# Patent

## Title
Method for producing water-absorbing polymer particles by suspension polymerisation

## Abstract

A process for producing water-absorbing polymeric particles by a suspension polymerization with recycling of the hydrophobic solvent, the hydrophobic solvent comprising branched saturated aliphatic hydrocarbons.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This is the U.S. national phase of International Application No. PCT/EP2008/050105, filed Jan. 8, 2008, which claims the benefit of European patent Application No. 07100421.2, filed Jan. 11, 2007.
The present invention relates to a process for producing water-absorbing polymeric particles by a suspension polymerization with recycling of the hydrophobic solvent, the hydrophobic solvent comprising branched saturated aliphatic hydrocarbons.
The production of water-absorbing polymeric particles is described in the monograph “Modern Superabsorbent Polymer Technology”, F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998 pages 69 to 117. The water-absorbing polymeric particles are typically produced by solution polymerization or suspension polymerization.
Water-absorbing polymers are used as aqueous solutions of absorbing products to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening.
The properties of water-absorbing polymers can be controlled via the degree of crosslinking. Gel strength increases and absorptive capacity decreases with increasing degree of crosslinking.
To improve their performance characteristics, for example Saline Flow Conductivity (SFC) in the diaper and Absorbency Under Load (AUL), water-absorbing polymeric particles are generally postcrosslinked. This increases the degree of crosslinking of the particle surface only, making it possible to decouple Absorbency Under Load (AUL) and Centrifuge Retention Capacity (CRC) to some extent at least.
JP S63-218702 describes a continuous process for producing water-absorbing polymeric particles by suspension polymerization.
It is an object of the present invention to provide art improved process for producing water-absorbing polymeric particles by suspension polymerization.
We have found that this object is achieved by a process for producing water-absorbing polymeric particles by a suspension polymerization in a hydrophobic solvent, comprising

    i) metering a monomer solution by means of at least one feed line into a stirred reactor,
    ii) removing the hydrophobic solvent from the resulting water-absorbing polymeric particles, and
    iii) recycling the removed hydrophobic solvent into the process,


wherein the hydrophobic solvent is at least partly replaced by fresh hydrophobic solvent, the fresh hydrophobic solvent is at least partly a hydrocarbon mixture and the hydrocarbon mixture comprises at least 1% by weight of branched saturated aliphatic hydrocarbons.
Hydrophobic solvents have a 23° C. solubility in water of less than 5 g/100 g, preferably less than 1 g/100 g and more preferably less than 0.5 g/100 g.
Fresh hydrophobic solvent herein refers to hydrophobic solvent which is supplied to the process from the outside. In contradistinction thereto, recycled hydrophobic solvent was already being used in the process.
The fresh hydrophobic solvent comprises preferably at least 50% by weight, more preferably at least 75% by weight and most preferably at least 90% by weight of a hydrocarbon mixture. The fresh hydrophobic solvent can also be a hydrocarbon mixture itself.
Branched saturated aliphatic hydrocarbons are hydrocarbons having tertiary and quaternary carbon atoms, such as 2-methylpentane, 3-methylpentane, 2,2-dimethylpentane, methylcyclopentane, 2,4-dimethylpentane, 2,2,3-trimethylbutane, 3,3-dimethylpentane, 2-methylhexane, 2,3-dimethylpentane, 1.1-dimethylcyclopentane, 3-methylhexane, cis-1,3-dimethylcyclopentane, trans-1,3-dimethylcyclopentane, 3-ethylpentane, trans-1,2-dimethylcyclopentane, 2,2,4-trimethylpentane, methylcyclohexane, cis-1,2-dimethylcyclopentane, 1,1,3-trimethylcyclopentane, 2,2-dimethylhexane, ethylcyclopentane, 2,5-dimethylhexane, 2,2,3-trimethylpentane, 2,4-dimethylhexane, trans-cis-1,2,4-trimethylcyclopentane, 3,3-dimethylhexane, trans-cis-1,2,3-trimethylcyclopentane, 2,3,4-trimethylpentane, 2,3,3-trimethylpentane, 1,1,2-trimethylcyclopentane, 2,3-dimethylhexane, 2-methyl-3-ethylpentane, 2-methylheptane, 4-methylheptane, 3-methyl-3-ethylpentane, 3,4-dimethylhexane, cis-trans-1,2,4-trimethylcyclopentane, cis-cis-1,2,4-trimethylcyclopentane, cis-1,3-dimethylcyclohexane, 3-methylheptane, cis-trans-1,2,3-trimethylcyclopentane, 3-ethylhexane, trans-1,4-dimethylcyclohexane, 1,1-dimethylcyclohexane, 2,2,5-trimethylhexane, trans-1-ethyl-3-methylcyclopentane, cis-1-ethyl-3-methylcyclopentane, trans-1-ethyl-2-methylcyclopentane, 2,2,4-trimethylhexane, 1-ethyl-1-methylcyclopentane, trans-1,2-dimethylcyclohexane, cis-cis-1,2,3-trimethylcyclopentane, trans-1,3-dimethylcyclohexane, cis-1,4-dimethylcyclohexane, isopropylcyclopentane, 2,4,4-trimethylhexane, cis-1-ethyl-2-methylcyclopentane, 2,3,5-trimethylhexane, 2,2-dimethylheptane, cis-1,2-dimethylcyclohexane, 2,2,3-trimethylhexane, 2,4-dimethylheptane, 4,4-dimethylheptane, ethylcyclohexane, n-propylcyclopentane, 2-methyl-4-ethylhexane, 2,6-dimethylheptane, 1,1,3-trimethylcyclohexane, 2,5-dimethylheptane, 3,5-dimethylheptane, 3,3-dimethylheptane, 2,3,4-trimethylhexane, 2,3-dimethylheptane, 3,4-dimethylheptane, 4-ethylheptane, 4-methyloctane, 2-methyloctane, 3-ethylheptane, 3-methyloctane, 1,1,2-trimethylcyclohexane and 2,4,6-trimethylheptane.
Particularly hydrocarbons having tertiary carbon atoms are known polymerization regulators. Polymerization regulators terminate a growing polymer chain and start a new polymerization chain with time delay. This intervention in the polymerization kinetics leads to shorter polymer chains. This is undesirable in the production of long-chain polymers, particularly in the production of water-absorbing polymers. Therefore, the production of water-absorbing polymers by suspension polymerization has hitherto only utilized hydrophobic solvents free of branched saturated aliphatic hydrocarbons.
The present invention rests on the discovery that low concentrations of branched saturated aliphatic hydrocarbons can be tolerated and that branched saturated aliphatic hydrocarbons are partly destroyed during the polymerization, the concentration of branched saturated aliphatic hydrocarbons decreasing with every cycle.
Therefore, in a suspension polymerization involving solvent recycling, solvent losses can also be made good with hydrophobic solvents comprising branched saturated aliphatic hydrocarbons.
Hydrophobic solvents comprising branched saturated aliphatic hydrocarbons are for example hydrocarbon mixtures, such as Nappar® 6 (ExxonMobil Chemical, BE) or Exxol® Heptan (ExxonMobil Chemical, BE).
The hydrocarbon mixture comprises preferably 2.5% to 80% by weight, more preferably from 5% to 50% by weight and most preferably from 10% to 20% by weight of branched saturated aliphatic hydrocarbons.
The hydrocarbon mixture was preferably in the range from 50 to 150° C., more preferably in the range from 60 to 120° C. and most preferably in the range from 70 to 90° C.
The fraction of branched saturated aliphatic hydrocarbons can be determined by gas chromatography, for example in accordance with ASTM D 5134.
The fraction of recycled hydrophobic solvent is preferably from 50% to 99% by weight, more preferably from 70% to 95% by weight and most preferably from 80% to 90% by weight.
The form of recycling is not subject to any restriction. For example, the hydrophobic solvent can be removed from the water-absorbing polymeric particles by distillation. Preferably, however, the hydrophobic solvent is removed from the water-absorbing polymeric particles by filtration. However, it is also possible to remove a portion of the hydrophobic solvent by filtration and a further portion of the hydrophobic solvent by distillation.
Accordingly, the hydrophobic solvent can be distillatively purified before recycling. The fraction of distillatively purified hydrophobic solvent is preferably less than 80%, more preferably less than 50% and most preferably less than 20%.
In a preferred embodiment of the present invention, the feed line for the monomer solution dips into the liquid in the reactor, i.e., the feed line ends below the liquid surface. The feed line ends preferably at least 20%, more preferably at least 50% and most preferably at least 70% below the liquid surface, the distance of the liquid surface from the deepest point of the reactor floor being 100%. The dipped feed line provides water-absorbing polymeric particles having a reduced content of residual solvent.
The reaction is preferably carried out under reduced pressure, for example at a pressure of 800 mbar. The boiling point of the reaction mixture can be adjusted to the desired reaction temperature via the pressure.
The water-absorbing polymeric particles have a Centrifuge Retention Capacity (CRC) of typically at least 15 g/g, preferably at least 20 g/g, more preferably at least 25 g/g, even more preferably at least 30 g/g and most preferably at least 35 g/g. The Centrifuge Retention Capacity (CRC) of the water-absorbing polymeric particles is typically less than 60 g/g, Centrifuge Retention Capacity (CRC) being determined in accordance with the EDANA (European Disposables and Nonwovens Association) recommended test method No. 441.2-02 “Centrifuge retention capacity”.
Useful hydrophobic solvents include all solvents known to a skilled person for use in suspension polymerization. Preference is given to using aliphatic hydrocarbons, such as n-hexane, n-heptane, n-octane, n-nonane, n-decane, cyclohexane or mixtures thereof.
Preference among these solvents is given to those having a boiling point between 50 and 150° C. Heptane and cyclohexane are particularly preferred. The ratio between the hydrophobic solvents and the monomer solution is preferably in the range from 0.9 to 1.1.
The average diameter of the monomer solution droplets is preferably at least 200 μm, more preferably in the range from 250 to 800 μm and most preferably in the range from 300 to 600 μm, the particle diameter, which is the volume average diameter, being determinable by light scattering. The diameter of the monomer solution droplets can be adjusted via the stirring energy introduced.
To disperse the aqueous monomer solution in the hydrophobic solvent and to disperse the resulting water-absorbing polymeric particles it is preferable to add dispersing assistants. These may be anionic, cationic, nonionic or amphoteric surfactants or natural, semisynthetic or synthetic polymers.
Examples of anionic surfactants are sodium polyoxyethylene dodecyl ether sulfate and sodium dodecyl ether sulfate. An example of a cationic surfactant is trimethylstearyl-ammonium chloride. An example of an amphoteric surfactant is carboxymethyl-dimethylcetylammonium. Examples of nonionic surfactants are sucrose fatty acid esters, such as sucrose monostearate and sucrose dilaurate, sorbitan esters, such as sorbitan monostearate, polyoxyalkylene compounds based on sorbitan esters, such as polyoxyethylene sorbitan monostearate.
Examples of natural or semisynthetic polymers are cellulose derivatives, such as cellulose ethers, for example ethylcellulose, and cellulose esters, for example cellulose acetates. Examples of synthetic polymers are polyvinyl alcohols, polyvinyl alcohol derivatives, maleic acid-butadiene copolymers and quaternary salts, such as styrenedimethylaminoethyl methacrylate.
The dispersing assistant is customarily dissolved or dispersed in the hydrophobic solvent.
The dispersing agent is used in amounts between 0.01% and 10% by weight, preferably between 0.2% and 5% by weight and more preferably between 0.5% and 2% by weight, based on the monomer solution.
Dispersing assistant identity and amount can be used to control the diameter of the monomer solution droplets.
The monomer solutions to be used in the process of the present invention comprise typically at least one ethylenically unsaturated monomer a), selectively at least one crosslinker b), at least one initiator c) and water d).
The monomers a) are preferably water soluble, i.e., the solubility in water at 23° C. is typically at least 1 g/100 g of water, preferably at least 5 g/100 g of water, more preferably at least 25 g/100 g of water and most preferably at least 50 g/100 g of water, and preferably have at least one acid group each.
Suitable monomers a) are for example ethylenically unsaturated carboxylic acids, such as acrylic acid, hydroxyethyl acrylate, acrylamidomethylpropanesulfonic acid, styrenesulfonic acid, methacrylic acid, maleic acid, fumaric acid and itaconic acid. Acrylic acid and methacrylic acid are particularly preferred monomers. Acrylic acid is most preferable.
Preferred monomers a) have at least one acid group, and the acid groups are typically in an at least partly neutralized state, the degree of neutralization being preferably in the range from 25 to 95 mol %, more preferably in the range from 50 to 80 mol % and even more preferably in the range from 60 to 75 mol %. Customary neutralizing agents can be used, preference being given to alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and also mixtures thereof. Instead of alkali metal salts it is also possible to use ammonium salts. Sodium and potassium are particularly preferred as alkali metals, but most preference is given to potassium hydroxide, potassium carbonate or potassium bicarbonate and also mixtures thereof.
The proportion of the total amount of monomers a) which is attributable to acrylic acid and/or its salts is preferably at least 50 mol %, more preferably at least 90 mol % and most preferably at least 95 mol %.
The concentration of the monomers a) in the monomer solution is preferably at least 35% by weight, more preferably at least 40% by weight and most preferably at least 45% by weight.
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 hydroquinone monomethyl ether (MEHQ) and/or tocopherols.
Tocopherol refers to compounds of the following formula:




where R1 is hydrogen or methyl, R2 is hydrogen or methyl, R3 is hydrogen or methyl and R4 is hydrogen or an acid radical of 1 to 20 carbon atoms.

Preferred R4 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 R1═R2═R3=methyl, especially racemic alpha-tocopherol. R1 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 hydroquinone 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 methacrylate, 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 2003/104299 A1, WO 2003/104300 A1, WO 2003/104301 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 DE 103 55 401 A1, or crosslinker mixtures as described for example in DE 195 43 368 A1, DE 196 46 484 A1, WO 90/15830 A1 and WO 2002/32962 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, butanediol 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 100 and 1000.
However, particularly advantageous crosslinkers b) are di- and triacrylates of 3- to 20-tuply ethoxylated glycerol, of 3- to 20-tuply ethoxylated trimethylolpropane, of 3- to 20-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 propoxylated glycerol, of 3-tuply propoxylated trimethylolpropane, and also of 3-tuply mixedly ethoxylated or propoxylated glycerol, of 3-tuply mixedly ethoxylated or propoxylated trimethylolpropane, of 15-tuply ethoxylated glycerol, of 15-tuply ethoxylated trimethylolpropane, of at least 40-tuply ethoxylated glycerol, of at least 40-tuply ethoxylated trimethylolethane and also of at least 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 2003/104301 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.
The amount of crosslinker b) is preferably less than 0.5% by weight, more preferably less than 0.05% by weight and most preferably less than 0.01% by weight, all based on the monomer a).
Useful initiators c) 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. In some cases, it is advantageous to use mixtures of various initiators, for example mixtures of hydrogen peroxide and sodium peroxodisulfate or potassium peroxodisulfate. Mixtures of hydrogen peroxide and sodium peroxodisulfate can be used in any proportion.
Particularly preferred initiators c) are azo initiators, such as 2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride and 2,2′-azobis[2-(5-methyl-2-imidazolin-2-yl)propane]dihydrochloride, and photoinitiators, such as 2-hydroxy-2-methylpropiophenone and 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, redox initiators, such as sodium persulfate/hydroxymethylsulfinic acid, ammonium peroxodisulfate/-hydroxymethylsulfinic acid, hydrogen peroxide/hydroxymethylsulfinic acid, sodium persulfate/ascorbic acid, ammonium peroxodisulfate/ascorbic acid and hydrogen peroxide/ascorbic acid, photoinitiators, such as 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propan-1-one, and also mixtures thereof.
The initiators are used in customary amounts, for example in amounts from 0.001% to 5% by weight and preferably from 0.01% to 1% by weight, based on the monomers a).
The preferred polymerization inhibitors require dissolved oxygen for optimum performance. Therefore, the monomer solution may be freed of dissolved oxygen prior to polymerization by inertization, i.e., by flowing an inert gas, preferably nitrogen, through it. Preferably, the oxygen content of the monomer solution is lowered to less than 1 weight ppm and more preferably less than 0.5 weight ppm prior to polymerization.
When the polymerization is carried out under sufficient reflux, inertization can be dispensed with. In this case, the dissolved oxygen is removed from the polymerization reactor together with evaporating solvent.
Advantageously, two or more stirred reactors are connected in series. The supplementary reaction in further stirred reactors can be used to increase monomer conversion and to reduce backmixing.
It is further advantageous in this embodiment when the first stirred reactor is not excessively large. As a stirred reactor increases in size, the size distribution of the dispersed monomer solution droplets will inevitably broaden. A comparatively small first reactor therefore makes it possible to produce water-absorbing polymeric particles having a particularly narrow particle size distribution.
The polymer dispersion claimed is advantageously azeotropically dewatered in at least one further vessel.
The dried water-absorbing polymeric particles have a water content of preferably below 15% by weight and in particular below 10% by weight. Water content is determined in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. 430.2-02 “Moisture content”.
The polymeric particles may be postcrosslinked to further improve their properties. Useful postcrosslinkers are compounds comprising two or more groups capable of forming covalent bonds with the carboxylate groups of the hydrogel. Suitable compounds are for example alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyepoxides, as described in EP 83 022 A2, EP 543 303 A1 and EP 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 450 922 A2, or R-hydroxyalkylamides, as described in DE 102 04 938 A1 and U.S. Pat. No. 6,239,230.
Furthermore, DE 40 20 780 C1 describes cyclic carbonates, DE 198 07 502 A1 2-oxazolidone and its derivatives, such as 2-hydroxyethyl-2-oxazolidone, DE 198 07 992 C1 bis- and poly-2-oxazolidinones. DE 198 54 573 A1 2-oxotetrahydro-1,3-oxazine and its derivatives. DE 198 54 574 A1 N-acyl-2-oxazolidones, DE 102 04 937 A1 cyclic ureas. DE 103 34 584 A1 bicyclic amide acetals, EP 1 199 327 A2 oxetanes and cyclic ureas and WO 2003/31482 A1 morpholine-2,3-dione and its derivatives as useful postcrosslinkers.
It is further possible to use postcrosslinkers comprising additional polymerizable ethylenically unsaturated groups, as described in DE 37 13 601 A1.
The amount of crosslinker is preferably in the range from 0.01% to 1% by weight, more preferably in the range from 0.05% to 0.5% by weight and most preferably in the range from 0.1% to 0.2% by weight, all based on the polymer.
In a preferred embodiment, polyvalent cations are applied to the particle surface in addition to postcrosslinkers.
Useful polyvalent cations for the process of the present invention include for example bivalent cations, such as the cations of zinc, magnesium, calcium and strontium, tervalent cations, such as the cations of aluminum, iron, chromium, rare earths and manganese, quadruvalent cations, such as the cations of titanium and zirconium. Useful counterions include chloride, bromide, sulfate, hydrogensulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate and carboxylate, such as acetate and lactate. Aluminum sulfate is preferred.
The amount used of polyvalent cation is for example in the range from 0.001% to 0.5% by weight, preferably in the range from 0.005% to 0.2% by weight and more preferably in the range from 0.02% to 0.1% by weight, all based on the polymer.
The postcrosslinking step can be carried out in two different ways.
First, the dewatered polymer dispersion may be filtered, selectively dried and only the resulting water-absorbing polymeric particles postcrosslinked (postcrosslinking A). This form of postcrosslinking is typically carried out by spraying a solution of the post-crosslinker and selectively a solution of the polyvalent cation onto the water-absorbing polymeric particles. Thermal drying follows the spraying step, and the postcrosslinking reaction can take place not only before but also during drying.
The spraying with a solution of the crosslinker 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 mixers include for example Lödige mixers, Bepex mixers, Nauta mixers, Processall mixers and Schugi mixers.
Contact dryers are preferable, shovel dryers more preferable and disk dryers most preferable as apparatus in which thermal drying is carried out. Useful dryers include for example Bepex dryers and Nara dryers. Fluidized bed dryers can be used as well.
Drying can take place in the mixer itself, by heating the jacket or by blowing warm air into the mixer. Also suitable is a downstream dryer, for example a tray dryer, a rotary tube oven or a heatable screw. A fluidized bed dryer is particularly advantageously used for mixing and drying.
Preferred drying temperatures range from 100 to 250° C., preferably from 120 to 220° C. and more preferably from 130 to 210° C. The preferred residence time at this temperature in the reaction mixer or dryer is preferably at least 10 minutes, more preferably at least 20 minutes and most preferably at least 30 minutes.
Secondly, the postcrosslinkers and selectively the polyvalent cations can also be added to the selectively dewatered polymer dispersion, preferably as an aqueous solution (postcrosslinking B). This is followed by thermal postcrosslinking. Since the reaction temperature in this version is limited by the boiling point of the hydrophobic solvent used as a dispersant, there is a preference for using reactive postcrosslinkers, such as di- or polyepoxides. After thermal postcrosslinking, the polymer is selectively azetoptropically dewatered, filtered and dried.
The process of the present invention can also be carried out advantageously as a continuous operation.
The water-absorbing polymeric particles are tested using the test methods described hereinbelow.
Methods:
Measurements should be carried out, unless otherwise stated, at an ambient temperature of 23±2° C. and a relative humidity of 50±10%. The water-absorbing polymers are thoroughly commixed before measurement.
Residual Monomers
The level of residual monomers in the water-absorbing polymeric particles is determined in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. 410.2-02 “Residual monomers”.
Centrifuge Retention Capacity (CRC)
The centrifuge retention capacity of the water-absorbing polymeric particles is determined in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. 441.2-02 “Centrifuge retention capacity”.
Absorbency Under Load (AUL 0.7 psi)
Absorbency under load is determined in accordance with EDANA (European Disposables and Nonwovens Association) recommended test method No. 442.2-02 “Absorption under pressure”, using a weight of 49 g/cm2 (0.7 psi) instead of 21 g/cm2 (0.3 psi).


EXAMPLES
Example 1
Comparative
The following solutions were made up in separate stirred tanks:
Monomer Solution:
















360.3 kg (5 kmol)
of acrylic acid



196.4 kg (3.5 kmol)
of potassium hydroxide



0.492 kg
of polyethylene glycol 300 diacrylate



538.8 kg
of water










Initiator Solution:
















0.476 kg (2 mol)
of potassium persulfate



 7.2 kg
of water










Organic Phase:













 1136 kg
of cyclohexane (about 1630 l)


11.36 kg
of ethylcellulose



(comprising 49.5% of ethoxy groups)









Procedure:
A 5 m3 stirred tank equipped with blade stirrer, heating/cooling jacket, reflux condenser and means for azeotropic removal of water was charged with the cyclohexane, and the appropriate amount of ethylcellulose was dispersed therein by stirring. This was followed by inertization with nitrogen and heating to reflux (80° C.).
Then, in the course of 1.5 hours, the monomer solution was metered into the stirred tank together with the initiator solution, the monomer and initiator solutions being mixed by a static mixer shortly before entry into the reactor. Reflux conditions were maintained during the entire period of monomer addition.
The diameter of the inlet tubes was dimensioned such that the monomer solution was introduced into the organic phase in a thin, nonturbulent jet having a very low speed of flow. Altogether, 10 inlet tubes were used. The 10 inlet tubes had a spacing of 2 cm, were disposed perpendicularly to the tangential direction of the stirring shaft and ended 0.2 m above the liquid surface.
The resulting suspension was supplementarily reacted for one hour. The residual water content of the polymeric particles obtained was then lowered to 40% by weight by azeotropic removal of water.
Then, 1.0 kg of ethylene glycol diglycidyl ether was added before supplementary reaction under reflux conditions for 2 hours. During this period, water was removed to give a solids content of about 80% by weight for the polymeric particles.
The suspension was then cooled down and the polymeric particles were filtered off. Further drying was carried out at 60° C. in a paddle dryer to a residual moisture content of 5% by weight.
The product thus obtained had the following properties:

















CRC:
35.6
g/g



AUL0.7 psi:
25.0
g/g



Residual monomers:
20
ppm



Residual solvent:
0.1
ppm



Average particle size:
380
μm



Particles <200 μm:
1.0%
by weight



Particles >600 μm:
2.0%
by weight










Example 2
Comparative
Example 1 was repeated. Cyclohexane was replaced by Nappar® 6. Nappar® 6 is a hydrocarbon mixture comprising less than 5% by weight of n-hexane, 10% to 15% by weight of n-heptane, 70% to 80% by weight of cyclohexane and 10% to 15% by weight of 2-methylpentane (isohexane).
The product thus obtained had the following properties:

















CRC:
35.7
g/g



AUL0.7 psi:
22.1
g/g



Residual monomers:
23
ppm



Residual solvent:
0.2
ppm



Average particle size:
390
μm



Particles <200 μm:
0.9%
by weight



Particles >600 μm:
2.3%
by weight










Examples 1 and 2 show that using hydrophobic solvents comprising branched hydrocarbons causes the sum total of CRC and AUL0.7 psi to fall significantly compared with the use of cyclohexane.
Example 3
Comparative
A stirred tank cascade was used to produce water-absorbing polymeric particles in a continuous suspension polymerization process. The stirred tank cascade consisted of five reactors in series each with an internal volume of 2 m3. The reactors were equipped with propeller stirrer, heating jacket and reflux condenser. In addition, each reactor was equipped with means for azeotropic removal of water. The reactors were blanketed with nitrogen. The reaction temperature was 80° C. The pressure setting in the reactors was such that the reactor contents boiled under reflux.
Per hour, a mixture of 1600 l of cyclohexane and 11.2 kg of ethylcellulose (comprising 49.5% of ethoxy groups) were metered from a first makeup tank into the first reactor.
Per hour, 1000 kg of a mixture of 9.85% by weight of acrylic acid, 35.1% by weight of potassium acrylate, 0.045% by weight of polyethylene glycol 300 diacrylate and water and 7.0 kg of a mixture of 6.2% by weight of potassium sulfate and water were withdrawn respectively from a second makeup tank and from a third makeup tank, mixed in a static mixer and metered via inlet tubes into the first reactor.
The inlet tubes into the first reactor ended 0.5 m below the liquid surface. The diameter of the inlet tubes was dimensioned such that the monomer solution was introduced in a thin, nonturbulent jet having a very low speed of flow. All together 10 inlet tubes were used. The 10 inlet tubes had a spacing of 2 cm and were disposed perpendicularly to the tangential direction of the stirring shaft.
The fill level of the reactors was kept constant by withdrawal of suspension. The suspension was removed from the reactors at half the height of the liquid surface.
Per hour, 15 kg of water were azeotropically removed in the third reactor.
Per hour, 10 kg of a mixture of 10% by weight of ethylene glycol diglycidyl ether and water were metered from a fourth makeup tank into the fourth reactor. Per hour, 90 kg of water were azeotropically removed in the fourth reactor.
Per hour, 80 kg of water were azeotropically removed in the fifth reactor.
The suspension was withdrawn from the fifth reactor and transferred to a knife-discharge centrifuge. In the knife-discharge centrifuge, the water-absorbing polymeric particles were separated from the cyclohexane. The water-absorbing polymeric particles separated off were dried at 60° C. in a paddle dryer down to a residual moisture content of 5% by weight.
The cyclohexane separated off in the knife-discharge centrifuge was returned without further treatment into the first makeup tank. Losses of cyclohexane were made good by addition of fresh cyclohexane.
The product thus obtained had the following properties once stable conditions were achieved:

















CRC:
34.6
g/g



AUL0.7 psi:
25.7
g/g



Residual monomers:
1.5
ppm



Residual solvent:
<0.1
ppm



Average particle size:
390
μm



Particles <200 μm:
0.8%
by weight



Particles >600 μm:
1.8%
by weight










Example 4
Example 3 was repeated. Cyclohexane was replaced by Nappar® 6. Nappar® 6 is a hydrocarbon mixture comprising less than 5% by weight of n-hexane, 10% to 15% by weight of n-heptane, 70% to 80% by weight of cyclohexane and 10% to 15% by weight of 2-methylpentane (isohexane).
The product thus obtained had the following properties once stable conditions were achieved:

















CRC:
34.8
g/g



AUL0.7 psi:
24.5
g/g



Residual monomers:
1.8
ppm



Residual solvent:
<0.1
ppm



Average particle size:
405
μm



Particles <200 μm:
1.0%
by weight



Particles >600 μm:
2.0%
by weight










Examples 3 and 4 show that the present invention's use of hydrophobic solvents comprising branched hydrocarbons leaves the sum total of CRC and AUL0.7 psi relatively unchanged compared with the use of cyclohexane.

## Claims

1. A process for producing water-absorbing polymeric particles from a reaction mixture by a suspension polymerization in a hydrophobic solvent, comprising:
i) metering a monomer solution using at least one feed line into a stirred reactor,
ii) removing the hydrophobic solvent from the resulting water-absorbing polymeric particles, and
iii) recycling the removed hydrophobic solvent into the process, wherein the hydrophobic solvent is a mixture of branched and unbranched saturated aliphatic hydrocarbons and is at least partly replaced by fresh hydrophobic solvent, the fresh hydrophobic solvent is at least partly a hydrocarbon mixture and the hydrocarbon mixture comprises 2.5% to 80% by weight of branched saturated aliphatic hydrocarbons.

2. The process according to claim 1 wherein a recycle fraction of the hydrophobic solvent is 70% to 95% by weight.

3. The process according to claim 1 wherein a recycle fraction of the hydrophobic solvent is 50% to 99% by weight.

4. The process according to claim 1 wherein the hydrocarbon mixture comprises 5% to 50% by weight of branched aliphatic hydrocarbons.

5. The process according to claim 1 wherein a recycle fraction of the hydrophobic solvent is 80% to 90% by weight.

6. The process according to claim 1 wherein the hydrocarbon mixture comprises 10% to 20% by weight of branched saturated aliphatic hydrocarbons.

7. The process according to claim 1 wherein the water-absorbing polymeric particles have a Centrifuge Retention Capacity of at least 15 g/g.

8. The process according to claim 1 wherein the monomer solution comprises at least 35% by weight of monomers.

9. The process according to claim 1 wherein the water-absorbing polymeric particles comprise less than 0.1% by weight of an interpolymerized crosslinker.

10. The process according to claim 1 wherein the water-absorbing polymeric particles comprise at least 50 mol % of at least partially neutralized polymerized acrylic acid.

11. The process according to claim 1 wherein the suspension polymerization is carried out continuously.

12. The process according to claim 1 wherein the reaction mixture is azeotropically dewatered, postcrosslinked, azeotropically dewatered again, filtered, and dried.

13. The process according to claim 1 wherein the reaction mixture is azeotropically dewatered, filtered, dried, and postcrosslinked.

14. The process according to claim 1 wherein the reaction mixture is supplementarily reacted in at least one further stirred reactor.

15. The process according to claim 1 wherein a dispersing assistant is used in the suspension polymerization.

16. The process according to claim 1 wherein a feed line in the stirred reactor ends below a liquid surface.

17. The process according to claim 1 wherein less than 80% by weight of the recycled hydrophobic solvent is distillatively purified during or after the removing ii) and before the recycling iii).

18. The process according to claim 1 wherein the hydrocarbon mixture boils in the range from 50 to 150° C.