EXPLANATION OF THE SYMBOLS

[0041] 1 : Porous glass plate

[0042] 2 : Glass filter

[0043] 3 : Introducing tube

[0044] 4 : Liquid-storing receptacle

[0045] 5 : Supporting ring

[0046] 6 : Physiological saline

[0047] 7 : Balance

[0048] 8 : Stand

[0049] 9 : Measuring sample (water-absorbent resin or liquid-diffusing member)

[0050] 10 : Load (0.41 kPa (0.06 psi))

[0051] 11 : Open-air-aspirating pipe

[0052] 12 : Introducing tube

[0053] 13 : Glass filter

[0054] 14 : Physiological saline

[0055] 15 : Liquid-storing receptacle

[0056] 16 : Balance

[0057] 17 : Filterpaper

[0058] 18 : Wire net

[0059] 19 : Plastic cylinder

[0060] 10 : Load (0.41 kPa (0.06 psi))

[0061] 20 : Load (2.07 kPa (0.3 psi))

[0062] 21 : Load (4.83 kPa (0.7 psi))

[0063] 31 : Liquid-permeable polyester nonwoven fabric

[0064] 32 : Water-absorbent resin

[0065] 33 : Liquid-diffusing member

[0066] 34 : Liquid-impermeable polyethylene film

[0067] 35 : Haetlon paper

[0068] 36 : Adhesive tape

DETAILED DESCRIPTION OF THE INVENTION

[0069] [1] Capillary Absorption Ability:

[0070] The capillary absorption ability as used in the present invention is generally an evaluation item that has been used hitherto in order to evaluate the absorbency of a material (e.g. paper and pulp) sucking up and absorbing a liquid by capillary action. The liquid amount as absorbed per a unit weight of a sample is measured in a state where the liquid-absorbing position is changed to various heights by using an apparatus as mentioned below, and thereby evaluated are the capillary absorption capacity and liquid suction ability of the sample. A specific measurement method of the capillary absorption capacity meaning the capillary absorption ability in the present invention is disclosed in examples as mentioned below in detail. Measurement methods based on the same principle are also disclosed in such as: Textile Research Journal Vol. 57, 356 (1967), “Absorbency” (Chatterjee, Textile, Science and Technology, Vol. 7, 1985), JP-A-052349/1996, and WO 99/47184.

[0071] In an evaluation method for a water-absorbent resin in the present invention, comprising the step of measuring an absorption capacity for a liquid as absorbed by the water-absorbent resin within a predetermined time in a state where the liquid-absorbing position height H1 is in a position higher than the liquid surface height H2 in a liquid-storing receptacle, the above-mentioned method is applied first to the water-absorbent resin. From the resultant value, it is found that the liquid absorption ability of the water-absorbent resin from other base material such as a liquid-diffusing member or a liquid-acquiring member can be judged correctly. In order to raise the accuracy of measurement and the relationship with the performance of the absorbent structure, the measurement is carried out. in a state where the height difference between the liquid-absorbing position height H1 and the liquid surface height H2 in the liquid-storing receptacle is favorably in the range of 20 to 60 cm, more favorably 30 to 50 cm

[0072] As to the capillary absorption ability in the present invention, there are two kinds of a capillary absorption capacity and a capillary absorption index. As to the capillary absorption capacity in the present invention, measured is an amount (capacity) of a liquid as absorbed by a sample within 30 minutes in a state where there is a height difference between the liquid-absorbing position and the liquid surface in a liquid-storing receptacle. When the height difference between the liquid-absorbing position and the liquid surface in the liquid-storing receptacle is 40 cm, it is defined as “a capillary absorption capacity at a height of 40 cm”, and when the height difference between the liquid-absorbing position and the liquid surface in the liquid-storing receptacle is 0 cm, it is defined as “a capillary absorption capacity at a height of 0 cm”.

[0073] In addition, the capillary absorption index in the present invention can be calculated by dividing a value of the capillary absorption capacity (as absorbed by a sample within 30 minutes in a state where there is a height difference between the liquid-absorbing position and the liquid surface in the liquid-storing receptacle) by a value of the capillary absorption capacity at a height of 0 cm (as absorbed by the sample within 30 minutes when the height difference with the liquid surface in the liquid-storing receptacle is 0 cm). “The capillary absorption index at a height of 40 cm” is calculated by dividing a value of the “capillary absorption capacity at a height of 40 cm” (when the height difference between the liquid-absorbing position and the liquid surface in the liquid-storing receptacle is 40 cm) by the value of the “capillary absorption capacity at a height of 0 cm” (when the height difference with the liquid surface in the liquid-storing receptacle is 0 cm).

[0074] Water-absorbent resins, which are on a market at present and used for sanitary materials in a large amount, are crosslinked poly(acrylic acid (salt)) polymers of which the major raw material is an acrylic acid (salt). The mechanism of absorbing a liquid is not by capillary absorption such as pulp, but it is fundamentally derived from the osmotic pressure difference between an absorbed liquid and the polymer itself that is a. polymer electrolyte. However, the ability of the water-absorbent resin to absorb a liquid from a liquid-diffusing member or liquid-acquiring member having excellent liquid suction ability in the vertical direction, wherein the liquid is kept in the above member, could not be expected at all from only the absorption properties generally known as the ability of the water-absorbent resin hitherto, such as absorption capacity, absorption rate, absorption capacity under a load, and liquid permeability of gel layer.

[0075] The present inventors took note of and considered the ability as called the capillary absorption ability even in the water-absorbent resin similar to the liquid-diffusing member or liquid-acquiring member. Then, they found out that: the capillary absorption ability is greatly different due to the kinds of water-absorbent resins; and further, by using a combination of a water-absorbent resin and a liquid-diffusing member or liquid-acquiring member wherein the water-absorbent resin has capillary absorption ability specifically in relation to the capillary absorption ability of the above liquid-diffusing member or liquid-acquiring member, the water-absorbent resin can favorably absorb and store a liquid from the liquid-diffusing member or liquid-acquiring member. Furthermore, they found out that: an absorbent structure as planed to maintain this relationship displays very excellent liquid absorption efficiency; and, in an absorbent article (e.g. disposable diaper) including such an absorbent structure, a water-absorbent resin is spread all over and used very effectively, and therefore the absorption ability of the entire diaper can be very greatly enhanced; and a thin-type easily-movable diaper including fewer members can be produced by adjusting this high absorption ability to absorption ability in a desirable practical level.

[0076] In order to cause the capillary absorption ability of the water-absorbent resin in the present invention, the balance between the capillary absorption ability and the absorption properties is thought to be very important, wherein the capillary absorption ability is derived from the physical shape of the water-absorbent resin and wherein the absorption properties are derived from the osmotic pressure of various polymers themselves as caused by carrying out surface-crosslinking treatment.

[0077] [2] Absorbent Structure Comprising Liquid-Diffusing Member and Water-Absorbent resin:

[0078] (2-1) Relationship Between Capillary Absorption Ability of Liquid-Diffusing Member and that of Water-Absorbent Resin:

[0079] The relationship between the capillary absorption ability of the liquid-diffusing member and that of the water-absorbent resin in the present invention is explained.

[0080] The water-absorbent resin usable in the present invention is a water-absorbent resin wherein when the capillary absorption index of the liquid-diffusing member at a height of 40 cm is referred to as A (A≧0.10), the capillary absorption index B of the water-absorbent resin at a height of 40 cm satisfies the following equation:

B/A≧ 0.7 (equation 1)

[0081] The value of the capillary absorption index B of the water-absorbent resin necessary in the present invention at a height of 40 cm is different depending upon the property of the liquid-diffusing member as used, namely, the capillary absorption index A of the liquid-diffusing member as used at a height of 40 cm. If the relationship B/A≧0.7 above is satisfied, a liquid from the liquid-diffusing member to the water-absorbent resin is favorably distributed, and the water-absorbent resin can favorably absorb and store the liquid. In the case where the B/A is less than 0.7, there are cases where: the water-absorbent resin difficultly absorbs the liquid from the liquid-diffusing member; and the liquid distribution ratio from the liquid-diffusing member is lowered; and the absorption quantity of the water-absorbent resin is not improved even if diapers include these absorbent structures. Therefore, the water-absorbent resin does not favorably work as the liquid-storing member. The water-absorbent resin favorably satisfies B/A≧1.3, more favorably B/A≧1.5. In addition, in the case where the B/A is more than 2.0, there is a case where the liquid diffusion ratio of the liquid-diffusing member is lowered, and it is necessary to pay attention to this matter. Incidentally, hereinafter, the value of the B/A may be referred to as a liquid-diffusion-and-storage coefficient 1.

[0082] In addition, another water-absorbent resin usable in the present invention is water-absorbent wherein when the capillary absorption capacity of the liquid-diffusing member at a height of 40 cm is referred to as C (C≧2.0 (g/g)), the capillary absorption capacity D of the water-absorbent resin at a height of 40 cm satisfies the following equation:

D/C≧ 0.7 (equation 2)

[0083] The value of the capillary absorption capacity D of the water-absorbent resin necessary in the present invention at a height of 40 cm is different depending upon the property of the liquid-diffusing member as used, namely, the capillary absorption capacity C of the liquid-diffusing member as used at a height of 40 cm. Even if the relationship D/C≧0.7 above is satisfied, a liquid from the liquid-diffusing member to the water-absorbent resin is favorably distributed, and the water-absorbent resin can favorably absorb and store the liquid. In the case where the D/C is less than 0.7, the water-absorbent resin difficultly absorbs the liquid from the liquid-diffusing member, and then the water-absorbent resin does not favorably work as the liquid-storing member. The water-absorbent resin favorably satisfies D/C≧1.3, more favorably D/C≧1.5. In addition, in the case where the D/C is more than 10, there is a case where the liquid diffusion ratio of the liquid-diffusing member is lowered, and it is necessary to pay attention to this matter. Incidentally, hereinafter, the value of the D/C may be referred to as a liquid-diffusion-and-storage coefficient 2.

[0084] In the present invention, it is more favorable that both the liquid-diffusion-and-storage coefficient 1 and the liquid-diffusion-and-storage coefficient 2 as mentioned above satisfy the present invention range. When only the one coefficient satisfies the range, the liquid absorption ability of the water-absorbent resin from the liquid-diffusing member may not be displayed favorably due to a condition of use, and therefore it is necessary to pay attention to his matter.

[0085] In addition, the present invention relates to an absorbent structure comprising a liquid-diffusing member and a water-absorbent resin having a specific relationship, but the absorbent structure also works as a liquid-transfer-and-absorption system comprising a liquid-diffusing member and a water-absorbent resin having a specific relationship. That is to say, the present invention can also provide: a liquid-transfer-and-absorption system, which is an absorbent structure comprising a liquid-diffusing member and a water-absorbent resin, with the system being characterized in that when the capillary absorption index of the liquid-diffusing member at a height of 40 cm is referred to as A (A≧0.10), the capillary absorption index B of the water-absorbent resin at a height of 40 cm satisfies the following equation:

B/A≧ 0.7 (Equation 1); and

[0086] a liquid-transfer-and-absorption system, which is an absorbent structure comprising a liquid-diffusing member and a water-absorbent resin, with the system being characterized in that when the capillary absorption capacity of the liquid-diffusing member at a height of 40 cm is referred to as C (C≧2.0 (g/g)), the capillary absorption capacity D at a height of 40 cm satisfies the following equation:

D/C≧ 0.7 (Equation 2)

[0087] (2-2) Liquid-Diffusing Member:

[0088] The liquid-diffusing member usable in the present invention is defined as a material displaying a capillary absorption index A of not less than 0.10 at a height of 40 cm and a capillary absorption capacity C of not less than 2.0 (g/g) at a height of 40 cm, and substantially having no hydrogel-formability. The liquid-diffusing member is a material for diffusing a liquid as added to the absorbent structure or absorbent article having the absorbent structure, over a wider area in the absorbent structure. Particularly, even in a mode of practical use, in order to enable such a function to be displayed sufficiently, the liquid-diffusing member has a porous structure and excellent liquid suction ability in the vertical direction. In addition, the liquid-diffusing member itself more favorably has a predetermined level of ability of retaining, absorbing and storing a liquid.

[0089] The liquid-diffusing member usable in the present invention is a member having excellent liquid diffusion ability and liquid suction ability, and it is necessary that the capillary absorption index A is not less than 0.10 at a height of 40 cm. The capillary absorption index A of such as flap pulp as used for disposable diapers hitherto is not more than 0.05 at a height of 40 cm according to the measurement method in the present invention. In such a material displaying a capillary absorption index A is less than 0.10, the liquid suction ability in the vertical direction is small, and it is difficult to diffuse a liquid over the entire face of the liquid-diffusing member or the entire absorbent structure, and the material of the entire absorbent structure cannot be efficiently used. The capillary absorption index A is favorably not less than 0.20 at a height of 40 cm, more favorably not less than 0.30, most favorably not less than 0.40.

[0090] In addition, the liquid-diffusing member usable in the present invention favorably displays a capillary absorption capacity of not less than 10 (g/g) at a height of 0 cm. The higher the capillary absorption capacity is at a height of 0 cm, the larger the liquid-transferring capacity of the liquid-diffusing member is. Also from the viewpoint of absorbing, retaining and storing a liquid, it can function, and therefore an excellent absorbent structure can be obtained. The capillary absorption capacity is more favorably not less than 20 (g/g) at a height of 0 cm, still more favorably not less than 30 (g/g).

[0091] It is necessary that another liquid-diffusing member usable in the present invention displays a capillary absorption capacity C of not less than 2.0 (g/g) at a height of 40 cm. The capillary absorption capacity C of such as flap pulp as used for disposable diapers hitherto is not more than 1.0 (g/g) at a height of 40 cm. In such a material displaying a capillary absorption capacity C is less than 2.0 (g/g) at a height of 40 cm, the liquid suction ability in tie vertical direction is small, and it is difficult to diffuse a liquid over the entire face of the liquid-diffusing member or the entire absorbent structure, and the material of the entire absorbent structure cannot be efficiently used. The capillary absorption capacity C is favorably not less than 5.0 (g/g) at a height of 40 cm, more favorably not less than 10.0 (gig).

[0092] In addition, similarly, another liquid-diffusing member usable in the present invention favorably displays a capillary absorption capacity of not less than 10 (g/g) at a height of 0 cm. The higher the capillary absorption capacity is at a height of 0 cm, the larger the liquid-transferring capacity of the liquid-diffusing member is. Also from the viewpoint of absorbing, retaining and storing a liquid, it can function, and therefore an excellent absorbent structure can be obtained. The capillary absorption capacity is more favorably not less than 20 (g/g) at a height of 0 cm, still more favorably not less than 30 (g/g).

[0093] The liquid-diffusing member usable in the present invention satisfies the above conditions, and is used together with a water-absorbent resin, and thereby they are used as an absorbent structure.

[0094] As to the relationship between both of them, as is aforementioned, it is necessary that: when the capillary absorption index of the liquid-diffusing member at a height of 40 cm is referred to as A, the capillary absorption index B of the above water-absorbent resin at a height of 40 cm satisfies B/A≧0.7, favorably B/A≧1.3; or when the capillary absorption capacity of the liquid-diffusing member at a height of 40 cm is referred to as C (C≧2.0 (g/g)), the capillary absorption capacity D of the above water-absorbent resin at a height of 40 cm satisfies D/C≧0.7, favorably D/C≧1.3. In addition, both of them more favorably satisfy the B/A≧0.7 and D/C≧0.7 at the same time, still more favorably the B/A≧1.3 and D/C≧1.3 at the same time.

[0095] In addition, the liquid-diffusing member usable in the present invention favorably displays a suction height of not lower than 30 cm, more favorably not lower than 40 cm, still more favorably not lower than 50 cm, wherein the suction height is ability of sucking up a liquid in the vertical direction as mentioned below. In the case where the suction height is not higher than 30 cm, the liquid diffusion ratio of the absorbent structure is low, and the entire absorbent structure cannot be utilized effectively.

[0096] The shape of the liquid-diffusing member can be a sheet, fibrous, particulate, or strip shape, but it is favorably a sheet shape in general. Then, the weight of the liquid-diffusing member per its unit area is in the range of about 50 to about 500 g/m 2 , about 100 to about 200 g/m 2 .

[0097] In addition, when the liquid-diffusing member has a difference of density, a slope of density, a difference of diffusion ability, and a slope of diffusion ability in the member, or when a second liquid-diffusing member not satisfying the present invention relationship is further used, it is favorable to make the capillary absorption ability of a portion of the liquid-diffusing member much closer to the water-absorbent resin satisfy the above relationship.

[0098] Examples of such a liquid-diffusing member include: porous polymers obtained by a process including the step of polymerizing a high-internal-phase emulsion (HIPE); fibrous materials having a predetermined density (e.g. cellulose pulps or nonwoven fabrics); and foam materials (e.g. urethane sponges and cellulose sponges). The liquid-diffusing member is favorably a member having excellent liquid suction ability, suction amount, and suction rate in the vertical direction. Among these, the porous polymers obtained by a process including the step of polymerizing a high-internal-phase emulsion (HIPE) as explained below are favorable.

[0099] a. Liquid-Diffusing Member Including Porous Polymers Obtained by a Process Including the Step of Polymerizing a High-Internal-Phase Emulsion (HIPE):

[0100] The porous polymer fitly usable as the liquid-diffusing member in the present invention can be obtained by a process including the step of polymerizing a high-internal-phase emulsion (HIPE), where the ratio of the water phase and the oil phase (W/O ratio) is not less than about 3/1 wherein the water phase is a dispersible (inner) phase and the oil phase is an outer phase. The method for producing the porous polymer from the HIPE is, for example, described in such as U.S. Pat. No. 5,189,070, U.S. Pat. No. 5,250,576, U.S. Pat. No. 5,252,619, U.S. Pat. No. 5,290,820, U.S. Pat. No. 5,358,974, U.S. Pat. No. 5,252,619, U.S. Pat. No. 5,670,101, and U.S. Pat. No. 6,204,298. The porous polymer as obtained in this way is in a state of low-density foam including continuous foam having a fine diameter. If conditions are selected, the polymer foam having desirable absorption properties (e.g. very excellent liquid diffusion and suction properties) can be produced.

[0101] The raw material of the HIPE as used includes: an oil phase containing a polymerizable monomer component and a surfactant; and a water phase containing water. Examples of the polymerizable monomer component include: a polymerizable monomer having one polymerizable unsaturated group in its molecule wherein the monomer can form a crosslinked structure by polymerization; and/or a crosslinkable monomer having at least two polymerizable unsaturated groups in its molecule. Furthermore, if necessary, polymerization initiators, salts, and other additives may also be included as arbitrary components that are comprised in the oil phase and/or water phase.

[0102] As to the polymerizable monomer, at least one portion thereof favorably includes a (meth)acrylate ester. Specific examples thereof include: arylene monomers, such as styrene; monoalkylene arylene monomers, such as styrene, ethylstyrene, α-methylstyrene, vinyltoluene, and vinylethylbenzene; (meth)acrylate esters, such as methyl (metlh)acrylate, ethyl (metli)acrylate, butyl (methi)acrylate, isobutyl (meth)acrylate, isodecyl (ineth)acrylate, 2-ethylehexyl (meth)acrylate, lauryl (meth)acrylate, stearyl (ineth)acrylate, cyclohexyl (meth)acrylate, and benzyl (meth)acrylate; chlorine-containing monomers, such as vinyl chloride, vinylidene chloride, and chloromethylstyrene; acrylonitrile compounds, such as acrylonitrile, and methacrylonitrile; and other monomers, such as vinyl acetate, vinyl propionate, N-octadecyl acrylamnide, ethylene, propylene, and butene. These may be used either alone respectively or in combinations with each other.

[0103] The above crosslinkable monomer may be a compound having at least two polymerizable unsaturated groups in its molecule, or a compound that can form a crosslinked structure by polymerization. There is no especial limitation on the crosslinkable monomer if it is polymerizable in a dispersible emulsion or water-drop-in-oil-type high-dispersible-phase emulsion in the same way as of the above polymerizable monomer. Specific examples of the crosslinkable monomer include: aromatic monomers, such as divinylbenzene, trivinylbenzene, divinyltoluene, divinylxylene, p-ethyl-vinylbenzene, divinylnaphthalene, divinylalkylbenzenes, divinylphenathrene, divinylbiphenyl, divinyldiphenylmethane, divinylbenzil, divinyl phenyl ether, and divinyl phenyl sulfide; oxygen-containing monomers such as divinylfuran; sulfur-containing monomers, such as divinyl sulfide and divinyl sulfone; aliphatic monomers, such as butadiene, isoprene, and pentadiene; ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-butanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol. di(meth)acrylate, octanediol di(meth)acrylate, decanediol di(meth)acrylate, trimethylolpropane di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaeryhritol di(meth)acrylate, pentaeryhritol tri(meth)acrylate, pentaerythritol tetra(meth)aciylate, dipentaerythritol di(meth)acrylate, dipentaerytlhritol tri(metli)acrylate, dipentaerythritol tetra(meth)acrylate, N, N′-methylenebis(meth)acrylamide, triallyl isocyanurate, triallylamine, tetraallyloxyethane, and ester compounds between polyhydric alcohols (e.g. hydroquinone, catechol, resorcinol, and sorbitol) and acrylic or methacrylic acid. These may be used either alone respectively or in combinations with each other.

[0104] The amount of the above crosslinkable monomer as used is favorably in the range of 0.1 to 90 weight %, more favorably 1 to 70 weight %, particularly favorably 5 to .50 weight %, of the weight of the entire polymerizable monomer component including the above polymerizable monomer and the above crosslinkable monomer.

[0105] In addition, there is no especial limitation on the surfactant as used in the oil phase if it can emulsify the water phase. Such as nonionic surfactants, cationic surfactants, anionic surfactants, and amphoteric surfactants that are publicly known hitherto can be used. Of the above, there is a case where the stability of the HIPE is improved when the nonionic surfactants are used together with the cationic surfactants.

[0106] The amount of the above surfactant as used is favorably in the range of 1 to 30 parts by weight, more favorably 3 to 15 parts by weight, per 100 parts by weight of the entire polymerizable monomer component including the polymerizable monomer and the crosslinkable monomer.

[0107] As to the above water, tap water, pure water, deionized water, and besides wasted water as obtained by producing the porous polymer can be used exactly or after carrying out definite treatment. The amount of the above water as used can be fitly selected due to desirable liquid distribution performance. Specifically, the porous ratio of the porous polymer is determined by changing the ratio of water phase/oil phase (W/O) of the HIPE. Therefore, the amount of the water as used is naturally determined if the W/O ratio is selected so that the porous ratio will accord with the aim.

[0108] The polyinerization initiators may be initiators usable in ordinary polymerization, and any of water-soluble polymerization initiators (e.g. azo compounds such as 2,2′-azobis(2-amidinopropaiie) dihydrochloride; persulfate salts, such as ammonium persulfate, potassium persulfate, and sodium persulfate; peroxides, such as potassium peracetate, sodium peracetate, potassium percarbonate, and sodium percarbonate) and oil-soluble polymerization initiators can be used. Furthermore, a redox polymerization initiator system as obtained by combining the above polymerization initiator and a reductant may be used. In this case, as to the polymerization initiator, any of the water-soluble initiator or the oil-soluble initiator. can be used, and the water-soluble redox polymerization initiator system may be used together with the oil-soluble redox polymerization initiator system.

[0109] The salts may be used if it is necessary to improve the stability of the HIPE. Specific examples of the salts include water-soluble salts such as halides, sulfate salts, and nitrate salts of alkaline or alkaline earth metals (e.g. calcium chloride, sodium sulfate, sodium chloride, and magnesium sulfate). These salts may be used either alone respectively or in combinations with each other. These salts are favorably added to the water phase. Of the above, multivalent metal salts are favorable in view of the stability of the HLPE when the polymerization is carried out.

[0110] The amount of such salts as used is favorably in the range of 0.1 to 20 parts by weight, more favorably 0.5 to 10 parts by weight, per 100 parts by weight of the water.

[0111] Furthermore, if the performance and function of other various additives are added and thereby they lead to improve the performance of the porous polymer as the liquid-diffusing member, such other various additives may fitly be used. For example, bases and/or buffers may be added in order to adjust a pH. Examples of such additives include active carbon, inorganic powders, organic powders, metal powders, deodorants, antimicrobial agents, anti-molding agents, perfumes, various polymers, and surfactants.

[0112] There is no especial limitation on the emulsification method of the HIPE usable in the present invention. For example, a uniform oil phase is prepared by stirring an oil-phase constituent component at a definite temperature wherein the component includes such as the polymerizable monomer component and surfactant, and the polymerization initiator and other additives that can be further added thereto if necessary. On the other hand, a uniform water phase is prepared by: stirring water with an objective amount as used while a water-phase constituent component is further added to the water wherein the component includes such as the polymerization initiators, salts, and other additives that can be further added thereto if necessary; and heating them at a predetermined temperature of the HIPE. Subsequently, the HIPE can stably be prepared by: combining the oil phase and the water phase, wherein the oil phase is a mixture of such as a polymerizable monomer component and surfactant, and the water phase is a mixture of such as water and a water-soluble salt as prepared in the above way; efficiently mix-stirring them at an emulsifying temperature of the HIPE to apply the optimum shearing stress; and then emulsifying them.

[0113] The ratio of water phase/oil phase (W/O) (weight ratio) can fitly be selected and is not especially limited. The ratio may be not less than 3/1 as defined before, but it is favorably in the range of 10/1 to 250/1, particularly favorably 10/1 to 100/1. The porous ratio of the porous polymer is determined by changing the W/o ratio, and thereby the liquid distribution ability, liquid suction ability, and liquid-retaining ability of the liquid-diffusing member can be changed. Therefore, when the liquid-diffusing member as an object of the present invention is produced, the W/O ratio is in the range of about 10/1 to about 100/1, more favorably about 20/1 to about 80/1.

[0114] There is no especial limitation on production apparatuses of the above HIPE. Examples of the hitherto known production apparatuses include stirrers having such as a propeller-type, paddle-type, and turbine-type blade, homomixers, pin mixers, line mixers, and static mixers. These may be used either alone respectively or in combinations with each other.

[0115] The emulsifying temperature of the HIPE in the emulsifying step in which the HIPE is formed is usually in the range of 40 to 110° C.

[0116] The HIPE as obtained by mixing the polymerization initiator is formed in a desirable mode. The molding shape is favorably a sheet shape in order to use the porous polymer as the liquid-diffusing member wherein the porous polymer is obtained in the present invention, but the HIPE is added to a cylindrical container to polymerize it and thereafter the resultant polymer may be cut into a sheet shape, or porous polymers having various modes (e.g. particulate, fibrous, and film shape) may be processed to a mode having liquid distribution ability as an end product. When the mode is a sheet shape, the thickness thereof is not limited, but the thickness as the mode of the end product is favorably not more than about 10 mm, more favorably not more than about 5 mm, still more favorably not more than about 3 mm, particularly favorably not more than about 1 mm, most favorably not more than about 0.5 mm. In the case where the thickness is considerably thick, the attachment feeling may be lowered when it is used as a liquid-diffusing member for absorbent articles.

[0117] There is no especial limitation on the method for polymerizing the HIPE, and hitherto known methods for polymerizing the HIPE can fitly be used. The HIPE is usually polymerized by heating with a static polymerization method under a condition that the structure in the HIPE is not destroyed. In this case, the batchwise polymerization in which this HIPE is polymerized every batch, or the continuous polymerization in which this HIPE is continuously polymerized by casting, for example, while feeding it into a heating zone may be carried out. The above polymerization temperature is usually in the range of 40 to 110° C. However, when the productivity is considered, the above polymerization temperature is favorably higher (e.g. favorably in the range of about 60 to about 110° C., more favorably about 80 to about 105° C.). The polymerization time is favorably in the range of several tens seconds to 30 minutes in order to obtain a porous polymer having uniform properties in view of the productivity. These detailed production methods are disclosed in such as Japanese Patent Application No. 203744/2000.

[0118] The porous polymer as obtained after the polymerization is usually dehydrated by compression, aspiration under reduced pressure, or a combination of these, and the polymer can be pressed in a mode such that the polymer is compressed to one half or third of the original thickness depending upon the its kind. For the purpose of such as further improving surface conditions of the porous polymer, the porous polymer may be washed in an aqueous solution or a solvent including pure water and an arbitrary additive, or thereafter if necessary, the polymer may be heat-dried by such as hot air, infrared ray, microwave. In addition, the water content of the resultant polymer may be adjusted by adding humidity. Furthermore, the polymer is cut to obtain a desirable shape and size for using as an end product, and then it may be processed to obtain a product in accordance with various uses.

[0119] b. Other Liquid-Diffusing Member:

[0120] Examples of other liquid-diffusing member usable in the present invention include: foaming structures including synthetic polymers, such as polyurethanes polystyrene, polyethylene, polypropylene, polyesters, poly(vinyl alcohol), butadiene-styrene rubbers (SBR), and nitrile-butadiene rubbers; fibrous aggregates as obtained by adhering to or combining with synthetic fibers, such as polyethylene, polypropylene, polyethylene terephthalate, and nylon; rayon fibers; and fibrous aggregates as obtained by adhering to under a pressure, adhering to, or combining with hydrophilic fibers, such as cellulose fibers (e.g. celluloses, cellulose acetate, and nitro cellulose), and polyamide fibers. The shape thereof can be a sheet, fibrous, or particulate shape, but it is favorably a sheet shape in general. Favorable are the fibrous aggregates as obtained by adhering to under a pressure, adhering to, or combining with hydrophilic fibers, such as cellulose fibers and rayon fibers. These liquid-diffusing members may be produced in a line when absorbent structures and absorbent articles are produced.

[0121] The necessary performance of the liquid-diffusing member in the present invention as shown in these a. and b. is mentioned above.

[0122] (2-3) Water-Absorbent Resin:

[0123] The present invention water-absorbent resin is a hydrophilic crosslinked polymer, namely a polymer (water-swellable water-insoluble hydrogel-formable polymer) having a property such that: when an aqueous liquid contact is in contact with such as a particulate polymer aforementioned, the above polymer particles are swollen by absorbing the above liquid in the particles, and a hydrogel including the aqueous liquid can be formed. In addition, the water-absorbent resin may include a mixture as obtained by adding an additive to the water-swellable water-insoluble hydrogel-formable polymer, wherein the amount of the additive is not more than 30 weight % relative to. the total amount of the above water-swellable water-insoluble hydrogel-formable polymer and the above additive.

[0124] Water-absorbent resins have hitherto been used as materials absorbing liquids due to the osmotic pressure difference between the inside and the outside of the resins, namely as liquid-storing members such as disposable diapers. However, the present inventors took note of that: even if the properties of the water-absorbent resins as known hitherto (e.g. absorption capacity, absorption capacity under a load) are identical, the absorbing behaviors are greatly different due to the kinds of resins when liquids are absorbed from such as the liquid-diffusing member. Then, the present inventors diligently considered and found out that: the capillary absorption ability is greatly different even in a water-absorbent resin itself; and the water-absorbent resin can receive and store a liquid from the liquid-diffusing member more favorably when the relationship between the capillary absorption ability of the liquid-diffusing member and the capillary absorption ability of the water-absorbent resin satisfies a specific condition.

[0125] The water-absorbent resin usable in the present invention is a water-absorbent resin, in which, as is mentioned above, when the capillary absorption index of the liquid-diffusing member at a height of 40 cm is referred to as A (A≧0.10), the capillary absorption index B of the above water-absorbent resin at a height of 40 cm satisfies B/A≧0.7, favorably B/A≧1.3, more favorably B/A≧1.40.

[0126] The value of the capillary absorption index B of the water-absorbent resin necessary in the present invention at a height of 40 cm is different depending upon the property of the liquid-diffusing member as used, namely, the capillary absorption index A of the liquid-diffusing member as used at a height of 40 cm. If the above relationship B/A≧0.7 is satisfied, a liquid from the liquid-diffusing member to the water-absorbent resin is favorably distributed, and the water-absorbent resin can favorably absorb and store the liquid. The water-absorbent resin displays a capillary absorption index B of not less than 0.4 at a height of 40 cm, favorably not less than 0.5, more favorably not less than 0.6.

[0127] In addition, the water-absorbent resin as used in the present invention favorably displays a capillary absorption capacity of not less than 30 (g/g) at a height of 0 cm. If the capillary absorption capacity at a height of 0 cm is higher, the water-absorbent resin can retain a large amount of liquid as sucked up from the liquid-diffusing member. Therefore, an excellent absorbent structure is obtained from the viewpoint of the liquid absorption ability. The water-absorbent resin favorably displays a capillary absorption capacity of not less than 40 (g/g) at a height of 0 cm, more favorably not less than 50 (g/g).

[0128] In addition, another water-absorbent resin usable in the present invention is a water-absorbent resin, in which, when the capillary absorption capacity of the liquid-diffusing member at a height of 40 cm is referred to as C (C≧2.0 (g/g)), the capillary absorption capacity D of the above water-absorbent resin at a height of 40 cm satisfies D/C≧0.7, favorably D/C≧1.3, more favorably D/C≧1.40.

[0129] The value of the capillary absorption capacity D of another water-absorbent resin necessary in the present invention at a height of 40 cm is different depending upon the property of the liquid-diffusing member as used, namely, the capillary absorption capacity C of the liquid-diffusing member as used at a height of 40 cm. Even if the above relationship D/C≧0.7 is satisfied, a liquid from the liquid-diffusing member to the water-absorbent resin is favorably distributed, and the water-absorbent resin can favorably absorb and store the liquid. The water-absorbent resin favorably displays a capillary absorption capacity D of not less than 15 (g/g) at a height of 40 cm, more favorably not less than 20 (gig), still more favorably not less than 25 (g/g), most favorably not less than 30 (gig).

[0130] In addition, similarly, the water-absorbent resin as used in the present invention favorably displays a capillary absorption capacity of not less than 30 (g/c) at a height of 0 cm. If the capillary absorption capacity at a height of 0 cm is higher, the water-absorbent resin can retain a large amount of liquid as sucked up from the liquid-diffusing member. Therefore, an excellent absorbent structure is obtained from the viewpoint of the liquid absorption ability. The water-absorbent resin favorably displays a capillary absorption capacity of not less than 40 (g/g) at a height of 0 cm, more favorably not less than 50 (g/g).

[0131] In addition, when the water-absorbent resin as used in the present invention displays. an absorption capacity of 20 to 50 g/g under a load of 2.07 kPa (0.3 psi), there are advantages in that the absorbency can favorably be maintained even if the absorbent structure is in a pressurized state. The water-absorbent resin more favorably displays an absorption capacity of 25 to 40 g/g.

[0132] The water-absorbent resin usable in the present invention satisfies the above conditions, and is used together with a liquid-diffusing member, and thereby they are used as an absorbent structure.

[0133] The shape of the water-absorbent resin can be a particulate, fibrous, sheet, or strip shape, but it is favorably a particulate shape in general. The water-absorbent resin is favorably a particulate resin, of which the raw material is an acrylic acid (salt) in a major proportion, and of which the weight-average particle diameter of the fundamental particles is not larger than 250 μm, and the water-absorbent resin favorably has narrow particle diameter distribution. In addition, as to the production method thereof, aqueous solution polymerization or reversed-phase suspension polymerization can be carried out, but the water-absorbent resin is favorably a resin obtained by the reversed-phase suspension polymerization. In addition, in view of handling, the water-absorbent resin comprised of the fundamental particles is granulated while the capillary absorption ability of the present invention is maintained, and the weight-average particle diameter may be outside of the above range.

[0134] In the present invention, a water-absorbent resin satisfying the above relationship and a water-absorbent resin not satisfying the above relationship may be used together as the water-absorbent resin, but only the water-absorbent resin satisfying the above relationship is favorably used in order to display the present invention effect to the maximum. In addition, the resin is favorably arranged so that the capillary absorption ability of a portion of the water-absorbent resin will satisfy the above relationship, wherein the portion is much closer to the liquid-diffusing member.

[0135] Examples of the water-absorbent resin usable in the present invention include water-swellable crosslinked polymers that can be obtained by polymerizing hydrophilic monomers. Of the above, crosslinked poly(acrylic acid (salt)) polymers of which the major component is derived from acrylic acid or its salt are favorable. Specific examples thereof include: partially-neutralized crosslinked poly(acrylic acid) polymers (e.g. U.S. Pat. No. 4,625,001, U.S. Pat. No. 4,654,039, U.S. Pat. No. 5,250,640, U.S. Pat. No. 5,275,773, and EP 456136); crosslinked partially-neutralized graft polymers of starch-acrylic acid (U.S. Pat. No. 4,076,663); copolymers of isobutylene-maleic acid (U.S. Pat. No. 4,389,513); saponified copolymers of vinyl acetate-acrylic acid (U.S. Pat. No. 4,124,748); hydrolyzed (co)polymers of acrylamide (U.S. Pat. No. 3,959,569); and hydrolyzed polymers of acrylonitrile (U.S. Pat. No. 3,935,099). As to the crosslinked poly(acrylic acid (salt)) polymers, the acid group in the polymers is favorably neutralized in a ratio of 50 to 90 mol %, and examples of the salt include alkaline metal salts, ammonium salts, and amine salts.

[0136] The water-absorbent resin usable in the present invention, particularly the crosslinked poly(acrylic acid (salt)) polymer as favorably used, may be obtained by copolymerizing monomers as used in a major proportion (e.g. acrylic acid or its salt), and besides, other monomers together if necessary. Examples of other monomers include: anionic unsaturated monomers, such as methacrylic acid, maleic acid, vinylsulfonic acid, styrenesulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonic acid, and salts thereof; nonionic hydrophilic-group-containing unsaturated monomers, such as acrylamide, methacrylamide, N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamiide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, and N-acryloylpyrrolidine; and cationic unsaturated monomers, such as N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimetliylaminopropyl (meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide, and quaternary salts thereof. The amount of the other monomers other than the acrylic acid as used is favorably in the range of 0 to 30 mol %, more favorably 0 to 10 mol %, relative to the entire monomer.

[0137] Examples of the method for introducing a crosslinked structure into the water-absorbent resin as used in the present invention include: a self-crosslinking-type method in which no crosslinking agent is used; and a method which involves copolymerizing or reacting with an internal-crosslinking agent having at least two polymerizable unsaturated groups or at least two reactive groups. Favorable is the method involving copolymerizing or reacting with the internal-crosslinking agent.

[0138] Specific examples of these internal-crosslinking agents include: N,N′-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane di(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylate methacrylate, ethylene-oxide-modified trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly(meth)allyloxyalkanes, (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, pentaerythritol, ethylenediamine, polyethylenimine, and glycidyl (meth)acrylate. In addition, these internal-crosslinking agents may be used in combinations with each other. Of the above, from the viewpoint of the absorption performance of the water-absorbent resin as obtained, it is favorable that the compound having at least two polymerizable unsaturated groups is essentially used. The amount as used is favorably in the range of 0.005 to 3 mol %, more favorably 0.01 to 1.5 mol %, of the aforementioned monomer component.

[0139] Incidentally, when the above polymerization is carried out, hydrophilic polymers (e.g. starch-cellulose, derivatives from starch-cellulose; polyvinyl alcohol, polyacrylic acid (salts), and crosslinked products of polyacrylic acid (salts)) and chain transfer agents (e.g. hypophosphorous acid (salts)) may be added.

[0140] When the above monomer including acrylic acid or its salt in a major proportion is polymerized in order to obtain the water-absorbent resin as used in the present invention, bulk polymerization or precipitation polymerization can be carried out. However, from the viewpoint of the performance or the easiness of controlling the polymerization, aqueous solution polymerization or reversed-phase suspension polymerization is favorably carried out by using the above monomer in the form of its aqueous solution. The above polymerization method is publicly known hitherto and disclosed in such as U.S. Pat. No. 4,625,001, U.S. Pat. No. 4,769,427, U.S. Pat. No. 4,873,299, U.S. Pat. No. 4,093,776, U.S. Pat. No. 4,367,323, U.S. Pat. No. 4,446,261, U.S. Pat. No. 4,683,274, U.S. Pat. No. 4,690,996, U.S. Pat. No. 4,721,647, U.S. Pat. No. 4,738,867, and U.S. Pat. No. 4,748,076.

[0141] In addition, when the polymerization is carried out, radical polymerization initiators (e.g. potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide, and 2,2′-azobis(2-amidinopropane) dihydrochloride), and active energy rays (e.g. ultraviolet rays and electron beams) can be used. In addition, when acidic radical polymerization initiators are used, reductants (e.g. sodium sulfite, sodium hydrogensulfite, ferrous sulfate, and L-ascorbic acid) may be used together to carry out redox polymerization. The amount of these polymerization initiators as used is usually in the range of 0.001 to 2 mol %, favorably 0.01 to 0.5 mol %.

[0142] The particle shape of the water-absorbent resin as obtained by the above polymerization is generally such as irregular pulverized, spherical, fibrous, bar, almost spherical, or flat shape.

[0143] In order to obtain the water-absorbent resin as used in the present invention wherein the water-absorbent resin displays an excellent capillary absorption index and a capillary absorption capacity at a height of 40 cm, its particle surface is favorably crosslinked with a surface-crosslinking agent.

[0144] Examples of the surface-crosslinking agent usable for surface-crosslinking the water-absorbent resin include: polyhydric alcohol compounds, such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymers, pentaerythritol, and sorbitol; epoxy compounds, such as ethylene glycol diglycidyl ether, polyethylene diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and glycidol; polyamine compounds, such as ethylenediamnine, diethylenetriamine, triethylenetetraamine, tetraethylenepentaamine, pentaethylenehexaamine and polyethylenimine, and their inorganic or organic salts (e.g. azetidinium salts); polyisocyanate compounds, such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; polyoxazoline compounds, such as 1,2-ethylenebisoxazoline; allcylene carbonate compounds, such as 1,3-dioxolan-2-one, 4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan -2-one, 4-hydroxymethyl-1 ,3-dioxolan-2-one, 1,3-dioxan-2-one, 4-methyl- 1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and 1,3-dioxopan-2-one; haloepoxy compounds, such as epichlorohydrin, epibromohydrin and α-methylepichlorolhydrin, and their polyamine adducts (for example, Kymene made by Hercules: registered trademark); silane coupling agents, such as γ-glycidoxypropyltrimethoxysilane, and γ-aminopropyltriethoxysilane; and polyvalent metallic compounds, such as hydroxides and chlorides of zinc, calcium, magnesium, aluminum, iron, and zirconium.

[0145] Of the above, the surface-crosslinking agent favorably includes a combination of surface-crosslinking agents of which the solubility parameters are different each other. The surface-crosslinking agent favorably includes a combination of: a first surface-crosslinking agent having a solubility parameter of not less than 25.6 [(J/cm 3 ) 1/2 ] (12.5 [(cal/cm 3 ) 1/2 ]); and a second surface-crosslinking agent having a solubility parameter of less than 25.6 [(J/cm 3 ) 1/2 ] (12.5 [(cal/cm 3 ) 1/2 ]). The solubility parameter of the surface-crosslinking agent is disclosed in such as U.S. Pat. No. 5,422,405.

[0146] The amount of the surface-crosslinking agent as used is favorably in the range of about 0.001 to about 5 parts by weight per 100 parts by weight of the water-absorbent resin. In the case where the amount is larger than 5 parts by weight or smaller than 0.001 part by weight, there is a case where it is difficult to obtain the surface-crosslinked layer in the range of the present invention.

[0147] Water may be used when the present invention surface-crosslinking agent is blended with the water-absorbent resin. The amount of water as used is also generally in the range of 0.5 to 10 parts by weight (excluding 0.5 part by weight), favorably l to 5 parts by weight, per 100 parts by weight of the water-absorbent resin in terms of solid content.

[0148] In addition, when the surface-crosslinking agent or its aqueous solution is blended, hydrophilic organic solvents or a third substance may be used. When the hydrophilic organic solvents are used, examples of the hydrophilic organic solvents include: lower alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol; ketones such as acetone;- ethers, such as dioxane, tetrahydrofuran, methoxy(poly)ethylene glycol; amides, such as ε-caprolactam, and N,N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; and polyhydric alcohols, such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol, polyglycerol, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymers, pentaerythritol, and sorbitol. The amount of the hydrophilic organic solvent as used is different according to factors such as the kind, particle diameter, or water content of the water-absorbent resin, but it is favorably smaller than 50 parts by weight, more favorably 0.1 to 10 parts by weight, per 100 pars by weight of the water-absorbent resin in terms of solid content. In addition, such as inorganic acids, organic acids, and poly(amino acids) shown in EP 0668080 may be allowed to exist as the third substance.

[0149] There is no especial limitation on blending methods which involve blending the water-absorbent resin and the surface-crosslinking agent, but examples thereof include: a method which involves immersing a water-absorbent resin in a hydrophilic organic solvent, and if necessary blending a surface-crosslinking agent as dissolved in water and/or the hydrophilic organic solvent; and a blending method which involves spraywise or dropwise adding a surface-crosslinking agent directly to a water-absorbent resin wherein the surface-crosslinking agent is dissolved in water and/or a hydrophilic organic solvent. In addition, the blending temperature, namely, both the temperature of the water-absorbent resin powder before blending and the temperature of the treating agent including the surface-crosslinking agent are controlled in a specific range, and thereby there is a case where the thickness or the weight ratio of the crosslinked layer is easily controlled in the range of the present invention. In addition, when the blending is carried out by using water, such as water-insoluble fine particulate powders and surfactants may be allowed to exist together.

[0150] After the water-absorbent resin and the surface-crosslinking agent are blended, the heat treatment is usually carried out to achieve the crosslinking reaction. The above heat-treating temperature depends also upon the surface-crosslinking agent as used, but the temperature of the water-absorbent resin powder is favorably adjusted to the range of 40 to 250° C. In the case, where the treating temperature is lower than 40° C., there is a case where a water-absorbing agent having excellent absorption properties cannot be obtained. In the case where the treating temperature is higher than 250° C., there is a case where the deterioration of the water-absorbent resin is caused, and the performance is lowered. Therefore, it is necessary to pay attention to this matter. The heat-treating time is in the range of about a minute to about 2 hours, favorably about 5 minutes to about an hour.

[0151] Of the matters as mentioned above, preferred examples of methods to obtain the water-absorbent resin which is usable in the present invention and displays an excellent capillary absorption index B and capillary absorption capacity D at a height of 40 cm include:

[0152] (1) a method which involve heat-treating a carboxyl-group-containing water-absorbent resin precursor having a weight-average particle diameter of not larger than 250 μm (favorably in the range of 40 to 200 μm, more favorably 70 to 150 μm), in the presence of a first surface-crosslinking agent having a solubility parameter of not less than 25.6 [(J/cm 3 ) 1/2 ] (12.5 [(cal/cm 3 ) 1/2 ]) and a second surface-crosslinking agent having a solubility parameter of less than 25.6 [(J/cm 3 ) 1/2 ] (12.5 [(cal/cm 3 ) 1/2 ]) wherein the surface-crosslinking agents are reactable with the carboxyl group;

[0153] (2) a method which involve: heat-treating a carboxyl-group-containing water-absorbent resin precursor in the presence of a surface-crosslinking agent at a water content of not more than 10% wherein the water-absorbent resin precursor is obtained by reversed-phase suspension polymerization and has a weight-average particle diameter of not larger than 250 μm (favorably in the range of 40 to 200 μm, more favorably 70 to 150 μm), so that the absorption capacity will be not less than 20 (g/g) under a load of 2.07 kPa (0.3 psi), favorably not less than 25 (g/c,), more not less than 30 (g/g); and thereafter treating the resultant water-absorbent resin with a solvent; and

[0154] (3) a method which involve: surface-crosslinking-treating a carboxyl-group-containing water-absorbent resin precursor having a weight-average particle diameter of 100 to 1,000 μm in the presence of a polyhydric alcohol or an alkylene carbonate; and thereafter classifying the resultant water-absorbent resin with a sieve having specific particle diameter distribution in order to obtain particles having a weight-average particle diameter of not larger than 300 μm (favorably in the range of 10 to 250 μm, more favorably 70 to 150 μm).

[0155] According to these methods, obtained is a water-absorbent resin displaying a capillary absorption capacity D of such as not less than 15 (g/g) at a height of 40 cm, favorably not less than 20 (g/g), most favorably not less than 25 (g/g), and it can favorably be used for the present invention. In addition, according to the above methods, obtained is a water-absorbent resin displaying a capillary absorption index B of such as not less than 0.4 at a height of 40 cm, favorably not less than 0.5, more favorably not less than 0.6, and it can favorably be used for the present invention.

[0156] Of the above, favorable is the water-absorbent resin of which the major proportion is comprised of a crosslinked poly(acrylic acid (salt)) polymer which is surface-crosslinking-treated by the above method (2) and has a weight-average particle diameter of not larger than 250 μm and is obtained by reversed-phase suspension polymerization. A hitherto unknown excellent resin displaying a capillary absorption capacity D of such as not less than 25 (g/g) at a height of 40 cm can be obtained. Incidentally, whether the surface-crosslinking treatment is carried out or not can be distinguished by such as a method that is disclosed in Japanese Patent Application No. 309105/1999.

[0157] Incidentally, various functions also can be given or enhanced to the present invention water-absorbent resin by further adding additives (e.g. water-insoluble fine-particulate inorganic powders, such as silicon dioxide, titanium dioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, calcium phosphate, barium phosphate, silicic acid or its salts, clay, diatom earth, zeolite, bentonite, kaolin, hydrotalcite, and active white salts; and deodorants, perfumes, antimicrobial agents, cationic polymer compounds such as polyamines, adhesives, pressure sensitive adhesives, foaming agents, pigments, dyes, manure, oxidants, reductants, and water) to the water-swellable water-insoluble hydrogel-formable polymer, and thereby by including the additives in the above water-swellable water-insoluble hydrogel-formable polymer or attaching them thereto. The amount of the above additive as used is favorably smaller than 30 weight %, more favorably smaller than 10 weight %, still more favorably smaller than 5 weight %, particularly favorably smaller than 1 weight %, relative to the total of the water-swellable water-insoluble hydrogel-formable polymer and the additive.

[0158] (2-4) Absorbent Structure:

[0159] The present invention absorbent structure is obtained by combining a liquid-diffusing member and a water-absorbent resin in order to satisfy liquid-diffusion-and-storage coefficient 1 and/or liquid-diffusion-and-storage coefficient 2 in the present invention.

[0160] In addition, the present invention absorbent structure is also obtained by using: a liquid-diffusing member displaying a suction height of not lower than 30 cm; and a water-absorbent resin, as a liquid-storing member, displaying a capillary absorption capacity D of not less than 15 (g/g) at a height of 40 cm.

[0161] Furthermore, the present invention absorbent structure is also obtained by using: a liquid-diffusing member displaying a suction height of not lower than 30 cm; and a water-absorbent resin as a liquid-storing member wherein the water-absorbent resin is surface-crosslinking-treated and has a weight-average particle diameter of not larger than 250 μn.

[0162] The present invention absorbent structure may comprise other materials in addition to the liquid-diffusing member and the water-absorbent resin, as long as the liquid-diffusion-and-storage system as aimed in the present invention is not hindered. Examples of other materials include hydrophilic fibers, nonwoven fabrics, papers, and tissue papers. Examples of the above hydrophilic fibers include: cellulose fibers as obtained from wood, such as mechanical pulps, chemical pulps, semi-chemical pulps, and dissolved pulps; and fibers, such as rayon and acetate. Among the above-exemplified fibers, the cellulose fibers are favorable. In addition, the hydrophilic fibers may include synthetic fibers, such as polyamide, polyesters, and polyolefin. Incidentally, the hydrophilic fibers are not limited to the above-exemplified fibers. Examples of the nonwoven fabrics include nonwoven fabrics of such as polyesters, polyethylene, polypropylene, nylon, and rayon, having a spun bond, chemical bond, or spunlace system.

[0163] The weight ratio of the water-absorbent resin and the liquid-diffusing member in the absorbent structure can be selected in an arbitrarily range, but the weight ratio of the water-absorbent resin is favorably in the range of 5 to 99 weight %, more favorably 20 to 90 weight %, still more favorably 30 to 80 weight % , relative to the total weight of the water-absorbent resin and the liquid-diffusing member.

[0164] Particularly, in the case where the weight ratio of the water-absorbent resin is in the range of 75 to 90 weight % relative to the total weight of the water-absorbent resin and the liquid-diffusing member, there are advantages in that: the amount of the liquid-diffusing member as used can be lowered relatively, and therefore a lighter and thinner absorbent structure can be produced in view of shape. In addition, in order to produce the absorbent structure in which the weight ratio of the water-absorbent resin is in the range of 75 to 90 weight % relative to the total weight of the water-absorbent resin and the liquid-diffusing member, a water-absorbent resin displaying a capillary absorption capacity D of not less than 15 (g/g) at a height of 40 cm is more favorably used as the water-absorbent resin. When using a water-absorbent resin displaying a capillary absorption capacity D of not less than 15 (g/g) at a height of 40 cm as the water-absorbent resin, the transfer and diffusion of the liquid from the liquid-diffusing member to the water-absorbent resin is favorably carried out. Therefore, the storage ability is not requested as the liquid-diffusing member, and the amount of the liquid-diffusing member as used is greatly decreased. Then, a porous polymer, which is obtained by a process including the step of polymerizing a high-internal-phase emulsion and of which the suction height is not lower than 30 cm, is favorably used as the liquid-diffusing member.

[0165] Examples of the arranging position of the water-absorbent resin include: a back face of the liquid-diffusing member, a front face of the liquid-diffusing member, a portion of a back face side of the liquid-diffusing member, a portion of a front face side of the liquid-diffusing member, a portion between the liquid-diffusing members, and an inner portion of the liquid-diffusing member, and these arranging methods may be combined. Of the above, the water-absorbent resin is favorably arranged at a back face side of the liquid-diffusing member, the water-absorbent resin more favorably exists in a layer form. In addition, the weight of the water-absorbent resin per its unit area is in the range of about 50 to about 500 g /m 2 .

[0166] Examples of the arranging state of the water-absorbent resin include: a state in which the water-absorbent resin uniformly exists over the entire surface of the liquid-diffusing member; a state in which the water-absorbent resin exists in a specific pattern; a state in which the water-absorbent resin exists with a slope of density; a state in which the water-absorbent resin exists only in the center of the liquid-diffusing member; and a state in which the water-absorbent resin exists only front and back the liquid-diffusing member.

[0167] In addition, the water-absorbent resin itself is converted to a sheet by the hitherto publicly known method, or scattered on a base material for fixing, or packed in a bag, or given adhesion. Thereafter, it may be combined with the liquid-diffusing member. Furthermore, the water-absorbent resin may adhere to the liquid-diffusing member by using an adhesive binder.

[0168] Examples of the above adhesive binder include: hot-melt adhesive fibers, such as polyolefin fibers (e.g. polyethylene, polypropylene, ethylene-propylene copolymers, and 1-butene-ethylene copolymers), and adhesive emulsions, and hot-melt adhesives. These adhesive binders may be used either alone respectively or in combinations with each other.

[0169] Incidentally, also as to the present invention absorbent structure, materials (e.g. water-insoluble fine-particulate inorganic powders, such as silicon dioxide, titanium dioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, calcium phosphate, barium phosphate, silicic acid or its salts, clay, diatom earth, zeolite, bentonite, kaolin, hydrotalcite, and active white salts; deodorants; perfumes; antimicrobial agents; cationic polymer compounds such as polyamines; foaming agents; pigments; dyes; hydrophilic short fibers; manure; oxidants; reductants; and water) are further added thereto, and further functions can also be given to the absorbent structure.

[0170] [3] Absorbent Structure Comprising Liquid-Acquiring Member and Water-Absorbent Resin Layer:

[0171] (3-1) Relationship Between Capillary Absorption Ability of Liquid-Acquiring Member and that of Water-Absorbent Resin (Layer):

[0172] The relationship between the capillary absorption ability of the liquid-acquiring member and that of the water-absorbent resin (layer) in the present invention is explained.

[0173] The water-absorbent resin layer as combined with the liquid-acquiring member favorably includes a water-absorbent resin in a scattering amount of not smaller than 250 g/m 2 , and it is composed so that the water-absorbent resin will form a substantially continuous layer when being swollen. In the case where the scattering amount is smaller than 250 g/m 2 , there is a tendency such that: the saturated absorption quantity of the absorbent structure is decreased; the liquid-acquiring layer cannot be dried sufficiently; the dry feeling is deteriorated; and the amount of wet back of the aqueous liquid is increased. The scattering amount in the water-absorbent resin layer is more favorably not smaller than 300 g/m 2 , still more favorably not smaller than 350 g/m 2 , particularly favorably not smaller than 400 g/m 2 .

[0174] The water-absorbent resin layer comprises only the water-absorbent resin or a mixture of the water-absorbent resin and other water-absorbent or hydrophilic materials. Examples of other water-absorbent or hydrophilic materials include: fibers, such as natural fibers, regenerated fibers, and synthetic fibers (e.g. pulps, rayon, polyesters, and nylon); and these hydrophilyzing-treated materials. The ratio of the water-absorbent resin in the water-absorbent resin layer is favorably not less than 70 weight % in view of thinning the absorbent structure and enabling the absorption amount to increase, more favorably not less than 80 weight %, still more favorably not less than 90 weight %. The water-absorbent resin layer particularly favorably comprises only the water-absorbent resin (namely, 100 weight %).

[0175] The water-absorbent resin usable in the present invention is a water-absorbent resin wherein when the capillary absorption index of the liquid-acquiring member at a height of 40 cm is referred to as E (E<0.1), the capillary absorption index B of the water-absorbent resin at a height of 40 cm satisfies the following equation:

B/E≧ 10 (equation 3)

[0176] In addition, the water-absorbent resin layer usable in the present invention is a water-absorbent resin wherein when the capillary absorption index of the liquid-acquiring-member at a height of 40 cm is referred to as E (E<0.1), the capillary absorption index F of the water-absorbent resin layer at a height of 40 cm satisfies the following equation:

F/E≧ 10 (equation 4)

[0177] The value of the capillary absorption index B or F of the water-absorbent resin or water-absorbent resin layer necessary in the present invention at a height of 40 cm is different depending upon the property of the liquid-acquiring member as used, namely, the capillary absorption index E of the liquid-acquiring member as used at a height of 40 cm. If the above relationship B/E≧10 or F/E≧10 is satisfied, a liquid from the liquid-acquiring member to the water-absorbent resin is favorably absorbed, and the water-absorbent resin can dry the liquid-acquiring member sufficiently. In the case where the B/E or F/E is less than 10, there are cases where: the water-absorbent resin cannot absorb the liquid from the liquid-acquiring member sufficiently; and the liquid-acquiring member is left in a wet feeling; and the next liquid cannot be received in a moment. The water-absorbent resin favorably satisfies B/E≧20 or F/E≧20, more favorably B/E≧30 or F/E≧30. Incidentally, hereinafter, the value of the B/E or F/E may be referred to as a liquid-acquirement-and-storage coefficient 1. In addition, the B means a capillary absorption index as determined by using a single water-absorbent resin, and the F means a capillary absorption index as determined by using a water-absorbent resin layer itself, when it is, for example, difficult to isolate the water-absorbent resin from the water-absorbent resin layer.

[0178] As to another absorbent structure usable in the present invention, the liquid-acquiring member displays a capillary absorption capacity G of not more than 1.0 (g/g) at a height of 40 cm, and the aforementioned water-absorbent resin displays a capillary absorption capacity D of not less than 5 (g/g) at a height of 40 cm.

[0179] In addition, as to another absorbent structure usable in the present invention, the liquid-acquiring member displays a capillary absorption capacity G of not more than 1.0 (g/g) at a height of 40 cm, and the aforementioned water-absorbent resin layer displays a capillary absorption capacity H of not less than 5 (g/g) at a height of 40 cm.

[0180] If the liquid-acquiring member and the water-absorbent resin or water-absorbent resin layer satisfy these relationships, a liquid from the liquid-acquiring member to the water-absorbent resin is favorably distributed, and the water-absorbent resin can dry the liquid-acquiring member sufficiently, and the liquid can be absorbed and stored therein.

[0181] In the case where the D or H is less than 5 (g/g), the water-absorbent resin difficultly absorbs the liquid from the liquid-acquiring member sufficiently, and the liquid-acquiring member is not dried, and the amount of wet back of the aqueous liquid is greatly increased. The value of the capillary absorption capacity D or