Abstract:
A through-air-drying (TAD) fabric for producing tissue paper and related products on a papermaking machine comprising a plurality of warp yarns interwoven with a plurality of weft yarns to produce pockets on a paper-side surface of the fabric. The weft yarns have a substantially triangular cross-section and are oriented with their flat surface facing a machine side surface of the fabric. The points interlacing with the warp as they pass over and under the weft yarns produce an increased pocket depth and volume in the TAD fabric.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the papermaking arts. More specifically, the present invention relates to through-air-drying (TAD) fabrics used in the manufacture of bulk tissue and towel, and of nonwoven articles and fabrics. 
         [0003]    2. Description of the Prior Art 
         [0004]    Soft, absorbent disposable paper products, such as facial tissue, bath tissue and paper toweling, are a pervasive feature of contemporary life in modern industrialized societies. While there are numerous methods for manufacturing such products, in general terms, their manufacture begins with the formation of a cellulosic fibrous web in the forming section of a paper machine. The cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric. 
         [0005]    The cellulosic fibrous web is then transferred to a through-air-drying (TAD) fabric or belt by means of an air flow, brought about by vacuum or suction, which deflects the web and forces it to conform, at least in part, to the topography of the TAD fabric or belt. Downstream from the transfer point, the web, carried on the TAD fabric or belt, passes through a through-air dryer, where a flow of heated air, directed against the web and through the TAD fabric or belt, dries the web to a desired degree. Finally, downstream from the through-air dryer, the web may be adhered to the surface of a Yankee dryer and imprinted thereon by the surface of the TAD fabric or belt, for further and complete drying. The fully dried web is then removed from the surface of the Yankee dryer with a doctor blade, which foreshortens or crepes the web and increases its bulk. The foreshortened web is then wound onto rolls for subsequent processing, including packaging into a form suitable for shipment to and purchase by consumers. 
         [0006]    As noted above, there are many methods for manufacturing bulk tissue products, and the foregoing description should be understood to be an outline of the general steps shared by some of the methods. For example, the use of a Yankee dryer is not always required, as, in a given situation, foreshortening may not be desired, or other means, such as “wet creping”, may have already been taken to foreshorten the web. 
         [0007]    It should be appreciated that TAD fabrics may take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it is dried. 
         [0008]    Those skilled in the art will appreciate that fabrics are created by weaving, and have a weave pattern which repeats in both the warp or machine direction (MD) and the weft or cross-machine direction (CD). Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam. It will also be appreciated that the resulting fabric must be uniform in appearance; that is, there are no abrupt changes in the weave pattern to result in undesirable characteristics in the formed paper sheet. In addition, any pattern marking imparted to the formed tissue will impact the characteristics of the paper. 
         [0009]    Contemporary papermaking fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric woven from monofilament and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts. 
         [0010]    The present application is concerned, at least in part, with the TAD fabrics or belts used on the through-air dryer of a bulk tissue machine although it may have other applications beyond this. However, the present application is primarily concerned with a TAD fabric. 
         [0011]    Such fabric may also have application in the forming section of a bulk tissue or towel machine to form cellulosic fibrous webs having discrete regions of relatively low basis weight in a continuous background of relatively high basis weight. Fabrics of this kind may also be used to manufacture nonwoven articles and fabrics, which have discrete regions in which the density of fibers is less than that in adjacent regions whereby the topography of the nonwoven article is changed, by processes such as hydroentanglement. 
         [0012]    The properties of absorbency, strength, softness, and aesthetic appearance are important for many products when used for their intended purpose, particularly when the fibrous cellulosic products are facial or toilet tissue, paper towels, sanitary napkins or diapers. 
         [0013]    Bulk, cross directional tensile, absorbency, and softness are particularly important characteristics when producing sheets of tissue, napkin, and towel paper. To produce a paper product having these characteristics, a fabric will often be constructed so that the top surface exhibits topographical variations. These topographical variations are often measured as plane differences between strands in the surface of the fabric. For example, a plane difference is typically measured as the difference in height between a raised weft or warp yarn strand or as the difference in height between MD knuckles and CD knuckles in the plane of the fabric&#39;s surface. Often, the fabric surface will exhibit pockets in which case plane differences may be measured as a pocket depth. 
         [0014]    Additionally, drying capability of an industrial fabric is very essential for its use in processes such as TAD. Typically, a standard TAD fabric design in the papermaking industry for making paper towel, which is a 5-shed, 3×2 weave pattern. This design exhibits higher sheet caliper and absorbency, which allows lower sheet basis weight. The other design that is typically used in toilet tissue production is a 5-shed, 4×1 weave pattern which has demonstrated to result in a higher sheet softness. Both designs have proven to be robust in the hot, humid, TAD environment with better sheet properties. Fabric designers realize that pocket depth formed by the weave pattern is also important so multilayer thicker fabrics have been tried. However, these multilayer designs pose some serious drawbacks, such as increased fabric water content as they generally carry more water, which results in higher drying time. The primary mechanism for producing low density high caliper tissue webs with the TAD process is the pocket depth of the fabric. Therefore, it is the pocket depth of the fabric that dictates the caliper of the tissue web. A close study of the designs discussed above showed that both warp and weft yarns are primarily responsible for the creation of the depth of the pocket, thus limiting sheet caliper generation. Particularly, in single layer designs, the weft yarns show better control of pocket depth than the warp yarns. It is therefore observed that changing the profile of the weft yarns to a triangle or substantially triangular shaped cross-section instead of the conventional round yarns results in an increase of pocket depth, leading to higher sheet caliper and other desirable sheet characteristics. 
         [0015]    The present invention provides an improved TAD fabric which exhibits favorable characteristics for the formation of tissue paper and related products. 
       SUMMARY OF THE INVENTION 
       [0016]    Accordingly, the present invention is a TAD fabric, although it may find application in the forming, pressing and drying sections of a paper machine. As such, it is a papermaker&#39;s fabric which comprises a plurality of warp yarns interwoven with a plurality of weft yarns. 
         [0017]    The present invention is preferably a TAD fabric comprising a plurality of warp yarns interwoven with a plurality of weft yarns to produce a paper-side surface pattern characterized by pockets of higher depth and volume for the same mesh and count. In the fabric according to the present invention, the weft yarns have a triangular cross-section or substantially triangular shaped cross-section and are oriented with their flat surface facing a machine side surface of the fabric. The points interlacing with the warp as they pass over and under the triangular shaped weft yarns produce increased pocket depth and volume in the TAD fabric. 
         [0018]    It is therefore an object of the present invention to increase pocket depth and pocket volume of an industrial fabric in order to improve sheet properties such as sheet caliper, bulk and absorbency in TAD or other sheet forming type processes that utilize a TAD or structured fabric to imprint a pattern into the sheet. 
         [0019]    It is another object of the present invention to increase the air permeability of the fabric and thus a more efficient operation. 
         [0020]    It is a further object of the present invention to improve sheet drying rate and therefore reduce energy consumption. 
         [0021]    It is yet another object of the present invention to improve the cleanability of the fabric. 
         [0022]    The present invention will now be described in more complete detail with frequent reference being made to the drawing figures, which are identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0023]    For a more complete understanding of the invention, reference is made to the following description and accompanying drawings, in which: 
           [0024]      FIG. 1A  shows a paper side view and a surface depth view highlighting the relative pocket sizes on the paper side surface of a preferred embodiment of the present invention. 
           [0025]      FIGS. 1B and 1C  show cross-sectional views of a fabric incorporating the teachings of the present invention; 
           [0026]      FIG. 1D  shows a cross-sectional view of a standard TAD fabric; and 
           [0027]      FIG. 2  shows a “house” shaped cross-section of a yarn. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    The present invention is preferably a TAD fabric having improved pocket depth and pocket volume on the paper side surface of the fabric. The pocket sizes are a function of the weave pattern, mesh count, and yarns used in the pattern. Pocket sizes can be characterized by an MD/CD dimension and/or by a pocket depth. The pockets are formed/bounded by weft yarns and warp yarns which are raised from the base plane of the fabric surface, produced by the weave pattern utilized. Pocket size and depth affect resultant sheet properties such as absorbency amongst others. 
         [0029]      FIG. 1A  shows a paper side view and a surface depth view highlighting the relative pocket sizes on the paper side surface of a preferred embodiment of the present invention. As shown in  FIG. 1A  a fabric  50  according to this embodiment may be formed using weft yarns  20  having a triangular cross-section. While we refer to weft yarns as having a triangular cross-section in reality the cross-section would be that shown in  FIG. 1B . As can be seen therein the weft yarns  20  have a somewhat or substantially triangular cross-section with slightly rounded edges  22 . While an equilateral triangular shape is shown having sides  24 , other triangular shapes suitable for the purpose may also provide the desired results. In  FIG. 1A  the triangular weft yarns  20  are shown to run horizontally and the warp yarns  10  run vertically. Weft yarns  20  may be oriented within fabric  50  in a manner such that a flat surface or side  24  of the triangle is facing the machine side of fabric  50  and a pointed side of the triangle is facing the paper or surface side of fabric  50 , with the points interlacing with the warp yarns  10  as they pass over and under the triangular weft yarns  20  producing increased pocket depth.  FIG. 1C  also shows the warp yarn  10  contour for the fabric pattern according to this embodiment. Note as to warp yarns  10  they are shown having a circular cross-section. Other shaped cross-sections suitable for the purpose are possible. As seen in this contour, the fabric  50  has deeper pockets  30 ,  40 , which are correspondingly highlighted on the paper side surface of fabric  50 . It can be observed that the raised weft yarns  20  and raised warp yarns  10  indicated in the paper side surface of the fabric  50  form the pockets  30 ,  40  at points where they interweave with each other or points interlacing with the warp as they pass over and under the triangular weft yarns  20 , producing increased pocket depths. 
         [0030]    Orientation of the triangular weft yarns in this manner (flat surface facing the machine side) will also greatly change the bottleneck profile for both the 5-shed weave designs discussed in the background of the invention. This means, for a given mesh and count, the air permeability of the fabric will also increase. Therefore, by keeping the same mesh and count, the fabric according to the present invention will maintain its robustness in the hot, humid TAD environment, as well as result in increased sheet caliper and absorbency or softness, overcoming the drawbacks of the prior art. 
         [0031]    In this regard for point of comparison, there is shown in  FIG. 1D  a cross-sectional view of a standard TAD fabric woven in the same weave pattern as that shown in  FIG. 1B  with, however, using yarns having circular cross-section yarns. The weft yarns have been designated  20 ′ and the warp yarns designated  10 ′. If one compares the pocket areas formed on  FIG. 1D  at  30 ′ and  40 ′ to the pockets  30  and  40  in  FIG. 1B  one can see that the pockets created are larger in the latter due to the substantially triangular shaped cross-section yarns. This can be seen, for example, in the open area between adjacent yarns which has been designated “X” in  FIG. 1B  and “Y” in  FIG. 1D . Accordingly for the same linear density of yarns, larger pockets are formed in the fabric shown in  FIG. 1B  with the attendant advantages. 
         [0032]    Note the fabric according to the present invention may be formed using any weave pattern, such as for example, plain, twill, sheet surface having floats weft or warp dominant or combinations thereof. The present invention is intended to cover other fabric patterns having different sizes and shapes of pockets, different pocket depths, and different yarn contours. Accordingly, the present invention should not be construed as being limited to the preferred embodiment disclosed above. 
         [0033]    The fabric according to the present invention preferably comprises only monofilament yarns, preferably of polyester, nylon, polyamide, or other polymers. Any combination of polymers for any of the yarns can be used as will be appreciated by one of ordinary skill in the art. The CD yarns of the fabric may have a triangular cross-sectional yarns of different sizes and may alternate with yarns having different non-triangular cross-sections such as circular or other shapes. Such alternation can be single or in pairs or other combinations of yarns in even or odd numbers in a manner suitable for the purpose Similarly, the MD yarns may have a circular cross-section with one or more different diameters. Further, in addition to triangular and circular cross-sectional shapes, other shapes are envisioned such as the “house” shaped yarn  60  shown in  FIG. 2 . Moreover some of the yarns, including the MD yarns may have other cross-sectional shapes such as a rectangular cross-sectional shape or a non-round cross-sectional shape such as triangular or substantially triangular. 
         [0034]    Modifications to the above would be obvious to those of ordinary skill in the art, but would not bring the invention so modified beyond the scope of the present invention. The claims to follow should be construed to cover such situations.