JPH0779829B2 - Absorbent manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention contains a highly absorbent resin, and is suitable for, for example, a disposable diaper that rapidly absorbs a large amount of urine and does not get wet with a diaper. More specifically, it efficiently draws out the potential absorption capacity of the super absorbent resin,
Moreover, the present invention relates to a method for manufacturing a thin and compact absorbent material having excellent performance capable of rapidly absorbing a large amount of water.

(Prior Art) Recently, an absorbent material using a highly absorbent resin is provided and used as a so-called disposable sanitary material such as a disposable diaper. Such an absorbent material will be described by taking a disposable diaper as an example.

As shown in FIG. 8, the disposable diaper generally has a water-permeable top sheet 1 on the side in contact with the skin of an infant, a sick person, an elderly person, a liquid absorption holding layer 2 in the middle, and an impermeable backing sheet 3 on the outside. It consists of Conventionally, the liquid absorption / retention layer 2 has used an absorbent material in which pulverized wood pulp fibers and fine particle-shaped high-absorbency resin are mixed and used as they are or in a layered manner. It wasn't enough right now. Therefore, by improving the internal structure of the absorbent material, a concentration (density) gradient in the thickness direction is provided in the distribution of the super-highly absorbent resin in the absorbent material, for example, the distribution side on the side in contact with the body is sparse and the side away from the body. The distribution of
The method of obtaining the following effects is proposed.

The swollen resin that absorbs urine sticks to each other to form a weir, which suppresses the formation of a so-called gel block that prevents the penetration of urine into the inner layer.

The part of low resin density serves as a reservoir for urine that is discharged in large quantities, and plays the role of a storage container until it is absorbed by the superabsorbent resin, which has a low absorption rate.

It prevents urine absorbing and gelling slimy resin from directly touching the body and giving an unpleasant feeling.

As described above, as a specific method for imparting a concentration (density) gradient in the thickness direction to the distribution of superabsorbent resin in the absorbent material, for example, as shown in FIG. A method has been proposed in which a mixture of the absorbent resin 4 and the wood pulp fiber 5 is laminated in multiple layers.

However, as in the present invention, in the case of an absorbent material in which an acrylic acid-based superabsorbent resin monomer solution is applied to a fibrous web and then polymerized to form a superabsorbent resin, high absorption in the thickness direction of the absorbent material is achieved. It was extremely difficult to provide a concentration (density) gradient of the functional resin.

(Problem to be Solved by the Invention) An object of the present invention is to provide a method for producing an absorbent material in which a highly absorbent resin is fixed to a fiber base material, in which the concentration of the highly absorbent resin in the thickness direction of the absorbent material ( It is to provide a method of providing a density gradient.

(Means for Solving the Problems) The present inventor has found that when the density of a fiber base material forming a fiber web is changed, the amount of non-adhesion of a monomer solution to the fiber web is changed, and a fiber layer having a different fiber base density is used. In the fibrous web formed by laminating the resin, the amount of the monomer solution attached changes depending on the fiber base material density of the fiber layer when the monomer solution is applied. The inventors have found that it is possible to control changes in the concentration (density) of the above, and have completed the present invention. It is considered that the amount of adhesion changes in this way because the higher the density of the fibrous web, the smaller the voids in the fibrous base material, and the capillary action strongly acts to accumulate a large amount of the monomer solution.

That is, the manufacturing method of the absorbent according to the present invention, the acrylic acid superabsorbent resin monomer solution is applied to the fiber web,
A method for producing an absorbent by polymerizing the same, wherein the fiber web is constituted by laminating a relatively high-density fiber base material layer and a low-density fiber base material layer, and applied to the web. The characteristic feature is that the monomer solution is accumulated in a relatively large amount in the high-density fiber base material layer by utilizing the difference in the density, and then the monomer is polymerized.

According to the method for producing an absorbent material of the present invention, the high-absorbency resin is sparse-dense or dense-sparse from the side in contact with the body,
This can be achieved by changing the density of the fiber base material of the individual fiber layers constituting the fiber web.

The density of the fibrous base material is a value defined by the surface area (cm ² ) of the fibrous base material contained in a unit volume (1 cm ³ ) of the fibrous layer. In the combination of fiber base materials having the same basis weight and the same fineness, the smaller the bulk, the higher the fiber base material density. Conversely, in the same basis weight and the same bulk, the smaller the fiber base material's fineness, the higher the fiber base material density.

In the present invention, the distribution of the concentration (density) of the superabsorbent resin in the absorbent material is not particularly limited, but as described above, when the side in contact with the body is sparse and the side away from the body is dense, the absorption performance is high. Is particularly improved, which is preferable. For example, as shown in FIG. 1, when the fibrous web is composed of two layers of fibrous webs (A) and (B), the fibrous layer has a sparse resin concentration (density) distribution on the side in contact with the body. The density of the fiber base material F of (A) is 5 to 25 (cm ² ), and the density of the fiber base material F of the fiber layer (B) on the side far from the body is dense.
It should be 10 to 50 (cm ² ), and the fiber base material densities of both should be different by at least 5 (cm ² ). When the fibrous web is composed of three or more fibrous layers, the fibrous layer on the side in contact with the body and the fibrous base material density of the fibrous layer farthest from the body are made the same as those of the fibrous layer (A) and the fibrous layer (B), respectively, This 2
The fiber base material density may be appropriately changed so that the resin concentration (density) increases between the two fiber layers as the distance from the side in contact with the body increases.

As another example, the distribution of the superabsorbent resin in the absorbent material is closely distributed on the side in contact with the body, and then the intermediate layer is sparsely distributed.
And when the side farthest from the body is densely constructed again,
(A) The flexibility of the absorbent material is increased, the texture is softened, the adhesion to the body is improved, and diaper leakage is suppressed, (b)
Since the intermediate layer, in which the resin is sparsely distributed, acts as a urine reservoir, the urine discharged in a large amount at a time can be rapidly taken into the absorbent material and quickly absorbed in the upper and lower resin tanks. (C) When the superabsorbent resin absorbs urine and swells, it destroys the fiber skeleton (mesh of the network), so the swollen gel tends to fall apart, but if an absorbent layer is provided, it will swell. The effect of improving the morphological stability of can be obtained.

Furthermore, if the distribution of the super absorbent resin in the absorbent material is sparse on the side in contact with the body, then the intermediate layer is made dense and the side farthest from the body is sparse again, (a) gel block on the side in contact with the body The formation of urine can be suppressed and absorption performance can be improved. (B) The two places on the side in contact with the body and on the side farthest from the body become reservoirs for urine and large amounts of urine are absorbed by the resin. You can save it until you wait, (C)
The swollen middle layer gel is blocked by the loosely distributed layers of the superabsorbent resin above and below and does not appear on the absorbent surface.
Since the distribution of the high-absorbency resin on both the front and back surface layers of the absorbent material is sparse, it is difficult for the resin, which becomes sticky in the water-containing state during the absorbent manufacturing process, to come into direct contact with devices such as rollers and belts. It is possible to obtain an effect such that it is possible to prevent the absorbent material from sticking to each other to cause a trouble during manufacturing.

In short, depending on the purpose, the distribution of super absorbent resin in the absorbent material,
That is, a combination of the fiber base material densities of the fiber layers forming the fiber web may be selected.

Next, the absorbent material of the present invention will be described in more detail by focusing on its manufacturing method (FIG. 2). The fibrous web 6 is coated by a nip type coating roll 7 with a monomer solution 8 to which one initiator of a redox type (redox type) polymerization initiator and a crosslinkable monomer are added, and then the other initiator solution 9 is applied. Is sprayed and then introduced into the polymerization tank 10 maintained at 60 to 100 ° C. The polymerization initiator added to the monomer solution comes into contact with the other sprayed initiator to start the radical cassette polymerization reaction, and the monomer solution on the fiber base material is polymerized from the surface portion of the droplet while generating heat. The reaction is completed in a few seconds. The superabsorbent resin 11 thus obtained is a water-containing polymer containing approximately 10 to 30% of water, and contains multiple unreacted residual acid monomers. To complete the polymerization. When irradiating with electron beam and ultraviolet ray, moisture should be 20-
If adjusted to 30%, the unreacted residual acid monomer can be efficiently reduced. Therefore, especially when performing electron beam irradiation followed by ultraviolet irradiation, spray water with an appropriate means such as a rotating brush 13 in the middle. Perform humidity control.

Thereafter, as shown in FIG. 3, a solution of a cross-linking agent having two or more functional groups capable of reacting with a carboxylic acid group in the molecule is sprayed on the water-containing polymer by an appropriate means such as a rotating brush 17. , Leading to the heating device 18 to form crosslinks on the resin surface at 100 to 200 ° C. to improve the absorption performance.

The fiber web composed of at least two fiber layers used in the present invention preferably has a total basis weight of 10 to 100 g / m ² . If it is less than 10 g / m ² , it is difficult to attach the monomer solution of the superabsorbent resin in the amount required for the absorbent material of the disposable diaper. ^{On the other hand} , if it exceeds 100 g / m ² , the air permeability becomes poor, which is economically disadvantageous.

Further, in each fiber layer constituting the fiber web, the volume (cc) of the fiber layer per 1 g of the fiber base material, that is, the specific volume (cc /
g) is preferably 0.8 × 10 ^{2 to} 1.6 × 10 ² (cc / g). 0.8 x 10
^If it is less than ² cc / g, the monomer solution will form a film and adhere between the fiber base materials, and if it exceeds 1.6 × 10 ² (cc / g),
The thickness of the obtained absorbent material becomes too large, which makes it difficult to handle in the subsequent steps.

The air permeability of the fiber web is 10cc / sec (100mmH ₂ O)
The above is preferable, and if it is not filled with 10 cc / Sec (10 mH ₂ O), the rapid heat generation of the polymerization during the polymerization of the applied monomer is not radiated, and the temperature of the monomer rapidly rises, causing the self-crosslinking abnormality. A reaction occurs, and an absorbent resin having a low absorption performance is formed on the fiber.

In the present invention, the density of the fiber base material of the fiber layer can be appropriately changed, specifically, depending on the kind of the polymer of the fiber base material forming the fiber layer, the fineness, and the basis weight and bulk of the fiber layer itself, that is, the specific volume. . These fiber layers are formed by uniformly blending a highly elastic base fiber and a heat-fusible binder fiber in an appropriate ratio, and forming a web using a means such as carding.
Then, it can be easily obtained by heat fusion with each other, but in particular, the kind of polymer of the fiber base material constituting the base fiber, the combination of fineness and its composition ratio, and the fiber base material constituting the binder fiber. It can be freely controlled by the fineness and the composition ratio thereof.

The base fiber has a fineness of 2 to 20 denier and a fiber length of 32 to 12
It is preferably in the range of 8 mm. If the fineness is less than 2 denier, large voids with good air permeability and high compression elasticity required in the present invention cannot be obtained, and if it exceeds 20 denier, it becomes difficult to sufficiently increase the amount of the monomer solution attached. When the fiber length is less than 32 mm, the entanglement between the fibers is weak, and the web is cut off before the heat treatment. When the fiber length exceeds 128 mm, the entanglement is too strong, which makes opening and carding at high speed difficult.

Such a bulky and highly elastic base fiber can be easily obtained by using, for example, a tubular structure having a hollow center.

Hereinafter, the base fiber will be described first.

In the present invention, when a part of the base fiber is formed into a modified cross-section fiber having at least one concave portion continuous along the fiber axis on the fiber surface, the state of adhesion of the monomer solution to the fiber surface becomes a fine morphology, and during polymerization, It is possible to mitigate the rapid temperature rise in the. That is, the initiator solution applied to the fiber surface rapidly diffuses along the recesses to form a fine shape, and the fiber surface area becomes large in the irregular cross section, so that the adhesion of the monomer solution is improved. In the present invention, in addition to the fiber base material density, the concentration (density) of the superabsorbent resin may be changed by combining such measures.

As the base fiber, one or more thermoplastic polymers may be selected from polyester, polyamide, polypropylene, polyethylene fiber and the like, and polyester fiber is particularly preferable because it has excellent fiber performance.

When a polyester fiber is used as the base fiber, when a composite yarn-proof fiber made of a low-melting polyester polymer and polyethylene terephthalate is used, the polyester polymers are compatible with each other by heat treatment, so that a bulky and air-permeable fiber web is obtained. Obtainable. Such a low-melting polyester polymer has a good fiber-forming property and has a temperature of 80 to 180 ° C.
There is no particular limitation as long as it is melted and softened by. From the viewpoint of ease of fiber production and fiber physical properties, it is preferable to use a copolymer of terephthalic acid, isophthalic acid and ethylene glycol as the low melting point polyester polymer.

Next, the binder fiber will be described.

The binder fiber has a fineness of 1.5 to 6 denier and a fiber length of 3
It is preferably in the range of 2 to 128 mm. When the fineness of the binder fiber is reduced, the number of fibers increases and the number of entangled adhesive points increases, the morphological stability of the fiber web improves, and the amount of the monomer solution attached increases, but on the contrary, the porosity decreases and the air permeability decreases. The sex becomes worse. On the contrary, when the fineness is increased, the number of fibers is decreased and the entanglement adhesion points are decreased, so that the bulkiness, compression elasticity and morphological stability of the fiber web are impaired. If the fiber length is less than 32 mm, the web will break before heat treatment,
If it exceeds 128 mm, the opening carding property is deteriorated. The compounding ratio of the binder fiber in the fibrous web is preferably 5 to 50% by weight. If it is less than 5% by weight, the bulkiness and morphological stability of the fibrous web will be insufficient, and if it exceeds 50% by weight, the fibrous web will be thin and hard and it will not be possible to obtain a fibrous web having good porosity. .

Binder fibers cannot stabilize the morphology of the fibrous web if the entire fiber is melted by heat treatment, so a composite spinning that combines a polymer component with a low melt softening point that melts by heat treatment and a polymer component that does not melt melt. Good to use fiber. The heat fusion type composite spun fiber may be a core-sheath type, a side-by-side type, or another type, and the softening point of a polymer component having a low melt softening point is the softening point of another polymer component. Than at least 30 ℃
If it is lower than the above, there is no particular limitation. Examples of such a combination of thermoplastic polymers include, for example, low melting point polyester polymers and polyethylene terephthalate, polyethylene and polyethylene terephthalate, polyethylene and polyamide, polyethylene and polypropylene, polypropylene and polyethylene terephthalate, polypropylene and polyamide, and the like. However, the combination is not limited to these. Furthermore, two or more kinds of fibers may be selected from these combinations to form the binder fiber. For example, a combination of a composite spun fiber made of a low melting point polyester polymer and polyethylene terephthalate and a composite spun fiber made of polyethylene and polyethylene terephthalate can be mentioned.

In the case of the core-sheath type composite spun fiber, the ratio of the low melting point polymer component arranged in the sheath part and the polymer arranged in the core part is 10:90.
~ 90:10 is fine. If it is less than 10:90, the core-sheath type composite spinning is difficult, and if it exceeds 90:10, the fiber performance is deteriorated.

In the present invention, when the binder fiber is a hollow core-sheath type composite spun fiber having a fiber cross section illustrated in FIGS. 4 (a) to (f), the bulkiness and the compression elastic modulus of the fiber web are remarkably improved. It is extremely useful because it can In FIG. 4, reference numeral 20 is a hollow portion, 21 is a sheath polymer, and hatched portion 22 is a core polymer.

In the present invention, when a fiber base material whose surface is treated with a hydrophilicity-imparting agent is mixed with the fiber layer composed of the base fiber and the binder fiber, the non-adhesiveness of the acrylic acid-based monomer solution can be increased. As such a hydrophilicity-imparting agent, a known surfactant having a large wetting effect may be used. For example, nonionic surfactants such as polyoxyethylene alkyl ether and polyoxyethyleneoxypropylene block copolymer, and fatty acids. Anionic surfactants such as salts, alkyl naphthalene sulfonates, dialkyl succinates, alkyl sulfate ester salts and higher alcohol sulfate ester salts, and cationic surfactants such as alkyl amine salts and alkyl quaternary ammonium salts. Can be used. Further, when a fibrous base material in which a hydrophilic group-containing oligomer is coated on the fiber surface is used instead of the surfactant having a large wetting effect, durable hydrophilicity can be imparted to the fibrous web.

On the contrary, when the fiber layer is mixed with a fiber base material whose surface is treated with a water-repellent agent, the adhesion of the acrylic acid-based monomer solution can be reduced. As such a water repellent, for example, an organic acid salt or other compound such as Zn, Sn, or Pb as a catalyst for promoting oxidative condensation in an emulsion solution of a silicon-based monomer such as methyl hydrogen polysiloxane and / or dimethyl polysiloxane It is well known that the water-repellent effect is added, and when applied to a fiber substrate, dried, and then cured, a durable water-repellent effect is obtained. Furthermore, a similar water-repellent effect can be obtained by applying an emulsion solution of a fluorocarbon-based monomer composed of perfluoromonocarboxylic acid chromium complex salt and acrylic acid fluorinated alkyl ester to a fiber substrate, followed by drying and then curing. it can. It is sufficient that the above hydrophilic agent and water repellent are attached to the fiber base material in an amount of 0.05 to 2.0% by weight.

The fiber layer used in the present invention is formed by carding, air-laying, other known techniques, or a combination thereof. Among them, the carding method can obtain a fiber web having large bulk and high compression elasticity. A take-off device (card web take-out device) that can produce a wide sheet at high speed and with high productivity, can easily stack two or more web sheets, and can easily adjust the basis weight. A condensing roll (or compression roll) can be attached to adjust the degree of orientation of each fiber substrate.
The obtained fibrous web is most preferable because it has many advantages such as being homogeneous.

The lamination of the fiber layers in the present invention can be easily achieved by arranging two or more card opening machines in series and sequentially stacking the fiber layers formed by the respective card opening machines. Then, after laminating the fiber layers, a heat treatment described later is performed to obtain a fiber web in which two or more fiber layers are integrated.

As the card opening machine, known roller cards, flat cards and the like can be used. Fig. 5 shows an example of a flat card. In the figure, reference numeral 23 is a cylinder, and 24 is a flat cloth, and the surface velocity thereof is slightly moving or stationary, and serves to arrange the thick fiber layers on the cylinder 23. In the figure, reference numeral 25 is a dish plate, 26 is a feed roller,
27 is a take-in roller, 28 and 29 are doffer rollers, 30
Denote take-off devices. An example of such a card is a tandem master card manufactured by Holingworth, USA. Further, the heat treatment of the card web can be performed by using a known air-through type dryer for dry non-woven fabric in which hot air penetrates in the thickness direction of the web. The method of manufacturing dry non-woven fabric is to mix and disperse heat-fusible binder fibers that melt and adhere to the base fiber at a relatively low temperature, and heat-treat it in a completely dry state. Compared with the method of heat-treating the web in a wet state, it is possible to form a fibrous web having a larger bulkiness and a higher waist (compressive elasticity), and there is a feature that there is no safety and health problem of the drug on the skin. The heat treatment conditions are as follows: the softening point of the low melting point polymer of the binder fiber to the softening point + 80 ° C, the hot air velocity of the air through is 0.2 to 3.0 m / sec, and the heat treatment time is in the range of 1 to 30 sec. Is enough.

Next, the acrylic acid-based superabsorbent resin monomer solution used in the present invention will be described. This monomer solution is preferably an aqueous solution of acrylic acid or a mixture of acrylic acid and methacrylic acid, in which 20 to 95% of all carboxyl groups are partially neutralized with an alkali metal salt or an ammonium salt. If the partial neutralization concentration is too high, it is difficult to increase the concentration of the aqueous solution, and thus the crosslinking reaction is suppressed and the proportion of the water-soluble resin portion increases, so that a occlusive gel film is formed on the resin surface. Also, the polymerization reaction rate decreases,
Furthermore, the swollen gel exhibits weak alkalinity, which is unfavorable for safety and hygiene. Conversely, if the degree of partial neutralization is too low,
The absorption capacity of the resin will be significantly reduced. To neutralize the acrylic acid-based monomer, an alkali metal hydroxide, bicarbonate, ammonium salt, or the like can be used, but sodium hydroxide or water is preferred because of its industrial availability, price, and safety. Potassium oxide is preferable, and potassium hydroxide is particularly preferable because it can increase the concentration of the acrylic acid monomer aqueous solution. The concentration of these acrylic acid-based monomer solutions is preferably at least 35% by weight or more, and the higher the concentration, the higher the degree of resin polymerization and the more the amount of monomer adhering to the fibrous web. Unless otherwise noted, it is preferable to adopt a concentration slightly lower than the saturation concentration at the operating temperature.

The monomer solution preferably has a viscosity in the range of 2 to 200 cps. When the viscosity is less than 2 cps, the monomer solution once attached to the fiber base material immediately slides down along the fiber axis and wraps the fiber base material in a long and thin shape in a sheath shape. No film is formed. Conversely, 200c
If it exceeds ps, the monomer solution applied to the fibrous web will form a film, which is not preferable.

The absorbent of the present invention is a fiber having a basis weight of 10 to 100 g / m ² and an effective urine absorption amount (under a pressure of 0.5 psi) of 15 to 45 cc of superabsorbent resin fixed per 1 g. The absorbent used for disposable diapers is a rectangular sheet (width: about 0.06m ² ) with a width of about 15 cm and a length of about 40 cm. To absorb and hold about 50 cc of baby urine 4-5 times at a time, The amount of the superabsorbent resin needs to be at least twice the weight of the fiber web.

In the present invention, as the cross-linking agent, for example, a cross-linking monomer having two or more double bonds in the molecule capable of copolymerizing with an acrylic acid-based monomer such as ethylene glycol diacrylate and N, N′-methylbisacrylamide is used as the acrylic resin. The morphology of the swollen gel can be stabilized by adding 0.01 to 1.0% by weight to the acid-based monomer to form intermolecular crosslinks.

Further, in the present invention, as a viscosity modifier in a monomer solution, polyacrylic acid soda, a water-soluble polymer such as polyvinyl alcohol, a high-molecular-weight polysaccharide such as hydroxyethyl cellulose, an ultrafine particle inorganic such as colloidal silica as a thixotropic agent. It is also possible to use at least one compound selected from the compounds to improve the adhesion of the monomer solution to the fiber substrate.

In the present invention, the acrylic acid superabsorbent resin monomer is resinified by a so-called redox type (redox type) polymerization initiation method in which radicals are generated by a combination of a peroxide substance and a reducing substance to initiate polymerization. , This polymerization initiation method has less radical generation activation energy than the thermal decomposition type radical polymerization method, and it is possible to explosively polymerize at relatively low temperature. Is extremely suitable for producing an absorbent by coating the above composition and completing the polymerization in an extremely short time (several seconds).

The peroxide substance is at least one compound selected from hydroperoxide compounds such as t-butyl hydroperoxide, azo compounds such as azobisisobutyronitrile, hydrogen peroxide, and potassium persulfate. Good to configure.

The reducing substance is a sulfite compound such as sodium hydrogen sulfite, a reducing organic acid compound such as L-ascorbic acid,
It is preferable to be composed of at least one compound selected from salt compounds such as ferrous sulfate, but it is not limited to these.

These polymerization initiators are preferably highly water-soluble, and the amount used varies depending on the combination of the initiators and the polymerization temperature, but generally 0.01 to 2.0% by weight based on the monomer.
It is good to use in the range of. If it is less than 0.01% by weight, the polymerization reaction does not proceed sufficiently, and if it exceeds 2.0% by weight, the effect does not appear even if the amount of the initiator is increased.

By the way, the application of the acrylic acid-based monomer solution to the fiber web is performed by kiss coating using a coating roll,
In addition, it is possible to efficiently perform the fiber web running at a high speed by the scattering by the rotating brush, and it is possible to easily control the coating amount. Among them, the nip type coating roll is most preferable.

After applying the monomer solution to the fibrous web as described above, an initiator is added over the monomer solution to initiate polymerization. At this time, it is preferable that the aqueous solution of the initiator is made into fine particles using a spraying device and added to the surface of the monomer solution.
The higher the concentration of the initiator is, the more preferable it is. However, in order to avoid clogging of the injection nozzle, it is preferable to keep the concentration lower than the saturation concentration at a predetermined temperature. Further, it is preferable to dispose a large number of injection nozzles in order to uniformly spray the entire fibrous web. It is sufficient to add the initiator in an amount of 0.01 to 2.0% by weight based on the amount of the monomer. The monomer solution applied to the fibrous web is
After that, it is introduced into a polymerization tank maintained at 60 to 100 ° C and a polymerization reaction is carried out.

The shape of the reaction vessel used is not particularly limited, but in the case of finishing the polymerization in a short time while running the fiber web discontinuously, as shown in FIG. Alternatively, it is preferable to use a polymerization tank capable of keeping the temperature at 60 to 100 ° C and the humidity at 80% or more by hot air saturated with water while traveling from the bottom to the top.

When the temperature in the reaction tank is lower than 60 ° C, the basic molecular weight of the obtained resin is large and the absorption performance of the resin is improved, but it takes a long time to complete the polymerization reaction, so 60 ° C or higher is desirable. On the other hand, if it exceeds 100 ° C., a self-crosslinking type polymerization reaction occurs, and the absorption performance of the resin decreases, which is not preferable.

Usually, in the polymerization of an acrylic acid-based monomer, it is not uncommon that a large reaction heat is generated as the reaction progresses and a local rapid temperature rise is caused to cause an abnormal reaction, but in the present invention, it is used as a carrier for a monomer solution. Since the fiber web is bulky and has good air permeability, the water in the monomer solution is efficiently vaporized out of the system during the polymerization to cool the reaction heat by the latent heat of vaporization, which makes the temperature distribution uniform, It is possible to prevent the trouble that the self-crosslinking proceeds to a high degree only by the local temperature rise and the absorption performance is deteriorated. In the present invention, the polymerization reaction accompanied by a rapid heat generation is homogeneously completed in an extremely short time. As described above, this is because a short fiber of monomer solution is formed into a bulky and air-permeable fibrous web. It is possible only when used.

The polymerization reaction of the monomers is usually completed in a few seconds (3 to 15 seconds), and about 90 to 95% by weight of the acid monomer is converted into a superabsorbent resin. The resin thus obtained is a water-containing polymer containing about 20% by weight of water, and contains 5 to 10% by weight inside.
With unreacted residual acid monomer. The residual acrylic acid or the mixture monomer of acrylic acid and methacrylic acid not only lowers the absorption performance of the superabsorbent resin but also irritates the baby's skin and induces rashes and sores, which is not preferable in terms of safety and hygiene. . Therefore, these residual monomers are
It is necessary to reduce it to a safe value by the well-known electron beam and / or UV irradiation.

In the present invention, if the crosslink density in the surface layer of the superabsorbent resin is increased, it is possible to prevent a so-called gel block in which a paste-like gel is formed in the surface layer, whereby urine quickly reaches the inside of the resin. To penetrate,
The absorption performance can be further improved. Examples of such a cross-linking agent include polyglycidyl ethers such as ethylene glycol diglycidyl ether, polyols such as diethylene glycol, and polyamines such as ethylenediamine, which have two or more functional groups capable of reacting with carboxylic acid in the molecule. To be When these cross-linking agents are added to the water-containing polymer and heated, it is possible to form extremely uniform intermolecular cross-links on the surface layer of the water-containing polymer, and the mesh size of the network structure formed by the cross-linking can be adjusted. You have some control.

The bulkiness (cc / g) and compression modulus (%) of the fibrous web used in the present invention are measured as follows.

Bulkiness (cc / g) is obtained by cutting a fibrous web into squares with a side of 10 cm (the sides are aligned in the machine direction and the width direction, respectively). On top of it, put a methylmethacrylate resin plate and weight on it, apply a load of 0.5 g / cm ² for 10 minutes,
The volume V ₁ (cc) of the fibrous web layer at that time was measured.
V ₁ is divided by the weight of the pre-weighed fibrous web layer.

On the other hand, the compression modulus (%) is a load of 35 g / cm ² to the fibrous web layer measured bulky V ₁ (cc), to measure the volume V ₂ (cc) when left for 10 minutes, V ₁ , V ₂ to obtain the compression modulus (%) according to the following formula.

Compressive modulus (%) = V ₂ / V ₁ × 100 Further, the tensile strength of the fibrous web used in the present invention is a rectangle having a length of 15 cm and a width of 2.5 cm (the long side is aligned with the machine direction and the width direction). ) Is used for measurement.
Hold both ends of the test piece with chucks using "Tensilon",
Adjust the test length to 10 cm. Then, elongation is performed at an elongation rate of 100% / min to obtain a stress-elongation relationship curve. The elongation strength (g / 25 mm) and the elongation (%) when the test piece breaks are read from this relational curve.

Further, the tensile strength and elongation of the fibrous web are obtained in the machine direction and the width direction, respectively.

The absorption performance of the absorbent material obtained in the present invention, that is, the absorption capacity and the water retention capacity, was vacuum-dried at 80 ° C for 2 hours, then at 25 ° C and 60%.
A test piece prepared by cutting a humidity-controlled sample in an atmosphere of RH% for 8 hours into a square of 10 cm on each side (the directions of the sides match the machine direction and the width direction, respectively) is measured. First, the weight [a] (g) of the test piece was weighed, and then the sixth
As shown in the figure, the test piece 31 is put in a bag 32 of 250 mesh nylon cloth having a length of 20 cm and a width of 15 cm, and a test solution (saline solution: 0.9 wt% NaCl) 33 prepared in advance is used. The test solution is absorbed by immersing it in a container 34 having a shallow bottom.
After soaking for 1 hour, the test piece 31 was taken out together with the nylon cloth bag 32, placed on a wire mesh 35 of 10 mesh, and a methylmethacrylate resin plate 36 and a weight 37 were placed thereon, as shown in FIG. Drain for 15 minutes under a pressure of 35 g / cm ² . After that, the test piece 31 is taken out and the weight [b] (g) is weighed. Return the test piece 31 to the nylon cloth bag 32 again, and put it on the side wall of the rotary tank of the centrifugal dehydrator together with the nylon cloth bag 32.
After centrifugal dehydration for 90 seconds with a centrifugal force of 150 G, the test piece 31
Is taken out from the nylon cloth bag 32, and the weight [c] (g) is weighed.

From the above [a], [b], and [c], the absorption capacity and the water retention capacity are determined according to the following formulas.

(Function) The absorbent material of the present invention is obtained by applying the acrylic acid-based superabsorbent resin monomer solution to a fiber web formed by laminating at least two fiber layers having different fiber substrate densities, and then polymerizing the solution. Since the super absorbent resin is fixed to the fiber base material, it is possible to change the concentration (density) gradient of the super absorbent resin in the thickness direction of the absorbent material by changing the fiber base material density of the fiber layer. . By selecting the gradient of resin concentration (density), (i) avoiding deterioration of absorption performance due to gel block between resins, and (ii) temporary absorption material until a large amount of discharged urine is absorbed by the resin. (Iii) Prevent the gelling and slimy resin from separating from the absorbent material, and (iv) Prevent the resin from swelling and breaking the fiber skeleton of the absorbent material. It is possible to obtain excellent effects such as

(Examples) Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example-1) 70% by weight of a base fiber made of polyethylene terephthalate fiber (having an intrinsic viscosity of 0.57, a fineness of 6 denier and a fiber length of 51 mm)
And a core-sheath in which a low-melting point polyester polymer (intrinsic viscosity 0.35) is used as the sheath and polyethylene terephthalate (intrinsic viscosity 0.57) is used as the core so that the volume of the sheath: core is 1: 1. One side of a card opening machine in which two fibers are arranged in series after being uniformly mixed and opened using 30% by weight of a binder fiber composed of a type composite spun fiber (fineness 4 denier, fiber length 51 mm) [A] To supply web sheet [I]
(The web to be layer A in FIG. 1) was formed at a speed of 50 m / min. Separately, in place of the above-mentioned core-sheath type composite spun fiber [A], high-density polyethylene (MI = 20, 190 ° C.) is used as the sheath part, and polyethylene terephthalate (intrinsic viscosity 0.57) is used as the core part. : Using 70% by weight of the base fiber by using 30% by weight of the binder fiber composed of the core-sheath type composite spun fiber (fineness 4 denier, fiber length 51 mm) [B] arranged so that the volume of the core is 1: 1. After uniform cotton opening, the web was fed to the other of the two card opening machines to form a web sheet [II] (web to be layer B in FIG. 1) at a speed of 50 m / min.

As described above, after stacking the web sheets [I] and [II] formed from the two card opening machines arranged in series, the card webs are continuously applied to the flat belt type air-through heat treatment apparatus. Guide, during the card web
Hot air at 160 ° C was passed for 10 seconds to melt the binder fibers and bond the fibers to each other to form a fiber web having good morphological stability.

The performance of the fibrous web thus obtained is shown in Table 1.

(Example-2) While running the fibrous web obtained in Example-1 at 50 m / min, hydrogen peroxide was added to this in an amount of 1.67% by weight relative to the monomer, and the monomer solution heated to 40 ° C had a monomer weight of 200g
/ m ² to the fiber web using a nip-type coating roller, then sprayed 5% by weight L-ascorbic acid aqueous solution to the monomer solution to 0.34% by weight, immediately Polymerization was carried out by introducing it into a polymerization tank whose atmosphere was kept at 80 ° C and humidity of 80% or more.

Here, the monomer solution is a partially neutralized acrylic acid monomer aqueous solution having a concentration of 65 wt% obtained by neutralizing 60% of all the carboxyl groups of acrylic acid with potassium hydroxide, and NN′-methylbisacrylamide as a crosslinking monomer. Against 0.
The one added was 085% by weight.

The polymerization reaction started immediately after the monomer solution was applied to the fibrous web, and ended in about 8 seconds while generating heat. The residual monomer in the superabsorbent resin obtained by this polymerization reaction, that is, the partial potassium salt of polyacrylic acid was measured and found to be 5000 ppm.

Then, as shown in FIG. 2, by using the accelerated electron beam irradiation devices 12, 12 which face each other in an alternating manner, 2
Irradiate a total of 4 megarads of electron beams, one for each megarad (Mrad),
The residual monomer was polymerized and crosslinked. Further, water is sprayed on this resin-coated web to keep the water content of 30% against the resin.
After adjusting the weight ratio to be 100% by weight, both sides of the fibrous web were irradiated with ultraviolet rays of 100 millijoules / cm ² by using the UV irradiation devices 14, 14 shown in FIG. Were all polymerized.

The performance of the absorbent thus obtained is shown in Table-2.

(Comparative example) 70% by weight of a base fiber made of polyethylene terephthalate fiber (having an intrinsic viscosity of 0.57, a fineness of 6 denier and a fiber length of 51 mm)
And a low melting point polyester polymer (intrinsic viscosity 0.35) was used for the sheath and polyethylene terephthalate (intrinsic viscosity 0.57) was used for the core so that the volume of the sheath: core was 1: 1. Using the same core-sheath type composite spun fiber (fineness 4 denier, fiber length 51 mm) [A] as used in Example-1, 30% by weight of binder fiber was uniformly mixed and opened, and then the card was opened. Web sheet [I] supplied to fiber machines
Only formed at a speed of 50 m / min.

Hereinafter, the completely same operation as Example-2 was performed and the absorber was manufactured.

The properties of the fibrous web and absorbent thus obtained are shown in Tables 1 and 2.

(Effects of the Invention) The method for producing an absorbent according to the present invention comprises at least two or more fiber layers having different fiber base material densities as shown in the fiber layers [I] and [II] of the examples in Table 1. Since the acrylic acid-based superabsorbent resin monomer solution was applied to the laminated fibrous web and then polymerized to fix the superabsorbent resin to the fiber base material, as shown in the examples of Table 2, It is possible to change the concentration (density) gradient of the superabsorbent resin in the thickness direction of the absorbent by changing the density of the fiber base material of the layer, and if a suitable resin concentration (density) gradient is carried, (i) Avoiding deterioration of absorption performance due to gel block between resins, (ii) storing a large amount of excreted urine temporarily in an absorbent until it is absorbed by resin, and (iii) gelling and null nulling Prevent the resin from separating from the absorbent, iv) resin is suppressed to become loose broken fibers skeleton of the absorbent material swells, it is possible to obtain an excellent effect such.

[Brief description of drawings]

FIG. 1 is a cross-sectional configuration diagram showing an example of an absorbent material obtained by the method of the present invention, FIG. 2 is a configuration diagram showing an example of a manufacturing apparatus for carrying out the method of the present invention, and FIG. Configuration diagram of a cross-linking agent processing apparatus used for implementation, FIG.
(F) is a cross-sectional configuration diagram of a hollow core-sheath composite yarn suitable for use in carrying out the method of the present invention, FIG. 5 is a configuration diagram showing an example of a flat card device, and FIGS. 6 and 7 are FIG. 8 is a schematic configuration diagram of an apparatus used to measure the absorption performance of an absorbent material, FIG. 8 is a sectional configuration diagram of a disposable diaper, and FIG. 9 is a sectional configuration diagram of a conventional absorbent material. 6 ... Fiber web, 8 ... Monomer solution 11 ... Super absorbent resin, F ... Fiber base material A, B ... Fiber layer

Claims (1)

[Claims] 1. A method for producing an absorbent material by applying an acrylic acid-based superabsorbent resin monomer solution to a fibrous web and polymerizing the solution, wherein the fibrous web comprises a fibrous base material layer having a relatively high density. It is constituted by laminating a low-density fiber substrate layer, and the monomer solution applied to the web is relatively accumulated in the high-density fiber substrate layer by utilizing the difference in the density. After that, the monomer is polymerized, and the production method is characterized.