Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ARC RESISTANT AND SMOOTH WIRE
Related Applications:
This application claims the benefit of priority from U.S. Provisional
Patent Application No. 61/005,718, filed on December 7, 2007 and
U.S. Provisional Patent Application No. 61/131,629, filed on June 10,
2008, the entirety of which are incorporated by reference.
Field of the Invention:
This application relates to cable construction. More particularly, the
present invention relates to a layered insulation for cables.
Background:
In the field of high performance airframe wires, such as those used in
commercial and military airplanes, these wires must meet stringent
performance ratings. Generally, improving the performance attributes
while maintaining or reducing the weight of the wire is a primary goal
in the industry.
Some of the critical performance attributes that must be maintained
by the wire/insulation combination include thermal mechanical
performance, arc resistance, UV-Laser mark-ability, abrasion
resistance, dynamic cut-through resistance and smooth surfacing.
In the prior art, polyimide tapes, including pure polyimide tapes and
Teflon TM (fluoropolymer) coated polyimide tapes, have been used as
primary insulators. The polyimide provides a number of advantageous
properties, including good mechanical and insulating properties.
However, the polyimides need to be applied as tapes instead of by
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melt extrusion processing because the high molecular weight
polyimidies, needed for the performance characteristics, make it very
difficult to extrude.
These polyimide tapes, aside from the majority of their advantageous
properties, do suffer from poor dry and wet arc resistance tracking.
Although later versions of the polyimidie tapes have fluoropolymer
coatings with improved arc tracking, they are still not ideal for meeting
the desired arc resistance standards.
Fluoropolymers (including TeflonTM) are known to have a good arc
resistance properties and are thus commonly used in a second layer
over the primary insulation in wires for airframe applications. For
example unsintered PTFE (Polytetrafluoroethylene - Teflon TM) tapes
may be applied over the primary polyimide insulation. An Example of
the prior art cable according to this construction, such as an airframe
wire according to the industry standard AS22759/80-82, and 86-92,
is found in Figure 1.
However, another concern with airframe wires is the desire that they
be both smooth and printable (e.g. using UV laser printing). In order
to make the outer PTFE layer printable, additives, particularly titanium
dioxide, and other related materials (pigments) are added to the PTFE
insulation tape. These added materials, although they allow for easy
UV laser marking, often times reduce other essential properties of the
cable, including arc resistance properties. For example, the common
additive of titanium dioxide is used because it enhances the UV laser
markability (enhances marking contrast level) of fluoropolymers
(PTFE), but it is also good at sustaining electrical arcs and thus
simultaneously reduces the arc resistance of the wire.
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Another one of the performance attributes that is desirable in this
industry is a smooth surface wire which at least maintain the current
AS22759/80-82, and 86-92 performance levels (industry standards for
airframe cables). However, because of the tape constructions, the
wire are not edgeless and smooth and because of the impurities added
to the PTFE for marking purposes as noted above, they do not perform
well in the wet arc resistance tests. Thus, due to these limitations
typical current specifications for air frame wires only require that the
wire pass "wet-arc track" at 90% and 85% for medium and thin wall
respectively.
Objects and Summary:
The present invention overcomes the drawbacks associated with the
prior art by providing a wire having a conductor coated with a primary
insulation layer and a secondary insulation layer. Over the secondary
insulation layer a third thin layer is applied, where this third layer is
provided with a various marking an color additives so as to make the
outer surface both smooth and markable as well as to remove the
need for placing such additives in the primary and secondary insulation
layers.
By separating and restricting the UV laser marking and coloring
functionality to the outside layer of the insulation system of the wire,
the overall insulation system may be formulated for improved
mechanical and electrical performance. For example, doping only the
outer thin third layer of PTFE insulation with titanium dioxide and
coloring additives and leaving the secondary insulation layer as natural
(pure and free of additives) PTFE.
In one arrangement of the invention, the doping of the thin third layer
of insulation may be achieved by adding the third layer through melt
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extrusions, wrapping, coating or by treating the outer surface of
secondary insulation layer by other means. One example, is a fusion
process where the UV laser sensitizer (titanium dioxide) and the
coloring additives are added to the outer third layer of insulation.
By separating the outer third marking layer (or coating insulation
layer) from the secondary insulation layer, this allows the secondary
layer to be made without the need for marking additives, allowing for
even better overall arc resistance of the total insulation layer. In fact,
the pure PTFE secondary insulation tape may be made thinner allowing
for more latitude in the formation of the primary insulation layer, such
as in the composition of the tape used.
Furthermore, this improved arc tracking performance from the PTFE
secondary insulation layer, allows for the overall polyimide
concentration in the insulation of the wire, such as the polyimide
component of the primary insulation tape layer relative to the
fluoropolymer component, to be increased, thus achieving better
thermal and mechanical performance without compromising the arc
resistance performance.
Such characteristics allow for greater design choice in meeting certain
desired mechanical characteristics while still being able to pass the
necessary performance standards. For example, this arrangement
enables the cable to pass arc resistance testing at the rate of 95% and
higher and meet other requirement of AS22759/80-/82 and
AS22759/86-/92.
To this end, the present invention provides for a wire having a
conductor, a primary insulation layer and a secondary insulation layer.
A third insulation layer is applied over said second insulation layer,
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where said third insulation layer includes the only marking additives
used in the three insulation layers of said wire.
5 Brief Description of the drawings:
The present invention can be best understood through the following
description and accompanying drawings, wherein:
Figure 1 is a prior art airframe wire (AS22759/80-82, 86-92);
Figure 2 illustrates a multi-layer insulation air frame cable, in
accordance with one embodiment of the present invention; and
Figure 3 illustrates a multi-layer insulation air frame cable, in
accordance with another embodiment of the present invention.
Detailed Description:
In one embodiment of the present invention, a wire (or cable) 10 is
provided having a conductor core 12 and insulation layer 20. As noted
above, wire 10 is typically for use in airframe applications, however
the invention is not limited in this respect.
Conductor core 12 for wire 10 as shown in Figure 1, is a plurality of
stranded conductor elements 14. To illustrate the salient features of
the present invention, each conductor element 14 is comprised of 19
strands of 32 AWG nickel plated copper (19x32, 20 American Wire
Gauge NPC conductor). However, it is understood that individual
conductor elements 14 may be made from any other conductor
materials that are suitable for the desired application. Moreover, the
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features of the present invention related to the insulation layer 20,
described below, may be applied to single conductor core 12 of solid
(only one element) construction.
In one embodiment of the present invention, as shown in Figure 1,
insulation layer 20 is comprised of a first primary insulation layer 22, a
secondary insulation layer 24 and a coating layer 26.
In one arrangement, primary insulation layer 22 is preferably formed
using composite tape (e.g. DuPontTM 120 TW T 561) having an overall
thickness of substantially 0.0012." Such a tape may be constructed of
a polyimide substrate coated with modified PTFE
(Polytetrafluoroethylene) on both sides of the polyimide base. In
another arrangement, a tape used for primary insulation layer may
simply be an un-coated polyimide tape.
In one arrangement of the present invention, to form layer 22, such a
tape is helically wrapped at substantially 51% - 54% overlap,
preferably 53% overlap, over conductor core 12. The wrapping at
about 50% overlap translates into a thickness of primary insulation
layer being approximately two times the thickness of the tape. Such
a design for primary insulation later 22 is used in exemplary fashion to
demonstrate the salient features of the invention. However, it is
understood that other manners of applying primary insulation layer 22
to conductor core 12 may also be utilized within the context of the
present invention.
Secondary insulation layer 24, is preferably formed using a wrapped
tape made from natural unsintered PTFE which is substantially 0.0015"
- 0.0020" in thickness, but may be in the range of 0.0005" - 0.004".
Ideally, tapes used for secondary insulation layer 24 are made of
natural PTFE with no additives, however they may be made from
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modified PTFE with additives of 4% or less of titanium dioxide and/or
10% or less of other additives by weight. Such tapes may be in the
form of skived, unsintered cast or expanded form. Although ideally,
the PTFE tape would have no additives to take advantage of the
benefits provided by the third insulation layer 26 as explained below, it
is possible that some small amount of additives (or possible
impurities) may still be present without compromising the benefits of
the present invention.
Like the tape from primary layer 22, this tape is also helically wrapped
at substantially 51% - 54% overlap, preferably 53% overlap, over
primary insulation layer 22. Unlike the prior art, secondary insulation
layer does not contain any additives, such as marking additives and
thus provides very high arc-resistance properties to over all cable
insulation 20. Such a design for secondary insulation later 24 is used
in exemplary fashion to demonstrate the salient features of the
invention. However, as with primary insulation layer 22, it is
understood that other manners of applying secondary insulation layer
24 over primary insulation layer 22 may also be utilized within the
context of the present invention.
In one embodiment of the present invention, a third insulation layer
26 is applied over the outside of secondary insulation layer 24.
Advantageously, it may be applied by means of a fusion where the
PTFE is applied using a surface treatment and then fused to the outer
surface of secondary insulation layer 24. In this arrangement, third
insulation layer 26 is preferably applied in several iterations, each
iteration preferably being a layer of substantially 0.0001" - 0.0004" in
thickness. Each application iteration may be in the range of 0.00005"
- 0.001". It is also understood that third layer 26 may also be applied
in other manners, such as melt extrusions, wrapping or coating.
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The fusion coating material (PTFE fusion coating layer 26) is
formulated to give color to the wire, and to have wire 10 UV laser
printable on its surface. For this purpose, the PTFE layer may include
the marking additives, such as titanium dioxide. Although this has the
above identified drawbacks associated with arc resistance properties,
this impact is minimized because third insulation layer 26 is so small
relative to the overall thickness of secondary insulation layer 24.
Moreover, outer coating or third insulation layer 26, being applied by
fusion coating, has the additional advantage that it smoothes over the
helical indentations in secondary insulation (which is a wrapped PTFE
tape) which not only makes it easier for marking, but also removes
the edges (caused by the wrapping of the tape) so as to improve
abrasion resistance and to avoid instances of the tape insulation being
caught or unwound accidentally by physical environmental hazards.
In another embodiment of the present invention, using the same
illustration from Figure 2, primary insulation layer 22 is made from a
tape having a polyimide substrate that is coated with modified PTFE on
both sides of the polyimide base film. This tape may then be helically
wrapped at substantially 63% - 70% overlap, preferably 65% overlap,
over conductor core 12. In this arrangement, the wrapping of the
tape for primary insulation layer 22 at substantially 65% overlap, not
only results in an overall thickness of primary insulation layer to be
three times the thickness of the tape, but it also results in a more
smoothly contoured outer surface.
Secondary insulation layer 24, wrapped thereover, is formed from
natural unsintered PTFE tape which is substantially 0.0015" in
thickness or less. Again, this tape is helically wrapped at substantially
65% overlap over primary insulation layer 22. Because secondary
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insulation layer 24 is made of pure PTFE with no marking additives, it
may be made using a lesser thickness than normal because it can
achieve the desired insulation and mechanical properties with lesser
thickness owing to its pure, non-marking additives formulation.
Additionally, the winding at substantially 65% results in a more
smoothly contoured outer surface.
As with the first arrangement, an third coating layer 26 is applied as a
final outer layer over primary insulation layer 24 for example, by
means of PTFE fusion coating described above. This third insulation
layer 26 may be applied in several iterations, each of substantially
0.0001" - 0.0004" in thickness.
In another embodiment of the present invention, wire 10 has identical
conductor core 12, primary insulation layer 22 and secondary
insulation layer 24 as shown in Figure 2. In this embodiment, third
insulation layer (or coating layer) 26 is a helically wrapped PTFE tape
of substantially 0.0015" thickness, applied at substantially 10%
overlap. As with the above described third insulation layer 26, the
PTFE used for this layer is formulated to give color to wire 10 and to
have wire 10 UV laser printable on its surface, such as by the inclusion
of titanium dioxide additives. Although this has the above identified
drawbacks associated with arc resistance properties, the impact is
minimized because third insulation layer 26 is so small relative to the
overall thickness of secondary insulation layer 24.
In another embodiment of the present invention, as shown in Figure 3,
an additional internal fusion coating layer 23 may be added between
primary insulation layer 22 and secondary insulation layer 24. In this
arrangement, after using the PTFE coated polyimide tape for primary
insulation layer 22, as with the above examples.
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Thereafter, a fusion coating layer 23 is applied over primary insulation
layer 24. This inner fusion coating layer 23 is preferably applied in
multiple iterations, where each iteration is preferably substantially
0.0001" - 0.0005" in thickness. Each application iteration may be in
5 the range of 0.00005" - 0.001". Inner coating layer 23 is
advantageously arranged to covers and seal primary insulation layer
22 while at the same time providing smooth and edgeless surface as a
base for secondary insulation layer 24. This improves the overall
smoothness of the outer surface of cable 10. After the application of
10 inner coating layer 23, secondary insulation layer 24 and fusion
coating layer 26 may be applied as above.
As such, according to the above described embodiments, outer fusion
coating layer 26 provides an opportunity for wire 10 designers to have
an optimum formulation that allows for good laser contrast levels used
for marking. The high laser print contrast level on the surface of wire
10 is highly desirable in the industry. By moving the additives into a
thinly applied third insulation layer 26, such as one applied by fusion
coating, leaving secondary insulation layer 24 as a pure PTFE tape,
wire 10 is able to maintain this maximum laser contrast level without
losing the "Wet Arc Resistance" performance. Third insulation layer 26
also provides the surface smoothness to wire 10 which is also highly
desired in the industry.
The separation of the marking components into a thin fusion applied
third insulation layer 26 from the underlying secondary insulation layer
24 made from PTFE tape also allows designers of wire 10 additional
freedom to explore new wire designs with unique properties.
For example, one such performance attribute is the "dynamic cut-thru"
performance wire. The "dynamic cut-thru" performance, designed to
measure the ability of a cable or wire insulation to resist being cut by
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a sharp edge and thus shorting in an in-service environment, is
primarily determined by amount of polyimide material present in the
insulation 20, such as in primary insulation layer 22. However,
polyimide material is a detriment to the "Wet Arc Resistance"
performance of cable 10, thus necessitating the PTFE coating on
primary tape as well as secondary insulation layer 24. With
arrangement of the present invention, which has greatly improved
"wet arc resistance" performance because of the pure PTFE secondary
insulation layer 24, the polyimide component in primary insulation
layer 22 may be increased in order to increase the mechanical
property of the insulation (i.e. dynamic cut-thru) while still meeting
the "wet arc resistance" performance standards. This flexibility is not
possible with current existing wire system, such as those specified in
AS22759/80-/82, /86-/92 and BMS-1360.
Another advantage of the present invention is that that the separate
and discrete design of the secondary insulation layer 24 of pure PTFE
tape and the thin third insulation layer 26 provides freedoms to use
alternative fluoropolymers in third layer 26 in wire 10 such as PFA
(Perfluoroalkoxy), MFA (Perfluoromethylvinylether), ETFE (Ethylene
Tetrafluoroethylene or TefzelTM), and FEP (Fluorinated Ethylene
Propylene) to achieve better abrasion and dynamic cut-through
resistant properties while maintaining desirable arc-tracking
performance. The discrete coating layer 26 also allows engineers to
design a formulation using nano-fillers in this layer without the
problem of fully exfoliating the nano-fillers (nano fillers are often
agglomerated, and difficult to disperse in compounding process) which
could further improve this fusion coating layer 26 regarding its own
mechanical, thermal mechanical, and flame performances while
maintaining the good arc-resistance based on the primary and
secondary layers 22 and 24.
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While only certain features of the invention have been illustrated and
described herein, many modifications, substitutions, changes or
equivalents will now occur to those skilled in the art. It is therefore, to
be understood that this application is intended to cover all such
modifications and changes that fall within the true spirit of the
invention.
