JPH06192319A - Production of water-absorbing resin - Google Patents

Abstract

PURPOSE:To obtain a water-absorbing resin having low residual monomer content and useful for sanitary material, water-retaining agent for agricultural and horticultural use, etc., by incorporating water-absorbing resin powder with a substance reactive with the residual monomer of the resin, heating the mixture under specific condition while keeping the water-content and drying the product. CONSTITUTION:Sodium acrylate, acrylic acid and N,N'-methylenebisacrylamide are dissolved in deionized water, nitrogen gas is passed through the solution to expel dissolved oxygen, the solution is incorporated with sodium persulfate and L-ascorbic acid and polymerized by standing polymerization at 30 deg.C and the obtained water-containing gelatinous polymer is dried with hot air at 150 deg.C, crushed with a hammer mill and sieved with a 20-mesh sieve to obtain water- absorbing resin powder. The resin powder is incorporated with a substance (e.g. sulfurous acid) capable of reacting with the residual monomer in the resin powder, the water-content is adjusted to 25-55wt.%, the powder is heated at 120-200 deg.C for >=10min while keeping the water-content of the resin composition and the treated product is dried at <120 deg.C to obtain the objective water- absorbing resin.

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, and more specifically, to remarkably reduce the content of monomers remaining in the water-absorbent resin without adversely affecting various properties of the water-absorbent resin. The present invention relates to a manufacturing method of.

[0002]

2. Description of the Related Art In recent years, water absorbent resins have been used as a constituent material of sanitary materials that absorb body fluids such as sanitary cotton, disposable diapers, and other water-retaining agents for agriculture and horticulture.

Examples of such water absorbent resins include crosslinked polyacrylic acid partially neutralized products, hydrolyzates of starch-acrylonitrile graft polymers, partially neutralized products of starch-acrylic acid graft polymers, and vinyl acetate. Known are saponified products of acrylic acid ester copolymers, hydrolyzed products of acrylonitrile copolymers or acrylamide copolymers, and crosslinked products thereof.

However, in these water-absorbent resins, unpolymerized residual monomers are usually present. Especially when this water-absorbent resin is used in sanitary materials, food fields, etc., its content should be reduced as much as possible. Preferably.

Attempts to reduce residual monomers in polymers have hitherto been made in the field of water-soluble polymers such as polymer flocculants. As the method, as described in JP-A-56-103,207,
(1) A method of increasing or dividingly adding a polymerization initiator,
The method is roughly classified into three methods: (2) extraction method with a solvent or the like, and (3) method of leading the residual monomer to another derivative. These techniques can also be applied to reduce residual monomers in the water absorbent resin.

Examples of applying the method (1) to a water absorbent resin include JP-A-56-72005 and JP-A-1-
No. 24,808 has been reported, but when this method is applied to a water-absorbent resin, the molecular weight of the polymer is reduced as in the case of a water-soluble polymer, and the absorption capacity is reduced due to excessive self-crosslinking reaction. An increase in the amount of water-soluble components is observed, and the residual monomer is reduced, but the properties of the water-absorbent resin are deteriorated.

As the method (2), Japanese Patent Laid-Open No. 1-
Although Japanese Patent No. 292,003 has been reported, this method is also effective in reducing the residual monomer, but the use of a large amount of solvent and its recovery cost are very high, and it is difficult to say that this is a preferable method.

As a method for converting the residual monomer of (3) into another derivative, in the case of a water-soluble polymer, a method of adding amine or ammonia etc. (Japanese Patent Publication No. 33-2646) and bisulfite salt. , A method of adding sulfite, pyrosulfite, etc. (JP-A-56-103,207) and the like, and an example in which they are directly applied to a water-absorbent resin has been reported (JP-A-1- 62,317 and West German Patent Publication 3724709). However, according to the results obtained by the present inventors, JP-A-1-6
In the method described in Japanese Patent No. 2,317, the reduction level of the residual monomer is still high, and the German Patent Publication 37247.
The same is true for the method of JP-A-09-09, in which a substance capable of reacting with the residual monomer is added and mixed to obtain a water content of 20 to 90%.
The method is characterized in that it is dried at a high temperature after it is prepared as described above.
It was hard to say that it was still sufficiently reduced at the pm level.

[0009]

The present invention has been made in view of the above situation.

Accordingly, it is an object of the present invention to provide a new method for producing a water absorbent resin. Another object of the present invention is to provide a production method for significantly reducing the residual monomer content in the water absorbent resin without adversely affecting the water absorption characteristics.

[0011]

Means and Actions for Solving the Problems The present inventors have
As a result of earnestly studying to achieve the above-mentioned object, these various objectives were found to include (a) a water-absorbent resin powder to which a substance capable of reacting with a residual monomer in the water-absorbent resin powder was added.
To 55% by weight of water-absorbent resin composition (b) Water-absorbent resin treatment by heating the water-absorbent resin composition at a temperature of 120 to 200 ° C. for 10 minutes or more while maintaining the water content range of the water-absorbent resin composition. And (c) a step of drying the treated water-absorbent resin at a temperature of less than 120 ° C.

The present invention will be described in more detail below.

The water-absorbent resin powder used in the present invention is
It absorbs a large amount of water in water and swells to form a hydrated gel (hydrogel). The conventionally known ones mentioned above can be mentioned, but the necessity and effect of reducing residual monomers are considered. It is particularly preferable to use a powder of a crosslinked polyacrylic acid partially neutralized product.

As the form of the water-absorbent resin powder, a powdery form obtained by polymerization, drying and pulverization is preferable, and a form in which the surface portion and its vicinity are cross-linked is more preferable. And the residual monomer in the water absorbent resin powder is 1000 p
A water-absorbent resin powder of less than pm, preferably less than 500 ppm, more preferably less than 300 ppm can be suitably used.

In the present invention, a water-absorbent resin composition having a predetermined composition is first prepared in the step (a). The substance capable of reacting with the residual monomer in the water-absorbent resin used at that time is combined with the monomer. Nitrogen-containing compounds such as ammonia, ammonium salts, hydroxylamine and amino acids; sulfurous acid (salt),
Examples thereof include reducing substances such as hydrogen sulfite, phosphorous acid (salt), hypophosphorous acid (salt), and thiosulfuric acid (salt). Among these, reducing substances such as sulfurous acid (salt), hydrogen sulfite, pyrosulfurous acid (salt), and thiosulfuric acid (salt) are preferable in view of the residual monomer reduction rate.

In the step (a), the water content of the water absorbent resin must be in the range of 25 to 55% by weight.
The water content may be adjusted at any time, for example, before or after the addition of a substance capable of reacting with the residual monomer in the water-absorbent resin to the water-absorbent resin, or during addition. When producing a water-absorbent resin, it is often carried out by aqueous solution polymerization of a water-soluble monomer or reverse phase suspension polymerization, and these are generally carried out in the presence of water. The water-absorbent resin obtained by such a polymerization method has a water content of 2
When it is in the range of 5 to 55% by weight, it may be used as it is in the step (a). At that time, when a substance capable of reacting with the residual monomer in the water absorbent resin is added immediately before the completion of the polymerization reaction, it may be considered that the step (a) is included in the polymerization reaction, The water absorbent resin composition thus obtained may be subjected to the step (b) subsequently.
It goes without saying that the water-soluble monomer may be polymerized under conditions outside this range, and then water may be added or drying may be carried out in order to adjust to this range. Further, when the water absorbent resin is a dry powder, it is necessary to add water in order to adjust the water content to the range of 25 to 55% by weight. In this case, an aqueous liquid obtained by dissolving a substance capable of reacting with the residual monomer in water may be added, or water may be added after adding the substance capable of reacting with the residual monomer, or the substance may react with the residual monomer after adding water. Substances may be added.

If the water content is less than 25% by weight, the reduction rate of the residual monomer is low. On the other hand, if the water content is 55% by weight or more, the reduction rate of the residual monomer is low. It requires a large amount of energy and is economically disadvantageous. Preferably, it is 35 to 45% by weight.

The water content of 2 thus obtained in step (a)
The water absorbent resin composition of 5 to 55% by weight is heated at a temperature of 120 to 200 ° C. for 10 minutes or more in the step (b) while keeping the water content in this range to give a water absorbent resin treated product. In the invention, it is very important to heat while keeping the water content in the range of 25 to 55% by weight, and as a result, the residual monomer is significantly reduced. At this time, the temperature is 12
It is suitable to keep at a high temperature of 0 ° C. to 200 ° C., preferably 140 ° C. to 160 ° C. for 10 minutes or longer, preferably 30 minutes or longer, more preferably 1 hour or longer. The upper limit of the heating time is not particularly limited, but is usually 10 hours or less.
Even if the heat treatment is performed for more than 10 hours, no particular improvement in the effect is recognized, and rather the water absorbent resin may be deteriorated. It is preferably 5 hours or less, more preferably 3 hours or less. This treated water-absorbent resin is then subjected to 1 by the step (c).
Dry at temperatures below 20 ° C. According to the conventional method, the drying is generally performed at a high temperature, but when the drying is performed at a high temperature, the residual monomer amount once reduced is increased again, which is not preferable. There must be. 50 to 1 to enhance the effect
The temperature is preferably 00 ° C, more preferably 60 to 80 ° C.

The heating means used in the step (b) is not particularly limited as long as it is within the range of the above conditions, and typical heating methods include heating in a closed state, conduction heating in an open system, and The method of spraying and heating steam is illustrated.
Devices that can be used for heating in a closed state are, for example, autoclaves, (pressure resistant) groove type stirring heaters, (pressure resistant).
Rotary heater, (pressure resistant) disk heater, (pressure resistant) fluidized bed heater, (pressure resistant) air flow heater, (pressure resistant) infrared heater, (pressure resistant) hot air heater, (pressure resistant) micro A wave heater and the like can be mentioned.

As the apparatus used for drying in the step (c), a conventionally known apparatus can be used, but when the heating apparatus used in the step (b) is also used for the drying means depending on operating conditions, The heating device used in the step and the drying device used in the step (c) may be the same. And
Both the step (a) and the step (b) may be carried out continuously or batchwise.

In the present invention, in the step (b) of heating the water-absorbent resin composition, the treatment is performed at a temperature as high as possible.
In the step of drying the water-absorbent resin-treated product of (c), it is very important to perform it at a temperature as low as possible.

In the present invention, additives may be added for the purpose of achieving the object and effect of the present invention at a higher level.

Examples of such additives include inorganic and / or organic water-insoluble fine particle powders, surfactants and dispersion media such as organic solvents.

Examples of the water-insoluble particulate powder include silicon dioxide, titanium dioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, calcium phosphate, barium phosphate, clay, diatomaceous earth, zeolite, bentonite, kaolin, hydrotalcite, Inorganic water-insoluble fine particle powder such as activated clay, cellulose powder, pulp powder, ethyl cellulose, ethyl hydroxyethyl cellulose,
Cellulose acetate butyrate, modified starch, chitin, rayon, polyester, polyethylene, polypropylene, polyvinyl chloride, polystyrene, nylon, polymethylmethacrylate, melamine resin, melamine-benzoguanamine resin, activated carbon, organic leaves such as tea leaves The powdery particles and the like can be exemplified, and of these, one kind or two or more kinds can be used.

The particle diameter of these water-insoluble particulate powders is 1000 μm or less, more preferably 50 μm or less, and most preferably 10 μm or less. Among these water-insoluble fine particle powders, inorganic water-insoluble fine particle powders are more preferable, and among them, silicon dioxide, titanium dioxide, aluminum oxide, zeolite, kaolin and hydrotalcite are preferable. Further, inorganic powder having a deodorizing function such as sepiolite can be particularly preferably used in the present invention. Furthermore, among the organic water-insoluble particulate powder, pulp,
Cellulose powder, methylmethacrylate polymer having a particle size of 5 μm or less, such as polymethylmethacrylate powder and activated carbon are preferable. Among these, those partially hydrophobized can also be preferably used in the present invention.

As the surface active agent, an anionic surface active agent, a nonionic surface active agent, a cationic surface active agent,
Examples thereof include amphoteric surfactants and polymer surfactants, and of these, one kind or two or more kinds can be used.

Examples of the anionic surfactant include higher alcohol sulfate ester salt, alkylnaphthalene sulfonate, alkylpolyoxyethylene sulfate salt, dialkylsulfosuccinate, and the like, and nonionic surfactants include sorbitan fatty acid ester. Polyoxyethylene sorbitan fatty acid ester (poly) glycerin fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenol ether, polyoxyethylene acyl ester, sucrose fatty acid ester, etc. Quaternary ammonium salts, alkylamine salts, amphoteric surfactants such as alkyl betaine and lecithin, and polymer surfactants such as lipophilic carboxyl groups. Polymers and ethylene oxide having - propylene Lee oxide block polymers, etc. can be used respectively.

Of these surfactants, water-soluble and / or water-dispersible surfactants are preferred as those which do not impair the absorbability of the final product and are excellent in the effect of reducing the amount of residual monomers. Among them, anionic surfactants or nonionic surfactants having HLB of 7 or more, preferably HLB of 10 or more can be preferably used.

Examples of the organic solvent include hydrophilic organic solvents such as methanol, ethanol, isopropanol, butanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, acetone and methyl ethyl ketone. , N-pentane, n-heptane, n-
Octane, cyclohexane, cyclooctane, methylcyclohexane, decalin, benzene, ethylbenzene,
Hydrophobic organic solvents such as toluene and xylene can be exemplified, but hydrophilic organic solvents are preferable.

In carrying out the present invention, when a water-absorbent resin powder is used in the form of a powdery material after dry pulverization or one in which the surface portion and the vicinity thereof are cross-linked, these water-insoluble particulate powders are used. In some cases, the use of a surfactant or an organic solvent may reduce the residual monomer. Generally, the amount of these used is the water absorbent resin 10 to be treated.
0.01 parts by weight of water-insoluble particulate powder to 0 parts by weight
˜5 parts by weight, 0.01 to 5 parts by weight for the surfactant, and 0.01 to 8 parts by weight for the organic solvent are preferably used.

Further, similarly, a deodorant, a fragrance, a drug, a plant growth aid, a bactericide, a foaming agent, a pigment, a dye, a hydrophilic short fiber, a fertilizer and the like are interposed in the water absorbent resin obtained by the present invention. As a result, a new function can be imparted to the resulting water absorbent resin.

[0032]

The present invention will be further described below with reference to examples, but the scope of the present invention is not limited to these examples. In the following examples, "parts" are by weight unless otherwise specified.

The residual monomer amount and water content were determined as follows.

(1) Residual monomer 100 ml of deionized water was added to a beaker of 200 ml,
1.0 g of a water-absorbent resin was added thereto with stirring to make the deionized water entirely gel. After 1 hour, the gel was contracted by adding 5 ml of a phosphoric acid aqueous solution, and after stirring, the water-absorbent resin dispersion was filtered using a filter paper, and the filtrate was analyzed by high performance liquid chromatography.

On the other hand, a calibration curve obtained by similarly analyzing a monomer standard solution having a known concentration was used as an external standard, and the amount of residual monomer in the water absorbent resin was determined in consideration of the dilution ratio of the filtrate. . (Amount of residual monomer is all converted to solid content.) (2) Water content The water content of the water absorbent resin composition was defined as the water content by the loss on drying after standing for 3 hours in a dryer at 180 ° C.

[0036]

[Reference Example] 141 parts of sodium acrylate, 36.1 parts of acrylic acid and 0.093 part of N, N ″ -methylenebisacrylamide were dissolved in 329 parts of deionized water, and nitrogen gas was blown into the solution to dissolve dissolved oxygen. I kicked it out. An aqueous solution of this monomer was kept at 30 ° C. Then, 0.6 part of sodium persulfate and 0.01 part of 1-ascorbic acid were added and the mixture was allowed to stand to polymerize to obtain a gel-like hydropolymer.

This was dried with hot air at 150 ° C., then pulverized with a hammer type pulverizer and sieved with a 20-mesh wire net to obtain a water-absorbent resin powder (a) which passed through 20 mesh. The residual monomer content in the water absorbent resin was 223 ppm.

Further, the water absorbent resin powder (a) 100
An aqueous liquid consisting of 1 part by weight of glycerin, 4 parts by weight of water and 4 parts by weight of ethanol was mixed with 1 part by weight, and the mixture was mixed at 180 ° C. for 30 days.
By heat treatment for minutes, a water absorbent resin powder (b) having a surface portion and the vicinity thereof cross-linked was obtained.

The residual monomer content in the water absorbent resin powder (b) was 230 ppm.

[0040]

Example 1 Water-absorbent resin powder (b) 10 obtained in Reference Example
1.0 part of silicon dioxide (manufactured by Nippon Aerosil Co., Ltd., Aerosil 200) was mixed with 0 part. As a reducing substance, 1 part of sodium hydrogen sulfite was dissolved in 60 parts of water and 8 parts of isopropanol to prepare an aqueous solution of the reducing substance. This aqueous liquid was added to and mixed with the mixture of the water absorbent resin powder (b) and silicon dioxide using a high speed paddle type mixer. In addition,
At this time, the water content of the water absorbent resin composition was 37.5% by weight.

Next, this water absorbent resin composition was treated with an inner diameter of 10c.
After putting it in a cylindrical container of m and a length of 10 cm and sealing it, 1
Heat treatment was performed for 1 hour in a heating furnace at 50 ° C. The water content of the water absorbent resin composition after heat treatment (water absorbent resin treated product) was 28.9% by weight, and the residual monomer was 0.89 ppm. Then, the water-absorbent resin composition was taken out from the container and
It was dried with hot air at 0 ° C. for 3 hours and sieved with a 20-mesh wire net to obtain a water-absorbent resin that passed through 20-mesh. The residual monomer of this water absorbent resin was 0.92 ppm.

[0042]

Example 2 Water-absorbent resin powder (b) 10 obtained in Reference Example
In 0 part, 0.7% of sodium dialkylsulfosuccinate (Perox OT-P manufactured by Kao Corporation) which is a surfactant is added.
The parts were mixed. As a reducing substance, 1 part of sodium hydrogen sulfite was dissolved in 33 parts of water, and the mixture of the water-absorbent resin powder (b) and the surfactant was added and mixed using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 25.0% by weight.

Next, this water-absorbent resin composition was treated with an inner diameter of 10c.
After putting it in a cylindrical container of m and a length of 10 cm and sealing it, 1
Heat treatment was performed in a heating furnace at 40 ° C. for 10 hours. The water content of the water absorbent resin composition after heat treatment (water absorbent resin treated product) was 24.8% by weight, and the residual monomer was 0.83 ppm. Then, take out the water-absorbent resin composition from the container,
Heat in a fluidized bed dryer at a temperature of 60 ° C. for 5 hours, and
It was sieved with a 0-mesh wire mesh to obtain a water-absorbent resin of 20-mesh passing product. The residual monomer of this water absorbent resin is 0.8
It was 4 ppm.

[0044]

Example 3 Water-absorbent resin powder (b) 10 obtained in Reference Example
To 0 part, 1.0 part of hydrophobized silicon dioxide (manufactured by Nippon Aerosil Co., Ltd., Aerosil R972) was mixed.
As a reducing substance, 2 parts of sodium thiosulfate was dissolved in 67 parts of water to prepare an aqueous solution of the reducing substance. This aqueous liquid was added to and mixed with a mixture of the water absorbent resin powder (b) and hydrophobized silicon dioxide using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 40.2% by weight.

Next, this water-absorbent resin composition was treated with an inner diameter of 10c.
After putting it in a cylindrical container of m and a length of 10 cm and sealing it, 1
Heat treatment was performed for 30 minutes in a heating furnace at 60 ° C. The water content of the water absorbent resin composition after heat treatment (water absorbent resin treated product) was 34.2% by weight, and residual monomer was 0.56 ppm. Then, take out the water-absorbent resin composition from the container,
It was dried with hot air at 70 ° C. for 5 hours and sieved with a 20-mesh wire net to obtain a water-absorbing agent of 20-mesh passing product. The residual monomer content of this water absorbing agent was 0.56 ppm.

[0046]

Example 4 Water-absorbent resin powder (b) 10 obtained in Reference Example
To 0 part, 2.0 parts of aluminum oxide (manufactured by Degussa Co., Ltd., aluminum oxide C) was mixed. As a reducing substance, 1 part of sodium sulfite, 120 parts of water, and 2 parts of ethanol
The solution was dissolved in 1 part to prepare an aqueous liquid of a reducing substance. This aqueous liquid was added to and mixed with the mixture of the water absorbent resin powder (b) and aluminum oxide using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 54.7% by weight.

Next, this water absorbent resin composition was treated with an inner diameter of 10c.
After putting it in a cylindrical container of m and a length of 10 cm and sealing it, 1
Heat treatment was performed in a heating furnace at 80 ° C. for 10 hours. The water content of the water absorbent resin composition after heat treatment (water absorbent resin treated product) was 35.2% by weight, and the residual monomer was 0.71 ppm. Then, take out the water-absorbent resin composition from the container,
Heat using a fluid bed dryer at a temperature of 80 ° C. for 3 hours, and
It was sieved with a 0-mesh wire mesh to obtain a water-absorbent resin of 20-mesh passing product. The residual monomer of this water absorbent resin is 0.7
It was 5 ppm.

[0048]

Example 5 Water-absorbent resin powder (b) 10 obtained in Reference Example
2.0 parts of activated carbon was mixed with 0 parts. As a reducing substance, 1 part of sodium sulfite, 54 parts of water, and 2 parts of isopropanol
The solution was dissolved in 1 part to prepare an aqueous liquid of a reducing substance. This aqueous liquid was added to and mixed with the mixture of the water-absorbent resin powder (b) and activated carbon using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 35.0% by weight.

Next, the water-absorbent resin composition was put in a heating furnace of a type capable of blowing steam, and heat-treated for 5 hours while blowing steam at 120 ° C. The water content of the water absorbent resin composition after heat treatment (water absorbent resin treated product) is 40.2% by weight,
The residual monomer was 0.53 ppm. Then, the water-absorbent resin composition was taken out of the container, dried with hot air at 50 ° C. for 10 hours, and sieved with a 20-mesh wire net to obtain a 20-mesh water-absorbent resin. The residual monomer of this water absorbent resin was 0.53 ppm.

[0050]

Example 6 Water-absorbent resin powder (b) 10 obtained in Reference Example
2.0 parts of polymethylmethacrylate fine powder was mixed with 0 parts. As a reducing substance, 1 part of sodium hydrogen sulfite was dissolved in 60 parts of water, and the mixture of the water absorbent resin powder (b) and polymethylmethacrylate was added and mixed using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 37.5% by weight.

Next, this water absorbent resin composition was treated with an inner diameter of 10c.
After putting it in a cylindrical container of m and a length of 10 cm and sealing it, 2
Heat treatment was performed for 10 minutes in a heating furnace at 00 ° C. The water content of the water absorbent resin composition (water absorbent resin treated product) after the heat treatment was 35.2% by weight, and the residual monomer was 0.27 ppm. Then, take out the water-absorbent resin composition from the container,
Heat in a fluid bed dryer at a temperature of 120 ° C. for 2 hours,
The mixture was sieved with a 20-mesh wire mesh to obtain a water-absorbent resin having a 20-mesh flow rate. The residual monomer of this water absorbent resin is 0.
It was 93 ppm.

[0052]

Comparative Example 1 Water-absorbent resin powder (b) 10 obtained in Reference Example
An aqueous solution prepared by dissolving 0.3 part of sodium hydrogen sulfite in 0 part of 5 parts of water was added and mixed for 5 minutes using a high speed paddle type mixer. The residual monomer of the obtained comparative water absorbent resin was 2
It was 1.0 ppm.

[0053]

Comparative Example 2 Water-absorbent resin powder (b) 10 obtained in Reference Example
To 0 part, 1 part of hydroxylamine hydrochloride was dissolved in 50 parts of water, and the mixture was added and mixed using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 33.3% by weight. This water-absorbent resin composition was taken out, and 120
It was dried with hot air at 0 ° C. for 2 hours and sieved with a 20-mesh wire net to obtain a 20-mesh-passing water-absorbing agent. The water content of this comparative water absorbent resin was 2.7% by weight, and the residual monomer content was 80.3.
It was ppm.

[0054]

Comparative Example 3 Water-absorbent resin powder (b) 10 obtained in Reference Example
To 0 part, 5 parts of sodium hydrogen sulfite was dissolved in 300 parts of water, and the mixture was added and mixed using a high speed paddle type mixer. In addition,
At this time, the water content of the water absorbent resin composition was 75.2% by weight. The water-absorbent resin composition was taken out, dried with hot air at 120 ° C. for 2 hours, and sieved with a 20-mesh wire net to give 20
A water absorbent of the mesh passing product was obtained. The water content of this comparative water absorbent resin was 3.3% by weight, and the residual monomer was 33.4 pp.
It was m.

[0055]

Comparative Example 4 Water-absorbent resin powder (b) 10 obtained in Reference Example
To 0 part, 1.0 part of hydrophobized silicon dioxide (manufactured by Nippon Aerosil Co., Ltd., Aerosil R972) was mixed.
As a reducing substance, 2 parts of sodium thiosulfate was dissolved in 20 parts of water to prepare an aqueous solution of the reducing substance. This aqueous liquid was added to and mixed with a mixture of the water absorbent resin powder (b) and hydrophobized silicon dioxide using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 16.7% by weight. Next, the water absorbent resin composition is taken out and 1
It was dried with hot air at 10 ° C. for 10 hours and sieved with a 20-mesh wire net to obtain a 20-mesh-passed water absorbent resin for comparison.
The water content of this comparative water absorbent resin was 8.6% by weight, and the residual monomer was 62.5 ppm.

[0056]

Comparative Example 5 Water-absorbent resin powder (b) 10 obtained in Reference Example
To 0 part, 1 part of sodium thiosulfate as a reducing substance was dissolved in 1900 parts of water to prepare an aqueous solution of the reducing substance.
This aqueous liquid was added to and mixed with the water-absorbent resin powder (b) using a double-arm kneader. The water content of the water absorbent resin composition at this time was 95.0% by weight.

Next, this water-absorbent resin composition was treated with an inner diameter of 10c.
After putting it in a cylindrical container of m and a length of 10 cm and sealing it, 1
Heat treatment was performed for 1 hour in a heating furnace at 50 ° C. The water content of the water absorbent resin composition after heat treatment (water absorbent resin treated product) was 76.2% by weight, and the residual monomer was 14.2 ppm. Then, take out the water-absorbent resin treated product from the container, 1
It was dried with hot air at 50 ° C. for 3 hours and sieved with a 20-mesh wire net to obtain a 20-mesh-passed water absorbent resin for comparison. The residual monomer of this comparative water absorbent resin was 31.6 ppm.

[0058]

Comparative Example 6 Water-absorbent resin powder (b) 10 obtained in Reference Example
To 0 part, 2.0 parts of aluminum oxide (manufactured by Degussa Co., Ltd., aluminum oxide C) was mixed. As a reducing substance, 1 part of sodium sulfite was dissolved in 40 parts of water and 2 parts of ethanol to prepare an aqueous solution of the reducing substance. This aqueous liquid was added to and mixed with the mixture of the water absorbent resin powder (b) and aluminum oxide using a high speed paddle type mixer. In addition,
At this time, the water content of the water absorbent resin composition was 28.6% by weight.

Next, this water-absorbent resin composition was treated with an inner diameter of 10c.
8 m after being put in a cylindrical container of 10 cm in length and sealed
Heat treatment was performed for 30 minutes in a heating furnace at 0 ° C. The water content of the water absorbent resin composition after heat treatment (water absorbent resin treated product) was 28.5% by weight, and the residual monomer was 21.7 ppm. Then, the water absorbent resin treated product was taken out of the container and heated at a temperature of 80 ° C. for 3 hours using a fluidized bed dryer,
It was sieved with a mesh wire mesh to obtain a comparative water-absorbent resin having a 20-mesh flow rate. The residual monomer of this comparative water absorbent resin was 21.8 ppm.

[0060]

Comparative Example 7 Water-absorbent resin powder (b) 10 obtained in Reference Example
2.0 parts of activated carbon was mixed with 0 parts. As a reducing substance, 1 part of monoethanolamine was dissolved in 25 parts of water and 2 parts of isopropanol to prepare an aqueous liquid of the reducing substance. This aqueous liquid was added to and mixed with the mixture of the water-absorbent resin powder (b) and activated carbon using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 20.0% by weight.

Next, this water absorbent resin composition was treated with an inner diameter of 10c.
It was placed in a cylindrical container having a length of 10 m and a length of 10 cm, and the container was sealed and left at room temperature for 10 hours. The water content of the water absorbent resin composition (treated water absorbent resin) after standing was 19.6% by weight, and the residual monomer was 28.2 ppm. Then, the treated water-absorbent resin was taken out of the container, dried with hot air at 100 ° C. for 3 hours, and sieved with a 20-mesh wire net to obtain a 20-mesh-passed water absorbent resin for comparison. The residual monomer of this comparative water absorbent resin was 30.5 ppm.

[0062]

Comparative Example 8 The treated water absorbent resin (water content 28.9% by weight) obtained in Example-1 was dried at 150 ° C. for 3 hours to obtain a comparative water absorbent resin. The residual monomer of this comparative water absorbent resin was 11.2 ppm.

[0063]

Comparative Example 9 The treated water-absorbent resin (water content 24.8% by weight) obtained in Example-2 was dried at 180 ° C. for 2 hours to obtain a comparative water-absorbent resin. The residual monomer of this comparative water absorbent resin was 19.3 ppm.

[0064]

As is clear from the above Examples and Comparative Examples, in order to reduce the residual monomer in the water absorbent resin powder, the amount of water when adding a substance capable of reacting with the residual monomer and the subsequent heat treatment. It is very important that the change in water content during the process stays within a specific water content range and the temperature and time at that time are very important.Even if the amount of added water is too small or too large, the residual monomer is not efficiently reduced. However, even if the amount of added water is optimum, the residual monomer is not efficiently reduced if it is immediately dried or dried at high temperature.

From the above, in order to efficiently reduce the residual monomer, an appropriate amount of water added when adding a substance capable of reacting with the residual monomer is added, and heating is performed while maintaining the water content within a specific range. Further, by drying at a temperature as low as possible after that, it is possible to reduce the residual monomer very efficiently, and according to the present invention, it was found that a water absorbent resin containing almost no residual monomer was obtained, and a highly safe water absorbent resin was obtained. It has become possible to produce stably and efficiently.

Therefore, the water-absorbent resin obtained by the present invention makes use of its characteristics to absorb sanitary materials such as disposable diapers and sanitary napkins, medical water retention agents, agricultural and horticultural water retention agents, and other industrial dehydration agents. It can be suitably used for applications requiring water absorption and water retention of agents and the like.

Claims (8)

[Claims] 1. A method for producing a water-absorbent resin, comprising at least the following steps (a), (b) and (c), wherein (a) the water-absorbent resin powder has residuals in the water-absorbent resin powder: Water content of 25-55% by weight
(B) a step of heating the water absorbent resin composition at a temperature of 120 to 200 ° C. for 10 minutes or more to obtain a water absorbent resin treated product while maintaining the water content of the water absorbent resin composition in the above water content range. (C) A method for producing a water-absorbent resin, comprising the step of drying the treated water-absorbent resin at a temperature of less than 120 ° C. 2. The method according to claim 1, wherein the water absorbent resin powder is a crosslinked polyacrylic acid partially neutralized product. 3. The substance capable of reacting with the residual monomer in the water absorbent resin powder is at least one selected from the group consisting of sulfite (salt), hydrogen sulfite (salt), pyrosulfite (salt) and thiosulfate (salt). The manufacturing method according to claim 1, wherein 4. The heating temperature in step (b) is 150 to
The production method according to claim 1, which is 170 ° C. 5. The production method according to claim 1, wherein the heating time in step (b) is 1 hour or more. 6. The drying temperature in step (c) is 60-8.
The production method according to claim 1, which is 0 ° C. 7. The water-absorbent resin composition in step (a) further comprises at least one additive selected from the group consisting of organic and / or inorganic water-insoluble particulate powder, a surfactant and an organic solvent. The manufacturing method according to claim 1, further comprising: 8. The production method according to claim 1, wherein in the step (c), the drying is performed up to a water content of 15% by weight.