Water resistant communications cable

Abstract: A communications cable comprising a core of at least one transmission media and a plastic jacket includes provisions for preventing the movement of water within the cable. A water blocking yarn (33) or strip (35) is interposed between the core and the jacket (32) and extends either linearly along the cable or is wrapped helically about a portion of the sheath system. In any plane along the cable which is transverse to the longitudinal axis of the cable, the yarn or the strip extends about an insubstantial portion of an inner periphery of the cable. As a result, any desired bond between the jacket and an underlying element of the cable sheath system is discontinuous for only an insignificant portion of the peripheral surface of contact. The yarn may be one which has been treated with a superabsorbent material whereas the strip may comprise a substrate strip which has been impregnated with a superabsorbent material which upon contact with water swells and inhibits the movement of water within the cable.

Description: Technical Field This invention relates to a water resistant communications cable. Background of the Invention In the cable industry, it is well known that changes in ambient conditions lead to differences in vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level inside the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. High moisture levels inside a cable sheath system may have a detrimental effect on the transmission characteristics of the cable. Furthermore, water may enter the cable because of damage to the cable which compromises its integrity. For example, lightning or mechanical impacts may cause openings in the sheath system of the cable to occur, allowing water to move toward a core of the cable, and, if not controlled, to move longitudinally into splice closures. There are some splice closures available commercially in which the cable jacket is terminated inside the closure. Hence, if water is able to travel longitudinally along the cable, it could enter the splice closure, possibly causing a degradation in transmission. Lately, optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not detrimental to its performance, passage of the water along the cable interior to connection points or terminals or associated equipment may cause problems and should be prevented. Also, in some climates, the development of ice within an optical fiber cable may have a crushing influence on the optical fibers in the core which may affect adversely the attenuation thereof. In the prior art, a metallic shield has been used to protect a cable against water ingress. Filling materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of a filling material causes housekeeping problems, inhibits manufacturing line speeds because of the need to fill carefully interstices of the core and presents problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core. In other arrangements, an atactic material may be used to flood the outer surface of a metallic shield. Presently, many commercially available cables also include a water-swellage tape. Such a tape generally is laminated, including a water swellable powder which is trapped between two cellulosic tissues. Although such a tape provides suitable water protection for the cable, it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware. Another factor that must be considered with respect to a water blocking system for a cable is the bonding of the plastic cable jacket to an underlying metallic shield. Where such adhesion is important to the performance of the cable, care must be taken not to interpose a water blocking member therebetween which would impair the desired adhesion. In the past, it has been commonplace to dispose a water blocking tape between a shield of the sheath system and the core or core tube, if any. Although the use of a water blocking tape is placed typically between a metallic shield and a core tube, such an arrangement does not prevent the longitudinal flow of water between other elements of the sheath system and into closures, for example. EP-A-0 174 424 discloses a water blocked optical fiber cable in which water absorbing fibrous braid is placed adjacent each optical fiber as well as in the cable sheath to absorb any water which may be present. EP-A-0 107 433 discloses a cable in which two covering layers surround the core, each layer being a sheathing or jacketing layer, and in which a water blocking layer lies between the covering layers. The water blocking layer comprises a wrapping of flexible and compressible powder pregnable material. DE-A-30 05 875 discloses a cable in which two strips of water blocking material lie between outer strength members (or an outer metallic shield) and the outer coating of the core. The strength members lie within a water absorbing layer. EP-A-0 013 477 discloses a system for water proofing telephone cable. Longitudinal strands with water sensitive swelling agent are adjacent insulating conductors, and plug the cable in the event of water intrusion. Seemingly, the prior art does not disclose a cable which is provided with a system which prevents substantially the flow of water longitudinally along the cable sheath system. What is needed and what does not appear to be available in the marketplace is a cable water blocking system which is relatively inexpensive and which does not compromise any desired bonding between members of the cable sheath system. Such a system should be one which is easily provided during the cable manufacturing process. GB-A-2,186,098 discloses a communications cable according to the preamble of claim 1. According to the present invention, there is provided a cable as defined in claim 1. The foregoing problems of the prior art have been overcome with the cable of this invention. That cable includes a core which may include optical fibers or metallic conductors and a plastic jacket. Interposed between the core and the jacket is an elongated water blocking member. The water blocking member has a relatively high tensile strength and is relatively thin. The water blocking member may be in the form of a strip or a yarn which covers only an insubstantial portion of an inner periphery of the jacket. In this way, the strip or the yarn separates only an insubstantial portion of the jacket from other portions of the sheath system. Hence, since adhesion between the jacket and the other portions of the sheath system is desired, that adhesion is not compromised by the water blocking member. Further. the water blocking member extends linearly or helically along the cable. In a cable in which strength members extend linearly within the cable, the strip or yarn may be wrapped helically about a core tube along an outer surface of which extends the strength members. In a cable in which the strength members extend helically about the cable core, the yarn or strip extends linearly or is wrapped in a helical direction opposite to that of the strength members and is disposed between the strength members and the core. Brief Description of the Drawing FIG. 1 is a perspective view of a communications cable having a sheath system which includes a water blocking member with various layers of the sheath system broken away and some of the layers exaggerated in thickness for purposes of clarity; FIG. 2 is an end sectional view of the cable of FIG. 1 which illustrates some elements of the cable in greater detail; FIG. 3 is a perspective view of the cable of FIG. 1 with a yarn therein being applied helically; FIG. 4 is a perspective view of another embodiment of the cable of FIG. 1 which includes a water blocking strip; FIGS. 5 and 6are perspective and end views of a cable which includes a water blocking strip; and FIGS. 7 and 8 are perspective and end views of another embodiment of the cable of FIGS. 5and 6which includes water blocking yarns. Detailed Description Referring now to FIGS. 1 and 2, there is shown a communications cable which is designated generally by the numeral 20. It has a longitudinal axis 21 and includes a core 22 comprising one or more optical fibers 24-24 and is filled with a material 25 such as that disclosed in U.S. patent 4,701,016. The core is enclosed by a sheath system 26 which includes a core tube 28 which encloses the optical fibers, a corrugated metallic shield system 29 and one or more strength members 30-30 and an outer jacket 32. The shield system is bonded to the jacket 32. Such a cable is disclosed and claimed in U.S. patent 4,765,712. Disposed between the shield and the core tube is a water blocking tape 31. The water blocking tape is a laminate comprising two relatively porous substrate tapes having a water swellable powder therebetween. In the cables of this invention, additional provisions are made for preventing the flow of water longitudinally along the cable into closures. Between the optical fibers 24-24 and the jacket 32 is disposed a water blocking member such as a yarn or strip which has been treated with a water blockable material such as a water swellable material. In the cable depicted in FIGS. 1 and 2, the strength members 30-30 extend helically about the metallic shield 29. The jacket 32 is bonded to the underlying shield with the strength members being embedded partially in the plastic of the jacket. In such a cable, when the strength members 30-30 engage the metallic shield 29, water may travel along the cable adjacent to the intersection of the strength members and the crests of the corrugated shield. Such flow is prevented substantially by causing a yarn or strip which covers only an insubstantial portion of the periphery of the shield to be disposed between the shield and the strength members. In a preferred embodiment, the water blocking member is a yarn 33 (see FIG. 1) such as a 5 denier yarn, for example, which has been treated with a water swellable material. Such a yarn is disclosed in U.S. patent 4,366,206 which is incorporated by reference hereinto. Yarn suitable for use in a cable 20 is manufactured by Japan Exlan Co., Ltd. of Osaka, Japan, under the trade designation "LANSEAL-F" super absorbent fiber and is available commercially from Chori America, Inc. Obviously, a single yarn extending longitudinally of the cable in linear or helical fashion detracts very little from any bond between the jacket and any underlying shield. The yarn 33 may extend substantially linearly along the cable parallel to its longitudinal axis so that each helically wound strength member crosses the yarn on each convolution. In the alternative, the yarn 33 may be wound helically in an opposite direction from that of the helically wound strength members (see FIG. 3). It is important that the water blocking member in any given plane transverse of the longitudinal axis 21 of the cable extend about only an insubstantial portion of an inner periphery of the cable in that plane. In this way, since it is desired to have an adhesive bond between the jacket and an underlying element of the sheath system such as a shield, for example, the yarn or tape disturbs only an insignificant portion of that bonding area. For the cable depicted in FIG. 4, a water blocking, longitudinally extending strip 35 is positioned between the metallic shield 29 and the strength members 30-30. Because the strength members are wrapped helically about the shield, the water blockable strip extends linearly along the shield prior to the application of the strength members. In the alternative, it may be wrapped helically about the core in a helical direction opposite to that of the strength members as in the yarn in FIG. 3. Care must be taken in this instance also not to isolate completely the jacket from the shield because of the bonding therebetween which is desired. Indeed in some applications, the outer surface of the shield 29 is provided with an adhesive material to bond to the jacket. The water protection of the cable must be accomplished without compromising the bonding of portions of the sheath system. Typically, for a cable having an outer diameter of about 1.27 cm, a strip having a width of about 0.95 cm is disposed between the shield and the jacket. Hence, a substantial portion of the periphery of the shield is still available for bonding to the jacket. The water blocking strip 35 may comprise a laminate in which a water swellable powder is disposed between two tapes or, as in a preferred embodiment, it may be a water blockable member such as that disclosed in the hereinbefore-identified C. J. Arroyo application filed of even date herewith. Should the strip 35 be made in accordance with the hereinbefore-identified C. J. Arroyo application filed of even date herewith, the tape comprises a substrate or carrier strip 37 made of a hydrophobic material and treated with a water blocking material. Advantageously, the treated tape is hydrophilic. A hydrophilic material is one that has a strong affinity for water in that it absorbs water easily. In the preferred strip embodiment, the substrate strip 37 is a spunbonded non-woven polyester material and includes a web structure comprised of randomly-arranged fibers which are bonded primarily at the filament crossovers. Continuity of the fibers of the web, while not necessary to the invention, will provide the web with an increased tensile strength. The fibers may be formed of any plastic resin, or other appropriate material, which is substantially nonhygroscopic, and which has the capability of maintaining its shape in the presence of the temperatures encountered during the extrusion of the jacket 32. The fibers of the web structure are arranged so that air cells or pockets are formed. A polyethylene terephthalate fiber product, formed into a web structure as described above has been identified under the registered trademark "Reemay" by the E. I. du Pont de Nemours and Company, Incorporated of Wilmington, Del. Presently, the Reemay® web structure is available in various thicknesses and densities from Reemay, Inc. of Old Hickory, Tennessee. The properties of Reemay® tapes are further defined and described in Bulletin R-1, dated March, 1986, entitled "Properties and Processing of Reemay® Spunbonded Polyester" from E. I. du Pont de Nemours and Company, Incorporated, Wilmington, Delaware. Although in a preferred embodiment, a spunbonded polyester strip is used, others also are acceptable. For example, the substrate strip which is to be impregnated may be a nylon spunbonded fabric, non-woven glass, polypropylene melt blown non-woven fabric, polyurethane spunbonded fabric or TCF cellulose fabric, for example. Another important characteristic of the strip 37 is the stiffness of the body of the material which comprises the strip. Within limits, as the material of the strip 37 is made increasingly stiffer, it is still relatively easy to form the tape longitudinally about the cable core 22 without wrinkling. However, it is important that the material be supple and flacid enough to conform readily to the contour of the outer surface of the core so that a minimal peripheral dimension is presented over which the dielectric jacket is applied. As a desirable consequence, a minimum overall diameter is achieved for the cable which will meet all the necessary requirements. Stiffness of the material for the strip 37 is controlled by a combination of factors such as the number of fibers per unit volume, thickness of the material, size of the fibers and the amount and type of binder used in the material. Increasing the thickness of the material obviously increases the cost of the material per unit of surface area of cable covered. Increasing the number of the fibers per unit volume or increasing the amount of binder tends to decrease the ability of the material to prevent heat transfer. Thus, at least four factors, formability of the strip 35, cost of the strip, insulative capability of the strip, and its water blocking capability must be considered and balanced in providing the proper material for use on a particular cable. In a preferred embodiment, the spunbonded polyester strip 37 combines the thermal, chemical and mechanical properties of polyester fibers with a spunbonded structure to provide a strip which is suitable for use in a communications cable. These properties include a relatively high tensile strength and elongation, excellent tear strength, and resistance to temperatures as high as about 204°C. In order to render the substrate strip 37 swellable upon contact with moisture, the strip 37 is impregnated with a suitable water swellable material which herein is referred to as a superabsorbent material. As will be recalled, the substrate strip 37 impregnated with a water swellable material is designated with the numeral 35. Superabsorbents are hydrophilic materials which can absorb and retain water under pressure without dissolution in the fluid being absorbed. See J. C. Djock and R. E. Klern "Review of Synthetic and Starch-Graft Copolymer Superabsorbents" prepared for the Absorbent Products Conference held November 16-17, 1983 in San Antonio, Texas and incorporated by reference hereinto. Properties such as enzyme stability, biodegradability, absorbent capacity and rate of uptake are used to characterize a superabsorbent material. One of the early superabsorbents was a saponified starch graft polyacrylonitrile copolymer. See U.S. patent 3,425,971. The above-identified patent disclosed saponifying starch-graft polyacrylonitrile copolymers with aqueous bases. The two major superabsorbents which are available today are cellulosic or starch-graft copolymers and synthetic superabsorbents. There are two major broad classes of synthetic superabsorbents. These are the polyelectrolytes and the nonelectrolytes. The polyelectrolytes are the most important and fall into four classes - polyacrylic acid superabsorbents, polymaleic anhydride-vinyl monomer superabsorbents, polyacrylonitrile-based superabsorbents and polyvinyl alcohol superabsorbents. Of these, the polyacrylic acid and polyacrylonitrile-based superabsorbents are the most common. As with cellulosic-graft copolymer superabsorbents, the capacity of synthetic superabsorbents decreases with increasing salinity. The polyacrylic acid class of superabsorbents includes both homopolymers and copolymers of acrylic acids and acrylate esters. The monomer units usually are polymerized to produce a water-soluble polymer which is then rendered insoluble by ionic and/or covalent cross-linking. Cross-linking of the polymer may be accomplished with a multivalent cation, radiation, or with a cross-linking agent. The absorbency of the product is determined by the number of ionizable groups, usually carboxylates, and the cross-linking density. The cross-linking density affects not only the absorbency, but also the time it takes to absorb and the strength of the gel formed. Generally, the higher the cross-linking density, the stronger is the gel which is formed. The time to reach absorbent capacity decreases as the cross-linking density increases, and the absorbent capacity decreases. The spunbonded substrate strip 37 may be impregnated with any of several water blocking superabsorbent materials. In a preferred embodiment, it is impregnated with a superabsorbent material which is derived from an aqueous solution comprising acrylate polymeric material which combines acrylic acid and sodium acrylate functionalities and water. The impregnating material of the preferred embodiment comprises a sodium salt of polyacrylic acid in which all the carboxylic groups may or may not be reacted with sodium. In other words, it is saponified in whole or in part. The level of saponification which may fall within a relatively wide range depends on desired properties. After the substrate strip 37 has been impregnated, the superabsorbent material is dried to provide a film on the tape. It is desirable to impregnate the strip 35 with a film of the impregnating material instead of a powder. The impregnated tape 35 has a density of about .037 to .061 kg/m² which includes the density of the untreated tape 37 increased about 10 to 80%, i.e. add-on, by the treating material. In another embodiment, a Reemay® spunbonded polyester tape is impregnated with an aqueous solution comprising acrylates and acrylamide polymer powders mixed with water to provide a strip 35. The strip impregnated with such a composition has a density which may represent an increase as high as about 80% of the density of the untreated strip 37. In each of the embodiments just described, the impregnating material is a mixture of water and a superabsorbent material in which the mixture comprises about 4 to 7% solids when the impregnating material is an aqueous solution and applied. In general, the strip 37 may be impregnated with (1) a material comprising polyacrylic acid, (2) a material comprising polyacrylamide (3) blends of (1) and (2) or salts thereof or (4) copolymers of acrylic acid and acrylamides and salts thereof as well as other similar superabsorbents. Advantageously, in response to contact with water, the superabsorbent material in a cable structure swells to block the flow of water in a longitudinal direction. The superabsorbent material also forms a gel and changes the viscosity of the ingressed water at the point of contact with the superabsorbent material, making it more viscous and consequently developing more resistance to water flow. As a result, the flow of water longitudinally along a cable from a point of entry is reduced substantially. The substrate strip 37 also possesses specific properties such as porosity and thickness which enhance its use as a water blocking element for a cable, particularly for a communications cable. Of importance is the need for the tape to be made of a material which has a relatively high porosity. It has been found that the water blockability of the strip 37 increases as the porosity of the tape increases. Porosity may be measured by air permeability in units of cubic feet per minute at a specified water pressure. The water blocking capability of a Reemay® spunbonded polyester impregnated strip is a surprising result. Inasmuch as the Reemay material has a relatively high porosity, it would be expected that moisture would penetrate it rather easily. In at least one catalog which discloses a superbonded polyester tape having a relatively high porosity, mention is made that the tape has a relatively low moisture pickup, presumably by surface tension. This seemingly would lead one away from its use as a water blockable tape for use in communications cables. Porosity of the strip 37 enhances the water blockability of the treated strip 35. Evidently, because it is so porous, it is capable of accepting a substantial quantity of the impregnating material. As a result, entering water contacts a substantial area of the impregnating material which is water blocking. There is a surprisingly fast reaction between the water blocking material and the water causing the water blocking material to swell and block off further longitudinal movement of the water along the cable. The porosity of the untreated strip 37 decreases with increasing thickness for a given denier. In a preferred embodiment, the Reemay® tape is style 2014 which has a porosity of 4.1 m³/m² · s at 124 Pa. Reemay tape marketed under code designation 2014 has a density of 0.037 kg/m², has a thickness of 0.02 cm and is formed of substantially straight polyethylene terephthalate fibers. Reemay tape having a code designation of 2024 has a density of 0.078 Kg/m², has a thickness of 0.03 cm, has a porosity of 1.5 m³/m² · s at 124 Pa and also is formed of substantially straight polyethylene terephthalate fibers. Reemay tape having a code designation of 2415 has a density of 0.039 kg/m², has a thickness of 0.036 cm, has a porosity of 3.6 m³/m² · s at 124 Pa and is formed of crimped polyethylene terephthalate fibers. Obviously, various other grades and thicknesses of Reemay® spunbonded polyester tape or other similar materials may be used. Material densities of up to about 2 ounces per square yard are practical values. Practical material thicknesses may range from 0.013 am to 0.03 cm thick. These values by no means limit the invention but represent the presently preferred ranges. The porosity of the substrate strip 35 must be balanced against other properties. For example, because the strip is to be embodied in a cable, it is beneficial for the strip to have a relatively high tensile strength. For a given strip width, the tensile strength decreases as the thickness decreases. Although a larger thickness is desired insofar as tensile strength is concerned, a larger thickness may result in less porosity, at least for those strips which are available commercially. Therefore, these two properties must be balanced against each other to arrive at a final thickness. As mentioned hereinbefore, the style 2014 preferred Reemay® tape has a thickness of 0.02 cm which is suitable for use in the cables of this invention. Thickness of the strip 37 also is important from another standpoint. In order to allow the cable to be terminated by standard size hardware, the diameter of the cable must be maintained within a desired range. Accordingly, the thickness of each element of the sheath system must be considered. Therefore, the thickness is established while being mindful of the porosity and the tensile strength. The strip 37 must not be too thin, else the tensile strength is affected adversely, and if too thick. the porosity and overall cable outside diameter are affected adversely. As mentioned earlier, cable armor may be wrapped with a water blockable yarn instead of an impregnated tape. In a preferred embodiment, yarn such as that disclosed in U.S. patent 4,366,206, is wrapped helically about the shield with a pitch of about 22.9 cm. Going now to FIGS. 5 and 6, there is shown a cable 60 having two substantially straight longitudinally extending strength members 62-62. The strength members are in engagement with an underlying metallic shield 64 and cross periodically an impregnated helically wrapped strip 35 such as a smaller width of the tape 31 in FIG. 1. The shield 64 encloses a core tube 68 and a core 70 comprising at least one transmission medium 72 such as an optical fiber. The core 70 may or may not be filled with a waterproofing material 74. The shield 64 is enclosed by a plastic jacket 78. For this arrangement, a water blocking tape 76, such as the tape 31, is wrapped about the core tube prior to the introduction of the shield. The peripheral coverage of the strip 35 is caused to be relatively small so that the interference with bonding between the shield and the overlying jacket 78 is relatively insignificant. In an alternative embodiment of the cable of FIGS. 5 and 6 water swellable yarns 33-33 are caused to extend parallel to the strength members 62-62 (see FIGS. 7 and 8). As can be seen in the drawings, two longitudinally extending. substantially linear water swellable yarns 33-33 are associated with each strength member 62 and are parallel thereto. Each yarn 33 is nestled in a nip between a portion of the circumference of the associated strength member and a portion of the periphery of the adjacent corrugated metallic shield 64.

Claims: A communication cable (20), which comprises a core (22) having a longitudinal axis (21) and comprising at least one transmission medium (24;72); an inner metallic protective layer (29;64) enclosing said core; at least one rod-like strength member (30;62) engaging the outer surface of said metallic layer; and an outer jacket (32;78) which comprises a plastic material bonded to the inner metallic layer with said at least one rod-like strength member partially embedded in the plastic material, said cable being characterized in that it further comprises a longitudinally extending water blocking member (33;35), which is positioned such that it is immediately adjacent to both the outer surface of the inner metallic protective layer and said at least one rod-like strength member and in that the water blocking member covers a relatively small surface area of the inner surface of said plastic jacket so as not to significantly impair the bonding between the plastic jacket and the inner metallic layer. The cable of claim 1, wherein said water blocking member extends linearly parallel to the longitudinal axis. The cable of claim 1, wherein said water blocking member extends helically about the longitudinal axis. The cable of claim 1, wherein said water blocking member is a yarn (33) which has been treated with a water swellable material. The cable of claim 1, wherein said water blocking member is a longitudinal strip (35) of material which is water swellable upon contact with water. The cable of claim 1, wherein the inner metallic protective layer (29;64), is a corrugated metallic shield, and wherein the longitudinally extending water blocking member comprises a non-metallic material which is disposed in engagement with the outer surface of said shield between said shield and said jacket, said member being treated with a water blocking material which is swellable upon contact with moisture to block entry of moisture into said core and movement longitudinally along said cable and having a thickness that is controlled to optimize the tensile strength of the member and a porosity which prior to its being impregnated is relatively high. The cable of claim 6, wherein said water blocking member is a strip (35) of material which has been treated with a water blocking material and wrapped helically about said shield. The cable of claim 7, wherein said strip comprises a spunbonded polyester material including continuous filament polyester fibers that are randomly arranged, highly dispersed and bonded at filament junctions and which has been impregnated with a water blocking material. The cable of claim 8, wherein said water blocking material is selected from the group consisting of (a) polyacrylic acid; (b) polyacrylamide; (c) blends of (a) and (b); (d) salts of (a) and (b); and (e) copolymers of (a) and (b). The cable of claim 6, wherein said cable includes a plurality of strength members which are wound helically about said shield. The cable of claim 10, wherein said water blocking member extends linearly generally parallel to the longitudinal axis. The cable of claim 10, wherein said water blocking member is wrapped helically about said shield in a direction opposite to the direction of helical wind of said strength members.

Inventors: ARROYO CANDIDO JOHN, DEBBAN HAROLD PAUL JR, PAUCKE WALTER J, ARROYO, CANDIDO JOHN, DEBBAN, HAROLD PAUL, JR., PAUCKE, WALTER J.
Applicants: AT & T CORP, AT&T CORP.
Application year: 1988
Main classification: G02B6
IPC codes: H01B7, G02B6