WO2008055856A1

WO2008055856A1 - Process for producing superabsorbents

Info

Publication number: WO2008055856A1
Application number: PCT/EP2007/061851
Authority: WO
Inventors: Norbert Herfert; Guy Thomas Woodrum
Original assignee: Basf Se
Priority date: 2006-11-09
Filing date: 2007-11-05
Publication date: 2008-05-15

Abstract

Superabsorbents are produced by a process comprising an acid neutralization step using alkali hydroxide containing phosphate ions.

Description

Process for producing superabsorbents

Description

The present invention concerns a process for producing superabsorbents. In particular, it concerns a process for producing superabsorbents using a comparatively low-purity grade of caustic without significant degrading effects on superabsorbent properties. Such low-purity grades of caustic (to be precise, solid sodium hydroxide or aqueous sodium hydroxide solution) contain rather high amounts of iron impurities.

Superabsorbents are known. Such materials are also commonly known by designations such as "high-swellability polymer", "hydrogel" (often even used for the dry form), "hydrogel-forming polymer", "water-absorbing polymer", "absorbent gel-forming material", "swellable resin", "water-absorbing resin" or the like. The materials in question are crosslinked hydrophilic polymers, in particular polymers formed from (co)polymerized hydrophilic monomers, graft (co)polymers of one or more hydrophilic monomers on a suitable grafting base, crosslinked ethers of cellulose or starch, crosslinked carboxy- methylcellulose, partially crosslinked polyalkylene oxide or natural products that are swellable in aqueous fluids, examples being guar derivatives, of which water-absorbing polymers based on partially neutralized acrylic acid are most widely used. The essential properties of superabsorbents are their ability to absorb and retain amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. A superabsorbent which is used in the form of a dry powder transforms into a gel on taking up a liquid, specifically into a hydrogel when as usual taking up water. By far the most important field of use of superabsorbents is the absorbing of bodily fluids.

Superabsorbents are used for example in diapers for infants, incontinence products for adults or feminine hygiene products. Examples of other fields of use are as water- retaining agents in market gardening, as water stores for protection against fire, for liquid absorption in food packaging or, in general, for absorbing moisture.

Processes for producing superabsorbents are also known. The acrylate-based superabsorbents which dominate the market are produced by radical polymerization of acrylic acid in the presence of a crosslinking agent (the "internal crosslinker"), the acrylic acid being neutralized to some degree in a neutralization step conducted prior to or after polymerization, or optionally partly prior to and partly after polymerization, usually by adding a alkali, most often an aqueous sodium hydroxide solution. This yields a polymer gel which is comminuted (depending on the type of reactor used, comminution may be conducted concurrently with polymerization) and dried. Usually, the dried powder thus produced (the "base polymer") is surface crosslinked (also termed surface "posfcrosslinked) by adding further organic or polyvalent cationic crosslinkers to generate a surface layer which is crosslinked to a higher degree than the particle bulk. Most often, aluminium sulphate is being used as polyvalent cationic crosslinker. Generally, a higher-purity grade of caustic has to be used for neutralization since the iron impurities contained in lower-purity grades lead to instant or eventual discoloration or the superabsorbent produced, which is unacceptable in the market for personal care products. Of the at least three grades of caustic available, "commercial" or "industrial" grade (the designation may vary according to the producer) has the highest iron level. The impurity level varies according to the production process and its specific embodiment in a given production plant. Other designations of caustic grades refer to the production processes: Caustic is obtained as one product of electrolytic chlorine and caustic production processes, and "diaphragm process", membrane process" or "mercury process" grades refer to the type of electrolysis cell used, which influences product purity. The most pure "mercury process" grade caustic also is the most costly. Due to its impurity level, "commercial" or "industrial" grade caustic, the cheapest grade, is not used for neutralization in superabsorbent production.

Frederic L. Buchholz und Andrew T. Graham (Hrsg.) in: ,,Modern Superabsorbent Polymer Technology", J. Wiley & Sons, New York, U.S.A. / Wiley-VCH, Weinheim, Germany, 1997, ISBN 0-471-19411-5, give a comprehensive overview over known processes for producing superabsorbents.

WO 02/22717 A1 discloses superabsorbents and processes for their production, in which surface crosslinking is achieved by reacting the base polymer with an organic crosslinker which is not a polyol, and a cation. The cation is preferably a polyvalent cation and may be used, among other forms, in the form of a phosphate.

WO 02/20068 A1 discloses surface crosslinking using certain combinations of polyol and cationic crosslinkers. The cation is preferably a polyvalent cation which may be used, among other forms, in the form of a phosphate.

WO 2005/01 1860 A2 describes a superabsorbent having a comparatively high content of fine particles which are affixed to the surface of larger particles by means of a thermoplastic melt adhesive. The fines may be superabsorbents fines, but, in another embodiment, are preferably organic non-superabsorbent fines or inorganic fines. Among others, metal salts of mono-, oligo- and polyphosphoric acids are mentioned, preferably alkaline earth salts, and in particular hydrates.

According to WO 2004/052949 A1 and WO 03/014172 A2, phosphates or phosphoric acid can be used as acidic catalysts for surface crosslinking. The former publication further outlines that iron (II) ions may be used reducing component of the redox initiator used for starting the polymerization. WO 2004/018005 A1 describes superabsorbent which may contain vinylphosphoric acid moieties in the acrylic acid network, and the use of clay additives to improve fluid acquisition rates. The clay contains inorganic metal components such as iron. WO 00/53644 A1 discloses superabsorbents which may contain allylphosphoric acid ester moieties in the acrylic acid network, and the use of metal ions such as iron as surface crosslinker.

The superabsorbents disclosed in WO 2004/024816 A1 comprise a nitrogen-containing polymer having protonated nitrogen atoms. This polymer is preferably produced by hydrolysing N-vinyl formamide at pH values of 9 to 14, but may also be produced by hydrolysing N-vinylformamide at acidic pH values using carboxylic or inorganic acids such as phosphoric acid. In one embodiment, the superabsorbent further contains fine particles of a salt having low solubility in water. Some examples of such salts are phosphates such as calcium, magnesium, lithium or zinc phosphate. Again, iron (II) ions may be part of the redox initiator system used for starting the polymerization of the non-nitrogen-containing part of this particular superabsorbent, and phosphoric acid may be used as a catalyst for surface postcrosslinking.

WO 02/47472 A1 discloses a method for avoiding malodours generated by bacterial degradation of secreted body fluids in absorbent articles such as superabsorbent- containing diapers. The method comprises adding urease inhibitor complexes formed from a divalent metal ion and a polyanionic, preferably amine-based chelating agent, for example ethylenediamine tetraacetic acid.

Japanese laid-open patent publication JP 05/086251 A discloses that the discoloration of superabsorbents caused by high temperature and humidity during storage can be avoided by blending the superabsorbent with phosphoric acid derivatives or salt, in particular 1-hydroxiethylidene-1 ,1-diphosphonic acid, ethylenediamine tetra(methylene phosphonic acid), diethylenetriaminepenta-(methylene phosphonic acid) and/or its alkali metal, ammonium or amine salts. WO 03/059962 A1 or its equivalent US patent application publication US 2005/0085604 A1 report that this discoloration can be inhibited by allowing a metal chelating agent to be present at any step of the production process of polyacrylic acid, and adding a reducing or an oxidizing agent prior to drying the polymerized wet gel. A range of metal chelating agents are disclosed, including agents derived from phosphoric acid such as pyrophosphoric acid or tripolyphosphoric acid and their salts.

US 5,700,350 relates to the negative effect of transition metals such as iron on chlorine-free pulp bleaching processes, and discloses a process for recycling chelating agents (in particular, ethylendiamine tetracetic acid) into the bleaching process by removing the chelated transition metal from the chelating agent in alkaline (caustic or soda) solution by substituting them with calcium, utilizing the low solubility of transition metal hydroxides as further thermodynamic driving force of this substitution.

US 4,558,145 describes that the chelate complex of iron (III) with ethylenediamine tetraacetic acid (having one carboxyl group still protonated) is obtainable by reacting the trisodium salt of ethylenediamine tetraacetic acid with iron (II) nitrate to form the iron(ll) chelate and then oxidizing iron (II) to iron (III) using nitric acid. US 4 500 494 discloses microencapsulated beta-diketones, 8-hydroxiquinolines and oximes for use as chelating agents for metal ion removal. US 4,064,044 describes a process for inhib- iting scale formation in aqueous systems by chelating scale-forming metal ions such as calcium or magnesium with methylene phosphonates of polyalkylenepolyamines polymerized with epoxihalides or dihalides.

It is an object of the present invention to provide a novel, improved or alternative proc- ess for producing superabsorbents. In particular, the object of the present invention is to provide a process for producing superabsorbents in which comparatively low-purity grades of caustic can be used without adverse effects on product quality.

We have found that this object is achieved by a process for producing superabsorbents comprising an acid neutralization step using alkali hydroxide containing phosphate ions. This process allows the use of "commercial" or "industrial" grade caustic, which means low-purity caustic and in particular caustic having a high iron content, without deteriorating effect on the superabsorbent. In particular, the superabsorbent will remain as colorless even under circumstances of heat and humidity, as a superabsorbent pro- duced using higher-purity grades of caustic.

Most usually, the alkali hydroxide used as neutralizing agent is sodium hydroxide ("caustic"), although there are instances of the use of lithium or potassium hydroxide. Neutralization is customarily achieved by admixing the neutralizing agent as an aque- ous solution or else as a solid material. For example, sodium hydroxide having a water content of distinctly below 50% by weight can be present as a waxy mass having a melting point of above 23°C. In this case, metering as piecegoods or melt at elevated temperature is possible. The amount of water used as solvent for the neutralizing agent depends on several factors, such as total water needed for processing the superabsor- bent, limitations or requirements of drying steps, which any expert takes into account when determining the amount of water to be used as a solvent for the neutralizing agent.

Phosphate is added to the caustic in the form of phosphoric acid or of a phosphate salt, or in the form of any substance that will turn into or release phosphate ions under the chosen processing conditions. It is also possible to add mixtures of these phosphates or phosphate sources. Any phosphate salt may be used, particularly useful are the alkali phosphates U3PO4, NasPCU or K3PO4, among them sodium phosphate is most preferred. Adding phosphoric acid to caustic of course yields sodium phosphate. Phospho- ric acid or phosphate is added to the hydroxide by any conventional method or apparatus for adding and mixing one substance to and into another. The easiest method is adding phosphoric acid or the phosphate to an aqueous solution of the hydroxide. Add- ing phosphate may be advantageous where equipment could be corroded by phosphoric acid. If the equipment tolerates phosphoric acid, phosphoric acid may be advantageous due to lower cost.

The amount of phosphate in the alkali hydroxide used for neutralization is generally at least 0.5% by weight, preferably at least 0.75% by weight, more preferably at least 1 % by weight. In principle, the upper limit is only determined according to economic considerations. Generally, the phosphate content will be not more than 20% by weight, preferably not more than 15% by weight and more preferably not more than 10% by weight, all values calculated as percentage of phosphate salt relative to alkali hydroxide, disregarding any solvent. In the most usual case, these percentages are percentages of sodium phosphate NasPCU relative to sodium hydroxide.

The superabsorbent in the present invention is a customary superabsorbent capable of absorbing and retaining amounts of water equivalent to many times its own weight under a certain pressure. In general, it has a centrifugal retention capacity (CRC, method of measurement see hereinbelow) of at least 5 g/g, preferably at least 10 g/g and more preferably at least 15 g/g. Preferably, the superabsorbent is a crosslinked polymer based on partially neutralized acrylic acid, and more preferably it is surface post- crosslinked. A "superabsorbent" can also be a mixture of chemically different individual superabsorbents in that it is not so much the chemical composition which matters as the superabsorbing properties.

Processes for producing superabsorbents, including surface-postcrosslinked superab- sorbents, are known. Synthetic superabsorbents are obtained for example by polymerization of a monomer solution comprising

a) at least one ethylenically unsaturated acid-functional monomer, b) at least one crosslinker, c) optionally one or more ethylenically and/or allylically unsaturated monomers co- polymerizable with the monomer a), and d) optionally one or more water-soluble polymers onto which the monomers a), b) and if appropriate c) can be at least partly grafted.

Suitable monomers a) are for example ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid, or derivatives thereof, such as acrylamide, methacrylamide, acrylic esters and methacrylic esters. Acrylic acid and methacrylic acid are particularly preferred monomers. Acrylic acid is most preferable.

The monomers a) and especially acrylic acid comprise preferably up to 0.025% by weight of a hydroquinone half ether. Preferred hydroquinone half ethers are hydro- quinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol refers to compounds of the following formula:

where R³ is hydrogen or methyl, R⁴ is hydrogen or methyl, R⁵ is hydrogen or methyl and R⁴ is hydrogen or an acid radical of 1 to 20 carbon atoms.

Preferred R⁶ radicals are acetyl, ascorbyl, succinyl, nicotinyl and other physiologically tolerable carboxylic acids. The carboxylic acids can be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R³ = R⁴ = R⁵ = methyl, especially race- mic alpha-tocopherol. R⁶ is more preferably hydrogen or acetyl. RRR-alpha-Tocopherol is preferred in particular.

The monomer solution comprises preferably not more than 130 weight ppm, more preferably not more than 70 weight ppm, preferably not less than 10 weight ppm, more preferably not less than 30 weight ppm and especially about 50 weight ppm of hydro- quinone half ether, all based on acrylic acid, with acrylic acid salts being arithmetically counted as acrylic acid. For example, the monomer solution can be produced using an acrylic acid having an appropriate hydroquinone half ether content.

Crosslinkers b) are compounds having at least two polymerizable groups which can be free-radically interpolymerized into the polymer network. Useful crosslinkers b) include for example ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacry- late, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane as described in EP 530 438 A1 , di- and triacrylates as described in EP 547 847 A1 , EP 559 476 A1 , EP 632 068 A1 , WO 93/21237 A1 , WO 03/104299 A1 , WO 03/104300 A1 , WO 03/104301 A1 and DE 103 31 450 A1 , mixed acrylates which, as well as acrylate groups, comprise further ethylenically unsaturated groups, as described in DE 103 31 456 A1 and WO 04/013064 A2, or crosslinker mixtures as described for example in DE 195 43 368 A1 , DE 196 46 484 A1 , WO 90/15830 A1 and WO 02/032962 A2.

Useful crosslinkers b) include in particular N,N'-methylenebisacrylamide and N,N'-methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids of polyols, such as diacrylate or triacrylate, for example butanediol diacrylate, buta- nediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate and also trimethylolpropane triacrylate and allyl compounds, such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid and also vinylphosphonic acid derivatives as described for example in EP 343 427 A2. Useful crosslinkers b) further include pentaerythritol diallyl ether, pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol diallyl ether, glycerol triallyl ether, polyallyl ethers based on sorbitol, and also ethoxylated variants thereof. The process of the present invention may utilize di(meth)acrylates of polyethylene glycols, the polyethylene glycol used having a molecular weight between 300 and 1000.

However, particularly advantageous crosslinkers b) are di- and triacrylates of 3- to 15-tuply ethoxylated glycerol, of 3- to 15-tuply ethoxylated trimethylolpropane, of 3- to 15-tuply ethoxylated trimethylolethane, especially di- and triacrylates of 2- to 6-tuply ethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane, of 3-tuply pro- poxylated glycerol, of 3-tuply propoxylated trimethylolpropane, and also of 3-tuply mix- edly ethoxylated or propoxylated glycerol, of 3-tuply mixedly ethoxylated or propoxylated trimethylolpropane, of 15-tuply ethoxylated glycerol, of 15-tuply ethoxylated trimethylolpropane, of 40-tuply ethoxylated glycerol, of 40-tuply ethoxylated trimethylolethane and also of 40-tuply ethoxylated trimethylolpropane.

Very particularly preferred for use as crosslinkers b) are diacrylated, dimethacrylated, triacrylated or trimethacrylated multiply ethoxylated and/or propoxylated glycerols as described for example in WO 03/104301 A1. Di- and/or triacrylates of 3- to 10-tuply ethoxylated glycerol are particularly advantageous. Very particular preference is given to di- or triacrylates of 1- to 5-tuply ethoxylated and/or propoxylated glycerol. The triacrylates of 3- to 5-tuply ethoxylated and/or propoxylated glycerol are most preferred. These are notable for particularly low residual contents (typically below 10 weight ppm) in the water-absorbing polymer and the aqueous extracts of the water-absorbing polymers produced therewith have an almost unchanged surface tension (typically at least 0.068 N/m) compared with water at the same temperature.

Examples of ethylenically unsaturated monomers c) which are copolymerizable with the monomers a) are acrylamide, methacrylamide, crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethyl- aminopropyl acrylate, dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate and dimethylami- noneopentyl methacrylate.

Useful water-soluble polymers d) include polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, polyethyleneimines, polyglycols, polymers formally constructed wholly or partly of vinylamine monomers, such as partially or completely hydrolyzed polyvinylamide (so-called "polyvinylamine") or polyacrylic acids, preferably polyvinyl alcohol and starch.

The polymerization is optionally carried out in the presence of customary polymerization regulators. Suitable polymerization regulators are for example thio compounds, such as thioglycolic acid, mercapto alcohols, for example 2-mercaptoethanol, mercap- topropanol and mercaptobutanol, dodecyl mercaptan, formic acid, ammonia and amines, for example ethanolamine, diethanolamine, triethanolamine, triethylamine, morpholine and piperidine.

The monomers (a), (b) and optionally (c) are (co)polymerized with each other in the presence of the water-soluble polymers d), in 10% to 80%, preferably 20% to 50% and especially 30% to 45% by weight aqueous solution in the presence of polymerization initiators. Useful polymerization initiators include all compounds that disintegrate into free radicals under the polymerization conditions, examples being peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds and the so-called redox initiators. The use of water-soluble initiators is preferred. It is advantageous in some cases to use mixtures of various polymerization initiators, examples being mixtures of hydrogen peroxide and sodium or potassium peroxodisulfate. Mixtures of hydrogen peroxide and sodium peroxodisulfate can be used in any desired ratio. Suitable organic peroxides are for example acetylacetone peroxide, methyl ethyl ketone peroxide, tert- butyl hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl per- pivalate, tert-butyl perneohexanoate, tert-butyl perisobutyrate, tert-butyl per-2- ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzo- ate, tert-butyl per-3,5,5-trimethylhexanoate and tert-amyl perneodecanoate. Further suitable polymerization initiators are azo initiators, for example 2,2'-azobis(2- amidinopropane) dihydrochloride, 2,2'-azobis(N,N-dimethylene)-isobutyramidine dihy- drochloride, 2-(carbamoylazo)isobutyronitrile and 4,4'-azobis(4-cyanovaleric acid). The polymerization initiators mentioned are used in customary amounts, for example in amounts of from 0.01 to 5 mol%, preferably 0.1 to 2 mol%, based on the monomers to be polymerized.

The redox initiators comprise, as oxidizing component, at least one of the above- indicated per compounds and a reducing component, for example ascorbic acid, glucose, sorbose, ammonium bisulfite, ammonium sulfite, ammonium thiosulfate, ammonium hyposulfite, ammonium pyrosulfite, ammonium sulfide, alkali metal bisulfite, alkali metal sulfite, alkali metal thiosulfate, alkali metal hyposulfite, alkali metal pyrosulfite, alkali metal sulfide, metal salts, such as iron(ll) ions or silver ions or sodium hydroxy- methylsulfoxylate. The reducing component of the redox initiator is preferably ascorbic acid or sodium pyrosulfite. 1 • 10"⁵ to 1 mol% of the reducing component of the redox initiator and 1 • 10 ⁵ to 5 mol% of the oxidizing component are used based on the amount of monomers used in the polymerization. Instead of the oxidizing component or in addition it is also possible to use one or more water-soluble azo initiators.

A redox initiator consisting of hydrogen peroxide, sodium peroxodisulfate and ascorbic acid is preferably used. These components are used for example in the concentrations of 1 • 10^"2 mol% of hydrogen peroxide, 0.084 mol% of sodium peroxodisulfate and 2.5 • 10^"3 mol% of ascorbic acid, based on the monomers.

It is also possible to initiate the polymerization by the numerous other known means to initiate polymerizations. On example is initiating polymerization by irradiating with radiation of sufficiently high energy, in particular ultraviolet light. Usually, when initiating polymerization by ultraviolet light, compounds are added which decompose into radicals upon irradiation by ultraviolet light. Examples of such compunds are 2-hydroxi-2- methyl-1-phenyl-1-propanone and/or alpha,-alpha-dimethoxi-alpha- phenylacetophenone.

The aqueous monomer solution may comprise the initiator in dissolved or dispersed form. However, the initiators may also be added to the polymerization reactor separately from the monomer solution.

The preferred polymerization inhibitors require dissolved oxygen for optimum effect. Therefore, the polymerization inhibitors can be freed of dissolved oxygen prior to polymerization, by inertization, i.e., by flowing an inert gas, preferably nitrogen, through them. This is accomplished by means of inert gas, which can be introduced cocur- rently, countercurrently or at entry angles in between. Good commixing can be achieved for example with nozzles, static or dynamic mixers or bubble columns. The oxygen content of the monomer solution is preferably lowered to less than 1 weight ppm and more preferably to less than 0.5 weight ppm prior to polymerization. The monomer solution is optionally passed through the reactor using an inert gas stream.

The preparation of a suitable polymer as well as further suitable hydrophilic ethyleni- cally unsaturated monomers a) are described for example in DE 199 41 423 A1 , EP 686 650 A1 , WO 01/45758 A1 and WO 03/104300 A1.

Superabsorbents are typically obtained by polymerization of an aqueous monomer solution and optionally a subsequent comminution of the hydrogel. Suitable methods of making are described in the literature. Superabsorbents are obtained for example by

• gel polymerization in the batch process or tubular reactor and subsequent com- minution in meat grinder, extruder or kneader, as described for example in

EP 445 619 A2 and DE 198 46 413 A1 ; • polymerization in kneader with continuous comminution by contrarotatory stirring shafts for example, as described for example in WO 01/38 402 A1 ;

• addition polymerization on belt and subsequent comminution in meat grinder, extruder or kneader, as described for example in EP 955 086 A2, DE 38 25 366 A1 or US 6,241 ,928;

• emulsion polymerization, which produces bead polymers having a relatively narrow gel size distribution, as described for example in EP 457 660 A1 ;

• in situ addition polymerization of a woven fabric layer which, usually in a continuous operation, has previously been sprayed with aqueous monomer solution and subsequently been subjected to a photopolymerization, as described for example in WO 02/94328 A2, WO 02/94329 A1.

The cited references are expressly incorporated herein for details of process operation. The reaction is preferably carried out in a kneader or on a belt reactor.

Continuous gel polymerization is the economically preferred and therefore currently customary way of manufacturing superabsorbents. The process of continuous gel polymerization is carried out by first producing a monomer mixture by admixing the acrylic acid solution with the neutralizing agent, optional comonomers and/or further auxiliary materials at different times and/or locations and then transferring the mixture into the reactor or preparing the mixture as an initial charge in the reactor. The initiator system is added last to start the polymerization. The ensuing continuous process of polymerization involves a reaction to form a polymeric gel, i.e., a polymer swollen in the polymerization solvent - typically water - to form a gel, and the polymeric gel is already com- minuted in the course of a stirred polymerization. The polymeric gel is subsequently dried, if necessary, and also chipped ground and sieved and is transferred for further surface treatment.

The acid groups of the hydrogels obtained are partially neutralized in an acid neutrali- zation step, generally to an extent of at least 25 mol%, preferably to an extent of at least 27 mol% and more preferably at least 40 mol% and generally to an extent of not more than 85 mol%, preferably not more than 80 mol%, and more preferably not more than 75 mol%, for which the phosphate-containing alkali hydroxide according to this invention is used as neutralizing agent.

Neutralization can also be carried out after polymerization, at the hydrogel stage. But it is also possible to carry out the neutralization to the desired degree of neutralization wholly or partly prior to polymerization. In the case of partial neutralization and prior to polymerization, generally at least 10 mol%, preferably at least 15 mol% and also gen- erally not more than 40 mol%, preferably not more than 30 mol% and more preferably not more than 25 mol% of the acid groups in the monomers used are neutralized prior to polymerization by adding a portion of the neutralizing agent to the monomer solution. The desired final degree of neutralization is in this case only set toward the end or after the polymerization, preferably at the level of the hydrogel prior to its drying. The monomer solution is neutralized by admixing the neutralizing agent. The hydrogel can be mechanically comminuted in the course of the neutralization, for example by means of a meat grinder or comparable apparatus for comminuting gellike masses, in which case the neutralizing agent can be sprayed, sprinkled or poured on and then carefully mixed in. To this end, the gel mass obtained can be repeatedly meat-grindered for ho- mogenization.

Neutralization of the monomer solution to the desired final degree of neutralization prior to polymerization by addition of the neutralizing agent or conducting the neutralization after polymerization is usually simpler than neutralization partly prior to and partly after polymerization and therefore is preferred.

The as-polymerized gels are optionally maintained for some time, for example for at least 30 minutes, preferably at least 60 minutes and more preferably at least 90 minutes and also generally not more than 12 hours, preferably for not more than 8 hours and more preferably for not more than 6 hours at a temperature of generally at least 50⁰C and preferably at least 70⁰C and also generally not more than 130⁰C and pref- erably not more than 100⁰C, which further improves their properties in many cases.

The neutralized hydrogel is then dried with a belt or drum dryer until the residual moisture content is preferably below 15% by weight and especially below 10% by weight, the water content being determined by EDANA (European Disposables and Nonwov- ens Association) recommended test method No. 430.2-02 "Moisture content". The dry superabsorbent consequently contains up to 15% by weight of moisture and preferably not more than 10% by weight. The decisive criterion for classification as "dry" is in particular a sufficient flowability for handling as a powder, for example for pneumatic conveying, pack filling, sieving or other processing steps involved in solids processing technology. Optionally, however, drying can also be carried out using a fluidized bed dryer or a heated plowshare mixer. To obtain particularly colorless products, it is advantageous to dry this gel by ensuring rapid removal of the evaporating water. To this end, dryer temperature must be optimized, air feed and removal has to be policed, and at all times sufficient venting has to be ensured. Drying is naturally all the more simple - and the product all the more colorless - when the solids content of the gel is as high as possible. The solvent fraction at addition polymerization is therefore set such that the solid content of the gel prior to drying is therefore generally at least 20% by weight, preferably at least 25% by weight and more preferably at least 30% by weight and also generally not more than 90% by weight, preferably not more than 85% by weight and more preferably not more than 80% by weight. It is particularly advantageous to vent the dryer with nitrogen or some other nonoxidizing inert gas. Optionally, however, simply just the partial pressure of oxygen can be lowered during drying to prevent oxidative yellowing processes. But in general adequate venting and removal of the water vapor will likewise still lead to an acceptable product. A very short drying time is generally advantageous with regard to color and product quality.

The dried hydrogel (which is no longer a gel (even though often still called that) but a dry polymer having superabsorbing properties, which comes within the term "superab- sorbent") is preferably ground and sieved, useful grinding apparatus typically including roll mills, pin mills, hammer mills, cutting mills or swing mills. The particle size of the sieved, dry hydrogel is preferably below 1000 μm, more preferably below 900 μm and most preferably below 850 μm and preferably above 80 μm, more preferably above 90 μm and most preferably above 100 μm.

Very particular preference is given to a particle size (sieve cut) in the range from 106 to 850 μm. Particle size is determined according to EDANA (European Disposables and Nonwovens Association) recommended test method No. 420.2-02 "Particle size distribution".

The dry superabsorbing polymers thus produced are typically known as "base polymers" and are then preferably surface postcrosslinked. Surface postcrosslinking can be accomplished in a conventional manner using dried, ground and classified polymeric particles. For surface postcrosslinking, compounds capable of reacting with the functional groups of the base polymer by crosslinking are applied, usually in the form of a solution, to the surface of the base polymer particles. Suitable postcrosslinkling agents are for example:

• di- or polyepoxides, for example di- or polyglycidyl compounds such as phos- phonic acid diglycidyl ether , ethylene glycol diglycidyl ether, bischlorohydrin ethers of polyalkylene glycols,

• alkoxysilyl compounds, • polyaziridines, compounds comprising aziridine units and based on polyethers or substituted hydrocarbons, for example bis-N-aziridinomethane,

• polyamines or polyamidoamines and also their reaction products with epichloro- hydrin,

• polyols such as ethylene glycol, 1 ,2-propanediol, 1 ,4-butanediol, glycerol, methyl- triglycol, polyethylene glycols having an average molecular weight Mw of 200 -

10 000, di- and polyglycerol, pentaerythritol, sorbitol, the ethoxylates of these polyols and also their esters with carboxylic acids or carbonic acid such as ethylene carbonate or propylene carbonate,

• carbonic acid derivatives such as urea, thiourea, guanidine, dicyandiamide, 2- oxazolidinone and its derivatives, bisoxazoline, polyoxazolines, di- and polyisocy- anates, • di- and poly-N-methylol compounds such as for example methylenebis(N- methylolmethacrylamide) or melamine-formaldehyde resins,

• compounds having two or more blocked isocyanate groups such as for example trimethylhexamethylene diisocyanate blocked with 2,2,3,6-tetramethylpiperidin- 4-one.

If necessary, acidic catalysts can be added, examples being p-toluenesulfonic acid, phosphoric acid, boric acid or ammonium dihydrogenphosphate.

Particularly suitable postcrosslinking agents are di- or polyglycidyl compounds such as ethylene glycol diglycidyl ether, the reaction products of polyamidoamines with epichlorohydrin, 2-oxazolidinone and N-hydroxyethyl-2-oxazolidinone.

Surface postcrosslinking (often just "postcrosslinking") is typically carried out by spray- ing a solution of the surface postcrosslinker (often just "postcrosslinker") onto the hy- drogel or the dry base polymer powder.

The solvent used for the surface postcrosslinker is a customary suitable solvent, examples being water, alcohols, DMF, DMSO and also mixtures thereof. Particular pref- erence is given to water and water-alcohol mixtures, example being water-methanol, water-1 ,2-propanediol and water-1 ,3-propanediol.

The spraying with a solution of the postcrosslinker is preferably carried out in mixers having moving mixing implements, such as screw mixers, paddle mixers, disk mixers, plowshare mixers and shovel mixers. Particular preference is given to vertical mixers and very particular preference to plowshare mixers and shovel mixers. Useful and known mixers include for example Lodige^®, Bepex^®, Nauta^®, Processall^® and Schugi^® mixers. Very particular preference is given to high speed mixers, for example of the Schugi-Flexomix^® or Turbolizer^® type.

The spraying with the crosslinker solution can be optionally followed by a thermal treatment step, essentially to effect the surface-postcrosslinking reaction (yet usually just referred to as "drying"), preferably in a downstream heated mixer ("dryer") at a temperature of generally at least 50⁰C, preferably at least 80⁰C and more preferably at least 80⁰C and also generally not more than 300⁰C, preferably not more than 250⁰C and more preferably not more than 200⁰C. The average residence time (i.e., the averaged residence time of the individual particles of superabsorbent) in the dryer of the superabsorbent to be treated is generally at least 1 minute, preferably at least 3 minutes and more preferably at least 5 minutes and also generally not more than 6 hours, preferably 2 hours and more preferably not more than 1 hour. As well as the actual drying taking place, not only any products of scissioning present but also solvent fractions are removed. Thermal drying is carried out in customary dryers such as tray dryers, rotary tube ovens or heatable screws, preferably in contact dryers. Preference is given to the use of dryers in which the product is agitated, i.e., heated mixers, more preferably shovel dryers and most preferably disk dryers. Bepex^® dryers and Nara^® dryers are suitable dryers for example. Fluidized bed dryers can also be used. But dry- ing can also take place in the mixer itself, by heating the jacket or blowing a preheated gas such as air into it. But it is also possible for example to utilize an azeotropic distillation as a drying process. The crosslinking reaction can take place not only before but also during drying.

A particularly preferred embodiment of the present invention additionally comprises modifying the hydrophilicity of the particle surface of the base or surface crosslinked polymers through formation of complexes. Complexes are formed on the outer shell of the particles by spraying with solutions of bi- or more highly valent cations, the cations being capable of reacting with the acid groups of the polymer to form complexes. Ex- amples of bi- or more highly valent cations are polymers formally constructed wholly or partly of vinylamine monomers, such as partially or completely hydrolyzed polyvinyla- mide (so-called "polyvinylamine") whose amine groups are always - even at very high pH values - partly present in a state of protonation to ammonium groups, or metal cations such as Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Ti⁴⁺, Mn²⁺, Fe^2+/3+, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Y³⁺, Zr⁴⁺, La³⁺, Ce⁴⁺, Hf⁴⁺, and Au³⁺. Preferred metal cations are Mg²⁺, Ca²⁺, Al³⁺, Ti⁴⁺, Zr⁴⁺ and La³⁺, and particularly preferred metal cations are Al³⁺, Ti⁴⁺ and Zr⁴⁺. The metal cations can be used not only alone but also in admixture with each other. Of the metal cations mentioned, any metal salt can be used that has sufficient solubility in the solvent to be used. Metal salts with weakly complexing anions such as for example chlo- ride, nitrate and sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate and carboxylate, such as acetate and lactate, are particularly suitable. It is particularly preferred to use aluminum sulfate. Useful solvents for the metal salts include water, alcohols, DMF, DMSO and also mixtures thereof. Particular preference is given to water and water-alcohol mixtures such as for example water-methanol, water-1 ,2-propanediol and water-1 ,3-propanediol.

The treatment of the base polymer with solution of a uni- or multivalent cation is carried out in the same way as that with surface postcrosslinker, including the selective drying step. Surface postcrosslinker and polyvalent cation can be sprayed onto the base polymer in a conjoint solution or as separate solutions. The spraying of the metal salt solution onto the particles of superabsorbent can take place not only before but also after the surface-postcrosslinking operation. In a particularly preferred process, the spraying with the metal salt solution takes place in the same step as the spraying with the crosslinker solution, the two solutions being dispensed separately in succession or simultaneously through two nozzles, or crosslinker solution and metal salt solution can be conjointly sprayed through one nozzle. When a drying step is carried out after surface postcrosslinking and/or treatment with complexing agent, it is advantageous but not absolutely necessary to cool the product after drying. Cooling can be carried out continuously or discontinuously, conveniently by conveying the product continuously into a cooler downstream of the dryer. Any ap- paratus known for removing heat from pulverulent solids can be used, in particular any apparatus mentioned above as a drying apparatus, provided it is supplied not with a heating medium but with a cooling medium such as for example with cooling water, so that heat is not introduced into the superabsorbent via the walls and, depending on the design, also via the stirrer elements or other heat-exchanging surfaces, but removed from the superabsorbent. Preference is given to the use of coolers in which the product is agitated, i.e., cooled mixers, for example shovel coolers, disk coolers or paddle coolers, for example Nara^® or Bepex^® coolers. The superabsorbent can also be cooled in a fluidized bed by blowing a cooled gas such as cold air into it. The cooling conditions are set such that a superabsorbent having the temperature desired for further process- ing is obtained. Typically, the average residence time in the cooler will be in general at least 1 minute, preferably at least 3 minutes and more preferably at least 5 minutes and also in general not more than 6 hours, preferably 2 hours and more preferably not more than 1 hour, and cooling performance will be determined such that the product obtained has a temperature of generally at least 0⁰C, preferably at least 10⁰C and more preferably at least 20⁰C and also generally not more than 100⁰C, preferably not more than 80⁰C and more preferably not more than 60⁰C.

Optionally, a further modification of the superabsorbent can be effected by admixing finely divided inorganic solids, for example silica, alumina, titania and iron(ll) oxide, which further enhances the effects of the surface aftertreatment. It is particularly preferred to admix hydrophilic silica or an alumina having an average primary particle size in the range from 4 to 50 nm and a specific surface area of 50 - 450 m²/g. Finely divided organic solids are preferably admixed after the surface modification through crosslinking/complexing, but can also be carried out before or during these surface modifications.

Optionally, superabsorbent is provided with further customary additives and auxiliary materials to influence storage or handling properties. Examples thereof are colorations, opaque additions to improve the visibility of swollen gel, which is desirable in some applications, additions to improve the flowability of the powder, surfactants or the like. The superabsorbent is often admixed with dustproofing or dustbinding agents. Dust- proofing or dustbinding agents are known in that for example polyether glycols such as polyethylene glycol having a molecular weight in the range from 400 to 20 000 g/mol, polyols such as glycerol, sorbitol, neopentylglycol or trimethylolpropane, which are op- tionally 7- to 20-tuply ethoxylated, are used. Similarly, a final water content can be set for the superabsorbent, if desired, by adding water. The solids, additives and auxiliary materials can each be added in separate processing steps, but one convenient method may be to add them to the superabsorbent in the cooler, for example by spraying the superabsorbent with a solution or adding them in finely divided solid or in liquid form, if this cooler provides sufficient mixing quality.

The surface-postcrosslinked superabsorbent is optionally ground and/or sieved in a conventional manner. Grinding is typically not necessary, but the sieving out of agglomerates which are formed or undersize is usually advisable to set the desired particle size distribution for the product. Agglomerates and undersize are either discarded or preferably returned into the process in a conventional manner and at a suitable point; agglomerates after comminution. The superabsorbent particle size is preferably not more than 1000 μm, more preferably not more than 900 μm, most preferably not more than 850 μm, and preferably at least 80 μm, more preferably at least 90 μm and most preferably at least 100 μm. Typical sieve cuts are for example 106 to 850 μm or 150 to 850 μm.

We have further found superabsorbent produced by the process of the present invention and hygiene articles comprising the superabsorbent produced by the process of the present invention. Hygiene articles in accordance with the present invention are for example those intended for use in mild or severe incontinence, such as for example inserts for severe or mild incontinence, incontinence briefs, also diapers, training pants for babies and infants or else feminine hygiene articles such as liners, sanitary napkins or tampons. Hygiene articles of this kind are known. The hygiene articles of the present invention differ from known hygiene articles in that they comprise the superabsorbent of the present invention. We have also found a process for producing hygiene articles, this process comprising utilizing at least one superabsorbent of the present invention in the manufacture of the hygiene article in question. Processes for producing hygiene articles using superabsorbent are otherwise known.

The present invention further provides for the use of the composition of the present invention in training pants for children, shoe inserts and other hygiene articles to absorb bodily fluids. The composition of the present invention can also be used in other technical and industrial fields where liquids, in particular water or aqueous solutions, are absorbed. These fields are for example storage, packaging, transportation (as con- stituents of packaging material for water- or moisture-sensitive articles, for example for flower transportation, also as protection against mechanical impacts); animal hygiene (in cat litter); food packaging (transportation of fish, fresh meat; absorption of water, blood in fresh fish or meat packs); medicine (wound plasters, water-absorbing material for burn dressings or for other weeping wounds), cosmetics (carrier material for phar- machemicals and medicaments, rheumatic plasters, ultrasonic gel, cooling gel, cosmetic thickeners, sun protection); thickeners for oil-in-water and water-in-oil emulsions; textiles (moisture regulation in textiles, shoe inserts, for evaporative cooling, for exam- pie in protective clothing, gloves, headbands); chemical engineering applications (as a catalyst for organic reactions, to immobilize large functional molecules such as enzymes, as adhesion agent in relation to agglomerations, heat storage media, filter aids, hydrophilic component in polymeric laminates, dispersants, superplasticizers); as auxil- iaries in powder injection molding, in building construction and engineering (installation, in loam-based renders, as a vibration-inhibiting medium, auxiliaries in tunnel excavations in water-rich ground, cable sheathing); water treatment, waste treatment, water removal (deicing agents, reusable sandbags); cleaning; agritech (irrigation, retention of melt water and dew deposits, composting additive, protection of forests against fun- gal/insect infestation, delayed release of active components to plants); for firefighting or for fire protection; coextrusion agents in thermoplastic polymers (for example to hydro- philcize multilayered films); production of films and thermoplastic moldings able to absorb water (for example rain and dew water storage films for agriculture; superabsor- bent-containing films for keeping fruit and vegetables fresh which are packed in moist films; superabsorbent-polystyrene coextrudates, for example for food packaging such as meat, fish, poultry, fruit and vegetables); or as carrier substance in formulations of active components (pharma, crop protection).

Test methods

Centrifuge retention capacity (CRC):

Centrifuge retention capacity (CRC) is determined by EDANA (European Disposables and Nonwovens Association, Avenue Eugene Plasky 157, 1030 Brussels, Belgium) recommended test method No. 441.2-02 "Centrifuge Retention Capacity", which is available from EDANA at the address given.

Whiteness Index

The whiteness index is determined using a spectrophotometer, model LabScan XE obtained from Hunter Associates Laboratory Inc., Reston, Virginia, U.S.A. according to this company's standard Wl E313 (defined by ASTM designation E313-73).

Examples

Mixtures of 21 1 g commercial grade sodium hydroxide having an iron content of 7 ppm each with the amounts of sodium phosphate Na3PO4 or phosphoric acid stated in table 1 below were prepared by thorough mixing.

Each of these mixtures was added to 1000 g of unneutralized internally crosslinked polyacrylic acid gel and mixed three times in a gel chopper (meat grinder). The gels were then dried, milled and sieved to obtain a particle size fraction of 106 to 850 Micrometer. The whiteness index of these samples were determined.

Table 1 is a compilation of the results. For comparison, a whiteness index of a su- perabsorbent prepared likewise using diaphragma grade sodium hydroxide having an iron content of 1.5 ppm without addition of phosphate is 35.

Table 1

The results summarized in table 1 demonstrate that the process of this invention provides superabsorbents that are at least as white as those produced by a conventional process.

Claims

We claim:

1. A process for producing superabsorbents comprising an acid neutralization step using alkali hydroxide containing phosphate ions.

2. The process of claim 1 in which the alkali hydroxide is sodium hydroxide.

3. The process of claim 1 in which phosphoric acid or a phosphate is added to the alkali hydroxide prior to its use in the neutralization step.

4. The process of claim 2 in which phosphoric acid or a phosphate is added to the sodium hydroxide prior to its use in the neutralization step.

5. The process of claim 3 in which the phosphate is sodium phosphate.

6. The process of claim 4 in which the phosphate is sodium phosphate.

7. The process of any of claims 1 to 6, in which at least 0,5 wt.-% of phosphate, based on alkali hydroxide, are added to the alkali hydroxide.

8. The superabsorbent produced by the process of any of the preceding claims.

9. A hygiene article comprising the superabsorbent of claim 8.

10. Process for producing hygiene articles, comprising utilizing at least one superabsorbent of claim 8 in the manufacture of the hygiene articles.