GB2099639A - Pressure dam cable - Google Patents

Abstract

A pressure dam cable having an improved gas-tightness is formed by stripping off the sheath 2, covering the sheath-stripped portion and its vicinity with a protective sleeve 4 and filling a resin 5 inside the sleeve; the insulation of an insulated cable conductor is made of a compound containing a polyolefin and an ethylene-glycidyl methacrylate- vinyl acetate terpolymer or the crosslinked product of the compound, and a tape made of the terpolymer is previously wound and melt-adhered onto the sheath in the vicinity of the both ends of the sheath-stripped portion in the pressure dam portion of the cable. <IMAGE>

Description

SPECIFICATION Pressure dam cable The present invention relates to an improved pressure dam cable which is employed for preventing leakage of gas from a gas-pressurized communication cable into a non gas-pressurized communication cable by connecting it between the gas-pressurized cable and the non gas-pressurized cable.

In general, the pressure dam of a pressure dam cable includes a cable body, a protective sleeve provided to cover the portion where a sheath for the cable is stripped off, and a filler which is gas-tightly filled in the protective sleeve. Therefore, as channels of gas leakage from a gas-pressurized cable into a non gas-pressurized cable or the atmosphere through the pressure dam cable, there are mentioned channels (a) between a conductor and an insulation of the cable, (b) between the insulation and the filler, and (c) between the sheath and the filler. In addition, the gas leakage may take place (d) between the protective sleeve and the filler or (e) between the sheath and the protective sleeve.However, if the gas-tightness in the above-mentioned portions (a), (b) and (c) is complete, the gas leakage is sufficiently prevented and, therefore, various proposals have been made on the improvement of the gas-tightness in the above-mentioned portions (a), (b) and (c).

For instance, in order to improve the gas-tightness between a conductor and an insulation of a cable, it is proposed to form an insulation from a compound of a polyethylene and an ethylene-vinyl acetate copolymer. However, the adhesiveness of the insulation is not sufficient as mentioned after.

In order to improve the gas-tightness between an insulation of a cable and a filler, namely the adhesiveness between them, there is proposed an insulated conductor having an insulation of a two layer structure where a polyolefin insulation layer is formed on a conductor and it is covered with an insulation layer made of a copolymer of ethylene as a main component (e.g. a copolymer of ethylene and methacrylic acid) or a metal salt of the copolymer. In case of employing such an insulated conductor, the adhesiveness to a filling resin is improved. However, the preparation steps are complicated because the insulation is composed of two layers, and also the adhesiveness of the insulation to the conductor is bad because a usual polyolefin is used as a material of the under insulation layer formed on the conductor.It is also possible to make the insulation of only the abovementioned ethylene copolymer, but in that case, electric characteristics are lowered.

Also, in order to improve the gas-tightness between a filling resin and a sheath, there is proposed a pressure dam cable in which a laminated tape is wound round the sheath in the neighborhood of the sheath-stripped ends so as to bring the plastic surface of the tape into contact with the sheath surface and is melt-adhered, while the metal foil surface of the tape is brought into contact with the filling resin.

According to this structure, the laminated tape adheres well not only to a plastic sheath, but also to a filler such as an epoxy resin, and accordingly the gas-tightness between the sheath and the filling resin is improved. However, this proposal has the disadvantages that because the tape used is composed of composite materials, the melt-adhesion procedure is complicated and the preparation cost rises. It is also proposed to use a non-woven fabric tape impregnated with an epoxy resin, but this tape not only is expensive, but also cannot satisfactorily improve the adhesiveness to the sheath.

There is also proposed a pressure dam cable in which a cylindrical elastic body is fitted round the outer circumference of a filling portion so as to pressurize the filling portion by utilizing the springback of the cylindrical elastic body. This cable also has the disadvantages of complication in structure and cost up.

It is an object of the present invention to provide a pressure dam cable capable of completely preventing a gas from leaking through all channels of gas leakage.

A further object of the invention is to provide a pressure dam cable which is simple in structure and can be prepared inexpensively.

These and other objects of the present invention will become apparent from the description hereinafter.

The present invention provides a process dam cable comprising an insulated cable conductor, a plastic cable sheath, a protective sleeve which covers the portion and its vicinity where the sheath is stripped off, and a resin filled inside the protective sleeve, said pressure dam cable being improved in that the insulation of said insulated cable conductor is made of a compound containing a polyolefin and an ethylene-glycidyl methacrylate-vinyl acetate terpolymer or a cross-linked product of the compound, and a tape made of a compound containing an ethylene-glycidyl methacrylate-vinyl acetate terpolymer as a main component is wound and melt-adhered onto the sheath in the neighborhood of the sheathstripped ends in the pressure dam portion.

Fig. 1 is an illustrative view of a pressure dam cable according to the present invention; and Fig. 2 is a cross section view of an insulated cable conductor.

Referring to Figs. 1 and 2, numeral 1 is insulated conductors of a cable, and numeral 2 is a plastic sheath of the cable, e.g. a polyethylene sheath or a laminated sheath. The insulated cable conductor 1 includes a conductor 11 and an insulation 1 2 made of a compound containing a polyolefin and an ethylene-glycidyl methacrylate-vinyl acetate terpolymer or a cross-linked product of the compound. A part of the sheath 2 is stripped off, and a tape made of a compound containing the ethylene-glycidyl methacrylate-vinyl acetate terpolymer as a main component is wound round the outer circumference of the sheath 2 in the neighborhood of the both ends of the sheath-stripped portion and is melt-adhered together by heating.The sheath-stripped portion is covered with a protective sleeve 4 such as a heat shrinkable tube, and a resin is filled inside the protective sleeve 4 as a filler 5. Numeral 6 is a shield bonding wire which connects between metal foils of the laminated sheath, and numeral 7 is a temporary dam made of an epoxy resin for preventing the filling resin 5 from flowing into the cable in the longitudinal direction.

A terpolymer of 80 to 90% by mole of ethylene, 6 to 14% by mole of glycidyl methacrylate and 4 to 6% by mole of vinyl acetate is employed in the present invention as a material of the insulation 12 of the insulated conductor 1 and as a material of the tape 3 to be wound round the cable sheath 2. The ethylene-glycidyl methacrylate-vinyl acetate terpolymer has an excellent adhesiveness to not only a resin filler such as an epoxy resin, but also various other substances, e.g. metals such as copper and aluminum, as compared with the before-mentioned ethylene copolymer, e.g. ethylene-methacrylic acid copolymer, or the metal salt thereof as used in a conventional cable as a material of the insulation.

Therefore, the terpolymer can provide an excellent gas-tightness, and gas leakage can be completely prevented.

In the following Table 1, there are shown the results of a peeling test conducted to compare the adhesiveness to an epoxy resin with respect to the ethylene-glycidyl methacrylate-vinyl acetate copolymer (commercial name "Bondfast" made by Sumitomo Chemical Co., Ltd.) used in the cable of the present invention and a metal salt of an ethylene-methacrylic acid copolymer (commercial name "SURLYN A" made by E. I. du Pont de Nemours and Company). The peeling test was made by casting an epoxy resin onto a sheet of the polymer to be tested having a thickness of about 1 mm., hardening and heating the epoxy resin under pressure, and peeling it from the sheet at a tensile speed of 20 mm./min.

TABLE 1 Polymer Peeling strength (kg./cm.) Ethylene-glycidyl methacrylate- 1 4 to 1 7 vinyl acetate terpolymer Metal salt ofethylene- 5.0 to 8.0 methacrylic acid copolymer As is clear from Table 1, the terpolymer used in the present invention has an adhesive strength of about 2 to about 3 times that of the comparative metal salt of ethylene-methacrylic acid copolymer.

The ethylene-glycidyl methacrylate-vinyl acetate terpolymer improves the adhesiveness of a polyolefin insulation to a conductor and a resin filler. In the following Table 2, there are shown the results of a conductor pulling test which was made to examine the adhesiveness of an insulation to a conductor by employing insulated conductors in which a conductor of 0.5 mm. in diameter is covered with the insulation of 0.1 5 mm. in thickness having the composition shown in Table 2. The conductor pulling test was made according to JIS C 3403-1 956 by stripping off the insulation of the insulated conductor having a strength of about 200 mm. at one end thereof in a length of about 50 mm. to expose the conductor, pulling out the conductor at a rate of about 200 mm./min. through a die having a larger diameter somewhat than the diameter of the conductor, and measuring the value at the time when the insulation begins to move along the conductor and the resistance reaches the maximum.

TABLE 2 Component of insulating material Run Ethylene-glycidyl Conductor No. Polyethylene methacrylate-vinyl Ethylene-vinyl pulling-out acetate terpolymer acetate copolymer resistance part by weight kg./cm.

1 5 100 5 1.85 to 2.15 2 100 10 1.80to2.45 3 100 5 0.56 to 0.98 4 100 10 0.61to1.11 5 100 0.51 to 0.71 As is clear from'Table 2, the insulated conductor of Run Nos. 1 and 2 as used in the cable of the present invention has a conductor pulling-out resistance of about 2 to about 3 times that of the conventional insulated conductor of Run Nos. 3 and 4, and is very excellent in adhesiveness of the insulation to the conductor.

In the present invention, the insulation 1 2 of the insulated conductor 1 is formed from a compound of a polyolefin and an ethylene-glycidyl methacrylate-vinyl acetate terpolymer. The terpolymer is employed in an amount of at least 3% by weight, preferably 3 to 60% by weight, based on the total amount of the insulating compound. The insulating compound containing at least 3% by weight of the terpolymer provides the insulation 12 having a sufficient adhesiveness to the filler 5 such as an epoxy resin and the conductor 11 such as copper or aluminum. Therefore, it is not necessary to form the insulation into a two layer structure, and also satisfactory electric characteristics are obtained.

The larger the content of the terpolymer in the insulation, the stronger the adhesiveness of the insulation to the filler and the conductor and, therefore, the better the gas-tightness. When the content of the terpolymer is more than 60% by weight, the electric characteristics and processability are lowered. Polyolefins usually employed for the insulation of the conductor are employed in the present invention, e.g. polyethylene, ethylene-propylene copolymers and polypropylene.

The insulation 12 may be a cross-linked polyolefin. The cross-linking is made in a usual manner, for instance, by means of a peroxide which is incorporated in the compound of a polyolefin and the terpolymer. In case of filling and molding a resin by a heat molding method in the formation of the pressure dam, there is the possibility of melting of the insulation by the heat upon heat molding. The cross-linking of a polyolefin is advantageous, since the melting point is raised.

In the present invention, in order to prevent the gas leakage between the sheath 2 and the' filler 5, a tape 3 made of the ethylene-glycidyl methacrylate-vinyl acetate terpolymer is wound round the sheath 2 and melt-adhered thereto by heating. Since the tape causes the sheath 2 and the filler 5 to closely adhere to each other through it, the leakage of a gas from the interface between them can be prevented. Moreover, the terpolymer tape used in the present invention has a larger adhesiveness than a laminated tape or an epoxy resin impregnated non-woven fabric as used in a conventional pressure dam cable, and is inexpensive, and the melt-adhesion procedure is simple. A plastic sheath such as a polyethylene sheath, a polyvinyl chloride sheath or a laminated aluminum polyethylene sheath is employed as cable sheath 2.

The pressure dam is formed in a usual manner. For instance, the terpolymer tape is wound round the sheath at two places at a suitable interval, e.g. about 20 cm., and is heat-molten to adhere to the sheath, and then the sheath between the tapes is stripped off, thus causing the insulated conductors 1 to expose. After providing a temporary dam 7 as occasion demands, a protective sleeve 4 is provided on the sheath 2 to cover the sheath-stripped portion and the tape 3. As a protective sleeve, a heatshrinkable tube is easy to use and to form the protective sleeve, In case that the filling of a resin is conducted by a heat molding method, a heat-shrinkable tube made of an ethylene-propylene polymer which has a large shrinkability is preferably employed.In case that the filling of a resin is conducted by a casting method, the material of the heat-shrinkable tube is not particularly limited. The heat-shrinkable tube is fitted on the sheath 2 to cover the sheath-stripped portion. One end of the heat-shrinkable tube, which is on the temporary dam locating side in the case where the temporary dam has been provided or is on the either side in the case where the temporary dam has not been provided, is heat-shrinked to closely adhere to the sheath 2 at the outside 8 of the tape 3. A necessary amount of a resin as a filler 5 is then introduced from the another side of the heat-shrinkable tube.

The filling and forming of a resin may be carried out by either a casting method or a heat molding method. A known filling compound is employed for forming the filler 5. In case of the casting method, resins such as an epoxy resin, an urethane resin and a silicone resin are employed as a filling resin, and in case of the heat molding method, resins having a lower melting point than the insulation material, such as an ethylene-vinyl acetate copolymers are employed as a filling resin. In case of the former resin, the open end of the heat-shrinkable tube is brought into close contact with the sheath 2 at the outside 9 of the tape 3 by heat-shrinking it after hardening the resin.In case of the latter resin, the open end of the tube is heat-shrinked immediately after introducing the resin into the tube, and the entire dam portion is then heated to melt the resin and is cooled to solidify it.

The present invention is more specifically described and explained by means of the following Examples, in which all % and parts are by weight unless otherwise noted. It is to be understood that the present invention is not limited to the Examples and various changes and modifications may be made in the invention without departing from the spirit and scope thereof.

EXAMPLES 1 to 5 AND COMPARATIVE EXAMPLES 1 to 3 In the following manner, a pressure dam was provided on each of cables which was manufactured by assembling to form a cable core, 200 pairs of pair twinning of insulated conductors consisting of an annealed copper wire of 0.5 mm. in diameter and a polyethylene insulation of 0.15 mm. in thickness containing or not containing an ethylene-glycidyl methacrylate-vinyl acetate terpolymer (85 :10 : 5 by mole) (commercial name "Boundfast" made by Sumitomo Chemical Co., Ltd.) and providing a plastic winding tape, a laminated tape and a polyethylene sheath on the cable core in that order.

A tape made of the Bondfast (hereinafter referred to as "terpolymer tape") was wound round the sheath in the vicinity of the center of the cable at an interval of about 20 cm., and was melt-adhered to the sheath. The sheath located between the tapes wound was stripped off to expose the insulated conductors. The sheath-stripped portion was covered with a polyvinyl chloride (PVC) heat-shrinkable tube or an ethylene-propylene rubber (EPR) heat-shrinkable tube as a protective sleeve, and one end of the tube was heat-shrinked to bring into close contact with the sheath at the position outside the tape adhering portion.From the another'open end of the tube, there was introduced a filler A which was a filling compound containing an epoxy resin (commercial name "No. 2-A resin" made by Terada Kogyo Kabushiki Kaisha) and 35 parts of a hardener (commercial name "No. 2-A hardener" made by Terada Kogyo Kabushiki Kaisha) per 100 parts of the epoxy resin, or a filler B which was a filling compound containing a powder of a copolymer of about 97% of ethylene and about 3% of vinyl acetate and 2 parts of dicumyl peroxide per 100 parts of the powder. In case of the filler A, the resin filled in the tube was hardened and the open end of the tube was then heated to shrink and bring into close contact with the sheath at the position outside the tape adhering portion.In case of the filler B, the open end of the tube was heat-shrinked in the same manner as above immediately after filling the resin in the tube. The entire dam portion was then heated to melt the filler B, and then cooled to solidify.

The thus prepared pressure dam cables were subjected to the following heat cycle tests to determine the gas-tightness of the cables as a whole.

Heat cycle test A Air was charged in a cable with pressure dam from one end of the cable so that the gas pressure in the cable was 1.0 kg./cm.2 at 200 C. The gas-filled cable was heated and cooled. The heating from 0 to 400 C. and the cooling from 400 to OOC. were made one cycle. The specimen showing no leakage of air in 10 cycles was regarded as good.

Heat cycle test B Air was charged in the same manner as in the test A. Heating and cooling between -200C. and 600 C. were made one cycle, and the specimen showing no leakage of air in 10 cycles was regarded as good.

Heat cycle test C Air was charged in a cable with pressure dam so that the gas pressure in the cable was 1.0 kg./cm.2 at 300 C. Heating and cooling between 300 C. and 700 C. were made one cycle, and the specimen showing no leakage of air in 100 cycles was regarded as good.

In the above heat cycle tests A, B and C, three specimens were tested in each test, and also the specimen was retained for at least 30 minutes at both minimum and maximum temperatures in each cycle.

The results of the heat cycle test are shown in Table 3, in which the estimation is made according to the following criteria.

All specimens are good.

A part of specimens is bad.

Ail specimens are bad.

TABLE 3

Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Com. Com. Com.

Ex. 1 Ex. 2 Ex. 3 Content of terpolymer in insulation (%) 3 10 30 30 50 0 10 30 Filler A A A B A A A A Presence of terpolymer tape yes yes yes yes yes yes no no Protective sleeve PVC PVC EPR EPR EPR EPR EPR EPR Heat cycle test Test A O 0 0 0 0 x x x Test B x A O O 0 0 - Test C x x x 0 O As shown in Table 3, all of the pressure dam cables according to the present invention in which the insulation contains at least 3% of an ethylene-glycidyl methacrylate-vinyl acetate terpolymer and the terpolymer tape is wound and melt-adhered to the sheath in the vicinity of the sheath-stripped ends, stand at least the heat cycle test A. On the other hand, the pressure dam cables of Comparative Examples which do not satisfy either one of the above conditions, namely lack of the terpolymer in the insulation or lack of the terpolymer tape, do not stand the heat cycle test A for the reasons that the cable of Comparative Example 1 is bad in adhesiveness between the insulation and the filler and adhesiveness between the conductor and the insulation, and the cables of Comparative Examples 2 and 3 are bad in adhesiveness between the sheath and the filler.

Also, the larger the content of the terpolymer in the insulation, the stronger the adhesiveness of the insulation to the filler and the conductor, and accordingly the cable can stand the severer test.

Vv'ith respect to the filler, it is also observed in Table 3 that the ethylene-vinyl acetate copolymer shows a somewhat larger adhesiveness to the insulation than the epoxy resin, though the epoxy resin is simple in procedure.

As is clear from the above test results, the present invention has the advantages that it is not necessary to form the insulation into two layers or to use an expensive tape or elastic protective sleeve, the structure is simple, the production is easy and a pressure dam cable having an excellent gastightness can be inexpensively produced.

In addition to the elements and ingredients used in the Examples, other elements and ingredients can be used in the Examples as set forth in the specification to obtain substantially the same results.

Claims (4)

1. In a pressure dam cable comprising an insulated cable conductor, a plastic cable sheath, a protective sleeve which covers the portion and its vicinity where the sheath is stripped off, and a resin filled inside the protective sleeve, said pressure dam cable being improved in that the insulation of said insulated cable conductor is made of a compound containing a polyolefin and an ethylene-glycidyl methacrylate-vinyl acetate terpolymer or a cross-linked product of said compound, and a tape made of a compound containing an ethylene-glycidyl methacrylate-vinyl acetate terpolymer as a main component is wound and melt-adhered onto the sheath in the neighbourhood of the sheath-stripped ends in the pressure dam portions.

2. The pressure dam cable of Ciaim 1, wherein said compound for the insulation contains at least 3% by weight of an ethylene-glycidyl methacrylate-vinyl acetate terpolymen

3. The pressure dam cable of Claim 1, said polyolefin is a polyethylene.

4. A pressure dam cable disclosed herein substantially as described in any one of the examples.