Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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WO98/19860 PCT~S97/19725
PAPER ~A~RTN~ rT.~l~l~s CON~L~ ) OF ~PAND~n PTFE
FT~T-n OF T~F TNVENTION
This invention relates to paper machine
clothing suitable for use in the forming, pressing
or drying sections of a paper making machine. More
particularly, it relates to such fabrics made from
monofilaments of synthetic polymer resins,
specifically expanded PTFE.
BACR~ROUND OF T~F lNv~:NlION
The modern papermaking machine is in essence a
device for removing water from the paper furnish,
which is a slurry of papermaking constituents. The
water is removed sequentially in three stages or
sections of the machine. In the first section, also
known as the forming section, the furnish is
deposited on a moving forming wire and water is
drained through the wire to leave a paper sheet or
web having a solids content of about 18 to 25
percent by weight. The formed web is carried into a
wet press felt section and passed through one or
more nip presses on a moving press felt to remove
sufficient water to form a sheet having a solids
content of 36 to 50 percent by weight. This sheet
is transferred to the dryer section of the
papermaking machine where dryer felts press the
paper sheet against steam heated cylinders to obtain
a 92 to 96 percent solids content.
On papermaking machines, endless belts are
employed in the various sections to carry the sheet
or web of paper. There are a wide variety of forms
of the endless belts, some fabricated from metal and
others from textile material such as cotton, cotton
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and asbestos, asbestos and synthetic fibrous or
filamentous materials. The selection of a given
material is dependent to some degree upon the use to
which the fabric will be put, i.e., as a forming
fabric, dryer felt, etc.
One form of belt which has been used
extensively as a forming wire in the forming section
of the papermaking machine is one fabricated from an
open weave of synthetic, polymeric resin
monofilaments. Such fabrics generally perform well
in the forming section although there are certain
limitations. For example, the relatively open
weaves, particularly when run at highest speeds,
lack dimensional stability. This shortens the
overall life of the forming wire which is subject to
abrasion as it shifts in position on the machine.
In addition, the relatively open weaves are less
than fully supportive of the furnish fibers
deposited on the wire. Ideally, the fiber and sheet
supporting properties of a wire should be increased
without significant decrease of water removal
through drainage.
Press fabrics operate in the press section of
the paper making machine. In this section, the
paper sheet is transported by the press fabric and
the sheet and fabric are pressed between the nip of
the press rolls, which exerts enormous pressure on
the sheet and fabric, thereby dewatering the sheet
by pressing out the liquid. Also, temperatures in
this section of the machine are relatively high.
The press fabric generally consists of a base layer
and a batt layer of staple fibers needled to the
base. Synthetic materials have become the norm,
with polyamide being a particularly favored
material.
The fabric in the drying section of the machine
together with its sheet of paper tends to be
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subjected to elevated temperatures in a rigorous
chemical environment. Dryer fabrics or "dryer
screens" employed in the paper making industry have,
traditionally, been formed from a variety of
materials such as poly (ethylene terephthalate),
polyphenylene sulfide and polypropylene. Each
material has different properties and pricing. An
important property for any material used as a dryer
screen in a paper making machine is that the
material should have good hydrolytic stability and
good dimensional stability.
Polypropylene is the cheapest material
presently available; it has excellent hydrolytic
stability, but poor dimensional stability at
elevated temperature, and as a result it has only
limited use.
Poly~ethylene terephthalate) (PET) is
moderately priced, has exceptional dimensional
stability and reasonable hydrolytic stability.
Poly(ethylene terephthalate) is the predominant
material currently used in the market place and in
most cases, the hydrolytic stability of
poly(ethylene terephthalate) can be improved by the
addition of carbodiimide stabilizers. Polyphenylene
sulfide has excellent dimensional and hydrolytic
stability, but is extremely high priced, is more
difficult to work, and tends to suffer from brittle
fracture problems in the crystalline state due to
normal flexing experienced on the paper machine.
sT~ Uy OF T~ Thv~ Ll ON
The present invention is an article of paper
making machine clothing suitable for use in the
forming, pressing or drying sections of a paper
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WO98/19860 PCT~S97/1972S
ma~ing machine wherein the article is comprised of a
fibre structure constructed of yarns of expanded
poly(tetrafluoro ethylene~ ("expanded PTFE").
Expanded PTFE is a material available from W.L.
Gore and Associates, Inc., Newark DE, USA. As
described in U.S. Patent No. 3,953,566, expanded
PTFE is a highly porous material characterized by a
microstructure of nodes interconnected by fibrils.
The disclosure of the '566 patent is incorporated
herein by reference. The material, in addition to
its high porosity, exhibits high tensile strength.
BRIEF DESCRIPTTON OF TFR FIGU~7~-~
Figure l shows the results of an abrasion test
performed on the preferred embodiment.
Figures 2a-2d show SEM's of the filament used
in the preferred embodiment.
Figures 3a-3b show the degree of abrasion for
the prior art.
Figures 4a-4b show the degree of abrasion for
the preferred embodiment.
Figures 5a-5b show the degree of abrasion for
the prior art.
Figures 6a-6b show the degree of abrasion for
the preferred embodiment.
DETATT~n DESCRIPTION OF T~T~' p~ T.~n T~'~RODTM~NT
The Applicants have found that paper machine
clothings comprised of expanded PTFE filaments
provide industrial fabrics that exhibit excellent
abrasion resistance, contamination resistance, and
crease resistance. Tensile properties of expanded
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PTFE woven goods equal or exceed that of currently
available polyesters or polyamides. The enhanced
tensile properties relative to these other materials
are believed to improve the dimensional stability of
the fabrics. Also, the use of expanded PTFE
monofilaments allows higher use temperatures, up to
288~C, than temperatures at which conventional
materials could be used.
With the fabrics of the present invention, many
of the above-described shortcomings of the prior art
are eliminated or reduced in severity. Forming
wires constructed according to the invention may be
fabricated from an all monofilament fabric which
provide an exceptionally smooth surface to contact
the paper sheet. A maximum degree of support is
achieved. As a result, relatively mark free paper
product is obtained, while the desired advantages of
an all monofilament fabric are retained, such as an
efficient degree of water drainage. In addition,
the structure of the fabrics and the wires of the
invention is such that a greater dimensional
stability is achieved in both the machine and cross-
machine directions. This reduces yarn abrasion and
increases operating life. The overall operating
life of the forming fabric is significantly
increased over prior art fabric.
One of the more important features of paper
machine clothing in accordance with the present
invention is its potential use in high temperature
sections of a paper making machine, in particular
dryer fabrics and dryer screen fabrics, since the
material from which it is made is not readily
hydrolyzed. That is, it exhibits good resistance to
hydrolysis. PTFE is known to be a highly inert,
water repellant material.
It will be appreciated by the person skilled in
the art that with the tendency towards ever higher
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temperatures in the forming and pressing sections of
a paper making machine, paper machine clothings
constructed of expanded PTFE can be used in all
sections of the paper making machine. In the
forming section it is possible to form an open weave
using monofilament materials which allow for
adequate support of the solid materials in the
furnish and yet allow sufficient dewatering to
produce a coherent sheet preparatory to pressing.
In the pressing section, by providing both the
support layer and at least a portion of the surface
layer of the pressing fabric in accordance with the
present invention, pressing fabrics much more
tolerant of high temperature operation are pro~uced.
The invention, therefore, is concerned not only
with the production of paper machine clothing (PMC)
materials which may be of woven or spiral or of
other suitable monofilament structures, in which
monofilaments may extend in both the machine
direction and the cross direction of the fabric, but
also include other PMC structures. Such material
may be used to produce PMC fabrics comprised of
staple, multifilament, and/or monofilament fibres.
Typical range of slzes of monofilaments used in
press fabrics and dryer fabrics are 0.20 mm - 1.27
mm in diameter or the equivalent mass ln cross-
section in other cross-section shapes, e.g. square
or oval.
For forming fabrics finer monofilaments are
used, e.g. as small as 0.05 mm, while special
Industrial applications may use monofilaments up to
3.8 mm.
Filaments of expanded PTFE can be formed by
helically winding expanded PTFE sheet material. See
U.S. Patent No. 5,281,475, the disclosure of which
is incorporated herein by reference.
Filaments are woven into paper machine belts
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according to conventional weaving techniques. The
type of weave and density thereof will depend on the
type of paper and papermaking operation for which
the belt is to be used. Single layer and
multilayered constructions are suitable, and when
formed of a woven construction, the fabrics may be
woven endless or in flat form. The following
examples demonstrate the advantages of the present
invention.
Exam~le 1
A woven single layer fabric was constructed of
expanded PTFE monofilaments (obtained from W.L. Gore
& Associates) as the shute yarns and Hoescht 906Y
polyethylene terephthalate material as the warp
yarns. The woven fabric was a 62 x 46 five harness
single layer straight twill fabric woven with 0.30
mm monofilament shute in a 2x3 configuration. For
comparison, a woven fabric containing 0.30 mm
Hoescht 906Y HMW PET monofilament as warp and shute
was prepared in the same configuration as the
expanded PTFE containing fabric. The coefficient
of friction of the woven samples was calculated from
the force needed to drag a steel plate over the
samples, with the test being performed in the
machine direction. The friction values obtained
were 50~ lower for the expanded PTFE material on
both the short and long warp knuckle sides. It was
observed that the expanded PTFE samples felt very
slippery.
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WO98119860 PCT~S97119725
Friction Values
Long Warp Short Warp
Knuckle Down Knuckle Down
Expanded PTFE O.lO 0.l2
Hoechst PET 0.20 0.2l
Fxample 2
The fabric samples described above were tested
for abrasion resistance. The samples were tested
with the long shute knuckle down to maximize the
exposure of the expanded PTFE to the wear cylinder.
A force transducer was used to measure the force
exerted on the fa~ric during the test.
In the first running of the test samples were
run to failure. After 40 minutes the PET sample had
lost all of the wear side exposed shutes, as seen in
Fig.3. All that remains are short sections of shute
on the paper side. The sample containing expanded
PTFE was run for 240 minutes without failure.
Inspection of the expanded PTFE sample showed that
it was entirely plugged by smeared, expanded PTFE
together with a little of the calcium carbonate
filler from the test, as seen in Figure 4. Although
the sample appeared to be intact, there was no
strength remaining in the cross-machine direction
and the sample easily pulled apart.
A second run was made where the samples were
removed for caliper measurement after 20, 30, and 40
minutes. As seen in Fig. l, the PE~ fabric lost
caliper much more quickly than the expanded PTFE
sample.
Unlike the first test, the PET sample was still
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intact after 40 minutes, but the shute strands are
heavily worn. Photos from the test are shown in
Figures 5 and 6. The expanded PTFE strands have
become somewhat frayed at the edges. There is no
wear to the warp strands of the expanded PTFE sample
after 40 minutes.
