Note: Descriptions are shown in the official language in which they were submitted.
WO90/1~65 PCT/US90/029
FORMING FABRIC HAVING A
NONWOVEN SURFACE COATING
Field of the Invention
This invention relates to improved forming
fabrics for papeL ~ki ng.
Backqround of the Invention
It has long been known that to provide ~; L
resistance to abrasion in forming fabrics, large
wear resistant yarns must be used. However, when
such yarns are used in single layer woven fabrics,
the resultant meshes are not fine enough to prevent
the loss of fines through the fabric and paper
quality suffers in terms of wire mark and noti~eable
sheet two si~e~n~ss.
In the case of single layer woven fabrics for
paper ~king, a delicate balance must be made between
-ki~ the fabric mesh too fine with consequent
reduced wear life, and making the fabric too coarse
at the ~Ypenge of sheet quality.
To overcome these drawbacks, there has been an
ever increasing trend in recent years toward
replacement of single layer forming fabrics with
multiple layer forming fabrics. Such multiple layer
woven forming fabrics may be woven with fine mesh in
the sheet contact side and coarse mesh in the wear
side of the fabric.
While providing at least a partial solution to
the operational drawbacks of single layer woven
forming fabrics, the multi-layer forming fabrics are
much more costly and time consuming to manufacture
since they invariably contain finer yarns and more
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WO 90/14465 PCI'/US90/02964
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of them than in the eguivalent single layer fabrics.
In some cases, new machinery may be needed to weave
the more sophisticated multi-layer fabrics. In
fact, at ~ -sL every step in the manufacturing
sequence, the process is slowed down because of the
higher number of yarns involved and the need for
greater attention to detail to avoid fabric damage.
This trend toward more intricate weaves and
multi-layer fabrics has taken place as a direct
result of the need to achieve higher paper quality
standards at ever increasing paper machine speeds.
Multi-layer fabrics enable the fabric designer
to provide the papermaker with a fabric which can be
designed for improved sheet support and fines
retention in the sheet contact side, and on the
machine contact side, for long wear life.
At the start of this trend, double layer forming
fabrics led the way. Now triple layer fabrics are
seen as even better in meeting the need where sheet
quality is of prime concern.
But in spite of this increased weaving
sophistication, a significant percentage of finers
are still lost during formation, because it has not
been found practical to weave a forming fabric in
sufficiently fine a mesh to prevent the escape of
the smallest particles that exist in the furnish.
Only after an initial precoat of-long fibers
forms above or between adjacent strand members does
the particle capture efficiency begin to increase.
On many fG~ ~n~ ma~hin~s, the sheet comes under such
severe agitation during this formation phase that
the initial sheet precoat is disrupted repeatedly,
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WO90/1~6~ PCT/US90/02964
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causing recurrent losses of fines during much of
this early dewatering phase.
During sheet formation, the forming fabric
surface must be initially bridged over by particles
in the furnish that are at least as long as the
space between adjacent fabric elements. Until this
happens, pore spaces between yarns serve as ready
passageways for egress of small stock particles
called fines.
After the initial bridging takes place, it is
c~ o~ly assumed that the precoat forms up more or
less permanently to thenceforth improve the particle
capture efficiency of the combination of fabric and
precoat: however, recent studies have shown that
this is not always the case. Owing to high machine
speeds and to disturbances during formation,
turbulence takes place, with the result that the
initial precoat is disrupted more than once, each
time with the loss of more fines until the precoat
again becomes established to prevent further loss of
fines through the mesh.
The fines that are lost during sheet formation
are fed back into the headbox, which increases the
fines concentration of the furnish. This constant
recycling can reduce the drainage rate for the
machine. With regard to such loss of fines, it is
known that under some conditions even small
differences in fabric mesh can effect fines
retention significantly.
One of the drawbacks of multi-layer woven
forming fabrics is that particle matter penetrating
through the relatively fine mesh of the sheet
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WO90/14465 PCT/US90/029~
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contact layer frequently lodges between the multiple
layers of t~e fabric. To prevent the buildup of
particle matter, intense showers are needed. Paper
machines without such showering capability,
particularly older, open draw mach;n~, may have
extreme difficulty in n1nning these multi-layer
fabrics without incurring early fillup and plugging
of the fabric. Thus, in addition to the premium
papermakers must pay for the multi-layered fabrics,
they frequently have to add expensive auxiliary
showering equipment to be able to operate these
fabrics.
Perfection of the forming fabric is prerequisite
to the production of high quality paper. In the
case of endless woven forming fabrics, imperfections
in the woven edges can cause sheet breakdown and
disruption of operations. In the case of joined
forming fabrics, the fabric must be very
painstakingly joined into the form of an endless
belt in order to operate on the paper machine. Even
small discrepancies in the perfection of the fabric
join may result in sheet breaks or loss of sheet
quality. The presence of even slight imperfections
in the weave may be cause for rejection of a very
~Yr~ncive f;niche~ forming fabric.
In the field, forming fabrics may be expected to
operate satisfactorily for periods of from one to
several months - unless they are subjected to some
form of accidental damage. Then, efforts are made
to restore the fabric to its original
characteristics by pat~hing the damaged'area. In
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most cases, this ls a time consuming task, and often, it is
not posslble to make a perfect repalr.
Formlng fabrics effectively form a paper sheet by
capturlng paper flbers on thelr sheet contact surface. They
perform this function more or less efficiently based upon the
fineness of the mesh. One way of expresslng thls flneness is
the strand to strand distance. In even the flnest forming
fabrlcs, woven ln multl-layer weaves, the strand to strand
distance exceeds the length of the flnes by a very substantlal
margin.
Dryer fabrics for paper machlnes have for many years
been supplied as flat belts and ~olned together after
lnstallation on the dryer with the ald of a pln seam. More
recently, wet press felts have been furnlshed ln flat form and
successfully ~olned together on the paper machine. Such ease
of lnstallation and seamlng in place on the paper machine has
never been possible for forming fabrics owing to the need for
uniformity in the sheet contact surface of the forming fabric.
Summary of the Invention
In view of the foregoing, the present invention aims
to signlficantly improve the fines retention of single layer
forming fabrics by providing them with a nonwoven sheet
contact layer which will act as the equivalent of a permanent
precoat to reduce the loss of fines through the fabric.
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The inventlon provides an improved forming fabric
for a papermaklng machine havlng a machlne engaglng surface
and a sheet support surface wherein the improvement comprises
machine direction and cross machlne direction yarns
interwoven to form a base fabric, said base fabric havlng a
machine engaglng surface and an opposite surface, said
opposite surface extending in a plane; and
a non-woven array of fibrous strands adhered to said
opposlte surface and forming fluld flow passageways between
sald flbrous strands whlch extend from one surface of sald
array of flbrous strands to an opposlte surface of sald array
of flbrous strands wlthout flrst passlng through any other
fluld flow passageways located between said one surface and
said opposite surface of said array of fibrous strands,
substantially all of said flbrous strands extending ln the
dlrectlon ln whlch sald plane extends, flrst portlons of sald
opposlte surface belng covered by said fibrous strands and
second portions of sald opposlte surface not being covered by
said fibrous strands, the sum of the areas of said first
portions being less than the sum of the areas of said second
portions, said flrst portions and said second portions
collectively forming said sheet support surface, said sheet
support surface having structural elements formed by sald
yarns and sald flbrous strands, the average span between
66601-68
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ad~acent structural elements ln said sheet support surface
belng smaller than the average span between ad~acent yarns in
sald base fabrlc.
The invention enables a paper mill not equipped with
hlgh pressure showers to operate an improved single layer
forming fabrlc which will provide higher fines retention than
previous single layer forming fabrics.
The invention also provides means to improve the
performance of multi-layer formlng fabrics still further
through the addltlon of a permanent nonwoven precoat as the
sheet contacting surface of these fabrlcs.
The lnvention can provide a nonwoven sheet contact
surface layer for nonwoven forming fabrics such as the type
described ln U.S. Patent No. 4,740,409.
The lnventlon helps to reduce the loss of flnes
through the surface of any type of formlng fabric, thereby
reducing sheet two sidedness~and adding to sheet quality and
to reduce the occurrence of wire mark in the paper sheet.
Penetration of fines through the sheet contact layer
of the formlng fabric, can also be reduced, thereby reducing
fillup during operation on the paper machine.
The inventlon acts to submerge and cover up any
underlylng imperfectlons ln the base fabrlc so that such
abnormalltles as may exlst do not adversely lmpalr the
performance of the fabrlc nor the quallty of the paper sheet
made on the fabrlc.
Furthermore the sheet contact surface may readlly be
repalred ln the field followlng accidental damage.
66601-68
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The lnvention can provlde a permanent precoat in the
form of a nonwoven fabric sheet contacting surface havlng a
strand to strand dlstance less than can be achieved by present
slngle or multlple layer weavlng methods.
And flnally, the lnventlon can provlde a sheet
contact surface layer whlch can obscure an underlylng base
fabrlc seam, thereby enabllng formlng fabrlcs to be iolned ln
the fleld for faster lnstallatlon.
The preferred nonwoven sheet contact layer of the
formlng fabrlc ls further characterlzed ln that substantlally
all of lts structural elements encompass fluld flow
passageways that vent dlrectly to elther the sheet slde or to
the base fabrlc slde of the nonwoven sheet contact layer
wlthout flrst passlng through further strata of fluld flow
passageways.
The structural elements of the nonwoven layer reside
substantially entlrely ln the transverse plane, whlch ls the
plane of the surface of the fabric. Between these structural
members are vold spaces whlch act as fluld flow passageways.
Flulds can readlly move upward or downward ln the vertical
plane through these fluld flow passageways whlch are deflned
by the sldes of the transversely dlsposed
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WO90/14465 PCT/US90/02964
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structural elements which comprise the nonwoven
sheet contact layer. Also, dependent upon the
arrangements of the structural members, passageways
may not extend entirely through the contact layer
and may thu~ only be in contact with one of the
transversely disposed surfaces.
The transversely disposed nonwoven sheet contact
layer structural members define the limits of pore
spaces located therebetween which act as fluid flow
passageways, allowing liquid to move upward or
downward in the vertical plane.
The nonwoven sheet contact layer may be
comprised of fibers, filaments, monofilaments, open
cell foam, or the like, adhered to the base fabric
layer by adhesive ho~; ng, fusion bonding or other
means.
Brief Description of the Drawinqs:
This invention may be clearly understood by
reference to the attached drawings in which:
Figure 1 is a fragmentary plan view of a forming
fabric embodying the prior art;
Figure 2 is a sectional view along the line 2-2
in Figure 1.
Figure 3 is a sectional view along the line 3-3
of Figure 1;
Figure 4 is a fragmentary plan view of one
forming fabric of the present invention;
Figure 5 is a sectional view along the line 5-5
in Figure 4;
Figure 6 is a sectional view along the line 6-6
in Figure 4;
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WO90/14465 PCTIUS90/029~
Figure 7 is a perspective view of another fabric
of the invention;
Figure 8 is a sectional view along the line 8-8
in Figure 7;
S Figure 9 is a sectional view along the line 9-9
in Figure 7;
Figure lo is a perspective view of another
fabric of the invention;
Figure 11 is a sectional view along the line 11-
11 in Figure 10;
Figure 12 is a sectional view along the line 12-
12 in Figure 10;
Figure 13 is a perspective view of another
fabric of the invention;
Figure 14 is a sectional view along the line 14-
14 in Figure 13; and
Figure 15 is a sectional view along the line 15-
15 in Figure 13.
Detailed Descri~tion of the Invention
Figure 1 depicts a known single layer woven
forming fabric 22 of the type described in U.S.
Patent No. 3,858,623, which is incorporated herein
by reference, and which fabric comprises
monofilament yarns woven endless so that the warp
yarns 24 extend in the cross machine direction and
filing yarns 26 extend in the machine direction,
which is depicted by the arrow T.
In this prior art fabric, there are 32 yarns/cm
in the machine direction providinq a center to
3 0 center distance o~ 0. 31 mm and 20 yarns/cm in the
cross-machine direction providing a center to center
distance of 0.5:mm. Typically, there are between 20
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WO90/14465 PCT/US90/029~
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and 30 strands/cm in single layer fabrics, so that
the average center to center distance between
strands ranges from about 0.33 mm to o.5 mm.
This prior art single layer fabric is woven in a
four harness satin weave, where cross machine
direction yarns interlace with every fourth machine
direction yarn. As can clearly be seen in Figures 2
and 3, this interlacing pattern prevents the yarns
from forming a flat surface for sheet formation.
For example, it is not likely that a fiber may
easily bridge between adjacent cross-machine
direction yarns at positions 30 and 32 since they do
not reside in a single plane.
The average length of softwood fibers is a~out
3 mm to 3.5 mm, whereas hardwood fibers average
about 1.0 mm. It is known that fibers must be
considerably longer than the center to center
distance between su~o~L strands in the sheet
contact surface to achieve an appreciable
probability of being ret~in~d on the strands.
In one theoretical study, average fiber length
was kept in the same length range as the strand to
strand distance and this distance was the same in
the machine direction and cross-machine direction.
All the strands were assumed to lie in the same
plane. Under these assumptions, it was calculated
that: where the strand to strand distance was equal
to the fiber length, only about 15% initial
retention would occur; where the center to center
distance was half the fiber length, initial
retention would increase to 75%; and where the
center to center distance was one quarter the
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WO90/1446~ PCT/US90/029~
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average fiber length, initial retention would
increase to over 90%. Strand frequency may vary in
the range of one quarter to one tenth fiber length
with little further effect upon retention, according
S to this study. (Refer to Helle, Torbjorn, "Fiber
Web Support of the Forming Wire", Tappi Journal,
January, 1988.) For example, on the basis of this
theoretical treatment, if it is assumed that the
average fiber length is l.O mm, then to achieve over
90% retention, strand to strand distance would have
to be about 0.25 mm, which is equivalent to a strand
frequency of 40 strands/cm - well above the usual
range for single layer fabrics. In addition, such
strands would have to lie in the same plane to
achieve this efficiency.
Actual therm~ ?chAnical pulps contain widely
varying fiber lengths. For an average of several
pulps, it was found that whereas 52% of the weighted
average fiber length excee~eA 1.3 mm, 35% of the
weighted average fiber length was less than 0.4 mm
long. Included in this 35% are the fines, defined
as fibers less than 75 micrometers long. It is
generally conce~e~ that woven strands do not
efficiently capture fines and that the fines are
ret~;ne~ on the sheet fo. ; ng surface by the
combination of fabxic and long fibers that bridge
between strand 5~GL L members to establish a
precoat. To further illustrate the relationship of
the forming fabric surface to the fines, a single
fines fiber 28 measuring 75 micrometers in length is
shown to scale in Figures 1, 2, and 3. It should be
recalled that this 75 micrometer length is the upper
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WO90/14465 PCTIUS90/029~
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limit for the fines fraction, and that many fines
are much shorter than this.
From all of the foregoing, it should be clear
that when strand to strand distances in the
supporting sheet contact layer are significantly
less than the average fiber length, better initial
fines retention will occur, and that even small
differences in the strand to strand distance through
the critical range can vastly effect initial fines
retention and sheet quality.
It should also be clear that a substantial part
of the fines cont~ine~ in any furnish will likely
pass through this prior art fabric, where strand to
strand distances are much larger than the length of
the fines.
Figure 4 shows one embodiment of the invention
where a nonwoven sheet contact layer 34 comprised of
randomly disposed continuous filaments 36 is adhered
to the sheet forming side of the same prior art
fabric 22 to provide a closer mesh sheet contact
layer having a r~uçe~ strand to strand distance
than the prior art fabric had in its original state.
The filaments in this no-,~uven sheet contact layer
are adhered to the topmost surfaces of the base
fabric layer at each point of contact 38. The
fil. -nts are adhered to each other at each point of
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contact 40. The porous passa~ ~y~ 42 formed
between filament members vent directly to either the
sheet side 44 or to the base fabric side 46 of the
nonwoven sheet contact layer.
Means for pro~l~cin~ such continuous filament
randomly disposed arrays are well known. Such means
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WO90/14465 PCTJUS90/029
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include, for example, the random array hot melt
filament applicator known as the Acu-Fiber system
made by Acumeter Laboratories, Marlborough,
Massachusetts USA. In the preferred embodiment, the
filaments would be comprised of a thermoplastic
material that would adhere to the base fabric
material. For example, in the case where the base
fabric was comprised of polyester, the nonwoven
monofilaments would preferably be comprised of
thermally bondable lower melting temperature
polyester, a copolyester, or some other fuseable
thermoplastic that would adhere well to the
polyester base fabric and would be suitable for use
on paper mach;nes.
The diameter of the filaments may range from
less than 25 mi~ Lers to more than 150
micrometers, but will in most cases fall within this
range, since fibers smaller than 25 micrometers may
not provide adequate life on the paper machine and
fibers above 150 micrometers may not allow for the
desired close strand to strand spacing without
eYc~s.ively limiting the size of fluid fIow
passageways between strand member~.
Using the randomly ~icpose~ pattern may at first
consideration-seem to be a disadvantage, when
- -red to the stringent pattern uniformity
requirements in woven base fabrics. However, there
are two reAConc why the random pattern may be
tolerated without quality sacrifice in this nonwoven
sheet contact layer.
First, pattern perfection in woven fabrics is
needed because the average strand to strand distance
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~O90/1~65 PCT/US90/029~
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is large in comparison to the fiber length of the
furnish, and as already shown, small differences in
such large spans between support strands can lead to
substantial differences in fines retention. In
c~ rison, the average strand to strand distance in
the nonwoven sheet contact layer must be smaller
than in the base fabric layer to limit fines passage
into the base layer, and also, to provide improved
sheet support with resultant improved fines
retention. Furthermore, by controlling the strand
disposition without consideration of weaving
constraints, it is entirely within reach to provide
a strand to strand average spacing well above the
most sensitive range, such that variations in actual
strand to strand distance will in ~ L all cases,
still yield spans that are readily bridgeable and,
hence, conducive to a rapid establishment of a
precoat.
Second, the most serious weave imperfections
involve conditions which span across major
ions of the fabric. Owing to the woven nature
of the fabrics, suGh defects are usually aligned
with either the machine direction or cross-machine
direction yarn systems. Any woven machine direction
imperfection that involves an irregular span between
support strands could result in a continuous paper
sheet defect. Any cross - rh i ne direction weave
imperfection involving a change in support~strand
frequency could cause sheet breaks. In the case of
the random nonwoven sheet contact surface: first,
the OC~ul L ence of a larger than ordinary gap between
support stran~s is not likely to result in a
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WO90/1~65 PCT/USgO/029~
7'~ ?~
significant sheet defect because such a defect would
not likely occur in a full width or full length
pattern that would be readily detectable in the
sheet; and second, by reducing the average span
between strands to less than the average distance
between strands in the base fabric, the importance
of small differences between support strand
distances is lessened.
Figure 7 shows an example of a sheet contact
layer adhered to a woven single layer forming fabric
22 of the same prior art, where the layer is
comprised of a blend of fine staple fibers 48
combined with coarse fibers 50. These fibers are
entangled with each other and adhered to each other
and to the base fabric at points of contact.
The fibers lie in a single layer, such that
substantially all of the fluid flow passageways
between fiber strands are in fluid c lication
with either the sheet side or the underside of the
nonwoven layer. Furthermore, the average span
measured in the transverse plane, i.e., in the plane
of the fabric surface, measured between adjacent
structural ~rs in the nonwoven layer is smaller
than the average span between adjacent structural
members in the base fabric layer to which the
nonwoven layer is bonded.
In the preferred case, substantially all of the
pore spaces within the nonwoven sheet contact layer
vent directly to either surface of the nonwoven
layer without first passing through an additional
series of pores which do not vent directly to the
nonwoven layer surfaces. - -
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WO90/1446S PCT/US90~029~.
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From the foregoing, it should be readily
apparent that many ordinary nonwoven fabrics would
not be suitable for incorporation in the structure
of this invention. For ~YA, le, nonwoven materials
-ch~nically bonded together by needle p~lnch~ng
would of necessity contain a multiplicity of
superimposed fibrous layers which might serve to
entrap fines within internal pore spaces.
It has been found that flock fibers of from 0.1
to 1 centimeter in length can be applied uniformly
to the surface of a base fabric by first coating the
fabric surface with a tacky conductive adhesive and
then applying the fibers by known electrostatic
flock application means. The flock fibers will at
first be disposed principally in the vertical plane
with respect to the fabric surface. Subsequent
treatment with heat and pressure is ~ee~e~ to
consolidate the flock into the peL ~n~tly adhered
transverse oriented porous sheet contact layer of
the invention.
In the preferred case, such flock fibers would
be adhered to each other and to the base fabric by
fusion bon~ing means. Bi_ -nt..fibers such as
those made by ~oe~hct Celanese Corporation,
Charlotte, North Carolina USA, having a polyester
core and a lower melting t~ -rature copolyester
sheath, may be ;~e~lly suited for this capping
layer, although other fiber types may-also prove
useful.
Other means of forming a suitable nonwoven layer
include ~epo~;tion from a wet fiber slurry, such as
is illustrated in Figure lO. Here, a wet formed
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WO90~1~65 PCT/US90/029
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random fiber nonwoven array 52 is adhered to the
same known forming fabric as described earlier.
Non-crimped short fibers 54 under 1 cm in length are
first disbursed in a wet slurry containing a dilute
resin binder. Formation is achieved by well known
means, except at slower speed. If at least some of
the web fibers are of lower melting temperature, and
posses high melt viscosity, the web adhesion to the
base fabric may be Pnh~ncP~ by fusing such fibers to
the base fabric. This example illustrates very
short strand to strand distances, with the
likelihood of affording high initial fines
retention.
As shown in Figure 10, the random nonwoven fiber
coverage on the base fabric results in some
localized spots where fiber coverage is a~ove
average and other localized ~pots where fiber
coverage is below average. It has been found
through experimentation that such localized fiber
distribution differences have little if any effect
upon sheet quality because they only extend over
small areas and because even in the case where the
distance between ~u~o~ strands is below average
for the nonwoven layer, the span distance is still
smaller than in the underlying base fabric.
Fig~re 13 shows a nor.woven sheet contact layer
comprised of two sets of parallel diagonally
oriented monofilament strands attached
perpendicularly to each other and to-the same known
base fabric by fusion hon~in~ means. -In this
example, monofilaments 56 are adhered directly to
the surface of the woven fabric 22. Monofilaments
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58 in the second diagonal set adhere primarily to
the contacting surfaces 60 of the first diagonal
set. Notice that strand to strand distance in the
transverse plane of the nonwoven sheet contact
surface is less than in the base fabric layer
adjacent to the nonwoven layer, and that the
nonwoven layer contains fluid flow passageways that
vent directly either to the sheet side or to the
base fabric side of the nonwoven layer.
Many variations in the processes and materials
may be made without departing from the scope of this
invention. For example, in considering the
formation of a nonwoven precoat on the surface of a
for ing fabric from a wet slurry, it is not
nec~s~ary to restrict such materials to textile
fibers only.
What is desired is a reduction in the average
distance between sheet ~u~Gl ~ surfaces, and any
durable particle which can accomplish this purpose
without adversely reducing drainage rate may be
considered an appropriate candidate for this
~L~ose. Thus, very short fibers, fibrils, and even
particles of nondescript shape may be considered
appropriate.
For example, such particles may be suspended in
a resinous slurry which when drained through the
forming fabric by known means will produce a coating
of particles that bridge across interstices between
~u~G~ L surfaces in the base fabric. These small
particles could be hon~ed in place by drying and
curing the resin binder, although adhesion may be
greatly enh~nce~ if the particles are themselves
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fusible under the influence of elevated temperature
or other means.
The result will be a permanently adhered fine
pore size nonwoven precoat adjacent the top surface
of the underlying base fabric such that, during
sheet formation, the precoat will limit the escape
of fines and improve sheet quality.
Although not illustrated, it is also possible to
produce fabrics of the invention through the
adhesion or direct formation of porous reticulated
plastic foam as the sheet contact layer, provided
that the average distance between support members in
the nonwoven layer is less than the average distance
between support ~ h~rs in the base layer, and that
fluid flow passageways between nonwoven members are
in communication with the sheet contact surface, the
base fabric surface, or both.
E X A M P L E ~
Several prototype fabrics of the invention were
constructed and evaluated compared to the original
control fabrics using the dynamic drainage jar,
described by Britt et al. in their July 1986 Tappi
Journal article entitled "Observations on Water
Removal in Papermaking".
Experimental fabric A utilized a woven double
layer forming fabric treated with a light coating of
3 denier nylon flock that had been cut to a precise
O.75 mm length. The flo~;n~ was applied by
3C electrostatic means to the adhesive treated sheet
contact sur~ace of the double cloth fabric. The
adhesiv~ was-then dried, and the flock was hot
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WO90/1446~ PCT/US~/029
pressed to form the fibers into a porous smooth
nonwoven transverse oriented sheet contact layer
that obscured about 30% of the woven fabric surface
layer in contact with the nonwoven layer.
Experimental fabric B was prepared in a like
manner as fabric A and utilized the same woven
double cloth base fabric and the same flock fiber,
the exception being that a slightly heavier coating
of fibers was applied such that the completed
transverse fabric surface layer obscured about 40%
of the woven fabric surface layer in contact with
the nonwoven layer.
In tests utilizing a C~na~iAn 50/50 blend of
hardwood and softwood stock with 10% total fines, it
was found that while making 35 gram per square meter
sheet, the original double cloth fabric provided 33
fines fraction retention versus 63% fines fraction
retention for fabric A. When the same stock was
used to produce 50 grams per square meter sheet, the
original double cloth yielded only 18% fines
fraction retention versus 48% fines fraction
retention for fabric A.
Further testing was carried out at a sheet -
weight of 35 grams per sguare meter utilizing a US
manufacturer's top liner stock having 27~ fines
fraction. The same double cloth CGn~LO1 fabric
yielded 28% fines fraction-retention, versus 41~ for
experimental fabric A and 51~ for experimental
fabric B. The time to form the sheet and the total
vacuum pressure applied during sheet formation were
both monitored. There was no significant increase
in sheet forming time or vacuu~ pressure level
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WO90/14465 PCT/US9OtO29~
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during sheet formation with the experimental fabrics
compared to the control fabrics. Sheet smoothness
and formation were observed visually to be of equal
quality between the control and experimental fabric
materials. The sheet released equally well from
these three materials, and there was no visual
indication of fillup within the very thin transverse
oriented capping layer of the experimental fabrics.
These results indicate that fabrics of the invention
can significantly increase fines retention under the
dynamic sheet formation conditions that occur during
the papermaking process.
The embo~i -rLs which have been described herein
are but some of se~eral which utilize this invention
and are set forth here by way of illustration but
not limitation. It is obvious that many other
'-o~ ts which will be readily apparent to those
skilled in the art may be made without departing
materially from the spirit and scope of this
invention.
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