Method for producing superabsorbent polymer particles with long-term color stability

Abstract

The present invention relates to a method for producing long-term color-stable superabsorbent polymer particles, comprising the steps of polymerizing a monomer solution, drying the resulting polymer gel, optionally grinding and classifying the resulting dried polymer gel, and thermally surface postcrosslinking the resulting polymer particles and cooling, wherein a thermal surface postcrosslinker and hydrogen peroxide are added to the polymer particles prior to the thermal surface postcrosslinking.

Description

[Technical Field]

[0001] The present invention relates to a method for producing superabsorbent polymer particles with long-term color stability, comprising the steps of polymerizing a monomer solution, drying the resulting polymer gel, and optionally grinding the resulting dried polymer gel. Classification The present invention relates to a method comprising the steps of: preparing the polymer particles, and thermally post-crosslinking the obtained polymer particles and cooling them, wherein a thermal surface post-crosslinker and hydrogen peroxide are added to the polymer particles before thermal post-crosslinking. [Background technology]

[0002] Superabsorbent polymer particles are also used to manufacture diapers, tampons, sanitary napkins, and other hygiene products, as well as water-retaining agents in market horticulture. Superabsorbent polymer particles are also often called "absorbent resins," "superabsorbent materials," "superabsorbent polymers," "absorbent polymers," "absorbent gelling materials," "hydrophilic polymers," or "hydrogels."

[0003] The production of superabsorbent polymer particles is described in the monograph "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103.

[0004] The properties of superabsorbent polymer particles can be adjusted, for example, by the amount of crosslinking agent used. As the amount of crosslinking agent increases, the centrifugal retention capacity (CRC) decreases, reaching 21.0 g / cm³. 2 The absorption under pressure (AUL) exceeds the maximum value.

[0005] Application characteristics, for example, the permeability of the swollen gel base (SFC) in diapers, and 49.2 g / cm³ 2To improve absorbency under pressure (AUHL), superabsorbent polymer particles are generally post-crosslinked on the surface. This increases the degree of crosslinking on the particle surface, resulting in a absorption rate of 49.2 g / cm³. 2 The pressure absorption and centrifugal retention capacity (CRC) of (AUHL) can be at least partially separated. This post-surface crosslinking can be carried out in an aqueous gel phase. However, preferably, dried, pulverized, and sieved polymer particles (base polymer) are surface coated with a post-surface crosslinking agent, thermally post-crosslinked, and dried. A crosslinking agent suitable for this purpose is a compound that can form covalent bonds with at least two carboxylate groups of the superabsorbent polymer particles.

[0006] A common problem with superabsorbent polymer particles is discoloration, which occurs during storage in high temperatures or high humidity. These conditions are frequently encountered in tropical or subtropical countries. Under these conditions, superabsorbent polymer particles tend to yellow, and may even turn brown or nearly black. This discoloration of colorless superabsorbent polymer particles is unsightly and undesirable, especially in thin sanitary products where discoloration is desired, and consumers will reject such products. While the exact cause of the discoloration is not fully understood, it appears that reactive compounds, such as monomers remaining after polymerization, the use of certain initiators, impurities in the monomers or neutralizers, post-crosslinking agents, or stabilizers in the monomers used, may be contributing factors.

[0007] EP1770113A1 discloses post-surface crosslinking in the presence of peroxides to reduce residual monomers.

[0008] WO2006 / 062258A2 discloses a method for post-crosslinking of a surface by adding a water-soluble polymerization initiator and irradiating the mixture with active energy rays. This method does not require heating.

[0009] WO2010 / 096595A2 discloses a superabsorbent polymer having antimicrobial properties. The superabsorbent polymer is coated with 0.5 to 20% by weight of hydrogen peroxide and dried.

[0010] EP2915548A1 discloses a method for producing superabsorbent particles, including the addition of peroxides after polymerization. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] EP1770113A1 [Patent Document 2] WO2006 / 062258A2 [Patent Document 3] WO2010 / 096595A2 [Patent Document 4] EP2915548A1 [Non-patent literature]

[0012] [Non-Patent Document 1] "Modern Superabsorbent Polymer Technology", FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103 [Overview of the project]

[0013] The object of the present invention was to provide a method for producing superabsorbent polymer particles having improved long-term color stability. [Means for solving the problem]

[0014] The objective is a method for producing superabsorbent polymer particles with long-term color stability, a) Partially neutralized acrylic acid, b) Depending on the case, at least one crosslinking agent, and c) at least one initiator polymerizing a monomer solution containing the same, drying the resulting polymer gel, optionally grinding and Classification performing steps such as, and thermally post-crosslinking and cooling the resulting polymer particles, and a thermal surface post-crosslinking agent and 0.014 to 0.095% by weight of hydrogen peroxide based on the polymer particles are added to the polymer particles before the thermal surface post-crosslinking, which is achieved by a method.

[0015] The thermal surface post-crosslinking agent is preferably added to the polymer particles before the thermal surface post-crosslinking in an amount of 0.005 to 5% by weight, more preferably 0.010 to 2% by weight, and most preferably 0.015 to 1% by weight based on the polymer particles.

[0016] Suitable surface post-crosslinking agents are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, bifunctional or polyfunctional alcohols, β-hydroxyalkylamides, cyclic carbonates, 2-oxazolinone and its derivatives, bis- and poly-2-oxazolinone, 2-oxotetrahydro-1,3-oxazine and its derivatives, cyclic ureas, bicyclic amidoacetals, oxetanes, cyclic ureas, and morpholine-2,3-dione and its derivatives.

[0017] Hydrogen peroxide is preferably added to the polymer particles before the thermal surface post-crosslinking in an amount of 0.016 to 0.080% by weight, more preferably 0.018 to 0.065% by weight, and most preferably 0.020 to 0.050% by weight based on the polymer particles.

[0018] The temperature of the polymer particles during the thermal surface post-crosslinking is preferably 110 to 220 °C, more preferably 120 to 200 °C, and most preferably 130 to 190 °C. A lower temperature may be possible during the temperature rise of the polymer particles.

[0019] The residence time of polymer particles during thermal post-crosslinking is preferably 10 to 120 minutes, more preferably 15 to 90 minutes, and most preferably 20 to 60 minutes. This residence time is the residence time of polymer particles in the apparatus used for thermal post-crosslinking.

[0020] In preferred embodiments of the present invention, the monomer solution contains a polymerization inhibitor in an amount of preferably 0.001 to 0.020% by weight, more preferably 0.002 to 0.015% by weight, and most preferably 0.004 to 0.010% by weight, based on the acrylic acid before neutralization.

[0021] Suitable polymerization inhibitors include, for example, hydroquinone monomethyl ether (MEHQ) and alpha-tocopherol (vitamin E). Hydroquinone monomethyl ether (MEHQ) is preferred.

[0022] In preferred embodiments of the present invention, the chelating agent is added to the polymer particles during or after cooling, preferably in an amount of 0.005 to 1.0% by weight, more preferably 0.010 to 0.7% by weight, and most preferably 0.015 to 0.5% by weight, based on the polymer particles.

[0023] Suitable chelating agents include, for example, citric acid, tartaric acid, iminodiacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetic acid, nitrilotripionic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, triethylenetetraaminehexaacetic acid, N,N-bis(2-hydroxyethyl)glycine, trans-1,2-diaminocyclohexanepentaacetic acid, and 1-hydroxyethane-1,1-diphosphonic acid, as well as salts thereof. 1-hydroxyethane-1,1-diphosphonic acid and / or its salts are preferred.

[0024] Surface post-crosslinking is performed by heating. Irradiation with active energy rays is not required.

[0025] This invention is based on the discovery that the long-term color stability of superabsorbent polymers can be increased by treating them with hydrogen peroxide before thermal post-crosslinking. Too much hydrogen peroxide has an adverse effect on the high load absorption capacity (AUHL).

[0026] Adding hydrogen peroxide after thermal crosslinking, or using other peroxides, is relatively ineffective.

[0027] The present invention further provides superabsorbent polymer particles obtained by the method according to the present invention.

[0028] The superabsorbent polymer particles produced by the method according to the present invention preferably have a centrifugal retention capacity (CRC) of at least 40 g / g, more preferably at least 41 g / g, and most preferably at least 42 g / g. The centrifugal retention capacity (CRC) of superabsorbent polymer particles is typically less than 60 g / g.

[0029] The superabsorbent polymer particles produced by the method according to the present invention preferably have an AUHL of at least 20 g / g, more preferably at least 22 g / g, and most preferably at least 24 g / g. The AUHL of the superabsorbent polymer particles is typically less than 50 g / g.

[0030] The superabsorbent polymer particles produced by the method according to the present invention have a yellowness index (YI)(2 / C) of YI D1925, preferably 54 or less, more preferably 52 or less, and most preferably 50 or less, after aging for 14 days at 70°C and 80% relative humidity.

[0031] The manufacturing process for superabsorbent materials is described in detail below.

[0032] Superabsorbent materials are manufactured by polymerizing monomer solutions and are typically insoluble in water.

[0033] A suitable crosslinking agent b) is a compound having at least two groups suitable for crosslinking. Such groups include, for example, ethylenically unsaturated groups that can be polymerized to polymer chains by free radicals, and functional groups that can form covalent bonds with acidic groups of acrylic acid. In addition, polyvalent metal salts that can form coordinate bonds with at least two acidic groups of acrylic acid are also suitable as crosslinking agents b).

[0034] The crosslinking agent b) is preferably a compound having at least two polymerizable groups that can be polymerized into a polymer network by free radicals. Suitable crosslinking agents b) include, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, EP0547847A1, EP0559476A1, EP0632068A1, WO93 / 21237A1, and WO2003 / 104299A. 1. Di- and triacrylates as described in WO2003 / 104300A1, WO2003 / 104301A1, and DE10331450A1, mixed acrylates containing further ethylenically unsaturated groups along with acrylate groups as described in DE10331456A1 and DE10355401A1, or mixtures of crosslinking agents as described, for example, DE19543368A1, DE19646484A1, WO90 / 15830A1, and WO2002 / 032962A2.

[0035] Preferred crosslinking agents b) are pentaerythrityl trialyl ether, tetraallyloxyethane, methylenebismethacrylamide, tuply ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate, and triallyamine.

[0036] A particularly preferred crosslinking agent b) is polyethoxylated and / or propoxylated glycerol, esterified with acrylic acid or methacrylic acid to give a di- or triacrylate, as described, for example, in WO2003 / 104301A1. Di- and / or triacrylates of 3-10 times ethoxylated glycerol are particularly advantageous. Di- or triacrylates of 1-5 times ethoxylated and / or propoxylated glycerol are very particularly preferred. Most preferred are triacrylates of 3-5 times ethoxylated and / or propoxylated glycerol, especially triacrylates of 3 times ethoxylated glycerol.

[0037] The amount of crosslinking agent b) is preferably 0.05 to 1.5% by weight, more preferably 0.1 to 1% by weight, and most preferably 0.3 to 0.6% by weight, based on the amount of acrylic acid before neutralization in each case. As the crosslinking agent content increases, the centrifugal retention capacity (CRC) decreases to 21.0 g / cm³. 2 The pressure absorption capacity passes through the maximum value.

[0038] The initiator c) used may be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Preferred redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. A mixture of a thermal initiator and a redox initiator, such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid, is preferred. However, the reducing component used is preferably disodium 2-hydroxy-2-sulfonatoacetate, or a mixture of disodium 2-hydroxy-2-sulfinatoacetate, disodium 2-hydroxy-2-sulfonatoacetate, and sodium bisulfite. Such mixtures are available as Brueggolite® FF6 and Brueggolite® FF7 (Brueggemann Chemicals (Heilbronn; Germany)).

[0039] Typically, an aqueous monomer solution is used. The water content of the monomer solution is preferably 40-75% by weight, more preferably 45-70% by weight, and most preferably 50-65% by weight. A monomer suspension, i.e., a monomer solution containing excess sodium acrylate, can also be used. As the water content increases, the energy requirement for subsequent drying increases, and as the water content decreases, the heat of polymerization may not be removed sufficiently.

[0040] For optimal action, preferred polymerization inhibitors require dissolved oxygen. Therefore, the monomer solution may have its dissolved oxygen removed before polymerization by inertization, i.e., by passing an inert gas, preferably nitrogen or carbon dioxide, through it. The oxygen content of the monomer solution is preferably reduced to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, and most preferably to less than 0.1 ppm by weight, before polymerization.

[0041] To better control the polymerization reaction, any known chelating agent may be added to the monomer solution or suspension, or to the raw materials thereof. Suitable chelating agents include, for example, phosphoric acid, diphosphate, triphosphate, polyphosphate, citric acid, tartaric acid, or salts thereof.

[0042] Further preferred examples include iminodiacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetic acid, nitrilotripionic acid, ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, triethylenetetraaminehexaacetic acid, N,N-bis(2-hydroxyethyl)glycine, and trans-1,2-diaminocyclohexanetetraacetic acid, as well as salts thereof. The amount used is typically 1 to 30,000 ppm, preferably 10 to 1,000 ppm, preferredly 20 to 600 ppm, more preferably 50 to 400 ppm, and most preferably 100 to 300 ppm, based on monomer a).

[0043] Polymerization of the monomer solution is performed. Suitable reactors are, for example, a kneading reactor or a belt reactor. In a kneading reactor, the polymer gel formed in the polymerization of the aqueous monomer solution or suspension is continuously pulverized by an inverting stirring shaft, for example, as described in WO2001 / 038402A1. Polymerization on a belt is described, for example, in DE3825366A1 and U.S. Patent No. 6,241,928. Polymerization in a belt reactor forms a polymer gel, which must be pulverized in a further process step, for example, in an extruder or kneading reactor.

[0044] To improve drying properties, the pulverized polymer gel obtained by the kneader can be further extruded.

[0045] The acidic groups of the resulting polymer gel are typically partially neutralized. Neutralization is preferably carried out at the monomer stage. This is typically achieved by mixing with a neutralizing agent, either as an aqueous solution or, preferably, as a solid. The degree of neutralization is preferably 50-85 mol%, more preferably 60-80 mol%, and most preferably 65-75 mol%, and conventional neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates, and mixtures thereof, can also be used for neutralization. Ammonium salts can also be used instead of alkali metal salts. Particularly preferred alkali metals are sodium and potassium, but sodium hydroxide, potassium hydroxide, and mixtures thereof are also very particularly preferred.

[0046] The resulting polymer gel is dried. The drying oven is not limited in any way. However, drying of the polymer gel is preferably carried out by a belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, more preferably 1 to 7% by weight, and most preferably 2 to 5% by weight, and the residual moisture content is determined by EDANA recommended test method No. WSP230.2(05) "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel will have a glass transition temperature T that is too low.g It may have further processing, but this is only difficult. If the residual moisture content is too low, the dried polymer gel becomes too brittle, and in the subsequent grinding step, an undesirable large amount of polymer particles (fine powder) with an excessively small particle size is obtained. The solid content of the gel before drying is preferably 25-90% by weight, more preferably 35-70% by weight, and most preferably 40-60% by weight. However, a fluidized bed dryer or a paddle dryer may also be used for drying purposes if necessary.

[0047] Next, the dried polymer gel is crushed and Classification The apparatus used for grinding is typically a single-stage or multi-stage roll mill, preferably a two-stage or three-stage roll mill, a pin mill, a hammer mill, or a vibratory mill.

[0048] The average particle size of the polymer particles extracted as the product fraction is preferably at least 200 μm, more preferably 250-600 μm, and very specifically 300-500 μm. The average particle size of the product fraction can be determined by EDANA recommended test method No. WSP220.2(05) "Particle Size Distribution," but the average particle size was determined by plotting the percentage by mass of the sieved fraction in cumulative form. The average particle size here is the mesh size value that produces a cumulative 50% by weight.

[0049] The proportion of particles having a particle size of at least 150 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0050] Polymer particles with excessively small particle sizes reduce saline flow conductivity (SFC). Therefore, the proportion of excessively small polymer particles ("fine powders") should be reduced.

[0051] Therefore, excessively small polymer particles are typically removed and reused in the process. This is preferably done before, during, or immediately after polymerization, i.e., before the polymer gel dries. Excessively small polymer particles can be moistened with water and / or an aqueous surfactant before or during reuse.

[0052] In a later process step, for example, after a post-crosslinking surface or another coating step, the excessively small polymer particles may be removed. In this case, the reused excessively small polymer particles are post-crosslinked or coated by another method, for example, fumed silica or precipitated silica.

[0053] When using a kneading reactor for polymerization, excessively small polymer particles are preferably added to the last third of the polymerization.

[0054] If excessively small polymer particles are added to the monomer solution at a very early stage, for example, the centrifugal retention capacity (CRC) of the resulting superabsorbent decreases. However, this can be compensated for, for example, by adjusting the amount of crosslinking agent b) used.

[0055] If excessively small polymer particles are added at a very late stage, for example, not until they reach an apparatus connected downstream of the polymerization reactor, such as an extruder, then while the excessively small polymer particles can be incorporated into the resulting polymer gel, it is only difficult. However, insufficiently incorporated excessively small polymer particles will detach again from the dried polymer gel during grinding, and therefore, Classification This process increases the amount of excessively small polymer particles that are extracted and reused.

[0056] The proportion of particles having a maximum particle size of 850 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0057] The proportion of particles having a maximum particle size of 600 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0058] Excessively large polymer particles reduce the free swelling rate (FSR). Therefore, the proportion of excessively large polymer particles should also be kept low.

[0059] Therefore, excessively large polymer particles are typically removed and reused for grinding the dried polymer gel.

[0060] However, the aqueous monomer solution can also be formed into droplets, and the resulting droplets can be polymerized in a heated carrier gas stream. Here, the polymerization and drying process steps can be combined, as described in WO2008 / 052971A1 and WO2011 / 026876A1.

[0061] For this purpose, the monomer solution is metered and supplied to the reaction chamber through at least one pore for forming droplets. The pore may be, for example, on a droplet formation plate.

[0062] The droplet formation plate is a plate having at least one hole through which the liquid passes from above. The droplet formation plate or the liquid can be vibrated, thereby generating a chain of monodisperse droplets, ideally in each hole, on the underside of the droplet formation plate. In a preferred embodiment, the droplet formation plate is not shaken.

[0063] The number and size of the pores are selected based on the desired capacity and droplet size. The droplet diameter is typically 1.9 times the pore diameter. Importantly, the liquid to be dropleted should not pass through the pore too quickly, and the pressure loss at both ends of the pore should not be too large. Otherwise, the liquid will not droplet, and rather, due to its high kinetic energy, the liquid jet will break apart (spray). The Reynolds number, based on the throughput per pore and the pore diameter, is preferably less than 2000, more preferably less than 1600, particularly preferably less than 1400, and most preferably less than 1200.

[0064] The droplet plate preferably has at least 5 holes, more preferably at least 25, most preferably at least 50, preferably up to 750, more preferably up to 500, and most preferably up to 250 holes. The diameter of the holes is selected according to the desired droplet size.

[0065] The diameter of the pores is preferably 50 to 500 μm, more preferably 100 to 300 μm, and most preferably 150 to 250 μm.

[0066] The temperature of the monomer solution as it passes through the pores is preferably 5 to 80°C, more preferably 10 to 70°C, and most preferably 30 to 60°C.

[0067] The distance between pores is preferably 10 to 50 mm, more preferably 12 to 40 mm, and most preferably 15 to 30 mm. Excessively short distances will lead to aggregation.

[0068] The carrier gas flows through the polymerization reactor. This carrier gas can be guided through the reaction chamber in parallel or counter-flow, preferably in parallel, i.e., upward from the bottom, relative to the free-falling monomer solution droplets. After one pass, the carrier gas is preferably reused in the reaction chamber as a cycle gas, at least partially, preferably in the range of at least 50%, more preferably in the range of at least 75%. Typically, a portion of the carrier gas, preferably up to 10%, more preferably up to 3%, and most preferably up to 1%, is discharged after each pass.

[0069] The oxygen content of the carrier gas is preferably 0.5 to 15 volume%, more preferably 1 to 10 volume%, and most preferably 2 to 7 by weight.

[0070] In addition to oxygen, the carrier gas preferably contains nitrogen. The nitrogen content of the carrier gas is preferably at least 80% by volume, more preferably at least 90% by volume, and most preferably at least 95% by volume. Further preferred carrier gases are carbon dioxide, argon, xenon, krypton, neon, and helium. Gas mixtures may also be used. The carrier gas may also be loaded with vapor and / or acrylic acid vapor.

[0071] The gas velocity is preferably set so that the flow within the polymerization reactor is guided and, for example, there is no convection opposing the overall flow direction, and is typically 0.1 to 2.5 m / s, preferably 0.3 to 1.5 m / s, more preferably 0.5 to 1.2 m / s, particularly preferably 0.6 to 1.0 m / s, and most preferably 0.7 to 0.9 m / s.

[0072] The carrier gas flowing through the reactor is preheated to the reaction temperature upstream of the reactor.

[0073] Advantageously, the gas inlet temperature is adjusted so that the gas outlet temperature, i.e., the temperature at which the carrier gas exits the reaction chamber, is typically 90–150°C, preferably 100–140°C, more preferably 105–135°C, particularly preferably 110–130°C, and most preferably 115–125°C.

[0074] The reaction can be carried out under high pressure or low pressure, with a reduction of up to 100 mbar relative to the ambient pressure being preferred.

[0075] The reaction off-gas, i.e., the gas leaving the reaction chamber, can be cooled, for example, in a heat exchanger. This condenses the water and unconverted monomer a). The reaction off-gas can then be reheated, at least partially, and reused in the reactor as a cycle gas. A portion of the reaction off-gas can be discharged and replaced with a new carrier gas, in which case the water and unconverted monomer a) present in the reaction off-gas can be removed and reused.

[0076] A particularly preferred option is an integrated heating system, which means that some of the waste heat from cooling the off-gas is used to heat the cycle gas.

[0077] The reactor may be trace-heated. The trace heating is adjusted so that the wall temperature is at least 5°C above the internal reactor temperature, ensuring that condensation on the reactor walls is prevented.

[0078] The reaction product is then thermally post-treated and, if applicable, dried to the desired moisture content.

[0079] To improve the properties, the polymer particles (base polymer) are subsequently thermally post-crosslinked. Suitable post-crosslinking agents are compounds containing groups that can form covalent bonds with at least two acid groups of the polymer particles. Suitable compounds include, for example, polyfunctional amines, polyfunctional amide amines, and polyfunctional epoxides described in EP0083022A2, EP0543303A1, and EP0937736A2, difunctional or polyfunctional alcohols described in DE3314019A1, DE3523617A1, and EP0450922A2, or β-hydroxyalkylamides described in DE10204938A1 and U.S. Patent No. 6,239,230.

[0080] The following are also listed as suitable post-crosslinking agents: cyclic carbonates in DE4020780C1, 2-oxazolidinones and their derivatives in DE19807502A1, such as 2-hydroxyethyl-2-oxazolidinone, bis- and poly-2-oxazolidinones in DE19807992C1, 2-oxotetrahydro-1,3-oxazines and their derivatives in DE19854573A1, N-acyl-2-oxazolidinone in DE19854574A1, cyclic ureas in DE10204937A1, bicyclic amide acetals in DE10334584A1, oxetanes and cyclic ureas in EP1199327A2, and morpholine-2,3-diones and their derivatives in WO2003 / 031482A1.

[0081] Preferred post-crosslinking agents include ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamide and epichlorohydrin, and mixtures of propylene glycol and 1,4-butanediol.

[0082] Particularly preferred post-crosslinking agents are 2-hydroxyethyl oxazolidine-2-one, oxazolidine-2-one, and 1,3-propanediol.

[0083] In addition, a post-crosslinking agent containing additional polymerizable ethylenically unsaturated groups, as described in DE3713601A1, can also be used.

[0084] The amount of post-crosslinking agent is preferably 0.005 to 5% by weight, more preferably 0.01 to 2% by weight, and most preferably 0.015 to 1% by weight, based on the polymer particles in each case.

[0085] In a preferred embodiment of the present invention, the polyvalent cation is applied to the particle surface in addition to the post-crosslinking agent, either before, during, or after post-crosslinking.

[0086] Polyvalent cations that can be used in the method according to the present invention include, for example, divalent cations such as zinc, magnesium, calcium, iron, and strontium cations, trivalent cations such as aluminum, iron, chromium, rare earth elements, and manganese cations, and tetravalent cations such as titanium and zirconium cations. Possible counterions are chloride ions, bromide ions, sulfate ions, hydrogen sulfide ions, carbonate ions, bicarbonate ions, nitrate ions, phosphate ions, hydrogen phosphate ions, dihydrogen phosphate ions, and carboxylate ions such as acetate ions and lactate ions. Aluminum sulfate and aluminum lactate are preferred. In addition to metal salts, polyvalent amines can also be used as polyvalent cations.

[0087] The amount of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight, and more preferably 0.02 to 0.8% by weight, based on the polymer particles.

[0088] Post-surface crosslinking is typically carried out by spraying a solution of the post-surface crosslinking agent onto dry polymer particles. After spraying, the polymer particles coated with the post-surface crosslinking agent are dried by heat, but the post-surface crosslinking reaction can occur either before or during drying.

[0089] The spray application of the post-crosslinking agent solution is preferably carried out using a mixer with a movable mixing device, such as a screw mixer, a disc mixer, and a paddle mixer. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly preferred. The distinction between horizontal and vertical mixers is made by the position of the mixing shaft; that is, horizontal mixers have a horizontally mounted mixing shaft, and vertical mixers have a vertically mounted mixing shaft. Suitable mixers include, for example, the horizontal Pflugschar® mixer (Gebr. Loedige Maschinenbau GmbH; Paderborn; Germany), the Vrieco-Nauta continuous mixer (Hosokawa Micron BV; Dootinghem; Netherlands), the Processall Mixmill mixer (Processall Incorporated; Cincinnati; USA), and the Schugi Flexomix® mixer (Hosokawa Micron BV; Dootinghem; Netherlands). However, it is also possible to spray a post-crosslinking agent solution onto a fluidized bed.

[0090] Post-crosslinking agents are typically used in aqueous solution form. The penetration depth of the post-crosslinking agent into polymer particles can be adjusted by the content and total amount of the non-aqueous solvent.

[0091] When water is used in excess as a solvent, it is advantageous to add a surfactant. This improves the wetting behavior and reduces the tendency to form clumps. However, it is preferable to use solvent mixtures, such as isopropanol / water, 1,3-propanediol / water, and propylene glycol / water, with a preferred mass mixing ratio of 20:80 to 40:60.

[0092] Thermal post-crosslinking of the surface is preferably carried out in a contact dryer, more preferably a paddle dryer, and most preferably a disc dryer. Suitable dryers include, for example, the Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Rheingarten; Germany), the Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Rheingarten; Germany), and the Nara Paddle Dryer (NARA Machinery Europe; Frächen; Germany). Furthermore, a fluidized bed dryer may also be used.

[0093] Thermal post-crosslinking of the surface can be performed within the mixer itself by heating the jacket or blowing in warm air. Downflow dryers, such as rack dryers, rotary tubular furnaces, or heatable screws are equally suitable. Mixing and drying in a fluidized bed dryer is particularly advantageous.

[0094] The post-surface crosslinking temperature is preferably in the range of 110 to 220°C, more preferably 120 to 200°C, and most preferably 130 to 190°C. The residence time of polymer particles during thermal post-surface crosslinking is preferably 10 to 120 minutes, more preferably 15 to 90 minutes, and most preferably 20 to 60 minutes.

[0095] Next, the surface-crosslinked polymer particles are again Classification This allows for the removal of excessively small and / or excessively large polymer particles, which are then reused in the process.

[0096] To further improve the properties, the surface-crosslinked polymer particles can be coated or re-humidified.

[0097] Re-humidification is preferably carried out at 30-80°C, more preferably at 35-70°C, and most preferably at 40-60°C. At excessively low temperatures, the superabsorbent material tends to form clumps, and at high temperatures, the water has already visibly evaporated. The amount of water used for re-humidification is preferably 1-15% by weight, more preferably 2-10% by weight, and most preferably 3-5% by weight, based on the polymer particles. Re-humidification increases the mechanical stability of the polymer particles and reduces their tendency to become electrostatically charged.

[0098] Suitable coatings for improving free swelling rate and saline flow induction (SFC) are, 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 are, for example, polyols. Suitable coatings for suppressing the undesirable caking tendency of polymer particles are, for example, fumed silica such as Aerosil® 200, or precipitated silica such as Sipernat® D17, and surfactants such as Span® 20.

[0099] method Unless otherwise specified, measurements shall be performed at an ambient temperature of 23±2°C and relative atmospheric humidity of 50±10%. The superabsorbent polymer shall be thoroughly mixed before measurement.

[0100] Residual monomers (RAA) The residual monomers in superabsorbent polymer particles are determined by EDANA recommended test method No. WSP210.2(04) "Determination of the Amount of Residual Monomers in Superabsorbent Materials".

[0101] Particle size distribution The particle size distribution of superabsorbent polymer particles is determined by EDANA recommended test method No. WSP220.2(05) "Determination of Polyacrylate Superabsorbent Powders and Particle Size Distribution - Sieve Fractionation".

[0102] Roundness Roundness is determined by the PartAn® 3001 L Particle Analysator (Microtrac Europe GmbH; Meerbusch; Germany).

[0103]

number

[0104] For measurement, superabsorbent polymer particles are introduced through a funnel and transferred to a drop shaft with a measurement channel. While the particles fall through an illuminated wall, they are selectively recorded by a camera. The recorded images are evaluated by software according to selected parameters.

[0105] Moisture content (MC) The moisture content of superabsorbent polymer particles is determined by EDANA recommended test method No. WSP230.2(05) "Moisture Content - Weight Loss Upon Heating".

[0106] Centrifugal retention capacity (CRC) The centrifugal retention capacity of superabsorbent polymer particles is determined by the EDANA recommended test method No. WSP241.2(05) "Gravimetric Determination of Fluid Retention Capacity in Saline Solution After Centrifugation," but for large centrifugal retention capacities, a large tea bag must be used.

[0107] Load absorption capacity (AUL) The absorption capacity of superabsorbent polymer particles under load is determined by EDANA recommended test method No. WSP242.2(05) "Gravimetric Determination of Absorption Under Pressure".

[0108] High load absorption capacity (AUHL) The absorption capacity of the superabsorbent polymer particles under high load is 21.0 g / cm³. 2 Instead of the weight, use 49.2g / cm³ 2 Except for using the weight, the determination is made in the same manner as EDANA recommended test method No. WSP242.2(05) "Gravimetric Determination of Absorption Under Pressure".

[0109] flow rate The flow rate of superabsorbent polymer particles is determined by EDANA recommended test method No. WSP250.2(05) "Gravimetric Determination of Flowrate".

[0110] bulk density The bulk density of superabsorbent polymer particles is determined by EDANA recommended test method No. WSP260.2(05) "Gravimetric Determination of Density".

[0111] Extractables (Ext.16h) The content of extractable components in the superabsorbent polymer particles is determined by the EDANA recommended test method No. WSP270.2(05) "Determination of Extractable Polymer Content by Potentiometric Titration".

[0112] Caking (40℃ / 80% relative humidity / 1 hour) Place 5 g of superabsorbent polymer particles in an aluminum weighing dish (57 mm × 15 mm) and store at 40 °C and 80% relative humidity for 1 hour. Cool the sample to ambient temperature and weigh it. After sieving through a sieve with a 1.68 mm pore size (ASTM No. 12, sieve diameter over 57 mm and less than 100 mm), weigh the amount that passed through the sieve to determine the mass of the non-caking polymer particles. The sieving process is as described below.

[0113] Carefully take the aluminum dish containing the hydrated polymer and hold it vertically with one hand. Invert the sieve pan assembly above the dish and gently invert the sieve, the pan, and the weighing dish containing the polymer in one continuous motion so that the dish is upside down on the sieve mesh. Place a lid on the sieve mesh containing the aluminum weighing dish and place the assembly on a sieve shaker. Vibrate the sieve assembly for 1 minute at an amplitude of 0.20 mm under the control of a Retsch (registered trademark) Vibratory Sieve Shaker AS 200.

[0114] Next, the percentage of non-caking particles is calculated using the following formula

[0115] [Number] [where W d is the weight of the aluminum dish, W HYD is the weight of the hydrated polymer + aluminum dish before sieving, W PAN is the weight of the collection pan, and W UNC is the weight of the collection pan and the hydrated polymer] and is determined by

[0116] Color values ​​(CIE color metric [L, a, b]) Color values ​​are measured using the "LabScan XE Spectrometer" (HunterLab; Reston; USA) according to the CIELAB procedure (Hunterlab, Vol. 8, 1996, No. 7, pp. 1-4). Colors are represented by three-dimensional coordinates L, a, and b. L describes lightness, where L=0 is black and L=100 is white. The values ​​of a and b describe the position of the color on the red / green and yellow / blue color axes, respectively. A positive a value represents red, a negative a value represents green, a positive b value represents yellow, and a negative b value represents blue.

[0117] The yellowness index (YI) (2 / C) of YI D1925 is measured according to ASTM D-1925, 2 degrees / 3.℃. The higher the value, the darker and more yellow the color.

[0118] The tests were conducted using tissue culture dishes (35 mm in diameter and 10 mm in height) and a 0.5-inch port plate opening.

[0119] Color values ​​are measured according to the tristimulus method specified in DIN 5033-6. [Examples]

[0120] Example 1 (Base Polymer) This example was conducted in the same manner as Example 1 in WO2016 / 134905A1.

[0121] This method was carried out in a simultaneous spray drying plant having an integrated fluidized bed (27), as shown in Figure 1 of WO2016 / 134905A1. The reaction zone (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3 m and a weir height of 0.25 m.

[0122] The dry gas was supplied through a gas distributor (3) at the top of the spray dryer. The dry gas was partially reused (dry gas loop) via a cyclone and condenser (12) as a dust separation unit (9). The dry gas was nitrogen with 1% to 4% residual oxygen. Before starting polymerization, the dry gas loop was filled with nitrogen until the residual oxygen was below 4%. The gas velocity of the dry gas in the reaction zone (5) was 0.79 m / sec. The pressure inside the spray dryer was 4 mbar below the ambient pressure.

[0123] The temperature of the gas exiting the reaction zone (5) was measured at three points around the circumference of the end of the cylindrical section of the spray dryer, as shown in Figure 3 of WO2016 / 134905A1. The average temperature (spray dryer outlet temperature) was calculated using the three individual measurements (43). The drying gas loop was heated and the addition of the monomer solution was started. From this point, the spray dryer outlet temperature was controlled to 114°C by adjusting the gas inlet temperature through the heat exchanger (20). The gas inlet temperature was 167°C.

[0124] The product deposited in the internal fluidized bed (27) reached the weir height. A conditioned internal fluidized bed gas with a temperature of 105°C was supplied to the internal fluidized bed (27) through line (25). The gas velocity of the internal fluidized bed gas in the internal fluidized bed (27) was 0.65 m / sec. The residence time of the product was 150 minutes. The temperature of the superabsorbent polymer particles in the internal fluidized bed (27) was 71°C.

[0125] The exhaust gas from the spray dryer was filtered through a cyclone acting as a dust separation unit (9) and sent to a condenser (12) for quenching / cooling. By controlling the condenser (12) to a constant packing level, excess water was pumped out of the condenser (12). The water in the condenser (12) was cooled by a heat exchanger (13) and pumped in a countercurrent to the gas. To wash away acrylic acid vapor, the water in the condenser (12) was made alkaline by adding a sodium hydroxide solution.

[0126] The gas exiting the condensing tower (12) was split into a dry gas inlet pipe (1) and a regulated internal fluidized bed gas (25). The gas temperature was controlled by heat exchangers (20) and (22). The high-temperature dry gas was supplied to the simultaneous spray dryer through a gas distributor (3). The gas distributor (3) consists of a series of plates and provides a pressure drop of 2 to 4 mbar depending on the amount of dry gas.

[0127] The product was discharged from the internal fluidized bed (27) through a rotary valve (28) to a sieve (29). The sieve (29) was used to separate overs / clumps with a particle diameter of more than 800 μm.

[0128] A monomer solution was prepared by first mixing glycerol triacrylate (internal crosslinking agent) which had been ethoxylated three times with acrylic acid, secondly with a 37.3 wt% sodium acrylate solution, and thirdly with an aqueous solution of disodium 1-hydroxyethane-1,1-diphosphonic acid (HDPA). The temperature of the resulting monomer solution was controlled to 10°C by using a heat exchanger and pumping it through a loop. A filter unit with a mesh size of 250 μm was used in the loop after pumping. The initiator was metered and supplied to the monomer solution upstream of the droplet maker via lines (33) and (34) using stationary mixers (31) and (32), as shown in Figure 1 of WO2016 / 134905A1. A sodium peroxodisulfate solution at a temperature of 20°C was added via line (33), and together with it, a [2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solution was added via line (34). Each initiator was pumped into the loop and added to each droplet unit via a control valve. After the stationary mixer (32), a second filter unit with a mesh size of 140 μm was used. Three droplet units were used to add the monomer solution to the top of the spray dryer, as shown in Figure 4 of WO2016 / 134905A1.

[0129] As shown in Figure 5 of WO2016 / 134905A1, the droplet formation unit consisted of an outer pipe (47) having an opening to a droplet formation cassette (49). The droplet formation cassette (49) was connected to an inner pipe (48). The inner pipe (48), which has a PTFE block (50) at its end as a seal, can be pushed into and pushed out of the outer pipe (47) during process operation for maintenance purposes.

[0130] The droplet cassette (49) had 256 perforations with a diameter of 170 μm and a perforation spacing of 15 mm. The droplet cassette (49) consisted of a channel (56) without stagnant volume, which was essential for uniformly distributing the pre-mixed monomer and initiator solution, and a droplet plate (53). The droplet plate (53) had a sloping configuration with an angle of 3°. The droplet plate (53) was made of stainless steel and had a length of 630 mm, a width of 128 mm, and a thickness of 1 mm.

[0131] The feed to the spray dryer consisted of 10.45% by weight acrylic acid, 33.40% by weight sodium acrylate, 0.018% by weight tripethoxylated glycerol triacrylate, 0.108% by weight disodium 1-hydroxyethane-1,1-diphosphonic acid (HDPA), 0.072% by weight [2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride], 0.072% by weight sodium peroxodisulfate solution (in water, 15% by weight), and water. The degree of neutralization was 71%. The feed rate per puncture was 1.4 kg / hour.

[0132] The obtained superabsorbent polymer particles were analyzed. The conditions and results are summarized in Tables 1-3.

[0133] Example 2 (Not the present invention) 1200 g of superabsorbent polymer particles (base polymer) prepared in Example 1 were placed in a laboratory Ploughscher mixer (Model MR5, Gebrueder Loedige Maschinenbau GmbH, Paderborn, Germany). A post-crosslinking agent solution was prepared in a beaker by mixing 30 g of ethylene carbonate, 1.2 g of aluminum sulfate, and 60 g of deionized water, as shown in Table 4. The aqueous solution was sprayed onto the polymer particles using a spray nozzle within 1 minute at a mixer speed of 200 rpm. Mixing was continued for an additional 5 minutes. The product was removed and transferred to another Ploughscher mixer (Model MR5, Gebrueder Loedige Maschinenbau GmbH, Paderborn, Germany) that had been preheated to 150°C. After mixing for a further 40 minutes at 150°C, the product was removed from the mixer and sieved through an 850 μm sieve.

[0134] As shown in Table 8, a rehumidified solution was prepared in a beaker by mixing 3 g of aluminum lactate, 25 mg of sorbitan monododecanoate (Span® 20), and 80 g of deionized water. 1000 g of this product was transferred to another Plauschär mixer (Model MR5, Gebrueder Loedige Maschinenbau GmbH, Paderborn, Germany) preheated to 80°C. After reaching the final temperature of 80°C, the heating of the Plauschär mixer was turned off, and the aqueous rehumidified solution was sprayed onto the polymer particles by a spray nozzle within 2 minutes at a mixer speed of 200 rpm. Mixing was continued for an additional 13 minutes. The product was removed and sieved through an 850 μm sieve.

[0135] The samples were analyzed. The formulation, conditions, and results are summarized in Tables 4 and 5.

[0136] Example 3 (The present invention) This example was carried out in the same manner as Example 2, except that an additional 0.18 g of hydrogen peroxide was added to the post-crosslinking agent solution.

[0137] Example 4 (The present invention) This example was carried out in the same manner as Example 2, except that an additional 0.36 g of hydrogen peroxide was added to the post-crosslinking agent solution.

[0138] Example 5 (The present invention) This example was carried out in the same manner as Example 2, except that an additional 0.72 g of hydrogen peroxide was added to the post-crosslinking agent solution.

[0139] Example 6 (The present invention) This example was carried out in the same manner as Example 2, except that an additional 1.08 g of hydrogen peroxide was added to the post-crosslinking agent solution.

[0140] Example 7 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 1.80 g of hydrogen peroxide was added to the post-crosslinking agent solution.

[0141] Example 8 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 3.60 g of hydrogen peroxide was added to the post-crosslinking agent solution.

[0142] Example 9 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 0.18 g of sodium persulfate was added to the post-crosslinking agent solution.

[0143] Example 10 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 0.36 g of sodium persulfate was added to the post-crosslinking agent solution.

[0144] Example 11 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 0.72 g of sodium persulfate was added to the post-crosslinking agent solution.

[0145] Example 12 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 0.15 g of hydrogen peroxide was added to the rehumidified solution.

[0146] Example 13 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 0.30 g of hydrogen peroxide was added to the rehumidified solution.

[0147] Example 14 (Not the present invention) This example was carried out in the same manner as Example 2, except that an additional 0.60 g of hydrogen peroxide was added to the rehumidified solution.

[0148] [Table 1]

[0149] [Table 2]

[0150] [Table 3]

[0151] [Table 4]

[0152] [Table 5]

[0153] Figure 1 shows the yellowness index (YI D1925) after 14 days of storage in a weathering test cabinet at 70°C and 80% relative humidity, depending on the hydrogen peroxide content in the post-crosslinking agent solution (Examples 2-8).

[0154] Figure 2 shows the high load absorption capacity (AUHL) which depends on the hydrogen peroxide content in the post-crosslinking agent solution (Examples 2-8).

Claims

1. A method for producing superabsorbent polymer particles with long-term color stability, a) Partially neutralized acrylic acid, b) at least one crosslinking agent, and c) At least one initiator The process includes the steps of polymerizing a monomer solution containing, drying the resulting polymer gel, and thermally post-crosslinking and cooling the resulting polymer particles. A method comprising adding a post-thermal crosslinking agent and 0.016 to 0.095% by weight of hydrogen peroxide, based on the polymer particles, to the polymer particles before post-thermal crosslinking.

2. The method according to claim 1, wherein 0.005 to 5% by weight of a thermal post-crosslinking agent is added to the polymer particles before post-crosslinking, based on the polymer particles.

3. The method according to claim 1 or 2, wherein 0.02 to 0.05% by weight of peroxide, based on the polymer particles, is added to the polymer particles before post-crosslinking.

4. The method according to any one of claims 1 to 3, wherein the post-crosslinking temperature is 130 to 190°C.

5. The method according to any one of claims 1 to 4, wherein the residence time of the post-bridge is 20 to 60 minutes.

6. The method according to any one of claims 1 to 4, wherein the monomer solution contains a polymerization inhibitor.

7. The method according to any one of claims 1 to 6, wherein the monomer solution contains 0.004 to 0.010% by weight of a polymerization inhibitor based on acrylic acid.

8. The method according to claim 6 or 7, wherein hydroquinone monomethyl ether is used as a polymerization inhibitor.

9. The method according to any one of claims 1 to 8, wherein at least one chelating agent is added to polymer particles during or after cooling.

10. The method according to any one of claims 1 to 9, wherein at least one chelating agent is added to the polymer particles in an amount of 0.005 to 1% by weight, based on the polymer particles, during or after cooling.

11. The method according to claim 9 or 10, wherein 1-hydroxyethane-1,1-diphosphonic acid and / or a salt thereof is used as a chelating agent.

12. The method according to any one of claims 1 to 11, wherein the post-crosslinking does not involve irradiation with active energy rays.

13. Superabsorbent polymer particles obtained by the method according to any one of claims 1 to 12, having a centrifugal retention capacity of at least 40 g / g and a high load absorption capacity of at least 20 g / g, and having a yellowness index of 54 or less after aging for 14 days at 70°C and 80% relative humidity, wherein the amount of crosslinking agent b) is 0.05 to 1.5% by weight based on acrylic acid before neutralization, and 0.005 to 5% by weight of thermal post-crosslinking agent and 0.016 to 0.080% by weight of hydrogen peroxide are added to the polymer particles before post-crosslinking.

14. The superabsorbent polymer particles according to claim 13, having a yellowness index of 50 or less after 14 days of aging at 70°C and 80% relative humidity.

15. The method according to any one of claims 1 to 12, wherein the obtained polymer particles are thermally post-crosslinked and the obtained dried polymer gel is pulverized and classified before the step of cooling.

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