US3590141A

US3590141A - Electric cable having improved resistance to moisture

Info

Publication number: US3590141A
Application number: US799674A
Authority: US
Inventors: Raymond C Mildner
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1969-02-17
Filing date: 1969-02-17
Publication date: 1971-06-29
Anticipated expiration: 1988-06-29
Also published as: DE2005881B2; GB1258221A; FR2031461A1; DE2005881A1; FR2031461B1; BE746016A; NL7001311A

Abstract

An electric cable having a cylindrical protective metal shield formed by overlapping the edges of a metal strip to form a longitudinal seam and an outer plastic jacket is rendered resistant to moisture by means of a sheath in the form of a layer of a hygroscopic material such as paper or a blend of a polymer and a siccative drying agent and a metal shield surrounding the layer of hygroscopic material and having the edges thereof in overlapping relationship to form a longitudinal seam bonded together by means of an adhesive polymer. The sheath is disposed within the cable between the protective metal shield and the outer plastic jacket such that the layer of hygroscopic material is in contact with the protective metal shield and the metal shield of the sheath is in contact with the outer plastic jacket.

Description

United States Patent [72] Inventor Raymond C. Mildner Midland, Mich. [21] Appl. No. 799,674 [22 Filed Feb. 17, 1969 [45] Patented June 29, 1971 [73] Assignee The Dow Chemical Company Midland, Mich.

[541 ELECTRIC CABLE HAVING IMPROVED RESISTANCE T0 MOISTURE 18 Claims, 3 Drawing Figs, [52] US. Cl 174/105, 174/28,174/36, 174/107, 174/113 [511' Int. Cl. H0lb7/28 [50] Field of Search 174/26- -29, 36, 34,102,105,107,1l3,1l0.4, 110.8, 113, 116, 121.1, 121 5 6] References Cited UNITED STATES PATENTS 2,056,085 9/1936 Alles 174/105 2,186,793 1/1940 Wodtke 174/116 3,211,821 10/1965 Wakefield 174/26 3,272,91 1 9/1966 Rollins et a]. 174/106 3,315,025 4/1967 Tomlinson 174/107 3,328,514 6/1967 Cogelia 174/113 3,332,138 7/1967 Garner ..l14/SHIELD DIGEST 3,365,534 l/1968 Volk 174/29 FOREIGN PATENTS 734,163 7/1955 Great Britain 174/28 Primary Examiner- Lewis I-I. Myers Assistant Examiner-A. T, Grimley Attorneys-Griswold and Burdick, Richard G. Waterman,

Lester .I. Dankert and Ralph M. Mellom ing the edges thereof in overlapping relationship to form a lon-.

gitudinal seam bonded together by means of an adhesive polymer. The sheath is disposed within the cable between the protective metal shield and the outer plastic jacket such that the layer of hygroscopic material is in contact with the protective metal shield and the metal shield of the sheath is in contact with the outer plastic jacket.

Re /u/'n conduc /o/ ELECTRIC CABLE HAVING IMPROVED RESISTANCE T MOISTURE This invention relates to electric cables. In one aspect, this invention relates to electric cables having improved resistance to moisture. In another aspect, this invention relates to coaxial and twisted pair cables wherein the insulating medium between the conductors is protected against the penetration of moisture into the cables.

The efficiency and indeed the ability of a cable to effectively transmit signals within the audio and video frequency ranges and to transmit electric current in power distribution systems is dependent to a large extent upon the condition of the cable. While many factors affect the condition of a cable when it is in service, the humidity or amount of moisture within the cable is probably the single most important factor governing the ability of a cable to effectively transmit electric signals. Since many cables are used in areas of high humidity such as when they are buried in the earth near or below the water table or when they are used above ground and thus subjected to rain and snow, it is necessary to construct cables which are impermeable to water and other forms of moisture.

The advent of outer plastic jackets on cables has contributed substantially to preventing water from permeating into the cable. Although the outer plastic jacket appears impervious to moisture, such is not the case as evidenced by the presence of moisture within the cable determined by actual measurement.

More recent developments in cable construction employ a cylindrical protective metal shield interiorally of the plastic jacket to assist in preventing moisture from permeating into the working parts of the cable. The cylindrical metal shield is formed from a strip of metal folded around the cable core such that the edges overlap to form a' longitudinal seam which is bonded together with an adhesive polymer. Although the protective metal shield does reduce moisture permeation into the cable, and is probably sufficient in some types of cables such as those designed to transmit signals at low (audio) frequencies the adhesive polymer used to bond the longitudinal seam is not impervious and will permit minute quantities of moisture to pass into the cable where it will collect in the insulation between the conductors. When the insulation is of a type sensitive to moisture, such as a foamed plastic material, the presence of these minute quantities of moisture can produce disastrous results. And, while the moisture may be in vapor form initially, a decrease in ambient temperature will cause the vapor to condense in the insulation and produce droplets of water which can completely destroy the signal being transmitted through the conductors. While hermetic sheaths such as those fashioned from lead or aluminum can be employed to prevent moisture permeation in place of the pro tective metal shield having the longitudinal seam, this type of protection has many disadvantages in that it is much more expensive, the resulting cable is much heavier, and the cable loses its flexibility.

While it is of course desirable to maintain the interior of all types of cables free of moisture, it is particularly important to keep the interior of air spaced or plastic foam insulated coaxial and twisted pair cables dry since even minute quantities of moisture within the electric field between the conductors in these types of cables reduces substantially the efiiciency of the signals being transmitted and frequently the moisture will shunt the signal across the conductors thereby causing complete failure. Moreover, if the relative humidity of the air within the cable increases by the permeation of even minute quantities of moisture, a small drop in ambient temperature can cause condensation of the moisture within the cable thus producing disastrous results on the electrical performance of the cable.

According to this invention, the problems incident to the unavoidable penetration of moisture through the outer plastic jacket and protective metal shield of a cable are overcome by means of a sheath comprising, in combination, a layer of hygroscopic material disposed around the protective metal shield interiorly of the outer jacket to serve as a sink for absorbing moisture and a metal shield surrounding the layer of hygroscopic material and disposed between the hygroscopic material and the outer jacket. The cable construction of this invention thus has the protective metal shield located as an inner shield relative to the sheath and the outer plastic jacket. Similarily, the metal shield of the sheath is located as an outer shield relative to the inner protective metal shield. The hygroscopic layer and the metal shield surrounding the hygroscopic layer function in combination to prevent moisture from migrating into the working parts of the cable.

The metal shield between the hygroscopic layer and the outer jacket is formed by longitudinally folding a metal strip such that the edges thereof are in overlapping relationship to form a longitudinal seam which is bonded together by means of an adhesive polymer. An important requirement of the sheath of this invention is that the metal shield element of the sheath be reasonably resistant to the permeation of moisture to obviate the necessity of having to use a very thick hygroscopic layer which would otherwise be necessary to accommodate large quantities of moisture. The sheath of this invention is thus distinguishable from the concept of employing concentric plastic layers with a layer of moisture absorbent material between them. In the latter construction, the layer of absorbent material would have to be of considerable thickness in order to accommodate the large quantities of moisture which would pass through the outer plastic layer of the concentric plastic layers.

Accordingly, it is the object of this invention to provide an electric cable of improved resistance to moisture.

Another object of this invention is to provide a layer of hygroscopic material in a cable 'to serve as a sink for absorbing moisture.

A further object of this invention is to provide a cable which performs satisfactorily during exposure to moisture.

These and other objects of the invention will become apparent to one skilled in the art after studying the following detailed description, the appended claims, and the accompanying drawings wherein:

FIG. I is a partial section in isometric view of a cable constructed according to one embodiment ofthe invention;

FIG. 2 is a cross section of a cable constructed according to another embodiment of the invention; and

FIG. 3 is a cross section of a cable constructed according to yet another embodiment of the invention.

While the cables illustrated in the drawings are intended to show specific cable constructions embodying the novel concept of the invention, the drawings are for illustration purposes only and many other cable constructions within the spirit and scope of the invention are possible. Similarily, the cables illustrated in the drawings are not necessarily drawn to scale nor do they necessarily illustrate the relative size of the several elements.

A cable of the kind with this invention is concerned is one which comprises at least one conductor for transmitting electric signals, insulating means surrounding the conductor, a protective metal shield surrounding the insulating means, and an outer jacket of plastic material. The objects of this invention are realized in the cable described above by the improvement which comprises a sheath comprising, the combination of, a layer of hygroscopic material disposed around the protective metal shield interiorly of the outer jacket to serve as a sink for absorbing moisture and a metal shield surrounding the layer of hygroscopic material and disposed between the layer of hygroscopic material and the outer plastic jacket. The metal shield surrounding the layer of hygroscopic material is fashioned such that the edges thereof lie in overlapping relationship to form a longitudinal seam which is bonded together by means of an adhesive polymer. In a modification of the cable constructions described above, a layer of plastic material such as a polyolefin or the like is disposed between the protective metal shield surrounding the insulating means and the layer of hygroscopic material.

Any hygroscopic material suitable to serve as a sink for absorbing moisture which migrates'through the outer plastic jacket and which migrates to a much less degree through the longitudinal seam of the metal shield can be used in the practice of the invention. Exemplary hygroscopic materials which can be used include paper, cloth, and blends of polymer material and compatible siccative drying agents such as calcium chloride, sodium sulfate, sodium chloride, and the like. Paper is generally preferred as the hygroscopic material because it is generally inert to moisture, relatively inexpensive, and freely available. Further, the paper is quite porous and thus can accommodate the moisture by swelling without imposing dangerous stresses on the cable. Any suitable polymer having a comparatively low modulus which is capable of accommodatinglarge amounts of the siccative drying agent without becoming brittle can be used in the practice of the invention. Exemplary polymer materials which can be blended with the siccative drying agent to form the hygroscopic material includes chlorinated polyethylene, copolymers of ethylene and isobutyl acrylate, and the like. Since a corrosive environment is likely to occur when the siccative drying agent absorbs moisture, a suitable corrosion inhibitor can be admixed with the polymer material and the siccative drying agent if desired.

While the amount of siccative drying agent blended with the Polymer material is largely a matter of personal choice dictated by such factors as economics and the like, it is generally preferred that the drying agent by employed in an amount between about and about 80 weight percent based upon the total weight of the blend.

The metal shield surrounding the layer of hygroscopic material which functions in combination with the hygroscopic material to improve the resistance to moisture of the cables constructed in accordance with this invention is fashioned of any suitable metal such as, for example, aluminum, copper, bronze, steel, composites of two or more of the foregoing metals, and the like. The metal shield can be of any suitable and convenient thickness such as, for example, between about 2 and about 20 or more mils.

It is generally preferred that the metal shield be adhesively bonded to the outer jacket by means of an adhesive disposed as a layer over substantially the entire area of contact between the metal shield and the outer jacket. In that embodiment of the invention wherein a layer of plastic material is disposed between the protective metal shield and the layer of hygroscopic material, it is generally preferred that the protective metal shield be bonded to the plastic layer by means of an adhesive disposed as a layer over substantially the entire area of contact between them. It is also evident that the optional layer of plastic material can contain the siccative drying agent and thus serve as the hygroscopic layer if desired.

Any suitable adhesive can be used to bond the several elements of the cables together. Exemplary adhesives include polymers of an olefin such as, for example, ethylene, propylene, and the like, and an ethylenically unsaturated carboxylic acid having between 3 and 8 carbon atoms per molecule such as, for example, acrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, isocrotonic acid, tiglic acid, angelic acid, senecioic acid, and the like.

Both random and graft copolymers of the olefin and the ethylenically unsaturated carboxylic acid can be used in the practice of the invention. These adhesive copolymers can be obtained commercially or they can be prepared by processes well known in the art. While the invention is not to be bound by any particular technique for preparing the adhesive copolymers, one exemplary technique for producing random copolymers, one exemplary technique for producing random copolymers involves subjecting a mixture of the olefin monomers and acid monomers to a high pressure such as between about 500 and about 1000 atmospheres and to an elevated temperature such as between about 100 and about 400 C. in the presence of a suitable free radical initiator such as ditertiary butyl peroxide. Reaction conditions can be varied to produce random copolymers having the desired molecular weight. Exemplary techniques for producing the graft copolymers which can beused in the practice of this invention are outlined in US. Pat. Nos. 3,177,269 and 3,270,090, the disclosures of which are both specifically incorporated herein by reference.

Referring now to the drawings, wherein like reference numerals are used to denote like elements whenever convenient the invention will be described in more detail in connection with several embodiments illustrative of the invention.

FIG. 1, a coaxial cable indicated generally by reference numeral 1 comprises a conductor 2 of copper or the like disposed substantially in the center of the cable and insulating means comprising a plurality of disc-shaped spacers 3 transverse to the conductor 2 and longitudinally spaced thereon. A protective metal shield 4 fashioned of copper, aluminum, or the like and having a longitudinal seam 5 formed by overlapping the edges of a metal strip surrounds the insulating means. In the coaxial cable illustrate in FIG. 1, protective metal shield 4 serves as a return conductor. A plastic layer 7 of a polyolefin such as polyethylene, polypropylene, or the like surrounds the protective metal shield 4 and is optically bonded thereto by means of an adhesive layer 8 applied to the outer surface of the protective metal shield 4. The plastic layer 7 is an optional feature of the cable construction illustrated in FIG. 1 and the coaxial cable 1 can be fabricated without it if desired.

A layer 9 of hygroscopic material fashioned of, for example, paper, textile cloth, blend of polymer material and siccative drying agent, or the like is disposed around the plastic layer 7 and can optionally be bonded thereto by means of an adhesive layer 11 coated on the outer surface of the plastic layer 7. A metal shield 12 surrounds the layer 9 of hygroscopic material and is formed such that the edges thereof lie in overlapping relationship to form a longitudinal seam 13 which is bonded together by means of an adhesive polymer disposed as a bead or the like along the edges of the metal shield. The metal shield 12 is fashioned of any suitable metal such as, for example, aluminum, copper, bronze, steel, composites of two or more of the foregoing metals, or the like.

An outer plastic jacket 14 of a polyolefin material such as polyethylene, polypropylene, or the like surrounds the metal shield 12 and is preferably bonded thereto by means of an adhesive polymer layer 16 applied to the outer surface of the metal shield.

The cable illustrated in FIG. 1 can be fabricated by suitable techniques well known in the art. The disc-shaped spacers 3 are provided with radial slits (not shown) which allow then to be mounted on the conductor 2 as the conductor passes into a cable fabrication apparatus. The protective metal shield 4 surrounding the spacers 3 is formed by longitudinally folding a metal strip and overlapping the edges thereof to provide the longitudinal seam 5 which is bonded together by an adhesive. The optional plastic layer 7 is then extruded over the metal shield 4 by passing the same through a conventional extrusion apparatus. The layer 9 of hygroscopic material is positioned around the plastic layer 7 by any suitable technique depending upon the nature of the hygroscopic material. When paper or a textile cloth is employed as the hygroscopic material, it is hellcally wrapped or longitudinally folded over the plastic layer 7. When the hygroscopic material comprises a polymer blend and a siccative drying agent it is disposed around the plastic layer 7 by extrusion or the like. The metal shield 12 is disposed around the layer 9 of hygroscopic material by longitudinally folding a metal strip to provide the longitudinal seam 13. The outer plastic jacket 14 is then extruded upon the metal shield 12 by suitable extrusion apparatus.

When the protective metal shield 4 and the metal shield 12 are provided with adhesive coatings to form a strong bond with their respective adjacent elements in the cable, such adhesive coatings can be applied by extruding the adhesive onto the metal surfaces, depositing the adhesive from solution or a latex, film lamination, or the like.

While the disc-shaped spacers 3 are illustrated as the insulation means in FIG. 1, it is evident that other forms of insulation can be used such as a helical strip of insulating material or a foamed polyolefin such as foamed polyethylene, foamed polypropylene, or the like. When a foamed polyolefin is employed as the insulation means, it is preferable applied by passing the conductor 2 through an extruder which extrudes the polyolefin containing a blowing agent under conditions of temperature and pressure to allow the blowing agent toexpand the polyolefin upon leaving the extruder and thereby form a cellular layer.

The protective metal shield 4 in the cable structure illustrated in FIG. l is generally fashioned of a material such as copper or the like so that it serves as a conductor for transmitting electric signals.

FIG. 2 of the drawings illustrates a cable shown generally by reference numeral 21 comprising two coaxial cables shown generally by

reference numerals

22 and 23 which are arranged in a generally parallel relationship. Each of the

coaxial cables

22 and 23 comprises a first conductor 24 for transmitting electric signals, insulating means 26 surrounding the first conductor 24, a second or return conductor 27 in a form ofa cylinder positioned around the insulating means 26 with the edges thereof in overlapping relationship to form a longitudinal seam 28 which is preferably bonded together by means of an adhesive, and an outer jacket 29 of plastic material surrounding the conductor 27. Conductor 24 is

coaxial cables

22 and 23 is preferably fashioned of copper or the like. The insulation means 26 comprises a foamed polyolefin such as foamed polyethylene or the like. The insulating means can optionally comprise the disc-shaped spacers 3 described in connection with FIG. 1 of the drawings. The return conductor 27 in each of the

coaxial cables

22 and 23 is preferably fashioned of a highly conductive material such as copper or the like. The outer plastic jacket 29 in each of the coaxial cables is a suitable polyolefin such as, for example, polyethylene.

An optional layer 31 of plastic material which can be either unfoamed or foamed polyethylene or the like is disposed between the

coaxial cables

22 and 23 and a layer 9 of hygroscopic material. A metal shield 12 surrounding the layer of hygroscopic material has the edges thereof in overlapping relationship to form a longitudinal seam 13 which is bonded together by means of an adhesive polymer. An outer jacket M of a suitable plastic material such as a polyolefin or the like surrounds the metal shield 12.

As described in connection with FIG. 1 of the drawings, the several elements of the cable illustrated in FIG. 2 can be bonded together by means of an adhesive copolymer of an olefin and an ethylenically unsaturated carboxylic acid. Thus, the return conductor 27 is optionally bonded to the insulation means 26 and the outer jacket 29. Similarly, the metal shield 12 is optionally adhesively bonded to the outer jacket 14 by means of the adhesive polymer.

While the cable 21 of FIG. 2 illustrates two

coaxial cables

22 and 23 disposed within the hygroscopic layer 9 and the metal shield 12, it is within the spirit and scope of the invention to employ a plurality of coaxial cables arranged in a generally parallel relationship. The layer 9 of the hygroscopic material, the metal shield 12, and the remaining elements of cable 21 are fabricated of the same materials described in connection with the cable illustrated in FIG. l. Cable 21 is fabricated by employing well known cable forming apparatus including cable shield forming means, cable jacket extruders, and the like.

The cable design shown by FIG. 3 of the drawing is illustrative of a twisted pair cable employing the inventive concept of the invention. A twisted pair cable shown generally by reference numeral 32 in FIG. 3 comprises a pair of conductor 33 individually insulated with a layer 34 of foamed polyolefin such as polyethylene or the like which are twisted together to form a twisted pair having opposed helical valleys defined by the outer surfaces of the layers 34. A layer 36 of a foamed polyolefin such as polyethylene surrounds the twisted pair and is disposed substantially throughout the helical valleys defined by layers 34 to provide a cylindrical cable core of foamed insulation. A. protective metal shield 37 surrounds the layer 36 and has the edges thereof in overlapping relationship to form a longitudinal seam 33. The edges which form seam 38 are preferably bonded together by means of an adhesive polymer. A layer 39 of solid plastic material such as a polyolefin is disposed between the protective metal shield 37 and a layer 9 of hygroscopic material. The plastic layer 39 is optional and can be omitted from the cable construction illustrated in FIG. 3 is desired. When the layer 39 of plastic material is not used, the layer 9 of hygroscopic material is in direct contact with the protective metal shield 37.

A metal shield 12 surrounds the layer 9 of hygroscopic material and is generally coextensive therewith. The metal shield 12 is formed with the edges thereofin overlapping relationship to form a longitudinal seam 13 which is bonded together by means of the adhesive copolymer. The metal shield 12 is fashioned ofa suitable metal such as, for example, aluminum, copper, bronze, steel, composites of two or more of the foregoing metals, or the like. An outer jacket 14 of plastic material surrounds the metal shield 12 and is fashioned of a suitable polymer material such as polyethylene, poly (vinylchloride), or the like.

The layer 9 of hygroscopic material is fashioned from any suitable material such as, for example, those materials described in connection with FIGS. 1 and 2 of the drawings. To add mechanical strength to the cable of FIG 3, it is generally preferred to employ an adhesive comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid disposed as a layer over substantially the entire area of contact between layer 36 of foamed polyolefin and protective metal shield 37 and over substantially the entire area of contact between metal shield 12 and outer jacket 14. While the cable design illustrated by FIG. 3 is a twisted pair cable, it is within the spirit and scope of the invention to employ any number of twisted pairs or twisted quads.

In cables employing the novel concept of this invention, it is generally preferred that some free space be provided between the protective metal shield and the metal shield surrounding the layer of hygroscopic material to allow the hygroscopic material to expand as an incident to the absorption of water. As previously indicated, paper in and of itself normally contains enough free space to accommodate this expansion. When the hygroscopic layer is fashioned of a blend of a polymer material and a siccative drying agent, the polymer material can be partially or completely foamed if desired. However, longitudinal passages in the vicinity of the hygroscopic layer should be avoided to prevent longitudinal movement ofthe moisture.

Although the invention has been described primarily in connection with coaxial cables, this has been done because problems associated with moisture penetration are more severe in these type of cables. It is evident that the invention is applicable to all types of cables and particularly those where moisture is a problem.

While, as previously indicated, the metal shield of the sheath can be constructed of any suitable metal, it is generally preferred to employ tin plated steel because of the better electromagnetic shielding which this material provides at low frequencies and because it is mechanically stronger than the softer metals such as copper or aluminum, so that it affords better mechanical protection to the cable.

The following Example illustrates the improved results obtained with the cable sheath of this invention. It must be understood that this Example is for illustration purposes only and should not be construed as limiting of the invention.

EXAMPLE An air-spaced coaxial cable having an insulated center conductor, a return conductor, and a layer ofpolyethylene in the form of an inner jacket surrounding the return conductor is provided with a sheath comprising a layer of hygroscopic paper 20 mils thick and a metal shield surrounding the hygroscopic paper layer. The metal shield is formed by longitudinally folding an 8-mil thick strip of aluminum around the layer of hygroscopic paper such that the edges of the strip overlap to form a longitudinal seam. The strip of aluminum which forms the metal shield has a 2-mil thick coating of an adhesive copolymer of ethylene and acrylic acid on the surface of the strip which becomes the exterior surface of the metal shield. The adhesive copolymer bonds the edges of the metal shield along the longitudinal seam and also bonds the metal shield to an outer jacket of polyethylene which is then extruded over the shield. The several elements of the cable are set forth in Table 1 below.

In the following calculations, which illustrate the resistance to moisture of a cable having a sheath constructed in accordance with this invention, the permeability (P) of low density polyethylene is taken as l l cc. at standard temperature and pressure per centimeter per second per cm. of Hg. at C. The calculations also employ an upper limit 50 percent relative humidity in the air space between the conductors. This upper limit may be accepted as the maximum tolerable relative humidity to avoid condensation within the air space when the temperature of the cable decreases.

The flow of moisture, F, through a layer of polyethylene in a cable, such as through the inner jacket or the outer jacket, is governed by the expression where: F cc of water/day/cm. of cable length P permeability of the polyethylene A p pressure differential across the polyethylene layer D Outer diameter of the polyethylene layer D inner diameter of the polyethylene layer in the several runs, the cable is immersed in water to provide an ambient relative humidity of 100 percent.

Control No. l

The flow of moisture (F) into a coaxial cable comprising only Elements 1, 2 and 3 of Table 1, except that the longitudinal seam of the return conductor is not bonded nor is the return conductor bonded to the inner jacket (i.e., the return conductor does not have an adhesive copolymer coating), is determined by the formula above to be 2.4 cc. per day per cm. of cable length. This corresponds to about 40 micrograms per day per'cm. of cable length. The volume of air between the conductors is about 2 cubic centimeters per centimeter length of cable. Since the density of saturated water vapor is 17.3 micrograms /cc. at 20 C., the air space between the conductors would have a relative humidity of percent in less than one day for a water vapor flow of 40 micrograms per day. Control No. 2

Experience has shown that by adhesively bonding the longitudinal seam in the return conductor and by bonding the return conductor to the jacket ofa coaxial cable, the moisture penetration through the jacket and the seam in the return conductor is improved by a factor of about 100. Thus, the flow of moisture (F) into a coaxial cable comprising only elements 1, 2, and 3 of Table l and having the longitudinal seam bonded with the adhesive copolymer and the return conductor bonded to the inner jacket is reduced to about 0.4 micrograms per day per cm. of cable length. The relative humidity RH within this cable will increase about 1.16 percent per day as determined by the expression The time, T, required to reach a relative humidity of 50 percent is about 60 days as determined by the expression.

100 100 T I175 It is evident that a cable of the type described having only 60 days of satisfactory operation is unsuitable. Control No. 3

A coaxial cable is made from elements 1, 2, 3, 5, and 6 only of Table 1 except no adhesive copolymer is employed in the construction (i.e., the same cable construction as outlined in control No. 1 except in this run the cable is provided also with an unbonded rnetal shield and an outer jacket). In this construction, the additional metal shield and outer jacket would serve to reduce moisture penetration by one-half. The air space between the conductors in this construction would achieve about 100 percent relative humidity in about 2 days. Control No. 4

A coaxial cable is made from the elements in control 3 except the longitudinal seams in the return conductor and in the metal shield are bonded with adhesive copolymer. The return conductor and metal shield are also adhesively bonded to their respective polyethylene elements. In this construction, the additional metal shield and outer jacket serves to reduce the penetration of moisture over the cable construction of Control No. 3 by one-half. Thus, the air in the air space of this cable construction will reach a relative humidity of 50 percent in about days.

=60 days.

CABLE CONSTRUCTION OF TABLE I A cable constructed according to the invention and employing all of the elements identified in Table l utilizes a hygroscopic paper weighing about 0.3 grams per cm. of cable length. The paper is of a type which will absorb water vapor equal to about 7.5 weight percent of its own weight when in equilibrium with air having a relative humidity of 50 percent or about 15 weight percent when in equilibrium with air havi-ng a relative humidity of 100 percent. Since a cable construction having a bonded metal shield and a bonded jacket will allow about 0.4 micrograms of water vapor to enter the hygroscopic paper per day, when the cable is submerged in water the relative humidity, RH of the hygroscopic paper will increase about 9 l0 percent per day as determined by the expression cent in the hygroscopic layer is about 7.7 l0" days (about 2 10 years) as detennined by the expression X 100 =about 9 X 10* (approx.

100 log =7.7 10 days 2(0 years) f Control If the longitudinal seam of the metal shield and the return conductor were left unbonded and the metal shield and return conductor were not bonded to their respective plastic ele ments, the time required to reach a relative humidity of 50 percent in the Cable Construction of Table I set forth above would be reduced by a factor of 100 (as related in Control No. 2) thus giving an operational life of only 770 days or just slightly more than 2 years. it is thus evident that the improvement of this invention is attributed to the combination of an adhesively bonded metal shield and the layer of hygroscopic material.

While the invention has been described in considerably detail, it must be understood that such description is for that purpose only and should not be construed as limiting of the invention.

What I claim is:

1. In a cable comprising at least one conductor for transmitting electric signals, insulating means surrounding said conductor, a protective metal shield surrounding said insulating means and characterized by having a longitudinal seam formed by overlapping the edges of a metal strip used to form said protective metal shield, and an outer jacket of plastic material, the improvement comprising a sheath comprising, in combination:

A. a layer of hygroscopic material disposed around said protective metal shield interiorally of said outer jacket to serve as a sink for absorbing moisture, and

B. a metal shield surrounding said layer of hygroscopic material and disposed between said layer of hygroscopic material and said outer jacket, said metal shield having the edges thereof in overlapping relationship to form a longitudinal seam bonded together by means of an adhesive polymer.

2. A cable according to claim 1 wherein said layer of hygroscopic material is fashioned of a material selected from the group consisting of paper, cloth, blends of polymer material and calcium chloride, blends of polymer material and sodium sulfate and blends of polymer material and sodium chloride.

3. A cable according to claim 1 wherein said metal shield surrounding said layer of hygroscopic material is fashioned of a metal selected from the group consisting of aluminum, copper, bronze, steel, and composites of two or more of the foregoing metals.

4. A cable according to claim 1 wherein said metal shield disposed between said layer of hygroscopic material and said outer jacket is adhesively bonded to said outer jacket by means of an adhesive comprising a polymer of an olefin and an ethylenically unsaturated carboxylic acid.

5. A cable according to claim 1 wherein said conductor is a single conductor of copper disposed substantially in the center of said cable, said insulating means comprises a plurality of disc-shaped spacers transversely positioned on said conductor and longitudinally spaced thereon, and said protective metal shield comprises a conductor fashioned of a material selected from the group consisting of copper and aluminum.

6. A cable according to claim 1 which includes a layer of plastic material disposed between said protective metal shield and said layer of hygroscopic material.

7. A cable according to claim 6 wherein an adhesive comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid is disposed as a layer over substantially the entire areas of contact a. between said protective metal shield and said layer of plastic material which is disposed between said protective metal shield and said layer of hygroscopic material, and

b. also between said metal shield, which is disposed between said layer of hygroscopic material and said outer jacket, and said outerjacket.

8. A cable according to claim 1 wherein said conductor is a single conductor of copper disposed substantially in the center of said cable, said insulating means comprises a foamed polyolefin, and said protective metal shield comprises a con ductor fashioned of a material selected from the group consisting of copper and aluminum.

jacket of plastic material,

9. A cable according to claim 8 wherein an adhesive comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid is disposed as a layer over substantially the entire area of contact between said metal shield, which is disposed between said layer of hygroscopic material and said outer jacket, and said outer jacket.

10. In a cable comprising a plurality of coaxial cables arranged in a generally parallel relationship, each of said coaxial cables comprising;

a. a first conductor for transmitting electric signals,

b. insulating means surrounding said first conductor,

0. a second conductor in the form of a cylinder positioned around said insulating means characterized by having a longitudinal seam formed by overlapping the edges of a metal strip used to form said second conductor, and

d. an outer jacket of plastic material surrounding said second conductor; and

an outer jacket of plastic material surrounding said plurality of coaxial cables, the improvement comprising a sheath comprising, in combination:

A. a layer of hygroscopic material surrounding said plurality of coaxial cable interiorally of said outer jacket and substantially coextensive with said plurality of coaxial cables to serve as a sink for absorbing moisture; and

B. a metal shield surrounding said layer of hygroscopic material and disposed between said layer of hygroscopic material and said outer jacket, said metal shield having the edges thereof in overlapping relationship to form a longitudinal seam bonded together by means of an adhesive.

11. A cable according to claim 10 which includes a layer of plastic material disposed between said plurality of coaxial cables and said layer of hygroscopic material.

12. A cable according to claim 10 wherein an adhesive comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid is disposed as a layer over substantially the entire areas of contact a. between said second conductor and said outer jacket in each of said coaxial cables, and

b. also between said metal shield and said outerjacket.

13, A cable according to claim 10 wherein a. said insulating means surrounding said first conductor in each of said coaxial cables comprises a foamed polyolefin,

b. said second conductor in each of said coaxial cables is fashioned ofa metal selected from the group consisting of aluminum and copper, and

c. said metal shield surrounding said layer of hygroscopic material is fashioned of a metal selected from the group consisting of aluminum, copper, bronze, steel, and composites of two or more of the foregoing metals.

14. A cable according to claim 10 wherein said layer of hygroscopic material is fashioned of a material selected from the group consisting of paper, cloth, blends of polymer material and calcium chloride, blends of polymer material and sodium sulfate, and blends of polymer material and sodium chloride.

15. ln a twisted pair cable comprising a pair of conductors individually insulated with a layer of foamed polyolefin and twisted together to form a twisted pair having opposed helical valleys defined by the outer surfaces of said layers of foamed polyolefin, a layer of foamed polyolefin surrounding said twisted pair and disposed substantially throughout the helical valleys therein, a protective metal shield surrounding said layer of foamed polyolefin and characterized by having a longitudinal seam formed by overlapping the edges of a metal strip used to form said protective metal shield, and an outer the improvement comprising a sheath comprising, in combination:

A. a layer of hygroscopic material disposed around said protective metal shield interiorally of said outer jacket and substantially coextensive with said protective metal shield to serve as a sink for absorbing moisture; and

16. A twisted pair cable according to claim 15 which includes a layer of plastic material disposed between said protective metal shield and said layer of hygroscopic material.

17. A twisted pair cable according to claim 15 wherein said layer of hygroscopic material is fashioned of a material selected from the group consisting of paper, cloth, blends of

Claims

1. In a cable comprising at least one conductor for transmitting electric signals, insulating means surrounding said conductor, a protective metal shield surrounding said insulating means and characterized by having a longitudinal seam formed by overlapping the edges of a metal strip used to form said protective metal shield, and an outer jacket of plastic material, the improvement comprising a sheath comprising, in combination: A. a layer of hygroscopic material disposed around said protective metal shield interiorally of said outer jacket to serve as a sink for absorbing moisture, and B. a metal shield surrounding said layer of hygroscopic material and disposed between said layer of hygroscopic material and said outer jacket, said metal shield having the edges thereof in overlapping relationship to form a longitudinal seam bonded together by means of an adhesive polymer.

7. A cable according to claim 6 wherein an adhesive comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid is disposed as a layer over substantially the entire areas of contact a. between said protective metal shield and said layer of plastic material which is disposed between said protective metal shield and said layer of hygroscopic material, and b. also between said metal shield, which is disposed between said layer of hygroscopic material and said outer jacket, and said outer jacket.

8. A cable according to claim 1 wherein said conductor is a single conductor of copper disposed substantially in the center of said cable, said insulating means comprises a foamed polyolefin, and said protective metal shield comprises a conductor fashioned of a material selected from the group consisting of copper and aluminum.

10. In a cable comprising a plurality of coaxial cables arranged in a generally parallel relationship, each of said coaxial cables comprising; a. a first conductor for transmitting electric signals, b. insulating means surrounding said first conductor, c. a second conductor in the form of a cylinder positioned around said insulating means characterized by having a longitudinal seam formed by overlapping the edges of a metal strip used to form said second conductor, and d. an outer jacket of plastic material surrounding said second conductor; and an outer jacket of plastic material surrounding said plurality of coaxial cables, the improvement comprising a sheath comprising, in combination: A. a layer of hygroscopic material surrounding said plurality of coaxial cable interiorally of said outer jacket and substantially coextensive with said plurality of coaxial cables to serve as a sink for absorbing moisture; and B. a metal shield surrounding said layer of hygroscopic material and disposed between said layer of hygroscopic material and said outer jacket, said metal shield having the edges thereof in overlapping relationship to form a longitudinal seam bonded together by means of an adhesive.

12. A cable according to claim 10 wherein an adhesive comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid is disposed as a layer over substantially the entire areas of contact a. between said second conductor and said outer jacket in each of said coaxial cables, and b. also between said metal shield and said outer jacket.

13. A cable according to claim 10 wherein a. said insulating means surrounding said first conductor in each of said coaxial cables comprises a foamed polyolefin, b. said second conductor in each of said coaxial cables is fashioned of a metal selected from the group consisting of aluminum and copper, and c. said metal shield surrounding said layer of hygroscopic material is fashioned of a metal selected from the group consisting of aluminum, copper, bronze, steel, and composites of two or more of the foregoing metals.

15. In a twisted pair cable comprising a pair of conductors individually insulated with a layer of foamed polyolefin and twisted together to form a twisted pair having opposed helical valleys defined by the outer surfaces of said layers of foamed polyolefin, a layer of foamed polyolefin surrounding said twisted pair and disposed substantially throughout the helical valleys therein, a protective metal shield surrounding said layer of foamed polyolefin and characterized by having a longitudinal seam formed by overlapping the edges of a metal strip used to form said protective metal shield, and an outer jacket of plastic material, the improvement comprising a sheath comprising, in combination: A. a layer of hygroscopic material disposed around said protective metal shield interiorally of said outer jacket and substantially coextensive with said protective metal shield to serve as a sink for absorbing moisture; and B. a metal shield surrounding said layer of hygroscopic material and disposed between said layer of hygroscopic material and said outer jacket, said metal shield having the edges thereof in overlapping relationship to form a longitudinal seam bonded together by means of an adhesive.

17. A twisted pair cable according to claim 15 wherein said layer of hygroscopic material is fashioned of a material selected from the group consisting of paper, cloth, blends of polymer material and calcium chloride, blends of polymer material and sodium sulfate, and blends of polymer material and sodium chloride.

18. A twisted pair cable according to claim 15 wherein an adhesive comprising a copolymer of an olefin and an ethylenically unsaturated carboxylic acid is disposed as a layer over substantially the entire areas of contact between a. said layer of foamed polyolefin surrounding said twisted pair and said protective metal shield, and b. also between said metal shield and said outer jacket.