Note: Descriptions are shown in the official language in which they were submitted.
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LOW SMOKE AND FLAME SPREAD CABLE CONSTRUCTION
(IR 2588)
: Back~round of the Invention
:
This invention relates generally to plastic jacketed
electrical cables and more specifically to a cable
.
construction employing poly(vinylidene fluoride) resin
materials.
Plenum cables are electrical power and signal carrying
cables which are located in the air spaces between the floors
of buildings and suspended ceiIings beneath the floors.:
Because these air spaces normally are continuous, if flamable
iJ7~t
materials are employed in electrical cable construction, the
cables can contribute to the rapid spread of fire and smoke
throughout the entire floor of the building. Therefore, where
flamable materials are included, the cables must be encased
in metal conduits, which are expensive. Pol~fluorinated
resins such as fluorinated ethylene propylene (FEP) have been
employed to provide flame-resistant and low-smoke producing
coatings so that metal conduits are not required. We have
now found a polyfluorinated resin containing cable
construction which has exceptionally low flame spread and
smoke production properties so that such cables are
especially suited for us~ in plenum cable systems. -
Brief Summary of the lnvention
In accordance with this invention, there is provided a
lS jacketed cable comprising a bundle of conductors having
insulating layers including-a poly(vinylidene fluoride)
resin, a wrapping of a fluorinated polymer impregnated glass
tape on the bundle, and a jacket of poly(vinylidene fluoride)
resin.
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Description of the Drawing
Figure 1 is an elevational side view, with parts broken
away~ of an embodiment of the cable of the invention.
Detailed Description
` The cable construction of the invention employs
poly(vinylidene fluoride) (PVDF) resin in combination with a
glass wrapping tap~ which provides a flame retardant and low
smoke electrical cable. The electrical cable, as illus~rated
in Figure 1, generally comprises~a plurality of individual
electrical conductors 13 of, for ex~mple, copper or aluminum
which each have an insulating layer 15 of polymer so that
they are electrically insulated from one another. These
wires are twisted into a bundle 17 and the bundle 17 lS held
together to form core 18 by a wrapping of tape 19. Tape 19
is of a polyfluorocarbon resin impregnated silica glass. A
glass tape of "E-glass" impregnated with 30 weight percent
poly(tetrafluoroethylene~ (PTFE) has been found to be
particulary suitable. Such materials are commercially ~
available for use as cable wrapping and besides holding the
bundle togetherj perform the additional function of
protecting the eonductor insulating layers 15 where the cable
jacket 21 is formed of a higher melting re~in. The jaclcet
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21 is then formed such as by extrusion using a cross head.
The polymer insulating and jacket layers are formed of a
poly(vinylidene fluoride) resin. Weight ratios of
poly(vinylidene fluoride) polymer to impregnated glass tape
of from about 6 to 1 ~o about 33 to 1 have been successfully
employed. Ratios greater than 33 to 1 would be expected to
produce increased smoke and flame spread. Exceptional low
smoke generation properties and low flame spread are obtained
when the ratio is about 22 to 1. The reason for the
surprisingly better flame spread and smoke generation
properties is not completely understood but it is believèd
that the property of HF generation by poly(vinylidene
fluoride) polymers at high temperatures combined with
absorption of HF by the silica glass tape may be involved
Other fluorinated polymers which have been employed in cable
construction such as poly(tetrafluoroethylene) (PTFE) and
fluorinated ethylene propylene (FEP) polymers do not have the
property of releasing HF. The low flame spread evidenced by
the cable construction of the invention is even more
surprising considering the fact that poly(vinylidene
fluoride) has a limiting oxygen index value (LOI) (ASTM D
2863) of about 44 as opposed to 95 for
poly(tetrafluoroethylene) and fluorinated ethylene-propylene
polymers. Because of the significantly lower LOI of
poly(vinylidene fluoride), the flame spread properties of the
cables of the invention would be expected to be inferior
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rather than superior to a cable including PTFE and FEP
polymers as jacket and insulating layers.
Although the specific examples of cable described herein
employ poly(vinylidene fluoride~ homopolymer, it should be
understood that cable constructions employing copolymers
containing a major portion of vinylidene fluoride and
possessing superior smoke and flame spread properties are
considered to be within the scope of the invention. The term
"poly(vinylidene fluoride) resin" as used herein encompasses
such copolymers. The polymers can also contain minor amounts
of additives such as pigments, plasticizers and extrusion
aids.
In order to further illustrate the cable construction of
the invention and compare its smoke generation and flame
lS spread properties with other cable constructions, a series of
cables were made and tested as described in the following
examples:
Example I
A telephone cable construction containing 25 pair of
conductors was manufactured by the following steps:
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1. Copper wire of 22 AWG was coated with KYNAR~ 460
grade poly(vinylidene fluoride) resin manufactured by
~: Pennwalt Corporation~containing 5 parts by weight per hundred
parts by weight of resin of color concentrate which was added
for identification. The wire was coated by the method known
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1 17~4~1
in the art as pressure extrusion. The insulation thickness
was 10 mils average with an 8 mil minimum.
2. Two insulated wires made by Step 1 were twisted
together with a 3 inch lay where the lay is defined as the
degree of twist or the length measured along the axis of a
wire or cable required for a single strand of wire to make
one complete turn about the axis.
3. 25 pair of wires twisted in Step 2 were then twisted
together to form a bundle with a 12 inch lay.
4. The bundle made by Step 3 was wrapped with a glass
tape (E-glass cloth impregnated with PTFE resin). The tape
was 0.025 inch thick and 1~ inches wide. The glass tape is
available commercially under the trademark FLUORO&LASS~, a
product of Oak Materials Group, Inc. The tape was wrapped on
the wire bundle with a 1.78 inch lay and ~ inch overlay. The
E glass composition is approximately, in weight /O: SiO2 54%,
; A12O3 14%, B203 10%, MgO 4.5% and CaO 17.5%. The PTFE resin
comprises about 30 weight percent of the total weight of
impregnated tape.
5. The core made by Step 4 was jacketed by a process
known in the art as tubing extrusion coating using KYNAR 460
grade poly~vinylidene fluoride) resin containing 1-2 parts
per hundred by weight of extrusion aid (which is a resin
consisting of, by weight, 99% KYNAR 460 grade resin and 1%
polytetrafluoroethylene resin) and 1 part per hundred by
weight of color concentrate. The wall thickness of the
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jacket was .045 inch average and a minimum of .027 inch. The
weight ratio of total poly(vinylidene fluoride) to glass tape
in this construction was calculated to be about 22 to 1 with
the weight of resin in the cable being about 29.5 gms/ft.
Example 2
The same cable construction was produced as in Example 1
except for Step 4 where -the tape used was a MYLAR~(Du Pont)
polyester film tape .001 inch thick and 1~ inch wide.
Example_3
The same cable construction was produced as in Example 2
except fluorinated ethylene propylene polymer insulated
conductor was made in Step 1.
Example 4
The same cable construction as Example 1 was produced
except fluorinated ethylene propylene polymer was used for
the jacket instead of poly(viny.idene fluoride).
Example S
A power limited fire protective signalling cable was
constructed with having 24 conductors of No. 22 AWG wire
employing a KYNAR 460 grade resin insulation and jacket. The
jacket was applied over MYLAR polyes~ter tape which was 0.001
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inch thick and 1.2 inches wide with a lap of ~ inch applied
over the conductor assembly.
Example 6
The same construction as Example 5 was produced except
that the PTFE impregnated E-glass binder tape as described in
Example 1 was used instead of the MYL~R polyester tape. The
weight ratio of KYNAR resin to glass tape was calculated to
be about 33 to 1 with the weight of resin in the cable being
about 33 gms/ft.
Samples of cables prepared by Examples 1-6 were tested
by a modified Steiner Tunnel test UL 723 (ASTM E84).
Comparison samples of polyvinyl chloride insulated and
jacketed cable were tested in both steel and aluminum
conduits for control purposes.
The Steiner Tunnel test was modified to adapt the UL 723
test procedure to adequately test cables. The standard flame
and draft conditions were used (240 fpm in the direction of
flame growth and a 300,000 Btu/hr 4~ foot long methane
igniting flame). The duration of the test was chosen as 20
minutes and the sample cables were supported on a 12 inch
wlde cable rack in the zone of maximum temperature and heat
concentration in a single layer which completely filled the
rack width. The maximum flame spread was recorded rather
than a flame spread factor. The smoke development was
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monitored by a photometer system in the test furnace exhaust
duct and the optical smoke density was calculated from the
light attenuation values. The results are given in Table I
below:
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I ~71481
It can be seen from the results reported in Table I that
~he preferred cable construction of Example 1 a~ about a 22
to 1 PVDF to glass resin ratio had surprisingly lower flame
spread than the other samples and produced little smoke. The
S cable construction of Example 6 at a 33 to 1 PVDF to glass
resin ratio was measurably better than the comparable cable
construction of Example 5, which used polyester tape, with
respect to smoke gerleration and was comparable in flame
spread. The cable construction of Example 1 was also
superior to the average reported values for comparable cables
formed with with an FEP resin insulation and jacket (3.0 ft.
flame spread and 0.30 optical peak for smoke generation) and
EGTFE (copolymer of ethylene and chlorotrifluoroethylene
resin insulation and jacket (4.0 ft. flame spread and 0.215
optical peak for smoke generation).
Example 7
A two pair telephone cable was prepared by coating
copper wire of 22 AWG with Kynar 460 grade resin insulaton
and jacket. The jacket was applied over Mylar polyester tape
which was .0025 inch thick and 1.5 inches wide applied over
the conductor assembly.
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1 171481
Example 8
The same construction as Example 7 was produced except
that the PTFE impregnated E-glass binder tape as described in
Example 1 was used instead of the Mylar polyester tape. The
weight ratio of Kynar resin to glass tape was calculated to
be about 6 to 1 with the weight of resin in the cable being
about 5 gms/ft.
Samples of cables prepared by Examples 7 and 8 were
tested by the modified Tunnel test with about 65 lengths used
to fill the rack. In two tests, the cables of Example 7 gave
flame spreads of 3.0 and 3.5 feet, average optical smoke
densities of .03 and .04 and smoke peaks of 0.12 and 0.25
respectively. In two tests, the cables of Example 8 gave
flame spreads of 2.0 and 2.5 feet, average optical smoke
densities of .02 and .02 and peaks of .08 and .10
respectively, thus demonstrating that for the cable
configuration having two pairs of conductors the construction
using glass tape was superior to the comparable one using
polyester tape both with respect to smoke generation and
flame spread. Because of the smaller cable diameter in these
examples, the mass of resin in the rack was only about 325
gms/ft (65 cables x 5 gms/ft per cable) compared to from
about 675 to 825 gms/ft for the tests of the cable of
Examples 1-6 so that the smoke results for Examples 7 and 8
would be expected to be lower than those of Examples 1-6
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1171~1
because of a smaller mass of resin being subjected to the
flame .
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