JP2001079829A - Water absorbing resin and its preparation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a water absorbing resin with excellent physical properties prepared by surely mixing fine powder with a water-containing gel-like polymer and a method for preparing the water absorbing polymer by which the water- absorbing polymer can be highly efficiently prepared. SOLUTION: Fine powder of a water-absorbing gel-like polymer with a smaller size than a specified range generated in the manufacturing process of the water absorbing resin and fine powder of an additive for improving various characteristics of a water absorbing resin with the roughly ground water-containing gel-like polymer are fed from a feeding hole 14 of a screw type extruder and they are kneaded in a uniform pressure part in a casing 11. Then, while compression force to the water-containing gel-like polymer is elevated at a compression part in the neighborhood of an extrusion hole provided with a porous plate 17, the water-containing gel-like polymer is extruded. It is possible by this method to surely and sufficiently mix the fine powders into the water-containing gel-like polymer and to make them into fine granules.

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 in which fine powder is mixed with a hydrogel polymer, and more particularly to a method for producing fine powder generated in the process of producing a water-absorbent resin. The present invention relates to a water-absorbent resin that recycles the fine powder by mixing with a hydrogel polymer as a raw material, and a method for producing the same.

[0002]

2. Description of the Related Art It is well known that a water-containing gel polymer (hereinafter simply referred to as a water-containing gel) can be obtained as a water-absorbing polymer by aqueous solution polymerization of a water-soluble ethylenically unsaturated monomer. I have. This hydrogel is obtained as a semi-solid and highly elastic gel-like material, which is dried to a powder state to be used as a water-absorbing resin, that is, a water-absorbing agent.

[0003] The hydrogel is obtained as a lump or as an aggregate of hydrogel particles. The lump or aggregate hydrogel is pulverized using a pulverizer such as a kneader or meat chopper. The obtained particulate hydrogel (crushed gel)
After being dried, it is further pulverized by a pulverizer into particles having a size (particle size) within a predetermined range. As a result, a particulate water-absorbing resin is obtained.

[0004] Here, in the pulverizing step after the pulverized gel is dried, particles smaller than the predetermined range, that is, fine powder, are generated in addition to the particles having a predetermined size. When this fine powder is contained in the water-absorbent resin, dusting occurs when handling the water-absorbent resin, which is not preferable for hygiene, and the absorption capacity under pressure of the water-absorbent resin is reduced or the liquid permeability is reduced. It is not preferable because physical properties are reduced. However, removing the fine powder lowers the yield in the production of the water-absorbent resin, and also necessitates the cost of discarding the fine powder, resulting in a significant cost increase. Therefore, various techniques for reusing the fine powder have been conventionally proposed.

For example, Japanese Patent Application Laid-Open No. 3-152104 discloses a technique in which water is sprayed on dried fine powder to hydrate the fine powder to form a hydrate, and then the hydrous gel is kneaded with shear stress and extruded. Have been. Also, JP-A-4-41
No. 532 discloses a technique in which a fine powder is sufficiently wetted to form an amorphous gel and then combined with a hydrous gel after polymerization. In addition, Japanese Patent Application Laid-Open No. 5-43610
In the official gazette, a mixture of fine powder and hydrogel during polymerization is
There is disclosed a technique for increasing the degree of polymerization of the hydrogel beyond that at the time of mixing. Further, Japanese Patent Application Laid-Open No. Hei 4-227934 discloses a technique of forming a dispersion of fine powder in a polymer gel, and then adding and mixing water to form a substantially uniform mixture.

[0006]

However, in the above-mentioned techniques, water is added to the fine powder to return to the hydrogel and then added to the hydrogel after polymerization, so that the production process is complicated and difficult. . Further, it is economically disadvantageous in that the fine powder is once returned with water, added to the hydrogel after polymerization, and further dried to obtain a water-absorbing resin. Further, at this time, the fine powder is once swollen with water and dried again. Therefore, in the finally obtained water-absorbent resin, the portion which is originally a fine powder is further heated in a hydrogel state, thereby causing a decrease in performance. As a result, the water absorbing properties of the entire water absorbing resin are reduced.

[0007] On the other hand, in the above technique, a dry fine powder is added to a hydrogel during the polymerization reaction as it is while mixing with a kneader or the like. Therefore, as in each of the above techniques, the possibility of lowering the physical properties of the water-absorbent resin is reduced, but since the polymerization reaction in Examples is a batch, it is difficult to continuously advance the production process. I have. Therefore, there is a problem that the production efficiency of the water-absorbent resin is reduced and the recycle efficiency of the fine powder is also reduced.

[0008] This technique has substantially the same problems as those described above, although there are differences in the method of adding fine powder and water.

The present invention has been made in view of the above problems, and an object of the present invention is to efficiently reuse fine powder generated in the process of producing a water-absorbent resin and to absorb water obtained by recycling the fine powder. An object of the present invention is to provide a method for producing a water-absorbing resin which is excellent in production efficiency without lowering the physical properties of the water-soluble resin.

[0010]

Means for Solving the Problems The present inventors polymerize a water-soluble ethylenically unsaturated monomer, preferably in the presence of a crosslinking agent to obtain a hydrogel, and then add fine powder to the hydrogel to obtain a hydrogel. By mixing and extruding while applying compressive force, the fine powder can be sufficiently mixed with the hydrogel, and it has been found that a high-quality water-absorbent resin can be continuously produced, and the present invention has been completed. .

That is, a method for producing a water-absorbent resin according to the present invention is directed to a method for producing a water-absorbent resin comprising mixing fine powder with a hydrogel polymer to solve the above-mentioned problems.
Using a screw-type extruder equipped with an extrusion port having a perforated plate, a hydrogel obtained by increasing the compressive force near the extrusion port while kneading the hydrogel polymer and fine powder in the screw-type extruder, It is characterized in that a mixture of the polymer and the fine powder is preferably continuously extruded from an extrusion port.

At this time, the fine powder has a size (particle size) smaller than a predetermined range, which is obtained in the process of producing the water absorbent resin.
A fine powder of a water-absorbing resin having the following is preferably used. As the above-mentioned predetermined range, the size of the fine powder is more preferably less than 212 μm, and further preferably less than 150 μm. Furthermore, it is particularly preferable that the proportion of the water-absorbent resin having a size smaller than the predetermined range is 90% by weight or more of the whole fine powder. Also, as the perforated plate,
0.8 mm to 28 mm, preferably 5 mm to 2 mm
A plurality of holes in a range of 4 mm are formed, and an opening ratio of the plurality of holes is in a range of 20% to 55%, particularly 25% to
It is preferable to use one set within the range of 35%. Further, the mixing ratio of the fine powder to the hydrogel polymer is preferably 50% by weight or less, more preferably 20% by weight or less, based on the solid content of the hydrogel polymer. It is particularly preferred that it is between 5% by weight and 20% by weight. Further, it is preferable that the water-absorbent resin obtained by the above method is further surface-crosslinked.

According to the above method, in the vicinity of the extrusion port,
The hydrogel and the fine powder are mixed while being compressed. Therefore, the fine powder which is usually difficult to mix is sufficiently mixed with the hydrogel. Moreover, since the hydrogel and the fine powder are continuously extruded while being mixed, the fine powder is more uniformly and firmly adhered to the surface of the hydrogel. Note that the term “continuous” in the present invention includes not only continuous continuous, but also intermittent continuous occurring at fixed intervals.

As a result, the fine powder can be mixed with the hydrogel in a reliable and sufficient manner, and the characteristics of the resulting water-absorbing resin can be improved. In addition, when the hydrogel after the fine granulation is dried and pulverized, the amount of regenerated fine powder can be reduced. In particular, when a fine powder of a water-absorbent resin is used as the fine powder, the fine powder can be more reliably reused, and the yield of the water-absorbent resin production can be improved. Also, once the fine powder is swollen with water,
Since it is not necessary to carry out an operation that causes deterioration of the physical properties of the finally obtained water-absorbent resin, a high-quality water-absorbent resin can be obtained.

Further, according to the above-mentioned method, the hydrogel is continuously refined while being mixed with the fine powder. for that reason,
Reuse of the fine powder and production of the water-absorbent resin can be efficiently performed.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below. Note that the present invention is not limited to this. In the method for producing a water-absorbent resin according to the present invention, fine powder is added to a hydrogel polymer obtained by polymerizing a water-soluble ethylenically unsaturated monomer, and further mixed while applying a compressive force to the hydrogel polymer. And extrude. In the present invention, the term “water-absorbent resin” refers to 10 times or more of its own weight, more preferably 20 to 2000 times.
Absorbs water (ion-exchanged water) two times, more preferably 50 to 1000 times, and forms a water-insoluble (water component is 50% by weight or less, more preferably 20% by weight or less, still more preferably 10% by weight or less) hydrogel. This indicates a water-swellable crosslinked product to be formed.

The hydrogel polymer used in the present invention (hereinafter simply referred to as a hydrogel) is obtained by polymerizing an ethylenically unsaturated monomer in an aqueous solution. By adding a cross-linking agent before or after polymerization, an anionic, cationic or nonionic cross-linked product can be obtained. In addition, instead of the ethylenically unsaturated monomer, ethylene oxide or ethyleneimine is used as a monomer, the monomer is subjected to ring-opening polymerization, and then, if necessary, crosslinking is performed to obtain a hydrogel. Is also good.

In the present invention, the hydrogel may be obtained by adding water to a dried or undried polymer, but is usually preferably obtained by polymerization of the following monomers.

The ethylenically unsaturated monomer used as a raw material of the hydrogel is a water-soluble monomer,
Specifically, for example, (meth) acrylic acid, β-acryloyloxypropionic acid, maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, cinnamic acid,
2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2- (meth) acrylamide-2-methylpropanesulfonic acid, vinylsulfonic acid, styrenesulfonic acid, allylsulfonic acid,
Acid group-containing monomers such as vinylphosphonic acid, 2- (meth) acryloyloxyethyl phosphoric acid, (meth) acryloxyalkanesulfonic acid, and alkali metal salts, alkaline earth metal salts, ammonium salts, and alkylamines thereof. Salts; dialkylaminoalkyl (meth) acrylates such as N, N-dimethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide, and salts thereof. Quaternaries (e.g., reactants with alkyl hydride, reactants with dialkyl sulfate, etc.); dialkylaminohydroxyalkyl (meth) acrylates and quaternaries; N-alkylvinylpyridinium halide; hydroxymethyl (meth) acrylate , Hydroxyalkyl (meth) acrylates such as -hydroxyethyl methacrylate and 2-hydroxypropyl (meth) acrylate; acrylamide, methacrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl ( (Meth) acrylamide, N, N
-Dimethyl (meth) acrylamide; alkoxypolyethylene glycol (meth) acrylate such as methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate; vinylpyridine, N-vinylpyridine, N-vinylpyrrolidone,
N-acryloylpiperidine; N-vinylacetamide; and the like. One of these ethylenically unsaturated monomers may be used alone, or two or more thereof may be appropriately mixed.

When a monomer containing an acrylic acid (salt) monomer as a main component among the ethylenically unsaturated monomers exemplified above is used, the water-absorbing properties and safety of the obtained hydrogel are further improved. It is preferable because it improves. Here, the acrylic acid (salt) monomer refers to acrylic acid and / or water-soluble salts of acrylic acid.

The term "water-soluble salts of acrylic acid" refers to a neutralization ratio in the range of 30 mol% to 100 mol%, preferably 5 mol%.
An alkali metal salt, an alkaline earth metal salt, an ammonium salt, a hydroxyammonium salt, an amine salt, and an alkylamine salt of acrylic acid in the range of 0 mol% to 99 mol% are shown. Among the water-soluble salts exemplified above, lithium salts, sodium salts and potassium salts are more preferred, and sodium salts are particularly preferred.

After polymerization of an unneutralized or partially neutralized acrylic acid (salt) monomer, for example, a monomer having a neutralization ratio of less than 30 mol%, a predetermined amount of water-containing gel is obtained. By adding a neutralizing agent to the neutralization rate, the neutralization rate becomes 30
It is also possible to obtain a hydrogel of mol% or more. Examples of the neutralizing agent include a carbonate (hydrogen) salt and an alkali metal salt. These neutralizing agents may be used alone, or two or more of them may be used as an appropriate mixture.

The above acrylic acid (salt) monomers may be used alone or in combination of two or more.
The average molecular weight (degree of polymerization) of the water-absorbent resin is not particularly limited.

By polymerizing a monomer composition containing the above-mentioned ethylenically unsaturated monomer as a main component in the presence of a crosslinking agent, not only a hydrated gel-like polymer but also a hydrated gel-like crosslinked polymer is obtained. Obtainable. Therefore, the above-mentioned monomer composition contains other monomers copolymerizable with the above-mentioned ethylenically unsaturated monomer (copolymerizable copolymer) to such an extent that the hydrophilicity of the obtained hydrogel crosslinked polymer is not impaired. Monomer). In the following description, not only a hydrogel polymer but also a hydrogel crosslinked polymer is simply referred to as a hydrogel.

Specific examples of the copolymerizable monomer include (meth) acrylates such as methyl (meth) acrylate, ethyl (meth) acrylate, and butyl (meth) acrylate; vinyl acetate, propion and the like. Hydrophobic monomers such as vinyl acid; and the like. These copolymerizable monomers may be used alone,
Further, two or more kinds may be appropriately mixed and used.

Examples of the crosslinking agent used for polymerizing the above monomer component include a compound having a plurality of vinyl groups in the molecule; a functional group capable of reacting with a carboxyl group or a sulfonic acid group in the molecule. A compound having a plurality of groups; a compound having a plurality of functional groups capable of reacting with a carboxyl group or a sulfonic acid group in a molecule; These crosslinking agents may be used alone or in combination of two or more.

Specific examples of the compound having a plurality of vinyl groups in the molecule include N, N-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, and (poly) propylene glycol diamine. (Meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, glycerin tri (meth) acrylate, glycerin acrylate methacrylate, ethylene oxide modified trimethylolpropane tri (meth) acrylate, pentaerythritol tetra ( (Meth) acrylate, dipentaerythritol hexa (meth) acrylate, N, N-diallylacrylamide,
Examples include triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, diallyloxyacetic acid, N-methyl-N-vinylacrylamide, bis (N-vinylcarboxylic acid amide), and tetraallyloxyethane.

Compounds having a plurality of functional groups capable of reacting with a carboxyl group or a sulfonic acid group in the molecule include (poly) ethylene glycol, (poly) propylene glycol, 1,3-propanediol, 2,2,2 4-
Trimethyl-1,3-pentanediol, (poly) glycerin, 2-butene-1,4-diol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexane Dimethanol,
Polyhydric alcohol compounds such as 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, oxyethyleneoxypropylene block copolymer, pentaerythritol and sorbitol; (poly) ethylene glycol diglycidyl ether, (poly) glycerol Epoxy compounds such as polyglycidyl ether, diglycerol polyglycidyl ether, (poly) propylene glycol diglycidyl ether, and glycidol; various compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyamidepolyamine, and polyethyleneimine. Trivalent amine compound,
And condensates of these polyamines and haloepoxy compounds; polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; γ-glycidic Silane coupling agents such as xypropyltrimethoxysilane and γ-aminopropyltrimethoxysilane; 1,3-dioxolan-2-
On, 4-methyl-1,3-dioxolan-2-one,
4,5-dimethyl-1,3-dioxolan-2-one,
4,4-dimethyl-1,3-dioxolan-2-one,
4-ethyl-1,3-dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one,
1,3-dioxan-2-one, 4-methyl-1,3-
Alkylene carbonate compounds such as dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one and 1,3-dioxopan-2-one; haloepoxy compounds such as epichlorohydrin; zinc, calcium, Hydroxides or chlorides of magnesium, aluminum, iron, zirconium and the like. These compounds having a plurality of functional groups in the molecule can also be used as a crosslinking agent at the time of surface crosslinking described below.

The amount of the cross-linking agent used is not particularly limited, but when the above-mentioned monomer component or polymer is cross-linked, 0.1 to 0.1% of the monomer component before the polymerization.
It is preferably in the range of 0001 mol% to 10 mol%, more preferably in the range of 0.001 mol% to 1 mol%, and more preferably in the range of 0.005 mol% to 0.5 mol%. More preferably, it is 0.01 mol% or more.
Most preferably, it is in the range of 0.2 mol%. That is, the lower limit of the amount of the crosslinking agent to be used is preferably 0.0001 mol% with respect to the above monomer component,
It is more preferably 001 mol%, further preferably 0.005 mol%, and most preferably 0.01 mol%. The upper limit of the amount of the crosslinking agent used is preferably 10 mol%, more preferably 1 mol%, and more preferably 0.5 mol%, based on the monomer components.
Is more preferable, and most preferably 0.2 mol%.

In the present invention, the method for polymerizing the above monomer component is not particularly limited, and various conventionally known polymerization methods such as bulk polymerization, precipitation polymerization, aqueous solution polymerization, and reverse phase suspension polymerization are used. A method can be adopted.
Among them, reversed-phase suspension polymerization and aqueous solution polymerization, particularly aqueous solution polymerization using the above monomer component as an aqueous solution, are preferred from the viewpoint of improving the water absorbing properties of the obtained water-absorbent resin and easily controlling the polymerization. Examples of the method of performing aqueous solution polymerization include casting polymerization performed in a mold, polymerization on a belt conveyor, and polymerization in a kneader having a stirring blade.

During the above-mentioned polymerization reaction, it is preferable to carry out the polymerization while allowing the monomer components to stand without stirring. Further, when the aqueous solution polymerization of the above ethylenically unsaturated monomer, any of continuous polymerization or batch polymerization may be employed, and any of normal pressure, reduced pressure, and pressurized It may be performed under pressure. Note that the polymerization reaction is preferably performed under a stream of an inert gas such as nitrogen, helium, argon, or carbon dioxide.

At the start of the polymerization in the above-mentioned polymerization reaction, for example, a polymerization initiator or an activation energy ray such as radiation, an electron beam, an ultraviolet ray, or an electromagnetic ray can be used. Specific examples of the polymerization initiator include inorganic compounds such as sodium persulfate, ammonium persulfate, potassium persulfate, and hydrogen peroxide; and t-butyl hydroperoxide, benzoyl peroxide, cumene hydroperoxide, and the like. An organic peroxide; a thermally decomposable radical polymerization initiator such as 2,2'-azobis (N, N '
-Methyleneisobutylamidine) or a salt thereof, 2,
Azo compounds such as 2′-azobis (2-methylpropionamidine) or a salt thereof, 2,2′-azobis (2-amidinopropane) or a salt thereof, and 4,4′-azobis-4-cyanovaleric acid; Benzophenone,
Photo-radical polymerization initiators such as acetophenone and benzyl, and derivatives thereof, and acylphosphine oxide-based compounds. In addition, the azo compound is used not only as a photoradical polymerization initiator but also as a thermally decomposable radical polymerization initiator.

These polymerization initiators may be used alone or in combination of two or more. When a peroxide is used as the polymerization initiator, for example, redox polymerization may be performed using a reducing agent such as sulfite, bisulfite, or L-ascorbic acid (salt) in combination. . The amount of these polymerization initiators can be appropriately determined as required, but is usually more preferably in the range of 0.001 mol% to 2 mol% with respect to the monomer component, More preferably, it is in the range of 0.005 mol% to 0.5 mol%. That is, the lower limit of the amount of the polymerization initiator to be used is more preferably 0.001 mol%, more preferably 0.005 mol%, based on the monomer component. The upper limit of the amount of the polymerization initiator to be used is more preferably 2% by mole, more preferably 0.5% by mole, based on the monomer component. If the amount of the polymerization initiator used is less than 0.001 mol% with respect to the monomer component, the induction period may be prolonged or the residual monomer may increase. On the other hand, when the content is more than 2 mol% with respect to the monomer component, the molecular weight of the polymer is reduced, and self-crosslinking and decomposition due to excess radicals are increased, and the water absorbing properties of the water absorbent resin may be reduced.

In the present invention, if the hydrogel obtained by polymerizing the above-mentioned monomer component contains bubbles inside,
It is particularly preferable because the water absorbing performance of the obtained water absorbing resin can be improved. The hydrogel containing bubbles therein can be easily obtained by polymerizing the monomer component in the presence of a crosslinking agent so as to contain bubbles. As such a polymerization method, a polymerization method in the presence of an azo-based initiator; a carbonate as a foaming agent (JP-A-5-23)
No. 7378, JP-A-7-185331); a polymerization method in which a water-insoluble blowing agent such as pentane or trifluoroethane is dispersed in a monomer (US Pat. No. 5,328,935; US Patent 533
No. 8766); a polymerization method using a solid particulate foaming agent (WO 96/17884); a method of polymerizing while dispersing an inert gas in the presence of a surfactant; Various methods can be employed.

When polymerizing the above monomer components in the presence of a crosslinking agent, it is preferable to use water as a solvent. That is, it is preferable that the monomer component and the cross-linking agent be an aqueous solution. This is to improve the water absorbing properties of the obtained water-absorbing resin and to efficiently perform foaming with a foaming agent.

The above aqueous solution (hereinafter referred to as an aqueous monomer solution)
The concentration of the monomer component therein is preferably in the range of 20% by weight to 60% by weight, and more preferably in the range of 25% by weight to 50% by weight, regardless of whether or not foaming is performed. When the concentration of the monomer component is less than 20% by weight, the amount of the water-soluble component of the obtained water-absorbent resin may increase, and the foaming by the foaming agent becomes insufficient, so that the water absorption rate can be improved. It may not be possible. On the other hand, when the concentration of the monomer component exceeds 60% by weight, it may be difficult to control the reaction temperature and foaming by the foaming agent.

As a solvent for the aqueous monomer solution, water and
An organic solvent soluble in water can be used in combination. Specific examples of the organic solvent include methyl alcohol, ethyl alcohol, acetone, dimethyl sulfoxide, and ethylene glycol monomethyl ether. These organic solvents may be used alone or in combination of two or more.

The foaming agent added to the above monomer aqueous solution includes:
Those that disperse or dissolve in the aqueous monomer solution can be used. As the foaming agent, specifically, for example, n-pentane, 2-methylpropane, 2,2-dimethylpropane, hexane, heptane, benzene, substituted benzene, chloromethane, chlorofluoromethane,
Volatile organic compounds which disperse or dissolve in the above monomer aqueous solution such as 1,1,2-trichlorotrifluoroethane, methanol, ethanol, isopropanol, acetone, azodicarbonamide, azobisisobutyronitrile; sodium bicarbonate And ammonium carbonate, ammonium bicarbonate, basic magnesium carbonate, calcium carbonate, and the like; ammonium nitrite; dry ice; and acrylates of amino group-containing azo compounds. The above foaming agents may be used alone or in combination of two or more.

The amount of the foaming agent to be used with respect to the monomer may be appropriately set according to the combination of the monomer and the foaming agent, and is not particularly limited. However, the amount is more preferably in the range of 0.001 to 10 parts by weight based on 100 parts by weight of the monomer. If the amount of the foaming agent is out of the above range, the water absorbing properties of the resulting water absorbing resin may be insufficient.

The water content of the water-containing gel obtained as described above is generally in the range of 10% by weight to 80% by weight, preferably in the range of 20% by weight to 75% by weight. If the water content is less than 10% by weight, pulverization of the hydrogel may be difficult, and in the case of a hydrogel containing bubbles, the bubbles may be crushed. On the other hand, if the water content is higher than 80% by weight, it takes too much time for drying after grinding.

The polymerization rate of the hydrogel is preferably at least 70 mol%, more preferably at least 90 mol%, further preferably at least 95 mol%. Then, the polymerization rate is further increased by a drying step or the like (preferably 99 mol% or more, more preferably 9 mol% or more).
9.9 mol% or more).

The hydrogel has a solid content of 20% by weight.
Preferably, it is in the range of from 60 to 60% by weight, more preferably in the range of from 25 to 50% by weight. If the solid content of the hydrogel is out of the above range, good drying cannot be achieved, which is not preferable.

In the case of aqueous polymerization or the like, the above-mentioned hydrogel is usually finely divided into a predetermined size to obtain a particulate hydrogel (hereinafter, referred to as a finely divided hydrogel) (a fine-graining step). ). Thereafter, the finely divided hydrogel is dried, pulverized, and classified to become a water-absorbing resin. Here, it is desirable that the hydrogel is refined to be uniform and not kneaded. This is to avoid a decrease in physical properties of the obtained water-absorbent resin.

In the method for producing a water-absorbent resin in the present embodiment, a screw-type extruder is used as the above-mentioned grain refining means. The screw-type extruder may be a single-screw or multi-screw as long as it has a screw rotating in a cylinder. The specific configuration of the screw-type extruder, a method of reusing fine powder using the same, and a method of refining a hydrogel will be described later.

It is preferable to carry out a crushing step of coarsely crushing (crushing) the hydrogel at the time of polymerization or after the polymerization, preferably the hydrogel after the polymerization, before the above-mentioned fine-graining step. By coarsely crushing the hydrogel, the hydrogel can be easily supplied and filled into the granulation means, and the granulation step can be performed more smoothly. The crushing means used in the crushing step is preferably a means capable of coarsely crushing a hydrogel without kneading, and examples thereof include a guillotine cutter and a kneader.

Since the size and shape of the crushed product of the hydrogel obtained in the crushing step vary depending on the polymerization method used, the size and shape of the hydrated gel are not particularly limited as long as the size and shape can be filled in the granulating means. Not something. However, in general,
The size and shape of the crushed product of the hydrogel are preferably such that the length of one side of the long portion is in the range of 5 mm to 500 mm, more preferably in the range of 10 mm to 150 mm, and more preferably in the range of 30 mm to More preferably, it is within the range of 100 mm. If the length of one side is less than 5 mm, the meaning of pulverization by the fine-graining means is reduced.
On the other hand, if the length of one side exceeds 500 mm, a large gap is likely to be formed when the hydrogel is filled in the granulating means, and the pulverization efficiency may be reduced.

The finely divided hydrogel obtained by the above-mentioned fine-graining step is usually and preferably dried thereafter (drying step). The drying method at this time is not particularly limited, and for example, a conventional drying method using a band dryer, a stirring dryer, a fluidized-bed dryer, or the like is suitably used, and preferably 80 to 230 ° C. And more preferably in the range of 150 ° C to 200 ° C. Usually, the water content of the hydrogel after drying is 15
The conditions such as the drying temperature and the drying time are selected so as to be at most 10% by weight, preferably at most 10% by weight.

[0048] The granulated hydrogel in a substantially dry state obtained after the drying step is pulverized to obtain particles having a size within a predetermined range (pulverizing step). The pulverization method at this time is not limited, and a conventional pulverization method using a roll mill or the like can be suitably used.

The pulverized product obtained in the pulverization step is classified (classification step). As a result, only particles having a size within a predetermined range are selected and used as a water-absorbing resin. The classification method at this time is not particularly limited, and for example, sieving using a sieve is preferably used. For example, the size range of the milled product is 212 μm
In the case of と し た 850 μm, the pulverized product is first sieved with a sieve having an opening of 850 μm, and then the pulverized product passed through the sieve is sieved with a sieve having an opening of 212 μm. This 212
The pulverized product remaining on the μm sieve becomes a water-absorbent resin within a desired range.

The particle size of the water-absorbent resin obtained after the classification step is preferably in the range of 150 μm to 850 μm, and more preferably in the range of 212 μm to 850 μm. Fine powder with a size of less than 150 μm, or 8
Large particles having a size of more than 50 μm are not preferable because they cause problems such as insufficient water absorption properties. In addition,
Since the classification efficiency is usually less than 100%, fine powder (for example, a passing product of a sieve having an opening of 150 μm) is reduced to 10%.
It is difficult to remove 0%. Therefore, in the present invention, that the particle size is within a certain range means that particles having a particle size within the range occupy most of all particles. For example, when the particle size of the water-absorbent resin is in the range of 150 μm to 850 μm, most of the water-absorbent resin has a particle size of 150 μm to 85 μm.
It means that the particles are 0 μm. Particles having a particle size within a certain range more preferably account for 90% by weight or more of all the particles, more preferably 95% by weight or more, and most preferably 98% by weight or more.

Here, large particles having a size exceeding 850 μm can be converted into particles having a size within a predetermined range by repeating the pulverizing step again. However, when the fine powder having a size of less than 150 μm (or less than 212 μm) is used as a water-absorbing resin, its physical properties and handleability are reduced. Specifically, it is easy to generate dust during work, causing the work environment to deteriorate and clogging,
Dama (mamako) is easily generated, which hinders the diffusion of liquid. On the other hand, if this fine powder is discarded as it is, the cost and labor for disposal are separately required, and the yield in the production of the water-absorbent resin decreases. Therefore, it is very preferable to collect and reuse the fine powder of the water absorbent resin.

As a method for reusing the fine powder, specifically, a method is used in which the fine powder is added to the hydrogel before the granulation and mixed. However, if the dried fine powder is added to the hydrogel as it is and mixed, the fine powder does not break down and the fine powders aggregate together to form particles (masamako), which are not sufficiently dispersed in the hydrogel, and are therefore reused. Is difficult. Then, once the fine powder is swollen with water and added to the hydrogel before the granulation, the fine powder can be sufficiently mixed with the hydrogel. However, since the water-containing gel (fine powder) once dried is returned to the gel and then dried, the water absorption such as a decrease in the water absorption capacity of the fine powder portion and an increase in the amount of soluble matter, which may be caused by deterioration due to reheating at that time, is considered. The properties are reduced, and as a result, the physical properties of the water-absorbent resin itself are reduced. That is, the physical properties of the finally obtained water-absorbent resin are reduced.
Therefore, it is very preferable that the fine powder of the water-absorbent resin is added to the hydrogel before granulation as it is while being dried.

As described above, the fine powder that can be used in the present invention is generated in the process of producing the water-absorbent resin, for example, the fine powder classified in the classification step, the heat treatment step described below, and the transport step of the water-absorbent resin. Fine powder, fine powder repaired by the bag filter in the pulverizing step, and the like. Also,
Fine powder of a water-absorbing resin generated in another production line or factory can also be used. The fine powder may be in a dry state or a wet state, but the water content of the fine powder is preferably 15% by weight or less, more preferably 10% by weight or less. Then, a fine gel generated by reverse phase suspension polymerization or gel pulverization may be used as fine powder.

Further, in the present invention, not only the above-mentioned fine powder of the water-absorbing resin but also a water-absorbing resin such as fluidity, water-absorbing property, antibacterial property, deodorant property, urine resistance and light resistance of the water-absorbent resin is desired. In order to improve various properties, various water-soluble or water-insoluble organic or inorganic fine powders can be added. The average particle size of the fine powder is preferably 150 μm or less, more preferably 50 μm or less, still more preferably 10 μm or less, and 1 μm or less.
It is most preferred that: As such fine powder, for example, silica, titanium oxide, cement, clay, talc, antibacterial agent, deodorant, ion exchange resin, chelating agent,
Examples include waxes, surfactants, water-soluble inorganic salts (alkali metal salts), fine powders of polyethylene, polypropylene, cotton, hemp, starch, and other polysaccharides. The addition amount of these fine powders is appropriately determined within a range of 50% by weight or less based on the solid content of the hydrogel.

In the present invention, other additives can be added to the hydrogel in addition to the various fine powders.
As additives, water, organic solvents, polymerization initiators, monomers,
Examples include a crosslinking agent, a surfactant, a deodorant, and an antibacterial agent. When the fine powder of the water-absorbing resin is added to the hydrogel and water or an aqueous solution (hereinafter simply referred to as an aqueous solution) is added, it is desirable that the amount of the aqueous solution added is small for the above-mentioned reason. The ratio of the aqueous solution to 1 part by weight of the fine powder is preferably 4 parts by weight or less,
It is more preferably at most 0.1 part by weight, most preferably at most 0.1 part by weight, most preferably at most 0.01 part by weight.

In the present invention, in reusing fine powder, the fine powder is added to the hydrogel before granulation, and then kneaded using a screw-type extruder, and compressed near the extrusion port of the screw-type extruder. A method of continuously extruding a hydrogel with increasing the force is used. Thereby, the dried fine powder can be uniformly mixed with the hydrogel.

As the screw type extruder, for example, as shown in FIG. 2, a casing 11, a base 12, a screw 13, a supply port 14, a hopper 15, an extrusion port 16,
A configuration including the perforated plate 17, the rotary blade 18, the ring 19, the reverse prevention member 20, the motor 21, the streak-like projection 22, and the like can be preferably used.

The casing 11 has a cylindrical shape, and a screw 13 is arranged inside the casing 11 along the longitudinal direction of the casing 11. Cylindrical casing 11
An extrusion port 16 for extruding and crushing the hydrogel is provided at one end of the, and at the other end,
A motor 21 for rotating the screw 13 and a drive system are provided. The table 1 is located below the casing 11.
2 are provided so that the screw extruder can be stably arranged on the floor. On the other hand, a supply port 14 for supplying a hydrogel is provided above the casing 11, and preferably a hopper 15 for facilitating the supply of the hydrogel is provided.

The shape and size of the casing 11 are not particularly limited as long as it has a cylindrical inner surface corresponding to the shape of the screw 13. The number of rotations of the screw 13 is not particularly limited because it varies depending on the shape of the screw-type extruder. However, as will be described later, the screw 13 rotates in accordance with the supply amount of the hydrogel.
Is preferably changed.

The direction of rotation of the screw 13 is not particularly limited. In the present invention, the motor 21
The screw 13 rotates clockwise when viewed from the end on the side where is connected.

3 (a) and 3 (b)
A perforated plate 17 having a plurality of holes 17a as shown in FIG. The porous plate 17 is detachably fixed to the extrusion port 16 by a ring 19. This is because the size of the finely divided hydrogel particles is determined by the diameter of the holes 17a of the perforated plate 17, and in order to adjust the size of the particles of the hydrogel, the perforated plates 1 having different diameters of the holes 17a are required.
This is because it is necessary to replace 7 appropriately.

The thickness of the perforated plate 17 is 1 mm to 20 mm.
Within the range. The diameter of the plurality of holes 17a is 1 m.
It is preferably within a range of m to 30 mm, and 5 mm to
It is more preferably within a range of 25 mm,
It is particularly preferred that it is within the range of 0 mm. When the diameter of the hole 17a is within the above range, a sufficient compressive force is applied to the hydrogel (and fine powder) in the vicinity of the extrusion port 16, and no excessive mechanical external force is applied to the hydrogel during extrusion. . Therefore, the fine powder can be sufficiently dispersed in the hydrogel, and the hydrogel can be finely granulated.

The aperture ratio of the perforated plate 17 is 20% to 55%.
%, More preferably within a range of 25% to 35%, and particularly preferably around 30%. When the opening ratio is less than 20%, the hydrogel is hardly extruded, and the productivity is reduced. Also,
Since the hydrogel is less likely to be extruded, the perforated plate 17
This is not preferable because the hydrogel may be excessively finely crushed at the pressure feeding portion. On the other hand, if it exceeds 55%, sufficient compressive force may not be applied to the hydrogel at the time of extrusion, and the fine powder may not be sufficiently dispersed in the hydrogel. The aperture ratio indicates a ratio of the total area of all the holes 17a to the total area of the perforated plate 17 (substantially the same as the cross-sectional area of the casing 11).

If the perforated plate 17 has the diameters of the holes 17a and the opening ratios thereof set as described above, the hydrogel and fine powder charged into the screw type extruder include:
A large compressive force is applied near the extrusion port 16.

More specifically, as shown in FIG.
The hydrogel and the fine powder charged into the casing 11 from 4 are not sufficiently mixed. The hydrogel and the fine powder supplied into the casing 11 are mixed (kneaded) by the rotation of the screw 13, but the rotation of the screw 13 causes the hydrogel to be conveyed to the extrusion port 16 side rather than the mixing. Is added. Therefore, a uniform pressure is applied to such an extent that the hydrogel is not compressed. Most of the inside of the casing 11 to which such a uniform pressure is applied to the hydrogel is referred to as a pressure equalizing section as shown in FIG.

On the other hand, the perforated plate 17 is
Is provided, the hydrogel is not easily extruded in the vicinity of the extrusion port 16 and is compressed by the rotation of the screw 13. Moreover, as described above, the hydrogel and the fine powder in the casing 11 are conveyed to the extrusion port 16 side by the rotation of the screw 13, so that the compressive force is further increased. As a result, the hydrogel and the fine powder are stirred and mixed while being compressed (see FIG. 1), and are extruded through the extrusion port 16 (perforated plate 1).
7). The vicinity of the extrusion port 16 to which such a large compression force is applied is defined as a compression section as shown in FIG.

In normal stirring / mixing, the hydrogel and the fine powder are not compressed (see the pressure equalizing section in FIG. 1), so that the fine powder added to the hydrogel is not sufficiently dispersed and easily becomes lumps. On the other hand, in the stirring and mixing by the screw-type extruder, the hydrogel and the fine powder are mixed while being compressed, so that the fine powder is stirred in a state of being in close contact with the hydrogel (see the compression section in FIG. 1).

Further, since the hydrogel and the fine powder are extruded while being mixed and finely divided, the fine powder is reliably and sufficiently dispersed in the hydrogel, and the fine powder is uniformly and uniformly dispersed on the surface of the finely divided hydrogel. It can be firmly fixed. That is,
The finely divided hydrogel extruded from the screw type extruder is a mixture of the hydrogel and the fine powder in which the fine powder is sufficiently dispersed in the hydrogel. Therefore, the fine powder can be effectively reused and a high-quality water-absorbing resin can be obtained.

Further, in the finely divided hydrogel obtained by recycling the fine powder, the fine powder is uniformly dispersed and firmly adheres to the surface of the hydrogel. Therefore, a large amount of fine powder does not regenerate even when crushed after drying. Therefore, it is possible to efficiently manufacture the water-absorbing resin without reducing the recycling rate of the fine powder.

Further, the screw type extruder used in the present invention is preferably provided with a reversion preventing member 20 at least near the extrusion port 16. Therefore, it is possible to apply a more sufficient compressive force in the vicinity of the extrusion port 16. The above-described reversing prevention member 20 is used to
When provided near the (perforated plate 17), the hydrogel does not return to the supply port 14 side even when the diameter of the hole 17a is considerably small. Therefore, a more sufficient compressive force is applied to the hydrogel in the vicinity of the extrusion port 16 to reliably disperse and mix the fine powder, and the hydrogel is extruded smoothly, thereby avoiding a decrease in productivity. can do.

The shape of the reversion preventing member 20 is not particularly limited as long as the hydrogel is prevented from reverting from the extrusion port 16 to the supply port 14 side. For example, as the return preventing member 20, a spiral band-shaped projection 20a as shown in FIG. 2, a concentric band-shaped projection 20b as shown in FIG.
Streaks 22 parallel to the direction of travel (see FIG. 2), granular,
Spherical or angular projections are exemplified.

As shown in FIG. 2, the reversion preventing member 20 needs to be provided at least near the extrusion port 16 in the casing 11 (in FIG.
a) as an example), but may be provided on the entire inner surface of the casing 11. By providing the above-mentioned back-stopping member 20 in the vicinity of the extrusion port 16, the back-flow of the hydrogel is prevented near the extrusion port 16. However, if the back-back prevention member 20 is provided on the entire inner surface of the casing 11, the casing The reversal of the hydrogel is avoided at all the portions in 11, the hydrogel can be satisfactorily compressed in the vicinity of the extrusion port 16, and the hydrogel can be granulated without applying a mechanical external force.

It is preferable that the reversing prevention member 20 is provided on the entire inner surface of the casing 11.
For example, as shown in FIG. 2, the shape of the reverse rotation preventing member 20 provided in the vicinity of the extrusion opening 16 may be different from the shape of the reverse rotation preventing member 20 provided in other positions. For example, in FIG. 2, a spiral band-shaped projection 20a is formed near the extrusion port, and in addition, a strip-shaped projection 22 parallel to the axial direction of the screw 13 is formed. The surface other than the vicinity of the extrusion opening 16 may be a smooth surface on which the above-mentioned streaky projections 22 are not provided.

The above-mentioned reversion preventing member 20 and screw 13
Is preferably in the range of 0.1 mm to 5 mm. If it is less than 0.1 mm, the back-up prevention member 2
0 is not preferable because the rotation of the screw 13 is hindered. On the other hand, if it exceeds 5 mm,
A value of 0 is not preferable because the reverse movement of the hydrogel cannot be prevented.

The end of the screw 13 on the side not connected to the motor 21 is close to the extrusion port 16, but the perforated plate 17 and the end of the screw 13 are provided between the perforated plate 17 and the end of the screw 13. The rotary blade 18 is arranged so as to operate substantially in contact with the surface of the rotary blade 17. This rotary blade 18
By using, the size of the particles of the hydrogel extruded from the perforated plate 17 can be made smaller and a uniform particle size distribution can be obtained.

The configuration of the rotary blade 18 is not particularly limited. For example, a cross-shaped configuration can be suitably used. The direction of rotation of the rotary blade 18 is not particularly limited. For example, in the present invention, the rotary blade 18 rotates in the same direction as the screw 13. Furthermore, the rotation speed of the rotary blade 18 is not particularly limited.

In the method for producing a water-absorbent resin according to the present invention, it is preferable that the hydrogel is filled into the casing 11 of the screw-type extruder in a predetermined amount or more, preferably almost full, and pulverized. By crushing in this way, kneading of the hydrogel in the casing 11 can be suppressed, and the productivity of the crushing process can be improved.

In the present invention, the predetermined amount of the hydrogel filled in the casing 11 is defined by the following filling rate. Casing 1 of the above screw type extruder
A is a processing amount per unit time when the crushing treatment is performed by completely filling the hydrated gel into 1, and the supply of the hydrated gel supplied in a state where the screw 13 rotates at the same rotation speed as at this time. When the amount is B, the filling rate C is given by the following equation (1).
Is defined.

C = (B / A) × 100 (1) In the present invention, it is preferable to carry out the pulverizing treatment by setting the above-mentioned filling rate C within a range of 30% to 100%.
It is particularly preferable to make the filling rate C close to 100%. If the filling rate C is less than 30%, an extra mechanical external force is applied to the hydrogel as the screw 13 rotates in the casing 11, and the hydrogel may be kneaded.

Here, depending on the production amount of the hydrogel, the above-mentioned filling rate C may be less than 30%, and mechanical external force is easily applied to the hydrogel. In order to avoid this problem, in the method for producing a water-absorbent resin according to the present invention, the filling rate C is 30% to 100% depending on the change in the supply amount of the hydrogel supplied to the screw-type extruder. % Of the screw 13 is changed.

Specifically, for example, when the hydrogel is continuously supplied, if the supply amount is small, the hydrogel is pulverized in such a state that the filling rate C becomes 30% or less. A condition will occur. Therefore, the rotational speed of the screw 13 is reduced in accordance with the width of the decrease in the supply amount (that is, the decrease in the filling rate C).

Since the hydrogel of the present invention has a crosslinked structure, the throughput A per unit time can be reduced by lowering the rotation speed of the screw 13. As a result, even when the supply amount is reduced, the filling rate C is reduced to 30%.
The hydrogel can be refined while maintaining the content in the range of 100%. In the present invention, the relationship between the number of rotations and the processing amount A is parameterized, the number of rotations of the screw 13 is changed according to the change in the supply amount of the hydrogel, and the filling rate C is reduced from 30% to
The hydrogel can be refined while maintaining the content within the range of 100%.

In the present invention, as described above, a change in the supply amount of the hydrogel is defined as a change in the filling rate C.
The supply amount is not limited to this, and if the supply amount can be specified by another parameter, the amount of reduction in the rotation speed of the screw 13 can be appropriately specified according to the parameter.

Further, as described above, the variation width of the rotation speed of the screw 13 corresponding to the variation of the supply amount of the hydrogel is not limited to a specific width, and the conditions for the fine graining,
For example, the shape of the screw type extruder used (the volume of the casing 11, the shape of the screw 13, the
The diameter of the hole 17a) and the physical properties of the hydrogel can determine the optimum width of change. therefore,
It is preferable that the number of rotations is appropriately determined according to the screw type extruder or hydrogel used.

In the recycling of the fine powder, the mixing ratio of the fine powder to the hydrogel is preferably at least 50% by weight or less based on the solid content of the hydrogel.
%, More preferably 5% by weight or less.
It is particularly preferred that it is in the range of weight%.

Usually, the amount of fine powder obtained after pulverizing, drying and pulverizing the hydrogel is about 20% by weight based on the solid content of the hydrogel before pulverization. Therefore, 2
If the fine powder is reused at a mixing ratio of 0% by weight, it is possible to avoid a decrease in the yield in the production of the water absorbent resin. A particularly preferable range is within the above range of 5% by weight to 20% by weight.

However, in the actual production of the water-absorbent resin, one production line A carries out a production process of merely collecting the fine powder generated, and another production line B carries out the production process of the fine powder collected on the production line A. May be reused in the grain refining process. In this case, it is possible to add a large amount of fine powder at a level that does not deteriorate the physical properties of the obtained water-absorbent resin in the fine-graining step of the production line B. Therefore, when the collection of the fine powder and the reuse of the fine powder are performed in different production lines, the mixing ratio of the fine powder may be in the range of 20% by weight to 50% by weight.

If the mixing ratio of the fine powder exceeds 50% by weight, the amount of the fine powder is too large, and the physical properties of the finally obtained water-absorbent resin are undesirably reduced.

Further, in the recycling of the fine powder, the method of charging the fine powder into the screw type extruder is not particularly limited. In the method for producing a water-absorbent resin according to the present invention, since the hydrogel is continuously finely divided, the crushed hydrogel is continuously charged. On the other hand, the fine powder is sufficiently mixed with the hydrogel in the vicinity of the extrusion port 16 and then finely divided, so that the fine powder may be continuously introduced as in the case of the crushed hydrogel, or the fine powder may be mixed. It may be inserted intermittently in consideration of time.

The fine powder and the hydrogel may be mixed before being charged into the screw type extruder.
That is, after dusting the crushed hydrogel,
It may be charged into a screw type extruder. However, in this case, the mixing state of the fine powder and the hydrogel in the obtained finely divided hydrogel does not change significantly as compared with the case where it is not mixed before being charged. In addition, since the number of steps for mixing the fine powder and the hydrogel increases, it also hinders simplification of the manufacturing process. Therefore, the fines and the hydrous gel do not need to be mixed before entering the screw extruder.

The temperature of the hydrogel fed into the screw type extruder and the temperature of the granulated hydrogel extruded from the screw type extruder are from 40 ° C. to 100 ° C.
Is preferably within the range. When the temperature is out of the above-mentioned temperature range, particularly when the temperature is cooled to 40 ° C. or less, it is not preferable because handling of the finely divided hydrogel becomes difficult.

The finely divided hydrogel obtained by pulverization by the above-mentioned screw type extruder has a sharp particle size distribution and is of good quality to which no extra mechanical external force is applied during pulverization. Here, the average particle size of the finely divided hydrogel is preferably in the range of 0.5 mm to 3 mm, more preferably in the range of 0.5 mm to 2 mm. When the hydrogel is hard, 1 mm to 2 mm Is particularly preferred.

In the present invention, a multi-stage screw type extruder may be used, or the same extruder may be used in order to improve the mixing property of the fine powder and control the particles of the finely divided hydrogel. It may be extruded several times.

The smaller the average particle size of the finely divided hydrogel, the more uniform and good the subsequent drying step can be made. However, when the hydrous gel is hard, it is pulverized not so finely but coarsely. Is preferred. This is because if the hard hydrated gel is too finely crushed, clogging or the like may occur in a subsequent drying step or the like, making it difficult to dry uniformly and possibly causing undried matter. In addition, even if the finely divided hydrogel is too large, undried matter is generated. Therefore, the average particle size of the finely divided hydrogel is 1 mm to 2 mm.
It is particularly preferable that the ratio is within the range.

The finely divided hydrogel is then subjected to the above-mentioned drying step, pulverizing step, and classifying step to become a particulate water-absorbing resin having a size within a predetermined range. The fine powder obtained after the pulverizing step and the classifying step is reused again by the above-described method.

In the method for producing a water-absorbent resin according to the present invention, the above-mentioned particulate water-absorbent resin is used as a base polymer,
This may be further subjected to surface cross-linking to finally obtain a surface cross-linked polymer of the water-absorbing resin. The method for producing the surface crosslinked polymer is not particularly limited, and examples thereof include a method in which the particulate water-absorbing resin obtained after the classification step is mixed with a surface crosslinking agent solution and heated. As the cross-linking agent used for performing the surface cross-linking, the compounds mentioned as the cross-linking agent used for polymerizing the monomer component can be used.

The surface crosslinking agent solution is obtained by mixing a surface crosslinking agent with a solvent. As the solvent, water is most preferable, but in addition to water, an organic solvent soluble in water can be used in combination. Examples of the organic solvent include the above-mentioned organic solvents. Further, as the surface crosslinking agent,
The same cross-linking agent used when polymerizing the above-mentioned monomer components can be used. These crosslinking agents may be used alone or in combination of two or more. Among them, ethylene glycol diglycidyl ether, ethylene glycol, propylene glycol, butanediol, ethylene carbonate, polyethylene imine, polyamidoamine-epichlorohydrin and the like are preferable. The amount of the surface cross-linking agent used is
It is more preferably in the range of 0.01% by weight to 10% by weight, and further preferably in the range of 0.05% by weight to 5% by weight.

The amount of the solution comprising water and an organic solvent soluble in water (when used in combination) is more preferably 50% by weight or less, and preferably in the range of 0.5% by weight to 20% by weight. More preferably, 1% by weight to 10% by weight
It is particularly preferable that the ratio is within the range.

When the surface crosslinking agent solution is mixed with the base polymer (particulate water-absorbing resin), the water-absorbing resin swells due to water in the surface crosslinking agent solution. Then, the swollen water-absorbing resin is dried by heating. The heating temperature (drying temperature) at this time is preferably 80 to 220 ° C. The heating time (drying time) is 10 to 120.
Minutes.

As described above, by further cross-linking the surface of the water-absorbent resin obtained by reusing the fine powder, the physical properties of the water-absorbent resin can be further improved.

The water-absorbing resin obtained by the production method according to the present invention as described above has excellent water-absorbing performance, and is suitable for sanitary use such as disposable diapers, sanitary napkins, incontinence pads, wound protection materials, wound healing materials and the like. Materials (body fluid absorbent articles); Absorbent articles such as urine for pets; water-retaining materials for building materials and soil;
Materials for civil engineering and construction such as packing materials, gel blisters, etc .; Food products such as drip absorbing materials, freshness preserving materials, and cold insulating materials; Various industrial products such as oil-water separating materials, anti-condensation materials, coagulating materials; plants and soil It has been suitably used for various purposes such as agricultural and horticultural articles such as water retention materials.

[0102]

EXAMPLES The method for producing a water-absorbent resin according to the present invention will be described more specifically based on the following Reference Examples, Examples and Comparative Examples, but the present invention is not limited thereto. In addition, the water absorption capacity, the water absorption capacity under pressure, and the soluble content of the water-absorbent resin in the following Examples and Comparative Examples were measured as follows.

(Water Absorption Ratio) About 0.2 g of the water-absorbent resin was accurately weighed, placed in a 5 cm square non-woven tea bag, and sealed by heat sealing. This tea bag was immersed in artificial urine at room temperature. After 1 hour, pull up the tea bag and centrifuge at 1300 rpm for 3 hours.
After draining for 5 minutes, the weight W of the tea bag
₁ (g) was measured. Separately, the same operation was performed without enclosing the water absorbent resin in the tea bag, and the weight W ₀ (g) of the tea bag at that time was determined as a blank. The water absorption capacity was calculated based on the following equation.

[0104]

(Equation 1)

Table 1 shows the composition of the artificial urine and the amounts thereof.

[0106]

[Table 1]

(Water absorption capacity under pressure) First, the configuration of a measuring device used for measuring the water absorption capacity under pressure will be described.

As shown in FIG. 5, the measuring device is a balance 10
1 and a container 1 of a predetermined capacity placed on the balance 101
02, an outside air suction pipe 103, a conduit 104 made of silicone resin, a glass filter 106, and a measuring unit 105 mounted on the glass filter 106. The container 102 has an opening 102a at its top.
Has an opening 102b on the side surface thereof. An outside air suction pipe 103 is fitted into the opening 102a, and a conduit 104 is attached to the opening 102b.

The container 102 contains a predetermined amount of artificial urine 112. The lower end of the outside air suction pipe 103 is submerged in the artificial urine 112,
3 keeps the pressure in the container 112 at approximately atmospheric pressure. The glass filter 106 is formed to have a diameter of 55 mm, and the position and the height with respect to the container 102 are fixed. This glass filter 106 and container 1
02 communicates with each other by a conduit 104.

The measuring section 105 has a filter paper 107, a support cylinder 109, a wire net 110 attached to the bottom of the support cylinder 109, and a weight 111. On the glass filter 106, a filter paper 107, a support cylinder 109
(That is, the wire mesh 110) are placed in this order, and the weight 1 is placed inside the support cylinder 109, that is, on the wire mesh 110.
11 are placed. The wire mesh 110 is made of stainless steel, and is formed in 400 mesh (mesh size 38 μm).

A predetermined amount and a predetermined particle size of the water-absorbing resin 115 are uniformly spread on the wire mesh 110 to measure the water absorption capacity under pressure of the water-absorbing resin. The height of the upper surface of the wire mesh 110, that is, the height of the contact surface between the wire mesh 110 and the water absorbent resin 115 is set to be equal to the height of the lower end surface 103a of the outside air suction pipe 103. The weight 111 is a wire mesh 110, that is, the water absorbent resin 1
The weight is adjusted so that a load of 50 g / cm ² (corresponding to about 4.8 kPa) can be applied to No. 15.

Next, a method of measuring the water absorption capacity under pressure will be described. First, predetermined preparation operations such as putting a predetermined amount of artificial urine 112 into the container 102 and fitting the outside air introduction pipe 103 into the container 102 were performed. Next, the filter paper 107 is placed on the glass filter 106, and in parallel with this operation, the inside of the support cylinder 109, that is, the wire mesh 11
Then, 0.9 g of the water-absorbent resin 115 was uniformly removed on the water-absorbent resin 115, and the weight 111 was placed on the water-absorbent resin 115.

Next, on the filter paper 107, the wire cylinder 110, that is, the support cylinder 109 on which the water-absorbent resin 115 and the weight 111 are placed is placed.
Was placed so as to coincide with the center part. And filter paper 10
7, the artificial urine 1 absorbed by the water-absorbent resin 115 over a period of 60 minutes from the time when the support cylinder 109 is placed on the urine 1
The weight W ₂ (g) of the sample No. 12 was measured using the balance 101. The same operation was performed without using the water-absorbent resin 115, and the weight at that time, that is, the weight of the artificial urine 112 absorbed by the filter paper 107 or the like other than the water-absorbent resin 115 was measured using the balance 101. , Blank weight W ₃
(G). These weights W ₂ and blank weights W ₃
The water absorption capacity under pressure (g / g) was calculated from the following equation.

[0114]

(Equation 2)

(Soluble) 0.5 g of water-absorbing resin was added to 1000
Dispersed in mL of deionized water, stirred for 16 hours and filtered through filter paper. Thereafter, the obtained filtrate is subjected to colloid titration using a cationic colloid reagent, and the amount of water-soluble resin soluble (% by weight) is determined by measuring the amount of colloid of the water-absorbent resin dispersed in the filtrate. Was.

As the screw type extruder, a casing 11 having an inner diameter of 210 mm and a length of 900 mm was used. In addition, four spiral band-like projections 20a are provided as the reversion preventing member 20 in the vicinity of the extrusion port 16 in the casing 11 described above. The height of the band-shaped projection 20a from the inner surface of the casing 11 was 9 mm.

Reference Example 1 A monomer aqueous solution containing 0.1 mol% of 70 mol% of partially neutralized sodium acrylate and polyethylene glycol diacrylate (based on partially neutralized sodium acrylate) was prepared. At this time, the concentration of sodium acrylate was 39% by weight. Nitrogen was blown into the monomer aqueous solution to reduce the dissolved oxygen concentration in the aqueous solution to 0.5 ppm or less.

Next, 0.12 g / mol of sodium persulfate (based on partially neutralized sodium acrylate) and 0.0005 g / mol of L-ascorbic acid (based on partially neutralized sodium acrylate) were sequentially added to carry out polymerization. Was. The polymerization initiation temperature was 18 ° C., and after 12 minutes, the temperature reached 90 ° C.

The hydrogel (solid content: 42% by weight) obtained after the polymerization was crushed to 30 to 100 mm. 100 parts by weight of the obtained crushed hydrogel (crushed product) was put into a screw-type extruder provided with spiral projections inside the body, and the hydrated gel was passed through a perforated plate having a pore diameter of 16 mm and an opening ratio of 35%. Extruded. The average particle size of the extruded hydrogel obtained by extrusion was about 2 mm, and no particles larger than 10 mm were observed.

The finely divided hydrogel was dried with hot air at 170 ° C. for 40 minutes to a solid content of 94% by weight and then roll-milled.
It was pulverized to 50 μm or less. The pulverized product was sieved with a JIS standard sieve having a sieve opening of 150 μm, and the upper portion of the sieve was obtained as a reference water absorbent resin (1).
In addition, particles that passed through the sieve, that is, particles having a particle size (size) of less than 150 μm, were obtained as fine powder (A). The ratio of the fine powder (A) was 15% by weight. Also, 15
Reference water-absorbent resin obtained by removing fine powder of weight% (1)
Had a water absorption capacity of 43 times and a soluble content of 6% by weight.

Example 1 In the same manner as in Reference Example 1, 30
A crushed hydrogel crushed to １００100 mm was obtained. 100 parts by weight of the crushed hydrogel and the fine powder (A) obtained in Reference Example 1
8 parts by weight were continuously charged into the screw-type extruder, and a hydrogel was extruded from a perforated plate having a hole diameter of 16 mm. The average particle size of the extruded finely divided hydrogel is about 2 mm,
No particles larger than 0 mm were found.

The finely divided hydrogel was dried with hot air at 170 ° C. for 40 minutes and pulverized to 850 μm or less with a roll mill to obtain a water-absorbent resin (1) of the present invention. The water absorbing capacity of the water absorbent resin (1) was 43 times, and the soluble matter was 6% by weight. The ratio of the fine powder having a particle size of less than 150 μm contained in the water absorbent resin (1) was 16% by weight.

Example 2 Fine powder (A) in Example 1
Was changed to 12 parts by weight in the same manner as in Example 1,
The water absorbent resin (2) according to the present invention was obtained. The water absorption capacity of the water absorbent resin (2) was 43 times, and the soluble matter was 6% by weight. In addition, the particle size of 150 μm contained in the water-absorbent resin (2)
The proportion of fines less than 17% by weight.

REFERENCE EXAMPLE 2 A monomer aqueous solution containing 0.04 mol% of 65 mol% of partially neutralized sodium acrylate and polyethylene glycol diacrylate (based on partially neutralized sodium acrylate) was prepared. At this time, the concentration of sodium acrylate was 35% by weight. Nitrogen was blown into the monomer aqueous solution to reduce the dissolved oxygen concentration in the aqueous solution to 0.5 ppm or less.

Subsequently, 0.12 g / mol of sodium persulfate (based on partially neutralized sodium acrylate) and 0.0005 g / mol of L-ascorbic acid (based on partially neutralized sodium acrylate) were sequentially added to carry out polymerization. Was. The polymerization initiation temperature was 22 ° C, and the temperature reached 85 ° C after 12 minutes.

The hydrogel obtained after the polymerization was mixed with 30 to 100
mm. The resulting crushed hydrogel (crushed product)
100 parts by weight were charged into the screw-type extruder, and a hydrogel was extruded from a perforated plate having a hole diameter of 9.5 mm and an opening ratio of 35%. The average particle size of the extruded hydrogel obtained by extrusion was about 2 mm, and no particles larger than 10 mm were observed.

The finely divided hydrogel was dried with hot air at 160 ° C. for 60 minutes and pulverized to 850 μm or less by a roll mill.
The pulverized product was sieved with a sieve having a sieve opening of 150 μm, and the upper portion of the sieve was obtained as a reference water absorbent resin (2). Also, particles that have passed the sieve,
That is, particles having a particle size of less than 150 μm were obtained as fine powder (B). The ratio of the fine powder (B) was 15% by weight. The reference water-absorbent resin (2) obtained by removing 15% by weight of fine powder had a water absorption ratio of 65 times and a soluble content of 12% by weight.

Example 3 In the same manner as in Reference Example 2, 30
A crushed hydrogel crushed to １００100 mm was obtained. 100 parts by weight of the crushed hydrogel and the fine powder (B) obtained in Reference Example 1
10 parts by weight and into the screw extruder continuously,
The hydrogel was extruded from a perforated plate having a pore size of 9.5 mm. The average particle size of the extruded hydrous gel was about 2 mm, and no particles larger than 10 mm were observed.

The finely divided hydrogel was dried with hot air at 160 ° C. for 60 minutes and pulverized to 850 μm or less with a roll mill to obtain a water-absorbent resin (3) of the present invention. The water absorption ratio of the water absorbing resin (3) was 64 times, and the soluble matter was 12% by weight. In addition,
The ratio of the fine powder having a particle size of less than 150 μm contained in the water absorbent resin (3) was 16% by weight.

Table 2 summarizes the above Examples and Reference Examples.
Shown in

[0131]

[Table 2]

As can be seen from the above Examples and Reference Examples, the water-absorbent resins (1) to (3) mixed with fine powder, obtained by the method for producing a water-absorbent resin according to the present invention, were produced by a usual production method. It can be seen that there is no increase in cost due to a decrease in the yield, the water absorption properties are not reduced, and the obtained material has excellent physical properties as compared with the obtained reference water absorbent resins (1) and (2) from which fine powder has been removed.

That is, the reference water-absorbent resin (1) obtained by removing 15% by weight of the fine powder of Reference Example (1) (the yield was reduced by 15% by weight) and 8% by weight of Examples (1) and (2). The water-absorbent resins (1) and (2), which regenerate 12% by weight fine powder and do not remove fine powder, have the same physical properties. And
Reference water-absorbent resin (2) obtained by removing 15% by weight of fine powder of Reference Example (2) (yield reduced by 15% by weight), and Example (3) in which 10% by weight of fine powder was regenerated and fine powder was not removed ) Has the same physical properties as the water absorbent resin (3).

Next, a case where the above water-absorbent resins (1) to (3) are used as a base polymer and the surface is cross-linked to obtain a water-absorbent resin will be described.

Example 4 The water-absorbent resin (1) obtained in Example 1 was sieved with a sieve having a sieve opening of 150 μm to remove fine powder having a particle size of less than 150 μm.

With respect to 100 parts by weight of the water-absorbent resin (1) as a base polymer from which fine powder was removed, 0.03 parts by weight of ethylene glycol diglycidyl ether, 3 parts by weight of water,
A surface crosslinking agent solution consisting of 1 part by weight of isopropyl alcohol was mixed. Then, it heated at 200 degreeC for 30 to 50 minutes, and obtained the water absorbent resin (4) as a surface crosslinked body concerning this invention. The water absorption capacity of the water absorbent resin (4) is 30.
, Water absorption capacity under pressure 27 times, soluble content 5% by weight
Met.

Example 5 A water absorbent resin (5) according to the present invention was obtained in the same manner as in Example 4 except that the water absorbent resin (2) obtained in Example 2 was used as a base polymer.
The water absorbency of the water absorbent resin (5) was 30 times, the water absorbency under pressure was 27 times, and the soluble matter was 5% by weight.

Comparative Example 1 Example 4 was repeated except that the reference water absorbent resin (1) obtained in Reference Example 1 was used as a base polymer.
Example 4 in the same manner as in Example 4, ie, without reusing the fine powder.
In the same manner as in the above, a comparative water absorbent resin (1) was obtained. Comparative water absorbent resin (1) has a water absorption capacity of 30 times, and a water absorption capacity under pressure of 2
Seven times, the soluble content was 5% by weight.

[Comparative Example 2] In Comparative Example 1, reference example 1
Comparative water-absorbent resin (2) was obtained in the same manner as in Comparative Example 1 except that fine powder having a particle size of 150 μm or less was not removed from the reference water-absorbent resin (1) obtained in the above. The comparative water absorbent resin (2) had a water absorption capacity of 29 times, a water absorption capacity under pressure of 25 times, and a soluble content of 6
% By weight.

Example 6 The water-absorbent resin (3) obtained in Example 3 was sieved with a sieve having a sieve opening of 150 μm to remove fine powder having a particle size of less than 150 μm.

With respect to 100 parts by weight of the water-absorbing resin (3) as a base polymer from which fine powder was removed, 0.03 parts by weight of ethylene glycol diglycidyl ether, 3 parts by weight of water,
A surface crosslinking agent solution consisting of 1 part by weight of isopropyl alcohol was mixed. Then, it heated at 190 degreeC for 30 to 50 minutes, and obtained the water absorbent resin (6) as a surface crosslinked body concerning this invention. The water absorption capacity of this water absorbing resin (6) is 40
The water absorption capacity under pressure was 30 times, and the soluble matter was 12% by weight.

Comparative Example 3 Example 6 was repeated except that the reference water absorbent resin (2) obtained in Reference Example 2 was used as the base polymer.
Example 6 in the same manner as in Example 6, that is, without reusing the fine powder.
In the same manner as in the above, a comparative water absorbent resin (3) was obtained. The water absorption capacity of the comparative water absorbent resin (3) is 41 times, and the water absorption capacity under pressure is 3
0 times, the soluble content was 12% by weight.

[Comparative Example 4] In Comparative Example 1, reference example 2
A comparative water-absorbent resin (4) was obtained in the same manner as in Example 6, except that fine powder having a particle size of 150 μm or less was not removed from the reference water-absorbent resin (2) obtained in. Comparative water absorbent resin (4) has a water absorption capacity of 38 times, a water absorption capacity under pressure of 27 times, and a soluble content of 1
It was 4% by weight.

Table 3 summarizes the results of the above Examples and Comparative Examples.

[0145]

[Table 3]

As can be seen from the above examples, it is understood that the water absorbing resins (4) to (6) of the present invention can exhibit more excellent physical properties by cross-linking the surface. Further, as is clear from comparison with the comparative water absorbent resins (1) to (4), the water absorbent resins (4) to (6) have excellent physical properties even when fine powder is added. You can see that.

That is, the water-absorbent resin (5) using the water-absorbent resin (2) whose base powder is obtained by recycling the fine powder of 12% by weight as a base polymer, and the yield is reduced because the fine powder is not reused. It shows the same physical properties as the comparative water absorbent resin (1) using the reference water absorbent resin (1) as a base polymer. Therefore, the water-absorbent resin (5) can be manufactured at low cost, and therefore, the reference water-absorbent resin (1)
Better.

In order to avoid an increase in cost due to a decrease in yield, the comparative water-absorbent resin (2) obtained by subjecting the base polymer to surface cross-linking without removing the fine powder and containing 15% by weight of the fine powder was used. As compared with the comparative water-absorbent resin (1) in which the surface cross-linking of the base polymer was performed after removing the fine powder, the physical properties were greatly reduced. Therefore, a method of cross-linking the surface of the base polymer without removing fine powder in order to avoid an increase in cost is not preferable because the physical properties of the obtained water-absorbent resin are reduced.

[0149]

As described above, the method for producing a water-absorbent resin according to the present invention is a method for producing a water-absorbent resin in which fine powder is mixed with a hydrogel polymer, the method comprising an extrusion port having a perforated plate. Using a screw type extruder, a mixture of the hydrogel polymer and the fine powder obtained by increasing the compressive force near the extrusion port while kneading the hydrogel polymer and the fine powder in the screw type extruder is used to extrude the mixture. It is a method of pushing out from.

[0150] Therefore, there is an effect that the fine powder can be surely and sufficiently mixed with the hydrogel. In particular, when the fine powder of the hydrogel is used as the fine powder, the fine powder can be surely reused to improve the yield of the water-absorbent resin production and reduce the physical properties of the finally obtained water-absorbent resin. Since it is not necessary to carry out such an operation, an effect that a high-quality water-absorbing resin can be obtained is achieved. In addition, there is an effect that the reuse of the fine powder and the production of the water absorbent resin can be efficiently performed.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a step of pulverizing a hydrogel using a screw-type extruder in a method for producing a water-absorbent resin according to one embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a configuration of a screw-type extruder used in a method for producing a water-absorbent resin according to one embodiment of the present invention.

3 (a) and 3 (b) are perspective views showing an example of a configuration of a perforated plate included in the screw-type extruder shown in FIG.

FIG. 4 is an explanatory view showing another example of a configuration of a band-shaped projection as a reversion preventing member provided in the screw-type extruder shown in FIG.

FIG. 5 is a schematic cross-sectional view showing a configuration of a measuring device used to measure a water absorption capacity under pressure, which is one of the physical properties of a water absorbent resin obtained by the method for producing a water absorbent resin.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Casing 13 Screw 14 Supply port 16 Extrusion port 17 Perforated plate 17a Hole 20 Reverse return prevention member 20a Strip projection (return prevention member) 20b Strip projection (return prevention member) 22 Stripe projection (return prevention member)

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B29C 47/30 C08J 3/24 Z C08J 3/12 CER 5/00 3/24 A61F 13/18 307B 5 / 00 // B29K 105: 16 105: 24 C08L 101: 00 (72) The inventor Takumi Hatsuda 992 Nishioki, Nishioki, Abashiri-ku, Himeji-shi, Hyogo 1 Inside Nippon Shokubai Co., Ltd. 992 Nishioki 1 Nippon Shokubai Co., Ltd. (72) Inventor Kishihiko Ishi ▲ saki ▼ 992 Nishioki Okihama, Abashiri-ku Himeji-shi Hyogo Prefecture 1 Nippon Shokubai Co., Ltd.

Claims (8)

[Claims] 1. A method for producing a water-absorbent resin in which fine powder is mixed with a hydrogel polymer, comprising the steps of: using a screw-type extruder having an extrusion port having a perforated plate; A method for producing a water-absorbent resin, comprising: extruding a mixture of a hydrogel polymer and fine powder obtained by increasing the compressive force in the vicinity of an extrusion port while kneading a coalesced product and a fine powder from the extrusion port. 2. The method for producing a water-absorbent resin according to claim 1, wherein the fine powder is a fine powder of a water-absorbent resin having a size less than a predetermined range, which is obtained in a process of producing the water-absorbent resin. . 3. The method for producing a water-absorbent resin according to claim 1, wherein the size of the fine powder is less than 150 μm. 4. The method according to claim 1, wherein the perforated plate is 0.8 mm to 28 mm.
4. A method according to claim 1, wherein a plurality of holes are formed within the range, and the aperture ratio of the plurality of holes is set in a range of 20% to 55%. A method for producing the water-absorbent resin according to the above. 5. The method according to claim 1, wherein the mixing ratio of the fine powder to the hydrogel polymer is 50% by weight or less based on the solid content of the hydrogel polymer. 4. The method for producing a water-absorbent resin according to claim 1. 6. The method for producing a water-absorbent resin according to claim 1, wherein the water-absorbent resin obtained by the above method is further subjected to surface crosslinking. 7. A water-absorbent resin produced by the production method according to claim 1. 8. A diaper comprising the water absorbent resin according to claim 7.