A PAPERMAKERS' FABRIC

Abstract: A papermakers' fabric, having a repeating pattern of floats on its paper support surface is provided with surface floater yarns (2,4,etc.) interspaced between adjacent yarns (1,3,etc.) to improve sheet support characteristics of the fabric. The floater yarns (2,4 etc.) are characterized by lack of interlacing with yarns (a,b,etc.) transverse thereto. The floater yarns (2,4, etc.) are "trapped" beneath the surface floats and are of a diameter generally less than the diameter of the adjacent yarns.

Description: The invention relates to woven papermakers' fabrics according to the preamble either of claim 1 or claim 2. In the conventional fourdrinier papermaking process, a water slurry or suspension of cellulose fibers, known as paper "stock" is fed onto the top of the upper run of a traveling endless belt. The belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the wet paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for drainage of the aqueous medium through its mesh openings, also known as drainage holes. In the conventional fourdrinier machine, the forming fabric also serves as a drive belt. Accordingly, the machine direction yarns are subjected to considerable tensile stress, and, for this reason, are sometimes referred to as the load-bearing yarns. Effective sheet support and lack of wire marking are important considerations in papermaking, especially in the formation of the wet web. The problem of wire marking is particularly acute in the formation of fine paper grades where the smoothness of the sheet side surface of the forming fabric is critical as it affects paper properties such as sheet mark, porosity, seethrough, pinholing and the like. Accordingly, paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing and like grades of fine paper have heretofore been formed on very fine woven forming fabrics or fine wire mesh forming fabrics. Such forming fabrics, however, are delicate, lack stability in the machine and cross machine directions, and are characterized by relatively short service life. Prior art workers have attempted to use somewhat coarser and stronger fabrics, taking steps to increase surface smoothness by various methods such as reduction in the amplitude of sheet side knuckles through sanding or calendering (e.g., U.S. 4,239,065), the use of flat machine direction yarns and the equalization of machine direction and cross-machine direction knuckle amplitude. There are further known a number of different approaches to improvement of sheet support. Fabrics are frequently inverted to take advantage of the fiber support orientation of the cross-machine direction (CMD) yarns. Sheet forming on the CMD yarns does not directly block the smallest of the drainage holes, those which exist between the machine direction (MD) yarns, and therefore, the fabric drains better and performance improves. Unfortunately, the CMD yarns are the most widely spaced yarns, and wire marking increases.In an attempt to improve sheet support yet avoid excessive wire marking, one prior approach has been to increase the picks or ends in the conventional weave patterns to improve sheet support. This approach, however, results in the reduction in the rate of drainage and fabric performance. Another approach has been the use of a duplex type fabric in order to maintain drainage capability. This latter approach has a disadvantage in that the thicker duplex fabric is less effective in its hydraulic performance and that less than half the yarns are on the surface for wear or sheet support. U.S. 4,182,381 discloses the provision of additional weft yarns, described as "floating", at the wear surface and further suggests that such additional weft yarns might be provided to advantage at the paper side of a dryer fabric. However, the yarns described as "floating" in U.S. 4,182,381 are interlaced by warp in a manner tending to force those yarns to the center of the fabric and, to the extent that the "floating" yarn is forced toward the center of the fabric, the fabric surface is rendered uneven and less suitable for use as a forming fabric. Specifically, with reference to Fig. 3 of U.S. 4,182,281 it is seen that warp No. 1 passes over "floating" weft No. 3 and immediately turns toward the opposite surface between wefts 3 and 4. Thus, warp No. 1 may be characterized as interlaced with weft 3. Likewise, warp 6 is interlaced with weft 4. These interlacings tend to force the weft toward the fabric center. papermakers' single layer fabric, comprising warp and weft yarns interwoven together to define the central plane of said fabric and to provide a repeating pattern of warp or weft floats, respectively, at the paper support surface, and additional sheet supporting floater yarns located between and parallel to said weft or warp yarns, respectively, wherein the said fabric is free of any interlacing between the said additional floater yarns and adjacent parallel yarns in that none of the yarns extending perpendicularly over the said additional floater yarns at the paper support surface passes between the said additional floater yarns and the adjacent parallel yarns, and in that none of the yarns extending perpendicularly to the said additional floater yarns is crimped around the said additional floater yarns. A similar weave pattern for a duplex paper maker's fabric is disclosed. With the aforementioned fabric each of the additional floater yarns have the same diameter as adjacent parallel yarns, because only then can the edge curling be avoided. To accomplish the same objective, i.e. preventing edge curl, US-A-4,281,688 discloses a reversible fabric with additional floater wear yarns which are always of the same diameter as the adjacent parallel yarns. Accordingly, it is the object of the invention to provide a papermakers' fabric, particularly a forming fabric, having both improved sheet support and sheet support surface smoothness. However, the present invention would also provide advantages in the conveying, press, and dryer sections. It is another object of the present invention to provide such a papermakers' fabric having excellent machine and cross-machine direction stability and long service life. These objects are solved by the characterizing features of either claim 1 or claim 2. This will be explained in more detail below: The present invention is based, in part, on a recognition that the performance of a fourdrinier papermaking machine improves when the sheet forms high on the sheet bearing surface of the forming fabric. Where the sheet forms high on the surface of the forming fabric, the sheet releases better, not being trapped within the web, and thus allows for higher machine speeds and higher paper machine efficiency. Additionally, when the sheet forms high on the fabric, wire mark is reduced, and drainage is improved. (See Kufferath, "Comparing Papermaking Wires by Drainage Performance", Pulp & Paper Canada, Vol. 80, No. 8, August 1979, pp. 72-78.) It has now been discovered that the objective of forming the paper web high upon the forming surface, with attendant improvement in sheet support and reduction of wire marking, can be achieved by providing floater surface yarns of relatively small diameter, which are free of interlacing and are arranged parallel to and interspaced between the conventional, larger diameter yarns. The terminology "free of interlacing", as used herein, has reference to the fact that no yarn passing over a given floater yarn passes between that floater and a yarn next adjacent and parallel to that floater. Thus, the floater yarns of the present invention truly float at the paper support surface in the sense that they are not urged toward the center of the fabric by any yarn passing thereover and directly down into the fabric as are all other (interlaced) weft and warp, yarns in the fabric. In the embodiment of the present invention the floater yarns are relatively small diameter yarns in the cross-machine direction (CMD) which are arranged parallel to and alternate with the larger diameter CMD yarns. In such an embodiment, the floater yarns bridge the holes formed by the machine direction (MD) yarns and are "trapped" within the surface of the fabric between the points where the MD yarns cross between adjacent CMD yarns and MD yarn surface floats which pass over the same two adjacent CMD yarns. The embodiments having CMD floater yarns provide one surface floater yarn for each CMD yarn in a monoplanar fabric or one surface floater for each adjacent yarn in the surface in a multiplex fabric. In the embodiment referred to above, the entire lengths of the floater yarns are located in and serve to define a continuous planar surface above and parallel to the central plane of the monoplanar fabric and below and parallel to a plane defined by the surface floats. Although less preferred, for reasons of economy, the present invention also provides a papermakers' multilayer fabric wherein parallel weft yarns define the central plane of the upper layer and the floater yams are located in and define the plane of a paper support surface located above and parallel to the central plane of a paper support surface just below the level of the surface floats. In both the monoplanar and multilayer versions, the CMD floater yarns are substantially uncrimped and their entire lengths run continuously through a single plane of the fabric. In both versions, the floater yarns are trapped between (1) the points in the central plane of the monoplanar fabric or the central plane of the upper layer of a multiplex fabric where the MD yarns cross, i.e., the plane passing the centers of the adjacent larger diameter CMD yarns, and (2) MD yarn surface floats. The fabric with floater yarns may be woven endless (= weft) or flat (= warp). A flat weave is preferred from the viewpoint of maintaining loom productivity, but the time required for seaming is increased in proportion to the number of floater warp yarns employed. On the other hand an endless weave eliminates the tedious process of seam formation but also reduces loom productivity by increasing the number of picks (weft) required for a given size fabric. Of course CMD floater yarns do not contribute to stretch resistance but they do offer significant advantages in that (1) an endless weave may be formed without a sacrifice of loom productivity and (2) a further increase in sheet support is provided. With regards to the latter advantage, a CMD surface yarn is considered the equivalent of approximately two MD surface yarns of like diameter in terms of sheet support. Thus, from the viewpoint of sheet support alone, those fabrics having CMD floater yarns represent the preferred embodiments of the present invention. In the preferred embodiments having CMD floater yarns, the fabrics are preferably multilayer to enhance stretch resistance. In these CMD embodiments an endless weave is preferred (CMD = warp) from the viewpoint of loom productivity. Preferred ways of carrying out the invention are described in detail below with reference to drawings in which: Fig. 1 is a cross-machine direction sectional view of a conventional 2/2 twill papermakers' fabric, modified by inclusion of CMD surface floater yarns; Fig. 2 is a plan view of the sheet support surface of a conventional 2/3 twill papermarkers' fabric, modified by inclusion of CMD surface floater yarns; Fig. 3 is a topographical plan view of a conventional multilayer papermakers' fabric, modified by inclusion of CMD surface floater yarns; Fig. 3A is a sectional view taken along line A-A in Fig. 3; and Fig. 3B is a sectional view taken along line B-B in Fig. 3. At the outset, the present invention may be described as a papermakers' fabric characterized by the presence of a repeating pattern of floats on its paper support surface, MD yarns interwoven with the CMD yarns and floater yarns interspaced between adjacent CMD yarns, the floater yarns being characterized by a lack of interlacings with the yarns transverse thereof. The floater yarns are preferably of a substantially smaller diameter than the diameter of the interwoven parallel yarns. In the CMD floater embodiments each of the smaller diameter, paper-supporting yarns of the fabrics of the present invention is essentially uncrimped. Further, while each yarn in the fabric transverse of the floater yarns forms float over a number of the floater yarns, no transverse yarn (MD yarn) is crimped around a floater yarn or interlaced with a floater yarn in a manner tending to pull it toward the center of the fabric. Where the floater yarns are CMD yarns, the entire lengths of the floater yarns run essentially straight through a plane between a "central plane", i.e., a plane passing through the centers of the larger diameter CMD yarns which alternate with the floaters, and a plane defined by the MD surface floats. The function of these floater yarns is to bridge the afore-mentioned MD yarn holes and to support the paper web at the fabric surface. The term "surface", as used herein, has reference to the paper sheet support surface. The warp and weft yarns used in the present invention are preferably synthetic yarns of materials conventionally used in such fabrics, such as polyamides (nylon), polyesters (dacron), and acrylic fibers (orlon, dinel and acrilan), or copolymers (sacran). Preferred polyesters include Kevlar and Kevlar 29 which are trademarks of E. I. DuPont de Nemours & Company for synthetic fibers which comprise poly(paraphenylene terephthalamide). The warp and weft yarns may be in the form of monofilament, multifilament or staple yarns or plied or wrapped yarns. Low modulus highly extensible yarns may be used for the floater, if a CMD yarn, to further enhance sheet support. The diameter of the floater yarns employed in the fabrics of the present invention is preferably less than that of the interwoven parallel yarns with which the floater yarns alternate so that the floater yarns can occupy the interstices or spaces which naturally occur between adjacent yarns in a conventional papermakers' weave. Preferably, the diameter of the floater yarns should be substantially smaller than that of the interwoven parallel yarns, e.g. 80% or less than that of the interwoven parallel yarn. More preferably the diameter of the floater yarns is 50-75% that of the interwoven parallel yarns. The inventor has found that smaller yarns are weakened by repeated cycles of tensioning (at the top run of the belt) and untensioning (at the lower run) and are so mobile that the fabric becomes dimensionally unstable. Virtually any conventional papermakers' weave pattern, other than a plain weave, may be modified by the further inclusion of floater yarns in accordance with the present invention. Any weave pattern characterized by the presence of surface floats will provide a space for the floater yarns of the present invention between those floats and the points where those yarns providing the surface floats cross in the central plane of a monolayer fabric of the central plane of the upper layer of a multilayer fabric. The weaves depicted in the figures of the drawings illustrate the preferred weave patterns which include the monoplanar 1/2 twill, 2/2 twill, 4-harness satin and, especially preferred, the 2/3 twill. While two or more surface floaters could theoretically be provided for each pick, loom productivity dictates a 1:1 ratio of floater yarns to adjacent yarns. The same consideration dictates preference for a monoplanar fabric. Fig. 1 shows 2/2 twill in accordance with the present invention wherein the floater yarns 2, 4, 5 and 8 (of which only 8 and 2 are shown) and warp yarns 21, 23, 25 and 27 are CMD yarns. Yarns A, B, C and D are the MD yarns. When utilized as CMD yarns in this manner, the floater yarns provide maximum sheet support. Although described here as woven endless, as in the case of those embodiments with MD floater yarns, such a fabric may also be woven flat. Fig. 2 shows a 2/3 twill in accordance with the present invention wherein the floater yarns 2, 4, 6, 8 and 10 alternate with CMD yarns 21, 23, 25, 27 and 29. A, B, C, D and E designate MD yarns. Fig. 3 shows a multiplex (duplex) fabric in accordance with the present invention wherein a plurality of surface floater yarns 1A-7A are parallel to and alternate with a plurality of weft yarns 31-37 which define the upper layer of the fabric and which are interwoven with warp a-g to provide a repeating pattern of machine direction floats at the paper support surface. Fig. 3 shows a repeating pattern of weft floats 2 and 3 yarns in length and warp floats 2 yarns in length. The floater yarns have a diameter approximately 70% that of the upper layer weft yarns. It should be noted that, as in the previous embodiments, the floater yarns are not interlaced with any warp yarn passing thereover. The entire lengths of the floater yarns pass through a layer having a central plane which is above the central plane of the multilayer fabric and above the central plane of the upper weft layer. In Figs. 3A and 3B the weft yarns of the lower layer are shown as 1', 2', 3', etc.

Claims: A papermakers' single layer fabric, comprising warp (a, b, c, d, ...., A, B, C, D, ...) and weft yarns (1, 3, 5, 7, ..., 21, 23, 25, 27, ...) interwoven together to define the central plane of said fabric and to provide a repeating pattern of warp or weft floats, respectively, at the paper support surface, and additional sheet supporting floater yarns (2, 4, 6, 8, ...) located between and parallel to said weft (1, 3, 5, 7, ..., 21, 23, 25, 27, ...) or warp (a, b, c, d, ...., A, B, C, D, ...) yarns respectively, wherein the said fabric is free of any interlacing between the said additional floater yarns (2, 4, 6, 8, ...) and adjacent parallel yarns in that none of the yarns extending perpendicularly over the said additional floater yarns (2, 4, 6, 8, ...) at the paper support surface passes between the said additional floater yarns (2, 4, 6, 8, ...) and the adjacent parallel yarns, and in that none of the yarns extending perpendicularly to the said additional floater yarns is crimped around the said additional floater yarns, characterized in that the additional floater yarns (2, 4, 6, 8, ...) are CMD yarns, the diameter of which is substantially smaller than the diameter of the adjacent interwoven parallel yarns, so that the said additional floater yarns (2, 4, 6, 8, ...) are interspaced between adjacent parallel yarns and define a plane above and parallel to the central plane of the interwoven surface yarns just below the level of the said surface yarns. A papermakers' multilayer fabric comprising an upper layer of weft yarns (11, 13, ..., 31, 32, 33, 34, 36, 37,...) defining the central plane of said upper layer, warp yarns (a, b, c, d, ...) interwoven with said weft yarns to form a repeating pattern of warp or weft floats, respectively, at the paper support surfaces and additional cross-machine direction sheet supporting floater yarns (2', 4', ..., 1A, 2A, 3A, 4A, 5A, ...) located between and parallel to said weft (11, 13, ..., 31, 32, 33, 34, 36, 37,...) or warp (a, b, c, d, ...) yarns respectively wherein the said fabric is free of any interlacing between the said additional floater yarns (2', 4', 6', 8', ...1A, 2A, 3A, 4A, 5A, ...) and adjacent parallel yarns in that none of the yarns extending perpendicularly over the said additional floater yarns 2', 4', 6', 8', ...1A, 2A, 3A, 4A, 5A, ...) at the paper support surface passes between the said additional floater yarns (2', 4', 6', 8', ...1A, 2A, 3A, 4A, 5A, ...) and the adjacent parallel yarns, and in that none of the yarns extending perpendicularly to the said additional floater yarns is crimped around the said additional floater yarns, characterized in that the additional floater yarns (2', 4', 6', 8', ...1A, 2A, 3A, 4A, 5A, ...) are CMD yarns, the diameter of which is substantially smaller than the diameter of the adjacent interwoven parallel yarns, so that the said additional floater yarns (2', 4', 6', 8', ...1A, 2A, 3A, 4A, 5A, ...) are interspaced between adjacent parallel yarns and define a plane above and parallel to the central plane of the interwoven surface yarns just below the level of the said surface yarns. The fabric according to claim 1 or 2, characterized in that said additional floater yarns (2, 4, etc.) have a diameter 75 - 50 % that of the adjacent interwoven parallel yarns. The fabric according to any of the preceding claims 1 to 3, characterized in that either the weft yarns or the warp yarns are cross-machine yarns.

Inventors: THOMPSON, CHARLES E.
Applicants: HUYCK CORPORATION
Application year: 1983
Main classification: D03D1
IPC codes: D21F1, D03D11, D21F7, D03D25, D03D1