JP7576986B2 - Water-absorbent sheet and absorbent article including same - Google Patents

Description

The present invention relates to an absorbent sheet and an absorbent article containing the same.

Super absorbent polymers (SAPs) are water-swellable, water-insoluble polymeric gelling agents that are used in a variety of applications, including hygiene materials such as paper diapers, sanitary napkins, and adult incontinence products, soil water retention agents for agricultural and horticultural use, and industrial water-stopping agents.

These absorbent articles are generally manufactured at paper diaper manufacturing plants as absorbent bodies that are individually molded for each absorbent article by mixing absorbent resin and fibrous material, and are processed into absorbent bodies of various shapes (for example, hourglass-shaped, fox-shaped, oval, etc. when viewed in plan) depending on the purpose. These manufacturing methods for absorbent bodies are the mainstream for paper diapers today, as they can be processed into any shape because they are individually molded, and the amount of fiber and absorbent resin can be easily adjusted for each absorbent article.

However, in recent years, paper diapers have begun to be manufactured using absorbents (called absorbent sheets, usually cut into rectangles about 10 cm wide and several tens of cm long) made by cutting a long absorbent sheet with absorbent resin fixed between two sheets during the manufacturing process of sanitary materials. By purchasing or manufacturing a long continuous absorbent sheet, paper diaper manufacturers can simplify the manufacturing process of paper diapers, and furthermore, by not using pulp, they can make paper diapers thinner. Absorbent sheets are constructed by sandwiching and fixing absorbent resin particles between upper and lower sheets (particularly nonwoven sheets), and usually after manufacturing a long continuous sheet, the long continuous sheet is cut into rectangles about 10 cm wide and several tens of cm long, which are then incorporated into paper diapers (for example, Patent Document 1).

Unlike conventional sanitary materials (paper diapers), paper diapers made with absorbent sheets have a short history, and the reality is that there has been almost no development of absorbent resins suitable for absorbent sheets or proposals for parameters, and the absorbent resins used in conventional paper diapers are used as is for absorbent sheets.

International Publication No. 2010/143635

The inventors have found that, due to the structure unique to the mainstream water-absorbent sheets, which are thin, the absorbed liquid is easily released in the direction of the absorbed liquid when pressure is applied to the water-absorbent sheet, which is known as "return." Return is also called re-wet. The inventors have found that the occurrence of backflow is more pronounced when liquid is introduced intermittently multiple times (especially three or more times) and the amount of liquid introduced is large. When backflow occurs, the skin in contact with the water-absorbent sheet comes into contact with the backflowing liquid and is exposed to a highly humid state. This not only causes discomfort to the user, but also makes the skin in contact with the water-absorbent sheet more susceptible to rashes.

In the course of diligent research into solving the problem of backflow when liquid is intermittently introduced multiple times (especially three or more times), the inventors conducted research and development to design an absorbent sheet so that absorbed liquid does not return upward (towards the skin), but discovered that doing so makes it more likely for leakage in the surface direction (side leakage) to occur. Side leakage is a phenomenon in which, for example, when an absorbent sheet is placed upright or tilted, liquid that is not instantly absorbed after reaching the absorbent layer leaks out from the absorbent sheet in the surface direction. The inventors discovered that this problem of side leakage becomes more pronounced when liquid is intermittently introduced multiple times (especially three or more times) and the amount of introduced liquid is large.

The present invention has been made in consideration of the above circumstances, and aims to provide a new absorbent sheet that can significantly reduce liquid leakage from the absorbent sheet due to backflow and lateral leakage, even if liquid is intermittently introduced multiple times (particularly, three or more times).

The present inventors have conducted extensive research to solve the above problems. As a result, they have found that the above problems can be solved by a water-absorbent sheet having a first substrate, a second substrate, and a water-absorbent layer located between the first substrate and the second substrate, the water-absorbent layer containing a particulate water-absorbing agent, the first substrate being located on the side where the aqueous liquid is introduced, and the initial absorption speed being adjusted to 2.2 to 3.0 seconds, and have completed the present invention.

The present invention provides a novel absorbent sheet that can significantly reduce liquid leakage from the absorbent sheet due to backflow and lateral leakage, even when liquid is intermittently introduced multiple times (particularly, three or more times).

1 is a schematic diagram illustrating a cross section of a water-absorbent sheet according to one embodiment of the present invention. 1 is a schematic diagram illustrating a cross section of a water-absorbent sheet according to one embodiment of the present invention. 1 is a schematic diagram illustrating a cross section of a water-absorbent sheet according to one embodiment of the present invention. 1 is a schematic diagram illustrating a cross section of a water-absorbent sheet according to one embodiment of the present invention. FIG. 1 is a schematic diagram showing an apparatus for measuring GPR. FIG. 2 is a plan view and a right side view showing a sample used in evaluating the amount of return, and shows how the water-absorbent sheet produced in the example is wrapped in a liquid-impermeable sheet. 1A and 1B are a plan view and a front view of a liquid injection tube used in evaluating the amount of backflow. FIG. 2 is a front view showing a liquid injection tube placed on the water-absorbent sheet used in the examples of the present application. FIG. 11 is a front view showing a state in which a sodium chloride aqueous solution is poured into a water-absorbent sheet from a liquid pouring tube using a funnel. FIG. 1 is a schematic diagram showing an apparatus used for evaluating the amount of leakage in the surface direction.

Hereinafter, the present invention will be described with reference to the best mode. Throughout this specification, singular expressions should be understood to include the concept of the plural, unless otherwise specified. Therefore, singular articles (e.g., in the case of English, "a", "an", "the", etc.) should be understood to include the concept of the plural, unless otherwise specified. In addition, it should be understood that the terms used in this specification are used in the sense commonly used in the field, unless otherwise specified. Therefore, unless otherwise defined, all technical terms and scientific and technical terms used in this specification have the same meaning as commonly understood by those skilled in the art to which this invention belongs. In the case of conflict, this specification (including definitions) will take precedence. The present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims.

1. Definitions or explanations of terms
[1-1. Water-absorbent sheet]
The absorbent sheet is in the form of a continuous sheet or a roll of the continuous sheet. When using the absorbent sheet, the continuous sheet is cut into an appropriate shape (e.g., rectangular) and then incorporated into an absorbent article (e.g., paper diaper, sanitary napkin, pad for incontinent person, etc.). Absorbent articles such as paper diapers (disposable diapers), sanitary napkins, and pad for incontinent person have an absorbent body that absorbs and retains body fluids such as urine and menstrual blood excreted from the body, a flexible liquid-permeable front sheet arranged on the side that contacts the body, and a liquid-impermeable back sheet arranged on the side opposite to the side that contacts the body. The absorbent sheet can be used as the absorbent body. Conventional paper diapers are absorbent articles that are individually molded for each paper diaper and have a shape that fits the buttocks. Therefore, such absorbents have technical properties that are different from the absorbent sheet of the present invention.

[1-2. Water-absorbing resin]
In the present specification, the term "water-absorbent resin" refers to a resin having a water swelling coefficient (CRC) of 5 g/g or more as defined by ERT441.2-02 and a water-soluble component content ( Ext) is 50 mass % or less.

The water-absorbent resin is preferably a hydrophilic cross-linked polymer obtained by cross-linking and polymerizing an unsaturated monomer having a carboxyl group. The shape of the water-absorbent resin may be sheet-like, fibrous, film-like, particulate, gel-like, or the like. The water-absorbent sheet according to one embodiment of the present invention uses a particulate water-absorbent resin.

In addition, in this specification, the term "water-absorbent resin" is a concept that also includes intermediates in the manufacturing process of water-absorbent resin. For example, a hydrogel-like cross-linked polymer after polymerization, a dried polymer after drying, and water-absorbent resin powder before surface cross-linking may also be referred to as a "water-absorbent resin."

Thus, in this specification, in addition to the water-absorbent resin itself, the water-absorbent resin composition and intermediate may be collectively referred to as the water-absorbent resin.

[1-3. Water absorbing agent, particulate water absorbing agent]
In this specification, the term "water-absorbing agent" refers to a composition (or absorbent gelling agent) for absorbing an aqueous liquid (sometimes simply referred to as "liquid" in this specification) containing a water-absorbing resin as a main component. Here, the aqueous liquid (liquid) is not limited to water, but is any liquid containing water. The aqueous liquid absorbed by the water-absorbent sheet according to one embodiment of the present invention is urine, menstrual blood, sweat, and other body fluids.

In this specification, the term "particulate water absorbing agent" refers to a particulate (powdered) water absorbing agent (corresponding to particulate water absorbing resin, since the water absorbing agent contains water absorbing resin as a main component). The concept of "particulate water absorbing agent" includes both a single particle of particulate water absorbing agent and an aggregate of a plurality of particulate water absorbing agents. In this specification, "particulate" means having a particle shape. Here, "particle" refers to a relatively small divided body of a substance, and has a size of several Å to several mm (see "Particle", McGraw-Hill Scientific and Technical Dictionary 3rd Edition, edited by the McGraw-Hill Scientific and Technical Dictionary Editorial Committee, Nikkan Kogyo Shimbun, 1996, p. 1929). In this specification, the term "particulate water absorbing agent" may be simply referred to as "water absorbing agent".

In this specification, the first particulate water absorbing agent and the second particulate water absorbing agent described below may be collectively referred to (i.e., a mixture of them), or one of them may simply be referred to as the "particulate water absorbing agent."

The particulate water absorbent contains a water absorbent resin as a polymer as a main component. In this specification, the particulate water absorbent is not limited to an embodiment in which the total amount (100% by mass) of the particulate water absorbent is only the water absorbent resin. As long as the above-mentioned CRC and Ext are satisfied, the particulate water absorbent may be a water absorbent resin composition containing additives added internally. The particulate water absorbent contains 60 to 100% by mass, preferably 70 to 100% by mass, more preferably 80 to 100% by mass, even more preferably 90 to 100% by mass, and particularly preferably 95 to 100% by mass of the water absorbent resin as a polymer. The remainder of the particulate water absorbent may optionally contain water, additives that can be added externally (such as inorganic fine particles), and the like. The particulate water absorbent used in the examples of this application contains 80 to 100% by mass of the water absorbent resin.

That is, the upper limit of the water-absorbing resin in the particulate water-absorbing agent is, for example, 100% by mass, 99% by mass, 97% by mass, 95% by mass, or 90% by mass. Preferably, the particulate water-absorbing agent further contains 0 to 10% by mass of components other than the water-absorbing resin, in particular additives that can be added externally (such as inorganic fine particles).

The preferred moisture content of the particulate water absorbing agent is 0.2 to 30% by mass. Even if water is contained, it can still be called a particulate water absorbing agent.

Examples of water-absorbing resins that are the main components of particulate water absorbents include polyacrylic acid (salt)-based resins, polysulfonic acid (salt)-based resins, maleic anhydride (salt)-based resins, polyacrylamide-based resins, polyvinyl alcohol-based resins, polyethylene oxide-based resins, polyaspartic acid (salt)-based resins, polyglutamic acid (salt)-based resins, polyalginic acid (salt)-based resins, starch-based resins, and cellulose-based resins. Of these, polyacrylic acid (salt)-based resins are preferably used as the water-absorbing resin.

[1-4. Polyacrylic acid (salt)]
In this specification, "polyacrylic acid (salt)" refers to polyacrylic acid and/or its salt. The polyacrylic acid (salt) is a polymer containing repeating units of acrylic acid and/or its salt (hereinafter referred to as "acrylic acid (salt)") as a main component, and further containing a graft component as an optional component. The polyacrylic acid (salt) is obtained by polymerization of acrylic acid (salt), hydrolysis of polyacrylamide, polyacrylonitrile, etc. Preferably, the polyacrylic acid (salt) is obtained by polymerization of acrylic acid (salt).

Here, "contains as a main component" means that the amount of acrylic acid (salt) used when polymerizing polyacrylic acid (salt) is usually 50 to 100 mol %, preferably 70 to 100 mol %, more preferably 90 to 100 mol %, and even more preferably substantially 100 mol %, based on the total amount of monomers (excluding the internal crosslinking agent) used in the polymerization.

[1-5. EDANA and ERT]
"EDANA" is an abbreviation for European Disposables and Nonwovens Associations. "ERT" is an abbreviation for EDANA Recommended Test Methods, a European standard (effectively a global standard) for measuring water-absorbent resins. In this specification, unless otherwise specified, the physical properties of the water-absorbent resin are measured in accordance with the 2002 edition of ERT.

[1-6. Others]
In this specification, the range "X to Y" means "greater than or equal to X and less than or equal to Y."

In this specification, unless otherwise noted, the unit of mass "t (ton)" means "metric ton." "ppm" means "ppm by mass." "Mass" and "weight," "parts by mass" and "parts by weight," "% by mass" and "% by weight," "ppm by mass" and "ppm by weight" are each treated as having the same meaning.

In this specification, "acid (salt)" means "acid and/or its salt." "(Meth)acrylic" means "acrylic and/or methacrylic."

In this specification, the unit of volume "liter" may be written as "l" or "L". "% by mass" may be written as "wt%". When measuring trace components, amounts below the detection limit may be written as N.D. (Non Detected).

[2. Water-absorbent sheet]
The water-absorbent sheet of the present invention has a first substrate, a second substrate, and a water-absorbent layer located between the first substrate and the second substrate, the water-absorbent layer contains a particulate water-absorbing agent, the first substrate is located on the side where an aqueous liquid is introduced, and the initial absorption speed is adjusted to 2.2 to 3.0 seconds. With this configuration, a novel water-absorbent sheet is provided that can significantly reduce liquid discharge from the water-absorbent sheet due to backflow and lateral leakage even if liquid is intermittently introduced multiple times (particularly, three or more times).

As described above, the inventors have found that when liquid is intermittently introduced multiple times, attempting to solve the problem of backflow makes side leakage more likely to occur. In other words, they have found that there is a trade-off between the specific return amount and the specific leakage amount. Here, the specific return amount and the specific leakage amount refer to the return amount and leakage amount, respectively, evaluated by the method described in the examples of this application.

In the process of intensively studying how to solve this trade-off problem, it was discovered that, phenomenologically, if the initial absorption of liquid by the absorbent sheet is too fast, side leakage is small but backflow is likely to occur, and if the initial absorption of liquid by the absorbent sheet is too slow, backflow is small but side leakage is likely to occur. In order to solve this trade-off, as described below, it was necessary to carry out trial and error with respect to the type of each substrate, the thickness of the substrate, the basis weight of the substrate, the type of absorbent agent, the type of adhesive, the amount of adhesive applied, the application method, etc., and complex and excessive consideration was required. Therefore, the absorption speed immediately after the introduction of the aqueous liquid was defined as the "initial absorption speed" as a specific physical property value of the absorbent sheet. In the present invention, the "initial absorption speed" is an index showing the degree of liquid compatibility of the first substrate with the aqueous liquid introduced into the absorbent body, and is a physical property that greatly affects the initial liquid capture ability in absorbent articles such as disposable diapers. In a conventional paper diaper using an absorbent containing pulp, even if the liquid compatibility of the top layer substrate corresponding to the first substrate of the present invention is somewhat poor, the liquid capture property of the paper diaper is not reduced because the liquid capture property of the pulp itself is very high. However, since the absorbent sheet is an absorbent that does not contain pulp, the liquid compatibility of the first substrate has a large effect on the liquid capture property of the paper diaper. Methods for evaluating the liquid compatibility of a substrate include a method of measuring the contact angle at the moment when a droplet of an aqueous liquid is dropped on the substrate, but this method can only determine the difference between extreme hydrophilicity and hydrophobicity, and cannot evaluate the slight difference required in the present invention. Therefore, the inventors of the present invention have examined various evaluation methods and have found that the time required from the time when a very small amount of colored aqueous liquid is dropped on the substrate at a constant speed to the time when the liquid is completely absorbed by the substrate surface and the color of the liquid disappears can be defined as the "initial water absorption speed", and that the above problem can be solved by keeping the "initial absorption speed" within a specific range.

In this way, the absorbent sheet of the present invention has an initial absorption speed adjusted to a specific range. This initial absorption speed is an index for evaluating how long it takes for the first aqueous liquid (urine water, 0.9% sodium chloride solution as a substitute for urine water) to be absorbed into the absorbent sheet when it is introduced from the first substrate side. Specifically, it is measured by the method described in the Examples. If the initial absorption speed exceeds 3.0 seconds, the aqueous liquid cannot penetrate into the substrate at the moment it contacts the first substrate, making it difficult to solve the problem of the specific leakage amount. On the other hand, if the initial absorption speed is less than 2.2 seconds, the aqueous liquid is instantly absorbed only in the vicinity of the point where it contacts the first substrate, and the aqueous liquid cannot be diffused throughout the absorbent sheet, resulting in insufficient liquid capture, making it difficult to solve the problem of the specific return amount.

Here, factors that influence the initial absorption rate include the type, structure, and physical properties of the first substrate; the type, physical properties, amount, and application method of the adhesive placed on the first substrate; and the physical properties of the particulate water absorbent (the first particulate water absorbent and the second particulate water absorbent). For example, if the hydrophilicity of the first substrate is high, the initial absorption rate will tend to be faster, and conversely, if the hydrophobicity is high, the initial absorption rate will tend to be slower. Also, if the hydrophilicity of the adhesive placed on the first substrate is high, the initial absorption rate will tend to be faster, and conversely, if the hydrophobicity is high, the initial absorption rate will tend to be slower. Also, the larger the adhesive droplets (i.e., the sparser the adhesive distribution density on the first substrate), the faster the initial absorption rate will be, and conversely, the smaller the adhesive droplets (i.e., the denser the adhesive distribution density on the first substrate), the slower the initial absorption rate will be. In addition, it is considered that if the content (amount used) of the adhesive placed on the first base material is small, the initial absorption speed tends to be fast, and conversely, if the content (amount used) of the adhesive placed on the first base material is large, the initial absorption speed tends to be slow. It is considered that if the CRC of the particulate water absorbing agent (particularly the first particulate water absorbing agent) is high, the initial absorption speed tends to be fast. It is considered that if the CRC of the particulate water absorbing agent (particularly the second particulate water absorbing agent) is high, the initial absorption speed tends to be fast. It is considered that if the surface tension of the particulate water absorbing agent (particularly the second particulate water absorbing agent) is high, the initial absorption speed tends to be fast. As described above, the value of the initial absorption speed is thought to depend on several factors, but those who wish to practice the present invention can adjust the initial absorption speed to 2.2 to 3.0 seconds by appropriately adjusting the type, structure, and physical properties of the first substrate; the type, physical properties, amount, and application method of the adhesive placed on the first substrate; and the physical properties of the particulate water absorbing agent (physical properties of the first particulate water absorbing agent and the second particulate water absorbing agent), with reference to the above findings and the examples described below.

<Specific adjustment method>
The type and structure of the first substrate used in the present invention is preferably a nonwoven fabric with fewer inter-fiber voids such as an air-laid nonwoven fabric, since it is easier to adjust the initial absorption rate than a nonwoven fabric with many inter-fiber voids such as an air-through nonwoven fabric. In addition, since the adhesive used in the present invention is hydrophobic and may deteriorate the liquid compatibility, it is preferable to use a hydrophilic substrate as the first substrate.

In the present invention, the amount of the first adhesive applied may be adjusted to within the range of 0.0005 to 0.3 times the amount of the first particulate absorbent. However, even if the type of first substrate is the same, the preferred amount varies depending on the basis weight of the nonwoven fabric and other conditions. Therefore, to be precise, the amount of adhesive applied must be appropriately adjusted for the selected first substrate.

In the present invention, the adhesive application method can be "solvent dissolution spraying" in which a liquid solution-type adhesive is sprayed, or "heat melt spraying" in which a solid hot melt adhesive is heated and melted and sprayed. When the adhesive application method is solvent dissolution spraying, although it cannot be said in general terms depending on the type of adhesive and the specifications of the nozzle spraying the adhesive, in the case of spraying with a large droplet diameter, specifically, when the discharge amount of the first adhesive to the first substrate is 0.5 g/sec or more, the preferred application amount of the first adhesive to the amount of the first particulate absorbent is 0.03 to 0.2 times. On the other hand, in the case of spraying with a small droplet diameter, specifically, when the discharge amount of the first adhesive to the first substrate is less than 0.5 g/sec, the preferred application amount of the first adhesive to the amount of the first particulate absorbent is 0.01 to 0.04 times.

In the present invention, when the adhesive application method is heat-melt spraying, the adhesive strength tends to be stronger than when the adhesive is dissolved in a solvent and sprayed, depending on the specifications of the nozzle used to spray the adhesive, so it is possible to adjust the amount of adhesive applied to be smaller. Specifically, the preferred amount of adhesive applied is 0.005 to 0.03 times the amount of the first particulate absorbent.

It should be noted that, as described in the present specification, the "specific return amount evaluation" that measures the amount of return when liquid is intermittently introduced multiple times is a different index from the "return amount evaluation" measured under general conditions. Similarly, it should be noted that the "specific leakage amount evaluation" that measures the amount of lateral leakage when liquid is intermittently introduced multiple times is a different index from the "lateral leakage evaluation" measured under general conditions. An absorbent sheet that has a good "return amount evaluation" measured under general conditions and a good "lateral leakage evaluation" measured under general conditions does not necessarily have a good "specific return amount evaluation" and "return amount evaluation."

Thus, the absorbent sheet according to one embodiment of the present invention is suitable as an absorbent article (e.g., a diaper) for use by infants who are just learning to run and whose bladders are still small during the daytime when they are actively moving around, and is also suitable as an absorbent article (e.g., a diaper) for use by infants who turn over a lot and have bad sleeping habits. Of course, the form of use is not limited to this. Furthermore, the mechanisms described in this specification do not limit the technical scope of the claims of this application.

The absorbent sheet according to one embodiment of the present invention can be made thinner than absorbents used in conventional absorbent articles. When the absorbent sheet is used in a disposable diaper, the thickness is, for example, at 40% RH to 50% RH, preferably 15 mm or less, more preferably 10 mm or less, even more preferably 7 mm or less, particularly preferably 5 mm or less, and most preferably 4 mm or less. On the other hand, the lower limit of the thickness is 0.2 mm or more, preferably 0.3 mm or more, and more preferably 0.5 mm or more, taking into account the strength of the absorbent sheet and the diameter of the particulate water absorbing agent. The thickness of the absorbent sheet used in the examples of this application was 3 mm to 5 mm under the above conditions.

The thickness of the absorbent sheet is measured using a large-type dial thickness gauge (thickness measuring device) (manufactured by Ozaki Seisakusho Co., Ltd., model number: J-B, measuring probe: anvil top and bottom φ50 mm). The absorbent body is divided into three equal parts in the longitudinal direction, and the measurement points are the center of each part (the intersection of a diagonal line drawn from the end of the absorbent body). For example, in the case of an absorbent sheet that is 36 cm long and 10 cm wide, the measurement points are 6 cm from the left end in the longitudinal direction and 5 cm from both ends in the width direction (left), 18 cm from the left end in the longitudinal direction and 5 cm from both ends in the width direction (center), and 30 cm from the left end in the longitudinal direction and 5 cm from both ends in the width direction (right). Each part is measured twice, and the thickness measurement value is the average of the total of six points. Specifically, the absorbent sheet is attached flat to a plate of a uniform thickness so that there are no wrinkles or distortions at the measurement point, and the plate is then set on the lower probe of the thickness gauge. Next, the upper probe of the thickness gauge is brought close to a height of 2-3 mm from the absorbent sheet, and then the handle is slowly released and the combined thickness of the absorbent sheet and plate is measured. The thickness of the absorbent sheet is determined by the formula: T1 = T2 - T0 (T0: thickness of plate (mm), T1: thickness of absorbent sheet (mm), T2: thickness of absorbent sheet and plate (mm)).

The thicknesses of the first substrate, second substrate, intermediate sheet, and wrapping sheet, described below, are also measured in the same manner.

The following describes the embodiments of the present invention in more detail with reference to the attached drawings. The absorbent sheet may have a structure in which the substrate on the liquid-absorbing side is in direct contact with aqueous liquids such as urine water, or the sheets may be laminated in any manner so long as it does not impede the initial water absorption rate of the absorbent sheet. In the description of the drawings, the same elements are given the same reference numerals, and duplicate descriptions are omitted. The dimensional ratios in the drawings are exaggerated for the convenience of explanation, and may differ from the actual ratios.

Figure 1 is a schematic diagram showing a cross section of a water-absorbent sheet 40 according to one embodiment of the present invention. In Figure 1, the arrow indicates the direction in which the aqueous liquid (liquid) to be absorbed is introduced. The first substrate 11 is located on the side where the liquid to be absorbed (liquid to be absorbed) is introduced. That is, the first substrate is arranged on the liquid discharge side (for example, the skin side in the case of a paper diaper). In this way, the aqueous liquid is in direct contact with the first substrate 11. A water-absorbent layer 12 is present between the first substrate 11 and the second substrate 13. The water-absorbent layer 12 includes a particulate water-absorbing agent 14a in contact with (or fixed to) the first substrate and a particulate water-absorbing agent 14b in contact with (or fixed to) the second substrate 13. Therefore, in the water absorbent sheet according to one embodiment of the present invention, the particulate water absorbing agent 14 (14a, 14b) is composed of a first particulate water absorbing agent 14a localized on the surface side of the first substrate 11 facing the second substrate 13, and a second particulate water absorbing agent 14b localized on the surface side of the second substrate 13 facing the first substrate 11. Some of the particulate water absorbing agents 14a, 14b may not be in contact with the substrates 11, 13 (or may not be fixed. They may be detached from the substrates 11, 13). Therefore, the water absorption "layer" does not only refer to a continuous body such as a sheet, but may be in any form as long as it exists with a certain thickness between the first substrate 11 and the second substrate 13. When the particulate water absorbing agents 14a, 14b are fixed to the substrates 11, 13, for example, as shown in FIG. 1, an adhesive 17 (first adhesive 17a, second adhesive 17b) may be used. The first adhesive refers to an adhesive disposed on the surface of the first substrate facing the second substrate, and the second adhesive refers to an adhesive disposed on the surface of the second substrate 13 facing the first substrate 11. In this specification, the first adhesive, the second adhesive, and the first' adhesive and the second' adhesive described later may be collectively referred to, or a part of them may be simply referred to as "adhesive". The method of manufacturing a water-absorbent sheet using an adhesive will be described in detail in [3. ]. The water-absorbent layer 12 may also have an intermediate sheet 16 as necessary. In the water-absorbent sheet 40 of FIG. 1, the first particulate water-absorbing agent 14a localized on the surface side of the first substrate 11 facing the second substrate 13 and the second particulate water-absorbing agent 14b localized on the surface side of the second substrate 13 facing the first substrate 11 are present so as to sandwich the intermediate sheet 16. In this intermediate sheet 16, the water absorbing agent 14 that was fixed by, for example, adhesive 17 (17a, 17b) may be detached and captured by the intermediate sheet 16.

As described above, the water-absorbent sheet 40 shown in FIG. 1 has an initial absorption speed adjusted to 2.2 to 3.0 seconds by appropriately adjusting the type, structure, and physical properties of the first substrate 11; the type, physical properties, amount, and application method of the first adhesive 17a placed on the first substrate; and the physical properties of the particulate water-absorbing agent 14 (physical properties of the first particulate water-absorbing agent 14a and the second particulate water-absorbing agent 14b).

2 is a schematic diagram showing a cross section of a water-absorbent sheet 40 according to one embodiment of the present invention. The water-absorbent sheet 40 of this embodiment has a wrapping sheet 18. The purpose of using the wrapping sheet is to maintain the shape of the water-absorbent sheet 40, which is a structure in which the particulate water-absorbing agent 14 is supported between the first substrate 11 and the second substrate 13, to prevent the particulate water-absorbing agent 14 supported between the first substrate 11 and the second substrate 13 from spilling (falling off) from the water-absorbent sheet 40, and to prevent the particulate water-absorbing agent 14 from directly contacting the skin when the particulate water-absorbing agent 14 permeates the first substrate 11 and migrates to the outer surface (surface that directly contacts the liquid) of the first substrate 11 (the wrapping sheet may be provided for purposes other than those described above). Therefore, in one embodiment of the present invention, the water-absorbent sheet is in a form in which the aqueous liquid contacts the first substrate 11 via the wrapping sheet. Although some wrapping sheets have meshes, the wrapping sheet of the embodiment of the present invention has meshes of a size that prevents the particulate water absorbing agent in the water absorbent sheet from migrating to the outside. In the embodiment in FIG. 2, the wrapping sheet 18 is disposed on the surface of the first substrate 11 on the side where the aqueous liquid is introduced. The wrapping sheet 18 is folded over so as to enclose the entire water absorption layer 12 and the second substrate 13. Thus, the wrapping sheet 18 covers the entire first substrate 11, the water absorption layer 12, and the second substrate 13. The wrapping sheet 18 does not need to cover the entire first substrate 11, the water absorption layer 12, and the second substrate 13. For example, the wrapping sheet 18 may be disposed on the first substrate 11, folded over so as to enclose the side of the water absorption layer 12 and the side of the second substrate 13, and folded over the surface of the second substrate 13 opposite to the surface that absorbs the liquid. In this case, the wrapping sheet 18 covers the first substrate 11, the water-absorbing layer 12, and the side surface of the second substrate 13, and also covers a part of the surface of the second substrate 13 opposite to the surface on which the water-absorbing layer 12 is provided (not shown). In one embodiment of the present invention, the wrapping sheet 18 is fixed to each substrate 11, 13 by using, for example, adhesives 17 (17c, 17d) as shown in Fig. 2. In Fig. 2, a first' adhesive 17c is disposed between the wrapping sheet 18 and the first substrate 11. Also, a second' adhesive 17d is disposed between the wrapping sheet 18 and the second substrate 13. In FIG. 2, the first' adhesive 17c and the second' adhesive 17d are disposed over the entire surface between the wrapping sheet 18 and each of the substrates 11 and 13, but as long as the wrapping sheet 18 is fixed to the absorbent sheet 40 by a method such as crimping, there may be some or all areas where the first' adhesive 17c and the second' adhesive 17d are not disposed.

As described above, the initial absorption speed of the water-absorbent sheet 40 shown in FIG. 2 is adjusted to 2.2 to 3.0 seconds by appropriately adjusting the type, structure, and physical properties of the first substrate 11; the type, physical properties, amount, and application method of the first adhesive 17a placed on the first substrate; the physical properties of the particulate water-absorbing agent 14 (physical properties of the first particulate water-absorbing agent 14a and the second particulate water-absorbing agent 14b); and the type, physical properties, amount, and application method of the first' adhesive 17c placed between the wrapping sheet 18 and the first substrate 11.

Figure 3 is a schematic diagram showing a cross section of a water-absorbent sheet 40 according to one embodiment of the present invention. The water-absorbent sheet 40 shown in Figure 3 is a modified version of the water-absorbent sheet 40 shown in Figure 2, and the second' adhesive 17d is substantially absent between the wrapping sheet 18 and the second substrate 13. In this specification, "substantially no adhesive is present" means that no adhesive is present at all, or that even if adhesive is present, the amount is 0.001 g or less per 1 g of the first and second particulate water-absorbing agents. With this configuration, the water-absorbent sheet can be efficiently wrapped by the wrapping sheet while achieving the intended effect of the present invention.

As described above, the water-absorbent sheet 40 shown in FIG. 3 has an initial absorption speed adjusted to 2.2 to 3.0 seconds by appropriately adjusting the type, structure, and physical properties of the first substrate 11; the type, physical properties, amount, and application method of the first adhesive 17a placed on the first substrate; the physical properties of the particulate water-absorbing agent 14 (physical properties of the first particulate water-absorbing agent 14a and the second particulate water-absorbing agent 14b); and the type, physical properties, amount, and application method of the first' adhesive 17c placed between the wrapping sheet 18 and the first substrate 11.

4 is a schematic diagram showing a cross section of the absorbent sheet 40 according to one embodiment of the present invention. The absorbent sheet 40 shown in FIG. 4 is a modified version of the absorbent sheet 40 shown in FIG. 2, in which substantially no adhesive (first' adhesive 17c) is present between the wrapping sheet 18 and the first substrate 11. This configuration allows liquid to be easily introduced into the absorbent sheet 40, and the initial absorption rate can be increased. In addition, in the absorbent sheet 40 of this embodiment, a portion of the adhesive (second' adhesive 17d) is present between the wrapping sheet 18 and the second substrate 13. This configuration allows the absorbent sheet to be efficiently wrapped by the wrapping sheet while achieving the intended effect of the present invention. In a further modified version, the absorbent sheet 40 shown in FIG. 4 may not substantially have the second' adhesive 17d present between the wrapping sheet 18 and the second substrate 13, as in the embodiment shown in FIG. 3 (not shown).

As described above, the water-absorbent sheet 40 shown in FIG. 4 has an initial absorption speed adjusted to 2.2 to 3.0 seconds by appropriately adjusting the type, structure, and properties of the first substrate 11; the type, properties, amount, and application method of the first adhesive 17a placed on the first substrate; the properties of the particulate water-absorbing agent 14 (the properties of the first particulate water-absorbing agent 14a and the second particulate water-absorbing agent 14b); and the amount of the first' adhesive 17c placed between the wrapping sheet 18 and the first substrate 11 (i.e., substantially 0).

Below, we will explain in detail each component that makes up the absorbent sheet.

[2-1. First substrate, second substrate and intermediate sheet]
The first substrate is a water-permeable sheet located on the side where the aqueous liquid (liquid) to be absorbed is introduced. The liquid to be absorbed is not limited to water, but may be urine, blood, sweat, feces, waste liquid, moisture, steam, ice, a mixture of water and an organic solvent and/or an inorganic solvent, rainwater, groundwater, etc., and is not particularly limited as long as it contains water. Preferred examples include urine, menstrual blood, sweat, and other body fluids.

The first substrate, the second substrate, and the intermediate sheet used in the absorbent sheet according to one embodiment of the present invention are preferably made of nonwoven fabric. The material of the nonwoven fabric is not particularly limited, but from the viewpoint of liquid permeability, flexibility, and strength of the absorbent sheet, pulp (cellulose) nonwoven fabric, polyolefin fiber (polyethylene (PE), polypropylene (PP), etc.), polyester fiber (polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), etc.), polyamide fiber (nylon, etc.), rayon fiber, pulp (cellulose) fiber, etc. are preferred. Nonwoven fabrics of other synthetic fibers and nonwoven fabrics manufactured by mixing synthetic fibers with cotton, silk, hemp fibers, etc. are also preferred. The nonwoven fabrics described above may be nonwoven fabrics containing only one type of the above-mentioned fiber, or nonwoven fabrics combining two or more types of fibers. The nonwoven fabrics used in the first substrate and the second substrate are preferably manufactured by the airlaid method. The nonwoven fabric used for the first and second substrates is preferably made of pulp (cellulose) fibers.

As described above, the nonwoven fabric used in the water-absorbent sheet according to one embodiment of the present invention is preferably a hydrophilic nonwoven fabric to enhance water permeability, but a hydrophilizing agent (such as a surfactant) may be used to hydrophilize the nonwoven fabric or the fibers that are the material of the nonwoven fabric.

Examples of hydrophilizing agents include stain release agents containing anionic surfactants (aliphatic sulfonates, higher alcohol sulfates, etc.), cationic surfactants (quaternary ammonium salts, etc.), nonionic surfactants (polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, etc.), silicone surfactants (polyoxyalkylene-modified silicones, etc.), polyester-based, polyamide-based, acrylic-based, and urethane-based resins. Examples of hydrophilizing agents include stain release agents containing anionic surfactants (aliphatic sulfonates, higher alcohol sulfates, etc.), cationic surfactants (quaternary ammonium salts, etc.), nonionic surfactants (polyethylene glycol fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, etc.), silicone surfactants (polyoxyalkylene-modified silicones, etc.), polyester-based, polyamide-based, acrylic-based, and urethane-based resins.

The first substrate used in the water-absorbent sheet according to one embodiment of the present invention is a water-permeable sheet because it is located on the side where the liquid to be absorbed is introduced. The second substrate and the intermediate sheet are also preferably water-permeable sheets having water permeability, and may be the same type or different types. The water permeability of the water-permeable sheet is preferably a water permeability coefficient (JIS A1218:2009) of 1×10 ⁻⁵ cm/sec or more. The water permeability coefficient is more preferably 1×10 ⁻⁴ cm/sec or more, even more preferably 1×10 ⁻³ cm/sec or more, even more preferably 1×10 ⁻² cm/sec or more, and even more preferably 1×10 ⁻¹ cm/sec or more. The water-absorbent sheet according to one embodiment of the present invention is preferably a hydrophilic nonwoven fabric. By being a hydrophilic nonwoven fabric, the desired effect of the present invention can be efficiently achieved.

The thickness of the first substrate and the second substrate is preferably as thin as possible within a range that allows the substrate to have strength as a water-absorbent sheet, and is appropriately selected from the range of 0.01 to 2 mm, further 0.02 to 1 mm, 0.03 to 0.6 mm, or 0.05 to 0.5 mm per substrate. The basis weight is preferably 5 to 300 g/m ² , more preferably 8 to 200 g/m ² , even more preferably 10 to 100 g/m ² , and even more preferably 11 to 50 g/m ² per substrate.

In the absorbent sheet according to one embodiment of the present invention, the nonwoven fabric used for the intermediate sheet is preferably an air-through nonwoven fabric. The nonwoven fabric used for the intermediate sheet is preferably bulky, specifically, 0.5 mm or more, more than 0.5 mm, 0.6 mm or more, more than 0.6 mm, 1 mm or more, or more than 1 mm. The nonwoven fabric described above may contain a small amount of pulp fibers to the extent that the thickness of the absorbent sheet is not increased. There is no particular upper limit on the thickness of the nonwoven fabric used for the intermediate sheet, but it is, for example, 4.9 mm or less, 4.8 mm or less, 4.7 mm or less, 4.6 mm or less, 4.0 mm or less, 3.0 mm or less, 2.5 mm or less, or 2.0 mm or less.

[2-2. Water absorption layer]
The water-absorbing layer in the water-absorbent sheet according to one embodiment of the present invention contains a particulate water-absorbing agent.

(Particulate water absorbing agent)
"surface tension"
Surface tension is the work (free energy) required to increase the surface area of a solid or liquid, expressed per unit area. The surface tension in this application refers to the surface tension of the aqueous solution when the particulate water absorbing agent is dispersed in a 0.90 mass % sodium chloride aqueous solution. The surface tension of the water absorbing agent is measured by the following procedure. That is, 50 ml of physiological saline adjusted to 20°C is placed in a thoroughly washed 100 ml beaker, and the surface tension of the physiological saline is first measured using a surface tensiometer (K11 automatic surface tensiometer manufactured by KRUSS). Next, a thoroughly washed 25 mm long fluororesin rotor and 0.5 g of the particulate water absorbing agent are placed in the beaker containing the physiological saline adjusted to 20°C after the surface tension measurement, and the mixture is stirred for 4 minutes under the condition of 500 rpm. After 4 minutes, the stirring is stopped, and after the particulate water absorbing agent containing the water is settled, the surface tension of the supernatant liquid is measured again by performing the same operation. In the present invention, a plate method using a platinum plate is adopted, and the plate is thoroughly washed with deionized water and heated and washed with a gas burner before each measurement.

In the water-absorbent sheet according to one embodiment of the present invention, the surface tension of at least one of the particulate water-absorbing agent (here, a mixture of the first particulate water-absorbing agent and the second particulate water-absorbing agent) is preferably 60 mN/m or more, 65 mN/m or more, 66 mN/m or more, 67 mN/m or more, 69 mN/m or more, 70 mN/m or more, 71 mN/m or more, and most preferably 72 mN/m or more. In particular, when the surface tension of the second particulate water-absorbing agent is 60 mN/m or more or 65 mN/m or more, the effect of reducing the specific return amount and the specific leakage amount in the water-absorbent sheet becomes remarkable. In the water-absorbent sheet according to one embodiment of the present invention, the upper limit of the surface tension of at least one of the particulate water-absorbing agent (here, a mixture of the first particulate water-absorbing agent and the second particulate water-absorbing agent) is not particularly limited, but is practically 73 mN/m or less.

In the water-absorbent sheet according to one embodiment of the present invention, the CRC (absorption capacity without pressure) of at least one of the particulate water-absorbing agent (here, a mixture of the first particulate water-absorbing agent and the second particulate water-absorbing agent) is, in the following order of preference, 30 g/g or more, 32 g/g or more, 33 g/g or more, 34 g/g or more, 35 g/g or more, 36 g/g or more, 37 g/g or more, and 38 g/g or more. In particular, when the CRC of the first particulate water-absorbing agent is 30 g/g or more or 35 g/g or more, the effect of reducing the specific return amount in the water-absorbent sheet becomes remarkable. Here, in particular, the CRC of the first particulate water-absorbing agent is preferably 30 g/g or more, and more preferably 35 g/g or more. By having such a lower limit, the effect of reducing the return can be obtained significantly. In particular, if the CRC of the second particulate water absorbing agent is less than 35 g/g, there is a risk that the effect of reducing backflow in the water absorbent sheet cannot be obtained. In the water absorbent sheet according to one embodiment of the present invention, there is no particular upper limit for the CRC of at least one of the particulate water absorbing agent (here, a mixture of the first particulate water absorbing agent and the second particulate water absorbing agent), the first particulate water absorbing agent, and the second particulate water absorbing agent, but it is preferably 40 g/g or less, 39 g/g or less.

In the water absorbent sheet according to one embodiment of the present invention, the AAP (absorption capacity under pressure) of at least one of the particulate water absorbing agent (here, a mixture of the first particulate water absorbing agent and the second particulate water absorbing agent), the first particulate water absorbing agent, and the second particulate water absorbing agent is preferably 25 g/g or more, 28 g/g or more, and 30 g/g or more in the following order of preference from the viewpoint of improving the performance of the water absorbent sheet (reducing the amount of return). There is no particular limit to the upper limit, but it is, for example, 37 g/g or less, 32 g/g or less. In the water absorbent sheet according to one embodiment of the present invention, when the AAP of the second particulate water absorbing agent is 28 g/g or more and 35 g/g or less, or less than 31 g/g, the effect of reducing the return and leakage in the surface direction in the water absorbent sheet becomes remarkable. The AAP of the particulate water absorbing agent is an abbreviation of Absorption Against Pressure defined by ERT442.2-02, and means the water absorption capacity under pressure of the particulate water absorbing agent. Specifically, it means the water absorption capacity (unit: g/g) after 0.9 g of the particulate water absorbing agent is swollen under a load of 2.06 kPa (21 g/cm ² , 0.3 psi) for 1 hour with a large excess of 0.9 mass % sodium chloride aqueous solution. Also, ERT442.2-02 refers to it as Absorption Under Pressure (AUP), but it is substantially the same.

"GPR"
Gel Permeation Rate (GPR)
In this specification, the "liquid permeability" of the particulate water absorbing agent refers to the flowability of a liquid passing between swollen gel particles under a load. As an index of this, the gel permeability rate (GPR) is used. The gel permeability rate (GPR) of the particulate water absorbing agent contained in the water absorbent sheet according to one embodiment of the present invention is measured by the following procedure, changing the measurement conditions with reference to the saline flow conductivity (SFC) test described in U.S. Pat. No. 5,849,405.

As a device for the measurement, the device 400 shown in FIG. 5 is used. The device 400 is roughly composed of a container 410 and a tank 420. The container 410 is provided with a cell 411 (inner diameter 6 cm), and the swelling gel 414 (a particulate water absorbing agent absorbed) can be stored inside the cell 411, and the liquid 423 can be introduced. In addition, by fitting a piston 412 into the cell 411, pressure can be applied to the swelling gel 414. Wire meshes 413a and 413b (No. 400 stainless steel wire mesh, mesh size 38 μm) are stretched on the bottom surface of the cell 411 and the bottom surface of the piston 412, so that the swelling gel 414 (and the particulate water absorbing agent) cannot pass through. Here, a 0.90 wt % sodium chloride aqueous solution is used as the liquid 423. The tank 420 stores the liquid 423 inside. The liquid 423 is introduced into the cell 411 through the L-shaped tube 422 with a cock. In addition, the glass tube 421 is inserted into the tank 420, and the inside of the glass tube 421 is filled with air. This allows the lower end of the glass tube 421 and the liquid level in the cell 411 to be the same. That is, while the liquid level of the liquid 423 in the tank 420 is above the lower end of the glass tube 421, it is possible to keep the liquid level in the cell 411 constant. In this measurement, the height difference between the lower liquid level of the liquid 423 in the tank 420 (i.e., the lower end of the glass tube 421) and the bottom surface of the swollen gel 414 was set to 4 cm. That is, according to the device 400, the liquid 423 with a constant hydrostatic pressure can be introduced into the cell 411. Since the piston 412 has a hole 415, the liquid 423 flows through the hole 415 and further flows through the swollen gel 414 layer, and flows out to the outside of the cell 411. The container 410 is placed on a stainless steel wire mesh 431 that does not prevent the passage of the liquid 423. Therefore, the liquid 423 flowing out of the cell 411 is finally collected in the collection container 432. The amount of the liquid 423 collected in the collection container 432 can be weighed by a top-loading balance 433.

The specific method for measuring the gel passage rate (GPR) is as follows. The following operations are performed at room temperature (20-25°C).

(1) Place particulate water absorbing agent (0.900 g) evenly into cell 411.

(2) The particulate water absorbent is allowed to absorb a 0.90% by mass aqueous solution of sodium chloride for 60 minutes without applying pressure to produce a swollen gel 414.

(3) Place a piston on top of the swollen gel 414 and apply a pressure of 0.3 psi (2.07 kPa).

(4) While maintaining the hydrostatic pressure at a constant value of 3923 dyne/cm2, liquid 423 is introduced into cell 411 and allowed to pass through the swollen gel 414 layer.

(5) The amount of liquid 423 passing through the swollen gel 414 layer is recorded at 5-second intervals for 3 minutes. In other words, the flow rate of the liquid 423 passing through the swollen gel 414 layer is measured. For the measurement, a top-loading balance 433 and a computer (not shown) are used.

(6) Average the flow rate from 1 minute to 3 minutes after the start of the flow of liquid 423, and calculate the gel permeation rate (GPR) [g/min].

In the water-absorbent sheet according to one embodiment of the present invention, the GPR of the first particulate water-absorbing agent is preferably 5 g/min or more, more preferably 35 g/min or more, and may be 55 g/min or more, 75 g/min or more, 95 g/min or more, or 110 g/min or more. In such an embodiment, the liquid to be absorbed is introduced from the first substrate side and then easily transferred to the second particulate water-absorbing agent, and the second particulate water-absorbing agent can be effectively utilized, resulting in an excellent result in the specific return amount evaluation. In the water-absorbent sheet according to one embodiment of the present invention, there is no particular limit to the upper limit of the GPR of the first particulate water-absorbing agent, but from the viewpoint of preventing side leakage, it is preferably 500 g/min or less, 400 g/min or less, 300 g/min or less, 200 g/min or less, 100 g/min or less, or 50 g/min or less.

In the water-absorbent sheet according to one embodiment of the present invention, the GPR of the second particulate water-absorbing agent is preferably 1 g/min or more, more preferably 3 g/min or more, and even more preferably 5 g/min or more. By having such a lower limit, gel blocking is suppressed, diffusibility is increased, and liquid is easily absorbed by the absorbent. In the water-absorbent sheet according to one embodiment of the present invention, there is no particular limit to the upper limit of the GPR of the second particulate water-absorbing agent, but it is 500 g/min or less, 400 g/min or less, or 300 g/min or less, 200 g/min or less, 100 g/min or less, 50 g/min or less, 30 g/min or less, 25 g/min or less, 20 g/min or less, 15 g/min or less, or 10 g/min or less. By having such an upper limit, the effect of preventing side leakage is improved.

"Particle shape"
In the water-absorbent sheet according to one embodiment of the present invention, the particulate water-absorbing agent is not limited in its particle shape, and may be, for example, a spherical particulate water-absorbing agent (and its granulated product). In a preferred embodiment, the particulate water-absorbing agent is preferably in an irregularly crushed form. Here, the irregularly crushed form refers to particles in a crushed form that are not uniform in shape. This is because the irregularly crushed form can be easily fixed to a substrate compared with spherical particles obtained by reversed-phase suspension polymerization or gas-phase polymerization. The particulate water-absorbing agent according to one embodiment of the present invention is preferably a crushed product in aqueous solution polymerization. On the other hand, in the case where the crushing step is not performed, typically spherical particles or granulated products of spherical particles obtained by reversed-phase suspension polymerization or droplet polymerization such as spraying and polymerizing a polymerization monomer are not in an irregularly crushed form. In the embodiment of the present invention, when the particulate water-absorbing agent has an irregularly crushed form, the shape of the water-absorbent sheet is more easily maintained than those having a high average roundness (for example, spherical). In the embodiment of the present invention, the average circularity of the particulate water absorbing agent is preferably 0.70 or less, more preferably 0.60 or less, and further preferably 0.55 or less.

The average circularity was calculated as follows. 100 or more particulate water absorbents were randomly selected, and each particulate water absorbent was photographed with an electron microscope (VE-9800, manufactured by Keyence Corporation) (magnification: 50x) to obtain images of the particulate water absorbent. The perimeter and area of each particle were calculated using the attached image analysis software. The following formula:

The circularity of each particle is calculated, and the average of the obtained values is calculated as the average circularity.

"Particle size"
The particle diameter of the particulate water absorbing agent (or particulate water absorbent resin, water absorbent resin particles) according to one embodiment of the present invention is a weight average particle diameter obtained according to a measurement method of “PSD” specified in ERT420.2-02, and may be 150 to 600 μm.

The method for producing the particulate water absorbing agent is not particularly limited as long as it is a method for producing a water absorbing agent having the desired physical properties, and it can be produced as appropriate by taking into consideration, for example, the publications described in the examples.

[2-3. Wrapping Sheet]
As described above, the water-absorbent sheet according to one embodiment of the present invention preferably has at least a wrapping sheet disposed on the surface of the first substrate. The wrapping sheet may be disposed on the surface of the first substrate, but it is more preferable that the wrapping sheet be disposed so as to cover the side surface of the first substrate and the side surface of the water-absorbent layer, and it is even more preferable that the wrapping sheet cover the side surface of the first substrate, the side surface of the water-absorbent layer, and the side surface of the second substrate, and cover a part or the whole surface of the second substrate opposite to the side where the liquid to be absorbed is introduced.

In one embodiment of the present invention, the thickness of the wrapping sheet is preferably 0.001 mm or more, more preferably 0.005 mm or more, even more preferably 0.01 mm or more, and particularly preferably 0.1 mm or more, for example, at 40% RH to 50% RH. The thickness of the wrapping sheet is preferably less than 0.9 mm, 0.8 mm or less, 0.7 mm or less, 0.6 mm or less, 0.5 mm or less, 0.4 mm or less, 0.3 mm or less, or 0.2 mm or less, for example, at 40% RH to 50% RH. In one embodiment of the present invention, the bulk density of the wrapping sheet is preferably 1 g/cm ³ or less, more preferably 0.5 g/cm ³ or less, and even more preferably 0.3 g/cm ³ or less. The bulk density of the wrapping sheet is preferably 0.1 g/cm ³ or more, more preferably 0.12 g/cm ³ or more, and even more preferably 0.13 g/cm ³ or more. In one embodiment of the present invention, the wrapping sheet has a basis weight of preferably 5 to 100 g/ ^m2 , more preferably 5 to 70 g/ ^m2 , and even more preferably 10 to 65 g/ ^m2 .

The thickness, bulk density, and basis weight of the wrapping sheet can be controlled by the materials that make up the wrapping sheet and the manufacturing method of the wrapping sheet, and the thickness and bulk density of the wrapping sheet are determined by the balance of these factors.

"Materials that make up wrapping sheets"
The material constituting the wrapping sheet is not particularly limited as long as it can achieve the purpose of providing the wrapping sheet described above, but examples include paper (sanitary paper, such as tissue paper, toilet paper, and towel paper), net, nonwoven fabric, woven fabric, film, etc.

The nonwoven fabric used is not particularly limited, but from the viewpoint of liquid permeability, flexibility, and strength when made into a water-absorbent sheet, examples include nonwoven fabrics made of polyolefin fibers such as polyethylene (PE) and polypropylene (PP), polyester fibers such as polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polyethylene naphthalate (PEN), polyamide fibers such as nylon, rayon fibers, and other synthetic fibers, as well as nonwoven fabrics made by mixing cotton, silk, hemp, pulp (cellulose) fibers, etc. Preferred materials for nonwoven fabrics that can be used as wrapping sheets include rayon fibers, polyolefin fibers, polyester fibers, pulp fibers, and fibers mixed with these, and polyolefin fibers are more preferred. These fibers may be hydrophilized.

The nonwoven fabric that can be used as the wrapping sheet is not particularly limited, and may be obtained by any method such as the air-through method, the airlaid method, the spunbond method, or the spunlace method, but is preferably obtained by the spunbond method (spunbond nonwoven fabric). A wrapping sheet that has water repellency is preferable so that the urine absorbed in the absorbent sheet does not seep out of the sheet (so-called backflow) even when weight is applied to the absorbent sheet (when pressure is applied), such as when an infant wearing an absorbent article such as a paper diaper is sitting on it, and for example, a spunbond nonwoven fabric is preferable. The manufacturing method of spunbond nonwoven fabric is a method in which continuous long fibers obtained by melting and spinning raw material resin are directly accumulated to form a fleece. Examples of raw material resins include polyethylene, polypropylene, and polylactic acid.

According to one embodiment of the present invention, the manufacturing method of the first substrate and the manufacturing method of the second substrate are different from the manufacturing method of the intermediate sheet and the manufacturing method of the wrapping sheet. By appropriately changing the manufacturing methods of each member constituting the water-absorbent sheet in this manner, the intended effect of the present invention can be efficiently achieved. According to one embodiment of the present invention, the first substrate and the second substrate are air-laid nonwoven fabrics, the intermediate sheet is an air-through nonwoven fabric, and the wrapping sheet is a spunbond nonwoven fabric. With this embodiment, the intended effect of the present invention can be efficiently achieved.

3. Method for producing water-absorbent sheet
The method for producing a water-absorbent sheet according to one embodiment of the present invention includes at least one of the following steps: (1) spraying a first particulate water-absorbing agent on a first substrate, (2) spraying a second particulate water-absorbing agent on a second substrate, (3) spraying the first particulate water-absorbing agent and/or the second particulate water-absorbing agent on an intermediate sheet, (4) spraying a first adhesive on the first substrate, (5) spraying a second adhesive on the second substrate, and (6) spraying at least one of the first adhesive and the second adhesive on the intermediate sheet. The following production methods (a) to (e) are given as examples of more specific production methods.

(a) A first particulate water absorbing agent (and preferably an adhesive) is evenly spread on a first substrate. An intermediate sheet is placed on top of the first particulate water absorbing agent and pressed. A second particulate water absorbing agent (and preferably an adhesive) is evenly spread on the surface of the intermediate sheet that does not face the first particulate water absorbing agent. An intermediate sheet is placed on top of the first particulate water absorbing agent and pressed (preferably under heating conditions that melt the hot melt adhesive or in a state in which the hot melt adhesive is molten).

(b) The second particulate water absorbing agent is uniformly sprayed on the intermediate sheet. Also, an adhesive is sprayed on the second substrate. Then, the surface of the intermediate sheet on which the second particulate water absorbing agent is sprayed and the surface of the second substrate on which the adhesive is sprayed are pressed together. Next, the first particulate water absorbing agent is uniformly sprayed on the surface of the intermediate sheet opposite to the surface on which the second particulate water absorbing agent is sprayed after the pressing. Also, an adhesive is sprayed on the first substrate. Then, the surface of the intermediate sheet on which the first particulate water absorbing agent is sprayed and the surface of the first substrate on which the adhesive is sprayed are pressed together. The pressing is preferably performed under heating conditions in which the hot melt adhesive melts, or in a state in which the hot melt adhesive is molten.

(c) Spray adhesive onto the second substrate. Next, evenly spray the second particulate water absorbing agent onto the adhesive. Next, place an intermediate sheet on top of the adhesive and press it together. Next, spray adhesive onto the surface of the intermediate sheet that does not face the second particulate water absorbing agent. Next, evenly spray the first particulate water absorbing agent onto the adhesive. Next, place the first substrate on top of the adhesive and press it together. The above pressing is preferably performed under heating conditions that melt the hot melt adhesive, or while the hot melt adhesive is in a molten state.

(d) Spray adhesive onto the second substrate. Next, evenly spray the second particulate water absorbing agent onto the adhesive. Next, place an intermediate sheet on top and press. Next, evenly spray the first particulate water absorbing agent onto the surface of the intermediate sheet that does not face the second particulate water absorbing agent. Also, spray adhesive onto the first substrate. Then, press the surface of the intermediate sheet on which the first particulate water absorbing agent has been sprayed onto the surface of the first substrate on which the adhesive has been sprayed. The pressing is preferably performed under heating conditions that melt the hot melt adhesive, or while the hot melt adhesive is in a molten state.

(e) An intermediate sheet is placed on the first substrate, and the second particulate water absorbing agent is sprayed on the surface of the intermediate sheet. Next, the second adhesive is sprayed on the surface of the second substrate. Then, the surface of the intermediate sheet on which the second particulate water absorbing agent is sprayed is placed on the surface of the second substrate on which the second adhesive is sprayed, and the two substrates are pressed together. Then, the first substrate is peeled off from the intermediate sheet. Next, the first particulate water absorbing agent is sprayed on the surface of the intermediate sheet opposite to the surface on which the second particulate water absorbing agent is disposed. Next, the first adhesive is sprayed on the surface of the peeled off first substrate. Then, the first substrate is pressed together with pressure so that the surface of the intermediate sheet on which the first particulate water absorbing agent is sprayed and the surface of the intermediate sheet on which the first adhesive is sprayed face each other to obtain a water absorbent sheet.

In addition, in an embodiment in which the absorbent sheet is provided with a wrapping sheet as shown in Figures 2 to 4, the manufacturing method of the absorbent sheet in one embodiment of the present invention includes a step of covering the first substrate, the absorbent layer, and the second substrate with the wrapping sheet. For example, a first' adhesive is sprayed as necessary on the surface of the first substrate on the side where the aqueous liquid is introduced, and the wrapping sheet is placed on at least a portion of the surface to cover the first substrate, the absorbent layer, and the second substrate.

In addition, in the case of water-absorbent sheets, methods for bonding substrates to each other, substrate and particulate water-absorbing agent, and substrate and wrapping sheet include bonding with water, water-soluble polymers, and various binders dissolved or dispersed in a solvent, and heat-sealing substrates to each other at the melting point of the material of the substrate itself. However, as mentioned above, the method of using an adhesive and pressing as necessary is preferable.

In one embodiment of the present invention, the adhesive is placed at least in one of the following locations: the surface of the first substrate facing the second substrate, the surface of the second substrate facing the first substrate, between the wrapping sheet and the first substrate, and between the second substrate and the wrapping sheet.

In one embodiment of the present invention, the content (non-volatile content, amount used) of the adhesive (solution type) disposed on the surface of the first substrate facing the second substrate, i.e., the first adhesive (solution type), is preferably 0.0005 to 0.3 times, 0.001 to 0.25 times, 0.01 to 0.2 times, 0.02 to 0.18 times, 0.02 to 0.15 times, or 0.025 to 0.12 times the mass of the first particulate water absorbing agent. This embodiment has a technical effect of efficiently controlling the initial absorption speed within a desired range. In one embodiment of the present invention, when the first adhesive is styrene butadiene rubber, the content (non-volatile content, amount used) is preferably 0.02 to 0.18 times the mass of the first particulate water absorbing agent. In one embodiment of the present invention, when the first adhesive is styrene butadiene rubber and the basis weight of the first substrate is 45 g/m ² or more, the content (non-volatile content, amount used) is preferably 0.03 times or more, 0.04 times or more, or 0.05 times or more, and preferably 0.1 times or less, 0.08 times or less, 0.075 times or less, or less than 0.07 times, relative to the mass of the first particulate water absorbing agent. In one embodiment of the present invention, when the first adhesive is styrene butadiene rubber and the basis weight of the first substrate is less than 45 g/m ² , the content (non-volatile content, amount used) is preferably 0.07 times or more, 0.08 times or more, or 0.09 times or more, and preferably 0.18 times or less, or 0.15 times or less, relative to the mass of the first particulate water absorbing agent. In one embodiment of the present invention, in the case where the first adhesive is an ethylene-vinyl acetate copolymer and the basis weight of the first base material is 45 g/m2 or ^more , the content (non-volatile content, amount used) thereof is preferably 0.01 times or more, or 0.015 times or more, and is preferably 0.06 times or less, 0.05 times or less, 0.04 times or less, or 0.03 times or less, relative to the mass of the first particulate water absorbing agent.

In one embodiment of the present invention, the content (non-volatile content, amount used) of the adhesive (hot melt type) placed on the surface of the first substrate facing the second substrate, i.e., the first adhesive (hot melt type), is, in order of preference, 0.0005 to 0.3 times, 0.001 to 0.25 times, 0.003 to 0.2 times, 0.004 to 0.2 times, 0.005 to 0.1 times, 0.005 to 0.05 times, 0.005 to 0.03 times, 0.006 to 0.03 times, 0.007 to 0.02 times, and 0.0075 to 0.015 times the mass of the first particulate water absorbing agent. This embodiment has the technical effect of efficiently controlling the initial absorption rate within a desired range.

Adjusting the amount of adhesive can be one of the important factors for adjusting the initial water absorption rate to a specific range, so it is important to determine the initial water absorption rate to be in a specific range.

In one embodiment of the present invention, the content (non-volatile content) of the adhesive (solution type) placed on the surface of the second substrate facing the first substrate, i.e., the second adhesive (solution type), is preferably 0.0005 to 0.3 times, 0.001 to 0.25 times, 0.01 to 0.2 times, 0.02 to 0.18 times, 0.02 to 0.15 times, or 0.025 to 0.12 times the mass of the second particulate water absorbing agent, in order of preference. This embodiment has the technical effect of efficiently controlling the initial absorption rate within a desired range.

In one embodiment of the present invention, the content (non-volatile content, amount used) of the adhesive (hot melt type) placed on the surface of the second substrate facing the first substrate, i.e., the second adhesive (hot melt type), is, in order of preference, 0.0005 to 0.3 times, 0.001 to 0.25 times, 0.003 to 0.2 times, 0.005 to 0.1 times, 0.006 to 0.05 times, or 0.007 to 0.02 times the mass of the second particulate water absorbing agent. This embodiment has the technical effect of efficiently controlling the initial absorption rate within a desired range.

In one embodiment of the present invention, the content (non-volatile content, amount used) of the adhesive placed between the wrapping sheet and the first base material, i.e., the first' adhesive, is preferably 0 to 0.1 times, more preferably 0 to 0.05 times, and even more preferably 0 to 0.03 times, the mass of the first particulate water absorbing agent. This embodiment has the technical effect of efficiently controlling the initial absorption rate within a desired range. Note that, in order to more efficiently achieve the desired effect of the present invention (particularly the effect of reducing the amount of return), it is preferable that the first' adhesive is not placed.

Therefore, in one embodiment of the present invention, the wrapping sheet is placed on the surface of the first substrate on the side where the aqueous liquid is introduced, and there is substantially no adhesive between the wrapping sheet and the first substrate. Adjusting the amount of adhesive can be one of the important factors for adjusting the initial water absorption rate to a specific range, so it is important to determine the initial water absorption rate to be within a specific range.

In one embodiment of the present invention, the adhesive placed between the wrapping sheet and the second substrate, i.e., the second' adhesive, may be used to the extent that it does not impede the object of the present invention. It is preferable not to use an adhesive. When an adhesive is used, the content (non-volatile content, amount used) of the adhesive is preferably 0 to 0.01 times, more preferably 0 to 0.008 times, and even more preferably 0 to 0.001 times the mass of the second particulate water absorbing agent. This embodiment has the technical effect of efficiently controlling the initial absorption rate to a desired range.

In one embodiment of the present invention, the adhesive may be a solution-based adhesive. A solution-based adhesive refers to an adhesive in which the adhesive is dissolved or dispersed in a solvent, and in one embodiment of the present invention, the solvent of the solution-based adhesive contains at least one of hexane selected from isohexane, cyclohexane, and normal hexane, and at least one of ethers such as dimethyl ether. The structure and composition of the main component of the solution-based adhesive are the same as those of the hot melt-based adhesive described below, with the difference being that the adhesive is in a pre-dissolved liquid state or a solid state.

Since it is necessary to remove the solvent after spraying, it is preferable to use a volatile organic compound with a low boiling point as the solvent for solution-based adhesives. Due to the time required for removing the solvent, the odor caused by the remaining solvent, and the need to improve the working environment for manufacturing sheets, hot-melt adhesives that do not contain substantially volatile organic compounds are preferable. When spraying solution-based adhesives, the solvent concentration is usually higher than that of heat-melt (hot-melt) adhesives. This is because a large amount of solvent is used to lower the viscosity of the adhesive resin and make it sprayable. In contrast, heat-melt adhesives are sprayed at high temperatures, and the viscosity drop due to the high temperature is large, which results in a lower solvent concentration.

It is preferable that the adhesive contained in the sheet contains a small amount of volatile organic compounds. Specifically, it is most preferable that the volatile organic compounds are below the detection limit. In one embodiment of the present invention, the volatile organic compounds remaining in the adhesive are below the detection limit. The adhesive containing volatile organic compounds below the detection limit may be at least one of the first adhesive, the second adhesive, the first' adhesive, and the second' adhesive. However, if the volatile organic compounds in the first adhesive are below the detection limit, the desired effect of the present invention can be achieved more efficiently. A volatile organic compound below the detection limit means 1 ppm or less of the adhesive. The remaining volatile organic compounds are measured using gas chromatography or the like.

The form and melting point of the hot melt adhesive can be selected as appropriate, and it may be in particulate, fibrous, net, or film form, or it may be in liquid form melted by heating.

The melting temperature or softening point of the hot melt adhesive is preferably 50 to 200°C, or 60 to 180°C. When a particulate adhesive is used, the particle diameter is about 0.01 to 2 times, 0.02 to 1 times, or 0.05 to 0.5 times the average particle diameter of the particulate water absorbent.

The hot melt adhesive used in the present invention can be appropriately selected, but preferably, one or more selected from ethylene-vinyl acetate copolymer adhesives, styrene-based elastomer adhesives, polyolefin-based adhesives, polyester-based adhesives, etc. can be appropriately used. Specifically, polyolefin-based adhesives include polyethylene, polypropylene, atactic polypropylene, and copolymerized polyolefins. Styrene-based elastomer adhesives include styrene-isoprene block copolymers (SIS), styrene-butadiene block copolymers (SBS), styrene-isobutylene block copolymers (SIBS), styrene-ethylene-butylene-styrene block copolymers (SEBS), styrene butadiene rubber (SBR), etc.; polyester-based adhesives include polyethylene terephthalate (PET), polybutylene terephthalate (PBT), copolymerized polyesters, etc.; ethylene-vinyl acetate copolymer adhesives include ethylene-vinyl acetate copolymer (EVA) adhesives; ethylene-ethyl acrylate copolymers (EEA), ethylene-butyl acrylate copolymers (EBA), etc. The selection of the adhesive can be one of the important factors for adjusting the initial water absorption rate to a specific range, so it is important to determine the initial water absorption rate to be within a specific range. In selecting an adhesive, a hot melt adhesive is preferable to a solution type adhesive because it can improve the leakage and backflow amounts of the obtained sheet even if the adhesive content ratio to the mass of the particulate water absorbent is low (even if only a small amount is used in sheet production). The assumed mechanism of action is that the hot melt adhesive is effectively attached (distributed unevenly) on the surface of the substrate because it is difficult for the adhesive to penetrate into the substrate.

In the method for producing a water-absorbent sheet according to one embodiment of the present invention, additives (such as deodorants, fibers, antibacterial agents, and gel stabilizers) may be appropriately blended. The amount of additives blended is preferably 0 to 50% by mass, and more preferably 0 to 10% by mass, relative to the mass of the particulate water-absorbing agent. In the above-mentioned production method, a particulate water-absorbing agent in which additives have been mixed in advance may be used, or additives may be added during the production process.

The dimensions of the absorbent sheet produced can be designed as appropriate. Typically, the width is 10 cm to 10 m, and the length is several tens to several thousand meters (in the form of a continuous sheet or roll). The absorbent sheet produced is cut according to the purpose (the size of the absorbent body to be used) and used.

In addition to the above examples, methods for producing absorbent sheets are disclosed, for example, in the following patent documents: WO 2012/174026, WO 2013/078109, WO 2015/041784, WO 2011/117187, WO 2012/001117, WO 2012/024445, WO 2010/004894, WO 2010/004895, WO 2010/076857, WO 2010/082373, WO 2010/113754, WO 2010/143635, WO 2011/043256, WO 2011/0 86841, WO 2011/086842, WO 2011/086843, WO 2011/086844, WO 2011/117997, WO 2011/118409, WO 2011/136087, WO 2012/043546, WO 2013/09 No. 9634, International Publication No. 2013/099635, JP 2010-115406, JP 2002-345883, JP 6-315501, JP 6-190003, JP 6-190002, JP 6-190001, JP 2-252558, JP 2-252560, and JP 2-252561. The methods of manufacturing water-absorbent sheets disclosed in these documents are also referred to as appropriate.

<Particularly preferred combinations of constituent elements of water-absorbent sheet>
The embodiments disclosed in this specification provide descriptions of particularly preferred combinations, which must be considered as all combinations being disclosed (i.e., as a basis for the legality of amendments in each country).

(Especially preferred embodiment)
The CRC of the first particulate water absorbing agent is 30 g/g or more, or 35 g/g or more; the CRC of the second particulate water absorbing agent is 35 g/g or more, 36 g/g or more, particularly 37 g/g or more, and 40 g/g or less, or 39 g/g or less; the surface tension of the particulate water absorbing agent (particularly, the second particulate water absorbing agent) is 60 mN/m or more, particularly 65 mN/m or more; both the first base material and the second base material are produced by an airlaid method using pulp fiber as a main component; the adhesive is applied by hot melt spraying (hot melt spraying (spraying)); and the first' adhesive is substantially absent.

The CRC of the first particulate water absorbing agent is 30 g/g or more, or 35 g/g or more; the GPR is 35 g/min or more and 200 g/min or less, 100 g/min or less, particularly 50 g/min or less; the CRC of the second particulate water absorbing agent is 35 g/g or more, 36 g/g or more, particularly 37 g/g or more, and 40 g/g or less, or 39 g/g or less; the AAP is 25 g/g or more, 28 g/g or more, or 30 g/g or more; or less or 32 g/g or less; GPR is 5 g/min or more, 30 g/min or less, 25 g/min or less, particularly 10 g/min or less; surface tension is 60 mN/m or more, particularly 65 mN/m or more; both the first substrate and the second substrate are made by the airlaid method with pulp fiber as the main component; the adhesive is applied by hot melt spraying (hot melt spraying (spraying)); and the first' adhesive is substantially absent.

In the particularly preferred embodiment described above, the surface tension of the particulate water absorbing agent (particularly the second particulate water absorbing agent) is 65 mN/m or more.

In the particularly preferred embodiment described above, the CRC of the first particulate water absorbing agent is 35 g/g or more.

In the particularly preferred embodiment, the CRC of the second particulate water absorbing agent is 36 g/g or more and 39 g/g or less.

In the particularly preferred embodiment described above, there is substantially no adhesive between the wrapping sheet and the first substrate.

In the particularly preferred embodiment, the amount of volatile organic compounds remaining in the first adhesive is below the detection limit.

4. Absorbent Articles
The absorbent article according to one embodiment of the present invention has a structure in which the water-absorbing sheet described in [2] is sandwiched between a liquid-permeable sheet and a liquid-impermeable sheet. Here, the liquid-permeable sheet is located on the first substrate side, and the liquid-impermeable sheet is located on the second substrate side. Thus, the absorbent article according to one embodiment of the present invention is an absorbent article that is sandwiched between a liquid-permeable sheet and a liquid-impermeable sheet, the liquid-permeable sheet being located on the first substrate side, and the liquid-impermeable sheet being located on the second substrate side. Specific examples of absorbent articles include paper diapers, incontinence pads, sanitary napkins, pet sheets, food drip sheets, and waterproofing agents for power cables.

As the liquid-permeable sheet and the liquid-impermeable sheet, any sheet known in the technical field of absorbent articles can be used without particular restrictions. In addition, the absorbent article can be manufactured by a known method.

The present invention will be described in more detail using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples. In addition, in the following manufacturing examples, measurement conditions, examples, and comparative examples, operations were performed under room temperature (25°C)/relative humidity conditions of 40-50% RH unless otherwise specified.

<Production Example>
By appropriately adjusting the CRC by the amount of the internal crosslinking agent with reference to the manufacturing examples, examples, and comparative examples described in the following patents, particulate water absorbing agents (1) to (7) of polyacrylic acid (salt) resin were obtained. The physical properties of the obtained particulate water absorbing agents are shown in Table 1.

International Publication No. WO 2014/034897, International Publication No. WO 2017/170605, International Publication No. WO 2016/204302, International Publication No. WO 2014/054656, International Publication No. WO 2015/152299, International Publication No. WO 2018/062539, International Publication No. WO 2012/043821.

[Method of measuring physical properties of particulate water absorbing agent]
<Weight average particle size>
The particle diameter of the particulate water absorbing agent (or particulate water absorbent resin, water absorbent resin particles) according to one embodiment of the present invention is a weight average particle diameter obtained according to the measurement method of "PSD" specified in ERT420.2-02. Here, in this specification, the measurement method of the weight average particle diameter is calculated in the same manner as "(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Distribution" described in U.S. Patent No. 7,638,570 based on the PSD obtained according to the measurement method of "PSD" specified in ERT420.2-02.

The weight average particle size of each particulate water absorbent is shown in Table 1.

<CRC (Water absorption capacity without pressure) (ERT441.2-02)>
"CRC" is an abbreviation for Centrifuge Retention Capacity, and means the water absorption capacity under no load of a particulate water absorbing agent (sometimes referred to as "water absorption capacity"). Specifically, 0.2 g of the particulate water absorbing agent was placed in a nonwoven bag, and then immersed in a large excess of 0.9% by mass sodium chloride aqueous solution for 30 minutes to allow free swelling, and then centrifuged (250 G) for 3 minutes. This refers to the water absorption capacity (unit: g/g) after draining.

<Surface tension>
In the present invention, the surface tension means the surface tension of an aqueous solution when the particulate water absorbing agent is dispersed in a 0.90 mass % sodium chloride aqueous solution.

50 ml of physiological saline adjusted to 20°C is placed in a thoroughly washed 100 ml beaker, and the surface tension of the physiological saline is measured using a surface tensiometer (K11 automatic surface tensiometer manufactured by KRUSS). In the present invention, a plate method using a platinum plate is adopted, and the plate is thoroughly washed with deionized water before each measurement, and heated and washed with a gas burner before use. Next, a thoroughly washed 25 mm long fluororesin rotor and 0.5 g of particulate water absorbing agent are placed in a beaker containing the physiological saline adjusted to 20°C after the surface tension measurement, and stirred at 500 rpm for 4 minutes. After 4 minutes, the stirring is stopped, and after the water-absorbed particulate water absorbing agent has settled, the surface tension of the supernatant liquid is measured again by performing the same operation. Here, the surface tension of the particulate water absorbing agent (unit: mN/m) is obtained from the surface tension of the supernatant liquid when the particulate water absorbing agent is dispersed in the physiological saline. The surface tension of each particulate water absorbent is shown in Table 1.

[Production Example of Acrylic Acid]
Commercially available acrylic acid (2000 ppm acrylic acid dimer, 500 ppm acetic acid, 500 ppm propionic acid, 200 ppm p-methoxyphenol ) was supplied to the bottom of a high-boiling impurity separation column having 50 perforated trays without downcomer, distilled at a reflux ratio of 1, and after removing dimers (acrylic acid dimers) composed of maleic acid and acrylic acid, crystallization was further carried out to obtain acrylic acid (20 ppm acrylic acid dimer, 50 ppm acetic acid, 50 ppm propionic acid, 1 ppm or less furfural, 1 ppm or less protoanemonin), and 50 ppm p-methoxyphenol was added after the distillation.

[Method for producing aqueous sodium acrylate solution]
1,390 g of the above acrylic acid was neutralized with 48% caustic soda at 20-40° C. according to Example 9 of US Pat. No. 5,210,298 to obtain an aqueous solution of sodium acrylate having a concentration of 37% and being 100% neutralized.

<Particulate water absorbing agent (1)>
5.03 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 9) was dissolved in 5500 g (monomer concentration: 38.0% by mass) of an aqueous solution of sodium acrylate having a neutralization rate of 75 mol% obtained by mixing the acrylic acid obtained in the above-mentioned acrylic acid production example, the aqueous sodium acrylate solution obtained by the above-mentioned method for producing an aqueous sodium acrylate solution using the acrylic acid, and deionized water to obtain a reaction solution. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having an internal volume of 10 L and two sigma-type blades, and the system was replaced with nitrogen gas while maintaining the reaction solution at 30°C to remove dissolved oxygen in the reaction solution. Next, 30.68 g of a 10% by mass aqueous solution of sodium persulfate and 37.76 g of a 1% by mass aqueous solution of L-ascorbic acid were added while stirring the reaction solution, and polymerization started about 1 minute later. 40 minutes after the start of polymerization, the hydrogel polymer was taken out. The obtained hydrogel polymer was fragmented into particles of about 2 to 4 mm. The fragmented hydrogel polymer was spread on a wire net with a mesh size of 50 mesh (300 μm) and dried with hot air at 175° C. for 65 minutes. The dried product was then pulverized using a roll mill, and further classified and mixed using a wire net with a mesh size of 600 μm to obtain an irregularly pulverized water absorbent resin (1-1) having an average particle size of 380 μm.

100 parts by mass of the obtained water absorbent resin (1-1) was mixed with 3.83 parts by mass of a surface cross-linking agent aqueous solution consisting of 0.03 parts by mass of ethylene glycol diglycidyl ether, 0.3 parts by mass of 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 3.0 parts by mass of water by spray mixing. The above mixture was heated for 45 minutes at a heat medium temperature of 210°C using a paddle-type mixing and heating machine to obtain a surface cross-linked water absorbent resin (1-2). 100 parts by mass of the obtained surface cross-linked water absorbent resin (1-2) was mixed with 1.0 part by mass of water by spray mixing, cured at 60°C for 1 hour in a sealed container, and then passed through a sieve with a mesh size of 710 μm to obtain a water absorbent resin (1-3). 0.3 parts by mass of Aerosil 200 (hydrophilic amorphous silica, manufactured by Nippon Aerosil Co., Ltd.) was added to the water absorbent resin (1-3) and mixed to obtain a water absorbent resin as a particulate water absorbent (1).

<Particulate water absorbing agent (2)>
The acrylic acid obtained in the above-mentioned acrylic acid production example, the aqueous sodium acrylate solution obtained by the above-mentioned method for producing an aqueous sodium acrylate solution using the acrylic acid, and deionized water were mixed to obtain an aqueous sodium acrylate solution having a neutralization rate of 75 mol%, and 5500 g (monomer concentration 36.0% by mass) was dissolved in 3.93 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 9) to obtain a reaction solution. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having an internal volume of 10 L and two sigma-type blades, and the system was replaced with nitrogen gas while maintaining the reaction solution at 30°C, and dissolved oxygen in the reaction solution was removed. Next, 29.07 g of a 10% by mass aqueous solution of sodium persulfate and 35.78 g of a 1% by mass aqueous solution of L-ascorbic acid were added while stirring the reaction solution, and polymerization started about 1 minute later. 40 minutes after the start of polymerization, the hydrogel polymer was taken out. The obtained hydrogel polymer was fragmented into particles of about 2 to 4 mm. The fragmented hydrogel polymer was spread on a wire net with a mesh size of 50 mesh (300 μm) and dried with hot air at 175° C. for 65 minutes. The dried product was then pulverized using a roll mill, and further classified and mixed using a wire net with a mesh size of 600 μm to obtain an irregularly pulverized water absorbent resin (2-1) having an average particle size of 350 μm.

100 parts by mass of the obtained water absorbent resin (2-1) was mixed by spraying with 4.03 parts by mass of a surface cross-linking agent aqueous solution consisting of 0.03 parts by mass of ethylene glycol diglycidyl ether, 1.0 parts by mass of propylene glycol, and 3.0 parts by mass of water. The above mixture was heated for 40 minutes at a heat medium temperature of 100°C using a paddle-type mixing and heating machine to obtain a surface cross-linked water absorbent resin (2-2). 100 parts by mass of the obtained surface cross-linked water absorbent resin (2-2) was mixed by spraying with 3.0 parts by mass of water, cured at 60°C for 1 hour in a sealed container, and then passed through a sieve with a mesh size of 710 μm to obtain a water absorbent resin (2-3). 0.3 parts by mass of Aerosil 90G (hydrophilic amorphous silica, manufactured by Nippon Aerosil Co., Ltd.) was added to the water absorbent resin (2-3) and mixed to obtain a water absorbent resin as a particulate water absorbent (2).

<Particulate water absorbing agent (3)>
The acrylic acid obtained in the above-mentioned acrylic acid production example, the aqueous sodium acrylate solution obtained by the above-mentioned method for producing an aqueous sodium acrylate solution using the acrylic acid, and deionized water were mixed to obtain an aqueous sodium acrylate solution having a neutralization rate of 75 mol%, and 5,500 g (monomer concentration 33.0% by mass) was dissolved in 5.82 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 9) to obtain a reaction solution. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having an internal volume of 10 L and two sigma-type blades, and the system was replaced with nitrogen gas while maintaining the reaction solution at 30°C, and dissolved oxygen in the reaction solution was removed. Next, 26.65 g of a 10% by mass aqueous solution of sodium persulfate and 32.79 g of a 1% by mass aqueous solution of L-ascorbic acid were added while stirring the reaction solution, and polymerization started about 1 minute later. After 40 minutes from the start of polymerization, 181.5 g of water-absorbent resin fine powder of 150 μm or less was added, and the kneader blade was rotated at high speed (130 rpm) for 10 minutes to break down the gel, and then the hydrogel polymer was taken out. The obtained hydrogel polymer was fragmented into particles of about 1 to 2 mm. The fragmented hydrogel polymer was spread on a wire mesh of 50 mesh (300 μm mesh size) and dried with hot air at 175° C. for 65 minutes. The dried product was then pulverized using a roll mill, and further classified and mixed using a wire mesh with 600 μm mesh size to obtain an irregularly pulverized water-absorbent resin (3-1) with an average particle size of 350 μm.

100 parts by mass of the obtained water-absorbent resin (3-1) was mixed with 3.83 parts by mass of a surface-crosslinking agent aqueous solution consisting of 0.03 parts by mass of ethylene glycol diglycidyl ether, 0.3 parts by mass of 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 3.0 parts by mass of water, by spraying. The above mixture was heated for 40 minutes at a heat medium temperature of 210°C using a paddle-type mixing and heating machine to obtain a surface-crosslinked water-absorbent resin (3-2). 100 parts by mass of the obtained surface-crosslinked water-absorbent resin (3-2) was mixed with 1.0 part by mass of water by spraying, cured at 60°C for 1 hour in a sealed container, and then passed through a sieve with a mesh size of 710 μm to obtain a water-absorbent resin (3-3). 0.3 parts by mass of Aerosil 200 (hydrophilic amorphous silica, manufactured by Nippon Aerosil Co., Ltd.) was added to the water-absorbent resin (3-3) and mixed to obtain a water-absorbent resin as a particulate water-absorbent agent (3).

<Particulate water absorbing agent (4)>
The acrylic acid obtained in the above-mentioned acrylic acid production example, the aqueous sodium acrylate solution obtained by the above-mentioned method for producing an aqueous sodium acrylate solution using the acrylic acid, and deionized water were mixed to obtain an aqueous sodium acrylate solution having a neutralization rate of 75 mol%, and 5500 g (monomer concentration 33.0% by mass) was dissolved in 4.04 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 9) to obtain a reaction solution. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having an internal volume of 10 L and two sigma-type blades, and the system was replaced with nitrogen gas while maintaining the reaction solution at 30°C, and dissolved oxygen in the reaction solution was removed. Next, 26.65 g of a 10% by mass aqueous solution of sodium persulfate and 32.79 g of a 1% by mass aqueous solution of L-ascorbic acid were added while stirring the reaction solution, and polymerization started about 1 minute later. After 40 minutes from the start of polymerization, 181.5 g of water-absorbent resin fine powder of 150 μm or less was added, and the kneader blade was rotated at high speed (130 rpm) for 10 minutes to break down the gel, and then the hydrogel polymer was taken out. The obtained hydrogel polymer was fragmented into particles of about 1 to 2 mm. The fragmented hydrogel polymer was spread on a wire mesh of 50 mesh (300 μm mesh size) and dried with hot air at 175° C. for 65 minutes. The dried product was then pulverized using a roll mill, and further classified and mixed using a wire mesh with 600 μm mesh size to obtain an irregularly pulverized water-absorbent resin (4-1) with an average particle size of 350 μm.

100 parts by mass of the obtained water absorbent resin (4-1) was mixed with 3.83 parts by mass of a surface cross-linking agent aqueous solution consisting of 0.03 parts by mass of ethylene glycol diglycidyl ether, 0.3 parts by mass of 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 3.0 parts by mass of water by spray mixing. The above mixture was heat-treated for 40 minutes at a heat medium temperature of 195°C using a paddle-type mixing heat treatment machine to obtain a surface-cross-linked water absorbent resin (4-2). 100 parts by mass of the obtained surface-cross-linked water absorbent resin (4-2) was mixed with 1.0 part by mass of water by spray mixing, and the mixture was cured for 1 hour at 60°C in a sealed container, and then passed through a sieve with a mesh size of 710 μm to obtain a water absorbent resin (4-3). 0.3 parts by mass of Aerosil 200 (hydrophilic amorphous silica, manufactured by Nippon Aerosil Co., Ltd.) was added to the water absorbent resin (9-3) and mixed to obtain a water absorbent resin as a particulate water absorbent (4).

<Particulate water absorbing agent (5)>
5.29 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 9) was dissolved in 5500 g (monomer concentration: 35.5% by mass) of an aqueous solution of sodium acrylate having a neutralization rate of 75 mol% obtained by mixing the acrylic acid obtained in the above-mentioned acrylic acid production example, the aqueous sodium acrylate solution obtained by the above-mentioned method for producing an aqueous sodium acrylate solution using the acrylic acid, and deionized water to obtain a reaction solution. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having an internal volume of 10 L and two sigma-type blades, and the system was replaced with nitrogen gas while maintaining the reaction solution at 30° C. to remove dissolved oxygen in the reaction solution. Next, 28.66 g of a 10% by mass aqueous solution of sodium persulfate and 35.28 g of a 1% by mass aqueous solution of L-ascorbic acid were added while stirring the reaction solution, and polymerization started about 1 minute later. 40 minutes after the start of polymerization, a hydrogel polymer was taken out. The obtained hydrogel polymer was fragmented into particles of about 2 to 4 mm. The fragmented hydrogel polymer was spread on a wire net with a mesh size of 50 mesh (300 μm) and dried with hot air at 175° C. for 65 minutes. The dried product was then pulverized using a roll mill, and further classified and mixed using a wire net with a mesh size of 600 μm to obtain an irregularly pulverized water absorbent resin (5-1) having an average particle size of 350 μm.

100 parts by mass of the obtained water absorbent resin (5-1) was mixed with 4.03 parts by mass of a surface cross-linking agent aqueous solution consisting of 0.03 parts by mass of ethylene glycol diglycidyl ether, 1.0 parts by mass of propylene glycol, and 3.0 parts by mass of water by spray mixing. The above mixture was heat-treated for 40 minutes at a heat medium temperature of 100°C using a paddle-type mixing heat treatment machine to obtain a surface-cross-linked water absorbent resin ( 5-2 ). 100 parts by mass of the obtained surface-cross-linked water absorbent resin (5-2) was mixed with 3.0 parts by mass of water by spray mixing, and the mixture was cured at 60°C for 1 hour in a sealed container, and then passed through a sieve with an opening of 710 μm to obtain a water absorbent resin (5-3). 0.3 parts by mass of Aerosil 90G (hydrophilic amorphous silica, manufactured by Nippon Aerosil Co., Ltd.) was added to the water absorbent resin (5-3) and mixed to obtain a water absorbent resin as a particulate water absorbent (5).

<Particulate water absorbing agent (6)>
The acrylic acid obtained in the above-mentioned acrylic acid production example, the aqueous sodium acrylate solution obtained by the above-mentioned method for producing an aqueous sodium acrylate solution using the acrylic acid, and deionized water were mixed to obtain an aqueous sodium acrylate solution having a neutralization rate of 75 mol%, and 5500 g (monomer concentration 38.0 mass%) was dissolved in 3.27 g of polyethylene glycol diacrylate (average number of moles of ethylene oxide added: 9) to obtain a reaction solution. Next, the reaction solution was supplied to a reactor formed by attaching a lid to a jacketed stainless steel double-arm kneader having an internal volume of 10 L and two sigma-type blades, and the system was replaced with nitrogen gas while maintaining the reaction solution at 30°C, and dissolved oxygen in the reaction solution was removed. Next, 30.68 g of a 10% by mass aqueous solution of sodium persulfate and 37.76 g of a 1% by mass aqueous solution of L-ascorbic acid were added while stirring the reaction solution, and polymerization started about 1 minute later. 40 minutes after the start of polymerization, the hydrogel polymer was taken out. The obtained hydrogel polymer was fragmented into particles of about 2 to 4 mm. The fragmented hydrogel polymer was spread on a wire net with a mesh size of 50 mesh (300 μm) and dried with hot air at 175° C. for 65 minutes. The dried product was then pulverized using a roll mill, and further classified and mixed using a wire net with a mesh size of 600 μm to obtain an irregularly pulverized water absorbent resin (6-1) having an average particle size of 350 μm.

100 parts by mass of the obtained water absorbent resin (6-1) was mixed with 3.83 parts by mass of a surface cross-linking agent aqueous solution consisting of 0.03 parts by mass of ethylene glycol diglycidyl ether, 0.3 parts by mass of 1,4-butanediol, 0.5 parts by mass of propylene glycol, and 3.0 parts by mass of water by spray mixing. The above mixture was heated for 45 minutes at a heat medium temperature of 195°C using a paddle-type mixing heat treatment machine to obtain a surface cross-linked water absorbent resin (6-2). 100 parts by mass of the obtained surface cross-linked water absorbent resin (6-2) was mixed with 1.0 part by mass of water by spray mixing, cured at 60°C for 1 hour in a sealed container, and then passed through a sieve with a mesh size of 710 μm to obtain a water absorbent resin (6-3). 0.3 parts by mass of Aerosil 200 (hydrophilic amorphous silica, manufactured by Nippon Aerosil Co., Ltd.) was added to the water absorbent resin (6-3) and mixed to obtain a water absorbent resin as a particulate water absorbent (6).

<Particulate water absorbing agent (7)>
[Removal of water-absorbent resin from commercially available disposable diapers]
A water-absorbing resin was taken out from a commercially available disposable diaper (NEW Doremi Pants (L size, Lot No. 8-1A042 1489), manufactured by OJI, purchased in 2001) that had been stored in a humidity-controlled environment. When taking out, only the water-absorbing resin was taken out so as not to be mixed with cotton-like pulp or the like. The water-absorbing resin taken out had a particle shape obtained by granulating spherical particles. This water-absorbing resin was designated as a particulate water-absorbing agent (7). The physical properties of the particulate water-absorbing agent (7) are shown in Table 1.

[Example]
In the following Examples and Comparative Examples, unless otherwise specified, operations were carried out under the conditions of room temperature (25° C.) and relative humidity of 40 to 50% RH.

[Example 1]
At room temperature, a nonwoven fabric A (corresponding to the first base material, mainly composed of pulp fiber, thickness 0.4 mm, produced by the airlaid method, basis weight: 47 g/ ^m2 ) was cut to 10 cm length and 40 cm width, on which an air-through nonwoven fabric C (corresponding to the intermediate sheet) mainly composed of olefin and having a thickness of 1.4 mm was placed, and 6 g (spreading amount: 150 g/ ^m2 ) of particulate water absorbing agent (2) (corresponding to the second particulate water absorbing agent) was uniformly spread on the surface of the nonwoven fabric C. The reason why the particulate water absorbing agent (2) was spread on the surface of the nonwoven fabric C while the nonwoven fabric A was placed under the air-through nonwoven fabric C is that the particulate water absorbing agent (2) that can pass through the nonwoven fabric C without being captured by the nonwoven fabric C is to be received by the nonwoven fabric A.

Next, 0.5 g of an adhesive (solution-type adhesive) (corresponding to the second adhesive) (Spray Glue 77, manufactured by 3M Japan Ltd.) containing a solvent mainly composed of a mixture of isohexane, cyclohexane, and normal hexane, liquefied petroleum gas, dimethyl ether, and other gases, and styrene-butadiene rubber, was evenly sprayed onto the surface of nonwoven fabric A (different from the ^above ) (corresponding to the second adhesive, spray amount: 12.5 g/ ^m2 ...

Then, the surface of nonwoven fabric C on which the particulate water absorbent was sprayed and the surface of nonwoven fabric A on which the adhesive was sprayed were placed face-to-face (in contact) and pressed together. After that, the nonwoven fabrics were turned upside down so that the bottom nonwoven fabric A (corresponding to the first substrate) was on top, and the upper nonwoven fabric A (corresponding to the first substrate) that was not pressed together was peeled off.

Next, 6 g (spreading amount: 150 g/m ² ) of the particulate water absorbing agent (1) (corresponding to the first particulate water absorbing agent) was uniformly spread on the surface of the nonwoven fabric C not facing the particulate water absorbing agent (2).

Next, 0.5 g of an adhesive (corresponding to the first adhesive) containing styrene butadiene rubber (Spray Glue 77, manufactured by 3M Japan Ltd.) was evenly sprayed onto the surface of the previously peeled nonwoven fabric A (corresponding to the first substrate) (spray amount: 12.5 g/ ^m2 ). Thereafter, the surface of nonwoven fabric C sprayed with the particulate water absorbing agent (1) and the surface of nonwoven fabric A sprayed with the adhesive were placed face to face (contacted), and pressed together to obtain a water absorbent sheet Z.

Finally, 0.1 g of an adhesive (corresponding to the first' adhesive) (Spray Glue 77, manufactured by 3M Japan Ltd.) containing styrene butadiene rubber was uniformly sprayed (spray amount: 2.5 g/m2) on the nonwoven fabric A of the water-absorbent sheet Z (the surface of the nonwoven fabric A opposite to the surface on which the particulate water-absorbing agent ( ¹ ) was sprayed), and then the nonwoven fabric was wrapped in a spunbond nonwoven fabric (wrapping sheet) having a thickness of 0.1 mm and mainly composed of olefin, cut to a length of 24 cm and a width of 40 cm, and pressed to bond the same, thereby obtaining a water-absorbent sheet (1).

[Examples 2 to 9, Comparative Examples 1 to 10]
A water-absorbent sheet was prepared in the same manner as in Example 1, except that the first particulate water-absorbing agent, the second particulate water-absorbing agent, the first substrate, the type of adhesive, the amount of the first adhesive, the amount of the second adhesive, and the amount of the first' adhesive were changed as shown in Table 1.

[Example 10]
A water-absorbent sheet (10) was obtained in the same manner as in Example 1, except that a hot melt adhesive (adhesive containing styrene-butadiene rubber) heated at 140° C. was applied using a glue applicator (Nordson Alta Blue Touch) instead of the spray glue 77.

[Examples 11 and 12, Comparative Example 11]
A water-absorbent sheet was prepared in the same manner as in Example 10, except that the first particulate water-absorbing agent, the second particulate water-absorbing agent, the first substrate, the type of adhesive, the amount of the first adhesive, the amount of the second adhesive, and the amount of the first' adhesive were changed as shown in Table 1.

[Method of evaluating water-absorbent sheets]
In the following evaluation methods, unless otherwise specified, the operations were carried out under the conditions of room temperature (25° C.) and relative humidity of 40 to 50% RH.

<Initial absorption rate>
A funnel (flow rate 7 ml/min) was placed at a height of 1.5 cm from the absorbent, and 1.00 g of physiological saline (0.9% sodium chloride aqueous solution) containing 20 ppm of Blue No. 1 reagent was poured into the funnel, and the time until the absorbent absorbed the liquid was measured. That is, the initial absorption rate (seconds) is the time displayed when a stopwatch is started immediately after pouring in the liquid and stopped when the absorbent has completely absorbed the liquid. N was 5 times, and the arithmetic average was taken as the initial absorption rate.

<Return amount (specific return amount evaluation)>
As shown in Fig. 6, the absorbent sheet 40, which was made to be 10 cm long and 40 cm wide, was wrapped in a liquid-impermeable sheet 21, which was 14 cm long and 40 cm wide, so that an opening was formed at the top. The absorbent sheet 40 wrapped in the liquid-impermeable sheet 21 was placed on a flat surface, and a liquid injection tube 41 (Fig. 7) was placed on top of it in the center of the absorbent sheet 40 as shown in Fig. 8. In this state, 80 g of a 0.9 wt % sodium chloride aqueous solution at 23 ° C. was poured into the liquid injection tube 41 using a funnel 42 capable of pouring liquid at a flow rate of 7 ml / sec (Fig. 9). In this case, the liquid was poured into the absorbent sheet 40 exposed from the liquid-impermeable sheet 21. 10 minutes after the liquid was poured in, 20 sheets of filter paper 43 (type No. 2, manufactured by ADVANTEC; circular with a diameter of 110 mm) whose weight had been measured in advance were placed in the center of the absorbent sheet 40, and a circular weight 44 (1200 g) with a diameter of 100 mm was further placed on it and held for 1 minute. After 1 minute, the weight 44 was removed, and the first amount of backflow (g) was measured from the weight increase of the filter paper 43. One minute after the weight 44 was removed, the same operation (pouring in the liquid → 10 minutes after pouring, placing the filter paper 43 and the weight 44 (1200 g) and holding for 1 minute → holding for 1 minute → removing the weight, measuring the amount of backflow) was repeated, and the second amount of backflow (g) and the third amount of backflow (g) were measured. The total of the measured amounts of backflow from the first to third times are shown in Table 2.

<Leak amount (diagonal evaluation) (specific leak amount evaluation)>
The leakage rate was measured using the device shown in FIG.

In summary, it is a mechanism that uses a commercially available experimental equipment stand 60 and pipes 61 to fix an acrylic plate 63 at an angle, then pours saline solution vertically from above using a funnel onto an absorbent sheet fixed onto the plate, and measures the amount of leakage. The detailed specifications are shown below.

The acrylic plate 63 had a length of 400 mm in the direction of the inclined surface, and was fixed by the stand 60 so that it made an angle of 20° with the horizontal. The acrylic plate 63 was 200 mm wide and 3 mm thick. The surface of the acrylic plate 63 was smooth, so liquid did not accumulate or get absorbed by the plate. The stand 60 was used to fix the funnel 64 vertically above the inclined acrylic plate 63. The funnel 64 used was one that could input liquid at a rate of 7 mL/sec.

A metal tray 65 was placed under the acrylic plate 63 to catch any test liquid that leaked out and record its mass to an accuracy of 0.1 g.

The inclined leakage test using such a device was carried out as follows. As shown in Figure 10, the back side of the absorbent sheet 40 cut to a size of 100 mm in length and 100 mm in width was attached to an acrylic plate 63.

A mark was made 1.5 cm below the top end of the absorbent sheet, and the funnel's inlet was fixed vertically above the mark at a distance of 15 ± 2 mm.

20 mL of saline was poured into the dropping funnel 64 at once. The amount of test liquid that flowed down the inclined acrylic plate 63 without being absorbed by the absorbent sheet 66 and entered the metal tray 65 was measured and recorded as the first leakage amount (mL) in the surface direction. After 10 minutes, 20 mL of saline was poured in the same way and the second leakage amount was measured, and after another 10 minutes, 20 mL of saline was poured in the same way and the third leakage amount was measured. The leakage amount in the table is the total value of the first to third leakage amounts.

<Application method> Solvent dissolving spray: X, Heat melting spray Y.

<First substrate> A: nonwoven fabric A (mainly composed of pulp fibers, thickness 0.4 mm, produced by airlaid method, basis weight: 47 g/m ² ), B: nonwoven fabric B (mainly composed of pulp fibers, thickness 0.4 mm, produced by airlaid method, basis weight: 42 g/m ² )).

<Adhesive> SBR: Styrene butadiene rubber, EVA: Ethylene-vinyl acetate copolymer.

<Measurement of non-volatile content of solvent-based adhesive>
The weight of the nonwoven fabric was measured immediately after spraying the solution-based adhesive (within 10 seconds after spraying), and the difference in weight from the nonwoven fabric before spraying was calculated to measure the amount of solution-based adhesive sprayed. Since the proportion of nonvolatile content in the amount sprayed measured in advance was 81.16% by mass (the nonvolatile content was measured from the remaining amount at 90°C for 1 hour), the nonvolatile content of the sprayed solution-based adhesive was calculated by multiplying the sprayed weight of the solution-based adhesive by 81.16% by mass.

<Measurement of non-volatile content of hot melt adhesive>
The weight of the nonwoven fabric was measured immediately after spraying the hot melt adhesive (within 10 seconds after spraying), and the difference in weight from the nonwoven fabric before spraying was calculated to measure the amount of hot melt adhesive sprayed. Since the ratio of nonvolatile content to the sprayed amount was essentially 100% by mass (the nonvolatile content was measured from the remaining amount at 90°C for 1 hour), the nonvolatile content of the sprayed hot melt adhesive was taken as the measured amount of hot melt adhesive sprayed.

The residual solvent in the solution-based adhesive (volatile organic compounds remaining in the adhesive) was more than 1 ppm in the adhesive.

[Evaluation rank]
A is the best, E is the worst (not practical)
(Leakage amount)
A: Less than 2.5 B: 2.5 or more but less than 3 C: 3 or more but less than 4 D: 4 or more but less than 5 E: 5 or more.

(Return amount)
A: Less than 4.5 B: 4.5 or more but less than 5 C: 5 or more but less than 5.5 D: 5.5 or more but less than 6 E: 6 or more.

40 Water-absorbent sheet 11 First substrate,
12 water absorption layer,
13 second substrate;
14 Particulate water absorbing agent,
14a: a first particulate water absorbing agent;
14b a second particulate water absorbing agent;
17 adhesives,
17a first adhesive;
17b second adhesive;
17c first' adhesive,
17d second'adhesive;
18 wrapping sheets,
21 Liquid impermeable sheet,
41 Liquid injection tube,
42 funnel,
60 stand,
61 Pipe,
63 Acrylic plate,
64 Funnel,
65 Metal trays,
400 devices,
410 container,
411 Cell,
412 piston,
413a, 413b wire mesh,
414 Swollen gel (a particulate water absorbing agent absorbed in water),
415 holes,
420 tank,
421 Glass tube,
422 Cock L-shaped tube with glass tube,
423 liquid,
431 Stainless steel wire mesh,
432 Collection vessel,
433 Top-pan balance.

Claims (8)

A first substrate;
A second substrate;
a water-absorbing layer located between the first substrate and the second substrate;
A water-absorbent sheet having
The water absorbing layer includes a particulate water absorbing agent,
The first substrate is located on a side where an aqueous liquid is introduced,
The initial absorption speed is adjusted to 2.2 to 3.0 seconds.
the particulate water absorbing agent comprises a first particulate water absorbing agent localized on a surface side of the first base material facing the second base material, and a second particulate water absorbing agent localized on a surface side of the second base material facing the first base material,
A water-absorbent sheet, comprising: a first adhesive disposed on a surface of the first substrate facing the second substrate; the first adhesive being a hot-melt type; and a content (non-volatile content) of the first adhesive being 0.005 to 0.05 times the mass of the first particulate water-absorbing agent . The water-absorbent sheet according to claim 1 , wherein the first particulate water-absorbing agent has a CRC of 30 g/g or more. The water-absorbent sheet according to claim 2 , wherein the first particulate water-absorbing agent has a CRC of 35 g/g or more. The water-absorbent sheet according to claim 1 , wherein the second particulate water-absorbing agent has a surface tension of 60 mN/m or more. The water-absorbent sheet according to any one of claims 1 to 4 , wherein the first adhesive contains a volatile organic compound at or below a detection limit. The water-absorbent sheet according to any one of claims 1 to 5 , which has a wrapping sheet. The water-absorbent sheet according to claim 6, wherein the wrapping sheet is placed on the surface of the first substrate on which an aqueous liquid is introduced, and there is substantially no adhesive between the wrapping sheet and the first substrate. A water-absorbent article comprising the water-absorbent sheet according to any one of claims 1 to 7 sandwiched between a liquid-permeable sheet and a liquid-impermeable sheet, the liquid-permeable sheet being positioned on the first substrate side, and the liquid-impermeable sheet being positioned on the second substrate side.