Method for producing superabsorbent resin and superabsorbent resin

Abstract

The present invention relates to a method for producing a superabsorbent polymer, more specifically, the method for producing a superabsorbent polymer of the present invention can effectively control the initiation and inhibition of a polymerization reaction to reduce the content of unreacted monomer in the final product.

Description

Technical Field

[0001] The present invention relates to a method for producing a superabsorbent resin. More specifically, it relates to a method for producing a superabsorbent resin that can significantly reduce the generation of unreacted monomers in the product.

Background Art

[0002] A superabsorbent resin (Super Absorbent Polymer, SAP) is a synthetic polymer material that has the function of absorbing about 500 to 1000 times its own weight of water, and is named differently such as SAM (Super Absorbency Material) and AGM (Absorbent Gel Material) by each developing company. Such superabsorbent resins began to be put into practical use in sanitary products and are now widely used as materials such as soil moisturizers for gardening, waterstops for civil engineering and construction, seedling-raising sheets, freshness retainers in the food distribution field, and materials for compresses.

[0003] Such superabsorbent resins are mainly widely used in the field of sanitary materials such as diapers and sanitary napkins. In the sanitary materials, the superabsorbent resin is generally contained in a state of being diffused in pulp. However, in recent years, efforts have continued to provide sanitary materials such as thinner diapers, and as part of this, the pulp content has been reduced, or even the development of so-called pulpless diapers in which no pulp is used has been actively progressing.

[0004] Thus, in the case of sanitary materials with a reduced pulp content or no pulp used, the superabsorbent resin is relatively contained at a high ratio, and the superabsorbent resin particles are unavoidably contained in multiple layers in the sanitary materials. In order for the overall superabsorbent resin particles contained in multiple layers to more efficiently absorb a large amount of liquid such as urine, the superabsorbent resin basically needs to have not only high absorption performance but also a fast absorption rate.

[0005] Such superabsorbent polymers are produced by a method of drying, pulverizing, and classifying a water-containing gel polymer produced by crosslinking and polymerizing a monomer containing a water-soluble ethylenically unsaturated carboxylic acid or a salt thereof, or by surface-crosslinking this again.

[0006] When crosslinking and polymerizing the above-mentioned monomer, an appropriate type of polymerization initiator or polymerization inhibitor is used, and the degree of progress of the polymerization reaction is adjusted through reaction process conditions, etc. In particular, for polymerization activation, methods such as removing dissolved oxygen present in the monomer mixture before introducing it into the polymerization reactor are known.

[0007] Also, due to the excellent physical properties of the finally produced superabsorbent polymer, it is necessary to accurately control the start and suppression of the reaction, etc. Due to the characteristics of the radical reaction, this is very difficult, and there is a problem that the physical properties of the superabsorbent polymer are deteriorated depending on the amount of the polymerization initiator or polymerization inhibitor used.

[0008] In particular, when the initiator and the monomer meet, the polymerization reaction starts immediately, polymerization starts in transfer lines such as pipes that are not polymerization reactors, making continuous operation difficult, and there is a possibility that the content of unreacted monomer in the final product increases.

[0009] Therefore, research is needed to efficiently control the start and suppression of the polymerization reaction.

Summary of the Invention

Problems to be Solved by the Invention

[0010] The purpose of this specification is to provide a method for producing a superabsorbent polymer that can efficiently control the start and suppression of the polymerization reaction when performing polymerization.

Means for Solving the Problems

[0011] This specification involves performing polymerization on a monomer composition containing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator, to form a polymer in which the water-soluble ethylenically unsaturated monomer having the acidic group and the internal crosslinking agent are crosslinked and polymerized (Step 1); neutralizing at least a part of the acidic groups of the polymer to form a hydrogel polymer (Step 2); granulating the hydrogel polymer in the presence of a surfactant (Step 3); and drying the neutralized and granulated polymer to produce dried superabsorbent resin particles (Step 4). In the step of forming the polymer, a first monomer composition containing the monomer and the internal crosslinking agent is transferred via a monomer transfer line, and the polymerization initiator is transferred via an initiator transfer line. Immediately before being introduced into the polymerization reactor, the monomer transfer line and the initiator transfer line are combined so that the first monomer composition and the initiator are mixed to form a second monomer composition, with the aim of providing a method for producing a superabsorbent resin.

[0012] This specification also provides a superabsorbent resin produced by the method for producing a superabsorbent resin described above.

Advantages of the Invention

[0013] According to the method for producing a superabsorbent resin of the present invention, the start and suppression of the polymerization reaction can be efficiently controlled, and the content of unreacted monomers in the final product can be reduced.

Embodiments for Carrying out the Invention

[0014] The terms used in this specification are merely used to illustrate exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates a different meaning. In this specification, terms such as "including", "comprising", or "having" are used to specify the presence of implemented features, steps, components, or combinations thereof, and it should be understood that they do not preclude the possibility of the presence or addition of one or more other features, steps, components, or combinations thereof in advance.

[0015] The present invention can be subjected to various modifications and can have various forms. Specific examples will be illustrated and described in detail below. However, this is not intended to limit the present invention to a specific disclosed form, and it should be understood to include all modifications, equivalents, or alternatives included in the spirit and technical scope of the present invention.

[0016] Hereinafter, the method for producing a superabsorbent resin and the superabsorbent resin will be described in more detail with reference to specific embodiments of the invention.

[0017] Prior to that, the technical terms used in this specification are for merely referring to specific embodiments and are not intended to limit the present invention. And the singular forms used herein include the plural forms as well, unless the context clearly indicates the contrary meaning.

[0018] According to an embodiment of the present invention, polymerization is carried out on a monomer composition containing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having the acidic group and the internal crosslinking agent are crosslinked and polymerized (step 1); neutralizing at least a part of the acidic groups of the polymer to form a hydrogel polymer (step 2); granulating the hydrogel polymer in the presence of a surfactant (step 3); and drying the neutralized and granulated polymer to produce dried superabsorbent resin particles (step 4). In the step of forming the polymer, the first monomer composition containing the monomer and the internal crosslinking agent is transferred via a monomer transfer line, and the polymerization initiator is transferred via an initiator transfer line. Immediately before being introduced into the polymerization reactor, the monomer transfer line and the initiator transfer line are combined, and the first monomer composition and the initiator are mixed to form a second monomer composition. A method for producing a superabsorbent resin is provided.

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

[0020] Also, the term "superabsorbent resin" means, depending on the context, a crosslinked polymer or a powder base resin composed of superabsorbent resin particles obtained by pulverizing the crosslinked polymer, or includes all products that have been made suitable for commercialization through additional processes such as drying, pulverization, classification, surface crosslinking, etc. for the crosslinked polymer and the base resin.

[0021] Also, the term "fine powder" 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.

[0022] Also, the term "chopping" means cutting the water-containing gel polymer into small pieces in millimeters to increase the drying efficiency, and is used separately from pulverizing to the normal particle level.

[0023] Also, the term "micronizing, micronization" means pulverizing the water-containing gel polymer to a particle size of several tens to several hundreds of micrometers, and is used separately from "chopping".

[0024] Conventionally, superabsorbent resins have been manufactured including the following steps. (Polymerization) A step of forming a water-containing gel polymer by crosslinking polymerization of a water-soluble ethylenically unsaturated monomer having at least partially neutralized acidic groups in the presence of an internal crosslinking agent and a polymerization initiator; (Chopping) A step of chopping the water-containing gel polymer; (Drying) A step of drying the chopped water-containing gel polymer; and (Pulverization / Classification) A step of classifying the dried polymer into normal particles and fine powder after pulverization.

[0025] Among the series of manufacturing processes as described above, in the polymerization stage, the polymerization reaction can proceed as soon as the monomer and the initiator meet. For example, when the monomer and the initiator component meet in the transfer line for supplying each reactant to the reactor, the polymerization reaction may proceed inside the transfer line, which may cause a problem that the transfer line is blocked.

[0026] Therefore, when the present inventor performs batch polymerization in the polymerization stage, the first monomer composition containing the monomer and the internal crosslinking agent is transferred through the monomer transfer line, and the polymerization initiator is transferred through the initiator transfer line, respectively. Immediately before being introduced into the polymerization reactor, the monomer transfer line and the initiator transfer line are combined so that the first monomer composition and the initiator are mixed to form a second monomer composition, thereby focusing on the fact that the problem of the closure of the transfer line can be solved, and the present invention has been completed.

[0027] On the other hand, in order to reduce the adhesiveness of the water-containing gel polymer in the chopping process, a method of adding a surfactant has been proposed. However, when a surfactant is added in the chopping process, due to the high water content of the water-containing gel polymer, the surfactant penetrates into the interior of the water-containing gel polymer rather than existing at the interface of the water-containing gel polymer, and there is a problem that the surfactant cannot perform its role correctly.

[0028] This is because the chopped particles form particles at the level of several millimeters or several centimeters compared to the polymer before chopping, so the surface area can be increased to a certain extent, but it is difficult to expect an effect that effectively improves the absorption rate. In order to improve the absorption rate, a method of increasing the surface area by increasing the mechanical force and kneading in the chopping stage can be considered. In this case, due to the adhesiveness peculiar to the polymer, excessive aggregation occurs, and after chopping, drying and pulverization, only the particle surface forms amorphous single particles with unevenness, and excessive kneading or pulverization may increase the water-soluble components instead.

[0029] As a result of repeated research to solve this problem, instead of polymerizing in a state where the acidic groups of water-soluble ethylenically unsaturated monomers are neutralized as in the usual method for producing superabsorbent resins, polymerization is first carried out in a state where the acidic groups are not neutralized to form a polymer. After granulating the hydrogel polymer in the presence of a surfactant, the acidic groups of the polymer are neutralized, or after neutralizing the acidic groups of the polymer to form a hydrogel polymer, the hydrogel polymer is granulated in the presence of a surfactant, or the acidic groups present in the polymer are neutralized simultaneously with granulation. It was confirmed that a large amount of surfactant is present on the surface of the polymer, which can sufficiently play a role in reducing the high adhesiveness of the polymer and preventing the polymer from aggregating excessively, and adjusting the aggregation state to a desired level.

[0030] Thereby, the polymer is produced as secondary particles in a form in which primary particles are aggregated. Then, as the grinding and drying processes proceed under milder conditions, the amount of fine powder generated during the process can be significantly reduced.

[0031] Also, when the polymer is granulated in the presence of the surfactant, the hydrophobic functional group portion contained in the surfactant imparts hydrophobicity to the surface of the ground superabsorbent resin particles, relaxing the frictional force between the particles, increasing the apparent density of the superabsorbent resin, and at the same time, the hydrophilic functional group portion contained in the surfactant can also bind to the superabsorbent resin particles so that the surface tension of the resin does not decrease. Thereby, the superabsorbent resin produced by the above-described production method can have a higher apparent density value while showing the same level of surface tension as the resin without using a surfactant.

[0032] Also, when polymerization is first carried out in an unneutralized state to form a polymer and then the acidic groups present in the polymer are neutralized, it is possible to form a polymer with a longer chain and achieve the effect of reducing the content of water-soluble components present in an incompletely cross-linked and uncross-linked state.

[0033] Since the water-soluble component has the property of being easily eluted when the superabsorbent resin comes into contact with a liquid, when the content of the water-soluble component is high, the eluted water-soluble component will remain on the surface of most of the superabsorbent resin, making the superabsorbent resin sticky and causing a decrease in liquid permeability. Therefore, it is important to maintain a low content of the water-soluble component from the aspect of liquid permeability.

[0034] According to one embodiment of the present invention, by performing polymerization in an unneutralized state, the content of the water-soluble component can be reduced, thereby improving the liquid permeability of the superabsorbent resin.

[0035] In addition, the superabsorbent resin produced according to one embodiment of the present invention can have a uniform particle size distribution, thereby providing a superabsorbent resin excellent in various absorption physical properties such as water retention ability and pressure absorption ability, rewetting characteristics, and absorption rate.

[0036] Hereinafter, the manufacturing method of the superabsorbent resin of one embodiment will be specifically described step by step.

[0037] Step 1: Overlay step First, polymerization is carried out on a monomer composition containing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having the acidic group and the internal crosslinking agent are crosslinked and polymerized.

[0038] The above step can consist of a step of preparing a monomer composition by mixing the water-soluble ethylenically unsaturated monomer having the acidic group, the internal crosslinking agent, and the polymerization initiator, and a step of polymerizing the monomer composition to form a polymer.

[0039] And the step of forming the polymer is carried out by continuous batch polymerization.

[0040] The water-soluble ethylenically unsaturated monomer may be any monomer commonly used in the production of superabsorbent resins. As a non-limiting example, the water-soluble ethylenically unsaturated monomer can be a compound represented by the following Chemical Formula 1:

[0041] [Chemical Formula 1] R-COOM’

[0042] In Chemical Formula 1 above, R is an alkyl group having 2 to 5 carbon atoms containing an unsaturated bond, M’ is a hydrogen atom, a monovalent or divalent metal, an ammonium group, or an organic amine salt.

[0043] Preferably, the monomer can be at least one selected from the group consisting of (meth)acrylic acid, and monovalent (alkali) metal salts, divalent metal salts, ammonium salts, and organic amine salts of these acids.

[0044] When (meth)acrylic acid and / or its salts are used as the water-soluble ethylenically unsaturated monomer in this way, a superabsorbent resin with improved water absorption can be obtained, which is advantageous. In addition, as the monomer, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, 2-acryloylethane sulfonic acid, 2-methacryloylethane sulfonic acid, 2-(meth)acryloylpropane sulfonic acid or 2-(meth)acrylamide-2-methylpropane sulfonic acid, (meth)acrylamide, N-substituted (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol (meth)acrylate, (N,N)-dimethylaminoethyl (meth)acrylate, (N,N)-dimethylaminopropyl (meth)acrylamide, etc. can be used.

[0045] Here, the water-soluble ethylenically unsaturated monomer has an acidic group. As described above, in the production of conventional superabsorbent resins, at least a part of the acidic groups are neutralized by a neutralizing agent and then crosslinked and polymerized to form a hydrogel polymer. Specifically, at the stage of mixing the water-soluble ethylenically unsaturated monomer having the acidic group, an internal crosslinking agent, a polymerization initiator, and a neutralizing agent, at least a part of the acidic groups of the water-soluble ethylenically unsaturated monomer are neutralized.

[0046] However, according to one embodiment of the present invention, polymerization is first carried out in a state where the acidic groups of the water-soluble ethylenically unsaturated monomer are not neutralized to form a polymer.

[0047] The water-soluble ethylenically unsaturated monomer (e.g., acrylic acid) in a state where the acidic groups are not neutralized is in a liquid state at room temperature, highly miscible with a solvent (water), and exists in a monomer composition as a mixed solution. However, the water-soluble ethylenically unsaturated monomer with neutralized acidic groups is in a solid state at room temperature, has different solubilities depending on the temperature of the solvent (water), and the solubility becomes lower as the temperature is lower.

[0048] Thus, the water-soluble ethylenically unsaturated monomer in a state where the acidic groups are not neutralized has a higher solubility or miscibility with respect to the solvent (water) than the monomer with neutralized acidic groups and is not precipitated even at a low temperature, which is advantageous for polymerization at a low temperature for a long time. Thereby, polymerization can be carried out for a long time using the water-soluble ethylenically unsaturated monomer in a state where the acidic groups are not neutralized, and a polymer having a higher molecular weight and a uniform molecular weight distribution can be stably formed.

[0049] In addition, it is possible to form a polymer with a longer chain, and the effect of reducing the content of the water-soluble component that exists in an incompletely polymerized and crosslinked state with incomplete polymerization and crosslinking can be achieved.

[0050] Further, when polymerization is first carried out to form a polymer with the acidic groups of the monomers not neutralized in this way, followed by granulation in the presence of a surfactant after neutralization, or neutralization after granulation in the presence of a surfactant, or neutralizing the acidic groups present in the polymer simultaneously with granulation, a large amount of the surfactant will be present on the surface of the polymer, and it can fully play a role in reducing the adhesiveness of the polymer.

[0051] The concentration of the water-soluble ethylenically unsaturated monomer in the monomer composition can be appropriately adjusted in consideration of the polymerization time, reaction conditions, etc., and can be about 20 to about 60% by weight, or about 20 to about 40% by weight.

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

[0053] Crosslinking at this stage is carried out without distinction between the surface and the interior. 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 structure newly crosslinked by the surface crosslinking agent, and the interior of the superabsorbent resin particles can maintain the structure crosslinked by the internal crosslinking agent as it is.

[0054] According to one embodiment of the present invention, the internal crosslinking agent can include any one or more of polyfunctional acrylate compounds, polyfunctional allyl compounds, or 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. These can be used alone or in a mixture of two or more.

[0056] Non-limiting examples of polyfunctional allyl compounds 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, etc. They can be used alone or in combination of two or more.

[0057] Non-limiting examples of polyfunctional vinyl compounds 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, etc. These can be used alone or in combination of two or more. Preferably, pentaerythritol triallyl ether can be used.

[0058] The above-mentioned polyfunctional allyl compounds or polyfunctional vinyl compounds can each bind the two or more unsaturated groups contained in the molecule to the unsaturated bond of the water-soluble ethylene-based unsaturated monomer or the unsaturated bond of other internal cross-linking agents, and form a cross-linked structure during the polymerization process. Different from acrylate compounds containing an ester bond (-(C=O)O-) in the molecule, the cross-linkage can be more stably maintained even during the neutralization process after the above-mentioned polymerization reaction.

[0059] Thereby, the gel strength of the produced superabsorbent resin becomes high, the process stability during the discharge process after polymerization becomes high, and the amount of water-soluble components can be minimized.

[0060] The crosslinking polymerization of the water-soluble ethylenically unsaturated monomer in the presence of such an internal crosslinking agent can be carried out in the presence of a polymerization initiator, a thickener, a plasticizer, a storage stabilizer, an antioxidant, etc. as required.

[0061] In the monomer composition, such an internal crosslinking agent can be used in an amount of 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 can be used in an amount of 0.01 part by weight or more, or 0.05 part by weight or more, or 0.1 part by weight or more, and 5 parts by weight or less, or 3 parts by weight or less, or 2 parts by weight or less, or 1 part by weight or less, or 0.7 part by weight or less based on 100 parts by weight of the water-soluble ethylenically unsaturated monomer. When the content of the upper internal crosslinking agent is excessively low, crosslinking is not sufficiently carried out, and it is difficult to achieve a strength above an appropriate level. When the content of the upper internal crosslinking agent is excessively high, the internal crosslinking density becomes high, and it is 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 polymerizing the water-soluble ethylenically unsaturated monomer is crosslinked by the internal crosslinking agent. Thus, when the polymer has a three-dimensional network structure, the water retention ability and the pressure absorption ability, which are various physical properties of the superabsorbent resin, can be significantly improved as compared with the case where the polymer has a two-dimensional linear structure that is not additionally crosslinked by the internal crosslinking agent.

[0063] According to one embodiment of the present invention, the step of polymerizing the monomer composition to form a polymer can be carried out in a batch type reactor.

[0064] In the production method of a normal superabsorbent resin, the polymerization method is roughly divided into thermal polymerization and photopolymerization by a polymerization energy source. Usually, when thermal polymerization is carried out, it can be carried out in a reactor having a stirring shaft such as a kneader. When photopolymerization is carried out, it can be carried out in a reactor equipped with a movable conveyor belt or in a container with a flat bottom.

[0065] On the one hand, in such a polymerization method, since the polymerization reaction time is generally short, about 1 hour or less, the molecular weight of the polymer is not large, and a polymer having a broad molecular weight distribution is formed.

[0066] 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 usually obtained hydrogel polymer is a sheet-like hydrogel 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 is usually obtained with a thickness of about 0.5 to about 5 cm.

[0067] However, when the monomer composition is supplied with the thickness of the sheet-like polymer being excessively thin, the production efficiency is low and not preferable. When the thickness of the sheet-like polymer is increased for productivity, the polymerization reaction is not uniformly carried out over the entire thickness, and it becomes difficult to form a high-quality polymer.

[0068] In addition, in the polymerization in a reactor having a stirrer shaft of the reactor 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 it is difficult to carry out uniform polymerization throughout the monomer composition, and there is a possibility of overall deterioration of physical properties.

[0069] However, according to one embodiment of the present invention, since polymerization is carried out in a batch reactor 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.

[0070] 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, for a time of 6 hours or more, than when the polymerization is carried out continuously in a reactor equipped with a conveyor belt. Despite the long polymerization reaction time as described above, since the polymerization is carried out on the water-soluble ethylenically unsaturated monomer in an unneutralized state, the monomer is not easily precipitated even when the polymerization is carried out for a long time, which is advantageous for carrying out the long-time polymerization.

[0071] On the other hand, in the polymerization in the batch reactor of the present invention, by utilizing a thermal polymerization method, the polymerization initiator uses a thermal polymerization initiator.

[0072] As the thermal polymerization initiator, one or more selected from the group of initiators consisting of persulfate initiators, azo initiators, hydrogen peroxide, and ascorbic acid can be used. Specifically, examples of persulfate initiators include sodium persulfate (Na2S2O8), potassium persulfate (K2S2O8), ammonium persulfate ((NH4)2S2O8), etc. 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), etc. More diverse thermal polymerization initiators are clearly shown on p203 of the book "Principle of Polymerization (Wiley, 1981)" by Odian and are not limited to the above examples.

[0073] When the concentration of the polymerization initiator is excessively low, the polymerization rate becomes slow, and a large amount of residual monomer may be extracted into the final product, which is not preferable. Conversely, when the concentration of the polymerization initiator is excessively high, the polymer chains forming the network become short, the content of water-soluble components increases, and the pressure absorption capacity decreases, etc., and the physical properties of the resin may deteriorate, which is not preferable.

[0074] The amount of the thermal polymerization initiator used may affect the physical properties of the base resin produced through subsequent processes, and particularly affects the water-soluble component content of the base resin. When the water-soluble component content increases, the physical properties of the finally produced superabsorbent resin deteriorate, and particularly the absorption under pressure (AUP) and the liquid permeability become poor. Also, when the amount of the thermal polymerization initiator is too small, the efficiency of the water-containing gel polymerization may decrease, and various physical properties of the finally produced superabsorbent resin may deteriorate.

[0075] Generally, the polymerization initiator described above is used in a form initially contained in a first monomer composition (mixture) containing a water-soluble ethylenically unsaturated monomer and an internal crosslinking agent. According to one aspect of the present invention, the initiator is prepared separately from the first monomer composition described above.

[0076] Specifically, in the step of forming the polymer, the first monomer composition containing the monomer and the internal crosslinking agent is transferred through a monomer transfer line, and the polymerization initiator is transferred through an initiator transfer line. Immediately before being introduced into the polymerization reactor, the monomer transfer line and the initiator transfer line are combined, and the first monomer composition and the initiator are mixed to form a second monomer composition.

[0077] By the method as described above, it is possible to prevent the problem that the polymerization reaction is started in the transfer line through which the polymerization reactant is supplied and the polymerization line is closed.

[0078] And at the stage where the monomer transfer line and the initiator transfer line are combined, the ratio (speed ratio) of the supply speed (m / s) of the initiator supplied from the initiator transfer line to the supply speed (m / s) of the first monomer mixture supplied from the monomer transfer line is preferably about 3.6 or more, or about 4.0 or more, or about 5.0 or more, or about 7.0 or more. The upper limit has no great significance, but it may be about 20 or less, or about 17 or less, or about 15 or less.

[0079] The supply rate as described above, that is, the linear velocity supplied from the transfer line, can be measured by the mass and density supplied per unit time (kg / hr; kg / m 3 ) or volume (m 3 / hr), and calculated using the cross-sectional area of the transfer line.

[0080] That is, it should be noted that the above-mentioned velocity ratio is not the velocity related to the supply amount during supply, but the ratio to the linear velocity in each transfer line.

[0081] When two fluids are mixed adjacent to each other, according to Bernoulli's principle, the pressure on the side of the fluid with a relatively slow velocity (for example, the monomer transfer line) becomes high, and the pressure on the side of the fluid with a relatively fast velocity (for example, the initiator transfer line) becomes low. Due to this pressure difference between the two, the substances contained in each fluid are mixed while diffusing. However, when the velocity range as described above is satisfied, instantaneous rapid diffusion occurs, and the monomer component and the initiator component can be mixed quickly and uniformly.

[0082] Thereby, it is possible to prevent the polymerization reaction from starting in the transfer line. At the same time, the monomer component and the initiator component are uniformly mixed, and the polymerization reaction inside the reactor can also proceed uniformly as a whole, thereby greatly reducing the residual monomer component in the polymer produced.

[0083] And at the stage where the monomer transfer line and the initiator transfer line are combined, the ratio (flow rate ratio) of the supply flow rate (kg / hr) of the initiator supplied from the initiator transfer line to the supply flow rate (kg / hr) of the first monomer mixture supplied from the monomer transfer line can be from about 0.01 to about 0.1.

[0084] On the other hand, in one embodiment of the present invention, polymerization can be started by charging together with a reducing agent that forms a redox couple with the initiator.

[0085] Specifically, when the initiator and the reducing agent are introduced into the polymer solution, they react with each other to form radicals.

[0086] The formed radicals will react with the monomers. Since the oxidation-reduction reaction between the initiator and the reducing agent is highly reactive, polymerization can be initiated even when only a small amount of the initiator and the reducing agent are introduced, without the need to raise the process temperature, enabling low-temperature polymerization and minimizing changes in the physical properties of the polymer solution.

[0087] The polymerization reaction using the oxidation-reduction reaction can occur smoothly even at or below room temperature (25°C). As an example, the polymerization reaction can be carried out at a temperature of 5°C or higher and 25°C or lower, or 5°C or higher and 20°C or lower.

[0088] In one embodiment of the present invention, when a persulfate-based initiator is used as the initiator, the reducing agent can be selected from the group consisting of sodium metabisulfite (Na2S2O5); tetramethylethylenediamine (TMEDA); a mixture of iron(II) sulfate and EDTA (FeSO4 / EDTA); sodium formaldehyde sulfoxylate; and disodium 2-hydroxy-2-sulfinoacetate, and one or more thereof can be used.

[0089] As an example, potassium persulfate can be used as the initiator and disodium 2-hydroxy-2-sulfinoacetate can be used as the reducing agent; ammonium persulfate can be used as the initiator and tetramethylethylenediamine can be used as the reducing agent; or sodium persulfate can be used as the initiator and sodium formaldehyde sulfoxylate can be used as the reducing agent.

[0090] In another embodiment of the present invention, when using a hydrogen peroxide-based initiator as the initiator, the reducing agent can 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-sulfinoacteate, and disodium 2-hydroxy-2-sulfoacteate.

[0091] That is, the second monomer composition further contains a reducing agent, and the reducing agent can be supplied together with the initiator through the initiator transfer line or supplied through a separate reducing agent transfer line.

[0092] And the ratio (speed ratio) of the supply speed (m / s) of the reducing agent to the supply speed (m / s) of the first monomer mixture supplied from the monomer transfer line can be about 3.5 or more, or about 4.0 or more, or about 5.0 or more, or about 6.0 or more, or about 6.5 or more. Although there is no great significance in its upper limit, it can be about 20 or less, or about 17 or less, or about 15 or less.

[0093] The technical significance of the supply speed ratio of the reducing agent is replaced by the explanation of the supply speed ratio of the initiator.

[0094] The monomer composition can further contain additives such as a thickener, a plasticizer, a storage stabilizer, and an antioxidant as needed.

[0095] And the monomer composition containing the monomer can be in a solution state dissolved in a solvent such as water. The solid content in such a solution-state monomer composition, that is, the concentrations of the monomer, the internal cross-linking agent, and the polymerization initiator can be appropriately adjusted in consideration of the polymerization time, reaction conditions, etc. For example, the solid content in the monomer composition can be 10 to 80% by weight, or 15 to 60% by weight, or 30 to 50% by weight.

[0096] At this time, the solvent that can be used can be used without limitation of its composition as long as it can dissolve the above-mentioned components. For example, 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, etc. One or more selected from these can be used in combination.

[0097] The polymer obtained in such a manner can polymerize by utilizing the unneutralized ethylenically unsaturated monomer to form a polymer having a high molecular weight and a uniform molecular weight distribution as described above, and the content of the water-soluble component can be reduced.

[0098] The polymer obtained in such a manner can be in a water-containing gel polymer state with a water content of 30 to 80% by weight. For example, the water content of the polymer can be 30% by weight or more, or 45% by weight or more, or 50% by weight or more, while being 80% by weight or less, or 70% by weight or less, or 60% by weight or less.

[0099] When the water content of the polymer is excessively low, it may be difficult to ensure an appropriate surface area in the subsequent grinding stage and the polymer may not be effectively ground. When the water content of the polymer is excessively high, the pressure applied in the subsequent grinding stage increases and it may be difficult to grind the polymer to the desired particle size.

[0100] On the other hand, throughout this specification, the "water content" refers to the content of water in the total polymer weight, which means the value obtained by subtracting the weight of the polymer in a dry state from the weight of the polymer. Specifically, it is defined as the value calculated by measuring the weight loss due to water evaporation in the polymer during the process of drying by raising the temperature of the polymer in a lump state through infrared heating. At this time, the drying conditions are such that the temperature is raised from room temperature to about 180°C and then maintained at 180°C. The total drying time is set to 40 minutes including 5 minutes for the temperature rise stage, and the water content is measured.

[0101] When a reducing agent is used, the reducing agent can be supplied together with the initiator to the initiator transfer line. At the stage where the monomer transfer line and the initiator transfer line merge, the ratio (speed ratio) of the supply speed of the reducing agent supplied from the initiator transfer line to the supply speed of the first monomer mixture supplied from the monomer transfer line can be 4.0 or more.

[0102] At the stage where the monomer transfer line and the initiator transfer line merge, the ratio (flow rate ratio) of the supply flow rate of the initiator supplied from the initiator transfer line to the supply flow rate of the first monomer mixture supplied from the monomer transfer line can be from about 0.01 to about 0.1.

[0103] And the monomer composition containing the monomer may be in a solution state dissolved in a solvent such as water, for example. The solid content in such a solution-state monomer composition, that is, the concentrations of the monomer, the internal cross-linking agent, and the polymerization initiator can be appropriately adjusted in consideration of the polymerization time and reaction conditions, etc. For example, the solid content in the monomer composition can be 10 to 80% by weight, or 15 to 60% by weight, or 30 to 50% by weight.

[0104] At this time, the solvent that can be used can be used without limitation of its composition as long as it can dissolve the above-mentioned components. For example, 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, and N,N-dimethylacetamide, etc., one or more selected from these can be used in combination.

[0105] The monomer composition can further contain additives such as a thickener, a reducing agent, a plasticizer, a storage stabilizer, and an antioxidant as needed.

[0106] Step 2: Neutralization step and Step 3: Fine-graining step Next, a step (step 2) of neutralizing at least a part of the acidic groups of the polymer is performed.

[0107] At this time, as the neutralizing agent, basic substances such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide that can neutralize acidic groups can be used.

[0108] Among the acidic groups contained in the polymer, the degree of neutralization, which refers to the degree of neutralization by the neutralizing agent, can be 50 to 90 mol%, or 60 to 85 mol%, or 65 to 85 mol%, or 65 to 75 mol%. The range of the degree of neutralization varies depending on the final physical properties. If the degree of neutralization is excessively high, the absorption capacity of the superabsorbent resin may decrease, the concentration of carboxyl groups on the particle surface may be excessively low, and surface crosslinking in subsequent processes may not be appropriately carried out, resulting in a decrease in absorption characteristics under pressure or liquid permeability. Conversely, if the degree of neutralization is excessively low, not only does the absorption power of the polymer significantly decrease, but it may also exhibit properties similar to those of elastic rubber, making it difficult to handle.

[0109] Simultaneously with the two steps, or before or after the completion of the two steps, a step of granulating the polymer is performed in the presence of a surfactant (step 3).

[0110] The step is a step of granulating the polymer in the presence of a surfactant. Instead of chopping the polymer into millimeter-sized pieces, the polymer is simultaneously shredded and aggregated into a size of several tens to several hundreds of micrometers. That is, by imparting appropriate adhesiveness to the polymer, secondary aggregated particles in the shape of aggregated primary particles shredded into a size of several tens to several hundreds of micrometers are produced. The water-containing superabsorbent resin particles, which are secondary aggregated particles produced in such a step, have a normal particle size distribution, a significantly increased surface area, and a significantly improved absorption rate.

[0111] After mixing the polymer and the surfactant in this way, it is possible to produce secondary aggregated particles that are shredded and aggregated in a state where the superabsorbent resin particles and the surfactant are mixed in the presence of the surfactant.

[0112] Here, the "water-containing superabsorbent resin particles" are particles with a moisture content (water content) of about 30% by weight or more, in which the polymer is shredded and aggregated into particles without a drying process. Therefore, they can have a water content of 30 to 80% by weight, similar to the polymer.

[0113] According to one embodiment of the present invention, the surfactant may be a compound represented by the following Chemical Formula 2 or a salt thereof, but the present invention is not limited thereto:

[0114]

Chemical formula

[0115] In Chemical Formula 2, A is an alkyl group having 5 to 21 carbon atoms, B1 is -OCO-, -COO-, or -COOCH(R1)COO-, B2 is -CH2-, -CH2CH2-, -CH(R2)-, -CH=CH-, or -C≡C-, wherein R1 and R2 are each independently an alkyl group having 1 to 4 carbon atoms, n is an integer of 1 to 3, C is a carboxyl group.

[0116] At this time, the surfactant is one or more selected from the group consisting of the carboxylic acid represented by Chemical Formula 2 and its metal salts. Specifically, the surfactant is one or more selected from the group consisting of the carboxylic acid represented by Chemical Formula 2, the alkali metal salt of the carboxylic acid represented by Chemical Formula 2, and the alkaline earth metal salt of the carboxylic acid represented by Chemical Formula 2. More specifically, the surfactant is one of the carboxylic acid represented by Chemical Formula 2, the alkali metal salt of the carboxylic acid represented by Chemical Formula 2, and the alkaline earth metal salt of the carboxylic acid represented by Chemical Formula 2.

[0117] In Chemical Formula 2, A is a hydrophobic moiety and can be a linear or branched alkyl group having 5 to 21 carbon atoms. However, when A is a linear alkyl group, it is more advantageous in terms of suppressing aggregation of the pulverized particles and improving dispersibility. When A is an alkyl group having less than 5 carbon atoms, there is a problem that the chain length is short and the aggregation control of the pulverized particles cannot be effectively carried out. When A is an alkyl group having more than 21 carbon atoms, there is a problem that the mobility of the surfactant decreases, it may not be effectively mixed with the polymer, or the unit price of the composition increases due to the increase in the cost of the surfactant.

[0118] Specifically, in Chemical Formula 2, A can be a linear alkyl having 5 to 21 carbon atoms, that is, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decanyl, n-undecanyl, n-dodecanyl, n-tridecanyl, n-tetradecanyl, n-pentadecanyl, n-hexadecanyl, n-heptadecanyl, n-octadecanyl, n-nonadecanyl, n-icosanyl, or n-henicosanyl.

[0119] More specifically, A may be a linear alkyl having 6 to 18 carbon atoms. For example, A may be -C6H 13 、-C 11 H 23 、-C 12 H 25 、-C 17 H 35 、or -C 18 H 37 and so on.

[0120] Also, in Chemical Formula 2, the (B1 - B2) moiety plays a role in improving the adsorption performance to the polymer surface, which may be insufficient only with the C moiety. When the carbon number of B2 is 3 or more, the distance between the B1 moiety and the C moiety becomes long, and the adsorption performance to the polymer may decrease.

[0121] At this time, R1 and R2 can each independently be a linear or branched alkyl group having 1 to 4 carbon atoms. More specifically, R1 and R2 can each independently be methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, or tert-butyl. However, from the aspect of the surfactant adsorbing to the superabsorbent resin particles, since it is advantageous that the surfactant molecular structure is not bulky, R1 and R2 can both be methyl.

[0122] Also, in Chemical Formula 2, n may be 1, 2, or 3. More specifically, n, which represents the number of (B1 - B2), is for reinforcing the adsorption performance of the (B1 - B2) portion to the C portion, and when considering the molecular length for the surfactant to effectively adsorb to the polymer, n is preferably 1.

[0123] Specifically, in Chemical Formula 2, B1 is

[0124]

Chemical Formula

[0125] and can be, where * is the bonding site with the adjacent atom.

[0126] For example, B1 can be

[0127]

Chemical Formula

[0128] and can be.

[0129] Also, in Chemical Formula 2, B2 is

[0130]

Chemical Formula

[0131] can be, where * is a bonding site with an adjacent atom. At this time, in terms of improving the adsorption performance of the surfactant to the crosslinked polymer together with the C part, B2 is

[0132] [Chemical formula]

[0133] preferably.

[0134] Also, in Chemical formula 2, the C part is a hydrophilic part and is a carboxyl group (COOH), provided that when the surfactant is a salt, it is a carboxylate group (COO-).

[0135] That is, the surfactant can be a compound represented by the following Chemical formula 2a:

[0136] [Chemical formula]

[0137] In Chemical formula 2a, M is H + , a monovalent cation of an alkali metal, or a divalent cation of an alkaline earth metal, k is 1 when M is H + or a monovalent cation of an alkali metal, and is 2 when M is a divalent cation of an alkaline earth metal, The descriptions of A, B1, B2 and n are as defined in Chemical formula 2 above.

[0138] More specifically, when the surfactant is an alkali metal salt of the carboxylic acid represented by Chemical formula 2, the surfactant can be represented by the following Chemical formula 2':

[0139] [Chemical formula]

[0140] In Chemical formula 2', M1 is an alkali metal, for example, sodium or potassium, and the descriptions of A, B1, B2 and n are as defined in Chemical Formula 2 above.

[0141] Also, when the surfactant is an alkaline earth metal salt of the carboxylic acid represented by Chemical Formula 2 above, the surfactant can be represented by the following Chemical Formula 2":

[0142]

Chemical formula

[0143] In one example, the surfactant can be any one carboxylic acid selected from the group consisting of the following:

[0144]

Chemical formula

Chemical formula

[0145] Or, the surfactant can be any one alkali metal salt selected from the group consisting of the following:

[0146]

Chemical formula

Chemical formula

[0147] Or, the surfactant can be any one alkaline earth metal salt selected from the group consisting of the following:

[0148] [Chemical formula] [Chemical formula]

[0149] In the above, M2 are each independently an alkaline earth metal.

[0150] For example, the surfactant can be any one of the compounds represented by the following chemical formulas 1-1 to 1-7, but is not limited thereto:

[0151] [Chemical formula]

[0152] According to another embodiment of the present invention, the surfactant can be a compound represented by the following chemical formula 3 or a salt thereof, but the present invention is not limited thereto:

[0153] [Chemical formula]

[0154] In the chemical formula 3, A1, A2 and A3 are each independently a single bond, a carbonyl,

[0155] [Chemical formula]

[0156] and at least one of these is a carbonyl or

[0157] [Chemical formula]

[0158] wherein m1, m2 and m3 are each independently an integer from 1 to 8,

[0159]

Chemical formula

[0160] are each linked to an adjacent oxygen atom,

[0161]

Chemical formula

[0162] are 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.

[0163] The surfactant is added by being mixed with the polymer so that the granulation step can be easily carried out without an aggregation phenomenon.

[0164] The surfactant represented by Chemical Formula 3 is a nonionic surfactant, and it also has excellent surface adsorption performance with respect to an unneutralized polymer due to a hydrogen bonding force, thereby realizing the intended aggregation control effect. On the other hand, in the case of an anionic surfactant that is not a nonionic surfactant, when it is mixed with a polymer neutralized with a neutralizing agent such as NaOH or Na2SO4, it is adsorbed through Na+ ions ionized in the carboxyl group substituent of the polymer, and when it is mixed with an unneutralized polymer, there is a problem that the adsorption efficiency with respect to the polymer is relatively reduced due to competition with the anions of the carboxyl group substituent of the polymer.

[0165] Specifically, in the surfactant represented by the chemical formula 3, the hydrophobic functional group is the R1, R2, and R3 parts (when not hydrogen) that are terminal functional groups, and the hydrophilic functional group further includes the part derived from glycerol in the chain and the terminal hydroxyl group (when An is a single bond and at the same time Rn is hydrogen, n = 1 to 3). However, the part derived from glycerol and the terminal hydroxyl group play a role in improving the adsorption performance of the hydrophilic functional group to the polymer surface. Thereby, the aggregation of the superabsorbent resin particles can be effectively suppressed.

[0166] In the chemical formula 3, the R1, R2, and R3 parts (when not hydrogen) that 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, and R3 parts (when not hydrogen) are alkyl or alkenyl having less than 6 carbon atoms, there is a problem that the aggregation control of the pulverized particles with a short chain length cannot be effectively carried out. When the R1, R2, and R3 parts (when not hydrogen) are alkyl or alkenyl having more than 18 carbon atoms, the mobility of the surfactant may decrease, and it may not be effectively mixed with the polymer, resulting in a problem that the unit price of the composition increases due to the increase in the cost of the surfactant.

[0167] Preferably, R1, R2, and R3 are hydrogen, or in the case of a linear or branched alkyl having 6 to 18 carbon atoms, they can also 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 can 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.

[0168] The surfactant can be selected from the compounds represented by the following Chemical Formulas 3-1 to 3-14:

[0169] [Chemical Formula] [Chemical Formula]

[0170] On the other hand, the surfactant can 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 polymer surface, and there may be a phenomenon of re-aggregation of particles 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 can 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, while being 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.

[0171] The method of mixing such a surfactant with the polymer is not particularly limited as long as it can uniformly mix them with the polymer, and can be appropriately selected and used. Specifically, the surfactant can be mixed dry, mixed in a solution state after being dissolved in a solvent, or mixed after melting the surfactant.

[0172] Among these, for example, the surfactant can be mixed in a solution state dissolved in a solvent. At this time, as the solvent, all types can be used without being limited to inorganic solvents or organic solvents, but considering the ease of the drying process and the cost of the solvent recovery system, water is most appropriate. Also, for the solution, methods such as putting the surfactant and the polymer into a reaction tank and mixing them, spraying the solution into a mixer with the polymer, or continuously supplying and mixing the polymer and the solution to a continuously operating mixer can be used.

[0173] 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 the step of granulating the polymer in the presence of a surfactant (step 3) can be performed sequentially, alternately, or simultaneously.

[0174] That is, after adding a neutralizing agent to the polymer to neutralize the acidic groups first, a surfactant is added to the neutralized polymer to granulate the polymer mixed with the surfactant (performed in the order of step 2 -> step 3), or a neutralizing agent and a surfactant can be added to the polymer simultaneously to perform neutralization and granulation on the polymer (performing steps 2 and 3 simultaneously). Or, the surfactant can be added first and then the neutralizing agent can be added (performed in the order of step 3 -> step 2). Or, the neutralizing agent and the surfactant can be added alternately. Or, the surfactant can be added first and granulated, then the neutralizing agent can be added for neutralization, and an additional surfactant can be further added to the neutralized hydrogel polymer to perform an additional granulation step.

[0175] On the other hand, for uniform neutralization of the entire polymer, it may be preferable to set a certain time difference between the addition of the neutralizing agent and the granulation step.

[0176] At least a part or a substantial amount of the surfactant can be present on the surface of the water-containing superabsorbent resin particles. 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 can be physically or chemically adsorbed on the surface of the superabsorbent resin. More specifically, the hydrophilic functional group of the surfactant can be physically adsorbed by intermolecular forces such as dipole-dipole interaction on the hydrophilic part of the surface of the superabsorbent resin. In this way, the hydrophilic part of the surfactant physically adsorbs on the surface of the superabsorbent resin particles and wraps the surface, and the hydrophobic part of the surfactant does not adsorb on the surface of the resin particles, so the resin particles can 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 at the stage of granulation after polymer formation. Compared with the case where the surfactant is added during the polymerization process and the surfactant is present inside the polymer, the surfactant can faithfully play its role as a surfactant, pulverization and aggregation are carried out simultaneously, and particles with a large surface area can be obtained in the form of aggregated fine particles.

[0177] According to one embodiment of the present invention, the step of granulating the polymer to produce water-containing superabsorbent resin particles can be carried out two or more times.

[0178] According to one embodiment of the present invention, the granulation step is performed by a granulating device, and the granulating device includes a body portion including a transfer space into which a polymer is transferred; a screw member installed rotatably inside the transfer space to move the polymer; a drive motor providing a rotational driving force to the screw member; a cutter member installed in the body portion to grind the polymer; and a porous plate including holes formed therein to discharge the polymer ground by the cutter member to the outside of the body portion. At this time, the size of the holes provided in the porous plate of the granulating device can be 1 mm to 20 mm, or 5 mm to 15 mm, or 5 mm to 12 mm.

[0179] According to one embodiment of the present invention, the primary and secondary granulation steps are respectively performed by primary and secondary granulating devices, and the primary and secondary granulating devices include a body portion including a transfer space into which a polymer is transferred; a screw member installed rotatably inside the transfer space to move the polymer; a drive motor providing a rotational driving force to the screw member; a cutter member installed in the body portion to grind the polymer; and a porous plate including holes formed therein to discharge the polymer ground by the cutter member to the outside of the body portion.

[0180] The sizes of the holes of the porous plates provided in the primary and secondary granulating devices may be the same as or different from each other.

[0181] On the other hand, according to one embodiment of the present invention, for ease of grinding, it is preferable that the size of the holes provided in the porous plate of the secondary granulating device is smaller than the size of the holes of the porous plate provided in the porous plate of the primary granulating device. For example, the size of the holes provided in the porous plate of the primary granulating device may be 1 mm to 6 mm, and the size of the holes provided in the porous plate of the secondary granulating device may be 0.5 mm to 6 mm.

[0182] Thus, when the polymer mixed with the surfactant is pulverized using a granulating device, a smaller particle size distribution is achieved, and the subsequent drying and pulverizing processes can be carried out under milder conditions, thereby improving the physical properties of the superabsorbent resin while preventing the generation of fine powder.

[0183] Step 4: Drying step Next, the step of drying the neutralized and granulated polymer to produce dried superabsorbent resin particles (step 4) is carried out.

[0184] The step is a step of drying the moisture of the water-containing superabsorbent resin particles, which is a polymer obtained by neutralizing at least a part of the acidic groups of the polymer and granulating the polymer in the presence of a surfactant.

[0185] In the conventional method for producing a superabsorbent resin, it is common to carry out the drying step until the water content of the superabsorbent resin is less than 10% by weight. However, according to an embodiment of the present invention, drying is carried out so that the water content of the superabsorbent resin is 10% by weight or more, for example, about 10 to about 20% by weight, or about 10 to about 15% by weight. However, the present invention is not limited thereto.

[0186] Therefore, the temperature in the dryer used in the drying step can be relatively low, for example, about 80°C to about 150°C, at about 150°C or lower. When the temperature in the dryer is excessively low, the drying time becomes excessively long, and when the drying temperature is excessively high, a superabsorbent resin having a water content lower than the desired water content can be obtained.

[0187] At this time, drying can be carried out in a moving type. Such a moving type drying is distinguished from stationary drying by the presence or absence of the flow of the substance during drying.

[0188] The so-called moving type drying refers to a method of drying while mechanically agitating the material to be dried. At this time, the direction in which the hot air passes through the material may be the same as or different from the circulation direction of the material. Alternatively, the material can be circulated inside the dryer, and the heat transfer fluid (heat medium oil) can be passed through a separate pipe outside the dryer to dry the material.

[0189] On the other hand, the so-called stationary drying refers to a method in which the material to be dried is stopped on a floor such as a porous iron plate through which air can pass, and hot air passes through the material from bottom to top for drying.

[0190] Therefore, from the aspect that uniform drying can be completed within a short time for drying at the above stage, it is preferable to dry the water-containing superabsorbent resin by the fluidized drying method.

[0191] As an apparatus capable of drying by such a fluidized drying method, a horizontal-type mixer, a rotary kiln, a paddle dryer, a steam tube dryer, or a generally used fluidized dryer can be used.

[0192] Step 5: Crushing step Next, the step of pulverizing the dried superabsorbent resin particles to produce superabsorbent resin particles is performed.

[0193] Specifically, the pulverizing step can be performed by pulverizing the dried superabsorbent resin particles to have a particle size at the normal particle level, that is, a particle diameter of 150 μm to 850 μm.

[0194] The pulverizers used for this purpose are specifically, for example, 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 are not limited to the above-mentioned examples.

[0195] Alternatively, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, etc. can also be used for the pulverizer, but are not limited to the above-mentioned examples.

[0196] On the other hand, in the production method of the present invention, in the granulation stage, superabsorbent resin particles with a smaller particle size distribution can be realized compared to the conventional chopping stage. When performing moving type drying, since the moisture content after drying is relatively high and maintained at 10% by weight or more, even if pulverization is performed under mild conditions with less pulverizing force, a superabsorbent resin with a very high content of normal particle sizes of 150 μm to 850 μm can be formed, and the fine powder generation ratio can be significantly reduced.

[0197] The superabsorbent resin particles produced as described above can contain superabsorbent resin particles having a particle size of 150 μm to 850 μm, that is, normal particles, at 80% by weight or more, 85% by weight or more, 89% by weight or more, 90% by weight or more, 92% by weight or more, 93% by weight or more, 94% by weight or more, or 95% by weight or more based on the total weight. The particle size of such resin particles can be measured by the EDANA WSP220.3 method of the European Disposables and Nonwovens Association (EDANA).

[0198] Further, the superabsorbent resin particles can contain fine powder having a particle size of less than 150 μm in an amount of about 20% by weight or less, or about 18% by weight or less, or about 15% by weight or less, or about 13% by weight or less, or about 12% by weight or less, or about 11% by weight or less, or about 10% by weight or less, or about 9% by weight or less, or about 8% by weight or less, or about 5% by weight or less, based on the total weight. This is in contrast to the case where a superabsorbent resin is produced by a conventional production method, which has fine powder in an amount of more than about 20% by weight to about 30% by weight.

[0199] Additional step After the step of pulverizing the superabsorbent resin particles, the step of classifying the pulverized superabsorbent resin particles by particle size can be further included.

[0200] Further, after pulverizing and / or classifying the superabsorbent resin particles, in the presence of a surface crosslinking agent, the step of forming a surface crosslinked layer on at least a part of the surface of the superabsorbent resin particles can be further included. By this step, the crosslinked polymer contained in the superabsorbent resin particles can be additionally crosslinked through the surface crosslinking agent, and a surface crosslinked layer can be formed on at least a part of the surface of the superabsorbent resin particles.

[0201] As the surface crosslinking agent, all surface crosslinking agents that have been conventionally used in the production of superabsorbent resins can be used without any special restrictions. For example, the surface crosslinking agent can be 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, propylene carbonate, and glycerol 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; and the like.

[0202] Specifically, one or more, or two or more, or three or more of the above-described surface crosslinking agents can be used as the surface crosslinking agent. For example, ethylene carbonate-propylene carbonate (ECPC), propylene glycol, and / or glycerol carbonate can be used.

[0203] Such a surface crosslinking agent can be used in an amount of about 0.001 to about 5 parts by weight based on 100 parts by weight of the superabsorbent resin particles. For example, the surface crosslinking agent can be used in an amount of 0.005 parts by weight or more, or 0.01 parts by weight or more, or 0.05 parts by weight or more, or 5 parts by weight or less, or 4 parts by weight or less, or 3 parts by weight or less based on 100 parts by weight of the superabsorbent resin particles. By adjusting the content range of the surface crosslinking agent within the above-described range, a superabsorbent resin showing excellent absorption properties can be produced.

[0204] Also, the step of forming the surface crosslinked layer can be carried out by adding an inorganic substance to the surface crosslinking agent. That is, in the presence of the surface crosslinking agent and the inorganic substance, a step of additionally crosslinking the surface of the superabsorbent resin particles to form a surface crosslinked layer can be carried out.

[0205] As such an inorganic substance, one or more inorganic substances selected from the group consisting of silica, clay, alumina, silica-alumina composite, titania, zinc oxide and aluminum sulfate can be used. The inorganic substance can be used in powder form or liquid form, and in particular, it can be used as alumina powder, silica-alumina powder, titania powder, or nanosilica solution. Also, the inorganic substance can be used in a content of about 0.001 to about 1 part by weight based on 100 parts by weight of the superabsorbent resin particles.

[0206] Also, there is no limitation on the configuration of the method of mixing the surface crosslinking agent with the superabsorbent resin composition. For example, the surface crosslinking agent and the superabsorbent resin composition can be put into a reaction tank and mixed, or the surface crosslinking agent can be sprayed onto the superabsorbent resin composition, or a method of continuously supplying and mixing the superabsorbent resin composition and the surface crosslinking agent into a continuously operated mixer can be used.

[0207] When mixing the surface crosslinking agent and the superabsorbent resin composition, water and methanol can be additionally mixed and added together. When water and methanol are added, there is an advantage that the surface crosslinking agent can be evenly dispersed in the superabsorbent resin composition. At this time, the contents of the added water and methanol can be appropriately adjusted to induce uniform dispersion of the surface crosslinking agent, prevent the aggregation phenomenon of the superabsorbent resin composition, and at the same time optimize the depth of surface penetration of the crosslinking agent.

[0208] The surface crosslinking step can be carried out at a temperature of about 80°C to about 250°C. More specifically, the surface crosslinking step can be carried out at a temperature of about 100°C to about 220°C, or about 120°C to about 200°C, for about 20 minutes to about 2 hours, or about 40 minutes to about 80 minutes. When the conditions of the above-described surface crosslinking step are satisfied, the surface of the superabsorbent resin particles can be sufficiently crosslinked and the pressure absorption capacity can be increased.

[0209] The means for raising the temperature for the surface crosslinking reaction is not particularly limited. A heat medium can be supplied, or a heat source can be directly supplied for heating. At this time, as the type of heat medium that can be used, heated fluids such as steam, hot air, and hot oil can be used, but it is not limited thereto, and the temperature of the supplied heat medium can be appropriately selected in consideration of the means of the heat medium, the heating rate, and the target heating temperature. On the other hand, as the heat source directly supplied, there are heating methods through electricity and heating methods through gas, but it is not limited to the above-described examples.

[0210] According to an embodiment of the present invention, after the step of forming a surface crosslinking layer on at least a part of the surface of the superabsorbent resin particles, a cooling step of cooling the superabsorbent resin particles on which the surface crosslinking layer is formed, a water addition step of introducing water into the superabsorbent resin particles on which the surface crosslinking layer is formed, and a post-treatment step of introducing an additive into the superabsorbent resin particles on which the surface crosslinking layer is formed can be further included, including any one or more of these steps. At this time, the cooling step, the water addition step, and the post-treatment step can be performed sequentially or simultaneously.

[0211] Additives introduced in the post-treatment step include a liquid permeability improver, an anti-caking agent, a fluidity improver, an antioxidant, etc., but the present invention is not limited thereto.

[0212] 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 produced.

[0213] According to another embodiment of the present invention, there is provided a superabsorbent resin produced by the above production method.

[0214] The superabsorbent resin produced by the above production method has a high absorption rate, a low fine powder content, and can have a water retention capacity (CRC) and a pressure absorption capacity (AUP), which are various absorption physical properties with respect to the superabsorbent resin produced by the conventional method, at the same level or higher.

[0215] In addition, it is possible to provide a superabsorbent resin having a narrow particle size distribution, a uniform particle size distribution, and excellent liquid permeability and rewetting properties by reducing the content of water-soluble components (EC).

[0216] Hereinafter, preferred examples are presented for the understanding of the present invention. However, the following examples are for illustrative purposes of the present invention, and the content of the present invention is not limited by the following examples.

[0217] <Example> An aqueous acrylic acid (AA) solution was used as the monomer component.

[0218] In order to remove the dissolved oxygen inside the monomer aqueous solution, nitrogen gas was used to carry out nitrogen purging at a temperature of 5°C at 1 L / min for about 1 hour.

[0219] As the internal crosslinking agent component, P-30 (Pentaerythritol dially ether) was mixed with the acrylic acid at about 3500 ppmw and used.

[0220] As the initiator component, it was used in another aqueous solution form in which a VA-086 weak 600 ppmw (relative to acrylic acid) which is an azo initiator and about 40 ppmw of hydrogen peroxide (relative to acrylic acid) were mixed so as to be supplied.

[0221] As a reducing agent component, it was used in another aqueous solution form mixed so that about 150 ppmw of ascorbic acid (compared to acrylic acid) and about 1.5 ppmw of iron sulfate (FeSO4) (compared to acrylic acid) were supplied with respect to acrylic acid.

[0222] When the concentrations of each aqueous solution solute are sorted separately, they are as shown in the following table.

[0223]

Table 1

[0224] The monomer aqueous solution, initiator aqueous solution, and reducing agent aqueous solution were all supplied through their respective transfer lines, and the initiator transfer line and reducing agent transfer line were configured to be sequentially combined with the monomer transfer line immediately before reaching the reactor.

[0225] The supply process conditions of the monomer transfer line, initiator transfer line, and reducing agent transfer line were sorted in the following table.

[0226]

Table 2

[0227] Under the above conditions, the monomer aqueous solution, initiator, and reducing agent were supplied (supply amount) to the reactor for 1 hour, and the polymerization reaction was carried out at a temperature of about 90 °C for about 6 hours to form a cross-linked polymer.

[0228] The obtained cross-linked polymer was dried / crushed to obtain a powder form, and the content of unreacted monomer in the polymer was analyzed for the sample according to the EDANA method, NWSP210.0.R215.

[0229] The above content was sorted in the following table.

[0230]

Table 3

[0231] Referring to Table 2 above, it can be confirmed that when a specific speed ratio is satisfied as in Examples 2 and 3, the content of the unreacted monomer significantly decreases.

[0232] As described above, according to Bernoulli's principle, the pressure on the side of the fluid with a relatively slow speed (monomer transfer line) increases, and the pressure on the side of the fluid with a relatively fast speed (initiator transfer line) increases. Instantaneous rapid diffusion occurs, which is considered to be due to the monomer component and the initiator component being quickly and uniformly mixed with each other.

Claims

1. Polymerizing a monomer composition containing a water-soluble ethylenically unsaturated monomer having an acidic group, an internal crosslinking agent, and a polymerization initiator to form a polymer in which the water-soluble ethylenically unsaturated monomer having the acidic group and the internal crosslinking agent are crosslinked and polymerized (Step 1); Neutralizing at least a part of the acidic groups of the polymer to form a hydrogel polymer (Step 2); Granulating the hydrogel polymer in the presence of a surfactant (Step 3); and Drying the neutralized and granulated polymer to produce dry superabsorbent resin particles (Step 4), In the step of forming the polymer, the first monomer composition containing the monomer and the internal crosslinking agent is transferred via a monomer transfer line, and the polymerization initiator is transferred via an initiator transfer line. Immediately before being introduced into the polymerization reactor, the monomer transfer line and the initiator transfer line are combined, and the first monomer composition and the initiator are mixed to form a second monomer composition; At the stage where the monomer transfer line and the initiator transfer line are combined, the ratio (speed ratio) of the supply rate (m / s) of the initiator supplied from the initiator transfer line to the supply rate (m / s) of the first monomer mixture supplied from the monomer transfer line is 3.6 or more, The second monomer composition further contains a reducing agent, The reducing agent is supplied together with the initiator via the initiator transfer line or supplied via another reducing agent transfer line, A method for producing a superabsorbent resin, wherein the ratio (speed ratio) of the supply rate (m / s) of the reducing agent to the supply rate (m / s) of the first monomer mixture supplied from the monomer transfer line is 3.5 or more.

2. The method for producing a superabsorbent resin according to claim 1, wherein the step of forming the polymer is carried out in a batch type reactor.

3. The method for producing a superabsorbent resin according to claim 1, wherein the steps 2 and 3 are carried out sequentially, simultaneously, or alternately.

4. The method for producing a superabsorbent resin according to claim 1, wherein at the stage where the monomer transfer line and the initiator transfer line are combined, the ratio (flow rate ratio) of the supply flow rate (kg / hr) of the initiator supplied from the initiator transfer line to the supply flow rate (kg / hr) of the first monomer mixture supplied from the monomer transfer line is 0.01 to 0.

1.

5. The method for producing a superabsorbent resin according to claim 1, wherein the step of drying the neutralized and granulated polymer is carried out in a moving type.

6. The method for producing a superabsorbent resin according to claim 5, wherein the fluidized drying is carried out using a horizontal-type mixer, a rotary kiln, a paddle dryer, or a steam tube dryer.

7. The method for producing a superabsorbent resin according to claim 1, wherein the step of drying the neutralized and granulated polymer is carried out at a temperature of 150°C or lower.

8. The method for producing a superabsorbent resin according to claim 1, wherein the water content of the superabsorbent resin particles obtained by drying the neutralized and granulated polymer is 10 to 30% by weight.

9. The method for producing a superabsorbent resin according to claim 1, wherein at least a part of the surfactant is present on the surface of the hydrogel polymer.

10. The method for producing a superabsorbent resin according to claim 1, wherein the surfactant contains one or more selected from the group consisting of a compound represented by the following chemical formula 2, a salt thereof, a compound represented by the following chemical formula 3, and a salt thereof. 【Chemical Formula 1】 In the above chemical formula 2, A is an alkyl group having 5 to 21 carbon atoms, B １ is -OCO-, -COO-, or -COOCH(R １ )COO-, B ２ is -CH ２ -, -CH ２ CH ２ -, -CH(R ２ )-, -CH=CH-, or -C≡C-, wherein R １ and R ２ are each independently an alkyl group having 1 to 4 carbon atoms, n is an integer of 1 to 3, C is a carboxyl group, 【Chemical Formula 2】 In the above Chemical Formula 3, A １ , A ２ and A ３ are each independently a single bond, a carbonyl group, 【Chemical Formula 3】 wherein at least one of these is a carbonyl group or 【Chemical Formula 4】 wherein m1, m2 and m3 are each independently an integer of 1 to 8, 【Chemical Formula 5】 are each connected to an adjacent oxygen atom, 【Chemical Formula 6】 is connected to adjacent R １ , R ２ and R ３ respectively, R １ , R ２ and R ３ are each independently hydrogen, a linear or branched alkyl group having 6 to 18 carbon atoms or a linear or branched alkenyl group having 6 to 18 carbon atoms, n is an integer of 1 to 9.

11. The method for producing a superabsorbent resin according to claim 1, wherein the superabsorbent resin particles contain 89% by weight or more of superabsorbent resin particles having a particle size of 150 μm to 850 μm with respect to the total weight of the superabsorbent resin particles.

12. The method for producing a superabsorbent resin according to claim 1, wherein the superabsorbent resin particles contain 20% by weight or less of superabsorbent resin particles having a particle size of less than 150 μm with respect to the total weight of the superabsorbent resin particles.

13. The method for producing a superabsorbent resin according to claim 1, further comprising a step of pulverizing the superabsorbent resin particles after the step of drying the neutralized and granulated polymer to produce the superabsorbent resin particles.

14. The method for producing a superabsorbent resin according to claim 13, further comprising a step of classifying the pulverized superabsorbent resin particles according to particle size after the step of additionally pulverizing the superabsorbent resin particles.

15. The method for producing a superabsorbent resin according to claim 1 or 14, further comprising a step of forming a surface crosslinked layer on at least a part of the surface of the superabsorbent resin particles.

16. After the step of forming a surface crosslinked layer on at least a part of the surface of the superabsorbent resin particles, a cooling step of cooling the superabsorbent resin particles having the surface crosslinked layer formed thereon; a water addition step of introducing water into the superabsorbent resin particles having the surface crosslinked layer formed thereon; and a post-treatment step of introducing an additive into the superabsorbent resin particles having the surface crosslinked layer formed thereon, further comprising one or more of the steps. The method for producing a superabsorbent resin according to claim 15.

17. The method for producing a superabsorbent resin according to claim 16, wherein the cooling step, the water addition step, and the post-treatment step are performed simultaneously.

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