JPH07242709A - Preparation of water-absorbent resin - Google Patents

Abstract

PURPOSE:To easily and inexpensively prepare a water-absorbent resin having a high water absorption ratio and a high water absorption capacity even under an applied pressure. CONSTITUTION:A water-absorbent carboxylated resin powder in a heated state is reacted with a cross-linking agent by using a gas stream to prepare a water- absorbent resin with improved properties. During the reaction of the surfaces of the resin particles, formation of agglomerates which inhibit the reaction and are present among the particles is suppressed to enable the resin having a high water absorption ratio even under an applied pressure to be prepared while keeping the high water absorption capacity thereof.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a water absorbent resin having excellent properties by subjecting it to a surface treatment by a specific method. More specifically, the present invention relates to a method capable of easily and inexpensively producing a water absorbent resin having a high water absorption capacity and having a high water absorbing capacity even under pressure, and the characteristics obtained by the method of the present invention are improved. The water-absorbent resin is widely used as sanitary products, civil engineering chemicals, and agricultural chemicals.

[0002]

2. Description of the Related Art In recent years, water absorbent resins have been used for sanitary napkins,
It is used as a sanitary agent such as paper diapers and breast milk pads, an agricultural agent such as a water retention agent for agricultural and horticultural purposes, and a civil engineering agent such as a sludge solidifying agent and drilling mud. Specific examples of such a water absorbent resin include, for example, a hydrolyzate of a starch-acrylonitrile graft copolymer, a neutralized product of a starch-acrylic acid graft copolymer, a polyvinyl alcohol cross-linked polymer, and a vinyl acetate-acryl. Hydrolyzates of acid ester copolymers, crosslinked products thereof, polyacrylic acid salt-based crosslinked products, maleic anhydride-based copolymer crosslinked products, and the like are known.

Among the applications of the water-absorbent resin, the one that is widely used at present is the sanitary paper diapers and the like.
In this application, the water-absorbent resin is expected to have a water absorption capacity to absorb body fluid even under pressure, a high water absorption capacity to absorb a large amount of the body fluid, and a water retention capacity to prevent the absorbed body fluid from reversing. Is mentioned. However, the water absorption capacity and the water absorption capacity under pressure have a contradictory relationship.If the water absorption capacity under pressure is improved, the water absorption capacity decreases, and conversely, if the water absorption capacity is increased, the water absorption capacity under pressure decreases. It is in the situation of doing. Also, the water retention capacity, unless both physical properties of water absorption capacity and water absorption capacity under pressure are high,
It is a general tendency that the performance cannot be sufficiently exhibited.

As a method for maintaining the water absorption capacity and improving the water absorption capacity under pressure, there is a method in which only the vicinity of the surface of the water absorbent resin having a high water absorption capacity is uniformly crosslinked, and a difference in the crosslinking gradient between the surface and the inside is provided. Are known. For example, a water-absorbent resin is dispersed in a hydrophilic organic solvent or in water and a hydrophilic organic solvent,
A method of crosslinking with a crosslinking agent (JP-A-57-44627, JP-A-58-42602) or a method of directly mixing a water-absorbing resin powder with a crosslinking agent or an aqueous solution of the crosslinking agent, and subjecting the surface to heat treatment to crosslink the surface. Method (JP-A-58-18023)
3, JP-A-59-62665, JP-A-61-4624
1) etc. are known.

However, in the above method using a hydrophilic organic solvent or a mixture thereof with water as a dispersion medium, there is a high possibility that the cross-linking agent is certainly dissolved in the dispersion medium to uniformly cross-link the surface of the water-absorbent resin. However, the cross-linking agent may penetrate into the water-absorbent resin and the water absorption capacity may be excessively reduced, which may reduce the water-retaining ability, which is not always satisfactory. In addition, there is a problem that there is a high risk of causing an explosion or a fire when heating in the reaction step and a drying step for removing the organic solvent or water, and there is a problem regarding safety such as environmental hygiene due to waste gas. Furthermore, it is not preferable from the economical point of view that an expensive organic solvent is used. Further, the latter method of directly mixing a cross-linking agent with the water-absorbent resin can be industrially carried out inexpensively, but the problem of increasing the amount of addition to the water-absorbent resin with the cross-linking agent alone, and an effective cross-linking agent Has a problem in that it is difficult to uniformly mix a large amount of viscous liquid and it is impossible to sufficiently crosslink the vicinity of the resin surface. Furthermore, in the method of spraying on the resin surface as an aqueous solution of the crosslinking agent, the crosslinking agent easily covers the entire surface of the water absorbent resin together with water, while the aqueous solution of the crosslinking agent easily penetrates to the inside of the water absorbent resin. However, there is a risk that the difference in crosslinking gradient between the surface of the water-absorbent resin and the inside may be impaired, and it is hard to say that this is the best method for exhibiting sufficient water-absorbing performance. Further, at the same time, agglomeration occurs between resin particles that have been swollen by water absorption, and this aggregate often impedes uniform mixing, and it is difficult to say that it is easy to uniformly crosslink the resin surface. is there. In addition, the water-absorbent resin in the form of agglomerates formed during this crosslinking reaction has an aqueous solution of the crosslinking agent incorporated between the resins, so that a concentration gradient of the crosslinking agent occurs on the surface of the particles, and the extremely low water absorption capacity due to crosslinking is obtained. Contrary to the part (1), cross-linking is insufficient, and a part having a low water absorption capacity under pressure is likely to be mixed, and it is difficult to obtain a material having sufficient water absorption capacity. Furthermore, these agglomerates almost always have a particle size of more than 1000 μm, and it is difficult to say that such particles have a sufficient water absorption rate, and such agglomerates remain in the product. Doing so is not preferable in terms of water absorption capacity and water absorption speed. In order to solve this problem, these agglomerates are classified and removed, or the classified and removed ones are re-crushed and returned to the product, but those operations are economical and workable. That is a big problem.

On the other hand, a method in which an aqueous solution of a cross-linking agent is mixed with a water-absorbent resin powder in the presence of an inert inorganic powder or sucrose fatty acid ester to carry out a reaction (JP-A-60-16395).
6, JP-A-60-255814, JP-A-3-2820
Although 3) is known, this method certainly suppresses agglomeration between resin particles due to the effect of the inorganic powder and the sucrose fatty acid ester, and it is highly possible that the aqueous solution of the crosslinking agent is uniformly mixed. In this case, on the contrary, since the aqueous solution of the cross-linking agent is absorbed by the inorganic powder existing before the surface treatment, the permeation of the aqueous solution of the cross-linking agent into the water-absorbent resin is not sufficiently controlled. Since it is not possible to obtain the difference in cross-linking gradient between the inside and the inside, it is difficult to exert sufficient water absorption performance. In addition, the use of an amount of inorganic powder required to suppress aggregation between resin particles causes a drawback that dust is likely to be generated from the water absorbent resin obtained, which is not preferable in terms of environmental hygiene. Further, although the use of sucrose fatty acid ester is good in terms of dust generation, it is expensive in itself and is not preferable for industrial implementation from the economical point of view.

Further, the water absorbent resin is continuously dispersed in the air without using the inorganic powder, and the cross-linking agent aqueous solution is 500 μm.
A method of forming droplets of m or less and mixing them (Japanese Patent Laid-Open No. 3-8400
4) is known, but if the droplets have a fine particle size of less than 200 μm, there is a risk that the wind power generated when continuously dispersing the water-absorbent resin in the air may cause insufficient contact with the resin. Yes, especially when it is carried out on an industrial scale, a large amount of gas and power are required to disperse the resin powder, but in such a state, it is necessary to bring fine droplets into uniform contact with the water absorbent resin. Is almost impossible. Further, when the heating is performed under heating, the fine droplets evaporate water before reaching the resin surface, which makes it difficult to perform sufficient crosslinking. Further, in the case where the particle diameter of the droplet is in the range of 200 to 500 μm, agglomeration occurs between the resin particles depending on the shape of the water absorbent resin and the state at the time of adding the aqueous solution of the cross-linking agent, so that the object is sufficiently achieved. Is a difficult method.

[0008]

As described above, various methods for cross-linking the vicinity of the surface of the water-absorbent resin to improve the balanced relationship in a well-balanced manner have been tried, but all of them have the above-mentioned problems. Therefore, it is hard to say that this is the most suitable method for obtaining a water-absorbent resin exhibiting a sufficient improvement effect.

An object of the present invention is to improve the above problems and provide an effective method for obtaining a water-absorbent resin having a high water-absorption capacity even under pressure while maintaining a high water-absorption capacity. . Furthermore, the present invention reduces the aggregates between particles that were by-produced during the surface treatment of the water-absorbent resin, and can omit the treatment step of the aggregates, which is an industrially highly economical water-absorbent resin. It is to provide a manufacturing method. A further object of the present invention is that it can be effectively carried out even in the case where a hydrophilic organic solvent or a flow aid fine particle that causes dust is not used in the surface treatment step, has excellent industrial safety, and is environmentally friendly. Another object of the present invention is to provide a method for producing a water-absorbent resin having improved properties which is not problematic.

[0010]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the inventors of the present invention have conducted extensive studies, and by reacting a heated water-absorbent resin powder with a gas flow mixed with a cross-linking agent,
The present invention has been completed by finding that the above-mentioned problems can be solved and a desired water-absorbent resin can be obtained.

That is, the present invention is a method for producing a water-absorbent resin having improved properties, which is characterized in that a water-absorbent resin powder having a carboxyl group in a heated state is mixed with a cross-linking agent using a gas flow to react. It is about.

The present invention will be described in more detail below. The water-absorbent resin used in the present invention has a carboxyl group, for example, a hydrolyzate of starch-acrylonitrile graft copolymer, a neutralized product of starch-acrylic acid graft copolymer, acrylonitrile copolymer. Alternatively, a hydrolyzate of an acrylamide copolymer, a hydrolyzate of a vinyl acetate-acrylic acid ester copolymer, or a crosslinked product thereof, a polyacrylic acid salt-based crosslinked product, a maleic anhydride-based copolymer crosslinked product, etc. Is mentioned.

Among such water-absorbent resins, those which are preferable in the present invention are generally obtained by homopolymerizing monomers of unsaturated carboxylic acids or salts thereof, or two or more kinds thereof, or (co) polymerizing with other unsaturated monomers. Examples of the unsaturated carboxylic acid salt-based monomer include acrylic acid, methacrylic acid, itaconic acid, maleic acid or their metal salts such as sodium and potassium. Other unsaturated monomers include 2-acrylamido-2-methylpropanesulfonic acid or its metal salts such as sodium and potassium; (meth) acrylamide, acrylonitrile, (meth) acrylic acid such as methyl (meth) acrylate. Esters; Vinyl acetate; Alkylaminoalkyl (meth) acrylates such as diethylaminoethyl (meth) acrylate and quaternary ammonium salts obtained by quaternizing them with alkyl halides such as methyl chloride, or treated with hydrochloric acid, sulfuric acid, etc. Examples thereof include tertiary amine salts, etc., but from the viewpoint of water absorption capacity, those obtained by (co) polymerizing unsaturated carboxylic acid salt-based monomers are more preferable, and particularly preferably, the ratio of acrylic acid and acrylic acid salt. Is 0 to 80:10
It is a copolymer of 0 to 20 mol% of an acrylate monomer. Of course, polyacrylic acid obtained by polymerizing acrylic acid may be partially neutralized so that the above molar ratio is obtained.

Further, as the water absorbent resin applied to the present invention, a polymer having no cross-linking structure can be effectively used. For example, when polymerizing the above-mentioned monomer, two or more unsaturated groups or a reactive group are used. It is more preferable that a small amount of a crosslinking agent having a functional group is reacted to form a crosslinked product having a crosslinked structure. Examples of the cross-linking agent used during this polymerization include:
N, N'-methylenebis (meth) acrylamide, N-methylol (meth) acrylamide, (poly) ethylene glycol di
(Meth) acrylate, (poly) propylene glycol di
(Meth) acrylate, glycerin tri (meth) acrylate, glycerin di (meth) acrylate, (meth) acrylic acid polyvalent metal salt, trimethylolpropane tri (meth) acrylate, triallylamine, triallyl phosphate
And glycidyl (meth) acrylate, (poly) ethylene glycol diglycidyl ether, glycerin tri (di) glycidyl ether and the like. Moreover, you may use these crosslinking agents in mixture of 2 or more types. The amount of these cross-linking agents used is generally 0.001% by weight to 0.5% by weight, preferably 0.001% by weight, based on the total amount of monomers.
It is about 01 to 0.3% by weight.

The polymerization initiator used in the polymerization includes water-soluble radical initiators generally used, such as peroxides such as sodium persulfate and ammonium persulfate, hydrogen peroxide, t-butyl hydroperoxide and the like. Examples thereof include azo compounds such as hydroperoxide and 2,2′-azobis-2-amidinopropane hydrochloride. These polymerization initiators can be used as a mixture of two or more kinds, and further, redox initiation using a sulfite such as sodium sulfite, L-ascorbic acid, erythorbic acid and a reducing agent for the salt in combination. Agent systems can also be used. The amount of the polymerization initiator used is generally 0.005 to the monomer.
It is 0.5% by weight.

The water-absorbent resin used in the present invention may be any one prepared by various polymerization methods such as aqueous solution polymerization, reverse phase suspension polymerization, precipitation polymerization and bulk polymerization, and particles of the water-absorbent resin obtained. The shape is also particularly limited, for example, a bead shape obtained by reverse phase suspension polymerization, a flake shape obtained by drum-drying a highly water-containing water-absorbent gel, or an indefinite shape obtained by pulverizing a massive water-absorbent resin. However, those prepared by an aqueous solution polymerization or reverse phase suspension polymerization, which has good workability and is easy to manufacture, are preferable, and those obtained by an aqueous solution polymerization are particularly preferable in view of safety and economy.

When the water-absorbent resin is polymerized by aqueous solution polymerization, reverse-phase suspension polymerization, etc., the monomer concentration can be arbitrarily set within a range up to the saturated concentration of the monomer. Since it is economically advantageous because the water-containing gel (crosslinked polymer in a water-containing state) can be easily shredded and dehydrated, it is generally 20% by weight or more, preferably 25% by weight.
It is above, and more preferably 30% by weight or more. It should be noted that, when polymerization is carried out at normal pressure at a monomer concentration of 35% by weight or more, sudden polymerization is likely to occur, which is dangerous, and in the case of aqueous solution polymerization, the resulting hydrous gel also self-destructs due to vapor pressure, but the reason is unknown. When the polymerization is carried out at a monomer concentration of 35% by weight or more, the pressure of a pressure-resistant container is applied so as to prevent boiling of water in the monomer, because the performance of the obtained water-absorbent resin becomes uneven. Is preferred.

The water-containing gel of the water-absorbent resin obtained by the aqueous solution polymerization or the reverse phase suspension polymerization is optionally shredded and then dried by a usual method. As the shredding method, a cutting device capable of extruding a rubber-like elastic body can be used, for example,
A cutter type shredder, a chopper type shredder, and a kneader type shredder can be illustrated. As a drying method, an ordinary dryer or a heating furnace can be used, and for example, a hot air dryer, a fluidized dryer, a gas stream dryer, an infrared dryer, a dielectric heating dryer or the like is used.

The water-absorbent resin thus dried is pulverized and classified if necessary to obtain a water-absorbent resin powder applicable to the present invention. As the crushing method, a device for crushing coarsely crushed particles such as ordinary ores can be used, and for example, a direct pressure crusher such as a jaw crusher or a cone crusher, a disk crusher such as a rotary crusher or an attrition mill. Examples thereof include impact crushers such as hammer mills and pulverizers, roller type crushers such as ring roller mills, ring roll mills and ball roller mills, and cylinder type crushers such as ball mills and rod mills. These crushers may be used alone, but it is more economical and workable to use them in combination in order to reduce production loss and obtain a water-absorbent resin powder with a sharp particle size distribution. From the point of, it is preferable.

The water-absorbent resin powder to be crosslinked in the present invention is not particularly limited in its particle size, but it is usually preferable to have a particle size distribution containing 90% by weight or more of particles having a particle size of 100 to 1000 μm. ~ 850
Those having a particle size distribution containing 90% by weight or more of particles of μm are particularly preferable. 1 particle with a particle size less than 100 μm
When it is present in an amount of more than 0% by weight, the water used for dispersing the cross-linking agent is quickly absorbed on the surface of the water absorbent resin even if a gas flow is present, which promotes aggregation between the water absorbent resin particles. In some cases, the penetration of the surface cross-linking agent into the water absorbent resin also progresses and the sufficient effect may not be obtained. On the other hand, when particles having a particle diameter of 1000 μm or more are present in an amount of more than 10% by weight, the water absorption rate of the resulting water absorbent resin may be slowed down. Even if the water absorption capacity and the water absorption capacity under pressure are not a problem, the hygienic It is not preferable when used as a product.

The cross-linking agent to be reacted with the water-absorbent resin in the present invention (hereinafter referred to as a surface cross-linking agent in distinction from the cross-linking agent at the time of polymerization) is preferably one which reacts with the functional group of the water-absorbent resin and is hydrophilic. More preferably a water-soluble compound, the type is not particularly limited as long as it is a water-soluble compound, for example, (poly) ethylene glycol diglycidyl ether, (poly) glycerol polyglycidyl ether,
Polyglycerol glycidyl ethers such as diglycerol polyglycidyl ether, (poly) propylene glycol diglycidyl ether, pentaerythritol polyglycidyl ether; (poly) ethylene glycol, diethylene glycol, (poly) glycerin, diethanolamine, polyoxypropylene, pentaerythritol, Polyhydric alcohols such as sorbitol; haloepoxy compounds such as epichlorohydrin and α-methylchlorohydrin; polyvalent amines such as ethylenediamine, diethylenetriamine, polyethyleneimine, triethylenetetramine; aluminum chloride, magnesium chloride, aluminum sulfate, magnesium sulfate Polyvalent metal salts such as; polyvalent isocyanates such as 2,4-toluylene diisocyanate and hexamethylene diisocyanate; Glutaraldehyde can be exemplified by polyhydric aldehydes such as glyoxal,
Among them, polyhydric glycidyl ethers and polyhydric alcohols are particularly preferable.

The amount of these surface-crosslinking agents used varies depending on the type, but is usually 0.01 to 2 with respect to the water-absorbent resin.
0% by weight is suitable, preferably 0.05 to 10% by weight. If this amount is less than 0.01% by weight, the effect of the present invention cannot be sufficiently exhibited, and if it is used in excess of 20% by weight, the water absorption capacity may be significantly reduced.

Regarding the method for mixing the surface-crosslinking agent and the water-absorbent resin used in the present invention, in order to exert the effect of the present invention better, a mixing method in which the surface-crosslinking agent is directly added to the water-absorbent resin, On the contrary, it is preferable to adopt a mixing method in which the water absorbent resin is directly added to the surface crosslinking agent. For example, (A) a method in which a water-absorbing resin powder is quantitatively flowed down in a cocurrent or countercurrent state and brought into contact with a gas flow in which a surface crosslinking agent is quantitatively sprayed (B) In a container in which a gas flow exists Method of contacting a certain amount of surface-crosslinking agent with a certain amount of water-absorbent resin powder that is agitated or vibrated by a spray (C) A certain amount that is laid on a belt and vibrates in a gas flow Examples of the method include showering a certain amount of the surface-crosslinking agent on the water-absorbent resin powder from above. By these methods, the cross-linking agent is mixed with the water-absorbent resin powder together with the gas flow, but since the water-absorbent resin powder to be mixed is in a heated state, the surface treatment reaction is uniform even without using a hydrophilic organic solvent or the like. Will be carried out. The water-absorbent resin powder is preferably heated to 80 ° C. or higher at the temperature of the powder itself, and more preferably heated to 100 to 200 ° C. When the temperature of the water-absorbent resin powder is less than 80 ° C., when the surface-crosslinking agent or the surface-crosslinking agent aqueous solution is mixed as droplets with the water-absorbent resin powder,
Even if a gas flow exists, aggregation between the water-absorbent resin particles may proceed, and a smooth surface cross-linking reaction may be hindered. In the present invention, as described above, the surface treatment reaction is carried out uniformly without using a hydrophilic organic solvent, but of course the water-absorbing property dispersed in an inert hydrophilic organic solvent. It is also possible to employ a method of mixing the surface cross-linking agent with the resin together with the gas flow.

In the methods (A), (B), (C), etc., in order to carry out the mixing of the surface cross-linking agent and the water-absorbent resin and the surface cross-linking reaction more uniformly, the surface cross-linking agent is dropped. It is preferable to add it to the water-absorbent resin powder in the state. Further, in order to more effectively carry out the mixing and surface cross-linking reaction, the surface cross-linking agent is preferably used as a dilute solution, and the solvent used for diluting is one that dissolves the surface cross-linking agent, Any material can be used without limitation as long as it does not affect the performance of the water absorbent resin. Examples thereof include water, lower alcohols such as methyl alcohol and ethyl alcohol; ketones such as acetone and methyl ethyl ketone; ethers such as dioxane and tetrahydrofuran. These may be used alone or in combination of two or more, but in this case as well, considering the economical efficiency and safety, water alone should be used rather than using an expensive organic solvent that is easy to catch fire or explode. Is particularly preferably used.

The amount of water used when the surface-crosslinking agent is used as an aqueous solution varies depending on the amount of the surface-crosslinking agent used.
Usually, 1 to 30% by weight is suitable for the water absorbent resin, and preferably 5 to 30% by weight. If this amount is less than 1% by weight, the effect of dilution does not appear and
If it is used in an amount of more than 1% by weight, even if a gas flow is present when the surface cross-linking agent and the water-absorbent resin powder are mixed, the resin particles may agglomerate during the surface treatment reaction, or the surface cross-linking agent may become deep in the water-absorbent resin. There is a risk that the effect of the invention will not be exhibited due to penetration.

When the surface-crosslinking agent aqueous solution is formed into droplets, the average diameter thereof is preferably in the range of 200 to 600 μm, more preferably 300 to 500 μm. Average diameter of droplets is 200μ
When it is less than m, it is blown out to the wall of the mixer or the outside of the system due to the wind force generated when dispersed and mixed in the water-absorbent resin powder or the gas flow existing in the reaction system, or the reaction system is in a heated state. The solvent water evaporates before reaching the internal water-absorbent resin, which may adversely affect surface cross-linking, which is not preferable. If the average diameter of the droplets is 600 μm or more, depending on the particle size distribution of the water-absorbent resin powder, aggregation between resin particles may proceed, which is not preferable. Average diameter 200 ~
There are no particular restrictions on the method for generating droplets in the range of 600 μm as long as the liquid can be atomized, but normally,
A spray type or shower ring type is preferable.

In the present invention, the surface cross-linking agent is mixed with the water-absorbent resin powder by a gas flow as described above. The method will be described in more detail below. The method is not particularly limited as long as it can uniformly mix the surface-crosslinking agent solution and the water-absorbent resin without disturbing the mixing. (1) A method in which a water absorbent resin is charged into a fluidized dryer type surface reaction device, a gas flow is introduced from the lower part of the device, and a surface cross-linking agent sprayed from an arbitrary position is mixed with the water absorbent resin (2) mixer type A method of charging a water-absorbent resin into the surface reaction apparatus of and mixing the water-absorbent resin with agitation of the water-absorbent resin by directly blowing a gas flow and a surface-crosslinking agent into the water-absorbent resin by using a nozzle or the like (3) Surface-crosslinking Examples include a method in which the surface-crosslinking agent and the gas flow are simultaneously sprayed and mixed from the same direction onto the water-absorbent resin by using a two-fluid nozzle (mixing or double-headed nozzle) with the agent or the surface-crosslinking agent aqueous solution.

In the present invention, it is preferable to use the surface cross-linking agent as a solution, and to use a gas flow in the range of 0.1 to 10 Nm ³ / kg with respect to the solution. When the gas flow rate is less than 0.1 Nm ³ / kg with respect to the surface cross-linking agent solution, it becomes difficult to quickly evaporate the solvent in the reaction system, especially the water present in the surface treatment of the water-absorbent resin. It is not preferable because the agglomeration between the two will progress. Even if the agglomeration between the resin particles does not proceed, if the solvent evaporation rate in the reaction system is slow, the difference in the crosslinking gradient between the surface of the water-absorbent resin and the inside of the water-absorbent resin becomes small, which leads to a reduction in the water absorption capacity, which is not preferable. At a gas flow rate of 10 Nm ³ / kg or more with respect to the surface cross-linking agent solution, the mixture of the surface cross-linking agent and the water-absorbent resin is hindered, and further, the solvent in the reaction system existing when the surface treatment of the water-absorbent resin is performed. Evaporate before contacting the water-absorbent resin,
On the contrary, it inhibits the surface reaction, which is not preferable.

The gas flow used in the present invention is not particularly limited as long as it is an inert gas which does not inhibit the reaction between the surface cross-linking agent and the water absorbent resin, and examples thereof include nitrogen, carbon dioxide, helium, oxygen and air. Can be mentioned. Of these, air is particularly preferable in consideration of handling and economy.

The water-absorbent resin obtained in the present invention is easy to handle as a powder as it is, but in order to further adjust the powder fluidity, fine particles acting as a flow aid are treated with the fine particles to finish the surface treatment in the present invention. Mixing afterwards is also appropriately performed. The fine particles to be used are not particularly limited as long as they are substances that absorb moisture after addition and are not sticky, for example, polymethylmethacrylate, polyvinyl chloride, polystyrene, polyethylene, ABS resin, polycarbonate, polypropylene, and other organic compound-based substances. Examples thereof include fine particles, or inorganic fine particles such as silicon dioxide (silica), alumina, titanium oxide, magnesium oxide, aluminum silicate, and magnesium silicate. Furthermore, not only the powder fluidity of the water-absorbent resin, but also crystalline fine particles having a layered structure such as hydrotalcite that suppresses the elution of residual monomers in the resin can be mentioned.

The particle size of these fine particles is preferably 100 μm or less, more preferably 50 μm or less.
Further, the addition amount of these fine particles varies depending on the particle diameter and the specific surface area of the fine particles, but it is usually 0.01 to 10% by weight, preferably 0.1 to 5% by weight based on the obtained water absorbent resin. % Range. If the fine particles used are within the range, without decreasing the water absorption capacity and the water absorption capacity under pressure of the water absorbent resin obtained in the present invention, depending on the powder fluidity and the added fine particles, the water absorbent resin It is possible to obtain a water-absorbent resin having excellent safety that suppresses elution of residual monomers therein. If this amount exceeds 10% by weight, not only the water absorption capacity under pressure and the water absorption capacity are reduced, but also these fine particles become dust during the handling of the powder, as in the prior art. However, it becomes scattered, which is not preferable in terms of environmental hygiene.

[0032]

Although the reaction mechanism of the present invention is within the estimated range, by mixing the surface cross-linking agent with the heated water-absorbent resin powder together with the gas flow, in the conventional reaction, aggregates between resin particles are formed. After contact with the water-absorbent resin, the solvent in the reaction system that had contributed to the water vaporized quickly evaporated to a level at which it did not participate in the formation of the agglomerate, and at the same time suppressed the formation of the agglomerate. When the surface cross-linking agent comes into contact with the surface, the penetration of the surface cross-linking agent is limited only to the vicinity of the surface of the water-absorbent resin, and as a result, the cross-linking gradient between the surface and the inside is large without being adversely affected by the agglomerate, and a uniform surface is obtained. It is believed that crosslinking occurs.

[0033]

The present invention will be described in detail below with reference to examples, but the scope of the present invention is not limited to these examples. In the following examples, weight% will be referred to as "%" unless otherwise specified.

Example 1 A temperature-controllable pressure-resistant container having an internal capacity of 10 liters was used.
% Aqueous sodium acrylate solution 4574 g, 98% acrylic acid 425 g acrylate monomer aqueous solution and N, N'-methylenebisacrylamide 2.1 g are charged, and while blowing nitrogen gas, the monomer temperature is 20 ° C.
Adjusted to. When nitrogen substitution was completed, 1.0 g of ammonium persulfate and 0.2 g of L-ascorbic acid were added as polymerization initiators, and at the same time, the pressure in the container was increased to 5 kg / cm ² . Polymerization starts in 3 minutes after the addition of the polymerization initiator,
The temperature of the reaction system reached the maximum in minutes. Take out the obtained hydrogel polymer from the container, and first use a cutter to
The pieces were roughly cut into cm squares and further cut into about 5 mm diameter with a meat chopper. The shredded hydrogel is dried for 60 minutes with a hot air dryer (temperature 130 ° C.) to obtain a water-absorbent resin powder (A-1) of about 2 mm square, which is further crushed with a roll crusher to obtain a particle diameter. To obtain a water absorbent resin powder (B-1) having a particle size of 150 μm to 1000 μm. A cylindrical container 1 having an inner diameter of 20 cm and a length of 40 cm, which has a raw material inlet 3 and a hot air inlet 4 shown in FIG. 1, and a jacket 2 is attached around the cylinder so as to be heated. A fluidized-drying type apparatus 1 equipped with a pressure spray nozzle 5 for spraying a surface crosslinking agent aqueous solution and a stainless net 6 impervious to a water absorbent resin was used for the surface reaction. The water-absorbent resin powder (B-1) previously obtained in this device
Add 1000g, heat the jacket to 120 ℃,
Further, a small amount of hot air of 120 ° C. was blown from the hot air blowing port 4 in the lower part of the apparatus to heat the water-absorbent resin powder (B-1) while flowing it so as to vibrate. When the temperature of the water absorbent resin powder (B-1) reached 90 ° C, a small amount of hot air at 120 ° C was adjusted to 0.5 Nm ³ / kg with respect to the surface crosslinking agent aqueous solution (0.92 Nm at the flow rate). ³ / Hr), an aqueous solution consisting of 3 g of polyethylene glycol diglycidyl ether (n = 9) and 150 g of water was sprayed with the pressure spray nozzle 5 for 5 minutes to give the water-absorbent resin powder (B-1). Mixed. The surface cross-linking agent and the water absorbent resin powder (B-1) reacted instantly by spraying, and the reaction was completed within 3 minutes after the spraying was completed. afterwards,
In order to completely remove the residual water, the hot-air blowing port 4 at the bottom of the device was adjusted to a hot air of 120 ° C to an air volume of 5 Nm ³ / Hr, and the surface-crosslinking agent aqueous solution was sprayed onto the water-absorbent resin powder.
(B-1) was dried for about 10 minutes. This was taken out from the surface reaction apparatus 1 and cooled to room temperature to obtain a surface-treated water absorbent resin (1). The water absorbent resin powder (B-1) and the surface-treated water absorbent resin (1) are shown below (a).
The particle size distribution, (b) water absorption capacity of salt water, and (c) water absorption under pressure for 30 minutes were evaluated. The results are shown in Table 1.

(A) Particle size distribution: Opening 1000 μm (1
6 mesh), 850 μm (18 mesh), 500 μm (30
Mesh), 250 μm (60 mesh), and 100 μm
(149 mesh) 20 cm diameter JIS standard sieve and pan are piled up, 100 g of water-absorbent resin powder is placed on the uppermost sieve, vibrated with a classifier for 10 minutes, and the weight collected in each sieve is measured. And indicated by weight%.
(B) Absorption capacity of salt water: 0.5 g of water-absorbent resin powder was precisely weighed and prepared with a 200-mesh nylon filter cloth 10
It was placed in a tea bag of cm × 10 cm and immersed in 200 ml of 0.9% sodium chloride aqueous solution (physiological saline) for 60 minutes. After that, pull up the teabag bag, 10
After draining for 10 minutes, the tea bag bag was blank-corrected, and the weight of the swollen gel was measured and calculated by the formula 1.

[0036]

[Equation 1]

(C) Water absorption under pressure: The water absorption under pressure was measured using the apparatus shown in FIG. Burette with measuring tube 3
At 0, a small tube 32 with an inner diameter of 2 mm was inserted to the center of the buret to take in air as small bubbles as shown in the figure, and a Teflon tube 35 was connected to the tip 34 of the buret to obtain a diameter of 60 mm. The glass filter 36 of 1 was attached. The burette is filled with artificial urine (2% urea, 0.9% sodium chloride, 500 ppm calcium chloride and 1000 ppm magnesium sulfate), the burette upper part 37 is plugged, and the upper surface of the filter plate 39 of the glass filter and the air intake thin tube 32 are filled. Were fixed on the same plane. On the upper surface of the filter plate 39, filter paper, 0.2 g of water-absorbent resin powder, and further filter paper are placed, and a weight 41 having a bottom diameter of 60 mm and a pressure of 20 g / cm ² is placed at the same time, and after placing these The amount of water absorbed by the water-absorbent resin after 30 minutes was measured and calculated by the following formula 2.

[0038]

[Equation 2]

Example 2 In Example 1, 120 ° C. at the time of spraying the surface crosslinking agent aqueous solution
Water absorbent resin (2) was obtained by repeating the same operation as in Example 1 except that the hot air of was adjusted to 5 Nm ³ / kg with respect to the aqueous surface-crosslinking agent solution (flow rate was 9.18 Nm ³ / Hr). It was This was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Example 3 The same procedure as in Example 1 was repeated except that the temperature of the water-absorbent resin powder (B-1) at the time of spraying the aqueous surface-crosslinking agent solution was 110 ° C. 3) was obtained. This was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Example 4 In Example 1, 120 ° C. at the time of spraying the surface crosslinking agent aqueous solution
Except that the hot air of was adjusted to 0.05 Nm ³ / kg with respect to the surface cross-linking agent aqueous solution (flow rate was 0.092 Nm ³ / Hr).
The same operation as in Example 1 was repeated to obtain a water absorbent resin (4). This was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Example 5 In Example 1, 120 ° C. at the time of spraying the surface crosslinking agent aqueous solution
Water absorbent resin (5) was obtained by repeating the same procedure as in Example 1 except that the hot air of was adjusted to 15 Nm ³ / kg (27.5 Nm ³ / Hr at the flow rate) with respect to the surface crosslinking agent aqueous solution. It was This was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Example 6 Example 1 was repeated except that the temperature of the water absorbent resin powder (B-1) at the time of spraying the aqueous solution of the surface cross-linking agent was changed to 70 ° C.
The same operation was repeated to obtain a water absorbent resin (6). This was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 1 The same operation as in Example 1 was repeated except that the water absorbent resin powder (B-1) at the time of spraying the aqueous solution of the surface cross-linking agent was kept at room temperature. 1) was obtained. This was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

[0045]

[Table 1]

Example 7 A cylindrical container having an inner diameter of 20 cm and a length of 40 cm, which was made of stainless steel and had a raw material inlet 9 and a hot air inlet 10 shown in FIG. A jacket 8 is attached, and a pressure spray nozzle 11 for spraying the surface cross-linking agent aqueous solution, a gas blow-in pipe 12 above it, and a stainless net 13 that does not allow water-absorbent resin to pass through are attached to the fluid-drying type. Device 7 was used for surface treatment. 1000 g of the water-absorbent resin powder (B-1) obtained in Example 1 was put into this apparatus, and the jacket was set to 120.
It is heated to ℃, and further 12 from the hot air blowing port 10 at the bottom of the device.
A small amount of hot air at 0 ° C. was blown in to heat the water-absorbent resin powder (B-1) while flowing it so as to vibrate. When the temperature of the water absorbent resin powder (B-1) reaches 100 ° C., air at room temperature (at a flow rate of 0.5 Nm ³ / kg with respect to the surface cross-linking agent aqueous solution is supplied from the gas injection pipe 12 above the spray nozzle. 0.46N
m ³ / Hr), and an aqueous solution consisting of 3 g of polyethylene glycol diglycidyl ether (n = 9) and 150 g of water was sprayed with the pressure spray nozzle 5 of one fluid for 10 minutes, and the water-absorbent resin powder ( B-1). After spraying, the surface cross-linking agent and the water absorbent resin powder (B-1) react instantly,
The reaction was completed within 3 minutes after the end of spraying. After that, the air blown from the upper gas blowing pipe 12 was stopped, and in order to completely remove the residual moisture, the hot air blown at 120 ° C from the hot air blowing port 10 at the lower part of the device was adjusted to 5 Nm ³ / Hr. Absorbent resin powder sprayed with the surface cross-linking agent aqueous solution
(B-1) was dried for about 10 minutes. This water absorbent resin powder
(B-1) was taken out from the surface treatment apparatus 7 and cooled to room temperature to obtain a surface-treated water absorbent resin (7). This was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

Example 8 In Example 7, the air blown from the air blowing pipe 12 above the spray nozzle 11 at the time of spraying the surface cross-linking agent aqueous solution was adjusted to be 5 Nm ³ / kg with respect to the surface cross-linking agent aqueous solution (flow rate). in 4.6 nm ³ / Hr) and except were got repeated water-absorbent resin (8) the same procedure as in example 4. This was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

Example 9 The procedure of Example 7 was repeated except that the temperature of the water-absorbent resin powder (B-1) at the time of spraying the aqueous surface-crosslinking agent solution was changed to 120 ° C. 9) was obtained. This was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

Example 10 To the water-absorbent resin powder (7) obtained in Example 7, 0.5% of silica (Tokusil Tokuyama Soda Co., Ltd.) was added and mixed to adjust the fluidity of the powder, and the water-absorbent property was obtained. Resin (10) was obtained. This was subjected to the same test as in Example 1, and the results are shown in Table 2.

Example 11 In Example 7, the air blown from the air blowing pipe 12 above the spray nozzle 11 at the time of spraying the surface cross-linking agent aqueous solution was adjusted to 0.05 Nm ³ / kg with respect to the surface cross-linking agent aqueous solution. A water absorbent resin (11) was obtained by repeating the same operation as in Example 4 except that the adjustment (flow rate of 0.046 Nm ³ / Hr) was performed. This was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

Example 12 In Example 7, the air blown from the air blowing pipe 12 above the spray nozzle 11 at the time of spraying the aqueous solution of the surface cross-linking agent was adjusted to be 15 Nm ³ / kg with respect to the aqueous solution of the surface cross-linking agent ( The same operation as in Example 4 was repeated except that the flow rate was 13.8 Nm ³ / Hr) to obtain a water absorbent resin (12). This was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

Example 13 Example 4 was repeated except that the temperature of the water absorbent resin powder (B-1) at the time of spraying the aqueous solution of the surface cross-linking agent was changed to 60 ° C.
The same operation as above was repeated to obtain a water absorbent resin (13). This was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

Example 14 In Example 7, except that 0.5% of silica (Tokushiru Tokuyama Soda Co., Ltd.) was added and mixed to the water absorbent resin powder (B-1) before spray-mixing the surface crosslinking agent aqueous solution. The same operation as in Example 7 was repeated to obtain a water absorbent resin (14).
This was evaluated in the same manner as in Example 1, and the results are shown in Table 2.

[0054]

[Table 2]

Example 15 A cylindrical container having an inside diameter of 20 cm and a length of 40 cm, which was made of stainless steel and had a raw material inlet 16 and a hot air inlet 17 shown in FIG. A jacket 15 is attached, and a two-fluid mixing spray nozzle 18 (liquid pipe 19 and gas pipe 20) for spraying the surface cross-linking agent aqueous solution is attached inside, and a net 21 made of stainless steel that does not allow water-absorbing resin to pass is attached to the fluid-drying type. Device 14 was used for surface treatment. 1000 g of the water-absorbent resin powder (B-1) obtained in Example 1 was put into the device 14, the jacket was heated to 120 ° C., and a slight amount of hot air of 120 ° C. was blown from the hot air blowing port 17 at the bottom of the device. Blow, water-absorbent resin powder (B
-1) was heated while flowing so as to vibrate. When the temperature of the water-absorbent resin powder (B-1) reaches 100 ° C., the nitrogen is heated to 100 ° C. so that the ratio of the amount of nitrogen to the aqueous solution of the surface cross-linking agent becomes 0.5 Nm ³ / kg. 1
8 from the gas pipe 20, and the surface cross-linking agent aqueous solution consisting of 3 g of polyethylene glycol diglycidyl ether (n = 9) and 150 g of water was pumped from the liquid pipe 19 of the two-fluid mixing spray nozzle 18 in 10 minutes. The water-absorbent resin powder (B-1) was sprayed (at a flow rate of 0.92 Nm ³ / Hr).
After spraying, the surface cross-linking agent and the water-absorbent resin powder (B-1) reacted instantly, and the reaction was completed within 3 minutes after the spraying was completed. After that, the heating nitrogen blown from the two-fluid mixing spray nozzle 18 was stopped, and the hot air blown at 120 ° C. from the hot air blow-in port 17 at the bottom of the device was changed to 5 Nm ³ / Hr in order to completely remove the residual moisture. The water absorbent resin powder (B-1) prepared and sprayed with the surface crosslinking agent aqueous solution was dried for about 10 minutes. The water absorbent resin powder (B-1) was taken out from the surface reaction device 14 and cooled to room temperature to obtain a surface treated water absorbent resin (15). This is evaluated in the same manner as in Example 1,
The results are shown in Table 3.

Example 16 In Example 15, the amount of nitrogen was adjusted so that the ratio of the amount of nitrogen in the two-fluid mixing spray nozzle 18 at the time of spraying the aqueous solution of the surface cross-linking agent to the aqueous solution of the surface cross-linking agent was 5 Nm ³ / kg. 4.6N
The same operation as in Example 15 was repeated except that m ³ / Hr) was obtained to obtain a water absorbent resin (16), the same evaluation as in Example 1 was performed, and the results are shown in Table 3.

Example 17 The procedure of Example 15 was repeated except that the temperature of the water-absorbent resin powder (B-1) at the time of spraying the aqueous surface-crosslinking agent solution was changed to 90 ° C. 17) was obtained.
This was evaluated in the same manner as in Example 1, and the results are shown in Table 3.

Example 18 In Example 15, the nitrogen content was adjusted so that the ratio of the nitrogen content of the two-fluid mixing spray nozzle 18 at the time of spraying the surface cross-linking agent aqueous solution and the surface cross-linking agent aqueous solution was 0.05 Nm ³ / kg ( A water absorbent resin (18) was obtained by repeating the same operation as in Example 15 except that the flow rate was 0.046 Nm ³ / Hr). This is Example 1
The same evaluations as above were performed, and the results are shown in Table 3.

Example 19 In Example 15, the amount of nitrogen was adjusted so that the ratio of the amount of nitrogen in the two-fluid mixing spray nozzle 18 at the time of spraying the aqueous solution of the surface cross-linking agent to the aqueous solution of the surface cross-linking agent was 20 Nm ³ / kg. 1
8.6 nm ³ / Hr) and except were the obtained repetitive absorbent resin (19) in the same manner as in Example 15, the same evaluation as in Example 1. The results are shown in Table 3.

Example 20 The procedure of Example 15 was repeated except that the temperature of the water-absorbent resin powder (B-1) at the time of spraying the aqueous solution of the surface-crosslinking agent was changed to 60 ° C. 19) was obtained.
This was evaluated in the same manner as in Example 1, and the results are shown in Table 3.

[0061]

[Table 3]

[0062]

EFFECTS OF THE INVENTION According to the present invention, the water-absorbent resin powder before the surface reaction is reacted at a high temperature, so that the formation of aggregates between the resin particles which inhibit the reaction during the surface reaction of the water-absorbent resin is reduced, It is possible to obtain a water absorbent resin having a high water absorbing capacity even under pressure while maintaining a high water absorption capacity. By further adjusting the particle size so that particles having a particle size of less than 100 μm do not exist, a more excellent water absorbent resin can be obtained. Further, the method of the present invention can be carried out without using fine particles of a flow aid which causes a hydrophilic organic solvent or dust, so that it is industrially safe and has excellent economical efficiency, and there is no problem in terms of environmental hygiene. It provides a method. Therefore, the water-absorbent resin obtained by the present invention is not only a raw material for sanitary products such as high-performance and highly safe paper diapers and sanitary napkins, but also in the field of agriculture,
It can be widely used in the field of civil engineering, medical field, etc.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a surface crosslinking reaction apparatus 1 used in Examples 1 to 6 of the present invention.

FIG. 2 is a cross-sectional view of a surface cross-linking reaction device 7 used in Examples 7 to 14 of the present invention.

FIG. 3 is a sectional view of a surface cross-linking reaction device 14 used in Examples 15 to 20 of the present invention.

FIG. 4 is a cross-sectional view of a water absorption measuring device under pressure used in the present invention.

[Explanation of numbers]

 DESCRIPTION OF SYMBOLS 1 ... Surface reaction device 2 ... Jacket 3 ... Raw material input port 4 ... Hot air blowing port 5 ... Pressurizing spray nozzle 6 ... Stainless steel mesh 7 ... Surface reaction device 8 ... ..Jacket 9 ... Raw material inlet 10 ... Hot air inlet 11 ... Pressurized spray nozzle 12 ... Gas inlet 13 ... Stainless mesh 14 ... Surface reaction device 15 ... Jacket 16 ... Raw material injection port 17 ... Hot air blowing port 18 ... Two-fluid mixing spray nozzle 19 ... Two-fluid mixing spray-nozzle liquid pipe 20 ... Two-fluid mixing spray-nozzle gas Piping 21 ... Stainless steel net 30 ... Burette with measuring pipe 31 ... Burette measuring pipe 32 ... Air inlet thin tube 33 ... Air inlet 34 ... Burette tip 35 ... Teflon tube 36 ... Las filter 37 ... buret top 38 ... stopper 39 ... glass filter plate 40 ... filter paper, sample, the layer 41 ... weight consisting filter paper

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Aoyama 1-1 Funami-cho, Minato-ku, Nagoya-shi, Aichi Toagosei Chemical Industry Co., Ltd. Nagoya Research Institute (72) Inventor Minoru Okada Minato-shi, Aichi Prefecture 1 of Funami-cho, Ward-ward Toagosei Chemical Industry Co., Ltd. Nagoya Research Institute (72) Inventor, Hotaro Yasuda 23, 23, Showa-cho, Minato-ku, Nagoya, Aichi Prefecture Toagosei Chemical Industry Co., Ltd. Nagoya Plant (72 ) Inventor Yoshikazu Mori 23 Toagosei Synthetic Chemical Industry Co., Ltd.Nagoya factory at 17 Showa-cho, Minato-ku, Nagoya city, Aichi prefecture

Claims (3)

[Claims] 1. A method for producing a water-absorbent resin having improved properties, which comprises mixing a water-absorbent resin powder having a carboxyl group in a heated state with a cross-linking agent using a gas flow to cause a reaction. 2. The water absorbent resin powder according to claim 1, wherein the water absorbent resin powder is 80.degree.
A method for producing a water-absorbent resin, which is characterized by being heated as described above. 3. The crosslinker according to claim 1 is used as a solution, and the flow rate of the gas stream is 0.1 to 1 with respect to the crosslinker solution.
A method for producing a water-absorbent resin, which is in the range of 0 Nm ³ / kg.