WO2004028686A2

WO2004028686A2 - Liquid absorbing polymer, a process and a vessel for its production

Info

Publication number: WO2004028686A2
Application number: PCT/CH2003/000589
Authority: WO
Inventors: Sahbi Belkhiria; Ibrahim M. Al-Alim
Original assignee: Polykem Sarl
Priority date: 2002-09-25
Filing date: 2003-08-28
Publication date: 2004-04-08
Also published as: WO2004028686A3; AU2003254691A1

Abstract

The invention relates to a superabsorbent polymer based on a cross-linked acrylic polymer derivative, in which the content of acrylic polymer containing a N atom, such as acrylamide or acrylonitrile, represents between 5 % and 95 % by weight, said superabsorbent comprising at least a hydrocolloidal polysaccharide in a amount not exceeding 5 parts by weight for 100 parts of acrylic units.

Description

LIQUID-ABSORBING POLYMER, A PROCESS AND A VESSEL FOR ITS PRODUCTION

The present invention relates to compositions of liquid-absorbing polymers, of a kind that is useful for various applications, including agricultural use. The present invention relates also to a process and a vessel for producing such liquid absorbing polymers.

Fluid-absorbing polymers or "superabsorbents" or "SAP" are generally hydrophilic crosslinked polymers able to swell and absorb amounts of water, saline solutions, physiological fluids, or various other liquids as high as 10 to 1'000 times their own weight and sometimes more. Most of them are traditionally prepared by polymerizing acrylic acid, and/or its salts and are mainly used in personal care and hygienic fields such as for preparing diapers, napkins, pads for incontinents, etc. They are also used in packing materials for perishable goods, protective covering for communication cables, damage-control operation in the environmental sector, and aids for watering indoors plants, allowing said plants to be watered with lower quantities and watering frequency. Other uses in dietetical, medical and chirurgical fields are described for instance in WO 89/07455.

It was progressively recognized however that, for more demanding usage, as for instance outdoor or open air usage, the acrylic acid based SAP (generally referred as polyacrylates) are less stable and less efficient than those based on acrylamide (generally referred as polyacrylamides), specially when applied in soils. Polyacrylates are capable of absorbing greater amounts of liquid than polyacrylamides but are more sensitive to presence of salts and rapidly break down.

On the other hand, for soil applications it is more and more desirable to employ polyacrylamides SAP having higher absorption capacities and absorption rates without altering their stability and their gel strength. This will allow their use in large-scale at lower concentrations in soils, and with lower SAP cost per m³ of soil.

"Gel strength" is a way to assess a key property of SAP, which is its ability not to release too easily and too quickly its water content when pressed or crushed

So far, attempting to increase absorption capacities of polyacrylamides has generally led in a decrease of their gel strength (and vice versa) and/or their stability, which is undesirable particularly for application on soils

The invention aims at coping with these drawbacks with peculiar compositions for superabsorbent polymers having both a high absorption capacity combined with a high gel strength and an improved stability, especially in soils.

The superabsorbent polymer according to the invention is based on an acrylic polymer and its derivatives, in which the content of acrylic units containing a N atom, such as an acrylamide or an acrylonitrile, represents between 5% and 95% by weight, preferably between 40% and 90%. Said polymer is cross-linked and not grafted and comprises at least a hydrocolloidal polysaccharide in amount' ^"not exceeding 5 parts by weight for 100 parts of acrylic units.

Said acrylic derivative can be selected among the acrylic "family", namely acrylic acid and their salts, acrylic esters, as well as methacrylic acid and their salts and more generally those other superior homologues that are water soluble or can partially or totally rendered water soluble, respectively the corresponding acrylic derivatives comprising a nitrogen atom such as acrylamide, acrylonitrile, methacrylamide, methacrylonitrile, etc.

To simplify the following description, we will however use the word "acrylic" indifferently for "acrylic" stricto sensu, as well as "methacrylic" and superior homologues as defined above.

Other water soluble α,β-ethylenically unsaturated carboxylic monomers such as maleic acid, maleic anhydride, fumaric acid, crotonic acid, citraconic acid, their esters (ex. (meth)acrylates of methyl, ethyl, n-butyl, 2-hydroxyethyl, etc.), respectively their N-substituted (alkyl)amides (ex. (meth)acrylamide, N- methylacrylamide, N-terbutylacrylamide, N,N-dimethylacrylamide, etc.), and/or their salts (ex. sodium (meth)acrylate, potassium (meth)acrylate, ammonium (meth)acrylate, etc.). may be combined in various proportions with the starting acrylic basis.

Other examples of water-soluble monoethylenically unsaturated monomers that could be used for producing superabsorbent composition include monomers containing sulfo group (vinyl sulfonic acid, their salts, etc.), esters obtained by reaction of organic oxides (ethylene oxide, propylene oxide, etc.) or carboxylic acids with alcohols and their derivatives, as well as (meth)acroleine, vinyl acetate, vinyl propionate, N-vinylpirrolidone, N-vinylformamide, N-vinycaprolactame and their derivatives.

Preferably, the hydrocolloidal polysaccharide is selected from the group consisting of gums such as tragacanth gum, guar gum, konjac gum, arabic gum, xanthan gum, tara gum, karaya gum, India gum, gellan gum, locust bean gum, alginates (such as sodium alginate, calcium alginate, propylene glycol alginate,...), carragheenates, pectin, agar, and their mixtures thereof.

It must be emphasized that the superabsorbent polymer must be a crosslinked polymer. A grafted polymer as described in EP 0 623 178 as a tannin agent for treating skins is not suitable due to its solubility into water and insufficient mechanical properties.

By contrast to the SAPs according to the invention, other grafted acrylic polymers such as those described in JP 56-07011 and JP 61-094655, even if they contain gums or alginates, are not multi purpose polymers and are not usable for agricultural use, due to their lack of stability in soils.

Nor is suitable for use in soils a crosslinked polymer as described in JP-

81-20019, which contains a gum, but which is made purely of acrylic acids and salts, without any content in acrylic derivatives containing a N atom.

Mixtures of at least two of such gums and having synergetic effect in increasing SAP absorption capacity and stability to salts are preferred. In that, It is also possible to use formulations already available for food ingredients market, especially as stabilizers for sectors of frozen desserts, bakery and confections, beverages, cultured systems, restructured foods, water and milk gels, etc. Such formulations usually referred as stabilizers generally consist on mixtures of various vegetable hydrocolloids (i.e. gums, alginates, ...) and/or other components such as gelatin, cellulose derivatives, starch derivatives, polyphosphates, etc. Usually, such stabilizing formulations are also mixed with at least one emulsifier such as mono- and diglycerides. Examples of such commercialized integrated mixtures of emulsifier and stabilizers (generally referred as stabilizers) are those produced by Palsgaard Co., FMC Corporation, Landwide Foods Inc. etc.

Moreover, said polymer may comprise as well a number of constituents which improve liquid (particularly water) absorption capacity and absorption rate and decrease the speed by which liquid is released from it without altering its gel strength and elasticity as well as polymer stability over the time. Starch, cellulose, and/or their derivatives (ex. cellulose ethers, methylcellulose, carboxymethylcellulose, hydroxymethylcellulose, hydroxypropylmethylcellulose,...) or other hydrosoluble polymers such as gelatin, polyvinylalcohol, poly(ethylene oxide), glycols may be used in combination with hydrocolloidal polysachharides.

In a preferred embodiment, the superabsorbent polymer according to the invention, comprises also at least one additive from the group consisting of mono-, di-, and/or triglycerides nonionic ester surfactants, and/or at least one silane, preferably halogeno-silanes solutions such as dimethyldichlorosilane (ex. Plusone^® of Pharmacia Biotech Co.), and their mixtures, in an amount not exceeding 5 parts by weight for 100 parts of (co)monomers acrylic units.

By such glyceride nonionic esters surfactants, it is intended to designate glycerides esters having, in addition to the surfacting action, the properties of oxidative resistance agent, repelling agent, stabilizing agent and/or lubrication agent. Examples of such nonionic esters that are available commercially are Cremao^® and Cremeol^® (Aarhus Olie Co.), Cetodan^® and Emuldan^® (Danisco Ingredients Co.), Radiamuls^® (Fina Chemicals), Myvacet^® (Eastman), etc. These surfacting agents may be used in combination with traditional anionic and/or nonionic surfactants that were reported to be useful in superabsorbent synthesis. Examples of such surfactants are sorbitan monolaurate, sodium alkyl sulfates, sodium alkyl-benzene sulfonates, polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenol ethers, polyoxyethylene sorbitan fatty acid esters, sugar fatty acid esters, etc.

Finally, superabsorbent compositions may comprise also further additives targeting specific applications of the superabsorbent such as in the agricultural field. Example of such additives are fertilizers, pesticides, anti bacterials, inert fillers such as earth, sand, compost, wood flours, cotton, cellulose floes, perlite, nylon, coal talc, clay fly ash, magnesium silicates, etc.

The superabsorbent according to the invention show improved absorption capacity, absorption rate, stability and handleability.

The superabsorbent composition may comprise also further additives that are generally reported in literature and that could be useful to improve the process of production (reaction, drying and post-treatment) and/or superabsorbent properties. By such improvements in the process of production, it is intended to designate improvements on solubility and homogeneity of reaction mixture components (ex. surfactants), on reduction of inhibition effects of traces of metals and/or oxygen (ex. chelating agents), on increase of reactants' conversion, on limitation of residuals and fraction of non-crosslinked polymers, on gel stickiness (inorganic powders such as silica, alumina, titanium dioxide, clays, insoluble silicates, carbonates, or sulfates), or on limitation of dust formation (wetting and recycling of SAP dusts, etc.).

By such improvements of superabsorbent properties, it is intended to designate improvements on its absorption capacity (ex. phosphite salts, methyl- • cellulose), its absorption rate (surfactants, surface crosslinkers, blowing agents to increase porosity such as sodium bicarbonate, alcohols, acetone, etc.), its gel strength (ex. phosphite salts, diglycidyl ethers, silica, alumina, titanium dioxide, aluminium sulfate or chloride, sodium aluminate, etc.), its stability to decomposition, to liquids, to oxidation, to photo-degradation, biodegradation, to heat, or its handleability (polyvalent metal salts or inorganic powders such as silica, alumina, titanium dioxide, clays, insoluble silicates, carbonates, or sulfates).

One preferred composition for the superabsorbent polymer is, for 100 parts by weight of total acrylic acid polymer derivatives, wherein derivatives having a nitrogen N atom represents from 40 to 90 % by weight, 0.01 to 2.5 parts by weight of a mixture of sodium alginate and/or a gum effect and 0.05 to 2.5 parts by weight of a mixture of at least one glyceride nonionic ester surfactant with dimethyldichlorosilane solution,

Most the increase in the absorption capacity for water and aqueous solutions seems to be given by the hydrocolloidal polysaccharide, preferably a mixture of gums together with sodium alginate, It must be noted that this increase is obtained without impairing the gel strength. Absorption rate is also increased.

In addition to increasing the SAP absorption rate, glycerides nonionic esters surfactants and silanes (repellant agents) are useful in the production of the superabsorbent polymer, by making less sticky the intermediate gel obtained during the polymerization process and as anti-gel blocking agent in the final product.

The invention is concerned as well with a method for producing a superabsorbent polymer. In such method, the polymerization reaction is carried out on a batch or continuous way, and comprises the steps of : a) preparing a solution or emulsion of acrylic monomer derivatives, in which part or whole of said monomer is an acrylic monomer containing a N atom, together with a crosslinking agent, the hydrocolloidal polysaccharide(s), if desired the triglyceride and/or silane additives, and a base

b) adding to said solution or emulsion polymerization initiator(s),

c) allowing the polymerization to proceed and complete under controlled temperature and efficient and continuous stirring, during which the solution or suspension turns into a gel which progressively beaks into crumbles of superabsorbent polymer,

d) recuperating the crumbles.

If necessary, the crumbles may be washed, dried and sized before conditionning.

The total monomer concentration is generally between 10 to 80% by weight of monomers in aqueous solution, preferably between 25 and 45%.

The addition of base, to partially hydrolyze or neutralize the monomers, is either added prior to polymerization and/or after the gel crumbles are formed. Standard bases, i.e. sodium, potassium or ammonium bases are generally used for partially hydrolyzing and/or neutralizing the monomers. Examples of such bases are NaOH, KOH, NH₄OH, (NH₄)₂ C0₃, Na₂C0₃, NaHC0₃, etc.

The crosslinking agent may be one or more of those that have at least two double bonds such as ethyleneglycol dimethacrylate, diethyleneglycol diacrylate, allylmethacrylate, 1 ,1,1-trimethylolpropane triacrylate, N,N'-methylenebisacrylamide, triethylene dimethacrylate, triallylamine, tetraallyloxyethane, etc.

The polymerization initiator may be at least one of any type of thermal, redox, or photo-initiators, preferably thermal and/or redox ones such as peroxides, persulfates, thiosulfates, and sulfites. Examples are sodium or ammonium peroxodisulfate, sodium hydrogensulfite, hydrogen peroxide together with ascorbic acid, etc.

Other additives, if they are desirable, are preferably added in step a), that is in the starting mixture prior to polymerization. Alternatively, depending on the other additives, they may be added as well in step b) or c), or even in the post-formulation of the SAP before storing or packaging.

Preferably also, both the preparation of the starting mixture under step a) and the addition of the polymerization under step b) are carried out under efficient and continuous stirring. At the same time, the stirring rate should be optimized to minimize shear stress of the forming gel while maintaining good homogeneity.

Alternatively, the production of the superabsorbent polymer may be conducted in reverse emulsion or suspension, in which case an non water miscible hydrocarbon(s) phase such as n-hexane, n-heptane, cyclohexane, ligroin, etc^", is used together in the presence of at least one surfactant, such as sorbitan monostearate, sorbitan monolaurate, sesquioleate, poly(oxyethylene) lauryl, nonyphenyl or stearyl ethers, sodium oleate, sodium sorbitan monopalmiate, etc.

The production is best carried out in deoxygenated medium or under inert gas, such as nitrogen, at atmospheric pressure or under vacuum.

Such a process of production, whether in solution or suspension, respectively in reverse emulsion, exhibits a number of advantages, for instance in terms of solubility and thermal and mass homogeneity, of reproducibility, of gel stickiness and particle size distribution, of limitation in fractions of non-crosslinked polymers, of SAP quality, and of limitation in dust formation.

It must be pointed out that, by adding a base in the starting mixture and allowing, when necessary, a certain period of time to spend before adding the polymerization initiator, some hydrolysis may occur, from the acrylic derivatives containing N (and esters) into acrylic salts. This is the reason why whole of acrylic monomer derivatives in the initial mixture may be acrylic derivatives containing N, the non-N acrylic derivatives being generated in situ.

It is expected that between 5 to 75% of the original acrylic derivatives containing N, can be hydrolyzed, depending on amount of base added, time, pH and temperature.

Once the polymerization is completed and amid the continuous stirring, the superabsorbent polymer is obtained in the form of gel particles or crumbles, which are recuperated, dried and eventually sized and post-treated with further additives. The SAP dried particles preferably have a particle size distribution between 0.2 and 3 mm with coarse particles up to 5 mm. Such an uneven distribution with coarse and fine particles is generally desirable when used in agriculture.

One example of such use is in deserts or arid zones, or in regions where regular heavy droughts are to be feared, for cultivation or reforestation. Then, when planting, the crumbles of superabsorbent polymer are blended with the plantation soil and the mixture is buried underground around the plant roots.

Of course, the superabsorbents polymers according to the invention can be used in several other fields, as those mentioned at the beginning of the present description.

The invention concerns moreover a vessel for producing the superabsorbent polymers, which is in the form of mixer/stirring/gel cutter reacting system provided with co-axial intermeshed baffles and blades, the space between baffles, blades and the vertical wall of the vessel being from 1 to 50 mm, preferably 2 to 10 mm. The stirring part may be of any type of impellers, turbines, propellers, etc.

One embodiment of such vessel is given on the enclosed drawing, a diametrical view, in which a series of blades 1 is formed as a U or an anchor (as shown) with the agitator shaft 2 and a series of baffles 3 are fixed on the vessel or reactor lid 4. The references 5 and 6 represent the vessel wall and vessel bottom, respectively. In other words, intermeshing blades and baffles are disposed like a comb. Reference 7 is the agitator base connecting the shaft with blades.

Blades and baffles are primarily organized as two buoys or plates, like two intermeshing combs, but the agitator and/or the baffle system may comprised a number of such plates, for instance two orthogonal plates or three plates disposed at 60° each.

This embodiment having such small distances between blades and baffles is quite efficient in preventing sticking of the reacting material. When crossing as illustrated on the drawing, blades and baffles clean out continuously their mutual surfaces

The system may be equipped with stoppers, thermocouples, and nitrogen-blanketing system and eventually with temperature control devices such as heating or refrigerating mantels, condenser, etc. The crumbles are generated by the "intermeshing" between baffles and blades. Rotation speed together with spaces between blades and baffles when crossing are selected to adjust the shearing rate of the forming gel and, as a consequence the size of the crumbles.

Blades and/or baffles, as vessel wall, may be coated with antifriction film materials such as fluoropolymers (ex. PTFE), PE, PP, etc.

Baffles may be hollow and used for the introduction of components (monomers, solvents, additives, nitrogen) used to lodge measurement devices (sensors, thermocouples, etc.).

The invention will be now further illustrated by reference to the following examples, the vessel used being of the type described and illustrated above.

EXAMPLES

COMPARATIVE EXAMPLE 1 A 0.5-liter jacketed polymerization reactor, consisting on mixer/gel-cutter reacting system as described above, is assembled. The system is equipped with stoppers, thermocouple, and nitrogen-blanketing system.

The monomer solution is prepared in the reactor by mixing 35.9 g of acrylamide, 29.3 g of acrylic acid, 135.7 g of water, 24.4 g of aqueous solution of NaOH 50%, 0.025 g of N, N'-methylenebisacrylamide, 0.2 g of EDTA, and 0.261 g of 5% aqueous solution of polyvinyl alcohol. The monomer solution is continuously homogenized by stirring and deoxygenated with nitrogen gas, bubbled through the solution.

While conserving the nitrogen blanketing, the solution temperature is rapidly raised to 40°C and the reaction is initiated by adding 1.05 g of aqueous solution (50 g/l) of sodium persulfate and 1.05 g of an aqueous solution (125 g/l) of sodium hydrogen sulfite. The exothermic reaction starts readily and the temperature gradually rises to reach about 75°C. The reaction mixture becomes more and more viscous and a sticky and rolling gel is progressively observed. However, due to the continual shearing, homogenization, and surface-cleanings, allowed by the rotating blades and their crossing with the shafts, the produced SAP gel was continuously cut to more and more finely divided crumbles. The produced gel is then held at 70°C for about 2 to 3 hours to allow reaction completion. The absorbent gel is then removed from the reactor, dried in an oven, and when necessary, crushed to the desired particle size.

COMPARATIVE EXAMPLE 2 In the same system as in comparative example 1 , the reaction solution is prepared by mixing 46.2 g of acrylic acid, 37.8 g of acrylamide, 153.7 g of water, 39 g of aqueous solution of NaOH 50%, 0.029 g of N, N'-methylenebisacrylamide, 0.27 g Of EDTA, and 0.336 g of 5% aqueous solution of polyvinyl alcohol. The monomer solution is continuously homogenized by stirring and deoxygenated with nitrogen gas, bubbled through the solution.

While conserving the nitrogen blanketing, the solution temperature is rapidly raised to 40°C and the reaction is initiated by adding 1.34 g of aqueous solution (50 g/l) of sodium persulfate and 1.34 g of an aqueous solution (125 g/l) of sodium hydrogen sulfite. The exothermic reaction starts readily and the temperature gradually rises to reach about 75°C. The reaction mixture becomes more and more viscous and a sticky and rolling gel is progressively observed. However, due to the continual shearing, homogenization, and surface-cleanings allowed by the rotating blades and their crossing with the shafts, the produced SAP gel was continuously cut to more and more finely divided crumbles. The produced gel is then held at 70°C for about 2 to 3 hours to allow reaction completion. The absorbent gel is then removed from the reactor, dried in an oven, and then eventually crushed to the desired particle size.

EXAMPLE 1

In the same system as in comparative example 1 , the reaction solution is prepared by mixing 30.9 g of acrylic acid, 37.8 g of acrylamide, 142.9 g of water, 25.7 g of aqueous solution of NaOH 50%, 0.025 g of N, N'-methylenebisacrylamide, 0.055 g of sodium alginate, 0.22 g of EDTA, 0.24g of dimethyldichlorosilane solution, and 0.29 g of 5% aqueous solution of polyvinyl alcohol. The reaction is then carried out as described in the comparative example 1. Absorption speed : 0.37 g/g/s as measured in NaCI 0.9% according to the Vortex time method.

EXAMPLE 2

Same experimental conditions as in example 1 , but with the substitution of sodium alginate by 0.057 g of a mixture of Locust bean gum and sodium alginate. Absorption speed (same method) : 0.36 g/g/s.

EXAMPLE 3

Same experimental conditions as in example 1 , but with the substitution of sodium alginate by 0.055 g of Palsgaard® stabilizer, a commercialized mixture of vegetable hydrocolloids for ice cream. Absorption speed (same method) : 0.37 g/g/s.

EXAMPLE 4

Using the same experimental conditions as in comparative example 2, but with the addition in the mixture of 0.06 g of Palsgaard® stabilizer. Absorption speed (same method) : 0.37 g/g/s.

EXAMPLE 5

In the same system as in comparative example 1 , the reaction solution is prepared by mixing 39.9 g of acrylic acid, 32.6 g of acrylamide, 141 g of water, 33.6 g of aqueous solution of NaOH 50%, 0.028 g of N, N'-methylenebisacrylamide, 0.044 g of Palsgaard® stabilizer, 0.23 g of EDTA, and 0.29 g of 5% aqueous solution of polyvinyl alcohol. The reaction is then carried out as described in the comparative example 2. Absorption speed (same method) : 0.38 g/g/s.

EXAMPLE 6

Same experimental conditions as in example 5, but with addition in the mixture of 0.25 g of dimethyldichlorosilane solution.

EXAMPLE 7

Same experimental conditions as in example 3, but with addition in the mixture of 1.09 g of Radiamuls® MCT.

The gel finely divided crumbles obtained have a whiter color and are totally not sticky. The inner wall of the reactor and the rotation axe surface were totally cleaned. Absorption speed (same method) : 0.57 g/g/s.

EXAMPLE 8

In the same system as in comparative example 1 , the reaction solution is prepared by mixing 8.3 g of acrylic acid, 60.7 g of acrylamide, 152 g of water, 6.6 g of aqueous solution of NaOH 50%, 0.024 g of N, N'-methylenebisacrylamide, 0.04 g of Palsgaard® stabilizer, 0.23 g of EDTA, and 0.21 g of 5% aqueous solution of polyvinyl alcohol. The reaction is then carried out as described in the comparative example 1. RESULTS

α pure water. Absorption capacity β after absorbing 200 g of pure water by 1 g of SAP, the force at the moment the cell intrudes into the gel (i.e. gel strength) is then measured using a rheometer (NMR-2002J)

It can easily be seen that the SAP according to the invention (examples 1 to 8) are far better water absorption capacities than those of comparatives examples 1 and 2 which do not contain any hydrocolloidal polysaccharide, and that such improvements are obtained without impairing gel strength.

By comparing example 8 to preceding examples, it is clear also that, despite its high content in N polymer, the corresponding SAP retains a high water absorption capacity, which is departing from the teachings of prior art.

The different SAPs were tested for planting trees on soils in arid zones.

The use of SAPs produced according to the invention yielded to higher nutrient and water-holding capacity in soils. The amount of irrigation water lost through percolation and evaporation was reduced significantly. Consequently, healthier trees with larger roots volume and higher survival and development rates were obtained.

Claims

1. A superabsorbent polymer based on a cross-linked acrylic polymer derivative, in which the content of acrylic polymer containing a N atom, such as acrylamide or acrylonitrile, represents between 5 % and 95 % by weight, said superabsorbent comprising at least a hydrocolloidal polysaccharide in a amount not exceeding 5 parts by weight for 100 parts of acrylic units.

2. A superabsorbent polymer according to Claim 1 , in which the hydrocqlloidal polysaccharide is selected from the group consisting of gums, such as tragacanthgum gum, guar gum, konjac, arabic gum, xanthan gum, tara gum, karaya gum, India gum, gellan gum, locust bean gum, alginates, carragheenates, pectin, agar, and their mixtures thereof.

3., A superabsorbent polymer according to Claim 1 or 2, which comprises also an additive from the group consisting of mono-, di-, and/or triglycerides nonionic ester surfactants, and silanes, such as dimethyldichlorosilane, and their mixtures, in an amount not exceeding 5 parts by weight for 100 parts of acrylic units.

4. A superabsorbent polymer according to Claim 1 comprising for 100 parts by weight of an acrylic polymer, wherein acrylic polymer having a nitrogen N atom represent from 40 to 90 % by weight, 0.01 to 2.5 parts by weight of a mixture of sodium alginate with a combination of gums and 0.05 to 2.5 parts by weight of a mixture of at least one glyceride nonionic ester surfactant with a silane.

5. A process for producing a superabsorbent polymer according to any of the preceding Claims comprising the steps of, under inert atmosphere and continuous stirring:

a) preparing a solution or emulsion of acrylic monomer derivatives, in which part or whole of said monomer is an acrylic monomer containing a N atom, together with a crosslinking agent, the hydrocolloidal polysaccharide(s), if desired the triglycehde and/or silane additives, and a base,

b) adding to said solution or emulsion at least one polymerization initiator,

c) allowing the polymerization to proceed and complete under controlled temperature and strong and continuous stirring, during which the solution or suspension turns into a gel which progressively beaks into crumbles of superabsorbent polymer,

d) recuperating the crumbles.

6. A process according to Claim 5, in which the polymerization is conducted in aqueous solution or reverse suspension or emulsion using at least one non-water miscible solvent such as n-hexane or cyclohexane and one or more surfactants.

7. A vessel for producing the superabsorbent polymers according to any Claims 1 to 4, said vessel being in the form of a mixer/stirring/gel cutter reacting system, in which said reacting system is provided with co-axial intermeshed baffles and blades, the space between baffles, blades and the vertical wall of the vessel being from 1 to 50 mm, preferably 2 to 10 mm.

8. A vessel according to Claim 7, which comprises a series of blades (1 ) formed as a U or an anchor with the agitator shaft (2) and as a series of baffles (3) fixed on the vessel or reactor lid (4).

9. Crumbles of superabsorbent as obtained by a process according to Claims 5 or 6, having an average particle size distribution between 0.2 and 3 mm with coarse particles up to 5 mm.

10. The use of crumbles according to Claim 9 in agriculture when planting wherein the crumbles are blended with the plantation soil and the mixture is buried underground around the plant roots.