Method for the production of superabsorbents

Abstract

The present invention relates to a method for producing superabsorbents, in which a monomer solution M is conveyed by means of a pump P, the pump P possesses a conduit L from the pressure side to the suction side of the pump P, and the monomer solution M, in the event of a shutdown, is circulated at least temporarily via the conduit L.

Description

[0001] Process for producing superabsorbents

[0002] The present invention relates to a process for producing superabsorbents, wherein a monomer solution M is conveyed by means of a pump P, the pump P has a line L from the pressure side to the suction side of the pump P and the monomer solution M is conveyed in a circuit via the line L at least temporarily in the event of a shutdown.

[0003] Superabsorbents are used in the manufacture of diapers, tampons, sanitary pads, and other hygiene products, as well as as water-retaining agents in agricultural horticulture. Superabsorbents are also known as water-absorbing polymers.

[0004] The production of superabsorbents is described in the monograph ''Modern Superabsorbent Polymer Technology”, FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103.

[0005] To improve application properties such as gel bed permeability (GBP) and absorption under a pressure of 49.2 g / cm 2 (AUL0.7 psi), superabsorbent particles are generally surface-crosslinked. This increases the degree of crosslinking of the particle surface, which increases absorption under a pressure of 49.2 g / cm 2 (AUL 0.7 psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled. This surface crosslinking can be carried out in the aqueous gel phase. Preferably, however, dried, ground, and sieved superabsorbent particles (base polymer) are coated on the surface with a surface crosslinker and thermally surface crosslinked. Suitable crosslinkers for this purpose are compounds that can form covalent bonds with at least two carboxylate groups of the superabsorbent particles.

[0006] WO 2007 / 028751 A1 relates to a neutralization process.

[0007] The object of the present invention was to provide an improved process for the production of superabsorbent particles, in particular trouble-free operation of the pumps used.

[0008] The object was achieved by a process for the production of superabsorbents, wherein at least one ethylenically unsaturated, acid-group-bearing monomer is at least partially neutralized with an aqueous base, the resulting aqueous monomer solution M is conveyed by means of a pump P, at least one crosslinker and at least one initiator are added to the aqueous monomer solution M before or after the pump P, the aqueous monomer solution M is then polymerized to form a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried on a circulating air belt dryer and the dried polymer gel is comminuted, classified and optionally thermally surface-postcrosslinked, characterized in that the pump P has a direct or indirect line L from the pressure side to the suction side of the pump P and the monomer solution M is conveyed in a circuit at least temporarily via the line L in the event of a shutdown.

[0009] The monomer solution M can be pumped via line L directly from the pressure side to the suction side of the pump P. However, it is also possible to pump the monomer solution M to any other location upstream of the pump P, for example, into the tank B of the neutralization process shown in Figure 1. The reference numerals have the following meaning:

[0010] W heat exchanger

[0011] R ring line

[0012] L Line

[0013] P1 pump in the ring line

[0014] P2 Pump to the polymerization reactor (inventive pump P)

[0015] B (buffer) tank

[0016] Zi supply line

[0017] Z2 supply line

[0018] Z3 supply line

[0019] The present invention is based on the finding that, when production is shut down, the monomer solution M remaining in the pump P can lead to polymerization due to the residual heat of the pump P. This can be avoided by continued, controlled operation of the pump P.

[0020] In the event of a shutdown, the monomer solution M is preferably circulated via line L for at least 60 minutes, particularly preferably at least 90 minutes, most preferably at least 120 minutes.

[0021] In the event of a shutdown, the delivery capacity of the pump P is preferably reduced by 50 to 99%, particularly preferably by 70 to 98%, most preferably by 90 to 95%.

[0022] In a particular embodiment of the present invention, the temperature in the pump P is measured and the delivery rate of the pump P is reduced to 0 in a temperature-controlled manner when it is switched off, so that the temperature in the pump P does not rise above 50°C.

[0023] Pumps P preferred according to the invention are centrifugal pumps and side channel pumps.

[0024] Unlike reciprocating and circulating piston pumps, which operate according to the positive displacement principle, centrifugal pumps and side-channel pumps operate according to the dynamic principle. A rotating impeller (the pumping element connected to the drive shaft) transfers work in the form of kinetic energy from the impeller to the monomer solution M being pumped. Downstream of the impeller, the kinetic energy is largely converted back into static pressure (pressure energy, according to the law of conservation of energy) in a diffuser and / or the volute casing. The impeller is essentially a simple disk with blades mounted on it.

[0025] The blades create vane channels, whose cross-section typically increases significantly from the inside to the outside due to their increasing circumference. These vane channels allow as much of the monomer solution (M) to be pumped to be thrown away as can flow into the center of the impeller. In contrast to piston pumps, the monomer solution (M) to be pumped flows continuously during operation in centrifugal and side-channel pumps.

[0026] In contrast to open impellers, closed impellers can also be used. The blade channels are simply covered by a second disc with an opening in the center.

[0027] The blade curvature typically follows the natural trajectory of a water droplet on a rotating, round, smooth disc, as seen by a co-rotating observer, if the water droplet is allowed to fall onto the center of the disc. This blade shape is referred to as a "backward-curved" blade. However, in principle, slightly forward-curved blades and helical, i.e., twisted, backward-curved blades, whose cutting edges extend into the impeller inlet and capture the monomer solution M like a ship's propeller, can also be used.

[0028] A centrifugal pump (a centrifugal pumping chamber) consists of the pump housing and the impeller, which rotates within the pump housing and is equipped with blades. The liquid F enters axially through the suction nozzle. It is directed radially outwards by centrifugal force and accelerated to high speed by the impeller. The pump housing has the task of collecting the monomer solution M from all blade channels so that it can be collected and passed on through the pressure outlets. At the same time, the pump housing has the task of converting the kinetic energy of the monomer solution M into pressure. This is usually done by exploiting the fact that an increase in cross-section reduces the velocity of the monomer solution M and thus causes an increase in pressure. Two designs of the pump housing are commonly used to increase the cross-section. Volute casings are often used in single-stage pumps or behind the last stage of multi-stage centrifugal pumps.This spiral surrounds the impeller. The cross-section expands toward the discharge outlet. This slows down the flow of the monomer solution M, resulting in a simultaneous increase in pressure.

[0029] Instead of the volute, fixed diffusers are also used, particularly in multi-stage pumps. The diffuser is installed in the pump casing and designed as an annular space. It encloses the impeller. Arranged in the diffuser are guide vanes which form channels that continually widen outwards. In this design, the monomer solution M is not thrown directly into the pump casing, but first flows through the vane channels of the diffuser. By widening in the direction of flow, they in turn slow the flow velocity and the resulting pressure build-up. The direction of the diffuser channels is normally opposite to that of the impeller channels and, on the inner circumference of the diffuser, corresponds to the direction of the outlet velocity of the monomer solution M from the impeller. A further task of the diffuser in two-stage centrifugal pumps is to collect the monomer solution M and guide it to the inlet of the second stage.

[0030] Of course, a combination of a diffuser and volute casing can also be used. This means that the monomer solution M is first collected in the diffuser before it can enter the volute casing.

[0031] Depending on the shape of the impellers and thus the outlet direction of the monomer solution M, a distinction is made between radial, semi-axial (also diagonal or helical impeller) and axial pumps (propeller pumps).

[0032] However, the pump chamber of the method according to the invention can also be designed as a multi-stage centrifugal pump, as described in "Pumps in the Fire Service," Part 1, "Introduction to Hydromechanics, Operation of Centrifugal Pumps," 4th edition 1998, W. Kohlhammer Verlag, Berlin. Single-stage centrifugal pumps are preferred according to the invention.

[0033] In a side channel pumping chamber, a narrow impeller with open blades rotates in the casing, in which, in addition to the blades, a side channel runs around most of the circumference. The monomer solution M to be pumped enters the blade chambers not along the axis, but through a slot in the end face, while at the same time the monomer solution M already in the chambers is driven outwards by centrifugal force. In the area of ​​the blade ends, the flow is deflected at the casing wall into the side channel, where it describes a helical path and after traveling a short distance re-enters the impeller. This process is repeated for a liquid particle on the way from the suction to the discharge nozzle, for example 10 to 50 times, depending on the throughput. In the blade chambers, the monomer solution M is accelerated not in the radial direction but also to the circumferential speed of the impeller.With this circumferential velocity and the superimposed circulation velocity, the fluid particle passes from the impeller into the side channel. As the screw travels further, the circulation component is slowed only slightly by wall friction, while the circumferential component is slowed significantly and essentially only as a result of the pressure buildup. The loss of kinetic energy of the resulting flow is continually compensated for in the impeller.

[0034] Side channel pumps are less efficient than centrifugal pumps, but generate higher discharge pressure. The magnetic coupling utilizes the attractive and repulsive forces between permanent magnets in both coupling halves for contactless and slip-free torque transmission. A can is located between the two magnet-equipped coupling halves, separating the product chamber from the surrounding area.

[0035] A canned motor is an electric motor in which the rotor and stator are separated by a can. The can is located in the gap between the stator and rotor of the motor.

[0036] The production of superabsorbents is explained in more detail below:

[0037] The superabsorbents are produced by polymerizing a monomer solution and are usually water-insoluble.

[0038] The ethylenically unsaturated, acid group-bearing monomers are preferably water-soluble, ie the solubility in water at 23°C is typically at least 1 g / 100 g water, preferably at least 5 g / 100 g water, particularly preferably at least 25 g / 100 g water, most particularly preferably at least 35 g / 100 g water.

[0039] Suitable monomers include, for example, ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is most preferred.

[0040] The ethylenically unsaturated, acid-containing monomers are usually partially neutralized. Neutralization is carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably from 40 to 85 mol%, more preferably from 50 to 80 mol%, most preferably from 60 to 75 mol%, and the usual neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Sodium and potassium are particularly preferred as alkali metals, but very particular preference is given to sodium hydroxide, sodium carbonate, or sodium bicarbonate, and mixtures thereof, in particular sodium hydroxide.

[0041] The monomers usually contain polymerization inhibitors, preferably hydroquinone hemiether, as storage stabilizers.

[0042] Suitable crosslinkers are compounds with at least two groups suitable for crosslinking. Examples of such groups include ethylenically unsaturated groups that can be radically polymerized into the polymer chain and functional groups that can form covalent bonds with the acid groups of the monomer. Polyvalent metal salts that can form coordinate bonds with at least two acid groups of the monomer are also suitable as crosslinkers.

[0043] Suitable crosslinkers 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 described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, WO 03 / 104301 A1 and DE 103 31 450 A1, mixed acrylates which, in addition to acrylate groups, contain further ethylenically unsaturated groups, as in DE 103 31 456 A1 and DE 10355 401 A1, or crosslinker mixtures as described, for example, in DE 195 43368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0044] The amount of crosslinker is preferably 0.05 to 1.5 wt. %, particularly preferably 0.1 to 1 wt. %, most preferably 0.15 to 0.6 wt. %, each calculated on the total amount of monomer used. With increasing crosslinker content, the centrifuge retention capacity (CRC) and the absorption under a pressure of 21.0 g / cm decrease. 2 (AUL0.3psi) passes through a maximum.

[0045] Any compound that generates radicals under the polymerization conditions can be used as initiators, for example, thermal initiators, redox initiators, and photoinitiators. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. Mixtures of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, are preferably used. The disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid, and sodium bisulfite is preferably used as the reducing component. Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0046] The water content of the monomer solution M is preferably from 40 to 75 wt.%, particularly preferably from 45 to 70 wt.%, most preferably from 50 to 65 wt.%. As the water content increases, the energy required for subsequent drying increases, and as the water content decreases, the heat of polymerization can only be dissipated insufficiently.

[0047] The temperature of the monomer solution M is preferably from 10 to 90°C, particularly preferably from 20 to 70°C, most preferably from 30 to 50°C.

[0048] The preferred polymerization inhibitors require dissolved oxygen for optimal effectiveness. Therefore, the monomer solution can be freed of dissolved oxygen before polymerization by inerting, i.e., by flowing an inert gas, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution before polymerization is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

[0049] Suitable reactors for polymerization include kneader reactors or belt reactors. In the kneader, the polymer gel formed during the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating agitator shafts, as described in WO 2001 / 038402 A1. Belt polymerization is described, for example, in DE 3825 366 A1 and US Pat. No. 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be comminuted, for example, in an extruder or kneader.

[0050] To improve the drying properties, the crushed polymer gel obtained by means of a kneader can be additionally extruded.

[0051] The polymer gel is then typically dried using a circulating air belt dryer until the residual moisture content is preferably 0.5 to 10 wt.%, particularly preferably 1 to 7 wt.%, most preferably 2 to 5 wt.%, wherein the residual moisture content is determined according to the EDANA-recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating." If the residual moisture content is too high, the dried polymer gel will have a glass transition temperature T too low. gand is difficult to process further. If the residual moisture content is too low, the dried polymer gel is too brittle, and undesirably large amounts of superabsorbent particles with too small a particle size ("fines") are produced in the subsequent comminution steps. The solids content of the polymer gel before drying is preferably between 25 and 90 wt. %, particularly preferably between 35 and 70 wt. %, most preferably between 40 and 60 wt. %. The dried polymer gel is then broken and optionally coarsely crushed.

[0052] The dried polymer gel is then usually ground and classified, whereby single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibrating mills can be used for grinding.

[0053] The average particle size of the superabsorbent particles separated as the product fraction is preferably from 150 to 850 pim, more preferably from 250 to 600 pim, and most particularly from 300 to 500 pim. The average particle size of the product fraction can be determined using the EDANA-recommended test method No. WSP 220.2 (05) "Particle Size Distribution," in which the mass fractions of the sieve fractions are plotted cumulatively and the average particle size is determined graphically. The average particle size is the mesh size value resulting for a cumulative 50 wt.%. The superabsorbent particles can be thermally surface-crosslinked to further improve their properties. Suitable surface-crosslinkers are compounds containing groups that can form covalent bonds with at least two carboxylate groups of the superabsorbent particles.Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083022 A2, EP 0 543303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 3523617 A1 and EP 0 450 922 A2, or ß-hydroxyalkyl amides, as described in DE 102 04 938 A1 and US 6,239,230.

[0054] The amount of surface postcrosslinker is preferably 0.001 to 2 wt.%, particularly preferably 0.01 to 1 wt.%, very particularly preferably 0.03 to 0.7 wt.%, in each case based on the superabsorbent particles.

[0055] In a preferred embodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the surface postcrosslinkers.

[0056] The polyvalent cations usable in the process according to the invention include, for example, divalent cations, such as the cations of zinc, magnesium, calcium, and strontium; trivalent cations, such as the cations of aluminum, iron, chromium, rare earths, and manganese; and tetravalent cations, such as the cations of titanium and zirconium. Possible counterions include chloride, bromide, hydroxide, sulfate, hydrogen sulfate, carbonate, hydrogen carbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate, and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0057] The amount of polyvalent cation used is, for example, 0.001 to 1.5 wt.%, preferably 0.005 to 1 wt.%, particularly preferably 0.02 to 0.8 wt.%, based in each case on the polymer.

[0058] Surface post-crosslinking is typically performed by spraying a solution of the surface post-crosslinker onto the dried superabsorbent particles. Following spraying, the superabsorbent particles coated with the surface post-crosslinker are thermally surface-crosslinked.

[0059] The spraying of a solution of the surface post-crosslinker is preferably carried out in mixers with moving mixing tools, such as screw mixers, disc mixers, and paddle mixers. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are most preferred. The distinction between horizontal and vertical mixers is made by the bearing of the mixing shaft, i.e. horizontal mixers have a horizontally mounted mixing shaft, and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, Horizontale Pflugschar® mixers (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Doetinchem; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and Schugi Flexomix® (Hosokawa Micron BV; Doetinchem; Netherlands). However, it is also possible to spray the surface post-crosslinker solution in a fluidized bed.Surface postcrosslinkers are typically used as an aqueous solution. The penetration depth of the surface postcrosslinker into the superabsorbent particles can be adjusted by varying the non-aqueous solvent content or the total solvent quantity.

[0060] Thermal surface post-crosslinking is preferably carried out in contact dryers, particularly preferably paddle dryers, and most preferably disc dryers. Suitable dryers include, 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). Fluidized-bed dryers can also be used.

[0061] Surface post-crosslinking can occur in the mixer itself, by heating the jacket or blowing in warm air. A downstream dryer, such as a tray dryer, a rotary kiln, or a heatable screw, is also suitable. Mixing and thermal surface post-crosslinking are particularly advantageous in a fluidized-bed dryer.

[0062] Preferred reaction temperatures are in the range from 100 to 250°C, preferably from 110 to 220°C, particularly preferably from 120 to 210°C, very particularly preferably from 130 to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually at most 60 minutes.

[0063] The surface-crosslinked superabsorbent particles can then be classified again, with superabsorbent particles that are too small and / or too large being separated and returned to the process.

[0064] The surface-crosslinked superabsorbent particles can be coated or remoistened to further improve their properties.

[0065] Rewetting is preferably carried out at 30 to 80°C, particularly preferably at 35 to 70°C, and most preferably at 40 to 60°C. At temperatures that are too low, the superabsorbent particles tend to clump together, and at higher temperatures, water evaporates noticeably. The amount of water used for rewetting is preferably from 1 to 10 wt.%, particularly preferably from 2 to 8 wt.%, and most preferably from 3 to 5 wt.%. Rewetting increases the mechanical stability of the superabsorbent particles and reduces their tendency to build up static charge. Rewetting is advantageously carried out in the cooler after thermal surface postcrosslinking. Suitable coatings for improving the swelling rate and gel bed permeability (GBP) include, for example, inorganic inert substances such as water-insoluble metal salts, organic polymers, cationic polymers, and divalent or polyvalent metal cations.Suitable coatings for dust binding include polyols. Suitable coatings to combat the undesirable caking tendency of superabsorbent particles include fumed silica, such as Aerosil® 200, precipitated silica, such as Sipernat® D17, and surfactants, such as Span® 20.

[0066] Examples

[0067] Example 1 (not according to the invention)

[0068] By continuously mixing deionized water, 50 wt.% sodium hydroxide solution, and acrylic acid, a monomer solution M was prepared (see Figure 1), resulting in a degree of neutralization of 72.0 mol%. The water content of the monomer solution M was 57.5 wt.%.

[0069] In the ring line R, partially neutralized acrylic acid was circulated by pump P1 through the heat exchanger W and tank B. Water was added via the feed line Zi, sodium hydroxide via the feed line Z3, and acrylic acid via the feed line Z2. The monomer solution M was pumped into the polymerization reactor by pump P2.

[0070] Magnetically coupled centrifugal pumps were used as pumps P1 and P2. The pump chamber and drive chamber were separated by a metal wall. The drive shaft was mounted horizontally in the pump chamber with a tungsten carbide plain bearing.

[0071] Triple ethoxylated glycerol triacrylate (approx. 85 wt.%) was used as the crosslinker. The amount used was 1.2 kg per t of monomer solution M.

[0072] To initiate the radical polymerization, 1.39 kg of a 0.25 wt.% aqueous hydrogen peroxide solution, 3.58 kg of a 15 wt.% aqueous sodium peroxodisulfate solution and 1.28 kg of a 1 wt.% aqueous ascorbic acid solution were used per t of monomer solution M.

[0073] The monomer solution M was transferred into a List Contikneter reactor with a volume of 6.3m 3 (LIST AG, Arisdorf, Switzerland). The throughput of the monomer solution M was approximately 20 t / h. The reaction solution had a temperature of 23.5°C at the inlet. Between the addition point for the crosslinker and the addition points for the hydrogen peroxide and sodium peroxodisulfate solutions, the monomer solution M was inertized with nitrogen. Ascorbic acid was dosed directly into the reactor.

[0074] After approximately 50% of the residence time, an additional 1,000 kg / h of superabsorbent particles with a particle size of less than 150 μm, which had been generated during the production process through comminution and classification, were added to the reactor. The residence time of the reaction mixture in the reactor was approximately 15 minutes.

[0075] The resulting polymer gel was fed onto the conveyor belt of a circulating air belt dryer using an oscillating conveyor belt. The circulating air belt dryer was 48 m long and its conveyor belt had an effective width of 4.4 m. On the circulating air belt dryer, the aqueous polymer gel was continuously circulated with an air / gas mixture (approx. 175°C) and dried. The residence time in the circulating air belt dryer was 37 minutes.

[0076] The dried polymer gel was ground using a three-stage roller mill and sieved to a particle size of 150 to 850 pim. Superabsorbent particles with a particle size of less than 150 pim were separated. Superabsorbent particles with a particle size greater than 850 pim were returned to the grinding process. Superabsorbent particles with a particle size in the range of 150 to 850 pim were thermally surface-crosslinked.

[0077] The superabsorbent particles were coated with a surface post-crosslinker solution in a Schugi Flexomix® (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried in a NARA Paddle Dryer (GMF Gouda, Waddinxveen, Netherlands) for 45 minutes at 176°C.

[0078] The following quantities were dosed into the Schugi Flexomix®:

[0079] 7.5 t / h superabsorbent particles

[0080] 348.75 kg / h surface post-crosslinker solution

[0081] The surface postcrosslinker solution contained 2.2 wt% 2-hydroxyethyl-2 oxazolidone, 2.2 wt% 1,3-propanediol, 29.0 wt% 1,2-propanediol, 3.2 wt% aluminum sulfate, 56.9 wt% water and 6.5 wt% isopropanol.

[0082] After drying, the surface-crosslinked superabsorbent particles were cooled to approximately 60°C in a NARA paddle cooler (GMF Gouda, Waddinxveen, Netherlands). The surface-crosslinked superabsorbent particles were coated with 124.5 kg of a 2.4 wt. % aqueous polyethylene glycol solution (polyethylene glycol with an average molecular weight of 400 g / mol). During production interruptions, pump P2 was switched off. Polymer was formed in pump P2.

[0083] Example 2 (not according to the invention)

[0084] The procedure is as in Example 1. If production is interrupted, pump P2 is completely emptied.

[0085] No polymer was formed in pump P2. Example 3 (according to the invention)

[0086] The procedure is as in Example 1. During a production interruption, pump P2 was not switched off; the monomer solution M was circulated via line L through tank B. No polymer was produced in pump P2.

Claims

Patent claims 1. A process for producing superabsorbents, wherein at least one ethylenically unsaturated, acid-group-bearing monomer is at least partially neutralized with an aqueous base, the resulting aqueous monomer solution M is conveyed by means of a pump P, at least one crosslinker and at least one initiator are added to the aqueous monomer solution M before or after the pump P, the aqueous monomer solution M is then polymerized to form a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried on a circulating air belt dryer and the dried polymer gel is comminuted, classified and optionally thermally surface-postcrosslinked, characterized in that the pump P has a direct or indirect line L from the pressure side to the suction side of the pump P and the monomer solution M is conveyed in a circuit at least temporarily via the line L in the event of a shutdown.

2. Process according to claim 1, characterized in that an ethylenically unsaturated carboxylic acid is used as the ethylenically unsaturated, acid group-bearing monomer.

3. Process according to claim 1, characterized in that acrylic acid is used as the ethylenically unsaturated, acid group-bearing monomer.

4. Process according to one of claims 1 to 3, characterized in that an alkali metal hydroxide, alkali metal oxides, an alkali metal hydrogen carbonate and / or an alkali metal carbonate is used as base.

5. Process according to one of claims 1 to 3, characterized in that sodium hydroxide is used as the base.

6. Process according to one of claims 1 to 5, characterized in that the ethylenically unsaturated, acid group-bearing monomer is neutralized to 40 to 85 mol%.

7. Process according to one of claims 1 to 5, characterized in that the ethylenically unsaturated, acid group-bearing monomer is neutralized to 60 to 75 mol%.

8. Process according to one of claims 1 to 7, characterized in that the water content of the monomer solution M is from 40 to 75 wt.%.

9. Process according to one of claims 1 to 8, characterized in that the water content of the monomer solution M is from 50 to 65 wt.%.

10. Process according to one of claims 1 to 9, characterized in that the aqueous monomer solution M has a temperature of 10 to 90°C.

11. Process according to one of claims 1 to 9, characterized in that the aqueous monomer solution M has a temperature of 30 to 50°C.

12. Method according to one of claims 1 to 11, characterized in that the power transmission to the drive shaft takes place by means of a magnetic coupling or a canned motor.

13. Process according to one of claims 1 to 12, characterized in that in the event of a shutdown, the monomer solution M is circulated via the line L for at least 60 minutes and the delivery rate of the pump P is reduced by 50 to 99%.

14. Process according to one of claims 1 to 12, characterized in that in the event of a shutdown, the monomer solution M is circulated via the line L for at least 120 minutes and the delivery rate of the pump P is reduced by 90 to 95%.

15. Method according to one of claims 1 to 12, characterized in that the temperature in the pump P is measured and the delivery capacity of the pump P is reduced to 0 in a temperature-controlled manner when it is switched off, so that the temperature in the pump P does not rise above 50°C.

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