Coaxial electric signal cable having a composite porous insulation

A crush-resistant high signal propagation velocity coaxial cable insulated with a low-density expanded PTFE insulation surrounded by an extruded closed-cell polymer foam.

FIELD OF THE INVENTION 
The invention pertains to insulated coaxial electric signal cables, 
particularly to those cables having a porous insulation, most particularly 
to those cables wherein the porous insulation comprises a fluorocarbon 
polymer. 
BACKGROUND OF THE INVENTION 
Low-density porous expanded polytetrafluoroethylene (PTFE), described in 
U.S. Pat. Nos. 3,953,566, 3,962,153, 4,096,227, 4,187,390, 4,902,423, and 
4,478,665, has been widely used to insulate electrical conductors to 
provide insulated conductors having improved properties of velocity of 
signal propagation, dielectric loss, and physical dimensions as compared 
to conductors insulated with full density polymer insulation. The high 
pore volume and low-density provide the improvements in the properties. 
A limitation to achieving extremely high signal propagation velocity 
through such insulated conductors lies in the open cell (nodes and 
fibrils) nature of ePTFE which is not inherently crush-resistant when it 
is manufactured to have a very high void content or pore volume to achieve 
low-density and low dielectric constant and therefore high velocity of 
signal propagation. 
Crushability of such an insulation can be improved by enclosing the 
insulation with a skin of thermoplastic polymer, but the velocity of 
signal propagation is reduced by the solid voidless insulation skin. 
Another method for providing crush-resistance to the cable insulation has 
been to foam thermoplastic polymers as they are being extruded around a 
conductor to yield a crush-resistant closed cell foam insulation around 
the conductor. The method is well known in the art and described in U.S. 
Pat. Nos. 3,072,583, 4,711,811, and 4,394,460 and in EP0442346 in which a 
foaming gas or liquid is injected into the molten polymer during 
extrusion. In these methods a foaming agent is used during the extrusion 
process to yield closed cell fluorocarbon polymer foams, which tend to be 
inherently crush-resistant. It is difficult, however, to produce a foam 
insulation having a high enough void content to provide insulated cables 
having high signal velocity propagation through them and at the same time 
provide adequate resistance to crushing. 
SUMMARY OF THE INVENTION 
The invention comprises a coaxial electric signal cable having a composite 
porous insulation comprising a layer of porous ePTFE insulation 
surrounding a signal conductor and this insulated conductor surrounded by 
a layer of closed-cell foam polymer insulation. The ePTFE insulation may 
be extruded or tape-wrapped onto the signal conductor and the closed-cell 
foam polymer insulation may be any customary insulation useful for 
conductor insulation which can be foamed by a foaming agent as it is 
extruded onto the ePTFE-clad conductor. A thermoplastic fluorocarbon 
polymer is preferred for the foamed closed-cell polymer, such as PFA, FEP, 
or the like, for example, and may also be polyester, polypropylene, or 
polyethylene. The foamed closed-cell polymer may be either extruded over 
the ePTFE layer or applied as a tape wrap. The composite insulation of the 
invention combines a microporous open-celled insulation of nodes and 
fibrils with a crush-resistant protective insulation of high closed-cell 
void-content which does not adversely affect the electrical properties of 
the ePTFE-clad conductor, particularly its signal propagation velocity. 
The insulated signal conductor having the two-layer composite insulation is 
provided with electrical shielding of a type customary for shielding in 
coaxial electric signal cables, such as metallized polymer tape, metal 
foil, served metal wires, or metal tubes, for example. The shielding is 
usually surrounded by a protective polymeric jacket, which may be 
tape-wrapped or extruded over the shielding. Such jackets may be of 
polyolefins, polyvinyl chloride, fluoropolymers, and the like, which may 
also be filled with conductive materials. The signal conductor and the 
shielding may be copper, copper alloy, noble metal-plated copper, 
aluminum, mu metal magnetic alloy, or other conductive metal. 
The insulated signal conductor having the two-layer composite insulation 
may be utilized as a twisted pair of insulated conductors without 
shielding and thus take advantage of the crush resistance and good 
dielectric properties of the composite insulation.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is now described in detail with reference to the drawings to 
more carefully delineate the details and scope of the invention. 
FIG. 1 is a cross-sectional view of a cable of the invention in which an 
electrical signal conductor 1 is surrounded by extrusion or tape-wrapping 
by a layer of preferably porous expanded polytetrafluoroethylene (ePTFE) 
insulation 2. The insulated conductor is surrounded by a layer of 
closed-cell polymer foam insulation 3 which is preferably extruded onto 
the ePTFE covered conductor by methods described above which embody 
extruding under heat and pressure a foamable thermoplastic onto a core 
while at the same injecting an unreactive gas or gasefiable liquid into 
the extruder barrel to effect foaming of the thermoplastic as it exits the 
extruder. A nucleating agent has been added to the thermoplastic polymer 
before extrusion so as to thereby maximize the number of voids formed and 
minimize their size. This procedure causes the foamed polymer layer 3 to 
be closed-celled with considerable strength against crushing. 
About 95% void content is about the maximum usefully attainable and the 
preferred range is about 50-90% void content, which will provide maximum 
signal propagation velocity with good crush-resistance in a coaxial signal 
cable. 
Other microporous polymers having very low dielectric constants may be 
substituted for the preferred ePTFE, such as polyethylene, polypropylene, 
fluorocarbons, for example. 
The center signal conductor 1 may be solid or stranded and may comprise 
copper, copper alloy, aluminum, aluminum-copper composite, carbon-filled 
polymer, metals coated with other metals by a plasma coating method, noble 
metal-plated copper and copper alloys, or tin and nickel-plated metals, 
for example. 
Foamable thermoplastic polymers which may be used for the closed-cell foam 
insulation 3 may include polyethylene, aromatic polyamide, polypropylene, 
fluorinated ethylene-propylene copolymers (FEP), perfluoroalkoxy 
tetrafluoroethylene polymers (PFA), chlorotrifluoroethylene polymers, 
ethylene-chlorotrifluoroethylene copolymers, polyvinylidene fluoride 
polymers, PTFE polymers containing fluorinated oxygen-containing rings, 
polystyrene, polyformaldehyde polyethers, vinyl polymer, aromatic and 
aliphatic polyamides, and ethylene-tetrafluoroethylene copolymers 
(Tefzel.RTM.). 
Foaming agents may be nitrogen, members of the Freon.RTM. series, carbon 
dioxide, argon, neon, methylene chloride, or low-boiling hydrocarbons, 
such as pentane, for example. Under extrusion conditions in a 
thermoplastic polymer, these will form the closed-cell voids in large 
numbers, particularly if a nucleating agent is used. 
To insure that the maximum number of minimum sized voids are formed, a 
nucleating agent to promote bubble formation is used. These may include 
particles of boron nitride, a magnesium, calcium, barium, zinc, or lead 
oxide or carbonate, alumina, silica gel, and titanium dioxide, for 
example. 
Surrounding the closed-cell foamed insulation 3 is a conductive shielding 
4, which may be wrapped, served, or extruded around insulation 3. Metal 
foils or metal-coated polymer tapes may be spiralled around insulation 3 
or conductive wire or tape served or braided around insulation 3. A soft 
conductive metal tube of copper, copper alloy, or aluminum may be drawn 
through a die around insulation 3. A silver-plated copper wire may be 
served around insulation 3. Conductive shielding 4 may comprise the same 
metals used above for the center conductor 1 and may also be mu metal 
magnetic alloy or conductive particle-filled polymer containing conductive 
carbon or metal particles, for example. Where a metal-coated polymer tape 
is used for the shielding 4, a spiralled or longitudinal drain wire 6 is 
often used adjacent and in contact with the shield to insure proper 
grounding of the shield. The drain wire may be of silver-plated copper, 
for example. 
Surrounding the shield 4 and alternative drain wire 6 is a protective 
jacket 5. Jacket 5 is usually an extruded thermoplastic, such as those 
listed above, and may contain conductive filler particles of carbon or 
metal. 
FIG. 2 describes a cable of the invention in a perspective cross-sectional 
view with layers successively peeled away to show the structure of the 
cable. Conductor 1 is surrounded by an ePTFE insulation layer 2, which is 
a turn surrounded by a closed-cell foam insulation 3 to provide crush 
strength to protect the microporous layer 2. The foam insulation 3 is 
shown wrapped spirally by a metal tape or metal-coated tape shielding 4. A 
drain wire 6 adds to the effective grounding of the shield. A protective 
polymer Jacket 5 in turn surrounds shield 4 and drain wire 6. 
EXAMPLE 
A 0.762 mm silver-plated copper wire was spirally-wrapped with an ePTFE 
tape having a density of 0.21 g/cc and a void content of about 90% as 
calculated, based on the density. A foamed fluoropolymer layer was 
extruded over the ePTFE. The density of the ePTFE layer and the foamed 
thermoplastic layer were measured by the following procedure. 
A small piece of cable was submerged in epoxy potting compound and placed 
in a vacuum chamber to pull air from the samples. The epoxy potting 
compound is allowed to cure and the samples then cross-sectioned and 
polished. 
A microscope with a video micrometer is then used to measure the diameter 
of the signal conductor, the diameter of the ePTFE core, and of the foamed 
thermoplastic polymer layer. A cross-sectional area can then be calculated 
for the ePTFE and the foamed thermoplastic polymer layer. An adjoining 12 
inch (30.48 cm) sample of the cable is then separated into its component 
parts and the ePTFE and the thermoplastic polymer layer weighed separately 
and the mass determined. The volume of each layer can be calculated by the 
cross-sectional area times the 12 inch (30.48 cm) length. The density is 
then calculated from the mass in grams for each layer divided by the 
volume in cubic centimeters. The density of the ePTFE layer averaged about 
0.21 g/cc., with a range of about 0.19 to about 0.28 g/cc. The wall 
thickness of the ePTFE layer was measured as about 0.294 mm. 
A crush-resistant layer of PFA was then extruded by a standard extruder for 
thermoplastic polymer extrusion onto the ePTFE wrapped conductor while 
Freon 113 was injected into the barrel of the extruder. The extruder had a 
30:1 length to diameter ratio. The PFA contained a boron nitride 
nucleating percent at about 0.79% by weight. Several samples were extruded 
having from about 0 to about 55% void content in the PFA layer. These void 
contents were confirmed by removing the PFA layer and measuring the 
density of the PFA layer. 
A spiral drain wire and aluminized polyester shield were applied in tandem 
by a tape-wrapping method known in the art. An extruded layer of FEP was 
added by a standard extrusion process to serve as an outer jacket. These 
samples were tested for velocity of signal propagation and the results 
compared with those of otherwise identical samples, having no outer 
jacket. The data from these measurements showed that as the void content 
of the PFA skin layer increased, the velocity of signal propagation of the 
cable increased correspondingly with little change of the ability of the 
PFA skin layer to prevent crushing of the ePTFE insulation core.