WO2013143943A1

WO2013143943A1 - Method for thermal surface post-crosslinking in a drum-type heat exchanger having an inverse screw flight

Info

Publication number: WO2013143943A1
Application number: PCT/EP2013/055827
Authority: WO
Inventors: Holger Barthel; Gerald GRüNEWALD; Norbert Herfert; Markus MüHL; Birgit Reinhard
Original assignee: Basf Se
Priority date: 2012-03-30
Filing date: 2013-03-20
Publication date: 2013-10-03
Also published as: JP2015512990A; CN104204039A; EP2831153A1

Abstract

The invention relates to a method for the thermal surface post-crosslinking of water-absorbing polymer particles, wherein the polymer particles are coated with an aqueous solution, the coated polymer particles are disaggregated, and the disaggregated polymer particles are thermally surface post-crosslinked by means of a drum-type heat exchanger having an inverse screw flight.

Description

Process for the thermal surface postcrosslinking in a drum heat exchanger with inverse spiral screw

description

The present invention relates to a process for the thermal surface postcrosslinking of water-absorbing polymer particles, wherein the polymer particles are coated with an aqueous solution, the deagglomerated polymer coated particles and the deagglomerated polymer particles are thermally surface postcrosslinked by means of a drum heat exchanger with inverse helix.

Water-absorbing polymer particles are used in the manufacture of diapers, tampons, sanitary napkins and other sanitary articles, but also as water-retaining agents in agricultural horticulture. The water-absorbing polymer particles are also referred to as superabsorbers.

The preparation of water-absorbing polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and AT. Graham, Wiley-VCH, 1998, pages 71-103.

The properties of the water-absorbing polymer particles can be adjusted, for example, via the amount of crosslinker used. As the amount of crosslinker increases, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm ² (AUL 0.3 psi) goes through a maximum.

To improve the application properties, such as permeability of the swollen gel bed (SFC) in the diaper and absorption under a pressure of 49.2 g / cm ²

(AUL0.7 psi), water-absorbing polymer particles are generally surface postcrosslinked. As a result, the degree of crosslinking of the particle surface increases, whereby the absorption under a pressure of 49.2 g / cm ² (AUL0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface postcrosslinking can be carried out in aqueous gel phase. Preferably, however, dried, ground and sieved polymer particles (base polymer) are coated on the surface with a surface postcrosslinker and thermally surface postcrosslinked. Crosslinkers suitable for this purpose are compounds which can form covalent bonds with at least two carboxylate groups of the water-absorbing polymer particles.

EP 1 757 645 A1 and EP 1 757 646 A1 disclose the surface postcrosslinking of water-absorbing polymer particles in a rotary tube.

DE 10 2007 024 080 A1 teaches the after-treatment of water-absorbing polymer particles, for example with water, in a rotating container. The object of the present invention was to provide an improved process for producing water-absorbing polymer particles, in particular an improved surface postcrosslinking.

The object was achieved by processes for thermal surface postcrosslinking of water-absorbing polymer particles, wherein the water-absorbing polymers by polymerization of an aqueous monomer solution or suspension containing a) at least one ethylenically unsaturated, acid group-carrying monomer which may be at least partially neutralized,

b) at least one crosslinker,

c) at least one initiator,

d) optionally one or more copolymerizable with the monomers mentioned under a) ethylenically unsaturated monomers and

e) optionally one or more water-soluble polymers are prepared, characterized in that the polymer particles coated with an aqueous solution, deagglomerated the coated polymer particles and the deagglomerated polymer particles are thermally surface postcrosslinked by means of a drum heat exchanger with inverse screw helix.

The coating and the deagglomeration are advantageously carried out in a horizontal mixer or the coating in a vertical mixer and the deagglomeration in a horizontal mixer.

In a preferred embodiment of the present invention, the coated polymer particles are dried or heated during deagglomeration.

The filling height of the drum heat exchanger is preferably 30 to 100%, particularly preferably 40 to 95%, very particularly preferably 65 to 90%, in each case based on the height of the screw helix.

The temperature of the water-absorbing polymer particles in the drum heat exchanger is preferably from 120 to 220 ° C, more preferably from 150 to 210 ° C, most preferably from 170 to 200 ° C, and / or the residence time of the water-absorbing polymer particles in the drum heat exchanger is preferably from 10 to 120 minutes, more preferably from 20 to 90 minutes, most preferably from 30 to 60 minutes.

The drum heat exchanger is usually heated electrically or by steam, preferably indirectly. Indirectly heated means that the heating takes place via the wall of the rotating drum. Another object of the present invention are suitable for carrying out the method according to the invention suitable devices. These are, in particular, a device for thermal surface postcrosslinking of water-absorbing polymer particles, comprising a heatable horizontal mixer and a drum heat exchanger with inverse spiral screw, and a device for thermal surface postcrosslinking of water-absorbing polymer particles, comprising a vertical mixer, a heatable horizontal mixer and a drum heat exchanger with inverse spiral screw. Heatable horizontal mixer and drum heat exchanger or vertical mixer, heatable horizontal mixer and drum heat exchanger are preferably connected directly in series.

In a preferred embodiment of the present invention, the drum heat exchanger is followed directly by a coolable horizontal mixer.

Immediately connected in series or immediately downstream means that the discharge from the one apparatus on the shortest possible route and without intermediate storage in the following apparatus passes.

The present invention is based on the finding that the result of the thermal surface postcrosslinking is very strongly influenced by the residence time. In particular, in the case of water-absorbing polymer particles produced by dropping a monomer solution, the permeability of the swollen gel bed (SFC) undergoes a pronounced maximum.

The thermal surface postcrosslinking in a drum heat exchanger with inverse screw helix allows a continuous thermal surface postcrosslinking with a narrow residence time distribution, a backmixing between the individual helices does not practically take place with proper loading. The proportion of water-absorbing polymer particles with too low and too high residence time and thus insufficient quality can be minimized.

The preparation of the water-absorbing polymer particles and the invention are explained in more detail below:

The water-absorbing polymer particles are prepared by polymerization of a monomer solution or suspension and are usually water-insoluble. The monomers a) are preferably water-soluble, ie the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g Water, more preferably at least 25 g / 100 g of water, most preferably at least 35 g / 100 g of water.

Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Very particular preference is given to acrylic acid.

Further suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrenesulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

However, it is also possible to use a plurality of monomers a), for example mixtures of acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid.

Impurities can have a significant influence on the polymerization. Therefore, the raw materials used should have the highest possible purity. It is therefore often advantageous to purify the monomers a) specifically. Suitable purification processes are described, for example, in WO 2002/055469 A1, WO 2003/078378 A1 and WO 2004/035514 A1. A suitable monomer a) is, for example, an acrylic acid purified according to WO 2004/035514 A1 with 99.8460% by weight of acrylic acid, 0.0950% by weight of acetic acid,

0.0332% by weight of water, 0.0203% by weight of propionic acid, 0.0001% by weight of furfurals, 0.0001

% By weight of maleic anhydride, 0.0003% by weight of diacrylic acid and 0.0050% by weight of hydroquinone monomethyl ether.

The proportion of acrylic acid and / or salts thereof in the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%.

The monomers a) usually contain polymerization inhibitors, preferably hydroquinone half ethers, as storage stabilizer.

The monomer solution preferably contains up to 250 ppm by weight, preferably at most 130 ppm by weight, more preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, particularly preferably at least 30 ppm by weight, in particular by 50% by weight . ppm, hydroquinone half ether, in each case based on the unneutralized monomer a). For example, an ethylenically unsaturated, acid group-carrying monomer having a corresponding content of hydroquinone half-ether can be used to prepare the monomer solution.

Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and / or alpha tocopherol (vitamin E).

Suitable crosslinkers b) are compounds having at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which are incorporated in the Polymer chain can be radically copolymerized, and functional groups that can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts which can form coordinative bonds with at least two acid groups of the monomer a) are also suitable as crosslinking agents b).

Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be incorporated in the polymer network in free-radically polymerized form. Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates, as in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 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, in addition to acrylate groups, contain 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 2002/032962 A2 ,

Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylenebismethacrylamide, 15-tuply ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine.

Very particularly preferred crosslinkers b) are the polyethyleneglyoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to form diioder triacrylates, as described, for example, in WO 2003/104301 A1. 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 glycerol, in particular the triacrylate of 3-times ethoxylated glycerol. The amount of crosslinker b) is preferably from 0.05 to 1, 5 wt .-%, particularly preferably 0.1 to 1 wt .-%, most preferably 0.2 to 0.6 wt .-%, each based on Monomer a).

As initiators c) it is possible to use all compounds which generate radicals under the polymerization conditions, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite. Preferably, mixtures of thermal initiators and redox initiators are used, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. As a reducing component but is preferably a mixture of the sodium salt of 2

Hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite used. Such mixtures are as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals, Heilbronn, Germany). However, it is also possible to use the di-sodium salt of 2-hydroxy-2-sulfonatoacetic acid alone as the reducing component. For example, acrylamide, methacrylamide, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, dimethylaminoethyl acrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate, dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, are ethylenically unsaturated monomers d) which are copolymerizable with the ethylenically unsaturated acid group-carrying monomers a).

As water-soluble polymers e) it is possible to use polyvinyl alcohol, polyvinylpyrrolidone, starch, starch derivatives, modified cellulose, such as methylcellulose or hydroxyethylcellulose, gelatin, polyglycols or polyacrylic acids, preferably starch, starch derivatives and modified cellulose.

Usually, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75% by weight, more preferably from 45 to 70% by weight, most preferably from 50 to 65% by weight. It is also possible monomer suspensions, i. Monomer solutions with excess monomer a), for example sodium acrylate, use. With increasing water content, the energy expenditure increases during the subsequent drying and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.

The preferred polymerization inhibitors require dissolved oxygen for optimum performance. Therefore, the monomer solution may be polymerized prior to polymerization by inerting, i. Flow through with an inert gas, preferably nitrogen or carbon dioxide, are freed of dissolved oxygen. Preferably, the oxygen content of the monomer solution before polymerization is reduced to less than 1 ppm by weight, more preferably less than 0.5 ppm by weight, most preferably less than 0.1 ppm by weight.

Suitable reactors are, for example, kneading reactors or belt reactors. In the kneader, the polymer gel resulting from the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating stirring shafts, as in

WO 2001/038402 A1. The polymerization on the belt is described, for example, in DE 38 25 366 A1 and US Pat. No. 6,241,928. During the polymerization in a belt reactor, a polymer gel is formed, which must be comminuted in a further process step, for example in an extruder or kneader.

To improve the drying properties, the comminuted polymer gel obtained by means of a kneader can additionally be extruded. However, it is also possible to drip an aqueous monomer solution and to polymerize the drops produced in a heated carrier gas stream. Here, the process steps polymerization and drying can be summarized, as described in WO 2008/040715 A2, WO 2008/052971 A1 and WO 201 1/026876 A1.

The acid groups of the polymer gels obtained are usually partially neutralized. The neutralization is preferably carried out at the stage of the monomers. This is usually done by mixing the neutralizing agent as an aqueous solution or preferably as a solid. The degree of neutralization is preferably from 25 to 95 mol%, particularly preferably from 30 to 80 mol%, very particularly preferably from 40 to 75 mol%, wherein the customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or Alkalimetallhydrogenkarbonate 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.

But it is also possible to carry out the neutralization after the polymerization at the stage of Polymergeis formed during the polymerization. Furthermore, it is 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 stage of Polymergeis. If the polymer gel is at least partially neutralized after the polymerization, the polymer gel is preferably comminuted mechanically, for example by means of an extruder, wherein the neutralizing agent can be sprayed, sprinkled or poured on and then thoroughly mixed in. For this purpose, the gel mass obtained can be extruded several times for homogenization.

The polymer gel is then preferably dried with a belt dryer until the moisture content is preferably 0.5 to 15 wt .-%, particularly preferably 1 to 10 wt .-%, most preferably 2 to 8 wt .-%, wherein the Moisture content according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". If the moisture content is too high, the dried polymer gel has too low a glass transition temperature T _g and is difficult to process further. If the moisture content is too low, the dried polymer gel is too brittle and in the subsequent comminution steps undesirably large amounts of polymer particles having too small a particle size ("fines") are produced. , more preferably from 35 to 70% by weight, most preferably from 40 to 60% by weight. Alternatively, a fluidized bed dryer or a paddle dryer can be used for drying. The dried polymer gel is then ground and classified, wherein multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibratory mills, can be used for grinding. The average particle size of the polymer fraction separated as a product fraction is preferably at least 200 μm, more preferably from 250 to 600 μm, very particularly from 300 to 500 μm. The average particle size of the product fraction can be determined by means of the EDANA recommended test method No. WSP 220.2-05 "Particle Size Distribution", in which the mass fractions of the sieve fractions are cumulatively applied and the average particle size is determined graphically. The mean particle size here is the value of the mesh size, which results for accumulated 50 wt .-%.

The proportion of particles having a particle size of greater than 150 μm is preferably at least 90% by weight, particularly preferably at least 95% by weight, very particularly preferably at least 98% by weight.

Polymer particles with too small particle size lower the permeability (SFC). Therefore, the proportion of too small polymer particles ("fines") should be low, so too small polymer particles are usually separated and recycled to the process, preferably before, during or immediately after the polymerization, ie before drying the polymer gel Polymer particles can be moistened with water and / or aqueous surfactant before or during the recycling process It is also possible to separate small polymer particles in later process steps, for example after surface postcrosslinking or another coating step. For example, if a kneading reactor is used for the polymerization, the polymer particles which are too small are preferably added during the last third of the polymerization.

If the polymer particles which are too small are added very early, for example already to the monomer solution, this lowers the centrifuge retention capacity (CRC) of the water-absorbing polymer particles obtained. However, this can be compensated for example by adjusting the amount of crosslinker b).

If the polymer particles which are too small are added very late, for example only in an apparatus downstream of the polymerization reactor, for example an extruder, then the polymer particles which are too small can only be incorporated into the resulting polymer gel with difficulty. Insufficiently incorporated too small polymer particles dissolve during the grinding again from the dried polymer gel are therefore separated again during classification and increase the amount of polymer particles which are too small to be recycled.

The proportion of particles having a particle size of at most 850 pm, is preferably at least 90 wt .-%, more preferably at least 95 wt .-%, most preferably at least 98 wt .-%.

The proportion of particles having a particle size of at most 600 μm is preferably at least 90% by weight, more preferably at least 95% by weight, most preferably at least 98% by weight.

Polymer particles with too large particle size reduce the swelling rate. Therefore, the proportion of polymer particles too large should also be low. Too large polymer particles are therefore usually separated and recycled to the grinding of the dried Polymergeis.

The polymer particles are surface-post-crosslinked to further improve the properties. Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US Pat. No. 6,239,230. Furthermore, in DE 40 20 780 C1 cyclic carbonates, in DE 198 07 502 A1 2-

Oxazolidinone and its derivatives, such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 992 C1 bis- and poly-2-oxazolidinone, in DE 198 54 573 A1 2-oxotetrahydro-1, 3-oxazine and its derivatives, in DE 198 54 574 A1 N-acyl-2-oxazolidinones, in DE 102 04 937 A1 cyclic ureas, in DE 103 34 584 A1 bicyclic amidoacetals, in EP 1 199 327 A2 oxetanes and cyclic see ureas and in WO 2003/031482 A1 morpholine 2,3-dione and its derivatives are described as suitable surface postcrosslinkers.

Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.

Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1, 3-propanediol.

Furthermore, it is also possible to use surface postcrosslinkers which contain additional polymerizable ethylenically unsaturated groups, as described in DE 37 13 601 A1 The amount of surface postcrosslinker is preferably 0.001 to 2 wt .-%, more preferably 0.02 to 1 wt .-%, most preferably 0.05 to 0.2 wt .-%, each based on the polymer particles. In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers before, during or after the surface postcrosslinking.

The polyvalent cations which can be used in the process according to the invention are, for example, divalent cations, such as the cations of zinc, magnesium, calcium, iron and strontium, trivalent cations, such as the cations of aluminum, iron, chromium, rare earths and manganese, tetravalent cations, such as the cations of Titanium and zirconium. As the counterion, hydroxide, chloride, bromide, sulfate, hydrogensulfate, carbonate, hydrogencarbonate, nitrate, phosphate, hydrogenphosphate, dihydrogenphosphate and carboxylate, such as acetate, citrate and lactate, are possible. It is also possible to use salts with different counterions, for example basic aluminum salts, such as aluminum monoacetate or aluminum monolactate. Aluminum sulfate, aluminum monoacetate and aluminum lactate are preferred. Apart from metal salts, polyamines can also be used as polyvalent cations. The amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, particularly preferably 0.02 to 0.8% by weight. in each case based on the polymer particles.

The surface postcrosslinking is usually carried out by coating the dried polymer particles with an aqueous solution of the surface postcrosslinker, for example by spraying the solution onto the polymer particles. The polymer particles coated with surface postcrosslinkers are subsequently deagglomerated and thermally surface postcrosslinked. The coating with the solution of the surface postcrosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Suitable mixers are, for example, Horizontal Pflugschar® mixers (Gebr. Lödige Maschinenbau GmbH, Paderborn, Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV, Doetinchem, the Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati, USA) and Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, The Netherlands). But it is also possible to spray the solution in a fluidized bed.

Deagglomeration is also preferably performed in mixers with agitated mixing tools, such as screw mixers, disc mixers, and paddle mixers. Advantageous suitable mixers are, for example, horizontal Pflugschar® mixers (Gebr. Lödige Maschinenbau GmbH, Paderborn, Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands) and Processall Mixmill Mixer (Processall Incorporated; Cincinnati, USA).

When coated with aqueous solutions, the water-absorbing polymer particles tend to clump together (agglomerate). In a vertical mixer, the water-absorbing polymer particles have a lower tendency to agglomerate during coating. The coating is therefore advantageously carried out in a vertical mixer. Furthermore, the resulting agglomerates can be redissolved by moderate mechanical stress. For this purpose, horizontal mixers are better suited due to the higher residence time. It is therefore also possible to perform coating and deagglomeration in a horizontal mixer. The distinction between horizontal mixer and vertical mixer is made by the storage of the mixing shaft, i. Horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft. The surface postcrosslinkers are used as aqueous solution. The penetration depth of the surface postcrosslinker into the polymer particles can be adjusted by the content of nonaqueous solvent or total solvent amount.

If only water is used as the solvent, it is advantageous to add a surfactant. As a result, the wetting behavior is improved and the tendency to clog is reduced. However, preference is given to using solvent mixtures, for example isopropanol / water, 1,3-propanediol / water and propylene glycol / water, the mixing mass ratio preferably being from 20:80 to 40:60. Advantageously, the water-absorbing polymer particles are dried and / or heated prior to the thermal surface postcrosslinking. This advantageously already takes place during deagglomeration, preferably in contact dryers, such as paddle dryers and disk dryers. Suitable contact dryers are, for example, Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH, Leingarten, Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH, Leingarten, Germany), Holo-Flite® dryers (Metso Minerals Industries, Inc., Danville, USA ) and Nara Paddle Dryer (NARA Machinery Europe, Frechen, Germany).

Coating with the aqueous solution increases the water content of the water-absorbing polymer particles. This relatively high water content precludes thermal surface postcrosslinking at higher temperatures. Therefore, it is advantageous to dry the water-absorbing polymer particles before the thermal surface postcrosslinking and to heat to the reaction temperature. The water content of the coated and deagglomerated polymer particles before thermal surface postcrosslinking is preferably less than 5% by weight, more preferably less than 2% by weight, most preferably less than 1% by weight. After deagglomeration, the optionally dried and / or heated polymer particles are transferred to the thermal surface postcrosslinking in a drum heat exchanger with inverse screw helix.

A drum heat exchanger with inverse spiral screw is a heated, horizontal and rotatable drum, wherein in the inner wall of the drum an inverse screw spiral for positive conveyance of the product is integrated. For example, the drum can be heated indirectly via the drum wall. Usually, the heating takes place electrically or with steam. Various wall temperatures can be set in the drum via several independent heating zones along the drum longitudinal axis.

A direct product heating in the interior of the drum heat exchanger by installing a burner or the introduction of hot flue gases is usually not used here.

Inasmuch as a very uniform radial temperature distribution of the product in the drum is required, radial mixing elements (e.g., in the form of cam lobes or flights) may be installed on the circumference of the drum inner wall in addition to the inverse screw flight. These support the radial mixing of the product within the separate zones, which form due to the inverse screw flight and are used in particular when working with large drum diameter and high filling height or height of the screw flight.

Figure 1 shows a drum heat exchanger with indirect heating

Figure 2 shows a section through the drum heat exchanger

The reference numbers of the figures have the following meanings:

A product infeed

B Product outlet

1 static jacket (insulated)

2 rotating drum

3 inverse spiral flights

4 heating zone 1 (electric or steam)

5 heating zone 2 (electric or steam)

6 Heating zone 3 (electric or steam)

a product

b Inner diameter of the drum

c height of the spiral helix The drum preferably has a length of from 3 to 30 m, more preferably from 5 to 25 m, most preferably from 7 to 20 m. The inner diameter of the drum is preferably from 0.3 to 10 m, more preferably from 0.4 to 5 m, most preferably from 1 to 3 m.

The screw helix has a height of preferably 0.05 to 1 m, more preferably from 0.1 to 0.8 m, most preferably from 0.2 to 0.6 m. The pitch of the screw flight is preferably 0.05 to 0.5 m, more preferably from 0.1 m to 0.4 m, most preferably from 0.15 to 0.3 m. The height of the screw helix is the distance between the drum inner wall and the highest point of the helix in the direction of the axis of rotation. The pitch of the helix is the displacement of the helix in the longitudinal direction in one full turn.

The peripheral speed of the drum is preferably from 0.02 to 0.5 m / s, more preferably from 0.03 to 0.3 m / s, most preferably from 0.04 to 0.15 m / s.

The maximum filling height of the drum heat exchanger with inverse spiral screw is the filling level at which barely any product reaches the next coil through the height of the screw spiral. Figure 2 shows the maximum fill level. This filling level corresponds to a filling level of 100%.

The inclination of the longitudinal axis of the drum heat exchanger with inverse helix relative to the horizontal is preferably +10 to -10 °, more preferably +5 to - 5 °, most preferably +1 to -1 °, the positive sign having a rising tendency in the conveying direction and the negative sign mean a slope sloping in the conveying direction.

In a preferred embodiment of the present invention, the water-absorbing polymer particles are cooled after the thermal surface postcrosslinking. The cooling is preferably carried out in contact coolers, such as blade coolers and disc coolers. Suitable coolers are, for example, Hosokawa Bepex® Horizontal Paddle Cooler (Hoorkawa Micron GmbH, Leingarten, Germany), Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH, Leingarten, Germany), Holo-Flite® coolers (Metso Minerals Industries, Inc., Danville ; USA) and Nara Paddle Cooler (NARA Machinery Europe; Frechen; Germany).

In the cooler, the water-absorbing polymer particles to 20 to 150 ° C, preferably 30 to 120 ° C, more preferably 40 to 100 ° C, most preferably 50 to 80 ° C, cooled. Subsequently, the surface-postcrosslinked polymer particles can be classified again, wherein too small and / or too large polymer particles are separated and recycled to the process. The surface-postcrosslinked polymer particles can be coated or post-moistened for further improvement of the properties. The post-wetting is preferably carried out at 30 to 80 ° C, more preferably at 35 to 70 ° C, most preferably at 40 to 60 ° C. If the temperatures are too low, the water-absorbing polymer particles tend to clump together and at higher temperatures water is already noticeably evaporating. The amount of water used for the rewetting is preferably from 1 to 10 wt .-%, particularly preferably from 2 to 8 wt .-%, most preferably from 3 to 5 wt .-%. By rewetting the mechanical stability of the polymer particles is increased and their tendency to static charge reduced. The post-moistening in the cooler is advantageously carried out after the thermal surface post-crosslinking. Suitable coatings for improving the swelling rate and the permeability of the swollen gel bed (SFC) are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers, and di- or polyvalent metal cations. Suitable coatings for dust binding are, for example, polyols. Suitable coatings against the unwanted caking tendency of the polymer particles are, for example, zinc oxide, zinc carbonate, fumed silica, such as Aerosil® 200, and surfactants, such as Span® 20.

The water-absorbing polymer particles produced by the process according to the invention have a moisture content of preferably 0 to 15 wt .-%, particularly preferably 0.2 to 10 wt .-%, most preferably 0.5 to 8 wt .-%, wherein the Moisture content according to the EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating".

The water-absorbing polymer particles produced by the process according to the invention have a proportion of particles having a particle size of from 300 to 600 μm, preferably at least 30% by weight, more preferably at least 50% by weight, very particularly preferably at least 70% by weight. ,

The water-absorbing polymer particles produced according to the process of the invention have a centrifuge retention capacity (CRC) of typically at least 15 g / g, preferably at least 20 g / g, preferably at least 22 g / g, more preferably at least 24 g / g, most preferably at least 26 g / g, on. The centrifuge retention capacity (CRC) of the water-absorbing polymer particles is usually less than 60 g / g. Centrifuge Retention Capacity (CRC) is determined according to the EDANA recommended Test Method No. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation". The water-absorbing polymer particles produced by the process according to the invention have an absorption under a pressure of 49.2 g / cm ² of typically at least 15 g / g, preferably at least 20 g / g, preferably at least 22 g / g, particularly preferably at least 24 g / g, most preferably at least 26 g / g, on. The absorption under a pressure of 49.2 g / cm ^{2 of} the water-absorbing polymer particles is usually less than 35 g / g. The absorption under a pressure of 49.2 g / cm ² is determined analogously to the EDANA recommended test method no. WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination", wherein instead of a pressure of 21, 0 g / cm ² a Pressure of 49.2 g / cm ^{2 is} set.

methods:

The standard test methods described below with "WSP" are described in: "Standard Test Methods for the Nonwovens Industry", 2005 edition, published jointly by the Worldwide Strategy Partners EDANA (Avenue Eugene Plasky, 157, 1030 Brussels, Belgium , www.edana.org) and INDA (1 100 Crescent Green, Suite 1 15, Cary, North Carolina 27518, USA, www.inda.org) This publication is available from EDANA and INDA. Unless otherwise stated, at an ambient temperature of 23 ± 2 ° C and a relative humidity of 50 ± 10%, the water-absorbing polymer particles are well mixed prior to measurement.

residual monomers

The residual monomer content of the water-absorbing polymer particles is determined according to the EDANA-recommended test method WSP No. 210.2-05 "Residual Monomers".

Centrifuge Retention Capacity

Centrifuge Retention Capacity (CRC) is determined according to the EDANA recommended Test Method No. WSP 241.2-05 "Fluid Retention Capacity in Saline, After Centrifugation".

Absorption under a pressure of 49.2 g / cm ² (absorption under load)

The absorption under a pressure of 49.2 g / cm ² (AUL0.7 psi) is determined analogously to the EDANA recommended test method no. WSP 242.2-05 "Absorption Under Pressure, Gravimetric Determination", whereby instead of a pressure of 21, 0 g / cm ² (AUL0.3psi) a pressure of 49.2 g / cm ² (AUL0.7psi) is set. extractable

The content of extractable constituents of the water-absorbing polymer particles is determined according to the EDANA-recommended test method No. WSP 270.2-05 "Extractable".

Saline Flow Conductivity The permeability (SFC) of a swollen gel layer under pressure of 0.3 psi (2070 Pa) is determined, as described in EP 0 640 330 A1, as gel layer permeability of a swollen gel layer of water-absorbing polymer particles. wherein the apparatus described in the aforementioned patent application on page 19 and in Figure 8 has been modified so that the glass frit (40) is no longer used, the punch (39) consists of the same plastic material as the cylinder (37) and now over the entire Support surface evenly distributed 21 holes of equal size. The procedure and evaluation of the measurement remains unchanged compared to EP 0 640 330 A1. The flow is automatically detected. The permeability (SFC) is calculated as follows: SFC [cc ³ s / g] = (Fg (t = 0) xL0) / (dxAxWP), where Fg (t = 0) is the flow rate of NaCl solution in g / s is the onsanalyse of the data Fg (t) is obtained of the flow determinations by extrapolation to t = 0 by a linear regression, L0 is the thickness of the gel layer in cm d, the density of the NaCl solution in g / cm ^3, a is the area the gel layer in cm ² and WP the hydrostatic pressure over the gel layer in dynes / cm ² .

Examples Example 1

A base polymer was prepared in a DC spray dryer with integrated fluidized bed (27) and external fluidized bed (29) according to Figure 1 of WO 201 1/026876 A1. The cylindrical part of the spray dryer (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3.0 m and a weir height of 0.4 m. The external fluidized bed (EFB) had a length of 3.0 m, a width of 0.65 m and a weir height of 0.5 m.

The drying gas was supplied via a gas distributor (3) at the tip of the spray dryer. The drying gas was partially passed through a fabric filter (9) and a wash column (12) returned (cycle gas). The drying gas used was nitrogen having an oxygen content of 1 to 5% by volume. Before the start of the polymerization, the plant was purged with nitrogen to an oxygen content of less than 5% by volume. The amount of gas in the cylindrical part of the spray dryer (5) was 16,170 kg / h. The pressure inside the spray dryer was 4 mbar below the ambient pressure.

The exit temperature of the spray dryer was measured at three points at the lower end of the cylindrical part as described in FIG. 3 of WO 201 1/026876 A1. The three individual measurements (47) were used to calculate the mean starting temperature. The cycle gas was warmed up and the metering of the monomer solution was started. From this point on the average starting temperature was regulated by adjusting the gas inlet temperature via the heat exchanger (20) to 1 16 ° C.

The product was collected in the internal fluidized bed (27) up to the height of the weir. Drying gas at a temperature of 132 ° C. was fed via the line (25) to the internal fluidized bed (27). The amount of gas in the internal fluidized bed (27) was 10,000 kg / h.

The exhaust gas of the spray dryer was fed via the fabric filter (9) of the wash column (12). The liquid level within the wash column (12) was kept constant by pumping off excess liquid. The liquid within the scrubbing column (12) was cooled by means of the heat exchanger (13) and countercurrently conveyed through the nozzles (1 1) so that the temperature inside the scrubbing column (12) was regulated to 45 ° C. To wash out acrylic acid from the exhaust gas, the liquid in the wash column (12) was rendered alkaline by the addition of sodium hydroxide solution.

The exhaust gas of the scrubbing column was divided into lines (1) and (25). The temperatures were controlled by means of the heat exchangers (20) and (22). The heated drying gas was fed to the spray dryer via the gas distributor (3). The gas distributor consisted of a number of plates and, depending on the amount of gas, had a pressure drop of 5 to 10 mbar.

The product was transferred from the internal fluidized bed (27) by means of the rotary valve (28) in the external fluidized bed (29). Via the line (40) the external fluidized bed (29) drying gas at a temperature of 60 ° C was supplied. Drying gas was air. The amount of gas in the external fluidized bed (29) was 2,500 kg / h.

The product was conveyed from the external fluidized bed (29) by means of the rotary valve (32) on the sieve (33). By means of the sieve (33), particles having a particle size of greater than 3,000 μm (agglomerates) were separated off. To prepare the monomer solution, acrylic acid was first mixed with triply ethoxylated glycerol intriacrylate (crosslinker) and then with 37.3% strength by weight aqueous sodium acrylate. By pumping over a heat exchanger, the temperature of the monomer solution kept at 10 ° C. In Umpumpkreis was behind the pump, a filter with a mesh size of 150 μηη. The initiators were added via lines (43) and (44) before the static mixers (41) and (42) to the monomer solution. Sodium peroxodisulfate was fed via line (43) at a temperature of 20 ° C and Brüggolit® FF7 (Brüggemann Chemicals, Heibronn, Germany) at a temperature of 5 ° C via line (44). Each initiator was pumped in a circle and metered via control valves in each dropletizer unit. Behind the static mixer (42) was a filter with a mesh size of 100 μηη. For dosing the monomer solution at the tip of the spray dryer three Vertropfereinheiten were used as described in Figure 4 of WO 201 1/026876 A1.

A Vertropferenheit consisted as described in Figure 5 of WO 201 1/026876 A1 from an outer tube (51) and a Vertropferkassette (53). The dropper cassette (53) was connected to an inner tube (52). The inner tube (52) had a PTFE gasket (54) at the end and could be pulled out for service during operation.

Figure 6 of WO 201 1/026876 A1 describes the internal structure of the dropletizer cassette. The temperature of the dropletizer cassette (61) was controlled to 25 ° C by means of cooling water in the channels (59). The dropper cassette had 256 holes. The holes had a diameter of 2.5 mm at the entrance and at the exit a diameter of 170 μηη. The holes were arranged in 6 rows, with the distance between the holes of a row 12.38 mm and the order of the rows was 14 mm. The dropletizer cassette (61) had a dead-space free flow channel (60) for homogeneously distributing the premixed monomer solution and the initiator solutions to the two dropletizer plates (57). The holes were distributed on two dropletizer plates (57), each with 128 holes, i. the two drip plates (57) each had three rows of holes. The two Vertropferplatten (57) had an angled arrangement with an angle of 3 °. Each dropletizer plate (57) was made of stainless steel (# 1 .4571 material) and had a length of 530 mm, a width of 76 mm and a thickness of 15 mm. The feed to the spray dryer contained 10.25% by weight of acrylic acid, 32.75% by weight of sodium acrylate, 0.035% by weight of trisubstituted ethoxylated glycerol triacrylate (about 85% by weight). 0.00285% by weight of Brüggolit® FF7 (Brüggemann Chemicals, Heibronn, Germany), 0.266% by weight of sodium peroxodisulfate and water. Brüggolit® FF7 was used as a 5% strength by weight aqueous solution and sodium peroxodisulfate was used as a 15% strength by weight aqueous solution. The degree of neutralization was 71%. The feed per bore was 1, 6 kg / h.

The resulting base polymer had a bulk density of 74.4 g / 100 ml, an average particle diameter of 392 μm, a particle diameter distribution width of 0.48, an average sphericity of 0.91, a centrifuge retention capacity (CRC) of 21.4 g / g , an absorption under a pressure k of 49.2 g / cm ³ (AUL0.7 psi) of 17.9 g / g and a residual monomer content of 2.75% by weight. Example 2

1300 g of base polymer from Example 1 were used for thermal surface postcrosslinking in a Pflugschar® mixer with heating jacket type Lö5 (Gebr Lödige Maschinenbau GmbH, Paderborn, Germany) at about 23 ° C and a shaft speed of 250 rpm by means of a two-component spray nozzle the following solution coated (in each case based on the base polymer):

0.10% by weight of 2-hydroxyethyl-2-oxazolidinone

0.10% by weight of 1,3-propanediol

1.00% by weight of 1,2-propanediol

1, 00 wt.% Water

3.00% by weight aqueous aluminum trilactate solution (22% strength by weight)

After spraying, the product temperature was raised to 185 ° C and the reaction mixture held for a total of 150 minutes at that temperature and a shaft speed of 60 revolutions per minute. After different times, samples were taken. All samples were sieved prior to analysis to a particle size of 150 to 850 μηη.

Tab. 1: Influence of the residence time

Example 3

1500 g of base polymer from Example 1 were used for thermal surface postcrosslinking in a Pflugschar® mixer with heating jacket of the type Lö5 (Gebr Lödige Maschinenbau GmbH, Paderborn, Germany) at about 23 ° C and a shaft speed of 250 rpm by means of a Two-substance spray nozzle coated with the following solution (based in each case on the base polymer):

1, 00 wt .-% 1, 3-propanediol

1, 00 wt .-% water

3.00% by weight aqueous aluminum trilactate solution (22% strength by weight)

After spraying, the product temperature was raised to 180 ° C. and the reaction mixture was kept at this temperature for a total of 120 minutes at a shaft speed of 60 revolutions per minute. After different times, samples were taken. All samples were sieved prior to analysis to a particle size of 150 to 850 μηη.

Tab. 2: Influence of the residence time

Examples 2 and 3 show the considerable influence of the residence time on the result of the surface postcrosslinking. The permeability (SFC) goes through a pronounced maximum.

Example 4

In a further experiment, the residence time distribution was determined in an apparatus hitherto used for thermal surface postcrosslinking. A paddle dryer type Nara Paddle Dryer 1, 6W (GMF Gouda, Waddinxveen, Netherlands) and Hysorb® M7055 (BASF SE, Ludwigshafen, Germany) was used. The speed was 37 rpm and the weir height 87% (at 100%, the weir is at the same height as the upper edge of the blade). The throughput of Hysorb® M7055 was 60 kg / h. At the entrance of the paddle dryer specially colored particles of the product Hysorb® M7055 (BASF SE, Ludwigshafen, Germany) were added for this experiment and the time distribution of the colored particles at the outlet of the paddle dryer was analyzed. The frequency distribution has been determined by integra- tion determines the cumulative frequency distributions τιο, τ ₅ ο and T90 (τιο corresponds to 10% of the cumulative frequency distribution, etc.).

The following widths of the frequency distribution were determined:

The mean residence time (τ ₅ ο) was 42 minutes.

Example 5 The procedure was as in Example 4. The throughput was increased from 60 kg / h to 80 kg / h. The following widths of the frequency distribution were determined: I 90 / T50 = 1, 31

The mean residence time (τ ₅ ο) was 34 minutes.

Example 6

The procedure was as in Example 4. The degree of filling was reduced by reducing the weir height from 87 to 62%.

The following widths of the frequency distribution were determined:

The mean residence time (τ ₅ ο) was 31 minutes. Example 7

The procedure was as in Example 4. Instead of a paddle dryer, a drum heat exchanger with inverse spiral screw was used. The drum heat exchanger had an inner diameter of 700 mm, a length of 6,000 mm and 60 helical turns. The spiral turns had a height of 100 mm and a pitch of 100 mm. The speed was 0.9 rpm, the throughput was 108 kg / h and the filling level was 100% of the height of the screw helix. The following widths of the frequency distribution were determined: T ₉ o / T50 = 1, 07

The mean residence time (τ ₅ ο) was 52 minutes.

Example 8

The procedure was as in Example 7. The speed was increased from 0.9 rpm to 2.5 rpm. The throughput was increased from 108 kg / h to 224 kg / h. The fill level was 72% of the helix height. The following widths of the frequency distribution were determined:

The mean residence time (τ ₅ ο) was 24 minutes.

Examples 4 to 8 prove the much closer residence time distribution of the drum heat exchanger with inverse spiral screw.

Claims

claims

1 . Process for the thermal surface postcrosslinking of water-absorbing polymer particles, wherein the water-absorbing polymers are prepared by polymerization of an aqueous monomer solution or suspension containing at least one ethylenically unsaturated, acid group-carrying monomer which may be at least partially neutralized,

at least one crosslinker,

at least one initiator,

optionally one or more ethylenically unsaturated monomers copolymerizable with the monomers mentioned under a) and

optionally one or more water-soluble polymers are prepared, characterized in that the polymer particles coated with an aqueous solution, the deagglomerated coated polymer particles and the deagglomerated polymer particles are thermally surface postcrosslinked by means of a drum heat exchanger with inverse screw helix.

A method according to claim 1, characterized in that the coating and the deagglomeration are carried out in a horizontal mixer.

A method according to claim 1, characterized in that the coating in a vertical mixer and the deagglomeration are carried out in a horizontal mixer.

4. The method according to any one of claims 1 to 3, characterized in that the coated polymer particles are dried during deagglomeration.

Method according to one of claims 1 to 4, characterized in that the coated polymer particles are heated during deagglomeration.

Method according to one of claims 1 to 5, characterized in that the filling height of the drum heat exchanger is 65 to 90%.

Method according to one of claims 1 to 6, characterized in that the temperature of the water-absorbing polymer particles in the drum heat exchanger of 170 to 200 ° C and / or the residence time of the water-absorbing polymer particles in the drum heat exchanger of 30 to 60 minutes.

Method according to one of claims 1 to 7, characterized in that the drum heat exchanger is heated electrically or with steam.

9. The method according to any one of claims 1 to 8, characterized in that the monomer a) is at least 95 mol% partially neutralized acrylic acid.

10. The method according to any one of claims 1 to 9, characterized in that the water-absorbing polymers have a Zentrifugeretentionskapazität of at least 26 g / g.

1 1. Device for thermal surface postcrosslinking of water-absorbing polymer particles, comprising a heatable horizontal mixer and a drum heat exchanger with inverse spiral screw.

12. The apparatus according to claim 1 1, wherein the horizontal mixer and the drum heat exchanger are connected directly one behind the other.

13. An apparatus for thermal surface postcrosslinking of water-absorbing polymer particles, comprising a vertical mixer, a heatable horizontal mixer and a drum heat exchanger with inverse screw helix.

14. The apparatus of claim 13, wherein the vertical mixer, the horizontal mixer and the drum heat exchanger are connected directly one behind the other.

15. Device according to one of claims 1 1 to 14, wherein the drum heat exchanger is followed directly by a coolable horizontal mixer.