Method for producing water absorbent resin particles

Abstract

Provided is a production method with which it is possible to obtain water absorbent resin particles that have excellent water absorption performance.　The present invention provides a method for producing water absorbent resin particles, the method comprising at least: a washing step for washing polymer particles that have a water content of not less than 10 mass% but less than 120 mass% with an organic solvent; and a surface crosslinking step for surface-crosslinking the polymer particles that have been washed in the washing step. This method for producing water absorbent resin particles comprises the washing step and the surface crosslinking step, whereby the obtained water absorbent resin particles have excellent water absorption performance.

Description

Method for producing water-absorbent resin particles

[0001] The present invention relates to a method for producing water-absorbent resin particles.

[0002] Owing to their excellent water absorption capacity, water-absorbent resin particles are widely used, for example, in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads. In addition, utilizing the water-blocking effect of water-absorbent resin particles, they are also used as water-blocking materials for communication cables such as optical cables, power cables, and the like.

[0003] It is known that water-absorbent resin particles are produced, for example, by methods using various polymerization techniques, and among these, a method for producing water-absorbent resin particles using reversed-phase suspension polymerization is widely known. In such a method, polymer particles are produced by one-stage or two-stage or more reversed-phase suspension polymerization, after the obtained polymer particles are dehydrated, if necessary, the polymer particles are subjected to surface crosslinking treatment (so-called post-crosslinking), and then subjected to treatments such as drying, to thereby obtain the target water-absorbent resin particles.

[0004] As described above, in the water-absorbent resin particles obtained by various polymerization methods, various additives, such as dispersion stabilizers and surfactants, are used during the production process. If such additives are not sufficiently removed, there is a risk of causing deterioration in various physical properties such as water absorption performance of the water-absorbent resin particles. Therefore, efficient washing of used additives and the like in the process of producing water-absorbent resin particles is extremely important for improving the physical properties of the water-absorbent resin particles.

[0005] For example, Patent Document 1 discloses a technique in which a water-absorbent resin is washed with water or a water-miscible organic solvent to improve the gel strength and water absorption capacity of the water-absorbent resin, and also improve the stickiness of the gel surface.

[0006] Japanese Unexamined Patent Publication No. 10-324750

[0007] However, various polymerization methods, such as reverse-phase suspension polymerization, use various additives, and washing with solvents alone is not always sufficient for thorough cleaning. As a result, there have been cases where the physical properties of the water-absorbing resin particles have deteriorated due to the effects of additives remaining after polymerization. In particular, surfactants used in the manufacturing process of water-absorbing resin particles can be difficult to remove completely depending on their type. Furthermore, if the water-absorbing resin particles are subjected to high-temperature treatment, such as the surface crosslinking treatment mentioned above, decomposition products of surfactants and other additives may be generated, which could lead to a further deterioration of the physical properties of the water-absorbing resin particles. From these perspectives, it is important to improve the water absorption performance (especially the 10-minute value of unpressurized DW) of the resulting water-absorbing resin particles by increasing the washing efficiency in the manufacturing process.

[0008] The present invention has been made in view of the above, and aims to provide a manufacturing method that can produce water-absorbing resin particles having excellent water absorption performance.

[0009] As a result of diligent research to achieve the above objective, the inventors of the present invention have discovered that the above objective can be achieved by a method comprising the steps of washing polymer particles with a predetermined water content using an organic solvent and surface crosslinking the polymer particles after washing, and have completed the present invention.

[0010] In other words, the present invention encompasses, for example, the subject matter described in the following sections: Section 1 A method for producing water-absorbent resin particles, comprising at least a washing step of washing polymer particles having a water content of 10% by mass or more and less than 120% by mass with an organic solvent, and a surface crosslinking step of surface crosslinking the polymer particles washed by the washing step. Section 2 The method for producing water-absorbent resin particles according to Section 1, wherein in the washing step, the polymer particles are repeatedly washed with the organic solvent. Section 3 The method for producing water-absorbent resin particles according to Section 1 or 2, further comprising a step of obtaining polymer particles by reverse-phase suspension polymerization before the washing step.

[0011] According to the method for producing water-absorbing resin particles of the present invention, water-absorbing resin particles having excellent water absorption performance can be obtained.

[0012] This is a schematic diagram of a measuring device for measuring the 10-minute value of unpressurized DW.

[0013] Embodiments of the present invention will be described in detail below. In this specification, the expressions "containing" and "including" include the concepts of "containing," "including," "substantially consisting of," and "consisting only of."

[0014] In the numerical ranges described in stages in this specification, the upper or lower limit of one stage of the numerical range can be arbitrarily combined with the upper or lower limit of another stage of the numerical range. In the numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with values ​​shown in the examples or values ​​that can be uniquely derived from the examples. Also, in this specification, numbers connected by "~" mean a numerical range that includes the numbers before and after "~" as the lower and upper limits. The concentrations and amounts used for each component described in this specification are based on the total amount of multiple substances if there are multiple substances corresponding to each component, unless otherwise specified.

[0015] 1. Method for producing water-absorbent resin particles The method for producing water-absorbent resin particles of the present invention comprises at least a washing step of washing polymer particles having a water content of 10% by mass or more and less than 120% by mass with an organic solvent, and a surface crosslinking step of crosslinking the polymer particles washed by the washing step.

[0016] According to the manufacturing method of the present invention, water-absorbing resin particles having excellent water absorption performance can be obtained. In particular, the manufacturing method of the present invention requires a step of washing polymer particles having a predetermined water content with an organic solvent, which allows for sufficient removal of additives such as surfactants used in the polymerization process, thereby suppressing a deterioration in the physical properties of the water-absorbing resin particles caused by the residue of such additives.

[0017] (Washing process) The washing process is a process for washing polymer particles having a water content of 10% by mass or more and less than 120% by mass with an organic solvent.

[0018] The polymer particles targeted for cleaning in this cleaning process are particles that constitute water-absorbent resin particles. As polymer particles, for example, a wide range of polymer particles applicable to water-absorbent resin particles can be used, and among these, polymers containing water-soluble ethylenically unsaturated monomers are preferred. The method for producing polymer particles is not particularly limited and they can be produced by various polymerization reactions, but it is preferable to produce them by a polymerization process that performs reverse-phase suspension polymerization as described later, and it is even more preferable to produce them through a post-polymerization treatment process that follows the polymerization process. Details of the polymerization process and the post-polymerization treatment process will be described later.

[0019] The polymer particles used in the washing process have a water content of 10% by mass or more and less than 120% by mass. That is, before the washing process begins, the water content of the polymer particles is 10% by mass or more and less than 120% by mass. If the water content of the polymer particles used in the washing process falls below 10% by mass, the polymer particles may not be washed sufficiently, and it may not be possible to improve the physical properties of the resulting water-absorbing resin particles. On the other hand, if the water content of the polymer particles used in the washing process is 120% by mass or more, the polymer particles are prone to degradation, and it may not be possible to improve the physical properties of the resulting water-absorbing resin particles.

[0020] The water content of the polymer particles used in the washing process is preferably 20% by mass or more, more preferably 25% by mass or more, even more preferably 30% by mass or more, particularly preferably 35% by mass or more, and also preferably 110% by mass or less, more preferably 100% by mass or less, even more preferably 90% by mass or less, even more preferably 80% by mass or less, and particularly preferably 70% by mass or less.

[0021] The method for measuring the water content of polymer particles used in the washing process will be described later.

[0022] In the cleaning process, the polymer particles are cleaned using an organic solvent. The type of organic solvent is not particularly limited, and for example, well-known organic solvents can be widely used in this invention.

[0023] Examples of organic solvents include hydrocarbon solvents, ether compounds, ester compounds, chlorinated hydrocarbons, ketone compounds, carbonate compounds, and alcohol compounds.

[0024] Examples of hydrocarbon solvents include aliphatic hydrocarbons having 6 to 8 carbon atoms such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such as benzene, toluene, and xylene. Among these hydrocarbon solvents, n-hexane, n-heptane, and cyclohexane are preferably used. Hydrocarbon solvents may be used individually or in combination of two or more types. As an example of a hydrocarbon solvent mixture, commercially available products such as Exsolheptane (manufactured by ExxonMobil: containing 75-85% by mass of heptane and its isomers) can also be used to obtain satisfactory results.

[0025] Examples of ether compounds include diethyl ether, dimethyl ether, tetrahydrofuran, methylphenyl ether (anisole), t-butyl methyl ether, and propyl ether.

[0026] Examples of ester compounds include ethyl acetate, vinyl acetate, n-butyl acetate, isobutyl acetate, t-butyl acetate, pentyl acetate, amyl acetate, isoamyl acetate, methoxybutyl acetate, n-propyl acetate, isopropyl acetate, methyl propionate, and ethyl propionate.

[0027] Examples of chlorinated hydrocarbons include chloroform, dichloromethane, and 1,2-dichloroethane.

[0028] Examples of ketone compounds include acetone and methyl ethyl ketone.

[0029] Examples of carbonate compounds include dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0030] Examples of alcohol compounds include methanol, ethanol, isopropyl alcohol, and t-butanol.

[0031] Other organic solvents include N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0032] In particular, the organic solvent used in the washing process can preferably have a relative permittivity of 45 or less. In this case, the washing efficiency can be increased, and the water absorption performance of the resulting water-absorbing resin particles (especially the 10-minute value of unpressurized DW) can be further improved. The relative permittivity of the organic solvent is more preferably 30 or less, even more preferably 25 or less, even more preferably 15 or less, and particularly preferably 8 or less. The lower limit of the relative permittivity is not particularly limited, but for example, it is 1 or more, preferably 1.5 or more.

[0033] The organic solvent used in the washing process has a dielectric constant within the above range and is preferably a hydrocarbon solvent, ether compound, ester compound, or alcohol compound. Among these, n-heptane (dielectric constant 1.8), ethyl acetate (dielectric constant 6.0), tetrahydrofuran (dielectric constant 7.6), acetone (dielectric constant 20.6), ethanol (dielectric constant 24.6), 2-propanol (dielectric constant 19.9), and anisole (dielectric constant 4.3) are examples of preferred organic solvents. N-heptane, ethyl acetate, and tetrahydrofuran are more preferred as organic solvents used in the washing process because they easily suppress the odor of the resulting water-absorbing resin particles and also improve washing efficiency.

[0034] The organic solvent used in the washing process may be a single type or a mixed solvent containing two or more types.

[0035] Furthermore, when polymer particles are produced by a polymerization reaction, if a solvent (for example, a hydrocarbon solvent as described later) is used in such a polymerization reaction, the solvent used in the washing step may be the same as the reaction solvent or may be different.

[0036] Next, a method for cleaning polymer particles with an organic solvent will be described in the cleaning process. In the cleaning process, for example, cleaning can be performed by bringing the polymer particles into contact with an organic solvent. In this case, for example, the polymer particles can be cleaned by adding an organic solvent to the polymer particles contained in a container and mixing the polymer particles with the organic solvent.

[0037] The method for bringing polymer particles into contact with an organic solvent is not particularly limited. For example, the polymer particles can be brought into contact with the organic solvent by stirring the polymer particles in the organic solvent. In this case, the cleaning efficiency tends to be higher.

[0038] When stirring polymer particles in an organic solvent, the polymer particles can be washed by mixing them with the organic solvent using a stirrer equipped with known stirring blades. Preferably, the stirring conditions are such that the tip speed of the stirring blade is 1.0 m / s or higher. In this case, the washing efficiency of the polymer particles can be increased, and the water absorption performance (especially the 10-minute value of unpressurized DW) of the resulting water-absorbing resin particles tends to be higher. The tip speed of the stirring blade is more preferably 1.5 m / s or higher, even more preferably 2.0 m / s or higher, and preferably 10.0 m / s or lower.

[0039] In the washing process, the temperature of the organic solvent used to wash the polymer particles is preferably 90°C or lower. In this case, it is easier to suppress changes in the water content of the polymer particles during the washing process, and as a result, the water absorption performance of the resulting superabsorbent resin particles (especially the 10-minute value of unpressurized DW) tends to be higher. The temperature of the organic solvent used to wash the polymer particles is preferably 20°C or higher, more preferably 30°C or higher, even more preferably 40°C or higher, and particularly preferably 50°C or higher.

[0040] In the washing process, the amount of organic solvent used when washing polymer particles can be appropriately set according to the amount of polymer particles to be washed. For example, if the amount of surfactant used in the synthesis of polymer particles is known, it is preferable to use 135 parts by mass or more of organic solvent per 1 part by mass of surfactant used. In this case, the surfactant is easier to remove, and the water absorption performance of the resulting superabsorbent resin particles (especially the 10-minute value of unpressurized DW) can be further improved.

[0041] The amount of organic solvent used per 1 part by mass of surfactant is more preferably 200 parts by mass or more, even more preferably 300 parts by mass or more, even more preferably 350 parts by mass or more, particularly preferably 400 parts by mass or more, and also preferably 1000 parts by mass or less, more preferably 800 parts by mass or less, even more preferably 700 parts by mass or less, and particularly preferably 600 parts by mass or less. As will be described later, washing with organic solvents can be repeated, and the amount of organic solvent used above refers to the amount used per washing.

[0042] If the amount of surfactant used in the synthesis of polymer particles is unknown, for example, the amount of organic solvent used per 100 parts by mass of polymer particles is preferably 10 parts by mass or more, more preferably 30 parts by mass or more, even more preferably 50 parts by mass or more, and also preferably 300 parts by mass or less, more preferably 200 parts by mass or less, and even more preferably 150 parts by mass or less.

[0043] In the washing process, the washing time for polymer particles with an organic solvent can be set within an appropriate range depending on the amount of organic solvent used and the temperature, etc. For example, it can be 1 minute or more, preferably 3 minutes or more, more preferably 5 minutes or more, even more preferably 120 minutes or more, preferably 90 minutes or less, more preferably 60 minutes or less, and even more preferably 40 minutes or less. Note that, as will be described later, washing with an organic solvent can be repeated, and the above washing time refers to the time per washing cycle.

[0044] When washing polymer particles by stirring them in an organic solvent, it is also possible to allow the polymer particles to stand still for a predetermined period of time after stopping the stirring. In this case, the standing time can also be included in the above washing time.

[0045] In the washing step, after washing the polymer particles with the organic solvent, the used organic solvent can be removed by an appropriate method. For example, the used organic solvent can be removed by decantation or filtration. The amount of the organic solvent removed may be, for example, at least 40% by mass based on the total mass of the used organic solvent, 90% by mass or more may be removed, or the entire amount may be removed.

[0046] As described above, the washing step can be completed by washing the polymer particles and removing the organic solvent, or after washing the polymer particles and removing the organic solvent, the polymer particles can be washed again using a fresh organic solvent (an organic solvent that has not been used for washing).

[0047] That is, in the washing step, the polymer particles can be repeatedly washed with the organic solvent. Specifically, when the operation from bringing the polymer particles into contact with the organic solvent for washing to removing the predetermined amount of the organic solvent as described above is defined as one washing operation, two or more such washing operations can be performed in the washing step. This allows the polymer particles to be washed easily, and particularly improves the water absorption performance (especially the 10-minute value of non-pressurized DW) of the resulting water-absorbent resin particles.

[0048] When the washing operation is repeatedly performed in the washing step, the number of repetitions is preferably 2 or more, more preferably 3 or more, and preferably 10 or less, more preferably 8 or less, still more preferably 6 or less, and particularly preferably 5 or less.

[0049] When the washing operation is repeatedly performed in the washing step, the type and amount of the organic solvent used in each washing operation and other washing conditions may all be the same, or may be partially different.

[0050] In the cleaning process, when the cleaning operation is repeated, the amount of organic solvent removed at the end of each cleaning operation is at least 40% by mass, as described above. That is, in each cleaning operation, it is preferable to replace at least 40% by mass of the total organic solvent with a new solvent.

[0051] The washing process can remove, for example, surfactants contained in polymer particles. In particular, in the present invention, by washing polymer particles whose water content has been adjusted to a predetermined range with an organic solvent, surfactants can be efficiently removed, and as a result, water-absorbing resin particles with excellent water absorption performance can be obtained. Although we do not necessarily want a restrictive interpretation, it is presumed that when polymer particles have a specific water content, surfactants present inside the polymer particles can more easily escape to the outside, making removal easier.

[0052] According to the above cleaning process, surfactants used in the polymerization process can be easily removed, for example. Examples of such surfactants include the various surfactants used in the polymerization process described later. In particular, even if surfactants with an HLB of 6 or less remain, they can be easily removed by the cleaning process. The HLB value is known to indicate the affinity of a surfactant for water and oil, and can be determined by the Griffin method.

[0053] Here, as an example, we will describe the washing process when the surfactant used in the polymerization process is a sucrose fatty acid ester. When sucrose fatty acid ester remains in the polymer particles, and the polymer particles are heat-treated, the heat can cause the sucrose fatty acid ester to denature. In particular, monoesters and diesters of sucrose fatty acid ester with a low degree of ester substitution are easily denatured by heat treatment, and if these remain in the polymer particles, it is likely to cause a deterioration in the physical properties of the resulting water-absorbing resin particles. In this respect, according to the washing process described above, even if monoesters or diesters of sucrose fatty acid ester with a low degree of ester substitution remain in the polymer particles, these are easily removed, and as a result, it is easier to prevent a deterioration in the physical properties of the water-absorbing resin particles.

[0054] (Surface Crosslinking Step) The method for producing water-absorbing resin particles of the present invention comprises a surface crosslinking step. The surface crosslinking step is a step for surface crosslinking the polymer particles that have been washed in the washing step. Surface crosslinking of the polymer particles adjusts the crosslinking density near the surface of the polymer particles, which tends to improve water absorption performance. The surface crosslinking step is preferably performed after the dewatering step described below. That is, the surface crosslinking step is preferably performed after the polymer particles that have been washed in the washing step have been subjected to the dewatering step.

[0055] In the surface crosslinking process, polymer particles can be crosslinked by adding a surface crosslinking agent (surface crosslinking reaction). This process of crosslinking with a surface crosslinking agent is sometimes called "post-crosslinking," and the surface crosslinking agent is sometimes simply called the post-crosslinking agent.

[0056] Examples of surface crosslinking agents include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, diethylene glycol, triethylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; halo-epoxy compounds such as epichlorohydrin, epibromhydrin, and α-methylepichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and 3-methyl-3-oxetane methanol and 3-ethyl-3-oxetane. Oxetane compounds such as methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; ethylene carbonate, propylene carbonate, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, and 4-ethyl Examples of surface crosslinking agents include carbonate compounds (e.g., alkylene carbonates) such as -1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these surface crosslinking agents, polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether are preferred.These surface crosslinking agents may be used individually or in combination of two or more types.

[0057] The amount of surface crosslinking agent used is preferably 0.00001 to 0.01 moles, more preferably 0.00005 to 0.005 moles, even more preferably 0.0001 to 0.005 moles, and even more preferably 0.0004 to 0.002 moles, per mole of the total amount of water-soluble ethylenically unsaturated monomers used in polymerization, if the water-soluble ethylenically unsaturated monomers used in polymerization are known. If the water-soluble ethylenically unsaturated monomers used in polymerization are not known, the total amount of water-soluble ethylenically unsaturated monomers (1 mole) can be interpreted as "the total amount of water-soluble ethylenically unsaturated monomers in the polymer particles (1 mole)".

[0058] The surface crosslinking agent may be added directly, as an aqueous solution, or, if necessary, as a solution using a hydrophilic organic solvent. Examples of hydrophilic organic solvents include lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as diethyl ether, dioxane, and tetrahydrofuran; amides such as N,N-dimethylformamide; and sulfoxides such as dimethyl sulfoxide. These hydrophilic organic solvents may be used individually, in combination of two or more, or as a mixed solvent with water.

[0059] When a surface crosslinking agent is added to polymer particles, the water content of the polymer particles is preferably 40% by mass or less. In this case, the reduction in surface area due to bonding between water-absorbing resin particles can be suppressed, and as a result, the water absorption performance of the resulting water-absorbing resin particles (especially the 10-minute value of unpressurized DW) can be improved.

[0060] When a surface crosslinking agent is added to polymer particles, the water content of the polymer particles is more preferably 30% by mass or less, even more preferably 20% by mass or less, and particularly preferably 10% by mass or less. When the water content of the polymer particles is 10% by mass or less when a surface crosslinking agent is added to polymer particles, it is possible to further improve the water absorption performance (especially the 10-minute value of unpressurized DW) of the resulting water-absorbing resin particles.

[0061] The method for adjusting the water content of polymer particles to 40% by mass or less when adding a surface crosslinking agent is not particularly limited. For example, the water content of polymer particles can be adjusted to 40% by mass or less when adding a surface crosslinking agent by the dehydration process described later.

[0062] The water content of polymer particles when a surface crosslinking agent is added can be calculated in the dehydration step described below using formula (2) (water content of polymer particles dehydrated in the dehydration step) described below.

[0063] The lower limit of the water content of polymer particles when a surface crosslinking agent is added is not particularly limited; for example, it may be 0% by mass or 1% by mass or more.

[0064] Therefore, in the method for producing water-absorbent polymer particles of the present invention, it is preferable to dehydrate the washed polymer particles in the dehydration step described below, before surface crosslinking the washed polymer particles in the surface crosslinking step. This is because such a dehydration step can reduce the water content of the polymer particles to a desired range.

[0065] The reaction temperature in the surface crosslinking reaction is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher, preferably 250°C or lower, more preferably 230°C or lower, even more preferably 220°C or lower, and particularly preferably 210°C or lower. The reaction time for the surface crosslinking reaction can be set within an appropriate range depending on the temperature, for example, preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0066] In particular, in the method for producing water-absorbing resin particles of the present invention, as described above, additives such as surfactants are sufficiently removed by the washing step. Therefore, even if the polymer particles are subjected to high-temperature treatment (for example, 175°C or higher) in the surface crosslinking step, decomposition products of surfactants and other substances, which have been a problem in the past, are less likely to occur. As a result, a decrease in the water absorption performance of the water-absorbing resin particles due to such decomposition products is less likely to occur. For this reason, the present invention allows for the selection of surface crosslinking agents that require treatment in a relatively high-temperature range. Examples of such surface crosslinking agents include the aforementioned carbonate compounds (for example, alkylene carbonates).

[0067] As described above, the method for producing water-absorbent resin particles of the present invention makes it possible to use surface crosslinking agents, which were previously difficult to use because they required high-temperature treatment and degraded the physical properties of the water-absorbent resin particles, and also provides a high degree of freedom in material design.

[0068] (Other steps) The method for producing water-absorbent resin particles of the present invention may include other steps, as long as it includes the washing step and the surface crosslinking step as essential steps as described above. Examples of such other steps include the polymerization step, the post-polymerization treatment step, and the dewatering step. In particular, the method for producing water-absorbent resin particles of the present invention preferably includes the polymerization step, the post-polymerization treatment step, the washing step, the dewatering step, and the surface crosslinking step in this order. The other steps will be described in detail below.

[0069] [Polymerization Process] The polymerization process is a step for obtaining polymer particles used in the method for producing water-absorbent resin particles of the present invention by polymerization. The polymerization method for obtaining polymer particles is not particularly limited, and various known polymerization methods can be used. Among these, it is preferable to obtain polymer particles by reverse-phase suspension polymerization, as this method allows for the stable acquisition of polymer particles.

[0070] In other words, the method for producing water-absorbent resin particles of the present invention preferably includes a step (polymerization step) of obtaining polymer particles by reverse-phase suspension polymerization before the washing step.

[0071] In reverse-phase suspension polymerization, aqueous solutions of water-soluble ethylenically unsaturated monomers can be polymerized in a dispersion medium containing a hydrocarbon solvent.

[0072] ≪Water-soluble ethylenically unsaturated monomers≫ The aqueous solution of water-soluble ethylenically unsaturated monomers used in reverse-phase suspension polymerization is a solution in which water-soluble ethylenically unsaturated monomers are dissolved in a solvent containing water.

[0073] As water-soluble ethylenically unsaturated monomers, a wide range of known monomers that can be used to synthesize general absorbent resin particles can be applied. Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid (hereinafter, "acry" and "methacry" are collectively referred to as "(meth)acry"; the same applies hereinafter) and its salts; 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate; amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, diethylaminopropyl(meth)acrylamide and their quaternary derivatives. These water-soluble ethylenically unsaturated monomers may be used individually or in combination (copolymerized) of two or more types. Among these, (meth)acrylic acid and its salts, (meth)acrylamide, and N,N-dimethylacrylamide are preferred due to their easy industrial availability, and (meth)acrylic acid and its salts are more preferred.

[0074] When acrylic acid and its salts are used as water-soluble ethylenically unsaturated monomers, the acrylic acid and its salts may be used as the main water-soluble ethylenically unsaturated monomers, and it is preferable that they be used in an amount of 70 to 100 mol% relative to the total number of moles of water-soluble ethylenically unsaturated monomers.

[0075] In reverse-phase suspension polymerization, the water-soluble ethylenically unsaturated monomer is used as an aqueous solution. The concentration of the water-soluble ethylenically unsaturated monomer in such an aqueous solution may be 20% by mass or more and below the saturation concentration, 25 to 90% by mass and below the saturation concentration, 30 to 75% by mass and below the saturation concentration, or 30 to 60% by mass and below the saturation concentration.

[0076] When the water-soluble ethylenically unsaturated monomer has an acidic group, such as (meth)acrylic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid, it is possible to use a solution in which the acidic group has been neutralized in advance with an alkaline neutralizing agent. Examples of such alkaline neutralizing agents include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium bicarbonate, potassium hydroxide, and potassium carbonate; and ammonia. In particular, these alkaline neutralizing agents may be used in aqueous solution form to simplify the neutralization operation. The above-mentioned alkaline neutralizing agents may be used individually or in combination of two or more. When neutralizing with an alkaline neutralizing agent, the degree of neutralization for all acidic groups of the water-soluble ethylenically unsaturated monomer may be adjusted to 50 to 100 mol%. The degree of neutralization is more preferably 50 to 90 mol%, even more preferably 70 to 80 mol%, and even more preferably 73 to 80 mol%.

[0077] The aqueous solution of the water-soluble ethylenically unsaturated monomer may contain the polymerization initiator described below, and the aqueous solution of the water-soluble ethylenically unsaturated monomer may also contain the internal crosslinking agent described below.

[0078] <<Thickener>> The aqueous solution of the water-soluble ethylenically unsaturated monomer may contain a thickener. In this case, it becomes easier to adjust the viscosity of the aqueous solution of the water-soluble ethylenically unsaturated monomer, which in turn makes it easier to adjust the droplet size during reverse-phase suspension polymerization to a desired range, and thus easier to adjust the median particle size of the resulting superabsorbent resin particles.

[0079] Examples of thickening agents include hydrophilic polymer dispersants such as hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, carboxymethylcellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide.

[0080] If the aqueous solution of the water-soluble ethylenically unsaturated monomer contains a hydrophilic polymer-based dispersant, its content can be adjusted within an appropriate range to achieve the desired viscosity. For example, it is preferably 1 part by mass or less, more preferably 0.8 parts by mass or less, and even more preferably 0.6 parts by mass or less, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer.

[0081] <Hydrogen Solvents> Examples of hydrocarbon solvents used in reverse-phase suspension polymerization include aliphatic hydrocarbons with 6 to 8 carbon atoms such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane; and aromatic hydrocarbons such as benzene, toluene, and xylene. Among these hydrocarbon solvents, n-hexane, n-heptane, and cyclohexane are particularly suitable because they are readily available industrially, have stable quality, and are inexpensive. Hydrocarbon solvents may be used individually or in combination of two or more types. Furthermore, suitable results can also be obtained by using commercially available hydrocarbon solvent mixtures such as exolheptane (manufactured by ExxonMobil: containing 75-85% by mass of heptane and its isomer hydrocarbons).

[0082] Regarding the amount of hydrocarbon solvent used, from the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature in the polymerization process, it is preferable to use 50 to 1500 parts by mass, and more preferably 100 to 1400 parts by mass, per 100 parts by mass of water-soluble ethylenically unsaturated monomer.

[0083] <Hydrophobic Polymer Dispersant> Reverse-phase suspension polymerization may be carried out in the presence of a hydrophobic polymer dispersant. Such a hydrophobic polymer dispersant is preferably dissolved or dispersed in the hydrocarbon solvent, for example. That is, it is preferable that the hydrophobic polymer dispersant has the property of dissolving or dispersing in the hydrocarbon solvent used. For example, the viscosity-average molecular weight of the hydrophobic polymer dispersant is 20,000 or less, preferably 10,000 or less, and more preferably 5,000 or less.

[0084] Specific examples of the hydrophobic polymer dispersant include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethylcellulose, ethyl hydroxyethylcellulose, maleated polybutadiene anhydride, maleated EPDM anhydride (ethylene / propylene / diene terpolymer), and the like.

[0085] Among these, the hydrophobic polymer dispersant is preferably at least one selected from the group consisting of maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, and oxidized ethylene-propylene copolymer.

[0086] In the polymerization process, only one type of hydrophobic polymer dispersant may be used, or two or more different types may be used.

[0087] The amount of hydrophobic polymer dispersant used is preferably 5 parts by mass or less, more preferably 3 parts by mass or less, and preferably 0.01 parts by mass or more, more preferably 0.05 parts by mass or more, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer. Preferred ranges include 0.01 to 5 parts by mass, 0.01 to 3 parts by mass, 0.05 to 5 parts by mass, and 0.05 to 3 parts by mass.

[0088] The hydrophobic polymer dispersant can be added to the hydrocarbon solvent. In this case, for example, by heating the hydrocarbon solvent to a predetermined temperature, some or all of the hydrophobic polymer dispersant can be dissolved or dispersed in the hydrocarbon solvent. Even if, after heating, the hydrocarbon solvent is cooled and some or all of the hydrophobic polymer dispersant precipitates and becomes a cloudy dispersion, there is no problem in adding the aqueous solution of the water-soluble ethylenically unsaturated monomer to the cloudy dispersion.

[0089] The hydrocarbon solvent may contain other components besides the hydrophobic polymer dispersant as needed. Examples of other components include chain transfer agents. Examples of chain transfer agents include known chain transfer agents used in reverse-phase suspension polymerization, such as thiols, thiolic acids, secondary alcohols, hypophosphorous acid, and phosphorous acid.

[0090] <<Surfactants>> Reverse-phase suspension polymerization can be carried out in the presence of surfactants. In this case, the dispersion stability of the water-soluble ethylenically unsaturated monomer in the hydrocarbon solvent is further improved.

[0091] Examples of surfactants that can be used include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkylallylformaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, polyethylene glycol fatty acid esters, alkyl glucosides, N-alkylgluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, phosphate esters of polyoxyethylene alkyl ethers, phosphate esters of polyoxyethylene alkylallyl ethers, sulfate esters of polyoxyalkylene alkyl ethers, and alkyl sulfate esters of polyoxyalkylene alkyl ethers. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred in terms of monomer dispersion stability. These surfactants may be used individually or in combination of two or more types.

[0092] The present invention's method for producing superabsorbent resin particles includes a washing step, which makes it easier to remove surfactants remaining on the polymer particles after polymerization. Therefore, regardless of which surfactant is used, the water absorption performance (especially the 10-minute value of unpressurized DW) of the resulting superabsorbent resin particles tends to be improved.

[0093] As mentioned above, it is preferable that the surfactant has an HLB of 6 or less, and particularly preferably 3 or less, in order to particularly improve the water absorption performance of the water-absorbing resin particles (especially the 10-minute value of unpressurized DW). When a surfactant with an HLB of 6 or less (especially 3 or less) is used, the resulting polymer particles tend to have an aggregated structure such as a grape cluster, and the presence of this structure in the water-absorbing resin particles makes it easier to improve the water absorption performance of the water-absorbing resin particles (especially the 10-minute value of unpressurized DW).

[0094] The amount of surfactant used is preferably 0.1 to 30 parts by mass, more preferably 0.3 to 20 parts by mass, even more preferably 0.3 to 10 parts by mass, and even more preferably 0.4 to 5 parts by mass, per 100 parts by mass of water-soluble ethylenically unsaturated monomer.

[0095] <<Polymerization Initiators>> In reverse-phase suspension polymerization, polymerization initiators, particularly radical polymerization initiators, can be widely used. Examples of radical polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, and hydrogen peroxide; and azo compounds such as 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2,2'-azobis[2-(N-phenylamidino)propane] dihydrochloride, 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, and 4,4'-azobis(4-cyanovaleric acid). Radical polymerization initiators can also be used in combination with reducing agents such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid to form redox polymerization initiators.

[0096] From the viewpoint of polymerization stability, the lower limit of the amount of polymerization initiator used in reverse-phase suspension polymerization may be 0.01 mmol or more, preferably 0.05 mmol or more, and more preferably 0.10 mmol or more, per mole of water-soluble ethylenically unsaturated monomer. Furthermore, from the viewpoint of polymerization stability, the upper limit of the amount of polymerization initiator used may be 20 mmol or less, preferably 10 mmol or less, and more preferably 5 mmol or less, per mole of water-soluble ethylenically unsaturated monomer.

[0097] Polymerization initiators used in reverse-phase suspension polymerization can be used individually or in combination of two or more.

[0098] For use in reverse-phase suspension polymerization, the polymerization initiator is preferably added to an aqueous solution of a water-soluble ethylenically unsaturated monomer.

[0099] <<Internal Crosslinking Agents>> In reverse-phase suspension polymerization during the polymerization process, internal crosslinking agents can be used. By using internal crosslinking agents, the resulting superabsorbent resin particles may have a structure in which the inside is crosslinked with the internal crosslinking agent. In this specification, the crosslinking agent used for crosslinking the inside of a polymer is referred to as an internal crosslinking agent to distinguish it from the surface crosslinking agent (also called a post-crosslinking agent) described later.

[0100] Examples of internal crosslinking agents include compounds or precursors having two or more functional groups that react with the functional groups of a water-soluble ethylenically unsaturated monomer, and compounds having two or more polymerizable unsaturated groups. Specific examples include carbonate compounds such as ethylene carbonate; glycidyl group-containing compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; and (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, glycidyl (meth)acrylate, etc. Other internal crosslinking agents include trialyl compounds; sucrose allyl ethers; di or tri(meth)acrylic acid esters of polyols such as (poly)ethylene glycol, (poly)propylene glycol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and (poly)glycerin; unsaturated polyesters obtained by reacting the above polyols with unsaturated acids such as maleic acid and fumaric acid; bisacrylamides such as N,N'-methylenebis(meth)acrylamide; di or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; di(meth)acrylic acid carbamyl esters obtained by reacting polyisocyanates such as tolylene diisocyanate and hexamethylene diisocyanate with hydroxyethyl (meth)acrylate; allyl starch; allyl cellulose; diallyl phthalate; N,N',N''-trialyl isocyanurate; and divinylbenzene. These internal crosslinking agents may be used individually or in combination of two or more. In this specification, for example, "polyethylene glycol" and "ethylene glycol" are collectively referred to as "(poly)ethylene glycol."

[0101] The amount of internal crosslinking agent used may be 0.001 mmol or more, preferably 0.005 mmol or more, more preferably 0.01 mmol or more, and even more preferably 0.02 mmol or more, per mole of water-soluble ethylenically unsaturated monomer used. Furthermore, the amount of internal crosslinking agent used may be 50 mmol or less, preferably 10 mmol or less, more preferably 5 mmol or less, and even more preferably 1 mmol or less, per mole of water-soluble ethylenically unsaturated monomer.

[0102] For use in reverse-phase suspension polymerization, the internal crosslinking agent is preferably added to an aqueous solution of a water-soluble ethylenically unsaturated monomer.

[0103] <<Inverted-Phase Suspension Polymerization>> In the polymerization process, inverted-phase suspension polymerization is carried out by stirring an aqueous solution of a water-soluble ethylenically unsaturated monomer in a dispersion medium containing the hydrocarbon solvent using a stirring blade. Such inverted-phase suspension polymerization is preferably carried out under an inert gas atmosphere such as nitrogen.

[0104] The polymerization time is preferably 0.1 hours or more, more preferably 20 hours or less, more preferably 10 hours or less, even more preferably 5 hours or less, and particularly preferably 3 hours or less.

[0105] The polymerization reaction temperature can be appropriately set according to the type and amount of water-soluble ethylenically unsaturated monomers, polymerization initiators, etc., and can be, for example, 20°C or higher, or 110°C or lower.

[0106] Examples of the impeller blades used in reverse-phase suspension polymerization include paddle blades, flat plate blades, propeller blades, anchor blades, turbine blades, Faudler blades, ribbon blades, Fullzone blades (manufactured by Kobe Steel Pantech Co., Ltd.), Maxblend blades (manufactured by Sumitomo Heavy Industries, Ltd.), and Supermix (manufactured by Satake Chemical Machinery Co., Ltd.).

[0107] Inverted-phase suspension polymerization can be carried out in one step (single step) or in two or more steps. Performing inverted-phase suspension polymerization in multiple steps means that, as described above, an aqueous solution of a water-soluble ethylenically unsaturated monomer is added to the polymer particles produced by inverted-phase suspension polymerization to carry out additional polymerization at least once. Depending on the number of these additional polymerization steps, it can be expressed as two steps, three steps, etc. Therefore, if no additional polymerization is performed, it is considered a single step.

[0108] In order to particularly enhance the water absorption performance of water-absorbing resin particles (especially the 10-minute value at unpressurized DW), it is preferable to carry out the reverse-phase suspension polymerization method in two or more stages. This is because when carried out in two or more stages, the resulting polymer particles tend to have an aggregated structure such as a grape cluster, and the presence of this structure in the water-absorbing resin particles makes it easier to improve the water absorption performance of the water-absorbing resin particles (especially the 10-minute value at unpressurized DW).

[0109] The method for carrying out reverse-phase suspension polymerization in multiple stages is not particularly limited, and for example, known multi-stage methods can be broadly adopted in the present invention. For example, when carrying out reverse-phase suspension polymerization in two or more stages, after carrying out the first stage of reverse-phase suspension polymerization, a water-soluble ethylenically unsaturated monomer is added to the reaction mixture obtained from the polymerization reaction of the first stage and mixed, and the second and subsequent stages of reverse-phase suspension polymerization are carried out in the same manner as the first stage. In the reverse-phase suspension polymerization of each stage from the second stage onward, it is preferable to carry out reverse-phase suspension polymerization by adding a radical polymerization initiator in addition to the water-soluble ethylenically unsaturated monomer, within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer as described above, based on the amount of water-soluble ethylenically unsaturated monomer added during the reverse-phase suspension polymerization of each stage from the second stage onward. In addition, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed in the polymerization of the second stage onward as well.

[0110] Through reverse-phase suspension polymerization, water-soluble ethylenically unsaturated monomers polymerize to produce particulate polymers, i.e., polymer particles. The resulting polymer particles are, for example, water-containing gel-like polymer particles, and are used in the washing process described later.

[0111] (Post-polymerization treatment step) Before subjecting the polymer particles obtained in the polymerization step to the washing step, chelating agents and reducing agents can be added in the post-polymerization treatment step. Adding chelating agents and reducing agents to the polymer particles tends to improve the water absorption performance of the resulting superabsorbent resin particles.

[0112] The type of chelating agent is not particularly limited, and for example, known metal chelating agents can be widely used. Specific examples of chelating agents include ethylenediaminetetraacetic acid, nitrilotriacetic acid, hydroxyethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, dihydroxyethylglycine, diethylenetriaminepentamethylenephosphonic acid, and salts thereof. When the chelating agent is in the form of a salt, the type of salt is not particularly limited, and examples include alkali metal salts such as sodium and potassium, alkaline earth metal salts such as magnesium and calcium, organic amine salts, ammonium salts, etc. The ligand of the chelating agent may form a salt, either entirely or partially. Only one type of chelating agent may be used, or two or more types may be used in combination.

[0113] When adding a chelating agent to polymer particles, the chelating agent can be added in the form of a solution dissolved in a solvent such as water, or it can be added in a solid state without using a solvent. Furthermore, the chelating agent can be added to the reaction solution after the reverse-phase suspension polymerization is complete.

[0114] The lower limit of the amount of chelating agent used is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and preferably 1% by mass or less, and more preferably 0.5% by mass or less, based on 100 parts by mass of the total amount of water-soluble ethylenically unsaturated monomers.

[0115] The type of reducing agent is not particularly limited; for example, well-known reducing agents can be widely used. Specific examples of reducing agents include sulfites, bisulfites, pyrosulfites, and dithionites. The type of salt is not particularly limited; for example, alkali metal salts such as sodium and potassium, alkaline earth metal salts such as magnesium and calcium, organic amine salts, and ammonium salts can be used. Only one type of reducing agent may be used, or two or more types may be used in combination.

[0116] When adding a reducing agent to polymer particles, the reducing agent can be added in a solution dissolved in a solvent such as water, or it can be added in a solid state without using a solvent. Furthermore, the reducing agent can be added to the reaction solution after the reverse-phase suspension polymerization is complete.

[0117] The lower limit of the amount of reducing agent used is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and more preferably 0.5% by mass or less, and more preferably 0.1% by mass or less, based on 100 parts by mass of the total amount of water-soluble ethylenically unsaturated monomers.

[0118] <<Dehydration Treatment>> In the method for producing water-absorbing resin particles of the present invention, it is necessary to adjust the water content of the polymer particles to the above range (10% by mass or more and less than 120% by mass) during the washing process. For this reason, it is desirable to adjust the water content of polymer particles obtained by reverse-phase suspension polymerization, or polymer particles to which chelating agents or reducing agents have been added, to the above range.

[0119] The method for adjusting the water content of polymer particles is not particularly limited. For example, polymer particles obtained by reverse-phase suspension polymerization contain a large amount of water (i.e., they are water-containing gel-like polymer particles), so the water content of the polymer particles can be adjusted to a desired range by using appropriate dehydration treatment. For example, the water content can be adjusted by performing dehydration treatment in the post-polymerization treatment step. Dehydration treatment in the post-polymerization treatment step can be performed on polymer particles after the addition of chelating agents or reducing agents, for example. Azeotropic distillation is one example of a dehydration treatment. Specifically, in the reverse-phase suspension polymerization described above, a hydrocarbon solvent is used, so the water content of the polymer particles can be adjusted by removing water from the system through azeotropic distillation of the hydrocarbon solvent and water.

[0120] The water content referred to here means the percentage of water content (mass%) based on the total mass of polymer particles (partially neutralized polyacrylic acid containing water) obtained after the dehydration treatment, minus the amount of water contained in the polymer particles. Specifically, the water content of the polymer particles means the percentage of the mass of water remaining in the polymer particles after the dehydration treatment relative to the total mass of solids of monomer components and additives used in the polymerization reaction, and is calculated by the following formula (1). As can be seen from the following formula (1), the mass of water remaining in the polymer particles is derived based on a value calculated by subtracting the mass of water removed from the system by the dehydration treatment from the mass of water used in the polymerization reaction, the mass of water produced in the neutralization reaction, and the mass of water contained in the additive aqueous solution added after the polymerization process. Water content = {(W1 + W2 + W3 - W4) / (W5 + W6)} × 100 ... (1) Here, W1 is the "mass of water used in the polymerization reaction", W2 is the "mass of water produced in the neutralization reaction", W3 is the "mass of water contained in the aqueous additive solution added after the polymerization step", W4 is the "mass of water removed from the system", W5 is the "mass of monomer components used in the polymerization reaction", and W6 is the "mass of solids of the additive". In equation (1), "mass of water removed from the system" (i.e., W4) means the total amount of water removed from the system in the polymerization step and the mass of water removed from the system in the dehydration step. Note that "additives" in equation (1) means anything other than the monomer components and solvent used in the polymerization step, and examples include thickeners, hydrophobic polymer dispersants, surfactants, polymerization initiators, internal crosslinking agents, oxidizing agents, reducing agents, chelating agents, radical chain inhibitors, antioxidants, etc. Among these additives, "additives added after the polymerization process" refers to additives added between the end of the polymerization process and the start of the surface crosslinking process, such as additives added in the post-polymerization treatment process. Specifically, these include oxidizing agents, reducing agents, chelating agents, radical chain inhibitors, and antioxidants.

[0121] The water content of the polymer particles can be controlled, for example, by adjusting the conditions of the azeotropic distillation described above. For example, by adjusting the amount of water expelled from the system, the water content of the polymer particles can be controlled to a desired range (10% by mass or more and less than 120% by mass).

[0122] Polymer particles with the water content adjusted as described above can be used in the aforementioned washing process.

[0123] [Dehydration Step] The method for producing water-absorbing resin particles of the present invention may include a dehydration step after the washing step. The dehydration step is a step for reducing the amount of water contained in the polymer particles (water-containing gel-like polymer particles).

[0124] The dehydration step can be performed after the washing step and before the surface crosslinking step. In this case, it becomes easier to adjust the water content of the polymer particles to a desired range when the surface crosslinking agent is added in the surface crosslinking step.

[0125] In the dehydration process, water, hydrocarbon solvents, etc., can be removed by distillation by applying energy such as heat from an external source. Specifically, azeotropic distillation of water and hydrocarbon solvents allows water to be extracted from the system while refluxing the hydrocarbon solvent.

[0126] In the dehydration process, dehydration by distillation may be carried out under atmospheric pressure. When dehydration is carried out under atmospheric pressure, the dehydration temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C.

[0127] In the dehydration step, it is preferable to subject the polymer particles to the surface crosslinking step. Therefore, in the dehydration step, it is preferable to adjust the water content of the polymer particles to 40% by mass or less.

[0128] The water content of polymer particles dehydrated in the dehydration step can be calculated using the following formula (2): Water content = {(P1 + P2 + P3 - P4) / (P5 + P6)} × 100 ... (2) Here, P1 represents the "mass of water used in the polymerization reaction", P2 represents the "mass of water produced in the neutralization reaction", P3 represents the "mass of water contained in the additive aqueous solution added after the polymerization step", P4 represents the "mass of water removed from the system", P5 represents the "mass of monomer components used in the polymerization reaction", and P6 represents the "mass of solids of the additive". In formula (2), "mass of water removed from the system" (i.e., P4) means the total amount of water removed from the system in the polymerization step and the mass of water removed from the system in the dehydration step. The rest is the same as in formula (1).

[0129] [Drying Step] The method for producing water-absorbent resin particles of the present invention may include a drying step. In the drying step, the amount of water and hydrocarbon solvent remaining in the water-absorbent resin particles can be further reduced by applying energy such as heat from an external source. In the present invention, it is preferable to include a drying step after the surface crosslinking step.

[0130] In the drying process, it is preferable to perform heat treatment under an airflow of nitrogen or the like from the viewpoint of increasing drying efficiency. The drying temperature is preferably 70 to 250°C, more preferably 80 to 180°C, even more preferably 80 to 140°C, and even more preferably 90 to 130°C. Furthermore, when the drying treatment is performed under reduced pressure, the drying temperature is preferably 40 to 160°C, and more preferably 50 to 110°C.

[0131] 2. Absorbent resin particles can be obtained by the method for producing absorbent resin particles of the present invention, which is equivalent to or better than absorbent resin particles. Such absorbent resin particles are preferably produced by a manufacturing method comprising the polymerization step, the post-polymerization treatment step, the washing step, the dewatering step, the surface crosslinking step, and the drying step in this order.

[0132] The water-absorbing resin particles obtained by the manufacturing method of the present invention are produced through at least the washing step and the surface crosslinking step, and therefore have a crosslinked structure on the surface of the polymer particles, and contain a small amount of additives such as surfactants. For this reason, water-absorbing resin particles with excellent water absorption performance can be obtained.

[0133] In particular, since the water-absorbing resin particles obtained by the method for producing water-absorbing resin particles of the present invention have had the surfactant sufficiently removed, even if the water-absorbing resin particles are subjected to high-temperature treatment, for example, decomposition products of the surfactant are less likely to be generated, and a deterioration in the physical properties of the water-absorbing resin particles due to such decomposition products is less likely to occur.

[0134] The median particle size of the water-absorbing resin particles obtained by the manufacturing method of the present invention is not particularly limited, but is preferably 200 μm or more, 250 μm or more, 300 μm or more, 320 μm or more, or 350 μm or more, and also preferably 700 μm or less, 600 μm or less, or 550 μm or less. Preferred ranges include median particle sizes of water-absorbing resin particles of 200 to 700 μm, 200 to 600 μm, 200 to 550 μm, 250 to 700 μm, 250 to 600 μm, 250 to 550 μm, 300 to 700 μm, 300 to 600 μm, 300 to 550 μm, 350 to 700 μm, 350 to 600 μm, and 350 to 550 μm.

[0135] The median particle size of the water-absorbent resin particles can be measured using a JIS standard sieve, and specifically, the value is measured by the method described in the examples.

[0136] The water-absorbing resin particles obtained by the manufacturing method of the present invention may consist only of polymer particles, or may contain additives as needed depending on the purpose. Examples of additives include surfactants, oxidizing agents, reducing agents, chelating agents, radical chain inhibitors, and antioxidants.

[0137] The water-absorbing resin particles obtained by the manufacturing method of the present invention may consist only of polymer particles, or they may contain inorganic powder and antibacterial agents. For example, the fluidity of the water-absorbing resin particles can be further improved by adding 0.05 to 5 parts by mass of amorphous silica as inorganic powder to 100 parts by mass of water-absorbing resin particles. It is preferable that the inorganic powder is hydrophilic or water-soluble.

[0138] 3. Absorbent material, absorbent article The superabsorbent resin particles obtained by the manufacturing method of the present invention constitute an absorbent material used in sanitary materials such as sanitary napkins and disposable diapers, and are suitably used in absorbent articles containing the absorbent material.

[0139] The absorbent material using the water-absorbent resin particles of the present invention contains the water-absorbent resin particles of the present invention. The absorbent material may further contain hydrophilic fibers. Examples of the structure of the absorbent material include a sheet-like structure in which water-absorbent resin particles are fixed on or between multiple nonwoven fabrics, a mixed dispersion obtained by mixing water-absorbent resin particles and hydrophilic fibers to a uniform composition, a sandwich structure in which water-absorbent resin particles are sandwiched between layered hydrophilic fibers, and a structure in which water-absorbent resin particles and hydrophilic fibers are wrapped in tissue. The absorbent material may also contain other components, such as adhesive binders such as heat-fusible synthetic fibers, hot-melt adhesives, and adhesive emulsions to enhance the shape retention of the absorbent material.

[0140] The content of water-absorbing resin particles in the absorbent material is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, even more preferably 20 to 90% by mass, and still more preferably 30 to 80% by mass.

[0141] Hydrophilic fibers include cellulose fibers such as cotton-like pulp, mechanical pulp, chemical pulp, and semi-chemical pulp obtained from wood; artificial cellulose fibers such as rayon and acetate; and fibers made from hydrophilized polyamide, polyester, polyolefin, and other synthetic resins. The average fiber length of hydrophilic fibers is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0142] The absorbent article of the present invention can be made by holding an absorbent material using the water-absorbent resin particles of the present invention between a liquid-permeable sheet (top sheet) through which liquid can pass and a liquid-impermeable sheet (back sheet) through which liquid cannot pass. The liquid-permeable sheet is placed on the side that comes into contact with the body, and the liquid-impermeable sheet is placed on the opposite side that comes into contact with the body.

[0143] Examples of liquid-permeable sheets include nonwoven fabrics such as air-through type, spunbond type, chemical bond type, and needle-punched type, as well as porous synthetic resin sheets made from fibers such as polyethylene, polypropylene, and polyester. Examples of liquid-impermeable sheets include synthetic resin films made from resins such as polyethylene, polypropylene, and polyvinyl chloride.

[0144] In specifying the inventions contained herein, the components (properties, structures, functions, etc.) described in each embodiment of this disclosure may be combined in any way. That is, this disclosure encompasses all subject matter consisting of any combination of the combinatable components described herein.

[0145] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to the embodiments of these examples. Unless otherwise specified, the examples, comparative examples, and measurements were carried out in an environment with a temperature of 25 ± 2°C and a humidity of 50 ± 10%.

[0146] <Production of superabsorbent polymer particles> (Example 1) [First polymerization step] A round-bottom cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirrer with a stirring blade having four inclined paddle blades with a blade diameter of 5 cm in two stages was used. 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (manufactured by Mitsui Chemicals, Inc., High Wax 1105A) as a dispersant were added to the separable flask and mixed. The mixture in the flask was heated to 80°C while being stirred with the stirrer, thereby dissolving the dispersant in n-heptane. The dispersion medium (n-heptane solution) containing the formed hydrocarbon solvent was cooled to 50°C.

[0147] Meanwhile, 92.0 g (1.03 mol) of an 80.5% by mass aqueous solution of acrylic acid as a water-soluble ethylenically unsaturated monomer was placed in a 300 mL beaker, and while cooling from the outside, 149.1 g of a 20.9% by mass aqueous solution of sodium hydroxide was added dropwise to the beaker to neutralize 76 mol% of the acrylic acid. To this, 0.0736 g (0.272 mmol) of potassium persulfate was added as a radical polymerization initiator, and 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent, and these were dissolved to prepare the first stage water-soluble ethylenically unsaturated monomer aqueous solution. This first stage water-soluble ethylenically unsaturated monomer aqueous solution was added to the n-heptane solution in a separable flask, and the resulting reaction mixture was stirred for 10 minutes. Separately, a surfactant solution was prepared by dissolving 0.736 g of sucrose stearate (Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370, HLB: 3), a surfactant, in 6.62 g of n-heptane. This surfactant solution was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring the reaction mixture at a stirrer speed of 550 rpm. Then, the separable flask was immersed in a 70°C water bath to raise the temperature of the reaction mixture, and the polymerization reaction was allowed to proceed for 60 minutes to obtain the first stage polymerization slurry.

[0148] [Second Polymerization Step] 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL beaker. While cooling from the outside, 161.6 g of a 27% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 76 mol% of the acrylic acid. 0.103 g (0.381 mmol) of potassium persulfate and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether were added to the beaker containing the neutralized acrylic acid aqueous solution and dissolved to prepare the second water-soluble ethylenically unsaturated monomer aqueous solution.

[0149] The first-stage polymerization slurry in the separable flask was cooled to 25°C while being stirred at a stirrer speed of 1000 rpm, and the entirety of the second-stage aqueous solution of water-soluble ethylenically unsaturated monomer was added thereto. After purging the separable flask with nitrogen for 30 minutes, the separable flask was again immersed in a 70°C water bath to raise the temperature of the reaction solution, and the second-stage polymerization reaction was carried out for 60 minutes to obtain water-containing gel-like polymer particles.

[0150] [Post-polymerization treatment step] To the reaction solution containing water-containing gel-like polymer particles, 4.91 g of a 4.5% by mass aqueous solution of sodium pentasodium diethylenetriaminepentaacetate and 2.58 g of 3.0% sodium sulfite were added under stirring. Subsequently, the separable flask was immersed in an oil bath set to 125°C, and 230.7 g of water was removed from the system by azeotropic distillation of n-heptane and water. After immersing the separable flask in a water bath set to 80°C, stirring was stopped, and the water-containing gel-like polymer particles were allowed to settle by standing for 5 minutes. After removing the separable flask from the water bath, 160.0 g of n-heptane from the supernatant in the separable flask was collected to obtain water-containing gel-like polymer particles for washing in the separable flask. The water content of the obtained water-containing gel-like polymer particles was calculated using equation (1) above, and this value is shown in Table 1 below as the "water content during washing".

[0151] [Washing Process] Using the water-containing gel-like polymer particles adjusted to the above-mentioned water content during washing, the washing process was carried out by sequentially performing the following operations (a), (b), and (c). Operation (a): 160.0 g of n-heptane was added as an organic solvent for washing to the separable flask containing the water-containing gel-like polymer particles obtained in the polymerization process. Operation (b): Next, the contents of the separable flask after operation (a) were maintained at 80°C and stirred at a tip peripheral speed of 2.6 m / s for 20 minutes. After that, stirring was stopped and the mixture was allowed to stand for 5 minutes to precipitate the water-containing gel-like polymer particles. Operation (c): 160.0 g of n-heptane was collected from the supernatant in the separable flask after operation (b). Performing operations (a), (b), and (c) described above once in sequence constitutes "one washing operation" (i.e., the operation from contacting the polymer particles with the organic solvent to washing them, and then removing the predetermined amount of the organic solvent as described above). In this washing process, this washing operation was performed a total of four times.

[0152] [Dehydration Process] A dehydration process was carried out on the polymer particles washed in the washing process. Specifically, 50.0 g of n-heptane was added to the separable flask containing the polymer particles washed in the washing process. The separable flask was immersed in an oil bath set at 125°C, and the pressure was maintained at 40 kPa. By distillation, n-heptane and water were removed from the system, and 231.1 g of polymer particles were obtained. The amount of water removed from the system in this dehydration process (hereinafter referred to as "amount of dehydration") was 66.3 g, and the result of calculating the water content using equation (2) above is shown in Table 1 below as "Water content at surface crosslinking".

[0153] [Surface Crosslinking Process] 15.0 g of the polymer particles dehydrated in the dehydration process were added to a separable flask, and while stirring at a stirrer speed of 500 rpm, 1.5 g (0.17 mmol) of a 1% by mass aqueous solution of ethylene carbonate was sprayed on as a surface crosslinking agent using a 3 mL syringe and sprayer (manufactured by Fuji Medical Co., Ltd.). The polymer particles were transferred to an aluminum petri dish with a diameter of 10.2 cm and heated at 200°C for 40 minutes using a hot air dryer (manufactured by ADVANTEC Co., Ltd.). These polymer particles were classified using a sieve with a mesh size of 850 μm to obtain 13.9 g of superabsorbent resin particles. The median particle size of these superabsorbent resin particles was 529 μm.

[0154] (Example 2) Except for changing the amount of water removed in the dewatering step to 28.5 g and changing the amount of polymer particles used in the surface crosslinking step to 17.3 g, 14.0 g of superabsorbent resin particles with a median particle size of 536 μm were obtained using the same method as in Example 1.

[0155] (Example 3) Except for changing the number of washing operations in the washing process to a total of two, changing the amount of water removed in the dewatering process to 25.1 g, and changing the amount of polymer particles used in the surface crosslinking process to 17.6 g, 13.4 g of superabsorbent resin particles with a median particle size of 397 μm were obtained using the same method as in Example 1.

[0156] (Example 4) In the polymerization step, the amount of water extracted by azeotropic distillation was changed to 143.2 g, the amount of n-heptane recovered in the polymerization step was changed to 150.0 g, the amount of n-heptane added in operation (a) of the subsequent washing step was changed to 150.0 g, the amount of n-heptane recovered in operation (c) was changed to 150.0 g, the amount of dewatering in the dewatering step was changed to 116.0 g, and the amount of polymer particles used in the surface crosslinking step was changed to 17.3 g. In the same manner as in Example 1, 14.0 g of superabsorbent resin particles with a median particle size of 521 μm was obtained.

[0157] (Example 5) Except for changing the organic solvent used for washing in the washing step to ethyl acetate, changing the temperature in operation (b) to 70°C, changing the amount of water removed in the dehydration step to 32.8 g, and changing the amount of polymer particles used in the surface crosslinking step to 17.0 g, 13.9 g of superabsorbent resin particles with a median particle size of 442 μm were obtained by the same method as in Example 1.

[0158] (Example 6) Except for changing the organic solvent used for washing in the washing step to tetrahydrofuran, changing the temperature in operation (b) to 60°C, changing the amount of water removed in the dewatering step to 35.2 g, and changing the amount of polymer particles used in the surface crosslinking step to 16.9 g, 13.8 g of superabsorbent resin particles with a median particle size of 473 μm were obtained by the same method as in Example 1.

[0159] (Comparative Example 1) [First Polymerization Step] A round-bottom cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. A stirrer with a stirring blade having four inclined paddle blades with a blade diameter of 5 cm in two stages was used. 293 g of n-heptane and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A) as a dispersant were added to the separable flask and mixed. The mixture in the flask was heated to 80°C while being stirred with the stirrer, thereby dissolving the dispersant in n-heptane. The resulting n-heptane solution containing the hydrocarbon solvent was cooled to 50°C.

[0160] Meanwhile, 92.0 g (1.03 mol) of an 80.5% by mass aqueous solution of acrylic acid as a water-soluble ethylenically unsaturated monomer was placed in a 300 mL beaker, and while cooling from the outside, 149.1 g of a 20.9% by mass aqueous solution of sodium hydroxide was added dropwise to the beaker to neutralize 76 mol% of the acrylic acid. To this, 0.0736 g (0.272 mmol) of potassium persulfate was added as a radical polymerization initiator, and 0.0101 g (0.058 mmol) of ethylene glycol diglycidyl ether was added as an internal crosslinking agent, and these were dissolved to prepare the first stage water-soluble ethylenically unsaturated monomer aqueous solution. This first stage water-soluble ethylenically unsaturated monomer aqueous solution was added to the n-heptane solution in a separable flask, and the resulting reaction mixture was stirred for 10 minutes. Separately, a surfactant solution was prepared by dissolving 0.736 g of sucrose stearate (Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370, HLB: 3), a surfactant, in 6.62 g of n-heptane. This surfactant solution was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring the reaction mixture at a stirrer speed of 550 rpm. Then, the separable flask was immersed in a 70°C water bath to raise the temperature of the reaction mixture, and the polymerization reaction was allowed to proceed for 60 minutes to obtain the first stage polymerization slurry.

[0161] [Second Polymerization Step] 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was placed in a 500 mL beaker. While cooling from the outside, 161.6 g of a 27% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 76 mol% of the acrylic acid. 0.103 g (0.381 mmol) of potassium persulfate and 0.0116 g (0.067 mmol) of ethylene glycol diglycidyl ether were added to the beaker containing the neutralized acrylic acid aqueous solution and dissolved to prepare the second water-soluble ethylenically unsaturated monomer aqueous solution.

[0162] The first-stage polymerization slurry in the separable flask was cooled to 25°C while being stirred at a stirrer speed of 1000 rpm, and the entirety of the second-stage aqueous solution of water-soluble ethylenically unsaturated monomer was added thereto. After purging the separable flask with nitrogen for 30 minutes, the separable flask was again immersed in a 70°C water bath to raise the temperature of the reaction solution, and the second-stage polymerization reaction was carried out for 60 minutes to obtain water-containing gel-like polymer particles.

[0163] To a reaction solution containing water-containing gel-like polymer particles, 4.91 g of a 4.5% by mass aqueous solution of sodium pentaacetate diethylenetriaminepentaacetate and 2.58 g of 3.0% sodium sulfite were added under stirring. Subsequently, the separable flask was immersed in an oil bath set to 125°C, and the pressure was maintained at 40 kPa. By distillation, n-heptane and water were removed from the system, yielding 266.8 g of polymer particles. The amount of water removed from the system was 248.2 g.

[0164] [Surface Crosslinking Process] 18.0 g of polymer particles obtained in the polymerization process were added to a separable flask, and while stirring at a stirrer speed of 500 rpm, 1.5 g (0.17 mmol) of a 1% by mass aqueous solution of ethylene carbonate was sprayed on as a surface crosslinking agent using a 3 mL syringe and sprayer (manufactured by Fuji Medical Co., Ltd.). The polymer particles were transferred to an aluminum petri dish with a diameter of 10.2 cm and heated at 200°C for 40 minutes using a hot air dryer (manufactured by ADVANTEC). These polymer particles were classified using a sieve with a mesh size of 850 μm to obtain 14.0 g of superabsorbent resin particles. The median particle size of these superabsorbent resin particles was 433 μm.

[0165] (Comparative Example 2) In the polymerization step, the amount of water extracted by azeotropic distillation was changed to 55.6 g, the amount of n-heptane recovered in the polymerization step was changed to 120.0 g, the amount of n-heptane added in operation (a) of the subsequent washing step was changed to 120.0 g, the amount of n-heptane recovered in operation (c) was changed to 120.0 g, the amount of dewatering in the dewatering step was changed to 197.0 g, and the amount of polymer particles used in the surface crosslinking step was changed to 17.7 g. In the same manner as in Example 1, 13.7 g of superabsorbent resin particles with a median particle size of 576 μm was obtained.

[0166] (Measurement Method) <Median Particle Size> 5 g of superabsorbent polymer particles were sieved using a continuous fully automatic ultrasonic vibration sieving analyzer (Robot Shifter RPS-205, manufactured by Seishin Corporation), sieves with JIS standard mesh sizes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 150 μm, and a receiving tray. The mass of the particles remaining on each sieve was calculated as a mass percentage of the total amount. The mass percentages of the particles remaining on each sieve were accumulated in order from the largest particle size, and the relationship between the sieve mesh size and the accumulated mass percentage of the particles remaining on the sieve was plotted on logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle size corresponding to an accumulated mass percentage of 50% was determined, and this was defined as the median particle size (μm).

[0167] [Measurement of 10-minute values ​​of unpressurized DW] The 10-minute values ​​of unpressurized DW (Demand Wetability) were measured using the measuring device shown in Figure 1. Three measurements were performed for each type of superabsorbent polymer particle, and the average value of the measured values ​​was calculated.

[0168] The measuring device comprises a burette section 1, a conduit 5, a measuring table 13, a nylon mesh sheet 15, a stand 11, and a clamp 3. The burette section 1 includes a burette tube 21 with markings, a rubber stopper 23 that seals the opening at the top of the burette tube 21, a cock 22 connected to the lower end of the burette tube 21, and an air inlet pipe 25 and a cock 24 connected to the lower part of the burette tube 21. The burette section 1 is fixed by the clamp 3. The flat measuring table 13 has a through hole 13a with a diameter of 2 mm formed in its center and is supported by a stand 11 with adjustable height. The through hole 13a of the measuring table 13 and the cock 22 of the burette section 1 are connected by a conduit 5. The inner diameter of the conduit 5 is 6 mm.

[0169] First, the stopcocks 22 and 24 of the burette section 1 were closed, and 0.9 mass% saline solution 50, adjusted to 25°C, was poured into the burette tube 21 through the opening at the top of the burette tube 21. The concentration of the saline solution is 0.9 mass%, which is the concentration based on the mass of the saline solution. After sealing the opening of the burette tube 21 with the rubber stopper 23, the stopcocks 22 and 24 were opened. The inside of the conduit 5 was filled with 0.9 mass% saline solution 50 to prevent air bubbles from entering. The height of the measuring platform 13 was adjusted so that the water level of the 0.9 mass% saline solution that reached the through-hole 13a was the same as the height of the top surface of the measuring platform 13. After the adjustment, the water level of the 0.9 mass% saline solution 50 inside the burette tube 21 was read on the scale of the burette tube 21, and that position was set as the zero point (reading at 0 seconds). A nylon mesh sheet 15 (100 mm x 100 mm, 250 mesh, approximately 50 μm thick) was laid near the through-hole 13a on the measuring platform 13, and a cylinder with an inner diameter of 30 mm and a height of 20 mm was placed in the center of the sheet. 1.00 g of water-absorbing resin particles 10a were uniformly scattered into this cylinder. The cylinder was then carefully removed, and a sample was obtained in which the water-absorbing resin particles 10a were dispersed in a circular pattern in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 on which the water-absorbing resin particles 10a were placed was quickly moved so that its center was at the position of the through-hole 13a, without causing the water-absorbing resin particles 10a to dissipate, and the measurement was started. The start of water absorption (0 seconds) was defined as the moment when the first air bubble was introduced into the burette tube 21 from the air inlet tube 25. The decrease in the 0.9 mass% saline solution 50 in the burette tube 21 (i.e., the amount of 0.9 mass% saline solution absorbed by the water-absorbing resin particles 10a) was read sequentially in 0.1 mL units, and the decrease in the 0.9 mass% saline solution 50 Wb (mL) 10 minutes after the start of water absorption by the water-absorbing resin particles 10a was read. From Wb, the 10-minute value of unpressurized DW was calculated using the following formula. Unpressurized DW is the amount of water absorbed per 1.00 g of water-absorbing resin particles 10a. 10-minute value of unpressurized DW (mL / g) = Wb / 1.00

[0170] [Saline Solution Water Retention Capacity] A cotton bag (membrane no. 60, 100 mm wide x 200 mm long) containing 2.0 g of superabsorbent polymer particles was placed in a 500 mL beaker. 500 g of 0.9 mass% sodium chloride aqueous solution (physiological saline solution) was poured into the cotton bag containing the superabsorbent polymer particles in one go, taking care not to let it spill. The top of the cotton bag was tied with a rubber band and left to stand for 30 minutes to allow the superabsorbent polymer particles to swell. After 30 minutes, the cotton bag was dewatered for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., model number: H-122) set to a centrifugal force of 167 G, and the mass Wc (g) of the cotton bag containing the swollen gel after dewatering was measured. The same procedure was performed without adding superabsorbent polymer particles, and the empty mass Wd (g) of the cotton bag when wet was measured. The physiological saline solution water retention capacity was calculated from the following formula: Physiological saline solution water retention capacity (g / g) = [Wc - Wd] / 2.0

[0171] Table 1 shows the conditions of the cleaning process carried out in each example and comparative example (type of cleaning organic solvent, water content during cleaning, number of cleaning operations), as well as the measurement results of water content at surface crosslinking, water retention, and 10-minute value of unpressurized DW.

[0172] Table 1 shows that superabsorbent polymer particles obtained by a manufacturing method comprising a washing step of washing polymer particles with a water content of 10% by mass or more and less than 120% by mass with an organic solvent, and a surface crosslinking step of surface crosslinking the polymer particles washed by the washing step, have excellent water absorption performance (especially the 10-minute value of unpressurized DW).

[0173]

[0174] The method for producing water-absorbing resin particles according to the present invention can provide excellent water absorption performance. This makes it possible to provide a material useful for environmental protection.

Claims

1. A method for producing water-absorbent resin particles, comprising at least a washing step of washing polymer particles having a water content of 10% by mass or more and less than 120% by mass with an organic solvent, and a surface crosslinking step of crosslinking the polymer particles washed by the washing step.

2. The method for producing water-absorbent resin particles according to claim 1, wherein in the washing step, the polymer particles are repeatedly washed with the organic solvent.

3. A method for producing water-absorbent resin particles according to claim 1 or 2, comprising a step of obtaining polymer particles by reverse-phase suspension polymerization before the washing step.

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