Water-absorbing resin particle production method

Abstract

In water-absorbing resin particle production including a step for mixing polymer particles and an additive in a solid phase state, the present invention provides a water-absorbing resin particle production method with which it is possible to suitably enhance the effect of the additive on water-absorbing resin particles.　This water-absorbing resin particle production method includes: a mixing step for mixing polymer particles and an additive in a solid phase state to obtain a mixture; and a water addition step for adding water to the mixture. The additive has a granular shape. The water solubility of the additive at 25°C is less than or equal to 0.035 g / 100 mL, and the amount of said water added in the water addition step is 0.1-15 parts by mass with respect to 100 parts by mass of the polymer particles.

Description

Method for producing superabsorbent polymer particles

[0001] The present invention relates to a method for producing superabsorbent resin particles, and more specifically, to a method for producing superabsorbent resin particles that constitute an absorbent material suitably used in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0002] Superabsorbent polymer particles have recently been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

[0003] Such water-absorbing resin particles include crosslinked polymers of water-soluble ethylenically unsaturated monomers, and more specifically, crosslinked polymers of partially neutralized polyacrylic acid. These are considered preferable water-absorbing resin particles because they possess excellent water absorption capabilities, and because acrylic acid, their raw material, is readily available industrially, they can be manufactured with consistent quality and at low cost, and they have numerous advantages such as being resistant to spoilage and deterioration (see, for example, Patent Document 1).

[0004] Absorbent products such as disposable diapers, sanitary napkins, and incontinence pads are primarily composed of an absorbent core located in the center that absorbs and retains bodily fluids such as urine and menstrual blood excreted from the body, a liquid-permeable top sheet located on the side that comes into contact with the body, and a liquid-impermeable back sheet located on the opposite side that comes into contact with the body. The absorbent core is usually composed of hydrophilic fibers such as pulp and water-absorbent resin particles.

[0005] International Publication No. 2015 / 152299

[0006] Superabsorbent polymer particles are produced, for example, by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, and then surface crosslinking to obtain polymer particles.

[0007] Furthermore, various additives may be added to the polymer particles that constitute the water-absorbing resin particles, depending on the purpose (e.g., Patent Document 1).

[0008] The inventors of this invention investigated and found that even when the same additive is applied to the surface of polymer particles, the effect of the additive on the water-absorbing resin particles varies greatly depending on the method of application.

[0009] The main objective of the present invention is to provide a method for producing superabsorbent polymer particles, which includes a step of mixing polymer particles and additives in a solid phase, and which can suitably improve the effect of the additives on the superabsorbent polymer particles.

[0010] The inventors diligently studied to solve the above problems. As a result, they found that in a method for producing water-absorbent resin particles, the method comprises a mixing step of mixing polymer particles and an additive in a solid state to obtain a mixture, and a water addition step of adding water to the mixture, wherein the shape of the additive is particulate, the water solubility of the additive at 25°C is set to a predetermined value or less, and furthermore, the amount of water added in the water addition step is set to a predetermined range, thereby suitably improving the effect of the additive on the water-absorbent resin particles. The present invention was completed based on these findings and further diligent studies.

[0011] In other words, the present invention provides an invention having the following configuration. Item 1. A method for producing superabsorbent polymer particles, comprising: a mixing step of mixing polymer particles and an additive in a solid state to obtain a mixture; and a water addition step of adding water to the mixture, wherein the shape of the additive is granular, the water solubility of the additive at 25°C is 0.035 g / 100 mL or less, and the amount of water added in the water addition step is 0.1 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the polymer particles. Item 2. The method for producing superabsorbent polymer particles according to Item 1, wherein the contact angle of the additive with respect to water is 90° or less. Item 3. The method for producing superabsorbent polymer particles according to Item 1 or 2, wherein the water content of the polymer particles subjected to the mixing step is 30% by mass or less. Item 4. The method for producing superabsorbent polymer particles according to any one of Items 1 to 3, wherein in the water addition step, water is added while stirring the mixture. Item 5. The method for producing water-absorbent resin particles according to any one of claims 1 to 4, wherein the polymer particles are formed by surface crosslinking of a polymer of water-soluble ethylenically unsaturated monomers.

[0012] According to the present invention, a method for producing superabsorbent polymer particles is provided that includes a step of mixing polymer particles and additives in a solid phase, and which can suitably improve the effect of the additives on the superabsorbent polymer particles.

[0013] This is a schematic diagram of the measuring apparatus for the 3-minute unpressurized DW value of superabsorbent polymer particles. This is a scanning electron microscope (SEM) image of the superabsorbent polymer particles obtained in Example 1 (scale bar is 50 μm). This is a scanning electron microscope (SEM) image of the superabsorbent polymer particles obtained in Comparative Example 1 (scale bar is 50 μm). This is a scanning electron microscope (SEM) image of the superabsorbent polymer particles obtained in Comparative Example 2 (scale bar is 50 μm).

[0014] In this specification, "comprising" includes "consisting essentially of" and "consisting of". In this specification, "(meth)acrylic" means "acrylic or methacrylic", "(meth)acrylate" means "acrylate or methacrylate", and "(poly)" means with or without the prefix "poly". In this specification, "water-soluble" means solubility of 5% by mass or more in water at 25°C.

[0015] In this specification, numbers enclosed in "~" represent a numerical range that includes the numbers before and after "~" as the lower and upper limits, respectively. If multiple lower and upper limits are listed separately, any lower and upper limits may be selected and enclosed in "~".

[0016] [Method for Producing Superabsorbent Polymer Particles] The method for producing superabsorbent polymer particles of the present invention comprises a mixing step of mixing polymer particles and an additive in a solid state to obtain a mixture, and a water addition step of adding water to the mixture, wherein the shape of the additive is granular, the water solubility of the additive at 25°C is 0.035 g / 100 mL or less, and the amount of water added in the water addition step is 0.1 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the polymer particles. By having these characteristics, the method for producing superabsorbent polymer particles of the present invention can suitably improve the effect of the additive imparted to the superabsorbent polymer particles. The method for producing superabsorbent polymer particles of the present invention will be described in detail below.

[0017] (Mixing process) In the mixing process, polymer particles and additives are mixed in a solid state to obtain a mixture.

[0018] From the viewpoint of suitably exhibiting the effects of the present invention, the polymer particles subjected to the mixing step are preferably polymers of water-soluble ethylenically unsaturated monomers, and more preferably polymers of water-soluble ethylenically unsaturated monomers that have been surface-crosslinked. These polymer particles can be obtained, for example, by a polymerization step of water-soluble ethylenically unsaturated monomers, and further by a surface crosslinking step, as described later.

[0019] From the viewpoint of more favorably exhibiting the effects of the present invention, the water content of the polymer particles is preferably 30% by mass or less, more preferably 25% by mass or less, and even more preferably 20% by mass or less. For example, the lower limit can be 0% by mass or more, 2% by mass or more, 5% by mass or more, and preferred ranges can be 0 to 25% by mass, 2 to 25% by mass, 5 to 20% by mass, etc. By having a water content of 30% by mass or less of the polymer particles, when the polymer particles are mixed with an additive having water solubility as described later, the penetration of the additive into the interior of the polymer particles and the reduction in the amount of additive present on the surface of the polymer particles (and furthermore, the amount of additive present on the surface of the water-absorbing resin particles produced) is suppressed, and the improvement of the properties of the water-absorbing resin particles by the additive can be more favorably exhibited. In this invention, the improvement of the properties of the water-absorbing resin particles by the additive can be, for example, the improvement of the properties of the 3-minute value of unpressurized DW as described later. The water content of the polymer particles is measured by the method described in the examples.

[0020] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the median particle size of the polymer particles is preferably 150 μm or more, 200 μm or more, 250 μm or more, 280 μm or more, 300 μm or more, 350 μm or more, etc., and also preferably 850 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less. The range of the median particle size of the polymer particles is preferably 150 to 850 μm, more preferably 200 to 600 μm, even more preferably 250 to 500 μm, even more preferably 300 to 450 μm, and even more preferably 320 to 400 μm.

[0021] The method for measuring the median particle size of polymer particles is the same as the method for measuring the median particle size of superabsorbent resin particles. The median particle size of superabsorbent resin particles can be measured using a JIS standard sieve, and specifically, the value is the value measured by the method described in the examples.

[0022] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the amount of saline solution that the polymer particles can hold is preferably 25 g / g or more, more preferably 30 g / g or more, even more preferably 35 g / g or more, and also preferably 60 g / g or less, more preferably 55 g / g or less, even more preferably 50 g / g or less, with preferred ranges being 25 to 60 g / g, 30 to 50 g / g, 35 to 50 g / g, etc.

[0023] The method for measuring the saline water retention capacity of polymer particles is the same as the method for measuring the saline water retention capacity of superabsorbent resin particles. The saline water retention capacity of the superabsorbent resin particles is the value measured by the method described in the examples.

[0024] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the 3-minute value of the unpressurized DW (Demand Wetability) of the polymer particles is preferably 0.5 mL or more, more preferably 1 mL or more, even more preferably 2 mL or more, and also preferably 55 mL or less, more preferably 50 mL or less, even more preferably 45 mL or less. Preferred ranges include 0.5 to 55 mL, 0.5 to 50 mL, 0.5 to 45 mL, 1 to 55 mL, 1 to 50 mL, 1 to 45 mL, 2 to 55 mL, 2 to 50 mL, 2 to 45 mL, etc.

[0025] The method for measuring the 3-minute unpressurized DW of polymer particles is the same as the method for measuring the 3-minute unpressurized DW of superabsorbent resin particles. The 3-minute unpressurized DW of superabsorbent resin particles is the value measured by the method described in the examples.

[0026] In the mixing step of the present invention, the shape of the additive mixed with polymer particles in a solid state is granular.

[0027] From the viewpoint of more favorably exhibiting the effects of the present invention, the median particle size of the additive is preferably 0.1 μm or more, more preferably 1 μm or more, even more preferably 3 μm or more, and also preferably 50 μm or less, more preferably 15 μm or less, and even more preferably 10 μm or less. Preferred ranges include approximately 0.1 to 50 μm, approximately 0.1 to 15 μm, approximately 0.1 to 10 μm, approximately 1 to 50 μm, approximately 1 to 15 μm, approximately 1 to 10 μm, approximately 3 to 50 μm, approximately 3 to 15 μm, and approximately 3 to 10 μm. The median particle size of the additive (D50 (median diameter), volume basis) is measured by a laser diffraction particle size distribution analyzer, and the specific measurement is described in the examples.

[0028] In the present invention, the water solubility of the additive mixed with the polymer particles in a solid state at 25°C is 0.035 g / 100 mL or less. In the water-absorbing resin particles of the present invention, by limiting the water solubility of the additive to such a value and setting the amount of water added in the water addition step described later to a predetermined amount, the effect of the additive on the water-absorbing resin particles can be suitably improved. That is, in the method for producing water-absorbing resin particles of the present invention, in the mixing step, the polymer particles and a granular additive having a predetermined water solubility are mixed in a solid state in advance to form a mixture, and a predetermined amount of water is added to the mixture, so that the additive adheres to the surface of the polymer particles with high uniformity. It is believed that the improvement in properties due to the additive applied to the water-absorbing resin particles is suitably exhibited because the additive adheres to the surface of the polymer particles with high uniformity.

[0029] The water solubility of the additive at 25°C should be 0.035 g / 100 mL or less, but from the viewpoint of more favorably exhibiting the effects of the present invention, the water solubility is preferably 0.010 g / 100 mL or less, more preferably 0.005 g / 100 mL or less, even more preferably 0.003 g / 100 mL or less, and also preferably 0.0001 g / 100 mL or more, more preferably 0.0005 g / 100 mL or more, and even more preferably 0.0010 g / 100 mL. The solubility of the additive in water at 25°C is measured in accordance with the column method of the OECD test guidelines.

[0030] Furthermore, from the viewpoint of more favorably exhibiting the effects of the present invention, the contact angle of the additive with water is preferably 120° or less, more preferably 100° or less, even more preferably 90° or less, also preferably 0° or more, more preferably 25° or more, even more preferably 30° or more, even more preferably 50° or more, and particularly preferably 70° or more. Preferred ranges include 0 to 120°, 0 to 100°, 0 to 90°, 20 to 120°, 20 to 100°, 20 to 90°, 25 to 120°, 25 to 100°, 25 to 90°, 30 to 120°, 30 to 100°, 30 to 90°, 50 to 120°, 50 to 100°, 50 to 90°, 70 to 120°, 70 to 100°, and 70 to 90°. The contact angle of the additive with water is the value measured by the method described in the examples.

[0031] In the present invention, the type of additive is not particularly limited as long as it is granular and satisfies the aforementioned water solubility requirements, and can be appropriately selected according to the purpose of improving the properties of the water-absorbent resin particles. For example, from the viewpoint of improving the 3-minute value of the unpressurized DW of surface-crosslinked polymer particles, preferred additives include tricalcium phosphate, calcium fluoride, barium sulfate, calcium sulfate, calcium sulfite, trilithium phosphate, calcium hydrogen phosphate, calcium nitrate, calcium carbonate, and magnesium diphosphate. Furthermore, from the viewpoint of improving the deodorizing effect of the polymer particles, preferred additives include silver-zinc zeolite, silver oxide, zinc, catechin, activated carbon, and zeolite. In addition, from the viewpoint of improving fluidity, examples of additives include silicon dioxide (silica), titanium dioxide, talc, multi-metal compounds containing two types of metal cations (divalent and trivalent) and hydroxyl groups having a hydrotalcite structure, aluminum hydroxide, aluminosilicates such as montmorillonite and zeolite, silicic acid, diatomaceous earth, bentonite, kaolin, and activated clay. The additives used in the mixing process may be one type or two or more types.

[0032] In the mixing process, the mixing ratio of polymer particles and additives can be appropriately adjusted according to the type and degree of the effect on improving the properties of the water-absorbent resin particles. For example, per 100 parts by mass of polymer particles, the amount of additive is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably 0.5 parts by mass or more, and also preferably 10 parts by mass or less, more preferably 5 parts by mass or less, even more preferably 3 parts by mass or less. Preferred ranges include 0.05 to 10 parts by mass, 0.05 to 5 parts by mass, 0.05 to 3 parts by mass, 0.1 to 10 parts by mass, 0.1 to 5 parts by mass, 0.1 to 3 parts by mass, 0.5 to 10 parts by mass, 0.5 to 5 parts by mass, and 0.5 to 3 parts by mass.

[0033] In addition, within the limit of not inhibiting the effects of the present invention, in the mixing step, in addition to additives having a water solubility of 0.035 g / 100 mL or less at 25°C, other additives such as additives having a water solubility exceeding 0.035 g / 100 mL and additives whose shape is not particulate (for example, liquid) may be further mixed. However, the mixing ratio of additives having a water solubility exceeding 0.035 g / 100 mL is preferably 2 parts by mass or less, more preferably 1 part by mass or less, and still more preferably 0.5 parts by mass or less with respect to 100 parts by mass of the polymer particles. Examples of the lower limit include 0 parts by mass and 0.01 parts by mass.

[0034] The temperature at which the mixing step is carried out is, for example, 10 to 90°C. Also, the mixing step can be carried out, for example, under atmospheric pressure.

[0035] In the mixing step, the polymer particles and the additives can be mixed using devices such as various stirrers having stirring blades and air current mixers. When using various stirrers having stirring blades, the stirring blades can be, for example, flat blades, lattice blades, paddle blades, propeller blades, anchor-type stirring blades, turbine blades, Faudler blades, ribbon blades, full zone blades, or max blend blades.

[0036] (Water addition step) In the method for producing the water-absorbent resin particles of the present invention, after the mixing step, a water addition step is carried out. In the water addition step, a predetermined amount of water is added to the mixture.

[0037] In the water addition step, the amount of water added is in the range of 0.1 part by mass or more and 15 parts by mass or less with respect to 100 parts by mass of the polymer particles. From the viewpoint of more preferably exerting the effects of the present invention, the amount of water added is preferably 0.2 parts by mass or more, more preferably 0.3 parts by mass or more, still more preferably 0.4 parts by mass or more, and is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, still more preferably 5 parts by mass or less. Preferred ranges include 0.2 to 10 parts by mass,​​​​​​​​​​​The water addition process is carried out at a temperature of, for example, 10 to 90°C. Furthermore, the water addition process can be performed, for example, under atmospheric pressure.

[0042] By performing the above mixing and water addition steps, the water-absorbing resin particles of the present invention are produced.

[0043] The water-absorbing resin particles of the present invention may contain other additives depending on the purpose (additives different from the granular additives with a water solubility of 0.035 g / 100 mL or less at 25°C mentioned above (i.e., additives with a water solubility exceeding 0.035 g / 100 mL, additives that are not granular in shape (e.g., liquid))). Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, radical chain inhibitors, antioxidants, 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 polymer particles. It is preferable that the other additives are hydrophilic or water-soluble.

[0044] The amount of saline solution that the superabsorbent resin particles obtained by the manufacturing method of the present invention can hold is preferably 25 g / g or more, more preferably 30 g / g or more, even more preferably 35 g / g or more, and also preferably 60 g / g or less, more preferably 55 g / g or less, even more preferably 50 g / g or less, with preferred ranges being 25 to 60 g / g, 30 to 50 g / g, 35 to 50 g / g, etc.

[0045] Furthermore, the 3-minute unpressurized DW (Demand Wetability) of the water-absorbing resin particles obtained by the manufacturing method of the present invention is preferably 18 mL or more, more preferably 20 mL or more, even more preferably 23 mL or more, and also preferably 55 mL or less, more preferably 45 mL or less, even more preferably 35 mL or less. Preferred ranges include 18-55 mL, 18-45 mL, 18-35 mL, 20-55 mL, 20-45 mL, 20-35 mL, 23-55 mL, 23-45 mL, 23-35 mL, etc.

[0046] The saline solution water retention capacity and the 3-minute unpressurized DW value of the superabsorbent resin particles were measured using the method described in the examples.

[0047] The median particle size of the water-absorbent resin particles is preferably, for example, 150 μm or more, 200 μm or more, 250 μm or more, 280 μm or more, 300 μm or more, or 350 μm or more, and also preferably 850 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less. In other words, the median particle size is preferably 150 to 850 μm, more preferably 200 to 600 μm, even more preferably 250 to 500 μm, even more preferably 300 to 450 μm, and even more preferably 320 to 400 μm.

[0048] 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.

[0049] Absorbent polymer particles may consist of single particles, or they may be in the form of aggregated fine particles (primary particles) (secondary particles). Examples of primary particle shapes include approximately spherical, irregularly shaped, or plate-like. In the case of primary particles produced by reverse-phase suspension polymerization, examples include approximately spherical single particles with a smooth surface shape such as a perfect sphere or ellipsoid.

[0050] Next, the polymer particles subjected to the mixing step (polymer particles mixed with additives in the mixing step) in the manufacturing method of the present invention will be described in detail.

[0051] (Polymer Particles) In the manufacturing method of the present invention, the polymer particles subjected to the mixing step are preferably polymers of water-soluble ethylenically unsaturated monomers, and more preferably are composed of surface-crosslinked polymers of water-soluble ethylenically unsaturated monomers, that is, surface-crosslinked polymers having structural units derived from water-soluble ethylenically unsaturated monomers. For the surface crosslinking treatment method, for example, the method described in <Surface Crosslinking Step> below can be used.

[0052] The preferred medium particle size for polymer particles is as described above.

[0053] Furthermore, polymer particles, like water-absorbing resin particles, may exist in forms consisting of single particles, or in forms where fine particles (primary particles) are aggregated (secondary particles). Examples of primary particle shapes include approximately spherical, irregularly shaped, and plate-like forms. In the case of primary particles produced by reverse-phase suspension polymerization, examples include approximately spherical single particles with a smooth surface shape such as a perfect sphere or ellipsoid.

[0054] Representative polymerization methods for water-soluble ethylenically unsaturated monomers include aqueous solution polymerization, emulsion polymerization, and reverse-phase suspension polymerization. In aqueous solution polymerization, polymerization is carried out by heating an aqueous solution of water-soluble ethylenically unsaturated monomers while stirring as needed. In reverse-phase suspension polymerization, polymerization is carried out by heating water-soluble ethylenically unsaturated monomers in a hydrocarbon dispersion medium while stirring.

[0055] An example of a method for producing polymer particles is described below.

[0056] Specific examples of methods for producing polymer particles include a method for producing polymer particles by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, the method comprising the steps of: carrying out polymerization in the presence of a radical polymerization initiator; drying the water-containing gel-like polymer obtained by polymerization; and surface crosslinking the polymer obtained by drying in the presence of a surface crosslinking agent. In addition, in the method for producing polymer particles of the present invention, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed to produce a polymer having an internal crosslinking structure.

[0057] <Polymerization Process> [Water-soluble ethylenically unsaturated monomers] Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid (in this specification, "acrylic" and "methacrylic" are collectively referred to as "(meth)acrylic"; the same applies hereinafter) and its salts; 2-(meth)acrylamide-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. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or its salts, (meth)acrylamide, and N,N-dimethylacrylamide are preferred, with (meth)acrylic acid and its salts being more preferred, from the viewpoint of being readily available industrially. These water-soluble ethylenically unsaturated monomers may be used individually or in combination of two or more types.

[0058] Among these, acrylic acid and its salts are widely used as raw materials for polymer particles, and in some cases, these acrylic acids and / or their salts are copolymerized with the aforementioned other water-soluble ethylenically unsaturated monomers. In this case, it is preferable that acrylic acid and / or its salts are used as the main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol% relative to the total water-soluble ethylenically unsaturated monomers.

[0059] The water-soluble ethylenically unsaturated monomer may be dispersed in a hydrocarbon dispersion medium in aqueous solution and subjected to reverse-phase suspension polymerization. By making the water-soluble ethylenically unsaturated monomer an aqueous solution, the dispersion efficiency in the hydrocarbon dispersion medium can be increased. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20% by mass to the saturation concentration or less. Furthermore, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 45% by mass or less. On the other hand, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 25% by mass or more, even more preferably 28% by mass or more, and even more preferably 30% by mass or more.

[0060] When the water-soluble ethylenically unsaturated monomer has an acidic group, such as (meth)acrylic acid or 2-(meth)acrylamide-2-methylpropanesulfonic acid, the acidic group may be neutralized beforehand with an alkaline neutralizing agent as needed. 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. These alkaline neutralizing agents may also be used in aqueous solution form to simplify the neutralization process. The above-mentioned alkaline neutralizing agents may be used individually or in combination of two or more types.

[0061] The degree of neutralization of a water-soluble ethylenically unsaturated monomer by an alkaline neutralizing agent is preferably 10 to 100 mol%, more preferably 30 to 90 mol%, even more preferably 40 to 85 mol%, and even more preferably 50 to 80 mol% as the degree of neutralization with respect to all acid groups of the water-soluble ethylenically unsaturated monomer.

[0062] [Radical polymerization initiators] Examples of radical polymerization initiators added to the polymerization process 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, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; and 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis[2-(N-Fe]. Examples of azo compounds include 2,2'-azobis[2-(N-allylamidino)propane] dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], and 4,4'-azobis(4-cyanovaleric acid). Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate, and 2,2'-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoint of being readily available and easy to handle. These radical polymerization initiators may be used alone or in combination of two or more. Furthermore, the radical polymerization initiator can also be used as a redox polymerization initiator in combination with reducing agents such as sodium sulfite, sodium bisulfite, ferrous sulfate, and L-ascorbic acid.

[0063] For example, the amount of radical polymerization initiator used is 0.00005 to 0.01 moles per mole of water-soluble ethylenically unsaturated monomer. By using such an amount, it is possible to avoid rapid polymerization reactions and complete the polymerization reaction within an appropriate time.

[0064] [Internal Crosslinking Agent] Examples of internal crosslinking agents include those that can crosslink the polymer of the water-soluble ethylenically unsaturated monomer used, such as (poly)ethylene glycol ["(poly)" means whether or not the prefix "poly" is present.] [The same applies hereafter], unsaturated polyesters obtained by reacting polyols such as (poly)propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and (poly)glycerin with unsaturated acids such as (meth)acrylic acid, maleic acid, and fumaric acid; bisacrylamides such as N,N-methylenebisacrylamide; di(meth)acrylic acid esters 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''-triallyl isocyanurate, divinyl Examples include compounds having two or more polymerizable unsaturated groups such as benzene; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; polyglycidyl compounds such as triglycidyl compounds; epihalohydrin compounds such as epichlorohydrin, epibromuhydrin, and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; and oxetane compounds such as 3-methyl-3-oxetane methanol, 3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol, 3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, and 3-butyl-3-oxetane ethanol. Among these internal crosslinking agents, polyglycidyl compounds are preferred, diglycidyl ether compounds are more preferred, and (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether are particularly preferred.These internal crosslinking agents may be used individually or in combination of two or more types.

[0065] The amount of internal crosslinking agent used is preferably 0.000001 to 0.02 moles, more preferably 0.00001 to 0.01 moles, even more preferably 0.00001 to 0.005 moles, and even more preferably 0.00005 to 0.002 moles per mole of water-soluble ethylenically unsaturated monomer.

[0066] [Hydrogen Dispersion Medium] Examples of hydrocarbon dispersion mediums 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 dispersion mediums, n-hexane, n-heptane, and cyclohexane are particularly suitable because they are readily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion mediums may be used individually or in combination of two or more types. Furthermore, suitable results can also be obtained by using commercially available hydrocarbon dispersion media such as exolheptane (manufactured by ExxonMobil: containing 75-85% by mass of heptane and its isomers).

[0067] The amount of hydrocarbon dispersion medium used is preferably 100 to 1500 parts by mass, and more preferably 200 to 1400 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer, from the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating control of the polymerization temperature. As will be described later, reverse-phase suspension polymerization is carried out in one stage (single stage) or in two or more stages, and the first-stage polymerization described above refers to the first-stage polymerization reaction in single-stage polymerization or multi-stage polymerization (the same applies hereinafter).

[0068] [Dispersion Stabilizers] (Surfactants) In reverse-phase suspension polymerization, dispersion stabilizers can be used to improve the dispersion stability of water-soluble ethylenically unsaturated monomers in hydrocarbon dispersion media. Surfactants can be used as such dispersion stabilizers.

[0069] 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, and phosphate esters of polyoxyethylene alkylallyl 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.

[0070] The amount of surfactant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0071] (Polymer-based dispersants) In addition, polymer-based dispersants may be used together with the surfactants mentioned above as dispersion stabilizers in reverse-phase suspension polymerization.

[0072] Examples of polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene terpolymer), maleic anhydride-modified polybutadiene, maleic anhydride-ethylene copolymer, maleic anhydride-propylene copolymer, maleic anhydride-ethylene-propylene copolymer, maleic anhydride-butadiene copolymer, polyethylene, polypropylene, ethylene-propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene-propylene copolymer, ethylene-acrylic acid copolymer, ethylcellulose, and ethyl hydroxyethylcellulose. Among these polymeric dispersants, it is particularly preferable to use 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, from the viewpoint of monomer dispersion stability. These polymeric dispersants may be used individually or in combination of two or more types.

[0073] The amount of polymeric dispersant used is preferably 0.1 to 30 parts by mass, and more preferably 0.3 to 20 parts by mass, per 100 parts by mass of the first-stage water-soluble ethylenically unsaturated monomer.

[0074] [Other Components] In the method for producing polymer particles, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer to carry out reverse-phase suspension polymerization, if desired. Other components may include various additives such as thickeners and chain transfer agents.

[0075] For example, reverse-phase suspension polymerization can be carried out by adding a thickening agent to an aqueous solution containing a water-soluble ethylenically unsaturated monomer. By adjusting the viscosity of the aqueous solution by adding a thickening agent in this way, it is possible to control the intermediate particle size obtained in reverse-phase suspension polymerization.

[0076] Examples of thickening agents that can be used include hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, carboxymethylcellulose, polyacrylic acid, partially neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide. It should be noted that, assuming the same stirring speed during polymerization, the higher the viscosity of the aqueous solution of the water-soluble ethylenically unsaturated monomer, the larger the primary and / or secondary particles obtained tend to be.

[0077] [Reverse-phase suspension polymerization] In reverse-phase suspension polymerization, for example, an aqueous monomer solution containing a water-soluble ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer. At this time, as long as it is before the polymerization reaction starts, the timing of adding the dispersion stabilizer (surfactant or polymer-based dispersant) does not matter; it can be before or after the addition of the aqueous monomer solution.

[0078] Among these methods, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in the resulting polymer particles, it is preferable to disperse a monomer aqueous solution in a hydrocarbon dispersion medium containing a polymer-based dispersant, and then further disperse a surfactant before carrying out polymerization.

[0079] Such reverse-phase suspension polymerization can be carried out in one or more stages. Furthermore, from the viewpoint of increasing productivity, it is preferable to carry it out in two to three stages.

[0080] When performing reverse-phase suspension polymerization in two or more stages, after performing the first stage of reverse-phase suspension polymerization, a water-soluble ethylenically unsaturated monomer is added to the reaction mixture obtained from the first stage polymerization reaction and mixed, and the second and subsequent stages of reverse-phase suspension polymerization are performed 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 add 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 and subsequent stages.

[0081] The reaction temperature for the polymerization reaction is preferably 20 to 110°C, and more preferably 40 to 90°C, from the viewpoint of improving economic efficiency by rapidly advancing polymerization and shortening the polymerization time, as well as easily removing the heat of polymerization and allowing the reaction to proceed smoothly.

[0082] <Drying Process> After the reverse-phase suspension polymerization described above, a drying process may be included in which water, hydrocarbon dispersion medium, etc., are removed by distillation by applying energy such as heat from an external source. When dehydrating a water-containing gel after reverse-phase suspension polymerization, the system in which the water-containing gel is dispersed in the hydrocarbon dispersion medium is heated, and the water and hydrocarbon dispersion medium are temporarily removed from the system by azeotropic distillation. At this time, if only the distilled hydrocarbon dispersion medium is returned to the system, continuous azeotropic distillation becomes possible. In this case, the temperature inside the system during drying is maintained below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of preventing resin degradation. Subsequently, the polymer is obtained by distilling off the water and hydrocarbon dispersion medium. By controlling the processing conditions of the drying process after polymerization and adjusting the amount of dehydration, it is possible to control the various properties of the obtained polymer particles.

[0083] In the drying process, the drying treatment by distillation may be carried out under atmospheric pressure or under reduced pressure. Furthermore, from the viewpoint of improving drying efficiency, it may be carried out under a stream of gas such as nitrogen. When the drying treatment is carried out under atmospheric pressure, 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. When the drying treatment is carried out under reduced pressure, the drying temperature is preferably 40 to 160°C, and more preferably 50 to 110°C.

[0084] <Surface Crosslinking Process> Next, it is preferable to crosslink the polymer particles of the present invention by adding a surface crosslinking agent to a polymer having an internal crosslinking structure obtained by polymerizing water-soluble ethylenically unsaturated monomers (surface crosslinking reaction). This surface crosslinking reaction is preferably carried out in the presence of the surface crosslinking agent after polymerization of the water-soluble ethylenically unsaturated monomers. In this way, by subjecting the polymer having an internal crosslinking structure to a surface crosslinking reaction after polymerization, the crosslinking density near the surface of the polymer particles can be increased, and polymer particles with improved performance such as water absorption capacity under load can be obtained.

[0085] 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 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-dioxolan-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; ethylene carbonate, propylene carbonate, 4,5-dimethyl-1, Carbonate compounds (e.g., alkylene carbonates) such as 3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-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-dioxolan-2-one are preferred. These surface crosslinking agents may be used individually or in combination of two or more types.

[0086] The amount of surface crosslinking agent used is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, and even more preferably 0.1 to 1 part by mass, per 100 parts by mass of polymer.

[0087] Regarding the method of adding the surface crosslinking agent, it may be added as is, 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.

[0088] When adding the surface crosslinking agent, the water content of the polymer is preferably 0.01 to 40% by mass, more preferably 0.05 to 30% by mass, even more preferably 0.1 to 25% by mass, and still more preferably 1 to 20% by mass.

[0089] The reaction temperature for the surface crosslinking reaction is preferably 50 to 300°C, more preferably 100 to 280°C, even more preferably 150 to 250°C, and even more preferably 170 to 220°C. The reaction time for the surface crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

[0090] Furthermore, the residual monomer content (amount of water-soluble ethylenically unsaturated monomers and their salts) contained in the polymer particles is, for example, 1 ppm or more, 5 ppm or more, 10 ppm or more, and also, for example, 100,000 ppm or less, 10,000 ppm or less, 5,000 ppm or less.

[0091] <Residual Monomer Treatment Process> As described above, water-absorbing resin particles can be produced, for example, by reverse-phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, and then surface crosslinking to obtain polymer particles.

[0092] Polymer particles that have undergone surface crosslinking may contain unreacted water-soluble ethylenically unsaturated monomers as residual monomers. Since there are concerns about skin irritation (skin roughness) due to these residual monomers, it is desirable to reduce the amount of residual monomers in the water-absorbent resin particles as much as possible. Therefore, it is preferable to perform a residual monomer treatment step after the surface crosslinking step.

[0093] The residual monomer treatment process can be carried out, for example, by mixing polymer particles, which are surface-crosslinked polymers of water-soluble ethylenically unsaturated monomers, with a component that promotes the polymerization reaction of residual monomers (such as a reducing inorganic salt or organic antioxidant) as needed, and then heat-treating the mixture.

[0094] Examples of reducing inorganic salts include sulfite compounds such as sodium sulfite, potassium sulfite, calcium sulfite, sodium bisulfite, potassium bisulfite, ammonium bisulfite, sodium pyrosulfite, and potassium pyrosulfite.

[0095] The amount of reducing inorganic salt added per 100 parts by mass of solid content of polymer particles is, for example, in the range of 0.1 to 1.5 parts by mass, with preferred ranges being approximately 0.2 to 1.0 parts by mass, 0.2 to 0.5 parts by mass, 0.3 to 1.0 parts by mass, and 0.3 to 0.5 parts by mass.

[0096] Preferred organic antioxidants include ascorbic acids, erythorbic acids, gallic acids, protocatechuic acids, benzimidazoles, and alkylhydroxyanisole compounds.

[0097] The amount of organic antioxidant added per 100 parts by mass of solid content of polymer particles may be, for example, in the range of more than 0 parts by mass and 0.1 parts by mass or less. Preferred ranges include approximately 0.0001 to 0.05 parts by mass, approximately 0.0001 to 0.015 parts by mass, approximately 0.0001 to 0.005 parts by mass, approximately 0.001 to 0.05 parts by mass, approximately 0.001 to 0.015 parts by mass, and approximately 0.001 to 0.005 parts by mass.

[0098] The heating temperature in the residual monomer treatment step should be set to a temperature at which the residual monomer contained in the polymer particles reacts with the reducing inorganic salt. Preferred ranges include approximately 50-150°C, 50-140°C, 50-130°C, 70-150°C, 70-140°C, 70-130°C, 80-150°C, 80-140°C, and 80-130°C.

[0099] The residual monomer content (amount of water-soluble ethylenically unsaturated monomers and their salts) contained in the water-absorbent resin particles produced by the manufacturing method of the present invention is preferably 300 ppm or less, more preferably 200 ppm or less, and even more preferably 150 ppm or less, with a lower limit of 0 ppm.

[0100] [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.

[0101] The absorbent material 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.

[0102] The basis weight of the water-absorbing resin particles in the absorbent material of the present invention is 50 g / m². 2 More than 400g / m 2 The following applies. The basis weight is preferably 100 g / m². 2 More comfortably, 120 g / m² 2 More preferably 140 g / m² 2 The above applies, and preferably 300 g / m². 2 More preferably, 250 g / m² 2 More preferably, 200 g / m 2 The following applies:

[0103] Examples of hydrophilic fibers include at least one selected from the group consisting of finely ground wood pulp, cotton, cotton linter, rayon, cellulose acetate, polyamide, polyester, and polyolefin. Examples 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 synthetic resins such as hydrophilized polyamide, polyester, and polyolefin. The average fiber length of hydrophilic fibers is usually 0.1 to 10 mm, or may be 0.5 to 5 mm.

[0104] The basis weight of the hydrophilic fibers in the absorber of the present invention is 0 g / m 2 or more and 800 g / m 2 or less. The basis weight is preferably 100 g / m 2 or more, more preferably 120 g / m 2 or more, still more preferably 140 g / m 2 or more, and is preferably 700 g / m 2 or less, more preferably 600 g / m 2 or less, still more preferably 500 g / m 2 or less.

[0105] As the content rate of the water-absorbing resin particles in the absorber, it is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, still more preferably 20 to 90% by mass, and even more preferably 30 to 80% by mass.

[0106] By holding the absorber using the water-absorbing 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 absorbent article of the present invention can be obtained. The liquid-permeable sheet is disposed on the side in contact with the body, and the liquid-impermeable sheet is disposed on the opposite side not in contact with the body.

[0107] Examples of the liquid-permeable sheet include air-through type, spunbond type, chemical bond type, needle punch type non-woven fabrics made of fibers such as polyethylene, polypropylene, and polyester, and porous synthetic resin sheets. Examples of the liquid-impermeable sheet include synthetic resin films made of resins such as polyethylene, polypropylene, and polyvinyl chloride. The liquid-permeable sheet is preferably at least one selected from the group consisting of thermal bond non-woven fabric, air-through non-woven fabric, spunbond non-woven fabric, and spunbond / meltblown / spunbond non-woven fabric.

[0108] The basis weight of the liquid-permeable sheet is preferably 5 g / m 2 or more and 100 g / m 2 or less, and preferably 10 g / m 2 or more and 60 g / m2 The following is more preferable. Furthermore, the liquid-permeable sheet may be embossed or perforated on its surface to improve the diffusion of the liquid. The embossing or perforation can be carried out by known methods.

[0109] Examples of liquid-impermeable sheets include sheets made from synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride; sheets made from nonwoven fabrics such as spunbond / meltblown / spunbond (SMS) nonwoven fabrics, which consist of a water-resistant meltblown nonwoven fabric sandwiched between high-strength spunbond nonwoven fabrics; and sheets made from composite materials of these synthetic resins and nonwoven fabrics (e.g., spunbond nonwoven fabrics, spunlace nonwoven fabrics). Liquid-impermeable sheets can also be made from synthetic resins primarily composed of low-density polyethylene (LDPE) resin. Liquid-impermeable sheets, for example, have a basis weight of 10 to 50 g / m². 2 It may be a sheet made of synthetic resin.

[0110] The absorbent article preferably comprises a laminate having an absorbent body containing water-absorbent resin particles and a core wrap sandwiching the absorbent body from above and below, a liquid-permeable sheet disposed on the upper surface of the laminate, and a liquid-impermeable sheet disposed on the side of the laminate opposite to the liquid-permeable sheet.

[0111] The present invention will be described in detail below with reference to examples and comparative examples. However, the present invention is not limited to the examples.

[0112] The water-absorbing resin particles obtained in the following examples and comparative examples were evaluated in the following tests. Unless otherwise specified, measurements were performed under conditions of 25±2°C and 50±10% humidity.

[0113] [Production of Polymer Particles] A round-bottom cylindrical separable flask with an inner diameter of 11 cm and a volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer (with a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm). A mixture was obtained by adding 293 g of n-heptane (hydrocarbon dispersion medium) and 0.276 g of maleic anhydride-modified ethylene-propylene copolymer (polymeric dispersant, Mitsui Chemicals, Inc., High Wax 1105A) to this separable flask. The dispersant was dissolved by heating this mixture to 80°C while stirring at a rotation speed of 200 rpm. The mixture was then cooled to 55°C.

[0114] Next, 92.0 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.03 mol) was placed in a 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 102.2 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% of the acrylic acid. After that, 0.0736 g (0.272 mmol) of potassium persulfate (water-soluble radical polymerization initiator), 0.0558 g of tetra(ethylene glycol) diacrylate (internal crosslinking agent), and 39.51 g of deionized water were added and dissolved to prepare the first stage monomer aqueous solution.

[0115] The above-mentioned first-stage monomer aqueous solution was added to the above-mentioned separable flask and stirred for 10 minutes. A surfactant solution was obtained by heating and dissolving 0.276 g of tetraglyceryl tristearate, 0.460 g of hexaglyceryl tristearate, and 0.368 g of decaglyceryl pentastearate in 6.62 g of n-heptane. The reaction solution was obtained by adding 7.724 g of the obtained surfactant solution to the separable flask. The separable flask system was then thoroughly purged with nitrogen while stirring the reaction solution at a rotation speed of 400 rpm. Subsequently, the reaction solution was heated by immersing the separable flask in a 70°C water bath, and the first-stage polymerization was carried out for 60 minutes to obtain the first-stage reaction mixture.

[0116] Next, 128.8 g of an 80.5% by mass acrylic acid aqueous solution (acrylic acid: 1.44 mol) was placed in another 500 mL Erlenmeyer flask. Subsequently, while cooling from the outside, 143.1 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize 75 mol% of the acrylic acid. Then, 0.1030 g (0.3810 mmol) of potassium persulfate, 0.0447 g of tetra(ethylene glycol) diacrylate (internal crosslinking agent), and 10.96 g of deionized water were added and dissolved to prepare the second stage monomer aqueous solution.

[0117] The reaction mixture for the first stage was stirred at 1000 rpm while the separable flask system was cooled to 27°C. Then, the entire volume of the aqueous solution for the second stage was added to the polymerization slurry for the first stage. The system was then purged with nitrogen for 30 minutes. The flask was then immersed again in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes to obtain the polymerization slurry for the second stage. After the polymerization of the second stage, the reaction mixture for the second stage was heated in an oil bath at 125°C, and 245.22 g of water was extracted by azeotropic distillation of n-heptane and water while refluxing the n-heptane. Subsequently, the n-heptane was evaporated to dry the mixture, and the dried polymer was obtained by passing it through a sieve with an 850 μm mesh size. This operation was repeated three times to obtain a total of 690 g of dried polymer.

[0118] <Surface Crosslinking Process> Next, 50 g of the polymer was weighed into a round-bottom cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. While stirring the dried polymer at 500 rpm, 0.005 g of 1,4-butanediol, 2.997 g of deionized water, and 1.983 g of 2-propanol were mixed to obtain a crosslinking agent solution. This solution was taken using a 20 mL syringe, a Fine Atomizer Oral (Yoshikawa Kasei Co., Ltd.) was attached to the tip of the syringe, and the solution was sprayed into the separable flask for 2 seconds. The mixture was then stirred for 100 seconds to obtain the mixture. The mixture was heated at 200°C for 40 minutes and then passed through a sieve with a mesh size of 850 μm. This surface crosslinking process was repeated 7 times, and the obtained materials were mixed to obtain polymer particles. The median particle size of the polymer particles was 340 μm.

[0119] <Residual Monomer Treatment Process> 30.00 g of polymer particles were weighed into a round-bottom cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. Next, while stirring at 500 rpm, a treatment solution consisting of 0.12 g of sodium sulfite, 0.0006 g of L-ascorbic acid, 6.0 g of water, and 1.5 g of 2-propanol was taken using a 20 mL syringe. A Fine Atomizer Oral (Yoshikawa Kasei Co., Ltd.) was attached to the tip of the syringe, and the solution was sprayed into the separable flask for 2 seconds. After stirring for 100 seconds, polymer particles containing the treatment solution (treatment solution-containing polymer particles) were obtained. The polymer particles containing the treatment solution were placed in a metal petri dish with a diameter of 120 mm and a depth of 25 mm. The entire metal petri dish containing the polymer particles containing the treatment solution was covered with aluminum foil and heated at 120°C for 30 minutes, and then air-cooled in a desiccator at 25°C and 25% humidity for 30 minutes. After air cooling, the polymer particles were classified using a sieve with an opening of 850 μm, and polymer particles that passed through the 850 μm sieve were obtained. This procedure was repeated 10 times, and 347.28 g of polymer particles were obtained. The median particle size of the polymer particles was 340 μm, the water content of physiological saline was 38 g / g, the 3-minute value of unpressurized dry weight was 3.2 mL, and the water content was 19.9%.

[0120] [Additive Mixing Process and Water Addition Process] (Example 1) 30 g of polymer particles and 0.3 g of tricalcium phosphate (Wako Pure Chemical Industries, Ltd., median particle size 8 μm, water solubility at 25°C 0.0025 g / 100 mL) were weighed into a round-bottom cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin. The polymer particles and tricalcium phosphate were stirred at 500 rpm for 60 seconds, and then 1.0 g of ion-exchanged water was sprayed into the separable flask for 1 second using a syringe tip fitted with a Fine Atomizer Oral (Yoshikawa Chemical Co., Ltd.), and then stirred for 100 seconds to obtain a mixture. The mixture was heated at 100°C for 40 minutes, and then passed through a sieve with an opening of 850 μm, and 0.2 parts by mass of silica were mixed in to obtain superabsorbent resin particles (1) with a median particle size of 346 μm. Figure 2 shows scanning electron microscope (SEM) images of the water-absorbing resin particles obtained in Example 1.

[0121] (Example 2) In the water addition method of Example 1, the syringe tip was changed from one with a Fine Atomizer Oral (Yoshikawa Kasei Co., Ltd.) attached to the tip to one with a needle attached to the tip, and the water was dropped into the separable flask in 1 second. The same procedure as in Example 1 was followed, and superabsorbent resin particles (2) with a median particle size of 373 μm were obtained.

[0122] (Example 3) The same procedure as in Example 1 was performed except that the amount of deionized water added was changed from 1 g to 0.15 g, and superabsorbent resin particles (3) with a medium particle size of 343 μm were obtained.

[0123] (Example 4) The same procedure as in Example 1 was performed, except that 0.3 g of tricalcium phosphate in Example 1 was replaced with 0.3 g of calcium fluoride (Wako Pure Chemical Industries, medium particle size 12 μm, water solubility at 25°C 0.0016 g / 100 mL), and superabsorbent resin particles (4) with a medium particle size of 348 μm were obtained.

[0124] (Comparative Example 1) In a round-bottom cylindrical separable flask with an inner diameter of 11 cm and equipped with an anchor-shaped stirring blade made of fluororesin, 30 g of polymer particles were stirred at 500 rpm. A dispersion of 0.3 g of tricalcium phosphate (Wako Pure Chemical Industries, medium particle size 8 μm) and 1.0 g of deionized water was added dropwise using a syringe with a needle attached to the tip, and the mixture was stirred for 100 seconds to obtain a mixture. The mixture was heated at 100°C for 40 minutes, and then passed through a sieve with an opening of 850 μm. 0.2 parts by mass of silica were added to obtain superabsorbent resin particles (5) with a medium particle size of 368 μm. A scanning electron microscope (SEM) image of the superabsorbent resin particles obtained in Comparative Example 1 is shown in Figure 3.

[0125] (Comparative Example 2) The same procedure as in Example 1 was performed, except that the deionized water from Example 1 was not added, and superabsorbent resin particles (6) with a medium particle size of 341 μm were obtained. Figure 4 shows a scanning electron microscope (SEM) image of the superabsorbent resin particles obtained in Comparative Example 2.

[0126] (Comparative Example 3) The same procedure as in Example 1 was followed except that the amount of ion-exchanged water was changed from 1.0 g to 6.0 g. However, the mixture agglomerated and could not be passed through a sieve with an 850 μm mesh size after heating at 100°C for 40 minutes, and no water-absorbing resin particles were obtained.

[0127] (Comparative Example 4) The same procedure as in Example 1 was performed except that 0.3 g of tripotassium phosphate in Example 1 was replaced with 0.3 g of trilithium phosphate (Wako Pure Chemical Industries, medium particle size 7 μm, water solubility at 25°C 0.058 g / mL) to obtain superabsorbent polymer particles (7).

[0128] (Reference Example 1) The polymer particles obtained in the above [Production of Polymer Particles] were used as water-absorbent resin particles and subjected to the evaluation described below.

[0129] [Evaluation] <Evaluation of Additives> (Median particle size of additives (laser diffraction)) The median particle size (D50 (median diameter), volume basis) of the additives used was measured using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, SALD2300).

[0130] (Contact angle of additive with respect to water) A 10 mm x 20 mm double-sided tape (Nitto Denko's re-peelable strong double-sided tape No. 5000NS) was attached to a glass plate (30 x 70 cm, 1 mm thick). 0.2 g of the additive was sprinkled onto the double-sided tape, and after pressing the additive down with a spatula, the excess additive was removed by turning the glass petri dish on its side to prepare the sample for measurement. The automatic contact angle meter (Kyowa Interface Chemical: DMo-601) consists of a sample placement stage that is movable in the vertical direction, a syringe unit installed above it, and a scope unit that allows horizontal observation of the stage. The contact angle was measured using such a contact angle meter in the following procedure. First, deionized water was drawn into the syringe (capacity 1 mL), and the sample for measurement was placed on the stage directly below the syringe. The stage was moved upward, and a droplet of deionized water (0.5 μL) was brought into contact with a smooth surface of the sample, causing the droplet to adhere to the sample. The contact angle between the ion-exchanged water droplet and the sample surface one second after application was determined by the θ / 2 method.

[0131] <Evaluation of Polymer Particles and Superabsorbent Polymer Particles> (Moisture Content of Polymer Particles) The tare weight (Wc) of an aluminum cup with a diameter of 48 mm and a depth of 30 mm was measured. 2.00 g of polymer or polymer particles was weighed onto aluminum foil that had been used to weigh the tare weight. The aluminum foil containing the polymer or polymer particles was placed in a hot air dryer at 105°C and heated for 2 hours. After heating, it was air-cooled for 15 minutes in a desiccator at 25°C and 25% humidity. After air-cooling, the weight (Wd) of the aluminum cup was measured, and the moisture content (mass%) of the polymer or polymer particles was calculated using the following formula. The results are shown in Table 1. Moisture content (mass%) = (2.00 - Wd + Wc) / 2.00 × 100

[0132] (Water retention capacity of superabsorbent polymer particles in physiological saline solution) The water retention capacity of superabsorbent polymer particles in physiological saline solution was measured using the following procedure. First, a cotton bag (membrane No. 60, 100 mm wide x 200 mm long) containing 2.00 g of superabsorbent polymer particles was placed in a 500 mL beaker. 500 g of physiological saline solution was poured into the cotton bag containing the superabsorbent polymer particles in one go, taking care not to cause spillage. The top of the cotton bag was then 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. The mass Wa [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 Wb [g] of the cotton bag when wet was measured. The water retention capacity of the superabsorbent polymer particles in physiological saline solution was calculated from the following formula. The results are shown in Table 1. Water retention capacity [g / g] = (Wa - Wb) / 2.00

[0133] (Median particle size of superabsorbent polymer particles) The median particle size of superabsorbent polymer particles was measured using the following procedure. Using a continuous fully automatic ultrasonic vibration sieving analyzer (Robot Shifter RPS-205, manufactured by Seishin Corporation), the particle size distribution of 5 g of superabsorbent polymer particles was measured using JIS standard sieves of 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm, and 180 μm, as well as a receiving tray. The relationship between the sieve opening and the cumulative mass percentage of particles remaining on the sieve was plotted on logarithmic probability paper by accumulating the particles on the sieve in descending order of particle size. By connecting the plots on the probability paper with a straight line, the particle size corresponding to a cumulative mass percentage of 50% was obtained as the median particle size.

[0134] (3-minute value of unpressurized DW (Demand Wetability) of superabsorbent polymer particles) The unpressurized DW of superabsorbent polymer particles was measured using the measuring device shown in Figure 2. Measurements were performed five times for one type of superabsorbent polymer, and the average value of the three measured values ​​excluding the lowest and highest values ​​was calculated. The measuring device comprises a burette section 1, a conduit 5, a measuring stand 13, a nylon mesh sheet 15, a stand 11, and a clamp 3. The burette section 1 comprises 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 tube 25 and a cock 24 connected to the lower part of the burette tube 21. The burette section 1 is fixed with a clamp 3. The flat measuring platform 13 has a through hole 13a with a diameter of 2 mm formed in its center and is supported by a height-adjustable frame 11. The through hole 13a of the measuring platform 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.

[0135] The measurement was performed in an environment with a temperature of 25°C and a humidity of 60 ± 10%. 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 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).

[0136] 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 superabsorbent resin 10a was uniformly scattered onto this cylinder. The cylinder was then carefully removed, and a sample was obtained in which the superabsorbent resin 10a was dispersed in a circular pattern in the center of the nylon mesh sheet 15. Next, the nylon mesh sheet 15 on which the superabsorbent resin 10a was placed was quickly moved so that its center was at the position of the through-hole 13a, without causing the superabsorbent resin 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.

[0137] 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 10a) was read sequentially, and the decrease in the 0.9 mass% saline solution 50 We (g) after 3 minutes from the start of water absorption by the water-absorbing resin 10a was read. From Wc, the 5-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 10a. Unpressurized DW value (mL / g) = We / 1.00

[0138]

[0139] 1. Burette section 3. Clamp 5. Conduit 10a. Absorbent resin 11. Stand 13. Measuring stand 13a. Through hole 15. Nylon mesh sheet 21. Burette tube 22. Cock 23. Rubber stopper 24. Cock 25. Air inlet tube

Claims

1. A method for producing superabsorbent polymer particles, comprising: a mixing step of mixing polymer particles and an additive in a solid state to obtain a mixture; and a water addition step of adding water to the mixture, wherein the shape of the additive is granular, the water solubility of the additive at 25°C is 0.035 g / 100 mL or less, and the amount of water added in the water addition step is 0.1 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the polymer particles.

2. The method for producing water-absorbent resin particles according to claim 1, wherein the contact angle of the additive with respect to water is 90° or less.

3. The method for producing water-absorbing resin particles according to claim 1 or 2, wherein the water content of the polymer particles subjected to the mixing step is 30% by mass or less.

4. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein in the water addition step, water is added while stirring the mixture.

5. The method for producing water-absorbent resin particles according to claim 1 or 2, wherein the polymer particles are formed by surface crosslinking of a polymer of water-soluble ethylenically unsaturated monomers.

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