MXPA00007062A - Secondary cross-linking of hydrogels by means of boric acid esters - Google Patents

Abstract

The invention relates to a method for the surface secondary cross-linking of water-absorbent polymers by treatment of the polymers with a solution for surface secondary cross-linking. According to said method the polymers during or after spraying are subjected to secondary cross-linking and dried by raising the temperature to 50-250°C and the cross-linking agent contains a boric acid ester with a bivalent or trivalent alcohol dissolved in an inert solvent. The invention also relates to water-absorbent polymers obtainable in accordance with the above method and to their use in hygiene articles, packing materials and non-woven materials.

Description

POST RETICULATION OF HYDROGELS USING BORIC ESTERS The present invention relates to a process for the post-cross-linking of gels or surfaces of water-absorbing hydrogels using boric esters of polyhydric alcohols, to water-absorbing polymers that can be obtained in this way and to their use in hygienic products and materials. for packaging. Hydrophilic highly hydrophilic hydrogels are, in particular, polymers composed of nitrilic monomers (co) polymerized or are (co) polymers grafted from one or more hydrophilic monomers on a suitable grafting base, cross-linked cellulose ethers or cross-linked starch ethers, cross-linked carboxymethylcellulose, partially cross-linked polyalkylene oxide or natural products that can swell in liquids aqueous: guar derivatives, for example. Hydrogels of this kind are used as products to absorb aqueous solutions in the production of diapers, tampons, sanitary napkins or other hygienic products, and as water retainers in the gardening market. To improve the service properties such as diaper rewet and AUL (absorbency under load), for example, the highly-swellable Mdrofilic hydrogels are generally subjected to post-surface or gel cross-linking. This post-crosslinking is known to those skilled in the art and is preferably carried out in the aqueous gel phase or as a post-surface crosslinking of the ground and screened polymer particles. The crosslinkers suitable for this purpose contain at least two groups which can form covalent bonds with the carboxyl groups of the hydrophilic polymer. Examples of suitable crosslinkers are diglycidyl or polyglycidyl compounds, such as diglycidyl phosphonate, alkoxysilyl compounds, polyaziridines, polyamines and polyamidoamines, and these compounds can also be used in mixtures with each other. (see for example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438). Polyamidoamines which are suitable as crosslinkers are described in particular in EP-A-0 349 935. An important disadvantage of these crosslinkers is their high reactivity. Although this is desirable in terms of chemical reaction, it carries a relatively high toxicological potential. In production operations, the processing of these crosslinkers requires special protection measures to comply with the requirements of governmental safety and hygiene provisions in the workplace. In addition, the use of polymers modified in this way in hygienic products appears to be objectionable.

Polyfunctional alcohols are also known crosslinkers. For example, EP-A-0 372 981, US-4 666 983 and US-5 385 983 teach the use of hydrophilic polyalcohols and the use of polyhydroxy surfactants. According to these documents, the reaction is carried out at temperatures of 120-250 ° C. The process has the disadvantage that the esterification reaction that causes the crosslinking is relatively slow even at these temperatures. The object was therefore to use compounds that are relatively slow to react but are reactive with carboxyl groups, to obtain as good but better post gel or surface crosslinking compared to the prior art. This objective was obtained with a very short reaction time and a very low reaction temperature. In theory, the prevailing reaction conditions should be the same as those obtained when highly reactive epoxides are used. It has now been surprisingly found that the esters of boric acid with polyhydric alcohols are highly suitable surface post-crosslinking agents. These esters are easily synthesized by the reaction of boric acid or boron oxide with alcohol. The invention offers a process for the surface post-crosslinking of water-absorbing polymers by treating the polymers with a post-surface cross-linking solution, the polymers being post-crosslinked and dried by means of an increase in temperature during or after the treatment, wherein the crosslinker consists of an ester of boric acid with a dihydric or polyhydric alcohol in solution in an inert solvent. A boric ester is a compound of the formula (B (OR) 3. The boric esters are formed, for example, in the reaction of boric anhydride B2O3 with alcohols, accompanied by the formation of boric acid, as follows: B203 + 3 ROH? B (0R) 3 + H3BO3 or in the case of a higher excess of alcohol, according to: B203 + 6 ROH? 2 B (OR) 3 + 3 H20 or by the reaction of boric acid with alcohols, which is accompanied by the elimination of water during the esterification reaction, in accordance with: B (OH) 3 + 3 ROH B (OR) 3 + 3 H20 The higher esters of boric acid can be obtained, for example by transesterification reactions: B (OR1) 3 + 3 R2OH? B (OR2) 3 + 3 R ** OH, the lower boiling alcohol R OH being separated from the mixture by distillation. The boric esters used in the process of the invention for a post-surface crosslinking are esters of bifunctional or polyfunctional alcohols. In the reaction of boric acid or boric anhydride or in the transesterification reaction with bifunctional or polyfunctional alcohols, cyclic compounds or polyesters can also be formed. Considering the reaction of ethylene glycol (Ri = H in the formulas la-Id) or 1, 2-propanediol in the formulas la-Id), it is possible that the following boric esters are formed: With a stoichiometric deficiency of alcohol, partially esterified boric acid is formed preferably first, for example or even the corresponding anhydride of these compounds; The complete esterification preferentially gives origin to the following products: together with a smaller amount of cyclic compounds and polyesters having the following repeat unit: H B- ^ "^ CR2- • C. (Id) Rl With the use of difunctional or polyfunctional alcohols in addition to ethylene glycol or 1,2-propanediol, the analogous boric esters are formed. The radical R is hydrogen or an alkyl group having preferably 1 to 12, especially 1 to 6, carbon atoms.

The post-crosslinking temperature is preferably 50-250 ° C, in particular between 50-200 ° C, especially between "100-180 ° C. To accelerate the reaction of the surface post-crosslinking solution, an acid catalyst can be added. The catalysts which can be used in the process of the invention are all inorganic acids, their corresponding anhydrides, and organic acids and their corresponding anhydrides, examples being boric acid, sulfuric acid, hydroiodic acid, phosphoric acid, tartaric acid, acetic acid and In particular, the polymeric forms, anhydrides and the acid salts of polybasic acids are also suitable, examples of which are boron oxide, sulfur trioxide, diphosphorus pentoxide and diacid ammonium phosphate. The invention is preferably done by spraying a surface post-crosslinker solution onto the polymer powder on a dry basis. of the spray application, the polymer powder is thermally dried, it being possible for the crosslinking reaction to take place before or during drying. Preference is given to the spray application of a crosslinker solution in reaction mixers and spray mixers or in mixing and drying systems such as, for example, Lódige mixers, @BEPEX mixers, mixers © NAUTA, mixers © SHUGGI or © PROCESSALL. In addition, it is also possible to use fluidized bed dryers. Drying can take place in the mixer itself, by heating the outer cover, or blowing hot air. In the same way, a downstream dryer is suitable, such as a drawer dryer, a rotary dryer or a heatable helix, for example. Otherwise, azeotropic distillation, for example, can be used as a drying technique. The residence time at the preferred temperature in the reaction mixer or dryer is from 5 to 90 minutes, preferably less than 30 minutes and, very particularly preferably less than 10 minutes. As the inert solvent preference is given to the use of water and mixtures of water with monohydric or polyhydric alcohols. However, it is also possible to use any miscibility organic solvent with unlimited water, as it can be certain esters and ketones, for example, that by themselves are not reactive under the conditions of the process. Where alcohol-water mixture is used, the alcohol content of this solution is, for example, 10-90% by weight, preferably 30-70% by weight, in particular 40-60% by weight. Any alcohol with unlimited miscibility with water can be used, such as mixtures of two or more alcohols (for example, methanol + glycerol + water). Particular preference is given to the use of the following alcohols in aqueous solution: methanol, ethanol, isopropanol, ethylene glycol and, with particular preference, 1,2-propanediol and also 1,3-propanediol. The surface post-crosslinking solution is used in a ratio of 1-20% by weight, based on the polymer mass. Particular preference is given to a solution amount of 2.5-15% by weight, with respect to the polymer. The crosslinker itself is used in an amount of 0.01-1.0% by weight, based on the polymer used. The water-absorbent polymer is preferably a polymeric acrylic acid or a polyacrylate. This water-absorbent polymer can be prepared according to a method known from the literature. Preference is given to polymers containing cross-linking comonomers (0.001-10 mol%); very particular preference is given, however, to polymers obtained by free radical addition polymerization using a polyfunctional, ethylenically unsaturated free radical crosslinker which also bears at least one free hydroxyl group (such as, for example, pentaerythritol trially ether) or trimethylolpropane diallyl ether). Highly swellable hydrophilic hydrogels for use in the process of the invention are in particular polymers composed of hydrophilic (co) polymerized monomers, or are (co) polymers grafted from one or more hydrophilic monomers on a suitable graft base, cellulose ethers crosslinked or crosslinked starch ethers, or natural products that can swell in aqueous liquids: guar derivatives, for example. These hydrogels are known to the person skilled in the art and are described, for example, in US-A-4 286 082, DE-C-27 06 135, US-A-4 340 706, DE-C-37 13 601, DE-C-28 40 010, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085, DE-A-39 17 846, DE-A-38 07 289, DE-A-35 33 337, DE- • A-35 03 458, DE-A-42 44 548, DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261, DE-A-35 11 086, DE-A-31 18 172, DE-A-30 28 043, DE-A-44 18 881, EP-A-0 801 483, EP-A-0 455 985, EP-A-0 467 073, EP-A-0 312 952, EP-A-0 205 874, EP-A-0 499 774, DE-A-26 12 846, DE- • A-40 20 780, EP-A-0 205 674, US-5 145 906, EP-A-0 530 438, EP-A- • 0 670 073, US-A-4 057 521, US-A-4 062 817, US-A-4 525 527, US-A-4 295 987, US-Ar-5 011 892, US-A-4 076 663 or US-Ar-4 931 497.

The content of the aforesaid patent documents are expressly incorporated herein by reference. ~~ ~~ Examples of the hydrophilic monomers suitable for preparing these highly swellable hydrophilic hydrogels are polymerizable acids, such as acrylic acid, acid, methacrylic, vinylsulfonic acid, vinylphosphonic acid, maleic acid including its anhydride, fumaric acid, itaconic acid, 2- acrylamido-2-ethylpropanesulfonic acid, 2-acrylamido-2-methylpropanephosphonic acid, alkali metal salts and ammonium salts of monomers containing acid groups and also their amides, hydroxyalkyl esters and esters containing aminoalkyl or ammonium and amides. In addition, water-soluble N-vinyl amides are also suitable, such as N-vinylformamide or even diallyldimethylammonium chloride. Preferred hydrophilic monomers are the compounds of the formula: R3 Rl wherein hydrogen, methyl or ethyl, 2 4 R is -COOR, a sulfonyl group, a phosphonyl group, a phosphonyl group esterified with C 1 -C 4 alkanol or a group of the formula: CH3 R5 (3), N CH2 H CH3 wherein R 3 is hydrogen, methyl, ethyl or a carboxyl group, R is hydrogen, alkali metal ion or ammonium ion, C 1 -C 4 aminoalkyl or C 1 -C 4 hydroxyalkyl, and R is a sulfonyl group, phosphonyl group, a carboxyl group or the alkali metal or ammonium salts of these groups. Examples of the C1-C4 alkanols are methanol, ethanol, n-propanol, isopropanol and n-butanol. Particularly preferred hydrophilic monomers are acrylic and methacrylic acid and the sodium, potassium and ammonium salts of these acids If desired, these acids may also be in partially neutralized form Suitable grafting bases for hydrophilic hydrogels obtainable by copolymerization Grafted olefinically unsaturated acids may be of natural or synthetic origin Examples are starch, cellulose and cellulose derivatives, and also other polysaccharides and oligosaccharides, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, and hydrophilic polyesters. Suitable polyalkylene oxides have, for example, the formula: XI 6 (CH2-CH-0) n- R7 (4) wherein R 6 and R 7 independent of each other are hydrogen, alkyl, alkenyl or acyl, X is hydrogen or methyl, and n is an integer from 1 to 10,000. R and R are preferably hydrogen, C 1 -C 4 alkyl, C 2 -C 6 alkenyl or phenyl. Particularly preferred hydrogels are polyacrylates, polymethacrylates and the graft copolymers described in US-A-4 931 _497, US-A-5 011 892 and US-A-5 041 496. The highly soluble hydrophilic hydrogels are preferably in a crosslinked form; that is, they include compounds having at least two double bonds that have been copolymerized in the polymer network. Particularly suitable crosslinkers are methylenebisacrylamide and methylene bismethacrylamide, esters of mono- or polycarboxylic acids unsaturated with polyols, such as diacrylate or triacrylate, the examples being the diacrylates and dimethacrylates of butanediol and ethylene glycol, and trimethylolpropane triaquilate, and also compounds of allyl, such as allyl (meth) acrylate, triallyl cyanurate, diallyl maleate, esters polyalilo, tetraalioxietano [sic], trialilamina, tetraaliletilendiamina, alilo esters of phosphoric acid and vinyl phosphonic acid derivatives as described, for example, in EP-A-0 343 427. However, in the process of the invention, particular preference is given to hydrogels prepared using polyallyl ethers as crosslinkers and by acid homopolymerization of acrylic acid. Suitable crosslinkers are pentaerythritol tri and tetraallyl ether, polyethylene glycol diallyl ether, monoethylene glycol diallyl ether, glycerol di- and triallyl ether, polyallyl ethers based on sorbitol and alkoxylated variants thereof. Highly swelling hydrophilic hydrogels can be prepared by traditional polymerization processes. Preference is given to polymerization by addition in aqueous solution by the processes known as gel polymerization. In this process, from 15 to 50% by weight of aqueous solutions of one or more hydrophilic monomers, and, if desired, of a suitable graft base, are polymerized in the presence of free radical initiator, preferably without mechanical mixing, using the Trommsdorff-Norrish effect (Makromol, Chem. 1 (1947) 169). The polymerization reaction can be carried out in the temperature range between 05C and 1505C [sic], preferably between 105C and 1005C [sic], at atmospheric pressure or under an increased or reduced pressure. The polymerization can also be carried out in an inert gas atmosphere, preferably under nitrogen. Polymerization can be initiated using high energy electromagnetic radiation or by the customary chemical polymerization initiators. Examples of these organic peroxides such as benzoyl peroxide, tertbutyl hydroperoxide, methyl ethyl ketone peroxide and eumeno hydroperoxide, azo compound such as azodiisobutyronitrile and peroxo inorganic compounds such as (NH4) 2S2? 8, K2S2? 8 or H202. These can, if desired, be used in combination with reducing agents such as sodium acid sulfite or iron (II) sulfate, or redox systems. The redox systems include a reducing component, which is generally aromatic aliphatic sulphinic acid, such as benzenesulfinic acid or toluensulfinic acid or derivatives of these acids, such as the Mannich addition products of sulfinic acid, aldehydes and amino compounds, as described in DE-C-13 01 566. The qualities of the polymers can be further improved by continuing the heating of the polymeric gels for a few hours within a temperature range from 50 to 130 ° C, preferably from 70 to 100 ° C. ° C. The resulting gels are neutralized to the extent of 0-100 mol% based on the monomer employed, preferably 25-200 mol% and particularly preferably 50-85 mol%, it being possible to use the customary neutralizing agents, preferably hydroxides of alkali metal or alkali metal oxides, and with particular preference sodium hydroxide, sodium carbonate or sodium hydrogen carbonate. Usually, the neutralization is effected by mixing the neutralizing agent as an aqueous solution or even, preferably, as a solid. For this purpose, the gel is comminuted mechanically by means of a chopper for example, and the neutralizing agent is sprayed, dispersed or poured on it and then mixed thoroughly. To effect homogenization, the resulting gel mass can pass through the chopper again a few times. The neutralized gel mass is then dried with a conveyor belt dryer or roller dryer until the residual moisture content is less than 10% by weight, preferably below 5% by weight. The dehydrated hydrogel is then crushed and sieved, the normal grinding apparatus being roller mills, barbed mills or vibrating mills. The preferred particle size of the sieved hydrogels is in the range of 45-1000 mm [sic], with particular preference 45-850 mm [sic] and very particularly preferably 100-850 mm [sic]. The invention further provides a water-absorbent polymer obtainable by the process described above. The invention further provides the use of the products produced by the process of the invention in hygienic products, packaging materials and non-woven goods. To ascertain the amount of post-surface crosslinking the anhydrous hydrogel is then tested using the test methods known from the prior art and described in the following: Methods: 1) Centrifuge Retention Capacity (CRC): This method measures the free swelling capacity of the hydrogel in a tea bag. Approximately 0.200 g of dehydrated hydrogel are sealed in a tea bag (format: 60 - m x 60 mm, Dexter 1234T paper) and soaked for 30 minutes in 0.9% by weight sodium chloride solution. The tea bag is then centrifuged for 3 minutes in a customary commercial centrifuge dryer (Bauknecht WS 130, 1400 rpm, basket diameter 230-mm). The amount of liquid absorbed is determined by weighing the centrifuged tea bag. The absorption capacity of the tea bag is taken into account by determining a white value (tea bag without hydrogel), which is deduced from the resulting weight (tea bag with swollen hydrogel). Retention CRC [g / g] = (weight of the tea bag - value-of-white - initial weight of the hydrogel) - * - initial weight of the hydrogel. 2) Absorbency under load (2068.5 Pa (0.3 psi) /3447.5 pa (0.5 psi) /4826.5 Pa (0.7 psi)) For absorbance under load, 0.900 g of the anhydrous hydrogel are evenly distributed over a cell's sieve bases measurement. The measuring cell consists of a Plexiglas cylinder (height = 50 mm, diameter = 60 mm) whose base is formed by adhering it on a steel mesh screen (mesh size 36 microns or 400 mesh). A covered plate is placed over the uniformly distributed hydrogel and loaded with an appropriate weight. The cell is then placed on a filter paper (black S &S 589 band, diameter = 90 mm) which is on a porous glass filter plate, this filter plate is on a petri dish (height = 30 mm , diameter = 200 mm) containing 0.9% sodium chloride solution of concentration by weight so that the liquid level at the beginning of the experiment is level with the upper edge of the glass frit. The hydrogel is then allowed to absorb the saline solution for 60 minutes. Subsequently, the entire cell with the swollen gel is removed from the filter plate and the apparatus is reweighed after removing the weight. The absorbance under load (AUL) is calculated as follows: AUL [g / g] = (- Wa) / (Ws) where Wb is the mass of the apparatus + gel after swelling, Wa is the mass of the apparatus + the initial weight of the gel before swelling, and Ws is the initial weight of the anhydrous gel. The device consists of the measuring cylinder and the covered plate.

Examples Preparation of boric esters Boric Ester 1 A three-neck flask with stirring, internal thermometer and reflux condenser is charged with 9 moles of ethylene glycol, and 1 mole of boric anhydride is slowly added to this initial charge. The solution is stirred at 80 ° C for two hours. Then the unreacted ethylene glycol and water are removed by distillation under reduced pressure. With cooling of form a white substance with waxy aspect.

Boric Ester 2 A three-necked flask with stirrer, internal thermometer and reflux condenser is charged with 4 moles of propanediol (1,2), and 1 mole of DORIC acid is added to this initial charge. The mixture is heated to boiling and the water is distilled under atmospheric pressure. Subsequently, the excess of 1,2-propanediol is distilled under reduced pressure. Again, a waxy white substance is obtained which solidifies with cooling. These boric esters are used according to the invention to crosslink superabsorbent polymers. The following examples illustrate the crosslinking action of the boric esters.

Example 1 In a 40 liter plastic bucket, 6.9 kg of pure acrylic acid are diluted with 23 kg of water. 45 g of pentaerythritol triallyl ether is added with stirring to this solution and the sealed cuvette becomes inert by passing nitrogen therethrough. The polymerization is then initiated by the addition of about 400 mg of hydrogen peroxide and 200 mg of ascorbic acid. After the end of the reaction the gel is mechanically crushed and sodium hydroxide solution is added in an amount sufficient to obtain a degree of neutralization of 75 mol%, based on the acrylic acid used. The neutralized gel is then dried on a roller dryer, crushed with a pin mill and finally isolated by sieving. This is the base polymer that is used in the subsequent examples.

This base polymer (1 kg) is spray coated with the surface post-crosslinking solution in a Lódige mixer in a two-stage process.

Step 1: First, a solution of boric ester 1 (0.5% by weight based on the base polymer) in ethylene glycol as a solvent (5% by weight based on the base polymer) is applied by spray.

Stage 2: Subsequently, the temperature of the heating jacket increases linearly from 50 to 200 ° C. As soon as the temperature of the product has reached 80-90 ° C, 5% by weight of additional water is sprayed (based on the base polymer). The process is terminated after about 30 minutes, and the hydrogel is screened again to remove lumps and can then be used, for example, as a water absorbing polymer in diapers. The measured values for CRC and AUL are indicated in the table.

Example 2 A base polymer is prepared according to Example 1 which is sprayed with the cross-linking solution in a Waring laboratory mixer. The composition of the solution is such that the following dosage is obtained, based on the base polymer employed: 0.5% by weight of boric ester 1, 4.5% by weight of propylene glycol and 4.5% by weight of water. Then the wet polymer is dried at 175 ° C for 60 minutes. The table indicates the measured values for CRC and AUL.

Example 3 A base polymer prepared according to Example 1 is sprayed with crosslinking solution in a Waring laboratory mixer. The composition of the solution is such that the following dosage, based on the base polymer employed, is obtained: 0.5% by weight of boric ester 2, 4.5% by weight of propylene glycol and 4.5% by weight of water. The wet polymer is then dried at 175 ° C for 60 minutes. The table indicates the common properties of the polymer.

Example 4 ~ A base polymer prepared according to Example 1 is sprayed with cross-linking solution in a Telschig laboratory mixer. The composition of the solution is such that the following dosage is obtained, based on the base polymer employed: 0.5% by weight of boric ester 72, 7% by weight of methanol and 3% by weight of water. The wet polymer is then dried at 150 ° C for 60 minutes. The table indicates the common properties of this water-absorbent polymer.

Table

Claims (1)

CLAIMS A process for the surface post-crosslinking of water absorbing polymers by treating the polymers with a surface post-crosslinking solution, the polymers being post-crosslinked and dried during or after the treatment by means of an increase in temperature, wherein the crosslinker it contains a boric acid ester having a dihydric or polyhydric alcohol dissolved in an inert solvent. The process as claimed in claim 1, wherein the water absorbing polymer is a polymeric acrylic acid or a polyacrylate, especially a polymeric acrylic acid or polyacrylate obtained by polymerization by the addition of free radicals in the presence of a polyfunctional free radical crosslinker, ethylenically unsaturated which may also carry one or more free hydroxyl groups. The process as claimed in claim 1 or 2, wherein the catalyst used for crosslinking consists of an inorganic acid, its anhydride, an organic acid or its anhydride. The process as claimed in claim 3, wherein the acid is boric, sulfuric, hydriodic, phosphoric, tartaric, acetic or toluene sulfonic acid or the polymeric forms, anhydrides or acid salts thereof. The process as claimed in one or more of claims 1 to 4, wherein the inert solvent is water, a mixture of water with organic solvents of unlimited solubility in water or a mixture of water with monohydric or polyhydric alcohols. The process as claimed in claim 5, wherein if an alcohol / water mixture is used, the alcohol content of this solution is 10-905 by weight, preferably 30-70% by weight. The process as claimed in claim 5 or 6, wherein the alcohol is methanol, ethanol, isopropanol, ethylene glycol, 1,2-propanediol or 1,3-propanediol. The process as claimed in one or more of claims 1 to 4, wherein the post surface cross-linking solution is employed in a proportion of

1-20% by weight, in particular 2.5-15% by weight, based on the polymer mass. A water absorbing polymer prepared by the process as claimed in one or more of claims 1 to 8. The use of a polymer prepared by the process as claimed in one or more of claims 1 to 8 in a hygienic product, packaging material or non-woven fabric.