JP5528714B2 - Method for producing water absorbent resin - Google Patents

Description

  The present invention relates to a method for producing a water absorbent resin. More specifically, the present invention relates to a method for producing a water absorbent resin, in which the surface treated water absorbent resin is heated.

  Conventionally, a water-absorbing agent mainly composed of a copolymer (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 the water-absorbing resin constituting such a water-absorbing agent include starch-acrylonitrile graft polymer hydrolyzate, starch-acrylic acid graft polymer neutralized product, vinyl acetate-acrylic acid ester copolymer kenken. Products, hydrolysates of acrylonitrile copolymers 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 rate, an excellent absorption rate, a high gel strength, an excellent suction force for sucking water from the substrate, and liquid permeability. As a technique aimed at improving various water absorption characteristics of the water absorbent resin, a method of treating the surface of the water absorbent resin with a crosslinking agent or a polymerizable monomer has been proposed.

  Examples of the surface treatment include surface cross-linking, pore formation, hydrophilization and hydrophobization of the water-absorbent resin, and surface cross-linking is preferable. The water-absorbent resin is a water-insoluble polymer having an internal crosslink produced by blending a polymerizable monomer with an internal cross-linking agent and a polymerization initiator and polymerizing. And the reactive functional group contained in a monomer exists in the surface of the water absorbing resin after superposition | polymerization. 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.

  Examples of surface cross-linking techniques that have been studied so far include, for example, a technique that uses a surface cross-linking agent in combination (Patent Document 1), and a technique that relates to a heating device that uses stirring means including a rotating shaft equipped with a plurality of stirring plates (Patent Document). 2), technology for surface cross-linking a water-absorbent resin having a high water content (Patent Document 3), technology for temperature increase control of the heating temperature for reacting the water-absorbent resin and the surface cross-linking agent (Patent Document 4), surface A technique of performing crosslinking twice (Patent Document 5) and a technique of bringing an aqueous solution containing a peroxide radical initiator into contact with the surface portion of the water absorbent resin and heating (Patent Document 6) are disclosed.

  Furthermore, the technique which surface-treats a water-absorbent resin by performing irradiation treatment of active energy rays, such as an ultraviolet-ray, instead of heat processing is also proposed (patent documents 7 and patent documents 8). In addition, as a technique aimed at improving various water absorption properties of the water absorbent resin, a method of heat-treating the surface of the water absorbent resin (Patent Document 9) and a method of polymerizing the water absorbent resin in two stages (Patent Document 10). ) A technique (Patent Document 11) using a plurality of heat treatment apparatuses is disclosed.

US Pat. No. 5,422,405 Japanese Patent Laid-Open No. 2004-352941 US Pat. No. 6,875,511 US Pat. No. 6,514,615 US Pat. No. 5,672,633 U.S. Pat. No. 4,783,510 International Publication No. 2006/62258 Pamphlet International Publication No. 2006/62253 Pamphlet US Pat. No. 5,206,205 JP 2005-213523 A US Patent Application Publication No. 2007/0149760

  However, when the water-absorbing resin is surface-treated using the techniques described in the above-mentioned patent documents, the water-absorbing properties of the surface-crosslinked water-absorbing resin obtained are not necessarily sufficient, particularly under pressure. Further improvement in water absorption characteristics (absorption capacity under pressure), absorption capacity (free swelling ratio) and liquid permeability has been demanded. 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 and liquid permeability increase when the crosslink density increases, but the free swelling ratio decreases. Further, in the water-absorbent resin particles having no surface cross-linkage, when the particles come into contact with the liquid, water absorption is not performed uniformly and a part that becomes a lump of the water-absorbent resin is formed, or the entire water-absorbent resin particles are formed. Since water does not diffuse, the absorption rate or the like may be extremely reduced. As described above, it has been difficult to obtain a water-absorbing resin having excellent absorption capacity under load, liquid permeability, and free swelling ratio at low cost.

  Then, an object of this invention is to provide the manufacturing method of the water absorbing resin which can improve the water absorption characteristic (especially free swelling magnification, liquid permeability) of a water absorbing resin.

  In view of the above problems, the present inventors conducted extensive research. As a result, it is possible to obtain a water-absorbing resin with improved water-absorbing properties (particularly, free swelling ratio and liquid permeability) by heating the water-absorbing resin under predetermined conditions after surface treatment such as surface crosslinking. As a result, the present invention has been completed.

  That is, in one embodiment of the present invention, (a) a step of surface-treating a water-absorbent resin, and (b) a water-absorbent resin having a water content of 1% by mass to 60% by mass obtained in the step (a), A method for producing a water-absorbent resin, comprising a step of heating at a temperature of 120 ° C. or more and 250 ° C. or less without adding a surface treatment agent.

  In another embodiment of the present invention, (a) a surface treatment of the water-absorbent resin, and (b) a water-absorbent resin having a water content of 1% by mass to 60% by mass obtained in the step (a). The step (b) is performed so that the free swelling ratio (GV) after correction of the water content of the water-absorbent resin is increased by 1 (g / g) or more before and after the step. And a method for producing a water-absorbent resin.

  Furthermore, another embodiment of the present invention includes (a) a surface treatment of the water absorbent resin, and (b) a step of heating the water absorbent resin obtained in the step (a), wherein the step (b) Is increased by 1 (g / g) or more from the value after the increase in the free swelling ratio (GV) after correction of the water content of the water-absorbent resin as compared with that before the step (b). g) A method for producing a water-absorbent resin, which is carried out until the amount decreases.

  According to the production method of the present invention, a surface-treated water-absorbing resin having excellent water absorption characteristics can be produced. In particular, according to the present invention, it is possible to improve both the free swelling ratio (GV) and the liquid permeability (SFC), which have been difficult to achieve in the past. Furthermore, physical damage (such as heat deterioration) that the water-absorbent resin is subjected to by the surface treatment can be reduced.

FIG. 1 is a schematic view of a measuring apparatus used for measurement of saline flow conductivity (SFC).

  Hereinafter, although the manufacturing method of the water absorbing resin which concerns on this invention is demonstrated, the technical scope of this invention is not restrained by these description, The range which does not impair the meaning of this invention except the following illustrations. And can be implemented with appropriate changes.

  In the present invention, “weight” is treated as a synonym for “mass”, and “weight%” is treated as a synonym for “mass%”. Unless otherwise specified, “physiological saline” means a 0.9 wt% sodium chloride aqueous solution, and “water content” means water absorption defined by a reduction of 180 ° C./3 hours described in the examples described later. It means the moisture content of the functional resin. The abbreviation “A to B” regarding the numerical range represents a numerical range from A to B.

(1) Water absorbent resin (base polymer)
The water-absorbent resin is a water-swellable water-insoluble polymer obtained by introducing a crosslinked structure into a hydrogel polymer. In the present invention, “water swellability” means that the free swelling ratio (GV) in physiological saline is 2 g / g or more, preferably 5 to 100 g / g, more preferably 10 to 60 g / g. It is. In the present invention, the value measured by the method described in the examples described later (value after moisture content correction) is adopted as the numerical value of the free swelling ratio (GV). In addition, “water-insoluble” means that the content of uncrosslinked water-soluble component (hereinafter also referred to as “eluting soluble content”) in the water-absorbent resin is 0 to 50% by mass, preferably It is 0-25 mass%, More preferably, it is 0-15 mass%, More preferably, it is 0-12 mass%. In addition, the value measured by the method of an Example shall be employ | adopted as a numerical value of an elution soluble part.

(A) Method for producing water-absorbing resin (base polymer) (a-1) Polymerization step for polymerizing monomer component In the present invention, the water-absorbing resin (base polymer) is, for example, polymerization for polymerizing the monomer component. Obtained by the process.

  In the polymerization step, first, it is preferable to obtain a hydrogel crosslinked polymer by polymerizing the monomer component in the presence of a crosslinking agent. The hydrogel crosslinked polymer may be of a self-crosslinking type that does not use a crosslinking agent, but is a crosslink having two or more polymerizable unsaturated groups or two or more reactive groups in one molecule. It is preferable that the agent is copolymerized or reacted. By performing internal crosslinking in this way, the water absorbent resin becomes water-insoluble.

(Monomer)
There is no restriction | limiting in particular about the kind of monomer which comprises water absorbing resin, For example, unsaturated monomers, such as an ethylenically unsaturated monomer, can be used. The ethylenically unsaturated monomer is not particularly limited and is preferably a monomer having an unsaturated double bond at the terminal, such as (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2- Anionic monomers such as (meth) acryloylpropane sulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid and salts thereof; (meth) acrylamide, N-substituted (meta ) Nonionic hydrophilic group-containing monomers such as acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (Meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, N, N- Methylamino propyl (meth) acrylamide, amino group-containing unsaturated monomers and their quaternized products and the like; and the like. As for these monomers, only 1 type may be used independently and 2 or more types may be used together. 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, and particularly preferably acrylic acid and / or a salt thereof. The proportion of acrylic acid (salt) is preferably from 50 to 100 mol%, more preferably from 70 to 100 mol%, particularly preferably from 90 to 100 mol%, based on all monomers.

  When an 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 the sodium, lithium, and potassium salts of acrylic acid.

  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.

  As the unsaturated monomer component, a partially neutralized product such as acrylic acid may be polymerized, or an acid group-containing monomer such as acrylic acid may be polymerized and then an alkali such as sodium hydroxide or sodium carbonate. The polymer may be neutralized with a compound.

  The preferred embodiment of the present invention has been described above by taking the case of using an ethylenically unsaturated monomer as the monomer component as an example, but the technical scope of the present invention is not limited to such a form. For example, the water-absorbent resin of the present invention may be a crosslinked polyamino acid, a crosslinked polysaccharide, a crosslinked polyester, a crosslinked polyacetal, and a complex thereof.

(Internal crosslinking agent)
Specific examples of the crosslinking agent (internal crosslinking agent) used in the polymerization step include, for example, N, N′-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, and (poly) propylene glycol di (meta). ) Acrylate, (ethylene oxide modified) trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, glycerin di (meth) acrylate, glycerin tri (meth) acrylate, (ethylene oxide modified) glycerin acrylate methacrylate, 3 -Methyl-1,3-butanediol di (meth) acrylate, pentaerythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acryl Rate, N, N-diallylacrylamide, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, diallyloxyacetic acid, bis (N-vinylcarboxylic acid amide), (ethylene oxide modified) tetraallyloxyethane, Examples include poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerin diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl (meth) acrylate. . Only one of these internal crosslinking agents may be used alone, or two or more thereof may be used in combination. In particular, in consideration of the water absorption characteristics of the resulting water-absorbent resin, it is preferable to use a compound having two or more polymerizable unsaturated groups as the internal crosslinking agent. Specifically, (poly) ethylene glycol di (meth) acrylate, (ethylene oxide modified) trimethylolpropane tri (meth) acrylate, glycerin di (meth) acrylate, (ethylene oxide modified) glycerin acrylate methacrylate, pentaerythritol tri ( (Meth) acrylate is preferable, and (poly) ethylene glycol di (meth) acrylate and (ethylene oxide-modified) trimethylolpropane tri (meth) acrylate are more preferable.

  The amount of the internal crosslinking agent used is preferably 0.0001 to 1 mol%, more preferably 0.0005 to 0.5 mol%, based on the total amount of monomer components used in producing the water absorbent resin. More preferably, it is 0.001-0.2 mol%, Most preferably, it is 0.003-0.1 mol%, Most preferably, it is 0.005-0.05 mol%. If the use amount of the internal cross-linking agent is 0.0001 mol% or more, internal cross-linking is introduced, and if the use amount of the internal cross-linking agent is 1 mol% or less, the gel strength of the resulting water-absorbent resin is excessively increased. Accordingly, a decrease in water absorption characteristics can be prevented. 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?

(Chain transfer agent / polymerization initiator)
In the polymerization, hydrophilic polymers such as starch-cellulose, starch-cellulose derivatives, polyvinyl alcohol, polyacrylic acid (salt), cross-linked polyacrylic acid (salt), and chains such as hypophosphorous acid (salt) A transfer agent may be added to the reaction system. The amount of the hydrophilic polymer used in the polymerization step is preferably 0 to 50% by mass, more preferably 0 to 30% by mass, and further preferably 0 to 10% by mass with respect to the total amount of the monomer components. It is. The amount of chain transfer agent used in the polymerization step is preferably 0.001 to 1 mol%, more preferably 0.005 to 0.5 mol%, based on the total amount of monomer components, Preferably it is 0.01-0.3 mol%.

  At the start of polymerization in the polymerization reaction, for example, a polymerization initiator or active energy rays such as radiation, electron beam, ultraviolet ray, electromagnetic ray, or the like can be used. Although it does not specifically limit as a polymerization initiator, A thermal polymerization initiator and a photoinitiator can be used.

  Thermal polymerization initiators include persulfates such as sodium persulfate, potassium persulfate and ammonium persulfate, peroxides such as hydrogen peroxide, t-butyl peroxide and methyl ethyl ketone peroxide; azonitrile compounds, azoamidine compounds, cyclic azoamidines Compounds, azoamide compounds, alkylazo compounds, 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, and the like. Can be mentioned. Examples of the photopolymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives, and azo compounds. As for these polymerization initiators, only 1 type may be used independently and 2 or more types may be used together. When a peroxide such as persulfate is used as the polymerization initiator, for example, redox polymerization is performed using a reducing agent such as sulfite, bisulfite, or L-ascorbic acid. Also good.

  The amount of the polymerization initiator used in the polymerization step is preferably 0.001 to 2 mol%, more preferably 0.01 to 0.5 mol%, based on the total amount of the monomer components.

  The temperature at the start of polymerization depends on the type of polymerization initiator used, but is preferably 15 to 130 ° C, more preferably 20 to 120 ° C. If the temperature at the start of polymerization is out of the above range, the residual monomer of the obtained water absorbent resin may increase, or excessive self-crosslinking reaction may proceed and the water absorption performance of the water absorbent resin may be reduced. is there. The polymerization time is usually 0.1 to 60 minutes.

(Polymerization method)
As the polymerization method, aqueous solution polymerization, reverse phase suspension polymerization, bulk polymerization, precipitation polymerization and the like can be employed. In consideration of performance and ease of control of polymerization, it is preferable to perform aqueous solution polymerization or reverse phase suspension polymerization using the monomer component including the monomer and the internal cross-linking agent as an aqueous solution, particularly aqueous solution polymerization. It is preferable to carry out.

  The concentration of the aqueous solution of the monomer component is usually 10 to 90% by mass, preferably 20 to 80% by mass, more preferably 30 to 70% by mass, and particularly preferably 40 to 60% by mass as the monomer. When the amount is less than 10% by mass, the amount of water in the obtained gel polymer (hydrogel) is large, which requires a heat amount and time for drying, which is disadvantageous. In addition, aqueous solutions include surfactants, polyacrylic acid (salts) and their cross-linked products (water-absorbent resins), high molecular compounds such as starch and polyvinyl alcohol, ethylenediaminetetraacetic acid, hydroxyethylethylenediaminetriacetic acid, dihydroxyethylethylenediaminedioxide. Acetic acid, 1,3-propanediaminetetraacetic acid, diethylenetriaminepentaacetic acid, triethylenetetraminehexaacetic acid, nitrilotriacetic acid, hydroxyethyliminodiacetic acid, L-aspartic acid-N, N-diacetic acid, aminotrimethylenephosphonic acid, hydroxy Various chelating agents such as ethanephosphonic acid and alkali metal salts thereof, various additives, and the like may be used in an amount of 0 to 30% by mass with respect to the monomer. In the present application, the aqueous solution includes a dispersion exceeding the saturation concentration, but is preferably polymerized at a saturation concentration or less.

  Such polymerization methods are described, for example, in US Pat. No. 462501, US Pat. No. 4,769,427, US Pat. No. 4,873,299, US Pat. No. 4,093,763, US Pat. No. 4,367,323, US Pat. US Pat. No. 4,446,261, US Pat. No. 4,683,274, US Pat. No. 4,690,996, US Pat. No. 4,721,647, US Pat. No. 4,738,867, US Pat. No. 4,748,076, US Pat. No. 6,906,159. It is described in the description.

  A water-containing gel-like crosslinked polymer is usually obtained by the polymerization process as described above. The solid content (water content) of the water-containing gel-like cross-linked polymer is appropriately determined based on the concentration of the monomer aqueous solution, water evaporation during polymerization, and the like.

(A-2) Drying / pulverizing / classifying step The hydrogel crosslinked polymer obtained in the polymerization step can be used as it is as a water-absorbent resin, but it is preferable to perform the drying step after the polymerization step. In the present invention, the water-absorbent resin includes not only the water-containing gel-like cross-linked polymer but also those obtained by drying and those obtained by further carrying out a pulverization step, a classification step, etc. after the drying step. To do.

(Drying process)
A drying process is a process of drying the water-containing gel-like crosslinked polymer obtained by the polymerization process. In the present invention, “drying” means that the solid content is increased by 10% by mass or more, or the water content is 25% by mass or less.

  There is no particular limitation on the drying means, and conventionally known drying means such as one or more of a band dryer, a stirring dryer, a fluidized bed dryer and the like can be suitably used. The water content of the water-absorbent resin after drying (defined by weight loss at 180 ° C. for 3 hours) is preferably 1 to 60% by mass, more preferably 5 to 60% by mass, and further preferably 5 to 40% by mass. Especially preferably, it is 7-30 mass%, Most preferably, it is 8-20 mass%.

  The drying temperature and drying time in the drying step are not particularly limited and are usually 70 to 250 ° C, preferably 90 to 220 ° C, more preferably 120 to 210 ° C, and even more preferably 150 to 200 ° C. The “drying temperature” is the temperature of the heat medium in the drying means, and when the heat medium temperature cannot be defined as in the case of drying using microwaves, the temperature of the water absorbent resin to be dried. Stipulated in If the drying temperature is 70 ° C. or higher, the drying time is prevented from becoming unnecessarily long. Moreover, if a drying temperature is 250 degrees C or less, deterioration of the water absorbing resin at the time of drying will be prevented. The drying time is appropriately set, and is usually 1 minute to 5 hours, preferably 10 minutes to 2 hours.

  For the purpose of making the massive hydrogel crosslinked polymer obtained in the polymerization step into particles, a gel granulation step of pulverizing the polymer may be performed before the drying step described above. By using a particulate hydrous gel, the surface area of the gel increases, so that the drying process described above can proceed smoothly. The pulverization can be performed by various cutting means such as a roller-type cutter, a guillotine cutter, a cutter mill, a slicer, a roll cutter, a shredder, or a scissor, alone or in combination, and is not particularly limited.

(Crushing process)
In a preferred embodiment, a pulverization step and a classification step are further performed after the drying step. The pulverization step is a step of pulverizing the dried product obtained in the drying step with a pulverizer to form particles. Examples of the pulverizer used in this pulverization step include a shear pulverizer and an impact pulverizer among the pulverization model names classified in Table 1.10 of the Powder Engineering Handbook (Edition of the Powder Engineering Society, first edition). It is classified as a high-speed rotary pulverizer, and those having one or more pulverization mechanisms such as cutting, shearing, impact and friction can be preferably used. Among the pulverizers corresponding to these models, the cutting and shearing mechanism is the main mechanism. It is particularly preferable to use a pulverizer. Examples thereof include a roll mill, a knife mill, a hammer mill, a pin mill, a jet mill, and the like, and it is preferable that a means for heating the inner wall surface of the pulverizer itself is provided.

(Classification process)
The classification step is a step of continuously classifying the dried product powder obtained in the pulverization step. The classification process is not particularly limited, but is preferably based on sieve classification (metal sieve, made of stainless steel). Preferably, in order to achieve the desired physical properties and particle size, the classification step uses a plurality of sieves simultaneously, and the classification step is performed before the surface treatment step described later, and further at two or more locations before and after. Are preferred. The continuous sieving classification process is preferably performed while heating or keeping the sieve.

(2) Surface treatment process The manufacturing method of the water absorbent resin which concerns on this invention includes the process (surface treatment process) which surface-treats a water absorbent resin. In the present invention, the surface treatment means that the surface of the water-absorbent resin or a region near the surface is chemically or physically modified. Here, chemical modification means a state of modification accompanied by some chemical bond, and physical modification means physical coating / attachment not accompanied by chemical bond. In addition, “near the surface” usually means a surface layer part having a thickness of several tens of μm or less or a surface layer part having a thickness of 1/10 or less of the whole, and this thickness is appropriately determined according to the purpose. Examples of the surface treatment include surface cross-linking, pore formation, hydrophilization, and hydrophobization of a water-absorbing resin (base polymer), and surface cross-linking is preferable. In the present invention, “surface crosslinking” means increasing the crosslinking density on the surface of the water absorbent resin. The surface cross-linking can be confirmed by a decrease in the free swelling ratio (value after correcting the water content) of the water absorbent resin (base polymer) after the surface cross-linking step.

  Usually, in the surface treatment step, the surface treatment of the water absorbent resin is performed using a surface treatment agent. In this specification, the “surface treatment agent” means a compound added for the purpose of modifying (surface treatment) the surface of the water-absorbent resin, and is applied to the surface of the water-absorbent resin like a surface cross-linking agent described later. It includes not only those that act chemically but also compounds such as radically polymerizable compounds and liquid permeability improvers.

  As the water-absorbing resin used in the surface treatment step, those produced by the above-mentioned method for producing the water-absorbing resin (base polymer) can be used, but the invention is not limited to these, and other methods are used. It may be what was done.

(Moisture content)
Although the water content of the water-absorbent resin used for the surface treatment step is not particularly limited, the water content of the water-absorbent resin before the surface treatment step can be about 1 to 60% by mass. If the water content of the water-absorbent resin is 1% by mass or more, it is necessary to efficiently improve the surface of the water-absorbent resin in the surface treatment step, and to improve the absorption capacity and liquid permeability under pressure to a desired level. Time for heat treatment can be shortened. On the other hand, if the water content of the water-absorbent resin is 60% by mass or less, the energy required in the heating process after the surface treatment process is prevented from excessively increasing. In the present invention, the surface treatment is performed on a water-absorbing resin having a relatively high water content as compared with the prior art. Surface treatment such as surface cross-linking is usually performed with stirring to introduce a uniform cross-linked structure. However, when the water content of the water-absorbing resin to be surface-treated is high, water is used as a cushion function (buffer). Function) and physical damage to the water-absorbent resin due to surface cross-linking can be reduced. From such a viewpoint, the water content of the water-absorbent resin is preferably 3 to 60% by mass, more preferably 5 to 60% by mass, still more preferably 5 to 40% by mass, and particularly preferably 7 to 30% by mass, most preferably 8-20% by mass.

  In the present invention, “the water content of the water-absorbing resin before the surface treatment step” means the water content of the water-absorbent resin immediately before the surface treatment is performed on the water-absorbent resin. Similarly, “water content of the water-absorbent resin after the surface treatment step” described later also means the water content of the water-absorbent resin immediately after the surface treatment of the water-absorbent resin. For example, as will be described later, in the case where the surface treatment process proceeds through a mixing treatment with a treatment liquid containing a surface crosslinking agent and the like and a heat treatment, the surface treatment (surface crosslinking) proceeds in the heat treatment. Therefore, the water content of the water-absorbent resin before (after) the surface treatment step means the water content immediately before (after) the heat treatment. “The water content of the water-absorbing resin before the surface treatment step” means the water content before the heat treatment, and “the water content after the surface treatment step” means the water content after the heat treatment. Alternatively, when the surface treatment process proceeds through mixing treatment of the water-absorbing resin and the treatment liquid containing the radical polymerizable compound, and irradiation with active energy rays and / or heat treatment, irradiation with active energy rays and / or Surface treatment proceeds in the heat treatment. Therefore, “the water content of the water-absorbing resin before the surface treatment step” means the water content before the irradiation with active energy rays and / or the heat treatment, and “the water content after the surface treatment step” means the active energy. It means the water content after irradiation and / or heat treatment. Also, in industrial production equipment, the water content of the water-absorbent resin when entering the reaction apparatus that performs the surface treatment reaction corresponds to the “water content before the surface treatment process”, and the water treatment resin exits from the reaction apparatus that performs the surface treatment reaction. The water content of the water-absorbent resin corresponds to the “water content after the surface treatment step”.

  As a method for adjusting the water content, for example, water may be added to adjust the concentration of the aqueous solution so that the water content of the water-absorbent resin is within the above-described range. Moreover, it is also possible to replace the addition of water by appropriately drying the water-containing gel-like crosslinked polymer obtained in the polymerization step and adjusting the water content in the polymer to about 1 to 60% by mass.

(Particle size distribution)
The shape of the water-absorbent resin used in the surface treatment process is preferably powder. More preferably, it is a powdery water-absorbing resin containing 90 to 100% by mass, particularly preferably 95 to 100% by mass of particles having a particle size in the range of 150 μm or more and less than 850 μm (specified by sieve classification). When the water-absorbing resin having a particle size of 850 μm or more is less than 10% by mass, it is preferable in that damage to the diaper back sheet can be prevented. On the other hand, when the water-absorbing resin having a particle size of less than 150 μm is less than 10% by mass, it is preferable from the viewpoint that scattering of fine powder and blockage of piping during production can be prevented. The weight average particle diameter (D50) is preferably 150 to 850 μm, more preferably 200 to 600 μm, and further preferably 300 to 500 μm. A weight average particle diameter of 150 μm or more is preferable from the viewpoint of safety and health. On the other hand, if the weight average particle diameter is 850 μm or less, it is preferable in that the touch when wearing a diaper is good. The logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.23 to 0.45, and more preferably 0.25 to 0.40. In addition, the value measured by the method as described in the Example mentioned later shall be employ | adopted as a numerical value of the logarithmic standard deviation ((sigma) ζ) of a weight average particle diameter or a particle size distribution.

(Neutralization rate)
The water-absorbing resin used in the surface treatment step preferably contains an acid group and has a predetermined neutralization rate (mol% of neutralized acid groups in all acid groups). Examples of the acid group include a carboxyl group, a sulfone group, and a phosphoric acid group. At this time, the neutralization rate of the obtained water-absorbent resin is preferably 25 to 100 mol%, more preferably 50 to 90 mol%, still more preferably 50 to 80 mol%, and particularly preferably 60. -75 mol%. The neutralization rate of the water-absorbent resin can be adjusted by polymerizing a monomer whose neutralization rate has been adjusted in advance, or, for example, by a neutralization method after acid polymerization (for example, US Pat. No. 6,187,872), etc. You may adjust the neutralization rate of the said polymer so that it may become a desired neutralization rate after manufacturing the polymer which has a low neutralization rate once.

(A) Surface treatment by polymerization of radically polymerizable compound In one embodiment of the present invention, the surface treatment step includes a step of mixing a water-absorbing resin and a treatment liquid containing a radically polymerizable compound (mixing step), and the radical It includes a step of polymerizing the polymerizable compound (polymerization step). In the present embodiment, the water-absorbent resin (base polymer) obtained in the polymerization step is usually mixed with an aqueous solution containing a radical polymerizable compound and water (hereinafter also referred to as “treatment liquid”) to obtain a water-absorbent resin. A composition is obtained. Then, the water-absorbing resin is surface-treated by polymerizing the radical polymerizable compound. Hereinafter, the mixing step and the polymerization step in the surface treatment step of the present embodiment will be described in detail.

(A-1) Mixing Step In the surface treatment method of the present embodiment, in the mixing step, the water absorbent resin and the treatment liquid containing the radical polymerizable compound are mixed to obtain a water absorbent resin composition. Specifically, a water-absorbing resin composition is obtained by mixing with a water-absorbing resin in an aqueous solution (treatment liquid) in which a radical polymerizable compound is dissolved in water. Preferably, the radical polymerizable compound is dissolved in water, and the radical polymerizable compound is used in the form of an aqueous solution in the absence of a hydrophobic organic solvent. According to such a form, since a radically polymerizable compound is uniformly disperse | distributed on the surface of a water absorbing resin, it is preferable. In the reaction system in the polymerization step of the present embodiment to be described later, it is essential that water is present together with the water-absorbent resin, but according to the above-described embodiment, the radical polymerizable compound is mixed with the water-absorbent resin in the form of an aqueous solution. Not only the radical polymerization initiator but also water can be simultaneously added to the reaction system. As a result, the operation required for the surface treatment can be simplified. In addition, you may employ | adopt the form which mixes a radically polymerizable compound directly with a water absorbing resin, and of course, after mixing a radically polymerizable compound and a water absorbing resin, you may mix the obtained mixture with water.

(Radically polymerizable compound)
The radically polymerizable compound means a compound that can be polymerized by radical polymerization, and specifically, for example, an ethylenically unsaturated monomer used in the production of the water absorbent resin in the above-mentioned “Water Absorbent Resin” column. The compounds mentioned as examples of the body (monofunctional radical polymerizable compound) and internal crosslinking agent (polyfunctional radical polymerizable compound) can be preferably used. In a preferred embodiment, from the viewpoint of ease of production, the compound used as the ethylenically unsaturated monomer and the internal cross-linking agent during the production of the water-absorbent resin is used as the radical polymerizable compound in this step. As the radical polymerizable compound, for example, only one of a monofunctional radical polymerizable compound and a polyfunctional radical polymerizable compound may be used, or both of them may be used in combination. Moreover, as a radically polymerizable compound, only 1 type may be used independently and 2 or more types may be used together.

  In a preferred embodiment, the radical polymerizable compound includes a monofunctional radical polymerizable compound and a polyfunctional radical polymerizable compound. By using a polyfunctional radical polymerizable compound in combination, the absorption capacity and liquid permeability under pressure can be further improved.

  The monofunctional radically polymerizable compound is preferably a monomer containing an acid group. Among radical polymerizable compounds, a compound containing an acid group (hereinafter, also simply referred to as “acid group-containing radical polymerizable compound”) is very excellent in terms of water absorption characteristics. Examples of the acid group include a carboxyl group, a sulfone group, and a phosphoric acid group. Specifically, (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, styrene Examples include sulfonic acid and / or a salt thereof. Among these, (meth) acrylic acid and 2- (meth) acrylamido-2-methylpropanesulfonic acid are more preferable, (meth) acrylic acid is even more preferable, and acrylic acid is particularly preferable in terms of water absorption characteristics. The proportion of acrylic acid is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, particularly preferably 90 to 100 mol% of all monomers. The acid group-containing radically polymerizable compound may be used alone or in a mixture of two or more.

  When the acid group-containing radically polymerizable compound is neutralized (in the form of a salt), the compound is preferably a monovalent salt selected from alkali metal salts, ammonium salts, and amine salts. Among these, an alkali metal salt is more preferable, and a salt selected from a sodium salt, a lithium salt, and a potassium salt is particularly preferable.

  The neutralization rate of the acid group-containing radically polymerizable compound is preferably 0 to 60 mol%. When the neutralization rate is in such a range, the reaction rate is improved in the polymerization reaction of the radical polymerizable compound in the polymerization step described later, and a water-absorbing resin having excellent water absorption characteristics can be obtained at a low temperature and in a short time. Conventionally, the neutralization rate was higher than the range of the present invention. This is because there was concern about the odor due to the decrease in water absorption ratio and the radical polymerizable compound remaining after the surface treatment. However, when an acid group-containing radical polymerizable compound having a neutralization rate lower than the neutralization rate of a radical polymerizable compound that has been used in the past is used contrary to conventional concerns, the surface treatment is unexpectedly performed rapidly. In addition, it has been found that the physical properties of the obtained water-absorbent resin are maintained. The neutralization rate of the acid group-containing radically polymerizable compound is more preferably 0 to 50 mol%, further preferably 0 to 40 mol%, particularly preferably 0 to 30 mol%, and still more preferably 0. It is -15 mol%, Most preferably, it is 0-10 mol%. In the above range, the conventional surface treatment time is several hours, whereas the surface treatment time is significantly reduced to several tens of minutes, preferably about several minutes. At the time of large-scale production of 1000 kg / hr (preferably during continuous production), a great economic effect is brought about.

  The neutralization rate of the acid group-containing radically polymerizable compound may be the same as or different from the neutralization rate of the water absorbent resin as the base polymer. It is preferably lower than the neutralization rate. Here, the relationship of the neutralization rate between the acid group-containing radically polymerizable compound and the water-absorbing resin is not particularly limited as long as the above relationship is satisfied. However, it is preferable that the neutralization rate of the acid group-containing radical polymerizable compound to be mixed is a numerical ratio of 0 to 80 mol% with respect to the neutralization rate of the water-absorbent resin as the base polymer, and 0 to 60 mol% Is more preferable, 0 to 40 mol% is further preferable, 0 to 20 mol% is particularly preferable, and 0 to 10 mol% is most particularly preferable. The “neutralization rate of the acid group-containing radical polymerizable compound” refers to the ratio of the neutralized acid group to the total acid groups of the acid group-containing radical polymerizable compound. “Total acid groups of acid group-containing radically polymerizable compound” and “neutralized acid groups” are those in each of the acid group-containing radically polymerizable compounds when there are two or more acid group-containing radically polymerizable compounds. Refers to the sum of acid groups and neutralized acid groups. For example, when acrylic acid and sodium acrylate are used in a molar ratio of 1: 1 as the acid group-containing radical polymerizable compound, the neutralization rate of the acid group-containing radical polymerizable compound is 50 mol%.

  A monofunctional radically polymerizable compound other than the acid group-containing radically polymerizable compound may be included. Such monofunctional radically polymerizable compounds include nonionic hydrophilic groups such as (meth) acrylamide, N-substituted (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, and 2-hydroxypropyl (meth) acrylate. Monomer; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide, Amino group-containing unsaturated monomers such as quaternized compounds thereof, and the like. The amount of the monofunctional radically polymerizable compound in such a case can be appropriately selected depending on the desired properties, but is preferably 0 to 100% by weight, more preferably 1 to 50% with respect to 100% by weight of the radically polymerizable compound. It is mass%, More preferably, it is 2-20 mass%, Most preferably, it is 3-10 mass%.

  Although it does not restrict | limit especially as a polyfunctional radically polymerizable compound, For example, the monomer which has two or more polymerizable unsaturated groups among the monomers illustrated as an internal crosslinking agent used by manufacture of a water absorbing resin. The body (crosslinkable unsaturated monomer) is preferably mentioned. Examples of the crosslinkable unsaturated monomer include polyethylene glycol diacrylate having an average ethylene oxide number of 2 to 50, trimethylolpropane tri (meth) acrylate, pentaerythritol triacrylate, 2,2,2-trisacryloyloxymethylethyl. Succinic acid, ethoxylated glycerol triacrylate, N, N′-methylenebis (meth) acrylamide, glycerol acrylate methacrylate, glycerol di (meth) acrylate, and the like are preferably used, and glycerol acrylate methacrylate and glycerol di (meth) acrylate are more preferably used. Is done. In order for the polyfunctional radical polymerizable compound to efficiently polymerize with the monofunctional radical polymerizable compound on the surface of the water absorbent resin, the water absorption of the polyfunctional radical polymerizable compound and the monofunctional radical polymerizable compound in the mixing step with the water absorbent resin. It is desirable that the diffusion behavior into the functional resin is similar. For this purpose, it is desirable that the polyfunctional radical polymerizable compound is similar to the monofunctional radical polymerizable compound used in terms of molecular weight and hydrophilicity. The amount of the crosslinkable unsaturated monomer to be used can be appropriately selected depending on the desired properties, but is preferably 0 to 20% by mass, more preferably 0.1 to 10% by mass with respect to 100% by mass of the radical polymerizable compound. %, Most preferably 0.5 to 5% by mass.

  In addition, the molar ratio of the usage-amount of the monofunctional radically polymerizable compound used in such a form and a polyfunctional radically polymerizable compound is the ethylenically unsaturated monomer and internal crosslinking agent at the time of manufacture of a water absorbing resin. The molar ratio may be the same or different. Preferably, a larger amount of polyfunctional radically polymerizable compound is used compared to the composition at the time of manufacturing the water absorbent resin. The amount of the polyfunctional radical polymerizable compound used is preferably 0.001 to 100 mol%, more preferably 0.01 to 50 mol%, still more preferably 0, based on the total amount of the monofunctional radical polymerizable compound. 0.05 to 30 mol%, particularly preferably 0.1 to 20 mol%, and most preferably 0.2 to 10 mol%. Such a configuration is advantageous in that the crosslink density on the surface of the water absorbent resin is efficiently improved.

  The amount of the radical polymerizable compound mixed with the water absorbent resin in the mixing step is preferably 0.1 to 50 parts by weight with respect to 100 parts by weight of the water absorbent resin (converted to a solid content of 100% by mass). More preferably, it is 0.5-30 weight part, More preferably, it is 1-20 weight part, Especially preferably, it is 2-15 weight part, Most preferably, it is 3-10 weight part. When the amount of the radical polymerizable compound is 0.1% by mass or more, the absorption performance under pressure of the water absorbent resin can be maintained at a sufficient value. On the other hand, when the amount of the radical polymerizable compound is 50% by mass or less, a decrease in the absorption capacity of the surface-treated water-absorbing resin can be prevented.

  The amount of water mixed with the water absorbent resin in the mixing step is preferably 1 to 50 parts by weight, more preferably 2 to 40 parts by weight with respect to 100 parts by weight of the water absorbent resin (converted to a solid content of 100% by mass). Parts, more preferably 3 to 30 parts by weight, particularly preferably 4 to 20 parts by weight, and most preferably 5 to 10 parts by weight. When water is mixed in such a range, the speed of the polymerization reaction in the polymerization step described later is increased, and a large amount of energy is not required in the heating step after the polymerization step, and the water-absorbing resin is less likely to be decomposed. preferable.

  The water-absorbent resin composition containing the water-absorbent resin and the treatment liquid containing the radical polymerizable compound is not limited to water and / or a hydrophilic solvent, as long as the effect of the present invention is not hindered. A component (for example, a radical polymerization initiator or a mixing aid described later) may be included.

(Radical polymerization initiator)
In the mixing step of the water absorbent resin and the radical polymerizable compound, a radical polymerization initiator is preferably added. Although it does not specifically limit as a radical polymerization initiator, Specifically, a thermally decomposable radical polymerization initiator and a photoinitiator are mentioned.

  The thermally decomposable radical polymerization initiator 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, more preferably 20 ° C. or higher and 100 ° C. or lower, Considering the temperature conditions for irradiating the active energy ray, those of 40 ° C. or more and 80 ° C. or less are particularly preferable. If the 10-hour half-life temperature is less than 0 ° C. (lower limit), it is unstable during storage.

  The thermal decomposable radical polymerization initiator is relatively inexpensive as compared with a compound marketed as a photopolymerization initiator, and strict light shielding is not always 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; hydrogen peroxide; 2,2′-azobis (2-amidinopropane) dihydrochloride, 2 , 2′-azobis [2-2 (-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis (2-methylpropionitrile) and the like. 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 load, liquid permeability, and free swelling capacity. Persulfate can be used in combination of not only one type but also two or more types having different counterions.

  In this embodiment, photopolymerization initiators such as oil-soluble benzoin derivatives, benzyl derivatives, acetophenone derivatives, oil-soluble ketone peroxides, peroxyketals, hydroperoxides, dialkyl peroxides, peroxyesters, peroxyesters. Oil-soluble organic peroxides such as carbonates may be used. Such a photopolymerization initiator may be a commercially available product, trade names of Ciba Specialty Chemicals: Irgacure (registered trademark) 184 (hydroxycyclohexyl-phenyl ketone), Irgacure (registered trademark) 2959 (1- [4- (2-hydroxyethoxy)) -Phenyl] -2-hydroxy-2-methyl-1-propan-1-one) and the like. The photopolymerization initiator used is preferably 0.001 to 1 part by weight, more preferably 0.005 to 0.5 part by weight with respect to 100 parts by weight of the water absorbent resin. More preferably, it is 01-0.1 weight part.

  As the radical polymerization initiator, either oil-soluble or water-soluble can be used. Oil-soluble radical polymerization initiators are characterized in that the decomposition rate is less affected by pH and ionic strength than water-soluble radical polymerization initiators. However, since the water-absorbent resin is hydrophilic, it is more preferable to use a water-soluble radical polymerization initiator in consideration of permeability to the water-absorbent resin. In addition, water-soluble means what melt | dissolves 1 mass% or more in water (25 degreeC), Preferably it is 5 mass% or more, More preferably, it melt | dissolves 10 mass% or more.

  The water-soluble radical polymerization initiator preferably includes a radical polymerization initiator selected from a persulfate, hydrogen peroxide, and an azo compound. Specifically, persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate; hydrogen peroxide; 2,2′-azobis-2-amidinopropane dihydrochloride, 2,2′-azobis [2-2 And water-soluble azo compounds such as (-imidazolin-2-yl) propane] dihydrochloride. Among these, the use of persulfate is particularly preferable in that the absorption capacity under pressure, liquid permeability, and free swelling ratio of the water-absorbent resin after the surface treatment are all excellent.

  In addition to or in place of the above radical polymerization initiator, a percarbonate such as sodium percarbonate; a peracetate such as peracetic acid or sodium peracetate may be further used. .

  Although there is no restriction | limiting in the usage-amount of a radical polymerization initiator, In this invention, the range of 0.01-20 weight part is preferable with respect to 100 weight part of water absorbing resin, More preferably, it is the range of 0.05-10 weight part. More preferably, it is in the range of 0.1 to 5 parts by weight. If the radical polymerization initiator is in such a range, a water-absorbing resin having excellent water absorption characteristics can be obtained, and the surface treatment reaction rate can be improved, so that productivity is excellent. Moreover, the radical polymerization initiator may be used alone or in combination of two or more.

  In the present embodiment, if necessary, other radical polymerization initiators such as a thermally decomposable radical polymerization initiator and a photopolymerization initiator are used in combination, and the use amount of the latter is 0 to 20 parts by weight with respect to 100 parts by weight of the water absorbent resin. , Preferably 0 to 15 parts by weight, particularly 0 to 10 parts by weight, and the use ratio thereof is less than the thermally decomposable radical polymerization initiator, for example, 1 of the weight ratio of the thermally decomposable radical polymerization initiator. / 2 or less, further 1/10 or less, particularly 1/50 or less.

(Mixing conditions)
The mixing conditions for mixing the above-described components (water-absorbing resin, radical polymerizable compound, water, and if necessary, radical polymerization initiator, mixing aid) are not particularly limited. For example, a method of spraying or dropping and mixing a treatment liquid containing a radical polymerizable compound and water (a radical polymerization initiator and a mixing aid as necessary) onto a water absorbent resin is preferable, and a method of spraying is more preferable. Since the water-absorbing resin has water-absorbing properties, it is possible to uniformly disperse the radical polymerizable compound and the radical initiator on the surface of the water-absorbing resin and to mix it with the water-absorbing resin by mixing in such a form. it can. In the case of the spraying method, the size of the droplets to be sprayed is preferably in the range of 0.1 to 300 μm, more preferably in the range of 0.1 to 200 μm, in terms of average particle diameter.

  The mixing temperature in the mixing step is preferably 0 ° C. or higher and lower than 120 ° C., 10 to 100 ° C., 20 to 80 ° C., or 30 to 70 ° C. in this order. At this time, if the mixing temperature is 0 ° C. or higher, problems due to water condensation can be suppressed and stable operation can be achieved. On the other hand, if it is less than 120 degreeC, a water absorbing resin and a surface treating agent can be mixed uniformly. 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, it is preferable to suppress excessive leakage of water vapor by sealing the mixing / irradiation system. Further, the temperature of the water-absorbing resin and water before the mixing step is also not particularly limited. For example, the temperature of the water-absorbing resin before the mixing step is 0 ° C or higher and lower than 120 ° C, 10 to 100 ° C, 20 to 80 ° C, It is preferable in order of 30-70 degreeC. If it is 0 degreeC or more, the problem by the dew condensation of water can be suppressed and it can drive | work stably. On the other hand, if it is less than 120 degreeC, water and a water absorbing resin can be mixed favorably, without evaporating excessively. The mixing time is not limited as long as uniform mixing is realized. 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 it is 0.1 second or longer, the water-absorbing resin, water, the radical polymerizable compound, and the like can be mixed uniformly. On the contrary, if the mixing time is 60 minutes or less, excessive water penetration into the water-absorbent resin can be prevented, and the surface treatment can proceed well.

  Moreover, it is preferable that the mixing means has a large mixing force in order to uniformly and reliably mix the components contained in the water absorbent resin composition. Examples of such a mixing apparatus include a cylindrical mixer, a double wall conical mixer, a high-speed stirring mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a double-arm kneader, and a pulverizer. A mold kneader, a rotary mixer, an airflow mixer, a turbulator, a batch-type Redige mixer, a continuous-type Redige mixer and the like are suitable. Among these, a continuous-type readyge mixer and a turbulizer are preferable because they can be stably continuously operated without being blocked.

(A-2) Polymerization step In this embodiment, in the polymerization step of the radical polymerizable compound, the radical polymerizable compound added to the water absorbent resin is polymerized on or near the surface of the water absorbent resin. Here, “polymerization” includes all physical or chemical actions performed by a radically polymerizable compound on a water-absorbent resin.

  In the present embodiment, the water absorbent resin is surface-treated in the polymerization step. Therefore, the “surface treatment temperature” in this embodiment means a temperature in the polymerization step. For example, when the polymerization step is performed by irradiation with active energy rays and / or heating described later, the temperature at the time of irradiation and / or heating of the active energy rays becomes the “surface treatment temperature”.

  In the present embodiment, the step of polymerizing the radically polymerizable compound is preferably performed in the absence of a hydrophobic organic solvent. If a hydrophobic organic solvent is present in the polymerization step of the radical polymerizable compound, a part of the treatment liquid is not absorbed by the water-absorbent resin and is dispersed in the hydrophobic organic solvent, which makes it difficult for the surface treatment to proceed. is there.

  The method for polymerizing the radical polymerizable compound to the water absorbent resin is not particularly limited. Preferably, in the polymerization step of the radical polymerizable compound, the mixture obtained in the mixing step (water absorbent resin composition) is irradiated with active energy rays and / or the mixture obtained in the mixing step (water absorbing property). A step of heating the resin composition). Specifically, the radically polymerizable compound mixed with the water absorbent resin is preferably polymerized on the surface of the water absorbent resin and / or in the vicinity thereof using radical generating means such as irradiation with active energy rays and / or heating.

By the polymerization, the vicinity of the surface of the water-absorbent resin is modified, and the properties desired during actual use of the water-absorbent resin such as absorption capacity under load and liquid permeability are enhanced. Hereinafter, the step of irradiating the mixture (water absorbent resin composition) with active energy rays and the step of heating the mixture will be described. However, this embodiment is not limited to the following form.
(A-2-1) Irradiation of active energy rays In this embodiment, the surface treatment of the water-absorbent resin can be performed by the presence of the radical polymerizable compound and the irradiation of the active energy rays, and the water-absorbing property is excellent in water-absorbing characteristics. A resin can be obtained. It is considered that the surface of the water absorbent resin is modified by such surface treatment.

  The active energy ray may be irradiated while mixing the water-absorbing resin, water, the radical polymerizable compound and, if necessary, the radical polymerization initiator, or may be irradiated after mixing two or more of these. However, from the viewpoint that uniform surface treatment can be performed, preferably, after obtaining a water absorbent resin composition containing a water absorbent resin, water, a radical polymerizable compound and, if necessary, a radical polymerization initiator, the water absorbent obtained. The resin composition is irradiated with active energy rays.

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 ray 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. When using ultraviolet rays, the irradiation conditions are preferably an irradiation intensity of 3 to 1000 mW / cm ² and an irradiation amount of 0.1 to 100 J / 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. The time for irradiating the active energy ray depends on the amount of the water-absorbing resin to be treated, but in the case of the method described in the examples described later, it may be preferably 0.1 minutes or more and less than 60 minutes, more preferably 0.8. It is 5 minutes or more and 40 minutes or less, more preferably 0.5 minutes or more and 30 minutes or less, and particularly preferably 1 minute or more and 20 minutes or less. When using a conventional surface cross-linking agent, the surface treatment time can be shortened when compared at the same cross-linking density, such as exceeding 60 minutes.

  When surface treatment is performed by irradiating active energy rays, it is not necessary to heat the surface. However, the irradiation with the active energy ray can also be performed under heating. As a result, a water-absorbing resin having excellent water absorption characteristics can be obtained. The heating temperature is preferably 0 ° C. or higher and lower than 120 ° C., more preferably 50 ° C. or higher and lower than 120 ° C., further preferably 50 to 100 ° C., particularly preferably 50 to 95 ° C., and most preferably 50 to 90 ° C. . In addition, radiant heat may be generated when active energy rays are irradiated. In this case, irradiation with active energy rays is performed under heating. In this embodiment, since the surface treatment is performed by irradiating active energy rays, the heating is auxiliary, and therefore the treatment temperature can be set lower than the conventional surface treatment temperature. In addition, as a heating method, a method of introducing a heated gas into the active energy ray irradiation device, a method of heating the periphery of the active energy ray irradiation device with a jacket, etc., by radiant heat when irradiating the active energy ray Examples of the heating method include a method of irradiating a preheated water-absorbing resin with an active energy irradiation beam.

  It is preferable to stir the water-absorbent resin during irradiation with active energy rays. The active energy ray can be uniformly irradiated to the mixture (water absorbent resin composition) of the radical polymerization initiator and the water absorbent resin by stirring. Preferably, in order to achieve uniform stirring, stirring is performed under a condition of 5 to 1000 rpm. The devices that can stir the water-absorbent resin during irradiation with active energy rays include vibration mixers, vibration feeders, ribbon mixers, conical ribbon mixers, screw mixers, air flow mixers, and batches. Examples thereof include a kneader, a continuous kneader, a paddle type mixer, a high-speed flow type mixer, a floating flow type mixer, a batch type redige mixer, and a continuous type redige 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 flow the water-absorbent resin composition, the pressure of a gas such as air may be used so as to be used for pneumatic transportation of powder. 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.

  In general, it is known that a reaction using a radical as an active species is inhibited by oxygen. However, in the manufacturing method of the present embodiment, the physical properties of the surface-treated water-absorbing resin were not lowered even when oxygen was present in the system. For this reason, it is not essential to make the atmosphere inert when irradiating active energy rays.

(A-2-2) Heating As described above, the radically polymerizable compound to be mixed with the water absorbent resin can be polymerized by heating. In the case where the polymerization is carried out by heating alone, it is not necessary to separately provide an active energy ray irradiation device, which is excellent in the design of the manufacturing apparatus. In addition, it is possible to improve the water absorption characteristics (particularly, the absorption capacity / liquid permeability under pressure) of the surface-treated water-absorbing resin obtained by a low-cost and safe technique.

  Specifically, first, in the mixing step, the water-absorbing resin is mixed with an aqueous solution (treatment liquid) containing a radical polymerizable compound, water, and, if necessary, a radical polymerization initiator, to obtain a water-absorbing material that is a mixture. A resin composition is obtained. Then, the water absorbent resin composition is subjected to heat treatment. Thereby, it is estimated that the surface of the water-absorbent resin is modified. However, the technical scope of the present invention is not limited only to such a form, and these may be added to the water-absorbent resin (base polymer) while performing heat treatment. There is no restriction | limiting in particular in the addition order of the component to comprise, and the timing of addition of each component, and heat processing. Moreover, what is necessary is just to prepare a water absorbing resin composition on the conditions similar to the time of irradiation of the active energy ray mentioned above, and a radical polymerization initiator is not an essential component.

  The temperature of the heat treatment is preferably 50 ° C. or higher and lower than 120 ° C., more preferably 50 to 100 ° C., further preferably 60 to 100 ° C., and particularly preferably 70 to 100 ° C. If the temperature of the heat treatment is 50 ° C. or higher, the surface treatment proceeds efficiently. On the other hand, when the temperature of the heat treatment is 100 ° C. or lower, the water absorbent resin can be prevented from being deteriorated by heat. By heating the water-absorbing resin in this way, it is possible to produce a surface-treated water-absorbing resin having excellent water absorption characteristics (particularly absorption capacity under load and liquid permeability) by a low-cost and safe technique. It becomes possible.

  There are no particular restrictions on the specific conditions of the atmosphere during the heat treatment, but it is preferable to heat in an atmosphere of relatively high humidity. Specifically, it is preferable to perform heating in saturated steam and / or superheated steam. The atmosphere in the case of heating at a temperature of 100 ° C. or higher is preferably filled with superheated steam, and it is more preferable to directly heat the water absorbent resin using superheated steam. Further, the pressure of the atmosphere during the heat treatment may be any of reduced pressure, normal pressure, and pressurized, and is not particularly limited, but is preferably 1013 to 43030 hPa, more preferably 1013 to 14784 hPa, and further preferably 1013 to 1313. 10498 hPa, particularly preferably 1013 to 4906 hPa. Furthermore, the relative humidity of the atmosphere is preferably 50 to 100% RH, more preferably 70 to 100% RH, still more preferably 90 to 100% RH, and particularly preferably 95 to 100% RH. Most preferably, it is 100% RH (saturated steam). The oxygen concentration in the atmosphere during the heat treatment is preferably 0 to 25% by volume, more preferably 0 to 15% by volume, still more preferably 0 to 10% by volume, and even more preferably 0. -5% by volume, particularly preferably 0-1% by volume, most preferably 0-0.5% by volume. Thus, when the oxygen concentration in the atmosphere is adjusted to a relatively low concentration, it is preferable because the oxidative deterioration of the water absorbent resin during heating can be prevented.

  The heating time for performing the heat treatment is not particularly limited, but is preferably 1 to 90 minutes, more preferably 2 to 60 minutes, further preferably 3 to 40 minutes, and particularly preferably 5 to 30 minutes. . If the heating time is 1 minute or longer, the surface of the water-absorbent resin is modified. On the other hand, if the heating time is 90 minutes or shorter, deterioration of the water-absorbent resin due to heating can be prevented.

  There is no particular limitation on the apparatus used for the heat treatment when the surface treatment of the water-absorbent resin is performed by heat treatment, and a known dryer can be used. For example, a conduction heat transfer type, a radiation heat transfer type, a hot air heat transfer type, a dielectric heating type dryer is preferably used, and specifically, a belt type, a groove type stirring type, a fluidized bed type, an air flow type, a rotary type. A kneading type, infrared type, or electron beam type dryer can be preferably used.

  During the heat treatment, it is preferable to stir the water absorbent resin. Heat can be uniformly applied to the mixture of the radical polymerization initiator and the water absorbent resin by stirring. Preferably, in order to achieve uniform stirring, stirring is performed under a condition of 5 to 1000 rpm. The devices that can stir the water-absorbent resin during the heat treatment irradiation include a vibratory mixer, a vibratory feeder, a ribbon mixer, a conical ribbon mixer, a screw mixer, an airflow mixer, and a batch type. Examples thereof include a kneader, a continuous kneader, a paddle type mixer, a high-speed fluidized mixer, a floating fluidized mixer, a batch-type Redige mixer, and a continuous Redige mixer.

  In addition, when performing the said surface treatment process, it is preferable to suppress an excessive leak of water vapor | steam by sealing a mixing / surface treatment system. According to such a form, the water-absorbent resin is surface-treated in a state where the water content is kept constant by a simple method. Specifically, mixing and / or surface treatment may be performed in a sealed container such as a plastic bag.

  By the surface treatment step, the surface of the water absorbent resin can be modified. The surface treatment process may be performed once or a plurality of times.

(B) Surface treatment using a surface crosslinking agent In another embodiment of the present invention, the surface treatment step is performed in the presence of a surface crosslinking agent. In the surface treatment step, when a surface cross-linking agent is present, the functional group on the resin surface and the surface cross-linking agent are chemically bonded to each other, whereby a stable surface cross-linked structure can be introduced to 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. As a result, a water absorbent resin excellent in water absorption characteristics can be produced.

  The surface cross-linking using the surface cross-linking agent can be performed by various methods, but is usually obtained by mixing the water-absorbing resin obtained in the polymerization step and the surface cross-linking agent (mixing step) and mixing. And a step of heating the water absorbent resin composition (heating step). Such a form has an advantage that a crosslinked structure can be efficiently introduced into the surface of the water-absorbent resin in a shorter time. Hereinafter, the mixing step and the heating step in the surface treatment step of the present embodiment will be described in detail. The technical scope of the present invention is not limited to the following forms, and there is no particular limitation on the order of addition of each component present in the reaction system and the timing of addition of each component and heat treatment. For example, you may add each component, such as a surface crosslinking agent, to a water absorbing resin, performing heat processing.

(B-1) Mixing step As a method of mixing the water-absorbing resin and the surface cross-linking agent, the surface cross-linking agent may be directly mixed with the water-absorbing resin, but the surface cross-linking agent may be water and / or a hydrophilic organic solvent. The form which mixes with a water absorbing resin in the state of the aqueous solution (processing liquid) dissolved in is preferably employ | adopted. As described in detail below, the heating step after the surface treatment step is preferably performed in the absence of a hydrophobic organic solvent. For this reason, it is preferable to use the surface cross-linking agent in the form of an aqueous solution in the absence of the hydrophobic organic solvent also in this step. Since the water-absorbing resin has water-absorbing properties, by mixing the surface cross-linking agent in the form of an aqueous solution, the surface cross-linking agent can be uniformly dispersed on the surface of the water-absorbing resin and can be uniformly mixed with the water-absorbing resin.

(Surface cross-linking agent)
Examples of the surface cross-linking agent include various organic cross-linking agents or inorganic cross-linking agents, but a cross-linking agent capable of reacting with a carboxyl group is preferable from the viewpoint of physical properties and handling properties. Specifically, from polyhydric alcohol compounds, epoxy compounds, polyvalent amine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, mono-, di- or polyoxazolidinone compounds, polyvalent metal salts, alkylene carbonate compounds, and oxetane compounds. It is preferably at least one selected from the group consisting of: an epoxy compound, a polyvalent amine compound, a mono-, di- or polyoxazolidinone compound.

  Specific examples include compounds exemplified in US Pat. No. 6,228,930, US Pat. No. 6,071,976, US Pat. No. 6,254,990, and the like. For example, mono, di, tri, tetra or polyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropylene glycol, 2,3,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin , 2-butene-1,4-diol, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, etc. Alcohol compounds; Epoxy compounds such as ethylene glycol diglycidyl ether and glycidol; Multivalents such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, and polyamidepolyamine Haloepoxy compounds such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin; condensates of the polyvalent amine compounds and the haloepoxy compounds; oxazolidinone compounds such as 2-oxazolidinone; ethylene carbonate, etc. Alkylene carbonate compounds; oxetane compounds such as mono-, di- or polyoxetanes; cyclic urea compounds such as 2-imidazolidinone, and the like, but are not particularly limited. These surface cross-linking agents may be used alone or in combination.

  The amount of the surface cross-linking agent used is preferably in the range of 0.001 to 10 parts by weight with respect to 100 parts by weight (mass part) of the water-absorbing resin, although it depends on the compounds used and combinations thereof. More preferably within the range of 0.01 parts by weight to 5 parts by weight.

  The amount of water used is preferably in the range of 0.5 to 20 parts by weight, more preferably 0.5 to 10 parts by weight with respect to 100 parts by weight of the water absorbent resin. Moreover, the usage-amount of a hydrophilic organic solvent is 0-10 weight part with respect to 100 weight part of water absorbing resin particles, Preferably it is the range of 0-5 weight part. As the hydrophilic organic solvent, a hydrophilic organic solvent that can be used as a mixing aid described later can be preferably used.

  The water-absorbent resin composition containing the water-absorbent resin and the treatment liquid containing the surface cross-linking agent is not limited to water and / or a hydrophilic solvent, as long as the effect of the present invention is not hindered. (For example, a mixing aid described later) may be included.

(Mixing conditions)
The mixing conditions for mixing the above-described components (water-absorbing resin, surface cross-linking agent, and, if necessary, water, hydrophilic organic solvent, and mixing aid) are not particularly limited. For example, it is preferable to prepare a solution (treatment liquid) in which the surface cross-linking agent is preliminarily mixed with water and / or a hydrophilic organic solvent, and spray or drop-mix the treatment liquid onto the water-absorbent resin, more preferably spraying. . In the case of the spraying method, the size of the droplets to be sprayed is preferably in the range of 0.1 to 300 μm, more preferably in the range of 0.1 to 200 μm, in terms of average particle diameter.

  The mixing temperature in the mixing step is preferably 0 ° C. or higher and lower than 120 ° C., 10 to 100 ° C., 20 to 80 ° C., or 30 to 70 ° C. in this order. At this time, if the mixing temperature is 0 ° C. or higher, problems due to water condensation can be suppressed and stable operation can be achieved. On the other hand, if it is less than 120 degreeC, a water absorbing resin and a surface treating agent can be mixed uniformly. The mixing time is not limited as long as uniform mixing is realized. 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 it is 0.1 second or longer, the water-absorbing resin, water, the radical polymerizable compound, and the like can be mixed uniformly. On the contrary, if the mixing time is 60 minutes or less, excessive water penetration into the water-absorbent resin can be prevented, and the surface treatment can proceed well.

  Moreover, it is preferable that the mixing means has a large mixing force in order to uniformly and reliably mix the components contained in the water absorbent resin composition. Examples of such a mixing apparatus include a cylindrical mixer, a double wall conical mixer, a high-speed stirring mixer, a V-shaped mixer, a ribbon mixer, a screw mixer, a double-arm kneader, and a pulverizer. A mold kneader, a rotary mixer, an airflow mixer, a turbulator, a batch-type Redige mixer, a continuous-type Redige mixer and the like are suitable. Among these, from the standpoints of mixing properties and throughput, continuous-type Redige mixers and Shugi mixers (high-speed stirring type mixers) are preferable.

(B-2) Heating process The water-absorbent resin composition containing each component described above is preferably heat-treated, and if necessary, cooled after that. By heating the water-absorbent resin composition in this way, the surface treatment of the water-absorbent resin is surface-crosslinked by a low-cost and safe method and has excellent water-absorbing properties (particularly absorption capacity under load and liquid permeability). It is possible to produce a water absorbent resin.

  In the present embodiment, the surface cross-linking reaction of the water absorbent resin proceeds in the heating step. Therefore, in the present embodiment, the “surface treatment temperature” means a temperature in the heating process.

  The heating temperature is preferably 50 ° C. or higher and lower than 120 ° C., more preferably 50 to 100 ° C., further preferably 60 ° C. to 100 ° C., and particularly preferably 70 to 100 ° C. If it is 50 degreeC or more, surface treatment can fully be advanced in a short time. On the other hand, if it is less than 120 degreeC, the thermal deterioration of a water absorbing resin can be suppressed. The heating time for the heat treatment is not particularly limited, but is preferably 1 to 90 minutes, more preferably 2 to 60 minutes, further preferably 3 to 60 minutes, and particularly preferably 5 to 30 minutes. . If the heating time is 1 minute or longer, a crosslinked structure is introduced on the surface of the water absorbent resin. On the other hand, if the heating time is 90 minutes or shorter, deterioration of the water absorbent resin due to heating can be prevented.

  Moreover, in the surface treatment process of this embodiment, in addition to the heat treatment described above, a treatment of irradiating active energy rays such as radiation, electron beams, ultraviolet rays, and electromagnetic rays may be used in combination.

  There is no particular limitation on the apparatus used for the heat treatment when the surface treatment of the water-absorbent resin is performed by heat treatment, and a known dryer can be used. For example, a conduction heat transfer type, a radiation heat transfer type, a hot air heat transfer type, a dielectric heating type dryer is preferably used, and specifically, a belt type, a groove type stirring type, a fluidized bed type, an air flow type, a rotary type. A kneading type, infrared type, or electron beam type dryer can be preferably used.

  In addition, when performing the said surface treatment process, it is preferable to suppress an excessive leak of water vapor | steam by sealing a mixing / surface treatment system. According to such a form, the water-absorbent resin is surface-treated in a state where the water content is kept constant by a simple method. Specifically, mixing and / or surface treatment may be performed in a sealed container such as a plastic bag.

  By the surface treatment step, a crosslinked structure can be introduced on the surface of the water absorbent resin. The surface treatment process may be performed once or a plurality of times.

(C) Mixing aid In the surface treatment step, a surface treatment agent (surface crosslinking agent or radical polymerizable compound) is present in the reaction system of the surface treatment together with the water-absorbent resin. For the purpose of improving, it is preferable that a mixing aid is further present in the reaction system. “Mixing aid” is a water-soluble or water-dispersible compound other than the radical polymerization initiator and the radical polymerizable compound, which suppresses aggregation of the water-absorbing resin due to water, and is capable of mixing the aqueous solution and the water-absorbing resin. If it can improve, it will not be restrict | limited. However, water is not included as a mixing aid. By adding the mixing aid, aggregation of the water absorbent resin with water is suppressed, and the aqueous solution and the water absorbent resin are uniformly mixed. As a result, the surface treatment such as the polymerization treatment of the radical polymerizable compound described later is performed uniformly on the water absorbent resin, and the entire water absorbent resin can be uniformly surface treated.

  Specific examples of the mixing aid include surfactants, water-soluble polymers, hydrophilic organic solvents, water-soluble inorganic compounds, inorganic acids (salts), and organic acids (salts). In the present specification, a water-soluble compound means a compound having a solubility in 100 g of water at room temperature of 1 g or more, preferably 10 g or more. By adding a mixing aid other than water, the water-absorbing resin is prevented from agglomerating with water and the aqueous solution and the water-absorbing resin can be mixed uniformly. Furthermore, the active energy rays can be evenly and evenly applied to the water absorbent resin, and the entire water absorbent resin can be uniformly surface treated.

  Examples of the surfactant include one or more surfactants selected from the group consisting of a nonionic surfactant having an HLB of 7 or more or an anionic surfactant. 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, dialkyl sulfosuccinates and the like. Among these, polyoxyethylene alkyl ether can be preferably used. 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 of the polyoxyethylene alkyl ether used as the mixing aid is 200 or more, the effect as the mixing aid can be effectively obtained. On the other hand, if the number average molecular weight is 100,000 or less, the solubility in water is sufficiently ensured, and the increase in the viscosity of the solution as the reaction system is suppressed, and the reaction system including the water-absorbing resin is mixed. Sex can be secured sufficiently.

  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. The number average molecular weight of these water-soluble polymers 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.

  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 and the like can be mentioned.

  Examples of inorganic acids (salts) include hydrochloric acid, sulfuric acid, phosphoric acid, carbonic acid, boric acid, and salts thereof (for example, alkali metal salts and alkaline earth metal salts). Examples of the organic acid (salt) 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).

  Of the above examples, polyoxyethylene alkyl ether (especially polyethylene glycol monomethyl ether), polyethylene glycol, water-soluble polyvalent metal salt, sodium chloride, ammonium hydrogen sulfate, ammonium sulfate, sulfuric acid and hydrochloric acid 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 the mixing aid added is not particularly limited as long as it can suppress aggregation of the water-absorbing resin with water and can improve the mixing property between the surface treatment agent and the water-absorbing resin. Preferably it is 0.0001-40 mass% with respect to quantity (100 mass% in conversion of solid content), More preferably, it is 0.001-10 mass%, Most preferably, it is 0.005-5 mass%. Yes, most preferably 0.01 to 1% by mass.

  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. The order of addition of the mixing aid is also not particularly limited, and after adding the mixing aid to the water-absorbent resin in advance, a method of adding and mixing the treatment liquid containing the surface treatment agent thereto, and the mixing aid as the treatment liquid It is also possible to use a method such as a method of dissolving in water and mixing at the same time as the water absorbent resin.

(D) Surface-treated water-absorbing resin By performing the surface treatment step on the water-absorbing resin, the surface-treated water-absorbing resin can be produced, and the water-absorbing property of the obtained water-absorbing resin is improved.

(Moisture content)
The water content of the water-absorbent resin after the surface treatment step is preferably 1 to 60% by mass, more preferably 1 to 50% by mass, still more preferably 3 to 40% by mass, and particularly preferably 5 to 5% by mass. 30% by mass, most preferably 7-20% by mass. Further, the amount of increase in moisture content before and after the surface treatment step [= (water content after the surface treatment step) − (water content before the surface treatment step)] is not particularly limited, but is preferably -20 to 40% by mass. More preferably, it is -10-30 mass%, More preferably, it is -5-20 mass%, Most preferably, it is -3-10 mass%. If the water content of the surface-treated water-absorbent resin and the amount of increase in water content are equal to or higher than the lower limit of the above range, the effects of the present invention are fully exerted, and water absorption performance such as absorption capacity under load and liquid permeability A surface-treated water-absorbing resin excellent in the above can be obtained. On the other hand, if these values are less than or equal to the upper limit of the above range, a decrease in water absorption characteristics accompanying an increase in moisture content can be prevented.

(Particle size distribution)
The shape of the water-absorbent resin obtained by the surface treatment step can be appropriately adjusted depending on the treatment conditions such as the shape of the water-absorbent resin before treatment, granulation / molding after treatment, etc., but is generally in the form of powder. Such powder has a weight average particle diameter (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 logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.23 to 0.45, and more preferably 0.25 to 0.35. According to such a form, since the outstanding water absorption characteristic is exhibited when it uses in a diaper, it is preferable.

  In the surface treatment process by polymerization of the radically polymerizable compound of the present invention, there is an effect of granulating fine powder generated during production of the water absorbent resin. For this reason, even if fine powder is contained in the water absorbent resin before surface treatment, the amount of fine powder contained in the surface treated water absorbent resin obtained can be reduced. The particle size distribution of the surface-treated water-absorbing resin obtained shifts to a higher particle size side as compared with the water-absorbing resin before the surface treatment. However, the shift ratio varies depending on the type and amount of the radical polymerization initiator, and when these are used as an aqueous solution, the ratio of water, the irradiation conditions of the active energy rays, the flow at the time of irradiation, and the like.

(Free swelling ratio GV)
The free swelling ratio (GV) (after moisture content correction) after the surface treatment step is preferably 5 to 1000 (g / g), more preferably 10 to 500 (g / g), and still more preferably 15 -100 (g / g), particularly preferably 18-70 (g / g), most preferably 20-50 (g / g).

(Saline flow inductive SFC)
The saline flow conductivity (SFC) after the surface treatment step is preferably 10 to 500 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ), more preferably 20 to 400 (unit: 10 ^{−). 7} · cm ³ · s · g ⁻¹ ), more preferably 30 to 300 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ), and particularly preferably 50 to 200 (unit: 10 ^{−). 7} · cm ³ · s · g ⁻¹ ). If it is in such a range, sufficient liquid permeability is ensured.

(Absorption capacity under pressure AAP)
Absorption capacity under pressure (AAP) after the surface treatment step (after moisture content correction) is preferably 10 g / g or more, more preferably 15 g / g or more, still more preferably 18 g / g or more, and particularly preferably 20 g / g or more. Most preferably, it is 22 g / g or more. The upper limit is not particularly limited, but is preferably 50 g / g or less.

  If the characteristics (particle size distribution, free swelling ratio, saline flow conductivity, absorption capacity under pressure) of the surface-treated water-absorbent resin are included in the above ranges, the effects of the present invention are sufficiently exerted and added. A surface-treated water-absorbing resin excellent in water-absorbing performance such as the rolling absorption capacity and liquid permeability can be obtained.

  By the surface treatment step, in the water-absorbent resin, a surface cross-link density near the surface is higher than the inside, preferably a uniform and high cross-link density is formed over the entire water-absorbent resin surface, and the resulting water-absorbent resin is excellent. However, by performing the heating process (blank process), which will be described later, after the surface treatment process, the characteristics (especially free swelling ratio and saline flow conductivity) desired for the water-absorbent resin are dramatically improved. Can be made.

(3) Heating process (empty baking process)
The heating step is a step of heating the water absorbent resin obtained in the surface treatment step. In the present invention, focusing on the “effect of heating after surface treatment on the water absorption properties of the water absorbent resin”, which has not been noted in the past, heating the water absorbent resin under predetermined conditions after surface treatment such as surface crosslinking Thus, it has been found that a water-absorbing resin having improved water absorption characteristics (particularly, free swelling ratio and liquid permeability) can be obtained. Hereinafter, the heating process after the surface treatment is also referred to as “empty baking process” in order to distinguish it from the heating process in the surface treatment process. In this step, the water-absorbent resin that has been surface-treated is heated and dried by heating the water-absorbent resin obtained in the surface treatment step.

  In the present invention, the baking step is performed without adding a surface treatment agent. That is, the empty baking step is performed in an environment where the radical polymerizable compound and the surface cross-linking agent are substantially absent. For this reason, substantial surface cross-linking reaction does not proceed in the baking step. Therefore, in the present invention, the empty baking process is the irradiation and heating of the active energy ray in the surface treatment process described above, the heating process in the presence of the surface cross-linking agent, the temperature adjustment process in the conventional surface cross-linking (temperature rising process, etc.) Clearly distinguished. However, a trace amount of a surface treatment agent such as a surface crosslinking agent or a monomer may remain in the reaction system after the surface treatment step, but it is preferable that the reaction system does not contain an amount that affects the water absorption characteristics of the water absorbent resin. Specifically, “the surface treatment agent is substantially absent” means that the amount of unreacted surface treatment agent present in the reaction system after the surface treatment step depends on the reactivity of the surface treatment agent. The amount of the water-absorbent resin (100 parts by mass) is preferably 0 to 1000 ppm by mass, more preferably 0 to 700 ppm by mass, still more preferably 0 to 500 ppm by mass, and particularly preferably. It is 0-100 mass ppm, Most preferably, it is 0-10 mass ppm. The “unreacted surface treatment agent present in the reaction system” means an unreacted surface treatment agent present in the water-absorbent resin to be subjected to the baking step. The amount of such an unreacted surface treating agent is a value measured by the method of an example described later. When the amount of the surface treatment agent exceeds the above upper limit value, the surface treatment (surface crosslinking reaction) proceeds and the water absorption property of the water absorbent resin may be deteriorated, which is not preferable. In addition, it is preferable that the surface treatment agent is sufficiently reacted in the surface treatment step described above, and the amount of the unreacted surface treatment agent remaining in the water-absorbent resin after the surface treatment step is within the above range. However, when an unreacted surface treatment agent is excessively present in the reaction system, the excess surface cross-linking agent on the surface may be removed by a method such as solvent washing.

  In the baking step, the amount of the hydrophobic organic solvent is preferably 0 to 20% by mass with respect to the amount of the water absorbent resin (100 parts by mass), particularly preferably in the absence of the hydrophobic organic solvent (0 to 1000). It is preferably carried out at a mass ppm). This is because the presence of a hydrophobic organic solvent may reduce the efficiency of heating. In the present invention, the “hydrophobic organic solvent” means an organic solvent having a solubility in water (20 ° C.) of 5 g / 100 g or less. Examples of the hydrophobic organic solvent include aliphatic hydrocarbon compounds such as n-pentane, cyclopentane, n-hexane, cyclohexane, n-heptane, methylcyclohexane, and n-octane; aromatic carbonization such as benzene, toluene, and xylene. A hydrogen compound etc. are mentioned.

(Moisture content)
In the present invention, the water content of the water-absorbent resin before the baking step is preferably 1 to 60% by mass, more preferably 1 to 50% by mass, further preferably 3 to 40% by mass, Especially preferably, it is 5-30 mass%, Most preferably, it is 7-20 mass%.

(Particle size distribution)
The shape of the water-absorbent resin to be subjected to the baking step is preferably powdery, more preferably 90 to 100% by weight, particularly preferably particles having a particle size in the range of 150 μm or more and less than 850 μm (specified by sieve classification). It is a powdery water absorbent resin containing 95 to 100% by weight. When the water-absorbing resin having a particle diameter of 850 μm or more is less than 10% by weight, it is preferable because uniform heating is achieved and damage to the diaper backsheet is prevented. On the other hand, when the water-absorbing resin having a particle diameter of less than 150 μm is less than 10% by weight, it is preferable because scattering of fine powder and blockage of piping during production can be prevented. Further, the weight average particle diameter (specified by sieve classification) is preferably 150 to 850 μm, more preferably 200 to 800 μm, and particularly preferably 300 to 600 μm. A weight average particle diameter of 150 μm or more is preferable from the viewpoint of safety and health. If water absorbent resin particles having a particle size of 850 μm or more are present in an aggregate of particles having a particle size close to the above-mentioned weight average particle size, the increase in the solid content of the large particles is slow. It is easy to become. Therefore, the content of particles having a particle size of 850 μm or more is preferably 0 to 20% by weight, more preferably 0 to 10% by weight, and particularly preferably 0 to 5% by weight. The logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.23 to 0.45, and more preferably 0.25 to 0.40.

  Conventionally, when surface cross-linking of a water-absorbent resin is formed, the ability to maintain liquid absorption even under pressure, that is, absorption capacity under pressure (AAP) and liquid permeability (SFC) is improved, but free swelling ratio (GV) ) Is known to decrease. According to the method of the present invention, the absorption ratio under pressure (AAP) and liquid permeability (SFC) are maintained or improved at a certain level by performing an empty baking step after a surface treatment step such as polymerization of a radical polymerizable compound. On the other hand, both free swelling ratio (GV) and liquid permeability (SFC) can be improved.

(Free swelling ratio GV)
In one embodiment of the present invention, the baking step is performed so that the free swelling ratio (GV) (after moisture content correction) of the water absorbent resin is increased by 1 (g / g) or more before and after the baking step. The amount of increase in the free swelling ratio (GV) is more preferably 1 to 20 (g / g), further preferably 2 to 15 (g / g), and particularly preferably 3 to 10 (g / g). It is. When the amount of increase in the free swell ratio (GV) of the water-absorbent resin due to the baking process is below the lower limit of the above range, the effects of the present invention may not be sufficiently exhibited. On the other hand, the amount of increase in the free swelling ratio of the water-absorbent resin due to baking is preferably as large as possible, but if the free swelling ratio is increased above a certain level, the absorption capacity under pressure (AAP) and liquid permeability (SFC) may be reduced. There is. From such a viewpoint, the increase amount of the free swelling ratio is preferably in the above range.

  In another embodiment of the present invention, after the baking step, the free swelling ratio (GV) (after moisture content correction) of the water-absorbent resin is increased by 1 (g / g) or more compared to before the baking step, This is performed until the value after the increase decreases by 1 (g / g) or more. More specifically, the free swelling ratio (GV) is preferably 1 to 20 (g / g), more preferably 2 to 10 (g), compared to the free swelling ratio (GV) value at the start of the baking step. / G), more preferably 3 to 8 (g / g), particularly preferably after 4 to 7 (g / g) increase, preferably compared to the free swelling ratio (GV) value after the increase It is preferable to perform an empty baking process so that it may reduce 1-10 (g / g), More preferably, 1-5 (g / g), More preferably, 2-4 (g / g). By controlling the time-dependent change of the free swelling ratio (GV) in the baking step within the above range, further improvement in absorption characteristics is achieved. In particular, the saline flow conductivity (SFC) can be remarkably improved by once increasing the free swelling ratio (GV) and then decreasing it. As a method for controlling the change with time of the free swelling ratio (GV), for example, by plotting the value of the free swelling ratio (GV) against the heating time, the relationship between the heating time and the GV value is grasped in advance, A condition for changing the GV as described above may be applied.

  In the present invention, the free swelling ratio (GV) before the baking process (at the start of the baking process) is preferably 10 to 50 (g / g), more preferably 15 to 45 (g / g). More preferably 18 to 40 (g / g), and particularly preferably 20 to 35 (g / g). If the free swelling ratio before the baking step is equal to or higher than the lower limit of the above range, a free swelling ratio (GV) suitable for use of sanitary materials such as diapers can be achieved after baking. On the other hand, if the free swelling ratio before the baking step is equal to or less than the upper limit of the above range, a decrease in absorption capacity under load (AAP) and liquid permeability (SFC) can be suppressed.

(Saline flow inductive SFC)
In one embodiment of the present invention, in the baking step, the saline flow conductivity (SFC) of the water absorbent resin is 5 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) before and after the baking step. ) Done to increase more. The amount of increase in saline flow conductivity (SFC) is more preferably 10 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, and still more preferably 15 (unit: 10 ⁻⁷ · cm ^3). · S · g ^-1 ) or more, particularly preferably 20 (unit: 10 ^-7 · cm ³ · s · g ^-1 ) or more, most preferably 30 (unit: 10 ^-7 · cm ³ · s). -It is more than g- ¹ ). When the amount of increase in the saline flow conductivity (SFC) of the water-absorbent resin by air baking is below the lower limit of the above range, the effects of the present invention may not be sufficiently exhibited. On the other hand, the higher the saline flow inductivity of the water-absorbing resin due to baking, the better. However, if the saline flow inductivity is increased to a certain level or more, the cost increases, the free swelling ratio (GV), the absorption capacity under pressure (AAP) ) May be reduced. For this reason, the increase amount of the saline flow conductivity (SFC) is preferably 200 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or less.

More specifically, the saline flow conductivity (SFC) before baking is preferably 10 to 500 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ), more preferably 20 to 400 ( (Unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ), more preferably 30 to 300 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ), particularly preferably 50 to 200 ( Unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ). If the saline flow conductivity before baking is not less than the lower limit of the above range, the saline flow conductivity (SFC) suitable for use of sanitary materials such as diapers can be achieved after baking. On the other hand, if the saline flow conductivity before baking is not more than the upper limit of the above range, the decrease in free swelling ratio (GV) and absorption capacity under pressure (AAP) can be suppressed. In addition, the value measured by the method as described in the Example mentioned later shall be employ | adopted as a value of a saline flow conductivity (SFC).

(Absorption capacity under pressure AAP)
The amount of increase in absorption capacity (AAP) under pressure of the water-absorbent resin in the baking step is preferably 0.5 g / g or more, more preferably 1.0 g / g or more, and even more preferably 2.0 g / g or more. It is preferable. The value measured by the method described in the examples (the value after the moisture content correction) is adopted as the value of the water absorption magnification under pressure.

(Heating conditions)
As a heating method, various methods such as heat drying, hot air drying, hot air drying, infrared drying, microwave drying, dehydration by azeotropy with a hydrophobic organic solvent, and high humidity drying using high-temperature steam are adopted. There is no particular limitation. When drying by hot air drying or hot air drying, as a drying method to be used, a method of drying in a stationary state, a method of drying in a stirring state, a method of drying in a vibrating state, or drying in a fluid state There are a method of performing drying, a method of performing drying with an air current, and the like. Among these, for the purpose of reducing physical damage to the water absorbent resin, it is preferable to perform hot air drying or hot air drying in a fluidized state or a stationary state.

  The heating device may be a normal dryer capable of heating the water-absorbent resin particles, and any of the known heating methods may be used, such as a batch type or continuous type, a direct heating type and / or an indirect heating type. It may be used. For example, belt-type stationary dryer, ventilated vertical dryer, cylindrical stirring dryer, grooved stirring dryer, rotary dryer, rotary dryer with water vapor tube, aerated rotary dryer, fluidized bed dryer, cone-type dryer Machine, vibration fluidized bed dryer, airflow dryer and the like. Among these, it is desirable to use a fluidized bed dryer. When a fluidized bed dryer is used, drying is performed under milder conditions as compared with drying with a stirring device using stirring blades. For this reason, it becomes possible to perform uniform drying while preventing physical damage due to stirring. More preferably, a heat transfer tube is used to efficiently heat the water absorbent resin particles. Industrially, it is desirable to use a continuous fluidized bed dryer having a plurality of drying chambers. Moreover, airflow classification can be performed by using fluidized bed drying. That is, by adjusting the air volume of the hot air used for drying, fine particles having a particle size equal to or smaller than a preferable particle size for the product can be blown off by the wind to control the particle size. In addition, the atmosphere during heating is preferably a low oxygen partial pressure in order to suppress coloring of the water absorbent resin, as in the drying step during the production of the water absorbent resin.

  The heating temperature is preferably 120 ° C. or higher and 250 ° C. or lower, more preferably 150 ° C. or higher and 250 ° C. or lower, further preferably 160 ° C. or higher and 220 ° C. or lower, and most preferably 170 ° C. or higher and 200 ° C. or lower. It is. If it is 120 degreeC or more, thermal efficiency will become favorable and the problem that a dryer becomes large can be solved because a drying rate becomes slow, and the water absorption characteristic of a water absorbing resin can be improved significantly. On the other hand, if it is 250 degrees C or less, the thermal deterioration of a water absorbing resin will be reduced.

  The heating time is set so that the water absorption characteristics (especially, free swelling ratio and saline flow inductivity) and water content of the water-absorbent resin are set to desired values, but the method of applying heat is not uniform depending on the drying method. In some cases, or when the dispersion of the residence time between particles is large in a continuous heating dryer, by continuing further heating, a water-absorbent resin having uniform physical properties between particles can be obtained.

  For example, when heat treatment is not performed in the surface treatment step, the heating time in this step is preferably 5 to 600 minutes, more preferably 10 to 300 minutes, and further preferably 15 to 180 minutes. Particularly preferred is 20 to 120 minutes.

  When heating in the baking step, surface treatment agents such as surface cross-linking agents are not added, but additives such as flow aids for the purpose of improving the uniformity of heating or preventing the increase of fine powder due to process damage. May be added. Examples of the flow aid used include surfactants and metal soaps. Moreover, the usage-amount of a flow aid is about 3 mass% or less per water absorbing resin (100 mass parts).

  Since the water-absorbing resin particles after heating are at a high temperature, cooling may be performed by providing a cooling chamber as appropriate and bringing them into contact with a refrigerant such as cold air or a cooling transmission surface. In addition, the classification process may be performed after the baking process, and when the classification is not performed before the baking process, or when the water-absorbent resin is subjected to process damage in the baking process, and fine powder is newly generated, etc. It is effective for.

  The mechanism by which the physical properties of the water-absorbent resin are improved in the baking process is unknown, but it is presumed that the baking causes a change in the cross-linked structure inside the water-absorbing resin, thereby achieving a dramatic improvement in physical properties. And since the baking process in this invention is performed on the mild conditions which do not accompany big stirring, it is thought that a particle shape is maintained even when a change arises in the crosslinked structure inside a water absorbing resin. Further, since the surface treatment agent is substantially absent in the baking step, deterioration of physical properties due to excessive increase in surface cross-linking density during heating can be avoided, and control to the intended physical properties is facilitated. Note that the above-described mechanism is merely based on speculation, and the technical scope of the present invention is not affected at all even if the operational effects of the present invention are obtained by different mechanisms.

  Therefore, in the present invention, after the surface treatment step, the heat treatment is performed under mild conditions different from the surface treatment. When the temperature rising process of the surface crosslinking is controlled as in Patent Document 4, as in Patent Document 5, When surface crosslinking is performed twice, or when simple heat treatment is performed as in Patent Document 9, the physical properties achieved by the present invention cannot be obtained.

(Characteristics of air-baked water-absorbent resin)
In the present invention, by applying the above-described treatment to the water absorbent resin, the properties desired for the water absorbent resin, for example, the absorption capacity, the absorption speed, the gel strength, and the suction force can be made extremely high. In particular, according to the method of the present invention, it is possible to obtain a water-absorbing resin excellent in absorption capacity under pressure, free swelling capacity, and saline flow conductivity.

(Moisture content)
The water content of the water absorbent resin after the baking step is preferably 0% by mass to 5% by mass, more preferably 0.1% by mass to 4% by mass, and still more preferably 0.1% by mass to 3% by mass. Hereinafter, it is particularly preferably 0.2% by mass or more and 2% by mass or less, and most preferably 0.2% by mass or more and 1% by mass or less. If the water content of the water-absorbing resin after the baking step is 5% by mass or less, the effect of the present invention is sufficiently exerted, and the surface treatment is excellent in water absorption performance such as absorption capacity under load and liquid permeability. A water absorbent resin is obtained. In addition, since 0% as a minimum requires a long time and may be accompanied by deterioration of a water absorbing resin, a minimum may be about 0.1%.

(Particle size distribution)
The shape of the water-absorbent resin obtained after the baking step can be appropriately adjusted depending on the processing conditions such as the shape of the water-absorbent resin before the treatment, granulation / molding after the treatment, etc., but is generally in a powder form. Such a powder has a weight average particle diameter (specified by sieve classification) of usually 10 to 1000 μm, preferably 200 to 600 μm, and preferably 150 to 850 μm in content of 90 to 100 wt. %, More preferably 95 to 100% by weight. The logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.23 to 0.45, and more preferably 0.25 to 0.35.

(Free swelling ratio GV)
The free swelling ratio (GV) after air baking (value after correction of moisture content) is preferably 10 to 60 (g / g), more preferably 15 to 50 (g / g), and still more preferably 20 -40 (g / g). If the free swelling ratio after baking is 60 g / g or less, a sufficient gel strength is ensured and a water-absorbing resin excellent in liquid permeability can be obtained. On the other hand, if it is 10 g / g or more, a water-absorbing resin that is sufficiently suitable for the use of sanitary materials such as diapers can be obtained.

(Saline flow inductive SFC)
The saline flow conductivity (SFC) after baking is preferably 10 to 500 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ), more preferably 20 to 400 (unit: 10 ^−7. cm ³ · s · g ^-1 ), more preferably 30 to 300 (unit: 10 ^-7 · cm ³ · s · g ^-1 ), and particularly preferably 50 to 200 (unit: 10 ^-7 ·). cm ³ · s · g ⁻¹ ). If the value of the saline flow conductivity (SFC) after baking is 10 or more (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, sufficient liquid permeability is ensured, and the absorbent is liquid. Can be spread sufficiently, and excretory fluids such as urine during use can be reliably absorbed. If the value of saline flow conductivity (SFC) after baking is 500 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or less, the diaper's significant cost due to a decrease in free swelling ratio (GV) You can prevent up.

(Absorption capacity under pressure AAP)
Absorption capacity under pressure (AAP) (value after correction of water content) of the water-absorbent resin after baking is preferably 20 g / g or more, more preferably 21 g / g or more, still more preferably 22 g / g or more, particularly preferably Is 23 g / g or more. The upper limit is not particularly limited, but about 40 g / g may be sufficient due to cost increase due to manufacturing difficulty.

(4) Liquid permeability improver The production method of the present invention preferably further includes a step of mixing a liquid permeability improver with the obtained water absorbent resin after the surface treatment step and the baking step. . By adding the liquid permeability improving agent, the water absorbent resin has a liquid permeability improving agent layer. Thereby, the obtained water absorbing resin is further excellent in liquid permeability (liquid permeability).

  Examples of the liquid permeability improver include polyamines, polyvalent metal salts, and water-insoluble fine particles. US Pat. No. 7,179,862, European Patent No. 11,656,313, US Pat. No. 7,157,141, US Pat. No. 6,831,142 , US Patent Application Publication No. 2004/176557, US Patent Application Publication No. 2006/204755, US Patent Application Publication No. 2006/73969, and Patent Application Publication No. 2007/106013. Technology is applied. Polyamines and water-insoluble fine particles are exemplified in International Publication No. 2006/082188, International Publication No. 2006/082189, International Publication No. 2006/082197, etc. Among these, a polyvalent metal salt is preferably used, and either a water-soluble polyvalent metal salt or a water-insoluble polyvalent metal salt may be used, but it is more preferable to use a water-soluble polyvalent metal salt. . Examples of the water-soluble polyvalent metal salt include at least one selected from the group consisting of aluminum sulfate, aluminum chloride, polyaluminum chloride, aluminum sulfate, potassium alum, calcium chloride, alkoxytitanium, ammonium zirconium carbonate, and zirconium acetate. It is done. Examples of water-insoluble polyvalent metal salts include aluminum hydroxide, aluminum lactate, aluminum phosphate, barium sulfate, barium phosphate, barium carbonate, calcium pyrophosphate, calcium phosphate, calcium carbonate, ferrous hydroxide, ferrous phosphate Iron, ferric pyrophosphate, ferrous carbonate, magnesium pyrophosphate, magnesium phosphate, cuprous chloride, manganese hydroxide, manganese sulfate, nickel hydroxide, nickel phosphate, lead sulfate, zinc oxide, lead phosphate And at least one selected from the group consisting of zinc hydroxide and zinc pyrophosphate.

  The amount of the liquid permeability improver used is preferably in the range of 0.001 to 5 parts by weight, more preferably in the range of 0.01 to 1 part by weight with respect to 100 parts by weight of the water absorbent resin. . If the usage-amount of a liquid permeability improver exists in the said range, the absorption capacity | capacitance under pressure (AAP) of a water absorbing resin and the physiological saline flow-inductivity (SFC) can be improved.

  The liquid permeability improver can be added as an aqueous solution if it dissolves in water, and as a powder or slurry if it does not dissolve. For example, a method of premixing with water and / or a hydrophilic organic solvent as necessary and then spraying or dropping and mixing the water-absorbent resin is preferable, and a method of spraying is more preferable.

  The water-absorbent resin obtained by the present invention has properties desired for the water-absorbent resin such as absorption capacity, liquid permeability, absorption speed, gel strength, and suction force (particularly, saline flow conductivity, free swelling capacity, absorption capacity under pressure). ) Is at a very high level. Therefore, the water-absorbent resin obtained by the present invention is a sanitary material that absorbs sanitary cotton, disposable diapers, sanitary products, incontinence pads, or other body fluids, medical fields such as medical supplies, agricultural and horticultural fields such as soil water retention agents, Suitable for various fields such as food fields such as freshness preservation, industrial fields such as anti-condensation materials and cold insulation materials, etc., depending on the purpose and function, silica, zeolite, antioxidant, surfactant, silicone oil, Chelating agents, deodorants, fragrances, chemicals, plant growth aids, bactericides, fungicides, foaming agents, pigments, dyes, fibrous materials (hydrophilic short fibers, pulp, synthetic fibers, etc.), fertilizers, etc. Other additives can be added. The amount of other additives added is preferably about 0.001 to 10% by mass with respect to the total weight of the product in various applications.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the technical scope of the present invention is not limited thereto. Hereinafter, for convenience, “mass%” may be simply referred to as “%”, and “liter” may be simply referred to as “L”. Unless otherwise specified, the operation was performed under conditions of room temperature (23 ± 1 ° C.) and humidity of 30% RH. Measurement methods and evaluation methods in Examples and Comparative Examples are shown below.

(1) Particle size distribution: mass average particle size (D50) and logarithmic standard deviation (σζ) of particle size distribution
10.0 g of the water-absorbent resin was charged into a JIS standard sieve (THE IIDA TESTING SIEVE: diameter 8 cm) having an opening of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 45 μm, and vibration classifier (IIDA SIEVE SHAK). TYPE: ES-65 type, SER No. 0501), classification was performed for 5 minutes, and the residual percentage R was plotted on logarithmic probability paper. The particle diameter corresponding to R = 50% by weight was read as the weight average particle diameter (D50).

  In addition, logarithmic standard deviation (σζ) of the particle size distribution was calculated according to the following formula, assuming that the particle sizes corresponding to R = 84.1 wt% and R = 15.9 wt% are X1 and X2, respectively. That is, the smaller the value of σζ, the narrower the particle size distribution.

(2) 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 uniformly spread on the bottom of the aluminum cup, and the weight W1 (g ) Was measured. Immediately after leaving this in a dryer (EYELA, constant temperature and temperature dryer (natural oven) NDO-450 manufactured by Tokyo Rika Kikai Co., Ltd.) adjusted to 180 ° C. for 3 hours (at least within 1 minute) The weight W2 (g) of the water-absorbent resin-containing aluminum cup was measured. And from these W1 and W2, the moisture content (mass%) was computed according to following Formula.

(3) Free swelling ratio (GV) (centrifuge retention capacity (CRC))
0.200 g of water-absorbing resin was made into a non-woven bag (made by Nankoku Pulp Industries Co., Ltd., trade name: Heaton paper, model: GSP-22, size: 85 mm × 60 mm, heat-sealed, and then brought to room temperature. After 30 minutes, the bag was pulled up and edena ABSORBENCY II 441.1-99 using a centrifuge (manufactured by Kokusan Co., Ltd., model: H-122). After draining for 3 minutes with the described centrifugal force (250 G), the weight W3 (g) of the bag was measured, and the same operation was performed without using the water absorbent resin, and the weight W4 (g) at that time The free swelling ratio (GV) (g / g) was calculated from the values of W3 and W4 according to the following formula.

(5) Saline flow conductivity (SFC)
The saline flow conductivity (SFC) is a value indicating the liquid permeability when the water absorbent resin swells. It shows that it has high liquid permeability, so that the value of SFC is large.

  The SFC was measured according to the saline flow conductivity (SFC) test described in US Pat. No. 5,849,405.

  Specifically, first, an apparatus shown in FIG. 1 was prepared. The apparatus will be described with reference to FIG. 1. A glass tube 32 is inserted into a tank 31, and a lower end of the glass tube 32 is a 0.69 mass% sodium chloride aqueous solution 33 and a swollen gel 44 in a cell 41. It was arranged so that it could be maintained at a height of 5 cm above the bottom of the. The 0.69 mass% sodium chloride aqueous solution 33 in the tank 31 was supplied to the cell 41 through the L-shaped tube 34. A cock 35 was installed in the L-shaped tube 34. Under the cell 41, a collection container 48 for collecting the liquid that has passed is disposed, and the collection container 48 is placed 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 was installed. There was 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 was attached to the bottom so that the water-absorbing resin or its swelling gel did not enter the hole 47. . The cell 41 was placed on a table on which the cell was placed, and the surface of the table in contact with the cell was installed on a stainless steel wire mesh 43 that did not hinder the permeation of the liquid.

  Using this device, the water-absorbent resin (0.900 g) uniformly placed in the container 40 is swollen in artificial urine under pressure of 0.3 psi (2.07 kPa) for 60 minutes, and the height of the gel layer of the gel 44 is increased. Was recorded. This artificial urine is composed of 0.25 g of calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, This is a mixture of 0.15 g of diammonium hydrogen phosphate and 994.25 g of pure water. Next, under a pressure of 0.3 psi (2.07 kPa), a 0.69 mass% sodium chloride aqueous solution 33 was passed through the gel layer swollen from the tank 31 with a constant hydrostatic pressure. This SFC measurement was performed at room temperature (20 to 25 ° C.). 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 mass (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. Fs (T = 0) was calculated by extrapolating the result of least squares of Fs (T) versus time to T = 0.

(5) 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 is uniformly sprayed thereon, and the outer diameter thereof is slightly smaller than 60 mm, and a load of 4.83 kPa can be uniformly applied to the water absorbent resin. A piston and a load, which were adjusted so that no gap was created between them and vertical movement was not hindered, were placed in this order, and the weight W5 (g) of this 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 mass% 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 Advantech 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. Specifically, after 1 hour, the measuring device set was lifted and its weight W6 (g) was measured. This mass measurement must be performed as quickly as possible and without vibrations. And the absorption capacity under load (AAP) (g / g) was calculated from W5 and W6 by the following formula.

(6) Measurement of elution solubles In a 250 mL capacity plastic container with a lid (diameter 6 cm × height 9 cm), weigh 184.3 g of physiological saline, add 1.00 g of water-absorbing resin therein, and add 16 hours By using a magnetic stirrer having a diameter of 8 mm and a length of 25 mm and stirring at a rotation speed of 500 rpm, an elution soluble component in the resin is extracted. This extract was filtered using one sheet of filter paper (manufactured by Advantech Toyo Co., Ltd., product name: JIS P 3801, No. 2, thickness: 0.26 mm, retention particle size: 5 μm), and 50. Weigh out 0 g to make a measurement solution.

  First, only physiological saline was titrated with 0.1N aqueous sodium hydroxide solution to pH 10, and then titrated with 0.1N hydrochloric acid to pH 2.7 to give an empty titration (respectively [bNaOH] and [bHCl], respectively). Called). 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.

(7) Measurement of amount of unreacted surface treatment agent present in reaction system Residual amount of radical polymerizable compound (mass ppm vs. water-absorbing resin) as a surface treatment agent is determined by using the filtrate obtained in (6) above. It can be measured by analyzing by liquid chromatography equipped with a UV detector. In addition, the amount of residual monomers in the water absorbent resin as the base polymer can also be measured by the same method.

  Moreover, the residual amount (mass ppm vs. water-absorbing resin) of the surface cross-linking agent as the surface treatment agent can be measured using, for example, the method described in US Pat. No. 5,981,070 and Japanese Patent Application Laid-Open No. 2003-313446.

(Production Example 1)
A mixed solution of 18.5 g / second of 48.5 mass% aqueous sodium hydroxide, 45.5 g / second of acrylic acid, and 19.8 g / second of water was continuously prepared. This mixed solution (flow rate: 78.6 g / sec), 48.5 mass% sodium hydroxide aqueous solution (flowrate: 23.3 g / sec), and 20 mass% polyethylene glycol diacrylate (average molecular weight 523) (flowrate: 0.00). 199 g / sec) was continuously supplied to the mixer to prepare an aqueous monomer solution (neutralization rate: 70.3 mol%). At this time, the temperature of the monomer aqueous solution was 95 to 100 ° C.

  To this prepared aqueous monomer solution (flow rate: 102.1 g / second), 46 mass% diethylenetriaminepentaacetic acid trisodium aqueous solution (flowrate: 0.0278 g / second) and 4.0 mass% aqueous sodium persulfate solution ( Flow rate: 0.635 g / sec) was added. Thereafter, the obtained aqueous monomer solution was continuously supplied onto an endless belt running at a speed of 7 m / min. The aqueous monomer solution continuously supplied onto the belt started to polymerize quickly. This obtained the strip | belt-shaped water-containing gel sheet.

  This hydrogel sheet is continuously coarsely crushed (subdivided) using a cutter mill having a screen with a diameter of 8 mm (trade name: “RC450”, manufactured by Yoshiko), and is about 1 to 3 mm in size. A crosslinked polymer (a) was obtained. At this time, the water content of the particulate water-containing crosslinked polymer (a) was 29.0% by mass.

  The particulate hydrous cross-linked polymer (a) was flowed at a flow rate of 280 kg / hr with a fluidized bed dryer (trade name: “CT-FBD-0.91 m2”, Nara Machinery Co., Ltd., fluidized bed length 800 mm / fluidized bed width. 550 mm = 1.45) and continuously dried. In addition, the inside of the fluidized bed was divided into two chambers in the length direction and had a three-chamber configuration (about 2: 1: 1 in the length direction). The temperature of the hot air was set to 130 ° C. in the first chamber near the inlet, and 190 ° C. in the second and third chambers far from the inlet. The speed of the hot air of the dryer is 2.4 m / second, the outlet of the dryer is provided at the upper and lower portions of the fluidized section, and the amount of the particulate hydrous crosslinked polymer (a) is at the upper outlet of the dryer. Controlled by the height of the weir. The height of the fluidized bed during drying was about 900 mm, and the average time for the particulate water-containing crosslinked polymer (a) to stay in the dryer was about 50 minutes.

  The obtained dried product was pulverized using a roll mill, and further classified using a sieve having an opening of 710 μm and a sieve having an opening of 150 μm to obtain a particulate water-absorbing resin (A) as a base polymer. .

  The particle size distribution of the obtained particulate water-absorbing resin (A) as the base polymer is shown in Table 1 below, and various evaluation results are shown in Table 2 below.

(Production Example 2)
The particulate water-containing crosslinked polymer (b) and the particulate water-absorbing resin (particulate water-absorbing resin ( B) was obtained. The water content of the particulate water-containing crosslinked polymer (b) and the particulate water-absorbing resin (B) as the base polymer were 28.8% by mass and 9.2% by mass, respectively.

  The particle size distribution of the obtained particulate water-absorbing resin (B) as the base polymer is shown in Table 1 below, and various evaluation results are shown in Table 2 below.

(Production Example 3)
In the same manner as in Production Example 1 except that the flow rate of 20% by mass polyethylene glycol diacrylate (average molecular weight 523) was changed to 0.232 g / sec, the particulate water-containing crosslinked polymer (c) and the particulate form as the base polymer were used. A water absorbent resin (C) was obtained.

  The moisture content of the particulate water-containing crosslinked polymer (c) and the particulate water-absorbing resin (C) as the base polymer were 29.3% by mass and 9.6% by mass, respectively.

  The particle size distribution of the obtained particulate water-absorbing resin (C) as the base polymer is shown in Table 1 below, and various evaluation results are shown in Table 2 below.

(Production Example 4)
In the same manner as in Production Example 1 except that the flow rate of 20% by mass polyethylene glycol diacrylate (average molecular weight 523) was changed to 0.265 g / sec, the particulate water-containing crosslinked polymer (d) and the particulate form as the base polymer A water absorbent resin (D) was obtained.

  The water content of the particulate water-containing crosslinked polymer (d) and the particulate water-absorbing resin (D) as the base polymer was 29.1% by mass and 9.6% by mass, respectively.

  The particle size distribution of the obtained particulate water-absorbing resin (D) as the base polymer is shown in Table 1 below, and various evaluation results are shown in Table 2 below.

[Surface treatment by polymerization of radically polymerizable compound]
(Example A1-1)
(1) Mixing step Particulate water-absorbing resin (A) 250 g as a base polymer is added to a 5 L Ledige mixer (Loedige, model M5R), and stirred at 300 rpm, NK ester 701A (made by Shin-Nakamura Chemical Co., Ltd. Glycerin acrylate methacrylate) 0.25 g, acrylic acid 10.0 g, water 17.5 g, polyethylene glycol monomethyl ether (number average molecular weight about 2,000) 0.025 g, and Irgacure 2959 (manufactured by Ciba Specialty Chemicals). A treatment liquid premixed with 025 g was sprayed at room temperature. The spraying of the treatment liquid took 16 seconds. The mixture was stirred and mixed at room temperature for 9 minutes from the start of spraying of the treatment liquid. In addition, the water content of the water absorbent resin after the mixing step was 15.0% by mass.

(2) Polymerization process of radically polymerizable compound Nine minutes after the start of spraying of the treatment liquid, stirring was temporarily stopped. After placing a quartz glass plate with a thickness of 3 mm in the opening, stirring was resumed at 300 rpm (required time for restarting was 1 minute), 1 kW metal halide lamp (USHIO, UVL-1500M2-N1) ) Is installed so that the distance between the center of the lamp and the quartz plate is 8 cm and irradiated with ultraviolet rays for 3 minutes at room temperature (illuminance meter). : Ushio Electric Co., Ltd. UV integrated light meter UIT-150, receiver type: UDV-S254, irradiation amount: 27 J / cm ² , illumination intensity: 150 mW / cm ² ), surface treatment was performed on particulate water-absorbing resin . As a result, a surface-treated water-absorbing resin (water content: 13.9 mass%, residual acrylic acid content: 360 mass ppm) was obtained.

(3) Heating process (empty baking process)
Next, 200 g of the surface-treated water-absorbing resin after irradiation with ultraviolet rays was put into a fluidized bed dryer (trade name: spray dryer Pulvis GB-22, manufactured by Yamato) in which the hot air temperature was previously set at 190 ° C. for 10 minutes. A water-absorbing resin (A1-1) that was heat-dried was obtained by heat-drying. In addition, the mantle heater was attached to the glass cell of the fluidized bed part of this dryer, and it set to 200 degreeC previously.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-1) are shown in Table 3 below. In addition, the amount of each component in the column of “treatment liquid” in Table 3 is mass% (wt%) with respect to the amount (500 g) of the water absorbent resin (A) as the base polymer.

  In addition, “GV after moisture content correction” and “AAP after moisture content correction” shown in Table 3 and Table 4 below were calculated by the following calculation formulas. Here, in the following formula, “GV before moisture content correction” is the free swelling ratio (GV) of the water-absorbent resin before measuring the moisture content in (2) above. "AAP of" is the absorption capacity (AAP) under pressure of the water-absorbent resin before measuring the water content in (2) above.

(Example A1-2)
The water-absorbent resin (A1-2) that was heat-dried by the same method as in Example A1-1, except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 20 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-2) are shown in Table 3 below.

(Example A1-3)
The water-absorbent resin (A1-3) that was heat-dried by the same method as in Example A1-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 30 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-3) are shown in Table 3 below.

(Example A1-4)
The water-absorbent resin (A1-4) that was heat-dried by the same method as in Example A1-1, except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 60 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-4) are shown in Table 3 below.

(Example A1-5)
Except that the heat drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 90 minutes, the water-absorbent resin (A1-5) that was heat-dried by the same method as in Example A1-1 Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-5) are shown in Table 3 below.

(Example A1-6)
The water-absorbent resin (A1-6), which was heat-dried by the same method as in Example A1-1, except that the heat-drying time of the water-absorbent resin after the surface treatment was 120 minutes in the fluidized bed dryer. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-6) are shown in Table 3 below.

(Example A1-7)
The water-absorbent resin (A1-7) that was heat-dried by the same method as in Example A1-1, except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 150 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbing resin (A1-7) are shown in Table 3 below.

(Example A1-8)
The water-absorbing resin (A1-8) that was heat-dried by the same method as in Example A1-1, except that the heat-drying time of the water-absorbing resin after the surface treatment was 180 minutes in the fluidized bed dryer. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-8) are shown in Table 3 below.

(Example A1-9)
The water-absorbent resin (A1-9) that was heat-dried by the same method as in Example A1-1, except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 240 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A1-9) are shown in Table 3 below.

(Comparative Example A1)
A comparative water absorbent resin (A1) was obtained in the same manner as in Example A1-1 except that the heating step was not performed after the surface treatment step. Various evaluation results of the obtained comparative water absorbent resin (A1) are shown in Table 3 below.

(Example A2-1)
Except for using 250 g of the particulate water-absorbing resin (B) as the base polymer, the water-absorbing resin surface-treated by the same method as in Example A1-1 described above (water content: 14.4% by mass, remaining Acrylic acid amount: 420 mass ppm) was obtained. In addition, the water content of the water absorbent resin after mixing of the treatment liquid (mixing step) was 14.8% by mass.

  A heat-dried water-absorbent resin (A2-1) was obtained in the same manner as in Example A1-1 described above except that this surface-treated water-absorbent resin was used.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A2-1) are shown in Table 3 below.

(Example A2-2)
The water-absorbent resin (A2-2) that was heat-dried by the same method as in Example A2-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 30 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A2-2) are shown in Table 3 below.

(Example A2-3)
The water-absorbent resin (A2-3) that was heat-dried by the same method as in Example A2-1, except that the heat-drying time of the water-absorbent resin after the surface treatment was 60 minutes in the fluidized bed dryer. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A2-3) are shown in Table 3 below.

(Example A2-4)
The water-absorbent resin (A2-4) that was heat-dried by the same method as in Example A2-1, except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 120 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A2-4) are shown in Table 3 below.

(Example A2-5)
The water-absorbent resin (A2-5) that was heat-dried by the same method as in Example A2-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 240 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A2-5) are shown in Table 3 below.

(Comparative Example A2)
A comparative water absorbent resin (A2) was obtained in the same manner as in Example A2-1 except that the heating step was not performed after the surface treatment step.

  Various evaluation results of the obtained comparative water absorbent resin (A2) are shown in Table 3 below.

(Example A3-1)
Except for using 250 g of the particulate water-absorbing resin (C) as the base polymer, the surface-treated water-absorbing resin (water content: 14.5% by mass, remaining, except for the above-described Example A1-1) Acrylic acid amount: 350 mass ppm) was obtained. In addition, the water content of the water-absorbent resin after mixing of the treatment liquid (mixing step) was 15.4% by mass.

  A heat-dried water-absorbent resin (A3-1) was obtained in the same manner as in Example A1-1 described above except that this surface-treated water-absorbent resin was used.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A3-1) are shown in Table 3 below.

(Example A3-2)
The water-absorbent resin (A3-2) that was heat-dried by the same method as in Example A3-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 30 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A3-2) are shown in Table 3 below.

(Example A3-3)
Except that the heat drying time in the fluidized bed dryer of the water-absorbing resin after the surface treatment was 60 minutes, the water-absorbing resin (A3-3) heat-dried by the same method as in Example A3-1 Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A3-3) are shown in Table 3 below.

(Example A3-4)
The water-absorbent resin (A3-4) that was heat-dried by the same method as in Example A3-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 120 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A3-4) are shown in Table 3 below.

(Comparative Example A3)
A comparative water absorbent resin (A3) was obtained in the same manner as in Example A3-1 except that the heating step was not performed after the surface treatment step.

  Various evaluation results of the obtained comparative water absorbent resin (A3) are shown in Table 3 below.

(Example A4-1)
Except for using 250 g of the particulate water-absorbing resin (D) as the base polymer, the water-absorbing resin surface-treated by the same method as in Example A1-1 described above (water content: 14.2% by mass, remaining Acrylic acid amount: 400 mass ppm) was obtained. In addition, the water content of the water absorbent resin after mixing of the treatment liquid (mixing step) was 15.5% by mass.

  A heat-dried water-absorbing resin (A4-1) was obtained in the same manner as in Example A1-1 described above except that this surface-treated water-absorbing resin was used.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A4-1) are shown in Table 3 below.

(Example A4-2)
The water-absorbent resin (A4-2) that was heat-dried by the same method as in Example A4-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 30 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A4-2) are shown in Table 3 below.

(Example A4-3)
The water-absorbent resin (A4-3) that was heat-dried by the same method as in Example A4-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 60 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A4-3) are shown in Table 3 below.

(Example A4-4)
The water-absorbent resin (A4-4) that was heat-dried by the same method as in Example A4-1 except that the heat-drying time of the water-absorbent resin after the surface treatment in the fluidized bed dryer was 120 minutes. Got.

  Various evaluation results of the obtained heat-dried water-absorbent resin (A4-4) are shown in Table 3 below.

(Comparative Example A4)
A comparative water absorbent resin (A4) was obtained in the same manner as in Example A4-1 except that the heating step was not performed after the surface treatment step.

  Various evaluation results of the obtained comparative water absorbent resin (A4) are shown in Table 3 below.

  As the water absorbent resins (A1-1) to (A4-4) of the present invention, water absorbent resins satisfying desired GV and AAP values were obtained in a relatively short time.

  More specifically, from the results shown in Table 3, it is confirmed that the absorption capacity under pressure (AAP) increases with heating and becomes an almost constant value, and the absorption capacity under pressure is maintained at a high value. . Moreover, it is confirmed that GV rises with the heating of the surface-treated water-absorbent resin and then decreases. On the other hand, it is confirmed that liquid permeability (SFC) decreases in the initial stage of heating, but gradually increases.

  Based on these findings, a water absorbent resin having desired characteristics can be obtained. For example, by adjusting the heating time, it is possible to obtain a water absorbent resin in which both GV and SFC are increased in addition to AAP. For example, in Examples (A1-5) to (A1-8) and Example (A2-5), water-absorbing resins in which all of AAP, GV, and SFC were improved by the heating step were obtained.

[Surface treatment using surface cross-linking agent]
(Example B1-1)
(1) Surface treatment step 500 g of the particulate water-absorbing resin (A) as a base polymer is added to a 5 L Readyge mixer (Loedige, model M5R) at 25 ° C. and a humidity of 50% RH, and stirred at 300 rpm. , 0.5 g of ethylene glycol diglycidyl ether (manufactured by Nagase ChemteX Corporation, Denacol EX-810) (0.1 part by mass with respect to 100 parts by mass of the particulate water absorbent resin (A)), 12.5 g of water (particles Treatment liquid in which 2.5 parts by mass with respect to 100 parts by mass of the water absorbent resin (A) and 5 g of isopropanol (1.0 part by mass with respect to 100 parts by mass of the particulate water absorbent resin (A)) were mixed in advance. Sprayed. Then, after stirring and mixing at room temperature for 1 minute, stirring was temporarily stopped, and the resulting mixture was placed in a polyethylene plastic bag. The water content of the water absorbent resin after mixing was 11.3% by mass. After sealing the plastic bag, the surface treatment was performed by heating at 80 ° C. for 30 minutes.

  The water-absorbent resin obtained by the above treatment is taken out from the plastic bag and pulverized until it passes through a JIS standard sieve having an opening of 850 μm to obtain a surface-treated water-absorbent resin (water content: 10.4% by mass). It was.

  The amount of the unreacted surface cross-linking agent (ethylene glycol diglycidyl ether) remaining in the water absorbent resin was less than 1 ppm by mass.

(2) Heating process (empty baking process)
Next, 200 g of the surface-treated water-absorbing resin obtained above was put into a fluidized bed dryer (trade name: Spray dryer GB-22, manufactured by Yamato Kagaku Co., Ltd.) in which the hot air temperature was set to 200 ° C. in advance. Heat-dried water-absorbent resin (B1-1) was obtained by heating and drying for 10 minutes. In addition, the mantle heater was attached to the glass cell of the fluidized bed part of this dryer, and it set to 200 degreeC previously.

  Various evaluation results of the obtained heat-dried water-absorbent resin (B1-1) are shown in Table 4 below. In addition, the amount of each component in the column of “treatment liquid” in Table 4 is mass% (wt%) with respect to the amount (500 g) of the water absorbent resin (A) as the base polymer.

(Example B1-2)
Except that the heat drying time of the water absorbent resin after the surface treatment in the fluidized bed dryer was set to 20 minutes, the water absorbent resin (B1- 2) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B1-2) are shown in Table 4 below.

(Example B1-3)
Except that the heat drying time in the fluidized bed dryer of the water-absorbent resin after the surface treatment was 40 minutes, the water-absorbent resin (B1- 3) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B1-3) are shown in Table 4 below.

(Example B1-4)
Except that the heat drying time of the water absorbent resin after the surface treatment in the fluidized bed dryer was 80 minutes, the water absorbent resin (B1- 4) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B1-4) are shown in Table 4 below.

(Example B1-5)
Except that the heat drying time of the water absorbent resin after the surface treatment in the fluidized bed dryer was set to 100 minutes, the water absorbent resin (B1- 5) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B1-5) are shown in Table 4 below.

(Example B1-6)
Except that the heat drying time in the fluidized bed dryer of the water-absorbent resin after the surface treatment was 120 minutes, the water-absorbent resin (B1- 6) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B1-6) are shown in Table 4 below.

(Example B1-7)
Except that the heat drying time in the fluidized bed dryer of the water absorbent resin after the surface treatment was 180 minutes, the water absorbent resin (B1- 7) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B1-7) are shown in Table 4 below.

(Comparative Example B1)
A comparative water absorbent resin (B1) was obtained by the same method as in Example B1-1 described above, except that the heat drying step was not performed after the surface treatment step. Various evaluation results of the obtained comparative water absorbent resin (B1) are shown in Table 4 below.

(Example B2-1)
As a treatment liquid in the surface treatment step, 0.5 g of ethylene glycol diglycidyl ether (manufactured by Nagase ChemteX Corporation, Denacol EX-810) (0.1 part by mass with respect to 100 parts by mass of the particulate water-absorbing resin (A)) 12.5 g of water (2.5 parts by mass with respect to 100 parts by mass of the particulate water absorbent resin (A)), 5 g of isopropanol (1.0 parts by mass with respect to 100 parts by mass of the particulate water absorbent resin (A)) And a liquid prepared by previously mixing 0.1 g of polyethylene glycol monomethyl ether (MeOPEG2000) (number average molecular weight of about 2,000) (0.02 parts by mass with respect to 100 parts by mass of the particulate water-absorbing resin (A)). Except for the above, a water-absorbent resin (water content: 11.6% by mass) surface-treated by the same method as in Example B1-1 was obtained. The water content of the water-absorbent resin after mixing the treatment liquid was 12.0% by mass.

  A heat-dried water-absorbing resin (B2-1) was obtained in the same manner as in Example B1-1 described above except that this surface-treated water-absorbing resin was used.

  Various evaluation results of the obtained heat-dried water-absorbent resin (B2-1) are shown in Table 4 below.

(Example B2-2)
Except that the heat drying time in the fluidized bed dryer of the water absorbent resin after the surface treatment was set to 20 minutes, the water absorbent resin (B2- 2) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B2-2) are shown in Table 4 below.

(Example B2-3)
Except that the heat drying time in the fluidized bed dryer of the water absorbent resin after the surface treatment was 40 minutes, the water absorbent resin (B2- 3) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B2-3) are shown in Table 4 below.

(Example B2-4)
Except that the heat drying time of the water absorbent resin after the surface treatment in the fluidized bed dryer was set to 80 minutes, the water absorbent resin (B2- 4) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B2-4) are shown in Table 4 below.

(Example B2-5)
Except that the heat drying time of the water absorbent resin after the surface treatment in the fluidized bed dryer was 120 minutes, the water absorbent resin (B2- 5) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B2-5) are shown in Table 4 below.

(Example B2-6)
Except that the heat drying time in the fluidized bed dryer of the water-absorbing resin after the surface treatment was 180 minutes, the water-absorbing resin (B2- 6) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B2-6) are shown in Table 4 below.

(Example B2-7)
Except that the heat drying time in the fluidized bed dryer of the water absorbent resin after the surface treatment was 240 minutes, the water absorbent resin (B2- 7) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B2-7) are shown in Table 4 below.

(Comparative Example B2)
A comparative water absorbent resin (B2) was obtained by the same method as in Example B2-1 described above, except that the heat drying step was not performed after the surface treatment step. Various evaluation results of the obtained comparative water absorbent resin (B2) are shown in Table 4 below.

(Example B3-1)
A water-absorbing resin heat-dried was obtained in the same manner as in Example B2-5 described above. To 100 parts by mass of this heat-dried water-absorbent resin, 1.0 part by mass of aluminum sulfate hydrate (13-14 hydrate) as a polyvalent metal salt is uniformly mixed for 10 minutes, and then heat-dried. Water absorbent resin (B3-1) was obtained. Various evaluation results of the obtained heat-dried water-absorbent resin (B3-1) are shown in Table 4 below.

(Example B3-2)
A water-absorbing resin dried by heating was obtained in the same manner as in Example B2-6 described above. Except for using this heat-dried water-absorbent resin, a heat-dried water-absorbent resin (B3-2) was obtained in the same manner as in Example B3-1. Various evaluation results of the obtained heat-dried water-absorbent resin (B3-2) are shown in Table 4 below.

  The water-absorbing resins (B1-1) to (B3-2) of the present invention provide water-absorbing resins that satisfy desired GV and AAP values in a short time compared to the comparative water-absorbing resins (B1) to (B2). It was.

  More specifically, from the results shown in Table 4, it is confirmed that the absorption capacity under pressure (AAP) increases with heating and becomes an almost constant value, and the absorption capacity under pressure is maintained at a high value. . Moreover, it is confirmed that GV rises with the heating of the surface-treated water-absorbent resin and then decreases. On the other hand, it is confirmed that liquid permeability (SFC) decreases in the initial stage of heating, but gradually increases.

  Based on these findings, a water absorbent resin having desired characteristics can be obtained. For example, by adjusting the heating time, it is possible to obtain a water absorbent resin in which both GV and SFC are increased in addition to AAP.

  It is also confirmed that the water absorption properties (GV and SFC) of the surface-treated water-absorbing resin can be further improved by adding a polyvalent metal salt (aluminum sulfate hydrate) after heating.

31 tanks,
32 glass tubes,
33 0.69 mass% sodium chloride aqueous solution,
34 L-shaped tube,
35 cock,
40 containers,
41 cells,
42, 43 stainless steel wire mesh,
44 swelling gel,
45 Glass filter,
46 piston,
47 holes,
48 collection containers,
49 Precision balance.

Claims (10)

A method for producing a water absorbent resin, comprising:
(A) a step of surface-treating a water-absorbing resin having a water content of 7 to 60% by mass; and (b) a water-absorbing resin having a water content of 7 to 60% by mass obtained in the step (a). the process of heating treatment agent temperature of 120 ° C. or higher 250 ° C. or less without adding only contains,
The step (a) is a step of mixing the water-absorbing resin and a treatment liquid containing a radical polymerizable compound to polymerize the radical polymerizable compound, or
The step (a) is a method for producing a water-absorbent resin, which is performed in the presence of a surface cross-linking agent . A method for producing a water absorbent resin, comprising:
(A) a step of surface-treating a water absorbent resin having a water content of 7 to 60% by mass; and (b) heating the water absorbent resin having a water content of 7% by mass to 60% by mass obtained in the step (a). Including steps,
Said step (b) have a row as free swelling capacity after correction of water content of the water-absorbing resin before and after the step (GV) increases 1 (g / g) or more,
The step (a) is a step of mixing the water-absorbing resin and a treatment liquid containing a radical polymerizable compound to polymerize the radical polymerizable compound, or
The step (a) is a method for producing a water-absorbent resin , which is performed in the presence of a surface cross-linking agent . The said process (b) is a manufacturing method of the water absorbing resin of Claim 1 or 2 performed by the quantity of the unreacted surface treating agent which exists in a reaction system at 1000 mass ppm or less. The method for producing a water absorbent resin according to any one of claims 1 to 3 , wherein the step (b) is performed such that the water content of the water absorbent resin is reduced to 5% by mass or less. The step (b) is performed so that the saline flow conductivity (SFC) of the water-absorbent resin is increased by 5 (unit: 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more before and after the step. The method for producing a water-absorbent resin according to any one of claims 1 to 4 , wherein the water-absorbent resin is produced. The method for producing a water-absorbent resin according to any one of claims 1 to 5 , wherein the heating temperature in the step (b) is 150 ° C or higher and 250 ° C or lower. The method for producing a water-absorbent resin according to any one of claims 1 to 6 , wherein the temperature of the surface treatment in the step (a) is 50 ° C or higher and lower than 120 ° C. The method for producing a water-absorbent resin according to any one of claims 1 to 7 , wherein the heating time in the step (b) is 5 to 600 minutes. The method for producing a water-absorbent resin according to any one of claims 1 to 8 , wherein the step (b) is performed in the absence of a hydrophobic organic solvent. After step (b)
The method for producing a water absorbent resin according to any one of claims 1 to 9 , further comprising a step of mixing a polyvalent metal salt with the water absorbent resin obtained in the step (b).

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