Note: Descriptions are shown in the official language in which they were submitted.
CA 02745116 2011-06-30
INDUSTRIAL TEXTILE INCLUDING POROUS BRAIDED YARNS
FIELD OF THE INVENTION
The invention relates to industrial textiles for filtration and separation
operations, in
particular to industrial textiles which are woven or otherwise assembled from
polymeric
thermoplastic monofilament yarns. At least a portion of the component yarns,
preferably the
product supporting side yarns, are braided yarns, either woven into the
structure by a
weaving process, or interconnected with other yarns in a non-woven structure,
or inserted
into helical coils which are assembled such that they are interconnected by
means of hinge
yarns. The industrial textiles of the invention are particularly suitable for
use in a
papermaking process.
BACKGROUND OF THE INVENTION
The industrial textiles of the invention can be used for many filtration and
conveyance
applications, and are particularly suitable for use in papermaking and similar
machines. In
the discussion below, some of the features of the invention are described with
particular
reference to papermaking fabrics, but it will be appreciated that the
invention is not limited
to such fabrics, and is applicable to a wide range of filtration and
conveyance operations.
In the papermaking process, a very dilute slurry of about 1% papermaking
fibers together
with a mixture of about 99% water and other papermaking components is ejected
at high
speed and precision from the slice opening of a headbox onto the paper side
(PS) of a
moving forming fabric. The fabric is guided and driven by a number of rolls
over various
drainage boxes and foils which assist in the removal of water through the
fabric so as to
leave behind a randomly dispersed, loosely cohesive network or web of
papermaking fibers.
At the end of the forming section, this web is transferred to the press
section, where further
water removal occurs by mechanical pressures as the web is conveyed on or
between a
series of press fabrics and is guided through one or more nips. The now self-
supporting but
still very wet web is then transferred to the dryer section of the papermaking
machine where
the remaining water is removed by evaporation. The resulting paper product may
then be
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subjected to various treatments before being finally wound onto a reel, cut to
size and
packaged for shipment.
It is widely recognized that the forming fabric plays a critical role in the
initial formation of
a paper web. Forming fabrics effectively form a paper sheet by capturing
papermaking
fibers and fines on the PS surface. A forming fabric must be rugged, so as to
withstand over
time the continuous moving contact to which its lower (machine side) surface
is exposed as
it is driven over the various stationary contact surfaces in the forming
section. It must be
stable, so that it does not crease or skew during operation. At the same time,
it must provide
an appropriate PS surface, which for smooth paper products is required to be
very fine, upon
which the individual fibers in the stock slurry are deposited, along with any
added fines and
fillers, so as to form a planar web which will eventually be consolidated into
a continuous
sheet following water removal in the downstream sections of the papermaking
machine. The
fineness of the fabric used in the papermaking process (i.e. the size of the
yarns, their
separation, the size of the openings in the mesh and number of support points
per unit area
provided by the fabric) will be dictated partly by the length of the
papermaking fibers used
in the stock and partly by the end use requirements of the paper product being
formed. In
even the finest forming fabrics, woven in multi layer weaves, the distance
between the
individual yams exceeds the length of the fines and at least a portion of the
fibers by a
substantial margin.
It is also known that increasing the fineness of the yams for the fabric, i.e.
by reducing their
diameter, together with increasing the yam count, provides increased support
for shorter
papermaking fibers; however, this leads to problems in providing sufficient
mechanical
stability for the fabric. A further result of using smaller diameter yarns is
that it can provide
a more open fabric structure, so that the sheet will be dryer on leaving the
couch at the end
of the forming section.
Papermaking fibers are increasingly derived from recycled materials, and such
fibers are
generally shorter in length than fibers obtained from virgin sources, e.g. 0.5
- 1.5mm for
recycle fibers, in contrast with 2 - 4mm for virgin. Papermaking stocks
increasingly contain
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significant percentages of such recycled fibers which must be supported by the
mesh of the
fabric upon which they are deposited if they are to provide benefit in the
papermaking
process. Increased support for the papermaking fibers can at present only be
provided by
decreasing the cross-sectional area of the yams from which the fabric is
woven, and
increasing the mesh (i.e. the density or number of yarns in each fabric
direction). A fine
mesh will provide more support points for the papermaking fibers, but will
also result in a
woven structure that is less rugged than a comparable fabric that is woven
using larger
yams. Thus, the use of finer yarns in these fabrics has resulted in thinner
textile structures
which are less mechanically stable and have reduced wear capability in
comparison to more
coarsely woven fabrics having larger yams, leading to the need to find other
means of
providing the required stability and wear capability.
In modern high speed papermaking machines in which the forming fabrics can be
moving at
speeds of I O0kph, or more, the minor pressure component perpendicular to the
fabric
surface exerts a significant level of force on the forming fabric, which can
cause excessive
impingement derived drainage of the stock over the initial portion of the
forming section.
This pressure component (the "impingement pressure"), which is minor on some
machines
but of increasing significance in many newer machines, and the turbulent
forces created by
stationary drainage elements, combined with the increased use of particulate
fillers and
shorter papermaking fibers, have the undesirable effect of reducing first pass
retention (i.e.
the amount of fines and fillers retained in the sheet) and increasing the
embedment of the
initial layers of the embryonic web into the PS surface of the forming fabric.
It is also well known that impingement drainage can cause sheet marking, low
retention by
the forming fabric of papermaking fibres, fines and fillers (i.e. low first
pass retention), and
plugging (i.e. sheet sealing) of the paper side layer of the forming fabric.
Unless the
structure of the forming fabric is designed to allow it to manage and control
impingement
drainage, further increases in machine speed and/or paper making machine
efficiency may
be limited or, in the case of gap formers, tied directly to improvements in
forming shoe or
forming board construction.
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Similarly, for other industrial filtration purposes as noted above,
impingement drainage will
have adverse effects on the efficiency of the filtration fabric in achieving
the particular
purpose for which it is being used. The shorter the fibers, and the lower the
fiber support
from the fabric, the lower the filtration efficiency.
The future demands of the paper industry will undoubtedly be towards ever
lighter basis
weight sheets which will be required to be made with ever decreasing fiber
lengths due to
recycling, at much greater paper machine speeds in order to reduce
manufacturing costs. In
order to achieve this, finer papermaking fabric structures will be required
than are currently
available, which will be woven or otherwise assembled using yarns of
increasingly smaller
cross-sectional area. The resulting fabric structures will be thinner and less
stable than those
woven using relatively larger size yarns. If such increases in paper machine
speeds and the
mechanical design of the newer high speed paper machines are to be
accommodated, this
will require much greater fabric stability, especially in the cross machine
direction, in order
to produce a uniform basis weight sheet of paper.
There is therefore a need for a novel fabric structure that is designed to
meet these new
requirements, and the problems of decreased fiber lengths, and to overcome the
disadvantages, discussed above, of the use of finer yarns. The present
invention seeks to
address these issues.
It has now been discovered that round or flat multi-yarn braids, comprised of
relatively
small diameter monofilaments of either circular or other cross-sectional
shapes, can be
woven or inserted into industrial textiles to provide enhancements to various
fabric
properties, particularly fiber support depending on the structure into which
they are
incorporated. The fabrics including braided yarns according to the invention
are particularly
advantageous for use in the forming section of a papermaking machine, but also
provide
significant advantages for a wide variety of intended applications.
For example, braided yarns woven as weft or cross-machine direction (CD) yarn
material
into the PS of a forming fabric or through-air dryer (TAD) fabric can provide
a very fine
paper forming surface in which the distance separating the individual yarn
components is
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much shorter than has previously been possible in woven structures formed
entirely from
single, interwoven monofilaments. Having such a fine structured paper side
including the
braided yams provides further consequential advantageous options. For example,
a non-
marking pin seam can now be provided for a forming fabric, because the braided
yarns will
cushion or mask the product from discontinuities resulting from the presence
of such a seam.
In addition, the increased exposed surface area of the braids now makes it
possible to bond
nonwoven scrims or webs onto the PS using suitable adhesives.
Further, braids woven or otherwise interconnected into a press felt base can
provide
improved anchoring of the batt fibers which are typically needled into the
base fabric so as
to provide positive attachment to that structure, as the braids will enhance
entanglement of
the batt fibers during the needling process. In addition, braided yarns may be
useful in
masking the interference pattern that is inherent with multiaxial designs.
Braids can also be used to "stuff' or reduce the air permeability of dryer
fabrics, in
particular those which are constructed by interengagement of a plurality of
helical coils
using hinge pins which are passed through channels where adjacent coils are
interengaged.
The braided yams are inserted in the so-called stuffer position of these
spiral fabrics, so as to
provide any required reduction in their air permeability.
DISCUSSION OF THE PRIOR ART
The use of compound yarn structures, such as twisted or cabled yarns, in the
construction of
filtration and papermaking fabrics is known. For example, US 3,158,984, US
3,351,205 and
US 3,510,005 (all to Butler et al.) disclose various filtration fabrics
including yams
comprised of twisted strands or filaments, secured together in a cabled or
similar
construction. Similarly, US 4,105,495 (Pai) discloses the use of filaments
which are twisted
about each other or about a central core yarn, to provide a stretch resistant
fabric. Other
examples of fabrics using yarns comprising twisted filaments include EP 934769
(Haasmas);
and US 6,699,367 (Gstrein et al.).
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For press fabrics, it is also known from US 5,049,425 (Essele) to construct a
porous pintle
from a braided, knitted or ply twisted yarn; and US 5,087,327 (Hood) discloses
a composite
yarn comprised of an at least partially soluble core about which a layer of
monofilaments is
either braided, knitted, or helically wound. It is also known from US
5,005,610 (Davenport)
to use braided yarns as the MD component in a press fabric.
It is known from US 4,650,709 (Lefferts) to use braided or woven tubes within
the coils of a
spiral or helical coil type fabric so as to improve the uniformity of the air
permeability of the
fabrics; and US 5,772,848 (Dutt) discloses a resin impregnated ENP or calender
belt having
a base fabric layer in the form of a braided structure. US 5,899,134 (Klein et
al.) discloses an
axially stable tubular braided fabric.
Further, US 6,790,796 (Smith et al.) discloses a forming fabric for nonwovens
manufacture
which includes striated or twisted/braided multistrand rough-surface yarns in
the PS to
prevent slippage of the web.
US 5,077,116 (Lefkowitz) suggests the construction of a forming fabric having
a nonwoven
surface sheet adhered to a base fabric layer, such that fluid flow passageways
in the
nonwoven layer would be smaller than those in the base fabric layer and allow
passage of
fluid from the sheet contact surface through to the base fabric. However, the
suggested
construction has been found to be unworkable in practice.
EP 1,255,892 (Senellart) discloses an industrial fabric wherein at least the
MD yarns are
multicomponent yarns including at least one thermofusible strand having a
melting point
that is lower than that of the remaining strands of the yarn; the
multicomponent yarns may
be of a knit, plied/twisted or braided construction.
However, none of these references suggests the use of braided yarns in the
manner of the
present invention, as described further below.
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SUMMARY OF THE INVENTION
The present invention seeks to provide an industrial textile for conveying a
product in a
machine direction, having a product supporting surface and comprising at least
one set of
cross-machine direction yams interconnected with at least one set of
transverse yams,
wherein at least some of the cross-machine direction yams comprise braided
yams, each
braided yam comprising first and second sets of component yams interconnected
in a
repeating braided pattern, wherein
(i) the first set of the component yams comprises at least three
longitudinally extending tri-
axial yams; and
(ii) the second set of the component yams comprises at least four carrier
yarns, each carrier
yam being braided with yams selected from at least some of the tri-axial yams
and at least
some others of the carrier yams to form an outer layer having a profiled outer
surface and
defining a longitudinal internal core.
The braided yams used in the fabrics of the invention can be constructed by
conventional
methods of industrial braid manufacture. They may include one or more core
yarns running
axially through the centre of the braid, but will preferably consist of at
least three and, more
preferably, at least six tri-axial yarns, which are those yarns of the braid
which are oriented
generally longitudinally along the axial direction but are located within the
yarn structure.
In a two layer braid, the tri-axial yams are interconnected with at least four
carrier yams
which are wrapped in opposing directions about the braid. In yarn braiding,
the carrier yams
are those which are paid off from yarn spools that orbit or revolve around the
axial center as
the yarn is braided, while the tri-axial yarns are those oriented in the
longitudinal or length
direction of the braid. Generally there will be at least twelve carrier yarns
used in
combination with at least six tri-axial yams. Such an arrangement is referred
to herein as
12C-6T, meaning there are twelve carrier yarns interconnected with six tri-
axial yarns. The
number of times the carrier yams are interconnected with the tri-axial yarns
is referred to
herein as the "pick count" and is expressed in PPI, or picks per inch. In the
fabrics of the
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invention, the PPI of the braided yarns will generally be at least twenty-five
(i.e. the braid
will have a minimum structure expressed as: 25 PPI-12C-6T, meaning twelve
carrier yarns
interconnected with six tri-axial yams at an interconnection rate of twenty-
five
interconnections per linear inch of braid). As many as twenty-four or more
yarns in total
may be used in the formation of a single braid for use in the fabrics of this
invention.
In some embodiments of the invention, the braided yams can be provided as a
three layer
construction, in which a third set of component yams is wrapped or otherwise
placed around
the tri-axial yams, as a middle layer, and is secured in the selected position
by an outer layer
of carrier yarns which are braided to each other but not to the third set of
yarns or the tri-
axial yams.
The number of yarns selected for the braid will depend on factors including
the intended end
use for the fabric; for example, a higher number of tri-axial yarns will in
general result in a
stiffer structure providing a higher fiber support.
The finished braid may have an overall circular cross-sectional shape, or it
may be flattened
or generally rectangular; preferably the braid has a generally circular cross-
sectional shape.
If flattened, the fabric manufacturer will generally wish to ensure that the
yams lie generally
flat in the weave of the fabric, without twists or kinks in their path.
The component yarns of the braid (i.e. the core, carrier and tri-axial yams)
can be formed
from any polymer such as is commonly used in monofilaments intended for use in
industrial
textiles. The component yarns can all be formed from the same polymer, or the
tri-axial
yams can be formed from a different polymer from the carrier yarns, or there
can be
differences between the yams of either set. For most applications, preferably
the chosen
polymer will be a polyester, such as PET, PEN, PBT and the like; or one or
more
polyamides, such as PA-6, PA-6/6, PA-6/10, etc. such as would commonly be used
in
industrial textiles. The component yarns may also have a bi-component
structure, such as a
conventional sheath-core construction, in which the outer sheath is comprised
of a low-melt
polymer adhesive and the core is comprised of a polymer having a higher melt
point; other
constructions are possible.
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The use of a low melt polymer adhesive as a component yam of the braid would
allow for
the optional attachment of a nonwoven substrate such as a scrim, or another
fabric structure,
onto the surface of the braid and fabric of which it is a component. For most
applications,
such scrim is preferably constructed of nylon or PET, having a basis weight of
at least 15
gsm; and is preferably secured to the fabric either by the use of bi-component
yarns within
the braids, as noted above, or by bonding to the braids and/or other yams in
the fabric by
means including, but not limited to, laser bonding, ultrasonic bonding,
chemical bonding,
heat activated bonding and mechanical entanglement. As a further option, the
nonwoven
scrim can include a thermoplastic heat activated adhesive component.
If braided yarns including one or more core yams are employed, the resultant
braided
structure will be expected to have advantageous mechanical properties and
increased
stiffness, and possibly lower outer diameter size variation.
Where the individual yarn components used in the braid have a circular cross-
section, their
diameter will generally preferably be in the vicinity of about 0.05mm, to
result in an outside
diameter of the braid similar to that found in modem industrial textiles,
generally in the
range of from about 0.4mm to about 1.0mm, with about 0.6mm being preferred.
However,
during weaving, the generally circular cross-sectional shape of the braided
yams will tend to
become flattened and will spread as they are interlaced with the warp yams.
This effect may
be exaggerated if the PS surface is calendered or if the fabric is passed
through a heated roll
nip.
Preferably, the braided yams represent between about 10% and 100% of the cross-
directional (or weft) yarn components in the fabrics of the present invention,
i.e. the braided
yams may be inserted as every fourth, third or second CD yarn, or all the CD
yams can be
braided yams. Where the braided yarns are incorporated in the entire body of
the fabric, i.e.
not merely at specific regions such as seaming areas, preferably they are
located on the
intended PS or material conveying surface of the fabric and are woven into the
fabric
structure as weft yarns interlaced with MD oriented warp yams, and preferably
are sufficient
in number to present a surface area of at least 10% of the PS surface. The
ratio of the
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number of PS braided yams to MS yams will be determined by the fabric
manufacturer in
accordance with need and the desired end properties of the textile. In three-
layer structures
(i.e. fabrics constructed with three layers of weft yarns, such as are
described by Danby et al.
in WO 09/103167), it may be preferred to locate the braided yams in the centre
layer of weft
so as to provide an increase in the Centre Plane Resistance (CPR) of the
fabric.
BRIEF DESCRIPTION OF THE FIGURES
The invention will now be described with reference to the drawings, in which
Figure 1 is a photograph showing one type of braided yam of the invention
adjacent a part of
a conventional forming fabric;
Figure 2 is a close-up photograph of the braided yarn shown in Figure 1;
Figure 3 is a photograph showing the braided yam of Figures 1 and 2 in cross-
section;
Figure 4 is a weave diagram for a 24-shed fabric in an embodiment of the
invention;
Figure 5 is a cross-sectional view along the warp yarns of a fabric of the
invention woven to
the weave pattern of Figure 4;
Figure 6 is a weave diagram for a 12-shed fabric in an embodiment of the
invention;
Figure 7 is a cross-sectional view along the warp yams of a fabric of the
invention woven to
the weave pattern of Figure 6;
Figure 8 is a graph showing the interrelationship of the number of picks per
inch (PPI) for
two embodiments of the braided yams of the invention in comparison to the void
volume of
the resulting yam;
Figure 9 is a photograph of part of a fabric woven according to the weave
pattern of Figure
4;
Figure 10 is a close up photograph of the fabric shown in Figure 9; and
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Figure 11 is a photograph of the MS of a fabric woven according to the weave
pattern of
Figure 6.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
DRAWINGS
Referring to Figures 1 to 3, Figure 1 is a photograph showing one type of
braided yarn of the
invention, shown adjacent a forming fabric to show the scale of the braided
yarns in
comparison to yarns currently used in conventional forming fabrics. Figure 2
is a close-up of
the braided yarn shown in Figure 1. Figure 3 is a photograph showing the
braided yarn of
Figures 1 and 2 in cross-section, from which it can be seen that the yarn is
coreless and
hollow. In this embodiment, the individual yarns of the braid are comprised of
polyamide-6.
Experimental forming fabrics were successfully woven in accordance with the
invention, in
which braided monofilament yarns were incorporated as weft yarns in their
product side
(PS) weave structure. Two fabric designs were woven, both of which were double
layer
constructions including two layers of weft yarns which were interwoven using
either one or
two systems of warp yarns; the PS well yarns in both designs were braided
yarns according
to the invention.
In the first weave design, shown in Figures 4 and 5, two systems of warp yarns
were used.
Figure 4 is a weave diagram of a 24-shed pattern of an embodiment having two
warp yarn
systems and two weft yarn systems, one being the braided yarns of the
invention, and the
other being solid well monofilament yarns. The first PS warp yarn system
interweaves with
both the PS braided yarns, to hold them into the fabric structure, and with
the MS well
yarns. The second warp yarn system interweaves only with the MS well yarns.
Figure 5 is an illustration of one pattern repeat showing a cross-section of a
fabric woven
according to the weave diagram of Figure 4, taken along the warp yarns to show
the
interweaving of the two warp yarn systems with the PS and MS well yarns of the
fabric
according to the weave pattern.
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The machine side layer (MS) was woven according to a simple over-3/under 1
design so as
to provide a mesh (warp) x knock (weft) count of approximately 90 x 70 yams
per inch; the
PS had mesh x knock of 90 x 23 due to the presence of braided yams. In this
design, the MS
well yams were monofilaments interwoven with both the MS and PS warp yams so
as to tie
the fabric structure together. The PS warp yams were interwoven according to
the same
pattern but inverted, so that they passed under three and over one of the
braided yams and
were interwoven with three of the MS well yams in each repeat.
In the second weave design, shown in Figures 6 and 7, only one warp yam system
was used,
in which the warp yams were interwoven with both the PS and MS well yams,
passing over
one PS braided well and beneath three MS monofilament well yams in the manner
shown in
cross-section in Figure 7. Figure 6 is a weave diagram of the 12-shed pattern
of this
embodiment; and Figure 7 is an illustration of one pattern repeat showing a
cross-section of
a fabric woven according to the weave diagram of Figure 6, taken along the
warp yams to
show the interweaving of the warp yams with the PS braided yarns and the solid
monofilament MS well yarns. The PS mesh x knock was 90 x 23 as in the first
design shown
in Figures 4 and 5.
In both these experimental fabric weave patterns, the warp yams were
rectangular 0.14 x
0.165 mm PET monofilaments and the well yams used in the MS weave were 0.22 mm
diameter round PET monofilaments.
In the experimental fabrics, all of the PS well yams were braided yams having
a 50 PPI
(picks per inch) construction, including 16 carrier yarns x 8 tri-axial yams,
as shown in
exemplary Figures 1, 2 and 3. In the braids used in the forming fabrics of the
experiment,
each individual tri-axial and carrier yam was the same and was a 0.06mm round
polyamide
(PA-6) monofilament. The braided yams were braided coreless on a horizontal
braider. The
outside diameter of the braids was approximately 0.60mm, and the braids had a
void volume
of approximately 72%. Variation in the void volume of braided yams as a
function of their
pick count is graphically represented in Figure 8, which is a graph showing
the
interrelationship of the number of PPI in the braided yams used in the present
invention, in
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comparison to the void volume of the resulting yarn. In the graph, the data
points associated
with the yarns used in the fabrics and yarns of Figures 1 to 7 are shown as
diamonds, while
data points for comparison yarns designated 12C-6T are shown as squares.
Figure 8 shows
that, as the PPI increases, the void volume of the resulting braid decreases.
For convenience, the following notation, as referred to previously, is adopted
to describe the
construction of the braided yarns used in the fabrics of this invention. The
braided
construction of Figures 1 to 7 can be described as:
50 ppi-16C-8T
Where
50 ppi = 50 picks per inch,
16C =16 carrier yarns (i.e. yarns from spools that orbit or revolve around the
center of the
braid during the braiding process), and
8T = 8 tri-axial yarns (i.e. the axial, or longitudinal yarns involved in the
braid).
A 50ppi-16C-8T braided yarn thus has 24 yarns in total, i.e. 16 carrier yarns
+ 8 tri-axial
yarns; and the PPI defines the braid density. In the experimental fabrics that
were woven
during trials, braids having a pick count as low as 25 PPI and as high as 200
PPI were used.
As in weaving, a relatively lower pick count is faster and more economical to
produce than
one having a comparatively higher pick count. The braid samples used in the
experimental
fabrics employed only one size of monofilament (0.06 mm); however, various
sizes can be
used in any carrier or tri-axial position (for example, relatively larger tri-
axial yarns might
be advantageous with respect to fabric stability or seam strength).
Figure 9 is a photograph of a fabric woven according to the pattern shown in
Figures 4 and 5
using the 50 PPI-16C-8T braided yarns.
Figure 10 is a close up photograph of the fabric shown in Figure 9,
illustrating the woven
position of a braided yarn in that structure.
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Figure 11 is a photograph of the MS of a fabric woven according to the pattern
shown in
Figures 6 and 7, showing a braided yarn in the fabric structure as seen from
the MS.
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