Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
This invention relates to the jacketing of cables such
as electric cables and more particularly to flame retardant cable
jackets.
~ACKGROUND OF THE INVENTION
Electric cables having a core of stranded conductors
or the like are coated with one or more layers forming a jacket to
protect the conductors from damage. One problem associated wi-th
cable jackets presently in use is their combustibility. To overcome
this problem flame retardants have been included in the jacketing
ma-terial bu-t they serve to create another problem under exposure to
flame or intense heat, namely the emission of .smoke and toxic gases
(especially carbon monoxide and halogen or phosphorous containing
gases). To avoid the hazard of high temperature smoke and gas emission
there are presently available flame retardant sprays, pastes and wrap
around tapes applicable to cables in situ for form coatings or coverings
on the cable jackets. However~ these coatings or coverings reduce
the flexibility of the cable twhich must be flexible to instal)
especially at low temperatures and they also require additional labour
to apply. me use of tapes additionally makes the cable bulky and difficult
to pull through ducts and they also have a tendency to unravel.
It is an object of the present invention to provide a
composition for use in the continuous coating of a jacketed cable to
give flame retardancy while inhibiting the emission of smoke and toxic
gases at high temperatures and retaining adequate flexibility at low
temperatures.
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It is a further object of the invention to provide
an improved flame retardant cable jacket having reduced emission
of smoke and toxic gases at high temperatures and adequate flex-
ibility at low temperatures.
STATEXENT OF THE INVENTION
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In its broadest aspect the invention consists of
a composition for use in the continuous coating of a jacketed cable
to fonn an outer layer thereon, comprising a polymer resin, a filler
and a stabilizer, proportioned whereby on heating the coating forms
a flame retardant char.
In another aspect the invention consists of a cable
CQmprising a core and a jacket, the jacket campris.ing an inner layer
and an outer layer comprising at least one polymer based resin, at
least one filler and at least one stabilizer, propo~tioned whereby,
on heating, the coating forms a flame retardant char.
DESCRIPTION OF THE DRAWINGS
.
An example embodiment of the invention is shown in the
accompanying drawings in which:
Figu~e 1 is a side view, partly broken away, showing a
cable jacketed according to the invention; and
Figure 2 is a segmental cross-sectional view taken along
line 2-2 of Figure 1.
DETAITT-~ DESCRIPTION
The example embodiment shown in the drawings consists
of a cable 10 comprising (1) a core 12 which has a plurality of
insulated conductors 14 bound by a core wrap 16, and (2) a jacket
18 having an inner layer 20 and an outer layer 22.
7'
Inner layer 20 of jacket 18 consists of a flexible
polymer based material such as polyvinylchloride (PVC). Other materials
suitable for the purpose are polyolefin, polyamide or rubber, all
well known in the trade and normally extruded continuously onto core
16. Outer layer 22 of ~acket 18 is also a polymer based resin with
the addition of a filler and a stabilizer. The resin of the outer
coating is preferably applied as a latex emulsion which also carries
a surfactant although this latter ingredient is substantially lost
by evaporation in the drying process. Additionally a plasticizer may
be included to improve the flexibility of outer layer 22 and an anti-
foam agent may be required to control the foam producing capacity of
the plasticizer or the filler.
The purpose of outer layer 22 is to present a char
forming outer surface of cable ~acket 18 where the flame retardant
components are concentrated, thus interfering minimally with the
strength a~d the cold temperature flexibility of the jacket. The
generation of foam in the char formir~ outer surface under heat further
enhances its effectlveness. The filler, and the plasticizer if present,
are employed to generate foam. If a plasticizer is added to improve
the flexibility of outer layer 22 is must be included in controlled
amounts because it reduces the ultimate strength of the outer layer.
Too much foam, however, is not tolerable and for this reason it may
be necessary to include an anti-foam agent in the composition used to
form outer layer 22.
The ingredients of the emulsion applied to form
outer layer 22 are present in the following range of proportions:
Ingredient Parts by dry wei~ht
Polymer resin (as latex emulsion) 100
Plasticizer (liquid additive) 0 - 50
Ingredient Parts by dry wei~ht
Fillers (flame retardant) 50 - 200
Stabilizers 0.5 - 10
Surfactants 0.5 - 2
Anti-foam agents 0 - 2.0
If a flame retardant plasticizer is not present
the polymer resin is halogenated to provide flame retardancy.
The following are two specific examples of latex
emulsion mixtures for applying to cable 10 to form outer layer 22:
Example I
Ingredient Parts by weight
Vinyledene chloride-butadiene latex
(resin) 230 parts (100 parts dry)
Al~minum trihydroxide (filler) 133
Antimony oxide (filler) 7
Mono-ammonium phosphate (filler) 4.5
Organo lead liquid (stabilizer) 4.5
Starch (filler, surfactant) 2
L2ad stearate (stabilizer, antifoamer)
Phenolic antioxidant (stabilizer) 0.5
Octyl alcohol~ (antifoamer) 0.5
Mixed linear alcohols* (surfactant) 0.1
Poly(dimethyl-siloxane) (antifoamer) 0.05
~ater* (carrier) 45
*No longer present after hea~ drying of the formulation~
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Example II
Ingredient Parts by we_~ht
Vinyledene chloride-butadiene
latex (resin) 183 (100 parts dry weight)
Chlorinated paraffin
(flame retardant plasticizer) 28
Activated azodicarbonamide (filler) 26
Pentaerythritol ~filler) 18
Monoammonium phosphate (filler) 39
Diammonium phosphate (filler) 56
Aluminum trihydroxide (filler) 21
Antimony oxide (filler) 10.5
Organo lead liquid (stabilizer) 6.5
Phenolic antioxidant (stabilizer) 1.3
Octyl alcohol* (antifoamer) 0.7
Mixed llnear alcohols* (surfactant) 0.3
Diethylene ~lycol monoether acetate*
(liquid additive O helps disperse solid
fillers) 4,0
Polydimethyl-slloxane (antifoamer) 0.07
*No longer present after heat drying of the formulation
Outer jacket 22 is thinner than inner Jacket 20.
The preferred thicknesses of the two jackets are in the following
ranges:
outer jacket 0.05 to 0.5 millirneters
inner jacket 0.2 to 3.0 millimeters
Notwithstanding these two thickness ranges the preferred ratio between
outer J'acket 22 and inner jacket 20 is in the range of 0.02 to 0.5.
The latex emulsion is preferably sprayed onto
inner ~acket 20 to form cable 10.
The cable of the invention exhibits (under conditions
of high heat flux) a low rate of flame spread and a lengthy time
to ignition, low cumulative evolution and low rate of evolution of
carbon monoxide and gases containing halogen or phosphorus~ and a
low rate of smoke output, while under low temperatures, for example -30C,
flexibility of the cable is retained.
A test of a cable of the invention was carried out
using as a sample an outer layer having the composition of ExampleI
and using a standard single layer jacketed cable for comparison. A
chamber wlth radiant heat panel and pilot flames (commonly called
the Ohio State University Release Rate Apparatus) was used. Such a
chamber is equipped with a window, a sealable entrance and a 15 cm
x 15 cm vertical plate to hold cable samples exposed to the radiant
panel. A thermocouple, a photo-electric detector, and a gas sampling
tube are included. Thirty minute exposures were performed at a
set incident heat flux from the radiant panel. The cable models
were stacked horizontally on the vertical plate. The samples were
about 30 cm from the panel. A pilot flame at the bottom of the
sam~le array was used to help lnduce combustion. Heat, smoke, and
carbon monoxide outputs were continuously monitored. Six inch cable
core lengths were wrapped with jacketing material of the desired
thickness. Several lengths were stacked and tied to the support.
The following results were produced by testing the
two samples:
Sample 1 Sample 2
Core: 50 pr FVC insulated 50 pr PVC insulated
24 A~G copper 24 AWG copper
Inner Jacket: 0.76 mm PVC 0.76 mm PVC
Outer Jacket: - 0.10 mm Flame
retardant outer jacket
(Example I)
~ASUR~ PARAMEr~
@ 30 kW/m2 HEAT FLUX Sample 1 Sample 2
Ignition time (sec)* 21 123
(~ Time to onset of net heat release from sample)
Cumulative Smoke Output after
300 sec. (smoke units)1.60 0.80
Cumulative Carbon Monoxide
0utput at-ter 300 sec.
(mL/Meter length) 4463 323
It will be appreciated that mor~ than one resin, filler
or stabilizer may be used in the composition of the invention.
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