Note: Descriptions are shown in the official language in which they were submitted.
WO 92/10607 PCT/GB91/02158
2~37~3~
IMPROVEMENTS IN AND RELATING TO PAPER MACHINE CLOTHING
DESCRIPTION
This invention relates to paper machine clothing and
has particular-reference to paper machine clothing for ~.:
use in the pressing and drying sections of a paper
machine, although the invention described herein is
e~ually app7icable in other sections of a paper making
machine.
A conventional paper making machine forms a web by
depositing a slurry of pulp fibres to be formed into a
paper sheet onto, for example, a travelling Foudrinier
wire. After initial dewatering on the Foudrinier wire,
the forming paper sheet or web is trans~erred to a
press section where the web passes through one or more
nips comprised of either roll or shoe combinations.
During this time, the sheet is consolidated and further
properties of the sheet may be developed. Thereafter,
the web pasRes over a series of heated dryer drums (and
optionally through a calender) and i6 then wound onto a
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WO 92/10607 PCT/GB91/02158~
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roll. There are many variations in the various
sections of the machine such as the forming or press
sections. In such a machine, the web usually arrives
at the press section with about 80% wet base moisture
S (a ratio of water to fibre ~ water) and leaves the
press section with approximately 60% moisture ratio (or
40% dryness). The remaining moisture has to be removed
by thermal evaporation in the dryer section as the web
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passes over a series of heated drums. In a typical
paper making machine, such as that manufacturing
newsprint, a significant number of dryer drums will be
employed, sometimes on the order of 50 to 70 drums per
machine. Each dryer drum is expensive to construct and
to operate and requires a provision of steam fittings
and a supply of steam or other heat source for,each
drum.
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In recent years, the principle of impulse drying has
introduced. Impulse drying is a method of dewatering a
paper sheet by incorporating both high temperatuIe and
pres~ure in a press section of a paper making machine.
During this process, the surface of the press roll is
usually heated to a temperature of 150C or greater,
and is in direct contact with the paper. With such a
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WO 92/10607 PCT/GB91/021S8
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process, the resulting paper sheet dryness is
significantly increased to 60% or higher. The
effectiveness of such a system is such that the number
of dryer drums in a paper making machine can be
S su~stantially reduced.
The principle of operation underlying impulse drying is
believed to be that the heated roll forms a zone of
steam in the paper sheet during pressing of the sheet
which drives out liquid water as the steam passes
through the sheet. Generation of this steam front, and
high temperature of the press roll provides rather
severe conditions for the press fabric.
In order to withstand this high temperature, the press
fabric should have sufficient thermal resistance to
allow continued exposure to high temperature steam and
occasional exposure to the high temperature press roll
directly during breaks in the paper sheet passing
therethrough.
In addition, this press fabric should adequately
release the paper sheet after the nip to proYide for
acceptable sheet runnabLlity. Also the press fabric
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WO 92/10607 2 0 3 7 8 3 ~ PCI'/GB9ltO2158 --
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should minimize rewet of the sheet as it exits the nip
to produce a sheet with maximum dryness. These
properties should be combined with sufficient
mechanical durability to withstand the severe
conditions of the impulse drying process. According to
traditional pressing theory, a press fabric should also
provide and maintain adequate permeability for the
water leaving the sheet.
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~ccording to the present invention, therefore, there is
provided an article of paper machine clothing which
comprises a first woven base layer and a second sheet
contacting layer carried thereby, characterised in that
the sheet contacting surface of said second layer
lS comprises a polyfluorocarbon polymer.
The polyfluorocarbon polymer may be in a fibrous form.
The said second layer may comprise a fibrous structure
of batt or staple fibre which is needled to the base
layer. The second layer may typically comprise a batt
of 100~ polyfluorocarbon polymers or may be a blend of
fibres of polyfluorocarbon with any high temperature
resistant fibres such, for example, as polyamide,
~;~ polyaramid, polyester, po}yimide, polyetherketone,
,
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WO 92/10607 PCT/GB91/02158
2~7~34
polybenzLmidazole or polyether imide. The
polyfluorocarbon may be polytetrafluoroethylene of the
type commercially available under the trade name
"TEFLON" ~PTFE).
s
In an alternative aspect of the pre~ent invention, the
sheet contacting surface may contain an effective
amount of polyfluorocarbon to achieve the desirable
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properties for the impulse drying process. The
effecti~e amount of polyfluorocarbon may be in the form
of a discrete surface layer, a coating on the surface
on the fibres constituting the said second layer, a
pre-formed nonwoven layer of polyfluorocarbon, or a
finely wo~en polyfluorocarbon fabric, each of which can
be needled or otherwise bonded to the surface to
incorporate it as part of the surface layer.
In another aspect of the present invention, the surface
layer incorporating polyfluorocarbon may be in a form
other than a textile form. The sheet contacting
surface of said second layer may comprise a film of
polyfluorocarbon. The layer may for example be formed
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by bondlng, l~minating or other~i~e att~ching a
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WO 92/10607 PCT/GB91/02158--
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polyfluorocarbon polymer film, to the remainder of the
second layer.
Either a monolithic, perforated or microporous film can
be employed. A surface layer can also be formed by
coating with a fluoro~arbon emulsion or suspension, by
spraying, dipping or other suitable methods. The
surface layer may also be formed by direct application
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of a fluorocarbon in particulate form.
In the practice of this invention, it has been noted
that after multiple compressions within the press nip
press fabrics have signîficantly reduced air
permeability, yet surprisingly still continue to
produce high sheet dryness values; the more so since
traditional press fabrics having such low air
permeabilities do not normally dewater the sheet
effectively. --
Following is a description by way of example only and
with reference to the accompanying drawings of methods
of carrying the invention into effect.
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WO 92/10607 PCT/~B91/02158
20378~4
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In the drawings:-
Figures 1 (a), (b) and (c) are a series of graphsrelating to Example 2.
s
Fig~re 2 is a photomicrograph of the sheet contacting
surface of a PTFE surfaced press fabric of Example 3.
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EXAMPLE 1
An experimental pilot press fabric, made up of 11 test
sections, was prepared usiAg conventional carding and
needling proce~ses. Each experimental section
represented either a candidate fibre or a test
structure. Each candidate fibre was prepared into a -
cA~ded web and a total of eight card web layers were
combined to conqtruct each experimental section. All
sections were needled onto a common woven base fabric.
The experimental felt was installed on a pilot impulse
drying machine and run at the following conditions:
Nip pressure: ~75 pli
Roll temperature: 160C (320F)
Fabric speed: 107 fpm
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WO 92tl0607 PCT/GB91/02158-`
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The handsheets used during this test were a newsprint
grade comprised of 71% groundwood, 2~% high yield
sulfite. Five of the eleven test sections in the
experimental felt were constructed with a paper
contacting surface comprised of at least 50% by weight
Teflon. Of the 11 sections tested during this study,
all five Teflon containing felts produced paper sheets
with the highest dryness, ranging between 70% and 7S%
~~ ~~ dryness. A11 of the other high temperature resistant
materials tested (such as polyara~id,
po~yetheretherketone, and an aromatic poly~ide 6T),
produced paper sheets with lower dryness values
(between 61% and 69%).
EXAMP~E 2
A series of experimental press fabrics were constructed
with varying percentages of Teflon fibre in the paper
contacting layers of the fabric. The blends ranged
from 0% Teflon to 100% Teflon, in increments of 25%.
Each fabric ~ample was conditioned at 21C for 500
cycles at 1000 psi, and then tested on a laboratory
impul~e dryer tester at the following conditions:
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WO 92/10607 2 ~ 3 ~ 8 3 ~ PcTrGB91/02lsx
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Pressure: 5500 KPa
Nip Residence: 45 msec
Upper Platen Temp: 190C
The paper stock used for these tests was 100~ sleached
Softwood Kraft with a 500 CSF prepared into 50 g/m2
handsheets with an ingoing dryness of 36%. The ingoing
felt moisture ratio for each fabric was 0.~5. During
testing, final paper dryness and felt moisture gain
were determined. In addition, each felt's air
permeability after conditioning was also measured.
Results of the study clearly show that as the
percentage of Teflon fibre in the paper contacting
layer increases, both paper dryness and felt moisture
gain increase significantly, while the press fabricls
air permeability decreases. Traditionally, lower air
permeability in a press fabric usually relates to lower
dewatering capability; however, in this case, the trend
is opposite. Under the test conditions discussed
above, approximately 10~ higher paper dryness values
are achieved with the sample having a 100% Teflon paper
contacting layer compared to the sample containing 0
Te~lon, while at the same time air permeability
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wo 92/10607 2 0 ~ 7 8 3 ~ PCT1GB91/02158-
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decreased from 12.2 cfm for the 0% Teflon sample to
4.2 cfm for the 100~ Teflon sample.
The results of this study are shown below in Table 1
and are displayed in Figure 1 of the accompanying
drawings.
Table l. Teflon B~end Pross Fa~ S~rucsure S~udy
. w . . .
No I Felt ¦ Itandstleet ¦ Felt Gain FsltAir !
Composition I ; I- - - Permeabi~i~y
~0 TAeflon ¦ Y Cry Final ¦ ~cfm~
l 1 ! o% T~flon ¦ 5~.0 ¦ 24.8 1
2 1 25% Teflon 1 60.4 1 25.0 1 11 9
3 ¦ 50% Teflon ¦ 6~8 ¦ 28.8 ¦ 12.1
.
4 ¦ 75% Teflon 6~ ¦ 29.9 ¦ 6.1
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1 1~0% Teflon ¦ 65.6 352 ¦ 4.2
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EXAMPLE 3
An experimental press fabric contain_ng a 100% Teflon
paper contacting layer was tested on a pilot paper
machine with an impulse drying roll and an extended nip
press shoe. The conditions used for this trial were as
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Wo 92~10607 2 0 3 7 ~ ~ L~ PCT/GB91/02158
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Maximum nip pressure: 600 pli
Machine speed: 820 m/min (2500fpm)
Hot Roll Temperature: 177C (350F)
Paper basis weight: 45-50 g/m2
Ingoing paper dryness: 25-36%
During this trial, a final paper dryness greater than
60~ was obtained. Subsequent to this trial, a sample
of the used press ~abric was returned to the laboratory
and examined. Visual examination showed that the
susface was heavily sealed and glazed. An SEN
photomicrograph of this surface ~see Figure 2 of the
- accompanying drawings) confirmed this condition. A
specimen was removed from this area of the fabric and
subsequently tested on a laboratory Lmpulse dryer
tester for dewatering using 50 g/m2 handsheets at
36% initial dryneqs. Regardless of this sealed surface,
: the results, surprisingly, showed paper dryness as high
as 65%. Through our experience, any other press fabrics
having a similar glazed or sealed surface would be
expected to produce a very low level o dewatering,
thus low paper dryness.
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