JPH06122708A - Production of water-absorbent resin - Google Patents

Abstract

PURPOSE:To obtain a highly water-absorbent acrylic resin which has a low residual-monomer content and which neither releases the residual monomer nor increases in monomer content after surface treatment or subsequent heat treatment. CONSTITUTION:A monomer in which 10-96mol% and 4-50mol% of the acid groups have been converted into the alkali metal salt form and the ammonium salt form, respectively, by neutralization is polymerized to obtain a polymer in the gel form. This polymer is heated as it is and/or after being dried. When the polymer is heated after being dried, surface crosslinking is conducted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a water absorbent resin containing an acid group-containing monomer as a main component. More specifically, the present invention relates to a method for producing a water-absorbent resin in which the amount of residual monomers is small and the amount of residual monomers is hardly increased even under various conditions of use. The water-absorbent resin of the present invention can be simply and inexpensively produced, and is excellent in performance and safety.

[0002]

2. Description of the Related Art In recent years, a water-absorbent resin that absorbs a large amount of water and gels has been developed as a kind of synthetic polymer, and is used in the field of sanitary materials such as paper diapers and sanitary napkins, the field of agriculture and forestry, and the field of civil engineering. Widely used.

Examples of such water-absorbent resins include crosslinked polyacrylic acid partially neutralized products, hydrolyzates of starch-acrylonitrile graft polymers, neutralized products of starch-acrylic acid graft polymers, and vinyl acetate-acrylic acid. Saponified product of ester copolymer, hydrolyzate of acrylonitrile copolymer or acrylamide copolymer, cross-linked polymer of cationic monomer, cross-linked copolymer of 2-acrylamido-2-methylpropanesulfonic acid and acrylic acid, Many cross-linked polymers of 2-sulfoethyl methacrylate are known, however, residual monomers of about 500 to 3000 ppm remain in these water-absorbent resins, and in recent years, reduction thereof has been strongly demanded.

Many methods for reducing residual monomers of water-absorbent resins and water-soluble resins have been proposed. For example, (a) a method of improving the polymerization rate itself by changing the polymerization conditions or radiation (Japanese Patent Application Laid-Open No. 5 (1999) -58242).
0-96689, JP-A-53-145895, JP-A-56-72005, JP-A-63-43930, JP-A-63-260906), and further a method for treating residual monomers after polymerization. As a method of adding additives such as amines and sulfites after the polymerization (b) (JP-A-50-4)
0689, JP-A-55-135110, JP-A-64
No. 62317, DE-3724707), and (c) a method of extracting the residual monomer with an organic solvent or the like (Japanese Patent Laid-Open No. 1-1990).
292003), (d) a method of adding a microorganism decomposing residual monomer (Japanese Patent Publication No. 29523/1985), (e)
Method of volatilizing residual monomer at high temperature (Japanese Patent Laid-Open No. 54-1 / 1979)
19588), and (g) a technique using a monomer obtained by a specific neutralization method or a monomer having a small amount of heavy metal (JP-A-2-20).
9906, JP-A-3-31306) and the like are known.

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

[0006] Usually, in the water-absorbent resin, cross-linking in the vicinity of the surface is performed after the polymerization in order to improve various physical properties.

Many methods for such surface treatment have been proposed, for example, a method using polyhydric alcohol (Japanese Patent Laid-Open No. 58-58).
-180233, JP-A-61-16903), a method using alkylene carbonate (DE-402078).
0C) and a method using glyoxal (JP-A-52-1)
17393), a method using a polyvalent metal (JP-A-51-
No. 136588, JP-A-61-257235, JP-A-62-7745), a method using a silane coupling agent (JP-A-61-211305, JP-A-61-252).
212, JP-A-61-264006), a method in which a water-absorbent resin is dispersed and crosslinked in a mixed solvent of water and a hydrophilic organic solvent (JP-A-57-44627), and a specific amount of water is allowed to coexist. To crosslink the water-absorbent resin (JP-A-58-117)
No. 222, JP-A-59-62665), a method of coexisting inorganic powder and water and crosslinking (USP-4568730).
8), a method of irradiating electromagnetic radiation (JP-A-63-439)
No. 30) and the like are known.

Among the many water-absorbent resins described above, the water-absorbent resin using acrylic acid is often used for the surface cross-linking because of the excellent effect of improving the water-absorbing property by the surface cross-linking.
However, the present inventors have found that in the surface cross-linking, in the water-absorbent resin in which acrylic acid is used as one kind of the acid group-containing monomer, the residual monomer is significantly increased to several tens to several 100 ppm, and , The increase in the residual monomer accounts for a large proportion of the residual monomer in the final product, and
The fact that the surface cross-linking effect is lowered is found.

[0009]

The present invention has been made in view of the above situation. Therefore, an object of the present invention is to provide a water-absorbent resin exhibiting excellent physical properties such as a high water-absorption capacity and having a small amount of residual monomer, and of a water-absorbent resin in which generation or increase of residual monomer is hardly seen. It is to provide a manufacturing method.

[0010]

Means and Actions for Solving the Problems As a result of intensive studies to achieve the above-mentioned object, as a result, a gel-like polymer obtained by polymerization of a monomer neutralized as an ammonium salt and an alkali metal salt was obtained. The inventors have found that the above problems can be solved by heat treatment and heat treatment of a mixture of a dried product of the gel polymer and a crosslinking agent, and completed the present invention.

That is, the present invention provides 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% are ammonium salts. A method for producing a water-absorbent resin, which comprises heat treatment. "And" by polymerization of a monomer in which 10 to 96 mol% of acid groups are neutralized as an alkali metal salt and 4 to 50 mol% are ammonium salts. The method for producing a water-absorbent resin, which comprises subjecting a dried product of the obtained gel polymer to surface cross-linking while being heat-treated. "

The present invention will be described in more detail below.

In the present invention, first, it is essential to prepare a monomer whose main component is an acid group-containing monomer whose acid group has been neutralized as an alkali metal salt and an ammonium salt. It is essential to obtain a gel polymer by polymerizing the body. (Note that the term “monomer” below is a general term for monomers containing an acid group-containing monomer as a main component used for polymerization of a water-absorbent resin.) Otherwise, the physical properties after polymerization and after surface crosslinking will be poor. In addition, when a monomer whose acid group is not neutralized as an ammonium salt is used, not only a large amount of residual monomer remains in the water absorbent resin after polymerization or after drying, but acrylic acid is used as one of the acid group-containing monomers. In that case, there is a problem in safety due to a peculiar phenomenon that residual monomers are further generated in the water-absorbent resin to increase during surface cross-linking or use at high temperature. Furthermore, when a monomer whose acid group is not neutralized as an alkali metal salt is used, problems such as deterioration of polymerizability and various physical properties, coloring, and a byproduct of carcinogenic substances and acrylamide may occur.

The proportion of the acid group-containing monomer as the main component in the present invention is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, further preferably 90 in the monomer.
~ 100 mol%. In the present invention, the acid group-containing monomer refers to the total of the acid group-containing monomer and its salt.

As the acid group-containing monomer used in the present invention, a monomer which becomes a water absorbent resin by polymerization is used without limitation, and examples thereof include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, and fumaric acid. , Crotonic acid,
Itaconic acid, vinyl sulfonic acid, styrene sulfonic acid,
2- (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid,
2- (meth) acryloylpropane sulfonic acid, sulfoethoxy polyethylene glycol mono (meth) acrylate and the like can be mentioned. Among these, acrylic acid,
Methacrylic acid, 2- (meth) acryloylethanesulfonic acid and 2- (meth) acrylamido-2-methylpropanesulfonic acid are preferable, and acrylic acid is more preferable.

In the present invention, the acid group-containing monomer is used as the main component of the monomer, but other hydrophilic monomer and / or hydrophobic monomer may be used in combination.

Examples of hydrophilic monomers that can be used in combination include:
Acrylamide, methacrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide,
N, N-methyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, methoxy polyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate, vinyl pyridine, N-vinyl Nonionic hydrophilic monomers such as pyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine;
N, N-dimethylaminoethyl (meth) acrylate,
N, N-diethylaminoethyl (meth) acrylate,
Examples thereof include cationic hydrophilic monomers such as N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide and quaternary salts thereof, which are selected from these groups. 1
One kind or two or more kinds can be used. Further, as hydrophilic monomers, methyl (meth) acrylate, ethyl (meth)
You may use the hydrophilic monomer which forms a water absorbent resin by hydrolysis of the functional group after a polymer like acrylate and vinyl acetate. Among these hydrophilic monomers that can be used in combination, methoxy polyethylene glycol (meth)
Acrylate, N, N-dimethylaminoethyl (meth)
Acrylate and its quaternary salts, acrylamide are preferred.

As the hydrophobic monomer which can be used in combination,
Styrene, vinyl chloride, butadiene, isobutene, ethylene, propylene, stearyl (meth) acrylate,
Examples thereof include lauryl (meth) acrylate.

When using a monomer containing no such acid group, 0 to 50 mol%, preferably 0 to 30 of all monomers are used.
It is good to use in the range of mol%, more preferably 0 to 10 mol%. In particular, when a hydrophobic monomer is used, attention should be paid to the water absorption capacity of the water absorbent resin obtained.

From the viewpoint of water absorption capacity and water absorption speed of the resulting water-absorbent resin, part or all of the acid groups in the monomer are converted to salts. Neutralization is performed on the monomer before polymerization in terms of the effect of improving the physical properties by surface crosslinking. The neutralization rate is 30 to 10
It is 0 mol%, preferably 40 to 95 mol%, and more preferably 50 to 80 mol%.

As the basic substance used for neutralization, alkali metal carbonate (hydrogen) salt, hydroxide, ammonia,
Examples thereof include various amino acids such as alanine and organic amines. From the viewpoint of various physical properties and reduction of residual monomers, sodium hydroxide and / or potassium hydroxide and ammonia are preferable, and sodium hydroxide and ammonia are particularly preferable. Further, urea may be added to the monomer as an ammonia precursor to perform polymerization.

In the present invention, further post-neutralization is not limited during or after the polymerization, but when a strong base such as sodium hydroxide is used for the post-neutralization, attention should be paid to hydrolysis of the crosslinking point. When polymerizing using a cross-linking agent such as polyvalent ester and further post-neutralizing, carbonic acid (hydrogen) of alkali metal is used for neutralization.
Weak bases such as salts and ammonia are preferably used.

In the present invention, the proportion of the ammonium salt in the acid groups mainly neutralized as the alkali metal salt and the ammonium salt is 4 to 50 mol%, preferably 10 to 40.
Mol%. The proportion of the alkali metal salt in the acid group is 10 to 96 mol%, preferably 20 to 80 mol%. Within such a range, a water-absorbent resin which is free from coloring and by-products of harmful substances, has a small amount of residual monomer, and is excellent in various physical properties can be obtained. The neutralization with a very small amount of polyvalent metal salt does not change the gist of the present invention.

In the present invention, the above-mentioned monomers are polymerized and cross-linked to obtain a water absorbent resin. The crosslinking method used is not particularly limited, for example, after obtaining a water-soluble resin by polymerizing the monomer of the present invention, a method of post-crosslinking by adding a crosslinking agent during or after the polymerization, radical Examples include radical cross-linking with a polymerization initiator and radiation cross-linking with an electron beam, etc., but in order to obtain a water-absorbing resin with excellent performance with good productivity, a predetermined amount of a cross-linking agent is added to a monomer in advance for polymerization. It is preferable to carry out a crosslinking reaction simultaneously with or after the polymerization.

The cross-linking agent used in the method in which a predetermined amount of the cross-linking agent is added to the monomer to carry out the polymerization and the cross-linking reaction is carried out at the same time as the polymerization or after the polymerization is N, N'-methylenebisacrylamide, (poly ) Ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, (poly) ethylene glycol di (β
-Acryloyloxypropionate), trimethylolpropane tri (β-acryloyloxypropionate), poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, ethylene glycol, polyethylene glycol, glycerin, pentaerythritol, Examples are ethylenediamine and polyethyleneimine. The amount used is usually 0.005 to 5 mol% with respect to the monomer, and more preferably 0.
It is from 0 to 1 mol%. Even among these cross-linking agents, due to the durability and water absorption properties of the resulting water-absorbent resin, and the handleability of the hydrogel at the time of production, the polymerizability of having two or more polymerizable unsaturated groups in the molecule It is preferable to essentially use a crosslinking agent.

When carrying out the polymerization of the above-mentioned monomers in the present invention, bulk polymerization or precipitation polymerization can be carried out, but the polymerization is carried out using the monomer as a solution in view of performance and easy control of the polymerization. It is preferable to carry out. As the polymerization solvent,
There is no particular limitation as long as it is a liquid in which the monomer dissolves, and water, methanol, ethanol, acetone, dimethylformamide, dimethylsulfoxide, etc. are exemplified, but water or an aqueous liquid is particularly preferable. The concentration of the monomer may exceed the saturated concentration, but it is usually in the range of 20% by weight to the saturated concentration, and more preferably 25 to 50% by weight.
If the concentration of the monomer is too high, various physical properties may be deteriorated, so caution is required.

In the polymerization, water-soluble chain transfer agents such as hypophosphite, thiols and thiolic acids, starch,
Cellulose, polyvinyl alcohol, polyacrylic acid,
Polymerization may be carried out by adding a hydrophilic polymer such as a crosslinked polyacrylate to the monomer. The amount used of them is usually within 5 parts by weight for the former and within 50 parts by weight for the latter.

Examples of the method for polymerizing the monomer in the present invention include polymerization with a radical polymerization initiator, radiation polymerization, electron beam polymerization, and ultraviolet polymerization with a photosensitizer. In order to obtain a water absorbent resin, polymerization with a radical polymerization initiator is preferable. Examples of the radical polymerization method include casting polymerization carried out in a mold, polymerization on a belt conveyor, various aqueous solution polymerizations such as polymerization carried out while subdividing a hydrous gel polymer, reverse phase suspension polymerization, reverse polymerization. Known polymerization methods such as phase emulsion polymerization, precipitation polymerization, bulk polymerization and the like can be exemplified, but reverse phase suspension polymerization or aqueous solution polymerization is particularly preferable. During the polymerization, continuous polymerization,
The palindromic polymerization may be distinguished, and the decompression, the pressurization, and the normal pressure may be distinguished. Further, a fibrous base material may coexist during the polymerization to form a water-absorbent composite. Further, the polymerization temperature is preferably maintained substantially in the range of 0 to 100 ° C. except for the initial stage of polymerization and the peak.

The radical polymerization initiator used for the polymerization is, for example, potassium persulfate, ammonium persulfate,
Persulfates such as sodium persulfate; Organic peroxides such as t-butyl hydroperoxide and cumene hydroperoxide; Hydrogen peroxide; Azo compounds such as 2,2′-azobis (2-amidinopropane) dihydrochloride Other examples include known initiators such as chlorite, hypochlorite, cerium salt, and permanganate. Among these, among these,
One or more selected from the group consisting of persulfates, hydrogen peroxide and azo compounds are preferable. Further, when an oxidizing radical polymerization initiator is used, a reducing agent such as a sulfurous acid (hydrogen) salt or L-ascorbic acid may be used in combination, and when an azo polymerization initiator is used, ultraviolet rays may be used in combination. .
These radical polymerization initiators may be added to the polymerization system all at once or may be added sequentially, but the amount used is usually 0.001 to 2 mol%, preferably 0, relative to the monomer. ．
It is from 0 to 1 mol%.

In the present invention, the heat treatment of the gel polymer obtained according to the above procedure and / or the heat treatment of the dried product at the time of surface cross-linking is essential.

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

The state of the polymer during the heat treatment is not particularly limited, and examples thereof include a gelled polymer after polymerization, a dispersion in an organic solvent, and a dried polymer. Further, the solid content of the crosslinked product during the heat treatment may be kept constant, or the solid content may be increased by volatilizing the solvent. Specific examples of the heat treatment time include a drying step, a surface cross-linking step, a reheating step after drying, a granulation step, and a step of adding an additive, but the heat treatment in the drying step is performed. It is preferable. That is, the gel polymer of the present invention is dried at the above temperature and heat-treated to improve the water absorption capacity and dramatically reduce residual monomers. Examples of the drying method used include known drying methods such as hot air drying, infrared drying, and azeotropic dehydration, and are not particularly limited.

An additive such as a surfactant, an inorganic fine powder or bisulfite may be added to the water absorbent resin after polymerization or after drying, or the particle size is adjusted by crushing or granulating. You may. For example, when a powdery water-absorbent resin is intended, the average particle size is 10 to 2000 μm, more preferably 1
00-1000 μm, most preferably 300-600 μm
It is adjusted to about m.

Further, the method for producing a water-absorbent resin in which the vicinity of the surface is cross-linked provided by the present invention is to subject the dried product of the gel polymer obtained by the above method to a heat treatment to cross-link the vicinity of the surface. To be achieved.

Although a dried product of a gel-like polymer is indispensable for surface cross-linking, a water-absorbent resin which has been subjected to heat treatment during drying according to the above-mentioned method is used for the purpose of reducing the surface cross-linking effect and residual monomers. Is preferred. Incidentally, the drying is usually performed to a solid content of 60% or more, preferably 90% or more. (Hereinafter, the dried product of the surface-crosslinked hydrogel is simply referred to as a water-absorbent resin.) In the present invention, cross-linking in the vicinity of the surface of the water-absorbent resin may be carried out by cross-linking by radiation or the like. It is performed by adding a crosslinking agent in the vicinity. As the cross-linking agent used, known cross-linking agents are used without particular limitation, and for example, ethylene glycol, propylene glycol, polyethylene glycol, propylene glycol,
Various polyhydric alcohols such as glycerin, polyglycerin, 1,6-hexanediol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethyleneoxypropylene block copolymer, pentaerythritol and sorbitol; ethylene glycol di Various polyvalent epoxy compounds such as glycidyl ether and polyethylene glycol diglycidyl ether; Various polyvalent amine compounds such as ethylenediamine and polyethyleneimine; 2,2-bishydroxymethylbutanol-tris (3- (1-aziridinyl) propionate) Polyvalent aziridine compounds such as;
1,3-dioxolan-2-one, 4-methyl-1.3
-Dioxolan-2-one, 4,6-dimethyl-1,3
Various alkylene carbonate compounds such as dioxan-2-one; various polyvalent aldehyde compounds such as glyoxal; polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate; polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; Haloepoxy compounds such as epichlorohydrin; polyvalent metal salts such as hydroxides and chlorides of aluminum, iron, zirconium and the like; and compounds having these functional groups in combination are also exemplified.

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

In the present invention, the amount of the crosslinking agent used is usually 0.01 to 20 parts by weight, preferably 0.05 to 10 parts by weight, based on 100 parts by weight of the solid content of the water absorbent resin. Is.

As a method for adding a crosslinking agent near the surface of the water absorbent resin, a known method is used. A method for directly adding the crosslinking agent to the water absorbent resin or a method for adding the crosslinking agent to the water absorbent resin dispersed in a solvent. There is a method. In the case of using the former method, an inorganic compound such as silicon oxide fine powder or a surfactant may coexist in order to uniformly add the crosslinking agent.

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

In the present invention, it is preferable that water is present when the heat treatment is carried out after mixing the water-absorbent resin and the cross-linking agent, from the effect of reducing the residual monomer and the effect of surface treatment. Therefore, when using a dried water absorbent resin, it is preferable to add water. Water may be added at the same time as the crosslinking agent or separately, but the amount thereof is usually 20 parts by weight or less, preferably 0.5 to 10 parts by weight based on 100 parts by weight of the solid content of the water absorbent resin. The range is parts by weight.

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

The heat treatment temperature is usually 100 to 300 ° C., preferably 120 to 260 ° C., more preferably 150 to 2
It is in the range of 50 ° C. If the temperature is less than 100 ° C, not only the heat treatment will take a long time to cause a decrease in productivity, but also uniform and strong surface cross-linking is difficult to be achieved, and further, the effect of reducing the residual monomer at the time of surface cross-linking is small, so that The remaining amount also increases. Further, it should be noted that at a high temperature exceeding 300 ° C, the water absorbent resin may be thermally deteriorated. The heating time is appropriately determined depending on the desired surface cross-linking effect, heating temperature, etc., but is usually 1 minute to 10 minutes.
It is a range of time.

In the conventional surface treatment method, when acrylic acid is used as one of the monomers in the water absorbent resin, the number of residual monomers is increased by several tens to several thousands ppm. But,
In the method of the present invention, not only the increase in the residual monomer is hardly observed by the heat treatment, but it may be decreased. Therefore, in the present invention, it is possible to obtain a water-absorbent resin which can achieve uniform and strong crosslinking in a short time by using high temperature and which has less residual monomer and residual crosslinking agent.

As a method for carrying out the heat treatment, known means are used. For (1) a method of directly adding a crosslinking agent to the water-absorbent resin and then heat-treating as it is, or (2) a water-absorbent resin dispersed in a solvent. Examples include a method of adding a cross-linking agent and then heat-treating the dispersion, and (3) a method of performing heat treatment by filtering from a dispersion medium. Among these methods, the method (1) is preferable because of the ease of heat treatment. The heat treatment device is not particularly limited, and a known device such as a hot air dryer, a fluidized bed dryer, or a Nauter type dryer is used. In order to evenly distribute the cross-linking agent on the surface,
It is also preferable to heat-treat using a higher temperature while volatilizing at least a part of the crosslinking agent such as polyhydric alcohol.

[0045]

The manufacturing method of the present invention is described in the following (1) to (5).
It has features such as.

(1) The residual monomer, which has been conventionally reduced by sacrificing the performance, productivity, cost and safety of the water-absorbent resin through complicated processes such as addition of additives, is simple and efficient. Can be reduced to

(2) Even if a high temperature is used in the manufacturing process, the amount of residual monomers during the manufacturing is small and the manufacturing can be performed with high productivity. Moreover, by performing the heat treatment, not only the residual monomer is reduced, but also various physical properties such as a water absorbent resin having a high water absorption capacity are obtained.

(3) Since there is no generation / increased amount of residual monomer under high temperature or long-term use after that, no matter how long-term use such as agriculture and horticulture or high-temperature use with hot water, etc. Highly safe even under conditions.

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

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

The water-absorbent resin thus obtained can be widely used in the fields of sanitary materials, foods, civil engineering, agriculture and the like.

[0052]

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

(1) Water absorption capacity A water-absorbent resin (0.2 g) was used as a non-woven tea bag bag (4
(0 * 150 mm) and then immersed in a 0.9 wt% sodium chloride aqueous solution. After 30 minutes, the tea bag type bag was pulled up and drained for a certain period of time, then the weight of the tea bag type bag was measured, and the water absorption capacity was calculated by the following formula.

[0054]

[Equation 1]

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

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

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

[0058]

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

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

Then, the above-mentioned acrylate 333 after neutralization
After adding 774 g of acrylic acid for 2 hours after distillation and 1063 g of ion-exchanged water to 8 g, 325 g of 25% aqueous ammonia solution was added, and further, as a cross-linking agent, 2.83 g of trimethylolpropane triacrylate (0.04 mol of the monomer). %), The concentration is 38% and the neutralization rate is 75% (sodium salt 55%, ammonium salt 20%).
The monomer (1) of was obtained.

[0061]

[Production Example 2] In the preparation of the monomer of Production Example 1, 2148 g of acrylate, 1132 g of acrylic acid, 1563 g of ion-exchanged water, 657 g of 25% aqueous ammonia solution, 2.86 g of trimethylolpropane triacrylate (compared to 0.04 g of monomer) The monomer (2) having a concentration of 38% and a neutralization ratio of 75% (sodium salt 35%, ammonium salt 40%) was obtained by changing the content of the monomer (2).

[0062]

Production Example 3 In the preparation of the monomer of Production Example 1, 3922 g of acrylic acid salt, 599 g of acrylic acid, 817 g of ion-exchanged water, 162 g of 25% aqueous ammonia solution, 2.81 g of trimethylolpropanone triacrylate (compared to 0.04 g of monomer). By changing to (mol%), the concentration of 3
A monomer (3) having a neutralization ratio of 75% (sodium salt 65%, ammonium salt 10%) was obtained at 8%.

[0063]

Production Example 4 In the preparation of the monomer of Production Example 1, 4500 g of acrylic acid salt, 425 g of acrylic acid, 575 g of ion-exchanged water, 2.79 g of trimethylolpropane triacrylate (0.04 mol% of monomer) ), A monomer (4) having a concentration of 38% and a neutralization ratio of 75% (sodium salt 75%) was obtained.

[0064]

Production Example 5 1446.3 g of 2-acrylamido-2-methylpropanesulfonic acid and 215.6 g of acrylic acid used in Production Example 1 were dissolved in 2313.3 g of ion-exchanged water, and then 48% by weight under ice cooling. The acid groups were neutralized by adding 415.8 g of an aqueous sodium hydroxide solution and further adding 101.8 g of a 25% aqueous ammonia solution. After neutralization,
Crosslinking agent N, N'-methylenebisacrylamide 1.54
By dissolving g (0.1 mol% with respect to the monomer), a monomer (5) having a neutralization ratio of 65 mol% (sodium salt 50%, ammonium salt 15%) and a concentration of 40% was obtained.

[0065]

[Production Example 6] In Production Example 5, the amount of ion-exchanged water used was changed to 2305.9 g, and 48% by weight aqueous sodium hydroxide solution 54 was added without using ammonia for neutralization.
The acid group was neutralized with only 0.5 g. Thereafter, the crosslinking agent was similarly dissolved to obtain a monomer (6) having a neutralization ratio of 65 mol% (sodium salt 65%) and a concentration of 40%.

[0066]

[Production Example 7] A distillation flask equipped with a stirrer was charged with 2500 g of ion-exchanged water and 1801 g of acrylic acid, and a 25% aqueous ammonia solution 1275 was added at a liquid temperature of about 10 ° C.
g was added dropwise into the flask over 100 minutes. After the dropwise addition, 2.96 g of trimethylolpropane triacrylate as a crosslinking agent (0.04 mol% based on the monomer)
Was added to obtain a monomer (7) having a concentration of 38% and a neutralization ratio of 75% (ammonium salt 75%).

[0067]

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

Polymerization started 1 minute after the catalyst was added,
After 6 minutes, it was subdivided into about 5 mm, and after stirring was continued for 44 minutes, the gel polymer was taken out.

5% of the gel polymer thus obtained having a neutralization ratio of 75% (sodium salt 60%, ammonium salt 15%)
It was spread on a 0 mesh wire mesh, dried with hot air at 180 ° C. for 60 minutes, and heat-treated. The dried product obtained was pulverized with a vibration mill and further classified with 20 mesh to obtain a water absorbent resin (1). The results are shown in Table 1.

[0070]

Examples 2 and 3 In Example 1, the hydrogel polymer after polymerization was dried and heat-treated at 150 ° C. and 120 ° C.
Water absorbing resin (2)
And (3) were obtained. As shown in the results in Table 1, the water absorption capacity decreased and the residual monomer increased by decreasing the drying temperature. The results are shown in Table 1.

[0071]

Examples 4 and 5 Polymerization and drying / heat treatment were carried out in the same manner as in Example 1, except that the monomers (2) and (3) were used instead of the monomer (1) used for the polymerization. By performing the above, water-absorbent resins (4) and (5) were obtained, respectively. The results are shown in Table 1.

[0072]

Example 6 SUS in which 4000 g of the monomer (5) was replaced with nitrogen, and then the inner surface was lined with a tetrafluoroethylene resin
Made of 316, put in a cast polymerization device that can open and close with an internal volume of 300 mm * 300 mm * 50 mm, and replace with nitrogen,
It was placed in a water bath at 30 ° C. Then, 0.15 mol% of ammonium persulfate and 0.02 mol% of sodium bisulfite were added to initiate polymerization. By the above operation, it took 4 hours from the preparation of the monomer (5) to the start of polymerization.

Five hours after the initiation of the polymerization, the gel polymer was taken out from the casting polymerization apparatus and further treated with a meat chopper for about 5 m.
After subdividing into m-diameter, the water-absorbent resin (6) was obtained by performing drying and heat treatment in the same manner as in Example 1. The results are shown in Table 1.

[0074]

Example 7 In place of the monomer (1) used for polymerization in Example 1, a monomer (4) and a monomer consisting of 142 g of urea as an ammonia precursor (10 mol% based on the monomer) were used. Polymerization was performed in the same manner except that it was used. The obtained gel polymer having a neutralization rate of 95% (ammonium salt: 20 mol%) was dried with hot air at 190 ° C. for 60 minutes, and thereafter, the same procedure as in Example 1 was carried out to obtain a water absorbent resin (7 ) Got. The results are shown in Table 1.

[0075]

Example 8 After mixing 100 parts of the water absorbent resin (1) with 1 part of glycerin, 2 parts of water, and 2 parts of ethyl alcohol,
By performing a heat treatment at 190 ° C. for 40 minutes, a water absorbent resin (8) was obtained. The results are shown in Table 2.

[0076]

Example 9 100 parts of a water absorbent resin (4), 2 parts of propylene glycol, 3 parts of water, and 2 parts of isopropyl alcohol
After mixing the parts, heat treatment was performed at 150 ° C. for 60 minutes to obtain a water absorbent resin (9). The results are shown in Table 2.

[0077]

Example 10 To 100 parts of the water absorbent resin (5), 0.1 part of ethylene glycol diglycidyl ether, 5 parts of water,
After mixing 1 part of isopropyl alcohol, the mixture was heated at 180 ° C. for 30 minutes to obtain a water absorbent resin (10). The results are shown in Table 2.

[0078]

Example 11 100 parts by weight of the water absorbent resin (6) was mixed with 0.1 part of ethylene glycol diglycidyl ether and 10 parts of water in the coexistence of 1 part of silicon inorganic fine powder (Aerosil), and then at 170 ° C. for 1 hour. By heat treatment, a water absorbent resin (11) was obtained. The results are shown in Table 2.

[0079]

Example 12 100 parts by weight of the water absorbent resin (7) was mixed with 1 part of aluminum sulfate, 1 part of glycerin and 8 parts of water and then heat-treated at 180 ° C. for 30 minutes to obtain the water absorbent resin (12). Obtained. The results are shown in Table 2.

[0080]

Comparative Example 1 Polymerization was performed in the same manner as in Example 1 except that the monomer (4) was used instead of the monomer (1). The gel polymer thus obtained, which had a neutralization ratio of 75% (sodium salt 75%), was dried and heat-treated in the same manner as in Example 1 and then pulverized and classified in the same manner to give the comparative water absorbent resin (1). Got The results are shown in Table 1.

[0081]

Comparative Example 2 A comparative water absorbent resin (2) was obtained in the same manner as in Example 1, except that the gel polymer after polymerization was dried under reduced pressure at 60 ° C. The results are shown in Table 1.

[0082]

Comparative Example 3 Polymerization was performed in the same manner as in Example 6 except that the monomer (6) was used instead of the monomer (5). The gel polymer having a neutralization ratio of 65% (sodium salt: 65 mol%) thus obtained was dried and heat-treated in the same manner as in Example 6 to obtain a comparative water absorbent resin (3). The results are shown in Table 1.

[0083]

Comparative Example 4 Polymerization was performed in the same manner as in Example 1 except that the monomer (7) was used instead of the monomer (1). The hydrogel polymer thus obtained having a neutralization ratio of 75% (ammonium salt: 75 mol%) was dried and heat-treated in the same manner as in Example 2 to obtain a comparative water absorbent resin (4). . The results are shown in Table 1. Unlike the other Examples and Comparative Examples, the comparative water absorbent resin (4) was colored, and 6 ppm of acrylamide, which was harmful to the residual monomer, was detected.

[0084]

[Comparative Example 5] The same procedure as in Example 8 was repeated except that the comparative water absorbent resin (1) was used instead of the water absorbent resin (1) as the water absorbent resin to be cross-linked in the vicinity of the surface.
Comparative water absorbent resin (5) was obtained. The results are shown in Table 2.

[0085]

[Comparative Example 6] The same procedure as in Example 9 was repeated except that the comparative water absorbent resin (4) was used in place of the water absorbent resin (4) as the water absorbent resin cross-linked in the vicinity of the surface,
Comparative water absorbent resin (6) was obtained. The results are shown in Table 2. The comparative water-absorbent resin (4) is more colored than the comparative water-absorbent resin (6), and in addition to the residual monomer, acrylamide is 12
detected in ppm.

[0086]

Comparative Example 7 Comparative Example 7 was carried out in the same manner as in Example 11, except that the comparative water absorbent resin (3) was used in place of the water absorbent resin (6) as the water absorbent resin to be crosslinked in the vicinity of the surface. A water absorbent resin (7) was obtained. The results are shown in Table 2.

[0087]

Comparative Example 8 In Example 8, the water absorbent resin (1)
After mixing glycerin, water, and ethyl alcohol,
A comparative water absorbent resin (8) was obtained by performing no heat treatment. The results are shown in Table 2.

As is clear from Table 1 and Table 2, in the method for producing a water-absorbent resin of the present invention, there are few residual monomers, and generation / increase of residual monomers is also observed in the surface treatment and the subsequent heat treatment. I can't. In addition, by heat treatment at a high temperature, a water absorbent resin having a high water absorption capacity can be obtained.

[0089]

[Table 1]

[0090]

[Table 2]

[0091]

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical display location C08F 20/58 MNG 7242-4J 22/02 MLT 7242-4J (72) Inventor Yoshio Irie Himeji, Hyogo Prefecture Niigata Catalyst Himeji Laboratory Co., Ltd. 1 at 992 Nishioki, Okihama, Aboshi-ku, Yokohama-shi

Claims (6)

[Claims] 1. A gel polymer obtained by polymerizing a neutralized monomer in which 10 to 96 mol% of an acid group is an alkali metal salt and 4 to 50 mol% is an ammonium salt. A method for producing a water absorbent resin, comprising: 2. A dried product of a gelled polymer in which 10 to 96 mol% of acid groups are neutralized as an alkali metal salt and 4 to 50 mol% is an ammonium salt and a cross-linking agent are mixed and heat-treated on the surface. A method for producing a water absorbent resin, which comprises cross-linking. 3. The method according to claim 1, wherein the heat treatment temperature is 150 to 250 ° C. 4. The method according to claim 1, wherein heat treatment is performed in the step of drying the gel polymer. 5. The acid group-containing monomer is (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2-
The method according to claim 1 or 2, which is one or more selected from the group consisting of (meth) acrylamido-2-methylpropanesulfonic acid. 6. The method according to claim 2, wherein one or more cross-linking agents consisting of a polyhydric alcohol compound, a polyhydric glycidyl compound, a polyhydric amine compound and an alkylene carbonate are added near the surface of the water absorbent resin and heat treated. Production method.