Fire-resistant plenum cable and method for making same

A new and improved method for preparing a fire resistant electrical-conductor is disclosed in which a length of a metal core conductor is first wrapped with a mica-impregnated glass tape to form a continuous first layer, and then an outer coat of a heat-curable platinum catalyzed silicone rubber composition is applied around the tape layer which is cured to form a fire resistant insulation for the electrical conductor. The insulated conductors of the invention provide circuit integrity under fire and may be useful in critical circuitry such as fire alarms and elevators.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for preparing a new and improved 
fire-resistant, insulated conductor and to new and improved insulated 
conductors made in accordance therewith. More particularly, it relates to 
a fire-resistant insulation prepared by helically wrapping a conductor 
with at least one layer of mica-impregnated glass tape in a half-lapped 
fashion to form a continuous first layer, and applying a continuous outer 
layer of a curable, platinum catalyzed silicone rubber composition thereto 
and curing said outer layer to form a hardened, fire-resistant insulation. 
In recent times, the hazards associated with fire in skyscrapers and other 
heavily populated buildings have become more appreciated. The need to 
provide circuit integrity to certain critical circuits, such as fire 
alarms and elevators, in the event of fire cannot be over-emphasized. 
It has long been known to use mica in insulating compositions. Having 
excellent dielectric properties and fire-resistance, this natural material 
is well suited for use in electrical insulation applications. Typically, 
mica insulations are provided in the form of mica tapes, such as described 
in U.S. Pat. No. 2,656,290. As described therein, individual mica flakes 
are bonded to one another, as well as to a pliable base sheet, and, if 
desired, also a cover sheet, by a liquid bonding agent which may be 
hardened by suitable additives. The bonded mica tape used for these 
purposes may be relatively narrow, having a width of 2 to 3 cm for 
example, or it may be used in sheets of greater width. A selected 
conductor is wrapped with the mica tape and the wrapped conductor is 
subjected to a vacuum and impregnated with a thin liquid impregnating 
resin. The resin and the bonding agent are specifically chosen such that 
the bonding agent, together with the hardners and polymerization 
accelerators present in the impregnating resin, combine completely with 
the impregnating resin to form a uniform hardened insulative coating. 
Considerable time and effort has been spent in making mica tape insulation 
which is flexible and exhibits good adhesion of mica to tape. The 
shortcoming with such tapes is that the vacuum impregnation step tends to 
be costly and care must be taken that the impregnating resin is fully 
dispersed throughout the windings to eliminate voids in the insulation 
which decrease the dielectric properties of the resulting insulation. 
Further, these compositions generally are not fire-resistant enough at 
high temperatures to be suitable in today's applications. 
More recently it has been known to use fluoropolymers, such as 
tetrafluoroethylene (TEFLON) and ethylenetetrafluoroethylene (TEFZEL) as 
insulative coatings. Such materials are effectively fire-resistant up to 
about 250.degree. C.-300.degree. C. which is an improvement, and have 
excellent dielectric strength. A serious shortcoming with these 
insulations however is that at high temperatures when these polymers burn, 
they may give off toxic gases rendering their use less than desirable in 
today's construction applications. 
Recently, elsewhere in the polymer art, there have been developed new and 
improved silicone elastomers such as those described in U.S. Pat. No. 
4,061,609, assigned to the same assignee as the present invention. The 
silicone elastomers described therein include a vinyl-containing 
polysiloxane, a hydrogen-containing siloxane, a platinum catalyst and 
further include an inhibitor compound containing at least one hydroperoxy 
radical added to improve processability. Similar but uninhibited silicone 
elastomers having a silica filler have been shown to be flame-retardant in 
U.S. Pat. No. 3,514,424. 
It has now been found that the platinum-catalyzed silicone elastomers 
described in the former patent are well suited to produce new and improved 
fire-resistant insulations in combination with a mica-impregnated glass 
tape. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a new and 
improved method for preparing fire-resistant electrical insulation. 
According to the invention a length of electrically conductive metal core 
conductor is provided, such as copper wire, which is first helically 
wrapped in a half-lapped fashion with a mica-impregnated glass tape to 
form a continuous first layer. A second outer coat of a curable, 
platinum-catalyzed silicone rubber composition is applied around said tape 
layer and cured to produce a continuous, tough, fire-resistant electrical 
insulation system around said conductor. 
The new and improved electrical insulations of the subject invention are 
fire-resistant and provide circuit integrity under fire at temperatures of 
1000.degree. C. or more. At these elevated temperatures the silicone 
rubber outer coat layer burns to a non-conductive silica ash which, in 
combination with the mica-impregnated glass tape, provides the electrical 
insulation required to maintain circuit integrity. The silicone rubber 
does not give off any toxic gases as it burns which is a distinct 
advantage over prior art fluoropolymer insulations. In addition, raw 
materials costs for the insulations of the present invention are greatly 
decreased over conventional fluoropolymer designs. Another advantage of 
the present insulations is that the silicone rubber coat layer may be 
applied through thin-wall extrusion methods thereby obviating the need for 
vacuum impregnation processing required in prior art insulations.

DETAILED DESCRIPTION OF THE INVENTION 
According to the subject invention, a new and improved method for preparing 
a flame-resistant insulated electrical conductor comprises providing a 
length of electrically conductive metal core conductor; overwrapping said 
core with a thin continuous layer, helically half-lapped, of a 
mica-impregnated glass tape; applying an outer coat of a curable 
platinum-catalyzed, silicone rubber composition around said tape layer; 
and then curing said outer coat to produce a continuous, tough 
fire-resistant electrical insulation for said conductor. 
Referring to FIG. 1, a new and improved electrically-conductive cable 10 is 
shown, insulated in accordance with the subject invention. As illustrated 
in FIG. 1, cable 10 is prepared by providing a length of an 
electrically-conductive metal core conductor 12. Core conductor 12 is 
overwrapped, in a helical half-lap fashion, with a mica-impregnated glass 
tape 14 forming a continuous first layer. A second outer coat of a 
platinum-catalyzed silicone rubber composition 16 is applied around tape 
layer 14 and is cured, thereby providing a tough, continuous, 
fire-resistant electrical insulation to core 12 of cable 10. 
Although the insulating method of the subject invention is well suited for 
insulating wires and cables, such as those made from copper or aluminum, 
any electrically-conductive metal core conductor may be insulated in 
accordance with this invention, as for example, high voltage coils, etc. 
In accordance with the subject invention the length of metal core conductor 
12 is helically overwrapped in half-lap fashion with mica-impregnated 
glass tape 14 to form a continuous first layer. Mica-impregnated glass 
tape insulations are well known in the art and generally comprise a 
backing or base layer of glass fiber sheet or glass cloth, an evenly 
distributed mica flake layer and a liquid thermosetting binding resin such 
as an alkyd, an epoxy, or a silicone resin. Typically, the pliable glass 
base sheet is passed below the mica dropping tower of a conventional mica 
layering machine. A solution of the selected liquid resinous binder 
dissolved in a volatile solvent is dripped upon the mica flakes in a 
quantity sufficient to wet them. The wetted mica layer is rolled to spread 
the solution of the binder between the mica flakes and the glass layer. 
The composite insulation is then heated to drive off the solvent, leaving 
substantially only the liquid resinous binder. 
In a preferred embodiment, the mica-impregnated glass tape will be a tape 
such as GEMAX.RTM. tape available from the General Electric Company. GEMAX 
tape is a thin laminated structure of mica paper supported by glass fabric 
or mat or glass roving laid parallel to each other in the direction of the 
tape. The entire structure is then bonded together by a silicone resin or 
other high temperature organic/inorganic binders resulting in a fully 
cured laminate. 
These tapes may be prepared by bonding the mica paper and its supporting 
structure together by a resin and then impregnating the mica paper with 
the desired high temperature resin. The resin is then cured to provide a 
smooth tack-free roll of material which can then be slit into rolls of 
tape of various widths. 
The platinum-catalyzed silicone rubber compositions 16 for use in the 
subject invention are suitably described in U.S. Pat. No. 4,061,609, 
incorporated herein by reference. As described therein, the 
platinum-catalyzed silicone rubber composition comprises: 
(a) 100 parts of a vinyl-containing base linear polysiloxane of the 
formula, 
EQU R.sub.a SiO.sub.4-a/2 (1) 
and blends of such polysiloxanes where R is selected from the class 
consisting of alkyl radicals of 1 to 8 carbon atoms, vinyl radicals, 
phenyl radicals, fluoroalkyl radicals of 3 to 10 carbon atoms and mixtures 
thereof wherein the vinyl radical concentration in said polymer is at 
least 0.005 mole percent and varies from 1.98 to 2.01; 
(b) at least 0.1 parts per million of platinum; 
(c) from 1 to 50 parts of a hydrogen containing polysiloxane; and 
(d) at least 0.007 parts of an inhibitor compound having at least one 
--C--O--O--H radical per 100 parts of said vinyl containing linear base 
polysiloxane. 
Preferably, the vinyl-containing linear base polysiloxane has the formula, 
##STR1## 
and has a viscosity that varies from 1,000 to 300,000,000, and more 
preferably varies from 1,000,000 to 200,000,000 centipoises at 25.degree. 
C., wherein in formula (2) the Vi is vinyl and R.sup.1 is selected from 
the class consisting of vinyl, phenyl, alkyl radicals of 1 to 8 carbon 
atoms, and fluoroalkyl radicals of 3 to 10 carbon atoms and mixtures 
thereof and where X varies from 2,500 to 11,000. There may be mixed into 
the base polymer also a vinyl-containing polysiloxane where the vinyl is 
appended to the silicon atoms in the internal part of the polysiloxane 
polymer chain. Such a vinyl-containing polysiloxane is preferably of low 
viscosity and acts both as a diluent and as a reinforcing agent for the 
final cure of the elastomer. It can be appreciated that the 
vinyl-containing base polymer may be one polymer or a blend of 
vinyl-containing polymers and more specifically a blend of the 
vinyl-containing polymer of formula (2), with other vinyl-containing 
polymers having vinyl units both on the terminal position of the 
polysiloxane chain as well as the internal positions on the polysiloxane 
chain. 
As the hydrogen-containing polysiloxane, any anhydride cross-linking agent 
normally utilized in SiH-olefin platinum-catalyzed reactions to form 
silicone elastomers or silicone polymers may be utilized in the instant 
case. The preferred hydride cross-linking agents for utilization in the 
formation of silicone elastomers are disclosed below. For instance, there 
may be utilized a hydride cross-linking agent composed of, 
##STR2## 
units and SiO.sub.2 units where the ratio of R.sup.3 to Si moieties varies 
from 1.1 to 1.9 and R.sup.3 is selected from the class consisting 
generally of any monovalent hydrocarbon radicals or halogenated monovalent 
hydrocarbon radicals of up to 10 carbon atoms. More preferably, R.sup.3 is 
selected from the class consisting of alkyl radicals of 1 to 8 carbon 
atoms, phenyl radicals and fluoroalkyl radicals of 3 to 10 carbon atoms. A 
specific desirable fluoroalkyl radical being trifluoropropyl. Generally, 
for any hydride cross-linking agent utilized in the instant invention, it 
is preferred that the hydride cross-linking agent have a hydride content 
broadly of 0.05 to 5% and more preferably of 0.1 to 1% by weight. 
Another necessary ingredient in the silicone rubber compositions is a 
platinum catalyst. Generally, there must be utilized at least 0.1 parts 
per million of a platinum catalyst in terms of parts of platinum metal. 
This platinum catalyst may be in any form. It may be a solid platinum 
metal deposited on a solid carrier or it may be a solubilized platinum 
complex. Any type of platinum catalyst will work in the instant invention. 
More preferably, the platinum complex is a solubilized platinum complex. 
Many types of platinum compounds for this SiH-olefin addition reaction are 
known and such platinum catalysts may be used herein. The preferred 
platinum catalysts are those platinum compound catalysts which are soluble 
in the present reaction mixture. The platinum compound can be selected 
from those having the formula (PtCl.sub.2 -Olefin).sub.2 and 
H(PtCl.sub.3)-Olefin) as described in U.S. Pat. No. 3,159,601 to Ashby. 
The olefin shown in the previous two formulas can be almost any type of 
olefin but is preferably an alkenylene having from 2 to 8 carbon atoms, a 
cycloalkenylene having from 5 to 7 carbon atoms or styrene. Specific 
olefins utilizable in the above formulas are ethylene, propylene, the 
various isomers of butylene, octylene, cyclopentene, cyclohexene, 
cycloheptene, etc. 
A further platinum-containing material usable in the composition of the 
present invention is the platinum chloride cyclopropane complex 
(PtCl.sub.2.C.sub.3 H.sub.6).sub.2 described in U.S. Pat. No. 3,159,662, 
Ashby. 
Still further, the platinum-containing material can be a complex formed 
from chloroplatinic acid with up to 2 moles per gram of platinum of a 
member selected from the class consisting of alcohols, ethers, aldehydes 
and mixtures of the above as described in U.S. Pat. No. 3,220,972, 
Lamoreaux. 
The preferred platinum compound to be used not only as a platinum catalyst 
but also as a flame-retardant additive is that disclosed in Karstedt, U.S. 
Pat. No. 3,814,730. Generally speaking, this type of platinum complex is 
formed by reacting chloroplatinic acid containing 4 moles of water of 
hydration with tetravinylcyclotetrasiloxane in the presence of sodium 
bicarbonate in an ethanol solution. 
The final basic ingredient in the instant composition is the inhibitor. 
Accordingly, in the present mixture there must be at least 0.007 parts per 
100 parts of the vinyl-containing polymer of an inhibitor compound which 
can be any organic or silicone compound containing at least one 
hydroperoxy radical. 
The structure of the hydroperoxy containing compound can have any desired 
structure as long as it contains a hydroperoxy radical in the molecular 
structure because it is such hydroperoxy radical that accomplishes the 
inhibiting activity for reasons that are not known. 
Other hydroperoxy inhibitor compounds that may be utilized in the instant 
invention are for instance, methylethylketone peroxides, cumene 
hydroperoxide, 1,1,3,3-tetramethylbutylhydroperoxide, and 2,5-dimethyl-2, 
5-dihydroperoxy hexane. 
Other compounds that may be utilized are t-butyl hydroperoxide, 
1-hydroxycyclohexyl hydroperoxide, decalin hydroperoxide, p-methane 
hydroperoxide and a pinane hydroperoxide. These compounds are manufactured 
and sold by Pennwalt Corp., Hercules, Inc., and Lucidol Chemical Co. Other 
addition-cured and peroxide-cured silicone rubber systems may be used such 
as those disclosed in U.S. Pat. No. 3,445,420 and U.S. Pat. No. 3,660,345. 
The above compounds are only exemplary and many other can be utilized since 
compounds containing hydroperoxy radicals are well known. 
The other additive which may be utilized in the instant invention is a 
filler and accordingly per 100 parts of the basic vinyl-containing polymer 
there may be utilized anywhere from 5 to 50 parts of a filler selected 
from the class of well known reinforcing fillers, such as fumed silica and 
precipitated silica, and extending fillers such as, titanium oxide. For 
instance, there may be utilized a filler in the broad range of 10 to 75 
parts which filler is selected from the class consisting of titanium 
oxide, lithopone, zinc oxide, zirconium silicate, silica aerogel, iron 
oxide, diatomaceous earth, calcium carbonate, fumed silica, cyclic 
polysiloxane treated silica, silazane treated silica, precipitated silica, 
glass fibers, magnesium oxide, chromic oxide, zirconium oxide, alpha 
quartz, calcined clay, asbestos, carbon, graphite, cork, cotton and 
synthetic fibers. The reinforcing fillers of fumed silica and precipitated 
silica are preferred, especially fumed and precipitated silica which have 
been treated with silicone compounds as is well known in the art. Also it 
is well known in the art that extending fillers may be used in combination 
with reinforcing fillers, treated or untreated, to get the proper balance 
in final physical properties in the silicone elastomer. Other additives 
may be utilized in the instant composition as is well known. For instance, 
there may be utilized additional flame-retardant additives; there may be 
utilized heat aging additives as well as pigments and process aids such as 
that disclosed in Konkle, U.S. Pat. No. 2,890,188. It is only necessary 
that the additive does not interreact with the hydroperoxy radical such 
that the hydroperoxy inhibitor compound loses its effectiveness. 
These are the basic ingredients of the instant composition. As far as the 
production of the polymers within the scope of the formulas (1) and (2), 
these are well known compounds. Reference is made to the patent of Jeram 
and Striker, U.S. Pat. No. 3,884,866, whose disclosure is hereby 
incorporated by reference. Such polymers are usually made by the 
equilibration of vinyl-containing cyclic polysiloxanes or non-vinyl 
containing chainstoppers at elevated temperatures to produce high 
viscosity vinyl-containing polymers. Such equilibration reactions are 
carried out with the use of alkali metal catalysts or, in the case of the 
production of low viscosity vinyl-containing polymers, by the use of acid 
catalysts such as, toluene sulfonic acid or acid-activated clay. In the 
case when the polymer is desired to contain some fluoroalkyl groups then a 
slightly different procedure is utilized such as, for instance, that 
disclosed in the issued patent of John Razzano, U.S. Pat. No. 3,937,684. 
The hydride cross-linking agents are also well known as disclosed in the 
above Jeram and Striker U.S. Pat. No. 3,884,866. Simply stated the hydride 
resins are simply produced by the hydrolysis of the appropriate 
hydrochlorosilanes in a two-phase hydrolysis system, that is, with a 
water-immiscible solvent and water, and separating the resulting 
hydrolyzate. 
In the case of fluorosilicone-containing hydride cross-linking agents, 
special procedures have to be utilized, for instance, those disclosed in 
the following Jeram patent applications: Jeram - Ser. No. 619,592, filed 
Oct. 6, 1975, entitled "Solvent Resistant Room Temperature Vulcanizable 
Silicone Rubber Compositions", whose disclosures are hereby incorporated 
into the present case by reference. 
In accordance with the subject invention, the mica-impregnated glass tape 
is first wrapped in a helical half-lap fashion to form a continuous first 
layer around a selected electrically conductive core material. The 
platinum-catalyzed silicone rubber composition is then, applied around the 
tape layer to form a continuous outer layer which is then cured to form 
the tough fire-resistant insulation of the invention. 
In preferred embodiments the silicone rubber composition is applied to the 
tape layer by thin-walled extrusion. The thin wall extrusion process is 
generally well known in the art. Basically, the process entails passing 
the extrudable material and the article to be coated through a series of 
machines arranged in an assembly line including, and in the following 
order, an extruder, a hot air vulcanizing unit (HAV), extended assembly 
line equipped with cool air jets and a cutter mechanism, and in this case, 
a winding machine for coiling the insulated wire onto spools for storage. 
The first step in the procedure is to introduce the silicone rubber 
composition into the extrusion machine. An extrusion machine generally 
includes a hopper section, a feed chamber equipped with a screw mechanism, 
a nozzle and a die. The silicone rubber composition is introduced to the 
hopper section of the extruding machine which is inclined so as to feed 
the silicone rubber material by gravity into the feed chamber. The screw 
mechanism rotates within the feed chamber and forces the silicone rubber 
towards the nozzle and the die. The nozzle contains the die and directs 
the silicone rubber composition thereto. The die is the opening through 
which the liquid material will pass. The shape of the die will determine 
the shape of the extrudate flowing therefrom. More particularly, the die 
acts as a negative template such that solid structures within the die 
produce hollow spaces within the extrudate. 
In the case of thin-walled extrusion for coating a wrapped conductor, the 
die contains a central solid form with an opening as is typical in a 
cross-head extruder. The conductor, prewrapped with mica-impregnated glass 
tape is fed concentrically through an opening in the center form of the 
die and extends from the downstream end of the form. The silicone rubber 
composition is pushed through the nozzle and around the periphery of the 
solid die form to connect with the taped outer surface of the wrapped 
conductor. 
The soft coated conductor is passed from the extruder machine to a HAV 
unit. The HAV unit is basically an elongated oven, where heating and 
therefore curing of the silicone rubber composition takes place. The 
temperatures in the HAV unit may be from 200.degree. C. to 500.degree. C. 
The silicone rubber composition should be completely cured upon leaving 
the HAV unit. The temperature of the HAV unit as well as the speed of the 
winding mechanism which draws the conductor through the assembly, and, in 
addition, the rate of flow of extrudate may be adjusted to insure that the 
outer layer of silicone rubber is continuous even, and fully cured upon 
leaving the unit. The cured coated conductor is then wound upon a spool 
for storage. 
The outer coat of silicone rubber may be applied to be as thin or thick a 
coating as desired, according to the extrusion die used. In preferred 
embodiments, this layer is generally from 5 to 50 mils thick and 
preferably from 8 to 15 mils thick. Other methods, other than thin walled 
extrusion, such as dipping or spray coating may be used to apply the 
silicone rubber layer to the taped conductor. 
The following examples are provided to better illustrate the subject 
invention. 
EXAMPLE I 
Insulated electrical conductors were made in accordance with the subject 
invention as follows: 
A 500-foot section of #22 AWG copper wire was selected and was carefully 
wrapped in a helical half-lapped fashion with GEMAX.RTM. mica-impregnated 
glass tape along its entire length. 
A TUFEL.RTM. silicone rubber composition available from the General 
Electric Company, was prepared and placed in the hopper section of a 
Davis-Standard 2 inch barrel diameter extrusion machine. The TUFEL.RTM. 
silicone rubber composition is a two-part system comprising the silicone 
rubber composition described above and in U.S. Pat. No. 4,061,609. One 
part of the system contains the platinum catalyst, the hydroperoxy 
inhibitor, the vinyl-containing base polysiloxane and fillers. The other 
part of the system includes the hydrogen containing polysiloxane 
cross-linking agent. The silicone rubber is prepared by thoroughly mixing 
the two parts together to form a heat curable silicone rubber composition. 
The prewrapped conductor wire was loaded into the extruder machine such 
that it extended from a loading drum, through the extruder, extrusion die, 
and HAV unit, and along the assembly line to the wind spool. With the 
TUFEL.RTM. silicone rubber composition loaded into the hopper section, the 
heating chamber of the extruder was set at room temperature. The winding 
mechanism was set to draw the prewrapped conductor through the extruder 
and HAV unit at a rate of 40 ft./minute. The HAV unit was set at a 
temperature of about 400.degree. C. The extruder machine and winding 
mechanism were started simultaneously and the silicone outer coat was 
applied to the wrapped conductor. The cured insulation around the 
conductor was approximately 10 mils thick. The insulated conductors 
prepared by this method provided improved fire resistance and circuit 
integrity under fire. 
The foregoing patents are all incorporated herein by reference. Although 
the present invention has been described with reference to a preferred 
embodiment, it is apparent that modifications and changes may be made 
therein by those skilled in the art. All such modifications are within the 
full intended scope of the invention as defined by the appended claims.