IE42120B1 - Absorbent cellulosic product - Google Patents

Abstract

An insoluble etherified cellulose graft copolymer is provided comprising etherified cellulose which is soluble group consisting of carboxyalkyl cellulose, sulfoalkyl cellulose and phosphonoalkyl cellulose and salts thereof. The etherified cellulose has grafted onto its cellulose backbone side chains of polymer moieties in sufficient quantities to render the grafted etherified cellulose insoluble. The products of this invention are used alone or mixed with other absorbent materials such as unmodified cellulose, in the manufacture of absorbent napkins, tampons, sponges and the like.

Description

The present invention relates to highly absorbent materials made by chemically modifying the naturally occurring structure of cellulose ethers having sufficiently high degrees of ether substitution to be soluble in water but being further modified to become highly absorbent insoluble cellulose ethers.

Cellulose fibre and regenerated cellulose are raw materials for many commercial absorbent products including, for example, such products for absorbing body fluids as catamenial napkins and tampons, diapers and surgical dressings. While, in the main, unmodified cellulose has proven useful in such products, in an effort to improve quality and economy, the art has searched for improved materials. Xt was discovered for example, that cellulose ethers such as carboxymethyl cellulose ethers such as carboxymethyl cellulose exhibited increased absorption and retention properties for body fluids, these desirable properties increasing with the degree of ether substitution (D.S.). Accordingly, such materials in, of course, insoluble form are useful in products for absorbing body fluids, such teachings being disclosed in U.S. Patent No, 3,005,456, issued to Graham on October 24, 1961. Further, in U.S.

Patent No. 3,589,364, issued to W. L. Dean and G. N. Ferguson on June 29, 1971, there is disclosed a particular form Of insoluble, carboxymethyl cellulose having a high degree of substitution of ether groups, 2120 and insolubilized by wet crosslinking the cellulose using methods such as are described in U.S. Patent No. 3,241,553 , issued to F. H. Steiger on March 22, 1966. In U.S. Patent 3,678,061, still another form of insoluble carboxymethyl cellulose is disclosed having a high degree of substitution, and in this case, insolubilized by an acid and heat treatment. U.S. Patent 3,256,372 to Adams et al. discloses grafting hydrophilic polymers to the cellulose backbone and, in Patent Specification No. (jtd/t.l r we disclose still another chemical modification of cellulose whereby a balanced quantity of ionic and nonionic copolymer moieties are grafted to the cellulose backbone and may be grafted to modified forms of cellulose as well.

Each of these references teach modified forms of cellulose which represent great improvements in absorption and retention capacities for body fluids over the properties of unmodified cellulose. Notwithstanding these prior art improvements, the search continues for still better absorbents.

It has now been discovered that a form of modified cellulose may be provided which far exceeds the absorption and retention properties of prior art modified cellulose and hence, is advantageously employed as an absorption media in an absorbent body for products such as catamenial napkins and tampons, diapers and dressings provided for absorbing and -342120 retaining body fluids. In accordance with this invention we provide a method of insolubilizing a water-soluble alkali metal or ammonium salt of carboxyalkyl cellulose, sulfoalkyl cellulose or phosphonoalkyl cellulose, comprising grafting onto the cellulose backbone of said salt, side chains of polymer moieties in sufficient quantity to render the grafted cellulose ether salt insoluble yet still absorbent.

A sufficient quantity of etherifying groups are present per anhydroglu,cose unit in the cellulose chain to render such ethers water-soluble if the grafted polymer groups were not present. Such a degree of ether substitution (D.S.) of at least 0.35 ether groups per anhydroglucose unit is generally sufficient. The salts of such ethers comprise the alkali metal and ammonia salts, e.g., Li, Na, K, or NH^ salts.

The cellulose ethers are rendered insoluble by grafting, to the cellulose backbone, side chains of homopolymer or copolymer moieties which side chains may be made up of hydrophilic, hydrophobic or both hydrophilic and hydrophobic polymeric moieties and should be present in sufficient quantities to render the ether water-insoluble. The quantity of side chains which must be grafted to the backbone will vary in accordance with the type of ether and the D.S. of the ungrafted cellulose ether. For example, in the case of carboxymethyl cellulose at a D.S. of 0.4, the side chains should constitute at least 10% by weight of the grafted cellulose ether. At a D.S. of 1.2, the side chains should be at least 25% by weight of the grafted cellulose ether. The side chains oould constitute as much as 90% or even more by weight.

While polymers that are hydrophilic and polymers that are hydrophobic or copolymers being both hydrophilic and hydrophobic may be grafted to the cellulose backbone, particular advantage accrues in using either the hydrophilic polymer or the hydrophilic/hydrophobio copolymers as with these, the resulting grafted cellulose ether exhibits even greater absorption and retention properties, this surprisingly being the case without sacrificing insolubility.

By this invention, a product is provided which is the result of having cellulose undergo the two chemical reactions of etherification and polymer grafting. The starting material for these reactions may be natural cellulose fibers, for example, wood pulp, hemp, bagasse and cotton or may be regenerated cellulose fiber such as rayon.

The etherified cellulose is carboxyalkyl cellulose, phosphonoalkyl cellulose or sulfoalkyl cellulose. Etherification is generally accomplished - 5 42120 by reacting cellulose with an etherification reagent in an alkaline dispersing media. Kor example, sodium carboxymethyl cellulose may be made by the process described by R. L, Whisler in Carbohydrate Chemistry, Vol. Ill (Cellulose), at pages 322-327, Academic Press, Inc. (1963) wherein there is disclosed methods of converting cellulosic materials, notably cotton linters, into carboxymethyl cellulose by reaction with chloracetic acid and aqueous sodium hydroxide in a propanol solution. The so-called slurry processes for manufacturing sodium carboxy10 methyl cellulose is described in U.S. Patent 3,345,855 issued to Russel Nelson on October 17, 1967 wherein the degree of etherification (D.S.) is controlled and can be varied in the range of from 0.4 to 1.6. This product is available conmercially in either powdered or fibrous form, although it should be noted that even higher D.S. products ranging from 2.5-2,77 can be made in accordance with the process described in the aforementioned publication by Whistler.

Phosphonoalkyl cellulose may be made in a similar manner employing a phosphonoalkylating reagent solution com20 prising a basic aqueous solution of a compound of the structure: OX X - R-P = 0 OX wherein X is a halogen and R is an alkylene radical. A preferred reagent is chloromethylphosphonic dichloride. -642130 Sulfoalkyl cellulose may be made in a similar manner employing sulfoalkylating reagent solution comprising a basic aqueous solution of a compound of the structure: II X - R - S - OY II wherein X is a halogen, R is an alkylene radical and Y is 5 chosen from a halogen, hydrogen, or alkali metal atom. A preferred reagent is chloroethylsulfonic acid.

The resulting cellulose ether should have sufficient etherifying groups to render it water-soluble in the absence of other treatment. A carboxymethyl cellulose ether with a D.S. of at least 0.35 will accomplish this result, particularly for the alkali metal salts of these ethers. As is well-known in the art, the D.S. may be controlled during the etherification process by controlling the reaction time and temperature and the proportions of reactants.

In accordance with this invention, the ethers have grafted onto the cellulose backbone thereof, side chains of polymer or copolymer moieties in sufficient quantity to render the grafted product insoluble in water. Such Bidecbains may comprise hydrophilic or hydrophobic moieties and, in fact, a given molecule of cellulose ether could be provided with various combinations of these kinds of polymers. -742120 Hydrophilic polymer moieties usable for this purpose may be, for example, poly(acrylic acid), sodium poly(acrylate), poly(methacrylic acid), potassium poly(methacrylate), poly(vinyl alcohol sulfate), poly(phosphoric acid), poly(vinyl amine), poly(4-vinyl pyridine) and hydrolyzed poly (acrylonitrile).

Also usable are hydrophobic polymers, for example, poly(methyl methacrylate), poly(ethyl methacrylate), poly(ethyl acrylate), poly(butyl acrylate), poly(vinyl acetate), poly(styrene), poly(butadiene) and poly(isoprene).

Copolymers of any of these groups are likewise usable and, in particular, still greater enhancement of absorption and retention properties are realized when at least a portion of the polymer moieties are chosen from the group characterized as hydrophilic.

The cellulose ethers may be combined with either the preformed homo- or copolymers in the first instance, or with the precurser monomers of these polymers with polymerization taking place in situ. In either event, the reactants may be dispersed, and the reaction carried out, in a vapor medium or a non-aqueous medium, for example, acetone,alcohols (e.g., methanol, ethanol, isopropanol), benzene and liquid ammonia . Preferably, however, the grafting reaction is carried out in an aqueous medium. -842120 When the grafting is carried out in a liquid medium, to promote dispersion and hence, more uniform polymerization of some monomers (e.g. butadiene), it is desirable to add a few drops of an emulsifier to the reaction mixture.

Examples of such an emulsifier are *Triton X-1OO (one of a class of acrylalkyl polyether alcohols, sulfonates, and sulfates sold by Rohm & Haas)*, sodium lauryl sulfate; lauryl bromethyl ammonium chloride; a cationic quaternary ammonium salt of the alkyl trimethylammonium chloride and dialkyl dimethylammonium chloride type wherein the average alkyl composition by weight is 90% dodecyl, 97, tetradecyl and 1.0% octadecyl and which is supplied as a solution of 337» active ingredient, 177. sodium chloride, and 507o water by Armour & Co. as Arquad 12; lauryl pyridinium chloride, and the like. (*Registered Trade Mark ).

The grafting reaction may be initiated with an ionic initiator (e.g., alkali hydroxides), a cationic initiator (e.g. a Lewis acid such as boron trifluoride), or even with radiation (ultraviolet, gramma, or electron beam radiation).

It is preferred, however, that the grafting be carried out by the free radical polymerization mechanism using a free radical initiator, for example, ceric ion, ferrous ion, cobaltic ion, (NH^SgOg and cuprous ion. The ceric ion initiator is preferred.

Because most free radical reactions are inhibited by the presence of oxygen, it is desirable to flush out essentially all the oxygen from the reaction vessels by bubbling an inert non-oxidizing gas such as nitrogen, helium and argon, through the system prior to the addition of the free radical initiator. -942120 The pH range used for the reaction depends on the particular initiator used. One could use anywhere from a highly acidic pH (0.8-2.3) to a highly basic pH (12-14), depending on the choice of initiator. For the preferred ceric ion initiator, the pH should be acidic, i.e., less than seven and preferably 0.8 to 2.3.

The grafting reaction may be carried out at temperatures ranging from room temperature (i.e. 20 to 30°C.) to the normal boiling point of the lowest boiling component of the mixture. If the reaction is carried out under greater than atmospheric pressure, the temperature could then be raised above the normal boiling point of the lowest boiling component of the mixture. The reaction mixture may also be cooled below room temperature, if desired.

A satisfactory product may be obtained by grafting either inherently hydrophilic or hydrophobic polymers or mixed copolymers to the cellulose In the manner described above. However, as is pointed out herein, particular advantage accrues to ethers grafted with at least partially hydrophilic chains and a satisfactory method of accomplishing this is to first graft, by the method described above, a polymer which is at least partially hydrolyzable and then subsequently hydrolyzing the product to produce an at least partially hydrophilic grafted chain. Such hydrolyzable polymer Is for example, polyacrylonitrile which can be hydrolyzed to hydrophilic alkali metal poly(acrylate). When copolymers of poly (acrylonitrile) -1042120 and relatively non-hydrolyzable polymer moieties, e.g. methyl methacrylate, ethyl acrylate and butadiene, are grafted to the cellulose ether and subsequently hydrolyzed in a controlled manner, a mixed polymer chain results which is partially hydrophilic and partially hydrophobic. Hydrolyzation is accomplished by reacting the grafted ethers, preferably under reflux, with an excess of a solution of a strong base, e.g., sodium hydroxide, potassium hydroxide and lithium hydroxide· The concentration of this solution may be from 1% to 50% by weight.

The weight percent of polymer grafted to the cellulose ether (based on the weight of grafted ether) depends to the major extent.upon the D.S. of the ether. Generally, the greater the D.S., the greater the weight percent of grafted polymer required to render the otherwise water-soluble cellulose ether insoluble. It has been found that this relationship is essentially independent of the nature of the polymer, i.e., the hydrophilicity or hydrophobicity of the polymer.

The following table illustrates this relationship using sodium carboxymethyl cellulose of varying D.S. as.exemplary cellulose ethers.

MINIMUM POLYMER GRAFTING TO INSOLUBILIZE SODIUM CARBOXYMETHYL CELLULOSE Degree of Substitution of CMC 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Minimum Polymer Add-On (%) by wt. 0 9 12 13 14 15 17 19 21 23 27 based on ungrafted CMC. -1142120 The grafted cellulose ethers of this invention have been found to be significantly greater in both absorption and retention properties over prior chemically modified forms of cellulose and even over the grafted unetherified cellulose -»«r described in Patent Specification No. if j jly j .

While the products of this invention may be used in the form of powders, it is preferred that, when they are used in a fibrous absorbent body in a product for absorbing body fluids, they retain the original fibrous structure Several methods of producing such a fibrous product will occur to one skilled in the art in view of the teachings herein.

For example, when the products of this invention are formed by first producing water-soluble alkali metal cellulose ethers and then grafting in an aqueous media, the ungrafted intermediate product can be temporarily water-insolubilized by treating it with an acid solution. Thereafter, the grafting can take place in a water media, while still preserving the fibrous structure. Subsequent to grafting, the product can be treated with an alkali metal hydroxide to convert the ether back to its original alkali metal salt form. Alternatively, the sequence of reaction steps could be varied to either first graft unetherified cellulose and then subsequently etherify or to both graft and etherify essentially simultaneously. In any event, the fibrous structure of the cellulose can be maintained. -1242120 The products of this invention may be used alone or mixed with unmodified cellulose, or other absorbent material, in the manufacture of absorbent napkins, tampons and sponges. Fibrous products of this invention either alone or in combination with other materials for example, untreated cellulose, may also be made into nonwoven fabrics or tissue, which fabrics or tissue are useful in the manufacture Of absorbent napkins, tampons and sponges.

The grafted cellulose ethers of this invention, their properties and the methods of preparation will be more fully understood from a consideration of the following examples which are given for the purposes of illustration.

EXAMPLE 1 A series of samples of commercially available sodium carboxymethyl cellulose powder having a D.S. of 0.41.2 are obtained from Hercules, Inc. of Wilmington, Delaware. Twenty grams of each sample is converted to the acid form by treating with 250 ml. of a methanol-nitric acid solution (100 ml. HNO3 in one liter MeOH) for twenty-four hours at 25°C. The re20 suiting acidified material is washed thoroughly with distilled water and then placed in a reactor with 1000 ml. of distilled water. Oxygen is removed from the system by purging with nitrogen for one half hour. Ten ml. of a ceric ammonium nitrate 1342120 solution (0.1 Molar Ce (IV) in 1 Nonnal nitric acid) was then added and, after five minutes, a predetermined quantity of acrylonitrile (varying from 5 to 30 ml.) is added. The reaction is allowed to proceed for two hours, at 25°C., under a nitrogen atmosphere. The resulting grafted cellulose ethers are transferred to a Buchner funnel and washed thoroughly with water and acetone. The washed ethers are then converted to the alkali metal salt form bv being treated with a 5% solution by weight of potassium hydroxide in methanol for 24 hours at 25°C. The product is washed with methanol and dried at 105°C.

The resulting series of hydrophobic-polymer grafted alkali metal cellulose ethers are tested to determine the weight percent of grafted polymer and the water and salt solution absorption and retention properties. The polymer concentration of these samples is determined by nitrogen analysis wherein the nitrogen content of the grafted ethers is obtained by the Kjeldahl method, using an ammonia electrode, this method being described in Official Methods and Analysis of the Association of Official Analytical' Chemists, Washington, D.C., 12th Edition, Edited by W. Horowitz, 1975, code no. 47.023.

Aqueous liquid absorbency is determined by the X0W Test, wherein an approximately one-half gram sample of the test material is accurately weighed and stirred into a beaker containing 100 ml. of the aqueous test fluid. After twenty minutes, the mixture is filtered through a piece of nylon trico fabric and allowed to drain for five minutes. The filtrate is collected and measured in the graduated cylinder. The X0W, the absorption capacity of the material expressed in units of grams of absorbed liquid per gram of test material is calculated as follows: -1442120 XOW « 100-filtrate (ml.) _ weight of test material (dry) Aqueous fluid retention capacity is determined by using a Porous Plate Testing Apparatus, as described in detail in Textile Res. J., 37, pp 356-366, 1967. Briefly, this test involves placing the test material in what is essentially a Buchner funnel having a porous bottom plate and holding the sample in place by applying thereon a standard weight to maintain a standardized confining pressure. The porous plate is placed in contact with a reservoir of liquid and the sample is allowed to absorb liquid through the porous plate until saturated.

By maintaining the sample at essentially the level of the reservoir, the liquid absorbed is subjected to essentially a zero hydraulic head with respect to the reservoir. To determine liquid retention, the saturated sample is elevated with respect to the liquid reservoir thereby Imposing a hydraulic head upon the liquid absorbed, the head arbitrarily chosen as 35.5 cm. of fluid. The apparatus is provided with means for directly measuring the volume of liquid retained under this hydraulic head. Retention values are reported as the volume retained per unit weight of the sample.

The results of these tests are tabulated in Table X using variously, aqueous salt solutions and water. As a control, ethers, as commercially obtained, are likewise tested and reported as unmodified. As a further control, ethers which are acidified following the above-described procedure and then converted back to the salt form without undergoing the grafting reaction are tested and also reported in Table I as, treated controls. -1542120 For comparative purposes, wood pulp fibers are also tested and reported as cellulose fiber. In all the Tables herein, percentages are by weight and add-on percentages are based on the weight of ungrafted ether.

Table I ABSORBENCY OF HYDROPHOBIC POLYMER (PAN) GRAFTED CMC (K-SALT) POWDER Absorption Properties Carboxymethyl Cellulose Powder DS Polymer Add-On .

Claims (10)

1. CLAIMS:1. A method of insolubilizing a water-soluble alkali metal or ammonium salt of carboxyalkyl cellulose, sulfoalkyl cellulose or phosphonoalkyl cellulose, comprising grafting 5 onto the cellulose backbone of said salt, side chains of polymer moieties in sufficient quantity to render the grafted cellulose ether salt insoluble yet still absorbent.

2. A method according to Claim 1, wherein said polymer moieties are grafted in a quantity sufficient to constitute 10 at least 10 per cent by weight of said grafted cellulose ether salt, the ether salt being a carboxymethyl cellulose ether salt with a degree of substitution of 0.4.

3. A method according to Claim 1 or 2, wherein said polymer moieties are hydrophilic. 15

4. A method according to Claim 1 or 2, wherein said polymer moieties are hydrophobic.

5. A method according to claim 1 or 2, wherein said polymer moieties are of copolymers comprising hydrophilic and hydrophobic polymer moieties. 20

6. A method according to Claim 1 or 2, wherein Said polymer moieties are of poly(acrylonitrile).

7. A method according to Claim 6, comprising hydrolyzing the grafted polyacrylonitrile moieties.

8. A method according to Claim 1 or 2, wherein said polymer 25 moieties are of copolymers of poly(acrylonitrile) and poly(ethyl acrylate).

9. A method according to Claim 8, comprising at least partially hydrolyzing said copolymer moieties.

10. A method of insolubilizing a water-soluble alkali metal 30 salt of a cellulose ether according to Claim 1 substantially as described in the foregoing Examples 1 to 4.