Note: Descriptions are shown in the official language in which they were submitted.
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WIRE
This invention relates to electrical wires, and
especially to wires that employ electrical insulation
based on aromatic polymers.
Electrical wire and cable that use aromatic
polymer insulation have been used for many years in
numerous applications. For example wires that employ
polyimide wraps or tapes usually bonded with fluoro-
polymer adhesive layers have been used extensively as
oircraEt wire, for both civil and military applica-
tions. Other examples of aromatic insulation that have
been used for equipment wire or "hook-up" wire, air
frame wire and in wire harnesses include aromatic
polyether ketones, polyether ether ketones, modified
polyphenylene oxide, and polyimide amides. Highly aro-
matic polymers have been used successfully in many
applications because they have a range of desirable
properties especially high strength and toughness,
abrasion resistance, temperature resistance, dielectric
strength and are often inherently highly flame-
retarded.
The combination of these properties has enabled
wlre and cable fabricated rom these polymers to ~e
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used in small lightweight constructions. Such
constructions have been used increasingly in both mili-
tary and civil applications due to the high density and
complexity of modern electrical systems.
However, these highly aromatic polymers suffer
from a major problem: they are particularly suscep-
tible to tracking. Tracking is a phenomenon associated
with the formation of permanent and progressive con-
ducting paths on the surface of the material by the
combined effects of an electrical field and external
surface pollution. Once commenced, the carbonaceous
conducting deposits often extend quickly in dendritic
fashion to give a characteristic "tree" pattern until
failure occurs across the surface. Electrical tracking
can occur when a damaged energised bundle of wires
become wet e.g. from electrolytes or condensation.
This tracking may lead to flashover and arcing that
causes additional wires in the bundle to become
damaged. A catastrophic cascade failure can result
from a fault to a single wire if adjacent wires that
are at a different electrical potential are also
susceptible ~o tracking or i the bundle is in contact
with a grounded structure. Tracking can occur at low
voltages e.g. lOOV a.c. or less but becomes less likely
as the voltage is rèduced.
A related phenomenon, to which these polymers are
also highly susceptihle, is that of breakdown due to
arcing.~ In this case a potential diference between
two conductors, or between a conductor in which the
in~ulation has been mechanically damaged, and ground,
an result in the formation of an arc between the con-
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ductors or between the conductor and ground. The high
temperature of the arc causes the polymer to degrade
extremely rapidly and form an electrically conductive
carbonaceous deposit which can extend rapidly, as with
wet tracking, and lead to catastrophic ~ailure in which
many or all of the wires in a ~undle are destroyed.
~rcing can occur at very low voltages, for example 24V
d.c. or lower, and since, unlike tracking, no electro-
lyte or moisture is involved, it is a particularly
hazardous phenomenonO Arcs may also be struck by
drawing apart two conductors between which a current is
passing as described for example by J.M. Somerville
"The Electric Arc", Methuen 1959.
Another phenomenon that can be associated with
tracking and arcing is erosion. In this case insu-
lating material is removed by a vapori~ation process
originated by an electrical discharge without the for-
mation o~ electrically conductive deposits so that
ailure of the insulation will not occur until complete
puncture of the insulation occurs.
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According to the present invention, there is pro-
vided an electrical wire which comprises an elongate
electrical conductor and electrical insulation that
comprises:
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(a) an inner insulating layer which comprises a poly-
ester that has both aromatic and aliphatic
moieties and has a molar carbon to hydrogen ratio
of not more than 1.15; and
~b) an outer insulating layer which comprises a
fluorinated polymer.
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The invention has the advantage that it enables a
wire to be formed that has a balance of properties such
as solvent resistance, scrape abrasion resistance,
toughness, weight and ability to strip in addition to
very high resistance to tracking and arcing.
The polyester preferably has a molar carbon to
hydrogen ratio of not more than 1.1 and especially not
more than 1Ø This will normally correspond to a car-
bonaceous char residue of not more than 15~, preferably
not more than 10%, most preferably not more than 5%,
especially not more than 2% and most especially
substantially 0% by weight.
The char residue of the polymer components in the
electrical wire according to the invention can be
measured by the method known as thermogravimetric ana-
lysis, or TGA, in which a sample of the polymer is
heated in nitrogen or other inert atmosphere at a
defined rate, e.g. 10C per minute to a defined tem-
perature and the residual weight, which is composed of
char, is recorded. The char residue is simply the
quantity of this residual char expressed as a percen-
tage of the initial polymer after having taken into
account any non polymeric volatile or non-volatile com-
ponentsO The char residue values quoted herein are
defined as having been measured at 850C.
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The polyesters that are used for layer (a) pre-
ferably include those based on a polyalkylene diol,
preferably having a least 3 carbon atomsl or a cyclo-
aliphatic diol and an aromatic dicarboxylic acid.
Preferred polyesters include polytetramethylene
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terephthalate, and cycloaliphatic diol/terephthalic
acid copolymers e.g. copolymers of -terephthalate and
isophthalate units with 1,4-cyclohexanedimethyloxy
units. The polyesters can include polyether esters,
for example polyether poly~ster block copolymers having
long chain units of the general formula:
O O
Il ~I
-OGO-C-R-C-
and short-chain ester units of the formula
O O
-ODO-C-R-C-
in which G is a divalent radical remaining after
the removal of terminal hydroxyl groups from a
polyalkylene oxide) glycol, preferably a poly (C2
to C4 alkylene oxide) having a molecular weight of
about 600 t~ 6000i R is a divalent radical
remaining after removal of carboxyl groups from at
least one dicarboxylic acid having a molecular
weight of less than about 300; and D is a divalent
radical remaining after removal of hydroxyl groups
from at least one diol having a molecular weight
less than 250.
Preferred examples of such copolyesters are the
polyether ester polymers derived from terephthalic
acid, polytetramethylene ether glycol and
1,4-butane diol. These are random block copoly-
mers having crystalline hard blocks with the
repeating unit:
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- ( CH2 ) 4-o-c~
and amorphous, elastomeric polytetramethylene
ether terephthalate soft blocks of repeating unit
[ O ( CH2 ) D~ ~O-C~ 11-
having a molecular weight of about 600 to 3000,
i.e. n = 6 to 40.
If desired the polyester may be blended with one
or more other polymers. ~For example the polyesters may
be used as blends with polyamides~ polyolefins such as
polyethylene, ethylene ethyl acrylate copolymers
styrene/diene block copolymers or ionomers.
The fluorinated polymer used in layer (b) pre-
f~rably 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
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with; one or more polymers which do not contain
fluorine. In one preferred class, the fluorocarbon
polymer comprises at least 50%, particularly at least
75% espclally 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 bei~g preferred. Such a fluoro-
carbon polymer may contain, for example, a fluorine-
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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 copolymers
of one or more fluorine-containing olefinically unsa-
turated monomers, or copolymers of one or more such
monomers with one or more olefins. The fluorocarbon
polymer usually has a melting point of at least 150C,
and will often have a melting point of at least 250~C,
e.g. up to 350C, the melting point being defined for
crystalline polymers as the temperature above which no
crystallinity exists in the polymer (or when a mixture
of crystalline polymers is used, in the major
crystalline component in the mixture). Preferably the
polymeric composition, prior to ~ross-linking, has a
viscosity of less than 104 Pa.s (105 poise) at a temp-
erature not more than 60~C above its melting point. A
preferred fluorocarbon polymer is a copolymer of ethy-
lene and tetrafluoroethylene and optionally one or more
other comonomers (known as ETFE polymers), 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; polyvinylidene
fluoride; copolymers of vinylidene fluoride with one or
both of hexafluoropropylene and tetrafluoroethylene, or
with hexafluoroisobutylene; and copolymers of tetra-
fluoroethylene and hexafluoropropylene. Alternatively
Cl-Cs perfluoroalkoxy substituted perfluoroethylene
homopolymers and copolymers with the above fluorinated
polymers may be used.
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The wire insulation, or at least the outer layer,
may be cross-linked, for example, by exposure to high
energy radiation.
Radiation cross-linking may be effected by expos-
ure to high energy irradiation such as an electron beam
or gamma-rays. Radiation dosages in the range 20 to
800 kGy, preferably 20 to 500 kGy, e g. 20 to 200 kGy
and particularly 40 to 120 kGy are in general
appropriate depending on the characteristics of the
polymer in question. For the purposes of promoting
cross-linking during irradiation, preferably from 0.2
to 15 weight per cent o a prorad such as a poly-
functional vinyl or allyl compound, for example,
triallyl cyanurate, triallyl isocyanurate (TAIC),
methylene bis acrylamide, metaphenylene diamine bis
maleimide or other crosslinking agents, for example as
described in U.S. patents Nos. 4,121,001 and 4,176,027,
are incorporated into the composition prior to irra-
~ ~ diation.
;~ The insulation may include additional additives,for example reinforcing or non-reinforcing fillers,
stabilisers such as ultra-violet stabilisers, antioxi-
dants, acid acceptors and anti-hydrolysis stabilisers,
pigmen~s, processing aids such as plasticizers, haloge-
nated or non-halogenated flame retardants e.g. hydrated
metal oxides such as alumina trihydrate or magnesium
hydroxide, or decabromodiphenyl ether, fungicides and
the like.
In many cases the wire insulation will consist
solely of the polyester inner layer and the fluoropo-
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27065-176
lymer outer layer. However, if desired one or more other layers
may be present. For example an additional inorganic arc-control
layer ~nay be provided directly on the conductor, formed for
example by deposition of an inorganic material on the conductor.
Alternatively or in addition a hiyhly aromatic polymer layer may
be provided between layers (a) and ~b) in order to improve for
example the high temperature properties of the in~ulation.
Examples of such aroma~ic polymers are disclosed in our copending
Canadlan Patent Application Serial No. 571,502 filed on July 8,
19~8.
The wires and cables according ko the invention may be
formed by conventional techniques. For example the polymers may
be blended with any addltional components, in a mixer, pelletised,
and then extruded onto a wire conductor~ O~her wires may be
formed by a tape-wrapping method although it is pre~erred for both
the fluoropolymer and the polyester layers to be melt extruded.
The wires may be used individually as equipment or
'`hook-up" wires, or airframe wires, or in bundles and harnesses,
hoth jacketted and unjacketted, and may be used in multiconductor
cables. The wires, harnesses or cahles may be unscreened or they
may be provided wlth a screen to protect them from electromagnetic
interference, as well known in the art. In addition flat cables
may be formed using the insulation materials according to the
~ invention, either employing ~lat conductors or round conductors.
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The invention will be described by way of example
with reference to the accompanying drawings in which:
Figure 1 is an isometric view oE part of an elec-
trical wire according to the invention;
Figure 2 is a schematic view of the test arrange-
ment for wet tracking; and
Figure 3 is a schematic view of the test arrange-
ment for dry arcing.
Referring initially to figure 1 of the accom-
panying drawings an electrical wire comprises a conduc-
tor 11 which may be solid or stranded as shown and is
optionally tinned. On the conductor an inner insu-
lating layer 12 ~primary insulation) has been extruded.
The insulation is formed from polybutylene terephtha-
late which contains about 5% by weight triallyl
isocyanurate crosslinking promotor. After the inner
layer 12 has been formed an outer layer 13 tprimary
jacket) formed from an ethylene-tetrafluoroethylene
copolymer, containing about 7% by weight triallyl iso-
cyanurate crosslinking promotor, is extruded on the
inner layer 12. Each layer has a wall thickness of
about 100 ~m. After both layers have been extruded the
insulation is irradiated by high energy electrons to a
dose of about 120 kGy.
The following Examples illustrate the invention.
In the Examples the following test procedures were
~ used:
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RK34 4
WET TR~CKING TEST
This test is designed to simulate the condition
occuring when a damaged wire bundle comes into contact
with an electrolyte. Vnder actual conditions, the
electrolyte may be moisture containing dust particles
or other ionic contaminant. Damage to the bundle may
occur through a number of reasons e.g. abrasion, hydro-
lysis of the insulation, ageing, etc. Current flow
through the electrolyte results in heating and evapora-
tion of the solution. This causes one or more dry
bands to appear across which the test voltage is
dropped, resulting in small, often intense, scin-
tillations which damage the insulation.
Figure l shows the sample set-up. A wire bundle 1
is prepared from seven 18cm lengths 2 of 20AWG tinned-
copper conductor coated with a layer of the material
under test. The bundle l is arranged with six wires
around one central wire and is held together using tie
wraps 3 so that the wires are not twisted. Two adja-
cent wires are notched circumferentially to expose
0~5mm bare conductor on each wire. The notches 4 are
arranged such that they are 5mm apart with the tie
wraps 5mm either side of them. One end of each wire is
stripped to enable connections to be made to the power
supply via insulated crocodile clips. The sample is
held at an angle of 30 degrees to the horizontal using
a simple clamp made of an electrically insulating resin
so that the damaged wires are uppermost and the
stripped ends are at the upper end of the bundle. A
piece of f ilter paper 5 20 x 10mm wide is wrapped
around the bundle approximately 2mm above the upper
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notch; this is best held in place with the upper tie
wrap.
A peristaltic pump conveys the electrolyte from
the reservoir to the sample via a dropping pipette 6,
and a power supply is prsvided to energise the bundle.
The electrolyte used is 2~ sodium chloride and
optionally O.02% in ammonium perfluoroalkyl carboxylate
surfactant in distilled or deionised watex. The pump
is set to deliver this solution at a rate of approxima-
tely lOOmg per minute through the pipette 6 which is
positioned lOmm vertically above the filter paper 5.
The power is supplied by a 3-phase 400Hz 115/200V
generator of at least SkVA capacity or a single phase
50Hz llSV transformer of at least 3kVA capacity. A
device for recording time to failure is provided which
records the time when either a wire goes open circuit,
or when a circuit breaker comes out. Leakage currents
can be followed with the use of current clamps
surrounding the wires and connected to a suitable
oscilloscope.
In the case of the three phase supply, adjacent
wires of the bundle are connected to alternate phases
of the power supply via 7.5A aircraft-type circuit
breakers e.g.~Klixon with the cen~ral wire connected
directly to neutral. In the case of the single phase
supply, alternate wires are connected to neutral with
the remaining wires including the central conductor to
live. A few drops of electrolyte are allowed to fall
onto the filter paper to ensure saturation prior to
starting the test. The power is switched on and the
timer started. The test is allowed to continue until:
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a) one or more circuit breakers come out;
b~ a wire becomes open circuit or
c) 8 hours have elapsed.
The condition of the final bundle and the time to
failure is noted in all cases. Where failure has
occurred due to breakers coming out, the power is then
reapplied and each breaker is reclosed in turn until
there is no further activity. The condition of the
bundle is again noted.
Failure due to the wire becoming open circuit
(result (b)) is indicative of erosion. If failure
occurs due to one or more circuit breakers coming out
(result (a)) then the absence of further crepitation on
resetting of the circuit breakers indicates failure due
to erosion, while further crepitation indicates
tracking failure.~ ~
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Dry Arc Test
his test is designed to simulate what happens
when a fault in a wire bundle causes arcing under dry
conditions. A graphite rod is used to initiate the arc
which causes thermal degradatiQn of the insulation.
Continuation of the fault current can only occur
through the wire bundle under test due to shorting
across adjacent phases through a conductive char, or
direct conductor-conductor contact such as might occur
if khe insulation is totally removed by the duration of
the arc.
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Figure 2 shows the sample set-up. A wire bundle
21 is prepared from seven 10cm lengths 22 of 20AWG
tinned-copper conductor coated with a layer of the
wire insulation under test. The bundle 22 is arranged
with six wires around one central wire and held
together with tie wraps spacad about 5cm apart. One of
the outer wires is notched circumferentially between
the tie wraps to expose 0.5mm bare conductor and one
end of each wire is stripped to enable connections to
be made via insulating crocodile clips.
A rod 23 is provided which is made of a spectro-
graphically pure graphite, diameter 4.6mm, with an
impurity level not more than 20ppm. It is prepared
before each test by sharpening one end using a conven-
tional pencil sharpener of European design to give an
angle of 10 degrees off vertical with a tip diameter of
0.4~0.1mm. A 100g weight 24 is clamped onto the top of
the rod 23 to maintain contact during the arc ini-
tiation and also acts as a device to limit the depth of
.
penetration of the rod by restricting its downward tra-
vel. The rod passes through a PTFE bush which allows
it to slide freely up and down.
The arrangement of levers enables precise posi-
tioning of the rod 23 on the wire bundle 21 which is
~; ; heId securely in place by means of a simple clamp 25
made of an electrically insulating resin and mounted on
a block 26 made of the ~ame material.
The power source can be either:
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a) a 3-phase 400Hz 115/200V generator of at
least 5kVA capacity
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b) a single phase 50Hz 115V transormer, at
least 3kVA capacity
c) 24V d.c. supplied by two 12V accumulators.
The fault current is detected by means of current
clamps surrounding the connecting leads and the voltage
at failure is measured using a 10:1 voltage probe. The
transducer signals are fed into a multi-channel digital
storage oscilloscope where they can be displayed and
manipulated to obtain power curves (voltage x current)
and energy (integration of power curve).
The wire bundle ~1 is positioned in the clamp 25
so that the notched wire is uppermost. Adjacent wires
of the bundle are connected to different phases of the
supply through 7.5A aircraft type circuit breakers, and
the central wire is connected directly to neutral. In
the case of single phase or d.c~ supplies, alternate
wires are connected to neutral or the negative ter-
minal, with the remaining wires, including the central
wire, connected through circuit breakers to live or the
postive terminal. The carbon rod is also connected to
neutral or the negative terminal and positioned so that
the point is in contact with the exposed conductor.
The gap between the lOOg weight and the PTFE bush is
adjusted to 0.4 mm using a suitable spacer to limit the
penetration of the rod into the sample. A voltage
probe is connected across the damaged wire and the rod,
and current clamps positioned on each of the three pha-
ses, or on the wires connected to the live side of the
supply. A protective screen is placed in front of the
test set-up and the power switched on. A material is
deemed~to pass this test if:
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a) no circuit breakers come out and the activity
is relatively non-eventful, or
b) there is no further activity on resetting the
breakers ater a non-eventful ~est.
In addition, non-tracking materials will have
relatively few spikes in the current trace with a
correspondingly low total energy consumed. Tracking
materials, on the other hand, show many spikes usually
on all three phases, which are accompanied by violent
crepitation and large energy consumption.
Examples
20 AWG tinned copper conductors were provided with
an extruded dual-wall insulation of approximately 100
micrometres wall thickness for each layer by means of a
20 mm Baughan extruder. The inner layer contained
approximately 5~ by weight triallyl isocyanurate cross-
linking promotor while the outer layer contained
approximately 7% triallyl isocyanurate. After extru-
sion the wire was irradiated with high energy electrons
to a dose of approximately 120 kGy in order to cross-
,
link the insulation. The ultimate elongation, tensilestrength, 125C cut through resistance, wet tracking
and dry arcing were measured, and the results are shown
in the TabIe.
Blends~ o~ polybutylene terephthalate with the
ionomer SSurlyn 9020) contained 80% PBT, 20% ionomer,
and blends with the butylene ether/butylene terephtha-
late~copolymer SBEBT) contained 70% PB~, 30% BEBT. All
percentages given are by weight.
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