CA2448140C - Superabsorbents, preparation thereof and use thereof - Google Patents

Abstract

The invention describes superabsorbents composed of surface-postcrosslinked polycarboxypolysaccharides having excellent age-stable absorption properties, even under load, high attrition resistance and bio-degradability and their use for absorbing water, aqueous or serous fluids and also blood. Also disclosed is a method of making which is impervious to changes in raw material quality and which provides consistent product quality.

Description

Superabsorbents, preparation thereof and use thereof The invention relates to superabsorbents based on surface-modified polycarboxypolysaccharides. The absor-bents according to the invention possess a high absorp-tion capacity and rate, even under pressure, for water and aqueous solutions; have no gel-blocking tendency;
and are mechanically robust. The absorption materials are age stable, toxicologically safe and biodegradable.
The invention further relates to a simple process for preparing them and to their use for absorbing water, aqueous solutions, dispersions and body fluids in human and animal hygiene articles, in food packaging materials, in culturing vessels and for soil improve-ment and also as cable sheathing.

Most of the absorption materials used today for their ability to take up large amounts of liquid (water, urine) in a short time are primarily lightly cross-linked synthetic polymers. They include, for example, polymers and copolymers based on acrylic acid or acryl-amide, which are not based on renewable raw materials and which are insufficiently biodegradable, if at all.
Superabsorbents were initially developed with the focus solely on a very high swellability on contact with liquid, known as absorption or free swelling capacity (FSC), but it was subsequently determined that it is not just the amount of liquid which is absorbed that is important but also the gel strength. Absorption capacity or else FSC on the one hand and gel strength of a crosslinked polymer on the other, however, are contrary properties, as is already known from USP 3,247,171 and also from USP Re 32 649. As a result, polymers having a particularly high absorption capacity have only little strength in the swollen gel state, so that a confining pressure, for example pressure due to the body of the wearer of a hygiene article, will cause the gel to deform and block further liquid distribution and absorption. According to USP Re 32 649, a balance

- 2 -should therefore be sought between the absorption capacity and the gel strength in order that, when such superabsorbents are used in a diaper structure, they ensure liquid absorption, liquid transport, diaper dryness and skin dryness.

Tnlhat matters in this connection is not just that the freely swollen polymer be able to retain the absorbed liquid under a subsequent application of a pressure, but also that the polymer be capable of absorbing liquids even against a simultaneously (i.e. during the liquid absorption process) exerted pressure of the kind encountered in practice when an infant or adult sits or lies on a sanitary article or when shearing forces are developed, for example as a result of motion of the legs. This specific absorption characteristic is refer-red to in Edana method 442.1-99 as "Absorbency Against Pressure" or AAP for short. The AAP value reported for a superabsorbent is crucially determined by the pressure employed, for example 21 g per cmz at 0.3 psi and 50 g per cm2 at 0.7 psi, but also by the ratio chosen for the measurement of the superabsorbent weight to area, for example 0.032 g per cm2, and also by the particle size distribution of a granular superabsorbent.

EP 0 538 904 Bl and US 5,247,072 describe superabsor-bents which are based on carboxyalkyl polysaccharides.
To turn the carboxyalkyl polysaccharide into a super-absorbent, the carboxyalkyl polysaccharide is dissolved in water, isolated by drying or precipitation and subsequently thermally crosslinked via internal ester bridges formed by the reaction of the hydroxyl groups of the polysaccharide skeleton with the acidic carboxyl groups. Since this crosslinking reaction is very sen-sitive to small changes in the pH, temperature or reaction time, the absorbents obtained have wildly fluctuating absorption properties. The materials are notable for a high absorbency under load value which,

3 -however, deteriorates to a fraction of the initial value after ageing for a few weeks.

US 5,550,189 describes absorbents based on carboxyalkyl polysaccharides that possess improved ageing stability owing to the addition of at least two-functional cross-linkers such as for example aluminium salts or citric acid. The absorbents are prepared from a conjoint homogeneous aqueous solution of carboxyalkyl poly-saccharide and crosslinkers, in which solution the components are present in low concentration and from which they are conjointly isolated and then thermally crosslinked. The synthesis of these absorbents is very energy and time intensive, since the aqueous solutions are really very weak. The improved ageing stability as it is reported in the majority of the illustrative embodiments does not meet actual service requirements.
EP 855 405 Al addresses the poor ageing stability of the absorption capacity of swellable starch maleates and proposes by way of solution to this problem adding mercapto compounds to the double bond of the maleic acid substituent. The absorption performance of the product, especially under a confining pressure, is very poor.

US 4,952,550 describes a method of making an absorbent based on carboxymethylcellulose by treating the carboxymethylcellulose in water or organic solvents with polyvalent metal salts and a hydrophobicity agent.
There is no thermal crosslinking step. According to the disclosure, the gel-blocking of these absorbents is reduced by the hydrophobicity agent.

The raw materials for preparing polysaccharide-based superabsorbents are frequently soluble in water and have to be converted into a water-insoluble form for use as superabsorbents for hygiene applications.
Numerous existing processes involve a homogeneous

4 -crosslinking for the absorbent material in order that the water solubility of the absorbent may be reduced.
This frequently has the disadvantage that such homogeneously crosslinked absorbents no longer have the desired absorption capacity for liquids, since the swellability is excessively constrained by the cross-linking of the polymer chains.

Furthermore, homogeneous crosslinking compromises the biodegradability of the absorbent, since the constrained swelling reduces the access for micro-organisms. In addition, the additionally introduced substituents inhibit enzymatic degradation [Mehltretter et al., Journal of the American Oil Chemists Society, 47 (1970) pages 522-524]. Attempts to ameliorate these disadvantageous properties have led to various surface treatment proposals.

US 5,811,531 describes the preparation of an absorbent on the basis of polysaccharides, such as xanthan, which contain uronic acid groups by reacting the polysac-charides at the surface with at least two-functional organic crosslinkers. According to the disclosure, the products possess better free-swell absorbing ability against salt solutions than carboxyalkylated poly-saccharides where the carboxyl groups are not attached directly to the saccharide units but via alkyl groups.
US 5,470,964 describes a process for preparing an absorbent providing improved absorbency under load that is based on polysaccharides containing acid groups and is surface crosslinked by polyvalent metal ions. The disadvantages of this process are that the improved absorbency under load is achieved by the crosslinking of a relatively thick surface layer and that, according to the disclosure, this is only possible through prior incipient swelling of the polysaccharide with a large amount of solvent. The incipiently swollen state then allows sufficiently deep penetration of the polyvalent

- 5 -metal ions into the surface. To achieve this, the polysaccharide is introduced into an excess of the aqueous metal salt solution such that the weight ratio of polysaccharide to water is from 1:2 to 1:40. The thick crosslinked surface layer does provide good absorbency under load values, but the free swell capacity and also the retention capacity of the absor-bent are disadvantageously reduced as a result. The process described has the further disadvantage that the polysaccharide portion added last to the crosslinker solution in the course of the manufacturing operation has less time to swell and encounters a lower cross-l.inker concentration, resulting in an inhomogeneous distribution of the crosslinker on the surface and hence wild fluctuations in the absorption properties.
US 4,043,952 discloses the surface treatment of water-swellable anionic polyelectrolytes with polyvalent metal ions in a dispersing medium in which the polymer is insoluble to improve the dispersibility of the water-absorbent products.

5a In one polymer embodiment, the invention provides a pulveruient surface-postcrosslinked polymer capable of absorbing water, an aqueous fluid, a serous fluid and blood and obtained by aqueously preswelling at least one partially neutralized, uncrosslinked, carboxyl group-containing polysaccharide and subsequently drying the polycarboxypolysaccharide, wherein the dried polycarboxypolysaccharide is surface-postcrosslinked by means of a surface crosslinker and has an absorbency against pressure (AAPo.7) value of at least 12.5 g/g.

In one process embodiment, the invention provides a process for preparing an absorbent polymer by crosslinking the surface of a polycarboxypolysaccharide with a surface crosslinker, comprising: forming a hydrogel from an uncrosslinked polycarboxypolysaccharide with water or an aqueous phase; and mechanically comminuting and drying the hydrogel, wherein the dried hydrogel is comminuted and classified to form a polymer powder, and wherein particles of the polymer powder are coated with a solution of a crosslinker and subsequently subjected to a surface postcrosslinking.

The broad aspect underlying the invention is to overcome the disadvantages arising from the state of the art.

It is an aspect of the present invention to provide biodegradable superabsorbent polymers based on renewable raw materials that are free of the defects described above. More particularly, the absorbents shall have very long term storage stability with very substantial retention of the absorption properties.
The absorbent particles shall also possess high mechanical robustness in order that the formation of fines in the course of processing operations such as for example screening or conveying may be avoided. Furthermore, with regard to the absorption performance, the absorbents shall not gel-block and shall possess not only a high absorption and retention capacity but also a high

- 6 -absorbency against pressure with regard to water and aqueous solutions. Moreover, for an effective absorp-tion and in-use performance, the absorbents shall have an overwhelmingly insoluble character even in an excess of aqueous solution.

It is a further aspect of the present invention to provide a process for preparing such superabsorbent polymers which is simple, economical and safe to carry out, which provides consistent product quality and which utilizes little solvent and ideally no organic solvent. Moreover, the processes shall not need toxico-logically suspect substances to carry out.

A further aspect according to the invention consists in improving the biodegradability of hygiene articles such as sanitary napkins, wound dressings, incontinence articles and diapers.

These aspects are achieved by a pulverulent surface-postcrosslinked addition polymer capable of absorbing water, aqueous or serous fluids and also blood and obtainable by surface crosslinking at least one partly neutralized carboxyl-containing polysaccharide, charac-.
terized in that the polycarboxypolysaccharide is aqueously preswollen in uncrosslinked form and redried before the surface crosslinking.

According. to the invention, the polysaccharide compo-nent used is a polycarboxypolysaccharide. Polycarboxy-polysaccharides are either derived frrom polysaccharides which inherently contain no carboxyl groups and are provided with carboxyl groups by subsequent modifica-tion or inherently already contain carboxyl groups and may optionally be provided with further carboxyl groups by subsequent modification.. The first group of poly-saccharides includes for example starch, amylose, amylopectin, cellulose and polygalactomannans such as guar and carob bean flour while the second group

7 -includes for example xanthan, alginates, and gum arabic.

The carboxyl groups, as mentioned, are either present inherently from the given molecular construction, for example due to uronic acid units in the polysaccharide molecule, or are introduced by subsequent modification with carboxyl-containing reagents or created by oxida-tion reactions. Of the polycarboxypolysaccharides where the carboxyl groups are introduced by subsequent modification, preference is given to the carboxyalkyl derivatives and especially to the carboxymethyl deriva-tives. Of the polycarboxypolysaccharides where the carboxyl groups are created by oxidation of the poly-saccharide molecule, preference is given especially to oxidized starches and derivatives thereof.

The polycarboxypolysaccharides to be used according to the invention are soluble or swellable in water and are used in non-crosslinked form.

The polycarboxypolysaccharides to be used according to the invention, as well as containing carboxyl groups, may be modified with further groups, especially with groups which improve the solubility in water, for example hydroxyalkyl and especially hydroxyethyl groups and also phosphate groups.

Particularly preferred polycarboxypolysaccharides are carboxymethylguar, carboxylated hydroxyethyl- or hydroxypropylcellulose, carboxymethylcellulose and carboxymethylstarch, oxidized starch, carboxylated phosphatestarch, xanthan and mixtures thereof. Par-ticular preference is given to carboxymethylcellulose.
In principle, polycarboxypolysaccharide derivatives having low and high degrees of carboxyl substitution are useful in the invention. In a preferred embodiment, they have an average degree of carboxyl substitution in

- 8 -the range from 0.3 to 1.5 and particular preference is given to polycarboxypolysaccharide derivatives having a degree of substitution in the range from 0.4 to 1.2.

The preferred water-soluble polycarboxypolysaccharide derivatives have a high average molecular weight for the molecular weight distribution dictated by the natural polymer construction and hence they also have a high solution viscosity in dilute aqueous solution like for example carboxymethylcellulose prepared from cotton linters. In the case of carboxymethylcellulose, useful derivatives have a 1% aqueous solution viscosity of more than 2 000 mPas. Preference is given to using carboxymethylcellulose having a 1% aqueous solution viscosity of more than 5 000 mPas and more preferably of more than 7 000 mPas.

Their method of making is such that polycarboxypoly-saccharides may include variable amounts of salt as a secondary constituent. Typical salt levels of carboxy-methylcelluloses in food grades are of the order of 0.5% by weight, while typical salt levels of carboxy-methylcelluloses in the case of technical grades range from about 2% by weight up to 25 to 50% by weight for products used as protective colloids. Although the absorbents according to the invention are very tolerant to a salt burden, the polycarboxypolysaccharides to be used should not include more than 15% by weight, preferably not more than 5% by weight and more prefer-ably not more than 2% by weight of salt.

The absorbents may be modified by addition of carboxyl-free polysaccharides. Preference is given to using strongly swelling polysaccharides, for example poly-galactomannans or hydroxyalkylcelluloses. The amounts of carboxyl-free polysaccharides used for modifying purposes are determined by the required performance profile and preference is given to using 20% by weight, preferably 10% by weight and more preferably 5% by

9 -weight, based on the polycarboxypolysaccharide.

The carboxyl groups of the polycarboxypolysaccharides are at least 80%, preferably at least 90% and most preferably 100% neutralized. Useful neutralizing agents are alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium car-bonate, sodium bicarbonate and potassium bicarbonate, ammonium hydroxide and amines.
The physical form of the polysaccharide derivatives used is immaterial to the properties of the absorbents according to the invention. The polysaccharide deriva-tives may therefore be used for example in the form of powders, micropowders, granules, fibres, flakes, beads or compacts, in which case the use of pulverulent materials having a particle size in the range from 1 to 2 000 pm is preferable for simplicity of metering and conveying.
The polycarboxypolysaccharide may be preswollen in an aqueous phase including, based on the polycarboxy-polysaccharide, 0.01 to 20% by weight and preferably 0.1 to 10% by weight of water-soluble auxiliaries and 0.01 to 20% by weight and preferably 0.1 to 10% by weight of antiblocking additive to improve the pro-cessibility of the hydrogel being formed and to remain in the product at least to some extent after drying.

Water-soluble auxiliaries for the purposes of the invention are selected from the group consisting of bases, salts and blowing agents. Blowing agents are selected from inorganic or organic compounds which release a gas under the influence of catalysts or heat, for example from azo and diazo compounds, carbonate salts, ammonium salts or urea.

Useful auxiliaries further include pH regulators such as for example alkali metal hydroxides, ammonia, basic

- 10 -salts such as for example alkali metal carbonates or alkali metal acetates. Useful auxiliaries further include neutral salts, for example alkali metal or alkaline earth metal sulphates or chlorides, to regu-late respectively the ionic strength of the solution and the salt content of the pulverulent absorbent resin.

The aqueous hydrogel may further have added to it water-miscible organic solvents, preferably water-miscible organic solvents which boil below 100 C. These volatile organic solvents very substantially escape again from the hydrogel in the course of the subsequent drying step. These solvents are then completely volatilized in the course of the subsequent surface postcrosslinking.

Useful antiblocking additives to further reduce the gel-blocking tendency of the pulverulent absorbent resin include for example native or synthetic fibre materials or other materials having a large surface area, for example from the group consisting of silica gels, synthetic silicas and water-insoluble mineral salts.
The absorbents according to the invention are surface postcrosslinked. Following thermal drying, comminution and classification of the hydrogel, this crosslinking of the surface of the polycarboxypolysaccharide powder is effected with covalent and/or ionic crosslinkers which react with surface moieties, preferably carboxyl, carboxylate or hydroxyl groups, preferably by heating.
Surface crosslinkers are used in an amount of 0.01-25%
by weight and preferably 0.1-20% by weight based on the polysaccharide.

Useful covalent surface postcrosslinking agents, which may be used alone or in combination with ionic cross-linkers, include crosslinkers which react with the - 1 1. -functional groups on the polycarboxypolysaccharide to form covalent bonds. A preferred embodiment comprises using crosslinkers capable of reacting with the hydroxyl groups of the absorbent resin, for example acid-functional substances.

Useful acid-functional substances include low molecular weight polycarboxylic acids and derivatives thereof, for example malonic acid, maleic acid, maleic anhydride, tartaric acid and polymeric polycarboxylic acids, for example based on (meth)acrylic acid and or maleic acid. Preference is given to the use of citric acid, butanetetracarboxylic acid and polyacrylic acid and particular preference is given to the use of citric acid. Citric acid is preferably used in an amount of 0.2-8% by weight and more preferably 0.3-6% by weight based on the polycarboxypolysaccharide. The polycarboxylic acids can also be used in partially neutralized form, for example due to partial neutra-lization with alkali metal hydroxides or amine bases.
Useful ionic postcrosslinking agents, which may be used alone or in combination with covalent postcrosslinking agents, include salts of at least divalent metal cations, for example alkaline earth metal ions such as Mgz+, Ca2+ and also Al3+, Ti4+, Fez+/Fe3+, Zn2+ or Zr4+, of which A13+, Ti4+ and Zr4+ are preferred and A13+ is particularly preferred. Aluminium salts are preferably used in an amount of 0.2-1.0% by weight and preferably 0.25-0.85% by weight based on the polycarboxypoly-saccharide.
The salts of the metal cations can be used not only alone but also mixed with each other. The metal cations in the form of their salts possess sufficient solu-bility in the solvent used, and particular preference is given to metal salts with weakly complexing anions such as for example chloride, nitrate, sulphate and acetate.

Useful postcrosslinking agents further include post-crosslinking agents capable of entering both covalent and ionic linkages, for example di- and polyamines which can function not only as covalent crosslinkers, via amide groups, but also as ionic crosslinkers, via ammonium salt complexes.

The covalent surface postcrosslinking may optionally be speeded by means of catalysts. Preferred catalysts are compounds which catalyse the esterification reaction between a carboxyl group and a hydroxyl group, for example hypophosphites, acetylacetonates, mineral acids, for example sulphuric acid, and Lewis acids.

Preference is given to using sulphuric acid and hypo-phospite. The weight ratio of surface postcrosslinker to crosslinking catalyst is 1:0001-1:1 and preferably 1:0.1-2:1.

The solution whereby the surface postcrosslinker is applied to the polycarboxypolysaccharide may optionally include one or more water-soluble auxiliaries to promote a homogeneous distribution of the crosslinker solution on the surface of the absorbent. In a prefer-red embodiment, the solution will include up to 40% by weight of these auxiliaries. Such auxiliaries, as well as water-miscible organic solvents such as for example ethanol, propanol, 2-propanol, acetone, glycerol, tetrahydrofuran and dioxane, also include water-soluble organic solids such as for example polyalkylene glycols, polyvinyl alcohols and polyacrylic acids.
Preference among organic solids is given to the use of polyethylene glycol. The preferred molecular weight range of the polyethylene glycol is not less than 1 000 and especially not less than 1 500.

In a preferred embodiment, the metal salts of divalent or higher cations function both as ionic surface cross-linkers and as auxiliaries for a homogeneous distribu-tion of the crosslinker solution on the surface.

The particulate absorbent resins according to the invention exhibit very good retention and absorption ability and a significantly improved absorbency for water and aqueous fluids against an external pressure in combination with an excellent ageing stability.

The excellent ageing stability shows itself in the fact that the absorbency against pressure (AAPo,7) value after ageing for 200 days under standard conditions is at least 80% of the initial absorbency against pressure (A.APo77) value.

The excellent absorption properties of the absorbents according to the invention show themselves in the fact that they can be made to have a retention of not less than 20 g/g coupled with an absorbency against pressure (AAPo.-i) value of at least 11 g/g and preferably of least 15 g/g and preferably to have a retention of not less than 25 g/g coupled with an absorbency against pressure (AAPo.7) value of at least 11 g/g and prefer-ably of at least 15 g/g.

The bulk density of the particulate absorbent resins according to the invention varies within the industrially customary range and is usually below 1 000 g/dm3. In a preferred embodiment, the product has a bulk density of less than 800 g/dm3 and more preferably of less than 600 g/dm3.

Another remarkable feature is the surprising attrition stability of the absorbents according to the invention.
Ball milling for 6 minutes (see "Mechanical Stability"
test method) produces less than 5% of fines of below 150 pm. This high attrition stability provides substan-tially dustless processing of the absorbents, for example in diaper manufacturing equipment in which the absorbents are exposed to mechanical stress in the course of conveyance.

Another remarkable feature is the biodegradability under composting conditions in that degradation to water and carbon dioxide is at least 40% after 90 days and continues thereafter.

The surface postcrosslinking according to the inven-tion, in contradistinction to prior art products, is concentrated on a slight outer layer of extreme stability. This is determined by measuring the surface crosslinking index (SCI), which is the difference between the crosslinker concentrations in the attrited fines and the nonattrited absorbent. The higher the SCI
index, the greater the amount of crosslinker removed with the fines off the absorbent, i.e. the greater the concentration in which the crosslinker is present on the outer layer of the absorbent. Absorbents according to the invention preferably have an SCI index of greater than 40. When absorbents have lower SCI values, the surface crosslinker has penetrated more deeply into the polymer particle, reducing the absorption properties.

The invention further provides a simply, economically and safely conductible process for preparing the mechanically stable, surficially postcrosslinked poly-mer particles having significantly improved absorption properties coupled with consistent product quality as per Claim 27 by crosslinking the surface of a poly-carboxypolysaccharide with a surface crosslinker, characterized in that a hydrogel is formed from an uncrosslinked polycarboxypolysaccharide with water, mechanically comminuted and dried, the dried hydrogel is comminuted and classified to form a polymer powder and in that the particles of the polymer powder are coated with a solution of a crosslinker and sub-sequently subjected to a surface postcrosslinking step.

The process according to the invention surprisingly affords particulate absorbent resins having very good retention and absorption ability and a significantly improved absorbency for water and aqueous fluids against an external pressure in combination with an.
excellent ageing stability and also a distinctly reduced solubility in aqueous solutions.

The process according to the invention completely unexpectedly yields age-stable superabsorbent resins which retain their very good absorption properties even when stored for a prolonged period, yet are continuously biodegraded under composting conditions.

The first step of the process according to the inven-tion converts the polycarboxypolysaccharide derivative together with a solvent into a solid hydrogel which optionally additionally includes further additives or auxiliaries. The solvent used is particularly prefer-ably water or a mixture of water with organic solvents such as for example ethanol, propanol, butanol, 2-propanol or acetone. In an embodiment, the poly-carboxypolysaccharide is presuspended in a mixture of water and organic solvent, if appropriate under eleva-ted temperature, and converted into the hydrogel after separation from the suspension.

The hydrogel is preferably prepared by mechanically mixing the polycarboxypolysaccharide derivative with the solvent component in a continuous or batch opera-tion. Suitable mixing means are for example batch kneaders such as trough kneaders, internal mixers or continuous kneaders such as single screw mixers or mixers having two or more screws.
To prepare the hydrogel, the level of polycarboxypoly-saccharide in the mixture of polycarboxypolysaccharide and water can vary within wide limits. In a preferred embodiment of the process, the level of polycarboxy-polysaccharide in the mixture of polycarboxypoly-saccharide and water is in the range from 5 to 65% by weight and more preferably from 5 to 55% by weight. To facilitate processing of the hydrogel, it can occasion-ally be necessary for the polycarboxypolysaccharide content not to exceed 45% by weight.

In a preferred embodiment, the solvent is added to the dry polycarboxypolysaccharide raw material in a con-tinuous operation, for example in an extruder, and the process is operated in such a way that the solvent is present in deficiency.

It was found that, surprisingly, the absorption proper-ties of the superabsorbents according to the invention are only minimally affected by the effectivity of the mixing or the homogeneity of the initially prepared hydrogel. The mixing of the individual components in a continuous mixing reactor with increasing throughput, for example, leads to less homogeneous hydrogels having increasing fractions of dry, nonswollen polymer frac-tions. It is believed that a subsequent swelling process takes place in the course of the further processing to pulverulent absorbent resins, so that the eventual absorption performance obtained is identical to that of completely homogeneously mixed gels.

The mixture of polycarboxypolysaccharide and water may according to the invention additionally include up to 30% by weight and preferably up to 20% by weight of one or more organic solvents miscible with water and immiscible with the polycarboxypolysaccharide.

The ratio of solid components to solvent components can vary within wide limits and is preferably chosen so that the resulting hydrogel has a firm and minimally tacky consistency. It is particularly advantageous for the swollen gel, having been conveyed using a mincer or extruder for example and shaped using a breaker plate, to be in the form of firm extrudates which have no tendency to mutual adherence even in the course of prolonged storage. Gel consistency can be specifically adjusted via the weight fraction of organic water-soluble solvent in the hydrogel. The lower the concen-tration of the polycarboxypolysaccharide derivative in the hydrogel, the higher the weight fraction of the organic solvent has to be in order that the preferred gel consistency may be obtained. When, for example, the polycarboxypolysaccharide derivative used is a high molecular weight carboxymethylcellulose having a 1%
aqueous solution viscosity of more than 4 000 mPas and the solvent used is pure water, the preferred gel consistency is obtained at a polymer content of more than 15% by weight based on the swollen gel. Reducing the polymer fraction within the gel to less than 15% by weight gives a soft and tacky gel which does not have the preferred consistency. However, on replacing 1-20%
and preferably 5-15% by weight of the water solvent with an organic water-miscible solvent such as for example 2-propanol, which is a coagulant for carboxymethylcellulose and decreases the solubility of the polymer in the solvent mixture, even hydrogels having a polymer fraction of less than 15% by weight will have the preferred gel consistency. Reducing the polymer fraction to less than 10% by weight requires that the fraction of the organic solvent be correspondingly further increased to more than 15% by weight in order that the preferred gel consistency may be obtained.

The presence of an organic water-soluble solvent in the swollen gel not only has a positive effect on gel con-sistency, but surprisingly also improves the absorption properties of the pulverulent superabsorbent signifi-cantly. This effect becomes clearly apparent even at low levels of less than 5% by weight based on the gel and shows itself in the absorbent resin particularly in a significantly higher absorption capacity for aqueous fluids against pressure.

The solvent or solvent mixture may optionally further include 0.01-20% by weight and preferably 0.1-10% by weight, based on the solids content, of one or more water-soluble auxiliaries from the group consisting of bases, salts and blowing agents to improve the processibility of the swollen gel or the absorption properties of the absorbent resin and also to suppress any crosslinking reaction during the drying operation.
Preferred auxiliaries are pH regulators such as for example alkali metal hydroxides, ammonia, basic salts such as for example alkali metal carbonates or acetates. Preferred auxiliaries further include neutral salts such as for example alkali metal or alkaline earth metal sulphates or chlorides for regulating the ionic strength of the solution and the salt content of the pulverulent absorbent resin. Additional auxiliaries used are preferably compounds which release gases under the action of catalysts or heat (blowing agents) and thus confer additional porosity on the hydrogel or absorbent resin whereby the absorption properties of the absorbent resin are additionally improved. Examples of blowing agents typically to be used are azo and diazo compounds, carbonate salts, ammonium salts and urea.

The hydrogel may if appropriate further include 0.01-20% by weight preferably 0.1-10% by weight of one or more antiblocking additives to further reduce the gel-blocking characteristics of the pulverulent absor-bent resin. Useful antiblocking additives include for example native or synthetic fibre materials or other materials having a large surface area, for example from the group consisting of silica gels, synthetic silicas and substantially water-insoluble mineral salts.

In the next step of the process according to the invention, the hydrogel is comminuted and dried to a low residual water content. The comminuting and drying step can immediately follow the preswelling step, but it is also possible to store the hydrogels for a prolonged period, for example several weeks, prior to further processing without the properties of the resulting superabsorbents according to the invention changing. Gel comminution particularly enlarges the ratio of gel surface area to gel volume, as a result of which the subsequent drying step requires substantially less energy input. The process of gel comminution is not subject to any limitation. In a particularly preferred embodiment, gel comminution is effected by pressing the gel through a breaker plate to form gel extrudates which may if appropriate be divided into shorter gel extrudates by a cutting tool.

As regards drying the hydrogel particles, various processes are known. Possible processes include for example vaporizative drying, evaporative drying, irradiative drying (example: infrared drying), high frequency drying (example: microwave drying), vacuum drying, freeze drying or spray drying. The drying can - accordingly be carried out for example according to the thin film drying process, for example using a biaxial can dryer; according to the plate drying process, whereby the hydrogel polymer particles are loaded on plates in multiple layers into a drying chamber in which hot air circulates; by the rotating drum process using can dryers; or by the conveyor belt process, hereinbelow also referred to as belt drying. Belt drying, where foraminous trays of a circle conveyor are load"ed in a tunnel with the material to be dried and the material is dried by blowing hot air through the tray holes during the passage through the tunnel, con-stitutes the most economical drying process for water-swellable hydrophilic hydrogels and therefore is preferred.

The moisture content of the polymer powder formed by drying the hydrogel is advantageously not above 30% by weight, preferably not above 15% by weight and more preferably not above 10% by weight.

The addition polymer gel is dried at temperatures above 70 C, preferably above 120 C and more preferably above 130 . The parameters such as the polymer content of the hydrogel, the pH of the solvent system, the method of mixing, the drying temperature and the drying time are interdependent and are preferably attuned to each other in such a way that no internal crosslinking of the hydrogel takes place during the drying step. If, for example, a solvent having a pH below 7 is used to make the hydrogel, some of the carboxylate groups present in the polysaccharide derivative are converted into the free acid form and are accordingly able, towards the end of the drying step in particular, to act as internal crosslinkers through an esterification with the hydroxyl groups. To control this fundamentally undesirable internal crosslinking, the drying in these cases preferably takes place at temperatures in the range of 70-100 C. The pH is usually set to 6 or higher. In a preferred embodiment of the invention, the hydrogel is prepared using a solvent having a pH of _ 7 and drying at temperatures of not less than 120 C, preferably from 130 to 160 C.

If the hydrogel is prepared in a continuous mixer, for example an extruder, the precursor products obtained at a pH of not less than 7 and which as yet have not been surface postcrosslinked may have high retention values of not less than 40 g/g, which turn out to be stable to heat treatment at 120 C for 60 minutes and which differ only minimally from products prepared at a higher pH.
If, by contrast, the hydrogels are prepared in a batch operation, the stability to heat treatment increases with increasing hydrogel pH. A preferred pH for hydro-gel formation in a batch operation is pH 10 or higher.

It was found that, surprisingly, the particularly pre-ferred drying temperatures of above 130 C, which bring about a partial hornification of the hydrogel par-ticles, provide superabsorbent polymers having a significantly higher absorption and retention ability coupled with comparable absorbency against an external pressure.

For the subsequent grinding of the dried hydrogel particles it is advantageous to cool the dried material to temperatures < 70 C, preferably < 60 C and more pre-ferably < 50 C in the last section of the preferred belt drying stage. The cooled dried hydrogel particles are initially prebroken, for example by means of a knuckle-type crusher. The thus precomminuted hydrogel particles are then ground, preferably by means of a roll mill in order that the production of fines may be minimized. In a particularly preferred embodiment, the grinding is carried out in two stages, first via a coarse roll mill and then via a fine roll mill, and the latter may in turn be carried out in one or two stages.
Screening is carried out subsequently to set the par-ticle size distribution, which is generally between 10 and 3 000 m, preferably between 100 and 2 000 pm and more preferably between 150 and 850 m. Oversize par-ticles may be resubmitted to grinding, while undersize particles may be recycled back into the forming operation.
The surface coating of the dried pulverulent addition polymer with 0.01 to 25% by weight and preferably 0.1 to 20% by weight based on the addition polymer of a postcrosslinker which is supplied in the form of a 0.01 to 80% by weight and preferably 0.1 to 60% by weight solution is carried out in suitable mixing assemblies.
These are for example Paterson-Kelly mixers, DRAIS
turbulence mixers, Lodige mixers, Ruberg mixers, screw mixers, pan mixers, fluidized bed mixers or Schugi mixers. The application of the crosslinker solution by spraying may be followed by a heat treatment step, preferably in a downstream dryer, at a temperature between 40 and 250 C, preferably 60-200 C and more preferably 80-160 C for a period of 5 minutes to 6 hours, preferably 10 minutes to 2 hours and more preferably 10 minutes to 1 hour to remove solvent frac-tions. The optimum duration of the subsequent heating operation can easily be determined for the individual crosslinker types in a few experiments. One limit for the duration is reached when the performance profile desired for the superabsorbent is destroyed again as a consequence of heat damage. The thermal treatment can be carried out in customary dryers or ovens; examples of suitable dryers and ovens are rotary tube ovens, fluidized bed dryers, pan dryers, paddle dryers and infrared dryers.

It has been determined to be advantageous in some instances for the aqueous solution of the surface postcrosslinker to be adjusted to a temperature of 15 C-100 C and preferably to 20 C-60 C before use.

Covalent surface postcrosslinking can be speeded if appropriate by means of catalysts. Preferred catalysts are compounds which catalyse the esterification reac-tion between a carboxyl group and a hydroxyl group, for example hypophosphites, acetylacetonates, mineral acids, for example sulphuric acid, and Lewis acids.
Preference is given to using sulphuric acid and hypo-phosphite. The weight ratio of surface postcrosslinker to crosslinking catalyst is 1:0.001-1:1 and preferably 1:0.1-2:1.

In a preferred embodiment, the crosslinking catalysts are mixed into the solution of the surface postcrosslinker.

The postcrosslinking solution may optionally include up to 70% by weight of one or more auxiliaries.
Auxiliaries are in particular water-soluble compounds which promote a homogeneous distribution of the crosslinker solution on the surface of the absorbent by slowing the penetration of the solvent into the interior of the superabsorbent particle and also reduce the solubility of the particle surface and hence the tendency of the moist superabsorbent particles to adhere to each other. Preferred auxiliaries, as well as water-miscible organic solvents such as for example ethanol, propanol, 2-propanol, acetone, glycerol, tetrahydrofuran and dioxane, also include water-soluble hydrophilic organic solids, especially polymers such as for example polyalkylene glycols, polyvinyl alcohols and preferably polyethylene glycols.

In a preferred embodiment, the metal salts of divalent or higher cations function both as ionic surface crosslinkers and as auxiliaries for a homogeneous distribution of the crosslinker solution on the surface.

The polymers according to the invention are notable for excellent absorption and retention ability for water, aqueous solutions and body fluids. At the same time, due to the controlled crosslinking of surface, the superabsorbent polymers possess a distinctly improved absorbency for aqueous solutions against an external pressure. In addition, the superabsorbents according to the invention, which are based on polycarboxypoly-saccharide derivatives, are stable in storage, free of residual monomer fractions, only minimally soluble in aqueous fluids and biodegradable.

The superabsorbents according to the invention are very useful as absorbents in hygiene articles such as for example infant and adult diapers, wound contact materials, sanitary napkins, tampons and the like. The polymers are especially suitable for use in hygiene articles which are to be composted after use, since the polymers have proved biodegradable in composting tests in accordance with ASTM method D 5338-92 of 15.12.1992;
in accordance with the CEN draft "Evaluation of the Ultimate Aerobic Biodegradability and Disintegration of Packaging Materials under Controlled Composting Conditions" of 6.5.1994; and in accordance with DIN 54900 Part 2 Method 3 of January 1997.

Absorbent hygiene products typically possess a general construction composed of a bodyfacing liquid-pervious topsheet (1), a liquid-absorbent layer (2) and a substantially liquid-impervious bodyremote outer layer (3). Further structures may optionally find application in the absorbent core to rapidly acquire and distribute body fluid (4). These structures are frequently but not necessarily used between the bodyfacing liquid-pervious topsheet (1) and the liquid-absorbent layer (2).
The liquid-pervious topsheet (1) is typically composed of a fibrous nonwoven or some other porous structure.
Useful materials for this topsheet (1) include for example synthetic polymers such as polyvinyl chloride, polyvinyl fluoride, polytetrafluoroethylene (PTFE), polyvinyl alcohols and derivatives, polyacrylates, polyamides, polyesters, polyurethanes, polystyrene, polysiloxanes or polyolefins (eg polyethylene (PE) or polypropylene (PP)) and also natural fibre materials and also any desired combinations of the aforementioned materials in the form of hybrid materials, composite materials or copolymers.

The liquid-pervious topsheet (1) has a hydrophilic character. It may also constitute a combination of hydrophilic and hydrophobic constituents. Preference is generally given to a hydrophilic finish for the liquid-pervious topsheet (1) in order that rapid seepage of body fluid into the liquid-absorbent layer (2) may be ensured, but partially hydrophobicized topsheets (1) are used as well.

Liquid-absorbent layer (2):
The liquid-absorbent layer (2) includes the super-absorbent powders or granules according to the invention and further components composed for example of fibrous materials, foam materials, film-forming materials or porous materials and also combinations of two or more thereof. Each of these materials can be of natural or synthetic origin and may have been prepared by chemical or physical modification of natural materials. The materials can be hydrophilic or hydrophobic, in which case hydrophilic materials are preferred. This applies especially to those compositions which are to efficiently acquire secreted body fluids and transport them in the direction of regions of the absorbent core which are more remote from the point of ingress of the body fluid.
Useful hydrophilic fibre materials include for example cellulosic fibres, modified cellulosic fibres (for example stiffened cellulosic fibres), polyester fibres (for example Dacron), hydrophilic nylon or else hydrophilicized hydrophobic fibres, for example surfactant-hydrophilicized polyolefins (PE, PP), polyesters, polyacrylates, polyamides, polystyrene, polyurethanes and others.

Preference is given to using cellulosic fibres and modified cellulosic fibres. Combinations of cellulosic fibres and/or modified cellulosic fibres with synthetic fibres such as for example PE/PP bicomponent fibres as used for example to thermobond airlaid materials or other materials are likewise customary. The fibre materials can be present in various use forms, for example as loose cellulosic fibres deposited or laid down from an air stream or from an aqueous phase, as a nonwoven or as a tissue. Combinations of various use forms are possible.

The superabsorbents according to the invention may optionally include further pulverulent substances, for example odour-binding substances such as cyclodextrins, zeolites, inorganic or organic salts and similar materials.

Useful porous materials and foam materials include for example polymer foams as described in DE 44 18 319 Al and DE 195 05 709 Al.

The liquid-absorbent layer (2) may be mechanically stabilized using thermoplastic fibres (for example bicomponent fibres composed of polyolefins), polyolefin granules, latex dispersions or hot melt adhesives.
Optionally, one or more layers of tissue are used for stabilization.

The liquid-absorbent layer (2) can be a single layer or be composed of a plurality of layers. Preference is given to the use of structures constructed of hydrophilic fibres, preferably cellulosic fibres, optionally a structure to rapidly acquire and distribute body fluid (4) such as for example chemically stiffened (modified) cellulosic fibres or high loft webs composed of hydrophilic or hydrophilicized fibres and also superabsorbent polymers.
The superabsorbent polymer according to the invention can be homogeneously distributed in the cellulosic fibres or stiffened cellulosic fibres, it can form a layer between the cellulosic fibres or stiffened cellulosic fibres, or the concentration of the superabsorbent polymer can have a gradient within the cellulosic fibres or stiffened cellulosic fibres. The ratio of the total amount of superabsorbent polymer and of the total amount of cellulosic fibres or stiffened cellulosic fibres in the absorbent core can vary between 0:100 and 70:30%, although one embodiment provides local concentrations of up to 100% of super-absorbent, for example in the case of gradiented or layered incorporation. Such structures featuring regions of high concentrations of absorbent polymer, the fraction of superabsorbent being between 60 and 100% and most preferably between 90% and 100% in certain regions, are described for example in US 5,669,894 and elsewhere.

It is optionally also possible to use at the same time two or more different superabsorbents which differ for example in the absorption rate, the permeability, the storage capacity, the absorbency against pressure, the particle size distribution or else the chemical composition. The various superabsorbents can be introduced into the absorbent core after blending with each other or else can be placed in the absorbent pad with local differentiation. Such a differentiated placing can be effected in the direction of the thickness of the absorbent core or in the direction of the length or in the direction of the width of the absorbent pad.
The liquid-absorbent layer (2) includes one or more of the above-described cellulosic fibre or stiffened cellulosic fibre layers containing superabsorbent polymers. A preferred embodiment utilizes structures composed of combinations of layers featuring homogeneous superabsorbent incorporation and additionally layered incorporation.

These aforementioned structures are optionally also supplemented by further layers of pure cellulosic fibres or stiffened cellulosic fibres on the bodyfacing side and/or else the bodyremote side.

The above-described structures can also repeat a number - 2$ -of times, in which case there may be a superposition of two or more identical layers or else a superposition of two or more different structures. The differences are in turn purely structural or else in the type of material used, for example the use of absorbent polymers differing in terms of properties or else the use of different pulp varieties.

Optionally, the entire absorbent pad or else individual layers of the liquid-absorbent layer (2) are separated from other components by layers of tissue or are in direct contact with other layers or components.

By way of example, the structure for rapid acquisition and distribution of body fluid (4) and the liquid-absorbent layer (2) can be separated from each other by tissue or else be in direct contact with each other. If there is no separate structure to rapidly acquire and distribute body fluid (4) between the liquid-absorbent layer (2) and the bodyfacing liquid-pervious top-sheet (1), but the fluid distribution effect is to be achieved for example by the use of a specific bodyfacing liquid-pervious topsheet (1), the liquid-absorbent layer (2) can optionally likewise be separated from the bodyfacing liquid-pervious top-sheet (1) by a tissue.

Instead of tissue it is optionally also possible for a nonwoven to be incorporated into the liquid-absorbent layer (2). Either component brings about the desired secondary effect of stabilizing and strengthening the absorbent core in the moist state.

Methods of making the liquid-absorbent layer:
Fibrous layers which contain superabsorbent and distribute and store liquid can be generated using a multiplicity of production processes. As well as the established conventional operations as generally subsumed by those skilled in the art under drum forming using forming wheels, forming pockets and product forms and correspondingly adapted metering means for the raw materials, customary methods for producing the abovementioned liquid stores include modern established processes such as airlaid (for example EP 850 615 column 4 line 39 to column 5 line 29, US 4,640,810) with all forms of metering, fibre laydown and consolidation such as hydrogen bonding (for example DE 197 50 890 column 1 line 45 to column 3 line 50), thermal bonding, latex bonding (for example EP 850 615 column 8 line 33 to column 9 line 17) and hybrid bonding, wetlaid (for example PCT WO 99/49905 column 4 line 14 to column 7 line 16), carding, meltblown and spunblown operations and similar operations for producing superabsorbent-containing nonwovens (within the meaning of the definition of EDANA of Brussels) singly and combined with and among each other.

Further processes include the production of laminates in the widest sense and also of extruded and coextruded, wet-consolidated and dry-consolidated and also subsequently consolidated structures. A combina-tion of these processes with and among each other is likewise possible.
Structures for rapid acquisition and distribution of body fluid (4):
A structure for rapid acquisition and distribution of body fluid (4) is composed for example of chemically stiffened (modified) cellulosic fibres or high loft webs composed of hydrophilic or hydrophilicized fibres or a combination of both.
Chemically stiffened, modified cellulosic fibres can be produced for example from cellulosic fibres which are reacted by means of crosslinkers such as for example C2-C8 dialdehydes, C2-C8 monoaldehydes having an additional acid function or C2-C9 polycarboxylic acids in a chemical reaction. Specific examples are glutaraldehyde, glyoxal, glyoxalic acid or citric acid.

Also known are cationically modified starch or polyamide-epichlorohydrin resins (eg KYMENE 557H, Hercules Inc., Wilmington, Delaware). The crosslinking provides and stabilizes a twisted, curled structure which has an advantageous effect on the rate of fluid acquisition.

Basis weight and density of absorbent articles:
The absorbent hygiene products can differ widely in basis weight and thickness and hence density. Typically the densities of the regions of the absorbent cores are between 0.08 and 0.25 g/cm3. The basis weights are between 10 and 1 000 g/m2, although it is preferable to provide basis weights between 100 and 600 g/m2 (see also US 5,669,894). The density varies in general along the length of the absorbent core. This is a consequence of a controlled metering of the cellulosic fibre or stiffened cellulosic fibre quantity or the quantity of the superabsorbent polymer, since these components are in preferred embodiments preferentially incorporated in the front region of the disposable absorbent article.
This controlled increase in the absorbent material in certain regions of the absorbent core can also be achieved for example by producing an appropriately sized airlaid or wetlaid sheet material composed of hydrophilic cellulosic fibres, optionally of stiffened cellulosic fibres, optionally of synthetic fibres (eg polyolefins) and also of superabsorbent polymers and subsequent backrolling or superposition.

The polymers according to the invention are also used in absorbent articles which are suitable for a wide variety of uses, for example by mixing with paper or fluff or synthetic fibres or by distributing the superabsorbents between substrates of paper, fluff or nonwoven textiles or by processing into base materials to form a continuous length. The polymers according to the invention are further used wherever aqueous fluids have to be absorbed, for example in cable sheaths, in food packaging, in the agricultural sector for plant cultivation and as water storage medium and also as a carrier for an active component to be released to the environment in a controlled manner.

The products according to the invention which have this excellent combination of very high absorption and retention values, excellent absorbency against pressure and biodegradability can be prepared without the use of toxicologically compromised substances. According to the invention, the polymers can be produced on a large industrial scale according to existing processes in a continuous or batchwise manner and with consistent product quality.

The invention is further concerned with structures for absorbing body fluids, comprising a polymer according to the invention. These aforementioned structures are preferably absorbent bodies. In another embodiment of the construction it is a sanitary napkin, a diaper or an incontinence product, wherein diapers are particularly preferred.

The invention is exemplified by some non-limiting examples as follows.

Examples Description of test methods used in examples:
Retention (TB) The retention values were determined by performing a tea bag test. The test solution used was a 0.9%
strength NaCl solution. 0.20 g of the test substance (screened off between 150 and 850 ~.un) were sealed into a tea bag and immersed in the test solution for 30 minutes. The tea bag was subsequently spun in a centrifuge, for example a commercially available laundry spindryer, at 1 400 rpm for 3 minutes. The amount of liquid absorbed was determined gravimetrically after subtraction of the blank value (weight of an empty tea bag after spinning) and converted to 1 g of test substance. The retention value corresponds to the amount of liquid absorbed in grams per gram of test substance.

Absorbency against a pressure of 0.3 or 0.7 psi (AAP) The ability to absorb a liquid against an external pressure (absorbency against pressure, AAP) was determined as per Edana method No 442.1-99. 0.90 g of the test substance (screened off between 150 and 850 la.m) was weighed into a test cylinder having an internal diameter of 60.0 mm and a 400 mesh screen base (concentration: 0.032 g/cm2) and uniformly distributed therein. Onto the test substance is placed a cylindrical weight (21 g/cmz = 0.3 psi or 50 g/cm2 =
0.7 psi) having an outer diameter of 59.2 mm. Filter plates covered with a filter paper are placed in a plastic dish. The plastic dish is filled with 0.9%
strength NaCl solution until the surface of the liquid is flush with the upper edge of the filter plates. The prepared measuring units are then placed on the filter plates. After a swell time of 60 minutes the measuring units are taken out and the weight is removed. The amount of liquid absorbed is determined gravimetrically and converted to 1 gram of test substance.
Extractables (EA) Extractable fractions in the biodegradable super-absorbent resins were determined by GPC analysis under the following test conditions.
Column material: HEMA BIO 40, column length: 300 mm, column diameter: 8 mm, eluent: 0.9% NaCl solution, flow rate: 1.0 ml/minute, temperature: room temperature, injection volume: 50 ul, running time 15 minutes, calibrating substance: low viscosity carboxymethyl-cellulose (Finnfix 4000G) 0.50 g of the test substance is admixed with 100 ml of 0.9% strength NaCl solution and stirred for 16 hours.
After filtration through a glass filter crucible (pore size 1) the filtrate was diluted with the eluent in a ratio of 1:10. This dilution was injected and the area value of the polymer peak determined. The soluble fraction of the test substance was calculated by means of a calibration curve prepared with the low viscosity carboxymethylcellulose under identical conditions.

Fluff-absorbent combination test (FACT) 2.0 g of cellulose fluff were weighed out on an analytical balance and formed into three fluff layers.
0.22 to 2 g (= 10 to 50% by weight) of superabsorbent resin were uniformly sprinkled between the fluff layers so as to create a fluff/SAP/fluff/SAP/fluff sandwich.
The fluff-superabsorbent pad was placed in a test apparatus having a screen base and weighted with a metal ring to stop the absorbent resin escaping from the test apparatus as it swells. The test specimen was loaded with a weight (21 g/cm2 or 50 g/cm2 corresponding to 0.3 psi and 0.7 psi respectively). The test specimen was then allowed to swell in 0.9% strength NaCl solution by capillary action while the absorption was recorded by electronic data processing. The test was deemed to have ended when less than 1 g of test liquid was absorbed in the course of 10 minutes. For every measurement, the amount of liquid absorbed was plotted against time in an absorption curve from which the following parameters were determined:
a) maximally attained end value in grams: Absmax b) time [min] at which the end value was attained: tmax c) time [min] at which x% of the final value was attained: tX%

Airlaid tests An airlaid machine was used to fabricate composite materials composed of one layer of tissue, a subsequent layer of a cellulose fluff-absorbent powder mix and a further layer of tissue. Round specimens 6 cm in diameter were die cut from the composite and used for the subsequent tests.
Retention A tea bag test was carried out to determine the retention values of the composite. The test solution used was a 0.9% strength NaCl solution. A die cut composite specimen was weighed, sealed into a tea bag and immersed in the test solution for 30 minutes. The tea bag was subsequently spun in a centrifuge, for example a commercially available laundry spindryer, at 1 400 rpm for 3 minutes. The amount of liquid absorbed was determined gravimetrically after subtraction of the blank value (weight of empty tea bag after spinning) and converted to 1 m2 of composite. The retention value corresponds to the amount of liquid absorbed in grams per square metre of airlaid composite.
Absorbency of a composite against a pressure of 20 or 50 g/cm2 (LAUL20/LAUL50) A die cut composite specimen was weighed into a test cylinder having an internal diameter of 60.0 mm and a 400 mesh screen base. A cylindrical weight (20 g/cm2 or 50 g/cm2) having an external diameter of 59.2 mm is placed onto the test substance. Filter plates are placed into a plastic dish and covered with a filter paper. The plastic dish is filled with 0.9% strength NaCl solution until the surface of the liquid is level with the upper edge of the filter plates. The prepared measuring units are then placed on the filter plates.
After a swell time of 60 minutes the measuring units are taken out and the weight is removed. The amount of liquid absorbed is determined gravimetrically and converted to 1 square metre of airlaid composite.

Mechanical stability 127 g of grinding media (cylindrical pieces of porcelain, U.S. Stoneware lh" O.D. *'A") and 10 g of a pulverulent superabsorbent resin having a particle size of 150 to 850 lun were weighed into a ball mill pot. The ball mill pot was sealed and rotated at 95 rpm on a roll mill for 6 minutes. The mechanically stressed superabsorbent was taken from the pot and analysed with regard to particle size distribution.

Surface crosslinking index (SCI) 127 g of grinding media (cylindrical pieces of porcelain, U.S. Stoneware lh" O.D. * 2/a") and 10 g of a surface-crosslinked superabsorbent resin having a particle size of 150 to 850 um were weighed into a ball mill pot. The ball mill pot was sealed and rotated at 95 rpm on a roll mill for 30 minutes. The mechanically stressed superabsorbent was taken from the pot and the particles having a particle size <150 um were screened out. The screened-off fines were digested with HN03 and H202 using a microwave and subsequently hydrolysed with water. The aluminium content was then determined photometrically via the yellowish red Alizarin S-aluminium complex. The SCI is calculated from the amount of A13+ added in the course of the surficial crosslinking, based on the superabsorbent resin (= CSp,p) , and the A13+ concentration of the fine particles found after mechanical exposure (= CF) in accordance with the equation SCI =(CF-CSAp)*100, where CF and CSAP are inserted in % A13+

Inventive examples and comparative examples:
All pretreated superabsorbent resins according to the invention were, unless otherwise stated, ground prior to surface coating and screened off to a particle size of 150 to 850 lun. The moisture content of all pulverulent absorbent resins was less than 10% by weight.

Inventive Example 1 In a make-up vessel, 100 g of carboxymethylcellulose (CMC) were suspended in a mixture of 244 g of 2-propanol and 156 g of DM water and refluxed for 1 hour. After the suspension had been cooled to room temperature, the carboxymethylcellulose was filtered off. A second make-up vessel was charged with 900 g of water which was adjusted to pH 9 with NaOH. The filtered-off carboxymethylcellulose was introduced into the second make-up vessel with vigorous stirring to form a firm hydrogel. After a swell time of 30 minutes, the swollen hydrogel was fed into a meat mincer equipped with a mincer plate and connminuted. The comrnznuted hydrogel was dried at 80 C in a circulating air cabinet for 12 hours. The dried hydrogel was coarsely comminuted and ground using a Retsch mill.
After the particle size fraction of 150 to 850 -~un had been screened off, the retention value of the uncross-linked precursor was determined. The table which follows lists the retention values thus determined for various commercially available carboxymethylcelluloses ( CMCs ) :
Sample CMC Viscosity D.S.Precursor No [mPas] retention [g/g]
1.1 Finnfixe 50,000'bl 8,200 (1%) 0.78 46.5 1.2 Cekol 50,000ibl 8,400 (1%) 0.72 47.0 1.3 Cekol 100,000'bl 10,000 (1%) 0.76 32.2 1.4 Tylose CB 30,000'r-3 >24,000 (2%) >0.85 45.3 1.5 Blanose 7HOFIal 2140 (1%) 0.85 36.8 1.6 Walocel VP-C-2204[e) >7500 (1%) 0.65-0.95 44.7 [a]: Degree of substitution as per manufacturer data, [b]: Noviant, [c] : Clariant, [d] : Aqualon, [e]: Wolff-Walsrode Inventive Example 2 A crosslinker solution was prepared from 1.29 g of citric acid monohydrate, 61 g of 2-propanol and 39 g of DM water. 10 g of each of the pulverulent precursors prepared according to Inventive Example 1) were each coated with 4 g of this crosslinker solution (corresponding to a citric acid concentration of 0.47%

based on CMC) and dried at 80 C for 2 hours. Surface crosslinking was completed by a subsequent annealing step for the stated period at 120 C. The annealing time was chosen so as to ensure a balanced ratio of retention to absorbency against pressure. The super-absorbents thus prepared had the following charac-teristic data:

No Precursor of Annealing TB AAPo.3 AAPo.7 inv. ex. [min] [g/g] [g/g] [g/g]
2.1 1.1 30 24.0 21.6 14.4 2.2 1.2 50 21.0 20.5 16.1 2.3 1.3 30 19.4 20.9 16.8 2.4 1.4 30 20.4 21.8 17.2 Inventive Example 3 Various crosslinker solutions were prepared by adding acetone to an aqueous solution of aluminium sulphate 18-hydrate in DM water:
A: 13 g of A12 (S04) 3*18 H20/100 g of DM water and 36.7 g of acetone B: 18 g of A12 (S04) 3*18 H20/100 g of DM water and 36.1 g of acetone 10 g of each of the precursors of Inventive Example 1 were each coated with 4 g of each crosslinker solution thus prepared by initially charging the pulverulent precursor and adding the crosslinker solution dropwise with stirring. The coated product was dried at 80 C for 2 hours and the dried products were measured for retention and APP:

No Precursor Cross- %Al3+/CMC TB AAPo.3 AAPo.7 of linker [g/g] [g/g] [g/g]
inv. ex. solution 3.1 1.1 B 0.38 24.1 18.9 13.6 3.2 1.2 A 0.28 22.6 20.5 16.1 3.3 1.3 B 0.38 18.3 17.9 14.8 3.4 1.6 B 0.38 22.8 18.6 14.1 = - 38 -Comparative Example 1 20 g of carboxymethylcellulose (Cekol 50,000, degree of neutralization 98.6%) were coated without prior swelling with 8 g of a solution of 40 g of A12(S04)3*18 H20 in 100 g of DM water and 21.6 g of acetone (0.8% of A13+ based on CMC) and dried at 80 C
for 2 hours, both steps being carried out as in Inventive Example 3). The characteristic absorption data were then determined:

Sample No 1.1 comp: TB = 21.1 g/g AAPo.7 = 10.9 g/g In contradistinction to Sample No 3.2, which was pretreated and postcrosslinked according to the invention, Comparative Example 1) required a distinctly larger amount of crosslinker (0.8% of A13+ based on CMC
instead of 0.28%), even though the two samples were based on the same starting material. The absorbency against pressure (0.7 psi) of Sample No 3.2 according to the invention is at 16.1 g/g significantly higher than that of Comparative Example No 1.1 comp. The pretreatment modifies the surface of the absorbents according to the invention in such a way that the result is a more effective distribution and action of the postcrosslinking agent and a significantly improved absorbency against pressure.

Inventive Example 4 Carboxymethylcellulose (Finnfix 50,000) was preswollen and dried, both steps being carried out as described in Example 1). 50 g of the uncrosslinked precursor thus prepared were weighed into a plastic cup and stirred using a household mixer. 3 g of a solution consisting of 3.33 g of citric acid monohydrate and 17.5 g of polyethylene glycol (1 500 g/mol) in 29.2 g of DM water were poured onto the pulverulent precursor in the course of 10 seconds, followed by stirring for a further 100 seconds. The coated absorbent was cured at 120 C for 45 minutes and showed the following characteristic data:
Sample No 4.1: TB = 22.2 g/g AAPo.7 = 13.3 g/g Inventive Example 5 Inventive Example 4) was repeated, except that the 3.0 g of coating solution used was composed of 5.5 g of polyacrylic acid (Mw 1 500 g/mol) and 0.39 g of sodium hydroxide in 10.0 g of DM water:
Sample No 5.1: TB = 23.2 g/g AAPo,7 = 13.0 g/g Inventive Example 6 Carboxymethylcellulose (Finnfix 50,000) was mixed with the stated amounts of guar bean flour. The powder mixture was preswollen and dried, both steps being carried out as described in Inventive Example 1). 10 g of each of the dried pulverulent precursors are coated with 4 g of a solution of 0.85 g of citric acid monohydrate in 99.15 g of 2-propanol and dried at 80 C
for 2 hours. The crosslinking reaction was then completed at 120 C for 30 minutes.

Sample % by weight TB AAPo.3 AAPO.7 No of guar bean flour [g/g] [g/g] [g/g]
6.1 5 22.4 19.1 12.7 6.2 10 22.5 17.5 13.1 6.3 20 16.2 16.5 12.8 Inventive Example 7 In a make-up vessel, 600 g of carboxymethylcellulose (Cekol 50,000, degree of neutralization 98.6%) were suspended in a mixture of 1 460 g of 2-propanol and 940 g of DM water and refluxed for 1 hour. The suspension was cooled down to room temperature and filtered. A second make-up vessel was charged with 5 000 g of DM water which were then adjusted to pH 9 with 2.5 g of 10% strength aqueous sodium hydroxide solution. The filter cake was introduced into the second make-up vessel with strong stirring and the hydrogel formed was comminuted after 1 hour in a meat mincer equipped with a mincer plate. The comminuted hydrogel was divided into two halves and dried on wire mesh at different temperatures. The dried hydrogels were coarsely comminuted, ground with a Retsch mill and screened off to a particle size of 150 to 850 pm. The screened-off pulverulent absorbents were measured for retention.

In a further run, the pH in the make-up vessel was adjusted to pH 11.3 with 11.5 g of 10% strength aqueous sodium hydroxide solution. The products resulting therefrom were coated with the crosslinking solution based on citric acid as per Inventive Example 2 and postcrosslinked at 120 C for 50 minutes.

Precursor Postcrosslinked product Sample pH Drying TB TB AA.Po.3 AAPo.7 No [ C/h] [g/g] [g/g] [g/g] [g/g]
7.1 9 80/12 45.2 7.2 9 150/2.25 10.6 7.3 11 80/12 45.2 21.0 20.5 16.1 7.4 11 150/2.25 55.6 28.2 20.5 16.0 This example demonstrates the effect of drying tempera-ture and pH on the internal crosslinking and also the surface hornification of the CMC. Sample 7.2 is internally crosslinked at high drying temperatures of 150 C and has a low retention value. When the drying temperature is lowered (Sample 7.1), there is no internal crosslinking. Internal crosslinking at high temperatures can be prevented by raising the pH, as Sample 7.4 shows. The comparison of Sample 7.3 with Sample 7.4 demonstrates the effect of a high temperature on the properties of the absorbent resins according to the invention. It is believed that drying at high temperatures gives rise to surface hornification, leading to comparable absorbencies against pressure at a significantly higher retention level.

To control internal crosslinking, the pH of the swelling medium for the pretreatment was hereinbelow adapted in such a way (unless otherwise stated) that drying temperatures of 150 C provided a precursor retention value of at least 40 g/g which did not decrease to below 40 g/g even when the dry precursor was annealed at 120 C for 60 minutes.
Inventive Example 8 Demineralized water is charged to a make-up vessel and mixed with different amounts of 2-propanol. The pH of the solvent is adjusted to a pH 11.7 with 4.7 g of 10%
strength NaOH per 1 000 ml and carboxymethylcellulose (Cekol 100,000, degree of neutralization 98.6%) is added with vigorous stirring so that the final concentration of the carboxymethylcellulose based on the entire batch is between 8 and 20% by weight. After a swell time of 1 hour, the swollen hydrogel was fed to a meat mincer equipped with a mincer plate and comminuted. The comminuted hydrogel was dried in a circulating air cabinet at 150 C for 2.5 hours. The dried hydrogel was coarsely comminuted and ground with a Retsch mill. After the particle size fraction of 150 to 850 um had been screened off, 50 g of each pulverulent precursor were placed in a plastic dish, stirred using a mixer and sprayed with 4.0 g of a 50%
strength aluminium sulphate x 14H20 solution in DM water, corresponding to 0.36% of A13+ based on CMC, in the course of 10 seconds. The coated powder was stirred for a further 110 seconds and subsequently dried at 150 C for 10 minutes. The characteristic absorption data of the dried surface-crosslinked absorbents were then determined.

Sample 2-Propanol CMC in TB AAPo.3 AAPo.7 Bulk No in solvent batch [g/g] [g/g] [g/g] density [wt%] [wt%] [g/dm3]

8.1 19.8 8 28.2 20.5 16.0 450 8.2 13.8 12 23.2 17.5 13.5 490 8.3 7.1 16 27.3 16.7 13.2 550 8.4 5 20 28.5 16.4 13.4 570 8.5 3 20 28.8 16.0 13.2 590 8.6 1 20 28.8 16.0 12.8 610 8.7 0 20 27.7 14.4 12.5 650 Inventive Example 8 shows that as little as less than 5% of isopropanol in the swelling medium will reduce the bulk density of the absorbents according to the invention and distinctly improve absorbency against pressure. The results suggest that the rapid vaporization of the solvent in the course of the drying of the precursors results in an increasingly porous particle structure, which is beneficial for the absorbency against pressure in particular.

Inventive Example 9 4 800 g of DM water are charged to a make-up vessel and admixed with sodium hydroxide (every 1 000 ml of water contain 4.7 g of 10% strength aqueous sodium hydroxide solution) until the pH is 12. 1 200 g of carboxymethyl-cellulose (Cekol 100 000, degree of neutralization 98.6%, NaCl content 0.74% by weight) are added with stirring to form a firm hydrogel.. After a swell time of 2 hours, the hydrogel was transferred into a mincer equipped with a mincer plate and comminuted. The comminuted hydrogel was dried at 150 C for 2 hours, coarsely comminuted and ground with a Retsch mill. The particle size fraction of 150 to 850 U.m was screened off and the characteristic precursor data were determined:
Sample No 9.1:
TB = 54.8 g/g AAPo.3 = 8.6 g/g AAPo.7 = 8.3 g/g 50 g of the screened-off precursor product No 9.1 were initially charged, coated with 6 g of a 50% strength solution of AlZ ( S04 ) 3x14H20 in DM water ( 0. 54% of A13+

based on CMC) with stirring and dried at 150 C for minutes:
Sample No 9.2:
TB = 27.7 g/g AAPo.3 = 14.4 g/g AAPc).7 = 12.5 g/g 50 g of the screened-off precursor product No 9.1 were initially charged, coated with 8 g of a solution of 16.67 g of citric acid monohydrate and 8.33 g of sodium hypophosphite in 25 g of a 37.5% strength solution of 10 polyethylene glycol 1 500 in DM water (5.1% of citric acid, 2.6% of sodium hypophosphite, 3% of PEG based on CMC) with stirring and postcrosslinked at 150 C for minutes.
Sample No 9.3:
15 TB = 23.0 g/g AAPO"3 = 14.3 g/g AAPa.7 = 11.6 g/g 50 g of the screened-off precursor product No 9.1 were initially charged, coated with 7 g of a solution composed of 67% by weight of a 50% strength 20 A12 (SO4) 3 x 14H2O solution in DM water and 33% by weight of a 40% citric acid monohydrate solution in DM water (0.43% of A13+ and 1.7% of citric acid based on CMC) with stirring and postcrosslinked at 140 C for 20 minutes:
Sample No 9.4:
TB = 26.3 g/g AAPo.3 = 14.3 g/g AAPo.7 = 12.1 g/g Inventive Example 10 0.5 g of each of the preswollen and crosslinked carboxymethylcellulose No 9.1 and the differently surface-crosslinked absorbents Nos 9.2 and 9.3 were transferred into 100 ml of 0.9% strength NaCl solution and stirred at room temperature for 16 hours.
Extractables were then determined by GPC
chromatography. The untreated raw material has a solubility of greater than 80% in this analysis.

Sample No 9.1 9.2 9.3 Extractables (%) 42 30 21 The pretreatment according to the invention is enough to bring about a distinctly reduced solubility for the precursor compared with the raw material used. The subsequent surface crosslinking further reduces the solubility.

Inventive Example 11 A: A pulverulent carboxymethylcellulose (Cekol 100 000, degree of neutralization 98.6%) was coextruded from a twin-screw extruder together with water at different pH values. The total throughput was 56 kg/hour and the carboxymethylcellulose fraction in the hydrogel was 20-25% by weight. The pH of the aqueous solvent was regulated by addition of sodium hydroxide. The extruder screws were equipped with additional kneading elements to improve the homogeneity of the hydrogel. The hydrogel formed was pressed through a breaker plate, the gel extrudates obtained were dried at 150 C and subsequently ground and screened off to 150-580 ~un. The screened pulverulent precursor product was analysed for its retention:

Sample pH of swelling TB
No medium [g/g]

11.1 7 42.9 11.2 8 41.8 11.3 9 41.0 11.4 10 41.1 B: A was repeated except that no kneading elements were used at a total throughput of 99 kg of hydrogel per hour. The homogeneity of the gel was visibly inferior than in A. Here and there the gel extrudates contained dry particles.
Sample pH of swelling TB
No medium [g/g]
11.5 7 43.1 11.6 8 45.2 11.7 9 42.7 11.8 10 41.6 C: B was repeated except that the total throughput was 97-102 kg of hydrogel per hour and the carboxy-methylcullose fraction in the hydrogel was varied between 20 and 45% by weight. The pH of the swelling medium was adjusted to 7.5 with sodium hydroxide in all cases:

Sample CMC fraction in TB
No hydrogel [wt%] [g/g]
11.9 23 44.5 11.10 29 41.7 11.11 35 43.1 11.12 40 45.2 11.13 45 42.4 50 g of each of the pulverulent precursor products of A, B and C were charged to a mixing reactor. The surface of the precursor products was coated with in each case 5 g of a 50% strength aluminium sulphate 14-hydrate solution with stirring. The coated products were each dried at 120 C for 20 minutes and analysed for their retention values and their absorbency against pressure:

Sample Precursor TB AAPo.3 AAPo.7 No No [g/g] [g/g] [g/g]
11.14 11.1 25.4 16.0 12.6 11.15 11.4 30.2 15.1 11.9 11.16 11.5 27.0 15.9 12.7 11.17 11.6 31.4 15.9 12.6 11.18 11.7 29.4 15.3 11.6 11.19 11.11 34.4 15.9 12.9 The results show that the absorbents according to the invention are obtainable on a large scale by continuous processes, the extrusion operation evidently not being subject to any limitations. Even comparatively less homogeneous gel extrudates, obtained for example by extrusion at high throughput and low water content without the use of kneading elements, do not lead to any impairment in the absorption properties of the absorbent resins according to the invention. Inventive Example 11, furthermore, demonstrates the conjoint influence of the pH and of the mixing technology on internal crosslinking. If, as in this case, the mixing operation used continuously supplies the dry raw material with the swelling medium and the swelling medium is not present in excess at any time, precursors which do not internally crosslink in the course of drying are obtained even at pH 7. It is believed that there is a relationship between the degree of solubilization of the raw material and the reactivity with regard to internal crosslinking. The preceding Inventive Examples 1 to 10 had the swelling medium introduced as an initial charge, ie the polysaccharides were present in greater dilution at the start of the swelling and were able to assume a more reactive preferred conformation. Raising the pH neutralized the free acid functionalities and hence controlled internal crosslinking.

Comparative Example 2 This example is carried out on the lines of US 5,470,964 or US 5,550,189. A reaction vessel was in each case charged with 200 g of an aqueous aluminium sulphate solution, the concentration of the aluminium sulphate x 14H2O being varied in the range from 0.25 to 0.75% by weight based on the solution. Each solution was admixed with 50 g of carboxymethylcellulose (Cekol 100 000, degree of neutralization 98.6%) with vigorous stirring. The solution very rapidly transformed into the gel state, so that homogeneous mixing was no longer possible towards the end of the addition period. The swollen hydrogel was dried at 80 C for 5 hours, coarsely comminuted and, after Retsch mill grinding, screened off to 150-850 pm. The screened particle size fractions gave the following characteristic data:
Sample [mg]A12 (S04) 3x14H20/ [A13+] */ TB AAPo.3 AAPo.7 No based on [g] CMC CMC [g/g] [g/g] [g/g]
2.1 comp 10 0.09 32.8 12.3 9.9 2.2 comp 15 0.14 27.9 13.5 10.7 2.3 comp 20 0.18 23.7 14.1 11.7 2.4 comp 40 0.36 16.3 14.4 12.4 2.5 comp 70 0.63 15.3 14.0 12.0 2.6# comp 15 0.14 12.3 12.5 10.2 2.6# comp: dried at 150 C for 30 minutes []*: in % by weight Comparative Example 2 shows that surface crosslinking with large amounts of solvent does not lead to the property profile of the absorbent resins according to the invention. Absorbency against a pressure (0.3 psi) values of > 14 g/g are obtainable only together with retention values of less than 25 g/g. Retention values >_ 25 g/g are achievable only at a low AA.Po,7 of less than 11 g/g. Higher drying temperatures lead in the case of Sample No 2.6 comp to substantial crosslinking and distinctly inferior absorption properties.
Absorbent No 9.2 according to the invention, which is based on the same raw material, by contrast, combines a retention value of 27.7 g/g with a high absorbency against pressure value (14.4 g/g in the case of [0.3 psi] and 12.5 g/g in the case of [0.7 psi]).

Comparative Example 3 The internal crosslinking of CMC under acidic pH
conditions is reproduced on the lines of EP 538 904 B1 or US 5,247,964. Carboxymethylcellulose (Cekol 100 000, degree of neutralization 98.6%) is coextruded as described in Inventive Example 11). The pH of the swelling medium used has been adjusted to pH 6 with sulphuric acid. The solids content of the hydrogel was 23-25% by weight while total throughput was varied. The swollen hydrogel was dried at 150 C for 60 minutes, in the course of which the polymer particles crosslinked internally owing to the acidic pH of the swelling medium. The dried polymer particles were ground, screened off to 150-850 um and analysed for their characteristic absorption data:

Sample Total hydrogel TB AAPo.3 AAPo.7 No throughput [g/g] [g/g] [g/g]
[kg/h]
3.1 comp 56 25.1 11.6 8.2 3.2 comp 99 28.2 14.3 8.9 This comparative example shows that precursors extruded at acidic pH give retention values of less than 30 g/g when dry owing to internal crosslinking. Unlike the absorbent resins according to the invention, the internally crosslinked products do not give an improved absorbency against pressure (AAPo,7).

Inventive Example 12 1 200 g of carboxymethylcellulose (Finnfix 50 000) were preswollen, dried, ground and screened to 150-850 lun, each step being carried out as described in Inventive Example 9). 50 g of each screened precursor was coated with a 50% strength solution of A12 (S04) 3 x 14H2O in DM water with stirring and dried at 120 C for 20 minutes. The surficially coated absorbents were analysed for their surface crosslinking index:

Sample g of 50% CSAP CF SCI
No Al2 (S04) 3 x 14H2O sln [Al3+] * [Al3+] *
50 g of precursor

12.1 4.00 0.36 0.78 42 12.2 5.00 0.45 0.90 45 12.3 7.00 0.64 1.20 56 []*: in % by weight Comparative Example 4 The absorbents prepared in Comparative Example 2 were analysed for their surface crosslinking index:

Sample CSAP CF SCI
No [Al3+] [Al3+]
4.1 comp 0.14 0.26 12 4.2 comp 0.18 0.36 18 4.3 comp 0.36 0.71 35 4.4 comp 0.64 0.99 35 []*: in % by weight Inventive Example 13 The absorbency against pressure performance of the pulverulent absorbent resin Sample No 11.17 (Inventive Example 11) was tested using the fluff-absorbent combination test against a synthetic superabsorbent (Z1030 product, synthetic pre- and postcrosslinked polyacrylic acid polymer having a degree of neutralization = 70%, AAPo.3 = 31.6 g/g and AAPo.7 =
24.4 g/g, from Stockhausen GmbH & Co. KG). The following characteristic data were determined from the absorption curve as a function of the concentration of superabsorbent (SAP):

FACT at 0.3 psi FACT at 0.7 psi Sample Parameter SAP conc. in pad SAP conc. in pad 10% 31% 50% 10% 31% 50%

AbSmax [g/g] 20.0 29.7 42.3 15.0 23.0 29.0 11.17 tmax [min] 16 27 84 16 25 47 t5oa [min] 1 3 9 1 2 7 Abs.x [g/g] 21.8 39.7 68.5 17.4 32.8 53.5 Z1030 tmax [min] 20 29 48 19 36 69 t50$ [min] 2 4 9 2 6 12 Inventive Example 13 shows that the absorbency of the absorbent resins according to the invention can be significantly proved by combination with a matrix material relative to a synthetic polyacrylate absorbent. Whereas Sample No 11.17 has less than 50% of the absorbency against a (0.3 and 0.7 psi) pressure value of a Z1030 product, this percentage increases to _ 55% in the case of 50% of SAP in the fluff-SAP mix and up to _ 86% in the case of 10% SAP in the fluff-SAP
mix.

Inventive Example 14 The surficially crosslinked pulverulent absorbent resin Sample No 11.15 of Inventive Example 11 was processed with different cellulose fluff quantities to form an airlaid composite article. A synthetic polyacrylate superabsorbent (Z 1030, Stockhausen) was processed into a composite article under identical conditions for comparison. The composites were characterized for their retention performance and their liquid absorbency against a pressure of 20 or 50 g/cm2:

Airlaid composites Fluff SAP Retention LAUL20 LAUL50 content (type) (0.3 psi) (0.7 psi) in com- Absolute Rela- Absolute Rela- Absolute Rela-posite [g/m2] tive* [g/m2] tive* [g/m2] tive*

11.15 6 821 86.6 7 784 65.0 6 337 67.0 50%

11.15 3 566 82.1 5 284 68.9 4 314 72.2 70%

11.15 1 219 86.7 3 210 81.7 2 568 80.2 90%

Characteristic data of pure pulverulent superabsorbent resins without SAP Retention AAPo.3 AAPo.7 fluff (type) Absolute Rela- Absolute Rela- Absolute Rela-[g/g] tive* [g/g] tive* [g/g] tive*

11.15 30.2 97.4 15.1 47.8 11.9 48.7 Z 1030 31.0 - 31.6 - 24.4 -*: Z 1030 product = 100 inventive Example 14 shows the performance improvement of absorbents according to the invention relative to synthetic superabsorbents in airlaid composites. The more homogeneous mixture of the matrix material with the absorbent is responsible for the fact that this relative performance improvement is even clearer than in Inventive Example 13 especially at high absorbent contents in the matrix.

Inventive Example 15 Ageing stability was characterized by storing the biodegradable superabsorbent resins for a prolonged period at room temperature and an average humidity of more than 50% and then measuring retention and absorbency against pressure.

Data after synthesis Data after storage Sample TB AAPo,3 AAPo.7 Age in TB AAPo3 3 AAPo.7 No [g/g] [g/g] [g/g] days [g/g] [g/g] [g/g]
2.3 19.4 20.9 16.8 513 19.2 19.4 15.4 2.4 20.4 21.8 17.2 384 20.5 22.0 15.8 8.1 28.2 20.5 16.0 222 32.6 19.0 14.1 9.2 27.7 14.4 12.5 225 25.5 14.1 11.4 Comparative Example 5 On the lines of the teaching of EP 538 904 or US 5,247,072, 1 980 g of DM water were charged to a make-up vessel and adjusted to pH 9 with NaOH. 20 g of carboxymethylcellulose (Blanose 7HOF, Aqualon) were added with stirring. The 2% solution was dried at 80 C
for 20 hours. The dried product was ground, screened off to a particle size of 150-850 pm and annealed at 150 C for a further 120 minutes. The absorption data of the product were determined directly after synthesis and also after storage at room temperature and a humidity of more than 50%:

Sample No 5.1 comp After synthesis: TB = 23.6 g/g AAPO.3 = 20.4 g/g After 10 days: TB = 23.9 g/g AAPO.3 = 15.1 g/g After 100 days: TB = 26.3 g/g AAPO.3 = 7.4 g/g After 200 days : TB = 29.2 g/g AAPO.3 = 7.2 g/g Comparative Example 5 shows that high AAPO.3 values are achievable directly after synthesis by drying and internal crosslinking in dilute CMC solutions. However, unlike the superabsorbents according to the invention, the products are not age stable and therefore are no longer usable as superabsorbents in commercial products after a short time.

Inventive Example 16 The particle size distribution of the pulverulent superabsorbents was determined before and after ball milling and to characterize the mechanical stability.
The data in the table which follows are based on the %
by weight content of the individual particle size fractions:

Sample Particle size fraction Particle size fraction No before ball milling after ball milling 150- 300- 600- 150- 300- 600- <150 pm 300 pm 600 pm 850 im 300 pm 600 pm 850 pm 8.1 12.8 53.6 33.6 16.0 55.6 27.4 1.0 11.15 19.6 53.7 26.7 21.6 51.9 25.7 0.8 11.17 23.3 52.2 24.5 23.5 52.6 22.5 1.3 11.19 11.0 61.6 27.4 24.6 50.7 23.4 1.3 This example demonstrates that the superabsorbents according to the invention are mechanically very robust and will have a similar particle size distribution and only very small <150 um fines fractions even after a mechanical stress of the kind occurring in product conveying for example. This ensures consistent product properties even after conveying and metering operations.
Comparative Example 6 Here Example 20 of US 4,043,952 is repeated using Blanose H carboxymethylcellulose. The CMC was reacted with 0.64 meq of Al cation per gram of CMC in methanolic suspension. The reaction product had the following characteristic data:
Retention: 28.9 g/g, AAP(O.3 psi): 9.2 g/g, AAP(0.7 psi):
7.4 g/g This comparative example shows that the CMC products of US 4,043,952, prepared in inert solvents and treated with polyvalent cations at the surface, have very low absorbency against pressure values.

Comparative Example 7 Example 1 of US 5,811,531 was repeated here by mixing xanthan gum with a small amount of an aqueous methanolic ethylene glycol diglycidyl ether solution and heating at 140 C. The product obtained has the following properties:
Retention: 29.3 g/g, AAP(0.3 psi): 7.9 g/g, AAP(0.7 psi):
5.7 g/g Again it is found that the process described in US 5,811,531 provides products having poor absorbency against pressure values.

The examples show that the polymers according to the invention combine a very high retention ability with a significantly improved ability to absorb water and aqueous fluids against an external pressure. They further combine good long term storage stability with good biodegradability under composting conditions. It has also been shown that only the process according to the invention, involving the preparation of a hydrogel followed by drying under conditions leading to hornification but not to internal crosslinking and subsequent surface crosslinking in minimal layer thickness, will provide the unique combination of high retention ability, high absorbency against an external pressure, stability in storage and biodegradability.
Inventive Examples 19) and 20) in particular further show that the polymers according to the invention, when used in an absorbent structure for the acquisition of body fluids through a combination with a matrix material such as for example cellulose fluff, develop a significantly higher absorbency for liquids even at high absorbent resin concentrations in the structure, especially against an external pressure, relative to a synthetic absorbent resin. Surface crosslinking a product which has not been preswollen does not lead to a comparable improvement in the absorbency against pressure (Comparative Examples 1, 6 and 7). Similarly, internal crosslinking starting from a hydrogel or dilute solution does not lead to the desired property profile (Comparative Examples 3 and 5). Surface cross-linking to a greater layer thickness falls far short of superabsorbents which are comparable to the products described (Comparative Example 2). On the contrary, the products show some absorbency against pressure only when retention is distinctly reduced. In addition, surface crosslinking of polymers to a greater layer thickness gives rise to appreciable problems with regard to the feasibility of the process (complete clumping of material and substantial inhomogeneities within the mixture).

Claims (47)

CLAIMS:

1. A pulverulent surface-postcrosslinked polymer capable of absorbing water, an aqueous fluid, a serous fluid and blood and obtained by aqueously preswelling at least one partially neutralized, uncrosslinked, carboxyl group-containing polysaccharide and subsequently drying the polycarboxypolysaccharide, wherein the dried polycarboxypolysaccharide is surface-postcrosslinked by means of a surface crosslinker and has an absorbency against pressure (AAP0.7) value of at least 12.5 g/g.

2. The polymer according to claim 1, wherein the polycarboxypolysaccharide is derived from starch, cellulose, polygalactomannan or a combination thereof.

3. The polymer according to claim 1 or 2, wherein the carboxyl groups of the polycarboxypolysaccharide are at least 80% neutralized.

4. The polymer according to any one of claims 1 to 3, wherein the carboxyl groups are attached to the polysaccharide at least partly in the form of carboxyalkyl groups.

5. The polymer according to any one of claims 1 to 4, wherein the polysaccharide has an average degree of carboxyl group substitution of 0.3 to 1.5.

6. The polymer according to any one of claims 1 to 5, wherein the uncrosslinked polycarboxypolysaccharide has a solution viscosity for a 1%
solution of more than 2 000 mPas.

7. The polymer according to any one of claims 1 to 6, further comprising a carboxylfree polysaccharide.

8. The polymer according to any one of claims 1 to 7, wherein the polycarboxypolysaccharide is preswollen in an aqueous phase containing (i) one or more water-soluble auxiliaries which are a base, a salt or a blowing agent, (ii) one or more antiblocking additives which are a natural fibre material, a synthetic fibre material, a silica gel, a synthetic silica or a water-insoluble mineral salt, or a combination of (i) and (ii).

9. The polymer according to claim 8, wherein the blowing agent used is a substance which releases a gas under the influence of a catalyst or heat.

10. The polymer according to claim 8 or 9, wherein the watersoluble auxiliaries and the antiblocking additives are each included in amounts of 0.01 to 20% by weight, based on the polycarboxypolysaccharide.

11. The polymer according to any one of claims 1 to 10, wherein an ionic crosslinker, a covalent crosslinker or a combination thereof is utilized for the surface postcrosslinking.

12. The polymer according to claim 11, wherein the ionic surface crosslinker is a salt of at least a divalent cation and the covalent surface crosslinker is an acid group-containing substance.

13. The polymer according to claim 12, wherein a polyvalent cation is formed from Mg2+, Ca2+, Al3+, Ti4+, Fe2+/Fe3+, Zn2+ or Zr4+, and the acid-functional substance is formed from a low molecular weight and polymeric polycarboxylic acid.

14. The polymer according to any one of claims 1 to 13, wherein the surface crosslinker is present in an amount of 0.01-25% by weight, based on the polycarboxypolysaccharide.

15. The polymer according to any one of claims 1 to 13, wherein the surface crosslinker is formed by a salt of an aluminium cation which is used in an amount of 0.2-1.0% by weight, based on the polycarboxypolysaccharide.

16. The polymer according to any one of claims 1 to 13, wherein the surface crosslinker is formed by citric acid used in an amount of 0.2-8% by weight, based on the polycarboxypolysaccharide.

17. The polymer according to any one of claims 11 to 13, wherein the covalent surface postcrosslinker is used in the presence of one or more crosslinking catalysts.

18. The polymer according to claim 17, wherein the crosslinking catalyst is an esterification catalyst which is a mineral acid, a Lewis acid, an acetylacetonate or a hypophosphite.

19. The polymer according to claim 17 and 18, wherein the ratio by weight of the surface postcrosslinker to the crosslinking catalyst is 1 : 0.001 - 1 : 1.

20. The polymer according to any one of claims 1 to 19, wherein the surface postcrosslinker is used in the presence of one or more water-soluble hydrophilic polymers.

21. The polymer according to claim 20, further comprising a hydrophilic polymer which is a polyalkyleneglycol or a polyvinylalcohol.

22. The polymer according to any one of claims 1 to 21, which has a retention of greater than or equal to 20 g/g.

23. The polymer according to claim 22, which has a retention of greater than or equal to 25 g/g.

24. The polymer according to any one of claims 1 to 21, which has an absorbency against pressure (AAP0.7) value not less than 80% of the initial value after ageing for 200 days under standard conditions.

25. The polymer according to any one of claims 1 to 21, which forms less than 5% by weight of fines having a particle size of below 150 µm after mechanical exposure due to roller milling for 6 minutes.

26. The polymer according to any one of claims 1 to 21, which has a surface crosslinking index (SCI) of greater than 40.

27. A process for preparing an absorbent polymer by crosslinking the surface of a polycarboxypolysaccharide with a surface crosslinker, comprising:

forming a hydrogel from an uncrosslinked polycarboxypolysaccharide with water or an aqueous phase; and mechanically comminuting and drying the hydrogel, wherein the dried hydrogel is comminuted and classified to form a polymer powder, and wherein particles of the polymer powder are coated with a solution of a crosslinker and subsequently subjected to a surface postcrosslinking.

28. The process according to claim 27, wherein mixing of polycarboxypolysaccharide and water is carried out in a continuous mixer.

29. The process according to claim 27, wherein mixing of polycarboxypolysaccharide and water is carried out in a batch mixer.

30. The process according to claim 28 or 29, wherein the mixture of polycarboxypolysaccharide and water has a pH of greater than or equal to 6.

31. The process according to claim 30, wherein the mixture of polycarboxypolysaccharide and water has a pH of greater than or equal to 10.

32. The process according to any one of claims 28 to 31, wherein the mixture of polycarboxypolysaccharide and water contains 5 to 65% by weight of polycarboxypolysaccharide.

33. The process according to any one of claims 28 to 32, wherein the mixture of polycarboxypolysaccharide and water further contains 0.01 to 20% by weight, based on the solids content, of one or more water-soluble auxiliaries which are a base, a salt or a blowing agent.

34. The process according to any one of claims 27 to 33, wherein the carboxyl groups of the polycarboxypolysaccharide are at least 80% neutralized.

35. The process according to any one of claims 28 to 34, wherein, in the mixture of polycarboxypolysaccharide and water, up to 30% by weight of the water is replaced by one or more water-miscible organic solvents which do not dissolve the polycarboxypolysaccharide.

36. The process according to any one of claims 28 to 35, wherein the mixture of polycarboxypolysaccharide and water further contains 0.01 to 20% by weight, based on the solids content, of one or more antiblocking additives.

37. The process according to any one of claims 27 to 36, wherein the drying of the hydrogel is effected at a temperature above 70°C.

38. The process according to any one of claims 27 to 37, wherein the hydrogel is dried to a moisture content of at most 30% by weight.

39. The process according to any one of claims 27 to 38, wherein 0.01-25% by weight of a covalent surface postcrosslinker, an ionic surface postcrosslinker or a combination thereof, based on the polymer powder, is added in the form of a 0.01-80% by weight aqueous solution.

40. The process according to claim 39, wherein the aqueous solution of the covalent surface postcrosslinker further contains a crosslinking catalyst.

41. The process according to claim 40, wherein the weight ratio of surface crosslinker to crosslinking catalyst is 1 : 0.001 - 1 :1.

42. The process according to any one of claims 27 to 41, wherein the surface postcrosslinking is carried out at a temperature of 40°C to 250°C.

43. A pulverulent polymer capable of absorbing water or an aqueous fluid, obtained by the process according to any one of claims 27 to 42.

44. A structure for absorbing a body fluid, comprising a polymer according to any one of claims 1 to 26 and 43.

45. Use of a polymer according to any one of claims 1 to 26 and 43, as an absorbent means for a liquid.

46. Use according to claim 45, in a structure for absorbing a body fluid, in a foamed or nonfoamed sheet material, in a packaging material, in a structure for plant cultivation or as a soil improver.

47. Use of the polymer according to any one of claims 1 to 26 and 43, as a carrier for an active component and the controlled release thereof.