Monomer composition for producing superabsorbent resin

Abstract

The present invention relates to a monomer composition for producing a superabsorbent resin. Since the monomer composition for producing a superabsorbent resin of the present invention has a high polymerization rate and can minimize the loss of bubbles during foamed polymerization, it can provide a superabsorbent resin having an excellent water absorption rate. In addition, the superabsorbent resin produced from the monomer composition for producing a superabsorbent resin of the present invention has a high gel strength and maintains a good form even after water absorption, so it can exhibit excellent water absorption performance despite external pressure.

Description

Technical Field

[0001] Cross - reference to related applications This application claims the benefit of priority based on Korean Patent Application No. 10-2023-0028619 filed on March 3, 2023 and Korean Patent Application No. 10-2024-0030792 filed on March 4, 2024, and all contents disclosed in the documents of the Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a monomer composition for producing a superabsorbent resin.

Background Art

[0003] Superabsorbent polymers (SAPs) are synthetic polymer substances having a function of absorbing about 500 to 1000 times their own weight of water. They began to be put into practical use as sanitary products, and now, in addition to sanitary products such as baby diapers and sanitary napkins, they are widely used in materials such as soil water retention agents for horticulture, water stop materials for civil engineering and construction, seedling sheets, freshness retainers in the food distribution field, and materials for ships.

[0004] Superabsorbent resins are generally produced by polymerizing a monomer composition prepared by neutralizing a water-soluble unsaturated ethylenic monomer such as acrylic acid with an alkali metal salt such as sodium salt or a basic compound such as caustic soda, and then adding a crosslinking agent, a polymerization initiator, and the like.

[0005] As a method for improving the water absorption rate of superabsorbent resins, a method is known in which a foaming agent is added to form bubbles in the monomer composition and then polymerization is carried out to form a porous structure in the superabsorbent resin to increase the surface area.

[0006] However, in the case of the method for producing a superabsorbent resin using an existing foaming agent, there is a problem that the bubbles generated by the foaming agent are lost before the monomer is polymerized, and ultimately, sufficient pores cannot be formed in the superabsorbent resin. Therefore, a method of reducing the loss of bubbles by using a foam stabilizer together with the foaming agent has been proposed, but there is a problem that the surface tension of the superabsorbent resin is reduced by the foam stabilizer.

[0007] For this reason, there is a demand for the development of a superabsorbent resin that exhibits a fast water absorption rate without a decrease in physical properties such as surface tension.

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention provides a monomer composition for producing a superabsorbent resin that can produce a superabsorbent resin having a fast water absorption rate and excellent gel strength.

Means for Solving the Problems

[0009] According to one embodiment of the present invention, there is provided a monomer composition for producing a superabsorbent resin, comprising an acrylic acid-based monomer having an acidic group and at least a part of the acidic group being neutralized; a crosslinking agent; clay; a carbonate-based foaming agent; and a polymerization initiator, when the monomer composition for producing a superabsorbent resin is irradiated with heat and / or light to proceed with a polymerization reaction, a monomer composition for producing a superabsorbent resin having a normalized gel point represented by the following formula (1) of 0.01 to 0.1 is provided.

[0010] [Formula (1)] Normalized gel point = Gelation point (seconds) / Total polymerization time (seconds)

[0011] In the above formula (1), the gelation point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for producing a superabsorbent resin according to the polymerization time and the loss modulus graph of the monomer composition for producing a superabsorbent resin according to the polymerization time.

[0012] According to another embodiment of the present invention, there is provided a method for producing a monomer composition for producing a superabsorbent resin, comprising the steps of mixing an acrylic acid-based monomer having an acidic group and at least a part of the acidic group neutralized, a crosslinking agent, a carbonate-based foaming agent, and a polymerization initiator; and adding clay to the mixture and performing shear mixing at a stirring speed of 6,500 rpm or more.

[0013] According to another embodiment of the present invention, there is provided a polymer produced from the monomer composition for producing a superabsorbent resin, wherein when the water content is 40% by weight to 80% by weight, the gel strength is 40,000 Pa to 60,000 Pa.

[0014] According to another embodiment of the present invention, there is provided a superabsorbent resin produced from the monomer composition for producing a superabsorbent resin, wherein the Performance Index (PI) represented by the following calculation formula 1 is 2.35 or more and T20 is less than 190 seconds:

[0015] [Calculation formula 1] Performance Index (PI) = (CRC / 27) + (AUP / 24) + (SFC / 30) - (T20 / 140)

[0016] In the above calculation formula 1, CRC is the centrifugal separation water retention capacity (g / g) of the superabsorbent resin for 30 minutes in a 0.9% by weight sodium chloride aqueous solution, AUP is the 0.7 psi pressurized water absorption capacity (g / g) of the superabsorbent resin for 1 hour in a 0.9% by weight sodium chloride aqueous solution, SFC is the flow conductivity (·10 -7 cm 3 ·s / g) of a 0.685% by weight sodium chloride aqueous solution, T20 is the time (seconds) required for 1 g of the superabsorbent resin to absorb 20 g of an aqueous solution of sodium chloride and an alcohol ethoxylate having 12 to 14 carbon atoms under 0.3 psi.

[0017] According to another embodiment of the present invention, there is provided a method for producing a superabsorbent resin, comprising the steps of irradiating the monomer composition for producing the superabsorbent resin with heat and / or light to polymerize it to produce a hydrogel polymer; drying, pulverizing and classifying the hydrogel polymer to form a base resin; and forming a surface crosslinked layer on the surface of the base resin in the presence of a surface crosslinking solution containing a surface crosslinking agent and a solvent.

Advantages of the Invention

[0018] The monomer composition for producing the superabsorbent resin of the present invention has a high rate of gelation after the start of polymerization and is excellent in the bubble stabilizing effect, and can minimize the loss of bubbles generated from the foaming agent. Therefore, the superabsorbent resin produced from the monomer composition for producing the superabsorbent resin of the present invention has a large number of pores inside and can exhibit an excellent water absorption rate.

[0019] In addition, the superabsorbent resin produced from the monomer composition for producing the superabsorbent resin of the present invention has a high gel strength and maintains its form well even after water absorption, and thus can exhibit excellent water absorption performance despite external pressure.

Embodiments for Carrying Out the Invention

[0020] The terms used in this specification are merely used to explain exemplary embodiments and are not intended to limit the present invention. 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 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.

[0021] Although the present invention can be modified in various ways and can have various forms, 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 it should be understood that all modifications, equivalents, or alternatives included within the spirit and technical scope of the present invention are encompassed.

[0022] As used herein, "clay" is used in the sense of encompassing single particles of phyllosilicate minerals and aggregates formed by the aggregation of a large number of such particles, and "clay dispersion" means a dispersion in which the above-mentioned clay is dispersed in a solvent.

[0023] As used herein, "(meth)acrylate" is used in the sense of including both acrylate and methacrylate.

[0024] As used herein, "base resin" or "base resin powder" refers to a polymer obtained by polymerizing a water-soluble ethylenically unsaturated monomer, drying and pulverizing it into particles or powder, and means a polymer in a state where surface modification or surface crosslinking is not performed.

[0025] Hereinafter, the present invention will be described in detail.

[0026] The monomer composition for producing a superabsorbent resin according to an embodiment of the present invention includes an acrylic acid-based monomer having an acidic group, at least a part of which is neutralized; a crosslinking agent; clay; a carbonate-based foaming agent; and a polymerization initiator. When the monomer composition for producing a superabsorbent resin is irradiated with heat and / or light to cause a polymerization reaction to proceed, the normalized gel point represented by the following formula (1) satisfies 0.01 to 0.1:

[0027] [Formula (1)] Normalized gel point = Gel point (seconds) / Total polymerization time (seconds)

[0028] In the above formula (1), the gel point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for producing a superabsorbent resin according to the polymerization time and the loss modulus graph of the monomer composition for producing a superabsorbent resin according to the polymerization time.

[0029] In one embodiment, the normalized gel point of the monomer composition for producing a superabsorbent resin is 0.1 or less, or 0.09 or less, or 0.08 or less, or 0.06 or less, and may be 0.01 or more, or 0.02 or more. The method for measuring the gel point will be specifically described in the examples below.

[0030] When a polymer is formed from the monomer composition for producing a superabsorbent resin, the liquid monomer composition changes into a water-containing gel-like polymer. Therefore, the polymerization rate can be measured by confirming the gel point, which is the time when the monomer composition gels. At this time, by comparing the "normalized gel point" from which the factors of the polymerization time affected by the amount of monomer, the amount of initiator, the initial polymerization temperature, and / or other polymerization conditions are removed, the polymerization rate of the monomer composition for producing a superabsorbent resin can be evaluated.

[0031] The monomer composition for producing a superabsorbent resin of the present invention is polymerized at a faster rate than existing monomer compositions, and thus the loss of bubbles generated from the foaming agent is minimized. Therefore, the water absorption rate of the superabsorbent resin produced can be improved without using an excessive amount of the foaming agent.

[0032] However, when the normalized gelation point is less than 0.01 and the polymerization rate is excessively fast, gelation may progress before the monomer composition is charged into the polymerization vessel, and the problem of blockage of the charging pipe may occur. Further, although the viscosity of the monomer composition rapidly increases until just before the gelation point, when the gelation point is less than 0.01, there is a problem that it is difficult to mix the monomer composition and it is difficult to form a uniform polymer. Further, when the normalized gelation point exceeds 0.1, the above-described effect of reducing the bubble loss rate cannot be ensured, and the effect of improving the water absorption rate of the superabsorbent resin cannot be expected.

[0033] Further, when the monomer composition for producing a superabsorbent resin according to one embodiment is irradiated with heat and / or light to advance the polymerization reaction, the rate of increase in gel strength (storage modulus) (ΔG’ / s, where ΔG’ is the difference between the storage modulus at the completion of polymerization and the storage modulus at the gelation point, and s is the time from the gelation point to the completion of polymerization) from the gelation point to the completion of polymerization may be 210 Pa / s or more.

[0034] Preferably, the rate of increase in the gel strength may be 215 Pa / s or more, or 220 Pa / s or more, or 230 Pa / s or more. On the other hand, the upper limit value of the rate of increase in gel strength is not particularly limited, but as an example, it may be 400 Pa / s or less, or 370 Pa / s or less, or 350 Pa / s or less. By showing such a fast polymerization rate and at the same time showing a high rate of increase in gel strength, the monomer composition for producing a superabsorbent resin of the present invention can produce a polymer having excellent gel strength.

[0035] Thus, not only the gelation point but also as the rate of increase in gel strength improves, the water-containing gel polymer obtained immediately after the completion of polymerization produced from the monomer composition for producing a superabsorbent resin may have a gel strength of 35,000 Pa or more, or 36,000 Pa or more, or 37,000 Pa or more when the water content is 40 to 80% by weight, or 40 to 60% by weight. On the other hand, the upper limit value of the gel strength of the water-containing gel polymer is not particularly limited, but as an example, it can satisfy 60,000 Pa or less, or 58,000 Pa or less, or 55,000 Pa or less.

[0036] The monomer composition for producing the superabsorbent resin contains an acrylic acid-based monomer; an alkali metal salt or an alkali compound capable of neutralizing the acrylic acid-based unsaturated monomer; a crosslinking agent; clay; a carbonate-based foaming agent; and a polymerization initiator, and satisfies the physical properties. Specifically, the monomer composition for producing the superabsorbent resin of the present invention contains clay as an additive, and such clay can be uniformly dispersed in the monomer composition, whereby the above-described characteristics can be exhibited.

[0037] The acrylic acid-based monomer is a compound represented by the following Chemical Formula 1:

[0038] [Chemical Formula 1] R 1 -COOM 1

[0039] In Chemical Formula 1, R 1 is an alkyl group having 2 to 5 carbon atoms containing an unsaturated bond, M 1 is a hydrogen atom, a monovalent or divalent metal, an ammonium group or an organic amine salt.

[0040] Preferably, the acrylic acid-based monomer contains one or more selected from the group consisting of acrylic acid, methacrylic acid, and their monovalent metal salts, divalent metal salts, ammonium salts and organic amine salts.

[0041] Here, the acrylic acid monomer may have an acidic group, and at least a part of the acidic group may be neutralized. Preferably, a monomer partially neutralized with an alkaline substance such as sodium hydroxide, potassium hydroxide, or ammonium hydroxide can be used. At this time, the degree of neutralization of the acrylic acid monomer may be 40 to 95 mol%, or 40 to 80 mol%, or 45 to 75 mol%. The range of the degree of neutralization can be adjusted according to the final physical properties. However, if the degree of neutralization is excessively high, the neutralized monomer may precipitate and polymerization may not proceed smoothly. Conversely, if the degree of neutralization is excessively low, not only does the water absorption capacity of the polymer decrease significantly, but it may also exhibit properties such as an elastic rubber that is difficult to handle.

[0042] The concentration of the acrylic acid monomer is about 20 to about 60% by weight, preferably about 40 to about 50% by weight, based on the monomer composition containing the raw material substances and solvent of the superabsorbent resin, and can be adjusted to an appropriate concentration considering the polymerization time and reaction conditions. However, if the concentration of the monomer is excessively low, the yield of the superabsorbent resin may be low, causing economic problems. Conversely, if the concentration is excessively high, problems such as partial precipitation of the monomer or low grinding efficiency during grinding of the polymerized hydrogel polymer may occur, and the physical properties of the superabsorbent resin may deteriorate.

[0043] As the crosslinking agent, any compound can be used as long as it enables the introduction of crosslinking bonds during the polymerization of the acrylic acid monomer. The crosslinking agent is also expressed as an "internal crosslinking agent" to distinguish it from the "surface crosslinking agent" for crosslinking the surface of the superabsorbent resin particles. Crosslinking by the crosslinking agent contained in the monomer composition is performed without distinction between the surface and the inside. However, when the surface crosslinking process of the superabsorbent resin particles obtained after polymerization, drying, grinding, and classification of the monomer composition proceeds, the particle surface of the finally produced superabsorbent resin has a structure crosslinked by the surface crosslinking agent, and the inside has a structure crosslinked by the internal crosslinking agent.

[0044] Specifically, as the crosslinking agent contained in the monomer composition, a crosslinking agent having one or more functional groups capable of reacting with the water-soluble substituent of the acrylic acid-based monomer and having one or more ethylenically unsaturated groups; or a crosslinking agent having two or more functional groups capable of reacting with the water-soluble substituent of the monomer and / or the water-soluble substituent formed by hydrolysis of the monomer can be used.

[0045] As non-limiting examples, the crosslinking agent may be N,N'-methylenebisacrylamide, trimethylolpropane tri(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol (meth)acrylate, butanediol di(meth)acrylate, butylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, hexanediol di(meth)acrylate, triethylene glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, dipentaerythritol pentaacrylate, glycerin tri(meth)acrylate, pentaerythritol tetraacrylate, triarylamine, ethylene glycol diglycidyl ether, propylene glycol, glycerin, or ethylene carbonate and other polyfunctional crosslinking agents may be used alone or in combination of two or more, and is not limited thereto.

[0046] Preferably, as the crosslinking agent, polyalkylene glycol di(meth)acrylate-based compounds such as polyethylene glycol (meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, or polypropylene glycol (meth)acrylate can be used. This is because when such a crosslinking agent is used, foaming with a carbonate-based foaming agent described later can be easily performed.

[0047] In the monomer composition, such a crosslinking agent can be used in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the acrylic acid-based monomer. For example, the crosslinking agent can be used in an amount of 0.01 part by weight or more, 0.05 part by weight or more, 0.1 part by weight, or 0.15 part by weight or more and 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 part by weight or less based on 100 parts by weight of the acrylic acid-based monomer. When the content of the crosslinking agent is excessively low, crosslinking does not occur sufficiently and it is difficult to achieve a strength above an appropriate level. When the content of the crosslinking agent is excessively high, the internal crosslinking density becomes high and it may be difficult to achieve the desired water retention ability.

[0048] In addition, the monomer composition contains a polymerization initiator for initiating the polymerization reaction of the monomer. The polymerization initiator is not particularly limited as long as it is generally used in the production of superabsorbent resins.

[0049] Specifically, depending on the polymerization method, a thermal polymerization initiator, an oxidation-reduction pair (Red-ox pair) initiator, or a photopolymerization initiator by UV irradiation can be used as the polymerization initiator. However, even in the photopolymerization method, a certain amount of heat is generated by irradiation such as ultraviolet irradiation, and a certain amount of heat is generated by the progress of the exothermic polymerization reaction. Therefore, a thermal polymerization initiator can also be additionally included.

[0050] The photopolymerization initiator can be used without limitation of its structure as long as it is a compound capable of forming radicals by light such as ultraviolet light.

[0051] As the photopolymerization initiator, for example, one or more selected from the group consisting of benzoin ether, dialkyl acetophenone, hydroxyl alkylketone, phenyl glyoxylate, Benzyl Dimethyl Ketal, acyl phosphine, and α-aminoketone can be used. On the other hand, as a specific example of acyl phosphine, commercial lucirin TPO, that is, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide can be used. For more diverse photoinitiators, it is well described in Reinhold Schwalm's "UV Coatings: Basics, Recent Developments and New Application (Elsevier 2007)" p.115 and is not limited to the above-mentioned examples.

[0052] The photopolymerization initiator may be contained at a concentration of about 0.001 to about 1.0% by weight based on the monomer composition. If the concentration of such a photopolymerization initiator is excessively low, the polymerization rate becomes slow, and if the concentration of the photopolymerization initiator is excessively high, the molecular weight of the superabsorbent resin may be small and the physical properties may become non-uniform.

[0053] In addition, 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)isobutylonitrile, 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 in "Principle of Polymerization (Wiley, 1981)" by Odian, p. 203, and are not limited to the examples described above.

[0054] The thermal polymerization initiator may be contained in an amount of about 0.001 to about 0.5% by weight based on the monomer composition. When the concentration of such a thermal polymerization initiator is excessively low, almost no additional thermal polymerization occurs and the effect of adding the thermal polymerization initiator is slight. If the concentration of the thermal polymerization initiator is excessively high, the molecular weight of the superabsorbent resin may be small and the physical properties may become non-uniform.

[0055] The redox pair initiator is a substance that undergoes a redox reaction with each other in the monomer composition. As an example, sodium metabisulfite and sodium persulfate; or ascorbic acid and hydrogen peroxide can be used as the redox pair initiator.

[0056] Since the redox pair initiator causes polymerization initiation before photopolymerization by the radicals generated in the redox process, it is preferably added to the monomer composition immediately before polymerization. The redox pair initiator (that is, each of the oxidizing agent and the reducing agent) can be used in an amount of about 0.001 to about 0.5% by weight based on the monomer composition.

[0057] The clay contained in the monomer composition is included to improve the polymerization rate and enhance the gel strength.

[0058] As the clay, swelling or non-swelling clay can be used. The swelling clay is a layered organic substance having water absorption, and montmorillonite, saponite, nontronite, laponite, beidellite, hectorite, vermiculite, magadiite, bentonite, etc. can be used. As the non-swelling clay, kaolin, serpentine, mica, etc. can be used. The clay can be used alone or in combination of two or more.

[0059] The clay having an average particle size (D50) of 0.025 μm or more, or 0.05 μm or more, or 0.1 μm or more, or 1 μm or more and 10 μm or less, or 8 μm or less, or 5 μm or less, or 3 μm or less can be used. When the average particle size (D50) of the clay is less than 0.025 μm, the effect of improving the gel strength of the superabsorbent resin cannot be sufficiently obtained. When the average particle size (D50) of the clay exceeds 10 μm, there may be a problem that it is difficult to achieve uniform dispersion in an aqueous system due to the strong interlayer attraction of the clay, and transparency cannot be ensured after dispersion.

[0060] At this time, "particle size Dn" means the particle size at the n% point of the cumulative particle number distribution according to the particle size, and D50 is the particle size at the 50% point of the cumulative particle number distribution according to the particle size. The average particle size (D50) of the clay can be measured by laser diffraction and dynamic light scattering methods using a particle size analyzer. 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, Mastersizer 3000 of Malvern). 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. D50 can be measured by calculating the particle diameter at the point where it becomes 50% of the cumulative particle number distribution according to the particle size in the measuring device.

[0061] The content of the clay is 0.1 part by weight or more, or 0.2 part by weight or more, or 0.25 part by weight or more based on 100 parts by weight of the acrylic acid monomer, and may be 10 parts by weight or less, or 7 parts by weight or less, or 5 parts by weight or less, or 3 parts by weight or less, or 1 part by weight or less. When the content of the clay is less than 0.1 part by weight with respect to 100 parts by weight of the acrylic acid monomer, the above-described normalized gelation point range cannot be satisfied. Further, if the content of the clay exceeds 10 parts by weight with respect to 100 parts by weight of the acrylic acid monomer, it is difficult to disperse it uniformly in the monomer composition, and the effect of improving the water absorption rate of the produced superabsorbent resin cannot be obtained.

[0062] On the other hand, in order to satisfy the above-described gelation point characteristics, it is necessary for the clay to be uniformly dispersed in the monomer composition. In this regard, it is preferable to use delaminated clay that is peeled by a physical or chemical method as the clay, or to peel the clay in the monomer composition.

[0063] Clay has a layered structure in which layers composed of small silicate plates about 1 nm thick are stacked multiple times due to strong van der Waals forces between them, and the distance between each layer is about 1 nm. Exfoliated clay is clay in which the silicate layers are exfoliated by a physical or chemical method, that is, the distance between the layers is widened to exceed 1 nm, and it is contained in a composition and exhibits more stable dispersibility.

[0064] As an example, as the clay, clay modified with a polymer dispersant containing two or more functional groups selected from the group consisting of an amine group, a carbonyl group, and a hydroxyl group can be used. Such clay has its layered structure exfoliated by the polymer dispersant, and its dispersion stability is significantly improved. Even when it is contained in a composition for producing a superabsorbent resin containing an alkali metal salt or a basic compound, the clay particles do not aggregate or settle and can be uniformly dispersed in the composition.

[0065] In this specification, the "two or more functional groups selected from the group consisting of an amine group, a carbonyl group, and a hydroxyl group" includes functional groups in which two or more of an amine group, a carbonyl group, and a hydroxyl group are combined, such as an amide group containing both an amine group and a carbonyl group, and a carboxyl group containing both a carbonyl group and a hydroxyl group. As an example, the "polymer dispersant containing an amine group and a carbonyl group" may contain an amine group and a carbonyl group in the molecule, respectively, or may contain an amide group in which an amine group and a carbonyl group are combined.

[0066] The polymer dispersant containing the functional group can be bonded to the surface of the clay by the non-bonding electrons of nitrogen and oxygen atoms, and charges can be induced in the polymer by a resonance phenomenon in a composition for producing a superabsorbent resin with a high ion concentration. Due to such characteristics, the polymer dispersant can exfoliate the layered structure of the clay and improve the dispersibility. In addition, since the polymer dispersant does not cause discoloration or reduce the physical properties of the superabsorbent resin, it can be suitably used in the production of the superabsorbent resin.

[0067] From this perspective, it is preferable that the polymer dispersant contains an amine group and a carbonyl group; or a carbonyl group and a hydroxyl group. More preferably, the polymer dispersant may contain an amide group and / or a carboxyl group.

[0068] Specific examples of the polymer dispersant include one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylamide, and polyacrylic acid. Preferably, polyvinylpyrrolidone can be used as the polymer dispersant.

[0069] The molecular weight of the polymer dispersant is not particularly limited. As an example, those having a weight average molecular weight of 2,500 g / mol or more, or 5,000 g / mol or more, or 10,000 g / mol or more and 200,000 g / mol or less, or 100,000 g / mol or less, or 40,000 g / mol can be used. If the weight average molecular weight of the polymer dispersant is less than 2,500 g / mol, when the polymer dispersant binds to the surface of the clay, it is difficult to sufficiently ensure the interlayer distance of the silicate of the clay, so there are problems in terms of ensuring dispersibility and long-term stability. If it exceeds 200,000 g / mol, there are engineering difficulties and it is not effective for surface modification of the clay. Therefore, it is preferable to satisfy the above range.

[0070] The weight average molecular weight of the polymer dispersant can be measured using gel permeation chromatography (GPC) using a polystyrene standard.

[0071] As an example, GPC analysis can be performed under the following conditions using a Waters PL-GPC220 instrument with a Polymer Laboratories PLgel MIX-B column with a length of 300 mm.

[0072] Column temperature: 160 °C Solvent: 1,2,4-trichlorobenzene Flow rate: 1 mL / min Sample: After preparing the concentration to 10 mg / 10 mL, supply in an amount of 200 μL. Using the calibration curve formed with polystyrene standards (9 kinds of standards with molecular weights of 2,000 / 10,000 / 30,000 / 70,000 / 200,000 / 700,000 / 2,000,000 / 4,000,000 / 10,000,000), induce the values of Mw and Mn.

[0073] The polymer dispersant is used in a content of more than 20 parts by weight to 200 parts by weight or less with respect to 100 parts by weight of clay to ensure the dispersibility of clay. Preferably, it can be used in a content of 23 parts by weight or more, or 25 parts by weight or more and 150 parts by weight or less, or 100 parts by weight or less.

[0074] In case the content of the polymer dispersant is 20 parts by weight or less with respect to 100 parts by weight of clay, the degree of surface modification of clay is insufficient and the interlayer spacing of clay cannot be widened sufficiently. Therefore, the dispersion stability of clay is not sufficient and clay particles can easily aggregate. Also, if the content of the polymer dispersant is excessive, more than 200 parts by weight with respect to 100 parts by weight of clay, the polymer dispersant may reduce the improvement effect of the physical properties that clay can provide, and the physical properties of the superabsorbent resin may deteriorate.

[0075] The clay whose surface is modified with the polymer dispersant can be produced by adding clay and the polymer dispersant to a solvent such as water, lower alcohols such as ethanol, and glycols, and stirring. Also, the clay whose surface is modified with the polymer dispersant can be added to the monomer composition for producing the superabsorbent resin in the state of a dispersion liquid dispersed in the solvent. As the solvent for producing the clay dispersion liquid, the solvent used for the monomer for producing the superabsorbent resin can be used, and the amount of the solvent used can be appropriately adjusted in consideration of the types and amounts of clay and the polymer dispersant, and the use of the clay dispersion liquid.

[0076] As an example, the dispersion containing clay surface-modified with the polymer dispersant may contain 1 part by weight or more, or 3 parts by weight or more and 20 parts by weight or less, or 10 parts by weight or less, or 8 parts by weight or less of clay in 100 parts by weight of the dispersion.

[0077] On the other hand, when using clay whose surface is not modified, the clay dispersion can be shear-mixed and added to the monomer composition, or the clay can be added to the monomer composition and then shear-mixing can be performed. By such shear-mixing, the clay can be physically exfoliated, whereby the clay can be more uniformly and stably dispersed in the monomer composition.

[0078] For the solvent for producing the clay dispersion containing the clay whose surface is not modified, and the content of the clay in the dispersion, reference can be made to the description of the dispersion containing the clay surface-modified with the polymer dispersant.

[0079] The shear-mixing may be carried out using equipment such as an in-line high shear mixer, a high shear batch mixer or a homogenizer, for example.

[0080] The shear-mixing may be carried out at a stirring speed of 6,500 rpm or more, or 9,000 rpm or more, or 12,000 rpm or more. The upper limit of the stirring speed is not particularly limited, but as an example, it may be 50,000 rpm or less, or 40,000 rpm or less, or 30,000 rpm or less.

[0081] Under such a stirring speed, the clay can be physically exfoliated by shear-mixing for at least 10 seconds or more, preferably 20 seconds or more, or 30 seconds or more. At this time, if the shear-mixing time becomes excessively long, there may be a problem of heat generation due to the shearing force, so the shear-mixing time is preferably carried out for 3 minutes or less, or 1 minute or less.

[0082] On the one hand, when adding clay to the monomer composition and then performing shear mixing, in order to achieve a more uniform dispersion degree, the non-surface-modified clay can be added to the monomer composition in the form of an aqueous dispersion. The clay aqueous dispersion can be produced by mixing water and clay using the above-mentioned shear mixing equipment at a stirring speed of 6,500 rpm or more, or 9,000 rpm or more, or 12,000 rpm or more. During such shear mixing, the upper limit of the stirring speed is not particularly limited. As an example, it may be 50,000 rpm or less, or 40,000 rpm or less, or 30,000 rpm or less. The clay aqueous dispersion can be produced by mixing for a time of 10 seconds or more, or 20 seconds or more, or 30 seconds or more and 5 minutes or less, or 3 minutes or less under the said stirring speed.

[0083] At this time, the concentration of the clay aqueous dispersion, that is, the content of clay in 100 parts by weight of the clay aqueous dispersion, may be 0.5 part by weight or more, or 1.0 part by weight or more, or 2.0 part by weight or more and 5.0 parts by weight or less, or 4.0 parts by weight or less, which may be appropriate for ensuring uniform dispersibility.

[0084] As described above, by using clay surface-modified with a polymer dispersant or physically exfoliating and using non-surface-modified clay, the clay can be uniformly dispersed in the monomer composition, and the monomer composition produced in this way can satisfy the above-mentioned gelation point characteristics.

[0085] On the other hand, whether the clay is "uniformly dispersed" in the monomer composition can be judged by whether layer separation of the monomer composition occurs when it is allowed to stand for 1 minute after mixing the clay. That is, if layer separation does not occur even after the monomer composition is allowed to stand for 1 minute, it can be expected that the clay is uniformly dispersed in the monomer composition, and thus the composition satisfies the above-mentioned gelation point characteristics.

[0086] The carbonate-based foaming agent plays a role in causing foaming during polymerization to form pores in the water-containing gel polymer and increase the surface area. As an example, one or more selected from the group consisting of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, calcium bicarbonate, calcium carbonate, magnesium bicarbonate, and magnesium carbonate can be used.

[0087] The carbonate-based foaming agent can be used in an amount of 0.005 to 1 part by weight based on 100 parts by weight of the acrylic acid-based monomer. When the content of the foaming agent is less than 0.005 part by weight, the role as a foaming agent is slight. When the content of the foaming agent exceeds 1 part by weight, the gel strength of the superabsorbent resin produced with an excessive number of pores in the crosslinked polymer decreases, and the density becomes small, which may cause problems in distribution and storage. For example, the carbonate-based foaming agent may be 0.01 part by weight or more, 0.05 part by weight or more, and 0.5 part by weight or less, 0.3 part by weight or less, or 0.2 part by weight or less based on 100 parts by weight of the acrylic acid-based monomer.

[0088] When manufacturing the monomer composition, surfactants such as alkyl sulfate compounds and polyoxyethylene alkyl ether compounds that are usually used as bubble stabilizers may not be used. For example, in the above-mentioned Step 1 and Step 2, cationic surfactants such as quaternary ammonium compounds like dodecyltrimethylammonium chloride and dodecyltrimethylammonium bromide; anionic surfactants such as alkyl sulfate compounds like sodium dodecyl sulfate, ammonium lauryl sulfate, sodium lauryl ether sulfate, or sodium myreth sulfate; or nonionic surfactants such as alkyl ether sulfate compounds like polyoxyethylene lauryl ether may not be used. Thereby, the problem that the surface tension of the superabsorbent resin becomes low due to the use of the surfactant can be prevented.

[0089] The monomer composition can further contain additives such as thickeners, plasticizers, storage stabilizers, and antioxidants as needed.

[0090] 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, crosslinking agent, and 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 may be 10 to 80% by weight, or 15 to 60% by weight, or 30 to 50% by weight.

[0091] When the monomer composition has a solid content within such a range, it is possible to adjust the grinding efficiency during the grinding of the polymer described later, while eliminating the need to remove unreacted monomers after polymerization by utilizing the gel effect phenomenon that appears in the polymerization reaction of the high-concentration aqueous solution.

[0092] At this time, the solvent that can be used can be used without any limitation on its composition as long as it can dissolve the above-described 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.

[0093] On the other hand, according to one embodiment of the present invention, there is provided a method for producing a superabsorbent resin, including the steps of irradiating the monomer composition for producing the superabsorbent resin with heat and / or light to polymerize it to produce a hydrogel polymer; drying, grinding, and classifying the hydrogel polymer to form a base resin; and a surface cross-linking step of forming a surface cross-linking layer on the surface of the base resin in the presence of a surface cross-linking solution containing a surface cross-linking agent and a solvent.

[0094] The method for polymerizing the monomer composition for producing the superabsorbent resin is not particularly limited. The polymerization method can be roughly divided into thermal polymerization and photopolymerization according to the polymerization energy source. Usually, when proceeding with thermal polymerization, it may be carried out in a reactor having a stirring shaft such as a kneader, and when proceeding with photopolymerization, it may be carried out in a reactor equipped with a movable conveyor belt. However, the above-described polymerization methods are only examples, and the present invention is not limited to the above-described polymerization methods.

[0095] As an example, as described above, for a reactor such as a kneader equipped with a stirring shaft, a water-containing gel polymer obtained by supplying hot air or heating the reactor to perform thermal polymerization may have a form of several centimeters to several millimeters when discharged from the discharge port of the reactor according to the form of the stirring shaft provided in the reactor. Specifically, the size of the obtained water-containing gel polymer can appear in various ways according to the concentration and injection rate of the monomer composition to be injected, but usually, a water-containing gel polymer with a weight average particle size of 2 to 50 mm is obtained.

[0096] Also, when photopolymerization is allowed to proceed in a reactor equipped with a movable conveyor belt as described above, the form of the water-containing gel polymer usually obtained may be a sheet-like water-containing gel polymer having the width of the belt. At this time, the thickness of the polymer sheet varies according to the concentration and injection rate of the monomer composition to be injected, but usually, it is preferable to supply the monomer composition so that a sheet-like polymer having a thickness of about 0.5 to about 5 cm can be obtained. When the monomer composition is supplied to such an extent that the thickness of the sheet-like polymer is excessively thin, the production efficiency is low and it is not preferable. When the thickness of the sheet-like polymer exceeds 5 cm, the polymerization reaction may not occur uniformly over the entire thickness due to the excessively thick thickness.

[0097] At this time, the normal water content of the water-containing gel polymer obtained by such a method may be about 40 to about 80% by weight. On the other hand, throughout this specification, the "water content" means the content of water in the total polymer weight, and means a value obtained by subtracting the weight of the polymer in the dry state from the weight of the polymer. Specifically, it is defined as a value measured and calculated by measuring the weight reduction due to water evaporation in the polymer during the process of drying by raising the temperature of the polymer by 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, and the total drying time is set to 20 minutes including 5 minutes in the temperature rising stage to measure the water content.

[0098] Next, a step of drying and pulverizing the hydrogel polymer to form a base resin in powder form is performed. Optionally, in order to enhance the efficiency of the drying step, a step of coarsely pulverizing before drying can be further carried out.

[0099] At this time, the pulverizer used is not limited in configuration. Specifically, it can include any one selected from the group of pulverizing devices consisting of 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, and a disc cutter, but is not limited to the above-described examples.

[0100] At this time, the pulverizing step can pulverize the polymer so that the particle size of the polymer is about 2 to about 10 mm. Pulverizing to a particle size of less than 2 mm is not technically easy due to the high water content rate of the hydrogel polymer, and there may also occur a phenomenon of aggregation between the pulverized particles. On the other hand, when pulverizing to a particle size exceeding 10 mm, the effect of increasing the efficiency of the subsequent drying step is slight.

[0101] Drying is performed on the polymer pulverized as described above or on the polymer immediately after polymerization that has not undergone the pulverizing step. At this time, the drying temperature in the drying step may be about 150 to about 250°C. If the drying temperature is less than 150°C, the drying time may become excessively long, and the physical properties of the finally formed superabsorbent resin may deteriorate. If the drying temperature exceeds 250°C, only the surface of the polymer may be excessively dried, and fine powder may be generated in the subsequent pulverizing process, and the physical properties of the finally formed superabsorbent resin may deteriorate. Therefore, preferably, the drying is performed at a temperature of about 150 to about 200°C, and more preferably at a temperature of about 160 to about 180°C.

[0102] On the other hand, in the case of drying time, it may be carried out for about 20 to about 90 minutes, but is not limited thereto, considering process efficiency and the like.

[0103] The drying method in the drying stage can be selected and used without limitation of its configuration as long as it is commonly used in the drying process of the water-containing gel polymer. Specifically, the drying stage can be advanced by methods such as hot air supply, infrared irradiation, extremely high frequency irradiation, or ultraviolet irradiation. The water content of the polymer after such advancement of the drying stage may be about 0.1 to about 5% by weight.

[0104] Next, a step of pulverizing the dried polymer obtained through such a drying stage is carried out.

[0105] The base resin, which is the polymer powder obtained after the pulverizing stage, may have a particle size of about 150 to about 850 μm. Specifically, a pin mill, a hammer mill, a screw mill, a roll mill, a disc mill, a jog mill, or the like can be used as the pulverizer for pulverizing to such a particle size, but the present invention is not limited to the above-described examples.

[0106] Then, in order to control the physical properties of the superabsorbent resin powder that is finally commercialized after such a pulverizing stage, the base resin obtained after pulverization is classified according to the particle size. Preferably, the polymer having a particle size of about 150 to about 850 μm is classified, and only the base resin having such a particle size can undergo the surface crosslinking reaction stage. Such a particle size can be measured by the method of the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP220.3.

[0107] Next, in the presence of a surface crosslinking agent, the surface of the base resin is additionally crosslinked to form a surface crosslinked layer. By this step, a superabsorbent resin is produced in which a surface crosslinked layer is formed on the surface of the base resin, more specifically, on at least a part of the surface of each base resin particle.

[0108] Surface crosslinking is a step of increasing the crosslinking density near the surface of superabsorbent polymer particles in relation to the crosslinking density inside the particles. Generally, the surface crosslinking agent is applied to the surface of the superabsorbent resin particles. Therefore, this reaction occurs on the surface of the superabsorbent resin particles, which improves the crosslinking property on the surface of the particles without substantially affecting the inside of the particles. Therefore, the surface crosslinked superabsorbent resin particles have a higher degree of crosslinking near the surface than inside.

[0109] As the surface crosslinking agent, all surface crosslinking agents conventionally used in the production of superabsorbent resins can 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, propylene carbonate, and glycerol carbonate; epoxy compounds such as ethylene glycol diglycidyl ether; oxazolidinone compounds such as 2-oxazolidinone; polyamine compounds; oxazoline compounds; mono-, di- or polyoxazolidinone compounds; or cyclic urea compounds; etc. can be included. Specifically, as the surface crosslinking agent, one or more, or two or more, or three or more of the above-described surface crosslinking agents can be used. For example, ethylene carbonate can be used as the surface crosslinking agent.

[0110] There is no limitation on the constitution of the method for mixing the surface crosslinking agent with the base resin. The surface crosslinking agent and the base resin powder can be put into a reaction tank and mixed, or the surface crosslinking agent can be sprayed onto the base resin powder. Methods such as continuously supplying the base resin and the surface crosslinking agent to a continuously operating mixer and mixing them can be used.

[0111] When adding the surface crosslinking agent, water can be mixed together and added in the form of a surface crosslinking solution. When adding water, there is an advantage that the surface crosslinking agent can be uniformly dispersed in the polymer. At this time, the content of the added water is preferably added at a ratio of about 1 to about 10 parts by weight with respect to 100 parts by weight of the base resin for the purpose of inducing uniform dispersion of the surface crosslinking agent, preventing the aggregation phenomenon of the polymer powder, and at the same time optimizing the surface penetration depth of the surface crosslinking agent.

[0112] The surface crosslinking bonding reaction is carried out by heating the base resin added with the surface crosslinking solution containing the surface crosslinking agent and the solvent at a temperature of about 100 to about 150 ° C, preferably about 110 to about 140 ° C for about 15 to about 80 minutes, preferably about 20 to about 70 minutes.

[0113] 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 types of heat media that can be used, heated fluids such as steam, hot air, and hot oil can be used, but the present invention is not limited to these, 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, examples of the directly supplied heat source include heating by electricity and heating by gas, but the present invention is not limited to the above-described examples.

[0114] The monomer composition for producing the superabsorbent resin gels at a high speed after the start of polymerization, and can minimize the loss of bubbles generated from the foaming agent. Therefore, the monomer composition for producing the superabsorbent resin can provide a superabsorbent resin exhibiting an excellent water absorption rate.

[0115] Specifically, the superabsorbent resin produced from the monomer composition for producing a superabsorbent resin has a Performance Index (PI) represented by the following calculation formula 1 of 2.35 or more and can satisfy that T20 is less than 190 seconds:

[0116] [Calculation formula 1] Performance Index (PI) = (CRC / 27) + (AUP / 24) + (SFC / 30) - (T20 / 140)

[0117] In the above calculation formula 1, CRC is the centrifugal separation water retention capacity (g / g) of the superabsorbent resin for 30 minutes with respect to a 0.9 wt% sodium chloride aqueous solution, AUP is the 0.7 psi pressurized water absorption capacity (g / g) of the superabsorbent resin for 1 hour with respect to a 0.9 wt% sodium chloride aqueous solution, SFC is the flow conductivity (·10 -7 cm 3 ·s / g) of a 0.685 wt% sodium chloride aqueous solution, T20 is the time (seconds) required for 1 g of the superabsorbent resin to absorb 20 g of an aqueous solution of sodium chloride and an alcohol ethoxylate having 12 to 14 carbon atoms under 0.3 psi.

[0118] The measurement methods of the above CRC, AUP, SFC, and T20 will be specifically described in the examples described later.

[0119] The superabsorbent resin produced from the monomer composition for producing a superabsorbent resin has a form in which the above-mentioned clay is uniformly dispersed inside and outside.

[0120] Preferably, the PI of the superabsorbent resin is 2.37 or more, 2.40 or more, or 2.41 or more, and may be 4.0 or less, or 3.0 or less. Further, T20 of the superabsorbent resin is 180 seconds or less, 160 seconds or less, 140 seconds or less, 130 seconds or less, or 125 seconds or less, and may be 100 seconds or more.

[0121] In addition, the superabsorbent resin produced from the monomer composition for producing a superabsorbent resin of the present invention has high gel strength and maintains a good form even after water absorption. Therefore, it can exhibit excellent water absorption performance despite external pressure.

[0122] Hereinafter, preferred embodiments are presented for the understanding of the present invention. However, the following embodiments are merely illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications can be made within the scope of the present invention and the scope of the technical idea. Needless to say, such changes and modifications belong to the scope of the appended claims.

[0123] [Examples] Comparative Example 1 (1) Production of monomer composition for producing superabsorbent resin To a 3 L glass container equipped with a stirrer and a thermometer, 450 g of acrylic acid, 3 g of PEGDA400 (polyethylene glycol diacrylate 400) as an internal crosslinking agent, and 0.04 g of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide were added and dissolved. Then, 580 g of 31.5% sodium hydroxide solution was added to produce a monomer mixture (neutralization degree: 70 mol%; solid content: 41 wt%).

[0124] To the monomer mixture, 0.1 part by weight of sodium bicarbonate (SBC) as a carbonate-based foaming agent was added per 100 parts by weight of acrylic acid to produce a monomer composition for producing a superabsorbent resin.

[0125] (2) Production of superabsorbent resin A part (86.6 g) of the monomer composition for producing a superabsorbent resin produced in (1) above was placed in a rheometer (TA Instruments, ARES) equipped with a UV-accessory, and ultraviolet rays were irradiated to perform UV polymerization for 180 seconds. Specifically, the temperature during polymerization was maintained at 60°C, and ultraviolet rays were irradiated for 60 seconds (irradiation dose: 100 mW / cm 2 ), and the reaction was allowed to proceed for an additional 120 seconds.

[0126] At this time, the storage modulus (G’) and loss modulus (G’’) of the monomer composition were measured from the polymerization start point and plotted against the polymerization time. The water content of the hydrogel polymer obtained after the polymerization reaction was measured, and as a result, it was confirmed to be 48%.

[0127] The remaining monomer composition for producing the superabsorbent resin that was not added to the rheometer was put into a polymerization chamber where the temperature was controlled at 60°C and a UV light source was present at the upper end. After that, it was irradiated with ultraviolet rays for 60 seconds, and the reaction was allowed to proceed for an additional 120 seconds to obtain a hydrogel polymer. After cutting the hydrogel polymer thus produced into a size of about 5 cm × 5 cm, it was put into a meat chopper to crush the polymer and obtain hydrogel crumbs having a size of 1 mm to 10 mm. Thereafter, the crumbs were dried in an oven where the air volume could be transferred up and down. Hot air at 180°C or higher was made to flow from the bottom to the top for 15 minutes and then from the top to the bottom for another 15 minutes for uniform drying so that the water content of the dried product after drying was 1% or less. After drying, it was crushed with a crusher and then classified to select a size of 150 to 850 μm to produce a base resin.

[0128] To 100 parts by weight of the produced base resin powder, a surface crosslinking solution containing 1.5 parts by weight of ethylene carbonate and 6 parts by weight of water with respect to 100 parts by weight of the base resin was sprayed, and it was stirred at room temperature to mix so that the surface crosslinking solution was uniformly distributed on the base resin powder. Next, the base resin powder mixed with the surface crosslinking solution was put into a surface crosslinking reactor to proceed with the surface crosslinking reaction.

[0129] In the surface crosslinking reactor, it was confirmed that the temperature of the base resin powder gradually increased from an initial temperature near 80°C, and after 30 minutes, it was operated to reach a reaction maximum temperature of 190°C. After reaching the reaction maximum temperature, it was allowed to react for an additional 15 minutes, and then the finally produced superabsorbent resin sample was taken. After the surface crosslinking step, it was classified with a standard sieve of ASTM standard to produce a superabsorbent resin of Comparative Example 1 having a particle size of 150 μm to 850 μm.

[0130] Example 1 450 g of acrylic acid, 3 g of PEGDA400 (polyethylene glycol diacrylate 400) as an internal crosslinking agent, and 0.04 g of photoinitiator diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide were added to a 3 L glass container equipped with a stirrer and a thermometer and dissolved. Then, 580 g of 31.5% sodium hydroxide solution was added to produce a monomer mixture (neutralization degree: 70 mol%; solid content: 41 wt%).

[0131] To the monomer mixture, an aqueous dispersion of kaolin (average particle size (D50) 0.6 μm) was added so that the amount of kaolin was 0.25 parts by weight with respect to 100 parts by weight of acrylic acid, and the mixture was dispersed by mixing at 6500 rpm for 3 minutes using a homogenizer (IKA, Ultra-turrax). The aqueous kaolin dispersion was prepared by mixing 96 g of water and 4 g of kaolin at 6500 rpm for 30 seconds using a homogenizer (IKA, Ultra-turrax).

[0132] Thereafter, 0.1 part by weight of sodium bicarbonate (SBC) was added and mixed with respect to 100 parts by weight of acrylic acid, and 0.12 g of hydrogen peroxide (H2O2), 0.04 g of ascorbic acid, and 2 g of sodium persulfate were added immediately before the start of polymerization to produce a monomer composition for producing a superabsorbent resin.

[0133] A superabsorbent resin was produced using the monomer composition in the same manner as in (2) of Comparative Example 1.

[0134] Example 2 A monomer composition for producing a superabsorbent resin and a superabsorbent resin were produced in the same manner as in Example 1, except that 0.5 part by weight of kaolin (average particle size (D50) 0.6 μm) was added with respect to 100 parts by weight of acrylic acid.

[0135] Example 3 A superabsorbent resin-producing monomer composition and a superabsorbent resin were produced in the same manner as in Example 1, except that 1 part by weight of kaolin (average particle diameter (D50): 0.6 μm) was added to 100 parts by weight of acrylic acid.

[0136] Example 4 A superabsorbent resin-producing monomer composition and a superabsorbent resin were produced in the same manner as in Example 1, except that 0.25 part by weight of bentonite (average particle diameter (D50): 0.8 μm) was added to 100 parts by weight of acrylic acid.

[0137] Example 5 A superabsorbent resin-producing monomer composition and a superabsorbent resin were produced in the same manner as in Example 1, except that 0.5 part by weight of bentonite (average particle diameter (D50): 0.8 μm) was added to 100 parts by weight of acrylic acid.

[0138] Example 6 A superabsorbent resin-producing monomer composition and a superabsorbent resin were produced in the same manner as in Example 1, except that 1 part by weight of bentonite (average particle diameter (D50): 0.8 μm) was added to 100 parts by weight of acrylic acid.

[0139] Example 7 To a 3-L glass container equipped with a stirrer and a thermometer, 450 g of acrylic acid, 3 g of an internal cross-linking agent PEGDA400 (polyethylene glycol diacrylate 400), and 0.04 g of a photoinitiator diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide were added and dissolved, and then 580 g of a 31.5% sodium hydroxide solution was added to produce a monomer mixture (neutralization degree: 70 mol%; solid content: 41% by weight).

[0140] Separately, into a flask, 2 g of polyacrylamide (Merck, weight average molecular weight of Polyacrylamide: 40,000 g / mol), 4 g of montmorillonite (average particle diameter (D50): 0.8 μm), and 96 g of water were charged and stirred at 6500 rpm for 30 seconds using a homogenizer (IKA, Ultra-turrax) to produce a 4% by weight clay dispersion.

[0141] To the monomer mixture, the clay dispersion was added such that the clay content was 0.5 parts by weight with respect to 100 parts by weight of acrylic acid. After adding and mixing 0.1 part by weight of sodium bicarbonate (SBC) with respect to 100 parts by weight of acrylic acid, 0.12 g of hydrogen peroxide (H2O2), 0.04 g of ascorbic acid, and 2 g of sodium persulfate were added immediately before the start of polymerization to produce a monomer composition for producing a superabsorbent resin.

[0142] Using the monomer composition, a superabsorbent resin was produced in the same manner as in (2) of Comparative Example 1.

[0143] Comparative Example 2 A superabsorbent resin was produced in the same manner as in Comparative Example 1, except that the temperature during polymerization was set to 50°C.

[0144] Comparative Example 3 A monomer composition for producing a superabsorbent resin and a superabsorbent resin were produced in the same manner as in Comparative Example 1, except that 0.12 g of hydrogen peroxide (H2O2) and 0.04 g of ascorbic acid were additionally added during the production of the monomer composition.

[0145] Comparative Example 4 A monomer composition for producing a superabsorbent resin and a superabsorbent resin were produced in the same manner as in Comparative Example 1, except that 0.2 part by weight of sodium bicarbonate (SBC) was added as a foaming agent with respect to 100 parts by weight of acrylic acid during the production of the monomer composition.

[0146] Comparative Example 5 A monomer composition for producing a superabsorbent resin and a superabsorbent resin were produced in the same manner as in Comparative Example 1, except that 0.3 part by weight of sodium bicarbonate (SBC) was added as a foaming agent with respect to 100 parts by weight of acrylic acid during the production of the monomer composition.

[0147] Comparative Example 6 A monomer composition for producing a superabsorbent resin and a superabsorbent resin were produced in the same manner as in Example 2, except that the stirring speed of the homogenizer was set to 300 rpm when the monomer mixture was mixed with the kaolin aqueous dispersion.

[0148] Experimental Example 1: Physical Property Evaluation of Monomer Composition for Producing Superabsorbent Resin From the G' plot and G'' plot obtained during the polymerization of each monomer composition for producing a superabsorbent resin in the above Examples and Comparative Examples, the gel point (GP), normalized gel point (Normalized GP), G' at the gel point (G' at GP), the final G' of the polymer after the completion of polymerization (final G'), and the acceleration rate of increase in G' from the gel point to the end point of polymerization (ΔG' / s) were determined, and the results are shown in Table 1 below.

[0149] The gel point is the time (seconds, s) at the intersection of the G' plot and the G'' plot, and the normalized gel point was calculated from Equation 1 below.

[0150] [Equation 1] Normalized gel point = gel point (seconds) / total polymerization time (seconds)

[0151] Experimental Example 2: Physical Property Evaluation of Superabsorbent Resin (1) Vortex Absorption Rate of Base Resin The vortex absorption rate (vortex time) of the base resins in the above Examples and Comparative Examples was measured by the following method. For the measurement of the absorption rate, a superabsorbent resin classified with a #30 - 50 sieve was used.

[0152] (i) First, 50 mL of 0.9% saline was placed in a flat-bottomed 100 mL beaker using a 100 mL graduated cylinder. (ii) Next, after placing the beaker in the center of a magnetic stirrer, a circular magnetic bar (diameter 8 mm, length 30 mm) was placed in the beaker. (iii) Thereafter, the stirrer was operated so that the magnetic bar stirred at 600 rpm, and the bottom of the vortex generated by stirring hit above the magnetic bar. (iv) After confirming that the temperature of the salt water in the beaker reached 24.0 °C, a 2 ± 0.01 g superabsorbent resin sample was added, and at the same time, a stopwatch was started. The time until the liquid surface became completely horizontal while the vortex disappeared was measured in seconds, and this was defined as the water absorption rate.

[0153] (2) Centrifuge Retention Capacity (CRC) According to the European Disposables and Nonwovens Association (EDANA) standard EDANA WSP241.3, the centrifuge retention capacity (CRC) was measured based on the water absorption multiple under no load. After uniformly placing a superabsorbent resin W0 (g, about 0.2 g) in a non-woven bag and sealing it, it was immersed in physiological saline, an aqueous solution of 0.9 wt% sodium chloride at room temperature. After 30 minutes, the bag was dehydrated at 250G for 3 minutes using a centrifuge, and then the mass W2 (g) of the bag was measured. Also, after performing the same operation without using the superabsorbent resin, the mass W1 (g) at that time was measured. Using each of the thus obtained masses, the CRC (g / g) was calculated by the following calculation formula 2 to confirm the water retention capacity.

[0154] [Calculation formula 2] CRC (g / g) = {[W2 (g) - W1 (g) - W0 (g)] / W0 (g)}

[0155] (3) Absorbing under Pressure (AUP) For the superabsorbent resins of the examples and comparative examples, the absorbing under pressure (AUP) was measured according to the method of the European Disposables and Nonwovens Association standard EDANA WSP242.3 - 10.

[0156] First, a 400-mesh stainless steel wire mesh was attached to the plastic cylindrical bottom with an inner diameter of 60 mm. Under the conditions of a temperature of 23 ± 2°C and a relative humidity of 45%, the resin W0 (g, 0.90 g) obtained in the examples and comparative examples was uniformly sprayed onto the wire mesh, and a piston with an outer diameter slightly smaller than 60 mm and no gap with the inner wall of the cylinder and whose up-and-down movement was not hindered was used to uniformly apply an additional load of 4.83 kPa (0.7 psi) thereon. At this time, the weight W3 (g) of the said device was measured.

[0157] A glass filter with a diameter of 125 mm and a thickness of 5 mm was placed inside a petri dish with a diameter of 150 mm, and physiological saline composed of 0.90 wt% sodium chloride was made to be at the same level as the upper surface of the glass filter. The said measuring device was placed on the glass filter, and the liquid was absorbed under the load for 1 hour. After 1 hour, the measuring device was lifted and its weight W4 (g) was measured.

[0158] Using each mass thus obtained, AUP (g / g) was calculated by the following calculation formula 3 to confirm the pressurized water absorption capacity.

[0159] [Calculation formula 3] AUP (g / g) = [W4 (g) - W3 (g)] / W0 (g)

[0160] In the said calculation formula 2, W0 (g) is the initial weight (g) of the superabsorbent resin, W3 (g) is the sum of the weight of the superabsorbent resin and the weight of the device capable of applying a load to the said superabsorbent resin, W4 (g) is the sum of the weight of the superabsorbent resin and the weight of the device capable of applying a load to the said superabsorbent resin after the superabsorbent resin has absorbed physiological saline under a load (0.7 psi) for 1 hour.

[0161] (3) Saline flow conductivity (SFC) It was measured and calculated by the method disclosed in columns 54 to 59 of US Patent Registration No. 5562646.

[0162] (4) T20 An aqueous solution was prepared by dissolving 9 g of sodium chloride and 0.1 g of Lorodac (main component: alcohol ethoxylate with 12 - 14 linear carbon atoms, CAS#68439 - 50 - 9) in 1 L of distilled water. Under a pressure of 0.3 psi, the time taken for 1 g of the superabsorbent resin to absorb 20 g of such an aqueous solution was calculated and measured. The specific measurement method for such T20 is described in detail on pages 13 - 18 of European Patent Publication No. 2535027.

[0163] (5) Performance Index (PI) Using CRC (g / g), AUP (g / g), SFC (·10 -7 cm 3 ·s / g), and T20 (seconds) measured by the above method, Performance Index (PI, unitless) was calculated according to the following calculation formula 1.

[0164] [Calculation formula 1] Performance Index (PI) = (CRC / 27) + (AUP / 24) + (SFC / 30) - (T20 / 140)

[0165]

Table 1

[0166]

Table 2

Claims

1. A monomer composition for producing a superabsorbent resin, comprising an acrylic acid-based monomer having an acidic group and at least a part of the acidic group neutralized; a crosslinking agent; clay; a carbonate-based foaming agent; and a polymerization initiator, wherein when the monomer composition for producing a superabsorbent resin is irradiated with heat and / or light to proceed with a polymerization reaction, the normalized gel point represented by the following formula 1 is 0.01 to 0.

1. A monomer composition for producing a superabsorbent resin. [Formula 1] Normalized gel point = gel point (seconds) / total polymerization time (seconds) In the above formula 1, the gel point is the polymerization time at the intersection of the storage modulus graph of the monomer composition for producing a superabsorbent resin according to the polymerization time and the loss modulus graph of the monomer composition for producing a superabsorbent resin according to the polymerization time.

2. The monomer composition for producing a superabsorbent resin according to claim 1, wherein when the monomer composition for producing a superabsorbent resin is irradiated with heat and / or light to proceed with a polymerization reaction, the rate of increase in gel strength from the gel point to the completion of polymerization is 210 Pa / s or more.

3. The monomer composition for producing a superabsorbent resin according to claim 1, wherein the clay is contained in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the acrylic acid-based monomer.

4. The monomer composition for producing a superabsorbent resin according to claim 1, wherein the clay has an average particle diameter of 0.025 μm to 10 μm.

5. The monomer composition for producing a superabsorbent resin according to claim 11, wherein the clay is modified with a polymer dispersant containing two or more functional groups selected from the group consisting of an amine group, a carbonyl group, and a hydroxyl group on the surface.

6. The monomer composition for producing a superabsorbent resin according to claim 5, wherein the polymer dispersant is one or more selected from the group consisting of polyvinylpyrrolidone, polyacrylamide, and polyacrylic acid.

7. A step of mixing an acrylic acid-based monomer having an acidic group and at least a part of the acidic group neutralized; a crosslinking agent; a carbonate-based foaming agent; and a polymerization initiator; and a step of adding clay to the mixture and shear-mixing at a stirring speed of 6,500 rpm or more; A method for producing a monomer composition for producing a superabsorbent resin according to claim 1.

8. A polymer produced from the monomer composition for producing a superabsorbent resin according to claim 1, wherein when the water content is 40% by weight to 80% by weight, the gel strength is 35,000 Pa to 60,000 Pa.

9. A superabsorbent resin produced from the monomer composition for producing a superabsorbent resin according to claim 1, wherein the Performance Index (PI) represented by the following calculation formula 1 is 2.35 or more and T20 is less than 190 seconds: [Calculation formula 1] Performance Index (PI) = (CRC / 27) + (AUP / 24) + (SFC / 30) - (T20 / 140) In the above calculation formula 1, CRC is the centrifugal retention capacity (g / g) of the superabsorbent resin for 30 minutes with respect to a 0.9 wt% sodium chloride aqueous solution, AUP is the 0.7 psi pressurized water absorption capacity (g / g) of the superabsorbent resin for 1 hour with respect to a 0.9 wt% sodium chloride aqueous solution, The SFC is the flow conductivity (·10 －７ cm ３ ·s / g) of an aqueous sodium chloride solution of 0.685% by weight, and T20 is the time (seconds) required for 1 g of the superabsorbent resin to absorb 20 g of an aqueous solution of sodium chloride and an alcohol ethoxylate having 12 to 14 carbon atoms under 0.3 psi.

10. A step of irradiating the monomer composition for producing a superabsorbent resin according to claim 1 with heat and / or light to polymerize and produce a hydrogel polymer; A step of drying, pulverizing and classifying the hydrogel polymer to form a base resin; and A method for producing a superabsorbent resin, comprising a step of forming a surface crosslinked layer on the surface of the base resin in the presence of a surface crosslinked solution containing a surface crosslinking agent and a solvent.

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