JP7847138B2 - Absorbent sheets and absorbent articles - Google Patents

Description

The present invention relates to absorbent sheets and absorbent articles, and more particularly to absorbent sheets suitably used in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads, and absorbent articles using absorbent sheets.

Superabsorbent polymers have recently been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

Such water-absorbing resins 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 resins 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).

Absorbent products such as disposable diapers, sanitary napkins, and incontinence pads primarily consist 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 a water-absorbent resin.

Japanese Patent Application Publication No. 3-227301

In such absorbent articles, the superabsorbent resin contained in the absorbent material is required to have a high water retention capacity. By increasing the water retention capacity of the superabsorbent resin, the phenomenon of liquid returning to the absorbent material after being absorbed (i.e., the return of liquid from the absorbent material, which is felt as an unpleasant wetness when the absorbent material is touched) can be improved. However, when using an absorbent article, if the absorbent surface of the absorbent article is sloped, the liquid that is repeatedly excreted may not be absorbed sufficiently, and despite the high water retention capacity of the superabsorbent resin, the liquid may leak out of the absorbent article, causing a problem.

Under these circumstances, the main objective of the present invention is to provide an absorbent sheet in which leakage is suppressed.

The inventors diligently studied to solve the above problems. As a result, they found that in an absorbent sheet comprising an absorbent layer and a nonwoven fabric sandwiching the absorbent layer above and below, leakage can be suppressed by using an absorbent resin having a predetermined saline solution retention capacity, water absorption capacity under a 4.14 kPa load, and a 5-minute value of unpressurized DW. The present invention was completed based on this finding and through further diligent study.

In other words, the present invention provides an invention having the following configuration.
Item 1. An absorbent sheet comprising an absorbent layer and a nonwoven fabric sandwiching the absorbent layer above and below,
The absorbent layer contains a water-absorbent resin,
The water-absorbing resin is a water-absorbing sheet having the following characteristics (A) to (C).
(A) The water content of physiological saline is between 45 g/g and 70 g/g.
(B) The water absorption rate under a load of 4.14 kPa is 13 ml/g or more.
(C) The unpressurized DW 5-minute value is 44 ml/g or more and 80 ml/g or less.
Item 2. The absorbent sheet according to Item 1, wherein the basis weight of the absorbent resin is 100 g/ ^m² or more and 450 g/ ^m² or less.
Item 3. The absorbent sheet according to item 1 or 2, wherein the absorbent layer is bonded to the nonwoven fabric via an adhesive.
Item 4. An absorbent article comprising the absorbent sheets described in Items 1 to 3.

According to the present invention, it is possible to provide an absorbent sheet that can suppress the occurrence of leakage. Furthermore, according to the present invention, it is also possible to provide an absorbent article using the absorbent sheet.

This is a schematic diagram of a measuring device used to measure the amount of saline solution absorbed by a water-absorbent resin under a 4.14 kPa load. This is a schematic diagram of a measuring device used for measuring the unpressurized DW of superabsorbent polymers. This is a schematic diagram illustrating the method of leakage testing (gradient absorption testing) for absorbent materials.

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

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 two lower and upper limits may be selected and enclosed in "~".

1. Absorbent Sheet The absorbent sheet of the present invention comprises an absorbent layer and a nonwoven fabric sandwiching the absorbent layer above and below. In the absorbent sheet of the present invention, the absorbent layer contains an absorbent resin, and the absorbent resin is characterized by having the following characteristics (A) to (C).
(A) The water content of physiological saline is between 45 g/g and 70 g/g.
(B) The water absorption rate under a load of 4.14 kPa is 13 ml/g or more.
(C) The unpressurized DW 5-minute value is 44 ml/g or more and 80 ml/g or less.

The absorbent sheet of the present invention, possessing these characteristics, can be used in absorbent articles to suppress leakage. The absorbent sheet of the present invention will be described in detail below.

In the absorbent sheet of the present invention, the absorbent layer may contain hydrophilic fibers or the like in addition to the absorbent resin, but it is preferable that it be substantially composed of only the absorbent resin. The statement that the absorbent layer is substantially composed of only the absorbent resin means that the content of the absorbent resin in the absorbent layer is 95% by mass or more, more preferably 98% by mass or more, more preferably 99% by mass or more, and more preferably 100% by mass.

The absorbent layer contained in the absorbent sheet of the present invention may be one layer or two or more layers. Preferably, the absorbent layer contained in the absorbent sheet is one or two layers. When the absorbent sheet contains two or more absorbent layers, it is preferable to place a liquid-permeable substrate, such as a nonwoven fabric, between each absorbent layer. This liquid-permeable substrate can be the same as the nonwoven fabric described later that sandwiches the absorbent layers above and below.

The structure of the absorbent sheet can be any structure in which an absorbent layer containing a water-absorbing resin is sandwiched between nonwoven fabrics. For example, it may be a sheet-like structure in which the absorbent layer is fixed between two sheets of nonwoven fabric, or a structure in which the absorbent layer is wrapped in nonwoven fabric.

The planar shape of the absorbent sheet is determined appropriately according to its application or the shape of the absorbent article. Examples include a roughly rectangular, elliptical, hourglass, or paddle-shaped sheet, and it may have notches or other features to improve fit. It may also have a block region in which multiple block structures, each partitioned by multiple vertical grooves extending in the vertical direction and multiple horizontal grooves extending perpendicular to the vertical direction, are arranged vertically. The internal structure of the absorbent sheet is also determined appropriately according to its purpose. For example, in addition to consisting of a single absorbent sheet, it may be a combination of multiple absorbent sheets (divided on a planar surface, divided vertically, etc.), or it may have a quantitative distribution gradient of absorbent resin and other components within the absorbent sheet (uniform distribution, quantitative distribution according to the liquid input area, etc.). Furthermore, the absorbent sheet may be embossed to provide a liquid channel.

The absorbent layer is preferably bonded to the nonwoven fabric via an adhesive. A hot-melt adhesive is preferred as the adhesive used to bond (fix) the absorbent layer to the nonwoven fabric.

In the absorbent sheet of the present invention, the basis weight of the absorbent resin contained in the absorbent layer is not particularly limited, but from the viewpoint of exhibiting the effects of the present invention more favorably, it is preferably 100 to 450 g/ ^m² , more preferably 120 to 350 g/ ^m² , and even more preferably 150 to 300 g/ ^m² .

The water-absorbing resin content in the water-absorbing sheet 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.

The absorbent article of the present invention can be created by holding an absorbent sheet between a liquid-permeable surface sheet (top sheet) through which liquid can pass and a liquid-impermeable back 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 side opposite to the body.

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

The basis weight of the liquid permeable sheet is preferably 5 g/ ^m² to 100 g/ ^m² , and more preferably 10 g/ ^m² to 60 g/ ^m² . Furthermore, the liquid permeable sheet may be embossed or perforated on its surface to improve liquid diffusion. Such embossing and perforation can be carried out using known methods.

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 may also have breathability, from the viewpoint of reducing stuffiness during wear and alleviating discomfort to the wearer. Liquid-impermeable sheets can also be made from synthetic resins mainly composed of low-density polyethylene (LDPE) resin. Liquid-impermeable sheets may, for example, be sheets made from synthetic resins with a basis weight of 10 to 50 g/ ^m² . Furthermore, to impart breathability to liquid-impermeable sheets, fillers may be added to the resin sheets, or embossing may be applied to the liquid-impermeable sheets. Calcium carbonate may be used as a filler.

The absorbent article preferably has a liquid-permeable sheet placed on the upper surface of the water-absorbing sheet, and a liquid-impermeable sheet placed on the side opposite to the liquid-permeable sheet. Furthermore, in the absorbent article, an absorbent material composed of a water-absorbing resin and hydrophilic fibers may be combined on the upper and/or lower surfaces of the water-absorbing sheet.

In addition to the liquid-permeable sheets, water-absorbing sheets, and liquid-impermeable sheets mentioned above, absorbent articles may also contain other components as appropriate for their intended use and function. Examples include core wraps, liquid-gathering and diffusion sheets, outer cover nonwoven fabrics, and leg gathers.

The core wrap is positioned, for example, to cover the outer perimeter of the absorbent sheet. The absorbent sheet is placed inside the core wrap. Examples of core wraps include tissue paper and nonwoven fabric. The core wrap has, for example, a main surface of the same size as the absorbent sheet. The absorbent sheet is shaped by being enclosed inside the core wrap. The method of shaping the absorbent sheet with the core wrap is not limited to this; for example, the absorbent sheet may be sandwiched between two separate pieces of core wrap, one above the other, or the core wrap may form a bag with the absorbent sheet placed inside.

(Liquid acquisition and diffusion sheet)
The absorbent article may include a liquid acquisition and diffusion sheet. The liquid acquisition and diffusion sheet may be placed, for example, on the underside of the liquid permeable sheet. This allows for the rapid movement of liquid that has permeated the liquid permeable sheet to the absorbent sheet side, or further reduces backflow. Adhesion between the liquid acquisition and diffusion sheet and the liquid permeable sheet may be achieved using hot melt adhesive, heat embossing, or ultrasonic welding. As the liquid acquisition and diffusion sheet, a nonwoven fabric may be used, or a resin film having numerous permeable pores may be used. As the nonwoven fabric, the same material as described in the section on liquid permeable sheets can be used, but a nonwoven fabric with higher hydrophilicity or a higher fiber density than the liquid permeable sheet is preferable because it exhibits superior liquid movement characteristics towards the absorbent sheet.

The liquid acquisition and diffusion sheet is usually positioned in the center with a width shorter than the absorbent sheet, but it may also be positioned across the entire width. The length of the liquid acquisition and diffusion sheet in the front-to-back direction may be approximately the same as the total length of the absorbent article, approximately the same as the total length of the absorbent sheet, or it may be a length corresponding to the area where the liquid is expected to be introduced.

(Outer cover nonwoven fabric)
Furthermore, the outer cover nonwoven fabric may be positioned on the side of the liquid-impermeable sheet opposite the absorbent sheet. The outer cover nonwoven fabric can be bonded to the liquid-impermeable sheet, for example, using an adhesive. The outer cover nonwoven fabric may be formed in one or more layers and may be made of a flexible material. The outer cover nonwoven fabric may be given a flexible texture, may have a pattern printed on it, or may have multiple joints, embossing, or a three-dimensional shape to appeal to consumers' purchasing intent or for other reasons.

(Leg gathers)
The absorbent article of the present invention may have leg gathers equipped with stretchable elastic members, positioned outside the widthwise ends of the absorbent sheet and installed substantially parallel to the longitudinal direction of the absorbent sheet. The length of the leg gathers is set to be approximately equal to or greater than the circumference of the wearer's legs. The stretch ratio of the leg gathers is set appropriately from the viewpoint of preventing leakage of discharged liquid while minimizing pressure during prolonged wear.

(Gathered front/back)
The absorbent article of the present invention may have front/back gathers provided with elastic members that are positioned near both ends in the longitudinal direction of the absorbent article and expand and contract in the width direction.

The absorbent article of the present invention is equipped with front/back gathers that can rise above the side edges in the width direction of the absorbent sheet. That is, on each of the longitudinal sides of the absorbent article, a sheet member for the front/back gathers, having a gather elastic member, is provided, thereby forming the front/back gathers.

The material used for the front/back gathers is usually a liquid-impermeable or water-repellent material, preferably a moisture-permeable material. Examples include a liquid-impermeable or water-repellent porous sheet, a liquid-impermeable or water-repellent nonwoven fabric, or a laminate of the porous sheet and the nonwoven fabric. Examples of the nonwoven fabric include thermal bond nonwoven fabric, spunbond nonwoven fabric, meltblown nonwoven fabric, spunlace nonwoven fabric, spunbond/meltblown/spunbond nonwoven fabric, etc. The basis weight of the material may be 5 to 100 g/ ^m² , 8 to 70 g/ ^m² , or 10 to 40 g/ ^m² .

Each component constituting the absorbent article of the present invention may be bonded together. For example, by bonding an absorbent sheet and a liquid permeable sheet, the liquid is more smoothly guided to the absorbent sheet, making it easier to obtain an absorbent article with excellent leakage prevention. When the absorbent sheet is covered or sandwiched by the core wrap 20, it is preferable that at least the core wrap 20 and the liquid permeable sheet 30 are bonded together, and it is even more preferable that the core wrap 20 and the absorbent sheet are bonded together. Known bonding methods include adhesives, heat sealing, and ultrasonic sealing. For example, a method of applying a hot melt adhesive to the liquid permeable sheet in a shape such as vertical stripes or spirals at predetermined intervals in the width direction, or a method of using a water-soluble adhesive selected from starch, carboxymethylcellulose, polyvinyl alcohol, polyvinylpyrrolidone, and other water-soluble polymers can be used. Furthermore, if the absorbent sheet contains heat-fusible synthetic fibers, a heat-fusible method may be employed.

The water-absorbing resin used in the water-absorbing sheet of the present invention is characterized by having the following properties (A) to (C). The water-absorbing sheet of the present invention, using a water-absorbing resin with these properties, can suppress leakage. The water-absorbing resin will be described in detail below.

(A) The water content of physiological saline is between 45 g/g and 70 g/g.
(B) The water absorption rate under a load of 4.14 kPa is 13 ml/g or more.
(C) The 5-minute value of the unpressurized DW is between 44 ml/g and 80 ml/g.

From the viewpoint of exhibiting the effects of the present invention more favorably, the content of the water-absorbing resin having the properties of (A) to (C) among the water-absorbing resins used in the water-absorbing sheet is preferably 30% by mass or more, more preferably 50% by mass or more, even more preferably 70% by mass or more, and even more preferably 90% by mass or more.

If the absorbent sheet of the present invention contains two or more absorbent layers, it is sufficient that at least one layer contains an absorbent resin having the properties of (A) to (C), and the absorbent resins contained in each absorbent layer may be the same or different. Furthermore, from the viewpoint of exhibiting the effects of the present invention more favorably, it is preferable that 50% or more of the total absorbent layers of the absorbent sheet contain an absorbent resin having the properties of (A) to (C), and it is more preferable that all layers contain an absorbent resin having the properties of (A) to (C).

From the viewpoint of exhibiting the effects of the present invention more favorably, the amount of saline solution that the water-absorbing resin can hold is preferably 46 g/g or more, more preferably 47 g/g or more, even more preferably 48 g/g or more, and also preferably 70 g/g or less, more preferably 68 g/g or less, and even more preferably 65 g/g or less.

Furthermore, from the viewpoint of exhibiting the effects of the present invention more favorably, the amount of water absorbed by the water-absorbing resin under a load of 4.14 kPa is preferably 14 ml/g or more, more preferably 15 ml/g or more, even more preferably 16 ml/g or more, and also preferably 33 ml/g or less, more preferably 27 ml/g or less, and even more preferably 23 ml/g or less.

Furthermore, from the viewpoint of exhibiting the effects of the present invention more favorably, the 5-minute value of the unpressurized DW of the water-absorbent resin is preferably 46 ml/g or more, more preferably 48 ml/g or more, even more preferably 50 ml/g or more, and also preferably 70 ml/g or less, more preferably 60 ml/g or less, and even more preferably 54 ml/g or less.

Furthermore, the amount of liquid absorbed by the water-absorbing resin until leakage occurs, as measured by a gradient absorption test, is preferably 185 g or more, more preferably 190 g or more, when the absorbent sheet contains one absorbent layer. Also, the amount of liquid absorbed until leakage occurs is, for example, 560 g or less, more preferably 555 g or less, or 550 g or less. When the absorbent sheet contains two absorbent layers, it is preferably 172 g or more, more preferably 176 g or more. Also, the amount of liquid absorbed until leakage occurs is, for example, 560 g or less, more preferably 555 g or less, or 550 g or less.

The methods for measuring the water retention capacity of the superabsorbent polymer in physiological saline solution, the water absorption capacity under a 4.14 kPa load, and the unpressurized DW 5-minute value, as well as the method for the gradient absorption test, are as described in the examples.

Superabsorbent polymers are composed of crosslinked polymers of water-soluble ethylenically unsaturated monomers, that is, crosslinked polymers having structural units derived from water-soluble ethylenically unsaturated monomers.

The water-absorbing resin used in the present invention may have various shapes. Examples of shapes for the water-absorbing resin include granular, approximately spherical, irregularly shaped crushed, plate-like, fibrous, flake-like, or aggregated forms of these resins. Preferably, the water-absorbing resin is in the form of granules, approximately spherical, irregularly shaped crushed, fibrous, or aggregated forms of these resins.

Furthermore, superabsorbent polymers may exist not only as single particles, but also as aggregates of fine particles (primary particles) (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 smooth surface shapes such as perfect spheres and ellipsoids. Primary particles of this shape have a smooth surface, resulting in high fluidity as a powder. Additionally, because the aggregated particles are densely packed, they are less likely to break under impact, resulting in a superabsorbent polymer with high particle strength.

From the viewpoint of more favorably exhibiting the effects of the present invention, the median particle size of the water-absorbent resin is preferably 200 μm or more, 250 μm or more, 280 μm or more, 300 μm or more, or 320 μm or more. Similarly, from the same viewpoint, the median particle size is preferably 700 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 200 to 700 μm, preferably 200 to 600 μm, more preferably 250 to 500 μm, even more preferably 300 to 450 μm, and even more preferably 320 to 400 μm.

The median particle size of the superabsorbent polymer can be measured using a JIS standard sieve, and specifically, the value is measured by the method described in the examples.

Typical 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. Reverse-phase suspension polymerization is preferred from the viewpoint of precise polymerization reaction control and broad particle size control.

A specific embodiment of the method for producing superabsorbent resins will be described below, using the reverse-phase suspension polymerization method as an example.

Specific examples of methods for producing superabsorbent resins include a method for producing superabsorbent resins by reverse-phase suspension polymerization of water-soluble ethylenically unsaturated monomers in a hydrocarbon dispersion medium, comprising the steps of polymerization in the presence of a radical polymerization initiator and post-crosslinking of the resulting water-containing gel in the presence of a post-crosslinking agent. Furthermore, in the method for producing superabsorbent resins, an internal crosslinking agent may be added to the water-soluble ethylenically unsaturated monomer as needed to obtain a water-containing gel having an internal crosslinking structure.

<Polymerization process>
[Water-soluble ethylenically unsaturated monomers]
Examples of water-soluble ethylenically unsaturated monomers include (meth)acrylic acid 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, and polyethylene glycol mono(meth)acrylate; amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and 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, and (meth)acrylic acid and its salts are 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.

Among these, acrylic acid and its salts are widely used as raw materials for superabsorbent resins, and in some cases, these acrylic acids and/or their salts are copolymerized with the other water-soluble ethylenically unsaturated monomers mentioned above. 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.

The water-soluble ethylenically unsaturated monomer is preferably dispersed in a hydrocarbon dispersion medium in an aqueous solution and subjected to reverse-phase suspension polymerization. By using an aqueous solution, the dispersion efficiency of the water-soluble ethylenically unsaturated monomer 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, from the perspective of easily obtaining the water absorption characteristics of the present invention, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 38% by mass or more, even more preferably 40% by mass or more, and even more preferably 42% by mass or more. On the other hand, 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 46% by mass or less.

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 monomer in which the acidic group has been neutralized in advance with an alkaline neutralizing agent, if necessary. 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 procedure. The above-mentioned alkaline neutralizing agents may be used individually or in combination of two or more types.

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 relative to all acid groups of the water-soluble ethylenically unsaturated monomer.

[Radical polymerization initiator]
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 and 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.

From the viewpoint of obtaining a water-absorbing resin that exhibits the effects of the present invention more favorably, the radical polymerization initiator mixed in the polymerization step preferably contains an azo compound and a peroxide. The molar ratio of peroxide to azo compound (peroxide/azo compound) is preferably in the range of 0.1 to 1.0, more preferably in the range of 0.2 to 0.8, and even more preferably in the range of 0.3 to 0.6.

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.

[Internal Crosslinking Agent]
Examples of internal crosslinking agents include those capable of crosslinking the polymer of the water-soluble ethylenically unsaturated monomer used, such as unsaturated polyesters obtained by reacting polyols such as (poly)ethylene glycol, (poly)propylene glycol, 1,4-butanediol, 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; allylated starch, allylated cellulose, diallyl phthalate, N,N' Examples include compounds having two or more polymerizable unsaturated groups such as N''-trialyl isocyanurate and divinylbenzene; polyglycidyl compounds such as diglycidyl compounds and triglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; 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, it is preferable to use unsaturated polyesters or polyglycidyl compounds, more preferably diglycidyl ether compounds, and especially (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether. These internal crosslinking agents may be used individually or in combination of two or more types.

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

[Hydroxide dispersion medium]
Examples of hydrocarbon dispersion media 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 media, n-hexane, n-heptane, and cyclohexane are particularly suitable because they are readily available industrially, have stable quality, and are inexpensive. These hydrocarbon dispersion media may be used individually or in combination of two or more types. As an example of a mixture of hydrocarbon dispersion media, 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.

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).

[Dispersing stabilizer]
(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.

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.

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.

(Polymer-based dispersant)
Furthermore, as a dispersion stabilizer used in reverse-phase suspension polymerization, polymer-based dispersants may be used in combination with the surfactants mentioned above.

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, ethyl cellulose, and ethyl hydroxyethyl cellulose. 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.

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.

[Other ingredients]
In the method for producing superabsorbent polymers, other components may be added to an aqueous solution containing a water-soluble ethylenically unsaturated monomer as desired, and reverse-phase suspension polymerization may be carried out. These other components may include various additives such as thickeners and chain transfer agents.

As an 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.

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.

[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 begins, the timing of adding the dispersion stabilizer (surfactant or polymer-based dispersant) does not matter; it can be added before or after the addition of the aqueous monomer solution.

Among these methods, from the viewpoint of easily reducing the amount of hydrocarbon dispersion medium remaining in the resulting water-absorbing resin, 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.

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.

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, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator and, if necessary, an internal crosslinking agent can be added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer, 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. Note that the amount of water-soluble ethylenically unsaturated monomer and the ratio of the polymerization initiator, internal crosslinking agent, etc. to the water-soluble ethylenically unsaturated monomer may be the same or different between the first stage and each stage from the second stage onward, as long as they are within the aforementioned range.

The reaction temperature for the polymerization reaction is preferably 20 to 110°C, and more preferably 40 to 90°C, from the viewpoint of rapidly advancing polymerization, shortening the polymerization time, thereby improving economic efficiency, and easily removing the heat of polymerization to ensure a smooth reaction.

<Post-crosslinking step>
Next, the water-absorbent polymer is obtained by adding a post-crosslinking agent to a water-containing gel-like substance having an internal crosslinked structure obtained by polymerizing water-soluble ethylenically unsaturated monomers and then crosslinking it (post-crosslinking reaction). This post-crosslinking reaction is carried out in the presence of the post-crosslinking agent after the polymerization of the water-soluble ethylenically unsaturated monomers. By performing such a post-crosslinking reaction, the crosslinking density near the surface of the water-absorbent polymer can be appropriately increased, making it possible to obtain a water-absorbent polymer with improved performance in terms of water absorption under load and unpressurized DW (downwater wattage).

Examples of post-crosslinking agents include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 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; epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples include halo epoxy compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 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; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these post-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 post-crosslinking agents may be used alone or in combination of two or more types.

The amount of post-crosslinking agent used is preferably 0.00001 to 0.01 moles, more preferably 0.00005 to 0.005 moles, and even more preferably 0.00001 to 0.001 moles, per mole of the total amount of water-soluble ethylenically unsaturated monomers used in polymerization.

Regarding the method of adding the post-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.

The timing of adding the post-crosslinking agent is flexible and can be anytime after polymerization of the water-soluble ethylenically unsaturated monomer. It is preferable to add the agent in the presence of 5 to 140 parts by mass of water per 100 parts by mass of the water-soluble ethylenically unsaturated monomer, more preferably 15 to 100 parts by mass, even more preferably 20 to 50 parts by mass, and still more preferably 22 to 28 parts by mass. The amount of water refers to the total amount of water contained in the reaction system and the water used as needed when adding the post-crosslinking agent. Adding the post-crosslinking agent when the amount of water is higher than 140 parts by mass tends to result in a lower water retention capacity. Adding the post-crosslinking agent when the amount of water is lower than 5 parts by mass tends to result in insufficient reaction of the post-crosslinking agent.

The reaction temperature in the post-crosslinking reaction is preferably 50 to 250°C, more preferably 60 to 180°C, even more preferably 60 to 140°C, and even more preferably 70 to 120°C. The reaction time for the post-crosslinking reaction is preferably 1 to 300 minutes, and more preferably 5 to 200 minutes.

<Drying process>
After the reverse-phase suspension polymerization described above, a drying step 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, a water-absorbent resin is obtained by distilling off the water and hydrocarbon dispersion medium. By controlling the processing conditions of the drying step after polymerization and adjusting the amount of dehydration, it is possible to control the various properties of the obtained water-absorbent resin.

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.

Furthermore, when performing the post-crosslinking process using the post-crosslinking agent described above, the drying process by distillation described above should be carried out after the completion of the post-crosslinking process. Alternatively, the post-crosslinking process and the drying process may be carried out simultaneously.

The water-absorbing resin may contain additives depending on the purpose. Examples of such additives include inorganic powders, surfactants, oxidizing agents, reducing agents, metal chelating agents, radical chain inhibitors, antioxidants, and antibacterial agents. For example, adding 0.05 to 5 parts by mass of amorphous silica as inorganic powder to 100 parts by mass of the water-absorbing resin can further improve the fluidity of the water-absorbing resin.

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

The superabsorbent resins obtained in the 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. The evaluation test methods are described below.

<Saline solution 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% by mass sodium chloride aqueous solution (physiological saline) 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 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, and the amount of physiological saline water it could hold was calculated using the following formula.
Physiological saline water retention capacity (g/g) = [Wa - Wb] / 2.0

<Amount of physiological saline absorbed under a 4.14 kPa load>
The amount of saline solution absorbed by superabsorbent polymer particles under load (at room temperature, 25°C ± 2°C) was measured using the measuring device Y shown in Figure 1. The measuring device Y consists of a burette section 61, a conduit 62, a measuring stand 63, and a measuring unit 64 placed on the measuring stand 63. The burette section 61 has a burette 61a extending vertically, a rubber stopper 61b positioned at the upper end of the burette 61a, a cock 61c positioned at the lower end of the burette 61a, an air inlet pipe 61d with one end extending into the burette 61a near the cock 61c, and a cock 61e positioned at the other end of the air inlet pipe 61d. The conduit 62 is installed between the burette section 61 and the measuring stand 63. The inner diameter of the conduit 62 is 6 mm. A hole with a diameter of 2 mm is made in the center of the measuring stand 63, and the conduit 62 is connected to it. The measuring unit 64 comprises a cylinder 64a (made of acrylic resin (plexiglass)), a nylon mesh 64b bonded to the bottom of the cylinder 64a, and a weight 64c. The inner diameter of the cylinder 64a is 20 mm. The mesh opening of the nylon mesh 64b is 75 μm (200 mesh). During measurement, the water-absorbing resin particles 65 to be measured are uniformly scattered on the nylon mesh 64b. The diameter of the weight 64c is 19 mm, and the mass of the weight 64c is 120 g. The weight 64c is placed on the water-absorbing resin particles 65, and a load of 4.14 kPa can be applied to the water-absorbing resin particles 65.

After placing 0.100 g of superabsorbent resin particles 65 into the cylinder 64a of the measuring device Y, a weight 64c was placed on top and measurement was started. Since the same volume of air as the saline solution absorbed by the superabsorbent resin particles 65 is quickly and smoothly supplied into the burette 61a through the air inlet tube, the decrease in the saline solution level inside the burette 61a represents the amount of saline solution absorbed by the superabsorbent resin particles 65. The scale of the burette 61a is marked from top to bottom in increments of 0.5 mL from 0 mL. The saline solution level was read from the scale Va of the burette 61a before absorption began and from the scale Vb of the burette 61a 60 minutes after absorption began, and the amount of saline solution absorbed under a load of 4.14 kPa was calculated using the following formula.
4. Absorption volume of physiological saline under a load of 4.14 kPa [mL/g] = (Vb - Va) / 0.100

<5-minute value of unpressurized DW (Demand Wetness)>
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 particle, and the average 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 stand 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 stand 13 and the cock 22 of the burette section 1 are connected by a conduit 5. The inner diameter of conduit 5 is 6 mm.

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% 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).

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 onto 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, and the decrease in the 0.9 mass% saline solution 50 Wc (g) after 5 minutes from the start of water absorption by the water-absorbing resin particles 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 particles 10a.
Unpressurized DW (mL/g) = Wc/1.00

<Medium particle size>
Ten 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 straight lines, the particle size corresponding to an accumulated mass percentage of 50% was determined and defined as the median particle size.

<Preparation of the test solution>
The test solution was prepared by dissolving inorganic salts in deionized water as described below, and then adding a small amount of Blue No. 1.
Test solution composition: Deionized water 5919.6 g
・NaCl 60.0g
・CaCl ₂・H ₂ O 1.8g
・MgCl ₂・6H ₂ O 3.6g
・Food blue No. 1 (for coloring)
1% Triton X-100 15.0g

<Leakage test (gradient absorption test)>
Figure 3 is a schematic diagram showing a method for evaluating the leakability of an absorbent article. A support plate 40 (here referred to as an acrylic resin plate, also called the inclined surface S1) with a length of 45 cm and a flat main surface was fixed by a frame 41 at an angle of 45 ± 2 degrees relative to the horizontal plane S0. In a room at a temperature of 25 ± 2 °C, a test absorbent article 100 was attached to the inclined surface S1 of the fixed support plate 40 with its longitudinal direction aligned with the longitudinal direction of the support plate 40. Next, a test solution 50 adjusted to 25 ± 1 °C was dripped from a dropping funnel 42 positioned vertically above the absorbent article, 8 cm above the center of the absorbent sheet in the absorbent article 100. For absorbent articles measuring 42 cm × 14 cm, 80 mL of test solution was dripped at a rate of 8 mL/second, and for articles measuring 32 cm × 12 cm, 30 mL of test solution was dripped at a rate of 8 mL/second. The distance between the tip of the dropping funnel 42 and the absorbent material was 10 ± 1 mm. The test solution was added a total of seven times under the same conditions, at 10-minute intervals starting from the first time the test solution was added.

If the test liquid that was not absorbed by the absorbent article 100 leaked from the bottom of the support plate 40, the leaked test liquid was collected in a metal tray 44 located below the support plate 40. The mass (g) of the collected test liquid was measured using a balance 43, and this value was recorded as the amount of leakage. The amount absorbed was calculated by subtracting the amount of leakage from the total amount of test liquid injected. The absorption rate was calculated using the following formula. The larger these values are, the less likely it is that liquid leakage will occur when worn. The density of the test liquid was assumed to be 1.0 g/mL.
Absorption rate (%) = Amount absorbed (g) / Amount of test solution added (g) × 100

<Manufacturing of water-absorbent resins>
(Manufacturing example 1)
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. The stirrer used had a two-stage configuration of four inclined paddle blades with a blade diameter of 5 cm. 252 g of n-heptane as a hydrocarbon dispersion medium and 0.736 g of maleic anhydride-modified ethylene-propylene copolymer (Mitsui Chemicals, Inc., High Wax 1105A) as a dispersant were added to the flask and mixed. The mixture in the flask was heated to 80°C while stirring at a stirrer speed of 300 rpm, thereby dissolving the dispersant in the n-heptane. The resulting solution was cooled to 50°C.

In a 300 mL beaker, 92.0 g (1.03 mol) of an 80.5% by mass acrylic acid aqueous solution as a water-soluble ethylenically unsaturated monomer was placed, and while cooling from the outside, 102.8 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize it to 75 mol%. Subsequently, 0.092 g of hydroxylethylcellulose (Sumitomo Seika Co., Ltd., HEC AW-15F) as a thickener, 0.0460 g (0.170 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.0276 g (0.102 mmol) of potassium persulfate as a peroxide, and 0.00184 g (0.0106 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 9.47 g of deionized water were added and dissolved to prepare the first monomer aqueous solution.

The prepared first-stage monomer aqueous solution was added to the reaction mixture in the separable flask and stirred for 10 minutes. Next, a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate (HLB: 3, Mitsubishi Chemical Foods Corporation, Ryoto Sugar Ester S-370) in 6.62 g of n-heptane was added to the reaction mixture, and the system was thoroughly purged with nitrogen while stirring at a stirrer speed of 600 rpm. After that, the flask was immersed in a 70°C water bath and the temperature was raised, and polymerization was carried out for 60 minutes to obtain the first-stage polymerization slurry.

Next, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution was taken into another 500 ml beaker as an ethylenically unsaturated monomer, and while cooling from the outside, 143.89 g of a 30% by mass sodium hydroxide aqueous solution was added dropwise to neutralize to 75 mol%. Then, 0.129 g (0.475 mmol) of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.0386 g (0.143 mmol) of potassium persulfate as a peroxide, 0.0116 g (0.0665 mmol) of ethylene glycol diglycidyl ether as an internal crosslinking agent, and 11.2 g of deionized water were added and dissolved to prepare the second monomer aqueous solution.

The separable flask system was cooled to 25°C while stirring at a stirrer speed of 1000 rpm. Next, the entire volume of the second stage monomer aqueous solution was added to the first stage polymerization slurry in the separable flask, and the system was purged with nitrogen for 30 minutes. After that, the flask was again immersed in a 70°C water bath to raise the temperature, and the polymerization reaction was carried out for 60 minutes.

To the reaction solution containing the hydrated gel polymer after the second polymerization stage, 0.393 g of a 45% by mass aqueous solution of sodium diethylenetriaminepentaacetate was added under stirring. Then, the flask was immersed in an oil bath set at 125°C, and 221.82 g of water was removed from the system by azeotropic distillation of n-heptane and water. Next, 4.42 g (0.507 mmol) of a 2% by mass aqueous solution of ethylene glycol diglycidyl ether was added to the flask, and the temperature inside the separable flask was maintained at 83°C for 2 hours.

Subsequently, the separable flask was immersed in an oil bath set to 125°C to remove n-heptane, thereby obtaining polymer particles (dried). These polymer particles were passed through a sieve with a mesh size of 850 μm, and 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Toxil NP-S, hydrophilic) relative to the mass of the polymer particles was mixed with the polymer particles to obtain 216.4 g of water-absorbing resin particles A containing amorphous silica. The performance of these water-absorbing resin particles is shown in Table 1.

(Manufacturing example 2)
The amount of n-heptane used as the hydrocarbon dispersion medium is 293 g, the azo compound 2,2'-azobis(2-amidinopropane) dihydrochloride dissolved in the first-stage monomer aqueous solution is not added, the amount of potassium persulfate is 0.0736 g (0.272 mmol), the amount of ethylene glycol diglycidyl ether added as an internal crosslinking agent is 0.0101 g (0.0579 mmol), the amount of ion-exchanged water is 39.9 g, the rotation speed during the first-stage polymerization is 550 rpm, and the second-stage monomer Except for not adding the azo compound 2,2'-azobis(2-amidinopropane) dihydrochloride, which is dissolved in a saturating aqueous solution, and instead adding 0.103 g (0.381 mmol) of potassium persulfate and 10.6 g of deionized water, and changing the amount of water removed from the system by azeotropic distillation to 247.9 g, and changing the amount of amorphous silica mixed with polymer particles (dried product) to 0.5 mass%, 228.0 g of superabsorbent polymer particles B were obtained in the same manner as in Production Example 1. The performance of the superabsorbent polymer particles is shown in Table 1.

(Manufacturing example 3)
Except for changing the amount of ethylene glycol diglycidyl ether, an internal crosslinking agent dissolved in the first-stage monomer aqueous solution, to 0.00276 g (0.0158 mmol), the amount of deionized water to be added to 40.7 g, and the amount of water removed from the system by azeotropic distillation to 266.4 g, 229.0 g of superabsorbent resin particles C were obtained in the same manner as in Production Example 2. The performance of the superabsorbent resin particles is shown in Table 1.

(Manufacturing example 4)
Except for using 293 g of n-heptane as the hydrocarbon dispersion medium, adding 0.0920 g (0.339 mmol) of the azo compound 2,2'-azobis(2-amidinopropane) dihydrochloride dissolved in the first-stage monomer aqueous solution, adding 0.00460 g (0.0264 mmol) of the internal crosslinking agent ethylene glycol diglycidyl ether, using 40.9 g of deionized water, setting the rotation speed during the first-stage polymerization to 550 rpm, and changing the amount of water removed from the system by azeotropic distillation to 234.2 g, 229.0 g of superabsorbent resin particles D were obtained in the same manner as in Production Example 1. The performance of the superabsorbent resin particles is shown in Table 1.

(Manufacturing example 5)
Except for changing the amount of water removed from the system by azeotropic distillation to 294.3 g and changing the amount of ethylene glycol diglycidyl ether aqueous solution used as a post-crosslinking agent to 13.25 g (1.52 mmol), 228.8 g of superabsorbent resin particles E were obtained in the same manner as in Production Example 3. The performance of the superabsorbent resin particles is shown in Table 1.

(Preparation of absorbent sheets and absorbent articles)
The following nonwoven fabrics were prepared.
• Airlaid nonwoven fabric (KNH Enterprise Co., Ltd., basis weight 40 g/ ^m² )
• Hydrophilic air-through nonwoven fabric (Rengo Nonwoven Products, basis weight 21 g/ ^m² )
• Air-through nonwoven fabric (Guangzhou Jinhan Nonwoven Fabric Co., Ltd., basis weight 40 g/ ^m² )
・Spunbond nonwoven fabric (TORAY POLYTECH NANTONG, basis weight 17 g/ ^m² )
・Spunlace nonwoven fabric (Kuraray Co., Ltd., basis weight 35 g/ ^m² )
• Liquid-impermeable sheet (made of polyethylene, basis weight 40 g/ ^m² )

(Example 1)
For the first and second sheets, airlaid nonwoven fabric cut to 42 cm x 14 cm was prepared. Using a hot melt coating machine (Harries Co., Ltd., pump: Marshall 150, table: XA-DT, tank temperature setting: 150°C, hose temperature setting: 165°C, gun head temperature setting: 170°C), 0.15 g of hot melt adhesive (Henkel Japan Ltd., ME-765E) was applied to the airlaid nonwoven fabric of the second sheet in 13 lines at 10 mm intervals along the longitudinal direction. The adhesive application pattern was a spiral stripe. Subsequently, 14.4 g of superabsorbent resin particles A were uniformly scattered on the adhesive-coated surface of the second sheet, excluding the short direction and the outer 1 cm area at both ends of the longitudinal direction. In the absorbent article, the basis weight of the superabsorbent resin particles was 300 g/ ^m² .

Hot melt adhesive was applied to the first sheet using the same procedure as described above. The side of the first sheet coated with hot melt adhesive and the side of the second sheet scattered with water-absorbing resin particles were aligned at both ends, then sandwiched from above and below with release paper, and pressed together using a laminating machine (Hashima Co., Ltd., Straight Linear Fussing Press, model HP-600LFS) at 110°C and 0.1 MPa, and the sheets were removed to obtain a water-absorbing sheet. Furthermore, a hydrophilic air-through nonwoven fabric of the same size as the water-absorbing sheet was placed on the upper surface of the water-absorbing sheet, and a polyethylene liquid-impermeable sheet with a basis weight of 40 g/ ^m² was placed on the lower surface of the water-absorbing sheet to prepare an absorbent article for testing. The resulting absorbent article has the following arrangement: hydrophilic air-through nonwoven fabric, first sheet, hot melt adhesive, absorbent layer consisting of water-absorbing resin particles A, hot melt adhesive, second sheet, and liquid-impermeable sheet.

(Comparative Example 1)
The absorbent article of Comparative Example 1 was prepared in the same manner as in Example 1, except that the water-absorbent resin particles were changed to water-absorbent resin particles B.

(Comparative Example 2)
The absorbent article of Comparative Example 2 was prepared in the same manner as in Example 1, except that the water-absorbent resin particles were changed to water-absorbent resin particles C.

(Comparative Example 3)
The absorbent article of Comparative Example 3 was prepared in the same manner as in Example 1, except that the water-absorbent resin particles were changed to water-absorbent resin particles D.

(Comparative Example 4)
The absorbent article of Comparative Example 4 was prepared in the same manner as in Example 1, except that the water-absorbent resin particles were changed to water-absorbent resin particles E.

(Example 2)
An absorbent article of Example 2 was prepared in the same manner as in Example 1, except that the absorbent article was 32 cm x 12 cm, the amount and number of hot melt adhesives were changed to 0.1 g and 11 sticks, and the amount of water-absorbing resin particles A was changed to 4.5 g. In the absorbent article, the basis weight of the water-absorbing resin particles was 150 g/ ^m² .

(Comparative Example 5)
The absorbent article of Comparative Example 5 was prepared in the same manner as in Example 2, except that the water-absorbent resin particles were changed to water-absorbent resin particles B.

(Comparative Example 6)
The absorbent article of Comparative Example 6 was prepared in the same manner as in Example 2, except that the water-absorbent resin particles were changed to water-absorbent resin particles C.

(Comparative Example 7)
The absorbent article of Comparative Example 7 was prepared in the same manner as in Example 2, except that the water-absorbent resin particles were changed to water-absorbent resin particles D.

(Comparative Example 8)
The absorbent article of Comparative Example 8 was prepared in the same manner as in Example 2, except that the water-absorbent resin particles were changed to water-absorbent resin particles E.

(Example 3)
After applying hot melt adhesive to the first sheet using the same procedure as in Example 1, 7.2 g of superabsorbent resin particles A were uniformly scattered on the first sheet, excluding the outer perimeter of 1 cm at both the short and long edges. An air-through nonwoven fabric cut to 40 cm x 12 cm was placed on the surface where the superabsorbent resin particles A were scattered, then sandwiched between release paper from above and below, and pressed together using a laminating machine using the same procedure as in Example 1. The release paper was then removed to obtain a laminate in which the air-through nonwoven fabric, an absorbent body consisting of superabsorbent resin particles A (upper absorbent layer), hot melt adhesive, and the first sheet were arranged in this order.

Next, hot melt adhesive was applied to the second sheet using the same procedure as in Example 1, and 7.2 g of water-absorbing resin particles A were uniformly scattered on the second sheet. The laminate, with its air-through nonwoven fabric surface and the surface of the second sheet on which the water-absorbing resin particles were scattered, was joined at both ends and sandwiched from above and below with release paper. The laminate was then pressed and bonded using a laminating machine in the same procedure as described above, the release paper was removed, and a water-absorbing sheet was prepared. Furthermore, a hydrophilic air-through nonwoven fabric of the same size as the water-absorbing sheet was placed on the upper surface of the water-absorbing sheet, and a polyethylene liquid-impermeable sheet with a basis weight of 40 g/ ^m² was placed on the lower surface of the water-absorbing sheet to prepare a test absorbent article. The resulting absorbent article has the following arrangement: hydrophilic air-through nonwoven fabric, first sheet, hot melt adhesive, absorbent body consisting of water-absorbing resin particles a (upper absorbent layer), air-through nonwoven fabric, absorbent body consisting of water-absorbing resin particles a (lower absorbent layer), hot melt adhesive, second sheet, and liquid-impermeable sheet. In the absorbent article, the basis weight of the water-absorbing resin particles was 150 g/ ^m² for the upper absorbent layer and 150 g/ ^m² for the lower absorbent layer.

(Example 4)
An absorbent article of Example 4 was prepared in the same manner as in Example 3, except that the water-absorbing resin particles used in the lower absorbent layer were changed to water-absorbing resin particles D.

(Example 5)
An absorbent article of Example 5 was prepared in the same manner as in Example 3, except that the water-absorbing resin particles used in the upper absorbent layer were changed to water-absorbing resin particles D.

(Comparative Example 9)
The absorbent article of Comparative Example 9 was prepared in the same manner as in Example 3, except that the water-absorbing resin particles used in the upper and lower absorbent layers were changed to water-absorbing resin particles B.

(Comparative Example 10)
The absorbent article of Comparative Example 10 was prepared in the same manner as in Example 3, except that the water-absorbing resin particles used in the upper and lower absorbent layers were changed to water-absorbing resin particles C.

(Comparative Example 11)
The absorbent article of Comparative Example 11 was prepared in the same manner as in Example 3, except that the water-absorbing resin particles used in the upper and lower absorbent layers were changed to water-absorbing resin particles D.

(Comparative Example 12)
The absorbent article of Comparative Example 12 was prepared in the same manner as in Example 3, except that the water-absorbing resin particles used in the upper and lower absorbent layers were changed to water-absorbing resin particles E.

(Example 6)
An absorbent article was prepared in the same manner as in Example 3, except that the first and second sheets were 32 cm x 12 cm, the amount and number of hot melt adhesives were 0.1 g and 11, the air-through nonwoven fabric was 30 cm x 10 cm, and the amount of water-absorbing resin particles A was changed to 2.3 g for the upper absorbent layer and 2.3 g for the lower absorbent layer. In the absorbent article, the basis weight of the water-absorbing resin particles was 75 g/ ^m² for the upper absorbent layer and 75 g/ ^m² for the lower absorbent layer.

(Example 7)
An absorbent article of Example 7 was prepared in the same manner as in Example 6, except that the water-absorbing resin particles used in the lower absorbent layer were changed to water-absorbing resin particles D.

(Example 8)
An absorbent article of Example 8 was prepared in the same manner as in Example 6, except that the water-absorbing resin particles used in the upper absorbent layer were changed to water-absorbing resin particles D.

(Comparative Example 13)
The absorbent article of Comparative Example 13 was prepared in the same manner as in Example 6, except that the water-absorbing resin particles used in the upper and lower absorbent layers were changed to water-absorbing resin particles B.

(Comparative Example 14)
The absorbent article of Comparative Example 14 was prepared in the same manner as in Example 6, except that the water-absorbing resin particles used in the upper and lower absorbent layers were changed to water-absorbing resin particles D.

(Example 9)
An absorbent article of Example 9 was prepared in the same manner as in Example 1, except that the nonwoven fabrics of the first and second sheets were changed to spunbond nonwoven fabrics.

(Comparative Example 15)
The absorbent article of Comparative Example 15 was prepared in the same manner as in Example 9, except that the water-absorbent resin particles were changed to water-absorbent resin particles B.

(Comparative Example 16)
The absorbent article of Comparative Example 16 was prepared in the same manner as in Example 9, except that the water-absorbent resin particles were changed to water-absorbent resin particles D.

(Example 10)
An absorbent article of Example 10 was prepared in the same manner as in Example 1, except that the nonwoven fabric of the first and second sheets was changed to spunlace nonwoven fabric.

(Comparative Example 17)
The absorbent article of Comparative Example 17 was prepared in the same manner as in Example 10, except that the water-absorbent resin particles were changed to water-absorbent resin particles B.

(Comparative Example 18)
The absorbent article of Comparative Example 18 was prepared in the same manner as in Example 10, except that the water-absorbent resin particles were changed to water-absorbent resin particles D.

The following evaluations were conducted on absorbent articles using absorbent sheets. The results are shown in Tables 2 to 6.

As can be seen from the results in Tables 1 to 6, the present invention provides an absorbent sheet that can suppress leakage and an absorbent article using the absorbent sheet. Furthermore, when manufacturing absorbents and absorbent articles having the configuration of the present invention, it is expected that absorbents and absorbent articles with equivalent performance can be provided even if the amount of absorbent resin used is reduced, potentially leading to cost reductions in manufacturing.

1 Burette section 3 Clamp 5 Conduit 10 Absorbent material 10a Water-absorbing resin particles 10b Hydrophilic fiber 11 Stand 13 Measuring stage 13a Through hole 15 Nylon mesh sheet 20 Core wrap 21 Burette tube 22 Cock 23 Rubber stopper 24 Cock 25 Air inlet tube 30 Surface sheet 31 Back sheet 40 Support plate 41 Stand 42 Dropping funnel 43 Balance 44 Metal tray 50 Saltwater 51 Test solution 61 Burette section 61a Burette 61b Rubber stopper 61c Cock 61d Air inlet tube 61e Cock 62 Conduit 63 Measuring stage 64 Measuring section 64a Cylinder 64b Nylon mesh 64c Weight 100 Absorbent article S ₀ Horizontal surface S ₁ Inclined surface

Claims (4)

An absorbent sheet comprising an absorbent layer and a nonwoven fabric sandwiching the absorbent layer above and below,
The absorbent layer contains a water-absorbent resin,
The water-absorbing resin is a water-absorbing sheet having the following characteristics (A) to (C).
(A) The water content of physiological saline is between 45 g/g and 70 g/g.
(B) The water absorption rate under a load of 4.14 kPa is 13 ml/g or more.
(C) The unpressurized DW 5-minute value is 44 ml/g or more and 80 ml/g or less. The absorbent sheet according to claim 1, wherein the basis weight of the absorbent resin is 100 g/ ^m² or more and 450 g/ ^m² or less. The absorbent sheet according to claim 1 or 2, wherein the absorbent layer is bonded to the nonwoven fabric via an adhesive. An absorbent article comprising the absorbent sheet described in claim 1 or 2.