Bibulous Cellulosic Fibers

Abstract

Bibulous cellulosic fibers for use in fibrous structures having an inherently absorptive function, for example surgical dressings, surgical sponges filters, catamenial napkins, tampons and diapers, which bibulous cellulosic fibers, and fibrous structures containing said bibulous fibers in effective amounts, are characterized by greatly improved absorbency for and retention of aqueous solutions, including physiological solutions. The bibulous cellulosic fibers are substantially water-insoluble, wet cross-linked fibrous salts of carboxymethyl cellulose having a sufficiently high degree of carboxymethyl substitution to insure water solubility in the absence of cross-linking. The bibulous cellulosic fibers are produced under conditions which preserve the fibrous form of the cellulose fiber raw material.

Parent Case Text

This application is a continuation-in-part of copending application, Ser. No. 713,143, filed Mar. 14, 1968 now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to the production of bibulous cellulosic fibers by chemically modifying cellulosic fibers to produce wet cross-linked and etherified cellulosic fibers which are substantially water-insoluble and manifest their original fibrous structure. More particularly, the invention relates to the provision of substantially water-insoluble, wet cross-linked fibrous salts of carboxymethyl cellulose which display improved absorbency and retention characteristics for aqueous solutions, including physiological solutions, and impart these characteristics to fibrous structures in which they are included.

Specifically, the invention, in one of its more important embodiments, consists of wet cross-linked fibrous salts of carboxymethyl cellulose which display their original cellulosic fibrous form and have a sufficiently high degree of substitution (DS) to be normally water-soluble, which fibrous salts of carboxymethyl cellulose have been wet cross-linked to achieve substantial water insolubility. Such substantially water-insoluble, wet cross-linked carboxymethyl cellulose fibers, sometimes referred to hereinafter as bibulous fibers for conciseness, are particularly adapted by their unique intrafiber absorbency for and retention of aqueous solutions for either use or admixture in effective amount in fibrous structures, including surgical dressing, surgical sponges, filters, catamenial napkins, tampons and diapers containing in effective amount the substantially water-insoluble, wet cross-linked carboxymethyl cellulose fibers of this invention. The disclosed bibulous fibers also exhibit a particular and unique absorbent affinity for dilute aqueous solutions of salts and other physiological solutions. In addition, the disclosed bibulous fibers exhibit the capability of absorbing and retaining virtually all of the undissolved water from substantially immiscible aqueous systems comprised of water together with at least one hydrocarbon such as chloroform, gasoline, benzene, toluene, kerosene, xylene, pentane, hexane, cyclohexane, and longer chain length saturated and unsaturated hydrocarbons of this type.

Several papermaking techniques and fiber selections intended to result in absorbent fibers, fibrous structures, waddings, nonwovens and papers have been developed and used heretofore. For example, the commercial grades of absorbent papers designated as blotting papers, filter papers, paper towelings and sanitary tissues are, in general, characterized by being produced in a soft, loosely felted condition to maximize absorbency. Such absorbent papers are generally unsized and are also often accorded a creping or other mechanical treatment after drying to further enhance and increase subjective softness and absorbency. The individual fibers from which such absorbent papers are produced are carefully selected for their contribution to the absorbency characteristics of such papers. Such individual fibers can be and are selected from rag fibers, high alpha wood and cotton linter fibers, ground wood fibers and mixtures thereof, and minor amounts of other fibers, for example crimped staple rayon fibers, can be incorporated to enhance structure. Further, such selected fibers in a furnish for producing absorbent papers are usually beaten lightly, if at all, to maximize the absorbency characteristics of the resulting papers, wadding and felted products.

Papermaking fibers, specifically cotton linter fibers and woodpulp fibers, have further been cross-linked without etherification prior to their inclusion in papermaking furnishes according to the teaching of U.S. Pat. No. 3,069,311, issued to John A. Harpham and Harry W. Turner on Dec. 18, 1962. Also, U.S. Pat. No. 3,241,553, issued to Fred H. Steiger on Mar. 22, 1966, discloses the cross-linking of cellulosic fibers, including cotton, wood pulp, cotton linter and regenerated cellulosic fibers, while they are in a wet swollen condition, to improve the fluid absorbency and fluid retention characteristics of fibrous structures from such fibers for body fluids. Cotton fabrics and fabrics woven of other cellulosic fibers, for example rayon and cellulose acetate, have also been accorded a measure of crease resistance by treatments with both a polyfunctional agent for cross-linking, for example epichlorohydrin and formaldehyde, and a monofunctional agent for either of etherification or esterification, for example monochloroacetic acid as disclosed in U.S. Pat. No. 2,971,815, issued to Austin L. Bullock and John D. Guthrie on Feb. 14, 1961.

Applicants have found, however, that the absorbency and retention characteristics of cellulosic fibers for aqueous solutions, including physiological solutions, can be greatly improved, i.e., about three times to in excess of about 20 times improved, over such characteristics of fibers produced according to teachings and suggestions previously available in the art. Such large scale improvements are generally realized, according to applicants' present disclosure, by converting cellulosic fibers into fibrous salts of carboxymethyl cellulose having a degree of substitution (DS) sufficient to insure water solubility in the absence of cross-linking. The carboxymethylation is accomplished either in an organic liquid media slurry process or in a so-called "dough" process which preserves the original fibrous form of the cellulose. The resulting carboxymethylated cellulosic fibers are then wet cross-linked to insure substantial water insolubility and retention of fibrous form in use. Applicants point out, however, that the order of performance of the carboxymethylation and wet cross-linking steps, whereby the bibulous fibers are produced, is not critical, and initial wet cross-linking followed by carboxymethylation or simultaneous performance of the two steps of carboxymethylation and wet cross-lining is within the purview of the present invention.

SUMMARY OF THE INVENTION

Inasmuch as fibers having increased absorbency and retention capacities for aqueous solutions, including physiological solutions, were desired for use in various types of fibrous structures, applicants further investigated their discovery that certain wet cross-linked cellulose derivative fibers exhibited the stated bibulous characteristics to an astonishingly high degree. Applicants' investigation revealed that the unusually absorbent and retentive fibers can be prepared by carboxymethylating native and regenerated cellulosic fibers to a DS which normally assures water solubility while preserving their initial fibrous form. The resulting fibrous sodium carboxymethyl cellulose is then wet cross-linked to produce the instant bibulous fibers. Wet cross-linking in the present instance comprises the step of treating fibrous salts of carboxymethyl cellulose, swollen with at least minor amounts of water, with a reagent polyfunctional with respect to cellulose, for example epichlorohydrin.

Although the wet cross-linking of unmodified cellulose fiber is known to result in about a two-fold improvement in the absorbency of fibrous structures, applicants were surprised to discover that the present bibulous fibers having a DS of about 0.4 to about 1.6, preferably about 0.6 to about 1.2, exhibited a manyfold increase in intrafiber absorbency and retention. Applicants were further surprised to discover that as minimum values increased over the minimum required DS to insure water solubility in a cellulosic fiber without wet cross-linking according to the instant invention, there existed a range of substantially water-insoluble wet cross-linked fibrous carboxymethyl products, at a DS of about 0.6 to about 1.6, which exhibited a particularly effective absorbency and retention of water, dilute salt and other physiological solutions.

It is, accordingly, a principal object of this invention to provide wet cross-linked fibrous salts of carboxymethyl cellulose which exhibit superior absorbency and retention characteristics with respect to water and aqueous solutions, including physiological solutions.

It is, accordingly, a principal object of this invention to provide wet cross-linked fibrous salts of carboxymethyl cellulose which exhibit superior absorbency and retention characteristics with respect to water and aqueous solutions, including physiological solutions.

It is a further object of the present invention to provide bibulous fibers for inclusion in papermaking furnishes, air-laid and water-laid nonwoven fabrics and in other fibrous pads and bandages whether they be of paper, woven or nonwoven origin.

It is yet another object of the present invention to provide wet cross-linked substantially water-insoluble fibers of carboxymethyl cellulose, having a DS sufficiently high to insure water solubility in the absence of wet cross-linking, for inclusion in the fibrous structure of a catamenial tampon having a substantially higher in-use absorptive capacity than that of prior art devices, which catamenial tampon provides increased protection and wearing time.

It is also an object of this invention to provide catamenial tampons, surgical dressings and other absorbent pads and fibrous structures either prepared from bibulous fibers or containing bibulous fibers in admixture with other fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantageous features of the present invention will become apparent as the following description of the instant bibulous fiber product, together with methods for its production and examples of its advantageous inclusion in useful fibrous structures, proceeds apace in this specification, as illustrated in part by the accompanying drawings wherein:

FIG. 1 is a graph which relates the DS of examples of the present wet cross-linked fibrous carboxymethyl cellulose fibers to their intrafiber absorbency as measured by Water Retention Value (WRV);

FIG. 2 is a graph which relates the DS of examples of the present wet cross-linked fibrous carboxymethyl cellulose fibers to their intrafiber absorbency as measured by Salt Water Retention Value (SRV).

The pertinent values of comparative fibers are also plotted on FIGS. 1 and 2 in order to illustrate the high degree of absorbency exhibited by the present bibulous fibers in comparison with other fibers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, applicants have found that the foregoing objectives can be met, as before stated, by preparing a water-soluble salt of carboxymethyl cellulose having a DS of about 0.4 to about 1.6, preferably about 0.6 to about 1.2, which retains the fibrous form of the original cellulose raw material with prior, concommitant or subsequent introduction of sufficient intrafiber cross-links into the wet or swollen form of said fibrous water-soluble salt of carboxymethyl cellulose to insure substantial water insolubility. Such substantially water-insoluble, wet cross-linked carboxymethyl cellulose fibers exhibit a WRV of about 900 to about 3,400 and a SRV of about 400 to about 1,500 in the stated DS range of about 0.4 to about 1.6. In the preferred DS range WRV's of about 1,100 to about 3,400 and SRV's of about 400 to about 1,500 are achieved. Fibrous salts of water-soluble carboxymethyl cellulose having DS's in excess of about 1.6 can be used, although, as a practical matter, the increased absorbency gained through the use of such high DS material is difficult to justify versus the increased chemical costs for etherification and cross-linking. Potassium sodium and lithium salts of fibrous water-soluble carboxymethyl cellulose are suitable for wet cross-linking in practicing the present invention; and, although the invention will hereinafter be illustrated in terms of fibrous water-soluble salts of sodium carboxymethyl cellulose (NaCMC), it will be appreciated and understood that other fibrous water-soluble salts of carboxymethyl cellulose, for example ammonium salts of fibrous water-soluble carboxymethyl cellulose, can be employed. The necessary wet cross-links can be introduced into water swollen fibrous NaCMC by treatment with a suitable cellulosic cross-linking reagent, selected from the group enumerated hereinbelow, but preferably epichlorohydrin. Suitable cellulosic fibers for carboxymethylation and wet cross-linking to form the present bibulous fibers are cotton, wood pulp and rayon fibers, although lesser used cellulosic fibers, for example cellulosic fibers attained from straw, esparto grass, bagasse and other cellulosic fibers will be found to increase in absorbency.

The minimum degree of substitution of about 0.4 required to assure water solubility in the fibrous NaCMC prior to wet cross-linking to achieve substantial water insolubility is obtainable by operation of the semidry processes disclosed in such patents as U.S. Pat. No. 2,510,355, issued to William F. Waldeck on June 6, 1950 and U.S. Pat. No. 2,553,725, issued to Linwood N. Rogers, William A. Mueller and Ernest E. Hembree on May 22, 1951, but preparation of the fibrous NaCMC of the herein disclosed DS for cross-linking is readily accomplished by operation of any of these so-called slurry processes for the manufacture of NaCMC, for example the slurry process taught in U.S. Pat. No. 3,345,855, issued to Russell Nelson on Oct. 17, 1967. Such slurry processes have the advantage, in the preparation of the instant bibulous fibers, that they can be easily controlled and operated to produce NaCMC having a DS in the range of about 0.4 to about 1.6 and in the preferred range of about 0.6 to about 1.2 in the fibrous form of the original cellulose raw material as required for production of the fibrous wet cross-linked NaCMC product of the present invention. The production of NaCMC fibers having DS's of less than about 0.8 can be accomplished in one etherification stage, but the achievement of DS's in excess of about 0.8 is more easily accomplished and controlled by the use of at least two etherification stages.

The fibrous water-soluble NaCMC with DS of about 0.4 to about 1.6 is then treated in the wet or swollen state with a suitable cellulosic cross-linking agent to introduce, engender or cause sufficient intrafiber cross-links to be formed so that the fibrous NaCMC becomes substantially insoluble in water. It is also possible to reduce the water solubility of bibulous fibers for particular applications by extracting crude bibulous fiber products with water prior to their use as fibers or admixture fibers in fibrous products. Suitable cross-linking reagents for use in producing the water-insoluble intrafiber cross-linked NaCMC product are, for example, epichlorohydrin and formaldehyde, and the type and amount of intrafiber cross-linking is derived from an amount of cross-linking agent sufficient to provide about one cross-link per about six to about 19 anhydroglucose units, with the cross-links having been introduced into the cellulose or cellulose derivative while it is in the wet state. In the practice of the invention with epichlorohydrin, for example, the required cross-linking is accomplished by reaction with about 3 percent to about 10 percent preferably about 3 percent to about 7 percent, by weight of epichlorohydrin based on the dry weight of the original cellulose fiber.

Although the general preparation of the bibulous cross-linked carboxymethyl cellulose fibers of the present invention has been described above in terms of preparing NaCMC of the prescribed DS and then introducing sufficient wet cross-links to insure substantial water insolubility, it is to be understood that the wet cross-linking and the one or more etherification steps can be carried out in any order or concomitantly. Applicants have found that cross-linking wet cellulose fibers to form wet cross-linked cellulose prior to one or more stages of etherification to obtain a desired DS has some materials handling advantages, although simultaneous etherification and wet cross-linking offers some economies in equipment and chemical usage.

As stated before, the bibulous fibers of the present invention are substantially water-insoluble. In practice, applicants have found that bibulous fibers having water solubilities of less than about 30 percent do not inhibit the penetration of liquid into fibrous structures in which they are present and do not interfere with the increased total absorbency of such structures. Actually, the present bibulous fibers would be found almost totally insoluble if the etherification and wet cross-linking reactions were homogeneous in nature. In practice, however, cellulosic fibers having varying degrees of polymerization enter into the wet cross-linking and etherification reactions, so that a degree of heterogeneity together with some solubility results in the resulting bibulous fibers. In applications of the bibulous fibers where essentially water-insoluble fibers are desired, it is possible to remove water-soluble fractions from the bibulous fibers by extraction with water prior to use.

Referring now to FIG. 1 of the accompanying drawings, which graphically relates to the carboxymethyl DS of fibrous NaCMC wet cross-linked to substantial water insolubility with epichlorohydrin to fiber absorbency in terms of the Water Retention Value/100 (WRV/100) as defined below, applicants have discovered that the ability of wet cross-linked NaCMC fibers to retain water is greatly improved as the DS of the cross-linked NaCMC exceeds about 0.4. The graph points are identified as bibulous fibers in the following examples. Higher DS's up to about 1.6 are also shown to be related to desirable and increased WRV values. Applicants further prefer to produce the present bibulous fibers by wet cross-linking NaCMC fibers having a DS of about 0.6 to about 1.2 because they have found, as illustrated by the graphed values of FIG. 1, that bibulous fibers in this DS range achieve the most advantage in increased absorbency versus production difficulties and chemical economies.

The graph points for illustrations 1, 2 and 3 are respectively the absorbency results for wood pulp fibers with no carboxymethylation (DS=0), wet cross-linked wood pulp fibers with no carboxymethylation, and wet cross-linked NaCMC fibers with a DS of 0.14. The inclusion of these illustrations in FIG. 1 serves to illustrate the large increase in absorbency achieved in the present bibulous fibers over previous absorbent fibers. In FIG. 1 the upper solid line represents the curve for bibulous fiber examples having a lower epichlorohydrin treatment of about 3 percent while the lower dotted line represents the curve for bibulous fiber examples having an epichlorohydrin treatment of about 5 percent. The area between the two lines encompasses advantageous bibulous fiber products, and increased amounts of epichlorohydrin treatment, as stated before, will result in only a slight downward displacement of the dotted line.

In a manner similar to the graphical illustration of FIG. 1, FIG. 2 graphically relates the carboxymethyl DS of fibrous NaCMC wet cross-linked to substantial water insolubility with epichlorohydrin to its fiber absorbency in terms of the Salt Water Retention Value/100 (SRV/100), as defined below. Illustrations 1, 2 and 3 are included for comparison as in FIG. 1, and it is noted that an aqueous solution containing 1 percent by weight of salt displays absorbent properties similar to physiological solutions such as urine, menstrual blood and blood.

As previously stated, the necessary wet cross-linking of the NaCMC fibers in the disclosed range of DS is accomplished by treatment of the fibrous NaCMC with a cross-linking reagent. Suitable cross-linking reagents for use in preparing the present fibers are, for example, bis-epoxypropyl ether, dichloroethane, divinyl sulfone, epichlorohydrin, ethylene glycol-bis-epoxypropyl ether, formaldehyde, vinyl cyclohexene dioxide, 1,3-dichloro-2-propanol, 1,3-di(.beta.-hydroxy-.GAMMA.-chloropropoxy)-2-propanol, 1,2-di(.beta.-hydroxy-.GAMMA.-chloropropoxy)ethane, 1,2:3,4-diepoxybutane, 1,2:5,6-diepoxyhexane, 2,3-dibromo-1-propanol, 2,3-dichloro-1-propanol and 2,2'-dichloroethyl ether, although other reagents polyfunctional with respect to cellulose can be employed. All of the cross-linking reagents listed above, with the exception of formaldehyde which requires acidic conditions, will cross-link carboxymethyl cellulose under alkaline conditions and result in cross-links containing at least one carbon atom.

Applicants prefer epichlorohydrin for wet cross-linking in forming the present bibulous fibers because both the wet cross-linking and etherification can be simultaneously accomplished in a single alkaline medium. The wet cross-linking necessary to result in substantial water insolubility can result from the treatment of NaCMC fibers, slurried in inert organic diluent and swollen in the presence of minor but swelling amounts of water, with about 3 percent to about 10 percent, preferably about 3 percent to about 7 percent, based on the weight of the original cellulose in the NaCMC of epichlorohydrin under alkaline conditions for about 24 hours at a temperature of about 25.degree. C. Formaldehyde cross-linking can e accomplished by treating swollen fibrous NaCMC fibers under acidic conditions with a dilute aqueous solution of formaldehyde.

Cross-linking of the cellulose raw material in its original fibrous and unmodified form can be accomplished by the foregoing treatment with epichlorohydrin or formaldehyde, and this treatment can be followed by etherification to result in the present bibulous fibers. Alternately, simultaneous cross-linking and etherification is, in general and for example, carried out by treating water-swollen cellulose fibers with epichlorohydrin and with monochloroacetic acid in the presence of a slight excess of sodium hydroxide in an inert organic diluent or slurry medium. Other carboxymethylating agents known in the art can be used to effect the required etherification.

In describing the characteristics of the bibulous fibers which are the subject of the present application, applicants have used several testing procedures. Included in these testing procedures are the determination of the Degree of Substitution (DS), the determination of the Water Retention Value (WRV), the determination of the Salt Water Retention Value (SRV), and the determination of the solubility in 1 percent NaCl. Determinations of these characteristics of bibulous and comparative fibers as reported herein were made according to the following procedural methods.

The DS of carboxymethyl cellulose is the number of carboxymethyl groups per anhydroglucose unit of cellulose. The analytical method employed herein was authored by Karin Wilson and published in Svensk Papperstidning, 63, pp. 714--15 (1960). This method has been accepted as giving correct results for grades of carboxymethyl cellulose varying widely in DS and purity. In this analytical method NaCMC is given an initial treatment with 1 N HC1 in order to destroy any carbonates present, and if the sample is other than NaCMC, it must be converted to NaCMC or the procedure must be modified. The 1 N HC1 treatment is followed by neutralization of the remaining HC1 with a 1 N aqueous solution of sodium hydroxide. The purified NaCMC, or wet cross-linked carboxymethyl cellulose fibrous material, in the sample is then isolated quantitatively by adding ethyl alcohol to result in a final alcohol concentration of 80 percent by volume. The carboxymethyl cellulose, precipitated from the 80 percent by volume ethyl alcohol solution, is then washed free of NaCl, first with 80 percent by volume ethyl alcohol and then with 95 percent by volume ethyl alcohol. The remaining precipitate sample, consisting of purified carboxymethyl cellulose, is subsequently dried, weighed and ashed at 575.degree. C. The sodium carbonate thus obtained is titrated to a methyl red end point with standard sulfuric acid. The DS is the calculated as follows:

The WRV's reported herein were determined by a procedure in which samples of bibulous fibers weighing from about 0.05 grams to about 0.30 grams are soaked in 100 milliliters of water in a covered container for at least 16 hours at room temperature. In this procedure for the determination of absorbency, subsequent to the soaking period, the soaked bibulous fibers are collected on a filter, lightly squeezed and then transferred to 80-mesh screen baskets which are supported one-half inch above the bottom of metal centrifuge tubes. The tubes are covered with plastic covers, and the samples are centrifuged at a relative centrifugal force of 1,500 to 1,700 gravities for 20 minutes. The centrifuged samples are removed rapidly from the screens by means of tweezers to tared weighing bottles and weighed. The weighed samples are then dried to constant weight at 110.degree. C. and reweighed. The WRV is calculated as follows:

Where

W = sample wet weight

D = sample dry weight

W-D = weight of absorbed water

The procedure used for the determination of the SRV's reported herein was the same as the method described above for use in the determination of WRV's, with the exceptions that aqueous sodium chloride solution containing 1 percent by weight of sodium chloride was used in the procedure instead of water, and the calculation, given below, corrects for the sodium chloride remaining on the dried fibers. The calculation is as follows:

The procedure used for determining the solubility in 1 percent NaCl employs a 0.2--0.3 gram sample of the bibulous fibers to be tested. The sample is washed with 80 percent by volume methanol and with 100 percent by volume methanol prior to drying the sample at 110.degree. C. and weighing it in a tared weighing bottle. The weighed sample is then washed into a beaker and soaked for an extended period (overnight) in 100 milliliters of aqueous solution containing 1 percent by weight of sodium chloride. The soaked fibers are then collected on a tared sintered glass filter, weighed while wet, dried at 110.degree. C. and weighed again. The calculation used to obtain the solubility in 1 percent NaCl is as follows:

Where

B = Initial dry weight

W = Wet weight of extracted sample

E = Extracted dry weight

W-E = Water evaporated from wet sample

(W-E)/99 = Weight salt deposited on sample

The following examples will serve to illustrate in detail the manner in which the bibulous fibers of this invention can be prepared and incorporated in useful fibrous structures. It will be understood, however, that the invention is not confined to the specific limitations or methods set forth in the individual examples, but rather to the scope of the appended claims.

EXAMPLE I

Eighteen hundred and sixty-five grams of sheeted purified wood pulp were steeped in a steeping press in 25 liters of aqueous sodium hydroxide solution containing 8 percent by weight of sodium hydroxide at room temperature for a period of 45 minutes. The steeped sheets were then pressed to a press weight ratio of alkali cellulose weight to weight of air dry cellulose (PWR) of 2.5, and the pressed sheets were shredded in a Baker-Perkins sigma blade shredder at room temperature for a period of 45 minutes. After shredding the alkali cellulose in the Baker-Perkins sigma blade shredder, 56.5 grams of epichlorohydrin were added to the shredded alkali cellulose in the shredder, and shredding was continued for an additional 45 minutes. The fibrous alkali cellulose, with epichlorohydrin thoroughly dispersed throughout, was then sealed in a plastic bag and remained therein for 18 hours at 25.degree. C. The resulting wet cross-linked cellulose fibers were then dispersed in water, neutralized with acetic acid, washed with water and centrifuged. The wet cross-linked cellulose fibers were then dried and found to be essentially insoluble in cadoxen and other solvents for cellulose. A sample of these wet cross-linked cellulose fibers was taken as illustration 2.

Three hundred and one grams (dry basis) of the dried cross-linked cellulose prepared above were thereafter slurried in 13.4 liters of 2-propanol and 1,602 milliliters of water in a 22-liter round-bottomed flask equipped with a mechanical stirrer. The resulting slurry was stirred and heated until the temperature reached 40.degree. C. When the slurry temperature reached 40.degree. C., 104.5 grams of sodium hydroxide dissolved in 204 grams of water were added to the mixture in the 22-liter flask during a period of 10 minutes. The heating and stirring were continued for 30 minutes while the temperature of the mixture was increased to 60.degree. C. At this time a solution of 112.5 grams of monochloroacetic acid in 600 milliliters of 2-propanol was added to the 22-liter flask, and the resulting final reaction mixture was stirred at 70.degree. C. for a period of 4 hours.

The reaction mixture was then cooled and neutralized with acetic acid, and the fibrous product was collected by deposition on a screen. The wet cross-linked carboxymethyl cellulose fibers, collected on the screen as bibulous fibers, were washed with 76 percent by weight (80/20 vol/vol) aqueous methanol. The washed wet cross-linked carboxymethyl cellulose fibers were then transferred to 100 percent methanol and dried therefrom. The resulting bibulous fibers were tested according to the methods and determinations detailed above, and were determined to have a DS of 0.47, an SRV of 590, a WRV of 1,650 and a solubility (1 percent NaCl) of 5.7 percent. The essentially water-insoluble cross-linked carboxymethyl cellulose fiber product was found to be highly absorbent and was judged suitable for inclusion in the fibrous structure of absorbent surgical dressings, catamenial napkins, tampons and bandage pads.

Additional examples of essentially water-insoluble cross-linked modified cellulosic fibers were prepared by using the cross-linking and single stage etherification procedure of example I with the exception that the amounts of etherifying and cross-linking agent were varied to result in higher and lower DS and cross-linking; these samples had the characteristics given in table I below. Table I also indicates the level of cross-linking by the amount of epichlorohydrin added. The fibers of Illustration 1 are wood pulp fibers without either etherification or wet cross-linking, while the fibers of illustration 3 are wet cross-linked, low DS wood pulp fibers. ##SPC1##

The bibulous fibers of examples I--VIII were highly absorbent with respect to aqueous solutions, including physiological solutions and blood, and were judged highly suitable for inclusion in fibrous structure of absorbent surgical dressings, catamenial napkins, tampons and bandage pads.

EXAMPLE IX

Eighteen hundred and sixty-five grams of sheeted purified wood pulp were steeped in a steeping press in 25 liters of 8 percent by weight sodium hydroxide at room temperature for 45 minutes. The steeped sheets were then pressed to a press weight ratio of alkali cellulose weight to weight of air dry wood pulp sheets (PWR) of 2.5, and the pressed sheets were shredded in a Baker-Perkins sigma blade shredder at room temperature for a period of 45 minutes. After shredding the alkali cellulose in the Baker-Perkins sigma blade shredder, 56.5 grams of epichlorohydrin (3 percent epichlorohydrin on cellulose) were added to the shredded alkali cellulose remaining in the shredded, and shredding was continued for an additional 45 minutes. The fibrous alkali cellulose, with epichlorohydrin thoroughly dispersed throughout, was then sealed in a plastic bag and remained therein for 18 hours at 25.degree. C. The resulting wet cross-linked cellulose fibers were then dispersed in water, neutralized with acetic acid, washed with water and centrifuged. The wet cross-linked cellulose fibers were then dried and found to be essentially insoluble in cadoxen and other solvents for cellulose.

Three hundred and nine grams (dry basis) of the dried cross-linked cellulose prepared above were thereafter slurried in 13,515 milliliters of 2-propanol and 1,654 milliliters of water in a 22-liter round-bottomed flask equipped with a mechanical stirrer. The resulting slurry was stirred and heated until the temperature reached 40.degree. C. When the slurry temperature reached 40.degree. C., 144 grams of sodium hydroxide in 169 grams of water were added to the heated slurry during a period of 10 minutes. The heating and stirring were continued for 30 minutes while the temperature of the mixture was increased to 65.degree. C. At this time a solution of 155 grams of monochloroacetic acid in 485 milliliters of 2-propanol was then added to the heated mixture in the 22 liter flask, and the temperature of the heated mixture was increased to 72.degree. C. for 1 hour. The slurry was thereafter allowed to cool to 56.degree. C. over a period of 2 hours.

At the end of the 2 hour cooling period an additional 65.6 grams of sodium hydroxide in 81 grams of water was added to the stirred and cooled mixture, and stirring was continued at 56.degree. C. for 30 minutes. At this time 77.5 grams of monochloroacetic acid in 245 milliliters of 2-propanol was added to the stirred mixture, and stirring was continued with additional heating to increase the temperature of the slurry to 69.degree.--72.degree. C. during a period of 3 hours.

The slurry was then brought to a neutral pH by the addition of acetic acid, and the resulting wet cross-linked carboxymethyl cellulose fibers were then collected on a screen. The collected bibulous fibers were washed with 76 percent by weight aqueous methanol (80/20 alcohol/water, vol/vol) solution. The washed bibulous fibers were then transferred to 100 percent methanol and dried therefrom.

The dried cross-linked bibulous fibers were tested according to the method detailed above, and were determined to have a DS of 0.90, an SRV of 1,460, a WRV of 3030 and a solubility (1 percent NaCl) of 16.1 percent. The bibulous fibers of example IX, judged on the basis of the above test procedures, were highly suitable for inclusion into fiber mixtures intended for use in tampons, surgical dressings and absorbent pads.

Additional examples of bibulous fibers were prepared by the two-stage etherification procedure of example IX at higher and lower DS's and cross-linking by regulating the amounts of cross-linking and etherification agents employed. These samples had the characteristics given in table II below. ##SPC2##

The wet cross-linked modified cellulosic fibers of examples IX--XVI have a higher DS than achieved with a single stage of etherification in examples I--VIII, and it is noted that the SRV and WRV values for the higher DS wet cross-linked modified celluloses also tend to be increased. The wet cross-linked modified cellulosic fibers of examples IX--XVI were found, like the wet cross-linked modified celluloses of examples I--VIII to be highly absorbent for aqueous solutions, including physiological solutions and blood, and were judged to be highly suitable for inclusion in the fibrous structure of absorbent surgical dressings, catamenial napkins, tampons, bandage pads and articles of like use. The substitution of potassium, lithium or ammonium salts of fibrous carboxymethyl cellulose in the preparation of the bibulous fibers of examples I--XVI and other bibulous fibers will result in fibers with similar characteristics.

EXAMPLE XVII

Thirty-two and two-tenths grams of purified wood pulp fibers were suspended in 1,428 milliliters of 2-propanol and 164 milliliters of water in a 2-liter round-bottomed flask and stirred together while the solution temperature was increased to 44.degree. C. Seventeen and five-tenths grams of sodium hydroxide in 31 milliliters of water were then added to the stirred mixture over a period of 10 minutes. The resulting fibrous slurry was stirred for 30 minutes during which time the slurry temperature was gradually raised to 65.degree. C. After the 30-minute stirring period a solution of 18.8 grams of monochloroacetic acid in 67.5 milliliters of 2-propanol was added to the slurry. The temperature of the solution was then increased to 71.degree.--74.degree. C., and the resulting slurry reaction mixture was stirred for 3 hours. After the 3-hour reaction period a small sample of the fibrous CMC was collected by screening, neutralized with acetic acid and washed with a 76 percent by weight aqueous methanol (80/20 alcohol/water, vol/vol) solution. At this point the fibrous carboxymethyl cellulose product was soluble in water and had a DS of 0.68, although it was maintained in fibrous form in the alcohol solution.

While the fibrous water-soluble carboxymethyl cellulose remained in slurry suspension, 3.9 grams of epichlorohydrin were added to the reaction mixture and the fibrous slurry was stirred for an additional 3.5 hours at 72.degree. C.

The resulting wet cross-linked carboxymethyl cellulose fibers were collected on a screen, neutralized with acetic acid, washed with 76 percent by weight methanol and thereafter dried from 100 percent methanol. The bibulous fiber product of this example XVII was determined to have a DS of 0.69, SRV of 1340, a WRV of 3,550 and a solubility (1 percent NaCl) of 19.6 percent. The bibulous fibers as shown by the test results were highly absorbent, with particular regard to their affinity for physiological solutions, and were eminently suitable for inclusion in tampons and surgical dressings.

EXAMPLE XVIII

Thirty-two and two-tenths grams of wood pulp fibers were slurried in 1,428 milliliters of 2-propanol and 164 milliliters of water contained in a 2-liter round-bottomed flask equipped with a mechanical stirrer. The fibrous slurry was stirred while raising the solution temperature to 44.degree. C. Seventeen and five-tenths grams of aqueous sodium hydroxide in 31 milliliters of water were added to the stirred mixture over a 10-minute period. The resulting mixture was stirred for 30 minutes during which time the solution temperature was gradually raised to 65.degree. C. After the 30-minute stirring period, a solution of 18.8 grams of monochloroacetic acid in 67.5 milliliters of 2-propanol was added to the solution. Five minutes after the addition of the monochloroacetic acid in the solution, 3.9 grams of epichlorohydrin were added, and the temperature of the solution was increased to 71.degree.--74.degree. C. while the resulting reaction mixture was stirred for 3.5 hours.

The resulting cross-linked modified cellulosic fibers were collected on a screen, neutralized with acetic acid, washed with 76 percent by weight methanol and thereafter dried from 100 percent methanol. The bibulous fiber product was determined to have a DS of 0.71, an SRV of 1,265 a WRV of 2,950 and a solubility (1 percent NaCl) of 14.1 percent.

The bibulous fibers of example XVIII were highly absorbent and hydrophilic in nature and were suitable for fibers for use in fibrous structures such as surgical dressings, tampons, catamenial napkins, etc. where absorbency for aqueous and physiological solutions is a desirable factor.

EXAMPLE XIX

In order to ascertain the effectiveness of the present bibulous fibers when included in effective amounts in catamenial dressings of the tampon type, test tampons were made including amounts of the presently disclosed fibers. The tampons were constructed from rayon staple fiber mats having a basis weight of about 2.5 ounces per square yard. The rayon staple fiber mat was in the form of garnetted, cross-lapped needle-punched matting prepared from crimped rayon staple fibers having a denier of 3 and a length of 19/16 inches.

The bibulous fibers of the present invention were in individual fibrous form, having been run through an attrition mill and sifted through a 14-mesh Rotap screen to break up fiber clumps.

The test tampons were prepared by cutting squares of the rayon mat to a dimension of 10 inches .times. 10 inches. A rayon mat-bibulous fiber stack was then developed by laying down a 10 inch .times. 10 inch square of the rayon mat and sifting 1.5 grams of the bibulous fibers uniformly thereon through a 14-mesh Rotap screen. After sifting the 1.5 grams of bibulous fibers on the rayon mat, another 10 inch .times. 10 inch square of rayon mat was laid on top of the sifted bibulous fibers, and another 1.5 grams of bibulous fibers were sifted onto the new rayon mat.

This procedure was repeated until there were four layers each of rayon mat and bibulous fibers. At this point the stack was capped with yet another square of rayon mat to form a pad containing five layers of rayon mat and four layers of sifted bibulous fibers. The so-formed pad or stack was calendered to imbed the bibulous fibers into the rayon matting, thus preventing a subsequent loss of bibulous fibers in processing.

Having preformed the rayon mat, bibulous fiber pad, individual rectangular tampon swatches having a dimension of 6 inches by 13/8 inches were cut from the pad. Each so-cut tampon swatches weighed about 3.25 grams. Each rayon layer in the tampon swatches weighed about 0.55 grams, and each bibulous fiber layer weighed about 0.125 grams. The percent of wet cross-linked carboxymethyl cellulose fibers in each cut tampon swatch was approximately 15 percent.

The tampon swatches were then further formed into finished tampons by attaching a withdrawal string to the middle of the tampon swatch and then folding over the cut tampon swatch to a 3-inch length. At this point in tampon preparation, the test swatches were conditioned to a moisture content of about 18 percent, and the final swatch weight before die pressing about 3.5 grams. The so-formed and folded tampon swatches comprising preformed fibrous bodies were compressed in a cylindrical die, and the sides of the tampons were roughened or abraded by rolling the tampons between two strips of card clothing, spaced 0.5 inches apart. The finished length of the tampons was approximately 1.80 inches and the diameter was approximately 0.45 inches. Comparative tampons were formed in the described manner from rayon alone, i.e., from five layers of rayon fiber mat, and from rayon and bibulous fibers.

The test tampons were placed for testing within the confines of a balloonlike rubber membrane fixed inside a glass jacket. The tampons were positioned so that about one-quarter inch of their proximal or internal ends were lying on top of a hypodermic needle which was positioned in the bottom of the rubber membrane and through which test fluid was emitted to the test sample in the artificial vagina formed by the rubber membrane. The distal end of the tampons, or the end with withdrawal string attached, was then about 2.5 inches from the artificial vagina opening and the withdrawal string extended outwardly through the opening. The test fluid had the following composition: ##SPC3##

This fluid is considered equal in average viscosity and solids content to menstrual and other physiological fluids.

In testing, water was admitted into the glass jacket to a hydrostatic pressure of 6 inches. The rubber membrane was thus collapsed and formed into a tight-fitting sheath around the test tampon. The test fluid was then admitted through the hypodermic needle at a rate which varied from about 1 milliliter to about 5 milliliters per minute, with the fluid supply regulated so that puddling during the test did not occur. The test was considered complete when test fluid formed a pool inside the rubber membrane at the distal end of the tampon. At this point the total weight of test fluid absorbed was obtained, and the absorption efficiency (weight absorbed/dry weight of test tampon) was calculated. In this manner the bibulous fiber products of example XIV and example VII above were tested for their effectiveness in tampons and the following table of data expresses this effectiveness. ##SPC4##

The data of table IV show that the test tampons containing bibulous fibers in an amount equal to about 15 percent of their weight were at least 37 percent more effective in total absorbency than the comparative test tampon not containing bibulous fibers. The inclusion of additional weight percentages of bibulous fibers will increase the total absorbency of such test products, but it is considered that the inclusion of 15 percent by weight bibulous fibers is sufficient to result in a tampon product which exhibits substantially more total absorbency than products presently available on the market and is characterized by a considerably reduced failure rate. Tampon failure is defined herein as failure to contain menstrual flow during use.

EXAMPLE XX

In order to ascertain the effectiveness of the present bibulous fibers when used in surgical dressings, catamenial napkins and absorbent pads, a 0.5 gram sample of the bibulous fibers of this invention, choosing specifically the bibulous fibers of example XI, were formed into an 2 inch .times. 2 inch absorbent pad. The bibulous fibers in the pad exhibited a DS of 0.79, an SRV of 1370, a WRV of 3480 and a solubility in 1 percent by weight aqueous NaCl of 16.6 percent.

The 2 inch .times. 2 inch absorbent pad of bibulous fibers was completely saturated with whole human blood by adding blood dropwise until excess blood flowed from the pad. The excess blood in the saturated pad was pressed out under a pressure of 1 pound per square inch on the fiber pad during a 3-minute period between blotters. The blotters were changed, to present fresh blotting surfaces, as they took up the excess blood in the pad. The weight of blood retained in the pad per gram of air dry bibulous fiber was calculated. The pad of this example IX formed of bibulous fibers retained 9.3 grams of blood per gram of bibulous fiber. Table V below compares the blood retention of the bibulous fibers of this example XI with other fibers commonly used in absorbent pads and tested by the procedure of this example: ##SPC5##

As shown by the blood retention results given in table V above, the bibulous fibers of example XI exhibited better than three times the blood retention capacity of the best of the other commonly employed fibers tested. When other bibulous fibers are prepared according to the present disclosure and tested for the retention of blood, menstrual fluids, 1 percent by weight aqueous salt solutions and other physiological solutions and for the retention of water, similarly enhanced and improved results will be obtained.

EXAMPLE XXI

In order to ascertain the effectiveness of the present bibulous fibers in absorbing and retaining undissolved water from substantially immiscible aqueous systems comprised of water together with at least one hydrocarbon, 7 grams of the bibulous fibers of example II were packed into a cylinder having an inside diameter of 2 inches and a height of 7 inches. Three and one-half liters of gasoline containing, as entrained fluid, 50 grams of water per liter of gasoline was passed through the cylinder. The gasoline passed through the cylinder while the water was absorbed and remained therein, as evidenced by a water content of 0.01 percent by weight in the gasoline passing though the cylinder. Similar results are obtained in substantially immiscible aqueous systems containing chloroform, toluene, benzene, xylene, kerosene, pentane, hexane and cyclohexane. Also systems containing mixtures of the aforementioned solvents, for example 1 liter of chloroform together with 2.5 liters of hexane, and 175 grams of water, are found to be amenable to water removal according to the procedure of example XXI. In certain instances flow rates were improved by combining the bibulous fibers with glass and other inert carrier fibers according to conventional dispersive practice in the formation of filter pads.

Since many apparent and widely different embodiments of the bibulous fibers of this invention can be made in physical form, chemical composition and use without departure from the spirit and scope of the invention, it will be understood that the invention is not limited to the herein disclosed specific embodiments thereof, except as defined in the appended claims.

Claims

Having thus described the invention, what we claim is:

1. Substantially water-insoluble, wet cross-linked carboxymethyl cellulose fibers, having a WRV of about 900 to about 3,400 and a SRV of about 400 to about 1,500 and manifesting their native cellulosic fiber structure, for use in fibrous structures for the absorption and retention of aqueous solutions, which substantially water-insoluble, wet cross-linked fibers of carboxymethyl cellulose have a degree of substitution of about 0.4 to about 1.6.

2. Substantially water-insoluble, epichlorohydrin wet cross-linked carboxymethyl cellulose fibers, having a WRV of about 900 to about 3,400 and a SRV of about 400 to about 1,500 and manifesting their native cellulosic fiber structure, for use in fibrous structures for the absorption of aqueous solutions, which substantially water-insoluble, epichlorohydrin wet cross-linked fibers of carboxymethyl cellulose have a degree of substitution of about 0.4 to about 1.6.

3. An absorbent fibrous structure, comprising a preformed absorbent pad, containing in effective amount the substantially water-insoluble carboxymethyl cellulose fibers of claim 2.

4. The absorbent fibrous structure of claim 3 which is a surgical dressing.

5. The absorbent fibrous structure of claim 3 which is a catamenial napkin.

6. The absorbent fibrous structure of claim 3 which is a diaper.

7. The absorbent fibrous structure of claim 3 which is a tampon.

8. The absorbent fibrous structure of claim 3 which is a filter for removing undissolved water from substantially immiscible aqueous systems comprised of water together with at least one hydrocarbon.

9. Substantially water-insoluble, wet cross-linked sodium carboxymethyl cellulose fibers, having a WRV of about 1,100 to about 3,400 and a SRV of about 400 to about 1,500 and manifesting their native cellulosic fiber structure, for use in fibrous structures for the absorption of physiological solutions, which substantially water-insoluble, wet cross-linked fibers of sodium carboxymethyl cellulose have a degree of substitution of about 0.6 to about 1.2.

10. A tampon, comprising a preformed fibrous body compressed in cylindrical form, containing in effective amount the substantially water-insoluble, wet cross-linked sodium carboxymethyl cellulose fibers of claim 9.