The present invention relates to an improved method for the preparation of carboxymethyl cellulose (CMC) absorbents and, more particularly, to such a method for the production of water-insoluble, partially acid form CMC's which are so useful.
The use of CMC as an absorbent material in catamenial devices or other absorbent dressings has been known in the literature for a number of years. See, for example, Masci et al U.S. Pat. Nos. 2,764,159 and 2,772,999 granted on Sept. 25 and Dec. 4, 1956, respectively; Ashton et al Pat. No. 2,766,137 granted Oct. 9, 1956; Graham U.S. Pat. No. 3,005,456 granted Oct. 24, 1961 and U.S. Pat. No. 3,055,379 granted Sept. 25, 1962; Burgeni et al U.S. Pat. No. 3,067,745 granted Dec. 11, 1962 and U.S. Pat. No. 3,187,747 granted June 8, 1965; and Lewing U.S. Pat. No. 3,371,666 granted Mar. 5, 1968.
Graham disclosed that only CMC's having a DS value (the number of carboxymethyl groups per anhydroglucose unit in the cellulose chain) less than about 0.35 are useful as absorbents, and described those materials having higher DS values as too soluble for such purpose. It has, however, subsequently been disclosed that CMC's having higher DS values are also suitable for use in absorbent dressings, particularly if insolubilized, e.g., by cross-linking. In this connection reference may be made, for example, to the aforesaid Ashton et al and Masci et al patents; Elliot U.S. Pat. No. 2,639,239 granted May 19, 1953; Dean et al U.S. Pat. No. 3,589,364 granted June 29, 1971; Ells et al U.S. Pat. No. 3,618,607, granted Nov. 9, 1971; Schoggen U.S. Pat. No. 3,678,031, granted July 18, 1972; Chatterjee U.S. Pat. No. 3,731,686, granted May 8, 1973; and Kaczmarzyk et al U.S. Pat. No. 4,044,766, granted Aug. 30, 1977.
The art further teaches that the insolubilized CMC's may be in either the salt form or partially in the acid form, i.e., that a portion, e.g., up to 85%, of the carboxylate groups of the sodium salt are converted to free carboxyl groups. See, for example, the aforesaid Ashton et al, Masci et al, Chatterjee and Kaczmarzyk et al patents, and Reid U.S. Pat. No. 3,379,720, granted Apr. 23, 1968. For purposes of the present invention the proportion of carboxyl groups in the acid form, as a percentage of the carboxylate moieties in the sodium carboxymethyl cellulose salt, is termed the degree of acidification (DA) of the particular CMC salt referenced.
One procedure which has been proposed for preparing water-insoluble CMC's in the partially acid form involves slurrying water-soluble CMC, e.g., sodium CMC, with hydrochloric acid in a water-miscible, organic solvent, e.g., isopropanol, and thereafter insolubilizing the isolated material merely by heat-catalyzed cross-linking. See, for example, the aforesaid Reid patent. Commercial application of such a procedure necessitates the application of solvent recovery techniques to facilitate economic production, and poses the risk of CMC losses in residues from distillation operations which may, for example, be utilized in such techniques. Moreover, the use of solvent recovery operations and the disposal of residues formed therein may additionally pose environmental pollution problems.
A principal object of the present invention is to provide an improved process for the preparation of water-insoluble, acid form CMC materials which may be readily utilized as absorbents for catamenial devices and other absorbent dressings.
It is a further object of the invention to provide such a process which may be commercially employed without substantial materials recovery problems and potential environmental risks as are inherent in the practice of the above-noted prior art procedure.
Other objects and advantages of the process of this invention will be apparent from the following description of preferred embodiments thereof.