically conductive rubber layer 15 is formed on one side thereof.

In each of Examples 5 to 7, a watertight stranded conductor, 500 mm.sup.2 in cross-sectional area, filled with an electrically conductive watertight compound (volume resistivity: 1.times.10.sup.4 .OMEGA..multidot.cm) consisting of 100 parts of butyl rubber (polysar-butyl), 15 parts of conductive carbon black (Ketjenblack EC), and 250 parts of calcium carbonate was covered with a conductor shield consisting of ethylene-propylene rubber (EPR) and electrically conductive carbon, an insulation layer consisting of EPR, and an insulation shield consisting of EPR and electrically conductive carbon to form a cable core. After the cable core was dried, a water impervious tape was longitudinally applied to it to form a water impervious layer. The water impervious tape consisted of a 50 .mu.m-thick lead foil laminated on both sides with a 100 .mu.m-thick adhesive EEA film. A hold tape of each material as shown in Table 2 below was wound around the structure with a 1/2 lap. A metal shield layer consisting of a copper wire having a diameter of 1.2 mm was formed around the hold tape layer. A jacket of polyethylene was formed as an outermost layer to provide a 66 kV water impervious ethylene-propylene rubber insulated power cable.

In each of Examples 8 to 10, a watertight stranded conductor, 500 mm.sup.2 in cross-sectional area, filled with an electrically conductive watertight compound (volume resistivity: 5.times.10.sup.3 .OMEGA..multidot.cm) consisting of 100 parts of butyl rubber (polysar-butyl), 20 parts of conductive carbon black (Ketjenblack EC), and 150 parts of calcium carbonate was covered with a conductor shield consisting of an ethylene-vinyl acetate copolymer (EVA) and electrically conductive carbon, an insulation layer of polyethylene, and an insulation shield consisting of EVA and electrically conductive carbon to form a cable core. After the cable core was dried, a water impervious tape was longitudinally applied around the cable core to form a water impervious layer. The water impervious tape consisted of a 50 .mu.m-thick lead foil laminated on both sides with a 100 .mu.m-thick adhesive EEA film. Each hold tape of a material as shown in Table 3 below was wound around the structure with a 1/2 lap. After a metal shield layer comprising a copper wire of 1.2 mm was formed around the hold tape layer, a jacket of polyvinyl chloride was formed as an outermost layer to provide a 66 kV water impervious polyethylene insulated power cable.

As shown in FIG. 2, a watertight conductor had stranded conductors 1 of 500 mm.sup.2 area which were filled with an electrically conductive watertight compound (volume resistivity: 1.times.10.sup.3 .OMEGA..multidot.cm) consisting of 100 parts of butyl rubber (polysar-butyl), 20 parts of conductive carbon black (Ketjenblack EC), and 100 parts of calcium carbonate. Conductor shield 2 consisting of ethylene-ethyl acrylate copolymer (EEA) and electrically conductive carbon, insulation layer 3 of crosslinked polyethylene, and insulation shield 4 consisting of EEA and electrically conductive carbon were formed around the watertight conductor to provide a cable core. After the cable core was dried at 60.degree. C. to make the amount of methane gas 0.2 cc or less per cc of the crosslinked polyethylene, a water impervious tape was longitudinally applied around the cable core to form water impervious layer 5, which is shown in detail in FIG. 1A. The water impervious tape was a 50 .mu.m-thick lead foil laminated on both sides with a 100 .mu.m-thick adhesive EEA films. Hold tape 6 (shown in detail in FIG. 1B) equal to that of Example 3 shown in Table 1 was wound around the resultant structure with a 1/2 lap. Metal shield layer 7 comprising a copper tape having a thickness of 0.1 mm and then hold tape layer 8 equal to that of Example 1 shown in Table 1 were formed. Jacket 9 of polyvinyl chloride was formed as an outermost layer to provide 66 kV water impervious crosslinked polyethylene insulated power cable.

The water impervious rubber or plastic insulated power cables in the above Examples and Comparative Examples were subjected to the following test.

The conductor of each power cable was powered and subjected to a heat cycle of a conductor temperature 130.degree. C..revreaction.R.T. ten times. Thereafter, the cables of each Example and Comparative Example were bent around a test cylinder at room temperature, unwound and the process repeated but this time they were bent in reverse direction. This cycle of operations was performed a total of five, ten and twenty times. Each cable was then disassembled, and the metal shield layer 7, the hold tape layer(s) 6,8), and the water impervious layer 5 of each cable were evaluated in accordance with the evaluation standards shown in Table 4.

The results of evaluation are shown in Table 5 below. In practice, if no abnormality is found in the water impervious layer, the hold tape layer, and the metal shield layer after bending 5 cycles along an arc of a diameter 10 times greater than the outer diameter of the cable, the cable is evaluated as being usable for practical applications.