Papermachine clothing yarn with soluble core

Abstract: A composite yarn 20 for use in a papermaker's fabric 10, for the press section of a papermaking or similar machine, is disclosed. The composite yarn 20 includes a core 24 substantially surrounded by a layer of monofilaments 26. The core is at least partially soluble. On the other hand, the monofilament layer is non-soluble and is either braided, knitted, or helically wound around the core. After the base fabric, woven from the composite yarns, has been needled, the soluble portion of the core is washed out, supplying an increase in void volume to the finished fabric.

Description: The invention relates to a composite yarn according to the preamble of claim 1 and as known, for example, from EP-A-303 798. It more specifically relates to the use of special composite yarns having soluble cores in the weaving of these fabrics in order to provide them with increased void volume and longitudinal extensibility and lower mark tendency. The press fabrics used to clothe the press sections of papermaking machines are crucial components in the paper manufacturing process. One of their functions is to support and carry the paper product being manufactured through the presses which act by means of compression to force or squeeze water from the wet paper sheet. In this respect, the fabric serves as a conveyor belt during the manufacturing process. The press fabrics also serve the function of finishing the surface of the paper sheet. That is, the surface of the press fabric is designed to be smooth and uniformly resilient so that, in the course of passing through the presses, a smooth, mark-free surface is imparted to the paper. Perhaps most importantly, the press fabrics accept the large quantities of water pressed from the wet paper. In order to fulfil this function, there literally must be somewhere for the water to go within the body of the fabric. Accordingly, a successful press fabric design provides both a certain amount of void volume, for the temporary storage of water, and channels or pathways between the strands of yarn from which it is woven. These will allow water to pass through the fabric from the sheet-carrying side to the other side and, in the vicinity of the press nip, to flow longitudinally through the fabric away from the nip. Contemporary press fabrics are available in a wide variety of styles designed to meet the requirements of the papermachines on which they are installed for the paper grades being manufactured thereon. Generally, they comprise a woven base fabric into which has been needled a batt of fine, nonwoven, fibrous material. The base fabrics can be woven from monofilament, plied monofilament, multifilament, and like yarns and can be single-layered or multi-layered. Typically, the spaces between machine direction (MD) and cross-machine direction (CD) yarns are intended to provide the requisite channels for water to flow in the plane of the fabric, and perpendicularly through the fabric, as well as to make up void volume for the temporary storage of water. After the base fabric has been woven, the batt is needled into the fabric structure. This provides the finished fabric with a smooth, even surface very much like that of the original press felts woven from wool. As a consequence of the needling process, some batt fibers extend perpendicularly through the plane of the fabric. These fibers, and the voids formed between them, will generally allow water, pressed from the wet paper sheet in the press nip, to pass through the body of the fabric and away from the paper. An undesirable effect of the needling process is the filling of much of the void volume within the base fabric. This makes the finished press fabric to some extent less capable of fulfilling the functions for which it has been designed. In addition, this reduced void volume will fill more quickly with trapped wood fiber and fine particles, both of which will make the fabric less permeable to water. As a consequence, press fabric life on the papermachine will be shorter than desirable. European Patent Publication No. 0 303 798 discloses a web of material, especially a papermaking machine felt, comprising a carrier material and fibers secured thereto. The fibers are stable under normal operating conditions but at least some of the fibers consist at least partly of a substance that is soluble in a particular solution in which the carrier material is stable. By applying such a solution, the fibers may be at least partially dissolved out so as to restore the original permeability of the felt, the dissolution being restricted to the part of the material web which is not load-bearing. The present invention provides a solution to the above-mentioned problem by permitting the re-introduction of void volume after the needling process has been completed by removing soluble material which forms at least part of the core of the composite yarns used in the weaving of the base fabric. The present invention provides a composite yarn for use in a press fabric for a papermaking or similar machine, comprising a core and an outer layer that substantially surrounds the core, characterised in that the outer layer is a monofilament layer and in that the core comprises at least one strand of a material that is substantially soluble in a solvent in which the outer monofilament layer is substantially insoluble so that the said strand(s) may be dissolved by the said solvent into solution without the outer layer also being removed by the solvent. It will be appreciated that any combination of materials may be used for the core and outer layer, respectively, provided that the materials differ substantially in their solubility in a particular solvent, which solvent will preferably be a readily available solvent that is not liable to damage the fabric and will usually be an organic solvent or an aqueous solvent, preferably an aqueous solvent having a pH value of not less than pH 4 and not more than pH 9. The monofilament layer may comprise non-soluble monofilament strands that are braided about said core, or that are knitted about said core, or may comprise at least one non-soluble monofilament strand that is helically wound around said core. The core may comprise a monofilament strand of a soluble material, or a plurality of monofilament strands, at least one of said monofilament strands being of a soluble material. Moreover, the core may comprise a plurality of multifilament strands of a soluble material, or may comprise a plurality of multifilament strands at least one of said multifilament strands being of a soluble material. The soluble material may be dissolved in water, preferably being selected from a group consisting of polyethylene oxide, polyvinyl alcohol, calcium alginate, soft acrylic, and hydroxymethyl cellulose. The soluble material may also be dissolved in a weak acidic solution having a pH in the range from 4.0 to 7.0, a preferred material being hydroxymethyl cellulose. Another suitable soluble material is one that may be dissolved in a weak alkaline solution having a pH in the range from 7.0 to 9.0; preferably, such a soluble material is selected from a group consisting of methylcellulose and a 65% methylmethacrylate/35% methacrylate copolymer. A further suitable soluble material is one that may be dissolved in an organic solvent, an especially preferred material being alkyl starch. The invention also provides a papermaking fabric for use in the press section of a papermaking or similar machine, the fabric being woven from yarns in longitudinal, or machine directions and transverse, or cross-machine, directions, wherein a yarn in one of the said directions comprises a composite yarn as hereinbefore described. The invention further provides a method of making a papermaking fabric for use in the press section of a papermaking machine comprising the steps of: (i) weaving a base fabric from a machine direction and/or a cross-machine direction yarn comprising a composite yarn as hereinbefore described; and (ii) needling a batt of fibrous material into the fabric structure. The fabric may then be treated or "washed" with an appropriate solvent to dissolve the soluble material of the core of the composite yarns so as to increase the void volume of the fabric. Preferably, all the washing is carried out after needling so as to provide the maximum void volume in the papermaker's fabric (i.e. corresponding to the volume of the solvent-removable yarn component). If the order were reversed, and the fabric washed before needling, the fabric would be completely filled during the needling operation, and no advantage would be gained from the use of the yarns of the present invention. The washing step is generally carried out by the fabric manufacturer as a final finishing step, but could be carried out after the fabric has been installed on the papermachine. Indeed, the latter might be preferred where the fabric is of the on-machine seamable (OMS) type, because good loop formation and integrity in the machine-direction yarns are important in such fabrics, and would be provided where the yarns are bulky and relatively stiff. Once the fabric is seamed on the machine, the solvent-removable portion of the yarn core can be removed. As the solvents are typically water, or weak acids or bases, this is not an especially hazardous operation. Where the washing step is performed by the fabric manufacturer, the solvent-removable material may, for example, be removed in a large-scale washing machine during the final cleaning of the fabric prior to shipment to a paper manufacturer. In accordance with the present invention, one includes in the weave of a papermaker's fabric, particularly one designed for use on the press section of a papermachine, a composite papermachine clothing (PMC) yarn which includes a core surrounded by an outer layer. The core may include monofilament, multifilament, or any combination thereof, yarns. The distinguishing feature of the present invention is that the core of the composite yarns includes soluble yarns, which are removed through the use of an appropriate solvent after the weaving of the papermaker's fabric. In contrast, the outer layer surrounding the core of the composite yarns is non-soluble, and may include yarns which are knitted, braided, or helically wound around the core. Accordingly, the present invention is a composite PMC yarn, which includes both soluble and non-soluble components, for use in weaving the base fabrics for papermachine clothing. In particular, the soluble components of the composite PMC yarns are found in the cores thereof, while the outer layers of the composite yarns are non-soluble. The cores may include both soluble and non-soluble components, or may be completely soluble. The reason for the soluble core is that, following the weaving process, the batt of fibrous material is needled into the structure of the fabric. As noted above, much of the void volume of the base fabric will be taken up by these fibers. However, void volume equal to that occupied by the soluble component of the core of the composite yarn is recovered by washing out or dissolving the soluble component. In this manner, the use of a soluble material allows one to ensure that the final fabric will have sufficient void volume to enable it to efficiently perform its function on the papermachine in spite of the tendency of the needling process to restrict void volume. The PMC yarns for the present invention have several different embodiments to be described further in the discussion to follow. Regardless of the specific embodiment however, these PMC yarns provide several distinct advantages associated with their structure. Once the soluble material has been washed out of the finished fabric, there will be provided channels, through the cores of the yarns, lying in the plane of the fabric and oriented in the longitudinal and transverse directions. This provides pathways for the flow of water longitudinally and transversely within the fabric when it is in the vicinity of the nip. When a given length of the press fabric, carrying a wet paper sheet, approaches a press nip, the water within the fabric will suddenly experience a great increase in pressure. In response to this increased pressure, it will flow from the sheet to and into the press fabric. The channels provided in the fabric through the yarn cores after the removal of the soluble component thereof allow the water to freely do so, and spare the wet paper sheet from potential damage, such as crushing. In a similar fashion, when a given length of the press fabric passes through the nip, water in the fabric is forced by the increased pressure in the direction of fabric motion through the same channels. A press fabric and a number of different types of composite yarns for use in such a fabric, all constructed in accordance with the present invention, will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of the press fabric; Figures 2a and 2b are cross-sectional views taken along line 2-2 of Figure 1 and being to a larger scale than Figure 1, and show, respectively, the fabric before and after the soluble core material of the composite PMC yarns has been washed out; Figures 3a and 3b show, respectively, two types of composite PMC yarns; and Figures 4a and 4b show, respectively, two further types of composite PMC yarns. With reference to Figure 1 of the accompanying drawings, a press fabric 10 is of the variety that is closed into endless form by means of a seam 12 when said press fabric 10 is being installed on a papermachine. However, the bases for press fabrics can also be woven in endless form. When such is the case, the press fabric 10 will neither have nor require a seam 12. Thus, PMC yarns constructed in accordance with the present invention can be used to weave the bases for press fabrics of either the seamed or "woven endless" varieties. As shown in Figure 1, the press fabric 10 has an outer surface 14 and an inner surface 16. The outer surface 14 is that which actually comes into contact with the wet fibrous paper sheet being processed. The inner surface 16, on the other hand, contacts machine components, such as press rolls and fabric support rolls, as the machine operates. During paper production, some water is pressed into and through the press fabric 10 from the outer surface 14, where it is pressed from the wet fibrous sheet, to the inner surface 16, and through that surface to a vented press roll. Another quantity of water is temporarily stored in the voids of the fabric structure. This latter quantity of water is usually removed by a device called a suction box. For the sake of completeness, the machine direction and the cross-machine direction are indicated in Figure 1 by the labels "MD" and "CD" respectively. By convention, these designations refer to the corresponding directions of the papermachine on which the press fabric 10 is installed. Figures 2a and 2b represent, on a greatly enlarged scale, the section view of the press fabric 10 taken as indicated in Figure 1. According to Figures 2a and 2b, the views shown are taken in the cross-machine direction. As such, the observer of both figures sees the MD yarns 20 of the press fabric 10 in cross section. CD yarns, while generally present in the press fabric 10, are not shown in the figures for the sake of clarity. In both Figures 2a and 2b, a batt 22 of fibrous material is shown as it would appear after being needled into the base of the press fabric 10 according to techniques conventionally applied in the industry. As can be most clearly seen in Figure 2a, the batt 22 occupies the space between the MD yarns 20 and the CD yarns (not shown), severely limiting the void volume there that is available for water. Further, the reduction of available void volume increases the likelihood that wood fiber and other fine particles from the sheets being processed into paper will become trapped therein and unduly restrict the permeability of the press fabric 10 which would shorten the useful life of the press fabric 10 if it were not for the provision of additional void volume in accordance with the invention. Turning particularly now to Figure 2a, the MD yarns 20, seen in cross section, are composite PMC yarns. In these yarns, a monofilament core 24 of soluble material, such as polyox (polyethylene oxide), which is soluble in water, is surrounded by a braided or knitted layer of non-soluble monofilaments 26. The view shown in Figure 2a represents the condition of the press fabric 10 before the washing, which will remove the monofilament core 24 of soluble material, has been performed. After the washing of the press fabric 10, it will assume the appearance represented by Figure 2b, which differs from Figure 2a by the absence of the monofilament cores 24 of soluble material in each of the MD yarns 20. The removal of this material in the washing process restores additional void volume to the interior of the press fabric 10 for the storage of water. Further, in the embodiment shown, the resulting tubular configuration of the MD yarns 20 provides longitudinal channels in the body of the press fabric 10 which can serve to conduct water away from the press nip in longitudinal directions to relieve water pressure build-up which can damage the still delicate wet paper sheets. Of additional benefit is the characteristic resilience of the now-hollow and tubular MD yarns 20. This enables the yarns to bear the compression within the nip, yet quickly spring back to original form when this pressure is removed. Several alternative types of PMC yarns will now be described in further detail. Figures 3a and 3b each show strands 28 of PMC yarn which include a layer 30 surrounding a core 32. In each of Figures 3a and 3b, the outer layer 30 is made of non-soluble monofilaments 34, intertwined by braiding or knitting around the core 32. In Figure 3a, the core 32 is a monofilament of a soluble material, such as polyox. In the washing process, this core 32 is completely removed. In Figure 3b, however, the core 32 has both soluble monofilaments 36 and non-soluble monofilaments 38. Following washing, the non-soluble monofilaments 38 will remain in the core 32 to provide added strength to the strand 28 as might be required by a particular papermachine fabric application. Alternatively, the core 32 may be a soluble multifilament or spun yarn, or may be a yarn of either of these types having both soluble and non-soluble filaments or fibers. Figures 4a and 4b show embodiments of a different variety. In each of these figures, the strand 40 is produced by twisting a non-soluble monofilament 42 around a core 44 in a helical or spiral fashion. The non-soluble monofilament 42 is not a natural spiral, however, since, if unravelled from around the core 44, it would be a straight monofilament once more. In other words, the non-soluble monofilament 42 does not naturally have the shape of a spiral or helix; rather it acquires such a shape by reason of its being twisted around the core 44. As was noted above in the discussion of Figures 3a and 3b, the core 44 in Figures 4a and 4b can be either single or composite. That is, in Figure 4a, the core 44 is a single strand of soluble monofilament. In Figure 4b, the core 44 includes both soluble monofilaments 46 and non-soluble monofilaments 48. As before, the embodiment of Figure 4b would be a PMC yarn of strength greater than that of the embodiment of Figure 4a, after the removal of the soluble component of the core 44. Also as before, the core 44 may be a soluble multifilament or spun yarn, or may be a yarn of either of these types having both soluble and non-soluble filaments or fibers. In general, the benefits of the PMC yarns of the present invention are that they provide increased void volume within the press fabrics in which they are incorporated. The yarns also have greater longitudinal extensibility by reason of their twisted, braided, or knitted, rather than straight, orientations. Finally, their use permits the production of press fabrics having a reduced tendency to mark the paper sheet being produced. This is because the yarns have a greater ability to flatten in response to compression perpendicular to the plane of the fabric to provide a smooth, even surface and a greater ability to spring back to original shape when the compression is removed. For example, use of a large number of these yarns in the top layer of a two- or three-layered base fabric will reduce base mark in the paper sheet. Since the yarn will "flatten" to some degree under pressure, its dimensional cross section presented to the paper sheet under compression in the press nip will be greater. Consequently, the applied pressure will be more uniformly distributed to the paper sheet. With proper selection of materials, one can maintain this property for an extended period of the fabric life. On the other hand, yarns could be designed to rapidly compact and produce a pseudo single-layered base for use in seamed press fabrics for board making positions. When one wraps or twists a soluble monofilament with another yarn, the yarn will appear to have a slight coil spring appearance in the base after the fabric is finished. This spring provides several running benefits to the fabric, such as an increased extensibility for multi-nip positions, better energy absorption in the nip due to a more resilient base, and the previously noted increase in void volume and water handling capability. With reference to the dimensions of the composite yarns, the diameters of the individual monofilament components thereof may be anywhere in the range from 0.10 mm to 0.40 mm, but preferably lie in the range from 0.10 mm to 0.20 mm. The diameters of the composite yarns would depend upon the number of strands used, as well as upon the "tightness" of the braid, knit, or helix. A representative diameter would be 0.70 mm, but could take on greater values, depending on the openness, ability to flatten, and resiliency of the composite yarns. The soluble component of the core of the yarns may be produced from a variety of materials. Polyox (polyethylene oxide), polyvinyl alcohol, and calcium alginate can be used to produce yarns which are soluble in water. In addition, "soft" acrylics, which are very soluble in warm water, are also quite suitable. With reference to the use of acrylics for this purpose, an acrylic film can be cast, dried, cured, and slit into narrow widths to form a monofilament yarn in dimensions similar to those of extruded monofilaments. The "monofilaments" produced in this manner may then be used as the soluble component of the composite PMC yarns. An example of a suitable acrylic is one of any of the family of Hycar resins supplied by B. F. Goodrich. Other substances from which the soluble component of the core of the yarns may be made are those which are soluble in a weak acid solution, having a pH in the range from 4.0 to 7.0, or those which are soluble in a weak basic or alkaline solution, having a pH in the range from 7.0 to 9.0. Other substances, soluble in organic solvent, may also be used. Examples of these materials include alkyl starch, which is soluble in organic solvents, such as alcohols or acetic acids. Another substance is methocel, or methylcellulose, which is soluble in alkaline environments. Still another example is hydroxymethyl cellulose, which is a water-soluble material, whose solubility is greatly enhanced in weak acidic solutions. Finally, a 65% methylmethacrylate/35% methacrylate copolymer is soluble in sodium hydroxide, an alkaline solution. All of these materials may be made into "monofilaments" by the "film" method described above for doing so for acrylics.

Claims: A composite yarn (28, 40) for use in a press fabric (10) for a papermaking or similar machine, comprising a core (32, 44) and an outer layer (30, 42) that substantially surrounds the core (32, 44), characterised in that the outer layer (30, 42) is a monofilament (34, 42) layer and in that the core (32, 44) comprises at least one strand (36, 46) of a material that is substantially soluble in a solvent in which the outer monofilament layer (30, 42) is substantially insoluble so that the said strand(s) (36, 46) may be dissolved by the said solvent into solution without the outer layer (30, 42) also being removed by the solvent. A composite yarn (28) as claimed in claim 1, wherein the monofilament layer (30) comprises non-soluble monofilament strands (34) that are braided or knitted about said core (32). A composite yarn (40) as claimed in claim 1, wherein said monofilament layer comprises at least one non-soluble monofilament strand (42) substantially helically wound around said core (44). A composite yarn (28, 40) as claimed in any one of claims 1 to 3, wherein said core (32, 44) comprises a monofilament strand of a soluble material, or comprises a plurality of monofilament strands (36, 38, 46, 48), at least one (36, 46) of which is a soluble material. A composite yarn (28, 42) as claimed in any one of claims 1 to 3, wherein said core comprises a plurality of multifilament strands, at least one of which is a soluble material, or comprises a plurality of multifilament strands of a soluble material. A composite yarn (28, 40) as claimed in any one of claims 1 to 5, wherein said soluble material is dissolvable in water, and is preferably selected from a group consisting of polyethylene oxide, polyvinyl alcohol, calcium alginate, soft acrylic, and hydroxymethyl cellulose. A composite yarn (28, 40) as claimed in any one of claims 1 to 5, wherein said soluble material is dissolvable in a weak acidic solution having a pH in the range from 4.0 to 7.0, and is preferably hydroxymethyl cellulose. A composite yarn (28, 40) as claimed in any one of claims 1 to 5, wherein said soluble material is dissolvable in a weak alkaline solution having a pH in the range from 7.0 to 9.0, and is preferably selected from a group consisting of methylcellulose and a 65% methylmethacrylate/35% methacrylate copolymer. A composite yarn (28, 40)as claimed in any one of claims 1 to 5, wherein said soluble material is dissolvable in an organic solvent and is preferably alkyl starch. A papermaking fabric (10) for use in the press section of a papermaking or similar machine, the fabric (10) being woven from yarns (20) in longitudinal, or machine directions and transverse, or cross-machine, directions, wherein a yarn (20) in one of the said directions comprises a composite yarn (28, 42) as claimed in any one of claims 1 - 9. A method of making a papermaking fabric (10) for use in the press section of a papermaking machine comprising the steps of: (i) weaving a base fabric from a machine-direction and/or a cross-machine direction yarn (20) comprising a composite yarn (28, 42) as claimed in any one of claims 1 to 9; and (ii) needling a batt (22) of fibrous material into the fabric structure. A method as claimed in claim 11, further comprising the step of treating the fabric (10) with a solvent to dissolve the soluble material of the core (32, 44) of the composite yarns (28, 42) so as to increase the void volume of the fabric (10). The use of a composite yarn (28, 40) as claimed in any one of claims 1 to 9 for the manufacture of a papermaking fabric (10) for use in the press section of a papermaking or similar machine.

Inventors: HOOD PAUL F, HOOD, PAUL F.
Applicants: ALBANY INT CORP, ALBANY INTERNATIONAL CORP.
Application year: 1990
Main classification: D02G3
IPC codes: D02G3