Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
2000-04-24
2001-12-18
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S109000, C162S111000
Reexamination Certificate
active
06331230
ABSTRACT:
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.
Moreover, from a manufacturing perspective, the throughdrying process is relatively energy intensive and therefore expensive compared to wet pressing. The high temperatures required for the throughdrying process also detrimentally affects the useful life of fabrics used in the manufacturing process.
Therefore, what is lacking and needed in the art is a method for manufacturing tissue products having improved softness, and in particular throughdried tissue products having improved softness, as well as a more economical method for manufacturing throughdried tissue products.
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.
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.
One particularly desirable means by which the web can be dewatered to about 30 percent consistency or greater 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 disclosed herein overcomes these difficulties and provides a practical apparatus for dewatering a wet web to consistency levels not previously thought possible at industrially useful speeds without thermal dewatering.
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 enh
Burazin Mark Alan
Hada Frank Stephen
Hermans Michael Alan
Hong Sung Ho
Charlier Patricia A.
Chin Peter
Kimberly--Clark Worldwide, Inc.
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