Paper making and fiber liberation – Processes and products – Multi-layer waterlaid webs or sheets
Reexamination Certificate
2002-09-27
2004-12-07
Fortuna, José A. (Department: 1731)
Paper making and fiber liberation
Processes and products
Multi-layer waterlaid webs or sheets
C162S111000, C162S129000, C162S130000, C428S153000
Reexamination Certificate
active
06827818
ABSTRACT:
BACKGROUND OF THE INVENTION
In the manufacture of tissue products such as bath tissue, a wide variety of product characteristics must be given attention in order to provide a final product with the appropriate blend of attributes suitable for the product's intended purposes. Among these various attributes, improving softness has always been a major objective for premium products. Major components of softness include stiffness and bulk (density), with lower stiffness and higher bulk (lower density) generally improving perceived softness.
Traditionally, tissue products have been made using a wet-pressing process in which a significant amount of water is removed from a wet laid web by pressing or squeezing water from the web prior to final drying. In particular, while supported by an absorbent papermaking felt, the web is squeezed between the felt and the surface of a rotating heated cylinder (Yankee dryer) using a pressure roll as the web is transferred to the surface of the Yankee dryer for final drying. The dried web is thereafter dislodged from the Yankee dryer with a doctor blade (creping), which serves to partially debond the dried web by breaking many of the bonds previously formed during the wet-pressing stages of the process. Creping generally improves the softness of the web, albeit at the expense of a significant loss in strength.
More recently, throughdrying has become a more prevalent means of drying tissue webs. Throughdrying provides a relatively noncompressive method of removing water from the 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 it is dry. The resulting dried web is softer and bulkier than a wet-pressed uncreped dried sheet because fewer papermaking bonds are formed and because the web is less dense. Squeezing water from the wet web is eliminated, although subsequent transfer of the web to a Yankee dryer for creping is still used to final dry and/or soften the resulting tissue.
While there is a processing incentive to eliminate the Yankee dryer and make an uncreped throughdried tissue, attempts to make throughdried tissue sheets without using a Yankee dryer (uncreped) have heretofore lacked adequate softness when compared to their creped counterparts. This is partially due to the inherently high stiffness and strength of an uncreped sheet, since without creping there is no mechanical debonding in the process. Because stiffness is a major component of softness, the use of uncreped throughdried sheets has been limited to applications and markets where high strength is paramount, such as for industrial wipers and towels, rather than for applications where softness is required, such as for bath tissue, premium household towels, and facial tissue in the consumer market.
SUMMARY OF THE INVENTION
It has now been discovered that tissues having properties particularly suitable for use as a bath tissue can be made using certain pretreated papermaking fibers in an appropriate process. A throughdrying tissue making process in which the tissue web is not adhered to a Yankee dryer and hence is uncreped is preferred. The resulting tissues of this invention are characterized by a unique combination of high bulk and low stiffness as compared to available creped bath tissue products and especially so as compared to prior uncreped throughdried products.
The stiffness of the products of this invention 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 Max Slope”) or by the machine direction Stiffness Factor (hereinafter defined), which further takes into account the caliper of the tissue and the number of plies of the product. In accordance with this invention, by overcoming the inherently high stiffness of uncreped throughdried sheets, an acceptably soft tissue with high bulk and low stiffness can be produced. In addition, the products of this invention can have a high degree of stretch of about 10 percent or greater, which provides in-use durability. Such soft, strong and stretchable tissue products with high bulk have heretofore never been made. While this invention is particularly applicable to bath tissue, it is also useful for other paper products where softness is a significant attribute, such as for facial tissue and household paper towels.
Hence in one aspect, the invention resides in a soft tissue having a Bulk (hereinafter defined) of about 9 cubic centimeters per gram or greater and an MD Max Slope of about 10 or less.
In another aspect, the invention resides in an a soft tissue comprising one or more uncreped throughdried plies and having a MD Max Slope of about 10 or less, preferably also having a Bulk of about 6 cubic centimeters per gram or greater.
In another aspect, the invention resides in a soft tissue having a Bulk of about 9 cubic centimeters per gram or greater and a MD Stiffness Factor of about 150 or less.
In another aspect, the invention resides in a soft tissue comprising one or more uncreped throughdried plies and having a MD Stiffness Factor of about 150 or less, preferably also having a Bulk of about 6 cubic centimeters per gram or greater.
In a further aspect, the invention resides in a method of making a soft tissue sheet comprising: (a) forming an aqueous suspension of papermaking fibers having a consistency of about 20 percent or greater; (b) mechanically working the aqueous suspension at a temperature of 140° F. or greater provided by an external heat source, such as steam, with a power input of about 1 horsepower-day per ton of dry fiber or greater to curl the fibers; (c) diluting the aqueous suspension of curled fibers to a consistency of about 0.5 percent or less and feeding the diluted suspension to a tissue-making headbox; (d) depositing the diluted aqueous suspension onto a forming fabric to form a wet web; (e) dewatering the wet web to a consistency of from about 20 to about 30 percent; (f) transferring the dewatered web from the forming fabric to a transfer fabric traveling at a speed of from about 10 to about 80 percent slower than the forming fabric; (g) transferring the web to a throughdrying fabric whereby the web is macroscopically rearranged to conform to the surface of the throughdrying fabric; and (h) throughdrying the web to final dryness.
The Bulk of the products of this invention is calculated as the quotient of the Caliper (hereinafter defined), expressed in microns, divided by the basis weight, expressed in grams per square meter. The resulting Bulk is expressed as cubic centimeters per gram. For the products of this invention, Bulks can be about 6 cubic centimeters per gram or greater, preferably about 9 cubic centimeters per gram or greater, suitably from about 9 to about 20 cubic centimeters per gram, and more specifically from about 10 to about 15 cubic centimeters per gram. The products of this invention derive the Bulks referred to above from the basesheet, which is the sheet produced by the tissue machine without post treatments such as embossing. Nevertheless, the basesheets of this invention can be embossed to produce even greater bulk or aesthetics, if desired, or they can remain unembossed. In addition, the basesheets of this invention can be calendered to improve smoothness or decrease the Bulk if desired or necessary to meet existing product specifications.
The MD Max Slope of the products of this invention can be about 10 or less, preferably about 5 or less, and suitably from about 3 to about 6. Determining the MD Max Slope will be hereinafter described in connection with FIG.
6
. The MO Max Slope is the maximum slope of the machine direction load/elongation curve for the tissue. The units for the MD Max Slope are kilograms per 3 inches (7.62 centimeters), but for convenience the MD Max Slope values are hereinafter referred to without the units.
The MD Stiffness Factor of the products of this invention can be ab
Bahlman Julia Smith
Burazin Mark Alan
Chen Fung-jou
Farrington, Jr. Theodore Edwin
Goerg Kristin Ann
Croft Gregory E.
Fortuna Jos'e A.
Kimberly--Clark Worldwide, Inc.
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