Note: Descriptions are shown in the official language in which they were submitted.
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INDUSTRIAL FABRICS
The present invention relates to industrial fabrics for use in phase
separation applications such as media for filtration and like fabrics for
papermaking.
When in use, such industrial fabrics suffer from the problems of fibre
shedding and wear. Also, rewetting of a paper sheet upon exit of the sheet
from
the press-nip of the papermaking machine is a recognized problem.
Various methods of alleviating the above drawbacks have been proposed.
In the papermaking felt described in US 5,372,876 (Appleton Mills), at
least one hydrophobic layer of co-joined synthetic, e.g. nylon filaments is
disposed
between a base fabric and the batt material and/or between at least
two batt layers. The various layers are joined by needling, In use, the batt
layer
receives water from the web, and such water is forced from the batt layer
through the hydrophobic layers) under pressure in the press-nip. Upon exiting
the press-nip, the pressure is relieved and the hydrophobic layer provides a
barrier which reduces backflow of water to the batt, thereby alleviating
rewetting.
In GB 2,285,935 (Scapa Group) the problem of fibre shedding and wear is
addressed by providing polymer coated paper machine clothing which reduces
fibre shedding and improves both abrasion resistance and surface smoothness.
The coating is applied by urging a polymer film coated release sheet on to the
surface of the base cloth of the fabric, curing the polymer by passing such
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through heated roller and then removing the release sheet to leave a permeable
coating.
In US 5, 118,557 (Albany) a thin layer of polymer foam is applied to the
surface of the press fabric, which is allowed to dry, this procedure is
repeated
several times to form a coating. This fabric then has an increased resistance
to
rewet of the paper and produces a paper sheet with increased surface
smoothness, due to increased surface contact area afforded by the foam coating
together with its controlled porosity and void volume.
In EP 0239207 (Asten) a 0.4mm thick scrim of low melt synthetic material
having a regular lattice configuration of 6mm squares is located between two
upper batt layers of a papermaker's felt. The lattice structure is designed in
order not to affect the moisture absorption of the felt and is provided to
prevent
excessive fibre migration as a result of the needling operation to join the
various
batt layers together. In a subsequent heat setting step the scrim, which has a
lower melting point than that of the batt material, is softened to undergo
deformation thereby adhering to the batt fibres and reducing the likelihood of
fibre migration or layer separation of the batt during use of the papermaker's
felt.
In US 4,199,401 (Asten) a coarse layer of batt is provided on the paper
contacting side, and a finer denier batt sandwiched between this and a base
layer of the papermaker's felt. This allows the water to migrate through the
felt
by capillary action fihereby reducing rewet.
In US 5,232,768 (Nordiska filt) a batt layer is provided on the paper
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contacting side, and a barrier layer is sandwiched between this and the base
layer of the press felt. The barrier layer may comprise filament threads as a
perforated film or sintered polymeric particles as a foam layer. The barrier
layer
provides a high resistance to water flowing back through it to the paper
contacting side.
In US 5,071,697 (Gulya) a permeable polymeric foam is secured to the
surface of a base substrate, with a thin outer layer of polymeric film being
bonded to the outer surface. This provides a flexible, tough skin to resist
abrasion and tearing during operation.
i0 In US 4,830,905 (Gulya) a method to reduce web rewet is described in
that a layer of closed cell polymeric foam is disposed on a face of the base
fabric. A fibrous batt layer is needled thereon, this needling action
penetrating
the foam and intersecting the cells. Whilst compressed in the nip, the
penetrated cell walls open up allowing water in, then on leaving the nip, the
walls once again close, so trapping water in the cells.
US 3,214,326 (Lee) describes a press felt, the objective of which is to
increase the quantity of water removed from the paper sheet, as well as to
reduce
rewet, whereby a barrier layer is attached to the upper, paper contacting
surface of
a woven base cloth. The barrier layer has been described as a fine, low
permeability, woven fabric.
US 3,399,1 11 (Beaumont) describes a 'supplemental belt', for use in
conjunction with a press felt, which runs on the machine-side. The
construction
includes at least one perforated film laminated to a foam or woven layer. One
of
the belt's purposes is to give good drainage and water removal
characteristics.
US 4,541,895 (Albert) describes a papermaker's fabric made up of a plurality
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of perforated plastic sheets, the size and distribution of the apertures being
variable.
US 4,550,588 (Lundstrom) describes an air impermeable felt/belt which has
been manufactured by filling a felt, except for the upper surface, which
retains a
chamois-like surface. A barrier layer, for example a non-woven layer or
additional
batt layer may be inserted below the surface to prevent the filler material
from
penetrating to the surface.
US 4,565,735 (Murka) describes a felt which consists of at least two types
of batt fibre, one being of lower melt material and being applied in a lower
quantity
than the other and then melted. The object is to give a felt with improved
wear and
compaction resistance.
It is an object of the present invention to provide an industrial fabric which
has improved resistance to rewet, and improved smoothness and wear resistance.
It is a further object to provide an industrial fabric which has improved
resistance to
fibre shedding, superior macro and micro scale pressure uniformity, more
economical
to apply and process more consistently.
In accordance with a first aspect of the present invention fihere is provided
a method of making an industrial fabric including the steps of providing a
base
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layer, at least one batt layer, and at least one polymeric film layer, the
method
further comprising the steps of needling the said layers and then
thermoforming the
at least one polymeric layer.
Needling results in perforation of the polymeric film layer enabling
anchoring of the batt fibres therethrough. The subsequent thermoforming of
that
layer leads to at least partial encapsulation of the surrounding batt fibres
and cross-
over points thereof. This gives improved locking of the fibres of the bast
together, so reducing shedding. i=urthermore, the combination of the bats
fibres
and polymeric film provide a resistance to re-wet and hence superior sheet
dryness when the fabric is used for paper machine clothing. It also blocks the
backflow of filtered substances in other applications. In comparison to the
papermaking fabric described in US 5,372,876 the present fabric when used as
a papermaking fabric has been found to provide a fabric with superior fibre
bonding and wear resistance, with enhanced surface/pressure uniformity and
contact area for sheet smoothness. It is intended that the term
"thermoforming"
not only covers the possibility of fully melting the plastics layer, but that
it should
cover the possibility of supplying enough thermal energy to soften, that is
deform,
that layer sufficiently to enable said at least partial encapsulation of the
batt fibres.
The polymeric film layer may be provided under at least one layer of batt
and/or on at least one layer of bats.
In a preferred embodiment the polymeric film layer is beneath a fine,
uniform, fibrous diffusion layer, which diffusion layer forms a surface layer.
This
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has the advantage that once needled and thermoformed, the polymeric partially
encapsulates and anchors the surface fibres, providing a reduction in shedding
in
this region and also an improved surface wear resistance.
For hot end use applications, polymeric film material with a higher melting
point may be used, for example a material such as an ether based polyurethane.
This has the advantage of minimal degradation thereby maintaining bonding
and fibre adhesion in the structure. The permeability of the fabric can be
controlled by varying the thickness and quantity of polymeric film layers
used, as
well as by adjusting the needling procedure, that is the number of punches per
unit area, and the amount and coarseness of the batt driven through. The heat
setting conditions which may include the use of compression from a calender
can also be used to adjust the permeability of the fabric to the required
level, by
altering the degree of tension/compressive forces present during high
temperature
applications.
In accordance with a second aspect of the present invention there is
provided an industrial fabric including a base layer, a batt layer and at
least one
low-melt polymeric film layer which has been needled into the batt layer and
subsequently thermoformed to at least partially encapsulate fibres of that
batt layer.
The polymeric film layer may have a melting point of less than
215°C.
Preferably the bait layer forms a surface layer of the fabric.
Preferably at least two batt layers are provided with the polymeric film
layer being located under at least one of the said batt layers. The polymeric
film
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layer may be provided as an elastomeric, thermoplastic polyurethane film. The
film layer may have a thickness in the region of 0.05mm. Polyurethane has the
advantage that it has a lower melting point than that of the nylon fibres
usually
used to form the various batt layers, whilst still having a melting point
which is
above 100°C thereby enabling continuous service of the fabric at
relatively high
temperatures without melting the polyurethane during use. Furthermore, being
an
elastomer, polyurethane maintains its original properties even after repeated
melt/cool process cycles and it's high melt viscosity prevents it from
"flowing"
excessively at high temperatures.
The polymeric film layer may be perforated and/or may include a filler.
The polymeric film layer may be of a multi-layer construction with at least
two
layers thereof having different characteristics.
The fabric may comprise a plurality of said polymeric film layers, and in a
preferred embodiment at least two of said layers have different
characteristics.
By way of example only specific embodiments of the invention will now be
described with reference to the accompanying drawings, in which:-
Fig. 1 is a schematic view showing the production of an industrial fabric
constructed in accordance with one aspect of the present invention;
Fig. 2 is a scanning electron microscope photograph showing a
detail of the constructed fabric of Fig. 1 illustrating the
needled and subsequently molten plastics film;
Fig.3 is a scanning electron microscope view of the paper
contacting or the like surface of the fabric of Fig. 2;
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Fig. 4 is a comparative graphical representation illustrating density
variations throughout the width of the constructed fabric; and
Fig. 5 is a schematic view of the layers in an industrial fabric
before they are needled and heat set constructed in accordance with
a second aspect of the present invention,
With reference to Fig. 1 (a), an industrial fabric constructed in accordance
with the invention comprises a base layer 2, a first upper batt layer 4, a
fine
0.051 mm (0.002 inch) thick elastomeric, thermoplastic polyurethane film layer
6,
an uppermost batt layer 8, and a lower batt layer 10. The polyurethane film
layer 6 has a lower melting point than the other layers. These layers are
joined,
as best illustrated in Fig. 1 (b) by needling 12 the layers together. Needling
causes the individual fibres of the batt layers to intermingle and to link
through
the base layer 2 to lock the layers together. Furthermore, the needling action
perforates the polyurethane film 6 to give a more open structure, through
which
the batt fibre is driven. As best illustrated in Fig. 1 (c) the upper paper
contacting
surface of the fabric 18 is then heated at 200°C using a cylinder 20,
this heat
being sufficient to melt the lower melting point needled polyurethane film 6
only.
This heat permeates through the fabric to melt the needled film 6 and causes
it
to flow upwards towards the upper surface 18 of the upper batt layer 8. As
best illustrated in Figs. 2 and 3, once cooled the then needled and melted
film 6
encapsulates the surrounding bait fibres and cross-over points thereof.
The fibres of the upper batt layer 8 thereby have a partial polyurethane
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coating which better interlocks the individual fibres thereof to provide a
more
cohesive structure less prone to fibre shedding. The gaps 22 within the coated
fibres nevertheless present a porous layer.
As best illustrated in Fig. 4, in the region of the needled and thermoformed
film layer 6 there is an increase in the density of the fabric, which means
that it is
more difficult for expressed water to pass through the fabric in this
particular region.
However, when in use the fabric passes through the nip and is placed under
very high pressure enabling expressed water, from the paper sheet carried on
the fabric, to be more readily forced into and through the fabric. When the
fabric
emerges from the nip, the pressure of the nip is relieved, the fabric
recovers,
and the needled and thermoformed film layer 6 thereby once again presents a
structure which is more difficult for the water to pass through. By this
means, the
water is not able to force its way back through the fabric, that is, through
this
denser region to the upper surface 18, so re-wet of a paper sheet is
minimised. The
encapsulated and anchored batt fibres of the present invention give greater
pressure
uniformity due to the more homogenous surface thereof thus enabling more
water to be squeezed out of a paper sheet transported thereon. In fact,
experimental results have demonstrated that a fabric constructed in accordance
with the present invention produced a paper sheet having an increase in sheet
dryness of 1.8%, when compared to a paper sheet produced by a fabric
constructed in accordance with US 5,372,876.
US 5,571,590 and US 5,731,063 (Appleton Mills) describe the fusing/butt
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joining of plastics film supplied in narrow rolls to form full size endless
loops.
The film of the present invention can be joined in the manner described in
these
prior patents, but has the additional advantage that the actual join will not
be
evident after heat and pressure has been applied during the heat thermo-
setting
process.
A fabric constructed in accordance with the present invention (Sample A)
was tested and compared to a control sample fabric (Sample B) which contained
equivalent layers, however the film had not been thermoformed in this control
sample, and a second control sample (Sample C) which contained equivalent
layers
with the exception that the film layer was omitted. The permeabilities of
Samples
A, B and C were measured on a Frazier permeameter with 12.7mm (0.5") water
gauge pressure. The permeabilities of Sample B and Sample C were 2.8
litres/m2/sec and 8 litres/mZ/sec respectively. This demonstrated that the
initial
addition of the thin film of polyurethane, which is needled but not
thermoformed,
results in a fabric which has a significant reduction in permeability thereby
undesirably reducing the flow of expressed water therethrough. However, the
permeability of Sample A, constructed in accordance with the present
invention, is
once again increased to 5.7 litres/m2/sec, giving a fabric with a permeability
not
significantly lower than control Sample C. Therefore, a fabric constructed in
accordance with the invention has made only a slight compromise in
permeability,
whilst having the advantage of reduced fibre shedding, a more homogenous
surface
and a reduced incidence of re-wet.
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It was expected that thermoforming of the polyurethane film would severely
reduce the permeability, however tests show that with a sufficiently thin film
the
fabric remained open due to the vertical and horizontal migration of the
polyurethane melt and it's ability to actually wet the batt fibres.
Although the film layer has been described as being provided between
two upper batt layers, the film could be provided on the uppermost surface 18
before it is joined by needling and then subsequently thermoformed. Although
two
upper batt layers have been described, several such layers may be provided
and also more than one film layer could be provided on, or between adjacent
such batt layers. Additional film layers may be provided between or on
adjacent
batt layers. Although a lowermost batt layer has been illustrated, this could
be
omitted, or equally consist of several such layers. A film may also be
provided
between the lower batt layer(s), or immediately adjacent the base layer.
Although in the described embodiment the base fabric has been illustrated as a
woven layer, this could be a non-woven layer, for example a porous film could
be employed.
In the embodiment of Fig. 5 the industrial fabric comprises a woven base
cloth 22, batt layers 24, 26, 28 and a plastics layer 30. Batt layer 24
comprises
a uniform, stiff, laminate non-woven batt structure which is substantially
aligned
in the machine direction (MD) of the fabric. The batt layer 24 is comprised of
0.1 mm (0.004") thick bi-axial non-woven fibres which both diffuses and masks
the base cloth. Batt layer 26 comprises substantially cross-machine (CD)
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orientated fibres having a dtex of 17 and which via needling bonds the base
cloth 22 and batt layer 24 together and also both diffuse and mask the coarser
machine direction orientated fibres of the batt layer 24. Batt layer 28
essentially
comprises a matrix of relatively fine batt fibres of 3.3 dtex. The batt layer
28
supports the sheet and facilitates ease of water movement from the sheet into
the press fabric.
Although the film layer has been described as being an elastomeric,
thermoplastic polyurethane film, the film layer could comprise other types of
plastics, for example other types of thermoplastic polymers; thermoplastic
resin
and/or elastomer, or a cross linkable resin andlor elastomer. The film layer
may
also contain fillers such as release agents for example fluorinated polymers
and
polysiloxanes, or inorganic fillers, adhesion promoters, foamable fillers etc.
The
film layer could be of a multi-layer construction with each layer providing
unique
properties such as melting temperature, elasticity, hydrophilic and
hydrophobic
characteristics influencing water movement in and out of the composite
structure, barrier properties etc. The film layer may also be of a multi-layer
construction with varying hardness. The film may be pre-perforated. The type
of material and properties thereof can be selected depending on the required
use of the fabric in terms of level of permeability required or the possible
degree
of hydrophobic properties required.
Although the specific example has been described in relation to a fabric
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suitable for use as a papermaker's belt, which could be seamed or endless,
such
fabrics could be used in other phase separation applications, such as
filtration.
Although specific thickness of film has been described, it is to be
understood that other thickness of film could be employed.
