Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to electrically insulated wire.
Problems exist in the use of electrically insulated wires
having electrical conductors with a maximum size possibly around 19
gauge but normally in the region of 22 to 26 gauge. For instance, in
telecommunications cable, the insulated wires essentially are flexible
to enable them to be bent into tortuous paths for instance in
switchboard or main frame installations. Many cables are used in such
installations and they must remain flexible to enable them to be flexed
into different positions during changing of other cables during repair
or maintenance procedures. Electrical wires to be used for connecting
within and hooking up electrical appliances need to be flexible for
similar reasons. A problem which exists with electrically insulated
wires in which flexibility to this degree is provided, is that the
wires are lacking in abrasion resistance when brought into contact with
or are drawn along frame members.
Further, where wires are to be soldered together or to
terminals, the insulation is crosslinked to give it the short term heat
resistance required when being touched momentarily by a hot soldering
iron. While crosslinking also makes the insulation abrasion resistant,
it detracts from its electrical insulating properties and increases its
rigidity to render it too inflexible to be used without difficulty in
small spaces where bending of the wire is required.
Small gauge electrically insulated wiring is also used
for house and other building wiring. Such wiring requires that the
finished cable containing such wiring should pass current overload and
fire resistance tests. Polyvinylchloride insulation will soften and
flow in overload tests in which temperatures at the insulation surface
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reach 200C. These may lead to exposure of conductors. Hence, as an
alternative insulating material for building wire, 612 nylon is being
used. This has certain disadvantages. For instance, nylons absorb
water which, if it is converted to steam during extrusion of insulating
layers, causes pinholes in the insulation. Nylon is also expensive and
extremely flammable. The use of nylon also makes it difficult to use
fastening push clips to attach wires to supporting surfaces as the
hardness of the nylon resists surface piercing by the clips during
attachment. There is also the problem that the nylon tends to separate
from any material lying beneath it when it is subjected to compression
as during attachment of push clips or even bending of wires along their
desired paths.
The present invention provides an electrically insulated
wire which has the required flexibility to eanble it to be used as
telecommunications cable and for electrical appliances. While
retaining its insulating properties, the insulation also has improved
abrasion resistance and short term high temperature resistance. These
latter properties also render the flexible wire suitable for building
wiring and where soldering is required.
According to the present invention, an electrically
insulated wire is provided comprising a conductor covered by two layers
of insulation, said two layers consisting of an inner layer of
non-crosslinked and non-crosslinkable polymeric material and an
irradiated crosslinked polymeric skin layer compatible with the inner
layer and surrounding the inner layer.
In the above defined wire according to the invention, the
desired flexibility and electrical insulating characteristics are
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provided by the inner non-crosslinked layer while the abrasion
resistance and short term heat resistance is afforded by the skin
layer. The materials of the two layers are a matter of choice and
depend upon particular requirements. For instance, the inner layer may
be chosen from materials including polyvinylchloride, polyolefins and
polyethylene. In a preferred construction, the layers are made from a
compound incorporating polyvinylchloride. It is essential that the
inner layer is non-crosslinkable. As already stated, the inner layer
provides desirable electrical insulating characteristics. The addition
of cross-linking materials to the inner layer would detract from these
desirable characteristics. Also, as there is no intention of
cross-linking the inner layer, the addition of cross-linking materials
should be avoided as in the non-crosslinked material they would be free
to migrate through the insulation thus appearing at the surface of the
outer layer to render the wire tacky. A tacky surface presents
problems during the life of a wire such as adhesion to insulation on
other wires and to switchboard and main frame installations. Such
migration would also soften the outer layer and thus detract from the
abrasion and short term heat resistance of the outer layer.
The invention also includes a method of making an
electrically insulated wire in which a conductor covered with an inner
layer of non-crosslinkable and non-crosslinked polymeric material is
fed through an extruder to form a skin layer of crosslinkable polymeric
material, compatible with the inner layer, around the inner layer, and
the skin layer is then irradiated to crosslink it.
Embodiments of the invention will now be described, by
way of example, with reference to the accompanying drawings, in which:-
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SS
Figure 1 is a cross-section through an electrically
insulated wire; and
Figure 2 is a diagrammatic illustration of a means for
producing the wire of Figure 1.
In a first embodiment, an electrically insulated wire, as
illustrated in Figure 1, comprises a copper electrical conductor 10,
having formed thereon an inner insulating layer 11 of a non-crosslinked
and non-crosslinkable polyvinylchloride composition, and an irradiation
crosslinked polyvinylchloride composition skin layer 12.
The two polyvinylchloride compositions are essentially
the same but for the skin layer, methacry1ate and acrylate monomers are
incorporated into the compositon at ~he dry blending stage. By
subsequently exposing the insulation coated wire to an electron beam
readiation, the skin 7ayer is crosslinked.
The above described wire is useful for a variety of
purposes. For instance, it may be used as a wire in a cable pair in
telecommunications cable. The inner layer 11 of insulation, of
non-crosslinked polyvinylchloride is extremely flexible and may enable
the wire to be bent without difficulty for main frame wiring or
switchboard installations. Also, because of the lack of curing action,
the electrical insulating properties of the polyvinylchloride are
retained. The thickness of the layer 11 is, in fact, consistent with
providing the required electrical insulating properties and for a
conductor thickness from 19 - 26 gauge, the inner layer 11 may be
around 0.010 inches thick. For the above reasons, the wire is also
useful for electrical appliance wiring.
ss
The outer or skin layer, because it is crosslinked,
becomes harder to provide abrasion resistant properties useful for all
applications and also has flame retardant and heat resistance
properties. Thus, the skin layer protects the inner layer and makes
the wire useful where conductor soldering is required as the skin layer
will resist short term high temperature treatment by momentarily being
touched by a hot soldering iron. While crosslinking of the skin layer
detracts from its electrical insulation properties, this is important,
as these properties are provided by the inner layer 11.
The wire is also useful for building wire and is capable
of withstanding the standard electrical overload tests and the skin
layer makes it fire retardant. While the overload tests could tend to
soften the material of layer 11, the skin layer 12 remains unaffected
by the heat thereby preventing softened material of layer 11 from
flowing away to expose the conductor. The flexibility of the wire also
is of assistance for use as building wire because it is easier to
position than conventional building wire.
Further, because of the layer 11 being non-crosslinked,
it is softer than the skin layer. Hence, it is a relatively simple
operation to apply push clips against the wire to hold it to supporting
structures as the skin layer is deformable by plastic displacement of
the inner layer by a compressive force applied by the clips.
Separation between the layers does not take place during this
deformability as the materials of the layers are compatible.
The wire remains flexible down to quite low tempertures,
for example down to -30C. It therefore satisfactorily replaces
present nylon insulated building wires while being cheaper and avoiding
the flammability and tendency for having pinholes of nylon.
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The thickness of the skin layer should be chosen to
provide sufficient abrasion, heat resistance and fire retardance
properties while not detracting unduly from the flexibility character-
istics of the wire. Thus, the skin layer should be as thin as possible
while providing the required properties to the wire. Hence, in a wire
with conductor up to 19 gauge thickness and an inner layer of around
0.020 inches thickness, the skin layer may have a thickness of around
0.003 inches.
Four compounds suitable for the skin layer 12 with
varying qualities of heat resistant and good flame retardation are:-
phr phr phr phr
_ 2 3 4
Polyvinylchloride (Kvalue 65) 100 100 100 100
TEGDM 5 5 5 10
TMPTM 15 15 15 10
TOTM 40 20 40 40
Tribase (stabilizer) 7 7 - 7
Santonox R 0.15 0.15 0.15 0.15
Mark 1900 - - 1.5
TFGDM = Difunctional methacrylate monomer
TMPTM = Trifunctional methacrylate monomer
TOTM = Trioctyl Trimaletate - plasticizer
Mark 1900 = Methyltin stabllizer.
Figure 2 illustrates diagrammatically a process for the
production of the wire of Figure 1. The conductor, for example a
copper wire 14, is fed off a reel 15. The wire 14 is often of a
diameter larger than the desired diameter of the conductor and in this
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case is fed through a drawing machine 16 which reduces the wire size to
form the conductor 10. The conductor is then fed through an annealing
furnace 17 and then through two extruders 18, 19 in tandem where the
polyvinylchloride inner PVC layer 11 is applied in extruder 18 and the
polyvinylchloride crosslinkable layer 12 is app7ied in extruder 19. A
transfer tube 20 connects the extruders 18 and 19 and initially this
may be heated, but this is not necessary after the process has run for
a short time. The transfer tube maintains the first layer of
insulation at a temperature which assists in the second layer bonding
to the first layer. From the extruder 19 the coated conductor passes
through a cooling region 21 and then through an irradiation apparatus
22.
Irradiation causes crosslinking in the skin of the
insulation for the whole or part of its depth to produce the skin
thickness required.
In a second embodiment, a wire is of similar
cross-section to that shown in Figure 1 and is produced by the process
described for the first embodiment. The electrically insulated wire of
the second embodiment is different, however, in that its inner layer 10
if formed of a non-crosslinked and non-crosslinkable polyethylene
composition and the skin layer is formed of a crosslinked polyethylene
composition.
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