CA1131727A - Conductor insulated with pulp over a cellular layer - Google Patents

Abstract

16.
CONDUCTOR INSULATED WITH PULP OVER A CELLULAR LAYER

Abstract of the Disclosure A pulp-insulated conductor having excellent dielectric properties notwithstanding a substantially reduced diameter includes an electrical conductor enclosed in an insulative cover comprising a pulpous material and a layer having an effectively cellular structure interposed between the electrical conductor and the pulpous material. The pulpous material has a relatively low moisture content and is disposed concentrically about the conductor. The interposed layer includes a coating comprising a material which when covered with the pulpous material is capable of creating an adhesive bond between the pulpous material and the conductor to form an insulative cover having substantial integrity, with the coating having a cellular structure when the pulpous material is adhered to the conductor.

Description

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CONDUCTOR I~lSULATED h~ITH PULP OVER A CELLULAR LAYER

Technical Field This invention relates to a conductor 5 insulated with pulp over a cellular layer, and, more particularly, it relates to an insulated conductor which comprises a conductor, and a composite insulative cover having excellent dielectric properties and comprising an effectively cellular coating and a pulpous material which 10 is adhered to the conductor by the coating to form pulp insulation having substantial integrity.
Background of the Invention One of the principal insulation materials for conductors used in telephone communication systems is 15 a pulpous material which has acceptable dielectric properties at voice and carrier frequencies.
Pulp insulation is low in cost and provides more conductors for a given cable diameter than most other kinds of cable because of its relatively low dielectric 20 constant. Also, it has continuing availability as opposed to the increasing dependence on foreign sources for a petroleum derivative from which plastic insulating materials are made. A further advantaqe of pulp is its ability to absorb moisture which localizes water at a 25 fault point and facilitates the use of routine electrical tests to accurately locate the fault. A detailed description of a pulp-insulating process can be had by referring, for example, to an article "Manufacturing Pulp Cable", on pages 86 ~ 94 of the July ~ October, 1971 issue of The Western Electric Engineer.
One of the problems with pulp insulating is the occurrence of uninsulated areas along the conductors which may occur either because of a lack of adherence of the pulp to the conductors during insulating 35 or because of the abuse to which the insulation is subjected in steps of a cable-making process subsequent to insulating. Since spare pairs of conductors must be provided in a cable to supplant conductors which have ' ~

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such defects and are unusable for telecommunications, the size of the cable cross-section is increased which necessitates additional plastic jacketing material and underground duct capacity. The frequency of uninsulated 5 areas along the conductor may be reduced by increasing the thickness of the pulp insulation, but this alternative undesirably increases the size of the cable.
Another problem occurs when two pulp-insulated conductors, each having a 10 substantially circular cross-section, are associated together to form a twisted pair, with a distance by which their centers are separated having an inversely proportional effect on mutual capacitance. Because the crush resistance of conventional pulp-insulation having a 15 residual moisture content is relatively low, one or both of the conductors may have its insulation deformed when subjected to the rigors of manufacturing processes such as, for example, twisting. This generally causes the distance between conductor centers to be decreased with 20 an accompanying undesirable increase in mutual capacitance. While this problem could be overcome by reducing the residual moisture content, the resulting pulp insulation may have unacceptable flexibility and endurance characteristics.
The aforementioned problems have been overcome by an invention disclosed in a copending Canadian patent application No. 336,439 in which an electrical conductor is enclosed in a coating which is capable of being treated to render the coating 30 substantially insoluble in water, and which when covered with pulpous material is capable of being further treated to create an adhesive bond between the pulpous materia and the conductor.
The above~identified Canadian patent 35 application No. 335,439 provides consistent adhesion of the pulp insulation to the conductor and permits increased drying of the pulp to provide improved mechanical and dielectric properties. However, the , :

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addition of the coating material between the conductor and the pulp increases the effective dielectric constant of the composite insulation, thereby countering the effect of the increased drying. Some further improvement 5 is therefore desirable with respect to efforts which have been made recently to accomodate larger conductor pair cables such as, for example, 3000 pairs, within existing ducts which saves the costs involved in new installation of outside telephone plant. The dielectric properties of J0 the composite insulation would deteriorate if the thickness of the pulp insulation were to be reduced in order to provide an increased number of conductors in a given core size.
Therefore, in order to realize the 15 mechanical advantages of coated, pulp-insulated conductors and at the same time reduce their size, the dielectric constant of the composite insulation must be reduced. A similar problem was faced and solved in the manufacture of plastic insulated conductor cable where a 20 cellular plastic insulation is extruded over the conductor after which a solid plastic material is extruded about the cellular plastic insulation to form an abrasion-resistant skin layer. Since the cellular plastic insulation has a lower dielectric constant than the solid 25 plastic insulation, the wall thickness of the cellular plastic insulation may be reduced over that of a solid plastic insulation for a particular dielectric property.
In pulp-insulating, however, any further reduction in the density of the pulpous material such as, 30 for example by a further reduction in its moisture content or the expansion of the pulpous material itself would degrade its mechanical properties. Prior efforts have been made to decrease the diameter-over~dielectric (DOD) of pulp insulation by an improvement in dielectric 35 properties through the use of pulpous material having plastic fibers introduced thereinto; however, the smooth plastic fibers do not bond well to the ragged pulp fibers which impairs the mechanical qualities of the insulation.

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`"` 1131'7~7 The prior art includes U.S. patents 1,615,422 and

2,440,802, which discloses the use of a binder material interposed between the conductor and the pulp. U.S.
patent 504,397 shows an electric conductor with a molded pulp cover having longitudinal or transverse air spaces formed therein and U.S. patent 3,480,725 shows a conductor provided with an insulating layer comprising pulp and hollow synthetic particles of a thermoplastic material.
Seemingly, the prior art of pulp insulating does not offer a solution to the problem of the provision of improved dielectric properties for pulp insulation in order to be able to decrease the DOD.
Optimally, the advantageous pulp-insulated structure which is disclosed and claimed in the previously identi-fied copending Canadian patent application No. 336,439 is retained while the dielectric properties of the overall pulp insulation are improved so that the electrical pro-perties of the pulp-insulated conductor could be at least maintained while reducing the diameter-over-dielectric.
This must be accomplished, however, without impairing the water-blocking properties and the mechanical properties of the pulp insulation.
Summary of the Invention According to the invention there is provided an insulated conductor, comprising an elongated metallic conductor, and a pulpous material which encloses the electrical conductor, said pulpous material having a relatively low moisture content and being disposed substantially concentrically about the conductor, said insulated conductor characterized by: a coating having an effectively cellular structure, which is interposed in a layer between the conductor and the pulpous material and which covers at least portions of the conductor, the coating comprising a material which when covered with the pulpous material is capable of creating an adhesive bond between the pulpous material and the conductor to form , .

1~317~7 - 4a -an insulative cover having substantial integrity, with the coating having a cellular structure when the pulpous material is adhered to the conductor.
The foregoing problems of optimizing size and di-electric properties are overcome by the pulp-insulated conductor of this invention that includes an elongated metallic conductor, and an effectively cellular conductor coating which is interposed between the conductor and a pulpous material. The pulpous material has a relativeiy low moisture content, and is disposed substantially con-centrically about the conductor. The coating comprises a material which when covered with pulpous material is capable of creating an adhesive bond between the pulpous material and the conductor to form an insulative cover having substantial integrity. The material which is coated over the electrical conductor is capable of being expanded to form, or is applied to effectively create, a cellular .~ .

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structure when the pulpous material is adhered to the conductor and which together with the pulpous material provides a low composite dielectric constant which has a uniform average value along portions of the length of the 5 conductor.
The insulated conductor in accordance with this invention having a composite insulation comprising pulp over an effectively cellular layer has a composite dielectric constant which is less than that of 10 pulp insulation thereby peemitting a reduction in the diameter.over~dielectric of the insulated conductor for the same capacitance characteristics. Alternatively, the diameter~.overrdielectric could be maintained the same as that for a conductor without a cellular layer to provide 15 a low capacitance conductor for high frequency transmission.
Brief Des~ on of the Drawin~s Other features of the present invention will be more readily understood from the following 20 detailed description of specific embodiments thereof when read in conjunction with the accompanying drawings, in which:
FIG. 1 is a cross sectional end view of a pulp-.insulated conductor in accordance with this ~5 invention; and FIG. 2 is a graph showing a plot of dielectric constant versus percent expansion;
FIG. 3 is a perspective view of another embodiment of an expanded layer of the insulated 30 conductor;
FIG. 4 is a cross sectional end view of the conductor shown in FIG. 3 and taken along lines ~4 thereof; and FIG. 5 is a cross~sectional end view of 35 still another embodiment of a pulp-insulated conductor in accordance with this invention.
Detailed Description Referring now to FIG. ], there is shown a ,, :

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pulp~insulated conductor ~0 which includes a copper conductor 11 enclosed in a layer 12 which may be coated over the conductor 1] and an outer pulp insulative cover 13. The layer 12 has an effectively cellular structure 5 SQ that the insulated conductor 10 has superior dielectric properties while the water-blocking and mechanical properties of pulp insulation are retained.
The layer ]2 is comprised of an expandable material that is partially curable, because 10 once fully cured, it would not adhere to or further react with pulpous material which is applied subsequently to the conductor to form the pulp cover 13 and adhere it to the conductor 11. Also, the material which comprises the layer 12 may be one which when partially cured is 15 rendered substantially insoluble in water to prevent its removal from the conductors ]1-~1 as they are advanced through a water pulp slurry. Further, the material of the layer 12 must be such that after it has been initially treated, it is essentially non-tacky so that it 20 does not peel off the conductor 11 when, for example, the conductor is moved into and out of engagement with portions of an apparatus (not shown) for applying the pulp cover 13.
An acrylic adhesive, and, more particularly, 25 an aqueous emulsion of an acrylic latex polymer as dis-closed in the hereinbefore-identified Canadian patent application No. 336,439, may be used for the layer 12. An aqueous emulsion should be specially suitable since it fa~
cilitates the cleaning of the equipment, there is no need 30 to be concerned about other solvents, and it is compatible with the pulp stock system. An ethyl acrylate-methyl acry-late copolymer such as that marketed by Rohm and Haas Com-pany under the designation HA-12 and having a solid content of 45% has been found to be suitable. It is nonionic and 35 cross-linkable with the cross71inking capable of being controlled so that water insolubility can be realized while remaining essentially nonttacky. Another acrylic latex material such as, for example, HAç16 from Rohm and .

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Haas is also acceptable.
A layer of binder material such as the aforementioned acrylic adhesive may be applied to the conductor 11 and partially cured by coating methods and 5 apparatus such as, for example, those disclosed and claimed in detail in the hereinbefore-identified copending Canadian patent application No. 336,439.
The following equations are used to determine the effects of a binder layer on the dielectric 10 properties of a pulp-insulated conductor. First, for a conductor 11 having only a cover of pulp, Ccoax = DOD (1) 15 in which = dielectric constant of pulp insulation, DOD =
diameter~over~dielectric and DW = diameter of conductor 11.
For a conductor 11 having a pulp cover 13 and a layer of binder material interposed between the conductor and the pulp, coax = 24.14 ~ ;
i 1gl0(DW+2t) + olgl0 ~DW ) (2) in which o = dielectric constant of outer layer of pulp, i = dielectric constant of inner layer, and t =
25 thickness of inner layer with DOD and DW designating the same parameters as in equation (]).
For a DOD of 0.07874 cm and a DW =
0.04039 cm and a coaxial capacitance of 157.4 pf/meter, equation (1) yields a dielectric constant ~ of 1.89 for a 30 conductor insulated only with pulp. Since the dielectric constant for a suitable binder material for the layer 12, which in a final state on the conductor 1~ is not effectively cellular, may be in the range of 3.0 to 5.0, the coaxial capacitance will be increased over the 157.4 35 pf/meter. For example, for t = 0.000508 cm, o = ] 89 and i = 3-0~ Ccoax is determined from equation ~2) to be 159.4 pf/meter which is about a 1.32% increase. Should i have a value of 5.0, Ccoax = 161 which is about 2.3%

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higher than that for a pulp only insulated conductor.
Notwithstanding the higher dielectric constant of the unexpanded binder material, the thickness of the pulp insulation can be reduced over that on a 5 conductor not coated with a binder without increasing the mutual capacitance to unacceptable levels. This result is possible because the moisture content of the pulp insulation may be reduced further than that which was possible before the hereinbefore-identified copending 10 Canadian patent application No. 336,439 when drying of the pulpous material without a binder layer resulted in a brittle pulp insulative cover. However, any further reduction in moisture content in order to achieve still further reductions in DOD would lead to a degradation of 15 the mechanical properties.
For maximum conductor pair pulp-insulated conductor cable, an objective, for example, might be to associate together 3600 pairs of 0.40 mm thick pulp~insulated conductors. In an arrangement such as 20 this, it is necessary to achieve a 9% outer cable diameter reduction, for example, going from 8.64 cm to 7.87 cm in order for the cable to be received in an existing 8.89 cm diameter underground duct. With the 9%
diameter reduction, it is hoped to reduce the 25 diameter~over~dielectric of standard pulp insulation of about 0.07874 cm to about 0.07165 cm. In order to increase the number of conductor pairs in a cable so as to be able to utilize existing ducts, a substantial reduction in DOD of the conductors ]0~10 is required 30 while not impairing the dielectric characteristics of the insulation. This optimization of size and properties may be realized by using the layer 12 which at least in a final state has an effectively cellular structure so that its dielectric constant is substantially less than that 35 of the pulp cover 13. This invention which results in a product having a reduced diametersover-dielectric with excellent electrical properties is brought about by using a dual layer of insulation-pulp over an effectively .

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In order to further characterize the material which comprises the layer 12 so that i~ will have a dielectric constant which will be less than that 5 of the pulp cover 13, attention is drawn to the graph shown in FIG. 2. In FIG. 2 are shown p~ots of dielectric constant, ~ , versus percent expansion, %E, for various copolymer binder materials, such as for example, plot 21 which has a relatively high dielectric constant at zero 10 percent expansion down to one designated 23 which has a relatively low dielectric constant at zero percent expansion. A plot designated 22 represents a graph of versus %E for typical plastic insulated conductors in which a percent expansion of 45% yields a dielectric 15 constant of ].64 which is less than that of pulp. In this specification, the term "percent expansion" is interpreted to mean the percent of the cross sectional area that is comprised of voids.
From FIG. 2, it can be seen that the 20 lower the dielectric constant of an unexpanded material i.e. 0% expansion, the lower the percent expansion which is required in order to achieve a reduced dielectric constant. For example, a binder material characterized by the plot 21 having a dielectric constant of about 5 25 must be expanded about 90% in order to result in a material having a dielectric constant of 1.4. On the other hand, a binder material characterized by the plot 23 having a dielectric constant of about 3.0 need only be expanded 80% in order to have final dielectric constant 30 of about 1.4.
Resorting now to the use of equation (2) and the graph of FIG. 2, the values of the dielectric constant ~i and % expansion for the inner layer 12 which are necessary to achieve a DOD of 0.07165 cm with 35 different inner layer thicknesses are determined. For example, using ti = 0.00254 cm, equation (2) yields an ~i of 1.07. Going now to FIG. 2, it can be seen that a binder material which has an original dielectric constant - ~3~Z~

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of 3.0 would have to be expanded about 93% to achieve a dielectric constant of ].07. This same approach is used to determine that for a 0.00508 cm thick binder layer, ~i = 1.35 and requires 68~ expansion; for 0.00762 cm, 5 i = 1.47 with 58% expansion; 0.01016 cm, 1.55 at 52%
expansion; 0.0127 cm, i=1.59 at 48%; and 0.01575 cm, ~i = 1-63 at 45%.
It should be recognized that in the above examples in which the overall diameter of the 10 pulp~insulated conductor is about 0.07165 cm, the diameter of the conductor itself i5 0.04039 cm. With the combined thickness of the binder and the pulp being about 0.0156 cm, the pulp insulation would have a thickness of about 0.01052 cm when the binder layer 12 is about 15 0.005~8 cm.
An optimum result would be the smallest thickness binder layer 12 with the greatest percent expansion. It would seem then that the overall dielectric properties of the pulp insulation may be 20 improved while adhering the pulp insulation to the conductor by using an acrylic material which has a maximum dielectric constant of 2 to 3, and one which is capable of being expanded in a range of from about 68 to 80% in a layer 12 about 0.0050~ cm thick.
Advantageously, the introduction of another parameter, i.e. percent expansion, to characterize the pulp insulated conductor permits an optimization in different ways in order to accomplish different ob~ectives. For example, the effectively 30 cellular layer 12 permits a reduction in DOD of the insulated conductor with constant capacitance characteristics. In the alternative, an insulated conductor 10 in accordance with this invention could include the effectively cellular layer 12 with an 35 unchanged thickness pulp cover 13, in order to improve the transmission characteristics of the insulated conductor at higher fre~uencies.
The expansion of the material of the :

1~3~'7Z7 layer 12 may be accomplished in any one of several ways such as, for example, by an expansion technique with the injection of air or gas or the use of chemical blowing agents such as mixing the binder material with an expan-sion medium, such as, for example, azo-dicarbonamide. The expanding medium may be one which decomposes under heat during the partial curing of the coating which comprises the layer 12 which creates nucleating sites by releasing gas and heat and releases additional gas to cause cells to grow in size.
Pulp-slurry is formed into ribbons and the conductors 11-11 having a partially cured coating thereon are guided so that one conductor is embedded in the center of each ribbon as formed. After forming the wet, paper-pulp ribbons with the adhesively coated conductors embedded therein, the moisture content of each of the pulp ribbons is reduced to about 70%. Then a wet ribbon is turned around each of the conductors to form the cover 13 of pulp insulation so that the conductor 11 is enclosed within superimposed concentric layers of cellular material and pulp, respectively. The wet insulation is dried to a final moisture content of about 3 to 6% and the cover 13 is further treated by passing the pulp-insulated conductor 10 through an atmosphere having an elevated temperature which is in the range of about 540 C to about 680 C. An apparatus for drying pulp insulated conductors is shown in U.S. patent 3,829,985.
The exposure for a predetermined time to the elevated temperatures tackifies the layer 12 and permits pulpous material to penetrate into and become adhered thereto.
This results in a pulp-insulated conductor 10 in which the interface between the initial latex adhesive layer 12 and the pulp layer 13 may not be as well defined as that shown in FIG. 1. Advantageously, the pulp-insulated conductor 10 has greatly improved crush resistance and flexibility endurance thereby HEA~D-7 ~3~727 assuring substantial insulation integrity and avoiding the formation of bare wire during the subsequent processes to which the pulp-insulation is subjected.
The final pulp-insulated conductor 10 5 effectively has a dual insulation - an intermediate expanded layer 12 covered by pulp. The expanding medium for the coating of the layer 12 may be one which is activated during the drying of the pulp insulation rather than during the partial drying of the coating before the 10 pulp is applied. The dual insulation of thisinvention has superior adhesion to the conductor 11 while the moisture content can be further reduced to the 3 to 6% range which prevents compacting of the insulation during twisting.
The capability of maintaining suitable distances between 15 centers of conductor pairs and the desired lowering effect on the dielectric constant by the expansion of the layer 12 results in a superior product. It should be noted that the composite dielectric constant for the insulated conductor of this invention has a uniform 20 average value along portions of the length o the conductor. The portions of the length are many times greater than the DOD and vary according to the relative size of the cells in the effectively cellular layer 12.
Referring now to FIGS. 3 and 4, there is 25 shown another embodiment of this invention which includes a conductor 21 being covered with an intermediate layer 32 of a material in such a way as to cause openings 33-33 to be formed thereon and spaced accordingly along the conductor 1l. This may be accomplished by an applicator 30 (not shown) having facilities for applying a coating on the conductor 21 in such a way that openings of a random size are produced randomly along the length and about the periphery of the conductor. This produces an effectively expanded or cellular layer 32 and it together with the 35 conductor 21 are embedded in a ribbon of pulpous material which is thenformed about the conductor and treated as hereinbefore described.
A still further embodiment is shown in . .
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FIG. 5 and includes a conductor 41 having stripes 42~42 formed longitudinally along the conductor and spaced about its periphery. The material which comprises the stripes may be that disclosed in copending Canadian S patent application No. 336,439 and is partially cured~
enclosed with a pulpous material 44 and then further dried. The result is a pulp-insulated conductor having the structure shown in FIG. 5 with an effectively cellular structure comprising longitudinal cells 43-43 10 created between the conductor and the pulp insulation.

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Claims (10)

Claims:

1. An insulated conductor, comprising an elongated metallic conductor, and a pulpous material which encloses the electrical conductor, said pulpous material having a relatively low moisture content and being disposed sub-stantially concentrically about the conductor, said insulated conductor characterized by:
a coating having an effectively cellular structure, which is interposed in a layer between the conductor and the pulpous material and which covers at least portions of the conductor, the coating comprising a material which when covered with the pulpous material is capable of creat-ing an adhesive bond between the pulpous material and the conductor to form an insulative cover having substantial integrity, with the coating having a cellular structure when the pulpous material is adhered to the conductor.

2. The insulated conductor in accordance with claim 1, characterized in that the interposed layer is comprised of an expanded material.

3. The insulated conductor in accordance with claim 2, characterized in that the material in the interposed layer has a percent expansion which is in the range of about 50 to 90%.

4. The insulated conductor in accordance with any of claims 1-3, characterized in that the interposed layer is comprised of stripes of material spaced about the periphery of the conductor and extending longitudinally therewith.

5. The insulated conductor in accordance with any of claims 1-3, characterized in that the thickness of the interposed layer is less than one-half the combined thick-ness of the interposed layer and the pulpous material.

6. The insulated conductor in accordance with claims 1-3, characterized in that the cellular structure of the interposed layer comprises a plurality of cells formed longitudinally along the conductor and opening to the pulpous material.

7. The insulated conductor in accordance with claims 1-3, characterized in that the dielectric constant of the in-terposed layer is substantially less than the dielectric constant of pulp insulation to permit a reduction in the diameter-over-dielectric of the insulated conductor while maintaining the capacitance characteristics of the insu-lated conductor.

8. The insulated conductor in accordance with claims 1-3, characterized in that the dielectric constant of the interposed layer is substantially less than that of the pulp insulation to provide a low capacitance conductor for high frequency transmission.

9. A communications cable, said cable comprising a plurality of individually insulated conductors each of which comprises:
an elongated metallic wire-like conductor; and an insulation cover concentrically disposed about and in engagement with said metallic conductor, said cover having a thickness which is substantially less than the thickness of a pulpous insulation material which has a coaxial capacitance equal to that of said insulation cover, said cover comprising:
a pulpous material which encloses said wire-like conductor; and a layer of an adhesive-like material having an effectively cellular structure which is interposed be-tween said conductor and said pulpous material and which covers at least portions of said conductor for adhering said pulpous material to said conductor, said layer com-prising a material having a cellular structure in which the percent voids is in the range of about 50% to 90%.

10. The cable of claim 9, wherein the adhesive-like layer is characterized by a thickness of about 0.005 cm and a percent voids of about 70.