JP2018119142A - Method for producing crosslinked polymer and method for producing water-absorbing resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a hydrous gel of a crosslinked polymer having suitable mechanical physical properties, in order to more efficiently conduct crushing treatment of the hydrous gel of the crosslinked polymer, without causing the deterioration of the water-absorbing ability (e.g. water retention ability) of a water-absorbing resin as an end product.SOLUTION: A method for producing a crosslinked polymer has the step in which: in an aqueous solution containing a water-soluble ethylenic unsaturated monomer and an inorganic filler, the water-soluble ethylenic unsaturated monomer is polymerized.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a crosslinked polymer as a raw material for a water absorbent resin. Furthermore, it is related with the manufacturing method of the water absorbing resin using the said crosslinked polymer.

  In recent years, water-absorbent resins have been widely used in various absorbent articles such as sanitary materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water retention agents and soil conditioners, and industrial materials such as water-stopping agents and anti-condensation agents. It is used. Many types of water-absorbing resins are known depending on the application, but water-absorbing resins made of a polymer of a water-soluble ethylenically unsaturated monomer are mainly used. Examples of the production method include a method in which a crosslinked polymer is produced by aqueous solution polymerization, suspension polymerization or the like and then dried.

JP-A-62-2223203 JP-A-9-124710

  A cross-linked polymer obtained by aqueous polymerization of a water-soluble ethylenically unsaturated monomer is obtained as a mass of viscous agar-like or jelly-like hydrogel, depending on the type of polymerization reactor. . Thereafter, the water-containing gel of the crosslinked polymer is subjected to a drying treatment to remove water in order to make the water-absorbent resin, but in order to efficiently perform this drying treatment, the water-containing gel of the crosslinked polymer prior to the drying treatment is Usually it needs to be crushed. However, when the water-containing gel of the crosslinked polymer is a viscous material, the water-containing gel of the cross-linked polymer has high toughness, spreadability, etc., and the water-containing gel is not easily subdivided even through a crushing device. In addition, the crushed gel particles adhere to each other to form a lump again, or the hydrated gel particles adhere to the crushing device and stay for a long time until they are crushed to a desired size. Exposure to the mechanical load of this leads to deterioration of the crosslinked polymer. For this reason, in order to perform the crushing treatment of the water-containing crosslinked polymer more efficiently without lowering the water absorption performance (water retention ability, etc.) of the water absorbent resin finally obtained, the crosslinking having suitable mechanical properties There is a need to obtain polymeric hydrogels in polymerization.

  In Patent Document 1, the presence of a binary metal hydroxide having an anion exchange ability during azeotropic dehydration of a water-absorbing polymer containing an alkali acrylate obtained by polymerizing monomers as a constituent of a polymer. A method for producing a highly-expandable water-absorbing polymer, characterized by crosslinking with a crosslinking agent having two or more functional groups and then drying, has been proposed. However, in the method described in Patent Document 1, since the hydrogel is dried at the time of azeotropic dehydration, it can be practically performed only by suspension polymerization, and has a crosslinking agent and anion exchange ability after polymerization. The process of adding a binary metal hydroxide is essential.

  In Patent Document 2, in order to increase the absorption capacity (water retention capacity) of the water-absorbing resin without applying pressure, the water-absorbing water obtained by further surface cross-linking the water-absorbing resin precursor obtained by aqueous solution polymerization in the presence of phosphite. A functional resin is disclosed. However, in such a production method, since the crosslinking inside the water-absorbent resin precursor at the time of polymerization is weakened, the resulting hydrogel is highly adhesive. That is, if an attempt is made to increase the absorption capacity under no pressure, it becomes difficult to crush the water absorbent resin precursor.

As a result of intensive studies to solve the above problems, the present inventors polymerize a water-soluble ethylenically unsaturated monomer in an aqueous solution containing a water-soluble ethylenically unsaturated monomer and an inorganic filler. It has been found that the above problems can be solved.
That is, the present invention
Provided is a method for producing a crosslinked polymer, comprising a step of polymerizing the water-soluble ethylenically unsaturated monomer in an aqueous solution containing a water-soluble ethylenically unsaturated monomer and an inorganic filler.
Furthermore, the manufacturing method of the water absorbing resin using the said crosslinked polymer is also provided.

  The crosslinked polymer obtained by the production method of the present invention is likely to be crushed in a hydrogel state. That is, it shows high brittleness. Furthermore, the adhesion between the particles after crushing and the adhesion to the crushing apparatus are also low. Therefore, according to the production method of the present invention, since the hydrogel is efficiently crushed, the mechanical load until it is crushed to a desired size is small, and the crosslinked polymer is hardly deteriorated. The water-absorbent resin obtained from this crosslinked polymer has a feature of excellent water absorption performance.

  Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments.

  The present invention relates to a method for producing a crosslinked polymer obtained by polymerizing a water-soluble ethylenically unsaturated monomer, characterized by polymerizing in the presence of an inorganic filler. . In the present invention, the water-soluble ethylenically unsaturated monomer solution (hereinafter also referred to as “monomer aqueous solution”) containing at least a water-soluble ethylenically unsaturated monomer and an inorganic filler is used. By polymerizing the saturated monomer, a crosslinked polymer in a hydrogel state can be obtained. The obtained hydrogel of the crosslinked polymer is crushed, dried, and pulverized as necessary to obtain a water absorbent resin. The water absorbent resin may be surface-crosslinked with a surface cross-linking agent.

<Aqueous monomer solution>
The aqueous monomer solution in the present invention contains at least a water-soluble ethylenically unsaturated monomer and an inorganic filler.

[Water-soluble ethylenically unsaturated monomer]
As the water-soluble ethylenically unsaturated monomer used in the present invention, for example, (meth) acrylic acid (in this specification, “acryl” and “methacryl” are collectively referred to as “(meth) acryl”). The same shall apply hereinafter.), Carboxylic acid monomers such as maleic acid, maleic anhydride, fumaric acid and the like, α, β-unsaturated carboxylic acids and salts thereof; (meth) acrylamide, N, N-dimethyl (meth) acrylamide Nonionic monomers such as 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate; N, N-diethylaminoethyl (meth) acrylate, N, N-diethylamino Amino group-containing unsaturated monomers such as propyl (meth) acrylate and diethylaminopropyl (meth) acrylamide, and quaternized products thereof Vinylsulfonic acid, styrene sulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloyl ethane sulfonic acid and sulfonic acid monomers such as a salt thereof and the like. These water-soluble ethylenically unsaturated monomers may be used alone or in combination of two or more.

  From the viewpoint of the interaction between the crosslinked polymer and the inorganic filler, it is preferable to use a monomer having a charge opposite to the charge on the surface of the inorganic filler as the water-soluble ethylenically unsaturated monomer. Among them, it is preferable to select a monomer having an anionic functional group (for example, a carboxylic acid monomer or a sulfonic acid monomer). Specifically, (meth) acrylic acid and its salt, 2- (meth) acrylamido-2-methylpropanesulfonic acid and its salt are preferable, and (meth) acrylic acid and its salt are more preferable. In some cases, (meth) acrylic acid and a salt thereof are copolymerized with another water-soluble ethylenically unsaturated monomer. In this case, the (meth) acrylic acid and a salt thereof may be used as a main water-soluble ethylenically unsaturated monomer in an amount of 70 to 100 mol% of the total water-soluble ethylenically unsaturated monomer. Preferably, 80 to 100 mol% is used.

  When the water-soluble ethylenically unsaturated monomer has an acid group such as (meth) acrylic acid or 2- (meth) acrylamido-2-methylpropanesulfonic acid, the acid group is previously alkaline if necessary. You may use what was neutralized with the summing agent. Examples of such an alkaline neutralizer include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate; ammonia and the like. These alkaline neutralizing agents may be used in the form of an aqueous solution in order to simplify the neutralization operation. The above alkaline neutralizing agents may be used alone or in combination of two or more. The neutralization of the acid group may be performed before the polymerization of the water-soluble ethylenically unsaturated monomer as a raw material, or may be performed during or after the polymerization.

  About the degree of neutralization of the water-soluble ethylenically unsaturated monomer with the alkaline neutralizer, the water absorption performance is increased by increasing the osmotic pressure of the resulting water-absorbent resin, and it is due to the presence of excess alkaline neutralizer. From the standpoint of preventing problems in safety and the like, the neutralization degree for all acid groups of the water-soluble ethylenically unsaturated monomer is usually preferably 10 to 100 mol%, and 30 More preferably, it is -90 mol%, More preferably, it is 40-85 mol%, More preferably, it is 50-80 mol%.

  The concentration of the water-soluble ethylenically unsaturated monomer in the water-soluble ethylenically unsaturated monomer aqueous solution may be usually 20% by mass or more and saturated concentration or less, preferably 25 to 70% by mass, and preferably 30 to 30%. 50 mass% is more preferable.

[Inorganic filler]
The inorganic filler in the present invention is an inorganic compound mainly composed of a metal oxide, a metal hydroxide, a salt of a metal and an organic acid or an inorganic acid, for example, a composite hydroxide, particularly a divalent Examples thereof include layered double hydroxides having metal hydroxide and trivalent metal ions as main structural units. The inorganic filler may be partially organically treated.

  The layered double hydroxide replaces a part of divalent metal ions with trivalent metal ions so that the hydroxide layer is positively charged and the negatively charged anion is sandwiched between the hydroxide layers. It becomes a laminated structure. As such a composite hydroxide, for example, hydrotalcite, allophane, imogolite, kaolinite, halloysite and the like can be used, and among these, hydrotalcite is preferably used.

  Examples of anions between layers include hydroxide ions, halide ions (eg fluoride ions, chloride ions, bromide ions, iodide ions, etc.), lactate ions, carbonate ions, nitrate ions, sulfate ions, organic carboxyls. An acid ion, a complex ion, etc. are mentioned.

  As an anion between layers contained in hydrotalcite, lactate ion, carbonate ion, nitrate ion, organic carboxylate ion, etc. can be preferably used, and lactic acid type hydrotalcite, carbonate type hydrotalcite, nitrate type hydrotalcite, respectively. It is called talcite or carboxylic acid type hydrotalcite. Of these, lactic acid hydrotalcite is preferably used from the viewpoint of dispersibility in water.

  The amount of the inorganic filler is preferably 0.1 to 10 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer, from the viewpoint that the crosslinked polymer hydrogel exhibits appropriate brittleness. 3 to 7.5 parts by mass are more preferable, 0.5 to 6 parts by mass are further preferable, and 1 to 5 parts by mass are more preferable.

  The inorganic filler is preferably dispersed in the aqueous monomer solution, and the inorganic filler can be dispersed in the aqueous monomer solution using a known technique. For example, a stirring type homogenizer, a pressure type homogenizer, an ultrasonic homogenizer, or the like can be used.

  From the viewpoint of dispersibility, the average particle size of the inorganic filler before dispersion may be 500 μm or less, for example, 250 μm or less, preferably 50 μm or less. The average particle size of the inorganic filler before dispersion is 0.01 μm or more. For example, it is 0.05 μm or more, preferably 0.1 μm or more.

  The crosslinked polymer in the present invention exhibits high brittleness in the state of a hydrous gel and is excellent in crushing treatment efficiency. On the other hand, polymerization is performed in a state where the inorganic filler is not present in the aqueous monomer solution, and after the polymerization, the operation of adding the inorganic filler from the outside of the obtained polymer is sufficiently obtained. It is not possible. Therefore, the effect of the present invention is not based only on the mere interaction between the polymer and the inorganic filler, but the functional group inside the crosslinked polymer interacts with the inorganic filler, so that the entanglement, folding, chain length, crosslinking This is presumably due to changes in density. Thus, it can be said that it is impractical for a person skilled in the art to directly and clearly specify a substance obtained by a production method involving a polymer reaction composed of complicated raw materials based on its structure or characteristics. .

[Internal cross-linking agent]
The crosslinked polymer of the present invention may be obtained by self-crosslinking type internal crosslinking in the presence of persulfate or the like, but is preferably obtained by internal crosslinking with an internal crosslinking agent in the monomer aqueous solution. As the internal crosslinking agent, for example, a compound having two or more polymerizable unsaturated groups is used. For example, (poly) ethylene glycol [In the present specification, for example, “polyethylene glycol” and “ethylene glycol” are collectively referred to as “(poly) ethylene glycol”. The same shall apply hereinafter), (poly) propylene glycol, trimethylolpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, and di- or tri (meth) acrylic acid esters of polyols such as (poly) glycerin; Unsaturated polyesters obtained by reacting unsaturated acids such as maleic acid and fumaric acid; bisacrylamides such as N, N′-methylenebis (meth) acrylamide; reacting polyepoxide with (meth) acrylic acid Di- or tri (meth) acrylic acid esters obtained; di (meth) acrylic acid carbamyl esters obtained by reacting polyisocyanate such as tolylene diisocyanate or hexamethylene diisocyanate with hydroxyethyl (meth) acrylate; Lil starch; allylated cellulose; diallyl phthalate; N, N ', N "- triallyl isocyanurate; divinylbenzene.

  As the internal cross-linking agent, a compound having two or more reactive functional groups can be used in addition to the compound having two or more polymerizable unsaturated groups. For example, glycidyl group-containing compounds such as (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether; (poly) ethylene glycol, (poly) propylene glycol, (poly) Examples include glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and glycidyl (meth) acrylate. Among these, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) glycerin diglycidyl ether, and N, N′- from the viewpoint of excellent reactivity at low temperatures. Methylene bisacrylamide is preferred. These internal crosslinking agents may be used alone or in combination of two or more.

  When using the internal cross-linking agent, the amount used is 0.0001 with respect to 100 moles of the water-soluble ethylenically unsaturated monomer in order to sufficiently enhance the water-absorbing performance such as the water-holding ability of the resulting water-absorbent resin. Mol or more is preferable, 0.001 mol or more is more preferable, 0.003 mol or more is more preferable, and 0.005 mol or more is more preferable.

  By adjusting the mechanical properties of the hydrogel of the crosslinked polymer by increasing the amount of the internal cross-linking agent, it is possible to obtain a hydrous gel that can be easily crushed, but when the amount of the internal cross-linking agent is increased. Further, water absorption performance such as water retention ability of the water-absorbing resin obtained is lowered. Therefore, the amount of the internal cross-linking agent is, for example, 0.50 mol or less, particularly 0.25 mol or less, preferably 0.05 mol or less, relative to 100 mol of the water-soluble ethylenically unsaturated monomer. More preferably 0.025 mol or less, and still more preferably 0.013 mol or less.

  In the case of using an internal cross-linking agent, the amount of the internal cross-linking agent is preferably relative to 100 g of the inorganic filler from the viewpoint of obtaining more suitable hydrophysical mechanical properties of the water-containing gel or excellent water absorption performance of the water-absorbent resin. It is 0.05-500 mmol, More preferably, it is 0.1-100 mmol, More preferably, it is 0.5-50 mmol.

[Other ingredients]
The monomer aqueous solution may contain additives such as a chain transfer agent and a thickener as necessary. Examples of the chain transfer agent include thiols, thiolic acids, secondary alcohols, hypophosphorous acid, phosphorous acid and the like. These may be used alone or in combination of two or more. Examples of the thickener include carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, polyethylene glycol, polyacrylic acid, polyacrylic acid neutralized product, polyacrylamide and the like.

<Method for producing crosslinked polymer>
The crosslinked polymer in the present invention is produced as a state in which the functional group in the crosslinked polymer and the inorganic filler interact by adding a polymerization initiator to the monomer aqueous solution and causing the polymerization reaction to proceed. As a polymerization method of the water-soluble ethylenically unsaturated monomer, a typical polymerization method such as an aqueous solution polymerization method, an emulsion polymerization method, a reverse phase suspension polymerization method, or the like is used. An aqueous solution polymerization method will be described as an example of the polymerization method of the water-soluble ethylenically unsaturated monomer in the present invention.
In the present specification, the crosslinked polymer includes an organic-inorganic composite crosslinked polymer containing an inorganic filler. The organic-inorganic composite cross-linked polymer is composed of an organic component (for example, a portion composed of a repeating unit derived from a water-soluble ethylenically unsaturated monomer) and an inorganic component (for example, an inorganic filler). This refers to a crosslinked polymer in which an organic component and an inorganic component are combined at the nano level or molecular level. Further, in the present specification, the crosslinked polymer includes a crosslinked polymer in a hydrated state (for example, a crosslinked polymer in a hydrated gel state).

[Aqueous solution polymerization method]
Examples of aqueous solution polymerization include static polymerization in which a monomer aqueous solution is polymerized in a static state, and stirring polymerization in which polymerization is performed in a stirrer. A water-soluble organic solvent other than water may be blended as appropriate.

  Polymerization is started by adding a polymerization initiator to the monomer aqueous solution and performing heating, light irradiation, or the like as necessary. As the polymerization initiator, a water-soluble radical polymerization initiator is preferably used, and the water-soluble radical polymerization initiator is used in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid. It can also be used as a redox polymerization initiator.

  Examples of the peroxide include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t -Peroxides such as butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide. Among these peroxides, it is preferable to use potassium persulfate, ammonium persulfate, sodium persulfate, hydrogen peroxide from the viewpoint of obtaining a water-absorbing resin having excellent water absorption performance, and further, potassium persulfate, More preferably, ammonium persulfate or sodium persulfate is used. These peroxides may be used alone or in combination of two or more.

  Examples of the azo compound include 2,2′-azobis [2- (N-phenylamidino) propane] dihydrochloride, 2,2′-azobis {2- [N- (4-chlorophenyl) amidino] propane}. Dihydrochloride, 2,2′-azobis {2- [N- (4-hydroxyphenyl) amidino] propane} dihydrochloride, 2,2′-azobis [2- (N-benzylamidino) propane] dihydrochloride 2,2′-azobis [2- (N-allylamino) propane] dihydrochloride, 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis {2- [N- (2-Hydroxyethyl) amidino] propane} dihydrochloride, 2,2′-azobis [2- (5-methyl-2-imidazolin-2-yl) propane] dihydrochloride, 2,2′-azobis [2 -(2-Imidazoline- -Yl) propane] dihydrochloride, 2,2'-azobis [2- (4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl) propane] dihydrochloride, 2,2 '-Azobis [2- (5-hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl) propane] dihydrochloride, 2,2'-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, 2,2′-azobis (2-methylpropionamide) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) Propane] disulfate dihydrate, 2,2'-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate, 2,2'-azobis [2-methyl-N- (2-Hydroxyethyl) propionamide] And the like azo compounds. From the viewpoint of easily obtaining a water-absorbing resin having excellent water-absorbing performance, 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis {2- [1- (2-hydroxy) Ethyl) -2-imidazolin-2-yl] propane} dihydrochloride and 2,2′-azobis [N- (2-carboxyethyl) -2-methylpropionamidine] tetrahydrate are preferred. These azo compounds may be used alone or in combination of two or more.

  The amount of the polymerization initiator used is 0.005 to 100 mol of the water-soluble ethylenically unsaturated monomer used for the polymerization from the viewpoint of avoiding a rapid polymerization reaction and shortening the polymerization reaction time. One mole is preferred.

  The reaction temperature in the polymerization process varies depending on the polymerization initiator used, so it cannot be determined unconditionally. However, the polymerization proceeds rapidly and the polymerization time is shortened to increase productivity and make the heat of polymerization easier. From the viewpoint of performing the reaction smoothly, the temperature is preferably 0 to 120 ° C, more preferably 10 to 100 ° C. Although the reaction time of a superposition | polymerization process is suitably set according to the kind and quantity of a polymerization initiator to be used, reaction temperature, etc., 1 to 200 minutes are preferable and 5 to 100 minutes are more preferable.

[Hydrogel]
After the polymerization in the production process of the crosslinked polymer of the present invention, the crosslinked polymer is obtained in the state of a hydrogel. 30-70 mass% is preferable, as for the moisture content of the water-containing gel of a crosslinked polymer, 40-70 mass% is more preferable, and 50-70 mass% is further more preferable. The water content of the hydrogel was measured by the method described in the following examples.

  It is sufficient that the inorganic filler contained in the hydrogel of the crosslinked polymer and the functional group inside the crosslinked polymer have some interaction, and the interaction is represented by, for example, coordination bond, hydrogen bond, etc. Includes supramolecular bonds, covalent bonds, ionic bonds, and the like.

<Method for producing water-absorbent resin>
A water-absorbent resin is obtained by drying a crushed hydrogel of the above-mentioned crosslinked polymer and then crushing it if necessary. The water absorbent resin may be surface-crosslinked with a surface cross-linking agent.

[Crushing process and crushing process]
The method for producing a water-absorbent resin of the present invention includes a step of crushing a hydrogel of a crosslinked polymer. From the viewpoint of the drying efficiency of the water-containing gel of the crosslinked polymer or the water absorption performance of the resulting water-absorbent resin, it is preferable that the water-containing gel of the cross-linked polymer is crushed and then dried and further pulverized. From the viewpoint of work efficiency and the like, it is preferable that the hydrogel of the crosslinked polymer obtained by polymerization is subjected to crushing treatment as it is. The step of crushing the hydrogel of the crosslinked polymer is preferably a treatment for crushing to a particle size of about 0.1 to 5 mm. And it is preferable that the grinding | pulverization process after the said drying process is a process grind | pulverized until it becomes a powder form with a particle diameter of about 500 micrometers or less.

  A crushing apparatus is not specifically limited, Well-known crushing apparatuses, such as a kneader, a meat chopper, and a cutter mill, can be used. The crushing device may be the same type as the crushing device, or a different one may be used.

  The water-containing gel of the crosslinked polymer in the production method of the present invention is highly brittle and further has low adhesiveness between the particles after crushing, so that the water-containing gel of the crosslinked polymer can be efficiently crushed.

[Drying process]
The method for producing a water-absorbent resin of the present invention includes a drying step of performing a drying process for removing a solvent such as water and an organic solvent contained in the hydrogel of the crosslinked polymer after the crushing process by evaporation. The method of a drying process is not specifically limited, What is necessary is just to remove a solvent from a water-containing gel by general methods, such as natural drying, heat drying, spray drying, and freeze-drying. The drying step can be performed under normal pressure, reduced pressure, or an air stream such as nitrogen in order to increase drying efficiency, and these methods may be used in combination. The drying temperature when the drying step is performed at normal pressure is preferably 70 to 250 ° C, more preferably 80 to 200 ° C. The drying step is preferably performed after polymerization, particularly after the crushing step, and may be performed simultaneously with the surface crosslinking step or after the surface crosslinking step. The drying step is performed until the water content of the crosslinked polymer is preferably 10% by mass or less.

[Surface cross-linking step]
The cross-linked polymer is further reacted by adding a cross-linking agent (denoted as a surface cross-linking agent) containing two or more functional groups having reactivity with a functional group derived from a water-soluble ethylenically unsaturated monomer. (It is expressed as surface cross-linking treatment). By performing a surface crosslinking treatment by adding a surface crosslinking agent after the polymerization, the crosslinking density in the vicinity of the surface of the water-absorbent resin is increased, so that the water-absorbing performance such as water retention ability of the obtained water-absorbent resin can be enhanced.

  Examples of the surface crosslinking agent include compounds having two or more reactive functional groups. For example, polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, polyglycerin; (poly) ethylene glycol diglycidyl ether, (poly) Polyglycidyl compounds such as ethylene glycol triglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, (poly) glycerol polyglycidyl ether; epichlorohydrin, epibromohydrin Haloepoxy compounds such as α-methylepichlorohydrin; 2,4-tolylene diisocyanate, hexamethylene diisocyanate and the like Compounds having two or more reactive functional groups such as cyanate compounds; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol Oxetane compounds such as 3-ethyl-3-oxetaneethanol and 3-butyl-3-oxetaneethanol; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; bis [N, N-di ( [beta] -hydroxyethyl)] adipamide and the like. Among these, (poly) ethylene glycol diglycidyl ether, (poly) ethylene glycol triglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, (poly) propylene glycol polyglycidyl ether, (poly) Polyglycidyl compounds such as glycerol polyglycidyl ether and / or polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, polyoxyethylene glycol, polyoxypropylene glycol are preferred, and polyglycidyl compounds are more preferred. preferable. These surface crosslinking agents may be used alone or in combination of two or more. For example, a polyglycidyl compound and polyols may be used in combination.

  The amount of the surface crosslinking agent is preferably from the viewpoint of increasing the crosslinking density in the vicinity of the surface of the water-absorbent resin as necessary and usually with respect to 100 mol of the total amount of the water-soluble ethylenically unsaturated monomer used for the polymerization. It is 0.0001-1 mol, More preferably, it is 0.001-0.5 mol.

  The surface crosslinking agent may be added after the polymerization of the water-soluble ethylenically unsaturated monomer, and the crosslinked polymer hydrogel, the crushed crosslinked polymer hydrogel, or after drying these hydrogels You may carry out with respect to the crosslinked polymer of this, and the water absorbing resin after a grinding | pulverization process. As for the method for adding the surface cross-linking agent, for example, the surface cross-linking agent solution may be added to the water-absorbing resin, or the surface cross-linking agent solution may be spray-added to the water-absorbing resin. From the viewpoint of uniformly dispersing the surface cross-linking agent, the surface cross-linking agent is preferably added as a surface cross-linking agent solution by dissolving the surface cross-linking agent in a solvent such as water. In addition, the surface cross-linking step may be performed once or divided into two or more times.

  The surface crosslinking step is preferably performed in the presence of water in the range of 1 to 200 parts by mass with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer. The water content can be adjusted as appropriate by using water or / and a water-soluble organic solvent such as an alcohol solvent. The solid content of the water-absorbent resin in the hydrogel can be calculated from the charged amount of the water-soluble ethylenically unsaturated monomer used for the polymerization reaction. That is, the water content contained in the water-containing gel in each step after the polymerization step can be calculated by subtracting the water content removed from the water-containing gel after polymerization from the water content contained in the monomer aqueous solution. Thus, by adjusting the amount of water during the surface cross-linking step, cross-linking in the vicinity of the particle surface of the water-absorbent resin can be more suitably performed.

  The reaction temperature of the surface cross-linking treatment is appropriately set according to the surface cross-linking agent to be used, and may be 20 to 250 ° C. The reaction time is preferably 1 to 200 minutes, more preferably 5 to 100 minutes.

The particle size of the water-absorbent resin is such that 90% or more of the total weight is preferably 850 μm or less, more preferably 700 μm or less, further preferably 600 μm or less, and even more preferably 500 μm or less. preferable. In order to adjust the particle size, a known method may be used. For example, a sieve or the like can be used.

  Hereinafter, the present invention will be described in more detail on the basis of Examples and Comparative Examples, but the present invention is not limited to only these Examples.

  The moisture content, compressive strength and adhesive strength of the crosslinked polymer hydrated gel in each Example and Comparative Example, and the physiological saline water retention capacity of the water-absorbent resin were evaluated by the following methods.

(Water content of water-containing gel of crosslinked polymer)
A hydropolymer gel containing 2.0 g of a crosslinked polymer was accurately weighed (Wb (g)) in an aluminum wheel case (No. 8) having a constant weight (Wa (g)) in advance. The sample was dried for 2 hours in a hot air dryer (ADVANTEC) with an internal temperature set to 105 ° C., then allowed to cool in a desiccator, and the mass after drying (Wc (g)) was measured. From the following formula, the water content of the hydrogel of the crosslinked polymer was calculated.
Moisture content (mass%) = [(Wb−Wa) − (Wc−Wa)] / (Wb−Wa) × 100

(Compressive strength of crosslinked polymer hydrogel)
The crosslinked polymer hydrogel after polymerization was cooled to 25 ° C., and then cut to obtain a sample of length × width × thickness of 25 × 25 × 20 mm. Using a small compression / tensile tester (“Eztest / CE” manufactured by Shimadzu Corporation), based on the constant speed strain method shown below, the load required for the sample thickness to be displaced by 5 mm (or less than 5 mm) When the sample was torn due to displacement, the load required for tearing was measured. This measurement was performed 5 times, and the average value was defined as the compressive strength [N] of the hydrogel of the crosslinked polymer.
Displacement speed: 10 [mm / min], load limit (upper limit): 15 [N], displacement limit (upper limit): 15 [mm], load cell: 20 [N], indenter: 2 [mmφ] (3.14 mm ²⁾ )
The value of the compressive strength is one of the indexes indicating the strength of the crosslinked polymer, and the higher the compressive strength, the easier the gel is crushed.

(Adhesion strength of crosslinked polymer hydrogel)
After cooling the water-containing gel of the crosslinked polymer after polymerization to 25 ° C., two 50 × 20 × 5 mm samples were cut out. Both samples were overlapped at an area of 30 × 20 mm and then compressed with a 5 kg weight for 30 seconds to obtain a measurement sample. Using a small compression / tensile tester (“Eztest / CE” manufactured by Shimadzu Corporation), grab the top and bottom of the sample for measurement and calculate the load required to peel off the adhered gel based on the constant-speed strain method shown below. It was measured. This measurement was performed three times, and the average was taken as the adhesive strength [N] of the hydrogel of the crosslinked polymer.
Displacement speed: 100 [mm / min], load limit (upper limit): 15 [N], load cell: 20 [N]
The value of the adhesive strength is one of the indexes indicating the degree of adhesion to the device, and the smaller the adhesive strength, the less the gel adhesion to the cutting blade of the crushing device. In addition, the value of the adhesive strength is one of the indexes indicating the adhesion between the crushed gel particles, and the smaller the adhesive strength, the more difficult the gel particles crushed during the crushing process to form a lump. .

(Saline retention capacity of water-absorbent resin)
In a beaker for 500 mL, weigh out 500 g of 0.9% by mass saline (saline) and stir at 600 rpm using a magnetic stirrer bar (no 8 mmφ × 30 mm ring). Was dispersed so that no mamako would occur. The mixture was allowed to stand for 30 minutes with stirring to sufficiently swell the water absorbent resin. After that, it was poured into a cotton bag (Membroad No. 60, width 100 mm x length 200 mm), the upper part of the cotton bag was tied up with a rubber band, and the centrifugal force was set to 167G. : H-122), the cotton bag was dehydrated for 1 minute, and the mass Wd (g) of the cotton bag containing the swollen gel after dehydration was measured. The same operation was performed without adding the water-absorbent resin, the empty mass We (g) when the cotton bag was wet was measured, and the physiological saline water retention capacity was calculated from the following equation.
Saline retention capacity (g / g) = [Wd-We] (g) / mass of water-absorbent resin (g)

[Example 1]
In a beaker, 35.7 mass% sodium acrylate aqueous solution 679.8 g, acrylic acid 100.2 g, and 2% polyethylene glycol diacrylate (average repeat unit of ethylene oxide: 9) aqueous solution 21.3 g, ion-exchanged water 170. A monomer aqueous solution in which 4 g and 14.1 g of lactic acid type hydrotalcite were mixed was prepared.
After transferring 745.5 g of the monomer aqueous solution to a stainless steel vat and adjusting the liquid temperature to 18 ° C. while stirring at 600 rpm, the inside of the system was replaced with nitrogen.
Subsequently, 1.2 g of 5% sodium persulfate aqueous solution and 1.2 g of 5% 2,2′-azobis (2-amidinopropane) dihydrochloride aqueous solution were added dropwise and stirred, and then 0.05% L-ascorbine. Immediately add 1.1 g of acid aqueous solution and then 1.2 g of 0.035% hydrogen peroxide aqueous solution.
Polymerization started immediately after the addition of the aqueous hydrogen peroxide solution, and the temperature of the aqueous monomer solution rose to 85 ° C. after 20 minutes. Thereafter, the vat was immersed in an 80 ° C. water bath for 10 minutes to obtain a crosslinked polymer in a water-containing gel state (water content: 61% by mass). In addition, when a part of obtained crosslinked polymer of the hydrogel state was cut | judged and the compressive strength and the adhesive strength were measured, the compressive strength was 11.4N and the adhesive strength was 1.4N.
The remaining crosslinked polymer in the hydrogel state was crushed with a double-arm kneader, and then air-dried at 180 ° C. for 30 minutes. The obtained dried crosslinked polymer was further pulverized by a jaw crusher type pulverizer, and then passed through a 500 μm sieve and classified on the 106 μm sieve.
A solution obtained by mixing 30 g of the obtained classified product in a separable flask and mixing 0.02 g of ethylene glycol diglycidyl ether, 0.3 g of propylene glycol, 1.0 g of ion-exchanged water, and 0.3 g of isopropanol while stirring at 500 rpm. The solution was added dropwise and further stirred for 1 minute. Next, the mixture was subjected to surface crosslinking treatment by heating at 180 ° C. for 40 minutes, and passed through a 500 μm sieve to obtain 29 g of a water-absorbing resin. The obtained water-absorbent resin had a physiological saline water retention capacity of 46 (g / g).

[Comparative Example 1]
Except that hydrotalcite was not added to the aqueous monomer solution, the same operation as in Example 1 was performed to obtain a crosslinked polymer in a hydrogel state (water content: 59% by mass). In addition, when a part of the obtained crosslinked polymer in the hydrogel state was cut and the compressive strength and the adhesive strength were measured, the compressive strength was 7.4 N and the adhesive strength was 2.8 N. Compared with Example 1, the compressive strength decreased and the adhesive strength increased significantly.
Further, the same operation as in Example 1 was performed, and 27 g of a water-absorbent resin was obtained by subjecting the dried product of the crosslinked polymer to a surface crosslinking treatment. The obtained water-absorbent resin had a physiological saline water retention capacity of 45 (g / g).

[Comparative Example 2]
A crosslinked polymer in a hydrous gel state was prepared in the same manner as in Comparative Example 1 except that the 2% polyethylene glycol diacrylate aqueous solution added to the monomer aqueous solution was changed to 63.9 g and the ion exchange water was changed to 149.1 g. (Water content: 63% by mass) was obtained. In addition, when a part of obtained crosslinked polymer of the hydrogel state was cut | judged and the compressive strength and adhesive strength were measured, the compressive strength was 11.4N and the adhesive strength was 1.6N.
Furthermore, the same operation as Example 1 was performed, and 28 g of water-absorbent resins were obtained by subjecting the dried product of the crosslinked polymer to a surface crosslinking treatment. The obtained water-absorbing resin had a physiological saline water retention capacity of 36 (g / g), and the water absorption performance was lower than that of Example 1.

Since the water-absorbent resin obtained by the production method of the present invention exhibits excellent water-absorbing performance, it can be suitably used for absorbent articles such as paper diapers and sanitary products.
Therefore, the method for producing a crosslinked polymer of the present invention and the method for producing a water-absorbing resin using the above-mentioned crosslinked polymer are advantageously used industrially.

Claims (7)

  The manufacturing method of a crosslinked polymer including the process of superposing | polymerizing the said water-soluble ethylenically unsaturated monomer in the aqueous solution containing a water-soluble ethylenically unsaturated monomer and an inorganic filler.   The manufacturing method of the crosslinked polymer of Claim 1 whose quantity of the said inorganic filler is 0.1-10 mass parts with respect to 100 mass parts of said water-soluble ethylenically unsaturated monomers in the said aqueous solution.   The method for producing a crosslinked polymer according to claim 1 or 2, wherein the inorganic filler is at least one selected from the group consisting of hydrotalcite, allophane, imogolite, kaolinite, and halloysite.   The manufacturing method of the crosslinked polymer in any one of Claims 1-3 whose ratio of (meth) acrylic acid and its salt is 70-100 mol% among the said water-soluble ethylenically unsaturated monomers.   The manufacturing method of the crosslinked polymer in any one of Claims 1-4 with which the said crosslinked polymer is obtained in the state of the water-containing gel whose moisture content is 30-70 mass%.   A method for producing a water-absorbent resin, comprising: a step of crushing a water-containing gel of a crosslinked polymer obtained by the method according to claim 1; and a step of drying the crushed water-containing gel.   The crosslinked polymer obtained by the manufacturing method in any one of Claims 1-5.