JP6063440B2 - Method for producing water absorbent resin particles - Google Patents

Description

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

  Water-absorbent resin particles have been used in applications such as sanitary materials such as disposable diapers and sanitary products, agricultural and horticultural materials such as water retention materials and soil improvement materials, industrial materials such as water-stopping materials for cables and anti-condensation materials. In addition to these, in recent years, animal excrement treatment materials such as pet sheets, dog or cat toilet compositions, simple toilets, fragrances, meat drip absorption sheets, formulations for moisturizing cosmetics, etc., water absorbent resin particles The field where is applied is expanding further. The water-absorbent resin particles used for such applications are required to have, for example, a high water absorption amount, an excellent water absorption rate, and an appropriate particle size according to the application.

  Compared to certain hygienic materials such as adult diapers, incontinence pads, toilet training pants and many daily napkins, water-absorbent resin particles used in cable waterproofing materials, pet seats, simple toilets, etc. It is assumed that a large amount of body fluid etc. is discharged vigorously. For this reason, in such applications, improvements in absorption capacity and absorption rate have been emphasized. The absorption capacity can be dealt with by the amount of water-absorbing resin particles used, but the absorption rate largely depends on the inherent characteristics of the water-absorbing resin particles. Therefore, various studies have been made so far in order to achieve an excellent water absorption speed in the water absorbent resin particles.

  For example, with respect to the aqueous solution polymerization method, a method of crosslinking the vicinity of the surface of a porous resin obtained in the presence of a foaming agent (see Patent Document 1) has been proposed.

  As for reverse phase suspension polymerization, an acrylic acid / acrylate aqueous solution is suspended in an alicyclic or aliphatic hydrocarbon solvent in the presence of a surfactant of HLB 8 to 12, and acrylic acid / acrylate is obtained. Have been proposed (see Patent Document 2), and a method of polymerizing water-soluble ethylenically unsaturated monomers in the presence of water-absorbing resins having different water absorption rates (see Patent Document 3). Yes.

International Publication No. 97/003114 JP 56-131608 A Japanese Patent Laid-Open No. 9-151224

  In addition, generally, in order to improve the water absorption speed of the water absorbent resin particles, it is also effective to some extent to reduce the particle diameter of the water absorbent resin particles. However, if the particle diameter of the water-absorbent resin particles is reduced, fluidity tends to deteriorate and handling as particles tends to be difficult. Therefore, a method for increasing the water absorption speed of the water-absorbent resin particles while having an appropriate particle diameter is required.

  Here, according to the study by the present inventors, the water-absorbent resin particles disclosed in Patent Document 1 are not satisfactory in terms of both the performance of the particle diameter and the water absorption speed. Further, although the water-absorbent resin particles disclosed in Patent Documents 2 and 3 have a relatively large particle diameter and excellent handling properties and water absorption speed, the hydrocarbon dispersion medium used at the time of manufacture is contained inside the resin particles as a volatile component. For example, an odor may be generated after water absorption, and the odor when using the product may impair the comfort of the user (wearer, etc.). In addition, from the viewpoint of environmental considerations in the manufacturing process and the improvement of water absorption performance, a small amount of residual volatile components is required.

  Accordingly, an object of the present invention is to provide a method that enables the production of water-absorbing resin particles having excellent handling properties and a sufficiently high water absorption rate and having a small amount of residual volatile components.

  The present invention relates to the following method for producing water absorbent resin particles.

Item 1.
An aqueous liquid containing water and a water-soluble ethylenically unsaturated monomer dissolved in the water is suspended in an oily liquid containing a surfactant having a HLB of 6 or more and a hydrocarbon dispersion medium. Preparing a polymerization step comprising polymerizing the water-soluble ethylenically unsaturated monomer while supplying an inert gas into the suspension,
A water absorption ratio in which a gas supply ratio represented by a ratio of a volume of the inert gas supplied per minute to a total volume of the aqueous liquid and the hydrocarbon dispersion medium is 0.1 to 3.0. A method for producing resin particles.

Item 2.
An aqueous liquid containing water and a water-soluble ethylenically unsaturated monomer dissolved in the water is suspended in an oily liquid containing a surfactant having a HLB of 6 or more and a hydrocarbon dispersion medium. Preparing a first polymerization step comprising polymerizing the water-soluble ethylenically unsaturated monomer while supplying an inert gas into the suspension;
After the first polymerization step, water and an additional water-soluble ethylenically unsaturated monomer dissolved in the water, which are separate from the aqueous liquid already introduced into the suspension An aqueous liquid is added to the reaction mixture after polymerization to suspend it, and then the water-soluble ethylenically unsaturated monomer is polymerized once while supplying an inert gas into the suspension. Two or more times, the second and subsequent polymerization steps, and
Supplied per minute with respect to the total volume of the aqueous liquid and the hydrocarbon dispersion medium introduced into the suspension in the polymerization step of the nth time (n is an integer of 1 or more). The method for producing water-absorbent resin particles, wherein the gas supply ratio in the n-th polymerization step, expressed by the ratio of the inert gas, is 0.1 to 3.0.

Item 3.
Item 3. The method for producing water-absorbent resin particles according to Item 1 or 2, wherein the surfactant comprises at least one selected from the group consisting of sorbitan fatty acid ester, polyglycerin fatty acid ester, and sucrose fatty acid ester.

Item 4.
Item 4. The water absorbing property according to any one of Items 1 to 3, wherein the water-soluble ethylenically unsaturated monomer includes at least one selected from the group consisting of acrylic acid and salts thereof, methacrylic acid and salts thereof, and acrylamide. A method for producing resin particles.

Item 5.
Item 5. The item according to any one of Items 1 to 4, wherein the hydrocarbon dispersion medium contains at least one selected from the group consisting of an aliphatic hydrocarbon having 6 to 8 carbon atoms and an alicyclic hydrocarbon having 6 to 8 carbon atoms. Of producing water-absorbent resin particles.

  The water absorbent resin particles obtained by the production method of the present invention have an excellent water absorption rate. Moreover, since the water-absorbent resin particles obtained by this method have an appropriate particle size, they are excellent in handling properties. Furthermore, since the amount of residual volatile components is small, problems such as odor after water absorption can be reduced.

  A method for producing water-absorbing resin particles according to this embodiment will be described below. However, the present invention is not limited to the following embodiments.

  In the production method according to the present embodiment, an aqueous liquid containing water and a water-soluble ethylenically unsaturated monomer dissolved in the water is converted into an oily liquid containing a surfactant having a HLB of 6 or more and a hydrocarbon dispersion medium. A polymerization step comprising polymerizing a water-soluble ethylenically unsaturated monomer while suspending in the suspension to prepare a suspension and supplying an inert gas into the suspension. In the polymerization step, the water-soluble ethylenically unsaturated monomer is polymerized by reverse phase suspension polymerization.

  Examples of the water-soluble ethylenically unsaturated monomer include ethylenically unsaturated monomers containing at least one functional group selected from the group consisting of a carboxyl group, a sulfo group, an amide group, an amino group, and the like. . Examples of the water-soluble ethylenically unsaturated monomer include (meth) acrylic acid (hereinafter referred to as “(meth) acryl” together with “acryl” and “methacryl”) and salts thereof (alkali salts), 2- (meth) acrylamide-2-methylpropanesulfonic acid and its salt (alkali salt), (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meta ) Acrylamide, polyethylene glycol mono (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, and diethylaminopropyl (meth) acrylamide. When the water-soluble ethylenically unsaturated monomer contains an amino group, the amino group may be quaternized. The functional group such as a carboxyl group and an amino group of the monomer can function as a functional group that can be crosslinked in a post-crosslinking step described later. These water-soluble ethylenically unsaturated monomers may be used individually by 1 type, and may be used in combination of 2 or more type.

  Among these, from the viewpoint of industrial availability, the water-soluble ethylenically unsaturated monomer is composed of acrylic acid and its salt, methacrylic acid and its salt, acrylamide, methacrylamide and N, N-dimethylacrylamide. It may be at least one compound selected from the group, or at least one compound selected from the group consisting of acrylic acid and its salt, methacrylic acid and its salt, and acrylamide. Furthermore, from the viewpoint of further safety, the water-soluble ethylenically unsaturated monomer may be at least one compound selected from the group consisting of acrylic acid and its salt, and methacrylic acid and its salt. .

  The water-soluble ethylenically unsaturated monomer is introduced into the oily liquid (O) in the state of an aqueous liquid (W) dissolved in an aqueous solvent containing water. The concentration of the water-soluble ethylenically unsaturated monomer in this aqueous liquid may be in the range of 20% by mass to the saturated concentration. In addition, from the viewpoint that the W / O type reversed phase suspension state is good and it is easy to obtain a suitable particle size, and the swelling performance of the resulting water-absorbent resin particles is further enhanced, the water-soluble ethylenically unsaturated monomer The concentration may be 25 to 50 mass%, 30 to 45 mass%, or 35 to 42 mass%.

  When the water-soluble ethylenically unsaturated monomer has an acid group such as (meth) acrylic acid and 2- (meth) acrylamido-2-methylpropanesulfonic acid, the acid group is in an alkaline state such as an alkali metal salt. The salt may be formed by neutralization with a hydrating agent. Examples of the alkaline neutralizer include aqueous solutions such as sodium hydroxide, potassium hydroxide and ammonia. These alkaline neutralizers may be used alone or in combination of two or more.

  The degree of neutralization with respect to all acid groups by the alkaline neutralizer increases the osmotic pressure of the resulting water-absorbent resin particles, thereby increasing the swelling capacity, and suppressing the remaining of the excess alkaline neutralizer to improve safety, etc. From the viewpoint of preventing the problem from occurring, it may be 10 to 100 mol%, 30 to 90 mol%, 50 to 80 mol%, or 60 to 78 mol%.

  The aqueous liquid containing a water-soluble ethylenically unsaturated monomer may contain a radical polymerization initiator. This radical polymerization initiator may be water-soluble. Examples of the radical polymerization initiator 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 , Peroxides such as t-butyl peroxyacetate, t-butyl peroxyisobutyrate, t-butyl peroxypivalate, and hydrogen peroxide; and 2,2′-azobis (2-methylpropionamidine) 2 Hydrochloride, 2,2′-azobis [2- (N-phenylamidino) propane] dihydrochloride, 2,2′-azobis [2- (N-allylamidino) propane] dihydrochloride, 2,2′- Azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, 2 2'-azobis {2-methyl-N- [1,1-bis (hydroxymethyl) -2-hydroxyethyl] propionamide}, 2,2'-azobis [2-methyl-N- (2-hydroxyethyl) -Propionamide], and azo compounds such as 4,4′-azobis (4-cyanovaleric acid). These radical polymerization initiators may be used alone or in combination of two or more.

  The usage-amount of a radical polymerization initiator is 0.005-1 mol normally with respect to 100 mol of water-soluble ethylenically unsaturated monomers. When the amount of the radical polymerization initiator used is 0.005 mol or more, the polymerization reaction does not require a long time and is efficient. When the amount of the radical polymerization initiator used is 1 mol or less, a rapid polymerization reaction tends not to occur.

  A radical polymerization initiator can also be used as a redox polymerization initiator in combination with a reducing agent such as sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid.

  The aqueous liquid containing the water-soluble ethylenically unsaturated monomer may contain a chain transfer agent in order to control the water absorption performance of the water absorbent resin particles. Examples of the chain transfer agent include hypophosphite, thiol, thiolic acid, secondary alcohol, and amine.

  The aqueous liquid containing a water-soluble ethylenically unsaturated monomer can also contain a water-soluble thickener. Examples of the water-soluble thickener include hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC); hydroxyalkylalkyl celluloses such as hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose and hydroxyethyl ethyl cellulose; carboxymethyl cellulose and the like Carboxyalkylcellulose; Carboxyalkylhydroxyalkylcellulose such as carboxymethylhydroxyethylcellulose. Among these, the water-soluble thickener may be at least one compound selected from hydroxyalkylcellulose, hydroxyalkylalkylcellulose and carboxyalkylhydroxyalkylcellulose, and at least one selected from hydroxyethylcellulose and hydroxypropylcellulose. It may be a seed compound. Hydroxyethyl cellulose and hydroxypropyl cellulose have high solubility in an aqueous liquid, can easily develop the thickening effect of the aqueous liquid, and can further reduce the amount of residual volatile components of the water-absorbent resin particles.

  The aqueous liquid containing a water-soluble ethylenically unsaturated monomer may contain a hydrophilic polymer dispersant. Examples of the hydrophilic polymer dispersant include polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), polypropylene glycol, polyethylene glycol / polypropylene glycol block copolymer, polyglycerin, polyoxyethylene glycerin, Examples thereof include polyoxypropylene glycerin, polyoxyethylene / polyoxypropylene glycerin copolymer, and polyoxyethylene sorbitan fatty acid ester. Among these, the hydrophilic polymer dispersant may be at least one compound selected from the group consisting of polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, polypropylene glycol and polyglycerin, and the effect of reducing the amount of residual volatile components From the viewpoint, polyvinyl pyrrolidone or polyvinyl alcohol may be used. These hydrophilic polymer dispersants may be used alone or in combination of two or more.

  The oily liquid used in the present embodiment includes a surfactant having an HLB of 6 or more and a hydrocarbon dispersion medium.

  6-16, 7-16, 8-12, or 8.5-10.5 may be sufficient as HLB of surfactant. When the HLB of the surfactant is within these ranges, the state of W / O type reverse phase suspension becomes better, and particles having a more preferable particle diameter and a better water absorption rate tend to be obtained. .

  Examples of the surfactant include sorbitan fatty acid ester, (poly) glycerin fatty acid ester (“(poly)” means both with and without the prefix “poly”; the same applies hereinafter), Sugar fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene glycerin fatty acid ester, sorbitol fatty acid ester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkyl phenyl ether, polyoxyethylene castor oil, polyoxy Ethylene hydrogenated castor oil, alkylallyl formaldehyde condensation polyoxyethylene ether, polyoxyethylene polyoxypropylene block copolymer, polyoxyethylene polyoxypropyl alkyl ether , And nonionic surfactants such as polyethylene glycol fatty acid esters; fatty acid salts, alkylbenzene sulfonates, alkylmethyl taurates, polyoxyethylene alkylphenyl ether sulfates, polyoxyethylene alkyl ether sulfonates, polyoxyethylene Examples include anionic surfactants such as alkyl ether phosphates and polyoxyethylene alkyl allyl ether phosphates. Among these, sorbitan fatty acid ester and polyglycerin are preferable from the viewpoint that W / O type reverse phase suspension is good, water-absorbing resin particles are easily obtained with a suitable particle size, and are easily available industrially. At least one compound selected from the group consisting of fatty acid esters and sucrose fatty acid esters can be used. Furthermore, sorbitan fatty acid ester can be used from a viewpoint that the said various performance of the water absorbent resin particle obtained improves. These surfactants may be used alone or in combination of two or more.

  The amount of the surfactant used is an aqueous solvent, a water-soluble ethylenic solvent, from the viewpoint of stabilizing the state of the W / O type reverse phase suspension and selecting an effective amount that can provide a suspension stabilizing effect. 0.1-5 parts by mass, 0.2-3 parts by mass, or 0.4-2 parts by mass with respect to a total of 100 parts by mass of various additives such as saturated monomers, radical polymerization initiators and chain transfer agents Part.

  For the purpose of stabilizing the state of the W / O type reverse phase suspension, the oily liquid may further contain a hydrophobic polymer dispersant. By using a surfactant and a hydrophobic polymer dispersant in combination, the state of W / O type reverse phase suspension can be further stabilized. Examples of the hydrophobic polymer dispersant include maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene / propylene copolymer, maleic anhydride-modified EPDM (ethylene-propylene-diene-terpolymer), Maleic anhydride modified polybutadiene, ethylene-maleic anhydride copolymer, ethylene-propylene-maleic anhydride copolymer, butadiene-maleic anhydride copolymer, oxidized polyethylene, ethylene-acrylic acid copolymer, ethylcellulose, and Ethyl hydroxyethyl cellulose is mentioned. Among these, from the viewpoint of the stability of the W / O type reverse phase suspension, the hydrophobic polymer dispersion medium is maleic anhydride-modified polyethylene, maleic anhydride-modified polypropylene, maleic anhydride-modified ethylene-propylene copolymer, Oxidized polyethylene or ethylene-acrylic acid copolymer may be used. These hydrophobic polymer dispersants may be used alone or in combination of two or more.

  The amount of the hydrophobic polymer dispersant used is 0.1 to 5 parts by mass, 0 with respect to 100 parts by mass in total of the aqueous solvent, the water-soluble ethylenically unsaturated monomer, the radical polymerization initiator, and various additives. It may be 2 to 3 parts by mass, or 0.4 to 2 parts by mass.

  Examples of the hydrocarbon dispersion medium include chain aliphatic carbonization such as n-hexane, n-heptane, 2-methylhexane, 3-methylhexane, 2,3-dimethylpentane, 3-ethylpentane, and n-octane. Hydrogen; cycloaliphatic carbonization such as cyclohexane, methylcyclohexane, cyclopentane, methylcyclopentane, trans-1,2-dimethylcyclopentane, cis-1,3-dimethylcyclopentane, and trans-1,3-dimethylcyclopentane Hydrogen; aromatic hydrocarbons such as benzene, toluene, and xylene. These hydrocarbon dispersion media may be used alone or in combination of two or more. These hydrocarbon dispersion media may contain at least one compound selected from the group consisting of C6-C8 chain aliphatic hydrocarbons and C6-C8 alicyclic hydrocarbons. . The viewpoint that W / O type reverse phase suspension state is good, water-absorbing resin particles having an excellent water absorption rate are easily obtained with a suitable particle size, are easily available industrially, and have a stable quality. Therefore, the hydrocarbon dispersion medium may contain n-heptane and cyclohexane. From the same point of view, as the mixture of the hydrocarbon dispersion medium, for example, commercially available Exol heptane (exxon mobile company: containing n-heptane and 75 to 85% isomer hydrocarbon) is used. it can.

  The amount of the hydrocarbon dispersion medium used is 50 with respect to 100 parts by mass of the water-soluble ethylenically unsaturated monomer contained in the aqueous liquid from the viewpoint of appropriately removing the heat of polymerization and easily controlling the polymerization temperature. -650 mass parts, 70-550 mass parts, or 100-450 mass parts may be sufficient. When the amount of the hydrocarbon dispersion medium used is 50 parts by mass or more, the polymerization temperature tends to be easily controlled. When the amount of the hydrocarbon dispersion medium used is 650 parts by mass or less, the productivity of polymerization tends to be improved, which is economical.

  In the synthesis of water-absorbing resin particles containing a water-soluble ethylenically unsaturated monomer as a constituent unit, it is desirable that the polymerization reaction is carried out in a reducing atmosphere. In general, for the purpose of removing dissolved oxygen in the reaction system containing the monomer, the polymerization proceeds by supplying an inert gas into the suspension and applying energy such as heat and light as necessary. Thus, water-absorbing resin particles are obtained.

  According to the knowledge of the present inventors, excellent handling is achieved by controlling the supply ratio of the inert gas within a specific range after mixing the aqueous liquid containing the water-soluble ethylenically unsaturated monomer with the oily liquid. And water absorption resin particles having a sufficiently high water absorption speed and a small amount of residual volatile components can be produced. Specifically, the ratio of the volume of inert gas supplied per minute (C) to the sum of the volume of aqueous liquid (A) and the volume of hydrocarbon dispersion medium (B) (= C / (A + B) ) Is controlled within a range of 0.1 to 3.0. When this gas supply ratio is less than 0.1, polymerization tends to take a long time. If the gas supply ratio exceeds 3.0, the dispersion of the unsaturated monomer may be insufficient, or coarse particles may be generated and the residual volatile components may increase. From the same viewpoint, the gas supply ratio may be 0.12 to 2.5, 0.15 to 2.0, 0.18 to 1.5, or 0.2 to 1.2. By setting the gas supply ratio within the above range, it is possible to efficiently reduce the remaining volatile components.

  The method according to the present embodiment solves the novel problem of reducing the amount of residual volatile components by setting the gas supply ratio to a specific range different from less than 0.1 which has been conventionally performed. . The mechanism is not necessarily clear, but it is presumed as follows. In the synthesis of water-absorbing resin particles by reverse phase suspension polymerization, the main factor for the increase in residual volatile components is considered to be inclusion of hydrocarbon dispersion medium in monomer droplets by O / W / O type emulsification. A liquid containing a monomer once generated when an aqueous liquid of a monomer is suspended in an oily liquid by setting the supply ratio of an inert gas used for deoxygenation for initiating polymerization within a specific range. The polymerization proceeds while the O / W / O emulsified fine droplets in the vicinity of the surface of the droplets are appropriately replaced with bubbles, and it is assumed that the residual volatile components are reduced.

  In order to increase the effect of reducing residual volatile components more efficiently in the polymerization reaction, the inert gas controls the polymerization at least from the start of the polymerization of the monomer until the formation of the water-insoluble hydrous gel is completed. From the viewpoint of simplicity, the mixing of the aqueous liquid containing the monomer and the oily liquid containing the hydrocarbon solvent is from the start of energy supply related to the initiation of polymerization such as heat and light to the arrival of the exothermic peak due to the polymerization. The gas may be supplied at the specific gas supply rate from the completion time to the end of the polymerization.

  The inert gas used in the present embodiment is not particularly limited. For example, the inert gas may include nitrogen, argon, or both. In particular, nitrogen can be selected from the viewpoint of industrial availability and cost.

  The temperature of the oily liquid before forming a suspension with the aqueous liquid may be 40 ° C. or higher, 45 to 110 ° C., 50 to 100 ° C., 55 to 90 ° C., or 60 to 85 ° C. The temperature of the suspension from the start of mixing the hydrocarbon dispersion medium and the aqueous liquid containing the water-soluble ethylenically unsaturated monomer to the end of mixing all of these is 35 ° C. or higher, 40-100 ° C. It may be maintained at 45-90 ° C, 50-85 ° C, or 55-85 ° C. When the temperature of the suspension in the mixing process is within these ranges, the remaining volatile components can be more effectively reduced.

  Since the polymerization temperature at the time of carrying out the reverse phase suspension polymerization varies depending on the type of the water-soluble radical polymerization initiator used, it cannot be generally determined. Usually, the reaction temperature is 20 to 110 ° C. or 40 to 90 from the viewpoint of shortening the polymerization time by allowing the polymerization to proceed rapidly, removing the heat of polymerization, and performing the reaction smoothly. It may be ° C. The polymerization time is usually 0.5 to 4 hours.

  In the polymerization step, the water-soluble ethylenically unsaturated monomer is polymerized to produce a particulate hydrogel polymer. Usually, the obtained polymer (hydrogel polymer) can be obtained in various forms such as a spherical shape, a granular shape, a crushed shape, a confetti shape, and an aggregate thereof. From the viewpoint of improving the specific surface area and the water absorption rate, the hydrogel polymer may be in the form of granules, or in the form of granules having many protrusions on the surface.

  After forming the hydrogel polymer by the first polymerization step (reverse phase suspension polymerization) as described above, an additional aqueous liquid containing a water-soluble ethylenically unsaturated monomer is added to the reaction mixture. You may further implement the superposition | polymerization process (reverse phase suspension polymerization) which superpose | polymerizes a monomer once or twice or more. By performing the reverse phase suspension polymerization a plurality of times, it is possible to achieve a further reduction in the amount of residual volatile components. When performing reverse phase suspension polymerization a plurality of times, the total number of polymerization steps may be 2 or 3 from the viewpoint of increasing productivity while reducing the amount of residual volatile components.

  Specifically, the reaction mixture containing the hydrogel polymer produced in the first polymerization step is cooled as necessary, and is 45 ° C or higher, 50-100 ° C, 55-90 ° C, 60-85 ° C, Or you may adjust to 65-80 degreeC.

  Then, the reaction mixture containing the hydrogel polymer is mixed with an aqueous solvent, a water-soluble ethylenically unsaturated monomer and, if necessary, an additional aqueous liquid (second aqueous liquid) containing a radical polymerization initiator. Then, the droplet-like second aqueous liquid is dispersed in the reaction mixture. In the second polymerization step, the temperature of the suspension from the start of mixing the reaction mixture containing the hydrogel polymer and the second aqueous liquid to the end of mixing all of these is 35 ° C. or higher. , 40-90 ° C, 45-85 ° C, or 50-80 ° C. The temperature of the reaction mixture and the second aqueous liquid before mixing, and the temperature of the suspension after mixing may also be in the above ranges. Thereby, it is possible to reduce the amount of residual volatile components of the obtained water-absorbent resin particles particularly efficiently. The third and subsequent polymerization steps can also be performed in the same manner as the second polymerization step using an additional aqueous liquid (third aqueous liquid or the like).

  In the case where a plurality of polymerization steps are performed, the above-described gas supply ratio is the water-soluble ethylenic content introduced into the suspension in the n-th polymerization step (n is an integer of 1 to 3, for example). Ratio of the supply volume (C) of the inert gas per minute to the sum of the volume (An) of the aqueous liquid containing the unsaturated monomer (nth aqueous liquid) and the volume (B) of the hydrocarbon dispersion medium (= C / (An + B)). The gas supply ratio is 0.1 to 3.0. The gas supply ratio may be 0.12-2.5, 0.15-2.0, 0.18-1.5, or 0.2-1.2. In each polymerization step after the second time, the residual volatile components can be efficiently reduced by setting the gas supply ratio after introducing the aqueous liquid containing the water-soluble ethylenically unsaturated monomer within these ranges. It becomes possible.

  The water-soluble ethylenically unsaturated monomer aqueous solution and radical polymerization initiator contained in the aqueous liquid introduced in the second and subsequent polymerization steps are, for example, the same as those described above in the description of the first polymerization step. Types of compounds can be used in similar amounts. The aqueous liquid in each polymerization step may be composed of the same monomer or the like, or may be composed of a different monomer or the like.

  The amount of the water-soluble ethylenically unsaturated monomer contained in the aqueous liquid introduced in the second and subsequent polymerization steps is the water solubility contained in the aqueous liquid (first aqueous liquid) in the first polymerization step. 20-250 mass parts, 40-200 mass parts, or 60-150 mass parts may be sufficient with respect to 100 mass parts of ethylenically unsaturated monomers. When the amount of the water-soluble ethylenically unsaturated monomer in the aqueous liquid after the second time is 20 parts by mass or more, the amount of residual volatile components of the resulting water-absorbent resin particles tends to be further reduced, There exists a tendency which can suppress that the particle diameter of the water-absorbent resin particle obtained becomes excessive because the usage-amount is 250 mass parts or less.

  The concentration of the water-soluble ethylenically unsaturated monomer in the aqueous liquid introduced in the second and subsequent polymerization steps is determined from the viewpoint of productivity improvement, from the viewpoint of productivity, aqueous solvent, water-soluble ethylenically unsaturated monomer, radical polymerization 1 mass% or more, 2-25 mass%, 3-20 mass%, or 4-15 mass% may be higher than the density | concentration of a 1st aqueous liquid on the basis of the sum total of an initiator and various additives.

  A hydrogel polymer is generated by the first polymerization step. The manufacturing method according to the present embodiment may further include an intermediate crosslinking step of crosslinking the produced hydrogel polymer before the second and subsequent polymerization steps. The intermediate crosslinking of the hydrated gel polymer is performed, for example, by mixing and heating the hydrated gel polymer and the following intermediate crosslinking agent.

  The intermediate crosslinking agent has a functional group capable of reacting with a functional group contained in the water-soluble ethylenically unsaturated monomer (for example, a carboxyl group in the case of acrylic acid), and may be a water-soluble compound. . Examples of the intermediate crosslinking agent include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; (poly) ethylene glycol diglycidyl Compounds having two or more epoxy groups such as ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; Compounds having two or more isocyanate groups such as 4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3- Oxetane compounds such as xetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; 1,2-ethylene Examples thereof include oxazoline compounds such as bisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide. Among these, the intermediate crosslinking agent may be a compound having two or more functional groups in the molecule that can react with the functional group of the water-soluble ethylenically unsaturated monomer. Examples of such a compound include the above polyol, a compound having two or more epoxy groups, a haloepoxy compound, and a compound having two or more isocyanate groups. These may be used alone or in combination of two or more.

  Among the intermediate crosslinking agents, a compound having two or more epoxy groups can be selected from the viewpoint of excellent reactivity. Among these, from the viewpoint of high solubility in water and good handleability as an intermediate crosslinking agent, the intermediate crosslinking agent is ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, glycerin diglycidyl ether, polyethylene glycol diglycidyl ether. , And at least one compound selected from the group consisting of polyglycerol glycidyl ethers, and from the viewpoint of improving the various performances of the water-absorbent resin particles obtained, the intermediate crosslinking agent is ethylene glycol diglycidyl ether, And at least one compound selected from propylene glycol diglycidyl ether.

  The mixing amount of the intermediate crosslinking agent is 0.0001 to 0.03 mol, 0.0005 to 0 with respect to 100 mol of the water-soluble ethylenically unsaturated monomer used to produce the hydrogel polymer. It may be 0.02 mol or 0.001 to 0.015 mol. When the mixing amount of the intermediate crosslinking agent is 0.0001 mol or more, the water-soluble ethylenically unsaturated monomer in the aqueous liquid in the second polymerization step is added to the hydrogel polymer after the intermediate crosslinking step. Absorption is suppressed, and there is a tendency that a decrease in water absorption rate and swelling performance can be suppressed. Moreover, there exists a tendency which can suppress the fall of the water absorption performance of the water absorbing resin particle by excessive bridge | crosslinking as the mixing amount of an intermediate crosslinking agent is 0.03 mol or less. The intermediate crosslinking reaction may have a reaction temperature of 60 ° C. or higher, or 70 ° C. to the boiling point of the hydrocarbon dispersion medium used in the first polymerization step. The reaction time of the intermediate cross-linking reaction varies depending on the reaction temperature, the type and amount of the intermediate cross-linking agent, and cannot be determined unconditionally, but is usually 1 to 200 minutes, 5 to 100 minutes, or 10 to 60 minutes. It may be.

  When mixing the hydrogel polymer and the intermediate crosslinking agent, water may be added as a solvent for dissolving the intermediate crosslinking agent in order to uniformly disperse the intermediate crosslinking agent, and a hydrophilic solvent is added. May be. Examples of the hydrophilic solvent include lower alcohols such as methyl alcohol, ethyl alcohol and isopropyl alcohol, ketones such as acetone and methyl ethyl ketone, ethers such as dioxane and tetrahydrofuran, amides such as N, N-dimethylformamide, and dimethyl sulfoxide. Examples include sulfoxide. These solvents may be used alone or in combination of two or more as necessary.

  Also in the second and subsequent polymerization steps, the water-soluble ethylenically unsaturated monomer is polymerized to further produce a particulate hydrous gel polymer. Usually, the obtained polymer (hydrogel polymer) can be obtained in various forms such as a spherical shape, a granular shape, a crushed shape, a confetti shape, and an aggregate thereof. In this embodiment, the specific surface area and From the viewpoint of improving the water absorption rate, the hydrogel polymer may be granular, or may be granular having many protrusions on the surface. You may perform the above-mentioned intermediate bridge | crosslinking between each polymerization process after the 2nd time.

  Based on the method for producing water-absorbent resin particles according to one of the embodiments, the productivity of the water-absorbent resin particles can be increased by carrying out reverse phase suspension polymerization twice or more in one process. It becomes possible to improve. Surprisingly, it is possible to greatly reduce the amount of residual volatile components of the water-absorbent resin particles obtained.

  Although the mechanism of reducing the amount of residual volatile components by carrying out the second and subsequent polymerizations is not clear, the hydrogel polymer obtained by the first polymerization when carrying out the second and subsequent polymerizations Is present to stabilize the separation and coalescence of the aqueous droplets in the W / O type reversed phase suspension system, that is, the hydrocarbon dispersion medium is included in the aqueous droplets when the aqueous droplets collide with each other. This is presumed to be because the frequency of the formation of the O / W / O structure is suppressed.

  The method for producing water-absorbing resin particles according to this embodiment may further include a post-crosslinking step of cross-linking the hydrogel polymer obtained in the final polymerization step. Furthermore, the method is such that the water in the hydrogel polymer is 100% by mass based on the water-soluble ethylenically unsaturated monomer-derived component (polymer solid content) constituting the hydrogel polymer. A primary drying step for adjusting the mass percentage (water content of the hydrogel polymer) to be, for example, 20 to 130% by mass may be provided before the post-crosslinking step.

  Although it does not specifically limit as a drying method of the said primary drying process, For example, it is azeotropic distillation by heating from the outside in the state which dispersed the (a) hydrogel polymer in the oil-based liquid (hydrocarbon dispersion medium). A method of removing water by refluxing the hydrocarbon dispersion medium, (b) A method of taking out the hydrogel polymer by decantation and drying under reduced pressure, (c) Filtering off the hydrogel polymer with a filter and drying under reduced pressure And the like. Especially, the method of (a) can be selected from the simplicity in a manufacturing process.

  As described above, a water-absorbent resin having better water-absorbing performance by post-crosslinking the water-containing gel polymer adjusted so that the water content of the water-containing gel polymer is, for example, 20 to 130% by mass. Particles are obtained.

  The post-crosslinking agent has a functional group that can react with a functional group contained in the water-soluble ethylenically unsaturated monomer (for example, a carboxyl group in the case of acrylic acid), and may be a water-soluble compound. . Examples of post-crosslinking agents include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; (poly) ethylene glycol diglycidyl Compounds having two or more epoxy groups such as ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; Compounds having two or more isocyanate groups such as 4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3-o Oxetane compounds such as cetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol; 1,2-ethylenebis Oxazoline compounds such as oxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide. Among these, the post-crosslinking agent may be a compound having in the molecule two or more functional groups capable of reacting with the functional group of the water-soluble ethylenically unsaturated monomer. Examples of such a compound include the above polyol, a compound having two or more epoxy groups, a haloepoxy compound, and a compound having two or more isocyanate groups. These may be used alone or in combination of two or more.

  Among the post-crosslinking agents, a compound having two or more epoxy groups can be selected from the viewpoint of excellent reactivity. Among them, from the viewpoint of high solubility in water and good handleability as a post-crosslinking agent, the post-crosslinking agent is ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, glycerin diglycidyl ether, polyethylene glycol diglycidyl ether. And at least one compound selected from the group consisting of polyglycerol glycidyl ether. From the viewpoint of improving the various performances of the water-absorbing resin particles obtained, the post-crosslinking agent may be at least one compound selected from ethylene glycol diglycidyl ether and propylene glycol diglycidyl ether.

  The mixing amount of the post-crosslinking agent is 0.0001 to 1 mol, 0.0005 to 0.5 with respect to 100 mol of the water-soluble ethylenically unsaturated monomer used to produce the hydrogel polymer. Mole, 0.001 to 0.1 mol, or 0.005 to 0.05 mol may be used. When the mixing amount of the post-crosslinking agent is 0.0001 mol or more, the effect of crosslinking is exhibited, and the water-absorbing resin particle surface at the time of water absorption does not become viscous, and the water absorption rate of the water-absorbing resin particles is further improved. When the amount is 1 mol or less, crosslinking does not become excessive, and the water absorption amount of the water absorbent resin particles tends to be further improved.

  Mixing of the hydrogel polymer and the post-crosslinking agent can be performed after adjusting the moisture content of the hydrogel polymer to a specific range (primary drying step). Thus, by controlling the moisture content of the hydrogel polymer during mixing of the hydrogel polymer and the postcrosslinking agent, the postcrosslinking reaction can proceed more suitably.

  20-130 mass%, 25-110 mass%, 30-90 mass%, 35-80 mass%, or 40-70 mass% may be sufficient as the moisture content of the hydrogel polymer in a post-crosslinking process. By setting the moisture content of the hydrogel polymer within the above range, it is possible to further improve the water absorption performance by a post-crosslinking reaction while shortening the primary drying step and increasing the production efficiency.

  The water content of the hydrogel polymer is the amount obtained by subtracting the amount of water extracted outside in the primary drying step from the amount of water contained in the aqueous liquid before polymerization in the entire polymerization step (the amount of water in the primary dry gel). ) And the amount of water added to the reaction mixture as necessary together with the intermediate or post-crosslinking agent, the component derived from the water-soluble ethylenically unsaturated monomer constituting the hydrogel polymer It can obtain | require by calculating the ratio of the moisture content of a water-containing gel-like polymer with respect to the mass of.

  The mass of the component derived from the water-soluble ethylenically unsaturated monomer constituting the hydrogel polymer is calculated from the total mass of the water-soluble ethylenically unsaturated monomer used in the polymerization reaction as the theoretical polymer solid content. Can be obtained by calculation.

  In mixing the hydrogel polymer and the post-crosslinking agent, water may be added as a solvent for dissolving the post-crosslinking agent in order to uniformly disperse the post-crosslinking agent, and a hydrophilic solvent is added. May be. Examples of the hydrophilic solvent include lower alcohols such as methyl alcohol, ethyl alcohol and isopropyl alcohol, ketones such as acetone and methyl ethyl ketone, ethers such as dioxane and tetrahydrofuran, amides such as N, N-dimethylformamide, and dimethyl sulfoxide. Examples include sulfoxide. These solvents may be used alone or in combination of two or more as necessary.

  The mass ratio between the water content of the gel polymer after the primary drying and the amount of solvent added is from the viewpoint of uniformly dispersing the post-crosslinking agent while rationally shortening the drying step and improving the economics of the process. 100: 0-60: 40, 99: 1-70: 30, 98: 2-80: 20, or 98: 2-90: 10.

  The reaction temperature of the post-crosslinking reaction is 60 ° C. or higher, and may be 70 to 200 ° C. or 80 to 150 ° C. When the reaction temperature is 60 ° C. or higher, the post-crosslinking reaction is promoted, and there is a tendency that excessive time is not required for the reaction. There exists a tendency which can suppress the fall of water absorption performance.

  The reaction time of the post-crosslinking reaction varies depending on the reaction temperature, the type and amount of the post-crosslinking agent, and cannot be determined unconditionally, but is usually 1 to 300 minutes and may be 5 to 200 minutes. .

  The method for producing the water-absorbent resin particles of the present embodiment includes a secondary drying step in which water, hydrocarbon dispersion medium, and the like are removed by distillation by applying energy such as heat from the outside after the post-crosslinking reaction. May be provided. By performing such secondary drying, water-absorbing resin particles having excellent fluidity tend to be obtained.

  The method of secondary drying is not particularly limited. For example, (a) a mixture of resin particles after being dispersed in an oily liquid (hydrocarbon dispersion medium) and then subjected to a cross-linking reaction is distilled to obtain water and a hydrocarbon dispersion medium. And the like, (b) a method of taking out resin particles by decantation and drying under reduced pressure, (c) a method of separating resin particles by a filter and drying under reduced pressure. Among these, the method of (a) can be selected from the simplicity in a manufacturing process.

  According to the manufacturing method concerning this embodiment, the water-absorbent resin particle concerning embodiment which is demonstrated below can be obtained. The water-absorbent resin particles have a low residual volatile component amount while having an appropriately sized particle diameter and an excellent water absorption rate.

  The median particle diameter of the water absorbent resin particles may be 100 to 600 μm. By having such a median particle diameter, it is possible to maintain good handling properties at the time of manufacturing the absorber and to design the absorber thinly. The median particle size may be 110 to 500 μm, 120 to 400 μm, or 120 to 350 μm. The method for measuring the median particle size will be described in detail in Examples described later.

  The water absorption speed of the physiological saline of the water absorbent resin particles may be 1 to 20 seconds. Since the water-absorbent resin particles have such an excellent water absorption speed, it is possible to prevent liquid leakage when the water-absorbent resin particles are used for absorbent articles, water-stopping materials, and the like. The water absorption rate may be 1 to 15 seconds, 2 to 10 seconds, 2 to 8 seconds, or 2 to 6 seconds. The method for measuring the water absorption rate will be described in detail in the examples described later.

  The residual volatile component amount of the water absorbent resin particles may be 1.5% by mass or less. In this way, the water-absorbent resin particles have a small amount of residual volatile components, for example, to suppress the generation of odor when the water-absorbent resin particles absorb water and improve the comfort of the wearer of the absorbent article Can do. Moreover, even if the said residual volatile component amount is 1.3 mass% or less, 0.001-1.0 mass%, 0.01-0.8 mass%, or 0.1-0.5 mass%. Good. The method for measuring the amount of residual volatile components will be described in detail in the examples described later.

  Although the water absorption amount of the physiological saline of the water absorbent resin particles is not particularly limited, it can be increased from the viewpoint of increasing the absorption capacity of the absorbent article. 30-90 g / g, 35-80 g / g, 45-75 g / g, 50-70 g / g, or 55-65 g / g may be sufficient as the water absorption amount of the physiological saline of a water absorbing resin particle. In addition, the measuring method of the water absorption amount of the physiological saline will be described in detail in Examples described later.

  Depending on the purpose, the water-absorbent resin particles obtained by the method for producing water-absorbent resin particles of the present embodiment may contain additives such as a heat-resistant stabilizer, an antioxidant, and an antibacterial agent. The amount of the additive varies depending on the use of the water-absorbent resin particles, the kind of the additive, and the like, but is 0.001 to 10 parts by mass, 0.01 to 5 parts by mass with respect to 100 parts by mass of the water-absorbent resin particles, Or 0.1-2 mass parts may be sufficient.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples.

[ Reference Example 1 ]
A round bottom cylindrical separable flask (hereinafter referred to as “round bottom flask”) having an inner diameter of 100 mm equipped with a reflux condenser, a dropping funnel, a nitrogen gas introduction tube, and a stirrer was prepared. As the stirrer, an apparatus having a two-stage inclined four-paddle blade with a blade diameter of 50 mm and a stirring blade coated on the surface with a fluororesin was used. To this round bottom flask was added 660 mL of n-heptane as a hydrocarbon dispersion medium, and 1.10 g of sorbitan monolaurate (manufactured by NOF Corporation, trade name: Nonion LP-20R; HLB8.6) as a surfactant was added. The temperature was raised to 70 ° C., and the surfactant was dissolved in n-heptane.

  On the other hand, 92 g (1.03 mol) of an 80.5 mass% acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer to a 300 mL beaker. While cooling the aqueous acrylic acid solution with ice water, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was added dropwise to the beaker to neutralize 75 mol% of the acrylic acid in the aqueous solution. Thereafter, 0.10 g (0.00037 mol) of potassium persulfate as a water-soluble radical polymerization initiator was added to the beaker and dissolved to prepare an aqueous liquid containing acrylic acid. This aqueous liquid had a monomer solid content of 91 g, a water content of 148.6 g, and a volume of 209 mL.

  The total amount of the aqueous liquid was added to n-heptane in a round bottom flask with the rotation speed of the stirrer set at 700 rpm to prepare a reverse phase suspension as a reaction liquid. The temperature of the resulting reaction solution was 47 ° C. At the same temperature, 99.8% nitrogen gas was supplied into the reaction solution at a volume of 200 mL per minute for 30 minutes to replace the inside of the reaction system. Thereafter, while the nitrogen gas was supplied at the same rate, the round bottom flask was immersed in a 70 ° C. water bath and the temperature was raised. As a result, the polymerization reaction started, and an exothermic peak was observed after 20 minutes. After that, heating was continued, and supply of nitrogen gas was stopped after 1 hour from the start of temperature increase to obtain a hydrogel polymer that had been formed.

  Next, the temperature in the system was raised using an oil bath at 120 ° C., and water and n-heptane were azeotroped to extract 111.7 g of water out of the system while refluxing n-heptane. Primary drying). Thereafter, 4.14 g (0.00048 mol) of a 2% by mass ethylene glycol diglycidyl ether aqueous solution was added to the round bottom flask as a post-crosslinking agent. The water content in the round bottom flask at this time was 40.9 g, and the water content of the hydrogel polymer after primary drying (after cross-linking) was 45% by mass. The mixture containing the postcrosslinking agent was held at about 80 ° C. for 2 hours (postcrosslinking step).

  Thereafter, n-heptane was evaporated at 120 ° C. and dried (secondary drying step) to obtain 88.2 g of granular water-absorbing resin particles.

[Example 2]
A round bottom cylindrical separable flask (hereinafter referred to as “round bottom flask”) having an inner diameter of 100 mm equipped with a reflux condenser, a dropping funnel, a nitrogen gas introduction tube, and a stirrer was prepared. As the stirrer, an apparatus having a two-stage inclined four-paddle blade with a blade diameter of 50 mm and a stirring blade coated on the surface with a fluororesin was used. To this round bottom flask was added 660 mL of n-heptane as a hydrocarbon dispersion medium, and 1.10 g of sorbitan monolaurate (manufactured by NOF Corporation, trade name: Nonion LP-20R; HLB8.6) as a surfactant was added. The temperature was raised to 75 ° C., and the surfactant was dissolved in n-heptane.

  On the other hand, 92 g (1.03 mol) of an 80.5 mass% acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer to a 300 mL beaker. While cooling the aqueous acrylic acid solution with ice water, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was added dropwise to the beaker to neutralize 75 mol% of the acrylic acid in the aqueous solution. Thereafter, 0.10 g (0.00037 mol) of potassium persulfate as a water-soluble radical polymerization initiator was added to the beaker and dissolved to prepare an aqueous liquid containing acrylic acid. This aqueous liquid had a monomer solid content of 91 g, a water content of 148.6 g, and a volume of 209 mL.

  The total amount of the aqueous liquid was added to n-heptane in a round bottom flask with the rotation speed of the stirrer set at 700 rpm to prepare a reverse phase suspension as a reaction liquid. The temperature of the obtained reaction liquid became 55 degreeC. At the same temperature, 99.8% nitrogen gas was supplied into the reaction solution at a volume of 400 mL per minute for 30 minutes to replace the inside of the reaction system. Thereafter, while the nitrogen gas was supplied at the same rate, the round bottom flask was immersed in a 70 ° C. water bath and the temperature was raised. As a result, the polymerization reaction started, and an exothermic peak was observed after 20 minutes. After that, heating was continued, and supply of nitrogen gas was stopped after 1 hour from the start of temperature increase to obtain a hydrogel polymer that had been formed.

Next, the same operation as in Reference Example 1 was performed to obtain 89.1 g of granular water-absorbent resin particles.

[Example 3]
A round bottom cylindrical separable flask (hereinafter referred to as “round bottom flask”) having an inner diameter of 100 mm equipped with a reflux condenser, a dropping funnel, a nitrogen gas introduction tube, and a stirrer was prepared. As the stirrer, an apparatus having a two-stage inclined four-paddle blade with a blade diameter of 50 mm and a stirring blade coated on the surface with a fluororesin was used. To this round bottom flask was added 530 mL of n-heptane as a hydrocarbon dispersion medium, and 1.65 g of sorbitan monolaurate (manufactured by NOF Corporation, trade name: Nonion LP-20R; HLB8.6) as a surfactant. The temperature was raised to 72 ° C., and the surfactant was dissolved in n-heptane.

  On the other hand, 92 g (1.03 mol) of an 80.5 mass% acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer to a 300 mL beaker. While cooling the acrylic acid aqueous solution with ice water, 147.6 g of a 20.9 mass% sodium hydroxide aqueous solution was added dropwise to the beaker to neutralize 75 mol% of the acrylic acid in the aqueous solution. Thereafter, 0.10 g (0.00037 mol) of potassium persulfate as a water-soluble radical polymerization initiator was added to the beaker and dissolved to prepare a first aqueous liquid containing acrylic acid. This first aqueous liquid had a monomer solid content of 91.0 g, a water content of 148.5 g, and a volume of 209 mL.

  The total amount of the first aqueous liquid was added to n-heptane in the round bottom flask at a rotation speed of the stirrer of 500 rpm to prepare a reverse phase suspension as a reaction liquid. The temperature of the obtained reaction liquid became 51 degreeC. At the same temperature, 99.8% nitrogen gas was supplied into the system at a volume of 500 mL per minute for 30 minutes to replace the inside of the reaction system. Thereafter, the temperature was raised by immersing the round bottom flask in a 70 ° C. water bath while supplying nitrogen gas at the same rate. The polymerization reaction started, and an exothermic peak was observed 18 minutes later. After that, heating was continued, and supply of nitrogen gas was stopped after 1 hour from the start of temperature increase to obtain a hydrogel polymer that had been formed (first polymerization step). After the polymerization, 1.24 g (0.00014 mol) of a 2% by mass aqueous ethylene glycol diglycidyl ether solution was added as an intermediate crosslinking agent, and a crosslinking reaction was performed at 75 ° C. for 30 minutes (intermediate crosslinking step).

  Next, separately from the first polymerization step, 92 g (1.03 mol) of an aqueous acrylic acid solution containing 80.5% by mass of acrylic acid as a water-soluble ethylenically unsaturated monomer was added to a 300 mL beaker. It was. While cooling the acrylic acid aqueous solution with ice water, 114.7 g of 26.9 mass% sodium hydroxide aqueous solution was dropped into a beaker, and 75 mol% of the acrylic acid in the aqueous solution was neutralized. Thereafter, 0.10 g (0.00037 mol) of potassium persulfate as a water-soluble radical polymerization initiator was added to the beaker and dissolved to prepare a second aqueous liquid. This second aqueous liquid had a monomer solid content of 91.0 g, a water content of 115.9 g, and a volume of 177 mL.

  The reaction mixture after completion of the crosslinking reaction with the intermediate crosslinking agent was cooled to 70 ° C. while stirring at a rotation speed of a stirrer of 1000 rpm. After dropwise addition of the entire amount of the second aqueous liquid in the cooled round bottom flask, the system temperature was maintained at the temperature (55 ° C.) when the dropping was completed, and 99.8% nitrogen gas was introduced into the system at the same temperature. Was supplied at a volume of 500 mL per minute for 30 minutes to replace the inside of the reaction system. Thereafter, the temperature was raised by immersing the round bottom flask in a 70 ° C. water bath while supplying nitrogen gas into the reaction solution at the same rate. The polymerization reaction started, and an exothermic peak was observed after 15 minutes. After that, heating was continued, and after 1 hour from the start of temperature increase, the supply of nitrogen gas was stopped to obtain a hydrogel polymer that was completely formed (second polymerization step).

  Immerse the round bottom flask in a 120 ° C. oil bath, raise the temperature of the suspension containing the hydrogel polymer after the second polymerization step, and azeotrope water and n-heptane. Then, 208.6 g of water was extracted out of the system while refluxing n-heptane (primary drying step). Thereafter, 8.28 g (0.00095 mol) of a 2% by mass aqueous ethylene glycol diglycidyl ether solution was added as a post-crosslinking agent to the round bottom flask to obtain a mixture containing the post-crosslinking agent. The water content in the round bottom flask at this time was 63.9 g, and the water content of the hydrogel polymer after primary drying (after cross-linking) was 35 mass%. After preparing a mixture containing a post-crosslinking agent, the mixture was held at about 80 ° C. for 2 hours (post-crosslinking step).

  Then, 191.1 g of granular water-absorbing resin particles were obtained by evaporating n-heptane at 120 ° C. and drying (secondary drying step).

[Comparative Example 1]
A round bottom cylindrical separable flask (hereinafter referred to as “round bottom flask”) having an inner diameter of 100 mm equipped with a flow cooler, a dropping funnel, a nitrogen gas inlet tube, and a stirrer was prepared. As the stirrer, an apparatus having a two-stage inclined four-paddle blade with a blade diameter of 50 mm and a stirring blade coated on the surface with a fluororesin was used. To this round bottom flask was added 660 mL of n-heptane as a hydrocarbon dispersion medium, and 1.10 g of sorbitan monolaurate (manufactured by NOF Corporation, trade name: Nonion LP-20R; HLB8.6) as a surfactant, The temperature was raised to 45 ° C., and the surfactant was dissolved in n-heptane.

  On the other hand, 92 g (1.03 mol) of an 80.5 mass% acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer to a 300 mL beaker. While cooling the aqueous acrylic acid solution with ice water, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was added dropwise to the beaker to neutralize 75 mol% of the acrylic acid in the aqueous solution. Thereafter, 0.10 g (0.00037 mol) of potassium persulfate as a water-soluble radical polymerization initiator was added to the beaker and dissolved to prepare an aqueous liquid containing acrylic acid. This aqueous liquid had a monomer solid content of 91 g, a water content of 148.6 g, and a volume of 209 mL.

  The rotation speed of the stirrer was set to 700 rpm, and the entire amount of the aqueous liquid was added to the round bottom flask to prepare a reverse phase suspension as a reaction liquid. The temperature of the reaction solution was 47 ° C. At the same temperature, 99.8% nitrogen gas was supplied into the system at a volume of 80 mL per minute for 30 minutes to replace the inside of the reaction system. Thereafter, while the nitrogen gas was supplied at the same rate, the round bottom flask was immersed in a 70 ° C. water bath and the temperature was raised. As a result, the polymerization reaction started, and an exothermic peak was observed 30 minutes later. After that, heating was continued, and supply of nitrogen gas was stopped after 1 hour from the start of temperature increase to obtain a hydrogel polymer that had been formed.

  Next, the temperature of the system was raised using an oil bath at 120 ° C., and water and n-heptane were azeotroped to extract 127.6 g of water out of the system while refluxing n-heptane. Primary drying step). Thereafter, 5.52 g (0.00063 mol) of a 2% by mass aqueous ethylene glycol diglycidyl ether solution was added to the round bottom flask as a post-crosslinking agent to obtain a mixture containing the post-crosslinking agent. The water content in the round bottom flask at this time was 26.3 g, and the water content of the hydrogel polymer after primary drying (after cross-linking) was 29% by mass. After preparing a mixture containing a post-crosslinking agent, the mixture was held at about 80 ° C. for 2 hours (post-crosslinking step).

  Thereafter, n-heptane was evaporated at 120 ° C. and dried (secondary drying step) to obtain 87.4 g of granular water-absorbing resin particles.

[Comparative Example 2]
A round bottom cylindrical separable flask (hereinafter referred to as “round bottom flask”) having an inner diameter of 100 mm equipped with a flow cooler, a dropping funnel, a nitrogen gas inlet tube, and a stirrer was prepared. As the stirrer, an apparatus having a two-stage inclined four-paddle blade with a blade diameter of 50 mm and a stirring blade coated on the surface with a fluororesin was used. To this round bottom flask was added 660 mL of n-heptane as a hydrocarbon dispersion medium, and 1.10 g of sorbitan monolaurate (manufactured by NOF Corporation, trade name: Nonion LP-20R; HLB8.6) as a surfactant, The temperature was raised to 75 ° C., and the surfactant was dissolved in n-heptane.

  On the other hand, 92 g (1.03 mol) of an 80.5 mass% acrylic acid aqueous solution was added as a water-soluble ethylenically unsaturated monomer to a 300 mL beaker. While cooling the aqueous acrylic acid solution with ice water, 147.7 g of a 20.9 mass% aqueous sodium hydroxide solution was added dropwise to the beaker to neutralize 75 mol% of the acrylic acid in the aqueous solution. Thereafter, 0.10 g (0.00037 mol) of potassium persulfate as a water-soluble radical polymerization initiator was added to the beaker and dissolved to prepare an aqueous liquid containing acrylic acid. This aqueous liquid had a monomer solid content of 91 g, a water content of 148.6 g, and a volume of 209 mL.

  The rotation speed of the stirrer was set to 700 rpm, and the entire amount of the aqueous liquid was added to the round bottom flask to prepare a reverse phase suspension as a reaction liquid. The temperature of the reaction solution was 34 ° C. At the same temperature, 99.8% nitrogen gas was supplied into the reaction solution at a volume of 2700 mL per minute for 30 minutes to replace the inside of the reaction system. Thereafter, while the nitrogen gas was supplied at the same ratio, the round bottom flask was immersed in a 70 ° C. water bath and the temperature was raised. As a result, the polymerization reaction started, and an exothermic peak was observed after 10 minutes. After that, heating was continued, and supply of nitrogen gas was stopped after 1 hour from the start of temperature increase to obtain a hydrogel polymer that had been formed.

Next, the same operation as in Reference Example 1 was performed to obtain 87.1 g of granular water-absorbent resin particles.

<Evaluation>
The following evaluation was performed on the water-absorbent resin particles obtained in Examples , Reference Examples and Comparative Examples. The evaluation results are shown in Table 2 together with the gas supply ratio in each polymerization step.

(1) Residual Volatile Components (a) Preparation of Calibration Curve The hydrocarbon dispersion medium, dimethylformamide (DMF), and 25% by mass phosphoric acid aqueous solution used in Examples , Reference Examples and Comparative Examples are placed in a stoppered glass container. Prepared. These were cooled and used as necessary to suppress errors due to volatilization during measurement.

  First, 0.15 g of the above hydrocarbon dispersion medium was precisely weighed into a volumetric flask having an internal volume of 200 mL, and DMF was added so that the total volume became 200 mL. Next, the standard solution 1 was accurately weighed into a 20 mL volumetric flask with a 10 mL hole pipette, DMF was added to a total of 20 mL, and the standard solution 1 was diluted in half.

  Similarly, standard solution 3 is prepared by diluting standard solution 2 in half, standard solution 4 is prepared by diluting standard solution 3 in half, and standard solution 4 is prepared by diluting standard solution 4 in half. 5 was prepared.

  4 mL of the standard solution 1 was added to a vial bottle (SMI-Lab Ltd., VZH-20CR-100) having an internal volume of 20 mL, and 5 mL of 25 mass% phosphoric acid aqueous solution was further added. The vial was quickly sealed using a septum rubber and an aluminum cap, and the vial was shaken for 1 minute. The same operation was performed for the standard solutions 2 to 5 to prepare a calibration curve solution.

  The vial was heated at 110 ° C. for 2 hours with shaking and stirring, and then 1 mL of the gas phase was injected into the gas chromatograph to obtain a chromatogram for each calibration curve solution. A calibration curve was prepared using the mass of the hydrocarbon dispersion medium and the peak area of the chromatogram precisely weighed for preparing the calibration curve solution. When a plurality of peaks derived from the hydrocarbon dispersion medium were observed, a calibration curve was created using the sum of the peak areas.

(B) Measurement of residual volatile component amount DMF and 25 mass% phosphoric acid aqueous solution were prepared. 0.10 g of the water-absorbent resin particles obtained in Examples , Reference Examples and Comparative Examples were precisely weighed into vials having an internal volume of 20 mL. To this vial, 4 mL of DMF and 5 mL of 25 mass% phosphoric acid aqueous solution were added. The vial was quickly sealed using a septum rubber and an aluminum cap, and the vial was shaken for 1 minute. While heating the vial at 110 ° C. for 2 hours while shaking the vial, 1 mL of the gas phase was injected into the gas chromatograph to obtain a chromatogram.

  From the peak area of the obtained chromatogram and the calibration curve created earlier, the amount of the hydrocarbon dispersion medium contained in the water-absorbent resin particles (0.10 g of the exact balance) is calculated and converted to 1 g of the water-absorbent resin particles. The value obtained was defined as the residual volatile component amount (% by mass).

The following are gas chromatographic conditions.
Device: GC-2014 (manufactured by Shimadzu Corporation)
Headspace autosampler: HT200H (manufactured by Hamilton Company)
Filler: Squalane 25% Shimalite (NAW) (101)
80-100 mesh
Column: 3.2mmφ × 2.1m
Column temperature: 80 ° C
Inlet temperature: 180 ° C Detector temperature: 180 ° C Detector: FID
Carrier gas: N ₂
Vial heating temperature: 110 ° C
Syringe set temperature: 130 ° C

(2) Odor sensory test (6-level odor intensity display method)
The odor derived from the hydrocarbon dispersion medium during swelling of the water-absorbent resin particles was evaluated by the following method. 20.0 g of a 0.9 mass% sodium chloride aqueous solution (physiological saline) at 25 ° C. was added to a glass bottle with a lid (mayonnaise bottle) having an internal volume of 140 mL, and a rotor having a length of 3 cm was added and stirred. 2.0 g of water-absorbent resin particles were put in a glass bottle with a lid and sealed. The odor derived from the hydrocarbon dispersion medium in the glass bottle was determined by five analysts according to the evaluation criteria shown in Table 1, and the average value was taken as the odor evaluation result.

(3) Absorption amount of physiological saline (g / g)
A mixed solution of 500 g of physiological saline and 2.0 g of water absorbent resin particles was stirred at room temperature for 60 minutes. The mixture solution was filtered using a JIS Z8801-1 standard sieve having a mass Wa (g) and an opening of 75 μm. With the sieve tilted at an inclination angle of about 30 degrees with respect to the horizontal, the filter cake was left on the sieve for 30 minutes. The total weight Wb (g) of the absorbed water-absorbing resin particles and the sieve was measured, and the amount of water absorption was determined by the following equation.
Absorption amount of physiological saline = (Wb−Wa) /2.0

(4) Saline water absorption rate (seconds)
The water absorption rate was measured in a room adjusted to 25 ° C. ± 1 ° C. 50 ± 0.1 g of physiological saline adjusted to a temperature of 25 ± 0.2 ° C. in a constant temperature water bath was stirred with a magnetic stirrer bar (without an 8 mmφ × 30 mm ring), and a vortex was generated at a rotational speed of 600 rpm. . 2.0 ± 0.002 g of water-absorbent resin particles are added to the physiological saline at once, and the time (seconds) from the addition of the water-absorbent resin particles to the time when the liquid surface vortex converges is measured. The time was defined as the water absorption rate of the water absorbent resin particles.

(5) Medium particle diameter 100 g of water-absorbent resin particles were mixed with 0.5 g of amorphous silica (Degussa Japan Co., Ltd., trade name: Sipernat 200) as a lubricant. When the water-absorbent resin particles are passed through a JIS Z8801-1 standard sieve having a sieve opening of 250 μm and the amount remaining on the sieve is 50% by mass or more with respect to the total amount, When the combination was less than 50% by mass, the median particle diameter was measured using a combination of sieves (B) below.
(A) From above, a JIS standard sieve has an opening of 850 μm, an opening of 600 μm, an opening of 500 μm, an opening of 425 μm, an opening of 300 μm, an opening of 250 μm, an opening of 150 μm. Combined in order of sieve and saucer.
(B) JIS standard sieve from above, sieve with 425 μm opening, sieve with 250 μm opening, sieve with 180 μm opening, sieve with 150 μm opening, sieve with 106 μm opening, sieve with 75 μm opening, 45 μm opening Combined in order of sieve and saucer.

  Among the combined sieves, the water-absorbing resin particles were put into a sieve located at the uppermost stage, and the water-absorbing resin particles were classified by shaking for 20 minutes using a low-tap shaker.

  After classification, the mass of the water-absorbent resin particles remaining on each sieve is calculated as a percentage by mass with respect to the total amount, and by integrating in order from the larger particle diameter, the water-absorbent resin particles remaining on the sieve and the sieve The relationship between the mass percentage and the integrated value was plotted on a logarithmic probability paper. By connecting the plots on the probability paper with a straight line, the particle diameter corresponding to an integrated mass percentage of 50 mass% was defined as the median particle diameter.

(6) Handling property The powder handling property of the water-absorbent resin particles was visually evaluated according to the standards by five analysts. The evaluation selected by three or more analysts was defined as the powder handling property of the water-absorbent resin particles.
Good: Less dusting. Fluidity is moderate, and operations such as weighing and cleaning are easy.
Defect: Much powdered. Since the fluidity is low, operations such as weighing and cleaning are difficult.

As shown in Table 2, the water-absorbent resin particles obtained in Reference Example 1 and Examples 2-3 are all excellent in water-absorbing performance such as water-absorbing speed while having an appropriate particle diameter, and residual volatilization. It turns out that there are few ingredients. On the other hand, it can be seen that the water-absorbent resin particles obtained in the comparative example are not sufficient in any of these characteristics. In particular, the amount of residual volatile components when the supply ratio of the inert gas is lower than specified (Comparative Example 1) and when the inert gas is excessively supplied (Comparative Example 2) is larger than that of the example. Therefore, the effect of the present invention is obvious.

  The water-absorbent resin particles obtained by the production method according to the present embodiment are used for sanitary materials such as paper diapers, sanitary goods, pet sheets, water and water-resisting materials, soil improvement materials and other agricultural and horticultural materials, and power and communication cables It can be used in various fields such as for industrial materials such as materials and anti-condensation materials. In particular, the water-absorbent resin particles are preferably used in the fields of adult diapers, incontinence pads, toilet training pants and specific sanitary materials such as daily napkins, waterproofing materials for cables, pet sheets, simple toilets, etc. It is done.

Claims (5)

An aqueous liquid containing water and a water-soluble ethylenically unsaturated monomer dissolved in the water is suspended in an oily liquid containing a surfactant having a HLB of 6 or more and a hydrocarbon dispersion medium. Preparing a polymerization step comprising polymerizing the water-soluble ethylenically unsaturated monomer while supplying an inert gas into the suspension,
A water absorption ratio represented by a ratio of a volume of the inert gas supplied per minute to a total volume of the aqueous liquid and the hydrocarbon dispersion medium is 0.46 to 3.0. A method for producing resin particles. An aqueous liquid containing water and a water-soluble ethylenically unsaturated monomer dissolved in the water is suspended in an oily liquid containing a surfactant having a HLB of 6 or more and a hydrocarbon dispersion medium. Preparing a first polymerization step comprising polymerizing the water-soluble ethylenically unsaturated monomer while supplying an inert gas into the suspension;
After the first polymerization step, water and an additional water-soluble ethylenically unsaturated monomer dissolved in the water, which are separate from the aqueous liquid already introduced into the suspension An aqueous liquid is added to the reaction mixture after polymerization to suspend it, and then the water-soluble ethylenically unsaturated monomer is polymerized once while supplying an inert gas into the suspension. Two or more times, the second and subsequent polymerization steps, and
Supplied per minute with respect to the total volume of the aqueous liquid and the hydrocarbon dispersion medium introduced into the suspension in the polymerization step of the nth time (n is an integer of 1 or more). The method for producing water-absorbent resin particles, wherein the gas supply ratio in the n-th polymerization step represented by the ratio of the inert gas is 0.46 to 3.0.   The method for producing water-absorbent resin particles according to claim 1 or 2, wherein the surfactant comprises at least one selected from the group consisting of sorbitan fatty acid esters, polyglycerin fatty acid esters, and sucrose fatty acid esters.   The water-soluble ethylenically unsaturated monomer includes at least one selected from the group consisting of acrylic acid and salts thereof, methacrylic acid and salts thereof, and acrylamide. Of producing water-absorbent resin particles.   The hydrocarbon dispersion medium includes at least one selected from the group consisting of aliphatic hydrocarbons having 6 to 8 carbon atoms and alicyclic hydrocarbons having 6 to 8 carbon atoms. A method for producing the water-absorbent resin particles according to Item.