Patent Application
20200270812
In the manufacture of tissue products, particularly absorbent tissue products, there is a continuing need to improve the physical properties and final product appearance. It is generally known in the manufacture of tissue products that there is an opportunity to mold a partially dewatered cellulosic web on a papermaking belt specifically designed to enhance the finished paper product's physical properties. Such molding can be applied by fabrics in an uncreped through-air dried process as disclosed in U.S. Pat. No. 5,672,248 or in a wet pressed tissue manufacturing process as disclosed U.S. Pat. No. 4,637,859. Wet molding typically imparts desirable physical properties independent of whether the tissue web is subsequently creped, or an uncreped tissue product is produced.
However, absorbent tissue products are frequently embossed in a subsequent operation after their manufacture on the paper machine, while the dried tissue web has a low moisture content, to impart consumer preferred visually appealing textures or decorative lines. Thus, absorbent tissue products having both desirable physical properties and pleasing visual appearances often require two manufacturing steps on two separate machines. Hence, there is a need for a single step paper manufacturing process that can provide the desired visual appearance and product properties. There is also a need to develop a paper manufacturing process that not only imparts visually discernable pattern and product properties, but which does not affect machine efficiency and productivity.
Previous attempts to combine the above needs, such as those disclosed in International Application Nos. PCT/US13/72220, PCT/US13/72231 and PCT/US13/72238, have utilized through-air drying fabrics having a pattern extruded as a line element onto the fabric. The extruded line element may form either discrete or continuous patterns. While such a method can produce textures, extrusion techniques are limited in the types of lines that may be formed resulting in reduced permeability of the through-air drying fabric. The reduced permeability in-turn decreases drying efficiency and negatively affects tissue machine efficiency and productivity.
As such, there remains a need for articles of manufacture and methods of producing tissue products having visually discernable patterns with improved physical properties without losses to tissue machine efficiency and productivity.
The present inventors have now discovered a means of improving tissue web drying by supporting the nascent web on a papermaking fabric comprising a plurality of structuring elements disposed on a carrier structure. More specifically the present inventors have discovered that certain desirable tissue product properties such as caliper and smoothness may be optimized without negatively affecting drying of the tissue product by providing a papermaking fabric having structuring elements having a perimeter less than 3.6 mm, such as less than about 3.0 mm, such as from about 1.4 to 3.6 mm, such as from about 1.6 to about 2.4 mm, wherein the structuring elements cover more than about 28 percent, such as from about 28 to about 35 percent, of the surface area of the web contacting surface of the fabric. In this manner the amount of the nascent web that contacts the structuring elements and is molded into a smooth surface is maximized without exacerbating the negative affect to drying commonly associated with occluding a large portion of the surface area of the papermaking fabric.
Accordingly, it has now been discovered that relatively narrow structuring elements, such as elements having a width of 0.7 mm or less, such as from 0.3 to 0.7 mm, have limited negative affect on drying, particularly the normalized drying rate, even when the elements cover a relatively large percentage of the papermaking surface area, such as more than 28 percent and in some instances more than 30 percent. This is counter to what was previously believed. Previously, it was believed that an increase in the percentage of fabric covered by structuring elements resulted in a commensurate reduction in heat transfer, based on the theoretical drying rate:
DR=q/φ=hA(Tsupply−Tsheet)
Where q is the heat transfer in W/m2, φ is the latent heat of the water dried in j/g, DR is the drying rate in g/s m2, h is the heat transfer coefficient in W/m2 C, A is the area open to the flow in m2/m2, and T is temperature. In view of the foregoing, it was believed that when the coverage area (A) is reduced the drying rate should be reduced by the same amount. It has now been discovered, however, that the dominant factor affecting drying rate is the relative size of the structuring member, and that the coverage area may be increased so long as the size of the structuring element is optimized.
Thus, in certain embodiments the present invention provides a papermaking belt comprising a woven carrier structure having a machine contacting surface and an opposite web contacting surface and a plurality of structuring elements, which may be formed from a liquid and air impervious material such as silicone or polyurethane, disposed on the web contacting surface. The structuring elements are preferably shaped and sized to enable molding of the nascent web and to minimize negative impacts to drying and as such generally have a cross-sectional perimeter less than 3.6 mm, such as less than about 3.0 mm, such as less than about 2.4 mm, such as less than about 2.0 mm, such as from about 1.4 to 3.6 mm, such as from about 1.6 to about 3.0 mm.
When viewed in the cross-section perpendicular to the X-Y plane of the papermaking belt the structuring elements may have any number of different cross-sectional shapes such as, for example, polygonal, semicircular or elliptical. In certain preferred embodiments the structuring member has a polygonal cross-sectional shape such as, for example, a trapezoid, a parallelogram, a rectangle, a rhombus, or a square. Regardless of the cross-sectional shape, the structuring elements generally have a cross-sectional perimeter less than 3.6 mm, such as less than about 3.0 mm, such as less than about 2.4 mm, such as less than about 2.0 mm, such as from about 1.4 to 3.6 mm, such as from about 1.6 to about 3.0 mm.
Structuring elements can provide a means for deflecting papermaking fibers in the Z-direction as the nascent web is molded and dried while supported by the fabric. The amount of fiber deflection and the physical properties of the resulting tissue web such as caliper, density and surface topography may be affected to some extent by the size and shape of the structuring elements. Thus, in certain embodiments, it may be preferred that the structuring member have a Z-directional height greater than about 0.4 mm, such as greater than about 0.5 mm, and more preferably greater than about 0.6 mm, such as from about 0.4 to about 1.2 mm and more preferably from about 0.4 to about 0.8 mm. In other embodiments the structuring member have width, generally measured in the cross-machine direction (CD) across the widest portion of the element and parallel to the upper surface plane of the carrier structure, of 0.7 mm or less, such as less than about 0.6 mm, such as less than about 0.5 mm, such as from about 0.4 to 0.7 mm.
In other instances fiber deflection and the physical properties of the resulting tissue web such as caliper, density and surface topography, as well as the effective drying of the web may be affected by the aspect ratio of the structuring member, or the ratio of the width to Z-directional height. For optimal drying and physical properties the aspect ratio, which is generally the ratio of the element height to the element width, may be from about 2:1 to 2:3, such as from 1.5:1 to about 1:1.
In addition to the size and the shape of the structuring member the physical properties of the resulting tissue web, as well as the drying of the web, may be influenced by the relative percentage of the carrier structure that is covered by the structuring elements. For example, it may be desirable to provide the finished tissue web with a plurality of relatively smooth, elevated portions that are brought in contact with a user's skin in-use, yet at the same time minimize the amount of the web that is contacted by the structuring elements, which are generally impermeable to air and water, so as not to impede drying. By providing a structuring member having a cross-sectional perimeter less than 3.6 mm, such as from about 1.4 to 3.6 mm, such as from about 1.6 to about 2.4 mm, it has been discovered that the relative area of the carrier structure that may be covered by structuring elements may be relatively high such as greater than about 28 percent, such as greater than about 30 percent and more preferably greater than about 32 percent, such as from about 28 to about 35 percent and more preferably from about 30 to about 32 percent, without negatively affecting drying of the nascent web. As such a finished tissue web having a relatively high degree of relatively smooth, elevated portions may be produced without negatively affecting drying of the nascent web.
As used herein, the term “papermaking fabric” means any woven fabric used for making a cellulosic web such as a tissue sheet, either by a wet-laid process or an air-laid process. Specific papermaking fabrics within the scope of this invention include forming fabrics; transfer fabrics conveying a wet web from one papermaking step to another, such as described in U.S. Pat. No. 5,672,248; molding, shaping, or impression fabrics where the web is conformed to the structure through pressure assistance and conveyed to another process step, as described in U.S. Pat. No. 6,287,426; creping fabrics as described in U.S. Pat. No. 8,394,236; embossing fabrics as described in U.S. Pat. No. 4,849,054; structured fabric adjacent a wet web in a nip as described in U.S. Pat. No. 7,476,293; or through-air drying fabric as described in U.S. Pat. Nos. 5,429,686, 6,808,599 B2 and 6,039,838. The fabrics of the invention are also suitable for use as molding or air-laid forming fabrics used in the manufacture of non-woven, non-cellulosic webs, such as baby wipes.
As used herein the term “machine direction,” designated MD, is the direction parallel to the flow of the fibrous web through the web-making equipment.
As used herein the term “cross machine direction,” designated CD, is the direction perpendicular to the machine direction in the X-Y plane.
As used herein the term “width” when referring to a structuring member generally refers to the widest portion of a cross-sectional portion of the element in the cross-machine direction (CD). Generally width is measured at the widest point of the element and parallel to the upper surface plane of the carrier structure.
As used therein the term “height” when referring to a structuring member is the Z-direction height of a member extending from the carrier structure and is generally measured between the upper surface plane of the carrier structure and the upper surface plane of the element.
As used herein the term “aspect ratio” when referring to a structuring member is the ratio of the element height to the element width.
As used herein the terms “effective perimeter” and “perimeter” when referring to a structuring member is the total perimeter of a cross-sectional portion of the element. Generally the perimeter of a cross-sectional portion of the element is a continuous line forming the boundary of a closed geometric figure.
As used herein the term “coverage area” generally refers to percentage of the carrier structure's upper surface area that is covered by structural elements as measured using a Keyence VHX-5000 Digital Microscope (Keyence Corporation, Osaka, Japan) and described in the Test Methods section below.
As used herein the term “line element” refers to structuring elements in the shape of a line, which may be a continuous, discrete, interrupted, and/or partial line with respect to the carrier structure on which it is present. The line element may be of any suitable shape such as straight, bent, kinked, curled, curvilinear, serpentine, sinusoidal, and mixtures thereof that may form regular or irregular periodic or non-periodic lattice work of structures wherein the line element exhibits a length along its path of at least 10 mm. In one example, the line element may comprise a plurality of discrete elements, such as dots and/or dashes for example, that are oriented together to form a line element.
As used herein the term “continuous” when referring to a structuring member generally refers to an element disposed on a carrier structure useful in forming a tissue web that extends without interruption throughout one dimension of the carrier structure.
As used herein the term “discrete element” when referring to a structuring member generally refers to separate, unconnected elements disposed on a carrier structure useful in forming a tissue web that do not extend continuously in any dimension of the support structure or the tissue web as the case maybe.
As used herein the term “curvilinear element” when referring to a structuring member generally refers to any structuring member that contains either straight sections, curved sections, or both that are substantially connected visually. Curvilinear structuring elements may appear as undulating lines, substantially connected visually, forming signatures or patterns.
As used herein “decorative pattern” refers to any non-random repeating design, figure, or motif. It is not necessary that the curvilinear structuring elements form recognizable shapes, and a repeating design of the curvilinear structuring elements is considered to constitute a decorative pattern.
With reference now to
As used herein, the term “Z-direction” designates any direction perpendicular to the X-Y plane. Analogously, the term “Z-dimension” means a dimension, distance, or parameter measured parallel to the Z-direction and can be used to refer to dimensions such as the height of discrete primary elements or the thickness (or height or caliper), of the secondary elements. It should be carefully noted, however, that an element that “extends” in the Z-direction does not need itself to be oriented strictly parallel to the Z-direction; the term “extends in the Z-direction” in this context merely indicates that the element extends in a direction which is not parallel to the X-Y plane. Analogously, an element that “extends in a direction parallel to the X-Y plane” does not need, as a whole, to be parallel to the X-Y plane; such an element can be oriented in the direction that is not parallel to the Z-direction.
One skilled in the art will also appreciate that any given structuring member may not necessarily have an upper surface that is substantially flat throughout its entire length, yet the upper most portion of the member may generally define a plane. Irregularities in the upper surface of structuring elements may result from the elements being manufactured by depositing a polymeric material, which may be flowable to a certain extent, onto a woven carrier structure having an upper surface of which is not entirely flat, but has a degree of texture. Nonetheless, as illustrated in
As shown in
Although each of the structuring elements 40 have similar shapes and dimensions, the invention is not so limited, and a variety of different shapes and sizes may be employed. For example, each of the line elements can be individually sized, shaped, and spaced. The illustrated structuring elements 40 are continuous line elements, each having a generally flat distal portion 48 (portion distal from the carrier structure 30) providing the papermaking fabric 10 with a relatively uniform second upper surface plane 74 (as illustrated in
There are virtually an infinite number of shapes, sizes, spacing and orientations that may be chosen for the structuring elements. The actual shapes, sizes, orientations, and spacing can be specified and manufactured by additive manufacturing processes based on the desired properties of the finished tissue web such as caliper, sheet bulk, surface smoothness and aesthetic appearance. The improvement of the present invention is that the shapes, sizes, spacing, and orientations of the structuring element are such that they provide the finished tissue web with desirable physical properties, such as caliper, sheet bulk and surface smoothness, without negatively affecting drying of the tissue web. As such the structuring elements are generally designed such that a sufficient amount of the nascent web is contacted by and molded into the elements, but the amount of the web that is effectively rendered impermeable because of its contact with the elements is minimized.
For optimal molding of the web and minimal negative impact to drying, the shape of the elements may be modified such that the cross-sectional perimeter is less than 3.6 mm, such as less than about 3.0 mm, such as less than about 2.4 mm, such as less than about 2.0 mm, such as from about 1.4 to 3.6 mm, such as from about 1.6 to about 3.0 mm. In other embodiments the aspect ratio may be modified to promote drying and impart the resulting web with the desired physical properties and aesthetic appearance. For example, optimal drying and physical properties of the tissue product may be obtained by using a through-air drying fabric having structuring elements with an aspect ratio, which is generally the ratio of the element height to the element width, from about 2:1 to 2:3, such as from 1.5:1 to about 1:1.
With continued reference to
Generally the structuring element 40 is disposed on the web-contacting surface 64 for cooperating with, and structuring of, the wet fibrous web during manufacturing. In a particularly preferred embodiment the web contacting surface 64 comprises a plurality of spaced apart three-dimensional elements 40 distributed across the web-contacting surface 64 of the carrier structure 30 such that the relative area of the carrier structure covered by the elements may be relatively high such as greater than about 28 percent, such as greater than about 30 percent and more preferably greater than about 32 percent, such as from about 28 to about 35 percent and more preferably from about 30 to about 32 percent, without negatively affecting drying of the nascent web. As such a finished tissue web having a relatively high degree of relatively smooth, elevated portions may be produced without negatively affecting drying of the nascent web.
In addition to structuring elements 40 the web-contacting surface 64 preferably comprises a plurality of continuous landing areas 60. The landing areas 60 are generally bounded by the elements 40 and coextensive with the upper surface plane 72 of the web contacting surface 64. Landing areas 60 are generally permeable to liquids and allow water to be removed from the cellulosic tissue web by the application of differential fluid pressure, by evaporative mechanisms, or both when drying air passes through the embryonic tissue web while on the papermaking belt 10 or a vacuum is applied through the belt 10. Without being bound by any particularly theory, it is believed that the arrangement of elements and landing areas allow the molding of the embryonic web causing fibers to deflect in the z-direction and generate the caliper of, and patterns on the resulting tissue web.
The carrier structure 30 has two principle dimensions—a machine direction (“MD”), which is the direction within the plane of the belt 10 parallel to the principal direction of travel of the tissue web during manufacture and a cross-machine direction (“CD”), which is generally orthogonal to the machine direction. The carrier structure 30 is generally permeable to liquids and air. In one particularly preferred embodiment the carrier structure is a woven fabric. The carrier structure may be substantially planar or may have a three dimensional surface defined by ridges. In one embodiment the carrier structure is a substantially planar woven fabric such as a multi-layered plain-woven fabric 30 having base warp yarns 32 interwoven with shute yarns 34 in a 1×1 plain weave pattern. One example of a suitable substantially planar woven fabric is disclosed in U.S. Pat. No. 8,141,595, the contents of which are incorporated herein in a manner consistent with the present invention. In a particularly preferred embodiment, the carrier structure comprises a substantially planar woven fabric wherein the plain-weave load-bearing layer is constructed so that the highest points of both the load-bearing shutes 34 and the load-bearing warps 32 are coplanar and coincident with the upper surface plane 72 of the web contacting surface 64.
A plurality of structuring elements 40 that may, such as in the embodiments illustrated in
The structuring elements 40 generally extend in the z-direction (generally orthogonal to both the machine direction and cross-machine direction) above the upper surface plane 72 of the web contacting surface 64. As noted previously, in certain embodiments, the elements 40 may have straight, parallel sidewalls 45, 47 providing the structuring elements 40 with a width (w), and a height (h) and the elements 40 may be similarly sized.
In certain embodiments the elements may have a Z-directional height greater than about 0.4 mm, such as greater than about 0.5 mm, and more preferably greater than about 0.6 mm, such as from about 0.4 to about 1.2 mm and more preferably from about 0.4 to about 0.8 mm. In a particularly preferred embodiment the height of the elements is substantially similar and ranges from about 0.4 to about 1.2 mm and more preferably from about 0.4 to about 0.8 mm.
Further, the structuring elements 40 may have a width (w), generally measured in the cross-machine direction (CD) across the widest portion of the element and parallel to the upper surface plane of the carrier structure, of 0.7 mm or less, such as less than about 0.6 mm, such as less than about 0.5 mm, such as from about 0.4 to 0.7 mm.
While the height (h) and width (w) of the elements may be varied, it is generally preferred that the elements have a cross-sectional perimeter less than 3.6 mm, such as less than about 3.0 mm, such as less than about 2.4 mm, such as less than about 2.0 mm, such as from about 1.4 to 3.6 mm, such as from about 1.6 to about 3.0 mm. In other instances the elements have a height (h) and width (w) such that the aspect ratio is from about 2:1 to 2:3, such as from 1.5:1 to about 1:1.
In a particularly preferred embodiment the structuring elements 40 have planar sidewalls 45, 47 such that the cross-section of the element has an overall square or rectangular shape. However, it is to be understood that the design element may have other cross-sectional shapes, such as a trapezoid or a parallelogram, which may also be useful in producing high bulk tissue products according to the present invention. Accordingly, in a particularly preferred embodiment the structuring elements 40 preferably have planar sidewalls 45, 47 and a square cross-section where the width (w) and height (h) are equal and are 0.7 mm or less, such as less than about 0.6 mm, such as less than about 0.5 mm, such as from about 0.4 to 0.7 mm.
The spacing and arrangement of the structuring elements relative to one another may vary depending on the desired tissue product properties and appearance. In one embodiment a plurality of elements extend continuously throughout one dimension of the belt and each element in the plurality is spaced apart from the adjacent element. Thus, the elements may be spaced apart across the entire cross-machine direction of the belt, may endlessly encircle the belt in the machine direction, or may run diagonally relative to the machine and cross-machine directions. Of course, the directions of the elements alignments (machine direction, cross-machine direction, or diagonal) discussed above refer to the principal alignment of the elements. Within each alignment, the elements may have segments aligned at other directions, but aggregate to yield the particular alignment of the entire elements.
Generally the elements are spaced apart from one another so as to define a landing area there-between. In use, as the embryonic tissue web is formed fibers are deflected in the z-direction by the continuous elements, however, the spacing of elements is such that the web maintains a relatively uniform density. This arrangement provides the benefits of improved web extensibility, increased sheet bulk, better softness, and a more pleasing texture.
If the individual elements are too high, or the landing area is too small, the resulting sheet may have excessive pinholes and insufficient compression resistance and cross-machine direction physical properties, such as stretch, and be of poor quality. Further, tensile strength may be degraded if the span between elements greatly exceeds the fiber length. Conversely, if the spacing between adjacent elements is too small the tissue will not mold into the landing areas without rupturing the sheet, causing excessive sheet holes, poor strength, and poor paper quality.
In addition to varying the spacing and arrangement of the elements along the carrier structure, the shape of the element may also be varied. For example, in one embodiment, the elements are substantially sinusoidal and are arranged substantially parallel to one another such that none of the elements intersect one another. As such, in the illustrated embodiment, the adjacent sidewalls of individual elements are equally spaced apart from one another. In such embodiments, the center-to-center spacing of design elements (also referred to herein as pitch or simply as p) may be greater than about 1.0 mm apart, such as from about 1.0 to about 20 mm apart and more preferably from about 2.0 to about 10 mm apart. In one particularly preferred embodiment the continuous elements are spaced apart from one-another from about 2.5 to about 4.0 mm apart. This spacing will result in a tissue web which generates maximum caliper when made of conventional cellulosic fibers. Further, this arrangement provides a tissue web having three dimensional surface topography, yet relatively uniform density.
In other embodiments the continuous elements may occur as wave-like patterns that are arranged in-phase with one another such that the pitch (p) is approximately constant. In other embodiments elements may form a wave pattern where adjacent elements are offset from one another. Regardless of the particular element pattern, or whether adjacent patterns are in or out of phase with one another, the elements are separated from one another by some minimal distance. Preferably the distance between continuous elements is greater than 0.5 mm and in a particularly preferred embodiment greater than about 1.0 mm and still more preferably greater than about 2.0 mm such as from about 2.0 to about 6.0 mm and still more preferably from about 2.5 to about 4.0 mm.
Where the continuous elements are wave-like, the elements have an amplitude (A) and a wavelength (L). The amplitude may range from about 2.0 to about 200 mm, in a particularly preferred embodiment from about 10 to about 40 mm and still more preferably from about 18 to about 22 mm. Similarly, the wavelength may range from about 20 to about 500 mm, in a particularly preferred embodiment from about 50 to about 200 mm and still more preferably from about 80 to about 120 mm.
While in certain embodiments the structuring elements are continuous the invention is not so limited. In other embodiments the elements may be discrete. For clarity, the discrete elements will be referred to herein as protuberances. Generally the protuberances are discrete and spaced apart from one another. Each protuberance is joined to a carrier structure and extends outwardly from the web contracting plane of thereof. In this manner the protuberances contact the tissue web during manufacture.
The protuberances may have a square horizontal and lateral (relative to the plane of the carrier structure) cross-sectional shape, however, the shape is not so limited. The protuberance may have any number of different horizontal and lateral cross-sectional shapes. For example, the horizontal cross-section may have a rectangular, circular, oval, polygonal or hexagonal shape. A particularly preferred protuberance has planar sidewalls which are generally perpendicular to the plane of the carrier structure. Alternatively, the protuberances may have a tapered lateral cross-section formed by sides that converge to yield a protuberance having a base that is wider than the distal end.
The individual protuberances may be arranged in any number of different manners to create a decorative pattern. In one particular embodiment protuberances are spaced and arranged in a non-random pattern so as to create a wave-like design. In the illustrated embodiment spaced between the decorative patterns are landing areas that provide a visually distinctive interruption to the decorative pattern formed by the individual spaced apart protuberances. In this manner, despite being discrete elements, the protuberances are spaced apart so as to form a visually distinctive curvilinear decorative element that extends substantially in the machine direction. Taken as a whole the discrete elements form a wave-like pattern.
In other embodiments the protuberances may be spaced and arranged so as to form a decorative figure, icon or shape such as a flower, heart, puppy, logo, trademark, word(s), and the like. Generally the design elements are spaced about the support structure and can be equally spaced or may be varied such that the density and the spacing distance may be varied amongst the design elements. For example, the density of the design elements can be varied to provide a relatively large or relatively small number of design elements on the web. In a particularly preferred embodiment the design element density, measured as the percentage of background surface covered by a design element, is from about 10 to about 35 percent and more preferably from about 20 to about 30 percent. Similarly the spacing of the design elements can also be varied, for example, the design elements can be arranged in spaced apart rows. In addition, the distance between spaced apart rows and/or between the design elements within a single row can also be varied.
In certain embodiments the plurality of protuberances defining a given design element may be spaced apart from one another so as to define landing areas there between. The landing areas are generally bounded by the designs and coextensive with the top surface plane of the carrier structure. Landing areas are generally permeable to liquids and allow water to be removed from the cellulosic tissue web by the application of differential fluid pressure, by evaporative mechanisms, or both when drying air passes through the embryonic tissue web while on the papermaking belt or a vacuum is applied through the belt.
The elements may be formed from a polymeric material, or other material, applied and joined to the carrier structure in any suitable manner. Thus in certain embodiments elements are formed by extruding, such as that disclosed in U.S. Pat. No. 5,939,008, the contents of which are incorporated herein by reference in a manner consistent with the present invention, or printing, such as that disclosed in U.S. Pat. No. 5,204,055, the contents of which are incorporated herein by reference in a manner consistent with the present invention, a polymeric material onto the carrier structure. In other embodiments the design element may be produced, at least in some regions, by extruding or printing two or more polymeric materials. In certain instances the polymeric material may be silicone or polyurethane, or a combination thereof.
The papermaking fabrics of the present invention are particularly useful in making through-air dried tissue webs and products. Through-air drying manufacturing processes are well known in the art and may be either creped through-air drying (CTAD) or uncreped through-air drying (UCTAD) processes. In one embodiment the fabrics are useful in an UCTAD manufacturing process such as that described in U.S. Pat. No. 5,607,551. In that process a twin wire former having a papermaking headbox, such as a layered headbox, injects or deposits a stream of an aqueous suspension of papermaking fibers onto a forming fabric positioned on a forming roll. The forming fabric serves to support and carry the newly-formed wet web downstream in the process as the web is partially dewatered to a consistency of about 10 dry weight percent. Additional dewatering of the wet web can be carried out, such as by vacuum suction, while the wet web is supported by the forming fabric.
The wet web is then transferred from the forming fabric to a transfer fabric. In one embodiment, the transfer fabric can be traveling at a slower speed than the forming fabric in order to impart increased stretch into the web. This is commonly referred to as a “rush” transfer. Preferably the transfer fabric can have a void volume that is equal to or less than that of the forming fabric. The relative speed difference between the two fabrics can be from 0 to 60 percent, more specifically from about 15 to 45 percent. Transfer is preferably carried out with the assistance of a vacuum shoe such that the forming fabric and the transfer fabric simultaneously converge and diverge at the leading edge of the vacuum slot.
The web is then transferred from the transfer fabric to the through-air drying fabric with the aid of a vacuum transfer roll or a vacuum transfer shoe, optionally again using a fixed gap transfer as previously described. The through-air drying fabric can be traveling at about the same speed or a different speed relative to the transfer fabric. If desired, the through-air drying fabric can be run at a slower speed to further enhance stretch. Transfer can be carried out with vacuum assistance to ensure deformation of the sheet to conform to the through-air drying fabric, thus yielding desired bulk and texture.
The side of the web contacting the through-air drying fabric is typically referred to as the “fabric side” of the paper web. The fabric side of the paper web, as described above, may have a shape that conforms to the surface of the through-air drying fabric after the paper web is dried in the throughdryer. The opposite side of the paper web, on the other hand, is typically referred to as the “air side.”
The level of vacuum used for the web transfers can be from about 3 to about 15 inches of mercury (75 to about 380 millimeters of mercury), preferably about 5 inches (125 millimeters) of mercury. The vacuum shoe (negative pressure) can be supplemented or replaced by the use of positive pressure from the opposite side of the web to blow the web onto the next fabric in addition to or as a replacement for sucking it onto the next fabric with vacuum. Also, a vacuum roll or rolls can be used to replace the vacuum shoe(s).
While supported by the through-air drying fabric, the web is dried to a consistency of about 94 percent or greater by the throughdryer and thereafter transferred to a carrier fabric. The dried basesheet is transported to the reel using the carrier fabric. Suitable carrier fabrics for this purpose are Albany International 84M or 94M and Asten 959 or 937, all of which are relatively smooth fabrics having a fine pattern. Optionally the base sheet may be subjected to additional converting steps such as reel calendering, off-line calendering or embossing.
Fabric images were acquired and analyzed using a Keyence VHX-5000 Digital Microscope (Keyence Corporation, Osaka, Japan) equipped with VHX-5000 Communication Software Ver. 1.5.1.1. The lens is an ultra-small, high performance zoom lens, VH-Z20R/Z20T.
Structuring element dimensions were measured using the Keyence software. For example, element height was measured by first drawing a line approximately along the top surface plane of the carrier structure with the line tangent to at least two filaments forming the web contacting surface of the carrier structure. A second parallel line has been drawn approximately along the top surface plane of the structuring element with the line tangent to the top surface of the element. With the two lines drawn, each corresponding to a surface plane of the fabric, the digital microscope software was instructed to calculate the distances between the planes.
Element width was measured by determining the widest portion of the element and using the software to draw a first line through the widest point, the line being substantially parallel to the web contacting surface plane of the carrier structure. A pair of lines were then drawn perpendicular to the first line tangent to the point that the first line intersected the element, the digital microscope software was instructed to calculate the distances between the pair of lines.
The surface area of the carrier structure covered by the structuring elements was measured using a Keyence Microscope and image analysis software described above. The sample of carrier structure for measurement should be an undamaged, flat fabric swatch approximately 3×3 inches in size.
An image of the fabric was acquired at a magnification of 20× and from the on-screen menu “Measure” was selected, followed by selection of “Auto” area measurement, then the “Color” option was selected and a measurement was taken. Once a measurement was taken the structuring elements were filled using the “Fill” and “Eliminate Small Grains” features, followed by selecting a Shaping step. If there are areas of the structuring elements that needed to be filled in, or otherwise edited to create an accurate 2-D highlight of the structuring elements, an accurate area representation was created by selecting “Edit”, “Fill.” The results were than tabulated by selecting the “Next” to proceed to the Result Display step where “Measure Result” was selected and the calculated Area Ratio Percent was displayed.
To evaluate the effect of structuring element size and coverage area on the drying of tissue webs several different through-air drying fabrics were used to manufacture a single ply uncreped through-air dried (“UCTAD”) tissue web as generally described in U.S. Pat. No. 5,607,551, the contents of which are incorporated herein in a manner consistent with the present invention. Tissue webs having a target bone dry basis weight of about 40 grams per square meter (gsm).
In all cases the base sheets were produced from a furnish comprising northern softwood kraft and eucalyptus kraft using a layered headbox fed by three stock chests such that the webs having three layers (two outer layers and a middle layer) were formed. The two outer layers comprised eucalyptus (each layer comprising 30 percent weight by total weight of the web) and the middle layer comprised softwood and eucalyptus. The amount of softwood and eucalyptus kraft in the middle layer was maintained for all inventive samples—the middle layer comprised 29 percent (by total weight of the web) softwood and 11 percent (by total weight of the web) eucalyptus. Strength was controlled via the addition of starch and/or by refining the softwood furnish.
The tissue web was formed on a Voith Fabrics TissueForm V forming fabric, vacuum dewatered to a consistency ranging from about 30 to about 33 percent and then subjected to rush transfer when transferred to the transfer fabric. The transfer fabric was the fabric described as “Fred” in U.S. Pat. No. 7,611,607 (commercially available from Voith Fabrics, Appleton, Wis.).
The web was then transferred to a through-air drying fabric comprising a printed silicone pattern disposed on the sheet contacting side. The silicone formed a wave-like pattern on the sheet contacting side of the fabric. Inventive papermaking fabrics are shown in
The tissue web was dried to a final consistency of about 98 percent by passing the web over first and second through-air dryers while supported by the through-air drying fabric. The exhaust temperature of the first through-air dryer was controlled to about 300° F. It was discovered that through-air drying fabrics having narrower structuring elements, which are impermeable to air and water, were able to produce tissue webs having a sufficient degree of smooth flat surfaces without negatively affecting drying. The plot shown in
In addition to varying the width of the elements the percent of coverage area was varied by altering the spacing of the elements relative to one another. In each case the structuring elements had a substantially square cross-sectional shape and an aspect ratio of 1:1. It was expected that the main factor affecting heat transfer coefficient would be the relative area of the fabric occluded by the elements and that for every one percent increase in coverage there would be a one percent reduction in heat transfer. It was surprisingly discovered however, that the dimensions of the elements was a dominant factor affecting drying rate, and that using elements having a width of 0.7 mm or less the coverage area could be increased with very little adverse effect on drying rate.
The drying benefit may also be determined by calculating the Drying Loss Factor that compares the loss in drying rate measured for a given fabric having impermeable structuring elements (DRoccluded) to the drying rate of a fabric devoid of such elements (DRopen):
Assuming that the area of the web in contact with the structuring elements are not drying at all, then the Drying Loss Factor will equal 1, if it is drying just as well as the rest of the web then the Drying Loss Factor will be 0.
Accordingly, in a first embodiment the present invention provides a papermaking fabric comprising a woven carrier structure having a machine and a cross-machine direction and a first side with a plurality of substantially machine direction oriented structuring elements disposed thereon, the structuring elements having a cross-sectional height and width, wherein the width is 0.7 mm or less and the aspect ratio is from about 2:1 to about 2:3
In a second embodiment the present invention provides the papermaking fabric of the first embodiment wherein the structuring elements are continuous line elements and have a polygonal cross-sectional shape.
In a third embodiment the present invention provides the papermaking fabric of the first or the second embodiments wherein the structuring elements have a cross-sectional shape selected from the group consisting of a trapezoid, a parallelogram, a rectangle, a rhombus, and a square.
In a fourth embodiment the present invention provides the papermaking fabric of the first through the third embodiments wherein the structuring elements have a height from about 0.4 to 0.7 mm.
In a fifth embodiment the present invention provides the papermaking fabric of the first through the fourth embodiments wherein the structuring elements have a perimeter from about 1.6 to about 2.4 mm.
In a sixth embodiment the present invention provides the papermaking fabric of the first through the fifth embodiments wherein the structuring elements are impermeable to air and water and comprise silicone or polyurethane.
In a seventh embodiment the present invention provides the papermaking fabric of the first through the sixth embodiments wherein the structuring elements cover from about 28 to about 32 percent of the surface area of the first side of the carrier structure.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US18/33611 | 5/21/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62509310 | May 2017 | US |
