JP2001192464A - Surface-cross-linked water-absorbing resin and method for cross-linking surface of water-absorbing resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface-cross-linked water-absorbing resin which can exhibit an excellent absorbing characteristic, even when the wt.% (resin concentration) of the water-absorbing resin is enhanced on the employment of the resin as a sanitary material or the like, to provide a new surface-cross-linking method for producing the resin, and to provide the use of the resin. SOLUTION: This surface-cross-linked water-absorbing resin characterized in that the thickness of a surface-cross-linked layer is 50 nm to 1,000 nm and/or in that the weight rate of the surface-cross-linked layer on the basis of the total amount of the resin is 0.3 to 3%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-crosslinked water-absorbent resin suitably used for sanitary materials such as paper diapers (disposable diapers) and sanitary napkins, so-called incontinence pads, and the surface of the water-absorbent resin for obtaining the same. The present invention relates to a crosslinking method and an absorbent article using the surface-crosslinked water-absorbent resin as a water-absorbing agent.

[0002]

2. Description of the Related Art In recent years, sanitary materials such as disposable diapers and sanitary napkins, so-called incontinence pads, have been used as constituent materials thereof.
Water-absorbing resins are widely used for the purpose of absorbing body fluids. Examples of the water-absorbing resin include a crosslinked product of a partially neutralized polyacrylic acid, a hydrolyzate of a starch-acrylic acid graft polymer, a saponified vinyl acetate-acrylate copolymer, an acrylonitrile copolymer or acrylamide. A hydrolyzate of a copolymer or a crosslinked product thereof, a crosslinked product of a cationic monomer, and the like are known.

[0003] The characteristics of the above water-absorbent resin are as follows: excellent water absorption and absorption rate when in contact with aqueous liquid such as body fluid, liquid permeability, gel strength of swollen gel, base material containing aqueous liquid Suction force for sucking water from the water. However, the relationship between these various properties does not always show a positive correlation. For example, the higher the absorption capacity under no pressure, the lower the absorption property under pressure. As a method for improving the water absorbing properties of the water-absorbent resin in a well-balanced manner, a technique of crosslinking the vicinity of the surface of the water-absorbent resin is known, and various methods have been disclosed so far.

For example, a method using a polyhydric alcohol as a crosslinking agent (JP-A-58-180233, JP-A-61-16903), polyhydric glycidyl compounds, polyhydric aziridine compounds, polyhydric amine compounds, Method using polyvalent isocyanate compound (JP-A-59-189103)
), A method using glyoxysal (Japanese Unexamined Patent Publication No.
JP-A-117393), a method using a polyvalent metal (JP-A-51-136588, JP-A-61-25723).
No. 5, JP-A-62-7745), a method using a silane coupling agent (JP-A-61-211305, JP-A-61-252212, JP-A-61-252212).
264006), a method using an alkylene carbonate (DE 4020780), and the like. Also, a method in which an inert inorganic powder is present during the crosslinking reaction for the purpose of improving the dispersibility of the crosslinking agent (Japanese Patent Application Laid-Open No.
0-163956, JP-A-60-255814), and a method in which a dihydric alcohol is present (JP-A-1-163954).
No. 292004), a method in which water and an ether compound are present (Japanese Patent Application Laid-Open No. 2-153903), and a method in which an alkylene oxide adduct of a monohydric alcohol, an organic acid salt, a lactam and the like are present (European Patent No. 555,592). ), A method in which phosphoric acid is present (Japanese Patent Application Laid-Open No. Hei 8-50851)
No. 7) is also known.

[0005] However, it has been difficult for conventional techniques of simply cross-linking the vicinity of the surface of the water-absorbent resin to sufficiently meet the performance requirements of the water-absorbent resin which has been increasingly sophisticated in recent years. For example, in recent years, in sanitary materials,
While the amount of the water-absorbing resin used increases, the thickness tends to be reduced. Various properties desired for the water-absorbing resin used for the constituent material of the so-called high resin concentration sanitary material in which such reduction in thickness has been achieved include an absorption capacity under no pressure and an absorption capacity under high pressure. Although absorption properties such as liquid permeability are mentioned, even if these values are pursued while maintaining a balance by the surface cross-linking, there still exists some degree of conflicting correlation between the physical properties, and emphasis is placed on certain physical properties. It has been found that the produced resin may not show sufficient physical properties in sanitary materials depending on use conditions.

That is, in order for the water-absorbent resin to sufficiently exhibit its performance in a sanitary material with a reduced thickness, and to always maintain a high absorption property, it is necessary to use a crosslinking agent such as the one described above. If the surface cross-linking conditions are set simply by limiting the type or pursuing the absorption characteristics such as absorption capacity under no pressure, absorption capacity under high pressure, and liquid permeability as a measure of surface cross-linking, the entire system is still in its entirety. The current situation is that satisfactory performance has not been stably imparted to sanitary materials.

[0007]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and the problem is that even when the resin concentration of the water-absorbing resin is increased when the resin is used as a sanitary material or the like, it is excellent. An object of the present invention is to provide a surface crosslinked water-absorbent resin capable of exhibiting absorption characteristics, a new surface crosslink method for obtaining the same, and an absorbent article using the surface crosslinked water absorbent resin as a water absorbing agent. .

[0008]

Means for Solving the Problems In order to achieve the above object, the present inventor has set forth a conventional surface crosslinked water-absorbing resin.
In particular, as a result of a detailed study of the surface cross-linked structure, it has been found that in the conventional surface cross-linked water-absorbent resin, there is still a problem in controlling the thickness of the surface cross-linked layer, and the water absorption performance may be reduced from a certain point in time. did. If the surface cross-linking layer is too thick, the surface cross-linking layer is considered to hinder the water absorbing ability of the water-absorbing resin. Therefore, a cross-linking method that does not greatly reduce the water absorption after surface cross-linking has been proposed. However, the present inventor has observed in detail that even the conventional crosslinked layer, which is considered to be relatively thin, obtained in this way does not significantly impede the water absorbing ability of the water absorbent resin. , Is still too thick,
It was found that cracks occurred in the surface crosslinked layer during and after swelling. Due to this crack, part of the effect of the surface cross-linking treatment on the water-absorbent resin was lost particularly over time.

As described above, the present inventor has studied the surface cross-linking structure of the water-absorbent resin in detail, and as a result, it has been found that the absorption capacity under no pressure and the absorption capacity under high pressure as described above have been improved. Rather than focusing on absorption properties such as liquid properties, focus on the thickness of the surface cross-linking layer formed on the surface of the water-absorbent resin particles. Or by controlling the weight% of the surface cross-linked layer with respect to the entire water-absorbent resin to a specific range, so that when used for sanitary materials, etc., even when the resin concentration is increased, it always shows excellent absorption characteristics. It was conceived that the resulting surface crosslinked water-absorbent resin (water-absorbing agent) could be obtained.

As a result of various studies by the present inventor on the thickness of the surface cross-linked layer and the water absorption performance, the thickness of the surface cross-linked layer was 1000
It was found that it was necessary to be not more than 50 nm, and when considering the crack at the time of swelling, it was necessary to be 50 nm or more. From the thickness of these crosslinked layers, it was also found that the weight percentage of the surface crosslinked layer with respect to the entire water-absorbent resin was required to be 0.3 to 3% by weight. Here, the thickness and the weight% of the surface cross-linked layer mentioned above are determined by subjecting the water-absorbent resin as the base polymer to surface swelling by subjecting the surface cross-linked water-absorbent resin to saturated swelling and solubilizing the inside of the resin. Is the average thickness of the surface cross-linked layer remaining after
Mean weight percent. The surface crosslinked layer preferably covers the entire surface of the water-absorbent resin particles as the base polymer uniformly, but may cover only a part thereof.

Next, the present inventors have studied in detail a method for facilitating such thickness control. That is, as a result of studying the surface crosslinking method of the water-absorbent resin, which can stably produce a surface-crosslinked water-absorbent resin with sufficient control of the thickness of the surface-crosslinked layer as described above, the surface-crosslinking treatment was carried out. The surface cross-linked water-absorbent resin obtained by performing the surface cross-linking treatment is saturated and swelled to satisfy the condition that the thickness of the surface cross-linked layer remaining after solubilizing the inside of the resin is 50 nm or more and 1000 nm or less, and / or Alternatively, the cross-linking treatment is performed under such a condition that the weight% of the surface cross-linked layer with respect to the entire water-absorbent resin is 0.3 to 3% by weight. It is easy to control by keeping each temperature of the liquid containing the surface cross-linking agent within a certain range, and the temperature difference between the water-absorbing resin and the liquid containing the surface cross-linking agent is within a certain range. Also easy to control it to be kept.

The technique for improving the performance of the surface-crosslinked water-absorbent resin found by the present inventors is completely different from the techniques which have been studied so far, and forms a surface-crosslinked layer having a specific range of thickness or weight%. This is the most significant feature, and is particularly effective as a technique for improving the performance of the water absorbent resin. Thus, the present invention has been completed. Therefore,
The surface crosslinked water-absorbent resin according to the present invention has a thickness of the surface crosslinked layer of 50 nm or more and 1000 nm or less, and / or a weight ratio of the surface crosslinked layer to the entire resin of 0.3 to 3%. Features.

The method for cross-linking the surface of a water-absorbent resin according to the present invention is a method of cross-linking the surface of the water-absorbent resin by bringing a surface cross-linking agent into contact with the water-absorbent resin as a base polymer. Satisfying the condition that the thickness of the surface cross-linked layer remaining after the surface cross-linking water-absorbent resin obtained by performing the surface cross-linking treatment on the water-absorbent resin is saturated and swelled and the inside of the resin is solubilized is 50 nm or more and 1000 nm or less. And / or the weight% of the remaining surface crosslinked layer
Is carried out under a crosslinking treatment condition that satisfies a condition of 0.3 to 3% by weight based on the standard surface crosslinked water-absorbent resin. Specifically, the surface-crosslinking agent-containing liquid is brought into contact with the water-absorbent resin. At this time, the temperature of the water-absorbent resin is set to 5 to 20 ° C. and / or the temperature of the surface crosslinking agent-containing liquid is set to 0 to 20 ° C. The temperature difference between the water-absorbent resin and the surface-crosslinking agent-containing liquid when the surface-crosslinking agent-containing liquid is brought into contact with the water-absorbing resin is set to 0 to 20 ° C.

An absorbent article according to the present invention is an absorbent article comprising a water-absorbing agent and a fiber base, wherein the amount of the water-absorbing agent is at least 40% by weight based on the total weight of the water-absorbing agent and the fiber base material,
The surface-crosslinked water-absorbing resin according to the present invention is used as the water-absorbing agent.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail. The surface crosslinked water-absorbent resin according to the present invention has a surface crosslinked layer having a thickness of 50 nm or more,
0 nm or less and / or the weight ratio of the surface crosslinked layer to the whole resin is 0.3 to 3%. The average particle size of the water absorbent resin is not limited, but may be 10
It is preferably from 0 to 1000 μm. The surface-crosslinked water-absorbent resin according to the present invention is thus one in which the thickness of the surface-crosslinked layer of the water-absorbent resin is controlled by the actual thickness or the weight ratio. Absorbent Polymer Technology (M
oder Superabsorbent Poly
mer Technology) ”(Dow Chem
ical company, 1997 WILEY-
VCH) has been proposed for measuring the thickness.
According to this, it is described that the thickness of the surface crosslinked layer is about 100 μm. Japanese Patent Laid-Open Publication No. Hei 6-20046 discloses that the crosslinking depth of the surface crosslinking is 40% of the base particle radius.
% Or less, preferably 20% or less, more preferably 10%
It is also described that, according to this, assuming that the average particle radius is 200 μm (average particle size 400 μm), the thickness of the crosslinked layer is 80 μm or less, preferably 40 μm or less, more preferably 20 μm or less. become.

The present inventors have actually isolated (obtained as a shell) the surface cross-linked layer of the water-absorbent resin for the first time, and have determined the relationship between this thickness and the absorption characteristics, and the weight% of the cross-linked layer with respect to the entire water-absorbent resin.
As a result of intensive studies on the relationship between the surface absorption layer and the absorption characteristics, the surface crosslinked layer is much thinner than the thickness reported so far, when controlled in submicron units, or when the weight% of the crosslinked layer with respect to the entire water absorbent resin is When a very small amount is controlled within a specific range, the obtained surface-crosslinked water-absorbent resin (water-absorbing agent) always shows excellent absorption characteristics even in an absorbent article having a resin concentration as high as 40% by weight or more. That is what I found.

The surface crosslinked layer can be isolated by solubilizing the inside by utilizing the difference between the stability of the surface layer and the stability of the inside, and the following method can be used. That is, the basic dye methylene blue 0.0050
(G / l) in 500 ml of deionized water
0.06 g of surface-crosslinked water-absorbing resin (or water-absorbing agent)
And leave overnight. This saturated swelling gel is filtered through a 90-mesh wire mesh, the obtained gel is put in a sealable polyethylene bag, the opening is sealed, and the bag is put into a beaker containing hot water of 90 ° C. and heated at 90 ° C. for 18 hours. . As a result, the inside of the resin is selectively solubilized, and the surface crosslinked layer is not solubilized and remains as a hydrogel, and a thin film colored with a basic dye remains as shown in FIG. If the internal solubilization is insufficient, the heating time may be further extended. This indicates that surface crosslinked layers having different swelling ratios and different crosslink densities exist near the particle surface. The surface crosslinked layer and the water-soluble polymer generated by solubilization are separated by an operation such as decantation or centrifugation. The thickness of the surface cross-linked layer is determined by drying the obtained surface cross-linked layer and analyzing the cross section thereof using a scanning electron microscope (SEM).

Other methods of measuring the thickness of the surface cross-linked layer include, for example, a method of mechanically cutting the surface of the water-absorbent resin and examining a change in composition from the surface, and a swelling ratio between the surface and the inside during swelling. And a method of determining the thickness by detecting the difference between the two. The weight% of the surface cross-linked layer is similarly used to solubilize the inside and determine the weight% of the surface cross-linked layer remaining after solubilization with respect to the whole original resin, and the following method can be used. 100 g of deionized water is added to a 500 ml glass bottle having a lid, and 0.100 g of a surface-crosslinked water-absorbing resin (or water-absorbing agent) is added.
Heat solubilization at 18 ° C. for 18 hours. If the internal solubilization is insufficient, the heating time may be further extended.
After adding the water evaporated during heating, 0.100 g / l
25.0 ml of an aqueous methylene blue solution was added, and the mixture was further stirred for 22 hours.
Centrifuge at 00 rpm for 20 minutes, and measure the absorbance at 650 nm of the obtained supernatant. As a standard product, a crosslinked product of sodium polyacrylate having the same neutralization ratio as the above water-absorbent resin having a crosslink density corresponding to the surface crosslinked layer (for example, a monomer concentration at the time of polymerization of about 40% by weight and a crosslinking agent such as methylenebisacrylamide or the like) Of a water-soluble sodium polyacrylate having the same neutralization ratio as 0.1 / 99.9 and 1/9, respectively.
9, 2/98, 5/95 (% by weight) so that the total weight of each was adjusted to 0.100 g, and mixed with 100 g of deionized water to obtain a dispersion of methylene blue (0.1%).
(100 g / l), and the mixture was stirred for 22 hours in the same manner as described above, followed by centrifugation, and the absorbance at 650 nm of the supernatant was measured. The crosslinked layer is 0.1% by weight, 1% by weight,
This corresponds to the absorbance when 2% by weight and 5% by weight remain.
Based on the absorbance calibration curve obtained from these, the weight% of the surface crosslinked layer can be determined from the absorbance of the measurement sample.

In the present invention, preferable conditions for performing the surface cross-linking treatment are preferable because a standard water-absorbing resin described below is prepared as a base polymer and can be easily determined by selecting and determining the same. May be performed under similar processing conditions. That is, the standard water-absorbent resin of the present invention is a spherical water-absorbent resin having a structure in which the inside is easily solubilized and having a water content of 5 to 10% and a particle size of 150 to 250 μm. Hydroxyethyl cellulose, polyethylene glycol diacrylate (n = 8) about 0.05 mol% (based on monomer) in an aqueous solution of partially neutralized acrylic acid having a humidity of 75 mol%
In addition, about 0.016 mol% of 2,2′-azobis (2-aminodipropane) dihydrochloride is dissolved as an initiator, reverse phase suspension polymerization is performed in cyclohexane in which sucrose fatty acid ester is dissolved, and azeotropic dehydration is performed. , Dried, classified and adjusted to a water content of 5 to 10% by weight and a particle size of 150 to 250 µm. The amount of the cross-linking agent, the amount of the initiator and the like may be optimized depending on the easiness of solubilization inside.

In the method for treating the surface of a water-absorbent resin according to the present invention, for example, such a standard water-absorbent resin is subjected to surface crosslinking, and a standard surface-crosslinked water-absorbent resin is first obtained. The thickness of the surface cross-linked layer remaining after solubilizing the inside, or the weight% of the surface cross-linked layer with respect to the entire resin is measured by the above-described measuring method, and the condition that the thickness of the remaining surface cross-linked layer is 50 to 1000 nm, or The surface cross-linking method is selected so that the weight% of the surface cross-linked layer to be formed is 0.3 to 3% by weight based on the standard surface cross-linked water-absorbent resin, and the same method is applied to the water-absorbent resin as a general base polymer. However, it is not always necessary to use this standard water absorbent resin.

In the present invention, a surface crosslinked water-absorbent resin may be produced by using this standard water-absorbent resin as a base polymer. The surface cross-linking conditions to be selected in order that the thickness and the weight% of the surface cross-linking layer satisfy the range of the present invention include:
The temperature of the water-absorbent resin used as the base polymer or the temperature / composition of the surface cross-linking agent-containing liquid, a method of mixing and contacting the surface cross-linking agent-containing liquid with the water-absorbent resin as the base polymer, mixing temperature, heat treatment temperature, The heat treatment time and the like can be mentioned, and the surface crosslinking method is selected by adjusting these crosslinking conditions. When a standard water-absorbing resin is used, the method is achieved by applying the same method to a water-absorbing resin as a general base polymer.

The method for cross-linking the surface of a water-absorbent resin according to the present invention comprises contacting a liquid containing a surface cross-linking agent with a water-absorbent resin as a base polymer under conditions selected and determined as described above. May be subjected to a cross-linking treatment. Among them, as a surface cross-linking agent-containing liquid, a temperature of 0 to
A method using a liquid containing a surface crosslinking agent at 20 ° C. is preferred. Examples of the surface crosslinking agent that can be used for the surface crosslinking of the water absorbent resin of the present invention include ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, and dipropylene. Glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerin,
Polyglycerin, 2-butene-1,4-diol, 1,
3-butanediol, 1,4-butanediol, 1,5
-Pentanediol, 1,6-hexanediol, 1,
Polyhydric alcohol compounds such as 2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol and sorbitol; ethylene glycol Epoxy compounds such as diglycidyl ether, polyethylene diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycidol; ethylenediamine, diethylenetriamine, triethylene Tetramine, tetraethylenepentamine, pentaethyl Polyamine compounds such as hexamine and polyethyleneimine, and inorganic or organic salts thereof (e.g., aztinium salts); polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 1,2-ethylene Polyvalent oxazoline compounds such as bisoxazoline; 1,3-dioxolan-2-one, 4-methyl-1,3-dioxolane-
2-one, 4,5-dimethyl-1,3-dioxolane-
2-one, 4,4-dimethyl-1,3-dioxolane-
2-one, 4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2
-One, 1,3-dioxan-2-one, 4-methyl-
1,3-dioxan-2-one, 4,6-dimethyl-
Alkylene carbonate compounds such as 1,3-dioxan-2-one and 1,3-dioxopan-2-one; haloepoxy compounds such as epichlorohydrin, epibromohydrin, α-methylepichlorohydrin, and the like Polyamine adducts (eg, Hercules Sponge; registered trademark); silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; zinc, calcium, magnesium, aluminum, iron, zirconium And polyvalent metal compounds such as chlorides.

Heretofore, the end point of the surface crosslinking reaction has been determined while controlling the physical properties of the water-absorbing resin and the water-absorbing agent. In particular, when the surface crosslinking agent forms an ester bond derived from dehydration by a crosslinking reaction such as a polyhydric alcohol compound or an alkylene carbonate compound, it has been difficult in some cases to determine the end point of the conventional reaction. However, according to the novel method of surface cross-linking a water-absorbent resin of the present invention, the thickness of the cross-linked layer is formed within a very thin specific range, or the weight% of the surface cross-linked layer with respect to the whole resin is reduced to a very small amount by a specific amount. Since the water absorbing agent is controlled within the range, a water absorbing agent having the same performance can be obtained more stably, and an absorbent article having excellent water absorbing properties can be stably obtained by using this method. The surface treatment method of the present invention is particularly advantageous when a polyhydric alcohol is used as the surface cross-linking agent.

The amount of the surface cross-linking agent used is about 0.001 to 5 parts by weight based on 100 parts by weight of the water-absorbing resin as the base polymer. If the amount exceeds 5 parts by weight or 0.001
If the amount is less than parts by weight, it may be difficult to obtain a surface crosslinked layer within the range of the present invention. Water may be used when mixing the surface cross-linking agent of the present invention with a water-absorbing resin as a base polymer. The amount of water used is generally more than 0.5, preferably 10 parts by weight or less, more preferably 1 part by weight to 5 parts by weight, based on 100 parts by weight of the solid content of the water absorbent resin.

When mixing the surface crosslinking agent or its aqueous solution, a hydrophilic organic solvent or a third substance may be used. When a hydrophilic organic solvent is used, for example, lower alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, t-butyl alcohol; ketones such as acetone; Ethers such as dioxane, tetrahydrofuran and methoxy (poly) ethylene glycol; amides such as ε-caprolactam and N, N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol and tetra Ethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, polypropylene Glycol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexane Examples include dimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, polyhydric alcohols such as pentaerythritol and sorbitol. The amount of the hydrophilic organic solvent used depends on the type and particle size of the water-absorbent resin, the water content, and the like, but is preferably 50 parts by weight or less based on 100 parts by weight of the solid content of the water-absorbent resin,
More preferably, it is in the range of 0.1 to 10 parts by weight. Further, inorganic acids, organic acids, polyamino acids and the like described in European Patent No. 0668080 may be present as the third substance.

The mixing method for mixing the water-absorbing resin as the base polymer and the surface cross-linking agent is not particularly limited. For example, the water-absorbing resin as the base polymer is immersed in a hydrophilic organic solvent, and if necessary, water and / or water is added. Alternatively, a method of mixing a surface cross-linking agent dissolved in a hydrophilic organic solvent, a method in which a surface cross-linking agent dissolved in water and / or a hydrophilic organic solvent is directly sprayed or dropped on a water-absorbent resin as a base polymer and mixed. Methods and the like can be exemplified. Also mixing temperature,
That is, by controlling both the temperature of the water-absorbent resin powder before mixing and the temperature of the treating agent containing the surface cross-linking agent to a specific range, the thickness and weight% of the surface cross-linked layer can be easily controlled to the range of the present invention. . The temperature of the water-absorbent resin before mixing is generally 0 to 60 ° C, preferably 5 to 2O0C.
The temperature of the surface crosslinking agent-containing liquid is generally 0 to 30 ° C, preferably 0 to 20 ° C, more preferably 5 to 10 ° C. In the case of mixing using water, a fine powder insoluble in water, a surfactant or the like may coexist.

After mixing the water-absorbing resin as the base polymer and the surface cross-linking agent, the mixture is usually subjected to a heat treatment to carry out a cross-linking reaction. The heat treatment temperature is preferably from 40 ° C. to 250 ° C., although it depends on the surface crosslinking agent used. When the treatment temperature is lower than 40 ° C., a uniform crosslinked structure is not formed, so that the water-absorbing agent having a surface crosslinked layer having a specific thickness of the present invention may not be obtained. If the treatment temperature exceeds 250 ° C., the water-absorbent resin may be degraded and the performance may be reduced, so caution is required.
The heat treatment time is about 1 minute to 2 hours, preferably 5 minutes to 1 hour.
About an hour. The present invention measures the thickness of the surface crosslinked layer remaining after solubilizing the inside of the surface crosslinked water-absorbent resin obtained as described above and the weight% of the surface crosslinked layer, and the thickness is 50 to 50%.
So as to be 1000 nm or by weight% of the surface crosslinked layer
The surface cross-linking conditions such as the composition of the treating agent, the mixing temperature, the mixing conditions, the heat treatment temperature, and the heat treatment time are selected so that the amount is 0.3 to 3% by weight based on the standard surface cross-linked water-absorbent resin. When a standard water-absorbent resin is used as the base polymer, the same treatment is performed on the water-absorbent resin as the base polymer. By using a standard water absorbent resin as the base polymer in the present invention,
The surface cross-linked layer can be easily isolated, and unlike the case of a water-absorbent resin as a general base polymer having various shapes and properties, the target cross-linked layer thickness and the weight% of the surface cross-linked layer can be easily determined. May be possible. Then, by applying this same method to a water-absorbing resin as a general base polymer having various shapes and properties, it is possible to form a surface crosslinked layer having a preferable thickness for any water-absorbing resin. I can do it. Further, by controlling the thickness of the surface cross-linking layer and the weight% thereof to specific ranges, the magnification of the resin, the adhesiveness between the resin particles, and the elasticity are determined, whereby the diffusion, absorption amount, and flow of the liquid under pressure are determined. It is considered that the properties and the like are optimized. Therefore, it is considered that the water-absorbent resin (water-absorbing agent) of the present invention exhibits excellent performance even when used in an absorbent article having a high resin concentration. Of course, the target base polymer may be a standard water-absorbing resin.

When the thickness of the surface cross-linked layer exceeds 1000 nm, swelling inside is restricted by the cross-linked layer, and the absorption amount is reduced. Further, the crosslinked layer is easily peeled and collapsed by swelling due to a shear force or the like during use, and a soluble component inside the resin is easily eluted. If the thickness is less than 50 nm, the surface cross-linked layer is too thin, and the physical properties are greatly reduced due to impact force, shear force, etc., and the surface cross-linked layer is easily formed during use, resulting in a slimy feeling and liquid permeability. Inferior to etc. The thickness of the surface crosslinked layer is preferably from 100 to 600 nm, more preferably from 200 to 500 nm. Similarly, when the weight% of the surface crosslinked layer exceeds 3% by weight with respect to the resin, the internal swelling is regulated by the crosslinked layer, and the absorption amount is reduced. In a state where the resin concentration is high, the crosslinked layer is easily peeled and collapsed by swelling due to a shear force at the time of use, and a soluble component inside the resin is eluted to give a slimy feeling, resulting in poor dry feeling. If the weight% is less than 0.3% by weight with respect to the resin, the surface cross-linked layer is too thin, and the physical properties are greatly reduced due to impact force, shear force, etc.
Similarly, a slimy feeling is produced, resulting in poor liquid permeability and the like. Preferably 0.5 to 2% by weight, more preferably 0.5 to 2% by weight.
1.0% by weight.

The water-absorbing resin as a base polymer that can be used in the surface crosslinking method of the present invention is not particularly limited, but typically contains acrylic acid and / or a salt thereof (neutralized product) as a main component. It is obtained by polymerizing and cross-linking a hydrophilic monomer.
It is a conventionally known resin that absorbs a large amount of water as much as 00 times and forms a hydrogel, and its shape is not particularly limited, such as a sphere, an irregular shape, and a granular shape. Preferably, an amorphous crushed one is suitable. As the water-absorbing resin as the base polymer, those having an uncrosslinked water-soluble component in the resin of usually 25% by weight or less, preferably 15% by weight or less, more preferably 10% by weight or less are used. .

Examples of the above acrylate include alkali metal salts, ammonium salts and amine salts of acrylic acid. The water-absorbent resin as the base polymer has acrylic acid as a constituent unit in an amount of 10 mol% to 4 mol%.
0 mol% and acrylate 90 mol% to 60 mol%
(However, the total amount of both is 100 mol%) is preferable. When a water-absorbent resin is obtained by polymerizing a hydrophilic monomer containing acrylic acid and / or a salt thereof as a main component, if necessary, a simple substance other than acrylic acid may be used in combination with the acrylic acid or a salt thereof. May be contained. The monomer other than acrylic acid is not particularly limited, but specifically, for example, methacrylic acid, maleic acid, vinylsulfonic acid, styrenesulfonic acid, 2- (meth) acrylamide-2-methyl Anionic unsaturated monomers such as propanesulfonic acid, 2- (meth) acryloylethanesulfonic acid and 2- (meth) acryloylpropanesulfonic acid and salts thereof; acrylamide;
Methacrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2- Hydroxypropyl (meth) acrylate, methoxypolyethylene glycol (meth)
Acrylate, polyethylene glycol mono (meth) acrylate, vinylpyridine, N-vinylpyrrolidone,
Nonionic unsaturated monomers containing a hydrophilic group such as N-acryloylpiperidine and N-acryloylpyrrolidine;
-Dimethylaminoethyl (meth) acrylate, N, N
-Diethylaminoethyl (meth) acrylate, N, N
-Dimethylaminopropyl (meth) acrylate, N,
N-dimethylaminopropyl (meth) acrylamide,
And cationic unsaturated monomers such as quaternary salts thereof. These monomers may be used alone or as a mixture of two or more.

When a monomer other than acrylic acid is used in the method for treating a surface of a water-absorbent resin of the present invention, the monomer other than acrylic acid is the total of acrylic acid and its salt used as a main component. Preferably 30 mol%, based on the amount
Or less, more preferably 10 mol% or less. By using a monomer other than the above-mentioned acrylic acid in the above-mentioned ratio, the water-absorbing properties of the obtained resin are further improved, and the water-absorbing resin can be obtained at a lower cost. When polymerizing the above-mentioned hydrophilic monomer having acrylic acid or a salt thereof as a main component in order to obtain a water-absorbent resin as a base polymer used in the surface treatment method of the water-absorbent resin of the present invention, bulk polymerization or Although precipitation polymerization can be performed, it is preferable to perform aqueous solution polymerization or reverse phase suspension polymerization using the above hydrophilic monomer as an aqueous solution in terms of performance and ease of polymerization control. When the hydrophilic monomer is used as an aqueous solution, the concentration of the monomer in the aqueous solution (hereinafter, referred to as an aqueous monomer solution) is not particularly limited, but may be from 10% by weight to 70% by weight. %, Preferably 20%
More preferably, it is in the range of 40% by weight to 40% by weight. Also,
When performing the aqueous solution polymerization or the reverse phase suspension polymerization, a solvent other than water may be used in combination as necessary, and the type of the solvent used in combination is not particularly limited.

When the above polymerization is initiated, for example, potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide,
A radical polymerization initiator such as 2,2′-azobis (2-aminodipropane) dihydrochloride can be used. Further, a redox initiator can be obtained by using a combination of a reducing agent that promotes the decomposition of the polymerization initiator and combining the two. As the above reducing agent, for example,
Examples thereof include (bis) sulfite (salt) such as sodium sulfite and sodium bisulfite, L-ascorbic acid (salt), reducing metals (salts) such as ferrous salt, and amines, but are particularly limited. is not.

The amount of the polymerization initiator used is usually 0.0
01 mol% to 2 mol%, preferably 0.01 mol% to
0.1 mol%. The amount of these polymerization initiators used is 0.1.
If the amount is less than 001 mol%, unreacted monomers increase, and therefore, the amount of residual monomers in the obtained water-absorbing resin increases, which is not preferable. On the other hand, when the use amount of these polymerization initiators exceeds 2 mol%, the amount of water-soluble components in the obtained water-absorbent resin increases, which may not be preferable. Instead of using the polymerization initiator, the polymerization reaction may be started by irradiating the reaction system with an active energy ray such as radiation, an electron beam, or an ultraviolet ray. The reaction temperature in the polymerization reaction is not particularly limited, but is preferably in the range of 20C to 90C. Also, the reaction time is not particularly limited, and may be appropriately set according to the type of the hydrophilic monomer and the polymerization initiator, the reaction temperature, and the like.

The water-absorbing resin as the base polymer used in the method for treating the surface of a water-absorbing resin of the present invention may be a self-crosslinking type which does not use a crosslinking agent. Those obtained by copolymerizing or reacting the above-mentioned polymerizable unsaturated groups or internal crosslinking agents having two or more reactive groups are more preferable. Specific examples of these internal crosslinking agents include, for example, N, N-methylenebis (meth) acrylamide, (poly) ethylene glycol (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) Acrylate, glycerin tri (meth) acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri (meth) acrylate, pentaerythritol hexa (meth) acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, Poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene Glycol, propylene glycol, glycerol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethylenimine, and glycidyl (meth) acrylate.

These internal cross-linking agents may be used alone or in combination of two or more. Further, these internal crosslinking agents may be added to the reaction system all at once, or may be added in portions. When using two or more types of internal cross-linking agents, in consideration of the water absorption properties of the resulting water-absorbent resin,
It is preferable to use a compound having two or more polymerizable unsaturated groups. The use amount of these internal crosslinking agents is preferably in the range of 0.005 mol% to 2 mol%, and more preferably in the range of 0.01 mol% to 1 mol%, based on the hydrophilic monomer. More preferably, When the use amount of the internal crosslinking agent is less than 0.005 mol% or more than 2 mol%, there is a possibility that a water-absorbing resin having desired water-absorbing properties may not be obtained.

When the crosslinked structure is introduced into the water-absorbent resin by using the internal crosslinking agent, the internal crosslinking agent is reacted during or after the polymerization of the hydrophilic monomer, or after the polymerization and neutralization. What is necessary is just to add to a system. In the above polymerization, various foaming agents such as carbonic acid (hydrogen) salt, carbon dioxide, azo compound, and inert organic solvent; starch / cellulose, starch / cellulose derivatives, polyvinyl alcohol, polyacrylic acid ( A hydrophilic polymer such as a salt) and a crosslinked polyacrylic acid (salt); various surfactants; and a chain transfer agent such as hypophosphorous acid (salt) may be added.

When the water-absorbent resin obtained by the above polymerization reaction is in a gel state, the water-absorbent resin is usually dried and pulverized if necessary. The water content of the water-absorbent resin as a base polymer that can be used in the surface treatment method of the water-absorbent resin of the present invention is not particularly limited, but is usually 1% by weight to 20% by weight, preferably 3% by weight to 15% by weight. The content is most preferably 5% by weight or more and 10% by weight or less. The particle shape of the water-absorbent resin as a base polymer that can be used in the surface treatment method of the water-absorbent resin of the present invention is not particularly limited, such as a spherical shape, a crushed shape, and an irregular shape. belongs to. The amorphous crushed water-absorbent resin has heretofore been difficult to perform particularly uniform surface cross-linking due to its shape, and in some cases, the performance expected when used in an absorbent article is difficult to develop. According to the method described above, a relatively uniform water absorbing agent having a specific thickness can be obtained, so that stable performance of the absorbent article can be exhibited.

The average particle size of the water-absorbing resin as the base polymer is usually 100 to 1000 μm, preferably 1 to 1000 μm.
It is 50 to 600 μm, more preferably more than 200 μm and 500 μm or less. The absorbent article of the present invention,
A surface crosslink comprising a water-absorbing agent and a fiber base, wherein the amount of the water-absorbing agent is at least 40% by weight based on the total weight of the water-absorbing agent and the fiber base material, and the water-absorbing agent has a specific thickness or weight according to the present invention; Water-absorbing resin, preferably 100-1000 μm in average particle size
It is characterized in that a surface cross-linked water-absorbent resin having a water-absorbent resin of m as a base polymer is used.

When the content of the water-absorbing agent is less than 40% by weight, the absorbent article becomes relatively thick, and the absorption characteristics such as the return amount are inferior. When the average particle diameter of the base polymer of the water absorbing agent is out of 100 to 1000 μm,
When introduced into an absorbent article, problems such as workability, such as generation of dust, and a problem such as discomfort during use due to physical size due to high resin concentration may occur. When the thickness of the surface cross-linked layer exceeds 1000 nm and / or the weight exceeds 3% by weight, the above-mentioned problem that the cross-linked layer regulates the swelling inside and the absorption amount of the absorbent article decreases. In addition, when the resin concentration is high, the crosslinked layer easily peels and collapses when swelling due to the shearing force during use, and the soluble components inside the resin are eluted and the slimy feeling is felt, resulting in poor dry feeling. There is also a problem that the amount of absorption under pressure also decreases. On the other hand, when the thickness of the surface cross-linked layer is less than 50 nm and / or the weight is less than 0.3% by weight, the surface cross-linked layer is too thin, and the physical properties due to impact force, shear force and the like are reduced. Due to the above-described problem of a large decrease, the surface cross-linked layer is easily formed during the production and use of the diaper, and similarly, a slimy feeling is produced, the liquid permeability is poor, and the absorption amount under pressure is reduced.

The absorbent article of the present invention may be composed only of a water-absorbing agent and a fiber base, or may be a liquid-impermeable sheet or a liquid-permeable liquid-permeable sheet formed of a water-absorbing agent and a fiber base. It may be in the form of a so-called paper diaper or sanitary napkin covered with. The water-absorbent resin of the present invention, the absorbent article further include a disinfectant, a deodorant, an antibacterial agent, a fragrance, various inorganic powders, a foaming agent, a pigment, a dye, a hydrophilic short fiber, a fertilizer, an oxidizing agent, a reducing agent, and water. , Salts and the like, whereby various functions can be imparted to the water absorbing agent and the absorbent article.

[0041]

The present invention will be described in more detail with reference to the following Examples and Comparative Examples, but the present invention is not limited thereto. The method for measuring the thickness of the surface crosslinked layer and the method for obtaining the weight% of the surface crosslinked layer are as described above. (1) Absorbency of absorbent article 75 parts by weight of a water-absorbing agent and 25 parts by weight of ground wood pulp were dry-mixed using a mixer. The resulting mixture is
After forming the web into a web having a size of 100 mm × 100 mm, the web was pressed at a pressure of 2 kg / cm ² (about 196 kPa) for 1 minute so that the basis weight was about 0.035 g / cm 2.
An absorbent article used for the measurement of ² was obtained.

Next, a glass filter having a diameter of 120 mm was placed on a petri dish, and a filter paper was placed thereon. A sheet made of polyethylene terephthalate and having a rectangular opening of 12.5 mm × 100 mm in the center and formed in a rectangular shape of 0.1 mm thick on the filter paper, the opening of which is located at the center of the glass filter. It was placed so as to be positioned. Then, the inner dimensions are 100mm x 10
A supporting rectangular tube having a thickness of 0 mm was placed so that the center portion thereof coincided with the center portion of the glass filter. The Petri dish was filled with physiological saline so that the liquid level was slightly higher than the upper part of the glass filter. Thereafter, the absorbent article was placed on a sheet inside the support rectangular tube, and a weight was placed on the absorbent article. The weight of the weight is adjusted so that a load of 20 g / cm ² (about 1.96 kPa) can be uniformly applied to the absorbent article.
The operation of placing the absorbent article and the weight was performed quickly. 60 minutes after the absorbent article was placed on the sheet, the absorbent article was removed and the weight W of the physiological saline absorbed
1 (g) was measured, and the absorbency (g / g) of the absorbent article 60 minutes after the start of water absorption according to the following equation from the above weight W1.
Was calculated.

Absorbency of absorbent article (g / g) = weight W
1 (g) / weight of absorbent article (g) (2) Absorption of absorbent article 70 parts by weight of a water-absorbing agent and 30 parts by weight of ground wood pulp were dry-mixed using a mixer. The resulting mixture is then added to 4
A web having a size of 120 mm × 350 mm was formed on a wire screen having a size of 00 mesh (mesh size 38 μm) by air forming using a batch type air forming apparatus (basis weight: about 0.05 g / cm ² ). Further, the web is subjected to a pressure of 2 kg / cm ² (about 196 kP).
An absorbent is obtained by pressing for 5 seconds in a), and a back sheet (liquid-impermeable sheet) made of liquid-impermeable polypropylene and having a so-called leg gather, the above-described absorber, and liquid-permeable A top sheet (liquid permeable sheet) made of polypropylene was adhered to each other in this order using a double-sided tape, and two so-called tape fasteners were attached to the adhered product to obtain an absorbent article.

After the absorbent article is mounted on a so-called Kewpie doll, and the doll is placed in a prone position, a tube is inserted between the absorbent article and the doll, and the urine is placed at a position corresponding to the position where urination is to be performed in the human body. Each time, 50 ml of physiological saline was sequentially injected at intervals of 60 minutes. The injection operation was terminated when the injected saline solution was not absorbed by the absorbent article and leaked, and the amount of the saline solution injected up to this time was measured. After the above measurement was repeated four times, the average of the measured values obtained was determined, and this value was defined as the absorption amount of the absorbent article.

Reference Example 1 Synthesis of Standard Water-Absorbent Resin as Base Polymer 8000 parts of cyclohexane was placed in a reactor equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube and a dropping funnel, and used as a dispersant. Sucrose fatty acid ester (DK-ester F-50, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) 2
5.2 parts were added and dissolved, and dissolved oxygen was expelled by blowing nitrogen gas. Separately, neutralization rate of 30% monomer concentration 7
9.0 parts of hydroxyethyl cellulose (HEC Daicel SP-) was added to 3000 parts of a 5% aqueous solution of partially neutralized acrylic acid.
850, manufactured by Daicel Chemical Industries, Ltd., 2.43 parts of polyethylene glycol diacrylate (n = 8) and 0.44 part of 2,2′-azobis (2-aminodipropane) dihydrochloride as an initiator were dissolved. Thereafter, dissolved oxygen present in the aqueous solution was expelled by blowing nitrogen gas.

Next, the monomer aqueous solution was added to the above reaction vessel, stirred and dispersed, and then heated to 60 ° C. to start a polymerization reaction. After keeping the temperature at this temperature for 2 hours to complete the polymerization, the water in the hydrous gel was distilled off by azeotropic dehydration with cyclohexane, followed by filtration, drying at 80 ° C. under reduced pressure, and using a wire mesh having openings of 150 μm and 250 μm. Then, a standard water-absorbent resin as a spherical base polymer having a water content of 6.4% in a particle size range of 150 to 250 μm was obtained. (Reference Example 2 Synthesis of irregularly crushed base polymer 1) To 5500 parts of a 33% by weight aqueous solution of sodium acrylate (neutralization ratio: 75 mol%), polyethylene glycol diacrylate (n = 8) as a crosslinking agent 3.92 A portion was dissolved to prepare a reaction solution. Next, the reaction solution was degassed for 30 minutes under a nitrogen gas atmosphere. Next, the above reaction solution was supplied to a polymerization reactor having stirring blades, and the system was purged with nitrogen gas while maintaining the reaction solution at 30 ° C. Subsequently, while stirring the reaction solution, 2.5 parts of sodium persulfate and 0.125 part of L-ascorbic acid were added, and polymerization started about 1 minute later. Then, polymerization was carried out at 30 to 90 ° C., and the hydrogel polymer was taken out 60 minutes after the initiation of the polymerization.

The resulting hydrogel polymer was finely divided into a diameter of about 7 mm. The finely divided hydrogel polymer was spread on a 50-mesh wire net and dried with hot air at 150 ° C. for 90 minutes. Next, the dried product is pulverized, and further classified by a 20-mesh wire net to form an irregularly crushed base polymer having an average particle diameter of 360 μm and a particle size of less than 106 μm in a proportion of 2% by weight. Water absorbent resin (1) was obtained. (Reference Example 3 Synthesis of spherical base polymer) 8000 parts of cyclohexane was placed in a reactor equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen gas inlet tube and a dropping funnel, and sucrose fatty acid esters (Daiichi Industrial Pharmaceutical Company, DK
-Ester F-50) was dissolved by adding 25.2 parts, and dissolved oxygen was expelled by blowing nitrogen gas. Separately, 9.0 parts of hydroxyethyl cellulose (HEC Daicel SP-850, manufactured by Daicel Chemical Industries, Ltd.) and polyethylene glycol diacrylate (n) were added to 3000 parts of an aqueous solution of a partially neutralized acrylic acid having a monomer concentration of 30% and a neutralization rate of 75%.
= 8) After dissolving 2.43 parts and 0.44 part of 2,2'-azobis (2-aminodipropane) dihydrochloride as an initiator, nitrogen gas was blown thereinto to dissolve dissolved oxygen present in the aqueous solution. I was kicked out.

Next, the monomer aqueous solution was added to the above reaction vessel, stirred and dispersed, and then heated to 60 ° C. to start a polymerization reaction. After maintaining the temperature at this temperature for 2 hours to complete the polymerization, the water in the hydrous gel was then distilled off by azeotropic dehydration with cyclohexane, filtered, dried at 80 ° C. under reduced pressure, and passed through a wire mesh with openings of 850 μm. Water content 8.1%
To obtain a water-absorbing resin (2) as a spherical base polymer. Example 1 0.03 parts by weight of ethylene glycol diglycidyl ether, 0.9 parts by weight of propylene glycol, and 3 parts of water were used as crosslinking agents with respect to 100 parts by weight of the standard water-absorbent resin as the base polymer obtained in Reference Example 1. A liquid containing a surface cross-linking agent consisting of 0.0 parts by weight and 3.0 parts by weight of isopropyl alcohol was sprayed and mixed. The powder temperature of the standard water-absorbent resin before mixing was 15 ° C, and the liquid temperature of the surface crosslinking agent-containing liquid was 10 ° C. The resulting mixture is 195
Heat treatment was performed at 40 ° C. for 40 minutes to obtain a surface crosslinked water-absorbent resin (1). The thickness and weight% of the surface crosslinked layer of the surface crosslinked water-absorbent resin (1) were measured, and the results are shown in Table 1.

(Example 2) Using the water-absorbing resin (1) as a base polymer obtained in Reference Example 2, the same surface cross-linking treatment as in Example 1 was carried out to obtain a surface-crosslinked water-absorbing resin (2). . Thickness of the surface cross-linked layer of the surface cross-linked water-absorbent resin (2),
The weight% was measured and the results are shown in Table 1. Absorbent articles were prepared using the surface-crosslinked water-absorbent resin (2) as the water-absorbing agent (1), and the absorption capacity and the absorption amount were determined as the absorbent article (1). The results are shown in Table 2. (Example 3) 0.9 parts by weight of 1,4-butanediol as a crosslinking agent, 3.0 parts by weight of water, and isopropyl alcohol were added to 100 parts by weight of the standard water-absorbent resin as the base polymer obtained in Reference Example 1. A surface cross-linking agent-containing liquid consisting of 3.0 parts by weight was atomized and sprayed and mixed. The powder temperature of the standard water-absorbent resin before mixing was 15 ° C, and the liquid temperature of the surface crosslinking agent-containing liquid was 12 ° C. The resulting mixture is heated at 195 ° C. for 40
By performing a heat treatment for a minute, a surface crosslinked water-absorbent resin (3) was obtained. The thickness and weight% of the surface crosslinked layer of the surface crosslinked water-absorbent resin (3) were measured, and the results are shown in Table 1.

Example 4 Using the water-absorbing resin (1) as a base polymer obtained in Reference Example 2, the same surface cross-linking treatment as in Example 3 was carried out to obtain a surface-crosslinked water-absorbing resin (4). . The thickness of the surface crosslinked layer of the surface crosslinked water-absorbent resin (4),
The weight% was measured and the results are shown in Table 1. Absorbent articles were prepared using the surface-crosslinked water-absorbent resin (4) as the water-absorbing agent (2), and the absorption capacity and absorption amount were determined as the absorbent article (2). The results are shown in Table 2. (Example 5) 2.0 parts by weight of propylene glycol as a crosslinking agent, 4.0 parts by weight of water, 1.0 part by weight of isopropyl alcohol based on 100 parts by weight of the standard water-absorbent resin as the base polymer obtained in Reference Example 1. The surface cross-linking agent-containing liquid consisting of parts was sprayed and mixed. The powder temperature of the standard water-absorbent resin before mixing is 15 ° C, and the liquid temperature of the surface cross-linking agent-containing liquid is 1
5 ° C. The resulting mixture was heat-treated at 195 ° C. for 60 minutes to obtain a surface-crosslinked water-absorbent resin (5). The thickness and weight% of the surface crosslinked layer of the surface crosslinked water-absorbent resin (5) were measured, and the results are shown in Table 1.

(Example 6) Using the water-absorbing resin (1) as a base polymer obtained in Reference Example 2, the same surface cross-linking treatment as in Example 5 was performed to obtain a surface-crosslinked water-absorbing resin (6). . Thickness of the surface cross-linked layer of the surface cross-linked water-absorbent resin (6),
The weight% was measured and the results are shown in Table 1. Absorbent articles were prepared using the surface-crosslinked water-absorbent resin (6) as the water-absorbing agent (3), and the absorption capacity and absorption amount were determined as the absorbent article (3). The results are shown in Table 2. (Example 7) 100 parts by weight of the standard water-absorbing resin as the base polymer obtained in Reference Example 1 was composed of 1.0 part by weight of glycerin as a crosslinking agent, 3.0 parts by weight of water, and 30 parts by weight of isopropyl alcohol. Was added dropwise and mixed. The powder temperature of the standard water-absorbent resin before mixing was 15 ° C, and the liquid temperature of the surface crosslinking agent-containing liquid was 15 ° C.
The resulting mixture was heated at 195 ° C. for 40 minutes to obtain a surface crosslinked water-absorbent resin (7). The thickness and weight% of the surface crosslinked layer of the surface crosslinked water-absorbent resin (7) were measured, and the results are shown in Table 1.

Example 8 Using the water-absorbing resin (1) as a base polymer obtained in Reference Example 2, the same surface cross-linking treatment as in Example 7 was performed to obtain a surface-crosslinked water-absorbing resin (8). . Thickness of the surface crosslinked layer of the surface crosslinked water-absorbent resin (8),
The weight% was measured and the results are shown in Table 1. Absorbent articles were prepared using the surface-crosslinked water-absorbent resin (8) as the water-absorbing agent (4), and the absorption capacity and absorption amount were determined as the absorbent article (4). The results are shown in Table 2. (Example 9) 0.1 part by weight of ethylene glycol diglycidyl ether as a crosslinking agent was added to 100 parts by weight of a standard water-absorbing resin as a base polymer obtained in Reference Example 1.
A liquid containing a surface crosslinking agent consisting of 3.0 parts by weight of water and 3.0 parts by weight of isopropyl alcohol was atomized and sprayed and mixed. The powder temperature of the standard water absorbent resin before mixing is 15 ° C,
The liquid temperature of the surface crosslinking agent-containing liquid was 8 ° C. The resulting mixture was heat-treated at 195 ° C. for 60 minutes to obtain a surface-crosslinked water-absorbent resin (9). The thickness and weight% of the surface crosslinked layer of the surface crosslinked water-absorbent resin (9) were measured, and the results are shown in Table 1.

(Example 10) Using the water-absorbing resin (1) as a base polymer obtained in Reference Example 2, the same surface cross-linking treatment as in Example 9 was performed, and the surface-crosslinked water-absorbing resin (1) was used.
0) was obtained. The thickness and weight% of the surface crosslinked layer of the surface crosslinked water-absorbent resin (10) were measured, and the results are shown in Table 1. Absorbent articles were prepared using the surface-crosslinked water-absorbent resin (10) as the water-absorbing agent (5), and the absorption capacity and absorption were determined as the absorbent article (5) of the present invention. The results are shown in Table 2. (Example 11) The same surface cross-linking treatment as that performed on the standard water-absorbent resin as the base polymer obtained in Reference Example 1 in Example 1 was carried out, and the water absorption as the base polymer obtained in Reference Example 3 was performed. The surface-crosslinked water-absorbent resin (11) was obtained by using the water-soluble resin (2). The thickness of the surface cross-linked layer of the surface cross-linked water-absorbent resin (11) is 380 μm, and the weight% is 0.1%.
It was 95% by weight.

Using the surface-crosslinked water-absorbent resin (11) as the water-absorbing agent (6), the above-mentioned absorbent article (6) of the present invention was prepared, and the absorption capacity and absorption amount were determined. The results are shown in Table 2. (Comparative Example 1) It is composed of 1.0 part by weight of a glycerin as a crosslinking agent, 10 parts by weight of water, and 30 parts by weight of isopropyl alcohol based on 100 parts by weight of the standard water-absorbent resin as the base polymer obtained in Reference Example 1. The liquid containing the surface crosslinking agent was dropped and mixed. The powder temperature of the standard water-absorbent resin before mixing was 25 ° C, and the liquid temperature of the surface crosslinking agent-containing liquid was 25 ° C. The resulting mixture was subjected to a heat treatment at 195 ° C. for 60 minutes to obtain a comparative surface-crosslinked water-absorbent resin (1). The thickness and weight% of the surface crosslinked layer of the comparative surface crosslinked water absorbent resin (1) were measured, and the results are shown in Table 1.

(Comparative Example 2) Using the water-absorbent resin (1) as the base polymer obtained in Reference Example 2, the same surface cross-linking treatment as in Comparative Example 1 was performed to obtain a comparative surface-crosslinked water-absorbent resin (2). Obtained. The thickness and weight% of the surface crosslinked layer of the comparative surface crosslinked water absorbent resin (2) were measured, and the results are shown in Table 1. Absorbent articles were prepared using the comparative surface-crosslinked water-absorbent resin (2) as the comparative water-absorbing agent (1), and the absorption capacity and absorption amount were determined as the comparative absorbent article (1). The results are shown in Table 2. (Comparative Example 3) 0.03 parts by weight of ethylene glycol diglycidyl ether and propylene glycol 0 as a crosslinking agent were added to 100 parts by weight of the standard water-absorbing resin as the base polymer obtained in Reference Example 1. A liquid containing a surface cross-linking agent composed of 9.9 parts by weight, 3.0 parts by weight of water and 30 parts by weight of isopropyl alcohol was dropped and mixed. The powder temperature of the standard water-absorbent resin before mixing was 25 ° C, and the liquid temperature of the surface crosslinking agent-containing liquid was 25 ° C. The resulting mixture was heated at 195 ° C. for 80 minutes to obtain a comparative surface crosslinked water-absorbent resin.
(3) was obtained. The thickness and weight% of the surface crosslinked layer of the comparative surface crosslinked water-absorbent resin (3) were measured, and the results are shown in Table 1.

(Comparative Example 4) The same surface cross-linking treatment as in Comparative Example 3 was performed using the water-absorbing resin (1) as the base polymer obtained in Reference Example 2 to obtain a comparative surface-crosslinked water-absorbing resin (4). Obtained. The thickness and weight% of the surface crosslinked layer of the comparative surface crosslinked water-absorbent resin (4) were measured, and the results are shown in Table 1. Absorbent articles were prepared using the comparative surface-crosslinked water-absorbent resin (2) as the comparative water-absorbing agent (2), and the absorption capacity and absorption amount were determined as the comparative absorbent article (2). The results are shown in Table 2. (Comparative Example 5) 0.03 parts by weight of ethylene glycol diglycidyl ether as a crosslinking agent and 100 parts by weight of water were added to 100 parts by weight of the standard water-absorbing resin as the base polymer obtained in Reference Example 1. A liquid containing a surface crosslinking agent consisting of 0 parts by weight and 3.0 parts by weight of isopropyl alcohol was sprayed and mixed. The powder temperature of the standard water-absorbent resin before mixing was 25 ° C, and the liquid temperature of the surface crosslinking agent-containing liquid was 25 ° C. The obtained mixture was subjected to a heat treatment at 195 ° C. for 40 minutes to obtain a comparative surface-crosslinked water-absorbent resin (5). The thickness and weight% of the surface crosslinked layer of the comparative surface crosslinked water-absorbent resin (5) were measured, and the results were shown in Table 1.
It was shown to.

(Comparative Example 6) The same surface cross-linking treatment as in Comparative Example 5 was performed using the water-absorbing resin (1) as the base polymer obtained in Reference Example 2 to obtain a comparative surface-crosslinked water-absorbing resin (6). Obtained. The thickness and weight% of the surface crosslinked layer of the comparative surface crosslinked water-absorbent resin (6) were measured, and the results are shown in Table 1. Absorbent articles were prepared using the comparative surface-crosslinked water-absorbent resin (6) as the comparative water-absorbing agent (3), and the absorption capacity and absorption amount were determined as the comparative absorbent article (3). The results are shown in Table 2.

Comparative Example 7 The same surface cross-linking treatment as that performed in Comparative Example 1 for the standard water-absorbent resin as the base polymer obtained in Reference Example 1 was carried out, and the base polymer obtained in Reference Example 3 was obtained. The surface-crosslinked water-absorbent resin (7) for comparison was obtained by using the water-absorbent resin (2) as a sample. The thickness and weight% of the surface cross-linked layer of the comparative water absorbent resin (4) is 103
0 μm and 3.38%. Absorbent articles described above were prepared using the comparative surface crosslinked water-absorbent resin (7) as the comparative water absorbing agent (4), and the absorption capacity and absorption amount were determined as the comparative absorbent article (4). The results are shown in Table 2.

[0059]

[Table 1]

[0060]

[Table 2]

As is clear from the examples, the surface cross-linking is carried out using a standard water-absorbing resin, and the surface cross-linking method is such that the thickness of the obtained surface cross-linked layer or the weight% of the cross-linked layer falls within the scope of the present invention. When applied to a water-absorbent resin as a polymer, the thickness of the resulting surface cross-linked layer or the weight% of the cross-linked layer
The present invention also falls within the scope of the present invention, and an absorbent article using the water absorbing agent has excellent water absorption speed and water absorption. The absorption performance of the absorbent article obtained by the composition of the surface cross-linking agent varies greatly, and it is shown that the largest factor that determines the absorption performance of the absorbent article is the thickness of the surface cross-linked layer formed and the weight% thereof. It is clear from 1, 2 that if the thickness of the crosslinked layer or the weight% of the crosslinked layer is controlled within the range of the present invention, a water-absorbent resin having excellent performance can be obtained, which is preferable for an absorbent article having a high resin concentration. Can be used.

[0062]

The surface-crosslinked water-absorbent resin according to the present invention comprises:
The thickness of the surface cross-linking layer is in a very thin range, and the thickness of the surface cross-linking layer is controlled so as to have a specific thickness range and a specific weight ratio. In addition, when used as a sanitary material or the like, even when the weight% (resin concentration) of the water-absorbing resin is increased, excellent absorption characteristics are exhibited. The surface cross-linking method for a water-absorbent resin according to the present invention stably and easily obtains the surface cross-linked water-absorbent resin according to the present invention.

Since the absorbent article according to the present invention uses the above surface-crosslinked water-absorbent resin of the present invention as a water absorbing agent, the water absorbing agent is as high as 40% by weight or more with respect to the total amount of the fiber base material. However, excellent water absorption performance can be exhibited, and thinning can be easily achieved.

[Brief description of the drawings]

FIG. 1 is an optical micrograph of a surface crosslinked layer isolated from a water-absorbent resin (swelled state).

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) A61F 5/44 A41B 13/02 D C08L 101: 00

Claims (7)

[Claims] 1. A water-absorbent resin having a crosslinked surface, wherein the thickness of the surface crosslinked layer is 50 nm or more and 1000 nm or less, and / or the weight ratio of the surface crosslinked layer to the whole resin is 0.3. Surface-crosslinked water-absorbent resin, characterized in that the content of the water-absorbent resin is from 3% to 3%. 2. The water-absorbent resin has an average particle size of 100 to 1
The surface-crosslinked water-absorbent resin according to claim 1, which has a thickness of 000 µm. 3. A method for cross-linking the surface of a water-absorbent resin by bringing a surface cross-linking agent into contact with a water-absorbent resin as a base polymer, wherein the cross-linking is carried out by performing a surface cross-linking. The condition that the thickness of the surface cross-linked layer remaining after the water-absorbent resin is saturated and swollen and the inside of the resin is solubilized is 50 nm or more and 1000 nm or less, and / or the weight% of the remaining surface cross-linked layer is A method for cross-linking the surface of a water-absorbent resin, wherein the method is carried out under a cross-linking treatment condition that satisfies a condition of 0.3 to 3% by weight based on the cross-linked water-absorbent resin. 4. A method for cross-linking the surface of a water-absorbent resin by bringing a surface-crosslinking agent-containing liquid into contact with a water-absorbing resin as a base polymer. The temperature of the water-absorbent resin is 5 to 20
C. and / or the temperature of the surface cross-linking agent-containing liquid is set to 0 to 20 ° C. 5. A method for crosslinking a surface of a water-absorbent resin by bringing a surface-crosslinking agent-containing liquid into contact with a water-absorbing resin as a base polymer. Wherein the temperature difference between the water-absorbent resin and the surface-crosslinking agent-containing liquid is 0 to 20 ° C. 6. The water-absorbent resin has an average particle size of 100 to 1
The method for cross-linking a surface of a water-absorbent resin according to any one of claims 3 to 5, which is 000 µm. 7. An absorbent article comprising a water-absorbing agent and a fiber base, wherein the amount of the water-absorbing agent is at least 40% by weight based on the total weight of the water-absorbing agent and the fiber base material, wherein the water-absorbing agent is used. 1
Or an absorbent article using the surface crosslinked water-absorbent resin described in 2 above.