CA1104002A - Superabsorbent products - Google Patents

Abstract

Abstract of the Disclosure Products having high absorbency for use in absorbent dressings and the like are prepared by precipitating a water insoluble but water-swellable superabsorbent polymer onto the surface of a long fiber cellulose from an aqueous slurry and drying the resulting coated fibers by dehydration with a water-miscible nonsolvent for the polymer.

Description

This invention relates to materials having a high degree of absorbency for water and aqueous salt solutions.
More specifically, it relates to the preparation of such materials based on cellulose in a Eibrous form having a S coating of a superabsorbent material on its surface.
In recent years, considerable ef~ort has been expended pended toward finding or developing materials having great-er powers of absorbency for water and aqueous salt solu-tions than the conventional materials hitherto employed for use in absorbent products. Typical of the absorbent products to which reference is made are such things as diapers, bandages, hospital and nursery bed pads and canta-menial devices. To date, products of these types have been based primarily on cotton, rayon, wood pulp or materials of this nature.
A number of materials have been found which exhibit substantially better absorbency and retention properties than do those conventionally used in these applications.
Generally, these have been polymeric materials which are normally water-soluble but which are treated as, e.g., by cross-linking, to render them substantially water-insoluble but capable of absorbing 1 arge amounts of water or aqueous salt solutions. Many of these absorbent materials are cross-linked, water-swellable but water-insoluble, poly-saccharide derivatives. Exemplary of these are cross-linked sodium carboxymethyl cellulose, cross-linked partial free aci~ carboxymethyl cellulose ~carboxymethyl cellulose will hereafter be referred to as CMC), cross-linked syn-thetic polymers such as cross-linked acrylamide--sodium ~ 30 acrylate copolymers and cellulosics, and starches, grafted with vinyl materials such as acrylic acid salts, acryloni-

-2-trile, and acrylamide. Materials of this type will be referred to hereinafter as "superabsorbent materialsl'.
While materials of the type described are highly absorbent, they have not been totally successful when used as such in absorbent products. In many cases, their absorbency is so great that they form gels which retard or prevent further absorption of liquid. ~ost lack sufficient wlcking action to perform satisfactorily as absorbents.
Moreover, these materials are frequently provided in fine particle form, making it difficult to form stable blends of them with long ~iber cellulosic furnishes.
In accordance with this invention a cellulosic fiber having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solutions comprises a long-fiber cellulose in the form of separate and discre-te fibers having on their surfaces a coating of water-insol-uble, water-absorbent polymer in an amount equal to 15 to 90% by weight based on the total weight of the coated fiber.
Also according to the invention a method of preparing cellulosic fibers having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solu-tions comprises the steps of preparing an aqueous suspen-sion of separate and discrete long-fiber cellulose fibers containing a water-insoluble, water absorbent polymer, stirring this suspension until water-insoluble, water-absorbent polymer forms an aqueous gel. adding to this suspension an inert water-miscible diluent in which the polymer is neither soluble nor swellable to precipitate the polymer onto the surface of the long-fiber cellulose, and thereafter dehydrate in the coated fibers by contacting them with a water-miscible diluent in which the polymer is neither soluble nor swellable, removing the diluent and re-covering separate and discrete long-fiber cellulose fibers.
The discrete coated cellulose fibers thus produced exhibit extremely good absorbency properties as to both rate of absorption and to volume of fluid that can be absorbed.
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presentations of data showin~ the absorption properties of some hereinafter exemplifled embodiments of the invention, and comparison of products of this invention with physical blends of superabsorben~s and long fiber substrates having the same content of superabsorbent.
Applicable superabsorbent materials include any water-insoluble, water-swellable polymers, including synthetic polymers such as cross-linked acrylamide--sodium acrylate copolymers. ~he superabsorbent materials of choice in this invention are based on polysaccharides, either natural or synthetic. Materials of this class include, e.~., cross-linked, normally water~soluble cellulose derivatives which are cross~linked to water-insoluble, water-swellable com-pounds, such as cross-linked sodium CMC, and cross-linked hydroxyethyl cellulose, cross-linked partial free acid CMC, and cellulose, starch, and guar gum yrafted with acrylamide and acrylic acid salts in combination with divinyl com-pounds/ e.g., methylene-bis-acrylamide. The most preferred materials are the CMC derivatives, either cross-linked sodium CMC or partial free acid CMC. Both of these mate-rials are known to the art to be highly absorbent.
Sodium CMC can be cross-linked with any of a number of reagents which are difunctional with respect to cellu-lose. Cross-linking methods applicable to sodium CMC are discussed in, e.g., U.S. patents 3,168,421 and 3,589,364.
Reagents which are difunctional with respect to cellulose include formaldehyde, epichlorohydrin and diepoxide re-agents. Epichlorohydrin is a particularly useful cross-linker. Cross-linking can be accomplished by either the wet or dry method taught in the reference patents. Either technique produces a water-insoluble but bibulous, highly absorbent product which can be employed in the practice of this invention.
Partial free acid CMC is also a known material. It is substantially insoluble in water bu~ will absorb and retain large quantities of water. It is prepared from the conventional sodium salt of CMC by acidifying, as by the method described in U.S. patent 3,379,720. Upon drying, , ,. : .

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3,678,031. The latter reference teaches a method of pre-paring this product directly without first forming the com-pletely neutralized sodium salt. The product resulting from this process is likewise highly absorbent and suitable fQr use in the instant invention. When reference is made hereinafter to partial free acid CMC, it can be taken to indicate the dried, cross-linked watee-insolub1e material.
~ n carrying out ~he process of ~his invention, the superabsorbent material and long fiber substrate are added to a quanti~y of water or aqueous organic medium in excess of that which the superabsorbent material can absorb. Hy-dration of the superabsorbent occurs to the point where theindividual particles swell, forming an aqueous gel slurry.
Upon agitation, these superabsorbent particles are dis-persed throughout the aqueous suspension of long fiber sub-strate (e.g., chemical cotton, wood pulp, staple cotton, rayon, plant fibersj.
The long fiber cellulose can be chemical cotton, wood pulp, staple cotton, rayon or plan9c fibers, for example.
Usually, such fibers are about 2 tv 50 mm. in length. The fibers are discrete, i.e., in fibrous state rather than being incorporated into a fabric or other wet structure.
The superabsorbent can be added before, after or dur-ing the addition of a long fiber cellulose furnish. In either case, agitation is continued for a time sufficient to form a homogeneous mixture of discrete cellulose fi~ers in the gel and to allow the gel to impregnate the fibers.
~he agitation should be sufficiently strenuous to disperse the particles of gelled superabsorbent while not damaging the long fiber cellulose furnish.
The superabsorbent material is recovered from the gel state and precipitated onto the long flber cellulose fur~
nish by contacting the gel slurry while continuing agita-tion with a water-miscible, organic liquid which neither dissolves nor swells either the cellulose furnish or the ~ ~ 9 ~
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superabsorbent. Precipitation of the superabsorbent is followed by removal of excess liquid ~via centrifuging, vacuum and/or pressure filtration or pressing and decant-ing). Dehydration with the same water-miscible organic nonsolvent, or by another which meets the same limitations, then follows. Dehydra~ion can be carried out by contacting the product from the precipitation stage with successive lots of water-miscible nonsolvent. q~he product is then dried to remove the water-miscible organic nonsolvent.
Preferred water-miscible nonsolvents are the water-miscible ketones and lower alcohols.
The specified method of water removal is critical.
In practice, drying can be accomplished by simply evaporat-ing the water. Drying by this technique is difficult and very time-consuming, due to the affinity of the superabsor-ben~ material for water. A much more serious problem, how-ever, if drying is accomplished by simple evaporation, is that the gel particles tend to coalesce as the water evapo-rates, forming a matrix in which the cellulose Eibers are fused together rather than being separate and discrete.
This matrix is stiff and horny and unsatisfactory as an ab-sorbent material in many applications as it lacks absorbent capacity and rapid rate of absorption.
By contrast, when dehydration is carried out by the method specified herein, water is removed from the super-absorbent without the coalescence and matrix formation described above. Discrete fibers are recovered which are fluffy and soft to the touch. These exhibit excellent water absorbency both as to quantity and rate of absorp-tion. They also exhibit excellent wicking action suchthat absorbed water can be transmitted throughout a body of the fibers rapidly and relatively uniformly.
The superabsorbent material is added to the long fiber cellulose furnish to add-on levels of about 15 to 90~ and preferably from about 40 to 90% based on the weight of the coated long fiber cellulose. However, substantial improvements in absorption properties over the fibrous substrate occur even at superabsorbent add-on levels of 15 :. :,... .... :. -: . : :: :

~o 30% based on the weight of the coated long fiber. On the other hand, above ahout 90% add-on, the total absor-bency and the rate of absorption begin to decline.
In observing the tstal absorbency and the rate of absorption of the coated long fiber cellulose, it has been noted that the superabsoxbent material increases the absor-bency of the long fiber cellulose synergistically at add-on levels above about 50%. That is to sayr the coated long fiber cellulose will absorb as well or better than will the superabsorbent material alone.
The coated long fiber cellulose of this invention can be used as an absorbent medium alone or in combination with uncoated long fiber cellulose in a ratio such that the total concentration of superabsorbent is between about 15 and ~0% to form highly absorbent products for use in, e.g., medical, health care and catamenial products~ When used in combination with uncoated long fiber cei]ulose~ the wicking action of the coated fiber affords good total absorbency to a product prepared therewith. In fact, in most cases, the absorbency of a blend of coated and uncoated fibers is greater than that of a physical blend of superabsorbent and uncoated cellulose having an equiva-lent total content of superabsorbent material. Such a characteristic is of considerable economic importance as it enables one to prepare superior absorbent products with lower concentrations of the more expensive superabsorbent materialO
In evaluating the absorbent performance of the prod-ucts according to this invention, two tests axe used prin-cipal]~. These are referred to as the l'CAP test" whichmeasures absorbent capacity and initial rate of absorption, and the "Syringe test" which measures absorption rate and wicking ability.
The apparatus employed for the CAP test consists of a Buchner ~ritted glass funnel, with a rubber tube attached to its neck; the tube is attached at the other end to a 50 ml. burette The burette is filled with the test solution~
and the level of liquid is allowed to rise until it just . ,:: .: :: ,:: . , , - -,.,.; . . . . .-makes contact with the bottom of the frit in the funnelO
The level oE liquid in the burette can be anywhere from a to 60 cm. below the bottom of this frit. The test sample is placed on top of the frit and a weight exerting a pres-sure of from Ool to 0.4 psi is applied to the sample. Thetest is then begun, and the loss of fluid in the burette i5 monitored as a function of time to give the rate of absorption. When equilibrium is reached, the capacity is calculated by dividing the total fluid absorbed at equilib-rium, or at the end of 45 minutes, by the weight of thepolymer sample. The conditions used with the CAP test for this work we~e:
(1) Pressure exerted on ~he sample was 0.11 psi;
(2) All of the tests were done with the liquid in the burette 2 cm. below the fritted glass initially. This level was allowed to continually change as absorption occurred;
(3) Pore size of the frit was about 4-5.5 microns.
In carrying out the Syringe test, a 10 cc~ calibrated syringe is filled with 1.0 gram of test sample and com-pressed with the syringe plunger to give a uniform column of material. The volume to ~hich the material was com pressed varied with the bulk of the sample. For most fibrous samples, the compressed volume was 5 ml., but a few very bulky samples could be compressed only to about 8 cm.
Granular materials occupied a volume between 1 and 3 cm.
The syringe, without the plunger of a needle, is immersed to the 1 cc. mark in a beaker of dyed blue test solution. The rate of uptake of the test solution is ob- `
served, and either the time required for a 5-ml. rise or the volume attained at 30 minutes is recorded.
The invention is illustrated in the examples set forth below. In these examples absorbent properties are demon~
strated with a 1~ NaCl solution to simulate human body fluids. Other salt solutions or plain water could also be used depending upon the application contemplated for the product.

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In a Waring Blendo ~jar containing 400 ml. of water was dispersed l g. of Grade 85 Chemical Cotton (Hercules Incorporated, Wilmington, De]aware). To this was added g g. of partial free acid CMC (made from Grade 85 Chemical Cotton) and stirring was continued for 5 minutes. The slurry was then allowed to stand at room temperature for lO minutes. ~fter another minute of stirring at low speed, the aqueous slurry was transferred to a 2-liter beaker. To this was added, with agitation, 600 ml. of acetone. The Blendor jar was rinsed with 200 ml. of additional acetone which was also~added to the 2-liter beaker. After 10 min-!' utes oE low speed stirring in the: beaker, excess liquid was removed by alternately pressing and decanting superna-tant fluid. The sample was then steeped three times in 600 ml. aliquots of acetone for about 5 minutes each time.
Excess acetone was then removed via pressing and decanting and the sample was dried in vacuum at 60C~ for 1.5 hours.
-Example lA was repeated using 8.0 g. of partial free acid CMC and 2O0 g. chemical cotton.
lC
The procedu~e of ~xample~lA wa~ repeated except that 3 g. of chemical cottQn was used and 7 9. of partial free acid CMC.
-Example lC was repeated except that the water slurry was added to 800 ml. of acetone contained in the ~-liter beakerO
_ Example lA was repeated with 4 g. of chemical cotton and 6 g. of cross-linked partial free acid CMC.
lF
The procedure of Example lA was répeated with 5 g.
chemical cotton and 5 g. of partial free acid CMC.
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Example lA was repeated using 6 9. of chemical cotton and 4 9~ of partial free acid CMC.
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Example lA was repeated using 7 g. of chemical cotton and 3 g. of partial free acid CMCo lI
Example lA was repeated using 8.5 gO of chemical ~ cotton and 1.5 9. o~ partial free acid CMC.
Each of the above materials was tested for 1% NaCl solution absorbency using the tests described previously.
; Simu].taneous control tests were run employing uncoated chem.ical cotton and fibrous partial free acid CMCo Perti-nent data concerning these materials are recorded in Table I.

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The C~P test data in TabIe I show that the equilib-rium absorption capacity of the coated chemical cotton reaches a maximum at about 90~ add-on of par~ial free acid CMC. Also, coated samples containing between about 60 and 90~ partial free acid CMC have absorption capacities equal to or greater than the partial free acid itself. The coated products containing 40 to 90~ partial free acid CMC
have faster initial rates of absorption than the partial free acid CMC by itself.
Lower Syringe Test values for the coated samples indicate better wicking ability and faster rate of absorp-tion than the partial free acid CMC alone.
Example 2 In 400 ml. of water in a Waring Blendor ~6 g. of fluffed wood pulp was slurried along with 4 9. of partial free acid CMC made from shredded chemical cotton sheets.
After about 5 minutes agitation, the slurry was poured into 800 ml. of acetone and agitated at high speed with an air-driven agitator. Excess liquid wa~ removed, as de-scribed in Example 1, and the slurry was steeped three times in 600 ml. of acetone for about 5~10 minutes each time. After removal of excess acetone as described in Example 1, a sample was dried in vacuum at 60C. for about 1.5 hours.
-Example 2A was repeated using 5 9. of fluffed wood pulp and 5 g. of partial free acid CMC.

Example 2A was repeated using 4 g. of fluffed wood pulp and ~ g. of partial free acid CMC.

Example 2A was repeated using 3 g. of fluffed wood pulp and 7 g. of partial free acid CMC~

Example 2A was repeated using 2 g~ of fluffed wood pulp and 8 g. of partial free acid CMC.
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~ xamp]e 2A was repeated using 1 g. of fluffed wood pulp and 9 g. of partial free acid CMC.
These samples were tested for their absorbent charac-teristics using the CAP test and simultaneously runningcontro~ specimens of uncoated fluffed wood pulp and the : partial free acid CMC used for coating. The results are : recorded in Table II.

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Coated samples containing between about 60 and 90% partial free acid CMC have absorption capacities equal to or greater than the partial free acid CMC itself. The coated samples containing 40 to 90% partial free acid CMC have initial rates of absorption equal to or faster than the partial free acid CMC itself.
These results are in agreement with the CAP test data for the Example 1 series where a different cross-linked CMC and fibrous substrate were used.
Examele 3 lS To a wide-mouthed 32 oz. plastic bottle was added 400 ml. of water and 3.0 g. of staple cotton ~extra long fiber furnish). Then 7 g. partial free ~cid CMC made from Grade 85 Chemical Cotton was added. The bottle was sealed, placed on rollers and rolled for one hour. The contents were then transferred to a 2-liter beaker and the eotton fibers which had become matted and entangled were pulled apart by hand to give a more uniform slurry in the beaker, following which 600 ml. of acetone was poured into the beaker. The bottle was rlnsed with another 200 ml. of acetone which was also added to the beaker. The slurry was stirred by hand with a spatula for about 5 minutes, then allowed to stand at room temperature for 15 minutes.
Excess liquid was removed by pressing, following which the sample was dehydrated by steeping with three 600-ml. por-tions of acetone for 10 minutes each steep. After removalof excess acetone, the sample was dried in vacuum at 60C. for 1.5 hours. The resultant sample was tested for its absorbency characteristics using the CAP test and 1~ sodium chloride solution. A control was run simultan-eously using just the uncoated cotton. The control fromExample 1 series served as a control for the partial free acid CMC in this case~

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Example 3A was repeated using staple rayon instead of staple cotton as the long fiber furnish. A control was run simultaneously using just the uncoated rayon.
Pertinent data are recorded in Table III.

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The CAP tes~ data show that coated fiber Example 3A
containing 70~ partial free acid CMC as coating has about the same absorption capacity, but a faster initial rate of absorption than the partial free acid CMC itself. Example 3A also exhibited better wi.cking action than the partial `; ~ree acid CMC itself, having a lower Syring test value of 5 ml. in 34 seconds compared to 5 ml. in 14 minutes for the modified CMC.
The CAP test data show that the rayon staple control ` 10 has a ]ower absorption capacity than the stapie cotton.
-~ This difference is reflected in the lower absorption capa-city of Example 3B as compared to Example 3A. The wi~king action of Example 3B is comparable to that of Example 3A, however, and thus is superior to the partial free acid CMC
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; ~ This example compares a~ueous acetone solutions with water as the slurry medium. To a Waring Blendor~jar con-taining 200 g. o~ water was added 4 g. of Grade 85 Chemical Cotton and 6 g. of partial free acid CMC prepared from fine-cut chemical cotton. The slurry was s~irred at slow speed for about 5 minutes, let~stand for 10 minutes, stirred for one minute, and then transferred to a l-liter ~` 25 beaker. Four hundred ml. of acetone was added to precipi-tate the CMC on the chemical cotton. The slurry was stirred for 10 minutes with an air stirrer. Excess liquid was then removed by suction filtration on a coarse sintered glass filter to about 50% solids content and the wet pad of sample was steeped three times with 60 ml. of acetone each steep. The three steeps were carried out on the glass fil-ter also, allowing each 60 ml. lot of acetone to drain through the wet pad of sample for 5 minutes and on the glass filter also, then suction iltering for about 5 min-; 35 ut~s. After excess acetone was removed from the sample folJowing the third steep, the sample was dried in vacuum ;~ - at 60C. for 1.5 hours.

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.. ! ..: .. . .'.: ' :-;.. :! ' `: ,, , ' -: . , -: ~: .``., . ~, ` '` :`:'' ,. ' Example 4A was repeated except that the slurry medium was 200 g. of 20~ acetone in water and 266 ml. of acetone was used in the precipitation step.
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Example 4A was repeated using 200 g. of 30% aqueous acetone solution as the slurry medium and 202 ml. of acetone in the precipitation stepO

; lO Example 4A was repeated using 200 9. of 40% aqueous acetone solution as the slurry~medium and 137 mlO of acetone in the precipitation step.

Example 4A was repeated using 200 ~. of 50% aqueous acetone solution as the slurry medium and 72 ml. of acetone in the precipitation step.
Syringe test data are given in Table IVa. CAP test data are recorded in ~able IVb.
TABLE IVa Example No. Slurry Medium _Syringe Test Value*
4A Water 4.2 ml. in 30 minutes 4B 20% aqueous acetone 5 ml. in 16 minutes 4C 30% aqueous acetone 5 ml. in 6.5 minutes 4D 40% aqueous acetone 5 ml. in 7 minutes 4E 50% aqueous acetone 5 ml. in 1.25 minutes *Rate of climb o~ 1% NaCl solution in Syringe TABLE IVb Example Absorption/Time Interval*
~o. 1 3 S lO 15 20 25 _ 4A 2.0 4.9 6.9 8.6 8.8 8.8 8.8 4B 2.3 5.4 7.2 8.3 8.4 8.4 804 -' 4C 2.3 5.0 7.7 8.4 8~6 8.6 8.6 4D 2.5 5~7 7.4 8.0 8.1 8.1 8~1 4E 1.8 4.4 6.3 7.6 7.7 7.8 7.8 *Absorption of 1% NaCl solution (ml./g. of sample) at various tlmes in minutes ~; The Syringe test data in Table IVa indicate that coated products prepared with aqueous acetone as the s]urry medium have better wicking action than those pre-'~
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pared in water. However r at 40 and 50% aqueous acetone levels, t~e coated samp]es have slightly less absorption capacity than the coated sample prepared with water as the slurry medium, as shown in Table IVb.
s Exam~le 5 5~
To 400 ml. of water in a Waring Blendor*jar was added 5 g. of fluffed wood pulp and 5 g. of partial free acid CMC
made from shredded chemical cotton sheets. After 5 minutes stirring, the slurry was let stand for 10 minutes, stirred one minute and then the slurry was poured into 1700 ml. of methanol and stirred vigorously with an air-driven agita-tor. After draining off excess liquid, the slurry was steeped three times in methanol using 600 ml. of methanol for each steep. Steeps were of about 5 to 10 minutes dura-tion. The sample was dried in vacuum at 60C. for 1.5 hours.

Example 5A was repeated using 800 ml. of isopropanol as precipitant and steeping with 600 ml. of isopropanol 3 times.
These specimens were tested using the CAP test and Syringe test. Pertinent data are recorded in Table V.
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While both precipitants yielded good products, the isopropano] was preferred over methanol since it required only 800 ml. to accomplish the objective. Acetone is pre-ferred over both methanol and isopropanol, however, as less acetone is required than methanol, and acetone impar~s better absorption properties to the coated fiber than does isopropanol.

To a Waring Blendor~jar containing 400 ml. of water was added 6 g. of fluffed wood pulp and 4 g. of a densi-fied powdery CMC prepared from fine cut cellulose and cross-linked with epichlorohydrin according to the proce-dures taught in Dean et al, U.S. patent 3r589t364O The slurry was stirred for 5 minutes at low speed in the Blendor, and then let stand for 10 minutes at room temper-ature. After stirring for one minute, the slurry was poured into 800 ml. of acetone contained in a 2-liter beaker. The mixture was stirred with an air driven stirrer. After about 10 minutes stirring, excess liquid was removed via alternately pressing and decanting, and the sample was dehydrated by steeping in acetone three times (600 ml. of acetone per steep of about 10 minutes duration). Excess acetone was removed by alternately presising and decanting, and ~he same was dried in vacuo at ~,~ 25 60C. The absorbent characteristics of this material were tested according to the CAP test and Syringe test.
The results are recorded in Table VI.
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The CAP test and Syringe test data in Table VI show that Example 6, with 40% cross-linked CMC as coa~ing, has a higher absorption capacity and better wicking action than the cxoss-linked CMC itself.
Example 7 l 7A
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Eight grams of Grade 85 Chemical Cotton was slurried in 400 ml. of water contained in a Waring Blendor~jar.
Two grams of fine-particle, water-insoluble but water-~! .
swellabJe cross-linked grafted cellulose (acrylamide:sodium acrylate grafts cross-linked with methylene-bis-acrylamide) was added. The mixture was stirred at low speed in the Blendor for 5 minutes and then le~ stand 10 minutes. After another minute of stirring, the slurry was transferred to a 2-liter beaker and 800 ml. of acetone was added with stirring via a Lightning Air Stirrer. Stirring was contin-ued for 10 minutes, after which the slurry was left un~
stirred for 10 minutes. Excess liquid was then removed by a]ternately decanting and pressing. The sample was steeped in acetone three times, using 600 ~1. of acetone per steep of at least 10 minutes duration. After removal of excess acetone by decanting and pressing, the sample was dried in vacuo at 60C. for 1.5 hours.

~s 25 The procedure for Example 7A was repeated, replacing .~ the cross-linked grafted cellulose witX`2 g. of similarly cross-linked grafted starch powder and replacing the Grade 85 Chemical Cotton with 8 g. of fluffed wood pulp.

The procedure for Example 7A was repeated, replacing the cross-linked grafted cellulose with 2 9. of similarly cross-linked grafted guar gum in fine particle form.

The procedure for Example 7~ was repeated, replacing the cross-linked grafted cellulose with 3 g. o~ water-insoluble, but water-swellable, fine particle acrylamide--sodium acrylate copolmer cross-linked with methylene-bis-acrylamide and using 7 g. of ~rade 85 Chemical Cotton.
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24~ 2 The initial slurry was made up in 500 ml. of water in this case. The slurry became too thick to stir adequately in the Blendor, so the mixture was stirred with a spatula by hand ~or the required time~ -' :
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As shown by the CAP test and Syringe test data in Tab]e VII, coated fiber samples prepared according to the procedures in Examples 7A, 7B, 7C and 7D were superior to their corresponding fine particle superabsorbent in absorp-tion capacity, initial rate of absorption and wickingaction.
Example 8 ~r To a Waring Blendor jar containing 400 ml. of acetone was added 6 g. of Grade 85 Chemical Cotton and 4 g. of par-tial free acid CMC to form a blend containing 40% partialfree acid CMC in which the chemical cotton fibers were not coated. Stirring was continued at low speed for 2 minutes, following which excess acetone was removed and the sample was dried in vacuum at 60C.
The absorbency of this material was determined via the CAP test. Results of this test, as compared with Exam-ple lG, show that coated fiber containing 40% partial free acid CMC add-on is considerably more effective as an absor-bent medium in both initial rate and absorption capacity than is the blend of partial free acid CMC and chemical cotton of the same CMC content. The data are presented graphically in Fig. 1.
Exam~le 9 In a Waring Blendor~jar containing 400 ml. of acetone was blended 8 g. of a material similar -to that of Example lF containing 50% partial free acid CMC and 2 g. of Grade ; 85 Chemical COttO}I, resulting in a sample having a total partial free acid CMC content of 40 weight per cent.
Excess acetone was removed and the sample dried in vacuum ` at 60C.
; 9B
Example 9A was repeated using 6 g. of a material similar to that of Example lF and 4 g. of Grade 85 Chemical Cotton, resulting in a sample having a total `~ partial free acid CMC content of 30 weight per cent.
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-Example 8 was repeated using 7g. of Grade 85 Chemical , ~ .: : .
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Cotton and 3 g. of partial free acid CMC to form a blend containing 30 weight per cent of partial free acid CMC and containing no coated fibers.
Absorption characteristics of these materials were de-S termined using the CAP test~ The comparison of these data,along with data of Example 8, is presented graphically in Fig. 2. The data in this graph show the improved absor-bency (faster initial rate and higher absorption capacity) exhibited by a blend of the coated iber and chemical cot-ton as compared to a blend of partial free acid CMC withuncoated chemical cotton in which the total CMC content is the same.
Example 10 lOA
An aq~eous slurry was prepared as described in Example 7D. After the required stirringr the slurry was trans-ferred to an aluminum panj and the water removed by drying in vacuo at 60C. It required a total of 10 hours to dry to constant weight. The sample formed a dense, brittle 20 mat on drying, in contrast to the soft, fluffy material prepared by the Example 7D procedure where acetone was used to remove the water prior to drying from the acetone~
wet state.
The CAP test data in Table V~II show that drying from 25 the acetone-wet s~ate as in Example 7D is much superior to ~;
drying from the water-wet state as in this example, leading to a faster initial rate of absorption and a higher absorp-tion capacity. y lOB
An aqueous slurry was prepared as described for Exam-ple lF. After the required stirring, the slurry was placed in an aluminum pan, and the water removed by drying at 100C. in an air-draft oven. It required 7 hours to dry ; to constant weight. This sample, though not as dense and brittle as lOA, was brittle and hard. In contrast, the material of Example lD, dried from the acetone~wet state, was soft and fluffy.
Again, the CAP test data in Table VIII show that ;

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Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Cellulosic fibers having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solutions comprising a long fiber cellulose having on its surfaces a coating of water-insoluble, water-absorbent polymer in an amount equal to 15 to 90% by weight based on the total weight of the coated fiber and being in the form of separate and discrete fibers.

2. A cellulosic fiber having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solutions which comprises a long fiber cellulose in the form of separate and discrete fibers having on their surfaces a coating of water-insoluble, water-absorbent polymer in an amount equal to 15 to 90% by weight based on the total weight of the coated fiber, said water-absorbent polymer being selected from the class consisting of cross-linked sodium carboxymethylcellulose, cross-linked partial. free acid carboxymethylcellulose, cross-linked hydroxyethyl cellulose, cross-linked cellulose--acrylamide--acrylic acid copolymers, and cross-linked acrylamide--acrylic acid copolymer.

3. A product according to claim 2 wherein the modified cellulose is an epihalohydrin cross-linked carboxymethylcellulose.

4. A product according to claim 2 wherein the modified cellulose is partial free acid carboxymethyl-cellulose.

5. The product according to claim 2 wherein the long fiber cellulose is selected from the class consisting of wood pulp, chemical cotton and cotton staple.

6. The product according to claim 2 wherein the long fiber cellulose is rayon staple fiber.

7. A cellulosic fiber having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solutions which comprises a separate and discrete staple cotton fiber having on its surfaces a coating comprising about 50 to 90% by weight of partial free acid carboxymethylcellulose based on the total weight of the coated fibers.

8. A cellulosic fiber having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solutions which comprises a separate and discrete wood pulp fiber having on its surfaces a coating comprising about 40 to 90% by weight of partial free acid carboxymethylcellulose based on the total weight of the coated fiber.

9. A cellulosic fiber having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solutions which comprises a separate and discrete chemical cotton fiber having on its surfaces a coating comprising about 40 to 90% by weight of partial free acid carboxymethylcellulose based on the total weight of the coated fiber.

10. A blend of an untreated long fiber cellulose and a long fiber cellulose comprised of separate and discrete fibers coated with a water-insoluble, water-absorbent polymer and having a ratio of coated to uncoated fibers such that the total concentration of water-insoluble, water-absorbent polymer in the blend is about 15 to 80%.

11. A method of preparing cellulosic fibers having a rapid rate of absorption and a high absorption capacity for water or aqueous salt solutions which comprises preparing an aqueous suspension of separate and discrete long fiber cellulose fibers containing a water-insoluble, water-absorbent polymer, stirring the suspension until the water-insoluble, water-absorbent polymer forms an aqueous gel, adding to the suspension an inert water-miscible diluent in which the polymer is neither soluble nor swellable to precipitate the polymer onto the surface of the long fiber cellulose and thereafter dehydrating the coated fibers by contacting them with a water-miscible diluent in which the polymer is neither soluble nor swellable, removing the diluent and recovering separate and discrete long fiber cellulose fibers therefrom.

12. The process of claim 11 wherein the long fiber cellulose is selected from the group consisting of wood pulp, chemical cotton and cotton staple.

13. The process of claim 12 wherein the water-insoluble, water-absorbent polymer is a modified polysaccharide.

14. The process of claim 13 wherein the polysaccharide is cellulose.

15. The process of claim 14 wherein the modified cellulose is selected from the class consisting of epichlorohydrin cross-linked carboxymethylcellulose and partial free acid carboxymethylcellulose.

16. The process of claim 12 wherein the aqueous suspension is prepared in water containing up to about 50%
acetone.

17. The process according to claim 12 wherein the water-miscible diluent employed to precipitate the polymer onto the cellulose fiber is acetone and water-miscible diluent employed to dehydrate the coated fibers is also acetone.