Filled Electrical Cable

Nearly all distribution cable now being installed in domestic telephone systems is buried beneath the ground. This represents a large investment in cable and the reliability of that cable as measured by maintenance costs and service continuity is an important factor. Most of the cable is waterproofed, because the most antagonistic environmental factor for buried cable is water.

Attempts to waterproof buried cable began nearly one hundred years ago and were unsuccessful in a practical sense until the introduction of plastic insulated cable (PIC) during the 1950's. Specially sheathed cables, with dual plastic coatings encasing an aluminum sheath have been used successfully and are still being buried in dry environments. Pressurized cable also contends successfully with water problems. However, both of these approaches are deficient, the former leaves the cable vulnerable and the latter is expensive to maintain. Since 1970, large quantities of cable have been filled with waterproofing compounds. This approach followed the recognition that in PIC cable the localized intrusion of water into the cable sheath is not in itself a serious problem. Disruption or deterioration of service occurs when long lengths of cable become flooded. Flooding occurs because water that penetrates into a localized opening in the cable sheath is free to channel as far as gravity allows, often hundreds of feet. Not only does this upset the capacitance balance of the transmission lines, but it introduces more potential corrosion sites in proportion to the length of wire that is wetted. Corrosion typically occurs slowly, but the useful life of water soaked wires is obviously shorter than that of dry wires.

The solution that has been widely adopted is to fill the voids in the cable with an insoluble filling material that simply plugs the cable to channeling water. However, though the physical function of the cable filling material is straightforward, the choice of the material is not. Among the many considerations that are important for materials used in this application are the hydrophobic nature of the material, stability on aging, low temperature properties, flow characteristics at elevated temperatures, processing characteristics, handling characteristics, dielectric properties, toxicity and cost.

Materials that satisfy most of these criteria, and which have been used widely in this application, are described in U.S. Pat. Nos. 3,607,487 and 3,717,716 issued Sept. 21, 1971 and Feb. 20, 1973 respectively. These materials are essentially a petroleum jelly, mixed with a polymer, usually polyethylene, to impart consistency and prevent flowing at warm temperatures.

Similar hydrophobic filling materials have been proposed for filling splice closures. For example, U.S. Pat. No. 3,879,575 issued Apr. 22, 1975 describes a mixture of a low viscosity oil gelled by a styrene-isoprene-styrene copolymer, again with polyethylene added to impart consistency and reduce slump.

This invention is essentially an improvement in the petroleum jelly-polyethylene cable filling material. That material has been found objectionable because of its handling characteristics. Installation and maintenance of cable requires the cable to be spliced. Distribution cables may contain several hundred wire pairs. Each wire must be isolated and spliced to another wire. When splicing cables that are filled with the petroleum jelly-polyethylene material, operating personnel are unable to avoid soiling hands, equipment and clothing. Moreover, removing just sufficient material from the wires to effect the splice is time consuming and the task is generally undesirable. Handling at low temperatures is significantly more difficult necessitating, on occasion, use of a torch to preheat the cable or solvents to soften the encapsulated core. Although from a technology standpoint the problem is pedestrian, from an economic standpoint, recognizing that the cost of telephone installation and repair is labor intensive, the acceptability and ease of the splicing task is a significant matter.

The improved material according to the invention is a mixture of: (1) ASTM Type 104A (napthenic) oil having a minimum sp. gr. of 0.860, a minimum SUS viscosity at 210 degrees F. of 45, a maximum pour point ASTM D 97 of 10 degrees F. and a maximum of 5 percent aromatic oils, (2) a styrene-ethylene butylene-styrene block copolymer having a styrene-rubber ratio of approximately 0.4 and (3) polyethylene having a softening point of 110 degrees C. to 130 degrees C. The amounts of these ingredients have been critically formulated in the proportions described below to give a cable filling material that meets the functional requirements of the cabling technology and has handling characteristics superior to those of the prior art materials. Other styrene block copolymers with a saturated midblock, such as hydrogenated polybutadiene, are also satisfactory.

The FIGURE is a ternary compositional diagram giving the composition ranges of the cable filling material of the invention.

Various oil-polymer mixtures were formulated in arriving at the ingredients described above and the preferred proportions. The preferred proportions are indicated in the shaded area bounded by lines ABCDEF of the FIGURE. Some of the test data used to arrive at this preferred compositional prescription are given in the following Table:

The compositions were evaluated in terms of three major considerations. The viscosity measurement indicates the processability of the material. Cables are filled by injecting the filling material into the voids between the wire pairs after the pairs are twisted. Therefore it is important that the material have a proper viscosity. The filling process involves elevated temperature. From the standpoint of the processing equipment and the effectiveness of the filling process it is more desirable to lower the viscosity of the filling material than to raise the temperature. The operating temperature is limited to the vicinity of 130 degrees C. by the insulation commonly used. Therefore further variation is obtained by choice of the composition. A maximum of 100 cps at 130 degrees C. has been imposed on the composition for acceptable processing. The second criterion appearing in the table is the slump characteristics after two hours exposure to three temperatures, 50 degrees, 60 degrees and 70 degrees C. This measures the retention of the filling material in an acceptably rigid state at elevated service temperatures. Mechanical data is also presented for some examples. The mechanical properties indicated were found to be adequate in nearly every case although it is preferred that the composition have a tear strength of at least 0.15 lb/in, a tensile strength at break of at least 1.15 psi and a percent elongation of at least 90.

The mechanical characteristics of the materials can be summarized in a subjective manner that is perhaps more meaningful. The prior art petroleum jelly material is a grease-like substance whereas the materials described here have a consistency resembling a soft gum eraser.

An important physical property of the material, its handleability, was discussed earlier. This property was evaluated subjectively and was one basis for choosing the styrene ethylene-butylene-styrene block copolymer over a styrene-isoprene-styrene copolymer. The latter gives a tacky compound (example 16). Another is flow at elevated temperatures and is the basis for choosing ID composition limits. The compositions of Examples 7, 9, and 10 were especially desirable from a slump standpoint. The compositions of Examples 8 and 11 were considered just marginally adequate while the composition of Example 12 was considered inadequate.

Examples 13-15 were tests to determine the limitations on the polyethylene constituent. All of these compositions contained polyethylene with a softening point outside the range of 110 degrees-130 degrees C. (ASTM E-28) and slumped excessively.

Various additional modifications and extensions of this invention will become apparent to those skilled in the art. All such variations and deviations which basically rely on the teachings through which this invention has advanced the art are properly considered to be within the spirit and scope of this invention.