Method for surface thermal post-crosslinking of superabsorbent

Abstract

The invention relates to a method for the continuous thermal post-crosslinking of the surface of a superabsorbent, in which superabsorbent particles are coated by spraying a surface post-crosslinking agent solution, the coated superabsorbent particles are thermally post-treated in a contact dryer (1), the superabsorbent particles subjected to heat post-treatment are cooled in a contact dryer (2), characterized in that the superabsorbent particles have a temperature of 40 DEG C to 85 DEG C and a moisture content of 1.5 wt% to 6.0 wt% immediately before the surface post-crosslinker solution is sprayed, and the surface post-crosslinker solution contains 1.5 wt% to 4.5 wt% of water based on the superabsorbent particles.

Description

[0001] This invention relates to a method for continuous surface thermal post-crosslinking of superabsorbent particles, wherein the superabsorbent particles are coated by spraying a surface post-crosslinking agent solution, the superabsorbent particles having a temperature of 40°C to 85°C and a moisture content of 1.5 wt% to 6.0 wt% immediately before the spraying of the surface post-crosslinking agent solution, and the surface post-crosslinking agent solution containing 1.5 wt% to 4.5 wt% water based on the superabsorbent particles, the coated superabsorbent particles are subjected to thermal post-treatment in a contact dryer 1, and the thermally post-treated superabsorbent particles are cooled in a contact dryer 2.

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

[0003] The preparation of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", FLBuchholz and A.T. Graham, Wiley-VCH, 1998, pp. 71-103.

[0004] To improve application properties such as gel bed permeability (GBP) and absorbance at 49.2 g / cm² pressure (AUL 0.7 psi), superabsorbent particles are typically post-crosslinked to the surface. This increases the degree of crosslinking on the particle surface, thereby allowing at least partial decoupling of the absorbance at 49.2 g / cm² pressure (AUL 0.7 psi) from the centrifugal retention capacity (CRC). This post-crosslinking can be performed in an aqueous gel phase. However, it is preferable to coat the surface of the dried, milled, and sieved superabsorbent particles (basic polymer) with a post-crosslinking agent and perform thermal post-crosslinking. Suitable crosslinking agents are compounds capable of forming covalent bonds with at least two carboxyl groups of the superabsorbent particles.

[0005] Surface post-crosslinking has been described, for example, in EP 1 130 045 A2, US 2007 / 0106013 A1, US 2019 / 0329219A1, US 2019 / 0329220 A1 and US 2021 / 0269606 A1.

[0006] The purpose of this invention is to provide a method for surface thermal post-crosslinking of improved superabsorbent particles, particularly to improve absorption capacity (CRC) and permeability (GBP).

[0007] This objective is achieved by a continuous surface thermal post-crosslinking method for a superabsorbent, wherein superabsorbent particles are coated by spraying a surface post-crosslinking agent solution, the coated superabsorbent particles are subjected to thermal post-treatment in a contact dryer 1, and the thermally post-treated superabsorbent particles are cooled in a contact dryer 2. The superabsorbent particles have a temperature of 40°C to 85°C and a moisture content of 1.5 wt% to 6.0 wt% immediately before being coated with the surface post-crosslinking agent solution, and the surface post-crosslinking agent solution contains 1.5 wt% to 4.5 wt% water based on the superabsorbent particles.

[0008] The temperature of the superabsorbent particles in the crosslinking agent solution after spraying the surface is 40°C to 85°C, preferably 45°C to 82°C, particularly preferably 50°C to 78°C, and even more preferably 55°C to 75°C.

[0009] The moisture content of the superabsorbent particles in the crosslinking agent solution after spraying onto the surface is 1.5 wt% to 6.0 wt%, preferably 2.0 wt% to 5.5 wt%, particularly preferably 2.5 wt% to 5.0 wt%, and even more particularly preferably 3.0 wt% to 4.5 wt%. The moisture content of the water-absorbing superabsorbent particles is determined according to EDANA's recommended test method No. WSP 230.2 (05) "Moisture Content – ​​Weight Loss Upon Heating".

[0010] The surface post-crosslinking agent solution contains 1.5 wt% to 4.5 wt%, preferably 1.6 wt% to 4.2 wt%, particularly preferably 1.7 wt% to 3.9 wt%, even more particularly preferably 1.8 wt% to 3.6 wt%, and especially preferably 1.9 wt% to 3.3 wt% of water, each based on the superabsorbent particles.

[0011] The post-crosslinking agent solution of the present invention can be sprayed in one step, i.e., sprayed as a mixture containing all components. However, it is also conceivable to divide the post-crosslinking agent solution or its components into at least two sub-solutions (a first post-crosslinking agent solution and a second post-crosslinking agent solution) and spray them sequentially. According to the present invention, the temperature of the superabsorbent particles when spraying the post-crosslinking agent solution refers to the temperature of the superabsorbent particles when spraying the first post-crosslinking agent solution. According to the present invention, the moisture content of the superabsorbent particles when spraying the post-crosslinking agent solution refers to the moisture content of the superabsorbent particles when spraying the first post-crosslinking agent solution. According to the present invention, the moisture content of the post-crosslinking agent solution refers to the overall moisture content of the post-crosslinking agent solution, i.e., the sum of the moisture contents of all sub-solutions (the moisture contents of the first and second post-crosslinking agent solutions).

[0012] Contact dryers suitable for the continuous method of the present invention are, for example, paddle dryers and disc dryers. In a contact dryer, the material to be dried is conveyed and tumbled along a heated surface by a power tool. Contact dryers can also be used for cooling.

[0013] This invention is based on the understanding that the temperature and moisture content of superabsorbent particles, as well as the amount of water used, have a decisive influence on post-crosslinking of the surface. The method of this invention enables the preparation of superabsorbents with high absorption capacity (e.g., high centrifuge retention capacity (CRC)) and high permeability (e.g., high gel bed permeability (GBP)).

[0014] The average droplet diameter when the crosslinking agent solution is sprayed onto the surface can be from 100 µm to 4,500 µm, preferably from 150 µm to 3,500 µm, particularly preferably from 200 µm to 2,500 µm, even more particularly preferably from 250 µm to 1,500 µm, specifically preferably from 300 µm to 1250 µm, and especially preferably from 350 µm to 1000 µm. The average droplet diameter can be determined by light scattering. The average droplet diameter is also known as the average volume diameter (MVD) or D. V0.5 Or average mass diameter (MMD).

[0015] In the case of nozzles, droplet size is directly related to flow rate; that is, droplet size decreases as flow rate increases. Similarly, droplet size decreases when nozzle inlet pressure increases or spray angle increases.

[0016] The spraying of the post-crosslinking agent solution is not subject to any other restrictions. Hydraulic nozzles and dual-fluid nozzles can be used, for example.

[0017] Based on the superabsorbent particles, the surface post-crosslinking agent solution contains preferably 0.001 wt% to 2 wt%, particularly preferably 0.01 wt% to 1 wt%, and even more particularly preferably 0.03 wt% to 0.7 wt% of a surface post-crosslinking agent. Preferably, the surface post-crosslinking agent forms covalent bonds on the surface of the superabsorbent particles.

[0018] The superabsorbent particles are heated in the contact dryer 1 to a temperature preferably from 110°C to 220°C, particularly preferably from 120°C to 210°C, and even more preferably from 130°C to 200°C. The residence time of the superabsorbent particles in the contact dryer 1 is preferably from 10 minutes to 60 minutes, particularly preferably from 15 minutes to 55 minutes, and even more preferably from 20 minutes to 50 minutes.

[0019] Heat tracing and insulation can be applied to the contact dryer 1 and the connection with the contact dryer 2.

[0020] The superabsorbent particles are cooled in the contact dryer 2 to a temperature preferably from 30°C to 80°C, particularly preferably from 35°C to 70°C, and even more preferably from 40°C to 60°C. The residence time of the superabsorbent particles in the contact dryer 2 is preferably from 5 minutes to 50 minutes, particularly preferably from 10 minutes to 40 minutes, and even more preferably from 15 minutes to 30 minutes.

[0021] Gases with an oxygen content of less than 10 vol% can pass through the contact dryer 1.

[0022] The total gas flow rate is, for example, from 5 Nm³ / h to 60 Nm³ / h, preferably from 10 Nm³ / h to 50 Nm³ / h, particularly preferably from 15 Nm³ / h to 40 Nm³ / h, and even more particularly preferably from 20 Nm³ / h to 30 Nm³ / h, all per m³ of the internal volume of the contact dryer 1. 1 Nm³ corresponds to a gas volume of 1 m³ under the conditions of 273.15 K and 1,013.25 hPa.

[0023] Preferably, the waste gas flow is drawn from the contact dryer 1. In the contact dryer 1, the waste gas flow is deflected upwards by at least 75° from the horizontal product flow direction. Immediately after the deflection, the velocity of the waste gas flow is preferably less than 5 m / s, particularly preferably less than 2 m / s, and even more particularly preferably less than 1 m / s.

[0024] The preparation of superabsorbents will be described in detail below:

[0025] Superabsorbents are prepared by polymerizing monomer solutions and are typically insoluble in water.

[0026] The olefinic unsaturated monomer carrying the acid group preferably has water solubility, that is, its solubility in water at 23°C is 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, and even more particularly preferably at least 35 g / 100 g water.

[0027] Suitable monomers include, for example, olefinic unsaturated carboxylic acids such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is even more particularly preferred.

[0028] Unsaturated olefinic monomers carrying acid groups are typically partially neutralized. Neutralization is carried out at the monomer stage. This is usually achieved by incorporating a neutralizing agent in aqueous or preferably solid form. The degree of neutralization is preferably 40 mol% to 85 mol%, particularly preferably 50 mol% to 80 mol%, and even more preferably 60 mol% to 75 mol%, wherein commonly used neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates or alkali metal bicarbonates and mixtures thereof. Ammonium salts, rather than alkali metal salts, can also be used. Particularly preferred alkali metals are sodium and potassium, but even more particularly preferred are sodium hydroxide, sodium carbonate or sodium bicarbonate and mixtures thereof, especially sodium hydroxide.

[0029] Monomers typically contain polymerization inhibitors, and preferably contain hydroquinone semiethers, which act as storage stabilizers.

[0030] Suitable crosslinking agents are compounds having at least two suitable crosslinking groups. Such groups include, for example, olefinic unsaturated groups capable of free radical polymerization into the polymer chain, and functional groups capable of forming covalent bonds with the acid groups of the monomer. Furthermore, polyvalent metal salts capable of forming coordination bonds with at least two acid groups of the monomer are also suitable as crosslinking agents.

[0031] Suitable crosslinking agents include, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyl ammonium chloride, and tetraallylpropoxyethane as described in EP 0 530 438 A1, such as diacrylates and triacrylates 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 / 104301A1 and DE 103 31 450 A1, such as DE 103 31 456 A1 and DE103 55 401. Mixed acrylates containing olefinic unsaturated groups in addition to acrylate groups as described in A1, or mixtures of crosslinking agents as described, for example, in DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO 02 / 032962 A2.

[0032] Based on the total amount of monomers used, the amount of crosslinking agent is preferably from 0.05 wt% to 1.5 wt%, particularly preferably from 0.1 wt% to 1 wt%, and even more particularly preferably from 0.15 wt% to 0.6 wt%. As the crosslinking agent content increases, the centrifugal retention capacity (CRC) decreases, and the absorption rate (AUL) at 21.0 g / cm² pressure reaches its maximum value.

[0033] All compounds that generate free radicals under polymerization conditions can be used as initiators, such as thermal initiators, redox initiators, and photoinitiators. Suitable redox initiators include sodium persulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium persulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. A mixture of a thermal initiator and a redox initiator, such as sodium persulfate / hydrogen peroxide / ascorbic acid, is preferred. Preferably, the disodium salt of 2-hydroxy-2-sulfinacetic acid, or a mixture of the sodium salt of 2-hydroxy-2-sulfinacetic acid, the disodium salt of 2-hydroxy-2-sulfinacetic acid, and sodium bisulfite, is used as the reducing component. Commercially available examples of such mixtures include Brüggolite. ® FF6 and Brüggolite ® FF7 (Brüggemann Chemicals; Heilbronn; Germany).

[0034] The water content of the monomer solution is preferably from 40 wt% to 75 wt%, particularly preferably from 45 wt% to 70 wt%, and even more preferably from 50 wt% to 65 wt%. As the water content increases, the energy consumption in the subsequent drying process increases, and if the water content decreases, the heat of polymerization may not be fully dissipated.

[0035] The temperature of the monomer solution is preferably from 10°C to 90°C, particularly preferably from 20°C to 70°C, and even more particularly preferably from 30°C to 50°C.

[0036] To achieve optimal results, the preferred polymerization inhibitor requires dissolved oxygen. Therefore, dissolved oxygen in the monomer solution can be removed before polymerization by inerting, i.e., passing an inert gas, preferably nitrogen or carbon dioxide, through it. Preferably, the oxygen content of the monomer solution is reduced to less than 1 wppm before polymerization, particularly preferably to less than 0.5 wppm, and even more preferably to less than 0.1 wppm.

[0037] Suitable reactors for polymerization include, for example, kneading reactors or belt reactors. In a kneader, the polymer gel produced during the polymerization of an aqueous monomer solution or suspension is continuously pulverized by, for example, a counter-clockwise rotating stirring shaft, as described in WO 2001 / 038402 A1. Polymerization in a belt reactor has been described, for example, in DE 38 25 366 A1 and US 6,241,928. Polymerization in a belt reactor produces a polymer gel that must be pulverized, for example, in an extruder or kneader.

[0038] To improve drying performance, the pulverized polymer gel obtained by kneading can be additionally extruded.

[0039] The solid content of the polymer gel before drying is preferably 25 wt% to 90 wt%, particularly preferably 35 wt% to 70 wt%, and even more particularly preferably 40 wt% to 60 wt%.

[0040] The polymer gel is then dried using a circulating air belt dryer until the remaining moisture content is preferably 0.5 wt% to 10 wt%, particularly preferably 1 wt% to 7 wt%, and even more particularly preferably 1.5 wt% to 6 wt%, wherein the remaining moisture content is determined according to EDANA's recommended test method No. WSP 230.2-05 "Moisture Content – ​​Weight Loss Upon Heating". The dried polymer gel is then broken up, and optionally coarsely pulverized.

[0041] The dried polymer gel is then typically ground and sorted, which can usually be done using a single or multi-stage roller mill (preferably a two- or three-stage roller mill), pin mill, hammer mill, or vibratory mill.

[0042] The average particle size of the superabsorbent particles separated as product fractions is preferably at least 150 µm to 850 µm, particularly preferably 250 µm to 600 µm, and even more particularly preferably 300 µm to 500 µm. The average particle size of the product fraction can be determined using EDANA's recommended test method No. WSP 220.2 (05) "Partikel Size Distribution," in which the mass proportions of the sieves are cumulatively plotted, and the average particle size is determined graphically. Here, the average particle size is the mesh width value corresponding to a cumulative 50 wt%.

[0043] To further improve properties, the superabsorbent particles are thermally surface-crosslinked. Suitable surface-crosslinking agents are compounds containing groups capable of forming covalent bonds with at least two carboxyl groups of the superabsorbent particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional aminoamines, and 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 β-hydroxyalkylamides as described in DE 102 04 938 A1 and US 6,239,230.

[0044] In a preferred embodiment of the invention, in addition to the surface crosslinking agent, polyvalent cations are also applied to the particle surface.

[0045] The polyvalent cations applicable to the methods of this invention are, for example, divalent cations such as those of zinc, magnesium, calcium, and strontium; trivalent cations such as those of aluminum, iron, chromium, rare earth elements, and manganese; and tetravalent cations such as those of titanium and zirconium. Possible counterions include chlorides, bromides, hydroxides, sulfates, bisulfates, carbonates, bicarbonates, nitrates, phosphates, hydrogen phosphates, dihydrogen phosphates, and carboxylates such as acetates and lactates. Aluminum hydroxide, aluminum sulfate, and aluminum lactate are preferred.

[0046] Based on the polymer, the amount of polyvalent cation used is, for example, 0.001 wt% to 1.5 wt%, preferably 0.005 wt% to 1 wt%, and particularly preferably 0.02 wt% to 0.8 wt%.

[0047] Post-crosslinking of the surface is achieved by spraying a solution of the post-crosslinking agent onto dried superabsorbent particles. After spraying, the superabsorbent particles coated with the post-crosslinking agent are subjected to heat treatment.

[0048] The surface crosslinking agent solution is preferably sprayed in a mixer with a moving mixing tool, such as a spiral mixer, disc mixer, or paddle mixer. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are even more preferred. The difference between horizontal and vertical mixers lies in the support method of the mixing shaft; that is, horizontal mixers have a horizontally supported mixing shaft, while vertical mixers have a vertically supported mixing shaft. Suitable mixers include, for example, Horizontal Pflugschar. ® Mischer (Gebr. Lödige Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta Continuous Mixer (Hosokawa Micron BV; Duttingheim; Netherlands), Processall Mixmill Mixer (Processall Incorporated; Cincinnati; USA), and SchugiFlexomix ® (Hosokawa Micron BV; Duttingheim; Netherlands). However, a surface crosslinking agent solution can also be sprayed in a fluidized bed.

[0049] Post-crosslinking agents are typically used as aqueous solutions. The penetration depth of the post-crosslinking agent into the superabsorbent particles can be determined by the content of non-aqueous solvent or the total solvent volume.

[0050] The surface thermal post-crosslinking is carried out in a contact dryer, particularly preferably in a paddle dryer, and even more preferably in a disc dryer. Suitable dryers include, for example, the Hosokawa Bepex. ® Horizontal PaddleDryer (Hosokawa Micron GmbH; Rheingarten; Germany), Hosokawa Bepex ® Disc Dryer (Hosokawa Micron GmbH; Rhinegarten; Germany), Holo-Flite ® dryers (Metso MineralsIndustries Inc.; Danville; USA) and Nara Paddle Dryer (NARA Machinery Europe; Freising; Germany).

[0051] The superabsorbent particles that have undergone surface cross-linking can then be reclassified, wherein superabsorbent particles that are too small and / or too large are separated and returned to the method.

[0052] To further improve the properties, the superabsorbent particles that have undergone surface cross-linking can be coated or re-wetted.

[0053] Rewetting is preferably performed at a temperature of 30°C to 80°C, particularly preferably at 35°C to 70°C, and even more preferably at 40°C to 60°C. At excessively low temperatures, the superabsorbent particles tend to aggregate, while at higher temperatures, water evaporates significantly. The amount of water used for rewetting is preferably 1 wt% to 10 wt%, particularly preferably 2 wt% to 8 wt%, and even more preferably 3 wt% to 5 wt%. 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.

[0054] 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).

[0055] method :

[0056] Unless otherwise specified, all measurements were performed at an ambient temperature of 23±2℃ and a relative humidity of 50±10%. The superabsorbent particles were thoroughly mixed before measurement.

[0057] Moisture content

[0058] The moisture content of the superabsorbent granules was determined according to EDANA’s recommended test method No. WSP 230.2(05) “Moisture Content – ​​Weight Loss Upon Heating”.

[0059] Centrifuge retention capacity

[0060] The centrifugation retention capacity (CRC) was determined according to the test method recommended by EDANA No. WSP 241.2 (05) "Gravimetric Determination of Fluid Retention Capacity in Saline Solution After Centrifugation".

[0061] Absorption rate under load at a pressure of 21.0 g / cm²

[0062] The absorption rate at 21.0 g / cm² pressure (AUL 0.3 psi) was determined according to the test method recommended by EDANA.

[0063] WSP 242.2 (05) "Gravimetric Determination of Absorption

[0064] "Under Pressure" measurement.

[0065] Extractable polymers

[0066] The content of extractable polymers (16-hour extractables) was determined according to the test method recommended by EDANA.

[0067] WSP 270.2 (05) "Determination of Extractable Polymer Content by Potentiometric Titration" determination.

[0068] Gel bed permeability

[0069] As described in US 2005 / 0256757 (paragraphs

[0061] and

[0075] ), the gel bed permeability (GBP) of the swollen gel layer under a pressure load of 0.3 psi (2070 Pa) was determined, which is the gel bed permeability of the swollen gel layer formed as water-absorbing superabsorbent particles. According to US 2005 / 0256757, the weight of the superabsorbent particles has been adjusted from 0.9 g to 2.0 g. Example

[0070] Preparation of superabsorbent particles (SP)

[0071] The monomer solution was prepared by continuously mixing deionized water, 50 wt% sodium hydroxide solution, and acrylic acid, resulting in a neutralization degree of 71.0 mol%. The water content of the monomer solution was 60.5 wt%.

[0072] 3-ethoxylated glycerol triacrylate (approximately 85 wt%) was used as a crosslinking agent. The dosage was 1.42 kg per ton of monomer solution.

[0073] In addition, polyethylene glycol with an average molecular weight of 4,000 g / mol is added. The dosage is 1.75 kg per ton of monomer solution.

[0074] To initiate free radical polymerization, 0.91 kg of 0.25 wt% aqueous hydrogen peroxide solution, 4.30 kg of 15 wt% aqueous sodium persulfate solution, and 0.84 kg of 1 wt% aqueous ascorbic acid solution were added to each ton of monomer solution.

[0075] The monomer solution was fed into a List Contikneter reactor (LISTAG, Ariesdorf, Switzerland) with a volume of 6.3 m³. The throughput of the monomer solution was approximately 20 t / h. The temperature of the reaction solution at the inlet was 30 °C.

[0076] Between the points where the crosslinking agent is added and where the hydrogen peroxide and sodium persulfate solution are added, the monomer solution is inerted using nitrogen gas. Ascorbic acid is then added directly to the reactor.

[0077] After approximately 50% of the residence time, approximately 1,000 kg / h of superabsorbent particles with a particle size less than 180 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.

[0078] To facilitate drying, the resulting polymer gel is extruded.

[0079] The extruded polymer gel thus obtained is conveyed onto the conveyor belt of a circulating air belt dryer via a vibrating conveyor belt. This circulating air belt dryer has a length of 48 m. The conveyor belt of the circulating air belt dryer has an effective width of 4.4 m. The aqueous polymer gel is dried continuously in the circulating air belt dryer using an air / gas mixture.

[0080] In each case, the temperature of the air / gas mixture and the residence time in the circulating air belt dryer for drying the polymer gel were set such that the resulting superabsorbent particles had a moisture content of 0.9 wt%, 1.7 wt%, 4.1 wt%, and 12.4 wt%.

[0081] The dried polymer gel was pulverized using a three-stage roller mill and sieved to a particle size of 180µm to 700µm. Superabsorbent particles smaller than 180µm were separated. The superabsorbent particles larger than 700µm were then returned for further pulverization.

[0082] Superabsorbent particles with particle sizes ranging from 180µm to 700µm were subjected to surface thermal post-crosslinking.

[0083] Table 1 summarizes the characteristics of the superabsorbent particles used in each case prior to surface post-crosslinking.

[0084] Table 1 :

[0085]

[0086] Surface thermal crosslinking

[0087] Place 190g of super absorbent granules into an MP-LB model mixing container (Somakon Verfahrenstechnik UG, Lünen, Germany), and control the temperature at 30℃, 50℃, 70℃, 80℃ and 90℃.

[0088] Then, a first surface crosslinking agent solution is first sprayed using a dual-fluid nozzle at a rotation speed of 500 Upm. This solution contains 1.2 wt% isopropanol (IPA), 0.5 wt% ethylene carbonate (EC), 0.14 wt% N-(2-hydroxyethyl)-2-oxazolidinone:1,3-propanediol (50:50 mixture; HEONON) or 0.05 wt% ethylene glycol diglycidyl ether (EGDGE), and 0 wt%, 0.9 wt%, 2.0 wt%, and 5.1 wt% water, each based on superabsorbent particles.

[0089] Then, a second surface crosslinking agent solution in the form of an aqueous aluminum sulfate solution is sprayed on. This solution contains 0.37 wt% aluminum sulfate and 1.0 wt% water, each also based on superabsorbent particles. Mix for another 5 minutes.

[0090] Table 2 summarizes the conditions for post-crosslinking of the surface.

[0091] Table 2 :

[0092]

[0093] *) Comparative examples

[0094] **) Water = All water from the crosslinking agent solution of the first and second surfaces

[0095]

[0096] After applying the surface post-crosslinking agent solution, superabsorbent particles were uniformly distributed on a drying plate (40cm × 25cm) and subjected to surface thermal post-crosslinking treatment at 175℃ in a circulating air drying oven. Samples were taken at 20min, 30min, 40min, 50min, 60min, and 70min, and aggregates larger than 850μm were separated by sieving. The remaining surface post-crosslinked superabsorbent particles were analyzed. The results are summarized in Table 3.

[0097] Table 3 :

[0098]

[0099]

[0100] Table 3 (continued) :

[0101]

[0102]

[0103] *) Comparative examples

[0104] exist Figures 1 to 3 In the middle, the measurement results are plotted graphically.

[0105] Figure 1 The dependence of CRC and GBP on superabsorbent particles on different temperatures immediately preceding surface thermal crosslinking is shown. The study found that the CRC / GBP ratio was worst at 30°C and 90°C. The optimal temperature lies between these two temperatures.

[0106] Figure 2The dependence of CRC and GBP of the superabsorbent particles on their moisture content immediately preceding surface thermal crosslinking is shown. The optimal moisture content can be observed to be between 1.5 wt% and 6.0 wt%.

[0107] Figure 3 The dependence of CRC and GBP on different amounts of water in the post-crosslinking agent solution during surface thermal crosslinking is shown. It can also be seen that the optimal values ​​are between 1.5 wt% and 4.5 wt% of water.

Claims

1. A method for continuous surface thermal post-crosslinking of a superabsorbent, wherein superabsorbent particles are coated by spraying a surface post-crosslinking agent solution, the coated superabsorbent particles are subjected to thermal post-treatment in a contact dryer 1, and the thermally post-treated superabsorbent particles are cooled in a contact dryer 2, characterized in that... The superabsorbent particles have a temperature of 40°C to 85°C and a moisture content of 1.5 wt% to 6.0 wt% immediately before the surface post-crosslinking agent solution is sprayed, and the surface post-crosslinking agent solution contains 1.5 wt% to 4.5 wt% water based on the superabsorbent particles.

2. The method according to claim 1, characterized in that, The superabsorbent particles are at a temperature of 55°C to 75°C immediately after being sprayed onto the surface and before the crosslinking agent solution.

3. The method according to claim 1 or 2, characterized in that, The superabsorbent particles have a moisture content of 3.0 wt% to 4.5 wt% immediately after being sprayed onto the surface and before the crosslinking agent solution.

4. The method according to any one of claims 1 to 3, characterized in that, Based on the superabsorbent particles, the surface post-crosslinking agent solution contains 1.9 wt% to 3.3 wt% water.

5. The method according to any one of claims 1 to 4, characterized in that, Partially neutralized, cross-linked polyacrylic acid is used as a superabsorbent.

6. The method according to any one of claims 1 to 5, characterized in that, The surface post-crosslinking agent forms covalent bonds on the surface of the superabsorbent.

7. The method according to any one of claims 1 to 6, characterized in that, The average droplet diameter during the application of the crosslinking agent solution to the surface is between 250 µm and 1500 µm.

8. The method according to any one of claims 1 to 7, characterized in that, Based on the superabsorbent particles, the surface post-crosslinking agent solution contains 0.001 wt% to 2 wt% of surface post-crosslinking agent.

9. The method according to any one of claims 1 to 8, characterized in that, The superabsorbent particles are heated to a temperature of 110°C to 220°C in the contact dryer 1.

10. The method according to any one of claims 1 to 9, characterized in that, The residence time of the superabsorbent particles in the contact dryer 1 is 10 to 60 minutes.

11. The method according to any one of claims 1 to 10, characterized in that, The contact dryer 1 and its connection to the contact dryer 2, including the hopper and metering device, are all heat-traced and insulated.

12. The method according to any one of claims 1 to 11, characterized in that, The superabsorbent particles are cooled to a temperature of 30°C to 80°C in the contact dryer 2.

13. The method according to any one of claims 1 to 12, characterized in that, The residence time of the superabsorbent particles in the contact dryer 2 is 5 to 50 minutes.

14. The method according to any one of claims 1 to 13, characterized in that, A gas stream with an oxygen content of less than 10% by volume is passed through the contact dryer 1.

15. The method according to claim 14, characterized in that, The total gas flow rate is 10 Nm³ / h to 50 Nm³ / h per cubic meter of the internal volume of the contact dryer 1, and / or waste gas flow is drawn out from the contact dryer 1, in which the waste gas flow is deflected upward from the horizontal product flow direction by at least 75°, and the gas velocity of the waste gas flow is less than 5 m / s immediately after the deflection.

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