JPH0921048A - Water absorbent sheet - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a water-absorbent sheet which is flexible and has a high water-absorbing performance, and has a remarkably excellent water-absorbing rate when it is used as a multilayer structure absorbent body. A water absorbent sheet comprising synthetic fibers and pulp fibers having a crosslinked carboxyl group with a degree of substitution of 0.35 to 0.8, and having a region where no pulp fibers exist in the thickness direction of the sheet. However, the area per one of the regions is 0.
05-6 mm ² and the total area is 10-10% of the total area of the sheet.
It is 50% by area.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water absorbent sheet composed of a composite sheet containing synthetic fibers and pulp fibers. More specifically, the present invention is a water-absorbent sheet composed of synthetic fibers and pulp fibers having a region in which pulp fibers having a crosslinked carboxyalkyl group are not present in the thickness direction, and is flexible and has high liquid absorbency. It has a remarkably excellent water absorption rate when made into a multilayer structure absorbent, various sanitary materials,
The present invention relates to a water absorbent sheet that can be used in a wide range of fields such as agricultural materials, food packaging materials, and civil engineering / construction materials.

[0002]

2. Description of the Related Art In recent years, a group of materials called superabsorbent resins have been known as water-absorbing materials for aqueous solutions containing salts such as water or saline. Basically, these resin materials have a chemical structure in which a water-soluble polymer is slightly crosslinked to make it insoluble in water. Examples of such a highly water-absorbent material include, for example, starch obtained by graft-polymerizing acrylonitrile and then hydrolyzing it, starch obtained by graft-polymerizing an acrylic acid metal salt, and acrylic acid polymerized with a copolymerizable cross-linking agent. A large number of crosslinking resins, hydrolysates of methyl methacrylate-vinyl acetate copolymers, etc. have been proposed, and some of them have been put to practical use.

Cotton, pulp, paper, cloth, sponge, etc., which are known as traditional water-absorbent materials, absorb water by a capillary phenomenon, whereas the superabsorbent resin has the principle of water absorption. Because it is osmotic, it can absorb much more water than capillarity. Further, the highly water-absorbent resin has an excellent feature that it does not easily re-emit water even when pressure is applied in a water-absorbing state. Therefore, as applications of super absorbent resin, disposable paper diapers, sanitary materials such as sanitary products, soil water retention materials, agricultural material fields such as nursery sheets, food field such as food freshness-retaining materials, dehydration materials, and building dew condensation prevention. Widely used as a building material such as sheets.

On the other hand, the present inventors impregnated a composite sheet composed of synthetic fibers and cellulosic fibers with a mixed solution composed of an alkali, a carboxymethylating agent and a crosslinking agent,
By heating the cellulosic fibers to crosslinked carboxymethyl cellulose, a water-absorbent non-woven fabric that maintains a sheet shape even after absorbing water and has excellent wet strength has been proposed (Japanese Patent Application No. 5-217912). However, if the water-absorbent sheet as described above has only one sheet in use and the water-absorbing ability is insufficient, it is unavoidable to use it as an absorbent body having a multilayer structure of two or more sheets. The whole swells, resulting in so-called gel blocking that loses water permeability, and it is difficult for the liquid to diffuse and reach the second and subsequent water-absorbing sheets or the water-absorbing material used together with the water-absorbing sheet. Therefore, there is a drawback that it takes a considerably long time to absorb the liquid, and when the amount of the liquid to be absorbed is large, the liquid leaks out of the absorber.

[0005]

SUMMARY OF THE INVENTION In view of the above situation, the present inventors have found that even if a plurality of water absorbent sheets are stacked and used as an absorbent body having a multilayer structure, the permeability or diffusibility of a liquid due to gel blocking is not hindered. The water-absorbent material in the form of a sheet was earnestly studied. As a result, the known water-absorbent sheet, carboxyalkylating agent, alkali metal hydroxide,
In order to carry out the reaction by immersing the sheet containing cellulose in an aqueous reaction solution consisting of a cross-linking agent and water, there is a cross-linked carboxyl group, including its surface,
As a result, the amount of water absorption increases and is significantly improved, but such a sheet is used as an absorbent body having a multilayer structure,
It has been found that when a large amount of liquid is absorbed, gel blocking occurs as described above, the liquid permeability is impaired, and the liquid absorption rate is significantly deteriorated.

[0006] Therefore, the present inventors have found that the absorbent sheet which causes gel blocking is uniformly impregnated with a water-soluble reaction solution in the whole cellulose fibers of a sheet containing synthetic fibers and cellulose fibers and then crosslinked. A sheet consisting of synthetic fibers and cellulose fibers in which a specific amount of areas in which no specific size of cellulose fibers are present are scattered, focusing on the point that water absorption performance is imparted to the cellulose fibers by carrying out a carboxyalkylation reaction. , After impregnating the above water-soluble reaction solution, by carrying out the reaction of crosslinking and carboxyalkylation, a water-absorbent sheet having a region where the cellulose fibers having water-absorbing performance is not present on the sheet surface is obtained. This sheet allows some of the liquid to easily pass through the area where there are no cellulose fibers, even when two or more absorbent sheets with a multilayer structure are used. Because are sequentially transmitted Te, and have completed the present invention found that water absorption rate is significantly improved. The object of the present invention is not to impair the transfer performance (having both liquid diffusion and temporary storage properties) and water absorption performance even when absorbing a liquid, and especially when a multilayer structure absorbent body having two or more sheets is formed. It is to provide a water absorbent sheet having a remarkably excellent water absorption rate.

[0007]

The present invention comprises a synthetic fiber,
A water-absorbent sheet containing pulp fibers having a crosslinked carboxyalkyl group with a degree of substitution of 0.35 to 0.8, having a region where no pulp fibers are present in the thickness direction of the sheet, and one region in each region. Area of 0.05 to 6 mm ²
And the total area is 10 to 50 area% of the total area of the sheet.

As the synthetic fibers used in the present invention, known fibers can be used as they are. For example, polyolefin fibers, polyester fibers, polyamide fibers, polyacrylic acid ester fibers, polyurethane fibers and the like can be used. These synthetic fibers may be used alone or as a mixture, and may be obtained by a spunbond method in which continuous filaments are laminated in a sheet shape, a card method in which staples are laminated in a sheet shape, or a wet method. Examples of the non-woven fabric include melt-blown non-woven fabric, and in the case of woven fabric, the production method is not particularly limited. The fineness of the fiber is not particularly limited, but 0.02
It is preferably from 10 denier. When the fineness of the fiber exceeds 10 denier and becomes thicker, the flexibility of the nonwoven fabric decreases,
When used as a water absorbent sheet, the flexibility, texture, and processability are poor. When the fiber fineness is less than 0.02 denier, it becomes extremely difficult to produce fibers having stable properties.

The pulp fibers used in the present invention include pulp fibers produced from wood and non-wood pulp fibers produced from herbs. Pulp fibers produced from wood include chemical pulp fibers prepared by cooking softwood and hardwood wood by the craft method, sulfite method, soda method, polysulfite method, etc. The mechanical pulp fiber or the waste paper pulp fiber can be used alone or in a mixture, and can be used in an unbleached state or a bleached state. Non-wood pulp fibers produced from herbs include, for example, fibers obtained by pulping cotton, manila hemp, flax, straw, bamboo, bagasse, kenaf, mulberry, sanpei, etc. in the same manner as wood pulp.

In the present invention, a known method can be used as a method for producing a composite sheet comprising synthetic fibers and pulp fibers. For example, “Research Disclosure, 17060,
June 1978 ”, Japanese Patent Laid-Open No. 5-253160, Japanese Patent Laid-Open No. 5-5160
No. 2,770,53, Japanese Patent Laid-Open No. 5-285083, etc., the composite sheet obtained by the method of interlacing and unifying a nonwoven fabric made of synthetic fiber and a paper sheet made of pulp fiber by a water jet flow has a texture. , Excellent in workability. In this case, the synthetic fiber non-woven fabric needs to have strength enough to withstand the pressure of the water jet flow. When obtaining a composite sheet by entanglement by a water jet flow, a laminate of a nonwoven fabric and a paper sheet made of synthetic fibers is placed on a porous support made of a woven fabric with the paper sheet facing upwards, When jetting a water jet flow from the paper sheet side to the non-woven fabric side so as to pass therethrough, the synthetic fibers and the pulp fibers are entangled with each other, and the part having no holes forming the porous support, for example, in the case of a net In the knuckle portion where the warp yarn and the weft yarn intersect, the vertically jetted water jet flow collides with the knuckle portion and is then flowed in the horizontal direction, so that the synthetic fibers and pulp fibers present in this portion are
It is forced horizontally along with the water jet stream, resulting in areas free of pulp fibers in the thickness direction of the sheet.

When continuous synthetic long fibers are heat-sealed like a spunbonded nonwoven fabric, even if the fibers are pushed horizontally by the water jet flow, the synthetic fibers will be restored to their original state when the jet of the water jet flow is stopped. Since the force to return to the position acts, it is unlikely that areas where synthetic fibers do not exist will occur, but in the case of non-woven fabrics where synthetic fibers are not fixed, areas where cellulose fibers and synthetic fibers do not exist, that is, holes have been opened in the sheet It becomes a state. In the present invention, it is also possible to use a composite sheet formed by previously mixing short fibers made of synthetic fibers and pulp fibers to form a web by a dry method or a wet method. Also, in order to maintain the wet tensile strength of the composite sheet, in order to partially bond the synthetic fibers to each other, a method of spraying an adhesive and then heating and adhering it, mixing heat fusion fibers, and heating A method of bonding, a method of needle punching, a method of stitch bonding or the like can be used.

There is no particular limitation on the method of providing the region where the pulp fibers do not exist in the thickness direction of the composite sheet, other than the method using the water jet flow described above. After the composite sheet is produced, mechanical force is applied. The composite sheet may be perforated to provide areas where pulp fibers are absent. When mechanical force is used, for example, the composite sheet is passed through a pair of needle rolls and a receiving roll to provide a portion where no pulp fiber is present on the composite sheet. In this case, the area and number of regions where pulp fibers do not exist in the thickness direction of the sheet are determined by, for example, a combination of pin pitches of needle rolls of 3 to 7.7 mm and pin diameters of 0.25 to 2.76 mm. When the pin pitch exceeds 7.7 mm, the total area of the region where the pulp fibers do not exist becomes less than 10 area% of the total sheet area even if the pin diameter is within the above range, and the liquid is contained in the absorbent body of the multilayer structure using the absorbent sheet. It is not suitable because an excellent absorption rate cannot be obtained when contacted. However, a composite sheet obtained by laminating, interlacing, and integrating a nonwoven fabric made of synthetic fibers and a paper sheet made of pulp fibers by using a water jet entanglement method flow has a pulp fiber layer on one side and a synthetic fiber on the opposite side. It can be formed into a layer and is excellent in flexibility, texture, workability and the like, and is particularly preferably used in the present invention.

The method for providing a region where no pulp fibers are present on the composite sheet by the water jet flow is made of stainless steel,
The spunbonded non-woven fabric and the paper sheet are formed on a porous support made of a woven fabric such as plain weave, satin weave, and twill weave having a filament density of 8 to 30 mesh using a thin filament made of plastic or the like. Laminate and place so that the paper sheets are on top, then the pore size is 0.01-
20-150 kg / cm through a 0.3 mm nozzle hole
Water is jetted at a pressure of ² , and the jet of water is sprayed from the surface of the paper sheet so as to penetrate the non-woven fabric and allowed to pass. In this way, the area and number of the region where the pulp fiber does not exist in the sheet is determined by the mesh of the porous support used, the nozzle holes, the water pressure, the basis weight of the non-woven fabric and the paper sheet, etc. in the case of the water jet flow. When a pin roll is used, it is determined by the pin diameter and its pin pitch. In the present invention, the ratio of the total area of the area where the pulp fibers do not exist in the thickness direction of the sheet to the total area of the sheet is appropriately selected depending on the application to be used, but 0.05 to 6 mm per area. The range of ² and the total area of such regions are adjusted to a range of 10 to 50 area% of the total area of the sheet without impairing the water absorption amount and flexibility of the sheet, and It has been found that when a multi-layered absorber using a plurality of is used, the liquid absorption rate is extremely excellent.

If the area per area where pulp fibers do not exist is less than 0.05 mm ² , liquid permeability becomes extremely poor when a water absorbent sheet is used as a multilayer structure absorbent body, and conversely that area is present. When the value exceeds 6 mm ² , the liquid permeability is remarkably improved, but the amount of water absorption per sheet is decreased, so that the amount of the water-absorbent sheet used is increased and the economy is disadvantageous. Similarly, when the area of the region where the pulp fiber does not exist is less than 10% of the total area of the sheet, the liquid permeability in the multilayer structure support using the sheet becomes poor, and conversely, the pulp fiber does not exist. If the area of the region becomes larger than 50% of the sheet area, the amount of water absorption per sheet decreases, so that the required amount of the water absorbent sheet increases correspondingly and the bulkiness increases, which deteriorates usability. In the present invention, the blending ratio of the synthetic fibers and the pulp fibers that form the sheet is preferably 1 to 19 pulp fibers with respect to 1 synthetic fiber. When the ratio of the pulp fiber to the synthetic fiber is less than 1, the amount of the pulp fiber to the synthetic fiber is relatively small, and the water absorption amount of the obtained sheet is small. On the other hand, if the proportion of pulp fibers is more than 19, the entanglement between the synthetic fibers and the pulp fibers is less likely to occur, and when the sheet obtained after water absorption is made to absorb water, swollen pulp fibers fall off. It is not suitable because it becomes easy and the wet tensile strength decreases.

In order to make the composite sheet containing the pulp fiber of the present invention a water absorbent material, the pulp fiber constituting the sheet is made to contain an aqueous reaction liquid containing a carboxyalkylating agent, an alkali metal hydroxide and a crosslinking agent. There is a need. As the carboxyalkylating agent used in this aqueous reaction solution, an alkali metal salt of halocarboxylic acid is used. As the halocarboxylic acid alkali metal salt, monochloroacetic acid alkali metal salt, monobromoacetic acid alkali metal salt, α-
Chloropropionic acid alkali metal salts, β-chloropropionic acid alkali metal salts, α-bromopropionic acid alkali metal salts, β-bromopropionic acid alkali metal salts and the like having 3 or less carbon atoms are preferable, and these are used alone or as appropriate. Select and mix and use. In terms of cost, sodium monochloroacetate is cheap, so it is better to use it. Sodium hydroxide is most preferred as the alkali metal hydroxide, but other alkali metal hydroxides such as lithium hydroxide, potassium hydroxide, rubidium hydroxide, cesium hydroxide and the like can also be used.

As the cross-linking agent, a cross-linking agent that cross-links with the cellulose in the pulp fiber in an alkaline aqueous reaction solution, has a boiling point as high as possible, and is almost non-volatile. In the present invention, it is economical to reduce the decomposition of the carboxyalkylating agent by adopting the step of removing the water by evaporating the water after the aqueous reaction solution is contained in a part of the sheet containing the pulp fiber, and therefore, it is economical in this step. A low-boiling cross-linking agent that volatilizes in the interim is not preferred. Examples of the cross-linking agent usable in the present invention include ethylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, polyethylene glycol diglycidyl ether, polyepoxy compounds such as diepoxybutane, divinyl sulfone, and methylenebisacrylamide. Divinyl compounds such as dichloropropanol,
At least one kind of polyvalent halogen compounds such as dibromopropanol and dichloroacetic acid, and polyvalent aziridine compounds are appropriately selected and used. Epichlorohydrin, which is often used for cross-linking cellulose derivatives and can impart excellent pure water absorption performance, is not suitable for the present invention because of its low boiling point.

The above-mentioned carboxyalkylating agent, alkali metal hydroxide and crosslinking agent are dissolved in water to give an aqueous reaction solution. The proportion of water based on the total weight in this aqueous reaction solution cannot be uniquely defined because the solubility varies depending on the type of the carboxyalkylating agent or alkali metal hydroxide used, but the proportion of water within the range of solubility is A smaller amount is advantageous because less decomposition of the carboxyalkylating agent with water occurs and a lower energy cost is required for the subsequent evaporation of water after containing the aqueous reaction solution. However, if the concentration is too high, the aqueous reaction solution becomes unstable and handling becomes difficult. Therefore, in the present invention, the ratio of water to the total weight of the aqueous reaction solution is 60 to 60% when sodium hydroxide is used.
A carboxyalkylating agent, an alkali metal hydroxide, and a cross-linking agent are dissolved in water so as to be 90% by weight. Since the solubility of this water varies depending on the type of the carboxyalkylating agent, the alkali metal hydroxide, and the crosslinking agent used, the lower limit value varies slightly. The mixing molar ratio of the carboxyalkylating agent and the alkali metal hydroxide is such that the alkali metal hydroxide / carboxyalkylating agent is 0.8 to
Adjust to 1.2. The more it deviates from this range, one of the chemicals remains unreacted, which is uneconomical. The mixing amount of the cross-linking agent varies depending on the kind of the cross-linking agent or the reaction conditions, but is 0.1 to 30% by weight based on the weight of cellulose.

Then, the aqueous reaction solution prepared as described above is impregnated with the composite sheet prepared in advance as described above, whereby the pulp fibers are subjected to carboxyalkylation and cross-linking reaction to impart high water absorption. To do. The impregnation amount of the reaction solution is such that the molar ratio of the carboxyalkylating agent to the glucose residue of the pulp fiber contained in the sheet is 0.7 to 2.0 in terms of carboxyalkylating agent / glucose residue (mol / molar ratio). Is. When the molar ratio is less than 0.7, it becomes impossible to obtain a substitution degree higher than 0.35, and a material having a high water absorption cannot be obtained. On the contrary, the molar ratio is 2.0
Even if added more, the degree of substitution hardly increases, which is economically disadvantageous. Further, it is advantageous to repeat the reaction at least twice in order to increase the degree of substitution, but it is disadvantageous in terms of cost. The method of incorporating the aqueous reaction solution into the sheet containing pulp fibers is not particularly limited, but the sheet may be immersed in an excess amount of the reaction solution and then squeezed by a roll to remove the excess reaction solution. Alternatively, a pre-calculated amount of the reaction solution may be applied by a coater.

The sheet containing the aqueous reaction solution is 5
A portion of the water of the aqueous reaction solution contained in the sheet is evaporated and removed by heating with air blowing at a temperature condition of 0 to 150 ° C., and the ratio of water per content of the aqueous reaction solution is 2
It is adjusted to be 0 to 60% by weight. The method of evaporating is not particularly limited, but a method of evaporating water in the shortest possible time is desirable. Since the reaction rate of carboxyalkylation is quite fast, if the evaporation temperature is too long even in the above temperature range, for example, 30 minutes or more, the carboxyalkylation proceeds or ends during evaporation,
The effect of the present invention cannot be obtained. Carboxyalkylation hardly progressed under the above evaporation conditions (degree of substitution was 0.1
Hereinafter, for such evaporation, a method in which water is evaporated in a dryer or in a reaction vessel while applying hot air in the above temperature range is simple. In that case, the ratio of water in the aqueous reaction liquid contained in the sheet after evaporation of water is 20
If it is less than wt%, the degree of substitution will be drastically reduced and the resulting water-absorbent sheet will remarkably turn yellow, so it must be avoided. Further, when the proportion of water is higher than 60% by weight, the effect of imparting water absorption may be reduced.

Immediately after adjusting the proportion of water in the aqueous reaction solution in this manner, as described above, almost no carboxyalkylation has occurred. Therefore, the sheet is heated thereafter to cause carboxyalkylation and crosslinking. Allow the reaction to take place. In this case, even if water evaporates during heating and escapes to the outside of the system, the proportion of water in the aqueous reaction solution needs to be maintained within the above range. In this operation, the sheet containing the aqueous reaction solution in which the proportion of water is adjusted is continuously passed through the heating device, or the sheet is covered with a liquid-impermeable sheet,
Batch-type reaction temperature 50-110 ° C after closed
It can be easily carried out by heating in the range. If the reaction temperature is lower than 50 ° C, it takes a long time to complete the reaction. On the contrary, if the reaction temperature exceeds 110 ° C and becomes high,
The degree of substitution and the amount of water absorption of the resulting water absorbent sheet tend to be low, and moreover, the water absorbent sheet is often yellowed, which is not suitable.

The heating may be performed from the outside of the sheet containing the aqueous reaction solution, but when the amount of the sheet is large, the temperature range may naturally reach due to the reaction heat of carboxyalkylation. In such a case, the object may be achieved only by leaving it in a closed condition or under an open condition. The reaction time cannot be determined regardless of the reaction temperature, but is in the range of 10 minutes to 4 hours. If the reaction time is less than 10 minutes, the reaction will not be completed sufficiently even if the temperature is increased.
Since the reaction is completed even if the reaction time is longer than the time, it is meaningless to spend more reaction time. As described above, the pure water absorption of the pulp fiber having the cross-linking bond and the carboxyalkyl group is mainly determined by the substitution degree of the carboxyalkyl group and the cross-linking density. The pure water absorption theoretically increases as the substitution degree of the carboxyalkyl group increases, but
Practically, a substitution degree of 0.35 to 0.8 is suitable. The upper limit of the degree of substitution is about 1.2, and even if the degree of substitution exceeds 1.2, the amount of water absorption is almost saturated, and beyond that, it is economically wasteful. Further, a high degree of substitution exceeding 0.8 cannot be achieved unless the reaction is repeated at least twice, which is disadvantageous because the process becomes complicated and the production cost of the resulting water absorbent sheet increases. .

On the other hand, if the degree of substitution is less than 0.35, a sufficient amount of pure water absorption cannot be obtained. The degree of substitution within the scope of the present invention described above is the amount of the carboxyalkylating agent and the alkali metal hydroxide added to the cellulose in the pulp fiber, the proportion of water in the aqueous reaction solution after water is evaporated, the reaction temperature, and the reaction. It is determined by the total effect of time and the like, and if necessary, these factors are appropriately selected and adjusted in combination to obtain a suitable degree of substitution. The amount of pure water absorbed increases as the crosslink density decreases, but if it is too small, the gel strength upon absorption of water is weak and the water-soluble carboxyalkyl cellulose salt remains, which is not desirable because it becomes sticky.
On the contrary, if the crosslink density is too high, the amount of water absorption decreases, so it is necessary to set the optimum crosslink density depending on the application. In the present invention, the crosslinking density is adjusted by adjusting the addition rate of the crosslinking agent.

The sheet which has undergone the carboxyalkylation and the crosslinking reaction contains a large amount of an inorganic salt and a decomposed product of the carboxyalkylating agent, and therefore it is necessary to remove them for purification, and any known method can be used. May be used. For example, a method of removing impurities by washing with a mixture of water and an organic solvent compatible with water such as lower aliphatic alcohols such as methanol, ethanol and isopropyl alcohol can be preferably used. The present invention, as described in detail above, while providing a region containing no pulp fibers in the thickness direction of the sheet in a sheet containing synthetic fibers and pulp fibers, a carboxyalkylating agent in the region where pulp fibers are present, By impregnating an aqueous reaction solution prepared by dissolving an alkali metal hydroxide and a cross-linking agent in water, and then heating the mixture to carry out carboxyalkylation and a cross-linking treatment, water-absorbed pulp fibers are present. It is a water-absorbent sheet in which regions and regions in which pulp fibers do not exist and therefore have no water absorption are arbitrarily mixed.

When the water-absorbent sheet obtained as described above is used in a plurality of sheets and comes into contact with water as a multi-structured absorbent body, the sheet itself absorbs water quickly and at the same time, water that cannot be absorbed is Since a large number of pulp fibers existing in the thickness direction of the sheet pass through the area and the second and subsequent water-absorbing sheets that are next to each other pass one after another, there is interference due to swelling of the surface of the water-absorbing sheet. Gel blocking, in which water accumulates and does not migrate, does not occur at all, and therefore the liquid absorption efficiency is extremely high, and excellent morphological stability can be maintained even after liquid absorption. Therefore, the field of sanitary materials such as disposable paper diapers and sanitary products, the field of agricultural materials such as soil water retention materials, sheets for raising seedlings, the field of food such as food freshness-retaining materials and dehydration materials, and the field of building materials such as dew condensation prevention sheets for buildings. It is preferably used in a wide range of.

[0025]

The present invention will be described in more detail with reference to the following examples, but of course the present invention is not limited thereto. In Examples and Comparative Examples,% means% by weight unless otherwise specified.

Example 1 A long-fiber spunbonded nonwoven fabric was prepared in which polypropylene long-fibers were accumulated and which had a large number of point-fused areas in which the polypropylene long-fibers were self-fused to each other. The fineness of the long fibers constituting this nonwoven fabric was 2.5 denier, and the basis weight of the long fiber nonwoven fabric was 15 g / m ² . Softwood bleached kraft pulp fibers (NBK
The paper sheets obtained by wet papermaking using P) were laminated. This paper sheet had a basis weight of 80 g / m ² according to JIS P 8124 and a density of 0.50 g / cm ³ according to JIS P 8118. Then, the paper sheet was placed on the upper side and the long fiber non-woven fabric was placed on the lower side, and the filaments made of stainless steel were woven with a plain weave structure and placed on a transfer conveyor formed of a wire mesh of 10 mesh. While transferring this laminate at a speed of 25 m / min,
50 kg / cm ² using a water jet jet device in which nozzle holes with a hole diameter of 0.1 mm are arranged at intervals of 0.6 mm
A water jet stream was ejected with the water pressure of, to penetrate from the surface of the paper sheet toward the non-woven fabric side, followed by dehydration and drying with a dryer at 120 ° C.

As described above, a composite sheet was obtained in which the pulp fibers forming the paper sheet and the long fibers forming the non-woven fabric were entangled and integrated. Next, a cut sheet of this composite sheet having a size of 20 cm × 30 cm was prepared. Then, the sheet was immersed in an aqueous reaction solution consisting of sodium hydroxide 8.38%, sodium monochloroacetate 24.40%, ethylene glycol diglycidyl ether 0.68% and water 66.54% for 1 minute and then taken out. Then, it was sandwiched between filter papers and pressed, and adjusted so that 3.0 g of the reaction liquid (carboxyalkyl / glucose residue (mol / mol) = 1.3) was contained per 1 g of the absolute dry sheet.

Further, a thermoelectric hot-air dryer equipped with an exhaust facility is maintained at 50 ° C., and while exhausting, a sheet containing the impregnating liquid is put in this dryer for 5 minutes to evaporate water, and the sheet is contained. The proportion of water in the aqueous reaction solution was 35%.
This sheet was put in a polyethylene bag and put in a dryer kept at 50 ° C. for 3 hours to carry out carboxymethylation of pulp fibers and a crosslinking reaction. Then, this sheet 70
% Aqueous methanol solution, taken out, sandwiched between filter papers and pressed, and this operation was repeated 4 times to thoroughly wash the composite sheet. Finally, the composite sheet was immersed in a 100% methanol solution, taken out, sandwiched between filter papers, pressed, and then air-dried to obtain a water absorbent sheet. In the obtained sheet, regions in which pulp fibers did not exist were arranged alternately in the longitudinal direction and the lateral direction in the thickness direction of 3 to 4 mm ² / one oval sheet. The ratio of the area where the pulp fibers do not exist to the total area of this sheet is 35% by area.
Met. The area where no pulp fiber was present was measured after the sheet surface was photographed with a magnifying microscope.

The degree of substitution, the amount of pure water absorbed, the absorption rate of artificial urine, and the stiffness of the resulting water absorbent sheet were tested by the following methods. Test method (1) A test sample having a degree of substitution of 1 g was placed in a flask, and 50 ml of a methanol-hydrochloric acid solution (a mixed aqueous solution prepared by dissolving hydrogen chloride in a 70% aqueous methanol solution to a concentration of 1 mol / liter).
Was added, the mixture was allowed to stand for 1 hour, washed thoroughly with methanol to completely remove hydrochloric acid, and air-dried. Next, the air-dried sample was placed in a 300 ml Erlenmeyer flask, about 30 ml of 0.1N sodium hydroxide solution and 100 ml of pure water were added, and the mixture was slowly stirred for 1 hour, and then phenolphthalein was added with 0.1N hydrochloric acid solution. Was used as an indicator, and the substitution degree was calculated by the formulas (1) and (2). Y = 0.1A-0.1B (1) Substitution degree = 162Y / (1000W-80Y) (2) However, A: Amount of 0.1N sodium hydroxide solution (ml) B: 0.1N of hydrochloric acid Amount (ml) Y: Amount of carboxymethyl group (milliequivalent) W: Weight of carboxymethyl cellulose (g)

(2) Water Absorption of Pure Water The test sample was cut into a size of 10 cm × 10 cm (0.01 m ² ) and enclosed in a 250 mesh nylon net bag,
This was immersed in pure water obtained by distilling water deionized through an ion exchange resin for 10 minutes to absorb water, and then pulled up and suspended to drain water for 10 minutes,
The weight of the test sample is measured, and one dry test sample (0.01
The amount of absorbed water was expressed by the weight (g) of pure water absorbed per m ² ). (3) Absorption rate 10 cm x with a circular hole with a diameter of 2.5 cm in the center
After placing 10 cm-sized plates on the test sample in which two sheets are stacked, 20 ml of artificial urine is injected through the hole in the central part, and the time until absorption into the test sample is measured. The composition of the artificial urine used is as follows. Composition of artificial urine Urea 1.9% NaCl 0.8% CaCl2 0.1% MgSo4 0.1% Distilled water 97.1% (4) Sheet stiffness The stiffness of the test sample is shown in TAPPI 543 pm-84. It was measured by the method.

Example 2 In the method for obtaining a composite sheet by integrating long fibers and a paper sheet, except that a 14-mesh porous support woven with a plain weave structure of stainless steel filament was used as a transfer conveyor. A composite sheet was produced in the same manner as in 1, and crosslinking reaction and carboxyalkylation reaction were further performed to impart water absorbing performance to the pulp fiber to obtain a water absorbing sheet.
The obtained sheet, a region 2 to 3 mm ^2/1 single oval pulp fibers are not present were arranged alternately vertically and horizontally. The ratio of the area where pulp fibers do not exist to the total area of this sheet was 25 area%. The water absorbent sheet was tested in the same manner as in Example 1.

Comparative Example 1 In the method of integrating a long fiber and a paper sheet to obtain a composite sheet, except that a 50-mesh porous support prepared by weaving stainless steel filaments in a plain weave structure was used as a transfer conveyor. A composite sheet was produced in the same manner as in 1, and crosslinking reaction and carboxyalkylation reaction were further performed to impart water absorbing performance to the pulp fiber to obtain a water absorbing sheet.
The obtained sheet region 0.01~0.03mm ^2/1 single oval pulp fibers are not present were arranged alternately vertically and horizontally. The ratio of the area where pulp fibers do not exist to the total area of this sheet was 9 area%. The water absorbent sheet was tested in the same manner as in Example 1.

Comparative Example 2 In the method of integrating a long fiber and a paper sheet to obtain a composite sheet, except that a 6-mesh porous support woven with a plain weave structure of stainless steel filament was used as a transfer conveyor. A composite sheet was produced in the same manner as in 1, and crosslinking reaction and carboxyalkylation reaction were further performed to impart water absorbing performance to the pulp fiber to obtain a water absorbing sheet. The resulting sheet, regions 9 to 10 mm ^2/1 single oval pulp fibers are not present were arranged alternately vertically and horizontally. The ratio of the area where pulp fibers do not exist to the total area of this sheet was 52% by area. The absorbent sheet was tested as in Example 1.

The results obtained in Examples and Comparative Examples are shown in Table 1.

[0035]

[Table 1]

As can be seen from Table 1, the water absorbent sheet of the present invention is flexible, the multi-layer structure absorbent body made of this sheet has a large amount of pure water absorbed, and has an extremely excellent liquid absorption rate (Examples 1 to 1). 2). On the other hand, if the area where the pulp fiber does not exist and the ratio of the area to the entire sheet are small, the flexibility is poor, and the multilayer structure absorbent body made of this sheet has a large amount of pure water absorption, but the first layer Since the water-absorbent sheet of No. 2 causes gel blocking after water absorption and it takes a considerable time for the liquid to diffuse to the water-absorbent sheet of the second layer, it is not suitable as an absorber (Comparative Example 1). Further, when the ratio of the area where the pulp fiber of the water-absorbent sheet does not exist and the area of the area to the entire sheet is large, the flexibility of the sheet is excellent, and the water absorption speed is remarkably excellent when a multilayer structure absorbent body, Since the amount of pure water absorbed per unit area decreases, more water absorbent sheets must be used, which is uneconomical (Comparative Example 2).

[0037]

INDUSTRIAL APPLICABILITY The present invention has the effect of providing a water absorbent sheet which is flexible and has an excellent water absorption amount, and has a significantly improved water absorption rate when it is formed into a multilayer structure absorbent body.

Claims (1)

[Claims] 1. A synthetic fiber and a degree of substitution of 0.35 to 0.8
A water-absorbent sheet containing pulp fibers having a crosslinked carboxyalkyl group, has a region in which the pulp fibers do not exist in the thickness direction of the sheet, and the area of each region is 0.05 to 6 mm ² . And a total area of 10 to 50 area% of the total area of the sheet.