JP2007326912A - Method for producing modified water-absorbing resin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing modified water-absorbing resin excellent in production efficiency, absorbing capacity under pressure, absorbing rate, gel strength, liquid permeability, etc. <P>SOLUTION: The invention relates to the method for producing the modified water-absorbing resin comprising a process (a) obtaining a water-absorbing resin composition comprising mixing a water-absorbing resin, water and a water soluble radical polymerization initiator without adding an ethylenic unsaturated monomer, (b) a process irradiating the water-absorbing resin composition obtained in the mixing process, with an active energy ray, wherein the mixing amount of the water in the mixing process (a) is 20-100 pts.wt. per 100 pts.wt. of the water-absorbing resin, and the surface water-absorbing rate in the water-absorbing resin composition in the irradiating process (b) at least any time in the process is regulated at less than 3.0 wt.%. per the 100 wt.%. of the water-absorbing resin. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a modified water absorbent resin, and more particularly to a method for producing a modified water absorbent resin by irradiating the water absorbent resin with active energy rays.

  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 property is affected by the crosslink density, the relationship between the properties does not necessarily show a positive correlation, for example, the gel strength increases but the water absorption amount decreases as the crosslink density increases. In particular, the absorption magnification and the absorption rate, gel strength, suction force, and the like are in a contradictory relationship. For this reason, in the water-absorbent resin having improved absorption capacity, when the particles come into contact with the liquid, water absorption is not performed uniformly, and a part of the water-absorbent resin is formed or water is absorbed in the entire water-absorbent resin particles. Since it does not diffuse, the absorption rate may be drastically reduced.

  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). It is difficult to uniformly distribute the internal crosslinks in the polymer, and it is not easy to adjust the crosslink density. For this reason, after obtaining a polymer having a low crosslinking density and a water-soluble polyacrylic acid gel, a persulfate as a polymerization initiator is added and heated. In this Patent Document 1, it is possible to precisely control the crosslinking density by adjusting the amount of initiator added, and since the crosslinking is uniformly present in the polymer, excellent water absorption characteristics are obtained, and the adhesiveness is high. No water-absorbing resin was obtained.

  The persulfate used in Patent Document 1 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, radical crosslinking occurs at the same time as polymerization by irradiating an aqueous solution of a water-soluble vinyl monomer with light energy, 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, there is a method in which the surface of the water absorbent resin is treated with a crosslinking agent to increase the crosslink density of the surface of the water absorbent resin (for example, Patent Document 4 and Patent Document 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 introduce cross-linking between functional groups, the surface cross-linking density of the water-absorbing resin increases, and a water-absorbing resin having excellent water-absorbing properties even under pressure can do.

  Further, when the surface crosslinking agent is used, the crosslinking formation reaction requires a high temperature or a long period of time, and there is a problem such as remaining unreacted crosslinking agent. Therefore, an aqueous solution containing a peroxide radical initiator is brought into contact with the resin, There is also a method in which the resin is heated to introduce crosslinks to polymer molecular chains in the vicinity of the surface of the resin through decomposition of the radical initiator (Patent Document 6). In the examples, heating with 130 ° C. superheated steam was performed for 6 minutes to obtain a water-absorbing resin with improved water absorption ratio.

Here, when adding a surface cross-linking agent to surface cross-link the water-absorbent resin particles, there is a technique for improving the water-absorbing characteristics of the water-absorbent resin particles by controlling the water content and water content of the entire water-absorbent resin particles. It is known (for example, Patent Document 7 and Patent Document 8).
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 U.S. Pat. No. 4,541,871 US Pat. No. 4,507,438 J. et al. Phys. Chem. , 1975, 79, 2693, J. Am. Photochem. Photobiol. , A, 1988, 44, 243

  The purpose of introducing surface cross-linking to the water-absorbent resin is to produce a water-absorbent resin excellent in the balance between the absorption capacity and the absorption speed. In general, a cross-linking agent having at least two functional groups that can react with a functional group on the surface of the water absorbent resin needs to act on the water absorbent resin. 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. For this reason, much energy and time are required.

  In the production methods described in Patent Document 7 and Patent Document 8 described above, the moisture content and water content of the entire water-absorbent resin particles are controlled when a surface cross-linking agent is added to introduce a crosslinked structure on the surface of the water-absorbent resin particles. However, it is required to introduce surface crosslinking more efficiently and to produce a water-absorbing resin having more excellent water-absorbing properties, but it is not always possible to provide a satisfactory production method. is there.

  Under such circumstances, an object of the present invention is to provide a method for producing a modified water-absorbing resin that is excellent in production efficiency and excellent in absorption capacity under pressure, absorption speed, gel strength, liquid permeability, and the like. And

  The present inventors have studied in detail the method for producing a water-absorbent resin having a modified surface. In the process, the inventors have eagerly searched for the reason why a water-absorbing resin having sufficiently excellent water-absorbing characteristics cannot be efficiently manufactured by the conventional manufacturing method for controlling the water content and the water content of the entire water-absorbing resin particles. As a result, it was found that the surface of the particles needs to retain a certain amount of moisture in order to efficiently introduce a crosslinked structure on the surface of the water-absorbent resin particles. That is, in the conventional manufacturing method, even if the water content and the water content of the entire water-absorbent resin particles are controlled, if the surface of the particles is not moistened to some extent, the introduction of a crosslinked structure to the particle surface is efficient. It was found that a water-absorbing resin excellent in water absorption characteristics could not be produced. Based on such knowledge, the present inventors tried to perform surface cross-linking treatment with the surface of the water-absorbent resin particles moistened to some extent. In addition, it has been found that by adopting active energy ray irradiation instead of the conventional addition of a surface cross-linking agent as a means for introducing surface cross-linking, the cross-linking efficiency and the water-absorbing properties of the resulting water-absorbent resin can be improved. The invention has been completed.

  That is, the present invention relates to a method for producing a modified water-absorbing resin, in which a) a water-absorbing resin, water, a water-soluble radical polymerization initiator and / Or a mixing step of mixing a thermally decomposable radical polymerization initiator to obtain a water absorbent resin composition, and b) an irradiation step of irradiating the water absorbent resin composition obtained in the mixing step with active energy rays, The mixing amount of the water in the mixing step a) is more than 20 parts by weight and not more than 100 parts by weight with respect to 100 parts by weight of the water-absorbent resin, and at least one point in the irradiation step b) The surface water content of the water-absorbent resin in the water-absorbent resin composition is controlled to 3.0% by weight or more with respect to 100% by weight of the water-absorbent resin. It is a resin manufacturing method.

  According to the production method of the present invention, a uniform cross-linked structure can be introduced on the surface of the water-absorbent resin particles. As a result, the modified water-absorbent resin has extremely high properties desired for the water-absorbent resin, such as absorption capacity, absorption rate, gel strength, and suction force.

  Moreover, since the method for producing a modified water-absorbing resin of the present invention performs surface cross-linking by irradiation with active energy rays, the water-absorbing resin can be modified in a shorter time than a conventional method.

  The present invention relates to a method for producing a modified water-absorbent resin, comprising: a) a water-absorbent resin, water, a water-soluble radical polymerization initiator and / or heat without adding an ethylenically unsaturated monomer. A mixing step of mixing a decomposable radical polymerization initiator to obtain a water absorbent resin composition, and b) an irradiation step of irradiating the water absorbent resin composition obtained in the mixing step with active energy rays. The mixing amount of the water in the mixing step a) is more than 20 parts by weight and not more than 100 parts by weight with respect to 100 parts by weight of the water-absorbent resin, and at least at any point in the irradiation step b) A modified water-absorbent resin, characterized in that the water content of the surface of the water-absorbent resin in the water-absorbent resin composition is controlled to 3.0% by weight or more with respect to 100% by weight of the water-absorbent resin. It is a manufacturing method.

  Hereinafter, although the manufacturing method of the modified water-absorbing resin according to the present invention will be described in detail, the scope of the present invention is not limited to these descriptions, and the gist of the present invention is impaired except for the following examples. The present invention can be carried out with appropriate modifications within the range.

(A) Water-absorbing resin The water-absorbing resin that can be used in the present invention is a water-swellable, water-insoluble crosslinked polymer capable of forming a hydrogel. As will be described in detail below, the present invention relates to the surface moisture content of the water-absorbent resin in the water-absorbent resin composition at least at any point in the irradiation step with respect to 100% by weight of the water-absorbent resin. It is characterized by being controlled to 0% by weight or more. For this reason, in the mixing step, the water-absorbing resin is already in a water-containing state before mixing, and even when no water is added, the surface moisture content at the start of the irradiation step of the next step is 3.0% by weight or more, In the mixing step, it is not always necessary to add water separately. In the present invention, “water swellability” means that the centrifuge retention capacity in physiological saline is 2 g / g or more, preferably 5 to 100 g / g, more preferably 10 to 60 g / 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, as a numerical value of a centrifuge retention capacity, the value measured by the method as described in the Example mentioned later shall be employ | adopted. Moreover, the value measured by the following method shall be employ | adopted as a numerical value of an elution soluble part.

[Measurement method for soluble elution]
Measure 184.3 g of physiological saline in a plastic container with a lid of 250 mL capacity (diameter 6 cm x height 9 cm), add 1.00 g of water-absorbing resin to it, and magnetize it with a diameter of 8 mm and a length of 25 mm for 16 hours. By using a stir bar to stir at a rotation speed of 500 rpm, the soluble fraction eluted in the resin is 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. Is measured to obtain a measurement solution.

  First, the physiological saline alone was 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) are 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. Calculate according to the following formula. In the case of an unknown amount, the average molecular weight of the monomer is calculated using the neutralization rate obtained by titration according to the following formula.

  In the present invention, “modification” refers to all physical or chemical effects on the water-absorbent resin, and includes surface cross-linking, pore formation, hydrophilization, and hydrophobicity of the water-absorbent resin.

  The water-absorbing resin that can be used in the present invention is not particularly limited as long as it is obtained by polymerization using a conventionally known method using a monomer component that essentially contains an ethylenically unsaturated monomer. .

  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; or the like can be cited, and 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, particularly preferably acrylic acid and / or a salt thereof.

  When acrylate is used as the monomer, a monovalent salt of acrylic acid selected from alkali metal salts, ammonium salts, and amine salts of acrylic acid is preferable from the viewpoint of water absorption performance of the water absorbent resin. More preferred are alkali metal acrylates, and particularly preferred are acrylates selected from sodium salts, lithium salts, and potassium salts.

  When producing the water-absorbent resin, other monomer components other than the above-mentioned monomers can be used within a range not impairing the effects of the present invention. For example, an aromatic ethylenically unsaturated monomer having 8 to 30 carbon atoms, an aliphatic ethylenically unsaturated monomer having 2 to 20 carbon atoms, and an alicyclic ethylenically unsaturated monomer having 5 to 15 carbon atoms And hydrophobic monomers such as alkyl group (meth) acrylic acid alkyl ester having 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 absorbent resin used in the present invention becomes insoluble due to the formation of internal crosslinks. Such internal crosslinking may be a self-crosslinking type that does not use a crosslinking agent, but uses an internal crosslinking agent having two or more polymerizable unsaturated groups and / or two or more reactive functional groups in one molecule. Can be formed.

  Such an internal cross-linking agent is not particularly limited, and preferably, for example, N, N′-methylenebis (meth) acrylamide, N-methylol (meth) acrylamide, glycidyl (meth) acrylate, (poly) ethylene glycol diester (Meth) acrylate, (poly) propylene glycol di (meth) acrylate, glycerol tri (meth) acrylate, glycerol acrylate methacrylate, (meth) acrylic acid polyvalent metal salt, trimethylolpropane tri (meth) acrylate, triallylamine, tri List allyl cyanurate, triallyl isocyanurate, triallyl phosphate, ethylene glycol diglycidyl ether, (poly) glycerol glycidyl ether, polyethylene glycol diglycidyl ether, etc. It can be. These internal crosslinking agents may be used alone or in combination of two or more.

  The amount of the internal crosslinking agent used is preferably 0.0001 to 1 mol%, more preferably 0.001 to 0.5 mol%, based on the total amount of monomer components used in producing the water absorbent resin. More preferably, it is 0.005-0.2 mol%. When the amount is less than 0.0001 mol%, internal crosslinking is not introduced into the resin. On the other hand, when the amount exceeds 1 mol%, the gel strength of the water-absorbent resin becomes too high, and the water absorption ratio may be lowered. When a crosslinked structure is introduced into the polymer using the internal cross-linking agent, the internal cross-linking agent is added to the reaction system before, during or after the polymerization of the monomer, or after the polymerization or neutralization. What should I do?

  In order to obtain a water-absorbing resin, the monomer component including the monomer and the internal crosslinking agent may be polymerized in an aqueous solution. In this case, usable polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate; potassium peracetate, sodium peracetate, potassium percarbonate, sodium percarbonate, t-butyl hydroperoxide; Hydrogen peroxide; azo compounds such as 2,2′-azobis (2-amidinopropane) dihydrochloride, water-soluble radical polymerization initiators such as 2-hydroxy-2-methyl-1-phenyl-propane-1- There are photopolymerization initiators such as ON. Further, for example, a reducing agent such as sulfite, L-ascorbic acid or ferric salt may be combined with the water-soluble radical polymerization initiator and used as a redox initiator.

  Although there is no restriction | limiting in particular in the density | concentration of the monomer in the said monomer aqueous solution, Preferably it is 15 to 90 weight%, More preferably, it is 35 to 80 weight%. If it is less than 15% by weight, the resulting hydrogel has a large amount of water, which requires a heat amount and time for drying, which is disadvantageous.

  The polymerization method is not particularly limited, and well-known methods such as aqueous solution polymerization, reverse phase suspension polymerization, precipitation polymerization, bulk polymerization and the like can be employed. Among these methods, aqueous solution polymerization in which a monomer is dissolved in an aqueous solution for polymerization and reverse phase suspension polymerization are preferable from the viewpoint of ease of control of the polymerization reaction and performance of the obtained water-absorbent resin.

  When starting the above polymerization, the above polymerization initiator is used. In addition to the aforementioned polymerization initiator, active energy rays such as ultraviolet rays, electron beams, and γ rays may be used alone or in combination with the polymerization initiator. 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, and more preferably in the range of 20 to 120 ° C. If the temperature at the start of polymerization is out of the above range, there may be an increase in the residual monomer of the resulting water-absorbent resin or an excessive self-crosslinking reaction to deteriorate the water-absorbing performance of the water-absorbent resin.

  The reverse phase suspension polymerization is a polymerization method in which an aqueous monomer solution is suspended in a hydrophobic organic solvent. For example, U.S. Pat. Nos. 4,093,764, 4,367,323, 4,446,261, 4,683,274, and 5,244,735. In US patents such as The aqueous solution polymerization is a method of polymerizing an aqueous monomer solution without using a dispersion solvent. For example, U.S. Pat. Nos. 4,462,001, 4,873,299, 4,286,682, 4,973,632, 4,985,518, 5,124,416, 5,250,640 are used. No. 5,264,495, US Pat. No. 5,145,906, US Pat. No. 5,380,808, and European patents such as European Patent Nos. 081636, 09555086, and 0922717. Monomers and initiators exemplified in these polymerization methods can also be applied in the present invention.

  When aqueous solution polymerization is performed, a partially neutralized product such as acrylic acid is polymerized, or an acid group-containing monomer such as acrylic acid is polymerized and then polymerized with an alkali compound such as sodium hydroxide or sodium carbonate. Can also be neutralized. Therefore, the water-absorbing resin used in the present invention preferably contains an acid group and has a predetermined neutralization rate (mol% 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 is the range of 25-100 mol%, Preferably it is the range of 50-90 mol%, More preferably, it is 50-75 mol%, Most preferably, it is the range of 60-70 mol%.

  Therefore, a preferred embodiment of the present invention is a method for producing a modified water-absorbing resin, in which a) a water-absorbing resin, water, and a radical polymerization initiator are added without adding an ethylenically unsaturated monomer. B) irradiating the resulting mixture with active energy rays, and the water-absorbent resin contains acid groups and has a neutralization rate of 50 to 75 mol% (total acid groups). A method for producing a modified water-absorbing resin having a mole percent of neutralized acid groups therein is provided. After polymerization, it is usually a hydrogel crosslinked polymer. In the present invention, the water-containing gel-like crosslinked polymer can be used as it is as a water-absorbing resin, but is preferably dried to have the water content (wt%) (100-solid content (wt%)) described later. .

  In the present invention, the water-absorbing resin is a water-soluble radical polymerization initiator and / or a thermally decomposable radical polymerization initiator (hereinafter, collectively referred to as “radical polymerization initiator”) as described later. In addition, it is mixed with water to form a water absorbent resin composition. The water-absorbent resin is modified by irradiating the composition with active energy rays. Such modification is due to the action of an active radical polymerization initiator on the polymer main chain. Therefore, not only the water-absorbing resin obtained by polymerizing the water-soluble ethylenically unsaturated monomer described above, but also a water-absorbing resin such as a polyvinyl alcohol crosslinked product, a polyethylene oxide crosslinked product, a polyaspartic acid crosslinked product, or a carboxymethylcellulose crosslinked product. Can also be modified.

  On the other hand, the water-absorbent resin used in the present invention is preferably a powdery water-absorbent resin obtained by polymerizing a monomer mainly composed of acrylic acid (salt). The hydrogel polymer obtained by polymerization is preferably pulverized after drying to obtain a water-absorbing resin. The drying 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.

  As a pulverizer that can be used for such pulverization, for example, among the pulverization model names classified in Table 1.10 of the Powder Engineering Handbook (Edition of the Powder Engineering Society, first edition), a shear pulverizer, 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 can be preferably used. Among crushers corresponding to these models, cutting and shearing A crusher whose mechanism is the main mechanism can be used particularly preferably. For example, a roll mill (roll rotary type) pulverizer is preferable.

  The water-absorbent resin used in the present invention is preferably in the form of powder. More preferably, it is a powdery water-absorbent resin containing 90 to 100% by weight, particularly preferably 95 to 100% by weight of particles having a particle size in the range of 150 to 850 μm (specified by sieve classification). When the water-absorbent resin larger than 850 μm is used for, for example, the obtained modified water-absorbent resin in a diaper or the like, the touch may be poor and the top sheet of the diaper may be broken. On the other hand, if the particles smaller than 150 μm exceed 10% by weight, fine powder may be scattered or clogged at the time of use, and the water absorption performance of the modified water absorbent resin may be lowered. The weight average particle diameter is in the range of 10 to 1,000 μm, preferably 200 to 600 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. In addition, as a numerical value of the particle size, a value measured by the particle size distribution measuring method described in the following examples is adopted.

  In addition to or instead of the above, the water-absorbent resin used in the present invention can be obtained by obtaining a water-absorbent resin precursor having a low neutralization rate and adding a base to the water-absorbent resin precursor. preferable. Conventionally, polyfunctional surface treatment agents have been used for surface treatment (surface crosslinking). 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, When a water-absorbing resin in which COOH and -COONa are uniformly distributed is produced and used for surface crosslinking with a polyfunctional surface treating agent, uniform crosslinking is obtained. On the other hand, a water-absorbent resin obtained by polymerizing an acid-type ethylenically unsaturated monomer such as acrylic acid as a main component and then neutralizing the polymer with an alkali compound such as sodium hydroxide or sodium carbonate Has a low soluble content and high gel strength. However, when the surface is crosslinked with a polyfunctional surface treatment agent, -COOH and -COONa are not uniformly distributed, so that the water absorption property is deteriorated. For this reason, it is not desirable to subject the water-absorbent resin obtained by the latter method to surface cross-linking with a conventional polyfunctional surface treating agent. However, according to the method of the present invention, the water-soluble radical polymerization initiator or the thermally decomposable radical polymerization initiator does not react with —COOH, but forms a radical by extracting hydrogen of the main chain. Since the radicals are cross-linked by coupling, the cross-linking reaction is not affected by whether -COOH is uniformly distributed in the absorbent resin. Therefore, according to the method of the present invention, once a monomer / monomer mixture mainly composed of an acid-type ethylenically unsaturated monomer such as acrylic acid is polymerized, a low neutralization rate is obtained. After obtaining the water-absorbing resin precursor, it becomes possible to modify the water-absorbing resin obtained by neutralizing the water-absorbing resin precursor with an alkali compound such as sodium hydroxide or sodium carbonate. The modified water-absorbing resin obtained by the above can exhibit high gel strength and excellent water absorption characteristics.

  As described above, the water-absorbing resin is polymerized with a monomer / monomer mixture containing an acid-type ethylenically unsaturated monomer as a main component to form a water-absorbing resin precursor having a low neutralization rate. In the case where the base is obtained by adding a base to the water absorbent resin precursor after being obtained, the base may be added in any form of a solid or a liquid. Preferably, the base is added in the form of a liquid, in particular an aqueous solution. When the base is added in the form of an aqueous solution in this manner, the obtained water-absorbing resin is in a water-containing state, and therefore the step of adding the base to the water-absorbing resin precursor and the step of obtaining the water-absorbing resin composition are performed simultaneously. Can do. The base that can be used in the above embodiment is not particularly limited as long as it can neutralize the water-absorbing resin precursor having a low neutralization rate to a desired neutralization rate, and is a conventionally known inorganic or organic salt or acid. Can be used. Specifically, sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate, sodium phosphate, potassium phosphate, Examples thereof include ammonium phosphate, sodium borate, potassium borate, ammonium borate, sodium acetate, potassium acetate, ammonium acetate, sodium lactate, potassium lactate, ammonium lactate, sodium propionate, potassium propionate, and ammonium propionate. These bases may be used alone or in a suitable mixture of two or more. Among the above bases, a low neutralization water-absorbing resin precursor obtained by polymerizing a monomer / monomer mixture mainly composed of an acid-type ethylenically unsaturated monomer such as acrylic acid When neutralizing, monovalent cation hydroxide such as sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide; sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate, Monovalent cation carbonates such as ammonium hydrogen carbonate are preferred because they are easily available industrially, have high physical properties, and can be efficiently adjusted to a desired neutralization rate. In addition, the addition amount of the base in the said embodiment is not restrict | limited in particular, The water absorbing resin used by mixing process a) is suitably selected for the desired neutralization rate of the above preferable range.

  In the present invention, 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. It refers to a water-absorbing resin precursor (neutralization rate is 0). Specifically, the neutralization rate (mol% of neutralized acid groups in all acid groups) is preferably 0 to 50 mol%, more preferably Means about 0 to 20 mol%. Such a low neutralization rate water-absorbing resin precursor is a monomer mainly composed of a monomer having an acid group such as acrylic acid so as to achieve the neutralization rate in the above method. Since it is obtained by the same method as described above by using the mixture, the detailed description is omitted here.

  The water content before modification of the water absorbent resin used in the method for producing a modified water absorbent resin of the present invention is not particularly limited as long as the water absorbent resin has fluidity. Preferably, the water content after drying at 180 ° C. for 3 hours is 0 to 50 wt%, 0 to 40 wt%, 0 to 30 wt%, 0 to 20 wt%, 0 to 10 wt%, 0 to 5 wt% It is preferable in the order of the range.

  The water-absorbent resin used in the present invention is not limited to the one produced by the above method, and may be one prepared by another method. The water-absorbing resin obtained by the above method is a water-absorbing resin that is not surface-crosslinked, but the water-absorbing resin used in the method for producing a modified water-absorbing resin of the present invention is a polyhydric alcohol in advance. Further, a water-absorbent resin surface-crosslinked using a polyvalent epoxy compound, an alkylene carbonate, an oxazolidone compound or the like may be used.

(B) Water-absorbent resin composition In the method for producing a modified water-absorbent resin of the present invention, in step a), water is added to the water-absorbent resin without adding an ethylenically unsaturated monomer, A water-absorbing resin composition is obtained by mixing with a radical polymerization initiator (water-soluble radical polymerization initiator and / or thermally decomposable radical polymerization initiator).

  Conventionally, surface cross-linking of a water-absorbent resin has been generally performed by blending a surface cross-linking agent. If a surface cross-linking agent is blended, the functional group on the resin surface and the surface cross-linking agent are chemically strongly bonded, and thereby a stable surface cross-linking structure can be introduced on the resin surface. Further, the surface crosslinking distance can be easily 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, even if such a surface cross-linking agent is not added, the water-absorbing resin is modified only by using a radical polymerization initiator, and specifically, a cross-linking structure is introduced on the surface of the water-absorbing resin. can do. Furthermore, by irradiating the water absorbent resin composition obtained by mixing water with active energy rays, a crosslinked structure can be efficiently introduced on the surface of the water absorbent resin particles, and the obtained modification is obtained. The water absorption characteristics of the water-absorbing resin thus improved can also be improved. In addition to the advantages described above, by adding a relatively large amount of water to the water-absorbent resin in step a), a crosslinked structure can be efficiently introduced on the surface of the water-absorbent resin particles by step b) described later. Therefore, there is also an advantage that the irradiation time required for improving the absorption capacity under pressure (AAP) and the saline flow conductivity (SFC) to a desired level can be shortened.

  In the present invention, “without adding an ethylenically unsaturated monomer” means that when the water-absorbent resin composition contains an ethylenically unsaturated monomer, an active energy ray is applied in step b). This is to avoid the radical polymerization initiator activated by irradiation reacting with the ethylenically unsaturated monomer before acting on the surface of the water-absorbent resin, and is consumed.

  In step a), water is mixed with the water absorbent resin. At this time, the mixing of the water-absorbing resin and water may be performed by adding water alone, or may be performed by adding water in the form of an aqueous solution containing other components. . As the said aqueous solution, the aqueous solution containing the radical polymerization initiator mentioned later, the aqueous solution containing the mixing aid mentioned later similarly, etc. are illustrated, for example.

  The amount of water mixed with the water-absorbent resin in step a) is 20 parts by weight with respect to 100 parts by weight of the water-absorbent resin (converted to 100 parts by weight of solid content) as a value in the obtained water-absorbent resin composition. It is essential that the amount is 100 parts by weight or less. Thus, by adding a relatively large amount of water to the water-absorbent resin in step a), a crosslinked structure can be efficiently introduced on the surface of the water-absorbent resin particles by step b) described later. The irradiation time required to improve the reduction absorption factor (AAP) and saline flow conductivity (SFC) to a desired level can be shortened. Further, the amount of water mixed with the water-absorbent resin in step a) is 20 as a value in the obtained water-absorbent resin composition with respect to 100 parts by weight of the water-absorbent resin (converted to 100 parts by weight of solid content). It is preferable in the order of 70 parts by weight over 50 parts by weight, 50 parts by weight over 20 parts by weight, 40 parts by weight over 20 parts by weight, and 30 parts by weight over 20 parts by weight. At this time, if the amount of water is more than 100 parts by weight, a large amount of energy is required in the drying process after irradiation with active energy rays, which is disadvantageous. In addition, the water absorbent resin may be decomposed.

  Furthermore, in step a), in addition to the water described above, a water-soluble radical polymerization initiator and / or a thermally decomposable radical polymerization initiator are mixed as a radical polymerization initiator in the water-absorbent resin composition. Hereinafter, “water-soluble radical polymerization initiator and / or thermally decomposable radical polymerization initiator” may be collectively referred to as a radical polymerization initiator.

  In this step, when the “water-soluble radical polymerization initiator” is mixed with the water-absorbent resin, it can be easily and uniformly dispersed on the surface of the water-absorbent resin excellent in hydrophilicity and water absorbency. As a result, a water-absorbing resin having excellent water absorption characteristics can be produced.

  The water-soluble radical polymerization initiator used in the present invention is one that dissolves in water (25 ° C.) in an amount of 1 wt% or more, preferably 5 wt% or more, more preferably 10 wt% or more. Persulfates such as sodium and potassium persulfate; hydrogen peroxide; 2,2′-azobis-2-amidinopropane dihydrochloride, 2,2′-azobis [2-2 (-imidazolin-2-yl) propane] And water-soluble azo compounds such as dihydrochloride. Among these, in particular, when persulfate is used, the absorption capacity under pressure of the modified water-absorbent resin with respect to physiological saline (in this specification, simply referred to as “absorption capacity under pressure”), saline flow conductivity The water absorption characteristics such as are preferable in that they are all excellent.

  On the other hand, the thermally decomposable radical polymerization initiator used in the present invention is a compound that generates radicals by heating, and preferably has a 10-hour half-life temperature of 0 ° C. or higher and 120 ° C. or lower, preferably 20 ° C. or higher and 100 ° C. The following are more preferable, and when considering the temperature conditions for irradiating the active energy ray, those of 40 ° C. or higher and 80 ° C. or lower are particularly preferable. If the 10-hour half-life temperature is less than 0 ° C. (lower limit), it is unstable during storage, and if it exceeds 120 ° C. (upper limit), it may be chemically too stable to lower the reactivity.

  In this step, when a “thermally decomposable radical polymerization initiator” is mixed with the water-absorbent resin, the surface is modified at a low temperature and in a short time by irradiating active energy rays with such a polymerization initiator added. Quality, high gel strength and excellent water absorption properties. In the present invention, as the thermally decomposable radical polymerization initiator, both oil-soluble and water-soluble can be used. Oil-soluble thermally decomposable radical polymerization initiators are characterized in that the degradation rate is less affected by pH and ionic strength than water-soluble thermally decomposable radical polymerization initiators. However, since the water-absorbent resin is hydrophilic, it is more preferable to use a water-soluble thermally decomposable radical polymerization initiator in consideration of permeability to the water-absorbent resin.

  The thermal decomposable radical polymerization initiator is relatively inexpensive as compared with a commercially available compound as a photodegradable radical polymerization initiator, and strict light shielding is not necessarily required, so that the manufacturing process and the manufacturing apparatus can be simplified. Typical thermal decomposable radical polymerization initiators 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 the like, and 2,2′-azobis (2-amidinopropane) dihydrochloride having a 10-hour half-life temperature of 40 to 80 ° C., 2, Azo compounds such as 2′-azobis [2-2 (-imidazolin-2-yl) propane] dihydrochloride and 2,2′-azobis (2-methylpropionitrile) are preferred. Among these, the use of persulfate is particularly preferable in terms of excellent absorption capacity under pressure and saline flow conductivity.

  The amount of the radical polymerization initiator used is preferably in the range of 0.01 to 20 parts by weight, more preferably in the range of 0.1 to 15 parts by weight, particularly preferably 1 to 10 parts by weight with respect to 100 parts by weight of the water absorbent resin. Part range. If the mixing amount of the radical polymerization initiator is less than 0.01 parts by weight, the water absorbent resin may not be sufficiently modified even when irradiated with active energy rays. On the other hand, when the amount of the radical polymerization initiator mixed is more than 20 parts by weight, the water absorption performance of the modified water absorbent resin may be lowered.

  In the present invention, a water-soluble radical polymerization initiator and / or a heat-decomposable radical polymerization initiator is essential, and when these radical polymerization initiators are not used at all, for example, only an oil-soluble photopolymerization initiator is used. Excellent physical properties can be achieved compared to the case of using. The oil-soluble photopolymerization initiator referred to here is, for example, a compound having a solubility in water of less than 1% by weight.

  In the present invention, a water-soluble radical polymerization initiator and / or a heat-decomposable radical polymerization initiator are essential, but an initiator other than these radical polymerization initiators may be used in combination. As other polymerization initiators that can be used in combination, photopolymerization initiators such as oil-soluble benzoin derivatives, benzyl derivatives, and acetophenone derivatives, oil-soluble ketone peroxides, peroxyketals, hydroperoxides, dialkyl peroxides, Examples thereof include oil-soluble organic peroxides such as peroxyesters and peroxycarbonates. Such a photopolymerization initiator may be a commercially available product, and trade names Irgacure (registered trademark) 184 (hydroxycyclohexyl-phenylketone), Irgacure (registered trademark) 2959 (1- [4- (2-hydroxyethoxy) of Ciba Specialty Chemicals Co., Ltd. ) -Phenyl] -2-hydroxy-2-methyl-1-propan-1-one) and the like.

  When other initiators are used in combination with the present invention, the amount used is 0 to 20 parts by weight, preferably 0 to 15 parts by weight, particularly 0 to 10 parts by weight, based on 100 parts by weight of the water absorbent resin. The use ratio is less than the radical polymerization initiator, for example, 1/2 or less, more preferably 1/10 or less, especially 1/50 or less in weight ratio to the radical polymerization initiator. At this time, when a water-soluble radical polymerization initiator and a thermally decomposable radical polymerization initiator are used in combination, the amount of the radical polymerization initiator means the total amount thereof.

  In order to mix the radical polymerization initiator with the water-absorbent resin, the radical polymerization initiator may be mixed with the water-absorbent resin as it is, but preferably this is dissolved in water and the resulting aqueous solution is used as the water-absorbent resin. Mix. Since the water-absorbing resin has water-absorbing properties, the radical polymerization initiator is mixed in the form of an aqueous solution to uniformly disperse the radical polymerization initiator on the surface of the water-absorbing resin and to be mixed with the water-absorbing resin uniformly. Can do. In addition, as aqueous solution, the other solvent may be included in the range which does not impair the solubility of a radical polymerization initiator other than water.

  When the radical polymerization initiator is added in the form of an aqueous solution, the amount of water in the aqueous solution used is not particularly limited, but the amount of water in the resulting water-absorbent resin composition is adjusted so as to be in the preferred range described above. That's fine. In addition, the mixing form of the water to a water absorbing resin is not necessarily restricted to the case of mixing with the form of the aqueous solution containing a radical polymerization initiator. After mixing the radical polymerization initiator and the water absorbent resin, water or an aqueous solution may be mixed. Accordingly, the water-containing gel-like crosslinked product obtained by polymerizing the monomer components is directly mixed with the dried water-containing gel having a water content of 0 to 50% by weight to obtain a water-absorbing resin composition. Also good.

  On the other hand, in order to improve the mixing property of the water absorbent resin composition, it is preferable to add a mixing aid to the water absorbent resin composition. At this time, the timing of adding the mixing aid is not particularly limited, but it is preferable to add the mixing aid simultaneously with the mixing step a) or before the step a).

  Here, the mixing aid is a water-soluble or water-dispersible compound other than the ethylenically unsaturated monomer or the radical polymerization initiator, suppresses aggregation of the water-absorbent resin with water, Although it will not restrict | limit if it can improve the mixability of this, It is preferable that it is a water-soluble or water-dispersible compound. Specific examples of such water-soluble or water-dispersible compounds include surfactants, water-soluble polymers, hydrophilic organic solvents, water-soluble inorganic compounds, inorganic acids, inorganic acid salts, organic acids, and organic acids. Salt can be used. In the present specification, the water-soluble compound means a compound having a solubility in 100 g of water of 1 g or more at room temperature, preferably 10 g or more. By adding a mixing aid, the water-absorbing resin is prevented from agglomerating with water, and the aqueous solution and the water-absorbing resin can be uniformly mixed. The water-absorbing resin can be irradiated evenly and evenly, and the entire water-absorbing resin can be uniformly surface-crosslinked.

  In the case of using a mixing aid, the usage form of the mixing aid is not particularly limited, and it may be used in the form of a powder or may be used by dissolving, dispersing or suspending in a solution. Used in the form of an aqueous solution.

  Also, the order of addition of the mixing aid when using the mixing aid is not particularly limited, and after adding the mixing aid to the water-absorbent resin in advance, water or a radical polymerization initiator (in some cases these are added). And the like, and a method of dissolving the mixing aid in the aqueous solution and mixing it with the water-absorbent resin can also be used.

  As such a surfactant, at least one surfactant selected from the group consisting of a nonionic surfactant having an HLB of 7 or more or an anionic surfactant can be used. 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 salt, alkylnaphthalene sulfone Examples thereof include acid salts, alkyl polyoxyethylene sulfate salts, and dialkyl sulfosuccinates. Among these, polyoxyethylene alkyl ether is preferable. The number average molecular weight of the polyoxyethylene alkyl ether is preferably 200 to 100,000, and more preferably 500 to 10,000. When the number average molecular weight is too large, the solubility in water decreases, the amount added cannot be increased, and the viscosity of the solution also increases, so that the miscibility with the water absorbent resin is not good. On the other hand, when the number average molecular weight is too small, the effect as a mixing aid is inferior.

  Examples of the water-soluble polymer include polyvinyl alcohol, polyethylene oxide, polyethylene glycol, polypropylene glycol, polyacrylamide, polyacrylic acid, sodium polyacrylate, polyethyleneimine, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, dextrin, Examples include sodium alginate and starch. Among these, polyethylene glycol is preferable. Similar to polyoxyethylene alkyl ether, the number average molecular weight is preferably 200 to 100,000, more preferably 500 to 10,000.

  Examples of hydrophilic organic solvents include alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol and t-butyl alcohol; ketones such as acetone and methyl ethyl ketone; dioxane and alkoxy (poly) ethylene glycol , Ethers such as tetrahydrofuran; amides such as ε-caprolactam and N, N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, 1,3 -Propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, glycerin, 2-butene-1,4-dioe 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, And polyhydric alcohols such as triethanolamine, polyoxypropylene, pentaerythritol, and sorbitol; and one or more of these can be used.

  Examples of water-soluble inorganic compounds include alkali metal salts such as sodium chloride, sodium hydrogen sulfate, and sodium sulfate; ammonium salts such as ammonium chloride, ammonium hydrogen sulfate, and ammonium sulfate; alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; , Aluminum chloride, polyaluminum chloride, aluminum sulfate, potassium alum, calcium chloride, alkoxytitanium, ammonium zirconium carbonate, zirconium acetate and other polyvalent metal salts, and hydrogen carbonate, dihydrogen phosphate, hydrogen phosphate, etc. Non-reducing alkali metal salt pH buffer is exemplified.

  Examples of inorganic acids (salts) include hydrochloric acid, sulfuric acid, phosphoric acid, carbonic acid, boric acid, and salts thereof, such as alkali metal salts and alkaline earth metal salts, and examples of organic acids (salts) include Representative examples include acetic acid, propionic acid, lactic acid, citric acid, succinic acid, malic acid, tartaric acid and salts thereof, for example, alkali metal salts and alkaline earth metal salts.

  Among the above examples, at least one water-soluble or water-dispersed material selected from the group consisting of polyoxyethylene alkyl ether, polyethylene glycol, water-soluble polyvalent metal salt, sodium chloride, ammonium hydrogen sulfate, ammonium sulfate, sulfuric acid, and hydrochloric acid Compounds are preferably used as mixing aids.

  These mixing aids may be used alone or in the form of a mixture of two or more. Further, the amount of addition of the mixing aid is not particularly limited as long as it can suppress aggregation of the water-absorbing resin with water and can improve the mixing property of the aqueous solution and the water-absorbing resin. It is preferably added in an amount of 0.01 to 40 parts by weight, more preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the conductive resin.

  In step a) according to the present invention, the mixing conditions of the water-absorbent resin, water and radical polymerization initiator, and if necessary, the mixing aid are not particularly limited. For example, the mixing temperature in step a) is preferably in the order of 0 to 150 ° C, 10 to 120 ° C, 20 to 100 ° C, 30 to 90 ° C, and 40 to 70 ° C. At this time, if the mixing temperature exceeds 150 ° C., the water-absorbent resin is deteriorated by heat, or the water content of the surface of the water-absorbent resin in step b) reaches less than 3.0% by weight due to water evaporation or the like. There is a possibility not to. On the other hand, if the mixing temperature is less than 0 ° C., water may condense and the operation may not be performed stably. In addition, when performing a mixing process at high temperature, a radical polymerization initiator can act | action with a small irradiation amount with a heat | fever, and is preferable. For this reason, in such a case, in order to make the surface moisture content of the water-absorbent resin in step b) 3.0% by weight or more, excessive leakage of water vapor is caused by sealing the mixing / irradiation system. It is preferable to suppress. In addition, the temperature of the water absorbent resin and water before step a) is also not particularly limited. For example, the temperature of the water absorbent resin before step a) is 0 to 150 ° C, 10 to 120 ° C, 20 to 100 ° C, It is preferable in order of 50-100 degreeC. At this time, if the temperature of the water-absorbent resin before step a) exceeds 150 ° C., the water-absorbent resin may be deteriorated by heat. There is a possibility that it cannot be performed stably. The temperature of the water before step a) is preferably 5 to 80 ° C, more preferably 10 to 60 ° C, and particularly preferably 20 to 50 ° C. At this time, if the temperature of the water before step a) exceeds 80 ° C., the water evaporates excessively before the mixing step a), and a sufficient amount of water cannot be mixed with the water absorbent resin. The surface moisture content of the water-absorbent resin may not reach 3.0% by weight, and conversely if it is less than 5 ° C., water may condense and the operation may not be performed stably. Furthermore, the mixing time in step a) is not particularly limited as long as these can be mixed uniformly. Specifically, the mixing time is preferably 0.1 second to 60 minutes, more preferably 1 second to 30 minutes, even more preferably 2 seconds to 20 minutes, and most preferably 5 seconds to 10 minutes. At this time, if the mixing time is less than the lower limit, the water absorbent resin, water and radical polymerization initiator, and if necessary, the mixing aid may not be mixed uniformly, and conversely, the mixing time exceeds the upper limit. If the length is longer than necessary, excess water penetrates into the water-absorbent resin, so that the surface moisture content decreases to less than 3.0% by weight, and the surface treatment by irradiation with active energy rays may not proceed easily. There is. Further, even if the amount of water mixed with the water-absorbent resin is reduced more than necessary, the surface moisture content is similarly reduced to less than 3.0% by weight, which is not preferable.

  As a method for obtaining a water-absorbent resin composition by mixing a water-absorbent resin, water, and a radical polymerization initiator, an ordinary mixer such as a V-type mixer, a ribbon-type mixer, or a screw-type mixer is used. And a mixing method using a rotating disk type mixer, an airflow type mixer, a batch type kneader, a continuous type kneader, a paddle type mixer, a vertical type mixer and the like.

(C) Active energy rays In the production 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 an internal crosslink can be prepared by blending an internal crosslinking agent and a photopolymerization initiator in a polymerizable monomer component and irradiating active energy rays such as ultraviolet rays, electron beams, and γ rays. it can. In addition, as a surface cross-linking method for a water-absorbent resin, it is also known to use a surface cross-linking agent and promote surface reaction to form surface cross-linking under heating conditions. As such surface cross-linking of the water-absorbent resin, a compound having a plurality of functional groups in one molecule, such as polyhydric alcohol, polyhydric glycidyl ether, haloepoxy compound, polyhydric aldehyde, is used. Generally, when heated to 100 to 300 ° C., these functional groups react with carboxyl groups and the like on the surface of the water absorbent resin, and a crosslinked structure is formed on the surface of the water absorbent resin. However, in the present invention, a crosslinked structure can be formed on the surface of the water-absorbent resin by irradiation with a radical polymerization initiator and active energy rays without the presence of such a surface crosslinking agent or polymerizable monomer. There are features. In addition, this makes it possible to improve the absorption capacity under load (AAP) and saline flow conductivity (SFC) of the modified water absorbent resin.

  The production method of the present invention is characterized in that the surface moisture content of the water absorbent resin in the water absorbent resin composition is controlled to a predetermined value or more in the irradiation step of irradiating the water absorbent resin composition with active energy rays. Have

  Specifically, in the irradiation step, the surface moisture content of the water absorbent resin in the water absorbent resin composition is controlled to 3.0% by weight or more. Note that the surface moisture content only needs to be controlled to be 3.0% by weight or more at least at any point in the active energy ray irradiation process. It is not necessary to be controlled to 3.0% by weight or more over the period. Here, over the entire period from the start to the end of the irradiation process, when the water content of the surface of the water absorbent resin is less than 3.0% by weight, modification of the surface of the water absorbent resin by active energy ray irradiation (for example, The introduction of a crosslinked structure may not be performed efficiently. As a result, the resulting modified water-absorbent resin may not exhibit sufficiently good water absorption characteristics.

  As described above, the value of the surface moisture content may be controlled to 3.0% by weight or more at least at any point in the irradiation step, but is preferably 3.5% by weight or more, more preferably 4. It is controlled to 0% by weight or more. From the viewpoint of obtaining the effects of the present invention, the upper limit value of the surface moisture content is not particularly limited, and can be appropriately set according to the purpose. However, the surface moisture content is usually 60.0% by weight or less, preferably 50.0% by weight or less, more preferably 40.0% by weight or less, and further preferably 30.0% by weight or less. . If the surface moisture content is too high, the water absorbent resin adheres / aggregates between the particles, and the surface of the water absorbent resin may not be efficiently irradiated with active energy rays.

  In the preferable form of this invention, the surface moisture content at the time of the start of an irradiation process is controlled by the value within said range. This makes it possible to reliably obtain the effects of the present invention. Here, during the period in which the irradiation process is performed, the value of the surface moisture content may vary. That is, the value of the surface moisture content may be increased or decreased as compared with the time of starting the irradiation process. In any case, it is sufficient that the surface moisture content is controlled to a value within the above range at least at any point in the irradiation step, and preferably 30% or more, more preferably 60% of the entire period of the irradiation step. As described above, the surface moisture content is controlled to a value within the above range at 90% or more, particularly preferably 100% (that is, the entire period).

  In the present invention, the “surface moisture content” means the percentage by weight of the amount of water present in the vicinity of the surface of the water-absorbent resin particles, and the concept of the moisture content and moisture content of the entire particles. It is essentially different. And the value measured by the method as described in the Example mentioned later shall be employ | adopted as the numerical value. Briefly explaining the measurement method, by adding a hydrophilic organic solvent to the water-absorbent resin composition obtained in step a) to extract water, the amount of water in the extract is quantified by the Karl Fischer method. A value for the surface moisture content can be calculated.

  In the irradiation step, the method for controlling the surface moisture content value of the water-absorbent resin to a value within the above range is not particularly limited. For example, in order to achieve a preferable surface moisture content, in the technique of adding a sufficient amount of water to the water-absorbent resin composition obtained in step a), or in the mixing step a) and the irradiation step b). And a method of suppressing moisture from penetrating into the water-absorbent resin particles and evaporating into the atmosphere. Since the degree of penetration of moisture into the water-absorbent resin particles is affected by time and temperature, it is preferable to appropriately control the temperature in the system in the mixing step a). Further, in order to suppress the evaporation of moisture, it is preferable to make the system as close to a seal as possible in addition to controlling the mixing time and the temperature in the system. In the irradiation step b), for example, when a stirrer having a box or a cylindrical structure is used, the opening for irradiation is hermetically sealed by covering it with a member capable of transmitting active energy rays such as quartz glass. A close state can be achieved. Further, as a method for promoting the penetration of water into the water-absorbent resin, the mixing time in the mixing step a) is increased, the water-absorbent resin composition is placed in a closed system, or water is added to the composition. A technique of performing a heat treatment at a temperature below the boiling point is mentioned. On the other hand, as a method for promoting diffusion from the surface of water, an air stream is introduced into the water absorbent resin composition, the water absorbent resin composition is placed in an open system, or the boiling point of water with respect to the composition. A method of performing heat treatment at the above temperature can be mentioned.

  In order to control the surface moisture content of the water-absorbent resin within the above range, it is preferable to irradiate active energy rays while appropriately monitoring the surface moisture content. Accordingly, the water absorbent resin composition may be dried to a certain range, or a predetermined amount of water may be added to the water absorbent resin composition. In addition, when water is added, the permeation and diffusion of water from the surface of the water absorbent resin to the inside may be appropriately controlled, and the volatilization of water from the surface of the water absorbent resin may be appropriately controlled.

  In the present invention, the irradiation with the active energy ray may be performed during the mixing of the water absorbent resin, water and the radical polymerization initiator, or may be performed after mixing two or more of these. However, preferably, after obtaining a water-absorbing resin composition containing a water-absorbing resin, water and a radical polymerization initiator, it is possible to form uniform surface cross-linking, and then to the obtained water-absorbing resin composition, active energy rays. Irradiate.

  Examples of active energy rays include one or more of ultraviolet rays, electron beams, and gamma rays. Of these, ultraviolet rays and electron beams are preferable. Considering the influence of the active energy rays on the human body, ultraviolet rays are more preferable, ultraviolet rays having a wavelength of 300 nm or less, particularly preferably 180 to 290 nm are more preferable.

Irradiation conditions, 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 irradiation time of the active energy ray may be preferably from 0.1 minutes to less than 60 minutes, more preferably from 0.2 minutes to less than 30 minutes, and even more preferably from 1 minute to less than 15 minutes. When a conventional surface cross-linking agent is used, the time for the surface cross-linking treatment can be shortened as compared with the same cross-linking density, which may exceed 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, the water-absorbent resin is preferably treated at a temperature of less than 150 ° C, more preferably less than 120 ° C, still more preferably from room temperature to 100 ° C, particularly preferably from 50 to 100 ° C. Therefore, the treatment temperature can be set lower than the conventional surface treatment temperature.

  It is preferable to stir the water-absorbent resin during irradiation with active energy rays. By stirring, the active energy ray can be uniformly irradiated to the mixture of the radical polymerization initiator and the water-absorbing resin. The devices that can stir the water-absorbent resin during irradiation with active energy rays include vibration mixer, vibration feeder, ribbon mixer, conical ribbon mixer, screw mixer, airflow mixer, and batch type. Examples thereof include a kneader, a continuous kneader, a paddle type mixer, a high-speed flow type mixer, and a floating flow type mixer.

  Alternatively, the water-absorbent resin composition may be flowed in a device having a cylindrical shape or a box shape, and active energy rays may be irradiated from the periphery of the device. At this time, in order to cause the mixture to flow, the pressure of a gas such as air may be used as used for pneumatic transportation of the powder. Furthermore, you may incline a pipe | tube as needed. When air is used, it is preferable to humidify the air in order to prevent drying of the water absorbent resin composition. Irradiation of active energy rays can be uniformly surface-treated in a short time when performed from multiple directions. In addition, the material which comprises the said apparatus will not be restrict | limited especially if it is a material which does not inhibit irradiation of the active energy ray to a water absorbing resin composition, For example, quartz glass etc. are illustrated.

(D) Other treatments 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 and 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 method for producing a modified water-absorbent resin of the present invention, a liquid permeability improving agent may be added to the water-absorbent resin 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. Further, these liquid permeability improvers may be mixed with a radical polymerization initiator and added. In addition, as an additive, an antibacterial agent, a deodorant, a chelating agent, and the like may be appropriately added within the above range.

(E) Modified Water Absorbent Resin According to the method for producing a modified water absorbent resin of the present invention, the absorption capacity under pressure of the obtained water absorbent resin is improved. Conventionally, it is known that when surface cross-linking is formed, the free swelling ratio is slightly reduced, but the ability to maintain liquid absorption even under pressure, that is, the absorption capacity under pressure, is improved. According to the method of the present invention, the absorption capacity under pressure of 4.83 kPa of the water absorbent resin is increased by 1 g / g or more without using a surface crosslinking agent or an ethylenically unsaturated monomer. 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 increase in absorption capacity under pressure is preferably 8 g / g or more, more preferably 12 g / g or more, still more preferably 15 g / g or more, particularly preferably 20 g / g or more, and most preferably in terms of physical properties after modification. Is 22 g / g or more. The modified water-absorbent resin of the present invention has an absorption capacity under pressure of 4.83 kPa, preferably 8 to 40 g / g.

  Further, the centrifuge retention capacity (CRC) of the modified water absorbent resin is preferably 50 g / g or less, more preferably 40 g / g or less, and further 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.

The modified water-absorbent resin obtained by the present invention has a saline flow conductivity (SFC) of preferably 10 (× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, more preferably 30 ( × 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, more preferably 50 (× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, particularly preferably 70 (× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, and most preferably 100 (× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more. In addition, as these numerical values, values measured by the method described in Examples described later are adopted.

  Further, the modified water-absorbing resin obtained by the present invention is characterized in that the amount of residual monomer is extremely small. This is considered because the initiator 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 amount of residual monomer contained in the water-absorbing resin as the base polymer is 200 to 500 ppm. ). The amount of residual monomer in the water absorbent resin after modification 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).

  Further, the modified water-absorbing resin obtained by the present invention includes a modified water-absorbing resin obtained by a conventional reforming method in which a surface cross-linking agent is added to a water-absorbing resin as a base polymer and heated at a high temperature. Compared with less solid content. This is because the production method of the present invention does not require a high temperature for the reaction, so that the water contained in the aqueous solution added to the water-absorbent resin as the base polymer remains almost intact after the reaction. When the water content in the water-absorbent resin is large, not only the amount of fine powder with a particle size of 150 μm or less, which is unfavorable for health, is reduced by granulation, but also the prevention of static electricity generation on the particle surface that causes blocking during pneumatic transportation. It has effects such as reduction of physical properties due to physical damage during pneumatic transportation. The solid content of the water absorbent resin after modification is preferably 95% or less, more preferably 93% or less, and still more preferably 91% or less. The lower limit is not particularly limited, but if it is 70% or less, the absorption capacity per unit weight of the water-absorbent resin is lowered, which may not be preferable depending on the application.

  The shape of the surface-treated water-absorbent resin obtained by the present invention can be adjusted as appropriate depending on the treatment conditions such as the shape of the water-absorbent resin before treatment, granulation / molding after treatment, etc. . Such powder has a weight average particle size (specified by sieve classification) of 10 to 1,000 μm, preferably 200 to 600 μm, and preferably 150 to 850 μm in content with respect to the water-absorbent resin. -100% by weight, more preferably 95-100% by weight.

  The production method of the present invention has the effect of granulating fine powder generated during the production of the water-absorbent resin during surface crosslinking of the water-absorbent resin. For this reason, even if fine powder is contained in the water absorbent resin before modification, the fine powder contained is granulated when the method for producing the modified water absorbent resin of the present invention is performed. The amount of fine powder contained in the water absorbent resin can be reduced. The particle size distribution of the modified water-absorbing resin obtained shifts to the higher particle size side compared to the water-absorbing resin before the modification. However, the shift ratio depends on the type and amount of the radical polymerization initiator to be mixed with the water-absorbent resin, and when this is used as an aqueous solution, the ratio of water, irradiation conditions of active energy rays, how to flow during irradiation, etc. Change.

  The modified water-absorbing resin obtained by the method of the present invention is formed with uniform and high cross-linking density across the entire surface of the water-absorbent resin, and the properties desired for the water-absorbing resin, such as absorption capacity, absorption rate, The gel strength and suction force can be made extremely high. In addition, when surface-crosslinking the acrylic acid water-absorbing resin using a surface cross-linking agent such as polyhydric alcohol, polyvalent epoxy compound, alkylene carbonate, the speed and degree of surface cross-linking depends on the neutralization rate of the water-absorbing resin. It was dependent. Specifically, when the neutralization rate is low, surface crosslinking proceeds quickly, and when the neutralization rate is high, surface crosslinking is difficult. Moreover, in order to surface-crosslink the water-absorbing resin obtained by post-neutralization of polyacrylic acid, it has been necessary to uniformly post-neutralize after the surface cross-linking treatment. However, in the present invention, it is possible to produce a water-absorbing resin excellent in water-absorbing characteristics by modifying the water-absorbing resin without depending on the neutralization rate of the water-absorbing resin and the uniformity of post-neutralization. Since surface cross-linking depends on the action of the radical polymerization initiator on the main chain of the water-absorbent resin, it does not matter whether the carboxyl group is present in an acid or is a salt, and so on. it is conceivable that.

  In addition, when this invention is implemented in presence of an ethylenically unsaturated monomer, since a radical polymerization initiator will be consumed for superposition | polymerization of an ethylenically unsaturated monomer, it is not the intention of this invention.

  According to the present invention, when the water-absorbing resin is surface-treated, it can be sufficiently surface-treated even at a reaction temperature near room temperature, and the obtained surface-treated water-absorbing resin has an absorption capacity, an absorption rate, a gel strength. The properties desired for the water-absorbing resin such as suction force are at a very high level. Therefore, the water-absorbent resin obtained by the present invention is optimal as a sanitary material that absorbs sanitary cotton, disposable diapers, or other bodily fluids, or an agricultural water-absorbent resin.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these. Hereinafter, for convenience, “part by weight” may be simply referred to as “part”, and “liter” may be simply referred to as “L”. Measurement methods and evaluation methods in Examples and Comparative Examples are shown below.

(1) Particle size distribution: weight average particle size (D50) and logarithmic standard deviation of particle size distribution (σζ)
A water-absorbing resin or a particulate water-absorbing agent 10.0 g under the conditions of room temperature (20 to 25 ° C.) and humidity 50 RH%, the openings are 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm, 45 μm. JIS standard sieve (THE IIDA TESTING SIEVE: diameter 8cm), and classifying for 5 minutes with vibration classifier (IIDA SIEVE SHAKER, TYPE: ES-65 type, SER. No. 0501). Plotted on log probability paper. Thereby, the particle size corresponding to R = 50% by weight was read as the weight average particle size (D50).

  Further, when X1 is R = 84.1% by weight and X2 is each particle size when R = 15.9% by weight, the logarithmic standard deviation (σζ) is expressed by the following equation. That is, the smaller the value of σζ, the narrower the particle size distribution.

(2) Surface moisture content 500 g of a water-absorbing resin as a base polymer was added to a 5 L Ladige mixer (Loedige, model M5R), and stirred at 300 rpm, 5.0 g of ammonium persulfate, polyethylene glycol monomethyl ether (number average molecular weight) A processing solution in which 2.5 g of about 2,000) and 40 g of water were mixed in advance was sprayed. Stirring was continued for 3 minutes at room temperature, and then stirring was stopped. 1 g of the obtained mixture was added to a screw tube, 4 g of anhydrous methanol was added, and the mixture was shaken with an IKA mini shaker MS1 for 30 seconds, and then sucked with a syringe, and a filter (manufactured by Zeal Science Co., Ltd., water system 25A (pore size 0.45 μm)). ). The amount of water contained in the filtrate was measured by the following method using a Karl Fischer moisture meter (manufactured by Kyoto Electronics Industry Co., Ltd., model: MKS-1S).

<Moisture measurement with Karl Fischer moisture meter>
1. Measurement principle This is a volumetric moisture determination method using a Karl Fischer reagent in which water reacts quantitatively with iodine and sulfurous acid gas in the presence of methyl alcohol and pyridine as a titrant.

  A titration end point is detected by the Dead Stop method in which a slight constant current is passed between two platinum electrodes immersed in a solution to polarize the electrode and a potential change caused by excess iodine at the end point is detected.

  In order to quantify moisture by the Karl Fischer method, a sample is placed in a titration flask, the Karl Fischer reagent is titrated, and the moisture content of the sample is determined by the product of the titer and the titer of the Karl Fischer reagent.

2. Measurement method A measurement solvent (a mixture of 50 mL of acetic acid (special grade), 50 mL of buffer solution (HYDRANAL-Buffer), and 900 mL of anhydrous methanol) is charged to such an extent that the electrode of the Karl Fischer moisture meter is immersed, and the START key is pressed and Karl Fischer reagent is used. Titration is performed to make the inside of the titration flask anhydrous.

  The sample is put into a titration flask, and the START key is pressed to perform titration with the Karl Fischer reagent, and the sample weight (a) [mg] and Karl Fischer titration (b) [mL] are recorded. Measure a total of 3 times and obtain the average value.

  Substituting the sample weight (a) and Karl Fischer reagent titration amount (b) into the following formula (1), the water content (c) [wt%] in anhydrous methanol used for water extraction from the water absorbent resin mixture is obtained. .

F (the titer of the Karl Fischer reagent) is measured in advance using HYDRANAL-Composite 5K (about 5 mgH ₂ O / mL), and is obtained by substituting it into the following formula (2).

  From the moisture concentration (c) in anhydrous methanol used for moisture extraction from the water-absorbent resin mixture obtained from the above formula (1), the moisture concentration and treating agent contained in anhydrous methanol itself measured in advance are added. (C)-(d) = (e) obtained by subtracting the total water concentration (d) [wt%] derived from water contained in the previous water absorbent resin, and used for water extraction from the water absorbent resin mixture The amount of water (g) [mg] extracted from the water-absorbent resin mixture is obtained from the amount of anhydrous methanol (f) [mg] using the following formula (3).

  In addition, the amount of water (h) [mg] derived from the treatment liquid calculated in the water absorbent resin mixture (a) is the weight (i) [mg] of the treatment liquid added per 1,000 mg of the water absorbent resin. From the weight (j) [mg] of the water contained in the treatment liquid, it is obtained using the following formula (4).

  The ratio of the amount of water (g) extracted from the water-absorbent resin to the amount of water (h) derived from the processing liquid included in the water-absorbent resin mixture (a) is calculated from the following formula (5), and the extraction rate (k ) [% By weight].

  The product of the weight ratio (l) [wt%] of water contained in the treatment liquid added to the water absorbent resin to the water absorbent resin and the extraction rate (k) is obtained by the following formula (6), and the surface moisture content (m) [ % By weight].

(3) 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.

(4) Absorption capacity under pressure (AAP)
A stainless steel 400 mesh wire mesh (aperture size 38 μm) is fused to the bottom of a plastic support cylinder with an inner diameter of 60 mm, and water is absorbed on the wire mesh at room temperature (25 ± 2 ° C.) and humidity 50 RH%. 0.900 g of the water-soluble resin was uniformly sprayed, and the outer diameter was adjusted to be able to uniformly apply a load of 4.83 kPa to the water-absorbent resin. A piston and a load, in which no gap is generated between the wall surface and the vertical movement is not hindered, were placed in this order, and the weight W _a (g) of the measuring device set was measured.

  A glass filter with a diameter of 90 mm (manufactured by Mutual Riken Glass Co., Ltd., pore diameter: 100 to 120 μm) is placed inside a petri dish with a diameter of 150 mm, and a 0.9 wt% sodium chloride aqueous solution (saline) (20 to 25 ° C.) was added to the same level as the upper surface of the glass filter. On top of that, a sheet of filter paper having a diameter of 90 mm (manufactured by ADVANTEC Toyo Co., Ltd., product name: (JIS P 3801, No. 2), thickness 0.26 mm, retained particle diameter 5 μm) was placed so that the entire surface was wetted. 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, the set of measuring devices was lifted and its weight W _b (g) was measured. This weight measurement must be performed as quickly as possible and without vibrations. Then, the absorption capacity under pressure (AAP) (g / g) was calculated from W _a and W _b by the following formula.

(5) Total moisture content In an aluminum cup having a bottom diameter of 4 cm and a height of 2 cm, 1.00 g of water absorbent resin is spread evenly on the bottom of the aluminum bag cup, and the weight of the aluminum cup containing the water absorbent resin [W4 (G)] was measured. This was left in a hot air dryer adjusted to 180 ° C. for 3 hours, and the weight [W5 (g)] of the water-absorbent resin-containing aluminum cup immediately after removal from the hot air dryer (within at least 1 minute) was measured. did. And from these W4 and W5, the whole moisture content (weight%) was computed according to following Formula.

(6) Saline Flow Conductivity (also referred to as “SFC”)
SFC is a value indicating the liquid permeability during swelling of the water-absorbent resin particles. It shows that it has high liquid permeability, so that the value of SFC is large.

  This was performed according to the saline flow conductivity (SFC) test described in JP-T 9-509591.

  Specifically, SFC was measured using the apparatus shown in FIG. In the apparatus shown in FIG. 1, a glass tube 32 is inserted into the tank 31, and the lower end of the glass tube 32 is 5 cm from the bottom of the swollen gel 44 in the cell 41 with a 0.69 wt% sodium chloride aqueous solution 33. It is arranged so that it can be maintained at an upper height. The 0.69 wt% sodium chloride aqueous solution 33 in the tank 31 was supplied to the cell 41 through the L-shaped tube 34 with a cock. A collection container 48 for collecting the liquid that has passed is disposed under the cell 41, and the collection container 48 is installed on an upper pan balance 49. The inner diameter of the cell 41 is 6 cm. A 400 stainless steel wire mesh (aperture 38 μm) 42 is installed. There is a hole 47 sufficient for the liquid to pass through the lower part of the piston 46, and a glass filter 45 with good permeability is attached to the bottom so that the water-absorbing resin or its swelling gel does not enter the hole 47. The cell 41 is placed on a table on which the cell is placed, and the surface of the table in contact with the cell is installed on a stainless steel wire mesh 43 that does not hinder the permeation of the liquid.

  The artificial urine consists of calcium chloride dihydrate 0.25 g, potassium chloride 2.0 g, magnesium chloride hexahydrate 0.50 g, sodium sulfate 2.0 g, ammonium dihydrogen phosphate 0.85 g, phosphoric acid Hydrogen diammonium 0.15g and pure water 994.25g are added.

  As a measurement operation, a water-absorbing resin (0.900 g) uniformly placed in the container 40 is swollen for 60 minutes in an artificial urine under a pressure of 0.3 psi (2.07 kPa), and the height of the gel layer of the gel 44 is measured. Then, under a pressure of 0.3 psi (2.07 kPa), a 0.69 wt% sodium chloride aqueous solution 33 was passed through the swelled gel layer from the tank 31 at a constant hydrostatic pressure. Using a computer and a balance, the amount of liquid passing through the gel layer at 20 second intervals as a function of time was recorded for 10 minutes. The flow rate Fs (t) passing through the swollen gel 44 (mainly between the particles) was determined in units of [g / s] by dividing the increased weight (g) by the increased time (s). Let Ts be the time at which a constant hydrostatic pressure and a stable flow rate were obtained, use only the data obtained between Ts and 10 minutes for the flow rate calculation, and use the flow rate obtained between Ts and 10 minutes. The value of Fs (t = 0), ie the initial flow rate through the gel layer, was calculated. Specifically, Fs (t = 0) was calculated by extrapolating the result of least squares of Fs (t) versus time to t = 0.

here,
Fs (t): flow rate expressed in [g / s],
L0: height of the gel layer expressed in cm,
ρ: density of a 0.69 wt% sodium chloride aqueous solution (1.003 g / cm ³ ),
A: Area above the gel layer in the cell 41 (28.27 cm ² ),
ΔP: Hydrostatic pressure applied to the gel layer (4920 dyne / cm ² ), and the unit of SFC value is (10 ⁻⁷ · cm ³ · s · g ⁻¹ ).

(Production Example 1)
An aqueous solution of sodium acrylate having a neutralization rate of 60 mol% (monomer) in a reactor formed by attaching a lid to a stainless steel double-armed kneader with an internal volume of 10 L having two sigma type blades and a jacket (Concentration: 39% by weight), and 7.90 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 liquid was deaerated under a nitrogen gas atmosphere. Subsequently, 30.19 g of a 10 wt% aqueous solution of sodium persulfate as a polymerization initiator and 25.16 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 (A) was obtained.

  The particle size distribution of the obtained water absorbent resin (A) is shown in Table 1 below, and various evaluation results are shown in Table 2 below.

(Production Example 2)
An aqueous solution of sodium acrylate having a neutralization rate of 90 mol% (monomer) in a reactor formed by attaching a lid to a stainless steel double-armed kneader with an internal volume of 10 L and having two sigma-shaped blades Concentration: 39% by weight) was charged, and 6.11 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 liquid was deaerated under a nitrogen gas atmosphere. Subsequently, 28.02 g of a 10 wt% aqueous solution of sodium persulfate as a polymerization initiator and 23.35 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 (B) was obtained.

  The particle size distribution of the obtained water absorbent resin (B) is shown in Table 1 below, and various evaluation results are shown in Table 2 below. In Table 1 below, “850 μm or more” indicates the ratio (% by weight) of the water-absorbing resin remaining on the sieve having an opening of 850 μm after the classification operation. “45 μm or less” indicates the ratio (% by weight) of the water-absorbent resin that passed through a sieve having an opening of 45 μm after the classification operation. “X to y μm” indicates the ratio (% by weight) of the water-absorbent resin that passed through the sieve having an opening of x μm and remained on the sieve having an opening of y μm after the classification operation.

Example 1
500 g of the water-absorbent resin (A) as a base polymer was added to a 5 L Leedige mixer (manufactured by Loedige, model M5R), and stirred at 300 rpm, 12.5 g of ammonium persulfate, polyethylene glycol monomethyl ether (number average molecular weight of about 2, 000) 2.5 g and water 120 g in advance were sprayed. Stirring and mixing were continued at room temperature for 3 minutes, and the added water was permeated and diffused inside the particles. Then, the stirring was once stopped and the sample inlet of the proshear mixer was removed. The water content of the surface of the water absorbent resin composition (1) was 10.4% by weight.

  A quartz glass plate having a thickness of 3 mm was placed on the opening of the water absorbent resin composition (1), and then stirring was resumed at 300 rpm (required time for restarting was 30 seconds). 1 kW metal halide lamp ( A UV irradiation device (UV-152 / 1MNSC3-AA06) equipped with Ushio Electric's UVL-1500M2-N1) is installed so that the distance between the center of the lamp and the quartz plate is 8 cm, and 15 minutes at room temperature. Ultraviolet rays were irradiated to obtain a modified water absorbent resin (1).

  Various evaluation results of the resulting modified water absorbent resin (1) are shown in Table 2 below. In addition, “CRC after correction of total moisture content” and “AAP after correction of total moisture content” shown in Table 2 below were calculated by the following calculation formulas. In the following formula, “CRC before correction of total moisture content” is the centrifuge retention capacity (CRC) of the water-absorbent resin before measuring the total moisture content of (5) above. “AAP before moisture content correction” is the absorption capacity (AAP) under pressure of the water absorbent resin before measuring the total moisture content in (5) above.

(Example 2)
A water absorbent resin composition (2) having a surface moisture content of 12.5% by weight was obtained in the same manner as in Example 1 except that the amount of water in the treatment liquid was changed to 160 g. Further, the water absorbent resin composition (2) was irradiated with ultraviolet rays for 15 minutes in the same manner as in Example 1 to obtain a modified water absorbent resin (2).

  Various evaluation results of the obtained modified water absorbent resin (2) are shown in Table 2 below.

(Example 3)
A water absorbent resin composition (3) having a surface moisture content of 15.5% by weight was obtained in the same manner as in Example 1 except that the amount of water in the treatment liquid was changed to 200 g. Further, the water absorbent resin composition (3) was irradiated with ultraviolet rays for 15 minutes in the same manner as in Example 1 to obtain a modified water absorbent resin (3).

  Various evaluation results of the resulting modified water absorbent resin (3) are shown in Table 2 below.

Example 4
A water absorbent resin composition (4) having a surface moisture content of 12.5% by weight was obtained in the same manner as in Example 2 above. Further, the water absorbent resin composition (4) was irradiated with ultraviolet rays for 1 minute in the same manner as in Example 1 to obtain a modified water absorbent resin (4).

  Various evaluation results of the resulting modified water absorbent resin (4) are shown in Table 2 below.

(Example 5)
A water absorbent resin composition (5) having a surface water content of 6.5% by weight was obtained in the same manner as in Example 1 except that the water absorbent resin (B) was used as the base polymer. Further, the water absorbent resin composition (5) was irradiated with ultraviolet rays for 15 minutes in the same manner as in Example 1 to obtain a modified water absorbent resin (5).

  Various evaluation results of the resulting modified water-absorbing resin (5) are shown in Table 2 below.

(Example 6)
A water absorbent resin composition (6) having a surface moisture content of 6.7% by weight was obtained in the same manner as in Example 2 except that the water absorbent resin (B) was used as the base polymer. Further, the water absorbent resin composition (6) was irradiated with ultraviolet rays for 15 minutes by the same method as in Example 1 to obtain a modified water absorbent resin (6).

  Various evaluation results of the resulting modified water absorbent resin (6) are shown in Table 2 below.

(Example 7)
A water absorbent resin composition (7) having a surface moisture content of 9.6% by weight was obtained in the same manner as in Example 3 except that the water absorbent resin (B) was used as the base polymer. Further, the water absorbent resin composition (7) was irradiated with ultraviolet rays for 15 minutes by the same method as in Example 1 to obtain a modified water absorbent resin (7).

  Various evaluation results of the resulting modified water absorbent resin (7) are shown in Table 2 below.

(Comparative Example 1)
A comparative water absorbent resin composition (1) having a surface moisture content of 1.9% by weight was obtained in the same manner as in Example 1 except that the amount of water in the treatment liquid was changed to 20 g. Moreover, the comparative water-absorbent resin composition (1) was irradiated with ultraviolet rays for 15 minutes in the same manner as in Example 1 to obtain a modified comparative water-absorbent resin (1).

  Various evaluation results of the resulting modified comparative water absorbent resin (1) are shown in Table 2 below.

(Comparative Example 2)
A comparative water absorbent resin composition (2) having a surface moisture content of 4.5% by weight was obtained in the same manner as in Example 1 except that the amount of water in the treatment liquid was changed to 40 g. Further, the comparative water absorbent resin composition (2) was irradiated with ultraviolet rays for 15 minutes by the same method as in Example 1 to obtain a modified comparative water absorbent resin (2).

  Various evaluation results of the resulting modified comparative water absorbent resin (2) are shown in Table 2 below.

(Comparative Example 3)
A comparative water absorbent resin composition (3) having a surface moisture content of 7.9% by weight was obtained in the same manner as in Example 1 except that the amount of water in the treatment liquid was changed to 80 g. Moreover, the comparative water-absorbent resin composition (3) was irradiated with ultraviolet rays for 15 minutes by the same method as in Example 1 to obtain a modified comparative water-absorbent resin (3).

  Various evaluation results of the resulting modified comparative water absorbent resin (3) are shown in Table 2 below.

(Comparative Example 4)
A comparative water absorbent resin composition (4) having a surface water content of 4.5% by weight was obtained in the same manner as in Comparative Example 2 above. Further, the comparative water absorbent resin composition (4) was irradiated with ultraviolet rays for 1 minute by the same method as in Example 4 to obtain a modified comparative water absorbent resin (4).

  Various evaluation results of the resulting modified comparative water absorbent resin (4) are shown in Table 2 below.

(Comparative Example 5)
A comparative water absorbent resin composition (5) having a surface moisture content of 1.1% by weight was obtained in the same manner as in Comparative Example 1 except that the water absorbent resin (B) was used as the base polymer. Moreover, the comparative water-absorbent resin composition (5) was irradiated with ultraviolet rays for 15 minutes in the same manner as in Example 1 to obtain a modified comparative water-absorbent resin (5).

  Various evaluation results of the resulting modified comparative water absorbent resin (5) are shown in Table 2 below.

(Comparative Example 6)
A comparative water absorbent resin composition (6) having a surface water content of 5.2% by weight was obtained in the same manner as in Comparative Example 3 except that the water absorbent resin (B) was used as the base polymer. Moreover, the comparative water-absorbent resin composition (6) was irradiated with ultraviolet rays for 15 minutes in the same manner as in Example 1 to obtain a modified comparative water-absorbent resin (6).

  Various evaluation results of the resulting modified comparative water absorbent resin (6) are shown in Table 2 below.

  From the results shown in Table 2, according to the production method of the present invention, the surface water content of the water-absorbent resin is controlled to a predetermined value or more and is active against the water-absorbent resin composition containing water and a radical polymerization initiator. By irradiating energy rays, it is shown that the surface of the water-absorbent resin particles can be efficiently modified and a water-absorbent resin having excellent water absorption characteristics can be produced. Moreover, from the result of Example 4, by adding a large amount of water to the water-absorbent resin and irradiating the active energy beam with the surface moisture content of the water-absorbent resin controlled to a predetermined value or more, the active energy beam is obtained. It is also shown that even when the irradiation time is short, the surface of the water-absorbing resin particles can be efficiently modified and a water-absorbing resin having excellent water absorption characteristics can be produced.

  According to the present invention, when modifying a water-absorbent resin, it can be sufficiently surface-treated even at a reaction temperature near room temperature, and the resulting modified water-absorbent resin is excellent in water-absorbing properties, so that it can be used as a paper diaper or the like. Can be industrially useful.

It is the schematic of the measuring apparatus used for the measurement of a saline flow conductivity (SFC).

Claims (14)

A method for producing a modified water absorbent resin,
a) a mixing step of mixing a water-absorbent resin, water, and a water-soluble radical polymerization initiator without adding an ethylenically unsaturated monomer to obtain a water-absorbent resin composition;
b) an irradiation step of irradiating the water-absorbent resin composition obtained in the mixing step with active energy rays;
The mixing amount of the water in the mixing step a) is more than 20 parts by weight and not more than 100 parts by weight with respect to 100 parts by weight of the water-absorbent resin, and at least one point in the irradiation step b) The surface water content of the water-absorbent resin in the water-absorbent resin composition is controlled to 3.0% by weight or more with respect to 100% by weight of the water-absorbent resin. Production method of resin.   The method for producing a modified water-absorbent resin according to claim 1, wherein the water-soluble radical polymerization initiator is at least one selected from the group consisting of persulfate, hydrogen peroxide and a water-soluble azo compound. A method for producing a modified water absorbent resin,
a) a mixing step of mixing a water-absorbent resin, water, and a thermally decomposable radical polymerization initiator without adding an ethylenically unsaturated monomer to obtain a water-absorbent resin composition;
b) an irradiation step of irradiating the water-absorbent resin composition obtained in the mixing step with active energy rays;
The mixing amount of the water in the mixing step a) is more than 20 parts by weight and not more than 100 parts by weight with respect to 100 parts by weight of the water-absorbent resin, and at least one point in the irradiation step b) The surface water content of the water-absorbent resin in the water-absorbent resin composition is controlled to 3.0% by weight or more with respect to 100% by weight of the water-absorbent resin. Production method of resin.   The method for producing a modified water-absorbent resin according to claim 3, wherein the thermally decomposable radical polymerization initiator is at least one selected from the group consisting of persulfate, hydrogen peroxide and azo compounds.   The method for producing a modified water-absorbent resin according to any one of claims 1 to 4, wherein the surface moisture content at the start of the irradiation step is controlled to 3.0 wt% or more.   The modified amount according to any one of claims 1 to 5, wherein the mixing amount of the radical polymerization initiator in the mixing step is 0.01 to 20 parts by weight with respect to 100 parts by weight of the water absorbent resin. Manufacturing method of water-absorbent resin.   The method for producing a modified water-absorbing resin according to any one of claims 1 to 6, wherein in the mixing step, the radical polymerization initiator is mixed in the form of an aqueous solution.   The modified water-absorbing property according to any one of claims 1 to 7, wherein a mixing aid is added to the water-absorbing resin simultaneously with the mixing step a) or before the mixing step a). Production method of resin.   The water-absorbing resin contains acid groups and has a neutralization rate of 50 to 75 mol% (mol% of neutralized acid groups in all acid groups). A process for producing the modified water-absorbent resin described in 1.   The method for producing a modified water-absorbent resin according to any one of claims 1 to 9, wherein the active energy rays are ultraviolet rays.   The modified water-absorbent resin according to any one of claims 1 to 10, wherein the water-absorbent resin is a powder water-absorbent resin obtained by polymerizing a monomer mainly composed of acrylic acid (salt). Production method of resin.   The method for producing a modified water-absorbent resin according to any one of claims 1 to 11, wherein 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 modified water-absorbent resin according to any one of claims 1 to 12, wherein the modified water-absorbent resin has an absorption capacity under pressure with respect to 4.83 kPa physiological saline of 8 to 40 g / g. The manufacturing method. The modification | reformation of any one of Claims 1-13 whose salt solution flow-inductivity of the water absorbing resin after modification | reformation is 10 (* 10 < ^-7 > * cm < ³ > * s * g < ^-1 >) or more. Of the water-absorbing resin made.

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