WO1997043484A1 - Method and apparatus for making soft tissue - Google Patents

Abstract

An uncreped tissue sheet having improved softness results from supplementally dewatering a wet web to a consistency of greater than about 30 percent using noncompressive dewatering techniques (30) prior to a differential speed transfer (37) and subsequent throughdrying (44). An air press (30) particularly well suited for providing the supplemental noncompressive dewatering incorporates side (80) and/or end seals (78) to minimize escape of pressurized fluid.

Description

METHOD AND APPARATUS FOR MAKING SOFT TISSUE

Background of the Invention

There are many characteristics of tissue products such as bath and facial tissue that must be considered in producing a final product having desirable attributes that make it suitable and preferred for the product's intended purpose. Improved softness of the product has long been one major objective, and this has been a particularly significant factor for the success of premium products. In general, the major components of softness include stiffness and bulk (density), with lower stiffness and higher bulk (lower density) generally improving perceived softness.

While enhanced softness is a desire for all types of tissue products, it has been especially challenging to achieve softness improvements in uncreped throughdried sheets. Throughdrying provides a relatively noncompressive method of removing water from a web by passing hot air through the web until it is dry. More specifically, a wet-laid web is transferred from the forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until dry. The resulting dried web is softer and bulkier than a conventionally-dried uncreped sheet because fewer bonds are formed and because the web is less compressed. Thus, there are benefits to eliminating the Yankee dryer and making an uncreped throughdried product. Uncreped throughdried sheets are typically quite harsh and rough to the touch, however, compared to their creped counterparts. This is partially due to the inherently high stiffness and strength of an uncreped sheet, but is also due in part to the coarseness of the throughdrying fabric onto which the wet web is conformed and dried.

Therefore, what are lacking and needed in the art are a method and apparatus for manufacturing tissue products having improved softness, and in particular uncreped throughdried tissue products having improved softness.

Summary of the Invention

It has now been discovered that an improved uncreped throughdried web can be made by dewatering the web to greater than about 30 percent consistency prior to transferring the wet web from a forming fabric to one or more slower speed intermediate transfer fabrics before further transferring the web to a throughdrying fabric for final drying of the web. In particular, increasing the consistency of the uncreped throughdried web before the point of differential speed transfer has surprisingly been found to result in: (1) both higher machine direction and cross direction tensile properties, contributing to improved runnability of the web; and (2) reduced modulus, that is increased softness, when the tensile strength is adjusted to the normal value. This discovery allows for the manufacture of tissue products with lower modulus at given tensile strengths as compared even to tissue products produced by undergoing differential speed transfer at lower consistencies.

One particularly desirable means by which the web can be dewatered to greater than about 30 percent consistency comprises an air press located just upstream of the differential speed transfer. While pressurized fluid jets in combination with a vacuum device have previously been discussed in the patent literature, such devices have not been widely used in tissue manufacturing. Principally, this appears to be due to the fact that it had not been previously recognized that dewatering the web to greater than about 30 percent consistency in advance of the differential speed transfer would result in the improved product properties identified herein. Moreover, the disincentive to using such equipment is also believed to be attributable to the difficulties of actual implementation, including disintegration of the tissue web, pressurized fluid leaks, seal and/or fabric wear, and the like. The air press used in the present method overcomes these difficulties and provides one practical apparatus for achieving the desired consistency ahead of the differential speed transfer.

Hence, in one aspect the invention resides in a method of making a soft tissue sheet. The method includes the steps of: depositing an aqueous suspension of papermaking fibers onto an endless forming fabric to form a wet web; dewatering the wet web to a consistency of from about 20 to about 30 percent; supplementally dewatering the wet web using noncompressive dewatering means to a consistency of greater than about 30 percent; transferring the supplementally dewatered web to a transfer fabric traveling at a speed of from about 10 to about 80 percent slower than the forming fabric; transferring the web to a throughdrying fabric; and throughdrying the web to a final dryness.

The air press desirably provides a pressure differential across the wet web of from about 35 to about 60 inches of mercury. This may be achieved in part by an air plenum of the air press maintaining a fluid pressure on one side of the wet web of from about 5 to about 60 pounds per square inch, and particularly from about 5 to about 30 pounds per square inch. The pressurized fluid may be air at ambient temperature, heated air, steam, or the like. Overall, the air press may be operable to increase the consistency of the wet web by at least about 3 and preferably at least about 5 percent. Optional steam showers may be employed before the air press to increase post air press consistency. Another aspect of the invention resides in an air press for dewatering a wet web.

In one embodiment, the air press includes an air plenum comprising a plenum cover having a bottom surface and a vacuum box comprising a vacuum box cover having a top surface positioned in close proximity to the bottom surface of the plenum cover. The air press also includes means for supplying pressurized fluid to the air plenum and means for applying vacuum to the vacuum box. Side seal members of the air press are adapted to reside in contact with the air plenum and the vacuum box for minimizing the escape of the pressurized fluid. The side seal members are attached to one of the air plenum and the vacuum box, and are positioned in close proximity to side seal contact surfaces defined by the other of the air plenum and the vacuum box. The side seal members are adapted to flex into sealing contact with the side seal contact surface upon exposure to the pressurized fluid to enhance the seal effectiveness.

Optionally, the air press may include a position control mechanism that functions to maintain the air plenum in close proximity to the vacuum box. In particular, the position control mechanism desirably includes a rotatably mounted lever attached to the air plenum, and a counterbalance cylinder attached to the lever. The position control mechanism is adapted to rotate the lever to counteract pressure changes within the air plenum. In this way, the air plenum resides in close proximity to or in contact with the fabrics passing between the air plenum and the vacuum box, without clamping the fabrics therebetween. In another embodiment, the air press includes an air plenum comprising a plenum cover having a bottom surface, and means for supplying pressurized fluid to the air plenum. The air press also includes a vacuum box comprising a vacuum box cover having a top surface positioned in close proximity to the bottom surface of the plenum cover, and means for applying vacuum to the vacuum box. An arm that is pivotally mounted on the air plenum comprises first and second portions, with the first portion of the arm being disposed at least partially inside the air plenum. A sealing bar is formed from or mounted on the first portion of the arm. The air press also includes means for pivoting the arm in response to fluid pressure within the air plenum. In this embodiment, the sealing bar portion of the pivotable arm acts as an end seal to prevent the escape of pressurized fluid from between the air plenum and the vacuum box. The sealing bar may conform to fabric irregularities or misalignment of the supporting structure. The end seals, which are also referred to as cross direction or CD seals, improve containment of the pressurized fluid and thus result in more efficient operation of the air press. The loading of the end seals is controlled to maintain the sealing bar in contact with the underlying moving fabric, without causing undue wear of the fabric.

The improved results can be obtained without sacrifice of efficiency. The present method and apparatus are capable of functioning at commercially viable web speeds. For example, the forming fabric may be controlled to travel at speeds of at least about 2000 feet per minute (fpm), and more desirably at speeds of at least about 4000 fpm.

The intermediate transfer fabric or fabrics are traveling at a slower speed than the forming fabric during the transfer in order to impart stretch into the sheet. As the speed differential between the forming fabric and the slower transfer fabric is increased

(sometimes referred to as "negative draw" or "rush transfer"), the stretch imparted to the web during transfer is also increased. The transfer fabric can be relatively smooth and dense compared to the coarse weave of a typical throughdrying fabric. Preferably the transfer fabric Is as fine as can be run from a practical standpoint. Gripping of the web is accomplished by the presence of knuckles on the surface of the transfer fabric. In addition, it can be advantageous if one or more of the wet web transfers, with or without the presence of a transfer fabric, are achieved using a "fixed gap" or "kiss" transfer in which the fabrics simultaneously converge and diverge, which will be hereinafter described in detail. Such transfers not only avoid any significant compaction of the web while it is in a wet bond-forming state, but when used in combination with a differential speed transfer and/or a smooth transfer fabric, are observed to smoothen the surface of the web and final dry sheet.

The speed difference between the forming fabric and the transfer fabric can be from about 10 to about 80 percent or greater, preferably from about 10 to about 35 percent, and more preferably from about 15 to about 25 percent, with the transfer fabric being the slower fabric. The optimum speed differential will depend on a variety of factors, including the particular type of product being made. As previously mentioned, the increase in stretch imparted to the web is proportional to the speed differential. For an uncreped throughdried three-ply wiper having a basis weight of about 20 grams per square meter per ply, for example, a speed differential in the production of each ply of from about 20 to about 25 percent between the forming fabric and a sole transfer fabric produces a stretch in the final product of from about 15 to about 20 percent.

The stretch can be imparted to the web using a single differential speed transfer or two or more differential speed transfers of the wet web prior to drying. Hence there can be one or more transfer fabrics. The amount of stretch imparted to the web can hence be divided among one, two, three or more differential speed transfers.

The transfer is desirably carried out such that the resulting "sandwich" (consisting of the forming fabric/web/transfer fabric) exists for as short a duration as possible. In particular, it exists only at the leading edge of the vacuum shoe or transfer shoe slot being used to effect the transfer. In effect, the forming fabric and the transfer fabric converge and diverge at the leading edge of the vacuum slot. The intent is to minimize the distance over which the web is in simultaneous contact with both fabrics. It has been found that simultaneous convergence/divergence is the key to eliminating macrofolds and thereby enhances the smoothness of the resulting tissue or other product.

In practice, the simultaneous convergence and divergence of the two fabrics will only occur at the leading edge of the vacuum slot if a sufficient angle of convergence is maintained between the two fabrics as they approach the leading edge of the vacuum slot and if a sufficient angle of divergence is maintained between the two fabrics on the downstream side of the vacuum slot. The minimum angles of convergence and divergence are about 0.5 degree or greater, more specifically about 1 degree or greater, more specifically about 2 degrees or greater, and still more specifically about 5 degrees or greater. The angles of convergence and divergence can be the same or different. Greater angles provide a greater margin of error during operation. A suitable range is from about 1 degree to about 10 degrees. Simultaneous convergence and divergence is achieved when the vacuum shoe is designed with the trailing edge of the vacuum slot being sufficiently recessed relative to the leading edge to permit the fabrics to immediately diverge as they pass over the leading edge of the vacuum slot. This will be more clearly described in connection with the Figures. In setting up the machine with the fabrics initially having a fixed gap to further minimize compression of the web during the transfer, the distance between the fabrics should be equal to or greater than the thickness or caliper of the web so that the web is not significantly compressed when transferred at the leading edge of the vacuum slot. Increased smoothness is achieved by use of the air press upstream of the differential speed transfer. This is most preferably used in combination with a fixed gap carrier fabric section following drying. Calendering of the web is not necessary to obtain desirable levels of smoothness, but further processing of the sheet, such as by calendering, embossing or creping, may be beneficial to further enhance the sheet properties.

As used herein, "transfer fabric" is a fabric which is positioned between the forming section and the drying section of the web manufacturing process. Suitable transfer fabrics are those papermaking fabrics which provide a high fiber support index and provide a good vacuum seal to maximize fabric/sheet contact during transfer from the forming fabric. The fabric can have a relatively smooth surface contour to impart smoothness to the web, yet must have enough texture to grab the web and maintain contact during a rush transfer. Finer fabrics can produce a higher degree of stretch in the web, which is desirable for some product applications. Transfer fabrics include single-layer, multi-layer, or composite permeable structures. Preferred fabrics have at least some of the following characteristics: (1) On the side of the transfer fabric that is in contact with the wet web (the top side), the number of machine direction (MD) strands per inch (mesh) is from 10 to 200 and the number of cross-machine direction (CD) strands per inch (count) is also from 10 to 200. The strand diameter is typically smaller than 0.050 inch; (2) On the top side, the distance between the highest point of the MD knuckle and the highest point of the CD knuckle is from about 0.001 to about 0.02 or 0.03 inch. In between these two levels, there can be knuckles formed either by MD or CD strands that give the topography a 3-dimensional characteristic, (3) On the top side, the length of the MD knuckles is equal to or longer than the length of the CD knuckles; (4) If the fabric is made in a multi-layer construction, it is preferred that the bottom layer is of a finer mesh than the top layer so as to control the depth of web penetration and to maximize fiber retention; and (5) The fabric may be made to show certain geometric patterns that are pleasing to the eye, which typically repeat between every 2 to 50 warp yarns. Specific suitable transfer fabrics include, by way of example, those made by

Asten Forming Fabrics, Inc., Appleton, Wisconsin and designated as numbers 934, 937, 939 and 959. Particular transfer fabrics that may be used also include the fabrics disclosed in U.S. Patent 5,429,686 issued July 4, 1995, to Chiu et al., which is incorporated herein by reference. The void volume of the transfer fabric can be equal to or less than the fabric from which the web is transferred.

The forming process and tackle can be conventional as is well known in the papermaking industry. Such formation processes include Fourdrinier, roof formers (such as suction breast roll), gap formers (such as twin wire formers, crescent formers), or the like. Forming wires or fabrics can also be conventional, with the finer weaves with greater fiber support being preferred to produce a more smooth sheet or web. Headboxes used to deposit the fibers onto the forming fabric can be layered or nonlayered. The method disclosed herein can be applied to any tissue web, which includes webs for making facial tissue, bath tissue, paper towels, dinner napkins, or the like. Such tissue webs can be single-ply products or multi-ply products, such as two-ply, three-ply, four-ply or greater. One-ply products are advantageous because of their lower cost of manufacture, while multi-ply products are preferred by many consumers. For multi-ply products it is not necessary that all plies of the product be the same, provided at least one ply is in accordance with this invention. The webs can be layered or unlayered (blended), and the fibers making up the web can be any fibers suitable for papermaking.

Suitable basis weights for these tissue webs can be from about 5 to about 70 grams per square meter (gsm), preferably from about 10 to about 40 gsm, and more preferably from about 20 to about 30 gsm. For a single-ply bath tissue, a basis weight of about 25 gsm is preferred. For a two-ply tissue, a basis weight of about 20 gsm per ply is preferred. For a three-ply tissue, a basis weight of about 15 gsm per ply is preferred. In general, higher basis weight webs will require lower air flow to maintain the same operating pressure in the air plenum. The width of the slots of the air press are desirably adjusted to match the system to the available air capacity, with wider slots used for heavier basis weight webs.

The drying process can be any noncompressive drying method which tends to preserve the bulk or thickness of the wet web including, without limitation, throughdrying, infra-red irradiation, microwave drying, or the like. Because of its commercial availability and practicality, throughdrying is a well-known and preferred means for noncompressively drying the web. Suitable throughdrying fabrics include, without limitation, Asten 920A and 937A, and Velostar P800 and 103A. The throughdrying fabrics may also include those disclosed in U.S. Patent 5,429,686 issued July 4, 1995, to Chiu et al. The web is preferably dried to final dryness without creping, since creping tends to lower the web strength and bulk.

While the mechanics are not completely understood, it is clear that the transfer fabric and throughdrying fabric can make separate and independent contributions to final sheet properties. For example, sheet surface smoothness as determined by a sensory panel can be manipulated over a broad range by changing transfer fabrics with the same throughdrying fabric. Webs produced by the present method and apparatus tend to be very two-sided unless calendered. Uncalendered webs may, however, be plied together with smooth/rough sides out as required by specific product forms. Numerous features and advantages of the present invention will appear from the following description. In the description, reference is made to the accompanying drawings which illustrate preferred embodiments of the invention. Such embodiments do not represent the full scope of the invention. Reference should therefore be made to the claims herein for interpreting the full scope of the invention.

Brief Description of the Drawings

Fig. 1 representatively shows a schematic process flow diagram illustrating a method and apparatus according to the present invention for making uncreped throughdried sheets.

Fig. 2 representatively shows an enlarged top plan view of an air press from the process flow diagram of Fig. 1.

Fig. 3 representatively shows a side view of the air press shown in Fig. 2, with portions broken away and shown in section for purposes of illustration. Fig. 4 representatively shows an enlarged section view taken generally from the plane of the line 4 - 4 in Fig. 3.

Fig. 5 representatively shows an enlarged section view similar to Fig. 4 but taken generally from the plane of the line 5 - 5 in Fig. 3.

Fig. 6 representatively shows a side view of an alternative sealing system for the air press shown in Figs. 2 and 3, with portions broken away and shown in section for purposes of illustration.

Fig. 7 representatively shows an enlarged side view of a vacuum transfer shoe shown in Fig. 2. Fig. 8 representatively shows an enlarged side view similar to Fig. 7 but illustrating the simultaneous convergence and divergence of fabrics at a leading edge of a vacuum slot.

Fig. 9 is a generalized plot of load/elongation curve for tissue, illustrating the determination of the MD Slope.

Detailed Description of the Invention

The invention will now be described in greater detail with reference to the Figures. Similar elements in different Figures have been given the same reference numeral for purposes of consistency and simplicity. In all of the embodiments, illustrated, conventional papermaking apparatus and operations can be used with respect to the headbox, forming fabrics, web transfers, drying and creping, all of which will be readily understood by those skilled in the papermaking art. Nevertheless, various conventional components are illustrated for purposes of providing the context in which the various embodiments of the invention can be used.

One embodiment of a method and apparatus for manufacturing a tissue is representatively shown in Fig. 1. For simplicity, the various tensioning rolls schematically used to define the several fabric runs are shown but not numbered. A papermaking headbox 20 injects or deposits an aqueous suspension of papermaking fibers 21 onto an endless forming fabric 22 traveling about a forming roll 23. The forming fabric 22 allows partial dewatering of the newly-formed wet web 24 to a consistency of about 10 percent.

After formation, the forming fabric 22 carries the wet web 24 to one or more vacuum or suction boxes 28, which may be employed to provide additional dewatering of the wet web 24 while it is supported on the forming fabric 22. In particular, a plurality of vacuum boxes 28 may be used to dewater the web 24 to a consistency of from about 20 to about 30 percent. The Fourdrinier former illustrated is particularly useful for making the heavier basis weight sheets useful as wipers and towels, although other forming devices such as twin wire formers, crescent formers or the like can be used instead. Hydroneedling, for example as disclosed in U.S. Patent No. 5,137,600 issued August 11 , 1992 to Barnes et al., can optionally be employed to increase the bulk of the web. Enhanced dewatering of the wet web 24 is thereafter provided by suitable supplemental noncompressive dewatering means, for example selected from the group consisting of an air press, infra-red drying, microwave drying, sonic drying, throughdrying, and displacement dewatering. In the illustrated embodiment, the supplemental noncompressive dewatering means comprises an air press 30, described in greater detail hereinafter. The air press 30 desirably raises the consistency of the wet web 24 to greater than about 30 percent, particularly greater than about 31 percent, more particularly greater than about 32 percent, and even more particularly greater than about 33 percent. In particular embodiments, the wet web 24 has a consistency exiting the air press 30 and prior to subsequent transfer of from about 31 to about 36 percent. In particular embodiments, the air press 30 increases the consistency of the wet web 24 by at least about 3 and preferably at least about 5 percent. Desirably, a support fabric 32 is brought in contact with the wet web 24 in advance of the air press 30. The wet web 24 is sandwiched between the support fabric 32 and the forming fabric 22, and thus supported during the pressure drop created by the air press 30. Fabrics suitable for use as a support fabric 32 include almost any fabric including forming fabrics such as Albany International 94M. The wet web 24 is then transferred from the forming fabric 22 to a transfer fabric

36 traveling at a slower speed than the forming fabric in order to impart increased stretch into the web. Transfer is preferably carried out with the assistance of a vacuum transfer shoe 37 as described hereinafter with reference to Figs. 7 and 8. The surface of the transfer fabric 36 is relatively smooth in order to provide smoothness to the wet web 24. The openness of the transfer fabric 36, as measured by its void volume, is relatively low and can be about the same as that of the forming fabric 22 or even lower.

The transfer fabric 36 passes over rolls 38 and 39 before the wet web 24 is transferred to a throughdrying fabric 40 traveling at about the same speed, or a different speed if desired. Transfer is effected by vacuum transfer shoe 42, which can be of the same design as that used for the previous transfer. The web 24 is dried to final dryness as the web is carried over a throughdryer 44.

Prior to being wound onto a reel 48 for subsequent conversion into the final product form, the dried web 50 can be carried through one or more optional fixed gap fabric nips formed between carrier fabrics 52 and 53. The bulk or caliper of the web 50 can be controlled by fabric embossing nips formed between rolls 54 and 55, 56 and 57, and 58 and 59. 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. Nip gaps between the various roll pairs can be from about 0.001 inch to about 0.02 inch (0.025 - 0.51 mm). As shown, the carrier fabric section of the machine is designed and operated with a series of fixed gap nips which serve to control the caliper of the web and can replace or compliment off-line calendering. Alternatively, a reel calender can be employed to achieve final caliper or complement off-line calendering. The air press 30 is shown in greater detail by the top view of Fig. 2 and the side view of Fig. 3, the latter having portions broken away for purposes of illustration. The air press 30 generally comprises an upper air plenum 60 in combination with a lower vacuum or suction box 62. The terms "upper" and "lower" are used herein to facilitate reference to and understanding of the drawings and are not meant to restrict the manner in which the components are oriented. The sandwich of the wet tissue web 24 between the forming fabric 22 and the support fabric 32 passes between the air plenum 60 and the vacuum box 62.

The illustrated air plenum 60 is adapted to receive a supply of pressurized fluid through air manifolds 64 operatively connected to a pressurized fluid source such as a compressor or blower (not shown). The air plenum 60 is fitted with a plenum cover 66 which has a bottom surface 67 that resides during use in close proximity to the vacuum box 62 and in close proximity to or contact with the support fabric 32 (Fig. 3). The plenum cover 66 is formed with slots 68 (Fig. 5) extending perpendicular to the machine direction across substantially the entire width of the wet web 24 to permit passage of pressurized fluid from the air plenum 60 through the fabrics and the wet web. The vacuum box 62 is operatively connected to a vacuum source and fixedly mounted to a support structure (not shown). The vacuum box 62 comprises a cover 70 having a top surface 72 over which the forming fabric 22 travels. The vacuum box cover 70 is formed with a pair of slots 74 (Figs. 3 and 5) that correspond to the location of the slots 68 in the plenum cover 66. The pressurized fluid dewaters the wet web 24 as the pressurized fluid is drawn from the air plenum 60 into and through the vacuum box 62. The fluid pressure within the air plenum 60 is desirably maintained at about 5 pounds per square inch (psi) (0.35 bar) or greater, and particularly within the range of from about 5 to about 30 psi (0.35 - 2.07 bar), such as about 15 psi (1.03 bar). The fluid pressure within the air plenum 60 is desirably monitored and controlled to a predetermined level.

The bottom surface 67 of the plenum cover 66 is desirably gently curved to facilitate web control. The surface 67 is curved toward the vacuum box 62, that is curved about an axis disposed on the vacuum box side of the web 24. The curvature of the bottom surface 67 allows a change in angle of the combination of the supporting fabric 32, the wet web 24, and the forming fabric 22 resulting in a net downward force that seals the vacuum box 62 against the entry of outside air and supports the wet web 24 during the dewatering process. The angle of curvature allows the loading and unloading of the air press 30 as required from time to time, based on process conditions. The change in angle necessary is dependent on the pressure differential between the pressure and vacuum sides and is desirably above 5 degrees, and particularly within the range of 5 to 30 degrees, typically about 7.5 degrees.

The top and bottom surfaces 72 and 67 desirably have differing radii of curvature. In particular, the radius of curvature of the bottom surface 67 is desirably larger than the radius of curvature of the top surface 72 so as to form contact lines between the air plenum 60 and the vacuum box 62 at the leading and trailing edges 76 of the air press 30 With proper attention to the position of the supporting fabric 32 and the forming fabric 22 sandwich and loading and unloading mechanisms, the radii of curvature of these surfaces may be reversed. The leading and trailing edges of the air press 30 may also be provided with end seals 78 (Fig. 3) that are maintained in very close proximity to or contact with the support fabric 32 at all times. The end seals 78 minimize the escape of pressurized fluid between the air plenum 60 and the vacuum box 62 in the machine direction. Suitable end seals 78 may be formed of resilient plastic compounds or the like. With additional reference to Figs. 4 and 5, the air press 30 is desirably provided with side seal members 80 to prevent the loss of pressurized fluid along the side edges 82 of the air press. The side seal members 80 comprise a semi-rigid material that is adapted to deform or flex slightly when exposed to the pressurized fluid of the air plenum 60. The illustrated side seal members 80 define a slot 84 for attachment to the vacuum box cover 70 using a clamping bar 85 and fastener 86 or other suitable means. In cross section, each side seal member 80 is L-shaped with a leg 88 projecting upward from the vacuum box cover 70 into a side seal slot 89 formed in the plenum cover 66. Pressurized fluid from the air plenum 60 causes the legs 88 to bend outward into sealing contact with the outward surface of the side seal slot 89 of the plenum cover 66, as shown in Figs 4 and 5. Alternatively, the position of the side seal members 80 could be reversed, such that they are fixedly attached to the plenum cover 66 and make sealing contact with contact surfaces defined by the vacuum box cover 70 (not shown). In any such alternative designs, it is desirable for the side seal member to be urged into engagement with the sealing contact surface by the pressurized fluid. A position control mechanism 90 maintains the air plenum 60 in close proximity to the vacuum box 62 and in contact with the support fabric 32. The position control mechanism 90 comprises a pair of levers 92 connected by crosspieces 93 and fixedly attached to the air plenum 60 by suitable fasteners 94 (Fig. 3). The ends of the levers 92 opposite the air plenum 60 are rotatably mounted on a shaft 96. The position control mechanism 90 also comprises a counterbalance cylinder 98 operably connecting a fixed structural support 99 and one of the crosspieces 93. The counterbalance cylinder 98 is adapted to extend or retract and thereby cause the levers 92 to rotate about the shaft 96, which causes the air plenum 60 to move closer to or further from the vacuum box 62. In use, a control system causes the counterbalance cylinder 98 to extend sufficiently for the end seals 78 to contact the support fabric 32 and the side seal members 80 to be positioned within the side seal slots 89. The air press 30 is activated such that pressurized fluid fills the air plenum 60 and the semi-rigid side seal members 80 are forced into sealing engagement with the plenum cover 66. The pressurized fluid also creates an upward force tending to move the air plenum 60 away from the support fabric 32. The control system directs operation of the counterbalance cylinder 98 to offset this upward force based on continuous measurements of the fluid pressure within the air plenum 60 by the pressure monitoring system. The end seals 78 are thereby maintained in very close proximity to or contact with the support fabric 32 at all times. The control system counters random pressure drops or peaks within the air plenum 60 by proportionately decreasing or increasing the force applied by the counterbalance cylinder 98. Consequently, the end seals 78 do not clamp the fabrics 32 and 22, which would otherwise lead to excessive wear of the fabrics.

An alternative sealing system for the air press 30 is representatively shown in Fig. 6. The air plenum 100 is provided with a pivotable arm 102 defining or carrying a sealing bar 104 that is adapted to ride on the support fabric 32 across the width of the wet web 24 to minimize escape of pressurized fluid in the machine direction. While only one arm 102 is illustrated in Fig. 6, it should be understood that a second arm at the opposite end of the air plenum 100 may be employed and constructed in a similar manner. The sides of the air plenum 100 may incorporate side seal members 80 as described in relation to Figs. 2 - 5 or be fixedly mounted on the vacuum box 62 to minimize or eliminate side leakage of pressurized fluid.

The pivotable arm 102 desirably comprises a rigid material such as structural steel, graphite composites, or the like. The arm 102 has a first portion 106 disposed at least partially inside the air plenum 100 and a second portion 108 preferably disposed outside the air plenum. The arm 102 is pivotally mounted on the air plenum 100 by a hinge 110. A hinge seal 112 impervious to the pressurized fluid is attached to both the interior surface of a wall 114 of the air plenum 100 and the first portion 106 to prevent escape of the pressurized fluid. The sealing bar 104 is desirably a separate element mounted on the first portion 106 and motivated toward the support fabric 32 (not shown in Fig. 6) by contact of the pressurized fluid on the first portion. Suitable sealing bars 104 may be formed of a low-resistance, low friction coefficient, durable material such as ceramic, heat resistant polymers, or the like. A counterbalance bladder 120 having an inflatable chamber 122 is mounted on the second portion 108 of the arm 102 with brackets 124 or other suitable means. The chamber 122 is operably connected to a source of pressurized fluid such as air to inflate the chamber. The arm 102 and the bladder 120 are positioned so that the bladder when inflated (not shown) presses against the exterior surface of the wall 114 of the air plenum 100 causing the arm to pivot about the hinge 110. Alternatively, a mechanism using pressurized cylinders (not shown) could be used in place of the counterbalance bladder as a means for pivoting the arm 102.

A control system is operable to inflate or deflate the bladder 120 proportionally in response to the pressure of the fluid within the air plenum 100. For example, as pressure within the air plenum 100 increases, the control system is adapted to increase pressure within or inflation of the counterbalance bladder 120 so that the sealing bar 104 does not clamp down excessively against the support fabric 32.

The design of the vacuum transfer shoe 37 used in the transfer fabric section of the process (Fig 1) is more clearly illustrated in Figs. 7 and 8. The vacuum transfer shoe 37 defines a vacuum slot 130 (Fig. 7) connected to a source of vacuum and having a length of "L" which is suitably from about 0.5 to about 1 inch (12.7 - 25.4 mm). For producing uncreped throughdried bath tissue, a suitable vacuum slot length is about 1 inch (25.4 mm). The vacuum slot 130 has a leading edge 132 and a trailing edge 133, forming corresponding incoming and outgoing land areas 134 and 135 of the vacuum transfer shoe 37. The trailing edge 133 of the vacuum slot 130 is recessed relative to the leading edge 132, which is caused by the different orientation of the outgoing land area 135 relative to that of the incoming land area 134. The angle "A" between the planes of the incoming land area 134 and the outgoing land area 135 can be about 0.5 degrees or greater, more specifically about 1 degree or greater, and still more specifically about 5 degrees or greater in order to provide sufficient separation of the forming fabric 22 and the transfer fabric 36 as they are converging and diverging.

Fig. 8 further illustrates the wet tissue web 24 traveling in the direction shown by the arrows toward the vacuum transfer shoe 37. Also approaching the vacuum transfer shoe 37 is the transfer fabric 36 traveling at a slower speed. The angle of convergence between the two incoming fabrics is designated as "C". The angle of divergence between the two fabrics is designated as "D". As shown, the two fabrics simultaneously converge and diverge at point "P", which corresponds to the leading edge 132 of the vacuum slot 130. It is not necessary or desirable that the web be in contact with both fabrics over the entire length of the vacuum slot 130 to effect the transfer from the forming fabric 22 to the transfer fabric 36. As is apparent from Figure 8, neither the forming fabric 22 nor the transfer fabric 36 need to be deflected more than a small amount to carry out the transfer, which can reduce fabric wear. Numerically, the change in direction of either fabric can be less than 5 degrees. As previously mentioned, the transfer fabric 36 is traveling at a slower speed than the forming fabric 22. If more than one transfer fabric is used, the speed differential between fabrics can be the same or different. Multiple transfer fabrics can provide operational flexibility as well as a wide variety of fabric/speed combinations to influence the properties of the final product. The level of vacuum used for the differential speed transfers can be from about 3 to about 15 inches of mercury, preferably about 5 inches 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 24 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).

Examples

The following EXAMPLES are provided to give a more detailed understanding of the invention. The particular amounts, proportions, compositions and parameters are meant to be exemplary, and are not intended to specifically limit the scope of the invention.

As referenced in relation to the Examples, MD Tensile strength, MD Stretch, and CD Tensile strength are obtained according to TAPPI Test Method 494 OM-88 "Tensile Breaking Properties of Paper and Paperboard" using the following parameters: Crosshead speed is 10.0 in/min (254 mm/min); full scale load is 10 lb (4,540 g); jaw span (the distance between the jaws, sometimes referred to as the gauge length) is 2.0 inches (50.8 mm); and specimen width is 3 inches (76.2 mm). The tensile testing machine is a Sintech, Model CITS-2000 from Systems Integration Technology Inc., Stoughton,

Massachusetts, a division of MTS Systems Corporation, Research Triangle Park, North Carolina.

The stiffness of the Example sheets can be objectively represented by either the maximum slope of the machine direction (MD) load/elongation curve for the tissue (hereinafter referred to as the "MD Slope") or by the machine direction Stiffness (herein defined), which further takes into account the caliper of the tissue and the number of plies of the product. Determining the MD Slope will be hereinafter described in connection with Figure 9. The MD Slope is the maximum slope of the machine direction load/elongation curve for the tissue. The units for the MD Slope are kilograms per 3 inches (7.62 centimeters). The MD Stiffness is calculated by multiplying the MD Slope by the square root of the quotient of the Caliper divided by the number of plies. The units of the MD Stiffness are (kilograms per 3 inches) -microns⁰⁵.

Figure 9 is a generalized load/elongation curve for a tissue sheet, illustrating the determination of the MD Slope. As shown, two points P1 and P2, the distance between which is exaggerated for purposes of illustration, are selected that lie along the load/elongation curve. The tensile tester is programmed (GAP [General Applications Program], version 2.5, Systems Integration Technology Inc., Stoughton, MA; a division of MTS Systems Corporation, Research Triangle Park, NC) such that it calculates a linear regression for the points that are sampled from P1 to P2. This calculation is done repeatedly over the curve by adjusting the points P1 and P2 in a regular fashion along the curve (hereinafter described). The highest value of these calculations is the Max Slope and, when performed on the machine direction of the specimen, wilf be referred to herein as the MD Slope.

The tensile tester program should be set up such that five hundred points such as P1 and P2 are taken over a two and one-half inch (63.5 mm) span of elongation. This provides a sufficient number of points to exceed essentially any practical elongation of the specimen. With a ten inch per minute (254 mm/min) crosshead speed, this translates into a point every 0.030 seconds. The program calculates slopes among these points by setting the 10th point as the initial point (for example P1), counting thirty points to the 40th point (for example, P2) and performing a linear regression on those thirty points. It stores the slope from this regression in an array. The program then counts up ten points to the 20th point (which becomes P1) and repeats the procedure again (counting thirty points to what would be the 50th point (which becomes P2), calculating that slope and also storing it in the array). This process continues for the entire elongation of the sheet. The Max Slope is then chosen as the highest value from this array. The units of Max Slope are kg per three-inch specimen width. (Strain is, of course, dimensionless since the length of elongation is divided by the length of the jaw span. This calculation is taken into account by the testing machine program.) Example 1 - 4. To illustrate the invention, a number of uncreped throughdried tissues were produced using the method substantially as illustrated in Fig. 1. More specifically, Examples 1 - 4 were all three-layered, single-ply bath tissues in which the outer layers comprised disperged, debonded eucalyptus fibers and the center layer comprised refined northern softwood kraft fibers. Cenebra eucalyptus fibers were pulped for 15 minutes at 10% consistency and dewatered to 30% consistency. The pulp was then fed to a Maule shaft disperger. The disperger was operated at 160° F. (70° C.) with a power input of 2.2 HPD/T (1.8 kilowatt-days per tonne). Subsequent to disperging, a softening agent (Witco C6027) was added to the pulp in the amount of 7.5 kg per metric ton dry fiber (0.75 weight percent). Prior to formation, the softwood fibers were pulped for 30 minutes at 3.2 percent consistency, while the disperged, debonded eucalyptus fibers were diluted to 2.5 percent consistency. The overall layered sheet weight was split 35%/30%/35% for Examples 1 , 2 and 4 and 33%/34%/33% for Example 3 among the disperged eucalyptus/refined softwood/disperged eucalyptus layers. The center layer was refined to levels required to achieve target strength values, while the outer layers provided softness and bulk. For added dry and temporary wet strength, a strength agent identified as Parez 631 NC was added to the center layer.

These examples employed a four-layer Beloit Concept III headbox. The refined northern softwood kraft stock was used in the two center layers of the headbox to produce a single center layer for the three-layered product described. Turbulence generating inserts recessed about three inches (75 millimeters) from the slice and layer dividers extending about six inches (150 millimeters) beyond the slice were employed. The net slice opening was about 0.9 inch (23 millimeters) and water flows in all four headbox layers were comparable. The consistency of the stock fed to the headbox was about 0.09 weight percent.

The resulting three-layered sheet was formed on a twin-wire, suction form roll, former with forming fabrics being Appleton Mills 2164-B fabrics. Speed of the forming fabric ranged between 11.8 and 12.3 meters per second. The newly-formed web was then dewatered to a consistency of 25 - 26% using vacuum suction from below the forming fabric without air press, and 32 - 33% with air press before being transferred to the transfer fabric which was traveling at 9.1 meters per second (29 - 35% rush transfer). The transfer fabric was Appleton Mills 2164-B. A vacuum shoe pulling about 6 - 15 inches (150 - 380 millimeters) of mercury vacuum was used to transfer the web to the transfer fabric.

The web was then transferred to a throughdrying fabric traveling at a speed of about 9.1 meters per second. Appleton Mills T124-4 and T124-7 throughdrying fabrics were used. The web was carried over a Honeycomb throughdryer operating at a temperature of about 350° F. (175° C.) and dried to a final dryness of about 94 - 98% consistency.

The sequence of producing the Example sheets was as follows: Four rolls of the Example 1 sheets were produced. The consistency data reported in Table 1 is based on 2 measurements, one at the beginning and one at the end of the 4 rolls. The other data shown in Table 1 represents an average based on 4 measurements, one per roll. The air press was then turned on. Data just prior to and just after activation of the air press is shown in Table 3 (individual data points). This data shows that the air press caused significant increases in tensile values. The process was then modified to decrease the tensile values to levels comparable to the Example 1 sheets. After this process adjustment period, four rolls of the Example 2 sheets (this invention) were produced. Later, 4 rolls of the Example 3 sheets (this invention) were produced using a different throughdrying fabric and with the air press activated. The air press was shut off and the process adjusted to regain tensile strength values comparable to the Example 3 sheets. Four rolls of Example 4 sheets were then produced. The consistency data for each Example in Table 2 is an average based on 2 measurements, one at the beginning and one at the end of each set of 4 rolls. The other data in Table 2 is based on an average of 4 measurements per Example sheet, one per roll. In Table 2, the Example 4 data is presented in the left column and the Example 3 data is presented in the right column to remain consistent with Tables 1 and 3, which show data without the air press in the left column and data with the air press in the right column.

Tables 1 - 3 give more detailed descriptions of the process condition as well as resulting tissue properties for examples 1 - 4. As used in Tables 1 - 3 below, the column headings have the following meanings: "Consistency @ Rush Transfer" is the consistency of the web at the point of transfer from the forming fabric to the transfer fabric, expressed as percent solids; "MD Tensile" is the machine direction tensile strength, expressed in grams per 3 inches (7.62 centimeters) of sample width; "CD Tensile" is the cross-machine tensile strength, expressed as grams per 3 inches (7.62 centimeters) of sample width; "MD Stretch" is the machine direction stretch, expressed as percent elongation at sample failure; "MD Slope" is as defined above, expressed as kilograms per 3 inches (7.62 centimeters) of sample width; "Caliper" is the 1 sheet caliper measured with a Bulk Micrometer (TMI Model 49-72-00, Amityville, New York) having an anvil diameter of 4 1/16 inches (103.2 mm) and an anvil pressure of 220 grams/square inch (3.39 Kilo Pascals), expressed in microns; "MD Stiffness" is the Machine Direction Stiffness Factor as defined above, expressed as (kilograms per 3 inches) -microns⁰⁵; "Basis Weight" is the finished basis weight, expressed as grams per square meter; "TAD Fabric" means throughdrying fabric; "Refiner" is power input to refine the center layer, expressed as kilowatts; "Rush" is the difference in speed between the forming fabric and the slower transfer fabric, divided by the speed of the transfer fabric and expressed as a percentage; "HW/SW" is the breakdown of weight of hardwood (HW) and softwood (SW) fibers in the three-layered, single-ply tissues, expressed as a percent of total fiber weight; and "Parez" is the add-on rate of Parez 631 NC expressed as kilograms per metric ton of the center layer fiber.

Table 1

EXAMPLE 1 EXAMPLE 2

(No Air (With Air Press

Press) and Process Adjustment)

Consistency @ Rush Transfer (%) 25.2 - 26.1 32.5 - 33.4

MD Tensile (grams/3") 933 944

CD Tensile (grams/3") 676 662

MD Stretch (%) 24.5 24.7

MD Slope (kg/3") 4.994 3.778

Caliper (microns) 671 607

MD Stiffness (kg/3"-microns⁰⁵) 129 93

Basis Weight (gsm) 34.6 35.2

TAD Fabric T-124-4 T-124-4

Refiner (kW) 32 26

Rush (%) 32 29

HW/SW (%) 70/30 70/30

Parez (kg/mt) 4.0 3.2

Table 2

EXAMPLE 4 EXAMPLE 3

(No Air (With Air Press

Press) and Process Adjustment)

Consistency @ Rush Transfer (%) 24.6 32.4

MD Tensile (grams/3") 961 907

CD Tensile (grams/3") 714 685

MD Stretch (%) 23.5 24.4

MD Slope (kg/3") 5.668 3.942

Caliper (microns) 716 704

MD Stiffness (kg/3"-mιcrons⁰⁵) 152 105

Basis Weight (gsm) 35.0 35.1

TAD Fabric T- 124-7 T-124-7

Refiner (kW) 40 34.5

Rush (%) 35 31

HW/SW (%) 66/34 70/30

Parez (kg mt) 2.5 2 5

Table 3

Press) Press)

Consistency @ Rush Transfer (%) 25.2 32.5

MD Tensile (grams/3") 915 1099

CD Tensile (grams/3") 661 799

CD Wet Tensile 127 150

MD Stretch (%) 24 4 28.5

MD Slope (kg/3") 4 996 4 028

Caliper (microns) 665 630

MD Stiffness (kg/3"-mιcrons⁰⁵) 129 101

Basis Weight (gsm) 34.3 34.6

TAD Fabric T-124-4 T-124-4

Refiner (kW) 32 32

Rush (%) 32 32

HW/SW (%) 70/30 70/30

Parez (kg/mt) 4.0 4.0

As shown by the previous Examples, the air press produces significantly higher consistencies upstream of the differential speed transfer which result in smoother sheets as evidenced by lower modulus values Desirably, the modulus (MD Stiffness) of tissue products is at least 20 percent less than that of a comparable tissue product made without supplementally dewatering to a consistency of greater than about 30 percent. Further, the machine direction tensile of the tissue products is at least 20 percent greater, and the cross direction tensile of the tissue products is at least 20 percent greater, than that of a comparable tissue product made without supplementally dewatering to a consistency of greater than about 30 percent Additionally, the machine direction stretch of tissue products is at least 17 percent greater than that of a comparable tissue product made without supplementally dewatering to a consistency of greater than about 30 percent

The foregoing detailed description has been for the purpose of illustration Thus, a number of modifications and changes may be made without departing from the spirit and scope of the present invention For instance, alternative or optional features descπbed as part of one embodiment can be used to yield another embodiment. Additionally, two named components could represent portions of the same structure Further, various process and equipment arrangements as disclosed in PCT Patent Application Publication WO 95/00706 dated January 5, 1995, and U S Patent Application Serial No 08/330,166 filed October 27, 1994, by Engel et al and titled

"Method For Making Smooth Uncreped Throughdried Sheets", the disclosures of which are incorporated herein by reference, may be employed Therefore, the invention should not be limited by the specific embodiments described, but only by the claims

Claims

ClaimsWe claim:

1. A method of making a soft tissue sheet, comprising the steps of: depositing an aqueous suspension of papermaking fibers onto an endless forming fabric to form a wet web; dewatering the wet web to a consistency of from about 20 to about 30 percent; supplementally dewatering the wet web using noncompressive dewatering means to a consistency of greater than about 30 percent; transferring the supplementally dewatered web to a transfer fabric traveling at a speed of from about 10 to about 80 percent slower than the forming fabric; transferring the web to a throughdrying fabric; and throughdrying the web to a final dryness.

2. The method of claim 1, wherein the noncompressive dewatering means is selected from the group consisting of an air press, infra-red drying, microwave drying, sonic drying, throughdrying, and displacement dewatering.

3. The method of claim 1 , wherein the noncompressive dewatering means comprises an air press.

4. The method of claim 3, wherein the air press increases the consistency of the wet web by at least about 3 percent.

5. The method of claim 3, wherein the air press comprises an air plenum and fluid pressure within the air plenum is maintained within the range of about 5 to about 30 pounds per square inch.

6. The method of claim 3, 4 or 5, wherein the air press provides a pressure differential across the wet web of from about 35 to about 60 inches of mercury.

7. The method of claim 3, 4 or 5, wherein the air press dewaters the wet web to a consistency of greater than about 31 percent.

8. The method of claim 7, wherein the air press dewaters the wet web to a consistency of greater than about 32 percent.

9. The method of claim 3, 4 or 5, wherein the air press dewaters the wet web to a consistency of from about 31 to about 36 percent.

10. The method of claim 1, wherein dewatering the wet web to a consistency of from about 20 to about 30 percent is accomplished using a plurality of vacuum boxes.

11. The method of claim 3, wherein the wet web is sandwiched between the forming fabric and a support fabric when transported through the air press.

12. The method of claim 1, 3, or 4, wherein the forming fabric travels at a speed of at least about 2000 feet per minute.

13. The tissue product made by the method of claim 1.

14. The tissue product of claim 13, wherein the modulus of the tissue product is at least about 20 percent less than that of a comparable tissue product made by the method of claim 1 except without supplementally dewatering to a consistency of greater than about 30 percent.

15. The tissue product of claim 13, wherein the machine direction tensile of the tissue product is at least about 20 percent greater than that of a comparable tissue product made by the method of claim 1 except without supplementally dewatering to a consistency of greater than about 30 percent.

16. The tissue product of claim 13, wherein the cross direction tensile of the tissue product is at least about 20 percent greater than that of a comparable tissue product made by the method of claim 1 except without supplementally dewatering to a consistency of greater than about 30 percent.

17. The tissue product of claim 13, wherein the machine direction stretch of the tissue product is at least about 17 percent greater than that of a comparable tissue product made by the method of claim 1 except without supplementally dewatering to a consistency of greater than about 30 percent.

18. An air press for dewatering a wet web, comprising: an air plenum comprising a plenum cover having a bottom surface; means for supplying pressurized fluid to the air plenum; a vacuum box comprising a vacuum box cover having a top surface positioned in close proximity to the bottom surface of the plenum cover; means for applying vacuum to the vacuum box; and side seal members adapted to contact the air plenum and the vacuum box for minimizing escape of the pressurized fluid, the side seal members attached to one of the air plenum and the vacuum box and positioned in close proximity to side seal contact surfaces defined by the other of the air plenum and the vacuum box, the side seal members adapted to flex into sealing contact with the side seal contact surface upon exposure to the pressurized fluid.

19. The air press of claim 18, wherein the side seal members are attached to the vacuum box cover, and the plenum cover defines side seal slots and the side seal contact surfaces.

20. The air press of claim 18, further comprising end seals attached to the plenum cover.

21. The air press of claim 18 or 20, further comprising a position control mechanism adapted to maintain the air plenum in close proximity to the vacuum box.

22. The air press of claim 21 , wherein the position control mechanism comprises a rotatably mounted lever attached to the air plenum and a counterbalance cylinder adapted to rotate the lever.

23. The air press of claim 21 , further comprising a control system adapted to direct operation of the counterbalance cylinder in response to measurements of fluid pressure within the air plenum.

24. The air press of claim 18, 19 or 20, wherein the top and bottom surfaces are curved toward the vacuum box.

25. The air press of claim 24, wherein the top and bottom surfaces have differing radii of curvature.

26. An air press for dewatering a wet web, comprising: an air plenum comprising a plenum cover having a bottom surface; means for supplying pressurized fluid to the air plenum; a vacuum box comprising a vacuum box cover having a top surface positioned in close proximity to the bottom surface of the plenum cover; means for applying vacuum to the vacuum box; an arm pivotally mounted on the air plenum and comprising first and second portions, the first portion disposed at least partially inside the air plenum and comprising a sealing bar; and means for pivoting the arm in response to fluid pressure within the air plenum.

27. The air press of claim 26, further comprising a hinge seal impervious to pressurized fluid and attached to both the air plenum and the first portion.