JP2944447B2 - Water absorbing agent and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water absorbing agent and a method for producing the same. More specifically, the present invention relates to a water-absorbing agent which exhibits a high absorption capacity even under pressure and exhibits high absorption capacity and has high safety because no highly reactive cross-linking agent remains on the resin surface, and is suitable for sanitary materials and a method for producing the same. Things.

[0002]

2. Description of the Related Art In recent years, water-absorbent resins have been widely used as one of the constituent materials of sanitary materials such as paper diapers and sanitary napkins for the purpose of absorbing body fluids.

Examples of these water-absorbing resins include cross-linked polyacrylic acid partially neutralized products (JP-A-55-8430).
4, JP-A-55-108407, JP-A-55-13
No. 3413), a hydrolyzate of starch-acrylonitrile graft polymer (JP-A-46-43995), and a neutralized product of starch-acrylic acid graft polymer (JP-A-51-125)
No. 468), a saponified product of a vinyl acetate-acrylate copolymer (JP-A-52-14689), a cross-linked carboxymethyl cellulose (US Pat. No. 4,650,716,
No. 4,689,408), a hydrolyzate of an acrylonitrile copolymer or an acrylamide copolymer (Japanese Unexamined Patent Publication No.
-15959) or a crosslinked product thereof, a crosslinked polymer of a cationic monomer (JP-A-58-154709, JP-A-58-154710), a crosslinked isobutylene-maleic anhydride copolymer (US Pat. No. 4,389,513), 2
-Acrylamido-2-methylpropane sulfonic acid and acrylic acid copolymerized crosslinked product (European Patent No. 068189) are known.

[0004] Desirable characteristics of such a water-absorbent resin include a high absorption capacity when contacted with an aqueous liquid, an excellent absorption rate, a liquid permeability, a gel strength of a swollen gel, and the ability of water to be absorbed from a substrate containing an aqueous liquid. For example, a suction force for sucking up. However, the relationship between these properties does not necessarily indicate a positive correlation, and for example, there is a problem that the higher the absorption capacity, the lower the physical properties such as liquid permeability, gel strength, and absorption rate. .

Therefore, as a method of improving the water absorbing properties of such a 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 proposed so far.

For example, a method using a polyhydric alcohol as a crosslinking agent (JP-A-58-180233, JP-A-61-180233)
-16903), a method using a polyvalent glycidyl compound, a polyvalent aziridine compound, a polyvalent amine compound, and a polyvalent isocyanate compound (JP-A-59-189103), and a method using glyoxal (JP-A-52-117393).
No.), a method using a polyvalent metal (JP-A-51-13658)
No. 8, JP-A-61-257235, JP-A-62-77
No. 45), a method using a silane coupling agent (JP-A-61-211305, JP-A-61-252212,
JP-A-61-264006), a method using a monoepoxy compound (JP-A-61-98121), and a method using an epoxy group-containing polymer (US Pat. No. 4,758,617).
), A method using an epoxy compound and a hydroxy compound (JP-A-2-132103), 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 (JP-A-60-163965, JP-A-60-25)
No. 5814), a method in which a divalent alcohol is present (JP-A-1-292004), a method in which water and an ether compound are present (JP-A-2-153903), and a method in which a water-soluble polymer is present (particularly, Kaihei 3-126730), 1
In the presence of alkylene oxide adducts of monovalent alcohols, organic acid salts, lactams and the like (EP-555,692)
No.) is also known.

Indeed, although the balance of various physical properties of the water-absorbent resin is improved by these methods, it is still difficult to say that it is still sufficient, and there is a need for higher quality. In particular, in recent years, while maintaining a high absorption capacity under no pressure, which is one of the basic physical properties of conventional water-absorbent resins, a resin excellent in absorption characteristics under pressure, especially absorption capacity under pressure is required. It has become. Of course, the absorption capacity under no pressure and the absorption capacity under pressure are generally in an opposite relationship, and at present, it is not possible to sufficiently respond to these needs only by the above-mentioned methods known so far. Was.

In addition to these, there is a problem that a cross-linking agent used when cross-linking the vicinity of the surface of the water-absorbent resin remains on the resin surface. There is no problem if the cross-linking agent used is low in reactivity and high in safety, such as polyhydric alcohol, but if the cross-linking agent is high in reactivity, such as an epoxy compound, the vicinity of the surface When the cross-linking agent itself is irritating to the skin and remains on the resin surface, the cross-linking of the cross-linking agent easily occurs quickly, which poses a new problem in terms of safety when applied to sanitary materials.

Therefore, in order to reduce the amount of the cross-linking agent that cross-links near the surface in the resin, cross-linking near the surface of the resin is started under a specific water content, and a specific amount of water is further added during the reaction. (Japanese Patent Application Laid-Open No. 3-195705) is known. However, such a method not only complicates the process, but also does not have a sufficient reduction rate of the remaining amount.

[0010]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a high absorption capacity under pressure as well as under no pressure, a highly reactive cross-linking agent not remaining on the resin surface, excellent safety, and hygiene. An object of the present invention is to provide a water absorbing agent suitable for a material and a method for producing the same.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies to achieve the above object, and as a result, mixed a water-absorbing resin having a carboxyl group with a cross-linking agent capable of reacting with the carboxyl group. When a specific additive is used in the production of a cross-linking agent having an extremely excellent absorption capacity under pressure and a high reactivity while maintaining high absorption characteristics, especially the absorption capacity under no pressure, It has been found that even when used, a water-absorbing agent having a remarkably reduced amount of the crosslinking agent can be easily obtained, and the present invention has been completed.

That is, the present invention relates to a method for crosslinking a water-absorbent resin having a carboxyl group with at least one additive selected from the group consisting of an inorganic acid and an organic acid soluble in an aqueous liquid and a carboxyl group. A method for producing a water-absorbing agent, comprising mixing an epoxy compound per 100 parts by weight of a water-absorbing resin having a carboxyl group.
An epoxy compound having an absorption capacity under no pressure of 45 (g / g) or more and an absorption capacity under pressure of 20 g / cm ² of 30 (ml / g) or more obtained by adding 0.05 to 8 parts by weight. Of a water-absorbing agent having a residual amount of 2 ppm or less.

Hereinafter, the present invention will be described in more detail.

The water-absorbing resin used in the present invention is not particularly limited as long as it has a carboxyl group. The water-absorbing resin such as a partially neutralized polyacrylate crosslinked polymer, a starch-acrylic acid graft polymer, and a carboxymethylcellulose crosslinked product can be used. Is a conventionally known water-absorbing resin resin which absorbs a large amount of water, preferably physiological saline 10 to 100 times under no pressure, to form a substantially water-insoluble hydrogel.

In the present invention, typically, a water-absorbing resin obtained by polymerizing a hydrophilic monomer containing acrylic acid and / or a salt thereof as a main component in the presence of a crosslinking agent and a graft main chain, if necessary. Is preferably used. Examples of the acrylate include alkali metal salts, ammonium salts, and amine salts of acrylic acid, and the degree of neutralization of a carboxyl group derived from acrylic acid as a constituent unit of the resin is 50 to 90 mol%.
Are preferred.

When the water-absorbing resin used in the present invention is obtained from a hydrophilic monomer containing acrylic acid and / or a salt thereof as a main component, the water-absorbing resin may be used in combination with the acrylic acid or a salt thereof and, if necessary, other monomer. The body may be copolymerized. (U.S. Pat. No. 5,338,810, EP 0574260).

Specific examples of other monomers include methacrylic acid, maleic acid, β-acryloyloxypropionic acid, vinylsulfonic acid, styrenesulfonic acid, 2- (meth) acrylamide-2-methylpropanesulfonic acid,
Anionic unsaturated monomers such as 2- (meth) acryloylethanesulfonic acid and 2- (meth) acryloylpropanesulfonic acid and salts thereof; acrylamide, methacrylamide, N-ethyl (meth) acrylamide, N-
n-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, N-acryloylpiperidine, N -Nonionic hydrophilic group-containing unsaturated monomer such as acryloylpyrrolidine; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) Examples thereof include cationic unsaturated monomers such as acrylate, N, N-dimethylaminopropyl (meth) acrylamide, and quaternary salts thereof. Further, a hydrophobic unsaturated monomer such as isobutylene or stearyl (meth) acrylate may be used as long as the water absorption is not impaired. The amount of these other monomers to be used is generally 0 to 50 mol%, preferably 0 to 30 mol%, more preferably 0 to 10 mol%, based on all monomers.

The water-absorbing resin used in the present invention has a cross-linked structure. However, the water-absorbing resin has two or more polymerizable unsaturated groups or two or more Desirable are those obtained by copolymerizing or reacting an internal crosslinking agent having a group or an internal crosslinking agent having both a polymerizable unsaturated group and a reactive group.

Specific examples of these internal crosslinking agents include, for example, N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, tri Methylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate,
Glycerin tri (meth) acrylate, glycerin acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, triary Lucyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerol diglycidyl ether,
Ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol,
Examples thereof include ethylenediamine, polyethyleneimine, and glycidyl (meth) acrylate. Further, two or more of these internal crosslinking agents may be used. Above all, it is preferable to use a compound having two or more polymerizable unsaturated groups as an internal cross-linking agent in view of the water absorbing properties of the obtained water-absorbing resin, and the amount thereof is based on the amount of the monomer component. The content is preferably 0.005 to 2 mol%, more preferably 0.01 to 1 mol%.

In the polymerization, starch or cellulose is used.
, Starch and cellulose derivatives, polyvinyl alcohol
, Hydrophilic polymers such as polyacrylic acid (salt) and cross-linked polyacrylic acid (salt), chain transfer agents such as hypophosphorous acid (salt), surfactants, and foaming agents such as carbonates May be added. The compound added to these monomers is
U.S. Pat. Nos. 4,076,663, 4,228,082, 4,432,040, 4,833,222, and 51187.
19, 5,149,750, 5,154,713 and 5,264,495, and European Patents 0,329,981 and 4,496,594.

When the above-mentioned monomer is polymerized in order to obtain the water-absorbing resin used in the present invention, bulk polymerization or precipitation polymerization can be carried out, but from the viewpoint of performance and ease of polymerization control. It is preferable to carry out aqueous solution polymerization and reverse phase suspension polymerization using the monomer as an aqueous solution. The concentration of the aqueous solution at that time is usually 10 to 70% by weight, preferably 20 to saturated concentration.

Incidentally, such a polymerization method is described, for example, in US Pat. Nos. 4,093,376, 4,367,323 and 4,446.
No. 261, No. 4552938, No. 462001,
4654393, 4683274, 4690
No. 996, No. 4721647, No. 4738867,
No. 4748076, No. 4769427, No. 4873
No. 229, No. 4985514, No. 5124416,
Nos. 5,247,225 and 5,250,640.

When the polymerization is started, potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide, 2,2'-azobis (2-amidinopropane) dihydrochloride, etc. , An active energy ray such as an ultraviolet ray or an electron beam, or the like, but it is preferable to use a radical polymerization initiator. When using an oxidizing radical polymerization initiator, sodium sulfite, sodium hydrogen sulfite,
Redox polymerization may be performed by using a reducing agent such as ferrous sulfate or L-ascorbic acid (salt) in combination. A plurality of these polymerization initiators and reducing agents may be used in combination, and the amount used is usually
0.001 to 2 mol%, preferably 0.1 to 2 mol%, based on the monomer.
01 to 0.5 mol%.

When the monomer is used as an aqueous solution, the gel polymer after polymerization is preferably dried. The drying methods used include hot air drying, drying with specific steam (US Pat. No. 490202), and microwave drying (US Pat.
75344), drying under reduced pressure, drying with a drum dryer, azeotropic dehydration in a hydrophobic organic solvent, and the like, preferably 70 to 300 ° C, more preferably 100 to 250 ° C.
Dry at ℃. Prior to drying, US Pat.
The gel polymer may be subdivided in advance by a method such as 75773, or the supply to the dryer may be controlled by a method such as US Pat. No. 5,229,487.

The shape of the water-absorbent resin obtained by the above polymerization can be variously used in the present invention, such as irregularly crushed, spherical, fibrous, rod-like, sheet-like, and substantially spherical shapes. In order to obtain a water-absorbing agent with maximum safety under pressure, exhibiting a high absorption under pressure as well as under no pressure, and leaving no unreacted cross-linking agent remaining, it is preferable as a starting material. Is spherical or amorphous crushed, particularly preferably amorphous crushed obtained by aqueous solution polymerization, the average particle diameter of which is 200 to 600 μm, and the weight of the resin having a particle diameter of less than 150 μm is 10% by weight or less. It is most preferable to use the water-absorbing resin of Average particle size is 200
If it is less than μm, the absorption capacity under pressure may be difficult to improve, and if it exceeds 600 μm, the absorption speed may be slow and it may take too long to reach the saturated absorption amount. When the resin having a particle diameter of less than 150 μm exceeds 10% by weight, the amount of the residual cross-linking agent may not be easily reduced.

The water absorbent resin used in the present invention has a water content of 1 to 50%, preferably 1 to 20%, more preferably 1 to 10%, and can be handled as a powder. When the water content exceeds 50%, the cross-linking agent used in the present invention penetrates into the water-absorbent resin and the absorption capacity is reduced even if at least one additive selected from the group consisting of inorganic acids and organic acids is used. In addition, absorption characteristics under pressure may not be improved.

In the present invention, for example, the water-absorbing resin having a carboxyl group obtained as described above is essentially required to contain at least one selected from the group consisting of inorganic acids and organic acids soluble in aqueous liquids. The additives are mixed with a crosslinking agent that can react with the carboxyl groups.

The additive used in the present invention is not particularly limited as long as it is an inorganic acid and / or an organic acid that can be dissolved in an aqueous liquid. Here, the aqueous liquid means water or a mixture of water and a hydrophilic organic solvent. Also, those having ordinary knowledge in the art, such as compounds having a pKa of more than 7.0,
It goes without saying that substances that are uniquely recognized as basic compounds and salts do not correspond to the above-mentioned inorganic acids and organic acids.

The organic acid referred to in the present invention is an acidic compound composed of elements consisting of carbon, hydrogen and oxygen, and is typically exemplified by organic carboxylic acids, organic sulfonic acids, and organic sulfinic acids. Thus, organic carboxylic acids are preferred. From the viewpoint of the effect, among the organic carboxylic acids, a saturated organic carboxylic acid is preferably used, and even more preferably, a hydroxy group-containing saturated organic carboxylic acid is used.

The inorganic acid referred to in the present invention is an acidic compound composed of an inorganic substance and hydrogen.
Preferably, an inorganic acid containing an element of the third period of the periodic table containing at least one of phosphorus, sulfur and chlorine as an inorganic substance;
More preferably, it contains an element of the third period of the periodic table,
It is a saturated inorganic acid. In the present invention, the term "saturated inorganic acid" refers to an inorganic acid in the most stable oxidized state of an inorganic substance in the case of an inorganic substance capable of being in a plurality of oxidation states.
It is an inorganic acid containing a positive (+5) element for sulfur, a hexavalent (+6) element for sulfur, and a monovalent (-1) element for chlorine.

Examples of such additives include diphosphate,
Inorganic acids such as tripolyphosphoric acid, phosphoric acid, sulfuric acid, and hydrochloric acid; anisic acid, benzoic acid, formic acid, valeric acid, citric acid, glyoxylic acid, glycolic acid, glycerin phosphate, glutaric acid,
Chloroacetic acid, chloropropionic acid, cinnamic acid, succinic acid, acetic acid, tartaric acid, lactic acid, pyruvic acid, fumaric acid, propionic acid, 3-hydroxypropionic acid, malonic acid, butyric acid, isobutyric acid, imidinoacetic acid, malic acid, isethion Organic acids such as acid, citraconic acid, adipic acid, itaconic acid, crotonic acid, oxalic acid, salicylic acid, gluconic acid, gallic acid, sorbic acid, gluconic acid, and p-oxybenzoic acid can be exemplified.

The use of a basic compound or a salt in place of such an inorganic or organic acid not only does not improve the absorption characteristics under pressure at all, but also leaves the residual amount of the crosslinking agent used for crosslinking near the surface. Is low. Further, the addition of an inorganic acid or an organic acid to the monomer has no effect on the achievement of the object of the present invention.

As the inorganic acid and the organic acid, from the viewpoint of the effect, an inorganic acid and an organic acid having an acid dissociation index (reciprocal value of the acid dissociation constant, pKa value) of 2.0 to 4.0 in an aqueous solution. It is preferable to use, for example, one or more selected from the group consisting of diphosphate, tripolyphosphate, phosphoric acid, citric acid, succinic acid, and lactic acid. Further, among the additives having an acid dissociation index (pKa value) of 2.0 to 4.0, an organic acid is preferable, and an organic carboxylic acid is more preferable. Further, the molecular weight of the inorganic acid and the organic acid is preferably 30 to 1000, more preferably 60 to 50.
It is in the range of 0.

Further, among the saturated organic acids and the saturated inorganic acids containing the elements of the third period of the periodic table, saturated organic carboxylic acids such as citric acid, succinic acid, and lactic acid; Organic carboxylic acids are more preferred. The acid dissociation index is described, for example, in the Chemical Handbook Basic Edition, Revised Third Edition, pages II-337 to 342, published by Maruzen Co., Ltd., and in the literature indicated in the footnote thereof.

In the present invention, the amount of the additive depends on the cross-linking agent used for cross-linking near the surface.
It is 5 to 8 parts by weight, preferably 0.01 to 5 parts by weight.
When the amount used exceeds 8 parts by weight, it is not only uneconomical, but also tends to be excessive in achieving the optimum crosslinking effect aimed at by the present invention, and a small amount of less than 0.005 part by weight. With the amount, it is difficult to obtain the effect of improving the absorption characteristics under pressure and the effect of reducing the remaining amount of the crosslinking agent. Preferably it is 0.1 to 3 parts by weight.

The crosslinking agent which can be used in the present invention and which can react with a carboxyl group is, for example, a known crosslinking agent used for the purpose. Examples thereof include ethylene glycol, diethylene glycol, propylene glycol and triethylene glycol. , Tetraethylene glycol, polyethylene glycol, propylene glycol, 1,3
-Propanediol, dipropylene glycol, 2,
2,4-trimethyl-1,3-pentadiol, polypropylene glycol, glycerin, polyglycerin,
-Butene-1,4-diol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-
Polyhydric alcohol compounds such as cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethyleneoxypropylene block copolymer, pentaerythritol, sorbitol; ethylene glycol diglycidyl ether, polyethylene diglycidyl ether, glycerol poly Epoxy compounds such as glycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, glycidol, γ-glycidoxypropyltrimethoxysilane; ethylenediamine, diethylenetriamine, triethylene Ethylenetetramine, tetraethylenepentamine, penta Chirenhekisamin, polyvalent amine compounds such as polyamides polyamines polyethyleneimine, as well as condensates of these polyvalent amine and haloepoxy compounds; 2,4-tolylene diisocyanate, a polyhydric isocyanate compound such as hexamethylene diisocyanate;
Polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltrimethoxysilane; 1,3-dioxolan-2-one,
-Methyl-1,3-dioxolan-2-one, 4,5-
Dimethyl-1,3-dioxolan-2-one, 4,4-
Dimethyl-1,3-dioxolan-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-1,3-dioxane-
Alkylene carbonate compounds such as 2-one and 1,3-dioxopan-2-one; haloepoxy compounds such as epichlorohydrin, epibromhydrin, α-methylepichlorohydrin; zinc, calcium, magnesium, aluminum and iron , Hydroxides such as zirconium, and polyvalent metal compounds such as chlorides; And among these crosslinking agents, one or more selected from a polyhydric alcohol, an epoxy compound, an alkylene carbonate compound, a polyvalent amine compound and an epihalohydrin condensate thereof are preferable.

Among the above crosslinking agents, it is particularly preferable to use an epoxy compound as an essential component of the crosslinking agent in order to achieve the effects of the present invention to the maximum. Further, in this case, the effect of the present invention tends to be further remarkable by using a combination of two or more kinds of crosslinking agents having different solubility parameter ranges as disclosed in Japanese Patent Application No. 4-336392. Sometimes. This Japanese Patent Application Hei 4-3363
As the first crosslinking agent in No. 92, it can react with a carboxyl group with a solubility parameter (SP value) of 12.5 [(cal / cm ³ ) ^1/2 ] or more. The solubility parameter (SP value) is generally used as a factor indicating the polarity of a compound. In the present invention, the solubility parameter of a solvent described in Polymer Handbook, 3rd edition (published by WILEY INTERSCIENCE) VII-527 to 539 is used. The value of -δ [(cal / cm ³ ) ^1/2 ] shall be applied, and for the cross-linking agents not described in this table, those described in VII-525 in the Small formula of VII-524 of the same handbook. Δ [(cal / c)
m ³ ) ^1/2 ].

Such a solubility parameter (SP value) 1
Examples of the first crosslinking agent capable of reacting with a carboxyl group at 2.5 [(cal / cm ³ ) ^1/2 ] or more include ethylene glycol, propylene glycol, glycerin, polyglycerin, pentaerythritol, sorbitol, ethylene carbonate, and propylene carbonate. And the like, and one or more selected from these groups can be used. Preferably, the solubility parameter (SP value)
13.0 to 18.0 [(cal / cm ³ ) ^1/2 ].

Further, among these first crosslinking agents, the molecular weight is preferably 200 or less, more preferably 50 to 150.
Is used.

The second crosslinking agent can react with a carboxyl group with a solubility parameter (SP value) of less than 12.5 [(cal / cm ³ ) ^1/2 ]. Specifically, diethylene glycol, triethylene glycol,
Tetraethylene glycol, dipropylene glycol,
Tripropylene glycol, 1,3-butanediol,
1,4-butanediol, 1,5-pentanediol,
2,4-pentanediol, 1,6-hexanediol-
, 2,5-hexanediol, trimethylolpropane, diethanolamine, triethanolamine, ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene Glycol diglycidyl ether, ethylene diamine, diethylene triamine, triethylene tetramine, 2,4-tolylene diisocyanate, hexamethylene diisocyanate, 4,5-dimethyl-1,3-
Examples thereof include dioxan-2-one, epichlorohydrin, epibromohydrin and the like, and one or more kinds selected from these groups can be used. Preferably, the solubility parameter (SP value) is 9.5 or more and 12.0 or more.
[(Cal / cm ³ ) ^1/2 ]. Further, among these second crosslinking agents, preferably, the molecular weight is 350
Hereinafter, those having 150 to 300 are more preferably used. In selecting the first crosslinking agent and the second crosslinking agent, the difference in the solubility parameter (SP value) between the first crosslinking agent and the second crosslinking agent is 2 [(cal / cm ³ ) ^1/2 ] or more. And more preferably 3 [(cal / cm ³ ) ^1/2 ] or more.

According to the present invention, the distribution of the cross-linking agent can be optimized by the effect of the additive.
A water-absorbing agent exhibiting a high absorption capacity under pressure can be obtained, and the residual amount of the cross-linking agent can be greatly reduced. However, when a highly reactive substance such as an epoxy compound is used, the effect of reducing the residual amount is particularly large. A highly safe water-absorbing agent with excellent properties and optimal for application to sanitary materials can be efficiently obtained in process.

The amount of the crosslinking agent used in the present invention varies depending on the type of the crosslinking agent used, but is usually 0.00 parts by weight based on 100 parts by weight of the solid content of the water-absorbent resin of the present invention.
01 to 10 parts by weight, preferably 0.005 to 8 parts by weight, more preferably 0.01 to 5 parts by weight, and most preferably 0.01 to 2 parts by weight, provided that the amount is within this range. A water-absorbent resin having excellent absorption characteristics under pressure can be obtained.
When the amount used exceeds 10 parts by weight, not only becomes uneconomical, but also tends to be excessive in achieving an appropriate crosslinking effect, and the crosslinking agent tends to remain even when the additive of the present invention is used. Conversely, if the amount is less than 0.001 part by weight, it is difficult to obtain the effect of improving the absorption characteristics under pressure.

When the first crosslinking agent and the second crosslinking agent are used in combination as described in the above-mentioned Japanese Patent Application No. 4-336392, the amount of the first crosslinking agent used is 100% of the solid content of the water-absorbing resin. It is usually in the range of 0.001 to 10 parts by weight, preferably 0.01 to 8 parts by weight, more preferably 0.1 to 5 parts by weight, and the amount of the second crosslinking agent used is usually 0 to 10 parts by weight. .001
10 to 10 parts by weight, preferably 0.001 to 1 part by weight, more preferably 0.005 to 0.5 part by weight.

In the present invention, it is preferable to use water when mixing the water-absorbing resin with the crosslinking agent / additive. In the present invention, the amount of water used depends on the type, particle size and water content of the water-absorbing resin, but the solid content of the water-absorbing resin is 10%.
0.5 to 5 parts by weight, preferably 0.5 to 5 parts by weight, per 0 parts by weight
The range is 3 parts by weight. When the amount of water exceeds 5 parts by weight, not only the absorption characteristics under pressure are hardly improved, but also the crosslinking agent may easily remain even when the additive of the present invention is used. Conversely, if the amount is less than 0.5 part by weight, the effect of improving the absorption characteristics under pressure may not be obtained.

In the present invention, when mixing the water-absorbing resin with the crosslinking agent / additive, a hydrophilic organic solvent may be used. As the hydrophilic organic solvent used, methyl alcohol, ethyl alcohol, n-propyl alcohol, i
so-propyl alcohol, n-butyl alcohol, i
lower alcohols such as so-butyl alcohol and t-butyl alcohol; ketones such as acetone; dioxane;
Ethers such as alkoxy (poly) ethylene glycol and tetrahydrofuran; amides such as N, N-dimethylformamide; and sulfoxides such as dimethylsulfoxide. In the present invention, the amount of the hydrophilic organic solvent used is optimally different depending on the type and particle size of the water-absorbing resin.
0 to 10 parts by weight, preferably 0.1 to 100 parts by weight.
It is in the range of 1 to 5 parts by weight.

In the present invention, the water-absorbent resin, the crosslinking agent, and the additives may be mixed in a state where the water-absorbent resin is dispersed in an organic solvent such as cyclohexane and pentane.
In order to maximize the features of the present invention, the following (1)
The methods (1) to (4) can be preferably exemplified.

(1) A method in which a crosslinking agent containing water and / or a hydrophilic organic solvent is mixed in advance with an additive as required, and then the mixture is sprayed or dropped on a water-absorbent resin.

(2) A method in which an additive is mixed in advance with the water-absorbent resin, and then, if necessary, a crosslinking agent containing water and / or a hydrophilic organic solvent is sprayed or dropped and mixed.

(3) A method in which a crosslinking agent containing water and / or a hydrophilic organic solvent is directly sprayed or dropped on a water-absorbent resin as required, and then an additive is mixed.

(4) A method in which a crosslinking agent containing water and / or a hydrophilic organic solvent, if necessary, and an additive are sprayed or dropped on the water-absorbing resin simultaneously and in parallel with two nozzles or the like.

As described above, for mixing the crosslinking agent and the additives into the water-absorbing resin, it is preferable to mix them as a solution using water or a hydrophilic organic solvent. Furthermore, these (1)-
Among the methods (4), the method (1) in which the crosslinking agent and the additive are mixed in advance is preferable from the viewpoint of the water absorbing property and the effect of reducing the amount of the remaining crosslinking agent. When water is used for mixing, a water-insoluble fine particle powder and a surfactant may coexist.

The preferred mixing device used for the mixing is
It is necessary to be able to produce large mixing power to ensure uniform mixing. Examples of the mixing device that can be used in the present invention include a cylindrical mixer, a double-walled conical mixer, a high-speed stirring mixer, a V-shaped mixer, a ribbon mixer, and a screw-type mixer. Machine, fluidized furnace rotary desk mixer, air flow mixer, double-armed kneader, internal mixer,
Examples include a pulverizing type kneader, a rotary mixer and a screw type extruder.

In the present invention, after the water-absorbent resin is mixed with the crosslinking agent and the additive, preferably, the crosslinking agent and the additive are mixed in advance and then mixed with the water-absorbent resin. Preferably by further performing a heat treatment,
Crosslink near the surface.

When performing the heat treatment in the present invention, the treatment temperature is preferably from 80 to 300 ° C. When the heat treatment temperature is lower than 80 ° C., not only the heat treatment takes a long time to cause a decrease in productivity, but also uniform cross-linking is not achieved, and a resin having high water absorption under pressure which is the object of the present invention is obtained. Not only may not be obtained, but also the crosslinking agent tends to remain. Although depending on the type of the crosslinking agent used, the heat treatment temperature is more preferably 100 to 230 ° C, further preferably 160 to 220 ° C.
Range.

The heat treatment can be carried out using a conventional dryer or heating furnace, for example, a groove-type mixing dryer, a rotary dryer, a desk dryer, a fluidized-bed dryer, a gas-flow dryer, and the like. An infrared dryer etc. can be illustrated.

As described above, when the water-absorbing agent obtained by the production method of the present invention uses an epoxy compound as a crosslinking agent, for example, 0.005 to 8 parts by weight of the epoxy compound per 100 parts by weight of the water-absorbing resin having a carboxyl group Is 45 (g / g) or more under no pressure obtained by adding
The absorption capacity under pressure at 0 g / cm ² is 30 (ml /
g) or more, preferably 35 (ml / g) or more and the residual amount of the epoxy compound is 2 ppm or less, and preferably the novel water-absorbing agent in which the epoxy compound is not detected. More preferably, the water-absorbing agent of the present invention contains 0.01 to 5% by weight of a saturated organic acid, more preferably a saturated organic acid containing a hydroxy group (based on the water-absorbing resin), from the viewpoint of safety and absorption characteristics.
Including. The water-absorbing agent of the present invention preferably has an average particle size of 2.
The resin having a particle diameter of from 00 to 600 μm and less than 150 μm has a shape of 10% by weight or less. The water-absorbing agent obtained in the present invention exhibits excellent absorption characteristics both under no pressure and under pressure, and the amount of the highly reactive crosslinking agent remaining on the resin surface is greatly reduced. Although the cause is not clear, the water-absorbent resin is mixed with the cross-linking agent in the presence of the additive of the present invention and heat-treated, so that the cross-linking in the vicinity of the surface of the water-absorbent resin is not caused without adversely affecting the cross-linking agent itself. It is speculated that the distribution of the agent is optimized and a uniform cross-linking reaction occurs promptly, so that the residual amount of the cross-linking agent is significantly reduced, and that a cross-link having a uniform density gradient is formed near the particle surface. You. In addition, if the shape of the water-absorbent resin is the irregularly crushed shape as described above, in addition to the above-described excellent absorption characteristics, the safety is high, and the liquid diffusibility is good, and Movement of
It can be a water absorbing agent that is less likely to fall off and is not suitable for conventionally known water absorbing resins, and is particularly suitable for sanitary materials.

Further, with respect to the water absorbing agent obtained in the present invention,
Add new compounds such as deodorants, fragrances, inorganic powders, foaming agents, pigments, dyes, hydrophilic short fibers, plasticizers, binders, surfactants, fertilizers, oxidizing agents, reducing agents, water, and salts. Functions may be added. Further, the water absorbing agent of the present invention may be further granulated or molded.

Such a compound is disclosed in US Pat.
No. 367, No. 4190563, No. 4500670,
No. 4693713, No. 482486, No. 4863
No. 989, No. 4929717, No. 4959060,
No. 4972019, No. 5078992, No. 5229
No. 488 and European Patent Nos. 0009977 and 04930.
No. 11 and the like. Granulation methods are exemplified in U.S. Pat. No. 4,734,478 and European Patent Nos. 0,450,922 and 4,800,313.

[0059]

According to the method of the present invention, it is possible to easily produce a water-absorbing agent which exhibits a high absorption capacity under pressure and has excellent safety and is suitable for sanitary materials.

The water-absorbing agent of the present invention has a high absorption capacity under no pressure and an extremely excellent absorption capacity under pressure, so that a highly reactive cross-linking agent does not remain on the resin surface, and furthermore the liquid diffusion It is particularly suitable for use in sanitary materials such as disposable diapers and sanitary napkins because it has characteristics such as properties, difficulty in moving from pulp and falling off.

[0061]

EXAMPLES The present invention will be described in detail with reference to the following examples, but the scope of the present invention is not limited to these examples.

Various properties of the water absorbing agent were measured by the following methods.

(A) Absorption capacity under no pressure 0.2 g of a water absorbing agent was added to a non-woven tea bag type bag (40 *
150 mm) and immersed in a 0.9% by weight aqueous sodium chloride solution (physiological saline). After 60 minutes, pull up the tea bag type bag, and after draining for a certain time,
Weight of tea bag type bag W ₁ Was measured. The same operation was performed without using a water absorbing agent, and the blank weight W _{0 at} that time was used.
Was calculated, and the absorption capacity was calculated by the following (Equation 1).

[0064]

(Equation 1)

(B) Absorption Capacity Under Pressure The absorption capacity under pressure is measured using the same apparatus as shown in FIG. 1 of Japanese Patent Application No. 4-336392. That is, the upper port 2 of the burette 1 is plugged, and the measuring table 4 and the air port 5 are set at the same height. 70 m diameter at the center of the measuring table 4
The filter paper 7 is placed on the glass filter 6 of m. Diameter 55
The non-woven fabric 8 is fixed to the lower end of the support cylinder 10 mm in diameter, and 0.76 g of a water absorbing agent is evenly sprayed on the non-woven fabric 8 to obtain an additional 20 g.
A load of 9 / cm ² is applied. The nonwoven fabric, the water-absorbing agent, and the load were placed on the filter paper 7 on the glass filter 6 together with the supporting cylinder, and the value (Aml) of a 0.9% by weight aqueous sodium chloride solution (physiological saline) absorbed over 30 minutes was measured. ,
The absorption capacity under pressure (ml / g) was determined from the following (Equation 2).

[0066]

(Equation 2)

(C) Amount of residual surface cross-linking agent (for epoxy compound) 2.0 g of water-absorbing agent was added to 100 ml of beaker, and 2 ml of a composition solution of methanol / water = 2/1% by weight was added. ,
Cover and leave for 1 hour. 5 ml of methanol is added to the above bead.
The mixture was filtered, and 1.0 g of the filtrate was placed in a 50 ml eggplant-shaped flask.
Add 5 ml. An air-cooled tube is attached to the eggplant flask, and the mixture is heated in a boiling water bath for 30 minutes. The reaction solution was filtered using filter paper, and the filtrate was subjected to high performance liquid chromatography.
Was analyzed.

On the other hand, a similar operation was carried out by adding a known amount of a crosslinking agent without using a water absorbing agent. The obtained calibration curve was used as an external standard, and the residual amount in the water absorbing agent was taken into account in consideration of the dilution ratio of the filtrate. The amount (ppm) of the surface crosslinking agent was determined.

(Reference Example) 1.58 g of N, N'-methylenebisacrylamide as an internal crosslinking agent was dissolved in 5500 g of an aqueous solution of sodium acrylate having a neutralization ratio of 75 mol% (monomer concentration: 33%). After degassing with nitrogen gas for 30 minutes, the mixture was supplied to a reactor covered with a stainless steel double-armed kneader having an inner volume of 10 L and having two sigma-type blades and keeping the monomer at a temperature of 30 ° C. The reaction system was further purged with nitrogen. Then, while rotating the wings, 2.4 g of ammonium persulfate and 0.12 g of l-ascorbic acid
When 1 minute was added, polymerization started 1 minute later, and 16 minutes later, the peak temperature in the reaction system reached 83 ° C., and the hydrogel polymer was subdivided into a diameter of about 5 mm. Thereafter, stirring was further continued, and the hydrogel polymer was taken out 60 minutes after the polymerization was started.

The granulated product of the obtained hydrogel polymer was treated with 5
It was spread on a 0-mesh wire net and dried with hot air at 150 ° C. for 90 minutes. The dried product was pulverized using a vibrating mill, further classified with a 20 mesh, and had an average particle diameter of 360 μm and 150 μm.
An amorphous crushed water-absorbent resin (A) having a resin having a particle diameter of less than 5% by weight and a water content of 6% by weight was obtained. (Example 1) Water absorbent resin (A) 100 obtained in Reference Example
Parts by weight, 0.05 parts of ethylene glycol diglycidyl ether as a crosslinking agent and lactic acid (pKa = 3.6) as an additive.
6) A composition liquid consisting of 0.5 part and 3 parts of water was mixed, and the resulting mixture was heated at 120 ° C. for 40 minutes. Table 1 shows the performance evaluation results of the obtained water absorbing agent (1).

(Example 2) 100 parts of the water-absorbent resin (A) obtained in the reference example was mixed with glycerin 0.5 as a first crosslinking agent.
Parts, 0.05 parts of ethylene glycol diglycidyl ether as a second crosslinking agent, and citric acid (pKa =
2.87) A composition solution consisting of 0.5 part, 3 parts of water and 1 part of isopropyl alcohol was mixed, and the resulting mixture was heated at 200 ° C. for 40 minutes. Table 1 shows the performance evaluation results of the obtained water absorbing agent (2).

Example 3 A water-absorbing agent (3) was obtained in the same manner as in Example 2, except that the additive was changed to 0.2 parts of phosphoric acid (pKa = 2.15). Table 1 shows the performance evaluation results of the water absorbing agent (3).

(Example 4) 100 parts of the water-absorbent resin (A) obtained in the reference example was mixed with 1 part of ethylene carbonate as a first crosslinking agent and glycerol polyglycidyl ether as a second crosslinking agent. 0.05 part, succinic acid (pK
a = 4.00) A composition liquid consisting of 0.2 part, 2 parts of water and 3 parts of isopropyl alcohol was mixed, and the resulting mixture was heated at 190 ° C. for 60 minutes. Table 1 shows the performance evaluation results of the obtained water absorbing agent (4).

Comparative Example 1 A comparative water absorbing agent (1) was obtained in the same manner as in Example 1, except that lactic acid was not used as an additive. Table 1 shows the performance evaluation results of the comparative water absorbing agent (1).

Comparative Example 2 A comparative water-absorbing agent (2) was obtained in the same manner as in Example 1 except that lactic acid as an additive was changed to 0.5 part of sodium lactate. Table 1 shows the performance evaluation results of the comparative water absorbing agent (2).

Comparative Example 3 A comparative water-absorbing agent (3) was obtained in the same manner as in Example 2 except that citric acid as an additive was changed to 0.5 part of sodium citrate.
Table 1 shows the performance evaluation results of the comparative water absorbing agent (3).

Comparative Example 4 A comparative water-absorbing agent (4) was obtained in the same manner as in Example 2, except that citric acid as an additive was changed to 0.5 part of monosodium succinate. Table 1 shows the performance evaluation results of the comparative water absorbing agent (4).

[0078]

[Table 1]

[Brief description of the drawings]

FIG. 1 is an apparatus for measuring absorption capacity under pressure used in the present invention. (Explanation of reference numerals) 1 ... burette 2 ... upper end of burette 3 ... stopper 4 ... measuring table 5 ... air intake 6 ... glass filter 7 ... filter paper 8 ...・ Non-woven fabric 9 ・ ・ ・ Load 10 ・ ・ ・ Support cylinder

Continuation of the front page (72) Inventor Yoshihiko Masuda 992, Nishioki, Okihama-shi, Aboshi-ku, Himeji-shi, Hyogo Nippon Shokubai Polymer Research Institute Co., Ltd. (56) References JP-A-4-106108 (JP, A) 4-292003 (JP, A) JP-A-5-230382 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C08F 8/00-8/50 C08J 3/24

Claims (17)

(57) [Claims] 1. A water-absorbing resin having a carboxyl group,
A method for producing a water-absorbing agent, comprising mixing at least one additive selected from the group consisting of a saturated inorganic acid and an organic acid soluble in an aqueous liquid and a crosslinking agent capable of reacting with a carboxyl group. 2. The method according to claim 1, wherein the additive and the crosslinking agent are mixed with the water-absorbent resin and then heat-treated at 100 to 230 ° C. 3. After preliminarily mixing the additive and the crosslinking agent,
The production method according to claim 1, wherein the water-absorbent resin is mixed. 4. The method according to claim 1, wherein the additive is at least one selected from saturated organic carboxylic acids and saturated inorganic acids containing an element of the third period of the periodic table. 5. An additive having an acid dissociation index (pKa value) of 2.
2. The method according to claim 1, wherein the number is from 0 to 4.0. 6. The method according to claim 4, wherein the additive is a saturated organic carboxylic acid. 7. The method according to claim 6, wherein the saturated organic carboxylic acid is at least one compound selected from the group consisting of citric acid, succinic acid, and lactic acid. 8. The water-absorbing resin having a carboxyl group,
The method according to claim 1, which is obtained by polymerizing a hydrophilic monomer mainly containing acrylic acid and / or a salt thereof. 9. The method according to claim 1, wherein the crosslinking agent capable of reacting with a carboxyl group is an epoxy compound. 10. The method according to claim 1, wherein 0.01 to 5 parts by weight of water is used based on 100 parts by weight of the water-absorbent resin during mixing. 11. The method according to claim 1, wherein the water-absorbent resin is amorphous and crushed, and the resin having an average particle diameter of 200 to 600 μm and a particle diameter of less than 150 μm is 10% by weight or less. 12. The method according to claim 1, wherein the water content of the water-absorbent resin is 1 to 10%. 13. The method according to claim 1, wherein the additive is used in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the water absorbent resin. 14. A water-absorbing resin 1 having a carboxyl group
The absorption capacity under no pressure obtained by adding 0.005 to 8 parts by weight of the epoxy compound per 00 parts by weight is 45 (g).
/ G) or more, the absorption capacity under pressure at 20 g / cm ² is 30 (ml / g) or more, and the residual amount of the epoxy compound is 2
A water-absorbing agent which is not more than ppm. 15. An average particle size of 200 to 600 μm and 15
The water-absorbing agent according to claim 14, wherein the resin having a particle size of less than 0 µm is 10% by weight or less. 16. The water-absorbing agent according to claim 14, wherein the absorption capacity under pressure at 20 g / cm ² is 35 (ml / g) or more and the epoxy compound is not detected. 17. Hydroxyl group-containing saturated organic acid 0.0
The water-absorbing agent according to claim 14, comprising 1 to 5% by weight (based on a water-absorbing resin).