U.S. patent application number 09/272081 was filed with the patent office on 2002-05-23 for paper machine for and method of manufacturing soft paper.
Invention is credited to KLERELID, INGVAR, LINDEN, ANDERS.
Application Number | 20020060042 09/272081 |
Document ID | / |
Family ID | 27355909 |
Filed Date | 2002-05-23 |
United States Patent
Application |
20020060042 |
Kind Code |
A1 |
KLERELID, INGVAR ; et
al. |
May 23, 2002 |
PAPER MACHINE FOR AND METHOD OF MANUFACTURING SOFT PAPER
Abstract
A paper machine for manufacturing soft paper comprises a press
section with a shoe press nip, through which a substantially
impermeable belt and a felt run with the fibrous web enclosed
between them, a drying cylinder and a transfer roll forming a nip
for transfer of the web to the drying cylinder. According to the
invention the belt comprises a carrier with a compressible polymer
coating having a hardness of 50 to 97 Shore A, which polymer
coating has a web-contacting surface with a degree of roughness in
unloaded state of R.sub.z=2-80 microns, measured in accordance with
ISO 4287, and a lower degree of roughness of R.sub.z=0-20 microns
when the polymer coating is compressed by linear load of 20-220
kN/m on the belt as measured in a non-extended press nip, the belt
and the web-carrying felt being arranged to run together not until
they enter the press section, the belt running directly to the
press section. Further, the belt runs from the shoe press to the
drying cylinder to carry the web to the transfer nip, the felt
running away from the belt before a water film formed in the nip on
the belt breaks up. A device is also provided for applying adhesive
on the drying cylinder.
Inventors: |
KLERELID, INGVAR; (KARLSTAD,
SE) ; LINDEN, ANDERS; (KARLSTAD, SE) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Family ID: |
27355909 |
Appl. No.: |
09/272081 |
Filed: |
March 19, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60088071 |
Jun 5, 1998 |
|
|
|
Current U.S.
Class: |
162/358.1 ;
162/358.4 |
Current CPC
Class: |
D21F 11/14 20130101;
D21F 11/006 20130101; D21F 3/045 20130101 |
Class at
Publication: |
162/358.1 ;
162/358.4 |
International
Class: |
D21F 003/04; D21F
011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 20, 1998 |
SE |
9800948-3 |
Claims
That which is claimed:
1. A paper machine for manufacturing a fibrous soft paper web,
including: a wet section having a headbox; a forming roll; and
inner and outer clothings running around the forming roll, between
which clothings the fibrous web is formed; a press section having
at least one shoe press having a shoe press roll and a counter
roll, said rolls forming an extended press nip, a substantially
impermeable belt and a press felt, the substantially impermeable
belt and the press felt running through the extended press nip with
the fibrous web enclosed therebetween; and a drying section having
a drying cylinder and a transfer member which, together with the
drying cylinder, forms a transfer nip for transfer of the fibrous
web to the drying cylinder, characterized in that a) the
substantially impermeable belt comprises a carrier with a
compressible polymer coating on the side facing the paper web,
having a hardness in the range from 50 to 97 Shore A, which polymer
coating has a web-contacting surface with a pressure-responsive,
recoverable degree of roughness, the web-contacting surface having
a degree of roughness in unloaded state of R.sub.z=2-80 microns,
measured in accordance with ISO 4287, Part I, and a lower degree of
roughness of R.sub.z=0-20 microns when the polymer coating is
compressed by linear load of 20-220 kN/m on the substantially
impermeable belt as measured in a non-extended press nip; b) the
press felt is arranged to carry the fibrous web to the press
section from the forming roll or from a point situated downstream
of the forming roll and upstream of the press section; c) the
substantially impermeable belt and the web-carrying press felt are
arranged to run together not until they enter the press section to
enclose the fibrous web between them, the substantially impermeable
belt being arranged to run directly to the press section; d) the
substantially impermeable belt is arranged to run from the shoe
press to the drying cylinder to carry the fibrous web to said
transfer nip; e) the press felt is arranged to run in a direction
away from the substantially impermeable belt at a point immediately
after the extended press nip and before a water film formed in the
extended press nip on the substantially impermeable belt breaks up;
and f) a device for applying adhesive is arranged before said
transfer nip to apply a continuous adhesive layer on the envelope
surface of the drying cylinder and/or on the fibrous web.
2. A paper machine as claimed in claim 1, characterized in that
said inner clothing in the wet section constitutes a felt, and that
this felt is arranged to run through the press section and there to
be used also as its press felt.
3. A paper machine as claimed in claim 1, characterized in that
said inner clothing in the wet section is a fabric, and that a
pick-up device is arranged in the loop of the press felt upstream
of the press section in order to transfer the fibrous web from the
fabric to the press felt.
4. A paper machine as claimed in claim 1, characterized in that the
press section also includes a roll press arranged upstream of the
shoe press and including a suction press roll arranged in the loop
of the press felt, and a counter roll arranged in the loop of the
substantially impermeable belt.
5. A paper machine as claimed in claim 4, characterized in that the
counter roll of the shoe press also is the counter roll of the roll
press.
6. A paper machine as claimed in claim 4, characterized in that the
press felt is guided away from the fibrous web in a side loop
around the suction press roll and at least one guide roll, which
side loop extends from the press nip of the roll press to the
extended press nip of the shoe press.
7. A paper machine as claimed in claim 1, characterized in that the
air permeability of the substantially impermeable belt is less than
6 m.sup.3/m.sup.2/minute, measured in accordance with the procedure
described in "Standard Test Method for Air Permeability of Textile
Fabrics, ASTM D 737-75, American Society of Testing and
Materials".
8. A paper machine as claimed in claim 1, characterized in that the
transfer member consists of a transfer roll arranged in the loop of
the substantially impermeable belt.
9. A paper machine as claimed in claim 1, characterized in that the
transfer member consists of the substantially impermeable belt that
runs around a predetermined part of the drying cylinder to produce
an extended transfer nip.
10. A method of manufacturing a fibrous soft paper web in a paper
machine, said paper machine including: a wet section having a
headbox; a forming roll; and inner and outer clothings running
around the forming roll, between which clothings the fibrous web is
to be formed; a press section having at least one shoe press having
a shoe press roll and a counter roll, said rolls forming an
extended press nip, a substantially impermeable belt and a press
felt, the substantially impermeable belt and the press felt running
through the extended press nip with the fibrous web enclosed
therebetween; and a drying section having a drying cylinder and a
transfer member which, together with the drying cylinder, forms a
transfer nip for transfer of the fibrous web to the drying
cylinder, characterized by a) performing the pressing with the aid
of a substantially impermeable belt that includes a carrier and a
compressible polymer coating on the side facing the paper web, said
polymer coating having a hardness in the range from 50 to 97 Shore
A and a web-contacting surface with a pressure responsive,
recoverable degree of roughness, the web-contacting surface having
a degree of roughness in unloaded state of R.sub.z=2-80 microns,
measured in accordance with ISO 4287, Part I, and a lower degree of
roughness of R.sub.z=0-20 microns when the polymer coating is
compressed by linear load of 20-220 kN/m on the substantially
impermeable belt as measured in a non-extended press nip; b) using
the press felt to carry the fibrous web to the press section from
the forming roll or roll from a point situated downstream of the
forming and upstream of the press section; c) running the
substantially impermeable belt and the web-carrying press felt
together not until they enter the press section to enclose the
fibrous web between them, and running. the substantially
impermeable belt directly to the press section; d) running the
substantially impermeable belt from the shoe press to the drying
cylinder to carry the fibrous web to said transfer nip; e) running
the press felt in a direction away from the substantially
impermeable belt at a point immediately after the extended press
nip and before a water film that is formed on the substantially
impermeable belt in the extended press nip breaks up; and f)
applying a continuous adhesive layer on the envelope surface of the
drying cylinder and web by means of an adhesive applying device at
a point before said transfer nip.
11. A method as claimed in claim 10, characterized by using a felt
as said inner clothing in the wet section, running that felt
through the press section, and using that felt also as the press
felt of the press section.
12. A method as claimed in claim 10, characterized by transferring
the fibrous web from said inner of a fabric, to the clothing, which
is in the form of a fabric, to the press felt by means of a pick-up
device arranged in the loop of the press felt upstream of the press
section.
13. A method as claimed in claim 10, characterized by dewatering
the fibrous web first in a roll press of the press section, said
roll press being arranged upstream of the shoe press and including
a suction press roll arranged in the loop of the press felt and a
counter roll arranged in the loop of the substantially impermeable
belt.
14. A method as claimed in claim 13, characterized by guiding the
press felt away from the fibrous web in a side loop running around
the suction press roll and at least one guide roll, said side loop
extending from the press nip of the roll press to the extended
press nip of the shoe press.
15. A method as claimed in claim 10, characterized by using a
substantially impermeable belt having an air permeability that is
less than 6 m.sup.3/m.sup.2/minute, measured in accordance with the
procedure described in "Standard Test Method for Air Permeability
of Textile Fabrics, ASTM D 737-75, American Society of Testing and
Materials".
16. A method as claimed in claim 10, characterized by using the
substantially impermeable belt also as said transfer member that
runs around a predetermined part of the drying cylinder to produce
an extended transfer nip.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/088,071, filed Jun. 5, 1998.
FIELD OF THE INVENTION
[0002] The present invention relates to paper machines for
manufacturing a fibrous soft paper web:
BACKGROUND OF THE INVENTION
[0003] A paper machine of the type described above is disclosed in
U.S. Pat. No. 5,393,384, see particularly FIG. 6. The paper machine
shown therein has a belt impermeable to water, which runs in a loop
through an extended press nip formed by a shoe press roll and a
counter roll. A press felt is conducted directly to the press nip,
where it runs together with the impermeable belt and the paper web.
The paper web is transferred from a forming fabric to the
impermeable belt, which is to carry the paper web on its under side
up to the press nip and thence to the drying cylinder. The
impermeable belt thus carries the paper web a relatively long
distance after the paper web has been transferred from the forming
fabric to the impermeable belt. There is therefore a risk of the
paper web not adhering sufficiently strongly along the entire
distance and thus becoming detached from the impermeable belt.
According to the '384 patent, the adhesion force between the
impermeable belt and the paper web is greater than that between the
press felt and the paper web. The impermeable belt in question is
not compressible and has a smooth, web-carrying surface. It is
generally known that such a smooth, impermeable belt obtains a film
of liquid on its smooth, web-carrying surface when the belt, press
felt and paper web pass together through a press nip and that,
after the press nip, the paper web therefore adheres to the
impermeable belt instead of to the press felt which does not have a
smooth surface, when the press felt and the impermeable belt run
away from each other. This situation is also utilized in U.S. Pat.
No. 4,483,745. Since, however, both the impermeable belt and the
drying cylinder in the paper machine according to U.S. Pat. No.
5,393,384 have smooth surfaces, with which the paper web is
intended to come into contact, there is considerable risk of the
paper web continuing to adhere to the smooth surface of the
impermeable belt after it is passed the nip at the drying cylinder
instead of being transferred to the smooth surface of the drying
cylinder as desired. Probably not even the application of large
quantities of adhesive on the envelope surface of the drying
cylinder would ensure adhesion of the paper web to the drying
cylinder.
[0004] DE-195 48 747 describes a paper machine for manufacturing
creped tissue paper, which is provided with a press comprising a
shoe press roll, a counter roll and a suction roll, the counter
roll forming a first press nip with the suction roll and a second
extended press nip with the shoe press roll. A felt passes through
the two press nips together with the paper web and then carries the
paper web with it to a Yankee dryer, to which the paper web is
transferred when the felt and the paper web pass around a transfer
roll forming a non-compressing nip with the Yankee dryer. Suction
zones for dewatering the felt are provided before and after the
first press nip, the suction zone before the press nip being
situated within the suction roll whereas the suction zone after the
press nip is in a side loop in which the felt runs alone and joins
the paper web again at the entry to the second press nip. One
drawback with such a paper machine is that the paper web is exposed
to rewetting by the wet felt before it reaches the Yankee
dryer.
[0005] U.S. Pat. No. 5,298,124 describes a compressible transfer
belt for use in a paper or board machine in order to eliminate open
draws of the paper web and to easily release the paper web so that
it can be transferred to a fabric or belt. The transfer belt
carries the paper web through the press section, which comprises
one or more press nips, and on to the drying section which
comprises a plurality of drying cylinders and a belt passing in a
loop around a transfer roll which forms a nip with the transfer
belt. Each press is also provided with a felt passing through its
press nip and enclosing the paper web between it and the transfer
belt. The impermeable transfer belt is also designed so that a
liquid film formed in the press nip between the transfer belt and
the paper web breaks up when the pressure on the transfer belt
ceases after the press nip, so that its release properties increase
and the paper web can thus more easily be transferred to a fabric
or another belt running in a loop. There is no proposal or
suggestion to having the transfer belt to carry the paper web to a
drying cylinder in a tissue machine.
[0006] U.S. Pat. No. 5,298,124 offers an excellent description of
the tasks a transfer belt cooperating with a press felt shall
perform in a satisfactory manner, and also of the properties and
design of such transfer belts, which then were described in patent
specifications U.S. Pat. Nos. 4,483,745, 4,976,821, 4,500,588,
5,002,638, 4,529,643 and CA-A-1,188,556. According to U.S. Pat. No.
5,298,124, for a transfer belt intended for cooperation with a
press felt the critical tasks are a) to remove the paper web from
the press felt without causing instability problems; b) to
cooperate with the press felt in one or more nips to ensure optimal
dewatering and high quality of the paper web; and c) to transfer
the paper web in a closed draw from a press in the press section to
a paper receiving fabric or belt in the following press or presses
of the press section or to a pick-up fabric in the drying section.
As mentioned, the transfer belt for the press section of a paper
machine described in U.S. Pat. No. 5,298,124 has a web-contacting
surface which is substantially impermeable to water and air and has
pressure-responsive microscale topography. Under influence of the
pressure in a press nip in the press section, the transfer belt is
compressed so that the microscale roughness of said surface is
decreased, whereupon the surface becomes much smoother and allows
the formation of a thin, continuous film of water thereon.
[0007] It has surprisingly been found that a belt of the type
described in U.S. Pat. No. 5,298,124 is also well suited for use in
a paper machine according to the invention, for transferring a soft
paper web in closed draw from a shoe press nip in the press section
to a Yankee dryer in the drying section of the soft paper machine.
As is known, a shoe press nip results in a high degree of
dewatering.
[0008] The object of the present invention is to provide an
improved paper machine and an improved method of manufacturing soft
paper, said paper machine and said method enabling the manufacture
of a fibrous web with high dry solids content before the drying
cylinder in order to enable a high production rate to be achieved
at reasonable cost, and to reliably transfer the fibrous web to the
drying cylinder although the fibrous web is carried to the drying
cylinder by an impermeable belt.
SUMMARY OF THE INVENTION
[0009] The paper machine according to the invention is
characterized in that
[0010] a) the substantially impermeable belt comprises a carrier
with a compressible polymer coating on the side facing the paper
web, having a hardness in the range from 50 to 97 Shore A, which
polymer coating has a web-contacting surface with a
pressure-responsive, recoverable degree of roughness, the
web-contacting surface having a degree of roughness in unloaded
state of R.sub.z=2-80 microns, measured in accordance with ISO
4287, Part I, and a lower degree of roughness of R.sub.z=0-20
microns when the polymer coating is compressed by linear load of
20-220 kN/m on the substantially impermeable belt as measured in a
non-extended press nip;
[0011] b) the press felt is arranged to carry the fibrous web to
the press section from the forming roll or from a point situated
downstream of the forming roll and upstream of the press
section;
[0012] c) the substantially impermeable belt and the web-carrying
press felt are arranged to run together not until they enter the
press section to enclose the fibrous web between them, the
substantially impermeable belt being arranged to run directly to
the press section;
[0013] d) the substantially impermeable belt is arranged to run
from the shoe press to the drying cylinder to carry the fibrous web
to said transfer nip;
[0014] e) the press felt is arranged to run in a direction away
from the substantially impermeable belt at a point immediately
after the extended press nip and before a water film formed in the
extended press nip on the substantially impermeable belt breaks up;
and
[0015] f) a device for applying adhesive is arranged before said
transfer nip to apply a continuous adhesive layer on the envelope
surface of the drying cylinder and/or on the fibrous web.
[0016] The method according to the invention is characterized
by
[0017] a) performing the pressing with the aid of a substantially
impermeable belt that includes a carrier and compressible polymer
coating on the side facing the paper web, said polymer coating
having a hardness in the range from 50 to 97 Shore A and a
web-contacting surface with a pressure responsive, recoverable
degree of roughness, the web-contacting surface having a degree of
roughness in unloaded state of R.sub.z=2-80 microns, measured in
accordance with ISO 4287, Part I, and a lower degree of roughness
of R.sub.z=0-20 microns when the polymer coating is compressed by
linear load of 20-220 kN/m on the substantially impermeable belt as
measured in a non-extended press nip;
[0018] b) using the press felt to carry the fibrous web to the
press section from the forming roll or from a point situated
downstream of the forming roll and upstream of the press
section;
[0019] c) running the substantially impermeable belt and the
web-carrying press felt together not until they enter the press
section to enclose the fibrous web between them, and running the
substantially impermeable belt directly to the press section;
[0020] d) running the substantially impermeable belt from the shoe
press to the drying cylinder to carry the fibrous web to said
transfer nip;
[0021] e) running the press felt in a direction away from the
substantially impermeable belt at a point immediately after the
extended press nip and before a water film that is formed on the
substantially impermeable belt in the extended press nip breaks up;
and
[0022] f) applying a continuous adhesive layer on the envelope
surface of the drying cylinder and/or on the fibrous web by means
of an adhesive applying device at a point before said transfer
nip.
[0023] It is surprising that a transfer belt according to U.S. Pat.
No. 5,298,124, which is intended for pressing in a press section
and usable for transferring a paper web from the press section to a
drying fabric, is useful with advantage for transferring a soft
paper web from a shoe press nip directly to a Yankee dryer or other
drying cylinder. As is well known, the conditions at a Yankee dryer
are completely different from those in a press nip. With a Yankee
dryer no pressing of the soft paper web occurs for direct
dewatering. Rather it is a question of supporting the soft paper
web to the envelope surface of the Yankee dryer so that the fibers
of the soft paper web adhere efficiently to the surface of the
Yankee dryer thereby achieving good heat transfer to the paper web.
This is exactly the effect which is achieved with the transfer belt
in the paper machine according to the invention and which is not
achieved with a press felt as described in DE-195 48 747 due to
rewetting of the paper web after the last press nip in the press
section which prevents satisfactory adhesion. Neither can it be
achieved, or only to a minor extent with a transfer belt as
described in U.S. Pat. No. 5,393,384 for the reason stated above.
The compressibility of the transfer belt used in the paper machine
according to the invention results in lower specific pressure at
the adhesion point, which in turn results in increased rate of
operation, i.e. higher production rate. This property also results
in increased vaporization of water from the soft paper web, i.e.
quicker drying of the soft paper web on the Yankee dryer, which
also contributes to higher efficiency in the process. The increased
efficiency can either be used to increase machine speed or to
reduce energy consumption while maintaining the same production
volume.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The invention is described in more detail in the following
with reference to the drawings.
[0025] FIG. 1 shows a paper machine according to a first embodiment
of the invention.
[0026] FIG. 2 shows a paper machine according to a second
embodiment of the invention.
[0027] FIG. 3 shows a paper machine according to a third embodiment
of the invention.
[0028] FIG. 4 shows a paper machine according to a fourth
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention now will be described more fully
hereinafter with reference to the accompanying drawings, in which
preferred embodiments of the invention are shown. This invention
may, however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
[0030] FIGS. 1-3 show schematically parts of a paper machine for
manufacturing a fibrous web 1 of soft paper such as tissue and
other sanitary paper products. Each of the paper machines comprises
a wet section 2, a press section 3 and a drying section 4.
[0031] The wet section 2 comprises a headbox 7, a forming roll 8,
an endless, carrying, inner clothing 9 and an endless, covering
outer clothing 10 consisting of a forming fabric. The inner and
outer clothings 9 and 10 run, each in its own loop, around a
plurality of guide rolls 11 and 12, respectively.
[0032] The drying section 4 comprises a drying cylinder 5 covered
by a hood 30. The drying cylinder is suitably a Yankee dryer. At
the outlet side of the drying section a creping doctor 21 is
arranged to crepe the fibrous web 1 off the Yankee dryer 5. An
application device 31 is provided for applying of a suitable
adhesive on the envelope surface of the Yankee dryer 5 immediately
before the transfer nip.
[0033] The press section 3 comprises a shoe press with a shoe press
roll 14 and a counter roll 19, these rolls 14 and 19 forming an
extended press nip with each other. The press section also
comprises an endless press felt 15 which runs in a loop around
guide rolls 6, and an endless, substantially impermeable transfer
belt 16. The substantially impermeable belt 16 runs in a loop
around the counter roll 19, a transfer roll 17 and a plurality of
guide rolls 18. The transfer roll 17 forms a transfer nip with the
Yankee dryer 5 with low linear pressure, i.e. about 30-60 kN,
through which transfer nip the substantially impermeable belt 16
thus passes.
[0034] In the embodiments shown in FIGS. 1 and 2 the press section
3 also includes a roll press, the rolls of which consisting of a
suction press roll 13 and said counter roll 19 to form a press nip
through which the substantially impermeable belt 16 and the press
felt 15 pass together with the fibrous web 1. After this initial
press nip, the press felt 15 is conducted away from the fibrous web
1 and the substantially impermeable belt 16 in a side loop around
the suction press roll 13 and two guide rolls 32. The press felt 15
rejoins the fibrous web 1 and the substantially impermeable belt 16
immediately before the extended press nip. If desired, suction
devices may be arranged within this side loop of the press felt 15
in order to increase the capacity of the press felt to absorb water
at the entrance to the extended press nip.
[0035] In the embodiments shown in FIGS. 1 and 3 the inner clothing
9 of the wet section 2 is a felt conveyed to the press section 3 to
be also used as press felt 15 and which thus runs in a loop back to
the forming roll 8.
[0036] In the embodiment shown in FIG. 2 the inner clothing 9 of
the wet section 2 is a fabric, in which case the press felt 15 runs
around a pick-up roll 20 arranged close to the loop of the fabric 9
so that press felt 15 and fabric 9 run in contact with each other
to transfer the fibrous web from the fabric 9 to the press felt 15.
The pick-up roll 20 may be provided with a suction shoe (not
shown). Alternatively the pick-up roll and suction shoe may be
replaced by a pick-up suction box.
[0037] FIG. 4 shows schematically parts of a paper machine
according to a further embodiment of the invention. it is similar
to that shown in FIG. 1 with the exception that the press felt 15
is not led in a side loop between the two press nips, but instead
accompanies the counter roll 19 so that the fibrous web 1 is held
enclosed between the substantially impermeable belt 16 and the
press felt 15. This embodiment can be used when there is little
risk of rewetting of the fibrous web.
[0038] In the embodiments according to FIGS. 1-4 the counter roll
19 is a smooth roll and is arranged in a loop of the substantially
impermeable belt 16. In an alternative embodiment (not shown) of
the press section according to FIG. 3, the positions of rolls 14
and 19 are reversed, i.e. the shoe press roll 14 is arranged in a
loop of the substantially impermeable belt 16 and the counter roll
19 in the loop of the press felt 15. In such a configuration the
counter roll may be a suction roll, a grooved roll or a
blind-drilled roll.
[0039] The substantially impermeable and elastically compressible
transfer belt 16 is of the type described in U.S. Pat. No.
5,298,124, for instance. The substantially impermeable belt 16 used
in the paper machines according to the invention is compressible
under the influence of the press forces prevailing in the press
nips. The substantially impermeable belt 16 thus assumes an
uncompressed state upstream and downstream of a press nip and a
compressed state when it passes a press nip, the surface, the
web-carrying surface, facing the fibrous web having a high degree
of roughness in the uncompressed state of the substantially
impermeable belt and a lower degree of roughness in the compressed
state of the substantially impermeable belt so that the
web-carrying surface in the compressed state of the substantially
impermeable belt is sufficiently smooth for a continuous liquid
film to be formed on the web-carrying surface when the
substantially impermeable belt, together with press felt and
fibrous web, passes through the press nip and so that the
web-carrying surface in the uncompressed state of the substantially
impermeable belt is sufficiently rough to permit the continuous
liquid film to break up after the substantially impermeable belt
has expended in thickness direction. More specifically, the
substantially impermeable and elastically compressible transfer
belt consists of an endless carrier, alternatively including a
joinable seam, with a polymer coating on its web-contacting surface
having a hardness of 50-97 Shore A, the polymer coating having a
degree of roughness in uncompressed state of R.sub.z=2-80 microns,
measured in accordance with ISO 4287, Part I, and being
compressible to a lower degree of roughness of R.sub.z=0-20 microns
when a linear load of 20-220 kN/m is applied on the substantially
impermeable belt, and also has the ability to be recovered to its
uncompressed degree of roughness when the pressure exerted on the
substantially impermeable belt ceases. The R.sub.z value is more
specifically the ten-point height which is defined in said ISO norm
as the average distance between the five highest peaks and the five
deepest valleys in the reference length measured from a line
parallel to the mean line and not crossing the surface profile. The
substantially impermeable transfer belt preferably has an air
permeability of less than 6 m.sup.3/m.sup.2/minute, measured in
accordance with the procedure described in "Standard Test Method
for Air Permeability of Textile Fabrics, ASTM D 737-75, American
Society of Testing and Materials".
[0040] As stated in said U.S. Pat. No. 5,298,124, the polymer
coating of the substantially impermeable belt may advantageously
comprise a polymer composition such as acrylic polymer resin,
polyurethane polymer resin and polyurethane/polycarbonate polymer
resin composition. The polymer coating may also contain particles
of filler with a different hardness from the polymer material and
which may consist of kaolin clay, polymer material or metal,
preferably stainless steel.
[0041] The transfer belt included in the paper machine according to
the invention is manufactured in the manner also described in U.S.
Pat. No. 5,298,124. Thanks to the use of the above polymer
composition a surface layer is achieved on the belt which is
elastically compressible. After the actual surface working in the
manufacturing process a surface is obtained which gives a
controlled topography. The compressibility and topography of the
surface layer are not noticeably affected by possible wear during
operation. The dense polymer surface is easy to keep clean and can
stand being cleaned with a doctor blade directly on the belt
surface. To ensure that the belt will not age at the edge portions
running against the Yankee dryer outside-the paper web, this must
be taken into consideration when selecting material and the
combinations which withstand heat best must be chosen. Otherwise
edge cooling must be effected by spraying water on the edge, for
instance, immediately before or after passage of the Yankee
dryer.
[0042] The carrier in the transfer belt is endless and includes
also types of base elements which can in some way be made endless.
The term also particularly covers an openable base element provided
with a seam, which element is made endless first when it is
installed in the paper machine with the aid of a suitable seam. The
carrier may consist, for instance, of a multi-layered woven fabric
produced from polymer monofilaments such as polyester, polyamide
and the like. The base element may also consist of a fibrous web
(non-woven) held together by binder, combined wound yarns, polymer
foil/film, warp knitting or the like. The requirement is that the
carrier is form-stable both in machine direction and transverse
thereto, so that it contributes to stability of the belt upon
mechanical stress in these directions. If the carrier is to be
completely enclosed in the polymer coating, this can either be
applied on one side with complete bleeding through to the reverse
side, or it may be applied first on one side and then on the other.
Configurations may occur requiring particularly thin belts, in
which case it may be advisable to apply the coating only from one
side with restricted through-bleeding. In this case it is essential
that the non-coated surface of the base element is wear-resistant
and easy to keep clean.
[0043] The paper machine according to the invention, with
single-felted press nip(s), produces a fibrous web with a high dry
solids content before the drying section, i.e. up to 55%, which
should be compared with the dry solids contents of up to 45%
achieved with paper machines in use today. This improvement can be
utilized either to run the paper machine at a higher production or
to reduce the energy consumption in the drying section. It also
enables a reduction in the diameter of the drying cylinder.
[0044] With the embodiments shown and described a guide roll may be
arranged if desired in the loop of the substantially impermeable
belt 16 immediately before the transfer roll 17.
[0045] With the embodiments shown and described a transfer member
is used consisting of the transfer roll 17. According to an
alternative embodiment (not shown), the transfer roll is replaced
by the substantially impermeable belt itself which is allowed to
run around a predetermined part of the drying cylinder, e.g. within
a sector angle of 30-60.degree., to form an extended transfer nip
with the drying cylinder.
[0046] Many modifications and other embodiments of the invention
will come to mind to one skilled in the art to which this invention
pertains having the benefit of the teachings presented in the
foregoing descriptions and the associated drawings. Therefore, it
is to be understood that the invention is not to be limited to the
specific embodiments disclosed and that modifications and other
embodiments are intended to be included within the scope of the
appended claims. Although specific terms are employed herein, they
are used in a generic and descriptive sense only and not for
purposes of limitation.
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