WO2015097036A1

WO2015097036A1 - Polysaccharide hydrogels

Info

Publication number: WO2015097036A1
Application number: PCT/EP2014/078226
Authority: WO
Inventors: Harald Keller; Alexander Wissemeier; Wolfgang Weigelt; Jorge SANZ-GOMEZ; Motonori Yamamoto
Original assignee: Basf Se
Priority date: 2013-12-23
Filing date: 2014-12-17
Publication date: 2015-07-02

Abstract

The present invention relates to a superabsorbent composition comprising a cross-linked polysaccharide and urea, wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L, and to a soil treatment product comprising the superabsorbent composition of the invention and at least one additional ingredient selected from the group consisting of fillers, nutrients, fertilizers, pesticides and combinations thereof, and to a process for the manufacture of the superabsorbent composition of the invention, and to the use of the composition for agricultural applications.

Description

POLYSACCHARIDE HYDROGELS

Description Field of invention

The present invention relates to a superabsorbent composition comprising a cross-linked polysaccharide and urea or a derivative thereof, wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L. The present invention further relates to a soil treatment product comprising the superabsorbent composition of the invention and at least one additional ingredient selected from the group consisting of fillers, nutrients, fertilizers, pesticides and combinations thereof, to a process for the manufacture of the superabsorbent composition of the invention, and to the use of the composition for agricultural applications.

Background of invention

Hydrogels are formed from superabsorbent polymers which can absorb and retain extremely large amounts of a liquid relative to their own mass. Such superabsorbent polymers are often also referred to as swellable polymers, hydrogel forming polymers, water absorbing polymers, gelforming polymers, and the like. Sometimes also the superabsorbent polymer in the dry form is referred to as hydrogel. In the context of the present invention, the term "hydrogel" will be used only in the context of the wetted state of a superabsorbent polymer, however, because in the dry state, the superabsorbent polymer is typically not present in the form of a gel, but in the form of a powder or a granulate having good flow properties.

An overview over superabsorbent polymers, their properties and methods of manufacturing them is provided by Frederic L. Buchholz and Andrew T. Graham in "Modern Superabsobent Polymer Technology", J. Wiley & Sons, New York, USA / Wiley VCH, Weinheim, Germany, 1997, ISBN 0-471 -1941 1 -5.

Superabsorbent polymers and compositions comprising superabsorbent polymers have become important materials for agricultural applications due to their capacity of absorbing large quantities of water. By using the superabsorbent polymers and superabsorbent compositions for soil treatment, the physiological properties of soils can be improved by increasing their capacity to hold water, reducing erosion and runoff, reducing the frequency of irrigation, increasing the efficiency of the water being used, increasing soil permeability and infiltration, reducing the tendency of the soil to get compacted, and helping plant performance. Most of the superabsorbent polymers used today are cross-linked synthetic polymers. They include, for example, polymers and copolymers based on acrylamide, which are not based on renewable raw materials and which are insufficiently biodegradable. For many applications, and in particular for agricultural applications, the biodegradation of the superabsorbent polymers is a preferred or required design variable to be addressed, however. In this context, polysaccharide-based and in particular cellulose-based superabsorbent polymers are considered highly attractive not only because of their biodegradability, but also because of the large availability of cellulose and the low cost of cellulose derivatives.

Depending on the polysaccharide derivatives used, a number of cross-linking agents and catalysts can principally be employed to form superabsorbent polymers thereof. Epichlorhydrin, aldehydes and aldehyde-based reagents, urea derivatives, carbodiimides and multifunctional car- boxylic acids are most widely used cross-linkers for polysaccharides. However, the water ab- sorption capacity of these cross-linked polysaccharides is often not satisfying compared to synthetic superabsorbent polymers. Good swelling properties are e.g. described for cross-linked polysaccharides obtainable by using epichlorhydrin as cross-linking agent.

In this regard, C. Chang et al. describe superabsorbent hydrogels based on cellulose for smart swelling and controllable delivery, which are prepared from sodium carboxymethylcellulose (CMC) and cellulose in an NaOH/urea aqueous system by using epichlorohydrin (ECH) as cross-linker. C. Chang et al. conclude that the experimental results would prove that the cellulose/CMC hydrogels exhibit superabsorbent capacity and high equilibrium swelling ratio (C. Chang, B. Duan, J. Cai, L. Zhang, European Polymer Journal 46 (2010) 92-100).

CN 101445609 A relates to a hygroscopic cellulose hydrogel and a preparation method thereof. The hydrogel is prepared by dissolving carboxymethylcellulose and cellulose, respectively, in a water solution of NaOH/urea, in which the concentration of NaOH is 6-10 wt.-%, and the concentration of the urea is 4-12 wt.-% to obtain a NaOH/urea mixed water solution of 1 -5 wt.-% of the carboxymethylcelluluse and a NaOH/urea mixed water solution of 1 -5 wt.-% of cellulose; mixing the NaOH/urea mixed water solution of the carboxymethylcellulose and the NaOH/urea mixed water solution of cellulose according to the weight ratio 9:1 -1 :9 and stirring evenly to obtain a mixed solution; adding the mixed solution to a certain amount of the cross-linker epoxy- cholorpropane; and, after the cross-linking reaction, placing the mixed solution at the gelatin temperature until the hygroscopic cellulose hydrogel is formed, wherein the water absorbing capacity is 50-1000 g/g. It should be emphasized however that epichlorhydrin is highly toxic in the unreacted state. Although unreacted chemicals are usually eliminated after cross-linking through extensive washing in distilled water, toxic cross-linking agents should be avoided, in order to preserve biocompati- bility of the resulting hydrogel, as well as to ensure an environmentally friendly sustainable pro- duction process. It is therefore desired to use alternative non-toxic cross-linking agents. At the same time, the polysaccharide-based superabsorbent polymers obtainable thereof should exhibit a satisfying water absorption capacity.

Accordingly, there remains a need for polysaccharide-based superabsorbent polymers obtainable from non-toxic cross-linking agents to further improve the safety of both, the final product and the manufacturing process. At the same time, it is desired to achieve a high water absorption capacity, in order to make the polysaccharide-based superabsorbent polymers competitive with acrylate-based superabsorbent polymers, which are presently on the market for agricultural applications.

It is therefore an object of the present invention to improve the water absorption capacity of a polysaccharide-based superabsorbent polymer, which is biodegradable and environmentally friendly in that it is obtainable from non-toxic cross-linking agents. Furthermore, it is an object of the present invention to provide a soil treatment product, which is biodegradable and environmentally friendly, and exhibits a satisfying water absorption capacity compared to non-biodegradable soil treatment products.

Summary of the invention

The above mentioned objects are achieved by providing a superabsorbent composition comprising a cross-linked polysaccharide and urea or a derivative thereof, wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges on each chain, wherein said ester bridges are spaced by a linker L.

It has surprisingly been found that by adding urea or a derivative thereof to a cross-linked polysaccharide according to the present invention, the water absorption capacity can be significantly improved. Furthermore, it is an advantage of the composition of the present invention that the cross-linked polysaccharides are completely biodegradable to water and carbon dioxide, and that e.g. urea in addition acts as a fertilizer. Furthermore, the cross-linked polysaccharide is obtainable not only from non-toxic polysaccharide derivatives, but also from non-toxic cross- linking agents because the formation of ester bridges does not require cross-linking agents, which contain epoxy groups or carbon-halide bonds as e.g. required for the formation of ether bridges with the hydroxyl groups of a polysaccharide. Furthermore, it should be emphasized as another advantage that ester bridges can be easily hydrolyzed in contrast to e.g. ether bridges. Therefore, the biodegradability is significantly improved.

According to the invention, the composition may be used as a component in a soil treatment product, e.g. for agricultural applications. Such a soil treatment product comprises the super- absorbent composition of the invention and at least one additional ingredient selected form the group consisting of fillers, nutrients, fertilizers, pesticides and combinations thereof, wherein the composition and the additional ingredient are preferably present in a weight ratio of from 80:20 to 20:80.

The soil treatment product of the invention is biodegradable and environmentally friendly because the presence of an acrylate-based superabsorbent polymer can be completely avoided. At the same time a comparable water absorption capacity can be achieved due to the addition of urea or a derivative thereof to the cross-linked polysaccharide in the superabsorbent composition contained in the soil treatment product. Urea itself further advantageously acts as fertilizer. The invention further relates to a process for preparing the superabsorbent compositions of the present invention comprising the steps of a1 ) dissolving the precursor P of the linker L in water to obtain a homogenous solution; b1 ) adding the polysaccharide to the homogeneous solution of step a1 ) to obtain a mixture; c1 ) drying the mixture of step b1 ) to obtain a dried mixture;

d1 ) heat treating the dried mixture of step c1 ) at a temperature of from 100°C to 180°C for a time period of from 10 min to 5 h to obtain a cross-linked polysaccharide;

e1 ) mixing the cross-linked polysaccharide of step d1 ) with an aqueous solution of urea or a derivative thereof to obtain a mixture;

f1 ) drying the mixture of step e1 ); or, alternatively, comprising the steps of a2) dissolving the precursor P of the linker L and urea or a derivative thereof in water to ob- tain a homogenous solution,

b2) adding the polysaccharide to the homogeneous solution of step a2) to obtain a mixture; c2) drying the mixture of step b2) to obtain a dried mixture; d2) heat treating the dried mixture of step c2) at a temperature of from 100°C to 180°C for a time period of from 10 min to 5 h to obtain a cross-linked polysaccharide.

Furthermore, the invention relates to the use of the superabsorbent compositions for agricultural applications.

Figures

Figure 1 : Water absorption capacities of cross-linked NaCMC with a low cross-link density alone (Example 1 a) and in combination with urea in a weight ratio of 50:50 (Example 2a), and water absorption capacities of cross-linked NaCMC with a medium cross-link density alone (Example 1 b) or in combination with urea in a weight ratio of 50:50 (Example 2b).

Detailed description of the invention

The superabsorbent composition according to the present invention comprises a cross-linked polysaccharide and a compound of formula (I),

R⁶ R⁸

(I) wherein in the compound of formula (I)

(a) X represents O or S, and

(b) R⁵, R⁶, R⁷ and R⁸ are independently selected from the group consisting of -H, (Ci-Ce)alkyl, (Ci-C6)alkenyl and (Ci-Ce)hydroxyalkyl, and

wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L.

In a preferred embodiment, the superabsorbent composition according to the present invention comprises a cross-linked polysaccharide and urea,

wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L, wherein the ester bridges are formed by reacting hydroxyl groups of the polysaccharide chains with a precursor P of the linker L comprising at least two carbonyl containing functional groups suitable for forming an ester bridge spaced by the aliphatic linker L, wherein the precursor P of the linker L is a compound of the following general formula (II):

(II) wherein

(a) R^a and R^b are independently selected from the group consisting of -halo, -OH, -OR¹, -NH2 and -N(R¹)₂ with R¹ being -(Ci-C₆)alkyl or -C(=0)(Ci-C₄)alkyl, or

(b) R^a-R^b together represent an oxygen bridge -0-, and

L is the aliphatic linker, which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR³, -NH₂, -N(R³)₂, -COOH, and -COOR⁴, with R³ being -(Ci- C₆)alkyl and R⁴ being -(Ci-C₆)alkyl or Na. As used herein, the expression "chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L" means that the polysaccharide chains have been cross-linked with each other by forming ester bridges to an aliphatic linker L. Accordingly, the polysaccharide chains are connected to each other in that each polysaccharide is connected to an ester bridge, and said ester bridges are connected to each other by the aliphatic linker L. Thus, two cross-linked polysaccharide chains of the cross-liked polysaccharide according to the invention can generically be illustrated as follows:

polysaccharide polysaccharide

chain _Chain

It should be emphasized that each polysaccharide chain may form several ester bridges which are linked to ester bridges of other polysaccharide chains via the aliphatic linker L so that a cross-linked network of polysaccharide chains is established.

As used herein, the term "aliphatic linker" refers to an alkyl, alkylene or alkynylene chain comprising from 1 to 10 carbon atoms, which may be linear or branched, and which may be unsub- stituted or substituted by at least one substituent selected from the group consisting of -OH, - OR³, -IMH2, -N(R³)₂, -COOH, and -COOR⁴, with R³ being -(Ci-C₆)alkyl and R⁴ being -(Ci-C₆)alkyl or Na. Preferably, the aliphatic linker is a (Ci-Cs)alkyl chain, which may be linear or branched, and which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR³, -NH₂, -N(R³)₂, -COOH, and -COOR⁴, with R³ being -(Ci-C₆)alkyl and R⁴ being -(Ci-Ce)alkyl or Na. More preferably, the aliphatic linker L is a linear (C2-C4) alkyl linker, which is optionally substituted with -OH, -OR³ and -COOH or -COOR⁴ with R³ being -(Ci-C₃)alkyl and R⁴ being -(Ci-Cs)alkyl or Na. Most preferably, the aliphatic linker L is a linear C3-alkyl linker, which is optionally substituted with -OH and -COOH or -COONa. Particularly preferably, the aliphatic linker is a linear C3-alkyl linker, which is substituted with one substituent -OH at the C2-atom and with one substituent -COOH or -COONa at the C2-atom.

As used herein, the term "ester bridge" covers a -C(=0)-0- moiety. The carbonyl group -C(=0)- of said -C(=0)-0- moiety may either stem from the polysaccharide chain, if e.g. carboxymethyl- cellulose is used and is reacted with a precursor P of the linker L comprising at least two hy- droxyl groups as functional groups, or the carbonyl group -C(=0)- of said -C(=0)-0- moiety may stem from a precursor P of the linker L comprising at least two carbonyl containing functional groups suitable for forming an ester bridge, which then react with the hydroxyl groups of the polysaccharide chain to form the -C(=0)-0-moiety. Thus, there are two possibilities for the connectivity of two cross-linked polysaccharide chains in terms of their cross-linkage via the ester bridges spaced by the aliphatic linker, i.e. the following connectivity 1 or 2.

polysaccharide polysaccharide polysaccharide polysaccharide chain chain chain chain

In a preferred embodiment of the present invention, the ester bridges are formed by reacting hydroxyl groups of the polysaccharide chains with a precursor P of the linker L comprising at least two carbonyl containing functional groups suitable for forming an ester bridge spaced by the aliphatic linker L.

Accordingly, it is preferred that the carbonyl group -C(=0)- of the -C(=0)-0-moiety in each es- ter bridge stems from the precursor P of the linker L. Thus, it is preferred that two cross-linked polysaccharide chains of the cross-linked polysaccharide according to the invention have the following connectivity 1.

polysaccharide polysaccharide

chain chain

It should be emphasized that ester bridges are particularly advantageous compared to e.g. ether bridges because they can be easily hydrolyzed, which improves the biodegradability of the cross-linked polysaccharides.

In view of the above, the term "precursor" has to be understood as a molecule comprising the linker L and at least two functional groups capable of forming the ester bridges with the poly- saccharide. As already indicated above, said functional groups may either be hydroxyl groups or carbonyl containing functional groups suitable for forming an ester bridge. It is preferred, however, that the precursor P comprises at least two carbonyl containing functional groups suitable for forming an ester bridge. When preparing the cross-linked polysaccharides according to the present invention, the precursor P of the linker L may thus be employed as cross-linking agent.

As used herein, the term "carbonyl containing functional group suitable for forming an ester bridge" refers to a functional group, which comprises a -C(=0)- moiety and is activated for forming an ester bridge if reacted with a hydroxyl group. Preferably, said "carbonyl containing func- tional group suitable for forming an ester bridge" is represented by -C(=0)-R^L, wherein R^L is a leaving group, which is substituted upon reaction with a hydroxyl group. For example, R^L may be selected from -halo, -OH, OR¹, -NH₂ and -N(R¹)₂ with R¹ being -(Ci-C₆)alkyl or -C(=0)(Ci- C4)alkyl. Since preferably at least two carbonyl containing functional groups suitable for forming an ester bridge are present in the precursor P, said groups may also be referred to as -C(=0)- R^a, -C(=0)-R^b, -C(=0)-R^c, -C(=0)-R^d, and so on. Two leaving groups together, e.g. R^a and R^b, may also together represent an oxygen bridge -0-, i.e. form a cyclic anhydride with two carbonyl groups, so that both carbonyl groups are activated for substitution by the one oxygen bridge -O-

In view of the above definition of the precursor P of said linker L as having at least two functional groups capable of forming the ester bridges of the polysaccharide, which are preferably car- bonyl containing functional groups suitable for forming an ester bridge, a precursor P of the linker L may e.g. be described by the general formula (II)

(II)

In a preferred embodiment of the present invention, the precursor P of the linker L is a compound of the following general formula (II)

wherein

(b) R^a-R^b together represent an oxygen bridge -0-, and

L is the aliphatic linker, which is preferably a linear or branched (Ci-Cs)alkyl chain, which is optionally substituted with at least one substituent selected from the group consisting of -OH, - OR³, -IMH2, -N(R³)₂, -COOH, and -COOR⁴, with R³ being -(Ci-C₆)alkyl and R⁴ being -(Ci-C₆)alkyl or Na.

In a more preferred embodiment of the present invention, the precursor P is a dicarboxylic acid or a tricarboxylic acid, preferably a tricarboxylic acid, more preferably citric acid. In this context, a dicarboxylic acid has to be understood as an aliphatic compound comprising two carboxylic acid groups -COOH as carbonyl containing functional groups -C(=0)-R^a and - C(=0)-R^b suitable for forming an ester bridge, and a linker L as defined above, wherein the optionally substituted alkyl, alkylene or alkynylene chain of the linker L is not substituted with a further carboxyl group such as an carboxylic acid group or an carboxylic acid ester group. On the other hand, a tricarboxylic acid has to be understood as an aliphatic compound comprising two carboxylic acid groups -COOH as carbonyl containing functional groups -C(=0)-R^a and - C(=0)-R^b suitable for forming an ester bridge, and a linker L as defined above, wherein the alkyl, alkylene or alkynylene chain of the linker L is mandatorily substituted with one -COOH group as further carbonyl containing functional group -C(=0)-R^c suitable for forming an ester bridge, but is not substituted with a further carboxyl group such as an carboxylic acid group or an carboxylic acid ester group.

Preferably, the dicarboxylic acid is an aliphatic compound comprising

(i) a linear or branched (Ci-Cs)alkyl chain, which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR³, -IMH2, and -N(R³)2, with R³ being - (Ci-C₆)alkyl, and

(ii) two carboxylic acid groups -COOH. Preferably, the tricarboxylic acid has to be understood as an aliphatic compound comprising

(i) a linear or branched (Ci-Cs)alkyl chain, which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR³, -IMH2, and -N(R³)2, with R³ being - (Ci-Ce)alkyl, and mandatorily substituted with at least one -COOH group, and

(ii) two carboxylic acid groups -COOH.

Particularly preferred dicarboxylic acids according to the present invention are e.g. malonic acid, succinic acid (butanedioic acid), glutaric acid, adipic acid and pimelic acid. Further preferred dicarboxylic acids are C₄ to C20 alpha, omega-dicarboxylic acids, for example C₄ to C10 alpha, omega-dicarboxylic acids. Further preferred dicarboxylic acids are unsaturated dicarboxylic acids such as itaconic acid, maleic acid and fumaric acid. Preferred tricarboxylic acids are citric acid, isocitric acid, aconitic acid, propane-1 ,2,3-tricarboxylic acid (tricarballylic acid, carballylic acid) and trimesic acid. The most preferred precursor P of the linker L is citric acid.

The chemical structure of citric acid is depicted below.

Citric acid can either exist in an anhydrous form or as a monohydrate. Both forms are encompassed by the term "citric acid", when used in the context of the present invention.

As used herein, the term "polysaccharide" refers to long carbohydrate molecules of monosaccharide units joined together by glycosidic bonds. In this context, the term "polysaccharide chain" is used to indicate that a single carbohydrate molecule may be considered as chain of monosaccharide units. Considering that the repeating monosaccharide units in polysaccharides are preferably six-carbon monosaccharides, polysaccharides may e.g. be represented by the general formula (C6Hio05)_n, wherein 40≤ n < 3000. Examples of polysaccharides include starch, glycogen, cellulose and chitin. Examples of monosaccharides are glucose, fructose and glyceraldehydes. A preferred polysaccharide according to the present invention is cellulose, which is based on repeated glucose units bonded together by β-glycosidic bonds.

According to the present invention, the term "polysaccharide" also covers derivatives of polysaccharides. For example, the hydroxyl groups of the polysaccharides may partly be modified, so that the hydrogen atom is replaced by an alkyl group such as an ethyl or methyl group, a hydroxyalkyl group such as a hyroxypropyl group, or a carboxyalkyl group such as a carboxy- ethyl group or a carboxymethyl group or the alkali salts thereof.

In a preferred embodiment of the present invention, the polysaccharide chains contain car- boxy(Ci-C3)alkyl groups, preferably carboxyethyl or carboxymethyl groups, more preferably carboxymethyl groups. Examples of natural polysaccharides containing carboxymethyl groups are inter alia alginate and pectin.

Accordingly, it is preferred that the hydroxyl groups of the polysaccharide chains are partly modified, so that the hydrogen atom is replaced by a carboxyethyl group or a carboxymethyl group or the alkali salts thereof, more preferably by a carboxymethyl group or the alkali salts thereof. If the carboxymethyl group is present in the form of its alkali salt, the carboxymethyl group is present in anionic form and an alkali metal is present in cationic form. Otherwise, the carboxymethyl group is present in protonated form. Accordingly, a carboxymethyl group may be represented by the formula -CH2COOH, if present in protonated form, or by the formula -ChbCOO-M*, wherein M represents an alkali metal, if in the form of the alkali salt. It is preferred according to the present invention that the polysaccharide comprises either protonated carboxymethyl groups or anionic carboxymethyl groups, preferably anionic carboxymethyl groups, more preferably sodiumcarboxymethyl groups. Most preferred polysaccharides according to the present are carboxymethylcellulose and sodiumcarboxymethylcellulose. Particularly preferably, the polysaccharide is sodiumcarboxymethylcellulose.

Carboxymethylcellulose may be represented by the following general formula (A):

(A) wherein R either represents hydrogen or -CH2COOH and 40≤ n < 3000, preferably 60≤ n < 2000, more preferably 80≤ n < 800, and wherein at least one R in the chain represented by general formula (A) represents -CH2COOH. Sodiumcarboxymethylcellulose may be represented by the following general formula (B):

(B) wherein R either represents hydrogen or -CH2COO^"Na⁺ and 40≤ n < 3000, preferably 60≤ n < 2000, more preferably 80≤ n < 800, and wherein at least one R in the chain represented by general formula (A) represents -CH2COOH.

Carboxymethylcellulose and sodiumcarboxymethylcellulose are particularly advantageous com- pared to cellulose because of their solubility in water. However, also mixtures of these cellulose derivatives with cellulose itself are encompassed by the polysaccharides of the present invention.

The properties of carboxymethylcellulose and sodiumcarboxymethylcellulose depend on the substitution grade, i.e. the extent to which the hydroxyl groups are modified, so that the hydrogen atom is replaced by a carboxymethyl group or a sodiumcarboxymethyl group. In other words, it is the ratio of the number of substituents R in the above formulae (A) and (B), which represent a carboxymethyl group or a sodiumcarboxymethyl group, relative to the number of substituents R, which represent hydrogen. According to the present invention, a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8.

Thus, in a preferred embodiment of the present invention, the polysaccharide chains are carboxymethylcellulose, preferably anionic carboxymethylcellulose, more preferably sodium carboxymethylcellulose, most preferably sodium carboxymethylcellulose with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8.

With regard to the preferred polysaccharide chains and the preferred precursors P of the linker L according to the present invention, it is preferred that the cross-linked polysaccharide is based on carboxymethylcellulose, preferably anionic sodium carboxymethylcellulose, more preferably sodium carboxymethylcellulose, most preferably sodium carboxymethylcellulose with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8, and a dicarboxylic acid or tricarboxylic acid, preferably a tricarboxylic acid, more preferably citric acid as precursor P of the linker L. For example, the cross-linked polysaccharide may be based on sodium carboxymethylcellulose with a substitution grade from 0.5 to 0.9 and a tricarboxylic acid as precursor P of the linker L. It is particularly preferred that the cross-linked polysaccharide is based on sodium carboxymethylcellulose with a substitution grade from 0.7 to 0.8 and citric acid as precursor P of the linker L. In other words, it is preferred that the cross-linked polysaccharide comprises anionic carboxymethylcellulose chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by a linear C3-alkyl linker, which is preferably substituted with -OH and - COOH or -COONa. It is more preferred that the cross-linked polysaccharide comprises sodium carboxymethylcellulose chains with a substitution grade from 0.5 to 0.9, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by a linear C3-alkyl linker, which is optionally substituted with -OH and -COOH or -COONa. It is particularly preferred that the cross-linked polysaccharide comprises sodium carboxymethylcellulose chains with a substitution grade from 0.7 to 0.8, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by a linear C3-alkyl linker, which is substituted with one substituent - OH at the C2-atom and with one substituent -COOH or -COONa at the C2-atom. In this context, it is also particularly preferred that the carbonyl group -C(=0)- of the -C(=0)-0- moiety of the ester bridge stems from the precursor P of the linker L, which corresponds to connectivity 1 indicated above. Cross-linking of the polysaccharide chains with the precursor P of the linker L may be achieved by reacting the polysaccharide chains with the precursor P of the linker L, i.e. the cross-linking agent, in a heat activated reaction.

In a preferred embodiment of the present invention, the cross-linked polysaccharide is formed by reacting the polysaccharide with the precursor P of the linker L in a weight ratio of from 200:1 to 50:1 , preferably from 150:1 to 75:1 , more preferably from 1 10:1 to 90:1 , most preferably from 100:1 to 98:1. Preferably the cross-linked polysaccharide is formed by reacting the polysaccharide with the precursor P of the linker L as cross-linking agent in the dry state at a temperature of from 100°C to 180°C for a time period of from 10 min to 5 h, more preferably at a temperature of from 120°C to 160°C for a time period of from 15 min to 2 h. The reaction is to be understood as a condensation reaction, which is favored not only at high temperature, but also in the absence of water. Preferably, the reaction is performed in the dry state, i.e. without dissolving the polysaccharide and the precursor P of the linker L, at a temperature of from 100°C to 180°C for a time period of from 10 min to 5 h, more preferably at a tem- perature of from 120°C to 160°C for a time period of from 15 min to 2 h. For example, the reaction may be performed by dissolving a suitable amount of citric acid in water; adding a suitable amount of sodiumcarboxymethylcellulose to obtain a highly viscous solution, wherein sodiumcarboxymethylcellulose and citric acid are present in a weight ratio of from 1 10:1 to 90:1 ; removing the water; and heating the dry mixture to a temperature of from 120°C to 160°C for a time period of from 15 min to 2 h.

The cross-linked polysaccharides according to the present invention may be characterized in terms of their cross-link density. The cross-link density can be influenced e.g. by the cross- linking temperature, the amount of the precursor P of the liner L, i.e. the cross-linking agent. The cross-link density may be determined by the swelling properties of the cross-linked polysaccharide. If a cross-linked polysaccharide absorbs from 0 to at most 100 g water per gram cross-linked polysaccharide after at least three days at a temperature of from 20 to 30°C, said cross-linked polysaccharide is classified as having a high cross-link density. If a cross-linked polysaccharide absorbs from more than 100 to at most 200 g water per gram cross-linked poly- saccharide after at least three days at a temperature of from 20 to 30°C, said cross-linked polysaccharide is classified as having a medium cross-link density. If a cross-linked polysaccharide absorbs more than 200 g water per gram cross-linked polysaccharide after at least three days at a temperature of from 20 to 30°C, said cross-linked polysaccharide is classified as having a low cross-link density.

In a preferred embodiment of the present invention, the cross-linked polysaccharide has a medium or a low cross-link density, preferably a low cross-link density. Accordingly, it is preferred that the cross-linked polysaccharide absorbs more than 100 g water per gram cross-linked polysaccharide, preferably more than 200 g water per gram cross-linked polysaccharide after at least three days at a temperature of from 20 to 30°C. In particular, it is preferred that the cross- linked polysaccharide absorbs from more than 100 to at most 400 g water per gram cross-linked polysaccharide after at least three days at a temperature of from 20 to 30°C, preferably from more than 200 to at most 300 g water per gram cross-linked polysaccharide after at least three days at a temperature of from 20 to 30°C.

With regard to cross-linked sodiumcarboxymethylcellulose as cross-linked polysaccharide, it is also preferred that said cross-linked sodiumcarboxymethylcellulose has a medium or low cross- link density, preferably a low cross-link density, i.e. absorbs more than 100 g water per gram, preferably more than 200 g water per gram after at least three days at a temperature of from 20 to 30°C. In particular, it is preferred that the cross-linked polysaccharide absorbs from more than 100 to at most 400 g water per gram cross-linked polysaccharide after at least three days at a temperature of from 20 to 30°C, preferably from more than 200 to at most 300 g water per gram cross-linked polysaccharide after at least three days at a temperature of from 20 to 30°C.

The composition of the present invention not only comprises a cross-linked polysaccharide as defined above, but also a compound of formula (I),

R⁶ R⁸

(I) wherein

(a) X represents O or S, and

(b) R⁵, R⁶, R⁷ and R⁸ are independently selected from the group consisting of -H, (Ci-Ce)alkyl, (Ci-C6)alkenyl and (Ci-C6)hydroxyalkyl.

As used herein, the term "urea and derivatives thereof" preferably covers all the compounds, which fall under the above generic formula (I). For example, "urea" refers to the compound of formula (I), wherein X is O and R⁵, R⁶, R⁷ and R⁸ are -H. The term "urea derivatives" refers to the remaining compounds covered by the above generic formula (I), including for example thiourea, wherein X is S and R⁵, R⁶, R⁷ and R⁸ are -H, or N-methylurea, wherein X is O, R⁵ is Ci- alkyl and R⁶, R⁷ and R⁸ are -H. If it is referred to "urea or a derivative thereof", it is referred to urea, i.e. a compound of formula (I), wherein X is O and R⁵, R⁶, R⁷ and R⁸ are -H, or any one of the remaining urea derivatives covered by the above generic formula (I).

It is preferred that in the compound of formula (I)

(I)

(a) X represents O or S, and (b) R⁵, R⁶, R⁷ and R⁸ are independently selected from the group consisting of -H, and (Ci- C₆)alkyl.

It is particularly preferred that the composition of the present invention comprises a cross-linked polysaccharide as defined above and urea as the compound of formula (I), i.e. a compound of formula (I)

R⁶ R⁸

(I) wherein

(a) X represents O, and

(b) R⁵, R⁶, R⁷ and R⁸ represent -H. Accordingly, it is most preferred according to the present invention that the compound of formula (I) is urea. Urea or carbamide is an organic compound with the chemical formula (Nhb)- (C=0)-(NH2), i.e. the molecule has two -Nhb groups joined by a carbonyl group -C(=0)-. Urea is commonly known as a nitrogen-release fertilizer. The high solubility of urea in water reflects its ability to engage in extensive hydrogen bonding with water. Without being bound to theory, it is assumed by the inventors that the ability of urea to form hydrogen bonds is also advantageous for improving the water absorption capacity of the cross-linked polysaccharides in the compositions of the present invention.

In a preferred embodiment of the present invention, the cross-linked polysaccharide and urea or a derivative thereof are present in a weight ratio of from 70:30 to 30:70, preferably from 65:35 to 40:60, more preferably from 60:40 to 50:50, and most preferably 57:43 in the composition according to the present invention. Preferably, the cross-linked polysaccharide and urea or a derivative thereof are together present in the composition in an amount of at least 60 wt.-%, more preferably at least 80 wt.-%, most preferably at least 90 wt.-%, particularly preferably at least 95 wt.-% based on the total weight of the composition.

In another preferred embodiment, cross-linked sodiumcarboxymethylcellulose and urea or a derivative thereof are present in a weight ratio of from 70:30 to 30:70, preferably from 65:35 to 40:60, more preferably from 60:40 to 50:50, and most preferably about 57:43 in the composition according to the present invention. Preferably, the cross-linked sodiumcarboxymethylcellulose and urea or a derivative thereof are together present in the composition in an amount of at least 60 wt.-%, more preferably at least 80 wt.-%, most preferably at least 90 wt.-%, particularly preferably at least 95 wt.-% based on the total weight of the composition. The presence of urea or a derivative thereof, in particular the presence of urea in the compositions of the present invention results in a significant improvement of the water absorption capacity of the cross-linked polysaccharides. For example, the water absorption capacity of a cross- linked polysaccharide, such as cross-linked sodium carboxymethylcellulose, having a high cross-link density is preferably improved by from 0% to 15 %, if the cross-linked polysaccharide and the urea are present in a weight ratio of 50:50 and after at least three days at a temperature of 20°C to 30°C. Furthermore, the water absorption capacity of a cross-linked polysaccharide, such as cross-linked sodium carboxymethylcellulose, having a medium cross-link density is preferably improved by from 15% to 35 %, preferably from 20% to 30%, more preferably from 23% to 27%, if the cross-linked polysaccharide and the urea are present in a weight ratio of 50:50 and after at least three days at a temperature of 20°C to 30°C. Moreover, the water absorption capacity of a cross-linked polysaccharide, such as sodium carboxymethylcellulose, having a low cross-link density is preferably improved by from more than 35% to 55%, preferably from 40% to 50%, more preferably from 42% to 46%, if the cross-linked polysaccharide and the urea are present in a weight ratio of 50:50 and after at least three days at a temperature of 20°C to 30°C.

It has been found that the water absorption capacity of the compositions of the present inventions increases with increasing amounts of urea or the derivative thereof until it reaches a maximum at a certain weight ratio of the cross-linked polysaccharide to urea or urea derivative in the range of 70:30 to 50:50. If urea is present in the composition, the water absorption capacity reaches a maximum at a weight ratio of the cross-linked polysaccharide to urea of 57:43. Accordingly, the most significant improvement of the water absorption capacity can be achieved at a weight ratio of cross-linked polysaccharide to urea of from 60:40 to 50:50, preferably of 57:43, if the cross-linked polysaccharide, which may e.g. be cross-linked sodiumcarboxymethylcellu- lose, has a low cross-link density.

In a preferred embodiment of the present invention, the composition is capable of absorbing water or an aqueous solution in an amount of at least 100 g, preferably in an amount of at least 300 g, more preferably at least 350 g per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 3 days. In another preferred embodiment, the composition is capable of absorbing water or an aqueous solution in an amount of from 100 g to 800 g, preferably in an amount of from 300 g to 650 g, more preferably from 350 g to 580 g, per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 3 days.

In another preferred embodiment, a composition comprising a cross-linked polysaccharide hav- ing a medium cross-link density, preferably cross-linked sodiumcarboxymethylcellulose having a medium cross-link density, and urea or a derivative thereof in a weight ratio of 50:50 is capable of absorbing water or an aqueous solution in an amount of at least 100 g, preferably at least 120 g, more preferably at least 150 g per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 3 days. In another preferred embodiment, a composition comprising a cross-linked polysaccharide having a medium cross-link density, preferably cross-linked sodiumcarboxymethylcellulose having a medium cross-link density, and urea or a derivative thereof in a weight ratio of 50:50 is capable of absorbing water or an aqueous solution in an amount of from 100 g to 300 g, more preferably in an amount of from 120 g to 250 g, most preferably from 150 g to 200 g, per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 3 days.

In another preferred embodiment, a composition comprising a cross-linked polysaccharide having a low cross-link density, preferably cross-linked sodiumcarboxymethylcellulose having a low cross-link density, and urea or a derivative thereof, preferably urea, in a weight ratio of 50:50 is capable of absorbing water or an aqueous solution in an amount of at least 300 g, preferably at least 320 g, more preferably at least 350 g per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 3 days. In another preferred embodiment, a composition comprising a cross-linked polysaccharide having a low cross-link density, preferably cross-linked sodiumcarboxymethylcellulose having a low cross-link density, and urea or a derivative thereof, preferably urea, in a weight ratio of about 50:50 is capable of absorbing water or an aqueous solution in an amount of from 300 g to 800 g, more preferably in an amount of from 320 g to 650 g, most preferably from 350 g to 580 g, per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 3 days. The present invention is also directed to a soil treatment product comprising the composition according to the present invention and at least one additional ingredient selected from the group consisting of fillers, nutrients, minerals, organic or inorganic fertilizers, wherein the fertilzers preferably comprise nitrates, sulfates, phosphates, ammonium compounds, calcium compounds, potassium compounds or combinations thereof, pesticides, fungizides, herbizides and combinations thereof, wherein the composition of the invention and the additional ingredient are preferably present in a weight ratio of from 80:20 to 20:80. Preferably, the composition accord- ing to the invention and the additional ingredient are together present in an amount of at least 90 wt.-% based on the total weight of the composition.

The composition according to the present invention and the soil treatment product according to the present invention are suitable for agricultural applications. For this purpose, the composition as well as the soil treatment product are preferably present in dry granular form, wherein the granulates exhibit good flow properties.

In the context of agricultural applications, it is particularly advantageous that the composition and the soil treatment product of the invention exhibit a particularly high biodegradability. Preferably, the composition or soil treatment product is biodegradable in soil by at least 20%, preferably at least 30%, more preferably by at least 45%, most preferably by at least 50% at a temperature of from 20°C to 30°C after 140 days, wherein the percentage value is calculated from the CO2 formation compared to the carbon content of the tested amount of the composition or soil treatment product. In particular, the percentage value defines the amount of carbon in mg, which has been converted the carbon dioxide, compared to the amount of carbon in mg in the tested sample of the composition or soil treatment product, which may be determined by elemental analysis. The present invention is also directed to a process for the manufacture of a composition according to the present invention comprising the steps of

a1 ) dissolving the precursor P of the linker L in water to obtain a homogenous solution; b1 ) adding the polysaccharide to the homogeneous solution of step a1 ) to obtain a mixture; c1 ) drying the mixture of step b1 ) to obtain a dried mixture;

f1 ) drying the mixture of step e1 ).

Alternatively, the process for the manufacture of a composition according to the present invention comprises the steps of:

a2) dissolving the precursor P of the linker L and urea or a derivative thereof in water to obtain a homogenous solution,

In a preferred embodiment of the process of the present invention, the weight ratio of the poly- saccharide and the precursor P of the linker L in step b) is from 200:1 to 50:1 , preferably from 150:1 to 75:1 , more preferably from 1 10:1 to 90:1 , most preferably from 100:1 to 98:1 .

In another preferred embodiment, the polysaccharide is sodium carboxymethylcellulose with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8.

In another preferred embodiment, the precursor P of the linker L is citric acid.

Accordingly, it is particularly preferred that the process of the present invention is performed with sodiumcarboxymethylcellulose with a substitution grade of from 0.7 to 0.8 as polysaccha- ride and with citric acid as precursor P of the linker L, wherein sodiumcarboxymethylcellulose and citric acid are used in a weight ratio of from 1 10:1 to 90: 1.

In another preferred embodiment, urea is used in the process of the present invention. In yet another preferred embodiment of the process of the present invention, the weight ratio of the cross-linked polysaccharide and urea or a derivative thereof, preferably urea, is from 70:30 to 30:70, preferably from 65:35 to 40:60, more preferably from 60:40 to 50:50, most preferably about 57:43. Accordingly, it is particularly preferred that the process of the present invention is performed with sodiumcarboxymethylcellulose with a substitution grade of from 0.7 to 0.8 as polysaccharide and with citric acid as precursor P of the linker L, wherein sodiumcarboxymethylcellulose and citric acid are used in a weight ratio of from 1 10:1 to 90:1 , and that the obtained cross- linked polysaccharide is then combined with urea in a weight ratio of from 60:40 to 50:50.

In a preferred embodiment, the aqueous solution of urea or a derivative thereof has a concentration of from 30 to 60% (w:w), preferably a concentration of about 50% (w:w). Preferably, an aqueous urea solution with a concentration of from 30 to 60 (w:w), more preferably 40% (w:w) is used.

In another preferred embodiment of the process of the present invention, the heat treating step is performed at a temperature of from 120°C to 160°C for a time period of from 15 min to 2 h. The present invention is also directed to the use of the composition or the soil treatment product according to the present invention for agricultural applications. In this context, the composition or soil treatment product of the invention is preferably used for improving plant growth and crop yield.

In a preferred embodiment, plant growth is accelerated by using the composition or soil treatment product of the invention in that the weight of a plant in treated soil is increased by at least 20%, preferably by at least 30%, most preferably by at least 40% compared to the weight of a plant in untreated soil, wherein the percentage value corresponds to the weight increase of the dry weight of the plant in treated soil after 3 weeks cultivation at a temperature of from 20°C to 30°C compared to the plant in untreated soil.

In a preferred embodiment, the composition or the soil treatment product according to the present invention may be used for improving the physiological properties of soils. This may e.g. be achieved by increasing their capacity to hold water, reducing erosion and runoff, reducing the frequency of irrigation, increasing the efficiency of the water being used, increasing soil permeability and infiltration, reducing the tendency of the soil to get compacted, and helping plant performance. In particular, the composition or soil treatment product may be used for improving the physiological properties of plant soil, garden soil, meadow soil, lawn soil, forest soil, field soil, for preparing soils for cultivating plants, and for recultivating of fields, which have become deserted.

It is preferred that the composition or the soil treatment product is used for absorbing and storing humidity in soils, e.g. in areas under cultivation of plants. Alternatively or additionally, it is preferred that the composition or the soil treatment product is used for improving the soil struc- ture by loosening the soil. Furthermore, the soil treatment product may also be used for uniformly distributing nutrients, minerals and fertilizers, wherein the nutrients, minerals and fertilizers are preferably released in a controlled manner over a time period of at least one month.

For the uses indicated above, the composition or the soil treatment product of the invention will preferably be added to the soil in an amount of 1 to 1000 kg/ha, preferably in an amount of 1 to 25 kg/ha field, or in an amount of from 0.1 to 100 kg/T soil.

The invention is further illustrated by the examples, which are not to be understood as limiting the invention, however.

E X A M P L E S A. Determination methods

The following definitions of terms and determination methods apply for the above general description of the invention including the claims as well as to the below examples unless otherwise defined.

a) Determining the water absorption capacity (tea bag analysis)

The water absorption capacity can be determined by the "tea bag analysis" using deionized water.

The cross-linked polysaccharide or superabsorbent composition is grinded and sieved, and the sieve fraction of 150 - 800 μηη is used for testing. The cross-linked polysaccharide or super- absorbent composition is dried and the residual moisture content is determined. 100 mg of the dry cross-linked polysaccharide or superabsorbent composition is placed in a first teabag 1 , and the teabag 1 is then sealed with a film sealer. Another 100 mg of the dry cross-linked polysaccharide or superabsorbent composition is placed in a second teabag 2, and the teabag 2 is then sealed with a film sealer. Both teabags 1 and 2 are placed in 700 ml deionized water and stored at ambient temperature. Three further teabags 3, 4 and 5 without cross-linked polysaccharide or superabsorbent composition are also placed in 700 ml deionized water and stored at ambient temperature.

After 24 hours, the teabags 1 and 2 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 1 and 2 is determined. Similarly, teabags 3, 4 and 5 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 3, 4 and 5 is determined and the average weight Wo is determined. After that, teabags 1 and 2 are again placed in 700 ml deionized water and stored at ambient temperature.

After 48 hours, the teabags 1 and 2 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 1 and 2 is determined. After that, teabags 1 and 2 are again placed in 700 ml deionized water and stored at ambient temperature. After 168 hours, the teabags 1 and 2 are taken out of the water and hanged out inclined for 10 minutes to let the water drain off. Then the weight of teabags 1 and 2 is determined.

The weight of the absorbed water is determined for the absorption times of 24 hours, 48 hours and 168 hours as follows:

Weight of absorbed water = Weight of teabag 1 - Weight of dry sample - Wo

Weight of absorbed water = Weight of teabag 2 - Weight of dry sample - Wo

Then, the weight of absorbed water is normalized to 1 g of dry cross-linked polysaccharide or superabsorbent composition.

The results are provided as the weight of absorbed water in gram per weight of the dry cross- linked polysaccharide or superabsorbent composition in gram [g (water)/g(cross-linked polysaccharide or superabsorbent composition)] after 24, 48 and 168 hours, respectively. b) Determining the biological degradability (biodegradability)

The mineralization of the cross-linked polysaccharide or superabsorbent composition is measured using the method and the manometric measurement system described by Robertz, M. ef al. ("Cost-effective method of determining soil respiration in contaminated and uncontaminated soils for scientific and routine analysis" published in: Wise, D.L., ef al. (eds.) Remediation Engineering of Contaminated Soil, 573 - 582, Marcel Dekker Inc., New York, Basel, 2000). The carbon mineralization is expressed as the difference in the accumulated soil respiration (CO2 formation) with the cross-linked polysaccharide or superabsorbent composition added minus without the cross-linked polysaccharide or superabsorbent composition added. Per measuring unit, 50 g of dry soil is used to which water is added up to 50% of its maximum water holding capacity. The amount of the cross-linked polysaccharide or superabsorbent composition added is equivalent to 50 mg C determined by elementary analysis. The soil used is a light textured soil from Limburgerhof, Germany, with pH 6.8. The results are the average of 4 replicates.

c) Determining the acceleration of plant growth (cylinder test)

With the aid of the test described hereinafter, the effects of the inventive compositions on the shoot and root growth of corn plants (plant growth) can be measured. The cross-linked polysaccharide or superabsorbent composition to be studied (0.01 -10 g/kg) is added to a water- moistened plant substrate and mixed in until homogeneously distributed. To determine the blank value, correspondingly moistened quartz sand is used. Then five precultivated corn seed- lings were planted into each pretreated substrate and cultivated at ambient temperature for about 3 weeks, in the course of which the plants are watered with a compound fertilizer solution once per week. The plants are removed from the pots along with the roots, the roots are cleaned by washing and the plants are assessed for appearance and size. Then the shoot and root are separated from each other in each case and both parts are weighed to determine their fresh weight. The shoots and roots are subsequently dried to constant weight and their dry weights are determined. The final weights for the shoots and roots of 5 identically treated plants in each case are used to calculate the mean values for fresh and dry weights.

B. Examples

Example 1

a) Preparation of cross-linked sodiumcarboxymethylcellulose with a low cross-link density:

Cross-linking of sodiumcarboxymethylcellulose with citric acid is performed in a heat activated reaction in the dry state.

22.850 kg water and 50.1 g citric acid are filled in a mixer (FM Lodige 130). The mixer is started with a rotation of 931/min. 7.550 kg sodiumcarboxymethylcellulose are added and the mixing process is continued for 1 h at 25°C. The mixture is filled into a fluidized bed dryer and dryed with a nitrogen flow at 81 °C for 45 min. Then, the temperature is increased to 120°C and the product is dryed at 120°C for 80 min to obtain the crosslinked sodiumcarboxymethylcellulose. For testing the water absorption capacity, 0.5 kg of the product is sieved and the sieve fraction with particle size of 150μηι - 800μηι is used for the tests. The result is provided in Figure 1 . b) Preparation of cross-linked sodiumcarboxymethylcellulose with a medium cross-link density:

22.850 kg water and 50.1 g citric acid are filled in a mixer (FM Lodige 130). The mixer is started with a rotation of 931/min. 7.550 kg sodiumcarboxymethylcellulose are added and the mixing process is continued for 1 h at 25°C. The mixture is filled into a fluidized bed dryer and dryed with a nitrogen flow at 138°C for 45 min. to obtain the crosslinked sodiumcarboxymethylcellulose.

For testing the water absorption capacity 0.5 kg of the product is sieved and the sieve fraction with particle size of 150μηη - 800μηη is used for the tests. The result is provided in Figure 1 . Example 2

a) Preparation of a superabsorbent composition comprising a cross-linked sodiumcarboxymethylcellulose with a low cross-link density and urea in a weight ratio of 50:50:

The cross-linked sodiumcarboxymethylcellulose as prepared in Example 1 a in an amount of 10 g is mixed with 25 g solution of urea in water in a concentration of 40 wt% to obtain a mixture, wherein the cross-linked sodiumcarboxymethylcellulose and the urea are present in a weight ratio of 50:50. Said mixture is then dried for 48 h by applying a temperature of 25°C and a reduced pressure of 30 mbar to obtain the superabsorbent composition 2a).

The water absorption capacity is determined by the "tea bag analysis". The result is provided in Figure 1 .

b) Preparation of a superabsorbent composition comprising a cross-linked sodiumcarbox- ymethylcellulose with a medium cross-link density and urea in a weight ratio of 50:50:

The cross-linked sodiumcarboxymethylcellulose as prepared in Example 1 b in an amount of 10 g is mixed with 25 g solution of urea in water in a concentration of 40 wt% to obtain a mixture, wherein the cross-linked sodiumcarboxymethylcellulose and the urea are present in a weight ratio of 50:50. Said mixture is then dried for 48 h by applying a temperature of 25°C and a re- duced pressure of 30 mbar to obtain the superabsorbent composition 2b).

The water absorption capacity is determined by the "tea bag analysis". The result is provided in Figure 1 . Figure 1 shows the water absorption capacities of super absorbent compositions comprising (1 ) cross-linked NaCMC with a low cross-link density alone (Example 1 a = curve with data points in the form of triangles) or in combination with urea in a weight ratio of 50:50 (Example 2a = curve with data points in the form of circles), (2) cross-linked NaCMC with a medium cross-link density alone (Example 1 b= curve with data points in the form of rhombs) or in combination with urea in a weight ratio of 50:50 (Example 2b = curve with data points represented by "x"), wherein the vertical y axis represents the water absorption in g/g and the horizontal x axis represents the number of days.

Claims

1 . A superabsorbent composition comprising a cross-linked polysaccharide and urea, wherein the cross-linked polysaccharide comprises polysaccharide chains, which are chemically cross-linked via ester bridges, wherein said ester bridges are spaced by an aliphatic linker L, wherein the ester bridges are formed by reacting hydroxyl groups of the polysaccharide chains with a precursor P of the linker L comprising at least two carbonyl containing functional groups suitable for forming an ester bridge spaced by the aliphatic linker L, wherein the precursor P of the linker L is a compound of the following general formula (II):

(II) wherein

(a) R^a and R^b are independently selected from the group consisting of -halo, -OH, - OR¹, -NH₂ and -N(R¹)₂ with R¹ being -(Ci-C₆)alkyl or -C(=0)(Ci-C₄)alkyl, or

(b) R^a-R^b together represent an oxygen bridge -0-, and

L is the aliphatic linker, which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR³, -NH₂, -N(R³)₂, -COOH, and -COOR⁴, with R³ being -(Ci-C₆)alkyl and R⁴ being -(Ci-C₆)alkyl or Na.

2. The composition according to claim 1 , wherein the aliphatic linker L is an alkyl, alkylene or alkynylene chain comprising from 1 to 10 carbon atoms, which is linear or branched, and which is unsubstituted or substituted by at least one substituent selected from the group consisting of -OH, -OR³, -NH₂, -N(R³)₂, -COOH, and -COOR⁴, with R³ being -(Ci-C₆)alkyl and R⁴ being -(Ci-C₆)alkyl or Na.

3. The composition according to claim 1 or 2, wherein the aliphatic linker L is a (Ci- Cs)alkyl chain, which is linear or branched, and which is optionally substituted with at least one substituent selected from the group consisting of -OH, -OR³, -NH₂, -N(R³)₂, -COOH, and -COOR⁴, with R³ being -(Ci-Ce)alkyl and R⁴ being -(Ci-Ce)alkyl or Na.

4. The composition according to anyone of the claims 1 to 3, wherein the precursor P of the linker L is a dicarboxylic acid or a tricarboxylic acid.

5. The composition according to claim 4, wherein the precursor P of the linker L is citric acid.

6. The composition according to any one of claims 1 to 5, wherein the polysaccharide chains contain carboxy(Ci-Cs)alkyl groups.

7. The composition according to any one of claims 1 to 5, wherein the polysaccharide chains contain carboxyethyl or carboxymethyl groups.

8. The composition according to any one of claims 1 to 7, wherein the polysaccharide chains are carboxymethylcellulose.

9. The composition according to any one of claims 1 to 7, wherein the polysaccharide chains are anionic carboxymethylcellulose, more preferably sodium carboxymethylcellulose, most preferably sodium carboxymethylcellulose with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8.

10. The composition according to any one of claims 1 to 9, wherein

(a) the cross-linked polysaccharide is formed by reacting the polysaccharide with the precursor P of the linker L as cross-linking agent in a weight ratio of from 200:1 to 50:1 , preferably from 150:1 to 75:1 , more preferably from 1 10:1 to 90:1 , most preferably from 100:1 to 98:1 ; and/or

(b) the cross-linked polysaccharide is formed by reacting the polysaccharide with the precursor P of the linker L as cross-linking agent in the dry state at a temperature of from

100°C to 180°C for a time period of from 10 min to 5 h.

1 1 . The composition according to any one of claims 1 to 10, wherein the cross-linked polysaccharide has a medium or a low cross-link density, preferably a low cross-link density.

12. The composition according to any one of claims 1 to 1 1 , wherein

(a) the cross-linked polysaccharide and urea or a derivative thereof are present in a weight ratio of from 70:30 to 30:70, preferably from 65:35 to 40:60, more preferably from 60:40 to 50:50, most preferably about 57:43; and/or

(b) the cross-linked polysaccharide and urea or a derivative thereof are together present in the composition in an amount of at least 60 wt.-%, preferably at least 80 wt.-%, more preferably at least 90 wt.-%, most preferably at least 95 wt.-% based on the total weight of the composition.

13. The composition according to any one of claims 1 to 12, wherein the composition is capable of absorbing water or an aqueous solution in an amount of at least 100 g, preferably in an amount of at least 300 g, more preferably at least 350 g per gram of the composition, at a temperature of from 20°C to 30°C for an absorption time of at least 3 days.

14. A soil treatment product comprising the composition according to any one of claims 1 to 13 and at least one additional ingredient selected from the group consisting of fillers, minerals, nutrients, fertilizers, pesticides and combinations thereof, wherein

(a) the composition according to any one of claims 1 to 12 and the additional ingredient are preferably present in a weight ratio of from 80:20 to 20:80; and/or

(b) the composition according to any one of claims 1 to 12 and the additional ingredient are preferably together present in an amount of at least 90 wt.-% based on the total weight of the composition.

15. Process for the manufacture of a composition according to any one of claims 1 to 13 comprising the steps of

a1 ) dissolving the precursor P of the linker L in water to obtain a homogenous solution;

b1 ) adding the polysaccharide to the homogeneous solution of step a1 ) to obtain a mixture;

c1 ) drying the mixture of step b1 ) to obtain a dried mixture;

d1 ) heat treating the dried mixture of step c1 ) at a temperature of from 100°C to 180°C for a time period of from 10 min to 5 h to obtain a cross-linked polysaccharide; e1 ) mixing the cross-linked polysaccharide of step d1 ) with an aqueous solution of urea or a derivative thereof to obtain a mixture;

f1 ) drying the mixture of step e1 ); or comprising the steps of a2) dissolving the precursor P of the linker L and urea or a derivative thereof in water to obtain a homogenous solution,

b2) adding the polysaccharide to the homogeneous solution of step a2) to obtain a mixture;

c2) drying the mixture of step b2) to obtain a dried mixture; d2) heat treating the dried mixture of step c2) at a temperature of from 100°C to 180°C for a time period of from 10 min to 5 h to obtain a cross-linked polysaccharide.

16. The process according claim 15, wherein

(a) the weight ratio of the polysaccharide and the precursor P of the linker L in step b) is from 200:1 to 50:1 , preferably from 150:1 to 75:1 , more preferably from 1 10:1 to 90:1 , most preferably from 100:1 to 98:1 ; and/or

(b) the polysaccharide is sodium carboxymethylcellulose with a substitution grade of from 0.3 to 1.0, preferably from 0.5 to 0.9, more preferably from 0.7 to 0.8; and/or

(c) the precursor P of the linker L is citric acid, and/or

(d) urea is used.

17. The process according to claim 15 or 16, wherein the weight ratio of the cross- linked polysaccharide and urea is from 70:30 to 30:70, preferably from 65:35 to 40:60, more preferably from 60:40 to 50:50, most preferably about 57:43.

18. Use of the composition or the soil treatment product according to any one of claims 1 to 13 for agricultural applications, preferably for improving the physiological properties of soils, more preferably for absorbing and storing humidity in soils, and/or for im- proving the soil structure by loosening the soil, wherein preferably plant growth is accelerated in that the weight of a plant in treated soil is increased by at least 20%, preferably by at least 30%, most preferably by at least 40% compared to the weight of a plant in untreated soil, wherein the percentage value corresponds to the weight increase of the dry weight of the plant in treated soil after 3 weeks cultivation at a temperature of from 20°C to 30°C compared to the plant in untreated soil.