JP7702074B2 - Water-absorbent sheet - Google Patents

Description

The present invention relates to absorbent sheets containing absorbent fibers, which can be used in a wide range of applications, including diapers, sanitary products, and cosmetic sheets, as well as in agriculture, horticulture, sanitary materials, civil engineering, food, and medical applications.

In absorbent articles such as diapers, sanitary products, and cosmetic sheets, pulp and superabsorbent polymers are used as absorbents that absorb moisture. In recent years, there has been a demand for improved comfort when worn and improved appearance when worn, and it has been proposed to reduce the thickness of the absorbent body, which is relatively bulky among the components of absorbent articles.

As examples of such thin absorbents, Patent Documents 1 and 2 report water-absorbent composites in which the proportion of superabsorbent polymer (SAP) is increased by adhering (supporting) the SAP to a nonwoven fabric substrate, etc., but because a nonwoven fabric substrate is used, there are limitations to how thin the material can be made.

Patent Document 3 also discloses an absorbent sheet in which a spread web containing absorbent fibers is embossed to form a sheet. However, this sheet adopts a sheet shape that is easy to handle and is capable of retaining a large amount of water (free water) between the fibers, so that the embossing process is used as a sheet forming method that does not inhibit the fibers from spreading in three dimensions when absorbing water, and is not thinned by applying high pressure.

Patent No. 3196933 JP 2013-39804 A JP 2005-82904 A

In the above-mentioned water-absorbent composites, in addition to fixing SAP to the nonwoven fabric substrate, the SAP layer is generally encapsulated in a nonwoven fabric sheet material in order to prevent granular or powdered SAP from leaking outside the absorbent article, but there is a problem that the nonwoven fabric limits how thin the absorbent article can be made. On the other hand, although it is possible to make the absorbent article thinner by using a thinner nonwoven fabric substrate, the amount of SAP that can be fixed is reduced, and sufficient water absorption performance cannot be obtained. In addition, the absorbent sheet of Patent Document 3 has unevenness due to the embossing process, so there is a limit to how thin the absorbent article can be made.

The present invention was made in consideration of the problems of the conventional technology as described above. That is, the present invention was made based on the discovery that the inherent water absorbency of polyacrylonitrile-based water absorbent fibers having carboxyl groups is not lost even when they are compressed under high pressure to form a thin sheet, and the object of the present invention is to provide a water absorbent sheet that combines sufficient water absorption performance with thinness.

The present invention has the following configuration.
(1) A water-absorbent sheet having a density of 0.4 to 1.4 g/ ^cm3 and a thickness of 0.035 to 1.0 mm, which contains 30 to 100% by weight of polyacrylonitrile-based water-absorbent fibers having carboxyl groups, wherein the sheet does not contain heat-fusible fibers, and the polyacrylonitrile-based water-absorbent fibers are uniformly pressed while maintaining their fibrous state.
( 2 ) The water-absorbent sheet according to (1), wherein the amount of carboxyl groups in the polyacrylonitrile water-absorbent fiber having carboxyl groups is 1.0 to 5.0 mmol/g.
( 3 ) The water-absorbent sheet according to (1) or ( 2 ), characterized in that the water-absorbent fiber of polyacrylonitrile type having a carboxyl group has a water-absorbency ratio of 2 to 300 times.

By adopting the above-mentioned method, it is possible to obtain a water-absorbent sheet that has a very high density and can be made thin. The water-absorbent sheet of the present invention can be used not only for sanitary products such as sanitary napkins and diapers, but also for applications requiring a thin water-absorbent material, such as water-stopping materials for pipes and cables, water-stopping sheets for industrial waste disposal sites, disaster prevention water-stopping sheets to be inserted into gaps in shutters and doors, and sheets for curing concrete.

1 is a scanning electron microscope photograph (magnification: 300 times) of the water-absorbent sheet of Example 1. 1 is a scanning electron microscope photograph (magnification: 300 times) of the water-absorbent sheet of Comparative Example 3.

The following describes an embodiment of the present invention in detail.

In diapers and sanitary products, it is desirable that the absorbent sheet is thin from the viewpoint of wearing comfort and appearance when worn. On the other hand, it is necessary to have a sufficient amount of absorbent material so as not to cause leakage. In order to achieve both the conflicting requirements of "thinness" and "high content of absorbent material", a water-absorbent sheet with high density is required. The present invention has found that a thin water-absorbent sheet with very high density can be obtained by using polyacrylonitrile-based water-absorbent fibers having carboxyl groups in the surface layer portion. The density of the water-absorbent sheet of the present invention is 0.25 to 1.4 g/cm ³ , preferably 0.4 to 1.0 g/cm ³ .

The polyacrylonitrile-based absorbent fiber having carboxyl groups used in the present invention has the characteristic that when immersed in water, the surface layer of the fiber, the inside of the fiber, or the entire fiber absorbs water and swells. The absorbency of the absorbent fiber is preferably 2 to 300 times, and more preferably 30 to 200 times, the weight of the fiber. If the absorbency is less than 2 times, the performance as an absorbent sheet is insufficient, and if the absorbency is more than 300 times, it becomes difficult to increase the gel strength when absorbing water.

There are no limitations on such polyacrylonitrile-based absorbent fibers as long as they satisfy the above-mentioned absorbency ratio, but specific examples include absorbent layers having salt-type carboxyl groups on the surface obtained by hydrolyzing the surface of acrylonitrile-based fibers, polyacrylonitrile-based absorbent fibers having a core-sheath structure with an acrylonitrile-based fiber part remaining in the center, and crosslinked polyacrylonitrile-based absorbent fibers having a core-sheath structure in which a crosslinked structure is introduced by covalent bonds into such absorbent fibers.

In particular, the above-mentioned crosslinked polyacrylonitrile-based water-absorbent fibers are more preferable because the acrylonitrile-based fiber portion remains in the center, which gives the fibers strong physical strength and good handleability during processing, and because there is little change in the length direction of the fiber when it swells, the dimensional stability of the product is good.

The introduction of a crosslinked structure by a covalent bond in such a polyacrylonitrile-based water-absorbent fiber may be performed before, simultaneously with, or after the hydrolysis of the above-mentioned fiber surface. In addition, the nitrile group of the acrylonitrile-based fiber or the carboxyl group generated by hydrolysis can be used to introduce the crosslinked structure. However, in the method using the carboxyl group generated by hydrolysis, the gel strength of the gel part generated on the fiber surface by hydrolysis is weak, and problems such as the gel falling off at the stage before the crosslinked structure is introduced or the flow out of a part of the carboxyl group-containing polymer that has not been crosslinked are likely to occur. In contrast, if a crosslinked structure is introduced using a nitrile group before or during hydrolysis, the gel strength after hydrolysis is high and the amount of the carboxyl group-containing polymer that has not been crosslinked is reduced, which is advantageous in terms of industrial handling and environmental impact. For this reason, the method of introducing a crosslinked structure using a nitrile group before or during hydrolysis is preferable.

The manufacturing method of crosslinked polyacrylonitrile absorbent fibers in which a crosslinked structure is introduced using nitrile groups will be described in detail below. First, the acrylonitrile polymer constituting the raw acrylonitrile fiber is preferably a polymer containing 80% by weight or more, preferably 85% by weight or more, of acrylonitrile. Examples of copolymerization monomers include vinyl halides and vinylidene halides such as vinyl chloride, vinyl bromide, and vinylidene chloride; ethylenically unsaturated carboxylic acids and their salts such as acrylic acid, methacrylic acid, maleic acid, and itaconic acid; (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, and butyl (meth)acrylate; vinyl esters such as vinyl acetate and vinyl propionate; ethylenically unsaturated sulfonic acids and their salts such as vinyl sulfonic acid, (meth)allyl sulfonic acid, and p-styrene sulfonic acid; and vinyl compounds such as (meth)acrylamide, vinylidene cyanide, and methacrylonitrile.

The molecular weight of the acrylonitrile polymer may be that used as a general clothing fiber, or may be a high molecular weight polymer used as a high strength fiber, but since it is more cost effective to use general clothing fibers, it is preferable to use a polymer with a weight average molecular weight of 200,000 or less.

Next, a method for obtaining absorbent fibers having the desired absorbency by using the acrylonitrile fibers as a starting material and introducing a crosslinking structure using nitrile groups will be described in detail. Examples of this method include a method in which a crosslinking process using nitrile groups is used, followed by a hydrolysis process, and a method in which crosslinking and hydrolysis are performed simultaneously using nitrile groups.

First, a method of performing a hydrolysis treatment after a crosslinking treatment using nitrile groups will be explained. The industrially preferred method for introducing a crosslinked structure using nitrile groups into acrylonitrile-based fibers is to treat the fibers in an aqueous solution with a crosslinking agent concentration of 0.1 to 10.0% by weight at a temperature of 50 to 120°C for 5 to 150 minutes. If the crosslinking agent concentration or treatment temperature falls below the lower limit, the amount of crosslinked structure introduced by covalent bonds will be insufficient, and conversely, if the crosslinking agent concentration or treatment temperature exceeds the upper limit, the amount of crosslinked structure introduced by covalent bonds will be too large, and in either case, it will be difficult to obtain absorbent fibers that fall within the absorbency range of the present invention.

The crosslinking agent is not particularly limited as long as it is a polyfunctional compound having two or more functional groups in one molecule that can chemically react with nitrile groups to form covalent bonds, but examples include polyfunctional compounds having two or more functional groups such as amino groups and epoxy groups. Specific examples include hydrazine hydrate, hydrazine sulfate, hydrazine hydrochloride, hydrazine nitrate, hydrazine bromate, diaminoethane, guanidine carbonate, 1,3-diaminopropane, and ethylene glycol diglycidyl ether.

The method for hydrolyzing the crosslinked acrylonitrile fiber thus obtained is to prepare a fiber to which an alkaline metal compound or an aqueous solution thereof is attached so that the amount of the alkaline metal compound is within the range of 2.5 to 10.0 mmol/g, preferably 5.0 to 10.0 mmol/g, based on the dry weight of the fiber, and to heat the fiber at a temperature of 80°C or higher for 5 to 180 minutes, preferably in a moist heat atmosphere of 100 to 150°C for 10 to 120 minutes. In this hydrolysis treatment, a crosslinking agent may be added as necessary, or the method of simultaneously treating crosslinking and hydrolysis described below may be used as is.

The alkaline metal compound used here refers to a substance that exhibits a pH of 7.5 or more when formed into a 1.0 wt % aqueous solution of the alkaline metal compound. Examples of such substances include hydroxides of alkali metals such as Na, K, Li, etc., and alkali metal salts of organic acids such as carbonic acid, acetic acid, and formic acid, such as Na, K, Li, etc. In addition, water is industrially preferred as a solvent for preparing an aqueous solution of an alkaline metal compound, but a mixed solvent of water and a water-miscible organic solvent such as alcohol, acetone, or dimethylformamide may also be used.

Next, a method of simultaneously carrying out crosslinking and hydrolysis using nitrile groups will be explained. An aqueous solution in which a crosslinking agent and an alkaline metal compound are both present is prepared so that the amount of alkaline metal compound is in the range of 2.5 to 10.0 mmol/g, preferably 5.0 to 10.0 mmol/g, and the amount of crosslinking agent is in the range of 0.1 to 1.5 wt %, preferably 0.5 to 1.0 wt %, based on the dry weight of the acrylonitrile-based fiber, and the fiber is then heated at a temperature of 80°C or higher for 5 to 180 minutes, preferably in a moist heat atmosphere of 100 to 150°C for 10 to 120 minutes.

The fibers hydrolyzed as described above should contain a carboxyl group amount of 1.0 to 5.0 mmol/g, preferably 1.5 to 4.0 mmol/g. If the carboxyl group amount is less than 1.0 mmol/g, satisfactory water absorbency may not be obtained, and if it exceeds 5.0 mmol/g, the fiber properties may be poor and handling may be difficult.

In this way, it is possible to produce crosslinked polyacrylonitrile-based absorbent fibers that incorporate a crosslinked structure using nitrile groups and have a water absorption rate in the range of 2 to 300 times.

The absorbent sheet of the present invention preferably contains 30 to 100% by weight, more preferably 40 to 100% by weight, and even more preferably 50 to 100% by weight of the polyacrylonitrile absorbent fiber used in the present invention described above. The higher the content of the absorbent fiber, the greater the amount of water retained per unit weight, making it possible to make the sheet thinner. On the other hand, if the content of the absorbent fiber is less than 30% by weight, the absorbency of the absorbent sheet per unit weight will be low, making it difficult to make the sheet thinner.

Fibers that can be mixed with the polyacrylonitrile-based absorbent fibers in the absorbent sheet of the present invention are not particularly limited, but from a cost perspective, it is preferable to use general-purpose fibers such as polyester, nylon, acrylic, polypropylene, polyethylene, vinylon, cotton, rayon, wool, and glass fibers.

The water-absorbent sheet of the present invention is one in which the above-mentioned polyacrylonitrile water-absorbent fibers are uniformly compressed while maintaining their fibrous state. In the present invention, "compression" refers to a state in which the fibers are plastically deformed by pressure and bonded, and "uniformly compressed" refers to a state in which there are no areas that are not compressed and no areas that are clearly weaker than other areas. From the viewpoint of plastic deformation, it is preferable to use thermocompression in which pressure is applied under heat.

The above-mentioned method for producing the water-absorbent sheet of the present invention includes a method in which the above-mentioned polyacrylonitrile-based water-absorbent fiber or a blend of the water-absorbent fiber and other fibers is made into a defibrated web using a carding machine, and then the defibrated web is directly heat-pressed, or a method in which the defibrated web is passed through a needle punching process and then heat-pressed. Here, from the viewpoint of uniform compression, it is preferable to use a smooth calendar roll or flat plate press machine for the heat pressing.

By heat pressing as described above, it is possible to make the absorbent sheet thinner, and the density of the sheet increases accordingly. That is, the density of the sheet is an index of thinning, and in the present invention, a preferable thinness for use is achieved when the density is 0.25 to 1.4 g/cm ³ , preferably 0.4 to 1.0 g/cm ^{3 .} If the density is less than 0.25 g/cm ³ , the sheet will be thick due to insufficient thinning, which may cause poor wearing comfort and poor appearance when worn. In addition, from the viewpoint of the density of the absorbent fiber itself used, it is difficult to make the density of the absorbent sheet exceed 1.4 g/cm ³ .

The temperature of the heat press is preferably 100 to 200°C, more preferably 120 to 180°C. If it is less than 100°C, the density of the sheet will not increase and the thinning may not be sufficient, which is not preferred. If it exceeds 200°C, the physical properties of the fibers may decrease, and the physical properties and water absorption performance of the sheet may also decrease, which is not preferred.

The pressure of the heat press is preferably 50 to 500 kgf/cm, more preferably 100 to 400 kgf/cm, in terms of linear pressure. If it is less than 50 kgf/cm, the density of the sheet will not increase and the thinning may be insufficient, which is not preferable. If it exceeds 500 kgf/cm, the amount of water absorption and the water absorption speed may slow down, which is also not preferable.

The thickness of the absorbent sheet of the present invention is preferably 0.035 to 1.0 mm, more preferably 0.06 to 0.5 mm, and even more preferably 0.06 to 0.3 mm. It is not easy to make the thickness less than 0.035 mm from the viewpoint of the fineness of the absorbent fibers used. On the other hand, if the thickness exceeds 1.0 mm, for example, when used in sanitary products, the effect of improving the wearing comfort and appearance when worn may not be sufficiently obtained.

The absorbent sheet of the present invention described above is made high density and thin by heat pressing, but the absorbency of the polyacrylonitrile absorbent fiber having carboxyl groups is not inhibited and is expressed. This is thought to be because, in the absorbent sheet of the present invention, the absorbent fiber is merely pressed and does not fuse, but retains its fibrous state.

The reason why polyacrylonitrile-based absorbent fibers with carboxyl groups have the above-mentioned properties is unclear, but it is believed that the acrylonitrile polymer contained in the fibers has a low glass transition point and softens at the heat press temperature, so the fiber shape is not destroyed even during heat pressing and the fibers deform into a flat shape. Furthermore, since the acrylonitrile polymer does not melt, it is believed that each fiber remains independent even after heat pressing.

Another advantage of the present invention is that by using polyacrylonitrile-based absorbent fibers with carboxyl groups, it is possible to form a sheet with sufficient strength without using heat-fusible fibers, which is extremely advantageous in practical use.

The present invention will be described in detail below with reference to examples, but the scope of the present invention is not limited to these examples. Parts and percentages in the examples are by weight unless otherwise specified.

<Method for measuring water absorption capacity>
Approximately 0.5 g of a sample was immersed in 300 ml of deionized water at 25° C. for 30 minutes, and then centrifugal dehydration (160 G x 5 minutes, where G is the acceleration of gravity) was performed to adjust the weight of the sample (Y1 (g)). The sample was then dried in a vacuum dryer at 80° C. until a constant weight was reached, and the weight of the fiber (Y2 (g)) was measured and calculated using the following formula.
Water absorption ratio (times) = (Y1 - Y2) / Y2

<Method for measuring amount of carboxyl groups>
After immersing the sample in a sulfuric acid aqueous solution of pH 2-3 for 30 minutes, it is thoroughly washed with water and dried in a dryer at 80°C. After that, about 0.4 g of the sample is weighed out (X1 (g)), which is placed in a solution of 0.5 g of sodium chloride dissolved in 100 ml of deionized water and stirred for 30 minutes. Next, 30 ml of 0.1 mol/L NaOH is dropped, and then several drops of phenolphthalein are dropped to confirm that the sample is colored red, and then the sample is stirred for 30 minutes. Then, the absorbent fiber and the dispersion are separated using a wire mesh to recover the dispersion. 0.1 mol/L HCl is dropped into the dispersion until the red color disappears (X2 (ml)). The amount of carboxyl groups is calculated according to the following formula:
Total amount of carboxyl groups (mmol/g)=(30-X2)×0.1/X1

<Method of measuring sheet density>
It was calculated from the sheet thickness (measured using a dial gauge (Code No. 2046F) manufactured by Mitutoyo) and the basis weight (g/m ² ).

<Preparation of polyacrylonitrile-based water-absorbent fiber A>
A mixed aqueous solution of 35% sodium hydroxide and 0.1% hydrazine was applied to the surface of an acrylic fiber having a fineness of 3.3 dtex and a fiber length of 51 mm in an amount equal to the weight of the acrylic fiber, and hydrolyzed at 108° C. for 15 minutes to prepare a polyacrylonitrile-based water-absorbent fiber A having a water-absorbent layer having a salt-type carboxyl group on the surface and a core-sheath structure with an acrylonitrile-based fiber part remaining in the center. The water absorption ratio of this polyacrylonitrile-based water-absorbent fiber A was 125 times, and the amount of carboxyl groups was 1.8 mmol/g.

<Preparation of polyacrylonitrile-based water-absorbent fiber B>
The same acrylic fiber as that used in the preparation of the polyacrylonitrile-based water-absorbent fiber A was immersed in a 2.0% hydrazine aqueous solution, and then held at 85°C for 40 minutes to introduce a crosslinked structure by covalent bonding, thereby obtaining a crosslinked acrylic fiber. Then, a mixed aqueous solution of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was attached to the fiber surface in an amount equal to the weight of the crosslinked acrylic fiber, and hydrolyzed at 113°C for 18 minutes to obtain a polyacrylonitrile-based water-absorbent fiber B having a water-absorbent layer having a salt-type carboxyl group on the surface and a core-sheath structure with an acrylonitrile-based fiber part remaining in the center. The water absorption ratio of this polyacrylonitrile-based water-absorbent fiber B was 10 times, and the amount of salt-type carboxyl groups was 3.2 mmol/g.

<Preparation of polyacrylonitrile-based water-absorbent fiber C>
The same acrylic fiber as that used in the preparation of the polyacrylonitrile-based water-absorbent fiber A was immersed in a 2.0% hydrazine aqueous solution, and then held at 85°C for 40 minutes to introduce a crosslinked structure by covalent bonding, thereby obtaining a crosslinked acrylic fiber. Then, a mixed aqueous solution of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was attached to the fiber surface in an amount of 1/3 of the weight of the crosslinked acrylic fiber, and hydrolyzed at 113°C for 18 minutes to obtain a polyacrylonitrile-based water-absorbent fiber C having a water-absorbent layer having a salt-type carboxyl group on the surface and a core-sheath structure with an acrylonitrile-based fiber part remaining in the center. The water absorption ratio of this polyacrylonitrile-based water-absorbent fiber C was 2 times, and the amount of salt-type carboxyl groups was 1.0 mmol/g.

<Preparation of polyacrylonitrile-based water-absorbent fiber D>
Using the same acrylic fiber as that used in the preparation of polyacrylonitrile-based water-absorbent fiber A, a mixed aqueous solution of 35% sodium hydroxide solution and 0.1% hydrazine solution was applied in an amount 1.5 times the weight of the acrylic fiber, and hydrolyzed at 108° C. for 15 minutes to prepare polyacrylonitrile-based water-absorbent fiber D having a water-absorbent layer having salt-type carboxyl groups on the surface and a core-sheath structure with an acrylonitrile-based fiber part remaining in the center. The water absorption ratio of this polyacrylonitrile-based water-absorbent fiber D was 300 times, and the amount of carboxyl groups was 2.5 mmol/g.

<Preparation of Polyacrylonitrile-Based Water-Absorbent Fiber E>
The same acrylic fiber as that used in the preparation of the polyacrylonitrile-based water-absorbent fiber A was immersed in a 2.0% hydrazine aqueous solution, and then held at 85°C for 40 minutes to introduce a crosslinked structure by covalent bonding, thereby obtaining a crosslinked acrylic fiber. Then, a mixed aqueous solution of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was attached to the fiber surface in an amount 1.5 times the weight of the crosslinked acrylic fiber, and hydrolyzed at 113°C for 18 minutes to obtain a polyacrylonitrile-based water-absorbent fiber E having a water-absorbent layer having a salt-type carboxyl group on the surface and a core-sheath structure with an acrylonitrile-based fiber part remaining in the center. The water absorption ratio of this polyacrylonitrile-based water-absorbent fiber E was 16 times, and the amount of salt-type carboxyl groups was 5.0 mmol/g.

(Examples 1 and 7)
A 50 g/ ^m2 defibrated web was made using polyacrylonitrile-based water-absorbent fiber A with a card defibrator, and the web was heat-pressed using a smooth metal calendar roll under the conditions in Table 1 to produce a water-absorbent sheet of the present invention. A scanning electron microscope photograph of the obtained water-absorbent sheet of Example 1 is shown in Figure 1. It can be seen that the fibers are compressed and dense, and the density is high, but the fibrous state is maintained.

Example 2
A 50 g/ ^m2 defibrated web was prepared using polyacrylonitrile-based water-absorbent fiber B with a card defibrator, and heat-pressed using a smooth metal calendar roll under the conditions in Table 1 to prepare a water-absorbent sheet of the present invention.

Example 3
Polyacrylonitrile-based water-absorbent fiber B and acrylic fiber having a fineness of 3.3 dtex and a fiber length of 51 mm were mixed in a weight ratio of 30:70, and a defibrated web of 50 g/ ^m2 was prepared using a card defibrator. This was then hot-pressed using a smooth metal calendar roll under the conditions in Table 1 to prepare a water-absorbent sheet of the present invention.

Example 4
A 50 g/ ^m2 defibrated web was prepared using polyacrylonitrile-based water-absorbent fiber C with a card defibrator, and heat-pressed using a smooth metal calendar roll under the conditions in Table 1 to prepare a water-absorbent sheet of the present invention.

Example 5
A 50 g/ ^m2 defibrated web was prepared using polyacrylonitrile-based water-absorbent fiber D with a card defibrator, and heat-pressed using a smooth metal calendar roll under the conditions in Table 1 to prepare a water-absorbent sheet of the present invention.

Example 6
A 50 g/ ^m2 defibrated web was prepared using polyacrylonitrile-based water-absorbent fiber E with a card defibrator, and heat-pressed using a smooth metal calendar roll under the conditions in Table 1 to prepare a water-absorbent sheet of the present invention.

(Comparative Example 1)
A 50 g/m ² defibrated web was prepared using the polyacrylonitrile water-absorbent fiber A with a card defibrator, and the defibrated web was directly used as a water-absorbent sheet without being subjected to heat pressing.

(Comparative Example 2)
A 50 g/m ² defibrated web was made using polyacrylonitrile-based water-absorbent fiber A with a card defibrator, and processed with a needle punch machine to produce a water-absorbent sheet without heat pressing.

(Comparative Example 3)
A 50 g/ ^m2 defibrated web was made with a card defibrator using a commercially available polyacrylic acid-based water-absorbent fiber that does not contain an acrylonitrile-based fiber portion (Teijin Frontier Co., Ltd., brand name: BELLOASIS, carboxyl group amount: 7.0 mmol/g), and a water-absorbent sheet was made by heat pressing using a smooth metal calendar roll under the conditions in Table 1. A scanning electron microscope photograph of the obtained water-absorbent sheet is shown in Figure 2. It can be seen that the fibers were destroyed by the heat press and the fiber shape was lost.

Table 1 shows the evaluation results for each example and comparative example.

For Examples 1 to 7, the sheets became high density by heat pressing, and the thickness was at a level that did not pose a practical problem. The absorbency was also good. On the other hand, for Comparative Examples 1 and 2, which did not use heat pressing, the density was low and the thickness was problematic for practical use. Furthermore, the absorbent sheet of Comparative Example 3 had low strength even in a dry state, making it difficult to handle.

Claims (3)

A water-absorbent sheet having a density of 0.4 to 1.4 g/ ^cm3 and a thickness of 0.035 to 1.0 mm, which contains 30 to 100% by weight of polyacrylonitrile-based water-absorbent fibers having carboxyl groups, wherein the sheet does not contain heat-fusible fibers, and the polyacrylonitrile-based water-absorbent fibers are uniformly pressure-bonded while maintaining their fibrous state. The absorbent sheet according to claim 1, characterized in that the amount of carboxyl groups in the polyacrylonitrile absorbent fibers having carboxyl groups is 1.0 to 5.0 mmol/g. 3. The water-absorbent sheet according to claim 1, wherein the polyacrylonitrile water-absorbent fiber having a carboxyl group has a water-absorbency of 2 to 300 times.