EP1056789A1

EP1056789A1 - Cross-linked polymers capable of swelling

Info

Publication number: EP1056789A1
Application number: EP99934290A
Authority: EP
Inventors: Rüdiger Funk; Volker Frenz; Uwe Stüven; Fritz Engelhardt; Thomas Daniel
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1998-02-21
Filing date: 1999-02-18
Publication date: 2000-12-06
Also published as: CA2319786A1; AU3253199A; DE19807504A1; ID28466A; WO1999042496A1; CN1291203A; BR9908078A; JP2002504568A

Abstract

The invention relates to a method for surface cross-linking particles of superabsorbent polymers, characterized by the use of compounds of formula (1) as cross-linking agents, where R and R' independently of each other are hydrogen or C1-C6 alkyl.

Description

Cross-linked swellable polymers

description

The present invention relates to the use of diglycol silicates as crosslinking agents in the production of hydrogels containing carboxyl groups with improved properties, and to the use of the hydrogels thus produced as so-called superabsorbers for absorbing aqueous liquids.

Superabsorbers are water-insoluble, crosslinked, carboxyl group-containing polymers which are capable of swelling and forming hydrogels in aqueous liquids and body fluids, e.g. Take up urine or blood and keep the amount of fluid absorbed under a certain pressure. They are also called superabsorbent polymers or SAP.

The preparation and use of such polymers capable of hydrogel formation is described in numerous patents, such as e.g. EP-A-0 316 792, EP-A-0 400 283, EP-A-0 343 427, EP-A-0 205 674 and DE-A-44 18 818.

In order to produce polymers which have hydrogels with a particularly high liquid absorption capacity, high gel strength and high absorption capacity under pressure, it has proven to be necessary to subject the polymer particles to a subsequent surface treatment. The higher crosslinking near the surface of particles of superabsorbent polymer (SAP) is described, for example, in US-A-4,734,478 and US-A-4,666,983. Substances are preferably used which contain two or more groups which can form covalent bonds with the carboxyl groups of the hydrophilic polymers on the surface of the particle. This is taught, for example, by EP-A-0 349 240.

Polyglycidyl ethers, haloepoxy compounds, polyols, polyamines or polyisocyanates can be used as crosslinking agents. The previously known systems for surface post-crosslinking can be divided into four groups with different mechanisms of action:

1. reactive, ring-opening compounds;

2nd bifunctional, condensation active compounds; 3. cross-linking by polyvalent metal ions; and

4. reactive cross-linking through a dehydration reaction.

EP-A-0 317 106 teaches a typical example of crosslinking with a reactive ring-opening compound through the use of polyglycidyl ethers, such as e.g. Ethylene glycol diglycidyl ether and EP-A-0 618 005 with the use of polyazetidinium salts. According to DE-A-4 020 780, improved absorption under pressure is achieved by surface-crosslinking treatment of a polymer with 0.1 to 5% by weight alkylene carbonate.

US-A-4 666 983 teaches the use of diisocyanates and polyisocyanates as possible compounds for the post-crosslinking of SAP surfaces. The reaction with hydroxyl groups creates carbamate structures.

Another possibility of cross-linking consists in the formation of ionic instead of covalent cross-linking points. According to EP-A-0 372 981, polyvalent metal ions such as e.g. Aluminum is suitable for ionic crosslinking of the carboxyl functions.

WO-A-94/09043 describes the use of polyhydroxy compounds, in particular organic polyhydroxy compounds such as e.g. Diethylene glycol or trimethylolpropane at elevated temperature with ester formation.

EP-A-0 610 013 describes organic esters of carboxyl compounds of a polyfunctional organic alcohol, wherein an only partial esterification of the hydroxyl groups is also described. The esters can also be applied to the SA surface in a mixture with water or an organic solvent.

EP-A-0 644 207 describes the surface crosslinking of (by using a blowing agent) porous SAP with polyhydroxy compounds. However, the crosslinking reaction is completely analogous to WO-A-94/09043, the difference lies exclusively in the base polymer before the surface postcrosslinking.

EP-A-0 509 708 teaches the use of mixtures of a polyhydroxy compound with a surfactant, characterized by an HLB value between 3 and 10. The surfactant does not contribute to the reaction but rather facilitates the distribution of the polyhydroxy component on the surface . Crosslinking with certain polyfunctional organic alcohols is also disclosed by EP-A-0 450 924.

As already explained in EP-A-0 610 013, the methods according to items 1-3 have some disadvantages. The crosslinking agents selected from these groups show a very high reactivity and they react directly to the polymer to be crosslinked. This high reactivity is very disadvantageous for an even distribution on the surface. This disadvantage cannot be compensated for by an increased mixing time. The application properties are not optimal due to this lack of distribution on the surface.

Crosslinking only with ionic groups is very difficult, since in the course of use a diffusion of the polyvalent ions into the SAP particles begins, and instead of surface crosslinking a stronger gel crosslinking is achieved which has no advantageous application properties.

In contrast to this, the use of organic alcohols or their esters for dehydrating crosslinking according to item 4 does not lead to these disadvantages. Due to the reduced reactivity compared to the polyglycidyl ethers, an even distribution in a mixing time on the surface of the SAP is possible. This enables uniform surface post-crosslinking, whereby the depth of penetration into the SAP grain can also be controlled via the mixing time and the solution conditions.

However, the use of alkylene carbonates or poorly reactive polyols has the disadvantage that it is necessary to crosslink at very high temperatures in order to obtain sufficient reactivity. At such high temperatures, a thermal post-crosslinking of the entire polymers can be determined, the centrifuge retention being adversely affected. The high temperatures can also lead to an undesirable thermal decomposition of the product, which leads to discoloration of the product.

A method of crosslinking the surface of a superabsorbent polymer by a silane coupling reaction e.g. EP-A-0 195 406 teaches gamma-glycidoxypropyltrimethoxysilanes. Compounds of the general formula

X (R) _m Si Y ₃ _ _m where X is a group that can react with carboxyl or carboxylate groups, Y is a hydrolyzable group and m = 0, 1 or 2.

The primary binding takes place through the reactive group X, the hydrolyzable group being converted to the silanol group in the presence of water, which in turn can form a Si-O-Si bond in a further step by dehydration. However, no compounds are disclosed which consist only of hydrolyzable groups.

The object of the present invention is to provide a new method for surface crosslinking, by means of which hydrogels with improved properties with regard to gel strength and water retention are obtained. The crosslinkers should have a medium reactivity which allows the system used to be distributed over the surface without the disadvantage of crosslinking only at very high temperatures.

Surprisingly, it was found that superabsorbent polymers with the required properties are obtained when the surface is crosslinked with diglycol silicates.

The invention therefore relates to a process for the surface crosslinking of particles of superabsorbent polymers, characterized by the use of compounds of the formula 1 as crosslinking agents

H ₂ C "• o- CHR '

Si (i:

RHC 0 ' ^• 0' CH ₂

wherein R and R 'are independent of one another or identical and denote hydrogen or Ci-Cg-alkyl. In formula (1), R and R 'preferably have the same meaning and in particular each represent H or CH ₃ .

The invention furthermore relates to water-swellable hydrogels based on (co) polymerized, hydrophilic monomers or based on natural hydrophilic polymers, a mixture of these two polymers or a copolymer which contains carboxyl groups, characterized in that they contain a compound of Formula 1 were superficially networked. 5

The crosslinking reaction generally takes place by transesterification in the range of higher temperatures. The crosslinking reaction can be accelerated by a catalyst, which means that it can be carried out at a lower temperature. Temperatures of 120 to 220 ° C are preferred for the process according to the invention.

The crosslinking is preferably carried out by transesterification of the compounds of formula 1 with ring opening with the reactive carboxyl groups of the monomers or the (co) poly eren.

The compounds of formula 1 are preferably used in amounts of 0.05 to 10% by weight, based on the total monomer weight or the total polymer weight. They are preferably used in the form of aqueous, alcoholic or aqueous-alcoholic mixtures.

Suitable natural polymers which can be crosslinked with compounds of the formula 1 to give hydrogels according to the invention can be used both in unrefined and in refined form.

Carboxymethylpolysaccharides, such as carboxymethylhydroxypropyl guar, carboxymethyl starch and alginates, are particularly suitable.

Particularly suitable copolymerizable hydrophilic monomers are acrylic acid, methacrylic acid, crotonic acid, 2-acrylamido-2-methylpropanesulfonic acid and phosphonic acid, vinylphosphonic acid, vinylphosphonic acid half-esters, their salts, acrylamide, N-vinylamides, vinyl acetate or mixtures thereof. The use of acrylic acid and its salts, e.g. Na, K and / or ammonium acrylate.

The polymerization can be carried out by any known reaction, radical polymerization in a homogeneous phase, e.g. in aqueous solution, a so-called gel polymerization. The precipitation polymerization from organic solvents, such as from alcohols, preferably tert, offers further possibilities for the synthesis of the hydrogels according to the invention. -Butanol or hydrocarbons, such as hexane or cyclohexane, or suspension, dispersion, emulsion or microemulsion polymerization, but also ionic polymerization.

The radical polymerization can be triggered by radical formers such as organic or inorganic peroxides and azo compounds. Examples are benzoyl peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, (NH ₄ ) ₂ S 0 ₈ , KS 0s, 6

H ₂ S 0 ₈ , H0 or azodiisobutyronitrile. Redox systems are also outstandingly suitable as polymerization initiators.

The polymerization can also be triggered by high-energy radiation.

The compounds of formula 1 can be used for crosslinking during the polymerization reaction and / or only after the polymerization reaction for subsequent crosslinking of the polymer. If the compound of formula 1 is first added to an uncrosslinked pr (co) polymer, this is usually done before drying by homogeneous mixing, for example by kneading an aqueous polymer gel in a kneader. Spraying from dilute solution onto a polymer powder or a polymer granulate is also possible. In this case, a suitable crosslinking of the polymers in the vicinity of the particle surface can be carried out by a suitable choice of the solvent system.

The hydrogels according to the invention are outstandingly suitable as absorbents for aqueous liquids, for the formulation of cosmetic preparations, as consolidators and / or binders of fibrous sheet-like structures containing reactive groups, as polymeric additives for aqueous drilling fluids and cement additives in petroleum production, and for use in hygiene articles , Packaging materials and nonwovens.

For use as so-called "super absorbent polymers" (SAP) for use in hygiene articles such as diapers, tampons or sanitary napkins, hydrogels according to the invention based on acrylic acid are particularly suitable, some of which may be present as alkali or ammonium salts. The neutralization can take place both before and after the polymerization. The hydrophilic monomers are polymerized to produce SAP, preferably in the presence of conventional crosslinkers containing at least two ethylenically unsaturated double bonds, e.g. N, N '-methylene bisacrylamide, triallylamine, 1,4-butanediol diacrylate, pentaerythritol trialkyl ether and / or tetraalkyl oxyethane.

Hydrogels according to the invention based on polysaccharides containing carboxyl groups are also outstandingly suitable for use as SAP.

By post-crosslinking, in particular on the surface with compounds of formula 1 of already pre-crosslinked polymers which are intended for use as SAP, their effectiveness can be speed with regard to absorption under pressure can be significantly improved.

The addition of acidic catalysts can accelerate the crosslinking reaction and / or lower the reaction temperature.

Compounds which are suitable as catalysts in the process according to the invention are inorganic oxo acids, their acidic salts and anhydrides, esters or partial esters, such as, for example, phosphoric acid, sulfuric acid, boric acid, polyphosphates, polyborates, polyphosphonates, phosphonic acid anhydrides, and also hydrogen sulfates, hydrogen phosphates . Also suitable are acids of the general formula HX where X can be a halogen, and all known Lewis acids, such as, for example, BF etherate or sulfonic acids, in particular organic sulfonic acid, such as, for example, p-toluenesulfonic acid. Hydrogen phosphates, boric acid and their anhydrides, esters or partial esters, and p-toluenesulfonic acid and BF ₃ etherate are particularly suitable. Preferably 0.05 to 3% by weight of catalyst, based on the mass of the polymer, is added.

Test methods

Certain test methods are used to characterize superabsorbent polymers, including the measurement of the extractable fractions after n hours and the absorption of 0.9% by weight NaCl solution with and without weight loading.

1. Extractable fractions, n hours (n = 1-16)

1 g SAP is dissolved in 200 ml 0.9% by weight NaCl solution and stirred with a magnetic stirrer for n (n = 1-16) hours. The supernatant solution is filtered and the polyacrylic acid or the polyacrylate is titrated. The pH electrode for the titration is calibrated at pH 2, 7 and 10. It is first titrated to a pH of 10 with approximately 0.1 N NaOH. The amount of NaOH consumed is called Va (in ml), the normality of the base is Na (n meq / ml). The solution titrated in this way is then adjusted to pH 2.7 with approximately 0.1 N HCl. The amount of HC1 consumed is called Vb, the normality of the acid is Nb (in meq / ml).

The same procedure is repeated with 0.9% by weight NaCl solution in order to determine the blank value of the titration. Vab denotes the amount of base required, Vbb the amount of acid required to titrate the pure salt solution.

The extractable parts are calculated according to: 8th

E = ([Wa + Wb] + 100) / W

where W represents the total mass of the superabsorbent sample and Wa = (Va-Vab) • Na • Ea • D Wb = (Mb • Eb • D) Mb = Mt-Ma

Mt = (Vb-Vbb) • Nb is and

Ea the equivalent mass of an unneutralized monomer in the polymer chain (72 daltons) Eb the equivalent mass of a neutralized monomer in the polymer chain (94 daltons) D = dilution factor = 4.

2. Centrifuge retention (CRC)

With this method, the free water absorption of the SAP is measured using the teabag method. Approx. 0.2 g SAP are welded into a tea bag and immersed in a 0.9% by weight NaCl solution for 20 minutes. The tea bag is then spun in a centrifuge for 5 minutes with a centrifugal force of 250 -G. A tea bag without SAP determines the blank value of the measurement:

Weight - blank value - weight retention CRC (g / g)

Weight

3. Absorption under pressure (20, 40, 60 g / cm ² )

When absorbed under pressure, 0.9 g SAP are evenly distributed on the bottom of a plexiglass cylinder. The cylinder has a height of 50 mm and an inner diameter of 60 mm. On the bottom of the cylinder is a wire mesh with a mesh size of 36 μm (400 mesh). A cover plate is placed over the super absorber and the sample is loaded with an appropriate weight (20, 40, 60 g / cm ² ). The superabsorbent is then allowed to swell under the appropriate pressure for 60 minutes.

The AUL (Absorbency under Load) under a pressure load is calculated according to:

AUL (g / g) = [(Wb-Wa) / Ws]

where Wa is the mass of the apparatus, Wb is the mass of the apparatus after water has been absorbed by the SAP, and Ws is the mass of the SAP. 9

4th Gel strength

The shear modulus is measured using a Carri-Med controlled stress rheometer. A 6 cm plate-plate geometry is used for the measurement. To determine the shear modulus, 2.5 g of SAP in 100 g of 0.9% by weight NaCl are allowed to swell for 24 hours and the shear modulus is then measured as a function of the frequency. The value at 10 Hz is given as a memory module.

Examples 1 and 2 describe the use of the compound of the formula 1, in which R = R '= H or CH ₃ , as a crosslinking agent during the polymerization reaction.

example 1

4780 g of demineralized water are placed in a polyethylene vessel with a capacity of 10 l, which is well insulated by foamed plastic material, 1696 g of sodium bicarbonate are suspended in it and 1994 g of acrylic acid are slowly metered in so that the reaction solution is not over-foamed, the temperature of the reaction solution rising to approx Cools 5-3 ° C. There are now 6 g of compound 2

H ₂ C "0 CH ₂

Si (2)

H ₂ C- Ό- CH ₂

and 10 a sodium Diisooctylsulfosuccinates g (Rewopol ^® V 2133 from Rewo, Steinau) and 2.5 g of N, N-methylenebisacrylamide was added. The initiators, a redox system consisting of 2.2 g of 2, 2'-azobisamidinopropane dihydrochloride, dissolved in 20 g of demineralized water, 4 g of potassium peroxodisulfate, dissolved in 150 g of demineralized water and 0 , 4 g of ascorbic acid, dissolved in 20 g of demineralized water, added in succession and stirred. The reaction solution is then left to stand without stirring, a solid gel being formed by the onset of polymerization, in the course of which the temperature rises to approximately 89 ° C. This is then crushed mechanically, dried at temperatures above 150 ° C and ground.

The product produced in this way was worked into a baby diaper in a conventional manner and was distinguished here by particularly good fluid retention. 10

Example 2

The procedure is analogous to Example 1, except that 6.0 g of compound 3 (= compound 1, where R = R '= CH ₃ ) are now used. 5 The resulting product is also ideal for use in baby diapers and is characterized by good fluid retention and distribution.

Example 3

10

Under adiabatic conditions, 1287 g of demineralized water cooled to 15 ° C. are placed in a 1.5 1 cylindrical wide-necked reaction flask and 255 g of acrylic acid and 1.28 g of tetra-allyloxyethane are dissolved therein. There will be nitrogen in the monomer

15 solution initiated (approx. 2 1 / min. For approx. 20 min.) In order to lower the oxygen content. At a content of 1.5 ppm 0 ₂ , 7.7 g of a 10% aqueous solution of 2, 2 '-azobis (2-amidino-propane) dihydrochloride are added. After further N introduction and at a 0 content of 1.3 ppm, 2.6 g of a%

20 H0 solution added and finally 6.4 g of a 0.1% ascorbic acid solution are added at a 0 ₂ ^_ content of 1.0 ppm. The onset of polymerization, in the course of which the temperature rises to approx. 65 ° C, results in a solid gel which is then mechanically comminuted. 400 g of the crushed

25 gels are mixed with 56.5 g of sodium hydroxide solution (50%) (degree of neutralization of acrylic acid 74 mol%), kneaded twice, mixed with 25 g of a 1% solution of compound 2, again kneaded twice, then at temperatures above 150 ° C dried, ground and sieved in a thin layer.

30

A product is obtained, essentially characterized by the following physical data, all measured with NaCl 0.9%:

Extractable proportions (1 h value) 2.1%, 35 absorption under pressure (20 g / cm ² ) = AUL 20 = 29.9 g / g.

Example 4a

Under adiabatic conditions, 2574 g of demineralized water cooled to 15 ° C. are placed in a 1.5 1 cylindrical wide-neck reaction flask and 510 g of acrylic acid and 1.52 g of triallylamine are dissolved therein. Nitrogen is introduced into the monomer solution (approx. 2 l / min. For approx. 20 min.) In order to reduce the oxygen content. At a content of 1.5 ppm 0, 7.7 g of a 45 10% strength aqueous solution of 2,2'-azobis (2-amidinopropane) dihydrochloride are added, after further N introduction and a 0 content of 1. 3 ppm, 2.6 g of a 1% H0 ₂ solution are added 11 and finally at a 0 content of 1.0 ppm 6.4 g of a 0.1% ascorbic acid solution are added. The onset of polymerization, in the course of which the temperature rises to approx. 65 ° C, results in a solid gel that is then mechanically comminuted. After drying this gel, the comparative product of Table I (Gel 4a) is obtained.

Example 4 b

To 300 g of a crushed, to 73 mol% neutralized

30% polymer gel from Example 4a, 0.26 g of a compound 1 in which R = R '= CH ₃ , dissolved in 40 ml of methanol, is added, homogeneously kneaded, comminuted, at 180 ° C. to a residual moisture of 3% dried, ground and sieved. The gel obtained in this way is referred to as example 4b after surface postcrosslinking.

The following examples show the effect of surface postcrosslinking according to the invention for a particulate system with carboxyl or carboxylate groups.

The increase in absorption under pressure loading is documented in Table I.

Example 5

0.51 g of compound 3 (= compound 1 with R = R '= CH ₃ ), dissolved in 40 ml of methanol, are added homogeneously to 300 g of a comminuted, 73 mol% neutralized 30% polymer gel from example 4a kneaded, crushed, dried at 180 ° C to a residual moisture of 3%, ground and sieved.

Example 6

0.39 g of compound 2, dissolved in 40 ml of methanol, are added to 300 g of a comminuted 30 molar polymer gel, neutralized to 73 mol%, from Example 4a, homogeneously kneaded, comminuted, at 180 ° C. to a residual moisture of 3 % dried, ground and sieved.

Example 7

0.76 g of compound 2, dissolved in 40 ml of methanol, are added to 300 g of a comminuted 30% strength polymer gel, neutralized to 73 mol%, from Example 4a, homogeneously kneaded, 12 shredded, dried at 180 ° C to a residual moisture of 3%, ground and sieved.

Example 8

To 300 g of a crushed to 73 mol% neutralized 30% polymer gel from Example 4a, 1.2 g of compound 2, dissolved in 40 ml of methanol, are added, homogeneously kneaded, crushed, at 180 ° C. to a residual moisture of 3 % dried, ground and sieved.

The products obtained in Examples 4 to 8 are characterized by the following data summarized in Table I:

Table I

Absorption gel strength * '

Extractable parts under pressure (20 g / cm ² )

16 h value

(%)

⁽ g / g ⁾ (Pa)

Starting polymer untreated, produced according to 9.8 18.4 510 Example 4a (comparative example)

Examples according to the invention:

4b 5.8 26.1 608

5 4.6 31.9 3210

6 4.3 33.1 3100

7 3.1 32.0 3500

8 3.7 34.6 3680

Example 9

Commercially available, partially neutralized, crosslinked polyacrylic acid heated to 45 ° C. for use as a superabsorbent in baby diapers, 0.2% by weight of compound 2 (= general formula 1 with R = R '= H), as a 10% solution in sprayed on a PETTERSON & KELLY mixer and mixed for 20 minutes at 185 ° C. After cooling to room temperature, the following values were found in Table II and improved compared to the starting product: 13

Table II

Extraction-absorption CRC binding 2 parts under pressure (g / g)

16 h values (60 g / cm ² )

% By weight (%) (g / g) 7.1 14.3 45 starting material used

Example 9 0.2 3.5 26.2 30

Example 10

100 g high molecular weight, uncrosslinked polyacrylic acid (molecular weight

Mw> 1 million) with a degree of neutralization of 53 mol%, in the form of a comminuted gel, prepared analogously to Example 4, but without a crosslinker, 120 g of carboxymethylguar and 100 g of 0.15% solution of compound 2 (formula 1 with R and R '= H), kneaded homogeneously, crushed, dried in an air stream at 180 ° C. for 15 minutes, ground and sieved. A water-swellable product with a water absorption capacity several times its own weight is obtained.

Examples 11 to 15 describe the preparation of water-swellable products with good absorbency by crosslinking polymers of various origins with compounds according to the invention, which is done in such a way that mixtures of polyacrylates and carboxymethylpolysaccharides or alginates are pasted in water with compounds of the formula 1 mixed, kneaded homogeneously, dried in an air stream at 180 ° C., ground and sieved.

Example 11

50 parts by weight Carboxymethyl cellulose, 50 parts by weight. Cross-linked polyacrylate, produced according to Example 4a, 0.6 parts by weight. a compound of formula 1 with R and R '= CH ₃ .

Example 12

30 parts by weight Carboxymethyl cellulose, 70 parts by weight. cross-linked polyacrylate, produced according to Example 4a, 0.35 part by weight. a compound of formula 1 with R and R '= H. 14

Example 13

45 parts by weight Carboxymethyl starch, 55 parts by weight. cross-linked polyacrylate, produced according to Example 4a, 0.56 part by weight. a compound of formula 1 in which R, R '= H.

Example 14

65 parts by weight of alginate, 45 parts by weight. Cross-linked polyacrylate, produced according to Example 4a, 1.8 parts by weight of a compound of the formula 1 in which R, R 'are CH ₃ .

Example 15

15 20 parts by weight Carboxymethyl cellulose, 20 parts by weight Carboxymethl - starch, 60 parts by weight cross-linked polyacrylate prepared according to Example 4a, 1.4 parts by weight a compound of formula 1 in which R = R '= H.

20 Examples 16-27

Commercially available, partially neutralized, crosslinked polyacrylic acid preheated to 30 ° C to 60 ° C for use as a superabsorbent in hygiene products, compounds of the formula 1 are added as 10% strength 25 aqueous or aqueous alcoholic solutions in a mixing unit and at 130 ° C - 200 ° C treated. In Examples 22 to 26, mixtures of the compounds of formula 1 were used.

30 After cooling to room temperature, the following values were found in Table III and improved compared to the starting product.

35

40

45 Table III above

Compound 1% by weight Extractable absorption under CRC ON proportions 16 h value pressure (60 g / cm ² )

R, R '= HR, R' = CH ₃ (%) (g / g) g / g) used - - 7.8 13.6 49 starting material

Example 16 0.3 3.2 27.1 29

Example 17 0.2 3.1 27.6 30

Example 18 0.25 3.0 27.6 28

Example 19 0.3 3.0 28.9 26 µl

Example 20 0.6 2.1 29.3 24

Example 21 1.5 2.0 20.0 18

Example 22 0.1 0.1 3.6 26.7 33

Example 23 0.2 0.2 2.8 27.5 30

Example 24 0.3 0.4 2.5 30.3 29

Example 25 0.5 0.5 2.1 29.9 28

Example 26 1.0 1.0 2.2 33.1 25

Example 27 0.25 3.5 26.2 32 o like this

©

WO 99/42496 -, _c PCT / EP99 / 01065 o

Examples 28-32

On commercially available partially neutralized, crosslinked polyacrylic acid for use as superabsorbents, compounds of the general formula 1 are added as aqueous or aqueous-alcoholic mixtures with the addition of 0.1 to 0.6% by weight, based on polymer, of acidic catalysts and at 120 ° C treated to 180 ° C. After cooling to room temperature, the following values, which are listed in Table IV, were found compared to the starting product.

Table IV

Compound 1 wt .-% catalyst reaction absorption CRC te p. vacuum

(60 g / cm ² )

R, R '= HR, R' = CH ₃ (% by weight) (° C) ⁽ g / g ⁾ ⁽ g / g ⁾ - - 13.6 49 starting material used

Example 28 0.2 NaH ₂ P0 ₄ 0.5 160 26.8 30

Example 29 0.6 H ₃ P0 ₄ 0.1 140 28.4 26

Example 30 0.3 H ₃ B0 ₃ 0, 6 165 27.1 28

Example 31 0.8 p-toluene 140 28.6 25.5 sulfonic acid 0.2 -J

Example 32 0.2 0.2 BF ₃ etherate 0.3 130 26.0 32

* 0 or

H

O

18th

The compounds mentioned in the description and in the claims

HC 0 \ ^ -0 CHR '

RH I ^si I (1) C 0 0 CH ₂

also include the stereoisomers, for example those of the formula

H ₂ C - 0 ^■ 0- CH ₂

Si

RHC 0 0 CHR '

Claims

19 patent claims

1. Process for the surface crosslinking of particles of superabsorbent polymers, characterized by the use of compounds of the formula 1 as crosslinking agents

H ₂ C 0 \ ^ -0 CHR '

I Si I (1)

RHC 0 '0 CH ₂

wherein R and R 'are independent of one another or the same and are hydrogen or -CC ₆ alkyl.

2. The method according to claim 1, characterized in that the superabsorbent polymer is a polyacrylate, a carboxymethyl polysaccharide or a mixture of polyacrylates and carboxymethyl polysaccharides.

3. The method according to claim 1 and / or 2, characterized in that the crosslinking takes place on the surface of the polymer particles.

4. The method according to one or more of claims 1 to 3, characterized in that the crosslinking is carried out in the temperature range from 120 ° C - 220 ° C.

5. The method according to one or more of claims 1 to 4, characterized in that the crosslinking is carried out with the addition of acidic catalysts.

6. The method according to claim 5, characterized in that the amount of catalyst is 0.05 to 3.0 wt.% Based on the polymer.

7. The method according to one or more of claims 1 to 6, characterized in that the compounds of formula 1 are applied in the form of aqueous mixtures.

8. The method according to one or more of claims 1 to 7, characterized in that the compounds of formula 1 are applied as aqueous-alcoholic mixtures.

9. The method according to one or more of claims 1 to 7, characterized in that polyacrylates are crosslinked in solid form as powder or granules.

10. Surface-crosslinked, superabsorbent polymers, produced by the process according to one or more of claims 1 to 9.

11. Use of the surface-crosslinked, superabsorbent polymers according to claim 10 in hygiene articles, packaging materials and nonwovens.