Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
2000-06-30
2002-09-24
Nguyen, Dean T. (Department: 3629)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S207000, C162S290000, C162S297000
Reexamination Certificate
active
06454904
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to methods for making paper products. More particularly, the invention concerns methods for making cellulosic webs having high bulk and absorbency on a modified conventional wet-pressed machine.
There are generally two different methods for making the base sheets for paper products such as paper towels, napkins, tissue, wipes and the like. These methods are commonly referred to as wet-pressing and throughdrying. While the two methods may be the same at the front end and back end of the process, they differ significantly in the manner in which water is removed from the wet web after its initial formation.
More specifically, in the wet-pressing method, the newly-formed wet web is typically transferred onto a papermaking felt and thereafter pressed against the surface of a steam-heated Yankee dryer while it is still supported by the felt. As the web is transferred to the surface of the Yankee dryer, water is expressed from the web and is absorbed by the felt. The dewatered web, typically having a consistency of about 40 percent, is then dried while on the hot surface of the Yankee dryer. The web is then creped to soften it and provide stretch to the resulting tissue sheet. A disadvantage of wet pressing is that the pressing step densifies the web, thereby decreasing the bulk and absorbency of the tissue sheet. The subsequent creping step only partially restores these desirable sheet properties.
In the throughdrying method, the newly-formed web is first dewatered using vacuum and then transferred to a relatively porous fabric and non-compressively dried by passing hot air through the web. The resulting web can then be transferred to a Yankee dryer for creping. Because the web is substantially dry when transferred to the Yankee dryer, the density of the web is not significantly increased by the transfer. Also, the density of a throughdried tissue sheet is relatively low by nature because the web is dried while supported on the throughdrying fabric. The disadvantages of the throughdrying method are the relatively high operational energy costs and the capital costs associated with the throughdryers.
Because the vast majority of existing tissue machines utilize the older wet-pressing method, it is of particular importance that manufacturers find ways to modify existing wet-pressed machines to produce the consumer-preferred low-density products without expensive modifications to the existing machines. Of course, it is possible to re-build wet-pressed machines to throughdried configurations, but this is usually prohibitively expensive. Many complicated and expensive changes are necessary to accommodate the throughdryers and associated equipment. In addition, the length of a through-air dried tissue machine is greater, requiring a building addition or modification. In some locations, building modifications are not practical or possible, or prohibitively expensive because of the interference with other existing equipment or limited area available on the site. Accordingly, there has been great interest in finding ways to modify existing wet-pressed machines without significantly altering the machine design.
As a specific example, an approach to modifying a crescent-former tissue machine is particularly desirable, as there are many existing crescent-former tissue machines that could benefit from the consumer-preferred low-density products that can be made with the improved process. Many older crescent-former tissue machines were provided with a lower felt run that could be easily adapted to serve as an additional fabric run required for certain embodiments of this invention. This invention discloses a simple method for modifying a crescent-former tissue machine.
One simple approach to modifying a wet-pressed machine to produce softer, bulkier tissue is described in U.S. Pat. No. 5,230,776 issued Jul. 27, 1993 to Andersson et al. The patent discloses replacing the felt with a perforated belt of wire type and sandwiching the web between the forming wire and this perforated belt up to the press roll. The patent also appears to disclose additional dewatering means, such as a steam blowing tube, a blowing nozzle, and/or a separate press felt, that may be placed within the range of the sandwich structure in order to further increase the dry solids content before the Yankee dryer. These extra drying devices are said to permit the machine to run at speeds at least substantially equivalent to the speed of throughdrying machines.
It is important to reduce the moisture content of the web coming onto the Yankee dryer, to maintain machine speed and to prevent blistering or lack of adhesion of the web. Referring to U.S. Pat. No. 5,230,776, the use of a separate press felt, however, tends to densify the web in the same manner as a conventional wet-pressed machine. The densification resulting from a separate press felt would thus negatively impacting the bulk and absorbency of the web.
Further, jets of air for dewatering the web are not per se effective in terms of water removal or energy efficiency. Blowing air on the sheet for drying is well known in the art and used in the hoods of Yankee dryers for convective drying. In a Yankee dryer hood, however, the vast majority of the air from the jets does not penetrate the web. Thus, if not heated to high temperatures, most of the air would be wasted and not effectively used to remove water. In Yankee dryer hoods, the air is heated to as high as 900 degrees Fahrenheit and high residence times are allowed in order to effectuate drying.
Thus, what is lacking and needed in the art is a practical method for making tissue sheets having high bulk and absorbency comparable to throughdried sheets on a modified, conventional wet-pressed machine.
SUMMARY OF THE INVENTION
It has now been discovered that a wet-pressed tissue can be made having bulk and absorbency properties equivalent to those of comparable throughdried products, while maintaining reasonable machine productivity. More particularly, wet-pressed cellulosic webs can be made by vacuum dewatering a wet web up to approximately 30 percent consistency, then using an integrally sealed air press to noncompressively dewater the sheet to 30 to 40 percent consistency. The wet web is desirably then transferred to a “molding” fabric substituted for the conventional wet-pressing felt in order to impart more contour or three-dimensionality to the wet web. The wet web is preferably thereafter pressed against the Yankee dryer while supported by the molding fabric and dried. The resulting product has exceptional wet bulk and absorbency exceeding that of conventional wet-pressed towels and tissue and equal to that of presently available throughdried products.
As used herein, “noncompressive dewatering” and “noncompressive drying” refer to dewatering or drying methods, respectively, for removing water from cellulosic webs that do not involve compressive nips or other steps causing significant densification or compression of a portion of the web during the drying or dewatering process.
The wet web is wet-molded in the process to improve the three-dimensionality and absorbent properties of the web. As used herein, “wet-molded” tissue sheets are those which are conformed to the surface contour of a molding fabric while at a consistency of about 30 to about 40 percent and then dried by thermal conductive drying means, such as a heated drying cylinder, as opposed to other drying means such as a throughdryer, before optional additional drying means.
The “molding fabrics” suitable for purposes of this invention include, without limitation, those papermaking fabrics which exhibit significant open area or three-dimensional surface contour sufficient to impart greater z-directional deflection of the web. Such 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 molding fabric that is in contact with the wet web (the top side), the number of machine dire
Hada Frank Stephen
Hermans Michael Alan
Charlier Patricia A.
Kimberly--Clark Worldwide, Inc.
Nguyen Dean T.
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