JP4263576B2 - Water absorbent sheet - Google Patents

Description

  The present invention relates to a water-absorbent sheet useful for the manufacture of disposable diapers, sanitary napkins and the like. The water-absorbent sheet has a high water absorption speed and a high water absorption amount while maintaining the flexibility of the fibrous base material.

  Conventionally, water-absorbent sheets used for paper diapers, sanitary napkins, and the like have been obtained by uniformly dispersing and adsorbing water-absorbent resin particles such as crosslinked polyacrylic acid on a fibrous base material such as paper, pulp, and nonwoven fabric. There is something. However, there are few water-absorbent resin particles in which the water-absorbent resin particles are securely fixed to the fibrous base material, and the water-absorbent resin particles easily fall off from the water-absorbent sheet. In addition, the operation is complicated in terms of handling powder particles.

  In order to solve the above problems, after spraying a monomer mixture aqueous solution composed of acrylic acid and acrylate onto a fibrous base material, the sprayed single amount is irradiated with ionizing radiation, fine particle ionizing radiation, etc. A water-absorbent sheet is known in which a body mixture is polymerized to fix a water-absorbent resin to a fibrous base material.

  Patent Document 1 discloses a method for producing a water-absorbent sheet similar to the above by UV-irradiating a monomer aqueous solution supported on a fibrous base material to polymerize the monomer. In the same publication, most of the monomers are polymerized by UV irradiation, and then irradiated with an electron beam so that no unreacted monomer remains after polymerization, and then UV irradiation is performed to perform polymerization conversion. It is described that it is preferable to increase the value. As described above, even when the UV polymerization method is employed, in order to produce a water-absorbent sheet in which unreacted monomers do not remain, the present situation is that a complicated production process must be employed.

  In Patent Document 2, a water-absorbing and water-holding ability is high by dropping and adhering a monomer aqueous solution droplet during polymerization onto a fibrous base material to complete redox polymerization on the fibrous base material. A method of manufacturing a sheet is described.

Patent Document 3 discloses that a similar water-absorbent sheet has a porosity of 50 to 99.5% as the fibrous base material, and the primary particle diameter of the water-absorbent resin particles carried on the base material. It is described that it is preferably 50 to 1000 μm, and the amount of the water absorbent resin supported per 1 m ^{2 of the} base material is preferably 10 to 500 g.

  According to the inventions described in Patent Documents 1 to 3, since the monomer aqueous solution is polymerized on the fibrous base material, the water-absorbing resin particles obtained are fixed to the fiber and integrated. For this reason, many of the problems of Patent Document 1 that occur when a powdery water-absorbing resin is dispersed and fixed to a fibrous base material have been solved.

However, the water-absorbent sheet described in each of the above publications has a problem that even if the water-absorbing resin fixing amount per 1 m ² of the base material is increased, the water-absorbing amount of the manufactured water-absorbent sheet does not increase in proportion thereto. is there. In other words, when the water-absorbing resin fixing amount is 100 g / m ² or more, even if the fixing amount is further increased, an increase in the water absorption amount corresponding to the increase cannot be obtained. There is a fundamental problem. Furthermore, when the fixing amount is large, unreacted monomers are likely to remain, and the flexibility of the water absorbent sheet is reduced.
JP-A-1-292103 (pages 1 to 3) JP 2000-328456 A (Claim 1) JP-A-9-67403 (Claims)

  In the present invention, a water-absorbent sheet having a high flexibility, which can greatly increase the amount of water absorption per water-absorbent resin, has an excellent water absorption rate and contains only a few unreacted monomers, is produced on an industrial scale. It is an object to provide a method.

  As a result of intensive studies to solve the above problems, the present inventors surprisingly have a specific range in which the breaking strength of the nonwoven fabric carrying the water-absorbent resin particles is smaller than the breaking strength of conventionally used nonwoven fabrics. In this case, the water-absorbent sheet produced using this nonwoven fabric was found to exhibit a large amount of water absorption, and the present invention was completed.

That is, the present invention is a water-absorbent sheet in which water-absorbent resin particles are supported on a non-woven fabric having a breaking strength of 80 to 200 g / 25 mm according to a breaking strength measuring method described later.

  The water absorbent sheet of the present invention is characterized in that a nonwoven fabric carrying water absorbent resin particles having a low breaking strength is used. The water-absorbent sheet of the present invention swells greatly in a sufficiently absorbed state, and its dimensions are greatly deformed. Since this water absorbent sheet has a low breaking strength of the nonwoven fabric, it easily deforms when the water absorbent resin particles absorb water and swells, and does not suppress any swelling of the water absorbent resin particles. As a result, it is considered that the water absorbent sheet of the present invention exhibits an extremely large water absorption amount. Furthermore, since the water-absorbent sheet of the present invention has the water-absorbing resin particles attached to the fibers constituting the nonwoven fabric independently in a so-called beaded state, the water absorption speed is high, the water absorption amount is large, and the flexibility is high. .

  The water absorbent sheet of the present invention is formed by carrying water absorbent resin particles on a nonwoven fabric. The water-absorbent resin particles are independently attached in a beaded manner to each fiber constituting the nonwoven fabric. By having such a shape feature, the water absorbent sheet of the present invention can achieve flexibility and a high water absorption rate.

  The water-absorbent resin particles are a polymer of a monomer mainly composed of acrylic acid and / or a salt thereof (hereinafter collectively referred to as an acrylic acid monomer). Monomers other than acrylic acid monomers can be used in combination, and specific examples thereof include methacrylic acid or a salt thereof, (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2-acrylamide-2-methyl. Examples thereof include propanesulfonic acid or a salt thereof.

Loading amount of water absorbent resin particles to be supported on the non-woven fabric 80 g / m ² or more, more preferably ^{100~1000g / m 2, 120~500g / m} 2 is particularly preferred. When the loading amount is less than 80 g / m ^{2, the} water absorption amount may be insufficient when used for diapers or the like. Moreover, when the carrying amount is less than 80 g / m ² , the water absorption amount of the water-absorbing sheet to be obtained is hardly affected by the breaking strength of the nonwoven fabric to be used, and the function and effect expected in the present invention are hardly exhibited. In other words, when the carrying amount exceeds 80 g / m ² , even if the carrying amount of the water-absorbing resin is the same by using a nonwoven fabric having relatively low breaking strength as defined in the present invention as the base material, Compared with the case of using a non-woven fabric having a breaking strength other than the above, the amount of water absorption can be increased.

  The average particle diameter of the water-absorbent resin particles carried on the nonwoven fabric is preferably 10 to 500 μm, and more preferably 30 to 300 μm.

  The breaking strength of the nonwoven fabric constituting the water-absorbent sheet of the present invention is 50 to 300 g / 25 mm, preferably 70 to 250 g / 25 mm, and more preferably 80 to 200 g / 25 mm.

  Here, the breaking strength of the nonwoven fabric in this invention shows what was calculated | required with the measuring method described below.

  That is, according to JIS P8113, using a non-woven fabric test piece cut into a rectangle having a length of 150 mm and a width of 25 mm, it is a value measured under measurement conditions of a test piece gripping interval of 100 mm and a pulling speed of 100 mm / min.

  Nonwoven fabrics are usually manufactured as rolls, and their breaking strength is generally different in the lateral direction (width direction) and the longitudinal direction (longitudinal direction). In the present invention, the breaking strength in the longitudinal direction is preferably 50 to 300 g / 25 mm, and more preferably, the breaking strength in both the transverse direction and the longitudinal direction is within this range.

  When the breaking strength is less than 50 g / 25 mm, the strength is too weak, there is a problem in handling, and the shape retention during water absorption is insufficient. When the breaking strength exceeds 300 g / 25 mm, swelling at the time of water absorption is suppressed and the desired water absorption amount is not exhibited.

  As will be described later, the breaking strength of the nonwoven fabric includes the base fiber and binder fiber material, fineness, fiber length, blending ratio of the binder fiber, and heat treatment conditions (temperature and time) for partially melting the binder fiber, as will be described later. It can adjust by selecting etc. suitably.

  Although there is no restriction | limiting in particular in the thickness of a nonwoven fabric, Usually 0.5-1.5mm (in the state before the raising process mentioned later) grade is suitable.

Basis weight of the nonwoven fabric is preferably 10 to 100 g / m ^2. When it is less than 10 g / m ^2, it becomes difficult to attach a sufficient amount of the monomer aqueous solution as a raw material for the water-absorbent resin particles to the nonwoven fabric during the production of the water-absorbent sheet. When the basis weight exceeds 100 g / m ² , the air permeability is deteriorated and the manufacturing cost is increased.

  Non-woven fabric is a generic term for non-woven fabric in a narrow sense, that is, a fiber web in which base fibers are fixed with a binder, a web that is carded or air-laid, and a pad with loose fibers. The nonwoven fabric used in the present invention is preferably one in which base fibers are heat-sealed with binder fibers. This nonwoven fabric is more preferable in this respect because it can be brushed as described later.

  The non-woven fabric is obtained by uniformly mixing base fibers and heat-sealable binder fibers, for example, at a ratio of base fibers / binder fibers = 40 to 90/60 to 10 (mass ratio), and using means such as carding. It can be easily manufactured by forming a web and then thermally fusing the base fibers and binder fibers together.

  The base fiber preferably has a fineness of 1 to 10 dtex and a fiber length of 32 to 128 mm. When using a base fiber having a fineness of less than 1 dtex, it is difficult to obtain a non-woven fabric with good air permeability and large voids required in the present invention. When it exceeds 10 dtex, it becomes difficult to attach a sufficient amount of the monomer solution to the fibers of the nonwoven fabric during the production of the water absorbent sheet, and a water absorbent sheet with poor flexibility tends to be easily obtained.

  When the fiber length is less than 32 mm, the entanglement between the fibers is weak, so that the web is cut before the heat treatment step for producing the nonwoven fabric, making it difficult to produce the nonwoven fabric. When the fiber length exceeds 128 mm, the entanglement between the fibers becomes excessively strong, and high-speed fiber opening and carding become difficult.

  The base fiber is preferably a thermoplastic polymer fiber such as polyester, polyamide, polypropylene, or polyethylene fiber, and particularly preferably a polyester fiber having excellent fiber performance. Moreover, you may mix and use these.

  When a composite spun fiber composed of low melting point polyester and polyethylene terephthalate is used as the base fiber, the polyesters are compatible with each other by heat treatment, so that a bulky and breathable fiber web can be obtained. Such low-melting polyester can be used without particular limitation as long as it has good fiber-forming properties and melts and softens at 80 to 180 ° C. From the viewpoint of easy fiber production and good fiber properties, the low melting point polyester is preferably a copolymer of terephthalic acid, isophthalic acid and ethylene glycol.

  The binder fiber melts by heat treatment and adheres to the base fiber, thereby imparting shape retention to the base fiber. The binder fiber preferably has a fineness of 1 to 10 dtex and a fiber length of 32 to 128 mm. When the fineness of the binder fiber is less than 1 dtex, the number of binder fibers constituting the nonwoven fabric increases, so that the number of entangled adhesion portions increases. As a result, the porosity is reduced and the air permeability is deteriorated. When the fiber length is less than 32 mm, the web breaks before the heat treatment step for producing the nonwoven fabric, making it difficult to produce the nonwoven fabric. If it exceeds 128 mm, the opening and carding properties are degraded.

  The blending ratio of the binder fiber in the nonwoven fabric is preferably 10 to 80% by mass. When the amount is less than 10% by mass, the bulkiness and shape stability of the nonwoven fabric are insufficient. When it exceeds 80% by mass, it is difficult to obtain a non-woven fabric having a thin texture and a hard non-woven fabric and having a high air permeability and a high porosity.

  If the binder fiber is melted by excessive heat treatment, the form of the nonwoven fabric cannot be stabilized. In order to avoid this problem, the binder fiber is preferably a composite spun binder fiber produced by combining a polymer component having a low melt softening temperature that is melted by heat treatment and a polymer component having a high melt softening temperature that is not melt softened by heat treatment. The softening temperature of the polymer component having a low melt softening temperature is preferably at least 30 ° C. lower than the softening temperature of the polymer component having a high melt softening temperature. By using such a binder fiber, the binder fiber can be prevented from being completely melted during the heat treatment. As the heat fusion type composite spinning binder fiber, a core-sheath type, a side-by-side type, and other shapes can be arbitrarily used.

  Examples of the combination of polymers constituting the composite spinning binder fiber include, for example, a low melting point polyester polymer and polyethylene terephthalate, polyethylene and polyethylene terephthalate, polyethylene and polyamide, polyethylene and polypropylene, polypropylene and polyethylene terephthalate, polypropylene and polyamide. There are many combinations. Further, two or more of these binder fibers may be used in combination. For example, a combination of a composite spinning binder fiber made of a low melting point polyester polymer and polyethylene terephthalate and a composite spinning binder fiber made of polyethylene and polyethylene terephthalate can be mentioned.

  In the case of the core-sheath type composite spun binder fiber, the ratio of the low melting point polymer component disposed in the sheath part and the polymer component disposed in the core part is preferably 10/90 to 90/10 (mass basis). . When the ratio of the sheath portion is less than 10/90, it is difficult to spin the fiber having the core-sheath structure. When the binder fiber exceeds 90/10, the fiber performance decreases.

  The web can be produced by carding, air laying, other known techniques, or a combination thereof. Among them, the carding method is capable of obtaining a web having a large bulk, being capable of forming a web at a high speed, being excellent in productivity, and being able to obtain a wide sheet, and being able to obtain a web having an easily adjusted basis weight. It has many advantages such as high homogeneity and is the most preferable production method.

  The web produced as described above is heat treated to give shape retention and obtain a nonwoven fabric. As a heat treatment method, a method using a known air-through dryer for producing dry nonwoven fabric in which hot air penetrates in the web thickness direction is preferable.

  Regarding heat treatment conditions, generally, the higher the heating temperature and the longer the heating time, the higher the breaking strength can be obtained. Usually, the heat treatment temperature is set to a temperature of the melting point of the low melting point polymer component of the binder fiber + (20 to 60 ° C.), for example, a low melting point polyethylene terephthalate or polyethylene having a melting point of 110 to 130 ° C. as the low melting point polymer. When used, the temperature may be set to 140 to 170 ° C., the hot air speed of the air-through dryer may be set to 0.2 to 3.0 m / second, and the heat treatment time may be set to 1 to 120 seconds.

  The non-woven fabric having a breaking strength of 50 to 300 g / 25 mm is a non-woven fabric production intermediate product containing the binder fiber at a ratio of 20 to 80% by mass as described above, from the melting point of the low melting point resin among the resins constituting the binder fiber. It can be produced by maintaining the temperature at 20 ° C. or higher for several seconds to 1 minute. Specifically, when the blending ratio of the binder fiber is 20% by mass, the heat treatment time is preferably about 1 minute, and when the blending ratio of the binder fiber is 80% by mass, the heat treatment time is several to 10 seconds is preferred.

  The base fiber material is non-hydrophilic in that the monomer aqueous solution adheres to the fiber in independent fine particles when the monomer aqueous solution is atomized and supported on the substrate surface as described later. Is preferred. Moreover, it is also preferable to use hydrophilic fibers such as rayon, cotton, and regenerated cellulose fibers in combination as a minor component. A non-woven fabric produced using such a non-hydrophilic resin fiber as a base fiber is preferable in that a water-absorbing resin obtained by polymerizing an aqueous monomer solution can be fixed to the fiber in the form of fine particles. If the paper itself does not have a certain degree of hydrophilicity, a paper diaper, a sanitary product and the like manufactured using the water-absorbing sheet obtained are liable to leak when used. In order to avoid this inconvenience, it is preferable that the nonwoven fabric has a moderately hydrophilic base fiber surface. Hydrophilic treatment agents include nonionic surfactants such as polyoxyethylene alkyl ether, polyoxyethylene phenol ether, polyoxyethylene sorbitan ester, polyoxyethylene alkyl fatty acid ester or polyoxyethylene oxypropylene block polymer, higher fatty acid salts, Examples thereof include anionic surfactants such as alkyl naphthalene sulfonate, dialkyl succinate and alkyl sulfate.

  As a specific hydrophilization treatment method, when the base fiber is spun, the above-mentioned surfactant, that is, a hydrophilizing agent may be mixed in advance with the spinning raw material resin, and then it may be spun. Further, a hydrophilizing agent dissolved in a solvent or the like may be sprayed on the fiber after spinning.

  Hereinafter, the manufacturing method of the water absorbent sheet of the present invention will be described.

  The water-absorbent sheet of the present invention is obtained by supporting a monomer aqueous solution mainly composed of acrylic acid and / or a salt thereof on the nonwoven fabric having a predetermined breaking strength and then supporting the nonwoven fabric on the nonwoven fabric. It can be suitably produced by a method of polymerizing a monomer.

  The nonwoven fabric may be used as it is, but it is preferable to use one that has been raised in advance by heating. The heating temperature for raising the hair is preferably near the softening point of the base fiber in the nonwoven fabric. Practically, the temperature is in the range of 70 to 150 ° C. Although the heating time varies depending on the heating temperature, it is usually several to 180 seconds. Specific examples of preferable heating conditions are 80 to 110 ° C. and 20 to 60 seconds.

  The heating means is not limited. For example, the nonwoven fabric may be passed through the heating furnace for a predetermined time, hot air may be blown onto the nonwoven fabric, or the nonwoven fabric may be heated by an infrared lamp or the like.

  A part of the fibers constituting the nonwoven fabric is fluffed on the substrate surface by the above heating method or the like. As a result, the volume of the nonwoven fabric normally swells to about 1.3 to 3.0 times that before the raising treatment. The degree of raising varies depending on the fiber length of the base fiber used in the raising. It is preferable to increase the volume of the nonwoven fabric by 1.5 to 3.0 times before the raising treatment by appropriately selecting the fiber length of the base fibers and the heating conditions and raising them.

  As a raising treatment means such as a nonwoven fabric or a fiber web, a raising treatment means using, for example, a roll with a needle thread is known in addition to heating. However, when a mechanical brushing treatment means other than heating is employed, it is difficult to carry a large amount of a monomer aqueous solution, which will be described later, on the fibers constituting the nonwoven fabric as fine particles. The body's water absorption and water absorption speed are inferior.

  An aqueous solution of a monomer mainly composed of acrylic acid and / or a salt thereof (acrylic monomer) is sprayed on the nonwoven fabric or the raised nonwoven fabric (usually in a sheet form). And supported on the base fiber and the binder fiber constituting the nonwoven fabric. A preferred monomer is a mixture of acrylic acid and acrylate in which 20 to 90 mole percent of acrylic acid has been converted to an alkali metal salt or ammonium salt. Monomers other than acrylic monomers can also be used in combination.

  Specific examples include methacrylic acid or a salt thereof, (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2-acrylamide-2-methylpropanesulfonic acid or a salt thereof, and the like. The preferable usage amount of such a monomer is 20 mol% or less based on the total amount with the acrylic acid monomer.

  The preferable concentration of the monomer in the monomer aqueous solution is 20 to 80% by mass, and more preferably 40 to 60% by mass. As the monomer concentration is higher, a water-absorbent sheet having a larger amount of water-absorbing resin fixed can be obtained, and the necessary heat energy can be reduced in the drying process after polymerizing the monomer. Therefore, it is preferable to use a monomer aqueous solution having a concentration as high as possible. A concentration near the saturated solubility of the monomer is also a preferable concentration.

  In addition to the monomer, it is preferable to add a crosslinking agent, a polymerization initiator, and the like to the monomer aqueous solution.

  Cross-linking agents include ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, and glycerin tri (meth) acrylate. N, N′-methylenebisacrylamide, diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate and the like.

  The addition ratio of the crosslinking agent is preferably 100 to 10000 ppm, particularly preferably 1000 to 5000 ppm with respect to the total mass of the monomers.

  The polymerization of the monomer can be performed by a general radical polymerization method. As a polymerization initiation method, a thermal polymerization method using a compound that generates radicals by heat as a polymerization initiator, or a method of initiating polymerization by irradiation with an active energy ray such as an ultraviolet ray or an electron beam can be employed. A thermal polymerization method or a method of irradiating ultraviolet rays in the presence of a photopolymerization initiator (hereinafter referred to as a UV irradiation polymerization method) is preferable, and a UV irradiation polymerization method is particularly preferable.

  Examples of the thermal polymerization initiator include water-soluble radical polymerization initiators such as hydrogen peroxide, ammonium persulfate, potassium persulfate, t-butyl hydroperoxide, and cumene hydroperoxide. These compounds may be used as a redox polymerization initiator in combination with a reducing compound such as sodium bisulfite, sodium thiosulfate, L-ascorbic acid or amine.

  The photopolymerization initiator used for polymerization by the UV irradiation polymerization method is not particularly limited, and any photopolymerization initiator that can generate radicals by ultraviolet rays can be used. An initiator can be appropriately selected according to the purpose and used. Specifically, azo compounds such as 2,2′-azobis (2-aminodipropane) salt, ketones such as 1-benzoyl-1-hydroxycyclohexane and benzophenone, benzoin and alkyl ethers thereof, benzyl ketals, and anthraquinone Examples thereof include derivatives.

  A benzoyl group such as 1-hydroxy-cyclohexyl-phenyl-ketone and an azo group such as 2,2'-azobis {2- (2-imidazolin-2-yl) propane} are preferable.

  The addition amount of the photopolymerization initiator is preferably 100 to 2000 ppm with respect to the monomer. When the concentration of the photopolymerization initiator is less than 100 ppm, sufficient polymerization does not occur, and when it exceeds 2000 ppm, the degree of polymerization of the resulting polymer is lowered.

  In addition to the photopolymerization initiator, a thermal decomposition type radical polymerization initiator is preferably used in combination. By using this initiator in combination, thermal polymerization also occurs in parallel with UV polymerization. As a result, the polymerization conversion rate increases and the remaining amount of unreacted monomer can be reduced. The amount of the thermal decomposition type radical polymerization initiator added is preferably from 100 to 5000 ppm, particularly preferably from 500 to 2000 ppm, based on the monomer.

  The polymerization temperature is preferably 50 to 80 ° C. Therefore, the thermal decomposition type radical polymerization initiator is preferably a compound that decomposes in water at 80 ° C. or less to generate radicals. Specific examples thereof include persulfates such as ammonium persulfate and sodium persulfate.

  A chain transfer agent, a surfactant or the like may be further added to the monomer aqueous solution as an additive as necessary.

  The monomer aqueous solution is sprayed (coated) on the nonwoven fabric in the form of a mist as described above. Thereby, the monomer aqueous solution can be carried in the form of independent fine particles on the fibers constituting the nonwoven fabric. As a method for atomizing the monomer aqueous solution, a known atomization technique can be used. Examples thereof include a dropping method, a droplet forming method using a spray nozzle, a droplet forming method using a rotating disk atomizer, and an ultrasonic method.

  The average diameter of the spray droplets is preferably 30 to 700 μm. When the average diameter is less than 30 μm, the droplet sprayed on the nonwoven fabric easily penetrates to the back side without adhering to the fiber, and the adhesion efficiency of the droplet is lowered. When the average diameter of the droplets exceeds 700 μm, the adhesion of the droplets becomes uneven, and as a result, the water absorption amount and the water absorption rate of the water-absorbing composite material obtained by polymerization may be insufficient. The monomer aqueous solution having a size of 30 to 700 μm is subjected to polymerization and drying steps to become water absorbent resin particles of approximately 10 to 500 μm, and is fixed to the fiber in a bead shape.

The amount of the monomer aqueous solution supported is an amount such that the water-absorbent resin particles obtained by polymerization are fixed to the nonwoven fabric in an amount of 80 g / m ² or more, preferably 100 to 1000 g / m ² , and 120 to 500 g / m ^2. m ² is more preferable.

  As described above, the nonwoven fabric carrying the aqueous monomer solution is irradiated with ultraviolet rays (UV), or the nonwoven fabric is heated to a predetermined temperature to polymerize the acrylic acid monomer in the presence of a crosslinking agent. Let

  In the polymerization, it is preferable to replace the atmosphere surrounding the monomer aqueous solution with an inert gas such as nitrogen gas so as to eliminate oxygen as much as possible.

As the ultraviolet lamp, a mercury lamp having a lamp input of 30 to 240 W / cm, such as a high pressure mercury lamp or a metal halide lamp, which can irradiate a wavelength of 250 to 450 nm is preferable. The amount of ultraviolet irradiation is 100 to 10000 mJ / cm ² , more preferably 2000 to 6000 mJ / cm ² . Many mercury lamps can be used side by side according to the required dose.

  By the UV irradiation, most of the monomers (about 90% or more) complete the polymerization in about 5 to 60 seconds. At this time, the polymerization temperature is estimated to be about 80 to 90 ° C. as the temperature of fine particles of the aqueous monomer solution applied to the nonwoven fabric. In this way, water-containing polymer particles containing about 15 to 30% by weight of water and 0.1 to 10% by weight of unreacted monomers are formed on the fibers.

  Hereinafter, the water-absorbent sheet of the present invention is obtained by further heating for an appropriate time to reduce the unreacted monomer and to dry the water-containing polymer particles.

  After producing the water-absorbent sheet by the above method, it is desirable to spray an aqueous solution of a crosslinking agent having a plurality of functional groups such as epoxy groups having reactivity with carboxyl groups (hereinafter referred to as surface treatment agent) on the water-absorbent sheet. . By this operation, the degree of crosslinking of the surface layer of the water-absorbent resin particles can be further increased.

  Examples of the surface treatment agent include glycidyl ethers such as ethylene glycol diglycidyl ether and polyethylene glycol diglycidyl ether, polyols such as diethylene glycol, triethylene glycol, propylene glycol, glycerin and pentaerythritol, and polyamines such as ethylenediamine. The addition amount of the surface treatment agent is preferably 100 to 1000 ppm with respect to the water absorbent resin particles.

  The obtained water-absorbent sheet is preferably further subjected to a heat compression treatment for the purpose of further improving the water absorption performance (water absorption speed, water absorption amount, liquid diffusibility, liquid reversion prevention property, etc.). The thermal compression is preferably performed using a hot press, a hot roll, an embossing roll, or the like.

  The heat compression temperature is preferably 50 to 150 ° C, more preferably 70 to 120 ° C. When the heat compression temperature is less than 50 ° C, a sufficient compression effect cannot be obtained. When the heat compression temperature is higher than 150 ° C, the fibers constituting the water absorbent sheet are melted by heat, and the flexibility of the resulting water absorbent sheet is impaired. This is not preferable.

  The hot compression pressure is preferably from 0.01 to 100 MPa, more preferably from 0.1 to 10 MPa. Although heat compression time changes with heat compression temperature and heat compression pressure, 1 to 100 second is preferable.

  When heat-compressing on an industrial scale, it is particularly preferable to use a heat roll. Specifically, the water-absorbent sheet is continuously guided between the rolls while the pair of rolls is pressurized to have a linear pressure of 1 to 100 kg / cm, and is thermally compressed between the rolls.

  The gap between the pair of rolls used for thermal compression is preferably 10 to 500 μm, although it depends on the thickness of the water-absorbent sheet to be thermally compressed. If it is less than 10 μm, the fiber may be cut, and if it exceeds 500 μm, the compression effect is insufficient. By heat compression, the water-absorbent sheet becomes 25 to 95% of the thickness before compression.

  In the pair of rolls, it is preferable that an uneven pattern is formed on at least one of the rolls. The depth of the concavo-convex pattern is 0.001 mm or more, preferably 0.01 to 1 mm. The concavo-convex pattern is preferably such that the concavo-convex pattern is repeated at intervals of 10 mm or less, or a pattern that fits in a circle with a diameter of 10 mm is continuously formed at intervals of 10 mm or less. In the case of a pattern with a repetition interval exceeding 10 mm or a pattern that does not fit in a 10 mm circle, the advantages of the present invention that are generated by compressing the fiber may not be sufficiently exhibited.

  Hereinafter, the present invention will be described more specifically with reference to examples.

(Example 1)
Tetraethylene glycol diacrylate (Aronix M-240, manufactured by Toagosei Co., Ltd.) as a cross-linking agent was added to a monomer aqueous solution (monomer total content 42 mass%) composed of 70 mol% sodium acrylate and 30 mol% acrylic acid. 0.05 parts by mass (based on monomer mass) was added, and the aqueous monomer solution was cooled to 20 ° C. Subsequently, nitrogen gas was blown into the monomer aqueous solution to reduce the dissolved oxygen concentration to 1 ppm or less. In the monomer aqueous solution, 0.02% by mass (based on monomer mass) of 1-hydroxy-cyclohexyl-phenyl-ketone as a photopolymerization initiator and 0.15% by mass of sodium persulfate (monomer mass) as a thermal polymerization initiator (Standard) was added and mixed to prepare an aqueous polymerizable monomer solution. After spraying the polymerizable monomer aqueous solution onto a low-strength air-through nonwoven fabric (breaking strength: 155 g / 25 mm) made of PET cotton, it is polymerized by irradiating ultraviolet rays using a high-pressure mercury lamp in a nitrogen atmosphere. (Ultraviolet light quantity: 3,500 mJ / cm). The obtained water-absorbing combination sheet was flexible with a water-absorbing resin fixed to 200 g / m ² .

(Example 2)
In Example 1, instead of the low-strength air-through nonwoven fabric having a breaking strength of 155 g / 25 mm, the same procedure as in Example 1 was used except that a nonwoven fabric having a breaking strength of 265 g / 25 mm was used. ^{2 A} fixed water-absorbent sheet was obtained.

(Comparative Example 1)
In Example 1, in place of the low-strength air-through nonwoven fabric having a breaking strength of 155 g / 25 mm, a commercially available nonwoven fabric having a breaking strength of 1780 g / 25 mm was used. A water-absorbent sheet having a / m ² adherence was obtained.

The water absorbent sheets of Examples 1 and 2 and Comparative Example 1 were evaluated by the test methods described below. The results are shown in Table 1. The artificial urine used for the test had the following composition.
Artificial urine (per 10 kg):
Urea / NaCl / MgSO ₄ .7H ₂ O / CaCl ₂ .2H ₂ O / pure water =
200g / 80g / 8.0g / 3.0g / 9709g
(Artificial urine water absorption)
A water-absorbent sheet cut into 6 cm × 7 cm and 200 ml of artificial urine were placed in a 300 ml beaker and left at room temperature for 30 minutes. Thereafter, the water-absorbent sheet swollen by absorbing artificial urine was taken out from the artificial urine, and the artificial urine adhered with a 200-mesh filter cloth was wiped off, and its mass was measured. The artificial urine water absorption A (kg / m ² ) was calculated according to the following formula.

A = (W1-W2) /0.42
In the formula, W1 represents the mass of the water absorbent sheet after water absorption, and W2 represents the mass of the water absorbent sheet before water absorption.

(Artificial urine water absorption speed)
A 300 ml beaker was filled with 200 ml of a water-absorbing sheet cut into 6 cm × 7 cm and artificial urine, and left at room temperature for 5 minutes to swell the polymer with artificial urine. Next, the artificial urine adhered with a 200-mesh filter cloth was wiped off, and its mass was measured. The artificial urine water absorption rate B (kg / m ² ) was calculated according to the following formula.

B = (W1-W2) /0.42
In the formula, W1 represents the mass of the water absorbent sheet after water absorption, and W2 represents the mass of the water absorbent sheet before water absorption.

実施例１及び比較例１において、３０分間人工尿に浸漬して膨潤した吸水性シートの寸法を表２に示した。実施例１の吸水性シートの寸法が大きく変化しているのが分る。

In Example 1 and Comparative Example 1, the dimensions of the water-absorbent sheet swollen by immersion in artificial urine for 30 minutes are shown in Table 2. It can be seen that the dimensions of the water-absorbent sheet of Example 1 are greatly changed.

Claims (1)

The average particle size of the nonwoven fabric having a breaking strength of 80 to 200 g / 25 mm , in which the base fiber is made of polyester fiber and heated at 80 to 110 ° C. for 2 to 60 seconds to increase the volume of the nonwoven fabric by 1.5 to 3.0 times. A water absorbent sheet comprising 80 g / m ² or more of 10 to 500 μm of water absorbent resin particles.