ANIONIC POLYMER

Abstract: The invention relates to an anionic polysaccharide having superabsorbent characteristics, its polysaccharide being substituted by sulphate groups and the polysaccharide being cross-linked to a sufficient extent that it remains insoluble in water. The polysaccharide is preferably cellulose. The anionic polysaccharide can be made by a process which comprises (i) reacting a polysaccharide with a pyridine -SO3 complex in a solvent; followed by (ii) reacting with a cross-linking agent to provide a degree of cross-linking sufficient that the product remains insoluble in water. The sulphated polysaccharides show superabsorbent properties largely independent of pH over a wide range and can be used in applications where it is desired to absorb salt containing aqueous liquids, for example in diapers and catamenials.

Description: ANIONIC POLYMER The present invention relates to an anionic polymer, more particularly a water absorbent polymer of the type commonly referred to as a "superabsorbent". The substances currently termed "superabsorbents" are typically slightly cross-linked hydrophilic polymers. The polymers may differ in their chemical nature but they have the property of being capable of absorbing and retaining even under moderate pressure amounts of aqueous fluids equivalent to many times their own weight. For example superabsorbents can typically absorb up to 100 times their own weight or even more of distilled water. Superabsorbents have been suggested for use in many different industrial applications where advantage can be taken of their water absorbing and/or retaining properties and examples include agriculture, the building industry, the production of alkaline batteries and filters. However, the primary field of application for superabsorbents is in the production of hygienic and/or sanitary products such as disposable sanitary napkins and disposable diapers either for children or for incontinent adults. In such hygienic and/or sanitary products, superabsorbents are used, generally in combination with cellulose fibres, to absorb body fluids such as menses or urine. However, the absorbent capacity of superabsorbents for body fluids is dramatically lower than for deionised water. It is generally believed that this effect results from the electrolyte content of body fluids and the effect is often referred to as "salt poisoning". The water absorption and water retention characteristics of superabsorbents are due to the presence in the polymer structure of ionisable functional groups. These groups are usually anionic and may be carboxyl groups, a high proportion of which are in the salt form when the polymer is dry but which undergo dissociation and solvation upon contact with water. In the dissociated state, the polymer chain will have a series of functional groups attached to it which groups have the same electric charge and thus repel one another. This leads to expansion of the polymer structure which, in turn, permits further absorption of water molecules although this expansion is subject to the constraints provided by the cross-links in the polymer structure which must be sufficient to prevent dissolution of the polymer. It is assumed that the presence of a significant concentration of electrolytes in the water interferes with dissociation of the functional groups and leads to the "salt poisoning" effect. The backbone polymer can be synthetic, for example as polyacrylate, or can be a natural polymer such as a polysaccharide, more particularly cellulose, which has been modified so that anionic groups are attached to the polymer backbone. Although the anionic groups are usually carboxyl a similar effect is theoretically possible with other anionic groups such as sulphate groups. Cellulose which has been modified to introduce sulphate groups has been reported in the literature. Concentrated sulphuric acid cannot be used to prepare sulphated cellulose since the result of treating cellulose with concentrated sulphuric acid is a soluble product, presumably resulting from hydrolysis of the cellulose backbone by the sulphuric acid. "Cellulose Chemistry and its Applications", Ed. T.P. Nevell and S.H. Zeronian, Halsted Press (Division of John Wiley & Sons) , 1985, page 350 reports sulphation of cellulose by direct action of aqueous sulphuric acid or sulphuric acid dissolved in a volatile organic solvent such as toluene, carbon tetrachloride or a lower alkanol . However, this reaction may also lead to hydrolysis of the cellulose chain and no use was suggested by Nevell and Zeronian for the products in question. Philipp & Wagenknecht in Carbohydrate Research, 164. 107-116, (1987) report the homogenous sulphation of cellulose using a pyridine-S0 3 complex in an N 2 0 4 -DMF system, the result being a product which is completely soluble in water. An object of the present invention is to provide a sulphated polysaccharide having superabsorbent properties. Another object of the invention is to provide a method for the production of such a sulphated polysaccharide. According to one aspect the present invention provides an anionic polysaccharide having superabsorbent characteristics, the polysaccharide being substituted by sulphate groups and the polysaccharide being cross-linked to a sufficient extent that it remains insoluble in water. Preferably the polysaccharide is cellulose. The present invention also provides a process for the production of an anionic polysaccharide having superabsorbent characteristics which comprises: (i) reacting a polysaccharide with a pyridine-S0 3 complex in a suitable solvent to provide a sulphated polysaccharide; and subsequently (ii) reacting the sulphated polysaccharide with a suitable cross-linking agent to provide a degree of cross- linking sufficient that the product remains insoluble in water. The process according to the invention has the advantage that sulphation takes place readily in a homogeneous phase reaction and the cross-linking step provides a product with superabsorbent properties and with the advantage that these properties are largely independent of pH over a range of about pH 3 to 10. The polysaccharide according to the invention is preferably based on cellulose, for example fibrous cellulose. The invention can be applied to fibrous cellulose derived by any chemical and/or mechanical treatment, for example cellulose fibres obtained from wood pulp purified by the sulphate process or the bisulphite process, cellulose fibres obtained from wood pulp by thermomechanical or mechanical treatment, beet cellulose, regenerated cellulose or cotton linters. Preferably the cellulose fibres are obtained from wood pulp purified by the sulphate process or as cellulose "fluff" derived from mechanical treatment or wood pulp and are of the type generally used for the preparation of absorbent pads in disposable products, for example sanitary napkins and towels and diapers. The invention may also be applied to non-fibrous cellulose, for example powdered or crystalline cellulose. The pyridine-S0 3 complex used in the first stage of the process is commercially available from manufacturers such as Aldrich & Merck and is also known as "sulphur trioxide pyridine complex" . Reaction is carried out in the presence of a solvent under anhydrous conditions for example at a temperature of 20-80°C for a period of 1 to 32 hours, for example, 6 to 24 hours. According to one embodiment the reaction is carried out at room temperature for about 12 hours. Suitable solvents include polar organic solvents, for example amides such as dimethylformamide (DMF) , sulphoxides such as dimethylsulphoxide (DMSO) , and heterocyclic compounds which may be saturated or unsaturated such as furan, tetrahydrofuran, dioxan and pyridine. The pyridine - S0 3 complex should generally be used in excess and the pyridine forming part of the pyridine-S0 3 complex may itself serve as the solvent. DMF is particularly preferred as the solvent. Water reacts with the pyridine -S0 3 complex so that in all cases care must be taken to maintain anhydrous conditions, for example, by drying the reagents and the solvents used and the reaction vessels. In general, the pyridine-S0 3 should be used in excess relative to the cellulose and, for example the ratio of cellulose anhydroglucose units to pyridine-S0 3 complex may be in the range 1:2 to 1:5. The degree of substitution ("ds") of the sulphonate cellulose product can be adjusted by varying the amount of pyridine-S0 3 complex and the ds will generally be in the range 0.1 to 1.5. The ds is a measure of the ratio of sulphate groups to cellulose anhydroglucose units and can be determined as described in WO 92/19652. According to a modification, the reaction with pyridine- S0 3 complex is carried out in the presence of dinitrogen tetroxide (N 2 0 4 ) . N 2 0 4 can be added to the reaction mixture, preferably before addition of the pyridine-S0 3 complex. Addition of dinitrogen tetroxide increases the solubility of the cellulose in the reaction system and can increase the yield of sulphated cellulose in the sense that a product of higher ds is obtained. The reaction with pyridine-S0 3 complex generally produces a soluble sulphated cellulose which can be purified for example by the following steps: neutralisation with alkali; precipitation from solution using an anti-solvent such as an excess of methanol; washing with distilled water; re-dissolving in distilled water and dialysis to remove low molecular weight fractions. The purified sulphated cellulose is then subjected to cross-linking to provide a product which is insoluble in water and shows superabsorbent properties. In principle any reagent capable of cross-linking cellulose can be used and a wide range of such reagents are known from the literature (see for example US-A-3589364, US- A-3658613, US-A-4066828, US-A-4068068) . However, the cross- linking agent should be capable of reacting with the sulphated cellulose under conditions which do not affect the sulphate groups. Some cross-linking agents require the use of severe alkaline conditions and sulphate groups may also react under these conditions. Preferred cross-linking agents, which will cross-link sulphated cellulose under conditions which do not destroy the sulphate groups, can be represented by the formula 2X e where R 1 , R 2 , R 4 and R 5 , which may be the same or different, are each monovalent organic radicals and R 3 is a divalent organic radical; and X θ is a suitable anion. Preferably R 1 , R 2 , R 3 , R 4 and R 5 are all saturated aliphatic or cycloaliphatic hydrocarbon radicals, i.e. alkyl in the case of R 1 , R 2 , R 4 and R 5 and alkylene in the case of R 3 . The terms alkyl and alkylene include radicals which may be or which may include cycloalkyl or cycloalkylene moieties. Each of R 1 , R 2 , R 3 , R 4 and R 5 is preferably a group containing 1 to 20 carbon atoms. Most preferably R 1 , R 2 , R 4 and R 5 are each methyl. Most preferably R 3 is propylene. X θ may be an inorganic or organic anion, for example halide (fluoride, chloride, bromide iodide) , nitrate, nitrite, phosphate, acetate, propionate, hydroxide. A particularly preferred cross-linking agent is 1,3-bis (glycidyldimethylammonium)propanedichloride. Other suitable cross-linking agents include epicholohydrin, formaldehyde, diepoxide, dicarboxylic acids, dialdehyde and diisocyanates. The conditions of the cross-linking reaction should be such as to ensure that the cross-linked sulphated polysaccharide (preferably cellulose) is insoluble in water. In the case of cellulose, the cellulose anhydroglucose units in the ratio of sulphated cellulose to cross-linking agent can be in the range of about 1:1 to 15:1. The reaction temperature can be, for example, in the range of about 4°C to 80°C and the reaction time for the cross-linking can be about 1 hour to 16 hours. Overall reaction time will usually be in the range of 4 to 34 hours. Cross-linking conditions will depend on the nature of the cross-linking agent but the reaction will generally be carried out in the presence of base. This leads to neutralisation of -S0 3 H groups in the sulphated cellulose and the groups are more stable in salt form. The sulphated polysaccharide according to the invention is useful as a superabsorbent and as an ion-exchanger. The product contains S0 3 H groups which are stronger acid groups than the C0 2 H groups found in most convention anionic superabsorbents so that equivalent absorbent capacity can be obtained at lower ds than with polymers containing C0 2 H groups. The absorbent according to the invention is particularly suitable for use in applications where it is desired to absorb salt containing aqueous liquids. Examples of such liquids include in particular menses and urine and the absorbent material can be used as the filling in catamenials and diapers, generally in admixture with a fibrous absorbent such as cellulose fluff. The absorbent according to the invention in acid form can also be used as an ion exchanger and superabsorbent in conjunction with a cationic superabsorbent in basic form as described in our copending Italian patent application No. TO94A000991 filed on 94/12/06 or in conjunction with an anion exchanger in basic form as described in our copending Italian patent application No. TO94A000889 filed on 94/11/10. As already indicated, the superabsorbent properties of the superabsorbent according to the present invention are largely independent of pH over quite a wide pH range (about 3 to 10) . This can have advantages in the use of the absorbent in diapers for example since although the mean pH of urine is around 6.5 the pH can vary within the range of about 5 to 7.3. However the absorbents according to the present invention show particular advantage in absorbing solutions at low pH (3 to 5) and can be particularly in the control of environmental pollution where acid solutions are involved, e.g. the absorbtion of acid leakage or spillage. The invention is illustrated by the following example. Example a. Sulphation of Cellulose 1. 2 g of bleached cellulose kraft pulp was added to 20 ml of anhydrous dimethylformamide (DMF) with stirring and stirring was continued at room temperature for 12 hours. 7.8 g of pyridine-S0 3 complex (Aldrich Chimica, Milan, Italy) was then added and after 4 hours at room temperature with continued stirring the temperature is raised to 70°C and maintained for 30 minutes. 200 ml distilled water were then added and the solution neutralized with IN NaOH. The soluble sulphated cellulose derivative thus obtained was precipitated by addition of a large excess of methanol, the precipitate was washed with distilled water and dialysed against distilled water for 1 to 3 days using a dialysis membrane with a molecular weight cut off point of 14,000 Da. The dialysed product was then lyophilised. 2. 4 g of bleached cellulose kraft pulp were placed in a 2 neck flask and 150 ml of anhydrous DMF were added. N 2 0 4 was bubbled in under mechanical stirring until a brown-green colour was obtained in the solution. 12 g pyridine-S0 3 complex were added (ratio of cellulose anhydroglucose units to pyridine-S0 3 complex 1:3) and the mixture maintained at 4°C for 16 hours with stirring. N 2 was then bubbled into the reaction vessel to eliminate residual N 2 0 4 from the reaction vessel. The polymer was precipitated by addition of a large excess of ethanol saturated with sodium acetate and the product separated by filtration using a G3 glass filter. The product was washed with ethanol, the polymer dissolved in water and the pH maintained at 7.5 by addition of acid (HC1) or alkali (NaOH) as required. The product was then reprecipitated using the procedure described above, re- dissolved in water and dialysed against distilled water for 3 days using a dialysis membrane with a molecular weight cut off point of 14,000 Da. The dialysed product was then lyophilised. b. Cross-linking 0.5 g (2.5 mmol) of the purified sulphated cellulose prepared in (1) above was mixed with 2.5 ml of 19% aqueous sodium hydroxide with stirring. 0.063 ml of 65% aqueous 1,3- bis(glycidyldimethyl-ammonium) propane dichloride and 2 ml of 1% aqueous NaCl was added at room temperature with stirring and stirring was continued at this temperature for 16 hours. The addition of NaCl and base leads to a product with anionic groups in the salt (Na + ) form. The product is then washed with distilled was to neutral pH, filtered and lyophilised. c. Test Results When tested with 1% NaCl solution, the sample had an absorbency (tea-bag test) of 54 (after draining) and 45 (after centrifugation at 60 g) . To illustrate the fact that absorbency is relatively unaffected by pH, the following figures were obtained for absorbency using the tea-bag test in 1% aqueous NaCl adjusted to different pH values using IN hydrochloric acid or IN sodium hydroxide. pH AFTER DRAINING AFTER CENTRIFUGATION 3 34.0 18.9 7 32.6 19.5 9.5 32.1 20.4 In each case the tea-bag test was performed by weighing about 0.3 g of the product into a tea-bag envelope which was itself then weighed and immersed in 150 ml of liquid (1% NaCl solution or distilled water) in a 250 ml beaker for 1 hour. The envelope was then removed from the liquid and allowed to drain for 10 minutes, weighed, and then centrifuged at 60 g for 10 minutes and weighed again. Absorbency is calculated as follows: where: A absorbency (after draining or centrifugation) ; 'wet weight of envelope containing sample after draining or centrifugation (grams) ; W,dry weight of envelope containing sample before immersion (grams) ; weight of sample used for the test (grams) .

Claims: CLAIMS 1. An anionic polysaccharide having superabsorbent characteristics, the polysaccharide being substituted by sulphate groups and the polysaccharide being cross-linked to a sufficient extent that it remains insoluble in water. 2. An anionic polysaccharide according to claim 1 which is cellulose. 3. An anionic polysaccharide according to claim 2 wherein the cellulose is fibrous cellulose. 4. An anionic polysaccharide according to any of claims 1 to 3 having a ds of 0.1 to 1.5. 5. An anionic polysaccharide according to any of claims 1 to 4 wherein the cross-linking agent is a compound of formula 2X " where R 1 , R 2 , R 4 and R 5 , which may be the same or different, are each monovalent organic radicals and R 3 is a divalent organic radical; and X " is a suitable anion. 6. An anion polysaccharide according to claim 5 wherein the cross-linkingagent is 1, 3-bis(glycidylmethylammonium)propane dichloride. 7. A process for the production of an anionic polysaccharide having superabsorbent characteristics which comprises: (i) reacting a polysaccharide with a pyridine-S0 3 complex in a suitable solvent to provide a sulphated polysaccharide; and subsequently (ii) reacting the sulphated polysaccharide with a suitable cross-linking agent to provide a degree of cross- linking sufficient that the product remains insoluble in water. 8. A process according to claim 7 wherein the polysaccharide is cellulose. 9. A process according to claim 8 wherein the cellulose is fibrous cellulose. 10. A process according to any of claims 7 to 9 wherein the reaction with pyridine-S0 3 complex is carried under anhydrous conditions in a solvent which is an amide, sulphoxide or heterocyclic compound. 11. A process according to any of claims 7 to 10 wherein the reaction with pyridine-S0 3 complex is carried out to provide a ds of 0.1 to 1.5. 12. A process according to any of claims 7 to 11 wherein the reaction with pyridine-S0 3 complex is carried out in the presence of dinitrogen tetroxide. 13. A process according to any of claims 7 to 12 wherein the cross-linking agent is a compound of formula CH 2 Rl_ 2X " where R 1 , R 2 , R 4 and R 5 , which may be the same or different, are each monovalent organic radicals and R 3 is a divalent organic radical; and X " is a suitable anion. 14. A process according to claim 13 wherein the cross- linking agent is 1,3-bis(glycidyldimethylammonium)propane dichloride. 15. A process according to any of claims 7 to 14 wherein cross-linking is carried out in the presence of base. 16. A process according to any of claims 7 to 15 wherein the ratio of cellulose anhydroglucose units to cross-linking agent is 1:1 to 15:1.

Inventors: CARLUCCI GIOVANNI, FORNASARI GIANCARLO, GAGLIARDINI ALESSANDRO, TORRI GIANGIACOMO, CARLUCCI, GIOVANNI, FORNASARI, GIANCARLO, GAGLIARDINI, ALESSANDRO, TORRI, GIANGIACOMO
Applicants: CARLUCCI GIOVANNI, FORNASARI GIANCARLO, GAGLIARDINI ALESSANDRO, PROCTER & GAMBLE, TORRI GIANGIACOMO, CARLUCCI, GIOVANNI, FORNASARI, GIANCARLO, GAGLIARDINI, ALESSANDRO, THE PROCTER & GAMBLE COMPANY, TORRI, GIANGIACOMO
Application year: 1995
Main classification: C07H5
IPC codes: A61L15, C08B3, C08B5, C08B15