Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
1999-08-09
2001-06-19
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S112000, C162S113000, C162S117000, C162S205000, C162S206000
Reexamination Certificate
active
06248211
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for making a throughdried tissue sheet exhibiting improved softness while retaining adequate strength. More particularly, the present invention relates to a method for making a creped throughdried tissue sheet using a resilient calender roll having an exterior covering formed of ethylene propylene diene polymer.
BACKGROUND OF THE INVENTION
A wide variety of product characteristics must be given attention in order to provide a tissue product with the appropriate blend of attributes suitable for the product's intended purposes. Contributing to this variety is the vast array of different product forms, such as facial tissue, bath tissue, napkins, and towels. Regardless of product form, however, improved softness of the product has long been one major objective, especially for premium products. In general, the major components of softness include stiffness and bulk, with lower stiffness and higher bulk generally improving perceived softness.
A throughdrying process can be used to improve the bulk of tissue products. Throughdrying is a relatively noncompressive method for removing water from a web. Specifically, a wet laid web is transferred from a forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until it is dried by hot air passing through the web.
Throughdried sheets can be quite harsh and rough to the touch, however, due to their inherently high stiffness and strength and also due to the coarseness of the throughdrying fabric. For this reason, creping has been used to improve the softness of throughdried tissue sheets. Creping removes some of the stiffness of the uncreped sheet, albeit at the expense of the sheet strength.
Despite the improvements in softness that can be gained from creping, however, additional improvements in softness would be beneficial to consumers. Therefore, what is lacking and needed in the art is a process for further improving the softness of creped throughdried tissue products.
SUMMARY OF THE INVENTION
It has now been discovered that the texture of creped, throughdried tissue products can be improved by processing the tissue web through a soft-nip calendering unit with the fabric side of the web oriented toward a resilient roll and the air side of the web oriented toward a smooth roll. More specifically, in one embodiment the invention concerns a method for making a throughdried tissue sheet comprising the steps of depositing an aqueous suspension of papermaking fibers onto an endless forming fabric to form a wet web, and then transferring the wet web to a throughdrying fabric. The wet web is described herein as having a fabric side, which is the surface that is in contact with the throughdrying fabric, and an opposite air side, which is the surface that is facing away from the throughdrying fabric. The wet web is then carried over a throughdryer to dry the web, transferred to a Yankee dryer, and creped from the Yankee dryer. The creped web is then calendered in a calendering unit comprising a smooth calendering roll and a resilient calendering roll having a Shore A surface hardness of about 75 to about 100 Durometer (approximately 55 to about 0 Pusey & Jones, respectively). During calendering, the creped web is oriented such that the fabric side is disposed toward the resilient calendering roll.
In another embodiment, the method of making a throughdried tissue sheet also comprises the step of embossing the creped web in an embossing unit comprising a pattern roll and a backing roll. In the embossing unit, the creped web is oriented such that the fabric side of the web is disposed toward the pattern roll.
The improved softness is particularly pertinent to creped throughdried tissue products, which typically have larger creping features than a comparable wet pressed sheet. Moreover, the oriented soft-nip embossing is advantageously used in conjunction with oriented embossing of the tissue web. In particular, the resilient backing roll of the embossing unit is against the opposite surface of the tissue sheet as is the resilient calendering roll.
The texture of tissue products has been examined to determine the attributes that result in a tissue product being considered soft. One attribute of a surface of a tissue that is indicative of the softness of the tissue is referred to as the Fuzzy attribute of the tissue surface. Contrariwise, attributes of a tissue surface that are counter-indicative of the softness of the tissue are referred to as the Gritty and Grainy attributes of the tissue surface.
The methods disclosed herein have been found to increase the Fuzzy attribute of the fabric side of the tissue product and reduce the Gritty and Grainy textures of the air side of the tissue product. The Fuzzy attribute is increased, particularly on the fabric side, as a result of frictional forces in the calendering nip caused by the speed differential at the contact point between the resilient calendering roll and the steel calendering roll. The creping process causes the air side of the tissue web to have pointed crepe structures such as ripples or ridges which result in Gritty and Grainy textures, and that contact between the steel calendering roll and the air side tends to flatten the pointed crepe structures present on the air side, thereby reducing the Gritty and Grainy textures. In addition, it is hypothesized that the clarity of the embossing pattern is improved because of an increase in opacity caused by calendering the sheet. The result of the selective orientation and distinctive treatment of the opposite surfaces of the creped throughdried tissue is an embossed tissue web with enhanced softness.
For purposes herein, a “tissue web” or “tissue sheet” is a cellulosic web suitable for making or use as a facial tissue, bath tissue, paper towels, napkins, or the like. It can be layered or unlayered, creped or uncreped, and can consist of a single ply or multiple plies. In addition, the tissue web can contain reinforcing fibers for integrity and strength. Tissue webs suitable for use in accordance with this invention are characterized by being absorbent, of low density and relatively fragile, particularly in terms of wet strength. Densities are typically in the range of from about 0.1 to about 0.3 grams per cubic centimeter. Absorbency is typically about 5 grams of water per gram of fiber, and generally from about 5 to about 9 grams of water per gram of fiber. Wet tensile strengths are generally about 0 to about 300 grams per inch of width and typically are at the low end of this range, such as from about 0 to about 30 grams per inch. Dry tensile strengths in the machine direction can be from about 100 to about 2000 grams per inch of width, preferably from about 200 to about 350 grams per inch of width. Tensile strengths in the cross-machine direction can be from about 50 to about 1000 grams per inch of width, preferably from about 100 to about 250 grams per inch of width. Dry basis weights are generally in the range of from about 5 to about 60 pounds per 2880 square feet. The tissue webs referred to above are preferably made from natural cellulosic fiber sources such as hardwoods, softwoods, and nonwoody species, but can also contain significant amounts of recycled fibers, sized or chemically-modified fibers, or synthetic fibers.
Tissue sheets that particularly benefit from the method of this invention are premium quality throughdried tissue sheets that have a relatively high degree of resiliency and low stiffness. The basis weight of the tissue sheet can be from about 5 to about 70 grams per square meter.
Referring now to the tissue making process of this invention, the forming process and tackle can be conventional as is well known in the papermaking industry. Such formation processes include Fourdrinier, roof formers such as a suction breast roll, gap formers such as twin wire formers and crescent formers, and other suitable formers. A twin wire former may be preferred for higher speed operation. Forming wires or fabrics can als
Jennings Richard Douglas
Salman Zeinab
Charlier Patricia A.
Chin Peter
Connelly Thomas J.
Kimberly--Clark Worldwide, Inc.
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