EP1838463B2 - Method for grading a particulate water-absorbing resin - Google Patents

Abstract

Process for classifying a particulate water-absorbing resin using a sieving apparatus at a reduced pressure compared with the ambient pressure and a sieving apparatus for classifying a particulate water-absorbing resin at a reduced pressure compared with the ambient pressure.

Description

The present invention relates to a method for classifying a particulate water-absorbent resin by means of a sieving apparatus at a pressure reduced from the ambient pressure.

The preparation of water-absorbent resins has been described many times, see for example " Modern Superabsorbent Polymer Technology ", FL Buchholz and AT Graham, Wiley-VCH, 1998, pp. 69-177 ,

Water-absorbent resins typically have a centrifuge retention capacity of 15 to 60 g / g, preferably at least 20 g / g, preferably at least 25 g / g, more preferably at least 30 g / g, most preferably at least 35 g / g. Centrifuge retention capacity (CRC) is determined according to the EDANA (European Disposables and Nonwovens Association) recommended test method no. 441.2-02 "Centrifuge retention capacity".

The preparation of water-absorbing resins usually comprises the steps of polymerization, drying, comminution, classification, postcrosslinking and, if appropriate, renewed classification.

A general overview of the classification is, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 4th Edition, Volume 2, pages 43 to 56, Verlag Chemie, Weinheim, 1972 , to find.

Especially in the classification of water-absorbing resins, however, there is the problem that the screening performance is reduced by agglomeration. So teaches EP-A-0 855 232 in that the sieves used must be kept in the heated state or thermally insulated.

US 2003/87983 teaches that when sieving at elevated temperature of the metal abrasion and thus the wear on the screening device greatly increases.

The object of the present invention was to find a simplified method for the classification of water-absorbing resins, which allows high screening performance and long equipment runtimes.

It has now been found that this object is achieved by classifying water-absorbing resins at reduced pressure relative to the ambient pressure, preferably at a pressure of at most 950 mbar, preferably at a pressure of at most 900 mbar, particularly preferably at a pressure of at most 800 mbar preferably at a pressure of at most 700 mbar, wherein the resin is overflown during the classifying with a gas, and wherein the gas flow in front of the screening device has a temperature of at least 40 ° C. The pressure is usually at least 10 mbar, preferably at least 50 mbar, preferably at least 100 mbar, more preferably at least 200 mbar, most preferably at least 300 mbar. Another aspect of the present invention is the screening device for carrying out the classification method according to the invention.

The screening devices which are suitable for the classification method according to the invention are not subject to any restrictions; plane sieve methods are preferred, tumble screening machines are very particularly preferred. The screening device is typically shaken to aid classification. This is preferably done so that the material to be classified is spirally guided over the sieve. This forced vibration typically has an amplitude of 0.7 to 40 mm, preferably 1.5 to 25 mm, and a frequency of 1 to 100 Hz, preferably of 5 to 10 Hz.

The water-absorbent resin is overflowed during the classifying with a gas stream, particularly preferably air. The amount of gas is typically from 0.1 to 10 m ³ / h per m ² screen area, preferably from 0.5 to 5 m ³ / h per m ² screen area, particularly preferably from 1 to 3 m ³ / h per m ² screen area, the gas volume being measured under standard conditions (25 ° C and 1 bar). The gas stream is before entering the screening device to a temperature of at least 40 ° C, preferably to a temperature of at least 50 ° C, preferably to a temperature of at least 60 ° C, more preferably to a temperature of at least 65 ° C, all more preferably at a temperature of at least 70 ° C. warmed up. The temperature of the gas stream is usually less than 120 ° C, preferably less than 110 ° C, preferably less than 100 ° C, more preferably less than 90 ° C, most preferably less than 80 ° C. The water content of the gas stream is typically not more than 5 g / kg, preferably not more than 4.5 g / kg, preferably not more than 4 g / kg, more preferably not more than 3.5 g / kg, most preferably not more than 3 g / kg. A gas stream with a low water content can be generated, for example, by condensing a corresponding amount of water from the gas stream having a higher water content by cooling.

In addition, the screening device can still be heated and / or thermally insulated, such as in EP-A-0 855 232 described. Typically, the screening device is operated at a temperature of 40 to 80 ° C.

1. i) mindestens ein ethylenisch ungesättigtes, säuregruppentragendes Monomer,
2. ii) mindestens eines Vernetzer,
3. iii) gegebenenfalls ein oder mehrere mit i) copolymerisierbare ethylenisch und/oder allylisch ungesättigte Monomere und
4. iv) gegebenenfalls ein oder mehrere wasserlösliche Polymere, auf die die Monomere i), ii) und ggf. iii) zumindest teilweise aufgepfropft werden können,
   wobei das dabei erhaltene Grundpolymer getrocknet, klassiert,
5. v) gegebenenfalls mit mindestens einem Nachvernetzer nachbehandelt, getrocknet und thermisch nachvernetzt

wird, hergestellt werden.The water-absorbing resins which can be used in the process according to the invention can be obtained by polymerization of a monomer solution comprising

1. i) at least one ethylenically unsaturated, acid group-carrying monomer,
2. ii) at least one crosslinker,
3. iii) optionally one or more ethylenically and / or allylically unsaturated monomers copolymerizable with i) and
4. iv) optionally one or more water-soluble polymers to which the monomers i), ii) and optionally iii) can be at least partially grafted,
   wherein the base polymer thereby obtained dried, classified,
5. v) optionally post-treated with at least one postcrosslinker, dried and thermally postcrosslinked

will be produced.

Suitable monomers i) 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 acid esters. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particular preference is given to acrylic acid.

The monomers i), in particular acrylic acid, preferably contain up to 0.025 wt .-% of a Hydrochinonhalbethers. Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and / or tocopherols.

wobei R¹ Wasserstoff oder Methyl, R² Wasserstoff oder Methyl, R³ Wasserstoff oder Methyl und R⁴ Wasserstoff oder ein Säurerest mit 1 bis 20 Kohlenstoffatomen bedeutet.Tocopherol is understood as meaning compounds of the following formula

wherein R ^{1 is} hydrogen or methyl, R ^{2 is} hydrogen or methyl, R ^{3 is} hydrogen or methyl and R ^{4 is} hydrogen or an acid radical having 1 to 20 carbon atoms.

Preferred radicals for R ⁴ are acetyl, ascorbyl, succinyl, nicotinyl and other physiologically acceptable carboxylic acids. The carboxylic acids may be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol with R ¹ = R ² = R ³ = methyl, in particular racemic alpha-tocopherol. R ⁴ is particularly preferably hydrogen or acetyl. Especially preferred is RRR-alpha-tocopherol.

The monomer solution preferably contains at most 130 ppm by weight, more preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight, particularly preferably around 50 ppm by weight, hydroquinone hemether, in each case on acrylic acid, wherein acrylic acid salts are mathematically taken into account as acrylic acid. For example, an acrylic acid having a corresponding content of hydroquinone half-ether may be used to prepare the monomer solution.

The water-absorbing polymers are crosslinked, ie the polymerization is carried out in the presence of compounds having at least two polymerisable groups which can be radically copolymerized into the polymer network. Suitable crosslinkers ii) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, as in EP-A-0 530 438 described, di- and triacrylates, as in EP-A-0 547 847 . EP-A-0 559 476 . EP-A-0 632 068 . WO-A-93/21237 . WO-A-03/104299 . WO-A-03/104300 . WO-A-03/104301 and in the German patent application with the file number 10331450.4 described, mixed acrylates containing in addition to acrylate groups further ethylenically unsaturated groups, as in the German patent applications with the file reference 10331456.3 and 10355401.7 or crosslinker mixtures, such as in DE-A-195 43 368 . DE-A-196 46 484 . WO 90/15830 and WO-A-02/32962 described.

Suitable crosslinkers ii) are, in particular, N, N'-methylenebisacrylamide and N, N'-methylenebismethacrylamide, esters of unsaturated monocarboxylic or polycarboxylic acids of polyols, such as diacrylate or triacrylate, for example butanediol or ethylene glycol diacrylate or methacrylate, and trimethylolpropane triacrylate and allyl compounds, such as allyl (meth) acrylate, triallyl cyanurate, maleic acid diallyl esters, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, allyl esters of phosphoric acid and vinylphosphonic acid derivatives, as described, for example, in US Pat EP-A-0 343 427 are described. Further suitable crosslinkers ii) are pentaerythritol di-, pentaerythritol tri- and pentaerythritol tetraallyl ethers, polyethylene glycol diallyl ether, ethylene glycol diallyl ether, glycerol and glycerol triallyl ethers, polyallyl ethers based on sorbitol, and ethoxylated variants thereof. Useful in the process according to the invention are di (meth) acrylates of polyethylene glycols, wherein the polyethylene glycol used has a molecular weight between 300 and 1000.

However, particularly advantageous crosslinkers ii) are di- and triacrylates of 3 to 20 times ethoxylated glycerol, 3 to 20 times ethoxylated trimethylolpropane, 3 to 20 times ethoxylated trimethylolethane, especially di- and triacrylates of 2 to 6-fold ethoxylated glycerol or trimethylolpropane, 3-fold propoxylated glycerin or trimethylolpropane, as well as the 3-times mixed ethoxylated or propoxylated glycerol or trimethylolpropane, the 15-fold ethoxylated glycerol or trimethylolpropane, and at least 40-times ethoxylated glycerol, trimethylolethane or trimethylolpropane.

Very particularly preferred crosslinkers ii) are the polyethoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form di- or triacrylates, as described, for example, in the earlier German application with file reference DE 10319462.2 are described. Particularly advantageous are di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol. Very particular preference is given to diacrylates or triacrylates of 1 to 5 times ethoxylated and / or propoxylated glycerol. Most preferred are the triacrylates of 3 to 5 times ethoxylated and / or propoxylated glycerin. These are characterized by particularly low residual contents (typically below 10 ppm by weight) in the water-absorbent 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 to water of the same temperature.

For example, acrylamide, methacrylamide, crotonic acid amide, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate, and dimethylaminoneopentyl methacrylate are exemplified by ethylenically unsaturated monomers iii) copolymerizable with monomers i).

As water-soluble polymers iv) it is possible to use polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, polyglycols or polyacrylic acids, preferably polyvinyl alcohol and starch.

The preparation of a suitable base polymer as well as other suitable hydrophilic ethylenically unsaturated monomers i) are described in DE-A-199 41 423 . EP-A-0 686 650 . WO-A-01/45758 and WO-A-03/104300 described.

The reaction is preferably carried out in a kneader, such as in WO-A-01/38402 described, or on a belt reactor, such as in EP-A-0 955 086 described, performed.

The acid groups of the hydrogels obtained are usually partially neutralized, preferably from 25 to 95 mol%, preferably from 27 to 80 mol%, particularly preferably from 27 to 30 mol% or from 40 to 75 mol%, using the usual neutralizing agents may, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogencarbonates and mixtures thereof. Instead of alkali metal salts and ammonium salts can be used. Sodium and potassium are particularly preferred as alkali metals, but most preferred are sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof. Usually, the neutralization is achieved by mixing the neutralizing agent as an aqueous solution, as a melt, or preferably as a solid. For example, sodium hydroxide with a water content well below 50 wt .-% may be present as a waxy mass with a melting point above 23 ° C. In this case, a dosage as general cargo or melt at elevated temperature is possible.

The neutralization can be carried out after the polymerization at the hydrogel stage. However, it is also possible to neutralize up to 40 mol%, preferably 10 to 30 mol%, particularly preferably 15 to 25 mol%, of the acid groups before the polymerization by adding a part of the neutralizing agent already to the monomer solution and the desired final degree of neutralization only after the polymerization is adjusted at the level of the hydrogel. The monomer solution can be neutralized by mixing in the neutralizing agent. The hydrogel can be mechanically comminuted, for example by means of a meat grinder, wherein the neutralizing agent can be sprayed, sprinkled or poured over and then thoroughly mixed. For this purpose, the gel mass obtained can be further gewolfft for homogenization. Neutralization of the monomer solution directly to the final degree of neutralization is preferred.

The neutralized hydrogel is then dried with a belt or drum dryer until the residual moisture content is preferably below 15 wt .-%, in particular below 10 wt .-%, wherein the water content according to the recommended by the EDANA (European Disposables and Nonwovens Association) Test Method no 430.2-02 "Moisture content" is determined. Alternatively, a fluidized bed dryer or a heated ploughshare mixer can be used for drying. To obtain particularly white products, it is advantageous in the drying of this gel to ensure rapid removal of the evaporating water. For this purpose, the dryer temperature must be optimized, the air supply and removal must be controlled, and it is in any case to ensure adequate ventilation. The drying is naturally simpler and the product is the whiter, if the solids content of the gel is as high as possible. The solids content of the gel before drying is therefore preferably between 30 and 80% by weight. Particularly advantageous is the ventilation of the dryer with nitrogen or other non-oxidizing inert gas. Optionally, however, it is also possible simply to lower only the partial pressure of the oxygen during the drying in order to prevent oxidative yellowing processes. As a rule, however, sufficient ventilation and removal of the water vapor also leads to an acceptable product. Advantageous in terms of color and product quality is usually the shortest possible drying time.

Another important function of the drying of the gel is the here still occurring reduction of the residual monomer content in the superabsorber. During drying, possibly remaining residues of the initiators decompose and lead to a copolymerization of residual monomers remaining. In addition, the tear vaporizing amounts of water remaining free steam-volatile monomers such as acrylic acid with, and thus also reduce the residual monomer content in the superabsorbent.

The dried hydrogel is thereafter ground and classified, wherein for grinding usually one- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibratory mills can be used.

To improve application properties, such as diaper fluid conductivity (SFC) and absorption under pressure (AUL), water-absorbent polymer particles are generally postcrosslinked. This postcrosslinking can be carried out in aqueous gel phase. Preferably, however, ground and sieved polymer particles (base polymer) are coated on the surface with a postcrosslinker, dried and thermally postcrosslinked. Crosslinkers suitable for this purpose are compounds which contain at least two groups which can form covalent bonds with the carboxylate groups of the hydrophilic polymer or which can crosslink at least two carboxyl groups or other functional groups of at least two different polymer chains of the base polymer.

Suitable postcrosslinkers v) are compounds which contain at least two groups which can form covalent bonds with the carboxylate groups of the polymers. Suitable compounds are, for example, alkoxysilyl compounds, polyaziridines, polyamines, polyamidoamines, di- or polyglycidyl compounds, as in EP-A-0 083 022 . EP-A-543,303 and EP-A-937 736 described, polyhydric alcohols, as in DE-C-33 14 019 . DE-C-35 23 617 and EP-A-450 922 described, or β-hydroxyalkylamides, as in DE-A-102 04 938 and US 6,239,230 described. Also suitable are compounds having mixed functionality, such as glycidol, 3-ethyl-3-oxetanemethanol (trimethylolpropane oxetane), as in EP-A-1 199 327 described, aminoethanol, diethanolamine, triethanolamine or compounds which form a further functionality after the first reaction, such as ethylene oxide, propylene oxide, isobutylene oxide, aziridine, azetidine or oxetane.

Furthermore are in DE-A-40 20 780 cyclic carbonates, in DE-A-198 07 502 2 Oxazolidone and its derivatives, such as N- (2-hydroxyethyl) -2-oxazolidone, in DE-A-198 07 992 Bis- and poly-2-oxazolidinone, in DE-A-198 54 573 2 Oxotetrahydro-1,3-oxazine and its derivatives, in DE-A-198 54 574 N-acyl-2-oxazolidones, in DE-A-102 04 937 cyclic ureas, in the German patent application with the file number 10334584.1 bicyclic amide acetals, in EP-A-1 199 327 Oxetane and cyclic ureas and in WO-A-03/031482 Morpholine-2,3-dione and its derivatives are described as suitable post-crosslinkers v).

The post-crosslinking is usually carried out so that a solution of the postcrosslinker is sprayed onto the hydrogel or the dry base polymer particles. Subsequent to the spraying, it is thermally dried, whereby the postcrosslinking reaction can take place both before and during the drying.

The spraying of a solution of the crosslinker is preferably carried out in mixers with agitated mixing tools, such as screw mixers, paddle mixers, disk mixers, plowshare mixers and paddle mixers. Vertical mixers are particularly preferred, plowshare mixers and paddle mixers are very particularly preferred. Suitable mixers are, for example, Lödige® mixers, Bepex® mixers, Nauta® mixers, Processall® mixers and Schugi® mixers.

The thermal drying is preferably carried out in contact dryers, more preferably paddle dryers, very particularly preferably disk dryers. Suitable dryers include Bepex® dryers and, Nara® dryers. Moreover, fluidized bed dryers can also be used.

The drying can take place in the mixer itself, by heating the jacket or blowing hot air. Also suitable is a downstream dryer, such as a hopper dryer, a rotary kiln or a heatable screw. However, it is also possible, for example, to use an azeotropic distillation as the drying process.

Preferred drying temperatures are in the range 50 to 250 ° C, preferably at 50 to 200 ° C, and particularly preferably at 50 to 150 ° C. The preferred residence time at this temperature in the reaction mixer or dryer is less than 30 minutes, more preferably less than 10 minutes.

The classification method according to the invention is preferably carried out after the drying of the base polymer, before the post-crosslinking and / or after the post-crosslinking. The water content of the water-absorbing resin after drying of the base polymer or before post-crosslinking is typically from 2 to 10% by weight and after post-crosslinking typically below 1% by weight, preferably below 0.1% by weight.

Examples example 1

In a Lödige ploughshare kneader type VT 5R-MK (5 l volume), 388 g of deionized water, 173.5 g of acrylic acid, 2033.2 g of a 37.3% strength by weight sodium acrylate solution (100 mol% neutralized) and 4, 50 g of 15-fold ethoxylated trimethylolpropane triacrylate (for example, Sartomer® SR9035) and rendered inert under bubbling nitrogen for 20 minutes. It was then started by addition (dilute aqueous solutions) of 2.121 g of sodium persulfate, 0.045 g of ascorbic acid and 0.126 g of hydrogen peroxide at 23 ° C. After the start, the temperature of the heating mantle was adjusted to the reaction temperature in the reactor by means of control. The crumbly gel finally obtained was then dried at 160 ° C for 3 hours in a convection oven. It was then ground and sieved to 250 to 850 microns. The water content was 2.7% by weight.

The ground base polymer was added to the sieve at the indicated temperature. The sieve could be operated at reduced pressure. In addition, the screen was covered with preheated air with defined water vapor content. The amount of air was 2 m ³ / h per m ² screen area. Ex. Temperature of the base polymer [° C] Pressure [mbar] Temperature of gas flow [° C] Water vapor content of the gas stream [g / kg] sieving 1 60 500 55 4 1 2 60 500 75 4 1 3 60 500 35 4 2 4 60 500 25 4 3 5 50 500 50 2 1 6 50 1013 50 2 2 7 60 500 60 2 1 8th 60 500 60 4 2 9 60 500 60 6 3 sieving: 1 slight caking on sieve and wall, no caking in the sieved product 2 low caking on sieve and wall, low caking in the sieved product 3 caking on sieve and wall, caking in sieved product

Claims (14)

1. A process for classifying a particulate water-absorbing resin using a sieving apparatus, which process comprises operating the sieving apparatus at a reduced pressure compared with the ambient pressure and a gas stream passing over the resin during the classifying process, the gas stream being incipiently heated to a temperature of not less than 40°C before entry into the sieving apparatus.
2. The process according to claim 1 wherein the sieving apparatus is operated at a pressure of not more than 950 mbar.
3. The process according to claim 2 wherein the sieving apparatus is operated at a pressure in the range from 300 to 700 mbar.
4. The process according to any one of claims 1 to 3 wherein the gas rate is in the range from 0.1 to 10 m³/h per m² of sieve area.
5. The process according to claim 4 wherein the gas stream is air.
6. The process according to any one of claims 1 to 5 wherein the gas stream has a temperature in the range from 40 to 120°C.
7. The process according to any one of claims 4 to 6 wherein the water content of the gas stream is less than 5 g/kg.
8. The process according to any one of claims 4 to 7 wherein the gas volume stream is in the range from 1 to 10 m³/h per m² sieve area, the gas volume being measured at a temperature of 25°C and a pressure of 1 bar.
9. The process according to any one of claims 1 to 8 wherein the sieving apparatus is partly or wholly thermally insulated.
10. The process according to any one of claims 1 to 9 wherein the temperature of the sieving apparatus is in the range from 40 to 80°C.
11. The process according to any one of claims 1 to 10 wherein the sieving apparatus vibrates.
12. The process according to claim 11 wherein the frequency of vibration is in the range from 1 to 100 Hz.
13. The process according to any one of claims 1 to 12 wherein the particulate water-absorbing resin is obtained by addition polymerization of a solution comprising acrylic acid and/or methacrylic acid.
14. The process according to claim 13 wherein the acrylic acid and/or methacrylic acid is at least 40% neutralized.