Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 93/01350 pcr/cA9l/oo247
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~ULTI-PLANE DEW~TERING FABRIC
This invention relates to dewatering fabrics used in the press section
of a papermaking machine. It is particularly concerned with a press section
fabric including multiple distinct layers of nattened monofilament warp
yarns confi~red to provide improved roll nip pressure unifo2Jnity, reduced
paper marking, increased paper dewatering and enhanced compaction
resistance.
. .
Pressure non-uniformity in a press nip can ari~e from two sources:
a) the pres~ rolls themselves, wbich may be grooved or drilled to provide
void space for the water e~presfied from the web in the nip; and b) the press
fabric it~elf, which, due to its woven structure (or other reasons) transmits
press nip pressures unevenly. In different situations, either of these two
sources may be the dominant source of non-uniformity and hence of paper
marking or low dewatenng. In some cases, for example, the pattern of the
grooves of the press roll can be seen clearly in the finished paper, while in
other cases, the marks of the woven structure of the fabric can be seen.
Sometimes both marks are present.
The first proposal to use flattened monofilament warp yarns in a
press section fabric was by Miller et al., U.S. 4,414,263. Miller et al.
advocated using ~lattened monofilament yarns with a relatively low aspect
ratio in an otherwise conventional weave. Vanations on the Miller et al.
fabric which are proposed by Stanley, U.S. 4,973,512 and U.S. 5,023,132.
An improved fabric using flattened monofilament warp yarns is described
by Jacks~s'W0 9V04374. In the Jackson fabric, a flattened monofilament
warp yarn with a somewhat higher aspect ratio i8 used at a high fill factor,
in a weave pattern which provides long exposed floats of flattened
monofilament warp yarns on the paper side of the fabric. This weave
construction provides a relatively flat, ~mooth, almost platform-like surface
on the paper side of the fabric. This relatively flat surface transfers the
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mechanical loads imposed by the press section rolls from the press fabric to
the paper web in a way that provides improved pre~sure uniformity.
The ma~mum benefit of a Jackson press section fabnc appears to be
obtained in a weave design that includes a ~ingle layer of weft ya~s. In
this type of fabric, the maximum float length for the flat warp yarns can be
achieved, thereby imparting the masimum smoothness and pressure
unifo~nity to the fabric.
.
However, it has now been found that a fabric containing a single
layer of weft yams as described by Jackson has certain disadvantage3. In
particular, it seems not to be entirely suitable for coDfigurations in which
at least one of the pres~ rolls has cavities, ~uch as grooves or holes in its
~ce. It appears that such a thin fabric when u~ed in a press felt i
.
unable to bndge these roll cavities adequately, and pressure non-uniformity
on the paper web results. If a double weft yarn layer fabnc design is used,
as IS described by Jackson, in order to bridge the press roll cavities better,
~the amount of flattened monofilament wa2p yarn~ which can be esposed on
the paper side of the fabnc is not as high as can be e~posed for a single
weft yarn fabric design. This is because the flattened monofilament warp
yarn~ follow a generally diagonal path within the fabric when they
interweave with t~e bottom weft yarns. This limits the length of float t~at
can be exposed on the paper side of the fabric.
Thus'there has been found to be a balance between enhanced
pressure uniformity on the paper side of the fabric, and press roll cavity
bridging on the press roll side of the fabric. If a double wef~ ya~ layer
fabric i8 used there iB the filrther disadvantage that the peak pressure in
the roll nip is not as bigh as is obtained with a single layer fabric, and
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therefore a douUe weflG yarn layer fabric will dewater the sheet of paper
less.
This invention seeks to overcome these problems and to provide an
improved fabric of the type descri~ed by Jackson which provides better roll
cavity bridgiIIg characteristics and better pressure unifo~ty. Further, this
invention also seeks to enhance the compaction resi~tance of the fa~ric.
Additionally,~ this invention seek~ to prov de a fabric in which these
Dnpro~rements can be realised without losing the ability to have long
e~posed floats of flattened monofilament warp yarns on the paper side of
the fabric.
These improvements are obtained by incorporating into the fabric at
least two distinct layers of flattened monofilament wa~p yarns, and at least
one layer of weft yarns. One layer of flattened monofilament warp yarn6
provides the long esposed floats generally on one side of the fabric.
Further, if a double weft yarn layer construction is used, the other warp
n layer(s) can be utilised to maintain good separation between the upper
and lower wef~ yarn layers (that is, the weft yarn layer~ adjacent the paper
side and the press roll side of the fabric, respectively) without having to
.
dimi~ish in any way- the amount of long esposed flattened monofilament
warp yarn floats on the paper side of the fabric. The separation thereby
obtained between two weft ya~n layers enhances fabric compaction
resistance and available void volume.
We have found that fabrics woven according to this invention reduce
marking by both the press rolls and-the press fabric, and the exact
oonfi~ration of the two layers of ~at warps can be adjusted for these
different circumstances. If the press roll grooves are 1he main source of
non-uniformity, it may be beneficial to locate the long floats close to the roll
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surface. In other cases, the long float~ may be beneficially located nearest
the paper side surface of the fabric.
In the fabrics of this invention, the term "warp yarn" is used to
designate the warp on the weaving loom, that i8 the set of yarns that travel
from the warp beam through the heddles of the loom, and wbich form the
shed through whi~h the wePc yarn i~ passed. If a fabric of this invention is
woven as an endless loop, the flattened monofilament waIps lie in the icross-
machine idirection of the paper making mai~hine. It i8 known to weave an
endless loop ~ncorporating a seam, or to insert a seam later, in order to
&cilitate installation in a press section. Alternatively, if a fabric of this
in~rention is woven as a "flat" fabric, a~ a continuous run, and then joined
into the required endless loop with a suitable seam, the flattened
monofilament warps lie in the machine direc~on of the paper making
macbine.
Thus, in a first broad aspect this`invention provides a woven
dewatering fabric, for the press section of a papermaking machine, having
a first side and a second side, one of which constitutes the paper side of the
fabric, and the other of which constitutes the press roll side of the fabric,
andha~inglo~gexposed floats of flattened monofilamentwarpyarns on the
first ~ide of the fabric, wherein:
(i) the aspect ratio of the flattened monofilament
- ' warp yarns is at least 1.5:1;
(ii) the fabric weave includes at least two distinct
layers of warp yarns each including flattened
monofilament warp yarns, one of whicb pro~ides
the exposed first side float;
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(iii) the fabrîe weave further include~ at least one
layer of wef~c yarn6; and
(iv~ the fill factor for each of the distiIlct layer~
including flattened monofilament warp yarns i~ at
lea~t 45%.
Preferably, the long exposed float~ on the first side of ~he
` ~ fabric satisfy a definition for 'float ratio' ~imilar to the one given by
JacJ~on, WO 9V04374, modified to fit`t~e fabrics of this i~rention. For t;he
fabrics of this invention it i8 preferred that the long espo~ed float6 on the
first ~ide of the fiablic ~ati~fy a float ratio fonnula x/y, wherein:
(i) "x" represents t;he number of wef~ ya~ in a wef~
y~ layer on the first ~ide of the fabnc in a single
weave patte~n repeat which are on t}-e in~ide of
the: ~bric with respect to a particular flattened
monofilament warp ya~, and supporting that
~varp yarn; and
- (ii) "y" representfi the total num~er of we~
yams in the wef~c yarn layer on the fir~t
side of the fabric in a ~ingle weave pattern
repeat;
and wherein for a m~ority of the fir~t side e~zposed floats of flattened
monofilam-ént warp yarns:
(iii) x i~ greater than one; and
~iY) x i8 greater tban one-half of y.
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In some application~, it i8 possible to use the fabrics of this invention
in a press roll section without a porow layer, such as a needled fibrous
batt, attached thereto. Conveniently, a porous layer, such as a fibrous batt,
- can be attached to the paper side, or to both the paper side and the press
roll side, of the fabric.
Prefernbly, the flattened monofilament wa~p yarn~ making up each
of the dill~na iayers of warp yar~s are situated substantially vertically
above one another in the ~Ivoven fabric.
The invention will now be discussed by way of referencs to the
attached ~Ygures in which:
Figures l, 2 and 3 show a single weft yarn l~yer fabric;
Figures 4 through 11 show alternative wea~re pattern~; -
Fi~res 12 and 13 show a wea ve pattern incorporating a binder tie
yarn;
Figures 14, 15 and 16 show fabrics carrJnng porous layers; and
Figure 17 sho~s the face of a fabnc.
In Figu~re 1 i~ shown A single we~ yarn layer fabric according to this
in rention. The fabric as sho~m includes three layers of yarn~:
(i) an upper, side A, set of flattened mono-filament
_, warp yarns 1, 3, 5 & 7;
(ii) a lower, side B, set of flattened monofilament
warp yarns 2, 4, 6 & 8; and
(iii) a ~ingle layer of weft ya~ns 1, 2, 3, 4, 6, 6, 7 & 8.
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Each set of warp yarns iB woven at à fill factor of at least 45%, and
preferably of about 85%. ThUB the combined fill factor for the two sets of
waIp yarns iB greater than 90%, and preferably iB about 170%. This high
fill factor for the nattened monofilament warp yarns provides the improved
pressure unifonnity on the paper side, improved press roll cavi1 y bridgiDg
on the press roll ~ide, and enhanced compaction resistance of the fabric.
Figures 2 and 3 8~0W schematically the weave pattern for the fabnc
of Figure 1. In these and later figures three con~rentions are used; it can be
seen that the yarns in Figure 1 are numbered to matdl Eigures 2 and 3.
First, in Figure 2 a filled-in ~quare indicates that at that point a warp yarn
pas~es over a wef~ yarn. Second, Fig~are 3 represents a cross secffon based
on Figure 2 aIong the line of 1 he walp yarn numbers given: thus in Figure
3 the walp yarns are numbers 1 & 2 from Figure 2, and the section in
Figure 3 iB the lef~hand side of the diagram in Figure 2. Third, 'A'
represents the fir~t side and 1~' the second side, of the fabric. In all of these
fabrics the side A include~ the long e~cposed float~.
Certain other features of the fabrics of this in~ention can be seen in
~igure 1.
Fir~t of all, only one set of yarns, 1, 3, 5 & 7, is e~posed on the side
A of the fabric. As shown, the two sets of waIp yarns are substantially one
over the other: thus warp ya~n 1 is abo~e warp yarn 2, and 80 on. ThiB is
largely a consequence of the fill factor used; generally, tbe higher the fill
factor the better thi~ correlation will be. It can also be seen that at some
points in this single weft yarn layer &bric walp yarns ~om both sets pas~
the same side of particular weft yarn: warp yarns 1 and 2 both pass o~rer
the side A of weft yarn 2, and pass under the side B of weft yarn 8. Fi~ally,
as shown this fabric is effectively the same on both faces, as the weave
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psttern~ for the two sets of warp yarns are mirror images of each other.
This is not necessary, and the weave pattern adopted for the press roll ~ide
B can be dif~erent to that of tbe paper side A.
In a fabr~c such as tbat in Figure 1 as the fabric iB the same on both
faces, it does not matter which of side A and side B i8 taken as the paper
~ide of the fabric, and which i8 taken a~ tbe pre~ roll side of t}le fabric. In
these Figures as noted above tbe convention i8 adopted that the first side
of the fabric, which includes the long e~cposed ~loats, is identified as side A.Generally, it is preferred that side A be tbe paper side ofthe fabric, but this
is not necessary. The fabrics of this invention function adequately in Bome
applications when side A is the press roll side of the fabnc.
.
The warp yarn weave pattern on the side A of the fabric preferably
utilizes the long exposed float concepts de~cribed by Jackson in WO
9V04374. That is, the weave pattern includes long e~ ed float~ of
flatbned monoSlament warp yan~ with an aspect ratio of at least 1.5:1 on
the~ side A of the fabric but not necessarily on the side B of the fabric.
These long e~cposed floats can be characterized by a "float ratio" e~cpressed
by the formula x/y as defined above. There is an upper limit to the float
ratio, which i8 the point at which fabric integrity becomes questionable. For
a fabric similar to that ~ho~n in F'igure 1, this seems to occur at a float
ratio of about 9l10. It al~o appears that if the values of s and y are both
low, or if x i8 clo~e to one half of y, then the dewatenng properties of the
fabric are im, paired.
.
The flcat ratio in a given f~bric need not be consta~t either along a
given nattened monoSlament warp yam, or for aU of the 9attened
monofilament warp yarns in a given weave. Further, not all of the e~po~ed
floats offlattened monofilament warp yarll8 need have a floatratio in which
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g
~c is both greater than one, and greater than one half of y, although
ma~cimum benefit will be obtained if these restrictions are obse2~red. AB i8
shown in Figure 17, which ~hows only the side A of the fabric, a fabnc can
be woven ~with varying float ratios. From the top downwards in Figure 17
the float ratios are 7/8, 5/8, 6/8, 418, 6/8, 3J8 and 5/8. In a similar wsy the
float length can be varied along a given flattened monofilament warp yarn,
to pronde in sequence a 112 unit, then a 5/6 UlIit, for e~cample. In such a
case the float ratio should be determined o~er the fi~ ngth of the pattern
repeat, and V2 combined with 5/6 in fact i~ 6/8.
The warp yarn wea~re pattern on the side B of the fabric i8 not BO
restricted, as i8 shown in the wea~re designs of Fi~res 4 t~ough 9. In ea~
of these, wbilst an adequate float ratio iB maintained for the set of warp
yarns pro~riding the side A long e~cposed float~, the path for the other wa~
yan~s can be quite dif~erent. Figures 6 and 7 show a second fabnc in which
~he wsTp yaTn weave patterns are mirror images of each other, while in
Figures 4, 6, 8 and 9 the patterns are quite dif~erent. The float ratio for
~ide A of each of these weaves ;B:
.. ~c
Figure 3 7 8 7/8
Figure 5 6 8 6/8
Figure 7 5 8 5/8
Figure 9 7 8 7/8
F~om these Eigures and the den~ed float ratios it can be seen how
the float ratio is calculated. In each case it ;8 ODIY the weft yar~s
supporting a warp yarn which are counted. In each of these ~igures, as is
indicated at X, there is a second warp yarn between the esposed float a~d
a weft yarn: that wef'c yarn is taken to be supporting the warp yarn float.
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The designs of ~igures 4, 5, 6 and 7 have flattened monofilament
warp yaIns on both sides of the fabric providing good paper side pre~AUre
uniformity and good press roll venting cavity bridging. Ln Figares 8 and 9
the design has flattened monofilament warp yarn floats on the side A of the
fabric, but unlike Figures 6 and 7 the ~econd warp yarn~ are highly
interlaced between each layer of wef~ yarns providing good paper side
pres~ure uni4rmity and good compaction re~istance.
In the designs of Figures 4, 5, 8 and 9 (and also Figures 10 and 11,
described below) the wa~p yarn sets have diffenng path length~. Designs
of this type may require the use of two, or more, warp yarn beams, as is
well known to those skilled in the art of weaving. The designs illustrated
are- all interwoven 80 that the result is a single fabric.
Figures 10 and 11 illustrates a design using two layers of wef~ yarns,
and three layers of flattened monofilament warp yarhs 1, 2 and 3. As
shown, the weave pattern for wa~p yarn 1, on the side A of t~e fab ic and
waIp yarn 3, on the side B of the fabric, are essentially similar, but this i8
not necessary: the pattern for wa~p yarn 3 can be different. The third layer
of warp yarns, 2, iB interlaced into the wefl; yarns between the other two.
Due to the high fill factors used for each warp yarn, in the woven fabric the
three warp yarns lie ~ubstantially above each other. In this fabric the float
ratio is 5/6.
Figures 12 and 13 illustrate a fabric in which the two distinct layers
_.. ~
of flattenedlmonofilament wa~p yarns weave separately into the two sets of
weft yarns. One set of flattened monofilament warp yarns ~veaves into the
gide A ~et of wefts (the even numbered ones) ant the other set of flattened
monofilament warp yarns weaves into the side B set of wefts (the odd
numbered ones). Again, although the weave pattems sho~vn are essentially
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the same for each ~et of flattened monofilament warp yarns, the pattern for
side B need not be the ~ame as that for side A. The float ratio is 6/6. These
two layers are then joined together in the weave by the separate binder
yarns Y. The~e separate binder yarns can be incorporated either in the
warp yarn direction (as shown in Figure 13) or in the weft yarn direction
(not shown). Generally binder yarns are small relative to the warp yarns
or weft yarn~, often serving only to hold the fabric together during weaving
and subsequent proces~ing, until a fiber batt layer is attached by needling
thus binding the entire structure together. In some case~ the binder yarns
are soluble yarns and are removed by washing after needling. In other
cases where no batt or other porous layer is used the binder yarns are
stronger and withstand use in the press section, to keep the two layer~ cf
the fabric ~om separating.
.
It is possible to we the fabrics of this invention on their own in a
press sec~on. As was noted above, cases where the fabric wea~re pattern is
such that the fabric is essentially the same on both faces, whether side A
or side B is the paper side of the fabric does not matter. But when an
unsymmetrical weave, such a3 that in Figure 5 or Figure 9 is used the
fabnc faces cease to be the same. ~ most applications it is preferred tbat
side A, the fir~t side which incorporates the long e~cposed floats of flattened
monofilament ~varp yarns, is taken a~ the paper ~ide of the fabric.
Alternatively, as shown in Figure 14, 15, and 16, the fabrics of this
i~ventio~ ~ay also be used with porous layers attached thereto. If only one
Iayer is used, generally it will be attached to the paper side of the fabric,
~uch as the layer 70 attached to side A of the fabric, or the layer 80
attached to side B of the fabric, in Figures 14 and 15. Two porous layers,
aB at 70 and 80 in Figure 16 may be attached one to ea¢h side of the fabric.
Again alt~ough this seemingly makes both faces of t}le fabric the same, it
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is still preferred that side A with the first set long e~cposed floats iB the
paper side of the fabric. Thus the single batt arrangement of Figure 14 iB
preferred rather than Fi~re 1~.
:
Typically a porous lsyer 70, 80 comprises a staple fiber batt, which
i8 generally attached to the fabric by needle punching.
~ . :
~- - If a batt iB used, some thoug~t should be given to the direction in
which it is laid, since it i~ known that the batt Sbers tend to be oriented
substantially in the direction the batt is laid onto the base fabric. For the
fabrics of this invention it i8 preferred that a batt iB laid with a high degreeof fiber orientation. There are, nevertheless, several options for the
direction of orientation, relative to the direction of the long e~posed floats
in side A of the fabric, regardless of whether side A i8 or i8 not the paper
side of the fabric.
For a fabric with a 6ingle batt layer, the orientation can be
sub~tantially parallel to, or substantially perpendicular to, the side A
e~posed floats.
For a fabric ~vith two batt layers, one on each side of the fabnc, the
onentation of each batt independently can be substantially parallel to, or
substantially perpendicular to, the side A esposed floats. Tbu~ there are
four options: both side A and side B batts oriented in the ~ame direction,
and either, parallel or perpendiculsr to the side A e~cposed floats, or the sideA and side B batt~ oriented substantially perpendicular to each other, and
one of them - either side A or side B - parsllel to the long e~posed floats.
In the fabric of this invention a high ~veft ysrn fill, as ~ell as a high
flattened monofilament warp yarn Sll of at least 45%, is recommended, in
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order to minimize the space between the flattened warp yarn 8UppOI't
points, such as the distsnce Z in Figure 3. The purpose of this high wePl
yarn fill i8 to reduce the bending of the nattened monofilament warp yarns
in a direction at right angles to their plane in the fabric under the influence
of the very high pressures exerted on the fabric in the pres~. Several
variables affect the weft yarn fill factor in a given fabric. Some of these are:
(a) ~ the degree of openness required in the fabnc;
(b) the thickness of the flattened monofilament
.
warp ysrns;
(c) the material used for the flattened
monoSlament warp yarns: a stiff material
with a high elastic modulus deflect~ less
than a 80ft one; and
(d) the degree of softness of the weft yarns: a
~oft deforma~le weft yarn provides better
support than a yarn which is hard and
makes contact with the warp yarns along a
..
thin line.
In practice? it appears tbat the weft yarn spacing, a~ at Z in Figure
3, should be less than 8 times the flattened monofilament warp yarn
thicknes~, and is preferably less than 5 t mes. ;~ -~
Generally pres~ fabrics are constructed ~om nylon monof~aments,
with a nylon staple fiber batt, although polyester and other matenals are
sometimes used. It is preferred to use nylon for this invention, but this
invention is not BO limited.
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~. 1 12124
The distinct warp yarn layers in t~e fabric of this invention are
woven from flattened monofilament yarns. In these ~attened monofilament
warp yarns, the aspect ratio, that is the ratio of wid~h to thicknes6, ~hould
be at least 1.5:1. If the thickness is made too low, the filament become~ too
thin and too fle~ible, and tends to transmit both knuckle markings from the
fabric weave and any pattern of cavities in tbe press ro~ls to the paper. A
suitable lower limit for the thickness appears to be at about 0.1 mm. If tbe
aspect ratio is made too low it becomes difficult to control the weanng
process and hence it iB desirable that the aspect ratio be at least 1.5:1, and
preferably at least 2.0:1. A desirable range for the aspect ratio iB from
about 4:1 to about 10:1. Currently available weaving equipment appear6
to be able to handle fibers with an aspect ratio of up to about 100:1. The
fiber cross section also need not be simply ~ubstantially rectangular. It i6
also of advantage to use the grooved flattened monofilament warp yarns
described by Jackson, W0 9V04374.
.
In the preceding discussion of the figures it is a6sumed that all of the
flattened monofilament warp yarn6 are the same. This i6 preferred, as it
simplifies weaving, but it i8 not nece6sary. Warp yarn6 which are not
flattened monofilaments can be included, but this iB not recommended,
especialb in the paper side of the fabric. The dewatering capabilitie6 of the
fabric will likely be impaired, and the risk of paper marking enhanced.
Consequently it is preferred that at least the majority, preferaUy
F.ubstantially all, and most preferably all of the warp yarns are flattened
monofilamen't yarns. It is al60 most convenient if the flattened
monofilaments in each of the different layers of warp ya~s are of
substantially the same width and thickness.
A fabric according to this invention can be woven either as a ~losed
endles~ loop, or as a continuous run of fabric which is then seamed in some
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way to provide the required }oop. In a closed loop fabric the flattened
monofilament warp yarns lie in the cro~s-machine direction, that i~
substan~ally at a right angle to the direction in which the paper web
move~. In a con~nuous run of fabric, the flattened monofilament warp
ya~ns lie in the machine direction of 1 he fabric substantially parallel to the
direction of movement of the paper web.
,
