Polytetrafluoroethylene multiconductor cable and process for manufacture thereof

A mutliconductor cable having separatable insulated conductors is provided having a plurality of insulated conductors spaced apart in planar relationship and at least the exterior layer is comprised of porous PTFE, wherein a contact area exists between adjacent exterior layers so that adjacent insulated conductors bond with each other. A multiconductor cable is also described comprising a plurality of conductors insulated in discrete clusters wherein each cluster is surrounded by an exterior layer of expanded porous PTFE which forms a bond with the adjacent exterior layer. A multiconductor cable comprising coaxial cables which are bonded together by the exterior layer of expanded porous PTFE is also described. A process of making the multiconductor cable is also described.

FIELD OF THE INVENTION 
This invention relates to an improved polytetrafluoroethylene 
multiconductor cable and the manufacture thereof. 
BACKGROUND OF THE INVENTION 
During the past 40 years, the development of increasingly sophisticated 
computer hardware has led to a need for improved electronic cables capable 
of carrying more signals at high frequencies and at lower signal levels 
than ever before. In addition, increased sophistication of automated test 
equipment, aircraft, weapon systems, and telecommunications equipment such 
as satellites have increased the demand for high signal density 
multiconductor cable that is increasingly miniaturized and at the same 
time lighter in weight and capable of withstanding extreme temperature 
variations. 
In these sophisticated systems, polytetrafluoroethylene (hereinafter PTFE) 
is preferred as an insulation because of its excellent dielectric 
properties, and thermal and chemical resistance. PTFE is also desirable as 
an insulation because of its ability to maintain mechanical integrity over 
a long period of time. 
Typically the manufacture of flat multiconductor or ribbon cables require 
lamination with films, woven threads, or thermoplastic adhesives such as 
FEP (a copolymer of tetrafluoroethylene and hexafluoropropylene) to hold 
the multiconductors together and maintain their position in a flat plane. 
Multiconductor flat or ribbon cable having at least one laminating film of 
PTFE insulation is known in the art. For example, U.S. Pat. No. 4,000,348 
discloses a process to make flat multiconductor cable involving the 
lamination of fluorocarbon and similar high temperature resins. In 
addition, the patent describes a process for bonding and sintering 
unsintered extruded polytetrafluoroethylene (PTFE) containing a 
multiconductor cable with thermoplastic fluorocarbon resins and in turn to 
other materials including PTFE. 
Alternatively, U.S. Pat. No. 4,443,657 discloses a cable construction 
having a plurality of conductors spaced apart in a planar relationship; a 
plurality of inner layers each surrounding one of the conductors, the 
inner layers formed of porous substantially unsintered 
polytetrafluoroethylene; a plurality of outer layers each substantially 
surrounding one of the inner layers, the outer layer being formed of 
impermeable sintered PTFE; and a plurality of webs, each connecting an 
outer layer to an adjacent outer layer, each of the webs being formed of 
impermeable sintered PTFE. 
These products have limited functionality because of the limited ability to 
route individual conductors. The tapes, adhesives, and webs necessary to 
hold the multiconductor cable together also increase the weight of the 
cable as well as limit the functionality of the cable to certain 
temperature ranges where the bonding properties degrade. 
There is a need for multiconductor cable that is capable of functioning 
over a wide temperature range and that is also lighter weight standard 
PTFE ribbon cable. There is also a need for a multiconductor cable 
construction where access to individual conductors is easily gained 
without disruption of the remaining conductors of the cable. Finally, it 
is also desired that previously achieved benefits from improvements in 
multiconductor cable construction such as flexibility and electrical 
performance are maintained. 
SUMMARY OF THE INVENTION 
A multiconductor cable is described having a plurality of conductors spaced 
apart in planar relationship and at least one layer of porous PTFE 
surrounding each of said conductors as an exterior layer wherein a contact 
area exists between adjacent exterior layers in which the porous PTFE 
unites with the porous PTFE of the adjacent exterior layer. The individual 
conductors may also have several layers of insulation surrounding the 
conductor and a final exterior layer of porous PTFE then surrounds the 
insulation. The conductors may be any electrically conductive material, 
and/or electromagnetic signal transmission fibers. The individual 
conductors may be color coded. 
Alternatively, additional embodiments include a multiconductor cable having 
a plurality of conductors insulated in discrete clusters so that each 
cluster has two or more conductors surrounded by either an exterior layer 
of expanded porous PTFE or a first layer of insulation and then an 
exterior layer of expanded porous PTFE. At least one of the individual 
conductors comprising each cluster should be insulated prior to the 
exterior layer of expanded porous PTFE being applied. Additionally, the 
conductors of a cluster may be twisted together before the exterior layer 
is applied. Multiple clusters are then united by bonding the exterior 
layers of expanded porous PTFE together. 
Another embodiment includes a multiconductor cable comprising a plurality 
of coaxial cables in which each coaxial cable comprises a conductor 
surrounded by a dielectric insulating material, a second conductor 
surrounding the dielectric material and surrounding the second conductor, 
either an exterior layer of expanded porous PTFE or one or more insulating 
layers then surrounding by expanded porous PTFE. Once again, the coaxial 
cables are united by a bond formed between the exterior layers of expanded 
porous PTFE of adjacent cables. 
A process to make multiconductor cable is also described having the steps 
of individually surrounding a plurality of conductors with an exterior 
layer of porous PTFE; aligning said conductors in parallel; pulling said 
parallel conductors over a curved shoe having a concave groove so that the 
conductors migrate towards the center of the groove wherein exterior 
layers of adjacent conductors are forced in contact with each other and 
are simultaneously heat treated to at least the crystalline melt point of 
the exterior layer so that a bond forms. 
Processes to make the clustered multiconductor cable and multiconductor 
coaxial cable are also provided.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The cable of the present invention provides for a plurality of conductors, 
each conductor insulated or surrounded by a least one outer layer of 
porous polytetrafluoroethylene (PTFE) preferably porous expanded PTFE and 
aligned in a planar relationship to form a flat multiconductor cable. The 
individual conductors with layers of porous PTFE surrounding them are 
attached to one another by bonds formed between adjacent layers of porous 
PTFE. The bonds formed between the porous PTFE layers thus eliminates the 
need for use of adhesives or tapes to bond individual conductors and 
insulation together. Alternatively, clusters of two or more conductors or 
coaxial cables may be used instead of single conductors. 
The resulting cables are lighter than conventional cables. The individual 
conductors of the cable are also easily separated for stripping, 
termination and routing. Because no adhesives are used, the inventive 
cable is not limited to temperature ranges often required for cables where 
adhesives are used. The multiconductor cable may also be color-coded so 
that individual conductors may be surrounded by porous PTFE containing a 
pigment. The cable also maintains desirable signal transmitting 
properties. 
The bond strength between the individual conductors depends on the contract 
area between the conductors. The contact are is the space where the porous 
PTFE surrounding adjacent conductors are in intimate contact and adhere 
together along the full length of the conductors. As can be seen from FIG. 
1, the contact area is shown by the contact plane "y" and the length of 
the insulated conductor "L". 
The contact area is proportional to the size of the wire and is preferably 
constructed to have dimensions which fall within the following range: 
EQU 10%.ltoreq.y/D.ltoreq.70% preferably 20%.ltoreq.y/D.ltoreq.30% 
where y is the contact plane and D is the overall diameter of the insulated 
conductor. Both parameters are shown in the cross-sectional view of the 
multiconductor cable depicted in FIG. 2. Typically, values of contact "y" 
less than 0.005 inches fail to create a strong enough bond to hold the 
wires together. Values greater than 30%, result in geometries that are not 
concentric thus possibly creating stripping problems for an individual 
wire. 
The bond strength between adjoining insulated conductors varies 
proportionally with the contact area. The bonding between adjacent 
conductors may be intermittent or continuous. In general, the greater the 
contact area, the greater the bond strength. The bond strength must be 
strong enough to hold the insulated conductors in their planar position 
during bonding, flexing and other types of handling. The bond strength 
need not be excessively high as easy separation of individual insulated 
conductors from the cable is desirable. 
The electrical characteristics of the inventive multiconductor cable remain 
consistent and perform similarly to that of flat ribbon cable. In 
addition, geometric positioning of conductors enables a wide range of 
transmission line properties such as impedance and capacitance to be 
achieved with the inventive cable. For example, conventional cables with 
webs, result in a characteristic impedance of about 125 ohms. The 
inventive cable allows the impedance to be reduced to 100 ohms because of 
the closer spacing between insulated conductors. 
In the final construction of the multiconductor cable, the density of the 
porous PTFE in the contact are may increase to greater than the initial 
density. 
The resulting cable may also be twisted using two or more parallel bonded 
conductors to form a twisted assembly with matched physical and electrical 
properties. 
FIG. 9 shows an illustration of a twisted assembly in which two insulated 
conductors 10, covered by a first layer of insulation 11 and then by an 
exterior layer of porous expanded PTFE 12 are bonded together and then 
twisted to form the assembly. 
In accordance with the present invention, the multiconductor cable includes 
a plurality of conductors spaced apart in planar relationship. FIG. 2 
shows a cross-section view of the multiconductor cable 1 that has a 
plurality of conductors 10 (four being shown in FIG. 2) positioned in 
parallel side-by-side relationship. The conductors 10 depicted in FIG. 2 
are single strand electrical signal carrying bare copper wire. Other 
conductor types, such as silver plated copper, nickel plated copper or 
other high temperature metals and electromagnetic signal transmission 
fibers of glass or high temperature plastic may be used and are considered 
within the scope of the present invention. 
Further in accordance with the present invention the improved 
multiconductor cable includes at least one layer of porous 
polytetrafluoroethylene (PTFE) 12 surrounding or insulating individual 
conductors 10 as its outermost layer. The porous PTFE 12 especially 
suitable for use in Cable 1 is porous expanded PTFE that has been produced 
by the process described in U.S. Pat. No. 3,953,566 and has properties 
described in U.S. Pat. No. 4,187,390. Preferably, the layer of porous PTFE 
used in the construction has not been heat treated to above its 
crystalline melt point (i.e. unsintered). 
Alternatively, and with particular reference to FIG. 1, the multiconductor 
cable may be comprised of individual conductors 10 that are first 
surrounded by conventional types of high temperature insulation 11 such as 
PTFE, porous PTFE, polyimide (Kapton.RTM.), polyetheretherketone, or 
polyimidesiloxane and then covered by an exterior layer of porous PTFE 12 
similar to that described above. The key to the invention is that the 
outermost (or exterior) layer of the individually insulated conductors be 
comprised of porous PTFE that has preferably not been heat treated. 
Further, in accordance with the present invention, and with particular 
reference to FIGS. 1 and 3, the method of fabricating the improved 
multiconductor cable includes first insulating the bare wires with desired 
layers of insulation of choice. The final, exterior layer of porous PTFE 
is then applied to the conductor using conventional technology and is 
preferably wrapped around the conductor. The insulated conductors are then 
aligned in parallel relationship to one another in area 19 (FIG. 3). The 
aligned conductors are simultaneously pulled with uniform tension over a 
shoe having a concave groove. The process can be seen schematically in 
FIG. 3 where the parallel wrapped conductors 15-18 are pulled over a shoe 
20. 
The shoe 20, of which a perspective is shown in FIG. 4 has a concave region 
or groove 22 on the exterior side of the shoe over which the conductors 
pass. The groove causes the insulated conductors to migrate towards each 
other. FIG. 5 shows an end view of the shoe. 
As the plurality of conductors are pulled over the concave shoe, the 
conductors merge within the concave region towards each other so that the 
exterior surfaces of adjacent insulated conductors contact and compress 
against each other. 
In the particular embodiment illustrated in FIG. 3, there are two shoes in 
present in a salt bath 21 heated to a temperature preferably above 
327.degree. C. It is not necessary that two shoes be provided. As the 
insulated conductors are contacted and compressed against each other, the 
exterior layers of porous PTFE are simultaneously heated to above the 
crystalline melt point of the exterior layer of insulation thereby 
coalescing the layers to form bonds between adjacent insulated conductors. 
Although a salt bath is illustrated as the means for heating the porous 
PTFE, other heating techniques are also suitable. 
FIG. 10 shows a cross-sectional view of the multiconductor cable 30 having 
a plurality of clustered conductors (four being shown in FIG. 10) 
positioned in parallel side by side relationship. The individual clusters 
32 shown in FIG. 10 each comprises a pair of twisted conductors 34, 
further in each conductor 35 is insulated with at least one layer of 
insulation 36. Alternatively, but not shown, a cluster may contain more 
than two conductors and in fact a preferable embodiment includes the 
clusters each having three twisted conductors. As previously described, 
the individual conductors comprising each cluster may be each covered with 
a layer of insulation 36 prior to the application of the exterior layer of 
expanded porous PTFE. Other embodiments but not shown may include clusters 
where only one of two conductors are covered with a layer of insulation. 
FIG. 10 also shows that the individual clusters are each surrounded by an 
exterior layer of expanded porous polytetrafluoroethylene 38. Although not 
shown, additional layers of insulation may first surround each cluster 
before the outermost layer of expanded porous PTFE is applied. Although 
somewhat exaggerated in FIG. 10, small air gaps 39 may be formed when the 
exterior layers of expanded porous PTFE or plurality of layers of 
insulation including the exterior layer of expanded porous PTFE is applied 
around the twisted pair cluster as the layer(s) maintain a circumference 
around the cluster and do not sag or conform to the exact shape of the 
twists. 
FIG. 11 shows a cross-sectional view of a multiconductor cable 40 having a 
plurality of coaxial cables (four being shown in FIG. 11) positioned in 
parallel side by side relationship. Each of the individual coaxial cables 
41 shown in FIG. 11 is comprised of a center conductor 42, a layer of high 
temperature dielectric material 44, a conductive shield 46 and an exterior 
layer of expanded porous polytetrafluoroethylene 48. The high temperature 
dielectric material 44 includes materials selected from the group 
consisting of polytetrafluoroethylene, polyimide (Kapton.RTM.), 
polyetheretherketone, polyimidesiloxane and preferably expanded porous 
polytetrafluoroethylene sold under the trademark GORE-TEX.RTM., 
commercially available from W. L. Gore & Associates, Inc. of Newark, Del. 
The conductive shield 46 may be selected from the group consisting of 
braided wires (woven), served wires (non-woven) that are helically-wrapped 
about the dielectric material, and a foil with or without a drain wire. 
Similar to the other embodiments, the exterior layer of expanded porous 
polytetrafluoroethylene serves as a "bonding agent" which forms a bond 
with the adjacent cables thereby forming a multiconductor coaxial cable. 
In the following examples, calculations were made to derive expected 
physical and electrical parameters in addition to actual measurements 
taken on the cables after construction. The following equations were used 
to calculate the specified parameters: 
Center Spacing=diameter of conductor+2 (.SIGMA. thickness of wall layers). 
Bond Thickness=0.25.times.center spacing. 
Bond Strength=(bond thickness).sup.2 .times.0.064 oz./mil..sup.2 
.times.[1.2 g/cc/2.15 g/cc.] 
Impedance=276/.sqroot.dielectric constant.times.log [2.times.center 
spacing/(0.97.times.conductor diameter)] 
Capacitance=1016.times..sqroot.dielectric constant/Impedance 
The following examples are illustrative only and are not intended to limit 
the scope of the present invention in any way. 
EXAMPLE 1 
Multiconductor Cable with AWG 24(19/36) Conductors 
Five AWG 24(19/36) silver plated copper conductors were used for this 
example. Each conductor was preinsulated with 0.006 inches of PTFE and 
then each helically wrapped with a layer of unsintered expanded PTFE. The 
wires were pulled side by side over a concave shoe in a salt bath at a 
temperature of above 370.degree.0 C. Adjacent wires were forced together 
due to the combined effects of their tensions and the profile of the shoe. 
Transverse forces caused the layers of expanded PTFE of adjacent wires to 
compress, changing the geometry of the contact area from a line to a 
plane. Simultaneously, the heat of the salt bath caused the expanded PTFE 
of adjacent wires to coalesce and form a bond strength necessary to keep 
the wires held together in a single plane. A continuous transverse force 
was provided to the assembly during the heat treatment to ensure a large 
contact area between wires and to compensate for any shrinkage of the 
expanded PTFE. The left side of FIG. 6 is a photo micrograph of a 
cross-section of two adjacent wires in the cable assembly taken at 
50.times. magnification. The photo on the right (FIG. 6) shows a close-up 
(at 500.times. magnification) of the contact area designated by the 
rectangle shown on the left photo. 
The following physical and electrical properties were both estimated by 
calculation and actually measured: 
______________________________________ 
Wire Description (single wire measurement - 
5 wires in cable) 
______________________________________ 
25 mil diameter conductor 
6 mil wall of PTFE 
1.6 mil wall of expanded porous PTFE 
______________________________________ 
Physical Properties 
Expected Actual 
______________________________________ 
Center Spacing (mils) 
40.2 39.8 +/- 2.0 
Bond Thickness "y" (mils) 
10.1 10.6.sup.a +/- 3.0 
Bond Strength (oz.) 
3.6 3.0.sup.b =/+ 0.5 
______________________________________ 
Electrical Properties 
Expected Actual 
______________________________________ 
Impedance (ohms) 114 100 
Capacitance (pf/ft) 
11 10.5 
Dielectric Constant 
1.6 1.5 
______________________________________ 
.sup.a Actual Bond Thickness measurements were made with an optical 
microscope at 50 X magnification equipped with an xy table and a digital 
readout. Thirtytwo measurements were taken and averaged to determine bond 
thickness. 
.sup.b Actual Bond Strength measurements were taken with a spring scale, 
in which a force was applied to separate a single conductor from the 
remaining assembly. 
EXAMPLE 2 
Multiconductor Cable with AWG 30(19/42) Conductors 
Twelve AWG 30(19/42) silver plated copper conductors were used for this 
example. Each conductor was preinsulated with 0.006 inches of PTFE and 
then each helically wrapped with a layer of unsintered expanded PTFE. The 
wires were pulled side by side over a concave shoe in a salt bath at a 
temperature of above 370.degree. C. Adjacent wires were forced together 
due to the combined effects of their tensions and the profile of the shoe. 
Transverse forces caused the layers of expanded PTFE of adjacent wires to 
compress, changing the geometry of the contact area from a line to a 
plane. Simultaneously, the heat of the salt bath caused the expanded PTFE 
of adjacent wires to coalesce and form a bond strength necessary to keep 
the wires held together in a single plane. A continuous transverse force 
was provided to the assembly during heat treatment to ensure a large 
contact area between wires and to compensate for any shrinkage of the 
expanded PTFE. 
The left side of FIG. 7 is a photomicrograph of a cross-section of two 
adjacent wires in the cable assembly taken at 100.times. magnification. 
The photo on the right (FIG. 7) shows a close-up (at 1000.times. 
magnification) of the contact area designated by the rectangle shown on 
the left photo. 
The following physical and electrical properties were both estimated by 
calculation and actually measured: 
______________________________________ 
Wire Description (single wire measurement - 
12 wires in cable) 
______________________________________ 
13 mil diameter conductor 
6 mil wall of PTFE 
1.6 mil wall of expanded porous PTFE 
______________________________________ 
Physical Properties 
Expected Actual 
______________________________________ 
Center Spacing (mils) 
28.2 27.2 +/- 1.0 
Bond Thickness "y" (mils) 
7.1 6.5.sup.a +/- 2.0 
Bond Strength (oz.) 
1.8 1.0.sup.b +/- 0.25 
______________________________________ 
Electrical Properties 
Expected Actual 
______________________________________ 
Impedance (ohms) 142 130 
Capacitance (pf/ft) 
9 8 
Dielectric Constant 
1.6 1.5 
______________________________________ 
.sup.a Actual Bond Thickness measurements were made with an optical 
microscope at 50 X magnification equipped with an xy table and a digital 
readout. Twentyone measurements were taken and averaged to determine bond 
thickness. 
.sup.b Actual Bond Strength measurements were taken with a spring scale, 
in which a force was applied to separate a single conductor from the 
remaining assembly. 
EXAMPLE 13 
Multiconductor Cable with AWG 26(7/34) Conductors 
Expanded PTFE as preinsulation 
Six AWG 26(7/34) silver plated copper conductors were used for this 
example. Each conductor was preinsulated with 0.015 inches of expanded 
PTFE and then each helically wrapped with a layer of unsintered expanded 
PTFE. The wires were pulled side by side over a concave shoe in a salt 
bath at a temperature of above 370.degree. C. Adjacent wires were forced 
together due to the combined effects of their tension and the profile of 
the shoe. The transverse force caused the layer of expanded PTFE of 
adjacent wires to compress, changing the geometry of the contact area from 
a line to a plane. Simultaneously, the heat of the salt bath caused the 
expanded PTFE of adjacent wires to coalesce and form a bond strength 
necessary to keep wires held together in a single plane. A continuous 
transverse force was provided to the assembly during heat treatment to 
ensure a large contact area between wires and to compensate for the 
shrinkage of the expanded PTFE. 
FIG. 8 is a photomicrograph showing a cross-section of two adjacent wires 
in the cable assembly (left side) taken at 50.times.0 and a close-up of 
the contact area (right side) at 500.times. magnification designated by 
the rectangle shown on the left photo. 
The following physical and electrical properties were both estimated by 
calculation and actually measured: 
______________________________________ 
Wire Description (single wire measurement - 
6 wires in cable) 
______________________________________ 
19 mil diameter conductor 
12 mil wall of PTFE 
1.6 mil wall of expanded porous PTFE 
______________________________________ 
Physical Properties 
Expected Actual 
______________________________________ 
Center Spacing (mils) 
46.2 43.8 +/- 2.0 
Bond Thickness "y" (mils) 
7.1 22.3.sup.a +/- 2.0 
Bond Strength (oz.) 
1.8 3.sup.b +/- .5 
______________________________________ 
Electrical Properties 
Expected Actual 
______________________________________ 
Impedance (ohms) 173 158 
Capacitance (pf/ft) 
7 5.5 
Dielectric Constant 
1.3 1.25 
______________________________________ 
.sup.a Actual Bond Thickness measurements were made with an optical 
microscope at 50 X magnification equipped with an xy table and a digital 
readout. Twentyone measurements were taken and averaged to determine bond 
thickness. 
.sup.b Actual Bond Strength measurements were taken with a spring scale, 
in which a force was applied to separate a single conductor from the 
remaining assembly.