JPWO2020067311A1 - Manufacturing method of water-absorbent resin and water-absorbent resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a water-absorbent resin having excellent absorption characteristics (small amount of liquid return) when used as a sanitary material.
SOLUTION: A monomer composition, an organic solvent, and a dispersion aid are continuously supplied to a disperser, and fine droplets containing the monomer composition are dispersed in the organic solvent. That is, the fine droplets dispersed in the organic solvent are supplied to the polymerization apparatus, and the monomer is polymerized to obtain a hydrogel polymer, and the organic separated from the hydrogel polymer. A method for producing a water-absorbent resin, which comprises supplying a solvent to the disperser again, wherein the heat resistance index of the dispersion aid is 60 mN / m or more.
[Selection diagram] Fig. 1

Description

The present invention relates to a method for producing a water-absorbent resin and a water-absorbent resin.

In recent years, in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads, a water-absorbent resin as a constituent material thereof has been widely used as a water-absorbing agent from the viewpoint of body fluid absorption. Many monomers and hydrophilic polymers are used as raw materials for the water-absorbent resin, but from the viewpoint of water-absorbing performance, polyacrylic acid using acrylic acid and / or a salt thereof as a monomer. (Salt) -based water-absorbent resin is the most produced industrially.

The characteristics that the water-absorbent resin should have include an excellent water absorption amount and water absorption rate when in contact with an aqueous liquid such as a body fluid, gel strength, and an attractive force for sucking water from a base material containing the aqueous liquid. Conventionally, a water-absorbent resin that has a plurality of physical properties in a specific range from these absorption characteristics and exhibits excellent performance (water absorption characteristics) when used as a sanitary material such as a disposable diaper or a sanitary napkin. Alternatively, various absorbent bodies and absorbent articles using the water-absorbent resin have been proposed.

The general method for producing a water-absorbent resin is roughly classified into an aqueous solution polymerization method and a reverse phase suspension polymerization method. According to the reverse phase suspension polymerization method, a pearl-like (spherical) water-absorbent resin can be obtained. The reverse phase suspension polymerization method is a method in which a monomer aqueous solution is suspended in an organic solvent to carry out polymerization. For example, there is a method of initiating polymerization after dispersing a monomer in an organic solvent in the form of droplets by mechanical stirring (Japanese Patent Laid-Open No. 61-192703, etc.). In such a method, it was necessary to add a large amount of dispersion aid when dispersing the solution containing the monomer in the organic solvent. As a result, a part of the dispersion aid remains in the water-absorbent resin obtained by the polymerization reaction, and the surface tension is lowered, so that the physical properties of the water-absorbent resin may be lowered. In addition, due to the batch polymerization operation, the production efficiency is poor and the quality may fluctuate.

On the other hand, in International Publication No. 2016/088848 (corresponding to US Patent Application No. 2017/267793) and International Publication No. 2016/182082, the purpose is to reduce the amount of dispersion aid added and improve production efficiency. Disclosed is a continuous polymerization method in which an organic solvent and a monomer aqueous solution are continuously mixed and dispersed at 70 ° C. or higher and at a specific flow rate ratio. Specifically, in International Publication No. 2016/182082, an organic solvent in which a small amount of an ester-based dispersion aid is dissolved is used as a dispersion aid, and a monomer aqueous solution is used as an organic solvent with a multi-fluid spray nozzle as a disperser. It is dispersed. Then, a hydrogel-like polymer is produced by polymerization, and the hydrogel-like polymer and the organic solvent are separated. The separated hydrogel polymer becomes a water-absorbent resin through a drying step and the like. On the other hand, the separated organic solvent is re-supplied to the spray nozzle and reused in the dispersion step and the polymerization step of the monomer aqueous solution (that is, the organic solvent is continuously circulated in the dispersion step, the polymerization step and the separation step). Make a phase).

In order to produce the water-absorbent resin industrially, the manufacturing process of the water-absorbent resin is usually operated continuously. However, when continuous operation is performed according to the specific embodiment described in International Publication No. 2016/182082, the surface tension of the obtained water-absorbent resin gradually decreases with the operation time, and excellent absorption when used as a sanitary material. It was found that the characteristics (small amount of liquid return) were impaired.

Therefore, an object of the present invention is to provide a production method capable of obtaining a water-absorbent resin having excellent absorption characteristics (small amount of liquid return) when used as a sanitary material even in continuous operation. Another object of the present invention is to provide a water-absorbent resin having a small amount of liquid return when used as a sanitary material.

The problem is that the monomer composition, the organic solvent, and the dispersion aid are continuously supplied to the disperser, and the fine droplets containing the monomer composition are dispersed in the organic solvent. That is, the fine droplets dispersed in the organic solvent are supplied to the polymerization apparatus, and the monomer is polymerized to obtain a hydrogel polymer, and the organic separated from the hydrogel polymer. The solution is a method for producing a water-absorbent resin, which comprises supplying the solvent to the disperser again, wherein the heat resistance index of the dispersion aid is 60 mN / m or more. Will be done.

Further, the above problem is solved by a water-absorbent resin obtained by reverse-phase suspension polymerization, which has a surface tension of 65 mN / m or more and a DRC of 5 min of 46 g / g or more. ..

It is the schematic which shows a part of the manufacturing process of the water-absorbent resin which concerns on one Embodiment of this invention. It is sectional drawing which shows an example of the disperser. It is sectional drawing which shows the other example of a disperser. It is sectional drawing which shows still another example of a disperser. It is sectional drawing which shows still another example of a disperser. It is a schematic diagram which shows the measuring apparatus of DRC 5min.

Hereinafter, the present invention will be described while showing the best mode. Throughout the specification, it should be understood that the singular representation also includes its plural concept, unless otherwise stated. Therefore, it should be understood that singular articles (eg, "a", "an", "the", etc. in English) also include the plural concept unless otherwise noted. It should also be understood that the terms used herein are used in the meaning commonly used in the art unless otherwise noted. Thus, unless otherwise defined, all terminology and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, this specification (including definitions) takes precedence. The present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims.

[1. Definition of terms〕
[1-1. Water-absorbent resin]
As used herein, the term "water-absorbent resin" refers to a water-soluble component having a water swellability (CRC) of 5 g / g or more as defined by ERT441.2-02 and a water-soluble component defined by ERT470.2-02. Ext) refers to a polymer gelling agent having an Ext) of 70% by weight or less.

In the present specification, the "water-absorbent resin" is not limited to the embodiment in which the total amount (100% by weight) is only the water-absorbent resin, and includes additives and the like if the above-mentioned CRC and Ext are satisfied. It may be a water-absorbent resin composition. Further, in the present specification, the "water-absorbent resin" is a concept including an intermediate in the manufacturing process of the water-absorbent resin. For example, a hydrogel polymer after polymerization, a dry polymer after drying, a water-absorbent resin powder before surface cross-linking, and the like may also be referred to as "water-absorbent resin".

As described above, in the present specification, in addition to the water-absorbent resin itself, the water-absorbent resin composition and the intermediate may be generically referred to as "water-absorbent resin".

[1-2. others]
In the present specification, "X to Y" indicating a range means "X or more and Y or less".

As used herein, "ppm" means "weight ppm".

As used herein, "~ acid (salt)" means "~ acid and / or a salt thereof". "(Meta) acrylic" means "acrylic and / or methacryl".

In the present specification, the unit of volume "liter" may be expressed as "l" or "L".

As used herein, the term "average" simply means the arithmetic mean.

As used herein, the term "continuous operation" refers to a series of steps (dispersion step, polymerization step, separation and recycling step) of the water-absorbent resin manufacturing process, preferably 5 hours or more, more preferably 1 day or more, and further. More preferably, it means to carry out for one month or more. However, even if the supply of the monomer composition to the disperser is interrupted, it shall be included in the category of "continuous operation" as long as the continuous phase circulates in the manufacturing process.

[2. Manufacturing method of water-absorbent resin]
In the method for producing a water-absorbent resin of the present invention, the monomer composition, the organic solvent, and the dispersion aid are continuously supplied to the disperser, and the monomer composition is contained in the organic solvent. Dispersing the contained fine droplets (hereinafter, also referred to as “dispersion step”), supplying the fine droplets dispersed in the organic solvent to the polymerization apparatus, and polymerizing the monomer to form a hydrogel-like weight. Obtaining a coalescence (hereinafter, also referred to as “polymerization step”) and supplying the organic solvent separated from the hydrogel polymer to the disperser again (hereinafter, also referred to as “separation and recycling step”). , And the heat resistance index of the dispersion aid is 60 mN / m or more. According to the manufacturing method, a water-absorbent resin having excellent absorption characteristics (small amount of liquid return) when used as a sanitary material can be obtained even if continuous operation is performed.

In order to industrially produce a water-absorbent resin having excellent absorption characteristics when used as a sanitary material, continuous operation of the water-absorbent resin manufacturing process is being studied. Therefore, when the present inventors attempted continuous operation for the specific embodiment described in International Publication No. 2016/182082, there is a problem that the surface tension of the obtained water-absorbent resin decreases with the operation time. found. When the surface tension of the water-absorbent resin is lowered, the effect of excellent absorption characteristics (small amount of liquid return) when used as a sanitary material is impaired. In a specific embodiment of WO2016 / 182082, an ester-based dispersion aid is added as a dispersion aid to an organic solvent which is a continuous phase, and the continuous phase is heated and circulated in the dispersion step, the polymerization step, and the separation step. ing. Therefore, when continuous operation is performed, the ester-based dispersion aid may be decomposed by heating, and fatty acids, which are decomposition products, may be accumulated in the continuous phase, which may adversely affect the surface tension of the obtained water-absorbent resin. The sex was guessed. Based on the above speculation, as a result of diligent studies on the dispersion aid, the present inventors suppressed the decrease in surface tension even when continuous operation was performed by using the dispersion aid having a heat resistance index of 60 mN / m or more. Therefore, they have found that a water-absorbent resin having excellent absorption characteristics (small amount of liquid return) can be obtained when used as a sanitary material, and have completed the present invention.

Hereinafter, the present embodiment will be described with reference to the attached drawings. In the description of the drawings, the same elements are designated by the same reference numerals, and duplicate description will be omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios.

FIG. 1 is a schematic view showing a part of a water-absorbent resin manufacturing process according to an embodiment of the present invention. Although the piping system is provided with a plurality of valves for adjusting the flow rate and pressure, the illustration of these valves is omitted in FIG.

As shown in FIG. 1, in the process of producing the water-absorbent resin, a mixing device 10, a dispersing device 12, a polymerization device 14, a separating device 16, a liquid feed pump 18, a heat exchanger 20, a drying device 22, and these devices are used. The pipes 31 to 36 for connecting the above are included. A pipe 37 for discharging the polymer after drying is connected to the drying device 22. The structure of the disperser 12 will be described in detail later. The polymerization apparatus 14 is composed of, for example, a vertical reaction column. The monomer supplied to the polymerization apparatus 14 is polymerized to obtain a hydrogel-like polymer (hereinafter, also referred to as “hydrogen gel”). The separation device 16 is composed of, for example, a screw press or a continuous centrifuge, and extracts a hydrogel to perform solid-liquid separation. The drying device 22 is composed of, for example, a paddle dryer, a fluidized bed dryer, a rotary dryer, and a steam tube dryer, and stir-drys the hydrogel. The pipe 35 branches from the pipe 34 from the heat exchanger 20 to the polymerization device 14 and is connected to the dispersion device 12.

A pipe 41 for supplying the monomer solution and a pipe 42 for supplying the polymerization initiator are connected to the mixing device 10. A pipe 43 for supplying a dispersion aid is connected to the pipe 33 from the liquid feed pump 18 to the heat exchanger 20. A pipe 44 for supplying a drying aid is connected to the pipe 36 from the separating device 16 to the drying device 22.

An example of a method for producing a water-absorbent resin will be described with reference to FIG. The method for producing the water-absorbent resin includes any mixing step; dispersion step; polymerization step; separation and recycling step, and optionally includes a drying step and the like after the separation and recycling step.

First, the disperser 12, the polymerization device 14, the separation device 16, the heat exchanger 20, and the pipes 32, 33, 34, and 35 connecting these devices are filled with an organic solvent. Next, the liquid feed pump 18 is operated to circulate the organic solvent. A part of the organic solvent is also supplied to the dispersion device 12 via the pipe 35. The dispersion aid is supplied to the organic solvent flowing through the pipe 33 via the pipe 43. The organic solvent in each device and piping is heated to a predetermined temperature in the heat exchanger 20.

Next, the separately prepared monomer solution and the polymerization initiator are continuously supplied to the mixing apparatus 10 via the pipes 41 and 42, respectively, and mixed to prepare a monomer composition (mixing step). The mixing device 10 is not particularly limited, and examples thereof include a line mixer and the like.

Then, the monomer composition is continuously supplied to the dispersion device 12 via the pipe 31. The monomer composition and the organic solvent are continuously and separately supplied to the disperser 12. The monomer composition is dispersed in the organic solvent in the form of fine droplets by the dispersion device 12 (dispersion step). As described above, in the present embodiment, the monomer is continuously dispersed in the organic solvent.

The monomers dispersed in the form of fine droplets are continuously charged into the organic solvent of the polymerization apparatus 14, and the polymerization reaction is started in the polymerization apparatus 14 (polymerization step). In the polymerization apparatus 14, fine droplets made of the monomer composition move due to the movement of the circulating organic solvent. While moving, the fine droplets are transformed into a hydrogel by a polymerization reaction. The moving direction of the droplet and the hydrogel is the same as the moving direction of the organic solvent (parallel flow). In the present invention, a polymerization method for obtaining a hydrogel by initiating a polymerization reaction in a state where droplets containing a monomer solution are dispersed or suspended in a liquid phase (continuous phase) composed of an organic solvent is a liquid phase liquid. It is called drop (suspension) polymerization.

Subsequently, the hydrogel obtained by the liquid phase droplet polymerization is continuously discharged from the polymerization apparatus 14 together with the organic solvent, and continuously supplied to the separation apparatus 16. In the separation device 16, the hydrogel and the organic solvent are continuously separated (separation step). The separated hydrogel is continuously supplied to the next step (drying device 22) via the pipe 36 (drying step). The separated organic solvent is sent to the liquid feed pump 18 via the pipe 32, and is re-supplied to the disperser 12 via the pipe 33, the heat exchanger 20, the pipe 34, and the pipe 35 (recycling step). The organic solvent is also supplied to the polymerization apparatus 14 again via the pipe 34.

In the drying device 22, the water contained in the hydrogel and the organic solvent that could not be completely separated in the separating device 16 are removed to obtain a particulate dry polymer. The particulate dry polymer is discharged from the pipe 37 and supplied to the next step (cooling device or the like). Although not shown, the organic solvent removed by the drying apparatus 22 is resupplied to the polymerization apparatus 14.

In the present invention, continuous polymerization (continuous production method) is adopted. The continuous production method is a hydrogel formed by continuously feeding a monomer solution or a monomer composition containing a monomer solution to an organic solvent in a polymerization apparatus, polymerizing the mixture, and performing a polymerization reaction. This is a method of continuously discharging the organic solvent from the polymerization apparatus. This method is referred to as liquid phase droplet continuous polymerization. In this case, since each process and each operation between processes can be continuously performed, troubles such as blockage due to stopping and re-operation of each device can be avoided. Since the continuous polymerization is a form in which the monomer composition is supplied from the dispersion device to the polymerization device, it is clearly distinguished from the form in which dispersion and polymerization are performed in one device (batch operation). ..

Hereinafter, each step will be described.

[2-1: Mixing step]
This step is an arbitrary step, and is a step of mixing a monomer solution and a polymerization initiator to obtain a monomer composition.

In this step, the method for preparing the monomer composition by mixing the monomer solution and the polymerization initiator is not particularly limited, but for example, (1) the monomer solution and the solution containing the polymerization initiator (1) Hereinafter, a method in which a "polymerization initiator solution") is prepared in advance and simultaneously supplied to a mixing device from different pipes for mixing, and (2) a previously prepared monomer solution is supplied to the mixing device. Later, a method of supplying the polymerization initiator to the mixing device and mixing the mixture can be mentioned.

The polymerization initiator may be in the form of a polymerization initiator solution in which the polymerization initiator is dissolved (dispersed) in a solvent. The solvent of the polymerization initiator solution is not particularly limited, but water is preferable.

The mixing device is not particularly limited, and examples thereof include a line mixer and a tank. From the viewpoint of storage stability and safety of the polymerization initiator, the mixing method (1) described above using a line mixer as a mixing device is preferable.

Hereinafter, the materials used in this step will be described.

"Monomer solution"
Monomer solution refers to a solution containing a monomer.

The solvent of the monomer solution is preferably water, a water-soluble organic solvent (for example, alcohol, etc.) and a mixture thereof, more preferably water or a mixture of water and a water-soluble organic solvent, and water. Is even more preferable. In the case of a mixture of water and a water-soluble organic solvent, the water-soluble organic solvent (for example, alcohol or the like) is preferably 30% by weight or less, and more preferably 5% by weight or less.

From the viewpoint of the water absorption performance of the obtained water-absorbent resin, the water-soluble ethylenically unsaturated monomer is preferable as the monomer. Examples of water-soluble ethylenically unsaturated monomers include (meth) acrylic acid, (anhydrous) maleic acid, itaconic acid, silicic acid, vinyl sulfonic acid, allyltoluenesulfonic acid, vinyltoluenesulfonic acid, and styrenesulfonic acid. , 2- (meth) acrylamide-2-methylpropanesulfonic acid, 2- (meth) acryloyl ethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) ) Acid group-containing unsaturated monomers such as acrylate, 2-hydroxyethyl (meth) acryloyl phosphate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate; (meth) acrylamide, N-ethyl (meth) ) Acrylamide, N, N-dimethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidin, N-acryloylpyrrolidin, N -Amid group-containing unsaturated monomers such as vinylacetamide; N, N-dimethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylamide, Amino group-containing unsaturated monomer such as N, N-diethylaminoethyl (meth) acrylate; mercapto group-containing unsaturated monomer; phenolic hydroxyl group-containing unsaturated monomer; lactam group-free such as N-vinylpyrrolidone Saturated monomer and the like can be mentioned.

In consideration of the stability of the monomer, a polymerization inhibitor may be added to the monomer solution as needed. As the polymerization inhibitor, for example, known polymerization inhibitors such as p-methoxyphenol, phenothiazine, and vitamin E can be used. When p-methoxyphenol is used, oxygen is used in combination as needed. The amount of the polymerization inhibitor used is preferably 0.1 ppm to 1000 ppm, more preferably 5 ppm to 500 ppm, based on the monomer.

When an acid group-containing unsaturated monomer having an acid group such as a carboxyl group is used as the monomer to produce a water-absorbent resin, a neutralized salt in which the acid group is neutralized can be used. In this case, the salt of the acid group-containing unsaturated monomer is preferably a salt with a monovalent cation, and more preferably at least one selected from an alkali metal salt, an ammonium salt and an amine salt. Alkali metal salts are even more preferred, at least one selected from sodium salts, lithium salts and potassium salts is even more preferred, and sodium salts are particularly preferred.

From the viewpoint of the water absorption performance of the obtained water-absorbent resin, the monomer is preferably an acid group-containing unsaturated monomer and / or a salt thereof, and more preferably (meth) acrylic acid (salt), (anhydrous). Maleic acid (salt), itaconic acid (salt), silica skin acid (salt), more preferably (meth) acrylic acid (salt), and particularly preferably acrylic acid (salt).

When an acid group-containing unsaturated monomer is used as the monomer, it is preferable to use it in combination with a neutralized salt of the acid group-containing unsaturated monomer from the viewpoint of the water absorption performance of the obtained water-absorbent resin. From the viewpoint of water absorption performance, the number of moles of the neutralizing salt (hereinafter referred to as "neutralization rate") with respect to the total number of moles of the acid group-containing unsaturated monomer and its neutralizing salt is preferably 40 mol% or more. It is more preferably 40 mol% to 95 mol%, further preferably 50 mol% to 90 mol%, even more preferably 55 mol% to 85 mol%, and particularly preferably 60 mol% to 80 mol%. That is, in one embodiment of the present invention, the monomer contains a mixture of an acid group-containing unsaturated monomer and a neutralizing salt thereof. In the present specification, the mixture of the acid group-containing unsaturated monomer and its neutralizing salt is also referred to as "partially neutralizing salt of the acid group-containing unsaturated monomer".

As a method for adjusting the neutralization rate, a method of mixing an acid group-containing unsaturated monomer and a neutralizing salt thereof; a method of adding a known neutralizing agent to the acid group-containing unsaturated monomer; Examples thereof include a method using a partially neutralized salt of an acid group-containing unsaturated monomer adjusted to a predetermined neutralization rate. Moreover, you may combine these methods.

The neutralizing agent used to neutralize the acid group-containing unsaturated monomer is not particularly limited, but is an inorganic salt such as sodium hydroxide, potassium hydroxide, sodium carbonate, ammonium carbonate, or an amino group. A basic substance such as an amine-based organic compound having an imino group or an imino group is appropriately selected and used. Two or more basic substances may be used in combination as the neutralizing agent.

The addition of the neutralizing agent may be carried out before the start of the polymerization reaction of the acid group-containing unsaturated monomer, or may be carried out on the hydrogel obtained after the completion of the polymerization reaction of the acid group-containing unsaturated monomer. May be good. For applications that may come into direct contact with the human body, such as absorbent articles such as disposable diapers, it is preferable to add a neutralizing agent before the start of the polymerization reaction.

In the production method according to the present invention, any one of the above-exemplified monomers may be used alone, or any two or more kinds of monomers may be appropriately mixed and used. Further, other monomers can be mixed as long as the object of the present invention is achieved.

When two or more kinds of monomers are used in combination in producing a water-absorbent resin, it is preferable to contain (meth) acrylic acid (salt) as a main component. In this case, the ratio of (meth) acrylic acid (salt) to the entire monomer used for polymerization is usually 50 mol% or more, preferably 70 mol% or more, from the viewpoint of the water absorption performance of the obtained water-absorbent resin. It is more preferably 80 mol% or more, still more preferably 90 mol% or more (upper limit is 100 mol%).

The concentration of the monomer in the monomer solution is not particularly limited as long as the monomer can be dissolved in the solvent, but is preferably 10% by weight to saturated concentration or less, and 20% by weight to saturated. The concentration or less is more preferable, 25 to 80% by weight is even more preferable, and 30 to 70% by weight is particularly preferable.

In the polymerization step, an internal cross-linking agent can be used if necessary. Examples of the internal cross-linking agent include conventionally known internal cross-linking agents having two or more polymerizable unsaturated groups or two or more reactive groups in one molecule. Examples of the internal cross-linking agent include N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, and the like. 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, tri. Allyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1 , 4-Butandiol, pentaerythritol, ethylenediamine, ethylenecarbonate, propylene carbonate, polyethyleneimine, glycidyl (meth) acrylate and the like. Only one kind of these internal cross-linking agents may be used, or two or more kinds thereof may be used.

Above all, it is preferable to use a compound having two or more polymerizable unsaturated groups as an internal cross-linking agent from the viewpoint of the water absorption characteristics of the obtained water-absorbent resin. The amount of the internal cross-linking agent used is usually 0.0001 to 5 mol%, more preferably 0.001 to 3 mol%, based on the physical characteristics of the desired water-absorbent resin. , Even more preferably 0.005 to 1.5 mol%.

Further, the substances exemplified below (hereinafter referred to as "other substances") can also be added to the monomer solution.

Specific examples of other substances include chain transfer agents such as thiols, thiolic acids, secondary alcohols, amines and hypophosphates; effervescent agents such as carbonates, bicarbonates, azo compounds and bubbles; ethylenediamine. Chelating agents such as 4 metal salts of acetic acid and metal salts of diethylenetriamine 5acetic acid; polyacrylic acid (salt) and cross-linked products thereof, starches, celluloses, starch-cellulose derivatives, thickeners such as polyvinyl alcohol and the like can be mentioned. Other substances may be used alone or in combination of two or more.

The amount of the other substance used is not particularly limited, but the total concentration of the other substance is preferably 10% by weight or less, more preferably 1% by weight or less, still more preferably 1% by weight or less, based on the monomer. Is 0.1% by weight or less.

"Polymerization initiator"
As the polymerization initiator, a thermal decomposition type polymerization initiator is preferably used. The pyrolysis-type polymerization initiator refers to a compound that is decomposed by heat to generate a radical, but from the viewpoint of storage stability of the pyrolysis-type polymerization initiator and production efficiency of a water-absorbent resin, a 10-hour half-life temperature (hereinafter referred to as “1”). , "T10") is preferably 0 ° C. to 120 ° C., more preferably 30 ° C. to 100 ° C., still more preferably 50 ° C. to 80 ° C., and a water-soluble compound is preferably used as the polymerization initiator.

Specific examples of the thermally decomposable polymerization initiator having T10 in the above range include persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; 2,2'-azobis (2-methylpropionamidine) dihydrochloride. , 2,2'-Azobis (2-amidinopropane) dihydrochloride, 2,2'-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, 2,2'-azobis (2-methylpro) Azo compounds such as pionitrile); peroxides such as hydrogen peroxide, t-butyl peroxide, methyl ethyl ketone peroxide and the like can be mentioned. Of these, two or more may be used in combination.

Among them, from the viewpoint of the handleability of the heat-decomposable polymerization initiator and the physical properties of the water-absorbent resin, the polymerization initiator is preferably persulfate, more preferably sodium persulfate, potassium persulfate, ammonium persulfate, and more preferably. Sodium persulfate is used.

The amount of the thermally decomposable polymerization initiator used is appropriately set according to the type of the monomer and the polymerization initiator, and is not particularly limited, but is preferably 0 with respect to the monomer from the viewpoint of production efficiency. It is 0.01 g / mol or more, more preferably 0.005 g / mol or more, still more preferably 0.01 g / mol or more. Further, from the viewpoint of improving the water absorption performance of the water-absorbent resin, it is preferably 2 g / mol or less, more preferably 1 g / mol or less.

Further, if necessary, it can be used in combination with other polymerization initiators such as a photodegradable polymerization initiator. Specific examples of the photodegradable polymerization initiator include benzoin derivatives, benzyl derivatives, acetophenone derivatives, benzophenone derivatives and the like.

When the above-mentioned pyrolysis-type polymerization initiator is used in combination with another polymerization initiator, the ratio of the pyrolysis-type polymerization initiator to the total polymerization initiator is preferably 60 mol% or more, more preferably 80 mol% or more. be.

Further, the thermal decomposition type polymerization initiator and the reducing agent can be used in combination to prepare a redox-based polymerization initiator. In the above-mentioned redox-based polymerization initiator, the pyrolysis-type polymerization initiator functions as an oxidizing agent. The reducing agent used is not particularly limited, but for example, (heavy) sulfite such as sodium sulfite and sodium hydrogen sulfite; reducing metal salt such as ferrous salt; L-ascorbic acid (salt), amines and the like. Can be mentioned.

That is, in one embodiment of the present invention, the monomer composition contains a partially neutralized salt of an acid group-containing unsaturated monomer, a pyrolysis-type polymerization initiator, and water. When such a monomer composition is used, excellent water absorption characteristics can be obtained.

"Monomer concentration of monomeric composition"
In the present invention, the concentration of the monomer in the monomer composition is selected according to the selected monomer, the type of organic solvent, etc., but the lower limit is preferably 10% by weight in terms of production efficiency. More preferably, it is 20% by weight or more, further preferably 30% by weight or more, and the upper limit is preferably 100% by weight or less, more preferably 90% by weight or less, and further. It is preferably 80% by weight or less, and even more preferably 70% by weight or less. From the viewpoint of the physical properties and productivity of the water-absorbent resin, the monomer concentration in the monomer composition is preferably 10% by weight to 90% by weight, more preferably 20% by weight to 80% by weight, still more preferably 30% by weight. It is from% by weight to 70% by weight.

Additives such as an internal cross-linking agent, a density adjusting agent, and a thickener can be added to the monomer composition as long as the object of the present invention is not impaired. The type and amount of the additive can be appropriately selected depending on the combination of the monomer and the organic solvent used.

[2-2. Dispersion process]
In this step, the monomer composition, the organic solvent, and the dispersion aid are continuously supplied to the disperser, and the fine droplets containing the monomer composition are dispersed in the organic solvent. It is a process.

Here, "to continuously supply the monomer composition, the organic solvent, and the dispersion aid to the dispersion device" means to the dispersion device 12 via a pipe as shown in FIG. , The monomer composition, the organic solvent, and the dispersion aid are allowed to flow in at least for a certain period of time. The “at least constant time” in the definition means, for example, 30 minutes or more, preferably 1 hour or more. In FIG. 1, the dispersion aid is flowed into the dispersion device 12 together with the organic solvent through the pipe 35, but may be flowed into the dispersion device 12 together with the monomer composition through the pipe 31. , The disperser 12 may flow through a pipe other than the pipes 31 and 35 (that is, apart from the organic solvent and the monomer composition). However, from the viewpoint of stably forming fine droplets, the form shown in FIG. 1 is preferable. That is, in a preferred embodiment of the present invention, a mixed solution of an organic solvent and a dispersion aid is continuously supplied to the dispersion device.

In this step, as shown in FIG. 1, it is preferable that the monomer composition and the organic solvent flow into the disperser through separate pipes. That is, it is preferable that the path through which the monomer composition flows into the disperser and the path through which the organic solvent flows into the disperser are independent of each other. The flow rate of the organic solvent flowing into the disperser can be appropriately adjusted according to the type of the disperser, the size of the polymerization device, and the like, but it may be appropriately adjusted so as to satisfy the space velocity (LHSV) in the polymerization device described later. Further, the flow rate of the monomer composition flowing into the disperser may be appropriately adjusted so as to satisfy the “monomer composition flow rate / organic solvent flow rate ratio” described later. Further, the flow rate of the dispersion aid flowing into the disperser is not particularly limited as long as fine droplets containing the monomer composition can be formed.

<Distributor>
The disperser used in this step is not particularly limited as long as it can form fine droplets containing a monomer composition in an organic solvent. For example, a high-speed rotary shear type stirrer (rotary mixer type, turbo mixer). Molds, discs, double cylinders, etc.), cylindrical nozzles such as needles, orifice plates with a large number of holes directly in the plate, spray nozzles, centrifugal atomizers such as rotary wheels, and the like. From the viewpoint of stably forming fine droplets, a high-speed rotary shear type stirrer can be preferably used as the disperser.

(High-speed rotary shear type stirrer)
According to the high-speed rotary shear type agitator, a flow path forming a shear field can be formed by relatively moving a pair of walls having facing surfaces facing each other across a gap, and a flow path forming a shear field can be formed. The monomer composition is continuously supplied into the organic solvent that circulates.

The "flow path" in the high-speed rotary shear type stirrer is a form in which a fluid (a fluid in which a monomer composition is supplied in an organic solvent) can flow through a gap between facing surfaces facing each other in a pair of walls. If so, the shape is not particularly limited.

The specific shape of the "wall" can have various shapes such as a planar shape, a blade shape, a disk shape, a hollow cylindrical shape, or a solid cylindrical shape, depending on the shape of the flow path.

The form in which the pair of walls move relatively is not particularly limited as long as it can form a flow path forming a shear field. For example, one wall can be a fixed wall and the other wall can be a movable wall. Further, both of the pair of walls can be configured as movable walls so that the moving speeds differ.

In the present invention, from the viewpoint of further miniaturizing the fine droplets containing the monomer composition dispersed in the organic solvent, it is preferable that the monomer composition is supplied in a relatively narrow flow path. From such a viewpoint, the size of the gap is preferably 5 mm or less, and preferably 2 mm or less. Further, in consideration of productivity, the size of the gap is preferably 0.1 mm or more, and more preferably 0.5 mm or more.

A high-speed rotary shear type stirrer will be described with reference to FIG. The dispersion device 12A illustrated in FIG. 2 is composed of a high-speed rotary shear type stirrer. In the disperser 12A, the monomer composition and the organic solvent are continuously and separately supplied, and fine droplets containing the monomer composition are dispersed in the organic solvent. As the high-speed rotary shear type stirrer other than FIG. 2, the dispersers 12B to 12C of FIGS. 3 to 4 can also be used in the present invention. When the member in the dispersion device 12A of FIG. 2 and the member in the dispersion devices 12B to 12C of FIGS. 3 to 4 are common, the subscript "A" is replaced with the subscript "A" of the member code attached to FIG. "B" to "C" are added, and duplicate explanations are omitted.

"Distributor 12A"
FIG. 2 is a cross-sectional view showing a disperser 12A according to an example. The disperser 12A is a rotary mixer type high-speed rotary shear type agitator. The disperser 12A includes a flow path 54A formed by a pair of walls 50A and 52A having facing surfaces 51A and 53A facing each other across a gap, and a drive unit 60A that relatively moves the pair of walls 50A and 52A. ,have. By relatively moving the pair of walls 50A and 52A by the drive unit 60A, the flow path 54A forming the shear field is formed. The disperser 12A further includes a first supply system 55A that continuously supplies the monomer composition to the flow path 54A and a second supply system 56A that continuously supplies the organic solvent to the flow path 54A. ing.

The pair of walls 50A and 52A have a cylindrical shape. One wall 50A is formed from a non-rotating outer cylinder having a central hole. The other wall 52A is formed from a solid inner cylinder rotatably arranged in the center hole of the outer cylinder. The drive unit 60A is composed of, for example, a motor and is connected to an inner cylinder. By operating the drive unit 60A, the inner cylinder is rotationally driven. As a result, one wall 50A constitutes a fixed wall and the other wall 52A constitutes a movable wall. The inner peripheral surface of the outer cylinder and the outer peripheral surface of the inner cylinder form facing surfaces 51A and 53A facing each other. The facing surfaces 51A and 53A facing each other have an uneven shape. The convex portion of the facing surface 51A has entered the recess of the facing surface 53A, and the convex portion of the facing surface 53A has entered the recess of the facing surface 51A. The flow path 54A has a bent shape.

The gap between the facing surfaces 51A and 53A is formed to a size that causes a desired shear field in the flow path 54A.

The bottom of the wall 52A has a downwardly tapered shape. A communication passage 58A that communicates the flow path 54A and the liquid discharge pipe 57A is formed between the bottom of the wall 52A and the bottom of the wall 50A. The gap of the communication passage 58A is larger than the gap of the flow path 54A. This makes it easier to lead the liquid discharged from the flow path 54A to the liquid discharge pipe 57A.

The liquid discharge pipe 57A is connected to the upper end of the polymerization apparatus 14. The inner diameter of the liquid discharge pipe 57A and the inner diameter of the polymerization apparatus 14 are formed to have substantially the same dimensions. This is to smooth the flow of the fluid from the disperser 12A to the polymerization device 14 and prevent retention. Since no retention occurs in the disperser 12A, it is possible to prevent the monomer composition from polymerizing to form a hydrogel. When a gel-like body is generated in the disperser 12A, the particle size of the generated droplets is difficult to be constant.

A pipe 31 is connected to the first supply system 55A. The monomer composition produced in the mixing device 10 is continuously supplied to the flow path 54A via the pipe 31 and the first supply system 55A. A pipe 35 is connected to the second supply system 56A. A part of the organic solvent circulated by the operation of the liquid feed pump 18 is continuously supplied to the flow path 54A via the pipe 35 and the second supply system 56A.

The drive unit 60A is operated to rotate the wall 52A. The facing surface 53A of the wall 52A moves with respect to the facing surface 51A of the facing wall 50A. While rotating the wall 52A, the monomer composition is continuously supplied to the flow path 54A through the pipe 31 and the first supply system 55A.

The organic solvent and the monomer composition are placed in the flow path 54A in which the pair of walls 50A and 52A having the facing surfaces 51A and 53A facing each other are relatively moved by the driving unit 60A. Supplied separately and continuously. A strong shearing force acts on the organic solvent flowing into the flow path 54A due to the speed difference between the facing surface 53A of the rotor side wall 52A and the facing surface 51A of the stator side wall 50A. The monomer composition is directly injected into the flow path 54A on which the shearing force acts, and is rapidly dispersed in the organic solvent in the form of droplets. Further, the droplet-like monomer composition is refined.

"Distributor 12B"
FIG. 3 is a cross-sectional view showing a disperser 12B according to still another example. The disperser 12B is a disk-type high-speed rotary shear type agitator.

One wall 50B is formed from a non-rotating casing. The other wall 52B is formed from a disk-shaped circular plate rotatably arranged in the casing. The drive unit 60B is connected to a circular plate. By operating the drive unit 60B, the circular plate is rotationally driven. As a result, one wall 50B constitutes a fixed wall and the other wall 52B constitutes a movable wall. The inner peripheral surface of the casing and the outer peripheral surface of the circular plate form facing surfaces 51B and 53B facing each other. Both the facing surfaces 51B and 53B have a peripheral surface shape. The flow path 54B has a cylindrical shape.

A part of the organic solvent circulated by the operation of the liquid feed pump 18 is continuously supplied to the flow path 54B via the pipe 35 and the second supply system 56B.

The drive unit 60B is operated to rotate the wall 52B. The facing surface 53B of the wall 52B moves with respect to the facing surface 51B of the facing wall 50B. While rotating the wall 52B, the monomer composition is continuously supplied to the flow path 54B through the pipe 31 and the first supply system 55B.

The organic solvent and the monomer composition are separately continuous in the flow path 54B in which the pair of walls 50B and 52B having the facing surfaces 51B and 53B facing each other are relatively moved by the driving unit 60B. Is supplied. A strong shearing force acts on the organic solvent flowing into the flow path 54B due to the speed difference between the facing surface 53B of the rotor side wall 52B and the facing surface 51B of the stator side wall 50B. The monomer composition is directly injected into the flow path 54B on which the shearing force acts, and is rapidly dispersed in the organic solvent in the form of droplets. Further, the droplet-like monomer composition is refined.

"Distributor 12C"
FIG. 4 is a cross-sectional view showing the disperser 12C according to an example. The disperser 12C is a double-cylindrical high-speed rotary shear stirrer.

The pair of walls 50C and 52C have a cylindrical shape. One wall 50C is formed from a non-rotating outer cylinder having a central hole. The other wall 52C is formed from a solid inner cylinder rotatably arranged in the center hole of the outer cylinder. The drive unit 60C is connected to the inner cylinder. By operating the drive unit 60C, the inner cylinder is rotationally driven. As a result, one wall 50C constitutes a fixed wall and the other wall 52C constitutes a movable wall. The inner peripheral surface of the outer cylinder and the outer peripheral surface of the inner cylinder form facing surfaces 51C and 53C facing each other. Both the facing surfaces 51C and 53C have a peripheral surface shape. The flow path 54C has a cylindrical shape. The bottom surface of the wall 50C is open. The bottom opening 59C of the wall 50C functions as a liquid discharge pipe.

A part of the organic solvent circulated by the operation of the liquid feed pump 18 is continuously supplied to the flow path 54C via the pipe 35 and the second supply system 56C.

The drive unit 60C is operated to rotate the wall 52C. The facing surface 53C of the wall 52C moves with respect to the facing surface 51C of the facing wall 50C. While rotating the wall 52C, the monomer composition is continuously supplied to the flow path 54C through the pipe 31 and the first supply system 55C.

The organic solvent and the monomer composition are placed in the flow path 54C in which the pair of walls 50C and 52C having the facing surfaces 51C and 53C facing each other are relatively moved by the driving unit 60C. Supplied separately and continuously. A strong shearing force acts on the organic solvent flowing into the flow path 54C due to the speed difference between the facing surface 53C of the rotor side wall 52C and the facing surface 51C of the stator side wall 50C. The monomer composition is directly injected into the flow path 54C on which the shearing force acts, and is rapidly dispersed in the organic solvent in the form of droplets. Further, the droplet-like monomer composition is refined. The liquid discharged from the flow path 54C falls as it is and is charged into the polymerization apparatus 14.

In the disperser 12C, the rotation speed of the wall 52C is not particularly limited, and for example, the rotation speed of the wall 52C may be derived in consideration of the structure and scale of the disperser so as to have the following suitable shear rate. .. The rotation speed of the wall 52C is, for example, 100 to 10,000 rpm, 500 to 9,000 rpm, and 1,000 to 8,000 rpm.

"Shear velocity in the flow path"
The shear rate in the flow path of the disperser is preferably 1,000 [1 / s] or more. When the shear rate is 1,000 [1 / s] or more, the shear rate is sufficient for the monomers in the flow path to be dispersed in the organic solvent, so that the dispersion is good and the primary particle size is increased. It becomes smaller. As the primary particle size becomes smaller, the specific surface area of the water-absorbent resin becomes larger, which leads to an improvement in the water absorption rate. Further, when the shear rate is 1,000 [1 / s] or more, it is possible to shorten the time for forming droplets. Furthermore, when the shear rate is 1,000 [1 / s] or more, the amount of the dispersion aid used at the time of dispersion can be reduced. From the above viewpoint, the shear rate in the flow path of the disperser is preferably 1,000 [1 / s] or more, more preferably 2,000 [1 / s] or more, and 3,000 [1 / s] or more. It is more preferably 1 / s] or more, and particularly preferably 3,500 [1 / s] or more. On the other hand, in order to operate the disperser stably, the shear rate is preferably 40,000 [1 / s] or less, more preferably 20,000 [1 / s] or less. It is more preferably 000 [1 / s] or less, and particularly preferably 6,000 [1 / s] or less. The shear rate in the flow path of the disperser is preferably 1,000 to 40,000 [1 / s], more preferably 2,000 to 20,000 [1 / s], and 3,000. It is even more preferably ~ 10,000 [1 / s], and particularly preferably 3,500 to 6,000 [1 / s]. When the disperser is a double cylinder type, the shear rate in the flow path of the disperser is preferably 1,000 to 40,000 [1 / s], preferably 2,000 to 20,000 [1]. / S] is more preferable, 3,000 to 10,000 [1 / s] is even more preferable, and 3,500 to 6,000 [1 / s] is particularly preferable.

Shear velocity is determined by rotor speed and flow path width (clearance, eg, outer cylinder radius and inner cylinder radius in the case of a double-cylindrical disperser).

Specifically, the shear rate is calculated as follows in the present specification.

Shear velocity [1 / s] = Moving velocity of relatively moving walls (rotors, rotors) in the disperser [m / s] / Gap (clearance) [m]
If the shape is complicated and it is difficult to define the moving speed, the moving speed shall be the maximum moving speed at the wetted part when one side is a fixed wall. When both are moving walls, the moving speed is the moving speed at the point where the difference between the moving speeds is maximized. When both of the pair of walls rotate, there is a difference in moving speed. If there are a plurality of gaps (clearances), the narrowest distance is used. If the shear rate varies depending on the position of the device, the maximum shear rate shall be the shear rate herein.

(spray nozzle)
The spray nozzle has a function of separately introducing a monomer composition and an organic solvent, passing them through the inside without contacting each other, and contacting and discharging them immediately before or after discharging from the spray nozzle. It is preferable to have.

Examples of the spray nozzle include a multi-fluid spray nozzle such as a two-fluid spray nozzle, a three-fluid spray nozzle, and a four-fluid spray nozzle; a multiple tube such as a double tube, a triple tube, and a quadruple tube; an ejector and the like. Examples of the two-fluid spray nozzle include a prefilming type, a plain jet type, a cross flow type, an external mixing type, an internal mixing type, and a Y jet type spray nozzle.

Commercially available products may be used as the multi-fluid spray nozzle, for example, Mini Atomize MMA manufactured by Kyoritsu Alloy Mfg. Co., Ltd., SETOJet manufactured by Ikeuchi Co., Ltd., Air Atomizing Nozzle SU-HTE91 manufactured by Spraying Systems Japan Co., Ltd., Examples include a micromizer manufactured by Niikura Kogyo Co., Ltd., a 4-fluid nozzle manufactured by Fujisaki Electric Co., Ltd., and a twin jet nozzle manufactured by Ohkawara Kakohki Co., Ltd.

The spray nozzle will be described with reference to FIG. The disperser 12D illustrated in FIG. 5 is composed of a two-fluid spray. The dispersion device 12D includes a first supply pipe 101 that continuously supplies the monomer composition, and a second supply pipe 102 that continuously supplies an organic solvent containing a dispersion aid. The monomer composition is sprayed from the first nozzle 103 and continuously supplied. The organic solvent containing the dispersion aid is sprayed from the second nozzle 104 and continuously supplied. The organic solvent containing the monomer composition and the dispersion aid is contacted and mixed outside the disperser 12D immediately after being discharged from the first nozzle 103 and the second nozzle 104, respectively (external mixing type). As a result, fine droplets containing the monomer composition in the organic solvent are generated. In the disperser 12D, the first nozzle 103 and the second nozzle 104 are preferably arranged so as to be immersed in the organic solvent filled in the polymerization apparatus. By arranging in this way, it is possible to obtain effects such as prevention of gas entrainment, prevention of blockage of the spray nozzle, and suppression of droplet coalescence.

An external mixing type spray nozzle such as the disperser 12D is preferable because internal blockage of the spray nozzle due to contact between the monomer composition and the organic solvent is unlikely to occur. As long as the internal blockage of the spray nozzle is avoided, the spray nozzle is a method in which the monomer composition and the organic solvent are brought into contact with each other and mixed immediately before being discharged from the spray nozzle (internal mixing type). May be good.

"Ratio of monomeric composition flow rate / total flow rate of organic solvent and dispersion aid"
In the present invention, the ratio of the flow rate of the monomer composition [ml / min] flowing into the disperser to the total flow rate [ml / min] of the organic solvent and the dispersion aid flowing into the disperser (the flow rate of the monomer composition [ml / min]. ml / min] / total flow rate of organic solvent and dispersion aid [ml / min]) is 0. It is preferably 01 or more, more preferably 0.02 or more, and even more preferably 0.03 or more. When a high-speed rotary shear type stirrer is used, a large amount of organic solvent is not required because the monomer is dispersed by applying a shearing force by a shearing field. Therefore, even within the above range, dispersion is performed well. The upper limit of the flow rate [ml / min] of the monomer composition / the total flow rate [ml / min] of the organic solvent and the dispersion aid is not particularly limited, but is preferably 1.00 or less, preferably 0.40 or less. More preferably, it is more preferably 0.20 or less.

Hereinafter, the materials used in this step will be described.

"Organic solvent"
Preferred organic solvents include at least one organic solvent selected from the group consisting of aliphatic hydrocarbons, aliphatic cyclic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. Specific examples include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane, and n-octane; aliphatic cyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, and decalin; benzene, toluene, xylene, and the like. Aromatic hydrocarbons; halogenated hydrocarbons such as chlorobenzene, brombenzene, carbon tetrachloride, 1,2-dichloroethane and the like are exemplified. Among these, n-hexane, n-heptane, and cyclohexane are preferable from the viewpoint of availability and quality stability. It is also possible to use it as a mixed solvent in which two or more kinds are mixed.

The temperature of the organic solvent supplied into the disperser is controlled so as to be Td, which will be described later. From the viewpoint of operation and polymerization efficiency, the boiling point of the organic solvent is preferably 70 ° C. or higher, more preferably 75 to 100 ° C., and even more preferably 80 to 95 ° C.

"Dispersion aid"
According to FIG. 1, the dispersion aid is added to the organic solvent flowing through the pipe 33 via the pipe 43. At this time, the form of addition of the dispersion aid is not particularly limited, and it may be added once, intermittently over a plurality of times, or continuously. Above all, continuous addition is preferable from the viewpoint of ensuring the supply of a sufficient amount of the dispersion aid in the dispersion step and the polymerization step and further suppressing the increase in the particle size of the obtained water-absorbent resin during the continuous operation. .. That is, a preferred embodiment of the present invention comprises continuously adding a dispersion aid to an organic solvent.

Here, "continuously adding the dispersion aid to the organic solvent" means that the dispersion aid is allowed to flow into the organic solvent flowing at a predetermined flow rate at a predetermined flow rate for at least a certain period of time. Specifically, as shown in FIG. 1, it means that a dispersion aid is flowed through a pipe 43 at a predetermined flow rate for at least a certain period of time into a continuous phase containing an organic solvent flowing through the pipe 33 at a predetermined flow rate. At this time, the ratio of the flow rate of the dispersion aid flowing from the pipe 43 [ml / min] to the flow rate [ml / min] of the continuous phase containing the organic solvent flowing through the pipe 33 (dispersion aid flow rate [ml / min] / continuous phase). The flow rate [ml / min]) is preferably 0.01 or more. Further, the “at least a certain period of time” in the definition means preferably during the polymerization step, for example, 30 minutes or more, preferably 1 hour or more.

When the dispersion aid is a solid or a liquid having low fluidity, it may be added by dissolving it in a solvent. As the solvent used at that time, the solvent used for the organic solvent or the monomer composition is preferable, and it is more preferable to use the same solvent as the organic solvent used for the polymerization.

In the present invention, the dispersion aid is a substance having a function of promoting the formation of fine droplets of the monomer composition in the dispersion device and stabilizing the dispersed state of the fine droplets containing the monomer composition. For example, a dispersant such as a surfactant, an auxiliary agent such as a builder, a stabilizer such as a protective colloid, and a composition containing these can be mentioned, as long as they satisfy the heat resistance index described later.

The heat resistance index of the dispersion aid used in the present invention is 60 mN / m or more, preferably 65 mN / m or more, and more preferably 68 mN / m or more. The upper limit of the heat resistance index of the dispersion aid is not particularly limited, but is, for example, 90 mN / m or less. Here, the "heat resistance index" is an index showing the heat resistance of the dispersion aid, and specifically, is the surface tension (mN / m) measured by the following method.

≪Measurement method of heat resistance index≫
18.4 g of propionic acid was placed in a 500 mL eggplant flask, and while ice-cooled from the outside, 31.6 g of a 23.6 wt% sodium hydroxide aqueous solution was added dropwise to 45% of 75 mol% partially neutralized propionic acid / Na salt. An aqueous solution was prepared. Further, 0.01 g of a dispersion aid and 100 g of n-heptane are added, a cooling tube is attached, the mixture is immersed in an oil bath at 90 ° C., and a stirring / reflux operation is performed. After 5 hours, the aqueous phase is separated by a liquid separation operation. 0.2 g of the aqueous phase separated into a 50 mL beaker was collected, diluted with 40 g of 0.9 wt% physiological saline, and surface tension at 20 ° C. using a surface tension meter (K11 automatic surface tension meter manufactured by KRUSS). (MN / m) is measured.

Examples of the dispersion aid having a heat resistance index of 60 mN / m or more include polyolefin-based dispersion aids. Examples of polyolefin-based dispersion aids include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, and maleic anhydride-modified ethylene / propylene / diene ternary copolymer (EPDM). , Acid-modified polyolefin such as maleic anhydride-modified polybutadiene, maleic anhydride / ethylene copolymer, maleic anhydride / propylene copolymer, maleic anhydride / ethylene / propylene copolymer, maleic anhydride / butadiene copolymer, Examples thereof include polyethylene, polypropylene, ethylene / propylene copolymer, oxidized polyethylene, oxidized polypropylene, oxidized ethylene / propylene copolymer, ethylene / acrylic acid copolymer and the like. Of these, acid-modified polyolefins are more preferable from the viewpoint of dispersion stability of the monomer composition. Of these, two or more may be used in combination.

As the dispersion aid, in addition to the above-mentioned dispersion aid having a heat resistance index of 60 mN / m or more, a known dispersion aid may be further used. When a known dispersion aid is used in combination, the heat resistance index of the mixture is preferably 60 mN / m or more. The method for calculating the heat resistance index of the mixture is calculated based on the content ratio of each dispersion aid. For example, when the dispersion aid (mN / m) having a heat resistance index A is contained in a% by weight with respect to the entire dispersion aid, and the dispersion aid having a heat resistance index B is contained in b% by weight based on the entire dispersion aid. The heat resistance index of the mixture is (A × a + B × b) / 100 (mN / m).

Further, from the viewpoint of reducing the surface tension of the obtained water-absorbent resin and suppressing the odor, it is preferable that the amount of the ester-based dispersion aid used is as small as possible. Specifically, the concentration of the ester-based dispersion aid in the organic solvent supplied to the disperser is preferably less than 0.005% by weight.

Examples of ester-based dispersion aids include sucrose fatty acid ester, polyglycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, and polyoxyethylene sorbitol fatty acid ester. Can be mentioned.

According to a preferred embodiment of the present invention, only the dispersion aid having a heat resistance index of 60 mN / m or more is used as the dispersion aid.

The acid value of the dispersion aid is preferably 10 to 100 mgKOH / g, more preferably 15 to 90 mgKOH / g, and even more preferably 20 to 80 mgKOH / g. The acid value of the dispersion aid shall be a value measured in accordance with JIS K 0070: 1992.

The weight average molecular weight of the dispersion aid is not particularly limited, but is, for example, 1,000 to 100,000. As the weight average molecular weight, a polystyrene-equivalent value measured by gel permeation chromatography (GPC) shall be adopted.

As the dispersion aid having the above heat resistance index of 60 mN / m or more, either a synthetic product or a commercially available product may be used. Commercially available products include Mitsui Chemicals Co., Ltd. High Wax (registered trademark) 1105A, 2203A, 210MP, 220MP, 310MP, 320MP, 405MP, 405MPF, 4051E, 4052E, 4202E, 4252E, 1120H, 1160H, etc. Registered trademarks) PP MA 1332, PP MA 6252, PE MA 4221, PE MA 4351 and the like.

The amount of the dispersion aid used is appropriately set according to the polymerization form, the monomer composition, the type of organic solvent, and the like. Specifically, the concentration of the dispersion aid in the organic solvent as a continuous phase (ratio of the content of the dispersion aid to the total amount of the organic solvent) is preferably 0.0001 to 2% by weight, more preferably 0. It is 0005 to 1% by weight.

According to the production method of the present invention, even if the amount of the dispersion aid used is small, fine droplets containing the monomer composition can be stably produced. That is, in one embodiment of the present invention, the amount of the dispersion aid added is 0.5% by weight or less with respect to the monomer composition. The amount of the dispersion aid added to the monomer composition includes the concentration of the dispersion aid solution, the flow rate of the dispersion aid flowing into the disperser [ml / min], and the flow rate of the monomer composition flowing into the disperser [ml]. By adjusting the ratio of [/ min], it can be controlled within a desired range.

[2-3. Polymerization process]
This step is a step of supplying fine droplets dispersed in an organic solvent to a polymerization apparatus and polymerizing the monomers to obtain a hydrogel polymer.

(Polymerization device)
The shape of the polymerization apparatus in which the polymerization reaction is carried out is not particularly limited, but preferably, the monomer (composition) is a droplet-like dispersed phase in an organic solvent which is a continuous phase formed in the polymerization apparatus. It is a shape that can carry out a polymerization reaction while moving. Examples of such a polymerization apparatus include a polymerization apparatus in which tubular reaction tubes are arranged in a vertical type, a horizontal type, or a spiral type. When the reaction tube is of the vertical type, the ratio (L / D) of the inner diameter D (mm) of the reaction tube to the length L (mm) is preferably 2 to 100,000, more preferably 3 to 50,000. , More preferably 4 to 20,000.

By setting the ratio (L / D) within the above range, the fine droplets containing the monomer composition move satisfactorily inside the polymerization apparatus, so that the variation in the residence time of the droplets is reduced. .. Further, since the particle size of the finally obtained gel-like polymer is less varied, the physical characteristics of the obtained water-absorbent resin are also improved.

Further, the polymerization apparatus may be provided with a temperature adjusting means so that the continuous phase inside the polymerization apparatus can be heated or cooled from the outside, if necessary. The temperature adjusting means keeps the temperature of the continuous phase in the polymerization apparatus within a predetermined range. The temperature adjusting means is not particularly limited, and examples thereof include installation of a jacket in a polymerization apparatus, installation of a heater, installation of a heat insulating material and a heat insulating material, supply of hot air and cold air, and the like. When the organic solvent is resupplied to the polymerization apparatus, the organic solvent is heated by the heat exchanger.

Further, as the material of the polymerization apparatus, copper, titanium alloy, stainless steel such as SUS304, SUS316, SUS316L, fluororesin such as PTEE, PFA, FEP and the like can be used. Among them, from the viewpoint of the adhesiveness of the obtained gel-like polymer, a fluororesin is preferably used, and more preferably, a fluororesin or the like is used on the inner wall surface of the polymerization apparatus.

"Polymerization temperature"
In the production method according to the present invention, the temperature of the organic solvent forming a continuous phase in the polymerization apparatus (hereinafter, referred to as “Td”) is defined as the polymerization temperature.

Since the monomer composition is dispersed in a continuous phase in the form of droplets, the temperature of the monomer composition rises rapidly due to heat transfer from the continuous phase. When the polymerization initiator contained in the droplets is a thermal decomposition type polymerization initiator, the thermal decomposition type polymerization initiator is decomposed and radicals are generated as the temperature rises. The generated radical initiates the polymerization reaction, and a gel-like polymer is formed as the polymerization reaction progresses.

When the continuous phase in the polymerization apparatus is circulating, the formed gel-like polymer moves inside the polymerization apparatus by the circulating continuous phase and is discharged from the polymerization apparatus together with the organic solvent forming the continuous phase.

When the monomer composition contains a thermal decomposition type polymerization initiator, the Td is preferably 70 ° C. or higher, more preferably 75 ° C. or higher, still more preferably 80 ° C. or higher from the viewpoint of the polymerization rate. The upper limit of Td is not particularly limited, but from the viewpoint of safety, it is appropriately selected within a range not exceeding the boiling point of the organic solvent forming a continuous phase.

"Polymerization time"
In the method for producing a water-absorbent resin according to the present invention, the "polymerization time" starts from the time when the monomer composition is put into the polymerization apparatus, and the gel-like polymer obtained by the polymerization reaction is discharged from the polymerization apparatus. It is the time specified with the time to polymerize as the end point. For example, when the monomer composition is continuously supplied to the polymerization apparatus in the form of droplets and the formed gel-like polymer is continuously discharged from the polymerization apparatus, the droplets of one monomer composition are used. Means the time required to reach the end point from the start point. In other words, the time from the start of supply of the monomer composition to the polymerization apparatus to the discharge of the first gel-like polymer from the polymerization apparatus is the polymerization time. The polymerization time corresponds to the residence time of the droplet in the polymerization apparatus.

The polymerization time is controlled according to the type of the monomer and the polymerization initiator, but is preferably 60 minutes or less, more preferably 30 minutes or less, still more preferably 20 minutes or less, particularly from the viewpoint of production efficiency. It is preferably controlled to 10 minutes or less, most preferably 5 minutes or less. The lower limit of the polymerization time is not particularly limited, but from the viewpoint of heat transfer efficiency from the continuous phase when the droplets of the monomer composition supplied into the polymerization apparatus are heated to the polymerization temperature. Therefore, it is preferably controlled for 30 seconds or more. By controlling the polymerization time within the above range, the size of the polymerization apparatus can be reduced, which is preferable.

"Space velocity in polymerization equipment (LHSV)"
In the method for producing a water-absorbent resin according to the present invention, the space velocity (LHSV) (unit: hr ^-1 ) in the polymerization apparatus represents the passage velocity of the monomer composition (hydrous gel) and the organic solvent in the polymerization apparatus. It is an index and is an index that serves as a guide when controlling the polymerization time.

From the viewpoint of polymerization rate to prevent contact of different hydrogel, the lower limit of space velocity in the polymerization system (LHSV) is preferably from ^{2 hr -1} or more, more preferably ^{3 hr -1} or more, ^{4hr -1} or more is more preferable. From the viewpoint of DRC5min of and the water-absorbent resin (residual monomer amount of the water-absorbing resin particles) polymerization of the resulting water-containing gel, the upper limit of space velocity in the polymerization system is preferably 30 hr ^-1 or less, 15hr ^-1 or less more preferably, even more preferably 12hr ^-1 or less, 10 hr ^-1 or less is particularly preferred. That is, in one embodiment of the present invention, the space velocity (LHSV) in the polymerization apparatus is preferably 2 to 30 hr ^-1 , more preferably 3 to 15 hr ^-1 , and even more preferably 3 to 12 hr ^-1. Is. The space velocity (LHSV) (unit: hr ^-1 ) in the polymerization apparatus is the volumetric flow rate Qm (unit: m ³ / hr) of the monomer composition (hydrous gel) supplied to the polymerization apparatus, the organic solvent, and the dispersion. It is a value obtained by dividing the total volumetric flow rate Qs (unit: m ³ / hr) of the auxiliary agent by the volume V (unit: m ³ ) of the polymerization apparatus, and can be calculated by the following formula.

[2-4. Separation and recycling process]
This step is a step of separating the hydrogel polymer discharged from the polymerization apparatus and the organic solvent in the above polymerization step to obtain a gel polymer (hydrogen gel), and supplying the separated organic solvent to the disperser again. Is.

In this step, the type and structure of the separation device for separating the hydrogel polymer and the organic solvent are not particularly limited, but for example, known methods such as filtration, sedimentation, centrifugation, and squeezing can be used. ..

In this step, it is preferable to supply the organic solvent separated from the hydrogel polymer to the disperser again while maintaining the temperature at 70 ° C. or higher. As a result, the temperature of the organic solvent which is a continuous phase can be maintained at 70 ° C. or higher, so that the process can be rapidly shifted from the dispersion step to the polymerization step. Therefore, the polymerization reaction can be carried out while suppressing the coalescence of the fine droplets containing the monomer composition produced in the dispersion step, and a fine water-containing gel can be obtained. That is, in one embodiment of the present invention, the temperature of the organic solvent resupplied to the disperser is 70 ° C. or higher. When the organic solvent was re-supplied to the disperser, it was separated from the hydrogel polymer by passing through a heat exchanger (for example, the heat exchanger 20 in FIG. 1) and appropriately adjusting the temperature of the heat exchanger. The organic solvent can be resupplied to the disperser while maintaining the temperature above 70 ° C.

"Shape of hydrogel polymer"
In the present invention, the shape of the obtained hydrogel polymer is spherical. The particle size of the hydrogel polymer (hereinafter referred to as "gel particle size") is appropriately adjusted according to the use of the obtained water-absorbent resin and the like.

The above-mentioned "spherical shape" is a concept including a shape other than a true sphere (for example, a substantially spherical shape), and the ratio of the average major axis to the average minor axis of the particles (also referred to as "sphericity") is preferably 1.0. Means particles that are ~ 3.0. The average major axis and the average minor axis of the particles are measured based on images taken with a microscope. In the present invention, the hydrogel-like polymer may be formed as an aggregate of fine spherical gels, or may be obtained as a mixture of the fine spherical gel and the aggregates of the spherical gels.

When the hydrogel polymer is an aggregate of spherical gels, the particle size of each spherical gel constituting the aggregate is referred to as a primary particle size. In the present invention, the average primary particle size is not particularly limited, but is preferably 1 to 200 μm, more preferably 5 to 100 μm, still more preferably 10 to 80 μm, particularly preferably from the viewpoint of suppressing the generation of fine particles in the drying step. Is 20 to 60 μm. As the average primary particle size of the hydrogel polymer (hydrogen gel), the value measured by the method described in the following Examples is adopted.

"Concentration of solid content of hydrogel polymer"
The solid content of the hydrogel polymer used in the drying step described later is not particularly limited, but from the viewpoint of drying cost, it is preferably 20% by weight or more, more preferably 30% by weight or more, still more preferably 40% by weight. % Or more, particularly preferably 45% by weight or more. The upper limit of the solid content of the hydrogel polymer is not particularly limited, but is preferably 90% by weight or less, more preferably 80% by weight or less, still more preferably 70% by weight or less, and particularly preferably 60% by weight or less. be. By subjecting the hydrogel polymer having a solid content in the above range to the drying step described later, the effect of the present invention becomes remarkable.

[2-5. Other processes]
In addition to the above-mentioned steps, the method for producing the water-absorbent resin according to the present invention includes a drying step, a crushing step, a classification step, a surface cross-linking step, a granulation step, a fine powder removing step, a granulation step and a fine powder, if necessary. A reuse step can be included. Further, a transportation process, a storage process, a packing process, a storage process, and the like may be further included. From the viewpoint of obtaining a water-absorbent resin having an excellent absorption ratio under pressure (AAP), the method for producing a water-absorbent resin according to the present invention includes a drying step and a surface cross-linking in addition to the above-mentioned dispersion step, polymerization step and dispersion and recycling step. It is preferable to have a further step.

(Drying process)
This step is a step of drying the hydrogel polymer to obtain a water-absorbent resin powder. The hydrogel polymer may be subjected to a drying step after being adjusted to a desired particle size or particle size distribution by crushing or granulating.

As known methods for drying the hydrogel polymer, for example, drying by conduction heat transfer, drying by convection heat transfer (for example, hot air), drying by reduced pressure, drying using infrared rays, and microwaves are used. Examples thereof include drying with hot water, drying by co-boiling dehydration with a hydrophobic organic solvent, and drying with superheated steam using high-temperature steam (for example, superheated steam).

However, in the present invention, a stirring type conduction heat transfer drying having high drying efficiency and easy recovery of liquid components such as an organic solvent is preferable, and a continuous stirring type drying device using an indirect heating method is more preferable. used.

In the present invention, a gel fluidizing agent is preferably added to the hydrogel polymer when it is dried. The addition of the gel fluidizing agent is particularly effective when treating the particulate hydrogel in the heat treatment step of the drying step.

The amount of the gel fluidizing agent added is appropriately set according to the water content of the hydrogel or the particulate hydrogel and the type of the gel fluidizing agent. The amount added is preferably 0.001% by weight to 0.5% by weight, more preferably 0.01% by weight to 0.3% by weight, still more preferably 0.02% by weight, based on the solid content of the hydrogel. % To 0.2% by weight.

As this gel fluidizing agent, for example, a surfactant disclosed in JP-A-8-134134 can be used.

Specifically, as surfactants used in gel fluidizers, triethanolamine lauryl sulfate, sodium polyoxyethylene lauryl sulfate, sodium lauryl phosphate, N-palm oil fatty acid acyl-L-monosodium glutamate, dodecylbenzene. Anionic surfactants such as sodium sulphonate, sodium alkylnaphthalene sulphonate; 1: 1 type palm oil fatty acid diethanolamide, polyethylene glycol monostearate, polyoxyethylene sorbitan monolaurate, octylphenol polyoxyethylene, sorbitan monooleate, Nonionic surfactants such as polyoxyethylene sorbitan monostearate; Cationic surfactants such as stearyltrimethylammonium chloride, lanolin fatty acid ethyl sulfate aminopropylethyldimethylammonium, lauryltrimethylammonium chloride, distearyldimethylammonium chloride; palm Oil fatty acids Amidopropyl dimethylaminoacetic acid betaine, lauryldimethylaminoacetic acid betaine, 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaine, lauryl betaine and other amphoteric surfactants; cationized cellulose, polyethylene glycol, polypropylene In addition to polymer surfactants such as glycol, known silicone-based surfactants and fluorine-based surfactants can be mentioned.

Further, as described above, the shape of the hydrogel polymer formed by the production method according to the present invention is spherical. By drying the spherical hydrogel polymer with the above-mentioned stirring type drying device, a dry polymer composed of spherical particles can be obtained. The dried polymer composed of spherical particles obtained in this drying step can be used as it is as a water-absorbent resin for various purposes. Further, when the water-absorbent resin is produced by this production method, the spherical dry polymer obtained in the drying step can be subjected to the surface cross-linking step described later. In this case, the dry polymer used in the surface cross-linking step described later is referred to as "water-absorbent resin powder" for convenience.

In the present invention, the drying temperature and the drying time are appropriately adjusted by using the solid content ratio as an index according to the use of the obtained water-absorbent resin. For example, in the case of a water-absorbent resin, the solid content is preferably 85% by weight or more, more preferably 90% by weight to 98% by weight, from the viewpoint of water absorption performance. The solid content of the water-absorbent resin is a value calculated based on the weight loss when the sample (water-absorbent resin) is dried at 180 ° C. for 3 hours.

(Crushing process, classification process)
The particulate dry polymer obtained in the above drying step is a water-absorbent resin whose particle size or particle size distribution is controlled by undergoing a pulverization step and a classification step, if necessary.

In the above crushing step, for example, a high-speed rotary crusher such as a roll mill, a hammer mill, a screw mill, a pin mill, a vibration mill, a knuckle type crusher, a cylindrical mixer and the like are appropriately selected and used.

In the above classification step, for example, sieve classification using a JIS standard sieve (JIS Z8801-1 (2000)), air flow classification, and the like are appropriately selected and used.

(Surface cross-linking process)
This step is a step of subjecting the water-absorbent resin powder obtained in the above drying step to surface cross-linking with a surface cross-linking agent. Specifically, this step is a step of providing a portion having a high cross-linking density on the surface layer of the water-absorbent resin powder by adding a surface cross-linking agent to the water-absorbent resin powder and then heat-treating it.

The surface cross-linking agent is not particularly limited, and for example, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl. -1,3-Pentanediol, Polypropylene glycol, Glycerin, Polyglycerin, 2-Buten-1,4-diol, 1,3-Butanediol, 1,4-Butanediol, 1,5-Pentanediol, 1,6 -Hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymer, pentaerythritol, sorbitol, etc. Hyvalent alcohol compounds; ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol polyglycidyl ether, glycidol, sorbitol polyglycidyl Epoxy compounds such as ethers, pentaerythritol polyglycidyl ethers, trimethylolpropane polyglycidyl ethers, neopentyl glycol diglycidyl ethers, 1,6-hexanediol diglycidyl ethers; ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, penta Polyvalent amine compounds such as ethylene hexamine and polyethylene imine and their inorganic or organic salts; polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; aziridine compounds such as polyaziridine; 1,2-ethylene Polyvalent oxazoline compounds such as bisoxazoline, bisoxazoline, polyoxazoline; carbonate derivatives such as urea, thiourea, guanidine, dicyandiamide, 2-oxazolidinone; 1,3-dioxolan-2-one (ethylene carbonate), 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-o , 4-Hydroxymethyl-1,3-dioxolan-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3 -Alkylene carbonate compounds such as dioxane-2-one and 1,3-dioxopan-2-one; haloepoxy compounds such as epichlorohydrin, epibromhydrin and α-methylepichlorohydrin and their polyvalent amine additions. Oxetane compounds; silane coupling agents such as γ-glycidoxypropyltrimethoxysilane and γ-aminopropyltriethoxysilane; hydroxides such as zinc, calcium, magnesium, aluminum, iron and zirconium, chlorides and sulfates. Polyvalent metal compounds such as salts, nitrates or carbonates; and the like. Of these, two or more may be used in combination. Among the above surface cross-linking agents, one or two or more selected from polyvalent metal ions, epoxy compounds, oxazoline compounds, and alkylene carbonate compounds are preferable.

The amount of the surface cross-linking agent added is preferably 0.01 to 5% by weight with respect to the solid content of the water-absorbent resin.

The surface cross-linking agent may be added as it is, but it is preferably added as a solution dissolved in water or an organic solvent from the viewpoint of ease of addition. The concentration of the surface cross-linking agent in the solution can be adjusted as appropriate, and is, for example, 1 to 50% by weight.

The heat treatment can be appropriately performed using a known heating means. The temperature of the heat treatment is not particularly limited, but is, for example, 100 to 250 ° C. The heat treatment time is also not particularly limited, but is, for example, 10 to 120 minutes.

After the heat treatment, the cooling treatment may be performed. Cooling conditions can be adjusted accordingly.

(Granulation process)
The "granulation step" means a step of loosening the loosely aggregated water-absorbent resin powder through the surface cross-linking step to adjust the particle size. The sizing step includes a fine powder removing step after the surface cross-linking step, a gel crushing step, and a classification step.

(Fine powder reuse process)
The “fine powder reuse step” means a step of supplying the fine powder generated in each of the above steps as it is or after granulating the fine powder to any of the steps.

[3. Applications of water-absorbent resin]
The use of the water-absorbent resin of the present invention is not particularly limited, but is preferably a water-stopping material, a paint, an adhesive, an anti-blocking agent, a light diffusing agent, a matting agent, an additive for a decorative board, and an additive for artificial marble. , Additives for resins such as additives for toners. The use as a water-absorbent resin is not particularly limited, but preferably, it may be used as an absorber for absorbent articles such as paper diapers, sanitary napkins, and incontinence pads. In particular, it can be used as an absorber for high-concentration disposable diapers, which have had problems such as odor and coloring derived from raw materials. Further, since this water-absorbent resin has excellent water absorption time and the particle size distribution is controlled, a remarkable effect can be expected when it is used in the upper layer portion of the absorber.

Further, as a raw material for the absorber, an absorbent material such as pulp fiber can be used together with the water-absorbent resin. In this case, the content (core concentration) of the water-absorbent resin in the absorber is preferably 30% by weight to 100% by weight, more preferably 40% by weight to 100% by weight, still more preferably 50% by weight to 100% by weight. %, More preferably 60% by weight to 100% by weight, particularly preferably 70% by weight to 100% by weight, and most preferably 75% by weight to 95% by weight.

By setting the core concentration in the above range, when the absorber is used in the upper layer of the absorbent article, the absorbent article can be kept in a clean white state. Further, since the absorber is excellent in diffusivity of body fluids such as urine and blood, efficient liquid distribution is expected to improve the absorption amount.

[4. Physical characteristics of water-absorbent resin]
"Particle shape of water-absorbent resin"
The present invention relates to the above [2. A water-absorbent resin produced by the production method described in [Method for producing a water-absorbent resin] is also provided. Further, in the present invention, polymerization is carried out by so-called reverse phase suspension polymerization. The water-absorbent resin thus obtained is usually spherical polymer particles. Here, the "spherical shape" includes a shape other than a true spherical shape. Specifically, "spherical" means particles having a ratio (also referred to as sphericity) of the average major axis to the average minor axis of the particles, preferably 1.0 to 3.0. The average major axis and the average minor axis of the particles are measured based on the images observed under the microscope. In the present invention, the "spherical polymer particles" are not limited to existing as single particles, and may form aggregates of spherical polymer particles.

The spherical polymer particles in the present invention are designed by selecting a polymerizable monomer according to its use and purpose. For example, when producing a powdery or particulate water-absorbent resin as spherical polymer particles, the polymerizable monomer typically used is (meth) acrylic acid and / or a salt thereof.

Since the particle shape is spherical, particularly spherical agglomerates, the water absorption rate of the water-absorbent resin tends to be higher than that of the indefinite shape.

"Average primary particle size"
From the viewpoint of further improving the water absorption rate, the upper limit of the average primary particle size of the water-absorbent resin is preferably less than 100 μm, more preferably 80 μm or less. The lower limit of the average primary particle size of the water-absorbent resin is not particularly limited, but is usually 10 μm or more. The average primary particle size of the water-absorbent resin is measured by the method described in Examples.

"CRC"
"CRC" is an abbreviation for Centrifuge Retention Capacity (centrifuge holding capacity), and means a water absorption ratio (sometimes referred to as a "water absorption ratio") of a water-absorbent resin under no pressure. The CRC (centrifuge holding capacity) was measured according to the EDANA method (ERT441.2-02). Specifically, 0.2 g of a water-absorbent resin is placed in a non-woven fabric bag, and then immersed in a large excess of 0.9 wt% sodium chloride aqueous solution for 30 minutes for free swelling, and then a centrifuge (250 G). ) For 3 minutes, the water absorption ratio (unit: g / g) after draining.

In addition, "EDANA" is an abbreviation for European Disposables and Nonwovens Associations. Further, "ERT" is an abbreviation for EDANA Recommended Test Methods, which is a European standard that defines a method for measuring a water-absorbent resin. In the present invention, unless otherwise specified, the physical characteristics of the water-absorbent resin are measured in accordance with the original ERT (revised in 2002).

The CRC (centrifuge holding capacity) of the water-absorbent resin is preferably 15 g / g or more, more preferably 30 g / g or more, still more preferably 35 g / g or more, still more preferably 38 g / g or more. The upper limit is not particularly limited, and a higher CRC is preferable, but from the viewpoint of balance with other physical properties, it is preferably 70 g / g or less, more preferably 60 g / g or less, still more preferably 50 g / g or less.

When the CRC is less than 15 g / g, the amount of absorption is small and it is not suitable as an absorber for absorbent articles such as disposable diapers. Further, when the CRC exceeds 70 g / g, the rate of absorbing body fluids such as urine and blood decreases, so that it may not be suitable for use in high water absorption rate type disposable diapers and the like. The CRC can be controlled by changing the type and amount of the internal cross-linking agent, the surface cross-linking agent, and the like.

"DRC 5min"
"DRC" is an abbreviation for Dunk Retention Capacity (immersion holding capacity), and "DRC 5min" means a 5-minute value of immersion holding capacity (water absorption ratio under no pressurization in 5 minutes). Specifically, 1.0 g of the water-absorbent resin was uniformly sprayed on a cylindrical cell having a mesh on the bottom surface in the same manner as in the measurement of AAP below, and brought into contact with a 0.9 wt% sodium chloride aqueous solution for 5 minutes. It refers to the water absorption ratio (unit: g / g) after free swelling.

From the viewpoint of the amount of liquid returned when used as a sanitary material, the lower limit of DRC 5 min of the water-absorbent resin is preferably 46 g / g or more, more preferably 47 g / g or more, still more preferably 50 g / g or more, particularly preferably. Is 52 g / g or more. The upper limit of DRC 5 min of the water-absorbent resin is not particularly limited, but is usually 70 g / g or less.

"AAP"
"AAP" is an abbreviation for Absorption Against Pressure, and means the water absorption ratio under pressure of a water-absorbent resin. Specifically, 0.9 g of a water-absorbent resin was swollen with a large excess of 0.9 wt% sodium chloride aqueous solution under a load of ^{2.06 kPa (21 g / cm 2, 0.3 psi) for 1 hour.} It refers to the subsequent water absorption ratio (unit: g / g). In this specification, it is defined as a value measured by changing the load condition to 4.83 kPa (about 49 g / cm ^{2, corresponding to about 0.7 psi).}

From the viewpoint of water absorption characteristics when used as a sanitary material, the lower limit of AAP of the water-absorbent resin is preferably 20 g / g or more, more preferably 23 g / g or more. The upper limit of AAP of the water-absorbent resin is not particularly limited, but is usually 40 g / g or less.

"surface tension"
From the viewpoint of the amount of liquid returned when used as a sanitary material, the lower limit of the surface tension of the water-absorbent resin is preferably 65 mN / m or more, more preferably 67 mN / m or more, and further preferably 70 mN / m or more. The upper limit of the surface tension of the water-absorbent resin is not particularly limited, but is usually 73 mN / m or less. The surface tension of the water-absorbent resin is measured by the method described in Examples.

Therefore, the water-absorbent resin according to the embodiment of the present invention is a water-absorbent resin obtained by reverse-phase suspension polymerization, having a surface tension of 65 mN / m or more and a DRC of 5 min of 46 g / g or more. .. The water-absorbent resin has excellent absorption characteristics (small amount of liquid return) when used as a sanitary material.

Further, the water-absorbent resin according to the embodiment of the present invention is described in the above [2. A method for producing a water-absorbent resin], which is a water-absorbent resin produced by the production method, having a surface tension of 65 mN / m or more and a DRC of 5 min of 46 g / g or more.

The effects of the present invention will be described with reference to the following Examples and Comparative Examples, but the present invention is not limited to these explanations and can be obtained by appropriately combining the technical means disclosed in each Example. Examples of these are also included in the scope of the present invention. The physical properties of the water-containing gel, the water-absorbent resin powder, the water-absorbent resin, and the absorber were measured by the following methods. In addition, although the display of "parts" or "%" may be used in the examples, it represents "parts by weight" or "% by weight" unless otherwise specified. Unless otherwise specified, each operation is performed at room temperature (25 ° C.).

"Average primary particle size"
Scanning electron micrographs (SEMs) of water-absorbent resin or water-absorbent resin powder were taken. 50 primary particles are arbitrarily selected from the photograph, and the average value of the primary particle diameter of each particle is calculated by measuring and averaging the major axis and minor axis of each particle as the primary particle diameter. The average value was taken as the average primary particle size of the water-absorbent resin.

"CRC"
The CRC of the water-absorbent resin was measured according to the EDANA method (ERT441.2-02).

"Amount of residual monomer"
The amount of residual monomer of the water-absorbent resin was measured according to the EDANA method (ERT410.2-02).

"surface tension"
50 ml of physiological saline adjusted to 20 ° C. is placed in a thoroughly washed 100 ml beaker, and the surface tension of the physiological saline is first measured using a surface tension meter (K11 automatic surface tension meter manufactured by KRUSS). bottom. In this measurement, it was confirmed that the value of surface tension was in the range of 71 to 75 [mN / m]. Next, 0.5 g of a sufficiently washed 25 mm long fluororesin rotor and water-absorbent resin (1) was placed in a beaker containing physiological saline adjusted to 20 ° C. after measuring the surface tension, and 500 rpm. The mixture was stirred for 4 minutes under the conditions of. After 4 minutes, the stirring was stopped, and after the water-containing water-absorbent resin had settled, the surface tension of the supernatant was measured again by performing the same operation. In the present invention, a plate method using a platinum plate was adopted, and the plate was sufficiently washed with deionized water before each measurement and then heated and washed with a gas burner before use.

"Moisture content"
The water content of the water-absorbent resin was measured according to the EDANA method (ERT430.2-02). In the present invention, the sample amount was changed to 1.0 g and the drying temperature was changed to 180 ° C. for measurement.

"Particle size"
The particle size (particle size distribution, weight average particle size (D50), logarithmic standard deviation (σζ) of the particle size distribution) of the water-absorbent resin is described in columns 27 and 28 of US Pat. No. 7,638,570. Measurement was performed in accordance with "Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of Particle Diameter Division".

"DRC 5min"
The DRC 5 min (immersion holding capacity 5 minutes value) of the water-absorbent resin (1) was measured by the method described in International Publication No. 2017/170605 (US Patent Application Publication No. 2019/1114111).

Specifically, using the device shown in FIG. 6, a wire mesh 201 (mesh size 38 μm) made of stainless steel 400 mesh is fused to the bottom of a plastic support cylinder 200 having an inner diameter of 60 mm, and the room temperature (20 to 25 ° C.) ), The water-absorbent resin (1) 202 1.000 ± 0.005 g was uniformly sprayed on the wire mesh 201 under the condition of a relative humidity of 50% RH, and the weight Wa (g) of this set of measuring devices was measured.

A glass filter 204 with a diameter of 120 mm (manufactured by Mutual Rikagaku Glass Mfg. Co., Ltd., pore diameter: 100 to 120 μm) is placed inside a circular or square Petri dish 203 with a bottom area of 400 cm ^{2, and 0.90 wt% saline solution is placed.} 206 (23 ± 0.5 ° C) at the same level as the upper surface of the glass filter (a state in which the liquid slightly floats on the outer periphery of the glass filter due to surface tension, or about 50% of the surface of the glass filter is covered with the liquid. It was added so that it would be in the state of being). On top of that, one filter paper 205 with a diameter of 110 mm (ADVANTEC Toyo Co., Ltd., product name: (JIS P 3801, No. 2), thickness 0.26 mm, reserved particle size 5 μm) is placed so that the entire surface of the filter paper gets wet. bottom.

The set of the measuring devices was placed on the damp filter paper to absorb the liquid (the liquid temperature is strictly controlled to 23 ± 0.5 ° C. even during the measurement). Exactly after 5 minutes (300 seconds), the set of measuring devices was lifted and its weight Wb (g) was measured. Then, DRC5min (g / g) was calculated from Wa and Wb according to the following formula.

"Evaluation of absorber (return amount)"
After dry mixing 2 g of the water-absorbent resin and 2 g of crushed wood pulp using a mixer, the obtained mixture was spread on a wire screen of 400 mesh (opening 38 μm) to form a web having a diameter of 90 mm. The web was then pressed at a pressure of 196.14 kPa (2 [kgf / cm ² ]) for 1 minute to create an absorber. The absorber (diameter 90 mm / core concentration 50%) is placed on the bottom of a SUS petri dish having an inner diameter of 90 mm, a non-woven fabric having a diameter of 90 mm is placed on the bottom, and a load of 4.8 kPa is evenly applied to the absorber. The adjusted piston and weight were placed in. The piston and weight used had a liquid inlet having a diameter of 5 mm at the center. Then, 50 mL of physiological saline (0.90 wt% sodium chloride aqueous solution) was poured from the liquid inlet to allow the absorber to absorb the liquid. After 5 minutes, the piston and weight were removed, and 30 sheets of filter paper (ADVANTEC Toyo Co., Ltd., product name: JIS P 3801, No. 2) with an outer diameter of 90 mm whose total weight was measured in advance were placed, and the load was further applied evenly. The piston and weight (total weight 20 kg) were quickly placed. After 1 minute, the piston, the weight, and the filter paper were removed, the total weight of the filter paper was measured, and the weight before the measurement was subtracted to determine the amount of liquid (g) absorbed by the filter paper. This liquid amount was defined as the liquid return amount (g).

[Example 1]
A series of steps 2 to 5 below are operated according to the manufacturing process shown in FIG. 1 to prepare a water-containing gel (1), and then the obtained water-containing gel (1) is dried to obtain a water-absorbent resin (1). Manufactured. The specific operation time was 10 hours after the start of feeding the monomer composition to the disperser in step 2 below.

First, n-heptane (density: 0.68 g / ml) was charged as an organic solvent into the disperser 12, the polymerization device 14, the separation device 16, and the piping connecting them.

Subsequently, the liquid feed pump 18 was operated to start the circulation of the organic solvent at a flow rate of 300 ml / min. The entire amount of the organic solvent was charged into the polymerization device 14 via the dispersion device 12. Further, the heat exchanger 20 was operated and heated so that the temperature of the circulating organic solvent was 90 ° C.

Next, separately, maleic anhydride-modified polyethylene (acid value: 60 mgKOH / g) was mixed with n-heptane as a dispersion aid and heated to 90 ° C. to dissolve it, and a 0.030 wt% dispersion aid solution was dissolved. (1) was prepared. Subsequently, the dispersion aid solution (1) obtained by the above operation was added to n-heptane flowing through the pipe 33 for 30 minutes at a flow rate of 50 ml / min via the pipe 43. The ratio of the content of maleic anhydride-modified polyethylene to the total amount of the organic solvent before the start of polymerization was 0.005% by weight. The heat resistance index of the dispersion aid measured by the above method was 71 mN / m.

(1. Mixing process)
Acrylic acid, 48.5% by weight sodium hydroxide aqueous solution and ion-exchanged water are mixed, and polyethylene glycol diacrylate (average degree of polymerization: 9) and diethylenetriamine 5 acetate / 3 sodium are further added to form a monomer. An aqueous solution (1) was prepared. Separately, a 6 wt% sodium persulfate aqueous solution (1) was prepared by mixing sodium persulfate and ion-exchanged water.

Subsequently, the monomer aqueous solution (1) and the sodium persulfate aqueous solution (1) obtained by the above operation were supplied to the mixing device 10 to prepare the monomer composition (1). The monomer concentration of the monomer composition (1) was 43% by weight, and the neutralization rate was 75 mol%. The internal cross-linking agent polyethylene glycol diacrylate is 0.020 mol% with respect to the monomer, the chelating agent diethylenetriamine-5 acetate / 3 sodium is 200 ppm with respect to the monomer, and the polymerization initiator is persulfate. Sodium (T10 70 ° C.) was 0.1 g / mol with respect to the monomer.

(2. Dispersion process)
As the disperser, a double-cylindrical high-speed rotary shear stirrer (dispersor 12C) shown in FIG. 4 was used. The inner diameter of the casing (inner diameter of the outer cylinder 50C) is 25 mm, the outer diameter of the rotor (outer diameter of the inner cylinder 52C) is 22 mm, and the effective rotor length (from the monomer aqueous solution inlet 55C to the outlet) is 65 mm. As the polymerization apparatus, a tube made of PFA (perfluoroalkoxy alkane) (inner diameter: 25 mm, total length: 10 m) was vertically arranged.

The mixed solution of the above organic solvent and dispersion aid was sent to the pipe 35 of the dispersion device 12C at a flow rate of 300 mL. Thirty minutes after the completion of the addition of the dispersion aid solution (1) before the start of polymerization, the rotor (inner cylinder 52C) was rotated to a rotation speed of 7,200 rpm (shear velocity 5729 [1 / s]), and then the rotor (inner cylinder 52C) was rotated. The monomer composition (1) was sent to the pipe 31 of the disperser 12C at a flow rate of 40 ml / min (47.2 g / min). The supplied monomer composition (1) was dispersed in the organic solvent in the form of fine droplets by a disperser.

(3. Polymerization step)
2. The dispersion liquid obtained in 1) was supplied to the polymerization apparatus 14.

The droplets composed of the monomer composition (1) fall into a fine spherical hydrogel (1) as the polymerization reaction progresses while falling in the polymerization apparatus filled with the organic solvent which is the continuous phase. changed. These tiny spherical gels adhered to each other as they fell to form aggregates. Then, a water-containing gel (1) having a diameter of about 1 cm, which was composed of agglomerates of minute spherical gels, was confirmed in the vicinity of the discharge port of the polymerization apparatus. The space velocity (LHSV) in the polymerization apparatus 14 was ^{4.2 hr-1.}

The hydrogel (1) obtained by the above series of operations was continuously discharged from the polymerization apparatus 14 together with the organic solvent.

(4. Separation and recycling process)
The hydrogel (1) discharged from the polymerization apparatus 14 and the organic solvent were continuously supplied to the separation apparatus 16 as they were. In the separation device, the hydrogel (1) and the organic solvent were separated. The organic solvent separated by the separation device is supplied to the heat exchanger 20 via the pipe 32, the liquid feed pump 18, and the pipe 33, and heat is exchanged so that the set temperature (organic solvent temperature) becomes 90 ° C. After adjusting the temperature in the vessel 20, the temperature was adjusted to 70 ° C. or higher through the pipe 35 and supplied to the disperser 12 and the polymerization apparatus 14. At that time, the above-mentioned dispersion aid solution (1) was applied to the disperser through the pipe 43 as a replenishing dispersion aid in a continuous phase containing an organic solvent flowing through the pipe 33 at a flow rate of 5 ml / min. 10 minutes after the start of the liquid feeding of the body composition, the continuous addition was started. That is, the dispersion aid flow rate [ml / min] / continuous phase flow rate [ml / min] was 0.017. The amount of the dispersion aid (maleic anhydride-modified polyethylene) added is 0.005% by weight based on the monomer composition (1).

The water-containing gel (1) obtained by the above operation had a shape in which minute spherical water-containing gels adhered and aggregated.

(5. Drying process)
The hydrogel (1) discharged from the separation device 16 was continuously supplied to the indirect heating type stirring / drying device as it was, and an ethanol solution (concentration 20% by weight) of polyethylene glycol 400 (PEG400) prepared in advance was added. .. The amount of the PEG400 ethanol solution with respect to the hydrogel (1) was 2.5% by weight. Subsequently, the heat medium temperature of the drying apparatus was adjusted to 180 ° C., and the hydrogel (1) was continuously dried while being mixed with PEG400 to obtain a particulate dried polymer (1). The obtained dry polymer (1) was continuously supplied to a sieving apparatus having a metal sieve net (JIS standard sieve) having an opening of 850 μm and 150 μm for classification, and the water-absorbent resin powder (1) was sampled. The surface tension of the water-absorbent resin powder (1) sampled 1 hour after the start of the polymerization step was 69 mN / m, and the surface tension of the water-absorbent resin powder (1) sampled 5 hours later was 69 mN / m. No decrease in surface tension was observed.

The above process 1. ~ 5. Was operated for 10 hours, and the mixture was mixed for 1 hour immediately after the start of polymerization in which the emission amount was not stable, and after the polymerization was stopped, to obtain a water-absorbent resin powder (1).

[Example 2]
In Example 1, instead of the dispersion aid solution (1), a maleic anhydride-modified ethylene-propylene copolymer (acid value: 30 mgKOH / g) as a dispersion aid was mixed with n-heptane and brought to 90 ° C. A water-absorbent resin powder (2) was obtained in the same manner as in Example 1 except that the dispersion aid solution (2) (concentration: 0.30% by weight) obtained by heating and dissolving was used. For the dispersion aid, the heat resistance index measured by the above method was 72 mN / m. The ratio of the content of the maleic anhydride-modified ethylene / propylene copolymer to the total amount of the organic solvent before the start of the polymerization was 0.05% by weight. The amount of the maleic anhydride-modified ethylene / propylene copolymer added continuously to the organic solvent during the polymerization was 0.05% by weight with respect to the above-mentioned monomer composition (1). The surface tension of the water-absorbent resin powder (2) sampled 1 hour after the start of polymerization was 70 mN / m, and the surface tension of the water-absorbent resin powder (2) sampled 5 hours later was 70 mN / m. No decrease in surface tension was observed.

[Example 3]
In the first embodiment, the dispersion step was carried out by using the two-fluid spray nozzle (dispersor 12D) shown in FIG. 5 instead of the double-cylindrical high-speed rotary shear stirrer, in the same manner as in the first embodiment. , A water-absorbent resin powder (3) was obtained.

Specifically, a two-fluid spray nozzle (external mixing type, spray nozzle inner diameter: 1.0 mm, type: SETOJet, air consumption category 075, injection volume 10, manufactured by Ikeuchi Co., Ltd.) was used as the disperser. The two-fluid spray nozzle has a first supply pipe 101 that continuously supplies the monomer composition, and a second supply pipe 102 that continuously supplies a mixed solution of the organic solvent and the dispersion aid. The monomer composition is spray-dispersed from the first nozzle 103, and the mixed solution of the organic solvent and the dispersion aid is spray-dispersed from the second nozzle 104 and continuously discharged to the polymerization apparatus. At this time, the position of the two-fluid spray was adjusted so that the tip of the nozzle of the two-fluid spray was immersed in the organic solvent contained in the polymerization apparatus. Further, the flow rate of the circulating mixture of the organic solvent and the dispersion aid is changed to 1000 ml / min, and the path of the circulated mixture of the organic solvent and the dispersion aid is passed through the disperser (two-fluid spray nozzle). It was branched into a route to be charged into the polymerization apparatus and a route to be charged directly into the polymerization apparatus. At this time, the flow rate of the mixed solution of the organic solvent and the dispersion aid charged into the polymerization apparatus via the dispersion apparatus (two-fluid spray nozzle) is set to 800 ml / min, and the organic solvent and the dispersion aid directly charged into the polymerization apparatus. The flow rate of the mixed solution was set to 200 ml / min. Next, the monomer composition (1) prepared in the mixing step was promptly sent to the first supply pipe 101 of the two-fluid spray. Then, using the above-mentioned two-fluid spray, the monomer composition (1) was put into the organic solvent filling the inside of the above-mentioned polymerization apparatus at a flow rate of 40 mL / min (47.2 g / min).

The monomer composition (1) charged by the two-fluid spray was dispersed in the organic solvent in the form of fine droplets. The space velocity (LHSV) in the polymerization apparatus 14 was ^{12.7 hr-1.}

The surface tension of the water-absorbent resin powder (3) sampled 1 hour after the start of polymerization was 68 mN / m, and the surface tension of the water-absorbent resin powder (3) sampled 5 hours later was 68 mN / m. No decrease in surface tension was observed.

[Example 4]
In Example 1, instead of the dispersion aid solution (1), maleic anhydride-modified polypropylene (acid value: 18 mgKOH / g) as a dispersion aid was mixed with n-heptane and heated to 90 ° C. to dissolve it. A water-absorbent resin powder (4) was obtained in the same manner as in Example 1 except that the dispersion aid solution (4) (concentration: 0.030% by weight) was used. For the dispersion aid, the heat resistance index measured by the above method was 70 mN / m. The content of polypropylene maleic anhydride with respect to the total amount of the organic solvent before the start of polymerization was 0.005% by weight. The amount of polypropylene maleic anhydride added continuously to the organic solvent during the polymerization was 0.005% by weight with respect to the above-mentioned monomer composition (1). The surface tension of the water-absorbent resin powder (4) sampled 1 hour after the start of polymerization was 67 mN / m, and the surface tension of the water-absorbent resin powder (4) sampled 5 hours later was 67 mN / m. No decrease in surface tension was observed.

[Comparative Example 1]
In Example 3, instead of the dispersion aid solution (1), a sucrose fatty acid ester (HLB value 6) as a dispersion aid is mixed with n-heptane and heated to 90 ° C. to dissolve the dispersion aid. A comparative water-absorbent resin powder (1) was obtained in the same manner as in Example 3 except that the agent solution (3) (concentration: 0.030% by weight) was used. The heat resistance index of the dispersion aid measured by the above method was 56 mN / m. The surface tension of the comparative water-absorbent resin powder (2) sampled 1 hour after the start of polymerization was 67 mN / m, and the surface tension of the comparative water-absorbent resin powder (2) sampled 5 hours later was 60 mN / m. A decrease in surface tension was observed in.

Table 1 shows various physical properties measured for the obtained water-absorbent resin powders (1) to (4) and the comparative water-absorbent resin powder (1).

[Example 5]
(6. Surface cross-linking process)
From 0.015 parts by weight of ethylene glycol diglycidyl ether, 1.0 part by weight of propylene glycol, and 3.0 parts by weight of ion-exchanged water with respect to 100 parts by weight of the water-absorbent resin powder (1) obtained in Example 1. The surface cross-linking agent solution was sprayed with a spray nozzle and mixed uniformly using a continuous high-speed mixer. Then, the water-absorbent resin powder (1) containing the surface cross-linking agent was introduced into a heat treatment machine whose ambient temperature was adjusted to 195 ° C. ± 2 ° C., heat-treated for 30 minutes, and then until the powder temperature reached 60 ° C. The mixture was forcibly cooled to obtain a water-absorbent resin (1').

(7. Grain size adjustment process)
Next, the water-absorbent resin (1') obtained in the above step was classified by a sieving device having a metal sieve (JIS standard sieve) having an opening of 850 μm. The residue on the metal sieve having an opening of 850 μm was pulverized again and then mixed with the material passing through the metal sieve having an opening of 850 μm. By the above operation, a sized water-absorbent resin (4) having a total particle size of less than 850 μm was obtained. Table 2 shows various physical characteristics of the obtained water-absorbent resin (4).

[Example 6]
In Example 5, the water-absorbent resin (5) was obtained in the same manner as in Example 5 except that the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (2) obtained in Example 2. ..

[Example 7]
In Example 5, the water-absorbent resin (6) was obtained in the same manner as in Example 5 except that the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (3) obtained in Example 3. ..

[Comparative Example 2]
In Example 5, the comparative water-absorbent resin (2) was used in the same manner as in Example 5 except that the water-absorbent resin powder (1) was changed to the comparative water-absorbent resin powder (1) obtained in Comparative Example 1. Obtained.

[Example 8]
In Example 1, polyethylene glycol diacrylate, which is an internal cross-linking agent, was changed to 0.010 mol% with respect to the monomer, and lauryldimethylamino was added instead of the ethanol solution of polyethylene glycol 400 (PEG400) added in the drying step. Same as in Example 1 except that the betaine acetate aqueous solution (concentration 3.1% by weight) was changed and the amount of the lauryldimethylaminoacetate betaine aqueous solution with respect to the hydrogel (1) was changed to 0.5% by weight. The water-absorbent resin powder (7) was obtained. Further, in Example 5, a water-absorbent resin (7) was obtained in the same manner as in Example 5 except that the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (7).

[Example 9]
In Example 5, the water-absorbent resin (8) was obtained in the same manner as in Example 5 except that the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (4) obtained in Example 4. ..

[Comparative Example 3]
As a reflux condenser, a dropping funnel, a nitrogen gas introduction pipe, and a stirrer, a 1-liter four-necked cylindrical round-bottom separable flask equipped with a stirring blade having four inclined paddle blades having a blade diameter of 50 mm in two stages was prepared. 550 ml of n-heptane was placed in this flask, 2.76 g of maleic anhydride-modified polyethylene used in Example 1 was added and dispersed, the temperature was raised to 50 ° C., the dispersion aid was dissolved, and then the temperature was cooled to 30 ° C. Separately, 92 g of an 80% by weight acrylic acid aqueous solution was taken in a 500 ml triangular flask, and 152.5 g of a 20.1% by weight sodium hydroxide aqueous solution was added dropwise from the outside while cooling with ice to neutralize 75 mol%. Then, 18.4 mg of ethylene glycol diglycidyl ether was added, and 0.11 g of potassium persulfate was further added and dissolved. This aqueous solution of partially neutralized acrylic acid is added to a four-necked flask, dispersed by a stirrer, the inside of the system is sufficiently replaced with nitrogen, the temperature is raised, and the bath temperature is maintained at 70 ° C. for the first stage polymerization. The reaction was carried out for 30 minutes. Then, the reaction solution was cooled to 20 ° C., and the same amount of the acrylic acid partially neutralized salt aqueous solution prepared in the same manner as described above was added dropwise to the system to absorb it for 30 minutes, and at the same time, the inside of the system was sufficiently replaced with nitrogen. The temperature was raised, the bath temperature was maintained at 70 ° C., and the second-stage polymerization reaction was carried out for 30 minutes. This reaction solution was distilled to remove water and n-heptane to obtain a comparative water-absorbent resin powder (3). Further, a comparative water-absorbent resin (3) was obtained in the same manner as in Example 5 except that the water-absorbent resin powder (1) was changed to the comparative water-absorbent resin powder (3) in Example 5.

Table 2 shows various physical properties measured for the obtained water-absorbent resins (4) to (8) and the comparative water-absorbent resins (2) to (3).

As shown in Table 1, according to the production method of the present invention, the surface tension of the water-absorbent resin powder does not decrease with time even after continuous operation. Further, as shown in Table 2, the water-absorbent resin obtained by surface-crosslinking the water-absorbent resin powder has an excellent balance between DRC 5 min and surface tension, and is excellent in water absorption when used as a sanitary material. It is within the range where the characteristics are shown. It can be seen that by using the water-absorbent resin of the present invention, a high-performance absorber can be obtained with a small amount of liquid return. On the other hand, when a dispersion aid having a heat resistance index of less than 60 mN / m is used (Comparative Example 1), the surface tension of the water-absorbent resin powder decreases with time when continuous operation is performed. Further, the water-absorbent resin obtained by surface-crosslinking the water-absorbent resin powder has a low surface tension. Further, when a batch operation in which the monomer composition, the organic solvent, and the dispersion aid are not continuously supplied to the disperser is performed (Comparative Example 3), the DRC of the obtained water-absorbent resin is 5 min. Is low. According to Table 2, it can be seen that the amount of liquid return increases when the water-absorbent resin obtained in any of the comparative examples is used as the absorber.

This application is based on Japanese Patent Application No. 2018-182114 filed on September 27, 2018 and Japanese Patent Application No. 2019-128663 filed on July 10, 2019, the contents of which are disclosed. , Referenced and incorporated as a whole.

10 Mixing device,
12, 12A-12D Disperser,
14 Polymerization equipment,
16 Separator,
18 Liquid feed pump,
20 heat exchanger,
22 Drying device,
31-37, 41-44 piping,
50A, 52A A pair of walls,
50B, 52B Pair of walls,
50C, 52C Pair of walls,
51A, 53A facing surface,
51B, 53B facing surface,
51C, 53C facing surface,
54A-54C flow path,
60A-60C drive unit,
55A-55C 1st supply system,
56A to 55C 2nd supply system 101 1st supply pipe 102 2nd supply pipe 103 1st nozzle 104 2nd nozzle 200 Supporting cylinder 201 Wire mesh 202 Water-absorbent resin 203 Petri dish 204 Glass filter 205 Filter paper 206 Saline.

Claims (16)

The monomer composition, the organic solvent, and the dispersion aid are continuously supplied to the disperser to disperse the fine droplets containing the monomer composition in the organic solvent.
The fine droplets dispersed in the organic solvent are supplied to the polymerization apparatus, and the monomers are polymerized to obtain a hydrogel polymer.
Resupplying the organic solvent separated from the hydrogel polymer to the dispersion device, and
It is a method for producing a water-absorbent resin having
A method for producing a water-absorbent resin, wherein the heat resistance index of the dispersion aid is 60 mN / m or more. The method for producing a water-absorbent resin according to claim 1, wherein the dispersion aid is continuously added to the organic solvent. The method for producing a water-absorbent resin according to claim 1 or 2, wherein the temperature of the organic solvent resupplied to the disperser is 70 ° C. or higher. The method for producing a water-absorbent resin according to any one of claims 1 to 3, wherein the organic solvent separated from the hydrogel polymer is resupplied to the disperser while maintaining the temperature at 70 ° C. or higher. The method for producing a water-absorbent resin according to any one of claims 1 to 4, wherein the dispersion aid is an acid-modified polyolefin. The method for producing a water-absorbent resin according to any one of claims 1 to 5, wherein the concentration of the ester-based dispersion aid in the organic solvent supplied to the disperser is less than 0.005% by weight. The method for producing a water-absorbent resin according to any one of claims 1 to 6, wherein the amount of the dispersion aid added is 0.5% by weight or less based on the monomer composition. The method for producing a water-absorbent resin according to any one of claims 1 to 7, wherein the monomer is a water-soluble ethylenically unsaturated monomer. The invention according to any one of claims 1 to 8, wherein the monomer composition contains a partially neutralized salt of an acid group-containing unsaturated monomer, a thermally decomposable polymerization initiator, and water. A method for producing a water-absorbent resin. The method for producing a water-absorbent resin according to any one of claims 1 to 9, wherein the space velocity (LHSV) in the polymerization apparatus is 2 to 30 hr- ^1. Further, the hydrogel-like polymer is dried to obtain a water-absorbent resin powder.
The method for producing a water-absorbent resin according to any one of claims 1 to 10, wherein the water-absorbent resin powder is surface-crosslinked with a surface cross-linking agent. A water-absorbent resin produced by the production method according to any one of claims 1 to 11. A water-absorbent resin produced by the production method according to any one of claims 1 to 11, wherein the surface tension is 65 mN / m or more and the DRC 5 min is 46 g / g or more. .. A water-absorbent resin obtained by reverse-phase suspension polymerization.
A water-absorbent resin having a surface tension of 65 mN / m or more and a DRC of 5 min of 46 g / g or more. The water-absorbent resin according to any one of claims 12 to 14, which has an absorption ratio (AAP) of 20 g / g or more under pressure. The water-absorbent resin according to any one of claims 12 to 15, wherein the average primary particle size is less than 100 μm.

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