Note: Descriptions are shown in the official language in which they were submitted.
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1007-276
PRESS FELT WITH IMPROVED DRAINAGE
BACKGROUND OF THE INVENTION.
This invention is concerned with composite fabrics known as
"press felts" used in the press section of a paper making
machine.
In the press section of a paper making machine, the wet
paper web is carried by at least one press felt, upon which the
wet paper is passed through at least one nip between either a
pair of rotating opposed rollers, or between a single rotating
roller and a device known as a "shoe", in what is known as a shoe
press. For simplicity, in the following discussion reference
will mainly be made to a conventional two roll press, but it is
to be understood that it is not limited only to that device; the
comments and concepts are equally applicable to a shoe press. In
the nip or nips, the press felt carrying the wet paper web is
exposed to compressive pressures ranging from about 3MPa to at
least about l8MPa. The applied pressure serves to expel a
proportion of the water in the wet paper web; the expelled water
is the carried away by the press felt. In the press section, the
paper consistency, which is defined as the percentage by weight
of paper making solids in the web, will increase from a value as
low as 15$ on entry to the press section, to a value of the order
of 60~ .
In addition to resisting the imposed compressive forces and
permitting transport of the expelled water, the press felt must
also be able to resist and contain the forces imposed upon it in
order to move the press felt, and the paper web carried by it,
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through the paper making machines: modern paper making machines
can operate at a linear speed in excess of 75kph.
In the following discussion, two terms are used to have
particular meanings:
(i) the term "paper side surface" refers to a surface of a
layer or fabric which is toward the wet paper web carried by the
press felts and
(ii) the term "machine side surface" refers to a surface of
a layer or fabric which is away from the wet paper web carried by
the press felt.
The known press felts are generally comprised of essentially
two components, a core fabric and an absorbent layer or layers.
The core fabric is frequently a woven fabric, although other
structures are known. The core fabric is structured to
accommodate the mechanical stresses imposed in order to move the
press felt, to resist undue compaction as it passes through the
nip or nips, and to support the absorbent layer. The absorbent
layer is generally at least one layer of fibrous batt, typically
applied to the core fabric by a needle punching process. The
absorbent layer may also comprise a porous foam or other material
having sufficient strength, resistance to compaction, and void
volume so as to be effective in a press felt. If an absorbent
layer is attached to only one surface of the core fabric, it will
typically be applied to the paper side surface of the core
fabric.
If more than one layer of batt is applied to either or both
of the machine side and the paper side of the core fabric,
successive batt layers may be comprised of relatively finer
fibres. This will increase the smoothness of the paper side of
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the press felt. It is also known to mix yarn sizes within one or
more layers.
It is also known that the paper side surface of a press felt
should provide as high a degree as possible of contact between
the paper side surface of the press felt and the wet paper web,
so as to provide, amongst other things, good support to the wet
paper web in the press nip. A high degree of contact also
promotes uniform dewatering of the wet paper web. A common means
of improving the degree of contact between the paper side surface
of the batt and the wet paper web is to needle, or otherwise
attach, a so-called capping layer of fine batt fibres onto the
paper side surface of the batt.
When the batt is applied to the core fabric, and attached to
it typically by needle punching, in theory the batt fibres are
more or less randomly oriented so that on a macro scale the batt
appears to be isotropic. On a micro scale, at the paper side
surface of the batt that is in contact with the wet paper web in
the nip, this is not the case. At the interface between the
paper side surface of the batt and the wet paper web, the
distribution of contact points is found to be irregular. There
are regions in the paper side surface of the batt which provide
relatively large numbers of contact points between the paper side
surface of the batt and the wet paper web, and there are other
regions in which the number of contact points is fewer.
It has now been recognised that the observed micro scale
anisotropic distribution of contact points between the paper side
surface of the batt and the wet paper web results in uneven
application of the roll pressure to the wet paper web carried on
the press felt in the roll nip, so that the pressure applied to
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the wet paper web along the length of the notional contact line
comprising the nip between, for example, a pair of rotating
rollers, varies along the length of the contact line, across the
press. This variation results in uneven water removal from the
wet paper web. It also follows that as the press felt carrying
the wet paper web moves through the nip, the pressure
distribution along the contact line is constantly varying, due to
the anisotropic distribution of the batt fibres at the interface
between the batt paper side surface and the wet paper web. It
has been observed that the anisotropic nature of the batt can
cause less than optimum dewatering of the wet paper web, and can
also mark the wet paper web. It has also been observed that the
use of a surface layer of batt of smaller fibres does not solve
this problem. Although this surface layer of smaller fibre batt
does appear to provide somewhat better support to the wet paper
web in the nip, there are several disadvantages. The support
provided to the wet paper, particularly for both light weight
paper sheets, and relatively dry paper sheets (which are thinner)
is far from optimal, as the points of support are still somewhat
randomly scattered. Furthermore, the use of smaller fibres
results in high press felt wear, thus unduly diminishing the
working life of the press felt.
Hitherto it has been assumed that the various processes
involved in creating an absorbent batt layer, such as carding,
needling, cross-lapping and others, would inherently ensure that
the batt paper side surface fibres forming the interface between
the press felt and the wet paper web are both randomly oriented
and evenly distributed within the batt. However, microscopic
examination of the paper side surface of a typical batt layer
forming part of a press felt shows that this is not the case.
Although the fibres are more or less randomly oriented, there is
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a very wide distribution in the spacing between the pints of
contact between the paper side surface of the batt and the wet
paper web.
It has been proposed to mitigate these effects by using
thinner fibres in greater number within the batt. While this
does appear to result in more void spaces within the batt, the
water passages through the batt are also reduced in size, which
diminishes the water removal capacity of the press felt.
BRIEF SUMMARY OF THE INVENTION.
It has now been discovered that improved press section
efficiency, in terms of the amount of water removed from the wet
paper web, can be obtained by providing a relatively uniform
porous surface layer on the paper side of the press felt, in
which the distribution of the contact points is both effectively
uniform, and the contact points are located relatively close
together. This substantially uniform porous layer is constructed
and arranged to provide an interface between the paper side
surface of the press felt and the wet paper web which has a very
uniform distribution of more or less equally spaced contact
points, together with a more or less uniformly spaced drainage
openings through the porous layer into the batt layer. The
porous layer appears to be most effective when the distance
between adjacent contact points between the wet paper web and the
paper side surface of the porous layer is approximately equal to,
or is less than, the thickness of the wet paper web at mid-nip as
it passes through the press.
It has now been discovered that the anisotropic nature of
the paper side batt surface of a press felt, forming the
interface between the press felt and the wet paper web, can be
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overcome at least in part by providing a relatively smooth and
uniformly porous surface layer on the paper side surface of the
press felt. The provision of a uniform porous layer at the
interface appears to bridge over the contact areas of the batt
paper side surface and thus supports the wet paper web better, so
as to improve pressing efficiency and observed paper consistency
on exit from the press section.
Thus in its broadest embodiment this invention seeks to
provide a composite press felt, comprising at least three layers
wherein:
(i) the first layer comprises a core fabric, having a
machine side surface and a paper side surface
(ii) the second layer comprises at least one absorbent layer
having a machine side surface and a paper side surface which is
attached on its machine side surface to the paper side surface of
the core fabrics and
(iii) the third layer comprises a substantially smooth and
substantially uniformly porous layer having a machine side
surface and a paper side surface, which is attached on its
machine side surface to the paper side surface of the second
layer.
Preferably, the core fabric is a woven fabric. More
preferably, the core fabric comprises a single layer or a
multilayer woven fabric.
Preferably, the second layer comprises one or more layers of
batt material attached by needling to the first layer. More
preferably, the second layer comprises at least two layers of
batt material.
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Preferably, the third layer is chosen from the group
consisting of: a fine woven fabric; a porous film sheet; and a
porous film obtained by heating a layer of an at least partially
fusible powder material applied to the paper side surface of the
third layer.
Preferably, the distance between adjacent contact points
between the wet paper web and the paper side surface of the
porous layer is approximately equal to the thickness of the wet
paper web at mid-nip as it passes through the press. More
preferably, the distance between adjacent contact points between
the wet paper web and the paper side surface of the porous layer
is from about 25$ to about 100$ of the thickness of the wet paper
web at mid-nip as it passes through the press.
BRIEF DESCRIPTION OF THE DRAWINGS.
In the attached drawings:
Figure 1 shows a micrograph of the surface of a batt
layer;
Figure 2 shows a micrograph of the surface of a fabric
porous layer;
Figure 3 shows a schematic cross section of a press
felt of this invention when compressed; and
Figure 4 shows a schematic diagram of a press felt
testing device.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring first to Figure 1, there is shown a photograph of
the paper side surface of a press felt that is exposed to 6.3 MPa
pressure so as to duplicate the effect experienced by the press
felt at the midpoint of a typical press nip. The photograph is
a 100x magnification of the batt surface, which is comprised of
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very fine, 6.7 dtex fibres the batt has been needled to a
standard, double layer woven base fabric. The dark regions of
the photograph show areas of contact between the batt and the
nip, while the white areas are effectively drainage openings into
the batt structure below this surface. The photograph thus
depicts the interface between the paper side surface of the batt
and wet paper web as they pass together through the press nip.
On a macroscopic scale, the batt fibres appear to be
randomly located and thus uniformly dispersed. Figure 1 shows
that, on a microscopic scale, the batt fibres 1 are not randomly
and uniformly dispersed; the contact areas and drainage openings
on the surface interface of the bait and the wet paper web are in
fact unevenly dispersed on the paper side surface of the batt.
The photograph clearly shows a concentration of contact points at
A, an absence of contact points at C, and an intermediate number
at B. The consequence of this uneven distribution is that, on a
microscopic scale, some regions of the web will be exposed to
pressure in the nip, such as those above the points A, while
others, such as those above C, will experience little or no
pressure, thus leading to uneven dewatering of the web and less
than optimum consistency at nip exit.
Figure 2 is a 100x magnification photomicrograph of the
paper side surface of a uniformly porous surface layer such as
would be used in the press felts of the present invention. In
this photograph, the porous layer is a 400 mesh (157.5 strands
per cm) stainless steel woven fabric, the individual strands of
which are 26 microns in diameter (roughly equivalent to a size of
dtex). This fabric was obtained from F.P. Smith Wire Cloth
Co. of Northlake, IL. It was then attached to a rubber backing
and pressed at 6.3 MPa using the same laboratory press as was
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used to obtain the photograph of Figure 1. The rubber backing
was provided to assist in simulating the compression
characteristics of a press felt upon which the porous layer would
normally be attached in the fabrics of this invention. In use,
the porous layer would be laid over the absorbent layer.
However, Figure 2 is intended to illustrate the uniform
distribution of equally spaced contact points and drainage
openings at the interface between the fabric and a web in the
nip.
It will be seen from Figure 2 that the contact points 3
between the fabric and the web, and hence the drainage openings
between these contact points, are uniformly arranged. It is thus
apparent that a paper web that is pressed in a nip using a press
felt having a paper side surface that is provided with a uniform
distribution of contact points and drainage openings will
experience a more even and uniform distribution of pressure than
a web that is pressed using a batt as shown in Figure 1. This
even distribution of pressure, combined with the uniform
distribution of drainage openings in the paper side surface of
the porous layer, will provide more even dewatering of the web,
thus a higher paper consistency upon exiting the nip, than a
press felt provided with a batt having typical surface
characteristics with uneven distribution of contact points.
The effect of compression on a batt provided with a
uniformly porous layer in accordance with this invention is shown
diagrammatically in Figure 3. Figure 3 is intended to represent
a high magnification cross section taken from the paper side
surface toward the machine side surface through a uniformly
porous fabric located on the paper side surface of a press felt
batt. In this Figure, the uniformly porous fabric is indicated
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generally at 7, the press felt batt is 2, contact regions in the
porous fabric are shown as 3, drainage openings in this fabric
are indicated at 4, while 5 designates the random fibres in the
paper side surface of the batt shown generally as 2. It will be
understood that the porous fabric 1 may be a woven structure, a
perforated film, or porous film obtained by heating a layer of at
least partially fusible powder material. Microscopic regions in
the batt having relatively lower elevation are shown as B, while
those regions closest to the interface with the uniform porous
layer are indicated as A. The batt and porous layer are shown as
being under compression as if in a press nip. It will be seen
that, following compression in the nip, the added porous layer 7
effectively bridges the lowered areas B in the compressed batt 2,
and provides uniformly distributed support as at 8 for the wet
paper web while under compression in the press nip to remove
water from it through drainage openings 4.
The porous layer in the press felts of this invention can
take several forms, and can be attached to the batt surface in
several ways.
In one embodiment, the porous layer comprises a fabric which
is applied to the exposed face of the batt, and attached to the
batt by any suitable means, such as gluing. Alternatively, a
relatively narrow fabric strip can be used, which is attached to
the core fabric using a spiral winding technique, essentially as
described by Best et al, US 5,268,076 or by Rexfelt, US
5, 360, 656.
In a second embodiment, a porous film is used, which can be
applied by the same techniques as are used for a porous fabric
layer. The film as applied can also be non-porous, and is
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rendered porous after attachment to the batt either by mechanical
means or by removal of a labile component under the influence of
heat or a solvent.
In a third embodiment, a powder material is applied to the
batt porous layer surface, and converted into a porous film by
heating.
In both the second and third embodiments the film can be
applied to-the full width of the pressfelt, or a variation of the
Best et al. or Rexfelt techniques can be employed.
A laboratory test was carried out in which comparison was
made between the same press felt both with, and without, a
surface fabric according to this invention. The essential parts
of the press roll test unit are shown diagrammatically in Figure
4. The test unit 10 contains a pair of opposed rolls 11, 12
which are urged together to provide the required nip pressure by
a force applied essentially in the direction of the arrow 13.
The test sample of press felt 14 is looped around only the lower
roll 12. The test samples of paper are contained in the wet tray
15, and transferred by hand onto the press felt under test. The
press felt sample circulates past the guides 16, 17 to maintain
its alignment in the test unit. The doctor blade 20 serves to
remove excess surface water from the surface of the roll 11 after
is has passed through the roll nip. The pair of rolls 11, 12
rotate in the directions indicated by the arrows 18, 19. Both
the speed of rotation of the rolls, and the applied load, can be
separately varied.
The test procedure is as follows. A supply of paper,
typically a sheet (or sheets) of a newsprint grade of about 50
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gsm, about 38mm x 25mm in size and of a known weight per sheet,
is presoaked overnight in de-ionised water. The press felt
sample is-cut to provide a belt of appropriate length and about
100mm in width. The belt is then placed on the test unit and
preconditioned by circulation through the roll nip saturated with
water and under a line load of 26kN/m. The belt typically
completes from about 9,000 - 10,000 rotation cycles prior to
testing. The felt is then allowed to dry until a moisture level
of 30 - 35$ is reached. The wet paper sample is blotted to
remove excess free water, placed on the test felt, and passed
through the roll nip at the desired nip load and press felt
speed. After removal from the top roll 11 by the doctor blade,
the wet sheet is reweighed to determine its consistency. In this
test, the porous layer was a layer of stainless steel fabric
woven in a plain weave with a mesh count of about 16 wires/mm,
woven from wires with a diameter of about 0.028mm. Comparison of
the exit paper consistency from the test unit with the fabric in
place, compared to paper consistency without the fabric in place,
showed a reproducible improvement in paper consistency of between
3~ and at least 5~.
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