Note: Descriptions are shown in the official language in which they were submitted.
S74
MP1088FF
This invention relates to insulated electrical
conductors.
It is known to use cross-linked polymeric com-
positions as electrical insulation on a wire or otherconductor. Known insulated wires include wires coated
with a layer of a radiation crosslinked fluorocarbon
polymer, particularly an ethylene/tetrafluoroethylene
copolymer (often referred to as an ETFE polymer), which
are extensively used for the wiring in aircraft.
Military Specification No. MIL-W-22759 sets various
standards for such insulated wires. Reference may be
made for example to U.S. Patents Nos. 3,763,222,
3,840,619, 3,894,118, 3,911,192, 3,970,770, 3,985,716,
3,995,091, 4,031,167, 4,155,823 and 4,353,961. Such
wires have the significant disadvantage that if the
outer surface of the insulation is damaged, subsequent
flexing of the wire causes the damage to propagate
through the insulation, at a rate which is highly unde-
sirable. This disadvantage is especially serious whenthe insulated wire is to be used in an aircraft or in
other high performance situations where the consequen-
ces of insulation failure can be so serious. A quan-
titative measure of this disadvantage can be obtained
from a notch propagation test such as that described
below.
We have discovered that this disadvantage can be
substantially mitigated by insulation which comprises
an inner layer of a polymer which has little or no
t~
MP1088FF
--2--
cross-linking and an outer layer of polymer which has a
relatively high level of cross-linking. Furthermore,
this improvement is obtained with little or no substan-
tial deterioration of other important properties of the
insulation, for example, resistance to scrape abrasion,
resistance to crossed wire abrasion and resistance to
cut-through.
In one aspect, the present invention provides
an insulated electrical conductor, especially a wire,
which comprises
(l) an electrical conductor; and
(2) electrical insulation which comprises
(a) an inner electrically insulating layer
which (i) is composed of a first melt-
processed, cross-linked polymer com-
position wherein the polymer has a
melting point of at least 200C, and (ii)
has a first Mloo value of 0 to 24.5 kg/cm2
(0 to 350 psi);
and
(b) an outer electrically insulating layer
which (i) is separated from the conductor
by the inner layer, (ii) is composed of a
second melt-processed cross-linked poly-
meric composition wherein the polymer has
a melting point of at least 200C, and
(iii) has a second Mloo value which is at
least 24.5 kg/cm2 (350 psi) and at least
~5S74
MP1088FF
3.5 kg/cm2 (50 psi) higher than the first
Mloo value.
The Mloo values given herein are modulus values
measured at a temperature above the melting point of
the polymer by the procedure described in detail below,
and therefore reflect the level of cross-linking in the
layer.
A preferred method of making such an insulated
conductor comprises
(1) melt-shaping the first polymeric composition
to form the first layer;
(2) melt-shaping the second polymeric composition
to form the second layer in contact with the
first layer, the second composition containing
a radiation cross-linking agent;
(3) maintaining contact between the first and
second layers under conditions such that part
of the radiation cross-linking agent migrates
from the second layer into the first layer;
and
(4) irradiating the first and second layers to
effect cross-linking thereof.
The polymeric component in the polymeric com-
positions used in the present invention preferably
comprises, and more preferably consists essentially of,
i;S74
MP1088FF
--4--
a melt-shapeable crystalline polymer having a melting
point of at least 200C, preferably at least 250C, or
a mixture of such polymers. The term "melting point"
is used herein to denote the temperature above which no
crystallinity exists in the polymer (or, when a mixture
of crystalline polymers is used, in the major
crystalline component of the mixture). Particularly
preferred polymers are fluorocarbon polymers. The term
"fluorocarbon polymer" is used herein to denote a
polymer or mixture of polymers which contains more than
10%, preferably more than 25~, by weight of fluorine.
Thus the fluorocarbon polymer may be a single fluorine-
containing polymer, a mixture of two or more fluorine-
containing polymers, or a mixture of one or more
fluorine-containing polymers with one or more polymers
which do not contain fluorine. Preferably the fluoro-
carbon polymer comprises at least 50~, particularly at
least 75%, especially at least 85%, by weight of one or
more thermoplastic crystalline polymers each containing
at least 25% by weight of fluorine, a single such
crystalline polymer being preferred. Such a fluorocar-
bon polymer may contain, for example, a fluorine-
containing elastomer and/or a polyolefin, preferably a
crystalline polyolefin, in addition to the crystalline
fluorine-containing polymer or polymers. The fluorine-
containing polymers are generally homo-or co-polymers
of one or more fluorine-containing olefinically
unsaturated monomers, or copolymers of one or more such
monomers with one or more olefins. The fluorocarbon
polymer has a melting point of at least 200C, and will
often have a melting point of at least 250C, e.g. up
to 300C. Preferably the polymeric composition has a
1 2~5S74
MP1088FF
--5--
viscosity of less than 105 poise at a temperature not
more than 60C above its melting point. A preferred
fluorocarbon polymer is a copolymer of ethylene and
tetrafluoroethylene and optionally one or more other
comonomers, especially a copolymer comprising 35 to 60
mole percent of ethylene, 35 to 60 mole percent of
tetrafluoroethylene and up to 10 mole percent of one or
more other comonomers. Other specific polymers which
can be used include copolymers of ethylene and
chlorotrifluoroethylene; copolymers of vinylidene
fluoride with one or both of hexafluoropropylene and
tetrafluoroethylene, or with hexafluoroisobutylene; and
copolymers of tetrafluoroethylene and hexafluoropropy-
lene.
The polymeric composition can optionally contain
suitable additives such as pigments, antioxidants, thermal
stabilisers, acid acceptors and processing aids.
The first and second compositions preferably
contain the same polymeric components, and more pre-
ferably are substantially the same in all respects
except for the level of cross-linking.
The conductor is preferably a metal (e.g. cop-
per) wire, which may be stranded or solid. The wire
may be for example from 10 to 26 AWG in size. The
first layer preferably contacts the conductor. The
second member is preferably also in the form of a layer
which has the same general shape as the first layer or
which serves to join together a number of wires each of
which is surrounded by a first layer, thus forming a
12~SS~
MP1088FF
--6--
ribbon cable. The layers are preferably in direct con-
tact, but may be joined together by a layer of adhe-
sive.
The first and second members are preferably
formed by melt-extrusion, particularly by sequential
extrusion, which may be tubular or pressure extrusion,
so that the layers are hot when first contacted, in
order to promote migration of the cross-linking agent.
The polymeric compositions should preferably be
selected so that at least the outer layer has a tensile
strength of at least 3,000 psi (210 kg~cm2); and since
a higher tensile strength is usually desired in the
cross-linked product and there is frequently a loss of
tensile strength during the irradiation step, a higher
initial tensile strength is preferred, e.g. greater
than 6,000 psi (420 kg/cm2), preferably at least 7,000
psi (490 kg/cm2), particularly at least 8,000 psi (560
kg/cm2 ) .
The thickness of the inner layer is generally
0.0075 to 0.038 cm (0.003 to 0.015 inch), preferably
0.007S to 0.0225 cm (0.003 to 0.009 inch). The
thickness of the outer layer is generally 0.01 to 0.063
cm (0.004 to 0.025 inch), preferably 0.01 to 0.023 cm
(0.005 to 0.009 inch).
Preferred radiation cross-linking agents con-
tain carbon-carbon unsaturated groups in a molar per-
centage greater than 15, especially greater than 20,particularly greater than 25. In many cases the cross-
linking agent contains at least two ethylenic double
lZ~5574
-7- 26775-78
bonds, which may be present, for example, in allyl,
methallyl, propargyl or vinyl groups. we have obtained
excellent results with cross-linking agents containing
at least two allyl groups, especially three or four
allyl groups. Particularly preferred cross-linking
agents are triallyl cyanurate (TAC) and triallyl isc-
cyanurate (TAIC); other specific cross-linking agents
include triallyl trimellitate, triallyl trimesate,
tetrallyl pyromellitate, the diallyl ester of 1,1,3-
trimethyl-5-carboxy-3-(~-carboxyphenyl) indan. Other
cross-linking agents which are known for incorporation
into fluorocarbon polymers prior to shaping, for
example those disclosed in U.S. Patents referenced
above, can also be used. Mixtures of cross-linking
agents can be used.
In the preferred method of preparing articles
of the invention, in which cross-linking agent migrates
from the second layer to the first layer, the first
composition as extruded contains little or no cross-
linking agent (e.g. 0 to 2% by weight, preferably 0%),
and the second composition as extruded contains more
than is desired during the cross-linking step, e.g. at
least 5%, preferably 5 to 25~, particularly 7 to 12~,
by weight. The time for which the layers should be
maintained in contact prior to cross-linking depends
upon the extent of migration which is needed and the
temperature during such contact, which i5 preferably S
to 15CC below the melting point of the polymer (or the
lower melting polymer if there are two or more polymers
in the layers). At the time of irradiation, the inner
layer preferably contains 0 to 3~ by weight of cross-
5574
MP1088FF
linking agent and the outer layer preferably contains 3to 10% by weight of crosslinking agent.
The dosage employed in the irradiation step is
generally below 50 Mrads to ensure that the polymer is
not degraded by excessive irradiation, and the dosages
preferably employed will of course depend upon the
extent of cross-linking desired, balanced against the
tendency of the polymer to be degraded by high doses of
irradiation. Suitable dosages are generally in the
range 2 to 40 Mrads, for example 2 to 30 Mrads, pre-
ferably 3 to 20 Mrads, especially 5 to 25 or 5 to 20
Mrads, particularly 5 to 15 Mrads. The ionising
radiation can for example be in the form of accelerated
electrons or gamma rays. Irradiation is generally
carried out at about room temperature, but higher tem-
peratures can also be used.
The inner layer need not be cross-linked at all,
but is preferably cross-linked so that it has an Mloo
value of 2.8 to 17.5 kg/cm2 (40 to 250 psi), par-
ticularly 3.5 to 10.5 kg/cm2 (50 to 150 psi). The
elongation of the inner layer is preferably at least
100%, particularly at least 125%, eg. at least 150%,
especially 200 to 300%.
The outer layer is preferably cross-linked so
that it has an Mloo value of at least 28 kg/cm2 (400
psi), particularly at least 31.5 kg/cm2 (450 psi), with
yet higher values of at least 42 kg/cm2 (600 psi) being
valuable in many cases. The elongation of the outer
layer is preferably 40 to 150%, particularly 50 to
120%.
~295574
MP1088FF
_g_
The various physical properties referred to in
this specification are measured as set out below.
The Notch Propagation Values are measured on a
piece of insulated wire about 30 cm (12 inch) long. A
notch is made in the insulation, about 5 cm (2 inch)
from one end, by means of a razor blade at right angles
to the axis of the wire. The depth of the notch is
controlled by mounting the razor blade between two
metal blocks so that it protrudes by a distance which
is 0.01 cm (0.004) inch or, if the insulation comprises
two layers and the outer layer has a thickness t which
is less than 0.018 cm (0.007 inch) thick, by a distance
which is tt-0.0051) cm [(t-0.002) inch]. The end of
the wire closer to the notch is secured to a horizontal
mandrel whose diameter is three times the outer
diameter of the insulation. A 0.675 kg (1.5 lb) weight
is secured to the other end of the wire so that the
wire hangs vertically. The mandrel is then rotated
clockwise, at about 60 revolutions a minute, until most
of the wires has wrapped around the mandrel. The
mandrel is then rotated, counterclockwise, until the
wire has unwrapped and most of the wire has again been
wrapped around the mandrel. The mandrel is then
rotated clockwise until the wire has unwrapped and most
of the wire has again been wrapped around the mandrel.
This sequence is continued until visual observation of
the notched area shows the conductor to be exposed.
If, at this time, the conductor is broken (or some or
all of the strands of a stranded wire conductor are
broken) then the failure is attributable to that
breakage, not to propagation of the notch through the
5~4
MP1088FF
--10--
insulation. The number of cycles (half the number of
times the rotation of the mandrel is reversed) is
recorded.
The Mloo _alues are determined by a static
modulus test as described in U.S. Patent No. 4,353,961
carried out at about 40C above the melting point of
the polymer, (e.g. at about 320C for ETFE polymers).
The tensile strenqths and elonqation_ are
determined in accordance with ASTM D 638-72 (i.e. at
23C) at a testing speed of 50 mm (2 inch) per minute.
The cross-wire abrasion values, the
cut-through resistance values, and the scra~ abrasion
values, are measured by the tests described in U.S.
Patent No. 4,353,961.
The invention is illustrated by the following
Examples, in which Example 1 is a comparative Example.
Exampl_ 1
A 20 AWG (19/32) stranded tin-coated copper wire
was insulated by melt-extruding over it, by sequential
extrusion, an inner insulating layer 0.01 to 0.0125 cm
(0.004 to 0.005 inch) thick and an outer insulating
layer .018 to 0.020 cm (0.007 to 0.008 inch) thick.
The layers were composed of the following compositions:
S74
-11- MP1088FF
% by weight
Inner Outer
ETFE polymer 94.6 89.8
(Tefzel T~ from duPont)
Additives 0.8 3.2
Triallyl isocyanurate 4.6 7.0
The polymeric insulation was cross-linked by irradiating
it to a dosage of 14 Mrads.
Example 2
The procedure of Example 1 was followed except that
the composition of the inner layer was
ETFE polymer 99.2
(TefzelT~ from duPoint)
Additives 0.8
Triallyl isocyanurate
The products of the Examples were subjected to the
various tests described above and the following results
(averaged for a number of specimens) were obtained.
~ ;~g~5~4
-12- MP1088FF
Example 1 Example 2
Tensile strength kg/cm2 (psi) 475.3 (6790) 508.9 (7270)
Elongation
Inner Layer 35250
Outer Layer 75 70
Notch Propagation (cycles) 43 90*
Range for 10 specimens (62) (42)
Cut Through Resistance 49 44
Range for 10 specimens (32) (29)
Scrape Abrasion Resistance 58 54
Range for 10 specimens (38) (38)
Ml o o kg/cm2 ( ps i )
Inner Layer 48.6 (694) 7.9 (113)
Outer Layer 50.7 (725) 49.6 (708)
Crossed Wire Abrasion
(cycles x 10-6)
at load of 1.4 Kg 0.137 0.236
1.2 0.252 0.424
1.0 0.520 0.851
0.8 1.261 1.996
0.6 3.950 5.984
0.4 19.750 28.137
*In most of the specimens, the cause of failure was breakage
of the conductor strands.
