JP3357093B2 - Method for producing 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 producing a water-absorbent resin containing acrylic acid as a main component. More specifically, the present invention relates to a method for producing a water-absorbent resin in which the amount of residual monomer is small and the amount of residual monomer is hardly increased even under various use conditions. The water-absorbent resin of the present invention can be easily and inexpensively manufactured, and is excellent in performance and safety.

[0002]

2. Description of the Related Art In recent years, a water-absorbent resin which absorbs a large amount of water and forms a gel has been developed as a kind of synthetic polymer, and has been used in the fields of sanitary materials such as disposable diapers and sanitary napkins, agriculture and forestry, and civil engineering. Widely used.

Examples of such water-absorbing resins include crosslinked products of partially neutralized polyacrylic acid, hydrolysates of starch-acrylonitrile graft polymer, neutralized products of starch-acrylic acid graft polymer, and vinyl acetate-acrylic acid. Saponified ester copolymer, hydrolyzed acrylonitrile copolymer or acrylamide copolymer, crosslinked polymer of cationic monomer, copolymerized crosslinked product of 2-acrylamido-2-methylpropanesulfonic acid and acrylic acid, Many are known, such as a crosslinked polymer of 2-sulfoethyl methacrylate.

[0004] Among these water-absorbing resins, acrylic acid-based water-absorbing resins using acrylic acid as a main component of a monomer are mainly used because of the availability of raw materials and excellent water-absorbing properties. On the other hand, about 500 to 3000 ppm of residual monomer remains in the acrylate-based water-absorbing resin, and in recent years, its reduction has been strongly demanded.

Many methods have been proposed for reducing the residual monomers of the water-absorbing resin and the water-soluble resin. For example, (a) a method of improving the polymerization rate itself by changing polymerization conditions or radiation (Japanese Patent Laid-Open No.
0-96689, JP-A-53-145895, JP-A-56-72005, JP-A-63-43930, JP-A-63-260906) and a method of treating a residual monomer after polymerization. (B) a method of adding an additive such as an amine or a sulfite after polymerization (Japanese Patent Application Laid-Open No. 50-4).
0689, JP-A-55-135110, JP-A-64
-62317), (c) a method of extracting residual monomers with an organic solvent or the like (JP-A-1-292003), (d)
Method of adding microorganisms that decompose residual monomers (Japanese Patent Publication No. Sho 6
No. 0-29523), (e) a method of volatilizing residual monomers at high temperature (Japanese Patent Application Laid-Open No. 54-119588), and (g) a monomer obtained by a specific neutralization method or a monomer containing less heavy metal. Technology (Japanese Unexamined Patent Publication No. 2-209906,
No. 1306).

However, the method (a) is insufficient in the effect of reducing the water content, and furthermore, the physical properties of the water-absorbent resin are deteriorated due to severe polymerization and post-treatment conditions. In the methods (b) and (c), additives and organic solvents used remain in the water-absorbing resin, and there is a problem in safety. Since the method (d) uses a microorganism, it is industrially difficult and is not preferable in terms of safety.
The method (e) has almost no effect because the acrylate does not evaporate. The method (g) has no particular problem in terms of safety, but its reduction effect is insufficient. All of these methods were insufficient.

Furthermore, in these methods, the unique phenomenon that the present inventors have found this time with respect to the acrylate-based water-absorbing resin, that is, even if the residual monomer is once reduced to about several hundred ppm, the production process is not completed. Or in subsequent use,
Furthermore, it was impossible at all to suppress the phenomenon that several tens to several thousand ppm of residual monomers are generated and increased.
Therefore, even for a water-absorbent resin with less residual monomer,
Furthermore, in the case where a heating step is required, in the case of a long time such as agricultural and horticultural use, or in the case of using under a high temperature such as hot water, a large amount of residual monomer is actually generated and increased.

[0008] Usually, in the case of a water-absorbent resin, cross-linking near the surface is further carried out after polymerization in order to improve various physical properties.

Many surface treatment methods have been proposed, for example, a method using a polyhydric alcohol (Japanese Patent Laid-Open No. 58-1983).
-180233, JP-A-61-16903), a method using an alkylene carbonate (DE-402078).
0C), a method using glyoxal (JP-A-52-1)
17393), a method using a polyvalent metal (Japanese Patent Laid-Open No.
JP-A-136588, JP-A-61-257235, JP-A-62-7745), a method using a silane coupling agent (JP-A-61-211305, JP-A-61-252).
No. 212, JP-A-61-264006), a method in which a water-absorbent resin is dispersed in a mixed solvent of water and a hydrophilic organic solvent to form a crosslink (JP-A-57-44627), and a specific amount of water is allowed to coexist. Cross-linking the water-absorbing resin by heating (JP-A-58-117)
No. 222, JP-A-59-62665), a method of coexisting inorganic powder and water for crosslinking (US Pat. No. 4,568,730).
8), a method of irradiating electromagnetic radiation (JP-A-63-439)
No. 30).

However, the present inventors have found that even in the above-mentioned surface cross-linking, the remaining monomer of the water-absorbing resin is similarly tens to ten.
It has been found that the amount has increased significantly to several hundred ppm, and that the increase in the residual monomer accounts for a large proportion of the residual monomer in the final product, and also causes a decrease in the surface crosslinking effect.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation. Therefore, an object of the present invention is to reduce residual monomers, and even after polymerization,
In the acrylate-based water-absorbent resin that shows a unique phenomenon that the residual monomer is further generated and increased during the manufacturing process and use, it shows excellent physical properties such as high water absorption ratio, the residual monomer is small, and the residual monomer An object of the present invention is to provide a method for producing a water-absorbent resin, which hardly generates or increases.

[0012]

As a result of diligent studies to achieve the above object, the present inventors have found that a water-absorbent resin having a large amount of residual monomer and a water-absorbent resin having a large amount of residual monomer generated and increased. Found that about 1% of β-hydroxypropionic acid was contained as a trace component other than the residual monomer, and that there was a correlation between the content of β-hydroxypropionic acid and the residual monomer.

As a means for solving the above problems, the present inventors have considered the importance of β-hydroxypropionic acid contained in a small amount in an acrylic acid (salt) monomer, which has not received any attention. Attention was paid to a technique of controlling the content thereof, and a heat treatment of a gel polymer neutralized as an ammonium salt and an alkali metal salt obtained by polymerization of the monomer, and / or a dried product thereof The above problem was solved by a method of performing heat treatment at the time of surface cross-linking, and the present invention was completed.

That is, according to the present invention, there is provided a method wherein “a partially neutralized or fully neutralized acrylate is used as a main component, and the total content of β-hydroxypropionic acid and its salt (to the total monomer) is 10% .
A crosslinked acrylate which is obtained by heat-treating a gel-like polymer obtained by polymerization of a monomer of 00 ppm or less or a dried product thereof.
A method for producing a crosslinked acrylate-based water - absorbent resin, wherein an acid group in the polymer at the time of the heat treatment is changed to an alkali metal salt and an ammonium salt. ”And“ partially or completely neutralized acrylates, and the total content of β-hydroxypropionic acid and salts thereof (based on all monomers ) is 1000
A crosslinked acrylate-based absorbent in which the crosslinked surface is further crosslinked by mixing and drying a gelled polymer obtained by polymerization of a monomer of not more than 1 ppm and a crosslinker with a crosslinking agent and heat-treating the mixture. a method of manufacturing a water-based resin, the heat treatment time of the acid groups in the polymer, characterized in that to keep the alkali metal salts and ammonium salts, the production method of crosslinked acrylic acid salt-based water-absorbing resin. About.

[0015] The present invention also provides a " crosslinked acrylate-based absorbent".
Due to the surface cross-linking of the aqueous resin powder,
Crosslinked acrylate-based water-absorbent resin powder that has been crosslinked
Wherein the content of the residual monomer after surface crosslinking is 100
ppm or less, and the content of the residual monomer after further heating at 180 ° C. for 3 hours is 100 ppm or less.
Crosslinked acrylate water-absorbent resin powder . About. Hereinafter, the present invention will be described in more detail.

In the present invention, it is essential to prepare a monomer having a partially or completely neutralized acrylate as a main component and having a β-hydroxypropionic acid content of 1,000 ppm or less. Then, it is essential to obtain a gel polymer in which the acid groups in the polymer are neutralized as alkali metal salts and ammonium salts by polymerization of the monomer. In addition, the monomer referred to below is a general term for a monomer containing acrylic acid as a main component used for polymerization of a water-absorbing resin.

When a partially or completely neutralized acrylate (hereinafter simply referred to as acrylate) is not used as a main component of the monomer, various physical properties after polymerization and after surface cross-linking are inferior. Further, the content of β-hydroxypropionic acid is 1000 p.
When a monomer exceeding pm is used, or when the acid group of the polymer at the time of heating the gel polymer or its dried product is not neutralized as an ammonium salt, the water-absorbent resin after polymerization or drying is used. In addition to the large amount of residual monomer, there is a unique phenomenon that the residual monomer is further generated and increased in the water-absorbent resin at the time of surface cross-linking or use at a high temperature, and there is a problem in safety. Further, when a gel polymer or a dried product thereof in which the acid group of the polymer is not neutralized as an alkali metal salt is used, a decrease in polymerizability and various physical properties, coloring, a carcinogenic substance,
This may cause problems such as by-products of acrylamide.

The proportion of the acrylate as the main component in the present invention is preferably from 50 to 100 mol%, more preferably from 70 to 100 mol%, even more preferably from 90 to 100 mol% in the monomer.
100 mol%. Hereinafter, the partially neutralized acrylate referred to in the present invention is a mixture of acrylic acid and a completely neutralized acrylate.

In the present invention, an acrylate is used as a main component of the monomer. In addition, a hydrophilic monomer and / or a hydrophobic monomer other than the acrylate are used as a second monomer. You may use together as a body.

The second monomer used in the present invention is not particularly limited as long as it can be copolymerized with an acrylate, and examples of the hydrophilic monomer include methacrylic acid, maleic acid, and anhydride. Maleic acid, fumaric acid, crotonic acid, itaconic acid, vinylsulfonic acid, styrenesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloyl Propanesulfonic acid,
Acid group-containing hydrophilic monomers such as sulfoethoxy polyethylene glycol mono (meth) acrylate and salts thereof; acrylamide, methacrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (Meth) acrylamide, N, N-methyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate, vinyl Nonionic hydrophilic monomers such as pyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethyl Cationic hydrophilic monomers such as minoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide and quaternary salts thereof can be exemplified. And one or more selected from these groups. In addition, as a hydrophilic monomer, a hydrophilic monomer which forms a water-absorbent resin by hydrolysis of a functional group after a polymer such as methyl (meth) acrylate, ethyl (meth) acrylate, or vinyl acetate is used. May be used.

Among these hydrophilic monomers that can be used in combination, methacrylic acid (salt), 2- (meth) acrylic acid (salt),
(Meth) acryloylethanesulfonic acid (salt), 2-
(Meth) acrylamide-2-methylpropanesulfonic acid (salt), methoxypolyethylene glycol (meth) acrylate, N, N-dimethylaminoethyl (meth) acrylate and its quaternary salt, and acrylamide are preferred.

The hydrophobic monomers that can be used together include:
Styrene, vinyl chloride, butadiene, isobutene, ethylene, propylene, stearyl (meth) acrylate,
Lauryl (meth) acrylate and the like.

When these second monomers are used, 0 to 50 mol%, preferably 0 to 30 mol%, of the monomers,
More preferably, it is used in the range of 0 to 10 mol%. In particular, when using a hydrophobic monomer, attention must be paid to the water absorbing ability of the resulting water absorbing resin.

In the present invention, among monomers containing acrylate as a main component, β-hydroxypropionic acid and salts thereof in all the monomers (hereinafter, both are simply referred to as β-hydroxypropionic acid). ) Must be reduced to 1000 ppm or less per solid content. The content of β-hydroxypropionic acid is 500 pp
m or less, more preferably 300 ppm or less, and most preferably 100 ppm or less. In addition, 1-10
A content of about ppm does not cause any particular problem, and reduction to 1 ppm or less is uneconomical.

When the content of β-hydroxypropionic acid is 1
Means for preparing the monomer of 000 ppm or less is not particularly limited, but one example of a preferred specific embodiment is as follows.

Acrylic acid is used for neutralization and monomer preparation within as short a time as possible after distillation purification, for example, within 24 hours, preferably 12 hours, and more preferably within 6 hours. Also,
In the neutralization step, the state where the neutralization rate exceeds 100 mol% is passed at least for one period. Next, while adjusting to a predetermined neutralization rate with the acrylic acid, a second monomer is added if necessary to prepare a monomer as a polymerization component, and the obtained monomer is prepared as short as possible, for example, The polymerization may be carried out within 24 hours, preferably 12 hours, more preferably 6 hours, particularly preferably within 2 hours after the preparation of the monomer. In addition, acrylic acid after distillation purification is used until neutralization and monomer preparation.
It is preferred to keep the temperature as low as possible, for example at 30 ° C. or lower, preferably at a freezing point of ２５25 ° C. The neutralization is preferably performed at a low temperature for a short time. If acrylic acid after distillation is kept for a relatively long time, it is preferable to keep it in a non-aqueous state. Further, the monomer after completion of preparation has a solidification point of 40 to 40 or more.
It should be stored at or below 0 ° C, preferably 0-30 ° C.

Acrylic acid is stored and shipped after the final distillation at the acrylic acid plant. Therefore, it takes at least four to five days, usually several tens to several months, before it is actually used industrially by the user. It is the present situation. Further, since a large amount of the monomer is prepared and stored in the factory, an average of three days or more elapses from the completion of the monomer preparation to the actual use of the monomer. Actually, several hours are required for deaeration time and temperature adjustment time. However, the present inventor believes that the longer the time after the purification by distillation of acrylic acid and the longer the time from the completion of the preparation of the monomer to the polymerization, the β-hydroxypropionic acid and the residual monomer increase. Thus, in the present invention, the monomer after completion of the preparation is preferably subjected to polymerization as quickly as possible.

The simplest method of neutralizing acrylic acid under conditions in which the neutralization rate exceeds 100 mol% at least at one time during the neutralization step is as follows: (1) Neutralization reaction system There is a method in which a basic substance is gradually added to a certain amount of acrylic acid while cooling. Further, as another method, disclosed in Japanese Patent Application Laid-Open No. 2-209906,
(2) After the neutralization rate of the acrylate in the neutralization system is maintained at less than 100 mol% from the start of neutralization, and after the neutralization rate exceeds 100 mol% during the neutralization, And a method of finally adjusting the neutralization ratio to 30 to 100 mol%. Of these methods, the method (1) requires a low temperature and sacrifices the neutralization rate in order to reduce β-hydroxypropionic acid. In the neutralization method (2), β-hydroxypropionic acid can be easily reduced with high productivity as long as the acrylic acid is used.

In the present invention, an acid group-containing monomer such as acrylic acid is partially neutralized or completely neutralized in view of various physical properties of the obtained water-absorbent resin and residual monomers. The neutralization ratio of the monomer is preferably 30 to 100 mol%, more preferably 4 to 100 mol%.
0-95 mol%, even more preferably 50-80 mol%
It is.

Examples of the basic substance used for neutralizing the monomer include alkali metal carbonates (hydrogen) and hydroxides, various amino acids such as ammonia and alanine, and organic amines. From the viewpoint of monomer reduction, sodium hydroxide and / or potassium hydroxide and ammonia are preferred, and sodium hydroxide and ammonia are particularly preferred. Further, urea may be added to a monomer as an ammonia precursor to carry out polymerization. The gel polymer obtained from the partially neutralized monomer is not limited to be further neutralized during or after the polymerization, but when a strong base such as sodium hydroxide is used for the post-neutralization. Attention must be paid to the hydrolysis of the crosslinking points. In the case of further neutralization using a crosslinking agent such as a polyvalent ester, a weak base such as ammonia or an alkali metal carbonate (hydrogen) is preferably used for neutralization.

In the present invention, the proportion of ammonium salt in the acid groups mainly neutralized as alkali metal salt and ammonium salt is 4 to 50 mol%, preferably 10 to 40 mol%.
Mol%. The proportion of the alkali metal salt in the acid group is 10 to 96 mol%, preferably 20 to 80 mol%. Within such a range, a water-absorbing resin excellent in various physical properties, free of coloring and by-products of harmful substances and having little residual monomer can be obtained. The neutralization with a very small amount of a polyvalent metal salt does not change the gist of the present invention. In addition, the range of the ratio of each salt specified above has an important meaning when heating the gel polymer or its salt, and is set to a lower ratio at the time of polymerization and at the time of heating. Adjustment to the above specified ratio is no problem. However, it is preferable to preliminarily set the above-mentioned specification at the time of polymerization because it is easy and the surface treatment effect is improved.

In the present invention, a water-absorbent resin is obtained by polymerizing and cross-linking a simple substance having a β-hydroxypropionic acid content of 1000 ppm or less.

The crosslinking method to be used is not particularly limited. For example, after a water-soluble resin is obtained by polymerizing the monomer of the present invention, a crosslinking agent is added during or after polymerization to carry out post-crosslinking. Method, radical crosslinking by radical polymerization initiator, radiation crosslinking by electron beam, etc.
In order to obtain a water-absorbent resin having excellent performance with good productivity, it is preferable to add a predetermined amount of a crosslinking agent to a monomer in advance to carry out polymerization, and to carry out a crosslinking reaction simultaneously with or after the polymerization.

As a crosslinking agent used in a method in which a predetermined amount of a crosslinking agent is previously added to a monomer to carry out polymerization and a crosslinking reaction is carried out simultaneously with or after the polymerization, N, N'-methylenebisacrylamide, (poly ) Ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, (poly) ethylene glycol di (β
-Acryloyloxypropionate), trimethylolpropane tri (β-acryloyloxypropionate), poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, ethylene glycol, polyethylene glycol, glycerin, pentaerythritol, Examples include ethylenediamine and polyethyleneimine. The amount used is usually 0.005 to 5 mol%, more preferably 0.1 to 5 mol%, based on the monomer.
01 to 1 mol%. Among these cross-linking agents, polymerizable compounds having two or more polymerizable unsaturated groups in the molecule due to the durability and water-absorbing properties of the resulting water-absorbent resin, and the handleability of the hydrogel at the time of production. It is preferable to use a crosslinking agent indispensably.

In carrying out the polymerization of the above-mentioned monomer in the present invention, bulk polymerization or precipitation polymerization can be carried out. However, from the viewpoint of performance and easy control of polymerization, the polymerization is carried out using the monomer as a solution. It is preferred to do so. As the polymerization solvent,
There is no particular limitation as long as it is a liquid in which the monomer is dissolved, and examples thereof include water, methanol, ethanol, acetone, dimethylformamide, and dimethylsulfoxide. Water or an aqueous liquid is particularly preferable. The concentration of the monomer may exceed the saturation concentration, but is usually in the range of 20% by weight to the saturation concentration, more preferably 25 to 50% by weight.
If the monomer concentration is too high, various physical properties may be reduced, and caution is required.

In the polymerization, a water-soluble chain transfer agent such as hypophosphite, thiols, thiolic acids, starch,
Cellulose, polyvinyl alcohol, polyacrylic acid,
Polymerization may be carried out by adding a hydrophilic polymer such as a crosslinked polyacrylate to the monomer. Their usage is usually less than 5 parts by weight for the former and less than 50 parts by weight for the latter.

The method for polymerizing the monomer in the present invention includes, for example, polymerization with a radical polymerization initiator, radiation polymerization, electron beam polymerization, and ultraviolet polymerization with a photosensitizer. In order to obtain a water-absorbent resin, polymerization using a radical polymerization initiator is preferred. Such radical polymerization methods include, for example, casting polymerization performed in a mold, polymerization on a belt conveyor, various aqueous solution polymerization such as polymerization performed while subdividing a hydrogel polymer, reverse phase suspension polymerization, reverse polymerization, and the like. Known polymerization methods such as phase emulsion polymerization, precipitation polymerization, bulk polymerization and the like can be exemplified, but reverse phase suspension polymerization or aqueous solution polymerization is particularly preferred. During the polymerization, continuous polymerization,
There is no particular limitation on the distinction between palindrome polymerization and the reduced pressure, pressurized pressure, and normal pressure. Further, a fibrous base material and the like may coexist during the polymerization to form a water-absorbing composite. Further, the polymerization temperature is preferably maintained substantially in the range of 0 to 100 ° C. except for the initial stage of polymerization and peaks.

As the radical polymerization initiator used in the polymerization, for example, potassium persulfate, ammonium persulfate,
Persulfates such as sodium persulfate; organic peroxides such as t-butyl hydroperoxide and cumene hydroperoxide; hydrogen peroxide; azo compounds such as 2,2'-azobis (2-amidinopropane) dihydrochloride Other known initiators such as chlorite, hypochlorite, ceric salt, and permanganate, among these,
One or two or more selected from the group consisting of persulfate, hydrogen peroxide and azo compounds are preferred. When an oxidizing radical polymerization initiator is used, a reducing agent such as a sulfite (hydrogen) salt or L-ascorbic acid may be used in combination, and when an azo polymerization initiator is used, ultraviolet light may be used in combination. .
These radical polymerization initiators may be added to the polymerization system all at once or sequentially, but the amount of the radical polymerization initiator is usually 0.001 to 2 mol%, preferably 0 to 2 mol%, based on the monomer. .
01 to 1 mol%.

In the present invention, it is essential to heat-treat the gel polymer obtained according to the above-described procedure or a dried product thereof, or to perform a heat treatment at the time of surface cross-linking of the dried gel polymer.

The heat treatment temperature is usually 100 to 300 ° C., preferably 120 to 260 ° C., and more preferably 150 to 2 ° C.
It is in the range of 50 ° C. When the heat treatment temperature is low, not only the effect of reducing the residual monomer is low, but also a decrease in various physical properties such as a decrease in water absorption may be observed. The heating time is usually 1 minute to 10 hours, preferably 10 minutes to 5 hours. In order to further achieve the object of the present invention, the polymer at the time of the heat treatment is preferably a film or a particle having a large surface area. For example, the average particle size is 0.05 to 10 mm,
Preferably, a powder or gel polymer of 0.1 to 5 mm is exemplified.

The state of the polymer at the time of the heat treatment is not particularly limited, and examples thereof include a gel polymer after polymerization, a dispersion in an organic solvent, and a polymer in a dry state. Further, the solid content of the crosslinked body during the heat treatment may be kept constant, or the solid content may be increased by evaporating the solvent. Specific examples of the timing of performing the heat treatment include, for example, a drying process, a surface crosslinking process, a reheating process after drying, a granulation process, and a process of adding an additive. Is preferred. That is, the gel polymer of the present invention is dried at the above-mentioned temperature and subjected to a heat treatment, thereby improving the water absorption capacity and dramatically reducing residual monomers. Known drying methods such as hot-air drying, infrared drying, azeotropic dehydration and the like can be used without any particular limitation.

[0042] Additives such as a surfactant, an inorganic fine powder, and a sulfurous acid (hydrogen) salt may be added to the water-absorbent resin after polymerization or drying, or pulverization or granulation may be carried out. May be adjusted. For example, when the purpose is a powdery water-absorbing resin, the average particle size is 10 to 2000 μm, more preferably 100 to 1000 μm, and most preferably 300 to 60 μm.
It is adjusted to about 0 μm.

Further, the method of the present invention for producing a water-absorbent resin having a crosslinked surface is provided by subjecting the dried gel polymer obtained by the above method to a heat treatment to crosslink the vicinity of the surface. Achieved.

For the surface cross-linking, a dried product of the gel polymer is indispensably used. However, it is preferable to perform a heat treatment before the surface cross-linking according to the above-mentioned method in order to effect the surface cross-linking and reduce the residual monomer. In addition, usually, drying is 60% of solid content.
Above, preferably up to 90% or more. (Hereinafter, a dried product of the hydrogel that is surface-crosslinked is simply referred to as a water-absorbent resin powder.) In the present invention, the crosslink near the surface of the water-absorbent resin powder may be crosslinked by radiation or the like. ,
This is performed by adding a crosslinking agent near the surface. As the crosslinking agent used, a known crosslinking agent is used without any particular limitation.
Ethylene glycol, propylene glycol, polyethylene glycol, propylene glycol, glycerin,
Polyglycerin, 1,6-hexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethyleneoxypropylene block copolymer, pentaerythritol,
Various polyhydric alcohols such as sorbitol; various polyepoxy compounds such as ethylene glycol diglycidyl ether and polyethylene glycol diglycidyl ether; various polyamine compounds such as ethylene diamine and polyethylene imine; 2,2-bishydroxymethylbutanol Polyvalent aziridine compounds such as tris (3- (1-aziridinyl) propionate); 1,3-dioxolan-2-one, 4-methyl1.3-dioxolane-
2-one, 4,6-dimethyl-1,3-dioxane-2
Various alkylene carbonate compounds such as -one; various polyhydric aldehyde compounds such as glyoxal; 2,4-
Polyvalent isocyanate compounds such as tolylene diisocyanate; polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; haloepoxy compounds such as epichlorohydrin; polyvalent compounds such as hydroxides and chlorides such as aluminum, iron and zirconium; Metal salts; and compounds having these functional groups in combination can also be exemplified.

In the present invention, among these crosslinking agents, polyhydric alcohols, polyhydric glycidyl compounds, polyhydric amines,
One or more selected from the group consisting of alkylene carbonates is more preferably used as a crosslinking agent, and a polyhydric alcohol suitable for heat treatment at a high temperature is most preferred.

In the present invention, the amount of the crosslinking agent used is based on 100 parts by weight of the solid content of the water-absorbent resin powder.
Usually 0.01 to 20 parts by weight, preferably 0.05 to 10 parts by weight
It is in the range of parts by weight.

Known methods are used for adding a crosslinking agent to the vicinity of the surface of the water-absorbent resin powder, such as adding the crosslinking agent directly to the water-absorbent resin powder, or adding the crosslinking agent to the water-absorbent resin powder dispersed in a solvent. Is added. In the case of using the former method, an inorganic compound such as silicon oxide fine powder or a surfactant may be allowed to coexist in order to uniformly add a crosslinking agent.

When the crosslinking agent is added to the water-absorbent resin powder, the crosslinking agent may be added as a solution or a dispersion. As the solvent to be used, hydrophilic organic solvents such as methyl alcohol, ethyl alcohol, n-propyl alcohol, iso-propyl alcohol, acetone, and tetrahydrofuran, and water are preferable. The amount used is generally 0 to 20 parts by weight, preferably 0 to 8 parts by weight, based on 100 parts by weight of the solid content of the water-absorbent resin powder.

In the present invention, from the viewpoint of the residual monomer reduction effect and the surface treatment effect, it is preferable that water is present when the heat treatment is performed after mixing the water-absorbent resin powder and the crosslinking agent. Therefore, when using a dried water-absorbent resin powder, it is preferable to add water. The addition of water may be carried out simultaneously with the crosslinking agent or may be carried out separately, but the amount thereof is usually 20 parts by weight or less, preferably 0.5 to 100 parts by weight, based on 100 parts by weight of the solid content of the water-absorbent resin powder. The range is 10 parts by weight.

After mixing the water-absorbent resin powder and the cross-linking agent according to the above method, a heat treatment is indispensable in the surface treatment of the present invention.

The heat treatment temperature is usually 100 to 300 ° C., preferably 120 to 260 ° C., more preferably 150 to 2 ° C.
It is in the range of 50 ° C. If the temperature is lower than 100 ° C., not only does the heat treatment take a long time to cause a decrease in productivity, but also it is difficult to achieve uniform and strong surface crosslinking, and the residual monomer reduction effect at the time of surface crosslinking is small. The remaining amount also increases. At a high temperature exceeding 300 ° C., care must be taken because the water-absorbent resin may be thermally degraded. The heating time is appropriately determined depending on the desired surface crosslinking effect, heating temperature, and the like.
Time range.

In the conventional surface treatment method, the residual monomer is reduced to several tens to several tens by heat treatment, particularly high-temperature heat treatment.
It increased by 00 ppm. However, in the method of the present invention,
In the heat treatment, not only the increase in the residual monomer is hardly observed, but also the decrease may occur. Therefore, in the present invention, uniform and strong crosslinking can be achieved in a short time by using a high temperature, and a water-absorbing resin with less residual monomer and residual crosslinking agent can be obtained.

Known methods can be used for the heat treatment. Examples are (1) a method in which a crosslinking agent is directly added to the water-absorbent resin powder and heat treatment as it is, and (2) a water-absorbent resin dispersed in a solvent. A method of adding a cross-linking agent to a powder and then heat-treating the powder while dispersing the powder, and (3) a method of performing heat treatment by filtering from a dispersion medium are mentioned. Is preferred. In addition, there is no restriction | limiting in particular about a heat processing apparatus, A well-known apparatus, such as a hot-air dryer, a fluidized-bed dryer, and a Nauter-type dryer, is used.
Further, in order to uniformly distribute the cross-linking agent on the surface, it is also preferable to perform heat treatment at a higher temperature while volatilizing at least a part of the cross-linking agent such as a polyhydric alcohol.

[0054] The present invention relates to a novel water-absorbent resin, i.e., substantially dry state in residual monomer 100ppm or less, preferably 10ppm or less, and at 3 a further 180 ° C.
During pressurized heated even residual monomers is 100ppm or less, preferably also provides a water absorbent resin whose main component is acrylic acid is 10ppm or less.

In the conventional water-absorbent resin, the amount of the residual monomer was reduced apparently, and the generation of the residual monomer was significantly increased by heating. However, the water-absorbing resin of the present invention is residual monomer be heated pressurized between 3:00 at 180 ° C. and at 100ppm or less, rather residual monomer is reduced, can be safely used under any conditions.

Although the cause of β-hydroxypropionic acid in the monomer leading to the increase or generation of the residual monomer is unknown, the increase of the residual monomer and the melting point of the β-hydroxypropionate below the melting point (142 ° C.) of sodium do not occur. From the occurrence, it is estimated that the solid phase β-hydroxypropionic acid in the water-absorbent resin is extremely easily decomposed or accelerates the depolymerization of the water-absorbent resin.

[0057]

According to the production method of the present invention, the following (1) to (5)
It has such features.

(1) Conventionally, the residual monomer, which has been reduced by sacrificing the performance, productivity, cost, and safety of the water-absorbent resin through a complicated process such as addition of an additive, is simply and efficiently used. Can be reduced to

(2) Even if a high temperature is used in the production process, an increase in the amount of residual monomers during the production is small, and production can be performed with high productivity. In addition, by performing the heat treatment, not only the remaining monomers are reduced, but also excellent physical properties such as obtaining a water-absorbent resin having a high water absorption ratio are exhibited.

(3) Since there is no generation or increase of residual monomers during subsequent use at high temperature or for a long time , any use such as long-term use in agriculture and horticulture or use under high temperature with hot water is required. High safety under conditions.

(4) In addition to improving the polymerizability, the amount of residual monomers can be reduced even with a small amount of catalyst or under mild polymerization conditions, so that a more excellent water-absorbent resin can be obtained without sacrificing physical properties.

(5) Almost no increase in the residual monomer at the time of the surface treatment bridge is observed, but rather the decrease.

The water-absorbing resin thus obtained can be widely used in fields such as sanitary materials, food, civil engineering, and agriculture.

[0064]

Hereinafter, the present invention will be described with reference to Examples.
The scope of the present invention is not limited only to these examples. The various physical properties described in the examples are values measured by the following test methods.

(1) Water Absorption Ratio A non-woven tea bag type bag (4 g)
0 * 150 mm) and immersed in a 0.9% by weight aqueous sodium chloride solution. Thirty minutes later, the tea bag type bag was pulled up and drained for a certain period of time, then the weight of the tea bag type bag was measured, and the water absorption capacity was calculated by the following equation.

[0066]

(Equation 1)

(2) Residual monomer 0.5 g of the water-absorbing resin was dispersed in 1000 ml of deionized water and stirred. After 2 hours, the dispersion was filtered through Whatman furnace paper, and the remaining monomers in the filtrate were measured by using high performance liquid chromatography.

(3) Increase / decrease in residual monomer upon heating After the obtained water-absorbent resin was further heated at 180 ° C. for 3 hours, the residual monomer after heating was measured by measuring the residual monomer according to the method of (2). The amount was measured.

(4) Suction force 20 ml of artificial urine is poured into a dish on which tissue paper is placed, and 1 g of surface-crosslinked water-absorbent resin is dropped on the center of the dish, and artificial urine is absorbed through the tissue paper. After a lapse of 10 minutes, the weight of the swollen gel is measured and defined as a suction force (g / g).

[0070]

[Production Example 1] Acrylic acid obtained from an acrylic acid production site of Himeji Works of Nippon Shokubai Co., Ltd. was purified by distillation. The acrylic acid after the distillation purification was stored at a temperature of 30 ° C. for 2 hours, and the acrylic acid was neutralized according to the method described in JP-A-2-209906 shown below.

A distillation flask equipped with a stirrer was charged with 1944 g of ion-exchanged water. While maintaining the temperature of the neutralization reaction system in the flask at 20 to 40 ° C., 1390 g of acrylic acid
And 1480 g of 48% by weight sodium hydroxide were simultaneously dropped into the flask over a period of 100 minutes at a dropping ratio of sodium hydroxide / acrylic acid = 0.9 to 0.95. After completion of the dropwise addition, 160 g of a 48% by weight aqueous sodium hydroxide solution was further supplied to increase the neutralization rate of the neutralization reaction system in the flask to 102 mol%, and then the temperature of the neutralization reaction system was increased to 4%.
It was adjusted to 0 ° C. and aged for 30 minutes. After aging,
By supplying 28 g of acrylic acid to the neutralization reaction system over 1 minute, 5002 g of acrylate (1) having a neutralization rate of 100 mol% and a concentration of 37% was obtained.

Next, the neutralized acrylate (1) 33
After adding 774 g of acrylic acid and 1063 g of ion-exchanged water for 2 hours after distillation to 38 g, 325 g of a 25% aqueous ammonia solution was added, and as a crosslinking agent, 2.83 g of trimethylolpropane triacrylate (0.04 mol per monomer) was added. %), A neutralization rate of 75% at a concentration of 38% (sodium salt 55%, ammonium salt 20%)
(1) was obtained. When analyzed by liquid chromatography, the amount of β-hydroxypropionic acid in the monomer (1) was 25 ppm.

[0073]

[Production Example 2] In the preparation of the monomer in Production Example 1, after the acrylate (1) was obtained in the same manner, 2148 g of acrylate (1), 1132 g of acrylic acid, 1563 g of ion-exchanged water were obtained in the same manner. 657 g of a 25% aqueous ammonia solution,
2.86 g of trimethylolpropane triacrylate
(0.04 mol% with respect to monomer), a neutralization rate of 75% at a concentration of 38% (sodium salt 35%, ammonium salt 4
(0%) of the monomer (2). The amount of β-hydroxypropionic acid in the monomer (2) was 18 ppm.

[0074]

[Production Example 3] In the preparation of the monomer in Production Example 1, acrylic acid (1) was similarly obtained, and then 3922 g of acrylate (1), 599 g of acrylic acid, 817 g of ion-exchanged water, 25 Aqueous solution 162 g, trimethylolpropane triacrylate 2.81 g (based on monomer 0.04 mol%), concentration 38%, neutralization rate 75% (sodium salt 65%, ammonium salt 10%)
(3) was obtained. The amount of β-hydroxypropionic acid in the monomer (3) was 22 ppm.

[0075]

[Production Example 4] In the preparation of the monomer in Preparation Example 1, the acrylic acid used for the preparation of the acrylate and the monomer was replaced with acrylic acid 2 hours after the distillation generation, and 12 hours after the distillation generation. An acrylate (2) was obtained in the same manner except that acrylic acid was used. Thereafter, using acrylate (2) in the same manner as in Production Example 1, a neutralization rate of 75% (sodium salt 55%, ammonium salt 20
%) Of monomer (4). Here, β in the monomer (4)
The amount of -hydroxypropionic acid was 50 ppm.

[0076]

[Production Example 5] In the preparation of the monomer in Production Example 1, the acrylic acid used for preparing the acrylate and the monomer was replaced with acrylic acid 2 hours after the distillation generation, and then 24 hours after the distillation generation. An acrylate (3) was obtained in the same manner except that acrylic acid was used. Thereafter, using the acrylate (3) in the same manner as in Production Example 1, a neutralization ratio of 75% (55% sodium salt, 20% ammonium salt) at a concentration of 38%
%) Of the monomer (5). Here, β in the monomer (5)
The amount of -hydroxypropionic acid was 110 ppm.

[0077]

[Production Example 6] Commercial production of acrylic acid (Wako Pure Chemicals special grade)
Was purified by distillation. The acrylic acid after the distillation purification was converted into an 80% aqueous solution having a high freezing point and easy handling, then stored at a temperature of 30 ° C. for 2 hours, and neutralized according to the following method.

In a distillation flask equipped with a stirrer, 2272 g of deionized water and 164% by weight of sodium hydroxide 164 were added.
0 g was charged. Next, while maintaining the temperature of the neutralization reaction system at 10 ° C., the 80% acrylic acid aqueous solution 23 was added to the flask.
By supplying 61 g over 6 hours, an acrylic acid salt (4) having a neutralization rate of 75 mol% and a concentration of 37% was obtained.

Thereafter, a monomer (6) having a neutralization rate of 75% (55% sodium salt and 20% ammonium salt) at a concentration of 38% using acrylate (4) in the same manner as in Production Example 1
I got The amount of β-hydroxypropionic acid in the monomer (6) was 50 ppm.

[0080]

[Production Example 7] In the preparation of the monomer in Production Example 1, acrylic acid (1) was obtained in the same manner, followed by the same procedure as above, followed by 4500 g of acrylate (1), 425 g of acrylic acid, 575 g of ion-exchanged water, and polyethylene 3.39 g of glycol diacrylate (average n = 7) (0.03 mol% of monomer) gave monomer (7) having a concentration of 38% and a neutralization rate of 75% (sodium salt 75%). In addition, monomer (7)
The amount of β-hydroxypropionic acid therein was 30 ppm.

[0081]

[Comparative Production Example 1] In the preparation of the monomer of Production Example 1,
The acrylic acid used for the preparation of the acrylate and the monomer is replaced with the acrylic acid 2 hours after the distillation generation, and the acrylic acid (200 hours after the distillation generation) is used. 5) was obtained. Hereinafter, using the acrylate (5) in the same manner as in Production Example 1, the concentration was 38%.
Thus, a comparative monomer (1) having a neutralization ratio of 75% (55% sodium salt, 20% ammonium salt) was obtained. The amount of β-hydroxypropionic acid in the comparative monomer (1) was 1100 p.
pm.

[0082]

[Comparative Production Example 2] In the preparation of the monomer of Production Example 1,
The acrylic acid used for the preparation of the acrylate and the monomer was replaced with acrylic acid 2 hours after distillation generation, and the same procedure was carried out except that commercially available acrylic acid (Wako Pure Chemicals special grade) was used as it was. Acid salt (6) was obtained, and further, using acrylic acid salt (6), a comparative monomer (2) having a concentration of 38% and a neutralization ratio of 75% (sodium salt 55%, ammonium salt 20%) was obtained. . The amount of β-hydroxypropionic acid of the comparative monomer (2) was 3,200 ppm.

[0083]

Comparative Production Example 3 In Production Example 6, the acrylate (7) was obtained by increasing the temperature of the neutralization system from 10 ° C. to 50 to 60 ° C. in order to shorten the neutralization time. In addition,
The time required for neutralization was reduced from 6 hours to 40 minutes.

The comparative monomer (3) having a neutralization rate of 75% (55% sodium salt and 20% ammonium salt) at a concentration of 38% using acrylate (7) in the same manner as in Production Example 1 ) Got. The amount of β-hydroxypropionic acid in the monomer (5) was 1,300 ppm.

[0085]

[Comparative Production Example 4] In Production Example 6, 8 used for neutralization
Comparative monomer (4) was obtained in the same manner as in Production Example 7, except that an aqueous acrylic acid solution preserved for 200 hours after distillation purification was used as the 0% aqueous acrylic acid solution. The content of β-hydroxypropionic acid in the comparative monomer (4) was 2
It was 900 ppm.

[0086]

Comparative Production Example 5 Production Example 7 was the same as Production Example 7, except that the acrylic acid used in the preparation of the monomer was replaced with the acrylic acid 2 hours after distillation generation, and the acrylic acid used in Comparative Production Example 1 was used. Comparative monomer (5) having a β-hydroxypropionic acid content of 1600 ppm
I got

[0087]

Example 1 Monomer (1) 550 obtained in Production Example 1
After degassing 0 g with nitrogen gas for 30 minutes, the mixture was placed in a nitrogen-substituted reactor having an inner volume of 10 L and a jacketed stainless steel double-armed cutter (kneader) having two sigma-type blades and a lid. Then, while being heated through warm water at 35 ° C., 0.3 mol% of ammonium persulfate and 0.03 mol% of sodium bisulfite were added. In addition, it took 2 hours from the preparation of the monomer (1) by the above operation to the introduction of the catalyst and the initiation of the polymerization.

One minute after the addition of the catalyst, polymerization started,
After 6 minutes, the gel polymer was subdivided into a diameter of about 5 mm. After stirring was continued for 44 minutes, the gel polymer was taken out.

The obtained finely divided gel polymer was spread on a 50-mesh wire net, dried with hot air at 180 ° C. for 60 minutes, and heat-treated. The obtained dried product was pulverized with a vibration mill, and further classified with 20 mesh to obtain a water absorbent resin (1). Table 1 shows the results.

[0090]

Examples 2 and 3 In Example 1, the time from the completion of preparation of the monomer (1) to the introduction of the catalyst was changed to 6 hours and 24 hours. Hereinafter, similarly, polymerization, drying, heat treatment, and the like were performed to obtain water-absorbing resins (2) and (3). Table 1 shows the results.

[0091]

Examples 4 and 5 The same procedures as in Example 1 were carried out except that the temperature of the drying and heating treatment of the gelled polymer after polymerization was changed to 150 ° C. and 120 ° C., whereby the water-absorbing resins (4) and ( 5) was obtained. As shown in Table 1, the water absorption ratio and the residual monomer were increased by decreasing the drying temperature, and the residual monomer was increased. Table 1 shows the results.

[0092]

Examples 6 to 9 In the same manner as in Example 1, except that the monomer (1) used for the polymerization was changed to the monomers (2) to (5), the polymerization, drying and heat treatment were performed in the same manner. By performing the above, water-absorbing resins (6) to (9) were obtained, respectively. Table 1 shows the results.

[0093]

Example 10 SU after 4,000 g of monomer (6) was replaced with nitrogen and the inner surface was lined with tetrafluoroethylene resin.
It was placed in an openable and castable polymerization apparatus made of S316 and having an internal volume of 300 mm * 300 mm * 50 mm, and was replaced with nitrogen and placed in a water bath at 30 ° C. Next, 0.05 mol% of ammonium persulfate and 0.02 mol% of sodium bisulfite were added to initiate polymerization. In addition,
It took 4 hours from the preparation of monomer (6) to the start of polymerization by the above operation.

After 5 hours from the start of the polymerization, the gel polymer was taken out of the casting polymerization apparatus, and was further treated with a meat chopper for about 5 m.
After being subdivided into m-diameter, a water-absorbent resin (10) was obtained by performing drying and heat treatment in the same manner as in Example 1. Table 1 shows the results.

[0095]

Example 11 Polymerization was carried out in the same manner as in Example 1, except that the monomer (7) was used in place of the monomer (1) used for the polymerization. However, 160 g of a 25% aqueous ammonia solution (10 mol% based on acid groups) was added to the gel polymer 30 minutes after the start of the polymerization, and polymerization was carried out for 30 minutes. The resulting gel polymer having a neutralization rate of 85% (including 10 mol% of ammonium salt) was dried with hot air at 170 ° C. for 80 minutes, and then,
A water-absorbent resin (11) was obtained in the same manner as in Example 1. Table 1 shows the results.

[0096]

Example 12 In Example 1, instead of the monomer (1) used for the polymerization, a monomer (7) and a monomer comprising 142 g of urea (10 mol% based on the monomer) as an ammonia precursor were used. Polymerization was carried out in the same manner except for using. The resulting gel polymer having a neutralization rate of 95% (including 20 mol% of ammonium salt) was dried with hot air at 190 ° C. for 60 minutes, and thereafter, the same procedure as in Example 1 was carried out to obtain a water absorbent resin (12 )
I got Table 1 shows the results.

[0097]

Example 13 Water-absorbing resin (1) obtained in Example 1
After mixing 1 part of glycerin, 2 parts of water, and 2 parts of ethyl alcohol with 100 parts of the powder, the mixture was heated at 190 ° C. for 40 minutes to obtain a water-absorbent resin (13). Table 2 shows the results
Shown in

[0098]

Example 14 100 parts of the powder of the water absorbent resin (6)
After mixing 2 parts of propylene glycol, 3 parts of water and 2 parts of isopropyl alcohol, the mixture was heated at 150 ° C. for 60 minutes to obtain a water-absorbent resin (14). Table 2 shows the results.

[0099]

Example 15 To 100 parts of the powder of the water-absorbent resin (7),
After mixing 0.1 part of ethylene glycol diglycidyl ether, 5 parts of water and 1 part of isopropyl alcohol, 18 parts were mixed.
By performing a heat treatment at 0 ° C. for 30 minutes, the water-absorbing resin (1
5) was obtained. Table 2 shows the results.

[0100]

Example 16 100 parts of the powder of the water absorbent resin (8)
After mixing 0.1 part of ethylene glycol diglycidyl ether, 30 parts of methanol and 15 parts of water, the mixture was heated at 180 ° C. for 1 hour to obtain a water-absorbent resin (16). Table 2 shows the results.

[0101]

Example 17 To 100 parts of the powder of the water-absorbent resin (9),
After mixing 2.5 parts of ethylene carbonate, 2.5 parts of water and 2.5 parts of acetone, the mixture was heated at 230 ° C. for 1 hour to obtain a water-absorbent resin (17). Table 2 shows the results.

[0102]

Example 18 100 parts by weight of the water-absorbent resin (10) were mixed with 0.1 part of ethylene glycol diglycidyl ether and 10 parts of water in the presence of 1 part of a silicon inorganic fine powder (Aerosil). By performing a heat treatment for one hour, a water-absorbent resin (18) was obtained. Table 2 shows the results.

[0103]

Example 19 One part by weight of aluminum sulfate, one part by weight of glycerin and eight parts by weight of water were mixed with 100 parts by weight of a powder of a water-absorbent resin (11), followed by heat treatment at 180 ° C. for 30 minutes.
A water absorbing resin (19) was obtained. Table 2 shows the results.

[0104]

Comparative Example 1 In Example 1, except that the monomer (1) used for the polymerization was changed to the comparative monomer (1),
The comparative water-absorbent resin (1) was obtained by carrying out the same polymerization, drying and heat treatment. Table 1 shows the results.

[0105]

Comparative Example 2 In Comparative Example 1, the time from the completion of preparation of the monomer (1) to the introduction of the catalyst was changed to 24 hours. Hereinafter, comparative water-absorbent resin (2) was obtained by carrying out polymerization, drying and heat treatment in the same manner. Table 1 shows the results.

[0106]

Comparative Example 3 Comparative water-absorbent resin (3) was obtained in the same manner as in Comparative Example 1, except that the temperature of the drying and heating treatment of the gel polymer after polymerization was changed to 120 ° C. Table 1 shows the results.

[0107]

Comparative Examples 4 to 7 In Example 1, the monomers (1) used for the polymerization were compared with the comparative monomers (2) to (5), respectively.
The comparative polymerization water-absorbing resins (4) to (7) were obtained in the same manner as described above except that the polymerization, drying, and heat treatment were performed. Table 1 shows the results.

[0108]

Comparative Example 8 Polymerization was carried out in the same manner as in Example 11, except that no post-addition of an aqueous ammonia solution was performed on the gel polymer 30 minutes after the polymerization. The thus obtained gel polymer having a neutralization ratio of 75% (sodium salt: 75 mol%)
By performing drying and heat treatment in the same manner as in Example 11,
A comparative water absorbent resin (8) was obtained. Table 2 shows the results.

[0109]

Comparative Example 9 A comparative water-absorbent resin (9) was obtained in the same manner as in Example 1, except that drying of the gel polymer after polymerization was changed to drying under reduced pressure at 60 ° C. Table 1 shows the results.

[0110]

Comparative Example 10 The procedure of Example 13 was repeated except that the powder of the comparative water-absorbent resin (1) was used instead of the powder of the water-absorbent resin (1) as the water-absorbent resin powder whose surface vicinity was crosslinked. Then, a comparative water absorbent resin (10) was obtained. Table 2 shows the results.

[0111]

Comparative Examples 11 to 15 In Examples 14 to 19, the comparative water-absorbent resin (4) was used in place of the water-absorbent resin (6) to (10) as the water-absorbent resin powder whose surface vicinity was crosslinked.
By performing exactly the same except that the powders of (8) to (8) are used,
Comparative water absorbent resins (11) to (15) were obtained. Table 2 shows the results
Shown in

[0112]

Comparative Example 16 In Example 13, the water-absorbent resin (1
After mixing glycerin, water and ethyl alcohol with the powder of 3), the comparative water-absorbent resin (16) was obtained by not performing heat treatment. Table 2 shows the results.

As is clear from Tables 1 and 2, in the method for producing a water-absorbent resin of the present invention, the amount of residual monomer was small, and the generation and increase of residual monomer during surface treatment and subsequent heat treatment were also observed. I can't. Further, by performing the heat treatment at a high temperature, a water-absorbent resin having a high water absorption capacity can be obtained.

[0114]

[Table 1]

[0115]

[Table 2]

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yoshio Irie 992, Nishioki, Okihama-shi, Aboshi-ku, Himeji-shi, Hyogo Nippon Shokubai Himeji Laboratory Co., Ltd. (56) References JP-A-2-194010 (JP, A) 59-98107 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C08F 6/00-6/28

Claims (10)

(57) [Claims] (1) β-hydroxypropionic acid and a salt thereof containing a partially or completely neutralized acrylate as a main component
A method for producing a crosslinked acrylate-based water - absorbent resin, comprising subjecting a gel polymer obtained by polymerization of a monomer having a total content (based on all monomers ) of not more than 1000 ppm or a dried product thereof to heat treatment. A method for producing a crosslinked acrylate-based water - absorbing resin, wherein an acid group in the polymer at the time of the heat treatment is converted into an alkali metal salt and an ammonium salt. Wherein a main component acrylic acid salts of partially neutralized or completely neutralized, beta-hydroxy propionic acid and its salts
The total amount (to total monomer) is dried product with a surface cross-linking were mixed by heating and cross-linking agent gel polymer obtained by polymerization of the following monomers 1000 ppm, the surface near the
Production side of the crosslinked crosslinked acrylic acid salt-based water-absorbing resin in al
A method for producing a crosslinked acrylate-based water - absorbent resin, wherein an acid group in the polymer at the time of the heat treatment is converted into an alkali metal salt and an ammonium salt. 3. 10 to 40 mol% of acid groups in the gel polymer
3. The method according to claim 1, wherein is an ammonium salt. 4. The content of β-hydroxypropionic acid is 1
The method according to claim 1 or 2, wherein a monomer of not more than 00 ppm is polymerized. 5. The method according to claim 1, wherein the heat treatment temperature is 150 to 250 ° C. 6. The method according to claim 1, wherein a heat treatment is performed in the step of drying the gel polymer. 7. A polyhydric alcohol compound, a polyvalent glycidyl compound, a polyvalent amine compound, wherein the one or more crosslinking agents comprising <br/> alkylene carbonate to heat treatment in addition to the vicinity of the surface of the water-absorbent resin Item 3. The production method according to Item 2. 8. The crosslinked acrylate-based water-absorbent resin powder has a surface.
Cross-linking further cross-links near the surface
A crosslinked acrylate-based water-absorbent resin powder that has a surface crosslinked
The content of the remaining monomer after is 100 ppm or less,
And further 180 content of residual monomer after heating for 3 hours at ℃ is also 100ppm or less, crosslinked acrylic acid salt-based water-absorbent resin powder. 9. The crosslinked acrylate water-absorbent resin powder according to claim 8, wherein the content of the residual monomer after heating at 180 ° C. for 3 hours is lower than the content of the residual monomer before the heating. 10. The method according to claim 1, wherein said water-absorbent resin powder is obtained by aqueous polymerization.
The crosslinked acrylate-based water-absorbent resin powder according to claim 8 or 9 , which is a powder obtained by the above method.