Process for making an electrical cable with expandable insulation

Improved insulative cable assemblies and methods for producing them are disclosed. The insulative cable assemblies comprise a conductor housed within a polymeric insulation layer expanded with expandable thermoplastic microspheres. The use of a relatively inelastic outside sheath controls the expansion of the insulative layer and assures a snug fit between the insulation and the conductor. The insulation layer is highly resilient and resistant to dielectric changes due to compression or damage from mechanical manipulation of the cable. Additionally, the ability to expand the insulative layer after cable assembly is completed provides far greater freedom in cable processing without compromising low dielectric constant.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electrical wires and cables. Specifically, 
the present invention provides an improved electrical insulation for wires 
and cables and methods for producing the same. 
2. Description of Related Art 
One of the constant areas of concern in the production of electrical wires 
and cables is the need to balance high speed electrical transmission with 
efficient electrical insulation. One of the more effective insulative 
materials in this regard is insulation made from polytetrafluoroethylene 
(PTFE) and particularly insulation made from expanded PTFE (ePTFE), such 
as that disclosed in U.S. Pat. No. 3,953,566 issued Apr. 27, 1976, to 
Robert W. Gore. The ePTFE material has many properties which make it 
particular effective as electrical insulation, including a relatively low 
dielectric constant, chemical inertness, high strength, low electrical 
loss, and high use temperature (i.e. thermal stability). 
Using various polymer insulation, electrical insulation is commonly 
produced by either extruding or wrapping an insulative material around a 
center conductor. Although extruding an insulation around a wire is a very 
rapid and effective manufacturing technique, certain extrusion processes 
(e.g. some RAM extrusion techniques) have a constraint that when the 
polymer insulation is extruded on a wire it tends to have a relatively low 
"pull-out" resistance, The problem of "pull out" is compounded with 
materials like PTFE and ePTFE due to their inherent lubricity. 
As is disclosed in copending U.S. patent application Ser. No. 023,642, one 
promising improvement in insulation production comprises combining a PTFE 
material and thermoplastic expandable microspheres. Such microspheres, 
which are commercially available under the trademark EXPANCEL.RTM. from 
Nobel Industries Sweden, Sundsvall, Sweden, comprise a thermoplastic shell 
entrapping a volatile liquid such as isopentane. When subjected to heat or 
similar activation energy, the microspheres dramatically expand to many 
times their original size and retain such size when the activation energy 
is removed. By subjecting the composite of PTFE and microspheres to the 
required activation energy, an expanded PTFE can be produced. In addition 
to microsphere expansion, the PTFE can be further expanded mechanically 
(i.e. before, during or after microsphere expansion) to lower the density 
of the resulting material even further. These processes produce an 
electrical insulation with a very low dielectric constant and a high 
velocity of propagation through the conductor. 
While the PTFE and expandable microsphere material can be readily 
co-extruded over a conductor, it has been observed that when the 
expandable microspheres are activated, both the inside and outside 
diameters of the material will increase--causing the insulation to pull 
away from the conductor. This condition is unacceptable and represents an 
extreme case of poor wire pull-out resistance. 
Although a number of others have experimented with composites of polymers 
and microspheres to produce electrical insulation, none has suggested some 
ready means to produce a conductor using such insulation with strong wire 
pull-out resistance. Some examples of early attempts to combine polymer 
and microspheres for electrical insulation include: U.S. Pat. No. 
4,273,806 issued Jun. 16, 1981, to Stechler (employing naturally occurring 
microspheres of silica and alumina in a polymer such as polyolefin or 
polyester); U.S. Pat. No. 5,115,103 issued May 19, 1992, to Yamanishi et 
al. (employing silica or polymer microspheres in an ultraviolet (u.v.) 
radiation curable polymer such as fluoroacrylate, silicone, or silicone 
acrylate); and U.S. Pat. No. 5,128,175 issued Jul. 7, 1992, to Yamanishi 
et al. (employing heat expandable polymer microspheres in a u.v. curable 
polymer such as silicone acrylate, silicone, or fluorinated acrylate). In 
this latter case, the expandable microspheres are expanded in flowing 
liquid resin, which expands the insulation in a more even manner against a 
conductor, but does not provide a sufficiently open-cell structure. 
In Japanese Laid-Open Application JP 4,335,044, published Nov. 24, 1992, it 
is taught that an electrical insulation with a low dielectric constant can 
be produced by mixing PTFE and unexpanded thermoplastic expandable 
microspheres and then expanding the microspheres to create an electrical 
insulation around a conductor. While this material does deliver an 
open-cell structure with low dielectric constant, as is discussed above, 
co-extrusion of such material around a conductor leads to a loose 
attachment between the conductor and the insulation once expansion is 
performed in situ. 
An even more serious problem in the manufacture of low dielectric cables is 
the preservation of low dielectric constants throughout the cable 
manufacturing process. This is due in large part to a typical direct 
correlation between dielectric constant and physical and mechanical 
endurance of the cable (i.e. low dielectric constant insulations are 
typically characterized by low resistance to mechanical force due to the 
high void volume and low density required to achieve low dielectric 
constants). As a result, if care is not exercised during the manufacture 
and handling of low dielectric cable, the insulative properties of the 
cable can be dramatically compromised before it is even placed into use. 
In the past, a number of solutions have been proposed to address these 
concerns. For example, "kid-glove" handling techniques have been employed 
during the manufacture and production of electronic cable in order to 
reduce and regulate the forces which impact the dielectric material during 
cable manufacturing. Typical kid-glove techniques include: using reduced 
tension upon the cable components; reducing the number of rollers and 
similar equipment which might cause densification; and using special 
handling of insulation prior to wrapping of the wire or other steps which 
might apply load to the cable or its parts. Unfortunately nearly all such 
techniques tend to be costly in labor, equipment, and production 
throughput rates. 
Another approach to limit the damage caused to low-dielectric cables during 
manufacture uses self-supporting covers placed over insulated conductors 
to bear any load encountered during subsequent processing. While some 
improvement may be achieved through this method, increased dielectric 
constant generally will still occur due to pressures placed upon the 
insulative material during application of the cover. 
The problem of increased dielectric constant due to the cable manufacturing 
process itself is of particular concern in the production of multiple 
layered cables, such as coaxial cables. In these instances where a number 
of different insulative and conductive layers must be combined into a 
single cable, the likelihood of densification to one or more layers of 
insulation during this process is a distinct possibility. 
Accordingly, it is a purpose of the present invention to provide an 
electrical cable which incorporates a low dielectric constant which is not 
adversely affected by the manufacturing process itself. 
It is another purpose of the present invention to provide an electrical 
insulation of a polymer/expandable microsphere composite which snugly 
surrounds a conductor. 
It is a further purpose of the present invention to provide an electrical 
insulation which can be installed quickly and easily around an electrical 
conductor. 
It is another purpose of the present invention to provide a process for 
producing an insulated electrical conductor which produces a variety of 
high speed electrical cables, such as coaxial cables, with minimal need 
for special handing procedures. 
It is still another purpose of the present invention to provide a process 
for producing an insulated electrical conductor which expands polymer 
electrical insulation in situ around a conductor while producing a snug 
fit between the conductor and the insulation. 
These and other purposes of the present invention will become evident from 
review of the following specification. 
SUMMARY OF THE INVENTION 
The present invention is an improved cable assembly and method for 
constructing the same. The cable assembly of the present invention 
comprises one or more-electrical conductors, a composite insulation layer 
of an expandable polymer (such as polytetrafluoroethylene (PTFE)) and 
expandable thermoplastic microspheres, and a relatively inelastic sheath 
(i.e. coating, wrapping, shielding, tubing, braid, skin, etc.) jacketed 
around the conductor and the insulation layer, By expanding the 
polymer/microsphere composition within the sheath, the composition tends 
to remain in snug contact with the conductor. This produces a cable with 
excellent electrical transmission properties while being resistant to wire 
pull-out. Moreover, cables can be produced in accordance with the present 
invention with both low dielectric constants and minimal need for special 
handling techniques. 
The present Invention employs a variety of construction methods, including 
various processes for applying the insulation layer to the conductor and 
for jacketing the conductor and composition within the sheath, The present 
invention is particularly suited for high-speed production of cable 
assemblies, especially through co-extrusion processes. 
In another embodiment of the present invention, a high-speed cable is 
produced by expanding insulation surrounding a conductor within a mold, 
such as a rolling die, which produces a tight fit between the conductor 
and the insulation without the need for a permanent sheath surrounding the 
cable,

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides an improved insulation for wires and cables 
and methods for producing such insulation. 
As used herein, the term "cable" is intended to encompass any form of 
conductor (e.g. electrical or optical) housed within some form of 
insulation. The conductor may include a metallic (e.g. silver or copper), 
non-metallic (e.g. carbon or carbon impregnated) material, or fiber optic 
(e.g. glass or plastic) material. The cable may take the form of a 
conventional insulated wire, a coaxial cable, ribbon cable, fiber optic 
cable, differential pair cable, twisted pair cable, etc. 
As is explained in U.S. Pat. No. 3,953,566 to Robert W. Gore, by heating 
and rapidly mechanically expanding an unsintered fine powder PTFE article 
in one or two directions through mechanical means, a tetrafluoroethylene 
polymer material can be created with a micro-structure characterized by 
nodes interconnected with fibrils. Once sintered to establish expanded 
dimensional stability, this material is highly porous, has high strength, 
and has very good electric insulative properties. Among the numerous 
useful forms of this material is as films/membranes, tubes, rods, and 
continuous filaments. 
Cables can be formed from the expanded PTFE product through a variety of 
methods. Although this material may placed on a conductor via coextrusion, 
it is preferred that the expanded PTFE insulation be formed into a tape 
form and then tightly wrapped around the wire. This produces an 
effectively insulated wire which is securely housed within its insulation. 
In the present invention, PTFE or similar polymers (e.g. ultra-high 
molecular weight polyethylene) is expanded by employing expandable 
microspheres blended into the polymer composition. Such microspheres 
comprise a plastic coating surrounding an expandable liquid or gaseous 
volatile fluid. U.S. Pat. No. 3,615,972 issued Oct. 26, 1971, to Morehouse 
et al., teaches thermoplastic microspheres adapted to expand dramatically 
when exposed to heat. These microspheres are monocellular particles 
comprising a body of resinous material encapsulating a volatile fluid. 
When heated, the resinous material of thermoplastic microspheres softens 
and the volatile material expands--causing the entire microsphere to 
increase substantially in size. On cooling, the resinous material in the 
shell of the microspheres ceases flowing and tends to retain its enlarged 
dimension; the volatile fluid inside the microsphere tends to condense, 
causing a reduced pressure in the microsphere. 
Such thermoplastic microspheres are commercially available from Nobel 
Industries Sweden, Sundsvall, Sweden, under the trademark EXPANCEL.RTM.. 
These microspheres may be obtained in a variety of sizes and forms, with 
expansion temperatures generally ranging from 80.degree. to 130.degree. C. 
Expansion can usually be practiced between 80.degree. and 260.degree. C. 
or above, depending upon a number of factors, such as dwell time. A 
typical EXPANCEL microsphere has an initial average diameter of 9 to 17 
microns and an average expanded diameter of 40 to 60 microns. According to 
Nobel Industries, the microspheres have an unexpanded true density of 
1250-1300 kg/m.sup.3 and an expanded density below 20 kg/m.sup.3. 
It should be understood that the use of the term "expandable microsphere" 
herein is intended to encompass any hollow resilient container filled with 
volatile fluid which is adapted to expand. Although presently available 
microspheres are essentially ball-shaped particles adapted to expand when 
exposed to an energy source, it should be understood that such 
microspheres are quite resilient in their expanded form and can be 
compressed and released (e.g. through extrusion) to achieve the expansion 
required for the present invention. Additionally, it may be possible to 
form such products in a variety of other shapes, such as tubes, 
ellipsoids, cubes, particles, etc. As such, the term "expandable 
microsphere" in the context of the present invention is intended to 
include all applicable forms and uses of these products now known or later 
developed. 
In one presently preferred embodiment of the present invention, EXPANCEL 
type 091 DU is employed. This product comprises an off-white dry powder 
with a particle size between 5 and 50 microns. The shell of the 
microsphere comprises acrylonitrile or methacrylonitrile. The volatile 
liquid comprises isopentane. 
It has been found that by mixing a dry preparation of EXPANCEL microspheres 
with a dispersion of PTFE or similar polymer and then heating the 
resulting composition, the polymer will undergo expansion in 
three-dimensions to achieve a porous network of polymeric nodes and 
fibrils. A mixture of PTFE, in the form of paste, dispersion or powder, 
and microspheres, in the form of dry powder or solution, are mixed in 
proportions of 1 to 90% by weight microspheres, with 5 to 85% by weight of 
microspheres being preferred. It should be appreciated that a wide range 
of products may be created even with a percentage of microspheres of 
merely 0.1 to 5% by weight; similarly, for some uses, filled products may 
be created with a percentage of microspheres and/or other fillers between 
90 to 99 or more percent by weight. Mixture may occur by any suitable 
means, including dry blending of powders, wet blending, co-coagulation of 
aqueous dispersions and slurry filler, high shear mixing, etc. 
Once mixed, preferably the resulting composition is heated to a temperature 
of 80.degree. to 180.degree. C. for a period of 0.5 to 10 minutes to 
activate the microspheres. 
With currently available microsphere technology, if further density 
reduction is desired, the composition may be re-heated to a temperature of 
40.degree. to 240.degree. C. and mechanically expanded through any 
conventional means, such as those disclosed in U.S. Pat. No. 3,963,566. In 
fact, it should be appreciated that the present invention is believed to 
lend itself to incorporation with many existing mechanical expansion 
techniques, whether before, during and/or after microsphere expansion. 
The resulting products have proven to have superior properties. When formed 
around a wire conductor, the composition has proven to be an excellent 
electrical insulation, with typical dielectric constants of 1.04 to 1.25, 
and typical velocities of propagation of 89-98%. Moreover, unlike some 
presently employed low dielectric insulations which must be carefully 
handled to avoid damage or compromise of the insulative properties, the 
insulation of the present invention is particularly resistant to 
deformation and compromise from mechanical working. 
Depending on application, insulations can be created with virtually any 
dielectric constant within the above described ranges or greater. From 
experimentation to date, it has been shown that insulations with 
dielectric constants of lower than 1.12 and 1.06 can be readily produced 
using the present invention. As such, in conjunction with the other unique 
properties of this material, compositions made in accordance with the 
present invention have tremendous applications in a wide variety of 
electrical applications, and particularly in wires, high VP cable, 
sheathing, and similar applications. 
The compositions of the present invention have a number of other desirable 
properties, such as thermal insulation, buoyancy, padding, selective 
solvent permeability, moldability, extremely low densities, selective 
expandability, dimensional stability (even in an unsintered form), and 
exceptional elasticity/resilience to deformation. Additionally, many of 
these properties can be particularly useful in the construction of unique 
cable configurations, such as using selective expandability of the 
insulative material to produce cables with changes in impedance along its 
length. 
One of the desirable properties as an electrical insulation is the 
composition's ability to "puff" into a resilient cushion-like coherent 
mass with substantial proportions of open air spaces therein. The mass can 
expand in all dimensions and remains highly self-cohesive despite the 
dramatic increase in its dimensions and typical air space contents from 10 
to 50 to 80% or more. Although not necessary, by mechanically expanding 
the PTFE/microsphere composition In addition to the microsphere expansion, 
the resulting product becomes even less dense. 
This "puffed" mass has proven to be remarkably elastic and resilient to 
deformation. For instance, a typical expanded mass will withstand 
compression of 50% or more with nearly 100% rebound to original shape. 
The expansion ability of the present invention lends itself to readily 
producing many previously expensive or unavailable insulated conductors. 
For example, conductors with extremely low dielectric constants must be 
very carefully handled during the manufacturing process to avoid any 
densification of the insulation. Such densification can both increase the 
dielectric of the conductor and cause distortion to the electrical signal 
traveling through the conductor. These problems are particularly 
problematic in conductors incorporating multiple layers of conductors, 
insulation and/or shielding, and those requiring high signal throughput. 
Among the more difficult cables in this regard are high speed coaxial 
cables, which presently must be manufactured using "kid-glove" processing 
steps with absolutely minimal twisting, bending or other mechanical 
manipulating of the assembled cable once insulation is installed. 
The present invention can vastly simplify the processing steps required to 
produce such insulation by providing two important properties to the 
assembled cable. First, by providing an insulation with expandable 
microspheres embedded therein, the cable can be constructed using normal 
assembly techniques with little concern about densification. Once the 
cable has been completely assembled, the insulation is then expanded in 
place to produce its full insulative properties. This provides low 
dielectric cable without fear of densification during the assembly 
process. 
Second, the resilient nature of insulation produced in accordance with the 
present invention is far less prone to damage due to mechanical working of 
the cable. By employing expandable microspheres within the insulation, the 
insulation is less likely to become permanently crushed or damaged during 
normal assembly, shipping, and installation procedures. As a result, 
cables of the present invention retain their original dielectric constants 
more consistently than previous electrical insulators. 
The resilient properties of the present invention may be better understood 
by examination of scanning electron microscopic (SEM) images of expanded 
compositions made in accordance with the present invention. FIGS. 1 and 2 
show SEM images of compositions of the present invention enlarged 150 
times. As can be seen, the composition comprises many expanded 
microspheres 10 interconnected by polymeric fibrils 12 (i.e. thread-like 
elements) and nodes 14 (i.e. particles from which fibrils emerge). This 
"scaffold" or "lattice" structure of fibrils and nodes incorporating 
microspheres defines substantial areas of open air spaces within the 
composition. It is believed to be both the voids within the microspheres 
10 and these air spaces which create many of the unique properties of this 
composition. 
The particular relationship between the microspheres and the polymer can be 
better seen in the SEM image of FIG. 3. In this image, magnified 1280 
times, the microspheres 10 can be seen attached to and embedded within 
fibrils 12 and nodes 14. As is shown, the polymer actually becomes 
attached to the microspheres, apparently with some fibrils 16 extending 
directly from the microspheres 10 and some nodes 18 attached directly to 
the surface of the microspheres 10. 
Although the polymer/microsphere composition has proven to be a very 
effective electrical insulation, its application to a conductor tends to 
be limited to conventional methods for applying an expanded PTFE 
insulation, primarily through wrapping of an expanded tape around the 
conductor. Among the deficiencies of this process is that it is time 
consuming and tends to reduce the desirable low density (i.e. "puffed") 
nature of the product. 
It is preferred to coat the conductor with the polymer/microsphere 
composition before the microspheres are expanded and then expanding the 
microsphere in place on the conductor. Not only can the insulative jacket 
be applied faster and with less expense using this process, but the 
expansion of the insulation in situ is also believed to provide better 
electrical and mechanical properties. Regrettably, in some instances it 
has been found that the activation of the polymer/microsphere composition 
in place tends to expand the insulation layer away from the 
conductor--leaving the wire subject to pull-out. This condition is 
particularly evident when the insulation is co-extruded around the 
conductor and then expanded. 
The present invention addresses these concerns by constricting the in situ 
expansion of the polymer/microspheres around the conductor to force the 
composition to form a snug fit with the conductor. As is shown in FIG. 4, 
a conductor 20 is housed within a relatively inelastic sheath material 22, 
such as PTFE or acrylic. By partially filling the sheath 22 or coating the 
conductor 20 with the polymer/microsphere composition 24 of the present 
invention, the composition can be expanded to fill in the sheath 
thoroughly and provide an even coating around the conductor 20. An 
alternative coating may be created by heating the conductor itself to 
institute composition expansion. 
It has been found that so long as the sheath is less elastic than the 
expanding polymer/microsphere composition, the composition tends not to 
pull away from the conductor, but, rather, will expand to fill the space 
between the conductor and the sheath and produce a snug fit between the 
insulation and the conductor. 
in conjunction with the sheath, the unexpanded composition can be provided 
on the conductor in a variety of methods. Some illustrative examples of 
application methods include: 
(1) It has been determined that the composition of the present invention 
will adhere to the conductor by merely dipping the conductor in the 
composition. This allows the conductor to be first dip-coated and then 
placed within a sheath where expansion energy can be applied; 
(2) The sheath itself can be filled with the composition and then the wire 
can be inserted therethrough; 
(3) The composition can be extruded on the conductor and then bonded firmly 
together through application of expansion energy once the conductor and 
composition are placed in a sheath. The sheath may also be co-extruded in 
this process or added through an additional step; 
(4) The composition containing unexpanded microspheres can be formed as a 
tape which can be wrapped either loosely or tightly (depending on 
particular needs) around the conductor. Once wrapped and housed within a 
sheath, activation energy can be applied to form a snug fit of insulation 
around the conductor; and 
(5) The composition and a PTFE shell can be formed into a single pellet and 
then relatively easily co-extruded around a conductor. 
As is shown in FIG. 5, the preferred sheath 26 comprises a continuous tube 
which surrounds the composition 24 and conductor 20. Depending upon 
desired properties of the final product, the sheath can be created from 
either a conductive or non-conductive material. The sheath 26 may comprise 
any dimensionally stable material, such as a metal (e.g. copper, aluminum, 
or steel braided shielding), or a polymer such as polyethylene, polyolefin 
(e.g. PTFE), fluorinated ethylenepropylene (FEP), polystyrene, 
ethylene-chlorotrifluoroethylene (ECTFE), ethylene-tetrafluoroethylene 
(ETFE), or perfluoroalkoxy polymer (PFA). For most applications, ideally 
the sheath should have low dielectric constraint and good mechanical 
properties. 
The sheath may be applied by a wide variety of methods, such as through 
extrusion, wrapping, braiding, drawing, serving, dipping, plating, 
dropping and blocking, etc. Additionally, as is explained below, the 
sheathing can be installed only temporarily and then removed once the 
insulation is expanded in place (such as through use of a removable mold 
during the production of a cable). Preferably, the sheath comprises a 
braid, wrap, or extrusion applied around either unexpanded or pre-expanded 
insulation. 
Examples of some of these methods for applying the sheath are set forth in 
greater detail below. As is illustrated in FIG. 6, the sheath may also be 
applied as a tape 28 wrapped around the insulation 24. Although the tape 
may be merely folded around the composition 24, it is preferred to spiral 
wrap the sheath around the composition 24 in the manner shown to provide a 
secure fit. FIG. 6 further illustrates that multiple conductors 30a, 30b, 
30c can be readily provided within the cable without departing from the 
present invention. 
Shown in FIG. 7 is a cable 32 constructed with a continuous sheath 34 and a 
composition layer 36 comprising a tape wrapped around the conductor 38. As 
has been explained, a cable of this structure can be formed by providing a 
tape with unexpanded microspheres embedded therein, wrapping the tape 
around a conductor, jacketing the wrapped conductor in a sheath, and then 
applying activation energy to cause the tape to expand to completely fill 
the space between the conductor and the sheath. This construction method 
produces a very snug connection between the insulation and the conductor. 
It should be understood that the sheath may likewise be wrapped around the 
tape 36, as is shown in FIG. 6, to form an even tighter fit. 
One of the advantages in applying the composition as a tape is that it 
provides a ready means to provide further mechanical expansion of the 
tape. As has been explained, this permits the density of the insulation to 
be lowered further, decreasing the weight of the insulation layer and 
increasing its void content. It may also be possible to expand the 
composition mechanically while in place on the conductor. Such in situ 
mechanical expansion may be particularly viable with certain conductors, 
such as PTFE filled with a conductive filler, which will accommodate 
stretching along with the insulative layer. 
It has been determined that certain other polymers will also expand in the 
presence of expandable microspheres in the manner described to create a 
lattice of polymeric nodes and fibrils and air spaces. As is explained in 
the following examples, expansion to form polymeric nodes and fibrils and 
air spaces has been achieved by using similar processes in conjunction 
with ultra-high molecular weight polyethylene polymer. Applicants believe 
that comparable results may also be achieved by employing the present 
invention with other long-chain polymers, particularly those with a high 
modulus of crystallinity, e.g.,: polypropylene, polyvinyl alcohol (PVA), 
poly(ethylene terephthalate)(PET), etc. 
The preferred materials for an extruded sheath are those materials that 
demonstrate good strength at the temperatures that are typically used for 
puffing and offer low dielectric constant. However, it should be 
appreciated that the amount of insulative properties required is dependent 
upon whether the sheath is to function as part of a dielectric system or 
as an electrical shield. The best examples being fluoropolymer resins such 
as FEP or PFA that both have a dielectric constant of about 2.15, and PTFE 
with a dielectric constant of about 2.15 and expanded PTFE with a 
dielectric constant of about 1.2 to 1.5. All these resins soften at 
temperatures above 200.degree. C. making them good choices for this 
application. Alternative resins could be used for this application such as 
polyethylene (dielectric constant 2.2) if the insulation is puffed 
inductively by heating the wire so as not to melt the polyethylene. 
While the presently preferred embodiments of the present invention employ 
expanded insulation permanently mounted within a sheath, as has been 
noted, it is also contemplated that insulation of the present invention 
may be created with a removable sheath. In this instance, the sheath may 
comprise merely a removable covering, such as a jacket or mold that can be 
easily removed, stripped off or dissolved after the insulation has been 
expanded. 
For continuous processing, the sheath may comprise a mold that is 
automatically removed after expansion. One possible apparatus 40 for use 
in this respect is shown in FIGS. 8 through 10. The apparatus 40 comprises 
a pair of abutting rolling surfaces 42, 44, (e.g. metal belts) each having 
a series of corresponding grooves 46, 48 therein. Traveling within the 
corresponding grooves 46, 48, conductors 50 coated with unexpanded 
insulation 52 are fed through the apparatus 40 from a first end 54 to a 
second end 56. 
In order to expand the insulation, expansion energy is applied to the 
rolling surfaces 42, 44 at the first end 54 of the apparatus. For 
instance, a heating element can be included within the rolling surfaces 
capable of producing sufficient heat to quickly expand the insulation 52 
around and into tight contact with the conductor 50. At the second end 56 
of the apparatus, a completed cable 58 is removed with now unsheathed 
insulation 52 fully expanded and in tight contact with the conductor 50. 
If desired, a cooling element may be provided at the second end 56 of the 
apparatus to ease in the separation of the cable 58 from the grooved 
surfaces. As is shown, it is possible to produce numerous cables through 
this process simultaneously. 
There are many contemplated advantages of this expansion apparatus. For 
example, abutting rolling surfaces maintain the insulation in the same 
relative position with the mold during the entire expansion process, 
avoiding dragging or other possibly damaging mechanical manipulation of 
the cable. Additionally, the abutting rolling surfaces allows for cable to 
be produced with varying widths of insulation along its length (e.g. 
grooved, spiral, or undulating surfaces) to fulfill various operational or 
market needs. The straight wire path also assures minimal mechanical 
damage to the insulative layer during the expansion process. The apparatus 
may likewise be used to fully or partially expand insulation housed in a 
separate sheath to impart particular operational properties or to assure 
proper restraint of the insulative layer during expansion. 
Without intending to restrict the scope of the present invention, the 
following represent examples of how the present invention may be employed. 
EXAMPLE 1 
A slurry was mixed consisting of 20.94 g of PTFE in the form of a 60.0% 
dispersion and 27.88 g of EXPANCEL-091 DU and 3.5 g of distilled water. 
This yields a solids content of 43% PTFE to 57% EXPANCEL. The PTFE 
dispersion used was part number TE 30, an aqueous dispersion obtained from 
E. I. duPont de Nemours and Company, Wilmington, Del. 
A 24 gauge silver plated copper wire was "dipped" coated with the above 
slurry by inserting the wire through a small hole in the bottom of the mix 
container and drawing the wire upwards through the bath. The wire was then 
hung vertically to dry for approximately 30 minutes at ambient conditions. 
The 1.07 m (3.5 ft) section of dried coated wire was inserted inside an 
expanded PTFE tube (e.g. tubing produced according to Gore U.S. Pat. No. 
3,953,566) of the same length having an inside diameter (I.D.) of 2 mm, an 
outside diameter (O.D.) of 3 mm, and a porosity of 70%. The assembly was 
then placed in a convection oven for 3 minutes at 165.degree. C. in order 
to puff the wire coating so that it fills the annular space between the 
wire and the PTFE tubing. The sample was removed and allowed to cool. The 
density of the puffed coating was calculated to be 0.096 g/cc using the 
known weight and volume of the puffed coating. 
The wire assembly was then helically wrapped with aluminized polyester film 
and tested for velocity of propagation (V.P.). A Techtronics Model CSA 803 
was used in TDR mode (time domain reflectometry) to measure velocity of 
propagation. A cable length of 1.67 m (5.47 ft) was measured to have a 
signal delay of 3.461 ns/m (1.055 ns/ft), which converts to a signal speed 
of 2.889.times.10.sup.8 meters/sec. This signal speed divided by the speed 
of light in a vacuum (2.998.times.10.sup.8 m/s) yields a V.P. of 96.36%. 
From this value, the dielectric constant (Er) can be calculated using the 
equation: V.P.=1/.sqroot.Er. Er for this cable calculates to be 1.077. 
EXAMPLE 2 
A slurry was mixed consisting of 7.36 g of PTFE in the form of a 60.0% 
dispersion and 13.67 gms of EXPANCEL-091 DU and 4.5 g of distilled water. 
This yields a solids content of 35% PTFE to 65% EXPANCEL. The PTFE 
dispersion used was part number TE 30, an aqueous dispersion obtained from 
Dupont Company. 
A 33 gauge silver plated copper wire was dipped coated with the above 
slurry by inserting the wire through a small hole in the bottom of the mix 
container and drawing the wire upwards through the bath. The wire was then 
hung vertically to dry for approximately 30 minutes at ambient conditions. 
The 1.07 m (3.5 ft) section of dried coated wire was inserted inside a 43 
mil I.D. copper tube of the same length, The assembly was then placed in a 
convection oven for 4 minutes at 165.degree. C. in order to puff the wire 
coating so that it fills the annular space between the wire and the copper 
tubing, The sample was removed and allowed to cool. 
A Techtronics Model CSA 803 was used in TDR mode (time domain 
reflectometry) to measure velocity of propagation. A cable length of 0.988 
m (3.240 ft) was measured to have a signal delay of 3.402 ns/m (1.037 
ns/ft), which converts to a signal speed of 2.939.times.10.sup.8 
meters/sec, This signal speed divided by the speed of light in a vacuum 
(2.998.times.10.sup.8 m/s) yields a V.P. of 98.03%, From this value, the 
dielectric constant (Er) was calculated to be 1.041. 
EXAMPLE 3 
The following procedure was used for making films of ultra-high molecular 
weight polyethylene (UHMW-PE) containing 50% EXPANCEL-091 DU by weight. 
Solutions of UHMW-PE were prepared in a jacketed reaction vessel capable of 
maintaining solutions temperatures in excess of 130.degree. C. The vessel 
was purged with flowing nitrogen. The vessel was also fitted with a 
thermocouple for determining the solution temperature, and a stirring 
paddle. 
1. The vessel was initially heated to approximately 79.degree. C., at which 
time 200 g of reagent grade mixed xylenes were added, and equilibrated at 
temperature. 
2. To the stirring solvent, 4.0 g of Hostalon GUR 412 and EXPANCEL-091 DU 
were slowly added. In addition, 0.04 g of an antioxidant, (IRGANOX 1010) 
was added. 
3. With continued stirring, the temperature of the oil bath was raised to 
123.degree. C. Stirring was stopped when the solution temperature reached 
117.degree. C., and the stirring paddle was removed. The solution was 
allowed to equilibrate at temperature for 30 minutes. 
4. After equilibration, the hot solution was poured into a glass dish, and 
loosely covered with aluminum foil. The foil was removed after 30 minutes, 
and the solvent evaporated overnight. 
A piece of the resulting product from the above procedure was measured to 
be 4.70 cm long, 1.10 cm wide, and 0.358 cm thick, a weight of 0.570 g, 
and a calculated density of 0.308 g/cc. This piece was placed in a 
convection oven at 165.degree. C. for 5 minutes and removed. The sample 
had a puffed appearance. A piece was cut from the puffed sample and was 
measured to be 8.92 cm long, 2.04 cm wide, and 0.767 cm thick, a weight of 
0.534 g, and a calculated density of 0.038 g/cc. 
EXAMPLE 4 
A process was tested for continuous processing of a thermoplastic extruded 
covering over a wire with a dip coating of an insulation of the present 
invention. A 30 awg wire (0.010 inch) was coated with a mixture of PTFE 
and expandable microspheres. The mixture of PTFE and expandable 
microspheres comprised a slurry mixed in the manner of Example 1 with a 
solids content of 50% PTFE and 50% EXPANCEL DU 091. Coating was 
accomplished by drawing the wire through a cup filled with the mixture and 
exiting through an extrusion tip with a 0.762 mm (0.03 inch) inside 
diameter (I.D.). Water in the mixture was removed by running the coated 
wire through a drying tube at about 107.degree. C. 
The coated wire was then run through the crosshead of a conventional 
thermoplastic extruder where a polyethylene skin was applied using a 4.45 
mm (0.175") outside diameter (O.D.) tip and a 5.06 mm (0.199") I.D. die. 
Polyethylene has a melt temperature of about 162.degree. C. at 2.8 
revolutions per minute (RPM) screw speed in a screw extruder. The wire was 
drawn through the extruder at about 274 cm/min (9 ft/min). The diameter of 
the wire was measured using an in-line laser gauge, and found to be about 
2.03 mm (0.08"). The capacitance was measured in-line at 29 pf/m (8.9 
pf/ft). This calculates to a dielectric constant of about 1.1. 
The conductor was sandwiched within a simulated shield (comprising two 
insulated planar boards having an aluminum/MYLAR shield applied to the 
inside surfaces of each) and velocity of propagation (V.P.) was measured 
along it. After the sample was puffed, a V.P. of 95.4% was achieved, 
calculating to a dielectric constant of 1.099. 
EXAMPLE 5A 
A 5% EXPANCEL/95% PTFE by weight sample was made by the following method: A 
slurry of 7.8 g of EXPANCEL-091 DU obtained from Nobel Industries, 1519 
Johnson Ferry Road, Marietta, Ga. 30062, and 551.2 g of de-ionized water 
was prepared in a 2 liter baffled stainless steel container. While the 
slurry was agitated at 800 RPM, 148.2 g of PTFE in the form of a 20.0% 
dispersion was rapidly poured into the vessel. The PTFE dispersion was an 
aqueous dispersion obtained from ICI Americas, Inc. After 30 seconds, 2.2. 
g of a 0.4% solution of a cationic modified polyacrylimide was added to 
initiate the co-coagulation. After a total of 1 minute, 20 seconds, the 
mixer was stopped. The coagulum settled to the bottom of the vessel and 
the effluent was clear. 
The coagulum was dried at 110.degree. C. in a convention oven. The dried 
cake was chilled below 0.degree. C. The cake was then hand ground through 
a 0.635 cm mesh stainless steel screen. A sample of the screened powder 
was lubricated with mineral spirits at a ratio of 0.375 cc solvent per 
gram of powder. 
A 19.1 mm O.D..times.12.7 mm I.D. pellet was formed in a cylinder at 
8.3.times.10.sup.6 N/m.sup.2 material pressure. The pellet was then 
extruded onto a 26 gauge, 7 strand wire (O.D. 0.480 mm (0.0189")) using a 
0.813 mm (0.032") tip and a 1.207 mm (0.0475") die. 
EXAMPLE 5B 
A 15% EXPANCEL/85% PTFE by weight composition was made by using the same 
method as Example 5A except the following components amounts were used: 
A slurry of 23.4 g of EXPANCEL-091 DU and 613.6 g of deionized water; 
132.6 g of PTFE in the form of a 20.0% dispersion; 
2.1 g of a 0.4% solution of a cationic modified polyacrylimide; 
A sample of screened powder was lubricated with mineral spirits at a ratio 
of 0.375 cc solvent per gram of powder. 
Using the same pelleting and extrusion equipment and conditions as is set 
forth in Example 5A, the material was again extruded onto a 26 gauge 7 
strand wire. 
EXAMPLE 5C 
A 15% EXPANCEL/85% PTFE by weight composition was made in accordance with 
Example 5B. A pellet was also made in accordance with Example 5B. The 
pellet was then extruded onto a 26 gauge , 7 strand wire using a 1.473 mm 
(0.058") tip and a 1.740 mm (0.0685") die. 
EXAMPLE 6 
A PFA sheath was screw extruded over each of the above described Examples 
5A, 5B, and 5C, producing new Examples 6A, 6B, and 6C, respectively. In 
each instance, the conditions for application of the PFA skin were as 
follows: a tip with a O.D. of 4.45 mm (0.175"); a die with an I.D. of 5.51 
mm (0.217"); RPM of 3.0; line speed of 1067 cm/min (35 ft/min); and melt 
temperature of 353.degree. C. 
The samples were cross sectioned prior to puffing and sectioned again after 
being puffed for 3 minutes at 160.degree. C. FIG. 10 is a SEM showing a 
cross-section of the cable of Example 6B prior to expansion. The cable 
comprises a multiple strand metal conductor 60, an unexpanded insulative 
layer 62, and an outer sheath 64. A distinct gap 66 is visible between the 
conductor 60 and the insulative layer 62. 
FIG. 11 shows the cross-section of cable of Example 6B after puffing. In 
this instance, the insulation layer 62 is expanded into tight contact with 
the conductor 60. 
While particular embodiments of the present invention have been illustrated 
and described herein, the present invention should not be limited to such 
illustrations and descriptions. It should be apparent that changes and 
modifications may be incorporated and embodied as part of the present 
invention within the scope of the following claims.