Method for making high bulk wet-pressed tissue

Details Method for making high bulk wet-pressed tissue Method for making high bulk wet-pressed tissue

1996-05-10

2002-02-26

Chin, Peter (Department: 1731)

Paper making and fiber liberation

Processes and products

Non-uniform, irregular or configured web or sheet

C162S112000, C162S113000, C162S146000, C162S157600, C162S205000, C162S206000

Reexamination Certificate

active

06350349

ABSTRACT:

BACKGROUND OF THE INVENTION
In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels and the like, the tissue sheet is formed by depositing an aqueous suspension of papermaking fibers onto a forming fabric. The web is then transferred to a papermaking felt and dewatered as it passes through a pressure nip created between a pressure roll and a Yankee dryer as the wet web is transferred to the Yankee surface. Free water expressed from the web in the pressure nip is absorbed and carried away by the felt as the web transfers to the Yankee surface. The web is then final dried on the surface of the Yankee and subsequently creped to impart bulk and softness to the resulting tissue sheet. This method of making tissue sheets is commonly referred to as “wet-pressing” because of the method used to dewater the wet web.
The wet-pressing method has a couple of distinct drawbacks. First, pressing the tissue web while wet densifies the web significantly. As the web is dried, the dried sheet retains this high density (low bulk) until it is creped. Creping is necessary to attempt to undo what the wet-pressing has done to the sheet. In response to this situation, through-air-drying methods have been developed in which the newly-formed web is partially dewatered to about 30 percent consistency using vacuum suction. Thereafter the partially dewatered web is final dried without compression by passing hot air through the web while it is supported by a throughdrying fabric. However, through-air-drying is expensive in terms of capital and energy costs.
A second drawback, shared by conventional wet-pressing and through-air-drying processes is the high energy costs necessary to dry the web from a consistency of about 35 percent to a final dryness of about 95 percent. This second drawback has recently been addressed in the manufacture of high density paper products by the advent of the high intensity extended nip press. This device employs an extended nip length and heat to more efficiently dewater the wet web up to exit consistencies of about 60 percent. Such devices have been successfully used for making paperboard, but have not been used to make low density paper products such as tissues because the high pressures and longer dwell times in the extended nip press serve to further densify the sheet beyond that experienced by conventional tissue wet-pressing methods. This increase in density is detrimental to the quality of the resulting tissue products because creping cannot completely overcome the added increase in sheet density.
Therefore there is a need for a method of making wet-pressed tissue sheets that minimizes or eliminates the high densities imparted to wet-pressed tissue webs.
SUMMARY OF THE INVENTION
It has now been discovered that the reduction in bulk associated with wet-pressing can be substantially reduced by incorporating into the web certain fibers which have been found to greatly diminish web densification when subjected to the high pressures necessary for dewatering with high intensity extended nip presses. As a consequence, high intensity extended nip presses can be used to dewater tissue webs without the heretofore adverse consequence of imparting a high degree of densification to the web.
Hence in one aspect the invention resides in a method for making a bulky tissue sheet comprising: (a) depositing an aqueous suspension of papermaking fibers onto a forming fabric to form a wet tissue web, said papermaking fibers comprising at least about 10 dry weight percent modified wet-resilient fibers; (b) partially dewatering the wet web to a consistency of about 15 percent or greater; (c) compressing the partially dewatered web in a high intensity extended nip press to further dewater the web to a consistency of about 35 percent or greater; and (d) final drying the web, wherein the Bulk of the dewatered web prior to final drying is greater than (−0.02C+3.11), wherein “C” is the consistency of the web leaving the high intensity extended nip press, expressed as percent dryness, and Bulk is expressed as cubic centimeters per gram. For a given consistency, the wet tissue webs of this invention have greater bulk than comparable wet tissue webs that have been dewatered by conventional means. Furthermore, the consistency can be increased well beyond that attainable by conventional tissue dewatering and, in most instances, still have a higher bulk at higher consistencies than that of conventional wet tissue webs at substantially lower consistencies.
In another aspect, the invention resides in the combination of dewatering a tissue web using a high intensity extended nip press, which greatly reduces the bulk of the tissue web, followed by rush transferring the dewatered web to increase the bulk of the web back to levels suitable for tissue. More specifically, the invention resides in a method for making a bulky tissue sheet comprising: (a) depositing an aqueous suspension of papermaking fibers onto a forming fabric to form a wet tissue web; (b) partially dewatering the wet web to a consistency of about 15 percent or greater; (c) compressing the partially dewatered web in a high intensity extended nip press to further dewater the wet web to a consistency of about 35 percent or greater; (d) transfering the dewatered web to a first transfer fabric; (e) transfering the dewatered web from the first transfer fabric to a second transfer fabric travelling at a slower speed than the first transfer fabric (rush transfer) to increase the bulk of the wet web; and (f) drying the web. The web can be dried on a Yankee dryer and creped, or the web can be throughdried and left uncreped or creped.
As used herein, “modified wet-resilient fibers” are fibers that have been modified from their natural state and have the capability to recover after deformation in the wet state, as opposed to fibers that remain deformed and do not recover after deformation in the wet state. Examples of modified wet-resilient fibers include, without limitation, chemically cross-linked cellulosic fibers, heat-cured cellulosic fibers, mercerized fibers and sulfonated pulp fibers. These fiber modification methods are well known in the art. The amount of modified wet-resilient fibers in the fiber furnish can be about 10 dry weight percent or greater, more specifically from about 20 to about 80 percent, and still more specifically from about 30 to about 60 percent. The bulk benefits associated with using modified wet-resilient fibers increase as the amount of the modified wet-resilient fibers increases. Consequently the amount used must take into account the desireability for added bulk versus other desired properties, such as tensile strength, that other fibers may be better suited to provide.
A “high intensity extended nip press”, as used herein, is a water-removing pressing apparatus wherein the wet web is compressed in an extended nip formed between the arcuate surface of a backing roll and a pressing fabric or blanket. Typically the pressing fabric is supported by a press shoe having a concave surface. The backing roll can be heated to elevated temperatures or remain at ambient temperature. The length of the extended nip can be substantial, typically from about 5 to about 10 inches or more. Such devices permit the operator to vary conditions such as dwell time, pressure and temperature to effect greater water removal than can normally be obtained in a conventional roll press. Such an apparatus can remove substantially all of the free water in the sheet and a significant portion of the bound water as well. An example of such an apparatus is disclosed and described in U.S. Pat. No. 4,973,384 issued Nov. 27, 1990 to Crouse et al. entitled “Heated Extended Nip Press Apparatus”, which is herein incorporated by reference. In operating the high intensity extended nip press, the use of a heated press roll in the extended nip is optional, although preferred for maximum water removal.
The consistency (weight percent fiber or percent dryness) of the partially dewatered web entering the high intensity extended

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