WO2022255302A1

WO2022255302A1 - Water absorbent sheet and absorbent article

Info

Publication number: WO2022255302A1
Application number: PCT/JP2022/021927
Authority: WO
Inventors: 春香津留
Original assignee: 住友精化株式会社
Priority date: 2021-05-31
Filing date: 2022-05-30
Publication date: 2022-12-08
Also published as: CN117295477A; JPWO2022255302A1

Abstract

A water absorbent sheet which comprises an absorbent layer and nonwoven fabrics that are arranged so that the absorbent layer is sandwiched therebetween in the vertical direction, wherein: the absorbent layer contains a water absorbent resin; and the water absorbent resin has the characteristics (A) to (C) described below. (A) The saline holding capacity is from 45 g/g to 70g/g. (B) The water absorption under a load of 4.14 kPa is 13 ml/g or more. (C) The five-minute value of DW under no pressure is from 44 ml/g to 80 ml/g.

Description

Absorbent sheet and absorbent article

The present invention relates to water absorbent sheets and absorbent articles, and more particularly to water absorbent sheets suitable for sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads, and absorbent articles using water absorbent sheets.

In recent years, water absorbent resins have been widely used in the field of sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads.

As such a water absorbent resin, a crosslinked product of a polymer of a water-soluble ethylenically unsaturated monomer, more specifically a crosslinked product of a partially neutralized polymer of polyacrylic acid, has excellent water absorption capacity. In addition, since acrylic acid, which is the raw material, is easily available industrially, it can be manufactured at a constant quality and at a low cost, and it has many advantages such as being less prone to putrefaction and deterioration. It is said that it is a flexible resin (see, for example, Patent Document 1).

Absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads are composed of an absorbent body that absorbs and retains body fluids such as urine and menstrual blood excreted from the body, and a side that comes in contact with the body. It consists of a liquid-permeable surface sheet (top sheet) arranged on the front side and a liquid-impermeable back sheet (back sheet) arranged on the opposite side in contact with the body. In addition, the absorber is usually composed of hydrophilic fibers such as pulp and water-absorbent resin.

JP-A-3-227301

In such absorbent articles, the absorbent resin contained in the absorbent body is required to have a high water retention capacity. By increasing the water retention capacity of the water-absorbing resin, the phenomenon that the liquid once absorbed by the absorber returns back (that is, the liquid returns from the absorber, and when the absorber is touched by hand, it becomes uncomfortable wetness. feel) can be improved. However, when the absorbent article is used, if the absorbent surface of the absorbent article is inclined, the liquid that is repeatedly excreted is not sufficiently absorbed, and despite the high water retention capacity of the water absorbent resin, There may also be a problem that the liquid leaks out of the absorbent article.

Under such circumstances, the main object of the present invention is to provide a water absorbent sheet that suppresses the occurrence of leakage.

The inventors have diligently studied to solve the above problems. As a result, in a water-absorbent sheet comprising an absorbent layer and non-woven fabric sandwiching the upper and lower sides of the absorbent layer, the water-absorbent resin has a predetermined physiological saline water retention capacity, a water absorption capacity under a load of 4.14 kPa, and a non-pressurized It was found that the occurrence of leakage can be suppressed by using a material having a DW of 5 minutes. The present invention is an invention that has been completed through extensive research based on such findings.

That is, the present invention provides an invention having the following configurations.
Section 1. A water absorbent sheet comprising an absorbent layer and a nonwoven fabric sandwiching the upper and lower sides of the absorbent layer,
The absorbent layer contains a water absorbent resin,
The water absorbent sheet, wherein the water absorbent resin has the following properties (A) to (C).
(A) The physiological saline water retention capacity is 45 g/g or more and 70 g/g or less.
(B) The water absorption under a load of 4.14 kPa is 13 ml/g or more.
(C) The 5-minute DW value without pressure is 44 ml/g or more and 80 ml/g or less.
Section 2. Item 2. The water absorbent sheet according to Item 1, wherein the water absorbent resin has a basis weight of 100 g/m ² or more and 450 g/m ² or less.
Item 3. Item 3. The water absorbent sheet according to Item 1 or 2, wherein the absorbent layer is adhered to the nonwoven fabric via an adhesive.
Section 4. An absorbent article comprising the water absorbent sheet according to Items 1 to 3.

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

FIG. 2 is a schematic diagram of a measuring device used for measuring the water absorption amount of physiological saline under a load of 4.14 kPa of a water absorbent resin. It is a schematic diagram of a measuring device used for measuring the non-pressure DW of the water absorbent resin. FIG. 3 is a schematic diagram for explaining a method of a leak test (gradient absorption test) for absorbent articles.

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

In this specification, a numerical value connected by "-" means a numerical range including the numerical values before and after "-" as lower and upper limits. If multiple lower limits and multiple upper limits are listed separately, any lower limit and upper limit can be selected and connected with "-".

1. Water-absorbing sheet The water- absorbing sheet of the present invention is a water-absorbing sheet comprising an absorbent layer and non-woven fabric sandwiching the upper and lower sides of the absorbent layer. In the water absorbent sheet of the present invention, the absorbent layer contains a water absorbent resin, and the water absorbent resin has the following properties (A) to (C).
(A) The physiological saline water retention capacity is 45 g/g or more and 70 g/g or less.
(B) The water absorption under a load of 4.14 kPa is 13 ml/g or more.
(C) The 5-minute DW value without pressure is 44 ml/g or more and 80 ml/g or less.

By providing such features, the water absorbent sheet of the present invention can be used in absorbent articles to suppress the occurrence of leakage. The water absorbent sheet of the present invention will be described in detail below.

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

The absorbent layer contained in the water absorbent sheet of the present invention may be one layer, or may be two or more layers. The absorbent layer contained in the water absorbent sheet is preferably one or two layers. In addition, when the absorbent sheet includes two or more absorbent layers, it is preferable to arrange a liquid-permeable substrate such as a nonwoven fabric between the absorbent layers. As for this liquid-permeable substrate, the same non-woven fabric as will be described later that sandwiches the upper and lower sides of the absorbent layer can be used.

The structure of the water-absorbent sheet may be any structure in which the absorbent layer containing the water-absorbent resin is sandwiched between nonwoven fabrics, for example, a sheet-like structure in which the absorbent layer is fixed between two nonwoven fabrics. It may be a structure wrapped in .

The planar shape of the water-absorbent sheet is appropriately determined according to the application or the shape of the absorbent article, and examples thereof include substantially rectangular, oval, hourglass, and battledore shapes, and cuts are provided to improve fit. etc. Moreover, it may have a block region in which a plurality of block structures partitioned by a plurality of vertical grooves extending in the vertical direction and a plurality of horizontal grooves extending in the horizontal direction orthogonal to the vertical direction are arranged in the vertical direction. In addition, the internal structure of the water absorbent sheet is also appropriately determined according to the purpose. For example, in addition to consisting of a single water-absorbing sheet, multiple water-absorbing sheets (split on the plane, split in the vertical direction, etc.) are combined, or inside the water-absorbing sheet, water-absorbing resin and other For example, the component may have a quantitative distribution gradient (uniform distribution, quantitative distribution depending on the liquid input portion, etc.). In addition, the water absorbent sheet may be embossed into the water absorbent sheet in order to provide liquid flow paths.

The absorbent layer is preferably adhered to the nonwoven fabric via an adhesive. As the adhesive for adhering (fixing) the absorbent layer to the nonwoven fabric, a hot melt adhesive or the like is preferable.

In the water absorbent sheet of the present invention, the basis weight of the water absorbent resin contained in the absorbent layer is not particularly limited, but from the viewpoint of exhibiting the effects of the present invention more preferably, it is preferably 100 to 450 g/m ² , more preferably 120 g/m 2 . ~350 g/m ² , more preferably 150-300 g/m ² .

The content of the water absorbent resin in the water absorbent sheet is preferably 5 to 100% by mass, more preferably 10 to 95% by mass, further preferably 20 to 90% by mass, and 30 to 80% by mass. % by mass is even more preferred.

By holding the water-absorbing sheet between a liquid-permeable front sheet (top sheet) through which liquid can pass and a liquid-impermeable back sheet (back sheet) through which liquid cannot pass, It can be an absorbent article. The liquid permeable sheet is arranged on the side in contact with the body, and the liquid impermeable sheet is arranged on the opposite side in contact with the body.

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

The basis weight of the liquid-permeable sheet is preferably 5 g/m ² or more and 100 g/m ² or less, more preferably 10 g/m ² or more and 60 g/m ² or less. In addition, the surface of the liquid-permeable sheet may be embossed or perforated in order to improve the diffusibility of the liquid. The embossing and perforation can be carried out by known methods.

Examples of liquid-impermeable sheets include sheets made of synthetic resins such as polyethylene, polypropylene, and polyvinyl chloride, and spunbond/meltblown/spunbond (SMS) nonwoven fabrics in which a water-resistant meltblown nonwoven fabric is sandwiched between high-strength spunbond nonwoven fabrics. and sheets made of composite materials of these synthetic resins and nonwoven fabrics (for example, spunbond nonwoven fabrics and spunlaced nonwoven fabrics). The liquid-impermeable sheet may have air permeability from the viewpoint of reducing stuffiness when worn and reducing discomfort given to the wearer. As the liquid-impermeable sheet, a sheet made of synthetic resin mainly composed of low-density polyethylene (LDPE) resin can be used. The liquid-impermeable sheet may be, for example, a synthetic resin sheet having a weight per unit area of 10 to 50 g/m ² . Moreover, in order to impart air permeability to the liquid-impermeable sheet, for example, a filler may be added to the resin sheet, or the liquid-impermeable sheet may be embossed. Calcium carbonate or the like is used as the filler.

The absorbent article preferably has a liquid-permeable sheet arranged on the upper surface of the water-absorbent sheet, and a liquid-impermeable sheet arranged on the side opposite to the liquid-permeable sheet. Moreover, in the absorbent article, an absorbent body composed of a water absorbent resin and hydrophilic fibers may be combined with the upper surface and/or the lower surface of the water absorbent sheet.

In addition to the above-described liquid-permeable sheet, water-absorbent sheet, and liquid-impermeable sheet, the absorbent article may also include members as appropriate according to the application and function. Examples include core wraps, liquid acquisition and diffusion sheets, outer cover nonwoven fabrics, leg gathers, and the like.

For example, the core wrap is arranged so as to cover the outer periphery of the water absorbent sheet. A water absorbent sheet is placed in the core wrap. The core wrap includes tissue, nonwoven fabric, and the like. The core wrap has, for example, a main surface of the same size as the water absorbent sheet. The water absorbent sheet is shape-retained by enclosing it in a core wrap. The method of retaining the shape of the water-absorbent sheet by the core wrap is not limited to this. For example, the water-absorbent sheet may be sandwiched between two separate upper and lower core wraps, or the core wraps may form a bag body in which the water-absorbent sheet may be arranged. .

(liquid acquisition diffusion sheet)
The absorbent article may include a liquid acquisition diffusion sheet. A liquid acquisition and diffusion sheet may be placed, for example, on the underside of the liquid permeable sheet. As a result, the liquid that permeates the liquid-permeable sheet can be quickly moved to the water-absorbing sheet side, or the backflow can be further reduced. A hot-melt adhesive, heat embossing, or ultrasonic welding may be used for bonding between the liquid acquisition diffusion sheet and the liquid permeable sheet. As the liquid acquisition/diffusion sheet, a non-woven fabric or a resin film having a large number of through-holes can be used. As the non-woven fabric, the same material as described in the liquid-permeable sheet section can be used. It is preferable because it has excellent movement characteristics.

The liquid acquisition/diffusion sheet is usually arranged in the central portion with a width shorter than that of the water absorbent sheet, but may be arranged over the entire width. The length of the liquid acquisition/diffusion sheet in the front-rear direction may be substantially the same as the total length of the absorbent article, may be substantially the same as the total length of the water absorbent sheet, or may be within a range of lengths assuming the portion into which the liquid is introduced. good too.

(Outer cover non-woven fabric)
Further, the outer cover nonwoven fabric may be arranged on the side of the liquid impermeable sheet facing the water absorbent sheet. The outer cover nonwoven can be adhered to the liquid impermeable sheet using, for example, an adhesive. The outer cover nonwoven fabric may be formed of one or more layers and may be a soft material. The outer cover nonwoven fabric may be imparted with a soft touch, may have a pattern printed on it, or may have a plurality of joints so as to appeal to consumers' willingness to purchase or for other reasons. , embossed, or formed into a three-dimensional form.

(leg gathers)
The absorbent article of the present invention has leg gathers provided with stretchable elastic members that are arranged outside both widthwise end portions of the water absorbent sheet and installed substantially parallel to the longitudinal direction of the water absorbent sheet. may be The length of the leg gathers is set to be around the wearer's leg or longer. The elongation rate of the leg gathers is appropriately set from the viewpoint of preventing the leakage of discharged liquid and reducing the feeling of oppression when worn for a long time.

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

The absorbent article of the present invention has front/back gathers that can rise above the side edges in the width direction of the water absorbent sheet. That is, on both sides of the absorbent article in the longitudinal direction, a front/back gather sheet member having a gather elastic member is arranged to constitute a front/back gather.

The member for the front/back gathers is usually made of a liquid-impermeable or water-repellent material, preferably a moisture-permeable material. Examples thereof include a liquid-impermeable or water-repellent porous sheet, a liquid-impermeable or water-repellent nonwoven fabric, or a laminate of the porous sheet and the nonwoven fabric. Examples of the nonwoven fabric include thermal bonded nonwoven fabric, spunbond nonwoven fabric, meltblown nonwoven fabric, spunlace nonwoven fabric, spunbond/meltblown/spunbond nonwoven fabric, and the like. The basis weight of the member may be 5 to 100 g/m ² , 8 to 70 g/m ² , or 10 to 40 g/m ² .

Each member constituting the absorbent article of the present invention may be adhered. For example, by adhering the water absorbent sheet and the liquid permeable sheet, the liquid is more smoothly guided to the water absorbent sheet, making it easier to obtain an absorbent article with excellent leakage prevention. When the water absorbent sheet is covered or sandwiched by the core wrap 20, it is preferable that at least the core wrap 20 and the liquid permeable sheet 30 are adhered, and more preferably that the core wrap 20 and the water absorbent sheet are adhered. Adhesion methods include known methods such as adhesives, heat sealing, and ultrasonic sealing. For example, a hot-melt adhesive is applied to a liquid-permeable sheet at predetermined intervals in the width direction in a shape such as vertical stripes or spirals, starch, carboxymethylcellulose, polyvinyl alcohol, polyvinylpyrrolidone and other water-soluble adhesives. Examples include a method using a water-soluble adhesive selected from polymers. In addition, when the water absorbent sheet contains heat-sealable synthetic fibers, a method using heat-sealing thereof may be employed.

The water absorbent resin used in the water absorbent sheet of the present invention is characterized by having the following properties (A) to (C). The water absorbent sheet of the present invention using the water absorbent resin having such characteristics can suppress the occurrence of leakage. The water absorbent resin will be described in detail below.

(A) The physiological saline water retention capacity is 45 g/g or more and 70 g/g or less.
(B) The water absorption under a load of 4.14 kPa is 13 ml/g or more.
(C) The 5-minute value of non-pressurized DW is 44 ml/g or more and 80 ml/g or less.

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

When the water absorbent sheet of the present invention contains two or more absorbent layers, at least one layer may contain a water absorbent resin having properties (A) to (C). The water absorbent resins contained may be the same or different. Further, from the viewpoint of more preferably exhibiting the effects of the present invention, it is preferable that the water absorbent sheet contains a water absorbent resin having properties (A) to (C) in 50% or more of the total absorbent layer. More preferably, the layer contains a water absorbent resin having properties (A) to (C).

From the viewpoint of more preferably exhibiting the effects of the present invention, the water-absorbent resin preferably has a physiological saline water retention capacity of 46 g/g or more, more preferably 47 g/g or more, and still more preferably 48 g/g or more, Also, it is preferably 70 g/g or less, more preferably 68 g/g or less, and still more preferably 65 g/g or less.

Further, from the viewpoint of exhibiting the effects of the present invention more preferably, the water absorption amount of the water absorbent resin under a load of 4.14 kPa is preferably 14 ml/g or more, more preferably 15 ml/g or more, and still more preferably 16 ml. /g or more, preferably 33 ml/g or less, more preferably 27 ml/g or less, still more preferably 23 ml/g or less.

In addition, from the viewpoint of exhibiting the effect of the present invention more preferably, the 5-minute value of the non-pressurized DW of the water absorbent resin is preferably 46 ml/g or more, more preferably 48 ml/g or more, and still more preferably 50 ml/g. g or more, preferably 70 ml/g or less, more preferably 60 ml/g or less, and even more preferably 54 ml/g or less.

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

The method for measuring the physiological saline water retention capacity of the water-absorbing resin, the water absorption capacity under a load of 4.14 kPa, the non-pressurized DW 5-minute value, and the method for the gradient absorption test are as described in Examples.

The water absorbent resin is formed by cross-linking a polymer of water-soluble ethylenically unsaturated monomers, that is, a cross-linked polymer having structural units derived from water-soluble ethylenically unsaturated monomers.

The water absorbent resin used in the present invention may have various shapes. Examples of the shape of the water-absorbent resin include granular, substantially spherical, irregularly crushed, plate-like, fibrous, flake-like, and aggregated shapes of these resins. The water-absorbent resin is preferably in the form of granules, substantially spherical, crushed amorphous, fibrous, or aggregated forms of these resins.

The water absorbent resin may be in a form (secondary particles) in which fine particles (primary particles) are aggregated, in addition to the form in which each is composed of a single particle. Examples of the shape of the primary particles include a substantially spherical shape, an irregular crushed shape, and a plate shape. In the case of primary particles produced by reversed-phase suspension polymerization, a substantially spherical single particle shape having a smooth surface shape such as a perfect sphere, an ellipsoidal shape, or the like can be mentioned. Because of its smooth surface, it has high fluidity as a powder, and because it is easy for aggregated particles to be densely packed, it is difficult to break even when subjected to impact, and has high particle strength. Become.

The median particle size of the water absorbent resin is preferably 200 µm or more, 250 µm or more, 280 µm or more, 300 µm or more, or 320 µm or more from the viewpoint of more preferably exhibiting the effects of the present invention. From the same viewpoint, the median particle size is preferably 700 μm or less, 600 μm or less, 550 μm or less, 500 μm or less, 450 μm or less, or 400 μm or less. That is, the median particle size is preferably 200 to 700 μm, preferably 200 to 600 μm, more preferably 250 to 500 μm, further preferably 300 to 450 μm, further preferably 320 to 400 μm. is even more preferable.

The median particle size of the water-absorbing resin can be measured using a JIS standard sieve, and is specifically the value measured by the method described in the Examples.

As the method for polymerizing the water-soluble ethylenically unsaturated monomer, typical polymerization methods such as aqueous solution polymerization method, emulsion polymerization method, and reversed-phase suspension polymerization method are used. In the aqueous solution polymerization method, polymerization is carried out by heating an aqueous solution of a water-soluble ethylenically unsaturated monomer while stirring it if necessary. In the reversed-phase suspension polymerization method, polymerization is carried out by heating a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium with stirring. A reversed-phase suspension polymerization method is preferably used from the viewpoint of enabling precise control of the polymerization reaction and control of a wide range of particle sizes.

A specific embodiment of the method for producing a water-absorbing resin will be described below, taking the reverse phase suspension polymerization method as an example.

As a specific example of the method for producing a water absorbent resin, in the method for producing a water absorbent resin by reverse phase suspension polymerization of a water-soluble ethylenically unsaturated monomer in a hydrocarbon dispersion medium, the presence of a radical polymerization initiator and a step of post-crosslinking the water-containing gel obtained by the polymerization in the presence of a post-crosslinking agent. In the method for producing a water-absorbent resin, if necessary, an internal cross-linking agent may be added to the water-soluble ethylenically unsaturated monomer to form a hydrogel having an internal cross-linked structure.

<Polymerization process>
[Water-soluble ethylenically unsaturated monomer]
Water-soluble ethylenically unsaturated monomers include, for example, (meth)acrylic acid and its salts; 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts; (meth)acrylamide, N,N-dimethyl Nonionic monomers such as (meth)acrylamide, 2-hydroxyethyl (meth)acrylate, N-methylol (meth)acrylamide, polyethylene glycol mono (meth)acrylate; N,N-diethylaminoethyl (meth)acrylate, N , N-diethylaminopropyl (meth)acrylate, diethylaminopropyl (meth)acrylamide and other amino group-containing unsaturated monomers and quaternized products thereof. Among these water-soluble ethylenically unsaturated monomers, (meth)acrylic acid or a salt thereof, (meth)acrylamide, and N,N-dimethylacrylamide are preferable from the viewpoint of industrial availability. , (meth)acrylic acid and salts thereof are more preferred. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

Among these, acrylic acid and its salts are widely used as raw materials for water-absorbent resins. Sometimes used. In this case, acrylic acid and/or its salt is preferably used as a main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol % based on the total water-soluble ethylenically unsaturated monomers.

The water-soluble ethylenically unsaturated monomer is preferably dispersed in a hydrocarbon dispersion medium in the form of an aqueous solution and subjected to reversed-phase suspension polymerization. The water-soluble ethylenically unsaturated monomer can increase the dispersion efficiency in the hydrocarbon dispersion medium by forming an aqueous solution. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous solution is preferably in the range of 20 mass % to the saturated concentration. In addition, from the point that the water absorption property of the present invention is easily obtained, the concentration of the water-soluble ethylenically unsaturated monomer is more than 38% by mass. Preferably, it is 40% by mass or more, and even more preferably 42% by mass or more. On the other hand, the concentration of the water-soluble ethylenically unsaturated monomer is more preferably 55% by mass or less, even more preferably 50% by mass or less, and even more preferably 46% by mass or less.

When the water-soluble ethylenically unsaturated monomer has an acid group such as (meth)acrylic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, etc., the acid group is optionally alkalinized in advance. Those neutralized with a neutralizing agent may also be used. Examples of such alkaline neutralizers include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide and potassium carbonate; ammonia and the like. Moreover, these alkaline neutralizers may be used in the form of an aqueous solution in order to facilitate the neutralization operation. In addition, the alkaline neutralizing agent mentioned above may be used independently and may be used in combination of 2 or more types.

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

[Radical polymerization initiator]
Examples of radical polymerization initiators added to the polymerization step include persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate, methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, Peroxides such as t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxy isobutyrate, t-butyl peroxy pivalate, hydrogen peroxide, and 2,2'-azobis ( 2-amidinopropane) dihydrochloride, 2,2′-azobis[2-(N-phenylamidino)propane]dihydrochloride, 2,2′-azobis[2-(N-allylamidino)propane]dihydrochloride , 2,2′-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane}dihydrochloride, 2,2′-azobis{2-methyl-N-[1, 1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, 2,2′-azobis[2-methyl-N-(2-hydroxyethyl)-propionamide], 4,4′-azobis(4- cyanovaleric acid), and the like. Among these radical polymerization initiators, potassium persulfate, ammonium persulfate, sodium persulfate and 2,2′-azobis(2-amidinopropane) dihydrochloride are preferred from the viewpoint of easy availability and handling. be done. These radical polymerization initiators may be used alone or in combination of two or more. The radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

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

The amount of the radical polymerization initiator used is, for example, 0.00005 to 0.01 mol per 1 mol of the water-soluble ethylenically unsaturated monomer. By satisfying such a usage amount, rapid polymerization reaction can be avoided and the polymerization reaction can be completed in an appropriate time.

[Internal cross-linking agent]
Examples of the internal cross-linking agent include those capable of cross-linking the polymer of water-soluble ethylenically unsaturated monomers used, such as (poly)ethylene glycol, (poly)propylene glycol, 1,4-butanediol, tri Unsaturated polyesters obtained by reacting polyols such as diols and triols such as methylolpropane and (poly)glycerin with unsaturated acids such as (meth)acrylic acid, maleic acid and fumaric acid; N,N-methylene Bisacrylamides such as bisacrylamide; Di(meth)acrylic acid esters or tri(meth)acrylic acid esters obtained by reacting polyepoxide and (meth)acrylic acid; Di(meth)acrylic acid carbamyl esters obtained by reacting isocyanate with hydroxyethyl (meth)acrylate; allylated starch, allylated cellulose, diallyl phthalate, N,N',N''-triallyl isocyanate Compounds having two or more polymerizable unsaturated groups such as nurate and divinylbenzene; diglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether, triglycidyl polyglycidyl compounds such as compounds; epihalohydrin compounds such as epichlorohydrin, epibromhydrin and α-methylepichlorohydrin; compounds having two or more reactive functional groups such as isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl- Examples thereof include oxetane compounds such as 3-oxetaneethanol. Among these internal cross-linking agents, unsaturated polyesters or polyglycidyl compounds are preferably used, and diglycidyl ether compounds are more preferably used. Ether, (poly)glycerol diglycidyl ether is preferably used. These internal cross-linking agents may be used alone or in combination of two or more.

The amount of the internal cross-linking agent used is preferably 0.02 mol or less, more preferably 0.000001 to 0.01 mol, per 1 mol of the water-soluble ethylenically unsaturated monomer. More preferably 0.00001 to 0.005 mol, even more preferably 0.00005 to 0.002 mol.

[Hydrocarbon dispersion medium]
Examples of hydrocarbon dispersion media include those having 6 to 8 carbon atoms such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane and n-octane. Aliphatic hydrocarbons; alicyclic rings such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, trans-1,3-dimethylcyclopentane aromatic hydrocarbons such as benzene, toluene and xylene; Among these hydrocarbon dispersion media, n-hexane, n-heptane, and cyclohexane are preferably used because they are industrially readily available, stable in quality, and inexpensive. These hydrocarbon dispersion media may be used alone or in combination of two or more. As an example of the mixture of the hydrocarbon dispersion medium, a commercially available product such as Exsolheptane (manufactured by Exxon Mobil Co., containing 75 to 85% by mass of heptane and its isomer hydrocarbons) can also be used to obtain suitable results. be able to.

From the viewpoint of uniformly dispersing the water-soluble ethylenically unsaturated monomer and facilitating the control of the polymerization temperature, the amount of the hydrocarbon dispersion medium used is the water-soluble ethylenically unsaturated monomer in the first stage. It is preferably 100 to 1,500 parts by mass, more preferably 200 to 1,400 parts by mass, based on 100 parts by mass. As will be described later, the reversed-phase suspension polymerization is carried out in one stage (single stage) or in multiple stages of two or more stages, and the above-described first stage polymerization is the first stage in single stage polymerization or multistage polymerization. means the polymerization reaction of (the same applies below).

[Dispersion stabilizer]
(Surfactant)
In the reversed-phase suspension polymerization, a dispersion stabilizer can be used to improve the dispersion stability of the water-soluble ethylenically unsaturated monomer in the hydrocarbon dispersion medium. A surfactant can be used as the dispersion stabilizer.

Examples of surfactants include sucrose fatty acid esters, polyglycerin fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene glycerin fatty acid esters, sorbitol fatty acid esters, polyoxyethylene sorbitol fatty acid esters, and polyoxyethylene. Alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene castor oil, polyoxyethylene hydrogenated castor oil, alkyl allyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropylene block copolymers, polyoxyethylene polyoxypropyl alkyl ethers, Using polyethylene glycol fatty acid esters, alkyl glucosides, N-alkyl gluconamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, polyoxyethylene alkyl ether phosphates, polyoxyethylene alkyl allyl ether phosphates, etc. can be done. Among these surfactants, sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters are particularly preferred from the standpoint of dispersion stability of the monomer. These surfactants may be used alone or in combination of two or more.

The amount of the surfactant used is preferably 0.1 to 30 parts by mass, preferably 0.3 to 20 parts by mass, per 100 parts by mass of the water-soluble ethylenically unsaturated monomer in the first stage. Parts by mass are more preferred.

(Polymer dispersant)
Further, as the dispersion stabilizer used in the reversed-phase suspension polymerization, a polymeric dispersant may be used together with the surfactant described above.

Examples of polymeric dispersants include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene-terpolymer), anhydrous Maleic acid-modified polybutadiene, maleic anhydride/ethylene copolymer, maleic anhydride/propylene copolymer, maleic anhydride/ethylene/propylene copolymer, maleic anhydride/butadiene copolymer, polyethylene, polypropylene, ethylene/propylene Copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene/propylene copolymer, ethylene/acrylic acid copolymer, ethyl cellulose, ethylhydroxyethyl cellulose and the like. Among these polymeric dispersants, maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene/propylene copolymer, maleic anhydride/ Ethylene copolymer, maleic anhydride/propylene copolymer, maleic anhydride/ethylene/propylene copolymer, polyethylene, polypropylene, ethylene/propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene/propylene copolymer It is preferred to use polymers. These polymeric dispersants may be used alone or in combination of two or more.

The amount of the polymeric dispersant used is preferably 0.1 to 30 parts by mass, preferably 0.3 to 20 parts by mass, relative to 100 parts by mass of the water-soluble ethylenically unsaturated monomer in the first stage. Parts by mass are more preferred.

[Other ingredients]
In the method for producing a water absorbent resin, if desired, other components may be added to the aqueous solution containing the water-soluble ethylenically unsaturated monomer to carry out reverse phase suspension polymerization. As other components, various additives such as thickeners and chain transfer agents can be added.

As an example, reverse phase suspension polymerization can be performed by adding a thickener to an aqueous solution containing a water-soluble ethylenically unsaturated monomer. By adjusting the viscosity of the aqueous solution by adding a thickener in this way, it is possible to control the median particle size obtained in the reversed-phase suspension polymerization.

Examples of thickeners include hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyacrylic acid, (partially) neutralized polyacrylic acid, polyethylene glycol, polyacrylamide, polyethyleneimine, dextrin, sodium alginate, and polyvinyl alcohol. , polyvinylpyrrolidone, polyethylene oxide and the like can be used. If the stirring speed during polymerization is the same, the higher the viscosity of the aqueous solution of the water-soluble ethylenically unsaturated monomer, the larger the primary particles and/or secondary particles of the obtained particles tend to be.

[Reverse phase suspension polymerization]
For reversed-phase suspension polymerization, for example, an aqueous monomer solution containing a water-soluble ethylenically unsaturated monomer is dispersed in a hydrocarbon dispersion medium in the presence of a dispersion stabilizer. At this time, the dispersion stabilizer (surfactant or polymer dispersant) may be added before or after the addition of the aqueous monomer solution as long as it is before the polymerization reaction is started.

Among them, from the viewpoint that it is easy to reduce the amount of hydrocarbon dispersion medium remaining in the resulting water absorbent resin, after dispersing the aqueous monomer solution in a hydrocarbon dispersion medium in which a polymeric dispersant is dispersed, Polymerization is preferably carried out after dispersing the surfactant.

Such reversed-phase suspension polymerization can be carried out in one stage or in multiple stages of two or more stages. In addition, from the viewpoint of increasing productivity, it is preferable to carry out in 2 to 3 stages.

In the case of performing reversed-phase suspension polymerization in multiple stages of two or more stages, after performing the first-stage reversed-phase suspension polymerization, water-soluble ethylenically unsaturated monomers are added to the reaction mixture obtained in the first-stage polymerization reaction. The monomers are added and mixed, and reversed-phase suspension polymerization in the second and subsequent stages may be carried out in the same manner as in the first stage. In the reversed phase suspension polymerization in each stage after the second stage, in addition to the water-soluble ethylenically unsaturated monomer, a radical polymerization initiator and, if necessary, an internal cross-linking agent are added to the second and subsequent stages. Based on the amount of the water-soluble ethylenically unsaturated monomer added during the reverse phase suspension polymerization in each stage, added within the range of the molar ratio of each component to the water-soluble ethylenically unsaturated monomer described above Reversed-phase suspension polymerization can be carried out. If the amount of the water-soluble ethylenically unsaturated monomer and the ratio of the polymerization initiator, internal cross-linking agent, etc. to the water-soluble ethylenically unsaturated monomer are within the above ranges, Each step after the first step may be the same or different.

The reaction temperature for the polymerization reaction is 20 to 110° C. from the viewpoints of speeding up the polymerization, shortening the polymerization time, thereby improving economic efficiency, and facilitating the removal of the heat of polymerization to allow the reaction to proceed smoothly. and more preferably 40 to 90°C.

<Post-crosslinking step>
Next, the water-absorbing resin is crosslinked by adding a post-crosslinking agent to a hydrous gel-like material having an internal crosslinked structure obtained by polymerizing a water-soluble ethylenically unsaturated monomer (post-crosslinking reaction). This post-crosslinking reaction is carried out in the presence of a post-crosslinking agent after the polymerization of the water-soluble ethylenically unsaturated monomer. By applying such a post-crosslinking reaction, the cross-linking density in the vicinity of the surface of the water-absorbing resin is appropriately increased to obtain a water-absorbing resin with improved various performances such as water absorption under load and non-pressure DW. can be done.

Examples of post-crosslinking agents include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin; (poly)ethylene glycol diglycidyl ether, (poly) Polyglycidyl compounds such as glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, (poly)glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin, α -halo epoxy compounds such as methyl epichlorohydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol , 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol and other oxetane compounds; 1,2-ethylenebisoxazoline and other oxazoline compounds; ethylene carbonate and other carbonate compounds and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide. Among these post-crosslinking agents, (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, ( Polyglycidyl compounds such as poly)glycerol polyglycidyl ether are preferred. These post-crosslinking agents may be used alone or in combination of two or more.

The amount of the post-crosslinking agent used is preferably 0.00001 to 0.01 mol, preferably 0.00005 to 0.01 mol, per 1 mol of the total amount of the water-soluble ethylenically unsaturated monomers used in the polymerization. 005 mol, more preferably 0.00001 to 0.001 mol.

As for the method of adding the post-crosslinking agent, the post-crosslinking agent may be added as it is or as an aqueous solution, but if necessary, it may be added as a solution using a hydrophilic organic solvent as a solvent. Hydrophilic organic solvents include, for example, lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol and isopropyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as diethyl ether, dioxane and tetrahydrofuran; - amides such as dimethylformamide; sulfoxides such as dimethylsulfoxide; These hydrophilic organic solvents may be used alone, in combination of two or more, or as a mixed solvent with water.

The timing of addition of the post-crosslinking agent may be after the polymerization of the water-soluble ethylenically unsaturated monomer, and it is added in the range of 5 to 140 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. It is preferably added in the presence of water, more preferably in the presence of water in the range of 15 to 100 parts by mass, more preferably in the presence of water in the range of 20 to 50 parts by mass. Adding in the presence of water in the range of 28 parts by mass is even more preferable. The amount of water means the total amount of water contained in the reaction system and water used as necessary when adding the post-crosslinking agent. If the post-crosslinking agent is added while the water content is higher than 140 parts by mass, the water retention tends to be low. Also, if the post-crosslinking agent is added in a state where the amount of water is less than 5 parts by mass, the reaction of the post-crosslinking agent tends to be insufficient.

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

<Drying process>
After the reversed-phase suspension polymerization described above, a drying step may be included in which water, a hydrocarbon dispersion medium, and the like are removed by distillation by applying energy such as heat from the outside. When dehydrating the water-containing gel after reverse-phase suspension polymerization, by heating the system in which the water-containing gel is dispersed in the hydrocarbon dispersion medium, the water and the hydrocarbon dispersion medium are temporarily removed from the system by azeotropic distillation. Distill off. At this time, if only the hydrocarbon dispersion medium that has been distilled off is returned into the system, continuous azeotropic distillation becomes possible. In this case, the temperature in the system during drying is maintained at or below the azeotropic temperature with the hydrocarbon dispersion medium, which is preferable from the viewpoint of the resin being less likely to deteriorate. Subsequently, water and a hydrocarbon dispersion medium are distilled off to obtain a water absorbent resin. By controlling the treatment conditions of the drying step after polymerization to adjust the amount of dehydration, it is possible to control various performances of the resulting water absorbent resin.

In the drying process, the drying treatment by distillation may be performed under normal pressure or under reduced pressure. Moreover, from the viewpoint of increasing the drying efficiency, the drying may be carried out under an air stream of nitrogen or the like. When the drying treatment is performed under normal pressure, the drying temperature is preferably 70 to 250° C., more preferably 80 to 180° C., further preferably 80 to 140° C., further preferably 90 to 130° C. is even more preferred. When the drying treatment is performed under reduced pressure, the drying temperature is preferably 40 to 160°C, more preferably 50 to 110°C.

In addition, when performing the above-described post-crosslinking step using a post-crosslinking agent, the above-described drying step by distillation is performed after the completion of the subsequent cross-linking step. Alternatively, the post-crosslinking step and the drying step may be performed simultaneously.

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

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

The water absorbent resins obtained in Examples and Comparative Examples were evaluated by the following various tests. Unless otherwise specified, measurements were carried out in an environment of temperature 25±2° C. and humidity 50±10%. Each evaluation test method will be described below.

<Physiological saline water retention>
A cotton bag (Membrane No. 60, width 100 mm×length 200 mm) in which 2.0 g of the water absorbent resin particles was weighed was placed in a 500 mL beaker. Pour 500 g of 0.9% by mass aqueous sodium chloride solution (physiological saline) into a cotton bag containing water-absorbent resin particles at once to prevent lumps, tie the top of the cotton bag with a rubber band, and let stand for 30 minutes. to swell the water absorbent resin particles. After 30 minutes, the cotton bag is dehydrated for 1 minute using a dehydrator (manufactured by Kokusan Co., Ltd., product number: H-122) set to a centrifugal force of 167 G, and the cotton bag containing the swollen gel after dehydration. The mass Wa (g) of was measured. The same operation was performed without adding the water-absorbent resin particles, and the empty weight Wb (g) of the wet cotton bag was measured, and the physiological saline water retention capacity was calculated from the following formula.
Physiological saline water retention amount (g / g) = [Wa - Wb] / 2.0

<Physiological saline water absorption under 4.14 kPa load>
The amount of water absorbed by the physiological saline under the load of the water-absorbing resin particles (room temperature, 25° C.±2° C.) was measured using the measuring device Y shown in FIG. The measuring device Y is composed of a burette section 61 , a conduit 62 , a measuring table 63 , and a measuring section 64 placed on the measuring table 63 . The burette portion 61 includes a burette 61a extending in the vertical direction, a rubber stopper 61b arranged at the upper end of the burette 61a, a cock 61c arranged at the lower end of the burette 61a, and one end extending into the burette 61a near the cock 61c. It has an air introduction pipe 61d and a cock 61e arranged on the other end side of the air introduction pipe 61d. A conduit 62 is attached between the burette portion 61 and the measuring table 63 . The inner diameter of conduit 62 is 6 mm. A hole with a diameter of 2 mm is drilled in the central part of the measuring table 63 and the conduit 62 is connected. The measuring part 64 has a cylinder 64a (made of acrylic resin (Plexiglas)), a nylon mesh 64b adhered to the bottom of the cylinder 64a, and a weight 64c. The inner diameter of the cylinder 64a is 20 mm. The opening of the nylon mesh 64b is 75 μm (200 mesh). At the time of measurement, the water absorbent resin particles 65 to be measured are evenly spread over the nylon mesh 64b. The weight 64c has a diameter of 19 mm and a mass of 120 g. The weight 64 c is placed on the water absorbent resin particles 65 and can apply a load of 4.14 kPa to the water absorbent resin particles 65 .

After putting 0.100 g of the water-absorbing resin particles 65 into the cylinder 64a of the measuring device Y, the weight 64c was put thereon and the measurement was started. Since the same volume of air as the physiological saline absorbed by the water-absorbing resin particles 65 is rapidly and smoothly supplied to the inside of the burette 61a through the air introduction pipe, the water level of the physiological saline in the burette 61a decreases. is the amount of physiological saline absorbed by the water absorbent resin particles 65 . The scale of the burette 61a is marked from top to bottom in increments of 0 mL to 0.5 mL. , and the physiological saline water absorption amount under a load of 4.14 kPa was calculated from the following formula.
Physiological saline water absorption under 4.14 kPa load [mL / g] = (Vb - Va) / 0.100

<5-minute value of non-pressurized DW (Demand Wetability)>
The non-pressurized DW of the water absorbent resin particles was measured using the measuring device shown in FIG. The measurement was performed 5 times for one type of water absorbent resin particles, and the average value of the measured values at 3 points excluding the lowest and highest values was obtained. The measuring device has a burette part 1 , a conduit 5 , a measuring table 13 , a nylon mesh sheet 15 , a pedestal 11 and a clamp 3 . The burette part 1 includes a burette tube 21 with a scale, a rubber stopper 23 sealing an upper opening of the burette tube 21, a cock 22 connected to the tip of the lower part of the burette tube 21, and a lower part of the burette tube 21. It has an air introduction pipe 25 and a cock 24 connected to the . The burette part 1 is fixed with a clamp 3 . A flat plate-shaped measuring stand 13 has a through hole 13a with a diameter of 2 mm formed in its central portion, and is supported by a pedestal 11 whose height is variable. Through hole 13 a of measuring table 13 and cock 22 of burette portion 1 are connected by conduit 5 . The inner diameter of conduit 5 is 6 mm.

The measurement was performed in an environment with a temperature of 25°C and a humidity of 60±10%. First, the

cocks

22 and 24 of the burette part 1 were closed, and the 0.9 mass % saline solution 50 adjusted to 25° C. was introduced into the burette tube 21 through the upper opening of the burette tube 21 . The salt solution concentration of 0.9% by mass is the concentration based on the mass of the salt solution. After sealing the opening of the burette tube 21 with a rubber stopper 23, the

cocks

22 and 24 were opened. The interior of the conduit 5 was filled with a 0.9 mass % saline solution 50 so as not to introduce air bubbles. The height of the measuring table 13 was adjusted so that the height of the water surface of the 0.9 mass % saline solution reaching the inside of the through-hole 13 a was the same as the height of the upper surface of the measuring table 13 . After the adjustment, the height of the water surface of the 0.9% by mass saline solution 50 in the burette tube 21 was read from the scale of the burette tube 21, and the position was taken as the zero point (read value at 0 seconds).

A nylon mesh sheet 15 (100 mm×100 mm, 250 mesh, thickness of about 50 μm) was laid near the through-hole 13a on the measurement table 13, and a cylinder with an inner diameter of 30 mm and a height of 20 mm was placed in the center. 1.00 g of water-absorbent resin particles 10a were evenly dispersed in this cylinder. After that, the cylinder was carefully removed to obtain a sample in which the water absorbent resin particles 10a were circularly dispersed in the center of the nylon mesh sheet 15 . Next, the nylon mesh sheet 15 on which the water absorbent resin particles 10a were placed was moved so quickly that the center of the nylon mesh sheet 15 was located at the position of the through hole 13a so that the water absorbent resin particles 10a did not dissipate, and the measurement was started. . The time when air bubbles were first introduced into the burette tube 21 from the air introduction tube 25 was defined as the start of water absorption (0 second).

The amount of decrease in the 0.9% by mass saline solution 50 in the burette tube 21 (that is, the amount of the 0.9% by mass saline solution absorbed by the water absorbent resin particles 10a) is read sequentially, and the water absorption of the water absorbent resin particles 10a is started. The weight loss Wc (g) of the 0.9% by mass saline solution 50 after 5 minutes was read. From Wc, the 5-minute value of no-pressure DW was determined by the following formula. The non-pressurized DW is the water absorption amount per 1.00 g of the water absorbent resin particles 10a.
Unpressurized DW (mL/g) = Wc/1.00

<Median particle size>
10 g of the water-absorbent resin particles were subjected to a continuous fully automatic sonic vibration sieving measuring instrument (Robot Shifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.), and JIS standard openings of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 250 μm and It was sieved using a 150 μm sieve and a tray. The mass of particles remaining on each sieve was calculated as a mass percentage of the total mass. The mass percentage of particles remaining on each sieve was integrated in descending order of particle size, and the relationship between the sieve opening and the integrated value of the mass percentage of particles remaining on the sieve was plotted on logarithmic probability paper. . By connecting the plots on the probability paper with a straight line, the particle size corresponding to the cumulative mass percentage of 50% by mass was obtained, and this was taken as the median particle size.

<Preparation of test solution>
The test solution was prepared by adding a small amount of Blue No. 1 to ion-exchanged water and adding and dissolving the following inorganic salt so that it was present.
Test solution composition Deionized water 5919.6g
・ NaCl 60.0 g
- _CaCl2.H2O _1.8g
・3.6 _g of _MgCl2.6H2O
・Edible blue No. 1 (for coloring)
・ 1% - Triton X-100 15.0g

<Leakage test (gradient absorption test)>
FIG. 3 is a schematic diagram showing a method for evaluating the leakiness of absorbent articles. A 45 cm long support plate 40 (here, an acrylic resin plate, hereinafter also referred to as an inclined surface S1) having a flat main surface was fixed by a mount 41 in a state inclined at 45±2 degrees with respect to the horizontal plane S0. In a room at a temperature of 25±2° C., the test absorbent article 100 was attached onto the inclined surface S1 of the fixed support plate 40 with its longitudinal direction along the longitudinal direction of the support plate 40 . Next, a test liquid 50 adjusted to 25±1° C. was dropped from a dropping funnel 42 placed vertically above the absorbent article 100 toward a position 8 cm above the center of the water absorbent sheet in the absorbent article 100. For the absorbent article with a size of 42 cm x 14 cm, 80 mL was dropped at a time, and for the absorbent article with a size of 32 cm x 12 cm, 30 mL of the test liquid was dropped at a rate of 8 mL/sec. The distance between the tip of the dropping funnel 42 and the absorbent article was 10±1 mm. At intervals of 10 minutes from the start of the first injection of the test liquid, the test liquid was injected a total of 7 times under the same conditions.

When the test liquid that was not absorbed by the absorbent article 100 leaked from the lower part of the support plate 40 , the leaked test liquid was collected in the metal tray 44 arranged below the support plate 40 . The mass (g) of the collected test liquid was measured by the balance 43, and the value was recorded as the amount of leakage. The absorbed amount was calculated by subtracting the leakage amount from the total input amount of the test liquid. Also, the absorbance was calculated by the following formula. It is judged that the larger these numerical values are, the more difficult it is for liquid to leak when worn. In addition, the density of the test liquid was set to 1.0 g/mL.
Absorption rate (%) = absorption amount (g) / input amount of test solution (g) x 100

<Production of water absorbent resin>
(Production example 1)
A round-bottom cylindrical separable flask with an inner diameter of 11 cm and an internal volume of 2 L was prepared, equipped with a reflux condenser, a dropping funnel, a nitrogen gas inlet tube, and a stirrer. As a stirrer, a stirring blade having two stages of four inclined paddle blades with a blade diameter of 5 cm was used. 252 g of n-heptane as a hydrocarbon dispersion medium and 0.736 g of a maleic anhydride-modified ethylene/propylene copolymer (manufactured by Mitsui Chemicals, Inc., Hi-Wax 1105A) as a dispersant were added to the flask and mixed. The dispersant was dissolved in n-heptane by raising the temperature to 80° C. while stirring the mixture in the flask with a stirrer rotating at 300 rpm. The formed solution was cooled to 50°C.

In a beaker with an internal volume of 300 mL, 92.0 g (1.03 mol) of an aqueous acrylic acid solution of 80.5% by mass as a water-soluble ethylenically unsaturated monomer is added, and while cooling from the outside, 30% by mass of hydroxylation Neutralization of 75 mol % was carried out by dropping 102.8 g of sodium aqueous solution into the beaker. After that, 0.092 g of hydroxyl ethyl cellulose (Sumitomo Seika Co., Ltd., HEC AW-15F) as a thickener and 0.0460 g of 2,2'-azobis(2-amidinopropane) dihydrochloride as an azo compound (0.0460 g) were added. 170 mmol), 0.0276 g (0.102 mmol) of potassium persulfate as a peroxide, 0.00184 g (0.0106 mmol) of ethylene glycol diglycidyl ether as an internal cross-linking agent, and deionized water9. By adding and dissolving 47 g, a first-stage monomer aqueous solution was prepared.

The prepared first-stage monomer aqueous solution was added to the reaction solution in the separable flask and stirred for 10 minutes. Then, a surfactant solution prepared by heating and dissolving 0.736 g of sucrose stearate (HLB: 3, Mitsubishi Kagaku Foods Co., Ltd., Ryoto Sugar Ester S-370) as a surfactant in 6.62 g of n-heptane, It was further added to the reaction solution, and the inside of the system was sufficiently replaced with nitrogen while stirring at a rotation speed of 600 rpm. Thereafter, the flask was immersed in a water bath at 70° C. to raise the temperature, and polymerization was carried out for 60 minutes to obtain a first-stage polymerization slurry.

Next, 128.8 g (1.44 mol) of an 80.5% by mass acrylic acid aqueous solution is taken as an ethylenically unsaturated monomer in another beaker with an internal volume of 500 ml, and cooled from the outside while adding 30% by mass. 143.89 g of an aqueous sodium hydroxide solution was added dropwise to effect 75 mol % neutralization. After that, 0.129 g (0.475 mmol) of 2,2′-azobis(2-amidinopropane) dihydrochloride as an azo compound, 0.0386 g (0.143 mmol) of potassium persulfate as a peroxide, internal crosslinking 0.0116 g (0.0665 millimoles) of ethylene glycol diglycidyl ether and 11.2 g of ion-exchanged water were added and dissolved as agents to prepare a second monomer aqueous solution.

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

0.393 g of a 45% by mass pentasodium diethylenetriamine pentaacetate aqueous solution was added to the reaction liquid containing the water-containing gel-like polymer after the second-stage polymerization with stirring. After that, the flask was immersed in an oil bath set at 125° C., and 221.82 g of water was extracted from the system by azeotropic distillation of n-heptane and water. After that, 4.42 g (0.507 mmol) of 2% by mass ethylene glycol diglycidyl ether aqueous solution was added to the flask, and the internal temperature of the separable flask was kept at 83° C. for 2 hours.

After that, the separable flask was immersed in an oil bath set at 125°C to remove n-heptane to obtain polymer particles (dry product). The polymer particles are passed through a sieve with an opening of 850 μm, and 0.2% by mass of amorphous silica (Oriental Silicas Corporation, Tokusil NP-S, hydrophilic) relative to the mass of the polymer particles is added to the polymer particles. to obtain 216.4 g of water absorbent resin particles A containing amorphous silica. Table 1 shows the performance of the water absorbent resin particles.

(Production example 2)
293 g of n-heptane used as a hydrocarbon dispersion medium was used, and potassium persulfate was added without adding the azo compound 2,2′-azobis(2-amidinopropane) dihydrochloride to be dissolved in the first-stage monomer aqueous solution. 0.0736 g (0.272 mmol), 0.0101 g (0.0579 mmol) of the internal cross-linking agent ethylene glycol diglycidyl ether, 39.9 g of ion-exchanged water, and the number of revolutions during the first stage polymerization was 550 rpm, without adding the azo compound 2,2'-azobis(2-amidinopropane) dihydrochloride to be dissolved in the second monomer aqueous solution, 0.103 g (0.381 mmol) of potassium persulfate, 10.6 g of ion-exchanged water was added, and the amount of water extracted outside the system by azeotropic distillation was changed to 247.9 g. 228.0 g of water absorbent resin particles B were obtained in the same manner as in Production Example 1, except that the content was changed to 5% by mass. Table 1 shows the performance of the water absorbent resin particles.

(Production example 3)
0.00276 g (0.0158 mmol) of the internal cross-linking agent ethylene glycol diglycidyl ether dissolved in the first-stage monomer aqueous solution and 40.7 g of ion-exchanged water were added, and the water was removed from the system by azeotropic distillation. 229.0 g of water absorbent resin particles C were obtained in the same manner as in Production Example 2, except that the amount of water-absorbing resin particles C was changed to 266.4 g. Table 1 shows the performance of the water absorbent resin particles.

(Production example 4)
293 g of n-heptane used as a hydrocarbon dispersion medium, and 0.0920 g (0 .339 mmol), the amount of the internal cross-linking agent ethylene glycol diglycidyl ether added is 0.00460 g (0.0264 mmol), the amount of ion-exchanged water is 40.9 g, the number of revolutions during the first stage polymerization is 550 rpm, 229.0 g of water-absorbent resin particles D were obtained in the same manner as in Production Example 1, except that the amount of water extracted outside the system by azeotropic distillation was changed to 234.2 g. Table 1 shows the performance of the water absorbent resin particles.

(Production example 5)
Except for changing the amount of water to be drawn out of the system by azeotropic distillation to 294.3 g, and changing the amount of the ethylene glycol diglycidyl ether aqueous solution as a post-crosslinking agent to 13.25 g (1.52 mmol). obtained 228.8 g of water absorbent resin particles E in the same manner as in Production Example 3. Table 1 shows the performance of the water absorbent resin particles.

(Preparation of water absorbent sheet and absorbent article)
The following nonwoven fabrics were prepared.
- Air-laid nonwoven fabric (KNH Enterprise Co., Ltd., basis weight 40 g/m ² )
・Hydrophilic air-through nonwoven fabric (Rengo Non-Woven Products, basis weight 21 g/m ² )
・Air-through nonwoven fabric (Guangzhou Jinhan Nonwoven Co., Ltd., basis weight 40 g/m ² )
・Spunbond nonwoven fabric (TORAY POLYTECH NANTONG, basis weight 17 g/m ² )
・Spunlace nonwoven fabric (Kuraray Co., Ltd., basis weight 35 g/m ² )
・Liquid impermeable sheet (made of polyethylene, basis weight 40 g/m ² )

(Example 1)
Prepare air-laid nonwoven fabric cut to 42 cm × 14 cm as the first and second sheets, hot melt coating machine (Harry's Co., Ltd., pump: Marshal 150, table: XA-DT, tank setting temperature: 150 ° C., inside the hose Set temperature: 165 ° C., gun head set temperature: 170 ° C.), 0.15 g of hot melt adhesive (Henkel Japan Co., Ltd., ME-765E) is applied to the air-laid nonwoven fabric of the second sheet along the longitudinal direction, 13 strips were applied at intervals of 10 mm. The adhesive application pattern was a spiral stripe. After that, 14.4 g of water-absorbing resin particles A were evenly dispersed on the adhesive-applied surface of the second sheet on both ends of the widthwise direction and the lengthwise direction except for a range of 1 cm around the periphery. In the absorbent article, the weight per unit area of the water absorbent resin particles was 300 g/m ² .

A hot-melt adhesive was also applied to the first sheet in the same manner as described above. The surface of the first sheet coated with hot melt adhesive and the surface of the second sheet coated with water-absorbent resin particles are aligned, then sandwiched from above and below with release paper, and laminated by a laminating machine (Hashima Co., Ltd.). , Straight Linear Fussing Press, model HP-600LFS), and pressed under conditions of 110° C. and 0.1 MPa, and the release paper was removed to obtain a water absorbent sheet. Furthermore, by placing a hydrophilic air-through non-woven fabric of the same size as the water-absorbent sheet on the upper surface of the water-absorbent sheet and placing a polyethylene liquid-impermeable sheet with a basis weight of 40 g/m ² on the lower surface of the water-absorbent sheet, An article was made. The resulting absorbent article comprises a hydrophilic air-through nonwoven fabric, a first sheet, a hot-melt adhesive, an absorbent layer composed of water-absorbent resin particles A, a hot-melt adhesive, a second sheet, and a liquid-impermeable sheet in this order. are placed.

(Comparative example 1)
An absorbent article of Comparative Example 1 was produced in the same manner as in Example 1 except that the water absorbent resin particles were changed to water absorbent resin particles B.

(Comparative example 2)
An absorbent article of Comparative Example 2 was produced in the same manner as in Example 1 except that the water absorbent resin particles were changed to water absorbent resin particles C.

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

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

(Example 2)
Example 1 in the same manner as in Example 1 except that the absorbent article was 32 cm × 12 cm, the amount and number of hot melt adhesives were changed to 0.1 g and 11, and the amount of water absorbent resin particles A was changed to 4.5 g. 2 absorbent articles were made. In the absorbent article, the weight per unit area of the water absorbent resin particles was 150 g/m ² .

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

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

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

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

(Example 3)
After the hot melt was applied to the first sheet by the same operation as in Example 1, 7.2 g of the water absorbent resin particles A were applied to the portions of the first sheet excluding the range of 1 cm on both ends in the width direction and the length direction. distributed evenly. After placing an air-through nonwoven fabric cut to 40 cm × 12 cm on the surface on which the water absorbent resin particles A are dispersed, it is sandwiched from above and below with release paper, and pressed and laminated using a laminating machine in the same manner as in Example 1. , the release paper was removed to obtain a laminate in which the air-through nonwoven fabric, the absorbent body (upper layer absorbent layer) composed of the water absorbent resin particles A, the hot melt adhesive and the first sheet were arranged in this order.

Next, a second sheet was coated with a hot-melt adhesive by the same operation as in Example 1, and 7.2 g of water absorbent resin particles A were evenly dispersed on the second sheet. The air-through non-woven fabric surface of the laminate and the surface of the second sheet on which the water-absorbing resin particles are dispersed are aligned, and the laminate is sandwiched from above and below with release paper, and the same operation as above is performed using a laminator. They were laminated together by pressing, and the release paper was removed to produce a water absorbent sheet. Furthermore, by placing a hydrophilic air-through non-woven fabric of the same size as the water-absorbent sheet on the upper surface of the water-absorbent sheet and placing a polyethylene liquid-impermeable sheet with a basis weight of 40 g/m ² on the lower surface of the water-absorbent sheet, An article was made. The resulting absorbent article includes a hydrophilic air-through nonwoven fabric, a first sheet, a hot melt adhesive, an absorbent body (upper absorbent layer) composed of water-absorbent resin particles a, an air-through nonwoven fabric, and an absorbent body composed of water-absorbent resin particles a ( lower absorbent layer), hot-melt adhesive, second sheet, and liquid-impermeable sheet are arranged in this order. In the absorbent article, the basis weight of the water absorbent resin particles was 150 g/m ² for the upper absorbent layer and 150 g/m ² for the lower absorbent layer.

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

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

(Comparative Example 9)
An absorbent article of Comparative Example 9 was produced in the same manner as in Example 3 except that the water absorbent resin particles used in the upper absorbent layer and the lower absorbent layer were changed to the water absorbent resin particles B.

(Comparative Example 10)
An absorbent article of Comparative Example 10 was produced in the same manner as in Example 3 except that the water absorbent resin particles used in the upper absorbent layer and the lower absorbent layer were changed to water absorbent resin particles C.

(Comparative Example 11)
An absorbent article of Comparative Example 11 was produced in the same manner as in Example 3, except that the water absorbent resin particles used in the upper absorbent layer and the lower absorbent layer were changed to the water absorbent resin particles D.

(Comparative Example 12)
An absorbent article of Comparative Example 12 was produced in the same manner as in Example 3, except that the water-absorbent resin particles E used in the upper absorbent layer and the lower absorbent layer were changed.

(Example 6)
The first sheet and the second sheet are 32 cm × 12 cm, the amount and number of hot melt adhesives are 0.1 g and 11, the air through nonwoven fabric is 30 cm × 10 cm, the amount of water absorbent resin particles A is 2.3 g for the upper absorbent layer, An absorbent article was produced in the same manner as in Example 3, except that the lower absorbent layer was changed to 2.3 g. In the absorbent article, the basis weight of the water absorbent resin particles was 75 g/m ² for the upper absorbent layer and 75 g/m ² for the lower absorbent layer.

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

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

(Comparative Example 13)
An absorbent article of Comparative Example 13 was produced in the same manner as in Example 6 except that the water absorbent resin particles used in the upper absorbent layer and the lower absorbent layer were changed to the water absorbent resin particles B.

(Comparative Example 14)
An absorbent article of Comparative Example 14 was produced in the same manner as in Example 6, except that the water absorbent resin particles used in the upper absorbent layer and the lower absorbent layer were changed to the water absorbent resin particles D.

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

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

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

(Example 10)
An absorbent article of Example 10 was produced in the same manner as in Example 1, except that the nonwoven fabrics of the first and second sheets were changed to spunlaced nonwoven fabrics.

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

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

The following evaluations were performed on absorbent articles using water absorbent sheets. The results are shown in Tables 2-6.

As can be seen from the results in Tables 1 to 6, the present invention can provide a water absorbent sheet that can suppress the occurrence of leakage, and an absorbent article using the water absorbent sheet. In addition, when manufacturing an absorbent body and an absorbent article having the structure of the present invention, it can be expected to provide an absorbent body and an absorbent article that have the same various performances even if the amount of water absorbent resin used is reduced. Manufacturing costs may be reduced.

1 Burette Part 3 Clamp 5 Conduit 10 Absorber 10a Water Absorbent Resin Particles 10b Hydrophilic Fiber 11 Base 13 Measurement Stand 13a Through Hole 15 Nylon Mesh Sheet 20 Core Wrap 21 Burette Tube 22 Cock 23 Rubber Stopper 24 Cock 25 Air Inlet Tube 30 Surface Sheet 31 Back sheet 40 Support plate 41 Base 42 Dropping funnel 43 Balance 44 Metal tray 50 Salt solution 51 Test liquid 61 Burette 61a Burette 61b Rubber plug 61c Cock 61d Air introduction tube 61e Cock 62 Conduit 63 Measuring table 64 Measuring part 64a Cylinder 64b Nylon mesh 64c Weight 100 Absorbent article S ₀ horizontal surface S ₁ inclined surface

Claims

A water absorbent sheet comprising an absorbent layer and a nonwoven fabric sandwiching the upper and lower sides of the absorbent layer,
The absorbent layer contains a water absorbent resin,
The water absorbent sheet, wherein the water absorbent resin has the following properties (A) to (C).
(A) The physiological saline water retention capacity is 45 g/g or more and 70 g/g or less.
(B) The water absorption under a load of 4.14 kPa is 13 ml/g or more.
(C) The 5-minute DW value without pressure is 44 ml/g or more and 80 ml/g or less.
The water absorbent sheet according to claim 1, wherein the water absorbent resin has a basis weight of 100 g/m 2 or more and 450 g/m 2 or less.
The water absorbent sheet according to claim 1 or 2, wherein the absorbent layer is adhered to the nonwoven fabric via an adhesive.
An absorbent article comprising the water absorbent sheet according to claim 1 or 2.