WO2019167013A1

WO2019167013A1 - Process for preparing a superabsorbent aerogel

Info

Publication number: WO2019167013A1
Application number: PCT/IB2019/051665
Authority: WO
Inventors: Anna Borriello; Lucia BALDINO; Stefano Cardea; Ernesto Reverchon; Luigi Nicolais
Original assignee: Materias S.R.L.
Priority date: 2018-03-02
Filing date: 2019-03-01
Publication date: 2019-09-06
Also published as: IT201800003239A1; EP3759167A1

Abstract

The present invention relates to a process for preparing a superabsorbent aerogel comprising the following steps: - supplying or preparing a superabsorbent hydrogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel, wherein said crosslinked polysaccharides are selected from carboxymethyl starch; hydroxypropyl starch (HPS), oxidized starch, carboxymethyl cellulose (CMC) possibly salified preferably carboxymethyl cellulose sodium salt (NaCMC), hydroxyethyl cellulose (HEC), ethyl hydroxyethyl cellulose (EHEC), hydroxypropyl cellulose (HPC), and oxidized cellulose; - dehydrating said hydrogel by subsequent immersion in baths consisting of water and an increasing concentration of an organic solvent miscible in water, starting from 50% v/v of organic solvent; and - drying the resulting product with supercritical CO₂. The aerogel obtained through such process and the use thereof in different technical fields, e.g. in agriculture or in the personal hygiene sector, represent further objects of the invention.

Description

“PROCESS FOR PREPARING A SUPERABSORBENT AEROGEL”

DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a process for preparing a superabsorbent aerogel consisting of at least 90% by weight of crosslinked polysaccharides; the aerogel obtained through such process and the use thereof in different technical fields, e.g. in agriculture or in the personal hygiene sector, represent further objects of the invention.

BACKGROUND ART

Superabsorbent polymers (SAP) consist of polyelectrolytes or other polymer matrices that have both crosslinking sites and numerous hydrophilic groups.

The materials on which superabsorbent polymers (SAPs) are based can absorb distilled water hundreds of times their own weight, e.g. 400 to 500 times their own weight.

The crosslinking between polymer chains allows the formation of a three-dimensional nerwork, so that when the crosslinked polymer is placed in contact with water, it swells because of the absorption of water allowing the formation of a superabsorbent hydrogel; for the formation of the superabsorbent hydrogel the presence of hydrophilic groups inside the polymer molecule is also fundamental, as it is these groups that interact with the water molecules.

SAPs can be used in different fields; for example, in agriculture they are used in devices to control the release of water and/or nutrients and/or phytochemical products in the ground, in particular for growing in dry, desert areas and in all cases in which frequent irrigation cannot be performed. These products, mixed in dry form into the soil in the areas surrounding the roots of the plants, absorb water during irrigation and are able to withhold it, slowly releasing it together with the nutrients and phytochemical products useful for cultivation. SAPs are also used in absorbent products for personal hygiene and the home such as, for example, the absorbent layer in children's bibs, in sanitary towels and the like; in the sector of toys and gadgets, for example, in products whose size can change significantly in contact with water or aqueous solutions. Finally, SAPs are used in the biomedical sector, e.g. in biomedical and/or medical devices such as, for example, absorbent medications for treating exudative wounds such as ulcers or burns, or in the pharmaceutical sector, e.g. in slow release polymer films adapted for the release of liquids for use in ophthalmology; and in the body fluid management sector, e.g. for controlling the amount of liquids in the organism, e.g. in products that can promote the elimination of fluids from the body, such as in the case of oedema, chronic heart failure, dialysis, etc.

Currently, most SAPs are prepared starting from synthetic polymers, especially from acrylic acid and derivatives thereof. As acrylic acid is derived from crude oil, it has numerous disadvantages connected with this raw material, which is not renewable, is not biodegradable and is expensive. In the sector of the superabsorbent products industry, there is therefore a strongly felt need to find SAPs starting from raw materials that are more respectful of the environment as they are renewable and biodegradable, and whose cost is lower and less fluctuating than that of crude oil.

Therefore, research has focused on the study of different polysaccharides, which are cheap, abundant, renewable and biodegradable, as raw material for preparing SAPs. For example, as starting polymers, cellulose, starch, chitin and natural rubbers such as xanthan gum, guar gum and alginates have been studied. Generally, the reactions for preparing SAP polysaccharides are of two types: a) graft copolymerization of a suitable vinyl monomer on a polysaccharide in the presence of a crosslinking agent or b) direct crosslinking of a polysaccharide.

A review regarding superabsorbent hydrogels, both acrylic- and cellulose-based, was drawn up by Zohuriaan- Mehr MJ, et al., (Iranian Polymer Journal, 2008, 17(6): 451 -477).

The synthesis of a microporous hydrogel through the crosslinking of hydroxypropyl cellulose with epichlorohydrin and ammonia in sodium hydroxide aqueous solution was described by Yan, L, et al., (Clean, 2009, 37(4-5): 392-398). The product obtained by extracting the hydrogel with water in a Soxhlet extractor and then operating freeze-drying (cryodesiccation), is a dry resin.

The synthesis of a hydrogel through crosslinking of carboxymethyl cellulose sodium salt and hydroxyethyl cellulose with divinyl sulfone was described by Esposito, F., et al., (Journal Applied Polymer Science, 1996, 60: 2403-2407). The dry product was obtained by adopting three different drying methods: some samples were dried at room conditions (1 atm, 25°C), other samples were vacuum dried and others dried by extraction with acetone.

The degree of crosslinking of a superabsorbent hydrogel synthesized through crosslinking with divinyl sulfone of a mixture of hydroxyethyl cellulose and carboxymethyl cellulose sodium salt in an aqueous medium was studied by Lenzi, F., et al., (Polymer, 2003, 44: 1577-1588). The dry product was obtained by phase inversion extraction with acetone and subsequent vacuum treatment.

The synthesis of a superabsorbent hydrogel through crosslinking of carboxymethyl cellulose sodium salt and hydroxyethyl cellulose with divinyl sulfone linked to a molecular spacer was described by Sannino, A., et al., (Journal Applied Polymer Science, 2003, 90: 168-174). The dry product was obtained by adopting two different drying methods: some samples were dried at room conditions (1 atm, 25°C), other samples were dried by phase inversion extraction with acetone.

The synthesis of a superabsorbent hydrogel through crosslinking of carboxymethyl cellulose sodium salt and hydroxyethyl cellulose with citric acid was described by Demitri C., et al., (J Appl Polym Sci, 2008, 1 10: 2453- 2460). All the samples obtained were pre-dried at 30°C for 24 hours to remove the absorbed water and then kept for 24 hours at 80°C for the crosslinking reaction.

In all the processes described in the documents mentioned above, the drying step removes the liquid phase from the hydrogel without replacing it with a gas; therefore, the resulting dry product is a xerogel. In such processes, the extraction of the liquid causes a strong contraction of the dimensions of the starting hydrogel, with a reduction greater than 90% and reduced re-expansion capacity when in the presence of water again. Drying processes are known in literature in which the liquid phase of the hydrogel is removed and replaced with a gas; the resulting dry product is an aerogel. In such processes the contraction of the dimensions of the starting hydrogel is not very marked and the reduction is less than 15%. Furthermore, the aerogels maintain the property of reabsorbing high quantities of water.

For example, in Baldino L, et al., Polymers, 2016, 8(4): 106 the preparation of aerogels of interpenetrated natural polymers (IPN) consisting of alginate/gelatin is described; such preparation comprises repeated immersions of the hydrogel in a bath at increasing ethanol concentrations starting from 10% v/v and subsequent treatment with supercritical CO2.

Mallepally R. R. et al., The Journal of Supercritical Fluids, 2013, 79 202-208 described a method for preparing a superabsorbent alginate-based aerogel that comprises the use of solutions with an increasing ethanol concentration starting from 20% v/v and the subsequent treatment of the resulting alcogel through supercritical CO2. Flowever, this method allows a material to be obtained that can only absorb 20 grams of distilled water per gram of material.

WO2016032733 describes a method for preparing edible alginate-, pectin- or starch-based aerogels that comprises the use of solutions with an increasing ethanol concentration starting from 20% v/v and the subsequent treatment of the resulting alcogel through supercritical CO2.

GARCA-GONZALEZ C A ET AL, Carbohydrate Polymers 86(201 1 ): 1425-1438 is a review on polysaccharide- based biodegradable aerogels useful as vehicles for the release of drugs. This document shows the preparation of an alginate-based aerogel that comprises subsequent immersions of the hydrogel in a bath with an increasing ethanol concentration starting from 50% v/v proceeding with 100% (twice), and subsequent treatment with supercritical CO2. This document also reports the preparation of starch-, alginate- or agar-based aerogels that comprises immersions of the hydrogel in a bath with 100% ethanol or in a bath with an increasing ethanol concentration starting from 30% v/v or starting from 10% v/v, respectively, and subsequent treatment with supercritical CO2.

WO2014178797 describes the preparation of recycled cellulose-based aerogels comprising immersion of the hydrogel in a bath of 99% ethanol and subsequent freeze drying.

US5772646A describes the preparation of cellulose-based absorbent structures produced by microorganisms; such absorbent structures may also comprise CMC (carboxymethyl cellulose). Said preparation comprises immersions in a bath with an increasing concentration of ethanol starting from 20% v/v, or in a bath with 100% v/v of ethanol and subsequent treatment with supercritical CO2.

CN103205015A describes the preparation of cellulose-based aerogels that comprises the immersion of the sample in a bath of absolute ethanol or isopropyl alcohol or methanol and subsequent treatment with supercritical CO2.

WO201 1030170 describes the preparation of cellulose-based nanoparticles that comprises the immersion of the sample in a bath of ethanol anhydrous and subsequent treatment with supercritical CO2.

None of the documents mentioned above describes or suggests applying the water/ethanol exchange technique and subsequent treatment with supercritical CO2 to crosslinked polysaccharides selected from specific derivatives of starch and cellulose; furthermore, none of such documents describes or suggests applying to such polysaccharides water/ethanol exchange with increasing ethanol concentration starting from 50% v/v. Finally, none of such documents suggests that the process according to the invention would have allowed to obtained aerogels characterized by high rehydration capacity.

SUMMARY OF THE INVENTION

The present inventors have faced the problem of preparing a superabsorbent aerogel, especially in the case in which the aerogel is based on renewable and biodegradable materials, e.g. polysaccharides.

In particular, the present inventors set out to solve the problem of how to prepare such superabsorbent aerogel while maintaining the three-dimensional structure and a high rehydration capacity.

It is known to a person skilled in the art that the structure of a superabsorbent hydrogel is much more delicate than the structure of non-superabsorbent hydrogels.

Considering the teachings contained in the publication by Mallepally et al. (2013), a person skilled in the art would be induced not to apply the water/ethanol solvent exchange technique for preparing superabsorbent polysaccharide-based aerogels, as such technique led to aerogels with low capacity to reabsorb water.

The present inventors have verified the validity of the results of Mallepally et al. (2013) but then they have surprisingly found that the specific starting conditions of the water/ethanol solvent exchange can allow a superabsorbent polysaccharide-based aerogel to be obtained, which maintains the three-dimensional structure and a high capacity to reabsorb water.

In particular, the present inventors have found that by initiating the dehydration of a crosslinked polysaccharide- based superabsorbent hydrogel wherein the polysaccharide is selected from specific derivatives of starch and of cellulose, by subsequent immersion in baths consisting of water and increasing concentrations of an organic solvent that is miscible in water, starting from 50% v/v of organic solvent and then drying the resulting product with supercritical CO2, an aerogel is obtained that maintains the three-dimensional structure and that has a capacity to reabsorb distilled water of 500 grams of water/grams of material (example 8 of the present experimental part).

A person skilled in the art could think that the steps of replacing water with ethanol in an increasing percentage of the latter could even be eliminated and that it could be possible to proceed directly with ethanol only. The authors have verified this possibility, founding that by directly immersing the superabsorbent hydrogel in 100% v/v of ethanol, there was a change in morphology of the material: the hydrogel was transformed into a membrane due to the effect of a chemical/physical transformation, known as phase inversion. The membrane had pores with dimensions of a few tens of microns (Figure 1); on the contrary, the starting hydrogel was characterized by a nanoporous morphology, which was maintained in the aerogel correctly transformed into alcogel and dried through supercritical CO2 (Figure 2).

A first object of the present invention is represented by a process for preparing a superabsorbent aerogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component comprising the following steps: - supplying or preparing a superabsorbent hydrogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel;

- dehydrating said hydrogel by subsequent immersion in baths consisting of water and an increasing concentration of an organic solvent miscible in water, starting from 50% v/v of organic solvent; and

- drying the resulting product with supercritical CO2.

A second object of the present invention is represented by an aerogel obtained with the process according to the first object of the invention.

A third object of the invention is represented by a superabsorbent aerogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the aerogel, having a capacity to absorb in 24 hours at room temperature (1 atm, 25°C) from at least 490, preferably at least 500, at least 550, at least 600 grams of distilled water per gram of dry aerogel.

A fourth object of the invention is represented by the use of the aerogel according to the second or to the third object of the invention, in products for agriculture, personal hygiene, domestic or industrial use, in the toys or gadgets sector, in the pharmaceutical, biomedicine and biomedical sector.

FIGURES

Figure 1 represents the SEM image of the aerogel (16) obtained with a comparison process

Figure 2 represents the SEM image of the aerogel (5) obtained with the process according to the invention

Figure 3 represents the SEM image of the aerogel (15) obtained with a comparison process

DETAILED DESCRIPTION OF THE INVENTION

The term "hydrogel” as used herein indicates hydrophilic three-dimensional polymer networks, in which a liquid is dispersed, e.g. water or biological fluids.

The term "crosslinked” or "crosslinking” referring to polysaccharides, as used herein indicates both physical and chemical crosslinking. Physical crosslinking of polysaccharide chains takes place through hydrogen bonds or ionic interactions, and can be promoted, for example, by using inorganic salts; chemical crosslinking of polymer chains takes place through covalent bonds in the presence of crosslinking agents (GARCA- GONZALEZ C A ET AL, Carbohydrate Polymers 86(2011): 1425-1438 mentioned above).

The term "xerogel” as used herein indicates a product obtained from the drying of a hydrogel through removal of the liquid phase without this being replaced with a gas. In such processes, the extraction of the liquid causes a strong contraction of the dimensions of the starting hydrogel, with a reduction greater than 90%.

The term "aerogel” as used herein indicates a product obtained from the drying of a hydrogel through removal of the liquid phase and replaced with a gas. In such processes the contraction of the dimensions of the starting hydrogel is not very marked and the reduction is less than 15%, but above all a high rehydration capacity is maintained.

The expressions "superabsorbent polymer(s) (SAPs)” or "superabsorbent material (s)”, or superabsorbent polymer-based material(s), or "superabsorbent product(s),” as used herein indicate a polymer, or a material or a product having the capacity to absorb in 24 hours at room temperature (1 atm, 25°C) distilled water hundreds of times its dry weight, e.g. 100, 200, 300, 400, 500, 600 times its dry weight. The expression "superabsorbent hydrogel” as used here indicates a hydrogel which after drying and subsequent rehydration has the capacity to absorb in 24 hours at room temperature (1 atm, 25°C) distilled water hundreds of times its dry weight, e.g. 100, 200, 300, 400, 500, 600 times its dry weight.

The expression "superabsorbent aerogel” as used here indicates an aerogel having the capacity to absorb in 24 hours at room temperature (1 atm, 25°C) distilled water hundreds of times its dry weight, e.g. 100, 200, 300, 400, 500, 600 times its dry weight.

A first object of the present invention is represented by a process for preparing a superabsorbent aerogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component comprising the following steps:

supplying or preparing a superabsorbent hydrogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel;

dehydrating said hydrogel by subsequent immersion in baths consisting of water and an increasing concentration of an organic solvent miscible in water, starting from 50% v/v of organic solvent; and drying the resulting product with supercritical CO2.

According to the first object of the invention, the process preferably comprises the following steps:

supplying or preparing a superabsorbent hydrogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel, wherein said crosslinked polysaccharides are selected from carboxymethyl starch; hydroxypropyl starch (HPS); oxidized starch; carboxymethyl cellulose (CMC) possibly salified, preferably carboxymethyl cellulose sodium salt (NaCMC); hydroxyethyl cellulose (HEC); ethyl hydroxyethyl cellulose (EHEC); hydroxypropyl cellulose (HPC); and oxidized cellulose;

dehydrating said hydrogel by subsequent immersion in baths consisting of water and an increasing concentration of an organic solvent miscible in water, starting from 50% v/v of organic solvent; and

drying the resulting product with supercritical CO2.

According to the first object of the invention, preferably supplying a superabsorbent hydrogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel comprises using any commercially available hydrogel having these characteristics.

According to the first object of the invention, preferably preparing a superabsorbent hydrogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel comprises:

crosslinking said at least one polysaccharide.

crosslinking at least one polysaccharide in the presence of a crosslinking agent in an alkaline environment. According to the first object of the invention, the crosslinking is performed according to known methods such as, for example, according to the methods described in Esposito, F., et al.; Lenzi, F., et al.; Sannino, A., et al.; Demitri C., et al.; Yan, L, et al., mentioned above.

According to the first object of the invention, the crosslinking agent is preferably selected from divinyl sulfone (DVS), citric acid, acetaldehyde, formaldehyde, glutaraldehyde, diglycidyl ether, diisocyanates, dimethylurea, epichlorohydrin, oxalic acid, phosphoryl chloride, trimetaphosphate, trimethylomelamine, and polyacroleine; more preferably it is divinyl sulfone.

According to the first object of the invention, preferably, the degree of crosslinking of the hydrogel is comprised between 1.0 · 10 and 25.0 · 10 moles/cm³, more preferably it is comprised between 10.0 · 10 and 15.0 · 10- ⁴ moles/cm³, even more preferably it is 13.0 · 10 moles/cm³.

The degree of crosslinking was calculated with three different techniques, i.e. swelling in free water, uniaxial compression and solid state 13C NMR spectroscopy, as described in Lenzi, F et al., (2003) reported above and in example 10 of the present experimental part.

According to the first object of the invention, the hydrogel preferably consists of at least 95%, 98%, 99% or 100% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel.

According to the first object of the invention, the crosslinked polysaccharides are preferably selected from carboxymethyl starch; hydroxypropyl starch (HPS); oxidized starch; carboxymethyl cellulose (CMC) possibly salified, preferably carboxymethyl cellulose sodium salt (NaCMC); hydroxyethyl cellulose (HEC); ethyl hydroxyethyl cellulose (EHEC); hydroxypropyl cellulose (HPC); and oxidized cellulose.

In an embodiment according to the first object of the invention, the crosslinked polysaccharides consist of carboxymethyl cellulose sodium salt (NaCMC) and hydroxyethyl cellulose (HEC).

In a preferred embodiment according to the first object of the invention, the ratio by weight between NaCMC:HEC is comprised between 0 (100% HEC) and 3:1; more preferably it is selected from 1 :3, 1 :1 and 3:1; even more preferably it is 3:1.

According to the first object of the invention, the organic solvent used to dehydrate the hydrogel is preferably selected from acetone, N-methyl pyrrolidone, dimethyl sulfoxide and alcohols; more preferably it is ethanol.

In an embodiment according to the first object of the invention, the dehydration of the hydrogel takes place by subsequent immersion in baths consisting of water and an increasing concentration of ethanol, starting from 50% v/v, and proceeding one or more time each with 55%, or 60%, or 65%, or 70%, or 75%, or 80%, or 85%, or 90% or 95%, or 100% v/v.

In an embodiment according to the first object of the invention, the dehydration of the hydrogel takes place by subsequent immersion in baths consisting of water and an increasing concentration of ethanol, starting from 50% v/v, and proceeding with 70%, 90% and 100% v/v.

In a preferred embodiment according to the first object of the invention, the dehydration of the hydrogel takes place by subsequent immersion in baths consisting of water and an increasing concentration of ethanol, starting from 50% v/v, and proceeding with 100% v/v. In a preferred embodiment according to the first object of the invention, the dehydration of the hydrogel takes place by subsequent immersion in baths consisting of water and an increasing concentration of ethanol, starting from 50% v/v, and proceeding with 100% v/v (once).

According to the first object of the invention, the

the supercritical CO2 drying takes place at a pressure comprised between 90 and 350 bars, more preferably at a pressure comprised between 120 and 250 bars;

and/or

the supercritical CO2 drying takes place at a temperature comprised between 35 and 60°C, more preferably at a temperature comprised between 40 and 52°C;

and/or

the supercritical CO2 residence time in the vessel is comprised between 5 and 40 minutes, more preferably comprised between 10 and 25 minutes;

and/or

the supercritical CO2 drying process extends between 20 and 120 minutes, more preferably between 30 and 80 minutes.

According to a first object of the invention, the aerogel obtained preferably has a capacity to absorb in 24 hours at room temperature (1 atm, 25°C) from at least 490, preferably at least 500, at least 550, at least 600 grams of distilled water per gram of dry aerogel.

According to the first object of the invention, the aerogel obtained preferably consists of at least 95%, 98%, 99% or 100% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the aerogel.

As is known to a person skilled in the art, the degree of crosslinking does not change when passing from a hydrogel to the corresponding aerogel; therefore, according to the first object of the invention, preferably, the degree of crosslinking of the aerogel obtained is comprised between 1.0 · 1 CH and 25.0 · 10⁴ moles/cm³, more preferably it is comprised between 10.0 · 10 and 15.0 · 1CH moles/cm³, even more preferably it is 13.0 · 1 CH moles/cm³.

As is known to a person skilled in the art, the characteristics required by the superabsorbent product are different according to the specific application of use.

For example, products for personal hygiene and the home must have a high absorption speed, low re-wetting capacity and a low level of residual monomers. On the contrary, for products used in agriculture, the absorption speed is not very important, whereas a high capacity to absorb water under load and a low sensitivity to the saline concentration are important.

Furthermore, products for hygiene applications must be able to withhold absorbent fluids; on the contrary, products used in agriculture must be able to release the absorbed water.

Obviously, it is impossible for a superabsorbent product to possess all these characteristics at the same time. Therefore, a person skilled in the art chooses on a case-by-case basis between hydrogels on the market or between hydrogels prepared in literature those which possess the suitable characteristics for the specific application of use (Zohuriaan-Mehr MJ, et al., mentioned above).

For example, the dynamic mechanical properties and the swelling capacity of the HPC-based hydrogel may be varied as a function of the temperature at which the crosslinking takes place and/or the duration of the crosslinking.

In general, the swelling capacity of hydrogels can be modulated by varying the distance between crosslinking sites, e.g. by inserting polyethylene glycols as a spacer between polymer chains or by varying the molecular weight of said polyethylene glycol when DVS is used as the crosslinking agent, as described in Sannino A., et al., cited above. Furthermore, for applications in food products, drugs and biomaterials, crosslinking agents must be non-toxic; in the event that they are toxic, e.g. in the case of DVS and ECH, they must be effectively removed before obtaining the hydrogel. Alternatively, carbodiimide can be used as a crosslinking agent which is not incorporated into the crosslinking bonds and is converted into urea derivatives that can be washed away by the polymer structure or citric acid can be used, which is not toxic or expensive, as described in Demitri C., et al., 2008, cited above. If citric acid is used, the swelling speed of the hydrogel is influenced by the reaction time and the concentration of the citric acid (Demitri C., et al., 2008, cited above).

The second object of the invention is represented by the aerogel obtained with the process according to the first object of the invention.

According to the second object of the invention, the crosslinked polysaccharides are preferably selected from carboxymethyl starch; hydroxypropyl starch (FIPS; oxidized starch; carboxymethyl cellulose (CMC) possibly salified, preferably carboxymethyl cellulose sodium salt (NaCMC); hydroxyethyl cellulose (HEC); ethyl hydroxyethyl cellulose (EHEC); hydroxypropyl cellulose (HPC); and oxidized cellulose.

According to the second object of the invention, the aerogel obtained preferably has a capacity to absorb in 24 hours at room temperature (1 atm, 25°C) from at least 490, preferably at least 500, at least 550, at least 600 grams of distilled water per gram of dry aerogel.

According to the second object of the invention, the aerogel obtained preferably consists of at least 95%, 98%, 99% or 100% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the aerogel.

According to the second object of the invention, preferably said crosslinked polysaccharides were crosslinked in the presence of a crosslinking agent in an alkaline environment.

According to the second object of the invention, preferably, the degree of crosslinking of the aerogel obtained is comprised between 1.0 · 10 and 25.0 · 10 moles/cm³, more preferably it is comprised between 10.0 · 10 and 15.0 · 10 moles/cm³, even more preferably it is 13.0 · 10 moles/cm³.

A third object of the invention is represented by a superabsorbent aerogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the aerogel, having a capacity to absorb in 24 hours at room temperature (1 atm, 25°C) from at least 490, preferably at least 500, at least 550, at least 600 grams of distilled water per gram of dry aerogel. According to the third object of the invention, the crosslinked polysaccharides are preferably selected from carboxymethyl starch; hydroxypropyl starch (HPS); oxidized starch; carboxymethyl cellulose (CMC) possibly salified, preferably carboxymethyl cellulose sodium salt (NaCMC); hydroxyethyl cellulose (HEC); ethyl hydroxyethyl cellulose (EHEC); hydroxypropyl cellulose (HPC); and oxidized cellulose.

According to the third object of the invention, the aerogel preferably consists of at least 95%, 98%, 99% or 100% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the aerogel.

According to the third object of the invention, preferably said crosslinked polysaccharides were crosslinked in the presence of a crosslinking agent in an alkaline environment.

According to the third object of the invention, preferably, the degree of crosslinking of the aerogel is comprised between 1.0 · 10 and 25.0 · 10 moles/cm³, more preferably it is comprised between 10.0 · 10 and 15.0 · 10- ⁴ moles/cm³, even more preferably it is 13.0 · 10 moles/cm³.

A fourth object of the invention is represented by the use of the aerogel according to the third object of the invention, in products for agriculture, personal hygiene, domestic or industrial use, in the toys or gadgets sector, in the pharmaceutical, biomedicine and biomedical sector.

EXPERIMENTAL PART

A) PREPARATION OF AEROGELS

Example 1 - Preparation of aerogel (1)

In step a) first 0.5 g of hydroxyethyl cellulose (HEC) (Aldrich Chimica s.r.l. Milano) were dispersed in 0.1 litres of distilled water and then 1.5 g of carboxymethyl cellulose sodium salt (NaCMC) having an average molecular weight of about 700,000 Da were added (Aldrich Chimica s.r.l. Milano); finally, divinyl sulfone was added (Aldrich Chimica s.r.l. Milano) 0.04 mol/litre; in the reaction solution, the weight ratio of NaCMC:HEC was 3:1; the reaction solution comprised a total polymer concentration of 2% by weight with respect to the total weight of the solution; such solution was constantly stirred with an IKA magnetic stirrer at 20°C, for 24 hours.

In step b) 0.02 M of potassium hydroxide (KOH) were added to the mixture obtained in step a) at 20°C, so as to trigger the crosslinking reaction in an alkaline environment; the crosslinking reaction was continued for 24 hours at the end of which the hydrogel 1 , of a yellowish colour, was obtained.

In step c) the hydrogel obtained in step b) was dehydrated by subsequent immersion in a bath of hydroalcoholic solutions at an increasing ethanol concentration starting from 50% v/v and proceeding with 100% v/v; each bath comprised 100 mL of hydroalcoholic solution for each volume of gel and the equilibrium time of each bath was 4 hours; at the end of step c) the alcogel 1 was obtained.

In step d) the alcogel 1 obtained in step c) was dried with supercritical CO2. Specifically, the alcogel was inserted into a closed vessel into which a continuous flow of supercritical CO2 was let in; when the desired pressure value, 200 bar, and the desired temperature value, 40°C, were reached, the alcohol was removed; the CO2 remained in the vessel for 15 minutes; the drying process lasted for 80 minutes; finally, depressurization lasting about 20-30 minutes was performed to bring the system back to atmospheric pressure and recover the aerogel from the vessel. Example 2 - Preparation of aerogels (2) and (3)

In example 2 the process was carried out as described in example 1 , but varying in the reaction solution the ratio by weight of NaCMC: HEC; specifically in example 2a such weight ratio was 1 :3 and in example 2b it was 1 : 1 , providing the aerogel (2) and (3), respectively.

Example 3 - Preparation of aerogel (4)

In example 3 the process was carried out as described in example 1 , using hydroxyethyl cellulose (HEC) (Aldrich Chimica s.r.l. Milano) and divinyl sulfone (Aldrich Chimica s.r.l. Milano), i.e. 100% HEC.

Example 4 - Preparation of aerogels (5), (6), (7) and (8)

I n example 4 the process was carried out as described in examples 1 -3, varying the concentration of the reaction solution, which in this case comprised a total concentration of polymers of 4% by weight with respect to the total weight of the solution; specifically in example 4a the ratio by weight of NaCMC:HEC was 3: 1 , in example 4b such ratio by weight was 1 :3, in example 4c it was 1 : 1 and in example 4d it was 0, providing the aerogels (5), (6), (7) and (8), respectively.

Example 5 - Preparation of aerogels (9-14)

In example 5 the process was carried out as described in example 1 (ratio by weight NaCMC:HEC of 3: 1 , total polymer concentration of 2% by weight with respect to the total weight of the solution) and in example 4a (ratio by weight NaCMC:HEC of 3: 1 , total polymer concentration of 4% by weight with respect to the total weight of the solution), varying the concentration of DVS, specifically in examples 5a and 5d the concentration of DVS was 0.066, in examples 5b and 5e it was 0.1 and in examples 5c and 5f it was 0.13 mol/litre, providing the aerogels (9), (10), (1 1 ), (12), (13) and (14), respectively.

Example 6 - Preparation of the control aerogel (CONTR) (15)

In example 6, the hydrogel obtained after steps a) and b) of example 4a was dehydrated by subsequent immersion in a bath of hydroalcoholic solutions at an increasing ethanol concentration starting from 30% v/v and proceeding with 50, 70, 90 and 100% v/v; each bath comprised 100 mL of hydroalcoholic solution for each volume of gel and the equilibrium time of each bath was 4 hours; the alcogel obtained was dried with supercritical CO2 as described in example 1 , step d).

Example 7 - Preparation of the control aerogel (CONTR) (16)

In example 7, the hydrogel obtained after steps a) and b) of example 4a was dehydrated by immersion in a bath of 100% v/v ethanol; the bath comprised 100 mL of alcoholic solution for each volume of gel and the equilibrium time was 4 hours; the alcogel obtained was dried with supercritical CO2 as described in example 1 , step d). The following Table 1 shows the different experimental conditions applied in the preparation of the aerogel of the invention (INV) and of the comparison aerogels (CON). Table 1

(1 ) - PROCESS ACCORDING TO THE INVENTION: bath in hydroalcoholic solutions with increasing ethanol concentration starting from 50% v/v and proceeding with 100% v/v; supercritical CO2

(2) - COMPARISON PROCESS: bath in hydroalcoholic solutions with increasing ethanol concentration starting from 30% v/v and proceeding with 50, 70, 90 and 100% v/v; supercritical CO2

(3) - COMPARISON PROCESS: bath in 100% v/v ethanol solution; supercritical CO2

B) CHARACTERIZATION OF THE AEROGELS

Example 8 - Water absorption capacity

The distilled water absorption capacity at room temperature (1 atm, 25°C) by the aerogel (5) according to the invention prepared in example 4a and comparison aerogels (15) and (16) prepared in examples 6 and 7, respectively, was determined by applying the following formula:

Wa= (weight of aerogel t24 - weight of aerogel to) / weight of aerogel to

wherein

Wa is the absorption of distilled water per gram of dry aerogel

weight of aerogel t24 is the weight expressed in grams of aerogel swollen with water 24 hours of immersion in distilled water

weight of aerogel to is the weight expressed in grams of aerogel before immersion in distilled water

The weight of absorbed water was calculated at room temperature by weighing the samples of aerogel before immersion in distilled water (Dl) and then at established time intervals after 0.17minutes, 30 minutes, and 1.0, 3.0, 5.0, until 24 hours after immersion. An electronic microbalance having precision of ± 10 grams was used. The values reported both for the aerogel (5) according to the invention and for the comparison aerogels (15) and (16) are the result of 12 measurements, as their respective synthesis was repeated 4 times and 3 samples were taken from each synthesis batch.

As shown in Table 2, the aerogel (5) obtained through the process according to the invention, in which the bath in hydroalcoholic solution at increasing ethanol concentration takes place starting from 50% v/v, at room temperature shows very high rehydration capacity; in fact, in 24 hours it reaches a maximum absorption (Wa) of 503 grams of distilled water per gram of dry aerogel.

On the contrary, the aerogel (15) obtained through a process in which the bath in hydroalcoholic solution at increasing ethanol concentration takes place starting from 30% v/v, at room temperature shows much lower rehydration capacity; in fact, in 24 hours it reaches a maximum absorption (Wa) of 26.70 grams of distilled water per gram of dry aerogel. Also the aerogel (16) obtained through a process in which the bath is performed in 100% v/v ethanol solution, at room temperature shows even lower rehydration capacity; in fact, in 24 hours it reaches a maximum absorption (Wa) of 17 grams of distilled water per gram of dry aerogel.

Table 2

Example 9 - Morphology

The morphology of the aerogel (5) according to the invention prepared in example 4a and of the aerogels (15) and (16) prepared in examples 6 and 7, respectively, was studied through scanning electron microscopy (SEM) with scanning electron microscope (mod. LEO 1525 Carl Zeiss SMT AG, Oberkochen, Germany).

The operating principle of the test is based on the emission of an electron beam that hits the sample subjected to analysis. The response of the latter to bombardment can have different forms:

emission of retro diffused electrons: electrons belonging to the primary beam that undergo an elastic collision within the material and are bounced outside with energy proximal to the initial energy;

emission of secondary electrons: electrons originally linked to more outer atomic levels that receive from the incident beam sufficient energy to remove them.

Such emissions are used to study the morphology.

The electron beam can be generated with two methods: thermionic emission and field emission. In the first case, a metal filament (tungsten or lanthanum hexaboride) is crossed by current, being heated by the Joule effect. In this way, the electrons acquire the necessary energy to overcome the potential barrier that separates them from the vacuum. In the second case, an electric field is applied so as to allow the exit of electrons by tunnel effect. To prevent the electrons being able to interact with the air molecules, it is necessary to operate under vacuum conditions. The electron beam generated is accelerated by means of the application of a potential difference thereto and it passes through a system of electromagnetic lenses that have the task of focusing the beam reducing the dimensions thereof. Through the use of electrostatic deflectors it is possible to perform electron beam scanning on the surface of the sample and the signals are collected by an electron detector. In particular, the secondary electrons are detected by a special detector, converted into electric pulses and sent to a monitor where the scanning is performed. The result is a black and white image with similar characteristics to a photograph.

As the materials used are not conductive, the samples must be subjected to metallization with a layer of gold, with a thickness of 250 A. As shown in Figure 1 , the SEM image of the aerogel (16) obtained through a comparison process highlights a micro-porous morphology.

As shown in Figure 2, the SEM image of the aerogel (5) obtained through the process according to the invention, highlights a nano-filamentous and regular morphology characterized by empty nano-spaces that determine high absorption capacity, due to the fact that part of the water condenses in the empty nano-spaces whose dimension increases, allowing further absorption of water.

As shown in Figure 3, the SEM image of the aerogel (15) obtained through a comparison process highlights a nano-porous morphology, but with pores that are not interconnected.

Example 10 - Degree of crosslinking

The degree of crosslinking was calculated with three different techniques, i.e. swelling in free water, uniaxial compression and solid state ¹³C NMR spectroscopy, as described in Lenzi, F et al., (2003) reported above. The degree of crosslinking (d.c.) is defined as the numerical density of junctions that link the polymer chains in a permanent structure, according to such definition, the degree of crosslinking can be expressed as: d.c. = v/2 V wherein

v/2 is the number of chemically effective crosslinking bonds, and

V is the total volume of the polymer.

In reality, the swelling properties are connected to the number of elastically effective crosslinking bonds (Ve/2) which is not generally equivalent to the number of chemically effective bonds, as the number of elastically effective crosslinking bonds also includes bonds due to entanglements. However, in the case of a perfect network, obtained by joining pairs of segments of linear chains with chemical crosslinking bonds, the concentration of elastically effective chain elements, Ve/V, is equal to the concentration of all the chemically crosslinked polymer segments (v/V), therefore p_x= Ve/V= v/V=1/(v’M_c) wherein

p_x represents the number of moles of polymer units involved in the crosslinking per cm³ of dry network,

Ve/V represents the moles of elastically effective chains per unit of volume of the network

v/V represents the moles of chemically effective chains per unit of volume of the network

v’ represents the specific volume of the polymer and

Me is the average molecular weight between the crosslinking points.

Free swelling test at equilibrium.

Small portions of hydrogel 1 , obtained in step b) of example 1 (CMCNa/HEC 3/1 ; DVS 0,04 mol/l; polymers 2% by weight with respect to the total weight of the solution) were placed in distilled water at equilibrium, and then the samples removed from the water were weighed using a Mettler AE100 electronic balance with sensitivity of IO-⁵. Before measuring the weight, the samples were quickly and delicately buffered to remove the liquid water from the surface and placed in a sealable plastic container with a known weight. The samples were dried at 50°C under strong vacuum (10-3 Torr) to determine the dry weight of the polymer. The equilibrium swelling was expressed as the ratio between grams of absorbed water and grams of dry polymer. All the tests were performed at 25°C.

Likewise, the free swelling at equilibrium of the hydrogel 11 obtained in step b) of example 5c was tested (CMCNa/FIEC 3/1 ; DVS 0,13 mol/l; polymers 2% by weight with respect to the total weight of the solution).

The free swelling at equilibrium of the hydrogel 1 was 880±50 g/g; the free swelling at equilibrium of the hydrogel 11 was 140± 10 g/g; such data were used to calculate the fraction of volume of the polymer at equilibrium swelling.

The numbers of moles of polymer units involved in crosslinking per cm³ of dry network (p_x) was obtained by applying the following equation:

Px=- [ln(1- F2) + f₂₊cf₂]/ni + ίf₂/(Z₊n_Gh) / F2,G{(F2/F2,G)^1/3 - 0,5 (f₂/f2,_G)} wherein

In represents the logarithm function

F2 is the fraction of volume of the polymer at swelling equilibrium,

x is the Flory-Higgins interaction parameter (polymer/swelling solvent),

Vi is the molar volume of the swelling solvent,

i is the fraction of structural units of the polymer that carry ionized pairs,

z₊ is the number of electronic charges carried by the cations formed by the dissociation of the ionic groups present on the polymer structure

V_m is the molar volume of the repeating unit of the structure,

f 2_,r is the fraction of volume of polymer in the reaction mixture before complete swelling in distilled water, and is equal to 0.02 for all the samples.

The following table shows the values of the parameters that appear in the previous equation, calculated as described in paragraph 4.1 of the publication by Lenzi, F. et al. (2003), cited above.

^a assuming density of bulk polymer =1 g/cm³

It turns out that p_x of the hydrogel 1 and 1 1 is 3.54x10 ± 0.25x10 mol/cm³ and 10.60x10 ± 2.5x10 mol/cm³, respectively.

Uniaxial compression test

Small portions of hydrogel 1 , obtained in step b) of example 1 (CMCNa/HEC 3/1 ; DVS 0,04 mol/l; polymers 2% by weight with respect to the total weight of the solution) straight after their synthesis were placed between two plates of polymethylmethacrylate). The sample was immersed in a container containing distilled water at controlled temperature, thanks to a jacket connected to a thermostatic bath, to prevent the drying of the swollen network. The upper plate was in contact with a load cell (Buster type 8453, Max load 1000 N, sensitivity 0.1 N) attached to a mobile frame. The frame bearing the load cell could be moved in very small steps monitored by a mobile microscope (accuracy ± 2 micrometres). Each measurement consisted of imposing a constant load and measuring the associated compression force. Once the constant compression force value was obtained, it was considered as the equilibrium value and the load was increased again. All the tests were carried out at 25°C. The experiments were repeated 4 times for each sample and the average compression force values were reported as a function of the imposed deformation.

Likewise, the uniaxial compression of the hydrogel 1 1 obtained in step b) of example 5c was tested (CMCNa/HEC 3/1 ; DVS 0.13 mol/l; polymers 2% by weight with respect to the total weight of the solution).

In the event of small deformations, the compressive modulus (a_s) of the swollen network can be expressed as: s= RT(Ve/V₀)( f 2,r) ^2/3 (F 2,s) ^1/3 wherein

R is the universal constant of the gases;

T is the temperature expressed in Kelvin degrees;

(Ve/Vo) represents the moles of elastically effective chains per cm³ of dry polymer network;

f _2,r is the fraction of volume of polymer in the reaction mixture before complete swelling in distilled water, and is equal to 0.02 for all the samples;

(<|>2,s) represents the volumetric fraction of the polymer in the swollen state under compression.

Starting from the hypothesis that for low compression loads the volume of the polymer is constant during the compression test, the compression module in the null stress limit (a_s,o) can be expressed as: s,0 =RT(Ve/Vo)(<|>2,r) ^2/3(<|>2,so)^1/3 wherein (f2_,do) is the reciprocal of the swelling ratio, calculated experimentally.

From this formula, after measuring a_s,o and calculating f2_,G and f2_,5o_, it is possible to obtain V_e/Vo, i.e. the moles of elastically effective chains per cm³ of dry polymer network. As Ve/V = p_x, the number of moles of polymer units involved in the crosslinking per cm³ of dry network (px) is obtained. It follows that p_x of the hydrogel 1 is 2.4x1 CH ± 0.3x1 CH mol/cm³.

Solid state NMR test

Straight after their synthesis, finely pulverized samples of aerogel 1 obtained in example 1 (CMCNa/HEC 3/1 ; DVS 0,04 mol/l; polymers 2% by weight with respect to the total weight of the solution) were packed into a zirconia rotor of 4 mm and sealed with Kel-F caps. The solid state NMR ¹³C CP-Mass spectra were obtained with a Bruker AC-200, 120 W CW spectrophotometer with a M3205 pulse amplifier. The spin rate was always kept at 8 KHz. The p/2 pulse width was 3.1 microseconds and the recycle delay was 4 seconds. The spectra were obtained with 1024 data points in the time domain, zero-filled and Fourier transformed with a size of 2084 data points; 19000 scans were performed for each experiment. As a function of the contact time, two series of experiments were carried out, with contact time from 0.02 to 7 milliseconds. Magic angle spinning experiments were performed with a single pulse excitation (MAS-SPE), the recycle delay was 40 seconds, the ¹³C p/2 pulse width was 2.8 microseconds. A single p/2 pulse was applied in this experiment to excite the carbon spectrum that was recorded in the presence of MAS and dipolar decoupling (DD). The NMR resonance analysis was performed using the“GLI NFIT” deconvolution program. This program can perform the complete deconvolution of superposed lines both with a Gaussian and/or Lorentzian shape. The line widths, , the chemical shift, the area and their standard deviations were obtained.

The standard deviations on the areas obtained were about 10% of their nominal values. All the tests were performed at 25°C.

The use of solid state NMR offers the possibility to determine the degree of crosslinking by observing the resonances related to the carbon atoms responsible for the crosslinking of the network. In this case, the degree of crosslinking, expressed as a number of crosslinking bridges per polysaccharide ring, was calculated from the ratio from the area of resonance due to the methylene carbon atoms present on the crosslinking molecule that has reacted and the resonance area related to the anomeric carbon atoms of the polysaccharide ring.

Specifically, the degree of crosslinking (p_x), expressed as the ratio between the DVS moles that have reacted and the moles of polysaccharide rings, was calculated by applying the following equation: p_x=So(A)/2 / (So(C1 ) wherein

So(A)/2 is half the resonance area due to the methylene carbon atoms adjacent to the DVS sulfoxide group. So(C1 ) is the sum of the resonance areas due to anomeric carbon atoms.

It is known to a person skilled in the art that supercritical CO2, in certain operating conditions, forms together with the organic solvent a supercritical mixture having negligible surface tension; when it is applied to the drying of a non-superabsorbent hydrogel, such technique prevents the destruction of the nano-structure; furthermore, the supercritical CO2 completely removes the organic solvent present.

On the other hand it is known that superabsorbent hydrogels are more delicate structures than non superabsorbent hydrogels and must be able to expand hundreds of times.

Mallepally et al. (2013) cited above teaches that treating alginate-based superabsorbent hydrogels with aqueous solutions that have an increasing ethanol (EtOH) concentration starting from 20% v/v proceeding with 40-60-80% v/v and then processing the resulting alcogel with supercritical CO2 _, allows to obtain alginate aerogels having a very low distilled water absorption capacity; in fact, in 24 such aerogels reached a maximum absorption (Wa) of 20 grams of distilled water per gram of dry aerogel.

Instead, it is known that superabsorbent polymers can absorb distilled water hundreds of times their weight, e.g. from 400 to 500 times their dry weight (as stated by Mallepally et al., 2013 cited above).

Considering the teachings contained in the publication by Mallepally et al. (2013), a person skilled in the art would be induced not to apply the water/ethanol solvent exchange technique and subsequent treatment with supercritical CO2 for preparing superabsorbent polysaccharide-based aerogels, as such technique led to aerogels having low Wa.

It is therefore surprising that the present inventors have found that such a process that comprises different steps, such as repeated immersions of the superabsorbent hydrogel in a bath of hydroalcoholic solutions with an increasing ethanol concentration and subsequent supercritical CO2 drying, can be applied to the preparation of crosslinked polysaccharide-based superabsorbent aerogels wherein the polysaccharide is selected from specific derivatives of starch and cellulose.

In particular, it is unexpected that the present inventors have found that specific water/ethanol solvent exchange conditions allow to obtain a crosslinked polysaccharide-based aerogel having a capacity to reabsorb distilled water greater than 100 grams of water per gram of aerogel, such as for example is the case of the aerogel (5), that shows a Wa value greater than 500 grams of distilled water per gram of dry aerogel.

This result means that the process according to the specific conditions of the invention does not destroy the three-dimensional structure of the aerogel, allowing it to maintain a capacity to reabsorb distilled water that is hundreds of times its dry weight.

In fact, unexpectedly, the process of the invention, characterized by the application of the water/ethanol exchange technique and subsequent treatment with supercritical CO2 to specific crosslinked polysaccharides, selected from specific derivatives of starch and cellulose, allows an aerogel to be obtained which, when rehydrated, recovers its volume; therefore, the polymer network had only been folded onto itself downstream of the supercritical drying process, but had not been destroyed by the process itself.

As well as having a very high capacity to reabsorb water, the aerogel according to the invention has the further advantage of coming from raw materials that respect the environment, as they are renewable and biodegradable. Such raw materials are less polluting and have a lower and less fluctuating cost than those of acrylic-based raw materials, which being derived from crude oil do not have these advantages. Furthermore, acrylic-based superabsorbent aerogels also have high costs for the recycling thereof.

Claims

1. Process for preparing a superabsorbent aerogel comprising the following steps:

supplying or preparing a superabsorbent hydrogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the hydrogel, wherein said crosslinked polysaccharides are selected from carboxymethyl starch; hydroxypropyl starch (HPS); oxidized starch; carboxymethyl cellulose (CMC) possibly salified, preferably carboxymethyl cellulose sodium salt (NaCMC); hydroxyethyl cellulose (HEC); ethyl hydroxyethyl cellulose (EHEC); hydroxypropyl cellulose (HPC); and oxidized cellulose; dehydrating said hydrogel by subsequent immersion in baths consisting of water and an increasing concentration of an organic solvent miscible in water, starting from 50% v/v of organic solvent; and drying the resulting product with supercritical CO2.

2. Process according to claim 1 wherein the organic solvent used to dehydrate the hydrogel is selected from acetone, N-methyl pyrrolidone, dimethyl sulfoxide and alcohols; more preferably it is ethanol.

3. Process according to claim 1 or 2, wherein the dehydration of the hydrogel takes place by subsequent immersion in baths consisting of water and an increasing concentration of ethanol,

starting from 50% v/v, and proceeding with 70%, 90% and 100% v/v

or

starting from 50% v/v, and proceeding with 100% v/v.

4. Process according to any one of the preceding claims wherein

and/or

5. Process according to any one of the preceding claims, wherein the crosslinked polysaccharides consist of carboxymethyl cellulose sodium salt (NaCMC) and hydroxyethyl cellulose (HEC).

6. Process according to any one of the preceding claims wherein the ratio by weight between NaCMC:HEC is comprised between 0 (100% HEC) and 3:1 ; preferably it is selected from 1 :3, 1 : 1 and 3:1 ; even more preferably it is 3:1.

7. Aerogel obtained with the process described in any one of claims 1 to 6.

8. Aerogel according to claim 7 having a capacity to absorb in 24 hours at room temperature (1 atm, 25°C) from at least 490, preferably at least 500, at least 550, at least 600 grams of distilled water per gram of dry aerogel.

9. Superabsorbent aerogel consisting of at least 90% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the aerogel, having a capacity to absorb in 24 hours at room temperature (1 atm, 25°C) from at least 490, preferably at least 500, at least 550, at least 600 grams of distilled water per gram of dry aerogel wherein the crosslinked polysaccharides are selected from carboxymethyl starch; hydroxypropyl starch (HPS); oxidized starch; carboxymethyl cellulose (CMC) possibly salified, preferably carboxymethyl cellulose sodium salt (NaCMC); hydroxyethyl cellulose (HEC); ethyl hydroxyethyl cellulose (EHEC); hydroxypropyl cellulose (HPC); and oxidized cellulose.

10. Aerogel according to claim 7, 8 or 9 consisting of at least 95%, 98%, 99% or 100% by weight of crosslinked polysaccharides with respect to the total weight of the polymer component of the aerogel.

11. Use of the aerogel according to any one of the claims from 7 to 10, in products for agriculture, personal hygiene, domestic or industrial use, in the toys or gadgets sector, in the pharmaceutical, biomedicine and biomedical sector.