DE19807504A1 - Surface crosslinking of superabsorbing polymer particles used in hygiene articles, packaging materials and nonwovens - Google Patents

Abstract

1,4,6,9-Tetraoxa-5-silaspiro(4.4)nonanes (diglycol silicates) (I) are used as crosslinking agents for surface crosslinking of particles of superabsorbing polymer. 1,4,6,9-Tetraoxa-5-silaspiro(4.4)nonanes (diglycol silicates) of formula (I) are used as crosslinking agents for surface crosslinking of particles of superabsorbing polymer: R, R' = hydrogen (H) or 1-6 carbon (C) alkyl. An Independent claim is also included for surface-crosslinked superabsorbing polymer produced in this way.

Description

The present invention relates to the use of diglycol silicates as Crosslinker in the production of hydrogels containing carboxyl groups improved properties, and the use of the hydrogels thus produced as a so-called super absorber for the absorption of aqueous liquids.

Superabsorbers are water-insoluble, cross-linked, carboxyl-containing Polymers that are capable of swelling and formation of hydrogels aqueous fluids and body fluids such as B. to take urine or blood and the amounts of liquid absorbed back under a certain pressure hold. They are also called superabsorbent polymers or SAP.

The production and use of such capable of hydrogel formation Polymer is described in numerous patents, such as. B. EP-A-0 316 792, EP-A-0 400283, EP-A-0 343427, EP-A-0 205674 and DE-A-44 18 818.

To produce polymers that have particularly high hydrogels Liquid absorption capacity, high gel strength and high absorption capacity possess under pressure, it has been found necessary for the polymer particles to be subjected to a subsequent surface treatment. The higher one Networking near the surface of the SAP particles is for example in U.S. 4,734,478 and U.S. 4,666,983 (Nippon Shokubai). In doing so preferably substances used which contain two or more groups, with the carboxyl groups of the hydrophilic polymers on the surface of the Particles can form covalent bonds. This teaches for example 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;
• 2. bifunctional, condensation active compounds;
• 3. cross-linking by polyvalent metal ions; and
• 4. reactive cross-linking through a dehydration reaction.

A typical example of networking with a reactive, ring-opening Compound teaches EP-A-0 317 106 through the use of polyglycidyl ethers such as e.g. B. ethylene glycol diglycidyl ether and EP-A-0 618 005 with the use of Polyazetidinium salts. According to DE-A-40 20 780 an improved absorption under pressure by surface-crosslinking treatment of a polymer with 0.1 to 5 wt .-% alkylene carbonate reached.

US 4,666,983 teaches the use of diisocyanates and polyisocyanates as possible connections for post-networking of SAP interfaces. Through the Reaction with hydroxyl groups creates carbamate structures.

Another possibility of networking is the training of ionic Place of covalent cross-linking points. According to EP-A-0 372 981 polyvalent metal ions such as B. aluminum for ionic crosslinking Carboxyl functions suitable.

WO-94/09043 describes the use of polyhydroxy compounds, especially organic polyhydroxy compounds such. B. diethylene glycol or Trimethylolpropane at elevated temperature with ester formation.

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

EP-A-0 644 207 describes the surface crosslinking of (by use a blowing agent) porous SAP with polyhydroxy compounds. The Crosslinking reaction is, however, completely analogous to WO-A-94/09043, the The only difference is in the base polymer before Surface post-crosslinking.

EP-A-0 509 708 teaches the use of mixtures of one Polyhydroxy compound with a surfactant, characterized by an HLB value between 3 and 10. The surfactant does not contribute to the reaction but makes it easier the distribution of the polyhydroxy component on the surface.

Crosslinking with certain polyfunctional organic alcohols also disclosed by EP-A-0 450 924.

As already stated in EP-A-0 610 013, the methods according to the Points 1-3 some disadvantages. The selected from these groups Crosslinking agents show a very high reactivity and they react in the process immediately when applied 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 due to this lack Distribution on the surface is not optimal.

Networking only with ionic groups is very difficult since in the course of Diffusion of the polyvalent ions into the SAP particles begins and application thus achieving a strong gel crosslinking instead of a surface crosslinking that has no advantageous application properties.

In contrast, the use of organic alcohols or their Esters for dehydrating crosslinking according to item 4 do not have these disadvantages. Due to the reduced reactivity compared to the polyglycidyl ethers, a Even distribution in a mixing time on the surface of the SAP possible. This enables a uniform surface post-crosslinking, whereby  about the mixing time and the solution conditions also the depth of penetration into the Can control SAP grain.

The use of alkylene carbonates or low reactivity polyols however, the disadvantage that one has to crosslink at very high temperatures in order to to obtain sufficient reactivity. At such high temperatures you can determine a thermal post-crosslinking of the entire polymer, the Centrifuge retention is adversely affected. The high temperatures can continue to lead to undesired 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. B. with gamma-glycidoxypropyltrimethoxysilanes teaches EP-A-0 195 406. Compounds of the general formula

X (R) _m Si Y _3-m

where X is a group having carboxyl or carboxylate groups can react, Y represents a hydrolyzable group and m = 0.1 or 2.

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

The object of the present invention is to develop a new method for Provide surface cross-linking through which hydrogels with improved Preserve properties regarding gel strength and water retention become. The crosslinkers should have a medium reactivity, the one Distribution of the system used on the surface allowed without the disadvantage to have to crosslink only at very high temperatures.  

Surprisingly, it was found that in the case of surface crosslinking with Diglycol silicates superabsorbent polymers with the required properties be preserved.

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

wherein R and R 'independently of one another are hydrogen or C ₁ -C ₆ alkyl.

Another object of the invention are water-swellable hydrogels based (co) polymerized, hydrophilic monomers or based on natural hydrophilic Polymers, a mixture of these two polymers or a copolymer in which Carboxyl groups are included, characterized in that they have a Compound of formula 1 were crosslinked on the surface.

The crosslinking reaction is generally carried out by transesterification in the area higher temperatures. The crosslinking reaction can be carried out by a catalyst accelerated, which means that they are carried out at a lower temperature can. Temperatures of 120 to 220 ° C are for the inventive method prefers.

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) polymer.

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

Suitable hydrogels according to the invention with compounds of formula 1 crosslinkable natural polymers can be used both in unprocessed and in refined form can be used.

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

In particular, acrylic acid, as copolymerizable hydrophilic monomers, Methacrylic acid, crotonic acid, 2-acrylamido-2-methylpropanesulfonic acid and -phosphonic acid, vinylphosphonic acid, vinylphosphonic acid half-esters, their salts, Acrylamide, N-vinyl amides, vinyl acetate or mixtures thereof in question. Especially acrylic acid and its salts are preferred.

The polymerization can be carried out by any known reaction radical polymerization in a homogeneous phase, e.g. B. in aqueous Solution, a so-called gel polymerization. Other ways to synthesize the Hydrogels according to the invention offer precipitation polymerization from organic Solvents, such as from alcohols, preferably tert-butanol or Hydrocarbons such as hexane or cyclohexane or the suspension, Dispersion, emulsion or microemulsion polymerization, but also that 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 ₂ O ₈ , K ₂ S _{2 8} , H ₂ S ₂ O ₈ , H ₂ O ₂ 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 are used and / or only after Polymerization reaction for subsequent crosslinking of the polymer. Will the Compound of formula 1 only added to an uncrosslinked pre (co) polymer, see above this usually happens before drying by homogeneous mixing, for example by kneading an aqueous polymer gel in a kneader. Also Spray from dilute solution onto a polymer powder or a polymer granulate is possible. In this case, by appropriate choice of the solvent system targeted crosslinking of the polymers in the vicinity of the particle surface be performed.

The hydrogels according to the invention are outstandingly suitable as Absorbents for aqueous liquids, for the formulation of cosmetic Preparation, as a solidifier and / or binder of reactive groups, fibrous sheets, as polymeric additives for aqueous drilling fluids and Cement additives in oil production, as well as for use in hygiene articles, Packaging materials and nonwovens.

For use as so-called "Super Absorbing Polymers" (SAP) for Use in hygiene articles such as diapers, tampons or Sanitary towels are particularly suitable based hydrogels according to the invention of acrylic acid, some of which are present as alkali or ammonium salts can. The neutralization can take place both before and after the polymerization respectively.

Also hydrogels according to the invention based on carboxyl groups Polysaccharides are ideal for use as SAP.

By post-crosslinking, especially on the surface with compounds of Formula 1 of pre-crosslinked polymers for use as SAP are provided, their effectiveness with regard to absorption under pressure be significantly improved.  

The crosslinking reaction can be accelerated by adding acidic catalysts and / or the reaction temperature can be reduced.

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. B. BF etherate or sulfonic acids, especially organic sulfonic acid such. B. 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

For the characterization of superabsorbent polymers certain are Test methods used, this includes measuring the extractable fractions according to n Hours and the absorption of 0.9% by weight NaCl solution with and without Weight load.

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

1 g SAP is dissolved in 200 ml 0.9% by weight NaCl solution and with a Magnetic stirrer stirred 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 initially approx. 0.1 N NaOH titrated to pH 10. The amount of NaOH consumed is called as Va (in ml), the normality of the base with 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 consumed HCl is called Vb, the normality of the acid with Nb (in meq / ml).  

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

The extractable parts are calculated according to:

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 centrifuged at 250 g for 5 minutes. A tea bag without SAP determines the blank value of the measurement:

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

When adsorbed 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 equipment,
Wb the mass of the equipment after water absorption by the SAP, and
Ws is the bulk of the SAP.

4. Gel strength

The shear module is controlled using a Carri-Med controlled stress rheometer measured. A 6 cm plate-plate geometry is used for the measurement. For Determination of the shear modulus is allowed for 2.5 g of SAP in 100 g of 0.9% by weight NaCl for Swell for 24 hours and then measure the shear module depending on the Frequency. The value at 10 Hz is given as a memory module.

In Examples 1 and 2 the use of the compound with formula 1 is considered Crosslinkers described during the polymerization reaction.

example 1

In a polyethylene vessel with a capacity of 10 l, well insulated by foamed plastic material, 4780 g of deionized water are placed in it, 1696 g of sodium bicarbonate are suspended therein 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 cools down approx. 5-3 ° C. There are now 6 g of compound 2

and 10 g of a sodium diisooctyl sulfosuccinate (®Rewopol V 2133 from the company REWO, Steinau), and 2.5 g of methylene bisacrylamide were added. At a The temperature of 4 ° C is the initiators, a redox system consisting of 2.2 g 2,2'-azobisamidinopropane dihydrochloride, dissolved in 20 g of deionized water, 4 g Potassium peroxodisulfate, dissolved in 150 g of demineralized water and 0.4 g of ascorbic acid added in 20 g of demineralized water and stirred. The reaction solution is then left to stand without stirring, with the onset of polymerization, in the course of which increases the temperature to approx. 89 ° C, a solid gel is formed. This is then mechanically crushed at temperatures above 150 ° C dried and ground.

The product so produced was conventionally placed in a baby diaper incorporated and distinguished itself by a particularly good one Fluid retention.

Example 2

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

Example 3

Under adiabatic conditions, 1287 g of deionized water cooled to 15 ° C. are placed in a 1.5 l cylindrical wide-necked reaction flask and 255 g of acrylic acid and 1.28 g of tetraallyloxyethane 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 O ₂ , 7.7 g of a 10% strength aqueous solution of 2,2'-azobis (2-amidinopropane) dihydrochloride are added. After further introduction of N ₂ and with an O ₂ content of 1.3 ppm, 2.6 g of a 1% H ₂ O ₂ solution are added and finally with an O ₂ 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 which is then mechanically comminuted. 400 g of the comminuted gel are mixed with 56.5 g of sodium hydroxide solution (50% strength) (degree of neutralization of the acrylic acid 74 mol%), kneaded twice, mixed with 25 g of a 1% strength solution of compound 2, again kneaded twice, then at Temperatures above 150 ° C dried, ground and sieved in a thin layer.

A product is obtained, essentially characterized by the following physical data, all measured with NaCl 0.9%:
Extractable portions (1 h value) 2.1%, A
Absorption under pressure (20 g / cm ² ) = AUL 20 = 29.9 g / g.

Example 4

Under adiabatic conditions, 2574 g of deionized water cooled to 15 ° C. are placed in a 1.5 l 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 O ₂ , 7.7 g of a 10% strength aqueous solution of 2,2'-azobis (2-amidinopropane) dihydrochloride are added, after further N ₂ introduction and an O ₂ content of 1.3 ppm, 2.6 g of a 1% H ₂ O ₂ solution are added, and finally, with an O ₂ 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 which is then mechanically comminuted. After drying this gel, the comparison product of the table (Gel 4a) is obtained.

Example 4b

0.26 g of compound 1 (R = CH ₃ ), dissolved in 40 ml of methanol, is homogeneously kneaded, comminuted, at 180 ° C. to 300 g of a comminuted, 73 mol% neutralized 30% polymer gel from Example 4a dried, ground and sieved to a residual moisture of 3%. The gel obtained in this way is referred to as example 4b after surface postcrosslinking.

The following examples show the effect of the invention Surface post-crosslinking for a particulate system with carboxyl or Carboxylate groups. The increase in absorption under pressure is documented in Table 1.

Example 5

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

Example 6

To 300 g of a crushed to 73 mol% neutralized 30% polymer gel Example 4a 0.39 g of compound 2 (R = H), dissolved in 40 ml of methanol add, knead homogeneously, crushed, at 180 ° C up to one Residual moisture of 3% dried, ground and sieved.

Example 7

To 300 g of a crushed to 73 mol% neutralized 30% polymer gel Example 4a 0.76 g of compound 2 (R = H), dissolved in 40 ml of methanol  add, knead homogeneously, crushed, at 180 ° C up to one Residual moisture of 3% dried, ground and sieved.

Example 8

To 300 g of a crushed to 73 mol% neutralized 30% polymer gel Example 4a 1.2 g of compound 2 (R = H), dissolved in 40 ml of methanol add, knead homogeneously, crushed, at 180 ° C up to one Residual moisture of 3% dried, ground and sieved.

The products obtained in Examples 4 to 8 are as follows in Table I. summarized data marked:

Table I

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 the compound 2 (R = H), sprayed on as a 10% solution in a PETTERSON & KELLY mixer and mixed for 20 minutes at 185 ° C. After cooling to room temperature the following listed in Table II, compared to the starting product improved values found:

Table II

Example 10

100 g high molecular weight, uncrosslinked polyacrylic acid with a degree of neutralization 53 mol% in the form of a comminuted gel, produced analogously to Example 5 without Crosslinker is with 120 g of carboxymethyl guar and 100 g of 0.15% solution of Compound 1, (R = H) homogeneously kneaded, crushed, in an air stream at 180 ° C Dried, ground and sieved for 15 minutes. A water-swellable product is obtained with a water absorption capacity several times its own weight.

Examples 11 to 15 illustrate the production of water-swellable products with good absorbency through cross-linking of different polymers Described with the compounds of the invention, what happens so that mixtures of polyacrylates and carboxymethyl polysaccharides or Alginates pasted in water, mixed with compounds of formula 1, homogeneous  kneaded, dried in an air stream at 180 ° C, ground and sieved.

Example 11

550 parts by weight Carboxymethyl cellulose, 50 parts by weight of polyacrylate, 0.6 parts by weight I (R = CH)

Example 12

30 parts by weight Carboxymethyl cellulose, 70 parts by weight Polyacrylate, 0.35 part by weight I (R = H)

Example 13

45 parts by weight of carboxymethyl starch, 55 parts by weight Polyacrylate, 0.56 part by weight I (R = H)

Example 14

65 parts by weight of alginate, 45 parts by weight of polyacrylate, 1.8 parts by weight of I (R = CH)

Example 15

20 parts by weight Carboxymethyl cellulose, 20 parts by weight Carboxymethyl starch,
60 parts by weight of polyacrylate,
1.4 parts by weight I (R = H)

Examples 16-27

Commercially available, partially neutralized, cross-linked, preheated to 30 ° C to 60 ° C Polyacrylic acid for use as a superabsorbent in hygiene products Compounds of formula 1 as 10% aqueous or aqueous alcoholic Solutions added in a mixing unit and treated at 130 ° C-200 ° C.

After cooling to room temperature, the following were listed in Table III, in Improved values found compared to the starting product.  

Table III

Examples 28-32

On commercially available partially neutralized, cross-linked polyacrylic acid for use as Superabsorbents, compounds of general formula 1 as aqueous or aqueous-alcoholic mixtures with the addition of 0.1 to 0.6 wt .-% based on Polymer acid catalysts added and treated at 120 ° C to 180 ° C. After Cooling to room temperature were the following listed in Table IV Improved values found compared to the starting product.  

Table IV

The compounds mentioned in the description and in the claims

also include the stereoisomers, for example those of the formula

Claims (11)

1. Process for the surface crosslinking of particles of superabsorbent polymers, characterized by the use of compounds of formula 1 as crosslinking agents
wherein R and R 'independently of one another are hydrogen or C ₁ -C ₆ alkyl. 2. The method according to claim 1, characterized in that it is in the superabsorbent polymers around a polyacrylate Carboxymethylpolysaccharide 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 characterized in that the crosslinking in the temperature range of 120 ° C-220 ° C. is carried out. 5. The method according to one or more of claims 1 to 4, characterized characterized in that the crosslinking with the addition of acidic catalysts is carried out. 6. The method according to claim 5, characterized in that the Amount of catalyst 0.05 to 3.0 wt .-% based on the polymer.   7. The method according to one or more of claims 1 to 6, characterized characterized in that the compounds of formula 1 in the form of aqueous Mixtures are applied. 8. The method according to one or more of claims 1 to 7, characterized characterized in that the compounds of formula 1 as aqueous alcoholic Mixtures are applied. 9. The method according to one or more of claims 1 to 7, characterized characterized in that polyacrylates in solid form as powders or granules be networked. 10. Surface-crosslinked, superabsorbent polymers, manufactured according to the Method 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.