Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
1997-10-31
2001-02-13
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S111000, C162S112000, C162S113000, C162S205000, C162S206000, C162S207000
Reexamination Certificate
active
06187137
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to methods for making tissue products. More particularly, the invention concerns methods for making an uncreped tissue on a modified conventional wet-pressing machine.
In the art of tissue making, large steam-filled cylinders known as Yankee dryers are commonly used to dry a tissue web that is pressed onto the dryer cylinder surface while the tissue web is still wet. In conventional tissue making, the wet paper web is firmly pressed against the surface of the Yankee dryer. The compression of the wet web against the drum provides intimate contact for rapid heat transfer into the web. As the web dries, adhesive bonds form between the surface of the Yankee dryer and the tissue web, often promoted by sprayed-on adhesive applied before the point of contact between the wet web and the dryer surface. The adhesive bonds are broken when the flat, dry web is scraped off the dryer surface by a creping blade, which imparts a fine, soft texture to the web, increases bulk, and breaks many fiber bonds for improved softness and reduced stiffness.
Traditional creping suffers from several drawbacks. Because the sheet is pressed flat against the Yankee, the hydrogen bonds that develop as the web dries are formed between the fibers in a flat, dense state. Although creping imparts many kinks and deformations in the fibers and adds bulk, when the creped sheet is wetted, the kinks and deformations relax as the fibers swell. As a result, the web tends to return to the flat state set when the hydrogen bonds were formed. Thus, a creped sheet tends to collapse in thickness and expand laterally in the machine direction upon wetting, often becoming wrinkled in the process if some parts of the laterally expanding web are restrained, still dry, or held against another surface by surface tension forces.
Further, creping limits the texture and bulk that can be imparted to the web. Relatively little can be done with the conventional operation of Yankees to produce a highly textured web such as the throughdried webs that are produced on textured throughdrying fabrics. The flat, dense structure of the web upon the Yankee sharply limits what can be achieved in terms of the subsequent structure of the product coming off the Yankee.
Another drawback of traditional creping is that the doctor blades used to effect creping on papermaking machines are subject to wear due to contact with the surface of the rotating cylinder. As wear progresses, the effectiveness of the doctor blade is diminished, which leads to progressively more variability in the tissue properties. Creping blades are commonly replaced after a product property of particular importance, such as stretch, bulk, or machine direction tensile strength, has changed from predetermined target levels. Changing creping blades requires considerable down-time and slows production.
The foregoing drawbacks of traditional creping may be avoided by producing an uncreped throughdried tissue web. Such webs may be produced with a bulky three-dimensional structure rather than being flat and dense, thereby providing good wet resiliency. It is known, however, that uncreped tissue often tends to be stiff and lacks the softness of creped products. Additionally, throughdried webs sometimes suffer from pinholes in the web due to the flow of air through the web to achieve full dryness. Moreover, most of the world's paper machines use conventional Yankee dryers and tissue manufacturers are reluctant to accept the high cost of adding throughdrying technology or the higher operating costs associated with throughdrying.
Prior attempts to make an uncreped sheet on a drum dryer or Yankee have included wrapping the sheet around the dryer. For example, cylinder dryers have long been used for heavier grades of paper. In conventional cylinder drying, the paper web is carried by dryer fabrics which wrap the cylinder dryer to provide good contact and prevent sheet flutter. Unfortunately, such wrapping configurations are not practical for converting a modern creped tissue machine into an uncreped tissue machine. Typical creped tissue machines employ a Yankee dryer with a heated hood in which high velocity, high temperature air is used to dry the web at rates well above those possible with conventional cylinder dryers. Most dryer fabrics would deteriorate rapidly under the high temperatures of a dryer hood, and they would interfere with heat transfer to the web. Further, the design of a conventional Yankee hood does not allow an endless loop of fabric to wrap the web through the dryer hood, without prohibitively expensive modifications to the equipment.
Therefore, there is a need for a method for making an uncreped tissue having a three-dimensional structure and offering good wet resilience, high softness and flexibility using a conventional papermaking machine including a Yankee dryer and drying hood. More particularly, there is a need for an adhesion control system which adequately adheres the web to the dryer surface to promote conductive heat transfer and resist blowing forces, while being bound loosely enough to allow the web to be pulled off the dryer surface in uncreped mode without damage to the web.
SUMMARY OF THE INVENTION
In response to the needs described above, it has been discovered that a soft, high bulk, textured, wet resilient tissue web can be produced using a conventional Yankee dryer or cylinder dryers in place of large and expensive throughdryers in the production of wet-laid tissue. Indeed, existing wet-pressed creped tissue machines can be economically modified to produce high quality uncreped tissue with properties similar to throughdried materials. High-speed production of such a web with excellent runnability is made possible through an adhesion control system that is adapted to restrain the sheet on the Yankee during drying while still permitting removal after the sheet has been dried. The adhesion control system comprises an interfacial control mixture that can extend the upper limit of the speed of operation of the tissue machine without sheet failure. The interfacial control mixture is especially useful when the tissue sheet is dewatered to a consistency of at least 30 percent prior to the Yankee.
More specifically, the wet web is provided with a three-dimensional high bulk structure before being attached to the cylindrical dryer surface. This is desirably achieved through a combination of using specially treated fibers, such as curled or dispersed papermaking fibers, rush transferring the moist web from a faster to a slower moving fabric, and/or molding the web onto a structured, textured fabric. The three-dimensional structure is characterized by having a substantially uniform density because the sheet is molded on a three-dimensional substrate rather than creating regions of high and low density through compressive means. The three dimensionality of the structure is promoted by noncompressively dewatering the web before attachment to the Yankee.
Thereafter, the web is desirably attached to the Yankee or other heated dryer surface in a manner that preserves a substantial portion of the texture imparted by previous treatments, especially the texture imparted by molding on three-dimensional fabrics. In particular, the web is attached to the dryer surface using a foraminous fabric that promotes good contact while preserving a degree of texture. Such a fabric preferably has low fabric coarseness and is relatively free of isolated protrusions. The conventional manner used to produce wet-pressed creped paper is inadequate for preserving a three-dimensional structure, for in that method, a pressure roll is used to dewater the web and to uniformly press the web into a dense, flat state. For the present invention, the conventional substantially smooth press felt is replaced with a textured material such as a foraminous fabric and desirably a throughdrying fabric, a textured felt, a textured nonwoven or the like.
For best results, significantly lower pressing pressures can be used as compared to con
Chen Shan Liang
Druecke Frank Gerald
Hermans Michael Alan
Hu Sheng-Hsin
Kamps Richard Joseph
Charlier Patricia A.
Chin Peter
Gage Thomas M.
Kimberly--Clark Worldwide, Inc.
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