JP2007321010A - Method for surface treating water-absorbing resin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for providing a water-absorbing resin excellent in absorption magnification under pressure, absorption rate, gel strength, liquid-permeating property, etc., in a high efficiency and at a low cost. <P>SOLUTION: This method for surface-treating the water-absorbing resin comprises (a) a process of obtaining a water-absorbing resin composition by mixing a water-absorbing resin, a radically polymerizable compound and an organic solvent, and (b) a process of irradiating an active energy beam to the obtained water-absorbing resin composition. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a surface treatment method for a water absorbent resin, and more particularly to a surface treatment method for a water absorbent resin by irradiating an active energy ray to the water absorbent resin.

  Conventionally, a water-absorbing resin has been used as a constituent material of sanitary materials that absorb sanitary cotton, disposable diapers, or other body fluids. Examples of such a water-absorbing resin include a hydrolyzate of starch-acrylonitrile graft polymer, a neutralized product of starch-acrylic acid graft polymer, a saponified product of vinyl acetate-acrylic ester copolymer, and an acrylonitrile copolymer. Examples thereof include hydrolysates of polymers or acrylamide copolymers, cross-linked products thereof, and cross-linked products of partially neutralized polyacrylic acid. All of these have an internal crosslinking structure and are insoluble in water.

  Properties desired for such a water-absorbent resin include a high absorption ratio, an excellent absorption rate, a high gel strength, and an excellent suction force for sucking water from the substrate. However, since the water absorption characteristics are affected by the crosslink density, the relationship between the characteristics does not necessarily show a positive correlation, for example, the gel strength increases but the water absorption magnification decreases as the crosslink density increases. In particular, the absorption magnification and the absorption rate, gel strength, suction force, and the like are sometimes in a contradictory relationship. For this reason, in the water-absorbent resin with improved absorption capacity, when the particles come into contact with the liquid, water absorption is not performed uniformly, and a mass of the water-absorbent resin is formed or water is not diffused throughout the water-absorbent resin particles. The speed etc. may decrease extremely.

  A method of coating the surface of water-absorbent resin particles with a surfactant or a non-volatile hydrocarbon in order to alleviate such a phenomenon and obtain a water-absorbent resin having a high absorption rate and a relatively good absorption rate. There is. This method improves the dispersibility of water absorbed in the initial stage, but is not sufficiently effective in improving the absorption speed and suction force of individual particles.

  Further, as a method for producing a polyacrylic acid-based superabsorbent polymer with improved water absorption characteristics, an aqueous composition of a polymer having a partial alkali metal salt of polyacrylic acid as a main component and a low crosslinking density is used as a water-soluble polymer. There is a method of heating in the presence of an oxide radical initiator and introducing crosslinking by radical crosslinking (Patent Document 1). In general, it is difficult to evenly distribute internal crosslinks within the polymer, and it is not easy to adjust the crosslink density. For this reason, in Patent Document 1, after obtaining a polymer containing a water-soluble polyacrylic acid gel having a low crosslinking density, a persulfate as a polymerization initiator is added and heated. In this method, it is possible to precisely control the crosslinking density by adjusting the initiator addition amount, and since the crosslinking is present uniformly in the polymer, excellent water absorption characteristics are obtained and there is no tackiness. It is said that a water absorbent resin was obtained.

  In the technique described in Patent Document 1, the persulfate used is decomposed by heat, but is also decomposed by ultraviolet rays to generate radicals (Non-Patent Document 1). Since persulfate has an action as a polymerization initiator, if an aqueous solution of a water-soluble vinyl monomer is irradiated with light energy, radical crosslinking occurs simultaneously with polymerization, and a hydrogel can be produced (patent document) 2). In addition to the hydrophilic polymer component and the photopolymerization initiator, there is also a reaction system in which a crosslinking agent is further added to form internal crosslinking by ultraviolet irradiation (Patent Document 3).

  On the other hand, as a method for obtaining a water-absorbing resin having excellent water-absorbing properties, a method for increasing the cross-linking density on the surface of the water-absorbing resin is also known (for example, Patent Documents 4 and 5). Reactive functional groups exist on the surface of the water-absorbent resin as shown in the above example. If a surface cross-linking agent capable of reacting with such a functional group is added to the water-absorbing resin and cross-linking is introduced between the functional groups, the surface cross-linking density increases, and excellent water absorption characteristics are exhibited even under pressure.

  The purpose of introducing surface cross-linking to the water-absorbent resin is to provide a water-absorbent resin having an excellent balance of absorption characteristics. In order to introduce surface cross-linking into the water-absorbent resin, a method in which a cross-linking agent having at least two functional groups capable of reacting with functional groups present on the surface of the water-absorbent resin is allowed to act on the water-absorbent resin is often used industrially. Yes. Examples of such cross-linking agents include polyhydric alcohols, polyhydric glycidyl ethers, haloepoxy compounds, polyhydric aldehydes, polyhydric amines, polyvalent metal salts, etc., but generally have low reactivity. It is necessary to carry out the reaction at a high temperature, and in some cases, it may be left under heating for a long time. This consumes a lot of energy and time.

  In view of such a problem, a method of bringing an aqueous solution containing a peroxide radical initiator into contact with a resin, heating the resin, and introducing crosslinks to polymer molecular chains in the vicinity of the surface of the resin through decomposition of the radical initiator There is also (patent document 6). In the Example of the said patent document 6, it heats for 6 minutes with 130 degreeC superheated steam, and obtains the water absorbing resin which the water absorption magnification improved.

  Further, a technique in which an aqueous solution of a water-soluble ethylenically unsaturated monomer containing a polymerization initiator and a crosslinking agent is impregnated in the vicinity of the surface of a particulate water-absorbing resin dispersed in an organic solvent and polymerized by heat (Patent Document 7). ) And a technique of adding a particulate water-absorbing resin to a monomer dispersed in an organic solvent and polymerizing by heat (Patent Document 8) has been proposed.

  However, even if the surface treatment of the water-absorbent resin is performed using heat treatment as described in these patent documents, a certain amount of thermal energy and There is no change in the fact that a certain amount of time is required, and there is a problem that it is difficult to significantly reduce the manufacturing cost.

As a means for reducing the manufacturing cost in the surface treatment of the water-absorbent resin accompanied by such heat treatment, a technique using irradiation with active energy rays (for example, ultraviolet rays) has been proposed (Patent Document 9). According to the technique described in the patent document, after adding a treatment liquid containing a radical polymerizable compound to the particulate water-absorbing resin as a base polymer, the active energy rays are irradiated while flowing the particulate water-absorbing resin. Thus, the surface treatment of the water-absorbent resin can be performed even under a temperature condition of about room temperature, and the manufacturing cost can be reduced. In addition, characteristics such as absorption capacity of the water-absorbent resin surface-treated by the technique can also be maintained at a high level.
U.S. Pat. No. 4,910,250 JP 2004-99789 A International Publication No. 2004/031253 Pamphlet US Pat. No. 4,666,983 US Pat. No. 5,422,405 U.S. Pat. No. 4,783,510 JP-A-1-126314 Japanese Patent Laid-Open No. 9-151224 JP-A-2005-97585 J. et al. Phys. Chem. , 1975, 79, 2693, J. Am. Photochem. Photobiol. , A, 1988, 44, 243

  Conventionally, various methods have been proposed for the surface treatment technology for introducing a crosslinked structure on the surface of the water absorbent resin. However, the present situation is that further reduction in energy and time required during the surface treatment and further improvement in the absorption characteristics of the surface-treated water-absorbing resin are desired.

  In view of such a current situation, the present invention aims to provide means capable of providing a water-absorbing resin excellent in absorption capacity under load, absorption speed, gel strength, liquid permeability, etc. at high efficiency and at low cost. To do.

  In order to solve the above-mentioned problems, the present inventors have intensively studied. In the process, a mixture of the water-absorbing resin, radical polymerizable compound and solvent is obtained, and then irradiated with active energy rays such as ultraviolet rays, so that a more uniform cross-linked structure can be formed on the surface of the water-absorbing resin in a shorter time. I found that it could be introduced. Further, the present inventors have found that when an organic solvent is used as the solvent, the processing time can be further shortened and the uniformity of the introduced crosslinked structure can be further improved, and the present invention has been completed. It was.

  That is, the present invention is a surface treatment method for a water-absorbent resin, and (a) a step of obtaining a water-absorbent resin composition by mixing a water-absorbent resin, a radical polymerizable compound, and an organic solvent. And (b) irradiating the obtained water-absorbent resin composition with active energy rays.

  According to the surface treatment method for a water-absorbent resin of the present invention, a crosslinked structure can be introduced into the surface of the water-absorbent resin in a shorter time, and further has a more uniform crosslinked structure, absorption capacity under pressure, absorption rate, gel A water-absorbing resin excellent in strength, liquid permeability and the like can be produced at a low cost.

  Embodiments of the present invention will be described below.

  The present invention is a surface treatment method for a water-absorbent resin, wherein (a) a step of obtaining a water-absorbent resin composition by mixing a water-absorbent resin, a radical polymerizable compound, and an organic solvent (hereinafter referred to as a “water-absorbent resin composition”). And (b) a step of irradiating the obtained water-absorbent resin composition with active energy rays (hereinafter also simply referred to as “step (b)”). This is a surface treatment method for a conductive resin.

  Hereinafter, the surface treatment method of the water-absorbent resin of the present invention will be described in detail in the order of steps, but the technical scope of the present invention is not limited to the following forms, and the present invention is also applied to forms other than the following forms. The present invention can be implemented with appropriate modifications within a range not impairing the gist of the above.

[Step (a)]
(Water absorbent resin)
In this step, first, a water absorbent resin is prepared.

  The water-absorbing resin prepared in this step is a water-swellable and water-insoluble crosslinked polymer that can form a hydrogel. In the present invention, “water swellability” means that the centrifuge retention capacity is 2 g / g or more, preferably 5 to 100 g / g, more preferably 10 to 60 g / g. Further, “water-insoluble” means that the content of an uncrosslinked water-soluble component (water-soluble polymer; hereinafter also referred to as “eluting soluble component”) in the water-absorbent resin is 0 to 50% by weight. Meaning, preferably 25% by weight or less, more preferably 15% by weight or less, and particularly preferably 10% by weight or less. In addition, the value measured by the measuring method prescribed | regulated in the Example mentioned later shall be employ | adopted as a numerical value of centrifuge retention capacity and an elution soluble part.

  The water-absorbing resin prepared in this step is not particularly limited as long as it is a resin that essentially contains a structural unit derived from an ethylenically unsaturated monomer. For example, the acquisition route of the water absorbent resin is not particularly limited. When the desired water-absorbing resin is commercially available, a product purchased from the product may be used, or a water-absorbing resin produced by itself may be used.

  The ethylenically unsaturated monomer is not particularly limited and is preferably a monomer having an unsaturated double bond at the terminal, such as (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2- Anionic monomers such as (meth) acryloylpropane sulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid and salts thereof; (meth) acrylamide, N-substituted (meta ) Nonionic hydrophilic group-containing monomers such as acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (Meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, N, N- Methylamino propyl (meth) acrylamide, amino group-containing unsaturated monomers and their quaternized products and the like; and the like, may be used alone or two or more selected from these. Preferably, (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, salts thereof, N, N-dimethylaminoethyl (meth) acrylate, N , N-dimethylaminoethyl (meth) acrylate quaternized product, (meth) acrylamide is used, and (meth) acrylic acid and / or a salt thereof is particularly preferably used.

  When (meth) acrylate is used as the monomer, the monomer is selected from alkali metal salts, ammonium salts, and amine salts of (meth) acrylic acid from the viewpoint of water absorption performance of the water absorbent resin (meta It is preferably a monovalent salt of acrylic acid, more preferably an alkali metal salt of (meth) acrylic acid, particularly preferably a salt selected from sodium, lithium and potassium salts of (meth) acrylic acid. is there.

  The water-absorbent resin may have a structural unit derived from a monomer other than the above monomer as long as the effects of the present invention are not impaired. Examples of such monomers include aromatic ethylenically unsaturated monomers having 8 to 30 carbon atoms, aliphatic ethylenically unsaturated monomers having 2 to 20 carbon atoms, and fats having 5 to 15 carbon atoms. Examples thereof include hydrophobic monomers such as cyclic ethylenically unsaturated monomers and alkyl (meth) acrylic acid alkyl esters having an alkyl group of 4 to 50 carbon atoms. Generally the ratio of these hydrophobic monomers is the range of 0-20 weight part with respect to 100 weight part of said ethylenically unsaturated monomers. When the amount of the hydrophobic monomer exceeds 20 parts by weight, the water absorption performance of the resulting water absorbent resin may be deteriorated.

  The water-absorbing resin prepared in this step becomes water-insoluble due to the formation of internal crosslinks. Such internal cross-linking may be a self-crosslinking type that does not use a cross-linking agent, but is an internal cross-link having two or more polymerizable unsaturated groups and / or two or more reactive functional groups in one molecule. It may be formed by using an agent.

  Such an internal cross-linking agent is not particularly limited. For example, N, N′-methylenebis (meth) acrylamide, N-methylol (meth) acrylamide, glycidyl (meth) acrylate, (poly) ethylene glycol di (meth) Acrylate, (poly) propylene glycol di (meth) acrylate, glycerin tri (meth) acrylate, glycerine acrylate methacrylate, (meth) acrylic acid polyvalent metal salt, trimethylolpropane tri (meth) acrylate, triallylamine, triallyl cyanurate , Triallyl isocyanurate, triallyl phosphate, ethylene glycol diglycidyl ether, (poly) glycerol glycidyl ether, polyethylene glycol diglycidyl ether, and the like. Only one of these internal crosslinking agents may be used alone, or two or more thereof may be used in combination.

  The amount of the internal crosslinking agent used is preferably 0 to 5 mol%, more preferably 0.0001 to 1 mol%, and still more preferably 0, based on the total amount of monomer components used in producing the water absorbent resin. 0.001 to 0.5 mol%, particularly preferably 0.005 to 0.2 mol%. If it is less than 0.0001 mol%, internal crosslinking may not be introduced into the resin. On the other hand, if it exceeds 1 mol%, the gel strength of the water-absorbent resin becomes too high, and the water absorption ratio may decrease. When a cross-linked structure is introduced into the polymer using the internal cross-linking agent, the internal cross-linking agent is used before the polymerization of the monomer, during the polymerization, after polymerization, or after neutralization. Can be added to the reaction system.

  As a manufacturing method in the case of manufacturing a water-absorbing resin itself, for example, a method of polymerizing a monomer component containing a desired monomer can be mentioned. At this time, the method for producing the water-absorbent resin is not particularly limited, and conventionally known knowledge can be appropriately referred to. The polymerization method is not particularly limited, and for example, aqueous solution polymerization, reverse phase suspension polymerization, precipitation polymerization, bulk polymerization and the like can be employed. Among these methods, from the viewpoint of ease of control of the polymerization reaction and performance of the resulting water-absorbent resin, aqueous polymerization in which polymerization proceeds in a state where the monomer component is dissolved in an aqueous solution, or micelles in a hydrophobic organic solvent Reverse phase suspension polymerization in which polymerization proceeds is preferred. Regarding aqueous solution polymerization, for example, U.S. Pat. Nos. 4,625,001, 4,873,299, 4,286,082, 4,973,632, 4,985,518, No. 5,124,416, No. 5,250,640, No. 5,264,495, No. 5,145,906, No. 5,380,808, EP 081636, No. 0955506, No. 0922717 and the like. On the other hand, for the reverse phase suspension polymerization, for example, U.S. Pat. Nos. 4,093,776, 4,367,323, 4,446,261, 4,683,274, and 5,244. , 735, and the like. Polymerization forms (for example, monomers and polymerization initiators) described in these documents may be used in this step.

  When producing a water-absorbing resin by polymerizing monomer components, it is preferable to use a polymerization initiator. Here, the polymerization initiator that can be used is not particularly limited, and examples thereof include a water-soluble radical polymerization initiator, a thermal decomposition type radical polymerization initiator, and 2-hydroxy-2-methyl-1-phenyl-propane-1. -Photoinitiators, such as ON, etc. are mentioned. The specific forms of the water-soluble radical polymerization initiator and the thermal decomposition type radical polymerization initiator will be described later. Moreover, for example, a reducing agent such as sulfite, L-ascorbic acid or ferric salt may be combined with the water-soluble radical polymerization initiator or the thermal decomposition type radical polymerization initiator and used as a redox initiator.

  The concentration of the monomer component in the aqueous solution of the aqueous solution polymerization or reverse phase suspension polymerization is not particularly limited. However, for example, the concentration of the monomer component is preferably 20 to the total amount of the monomer aqueous solution. 80% by weight, more preferably 25-70% by weight, still more preferably 30-60% by weight, and most preferably 35-50% by weight. When this concentration is less than 20% by weight, the production efficiency is lowered, and the production cost may be increased. On the other hand, if this concentration exceeds 80% by weight, the water absorption characteristics may deteriorate.

  The amount of the hydrophobic organic solvent used in the reverse phase suspension polymerization is not particularly limited, but is preferably 50 to 5000 parts by weight, more preferably 100 to 3000 parts by weight with respect to 100 parts by weight of the monomer. More preferably, it is 200 to 1000 parts by weight. When the amount of the hydrophobic organic solvent used for the reverse phase suspension polymerization is less than 50 parts by weight, there is a possibility that the active energy rays are not uniformly irradiated to the water absorbent resin in the step (b). On the other hand, if the amount of the hydrophobic organic solvent exceeds 5000 parts by weight, the production efficiency may be reduced and the production cost may increase.

  In order to start the polymerization, the polymerization is generally started by exerting the action of the aforementioned polymerization initiator by heating or irradiation with electromagnetic waves. However, polymerization may be initiated by using active energy rays such as ultraviolet rays, electron beams, and γ rays alone. The temperature at the start of polymerization depends on the type of polymerization initiator used, but is preferably in the range of 15 to 130 ° C, more preferably in the range of 20 to 120 ° C. If the temperature at the start of polymerization is out of the above range, it is preferable because there is a possibility that the residual monomer of the obtained water-absorbent resin increases or excessive self-crosslinking reaction proceeds to lower the water-absorbing performance of the water-absorbent resin. Absent.

  In this step, it is preferable that the production of the water-absorbent resin by polymerization of the monomer is performed by reverse phase suspension polymerization. In reverse-phase suspension polymerization, polymerization proceeds in a hydrophobic organic solvent, so that it is possible to obtain a “water-absorbent resin composition” by adding a radical polymerizable compound to the reaction liquid after the polymerization is completed. Become. And the said water absorbing resin composition can be used in the process (b) mentioned later as it is. That is, the steps from polymerization to introduction of surface crosslinking can be performed in one pot. Therefore, according to such a method, there is an advantage that the surface treatment of the water-absorbent resin can be performed in a short time and at a low cost as compared with the conventional method.

  In addition, when manufacturing a water absorbing resin by reverse phase suspension polymerization, the specific form of the apparatus used for manufacture of resin is not specifically limited, A conventionally well-known knowledge can be referred suitably. For example, JP-A-51-150592, JP-A-3-41104, JP-A-3-296502, JP-A-9-157313, JP-A-2001-158802, JP-A-2003-26706 The polymerization reactors described in JP-A Nos. 2004-269593 and 2005-132957 can be used for the production of a water-absorbing resin by reverse phase suspension polymerization. Further, the polymerization reactor generally has a stirring blade.

  Here, when reverse phase suspension polymerization is performed, the polymerization is performed in a state where a dispersant is added to the reaction system. The specific form of the dispersant is not particularly limited, and conventionally known knowledge can be referred to as appropriate in the polymer production field, but as an example, a surfactant and a polymer protective colloid can be preferably used. Here, examples of the surfactant include a nonionic surfactant or a mixture of a nonionic surfactant and an anionic surfactant. Specifically, for example, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenol ether, polyoxyethylene acyl ester, sucrose fatty acid ester, higher alcohol sulfate ester Examples thereof include salts, alkyl naphthalene sulfonates, alkyl polyoxyethylene sulfates, and dialkyl sulfosuccinates. Of these, sorbitan fatty acid esters are preferably used. As for these surfactants, only 1 type may be used independently and 2 or more types may be used together.

  On the other hand, examples of the polymer protective colloid include ethyl cellulose, ethyl hydroxyethyl cellulose, oxidation-modified polyethylene, maleic anhydride-modified polyethylene, maleic anhydride-modified polybutadiene, and maleic anhydride-modified ethylene / propylene / diene / terpolymer. These polymer protective colloids may also be used alone or in combination of two or more.

  The amount of the dispersant added is not particularly limited as long as it can suppress aggregation of the water-absorbent resin and improve the dispersibility of the water-absorbent resin in the organic solvent, but for example, 100 parts by weight of the water-absorbent resin in the solvent. On the other hand, it is preferably 0.01 to 40 parts by weight, more preferably 0.05 to 10 parts by weight, and still more preferably 0.1 to 5 parts by weight.

  In the case of producing a water-absorbing resin, a partially neutralized product such as (meth) acrylic acid may be polymerized, or hydroxylated after polymerizing an acid group-containing monomer such as (meth) acrylic acid. The polymer may be neutralized with an alkali compound such as sodium or sodium carbonate. Furthermore, the water absorbent resin prepared in this step preferably contains an acid group and has a predetermined neutralization rate (mole% of neutralized acid groups in all acid groups). . Under the present circumstances, the neutralization rate (mol% of the neutralized acid group in all the acid groups) of the water absorbing resin obtained becomes like this. Preferably it is 25-100 mol%, More preferably, it is 50-90 mol% More preferably, it is 50 to 75 mol%, and most preferably 60 to 70 mol%. When a water-absorbing resin is produced, the polymer obtained by polymerization is usually a hydrogel crosslinked polymer.

  In this step, when the water-absorbing resin is produced by aqueous solution polymerization, the obtained water-containing gel-like crosslinked polymer may be used as it is as the water-absorbing resin, but it is preferably dried to obtain a water content (% by weight). ) (Calculated by 100-solid content (% by weight)). Here, the water content of the water-absorbent resin after drying is not particularly limited, but is preferably 1 to 40% by weight, more preferably 2 to 30% by weight, and further preferably 5 to 20% by weight. In this step, more preferably, a pulverization treatment and a classification treatment are further performed to obtain a powdery water absorbent resin having a desired particle size. In addition, drying of the water-containing gel-like crosslinked polymer obtained by polymerization may be performed, for example, using a dryer such as a hot air dryer, preferably at 100 to 220 ° C, more preferably at 120 to 200 ° C. . The powdery water-absorbing resin thus obtained becomes a water-absorbing resin composition through a mixing step with a radically polymerizable compound and an organic solvent, which will be described later, and is provided to step (b).

  Further, as the pulverizer used in the pulverization treatment, for example, the pulverizer described in Table 1.10 of the Powder Engineering Handbook (Edition of the Powder Engineering Society, first edition) can be used. It is classified into an impact crusher and a high-speed rotary crusher, and those having one or more crushing mechanisms such as cutting, shearing, impact and friction are preferably used. Among these crushing machines, cutting and shearing are mainly used. A pulverizer as a mechanism is particularly preferably used. For example, a roll mill (roll rotary type) pulverizer can be preferably used.

  In this step, when preparing a powdery water-absorbing resin, the particle size distribution of the powder is not particularly limited, but preferably 90 to 100 weight of particles having a particle size in the range of 150 to 850 μm (specified by sieve classification). %, Particularly preferably 95 to 100% by weight. When the water-absorbent resin particles having a particle size larger than 850 μm are modified through the step (b) described later and applied to, for example, a diaper, the touch is bad and the back sheet of the diaper is broken. There is. On the other hand, if the water-absorbing resin particles having a particle diameter smaller than 150 μm exceed 10% by weight, fine powder may be scattered during handling in a subsequent process, or the piping may be blocked during production. Moreover, the weight average particle diameter of the powdery water absorbent resin is preferably 10 to 1,000 μm, more preferably 200 to 600 μm, and further preferably 300 to 500 μm. If the weight average particle size is less than 10 μm, it may be unfavorable for safety and health. On the other hand, if it exceeds 1,000 μm, it may not be used for diapers or the like. In addition, when preparing a powdery water absorbent resin having such a particle size, granulation treatment for increasing the particle size and classification treatment for cutting unnecessary components (for example, sieving) Etc. may be performed.

  Further, the logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.45 or less, more preferably 0.40 or less, still more preferably 0.35 or less, and particularly preferably 0.30 or less. is there. Further, σζ is preferably 0.20 or more. Note that the smaller the value of σζ, the narrower the particle size distribution. Here, when σζ exceeds 0.45, the particle size distribution becomes broad, that is, a particle size larger than 850 μm or a particle size smaller than 150 μm increases, which is not preferable. On the other hand, in order to make σζ less than 0.20, a facility with extremely low productivity is required, which may increase the manufacturing cost. In addition, as a value of the particle size of the powdery water-absorbing resin, a value measured by a particle size distribution measuring method of an example described later is adopted.

  The water-absorbing resin prepared in this step is preferably obtained by obtaining a water-absorbing resin precursor having a low neutralization rate and then allowing a base to act on the water-absorbing resin precursor. This is due to the following reason.

  That is, in the conventional technique, a crosslinked structure is introduced on the surface of the water-absorbent resin by using a polyfunctional surface treating agent. This polyfunctional surface treating agent has a property of reacting with a carboxyl group (—COOH) in a water absorbent resin but not reacting with a salt thereof (for example, —COONa). For this reason, by polymerizing an ethylenically unsaturated monomer mixture (for example, a mixture of acrylic acid and sodium acrylate) that has been adjusted so that the abundance ratio of -COOH / -COONa is in an appropriate range, If a water-absorbing resin in which COOH and -COONa are uniformly distributed is produced and used for surface crosslinking with a polyfunctional surface treatment agent, a uniform crosslinked structure can be obtained.

  On the other hand, after polymerizing an acid type ethylenically unsaturated monomer such as (meth) acrylic acid as a main component, the obtained polymer is neutralized with an alkali compound such as sodium hydroxide or sodium carbonate. The water-absorbing resin obtained by the above has a high gel strength, but when a cross-linked structure is introduced on the surface using a polyfunctional surface treatment agent, -COOH and -COONa are not uniformly distributed. There is a problem that the water absorption characteristics are deteriorated. For this reason, it was not desirable to introduce a crosslinked structure onto the surface of the water-absorbent resin obtained by the latter method using a conventional polyfunctional surface treating agent.

  However, in the method of the present invention, the surface treatment of the water-absorbent resin in the presence of the radical polymerizable compound inhibits the progress of the crosslinking reaction due to the non-uniform distribution of —COOH in the absorbent resin. Can be minimized.

  In view of the above, according to the method of the present invention, a low neutralization rate is obtained by once polymerizing a monomer or a mixture thereof having an acid type ethylenically unsaturated monomer such as acrylic acid as a main component. Even when a water absorbent resin obtained by neutralizing the water absorbent resin precursor with an alkali compound such as sodium hydroxide or sodium carbonate is used, The water-absorbent resin that has been modified and surface-treated can exhibit high gel strength and excellent water absorption characteristics.

  The “low neutralization rate water-absorbing resin precursor” means that the neutralization rate (mol% of neutralized acid groups in all acid groups) is low or the acid groups are not neutralized (medium Means a water-absorbing resin precursor having a sum ratio of 0, specifically, the neutralization rate (mol% of neutralized acid groups in all acid groups) is 0 to 50 mol%, more preferably The thing of about 0-20 mol% is said. Such a water-absorbent resin precursor having a low neutralization rate is obtained by using a monomer having an acid group such as acrylic acid in the above-described method, preferably so that the neutralization rate is a value within the above-described range. Since it is obtained by the same method as described above by using a monomer as a main component or a mixture thereof, a detailed description is omitted here.

  The water-absorbing resin prepared in this step is not limited to that manufactured by the above-described method, and may be manufactured by another method. The water-absorbent resin obtained by the above-described method is a water-absorbent resin that is not surface-crosslinked, but as a water-absorbent resin for use in this step, a polyhydric alcohol, a polyvalent epoxy compound, an alkylene carbonate, You may prepare the water absorbent resin surface-crosslinked using the oxazolidone compound.

(Radically polymerizable compound)
In this step, in addition to the water absorbent resin prepared above, a radical polymerizable compound is further prepared.

  Conventionally, when a cross-linked structure is introduced into the surface of a water-absorbent resin, it is common to use a surface cross-linking agent. When the surface cross-linking agent is used, the functional group present on the resin surface and the surface cross-linking agent are chemically bonded to each other, whereby a stable surface cross-linking structure can be introduced on the resin surface. Moreover, the distance between crosslinking points can be adjusted by appropriately selecting the chain length of the surface crosslinking agent, and the crosslinking density can be controlled by adjusting the blending amount. However, in the present invention, the surface treatment (specifically, the surface of the water-absorbent resin is performed only by allowing the radically polymerizable compound to coexist with the water-absorbent resin without using such a surface cross-linking agent. It was found that the introduction of a crosslinked structure into

  The term “radical polymerizable compound” means a compound that can be polymerized by radical polymerization. Specifically, for example, the ethylenic unsaturated compound used in the production of the water absorbent resin in the column “Water absorbent resin” above. The compounds mentioned as examples of the monomer (monofunctional radical polymerizable compound) and the internal crosslinking agent (polyfunctional radical polymerizable compound) can be preferably used. Therefore, detailed description is omitted here.

  In this step, for example, only one of the monofunctional radical polymerizable compound and the polyfunctional radical polymerizable compound may be used, or both of them may be used in combination. Moreover, as a radically polymerizable compound, only 1 type may be used independently and 2 or more types may be used together. In a preferred embodiment, from the viewpoint of ease of production, the compound used as the ethylenically unsaturated monomer and the internal cross-linking agent during the production of the water-absorbent resin is used as the radical polymerizable compound in this step. In addition, the molar ratio of the usage-amount of the monofunctional radically polymerizable compound used in such a form and a polyfunctional radically polymerizable compound is the ethylenically unsaturated monomer and internal crosslinking agent at the time of manufacture of a water absorbing resin. The molar ratio may be the same or different. Preferably, a larger amount of polyfunctional radically polymerizable compound is used compared to the composition at the time of manufacturing the water absorbent resin. In this step, preferably, the radical polymerizable compound is a compound having two or more polymerizable unsaturated groups in one molecule (for example, two or more polymerizable unsaturated groups among the above-mentioned internal crosslinking agents). Compound). According to such a form, it is possible to further improve the absorption capacity under pressure.

  The amount of the radical polymerizable compound in the water absorbent resin composition obtained in this step is preferably in the range of 0.1 to 40 parts by weight, more preferably 0.5 to 20 parts by weight with respect to 100 parts by weight of the water absorbent resin. The range is particularly preferably 1 to 10 parts by weight, most preferably 2 to 6 parts by weight. When the amount of the radical polymerizable compound is 0.1 parts by weight or more, the absorption performance of the water absorbent resin under pressure can be maintained at a sufficient value. On the other hand, when the amount of the radical polymerizable compound is 40 parts by weight or less, a decrease in the absorption capacity of the surface-treated water-absorbing resin can be prevented. At this time, the ratio of the polyfunctional radically polymerizable compound to the total amount of the radically polymerizable compound used is preferably in the range of 0 to 50 parts by weight, more preferably 0 to 100 parts by weight of the ethylenically unsaturated monomer. The range is 0.01 to 20 parts by weight, more preferably 0.1 to 10 parts by weight.

(Organic solvent)
In this step, the water-absorbing resin and radical polymerizable compound prepared above are mixed with an organic solvent. Thereby, a water absorbing resin composition is obtained. The obtained water-absorbing resin composition is used in step (b) described later. However, as described above, when an organic solvent is used as the solvent for producing the water-absorbent resin, the solution after completion of the polymerization may be used as it is in this step. In such a form, the step of “adding and dispersing the water-absorbent resin in an organic solvent” may not be performed.

  The organic solvent mixed with the water-absorbing resin and the radical polymerizable compound in this step is not particularly limited, and may be hydrophobic or hydrophilic.

  The hydrophobic organic solvent means an organic solvent having a solubility in water of 20 g / 100 g (20 ° C.) or less. Examples of the hydrophobic organic solvent include aliphatic hydrocarbon compounds such as n-pentane, cyclopentane, n-hexane, cyclohexane, n-heptane, methylcyclohexane, and n-octane; aromatic carbonization such as benzene, toluene, and xylene. Examples include hydrogen compounds; aliphatic alcohols such as n-butyl alcohol, isobutyl alcohol, (deleted), n-amyl alcohol; aliphatic ketones such as methyl isobutyl ketone; aliphatic esters such as ethyl acetate.

  On the other hand, the hydrophilic organic solvent means an organic solvent having a solubility in water of more than 20 g / 100 g (20 ° C.). Examples of the hydrophilic organic solvent include lower aliphatic alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; aliphatic ketones such as acetone; ether compounds such as dioxane, tetrahydrofuran, and methoxy (poly) ethylene glycol. Amide compounds such as ε-caprolactam and N, N-dimethylformamide; sulfoxide compounds such as dimethyl sulfoxide; ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, di Propylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin, 2 Butene 1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, Examples include trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymers, polyhydric alcohols such as pentaerythritol and sorbitol. Here, considering the case where the method of reverse phase suspension polymerization is used for the production of the water-absorbent resin as a raw material, the reaction liquid at the end of the polymerization is used as it is as the “dispersion liquid” in this process. It is preferable from the viewpoints of simplification of the process and reduction of manufacturing costs. From such a viewpoint, the “organic solvent” used in this step is preferably one used as a reaction solvent for reversed-phase suspension polymerization in the production of a water-absorbent resin. Specifically, the above-described hydrophobic organic solvent is preferably used, more preferably a hydrocarbon compound, more preferably a saturated hydrocarbon compound, cyclohexane, n-hexane, n -Pentane, cyclopentane and n-heptane are particularly preferably used, and cyclohexane, n-hexane and n-pentane are most preferably used.

  The amount of the organic solvent used is not particularly limited, but is preferably 50 to 5000 parts by weight, more preferably 100 to 3000 parts by weight, and still more preferably 200 parts per 100 parts by weight of the water absorbent resin alone. ~ 1000 wt%. If the amount of the organic solvent in the dispersion is 50 parts by weight or more, the active energy ray can be uniformly irradiated to the water-absorbent resin in the step (b). On the other hand, when the amount of the organic solvent is 5000% by weight or more, problems such as a decrease in production efficiency and a corresponding increase in manufacturing cost can be suppressed. However, a water-absorbing resin composition in a form outside these ranges may be obtained.

(Other forms)
As above-mentioned, in this process, a water absorbing resin composition is obtained by mixing a water absorbing resin, a radically polymerizable compound, and an organic solvent. However, the water absorbent resin composition may further contain other components. In a preferred form, the water absorbent resin composition further includes a radical polymerization initiator. According to such a form, there is an advantage that physical properties such as liquid permeability of the water absorbent resin after the surface treatment are improved. Hereinafter, although the form in which the radical polymerization initiator is contained in the water-absorbent resin composition will be described in detail, the technical scope of the present invention is not limited to the following form.

  Here, in the present embodiment, the water-absorbing resin composition “contains a radical polymerization initiator” means that (1) the radical polymerization initiator used in the water-absorbing resin production process is in the produced water-absorbing resin. And (2) a form in which a radical polymerization initiator is separately added to a water absorbent resin composition obtained by mixing a water absorbent resin, a radical polymerizable compound and an organic solvent. It is a concept that includes forms. At this time, the addition of the radical polymerization initiator may be performed at any time of mixing the water absorbent resin, the radical polymerizable compound and the organic solvent.

  In this embodiment, there is no particular limitation on the specific form of the radical polymerization initiator contained in the water-absorbent resin, and conventionally known knowledge is appropriately referred to, but more preferably, water-soluble radical polymerization is used as the radical polymerization initiator. An initiator and / or a thermal decomposition type radical polymerization initiator is contained in the water absorbent resin.

  Here, the “water-soluble radical polymerization initiator” means an initiator that dissolves 1% by weight or more in water (25 ° C.), preferably 5% by weight or more, more preferably 10% by weight or more. is there. When the water-absorbent resin further contains such a water-soluble radical polymerization initiator, the polymerization initiator can be uniformly and easily dispersed on the surface of the water-absorbent resin excellent in hydrophilicity and water absorption. As a result, it is possible to improve the water absorption characteristics of the surface-treated water absorbent resin.

  Specific examples of the water-soluble radical polymerization initiator include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; potassium peracetate, sodium peracetate, potassium percarbonate, sodium percarbonate, t-butyl hydro Peroxides; hydrogen peroxide; water-soluble azo compounds such as 2,2′-azobis (2-amidinopropane) dihydrochloride. Of these, persulfates, hydrogen peroxide and water-soluble azo compounds are preferably used, and persulfates are particularly preferably used. In particular, when persulfate is used, the absorption capacity under pressure of the water-absorbent resin after the surface treatment with respect to physiological saline (in this specification, simply referred to as “absorption capacity under pressure”), centrifuge retention capacity with respect to physiological saline (Hereinafter, simply referred to as “centrifuge retention capacity”) is preferable in that all are excellent.

  In this embodiment, the content of the water-soluble radical polymerization initiator contained in the water absorbent resin composition is preferably 0.01 to 20 parts by weight, more preferably 0.1 to 100 parts by weight of the water absorbent resin. -15 parts by weight, particularly preferably 0.5-10 parts by weight, most preferably 1-5 parts by weight. If content of a water-soluble radical polymerization initiator is 0.01 weight part or more, the surface treatment of the water absorbing resin at the time of irradiating an active energy ray in a process (b) can be performed efficiently. On the other hand, if the content of the water-soluble radical polymerization initiator is 20 parts by weight or less, the water absorption performance of the surface-treated water-absorbing resin can be prevented from being lowered.

  The term “thermally decomposable radical polymerization initiator” means a compound that decomposes by heating to generate radicals, and preferably has a 10-hour half-life temperature of 0 ° C. or higher and 120 ° C. or lower, preferably 20 ° C. or higher. Those having a temperature of 100 ° C. or lower are more preferable, and those having a temperature of 40 ° C. or higher and 80 ° C. or lower are particularly preferable in consideration of temperature conditions for irradiating active energy rays. If the 10-hour half-life temperature is 0 ° C. (lower limit) or more, it is stable during storage, and if it is 120 ° C. (upper limit) or less, appropriate reactivity can be ensured. When the water-absorbent resin further contains such a thermal decomposition type radical polymerization initiator, surface treatment can be performed at a low temperature in a short time, and the surface-treated water-absorbent resin has high gel strength and excellent water absorption characteristics. Can be achieved.

  Here, as the thermal decomposition type radical polymerization initiator, either oil-soluble or water-soluble may be used. The oil-soluble thermal decomposition type radical polymerization initiator is characterized in that the decomposition rate is less affected by pH and ionic strength than the water-soluble thermal decomposition type radical polymerization initiator. However, since the water-absorbing resin is hydrophilic, it is more preferable to use a water-soluble thermal decomposition type radical polymerization initiator in consideration of permeability to the water-absorbing resin.

  The thermal decomposition type radical polymerization initiator is relatively inexpensive as compared with a compound marketed as a photodecomposition type radical polymerization initiator, and strict light shielding is not necessarily required, so that the manufacturing process and the manufacturing apparatus can be simplified. Specific examples of the thermal decomposition type radical polymerization initiator include persulfates such as sodium persulfate, ammonium persulfate and potassium persulfate; percarbonates such as sodium percarbonate; peracetates such as peracetic acid and sodium peracetate; Hydrogen peroxide; 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2-2 (-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis And azo compounds such as (2-methylpropionitrile). Among them, persulfates such as sodium persulfate, ammonium persulfate, potassium persulfate, and 2,2′-azobis (2-amidinopropane) dihydrochloride having a 10-hour half-life temperature of 40 to 80 ° C., 2 , 2′-azobis [2-2 (-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis (2-methylpropionitrile) and the like are preferred. Among these, the use of persulfate is particularly preferable in terms of excellent absorption capacity under pressure and centrifuge retention capacity.

  In this embodiment, the content of the thermal decomposition type radical polymerization initiator contained in the water absorbent resin is preferably 0.01 to 20 parts by weight, more preferably 0.1 to 20 parts by weight with respect to 100 parts by weight of the water absorbent resin. 15 parts by weight, more preferably 1 to 10 parts by weight. If content of a thermal decomposition type radical polymerization initiator is 0.01 weight part or more, the surface treatment of the water absorbing resin at the time of irradiating an active energy ray in a process (b) can be performed efficiently. On the other hand, when the content of the thermal decomposition type radical polymerization initiator is 20 parts by weight or less, the water absorption performance of the surface-treated water-absorbing resin can be prevented from being lowered.

  In this embodiment, the water-absorbing resin further contains a water-soluble radical polymerization initiator and / or a heat-decomposable radical polymerization initiator, and the water-absorbing resin comprises a water-soluble radical polymerization initiator and a heat-decomposable radical polymerization initiator. Other polymerization initiators may be further included. Other polymerization initiators include, for example, photopolymerization initiators such as oil-soluble benzoin derivatives, benzyl derivatives, and acetophenone derivatives, and oil-soluble ketone peroxides, peroxyketals, hydroperoxides, dialkyl peroxides, peroxygens. Examples include oil-soluble organic peroxides such as esters and peroxycarbonates. Such a photopolymerization initiator may be a commercially available product, trade name Irgacure (registered trademark) 184 (hydroxycyclohexyl-phenyl ketone) manufactured by Ciba Specialty Chemicals, Inc., trade name Irgacure 2959 (1- [4 -(2-hydroxyethoxy) -phenyl] -2-hydroxy-2-methyl-1-propan-1-one) and the like. These other polymerization initiators may also remain in the resin as they are from the production process of the water absorbent resin, or may be added separately.

  When other polymerization initiators are used in combination, the content thereof is preferably 0 to 20 parts by weight, more preferably 0 to 15 parts by weight, particularly preferably 0 to 0 parts by weight based on 100 parts by weight of the water absorbent resin. 10 parts by weight, most preferably 0 to 5 parts by weight. The content of the other polymerization initiator is preferably smaller than the total content of the water-soluble radical polymerization initiator and the thermal decomposition type radical polymerization initiator, and more preferably 1 by weight ratio with respect to the total content. / 2 or less, more preferably 1/10 or less, and particularly preferably 1/50 or less.

  In this embodiment, the water-absorbing resin further contains a radical polymerization initiator (in particular, a water-soluble radical polymerization initiator and / or a thermal decomposition type radical polymerization initiator), so that compared with a case where no radical polymerization initiator is used. Thus, excellent physical properties can be achieved in the surface-treated water-absorbent resin.

  In the present invention, as described above, an organic solvent is used as a dispersion medium for the water-absorbent resin, and in addition to these, a radically polymerizable compound is further mixed to obtain a water-absorbent resin composition. ) By irradiating the water absorbent resin composition with active energy rays, a uniform cross-linked structure can be introduced into the surface of the water absorbent resin in a short time. As described above, the water-absorbent resin may contain a radical polymerization initiator as it remains from the resin production process, or contains a radical polymerization initiator at the end of the resin production process. It may not be. In the latter case, the radical polymerization initiator is separately mixed in the organic solvent before or after or simultaneously with the mixing of the water absorbent resin and the organic solvent.

  As described above, the water-absorbent resin may contain a radical polymerization initiator as it remains from the production process, or may contain a radical polymerization initiator by being added separately. Thus, in this step, the concentration of these components in the resulting water-absorbent resin composition can be adjusted by adding a radical polymerizable compound or a radical polymerization initiator to the organic solvent. The form in which the radical polymerizable compound or radical polymerization initiator is added to the organic solvent is not particularly limited and may be added as it is, but it may be added in the form of a solution dissolved in water or a hydrophilic organic solvent. It is preferable to add in the form of an aqueous solution. Here, the specific form of the hydrophilic organic solvent is not particularly limited. For example, lower aliphatic alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; aliphatic ketones such as acetone; dioxane; Ether compounds such as tetrahydrofuran and methoxy (poly) ethylene glycol; amide compounds such as ε-caprolactam and N, N-dimethylformamide; sulfoxide compounds such as dimethyl sulfoxide; ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol Polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol Glycerol, polyglycerol, 2-butene 1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexane Examples include dimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymers, polyhydric alcohols such as pentaerythritol and sorbitol. In addition, when a radical polymerizable compound or a radical polymerization initiator is added in the form of an aqueous solution, the amount of water in the aqueous solution is not particularly limited and can be adjusted as appropriate, but preferably 1 with respect to 100 parts by weight of the water absorbent resin. It is -50 weight part, More preferably, it is 2-30 weight part, More preferably, it is 5-20 weight part. When both the radical polymerizable compound and the radical polymerization initiator are added in the form of a solution, a solution containing these may be prepared separately and added separately, or a solution containing both of these may be prepared. It may be prepared and added all at once.

  In this step, it is preferable to add a mixing aid in order to improve the dispersibility of the water absorbent resin in the resulting water absorbent resin composition. By adding a mixing aid, the aggregation of the water-absorbent resin can be suppressed, and the organic solvent and the water-absorbent resin can be uniformly mixed. Therefore, when irradiating active energy rays in the subsequent step (b), The active energy ray can be evenly applied to the water absorbent resin, and a uniform cross-linked structure can be introduced to the entire surface of the water absorbent resin. At this time, the timing of adding the mixing aid to the organic solvent is not particularly limited, and may be before or after mixing the water-absorbing resin and the organic solvent, or may be simultaneous with mixing. It is preferably before mixing of the resin and the organic solvent. Moreover, it may be before and after mixing of the organic solvent and the radical polymerizable compound, or may be simultaneous with mixing. In addition, when manufacturing a water absorbing resin by reverse phase suspension polymerization, the dispersing agent added at the time of superposition | polymerization can be used also as the said mixing adjuvant. That is, the range of the dispersing agent in reverse phase suspension polymerization and the mixing aid added in this step are the same. Accordingly, detailed description of the mixing aid is omitted here. However, even in such a case, it may be added to the solution obtained after reverse phase suspension polymerization for the purpose of using the above-mentioned dispersant as a mixing aid.

  The addition form in the organic solvent in the case of adding the mixing aid is not particularly limited, and may be added in the form of a powder, or may be added in a form dissolved or dispersed in an appropriate solvent.

  In addition, the order of addition of the mixing aid in the case of adding the mixing aid is also not particularly limited, and a method of adding the mixing aid to the water-absorbent resin in advance and then adding it to the organic solvent, the mixing aid Any method can be used, such as a method of adding a water-absorbing resin after adding to the organic solvent and a method of adding a mixing aid to the organic solvent in which the water-absorbing resin is dispersed.

  The mixing means used for mixing the water-absorbing resin and the organic solvent, mixing the radical polymerizable compound and the organic solvent, and, if necessary, mixing the organic solvent and the radical polymerization initiator, is a normal mixing method. Machines such as V-type mixers, ribbon-type mixers, screw-type mixers, rotating disk-type mixers, air-flow-type mixers, batch-type kneaders, continuous kneaders, paddle-type mixers, vertical mixers, stirring blades A reaction kettle equipped with

[Step (b)]
In this step, the active energy ray is irradiated to the water absorbent resin composition obtained in the step (a). Thereby, a crosslinked structure is introduced into the surface of the water absorbent resin in the water absorbent resin composition, and the surface treatment of the water absorbent resin is completed.

(Active energy rays)
In the surface treatment of a water-absorbent resin, it is known to irradiate active energy rays to improve the polymerization rate. For example, an insoluble water-absorbing resin having internal crosslinking is prepared by blending an internal crosslinking agent and a photopolymerization initiator with a polymerizable monomer component and irradiating active energy rays such as ultraviolet rays, electron beams, and γ rays. sell. It is also known to introduce a crosslinked structure on the surface of a water-absorbent resin by using a surface crosslinking agent to promote the reaction under heating conditions. In this case, a compound having a plurality of functional groups in one molecule, such as a polyhydric alcohol, a polyvalent glycidyl ether, a haloepoxy compound, or a polyvalent aldehyde, is used as the surface cross-linking agent. In general, when heated to 100 to 300 ° C., these functional groups react with a carboxyl group or the like present on the surface of the water absorbent resin, and a crosslinked structure is introduced on the surface of the water absorbent resin. However, the present invention is characterized in that a crosslinked structure can be introduced into the surface of the water-absorbent resin by irradiation with active energy rays even without such a surface crosslinking agent. In addition, this makes it possible to improve water absorption characteristics such as absorption capacity under load (AAP) of the water absorbent resin after the surface treatment (introduction of surface crosslinking).

  In the present invention, the irradiation with active energy rays may be performed during the formation of the water absorbent resin composition by mixing the water absorbent resin, the radical polymerizable compound, and the organic solvent, and is obtained by thoroughly mixing the two. The obtained water absorbent resin composition may be irradiated. However, a method of irradiating the water-absorbent resin composition obtained by thoroughly mixing these components with active energy rays is preferable in that uniform surface cross-linking can be formed. Further, it is more preferable that the water absorbent resin composition is always in a fluid state while the water absorbent resin is irradiated with active energy rays. In the present invention, when a water-absorbing resin is produced by reversed-phase suspension polymerization, the reaction solution may be subjected to azeotropic dehydration after the completion of the polymerization reaction. Alternatively, during the process, active energy rays may be irradiated to the reaction liquid (that is, the “dispersion liquid” in the present invention).

  Moreover, there is no restriction | limiting in particular about the moisture content of the water absorbing resin at the time of active energy ray irradiation, Preferably it is 0.1 to 50 weight%, More preferably, it is 2 to 30 weight%, More preferably, it is 5 to 20 % By weight.

  As the active energy rays, one or more of ultraviolet rays, electron beams and γ rays are used. Preferably, it is an ultraviolet ray or an electron beam. Of these, ultraviolet rays having a wavelength of 400 nm or less are more preferred, ultraviolet rays having a wavelength of 300 nm or less are more preferred, ultraviolet rays having a wavelength of 180 to 290 nm are particularly preferred, and ultraviolet rays having a wavelength of 200 to 250 nm are most preferred.

But not irradiation conditions particularly limited, for example, when ultraviolet rays are used, preferably, irradiation intensity 3~1000mW / cm ^2, irradiation amount is 100~10000mJ / cm ^2. Examples of the apparatus for irradiating ultraviolet rays include a high pressure mercury lamp, a low pressure mercury lamp, a metal halide lamp, a xenon lamp, and a halogen lamp. As long as ultraviolet rays are irradiated, preferably as long as ultraviolet rays of 300 nm or less are irradiated, other radiations and wavelengths may be included, and the method is not particularly limited. In the case of using an electron beam, preferably the acceleration voltage is 50 to 800 kV and the absorbed dose is 0.1 to 100 Mrad.

  In general, the time of irradiation with active energy rays is preferably 0.1 minutes or more and less than 60 minutes, more preferably 0.2 minutes or more and less than 30 minutes, and even more preferably 1 minute or more and less than 15 minutes. In some cases, the time required to form surface crosslinks having the same crosslink density can be greatly reduced as compared to the conventional heat treatment using a surface crosslinker that requires a treatment time exceeding 60 minutes.

  When surface treatment is performed by irradiating active energy rays, it is not necessary to heat the surface. However, when irradiated with active energy rays, radiant heat may be generated. In general, when the water-absorbing resin is irradiated with active energy rays in a state where the water-absorbent resin is heated to a temperature of preferably less than 150 ° C, more preferably less than 120 ° C, still more preferably room temperature to 100 ° C, and particularly preferably 50 to 100 ° C. Good. Therefore, even when heating, the treatment temperature can be set lower than the conventional surface treatment temperature.

  It is preferable to stir the water-absorbent resin composition during irradiation with active energy rays. By stirring, the active energy ray can be uniformly irradiated to the water absorbent resin. Stirring means for stirring the water-absorbent resin composition during irradiation with active energy rays includes a vibration mixer, a vibration feeder, a ribbon mixer, a conical ribbon mixer, a screw mixer, and an airflow type. Examples thereof include a mixer, a batch kneader, a continuous kneader, a paddle type mixer, a high-speed fluid mixer, a floating fluid mixer, and a reaction kettle equipped with stirring blades. In addition, when irradiating an active energy ray, stirring using these stirring means, it is necessary to fully reach an active energy ray to a water absorbing resin composition. As a method for achieving such an object, for example, a method in which the active energy ray transmitting portion in the stirring means is used as an opening, or a substantially transparent member that is not damaged by the transmission of the active energy ray ( For example, a method composed of quartz glass) is exemplified.

  After irradiation with active energy rays, the water-absorbent resin may be heat-treated at a temperature of 50 to 250 ° C. as necessary for drying or the like.

  Further, after the active energy ray irradiation, surface cross-linking may be formed using a conventionally known surface cross-linking agent such as polyhydric alcohol, polyhydric epoxy compound, alkylene carbonate or the like.

  In the water-absorbent resin surface treatment method of the present invention, a liquid permeability improver may be added to the water-absorbent resin composition before or after irradiation with active energy rays. Examples of such liquid permeability improvers include talc, kaolin, fullerite, bentonite, activated clay, barite, natural asphalt, strontium ore, ilmenite, perlite, and other mineral products; aluminum sulfate 14-18 water Salts (or anhydrides), potassium aluminum sulfate 12-hydrate, sodium aluminum sulfate 12-hydrate, aluminum compounds such as aluminum chloride, polyaluminum chloride, aluminum oxide, and aqueous solutions thereof; other polyvalent metal salts; hydrophilic Amorphous silica (eg, dry method: Reolosil QS-20, Tokuyama Corp., precipitation method: DEGUSSA Sipernat22S, Sipernat 2200), etc .; 50), silicon oxide-aluminum oxide complex, and the like oxide composite such as silicon oxide, magnesium oxide complex. Such a liquid permeability improver is 0 to 20 parts by weight, more preferably 0.01 to 10 parts by weight, and particularly preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the modified water absorbent resin. Mix the parts. In the liquid permeability improving agent, those that are soluble in water can be added as an aqueous solution, and those that are not soluble can be added as a powder or slurry. These liquid permeability improvers may be added in a mixture with a polymerization initiator. In addition, as an additive, an antibacterial agent, a deodorant, a chelating agent, and the like may be appropriately added within the above range.

[Surface-treated water-absorbing resin]
According to the surface treatment method for a water-absorbent resin of the present invention, a water-absorbent resin that has been surface-treated (specifically, introduction of a crosslinked structure on the surface) is obtained. And in the obtained water-absorbing resin, the absorption capacity under pressure improves compared with before surface treatment.

  Conventionally, when a crosslinked structure is introduced on the surface of a water-absorbent resin, the free swelling ratio is slightly reduced, but it is known that the ability to maintain liquid absorption even under pressure, that is, the absorption capacity under pressure is improved. . According to the surface treatment method of the present invention, the absorption capacity under pressure can be improved without employing a surface cross-linking agent or heat treatment. Specifically, before and after the surface treatment, the absorption capacity under pressure of 4.83 kPa (0.7 psi) of the water absorbent resin is increased by 1 g / g or more. This is considered to indicate that a crosslinked structure was introduced on the surface of the water absorbent resin by the method of the present invention. The absolute value of the absorption capacity under pressure (AAP) of the surface-treated water-absorbing resin is preferably 8 g / g or more, more preferably 12 g / g or more, and still more preferably as a value under a pressure of 4.83 kPa. 15 g / g or more, particularly preferably 20 g / g or more, and most preferably 22 g / g or more. The upper limit of the absorption capacity under pressure (AAP) of the modified water-absorbent resin is not particularly limited, but about 40 g / g may be sufficient in view of the possibility of an increase in manufacturing cost due to manufacturing difficulty.

  The centrifuge retention capacity (CRC) of the surface-treated water-absorbent resin is preferably 50 g / g or less, more preferably 40 g / g or less, and still more preferably 35 g / g or less. Although a lower limit is not specifically limited, Preferably it is 10 g / g or more, More preferably, it is 20 g / g or more, More preferably, it is 25 g / g or more. If the centrifuge retention capacity (CRC) exceeds 50 g / g, the gel strength may decrease, and the water absorption capacity under pressure may decrease accordingly. On the other hand, if the centrifuge retention capacity (CRC) is less than 10 g / g, sufficient absorption capacity cannot be obtained, and urine leakage may occur when used as a diaper.

  In addition, as the values of the absorption capacity under pressure (AAP) and the centrifuge retention capacity (CRC), values measured by the method described in Examples described later are adopted.

  Moreover, when the water-absorbent resin in the water-absorbent resin composition obtained in step (a) contains a radical polymerization initiator, there is a feature that the amount of residual monomer in the surface-treated water-absorbent resin is extremely small. This is considered because the radical produced | generated by irradiation of the active energy ray to a radical polymerization initiator reacts with the residual monomer of a water absorbing resin. Since the water-absorbing resin is used in sanitary materials such as disposable diapers, the smaller the residual monomer, the better from the viewpoint of odor and safety. Usually, the residual monomer contained in the water-absorbent resin as the base polymer is 200 to 500 ppm, but the residual monomer of the modified water-absorbent resin obtained by the present invention is often 200 ppm or less (lower limit is 0 ppm). is there. The residual monomer of the modified water absorbent resin is preferably 200 ppm or less, more preferably 150 ppm or less, and even more preferably 100 ppm or less (the lower limit is 0 ppm). However, the technical scope of the present invention is not limited only to the method in which the residual monomer amount is a value within the above range.

  The shape of the surface-treated water-absorbent resin obtained by the surface treatment method of the present invention can be appropriately adjusted depending on the treatment conditions such as the shape of the water-absorbent resin before treatment, granulation / molding after treatment, etc. It is in powder form. Such powder has a weight average particle size (specified by sieve classification) of preferably 10 to 1,000 μm, preferably 200 to 600 μm, and more preferably a content of particles having a particle size of 150 to 850 μm. It is -100 weight%, More preferably, it is 95-100 weight%. Further, the value of the logarithmic standard deviation (σζ) of the particle size distribution is not particularly limited, but is preferably 0.2 to 0.45.

  The surface treatment method of the present invention has an effect of granulating fine powder generated during the production of the water absorbent resin during the surface treatment of the water absorbent resin. For this reason, even if fine powder is contained in the water-absorbing resin before the surface treatment, if the method of the present invention is performed, the fine powder contained in the water-absorbent resin is granulated, so it is contained in the surface-treated water absorbent resin. The amount of fine powder produced can be reduced. That is, the particle size distribution of the surface-treated water-absorbing resin can be shifted to a higher particle size side as compared with the water-absorbing resin before the surface treatment. However, the rate of shift depends on the type and amount of radically polymerizable compound (and radical polymerization initiator if necessary) contained in the water-absorbent resin, the amount of organic solvent used in the dispersion, irradiation conditions of active energy rays, irradiation It changes depending on the way of flow of time. Moreover, it is not restricted only to such a form, Even if a particle size distribution does not change or it may shift to the low particle size side, it is contained in the technical scope of this invention.

  In the surface-treated water-absorbent resin obtained by the surface treatment method of the present invention, the crosslinked structure is uniformly and densely introduced over the entire surface of the water-absorbent resin, and the characteristics desired for the water-absorbent resin, For example, the absorption magnification, absorption speed, gel strength, suction force, etc. can be improved to a very high level. In addition, when a cross-linked structure is introduced into the surface of an acrylic acid water-absorbent resin using a surface cross-linking agent such as a polyhydric alcohol, a polyvalent epoxy compound, and an alkylene carbonate as in the prior art, The degree was dependent on the neutralization rate of the water absorbent resin. Specifically, when the neutralization rate is low, the introduction of surface crosslinking proceeds rapidly, and when the neutralization rate is high, it becomes difficult to introduce surface crosslinking. Moreover, in order to introduce a crosslinked structure on the surface of the water-absorbent resin obtained by post-neutralization of polyacrylic acid, it has been necessary to uniformly neutralize after the surface crosslinking treatment. However, in the present invention, when the water absorbent resin composition contains a radical polymerization initiator, the water absorbent resin is surface-treated without depending on the neutralization rate of the water absorbent resin and the uniformity of the post-neutralization, A water-absorbing resin excellent in water-absorbing properties can be produced. This is because the introduction of surface cross-linking depends on the action of the radical polymerization initiator on the main chain of the water-absorbent resin, so that it does not matter whether the carboxyl group is present as an acid or a salt. It is thought to be due to progress.

  According to the present invention, when the water-absorbent resin is surface-treated, it can be sufficiently surface-treated even at a reaction temperature near room temperature, and the obtained water-absorbent resin subjected to surface treatment (introduction of surface cross-linking) The properties desired for the water-absorbing resin, such as absorption speed, gel strength, and suction force, are at a very high level. Accordingly, the surface-treated water-absorbing resin obtained by the surface treatment method of the present invention is optimal as a sanitary material that absorbs sanitary cotton, disposable diapers, or other body fluids, and a water-absorbing resin for agriculture.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention more concretely, the technical scope of this invention is not limited only to the following form. In the following description, for convenience, “parts by weight” may be simply referred to as “parts”, and “liters” may be simply referred to as “L”. Unless otherwise specified, all operations were performed under conditions of room temperature (20 to 25 ° C.) and humidity of 50 RH%. The measurement method and evaluation method of various parameters in the water-absorbent resin before and after the surface treatment in Examples and Comparative Examples are shown below.

(1) Centrifugal Retention Capacity (also referred to as “CRC”)
CRC indicates the absorption rate for 30 minutes under no pressure with respect to a 0.90% by weight sodium chloride aqueous solution (hereinafter, also simply referred to as “physiological saline”).

  0.200 g of water-absorbent resin was uniformly put into a bag (85 mm × 60 mm) made of a nonwoven fabric (trade name: Heaton Paper, model: GSP-22, manufactured by Nankoku Pulp Industries Co., Ltd.) and heat sealed, and then at room temperature. It was immersed in a large excess (about 500 mL) of physiological saline. After 30 minutes, the bag was pulled up and drained for 3 minutes with a centrifugal force (250 G) described in edena ABSORBENCY II 441.1-99 using a centrifuge (manufactured by Kokusan Co., Ltd., model: H-122). The weight of the bag was measured. This weight was designated as W1 (g). Further, the same operation was performed without using the water-absorbent resin, and the weight at that time was defined as W0 (g). And CRC (g / g) was computed from the value of these W1 and W0 according to the following numerical formula.

(2) Absorption capacity 0.7 psi under pressure (Absorbency Against Pressure; also referred to as “AAP0.7”)
AAP0.7 indicates the absorption capacity for 60 minutes under pressure of 4.83 kPa (0.7 psi) against physiological saline.

  As shown in FIG. 2, a stainless steel 400 mesh wire mesh 101 (aperture 38 μm) is fused to the bottom of a plastic support cylinder 100 having an inner diameter of 60 mm, and 0.900 g of water absorbent resin is uniformly dispersed on the wire mesh. In addition, the outer diameter is slightly smaller than 60 mm, adjusted to be able to uniformly apply a load of 4.83 kPa (0.7 psi) to the water-absorbent resin, and between the inner wall surface of the support cylinder 100 A piston 103 and a load 104, in which no gap is generated and the vertical movement is not hindered, were placed in this order, and the weight of the measuring device set was measured. This weight was designated as W2 (g).

  A glass filter 106 having a diameter of 90 mm (manufactured by Reciprocal Chemical Glass Co., Ltd., pore diameter: 100 to 120 μm) is placed inside a petri dish 105 having a diameter of 150 mm, and physiological saline (20 to 25 ° C.) is placed on the upper surface of the glass filter. Added to the same level. On top of that, a sheet of filter paper 107 having a diameter of 90 mm (manufactured by ADVANTEC Toyo Co., Ltd., trade name: (JIS P 3801, No. 2), thickness 0.26 mm, reserved particle diameter 5 μm) is placed so that the entire surface is wetted. And excess liquid was removed.

  A set of measuring devices was placed on the wet filter paper, and the liquid was absorbed under load for a predetermined time. This absorption time was calculated from the start of measurement and was 1 hour later. Specifically, after 1 hour, a set of measuring apparatus was lifted and its weight was measured. This weight was designated as W3 (g). This weight measurement must be performed as quickly as possible and without vibrations. And AAP0.7 was computed from W2 and W3 according to the following numerical formula.

(3) Weight average particle diameter (also referred to as “D50”) and logarithmic standard deviation of particle size distribution (also referred to as “σζ”)
Sieve the water-absorbent resin using a JIS standard sieve (THE IIDA TESTING SIEVE, manufactured by Iida Seisakusho Co., Ltd., diameter: 8 cm) having an aperture of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm, 45 μm. did. Specifically, a water-absorbing resin was charged into this JIS standard sieve, and classification was performed for 5 minutes using a vibration classifier (manufactured by Iida Seisakusho, SIEVE SHAKER, model: ES-65, SER. No. 0501). The residual percentage R was then plotted on log probability paper. From this plot, the particle size corresponding to R = 50% by weight was read as the weight average particle size (D50). Further, the logarithmic standard deviation (σζ) of the particle size distribution was calculated according to the following formula, where the particle diameters corresponding to R = 84.1 wt% and R = 15.9 wt% were X1 and X2, respectively. Note that the smaller the value of σζ, the narrower the particle size distribution.

(4) Elution and soluble content In a 250 mL capacity plastic container with a lid (diameter 6 cm x height 9 cm), weigh 184.3 g of physiological saline solution, add 1.00 g of water-absorbing resin therein, and add a diameter of 16 hours. By using a magnetic stir bar having a length of 8 mm and a length of 25 mm and stirring at a rotation speed of 500 rpm, an elution soluble component in the resin was extracted. This extract was filtered using one sheet of filter paper (ADVANTEC Toyo Corporation, product name: JIS P 3801, No. 2, thickness: 0.26 mm, retention particle diameter: 5 μm), and 50.0 g of the obtained filtrate. Was measured and used as a measurement solution.

  First, the physiological saline alone is titrated to pH 10 with 0.1N aqueous sodium hydroxide solution, and then titrated to pH 2.7 with 1N hydrochloric acid (referred to as [bNaOH] and [bHCl], respectively). )

  By performing the same titration operation on the measurement solution, titration amounts (referred to as [NaOH] and [HCl], respectively) were obtained.

  For example, in the case of a water-absorbing resin composed of a known amount of acrylic acid and its sodium salt, the elution soluble content in the water-absorbing resin is determined based on the average molecular weight of the monomer and the titration amount obtained by the above operation. It calculated according to the following numerical formula. In the case of an unknown amount, the average molecular weight of the monomer was calculated using the neutralization rate obtained by titration according to the following formula.

(5) Moisture content 1.00 g of water absorbent resin was uniformly spread on the bottom of an aluminum cup having a bottom diameter of 4 cm and a height of 2 cm, and the weight of the cup containing the water absorbent resin was measured. This weight was designated as W4 (g). Next, this cup was left in a hot air dryer adjusted to 180 ° C. for 3 hours, and the weight of the cup containing the water absorbent resin immediately after removal from the hot air dryer (at least within 1 minute) was measured. This weight was designated as W5 (g). And the moisture content was computed from W4 and W5 according to the following numerical formula.

<Production Example 1>
5433 g of an aqueous solution of sodium acrylate having a neutralization rate of 70 mol% (monomer) in a reactor formed by attaching a lid to a stainless steel double arm type kneader with an internal volume of 10 L and having two sigma blades Concentration: 39% by weight) was charged, and 12.83 g of polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9) as an internal crosslinking agent was dissolved in the aqueous solution to prepare a reaction solution. Subsequently, this reaction solution was degassed for 30 minutes in a nitrogen gas atmosphere. Subsequently, 29.43 g of a 10 wt% aqueous solution of sodium persulfate as a polymerization initiator and 24.53 g of a 0.1 wt% aqueous solution of L-ascorbic acid were added to the reaction solution while stirring. As a result, polymerization started after about 1 minute. Then, polymerization was performed at 20 to 95 ° C. while pulverizing the generated gel, and a hydrous gel-like crosslinked polymer was taken out 30 minutes after the polymerization started. The obtained hydrogel crosslinked polymer had a particle size of 5 mm or less. The crushed water-containing gel-like crosslinked polymer was spread on a 50 mesh (mesh opening 300 μm) wire net and dried with hot air at 175 ° C. for 50 minutes. In this way, an agglomerated powdery aggregate that can be easily pulverized was obtained.

  The obtained powdery aggregate was pulverized using a roll mill, and further classified using a JIS standard sieve having an aperture of 710 μm. Next, the particles that passed through the sieve having an aperture of 710 μm by the above operation were classified using a JIS standard sieve having an aperture of 150 μm, and the particles that passed through the sieve having an aperture of 150 μm were removed. In this way, a water absorbent resin (A1) was obtained.

  Various evaluation results of the obtained water absorbent resin (A1) are shown in Table 1 below. In addition, Table 2 below shows the particle size distribution of the water-absorbent resin (A1) obtained. In addition, in the column of “Example” and “Comparative Example” in this specification, the value “after moisture content correction” was calculated according to the following mathematical formula.

<Example 1>
10 g of the water-absorbent resin (A1) obtained in Production Example 1 above was charged in a separable flask made of quartz and stirred at 400 rpm using a stirring blade, while 0.08 g of acrylic acid, 0.20 g of sodium acrylate, polyethylene glycol A treatment liquid in which 0.02 g of diacrylate (average number of ethylene oxide units: n = 9), 0.10 g of ammonium persulfate (manufactured by Wako Pure Chemical Industries, Ltd.), and 0.90 g of pure water was added.

  After stirring for 3 minutes after the addition of the treatment liquid, a solution prepared by dissolving 0.20 g of Rheodor SP-S10 (manufactured by Kao Corporation, sorbitan monostearate, nonionic surfactant) in 300 g of cyclohexane was added, Stirring was continued for an additional 7 minutes.

After changing the number of revolutions of the stirring blade to 200 rpm, irradiation intensity using an ultraviolet irradiation device (UV-152 / 1MNSC3-AA06) attached with a metal halide lamp (Ushio Electric Co., Ltd., UVL-1500M2-N1) Ultraviolet rays were irradiated at 60 mW / cm ² for 10 minutes at room temperature.

  Agitation was stopped and the supernatant solution was removed as much as possible by decantation. Under a reduced pressure of 70 ° C. and 3 Torr (about 400 Pa), the mixture remaining in the separable flask was dried (Yamato Scientific Co., Ltd., vacuum low-temperature dryer, model: DP33, vacuum pump: ULVAC Kiko Co., Ltd., model) : GVD-100A) and dried for 1 hour to obtain a water-absorbent resin (1) subjected to surface treatment (crosslinked structure introduced on the surface). The treatment conditions for this water absorbent resin (1) are shown in Table 3 below, and the water absorption properties are shown in Table 4 below.

<Example 2>
A water-absorbing resin (2) subjected to surface treatment (a cross-linked structure was introduced to the surface) was obtained by the same method as in Example 1 except that the ultraviolet irradiation time was set to 5 minutes. The treatment conditions of this water absorbent resin (2) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Example 3>
A water-absorbent resin (3) subjected to surface treatment (a cross-linked structure was introduced to the surface) was obtained by the same method as in Example 1 except that the ultraviolet irradiation time was 1 minute. The treatment conditions of this water absorbent resin (3) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Example 4>
Surface treatment (cross-linked structure was introduced on the surface) was carried out in the same manner as in Example 1 except that Rhedol SP-S10 was not used and the rotation speed of the stirring blade at the time of ultraviolet irradiation was 400 rpm. A water absorbent resin (4) was obtained. The treatment conditions of this water absorbent resin (4) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Example 5>
10 g of the water-absorbent resin (A1) obtained in Production Example 1 above was charged into a quartz separable flask and stirred at 400 rpm using a stirring blade, while 0.08 g of acrylic acid, 0.20 g of sodium acrylate, polyethylene glycol 0.02 g of diacrylate (average number of ethylene oxide units: n = 9), 0.00045 g of sodium persulfate (manufactured by Wako Pure Chemical Industries, Ltd.), 0.003 g of Irgacure (registered trademark) 2959 (manufactured by Nagase Sangyo Co., Ltd.), Then, a treatment liquid mixed with 0.90 g of pure water was added.

  After stirring for 3 minutes after the addition of the treatment liquid, a solution prepared by dissolving 0.20 g of Rheodor SP-S10 (manufactured by Kao Corporation, sorbitan monostearate, nonionic surfactant) in 300 g of cyclohexane was added, Stirring was continued for an additional 7 minutes.

After changing the number of revolutions of the stirring blade to 200 rpm, irradiation intensity using an ultraviolet irradiation device (UV-152 / 1MNSC3-AA06) attached with a metal halide lamp (Ushio Electric Co., Ltd., UVL-1500M2-N1) Ultraviolet rays were irradiated at 60 mW / cm ² for 10 minutes at room temperature.

  Agitation was stopped and the supernatant solution was removed as much as possible by decantation. Under a reduced pressure of 70 ° C. and 3 Torr (about 400 Pa), the mixture remaining in the separable flask was dried (Yamato Scientific Co., Ltd., vacuum low-temperature dryer, model: DP33, vacuum pump: ULVAC Kiko Co., Ltd., model) : GVD-100A) and dried for 1 hour to obtain a water-absorbent resin (5) that was surface-treated (a crosslinked structure was introduced on the surface). The treatment conditions for this water absorbent resin (5) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Example 6>
A water-absorbing resin (6) subjected to surface treatment (a cross-linked structure was introduced to the surface) was obtained by the same method as in Example 1 except that the irradiation time of ultraviolet rays was 5 minutes. The treatment conditions for this water absorbent resin (6) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Example 7>
A water-absorbing resin (7) subjected to surface treatment (a cross-linked structure was introduced to the surface) was obtained in the same manner as in Example 1 except that the irradiation time of ultraviolet rays was 1 minute. The treatment conditions for this water absorbent resin (7) are shown in Table 3 below, and the water absorption properties are shown in Table 4 below.

<Example 8>
Surface treatment (cross-linked structure was introduced on the surface) was carried out in the same manner as in Example 1 except that Rhedol SP-S10 was not used and the rotation speed of the stirring blades during UV irradiation was 400 rpm. A water absorbent resin (8) was obtained. The treatment conditions of this water absorbent resin (8) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Comparative Example 1>
10 g of the water-absorbent resin (A1) obtained in Production Example 1 above was charged in a separable flask made of quartz and stirred at 400 rpm using a stirring blade, while 0.08 g of acrylic acid, 0.20 g of sodium acrylate, polyethylene glycol A treatment liquid in which 0.02 g of diacrylate (average number of ethylene oxide units: n = 9), 0.10 g of ammonium persulfate (manufactured by Wako Pure Chemical Industries, Ltd.), and 0.90 g of pure water was added.

  Stirring was continued for 3 minutes after addition of the treatment liquid, and then 300 g of cyclohexane was added and stirring was continued for another 17 minutes.

  Agitation was stopped and the supernatant solution was removed as much as possible by decantation. Under a reduced pressure of 70 ° C. and 3 Torr (about 400 Pa), the mixture remaining in the separable flask was dried (Yamato Scientific Co., Ltd., vacuum low-temperature dryer, model: DP33, vacuum pump: ULVAC Kiko Co., Ltd., model) : GVD-100A) for 1 hour to obtain a comparative water absorbent resin (1). The treatment conditions for this comparative water absorbent resin (1) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Comparative example 2>
As the treatment liquid, 0.08 g of acrylic acid, 0.20 g of sodium acrylate, 0.02 g of polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9), sodium persulfate (manufactured by Wako Pure Chemical Industries, Ltd.) Comparative water absorption was carried out in the same manner as in Comparative Example 1 except that a solution obtained by mixing 0405 g, 0.003 g of Irgacure (registered trademark) 2959 (manufactured by Nagase Sangyo Co., Ltd.), and 0.90 g of pure water was used. Resin (2) was obtained. The treatment conditions for this comparative water absorbent resin (2) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Comparative Example 3>
10 g of the water-absorbent resin (A1) obtained in Production Example 1 above was charged in a separable flask made of quartz and stirred at 400 rpm using a stirring blade, while ethylene glycol diglycidyl ether (manufactured by Nagase ChemteX Corporation, Denacol EX) -810) A treatment liquid in which 0.003 g, 0.032 g of 1,4-butanediol, 0.05 g of propylene glycol, and 0.273 g of pure water were added.

  Stirring was continued for 3 minutes after addition of the treatment liquid, and then 300 g of cyclohexane was added and stirring was continued for another 2 minutes. Next, the separable flask was immersed in a water bath at about 85 ° C. and heated for 15 minutes (temperature of the cyclohexane solution: 73 ° C. after 5 minutes from the start of heating, and 78 ° C. after 15 minutes).

  Agitation was stopped and the supernatant solution was removed as much as possible by decantation. Under a reduced pressure of 70 ° C. and 3 Torr (about 400 Pa), the mixture remaining in the separable flask was dried (Yamato Scientific Co., Ltd., vacuum low-temperature dryer, model: DP33, vacuum pump: ULVAC Kiko Co., Ltd., model) : GVD-100A) for 1 hour to obtain a comparative water absorbent resin (3). The treatment conditions for this comparative water absorbent resin (3) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Comparative example 4>
Except for using a solution obtained by mixing 0.01 g of ethylene glycol diglycidyl ether (manufactured by Nagase ChemteX Corporation, Denacol EX-810), 0.05 g of isopropyl alcohol, and 0.3 g of pure water as the treatment liquid. Comparative water-absorbing resin (4) was obtained by the same method as in Comparative Example 3. The treatment conditions for this comparative water absorbent resin (4) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

<Comparative Example 5>
10 g of the water-absorbent resin (A1) obtained in Production Example 1 above was charged in a separable flask made of quartz and stirred at 400 rpm using a stirring blade, while 0.08 g of acrylic acid, 0.20 g of sodium acrylate, polyethylene glycol 0.02 g of diacrylate (average number of ethylene oxide units: n = 9), 0.00045 g of sodium persulfate (manufactured by Wako Pure Chemical Industries, Ltd.), 0.003 g of Irgacure (registered trademark) 2959 (manufactured by Nagase Sangyo Co., Ltd.), Then, a treatment liquid mixed with 0.90 g of pure water was added.

After stirring for 10 minutes after addition of the treatment liquid, irradiation was performed using an ultraviolet irradiation device (UV-152 / 1MNSC3-AA06) equipped with a metal halide lamp (Ushio Electric Co., Ltd., UVL-1500M2-N1). A comparative water absorbent resin (5) was obtained by irradiating with ultraviolet rays at room temperature for 1 minute at an intensity of 60 mW / cm ² . The treatment conditions for this comparative water absorbent resin (5) are shown in Table 3 below, and the water absorption characteristics are shown in Table 4 below.

  From the results shown in Table 4, it is shown that according to the surface treatment method for a water absorbent resin of the present invention, a crosslinked structure can be introduced on the surface of the water absorbent resin without using a conventional heat treatment. In addition, by using active energy ray irradiation instead of heat treatment, a highly uniform crosslinked structure can be introduced in a short time. Furthermore, the water-absorbent resin subjected to the surface treatment (introduction of surface crosslinking) produced by the surface treatment method of the present invention is excellent in water absorption properties such as absorption capacity under load and liquid permeability.

  According to the present invention, it is possible to introduce a crosslinked structure on the surface of the water-absorbent resin without using heat treatment, and the obtained surface-treated water-absorbent resin is excellent in water absorption characteristics, and thus used as a paper diaper or the like. Can be industrially useful.

It is the schematic of the measuring apparatus used for a measurement of absorption magnification under pressure (APP0.7).

Explanation of symbols

100 support cylinder,
101 wire mesh,
102 swelling gel,
103 piston,
104 load,
105 Petri dishes,
106 glass filter,
107 filter paper,
108 0.9 wt% sodium chloride aqueous solution (saline).

Claims (19)

A surface treatment method for a water absorbent resin,
(A) a step of obtaining a water-absorbing resin composition by mixing a water-absorbing resin, a radical polymerizable compound, and an organic solvent;
(B) irradiating the obtained water-absorbent resin composition with active energy rays;
A surface treatment method for a water-absorbent resin, comprising:   The surface treatment method according to claim 1, wherein the radical polymerizable compound is mixed in the form of an aqueous solution.   The surface treatment method according to claim 2, wherein the amount of water in the aqueous solution is 1 to 50 parts by weight with respect to 100 parts by weight of the water absorbent resin.   The surface treatment method according to claim 1, wherein the water absorbent resin composition further contains a radical polymerization initiator.   The surface treatment method according to claim 4, wherein the water absorbent resin composition is obtained by further mixing a radical polymerization initiator.   The surface treatment method according to claim 5, wherein the radical polymerization initiator is mixed in the form of an aqueous solution.   The surface treatment method according to claim 6, wherein a content of water in the aqueous solution is 1 to 40 parts by weight with respect to 100 parts by weight of the water absorbent resin.   The surface treatment method according to any one of claims 4 to 7, wherein the radical polymerization initiator is a water-soluble radical polymerization initiator or a thermal decomposition type radical polymerization initiator.   The content of the radical polymerization initiator in the water absorbent resin composition is 0.01 to 20 parts by weight with respect to 100 parts by weight of the water absorbent resin, according to any one of claims 4 to 8. Surface treatment method.   The surface treatment according to any one of claims 4 to 9, wherein the radical polymerization initiator is one or more selected from the group consisting of persulfate, hydrogen peroxide and a water-soluble azo compound. Method.   The surface treatment method according to claim 1, wherein the radical polymerizable compound includes a compound having two or more polymerizable unsaturated groups in one molecule.   The surface treatment method according to claim 1, wherein the organic solvent is a hydrophobic organic solvent.   The surface treatment method according to any one of claims 1 to 12, wherein the water-absorbent resin is obtained by reverse phase suspension polymerization.   The surface treatment method according to claim 1, further comprising a step of mixing a surfactant and / or polymer protective colloid as a mixing aid in the step of obtaining the water-absorbent resin composition.   The surface treatment method according to claim 14, wherein an amount of the mixing aid is 0.01 to 40 parts by weight with respect to 100 parts by weight of the water absorbent resin.   The water-absorbing resin contains acid groups and has a neutralization rate of 25 to 100 mol% (mol% of neutralized acid groups in all acid groups). The surface treatment method according to 1.   The surface treatment method according to claim 1, wherein the active energy rays are ultraviolet rays.   The surface treatment according to claim 1, wherein the water absorbent resin is a powdery water absorbent resin obtained by polymerizing a monomer component mainly composed of acrylic acid (salt). Method.   The water absorbent resin contains 90 to 100% by weight of particles having a particle size in the range of 150 to 850 μm, the weight average particle diameter of the water absorbent resin is 200 to 600 μm, and the logarithmic standard of the particle size distribution of the water absorbent resin The surface treatment method according to claim 1, wherein the deviation is 0.45 or less.