阅读说明：本技术 吸水性树脂的制造方法和吸水性树脂 (Process for producing water-absorbent resin and water-absorbent resin ) 是由 井村元洋 井上雅史 佐藤舞 本田耕士 田岛峻一 荒毛知幸 于 2019-09-26 设计创作，主要内容包括：提供在用于卫生材料时具有优异吸收特性(液体回流量少)的吸水性树脂的制造方法。一种吸水性树脂的制造方法,其具有：向分散装置中连续供给单体组合物、有机溶剂和分散助剂,使包含前述单体组合物的微细液滴分散在前述有机溶剂中；将分散在前述有机溶剂中的微细液滴供给至聚合装置中,将前述单体聚合而得到含水凝胶状聚合物；以及将从前述含水凝胶状聚合物中分离的前述有机溶剂再次供给至前述分散装置中,前述分散助剂的耐热性指数为60mN/m以上。(Provided is a method for producing a water-absorbent resin having excellent absorption characteristics (a small amount of liquid returned) when used in a sanitary material. A method for producing a water-absorbent resin, comprising: continuously supplying a monomer composition, an organic solvent and a dispersing aid to a dispersing device to disperse fine droplets containing the monomer composition in the organic solvent; supplying fine droplets dispersed in the organic solvent to a polymerization apparatus, and polymerizing the monomer to obtain a hydrogel polymer; and re-feeding the organic solvent separated from the water-containing gel-like polymer to the dispersing agent, wherein the heat resistance index of the dispersing aid is 60mN/m or more.)

1. A method for producing a water-absorbent resin, comprising:

continuously supplying a monomer composition, an organic solvent and a dispersion aid to a dispersing device to disperse fine droplets containing the monomer composition in the organic solvent;

feeding the fine droplets dispersed in the organic solvent to a polymerization apparatus, and polymerizing the monomer to obtain a hydrogel-containing polymer; and

the organic solvent separated from the water-containing gel-like polymer is supplied again to the dispersing device,

the heat resistance index of the dispersing aid is 60mN/m or more.

2. The method for producing a water-absorbent resin according to claim 1, comprising: continuously adding the dispersion aid to the organic solvent.

3. The method for producing a water-absorbent resin according to claim 1 or 2, wherein the temperature of the organic solvent resupplied to the dispersing device is 70 ℃ or higher.

4. 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 supplied to the dispersing device again while being maintained at 70 ℃ or higher.

5. The method for producing a water-absorbent resin according to any one of claims 1 to 4, wherein the dispersing aid is an acid-modified polyolefin.

6. 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 dispersing device is less than 0.005% by weight.

7. 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.

8. 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.

9. The method for producing a water-absorbent resin according to any one of claims 1 to 8, wherein the monomer composition comprises a partially neutralized salt of an unsaturated monomer having an acid group, a thermal decomposition type polymerization initiator, and water.

10. The method for producing a water-absorbent resin according to any one of claims 1 to 9, wherein a space velocity (LHSV) in the polymerization apparatus is 2 to 30hr^-1。

11. The method for producing a water-absorbent resin according to any one of claims 1 to 10, further comprising:

drying the hydrogel polymer to obtain a water-absorbent resin powder; and

the water-absorbent resin powder is subjected to surface crosslinking with a surface crosslinking agent.

12. A water-absorbent resin produced by the production method according to any one of claims 1 to 11.

13. A water-absorbent resin produced by the production method according to any one of claims 1 to 11, having a surface tension of 65mN/m or more and a DRC5min of 46g/g or more.

14. A water-absorbent resin obtained by reversed-phase suspension polymerization,

the surface tension is 65mN/m or more, and the DRC5min is 46g/g or more.

15. The water-absorbent resin according to any one of claims 12 to 14, wherein the absorption capacity under load (AAP) is 20g/g or more.

16. The water-absorbent resin according to any one of claims 12 to 15, which has an average primary particle diameter of less than 100 μm.

Technical Field

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

Background

In recent years, in sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads, water-absorbent resins, which are constituent materials of the sanitary materials, have been widely used as water-absorbing agents from the viewpoint of absorption of body fluids. Among the water-absorbent resins, various monomers and hydrophilic polymers are used as raw materials, but polyacrylic acid (salt) -based water-absorbent resins using acrylic acid and/or a salt thereof as a monomer are most industrially produced from the viewpoint of water absorption performance.

Examples of the properties to be possessed by the water-absorbent resin include excellent water absorption capacity, water absorption speed, gel strength, and suction force by sucking water from a base material containing an aqueous liquid when the water-absorbent resin comes into contact with the aqueous liquid such as a body fluid. Further, conventionally, there have been proposed a plurality of water-absorbent resins which have physical properties in which a plurality of these absorption properties fall within various specific ranges and exhibit excellent performance (water absorption properties) when used in sanitary materials such as disposable diapers and sanitary napkins, and an absorbent material and an absorbent article using the water-absorbent resins.

General methods for producing water-absorbent resins are roughly classified into aqueous solution polymerization and reversed-phase suspension polymerization. According to the reversed-phase suspension polymerization method, a water-absorbent resin in the form of beads (spheres) can be obtained. The reversed-phase suspension polymerization method is a method of carrying out polymerization by suspending an aqueous monomer solution in an organic solvent. For example, there is a method of dispersing a monomer in an organic solvent in the form of droplets by mechanical stirring and then starting polymerization (Japanese patent application laid-open No. 61-192703). In this method, when dispersing the monomer-containing solution into an organic solvent, a large amount of a dispersion aid needs to be added. As a result, a part of the dispersion aid may remain in the water-absorbent resin obtained by the polymerization reaction to lower the surface tension, thereby lowering the physical properties of the water-absorbent resin. Further, since the polymerization is carried out in a batch system, the production efficiency is poor and the quality may vary.

In this regard, international publication No. 2016/088848 (corresponding to U.S. patent application No. 2017/267793 specification) and international publication No. 2016/182082 disclose the following continuous polymerization processes: the organic solvent and the aqueous monomer solution are continuously mixed and dispersed at a flow ratio of 70 ℃ or higher and a specific flow ratio for the purpose of reducing the amount of the dispersion aid to be added and improving the production efficiency. 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 the dispersion aid, and a multi-fluid spray nozzle is used as a dispersing device to disperse an aqueous monomer solution into the organic solvent. Thereafter, a hydrogel polymer is produced by polymerization, and the hydrogel polymer is separated from the organic solvent. The separated hydrogel polymer is subjected to a drying step or the like to form a water-absorbent resin. On the other hand, the separated organic solvent is supplied to the spray nozzle again and reused in the dispersion step and the polymerization step of the aqueous monomer solution (that is, the organic solvent forms a continuous phase circulating in the dispersion step, the polymerization step, and the separation step).

Disclosure of Invention

In order to industrially produce a water-absorbent resin, generally, the production process of the water-absorbent resin is continuously operated. But it is clear that: when the continuous operation is performed according to the embodiment described in international publication No. 2016/182082, the surface tension of the resulting water-absorbent resin gradually decreases with the time of operation, and the excellent absorption characteristics (a small amount of liquid reflux) are impaired when used in a sanitary material.

Accordingly, an object of the present invention is to provide a method for producing a water-absorbent resin which can be continuously operated and has excellent absorption characteristics (a small amount of liquid returned) when used for a sanitary material. Further, an object of the present invention is to provide a water-absorbent resin having a small liquid reflux amount when used for a sanitary material.

The above problems are solved by a method for producing a water-absorbent resin, comprising: continuously supplying a monomer composition, an organic solvent and a dispersing aid to a dispersing device to disperse fine droplets containing the monomer composition in the organic solvent; supplying fine droplets dispersed in the organic solvent to a polymerization apparatus, and polymerizing the monomer to obtain a hydrogel polymer; and re-feeding the organic solvent separated from the water-containing gel-like polymer to the dispersing agent, wherein the heat resistance index of the dispersing aid is 60mN/m or more.

The above problem is also solved by a water-absorbent resin obtained by reversed-phase suspension polymerization, having a surface tension of 65mN/m or more and a DRC5min of 46g/g or more.

Drawings

FIG. 1 is a schematic view showing a part of a process for producing a water-absorbent resin according to an embodiment of the present invention.

Fig. 2 is a cross-sectional view showing an example of the dispersing device.

Fig. 3 is a sectional view showing another example of the dispersing device.

Fig. 4 is a sectional view showing another example of the dispersing device.

Fig. 5 is a sectional view showing still another example of the dispersing device.

FIG. 6 is a schematic diagram showing a DRC5min measurement apparatus.

Detailed Description

The present invention will be described below in detail with reference to the accompanying drawings. Throughout this specification, the singular expressions should be understood to include the plural concepts unless otherwise specified. Thus, articles in the singular (e.g., "a," "an," "the," etc. in the case of english) should be understood to also include the concept in the plural as long as it is not specifically recited. In addition, unless otherwise specified, terms used in the present specification should be understood to be used in the meaning generally used in the field. Accordingly, unless otherwise defined, all technical 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, the present specification, including definitions, will control. The present invention is not limited to the embodiments described below, and various modifications can be made within the scope of the claims.

[ 1. definition of terms ]

[1-1. Water-absorbent resin ]

In the present specification, the term "water-absorbent resin" means: a water-swelling capacity (CRC) of 5g/g or more as defined in ERT441.2-02 and a water-soluble component (Ext) of 70 wt% or less as defined in ERT 470.2-02.

In the present specification, the "water-absorbent resin" is not limited to the embodiment in which the total amount (100% by weight) of the water-absorbent resin is the same, and may be a water-absorbent resin composition containing additives or the like as long as the CRC and Ext described above are satisfied. In the present specification, the term "water-absorbent resin" refers to a concept including an intermediate in a process for producing a water-absorbent resin. For example, a hydrogel-containing polymer after polymerization, a dried polymer after drying, a water-absorbent resin powder before surface crosslinking, and the like may be sometimes referred to as "water-absorbent resin".

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

[1-2. other ]

In the present specification, "X to Y" indicating a range means "X or more and Y or less".

In the present specification, "ppm" means "ppm by weight".

In the present specification, "acid (salt)" means "acid and/or salt thereof". "(meth) acrylic" means "acrylic and/or methacrylic".

In the present specification, the unit "liter" of volume is sometimes expressed as "L" or "L".

In the present specification, the term "average" refers to an arithmetic average.

In the present specification, "continuous operation" means: the series of steps (dispersing step, polymerization step, separation and reuse step) of the process for producing a water-absorbent resin is preferably performed for 5 hours or longer, more preferably 1 day or longer, and still more preferably 1 month or longer. Among them, even in the case where the supply of the monomer composition to the dispersing device is interrupted, as long as the continuous phase is circulated in the manufacturing process, it is included in the category of "continuous operation".

[ 2. Process for producing Water-absorbent resin ]

The method for producing a water-absorbent resin of the present invention comprises: continuously supplying a monomer composition, an organic solvent and a dispersing aid to a dispersing device to disperse fine droplets containing the monomer composition in the organic solvent (hereinafter also referred to as a "dispersing step"); supplying fine droplets dispersed in the organic solvent to a polymerization apparatus, and polymerizing the monomer to obtain a water-containing gel-like polymer (hereinafter, also referred to as "polymerization step"); and re-supplying the organic solvent separated from the water-containing gel-like polymer to the dispersing agent (hereinafter also referred to as a "separation and reuse step") to obtain a dispersion aid having a heat resistance index of 60mN/m or more. According to this production method, a water-absorbent resin having excellent absorption characteristics (a small amount of liquid returned) when used in a sanitary material can be obtained even when a continuous operation is performed.

In order to industrially produce a water-absorbent resin having excellent absorption characteristics when used for sanitary materials, continuous operation of a production process of the water-absorbent resin is studied. Therefore, the present inventors have found that, when continuous operation is attempted for the embodiment described in international publication No. 2016/182082: the surface tension of the resulting water-absorbent resin has a problem of decreasing with the operation time. When the surface tension of the water-absorbent resin is lowered, the effect of excellent absorption characteristics (a small amount of liquid returned) is impaired when the water-absorbent resin is used for a sanitary material. In the embodiment of international publication No. 2016/182082, an ester-based dispersion aid as a dispersion aid is added to an organic solvent as a continuous phase, and the continuous phase is subjected to a heat cycle in a dispersion step, a polymerization step, and a separation step. The following possibilities can therefore be presumed: in the case of continuous operation, the ester-based dispersion aid is decomposed by heating, and fatty acid as a decomposition product accumulates in the continuous phase, which adversely affects the surface tension of the resulting water-absorbent resin. Based on the above presumption, the present inventors have conducted intensive studies on a dispersion aid and, as a result, have found that: the present inventors have completed the present invention by finding that a dispersion aid having a heat resistance index of 60mN/m or more can suppress the decrease in surface tension even when a continuous operation is performed, and can obtain a water-absorbent resin having excellent absorption characteristics (a small amount of liquid returned) when used in a sanitary material.

The present embodiment will be described below with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description thereof is omitted. In addition, the dimensional ratio of the drawings is exaggerated for the purpose of illustration and may be different from the actual ratio.

FIG. 1 is a schematic view showing a part of a process for producing a water-absorbent resin according to an embodiment of the present invention. Although a plurality of valves for adjusting the flow rate and the pressure are provided in the piping system, these valves are not shown in fig. 1.

As shown in FIG. 1, the process for producing a water-absorbent resin comprises a mixing apparatus 10, a dispersing apparatus 12, a polymerization apparatus 14, a separating apparatus 16, a liquid-feeding pump 18, a heat exchanger 20, a drying apparatus 22, and pipes 31 to 36 for connecting these apparatuses. A pipe 37 for discharging the dried polymer is connected to the drying device 22. The structure of the dispersing device 12 will be described in detail later. The polymerization apparatus 14 is constituted by, 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 "hydrogel"). The separator 16 is composed of, for example, a screw press or a continuous centrifuge, and takes out the hydrous gel 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 stirs and dries the water-containing gel. The pipe 35 branches off from the pipe 34 from the heat exchanger 20 to the polymerization apparatus 14, and is connected to the dispersion apparatus 12.

The mixing device 10 is connected to a pipe 41 for supplying a monomer solution and a pipe 42 for supplying a polymerization initiator. A pipe 43 for supplying the dispersion aid is connected to the pipe 33 from the liquid-feeding pump 18 to the heat exchanger 20. A pipe 44 for supplying the drying auxiliary agent is connected to the pipe 36 from the separator 16 to the dryer 22.

An example of a method for producing a water-absorbent resin will be described with reference to FIG. 1. The method for producing a water-absorbent resin includes an optional mixing step, a dispersing step, a polymerization step, a separation and reuse step, and optionally includes a drying step and the like after the separation and reuse step.

First, the dispersion apparatus 12, the polymerization apparatus 14, the separation apparatus 16, the heat exchanger 20, and the pipes 32, 33, 34, and 35 connecting these apparatuses are filled with an organic solvent. Subsequently, the liquid-feeding pump 18 is operated to circulate the organic solvent. A part of the organic solvent is also supplied to the dispersing device 12 through 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 apparatus and piping is heated to a predetermined temperature in the heat exchanger 20.

Subsequently, a separately prepared monomer solution and a polymerization initiator are continuously supplied to the mixing device 10 through 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.

Thereafter, the monomer composition is continuously supplied to the dispersing device 12 through the pipe 31. The monomer composition and the organic solvent are continuously supplied to the dispersing device 12, respectively. The monomer composition is dispersed in an organic solvent in the form of fine droplets by the dispersing device 12 (dispersing step). As described above, in the present embodiment, the monomer is continuously dispersed in the organic solvent.

The monomer dispersed in the form of fine droplets is continuously charged into the organic solvent in the polymerization apparatus 14, and the polymerization reaction is started in the polymerization apparatus 14 (polymerization step). In the polymerization apparatus 14, the movement of the organic solvent that is circulated causes the movement of fine droplets containing the monomer composition. The fine droplets move and become a hydrogel by a polymerization reaction. The droplets and the aqueous gel move in the same direction as the organic solvent (co-current flow). In the present invention, a polymerization method in which a polymerization reaction is started in a state in which droplets containing a monomer solution are dispersed or suspended in a liquid phase (continuous phase) containing an organic solvent to obtain a water-containing gel is referred to as liquid-phase droplet (suspension) polymerization.

Next, the hydrogel obtained by the liquid-phase droplet polymerization is continuously discharged from the polymerization device 14 together with the organic solvent, and is continuously supplied to the separation device 16. The separation apparatus 16 continuously separates the aqueous gel and the organic solvent (separation step). The separated hydrogel is continuously supplied to the subsequent step (drying device 22) via a pipe 36 (drying step). The separated organic solvent is sent to the liquid-sending pump 18 through the pipe 32, and is supplied again to the dispersing device 12 through 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 through the pipe 34.

In the drying device 22, the moisture contained in the hydrous gel and the organic solvent which is not separated cleanly in the separating device 16 are removed, and a dried polymer in the form of pellets is produced. The dried polymer in the form of pellets is discharged from the pipe 37 and supplied to a subsequent step (such as a cooling device). Although not shown, the organic solvent removed by the drying device 22 is supplied again to the polymerization device 14.

In the present invention, continuous polymerization (continuous production method) is employed. The continuous production method is a method comprising: the monomer solution or the monomer composition containing the monomer solution is continuously fed into the organic solvent in the polymerization apparatus to be polymerized, and the aqueous gel and the organic solvent formed by the polymerization reaction are continuously discharged from the polymerization apparatus. This process is referred to as liquid phase droplet continuous polymerization. In this case, since each operation between the respective steps can be continuously performed, it is possible to avoid a trouble such as a jam associated with a stop and a re-operation of each apparatus. Since continuous polymerization is a form in which a monomer composition is supplied from a dispersing apparatus to a polymerization apparatus, it is significantly different from a form in which dispersion and polymerization are carried out in one apparatus (batch operation).

Hereinafter, each step will be described.

[2-1: mixing procedure

This step is an optional step, and is a step of mixing the monomer solution with a polymerization initiator to obtain a monomer composition.

In the present step, the method for preparing the monomer composition by mixing the monomer solution and the polymerization initiator is not particularly limited, and examples thereof include the following methods: (1) a method of preparing a monomer solution and a solution containing a polymerization initiator (hereinafter referred to as "polymerization initiator solution") in advance, supplying the monomer solution and the solution to a mixing device from respective pipes at the same time, and mixing the monomer solution and the solution; (2) a method in which a monomer solution prepared in advance is supplied to a mixing device, and then a polymerization initiator is supplied to the mixing device and mixed.

The polymerization initiator may be in the form of a polymerization initiator solution obtained by dissolving (dispersing) the polymerization initiator in a solvent. The solvent of the polymerization initiator solution is not particularly limited, and is preferably water.

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

The materials used in this step will be described below.

"monomer solution"

Monomer solution refers to a solution comprising a monomer.

The solvent of the monomer solution is preferably water, a water-soluble organic solvent (e.g., alcohol), or a mixture thereof, more preferably water or a mixture of water and a water-soluble organic solvent, and still more preferably water. In the case of a mixture of water and a water-soluble organic solvent, the water-soluble organic solvent (e.g., alcohol) is preferably 30% by weight or less, more preferably 5% by weight or less.

From the viewpoint of the water absorbing performance of the resulting water-absorbent resin, the monomer is preferably a water-soluble ethylenically unsaturated monomer. Examples of the water-soluble ethylenically unsaturated monomer include acid group-containing unsaturated monomers such as (meth) acrylic acid, maleic acid (anhydride), itaconic acid, cinnamic acid, vinylsulfonic acid, allyltoluenesulfonic acid, vinyltoluenesulfonic acid, styrenesulfonic acid, 2- (meth) acrylamide-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2-hydroxyethyl (meth) acryloyl phosphate, methoxypolyethylene glycol (meth) acrylate, and polyethylene glycol mono (meth) acrylate; amide group-containing unsaturated monomers such as (meth) acrylamide, N-ethyl (meth) acrylamide, N-dimethyl (meth) acrylamide, N-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine and N-vinylacetamide; amino group-containing unsaturated monomers such as N, N-dimethylaminoethyl (meth) acrylate, N-dimethylaminopropyl (meth) acrylamide, and N, N-diethylaminoethyl (meth) acrylate; a mercapto group-containing unsaturated monomer; a phenolic hydroxyl group-containing unsaturated monomer; and lactam group-containing unsaturated monomers such as N-vinylpyrrolidone.

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, a known polymerization inhibitor such as p-methoxyphenol, phenothiazine, or vitamin E can be used. When p-methoxyphenol is used, oxygen may be used in combination as necessary. The amount of the polymerization inhibitor to be used is preferably 0.1 to 1000ppm, more preferably 5 to 500ppm, based on the monomer.

When an acid group-containing unsaturated monomer having an acid group such as a carboxyl group is used as a monomer to produce a water-absorbent resin, a neutralized salt obtained by neutralizing the acid group can be used. In this case, the salt of the unsaturated monomer having an acid group is preferably a salt with a monovalent cation, more preferably at least 1 selected from the group consisting of an alkali metal salt, an ammonium salt and an amine salt, further preferably an alkali metal salt, still further preferably at least 1 selected from the group consisting of a sodium salt, a lithium salt and a potassium salt, and particularly preferably a sodium salt.

From the viewpoint of the water absorbing performance of the water-absorbent resin to be obtained, the monomer is preferably an acid group-containing unsaturated monomer and/or a salt thereof, more preferably (meth) acrylic acid (salt), maleic acid (anhydride) (salt), itaconic acid (salt), and cinnamic acid (salt), still 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 preferably used in combination with a neutralized salt of the acid group-containing unsaturated monomer from the viewpoint of the water absorbing performance of the resulting water-absorbent resin. From the viewpoint of water absorption performance, the number of moles of the neutralized salt is preferably 40 mol% or more, more preferably 40 mol% to 95 mol%, even more preferably 50 mol% to 90 mol%, even more preferably 55 mol% to 85 mol%, and particularly preferably 60 mol% to 80 mol%, relative to the total number of moles of the acid group-containing unsaturated monomer and the neutralized salt thereof (hereinafter referred to as "neutralization ratio"). That is, in one embodiment of the present invention, the monomer comprises a mixture of an unsaturated monomer containing an acid group and a neutralized salt thereof. In the present specification, a mixture of an acid group-containing unsaturated monomer and a neutralized salt thereof is also referred to as "a partially neutralized salt of an acid group-containing unsaturated monomer".

Examples of the method for adjusting the neutralization ratio include: a method of mixing an unsaturated monomer containing an acid group with a neutralized salt thereof; a method of adding a known neutralizing agent to an unsaturated monomer having an acid group; a method of using a partially neutralized salt of an acid group-containing unsaturated monomer adjusted in advance to a predetermined neutralization rate. Further, these methods may be combined.

The neutralizing agent used for neutralizing the acid group-containing unsaturated monomer is not particularly limited, and an inorganic salt such as sodium hydroxide, potassium hydroxide, sodium carbonate, or ammonium carbonate; and basic substances such as amine-based organic compounds having amino groups and imino groups. As the neutralizing agent, 2 or more kinds of basic substances can be used in combination.

The addition of the neutralizing agent may be carried out before the polymerization reaction of the acid group-containing unsaturated monomer is started, or may be carried out after the polymerization reaction of the acid group-containing unsaturated monomer is completed, with respect to the resulting aqueous gel. In applications where there is a possibility that absorbent articles such as disposable diapers may come into direct contact with the human body, it is preferable to add the neutralizing agent before the start of the polymerization reaction.

In the production method of the present invention, any of the above-exemplified monomers may be used alone, or any 2 or more kinds of monomers may be appropriately mixed and used. Further, other monomers may be further mixed as long as the object of the present invention is achieved.

When 2 or more monomers are used in combination in the production of the water-absorbent resin, the (meth) acrylic acid (salt) is preferably contained as the main component. In this case, the proportion of the (meth) acrylic acid (salt) to the total monomers used for the polymerization is usually 50 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and still more preferably 90 mol% or more (the upper limit is 100 mol%) from the viewpoint of the water absorption performance of the resulting water-absorbent resin.

The monomer concentration in the monomer solution is not particularly limited as long as the monomer is soluble in the solvent, and is preferably 10 wt% or more and not more than the saturated concentration, more preferably 20 wt% or more and not more than the saturated concentration, further preferably 25 to 80 wt%, and particularly preferably 30 to 70 wt%.

In the polymerization step, an internal crosslinking agent may be used as needed. Examples of the internal crosslinking agent include conventionally known internal crosslinking agents having 2 or more polymerizable unsaturated groups and 2 or more reactive groups in 1 molecule. Examples of the internal crosslinking agent include N, N' -methylenebis (meth) acrylamide, (poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, glycerol tri (meth) acrylate, glycerol acrylate methacrylate, ethylene oxide-modified trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine, poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerol, 1, 4-butanediol, and the like, Pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine, glycidyl (meth) acrylate, and the like. These internal crosslinking agents may be used in only 1 kind, or may be used in 2 or more kinds.

Among them, from the viewpoint of the water absorption properties of the water-absorbent resin obtained, it is preferable to use a compound having 2 or more polymerizable unsaturated groups as the internal crosslinking agent. The amount of the internal crosslinking agent to be used may be appropriately determined depending on the desired physical properties of the water-absorbent resin, and is usually 0.0001 to 5 mol%, more preferably 0.001 to 3 mol%, and still more preferably 0.005 to 1.5 mol% based on the monomer.

Further, the following exemplified substances (hereinafter referred to as "other substances") may be added to the monomer solution.

Specific examples of the other substances include chain transfer agents such as thiols, thioalcohols, secondary alcohols, amines, and hypophosphites; foaming agents such as carbonates, bicarbonates, azo compounds, and bubbles; chelating agents such as metal salts of ethylenediaminetetraacetic acid and metal salts of diethylenetriaminepentaacetic acid; polyacrylic acid (salts) and their crosslinked materials, and thickeners such as starch, cellulose, starch-cellulose derivatives, and polyvinyl alcohol. The other substances may be used alone or in combination of 2 or more.

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

"polymerization initiator"

As the polymerization initiator, a thermal decomposition type polymerization initiator is preferably used. The thermal decomposition type polymerization initiator is a compound that decomposes by heat and generates a radical, and a water-soluble compound having a 10-hour half-life temperature (hereinafter referred to as "T10") of preferably 0 to 120 ℃, more preferably 30 to 100 ℃, and further preferably 50 to 80 ℃ is preferably used as the polymerization initiator, from the viewpoint of storage stability of the thermal decomposition type polymerization initiator and production efficiency of the water-absorbent resin.

Specific examples of the thermal decomposition type polymerization initiator having T10 in the above range include persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; azo compounds such as 2,2 '-azobis (2-methylpropionamidine) dihydrochloride, 2' -azobis (2-amidinopropane) dihydrochloride, 2 '-azobis [2- (2-imidazolin-2-yl) propane ] dihydrochloride, and 2, 2' -azobis (2-methylpropanenitrile); peroxides such as hydrogen peroxide, t-butyl peroxide, and methyl ethyl ketone peroxide. Of these, 2 or more kinds may be used in combination.

Among these, from the viewpoint of handling properties of the thermal decomposition type polymerization initiator and physical properties of the water-absorbent resin, a persulfate is preferably used as the polymerization initiator, sodium persulfate, potassium persulfate, ammonium persulfate, and sodium persulfate are more preferably used.

The amount of the thermal decomposition type polymerization initiator is not particularly limited and is suitably set depending on the kind of the monomer and the polymerization initiator, but is preferably 0.001 g/mole or more, more preferably 0.005 g/mole or more, and further preferably 0.01 g/mole or more relative to the monomer from the viewpoint of production efficiency. From the viewpoint of improving the water absorption performance of the water-absorbent resin, it is preferably not more than 2 g/mol, more preferably not more than 1 g/mol.

Further, other polymerization initiators such as a photodecomposition type polymerization initiator may be used in combination as necessary. Specific examples of the photodecomposition-type polymerization initiator include benzoin derivatives, benzil derivatives, acetophenone derivatives, and benzophenone derivatives.

When the above-mentioned thermal decomposition type polymerization initiator is used in combination with another polymerization initiator, the proportion of the thermal decomposition type polymerization initiator in the whole polymerization initiator is preferably 60 mol% or more, more preferably 80 mol% or more.

Further, the redox-type polymerization initiator can be prepared by using the thermal decomposition-type polymerization initiator in combination with a reducing agent. Among the redox polymerization initiators, the thermal decomposition type polymerization initiator functions as an oxidizing agent. The reducing agent to be used is not particularly limited, and examples thereof include sulfite (hydrogen) salts such as sodium sulfite and sodium bisulfite; reducing metal salts such as iron salts; l-ascorbic acid (salts), amines, and the like.

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

"monomer concentration of monomer composition"

In the present invention, the concentration of the monomer in the monomer composition is selected depending on the kind of the selected monomer and organic solvent, and the lower limit is preferably 10% by weight or more, more preferably 20% by weight or more, and still more preferably 30% by weight or more, and the upper limit is preferably 100% by weight or less, more preferably 90% by weight or less, still more preferably 80% by weight or less, and still more preferably 70% by weight or less, in terms of production efficiency. The monomer concentration in the monomer composition is preferably 10 to 90% by weight, more preferably 20 to 80% by weight, and still more preferably 30 to 70% by weight, from the viewpoint of the physical properties of the water-absorbent resin and productivity.

Additives such as an internal crosslinking agent, a density adjuster, and a thickener may be added to the monomer composition as long as the object of the present invention is not impaired. The kind and amount of the additive may be appropriately selected depending on the combination of the monomer and the organic solvent used.

[2-2. Dispersion step ]

This step is a step of continuously supplying a monomer composition, an organic solvent and a dispersing aid to a dispersing device and dispersing fine droplets containing the monomer composition in the organic solvent.

Here, "continuously supplying the monomer composition, the organic solvent, and the dispersion aid to the dispersing device" means: as shown in FIG. 1, the monomer composition, the organic solvent and the dispersing assistant are flowed into the dispersing device 12 through the piping for at least a predetermined period of time. The term "at least a certain time" in this definition means, for example, 30 minutes or more, preferably 1 hour or more. In fig. 1, the dispersion aid may be introduced into the dispersing device 12 together with the organic solvent through the pipe 35, may be introduced into the dispersing device 12 together with the monomer composition through the pipe 31, or may be introduced into the dispersing device 12 through a pipe other than the pipes 31 and 35 (i.e., separately from the organic solvent and the monomer composition). Among them, the form shown in fig. 1 is preferable from the viewpoint of stably forming fine droplets. That is, in a preferred embodiment of the present invention, a mixed liquid of an organic solvent and a dispersion aid is continuously supplied to a dispersing apparatus.

In this step, as shown in fig. 1, the monomer composition and the organic solvent are preferably flowed into the dispersing device through separate pipes. That is, the path through which the monomer composition flows into the dispersing device and the path through which the organic solvent flows into the dispersing device are preferably independent of each other. The flow rate of the organic solvent flowing into the dispersing device may be appropriately adjusted depending on the kind of the dispersing device, the size of the polymerization device, and the like, and may be appropriately adjusted so as to satisfy a space velocity (LHSV) in the polymerization device to be described later. The flow rate of the monomer composition flowing into the dispersing device may be appropriately adjusted so as to satisfy a "monomer composition flow rate/organic solvent flow rate ratio" described later. The flow rate of the dispersing aid flowing into the dispersing device is not particularly limited as long as fine droplets containing the monomer composition can be formed.

< dispersing apparatus >

The dispersing device used in this step is not particularly limited as long as it can form fine droplets containing the monomer composition in the organic solvent, and examples thereof include a high-speed rotary shear type stirrer (rotary stirrer type, turbine stirrer type, rotary disk type, double cylinder type, etc.), a cylindrical nozzle such as a needle, a perforated plate having a plurality of holes directly formed in the plate, a spray nozzle, a centrifugal atomizer such as a rotary wheel, and the like. From the viewpoint of stably forming fine droplets, a high-speed rotary shear mixer can be suitably used as the dispersing device.

(high-speed rotating shearing type mixer)

According to the high-speed rotary shear type stirrer, a flow path in which a shear field is formed by relatively moving a pair of walls having opposed surfaces opposed to each other with a gap therebetween can be formed, and the monomer composition can be continuously supplied to the organic solvent circulating through the flow path in which the shear field is formed.

The "flow path" in the high-speed rotary shear mixer is not particularly limited in shape as long as it 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 opposed facing surfaces of a pair of walls.

The specific shape of the "wall" may 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 of "relative movement of the pair of walls" is not particularly limited as long as it is a form capable of forming a flow path that exhibits a shear field. For example, one wall may be configured as a fixed wall and the other wall may be configured as a movable wall. In order to generate a difference in the moving speed, both the pair of walls may be configured as movable walls.

In the present invention, the monomer composition is preferably supplied into a relatively narrow flow path from the viewpoint of further reducing the size of fine droplets containing the monomer composition dispersed in the organic solvent. From this viewpoint, the size of the gap is preferably 5mm or less, more preferably 2mm or less. In view of productivity, the size of the gap is preferably 0.1mm or more, and more preferably 0.5mm or more.

Referring to fig. 2, a high-speed rotation shear type mixer will be described. The dispersing device 12A shown in fig. 2 is constituted by a high-speed rotary shear type stirrer. In the dispersing device 12A, the monomer composition and the organic solvent are continuously supplied, respectively, and the fine droplets containing the monomer composition are dispersed in the organic solvent. As a high-speed rotary shear type stirrer other than fig. 2, the dispersing devices 12B to 12C shown in fig. 3 to 4 can be used in the present invention. When the components in the dispersing device 12A in fig. 2 are used in common with the components in the dispersing devices 12B to 12C in fig. 3 to 4, the subscripts "B" to "C" are given instead of the subscript "a" given by the reference numeral in fig. 2, and redundant description is omitted.

"Dispersion unit 12A"

Fig. 2 is a sectional view showing an example of the dispersing device 12A. The dispersing device 12A is a rotary agitator type high-speed rotary shear type agitator. The dispersing device 12A includes: a flow path 54A formed by a pair of walls 50A, 52A having opposed surfaces 51A, 53A opposed to each other with a gap therebetween; and a driving unit 60A for relatively moving the pair of walls 50A, 52A. The flow path 54A in which a shear field appears is formed by relatively moving the pair of walls 50A and 52A by the driving unit 60A. The dispersing device 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.

The pair of walls 50A, 52A has a cylindrical shape. One wall 50A is formed by a non-rotating outer cylinder having a central bore. The other wall 52A is formed of a solid inner cylinder rotatably disposed in a center hole of the outer cylinder. The driving unit 60A is constituted by, for example, a motor, and is connected to the inner cylinder. The inner cylinder is rotationally driven by operating the driving unit 60A. Thus, the wall 50A as one constitutes a fixed wall, and the wall 52A as the other 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, 53A facing each other. The facing surfaces 51A and 53A facing each other have a concave-convex shape. The convex portion of the opposing surface 51A enters the concave portion of the opposing surface 53A, and the convex portion of the opposing surface 53A enters the concave portion of the opposing surface 51A. The flow path 54A has a curved shape.

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

The bottom of the wall 52A has a tapered shape toward the lower side. A 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 passage 58A is larger than the gap of the flow path 54A. This makes it easy to lead out 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 device 14 are formed to be substantially equal in size. This is because the liquid smoothly flows from the dispersing device 12A to the polymerization device 14 without being retained. By preventing the retention in the dispersing device 12A, the polymerization of the monomer composition can be suppressed and the hydrogel-containing body can be formed. When a gel is generated in the dispersing device 12A, the particle diameter of the generated droplets is difficult to be constant.

The 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 through the pipe 31 and the first supply system 55A. The pipe 35 is connected to the second supply system 56A. A part of the organic solvent circulated by the operation of the liquid-feeding pump 18 is continuously supplied to the flow path 54A through the pipe 35 and the second supply system 56A.

The driving unit 60A is operated to rotationally drive the wall 52A. The facing surface 53A of the wall 52A moves relative 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 continuously supplied to the flow path 54A in which the pair of walls 50A and 52A having the opposed surfaces 51A and 53A opposed to each other are relatively moved by the driving unit 60A. 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 where a shear force acts, and is rapidly dispersed in an organic solvent in a droplet form. Further, the droplet-shaped monomer composition is miniaturized.

"dispersing device 12B"

Fig. 3 is a sectional view showing another example of the dispersing device 12B. The dispersing device 12B is a disk-type high-speed rotary shear mixer.

One wall 50B is formed by a non-rotating housing. The other wall 52B is formed of a circular plate having a disk shape and disposed in a freely rotatable manner in the housing. The driving portion 60B is connected to the circular plate. The circular plate is rotationally driven by operating the driving unit 60B. Thus, the wall 50B as one constitutes a fixed wall, and the wall 52B as the other constitutes a movable wall. The inner peripheral surface of the housing and the outer peripheral surface of the circular plate form opposed surfaces 51B, 53B opposed to each other. The facing surfaces 51B and 53B each 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-feeding pump 18 is continuously supplied to the flow path 54B through the pipe 35 and the second supply system 56B.

By operating the driving unit 60B, the wall 52B is rotationally driven. The facing surface 53B of the wall 52B moves relative 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 continuously supplied to the flow path 54B in which the pair of walls 50B and 52B having the opposed surfaces 51B and 53B opposed to each other are relatively moved by the driving unit 60B. 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 where a shear force acts, and is rapidly dispersed in an organic solvent in the form of droplets. Further, the droplet-shaped monomer composition is miniaturized.

"Dispersion unit 12C"

Fig. 4 is a sectional view showing an example of the dispersing device 12C. The dispersing device 12C is a double cylinder type high-speed rotary shear mixer.

The pair of walls 50C, 52C have a cylindrical shape. One wall 50C is formed by a non-rotating outer cylinder having a central hole. The other wall 52C is formed of a solid inner cylinder rotatably disposed in a center hole of the outer cylinder. The driving unit 60C is connected to the inner cylinder. By operating the driving unit 60C, the inner cylinder is rotationally driven. Thus, the wall 50C as one constitutes a fixed wall, and the wall 52C as the other constitutes a movable wall. The inner peripheral surface of the outer cylinder and the outer peripheral surface of the inner cylinder form opposed surfaces 51C, 53C opposed to each other. The facing surfaces 51C and 53C each 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-feeding pump 18 is continuously supplied to the flow path 54C through the pipe 35 and the second supply system 56C.

The driving unit 60C is operated to rotationally drive the wall 52C. The facing surface 53C of the wall 52C moves relative to the facing surface 51C of the facing wall 50C. While the wall 52C is rotated, 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 continuously supplied to the flow path 54C in which the pair of walls 50C and 52C having the opposed surfaces 51C and 53C opposed to each other are relatively moved by the driving unit 60C. 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 where a shear force acts, and is rapidly dispersed in an organic solvent in a droplet form. Further, the droplet-shaped monomer composition is miniaturized. The liquid discharged from the flow path 54C directly falls and is charged into the polymerization apparatus 14.

In the dispersing device 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, the lot size, and the like of the dispersing device in order to realize a preferable shear rate as described below. The rotation speed of the wall 52C is, for example, 100 to 10,000rpm, 500 to 9,000rpm, 1,000 to 8,000 rpm.

Shear velocity in flow path "

The shear rate in the flow path of the dispersing device is preferably 1,000[1/s ] or more. By setting the shear rate to 1,000[1/s ] or more, the shear rate sufficient for dispersing the monomer in the flow path in the organic solvent is obtained, and therefore, the dispersion is good and the primary particle diameter is small. When the primary particle diameter is made smaller, the specific surface area of the water-absorbent resin becomes larger, which contributes to an increase in the water absorption rate. Further, by setting the shear rate to 1,000[1/s ] or more, the time for generating droplets can be shortened. Furthermore, by setting the shear rate to 1,000[1/s ] or more, the amount of the dispersing aid used can be reduced during dispersion. From the above-mentioned viewpoint, the shear rate in the flow path of the dispersing device is preferably 1,000[1/s ] or more, more preferably 2,000[1/s ] or more, still more preferably 3,000[1/s ] or more, and particularly preferably 3,500[1/s ] or more. On the other hand, in order to stably operate the dispersing apparatus, the shear rate is preferably 40,000[1/s ] or less, more preferably 20,000[1/s ] or less, still more preferably 10,000[1/s ] or less, and particularly preferably 6,000[1/s ] or less. The shear rate in the flow path of the dispersing device is preferably 1,000 to 40,000[1/s ], more preferably 2,000 to 20,000[1/s ], still more preferably 3,000 to 10,000[1/s ], and particularly preferably 3,500 to 6,000[1/s ]. Further, when the dispersion apparatus is a double cylinder type, the shear rate in the flow path of the dispersion apparatus is preferably 1,000 to 40,000[1/s ], more preferably 2,000 to 20,000[1/s ], still more preferably 3,000 to 10,000[1/s ], and particularly preferably 3,500 to 6,000[1/s ].

The shear rate is determined by the number of revolutions of the rotor and the width of the flow path (gap; for example, the outer cylinder radius and the inner cylinder radius in the case of a double-cylinder type dispersing device).

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

Shear rate [1/s ] is the moving speed [ m/s ]/clearance (m ]) of a wall (rotor ) moving relative to each other in a dispersing device

When the shape is complicated and the moving speed is difficult to define, the moving speed is set to the maximum moving speed of the liquid contact portion when one of the moving speeds is a fixed wall. When both are moving walls, the moving speed is set to the moving speed at the point where the difference in moving speed becomes maximum. When both the pair of walls rotate, a difference in moving speed is generated. When there are a plurality of gaps (clearances), the narrowest distance is used. When the shearing speed varies depending on the position of the apparatus, the maximum shearing speed is set as the shearing speed in the present specification.

(spray nozzle)

The spray nozzle preferably has the following functions: the monomer composition and the organic solvent are introduced separately so as to pass through the inside thereof without contacting each other, and are contacted and discharged immediately before or immediately after being discharged from the spray nozzle.

Examples of the ejection nozzle include a multi-fluid ejection nozzle such as a two-fluid ejection nozzle, a three-fluid ejection nozzle, or a four-fluid ejection nozzle; double tubes, triple tubes, quadruple tubes, and the like; an ejector, etc. Further, as the two-fluid ejection nozzle, ejection nozzles of a pre-filming (pre-filming) type, a plate-jet (plane-jet) type, a cross-flow type, an external mixing type, an internal mixing type, and a Y-jet type can be exemplified.

As the multi-fluid injection nozzle, commercially available ones can be used, and examples thereof include Mini atom injection MMA manufactured by Co-standing alloy manufacturing, SETOJet manufactured by IKEUCI, gas atomization nozzle SU-HTE91 manufactured by Spraying Systems, MICROZER manufactured by New Silo industries, quad fluid nozzle manufactured by Takazaki motors, and double injection nozzle manufactured by Kawasaki Productwork industries.

Referring to fig. 5, the injection nozzle will be described. The dispersing device 12D illustrated in fig. 5 is constituted by a two-fluid ejector. The dispersing device 12D has a first supply pipe 101 that continuously supplies the monomer composition and a second supply pipe 102 that continuously supplies the organic solvent containing the dispersing 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 solvents containing the monomer composition and the dispersing aid are mixed by contacting with each other outside the dispersing device 12D immediately after being discharged from the first nozzle 103 and the second nozzle 104, respectively (external mixing type). Thereby, fine droplets containing the monomer composition are generated in the organic solvent. In the dispersing device 12D, the first nozzle 103 and the second nozzle 104 are preferably disposed so as to be immersed in an organic solvent filled in the polymerization device. By such arrangement, effects such as prevention of gas entrainment, prevention of clogging of the ejection nozzle, and suppression of droplet confluence can be obtained.

An external mixing type spray nozzle such as the dispersing device 12D is preferable because the spray nozzle is less likely to be internally clogged due to contact between the monomer composition and the organic solvent. The spray nozzle may be a system in which the monomer composition and the organic solvent are contacted and mixed immediately before being discharged from the spray nozzle (internal mixing type) as long as internal clogging of the spray nozzle can be avoided.

"ratio of flow rate of monomer composition/total flow rate of organic solvent and dispersing aid"

In the present invention, the ratio of the flow rate of the monomer composition [ ml/min ] flowing into the dispersing device to the total flow rate [ ml/min ] of the organic solvent and the dispersing aid flowing into the dispersing device (the flow rate of the monomer composition [ ml/min ]/the total flow rate [ ml/min ] of the organic solvent and the dispersing aid) is preferably 0.01 or more, more preferably 0.02 or more, and still more preferably 0.03 or more. When a high-speed rotary shear type stirrer is used, the monomers are dispersed by applying a shear force by a shear field, and therefore a large amount of organic solvent is not required. Therefore, the dispersion is favorably carried out even in the above range. The upper limit of the flow rate [ ml/min ]/the total flow rate [ ml/min ] of the organic solvent and the dispersing aid is not particularly limited, but is preferably 1.00 or less, more preferably 0.40 or less, and still more preferably 0.20 or less.

The materials used in this step will be described below.

Organic solvent "

Preferred examples of the organic solvent include at least 1 organic solvent selected from the group consisting of aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons. Specific examples thereof include aliphatic hydrocarbons such as n-pentane, n-hexane, n-heptane and n-octane; alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane, and decalin; aromatic hydrocarbons such as benzene, toluene, and xylene; halogenated hydrocarbons such as chlorobenzene, bromobenzene, carbon tetrachloride, 1, 2-dichloroethane, and the like. Among these, n-hexane, n-heptane, and cyclohexane are preferable from the viewpoint of ease of acquisition and quality stability. The solvent mixture may be used in the form of a mixture of two or more kinds of solvents.

The temperature of the organic solvent supplied into the dispersion device is controlled so as to reach Td, which will be described later. The boiling point of the organic solvent is preferably 70 ℃ or higher, more preferably 75 to 100 ℃, and still more preferably 80 to 95 ℃ from the viewpoint of operation and polymerization efficiency.

Dispersing assistant "

According to fig. 1, a dispersion aid is added to an organic solvent flowing through a pipe 33 via a pipe 43. In this case, the addition method of the dispersion aid is not particularly limited, and may be a single addition, a plurality of intermittent additions, or a continuous addition. Among these, in the case of continuous operation, it is preferable to add the dispersion aid continuously from the viewpoint of ensuring supply of a sufficient amount of the dispersion aid in the dispersion step and the polymerization step and further suppressing increase in particle size of the water-absorbent resin to be obtained. That is, a preferred embodiment of the present invention has an operation of continuously adding the dispersion aid to the organic solvent.

Here, "continuously adding a dispersion aid to an organic solvent" means: the dispersion aid is allowed to flow into the organic solvent flowing at a predetermined flow rate for at least a predetermined time. Specifically, as shown in fig. 1, the following means: the dispersion aid is flowed into the continuous phase containing the organic solvent flowing through the pipe 33 at a predetermined flow rate for at least a predetermined time through the pipe 43. In this case, the ratio of the flow rate [ ml/min ] of the dispersion aid flowing from the pipe 43 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 flow rate [ ml/min ]) is preferably 0.01 or more. The "at least a certain time" in this definition is preferably a period during which the polymerization step is carried out, and is, for example, 30 minutes or longer, preferably 1 hour or longer.

When the dispersion aid is a solid or liquid having low fluidity, it can be dissolved in a solvent and added. The solvent used in this case is preferably the organic solvent or the solvent used for the monomer composition, and more preferably the same organic solvent as used for the polymerization.

In the present invention, the dispersing aid means: examples of the substance having a function of promoting the formation of fine droplets of the monomer composition or stabilizing the dispersed state of fine droplets containing the monomer composition in the dispersing device include dispersing agents such as surfactants, auxiliary agents such as builders (builders), stabilizers such as protective colloids, and compositions containing them, as long as the heat resistance index described below is satisfied.

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

Method for measuring Heat resistance index

In a 500mL eggplant type flask, 18.4g of propionic acid was taken, and 31.6g of a 23.6 wt% aqueous sodium hydroxide solution was added dropwise thereto while cooling with ice from the outside to prepare a 75 mol% aqueous solution of partially neutralized propionic acid/sodium salt of 45%. Further, 0.01g of a dispersing aid and 100g of n-heptane were added, and the mixture was immersed in an oil bath at 90 ℃ with a cooling tube and stirred under reflux. After 5 hours, the aqueous phase was separated by a liquid separation operation. 0.2g of the aqueous phase separated into 50mL beakers was diluted with 40g of 0.9 wt% physiological saline, and the surface tension (mN/m) at 20 ℃ was measured using a surface tension meter (K11 Autotensiometer manufactured by KRUSS).

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

As the dispersing aid, a known dispersing aid can be used in addition to the dispersing aid having a heat resistance index of 60mN/m or more. When a known dispersing aid is used in combination, the heat resistance index of the mixture is preferably 60mN/m or more. The heat resistance index of the mixture was calculated based on the content ratio of each dispersing aid. For example, when the dispersion aid having the heat resistance index a is contained in an amount of a% by weight based on the whole dispersion aid (mN/m) and the dispersion aid having the heat resistance index B is contained in an amount of B% by weight based on the whole dispersion aid (B/m), the heat resistance index of the mixture is (a × a + B × B)/100 (mN/m).

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

Examples of the ester-based dispersion aid include sucrose fatty acid esters, polyglycerol fatty acid esters, sorbitan fatty acid esters, polyoxyethylene glycerol fatty acid esters, sorbitol fatty acid esters, and polyoxyethylene sorbitol fatty acid esters.

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

The acid value of the dispersing aid is preferably 10 to 100mgKOH/g, more preferably 15 to 90mgKOH/g, and still more preferably 20 to 80 mgKOH/g. The acid value of the dispersion aid is a value measured in accordance with JIS K0070: 1992.

The weight average molecular weight of the dispersing aid is not particularly limited, and is, for example, 1,000 to 100,000. The weight average molecular weight is a value in terms of polystyrene measured by Gel Permeation Chromatography (GPC).

As the dispersion aid having a heat resistance index of 60mN/m or more, any of a synthetic product and a commercially available product can be used. Examples of commercially available products include HIWAX (registered trademark) 1105A, 2203A, 210MP, 220MP, 310MP, 320MP, 405MPF, 4051E, 4052E, 4202E, 4252E, 1120H, 1160H, and the like, manufactured by mitsui chemical corporation; and Licocene (registered trademark) PP MA 1332, PP MA 6252, PE MA 4221, PE MA 4351, manufactured by Clariant corporation, and the like.

The amount of the dispersion aid to be used is appropriately determined depending on the polymerization form, the monomer composition, the kind of the organic solvent, and the like. Specifically, the concentration of the dispersion aid in the organic solvent belonging to the continuous phase (the ratio of the content of the dispersion aid to the total amount of the organic solvent) is preferably 0.0001 to 2% by weight, and more preferably 0.0005 to 1% by weight.

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

[2-3. polymerization Process ]

This step is a step of supplying fine droplets dispersed in an organic solvent to a polymerization apparatus and polymerizing a monomer to obtain a hydrogel polymer.

(polymerization apparatus)

The shape of the polymerization apparatus for carrying out the polymerization reaction is not particularly limited, but is preferably a shape in which the monomer (composition) can move as a dispersed phase in the form of droplets in an organic solvent as a continuous phase formed in the polymerization apparatus to cause the polymerization reaction. Examples of such a polymerization apparatus include a polymerization apparatus in which a tubular reaction tube is arranged in a vertical, horizontal, or spiral manner. When the reaction tube is vertical, the ratio (L/D) of the inner diameter D (mm) to the length L (mm) of the reaction tube is preferably 2 to 100,000, more preferably 3 to 50,000, and still more preferably 4 to 20,000.

When the ratio (L/D) is within the above range, the fine droplets containing the monomer composition move well inside the polymerization apparatus, and thus variation in residence time of the droplets is reduced. Further, since variation in particle diameter of the finally obtained gel polymer is small, the properties of the water-absorbent resin obtained are also improved.

The polymerization apparatus may further include a temperature adjusting means for heating or cooling the continuous phase inside the polymerization apparatus from the outside as necessary. The temperature of the continuous phase in the polymerization apparatus is maintained within a predetermined range by the temperature adjusting means. The temperature adjusting means is not particularly limited, and examples thereof include a method of providing a jacket to the polymerization apparatus, a method of providing a heater, a method of providing a heat insulating material or a heat insulating material, and a method of supplying hot air or cold air. When the organic solvent is resupplied to the polymerization apparatus, the organic solvent is heated by a heat exchanger.

Further, as the material of the polymerization device, stainless steel such as copper, titanium alloy, SUS304, SUS316L, etc.; fluorine resins such as PTEE, PFA and FEP. Among these, from the viewpoint of adhesion of the obtained gel-like polymer, a fluororesin is preferable, and a material having a surface processed by a fluororesin process or the like on the inner wall surface of the polymerization apparatus is more preferably used.

"polymerization temperature"

In the production method of the present invention, the polymerization temperature is set to the temperature of the organic solvent forming the continuous phase (hereinafter referred to as "Td") in the polymerization apparatus.

Since the monomer composition is dispersed in the continuous phase in the form of droplets, the temperature of the monomer composition rapidly increases 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 decomposes with the temperature rise to generate radicals. The polymerization reaction is started by the generated radicals, and a gel-like polymer is formed as the polymerization reaction proceeds.

When the continuous phase in the polymerization apparatus circulates, the formed gel polymer moves inside the polymerization apparatus due to 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 ℃ or more, more preferably 75 ℃ or more, and further preferably 80 ℃ or more, from the viewpoint of the polymerization rate. The upper limit of Td is not particularly limited, and is appropriately selected within a range not exceeding the boiling point of the organic solvent forming the continuous phase from the viewpoint of safety.

Polymerization time "

In the method for producing a water-absorbent resin according to the present invention, the "polymerization time" means: the time is determined from the time of feeding the monomer composition into the polymerization apparatus as a starting point and from the time of discharging the gel-like polymer obtained by the polymerization reaction from the polymerization apparatus as an end point. For example, when the monomer composition is continuously supplied in the form of droplets to the polymerization apparatus and the formed gel-like polymer is continuously discharged from the polymerization apparatus, the time required from the start to the end of one droplet of the monomer composition is referred to. In other words, the time from the start of the 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. This polymerization time corresponds to the residence time of the droplets in the polymerization apparatus.

The polymerization time is controlled depending on the kind of the monomer and the polymerization initiator, and is preferably controlled to 60 minutes or less, more preferably 30 minutes or less, still more preferably 20 minutes or less, particularly preferably 10 minutes or less, and most preferably 5 minutes or less, from the viewpoint of production efficiency. The lower limit of the polymerization time is not particularly limited, and is preferably controlled to 30 seconds or more from the viewpoint of the efficiency of heat transfer from the continuous phase when the temperature of the droplets of the monomer composition supplied into the polymerization apparatus is raised to the polymerization temperature. It is preferable to control the polymerization time within the above range because the polymerization apparatus can be downsized.

"space velocity in polymerization plant (LHSV)"

In the process for producing a water-absorbent resin according to the present invention, the space velocity (LHSV) (unit: hr) in the polymerization apparatus^-1) Is an index showing the passing speed of the monomer composition (hydrogel) and the organic solvent in the polymerization apparatus, and is an index serving as a reference when controlling the polymerization time.

The lower limit of the space velocity (LHSV) in the polymerization apparatus is preferably 2hr from the viewpoint of preventing contact of water-containing gels having different polymerization rates^-1Above, more preferably 3hr^-1Above, more preferably 4hr^-1The above. From the viewpoints of the polymerization ratio of the resulting hydrogel (the amount of residual monomer in the water-absorbent resin particles) and the DRC of the water-absorbent resin of 5min, the upper limit of the space velocity in the polymerization apparatus is preferably 30hr^-1Less than, more preferably 15hr^-1The lower limit is more preferably 12hr^-1The lower, particularly preferably 10hr^-1The following. That is, in one embodiment of the present invention, the space velocity (LHSV) in the polymerization apparatus is preferably 2 to 30hr^-1More preferably 3 to 15hr^-1More preferably 3 to 12hr^-1. Note that the space velocity (LHSV) (unit: hr) in the polymerization apparatus^-1) Is the volume flow rate Qm (unit: m is³Hr), the total volume flow rate Qs of the organic solvent and the dispersing aid (unit: m is³/hr) divided by the volume of the polymerization apparatus V (unit: m is³) The obtained value can be calculated by the following equation.

[ mathematical formula 1]

LHSV[hr-1]＝{(Qm+Qs)/V}

[2-4. separation and Recycling Process ]

This step is a step of separating the hydrous gel-like polymer discharged from the polymerization apparatus in the above-mentioned polymerization step from the organic solvent to obtain a gel-like polymer (hydrous gel), and supplying the separated organic solvent again to the dispersing apparatus.

In this step, the type and structure of the separation apparatus for separating the water-containing gel-like polymer and the organic solvent are not particularly limited, and known methods such as filtration, sedimentation, centrifugal separation, and pressing can be used.

In this step, the organic solvent separated from the hydrogel polymer is preferably supplied to the dispersing device again while maintaining the temperature of the organic solvent at 70 ℃ or higher. This enables the temperature of the organic solvent as the continuous phase to be maintained at 70 ℃ or higher, and therefore, the transition from the dispersion step to the polymerization step can be made quickly. Therefore, the polymerization reaction can be carried out while suppressing coalescence of the fine droplets containing the monomer composition generated in the dispersion step, and a fine hydrogel can be obtained. That is, in one embodiment of the present invention, the temperature of the organic solvent to be resupplied to the dispersing device is 70 ℃ or higher. When the organic solvent is supplied again to the dispersing device, the temperature of the heat exchanger (for example, the heat exchanger 20 in fig. 1) is appropriately adjusted by passing the organic solvent through the heat exchanger, whereby the organic solvent separated from the hydrogel polymer can be supplied again to the dispersing device while maintaining the temperature of the organic solvent at 70 ℃.

"shape of Water-containing gel-like Polymer"

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

The term "spherical" refers to a concept including shapes other than true spheres (for example, substantially spherical), and is particles in which the ratio of the average major axis to the average minor axis (also referred to as "sphericity") of the particles is preferably 1.0 to 3.0. The average major diameter and the average minor diameter of the particles are measured based on an image taken by a microscope. In the present invention, the hydrogel polymer may be formed as an aggregate of fine spherical gels, or may be obtained as a mixture of fine spherical gels and the aggregate of 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, and particularly preferably 20 to 60 μm, from the viewpoint of suppressing the generation of fine particles in the drying step. The average primary particle diameter of the hydrogel-like polymer (hydrogel) is measured by the method described in the following examples.

Concentration of solid content of hydrogel-like Polymer "

The solid content fraction of the hydrogel polymer to be supplied to the drying step described later is not particularly limited, but is preferably 20% by weight or more, more preferably 30% by weight or more, further preferably 40% by weight or more, and particularly preferably 45% by weight or more, from the viewpoint of drying cost. The upper limit of the solid content fraction of the water-containing gel polymer is not particularly limited, but is preferably 90% by weight or less, more preferably 80% by weight or less, further preferably 70% by weight or less, and particularly preferably 60% by weight or less. The effect of the present invention is remarkable when the hydrogel polymer having the solid content fraction in the above range is supplied to a drying step described later.

[2-5. other procedures ]

The method for producing a water-absorbent resin according to the present invention may include, in addition to the above-described steps, a drying step, a pulverizing step, a classifying step, a surface crosslinking step, a size-adjusting step, a fine powder-removing step, a granulating step, and a fine powder-recycling step, as required. Further, the method may further include a transportation step, a storage step, a packaging step, a storage step, and the like. From the viewpoint of obtaining a water-absorbent resin having an excellent absorption capacity under load (AAP), the method for producing a water-absorbent resin according to the present invention preferably further comprises a drying step and a surface crosslinking step in addition to the dispersing step, the polymerization step, and the dispersing and reusing 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 disintegrated or granulated to adjust the particle size or particle size distribution to a desired value, and then subjected to a drying step.

Known methods for drying the hydrogel polymer include, for example, drying by conductive heat transfer, drying by convective heat transfer (for example, hot air), drying by reduced pressure, drying using infrared rays, drying using microwaves, drying by azeotropic dehydration with a hydrophobic organic solvent, and superheated steam drying using high-temperature steam (for example, superheated steam).

However, in the present invention, stirring-type conductive heat transfer drying is preferable which has high drying efficiency and facilitates recovery of liquid components such as organic solvents, and a continuous stirring-type drying apparatus using an indirect heating system is more preferable.

In the present invention, it is preferable to add a gel fluidizing agent to the hydrogel polymer during drying. The addition of the gel fluidizing agent is particularly preferable in the case of treating the particulate water-containing gel in the heat treatment step of the drying step.

The amount of the gel fluidizing agent to be added is appropriately determined depending on the water content of the hydrous gel or the particulate hydrous gel and the type of the gel fluidizing agent. The amount of the surfactant added is preferably 0.001 to 0.5 wt%, more preferably 0.01 to 0.3 wt%, and still more preferably 0.02 to 0.2 wt% based on the solid content of the hydrogel.

As the gel fluidizing agent, for example, a surfactant disclosed in Japanese patent laid-open No. 8-134134 can be used.

Specifically, examples of the surfactant used in the gel fluidizing agent include anionic surfactants such as triethanolamine lauryl sulfate, sodium polyoxyethylene lauryl sulfate, sodium lauryl phosphate, sodium N-coconut oil fatty acid acyl-L-glutamic acid monosodium salt, sodium dodecylbenzenesulfonate and sodium alkylnaphthalenesulfonate; 1:1 type coconut oil fatty acid diethanolamide, polyethylene glycol monostearate, polyoxyethylene sorbitan monolaurate, octyl phenol polyoxyethylene ether, sorbitan monooleate, polyoxyethylene sorbitan monostearate and other nonionic surfactants; cationic surfactants such as stearyl trimethyl ammonium chloride, ethyl lanolin sulfate fatty acid aminopropylethyl dimethyl ammonium, lauryl trimethyl ammonium chloride, and distearyl dimethyl ammonium chloride; amphoteric surfactants such as coconut oil fatty acid amide propyl dimethylamino acetic acid betaine, lauryl dimethyl glycine betaine, 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolium betaine, and lauryl betaine; cationic cellulose, polyethylene glycol, polypropylene glycol and other high molecular surfactants; and known silicon-based surfactants and fluorine-based surfactants.

As described above, the hydrogel polymer formed by the production method of the present invention has a spherical shape. By drying the spherical hydrogel polymer with the stirring type drying apparatus, a dried polymer containing spherical particles can be obtained. The dried polymer containing spherical particles obtained in the drying step may be used as it is for various purposes as a water-absorbent resin. In addition, in the case of producing the water-absorbent resin in this production method, the spherical dried polymer obtained in the drying step may be subjected to the surface crosslinking step described later. In this case, for convenience, the dried polymer to be subjected to the surface crosslinking step described later is referred to as "water-absorbent resin powder".

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

(grinding step and classifying step)

The particulate dried polymer obtained in the above-mentioned drying step is subjected to a pulverization step and a classification step as necessary to prepare a water-absorbent resin having a controlled particle diameter or particle size distribution.

In the above-mentioned pulverizing step, a high-speed rotary pulverizer such as a roll mill, a hammer mill, a screw mill, or a pin mill; a vibration mill, a KNUCKLE type pulverizer, a cylinder type agitator, etc.

In the classification step, for example, sieve classification using a JIS standard sieve (JIS Z8801-1(2000)), air stream classification, or the like can be suitably selected and used.

(surface crosslinking step)

This step is a step of subjecting the water-absorbent resin powder obtained in the drying step to surface crosslinking with a surface crosslinking agent. Specifically, this step is a step of adding a surface cross-linking agent to the water-absorbent resin powder and then performing a heat treatment to provide a portion having a high cross-link density on the surface layer of the water-absorbent resin powder.

The surface-crosslinking agent is not particularly limited, and examples thereof include polyhydric alcohol compounds such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, 1, 3-propanediol, dipropylene glycol, 2, 4-trimethyl-1, 3-pentanediol, polypropylene glycol, glycerin, polyglycerol, 2-butene-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, an ethylene oxide-propylene oxide block copolymer, pentaerythritol, and sorbitol; epoxy compounds such as 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 ether, pentaerythritol polyglycidyl ether, trimethylolpropane polyglycidyl ether, neopentyl glycol diglycidyl ether, 1, 6-hexanediol diglycidyl ether, and the like; polyamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, and polyethyleneimine, and inorganic salts or organic salts thereof; polyisocyanate compounds such as 2, 4-tolylene diisocyanate and hexamethylene diisocyanate; aziridine compounds such as polyaziridine; 1, 2-ethylenebisoxazoline, bisoxazoline, polyoxazoline and other polyoxazoline compounds; carbonic acid derivatives such as urea, thiourea, guanidine, dicyandiamide, 2-oxazolinone and the like; 1, 3-dioxolan-2-one (ethylene carbonate), 4-methyl-1, 3-dioxolan-2-one, 4, 5-dimethyl-1, 3-dioxolan-2-one, 4-dimethyl-1, 3-dioxolan-2-one, 4-ethyl-1, 3-dioxolan-2-one, alkylene carbonate compounds such as 4-hydroxymethyl-1, 3-dioxolan-2-one, 1, 3-dioxane-2-one, 4-methyl-1, 3-dioxane-2-one, 4, 6-dimethyl-1, 3-dioxane-2-one, and 1, 3-dioxepin (dioxopan) -2-one; halogenated epoxy compounds such as epichlorohydrin, epibromohydrin, and α -methyl epichlorohydrin, and polyamine adducts thereof; an oxetane compound; silane coupling agents such as gamma-glycidoxypropyltrimethoxysilane and gamma-aminopropyltriethoxysilane; and polyvalent metal compounds such as hydroxides, chlorides, sulfates, nitrates, and carbonates of zinc, calcium, magnesium, aluminum, iron, and zirconium. Of these, 2 or more kinds may be used in combination. Among the above surface crosslinking agents, 1 or 2 or more selected from polyvalent metal ions, epoxy compounds, oxazoline compounds and alkylene carbonate compounds are preferable.

The amount of the surface-crosslinking agent added is preferably 0.01 to 5% by weight based on the solid content of the water-absorbent resin.

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

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

After the heat treatment, a cooling treatment may be performed. The cooling conditions may be appropriately adjusted.

(finishing Process)

The "particle-sizing step" is a step of adjusting the particle diameter by disintegrating the water-absorbent resin powder which is gradually aggregated through the surface-crosslinking step. The size-adjusting step includes a surface-crosslinking step and a subsequent fine powder-removing step, a gel-breaking step, and a classifying step.

(Process for Recycling Fine powder)

The "fine powder recycling step" refers to a step of supplying the fine powder generated in each step as it is or granulating the fine powder and then supplying the granulated fine powder to an arbitrary step.

[ 3. use of Water-absorbent resin ]

The use of the water-absorbent resin of the present invention is not particularly limited, and examples thereof include additives for resins such as water-blocking preventing materials, paints, adhesives, antiblocking agents, light diffusing agents, matting agents, additives for decorative sheets, additives for artificial marble, and additives for toner. The use of the water-absorbent resin is not particularly limited, and preferable examples thereof include absorbent body uses of absorbent articles such as disposable diapers, sanitary napkins, and incontinence pads. In particular, it is useful as an absorbent for high-density paper diapers in which odor, coloring, and the like originating from the raw material are problematic. Further, the water-absorbent resin has an excellent water absorption time and a controlled particle size distribution, and therefore, a significant effect can be expected when it is used in the upper layer of the absorbent body.

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

When the absorbent body is used in the upper layer portion of the absorbent article, the absorbent article can be maintained in a white state with a clean feeling by setting the core concentration to the above range. Further, since the absorbent body is excellent in the diffusibility of body fluids such as urine and blood, it is expected that the absorption amount is increased by performing efficient liquid distribution.

[ 4. physical Properties of Water-absorbent resin ]

Particle shape of Water-absorbent resin "

The present invention also provides a water-absorbent resin produced by the production method described in [ 2] above-mentioned production method for a water-absorbent resin. In the present invention, the polymerization is carried out by so-called reversed-phase suspension polymerization. The water-absorbent resin thus obtained is usually in the form of spherical polymer particles. Here, "spherical" also includes shapes other than true spheres. Specifically, "spherical" refers to particles in which the ratio of the average major axis to the average minor axis (also referred to as sphericity) of the particles is preferably 1.0 to 3.0. The average major diameter and the average minor diameter of the particles were determined based on images observed by a microscope. In the present invention, the "spherical polymer particles" are not limited to the form of individual particles, and may be formed as an aggregate of spherical polymer particles.

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

When the particle shape is a spherical aggregate, particularly a spherical aggregate, the water absorbing rate of the water absorbent resin tends to be higher than that of the irregular shape.

"average primary particle diameter"

From the viewpoint of further improving the absorption rate, the upper limit of the average primary particle diameter of the water-absorbent resin is preferably less than 100 μm, and more preferably 80 μm or less. The lower limit of the average primary particle diameter 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 was measured by the method described in examples.

“CRC”

"CRC" is an abbreviation for the centrifugal Retention Capacity, and refers to the water absorption Capacity (sometimes referred to as "water absorption Capacity") of the water-absorbent resin without pressure. CRC (centrifuge Capacity) was determined according to EDANA method (ERT 441.2-02). Specifically, the water absorption capacity (unit; G/G) was determined by filling 0.2G of a water-absorbent resin in a nonwoven fabric bag, immersing the bag in a large excess of 0.9 wt% aqueous sodium chloride solution for 30 minutes to allow the bag to freely swell, and then controlling the water content in the bag by a centrifugal separator (250G) for 3 minutes.

Incidentally, "EDANA" is an abbreviation for European Disposables and Nonwovens associates. "ERT" is an abbreviation for EDANA Recommendated Test Methods, and is a European standard for specifying the method of measuring a water-absorbent resin. In the present invention, the physical properties of the water-absorbent resin are measured in accordance with ERT standard (revised 2002) unless otherwise specified.

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

When the CRC is less than 15g/g, the absorption amount is small, and the CRC is not suitable as an absorbent body of an absorbent article such as a disposable diaper. When the CRC exceeds 70g/g, the rate of absorption of body fluids such as urine and blood decreases, and therefore, the CRC may not be suitable for use in a high-rate-of-absorption type diaper. The CRC can be controlled by changing the kind and amount of the internal crosslinking agent, the surface crosslinking agent, and the like.

“DRC5min”

"DRC" is an abbreviation for Dunk Retention Capacity, "DRC 5 min" means a value of 5 minutes of the Retention Capacity (5 minutes and water absorption Capacity without pressure). Specifically, the water absorption capacity (unit; g/g) was determined by uniformly spreading 1.0g of a water-absorbent resin on a cylindrical dish having a mesh on the bottom surface, and allowing the water-absorbent resin to freely swell by contacting the water-absorbent resin with a 0.9 wt% aqueous solution of sodium chloride for 5 minutes in the same manner as in the measurement of AAP described below.

The lower limit of DRC5min of the water-absorbent resin is preferably 46g/g or more, more preferably 47g/g or more, still more preferably 50g/g or more, and particularly preferably 52g/g or more, from the viewpoint of the amount of liquid returned when used in a sanitary material. The upper limit of DRC5min of the water-absorbent resin is not particularly limited, but is usually 70g/g or less.

“AAP”

"AAP" is an abbreviation for adsorption age Pressure, and means the water Absorption capacity under Pressure of the water-absorbent resin. Specifically, the water-absorbent resin was used in an amount of 0.9g, which was an excess of 0.9 wt% of the aqueous sodium chloride solution, of 1 hour and 2.06kPa (21 g/cm)²0.3psi) under a load, and the water absorption capacity after swelling (unit; g/g). In the present specification, it is defined that the load condition is changed to 4.83kPa (about 49 g/cm)²Equivalent to about 0.7 psi).

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

Surface tension "

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

Accordingly, a water-absorbent resin according to one embodiment of the present invention is a water-absorbent resin obtained by reversed-phase suspension polymerization and having a surface tension of 65mN/m or more and a DRC5min of 46g/g or more. The water-absorbent resin has excellent absorption characteristics (a small amount of liquid returned) when used in a sanitary material.

Further, a water-absorbent resin according to an embodiment of the present invention is a water-absorbent resin produced by the production method described in [ 2] above, and has a surface tension of 65mN/m or more and a DRC5min of 46g/g or more.

Examples

The effects of the present invention will be described with reference to the following examples and comparative examples, but the present invention is not to be construed as being limited to these descriptions, and examples obtained by appropriately combining the technical means disclosed in the respective examples are also included in the scope of the present invention. The physical properties of the hydrogel, the water-absorbent resin powder, the water-absorbent resin, and the absorbent material were measured by the following methods. In the examples, "part" or "%" may be used, but unless otherwise specified, "part by weight" or "% by weight" is used. Unless otherwise specified, each operation was carried out at room temperature (25 ℃).

"average primary particle diameter"

A Scanning Electron Micrograph (SEM) of the water-absorbent resin or the water-absorbent resin powder was taken. The average primary particle diameter of the water-absorbent resin was determined by selecting 50 primary particles arbitrarily from the photograph, measuring the major and minor diameters of each particle, averaging the values to obtain the primary particle diameters, calculating the average primary particle diameter of each particle, and determining the average primary particle diameter.

“CRC”

CRC of the water-absorbent resin was measured according to EDANA method (ERT 441.2-02).

"residual monomer amount"

The residual monomer content of the water-absorbent resin was measured according to EDANA method (ERT 410.2-02).

Surface tension "

50ml of physiological saline adjusted to 20 ℃ was put into a well-washed 100ml beaker, and first, the surface tension of the physiological saline was measured using a surface tensiometer (K11 Autotensiometer manufactured by KRUSS Co.). In this measurement, it was confirmed that the surface tension value was in the range of 71 to 75[ mN/m ]. Subsequently, a well-cleaned 25mm long rotor made of a fluororesin and 0.5g of the water-absorbent resin (1) were put into a beaker containing physiological saline adjusted to 20 ℃ and subjected to surface tension measurement, and stirred at 500rpm for 4 minutes. After 4 minutes, the stirring was stopped, and after the water-absorbent resin containing water had settled, the same operation was repeated to measure the surface tension of the supernatant. In the plate method using a platinum plate according to the present invention, the plate was sufficiently washed with deionized water before each measurement, and was used after being heated and washed with a gas spray gun.

Water content "

The water content of the water-absorbent resin was measured by EDANA method (ERT 430.2-02). In the present invention, the sample amount was changed to 1.0g and the drying temperature was changed to 180 ℃.

Particle size "

The Particle size (Particle size Distribution, weight Average Particle size (D50), and Logarithmic Standard Deviation of Particle size Distribution (. sigma.. zeta.) of the water-absorbent resin were measured in accordance with "(3) Mass-Average Particle Diameter (D50) and Log arithmic Standard development (. sigma.. zeta.) of Particle Diameter Distribution described in columns 27 and 28 of U.S. Pat. No. 7638570).

“DRC5min”

The DRC of the water-absorbent resin (1) was measured for 5min (value of 5 minutes of immersion holding capacity) by the method described in International publication No. 2017/170605 (U.S. patent application publication No. 2019/111411).

Specifically, using the apparatus shown in FIG. 6, a 400-mesh metal mesh 201 (mesh size: 38 μm) made of stainless steel was welded to the bottom of a plastic supporting cylinder 200 having an inner diameter of 60mm, and 2021.000. + -. 0.005g of a water-absorbent resin (1) was uniformly dispersed on the metal mesh 201 at room temperature (20 to 25 ℃ C.) and a relative humidity of 50% RH, and the weight Wa (g) of the entire apparatus was measured.

The bottom area is 400cm²A glass filter 204 (made of chemical Nitro, pore diameter: 100 to 120 μm) having a diameter of 120mm was placed on the inner side of the round or square petri dish 203, and 0.90 wt% of a saline solution 206 (23. + -. 0.5 ℃) was added so that the upper surface of the glass filter became the same level (a state where the liquid slightly floats on the outer periphery of the glass filter due to surface tension or a state where about 50% of the surface of the glass filter is covered with the liquid). On which 1 sheet of 110mm diameter paper is placedThe filter paper 205 (product name of ADVANTEC Toyo Co.) (JIS P3801, No.2), 0.26mm thick, 5 μm retained particle size) was wetted over the entire surface thereof.

The entire measuring apparatus was placed on the wet filter paper to absorb the liquid (the liquid temperature was strictly controlled to 23 ± 0.5 ℃ during the measurement). Strictly speaking, after 5 minutes (300 seconds), the entire measuring apparatus was lifted up, and the weight wb (g) thereof was measured. DRC5min (g/g) was calculated from Wa and Wb according to the following formula.

[ mathematical formula 2]

DRC5min [ g/g ] ═ { (Wb-Wa)/(weight of Water-absorbent resin) }

Evaluation of absorbent body (liquid Return amount) "

After 2g of the water-absorbent resin and 2g of the wood-pulverized pulp were dry-mixed by using a stirrer, the resulting mixture was spread on a 400-mesh (38 μm-mesh) wire screen to form a fiber web (web) having a diameter of 90 mm. Next, the pressure was adjusted at 196.14kPa (2[ kgf/cm ]²]) The web was pressed for 1 minute under the pressure of (2) to prepare an absorbent body. The absorbent body (diameter 90 mm/core concentration 50%) was placed on the bottom of a SUS petri dish having an inner diameter of 90mm, a nonwoven fabric having a diameter of 90mm was placed thereon, and a piston and a weight were placed thereon so as to be adjusted to apply a load of 4.8kPa uniformly to the absorbent body. In addition, the piston and the weight were used as objects having a liquid inlet with a diameter of 5mm at the center. Then, 50mL of physiological saline (0.90 wt% aqueous sodium chloride solution) was injected from the liquid inlet to allow the absorbent body to absorb the liquid. After 5 minutes, the piston and the weight were removed, and 30 sheets of filter paper (ADVANTEC Toyo Co., Ltd., product name: JIS P3801, No.2) having an outer diameter of 90mm and a total weight measured in advance were placed thereon, and the piston and the weight to which a load was applied uniformly (total weight: 20kg) were quickly placed thereon. 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, thereby determining the amount of liquid (g) absorbed by the filter paper. This liquid amount was defined as a liquid reflux amount (g).

[ example 1]

A water-absorbent resin (1) was produced by preparing a water-containing gel (1) by performing a series of steps 2 to 5 described below according to the production process shown in FIG. 1, and then drying the water-containing gel (1). The specific operating time is 10 hours from the start of the supply of the monomer composition to the dispersion apparatus in step 2 described below.

First, n-heptane (density: 0.68g/ml) as an organic solvent was charged into the dispersing apparatus 12, the polymerizing apparatus 14, the separating apparatus 16 and the pipes connecting them.

Subsequently, the liquid-sending pump 18 was operated to start circulation of the organic solvent at a flow rate of 300 ml/min. The total amount of the organic solvent is charged into the polymerization apparatus 14 through the dispersing apparatus 12. Further, the heat exchanger 20 is operated to heat the organic solvent circulating as described above to 90 ℃.

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

(1. mixing Process)

An aqueous monomer solution (1) was prepared by mixing acrylic acid, a 48.5 wt% aqueous sodium hydroxide solution and ion-exchanged water, and further compounding polyethylene glycol diacrylate (average degree of polymerization: 9) and trisodium diethylenetriaminepentaacetate. Further, sodium persulfate and ion-exchanged water were separately mixed to prepare a 6 wt% aqueous solution of sodium persulfate (1).

Next, the aqueous monomer solution (1) and the aqueous sodium persulfate solution (1) obtained by the above operations are supplied to a mixing apparatus 10, thereby preparing a monomer composition (1). The monomer concentration of the monomer composition (1) was 43% by weight, and the neutralization rate was 75 mol%. Further, polyethylene glycol diacrylate as an internal crosslinking agent was 0.020 mol% with respect to the monomer, trisodium diethylenetriaminepentaacetate as a chelating agent was 200ppm with respect to the monomer, and sodium persulfate (T1070 ℃ C.) as a polymerization initiator was 0.1 g/mol with respect to the monomer.

(2. dispersing step)

As the dispersing device, a double cylinder type high-speed rotary shear mixer (dispersing device 12C) shown in fig. 4 was used. The inner diameter of the casing (inner diameter of the outer cylinder 50C) was 25mm, the outer diameter of the rotor (outer diameter of the inner cylinder 52C) was 22mm, and the effective rotor length (from the monomer aqueous solution inlet 55C to the outlet) was 65 mm. As the polymerization apparatus, a pipe made of PFA (perfluoroalkoxyalkane) (inner diameter: 25mm, total length: 10m) was vertically arranged.

The mixed liquid of the organic solvent and the dispersion aid was fed to the pipe 35 of the dispersing device 12C at a flow rate of 300 mL. 30 minutes after the end of the addition of the dispersion aid solution (1) before the start of polymerization, the rotor (inner cylinder 52C) was rotated so that the rotation speed became 7,200rpm (shear rate 5529[1/s ]), and then the monomer composition (1) was fed into the pipe 31 of the dispersing apparatus 12C at a flow rate of 40 ml/minute (47.2 g/minute). The supplied monomer composition (1) is dispersed in the organic solvent in the form of fine droplets by a dispersing device.

(3. polymerization Process)

The dispersion liquid obtained in the above step 2 is supplied to a polymerization apparatus 14.

The droplets containing the monomer composition (1) become a fine spherical hydrogel (1) as the polymerization reaction proceeds while they fall in a polymerization apparatus filled with the organic solvent as the continuous phase. These tiny spherical gels adhere to each other and form aggregates as they fall. In the vicinity of the outlet of the polymerization apparatus, a hydrogel-containing solution (1) having a diameter of about 1cm, which was formed of an aggregate of fine spherical gels, was observed. The space velocity (LHSV) in the polymerization apparatus 14 was 4.2hr^-1。

The aqueous gel (1) obtained by the above-mentioned series of operations is continuously discharged from the polymerization apparatus 14 together with the organic solvent.

(4. separation and Recycling Process)

The aqueous gel (1) and the organic solvent discharged from the polymerization apparatus 14 are directly and continuously supplied to the separation apparatus 16. In the separation apparatus, the aqueous gel (1) and the organic solvent are separated. The organic solvent separated in the separation apparatus is supplied to the heat exchanger 20 through the pipe 32, the liquid feeding pump 18, and the pipe 33, and after the temperature is adjusted by the heat exchanger 20 so that the set temperature (organic solvent temperature) reaches 90 ℃, the organic solvent is supplied to the dispersion apparatus 12 and the polymerization apparatus 14 through the pipe 35 while being maintained at 70 ℃ or higher. At this time, the dispersion aid solution (1) as a dispersion aid for replenishment was continuously charged into the continuous phase containing the organic solvent flowing through the pipe 33 at a flow rate of 5 ml/min for 10 minutes from the start of feeding the liquid monomer composition into the dispersion apparatus via the pipe 43. That is, the dispersing aid flow rate [ ml/min ]/continuous phase flow rate [ ml/min ] was 0.017. The amount of the dispersing aid (maleic anhydride-modified polyethylene) added was 0.005% by weight based on the monomer composition (1).

The water-containing gel (1) obtained by the above-described operation has a shape in which fine spherical water-containing gels adhere and aggregate.

(5. drying step)

The hydrogel (1) discharged from the separation device 16 was directly and continuously supplied to an indirect heating type stirring and drying device, and a previously prepared ethanol solution (concentration: 20 wt%) of polyethylene glycol 400(PEG400) was added thereto. The amount of the ethanol solution of PEG400 was 2.5% by weight with respect to the aqueous gel (1). Subsequently, the temperature of the heating medium of the drying apparatus was adjusted to 180 ℃, and the hydrogel (1) was continuously dried while mixing with PEG400, to obtain a granular dried polymer (1). The obtained dry polymer (1) was continuously fed to a sieving device having a metal mesh (JIS standard sieve) with mesh openings of 850 μm and 150 μm, and classified, 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 69mN/m, and the surface tension of the water-absorbent resin powder (1) sampled 5 hours after the start of the polymerization step was 69mN/m, and no decrease in the surface tension was observed with time.

The steps 1 to 5 were carried out for 10 hours, and the samples immediately after the start of polymerization and after the end of polymerization, in which the discharge amount was unstable, were removed and mixed to obtain a water-absorbent resin powder (1).

[ example 2]

A water-absorbent resin powder (2) was obtained in the same manner as in example 1 except that a dispersion aid solution (2) (concentration: 0.30% by weight) prepared by mixing a maleic anhydride-modified ethylene-propylene copolymer (acid value: 30mgKOH/g) as a dispersion aid into n-heptane and heating to 90 ℃ for dissolution was used in place of the dispersion aid solution (1) in example 1. The heat resistance index of the dispersion aid, as measured by the above method, was 72 mN/m. The proportion of the content of the maleic anhydride-modified ethylene-propylene copolymer relative to the total amount of the organic solvent before the start of polymerization was 0.05% by weight. The amount of the maleic anhydride-modified ethylene-propylene copolymer continuously fed to the organic solvent during the polymerization was 0.05% by weight based on the monomer composition (1). The surface tension of the water-absorbent resin powder (2) sampled 1 hour after the start of the polymerization was 70mN/m, and the surface tension of the water-absorbent resin powder (2) sampled 5 hours after the start was 70mN/m, and no decrease in the surface tension was observed with time.

[ example 3]

A water-absorbent resin powder (3) was obtained in the same manner as in example 1, except that in example 1, the dispersion step was carried out using a two-fluid injection nozzle (dispersion device 12D) shown in FIG. 5 in place of the double-cylinder high-speed rotary shear type stirrer.

Specifically, as the dispersion device, a two-fluid jet nozzle (external mixing type, jet nozzle inner diameter: 1.0mm, type: SETOJet, air consumption division: 075, jet amount: 10, manufactured by IKEUSHI corporation) was used. The two-fluid spray nozzle has a first supply pipe 101 for continuously supplying the monomer composition and a second supply pipe 102 for continuously supplying a mixed liquid of the organic solvent and the dispersion aid. The monomer composition is sprayed and dispersed from the first nozzle 103 and the mixed solution of the organic solvent and the dispersion aid is sprayed and dispersed from the second nozzle 104, and continuously discharged to the polymerization apparatus. At this time, the position of the two-fluid injection is adjusted so that the tip of the two-fluid injection nozzle is immersed in the organic solvent contained in the polymerization apparatus. Further, the flow rate of the mixed liquid of the organic solvent and the dispersion aid to be circulated was changed to 1000 ml/min, and the path of the mixed liquid of the organic solvent and the dispersion aid to be circulated was branched into a path to be fed into the polymerization apparatus by the dispersion apparatus (two-fluid spray nozzle) and a path to be directly fed into the polymerization apparatus. At this time, the flow rate of the mixed liquid of the organic solvent and the dispersion aid charged into the polymerization apparatus via the dispersion apparatus (two-fluid spray nozzle) was set to 800 ml/min, and the flow rate of the mixed liquid of the organic solvent and the dispersion aid charged directly into the polymerization apparatus was set to 200 ml/min. Next, the monomer composition (1) prepared by the mixing step is quickly fed into the first supply pipe 101 of the two-fluid ejector. Thereafter, the monomer composition (1) was charged into the organic solvent filled in the polymerization apparatus at a flow rate of 40 mL/min (47.2 g/min) using the above two-fluid injector.

The monomer composition (1) introduced through the two-fluid injector is dispersed in the organic solvent in the form of fine droplets. The space velocity (LHSV) in the polymerization apparatus 14 was set to 12.7hr^-1。

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

[ example 4]

A water-absorbent resin powder (4) was obtained in the same manner as in example 1 except that in example 1, a dispersion aid solution (4) (concentration: 0.030 wt%) was used in place of the dispersion aid solution (1), which was prepared by mixing maleic anhydride-modified polypropylene (acid value: 18mgKOH/g) as a dispersion aid into n-heptane and heating to 90 ℃ to dissolve the mixture. The heat resistance index of the dispersion aid, as measured by the above method, was 70 mN/m. The content of maleic anhydride polypropylene was 0.005% by weight based on the total amount of the organic solvent before the start of polymerization. The amount of the maleic anhydride polypropylene continuously charged into the organic solvent during the polymerization was 0.005% by weight based on the monomer composition (1). The surface tension of the water-absorbent resin powder (4) sampled 1 hour after the start of the polymerization was 67mN/m, and the surface tension of the water-absorbent resin powder (4) sampled 5 hours after the start was 67mN/m, and no decrease in the surface tension was observed with time.

Comparative example 1

A comparative water-absorbent resin powder (1) was obtained in the same manner as in example 3 except that a dispersion aid solution (3) (concentration: 0.030% by weight) was used in example 3, instead of the dispersion aid solution (1), which was prepared by mixing a sucrose fatty acid ester (HLB value: 6) as a dispersion aid into n-heptane and heating to 90 ℃. 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 the polymerization was 67mN/m, and the surface tension of the comparative water-absorbent resin powder (2) sampled 5 hours after the start of the polymerization was 60mN/m, and a decrease in the surface tension was observed with time.

The physical properties of the water-absorbent resin powders (1) to (4) and the comparative water-absorbent resin powder (1) are shown in Table 1.

[ Table 1]

[ example 5]

(6. surface crosslinking step)

With respect to 100 parts by weight of the water-absorbent resin powder (1) obtained in example 1, a surface cross-linking agent solution containing 0.015 part 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 was sprayed by a spray nozzle, and uniformly mixed using a continuous high-speed mixer. Thereafter, the water-absorbent resin powder (1) containing the surface-crosslinking agent was introduced into a heat treatment machine in which the atmospheric temperature was adjusted to 195 ℃. + -. 2 ℃ and heat-treated for 30 minutes, followed by forced cooling until the powder temperature reached 60 ℃ to obtain a water-absorbent resin (1').

(7. finishing Process)

Next, the water-absorbent resin (1') obtained in the above-mentioned step was classified by a sieving device having a metal sieve (JIS standard sieve) with a mesh opening of 850. mu.m. The residue on the metal sieve having a mesh opening of 850 μm was pulverized again and then mixed with the metal sieve having a mesh opening of 850 μm. By the above operation, a whole-particle water-absorbent resin (4) having a particle diameter of less than 850 μm in total was obtained. The physical properties of the resulting water-absorbent resin (4) are shown in Table 2.

[ example 6]

A water-absorbent resin (5) was obtained in the same manner as in example 5, except that in example 5, the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (2) obtained in example 2.

[ example 7]

A water-absorbent resin (6) was obtained in the same manner as in example 5, except that in example 5, the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (3) obtained in example 3.

Comparative example 2

A comparative water-absorbent resin (2) was obtained in the same manner as in example 5, except that in example 5, the water-absorbent resin powder (1) was changed to the comparative water-absorbent resin powder (1) obtained in comparative example 1.

[ example 8]

A water-absorbent resin powder (7) was obtained in the same manner as in example 1 except that in example 1, the internal crosslinking agent polyethylene glycol diacrylate was changed to 0.010 mol% with respect to the monomer, and the alcohol solution of polyethylene glycol 400(PEG400) added in the drying step was changed to a lauryl dimethylamino acetic acid betaine aqueous solution (concentration: 3.1 wt%), and the amount of the lauryl dimethylamino acetic acid betaine aqueous solution to the hydrogel (1) was changed to 0.5 wt%. A water-absorbent resin (7) was obtained in the same manner as in example 5, except that in example 5, the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (7).

[ example 9]

A water-absorbent resin (8) was obtained in the same manner as in example 5, except that in example 5, the water-absorbent resin powder (1) was changed to the water-absorbent resin powder (4) obtained in example 4.

Comparative example 3

A1 liter four-necked cylindrical round-bottomed separation flask equipped with a reflux condenser, a dropping funnel, a nitrogen introduction tube, and a stirring blade having 4 inclined blades with a blade diameter of 50mm in two stages as a stirrer was prepared. The flask was charged with 550ml of n-heptane, 2.76g of the maleic anhydride-modified polyethylene used in example 1 was added and dispersed, the temperature was raised to 50 ℃ and the dispersion aid was dissolved and then cooled to 30 ℃. Separately, 92g of an 80 wt% acrylic acid aqueous solution was weighed into a 500ml Erlenmeyer flask, 152.5g of a 20.1 wt% sodium hydroxide aqueous solution was added dropwise while cooling with ice from the outside, and after neutralization at 75 mol%, 18.4mg of ethylene glycol diglycidyl ether and 0.11g of potassium persulfate were added and dissolved. The aqueous solution of partially neutralized acrylic acid salt was charged into a four-necked flask, dispersed by a stirrer, and the inside of the system was sufficiently replaced with nitrogen, and then the temperature was raised to maintain the bath temperature at 70 ℃ to perform the first-stage polymerization reaction for 30 minutes. Thereafter, the reaction solution was cooled to 20 ℃, an equal amount of an acrylic acid partially neutralized aqueous salt solution prepared in the same manner as described above was added dropwise to the system, and the inside of the system was sufficiently replaced with nitrogen gas while allowing the solution to absorb the solution for 30 minutes, and then the temperature was raised to maintain the bath temperature at 70 ℃ to perform a second-stage polymerization reaction for 30 minutes. This reaction solution was distilled to remove water and n-heptane, to obtain a comparative water-absorbent resin powder (3). A comparative water-absorbent resin (3) was obtained in the same manner as in example 5, except that the water-absorbent resin powder (1) in example 5 was changed to the comparative water-absorbent resin powder (3).

The physical properties of the water-absorbent resins (4) to (8) and the comparative water-absorbent resins (2) to (3) are shown in Table 2.

[ Table 2]

(Table 2)

As shown in Table 1, according to the production process of the present invention, even when the continuous operation was carried out, no decrease in the surface tension of the water-absorbent resin powder with time was observed. Further, as shown in table 2, the water-absorbent resin obtained by surface-crosslinking the water-absorbent resin powder had an excellent balance between DRC5min and surface tension, and was within a range that exhibited excellent water absorption characteristics when used in a sanitary material. Therefore, the following steps are carried out: by using the water-absorbent resin of the present invention, an absorbent body having a small liquid reflux amount and high performance can be obtained. On the other hand, when a dispersion aid having a heat resistance index of less than 60mN/m was used (comparative example 1), the surface tension of the water-absorbent resin powder was observed to be lowered with time by continuous operation. Further, the water-absorbent resin obtained by surface-crosslinking the water-absorbent resin powder has a low surface tension. Further, when a batch operation was carried out without continuously supplying the monomer composition, the organic solvent and the dispersing aid to the dispersing apparatus (comparative example 3), the DRC of the water-absorbent resin obtained was low at 5 min. From table 2, it can be seen that: when the water-absorbent resin obtained in any of the comparative examples was used in an absorbent body, the liquid reflux amount increased.

The present application is based on japanese patent application No. 2018-182114, applied on 27/9/2018, and japanese patent application No. 2019-128663, applied on 10/7/2019, the disclosures of which are incorporated by reference in their entirety.

Description of the reference numerals

10 a mixing device,

12. 12A to 12D dispersion apparatus,

14 polymerization apparatus,

16 a separating device,

18 liquid feeding pump,

20 heat exchanger,

22 a drying device,

31 to 37, 41 to 44 pipes,

A pair of walls 50A and 52A,

A pair of walls 50B and 52B,

A pair of walls 50C and 52C,

51A and 53A opposed faces,

51B and 53B opposed surfaces,

51C and 53C opposed surfaces,

54A to 54C flow paths,

60A-60C driving part,

55A to 55C first supply system,

56A-55C second supply system

101 first supply pipe

102 second supply pipe

103 first nozzle

104 second nozzle

200 support cylinder

201 metal net

202 Water-absorbent resin

203 culture dish

204 glass filter

205 filter paper

206 saline solution.