Method for preparing superabsorbents

Abstract

The invention relates to a method for producing superabsorbents, in which a monomer solution M is conveyed by a pump P having a line L from the pressure side to the suction side of the pump P, and in the event of a stop, the monomer solution M is cyclically conveyed at least temporarily via the line L.

Description

[0001] The invention relates to a process for the production of superabsorbents, wherein a monomer solution M is conveyed by a pump P, the pump P having a line L from the pressure side to the suction side of the pump P, and in the event of a stop, the monomer solution M is at least temporarily conveyed in a circulating manner via the line L.

[0002] Superabsorbents are used for the manufacture of diapers, sanitary tampons, sanitary napkins and other hygiene products, but also as water-retaining agents in horticulture. Superabsorbents are also known as water-absorbing polymers.

[0003] The production of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", F. L. Buchholz and A. T. Graham, Wiley-VCH, 1998, pages 71 to 103.

[0004] In order to improve the application properties, such as gel bed permeability (GBP) and absorbency under load at 49.2 g / cm2(AUL0.7psi), the superabsorbent particles are usually surface postcrosslinked. By this, the degree of crosslinking of the particle surface is increased, so that the absorbency under load at 49.2 g / cm2(AUL0.7psi) can be at least partially decoupled from the centrifuge retention capacity (CRC). This surface postcrosslinking can be carried out in the aqueous gel phase. Preferably, however, the surface of the dried, ground and sieved superabsorbent particles (base polymer) is coated with a surface postcrosslinking agent and thermally surface postcrosslinked. Suitable crosslinking agents are compounds which can form covalent bonds with at least two carboxyl groups of the superabsorbent particles.

[0005] WO 2007 / 028751 A1 relates to a neutralization process.

[0006] It is an object of the present invention to provide an improved process for the production of superabsorbents, in particular for the trouble-free operation of the pumps used.

[0007] This object is achieved by a process for the production of superabsorbents, wherein at least one ethylene-unsaturated monomer containing acid groups is at least partially neutralized with an aqueous base, the resulting aqueous monomer solution M is conveyed by a pump P, at least one crosslinking agent 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 a polymer gel, the polymer gel is optionally extruded, the polymer gel is dried on a circulating 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 in the event of a stop, the monomer solution M is at least temporarily conveyed in a circulating manner via the line L.

[0008] The monomer solution M can be conveyed via line L directly from the pressure side to the suction side of the pump P. However, it is also possible to convey the monomer solution M at any other point upstream of the pump P, for example to the container B of the neutralization method shown. The reference signs have the following meanings here: Figure 1

[0009] W Heat exchanger R Annular line L Line P1 Pump in annular line P2 Pump for a polymerization reactor (pump P according to the invention) B (Buffer) vessel Z1 Supply line Z2 Supply line [Z3] Supply line

[0010] The present application is based on the insight that when production is stopped, the monomer solution M remaining in the pump P can polymerize by the residual heat of the pump P. This can be avoided by controlled continued operation of the pump P.

[0011] In the case of a stop, the monomer solution M is preferably conveyed in a circulating manner via line L for at least 60 minutes, particularly preferably for at least 90 minutes, very particularly preferably for at least 120 minutes.

[0012] In the case of a stop, the conveying capacity of the pump P is preferably reduced by 50% to 99%, particularly preferably by 70% to 98%, very particularly preferably by 90% to 95%.

[0013] In a particular embodiment of the present application, the temperature in the pump P is measured and, when stopped, the conveying capacity of the pump P is reduced to 0 in a temperature-controlled manner, so that the temperature in the pump P does not rise above 50°C.

[0014] The preferred pumps P according to the present application are centrifugal pumps and side channel pumps.

[0015] Unlike stroke and circulation plunger pumps, which work on the displacement principle, centrifugal pumps and side channel pumps work on the power principle. By rotating the impeller (conveying element connected to the drive shaft), work in the form of kinetic energy is transferred from the impeller to the monomer solution M to be conveyed. The kinetic energy is converted back into static pressure (pressure energy, law of conservation of energy) mainly after the impeller in the guide wheel and / or the spiral housing. The impeller is, in principle, a simple disc on which blades are mounted.

[0016] The blades form blade channels, the cross section of which usually increases greatly from the inside to the outside due to the increasing circumference. Through these blade channels, the monomer solution M to be conveyed can flow as much as possible into the center of the impeller. Thus, in contrast to piston pumps, the monomer solution M to be conveyed permanently flows during operation in centrifugal pumps and side channel pumps.

[0017] In contrast to open impellers, closed impellers can also be used. In this case, the blade channels are simply covered by a second disc with openings in the center.

[0018] ​When the water drop falls in the center of the disc, the blade curvature is usually similar to the natural path of the water drop on a rotating circular smooth disc from the perspective of a rotating observer. This blade shape is referred to as a "backward curved" blade. However, in principle, slightly forward curved blades and spiral blades, i.e. backward curved blades, whose edges protrude into the impeller inlet and grab the monomer solution M like a ship's propeller, can also be used.

[0019] A centrifugal pump (centrifugal pump chamber) consists of a pump housing and an impeller rotating in the pump housing, on which blades are mounted. The liquid F enters axially through the suction connection. It is deflected radially outwards under the influence of the centrifugal force and is accelerated to high speed in the process by the impeller. The task of the pump housing is to collect the monomer solution M from all the blade channels so that it can be collected through the pressure outlet. However, the pump housing at the same time has the task of converting the kinetic energy of the monomer solution M into pressure. For this purpose, the fact is usually exploited that the speed of the monomer solution M is reduced by an increase in cross section, which leads to an increase in pressure. In order to increase the cross section, there are usually two pump housing designs. Behind the last stage of a single-stage pump or a multi-stage centrifugal pump, a spiral housing is often used. This surrounds the impeller in the form of a spiral. The cross section widens in the direction of the pressure outlet. The monomer solution M flowing through is thus slowed down, which means that the pressure increases at the same time.

[0020] Instead of a spiral, fixed guide wheels are also used, in particular in the case of multi-stage pumps. The guide wheels are mounted in the pump housing and form an annular space. They surround the impeller. Guide vanes are arranged in the guide wheels and form channels that widen constantly outwards relative to one another. In this embodiment, the monomer solution M is not centrifuged directly into the pump housing, but first flows through the vane channels of the guide wheels. Due to the widening in the flow direction, it in turn leads to a slowing down of the flow rate and an increase in pressure resulting therefrom. The direction of the guide wheel channels is usually opposite to the direction of the impeller channels and corresponds to the direction of the speed of the monomer solution M leaving the impeller on the inner circumference of the guide wheel. In the case of a two-stage centrifugal pump, another purpose of the guide wheels is to collect the monomer solution M and direct it to the inlet of the second stage.

[0021] Of course, a combination of guide wheels and spiral housing can also be used. This means that the monomer solution M is first collected in the guide wheels before it enters the spiral housing.

[0022] Depending on the shape of the impeller and the outlet direction of the monomer solution M, it is possible to distinguish between radial pumps, semi-axial pumps (including diagonal pumps or screw pumps) and axial pumps (propeller pumps).

[0023] However, the pump chamber according to the method of the application can also be designed as a multi-stage centrifugal pump, as described in DIN EN 60 513-1, Pumps - Part 1: General requirements - Centrifugal pumps - Principles of operation, 4th edition 1998, W. Kohlhammer Verlag, Berlin. According to the application, a single-stage centrifugal pump is preferred.

[0024] In a side channel pump chamber, a narrow impeller with open blades rotates in a housing, wherein, in addition to the blades, a side channel extends over most of the circumference. The monomer solution M to be conveyed does not enter along the axis, but through a slit from the end face into the blade chamber, wherein, at the same time, the monomer solution M already in the chamber is driven outward by the centrifugal force. In the region of the blade end, the gas flow is deflected on the housing wall into the side channel, where it describes a spiral path and after a distance enters the impeller again. For liquid particles, depending on the flow velocity, this process is repeated, for example, 10 to 50 times on the path from the suction connection to the pressure connection. In the blade chamber, the monomer solution M is accelerated not only in the radial direction but also to the peripheral speed of the impeller. With this peripheral speed and the superimposed circulating speed, the liquid particles enter the side channel from the impeller. On the further spiral path, the circulating component is only slightly decelerated due to wall friction, while the peripheral component is significantly decelerated only due to the increase in pressure. The resulting loss of kinetic energy of the flow is repeatedly compensated in the impeller.

[0025] Side channel pumps are less efficient than centrifugal pumps, but produce higher conveying pressures.

[0026] Magnetic couplings use the attractive and repulsive forces between permanent magnets in two coupling halves to achieve contactless, slip-free torque transmission. Between the two magnet-equipped coupling halves there is a gap tube, which separates the product space from the surroundings.

[0027] A gap tube motor is an electric motor in which the rotor and stator are separated by a gap tube. The gap tube is located in the gap between the stator and the rotor of the motor.

[0028] The preparation of superabsorbers is explained in detail below.

[0029] Superabsorbers are prepared by polymerization of monomer solutions and are generally water-insoluble.

[0030] The acid group-carrying olefinically unsaturated monomers preferably have water solubility, i.e. a solubility in water at 23°C of generally at least 1 g / 100 g water, preferably at least 5 g / 100 g water, particularly preferably at least 25 g / 100 g water, further particularly preferably at least 35 g / 100 g water.

[0031] Suitable monomers are, for example, olefinically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Further particularly preferred is the use of acrylic acid.

[0032] The olefinically unsaturated monomers carrying acid groups are usually partially neutralized. The neutralization is carried out in the monomer stage. This is usually effected by mixing in a neutralizing agent in the form of an aqueous solution or, preferably, in solid form. The degree of neutralization is preferably from 40 to 85 mol%, particularly preferably from 50 to 80 mol%, further particularly preferably from 60 to 75 mol%, where customary neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal hydrogencarbonates and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Particularly preferred alkali metals are sodium and potassium, but further particularly preferably sodium hydroxide, sodium carbonate or sodium hydrogencarbonate and mixtures thereof, in particular sodium hydroxide, are used.

[0033] The monomers usually contain a polymerization inhibitor, preferably hydroquinone semiketal, which acts as a storage stabilizer.

[0034] Suitable crosslinking agents are compounds having at least two groups suitable for crosslinking. Such groups are, for example, olefinically unsaturated groups which can be free-radically polymerized into the polymer chain, and functional groups which can form covalent bonds with the acid groups of the monomers. Furthermore, polyvalent metal salts which can form coordination bonds with at least two acid groups of the monomers are also suitable as crosslinking agents.

[0035] Suitable crosslinking agents 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, diacrylates 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 containing, in addition to acrylate groups, also olefinically unsaturated groups as described in DE 103 31 456 A1 and DE 103 55 401 A1, or crosslinking agent mixtures as described, for example, in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0036] The amount of crosslinking agent is preferably from 0.05 to 1.5% by weight, particularly preferably from 0.1 to 1 % by weight, further particularly preferably from 0.15 to 0.6% by weight, based on the total amount of monomers used. With increasing crosslinking agent content, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm2(AUL0.3psi) passes through a maximum.

[0037] All compounds which generate free radicals under the polymerization conditions can be used as initiators, 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. Preference is given to using mixtures composed of thermal initiators and redox initiators, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid. Preferably, the disodium salt of 2-hydroxy-2-sulfinylethanoic acid or a mixture composed of the sodium salt of 2-hydroxy-2-sulfinylethanoic acid, the disodium salt of 2-hydroxy-2-sulfinylethanoic acid and sodium bisulfite is used as reducing component. Commercially available mixtures of this type are Bruggolite® FF6 and Bruggolite® FF7 (Bruggemann Chemicals; Heilbronn; Germany). ® FF6 and Bruggolite® FF7 (Bruggemann Chemicals; Heilbronn; Germany). ® FF6 and Bruggolite® FF7 (Bruggemann Chemicals; Heilbronn; Germany).

[0038] The monomer solution M preferably has a water content of from 40 to 75% by weight, particularly preferably from 45 to 70% by weight, further particularly preferably from 50 to 65% by weight. With increasing water content, the energy consumption in the subsequent drying process increases, and in the case of a reduced water content, the polymerization heat can not be sufficiently dissipated.

[0039] The monomer solution M preferably has a temperature of from 10 to 90°C, particularly preferably from 20 to 70°C, further particularly preferably from 30 to 50°C.

[0040] In order to achieve optimum results, the preferred polymerization inhibitors require dissolved oxygen. Therefore, prior to the polymerization, the dissolved oxygen in the monomer solution can be removed by means of inertization, i.e. by flowing an inert gas, preferably nitrogen or carbon dioxide, through the monomer solution. Preferably, the oxygen content of the monomer solution is reduced to less than 1 wppm prior to the polymerization, particularly preferably to less than 0.5 wppm, further particularly preferably to less than 0.1 wppm.

[0041] Suitable reactors for the polymerization are, for example, kneader reactors or belt reactors. In a kneader, the polymer gel produced in the polymerization of the aqueous monomer solution or suspension is continuously comminuted by means of, for example, counter-rotating stirring shafts, as described in WO 2001 / 038402 A1. The polymerization in a belt reactor is described, for example, in DE 38 25 366 A1 and US 6,241,928. The polymerization in a belt reactor produces a polymer gel which has to be comminuted, for example in an extruder or a kneader.

[0042] In order to improve the drying properties, the comminuted polymer gel obtained by means of a kneader can additionally be extruded.

[0043] Subsequently, the polymer gel is typically dried, usually by means of a circulating air belt dryer, until the residual moisture content is preferably from 0.5 to 10 wt.-%, particularly preferably from 1 to 7 wt.-%, further particularly preferably from 2 to 5 wt.-%, wherein the residual moisture content is determined in accordance with EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". In the case of too high residual moisture, the dried polymer gel has too low a glass transition temperature Tg g and is difficult to further process. In the case of too low residual moisture, the dried polymer gel is too brittle and produces an undesirably large amount of superabsorber particles of too small particle size ("fines") in the subsequent comminution step. The polymer gel prior to drying has a solids content of preferably from 25 to 90 wt.-%, particularly preferably from 35 to 70 wt.-%, further particularly preferably from 40 to 60 wt.-%. The dried polymer gel is subsequently broken up and optionally coarsely comminuted.

[0044] The dried polymer gel is subsequently typically ground and classified, wherein grinding can typically be carried out using a single- or multi-stage roll mill (preferably a two- or three-stage roll mill), a pin mill, a hammer mill or a vibration mill.

[0045] The average particle size of the superabsorber particles isolated as product fraction is preferably at least 150 pm to 850 pm, particularly preferably 250 pm to 600 pm, further particularly preferably 300 pm to 500 pm. The average particle size of the product fraction can be determined using the EDANA recommended test method No. WSP 220.2 (05) "Partikel Size Distribution", wherein the mass proportions of the sievings are cumulatively plotted and the average particle size is determined graphically. Here, the average particle size is the mesh width value corresponding to 50 wt.-% of the cumulative plot.

[0046] In order to further improve the properties, the superabsorber particles can be thermally postcrosslinked on the surface. Suitable surface postcrosslinkers are compounds containing groups which are capable of forming covalent bonds with at least two carboxyl groups of the superabsorber particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional amino amines, polyfunctional epoxides as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, difunctional or polyfunctional alcohols as described in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or ß-hydroxyalkyl amides as described in DE 102 04 938 A1 and US 6,239,230.

[0047] The amount of surface post-crosslinker, based on the superabsorbent particles, is preferably from 0.001 to 2 wt.-%, particularly preferably from 0.01 to 1 wt.-%, very particularly preferably from 0.03 to 0.7 wt.-%.

[0048] In a preferred embodiment of the present application, in addition to the surface post-crosslinker, also a polyvalent cation is applied to the particle surface.

[0049] The polyvalent cation which can be used in the process according to the present application is, for example, a divalent cation, such as the cations of zinc, magnesium, calcium and strontium, a trivalent cation, such as the cations of aluminum, iron, chromium, rare earths and manganese, and a tetravalent cation, such as the cations of titanium and zirconium. Possible counterions are chloride, bromide, hydroxide, sulfate, bisulfate, carbonate, bicarbonate, nitrate, phosphate, hydrogen phosphate, dihydrogen phosphate and carboxylates, such as acetate and lactate. Aluminum hydroxide, aluminum sulfate and aluminum lactate are preferred.

[0050] The amount of polyvalent cation, based on the polymer, is, for example, from 0.001 to 1.5 wt.-%, preferably from 0.005 to 1 wt.-%, particularly preferably from 0.02 to 0.8 wt.-%.

[0051] The surface post-crosslinking is usually carried out by spraying a solution of the surface post-crosslinker onto the dry superabsorbent particles. After the spraying, the superabsorbent particles coated with the surface post-crosslinker are subjected to thermal surface post-crosslinking.

[0052] The spraying of the solution of the surface post-crosslinker is preferably carried out in a stirrer with a moving stirring tool, such as in a screw stirrer, a disc stirrer and a paddle stirrer. The use of a horizontal stirrer, such as a paddle stirrer, is particularly preferred, and the use of a vertical stirrer is further particularly preferred. The difference between a horizontal stirrer and a vertical stirrer is in the way the stirring shaft body is supported, i.e. the horizontal stirrer has a horizontally supported stirring shaft body, and the vertical stirrer has a vertically supported stirring shaft body. Suitable stirrers are, for example, Horizontale Pflugschar ® Mischer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Dordrecht; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA) and Schugi Flexomix ®(Hosokawa Micron BV; Duttingheim; Netherlands). However, a surface crosslinking agent solution can also be sprayed in a fluidized bed.

[0053] Post-crosslinking agents are typically used as aqueous solutions. The penetration depth of the post-crosslinking agent into the superabsorbent particles can be adjusted by varying the content of the non-aqueous solvent or the total solvent volume.

[0054] Post-crosslinking of the hot surface is preferably carried out in a contact dryer, particularly preferably in a paddle dryer, and very particularly preferably in a disc dryer. Suitable dryers include, for example, the Hosokawa Bepex. ® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Rheingalten; Germany), Hosokawa Bepex ® Disc Dryer (HosokawaMicron GmbH; Rhinegarten; Germany), Holo-Flite ® Dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Freising; Germany). Fluidized bed dryers can also be used.

[0055] Post-crosslinking of the surface can be achieved within the mixer itself via a heated outer casing or by injecting hot air. This is also applicable to downstream dryers, such as rack dryers, rotary kilns, or heated worm gear dryers. Particularly advantageously, mixing and hot surface post-crosslinking can be carried out in fluidized bed dryers.

[0056] The preferred reaction temperature is in the range of 100°C to 250°C, more preferably 110°C to 220°C, particularly preferably 120°C to 210°C, and even more preferably 130°C to 200°C. The preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, even more preferably at least 30 minutes, and typically at most 60 minutes.

[0057] The superabsorbent particles that have undergone surface cross-linking can then be reclassified, wherein excessively small and / or excessively large superabsorbent particles are separated and fed back into the method.

[0058] To further improve the properties, the superabsorbent particles that have undergone surface cross-linking can be coated or re-wetted.

[0059] Rewetting is preferably performed at a temperature between 30°C and 80°C, particularly preferably between 35°C and 70°C, and even more preferably between 40°C and 60°C. At excessively low temperatures, the superabsorbent particles tend to aggregate, while at higher temperatures, the water has already evaporated significantly. The amount of water used for rewetting is preferably 1% to 10% by weight, particularly preferably 2% to 8% by weight, and even more preferably 3% to 5% by weight. Rewetting enhances the mechanical stability of the superabsorbent particles and reduces their tendency to become electrostatically charged. Preferably, rewetting is performed in a cooler after post-crosslinking on the hot surface.

[0060] Coatings suitable for improving swelling ratio and gel bed permeability (GBP) include, for example, inorganic inert materials (such as water-insoluble metal salts), organic polymers, cationic polymers, and divalent or polyvalent metal cations. Coatings suitable for binding dust include, for example, polyols. Coatings suitable for resisting the undesirable agglomeration tendency of superabsorbent particles include, for example, fumed silica (such as Aerosil). ® 200), precipitated silica (such as Sipernat) ® D17) and surfactants (such as Span) ® 20). Example

[0061] Example 1 (not according to the invention)

[0062] Monomer solution M was prepared by continuously mixing deionized water, 50% sodium hydroxide solution, and acrylic acid (see [link to original text]). Figure 1 This results in a neutralization degree of 72.0 mol%. The water content of monomer solution M is 57.5 wt%.

[0063] In the loop pipeline R, partially neutralized acrylic acid is circulated via heat exchanger W and container B through pump P1. Water is added via pipeline Z1, sodium hydroxide solution is added via pipeline Z3, and acrylic acid is added via pipeline Z2. The monomer solution M is then pumped into the polymerization reactor via pump P2.

[0064] The pumps used, P1 and P2, are magnetically coupled centrifugal pumps. The pump chamber and drive chamber are separated by a metal wall. The drive shaft is horizontally supported in the pump chamber by tungsten carbide sliding bearings.

[0065] 3-ethoxylated glycerol triacrylate (approximately 85% by weight) was used as a crosslinking agent. The dosage was 1.2 kg per ton of monomer solution M.

[0066] To initiate free radical polymerization, 1.39 kg of 0.25% by weight hydrogen peroxide aqueous solution, 3.58 kg of 15% by weight sodium persulfate aqueous solution, and 1.28 kg of 1% by weight ascorbic acid aqueous solution are used per ton of monomer solution M.

[0067] Monomer solution M was fed into a List Contikneter reactor (LISTAG, Ariesdorf, Switzerland) with a volume of 6.3 m³. The throughput of monomer solution M was approximately 20 t / h. The temperature of the reaction solution at the inlet was 23.5 °C.

[0068] The monomer solution M was inerted using nitrogen gas between the points where the crosslinking agent was added and where the hydrogen peroxide and sodium persulfate solution were added. Ascorbic acid was then added directly to the reactor.

[0069] After approximately 50% of the residence time, approximately 1,000 kg / h of superabsorbent particles with a particle size less than 150 µm, resulting from the crushing and sorting during the preparation process, are additionally added to the reactor. The residence time of the reaction mixture in the reactor is approximately 15 minutes.

[0070] The resulting polymer gel is conveyed onto the conveyor belt of a circulating air belt dryer via a vibrating conveyor belt. This circulating air belt dryer is 48 m long, and its conveyor belt has an effective width of 4.4 m. The aqueous polymer gel is continuously dried in the circulating air belt dryer using an air / gas mixture (approximately 175°C). The residence time in the circulating air belt dryer is 37 minutes.

[0071] The dried polymer gel was pulverized using a three-stage roller mill and sieved to a particle size of 150µm to 850µm. Superabsorbent particles smaller than 150µm were separated. Superabsorbent particles larger than 850µm were returned for further pulverization. The superabsorbent particles in the 150µm to 850µm particle size range were then subjected to thermal surface post-crosslinking.

[0072] Superabsorbent particles in Schugi Flexomix ® The sample was coated with a surface-crosslinking agent solution (Hosokawa Micron BV, Doetinchem, Netherlands) and then dried at 176°C for 45 minutes in a NARA paddle dryer (GMF Gouda, Waddinxveen, Netherlands).

[0073] Add the following amounts of material to the Schugi Flexomix ® :

[0074] 7.5 t / h Superabsorber particles 348.75 kg / h Surface post-crosslinker solution

[0075] The surface post-crosslinking agent solution contains 2.2 wt% 2-hydroxyethyl-2-oxazolidinone, 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.

[0076] After drying, the surface-crosslinked superabsorbent particles were cooled to approximately 60°C in a NARA paddle cooler (GMF Gouda, Waddingxveen, Netherlands). The surface-crosslinked superabsorbent particles were then coated with 124.5 kg of a 2.4% by weight aqueous solution of polyethylene glycol (polyethylene glycol with an average molecular weight of 400 g / mol).

[0077] In the event of a production interruption, pump P2 is shut down. Polymer is formed in pump P2.

[0078] Example 2 (not according to the invention)

[0079] The operation was performed as in Example 1. In the event of a production interruption, pump P2 was completely emptied. No polymer was formed in pump P2.

[0080] Example 3 (according to the invention)

[0081] The operation was carried out as in Example 1. In the event of a production interruption, pump P2 was not stopped, and monomer solution M was circulated to container B via pipeline L. No polymer was formed in pump P2.

Claims

1. A method for preparing a superabsorbent, wherein, At least one ethylene unsaturated monomer containing an acidic group is at least partially neutralized by an aqueous alkali, and the resulting monomer aqueous solution M is delivered by a pump P. At least one crosslinking agent and at least one initiator are added to the monomer aqueous solution M before or after the pump P, and then the monomer aqueous solution M is polymerized into a polymer gel. Optionally, the polymer gel is extruded, the polymer gel is dried on a circulating belt dryer, and the dried polymer gel is pulverized, graded, and optionally thermally surface-crosslinked. The pump P is characterized by having a direct or indirect pipeline L from the pressure side to the suction side of the pump P, and in the event of a stop, the monomer solution M is circulated at least temporarily via the pipeline L.

2. The method according to claim 1, characterized in that, Ethylene unsaturated carboxylic acids are used as monomers carrying ethylene unsaturated acid groups.

3. The method according to claim 1, characterized in that, Acrylic acid is used as a monomer to carry the unsaturated acid group of ethylene.

4. The method according to any one of claims 1 to 3, characterized in that, Alkali metal hydroxides, alkali metal oxides, alkali metal bicarbonates and / or alkali metal carbonates are used as alkalis.

5. The method according to any one of claims 1 to 3, characterized in that, Sodium hydroxide is used as an alkali.

6. The method according to any one of claims 1 to 5, characterized in that, The monomer carried by the ethylene unsaturated acid group is neutralized to 40 mol% to 85 mol%.

7. The method according to any one of claims 1 to 5, characterized in that, The monomer carried by the ethylene unsaturated acid group is neutralized to 60 mol% to 75 mol%.

8. The method according to any one of claims 1 to 7, characterized in that, The water content of the monomer solution M is 40% to 75% by weight.

9. The method according to any one of claims 1 to 8, characterized in that, The monomer solution M has a water content of 50% to 65% by weight.

10. The method according to any one of claims 1 to 9, characterized in that, The temperature of the monomer aqueous solution M is between 10°C and 90°C.

11. The method according to any one of claims 1 to 9, characterized in that, The temperature of the monomer aqueous solution M is 30°C to 50°C.

12. The method according to any one of claims 1 to 11, characterized in that, Force is transmitted to the drive shaft via a magnetic coupling or a gap tube motor.

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

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

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

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