JP5116088B2 - Water-absorbent resin having high water absorption ratio and low residual monomer, and method for producing the same - Google Patents

Description

  The present invention relates to a water-absorbent resin widely used in sanitary material fields such as disposable diapers and sanitary napkins, agricultural and forestry fields, civil engineering fields, and the like, and a method for producing the same. In particular, the present invention relates to a water-absorbing resin for sanitary materials such as disposable diapers and sanitary napkins that require a high water absorption ratio, and a method for producing the same. More specifically, a high water absorption ratio and a low residual monomer concentration are achieved by polymerizing an aqueous monomer solution containing an unsaturated carboxylic acid ammonium salt monomer by ultraviolet irradiation using a radical photopolymerization initiator and a peroxide. It can be done.

  In recent years, hygienic materials such as disposable diapers and sanitary napkins, so-called incontinence pads, have widely used water-absorbing resins for the purpose of absorbing bodily fluids as constituent materials. These water-absorbing resins include partially crosslinked polymers of acrylic acid-alkali metal acrylate (see, for example, Patent Document 1), starch-acrylonitrile hydrolysates (see, for example, Patent Document 2), and starch-acrylic acid partially crosslinked products. (See, for example, Patent Document 3), hydrolyzate of vinyl acetate-acrylic acid ester copolymer (see, for example, Patent Document 4), hydrolyzate of acrylonitrile copolymer or acrylamide copolymer (see, for example, Patent Document 5), etc. Is mentioned.

  Since these water-absorbing resins are used for sanitary materials, the amount of residual monomers is severely limited. For example, according to the “voluntary standards for water-absorbing resins for paper diapers” by the Water Absorbent Resin Industry Association, it is required to be 1000 ppm or less. It has been. In general, the monomer of the water-absorbing resin is water-soluble, and it is preferable to reduce the residual monomer as much as possible. Reduction of the residual monomer has been attempted by various methods. For example, there is a method in which a residual monomer is reduced by adding an inorganic salt or a reducing substance such as ascorbic acid before drying the gel polymer (for example, see Patent Document 6 or Patent Document 7). However, in this method, there is a concern that the substance to be added remains in the water-absorbent resin, and its safety may be a problem. Further, there are also methods in which residual monomers are reduced by examining polymerization methods and conditions such as a method using ultraviolet irradiation or radiation (for example, Patent Document 8, Patent Document 9, Patent Document 10, Patent Document 11, Patent Document 12). Patent Document 13 and Patent Document 14).

In particular, in Patent Documents 8 and 9, the residual monomer and water-soluble content can be reduced by a simpler method compared to the prior art by using a radical photopolymerization initiator and a peroxide in combination. However, even in this method, since sodium acrylate is used as the monomer, the residual monomer is present at 400 ppm or more, and it can be said that the reduction effect is insufficient. Other literature methods are unfavorable in some cases because of the concern of a decrease in physical properties of the water-absorbent resin, in particular, the water absorption capacity.
The method of extracting the residual monomer after the polymerization also has a problem in that the extraction solvent remains from the viewpoint of safety (for example, Patent Document 15).

  A method of heat-treating a gel polymer obtained by polymerizing a monomer in which 10 to 96 mol% of acid groups are neutralized as an alkali metal salt and 4 to 50 mol% as an ammonium salt is also being studied ( For example, Patent Document 16). However, from the description contents of Examples and Comparative Examples, since the water absorption ratio is low and the residual monomer is still large, neutralization of carboxylic acid is used in combination with alkali metal salt and ammonium salt to suppress acrylamide formation. In addition, the ammonium salt must be 4 to 50 mol%. Therefore, the water absorption magnification is equivalent to that of a normal polyacrylic acid sodium salt water absorbent resin.

  The water absorption capacity is considered to be the most important as the performance of the water absorbent resin. Studies to increase the water absorption ratio are also being conducted. As an example, there has been proposed a method for increasing the water absorption swelling power itself using a water-absorbent resin having a high charge density. Specifically, water-absorbing resins in which acidic groups are neutralized in the form of lithium salts are exemplified (for example, Patent Document 17). That is, by using a lithium salt with a light atomic weight, more functional groups can be introduced as compared with conventionally used sodium salts and potassium salts, and the water absorption swelling power itself is increased by increasing the charge density. Similarly, the water absorption ratio can be improved with an ammonium salt (for example, Patent Documents 18 and 19). However, the use of a lithium salt is disadvantageous because of its availability and is not preferred. Moreover, it can be said that there are still many residual monomers of the water-absorbing resin polymerized by the method using the normal peroxide disclosed in the above-mentioned literature or a combination of a peroxide and a reducing agent as an initiator.

  On the other hand, these water-absorbing resins do not have sufficient physical properties for sanitary materials as mere polymers, and various physical property improvements other than the water absorption magnification have been made. What is indicated as an index at that time is an excellent water absorption ratio, water absorption speed, gel strength, suction force for sucking water from a substrate containing an aqueous liquid, and the like when contacting an aqueous liquid such as a body fluid. However, the relationship between these characteristics does not necessarily show a positive correlation. For example, the higher the water absorption ratio, the lower the physical properties such as liquid permeability, gel strength, and absorption rate.

Therefore, as a method for improving various characteristics such as the water absorption speed of the water absorbent resin in a well-balanced manner, a technique for crosslinking the vicinity of the surface of the water absorbent resin is known, and various methods have been proposed so far.
For example, a method using a polyhydric alcohol as a crosslinking agent (see, for example, Patent Documents 20 and 21), a method using a polyvalent glycidyl compound, a polyvalent aziridine compound, a polyvalent amine compound, and a polyvalent isocyanate compound (see, for example, Patent Document 22). ), A method using glyoxal (see, for example, Patent Document 23), a method using a polyvalent metal (see, for example, Patent Documents 24 and 25), a method using a silane coupling agent (see, for example, Patent Documents 26, 27, 28), and the like. Are known.

  In addition, as an attempt to perform uniform surface crosslinking by applying a crosslinking agent to the surface of the water-absorbent resin during the crosslinking reaction, a method in which an inert inorganic powder is present when the crosslinking agent is added (see, for example, Patent Documents 29 and 30), Also known are a method in which a monohydric alcohol is present (see, for example, Patent Document 31), a method in which water and an ether compound are present (see, for example, Patent Document 32), and a method in which phosphoric acid is present (see, for example, Patent Document 33). .

Further, as an application example of the above method, there is also an example that discloses a method for producing a water absorbent resin that is excellent in water absorption rate and excellent in water absorption under pressure by polymerization in the presence of a blowing agent and surface crosslinking treatment. (For example, patent document 34).
Attempts to increase the water absorption speed of the water-absorbent resin include a method of reducing the particle size in order to increase the surface area, or a method of making a flake, granule, or porous (for example, Patent Document 35).
However, in general, when the water-absorbing resin is formed to have a small particle size, the water-absorbing resin is in a so-called Mako state due to contact with the aqueous liquid, and the water-absorbing resin itself becomes a water-stopping layer. Speed decreases. Even when the water-absorbent resin is formed into an aggregate, the aggregate itself usually becomes a macho state inside the aggregate due to contact with the aqueous liquid, and the absorption rate is reduced.

  As described above, although the balance of various properties such as the residual monomer of the water-absorbent resin, the water absorption ratio and the water absorption speed is improved by these methods, it is still not sufficient, and further quality improvement is demanded. In particular, when considering the necessary characteristics of the water-absorbent resin used in the absorbent article in a sanitary product that has been thinned using a large amount of water-absorbent resin, which is a recent trend, the above-mentioned conventional method still has a sufficient physical property level. It is the current situation that has not yet reached.

JP 55-84304 A Japanese Patent Publication No.49-43395 Japanese Patent Laid-Open No. 51-125468 JP-A-52-14689 Japanese Patent Publication No.53-15959 JP-A 64-62217 JP 56-103207 A Japanese Patent No. 3569322 Japanese Patent No. 3852515 JP 50-96689 A JP-A-56-72005 JP 63-43930 A JP-A-63-260906 JP-A-53-145895 Japanese Patent Laid-Open No. 1-29003 Japanese Patent No. 3259143 Japanese Patent Laid-Open No. 11-60975 JP 2004-315816 A JP 2005-200630 A JP 58-180233 A JP-A 61-16903 JP 59-189103 A JP 52-117393 A JP 51-136588 A JP-A-61-257235 JP-A-61-211305 JP-A 61-252212 JP 61-264006 A JP-A-60-163958 JP 60-255814 A JP-A-1-292004 Japanese Patent Laid-Open No. 2-153903 Japanese National Patent Publication No. 8-508517 Special table 8-509521 gazette JP-A-10-251308

In recent years, absorbent articles such as paper diapers have been desired to reduce residual monomers as much as possible and to reduce the amount of water-absorbent resin used from the viewpoint of safety. There is a need for improved performance and improved water absorption rate.
An object of the present invention is to provide a water-absorbent resin that can be suitably used for sanitary materials such as disposable diapers, sanitary napkins, incontinence pads, etc. and has a high balance between high water absorption performance and high water absorption speed, which has been difficult to achieve in the past. And it is providing the manufacturing method which can manufacture the water absorbing resin of this invention simply by an inexpensive method.

As a result of diligent studies to solve the above problems, the present inventors polymerized and produced a radical photopolymerization initiator containing a water-soluble unsaturated carboxylic acid ammonium salt and a peroxide by irradiation with ultraviolet rays. It was found that the water-absorbent resin produced by drying the water-absorbent resin and then performing heat treatment under specific conditions has a low residual monomer and a high water absorption ratio. Furthermore, acrylamide as a by-product is not detected from the water-absorbent resin produced by the production method of the present invention.
That is, the present invention is as described below.

[1] A method for producing a water-absorbent resin, comprising a polymerization step of polymerizing an aqueous monomer solution containing a water-soluble unsaturated carboxylic acid ammonium salt under the following conditions (1) and (2).
Polymerization conditions:
(1) The content of the water-soluble unsaturated carboxylic acid ammonium salt in the monomer aqueous solution is in the range of more than 50 mol% and not more than 100 mol% in all monomers.
(2) Irradiate with ultraviolet rays using a radical photopolymerization initiator and a peroxide.
[2] The method for producing a water-absorbent resin according to [1], wherein an aqueous monomer solution containing a water-soluble unsaturated carboxylic acid ammonium salt is polymerized under the following conditions (3) and (4): .
(3) The thickness of the monomer aqueous solution is 50 mm or less.
(4) The dissolved oxygen in the monomer aqueous solution is adjusted to 4 ppm or less.
[3] The method for producing a water-absorbing resin according to [1] or [2], wherein the water-soluble unsaturated carboxylic acid ammonium salt is an ammonium acrylate salt.
[4] The method for producing a water-absorbent resin according to the above [1] to [3], wherein the monomer aqueous solution is polymerized in a state containing a foaming agent.
[5] The method for producing a water-absorbent resin according to [4], wherein the foaming agent is a carbonate.
[6] A drying treatment step for drying the resin obtained by the polymerization step, and a heating treatment step for heat-treating the dried resin at a temperature higher by 10 ° C. or more than the drying conditions of the drying treatment step. The method for producing a water-absorbent resin according to the above [1] to [5].
[7] The method for producing a water absorbent resin according to [6], wherein the heat treatment is performed under the following conditions (5) and (6).
(5) 10 degreeC or more higher than the drying conditions of a water absorbing resin, and it carries out in 100-250 degreeC.
(6) The heat treatment is performed under heat treatment conditions such that the neutralization rate of the outer surface layer of the water-absorbent resin is less than 50 mol% and the neutralization rate of the resin center is 50 mol% or more.

The water-absorbing resin of the present invention has a good water-absorbing performance such as a water retention ratio of 40 g / g or more and a water absorption ratio of 10 g / g or more under pressure of 57 g / cm ² and a residual monomer content of 20 ppm or less. Suitable for hygiene materials.
In addition, in the method for producing the water absorbent resin of the present invention, the water absorbent resin of the present invention can be produced simply, for example, without an extraction step with a solvent and without adding additives that are likely to remain and affect various physical properties. It is very useful because it can.

The present invention is described in detail below.
<About the production method of the present invention>
The method for producing the water-absorbent resin of the present invention comprises polymerizing a monomer solution containing an unsaturated carboxylic acid ammonium salt by irradiating with ultraviolet rays in the presence of a radical photopolymerization initiator and a peroxide, In this method, the resulting resin is dried and then heat-treated.
The unsaturated carboxylic acid ammonium salt monomer used in the present invention is not particularly limited. (Meth) acrylic acid, itaconic acid, (anhydrous) maleic acid, crotonic acid, fumaric acid, 2- (meth) acryloylethanesulfone Examples thereof include ammonium salts such as acid and 2- (meth) acrylamido-2-methylpropanesulfonic acid, and preferably an ammonium salt of (meth) acrylic acid is used. The method for producing an unsaturated carboxylic acid ammonium salt may be a derivative such as neutralization of carboxylic acid or hydrolysis of an unsaturated amide compound or unsaturated nitrile compound by a microorganism.

  In this invention, other monomers other than unsaturated carboxylic acid ammonium salt can also be added. Other monomer components include methyl (meth) acrylate, ethyl (meth) acrylate, (meth) acrylamide, (meth) acrylonitrile, vinyl acetate, hydroxyethyl (meth) acrylate, methoxyethyl (meth) acrylate, hydroxypropyl (Meth) acrylate etc. are mentioned, Among these, 1 type, or 2 or more types can be added at 50 mol% or less in all the monomer components. You may add the compound used as a condensation type crosslinking agent with respect to the carboxyl group added as a crosslinking agent. Condensation type crosslinking agents include glycidyl ether compounds such as ethylene glycol diglycidyl ether, trimethylolpropane triglycidyl ether, (poly) glycerin polyglycidyl ether, diglycerin polyglycidyl ether, propylene glycol diglycidyl ether; (poly) glycerin, ( Poly) ethylene glycol, propylene glycol, 1,3-propanediol, polyoxyethylene glycol, triethylene glycol, tetraethylene glycol, diethanolamine, triethanolamine and other polyhydric alcohols; ethylenediamine, diethylenediamine, polyethyleneimine, hexamethylene Polyvalent amines such as diamines; polyvalent ions such as zinc, calcium, magnesium, aluminum That.

  There is no problem in copolymerizing a polymerizable crosslinking agent which is another crosslinking agent. Examples of polymerizable crosslinking agents include diethylene glycol diacrylate, N, N′-methylenebisacrylamide, polyethylene glycol diacrylate, polypropylene glycol diacrylate, trimethylolpropane diallyl ether, allyl glycidyl ether, pentaerythritol triallyl ether, and pentaerythritol diacrylate mono. Examples include stearate, bisphenol diacrylate, isocyanuric acid diacrylate, tetraallyloxyethane, diallyloxyacetate, and the like. Of these polymerizable crosslinking agents, N, N'-methylenebisacrylamide or trimethylolpropane triacrylate is particularly desirable.

  As another additive, a foaming agent is preferably added for the purpose of expanding the surface area of the water-absorbent resin. As the foaming agent, a known carbonate can be used. As carbonates, any carbonates or bicarbonates including salts or mixed salts are used in the present invention. Examples of carbonates more preferable for the present invention include sodium carbonate, sodium bicarbonate, potassium bicarbonate, carbonates. Examples thereof include ammonium, ammonium hydrogen carbonate, magnesium carbonate, calcium carbonate, barium carbonate and the like, and hydrates thereof, and one or more of them are used. In particular, preferred carbonates for the present invention are monovalent cations such as sodium, potassium, ammonium carbonates or bicarbonates. When a carbonate composed of a polyvalent cation species is used, the polymer having a carboxyl group is metal-crosslinked by the polyvalent cation species and adversely affects water absorption performance.

  The amount of carbonate added to the monomer aqueous solution is preferably 0.01 to 10% by weight, more preferably 0.1 to 5% by weight, based on the monomer component. When the amount of carbonate added is less than 0.01% by weight, the water-containing gel obtained by polymerization does not become a porous body. Moreover, when it adds exceeding 10 weight%, water soluble content will increase and the water retention capacity of resin will also be obstructed. Further, it is preferable to add the carbonate before the ultraviolet irradiation. As an addition method, the carbonate may be added as it is, or it may be dissolved in an arbitrary solvent and added as a carbonate solution.

  In the present invention, an antifoaming agent can be used for the purpose of controlling the foaming agent and the foaming time. As the antifoaming agent, generally known antifoaming agents, antifoaming agents, foam stabilizers and the like can be used, and one kind or a combination of two or more kinds can also be used. Specific antifoaming agents include fats and oils, fatty acids, lower alcohols, superabsorbent alcohols, metal soaps, silicones, hydrophobic silica / silicone compounds, fatty acid esters, polyglycols, poly Examples include glycol esters, polyethers, modified silicones, oil-soluble polymers, organophosphorus compounds, sulfated fatty acids, polyether derivatives, silica / modified silicone compounds.

  The solvent of the monomer solution is not particularly limited as long as it has excellent monomer solubility. Particularly preferred is water alone, but hydrophilic solvents such as ethanol, methanol, acetone and the like may be used alone or in combination. Moreover, you may add basic compounds, such as salts, such as sodium chloride, and ammonia for the purpose of pH control as needed. As a polymerization method, a known method such as solution polymerization can be used, but aqueous solution polymerization is preferable from the viewpoint of energy, such as not using an organic solvent. The type of the reactor is one in which polymerization is initiated by irradiation with ultraviolet rays, and is not particularly limited as long as the requirement is satisfied, and may be either a batch type or a continuous type. An apparatus such as an endless belt, which is a known reaction apparatus, may be used.

  In order to obtain the water-absorbent resin of the present invention, it is necessary to perform polymerization by irradiating with ultraviolet rays using a radical photopolymerization initiator and a peroxide as a polymerization initiation method. Examples of the radical photopolymerization initiator include benzoin, benzyl, acetophenone, benzophenone and derivatives thereof which are generally used for photopolymerization. Examples of derivatives include benzoin-based compounds such as benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, and acetophenone-based compounds such as diethoxyacetophenone, 2,2-dimethoxy-1,2-diphenyl. Ethan-1-one, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1- (4- (methylthio) phenyl) -2-monfolinopropane-1, 2-benzyl-2-dimethylamino-1- (4- Morpholinophenyl) butanone-1,2-hydroxy-2-methyl-1-phenylpropan-1-one, as benzophenone series, methyl O-benzoylbenzoate, 4-phenylbenzophenone, 4-benzoyl-4′-methyl Diphenyl Rufide, 3,3 ′, 4,4′-tetra (t-butylperoxycarbonyl) benzophenone, 2,4,6-trimethylbenzophenone, 4-benzoyl-N, N-dimethyl-N- [2- (1- Oxy-2-propenyloxy) ethyl] benzenemethananium bromide, (4-benzoylbenzyl) trimethylammonium chloride, 4,4′-dimethylaminobenzophenone, 4,4′-diethylaminobenzophenone, and the like.

  As other radical photopolymerization initiators, azo compounds can be used, and azonitrile compounds, azoamidine compounds, azoamide compounds, alkylazo compounds, and the like can also be used. However, in this case, since it is necessary to add a relatively large amount, it is preferable to use a radical photopolymerization initiator having a benzoyl group because it is difficult to increase the degree of polymerization. The addition amount of the photopolymerization initiator is preferably 0.0001 to 0.1% by weight, more preferably 0.001 to 0.01% by weight, based on the monomer component. When the addition amount of the photopolymerization initiator is less than 0.0001% by weight with respect to the monomer component, the polymerizability becomes extremely low. On the other hand, when it exceeds 0.1% by weight, low molecular weight tends to increase. Yes, the water-soluble content tends to increase.

  In the present invention, peroxide is used to reduce residual monomer. Examples of preferred peroxides for the present invention include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; hydrogen peroxide; organicization such as cumene hydroperoxide, t-butyl hydroperoxide, and peracetic acid. Well-known initiators, such as an oxide, are mentioned. One type or two or more types can be used in combination. The amount of peroxide added is preferably 0.001 to 10% by weight, more preferably 0.01 to 1% by weight, based on the monomer component. When the amount of peroxide added is less than 0.001% by weight, it is difficult to sufficiently reduce the residual monomer. When the amount added is 10% by weight or more, the water-soluble component increases and the obtained water-absorbent resin is obtained. May be colored.

  It is preferable to perform a deoxygenation operation in the monomer solution in advance before the start of polymerization. Specifically, dissolved oxygen is removed by bubbling with an inert gas for a sufficient time. Further, it is desirable that the atmosphere in the reactor is replaced with an inert gas such as nitrogen or helium. In the present invention, the dissolved oxygen in the monomer aqueous solution is preferably 4 ppm or less, more preferably 1 ppm or less. When the dissolved oxygen in the monomer aqueous solution exceeds 4 ppm, the polymerization start time is delayed, the reaction is not completed, and the residual monomer may increase. Furthermore, the inside of the reactor may be any of reduced pressure, normal pressure, and increased pressure.

  In the present invention, polymerization is initiated by irradiation with ultraviolet rays, and it is desirable that the ultraviolet rays be sufficiently transmitted into the unsaturated monomer aqueous solution. The thickness of the aqueous monomer solution is preferably 50 mm or less, more preferably 20 mm or less in order to control the reaction temperature (maximum temperature reached by the polymer) and to maintain sufficient UV transmission. When the thickness of the monomer aqueous solution exceeds 50 mm, ultraviolet irradiation is not performed uniformly, and the polymer may become non-uniform. Although there is no restriction | limiting in particular in the lower limit of the thickness of this monomer aqueous solution, When productivity is considered, 1 mm or more is preferable.

The amount of ultraviolet light is not particularly limited, but is usually 10 to 10,000 mJoul / cm ² . If the amount is less than this range, polymerization may be insufficient. If the amount is more than this range, excessive crosslinking may result in cleavage of the crosslinking point of the obtained polymer, which may increase the water-soluble content. Moreover, as a light source used for ultraviolet irradiation, a conventionally known light source can be used. For example, a mercury lamp, a metal hydride lamp or the like may be used in consideration of reaction conditions. The irradiation wavelength is not particularly limited, and light having a wavelength of 200 to 450 nm is usually used. Although the ultraviolet irradiation time is determined so as to be the above light amount, polymerization is started immediately after the irradiation is started under the above conditions, and the polymerization is usually completed in a short time of 10 to 180 seconds.

The polymerization start temperature is 0 to 30 ° C. The unsaturated monomer aqueous solution before the ultraviolet irradiation is preferably maintained at a temperature of 30 ° C. or lower, more preferably 0 to 20 ° C. When the aqueous monomer solution exceeds 30 ° C., the temperature of the reaction system becomes too high, so that the molecular weight may be lowered, the water retention capacity may be lowered, and the water-soluble content may be increased. There is no restriction | limiting in particular about this monomer aqueous solution temperature, What is necessary is just the temperature which this monomer aqueous solution does not freeze, and what is necessary is just 0 degreeC or more normally.
The concentration of the aqueous monomer solution is not particularly limited as long as the monomer can be dissolved, but is preferably 10 to 70% by weight, and particularly economical when ammonium acrylate is used as the monomer. From the viewpoint of ease of reaction control, 30 to 65% by weight is most preferable.

  When the water-soluble unsaturated monomer begins to polymerize, the temperature in the system rises, but in order to obtain an excellent water-absorbent resin, it is preferable to suppress the maximum temperature in the system to 120 ° C. or less, and more preferably Is to keep the temperature below 100 ° C. When the highest temperature in the system exceeds 120 ° C., the water-soluble content of the polymer obtained by polymerizing the monomer aqueous solution increases. In addition, the water retention capacity is poor. Various methods can be considered as a method for suppressing the maximum temperature during polymerization, for example, a method of cooling the polymer contact portion from the outside, a method of applying cold air to the polymer, etc., but these methods also increase the equipment, The aforementioned conditions, that is, the monomer aqueous solution concentration is set to 30 to 65%, the temperature of the monomer aqueous solution is set to 30 ° C. or less, and the thickness of the monomer aqueous solution is set to 50 mm or less, preferably 1 to 20 mm. It is desirable to keep the maximum temperature in the system at 120 ° C. or lower by adopting the condition of

  After the reaction, a water-containing gel is formed by solution polymerization. This is roughly crushed and then dried. After drying, it is pulverized to about several hundred μm and granulated. The particle size distribution is desirably in the range of 3000 μm to 1 μm, particularly preferably 1000 μm to 50 μm, and more preferably 900 to 100 μm. As a method for rough crushing, an apparatus capable of cutting and extruding a rubber-like elastic body can be used. For example, a cutter type cutting machine, a chopper type cutting machine, a kneader type cutting machine, etc. Can be achieved. When a cutter-type cutting machine is used, the polymer is less deteriorated due to the shear during gel cutting. In order to pulverize the dried gel, a conventionally known pulverization method can be employed. For example, it can be pulverized to a desired particle size by a vibration pulverizer, an impact pulverizer, a friction pulverizer, or the like.

  The drying method is not particularly limited, and vacuum drying and hot air drying are preferable. As the dryer that can be used in the present invention, a normal dryer or a heating furnace can be used, and examples thereof include a hot air dryer, a fluidized bed dryer, an air flow dryer, an infrared dryer, and a dielectric heating dryer. It is done. The drying temperature is preferably in the range of 70 to 180 ° C, particularly preferably 100 to 120 ° C.

After drying, the granulated water-absorbent resin is heat-treated, but may be continuously heated in the same dryer after the heating is completed, and may be a process independent of the drying process. It must be higher by 10 ° C. than the process. The heating condition is composed of two elements, that is, a heating temperature and a heating time, and has a great influence on the water absorption performance of the water absorbent resin. Although it varies depending on the required properties of the water-absorbent resin, it is preferably higher by 10 ° C. or higher than the drying conditions, more preferably 20 ° C. or higher, most preferably 30 ° C. or higher, and preferably 100 to 250 ° C. It is the range of this, Preferably it is 120-200 degreeC, More preferably, it is 150-170 degreeC. The heating time is preferably 10 minutes to 5 hours, most preferably 30 minutes to 1 hour. When heat treatment is performed under these conditions, it is possible to adjust the neutralization rate of the outer resin surface layer of the water-absorbent resin and the neutralization rate of the resin center to a preferable range. As this preferable range, the carboxyl group ammonia neutralization rate at the center of the resin is 50 mol% or more, preferably 60 mol% or more, most preferably 70 mol% or more, and the carboxyl group ammonia neutralization rate on the resin outer surface is less than 50 mol%. , Preferably 45 mol% or less, most preferably 40 mol% or less. The difference in the neutralization rate between the resin center and the resin outer surface is preferably 5 mol% or more, and more preferably 10 mol% or more.
In addition, there is no restriction | limiting in particular about a heat processing apparatus, Well-known apparatuses, such as a hot air dryer, a fluidized bed dryer, and a Nauter type dryer, are used.

<Method for obtaining unsaturated ammonium carboxylate by hydrolysis with microorganisms>
An unsaturated nitrile subjected to a hydrolysis reaction by a microorganism refers to a compound containing both an unsaturated bond and a cyan group in the molecule. It may contain many unsaturated bonds and cyan groups. The unsaturated bond means a bond containing a double bond (ethylene bond) or a triple bond (acetylene bond) in a bond between carbon atoms. Examples of such compounds include acrylonitrile, methacrylonitrile, crotonnitrile, cinnamate nitrile and the like. Of these, acrylonitrile and methacrylonitrile are preferable, and acrylonitrile is particularly preferable.

Moreover, the unsaturated amide used for the hydrolysis reaction by microorganisms is a compound containing both an unsaturated bond and a functional group represented by the general formula R—CONH ₂ (where R is an alkyl group, aryl group, etc.) in the molecule. I mean. Examples of such a compound include cinnamic acid amide, acrylamide, and methacrylamide, and acrylamide and methacrylamide are preferable, and acrylamide is particularly preferable.

  There are no particular limitations on the hydrolysis conditions of unsaturated nitrile and / or unsaturated amide by microorganisms, but microorganisms capable of producing an aqueous solution of unsaturated ammonium carboxylate having a concentration of 20% by weight or more are preferable. As such a microorganism, it is preferable to use at least one selected from the group consisting of Acinetobacter, Alkagenes, Corynebacterium, Rhodococcus, and Gordona. Among the microorganisms, microorganisms belonging to the genus Acinetobacter are preferable, and among them, the microorganism is Acinetobacter sp. AK226 strain (FERM BP-08590) or Acinetobacter sp. AK227 strain (FERM BP-08591). The microbiological properties of Acinetobacter sp. AK226 strain (FERM BP-08590) and Acinetobacter sp. AK227 strain (FERM BP-08591) are as shown below.

The unsaturated ammonium carboxylate aqueous solution produced by this microorganism hydrolysis method has a very small amount of impurities such as dimers and / or hydrates of unsaturated carboxylic acids. Therefore, this manufacturing method is a preferable method.
Specific examples of the impurities include, in the case of acrylic acid, β-acryloyloxypropionic acid which is a dimer of acrylic acid, β-hydroxypropionic acid which is a hydrate of acrylic acid, and salts thereof. Can be mentioned.

<About water absorbent resin>
The water-absorbent resin of the present invention is a water-absorbent resin in which 50 mol% or more of repeating units in the polymer molecular chain are composed of carboxyl group-containing units. Of the repeating units in the polymer molecular chain, the carboxyl group-containing unit needs to be 50 mol% or more, and is preferably 80 mol% or more, more preferably 90 mol% or more from the viewpoint of water absorption performance.

As the carboxyl group-containing units constituting the water-absorbent resin of the present invention, acrylic acid, methacrylic acid, itaconic acid, maleic acid, crotonic acid, fumaric acid, sorbic acid, cinnamic acid, and their anhydrides and their Examples thereof include units derived from monomers such as neutralized salts.
Examples of units that do not contain a carboxyl group constituting the water-absorbent resin of the present invention include acrylamide, methacrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, and N-isopropyl (meth) acrylamide. N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate, N-vinylpyrrolidone, Hydrophilic units derived from nonionic compounds such as N-acryloylpiperidine, N-acryloylpyrrolidine, (meth) acrylonitrile, styrene, vinyl chloride, butadiene, isobutene, ethylene, propylene Stearyl (meth) acrylate, the hydrophobic units derived from compounds such as lauryl (meth) acrylate.

  Further, the water-absorbent resin of the present invention may contain a unit that becomes a cross-linking agent between polymer molecular chains. For example, diethylene glycol diacrylate, N, N′-methylenebisacrylamide, polyethylene glycol diacrylate, polypropylene glycol diacrylate, trimethylolpropane diallyl ether, allyl glycidyl ether, pentaerythritol triallyl ether, pentaerythritol diacrylate monostearate, bisphenol Examples include units derived from diacrylate, isocyanuric acid diacrylate, tetraallyloxyethane, diallyloxyacetate, and the like.

  Moreover, the unit which becomes a crosslinking agent by condensing with the carboxyl group-containing unit may be contained. For example, glycidyl ether compounds such as ethylene glycol diglycidyl ether, trimethylolpropane triglycidyl ether, (poly) glycerin polyglycidyl ether, diglycerin polyglycidyl ether, propylene glycol diglycidyl ether; (poly) glycerin, (poly) ethylene glycol , Propylene glycol, 1,3-propanediol, polyoxyethylene glycol, triethylene glycol, tetraethylene glycol, diethanolamine, triethanolamine and other polyhydric alcohols; ethylenediamine, diethylenediamine, polyethyleneimine, hexamethylenediamine and other polyhydric alcohols And polyvalent ions such as zinc, calcium, magnesium, and aluminum.

  If the shape of the water-absorbent resin is in the form of particles, the spherical shape by suspension polymerization, the irregular shape obtained by crushing the aqueous solution polymer, the porous shape to increase the specific surface area, the shape in which multiple spherical particles are aggregated, etc. It is not limited. The particle size is not particularly limited, but a particle size of 50 μm or more is preferable because the generation of fine particles is often a problem in handling the water absorbent resin. More preferably, it is 100 micrometers or more. Classification of the water-absorbent resin can be achieved in a timely manner using a sieve.

One feature of the water-absorbent resin produced by the production method of the present invention is that it exhibits a high water absorption capacity. In the measurement of the water absorption capacity under no pressure by the Tea-bag method described later, it is preferably 55 g / g or more, more preferably 60 g / g or more. Similarly, it is preferably 40 g / g or more, more preferably 42 g / g or more, and most preferably 45 g / g or more in the measurement of the water retention ratio described later. Moreover, it is preferable that the water absorption capacity | capacitance under the pressurization at 57 g / cm < ² > is 10 g / g or more. If each numerical value is less than that value, it may not be possible to achieve thinning of disposable diapers in the field of sanitary materials, which has been demanded in recent years.

  The residual monomer of the water absorbent resin produced by the production method of the present invention shows a low value. The residual monomer of the water-absorbent resin by the production method of the present invention is preferably 20 ppm or less, more preferably 15 ppm or less, and most preferably 10 ppm or less. The residual monomer is preferably as small as possible. In addition, the water-absorbent resin produced by the production method of the present invention has a very low acrylamide content. The acrylamide of the water absorbent resin produced by the production method of the present invention is preferably 1 ppm or less. More preferably, it is 0.1 ppm or less.

<About neutralized salt>
The carboxyl group in the polymer molecular chain constituting the water absorbent resin of the present invention needs to be partially neutralized with ammonia. The neutralization rate of the entire carboxyl group with ammonia is preferably 50 mol% or more, more preferably 55 mol% or more, and most preferably 60 mol% or more.

  The water absorbent resin of the present invention has a distribution structure in which the carboxyl group ammonia neutralization rate inside the resin is higher than the carboxyl group ammonia neutralization rate on the outer surface of the resin. The carboxyl group ammonia neutralization rate of the resin center is 50 mol% or more, preferably 60 mol% or more, most preferably 70 mol% or more, and the carboxyl group ammonia neutralization rate of the resin outer surface is less than 50 mol%, preferably 45 mol% or less. Most preferably, it is 40 mol% or less. The difference in the neutralization rate between the resin center and the resin outer surface is preferably 5 mol% or more, and more preferably 10 mol% or more. It is preferable that the carboxyl group ammonia neutralization rate at the center of the resin is the above-mentioned numerical value because it is difficult for the water absorption capacity to decrease without pressure. Moreover, when the carboxyl group neutralization rate on the outer surface of the resin is the above-mentioned numerical value, it is preferable that the water absorption capacity under pressure is difficult to decrease.

The resin outer surface refers to a portion exposed to the outside of the resin. Moreover, the said resin center part means the innermost part from the resin outer surface of the said resin.
The water-absorbent resin has a core-shell structure inside the resin, and the carboxyl group neutralization rate averaged over the entire resin is preferably 30 mol% or more, and more preferably 50 mol% or more. If the average carboxyl group neutralization rate of the whole resin is extremely reduced, it may cause a decrease in water absorption capacity without pressure application, which is not preferable.
The core-shell structure inside the resin can be determined by measuring the neutralization rate of the carboxyl group between the resin outer surface and the resin center by the microscopic ATR method which is one of infrared absorption analysis methods.

The measurement of the neutralization rate of the carboxyl group on the resin outer surface directly measures the resin outer surface by the microscopic ATR method. The measurement of the resin center is performed by the micro ATR method after cleaving the resin to expose the center by using, for example, an ultramicrotome (ULTRACUT N manufactured by Reichert). As the measuring apparatus, for example, FTS-575 manufactured by Bio-Rad may be used.
Peaks of 1695 cm ⁻¹ (carboxylic acid νC = 0 baseline 1774 to 1616 cm ⁻¹ ) and 1558 cm ⁻¹ (carboxylate νCOO− baseline 1616 to 1500 cm ⁻¹ ) as indices defining the composition ratio of carboxylic acid and carboxylate The area ratio (1695/1558 cm ⁻¹ ) is calculated. Separately, 10 mol%, 30 mol%, 50 mol%, 70 mol%, 90 mol%, and 100 mol% of the total carboxylic acid were measured using partially crosslinked polyacrylic acid neutralized with ammonia as a standard sample, and the composition ratio was determined from the prepared calibration curve. .

Specific examples and comparative examples of the present invention are shown below, but the present invention is not limited to the following examples.
(1) Measuring method of water absorption capacity without pressure; Tea-bag method Water-absorbing resin A (g) (about 0.5 g) is uniformly placed in a non-woven teabag bag (7 × 9 cm), and the liquid temperature Immerse in 500 cc of physiological saline at 25 ° C. for 1 hour. After a predetermined time, the tea bag bag is pulled up and drained naturally for 10 minutes, and then the weight B (g) of the tea bag bag is measured. The same operation as a blank is performed using only a tea bag bag without adding a water-absorbent resin, and the weight C (g) is measured. The water absorption ratio is obtained from the following formula.
Water absorption magnification (g / g) = (B (g) -C (g)) / A (g)

(2) Absorption rate measurement method; vortex method 50 g of 0.9% physiological saline adjusted to 25 ° C. is measured in a 100 cc glass beaker. A 30 * 8 mm rotator is put here, put on a magnetic stirrer with a tachometer, and rotated at 600 rpm. Check the rotation speed with a non-contact tachometer. 2.00 g of water-absorbing resin is weighed and put into a beaker. The absorption time is defined as the time after the water-absorbing resin is charged until the liquid surface becomes flat.

(3) Measuring method of neutralization rate of carboxyl group of resin body outer surface and center part (i) Measuring device The measuring device used FTS-575 by Bio-Rad.
(Ii) Measurement conditions Microscopic ATR method (crystal plate Ge, single reflection)
Back Ground: Air room temperature measurement Aperture: 50 × 50 μm,
Accumulation count: 100 times From spectral data obtained by measurement, peak areas of 1695 cm ⁻¹ (carboxylic acid νC = 0 baseline 1774 to 1616 cm ⁻¹ ) and 1558 cm ⁻¹ (carboxylate νCOO− baseline 1616 to 1500 cm ⁻¹ ). The ratio (1695/1558 cm ⁻¹ ) is determined.
(Iii) Preparation of calibration curve Partially crosslinked polyacrylic acid obtained by neutralizing 10 mol%, 30 mol%, 50 mol%, 70 mol%, 90 mol%, and 100 mol% of the total carboxylic acid with ammonia was used as a sample for preparing a calibration curve. The calibration curve sample is cleaved, and the central part is performed 5 times per sample by the microscopic ATR method. A calibration curve (5th order polynomial approximate curve) was created from the -COOH / -COO-peak area ratio. The cleaving was performed with an ultramicrotome (ULTRACUT N manufactured by Reichert).
(Iv) Sample measurement Measurement was performed in the same manner as the calibration curve sample. The outer surface of the resin body was directly measured by the ATR method, and the central portion of the resin body was cleaved by an ultramicrotome and then measured by the ATR method. The outer surface of the resin body was measured 3 times per sample, and the central part of the resin body was measured 5 times per sample, and the average value was taken as the measurement result.

(4) Measuring method of residual monomer and acrylamide Weigh 1 g of water-absorbing resin in a 300 ml beaker, add 250 g of physiological saline, and stir for 2 hours. After a predetermined time, the membrane was filtered through a membrane filter, and the filtrate was analyzed by high performance liquid chromatography.
The analysis conditions of high performance liquid chromatography are as follows.
Column: Tosoh ODS80Ts
Column temperature: 40 ° C
Carrier: 10 mM phosphoric acid aqueous solution, flowing at 0.7 cc / min Detection: UV 207 nm
Driving amount: 50 μl

(5) Measurement of water absorption capacity under pressure To a cylindrical instrument made of acrylic resin (outer diameter 35.0mm, inner diameter 24.5mm, height 30mm, weight D (g)) with a 250 mesh nylon net on the bottom The water-absorbing resin E (g) (about 0.16 g) is put in a uniform manner, and nothing is applied at 0.0 psi as a weight, but at 0 g / cm ² , it is 21 g · cm ² , and 57 g / cm ² is 278. Place 3g weight (outer diameter 24.5mm). 60 cc of physiological saline is placed in a SUS petri dish (inner diameter: 120 mm), and a cylindrical instrument is placed in it for 1 hour. After a predetermined period of time, drain water with a paper towel, and measure the weight F (g) of the entire instrument with a balance. The water absorption magnification is obtained by the following equation (1).
<Formula (1)>
Water absorption ratio (g / g) = (F (g) −D (g) −weight of weight (g)) / E (g)

(6) Measurement of water retention magnification “Water retention magnification” represented by the following formula (2) is used as an index indicating the water retention capacity of the water absorbent resin.
The tea bag containing the water-containing resin immediately after the Tea-bag method absorption ratio measurement test is put into a centrifuge, dehydrated at 250 G for 3 minutes, and the weight is measured. The same operation is performed without using the water absorbent resin, and the weight is measured to obtain a blank. The water retention magnification is calculated according to the following equation (2). The measurement is performed three times, and the average value is set as the water retention magnification.
<Formula (2)>
Water-retaining capacity of water-absorbing resin (g / g) = {(weight of tea bag after water absorption and dewatered by centrifugal separator) − (weight of blank tea bag after water absorption) − (weight of water-absorbing resin)} / ( Weight of water absorbent resin)

[Production Example 1]
(Preparation of ammonium acrylate by neutralization of acrylic acid)
Acrylic acid manufactured by Wako Pure Chemical Industries, Ltd. was used as a reagent special grade. Acrylic acid was used after distillation before use to remove the polymerization inhibitor. Next, 100 g of acrylic acid was dissolved in 91.02 g of water. This aqueous solution was cooled in an ice bath, and while maintaining the liquid temperature at 30 ° C. or lower, 117.94 g of a 25 wt% aqueous ammonia solution was gradually added with stirring to obtain a 40 wt% aqueous ammonium acrylate solution.

[Production Example 2]
(Preparation of ammonium acrylate by hydrolysis of acrylonitrile)
(1) Preparation of biocatalyst Acinetobacter sp. AK226 (FERM BP-08590) having nitrilase activity is 0.1% sodium chloride, potassium dihydrogen phosphate 0.1%, magnesium sulfate heptahydrate 0.05%, sulfuric acid In a medium prepared by adjusting an aqueous solution containing iron heptahydrate 0.005%, manganese sulfate pentahydrate 0.005%, ammonium sulfate 0.1%, potassium nitrate 0.1% (both wt%) to pH = 7. Then, 0.5% by weight of acetonitrile was added as a nutrient source and cultured aerobically at 30 ° C. This was washed with a 30 mM phosphate buffer (pH = 7.0) to obtain a cell suspension (dry cell 15% by weight). Subsequently, 2.5% in a mixed solution of acrylamide, N, N′-methylenebisacrylamide, 5% N, N, N ′, N′-tetramethylethylenediamine aqueous solution, cell suspension, and 30 mM phosphate buffer. A potassium persulfate aqueous solution was mixed to obtain a heavy product. The final composition is 3% dry cell concentration, 52% 30 mM phosphate buffer (pH = 7), 18% acrylamide, 1% methylenebisacrylamide, 5% N, N, N ′, N′-tetramethylethylenediamine. An aqueous solution of 12% and a 2.5% potassium persulfate aqueous solution of 14% (both by weight) were used. The polymer was cut into about 1 × 3 × 3 mm square particles to obtain immobilized cells. The immobilized cells were washed with 30 mM phosphate buffer (pH = 7) to prepare an immobilized cell catalyst (hereinafter referred to as biocatalyst).

(2) Hydrolysis with a biocatalyst 400 g of distilled water is put into an Erlenmeyer flask with an internal volume of 500 ml, and 1 g of the above biocatalyst (corresponding to 0.03 g of dry cells) is put in a wire mesh basket in the liquid. After sealing with a rubber stopper, the inner temperature was kept at 20 ° C. by immersion in a constant temperature water bath, and the mixture was stirred with a stirrer.
Acrylonitrile was intermittently fed in an amount of 2% by weight (the acrylonitrile concentration was controlled at 0.5% by weight or more), and an accumulation reaction of ammonium acrylate was carried out. As a result, it was possible to accumulate up to 40% by weight.
The obtained aqueous solution of ammonium acrylate was colorless and transparent. In addition, when 5 L of the reaction solution was prepared under the same conditions and purified using a UF membrane (Asahi Kasei Pencil type module SIP-0013), no clogging or other phenomena were observed, and the entire solution could be processed. A highly pure 40 wt% ammonium acrylate aqueous solution was obtained.

[Production Example 3]
(Preparation of sodium acrylate by neutralization of acrylic acid)
Acrylic acid manufactured by Wako Pure Chemical Industries, Ltd. was used as a reagent special grade. Acrylic acid was used after distillation before use to remove the polymerization inhibitor. Next, 100.0 g of acrylic acid was dissolved in 43.20 g of water. This aqueous solution was cooled in an ice bath, and while maintaining the liquid temperature at 30 ° C. or lower, 166.7 g of 25 wt% sodium hydroxide aqueous solution was gradually added with stirring, and 40 wt% 75 mol% neutralized sodium acrylate aqueous solution. Got.

[Production Example 4]
(Preparation of ammonium acrylate by neutralization of acrylic acid)
Acrylic acid manufactured by Wako Pure Chemical Industries, Ltd. was used as a reagent special grade. Acrylic acid was used after distillation before use to remove the polymerization inhibitor. Next, 100 g of acrylic acid was dissolved in 150.73 g of water. This aqueous solution was cooled in an ice bath, and while maintaining the liquid temperature at 30 ° C. or lower, 59.03 g of a 30 wt% aqueous ammonia solution was gradually added with stirring to obtain a 38 wt% aqueous ammonium acrylate solution.

[Example 1]
300 g of 40% by weight ammonium acrylate aqueous solution of Production Example 1 and 0.0623 g of N, N′-methylenebisacrylamide are added. Next, 1.43 g of a 42 wt% glycerin aqueous solution was added by a syringe. Subsequently, 0.0067 g of 2,2-dimethoxy-1,2-diphenylethane-1-one and 0.0033 g of ammonium persulfate were added as a photopolymerization initiator, and the monomer aqueous solution was cooled to 10 ° C. The reaction system was deaerated by bubbling with gas, and purged with nitrogen. The dissolved oxygen was 1 ppm or less.
The aqueous solution was irradiated with ultraviolet rays for 2 minutes using a high-pressure mercury lamp (MENK-20-25XE, 20W, emission length 253 nm, 3 units, manufactured by SEN EngineeringCO., Ltd.) using an aqueous solution having a thickness of 20 mm. (Light quantity 684 mJ / cm ² ). The internal temperature started from 13 ° C and the maximum temperature reached was about 90 ° C. Thereafter, the gel is taken out and roughly crushed, and then dried for 2 hours using a 130 ° C. hot air dryer. After drying, the mixture is pulverized with a homogenizer, and 100 to 850 μm is collected by sieving. This was heated at 170 ° C. for 30 minutes in an inert oven under a nitrogen atmosphere. The water absorbent resin thus obtained is designated as water absorbent resin (1).

[Example 2]
300 g of 40% by weight ammonium acrylate aqueous solution of Production Example 2 and 0.0623 g of N, N′-methylenebisacrylamide are added. Subsequently, 0.0067 g of 2,2-dimethoxy-1,2-diphenylethane-1-one and 0.0033 g of ammonium persulfate were added as a photopolymerization initiator, and the monomer aqueous solution was cooled to 10 ° C. The reaction system was deaerated by bubbling with gas, and purged with nitrogen. The dissolved oxygen was 1 ppm or less.
The aqueous solution was irradiated with ultraviolet rays for 2 minutes using a high-pressure mercury lamp (MENK-20-25XE, 20W, emission length 253 nm, 3 units, manufactured by SEN EngineeringCO., Ltd.) using an aqueous solution having a thickness of 20 mm. (Light quantity 684 mJ / cm ² ). The internal temperature started from 13 ° C and the maximum temperature reached was about 90 ° C. Thereafter, the gel is taken out and roughly crushed, and then dried for 2 hours using a 130 ° C. hot air dryer. After drying, the mixture is pulverized with a homogenizer, and 100 to 850 μm is collected by sieving.
50 g of the obtained water-absorbing resin was charged into a beaker, 0.1250 g of ethylene glycol diglycidyl ether and 3.000 g of water were added as a cross-linking agent, heated at 130 ° C. for 15 minutes, dried, and surface-crosslinked water-absorbing resin was obtained. Obtained. Thereafter, 0.3000 g of silica was added and mixed.
This was heated at 180 ° C. for 10 minutes in an inert oven in a nitrogen atmosphere. The water absorbent resin thus obtained is designated as water absorbent resin (2).

[Example 3]
300 g of 40 wt% ammonium acrylate aqueous solution of Production Example 2 and 0.024 g of trimethylolpropane triacrylate are added. Subsequently, 0.0067 g of 2,2-dimethoxy-1,2-diphenylethane-1-one and 0.0033 g of ammonium persulfate were added as a photopolymerization initiator, and the monomer aqueous solution was cooled to 10 ° C. The reaction system was deaerated by bubbling with gas, and purged with nitrogen. The dissolved oxygen was 1 ppm or less. Then 1.2 g of sodium bicarbonate was mixed.
Using this high-pressure mercury lamp (SEN EngineeringCO., Ltd., MUMK-20-25XE, 20 W, emission length 253 nm, using three units), this aqueous solution was irradiated with ultraviolet rays for 2 minutes. (Light quantity 684 mJ / cm ² ). The internal temperature started from 13 ° C and the maximum temperature reached was about 90 ° C. Thereafter, the gel is taken out and roughly crushed, and then dried for 2 hours using a 130 ° C. hot air dryer. After drying, the mixture is pulverized with a homogenizer, and 100 to 850 μm is collected by sieving.
50 g of the obtained water-absorbing resin was charged into a beaker, 0.1250 g of ethylene glycol diglycidyl ether and 3.000 g of water were added as a cross-linking agent, heated at 130 ° C. for 15 minutes, dried, and surface-crosslinked water-absorbing resin was obtained. Obtained. Thereafter, 0.3000 g of silica was added and mixed.
This is heated at 180 ° C. for 10 minutes in an inert oven in a nitrogen atmosphere. The water absorbent resin thus obtained is designated as water absorbent resin (3).

[Example 4]
A water-absorbent resin was obtained in the same manner as in Example 1 except that the heating conditions in the inert oven in Example 1 were changed to 175 ° C. for 25 minutes. The water absorbent resin thus obtained is designated as water absorbent resin (4).
[Example 5]
A water-absorbent resin was obtained in the same manner as in Example 2, except that the thickness of the aqueous monomer solution was 30 mm and the heating condition in the inert oven was 45 minutes at 165 ° C. The water absorbent resin thus obtained is designated as water absorbent resin (5).

[Example 6]
A water-absorbent resin was obtained in the same manner as in Example 3, except that the thickness of the aqueous monomer solution was 10 mm in Example 3. The water absorbent resin thus obtained is designated as water absorbent resin (6).
[Example 7]
A water-absorbent resin was obtained in the same manner as in Example 3 except that sodium bicarbonate was changed to ammonium carbonate in Example 3. The water absorbent resin thus obtained is designated as water absorbent resin (7).

[Example 8]
A water absorbent resin was obtained in the same manner as in Example 3 except that the sieving in Example 3 was changed to 300 to 106 μm. The water absorbent resin thus obtained is designated as water absorbent resin (8).
[Example 9]
A water absorbent resin was obtained in the same manner as in Example 3 except that 1.2 g of sodium bicarbonate was changed to 2.4 g of ammonium bicarbonate in Example 3. The water absorbent resin thus obtained is designated as water absorbent resin (9).

[Example 10]
300 g of 40% by weight ammonium acrylate aqueous solution of Production Example 1 and 0.024 g of trimethylolpropane triacrylate are added. Subsequently, 0.0067 g of 2,2-dimethoxy-1,2-diphenylethane-1-one and 0.0033 g of ammonium persulfate were added as a photopolymerization initiator, and the monomer aqueous solution was cooled to 10 ° C. The reaction system was deaerated by bubbling with gas, and purged with nitrogen. The dissolved oxygen was 1 ppm or less. Then 1.2 g of sodium bicarbonate was mixed.
The aqueous solution was irradiated with ultraviolet rays for 2 minutes using a high-pressure mercury lamp (MENK-20-25XE, 20W, emission length 253 nm, 3 units, manufactured by SEN EngineeringCO., Ltd.) using an aqueous solution having a thickness of 20 mm. (Light quantity 684 mJ / cm ² ). The internal temperature started from 13 ° C and the maximum temperature reached was about 90 ° C. Thereafter, the gel is taken out and roughly crushed, and then dried for 2 hours using a 130 ° C. hot air dryer. After drying, the mixture is pulverized with a homogenizer, and 100 to 850 μm is collected by sieving.
The water absorbent resin thus obtained is designated as water absorbent resin (10).

[Comparative Example 1]
300 g of 40 wt% 75 mol% neutralized sodium acrylate aqueous solution of Production Example 3 and 0.036 g of trimethylolpropane triacrylate are added. Subsequently, 0.0067 g of 2,2-dimethoxy-1,2-diphenylethane-1-one and 0.0033 g of sodium persulfate were added as a photopolymerization initiator, and this monomer aqueous solution was cooled to 10 ° C., The reaction system was purged with nitrogen by bubbling with nitrogen gas.
The aqueous solution was irradiated with ultraviolet rays for 2 minutes using a high-pressure mercury lamp (MENK-20-25XE, 20W, emission length 253 nm, 3 units, manufactured by SEN EngineeringCO., Ltd.) using an aqueous solution having a thickness of 20 mm. (Light quantity 684 mJ / cm ² ). The internal temperature started from 13 ° C and the maximum temperature reached was about 90 ° C. Thereafter, the gel is taken out and roughly crushed, and then dried for 2 hours using a 130 ° C. hot air dryer. After drying, the mixture is pulverized with a homogenizer, and 100 to 850 μm is collected by sieving.
50 g of the obtained water-absorbing resin was charged into a beaker, 0.1250 g of ethylene glycol diglycidyl ether and 3.000 g of water were added as a cross-linking agent, heated at 130 ° C. for 15 minutes, dried, and surface-crosslinked water-absorbing resin was obtained. Obtained. Thereafter, 0.3000 g of silica was added and mixed.
This is heated at 180 ° C. for 10 minutes in an inert oven in a nitrogen atmosphere. The water absorbent resin thus obtained is referred to as comparative water absorbent resin (1).

[Comparative Example 2]
300 g of the 38 wt% 75 mol% neutralized ammonium acrylate aqueous solution of Production Example 4 and 0.1593 g of trimethylolpropane triacrylate are added. This aqueous monomer solution was adjusted to 30 ° C. and degassed by bubbling with nitrogen gas, and the reaction system was purged with nitrogen. Next, 0.9209 g of ammonium persulfate and 0.0557 g of sodium hydrogen sulfite were added to initiate polymerization. The internal temperature started from 30 ° C and the maximum temperature reached was about 100 ° C. Thereafter, the gel is taken out and coarsely crushed and then dried for 1 hour using a 180 ° C. hot air dryer. After drying, the mixture is pulverized with a homogenizer, and 100 to 850 μm is collected by sieving.
50 g of the obtained water-absorbing resin was charged into a beaker, 0.1250 g of ethylene glycol diglycidyl ether and 3.000 g of water were added as a cross-linking agent, heated at 130 ° C. for 15 minutes, dried and surface-crosslinked water-absorbing resin. Obtained. Thereafter, 0.3000 g of silica was added and mixed.
The water absorbent resin thus obtained is referred to as comparative water absorbent resin (2).

  Table 3 below shows a list of physical properties of each measured water absorbent resin.

Claims (7)

A method for producing a water-absorbing resin, comprising a polymerization step of polymerizing an aqueous monomer solution containing a water-soluble unsaturated carboxylic acid ammonium salt under the following conditions (1) and (2).
Polymerization conditions:
(1) The content of the water-soluble unsaturated carboxylic acid ammonium salt in the monomer aqueous solution is in the range of more than 50 mol% and not more than 100 mol% in all monomers.
(2) Irradiate with ultraviolet rays using a radical photopolymerization initiator and a peroxide. The method for producing a water-absorbent resin according to claim 1, wherein an aqueous monomer solution containing a water-soluble unsaturated carboxylic acid ammonium salt is polymerized under the following conditions (3) and (4).
(3) The thickness of the monomer aqueous solution is 50 mm or less.
(4) The dissolved oxygen in the monomer aqueous solution is adjusted to 4 ppm or less.   The method for producing a water-absorbent resin according to claim 1 or 2, wherein the water-soluble unsaturated carboxylic acid ammonium salt is an ammonium acrylate salt.   The method for producing a water-absorbent resin according to claim 1, wherein the monomer aqueous solution is polymerized in a state containing a foaming agent.   The method for producing a water-absorbent resin according to claim 4, wherein the foaming agent is carbonate.   A drying treatment step of drying the resin obtained by the polymerization step, and a heating treatment step of heat-treating the dried resin at a temperature higher by 10 ° C. or more than the drying conditions of the drying treatment step. Item 6. A method for producing a water absorbent resin according to any one of Items 1 to 5. The method for producing a water-absorbent resin according to claim 6, wherein the heat treatment is performed under the following conditions (5) and (6).
(5) 10 degreeC or more higher than the drying conditions of a water absorbing resin, and it carries out in 100-250 degreeC.
(6) The heat treatment is performed under heat treatment conditions such that the neutralization rate of the outer surface layer of the water-absorbent resin is less than 50 mol% and the neutralization rate of the resin center is 50 mol% or more.