Wet press papermakers felt

The disclosure is of a method of preparing a papermakers wet-press felt fabric, felts made therefrom and their use, as wet-press felts on papermaking machines. By the method of the invention, there is obtained a method of making a papermakers press felt of controlled porosity while at the same time maintaining a high level of void volume and permeability in a loaded nip of a paper machine press.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to papermakers felts and more particularly relates to 
a wet press felt for use in the press section of a papermaking machine and 
the method of its fabrication. 
2. Brief Description of the Prior Art 
The modern papermaker employs a highly sophisticated machine to make paper, 
which is named rather appropriately a "papermaking machine". The modern 
papermaking machine is in essence a device for removing water from the 
paper furnish. The water is removed sequentially in three stages or 
sections of the machine. In the first or forming section, the furnish is 
deposited on a moving forming wire and water drained through the wire to 
leave a paper sheet or web having a solids content of circa 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 
44 percent by weight. This sheet is then transferred to the dryer section 
of the papermaking machine where dryer felts press the paper sheet against 
hot, steam-heated dryer cylinders to obtain about 92 to 96 percent solids 
content. 
The clothing employed on the papermaking machine must perform a widely 
diverse range of functions, according to the position on the machine, 
i.e., forming, press or dryer section. In view of the diversity of 
functions, the clothing for use in each section of the machine must be 
manufactured to meet specific design requirements essential to the 
particular section. In the absence of meeting the specific felt design 
requirements demanded in each section of the machine, the overall 
operation of the machine will be unsatisfactory. Optimum operating lives 
of the felts will not be achieved, product quality may be adversely 
affected, machine speeds may be lowered or drying efficiency may be 
impeded. 
Those skilled in the art have long appreciated that the efficiency of water 
removal in the wet press section of the papermaking machine is critical to 
overall efficiency in the papermaking process. This is because, first a 
large amount of water must be removed from the sheet at the presses to 
realize a good drying economy. Secondly, greater efficiency in water 
removal creates a drier and hence stronger sheet less susceptible to 
breaking. A large variety of clothing constructions have been proposed as 
papermakers felts advantageously employed in the press section of a 
paper-making machine. In fact, there has been a continual evolution of 
clothing constructions, corresponding to improvements in the papermaking 
machine itself. This evolution began with the early woven felt, woven of 
spun yarn and then mechanically felted or fulled. A later development was 
found in the "Batt-on-Base" construction consisting of a woven fabric base 
and a batt surface attached by needling. The needled batt-on-base felts 
are widely used today and have been said to be the "standard of the 
industry". However, a wide variety of other constructions are available, 
including non-woven press felts. 
Important physical properties of a papermakers press felt are measured by 
four test measurements. They are: 
1. Saturated moisture: a measure of the amount of water absorbed by the 
felt under static conditions. Expressed as pounds of water absorbed per 
pound of felt, saturated moisture is an excellent indicator of the ability 
of a felt to receive water from the sheet in the nip. 
2. Vacuum dewatering: measures the ability of a felt or fabric running on a 
press to release water to a suction pipe. 
3. Air permeability: measured in a dry felt, is expressed as cfm/sq. ft. of 
felt at 0.5 in. water pressure (m.sup.3 /m.sup.2 per hr. at 10 mm water 
gauge). 
4. Flow resistance: the water permeability of the felt or fabric. 
Generally, the batt-on-base felts are advantageous in all four parameters, 
compared to the earlier conventional woven felt. However, as the speed of 
the papermaking machines has increased, so has the need for press felts 
which show an advantage in one or more of the desired physical properties. 
One type of press felt which has been suggested is a composite of a woven 
or non-woven fabric base bearing a surface layer of a flexible, open-cell, 
polymeric resin foam. This layer, acting like a sponge would enhance the 
removal of water from the paper sheet. In addition, the inherent thermal 
insulation provided by the foam layer would impart some protection to the 
underlying fabric structure which is normally exposed completely to the 
degradative, hot water being pressed from the paper sheet. These composite 
felts have also shown good resistance to compaction. Representative of the 
prior art concerned with the latter composite papermakers felts are the 
disclosures found in U.S. Pat. Nos. 1,536,533; 2,038,712; 3,059,312; 
3,399,111; and 3,617,442. In general, the papermakers felts of the prior 
art which comprise a composite laminate of a textile and a polymeric resin 
layer have not been completely satisfactory in regard to their resistance 
to wear, delamination and long term resistance to compaction. Apparently, 
the diverse nature of the two components enhances degradation of the 
overall composite. Further, the presence of a seam in the foam layer 
serves to provide a weak point in the construction. 
The composite structure of the papermakers felts of the present invention 
are an improvement over many of the prior art composite felts in regard to 
their resistance to wear, delamination and long term compaction 
resistance. They are virtually seam free. In addition, the method of their 
manufacture is an improvement over prior art manufacturing processes for 
composite felts. 
SUMMARY OF THE INVENTION 
The invention comprises a papermakers felt, which comprises; 
a base fabric of interwoven machine and cross-machine direction yarns; and 
a coating of a resilient, water-resistant, synthetic polymeric resin bonded 
to the base fabric; 
said resin coating being water permeable by virtue of a plurality of 
channels penetrating the body of the coating. 
The wet-press papermakers felts of the invention exhibit improved 
compression/recovery properties and are characterized in part by 
homogeneous, evenly distributed coating voids. 
The invention also comprises the method of fabricating the papermakers 
felts of the invention, wherein void size and distribution are highly 
controlled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
FIG. 1 is a cross-sectional side elevation, enlarged, of an intermediate 
fabric 10 useful in a preparation according to the method of the invention 
for preparing the wet-press felt fabric of the invention. The fabric 10 
comprises a base layer which may be any conventional press felt fabric. 
As shown in FIG. 1, the base layer 12 is preferably of interwoven machine 
direction (warp) 14 and cross-machine direction (weft) 16 textile yarns. 
The yarns may be spun yarns, spun from synthetic or natural staple fibers 
such as staple fibers of wool, cotton, polyolefins, polyamides, 
polyesters, mixtures thereof and the like. Alternatively, the yarns 14, 16 
may be multifilament yarns of the same synthetic or natural fiber 
materials. Preferably, the yarns 14, 16 are monofilament yarns of 
synthetic polymeric resins such as yarns of polyesters or polyamides and 
the like. 
The particular weave employed in providing the base layer is not critical 
and any conventional felt weave may be employed. Thus, the base layer may 
be a single layer or multi-layered weave construction and may include 
filling yarns or picks to control permeability of the fabric 10. 
Advantageously the denier of the yarns and the density of the weave is 
selected to provide a base layer weight of from about 4 to about 30 
oz./square yard for optimum strength. 
A top layer comprises a coating of a synthetic non-cellular, polymeric 
resin 20 containing a dispersion of solvent-removable chopped fibers 22. 
The resin 20 is curable or cross-linkable to a solvent-resistant state as 
shown in FIG. 2. The fabric 30 shown in FIG. 2 is the fabric 10 wherein 
resin 20 has been cured to obtain the cured resin 24. The resin 24 may be 
any solvent-resistant, cured resin of a synthetic polymeric resin 20. 
Representative of such resins 20, 24 are elastomeric resins of 
polyethylene, polyurethanes, including polyether and polyester 
polyurethanes, polyisocyanurates and the like. The method of preparing 
such resins and for coating them on substrates is well-known to those 
skilled in the art. The thickness of the coating or resin 24 is 
advantageously within the range of from 0.050" to 0.200". The 
solvent-removable fibers 22 are either synthetic polymeric resin staple or 
natural fibers, which may be dissolved with specific solvents, to which 
the resin 24 and yarns 14, 16 are solvent resistant. Representative of 
such solvent removable fibers are fibers of wool, ethyl cellulose, 
polystyrene, polycarbonate and polystyrenemethylmethacrylate which are 
readily dissolved in dry cleaning solvents or aqueous acid or alkali 
mediums (see U.S. Pat. No. 3,311,928). Fibers of polyvinyl alcohol may be 
used and are removable by dissolution in water; as are fibers of 
poly(ethylene oxide); see U.S. Pat. No. 4,097,652. Fibers of certain 
polyethylenes are also usable, being removable by dissolution in hot water 
(see U.S. Pat. Nos. 2,714,758 and 3,317,864). Wool fibers are inexpensive, 
and can be removed with 5% NaOH at 150.degree. F. to 212.degree. F. 
without damage to the base yarns or the resin 24. 
Alternatively, the solvent-removable fibers need not be chopped fibers 
admixed with the resin 20. Any other procedure may be followed whereby the 
fibers (or other solvent-removable material as hereinafter described) may 
be employed so as to leave void spaces or channels in the cured resin 24 
upon removal. For example, a tangle of mainly continuous solvent-removable 
yarns or filaments may be placed on the surface of a base structure. The 
resin 20 coating is then applied so as to penetrate the tangle and into a 
portion of the base structure, bonding the tangle to the base. Upon 
removal of the solvent-removable material, voids are left in the cured 
resin 24. The tangle might resemble a pot scrub pad. The density of the 
tangle would determine the degree of voids left. Also, the base fabric may 
be fabricated to include the solvent-removable fibers in such a way that 
the fibers project like pile or tufts. The tufts could be cut or left 
uncut. The pile side of the fabric can then be coated with the resin 20, 
penetrating at least part way into the base fabric. Upon removal of the 
solvent-removable fibers, voids are left in the cured resin 24. The 
density of the piles or tufts would determine the degree of voids. 
Although the use of solvent-removable fibers are preferred in the method of 
the invention, other solvent-removable materials may be used as the 
solvent-removable component. Representative of such, less preferred 
materials are solid granules or particles of solvent-removable, inert 
chemical components which may be dispersed homogeneously throughout the 
resin 20, 24 described above before curing. The term "inert" as used 
herein means that the chemical compound does not chemically react with the 
other components of the fabrics of the invention. Representative of such 
inert, solvent removable chemical compounds are dissolvable inorganic 
salts or the hydrates thereof or oxides thereof. The action of such a salt 
may generally be any of the alkaline metals and preferably any of the 
non-toxic alkaline earth metals, column lA and 2A, respectively, of the 
Periodic Table. 
The solvent removable components, whether a chemical compound in granular 
or particulate form or in the form of a textile fiber, is advantageously 
mixed and homogeneously dispersed with the resin 20 prior to coating the 
fabric substrate, employed in making the fabrics of the invention. The 
proportion of solvent removable component dispersed in the solvent 
resistant resins will depend on the volume of the solvent removable 
component and the desired void volume in the fabric of the invention. The 
optimum proportions may be determined by trial and error techniques. 
However, in general the proportions in the blend will be within the ratio 
of from about 10 to about 100 parts by weight of solvent removable 
component for each 100 parts by weight of the solvent resistant, resin 24. 
Thus, the fabricator has infinite control of the void volume and void 
distribution of the final fabric product in making the fabrics of the 
invention. 
In a final step of the method of the invention, the solvent fugitive or 
removable component is dissolved or leached out of the resin 24 layer of 
fabric 30 leaving void spaces in the fabric. This may be done by washing 
the fabric 30 in the appropriate solvent, under appropriate dissolution 
conditions. The resulting wet press felt fabric 40 as shown in FIG. 3 may 
then be dried and made into a belt 50 for use on a papermaking machine. 
FIG. 3 is a side elevation, enlarged, of a portion of an embodiment fabric 
40 of the invention, prepared as described above and wherein the 
solvent-removable fibers 22 have been dissolved away leaving open channels 
26 which penetrate the cured resin 24, making the fabric 40 water 
permeable via the voids created in the resin 24. The channels 24 receive 
water from carried wet paper, as it passes through the nip of the wet 
press on a papermakers machine. The received water is able to drain 
through the fabric 40 by gravity. 
FIG. 4 is a view-in-perspective of an embodiment wet press belt 50 made by 
making endless a fabric 40 made by the method of the invention. The fabric 
40 is made endless by joining the ends of the fabric 40 at seam 52, using 
conventional seaming techniques. The fabric 40 can be woven endless or 
joined to make felt endless. 
When the fabric of the invention is made up of an endless belt for use on a 
papermaking machine, the resulting controlled void volume felts display 
high density, high compaction resistance and less flow resistance under 
pressure than standard production felts and control samples. 
Those skilled in the art will appreciate that many variations of the 
above-described preferred embodiments may be made without departing from 
the spirit and scope of the invention. For example, the felts of the 
invention may be treated by heat-setting, with chemicals, etc., as 
conventionally done in the art to achieve particular properties. Also, 
those skilled in the art will appreciate that although the invention has 
been described herein in terms of a single type of wet felt press felt 
fabric, it applies to any textile felt construction, for example those 
described in U.S. Pat. Nos. 3,613,258 and 4,187,618.