Communication cable having reduced jacket shrinkage

[0002] The present invention is generally related to a communication cable and, more particularly, is related to the outer jacket of such cables.

[0003] Optical fiber cables have been used for many years to transmit information at high rates and very long distances. The transmission media of the optical fiber cable are hair-thin optical fibers protected from external forces and elements by precisely designed and manufactured cable structures. There are several relatively common cable structure families currently being used to protect these hair-thin optical fibers. Such cable structure families include the loose tube, the slotted core and the central core structures.

[0004] Optical fibers are relatively delicate compared to previous types of communication media. Typically made of glass, the fibers are not ductile and can be broken or cracked, either of which can destroy or degrade the signal being transmitted. Therefore, optical fibers are housed in rugged cable structures to protect the fibers from mechanical damage caused by heavy loads, sharp impacts or gnawing rodents. The quality of a signal transmitted through an optical fiber is also sensitive to tensile stress. Accordingly, tensile stiffness members are included in each type of optical fiber cable structure to carry the majority of tensile loads.

[0005] Plastic jacketing materials are extruded as the final layer of fiber optic cables in a continuous, high-speed sheathing operation. This layer is applied directly over the layers of components used to carry tensile loads and to shield the optical fibers from the environment. Once added, the jacketing plastic couples to the components in contact with them. Several types of strength members are used in cables, both metallic and dielectric. Heretofore, strength systems used in optical cables have had two primary functions: 1) to carry tensile loads and, 2) to restrict the contraction and expansion of the jacketing plastic. Tensile loads are applied to the cable during installation, e.g., while being pulled into ground conduits, or during service life, e.g., while suspended between telephone poles. Environmental temperature variations during product life can cause the jacketing plastic to expand and contract applying tensile and axial compressive loads on the strength systems. However, a more significant instance of contraction by the jacketing plastic occurs during manufacture of the cable. Current jacketing materials, including high-density polyethylene and medium-density polyethylene, exhibit sufficient post-extrusion shrinkage during cooling as to apply compressive axial loads on any adjacent components e.g., strength members extending axially along the cable. To resist these compressive and the other tensile loads, rigid strength rods, made from steel, epoxy/glass, epoxy/aramid, etc., have been embedded in cable jackets.

[0006] In general, metallic components, including wires and armoring tapes, have excellent tensile properties and sufficiently high compressive stiffness so as not to strain significantly under the axial compressive load exerted by the cooling jacket. Alternately, dielectric materials, such as aramid and glass fibers, which provide excellent tensile characteristics, provide little axial compressive resistance in their simplest, and therefore lowest cost, forms, e.g., as yarns. Because these yarns cannot resist compressive loads, the shrinking jacket buckles these dielectric tensile members as it cools. Then, when the final cable is loaded in tension, the tensile yarns will not support any tensile load until the cable has been elongated to the point where the yarns are straightened. This process leaves the optical fibers vulnerable to tensile loading until the tensile yarns pick up the load. As a result, current dielectric strength members are generally constructed as rigid composite rods made of high modulus and high strength fibers, such as glass or aramids, embedded in a hard epoxy matrix. However, these rigid dielectric rods can be very costly, and they, along with the metal wires, make the cable less flexible because their added compressive stiffness leads to higher flexural stiffness.

[0007] In illustration of this prior art, U.S. Pat. No. 5,131,064 to Arroyo, et al., discloses a cable having strength rods and a lightning-protective sheath system comprising a thermal barrier, which are disposed between the core of the cable and its plastic jacket. The thermal barrier comprises a textile of glass yams that have been woven into a unit and then sandwiched between a pair of tapes together with a waterblocking material such as a superabsorbent powder. The glass yarns undulate in the longitudinal direction, not only because of their weaving pattern, but also because the tape follows the undulations of a corrugated metallic shield. Such undulations preclude the tape from receiving any portion of the load until the cable has already been elongated. And since the disclosed tape has a very low tensile strength, 420 Newtons per centimeter of width, the cable's tensile strength effectively comes from rigid strength rods that are embedded in the plastic jacket. However, these rods are less flexible than the woven tape, thereby reducing the flexibility of the entire cable. Further, if a pair of rods are used and are positioned diametrically opposite each other on either side of the core, they make the cable inflexible in all but one plane and much more difficult to handle and install.

[0008] Another example is U.S. Pat. No. 4,730,894 to Arroyo, which discloses an optical fiber cable that includes a plurality of equally spaced strength members disposed on a carrier tape and held in place by an adhesive. Once a plastic jacket is extruded onto the strength members, they are coupled to the jacket and provide tensile strength to the cable. However, if the strength members are flexible, i e., they have essentially no compressive strength or stiffness, then they will shrink in the longitudinal direction after the plastic jacket cools and will not be able to receive any portion of the tensile load until the jacket is elongated. This is undesirable because excessive cable elongation can cause the tensile load to be transferred to the optical fibers. On the other hand, as stated previously, increased compressive stiffness correlates to increased flexural stiffness and, therefore, decreased cable flexibility, which makes cables more difficult to handle and to install. However, to protect the valuable optical fibers, cable flexibility generally has been sacrificed in the prior art.

[0009] Yet another example of prior art, U.S. Pat. No. 5,838,864 to Patel, discloses a cable with a dielectric strength member system that attempts to maximize the flexibility of the cable by using a flexible woven strength tape to carry to majority of the tensile loads. To control post-extrusion shrinkage, two rigid epoxy-fiber rods are embedded in the jacket, diametrically opposite one another, on either side of the core. However, as these rods do not have to carry tensile load, their size is minimized and, therefore, the overall cost of the strength system is reduced. Further, the volume of jacket material required to encase the smaller strength rods is less than for larger rods, further reducing the cost of the overall cable sheath. Still, the strength system is more expensive and complex than is desirable because of the need for two types of strength systems.

[0010] Another approach to preventing jacket shrinkage and buckled strength members from making the optical fibers vulnerable to tensile loads is to control the length of the optical fibers relative to that of the sheath components. Fibers can be protected during the elongation of the sheath if they also are not straight when the cable starts to stretch. Accordingly, most fiber optic cables contain excess fiber length relative to the final length of the cable sheath. However, there is a limit as to the amount of excess fiber length that can reasonably be introduced into a cable core without making the cable diameter prohibitively large or inducing optical signal loss by introducing excessive bends or undulations along a length of fiber. Unfortunately, the contraction of standard cable jacket materials can buckle the simple dielectric strength members as much as 1.5%, while acceptable amounts of excess fiber length may be less than 0.5%.

[0011] Therefore, since excess fiber length alone cannot be used to compensate for jacket shrinkage, the common default solution to this problem is to use the rigid dielectric strength rods to resist jacket shrinkage. As stated previously, unlike the metallic strength members, these dielectric rods are very costly, even when compared to their base component yarns and filaments, and rigid enough to reduce cable flexibility.

[0012] Therefore, there appears to be a fundamental conflict in providing an easy-handling, cost-effective dielectric cable sheath and a dielectric sheath that adequately protects the optical fibers. Known designs for best protecting the optical fibers make the cable more expensive, stiff, and difficult to handle. Known designs for making the cable flexible either require that the glass fibers have more excess length than is desirable, or expose the glass fibers to tensile loading and possible breakage.

[0013] However, if the jacket material had minimal post-extrusion shrinkage, ie., less than the excess length of the optical fibers, the more costly dielectric strength components would not be required. Accordingly, what is sought is a jacket material with minimal post-extrusion shrinkage that still meets the demands of ruggedness and flexibility for use in outdoor communication cables.

[0014] Heretofore, some prior art cables have incorporated nucleating agents, e.g., inorganic materials, salts of aliphatic monobasic or dibasic acids, or alkali metal or aluminum salts of aromatic or alicyclic carboxylic acids, and filler materials, e.g., talc, glass fiber, and glass spheres, into buffer or core tubes to give the desired properties of high strength, low shrinkage, good flexibility, improved processibility and low cost. An example of such a prior art cable is described in U.S. Pat. No. 5,574,816, (the '816 patent) issued to Yang, et al. Because the fillers or nucleating agents of the '816 patent were only added to the core or buffer tubes, the cables of that design still required tensile stiffness members with sufficient compressive stiffness to resist jacket shrinkage. As mentioned hereinbefore, it would be desirable to eliminate the need for the costly dielectric strength members. Furthermore, the '816 patent only added the fillers to a polyethylene-polypropylene copolymer resin and does not address the use of fillers that can be added to other types of resin.

[0015] Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.

[0016] The foregoing problems have been overcome by a communication cable including a jacket made of a resin and high aspect ratio fillers and, if needed, coupling agents added to the resin. The high aspect ratio fillers occupy a predetermined volume of the jacket so as to control post extrusion shrinkage. Adequate reduction in shrinkage can be achieved with embodiments of the invention including average filler particle sizes ranging from 5×10⁻⁶to 1×10⁻²meters, and aspect ratios ranging from approximately 10 to 3500.

[0017] Possible examples of the type of resins that may be incorporated into the jacket are polyethylenes, ethylene-vinyl acetate copolymers, copolymers of polypropylene and ethylene/propylene rubber, and ethylene-ethyl acrylate copolymers. The fillers may be either fibrous, platelike, or organic in nature. Possible examples of coupling agents, which can help bond the fillers to the resin, are organofunctional silanes, titanates, zirconates, epoxy coating and functionalized polyolefins.

[0018] Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.

[0019] The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

[0020] FIG. 1 is a cross-sectional end view of a standard dielectric cable design that can be used to measure post-extrusion jacket shrinkage.

[0021] FIG. 2 is a graph of Tensile Load versus Cable Elongation for both a low shrinkage jacket and a standard jacketing material.

[0022] FIG. 1 discloses a standard central-core dielectric cable 10 having a core tube 20 disposed within an outer jacket 30 comprising a resin, fillers, and a coupling agent. Disposed within core tube 20 are transmission media 40 and, optionally, a filling compound 50. A rip cord 60, for example Kevlar® yarn, facilitates removal of the outer jacket 30. Additionally, strength members 70 are disclosed here as ten glass rovings, 0.022 inches×0.054 inches. The purpose of the strength members 70 is to provide tensile stiffness when an axial load is applied to the cable 10 and to keep this load from being transmitted to the transmission media 40 within the central core 20.

[0023] Desirably, the strength members 70 possess negligible compressive stiffness as these components are very flexible and can be as much as 70% less in cost than rigid dielectric strength members, such as epoxy-glass rods, which have significant compressive as well as tensile stiffness. The strength members 70 may be impregnated glass rovings; however, other materials, metallic or non-metallic, such as glass or Kevlar® yarn can be used. Also, the strength members 70 can be composite structures. These strength members are an integral part of the jacket 30 and function together as a unit.

[0024] The specific design of these strength members and the complete sheath system are not critical to the novel aspects of the present invention. However, for the sake of completeness, various forms of acceptable sheath designs and constructions are provided in the following commonly assigned U.S. Pat. No. 4,078,853 to Kempf, et al.; U.S. Pat. No. 4,826,278 to Gartside, et al.; U.S. Pat. No. 4,844,575 to Kinard et al.; U.S. Pat. No. 5,082,348 to Gartside et al.; and U.S. Pat. No. 5,109,457 to Panuska et al., all of which are incorporated herein by reference.

[0025] As stated above with regard to the sheath system, the particular design and construction of the sheath is not critical to the implementation of the present invention. Nevertheless, one acceptable jacket design is an outer jacket 30 that comprises a resin and high aspect ratio fillers occupying a predetermined volume of the jacket. Additionally, the outer jacket 30 may comprise a coupling agent to aid adhesion between the filler and the resin, to aid in dispersion of the filler, and to improve processibility.

[0026] The coupling agents can either be added directly to the resin along with the fillers, or they can be first used to coat the fillers, before addition of the fillers to the resin of the jacket. Coupling agents may include organofunctional silanes, titanates, or zirconates. Other suitable coupling agents include epoxy coatings, available commercially from Baycomp Thermoplastic Composites, Inc., and functionalized polyolefins. Reported functional polyolefins include halogenated paraffins, carboxyl-modified polypropylenes, and maleated polypropylene. Halogenated compounds are known in the art and are described in Polymer Composites, v. 3, p. 231, by J. P. Trotignon, et al., 1982. The information regarding halogenated paraffins in this publication is hereby incorporated by reference. Carboxyl-modified polypropylenes are known in the art and are described in Polymer Engineering and Science, v. 15, p. 308, by K. Okuno and R. T. Woodhams, 1975. The information in this publication regarding polypropylenes is herein incorporated by reference. Maleated polypropylene is also known in the art and described in Polymer Composites, v. 5, p. 250, by 1. Chun and R. T. Woodhams, 1984. The information in this publication relating to maleated polypropylene is herein incorporated by reference.

[0027] Any type of high aspect ratio mineral or synthetic filler should be workable in the present invention, including fibrous or needle-like fillers, platelike fillers, and organic fillers. Examples of fibrous fillers are wollastonite, glass fiber, milled glass, chopped glass, and chopped carbon fiber. Examples of platelike fillers are mica, talc, glass flake, clays, and silica.

[0028] For fibrous, or needle-like fillers, the aspect ratio is taken from the fiber's length to its diameter. Thus, the equation describing aspect ratio is a=L/D, where a is aspect ratio, L is fiber length, and D is fiber diameter. For platelike fillers, the aspect ratio may be defined as a=D/t, where a is aspect ratio, D is particle diameter, and t is particle thickness, measured for a group of particles. If the particle is modeled as an ellipse, the diameter may be calculated as the average of the major and minor axes of the ellipse. This mode of calculating aspect ratio is known in the art and is described in Polymer Engineering and Science, v. 23, p. 766, by C. Busigin, G. M. Martinez, R. T. Woodhams and R. Lahtinen, 1983. Typically, aspect ratio of a particular sample is measured by optical microscopy and subsequent image analysis. The aspect ratio of individual members of a population is calculated by applying the above definitions to data for diameter, length, and thickness, and an average of these values is reported.

[0029] Fibrous fillers provide improved shear strain distribution when compared to platelike fillers, and thus yield better elongation when the cable is subjected to bending, when compared to equivalently loaded and coupled filler samples. Platelike fillers, for a given loading, are not as effective at controlling the shrinkage of the outer jacket upon extrusion. Fibrous fillers, in order to be effective in controlling shrinkage must also have good compressive properties, including resistance to buckling.

[0030] Very high aspect ratio fillers limit the elongation at break of the jacket. An acceptable jacket must have sufficient elongation to accommodate axial loading and bending-induced strains. For a one-dimensional analysis, strain=ΔL/L for axial loading. For bending-induced strains, Strain=(radial offset)/(bend radius).

[0031] To maintain the other desirable qualities of jacket 30, such as high elongation at break and cutability, volumes ranging from 1.5 to 25% of fillers are used. Assuming a perfect composite, ie., “perfect” coupling between the resin and the fillers, theoretical calculations indicate that the optimal aspect ratio for the fillers is 100:1 for fibrous fillers and 25 to 30:1 for platelike fillers. In reality, the degree of coupling between the fillers and the resin is less than perfect, and the fillers may slip relative to the resin when the composite is strained. As a result, the aforementioned coupling agent is needed. Typically, glass fibers are treated with silane and carbon fibers with an epoxy sizing. The mica and talc particles may or may not be coated with a coupling agent. Longer particle length, i.e., higher aspect ratios, can compensate somewhat for a poor bond between the filler and the resin.

[0032] Any high aspect ratio filler/coupling agent/resin combination may be feasible. Resin/filler combinations with reduced shrinkage are listed in Table 1.

[0033] A variety of base resins may be used in the present invention. Very effective resins appear to include linear low-density polyethylene (LLDPE), ethylene-vinyl acetate copolymer (EVA), and impact-modified isotactic polypropylene, which is typically a copolymer of polypropylene and ethylene/propylene rubber. However, high-density polyethylene (HDPE), medium-density polypropylene (MDPE), low-density polypropylene (LDPE), and ethylene-ethyl acrylate (E-EA) copolymers can all function as appropriate base resins for the jacket 30. Jacket 30 can also be made from polybutylene terephthalate (PBT), polycarbonate (PC), a layered combination of PBT and PC, or a polyamide such as Nylon-12. The PBT, PC or Nylon are good materials for making buffer or core tubes because they are materials that have high Young's modulus and low thermal expansion coefficients. However, such materials also have some disadvantages when compared to other materials such as polyethylene or polypropylene-polyethylene copolymers. These disadvantages include higher cost, lower flexibility, moisture sensitivity, and increased difficulty in handling and processing due to the mechanical properties of the materials.

[0034] It is noted that the strength members 70 are coupled to the cable jacket such that when a tensile force is applied to the jacket, the force is transferred to the strength members after minimal elongation of the cable. However, this will only happen when the flexible strands 70 are substantially straight, ie., they have a limited amount of excess length. If the strength members are applied such that they undulate in the longitudinal direction, then the length of the cable will have to increase significantly before the strength tape will pick up the load. In the meantime, the tensile force applied to the optical fibers may exceed their safe limit. And while optical fibers may rival the strength of steel for a given cross-section area, they have very small cross sections and are not ductile. It is important that the flexible strength members 70 be substantially straight so that the optical fibers will not be strained excessively at the cable's rated tensile load. In other words, the percent excess length of the flexible strands (ε_s) should be small enough to prevent the fiber from reaching its maximum allowable strain at the cable's rated tensile load.

[0035] FIG. 2 is a graph showing cable elongation, ie., strain, as a function of the tensile force, i e., load, applied to the cable for two different cables. The loads demonstrated in FIG. 2 are those that are expected during the standard service life of a cable. Curve 102 illustrates the situation wherein the standard plastic jacket is used and shrinks due to cooling after being extruded onto a cable structure similar to the one illustrated in FIG. 1. Because the strength members 70 have little or no compressive stiffness, and because they are coupled to the jacket, the strength members 70 buckle as the jacket shrinks. Thereafter, when a tensile load is applied to the cable, the strength members 70 are not available to resist the load until they straighten out. Such a situation is undesirable because the amount of excess fiber length (ε_f) needed to avoid excessive tensile loading is such that fiber-bending losses will occur.

[0036] Curve 101 graphically illustrates the present invention in terms of two different regions of cable operation. Region (A1) shows a rapid increase in cable elongation after a tensile load is first applied and the small undulations in the strength members 70 are straightening out; whereas region (B) shows a slow increase in cable elongation once the strength members 70 are handling the majority of the tensile load. The cable rapidly elongates in region (A1) because strength members 70 have been buckled due to shrinkage of the cable's plastic jacket and are returning to their unbuckled configuration as the tensile load is applied. The contribution of the low shrinkage jacket can be appreciated by comparing region (A1) of curve 101 with the corresponding region (A2) of curve 102 where standard jacketing material was used. The difference in cable elongation (ε₂−ε₁) as between curves 101 and 102, at a particular tensile load, is attributable to the fact that the jacket material shrinks less during cooling by that amount. Advantageously, this reduces the required amount of excess fiber by a like amount. In the preferred embodiment of the present invention, less than 1.0% excess fiber length is used.

[0037] Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention.