WET PRESS PAPERMAKERS FELT AND METHOD OF FABRICATION

Abstract: The disclosure is of a method of preparing a paperma­ kers 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 mak­ ing 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.

Description: This invention relates to a papermakers wet press felt according to the preamble of claim 1 as well as to methods of its fabrication. The modern papermaking machine is in essence a device for removing water from the paper furnish. In the first or forming section the furnish is deposited on a moving forcing wire and water drained through the wire to leave a paper sheet or web having a solids content of ca. 18 - 25 percent by weight. The formed web is carried into a wet press 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. The efficiency of water removal in the wet press section of the papermaking machine is critical to overall efficiency in the papermaking process because if a large amount of water is removed from the sheet at the presses less energy is necessary in the dryer section. Furthermore, greater efficiency in water removal creates a drier anbd hence stronger sheet less susceptible to breaking. A large variety of clothing constructions have been proposed as papermakers wet 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 kilograms of water absorbed per kilogram of felt, saturated moisture is an excellent indicator of the ability of a felt to receive water from the sheet in the press 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 m³/m² per hr. at 10 mm water gauge. 4. Flow resistance : the water permeability of the felt or fabric. DE-A-3 419 708 describes a process of manufacturing a wet press felt which is harder and denser than those of the prior art but at the same time maintaining a high level of void volume and permeability in the loaded nip of a papermachine press. The press felt includes a woven fabric base of interwoven machine-direction and cross-machine-direction yarns. A web of non-woven staple fibers is then needled into the woven fabric base. The web includes solvent-resistant fibers and solvent-removable fibers or particles. The whole structure is then compressed to make it denser and to reduce the break-in time of the felt fabric on the papermachine. After compaction the press felt fabric is washed with a solvent capable of dissolving the solvent-removable material. The removal of this material replaces void volume lost during the compaction of the press felt fabric. The resulting press felt is advantageous in all four parameters mentioned before. However, the whole process is somewhat complicated in that at first a woven fabric is provided, then a web of solvent-resistant fibers and solvent-removable fibers or particles is formed and needled into the fabric, thereafter the needled fabric is compressed and finally the compressed fabric is washed to remove the solvent-removable fibers or particle. US-A-3 617 442 shows a press felt which 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. However, a foam is subject to early degradation under the high stress in a press nip. It is the object of the present invention to provide a wet press felt of the species defined in the preamble of claim 1 which has all the advantages of the prior art felts discussed above and in addition a smoother surface, a better resistance to compaction and better abrasion resistance on its face and which can be manufactured with relatively low cost owing to considerably reduced number of method steps. This object is accomplished by the features of the characterizing part of claim 1. The wet-press papermakers felts of the invention have a smmother surface because it is formed by a coating of a resilient, water resistant, elastomeric, synthetic polymeric resin bonded to to a base layer, instead of a batt of staple fibers needled into a base layer. The coating has a better resistance to abrasion than a needled batt, whose fibers would tend to wear away with time. The press felts of the invention exhibit improved compression/recovery properties and have homogenous, evenly distributed coating voids. The invention also comprises methods of fabricating the papermakers felt of the invention, wherein void sizes and distribution are highly controlled. 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. Brief Description of the Drawings Figure 1 is a cross-sectional side elevation, enlarged, of a portion of an embodiment wet-press fabric of the invention, in an initial stage of fabrication before curing of the polymeric resin layer. Figure 2 is a view of a portion of the fabric shown in Figure 1, after curing of the polymeric resin layer. Figure 3 is a cross-sectional, side elevation of a portion of a preferred embodiment fabric of the invention. Figure 4 is an isometric view of a wet-press belt made from the fabric of Figure 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Figure 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 Figure 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 135 g/m² to about 710 g/m² (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 Figure 2. The fabric 30 shown in Figure 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 1.27mm to 5.08mm (.050" to .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. Patent 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. Patent 4,097,652. Fibers of certain polyethylenes are also usable, being removable by dissolution in hot water (see U.S. Patents 2,714,758 and 3,317,864). Wool fibers are inexpensive, and can be removed with 5% NaOH at 65.5 to 100°C (150°F. to 212°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 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 1A 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 Figure 3 may then be dried and made into a belt 50 for use on a papermaking machine. Figure 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. Figure 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. 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. Patents 3,613,258 and 4,187,618.

Claims: A papermakers wet press felt, which comprises a base layer which comprises a woven fabric of interwoven machine and cross-machine direction textile yarns (14,16) and a top layer (24) coated on the base layer, said coating comprising a resilient, water-resistant, elastomeric, synthetic polymeric resin bonded to the base layer, characterized in that the resin coating (20) is non-cellular and has homogenously distributed voids and is water permeable by virtue of a plurality of randomly oriented channels (26) penetrating the body of the coating top layer (24), said channels being produced by solvent-removable fibers (22) or particles removed with an appropriate solvent after curing the resin (20). The felt of claim 1, wherein said yarns (14,16) are monofilament yarns. A method of manufacturing a wet-press papermakers felt fabric, which comprises: a) providing a woven textile fabric of a wet-press fabric, b) coating said fabric with a curable composition which comprises a synthetic polymeric resin and a solvent-removable material, said resin being solvent-resistant upon curing, c) curing the coating, and d) dissolving the solvent removable material whereby channels are formed which penetrate the body of the coating. The method of claim 3 wherein the solvent-removable material is in the form of fibers. The method of claim 3 wherein the solvent-removable material is a granular or particulate form of a chemical compound. A method of manufacturing a wet-press papermakers felt fabric which comprises: a) providing a woven textile fabric base, b) arranging a tangle of mainly continuous solvent-removable yarns or filaments on the surface of said base, c) applying a synthetic polymeric resin so as to penetrate the tangle and into a portion of said base structure, said resin being solvent-resistant upon curing, d) curing the resin, and e) dissolving the solvent-removable yarns or filaments whereby channels are formed which penetrate the coating formed by said resin. The method of claim 6, wherein the solvent-removable yarns or fibers are included in the base fabric in such a way that they project like pile or tufts.

Inventors: DUTT, WILLIAM H.
Applicants: ALBANY INTERNATIONAL CORP.
Application year: 1986
Main classification: D21F7
IPC codes: D21F1, D21F7