High-density absorbent structure

Details High-density absorbent structure High-density absorbent structure

1998-03-13

2001-08-14

Fortuna, Jose (Department: 1731)

Paper making and fiber liberation

Processes and products

Plural fiber containing

C162S123000, C162S129000, C162S157600, C162S164100, C162S164600, C162S206000, C604S378000, C428S208000

Reexamination Certificate

active

06273996

ABSTRACT:

BACKGROUND OF THE INVENTION
In the manufacture of fiber-based materials used to absorb and contain fluids such as water, milk, coffee or urine, nasal discharge and other body exudates, it is commonly a property of the absorbent materials that they have a low density and corresponding high void volume. The high void volume allows for absorption and containment of the fluid, but leads to products that have a low overall density and occupy substantial volume. This inherent void volume is a characteristic of absorbent materials in that the fluid to be contained needs to reside in most cases in the void volume of the fiber web. Heretofore, most materials currently used in products designed to absorb and contain fluid, while having large void volumes, are inherently low in density and are therefore bulky.
The low density of the materials, while providing the desired performance in the product, also imposes some penalties in the form of bulky, low-density products. For the producer of products, the low density of the absorbent materials imparts a penalty in packaging and transporting of its products. For the converter, it imposes problems in storage, handling and shipping of its final products. For the retailer, it means using a large amount of shelf space for a relatively small number of products. For the ultimate consumer, it means handling, storing and using bulky, low-density materials. For such products as diapers, feminine pads and tissue and towel products, the functionality of the products as absorbents generally outweighs the disadvantages of the bulk imposed by the low density; however, given a choice, most consumers would prefer to have thinner, less bulky products. While the advantages to the consumer are obvious, there are many other advantages that thin, dense, but highly absorbent materials can provide to product design, performance and consumer appeal.
SUMMARY OF THE INVENTION
The present invention pertains to materials that are relatively high in density and are not bulky in the dry state, but when exposed to aqueous fluids such as water, coffee and milk and body fluids, such as blood, urine, nasal discharge and other body exudates, swell and increase their void volume to accommodate and contain the fluids. Such thin, dense materials should find application in the design of new, nonbulky consumer items. These materials could also form the basis for new and improved products used to contain fluids in the commercial and professional product areas.
In copending patent appliction Ser. No. 08/310,186 to Chen et al. filed Sep. 21, 1994, a low-density, wet-laid tissue sheet with exceptional wet resilience and absorbency was disclosed. One of the desirable attributes of this material is the resistance of its structure to collapse when the sheet is saturated with fluid. This attribute of wet collapse resistance imparts a significant improvement in fluid handling properties to these fiber webs. Their low density and ability to retain their low density when saturated with fluid is unique. These materials are finding wide use in developing consumer products for absorbent purposes. These materials are being used as components of diapers, bed pads and feminine hygiene products in addition to their application in the more traditional tissue and towel applications.
It has been found that the materials of this previous invention and similar wet-resilient structures can be compressed under suitable conditions to provide materials of relatively high density that retain their fluid handling and absorbing properties. When exposed to aqueous solutions and fluids, these materials expand and produce internal voids that can absorb and hold fluid. This property is especially useful in making absorbent materials that are thin when dry, but swell when wetted and are capable of holding large volumes of fluid. The swelling associated with exposure to fluids is largely the resumption of the original low-density structure. By using basesheets that have this property of wet collapse resistance, subsequent compression provides materials that are thin when dry but become thick when wet.
There are a number of applications for this compressed material in absorbent products. Incorporation of these materials in diapers, feminine hygiene products, and in other absorbent media is of particular value as these structures have been shown to efficiently absorb most types of fluids these products are normally expected to absorb and contain. In addition, towel and tissue products made from these compressed products retain their fluid handling properties but are less bulky and tend to be perceived as softer and more comfortable than conventional, non-compressed materials.
Hence, in one aspect, the invention resides in an absorbent structure comprising high yield pulp fibers having a wet:dry geometric mean tensile ratio of about 0.1 or greater, a density of about 0.3 grams per cubic centimeter or greater and an absorbent capacity of about 4 grams of water per gram of fiber or greater.
More specifically, the invention resides in a compressed uncreped through-air-dried web of papermaking fibers comprising high yield pulp fibers and a wet strength agent, said web having a density of about 0.3 grams per cubic centimeter or greater and an absorbent capacity of about 4 grams of water per gram of fiber.
In another aspect, the invention resides in a method of making an absorbent structure comprising: (a) forming a structure having a density of about 0.2 grams per cubic centimeter or less, said structure comprising high yield pulp fibers and having a wet:dry geometric mean tensile ratio of about 0.1 or greater and (b) compressing the structure to increase its density to about 0.3 grams per cubic centimeter or greater, wherein upon being saturated with distilled water, the density decreases about 20 percent or greater.
More specifically, the invention resides in a method of making an absorbent structure comprising: (a) forming an uncreped through-air-dried web comprising high yield pulp fibers and having a wet:dry geometric mean tensile ratio of about 0.1 or greater, and (b) calendering the web to increase its density to about 0.3 grams per cubic centimeter or greater, wherein upon being saturated with distilled water, the density decreases about 20 percent or greater.
In carrying out the method of this invention, compression of the relatively low-density sheet can be carried out by a number of methods. In the paper industry, it is well known that passing sheets through one or more rollers or nips will help compress and smoothen the surfaces of materials. The equipment used to do this is termed a calender or supercalender. The effect of calendering on low density sheets used in this invention depends upon the temperature, the pressure applied and the duration of the pressure. For purposes herein, calendering can be carried out at either at ambient or elevated temperatures. Suitable calendering pressures can be from about 50 to about 1400 pounds per linear inch (pli). Suitable temperatures can be from about 20° C. to about 240° C. The duration of calendering can be varied in conjunction with the nip pressure to produce the desired Caliper for the sheet.
In addition to calendering or supercalendering of the low density webs, the webs can be compressed using flat platten presses or fabric nips used to smooth and compact multi-wiper products as disclosed in U.S. Pat. No. 5,399,412 to Sudall et al. In this instance, the multi-ply wiper is carried on fabrics through a nip and the overall caliper of the multi-ply product is reduced. A similar process can be used to produce the sheets of the present invention. By inducing a pattern in the fabric or fabrics, the resulting sheet could have areas that are highly compressed and areas that are less compressed. The response of the resulting sheet to fluids would result in a regain of bulk, more or less uniformly, for the entire sheet.
Fibers useful for making the relatively low density sheets useful for purposes of this invention are wet resilient fibers that include high yield pulp fibe

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