High count transmission media plenum cables which include non-halogenated plastic materials

A cable which may be used in buildings in concealed areas such as in plenums or in riser shafts includes a core (22) which in a metallic conductor cable includes at least twenty-five transmission media each of which is enclosed with a non-halogenated plastic material. The core is enclosed with a jacket (28) which also is made of a non-halogenated plastic material. The non-halogenated plastic material of the insulation is selected from the group consisting of a polyetherimide and a silicone-polyimide copolymer, or a blend comprising the polyetherimide and the silicone-polyimide copolymer. For the jacket, the plastic material includes a blend composition of silicone-polyimide copolymer and a flame retardant and smoke suppressant system comprising titanium dioxide and zinc borate.

TECHNICAL FIELD 
This invention relates to relatively high count transmission media plenum 
cables which include non-halogenated plastic materials. More particularly, 
the invention relates to relatively high count transmission media plenum 
cables which are used in buildings and which include non-halogenated 
insulation and jacketing materials that exhibit flame spread and smoke 
generation properties acceptable by industry standards as well as an 
acceptable toxicity level and relatively low corrosivity. 
BACKGROUND OF THE INVENTION 
In the construction of many buildings, a finished ceiling, which is 
referred to as a drop ceiling, is spaced below a structural floor panel 
that is constructed of concrete, for example. Light fixtures as well as 
other items appear below the drop ceiling. The space between the ceiling 
and the structural floor from which it is suspended serves as a return-air 
plenum for elements of heating and cooling systems as well as a convenient 
location for the installation of communications cables including data and 
signal cables for use in telephone, computer, control, alarm and related 
systems. It is not uncommon for these plenums to be continuous throughout 
the length and width of each floor. Also, the space under a raised floor 
in a computer room is considered a plenum if it is connected to a duct or 
to a plenum. 
When a fire occurs in an area between a floor and a drop ceiling, it may be 
contained by walls and other building elements which enclose that area. 
However, if and when the fire reaches the plenum, and if flammable 
material occupies the plenum, the fire can spread quickly throughout an 
entire story of the building. The fire could travel along the length of 
cables which are installed in the plenum if the cables are not rated for 
plenum use. Also, smoke can be conveyed through the plenum to adjacent 
areas and to other stories. 
A non-plenum rated cable sheath system which encloses a core of insulated 
copper conductors and which comprises only a conventional plastic jacket 
may not exhibit acceptable flame spread and smoke evolution properties. As 
the temperature in such a cable rises, charring of the jacket material 
begins. Afterwards, conductor insulation inside the jacket begins to 
decompose and char. If the jacket char retains its integrity, it functions 
to insulate the core; if not, it ruptures either by the expanding 
insulation char, or by the pressure of gases generated from the insulation 
exposed to elevated temperature, exposing the virgin interior of the 
jacket and insulation to elevated temperatures. The jacket and the 
insulation begin to pyrolize and emit more flammable gases. These gases 
ignite and, because of air drafts within the plenum, burn beyond the area 
of flame impingement, propagating flame and generating smoke and possibly 
toxic and corrosive gases. 
As a general rule, the National Electrical Code (NEC) requires that 
power-limited cables in plenums be enclosed in metal conduits. The initial 
cost of metal conduits for communications cables in plenums is relatively 
expensive. Also, conduit is relatively inflexible and difficult to 
maneuver in plenums. Further, care must be taken during installation to 
guard against possible electrical shock which may be caused by the conduit 
engaging any exposed electrical service wires or equipment. However, the 
NEC permits certain exceptions to this requirement provided that such 
cables are tested and approved by an independent testing agent such as the 
Underwriters Laboratories (UL) as having suitably low flame spread and 
smoke-producing characteristics. The flame spread and smoke production of 
cable are measured using UL 910, Standard Test Method for Fire and Smoke 
characteristics of Electrical and Optical-Fiber Cables Used in 
Air-Handling Spaces. See S. Kaufman "The 1987 National Electric Code 
Requirements for Cable" which appeared in the 1986 International Wire and 
Cable Symposium Proceedings beginning at page 545. The UL 910 test is 
conducted in apparatus which is known as the Steiner Tunnel. 
The prior art has addressed the problem of cable jackets that contribute to 
flame spread and smoke evolution also through the use of fluoropolymers. 
These, together with layers of other materials, have been used to control 
char development, jacket integrity and air permeability to minimize 
restrictions on choices of materials for insulation within the core. 
Commercially available fluorine-containing polymer materials have been 
accepted as the primary insulative covering for conductors and as a 
jacketing material for plenum cable without the use of metal conduit. 
However, fluoropolymer materials are somewhat difficult to process. Also, 
some of the fluorine-containing materials have a relatively high 
dielectric constant which makes them unattractive for communications 
media. 
The problem of acceptable plenum cable design is complicated somewhat by a 
trend to the extension of the use of optical fiber transmission media from 
a loop to building distribution systems. Not only must the optical fiber 
be protected from transmission degradation, but also it has properties 
which differ significantly from those of copper conductors and hence 
requires special treatment. Light transmitting optical fibers are 
mechanically fragile, exhibiting low strain fracture under tensile loading 
and degraded light transmission when bent with a relatively low radius of 
curvature. The degradation in transmission which results from bending is 
known as microbending loss. This loss can occur because of coupling 
between the jacket and the core. Coupling may result because of shrinkage 
during cooling of the jacket and because of differential thermal 
contractions when the thermal properties of the jacket material differ 
significantly from those of the enclosed optical fibers. 
The use of fluoropolymers for optical fiber plenum cable jackets requires 
special consideration of material properties such as crystallinity, and 
coupling between the jacket and an optical fiber core which can have 
detrimental effects on the optical fibers. If the jacket is coupled to the 
optical fiber core, the shrinkage of fluoropolymer plastic material, which 
is semi-crystalline, following extrusion puts the optical fiber in 
compression and results in microbending losses in the fiber. Further, its 
thermal expansion coefficients relative to glass are large, thereby 
compromising the stability of optical performance over varying thermal 
operation conditions. 
Further, a fluoropolymer is a halogenated material. Although there exist 
cables which include halogen materials and which have passed the UL 910 
test requirements, there has been a desire to overcome some problems which 
still exist with respect to the use of halogenated materials such as 
fluoropolymers and polyvinyl chloride (PVC). These materials exhibit 
undesired levels of corrosion. If a fluoropolymer is used, hydrogen 
fluoride forms under the influence of heat, causing corrosion. For a PVC, 
hydrogen chloride is formed. 
In a more recently developed plenum cable, each transmission medium of a 
core of the cable is enclosed with a non-halogenated plastic material 
selected from the group consisting of a polyetherimide, a 
silicone-polyimide copolymer or blends of these two materials. A jacket 
encloses the core and is made of a non-halogenated plastic material which 
includes a silicone-polyimide copolymer constituent. The jacket may 
comprise as much as 100% by weight of the silicone-polyimide copolymer 
constituent. 
The just-described cable is acceptable for a plenum cable having a 
relatively low number of transmission media. However, there is a need to 
provide a plenum cable which includes a relatively high number of 
transmission media such as, for example, at least twenty-five metallic 
conductor pairs. 
The sought-after high number transmission media cable not only exhibits 
suitably low flame spread and low smoke producing characteristics provided 
by currently used cables which include halogenated materials but also one 
which meets a broad range of desired properties such as acceptable levels 
of corrosivity and toxicity. Such a cable does not appear to be available 
in the prior art. What is further sought is a cable which is characterized 
as having relatively low corrosive properties, and acceptable toxic 
properties, as well as low levels of smoke generation and one which may 
include a relatively high number of transmission media. 
SUMMARY OF THE INVENTION 
The foregoing problems of the prior art have been overcome with the cables 
of this invention. A cable of this invention comprises a core which 
includes a relatively high number of transmission media. For 
communications use, each transmission medium may be an optical fiber or a 
metallic conductor. For a metallic conductor cable, at least twenty-five 
pairs of metallic conductors are included. 
Each transmission medium is enclosed with a non-halogenated plastic 
material selected from the group consisting of a polyetherimide, a 
silicone-polyimide copolymer or blends of these two materials. A jacket 
encloses the core and is made of a non-halogenated plastic material which 
includes a silicone-polyimide copolymer constituent and a smoke 
suppressant and flame retardant system. The smoke suppressant and flame 
retardant system includes titanium dioxide in the range of about 0.5 to 15 
percent by weight and zinc borate in the range of about 0.5 to 15 percent 
by weight with the combination of zinc borate and titanium dioxide not 
exceeding about 20% by weight of the composition of the jacket. The jacket 
may comprise as much as 99% by weight of the silicone-polyimide copolymer 
constituent. 
Advantageously, the cables of this invention may be used in building 
plenums and/or risers. They are acceptable by UL 910 test requirements for 
flame spread and smoke generation. Further, they exhibit acceptable levels 
of toxicity and relatively low corrosivity.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown a cable which is designated 
generally by the numeral 20 and which is capable of being used in 
buildings in plenums. A typical building plenum 21 is depicted in FIG. 2. 
There, a cable 20 of this invention is disposed in the plenum. As can be 
seen in FIG. 1, the cable 20 includes a core 22 which comprises a 
relatively large number of transmission media and which may be enclosed by 
a core wrap (not shown). Each transmission medium may comprise a metallic 
insulated conductor or an optical fiber which includes at least one layer 
of coating material. The core 22 may be one which is suitable for use in 
data, computer, alarm and signaling networks as well as in voice 
communication. 
For purposes of the description hereinafter, the transmission medium 
comprises twisted pairs 24--24 of insulated metallic conductors 26--26. 
Although some cables which are used in plenums may include only one to 
four pairs, many such cables include twenty-five pairs and possibly 
one-hundred or more conductor pairs. 
Each insulated metallic conductor 26 includes a longitudinally extending 
metallic conductor portion 27. In order to provide the cable 20 with flame 
retardancy, low corrosivity, acceptable toxicity and low smoke generation 
properties, the metallic conductors are provided with an insulation 28 
comprising a plastic material which provides those properties. 
The metallic conductors each may be provided with an insulation cover 
comprising a polyetherimide. Polyetherimide is an amorphous thermoplastic 
resin which is available commercially, for example, from the General 
Electric Company under the designation ULTEM.RTM. resin. The resin is 
characterized by high deflection temperature of 200.degree. C. at 264 psi, 
a relatively high tensile strength and flexural modulus and very good 
retention of mechanical properties at elevated temperatures. It inherently 
is flame resistant without the use of other constituents and has a 
limiting oxygen index of 47. 
Polyetherimide is a polyimide having other linkages incorporated into the 
polyimide molecular chain to provide sufficient flexibility to allow 
suitable melt processability. It retains the aromatic imide 
characteristics of excellent mechanical and thermal properties. 
Polyetherimide is described in an article authored by R. O. Johnson and H. 
S. Burlhis entitled "Polyetherimide: A New High-Performance Thermoplastic 
Resin" which appeared beginning at page 129 in the 1983 Journal of Polymer 
Science. 
The insulation composition comprising a polyetherimide also includes an 
additive system which includes an antioxidant/thermal stabilizer, and a 
metal deactivator. See U.S. Pat. No. 5,074,640, which issued on Dec. 24, 
1991, in the names of T. G. Hardin, W. F. Moore, J. J. Mottine, Jr., J. D. 
Nielson and Lloyd Shepherd and which is incorporated by reference 
hereinto. Also included in the composition of the insulation may be a 
suitable lubricant. The additive system may be included in a color 
concentrate which is added to the polyetherimide at the feed zone of an 
extruder (not shown). Alternatively, it may be premixed with the 
polyetherimide constituent. 
In a preferred embodiment, the additive system includes about 0.15% by 
weight of an antioxidant/thermal stabilizer. It has been found that a high 
molecular weight hindered phenolic antioxidant/thermal stabilizer such as 
one available commercially from the Fairmount Chemical Company, Inc. under 
the trade designation Mixxim.RTM. AO-30 is suitable. The last mentioned 
material has the chemical name 1,1,3-tris 
(2-methyl-4-hydroxy-5-tert-butylphenyl)-butane. It is a non-staining, high 
molecular weight hindered phenolic compound which inhibits 
thermo-oxidative degradation. It provides excellent protection against 
oxidation when used at levels of 0.02 to 1% by weight. It has a melting 
point in the range 185.degree. C. to 190.degree. C. and a molecular weight 
of 544. It is disclosed in a product brochure available from the Fairmount 
Chemical Company with a revision data of Mar. 31, 1983. Generally, its 
prior art use has been as an antioxidant in products that are in contact 
with food. 
Combined with the antioxidant/thermal stabilizer is a metal deactivator in 
the amount of about 0.15% by weight. It has been found that a high 
molecular weight metal deactivator is suitable for inclusion in the 
composition of this invention. The metal deactivator inhibits degradation 
caused by copper or copper oxide, thereby reducing the adhesion of the 
plastic insulation to the metallic conductor. More particularly, a metal 
deactivator with the chemical name N, 
N'-bis[3-(3',5-di-tert-butyl-4'-hydroxyphenyl-propanyl]hydrazine, and 
available from the Ciba-Geigy Company as Irganox.RTM. MD-1024 metal 
deactivator, is used in the preferred embodiment. 
It has been found that the elongation of the insulation 28 may be increased 
by including titanium dioxide in the additive system. In a preferred 
embodiment, the titanium dioxide is included in the amount of about 0.2 to 
10% by weight. 
The additive system provides a synergistic effect for the protection of the 
insulation 28 during processing and long-term aging. In an embodiment 
which meets UL elongation tests, the range for each constituent of the 
additive system may be as high as about 1.0% by weight. 
As will be recalled, the higher the melt index of the plastic material to 
be extruded, the better the flow properties during extrusion. Tests were 
run to determine the melt index of off-the-shelf polyetherimide material. 
At temperatures of 390.degree. C., 340.degree. C. and 315.degree. C., the 
melt index ranges reported were 8-10, 1.5-2.5 and 0.7-1, respectively. For 
other materials used as insulation, the melt index is substantially 
higher. The melt index of Teflon.RTM. plastic material, for example, is in 
the range of about 24-29.5. Advantageously, the additive package system of 
the composition of this invention resulted in a melt index in the range of 
22-24 at 315.degree. C. which is significantly higher than the melt index 
of off-the-shelf polyetherimide. 
Further, the stabilizing additive system, in addition to providing 
protection from thermo-oxidative degradation during processing, also coats 
the inner surface of an extruder barrel and outer surfaces of pellets 
supplied to the extruder, thereby acting as a lubricant. This facilitates 
the use of reduced extrusion temperatures which helps to avoid degradation 
of the plastic material during extrusion. A 10.degree. to 30.degree. C. 
reduction in extrusion temperatures can be achieved. 
It has been found that polyetherimide has a relatively strong affinity for 
copper. As a result, when polyetherimide insulation is extruded over a 
copper conductor, adhesion of the insulation to the copper may be 
undesirably high. This high adhesion is indicative of some degradation of 
the insulation. 
In order to avoid this problem, insulation 28 may comprise additional 
constituents. For example, a relatively small amount by weight of a 
silicone-polyimide copolymer may be included in the additive system as a 
lubricant to improve the material processing and improve the physical 
properties. Silicone-polyimide copolymer is a flame-resistant non-halogen 
thermoplastic material. One such material is designated SILTEM.TM. 
copolymer and is available commercially from the General Electric Company. 
The silicone-polyimide content of such a blend composition may range from 
0% to 10%, with a preferred range of 0.5 to 2.0% by weight. High 
temperature sulfonamide plasticizers and high molecular weight stearate 
lubricants such as cerium stearate, have also been shown to be suitable 
for this application. 
It should be noted that the insulation 28 may comprise materials other than 
the polyetherimide. For example, the insulation may be a composition 
comprising a silicone-polyimide copolymer or a composition comprising a 
blend of a polyetherimide and a silicone-polyimide copolymer. A suitable 
silicone material is the above-mentioned SILTEM.TM. copolymer. The 
polyetherimide of the blend composition ranges from slightly above 0% to 
slightly below 100% by weight of the composition, and the 
silicone-polyimide copolymer ranges from slightly above 0% to slightly 
below 100% by weight of the composition. 
For optical fiber cables in which optical fibers are provided with a buffer 
layer, a silicone-polyimide copolymer is preferred as the material for the 
buffer layer. The silicone-polyimide copolymer has a lower modulus than 
the polyetherimide which reduces the possibility of inducing microbending 
loss into the optical fibers. 
About the core is disposed a jacket 29. The jacket 29 is comprised of a 
plastic material, which includes a silicone-polyimide copolymer 
constituent which may also be used as the insulation cover for the 
metallic conductors. The jacket 29 also may comprise a blend composition 
comprising a silicone-polyimide copolymer and a polyetherimide. 
Additionally, for the jacket, a system which does not exceed about 20% by 
weight is added to any of the singular materials or blends in order to 
enhance sufficiently the flame retardance and smoke suppression of the 
cable so that it can accommodate a relatively high number of transmission 
media. Among those systems which sufficiently enhance flame retardancy and 
smoke suppression are a blend composition of zinc borate ranging from 0.5 
to 15% and titanium dixoide ranging 0.5 to 15%. A preferred embodiment 
includes 1% by weight of zinc borate and 1% by weight of titanium dioxide. 
In the past, the cable industry in the United States has shield away from 
non-halogenated materials for use in plenum cables. These non-halogenated 
materials which possess desired properties seemingly were too inflexible 
to be used in such a product whereas those non-halogenated materials which 
had the desired amount of flexibility did not meet the higher United 
States standards for plenum cable. 
Surprisingly, the cable of this invention which includes non-halogenated 
insulation and jacketing materials not only meets acceptable industry 
standards for flame spread and smoke generation properties, but also it 
has relatively low corrosivity and an acceptable level of toxicity. The 
result is surprising and unexpected because it had been thought that 
non-halogenated materials which would have acceptable levels of flame 
spread and smoke generation were excessively rigid and that those which 
had suitable flexibility would not provide suitable flame spread and smoke 
generation properties to satisfy industry standards. The conductor 
insulation and the jacketing material of the claimed cable cooperate to 
provide a system which delays the transfer of heat to the transmission 
members. Because conductive heat transfer, which decomposes conductor 
insulation, is delayed, smoke emission and further flame spread are 
controlled. 
Flame spread and smoke evolution characteristics of cables may be 
demonstrated by using a well known Steiner Tunnel test in accordance with 
ASTM E-84 as modified for communications cables and now referred to as the 
UL 910 test. The UL 910 test is described in the previously identified 
article by S. Kaufman and is a test method for determining the relative 
flame propagation and smoke generating characteristics of cable to be 
installed in ducts, plenums, and other spaces used for environmental air. 
Tests have shown that heat is transferred to the cable core 22 principally 
by thermal radiation, secondly by conduction and finally by convection. 
During the Steiner Tunnel test, flame spread is observed for a 
predetermined time and smoke is measured by a photocell in an exhaust 
duct. For a cable to be rated as plenum, i.e. type CMP, according to the 
National Electric Code, flame spread must not exceed five feet. A measure 
of smoke evolution is termed optical density-which is an obscuration 
measurement over a length of time as seen by an optical detector. The 
lower the optical density, the lower and hence the more desirable is the 
smoke characteristic. A cable designated CMP must have a maximum smoke 
density which is 0.5 or less and an average smoke density which is 0.15 or 
less. 
Toxicity generating characteristics of cables may be demonstrated by a 
toxicity test developed by the University of Pittsburgh. In this test, a 
parameter referred to as LC.sub.50, which is the lethal concentration of 
gases generated from the burning of a material which causes a 50% 
mortality among an animal population, that is, 2 out of 4 mice, for 
example, is measured. LC.sub.50 is an indication of the toxicity of a 
material caused by the smoke generated by its burning. The higher the 
value of the LC.sub.50, the lower the toxicity. The higher the LC.sub.50 
value, the more material that must be burned to kill the same number of 
test animals. It is important to recognize that LC.sub.50 is measured for 
the plastic material used in the cable without the metallic conductors. 
The LC.sub.50 values for cables of this invention were higher than those 
for comparable cables which included halogenated materials. 
Low corrosion characteristics of the cables may be demonstrated by the 
measurement of the acid gases generated from the burning of the cable. The 
higher the percent acid gas generated, the more corrosive is the plastic 
material which encloses the transmission media. This procedure is 
currently used in a U.S. government military specification for shipboard 
cables. According to this specification, 2% acid gas, as measured in terms 
of percent hydrogen chloride generated per weight of cable, is the maximum 
allowed. Plenum cables of this invention showed 0% generation of acid gas. 
Test results for example cables of this invention as well as for similar 
plenum cables having halogenated materials for insulation and jacketing 
are shown in TABLE I hereinafter. Being plenum rated, the cables of TABLE 
I pass the UL 910 test for flame spread and smoke generation. 
Example cables were subjected to tests in a Steiner Tunnel in accordance 
with the priorly mentioned UL 910 test and exposed to temperatures of 
904.degree. C., or incident heat fluxes as high as 63 kw/m.sup.2. 
TABLE I 
______________________________________ 
HALO- NON 
GENATED HALOGENATED 
PLENUM CABLE EXAMPLE 
PROPERTY 1 2 3 4* 5* 
______________________________________ 
A. Smoke generation 
max optical 0.276 0.300 0.482 0.40 0.47 
density 
avg. optical 0.112 0.057 0.054 0.08 0.08 
density 
B. Corrosivity 
% acid-gas 42.20 30.79 0 0 0 
generation 
C. LC.sub.50 (grams) 
25 .+-. 7 
12 .+-. 2 
40 .+-. 5 
40 40 
D. Outside Diameter 
0.139 0.140 0.152 0.34 0.34 
(inch) 
E. Jacket thickness 
0.010 0.012 0.016 0.012 0.012 
(inch) 
______________________________________ 
*Twenty-Five Pair Metallic Conductor Cable 
Examples 1, 2, and 3 in TABLE I each included four pairs of 24 gauge copper 
conductors each having a 0.006 inch thick insulation cover. The insulation 
and jacket of Example Nos. 1 and 2 comprised a fluoropolymer. The 
insulation and the jacket of cables of Example 3 were comprised of 
non-halogenated plastic materials. For Example No. 3, the insulation and 
jacket each comprised a blend comprising 50% by weight of ULTEM.RTM. resin 
and 50% of SILTEM.TM. copolymer. For Example No. 4, the insulation was 
ULTEM plastic material and the jacket comprised a blend of 98% SILTEM 
copolymer, 1% titanium dioxide and 1% zinc borate. For example No. 5, the 
jacket comprised a blend of 98% SILTEM copolymer, 1% titanium dioxide and 
1% zinc borate. 
The cables of this invention include transmission media covers and jackets 
which have a range of thickness. But in each case, the cable passes the 
flame retardancy and smoke characteristics tests which are required today 
by the UL 910 test as well as provides relatively low corrosivity and 
acceptable toxicity. 
The sheath system 30 of this invention (a) delays the transfer of conducted 
heat to the core 22 which produces less insulation deterioration which in 
turn produces less smoke and therefore less flame spread; (b) effectively 
reflects the radiant energy present throughout the length of the UL 910 
test; (c) eliminates premature ignition at the overlapped seams; and (d) 
allows the insulation to char fully thereby blocking convective pyrolytic 
gas flow along the cable length. Further, it provides relatively low 
corrosivity and acceptable levels of toxicity. 
It is to be understood that the above-described arrangements are simply 
illustrative of the invention. Other arrangements may be devised by those 
skilled in the art which will embody the principles of the invention and 
fall within the spirit and scope thereof.