JP3522316B2 - Treatment method for water absorbent resin - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a water-absorbent resin, and more particularly, to remarkably reduce the content of monomers remaining in the water-absorbent resin without adversely affecting various properties of the water-absorbent resin. Regarding the processing 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 residual level of residual monomers is still high, and the German Patent Publication 37247
The same is true for the method of Japanese Patent Publication No. 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 being adjusted to 100%, but it was hard to say that even in this method, the residual monomer amount was still sufficiently reduced at a level of several tens of ppm.

[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 novel method for treating a water absorbent resin. Another object of the present invention is to provide a treatment 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 objects are a method for treating a water-absorbent resin, and the treatment step is at least the following steps (a) and (b), and (a) water-absorption: Of a water-absorbent resin composition having a water content of 10 to 70% by weight by adding a substance capable of reacting with a residual monomer in the water-absorbent resin to a water-soluble resin (b) Change rate of water content of the water-absorbent resin composition Of 20% or less while heating at a temperature of 100 to 200 ° C. for 10 minutes or more.

The present invention will be described in more detail below.

The water-absorbent resin used in the present invention is a resin that absorbs a large amount of water in water and swells to form a hydrogel, and examples thereof include the conventionally known resins described above. However, considering the necessity and effect of reducing the residual monomer, the method of the present invention is particularly effective for a crosslinked polyacrylic acid partially neutralized product.

The form of the water-absorbent resin is not particularly limited, and any of a gel-like substance after polymerization and before drying, a powdery substance after drying and crushing, and a substance in which the surface portion and the vicinity thereof are further subjected to crosslinking treatment. Can be 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 the monomer. And a nitrogen-containing compound such as ammonia, ammonium salt, hydroxylamine and amino acid; sulfurous acid (salt), hydrogen sulfite, phosphorous acid (salt), hypophosphorous acid (salt) ), Thiosulfate (salt), and other reducing substances. Among these, reducing substances such as sulfurous acid (salt), hydrogen sulfite, phosphorous acid (salt), hypophosphorous acid (salt), and thiosulfuric acid (salt) are preferable in view of the effect of reducing the residual monomer.

In the step (a), the water content of the water absorbent resin must be in the range of 10 to 70% 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 10
When it is in the range of up to 70% by weight, leave it as it is (a)
You may use for the process of. 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 a condition outside this range, and then water may be added or drying may be carried out to adjust to this range. When the water-absorbent resin is a dry powder, it is necessary to add water so that the water content is in the range of 10 to 70% 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 10% by weight, the reduction rate of the residual monomer is low. On the other hand, even if the water content is 70% by weight or more, the reduction rate of the residual monomer is low, and further, the water content is large, which leads to drying. It requires a large amount of energy and is economically disadvantageous. It is preferably 20 to 60% by weight, more preferably 30 to 50% by weight.

The water content of 1 thus obtained in step (a)
It is very important to heat the water-absorbent resin composition of 0 to 70% by weight while keeping the rate of change of the water content within the specified range, and the rate of change of the water content at this time is set to 20% or less. This significantly reduces the residual monomer.

According to the conventional method, the water-absorbent resin after the addition of the substance capable of reacting with the residual monomer was immediately dried, but in the present invention, the water-absorbent resin is exposed to a high temperature without being dried for a certain period.

By doing so, the residual monomers are significantly reduced. At this time, the temperature is 100 ° C to 2
00 ° C, preferably 120 ° C to 180 ° C, more preferably 1
At a high temperature of 40 ° C. to 160 ° C., the rate of change in water content is 20% or less, preferably 10% or less, more preferably 1% or less, 10 minutes or more, preferably 30 minutes or more, more preferably 1 It was found that the residual monomer can be remarkably reduced by keeping it for a time or more.

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 steam. The method of spraying and heating 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 heaters, (pressure resistant) disk heaters, (pressure resistant) fluidized bed heaters. , (Withstand pressure) air flow heater, (withstand pressure) infrared heater, (withstand pressure) hot air heater, (withstand pressure) microwave heater and the like.

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 particulate powders, surfactants and 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 preferable, and among them, silicon dioxide, titanium dioxide, aluminum oxide, zeolite, kaolin and hydrotalcite are more preferable.
Further, inorganic water-insoluble fine particle 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 salts, alkylnaphthalene sulfonates, alkylpolyoxyethylene sulfate salts, dialkylsulfosuccinates, and the like, and nonionic surfactants include sorbitan fatty acid esters. 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 powdered material after dry pulverization or a material in which the surface portion and the vicinity thereof are crosslinked is used as the form of the water-absorbent resin powder, 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, the water absorbent resin treated by the method of the present invention is added with a deodorant, a fragrance, a drug, a plant growth aid,
By interposing a bactericidal agent, a foaming agent, a pigment, a dye, a hydrophilic short fiber, a fertilizer, etc., a new function can be imparted to the resulting water absorbent resin.

[0033]

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 amount of residual monomer and the rate of change in water content were determined as follows.

(1) Add 100 ml of deionized water to a beaker containing 200 ml of residual monomer,
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. . (Amounts of residual monomers are all based on the solid content of the water-absorbent resin.) (2) Change rate of water content The water content of the water-absorbent resin composition is 180 ° C. after drying for 3 hours in a dryer. With the weight loss, the rate of change of water content was calculated by the following formula.

[0037]

[Equation 1]

[0038]

[Reference Example] 141 parts of sodium acrylate, 36.1 parts of acrylic acid and 0.093 parts of N, N'-methylenebisacrylamide were dissolved in 329 parts of deionized water, and nitrogen gas was blown into this to expel dissolved oxygen. It was 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 mesh to obtain a water-absorbing material which is a crosslinked polyacrylic acid partially neutralized product of 20 mesh. Resin (a) was obtained. Water absorbent resin (a)
The residual monomer content was 223 ppm.

Furthermore, 100 parts by weight of the water-absorbent resin (a) was mixed with an aqueous liquid consisting of 1 part by weight of glycerin, 4 parts by weight of water and 4 parts by weight of ethanol, and heat-treated at 180 ° C. for 30 minutes to obtain a surface portion. A water-absorbent resin (b) having a cross-linked structure was obtained. The residual monomer content in the water absorbent resin (b) was 230 ppm.

[0041]

Example 1 1.0 part of silicon dioxide (Aerosil 200 manufactured by Nippon Aerosil Co., Ltd.) was mixed with 100 parts of the water absorbent resin (b) obtained in Reference Example. 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 (b) and silicon dioxide 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, 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 the heat treatment was 37.1% by weight (water content change rate-1%), and the residual monomer was 0.4 ppm. Then, remove the water-absorbent resin composition from the container, 1
It was dried with hot air at 00 ° C. for 3 hours and sieved with a 20-mesh wire net to obtain a water-absorbing agent that passed through a 20-mesh. The residual monomer content of this water absorbing agent was 1.4 ppm.

[0043]

Example 2 To 100 parts of the water absorbent resin (b) obtained in Reference Example, 0.7 part of a sodium dialkylsulfosuccinate (Perex OT-P manufactured by Kao Corporation) as a surfactant was mixed. As a reducing substance, 1 part of sodium hydrogen sulfite was dissolved in 200 parts of water, and the mixture of the water absorbent resin (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 66.
It was 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, 2
Heat treatment was performed in a heating furnace at 00 ° C. for 10 hours. The water content of the water absorbent resin composition after the heat treatment is 55.3% by weight.
(Water content change rate -17%), residual monomer 1.2 ppm
Met. Then, the water-absorbent resin composition was taken out of the container, heated at a temperature of 80 ° C. for 3 hours using a fluidized bed dryer, and sieved with a 20-mesh wire net to obtain a 20-mesh passing water-absorbing agent. The residual monomer of this water absorbing agent is 1.3 p
It was pm.

[0045]

Example 3 Hydrophobized silicon dioxide (Nippon Aerosil Co., Ltd.) was added to 100 parts of the water absorbent resin (b) obtained in Reference Example.
Made, Aerosil R972) 1.0 part 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 the mixture of the water absorbent resin (b) and the hydrophobized silicon dioxide using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 16.5% 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 5 hours in a heating furnace at 80 ° C. The water content of the water absorbent resin composition after the heat treatment was 15.7% by weight (water content change rate -5%), and the residual monomer was 1.0 ppm. Then, remove the water-absorbent resin composition from the container, 1
It was dried with hot air at 00 ° C. for 3 hours and sieved with a 20-mesh wire net to obtain a water-absorbing agent that passed through a 20-mesh. The residual monomer content of this water absorbing agent was 1.9 ppm.

[0047]

Example 4 To 100 parts of the water absorbent resin (b) obtained in the reference example, 2.0 parts of aluminum oxide (aluminum oxide C manufactured by Degussa Co., Ltd.) 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 (b) and aluminum oxide using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 28.6% 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 carried out in a heating furnace at 20 ° C. for 10 hours. The water content of the water absorbent resin composition after the heat treatment is 28.6% by weight.
(Water content change rate 0%) and residual monomer was 0.9 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 of 20 mesh passing product. The residual monomer of this water absorbing agent was 1.0 ppm.

[0049]

Example 5 To 100 parts of the water absorbent resin (b) obtained in Reference Example, 2.0 parts of activated carbon was mixed. As a reducing substance, 1 part of monoethanolamine, 70 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 (b) and activated carbon using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 41.2% by weight.

Next, the water-absorbent resin composition was put into a heating furnace of a type capable of blowing steam, and heat-treated for 30 minutes while blowing steam at 140 ° C. The water content of the water absorbent resin composition after the heat treatment is 46.0% by weight (the water content change rate is 12%).
%) And residual monomer was 0.9 ppm. Then, the water-absorbent resin composition was taken out from the container, dried with hot air at 100 ° C. for 3 hours, and sieved with a 20-mesh wire net to obtain a water-absorbent agent of 20-mesh passage. The residual monomer content of this water absorbing agent was 1.5 ppm.

[0051]

Example 6 To 100 parts of the water absorbent resin (b) obtained in Reference Example, 2.0 parts of polymethylmethacrylate fine powder was mixed. As a reducing substance, 1 part of sodium hydrogen sulfite is added to 6 parts of water.
It was dissolved in 0 part, and the mixture of the water absorbent resin (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, 1
Heat treatment was performed for 10 minutes in a heating furnace at 00 ° C. The water content of the water absorbent resin composition after the heat treatment is 37.5% by weight.
(Water content change rate 0%) and residual monomer was 1.9 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 of 20 mesh passing product. The residual monomer content of this water absorbing agent was 1.9 ppm.

[0053]

Example 7 141 parts of sodium acrylate, 36.1 parts of acrylic acid, and 0.093 parts of N, N′-methylenebisacrylamide were dissolved in 329 parts of deionized water, and nitrogen gas was blown into this 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. The residual monomer of this gel polymer was 1.2%. In a double-arm kneader, 3 parts of sodium sulfite was added to and mixed with 100 parts by weight of the solid content of the gel polymer.

Next, the gel polymer having a water content of 65.0% by weight was taken out, placed in a cylindrical container having an inner diameter of 10 cm and a length of 10 cm and sealed, and then heat-treated in a heating furnace at 150 ° C. for 2 hours. It was The water content of the gel polymer after the heat treatment was 62.0% by weight (water content change rate-5%), and the residual monomer was 1.3 ppm. Then, the water-absorbent resin composition was taken out from the container, dried with hot air at 120 ° C. for 2 hours, pulverized with a pulverizer and sieved with a 20-mesh wire mesh to obtain 2
A water-absorbing agent that passed through 0 mesh was obtained. The residual monomer content of this water absorbing agent was 1.9 ppm.

[0055]

Example 8 Hydrophobized silicon dioxide (Nippon Aerosil Co., Ltd.) was added to 100 parts of the water absorbent resin (b) obtained in Reference Example.
Made, Aerosil R972) 1.0 part was mixed. As a reducing substance, 2 parts of sodium sulfite is dissolved in 70 parts of water,
An aqueous liquid of reducing substance was prepared. This aqueous liquid was added to and mixed with the mixture of the water absorbent resin (b) and the hydrophobized silicon dioxide using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time was 41.5% by weight.

Next, the water absorbent resin composition was put into a paddle dryer whose jacket was heated to 120 ° C., and after 10 minutes, a part of the water absorbent resin composition was taken out. At this time, the water content of the water absorbent resin composition was 34.2% by weight (water content change rate-17.6%), and the residual monomer was 0.81 ppm. Then, it was heated for 2 hours while being continuously dried. The water content of the water absorbing agent after the heat treatment was 8.5% by weight, and the residual monomer content was 0.92 ppm.

[0057]

Comparative Example 1 An aqueous solution prepared by dissolving 0.3 part of sodium hydrogen sulfite in 5 parts of water was added to 100 parts of the water absorbent resin (b) obtained in Reference Example and mixed for 5 minutes using a high speed paddle type mixer. The residual monomer of the obtained water absorbent for comparison was 21.0 p.
It was pm.

[0058]

Comparative Example 2 328 parts of acrylic acid and 2.6 parts of N, N′-methylenebisacrylamide were dissolved in 980 parts of deionized water, and 127.5 parts of sodium bicarbonate were used to obtain PH = 4.0.
Was prepared. Nitrogen gas was blown into this to expel dissolved oxygen. Azobisamidinopropane dihydrochloride (0.36 parts) obtained by dissolving an aqueous solution of this monomer in 120 parts of water, 0.73 parts of potassium persulfate, and 1.34 parts of sodium hydrogen sulfite.
Parts and 0.06 part of iron (II) gluconate were added and statically polymerized to obtain a gel-like water-containing polymer. This gel polymer was crushed, and 2 parts of hydroxylamine hydrochloride was added to and mixed with 100 parts by weight of the solid content of the polymer. This moisture content 7
7.0% by weight of the gel polymer was dried with hot air at 110 ° C. for 2 hours, then pulverized with a pulverizer and sieved with a 20-mesh wire net to obtain a 20-mesh-passing water absorbent for comparison. .
The water content of this comparative water absorbing agent was 4.2% by weight (water content change rate: -95%), and the residual monomer content was 32.1 ppm.

[0059]

Comparative Example 3 To 100 parts of the water absorbent resin (b) obtained in Reference Example, 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. In addition,
At this time, the water content of the water absorbent resin composition was 33.3% 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 was 2.7% by weight (water content change rate: -92%), and the residual monomer content was 80.3 ppm.

[0060]

Comparative Example 4 In 100 parts of the water absorbent resin (b) obtained in Reference Example, 5 parts of sodium bisulfite was dissolved in 300 parts of water,
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 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 obtain a water-absorbing agent of 20-mesh passing product. The water content of this comparative water absorbent was 3.3% by weight (water content change rate-96%), and the residual monomer was 33.4 ppm.

[0061]

Comparative Example 5 100 parts of the water-absorbent resin (b) obtained in Reference Example was mixed with hydrophobized silicon dioxide (Nippon Aerosil Co., Ltd.).
Made, Aerosil R972) 1.0 part 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 the mixture of the water absorbent resin (b) and the 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 was taken out, dried with hot air at 110 ° C. for 10 hours, and sieved with a 20 mesh wire mesh to obtain 2
A comparative water-absorbing agent that passed through 0 mesh was obtained. The water content of this comparative water absorbing agent was 8.6% by weight (water content change rate 49%).
The residual monomer was 62.5 ppm.

[0062]

Comparative Example 6 To 100 parts of the water absorbent resin (b) obtained in Reference Example, 1 part of sodium thiosulfate as a reducing substance was added to 19 parts of water.
It was dissolved in 00 parts to prepare an aqueous liquid of a reducing substance. This aqueous liquid was added to and mixed with the water absorbent resin (b) using a double arm type 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 the heat treatment was 76.2% by weight (water content change rate -20%), and the residual monomer was 14.2 ppm. Then, the water-absorbent resin composition was taken out of the container, dried with hot air at 150 ° C. for 3 hours, and sieved with a 20-mesh wire net to obtain a 20-mesh-passing water absorbent for comparison.
The residual monomer of this comparative water absorbing agent was 31.6 ppm.

[0064]

Comparative Example 7 To 100 parts of the water absorbent resin (b) obtained in the reference example, 2.0 parts of aluminum oxide (aluminum oxide C manufactured by Degussa Co., Ltd.) 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 (b) and aluminum oxide using a high speed paddle type mixer. The water content of the water absorbent resin composition at this time 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 the heat treatment was 28.5% by weight (water content change rate 0%), and the residual monomer was 21.7 ppm. Then, the water-absorbent resin composition was taken out of the container and heated at a temperature of 80 ° C. for 3 hours using a fluidized bed dryer,
The mixture was sieved with a mesh wire mesh to obtain a comparative water-absorbing agent that passed through 20 mesh. The residual monomer of this water absorbent for comparison is 2
It was 1.8 ppm.

[0066]

Comparative Example 8 To 100 parts of the water absorbent resin (b) obtained in Reference Example, 2.0 parts of activated carbon was mixed. As a reducing substance, 1 part of monoethanolamine, 25 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 (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 after standing was 19.6 wt% (water content change rate-2%), and the residual monomer was 28.2 ppm. Then, the water-absorbent resin composition 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-passing water absorbent for comparison. The residual monomer of this comparative water absorbing agent was 30.5 ppm.

[0068]

As is apparent from the above Examples and Comparative Examples, in order to reduce the residual monomer in the water absorbent resin, the amount of water when adding a substance capable of reacting with the residual monomer and the subsequent heat treatment step It is very important to keep the rate of change of water content within a certain range.Even if the amount of added water is too small or too large, the residual monomer does not decrease efficiently. The residual monomer is not efficiently reduced by drying.

From the above, in order to efficiently reduce the residual monomer in the water-absorbent resin, an appropriate amount of water added when adding a substance capable of reacting with the residual monomer is added so that the rate of change of water content falls within the specified range. It can be seen that the residual monomer can be reduced very efficiently by heating while keeping it at a low temperature. According to the present invention, a water absorbent having almost no residual monomer can be obtained, and a highly safe water absorbent can be stably and efficiently produced. It has become possible.

Therefore, the water-absorbing agent obtained by treating the water-absorbing resin by the method of the present invention is an absorbent of sanitary materials such as disposable diapers and sanitary napkins, medical water-retaining agent, agricultural and horticultural use by taking advantage of its characteristics. It can be suitably used for applications requiring water absorption and water retention such as water retention agents and other industrial dehydrating agents.

Continuation of the front page (56) Reference JP-A-4-106108 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C08F 6/00-6/28

Claims (9)

(57) [Claims] 1. A method for treating a water-absorbent resin, comprising at least the following steps (a) and (b), wherein (a) the water-absorbent resin is capable of reacting with residual monomers in the water-absorbent resin. Step (b) of adding a substance to obtain a water-absorbent resin composition having a water content of 10 to 70% by weight, while maintaining the rate of change of the water content of the water-absorbent resin composition at 20% or less, at a temperature of 100 to 200 ° C. A method for treating a water-absorbent resin, comprising a step of heating for at least minutes. 2. The treatment method according to claim 1, wherein the water absorbent resin to be treated is a crosslinked polyacrylic acid partially neutralized product. 3. The treatment method according to claim 1, wherein the substance capable of reacting with the residual monomer in the water absorbent resin is a reducing substance. 4. The step (a) comprises a water content of 30 to 50% by weight.
2. The treatment method according to claim 1, wherein the water absorbent resin composition is used. 5. The treatment method according to claim 1, wherein the rate of change of the water content in the step (b) is 1% or less. 6. The heating in step (b) is performed in a closed state and / or
Alternatively, the treatment method according to claim 1, which is performed in a high humidity atmosphere. 7. The heating temperature in step (b) is 140.
The treatment method according to claim 1, wherein the treatment temperature is 160 ° C. 8. The processing method according to claim 1, wherein the heating time in step (b) is 1 hour or more. 9. 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 processing method according to claim 1, further comprising: