Electric cable with combined radiation cross-linked and non-cross-linked insulation

A low-voltage cable with insulation having good fire self-extinguishing and life cycle properties and with improved heat resisting and low toxic gas generation properties. The cable is formed by extruding a mixture of a polymer which is substantially non-cross-linkable by irradiation, a monomer cross-linkable by irradiation and optionally, a fluidizing polymeric material which is cross-linkable by irradiation around a conductor and subjecting the so-formed cable to irradiation to cross-link the materials cross-linkable by irradiation.

The present invention relates to heat-resistant and fire non-propagating, 
low-voltage cables of the type commonly referred to as "building wires" 
and which are used in civil and military installations. 
The known heat-resistant and fire non-propagating cables usually include a 
conductor covered by a compound based upon cross-linked polymeric 
materials. 
Among the already known heat-resistant and fire non-propagating cables, 
those which present better characteristics, with respect to such 
properties, are those that have the conductor covering made from a 
compound based upon fluorinated-polymers, such as, ethylene 
tetrafluoroethylene copolymer and ethylene chlorotrifluoroethylene 
copolymer which, in addition to such properties, allow the conductor 
insulation to have a reduced thickness. However, the drawback of these 
known cables is that they emit toxic smoke during fires because of the 
fact that, when the insulating coverings burn, they generate gases 
containing fluorine and chlorine, and/or their mixtures. 
One object of this invention is to provide low-voltage, fire 
non-propagating electric cables which have an improved heat-resistance 
with respect to the known cables of the same type, which do not generate 
toxic gases during fires and which also have a conductor covering of a 
smaller thickness as compared to the conductor coverings obtained with the 
use of the copolymers set forth hereinbefore. 
The low voltage electric cable of the present invention comprises at least 
one conductor and at least one insulating covering layer for the conductor 
which is the result of irradiating a mixture of at least two polymeric 
materials, one of which is readily cross-linked through irradiation and 
the other of which is substantially not cross-linkable by irradiation, the 
latter polymeric materials being at least one of the polyarylates, 
aromatic polyethersulfones, aromatic polysulfones, aromatic polysulfides, 
aromatic polyetherimides, aromatic polyimides, aromatic polyamides and 
aromatic polyimideamides, the material which is readily cross-linked by 
irradiation is a monomer polymerizable by irradiation being at least one 
of triallycyanurate, triallyisocyanurate, trimethylolpropane 
trimethacrylate and ethoxylated bis-phenol-A-trimethacrylate.

In the drawing, the cable includes a conductor 1 formed either by a single 
wire, for example, of copper or aluminum, or by a plurality of wires made 
of copper or aluminum layed-up together. The conductor 1 has an insulating 
covering 2 which is the result of irradiating a mixture of the polymeric 
materials described hereinbefore and which has the characteristics 
described hereinafter. 
In the embodiment shown in the drawing, the conductor covering 2 is formed 
of a single layer, but the conductor covering 2 can be formed by a 
plurality of superimposed similar layers. 
The fundamental characteristic of a mixture for forming the 
conductor-covering 2, according to the invention, is that it includes at 
least two basic components prior to subjecting the mixture to irradiation. 
The first of these two essential components of the mixture is a polymeric 
material, called the "base polymer" herein, which is substantially not 
cross-linkable by the irradiation to which the mixture is subjected and 
which is one or more of the polyarylates, aromatic polyether sulfones, 
aromatic polysulfones, aromatic polysulfides, aromatic polyetherimides, 
aromatic polyimides, aromatic polyamides and aromatic polyimide-amides. 
Since all the above-listed polymers are predominantly aromatic in nature, 
they are substantially not cross-linkable through irradiation when they 
are subjected to the normally used amounts of radiation-energy used for 
cross-linking. 
The second essential component present in the mixture, prior to the 
cross-linking, is a monomer polymerizable by irradiation and which is one 
or more of triallylcyanurate, triallylisocyanurate, 
trimethylol-propanetrimethacrylate and ethoxylated 
bis-phenol-A-trimethacrylate. 
Prior to the cross-linking of the second component by irradiation, one of 
these monomers is present in the mixture in an amount in the range from 
about at least 5 to about 100 parts by weight with respect to 100 parts by 
weight of the base polymer and preferably, is present in an amount in the 
range from about 10 to about 30 parts by weight with respect to 100 parts 
by weight of the base polymer. 
A mixture containing the essential components, which is an extrudable 
compound, is extruded in a conventional way, over the conductor 1 in such 
a manner as to form a covering over it, and thereafter, it is subjected to 
cross-linking irradiation by passing the so-covered conductor through a 
device, of the per se known type, conventionally used for irradiation 
cross-linking. 
Due to the irradiation, the monomer present in the covering which is a 
poly-functional unsaturated monomer, undergoes polymerization. Since the 
monomer in the covering is uniformly distributed and also poly-functional, 
it forms a tridimensional polymeric net (when it is polymerized) which 
encloses in its meshes the base polymer which has not been cross-linked by 
the irradiation. 
Preferably, the mixture for forming the covering of a cable-conductor 
according to this invention, includes a third component which serves the 
purpose of fluidizing said mixture, during its extrusion for forming the 
covering. When said third, fluidizing component is present, such component 
is a polymer which is cross-linkable by the irradiation which is used to 
cross-link the second component and may be ethylene-propylene-diene 
polymer (EPDM) or silicone rubbers. As a consequence, during the 
cross-linking, by irradiation, of the conductor covering, even the 
fluidizing component is cross-linked which contributes to the forming of 
the polymeric meshes which enmesh the base polymer. The fluidizing 
component may be present in an amount up to 50 parts by weight per 100 
parts by weight of the base polymer and preferably, is present in 5 to 30 
parts by weight per 100 parts by weight of the base polymer. 
Two specific examples of mixtures of the conductor covering compound of the 
invention are as follows: 
EXAMPLE 1 
The conductor covering compound of this example contains only the 
fundamental components according to the invention. The recipe for this is 
as follows: 
__________________________________________________________________________ 
a. 
AROMATIC POLYETHERIMIDE such as, for example, 
100 
parts by weight 
that sold by General Electric 
under the trade name ULTEM 1000 
b. 
TRIMETHYLOL PROPANE TRIMETHACRYLATE 
15 parts by weight 
c. 
ANTIOXIDIZER such as that sold by Monsanto 
1.5 
parts by weight 
under the trade name SANTONOX R 
__________________________________________________________________________ 
EXAMPLE 2 
In addition to the essential components, the compound of this example also 
contains a further component for fluidizing the compound during its 
extrusion. The recipe for this is as follows: 
__________________________________________________________________________ 
a. 
AROMATIC POLYETHERIMIDE such as, for example, 
100 
parts by weight 
that sold by General Electric 
under the trade name ULTEM 100 
b. 
TRIMETHYLOL PROPANE TRIMETHACRYLATE 
10 parts by weight 
c. 
EPDM 5 parts by weight 
d. 
ANTIOXIDIZER such as that sold by Monsanto 
1 parts by weight 
under the trade name SANTONOX 
__________________________________________________________________________ 
With these said compounds two cables, according to the invention, have been 
made by extruding the compounds over a conductor in such a way as to form 
a covering for the latter and thereafter, subjecting the thus obtained 
cables, to an irradiation at 10 Megarads, by means of a known device, so 
as to cause the irradiation cross-linking of the cross-linkable component 
or components of the conductor covering formed by the compounds. 
Specifically, two cables were manufactured, each cable having a conductor 
with a cross-section of 1 mm.sup.2 and an irradiated covering with a 
thickness equal to 0.15 mm. 
Experimental tests were carried out on these two cables--for ascertaining 
their heat-resistant and non fire-propagating properties and also for 
checking the extent of any toxic gases which may be generated by the 
burning of said coverings. 
Similar experimental tests were also carried out on a prior art cable of 
the same category and having the same dimensions, the conductor covering 
of which was made of a radiation cross-linked compound based on an 
ethylene-tetrafluoroethylene copolymer. Said prior art cable is known to 
those skilled in the art as being one of the best known cables provided 
with a cross-linked conductor covering as far as fire-propagation 
resistance and heat-resistance are concerned. 
The test for checking the resistance to fire-propagation was carried out 
according to the U.L. STANDARDS 44. 
For this purpose, a length of cable was placed in a vertical position. 
Next, a flame was applied, for a period of 15 seconds, to the lower end of 
this cable length. After moving the flame away from the cable length, the 
period of time for the lighted conductor covering to extinguish by itself 
was noted, and the length of the conductor covering which actually 
suffered combustion was also measured. 
The determination of the heat-resistance was effected by means of the two 
tests established by the MIL-W-22759D STANDARD. 
The first of these two tests is the one that, in MIL-W-22759D is called 
"Dynamic Cut-Through Test", and it is carried out through the means of a 
special device provided for such test. The device in question includes a 
support upon which a length of cable is placed. 
Above the cable, and placed transversally to it, there is disposed a blade 
connected to an arm with the latter being hinged at one extremity to the 
structure of the device, while at its other extremity, the arm is provided 
with means which are capable of applying a weight, the amount of which 
increased by 200 g. per minute. 
The blade and the cable under test are inserted, in series, into an 
electrical circuit, and the whole is enclosed within a thermostatically 
regulated ambient set at a temperature established for a test, which, in 
this particular case, is 150.degree. C. 
After having inserted the cable into the above-described apparatus, the 
value of the weight applied to the arm which makes an incision in the 
conductor covering of a depth which will bring the blade into contact with 
the cable conductor itself is determined. The achievement of this 
situation is indicated by the flow of the current in the circuit wherein 
said elements are disposed in series. 
The second test, for determining the heat-resistance characteristics, is 
the one which, in MIL-W-22759D is called "Life Cycle Test". 
For this second test, a U-shaped length of cable is disposed around a 
mandrel having a diameter of 12 mm. and weights of 0.700 kg. are applied 
to the cable ends. 
The just desoribed unit is then housed inside an air-circulating furnace 
having a temperature of 300.degree. C., and it is left therein for 7 
hours. After such period, a cooling takes place which, within an hour, 
reduces the temperature of the unit to 20.degree. C. 
After this operation the cable-length is wound completely over a 12 cm 
diameter mandrel, first in one sense and then in the opposite sense, while 
subjecting it to traction by weights of 0.700 kg. applied to the ends. 
Successive to this treatment, the cable-length is immersed in a water 
solution containing 5% of a sodium-chloride solution, and after a 5 hour 
period of immersion, a voltage of 3 kV is applied between the extremities 
of the cable conductor and the solution, such voltage being applied to it 
for 5 minutes. 
The test, for determining the toxicity of the gases which are generated 
during the combustion of the cable-covering, is carried out, by the means 
described hereinafter, for drawing up a "Halogen Index" which, in this 
test, signifies the quantity of the halogenated compounds formed expressed 
as a percentage by weight of hydrofluoric acid with respect to 100 g. of 
the irradiated material which forms the covering of the conductor. 
The determination of this value is effectuated by means of burning a sample 
of 0.5 g. of the material forming the conductor covering of the cables 
according to the invention and of the covering of the above-mentioned 
"known", or prior art, cable and causing the gases thus obtained (for 
each) to bubble in a sodium hydroxide solution. The quantity of halogen 
ions which are present in the solution, is then determined by the methods 
set forth in the ASTM-D512 STANDARD. 
On the basis of these values, the actual quantity of the said halogens 
present, can then be determined by means of calculations known to those 
skilled in the art, and the "Halogen Index" can also be determined. 
The above-mentioned experimental tests were carried out on samples of the 
two cable lengths, according to the present invention, with their 
conductor covering formed with compounds cross-linked through irradiation 
(as given previously by way of example) and also on a sample of the prior 
art cable which is recognized as being one of the best examples of a cable 
as far as the heat-resistance of its conductor covering is concerned, the 
latter having a covering made from a compound based upon 
ethylene-polytetrafluoroethylene copolymer which is cross-linked through 
irradiation. 
The results of these Experimental Tests are given in the following Table: 
__________________________________________________________________________ 
EXAMPLE 1 EXAMPLE 2 Prior art irradiation 
irradiation 
irradiation 
treated ethylene- 
treated treated tetrafluoroethylene 
covering covering covering 
__________________________________________________________________________ 
FIRE-PRO- Self-Extinguishing 
Self-Extinguishing 
Self-Extinguishing 
PAGATION Time: less than 
Time: less than 
Time: less than 
according 5 seconds 5 seconds 5 seconds 
to the Length of cable 
Length of cable 
Length of cable 
STANDARD- tract burned: 
tract burned: 
tract burned: less 
UL 44 less than 20 cm 
less than 20 cm 
than 20 cm 
"DYNAMIC 19 Kg. 17 Kg. 9 Kg. 
CUT- 
THROUGH 
TEST" 
according 
to the 
STANDARD- 
MIL-W22759D1 
"LIFE CY- RESISTS RESISTS RESISTS 
CLE TEST" at 3 kV: for 
at 3 kV: for 
at 3KV: 
according 5 minutes 5 minutes for 5 minutes 
to the 
STANDARD- 
MIL-W22759D 
"HALOGEN INDEX 
0 0 45 
in weight of 
hydrofluoric 
acid per 100 g. 
of compound 
__________________________________________________________________________ 
From the results of the experimental tests set forth in the Table, it can 
be seen that with cables according to the present invention, the objects 
of the invention can be achieved. 
In fact, whereas, with respect to the resistance to fire-propagation, the 
cables of this invention have the same characteristics as those of a prior 
art cable having a covering formed by an ethylene-tetrafluoroethylene 
copolymer, the characteristics of heat-resistance of the cables of the 
invention provide better results, as compared to those of the prior art 
cable, with respect to "Dynamic Cut Through" which means that as compared 
to the prior art cable, the thickness of the conductor-covering can be 
reduced as a consequence of the high values obtained from the "Dynamic Cut 
Through". 
Furthermore, the test results of the tests for determining the toxicity of 
the gases that are generated during fires, show that with the known cables 
according to the present invention, as contrasted with the prior art 
cable, no danger is to be feared by reason of the formation of halogenated 
compounds during fires. 
An explanation for the results obtained with cables according to the 
invention may be as described hereinafter. 
With respect to heat resistance, the better performance of the cables 
according to the invention with respect to the known cables, could be due 
to the following reasons. 
Even if the base polymers of the compounds, forming the covering of a cable 
according to the invention have softening temperatures of lower than 
300.degree. C., the fact that they are enclosed inside the tridimensional 
net formed by a polymer obtained through the radiation polymerization of a 
poly-functional monomer allows for the unit to possess a considerable 
dimensional stability at high temperatures. Probably, this is because the 
net is formed in the presence of the base polymer, and hence, it results 
that the net is closely connected to it. 
Finally, the possibility of introducing substances having a high fluidizing 
action into the compounds forming the insulating covering of the cable 
conductor of the invention which are formed by polymers which are 
cross-linkable through irradiation, apart from the fact of aiding and 
speeding up the formation through extrusion of the conductor coverings, 
also contributes, along with the polymerizable monomers, in creating the 
cross-linked polymeric net which encases the base polymer of the compound. 
Although preferred embodiments of the present invention have been described 
and illustrated, it will be apparent to those skilled in the art that 
various modifications may be made without departing from the principles of 
the invention.