High absorbency lyocell fibers and method for producing same

This application is a Divisional application of U.S. Ser. No. 10/057,352, filed on Jan. 24, 2002. The complete disclosures of the aforementioned related U.S. patent applications are hereby incorporated herein by reference for all purposes.

This invention relates to a method for improving the absorbency characteristics of lyocell fibers, and more particularly to such a method which is useful in the preparation of absorbent materials for catamenial tampons, sanitary napkins, and other absorbent dressings.

There are several forms of rayon manufactured and used in various industries including viscose, cuprammonium, high-wet modulus and lyocell. However, lyocell is distinct from other rayons. For example, the degree of crystallinity of lyocell is at least twice that of rayon; lyocell consists of rather well defined fibrils which can separate due to wet abrasive action, but it is otherwise not very sensitive to water; and has higher tensile strength, especially wet tensile strength compared to other regenerated cellulose fibers. Thus, while viscose rayon has seen widespread use in absorbent articles, such as catamenial tampons, sanitary napkins, and other absorbent dressings, lyocell has not been used in any significant quantities for these articles.

Various techniques have been described in the literature for increasing the absorbency of cellulosic materials. Such techniques include, for example, the preparation of alloy fibers having matrices of regenerated cellulose and, uniformly dispersed therein, polyacrylates (e.g., Smith U.S. Pat. No. 3,884,287), acrylate/methacrylate copolymers (e.g., Allen et al U.S. Pat. No. 4,066,584; Meierhoefer U.S. Pat. No. 4,104,214; and Allen U.S. Pat. No. 4,240,937), alkylene vinyl ether/ethylene dicarboxylic acid copolymers (e.g., Denning U.S. Pat. No. 4,165,743), sulfonic acids (e.g., Allen U.S. Pat. No. 4,242,242, polyvinylpyrrolidone (e.g., Smith U.S. Pat. No. 4,136,697), cellulose sulfate (e.g., Smith U.S. Pat. No. 4,273,118), carboxymethylcellulose (e.g., Smith U.S. Pat. No. 4,289,824), or the like.

Viscose rayon or other regenerated cellulose polymer alloy fibers may be subjected during preparation to one or more hot, aqueous baths. For example, post-regeneration treatments in hot water baths at temperatures of from ambient (20°-25° C.) to as much as 100° C. have been described in various of the above patents. Such treatments have not, however, been disclosed as having any appreciable effects on the absorbency characteristics of the cellulosic materials.

Other treatments of cellulosic fibrous materials have been proposed in the literature for increasing their absorbency. Thus, the treatment of carboxymethylcellulose fibers in hot aqueous baths containing cross-linking agents has been proposed to effect wet cross-linking of the fibers, with consequent increase in the absorbency thereof (see, for example, Steiger U.S. Pat. No. 3,241,553; Ells U.S. Pat. No. 3,618,607; and Chatterjee U.S. Pat. No. 3,971,379). None of this literature, however, suggests the use of hot water treatments per se for improving the absorbency of the cellulosic materials thereof.

High Temperature Water (preferably deionized) Treatment of cellulosic fibrous materials has been described in Shah et al., U.S. Pat. No. 4,575,376. This treatment is at 95°-100° C., and materials actually subjected to this treatment appear to be limited to cotton, viscose rayon, and viscose rayon alloys. Due to the different water absorbency characteristics of viscose rayon from lyocell, the data provided in this reference does not necessarily suggest that the absorbency of lyocell would be improved as significantly as other cellulosic fibers in such a HTWT process.

Tyler et al., U.S. Pat. No. 4,919,681, purports to disclose a modified method of treating cellulose fibers in an acid solution having a pH of no more than 4. Again, the cellulosic fibers actually tested in this reference are limited, and the data does not necessarily suggest that the absorbency of lyocell would be improved as significantly as other cellulosic fibers in such a process.

The present invention provides high absorbency lyocell fibers to allow their use as a replacement for conventional viscose rayon fibers in absorbent articles.

In addition, it provides a new technique for treating lyocell fibrous materials to increase their absorbency.

This result is obtained in accordance herewith by a hydrothermal treatment, which comprises heating the lyocell fibers in the presence of water at temperatures within the range of up to about 100° C., for a period sufficient to increase the absorbency of the fibers to provide a Syngyna tampon absorbency of at least about 4.4 g/g (at a density of about 0.4 g/cm³) and drying the treated fibers.