JPS6253310A - Production of beadlike self-crosslinking water-absorbing polymer - Google Patents

Abstract

PURPOSE:To obtain the titled polymer, having improved water absorbing property and strength and useful for sanitary good, etc., by polymerizing an acrylic acid and a salt thereof under specific conditions using a copolymer of an alpha-olefin with an unsaturated polyfunctional carboxylic acid anhydride as a surfactant without using a crosslinking agent. CONSTITUTION:Acrylic acid or a mixture of acrylic acid with methacrylic acid and alkali metal salts thereof are polymerized in the presence of a water- soluble radical polymerization initiator, dispersing agent and a copolymer of an alpha-olefin with an alpha,beta-unsaturated polyfunctional carboxylic acid anhydride or a derivative thereof as a surfactant and water to give >=30wt%- saturated concentration range, based on the amount of the monomer after neutralization, monomer concentration based on the water present in the reaction system in the absence of a crosslinking agent by the water-in-oil type reversed phase suspension polymerization method. Thereby, the aimed polymer is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention has excellent water absorbency and high water absorption resistance.
The present invention relates to a method for producing a highly water-absorbent polymer having a large particle size and being easily pulverized.

[Industrial application field]

The polymer obtained by the production method of the present invention has excellent water absorption and swells when it absorbs a large amount of water, but it is insoluble in water, has a high water absorption strength, and has a large particle size. Since the polymer itself can be easily crushed, it can be advantageously used in the production of various absorbent materials or materials that are used in a swollen state after absorbing water.

[Prior art]

Conventionally, paper, pipes, nonwoven fabrics, sponge-like urethane resins, and the like have been used as water-retaining agents in sanitary napkins, paper diapers, various sanitary materials, and various agricultural materials. However, these materials absorb only 10 to 50 times their own weight, so in order to absorb or retain a large amount of water, a large amount of material is required, which not only makes them extremely bulky, but also makes them extremely bulky. However, there were drawbacks such as the fact that when pressurizing a material that has absorbed water, the water easily separates.

In order to improve the above-mentioned drawbacks of this type of water-absorbing material, various highly water-absorbing polymeric materials have been proposed in recent years. For example, starch graft polymers (Japanese Patent Publication No. 53-46199, etc.), cellulose modified materials, (Japanese Patent Publication No. 50-8037, etc.)
Publication No. 6, etc.), cross-linked products of water-soluble polymers (% Publication No. 43-2, etc.)
3462, etc.), self-crosslinking type acrylic acid alkali metal salt polymer (Japanese Patent Publication No. 30710/1983, etc.), etc., have been proposed.

However, these superabsorbent polymer materials are still opaque in terms of water absorption, and have low glue strength when absorbing water.
The above publication discloses that the polymer obtained by drying is extremely hard, difficult to crush easily, and requires a large mechanical crushing force, which makes it difficult to manufacture on a practical or industrial scale. There are problems.

The present inventors have already proposed a method for producing a water-absorbing material that improves the above-mentioned drawbacks of conventional water-absorbing materials (Japanese Patent Application No. 59-27
Publication No. 5308. Hereinafter referred to as the prior application. ) However, the water-absorbing material manufactured by the method disclosed in the above-mentioned prior application also has various drawbacks. That is, since a nonionic surfactant with an HLB of 3 to 6 is used as a surfactant in the water-in-oil reverse phase suspension polymerization method, the resulting polymer has an extremely fine particle size of 100 μm or less. It becomes like that.

For this reason, when handling powder, dust countermeasures are necessary.

Furthermore, the glue strength upon water absorption is still insufficient, and there has been a desire for a polymer with better shape retention.

[Problem that the invention seeks to solve]

The present invention improves the above-mentioned drawbacks, maintains high water absorbency, further improves the glue strength during water absorption, and produces a super water absorbent polymer with extremely large particle size that can be easily pulverized with high reproducibility. The purpose is to provide a method for manufacturing it efficiently.

[Means for solving problems]

(Structure of the Invention) As a result of various studies to solve the above-mentioned problems, the present inventors have discovered that alkaline acid monomers can be used in the presence of a water-soluble radical polymerization initiator, a dispersion medium, a surfactant, and water. In a polymerization method using a water-in-oil type reverse-phase suspension polymerization method, a copolymer of an α-olefin and an α,β-unsaturated polycarboxylic acid anhydride or a derivative thereof is used as a surfactant, and the polymerization reaction By setting the acrylic acid monomer concentration to the water present in the system at 30% by weight or more and polymerizing in the absence of a crosslinking agent, it has an extremely high water absorption capacity, specifically, a water absorption amount of 700 times or more than its own weight. A bead-shaped self-crosslinking type water absorbent/IJ Mama that has a high glue strength and an extremely large polymer particle size, specifically an average particle size of 100 μm or more and can be easily crushed. , which led to the completion of the present invention.

(Background and Features of the Invention) The copolymer of α-olefin and polyhydric carboxylic acid anhydride or its derivative used as a surfactant in the water-in-oil type reverse-phase suspension polymerization method of the present invention is Published in Japanese Unexamined Patent Publication No. 57-
No. 74309 discloses a method for producing bead-like water-soluble polymers, and this technique can be applied to the present invention.

However, the biggest difference between the present invention and the invention disclosed in JP-A-57-74309 is the generation.

7. Whether the limer is soluble or insoluble in water. Insolubility here refers to the fact that when a polymer is placed in water and stirred at room temperature, it absorbs water and swells to form a glue-like structure, but when the stirring is stopped and the polymer is left to stand, it loses fluidity and phase separation occurs. Stirring refers to the fact that it becomes a uniform aqueous solution by stirring, and this state continues even if the stirring is stopped and the solution is left at the same temperature.

The polymers produced according to the present invention are essentially insoluble in water and have the unique property of absorbing and retaining water. In order to provide this property, it is essential to cause a crosslinking reaction to insolubilize the polymer during its production. When a water-bathable polymer is mixed with the water-insoluble polymer according to the present invention, not only the water absorption performance will be reduced, but also the water absorption glue strength will be extremely reduced.
Water-soluble polymers are excluded from the purpose of the present invention because the polymer after water absorption becomes slimy, does not give a saran-like feel, and is difficult to handle.

In the present invention, acrylic acid or an acrylic acid/methacrylic acid mixture is used, and at least 50 moles of the total calgexyl groups are neutralized with an alkali metal salt, and the monomer concentration after neutralization with respect to water is 30% by weight. Person in charge or above
When using an acrylic acid/methacrylic acid mixture, the methacrylic acid concentration in the mixture must be 20 molar or less, and hydrogen peroxide or persulfate should be used as a water bath polymerization initiator. By using
It was found that the crosslinking reaction could be caused without using a crosslinking agent and the polymer could be insolubilized.

The polymer produced according to the present invention has characteristics of extremely large particle size, high water absorption, and extremely high water absorption strength.

Water-insoluble polymers with such characteristics are produced by using conditions that cause a crosslinking reaction in reverse-phase suspension polymerization as described above, and by using α-olefin and α-olefin as surfactants.
Production becomes possible for the first time by using a copolymer with β-unsaturated polycarinic acid anhydride or a derivative thereof.

In general, water absorption amount and water absorption gluing strength show contradictory tendencies. That is, in order to increase the amount of water absorption, it is necessary to reduce the crosslinking ratio for insolubilization of the polymer as much as possible, but on the other hand, the water absorption strength decreases. Therefore, in order to increase the water absorption glue strength, it is necessary to increase the crosslinking ratio, but in order to obtain a sufficiently satisfactory water absorption glue strength, the water absorption amount is usually set to a level that is not suitable for practical use. In reality, it is about 300 times or less. That is, by a method other than the method of the present invention, for example, a difunctional divinyl compound or a divinyl compound or a divinyl compound, etc. Obtained by water bath liquid polymerization or solution polymerization in the presence of a bifunctional compound capable of reacting with xyl groups. Even if the amount of cross-linking agent used in remer is minimized, the water absorption amount is at most about 1000 times its own weight, and the water absorption strength of the remer itself is extremely low, and if it is in the form of a paste, it will not be able to absorb enough water. If the amount of crosslinking agent is increased in order to obtain strength, the amount of water absorbed will be 300 times or less than its own weight.

In addition, JP-A-56-76419 discloses a water-in-oil type reverse phase suspension of an aqueous solution of an alkali metal acrylate with a concentration of 35% by weight to saturation containing hydroxyethylcellulose in the aqueous phase without using a crosslinking agent. A polymerization method has been proposed.

The water absorption capacity of illegally obtained polymers is 500 to 70% of its own weight.
Although it exhibits a relatively high value of 0 times, it has the disadvantage of extremely low water absorption strength and extremely poor durability. Well, fc1f obtained by this method! One of the characteristics of 1,1 mer is that it can be obtained in fairly large particles with a particle size of several hundred μm, but it is difficult to obtain it with good uniformity, and it is usually small, about 0 μm to about 500 μm. Some are quite large and have a wide distribution.

The present invention solves the above-mentioned conventional problems and has a high water absorbing strength without reducing the amount of water absorbed.Specifically, it has a water absorbing capacity of more than 700 times its own weight, and its average particle The present invention provides a method for producing a superabsorbent polymer having a diameter of 100 μm or more and a narrow distribution, and this is the most distinctive feature of the present invention.

(Detailed Description of the Invention) (1) Monomer 1 The monomer used in the polymerization reaction of the present invention is acrylic acid or a mixture of acrylic acid and methacrylic acid containing 20 mol methacrylic acid or less, and all of its cargexyl groups. The acrylic acid monomer is partially neutralized with an alkali metal salt at least 50 moles, preferably at least 60 moles. In the case of a mixture of acrylic acid and methacrylic acid, the amount of methacrylic acid can be up to 20 mol. Since the soluble portion increases, the content of methacrylic acid is preferably 10 molar or less.

As mentioned above, the degree of partial neutralization of acrylic acid or a mixture of acrylic acid and methacrylic acid (hereinafter sometimes referred to collectively as "acid monomers") is determined by its total cal? The range is such that 50 moles or more, preferably 60 moles or more of the xyl groups are alkali metal salts. If the degree of partial neutralization is too low, the water absorption amount will drop significantly and the obtained water absorption strength will also be extremely weak.

Alkali metal hydroxides, bicarbonates, etc. can be used to neutralize the harsh monomer, but alkali metal hydroxides are preferable, and specific examples include sodium hydroxide,
Potassium hydroxide and lithium hydroxide are mentioned. Sodium hydroxide is most preferred in terms of industrial availability, price, and safety.

(2) Water-soluble radical polymerization initiator The radical polymerization initiator used in the first production method of the present invention is persulfate such as potassium persulfate or ammonium persulfate, or hydrogen peroxide. These water-soluble radical polymerization initiators may be used in combination, or they may be used as a redox type polymerization initiator by combining reducing substances such as sulfites, amino acids, etc. good.

The amount of these persulfates and hydrogen peroxide used is 0.01 to 5% by weight, preferably 0.1 to 1% by weight, based on the neutralized acrylic acid monomer.

In addition, as a radical polymerization initiator, other water-soluble initiators, such as hydroperoxides such as butylhydronose oxide and cumene hydronooxide, 2,2'
-When using an azo compound such as azobis(2-amidiligulo-7) dihydrochloride, the resulting polymer becomes water-soluble.
, the purpose of the present invention cannot be achieved.

(3) Surfactant The surfactant used in the production method of the present invention is α
- A copolymer of an olefin and an α,β-unsaturated polycarboxylic acid anhydride or a derivative thereof. The α-olefin has 10 to 100 carbon atoms, preferably 16 to 60 carbon atoms.
It is.

Further, examples of the α,β-unsaturated polycarboxylic acid anhydride include maleic anhydride, citraconic anhydride, itaconic anhydride, etc. Among these, maleic anhydride is preferred. Derivatives of these copolymers include partially esterified or partially amidated copolymers. Examples of the partially esterified product include copolymer monomethyl ester, monoethyl ester, monobutyl ester, and the like. Further, examples of partially amidated copolymers include monoethylamide, monoethylamide, monobutylamide, and the like of copolymers.

Molecule of the above copolymer or its derivative #Fi5,000~
100,000. Preferably 10,000 to 50.0
It is 00.

Furthermore, in the present invention, the α-olefin/α,β-unsaturated polycarboxylic acid anhydride copolymer may be used in an acid anhydride state or in a partially or completely ring-opened state. There may be.

The amount of these surfactants used is 0.0
It is 1 to 10% by weight, preferably 0.05 to 5% by weight.

(4) Dispersion medium In principle, any liquid can be used as the dispersion medium used in the present invention as long as it does not participate in polymerization and does not mix with water. For example, aromatic hydrocarbons such as benzene, ethylbenzene, toluene, and xylene; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, and decalin; aliphatic hydrocarbons such as hexane, pen, tan, hegetane, and octane; benzene, brobenzene,
Examples include halodanized hydrocarbons such as dichlorobenzene. Among these, alicyclic hydrocarbons such as cyclohexane and methylcyclohexane, and aliphatic hydrocarbons such as hexane and hegetane are particularly preferred.

Further, these dispersion media can be used alone or in combination of two or more.

The amount of these dispersion media used is preferably 0.5 to 10 times the volume of the monomer aqueous solution, in order to make the polymerization reaction system a water-in-oil type, and from the standpoint of removing the heat of the polymerization reaction. It is desirable to increase the amount by 1 to 5 times.

(5) Water In the polymerization reaction solution of the present invention, water and acrylic acid monomer are present in the proportions described above (this point will be explained in more detail later).

In addition, water and a dispersion medium mean that, as mentioned above, the dispersion medium is an aqueous phase, that is,
It is desirable to use the monomer in an amount of 0.5 to 10 times, preferably 1 to 5 times, the volume of the monomer aqueous solution. If the ratio of water to dispersion medium is too high,
Since it is extremely difficult to remove the heat of polymerization, a stable water-in-surface-oil type suspension system cannot be obtained. Furthermore, if the ratio of water to the dispersion medium is too small, the monomer concentration per unit batch will be low, which will reduce productivity and be economically disadvantageous.

Note that water is generally added to the reaction system preferably in the form of water that dissolves the neutralizing agent that neutralizes the acid monomer, as described above, but a portion of the water may be added separately by an appropriate method. It is also possible to add In addition, some water is also generated by a neutralization reaction between an acid monomer and a neutralizing agent.

In the present invention, the acrylic acid monomer detailed above is used as a water-soluble radical polymerization initiator, a dispersion medium, a surfactant,
Polymerization is carried out by a water-in-oil reverse phase suspension polymerization method in the presence of water and water, but as mentioned above, the concentration of acrylic acid monomers relative to the water present in the polymerization reaction system at that time is the monomer concentration after neutralization. As such, the range from above 30 MMn2 to the saturation concentration is an important component of the present invention. That is, if the monomer concentration after neutralization is less than 30% by weight, the resulting polymer will have a large water-soluble portion and the resulting water absorption strength will be low, making it impossible to achieve the object of the present invention.

(6) Polymerization conditions Representative embodiments of the polymerization reaction of the present invention are as follows. That is, a polymerization initiator is added and dissolved in a monomer aqueous solution that has been neutralized in advance, and an inert gas such as nitrogen is introduced to perform deaeration. On the other hand, a surfactant is placed in a dispersion medium, heated slightly if necessary to dissolve it, and degassed by introducing an inert gas such as nitrogen.

The above monomer aqueous solution is poured into this and heated to a predetermined temperature. During this time, the aqueous solution portion of the reaction system becomes minute droplets and is dispersed and suspended in the dispersion medium.

After initiation of polymerization, cooling or heating is performed as appropriate depending on the state of heat generation.

The polymerization reaction temperature of the present invention is 60 to 100°C.

Preferably it is 60-80°C. If the reaction temperature is too low, the resulting polymer will have a large soluble portion and its water absorption performance will decrease; if the reaction temperature is too high, the water absorption performance will vary widely, and it will not be possible to produce a homogeneous polymer, but it will require high boiling point solvents. This has the disadvantage of requiring high temperatures for solvent recovery, etc.

The polymerization reaction of the present invention is carried out in a water-in-oil type suspension polymerization system, and as described above, it is necessary to combine an α-olefin and an α,β-unsaturated polyhydric cal. This objective can be achieved by combining means such as using a copolymer with phosphoric acid anhydride or a derivative thereof, appropriately adjusting the ratio of dispersion medium and water, and further performing appropriate stirring.

Stirring of the polymerization reaction system in the present invention is not only important for maintaining the desired stable water-in-oil type suspension system in the polymerization reaction system, but also improves the properties of the produced polymer. It is also important.

In other words, if the stirring is too strong, the fine human primary particles of the produced polymer may aggregate and become lumps, or the crosslinked structure may be broken and a partially water-soluble polymer may be produced. The variation in performance increases.

In addition, if the stirring is too weak, it will not become a stable dispersed system and abnormal polymerization will occur and the beads will not form.'-! The water absorption performance of the obtained 'polymer is also significantly reduced. Therefore, it is necessary to carry out appropriate stirring, but when using a polymerization reactor equipped with an ordinary stirrer for carrying out this type of suspension polymerization, the stirring speed is preferably 100 to 600 rpm, preferably 2.
By stirring at 00 to 400 rpm, bead-shaped polymers that have excellent water absorption and water absorption dullness strength and are easy to crush can be obtained with good reproducibility.

(7) Separation of N remer The polymer obtained by the production method of the present invention consists of wet, bead-like particles, and can be easily separated from the dispersion medium by decantation or evaporation. Then, the separated wet polymer is, for example, 1
Drying at temperatures below 20° C. results in powdered polymers or powdered polymers containing easily pulverized lumps. The polymer thus obtained usually has a diameter of 10
These particles are approximately 0 to 500 μm in diameter and contain a small amount of perfectly spherical primary particles whose surfaces are covered with a surfactant or secondary particles in which some of these primary particles are agglomerated. These secondary particles can also be easily pulverized by a small amount of mechanical force. This has great advantages in terms of polymer production and use.

[Effects of invention, etc.]

As mentioned above, the polymer obtained by the production method of the present invention has a high water absorption performance of 700 times or more than its own weight, has a high water absorption glue strength, and the polymer itself is a powder from the beginning or can be crushed easily. It can be easily made into powder.

Therefore, the polymer obtained by the production method of the present invention can be advantageously used in the production of sanitary napkins, paper diapers, etc., and other sanitary materials by utilizing its excellent water absorption performance.

It is also used in the production of various horticultural and agricultural materials, including soil conditioners and water retention agents, which have recently been attracting attention due to their excellent water absorption performance and relativity. can do.

Hereinafter, the present invention will be further explained in detail by giving Examples and Comparative Examples.

Note that the pure water absorption capacity and water absorption strength described in these examples are the results measured by the following test method.

A. In a beaker with a pure water absorption capacity of 12, polymer about 0.5.9 and pure water of 1.
Weigh and add each of , e, and mix. Leave for about 60 minutes, then add water. IJ Mama - fully swollen. Next, after draining with a 100-mesh filter, the amount of the filtrate is weighed, and the pure water absorption capacity is calculated according to the following formula.

B. Water-absorbing glue strength 200 times its own weight of pure water was added to the polymer to absorb water, and the strength of the water-absorbing glue was examined by pressing the elasticity of the obtained water-absorbing glue with a finger, and evaluated according to the following criteria.

×: Weak △: Slightly weak ○: Normal (standard) ■ = Slightly strong 01 Strong Example 1 ゛ Capacity 500m/! with stirrer, reflux condenser, thermometer, and nitrogen gas inlet pipe! Put 185 g of cyclohexane into a four-necked round-bottomed flask, and add 1.8 fi of α-olefin and maleic anhydride copolymer (part name: "Diacarte 30" manufactured by Mitsubishi Kasei ■, molecular weight approximately 10,000). The mixture was dissolved and the internal temperature was set to 65° C. under a nitrogen gas atmosphere.

Separately, 12.9 g of caustic soda in which 49 g of water was dissolved was added to 30 g of acrylic acid while externally cooling it to a conical flask having a capacity of 200,116 mm, thereby neutralizing 77.4% of the carboxylic groups.

In this case, the monomer concentration with respect to water corresponds to 40% by weight as a heptamer concentration after neutralization.

Next, 0.1 g of potassium persulfate was added and dissolved.

Add this 200I1 to the contents of the four-necked round bottom flask.
The contents of flask No. 1 were added, and polymerization was carried out at 65 to 70° C. for about 1 hour while stirring. Note that stirring was performed at 300 rpm.

When stirring was stopped after 1 hour of reaction, the wet polymer particles settled to the bottom of the flask and could be easily separated from the cyclohexane phase by decantation. Separated moisture 1
The polymer was transferred to a vacuum dryer and heated at 80 to 90°C to remove attached cyclohexane and water.

The resulting dry polymer was a powder containing free-flowing, easily grindable clumps.

Example 2 The procedure was the same as in Example 1, except that the Diacarte 30 used in Example 1 was treated with a large excess of methanol under reflux for about 8 hours, and the excess methanol was removed under reduced pressure. Polymerization and post-treatment were carried out according to the recipe.

The sheathed polymer was a powder with free-flowing, easily grindable clumps.

Example 3 The same procedure was carried out except that the Diacarte 30 used in Example 1 was treated with a large excess of butylamine under reflux at 70°C for about 8 hours, and the excess butylamine was removed under reduced pressure and the product was used as a surfactant. Polymerization and post-treatment were carried out using the same recipe as in Example 1.

The resulting polymer was a powder containing free-flowing, easily grindable clumps.

Example 4 In place of acrylic acid in Example 1, acrylic acid 28
The reaction was carried out in the same manner as in Example 1, except that a mixture of 2.9 g and 2.9 g of methacrylic acid was used, and the post-treatment was carried out in the same manner as in Example 1.

The resulting polymer was a powder containing free-flowing, easily grindable clumps.

Example 5 The amount of water used to dissolve caustic soda in Example 1 was reduced to 6
3.1,! i+, but otherwise the polymerization reaction was carried out in the same manner as in Example 1, and the same post-treatments were carried out. In this case, the monomer concentration with respect to water in the reaction system is 35% by weight as a seven-mer concentration after neutralization.

The resulting polymer was a powder containing free-flowing, easily grindable clumps.

Example 6 The amount of caustic 1z1 soda in Example 1 was set to 10.9,
The polymerization reaction was carried out in the same manner as in Example 1, except that the amount of water used for r=h of caustic soda was changed to 48.8 g, and the same post-treatment was carried out. In this case, 0.60% of acrylic acid is neutralized, and the monomer concentration d based on the water in the reaction system is 40% by weight as the monomer concentration after neutralization.

The resulting primer was a powder containing free-flowing, easily grindable clumps.

Comparative Example 1 Polymerization was carried out in the same manner as in Example 1 of JP-A-56-76419 to obtain a dry polymer. 5 with a section, stirrer, reflux condenser, dropping tube, and nitrogen gas inlet pipe.
0 Qcc 4 Lof flasks with hexane 230m/! Sorbitan monostearate 1.8. After nitrogen gas was blown into the reactor to drive out dissolved oxygen, the reactor was heated to 60 to 65°C. Separately, in a beaker, acrylic 8309 was cooled with ice from the outside and dissolved in water 499, and 134 g of 98% caustic soda was added to the solution. 78% of the xyl groups were neutralized.

The monomer concentration in the aqueous phase was 40% by weight.

Next, hydroxyethyl cellulose (degree of etherification 0.
8. Number of moles added: 1.9) After adding and dissolving 1.8.9, nitrogen gas was blown in to remove dissolved oxygen. The contents in the beakers were added to the four flasks and dispersed, the internal temperature was maintained at 60 to 65° C., and stirring was continued for 3 hours. The hexane was distilled off under reduced pressure, and the remaining swollen polymer portion was about 8
It was dried under reduced pressure at 0°C to obtain a powdery primer.

Table 1 shows the results of measuring the pure water absorption capacity, water absorption glue strength, and particle size of the dry polymers obtained in Examples 1 to 6 and Comparative Example 1.

As is clear from the results, the polymer obtained by the undiscovered method has a high water absorption capacity, a high water absorption gluing strength, a large particle size, and a relatively narrow particle size distribution.

Table 1

Claims (1)

[Claims] 1) Acrylic acid or an acrylic acid/methacrylic acid mixture and an alkali metal salt of acrylic acid or an acrylic acid/methacrylic acid mixture (hereinafter referred to as an acrylic acid monomer) are combined with a water-soluble radical polymerization initiator, In a method of polymerizing by a water-in-oil reverse phase suspension polymerization method in the presence of a dispersion medium, a surfactant, and water, an α-olefin and an α,β-unsaturated polycarboxylic acid anhydride are used as surfactants. using a copolymer or a derivative thereof, the concentration of the acrylic acid monomer relative to the water present in the polymerization reaction system is in the range of 30% by weight or more as the monomer amount after neutralization to the saturated concentration, and in the absence of a crosslinking agent. 2) A method for producing a bead-like self-crosslinking water-absorbing polymer, characterized in that the amount of methacrylic acid in the acrylic acid/methacrylic acid mixture is 20 mol% or less, according to claim 1. 3) A manufacturing method according to claim 1, wherein the acrylic acid monomer has 50 mol% or more of its total carboxyl groups neutralized with an alkali metal salt. The manufacturing method according to claim 1, wherein the water-soluble radical polymerization initiator is one or more selected from the group consisting of hydrogen peroxide and persulfates.