Method for producing superabsorbent polymer and superabsorbent polymer

Abstract

The present invention relates to a method for producing a superabsorbent polymer and the superabsorbent polymer, more particularly, to a method for producing a superabsorbent polymer and the superabsorbent polymer, which can suppress the generation of fine powder during the production process and realize an excellent absorption speed by producing a base resin powder having a relatively high moisture content by controlling process conditions in a drying step.

Description

Technical Field

[0001] Cross-reference to Related Application(s) This application claims the benefit of priority based on Korean Patent Application Nos. 10-2021-0079644 filed on June 18, 2021, 10-2021-0080336 filed on June 21, 2021, and 10-2022-0074722 filed on June 20, 2022, and all of the contents disclosed in the documents of the Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing a superabsorbent resin and a superabsorbent resin. More specifically, by controlling the process conditions of the drying step to produce a base resin powder with a relatively high water content, generation of fine powder during the manufacturing process is suppressed, and the present invention relates to a method for manufacturing a superabsorbent resin and a superabsorbent resin capable of realizing an excellent absorption rate.

Background Art

[0003] A superabsorbent polymer (SAP) is a synthetic polymer material having a function of absorbing about 500 to 1000 times its own weight of water, and is named with different names such as SAM (Super Absorbency Material) and AGM (Absorbent Gel Material) for each development company. The superabsorbent resin as described above has started to be put into practical use as a sanitary product, and is now widely used in materials such as a water retention agent for horticultural soil, a water stop material for civil engineering and construction, a seedling raising sheet, a freshness retainer in the food distribution field, and a material for wet tissues.

[0004] Such superabsorbent polymers are mainly widely used in the field of sanitary materials such as diapers and sanitary napkins. In the sanitary materials, it is common for the superabsorbent polymer to be contained in a state of spreading within pulp. However, recently, efforts have continued to provide sanitary materials such as thinner diapers, and as part of this, the pulp content has been reduced, or so-called pulpless diapers that do not use pulp at all have been actively developed.

[0005] Thus, in the case of sanitary materials with a reduced pulp content or no pulp used, the superabsorbent polymer is relatively contained in a high ratio, and the superabsorbent polymer particles are inevitably contained in multiple layers within the sanitary material. In order for the overall superabsorbent polymer particles contained in multiple layers to more efficiently absorb a large amount of liquid such as urine, the superabsorbent polymer basically needs to not only have high absorption performance but also exhibit a fast absorption rate.

[0006] Such superabsorbent polymers are generally produced through the steps of polymerizing monomers to produce a water-containing gel polymer containing a large amount of water, and pulverizing the water-containing gel polymer into resin particles having a desired particle size after drying. However, when passing through the step of pulverizing after drying the water-containing gel polymer as described above, a large amount of fine powder is generated, which has the problem of degrading the physical properties of the finally produced superabsorbent polymer.

[0007] On the other hand, when improving the water content rate of the superabsorbent polymer, the generation of fine powder due to product crushing is reduced, the cost of the product can be lowered, and the absorption rate can be improved.

[0008] In order to obtain such merits of high water content, conventionally, an additional water addition process facility was provided in the surface crosslinking facility, and water was introduced here. However, since it is an additional facility, the economic efficiency is low. When only water is used alone, the physical properties deteriorate due to the generation of scraps. When additives are used to prevent the generation of scraps, there is a problem that the physical properties of the final target deteriorate due to this substance.

[0009] To solve this problem, there is a need for technology that can produce products with high moisture content without additional water-adding equipment. Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, the present invention provides a method for producing a superabsorbent resin that can easily control the moisture content within a desired range without generating a large amount of fine powder and can realize an excellent absorption rate by producing a base resin powder with a relatively high moisture content by fluidizing and drying a polymerized hydrogel polymer under specific conditions, and also provides a superabsorbent resin. [Means for solving the problem]

[0011] In order to solve the above problem, according to one embodiment of the present invention, Step 1: forming a hydrogel polymer by crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal crosslinking agent and a polymerization initiator; A step (step 2) of pulverizing the hydrogel polymer in the presence of a surfactant to produce a mixture containing the pulverized hydrogel polymer; Step 3: drying the mixture at 100°C to 250°C in a moving type to form a base resin powder having a moisture content of 10% to 30% by weight; and (4) a step of thermally crosslinking the surface of the base resin powder in the presence of a surface crosslinking agent to produce highly water-absorbent resin particles. A method for producing a highly water-absorbent resin is provided.

[0012] According to one embodiment of the present invention, there is provided a superabsorbent polymer produced by the above-described method for producing a superabsorbent polymer. [Effects of the Invention]

[0013] According to the method for producing a superabsorbent resin of the present invention, in the step of drying the hydrogel polymer, by drying in a moving type at a relatively low temperature, a base resin with a relatively high water content is produced, generation of fine powder during the process is suppressed, and an excellent absorption rate can be realized.

Mode for Carrying Out the Invention

[0014] The terms used in this specification are used merely to explain exemplary embodiments and are not intended to limit the present invention.

[0015] Singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as "including," "comprising," or "having" are intended to specify the presence of implemented features, steps, components, or combinations thereof, and should be understood not to preclude in advance the presence or addition possibility of one or more other features, steps, components, or combinations thereof.

[0016] Terms such as first, second, third, etc. are used to describe various components, and these terms are used only for the purpose of distinguishing one component from another.

[0017] The present invention may be subject to various modifications and may have various forms, so specific embodiments will be illustrated and described in detail below. However, this is not intended to limit the present invention to a specific disclosed form, and should be understood to include all modifications, equivalents, and alternatives included in the spirit and technical scope of the present invention.

[0018] (Method for Producing Superabsorbent Resin) The method for producing a superabsorbent resin according to an embodiment of the present invention includes a step of forming a water-containing gel polymer by crosslinking and polymerizing a water-soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal crosslinking agent and a polymerization initiator (step 1); a step of granulating the water-containing gel polymer in the presence of a surfactant to produce a mixture containing the granulated water-containing gel polymer (step 2); a step of drying the mixture at 100°C to 250°C in a moving type to form a base resin powder having a water content of 10% by weight to 30% by weight (step 3); and a step of thermally crosslinking the surface of the base resin powder in the presence of a surface crosslinking agent to produce superabsorbent resin particles (step 4).

[0019] As used herein, the term "polymer" or "macromolecule" means a state in which a water-soluble ethylenically unsaturated monomer is polymerized, and can include all water content ranges or particle size ranges.

[0020] The term "superabsorbent resin powder" refers to a particulate substance containing a crosslinked polymer in which a water-soluble ethylenically unsaturated monomer containing an acidic group and at least a part of the acidic group is neutralized is polymerized and crosslinked by an internal crosslinking agent.

[0021] The term "superabsorbent resin" also means, depending on the context, a crosslinked polymer in which a water-soluble ethylenically unsaturated monomer containing an acidic group and at least a part of the acidic group is neutralized is polymerized, or a powder-like base resin composed of superabsorbent resin particles obtained by pulverizing the crosslinked polymer, or all of the crosslinked polymer and the base resin are made suitable for commercialization through additional processes such as surface crosslinking, micronization and regranulation, drying, pulverization, classification, etc.

[0022] The term "crosslinked polymer" means that it is crosslinked and polymerized in the presence of the water-soluble ethylenically unsaturated monomer and the internal crosslinking agent, and the "base resin powder" means a substance containing such a crosslinked polymer.

[0023] The term "fine powder" also means particles having a particle size of less than 150 μm among the superabsorbent resin particles. The particle size of such resin particles can be measured by the method of European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 220.3.

[0024] The term "chopping" also means cutting a water-containing gel polymer into small pieces in millimeters in order to improve drying efficiency, and is used separately from pulverizing to the micrometer or normal particle level.

[0025] The term "micronizing, micronization" also means pulverizing a water-containing gel polymer to a particle size of several tens to several hundreds of micrometers, and is used separately from "chopping".

[0026] Superabsorbent resins are generally produced through a step of polymerizing monomers to produce a water-containing gel polymer containing a large amount of water, and a step of pulverizing the water-containing gel polymer into resin particles having a desired particle size after drying. However, when passing through the step of pulverizing after drying the water-containing gel polymer as described above, a large amount of fine powder is generated, and there is a problem of degrading the physical properties of the finally produced superabsorbent resin.

[0027] Therefore, an attempt was made to improve the water content rate of the superabsorbent resin to reduce the generation of fine powder. In order to achieve a high water content rate, conventionally, additional water addition equipment was provided in the surface crosslinking equipment, and water was introduced here. However, since it is additional equipment, the economic efficiency is low. When only water is used alone, the physical properties deteriorate due to the generation of scraps. When additives are used to prevent the generation of scraps, there is a problem that the physical properties of the final target deteriorate due to this substance.

[0028] Therefore, the present inventors discovered that in the step of drying the hydrogel polymer, by drying it in a moving type at a relatively low temperature to produce a base resin powder with a relatively high moisture content, it is possible to suppress the generation of fine powder during the process and realize an excellent absorption rate, and thus completed the present invention.

[0029] Hereinafter, each step of the manufacturing method will be described in detail.

[0030] (Step 1: Polymerization Step) A method for preparing a superabsorbent resin according to an embodiment of the present invention includes a step (Step 1) of cross-linking a water-soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal cross-linking agent and a polymerization initiator to form a hydrogel polymer.

[0031] The step is a step of forming a hydrogel polymer by thermally polymerizing or photopolymerizing a monomer composition including a monomer mixture containing an internal crosslinking agent, a polymerization initiator, and a water-soluble ethylenically unsaturated monomer having an acidic group.

[0032] The water-soluble ethylenically unsaturated monomer having an acidic group may be any monomer commonly used in the production of superabsorbent resins. As a non-limiting example, the water-soluble ethylenically unsaturated monomer may be a compound represented by the following Chemical Formula 2:

[0033] [Chemical formula 2] R1-COOM 1

[0034] In the above Chemical Formula 2, R1 is an alkyl group having 2 to 5 carbon atoms and containing an unsaturated bond, M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0035] Preferably, the monomer may be at least one selected from the group consisting of acrylic acid, methacrylic acid, and monovalent metal salts, divalent metal salts, ammonium salts, and organic amine salts of these acids. In this way, when acrylic acid or a salt thereof is used as the water-soluble ethylenically unsaturated monomer, it is advantageous because a superabsorbent resin with improved water absorption can be obtained. Alternatively, the monomer may be at least one selected from the group consisting of anionic monomers such as maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethanesulfonic acid, 2-methacryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, or 2-(meth)acrylamido-2-methylpropanesulfonic acid, and salts thereof; nonionic hydrophilic monomers such as (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, or polyethylene glycol (meth)acrylate; and amino group-containing unsaturated monomers such as (N,N)-dimethylaminoethyl (meth)acrylate or (N,N)-dimethylaminopropyl (meth)acrylamide, and quaternized products thereof.

[0036] According to one embodiment of the present invention, when a water-soluble ethylenically unsaturated monomer having an unneutralized acidic group is used, the polymerized polymer has an acidic group, and therefore, a neutralization step may be included after the polymerization step.

[0037] That is, the step of forming the hydrogel polymer (Step 1) may be carried out by including a step of crosslinking a water-soluble ethylenically unsaturated monomer having an acidic group in the presence of an internal crosslinking agent and a polymerization initiator to form a polymer (Step 1-1), and a step of neutralizing at least a part of the acidic groups of the polymer to form a hydrogel polymer (Step 1-2).

[0038] A water-soluble ethylenically unsaturated monomer (e.g., acrylic acid) in which the acid group is not neutralized is in a liquid state at room temperature and exists in the form of a mixed solution in the monomer composition due to its high miscibility with the solvent (water). However, a water-soluble ethylenically unsaturated monomer in which the acid group is neutralized is in a solid state at room temperature and has different solubility depending on the temperature of the solvent (water), with the solubility decreasing as the temperature decreases.

[0039] Such water-soluble ethylenically unsaturated monomers in which the acidic group is not neutralized have higher solubility or miscibility in a solvent (water) than monomers in which the acidic group is neutralized, and therefore do not precipitate even at low temperatures, making them advantageous for long-term polymerization at low temperatures. As a result, long-term polymerization using the water-soluble ethylenically unsaturated monomers in which the acidic group is not neutralized can stably produce polymers having higher molecular weights and more uniform molecular weight distributions.

[0040] Furthermore, it is possible to form a polymer with a longer chain, and the content of water-soluble components that exist in an uncrosslinked state due to incomplete polymerization or crosslinking can be reduced.

[0041] In addition, the acidic groups of the monomers are first polymerized in an unneutralized state to form a polymer, and then the polymer is neutralized and then reacted in the presence of a surfactant. Fine Granulated, Fine When the acidic groups present in the polymer are neutralized during granulation, a large amount of surfactant is present on the surface of the polymer, and the surfactant can sufficiently play a role in reducing the stickiness of the polymer.

[0042] In the monomer composition containing the internal crosslinking agent, the polymerization initiator, and the water-soluble ethylenically unsaturated monomer having an acidic group, the concentration of the water-soluble ethylenically unsaturated monomer may be appropriately adjusted in consideration of the polymerization time, reaction conditions, etc., and may be about 20 to about 60% by weight, or about 20 to about 40% by weight.

[0043] According to another embodiment of the present invention, it may include a step of neutralizing at least a part of the acidic groups of a water-soluble ethylenically unsaturated monomer having acidic groups before the polymerization step.

[0044] That is, the step of forming the hydrogel polymer (Step 1) includes a step of neutralizing at least a part of the acidic groups of a water-soluble ethylenically unsaturated monomer having acidic groups (Step 1-1'), and in the presence of an internal cross-linking agent and a polymerization initiator, cross-linking and polymerizing the water-soluble ethylenically unsaturated monomer having at least a part of the neutralized acidic groups to form a hydrogel polymer (Step 1-2'). It may be carried out including these steps.

[0045] The step of neutralizing the acidic groups is carried out by mixing with a neutralizing agent capable of neutralizing the acidic groups. As examples of the neutralizing agent, basic substances such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide may be used.

[0046] At this time, the degree of neutralization of the acidic groups may be 40 to 95 mol%, or 40 to 90 mol%, or 45 to 80 mol%. The range of the degree of neutralization may vary depending on the final physical properties. However, if the degree of neutralization is excessively high, there may be a problem that the surface cross-linking reaction does not occur sufficiently and the absorption under pressure (AUP) decreases. Conversely, if the degree of neutralization is excessively low, not only the water absorption of the polymer is greatly inferior, but it may exhibit properties such as an elastic rubber that is difficult to handle.

[0047] On the other hand, according to an embodiment of the present invention, when the step of forming the hydrogel polymer (Step 1) is carried out in a step of neutralizing after polymerization (Step 1-1) (Step 1-2), the neutralization step may be carried out sequentially, simultaneously, or crosswise with the step of granulating the hydrogel polymer described later.

[0048] That is, a neutralizing agent may be sprayed onto the polymer to first neutralize the acidic groups of the polymer, and then a surfactant may be added to the neutralized polymer to pulverize the mixture containing the surfactant; a mixture containing the polymer and surfactant may be pulverized, and then a neutralizing agent may be added to neutralize the mixture; or a neutralizing agent and a surfactant may be added simultaneously to the polymer to neutralize and pulverize the polymer.

[0049] On the other hand, in order to ensure uniform neutralization of the entire polymer, it is preferable to leave a certain time lag between the addition of the neutralizing agent and the granulation step.

[0050] The hydrogel polymer obtained by this method may have a water content of 40% to 80% by weight. Preferably, it may be 45% by weight or more, 50% by weight or more, and 75% by weight or less, or 70% by weight or less. If the water content of the hydrogel polymer is too low, it may be difficult to ensure an appropriate surface area in the subsequent granulation step, and the polymer may not be granulated effectively. If the water content of the hydrogel polymer is too high, the pressure applied in the subsequent granulation step may increase, making it difficult to pulverize the polymer to the desired particle size.

[0051] Throughout this specification, the term "moisture content" refers to the amount of water contained in the total weight of the hydrogel polymer, calculated by subtracting the weight of the polymer in a dry state from the weight of the hydrogel polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to evaporation of water in the polymer during the drying process, in which the temperature of the polymer in a crumb state is increased using infrared heating. The drying conditions are as follows: the temperature is increased from room temperature to approximately 180°C and then maintained at 180°C; the total drying time is set to 40 minutes, including a 5-minute temperature increase step, and the moisture content is measured.

[0052] The term "internal cross-linking agent" used in this specification is a term used to distinguish it from a surface cross-linking agent for cross-linking the surface of the superabsorbent resin particles described later, and plays a role in introducing cross-linking bonds between unsaturated bonds of the water-soluble ethylenically unsaturated monomer described above to form a polymer containing a cross-linked structure.

[0053] The crosslinking in the above step is carried out regardless of whether it is on the surface or inside. However, when the surface crosslinking step of the superabsorbent resin particles described later is carried out, the surface of the finally produced superabsorbent resin particles can include a newly crosslinked structure by the surface crosslinking agent, and the inside of the superabsorbent resin particles can also maintain the structure crosslinked by the internal crosslinking agent as it is.

[0054] The internal crosslinking agent may include one or more of i) polyfunctional acrylate compounds, ii) polyfunctional allyl compounds, or iii) polyfunctional vinyl compounds.

[0055] Non-limiting examples of polyfunctional acrylate compounds include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol di(meth)acrylate, dipentaerythritol tri(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, glycerin di(meth)acrylate, and glycerin tri(meth)acrylate, etc. In the present invention, these may be used alone or in admixture of two or more.

[0056] Non-limiting examples of the polyfunctional allyl compound include ethylene glycol diallyl ether, diethylene glycol diallyl ether, triethylene glycol diallyl ether, tetraethylene glycol diallyl ether, polyethylene glycol diallyl ether, propylene glycol diallyl ether, tripropylene glycol diallyl ether, polypropylene glycol diallyl ether, butanediol diallyl ether, butylene glycol diallyl ether, hexanediol diallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, pentaerythritol tetraallyl ether, dipentaerythritol diallyl ether, dipentaerythritol triallyl ether, dipentaerythritol tetraallyl ether, dipentaerythritol pentaallyl ether, trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, glycerin diallyl ether, and glycerin triallyl ether. In the present invention, these may be used alone or in combination of two or more.

[0057] Non-limiting examples of the polyfunctional vinyl compound include ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, tetraethylene glycol divinyl ether, polyethylene glycol divinyl ether, propylene glycol divinyl ether, tripropylene glycol divinyl ether, polypropylene glycol divinyl ether, butanediol divinyl ether, butylene glycol divinyl ether, hexanediol divinyl ether, pentaerythritol divinyl ether, pentaerythritol trivinyl ether, pentaerythritol tetravinyl ether, dipentaerythritol divinyl ether, dipentaerythritol trivinyl ether, dipentaerythritol tetravinyl ether, dipentaerythritol pentavinyl ether, trimethylolpropane divinyl ether, trimethylolpropane trivinyl ether, glycerin divinyl ether, and glycerin trivinyl ether. In the present invention, these may be used alone or in admixture of two or more thereof.

[0058] In the above-mentioned polyfunctional acrylate compound, two or more acrylate groups contained in the molecule can each bind to the unsaturated bond of the water-soluble ethylene-based unsaturated monomer or the unsaturated bond of another internal cross-linking agent, and a cross-linked structure can be formed during the polymerization process.

[0059] In addition, in the above-mentioned polyfunctional allyl compound or polyfunctional vinyl compound, two or more unsaturated groups contained in the molecule can each bind to the unsaturated bond of the water-soluble ethylene-based unsaturated monomer or the unsaturated bond of another internal cross-linking agent, and a cross-linked structure can be formed during the polymerization process. Different from the acrylate compound containing an ester bond (-(C=O)O-) in the molecule, the cross-linking bond can be stably maintained even during the neutralization process after the above-mentioned polymerization reaction.

[0060] Thereby, the gel strength of the produced superabsorbent resin can be increased, and the process stability can be increased during the discharging process after polymerization.

[0061] The total content of the internal crosslinking agent may be 0.01 to 5 parts by weight based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. For example, the internal crosslinking agent may be used in an amount of 0.01 parts by weight or more, 0.05 parts by weight or more, 0.1 parts by weight or more, or 0.45 parts by weight or more, or 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, 1 part by weight or less, or 0.7 parts by weight or less, based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. If the content of the internal crosslinking agent is too low, crosslinking may not occur sufficiently, making it difficult to achieve strength above an appropriate level. If the content of the internal crosslinking agent is too high, the internal crosslink density may increase, making it difficult to achieve the desired water retention ability.

[0062] The polymer formed using such an internal crosslinking agent has a three-dimensional network structure in which the main chain formed by polymerization of the water-soluble ethylenically unsaturated monomer is crosslinked by the internal crosslinking agent. When the polymer has such a three-dimensional network structure, the water retention capacity and pressure absorption capacity, which are various physical properties of the superabsorbent resin, can be significantly improved compared to when the polymer has a two-dimensional linear structure that is not additionally crosslinked by the internal crosslinking agent.

[0063] The monomer composition may also contain a polymerization initiator commonly used in the production of superabsorbent resins. Non-limiting examples of the polymerization initiator include a thermal polymerization initiator or a photopolymerization initiator, depending on the polymerization method. However, since a certain amount of heat is generated by UV irradiation in photopolymerization and also by the progress of the exothermic polymerization reaction, a thermal polymerization initiator may also be included.

[0064] As the photoinitiator, for example, one or more compounds selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, Benzyl Dimethyl Ketal, acyl phosphine, and α-aminoketone may be used. On the one hand, specific examples of acyl phosphine include diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate, and the like. More diverse photoinitiators are disclosed in detail on page 115 of the book "UV Coatings: Basics, Recent Developments and New Application (Elsevier, 2007)" by Reinhold Schwalm and are not limited to the aforementioned examples.

[0065] As the thermal polymerization initiator, one or more compounds selected from the group consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid may be used. Specifically, examples of persulfate initiators include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), and the like. Examples of azo initiators include 2,2-azobis-(2-amidinopropane) dihydrochloride, 2,2-azobis-(N,N-dimethylene)isobutyramidine dihydrochloride, 2-(carbamoylazo)isobutylonitril, 2,2-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride, 4,4-azobis-(4-cyanovaleric acid), and the like. More diverse thermal polymerization initiators are disclosed on page 203 of the book "Principle of Polymerization (Wiley, 1981)" by Odian, and this can be referred to. For reference, as will be described later, when the polymerization step is carried out in a batch reactor, the aforementioned thermal polymerization initiator may be used by using the thermal polymerization method.

[0066] The polymerization initiator may be added at a concentration of 0.001 to 1 part by weight per 100 parts by weight of the water-soluble ethylenically unsaturated monomer. That is, if the concentration of the polymerization initiator is too low, the polymerization rate may be slowed and a large amount of residual monomer may be extracted into the final product, which is undesirable. Conversely, if the concentration of the polymerization initiator is too high, the polymer chains forming the network may be shortened, the content of water-soluble components may be increased, and the physical properties of the resin may be degraded, such as a decrease in pressure absorption capacity, which is undesirable.

[0067] Meanwhile, in one embodiment of the present invention, the polymerization can be initiated by adding a reducing agent that forms a redox couple with the polymerization initiator.

[0068] Specifically, when the polymerization initiator and the reducing agent are added to the polymer solution, they react with each other to form radicals.

[0069] The formed radicals react with the monomer, and the oxidation-reduction reaction between the polymerization initiator and the reducing agent is highly reactive, so polymerization can be initiated even with only a small amount of polymerization initiator and reducing agent added. There is no need to increase the process temperature, so low-temperature polymerization is possible, and changes in the physical properties of the polymer solution can be minimized.

[0070] The polymerization reaction using the oxidation-reduction reaction can occur smoothly at or below room temperature (25°C). For example, the polymerization reaction may be carried out at a temperature of 5°C to 25°C, or 5°C to 20°C.

[0071] In one embodiment of the present invention, when a persulfate-based initiator is used as the polymerization initiator, the reducing agent may be one or more selected from the group consisting of sodium metabisulfite (Na2S2O5); tetramethylethylenediamine (TMEDA); iron(II) sulfate (FeSO4); a mixture of iron(II) sulfate and EDTA (FeSO4 / EDTA); sodium formaldehyde sulfoxylate; and disodium 2-hydroxy-2-sulfinoacetate.

[0072] As an example, potassium persulfate may be used as the polymerization initiator and disodium 2-hydroxy-2-sulfinoacetate may be used as the reducing agent; ammonium persulfate may be used as the initiator and tetramethylethylenediamine may be used as the reducing agent; or sodium persulfate may be used as the initiator and sodium formaldehyde sulfoxylate may be used as the reducing agent.

[0073] In another embodiment of the present invention, when a hydrogen peroxide-based initiator is used as the initiator, the reducing agent may be one or more selected from the group consisting of ascorbic acid; sucrose; sodium sulfite (Na2SO3); sodium metabisulfite (Na2S2O5); tetramethylethylenediamine (TMEDA); a mixture of iron(II) sulfate and EDTA (FeSO4 / EDTA); sodium formaldehyde sulfoxylate; disodium 2-hydroxy-2-sulfinoacetate; and disodium 2-hydroxy-2-sulfoacetate.

[0074] In addition, the monomer composition may further contain additives such as a thickener, a plasticizer, a storage stabilizer, an antioxidant, etc., if necessary.

[0075] And such a monomer composition may be prepared in the form of a solution in which raw material substances such as the aforementioned water-soluble ethylenically unsaturated monomer, polymerization initiator, and internal crosslinking agent are dissolved in a solvent.

[0076] At this time, as the solvent that can be used, any solvent that can dissolve the aforementioned raw material substances may be used without limitation to its composition. For example, as the solvent, water, ethanol, ethylene glycol, diethylene glycol, triethylene glycol, 1,4-butanediol, propylene glycol, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monomethyl ether acetate, methyl ethyl ketone, acetone, methyl amyl ketone, cyclohexanone, cyclopentanone, diethylene glycol monomethyl ether, diethylene glycol ethyl ether, toluene, xylene, butyrolactone, carbitol, methyl cellosolve acetate, N,N-dimethylacetamide, or a mixture thereof may be used.

[0077] According to one embodiment of the present invention, the step of polymerizing the monomer composition to form a polymer may be carried out in a batch type reactor.

[0078] In the production method of a normal superabsorbent resin, the polymerization method is roughly divided into thermal polymerization and photopolymerization depending on the polymerization energy source. Usually, when thermal polymerization is carried out, it may be carried out in a reactor having a stirring shaft such as a kneader. When photopolymerization is carried out, it may be carried out in a reactor equipped with a movable conveyor belt or in a flat-bottomed container.

[0079] On the one hand, in the polymerization method as described above, a polymer having a wide molecular weight distribution with a small molecular weight is generally formed due to a short polymerization reaction time (for example, about 1 hour or less). On the other hand, when photopolymerization is carried out in a reactor equipped with a movable conveyor belt or a flat-bottomed container, the form of the hydrous gel polymer usually obtained is a sheet-like hydrous gel polymer having the width of the belt, and the thickness of the polymer sheet varies depending on the concentration of the monomer composition to be injected and the injection rate or injection amount, but usually, it is obtained with a thickness of about 0.5 to about 5 cm.

[0080] By the way, when the monomer composition is supplied to such an extent that the thickness of the polymer on the sheet is excessively thin, it is not preferable because the production efficiency is low. When the thickness of the polymer on the sheet is increased for productivity, the polymerization reaction does not occur evenly over the entire thickness, and it becomes difficult to form a high-quality polymer.

[0081] In addition, in the polymerization in a reactor having a stirring shaft equipped with the conveyor belt, since the polymerization product moves while a new monomer composition is supplied to the reactor and the polymerization is carried out continuously, polymers having different polymerization rates are mixed, and thus it becomes difficult to carry out uniform polymerization throughout the monomer composition, and there may be an overall decrease in physical properties.

[0082] However, as described above, by carrying out polymerization in a batch reactor according to an embodiment of the present invention in a fixed-bed type, there is little risk of mixing polymers having different polymerization rates, and thus a polymer having uniform quality can be obtained.

[0083] In addition, the polymerization step is carried out in a batch reactor having a predetermined volume, and the polymerization reaction is carried out for a longer time, for example, 3 hours or more, than when polymerization is carried out continuously in a reactor equipped with a conveyor belt. Despite such a long polymerization reaction time, since the capacity of the batch reactor can be adjusted to receive a monomer composition having a larger capacity than the reactor equipped with a conveyor belt, the overall productivity can also be maintained.

[0084] On the one hand, in the case of polymerization in a batch reactor as described above, the polymerization initiator may be the aforementioned thermal polymerization initiator by using a thermal polymerization method.

[0085] (Step 2: Granulation Step) Next, it includes the step (step 2) of granulating the water-containing gel polymer in the presence of a surfactant to produce a mixture containing the granulated water-containing gel polymer.

[0086] The step is a step of granulating the polymer in the presence of a surfactant, and instead of chopping the polymer into millimeter sizes, it is granulated and aggregated simultaneously at sizes of dozens to hundreds of micrometers. Fine That is, by imparting appropriate adhesiveness to the polymer, secondary aggregated particles in a shape where primary particles granulated to sizes of dozens to hundreds of micrometers are aggregated are produced. The water-containing superabsorbent resin particles, which are secondary aggregated particles produced in such a step, have a normal particle size distribution and a significantly increased surface area, and can significantly improve the absorption rate. Fine According to one embodiment of the present invention, the surfactant may be one or more selected from the group consisting of the compound represented by Chemical Formula 1 and its salts, but is not limited thereto. Fine

[0087]

[0088] <00003XX> # There seems to be an error in the original ID=24, assuming it should be <00003XX> as it's not a valid 7-digit tag. If it's a typo, this might need to be corrected in the original text.

Chemical formula

[0089] In Chemical Formula 1, A1, A2, and A3 are each independently a single bond, a carbonyl,

[0090]

Chemical formula

[0091] and provided that one or more of these are carbonyl or

[0092] [Chemical Formula]

[0093] wherein m1, m2, and m3 are each independently an integer from 1 to 8,

[0094] [Chemical Formula]

[0095] is each linked to an adjacent oxygen atom,

[0096] [Chemical Formula]

[0097] is each linked to adjacent R1, R2, and R3, R1, R2, and R3 are each independently hydrogen, a linear or branched alkyl having 6 to 18 carbon atoms, or a linear or branched alkenyl having 6 to 18 carbon atoms, n is an integer from 1 to 9.

[0098] The surfactant is mixed with the polymer and added so that the granulation (chopping) step can be easily performed without an aggregation phenomenon.

[0099] The surfactant represented by Chemical Formula 1 is a nonionic surfactant and has excellent surface adsorption performance by hydrogen bonding with an unneutralized polymer, and thus is suitable for realizing the intended aggregation control effect. On the other hand, in the case of an anionic surfactant rather than a nonionic surfactant, when mixed with a polymer neutralized with a neutralizing agent such as NaOH or Na2SO4, Na ionized in the substituent of the carboxy group of the polymer +When adsorbed through ions and mixed with an unneutralized polymer, there is a problem that the adsorption efficiency for the polymer is relatively reduced due to competition with anions of substituents of carboxy groups of the polymer.

[0100] Specifically, in the surfactant represented by the chemical formula 1, the hydrophobic functional group is the R1, R2, R3 part (when not hydrogen) which is the terminal functional group, and the hydrophilic functional group is the part derived from glycerol in the chain and the terminal hydroxyl group (A n is a single bond, and at the same time R n is hydrogen, n = 1 to 3). Further, the part derived from glycerol and the terminal hydroxyl group are hydrophilic functional groups, and play a role in improving the adsorption performance on the polymer surface. Thereby, aggregation of the superabsorbent resin particles can be effectively suppressed.

[0101] In the chemical formula 1, the R1, R2, R3 parts (when not hydrogen) which are hydrophobic functional groups are each independently a linear or branched alkyl having 6 to 18 carbon atoms or a linear or branched alkenyl having 6 to 18 carbon atoms. At this time, when the R1, R2, R3 parts (when not hydrogen) are an alkyl or alkenyl having less than 6 carbon atoms, since the chain length is short, there is a problem that the aggregation control of the pulverized particles cannot be effectively performed. When the R1, R2, R3 parts (when not hydrogen) are an alkyl or alkenyl having more than 18 carbon atoms, the mobility of the surfactant may decrease and may not be effectively mixed with the polymer, and there may be a problem that the unit price of the composition increases due to an increase in the cost of the surfactant.

[0102] Preferably, R1, R2, and R3 are hydrogen, or in the case of a linear or branched alkyl having 6 to 18 carbon atoms, they may be 2-methylhexyl, n-heptyl, 2-methylheptyl, n-octyl, n-nonyl, n-decanyl, n-undecanyl, n-dodecanyl, n-tridecanyl, n-tetradecanyl, n-pentadecanyl, n-hexadecanyl, n-heptadecanyl, or n-octadecanyl, or in the case of a linear or branched alkenyl having 6 to 18 carbon atoms, they may be 2-hexenyl, 2-heptenyl, 2-octenyl, 2-nonenyl, n-decenyl, 2-undecenyl, 2-dodecenyl, 2-tridecenyl, 2-tetradecenyl, 2-pentadecenyl, 2-hexadecenyl, 2-heptadecenyl, or 2-octadecenyl.

[0103] The surfactant may be selected from the compounds represented by the following Chemical Formulas 1-1 to 1-14.

[0104]

Chem.

Chem.

[0105] On the other hand, the surfactant may be used in an amount of 0.01 to 10 parts by weight based on 100 parts by weight of the polymer. When the amount of the surfactant used is excessively small, it may not be adsorbed evenly on the surface of the polymer, and thus particle re-aggregation may occur after pulverization. When the amount of the surfactant used is excessively large, various physical properties of the finally produced superabsorbent resin may deteriorate. For example, the surfactant may be used in an amount of 0.01 part by weight or more, 0.015 part by weight or more, or 0.1 part by weight or more, and 5 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, or 1 part by weight or less based on 100 parts by weight of the polymer.

[0106] The method of mixing such a surfactant with a polymer is not particularly limited as long as it can evenly mix them with the polymer, and it may be appropriately selected and used. Specifically, the surfactant may be mixed dry, mixed in a solution state after being dissolved in a solvent, or mixed after melting the surfactant.

[0107] Among these, for example, the surfactant may be mixed in a solution state dissolved in a solvent. At this time, the solvent is not limited to inorganic solvents or organic solvents, and all types may be used, but considering the ease of the drying process and the cost of the solvent recovery system, water is most appropriate. Also, the solution may be prepared by putting the surfactant and the polymer into a reaction tank and mixing them, a method of injecting the solution into the polymer in a mixer, a method of continuously supplying and mixing the polymer and the solution to a continuously operating mixer, etc.

[0108] On the other hand, according to one embodiment of the present invention, the step of neutralizing at least a part of the acidic groups of the polymer (step 2) and, in the presence of a surfactant, Fine the step of granulating to produce water-containing superabsorbent resin particles (step 3) may be carried out sequentially, alternately, or simultaneously.

[0109] That is, a neutralizing agent is added to the polymer to first neutralize the acidic groups, and then a surfactant is added to the neutralized polymer to mix the surfactant with the polymer, Fine followed by granulation, or a neutralizing agent and a surfactant may be added to the polymer simultaneously to perform neutralization and Fine granulation on the polymer. Alternatively, the surfactant may be added first and the neutralizing agent later. Or the neutralizing agent and the surfactant may be added alternately in a cross pattern. Or the surfactant may be added first and Fine granulated, then a neutralizing agent is added for neutralization, and an additional surfactant is further added to the neutralized water-containing gel polymer for Fine further granulation.

[0110] On the other hand, for uniform neutralization of the entire polymer, the addition of the neutralizing agent andFine It is preferable to have a certain time difference during the granulation process.

[0111] At least a part or a substantial amount of the surfactant may be present on the surface of the water-containing superabsorbent resin particles.

[0112] Here, the meaning that the surfactant is present on the surface of the water-containing superabsorbent resin particles means that at least a part or a substantial amount of the surfactant is adsorbed or bound to the surface of the water-containing superabsorbent resin particles. Specifically, the surfactant may be physically or chemically adsorbed on the surface of the superabsorbent resin. More specifically, the hydrophilic functional group of the surfactant may be physically adsorbed on the hydrophilic part of the surface of the superabsorbent resin by intermolecular forces such as dipole-dipole interaction. Thus, the hydrophilic part of the surfactant is physically adsorbed on the surface of the superabsorbent resin particles and surrounds the surface, and the hydrophobic part of the surfactant is not adsorbed on the surface of the resin particles, so the resin particles may be coated with the surfactant in the form of a kind of micelle structure. This is because the surfactant is not added during the polymerization process of the water-soluble ethylenic unsaturated monomer, but is added in the granulation step after the formation of the polymer. Fine Compared with the case where the surfactant is added during the polymerization process and the surfactant is present inside the polymer, it can faithfully play the role of the surfactant, and both pulverization and aggregation occur simultaneously, and particles with a large surface area can be obtained in the form of aggregated fine particles.

[0113] On the other hand, according to an embodiment of the present invention, the step of granulating the water-containing gel polymer (step 2) may be performed by extruding it through a perforated plate having a large number of holes.

[0114] Specifically, the granulation step may be performed by using a granulating device equipped with a perforated plate having a large number of holes and extruding the water-containing gel polymer mixed with the surfactant through the perforated plate.

[0115] Preferably, the Fine granulating device includes a body portion including a transfer space into which the water-containing gel polymer is transferred, a screw member rotatably provided inside the transfer space for moving the water-containing gel polymer, a drive motor for providing a rotational driving force to the screw member, a cutter member provided on the body portion for pulverizing the water-containing gel polymer, and a perforated plate provided outside the body portion for discharging the water-containing gel polymer pulverized by the cutter member and having a number of holes formed therein.

[0116] Preferably, the cutter member may include a perforated plate and a cutting knife disposed adjacent to the perforated plate on the outlet side of the body portion. After the mixture passes through the perforated plate, it may be pulverized and finely granulated by the cutting knife.

[0117] The cutter member may include a plurality of perforated plates and a plurality of cutting knives.

[0118] The size of the holes formed in the perforated plate may be 0.1 mm to 30 mm, preferably 0.5 mm to 25 mm, 1 mm to 20 mm, or 1 mm to 10 mm. By using the perforated plate having the size of the holes, finely granulated water-containing gel polymer particles having a desired particle size can be produced. On the other hand, when there are a plurality of cutter members, the size of the holes formed in the perforated plate of each cutter member can independently satisfy the above-mentioned range.

[0119] On the other hand, according to an embodiment of the present invention, the step (step 2) of finely granulating the water-containing gel polymer may be performed a plurality of times, preferably 1 to 6 times or 1 to 4 times.

[0120] The multiple pulverization steps may be performed using a single pulverization device including multiple cutter members, multiple pulverization devices, or multiple pulverization devices, one or more of which includes multiple cutter members. The hole sizes of the perforated plates used in the multiple pulverization steps may be the same or different.

[0121] Preferably, when the granulation step is carried out twice, the first granulation step and the second granulation step may be performed to granulate the hydrogel polymer particles granulated in the first granulation step so that the particles have an even smaller average particle size.

[0122] Meanwhile, according to one embodiment of the present invention, if polymerization is first performed in the polymerization step with the acidic groups of the monomers not neutralized to form a polymer that is not in a hydrogel state, and then neutralized to form a hydrogel polymer, the surfactant can be present in large amounts on the polymer surface in the granulation step, reducing the stickiness of the polymer and effectively controlling aggregation of the hydrogel polymer. Furthermore, if polymerization is performed with the acidic groups of the monomers not neutralized, it is possible to form a polymer with longer chains, and the content of water-soluble components that exist in an uncrosslinked state due to incomplete polymerization or crosslinking can be reduced.

[0123] In this case, a neutralizing agent may be sprayed onto a polymer containing unneutralized acidic groups to first neutralize the acidic groups of the polymer, and then a surfactant may be added to the neutralized polymer to pulverize the mixture containing the surfactant; a mixture containing the polymer and surfactant may be pulverized, and then a neutralizing agent may be added to neutralize the mixture; or a neutralizing agent and a surfactant may be added simultaneously to the polymer to neutralize and pulverize the polymer.

[0124] (Step 3: Moving Type Drying Step) Next, the mixture containing the granulated hydrogel polymer is dried at 100 to 250°C in a moving type manner to form a base resin powder having a moisture content of 10 to 30% by weight (step 3).

[0125] In the conventional method for producing a superabsorbent resin, it is common to perform the drying step until the water content of the base resin powder becomes less than 10% by weight. However, in the present invention, in the presence of a surfactant, by performing the granulation step, the aggregation of the granulated water-containing superabsorbent resin is controlled, and the drying is performed so that the water content of the superabsorbent resin particles to be dried satisfies 10% to 30% by weight. In the present invention, by performing the granulation step in the presence of a surfactant, even if the base resin powder is dried to have a relatively high water content within the above range, the aggregation between the base resin powders may not be minimized. As a result, the generation of fine powder during the subsequent process can be fundamentally prevented, and the absorption rate of the finally produced superabsorbent resin can be improved, which is preferable.

[0126] When the water content of the base resin powder is less than 10% by weight, it is difficult to effectively control the generation of fine powder, and there is a problem that a water addition step is essentially required in a subsequent surface crosslinking step or the like to increase the water content of the finally produced superabsorbent resin. Further, when the water content exceeds 30% by weight, there is a risk of partial aggregation, which may require an additional pulverization step.

[0127] The drying step is performed at a relatively low temperature of 100°C to 250°C in a moving type drying method. Such moving type drying is classified by the presence or absence of the flow of the substance during drying from fixed-bedtype drying, and it is preferable because it can prevent the aggregation phenomenon between the granulated water-containing gel polymer resin particles in the pulverized material to be dried and can complete the drying within a short time.

[0128] The so-called moving type drying refers to a method of drying while mechanically agitating the drying material. At this time, the direction in which the hot air passes through the substance may be the same as or different from the circulation direction of the substance. Alternatively, the substance may circulate inside the dryer, and the heat transfer fluid (heat medium flow) may be passed through a separate pipe outside the dryer to dry the substance. On the other hand, the fixed-bedtype drying refers to a method in which the substance to be dried is stopped on a bottom such as a porous iron plate through which air can pass, and hot air passes through the substance from bottom to top for drying.

[0129] If the drying temperature in the fluidized drying step is less than 100°C, the drying time may become excessively long, and there may be a risk that the particle size improvement, pulverization, and classification processes cannot be performed due to the un-dried polymer. If the drying temperature exceeds 250°C, only the surface of the polymer is dried, and it is difficult to achieve the target moisture content. When shortening the drying time to achieve the target moisture content, the inside may not be dried smoothly. The drying temperature is preferably carried out at 100°C to 150°C, 100°C to 140°C, 100°C to 130°C, or 110°C to 130°C. Within this range, the moisture content of the final superabsorbent resin can be controlled within the target range without the above-mentioned problems, and the absorption rate of the finally produced superabsorbent resin can be improved, so it is preferable.

[0130] On the other hand, the drying temperature may be the internal driving temperature into which the dried material of the fluidized drying device used is introduced. This may be adjusted by passing the heat transfer fluid (heat medium flow) through a separate pipe outside the dryer, but is not limited thereto.

[0131] On the other hand, the step of drying in the fluidized manner (Step 3) may be carried out by charging the mixture into a fluidized dryer rotating at a speed of 30 rpm to 300 rpm. For example, a mixture containing the granulated water-containing gel polymer may be charged into the fluidized dryer rotating within the above speed range, and the heat transfer fluid (heat medium flow) may be passed through a separate pipe outside the dryer to dry the mixture in the above temperature range.

[0132] When the rotational speed is less than 30 rpm, the drying time may become excessively long, smooth flow may not occur, and uniform drying may be difficult. When the rotational speed exceeds 300 rpm, problems of particle crushing due to increased friction between the polymer and the inside of the dryer may occur. Preferably, it may be carried out at a rotational speed of 50 rpm to 250 rpm, 55 rpm to 200 rpm, or 60 rpm to 100 rpm. Within this range, the moisture content of the final superabsorbent resin can be controlled within the target range without the above-mentioned problems, and the absorption rate of the finally produced superabsorbent resin can be improved.

[0133] For the step of drying in the fluidized manner (Step 3), a generally used fluidized dryer may be used without special restrictions. For example, it may be carried out using a fluidized dryer such as a Horizontal-type Mixer, a Rotary kiln, a Paddle Dryer, or a Steam tube dryer.

[0134] On the other hand, the step of drying in the fluidized manner (Step 3) may be carried out for 30 minutes to 120 minutes. Since there is little aggregation phenomenon among the fine-grained water-containing gel polymer resin particles in the pulverized material to be dried, the drying step is carried out at a relatively low temperature for a short time.

[0135] The drying step may preferably be carried out for 30 minutes to 90 minutes or 40 minutes to 60 minutes. Even when the drying process is carried out for such a short time under the above-mentioned low-temperature conditions, a superabsorbent resin having a high moisture content and an excellent absorption rate can be produced without the problem of non-uniformity of the moisture content between particles.

[0136] In one embodiment of the present invention, under the aforementioned conditions, the average particle size of the base resin powder produced through a fluidized drying process may be from 50 μm to 600 μm, preferably from 100 μm to 500 μm, from 200 μm to 500 μm, from 150 μm to 450 μm, or from 200 μm to 400 μm. By satisfying the particle size range, the polymer is produced as secondary particles in a form where primary particles are aggregated, and thereafter, by performing the grinding and drying processes under milder conditions, the amount of fine powder generated during the process can be significantly reduced.

[0137] In the present invention, the average particle size “Dn” means the particle size or particle diameter at the n% point of the cumulative particle number distribution according to the particle size. That is, D50 indicates the particle size at the 50% point of the cumulative particle number distribution according to the particle size, D90 indicates the particle size at the 90% point of the cumulative particle number distribution according to the particle size, and D10 indicates the particle size at the 10% point of the cumulative particle number distribution according to the particle size. The Dn can be measured using a laser diffraction method or the like. Specifically, after dispersing the powder to be measured in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac S3500), and when the particles pass through the laser beam, the difference in the diffraction pattern according to the particle size is measured to calculate the particle size distribution. By calculating the particle sizes at the 10%, 50%, and 90% points of the cumulative particle number distribution according to the particle size in the measuring device, D10, D50, and D90 can be measured.

[0138] (Step 4: Surface Crosslinking Step) Next, the method for producing a superabsorbent resin according to one embodiment of the present invention includes a step of thermally crosslinking the surface of the base resin powder in the presence of surface crosslinking to produce superabsorbent resin particles (step 4).

[0139] In the surface crosslinking step, in the presence of a surface crosslinking agent, a crosslinking reaction is induced on the surface of the base resin powder, so that the unsaturated bonds of the water-soluble ethylenic unsaturated monomer remaining on the surface without being crosslinked are crosslinked by the surface crosslinking agent, and a superabsorbent resin with a high surface crosslinking density is formed.

[0140] Specifically, in the presence of a surface crosslinking agent, a surface crosslinked layer may be formed in a heat treatment step. In the heat treatment step, the surface crosslinking density, that is, the external crosslinking density, will increase, while the internal crosslinking density will not change. The superabsorbent resin with the formed surface crosslinked layer will have a structure with a higher crosslinking density on the outside than on the inside.

[0141] On the other hand, according to an embodiment of the present invention, the surface crosslinking step is carried out at a relatively low temperature of 80°C to 120°C, and heat crosslinked at this temperature, so that it is possible to manufacture superabsorbent resin particles with an appropriate surface crosslinked layer having a target high water content and excellent absorption rate. Preferably, the surface crosslinking temperature may be 90°C to 110°C, or 95°C to 105°C.

[0142] More specifically, the surface crosslinking can be carried out by using the aforementioned temperature as the maximum reaction temperature and heat treating for 30 minutes to 80 minutes, or 40 minutes to 70 minutes at such a maximum reaction temperature to carry out the surface crosslinking reaction. Even if the surface crosslinking reaction is carried out at a relatively low temperature for a short time as described above, it is possible to effectively control the generation of fine powder without degrading the physical properties of the finally produced superabsorbent resin, so it is preferable.

[0143] By satisfying such surface crosslinking process conditions (especially the temperature rising conditions and the reaction conditions at the maximum reaction temperature), it is possible to manufacture a superabsorbent resin that appropriately satisfies physical properties such as a more excellent absorption rate.

[0144] The means for raising the temperature for the surface crosslinking reaction is not particularly limited. A heat medium may be supplied, or a heat source may be directly supplied for heating. At this time, as the type of heat medium that can be used, a heated fluid such as steam, hot air, or hot oil may be used, but it is not limited thereto. Also, the temperature of the heat medium to be supplied may be appropriately selected in consideration of the means of the heat medium, the rate of temperature rise, and the target temperature of temperature rise. On the other hand, examples of the heat source directly supplied include heating by electricity and heating methods by gas, but it is not limited to the examples described above.

[0145] On the other hand, as the surface crosslinking agent contained in the surface crosslinking agent composition, any of the surface crosslinking agents conventionally used in the production of superabsorbent resins may be used without particular limitation. For example, the surface crosslinking agent is one or more polyols selected from the group consisting of ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, 1,2-hexanediol, 1,3-hexanediol, 2-methyl-1,3-propanediol, 2,5-hexanediol, 2-methyl-1,3-pentanediol, 2-methyl-2,4-pentanediol, tripropylene glycol, and glycerol; one or more carbonate compounds selected from the group consisting of ethylene carbonate and propylene carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazoline compounds such as oxazolidinone; polyamine compounds; mono-, di- or polyoxazolidinone compounds; or cyclic urea compounds; etc. may be included. Preferably, the same one as the aforementioned internal crosslinking agent may be used. For example, a diglycidyl ether-based compound of an alkylene glycol such as ethylene glycol diglycidyl ether may be used.

[0146] In the surface crosslinking step, a surface crosslinking agent composition containing an alcohol-based solvent and water in addition to the surface crosslinking agent may be used.

[0147] Such a surface crosslinking agent may be used in an amount of 0.001 to 2 parts by weight based on 100 parts by weight of the base resin powder. Preferably, it may be used in an amount of 0.005 parts by weight or more, 0.01 parts by weight or more, or 0.02 parts by weight or more, and may also be used in a content of 0.5 parts by weight or less, 0.3 parts by weight or less. By adjusting the content range of the surface crosslinking agent to the aforementioned range, a superabsorbent resin exhibiting excellent physical properties such as absorption performance and liquid permeability can be produced.

[0148] On the other hand, the surface crosslinking agent is added to the base resin powder in the state of a surface crosslinking agent composition containing the same. However, there is no particular limitation on the configuration of the method for adding such a surface crosslinking agent composition. For example, the surface crosslinking agent composition and the base resin powder may be put into a reaction tank and mixed, or the surface crosslinking agent composition may be sprayed onto the base resin powder, or a method of continuously supplying and mixing the base resin powder and the surface crosslinking agent composition to a continuously operated mixer may be used.

[0149] And the surface crosslinking agent composition may further contain water and / or a hydrophilic organic solvent as a medium. Therefore, there is an advantage that the surface crosslinking agent and the like can be evenly dispersed on the base resin powder. At this time, the content of water and the hydrophilic organic solvent is adjusted for the purpose of inducing uniform dissolution / dispersion of the surface crosslinking agent, preventing the aggregation phenomenon of the base resin powder, and optimizing the surface penetration depth of the surface crosslinking agent, and may be applied by adjusting the addition ratio based on 100 parts by weight of the base resin powder.

[0150] On the other hand, in the method for producing a superabsorbent resin according to an embodiment of the present invention, in order to further improve liquid permeability and the like, an aluminum salt such as aluminum sulfate salt and other various polyvalent metal salts may be further used during surface crosslinking. Such a polyvalent metal salt may be contained on the surface crosslinking layer of the finally produced superabsorbent resin.

[0151] On the one hand, the superabsorbent particles produced according to an embodiment of the present invention can have a water content of 3.0 wt% to 10.0 wt%, preferably, can have a water content of 2.5 wt% to 8.5 wt% or 2.5 wt% to 7.5 wt%. Thus, being manufactured to have a high water content, even if the additional pulverization and classification steps are carried out without an additional water addition step or an additive mixing step, the generation of fine powder is significantly reduced, and excellent absorption properties, particularly the absorption rate, can be improved, which is preferable.

[0152] On the one hand, the superabsorbent resin produced according to an embodiment of the present invention can have a particle size of 150 to 850 μm. More specifically, at least 95% by weight or more of the base resin powder and the superabsorbent resin containing the same can have a particle size of 150 to 850 μm, and may contain 50% by weight or more of particles having a particle size of 300 to 600 μm, and the fine powder having a particle size of less than 150 μm may be less than 3% by weight.

[0153] (Additional step) The method for producing a superabsorbent resin according to an embodiment of the present invention may further include a step of pulverizing and classifying the base resin powder dried before the surface crosslinking step, if necessary.

[0154] Specifically, the pulverizing step may be performed by pulverizing the base resin powder to have a particle size at a normal particle level, that is, a particle size of 150 μm to 850 μm.

[0155] The pulverizer used for this purpose may specifically be a vertical pulverizer, a turbo cutter, a turbo grinder, a rotary cutter mill, a cutter mill, a disc mill, a shred crusher, a crusher, a chopper, or a disc cutter, etc., and is not limited to the examples described above.

[0156] Alternatively, as the pulverizer, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, etc. may be used, but it is not limited to the examples described above.

[0157] Also, according to an embodiment of the present invention, before or after the step of forming a surface crosslinked layer on at least a part of the surface of the base resin powder as necessary, a cooling step of cooling the superabsorbent resin particles, a water addition step of adding water to the superabsorbent resin particles, and a post-treatment step of adding an additive to the superabsorbent resin particles may be further included and performed. At this time, the cooling step, the water addition step, and the post-treatment step may be performed sequentially or simultaneously.

[0158] Examples of the additive added in the post-treatment step include a liquid permeability improver, an anti-caking agent, a fluidity improver, and an antioxidant, etc., but the present invention is not limited thereto.

[0159] By selectively performing the cooling step, the water addition step, and the post-treatment step, the water content of the final superabsorbent resin can be improved, and a higher quality superabsorbent resin product can be manufactured.

[0160] (Superabsorbent resin) According to another embodiment of the present invention, a superabsorbent resin produced by the above manufacturing method is provided.

[0161] The superabsorbent resin produced by the manufacturing method can achieve a high water content without any separate additional water addition process or additive addition process, resulting in a low fine powder content. The water retention capacity (CRC) and the pressure absorption capacity (AUP), which are absorption physical properties of the superabsorbent resin produced by the conventional method, are at the same level or higher, and the content of water-soluble components (EC) is low, thereby providing a superabsorbent resin with excellent liquid permeability, rewet characteristics, absorption rate, and the like.

[0162] Hereinafter, the operation and effects of the invention will be described in more detail through specific examples of the invention. However, such examples are merely presented as illustrations of the invention and do not determine the scope of the invention's rights.

[0163] [Examples and Comparative Examples] Example 1 (Step 1) 329.94 g of acrylic acid, 0.35 g of pentaerythritol triallyl ether as an internal crosslinking agent, and 725 g of water were stirred and mixed in a 2 L glass container equipped with a stirrer and a thermometer, and stirred while maintaining at 5°C. 1000 cc / min of nitrogen was introduced into the glass container containing the mixture for 1 hour to replace it with a nitrogen condition. Next, 4.42 g of a 0.3% hydrogen peroxide aqueous solution, 4.94 g of a 1% ascorbic acid aqueous solution, and 9.90 g of a 2% 2,2'-azobis-(2-amidinopropane) dihydrochloride aqueous solution were added as polymerization initiators, and at the same time, 4.90 g of a 0.01% iron sulfate aqueous solution was added as a reducing agent to start polymerization. After the temperature of the mixture reached 85°C, polymerization was carried out at 90 ± 2°C for about 3 hours to obtain a polymer.

[0164] (Step 2) 1000 g of the obtained polymer was passed through a granulating device equipped with a perforated plate having a large number of holes with a hole size of 6 mm four times to be granulated.

[0165] In the first fine granulation step, no separate additive was added. In the second fine granulation step, 400 g of a 32% NaOH aqueous solution was added, and granulation was carried out while neutralizing a part of the acidic groups of the polymer. In the third fine granulation step, 37.5 g of a 15% Na2SO4 aqueous solution was added, and granulation was carried out while neutralizing a part of the acidic groups of the polymer. In the fourth fine granulation step, 4 g of Glycerol Monolaurate was added in aqueous solution form.

[0166] (Step 3) Next, 1,000 g of the granulated mixture was charged into a rotary kiln fluidized dryer rotating at 120 rpm. Drying was carried out for 60 minutes while maintaining the internal temperature of the dryer at 105 °C to obtain resin powder. The obtained powder was pulverized using a two-roll mill (roll mill (GRAN-U-LIZER TM , MPE)) to obtain particles having a particle size of 150 μm to 850 μm. Only the base resin particles having a particle size of 150 μm to 850 μm were selectively recovered from the pulverized material using a classifier.

[0167] The water content of the base resin powder was 13.6 wt%.

[0168] (Step 4) Next, 8 g of water, 5 g of methanol, 0.08 g of ethylene glycol diglycidyl ether (EJ-1030S), 0.1 g of GK (polycarboxylate), 0.4 g of Als (aluminum sulfate), and 0.04 g of A200 (hydrophilic silica) were added to 100 g of the obtained base resin powder to produce a surface crosslinking liquid, which was mixed, and a surface crosslinking reaction was carried out at 100 °C for 50 minutes to produce a superabsorbent resin containing surface crosslinked superabsorbent resin particles.

[0169] Example 2 A superabsorbent resin was produced in the same manner as in Example 1, except that in Example 1, the fluidized drying conditions in Step 3 were carried out at 150 °C for 30 minutes, and the water content of the base resin powder was 12.8 wt%.

[0170] Example 3 A superabsorbent resin was produced in the same manner as in Example 1, except that in Example 1, the fluidized drying conditions in Step 3 were carried out at 180°C for 30 minutes, and the water content of the base resin powder was 11.2 wt%.

[0171] Comparative Example 1 (Step 1) 329.94 g of acrylic acid, 0.35 g of pentaerythritol triallyl ether as an internal crosslinking agent, and 725 g of water were stirred and mixed in a 2 L glass container equipped with a stirrer and a thermometer, and stirred while maintaining at 5°C. Nitrogen was introduced into the glass container containing the mixture at a rate of 1000 cc / min for 1 hour to replace it with a nitrogen atmosphere. Next, 4.42 g of a 0.3% aqueous hydrogen peroxide solution, 4.94 g of a 1% aqueous ascorbic acid solution, and 9.90 g of a 2% aqueous 2,2'-azobis-(2-amidinopropane) dihydrochloride solution were added as polymerization initiators, and at the same time, 4.90 g of a 0.01% aqueous iron sulfate solution was added as a reducing agent to initiate polymerization. After the temperature of the mixture reached 85°C, polymerization was carried out at 90 ± 2°C for about 3 hours to obtain a polymer.

[0172] (Step 2) 1000 g of the obtained polymer was passed through a granulating device equipped with a perforated plate having a large number of holes with a hole size of 6 mm four times to be granulated. No additional additives were added in the primary granulation step. In the secondary granulation step, 400 g of a 32% aqueous NaOH solution was added while granulating to neutralize a part of the acidic groups of the polymer. In the tertiary granulation step, 37.5 g of a 15% aqueous Na2SO4 solution was added while granulating to neutralize a part of the acidic groups of the polymer. No additional additives were added in the quaternary granulation step.

[0173] (Step 3) Next, 1000 g of the granulated mixture was put into an Air flow oven static dryer capable of transferring air flow up and down. While hot air at 190°C was introduced into the dryer up and down, drying was carried out for 40 minutes to obtain resin powder. The obtained powder was put into a two-roll mill (roll mill, (GRAN-U-LIZERTM Using MPE), it was pulverized to obtain particles having a particle size of 150 μm to 850 μm. Only the base resin particles having a particle size of 150 μm to 850 μm were selectively recovered from the pulverized product using a classifier.

[0174] The water content of the base resin powder was 2.3 wt%.

[0175] (Step 4) Next, 8 g of water, 5 g of methanol, 0.08 g of ethylene glycol diglycidyl ether (EJ-1030S), 0.1 g of GK (polycarboxylate), 0.4 g of Als (aluminum sulfate), and 0.04 g of A200 (hydrophilic silica) were added to 100 g of the obtained base resin powder to produce a surface crosslinking liquid, which was then mixed. A surface crosslinking reaction was carried out at 100 °C for 50 minutes to produce a superabsorbent resin containing surface-crosslinked superabsorbent resin particles.

[0176] Comparative Example 2 In Comparative Example 1, after the drying step in Step 3, 3 g of water was added to 100 g of the superabsorbent resin particles and uniformly mixed to perform a hydration step. Then, surface crosslinking was carried out in the same manner as in Comparative Example 1 to produce a superabsorbent resin containing the final superabsorbent resin particles.

[0177] Comparative Example 3 In Comparative Example 1, after the drying step in Step 3, 4 g of water, 0.05 g of PEG (polyethylene glycol molecular weight: 6000), and 0.05 g of Als (aluminum sulfate) were added to 100 g of the superabsorbent resin particles and uniformly mixed to perform a hydration step. Then, surface crosslinking was carried out in the same manner as in Comparative Example 1 to produce a superabsorbent resin containing the final superabsorbent resin particles.

[0178] Comparative Example 4 In Example 1, a superabsorbent resin was produced in the same manner as in Example 1, except that the fluidized drying conditions in Step 3 were carried out at 120 °C for 10 minutes, and the water content of the base resin powder was 31.7 wt%.

[0179] [Experimental Example] For the superabsorbent polymers produced in the above Examples and Comparative Examples, the physical properties were evaluated by the following methods, and the results are shown in Table 1.

[0180] Unless otherwise stated, all of the following physical property evaluations were carried out under constant temperature and humidity (23 ± 1 °C, relative humidity 50 ± 10 %), and physiological saline or saline means an aqueous solution of 0.9 wt% sodium chloride (NaCl).

[0181] (1) Moisture Content Evaluation The water content is the content of water in the total weight of the superabsorbent polymer, and was calculated by the following mathematical formula 1.

[0182] Specifically, in the process of drying by raising the temperature of the superabsorbent polymer through infrared heating, the weight loss due to water evaporation in the superabsorbent polymer was measured and calculated. At this time, the drying condition is a method of raising the temperature to 180 °C at room temperature and then maintaining it at 180 °C, and the total drying time was set to 40 minutes including a 5-minute temperature rise step. The weights of the superabsorbent polymer before and after drying were measured respectively, and calculated by the following mathematical formula 1.

[0183] [Mathematical formula 1] Water content (wt%) = [(Ao - At) / Ao] × 100

[0184] In the above formula, At is the weight of the superabsorbent polymer after drying, and Ao is the weight of the superabsorbent polymer before drying.

[0185] (2) Particle Distribution The superabsorbent polymers produced in the Examples and Comparative Examples were classified using standard sieves having scales of 850 μm (20 mesh), 600 μm (30 mesh), 300 μm (50 mesh), and 150 μm (100 mesh) according to ASTM standards. After measuring the weight of coarse particles having a size exceeding 850 μm, the content of the coarse particles was expressed as a percentage based on the total weight of the superabsorbent polymer particle sample (wt%).

[0186] Further, the average particle size was calculated by multiplying the percentage by the average particle size.

[0187] (3) Centrifuge Retention Capacity (CRC) For the superabsorbent resins produced in the examples and comparative examples, samples having a particle size of 150 to 850 μm among each superabsorbent resin were taken, and the centrifugal retention capacity (CRC) by the absorption magnification under no load was measured according to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP 241.3.

[0188] Specifically, resins classified by a #30 - 50 sieve were obtained from the resins respectively obtained through the examples and comparative examples. Such resin W0 (g) (about 0.2 g) was uniformly placed in a non-woven envelope and sealed, and then immersed in physiological saline (0.9% by weight) at room temperature. After 3 minutes, moisture was removed from the envelope for 3 minutes under the condition of 250G using a centrifuge, and the mass W2 (g) of the envelope was measured. Further, after performing the same operation without using the resin, the mass W1 (g) at that time was measured. The CRC (g / g) was calculated by the following formula using each obtained mass.

[0189] [Mathematical formula 2] CRC (g / g) = {[W2 (g) - W1 (g)] / W0 (g)} - 1

[0190] (4) Absorbency Under Pressure (AUP) For the superabsorbent resins produced in the examples and comparative examples, the pressure absorption capacity of 0.3 psi of each superabsorbent resin was measured according to the EDANA method NWSP 242.3. When measuring the pressure absorption capacity, the resin classification fraction at the time of the CRC measurement was used.

[0191] Specifically, a 400-mesh stainless steel wire mesh was attached to the bottom of a plastic cylinder with an inner diameter of 25 mm. Under the conditions of normal temperature and 50% humidity, 0 (g) (0.16 g) of the water-absorbing resin was uniformly sprayed onto the wire mesh, and then a piston capable of uniformly applying a load of 0.3 psi was placed thereon. The piston had an outer diameter slightly smaller than 25 mm, had no gap with the inner wall of the cylinder, and was configured such that its vertical movement was not obstructed. At this time, the weight W3 (g) of the said device was measured.

[0192] A glass filter with a diameter of 90 mm and a thickness of 5 mm was placed inside a Petri dish with a diameter of 150 mm, and physiological saline composed of 0.9 wt% sodium chloride was made to be at the same level as the upper surface of the glass filter. One sheet of filter paper with a diameter of 90 mm was placed thereon. The said measuring device was placed on the filter paper, and the liquid was absorbed under a load for 1 hour. After 1 hour, the measuring device was lifted, and its weight W4 (g) was measured.

[0193] Using each of the obtained masses, the pressure absorption capacity (g / g) was calculated by the following formula.

[0194] [Mathematical formula 3] AUP (g / g) = [W4 (g) - W3 (g)] / W0 (g)

[0195] The said measurement was repeated 5 times, and the average value and standard deviation were obtained.

[0196] (5) Vortex Absorption Rate The absorption rate (vortex time) was measured in seconds according to the method described in WO 1987 / 003208. When measuring the absorption rate, the resin obtained after the surface crosslinking was used without classification.

[0197] Specifically, 2 g of each resin was put into 50 mL of physiological saline at 23 °C, a magnetic bar (diameter 8 mm, length 30 mm) was stirred at 600 rpm, and the time until the vortex disappeared was measured and calculated in seconds.

[0198]

Table 1

[0199] As can be confirmed from the data in Table 1 above, the present invention controls the process conditions of the drying step, so that the finally produced superabsorbent resin is produced at a high moisture content, suppressing the generation of fine powder during the manufacturing process and realizing an excellent absorption rate.

Claims

1. In the presence of an internal crosslinking agent and a polymerization initiator, crosslinking polymerizing a water-soluble ethylenically unsaturated monomer having an acidic group to form a hydrogel polymer (Step 1-1); Neutralizing at least a part of the acidic groups of the polymer to form a hydrogel polymer (Step 1-2); In the presence of a surfactant, granulating the hydrogel polymer to produce a mixture containing the granulated hydrogel polymer (Step 2); Drying the mixture at 100°C to 250°C in a moving type to form a base resin powder having a water content of 10% by weight to 30% by weight (Step 3); In the presence of a surface crosslinking agent, thermally crosslinking the surface of the base resin powder to produce superabsorbent resin particles (Step 4), including The step of neutralizing at least a part of the acidic groups of the polymer (Step 1-2) and the step of granulating the hydrogel polymer in the presence of a surfactant to produce a mixture containing the granulated hydrogel polymer (Step 2) are carried out sequentially, alternately, or simultaneously, The surfactant is one or more selected from the group consisting of a compound represented by the following Chemical Formula 1 and its salts, a method for producing a superabsorbent resin. 【Chemical 1】 In Chemical Formula 1 above, A1, A2, and A3 are each independently a single bond, a carbonyl, [Chemical Formula 2] wherein at least one of these is a carbonyl or 【Chemical Formula 3】 wherein m1, m2, and m3 are each independently an integer from 1 to 8, 【Chemical 4】 is each linked to an adjacent oxygen atom, 【Chemical Formula 5】 is linked to adjacent R1, R2, and R3 respectively, R1, R2, and R3 are each independently hydrogen, a linear or branched alkyl having 6 to 18 carbon atoms, or a linear or branched alkenyl having 6 to 18 carbon atoms, n is an integer from 1 to 9.

2. The step of drying in the moving type (Step 3) is Performed by charging the mixture into a moving dryer rotating at a speed of 30 rpm to 300 rpm, The method for producing a superabsorbent resin according to Claim 1.

3. The step of drying in the moving type (Step 3) is The method for producing a superabsorbent resin according to claim 1, which is carried out using a fluidized dryer such as a horizontal-type mixer, a rotary kiln, a paddle dryer, or a steam tube dryer. The method for producing a superabsorbent resin according to claim 1.

4. The step of drying in a fluidized manner (step 3) is carried out for 30 minutes to 120 minutes. The method for producing a superabsorbent resin according to claim 1.

5. The step of granulating the water-containing gel polymer (step 2) is carried out by extruding the water-containing gel polymer through a perforated plate having a large number of holes formed therein. The method for producing a superabsorbent resin according to claim 1.

6. The size of the holes formed in the perforated plate is 0.1 mm to 30 mm. The method for producing a superabsorbent resin according to claim 5.

7. At least a part of the surfactant is present on the surface of the water-containing gel polymer. The method for producing a superabsorbent resin according to claim 1.

8. The surface crosslinking step (step 4) is carried out at 80°C to 120°C. The method for producing a superabsorbent resin according to claim 1.

9. The surface crosslinking step (step 4) is carried out for 30 minutes to 120 minutes. The method for producing a superabsorbent resin according to claim 1.

10. The water content of the superabsorbent resin particles is 3.0 wt% to 10.0 wt%. The method for producing a superabsorbent resin according to claim 1.

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