Patent Application
20120178330
The present invention relates to fabrics intended for use in industrial filtration processes, and is particularly concerned with papermakers forming fabrics which are used to drain and form a paper web in the forming section of papermaking machines. Although the invention is described below primarily in relation to papermakers fabrics, the invention is applicable to industrial filtration and similar processes which require fabrics having similar general properties to papermakers fabrics.
In modern high speed papermaking processes, a highly aqueous stock consisting of about 99% water and 1% papermaking solids is ejected at high speed and precision onto an endless moving forming fabric. A nascent web, which will be self coherent and consist of about 25% papermaking solids by the end of the forming section, is formed as the stock is drained through the fabric. This web is then transferred from the forming fabric into the press section where, together with at least one press fabric, it passes through one or more nips where additional fluid is removed by mechanical means. The web is then transferred into the dryer section of the papermaking machine where much of the remaining moisture is removed by evaporative means, the web being supported on one or more dryer fabrics as it is heated, for example by being passed in serpentine fashion over a series of heated rotating drums. The finished sheet is then reeled into large rolls at the end of the papermaking machine, and further finishing processes may be applied.
Forming fabrics are critical to the quality of the paper product that is ultimately produced on the papermaking machine. In simplest terms, these fabrics are designed to allow fluid from the stock to drain through the fabric in a controlled manner, while providing uniform support to the papermaking solids. The fabrics must also be very robust and dimensionally stable so as to survive the environmental forces to which they are exposed. In addition, the fabrics should be as thin as is possible, so as to minimize internal void volume and water carrying capacity. Considerable efforts have been made by various manufacturers of papermaking fabrics to decrease the thickness (or caliper) of their fabrics so as to minimize this interior void volume while, at the same time, maximizing fiber support.
The papermaking surfaces of modern forming fabrics are finely woven structures formed using very small diameter monofilament yarns in order to provide this requisite support for the papermaking components while allowing adequate fluid drainage. On its own, a fine woven structure would generally not be usable in a high speed papermaking process as it would lack sufficient mechanical stability and stiffness while in operation, thus causing problems such as fabric creasing and poor fabric guiding. It would also be difficult to provide a seam of sufficient strength to reliably join the fabric ends while in use on the machine; other mechanical issues, especially relating to wear, would also occur due to the small yarn size and fabric structure employed. By comparison, coarse mesh fabrics which employ relatively larger diameter yarns generally provide adequate stability and wear life while sacrificing good formation. Selection of an appropriate fabric design, mesh and yarn size by the fabric manufacturer for a given application usually represents a balance between desirable papermaking qualities (e.g. formation and drainage) and the structural properties of the fabric (e.g. stiffness and caliper).
To minimize this trade-off between sheet support and fabric stability, a variety of fabric structures have been developed over time. A comprehensive listing and description of these structures is provided by R. Danby and J. Perrault in Weaves of Papermaking Wires and Forming Fabrics, Pulp & Paper Technical Association of Canada [PAPTAC] Data Sheet G-18, Revised July 2009, a copy of which is incorporated here by reference. This Data Sheet G-18 lists the following forming fabric structures as those which are in current use:
Single layer designs—fabrics woven using one warp yarn system and one weft yarn system.
Semi Duplex or Extra Support Single Layer designs—fabrics woven using one warp yarn system and two weft yarn systems in which the weft yarns are not located directly over each other.
Double layer or Duplex—fabrics woven using one warp yarn system and two weft yarn systems in which the weft yarns of the two systems are usually vertically stacked directly over one another.
Extra Support Double Layer—double layer fabrics with additional weft yarns woven into one layer, usually the top papermaking surface.
Triple Weft—fabrics woven using one warp yarn system and three systems of weft yarns in which the weft are usually stacked vertically one over the other.
Standard Triple layer—fabrics woven using two warp yarn systems and two weft yarn systems to provide two independent fabric structures (top and bottom) that are stitched together during weaving, in the majority of cases using an extra weft yarn system.
Triple Layer Sheet Support Binder (SSB) or Intrinsic Weft or Paired Binders—fabrics woven using two warp and two weft (CD) yarn systems, in which a selected number of the weft yarns are woven into the fabric as interchanging pairs of intrinsic binder yarns. In these arrangements, when one yarn of the pair is being woven into a first fabric surface, the second yarn of the pair is being woven into the second fabric surface. These yarns then exchange positions within one repeat of the weave thereby providing an unbroken, continuous repeat of the weave in both surfaces, and tie the two surfaces together.
Triple Layer “Warp Tie”—fabrics woven using two weft yarn systems and two warp yarn systems in which at least a portion of the warp yarns are woven as interchanging pairs so that, as one yarn of the pair is woven into the first fabric surface, the other is woven into the second. In certain designs, some of the warp yarns of each of the two systems will be interwoven exclusively with weft yarns of one of either the first or second systems of weft yarns.
Triple Layer (WISS) Warp Integrated Sheet Support Binders—fabrics woven using two weft yarn systems and two warp (MD) yarn systems in which all (100%) of the warp yarns are woven as interchanging pairs, so that as one yarn of the pair is being woven into the first surface, the other yarn of the pair is woven into the second. In these fabrics, all of the warp yarns function to bind the surfaces together as well as to contribute to the woven structure of those surfaces.
The features of the present invention can advantageously be applied to each of the above described fabric structures, with the exception of single layer fabrics. The invention finds particular utility when applied in triple layer sheet support binder (SSB) fabrics, triple layer “warp tie” fabrics, and triple layer warp integrated sheet support (WISS) binder type fabrics.
A characteristic common to the fabric structures for which the present invention is applicable is that they include at least two layers or systems of weft yarns. This feature allows for each of the two fabric surfaces to be woven to differing fabric designs using differing materials. The fabric surfaces may be tied together using binder yarns which are part of the weave design in the manner described above; and if so, the binder yarns may be warp yarns or weft yarns. These fabrics are capable of providing high levels of fiber support and good mechanical stability and wear life.
As noted above, the forming fabric is installed on the papermaking machine as a continuous belt which is driven through the forming section at high speeds. Accordingly, the fabric must possess good mechanical stability, in particular cross-machine direction (CD) stability, in order to survive the rigors of the forming section environment. This problem has been recognized and addressed by various means in the past.
For example, one means of increasing CD fabric stability is to add additional weft yarns to the structure to create a triple weft fabric. Such fabrics are described in U.S. Pat. No. 4,379,735 (MacBean), U.S. Pat. No. 4,941,514 (Taipale), U.S. Pat. No. 5,164,249 (Tyler et al.), and U.S. Pat. No. 5,169,709 (Fleischer). Other similar structures are known and used. However, a problem associated with triple weft structures is that they are relatively thick, which increases fabric caliper and void volume. This increased thickness in comparison to other fabric designs adversely affects vacuum efficiency, and the water carried by these fabrics may also spot the sheet.
U.S. Pat. No. 6,902,652 (Martin) discloses a warp tie forming fabric with additional cross-machine direction (CD) packing yarns and paired intrinsic warp binder yarns. The CD packing yarns are additional weft yarns that are inserted between adjacent machine side (MS) weft yarns in the fabric weave. The packing yarns reduce the void volume on the machine side of the fabric without significantly disrupting the air permeability or increasing fabric caliper. The placement of the packing yarns also adds to the CD stability and seam strength of the fabric and reduces the lateral movement of the MS weft yarns.
U.S. Pat. No. 6,810,917 (Stone) discloses a forming fabric the PS and MS layers of which are interconnected by pairs of MS intrinsic weft binder yarns. Each of the binder yarn pair members in sequence interlaces with a portion of the MS warp yarns so as to complete an unbroken weft path in the MS weave pattern, and to provide an internal MS float. Each of the binder yarn pair members also interweaves with a PS warp yarn so as to bind the PS and MS layers together.
U.S. Pat. No. 7,637,291 (Boeck) discloses a forming fabric in which the MS layer is formed by binder weft yarns arranged as pairs; each pair is flanked by one non-binding MS weft yarn. WO 05/017254 (Hay et al.) discloses an intrinsic weft binder SSB type forming fabric having separate sets of MS and PS warp and weft interlaced by pairs of intrinsic weft binder yarns, the pair members forming a continuous PS weft path. One of the intrinsic weft binder pair members does not interlace with the MS warp and instead floats between the MS and PS layers before re-entering the PS layer to continue the PS weave pattern. In one embodiment, all of the PS weft yarns are intrinsic weft pair yarns including one “regular” binder pair member and one “binder top” weft pair member. The portion of the weft yarn path located between the MS and PS layers and formed by the binder top pair member is referred to as a “stiffening section” as it contributes to the CD stiffness of the fabric. U.S. Pat. No. 7,740,029 (Hodson et al.) discloses a papermakers fabric in which the weft yarns are arranged in groups of two or more and weave in adjacent side-by-side contact. There may be groups of weft yarns in between those weaving in side-by-side contact and these may have a differing weave path, or the paths may be the same.
It would therefore be advantageous to provide a forming fabric which offers the benefits of increased mechanical stability and CD stiffness in comparison with the known fabrics, without consequential disadvantages of undue increase in caliper or adverse effects on drainage or wear resistance, by improved weave patterns which are applicable as modifications to a wide variety of fabric structures.
As used herein, the term “complementary yarns” refers to two or more yarns, which for industrial fabrics in general may be warp yarns or weft yarns, and which are interwoven in a fabric so as to form a pattern equivalent to that followed by a single yarn in one repeat of the fabric weave. Such pairs of yarns only interweave with one layer of the fabric, and the term does not include yarns which function as binder yarns, i.e. which tie two fabric layers together by interweaving with yarns from both layers. Each member of a pair of complementary yarns alternates positions with the other member of that pair at exchange points as they interweave such that, as one yarn ceases interweaving with one layer and passes from that layer to be carried between the MS and PS layers, it is replaced by the other member of the pair which continues the weave pattern in that surface. The complementary yarns continue to exchange positions across the entire length or width of the pattern so as to form an unbroken yarn path in one surface of the fabric. In the fabrics of the present invention, the complementary yarns are pairs of weft yarns.
Further, as used herein, the term “float” refers to that portion of a component yarn which, in one repeat of the fabric weave, passes over or under a group of other yarns without interweaving with them.
The present invention is based on the discovery that it is possible to use, in the machine side layer of fabrics including at least two systems of weft yarns, pairs of machine side layer weft yarns arranged as pairs of complementary yarns to complete the MS fabric weave structure. In other words, the members of each weft yarn pair cooperate together by alternating with each other between interweaving with the MS warp yarns and being carried in the interior of the fabric, to form the weave pattern of the MS and effectively double the number of weft on the MS surface. This doubles the yarn mass in the MS layer and increases certain of the mechanical properties of the fabric, including stiffness, stability, wear resistance and Centre Plane Resistance (CPR). This feature is described by Danby et al. in U.S. Pat. No. 7,426,944, and refers to a reduced drainage area located along a notional centre plane through the fabric that is caused by the presence of long internal yarn floats; these floats may be from either, or both, the warp or weft yarns. The reduced drainage area in this centre plane of the fabric tends to resist the flow of fluid through the fabric and thereby retard the very high initial impingement drainage that occurs at or near the point of impingement of the stock jet onto the fabric.
Doubling the number of weft yarns in the MS layer, without reducing the size of those yarns, will increase the caliper or thickness of the resulting fabric. Over time, forming fabric manufacturers have strived to reduce fabric caliper so as to minimize the water carrying capacity of the fabric. Thin fabrics carry less water and are less prone to marking the sheet when the fabric passes around rolls at high speed in the papermaking process, causing water retained in the interior voids of the fabric to be released and spray onto the sheet.
In the fabrics of the present invention, it is possible to decrease the size of the MS complementary weft yarns in comparison to those which have been previously used in similar designs which are not so constructed and thereby to decrease fabric caliper, while at the same time retaining the prominence of the weft yarns on the MS surface, and thus not sacrificing the abrasion resistance of the fabric. This is because the number of weft yarns used in the fabrics of the present invention is double that which would be used in comparable designs. Further advantages of the fabrics of the invention include the ability to provide for increased stiffness and greater center plane resistance, while the use of decreased size for the MS complementary weft yarns allows for selection of desired values for fabric properties including air permeability and elastic modulus.
In the fabrics of this invention, the complementary weft yarns do not interweave with any of warp yarns forming the PS layer, but instead they alternate between remaining in the MS layer where they interweave solely with the MS warp yarns, and being carried between the PS and MS layers. Thus, the fabric structure can be tied into any selected PS weave by means of either intrinsic weft binder yarns in the manner described by Seabrook et al. in U.S. Pat. No. 5,826,627, or intrinsic warp binder yarns in the manner described by Danby et al. in U.S. Pat. No. 7,426,944. There can be zero, one, two or three MS warp yarns below each exchange point, i.e. the location where the complementary yarn pair members exchange positions with each other as they interweave across the fabric pattern so as to form an unbroken yarn path on the MS fabric surface. The MS surface of the fabric may be comprised entirely of the complementary yarn pairs (i.e. 100% of the MS weft yarns are complementary pairs), or there may be one, two, three or more “regular” (i.e. non-complementary) MS weft in between each complementary pair. The intrinsic binder yarns, either as pairs of weft yarns or pairs of warp yarns, will also interweave in the MS layer, but in general are recessed away from the MS surface.
In a broad embodiment, the invention seeks to provide a multilayer industrial fabric, woven according to an overall repeating weave pattern and having a paper side layer with a paper side surface and a machine side layer with a machine side surface, the fabric comprising at least one set of warp yarns and at least one set of paper side layer weft yarns and at least one set of machine side layer weft yarns, wherein at least some of the machine side layer weft yarns comprise complementary weft yarn pairs, each complementary weft yarn pair comprising a first member and a second member, which alternate with each other at exchange points to interweave only with selected warp yarns in the machine side layer such that for each complementary pair, when the first member of the pair is in the machine side surface of the fabric, the second member of the pair is carried between the machine side layer and the paper side layer.
Optionally, the machine side layer weft yarns comprise complementary weft yarn pairs and single weft yarns, in which case a ratio of complementary weft yarn pairs to all the machine side layer weft yarns can be at least 1 to 4, preferably at least 1 to 2, and more preferably at least 3 to 4. Alternatively, all the machine side layer weft yarns can comprise complementary weft yarn pairs.
For the fabrics of the invention, the repeating weave pattern can comprise a design selected from a double layer design, a semi duplex design, an extra support double layer design, a triple weft design, a triple layer sheet support binder design, a triple layer warp tie design and a triple layer warp integrated sheet support binder design.
In some embodiments, the paper side layer and the machine side layer are bound together by pairs of intrinsic weft binder yarns, in which case preferably the repeating weave pattern comprises a design selected from a double layer design, a semi duplex design, an extra support double layer design, a triple weft design, and a triple layer sheet support binder design, more preferably a triple layer sheet support binder design.
In other embodiments, the paper side layer and the machine side layer are bound together by pairs of intrinsic warp binder yarns, in which case preferably the repeating weave pattern comprises a design selected from a double layer design, a semi duplex design, an extra support double layer design, a triple weft design, a triple layer warp tie design and a triple layer warp integrated sheet support binder design, more preferably a triple layer warp integrated sheet support binder design.
In the fabrics of the invention, preferably each exchange point is located over a number of warp yarns in the machine side layer selected from 0, 1, 2 and 3.
Preferably, the complementary weft yarn pairs are polymeric monofilaments constructed of a material selected from polyesters, polyamides and blends thereof, and polymer blends. More preferably, the material is a polyester selected from polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and copolymers and blends thereof; or a polyamide selected from polyamide-6, polyamide-66, polyamide-6/10, polyamide-6/12; or a polymer comprising a blend of thermoplastic polyurethane and polyester.
In the fabrics of the invention, preferably each member of each complementary weft yarn pair has a cross-sectional shape selected from one of circular, elliptical, rectangular and square.
Although, as noted above, the invention is applicable to various industrial filtration fabrics, it is particularly advantageous for papermakers' forming fabrics.
The invention will now be described with reference to the drawings, in which
In the drawings, the sizes of some of the yarns, particularly the weft yarns 110 (
Referring first to
As can be seen from
As noted above, the two fabric layers 112 and 114 are tied together as an integrated structure by means of intrinsic weft binder yarns 126. Various possible arrangements of these yarns are well known, and one suitable arrangement is shown most clearly in
Also beginning at the left of
It can further be seen from
Further, it will be apparent to those skilled in the art that there may be zero, one, two, three, or more non-binding weft yarns such as weft yarn 8′, between each pair of intrinsic weft binder yarns such as weft binder yarns 4′ and 5′. In addition, it will also be apparent that there may be similar dedicated MS weft yarns in the fabric which are positioned between the pairs of complementary weft yarns, between the intrinsic weft yarn binder pair members, or at other locations as would be appropriate.
Referring now to
The fabric 100 shown in each of
In
Similarly, in the embodiment shown in
The embodiments illustrated in
It is not necessary that the complementary MS weft pair members be of the same size, shape or material constitution as the PS weft yarns 110, 710. The complementary weft yarns can be larger or smaller than the PS weft yarns; in certain instances, for example where fabric caliper is particularly important, it may be advantageous to downsize these weft yarns so that they contribute less to the fabric thickness. It may also be advantageous to use as complementary weft yarns monofilaments formed from one of the various polyesters, such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), or polyamides, such as polyamide-6, -66, -6/10, -12 and so on, or their respective blends or copolymers thereof, such as are known and would commonly be used in the industrial textile arts so as to maximize the wear life of the fabric; yarns formed from a blend of polyester and thermoplastic polyurethane such as described in U.S. Pat. No. 5,169,711 or U.S. Pat. No. 5,502,120 may also be beneficial.
A sample fabric of the invention was woven according to the weave pattern of
In Table 1 above, the PS & MS Mesh and Knocking are measured in the fabric following heatsetting at the tensions and temperatures indicated. Yarn sizes and processing conditions are as shown.
The data in Table 1 shows that Elastic Modulus of the experimental fabric is 10% higher than the comparison fabric that does not include the complementary weft yarn pairs of the present invention (8850 instead of 8100). This increase is likely due to the straighter path of the warp yarns in the fabric as a result of the yarn arrangement of the MS weft pair members. However, this increase in modulus is significant and was an unexpected benefit of the invention. However, the main benefit of the invention, that of increased fabric stiffness, is apparent from the data shown. The machine direction (MD) stiffness increased by 174% from 3.9 to 10.7 and the CD stiffness increased by 140% from 4.0 in the comparison fabric to 9.6 in the experimental which indicates that this fabric, which in almost all aspects is identical to the comparison fabric with the exception of the use of the complementary weft pairs in the MS, should be much stiffer when used on the papermaking machine. This should prevent or reduce problems such as creasing and similar issues associated with the dimensional stability of the fabric. Further, the MS crimp differential of the fabric of the invention is −0.0081 as compared to −0.0051 in the comparison fabric, indicating the weft yarns stand prouder from the MS surface of the fabric than those of the comparison fabric. This will prove beneficial with respect to the wear resistance properties of the fabric.
It will be noticed however that the air permeability of the experimental fabric is 20% lower than that of the comparison; this is due to the additional weft yarns in the MS surface. Further, although the caliper value of the comparison fabric is not provided, it is expected to be thinner than that of the experimental fabric. It is anticipated that both of these properties could be easily modified in the experimental fabric by replacing the MS weft yarns with smaller diameter yarns. This is not expected to adversely impact the wear resistance of the fabric due to the much higher wear volume present on the MS.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2680924 | Sep 2009 | CA | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA2010/001546 | 9/29/2010 | WO | 00 | 3/16/2012 |
