Paper making and fiber liberation – Processes and products – Non-uniform – irregular or configured web or sheet
Reexamination Certificate
1998-12-07
2001-02-13
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-uniform, irregular or configured web or sheet
C162S112000, C162S127000, C162S135000, C162S146000
Reexamination Certificate
active
06187140
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to processes for creping a cellulosic web and to paper wiping products prepared thereby.
Absorbent paper products such as paper towels, industrial wipers, and the like generally are designed to have high bulk, a soft feel, and high absorbency. Desirably, these paper wiping products will exhibit high strength, even when wet, and resist tearing. Further, such products should have good stretch characteristics, should be abrasion resistant, and should not prematurely deteriorate in the environment in which they are used.
In the past, many attempts have been made to enhance certain physical properties of paper wiping products. Unfortunately, steps taken to increase one property often adversely affect other characteristics. For example, in pulp fiber-based wiping products, softness may be increased by inhibiting or reducing interfiber bonding within the paper web. Inhibiting or reducing fiber bonding, however, adversely affects the strength of the product.
One process which has proven successful in producing paper towels and other wiping products is disclosed in U.S. Pat. No. 3,879,257 to Gentile et al., which patent is incorporated herein by reference in its entirety. Gentile et al. disclose a process for producing a soft, absorbent, single ply fibrous web having a laminate-like structure. The fibrous web is formed from an aqueous slurry of primarily lignocellulosic fibers under conditions which reduce interfiber bonding. A bonding material, such as a latex elastomeric composition, is applied to a first surface of the web in a spaced-apart pattern. The bonding material provides strength to the web and abrasion resistance to the surface. The bonding material may be applied in a like manner to a second surface of the web to provide additional strength and abrasion resistance. After applying bonding material to the second surface, the web may be brought into contact with a creping surface, such as the cylinder surface of a Yankee dryer. The bonding material will cause the web to adhere to the creping surface. The web then is creped from the creping surface with a doctor blade. Creping the web mechanically debonds and disrupts the fibers within the web, except where bonding material is present, thereby increasing the softness, absorbency, and bulk of the web. If desired, both sides of the web may be creped sequentially after the pattern of bonding material has been applied.
Gentile et al. describe the optional use in the process of one or more curing or drying stations before the web is wound into what is referred to as a parent roll. As a practical matter, curing or drying is necessary in order to prevent the layers in the parent roll from sticking or adhering to one another (a phenomenon referred to in the art as “blocking”). Moreover, unless the web is cooled before it is wound into the parent roll, spontaneous combustion may occur. As is well known by those having ordinary skill in the art, drying is an energy-intensive step, particularly when two curing or drying stations are employed. The presence of curing or drying stations also adds to the capital cost of the process equipment. Similarly, the need for a cooling station or chill roll adds to both capital and operating costs.
The presence of curing or drying stations also limits the types of noncellulosic fibers which may be present in the web. Such stations typically are operated at temperatures of the order of 150° C. These temperatures preclude the presence in the web of synthetic polymer fibers prepared from, by way of example only, polyolefins.
Thus, there is a need for a creping process which would permit the development of sufficient strength and other desirable attributes without an energy-intensive curing step. There also is a need for a creping process which would permit the use of a wider variety of synthetic polymeric fibers.
SUMMARY OF THE INVENTION
The present invention addresses some of the difficulties and problems discussed above by providing a method of increasing the wet strength of a creped sheet. The method involves providing a sheet which includes cellulosic fibers, which sheet has a first side and a second side; applying a low temperature-curing latex adhesive binder composition to the first side of the sheet in a fine, spaced-apart pattern occupying from about 20 to about 50 percent of the surface area of the sheet; adhering the first side of the sheet to a creping surface; and creping the sheet from the creping surface.
In general, the sheet has a basis weight of from about 40 to about 10 grams per square meter (gsm). The low temperature-curing latex adhesive binder composition is adapted to adhere the sheet to the creping surface. The composition includes a functional group-containing polymer in the form of a latex (sometimes referred to hereinafter as a functional group-containing latex), a functional group-reactive crosslinking agent, and a volatile base. In addition, the creping surface is heated at a temperature no greater than about 100° C. The low temperature-curing latex adhesive binder composition is adapted to have cured to a level, by the time the sheet is removed from the creping surface, which imparts to the creped sheet a cross-direction wet tensile strength which is at least about 50 percent that of an identical creped sheet which has been heated at about 150° C. for three minutes, in which the cross-direction wet tensile is tested in accordance with TAPPI Test Methods T494om-88 and T456om-87. In addition, the cross-direction wet tensile strength of the creped sheet is at least about 40 grams per centimeter.
In certain embodiments, the sheet may include up to about 20 percent by weight, based on the dry weight of cellulosic fibers, of synthetic polymer fibers. For example, the sheet may include from about 5 to about 10 percent by weight, based on the dry weight of cellulosic fibers, of synthetic polymer fibers. By way of example, the synthetic polymer fibers may be polyester fibers or polyolefin fibers. Examples of polyolefin fibers include polyethylene and polypropylene fibers.
In some embodiments, the functional groups of the functional group-containing latex will be carboxy groups. For example, the functional group-containing polymer may have an acid value of from about 15 to about 50 milligrams of potassium hydroxide per gram of polymer (mg KOH per g). As another example, the functional group-containing latex may be a polyacrylate. Also by way of example, the functional group-reactive crosslinking agent may be an aziridine oligomer having at least three aziridine groups. The functional group-reactive crosslinking agent may be present, by way of example, in an amount of from about 1 to about 8 percent by weight, based on the amount of the functional group-containing latex.
The present invention also provides a method of increasing the wet strength of a creped sheet, which method involves providing a sheet which includes cellulosic fibers, the sheet having a first side and a second side; applying a first low temperature-curing latex adhesive binder composition to the first side of the sheet in a fine, spaced-apart pattern occupying from about 20 to about 50 percent of the surface area of the sheet; applying a second low temperature-curing latex adhesive binder composition to the second side of the sheet in a fine, spaced-apart pattern occupying from about 20 to about 50 percent of the surface area of the sheet; adhering the second side of the sheet to a creping surface; and creping the sheet from the creping surface.
The sheet generally has a basis weight of from about 40 gsm to about 100 gsm.
The first low temperature-curing latex adhesive binder composition includes a first functional group-containing latex, a first functional group-reactive crosslinking agent, and a first volatile base. The second low temperature-curing latex adhesive binder composition is adapted to adhere the sheet to the creping surface and includes a second functional group-containing latex, a second functional group-reactive crosslinking agent, and a seco
Anderson Ralph
Davidson Christopher Lee
Larson Kenneth Curtis
Saffel Thomas C.
Weber Robert Emil
Alvo Steve
Kimberly--Clark Worldwide, Inc.
Sidor Karl V.
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