JP5390509B2 - Surface treatment method for water absorbent resin - Google Patents

Description

  The present invention relates to a surface treatment method for a water absorbent resin. More specifically, the present invention relates to a technique for improving manufacturing cost and safety while maintaining the water absorption characteristics of a water absorbent resin.

  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 has been proposed in which the surface of the water absorbent resin is treated with a crosslinking agent to increase the crosslinking density of the surface of the water absorbent resin. . For example, US Pat. No. 4,783,510 discloses a technique in which an aqueous solution containing a peroxide radical initiator is brought into contact with the surface portion of a water absorbent resin and heated. Japanese Patent Application Laid-Open No. 1-1113406 discloses a technique in which a water-absorbent resin particle is mixed with a cross-linking agent having two or more functional groups, and water-absorbing resin is cross-linked by bringing water vapor into contact therewith. Furthermore, in JP-A-1-297430, the surface of the water-absorbent resin particles is prepared by mixing a water-absorbent resin particle with a crosslinking agent having two or more functional groups and an organic solvent, and heating the mixture in an atmosphere of a specific humidity. A technique for introducing a cross-linked structure is disclosed. US Patent Application Publication No. 2007-0078231 discloses a technique for heating water-absorbent resin particles in the presence of a peroxide, an organic crosslinking agent (polyhydric alcohol) and water.

  All of these techniques are techniques for introducing a cross-linked structure on the surface of the water-absorbent resin by bringing a specific surface-crosslinking agent into contact with the water-absorbent resin and heating. On the other hand, the technique of surface-crosslinking a water-absorbent resin by performing irradiation treatment of active energy rays such as ultraviolet rays instead of heat treatment has been proposed. For example, International Publication WO2006 / 62258 pamphlet discloses a technique in which a water-absorbing resin and a water-soluble radical polymerization initiator are mixed and the resulting mixture is irradiated with active energy rays. Further, International Publication WO 2006/62253 pamphlet discloses a technique in which a predetermined amount of a radical polymerization initiator and a radical polymerizable compound are mixed in a water-absorbent resin, and the resulting mixture is irradiated with active energy rays. .

  According to the disclosure in these patent documents, the surface cross-linking density of the water-absorbent resin is increased by the reactive functional groups derived from the monomers present on the surface of the water-absorbent resin being cross-linked by the surface cross-linking agent. The water absorption characteristics can be improved.

  However, when the water-absorbent resin is surface-treated using the technology described in each of the above-mentioned patent documents, a cross-linking agent that has to be heated at a high temperature for a long time and increases in manufacturing cost or can react at a low temperature is employed. In some cases, the reactivity of the crosslinking agent is high and the safety is poor. In addition, the water-absorbing properties of the surface-crosslinked water-absorbing resin obtained are not necessarily sufficient, and further improvements in water-absorbing properties under pressure (absorption capacity under pressure) and liquid permeability are required. It was.

  Under such circumstances, the present invention is a surface-treated water-absorbent resin that is excellent in water absorption characteristics (especially absorption capacity under pressure and liquid permeability) at low temperature and in a short time by a low-cost and safe technique. An object of the present invention is to provide a means capable of manufacturing the above.

  In view of the above problems, the present inventors conducted extensive research. As a result, when surface-crosslinking the water-absorbent resin in the presence of a radical polymerization initiator and water, the surface cross-linking step is performed at a low cost by increasing the water content of the water-absorbent resin before and after the step. In addition, the present inventors have found that the water-absorbing properties (particularly, the absorption capacity and liquid permeability under pressure) of the surface-crosslinked water-absorbing resin obtained can be improved by a safe technique, and the present invention has been completed.

  That is, the surface treatment method for a water-absorbent resin according to the present invention is a surface treatment method for a water-absorbent resin including a surface cross-linking step of surface-crosslinking the water-absorbent resin in the presence of a radical polymerization initiator and water, In the surface crosslinking step, the surface crosslinking reaction system is not irradiated with an active energy ray having a wavelength equal to or less than the wavelength of ultraviolet rays, and the water content of the water-absorbent resin increases before and after the surface crosslinking step. It is characterized by doing so.

  According to the surface treatment method of the present invention, a surface-treated water-absorbing resin having excellent water absorption characteristics (especially absorption capacity under load and liquid permeability) can be produced at low temperature and in a short time by a low-cost and safe technique. Can be done.

  Still other objects, features and characteristics of the present invention will become apparent by referring to the following description and preferred embodiments illustrated in the accompanying drawings.

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

  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.

  Hereinafter, although the surface treatment method of the water-absorbent resin according to the present invention will be described, the technical scope of the present invention is not limited to these descriptions, and the gist of the present invention is not impaired except for the following examples. It can be implemented with appropriate changes within the scope.

  The present invention is a surface treatment method for a water-absorbent resin comprising a surface cross-linking step in which the water-absorbent resin is surface cross-linked in the presence of a radical polymerization initiator and water. The active energy ray having a wavelength equal to or shorter than the wavelength of ultraviolet rays is not irradiated, and the surface cross-linking step is performed so that the water content of the water-absorbent resin increases before and after the step.

(1) Water absorbent resin (base polymer)
The water-absorbent resin (base polymer) 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 centrifugal retention capacity (CRC) in physiological saline is 2 g / g or more, preferably 5 to 100 g / g, more Preferably it is 10-60 g / g. In addition, as a numerical value of CRC, the value measured by the method as described in the Example mentioned later shall be employ | adopted. In addition, “water-insoluble” means that the content of uncrosslinked water-soluble component (hereinafter also referred to as “elution soluble content”) in the water-absorbent resin is 0 to 50% by weight, preferably It is 0-25 weight%, More preferably, it is 0-15 weight%, More preferably, it is 0-10 weight%. In addition, the value measured by the following method shall be employ | adopted as a numerical value of an elution soluble part.

[Measurement method for soluble elution]
Weigh 184.3 g of physiological saline into a plastic container (diameter 6 cm x height 9 cm) with a capacity of 250 mL, add 1.00 g of water-absorbing resin to it, and magnetize it with a diameter of 8 mm and a length of 25 mm for 16 hours. By using a stir bar to stir at a rotation speed of 500 rpm, the soluble fraction eluted in the resin is extracted. This extract was filtered using one sheet of filter paper (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, the physiological saline alone is titrated with 0.1N aqueous sodium hydroxide solution to pH 10, and then titrated with 1N hydrochloric acid to pH 2.7 to give a blank titration (referred to as [bNaOH] and [bHCl], respectively). ) By performing the same titration operation on the measurement solution, titration amounts (referred to as [NaOH] and [HCl], respectively) are obtained.

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

(2) Method for producing water-absorbing resin (base polymer) (2-1) Polymerization step for polymerizing monomer component In the present invention, the water-absorbing resin is obtained, for example, by a polymerization step for polymerizing the monomer component. . As the polymerization method, aqueous solution polymerization, reverse phase suspension polymerization, bulk polymerization, precipitation polymerization and the like can be employed. In view of performance and ease of polymerization control, it is preferable to perform aqueous solution polymerization or reverse phase suspension polymerization using the monomer component as an aqueous solution, and particularly preferably aqueous solution polymerization.

  The concentration of the aqueous solution of the monomer component is preferably 20% by weight or more and a saturated concentration or less as a monomer, more preferably 30 to 70% by weight, and further preferably 35 to 60% by weight. If it is less than 20% by weight, the amount of water in the obtained polymer (hydrogel) is large, which requires a heat amount and time for drying, which is disadvantageous.

  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.

  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.

  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.

  In the polymerization step, first, a monomer component is polymerized in the presence of a crosslinking agent to obtain a hydrogel crosslinked polymer. 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.

  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, penta Erythritol tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, N, N-diallylacrylamide, triallyl cyanu , Triallyl isocyanurate, triallyl phosphate, triallylamine, diallyloxyacetic acid, bis (N-vinylcarboxylic acid amide), (ethylene oxide modified) tetraallyloxyethane, poly (meth) allyloxyalkane, (poly) Examples include 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 cross-linking agent used in the process is preferably 0.0001 to 1 mol%, more preferably 0.001 to 0.5 mol%, still more preferably 0 with respect to the total amount of the monomer components. 0.005 to 0.2 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.

  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 weight, more preferably 0 to 30% by weight, still more preferably 0 to 10% by weight based on 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.

  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. Here, the water-absorbent resin used in the present invention preferably contains an acid group and has a predetermined neutralization rate (mol% of neutralized acid groups in all acid groups). 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 75 mol%, and particularly preferably 60. -70 mol%.

  A water-containing gel-like cross-linked polymer is 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.

(2-2) Drying / Crushing / Classification Step In the present invention, the water-absorbent resin is obtained by drying the hydrogel crosslinked polymer obtained in the polymerization step in the drying step. Moreover, what carried out the grinding | pulverization process, the classification process, etc. after the drying process further to the particle form is also included by the concept of a water absorbing resin.

  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% or more, or the water content is 25% 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 0 to 25% by weight, more preferably 1 to 15% by weight, and further preferably 2 to 10% by weight. %.

  The drying temperature and drying time in the drying step are not particularly limited, and are usually 70 to 250 ° C, preferably 150 to 230 ° C, more preferably 160 to 180 ° 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 cutter, a guillotine cutter, a slicer, a roll cutter, a shredder, or a scissor, alone or in combination, and is not particularly limited.

  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.

  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 at the same time, and the classification step is performed before the surface cross-linking step described later, and further at two or more places before and after. Are preferred. The continuous sieving classification process is preferably performed while heating or keeping the sieve.

  The water-absorbent resin used in the present invention is preferably in the form of powder. More preferably, it is a powdery water-absorbent resin containing 90 to 100% by weight, particularly preferably 95 to 100% by weight of particles having a particle size in the range of 150 to 850 μm (specified by sieve classification). For example, if the water-absorbent resin having a size greater than 850 μm is used for a diaper or the like, for example, the obtained surface-crosslinked water-absorbent resin may have a poor touch and may break the top sheet of the diaper. On the other hand, when the amount of particles smaller than 150 μm exceeds 10% by weight, fine powder may be scattered or clogged at the time of use, and the water absorption performance of the surface-crosslinked water absorbent resin may be lowered. A weight average particle diameter becomes like this. Preferably it is 150-850 micrometers, More preferably, it is 200-600 micrometers, More preferably, it is 300-500 micrometers. 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 can be suitably used for diapers and the like. 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.

  The water absorbent resin used in the surface treatment method of the water absorbent resin of the present invention is not limited to those manufactured by the above-described method, and may be prepared by other methods. The water-absorbing resin obtained by the above-described method is usually a water-absorbing resin that is not surface-crosslinked, but the water-absorbing resin used in the present invention includes polyhydric alcohol, polyhydric epoxy compound, alkylene carbonate, It may be a water-absorbent resin that has been surface-crosslinked in advance using an oxazolidone compound or the like.

(3) Surface cross-linking step The surface cross-linking step is a step of surface cross-linking the water absorbent resin. “Surface cross-linking” means increasing the cross-linking density of the water-absorbent resin surface. Surface cross-linking can be confirmed by a decrease in CRC (value after correction of moisture content) from the water-absorbent resin (base polymer) before the surface cross-linking step.

  In this surface cross-linking step, the water-absorbing resin is surface cross-linked in the presence of a radical polymerization initiator and water. And this invention has the characteristics in the point which performs a surface crosslinking process so that the moisture content of a water absorbing resin may increase before and behind the said process. By performing surface cross-linking of the water-absorbent resin in this way, the water-absorbing properties (particularly, absorption capacity and liquid permeability under pressure) of the resulting surface-crosslinked water-absorbent resin are improved by a low-cost and safe technique. It becomes possible to make it.

(3-1) In the presence of radical polymerization initiator and water In order to introduce a crosslinked structure on the surface of the water absorbent resin, for example, the water absorbent resin may be heated in the presence of a radical polymerization initiator and water. The surface crosslinking step of the surface treatment method of the present invention is performed without irradiating the surface crosslinking reaction system with an active energy ray having a wavelength equal to or shorter than the wavelength of ultraviolet rays. This is because the resin may be deteriorated by irradiation with such active energy rays. Here, “irradiating an active energy ray having a wavelength equal to or less than the wavelength of ultraviolet rays” means that the active irradiation is performed using an irradiation device such as a metal halide lamp, and the light from natural light or illumination in a manufacturing facility is used. This is not included when the active energy rays contained in the light are inevitably irradiated.

  In the present invention, the water absorbent resin obtained in the polymerization step is usually mixed with an aqueous solution containing a radical polymerization initiator and water (hereinafter also referred to as “treatment liquid”) to obtain a water absorbent resin composition. Then, the water absorbent resin composition is subjected to heat treatment. Thereby, a crosslinked structure is introduced into the surface of the water absorbent resin. However, the technical scope of the present invention is not limited to such a form, and there is no particular limitation on the order of addition of each component existing in the reaction system and the timing of addition of each component and heat treatment. For example, the radical polymerization initiator and water may be separately added to the water absorbent resin, or these may be added to the water absorbent resin while performing a heat treatment.

  When heat treatment is performed after mixing the water-absorbent resin and the treatment liquid, the “water content of the water-absorbent resin before the surface cross-linking step” refers to the water content after the treatment liquid is mixed and before the heat treatment. As used herein, “water content after the surface crosslinking step” means the water content after the heat treatment. In addition, when mixing the water absorbent resin and the treatment liquid while performing the heat treatment, the “water content of the water absorbent resin before the surface crosslinking step” means the moisture content before the heat treatment, The “water content after the crosslinking step” means the water content after the heat treatment. Therefore, in an industrial production facility, the water content of the water-absorbent resin when entering the reaction apparatus for performing the surface cross-linking reaction corresponds to “the water content before the surface cross-linking process”, and the water exits from the reaction apparatus for performing the surface cross-linking reaction. The water content of the water-absorbent resin corresponds to the “water content after the surface crosslinking step”.

  Conventionally, surface cross-linking of a water-absorbent resin has been generally performed by blending a surface cross-linking agent. When a surface cross-linking agent is blended, the functional group present on the surface of the water-absorbent resin and the surface cross-linking agent are chemically strongly bonded, and thereby a stable surface cross-linking structure can be introduced on the surface of the resin. In addition, the surface crosslinking distance can be easily adjusted by appropriately selecting the chain length of the surface crosslinking agent, and the crosslinking density can be controlled by adjusting the blending amount. However, in the present invention, it is possible to introduce a crosslinked structure on the surface of the water-absorbent resin by treating the water-absorbent resin in the presence of a radical polymerization initiator and water without blending such a surface cross-linking agent. it can.

  In the present invention, the surface cross-linking step of the water absorbent resin is performed in the presence of a radical polymerization initiator. There is no restriction | limiting in particular about the radical polymerization initiator which can be used in the surface crosslinking process of this invention, A conventionally well-known knowledge can be referred suitably. Examples of the radical polymerization initiator include, but are not limited to, a water-soluble radical polymerization initiator and a thermally decomposable radical polymerization initiator. Hereinafter, the “water-soluble radical polymerization initiator and the thermally decomposable radical polymerization initiator” may be collectively referred to simply as “radical polymerization initiator”. The water-soluble radical polymerization initiator and the thermally decomposable radical polymerization initiator partially overlap.

  In the surface cross-linking step, when the “water-soluble radical polymerization initiator” is present together with the water-absorbing resin, the polymerization initiator can be uniformly dispersed on the surface of the water-absorbing resin excellent in hydrophilicity and water absorption. Thereby, a water-absorbing resin having excellent water absorption characteristics can be produced.

  The water-soluble radical polymerization initiator means one that dissolves 1% by weight or more in water (25 ° C.), preferably 5% by weight or more, more preferably 10% by weight or more. Specifically, persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate; hydrogen peroxide; 2,2′-azobis-2-amidinopropane dihydrochloride, 2,2′-azobis [2- ( And water-soluble azo compounds such as 2-imidazolin-2-yl) propane] dihydrochloride. In particular, when a persulfate or a water-soluble azo compound is used, the absorption capacity under pressure of the surface-treated water-absorbing resin with respect to physiological saline (in this specification, simply referred to as “absorption capacity under pressure”), Since water absorption characteristics, such as liquidity, can improve all, it is preferable.

  On the other hand, 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 to 120 ° C, more preferably 20 to 100 ° C. Those of ˜80 ° C. are particularly preferred. If the 10-hour half-life temperature is 0 ° C. or higher, it can be stored stably, and if it is 120 ° C. or lower, sufficient reactivity can be ensured.

  In the surface cross-linking step, surface cross-linking of the water-absorbing resin in the presence of the “thermally decomposable radical polymerization initiator” together with the water-absorbing resin can introduce the surface cross-linking at a low temperature and in a short time, resulting in high gel strength And excellent water absorption properties can be achieved. The thermally decomposable radical polymerization initiator may be oil-soluble or water-soluble. Oil-soluble thermally decomposable radical polymerization initiators are characterized in that the degradation rate is less affected by pH and ionic strength than water-soluble thermally decomposable radical polymerization initiators. However, since the water-absorbent resin is hydrophilic, it is more preferable to use a water-soluble thermally decomposable radical polymerization initiator in consideration of permeability to the water-absorbent resin.

  Thermally decomposable radical polymerization initiators are relatively inexpensive as compared with commercially available compounds as photodegradable radical polymerization initiators, and strict light shielding is not necessarily required, so that the production process and production apparatus can be simplified. Typical thermal decomposable radical polymerization initiators include persulfates such as sodium persulfate, ammonium persulfate and potassium persulfate; percarbonates such as sodium percarbonate; peracetates such as peracetic acid and sodium peracetate; Hydrogen peroxide; 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis And azo compounds such as (2-methylpropionitrile). Among them, persulfates such as sodium persulfate, ammonium persulfate, potassium persulfate, and 2,2′-azobis (2-amidinopropane) dihydrochloride having a 10-hour half-life temperature of 40 to 80 ° C., 2 , 2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis (2-methylpropionitrile) and the like are preferred. Among these, the use of persulfate is particularly preferable because the surface-treated water-absorbing resin obtained can have improved absorption capacity and liquid permeability under pressure. In the present invention, the “radical polymerization initiator” is not limited to the above-described form.

  The amount of the radical polymerization initiator contained in the water absorbent resin in the surface crosslinking step or mixed with the water absorbent resin before the surface crosslinking step is based on the amount of the water absorbent resin (100% by weight in terms of solid content), Preferably it is 0.01-20 weight%, More preferably, it is 0.05-10 weight%, More preferably, it is 0.1-10 weight%, Most preferably, it is 0.2-5 weight% . If the abundance of the radical polymerization initiator is 0.01% by weight or more, a crosslinked structure can be effectively introduced on the surface of the water absorbent resin. On the other hand, if the abundance of the radical polymerization initiator is 20% by weight or less, a decrease in water absorption characteristics of the surface-treated water-absorbing resin obtained can be suppressed.

  The form in which the radical polymerization initiator is present together with the water-absorbing resin in the surface cross-linking step may be a form in which the radical polymerization initiator is directly mixed with the water-absorbing resin, but the water-absorbing resin is in an aqueous solution in which this is dissolved in water. The form which mixes with is preferably employ | adopted. Since the water-absorbing resin has water-absorbing properties, it is possible to uniformly disperse the radical polymerization initiator on the surface of the water-absorbing resin by mixing the radical polymerization initiator in the form of an aqueous solution and to mix it with the water-absorbing resin uniformly. it can. The aqueous solution for adding the radical polymerization initiator may be water, other solvents or other components (for example, a radical polymerizable compound and a mixing aid described later) as long as the solubility of the radical polymerization initiator is not impaired. Etc.). Here, in the reaction system in the surface cross-linking step of the present invention, it is essential that water is present together with the water-absorbent resin, but according to the above-described embodiment in which the radical polymerization initiator is mixed with the water-absorbent resin in the form of an aqueous solution. In addition to the radical polymerization initiator, water can be simultaneously added to the reaction system. As a result, the operation required for surface crosslinking can be simplified.

  Water content of the water-absorbent resin before surface cross-linking treatment in the surface cross-linking step (when heat treatment is performed after mixing the water-absorbent resin and the treatment liquid, the water content after mixing of the treatment liquid and before the heat treatment) Is preferably 1 to 60% by weight, more preferably 5 to 50% by weight, still more preferably 7 to 45% by weight with respect to the amount of the water-absorbent resin (100% by weight in terms of solid content). It is particularly preferably 10 to 40% by weight. If the water content of the water-absorbent resin is 1% by weight or more, a crosslinked structure is efficiently introduced on the surface of the water-absorbent resin, and heating necessary to improve the absorption capacity and liquid permeability under pressure to a desired level. Processing time can be shortened. On the other hand, if the water content of the water-absorbent resin is 60% by weight or less, the energy required in the drying step after the surface cross-linking step is prevented from excessively increasing. As described above, when the radical polymerization initiator is added to the water absorbent resin in the form of an aqueous solution, the concentration of the aqueous solution should be adjusted so that the water content of the water absorbent resin after the addition becomes a value within the above range. That's fine. The amount of water added at this time is about 1 to 50% by weight, preferably 2 to 30% by weight, more preferably based on the amount of the water absorbent resin (100% by weight in terms of solid content). Is 3 to 20% by weight, more preferably 5 to 15% by weight. In addition, the mixing form of the water to a water absorbing resin is not necessarily restricted to the case of mixing with the form of the aqueous solution containing a radical polymerization initiator. After mixing the radical polymerization initiator and the water absorbent resin, water may be mixed separately. 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 10 to 60% by weight.

  As described above, in the surface cross-linking step of the present invention, it is not essential to use a surface cross-linking agent that has been conventionally used. However, in some cases, the surface cross-linking of the water-absorbing resin may be performed using a surface cross-linking agent. In other words, the surface cross-linking treatment (heat treatment) of the water absorbent resin may be performed in the presence of the surface cross-linking agent. According to such a form, there exists an advantage that a crosslinked structure can be efficiently introduce | transduced into the surface of a water absorbing resin in a short time.

  There is no restriction | limiting in particular about the surface crosslinking agent which can be used, A conventionally well-known knowledge can be referred suitably. Examples of surface crosslinking agents include, for example, polyhydric alcohol compounds, epoxy compounds, polyvalent amine compounds or condensates thereof with haloepoxy compounds, oxazoline compounds, mono-, di-, or polyoxazolidinone compounds, polyvalent metal salts. And alkylene carbonate compounds. Specific examples include US Pat. Nos. 6,228,930, 6071976, and 6254990. For example, mono, di, tri, tetra or polyethylene glycol, 1,2-propylene 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. Polyhydric alcohol compounds, epoxy compounds such as ethylene glycol diglycidyl ether and glycidol, ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine, polyamide polyamine A haloepoxy compound such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin; a condensate of the polyvalent amine compound and the haloepoxy compound; an oxazolidinone compound such as 2-oxazolidinone; Oxetane compounds; cyclic urea compounds; and alkylene carbonate compounds such as ethylene carbonate. Especially, it is preferable to use at least 1 sort (s) selected from an oxetane compound, a cyclic urea compound, or a polyhydric alcohol, More preferably, at least 1 sort (s) selected from a C2-C10 oxetane compound or a polyhydric alcohol is used. More preferably, a polyhydric alcohol having 3 to 8 carbon atoms is used.

  The amount of the surface cross-linking agent used is preferably 0.001 to 10% by weight with respect to the amount of the water-absorbent resin (100% by weight in terms of solid content), although it depends on the compounds used and combinations thereof. More preferably, it is 0.01 to 5 weight%.

(3-2) Radical polymerizable compound In the surface crosslinking step (3-1) above, the surface crosslinking of the water-absorbent resin is preferably performed in the presence of a radical polymerizable compound. According to such a form, a crosslinked structure can be more effectively introduced into the surface of the water absorbent resin without using a surface crosslinking agent.

  The term “radical polymerizable compound” means a compound that can be polymerized by radical polymerization. Specifically, for example, the ethylenic unsaturated compound used in the production of the water absorbent resin in the column “Water absorbent resin” above. The compounds mentioned as examples of the monomer (monofunctional radical polymerizable compound) and the internal crosslinking agent (polyfunctional radical polymerizable compound) can be preferably used. Therefore, detailed description is omitted here. 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, from the viewpoint of ease of production, the compound used as the ethylenically unsaturated monomer and the internal cross-linking agent during the production of the water-absorbent resin is used as the radical polymerizable compound in this step. In addition, the molar ratio of the usage-amount of the monofunctional radically polymerizable compound used in such a form and a polyfunctional radically polymerizable compound is the ethylenically unsaturated monomer and internal crosslinking agent at the time of manufacture of a water absorbing resin. The molar ratio may be the same or different. Preferably, a larger amount of polyfunctional radically polymerizable compound is used compared to the composition at the time of manufacturing the water absorbent resin. 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 0.5 to 10 mol%. Such a configuration is advantageous in that the crosslink density on the surface of the water absorbent resin is efficiently improved. In this step, the radical polymerizable compound is preferably a compound having two or more polymerizable unsaturated groups in one molecule (for example, two or more polymerizable unsaturated groups among the above-mentioned internal crosslinking agents). Compound). According to such a form, it is possible to further improve the absorption capacity and liquid permeability under pressure.

  The monofunctional radically polymerizable compound preferably contains an acid group and has a predetermined neutralization rate (mol% of neutralized acid groups in all acid groups). Among monofunctional radically polymerizable compounds, a compound containing an acid group 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.

  The acid group-containing monofunctional radical polymerizable compound to be mixed with the water-absorbent resin in the present invention is preferably a monomer containing an acid group among the ethylenically unsaturated monomers described above. 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, and acrylic acid is particularly preferable in terms of water absorption characteristics. The ratio of acrylic acid (salt) is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, and particularly preferably 90 to 100 mol% of the total monofunctional radically polymerizable compound. The acid group-containing radically polymerizable compound may be used alone or in a mixture of two or more.

  When the acid group-containing monofunctional 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.

  Further, when an ethylenically unsaturated monomer is used as the radical polymerizable compound, the neutralization rate of the monomer may be the same as or different from the neutralization rate of the water absorbent resin as the base polymer. It may be. However, the neutralization rate of the ethylenically unsaturated monomer as the radical polymerizable compound is preferably relatively small. According to such a form, the surface treatment can proceed in a short time. Specifically, the neutralization rate of the ethylenically unsaturated monomer as the radical polymerizable compound is preferably 0 to 90 mol%, more preferably 0 to 70 mol%, and still more preferably 0 to 90 mol%. 60 mol%, particularly preferably 0 to 50 mol%, even more preferably 0 to 20 mol%, most preferably 0 to 10 mol%, most particularly preferably 0 mol%. When the neutralization rate is in such a range, it is preferable in terms of improving the surface treatment speed. The “total acid groups of the acid group-containing radical polymerizable compound” and “neutralized acid group” are all acid group radical polymerizable compounds when there are two or more acid group-containing radical polymerizable compounds. It refers to the sum of the acid groups in it 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%.

  In the case where the surface cross-linking of the water-absorbing resin is performed in the presence of a radical polymerizable compound, the amount of the radical polymerizable compound present in the surface cross-linking reaction system is the amount of the water-absorbing resin (100% by weight in terms of solid content). Is preferably 0.5 to 50% by weight, more preferably 1 to 30% by weight, still more preferably 1.5 to 20% by weight, and even more preferably 2 to 15% by weight. Particularly preferred is 2.5 to 10% by weight, and most preferred is 3 to 7% by weight. When the amount of the radical polymerizable compound is 0.5% by weight 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 weight or less, a decrease in the absorption capacity of the surface-treated water-absorbing resin can be prevented.

(3-3) Mixing aid As described above, in the surface cross-linking step of the present invention, the radical polymerization initiator and water (and the surface cross-linking agent and radical polymerizable compound as necessary) are surfaced together with the water-absorbent resin. Although present in the cross-linking reaction system, it is preferable that a mixing aid is further present in the reaction system for the purpose of improving the mixing property of these existing 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. 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, surface cross-linking treatment such as heat treatment described later is performed uniformly on the water-absorbent resin, and the entire water-absorbent resin can be uniformly surface cross-linked.

  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).

  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).

  Among the above examples, polyoxyethylene alkyl 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. In addition, the amount of the mixing aid in the surface crosslinking reaction system is not particularly limited as long as it can suppress aggregation of the water-absorbent resin with water and can improve the mixability of the aqueous solution and the water-absorbent resin. Preferably it is 0.0001-40 weight% with respect to the quantity (100 weight% in conversion of solid content) of a water absorbing resin, More preferably, it is 0.001-10 weight%, Most preferably, it is 0.005-. 5% by weight, most preferably 0.01-1% by weight.

  The order of addition of the mixing aid in the case of using the mixing aid is also not particularly limited, and after adding the mixing aid to the water absorbent resin in advance, water and a radical polymerization initiator (in some cases, including these) A method of adding and mixing an aqueous solution) and a method of dissolving a mixing aid in an aqueous solution and mixing it with the water-absorbent resin can also be used.

(3-4) Mixing The mixing conditions in the case where the above-described components (water-absorbing resin, radical polymerization initiator, water, surface cross-linking agent, radical polymerizable compound, and mixing aid) are mixed in advance are particularly Not limited. For example, the mixing temperature is preferably 0 to 150 ° C., more preferably 10 to 120 ° C., further preferably 20 to 100 ° C., particularly preferably 30 to 90 ° C., and most preferably 40 to 70 ° C. The mixing time is not particularly limited, but is usually 0.1 to 60 minutes. The mixing means is not particularly limited, and a normal mixer such as a V-type mixer, a ribbon-type mixer, a screw-type mixer, a rotating disk-type mixer, an airflow-type mixer, a batch-type kneader, or a continuous type. A kneader, paddle type mixer, vertical mixer or the like can be used as the mixing means.

(3-5) Heat treatment Subsequently, the water-absorbent resin is surface-crosslinked. This surface cross-linking step is performed so that the water content of the water-absorbent resin increases before and after the step. By performing surface cross-linking of the water-absorbent resin in this way, the surface-treated water absorption is excellent in water absorption characteristics (especially absorption capacity under pressure and liquid permeability) at low temperature and in a short time by a low-cost and safe technique. It becomes possible to manufacture a functional resin.

  In order to introduce a cross-linked structure on the surface of the water-absorbent resin, for example, a surface cross-linking reaction system including the above-described components may be heated. Hereinafter, although these surface crosslinking processes are explained in full detail, it is not restrict | limited only to these specific forms.

  In order to surface-crosslink the water-absorbent resin by heat treatment, the reaction system containing the above-described components may be heated, and preferably heated in a relatively high humidity atmosphere. There are no particular restrictions on the specific conditions of such an atmosphere and the specific conditions of the heat treatment, so that the water content of the water absorbent resin increases before and after the surface crosslinking step, in other words, the water content of the water absorbent resin. What is necessary is just to determine suitably so that a rate may increase rather than before a surface crosslinking process.

  As an example of atmospheric conditions for performing the heat treatment, the temperature of the atmosphere is preferably 50 to 150 ° C, more preferably 70 to 130 ° C, still more preferably 90 to 110 ° C, and particularly preferably 95. ~ 100 ° C. The pressure of the atmosphere may be any of reduced pressure, normal pressure, and pressurized pressure, and is not particularly limited, but is preferably 1013 to 4906 hPa, more preferably 1013 to 2758 hPa, and further preferably 1013 to 1450 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). By setting the atmosphere during the heat treatment to saturated water vapor, it becomes possible to reliably increase the water content of the water-absorbent resin obtained by the surface cross-linking step than before the surface cross-linking. The oxygen concentration in the atmosphere 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 to 5% by volume. It is particularly preferably 0 to 1% by volume, and most preferably 0 to 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 30 minutes, and particularly preferably 5 to 15 minutes. It is. 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.

  There is no particular limitation on the apparatus used for the heat treatment when the surface of the water-absorbent resin is cross-linked by heat treatment, and a known dryer can be used, but it has a gas supply device for supplying a gas containing water vapor. Can be preferably used. For example, a conduction heat transfer type, a radiant heat transfer type, a hot air heat transfer type, a dielectric heating type dryer having a gas supply device is preferably used. Specifically, a belt type having a water vapor supply device, A grooved stirring type, fluidized bed type, airflow type, rotary type, kneading type, infrared type, and electron beam type dryer can be preferably used.

(3-6) Liquid permeability improver After the surface cross-linking treatment described above, a liquid permeability improver may be added to the surface-crosslinked water absorbent resin. Examples of such liquid permeability improvers include talc, kaolin, fullerite, bentonite, activated clay, barite, natural asphalt, strontium ore, ilmenite, perlite, and other mineral products; aluminum sulfate 14-18 water Salt (or anhydride), potassium aluminum sulfate 12 hydrate, sodium aluminum sulfate 12 hydrate, aluminum compounds such as aluminum chloride, polyaluminum chloride, aluminum oxide, zirconium sulfate, zirconium nitrate, zirconium acetate, zirconium carbonate, zirconium acetate Zirconium compounds such as ammonium and ammonium zirconium carbonate, and aqueous solutions thereof; other polyvalent metal salts; hydrophilic amorphous silica (eg, dry method: Reolosil QS-20, Tokuyama Corporation, precipitation method: D) EGUSSA Sipernat22S, Sipernat2200); silicon oxide / aluminum oxide / magnesium oxide complex (eg, ENGELHARD Attagel # 50), oxide oxides such as silicon oxide / aluminum oxide complex, silicon oxide / magnesium oxide complex and so on. The addition amount of such a liquid permeability improver is preferably 0 to 20% by weight, more preferably 0.8%, based on the amount of the surface-crosslinked water absorbent resin (100% by weight in terms of solid content). It is 01 to 10% by weight, and particularly preferably 0.1 to 5% by weight. The liquid permeability improver can be added as an aqueous solution if it is soluble in water, and as a powder or slurry if it is not soluble. In addition, as an additive, an antibacterial agent, a deodorant, a chelating agent, and the like may be appropriately added within the above range.

(4) Surface-treated water-absorbent resin In the present invention, the surface-treated water-absorbent resin is produced by applying the above-described treatment to the water-absorbent resin. The water-absorbing resin thus obtained is excellent in water absorption characteristics such as absorption capacity under load and liquid permeability.

  In the present invention, the water content of the water-absorbent resin increases before and after the surface cross-linking step, but the water content of the surface-treated water-absorbent resin obtained after the surface cross-linking step is preferably 1 to 50% by weight. More preferably, it is 2-45 weight%, More preferably, it is 3-40 weight%, Especially preferably, it is 5-30 weight%. Further, the amount of increase in moisture content before and after the surface crosslinking step is not particularly limited, but as an absolute value of the moisture content value (= “water content after the surface crosslinking step” − “water content before the surface crosslinking step”), Preferably it increases by 0.1 to 40% by weight, more preferably by 0.5 to 30% by weight, even more preferably by 1 to 20% by weight, particularly preferably by 2 to 15% by weight. 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.

  Conventionally, when surface cross-linking of a water-absorbent resin is formed, the centrifuge retention capacity (CRC) is slightly reduced, but the ability to maintain liquid absorption even under pressure, that is, the absorption capacity under pressure is improved. It has been. According to the method of the present invention, the absorption capacity under pressure (hereinafter also simply referred to as “AAP”) at 4.83 kPa of the water-absorbent resin is increased by 1 g / g or more even if the surface cross-linking agent is not essential. . This indicates that a crosslinked structure has been introduced on the surface of the water absorbent resin by the method of the present invention. In a preferred embodiment, the absorption capacity under pressure of the water-absorbent resin is preferably increased by 8 g / g or more, more preferably increased by 10 g / g or more, and further preferably increased by 12 g / g or more before and after the surface crosslinking step. To do. Moreover, the value of AAP of the obtained surface-treated water-absorbing resin is preferably 15 to 30 g / g, more preferably 18 to 25 g / g, and further preferably 20 to 24 g / g. In addition, as a value of AAP, a value (a value after moisture content correction) measured by a method described in Examples described later is adopted.

  Further, the centrifuge retention capacity (CRC) of the surface-treated water-absorbent resin is preferably 8 g / g or more, more preferably 15 g / g or more, still more preferably 20 g / g or more. Preferably it is 25 g / g or more. Although an upper limit is not specifically limited, Preferably it is 50 g / g or less, More preferably, it is 40 g / g or less, More preferably, it is 35 g / g or less. If CRC is 8 g / g or more, a water-absorbent resin that is sufficiently suitable for the use of sanitary materials such as diapers can be obtained. On the other hand, if CRC is 50 g / g or less, sufficient gel strength is ensured and the water absorbing resin excellent in liquid permeability will be obtained. In addition, as a CRC value, a value (value after moisture content correction) measured by a method described in an example described later is adopted.

Further, in the obtained surface-treated water-absorbent resin, the value of the saline flow conductivity (SFC) serving as a liquid permeability index is preferably 10 (unit: 10 ⁻⁷ × cm ³ × s × g ^−1). ) Or more, more preferably 15 (unit: 10 ⁻⁷ × cm ³ × s × g ⁻¹ ) or more, and further preferably 20 (unit: 10 ⁻⁷ × cm ³ × s × g ⁻¹ ) or more. Especially preferably, it is 50 (unit: 10 ⁻⁷ × cm ³ × s × g ⁻¹ ) or more. Although an upper limit is not specifically limited, Preferably it is 300 (unit: 10 <^-7> * cm < ^{3 >} * s * g <^-1> ) or less. If the value of the saline flow conductivity (SFC) is 10 (unit: 10 ⁻⁷ × cm ³ × s × g ⁻¹ ) or more, sufficient liquid permeability is secured, and the liquid is sufficiently distributed to the absorber. And excretory fluid such as urine during use can be absorbed reliably. In addition, as a value of SFC, a value measured by a method described in an example described later is adopted.

  The shape of the surface-treated water-absorbent resin obtained by the present invention can be appropriately adjusted depending on the treatment conditions such as the shape of the water-absorbent resin before treatment, granulation / molding after treatment, etc. is there. Such powder has a weight average particle size (specified by sieve classification) of 150 to 850 μm, preferably 200 to 600 μm, more preferably 300 to 500 μm, and has a particle size of 150 to 850 μm. However, it is preferably 90 to 100% by weight, more preferably 95 to 100% by weight, based on the total amount of the water-absorbent resin. 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.

  The surface-treated water-absorbent resin obtained by the method of the present invention has a higher cross-link density near the surface than the inside, preferably a uniform and high cross-link surface cross-link is formed over the entire water-absorbent resin surface. Desired characteristics such as absorption properties, absorption speed, gel strength, suction force, liquid permeability and the like can be set to extremely high levels.

  Since the surface-treated water-absorbent resin obtained by the method of the present invention has excellent characteristics as described above, the field of hygiene materials such as sanitary products, disposable diapers, and incontinence pads, medical fields such as medical supplies, Suitable for various fields such as agricultural and horticultural fields such as soil water retention agents, 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, oxidation Inhibitors, surfactants, silicone oils, chelating agents, deodorants, fragrances, drugs, plant growth aids, fungicides, fungicides, foaming agents, pigments, dyes, fibrous materials (hydrophilic short fibers, Pulp, synthetic fibers, etc.), other additives such as fertilizers can be added. The amount of other additives added is preferably about 0.001 to 10% by weight with respect to the total product weight in various applications.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention more concretely, the technical scope of this invention is not limited to these. Hereinafter, for convenience, “% by weight” 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. Thereby, the particle diameter corresponding to R = 50% by weight was read as the weight average particle diameter (D50).

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

(2) 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 (weight%) was computed according to following Formula.

(3) Centrifuge holding capacity (CRC)
0.200 g of water-absorbent resin was uniformly put into a bag (85 mm × 60 mm) made of a nonwoven fabric (trade name: Heaton Paper, model: GSP-22, manufactured by Nankoku Pulp Industries Co., Ltd.) and heat sealed, and then at room temperature. It was immersed in a large excess (about 500 mL) of physiological saline. After 30 minutes, the bag was pulled up and drained for 3 minutes with a centrifugal force (250 G) described in edena ABSORBENCY II 441.1-99 using a centrifuge (manufactured by Kokusan Co., Ltd., model: H-122). The weight W3 (g) of the bag was measured. Moreover, the same operation was performed without using a water absorbent resin, and the weight at that time was set to W4 (g). And CRC (g / g) was computed from the value of these W3 and W4 according to the following numerical formula.

(4) Absorption capacity under pressure (AAP)
A stainless steel 400 mesh wire mesh (aperture size 38 μm) is fused to the bottom of a plastic support cylinder with an inner diameter of 60 mm, and water is absorbed on the wire mesh at room temperature (25 ± 2 ° C.) and humidity 50 RH%. 0.900 g of the water-soluble resin 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 wt% sodium chloride aqueous solution (saline) (20 to 25 ° C.) was added to the same level as the upper surface of the glass filter. On top of that, a sheet of filter paper having a diameter of 90 mm (manufactured by 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.

(5) Saline flow conductivity (SFC)
The saline flow conductivity (SFC) is a value indicating the liquid permeability (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 unit of the SFC value is (10 ⁻⁷ × cm ³ × s × g ⁻¹ ).

  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. It was arranged so that it could be maintained at a height of 5 cm above the bottom of the. The 0.69 wt% sodium chloride aqueous solution 33 in the tank 31 was supplied to the cell 41 through the L-shaped tube 34. 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 wt% sodium chloride aqueous solution 33 was passed through the swelled gel layer from the tank 31 at 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.

(Production Example 1)
5433 g of an aqueous solution of sodium acrylate having a neutralization rate of 70 mol% (monomer) in a reactor formed by attaching a lid to a stainless steel double arm type kneader with an internal volume of 10 L and having two sigma blades (Concentration: 39% by weight) (24.2 mol) was charged, and 12.83 g (0.0246 mol) of polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9) as an internal crosslinking agent was dissolved in the aqueous solution. To prepare a reaction solution. Subsequently, this reaction liquid was deaerated under a nitrogen gas atmosphere. Subsequently, 29.43 g of a 10 wt% aqueous solution of sodium persulfate as a polymerization initiator and 24.53 g of a 0.1 wt% aqueous solution of L-ascorbic acid were added to the reaction solution while stirring. As a result, polymerization started after about 1 minute. Then, polymerization was performed at 20 to 95 ° C. while pulverizing the generated gel, and a hydrous gel-like crosslinked polymer was taken out 30 minutes after the polymerization started. The particle size of the obtained hydrogel crosslinked polymer was 5 mm or less. The crushed water-containing gel-like crosslinked polymer was spread on a 50 mesh (mesh opening 300 μm) wire net and dried with hot air at 175 ° C. for 50 minutes. In this way, an agglomerated powdery aggregate that can be easily pulverized was obtained.

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

(Production Example 2)
Production Example except that the amount of polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9) was changed to 11.55 g (0.0221 mol), and sieve openings 710 and 150 μm to be classified were changed to 850 μm and 212 μm. In the same manner as in Example 1, a water absorbent resin (B) was obtained.

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

(Example 1-1)
1. Add 500 g of the water-absorbent resin (A) as a base polymer to a 5 L Leedige mixer (Model M5R, manufactured by Loedige), and stir at 300 rpm with polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9) A treatment liquid in which 0 g (0.00192 mol), 27.4 g (0.381 mol) of acrylic acid, 45.0 g of water, and 5.0 g of ammonium persulfate were mixed in advance was sprayed. Stirring was continued for 3 minutes at room temperature, and the added water was permeated and diffused inside the particles. Then, the stirring was stopped once, and the sample inlet of the Leedige mixer was removed.

  10 g of water-absorbing resin (water content: 13.0% by weight) discharged from the Roedige mixer was uniformly spread on a circular stainless steel petri dish having a diameter of 9 cm and a depth of 1.5 cm.

  Next, the water-absorbent resin was put together with this petri dish into a water oven (manufactured by Sharp Corporation, trade name: Helcio, model name: AX-HC3), and heated in a steamed (strong) mode for 1 minute. In the treatment in this mode, the object to be treated is heated with saturated steam, and thus the heating temperature is 100 ° C. Moreover, the pressure in the water oven at the time of a heating was a normal pressure (1013 hPa), and the oxygen concentration in a water oven was 0.5 volume% or less. The water oven was preheated by operating in a steamed (strong) mode for 15 minutes immediately before the above operation.

  The water-absorbent resin obtained by the above treatment was pulverized until it passed through a JIS standard sieve having an opening of 850 μm to obtain a surface-treated water-absorbent resin (1-1). Various evaluation results of the obtained surface-treated water-absorbing resin (1-1) are shown in Table 2 below. In addition, the amount of each component in the column of “treatment liquid” in Table 2 is weight% (wt%) with respect to the amount (500 g) of the water absorbent resin (A) as the base polymer. In addition, “CRC after moisture content correction” and “AAP after moisture content correction” shown in Table 2 below were calculated by the following calculation formulas. Here, in the following formula, “CRC before moisture content correction” is the centrifuge retention capacity (CRC) of the water absorbent resin before measuring the moisture content in (2) above, “AAP before correction” is the absorption capacity under pressure (AAP) of the water-absorbent resin before measuring the water content in (2) above.

(Example 1-2)
A surface-treated water absorbent resin (1-2) was obtained in the same manner as in Example 1-1 described above, except that the heating time of the water absorbent resin in the water oven was 3 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (1-2) are shown in Table 2 below.

(Example 1-3)
A surface-treated water absorbent resin (1-3) was obtained in the same manner as in Example 1-1 described above except that the heating time of the water absorbent resin in the water oven was 5 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (1-3) are shown in Table 2 below.

(Example 1-4)
A surface-treated water absorbent resin (1-4) was obtained in the same manner as in Example 1-1 described above, except that the heating time of the water absorbent resin in the water oven was 10 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (1-4) are shown in Table 2 below.

(Example 1-5)
A surface-treated water absorbent resin (1-5) was obtained by the same method as in Example 1-1 described above, except that the heating time of the water absorbent resin in the water oven was 15 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (1-5) are shown in Table 2 below.

(Example 2-1)
As the treatment liquid, 1.0 g (0.00192 mol) of polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9), 4.0 g (0.056 mol) of acrylic acid, 10.0 g of sodium acrylate (0.0. 106 mol), 45.0 g of water, and 5.0 g of ammonium persulfate in advance, except that a liquid prepared in advance was used in the same manner as in Example 1-2. ) The water content of the water-absorbent resin discharged from the Raedige mixer after stirring and mixing the base polymer and the treatment liquid was 13.3% by weight. Various evaluation results of the obtained surface-treated water-absorbing resin (2-1) are shown in Table 2 below.

(Example 2-2)
A surface-treated water absorbent resin (2-2) was obtained by the same method as in Example 2-1 described above, except that the heating time of the water absorbent resin in the water oven was 5 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (2-2) are shown in Table 2 below.

(Example 2-3)
A surface-treated water-absorbing resin (2-3) was obtained in the same manner as in Example 2-1 described above, except that the heating time of the water-absorbing resin in the water oven was 10 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (2-3) are shown in Table 2 below.

(Example 2-4)
A surface-treated water absorbent resin (2-4) was obtained by the same method as in Example 2-1 described above except that the heating time of the water absorbent resin in the water oven was 15 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (2-4) are shown in Table 2 below.

(Example 3-1)
Example 1-3 described above except that 2.5 g of polyethylene glycol monomethyl ether (number average molecular weight: about 2000), 40.0 g of water, and 5.0 g of ammonium persulfate were used as the treatment liquid. A surface-treated water-absorbing resin (3-1) was obtained by the same method. The water content of the water-absorbent resin discharged from the Raedige mixer after stirring and mixing the base polymer and the treatment liquid was 12.8% by weight. Various evaluation results of the obtained surface-treated water-absorbing resin (3-1) are shown in Table 2 below.

(Example 3-2)
A surface-treated water absorbent resin (3-2) was obtained in the same manner as in Example 3-1 except that the heating time of the water absorbent resin in the water oven was 10 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (3-2) are shown in Table 2 below.

(Example 3-3)
A surface-treated water absorbent resin (3-3) was obtained in the same manner as in Example 3-1 except that the heating time of the water absorbent resin in the water oven was 30 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (3-3) are shown in Table 2 below.

(Example 4-1)
As the base polymer, 500 g of the water-absorbing resin (B) was used, and as the treatment liquid, NK ester 701A (manufactured by Shin-Nakamura Chemical Co., Ltd., glycerin acrylate methacrylate) 0.5 g (0.00234 mol), acrylic acid 20. 0 g (0.278 mol), water 35.0 g, polyethylene glycol monomethyl ether (number average molecular weight of about 2,000) 0.05 g, VA-044 (manufactured by Wako Pure Chemical Industries, Ltd., 2,2′-azobis (2 -(2-Imidazolin-2-yl) propane) dihydrochloride) A water-absorbed surface treated in the same manner as in Example 1-2, except that 0.5 g of a premixed solution was used. Resin (4-1) was obtained. The water content of the water-absorbent resin discharged from the Raedige mixer after stirring and mixing the base polymer and the treatment liquid was 10.5% by weight. Various evaluation results of the obtained surface-treated water-absorbing resin (4-1) are shown in Table 2 below.

(Example 4-2)
A surface-treated water absorbent resin (4-2) was obtained in the same manner as in Example 4-1 except that the heating time of the water absorbent resin in the water oven was 5 minutes. Various evaluation results of the obtained surface-treated water absorbent resin (4-2) are shown in Table 2 below.

(Example 4-3)
A surface-treated water absorbent resin (4-3) was obtained in the same manner as in Example 4-1 except that the heating time of the water absorbent resin in the water oven was 10 minutes. Various evaluation results of the obtained surface-treated water-absorbing resin (4-3) are shown in Table 2 below.

(Comparative Example 1)
As a treatment liquid, a comparison was made by the same method as in Example 3-3, except that a liquid in which 2.5 g of polyethylene glycol monomethyl ether (number average molecular weight: about 2000) and 40.0 g of water were mixed in advance was used. A water absorbent resin (1) was obtained. The water content of the water-absorbent resin discharged from the Laedige mixer after stirring and mixing the base polymer and the treatment liquid was 12.9% by weight. Various evaluation results of the obtained comparative water-absorbent resin (1) are shown in Table 2 below.

(Comparative Example 2-1)
A comparative water absorbent resin (2-1) was prepared in the same manner as in Example 3-3 described above except that the mode of the water oven was water oven cake (set temperature: 120 ° C.) and the heating time was 3 minutes. ) In the treatment in this mode, the mixture is heated by superheated steam that is further heated after vaporization. Moreover, the pressure in the water oven at the time of a heating was a normal pressure (1013 hPa), and the oxygen concentration in a water oven was 0.5 volume% or less. The water oven was preheated by operating for 15 minutes in the water oven / cake (set temperature: 120 ° C.) mode immediately before the above operation. Various evaluation results of the obtained comparative water absorbent resin (2-1) are shown in Table 2 below.

(Comparative Example 2-2)
A comparative water absorbent resin (2-2) was obtained in the same manner as in Comparative Example 2-1, except that the heating time of the water absorbent resin in the water oven was 5 minutes. Various evaluation results of the obtained comparative water absorbent resin (2-2) are shown in Table 2 below.

(Comparative Example 3)
As a means for heating the water-absorbent resin treated with the treatment liquid, a hot-air dryer (HISPEC HT320, manufactured by Enomoto Kasei Co., Ltd., manufactured by Enomoto Kasei Co., Ltd.) was used instead of a water oven, and heat treatment was performed at 120 ° C. for 30 minutes. A comparative water-absorbent resin (3) was obtained by the same method as in Comparative Example 2-1 described above except that. Various evaluation results of the obtained comparative water absorbent resin (3) are shown in Table 2 below.

(Comparative Example 4)
A comparative water absorbent resin (4) was obtained in the same manner as in Comparative Example 3 described above except that the heating temperature was 160 ° C. Various evaluation results of the obtained comparative water absorbent resin (4) are shown in Table 2 below.

(Comparative Example 5)
As the treatment liquid, 1.0 g (0.00192 mol) of polyethylene glycol diacrylate (average number of ethylene oxide units: n = 9), 4.0 g (0.056 mol) of acrylic acid, 10.0 g of sodium acrylate (0.0. 106 mol), 45.0 g of water, and 0.05 g of ammonium persulfate in advance were used, and a comparative water absorbent resin (5) was obtained by the same method as in Comparative Example 4 described above. The water content of the water-absorbent resin discharged from the Raedige mixer after stirring and mixing the base polymer and the treatment liquid was 12.5% by weight. Various evaluation results of the obtained comparative water-absorbing resin (5) are shown in Table 2 below.

(Comparative Example 6)
A comparative water absorbent resin (6) was obtained in the same manner as in Comparative Example 5 except that the heating temperature was 210 ° C. Various evaluation results of the obtained comparative water-absorbent resin (6) are shown in Table 2 below.

(Comparative Example 7)
A comparative water absorbent resin (7) was obtained in the same manner as in Comparative Example 5 described above except that the heating temperature was 80 ° C. and the amount of ammonium persulfate in the treatment liquid was 25.0 g. Various evaluation results of the comparative water absorbent resin (7) obtained are shown in Table 2 below.

  From the results shown in Table 2, surface-crosslinking the water-absorbent resin in the presence of a radical polymerization initiator and water so that the water content of the water-absorbent resin increases (for example, heating in saturated steam). It can be seen that the water-absorbing properties (particularly the absorption capacity under load (AAP) / liquid permeability (SFC)) of the surface-crosslinked water-absorbing resin obtained are improved. In addition, when the above surface crosslinking is performed under the condition where a surface crosslinking agent and a radical polymerizable compound such as acrylic acid (salt) are further present, the water absorption properties of the surface crosslinked water absorbent resin can be further improved. Recognize.

Claims (11)

A surface treatment method for a water-absorbent resin comprising a surface crosslinking step of surface-crosslinking a water-absorbent resin in the presence of a radical polymerization initiator and water,
In the surface cross-linking step, the surface cross-linking reaction system is not irradiated with an active energy ray having a wavelength equal to or shorter than the wavelength of ultraviolet rays, and
A surface treatment method for a water-absorbent resin, wherein the surface cross-linking step is performed so that the water content of the water-absorbent resin increases before and after the step.   The surface treatment method according to claim 1, wherein in the surface crosslinking step, the water absorbent resin is heated.   The surface treatment method according to claim 2, wherein the heating is performed in saturated steam.   The amount of the radical polymerization initiator in the surface crosslinking reaction system in the surface crosslinking step is 0.01 to 20% by weight based on the amount of the water absorbent resin. The surface treatment method according to item.   The surface treatment method according to claim 1, wherein the surface crosslinking step is performed in the presence of a radical polymerizable compound.   The surface treatment method according to claim 5, wherein an amount of the radical polymerizable compound in the surface crosslinking reaction system in the surface crosslinking step is 0.5 to 50 wt% with respect to the amount of the water absorbent resin. . The radical polymerizable compound includes an acid group-containing radical polymerizable compound,
The surface treatment method of Claim 5 or 6 whose neutralization rate of the said acid group containing radically polymerizable compound is 0-60 mol%.   The surface treatment method according to claim 1, wherein the water-absorbent resin is obtained by polymerization of an unsaturated monomer having acrylic acid (salt) as a main component. The surface treatment method according to any one of claims 1 to 8, wherein, in the surface cross-linking step, the water-absorbent resin is heated under an atmospheric condition of a temperature of 90 to 110 ° C. The surface treatment method according to any one of claims 1 to 9, wherein, in the surface cross-linking step, the water-absorbent resin is heated under an atmospheric condition of a relative humidity of 70 to 100% RH. The surface treatment method according to any one of claims 1 to 10, wherein an increase in water content before and after the surface crosslinking step is 0.1 to 40% by weight as an absolute value of a water content value.

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