Water-absorbent sheet

Abstract

To provide a water-absorbable sheet compatible of sufficient water-absorption performance and thinness by solving such problems that the slimming of a water- absorbable composite has limitations because of a covering since a highly water-absorbable polymer is generally covered with a nonwoven fabric sheet material and the like in order to fix the highly water-absorbable polymer to a nonwoven fabric substrate and the like and, in addition, to prevent the highly water-absorbable polymer from oozing outside of a water-absorbable composite in a prior water-absorbable composite, meanwhile the sufficient water-absorbing performance cannot be obtained since an amount of a fixable highly water-absorbable polymer decreases though slimming can be realized if a nonwoven fabric substrate and the like are made thin.SOLUTION: A water-absorbable sheet is a sheet-like matter including 30-100 wt.% of a polyacrylonitrile-based water-absorbable fiber having a density of 0.25-1.4 g / cm3, and a carboxyl group, where the polyacrylonitrile-based water-absorbable fiber is evenly pressed and joined while holding a fiber state.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a water-absorbent sheet containing water-absorbent fibers, and can be applied and developed in various fields such as diapers, sanitary products, beauty sheets, agricultural and horticultural, sanitary materials, civil engineering, food, and medical use.

Background Art

[0002] In water-absorbent articles such as diapers, sanitary products, and beauty sheets, pulp and superabsorbent polymers are used as absorbents for absorbing moisture. In recent years, an improvement in the feeling of wearing and the appearance during wearing has been demanded, and it has been proposed to thin a relatively bulky absorber among the constituent members of the absorbent article.

[0003] As such a thin absorber, Patent Documents 1 and 2 report a water-absorbent composite in which the ratio of SAP is increased by fixing (supporting) a superabsorbent polymer (SAP) to a nonwoven fabric substrate or the like. However, since a nonwoven fabric substrate is used, there is a limit to its thinning.

[0004] Further, Patent Document 3 discloses a water-absorbent sheet in which an open fiber web containing water-absorbent fibers is formed into a sheet by embossing. However, such a sheet employs an embossing process as a sheet-forming means that does not suppress the three-dimensional expansion of the fibers during water absorption in order to adopt a sheet shape excellent in handleability and retain a large amount of water (free water) between the fibers, and is not thinned by applying a high pressure.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

[0006] In the above-mentioned water-absorbent composites, in addition to fixing SAP to the nonwoven fabric substrate, the SAP layer is generally encapsulated with a nonwoven fabric sheet material in order to prevent granular or powdered SAP from leaking out of 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 making the nonwoven fabric substrate thinner, 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 embossing, so there is a limit to how thin the absorbent sheet can be made.

[0007] The present invention has been made in consideration of the problems of the prior art as described above. That is, the present invention has been made based on the discovery that the inherent water absorbency of polyacrylonitrile water absorbent fibers having carboxyl groups is not lost even when the fibers are compressed under high pressure to form a thin sheet, and the object of the present invention is to provide a water absorbent sheet which is both thin and has sufficient water absorption performance. [Means for solving the problem]

[0008] The present invention has the following configuration. (1) Density 0.4 ~1.4g / cm 3 and and having a thickness of 0.035 to 1.0 mm, A water-absorbent sheet comprising a sheet-like material containing 30 to 100% by weight of polyacrylonitrile-based water-absorbent fibers having carboxyl groups, the sheet-like material containing no heat-fusible fibers, and the polyacrylonitrile-based water-absorbent fibers being uniformly pressure-bonded while maintaining their fibrous state. ( 2 2) The water-absorbent sheet according to (1), characterized in that the amount of carboxyl groups in the polyacrylonitrile water-absorbent fiber having carboxyl groups is 1.0 to 5.0 mmol / g. ( 3The water absorption ratio of the polyacrylonitrile-based water-absorbent fiber having a carboxyl group is 2 to 300 times, which is characterized by (1) or ( 2 ) The water-absorbent sheet described in

Effect of the Invention

[0009] By adopting the above means, a water-absorbent sheet with a very high density and capable of being thinned can be obtained. Such a water-absorbent sheet of the present invention can be applied not only to sanitary articles such as sanitary products and diapers, but also to water-stopping materials for pipes and cables, water-stopping sheets for industrial waste disposal sites, disaster prevention water-stopping sheets inserted into gaps such as shutters and doors, and thin water-absorbent materials such as sheets for concrete curing. It can be developed for applications where it is required.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0011] Hereinafter, embodiments of the present invention will be described in detail.

[0012] In diapers and sanitary products, from the viewpoints of wearing comfort and appearance during wearing, it is desirable that the water-absorbent sheet is thin. On the other hand, it is necessary to have a sufficient amount of water-absorbent material so that leakage does not occur. In order to reconcile these conflicting requirements of "thinness" and "high content of water-absorbent material", a water-absorbent sheet with a high density is required. The present invention has found that by using a polyacrylonitrile-based water-absorbent fiber having a carboxyl group in the surface layer, a thin and very high-density water-absorbent sheet can be obtained. The density of such a water-absorbent sheet of the present invention is 0.25 to 1.4 g / cm 3 and preferably 0.4 to 1.0 g / cm 3 is.

[0013] The polyacrylonitrile-based water-absorbent fiber having a carboxyl group adopted in the present invention has the characteristics that when immersed in water, the surface layer part, the inside of the fiber, or the whole fiber absorbs water and swells. The water absorption ratio of the water-absorbent fiber is preferably 2 to 300 times, more preferably 30 to 200 times, based on the weight of the fiber. If the water absorption ratio is less than 2 times, the performance as a water-absorbent sheet is not sufficient, and if the water absorption ratio exceeds 300 times, it becomes difficult to strengthen the gel strength during water absorption.

[0014] The polyacrylonitrile-based water-absorbent fiber is not particularly limited as long as it satisfies the above-mentioned water absorption ratio. Specifically, a water-absorbent layer having a salt-type carboxyl group on the surface by hydrolyzing the surface of the acrylonitrile-based fiber, a polyacrylonitrile-based water-absorbent fiber having a core-sheath structure with an acrylonitrile-based fiber part remaining in the central part, and a crosslinked polyacrylonitrile-based water-absorbent fiber having a core-sheath structure in which a crosslinked structure by a covalent bond is introduced into such a water-absorbent fiber can be mentioned.

[0015] In particular, the above-mentioned crosslinked polyacrylonitrile-based water-absorbent fiber has a strong physical strength of the fiber because the acrylonitrile-based fiber part remains in the central part, has good handleability during processing, and has little change in the length direction of the fiber during swelling, so the dimensional stability of the product is good, which is more preferable.

[0016] The introduction of a crosslinked structure by a covalent bond in such polyacrylonitrile-based water-absorbent fibers may be carried out before, simultaneously with, or after the hydrolysis of the fiber surface described above. Further, in the introduction of the crosslinked structure, the nitrile group of the acrylonitrile-based fiber or the carboxyl group generated by hydrolysis can be utilized. However, in the method of utilizing the carboxyl group generated by hydrolysis, there are problems such as the gel strength of the gel part generated on the fiber surface by hydrolysis being weak, and the gel falling off at the stage before the introduction of the crosslinked structure, or a part of the carboxyl group-containing polymer that is not crosslinked flowing out. On the other hand, when a crosslinked structure is introduced by utilizing the nitrile group before or during hydrolysis, the gel strength after hydrolysis is high, and the carboxyl group-containing polymer that is not crosslinked is reduced, which is advantageous for industrial handling and environmental impact. Therefore, a method of introducing a crosslinked structure by utilizing the nitrile group before or during hydrolysis is preferred.

[0017] The manufacturing method of the crosslinked polyacrylonitrile-based water-absorbent fiber in which a crosslinked structure is introduced by utilizing the nitrile group will be described in detail below. First, as the acrylonitrile-based polymer constituting the acrylonitrile-based fiber as a raw material, a polymer containing 80% by weight or more, preferably 85% by weight or more of acrylonitrile is desirable. Examples of the comonomer include vinyl halides and vinylidene halides such as vinyl chloride, vinyl bromide, and vinylidene chloride; ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and itaconic acid and their salts; (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 such as vinyl sulfonic acid, (meth)allyl sulfonic acid, and p-styrenesulfonic acid and their salts; vinyl compounds such as (meth)acrylamide, vinylidene cyanide, and methacrylonitrile.

[0018] Here, as for the molecular weight of the acrylonitrile-based polymer, it may be one with a molecular weight to the extent used as general clothing fibers or one with a high molecular weight such as used for high-strength fibers, etc. However, since it is more cost-effective to use general clothing fibers, those with a weight-average molecular weight of 200,000 or less can be preferably used.

[0019] Next, a method of using the acrylonitrile-based fiber as a starting material and introducing a crosslinked structure using a nitrile group to obtain a water-absorbent fiber having a target water absorption ratio will be described in detail. As this method, a method of performing a hydrolysis treatment after a crosslinking treatment using a nitrile group and a method of simultaneously performing crosslinking and hydrolysis using a nitrile group can be mentioned.

[0020] First, a method of performing a hydrolysis treatment after a crosslinking treatment using a nitrile group will be described. As a method of introducing a crosslinked structure using a nitrile group into the acrylonitrile-based fiber, a means of treating 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 is industrially preferable. Here, when the crosslinking agent concentration and the treatment temperature are below the lower limit value, the amount of the crosslinked structure introduced by a covalent bond is insufficient. Conversely, when the crosslinking agent concentration and the treatment temperature exceed the upper limit, the amount of the crosslinked structure introduced by a covalent bond becomes too large. In either case, it is difficult to obtain a water-absorbent fiber within the range of the water absorption ratio of the present invention.

[0021] The crosslinking agent is not particularly limited as long as it is a polyfunctional compound having two or more functional groups capable of chemically reacting with a nitrile group to form a covalent bond. For example, polyfunctional compounds having two or more functional groups such as an amino group and an epoxy group can be mentioned. Specifically, hydrazine hydrate, hydrazine sulfate, hydrazine hydrochloride, hydrazine nitrate, hydrazine bromate, diaminoethane, guanidine carbonate, 1,3-diaminopropane, ethylene glycol diglycidyl ether, etc. can be mentioned.

[0022] The means for hydrolyzing the crosslinked acrylonitrile-based fiber thus obtained is to adjust the fiber to which an alkaline metal compound or its aqueous solution is attached so that the amount of the alkaline metal compound is in 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 heat the fiber at a temperature of 80°C or higher for 5 to 180 minutes, preferably heat it in a wet heat atmosphere of 100 to 150°C for 10 to 120 minutes. In such a hydrolysis treatment, a crosslinking agent may be added as necessary, or the method of simultaneously treating crosslinking and hydrolysis described later may be adopted as it is.

[0023] The alkaline metal compound used here refers to a substance whose pH of a 1.0 wt% aqueous solution of the alkaline metal compound is 7.5 or higher. Examples of such substances include hydroxides of alkaline metals such as Na, K, Li, and alkaline metal salts of organic acids such as carbonic acid, acetic acid, and formic acid of Na, K, Li, etc. As the solvent for preparing the aqueous solution of the alkaline metal compound, water is preferably used industrially, but a mixed solvent of a water-miscible organic solvent such as alcohol, acetone, dimethylformamide, etc. and water may also be used.

[0024] Subsequently, a method for simultaneously treating crosslinking and hydrolysis using a nitrile group will be described. An aqueous solution in which a crosslinking agent and an alkaline metal compound coexist is adjusted to the fiber to which the amount of the 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 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 heated at a temperature of 80°C or higher for 5 to 180 minutes, preferably heated in a wet heat atmosphere of 100 to 150°C for 10 to 120 minutes. It is desirable to adopt such means.

[0025] It is desirable that the hydrolyzed fibers as described above 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 physical properties may be poor and handling may be difficult.

[0026] In this way, it is possible to produce a crosslinked polyacrylonitrile-based water-absorbent fiber having a water absorption multiple in the range of 2 to 300 times, into which a crosslinked structure using a nitrile group is introduced.

[0027] The water-absorbent sheet of the present invention contains the polyacrylonitrile-based water-absorbent fiber employed in the present invention described above, preferably in an amount of 30 to 100% by weight, more preferably 40 to 100% by weight, and even more preferably 50 to 100% by weight. The higher the content rate of the water-absorbent fiber, the more the water retention amount per unit weight increases and the thinner the sheet can be made. On the other hand, when the content rate of the water-absorbent fiber is less than 30% by weight, it is difficult to make the sheet thinner because the water absorption amount per unit weight of the water-absorbent sheet decreases.

[0028] In addition, the fibers that can be mixed with the polyacrylonitrile-based water-absorbent fiber in the water-absorbent sheet of the present invention are not particularly limited, but from the viewpoint of cost, it is preferable to use general-purpose fibers such as polyester, nylon, acrylic, polypropylene, polyethylene, vinylon, cotton, rayon, wool, and glass fiber.

[0029] Further, the water-absorbent sheet of the present invention is such that the above-described polyacrylonitrile-based water-absorbent fiber is uniformly crimped while maintaining its fibrous state. In the present invention, "crimping" means a state of being joined by plastic deformation by applying pressure, and "uniformly crimped" means a state in which there is no clearly weakly crimped portion compared to the non-crimped portion or other locations. From the viewpoint of causing plastic deformation, thermocompression bonding in which pressure is applied under heating is desirable.

[0030] As the method for manufacturing the water-absorbent sheet of the present invention described above, 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 by a carding machine, and then the defibrated web is directly heat-pressed, or the defibrated web is passed through a needle punching process and then heat-pressed. Here, from the viewpoint of uniformly pressing, it is desirable to use a smooth calendar roll or a flat press machine for heat pressing.

[0031] By heat-pressing as described above, it becomes possible to reduce the thickness of the water-absorbent sheet, and accordingly, the density of the sheet increases. That is, the density of the sheet becomes an index of thickness reduction, and in the present invention, it is 0.25 to 1.4 g / cm 3 , preferably 0.4 to 1.0 g / cm 3 in the case of, it becomes a preferable thickness for use. When the density is less than 0.25 g / cm 3 , the thickness reduction is insufficient and the sheet becomes thick, and there is a possibility of poor wearing comfort and appearance defects during wearing. Also, from the viewpoint of the density of the water-absorbent fiber itself used, it is difficult to make the density of the water-absorbent sheet exceed 1.4 g / cm 3 .

[0032] The temperature of heat pressing is preferably 100 to 200 °C, more preferably 120 to 180 °C. If it is less than 100 °C, the density of the sheet may not increase and the thickness reduction may be insufficient, which is not preferable. Also, if it exceeds 200 °C, the physical properties of the fiber may deteriorate, and the physical properties and water absorption performance of the sheet may deteriorate, which is not preferable.

[0033] The pressure of heat pressing is preferably 50 to 500 kgf / cm as the linear pressure, more preferably 100 to 400 kgf / cm. If it is less than 50 kgf / cm, the density of the sheet may not increase and the thickness reduction may be insufficient, which is not preferable. Also, if it exceeds 500 kgf / cm, the water absorption amount and the water absorption speed may become slow, which is not preferable.

[0034] In addition, the thickness of the water-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 water-absorbent fibers used. On the other hand, when the thickness exceeds 1.0 mm, for example, when used in sanitary products, the wearing comfort and the effect of improving the appearance during wearing may not be sufficiently obtained.

[0035] Despite the fact that the water-absorbent sheet of the present invention described above has been made thin and dense by hot pressing, the water-absorbing performance of the polyacrylonitrile-based water-absorbent fiber having a carboxyl group is expressed without being inhibited. This is presumably because in the water-absorbent sheet of the present invention, the water-absorbent fibers are only pressed and do not fuse but remain in a fibrous state.

[0036] The reason why the polyacrylonitrile-based water-absorbent fiber having a carboxyl group has the above-described characteristics is not clear, but the acrylonitrile-based polymer contained in the fiber has a low glass transition temperature and softens at the hot pressing temperature. Therefore, it is considered that the fiber shape is deformed into a flat shape without being destroyed even during hot pressing. Furthermore, since the acrylonitrile-based polymer does not melt, it is considered that each fiber maintains an independent state even after hot pressing.

[0037] Another aspect of the effect of the present invention is that by adopting a polyacrylonitrile-based water-absorbent fiber having a carboxyl group, it is possible to form a sheet having sufficient strength without using a heat-fusible fiber, which is very advantageous in actual use.

Examples

[0038] Hereinafter, the present invention will be described in detail with reference to examples, but the scope of the present invention is not limited only to these examples. Parts and percentages in the examples are shown on a weight basis unless otherwise specified.

[0039] <Method for measuring water absorption ratio> After immersing about 0.5 g of the sample in 300 ml of deionized water at 25°C for 30 minutes, the weight (Y1 (g)) of the sample adjusted by centrifugal dehydration (160G × 5 minutes, where G is the acceleration of gravity) was measured. Next, the weight (Y2 (g)) of the fiber after drying the sample in a vacuum dryer at 80°C until it reached a constant weight was measured, and it was calculated by the following formula. Water absorption ratio (times) = (Y1 - Y2) / Y2

[0040] <Method for measuring the amount of carboxyl groups> After immersing the sample in a sulfuric acid aqueous solution with a pH of 2 to 3 for 30 minutes, it was thoroughly washed with water and dried in a dryer at 80°C. Then, about 0.4 g of the sample was weighed (X1 (g)), and this was put into a solution prepared by dissolving 0.5 g of sodium chloride in 100 ml of deionized water and stirred for 30 minutes. Subsequently, after dropping 30 ml of 0.1 mol / L NaOH, several drops of phenolphthalein were dropped and it was confirmed that the solution was colored red, and then stirring was continued for 30 minutes. Then, a wire mesh was used to separate the water-absorbing fiber and the dispersion liquid, and the dispersion liquid was recovered. 0.1 mol / L HCl was dropped into the dispersion liquid until the red color disappeared (X2 (ml)). The amount of carboxyl groups was calculated according to the following formula. Total amount of carboxyl groups (mmol / g) = (30 - X2) × 0.1 / X1

[0041] <Method for measuring the density of the sheet> The thickness of the sheet (measured using a Mitutoyo dial gauge (code No. 2046F)) and the basis weight (g / m 2 ) were used for calculation.

[0042] <Preparation of polyacrylonitrile-based water-absorbing fiber A> An aqueous mixed solution of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was adhered to the surface of an acrylic fiber with 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 hydrolysis was carried out at 108°C for 15 minutes to obtain a water-absorbing layer having salt-type carboxyl groups on the surface and a core-sheath structure with an acrylonitrile-based fiber part remaining in the center, thereby preparing polyacrylonitrile-based water-absorbing fiber A. The water absorption ratio of this polyacrylonitrile-based water-absorbing fiber A was 125 times, and the amount of carboxyl groups was 1.8 mmol / g.

[0043] <Preparation of Polyacrylonitrile-based Water-absorbing Fiber B> The same acrylic fiber as used in the preparation of polyacrylonitrile-based water-absorbing fiber A was immersed in a 2.0% hydrazine aqueous solution, held at 85 °C for 40 minutes to introduce a cross-linked structure by covalent bonding, and cross-linked acrylic fiber was obtained. Then, an aqueous solution mixture of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was adhered in an amount equal to the weight of the cross-linked acrylic fiber to the fiber surface, and hydrolyzed at 113 °C for 18 minutes to create a water-absorbing layer having salt-type carboxyl groups on the surface and a polyacrylonitrile-based water-absorbing fiber B having a core-sheath structure with a polyacrylonitrile-based fiber part remaining in the central part. The water absorption ratio of this polyacrylonitrile-based water-absorbing fiber B was 10 times, and the amount of salt-type carboxyl groups was 3.2 mmol / g.

[0044] <Preparation of Polyacrylonitrile-based Water-absorbing Fiber C> The same acrylic fiber as used in the preparation of polyacrylonitrile-based water-absorbing fiber A was immersed in a 2.0% hydrazine aqueous solution, held at 85 °C for 40 minutes to introduce a cross-linked structure by covalent bonding, and cross-linked acrylic fiber was obtained. Then, an aqueous solution mixture of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was adhered in an amount equal to 1 / 3 of the weight of the cross-linked acrylic fiber to the fiber surface, and hydrolyzed at 113 °C for 18 minutes to create a water-absorbing layer having salt-type carboxyl groups on the surface and a polyacrylonitrile-based water-absorbing fiber C having a core-sheath structure with a polyacrylonitrile-based fiber part remaining in the central part. The water absorption ratio of this polyacrylonitrile-based water-absorbing fiber C was 2 times, and the amount of salt-type carboxyl groups was 1.0 mmol / g.

[0045] <Preparation of Polyacrylonitrile-based Water-absorbing Fiber D> Using the same acrylic fiber as that used in the preparation of the polyacrylonitrile-based water-absorbent fiber A, a mixed aqueous solution of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was adhered so as to be 1.5 times the weight of the acrylic fiber, and hydrolyzed at 108 °C for 15 minutes to obtain a water-absorbing layer having salt-type carboxyl groups on the surface and a polyacrylonitrile-based water-absorbent fiber D having a core-sheath structure with a polyacrylonitrile-based fiber portion remaining in the central portion. 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.

[0046] <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, held at 85 °C for 40 minutes to introduce a cross-linked structure by covalent bonding, and a cross-linked acrylic fiber was obtained. Thereafter, a mixed aqueous solution of 35% sodium hydroxide aqueous solution and 0.1% hydrazine aqueous solution was adhered to the fiber surface so as to be 1.5 times the weight of the cross-linked acrylic fiber, and hydrolyzed at 113 °C for 18 minutes to obtain a water-absorbing layer having salt-type carboxyl groups on the surface and a polyacrylonitrile-based water-absorbent fiber E having a core-sheath structure with a polyacrylonitrile-based fiber portion remaining in the central portion. 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.

[0047] (Example 1, 7) Using the polyacrylonitrile-based water-absorbent fiber A, a defibrated web of 50 g / m 2 was made with a card defibrator, and hot pressing was performed under the conditions shown in Table 1 using a smooth metal calendar roll to prepare the water-absorbent sheet of the present invention. A scanning electron microscope photograph of the obtained water-absorbent sheet of Example 1 is shown in Fig. 1. It can be seen that the fibers are pressed and become dense, and the density is high, but the fiber state is maintained.

[0048] (Example 2) Using the polyacrylonitrile-based water-absorbent fiber B, a defibrated web of 50 g / m 2A defibrated web was produced, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention.

[0049] (Example 3) Polyacrylonitrile-based water-absorbent fiber B and acrylic fiber with a fineness of 3.3 dtex and a fiber length of 51 mm were mixed at a weight ratio of 30:70 and defibrated with a card defibrator to obtain a defibrated web of 50 g / m 2 A defibrated web was produced, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention.

[0050] (Example 4) Using polyacrylonitrile-based water-absorbent fiber C, a defibrated web of 50 g / m was produced with a card defibrator, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention. 2 A defibrated web was produced, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention.

[0051] (Example 5) Using polyacrylonitrile-based water-absorbent fiber D, a defibrated web of 50 g / m was produced with a card defibrator, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention. 2 A defibrated web was produced, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention.

[0052] (Example 6) Using polyacrylonitrile-based water-absorbent fiber E, a defibrated web of 50 g / m was produced with a card defibrator, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention. 2 A defibrated web was produced, and a heat press was performed under the conditions shown in Table 1 using a smooth metal calendar roll to produce the water-absorbent sheet of the present invention.

[0053] (Comparative Example 1) Using polyacrylonitrile-based water-absorbent fiber A, a defibrated web of 50 g / m was produced with a card defibrator, and the defibrated web was used as a water-absorbent sheet without performing a heat press. 2 A defibrated web was produced, and the defibrated web was used as a water-absorbent sheet without performing a heat press.

[0054] (Comparative Example 2) Using polyacrylonitrile-based water-absorbent fiber A, a defibrated web of 50 g / m was produced with a card defibrator, and 2A defibrated web was produced and processed with a needle punching machine to create a water-absorbent sheet without performing hot pressing.

[0055] (Comparative Example 3) Using a commercially available polyacrylic acid-based water-absorbent fiber (manufactured by Teijin Frontier Co., Ltd., brand name Bell Oasis, carboxyl group content 7.0 mmol / g) without an acrylonitrile-based fiber part, a defibrated web of 50 g / m 2 was produced, and hot pressing was performed under the conditions shown in Table 1 using a smooth metal calendar roll to create a water-absorbent sheet. A scanning electron microscope photograph of the obtained water-absorbent sheet is shown in Figure 2. It can be seen that the fibers were broken and the fiber shape was lost due to hot pressing.

[0056] Table 1 shows the evaluation results for each example and comparative example.

[0057]

Table 1

[0058] For Examples 1 to 7, hot pressing resulted in a high-density sheet, and the thickness was also at a level without practical problems. Also, the water absorbency was good. On the other hand, for Comparative Examples 1 and 2 without hot pressing, the density was low and the thickness was a problem in practical use. Also, the water-absorbent sheet of Comparative Example 3 had low strength even in the dry state and was difficult to handle.

Claims

1. The density is 0.4 to 1.4 g / cm ３ and the thickness is 0.035 to 1.0 mm, and it is a sheet-like material containing 30 to 100% by weight of polyacrylonitrile-based water-absorbent fibers having a carboxyl group. The sheet-like material does not contain heat-sealable fibers, and the polyacrylonitrile-based water-absorbent fibers are uniformly crimped while maintaining a fibrous state. A water-absorbent sheet characterized by being.

2. The water-absorbing sheet according to claim 1, characterized in that the amount of carboxyl groups in the polyacrylonitrile-based water-absorbing fiber having carboxyl groups is 1.0 to 5.0 mmol / g.

3. The water-absorbing sheet according to claim 1 or 2, characterized in that the water absorption magnification of the polyacrylonitrile-based water-absorbing fiber having carboxyl groups is 2 to 300 times.

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