CA2220368C - Single-jacketed plenum cable - Google Patents

Abstract

A communications cable having superior electrical characteristics and meeting the burn requirements for plenum applications has a core formed of one or more twisted wire pairs having primary insulation formed of a suitable material, such as high density polyethylene. The core is surrounded by a single outer jacket formed from a material having excellent heat/flame resistance characteristics and acceptable electrical characteristics that are substantially stable at relatively high temperatures, such as a foamed thermoplastic halogenated polymer, for example polyvinylidene fluoride material.

Description

SINGLE-JACKETED PLENUM CABLE
FIELD OF THE INVENTION
This invention relates to a communications cable suitable for plenum, riser, and other applications in building structures. More particularly, the present invention relates to an improved construction for a high-frequency communications cable that is capable of meeting rigorous burn requirements and is electrically stable during operation at substantially higher temperatures than prior art cables.
BACKGROUND OF THE INVENTION
It is common practice to route communication cables and the like for computers, data devices, and alarm systems through plenums in building constructions. If a fire occurs in a building which includes plenums or risers, however, the non-fire retardant plenum construction would enable the fire to spread very rapidly throughout the entire building. Fire could travel along cables installed in the plenum, and smoke originating in the plenum could be conveyed to adjacent areas of the building.
A non-plenum rated cable sheath system, which encloses a core of insulated copper conductors, and which comprises only a conventional plastic jacket, may not exhibit acceptable flame spread and smoke generation properties. As the temperature in such a cable rises due to a fire, charring of the jacket material may occur. If the jacket ruptures, the interior of the jacket and the insulation are exposed to elevated temperatures. Flammable gases can be generated, propagating flame and generating smoke.
Generally, the National Electrical Code requires that power-limited cables in plenums be enclosed in metal conduits. This is obviously a very expensive construction due to the cost of materials and labor involved in running conduit or the like through plenums. The National Electrical Code does, however, permit certain exceptions to the requirements so long as such cables for plenum use are tested and approved by an independent testing laboratory, such as the Underwriters Laboratory, as having suitably low flame spread and smoke-producing characteristics. The flame spread and smoke production characteristics of cable are measured per specification UL-910 plenum burn analysis.
With plenum cables, in addition to concerns about flammability and smoke production, the cables must also, of course, have suitable electrical characteristics for the signals intended to be carried by the cables. There are various categories of cable, such as Category 3, Category 4, Category 5, etc., with increasing numbers referring to enhanced or 1 S higher frequency electrical transmission capabilities. With Category 5, for example, extremely good electrical parameters are required, including low attenuation, structural return loss, and cross-talk values for frequencies up to 100 MHz. Unfortunately, cable materials which generally have the requisite resistance to flammability and smoke production also result in electrical parameters for the cable generally not suitable for the higher transmission rates, such as a Category 5 cable. Specifically, in the case of cables intended for Category 5, the cable core, in addition to passing the plenum burn test UL-910, must also pass physical property testing provided by the specification requirements UL-444, as well as meet Category 5 electrical requirements such as provided in Electronic Industries Association specification TIAIEIA-568A.
Currently, a cable construction which is available and which meets these requirements is provided in a configuration which includes fluorinated ethylene propylene (FEP) as insulation, with a low-smoke polyvinyl chloride (PVC) jacket. Such a cable construction 326169.6/29144.0800 2 meets the 100 MHz frequency operation requirements, and it has been demonstrated that such a cable construction can be suitable for operation at 155 Megabits or 150 MHz.
Unfortunately, FEP at times may be in short supply. Given the manufacturing capacity of FEP producers, only enough FEP is currently produced to meet approximately 50 percent of the demand for the volume of material required to construct high-category cables. Although it could be expected that the supply of FEP will continue to increase, it is apparent that the available quantity of FEP may not meet the demand for the material for use in plenum cables as the market is projected to increase at a rate of approximately 25 percent per year through 1999, particularly in anticipation of European and Scandinavian market demands for plenum cables.
Current riser cables utilize a foam/skin insulation. The insulation material construction is a foamed, high density polyethylene and PVC skin composite. A
jacketed and shielded cable of these insulation cores meets Category 3 electrical and the CMP burn requirements. However, developing a Category 5 cable is very difficult due to the extreme electrical parameters necessary, i.e., attenuation, structural return loss, and cross-talk values to 100 MHz. Furthermore, this core must pass elevated temperature attenuation requirements at 40 °C and 60 °C. The above-mentioned insulation composite with a PVC skin will not pass the elevated temperature attenuation requirements because the dielectric constant of PVC
increases with temperature.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of this invention to provide a cable construction suitable for high frequency electrical applications while at the same time being resistant to burning.
It is a more specific object of this invention to provide a cable design that meets Category 5 or higher electrical parameters, including elevated temperature attenuation requirements, while at the same time satisfying the burn rating standards for plenum cable.
326169.6/29144.0800 It is an additional object of this invention to provide a cable construction which meets the electrical and burn rating requirements and additionally meets various physical requirements such as cold bend, room temperature and aged tensile strength, elongation, and the like, required for plenum cables.
It is another object of this invention to provide such a cable construction meeting the above requirements, which does not utilize FEP, and which is suitable for operation up to 155 Megabits or 150 MHz.
A further object of the present invention is that it provides a cable construction having an outer jacket construction that exhibits electrically stable characteristics at substantially high temperatures, relative to the temperature requirements of currently available plenum cables.
Briefly, in accordance with one embodiment of the invention, a riser and plenum rated cable construction includes a plurality of twisted wire pairs utilizing a polyolefin primary insulation material and a single outer jacket for the cable construction formed of a thermoplastic halogenated polymer. To improve the electrical characteristics of the cable, the outer jacket is of a foamed construction.
BRIEF DESCRIPTION OF THE DRAB
A more complete understanding of the present invention may be derived by referring to the detailed descriprion and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and:
FIG. 1 is an elevation of a cable construction in accordance with the present invention with a portion of the outer jacket broken away for illustrative purposes;
FIG. 2 is a cross sectional view of a cable construction in accordance with the present invention in which a plurality of cable cores are enclosed as a composite in an outer jacket;
and 326169.6/29144.0800 FIG. 3 is a cross-section of one of the conductors in a twisted wire pair of the cable shown in FIG. 2.
DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS
As noted, FEP insulation with a low-smoke PVC jacket meets Category S
electrical requirements and the applicable physical and burn property tests for plenum rated cable.
While the electrical and physical property requirements for Category 5 and higher cable could be met with other plastics such as polyolefins or modified polyolefins, the plenum burn requirements, such as UL-910, could not be met since polyolefins burn readily.
If a polyolefin material was smoke suppressed and flame retarded, the ingredients necessary for flame protection would detract from the necessary electrical values of the polyolefin material, and would also detract from the physical property attributes of the material.
The CMP or plenum burn test is a severe test. The test takes place in a closed 1 S horizontal fixture or tunnel, with the ignition flame source being a 300,000 BTU/hour methane flame with a high heat flux, and a 240 foot/minute air draft. The test lasts 20 minutes, and the cable is stretched side to side across a 12 inch wide, 25 foot long wire mesh rack in the tunnel. To pass this test, flame spread must not exceed 5.0 feet after the initial 4.5 foot flame source; smoke generarion must not exceed a peak optical density of 0.5 (33%
light transmission); and the average optical density must not exceed 0.15 (70%
light transmission). The purpose of this optical smoke density parameter is to allow a person trapped in a fire the ability to see exit signs as well as visually discern a route or means of escape.
FIG. 1 shows an elevation of a cable construction in accordance with a preferred embodiment of the present invention for providing a cable meeting Category 5 electrical requirements and the applicable burn and smoke generation requirements, as well as the physical property requirements, for plenum-rated cable without the use of FEP.
Referring 326169.6/29144.0800 now to FIG. l, there is shown a cable which is designated gener211v by the reference numeral 5, which is suitable for use in building plenums and the like. Lz the specific e~cample shown in FIG. l, the cable ~ is illustrated as having four twisted pair of ~ansmission media, referred to as twisted pairs and indicated by reference numerals 6, 7, 8 and 9, forming what is generally referred to as the cable core. Ln accordance with this embodiment of the invention, the twisted pairs 6-9 have a polyolefin primary insulation, which has good electrical characteristics even though it readily burns. In a specific embodiment of the present invention, a foam/skin high density polyethylene (APE) is used for the primary insulation, which has the requisite electrical characteristics for high frequency cable applications.
In order to enable having the required resistance to burning, the cable construction in accordance with this invention is provided with an outer jacket 11 which is highly resistant to burning. Thermoplastic halogenated polymers have been found to be suitable materials, particularly thermoplastic fluorocarbon polymers. In a specific embodiment of the invention, polyvinylidene fluoride (PVDF) has been found to be quite suitable in terms of providing adequate flame and burn resistance to meet the applicable standards.
A cable construction consisting of only the core of twisted pairs with polyolefin insulation surrounded by a jacket of conventionally extruded thermoplastic fluorocarbon polymer (such as PVDF) meets the applicable burn standards, but does not meet the high frequency electrical standards for cable. Specifically, the less than optimal electrical characteristics of a conventionally manufactured fluorocarbon polymer jacket, and its proximity to the twisted pairs, degrade the cable's electrical characteristics.
In accordance with the present invention, a single outer foamed PVDF jacket 11 may be employed by cable 5 without any intermediate material between the cable core and the outer PVDF jacket 11. The particular foam construction of the outer PVDF
jacket 11 suitably enhances the electrical characteristics of the PVDF material, which typically exhibits very poor dielectric constant and dissipation factor values in a substantially solid or unfoamed state.

Although not shown in FIG. l, cable 5 may include a shield located within outer jacket 11. Preferably, such a shield substantially surrounds the cable core and is configured to enhance the electrical performance of the cable core. For example, the shield may be configured to protect the cable core from extraneous RF or electromagnetic fields and S signals. The shield may be formed from a metallic foil, such as aluminum or copper, and may be constructed according to any number of conventional methodologies. Such shields are known to those skilled in the art, and need not be described in detail herein.
Referring now to FIG. 2, there is shown a construction of a cable 10 in accordance with this invention, suitable for use in building plenums, and the like, i.e., indoor/outdoor rated cable, in which a plurality of cable cores are enclosed within a single foamed PVDF
outer jacket. In FIG. 2, the cable 10 comprises one or more wrapped cables 20, each of which may include a core 22. The core 22 may be one which is suitable for use in data, computer, alarm, and other signaling networks as well as communications. The core 22 is the transmission medium and is shown in FIG. 2 as comprising one or more twisted wire 1 S pairs, the pairs of which are referred to in FIG. 2 by reference numerals 24, 26, 28 and 30.
Cables which are used in plenums may include 25 or more conductor pairs, although some cables include as few as six, four, two or even a single conductor pair such as shown in FIG.
1. In the exemplary embodiment shown in FIG. 2, each of the cores 22 comprise four twisted conductor pairs, identified in FIG. 2 with reference numerals 24, 26, 28 and 30.
As shown in FIG. 2, each of the cables 20 preferably utilizes a foamed PVDF
inner jacket configured identified by reference numeral 23. The inner jacket 23 may be configured as described more fully hereafter. Those skilled in the art will appreciate that the inner jacket 23 is not a requirement of the present invention, and that any suitable wrapping element known to those skilled in the art may be employed by cable 10. Furthermore, the particular material utilized as the inner jacket 23 may be selected to enhance the electrical and/or physical properties of cable 10.
326169.6/29144.0800 7 As also shown in FIG. 2, a plurality of the cables 20 are disposed within an outer jacket 34 in this embodiment. In FIG. 2, three cables 20 are shown as enclosed in an outer jacket 34, although the invention is equally applicable to there only being one cable enclosed by an outer jacket (as shown in FIG. 1 ) and for there being more or less than three cables 20 disposed within the outer jacket 34.
FIG. 3 is a cross-section of one of the conductors in one of the twisted pairs, such as twisted pair 24. The conductor or transmission medium 24 includes a conductor surrounded by an insulating material 38. The insulating material 38 may have a skin portion indicated by reference numeral 40.
In accordance with a preferred embodiment of the invention, the primary insulation 38 surrounding conductor 36 in each wire in the twisted wire pairs, such as wire pair 24, is a foam/skin polyolefin dual extruded insulation, which is acceptable for Category 5 electrical characteristics. The reasons for using a foam/skin insulation such as foam 38 with skin 40 (FIG. 2), in addition to achieving improved electrical properties, is to effectively decrease 1 S the amount of polyolefin material available to burn.
It is important to keep the foam/skin pure, with no fillers, such that this insulation can match or exceed the electrical properties of FEP. For example, FEP has a dielectric constant of 2. I , with a dissipation factor of 0.0001; in accordance with a specific embodiment of the invention described herein, the insulation is a pure foam/skin HDPE having a dielectric constant of 1.8, with an equivalent dissipation factor of 0.0001. With this configuration, the velocity of propagation is even improved with the foam/skin at approximately 78% as opposed to approximately 75% for FEP. By comparison, a flame retardant polyolefin with fillers would have a velocity of propagation of 67%. Also, a 2x2 cable (two pairs of flame retardant polyolefins plus two pairs of FEP) would encounter velocity of propagation skew problems, which is the difference in the distribution of electrical flow between the two insulation types. There are no skew problems with the pure foam/skin HDPE.
Velocity of propagation considerations and skew factors are discussed more fully hereafter.
326169.6/29144.0800 In accordance with one specific embodiment of the present invention, the primary insulation is dual extruded, with foam insulation 38 being a HDPE. A suitable material is one produced and available from Union Carbide Corporation identified as DGDB-1351NT, although an equivalent suitable for mechanical foaming may be used. In accordance with S the specific embodiment of the invention, the skin portion 40 of wire 24 is also a HDPE
produced by Union Carbide Corporation and available therefrom and identified as DGDM-3364 NT. In such an insulation construction, the polyolefin skin 40 has to be of adequate thickness to protect the overall foam/skin primary insulation from crushing during twist. The degree of foaming, the foam thickness, and the skin thickness are dependent upon compliance with UL-444 physical property testing requirements.
In accordance with a specific embodiment of the invention, the conductor 36 in each wire 24 had a diameter range from 0.0194 inches to 0.0215 inches. In accordance with this specific embodiment, the insulating material 38 had a thickness of 0.0060 inches, and the skin 40 had a thickness of 0.0022 inches.
I S In accordance with one embodiment of this invention, each of the cables 20 may be provided with a substantially flame retardant core wrap rather than inner PVDF
jacket 23.
Such a construction may be desirable for a cable arrangement having a large number of insulated pairs, e.g., more than 12. A flame retardant core wrap may be employed to ensure that the cable arrangement satisfies the associated plenum burn requirements.
As previously mentioned, preferably the primary insulation of the transmission media is a foamed/skin construction of HDPE. One material which was found to be quite suitable in accordance with the invention is a polyethylene material known as DGDB-1351NT, and available under that designation from Union Carbide. When this material is foamed and dual extruded with a skin, DGDM 3364 NT also produced by Union Carbide Corporation, it has a dielectric constant at 1 MHz of I .80, a dissipation factor at 1 MHz of 0.0001, and an LOI
of 17 percent. LOI refers to the limiting oxygen index, the percent of oxygen in air at which the sample burns completely. The specific gravity of this material is 0.945, but this material 326169.6/29144.0800 9 does not char, and hence needs to be protected by additional materials to meet the burn test, in accordance with and as provided by this invention.
As described above, the outer jacket 11 or 34 in accordance with this invention is made of a foamed halogenated polymer, and can be a foamed PVDF material. One PVDF
material which has proved to be extremely suitable is known as SOLEF 31508-0009, available from Solvay Polymers, Inc. In an unfoamed state, this material has a dielectric constant of 8.40 at 1 MHz, a dissipation factor of 0.1850 at 1 MHz, and an LOI
of 100 percent (the ideal LOI). The specific gravity of the unfoamed material is 1.78, and it exhibits excellent char formation.
It should be appreciated that other materials, such as a PVDF alloy, may also be suitable for outer jackets 11 or 34. One such alloy that has been employed in a dual jacket embodiment is available from Solvay and identified as SOLEF 70109-X003. The dielectric constant of this material at 1 MHz is 5.20, the dissipation factor at 1 MHz is 0.1250, and the LOI is 85 percent. The specific gravity of this material is 1.64, and its char formation is 1 S excellent. The inventors contemplate that this and other PVDF alloys, including other suitable PVDF materials available from other commercial suppliers, may be foamed in accordance with the present invention.
During manufacturing of the preferred cable construction, an extrusion tool may be employed to ensure that outer jackets 11 and 34 are properly formed to meet physical and electrical requirements. With the exception of the extrusion tool having a die/core tube Land length of one to two inches, such extrusion tools and related processes are known to those skilled in the art and, therefore, need not be described in detail herein. In accordance with an exemplary manufacturing technique, a quench water trough is placed within approximately three inches from the extruder head to thereby quench the tube extruded jacket during draw-down. In addition, air (or another suitable gas) may be injected through the extruder head during draw-down to expand the jackets 11 and 34 and maintain their 326169.6/29144.0800 10 substantially round cross sectional shape throughout the extrusion process.
The use of such air injection prevents the foamed PVDF from collapsing during manufacturing.
In accordance with a first embodiment of the present invention, outer jackets 11 and 34 are formed by a chemical foaming process that utilizes a chemical foaming agent. In one exemplary embodiment, the outer jacket material is formed by introducing a chemical foaming agent to the PVDF (or other suitable material). Such chemical foaming techniques are known to those skilled in the material sciences and cable manufacturing arts. Of course, the specific amount of foaming agent may be varied depending upon the desired electrical and physical characteristics of the end product, the particular manufacturing processes and equipment used, the particular outer jacket material, or other application-specific variables.
In accordance with a second embodiment of the present invention, outer jackets and 34 are formed by gas injection, where the gas injected during the foaming process is preferably nitrogen. Such gas injection processes are known to those skilled in the art and, therefore, are not described in detail herein. In accordance with one exemplary embodiment, the amount of foaming agent/plastic carrier employed to electrically enhance the PVDF
jacket material falls within the range of approximately 1 to 10 percent by weight, and within a preferred range of about 3 to 8 percent by weight.
In accordance with another exemplary embodiment, outer jackets 11 and 34 are foamed to an expansion within the range of 5 to 30 percent, and within a preferred range of about 10 to 20 percent. In the context of this specification, the percent of expansion refers to the change in the specific gravity of the solid versus the foamed outer jacket material. The percent of expansion may be calculated by physically measuring the weight and dimensions of a sample portion of the foamed PVDF outer jacket and comparing the weight to a comparably sized amount of solid PVDF.
In the preferred embodiment, outer jacket 11 or 34 has a thickness within the range of 15 to 40 mils. The foamed PVDF outer jacket 11 or 34 is preferably about 25 mils thick.
In the preferred embodiment, the PVDF outer jacket 11, 34 is foamed from its inner surface 326169.6J29144.0800 11 to its outer surface with small, discrete cells. The uniformity and size of the foam cells suitably enhances the electrical characteristics of cables 5, 11. It should be noted that extrusion tools may be configured to impart a smooth (but not a skin) outer surface to cables 5, I 1. For example, the die tip of an exemplary extrusion tool may be heated to smooth the outer surface of the jacket after it has been foamed. In addition, the die Land length may be configured to suitably impose a higher pressure drop (and correspondingly higher foaming) as the PVDF material exits the die tip. In a preferred tooling embodiment, a die Land length of greater than one inch is utilized.
Those skilled in the art will appreciate that the specific thickness and surface texture of outer jacket 11 or 34 may vary depending upon the particular electrical and/or physical requirements of the cable. For example, the preferred embodiment of the present invention incorporates cores 22 and outer jacket 11 or 34 configured such that electrical performance of the cable is in compliance with TIA/EIA ~68A Category 5 cable standards.
The particular amount of foaming and the specific composition of outer jacket may be suitably selected to ensure that the electrical, physical, and burn characteristics of the cable meet all of the relevant requirements.
It should be appreciated that the use of a single outer jacket may reduce the manufacturing time and costs associated with a Category S cable, e.g., cable 5. The foamed PVDF construction of outer jacket 11 enables cable 5 to pass the required UL
bum tests and the Category 5 electrical tests without the need for an inner or intermediate jacket or a core wrap. Nonetheless, as previously described, in accordance with one aspect of the invention the core can be wrapped with an inner jacket of foamed PVDF material to provide further burn and smoke protection andlor to enhance the electrical performance of the cable.
A number of experimental cables were fabricated utilizing the materials set forth previously for insulation construction and outer cable jackets. The experimental cables which passed the UL-910 plenum burn test at an independent laboratory along with the relevant test data, are set forth in Table 1 below:
326169.6/29144.0800 12 UL-910 Steiner Tunnel Burn Results Foamed PVDF Single Jacket Cable Jacket Cable Thickness Peak Average Flame Construction (mils) Optical Densitx Qptical DensitX read ft (Requirements) (s0.5) (s0.15) (s5 ft) Cable #1 - 4 Pairs 24 Burn 1 0.19 0.07 2.5 Burn 2 0.25 0.07 3.5 Cable #2 - 4 Pairs 22 Burn 1 0.17 0.05 3.5 Burn 2 0.20 0.06 4.0 All of the above listed cables passed the plenum burn test as indicated, and also passed the Category 5 electrical requirements, as well as the UL-444 physical property test requirements.
Although an initial objective in accordance with the present invention focused on developing a cable construction that met the performance of existing cable using FEP
insulation, it has been unexpectedly found that cable constructed in accordance with the principles of this invention actually exceeds the performance of FEP insulated cable. In the prior art, in addition to cables utilizing, for example, four twisted pair, all having FEP
insulation, there have been constructions using a combination of insulation materials. These combination insulation constructions have been aimed at dealing with the shortage of FEP
material relative to the demand for high category cables. For example, one prior art construction utilized a cable containing three twisted pair of FEP insulated conductors with 326169.6/29144.0800 13 one twisted pair of olefin insulated conductors. Another prior art construction utilized a cable containing two twisted pair of FEP insulated conductors, and two twisted pair of olefin conductors.
When plenum cables are subjected to increased temperatures, the electrical characteristics of the cable (e.g., attenuation, structural return loss, and cross-talk) may drift by an undesirable amount. Indeed, Category 5 cables must pass elevated temperature attenuation requirements at 40 ° C and at 60 ° C; in accordance with current standards, the attenuation of Category 5 cables must be less than about 67.0 dB at room temperature, less than about 72.3 dB at 40°C, and less than about 77.7 dB at 60°C.
Although a cable utilizing FEP insulation and a low-smoke PVC jacket may meet these elevated temperature attenuation requirements, it may not remain electrically stable at much higher temperatures, e.g., greater than 100°C.
In accordance with the present invention, outer jackets 11 and 34 enable cables 5 and 10 to exhibit electrical stability (for purposes of performance tests) from room temperature to a temperature exceeding 60°C. In an exemplary embodiment, cables S
and 10 are electrically stable to at least about 121 °C, which is approximately the highest temperature that may be reached within a plenum. For example, although the attenuation of Category 5 cables must be less than about 94 dB at 121 °C, a prototype cable constructed in accordance with the present invention exhibited attenuation less than 70.0 dB at 121 °C. In addition to the enhanced attenuation performance, cables 5 and 10 also meet or exceed the electrical performance requirements associated with structural return loss and cross-talk from room temperature to 121 °C.
In all cables intended for high frequency transmission applications, the velocity of signal propagation (which should be as high as possible) is extremely important, as is the allowable skew. Skew refers to variations among twisted pair in a single cable of the velocity of propagation or other characteristics, and should be as small as possible to minimize data distortion. Table 2 represents the results of measurements of characteristics 326169.6/29144.0800 14 of 4 pair FEP, 3 pair FEP + 1 pair flame-retardant olefin, 2 pair FEP + 2 pair flame-retardant olefin, and 4 pair foam/skin HDPE in accordance with the present invention. In Table 2, the velocity of propagation is expressed in percent of the speed of light, and the delay is expressed in nanoseconds over a 100 meter cable run. The skew percent is determined by the ratio between the worst twisted pair characteristics and the best twisted pair characteristics. The references to BRN, GRN, BLU and ORN, are simply references to particular colors of twisted pair in a standard 4 twisted pair color standard.

Conductor Characteristics Cable Dielectric Velocity of Construction Insulation olor Constant Propagation (%1 ela ns 4 pr. FEP
FEP BRN 1.74 75.80 1.35 FEP GRN 1.76 75.40 1.36 FEP BLU 1.81 74.30 1.36 FEP ORN 1.83 73.90 1.39 Average 1.79 74.90 1.37 Skew 5.20% 2.80% 3.00%

3 pr. FEP
1 pr. Olefin Olefin BRN 1.99 70.90 1.43 FEP GRN 1.84 73.70 1.37 FEP BLU 1.90 72.50 1.39 FEP ORN 1.92 72.20 1.40 Average 1.91 72.30 1.40 Skew 8.20% 3.10% 4.40%

326169.6/29144.0800 1$

Cable Dielectric Velocity of Construction Insulation olor Constant Propagation (%1 Dela ns 2 pr. Olefin Olefin BRN 2.20 67.40 1.52 FEP GRN 1.79 74.70 1.3 8 FEP BLU 1.79 74.70 1.38 Olefin ORN 2.20 67.40 1.52 Average 2.00 71.05 1.45 Skew 22.90% 10.80% 10.10%

4 pr.
foam/skin Z O F/S BRN 1.59 79.20 1.30 F/S GRN 1.61 78.80 1.31 F/S BLU 1.64 77.90 1.32 F/S ORN 1.66 77.50 1.33 Average 1.63 78.35 1.32 Skew 4.40% 2.20% 2.30%
As shown by the above table, the dielectric constant, velocity of propagation, and delay time for cable constructed with foam/skin insulation in accordance with the present invention are all significantly better than FEP-only insulated cable, and vastly superior to those for composite FEP/olefin insulated cables. The skew for the cable of this invention is also significantly better than for FEP-only insulated cable. Such a cable construction is indeed suitable for operation at signal frequencies of 150 MHz or 155 Megabits.
In accordance with the present invention, an improved cable construction is achieved, which is a result of a novel combination of electrical and burn properties of materials.
Specifically, primary insulation of polyolefin, which in a specific example is foamed, such 326 t 69.6/29144.0800 16 as HDPE surrounded by a HDPE skin, is surrounded by a jacket of thermoplastic halogenated polymer, which in a specific example is a foamed PVDF material.
Although the specific examples discussed herein have, for purposes of completeness, included identification of specific suitable materials available from various manufacturers, S equivalent materials available now or hereafter can obviously be substituted with satisfactory results. It is intended, therefore, in the appended claims, to cover not only the specific materials and constructions which have been discussed herein, but also substitution of equivalent materials in the overall cable construction. For example, rather than the HDPE
foamlskin insulation, a polypropylene foam/skin insulation may be utilized to improve the crush resistance and the overall physical robustness of the cable. In addition, the present invention may employ an HDPE skin/foam/skin triple extruded insulation or a polypropylene skin/foam/skin insulation for improved velocity of propagation values.
326169.6/29144.0800 17

Claims (24)

1. The communications cable, comprising:
a core and an outer jacket surrounding said core and formed of a foamed polyvinylidene fluoride.

2. The communications cable of claim 1, wherein the foamed polyvinylidene fluoride is foamed to an amount within the range of 1 to 10 percent by weight.

3. The communications cable of claim 2, wherein the foamed polyvinylidene fluoride is foamed to an expansion within the range of 5 to 30 percent.

4. The communications cable of claim 1, wherein said at least one transmission medium comprises a twisted wire pair, and in which said plastic material serving as primary insulation on each wire in the twisted wire pair is a polyolefin material.

5. The communications cable of claim 4, wherein said polyolefin material is a high density polyethylene.

6. The communications cable of claim 5, wherein said high density polyethylene primary insulation is a foamed/skin composite construction.

7. The communications cable of claim 1, wherein said transmission medium comprises a plurality of twisted wire pairs with plastic material serving as primary insulation surrounding each wire in said twisted wire pairs.

8. The communications cable of claim 7, wherein the outer jacket is made of a thermoplastic fluorocarbon polymer.

9. The communications cable of claim 8, wherein the outer jacket is made of foamed polyvinylidene fluoride.

10. The communications cable of claim 9, wherein the foamed polyvinylidene fluoride is foamed to an amount within the range of 1 to 10 percent by weight.

11. The communications cable of claim 9, wherein the foamed polyvinylidene fluoride is foamed to an expansion within the range of 5 to 30 percent.

12. The communications cable of claim 7, wherein said plastic material serving as primary insulation on each wire in the twisted wire pairs is a polyolefin material.

13. The communications cable of claim 7, wherein said polyolefin material is a high density polyethylene.

14. The communications cable of claim 13, wherein said high density polyethylene primary insulation is a foamed/skin composite construction.

15. A communications cable, comprising:
a core which comprises at least one twisted pair and a polyolefin primary insulation material enclosing said at least one twisted pair; and an outer jacket surrounding said core and formed of a foamed polyvinylidene fluoride material.

16. The communications cable of claim 15, wherein the outer jacket has a thickness within the range of 15 to 40 mils.

17. The communications cable of claim 15, wherein the core and the outer jacket are configured such that electrical performance of the communications cable is in accordance with TIA/EIA 568A Category 5 cable standards.

18. The communications cable of claim 15, wherein the foamed polyvinylidene fluoride material is foamed to an amount within the range of 1 to 10 percent by weight.

19. The communications cable of claim 15, wherein the foamed polyvinylidene fluoride material is foamed to an expansion within the range of 5 to 30 percent.

20. The communications cable of claim 15, wherein the foamed polyvinylidene fluoride material is foamed by gas injection.

21. The communications cable of claim 15, wherein the foamed polyvinylidene fluoride material is chemically foamed with at least one chemical foaming agent.

22. The communications cable of claim 15, wherein the core and the outer jacket are configured such that a number of electrical characteristics of the communications cable are substantially stable to a temperature exceeding 60°C.

23. The communications cable of claim 22, wherein said number of electrical characteristics includes at least one of an attenuation characteristic, a structural return loss characteristic, and a cross-talk characteristic.

24. The communications cable of claim 15, further comprising a shield located within said outer jackets, said shield being configured to enhance electrical performance of said core.