JP7708768B2 - Method for producing water-absorbent resin particles - Google Patents

Description

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

Methods for synthesizing the crosslinked polymer that constitutes the water-absorbent resin particles include polymerization methods such as aqueous solution polymerization, which uses water during polymerization to obtain a lump-shaped hydrogel (see, for example, Patent Document 1). The lump-shaped hydrogel contains a large amount of water used during polymerization, and therefore needs to be dried.

JP 2016-30832 A

For example, when drying hydrous gel using a drying device, the hydrous gel may adhere to the metal surface of the drying device and be difficult to remove even after drying is completed. In order to improve the releasability of the metal surface of the drying device, metals that have been treated with resins such as fluororesin may be used. However, metals coated with resins such as fluororesin have problems such as a lower heat transfer rate and low heat resistance that makes them prone to deterioration compared to the original metal, so it is desirable for the crosslinked polymer to have high releasability after drying even on general metal surfaces that have not been treated with resin coatings or other methods to improve releasability.

The present invention aims to provide a method for producing water-absorbent resin particles that have excellent peelability from metal surfaces of the cross-linked polymer after drying.

The inventors have discovered that when drying a crosslinked polymer, controlling the degree of drying, particularly in the early stages of the drying process (15 minutes from the start of the drying process), is important in improving the peelability of the crosslinked polymer after the drying process.

The present invention provides a method for producing water-absorbent resin particles containing a crosslinked polymer, the method including a drying step of drying the crosslinked polymer, in which the crosslinked polymer has a dry weight basis moisture content of 100 mass% or more before the drying step, and the drying step is carried out so that an initial rate of change in the dry weight basis moisture content, as represented by the following formula, is 6.0%/min or less.
Initial rate of change of dry basis moisture content (%/min)=[{(dry basis moisture content before drying process−dry basis moisture content after 15 minutes of drying process)/dry basis moisture content before drying process}×100]/15

It is preferable that the drying process is carried out so that the initial rate of change of the dry basis moisture content is 5.6%/min or less.

It is preferable that the drying process is carried out so that the initial rate of change of the dry basis moisture content is 3.0%/min or more.

It is preferable that the method further includes crushing the crosslinked polymer before the drying step.

After the drying step, the method may further include grinding the crosslinked polymer.

The present invention provides a method for producing water-absorbent resin particles that have excellent peelability from metal surfaces after drying of the crosslinked polymer.

Hereinafter, the embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and can be modified in various ways within the scope of the present invention.

In this specification, "acrylic" and "methacrylic" are collectively written as "(meth)acrylic". "Acrylate" and "methacrylate" are also collectively written as "(meth)acrylate". "Polyethylene glycol" and "ethylene glycol" are collectively written as "(poly)ethylene glycol". The same applies to other expressions including "(poly)". In the numerical ranges described in stages in this specification, the upper limit or lower limit of a numerical range of a certain stage can be arbitrarily combined with the upper limit or lower limit of a numerical range of another stage. In the numerical ranges described in this specification, the upper limit or lower limit of the numerical range may be replaced with the value shown in the examples. "Water-soluble" means that the material has a solubility of 5% by mass or more in water at 25°C. The materials exemplified in this specification may be used alone or in combination of two or more types. When a plurality of substances corresponding to each component are present in the composition, the content of each component in the composition means the total amount of the plurality of substances present in the composition, unless otherwise specified. With respect to at least one (meth)acrylic acid compound selected from the group consisting of (meth)acrylic acid and its salts, the "content of (meth)acrylic acid compound" means the total amount of acrylic acid, acrylic acid salts, methacrylic acid and methacrylic acid salts. "Room temperature" means 25°C ± 2°C.

The method for producing water-absorbent resin particles containing the cross-linked polymer of this embodiment includes a drying step for drying the cross-linked polymer.

[polymerization]
In the production method according to the present embodiment, the crosslinked polymer can be obtained, for example, by polymerizing a monomer. The polymerization of the monomer can be carried out, for example, by using an aqueous monomer solution containing the monomer.

The manufacturing method according to this embodiment is suitable for the case where the crosslinked polymer obtained by polymerization is in the form of a hydrous gel and requires drying, for example, when an aqueous solution polymerization method is used. The case where an aqueous solution polymerization method is used is described below.

The monomer may include an ethylenically unsaturated monomer, and may include a water-soluble ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include unsaturated carboxylic acids such as (meth)acrylic acid, maleic acid, maleic anhydride, and fumaric acid, and carboxylic acid monomers such as salts thereof; nonionic monomers such as (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, and polyethylene glycol mono(meth)acrylate; amino group-containing unsaturated monomers such as N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide, and quaternized products thereof; and sulfonic acid monomers such as vinyl sulfonic acid, styrene sulfonic acid, 2-(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, and salts thereof. The ethylenically unsaturated monomer may contain at least one (meth)acrylic acid compound selected from the group consisting of (meth)acrylic acid and its salts. The ethylenically unsaturated monomer may contain both (meth)acrylic acid and a salt of (meth)acrylic acid. Examples of the salt of the unsaturated carboxylic acid (e.g., (meth)acrylic acid) include an alkali metal salt (e.g., a sodium salt, a potassium salt), an ammonium salt, and the like.

The acid group of an ethylenically unsaturated monomer having an acid group (e.g., (meth)acrylic acid) may be neutralized in advance with an alkaline neutralizing agent. Examples of the alkaline neutralizing agent include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, and potassium carbonate; and ammonia. The alkaline neutralizing agent may be used in the form of an aqueous solution to simplify the neutralization operation. The neutralization of the acid group may be carried out before, during, or after the polymerization of the ethylenically unsaturated monomer as the raw material.

The degree of neutralization of the ethylenically unsaturated monomer by the alkaline neutralizing agent is preferably 10 to 100 mol%, 30 to 90 mol%, 40 to 85 mol%, or 50 to 80 mol%, from the viewpoints of obtaining good water absorption performance by increasing the osmotic pressure, enhancing safety, and suppressing problems caused by the presence of excess alkaline neutralizing agent. The "degree of neutralization" refers to the degree of neutralization of all acid groups possessed by the ethylenically unsaturated monomer.

The content of the monomer (e.g., (meth)acrylic acid compound) may be 10% by mass or more, 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, or 35% by mass or more, based on the total mass of the aqueous monomer solution. The content of the monomer may be 60% by mass or less, 55% by mass or less, 50% by mass or less, less than 50% by mass, 45% by mass or less, less than 45% by mass, or 40% by mass or less. From the viewpoint of easily adjusting the dry weight basis moisture content before the drying process described below to an appropriate range, it is preferable that the monomer content in the aqueous monomer solution is 40% by mass or less.

From the viewpoints of suppressing side reactions such as self-crosslinking in the polymerization reaction, adjusting the hardness of the hydrogel to a level that makes it easy to crush in the crushing step described below, and suppressing deterioration of the solid content in the hydrogel in the drying step described below, the moisture content of the aqueous monomer solution is preferably 60 mass% or more. The moisture content of the aqueous monomer solution can be calculated by the following formula.
Water content (%) of monomer aqueous solution=100-solid content (%) of monomer aqueous solution
The solid content of the aqueous monomer solution is the ratio of the compound converted into the solid content constituting the crosslinked polymer to the total amount of the aqueous monomer solution. The amount of components other than the monomer (e.g., crosslinking agent, polymerization initiator) contained in the aqueous monomer solution is minute compared to the amount of the monomer, so the monomer content in the aqueous monomer solution may be the substantial solid content. The water content of the aqueous monomer solution may be, for example, 70% by mass or less, or 65% by mass or less.

The content of the (meth)acrylic acid compound may be 50 mol% or more, 70 mol% or more, 90 mol% or more, 95 mol% or more, 97 mol% or more, or 99 mol% or more based on the total amount of monomers contained in the aqueous monomer solution and/or the total amount of ethylenically unsaturated monomers contained in the aqueous monomer solution. The monomer contained in the aqueous monomer solution may be substantially composed of the (meth)acrylic acid compound, that is, 100 mol% of the monomers contained in the aqueous monomer solution may be the (meth)acrylic acid compound.

The aqueous monomer solution may contain a polymerization initiator. Polymerization of the monomer contained in the aqueous monomer solution may be initiated by adding a polymerization initiator to the aqueous monomer solution and, if necessary, heating or irradiating with light. Examples of the polymerization initiator include a photopolymerization initiator and a radical polymerization initiator, and a water-soluble radical polymerization initiator is preferred. From the viewpoint of easily improving water absorption performance, it is preferable that the polymerization initiator contains at least one type selected from the group consisting of an azo compound and a peroxide.

Azo compounds 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] dichloride. acid salt, 2,2'-azobis[2-(N-allylamidino)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-imidazolin-2-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-(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. From the viewpoint of easily obtaining good water absorption performance, the azo compound preferably contains at least one selected from the group consisting of 2,2'-azobis(2-methylpropionamide) dihydrochloride, 2,2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-azobis{2-[1-(2-hydroxyethyl)-2-imidazolin-2-yl]propane} dihydrochloride, and 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine] tetrahydrate.

Examples of peroxides include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; and organic peroxides such as methyl ethyl ketone peroxide, methyl isobutyl ketone peroxide, di-t-butyl peroxide, t-butyl cumyl peroxide, t-butyl peroxyacetate, t-butyl peroxyisobutyrate, and t-butyl peroxypivalate. From the viewpoint of easily obtaining good water absorption performance and easily reducing unreacted monomers contained in the water absorbent resin particles, it is preferable that the peroxide contains at least one selected from the group consisting of potassium persulfate, ammonium persulfate, and sodium persulfate.

The content of the polymerization initiator is preferably 0.001 mmol or more, 0.005 mmol or more, 0.01 mmol or more, 0.05 mmol or more, 0.1 mmol or more, or 0.15 mmol or more per mole of ethylenically unsaturated monomer (e.g., (meth)acrylic acid compound) from the viewpoint of easily increasing the water absorption performance and easily reducing the amount of unreacted monomer contained in the water absorbent resin particles. The content of the polymerization initiator is preferably 5 mmol or less, 4 mmol or less, 2 mmol or less, 1 mmol or less, 0.9 mmol or less, 0.7 mmol or less, 0.5 mmol or less, 0.4 mmol or less, or 0.3 mmol or less from the viewpoint of easily increasing the water absorption performance and easily avoiding a rapid polymerization reaction. From these viewpoints, the content of the polymerization initiator is preferably 0.001 to 5 mmol.

The aqueous monomer solution may contain a reducing agent. Examples of reducing agents include sodium sulfite, sodium hydrogen sulfite, ferrous sulfate, and L-ascorbic acid. A polymerization initiator and a reducing agent may be used in combination.

The aqueous monomer solution may contain an oxidizing agent, such as hydrogen peroxide, sodium perborate, superphosphoric acid and its salts, potassium permanganate, etc.

The aqueous monomer solution may contain an internal crosslinking agent. By using an internal crosslinking agent, the resulting crosslinked polymer can have a crosslinking structure due to the internal crosslinking agent in addition to the self-crosslinking structure due to the polymerization reaction.

Examples of internal cross-linking agents include compounds having two or more reactive functional groups (e.g., polymerizable unsaturated groups). Examples of the internal crosslinking agent include di- or tri(meth)acrylic acid esters of polyols such as (poly)ethylene glycol, (poly)propylene glycol, trimethylolpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, and (poly)glycerin; unsaturated polyesters obtained by reacting the above polyols with unsaturated acids (maleic acid, fumaric acid, and the like); glycidyl group-containing compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)glycerin polyglycidyl ether, and glycidyl (meth)acrylate; bisacrylamides such as N,N'-methylenebis(meth)acrylamide; di- or tri(meth)acrylic acid esters obtained by reacting polyepoxides with (meth)acrylic acid; polyisocyanates (tolylene diisocyanate, Examples of the internal crosslinking agent include di(meth)acrylic acid carbamyl esters obtained by reacting an alkyl ester such as aryl ester (e.g., diphenyl ether, hexamethylene diisocyanate) with hydroxyethyl (meth)acrylate; allylated starch; allylated cellulose; diallyl phthalate; N,N',N"-triallyl isocyanurate; divinylbenzene; pentaerythritol; ethylenediamine; and polyethyleneimine. From the viewpoint of easily increasing water absorption performance and excellent reactivity, the internal crosslinking agent preferably contains at least one selected from the group consisting of (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)glycerin diglycidyl ether, polyethylene glycol diacrylate, (poly)propylene glycol diacrylate, trimethylolpropane triacrylate, glycerol triacrylate, and trimethylolpropane diacrylate.

From the viewpoint of easily obtaining good water absorption performance, the content of the internal crosslinking agent is preferably 0.001 mmol or more, 0.005 mmol or more, 0.01 mmol or more, 0.05 mmol or more, 0.07 mmol or more, 0.09 mmol or more, 0.1 mmol or more, 0.11 mmol or more, or 0.13 mmol or more per mole of ethylenically unsaturated monomer (e.g., (meth)acrylic acid compound). From the viewpoint of easily obtaining good water absorption performance, the content of the internal crosslinking agent is preferably 5 mmol or less, 4.5 mmol or less, 4 mmol or less, 3.5 mmol or less, 3 mmol or less, 2.5 mmol or less, 2 mmol or less, 1.5 mmol or less, 1 mmol or less, 0.9 mmol or less, 0.8 mmol or less, 0.7 mmol or less, 0.5 mmol or less, 0.4 mmol or less, or 0.3 mmol or less. From these viewpoints, the content of the internal crosslinking agent is preferably 0.001 to 5 mmol.

The aqueous monomer solution may contain additives such as chain transfer agents, thickeners, and inorganic fillers as components different from the above-mentioned components, as necessary. Examples of chain transfer agents include thiols, thiolic acids, secondary alcohols, hypophosphorous acid, phosphorous acid, and acrolein. Examples of thickeners include carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, polyethylene glycol, polyacrylic acid, neutralized polyacrylic acid, and polyacrylamide. Examples of inorganic fillers include metal oxides, ceramics, and clay minerals.

Examples of polymerization methods for aqueous solution polymerization include a static polymerization method in which the aqueous monomer solution is polymerized without stirring (for example, in a static state); and an agitation polymerization method in which the aqueous monomer solution is polymerized while being stirred in a reaction vessel. In the static polymerization method, when polymerization is completed, a single block-shaped gel is obtained that occupies approximately the same volume as the aqueous monomer solution present in the reaction vessel.

The polymerization form may be batchwise, semi-continuous, continuous, etc. For example, when the static polymerization method is carried out as continuous polymerization, the polymerization reaction is carried out while continuously supplying the aqueous monomer solution to the continuous polymerization apparatus, and a gel can be obtained continuously.

The polymerization temperature varies depending on the polymerization initiator used, but from the viewpoints of increasing productivity by rapidly progressing the polymerization and shortening the polymerization time, as well as making it easier to remove the polymerization heat and carry out the reaction smoothly, a temperature of 0 to 130°C or 10 to 110°C is preferred. The polymerization time is set appropriately depending on the type and amount of the polymerization initiator used, the reaction temperature, etc., but 1 to 200 minutes or 5 to 100 minutes is preferred.

After the hydrogel (hydrogel-like cross-linked polymer) is obtained by polymerization, it may be immediately subjected to the drying step described below, or may be subjected to the drying step after a certain period of time (e.g., 10 minutes to 1 hour). The hydrogel may be placed in an environment of 25°C or higher and lower than the drying temperature (e.g., 75°C or lower) until it is subjected to the drying step.

[Rough crushing]
The crosslinked polymer is preferably crushed in advance before the drying step. That is, the production method according to the present embodiment preferably includes a step of crushing the crosslinked polymer before the drying step of the crosslinked polymer. By crushing the crosslinked polymer before the drying step, drying can be performed more efficiently. In the crushing step, for example, the lump-shaped hydrogel obtained by polymerization can be crushed.

For the coarse crushing, for example, a coarse crusher such as a kneader (pressure kneader, double-arm kneader, etc.), meat chopper, cutter mill, farmer mill, etc. can be used. In the coarse crushing step, the lump hydrogel may be cut into pieces of, for example, about 5 cm square in advance, and the cut hydrogel may be subjected to coarse crushing. When the polymerization step is carried out by stirring polymerization using a device such as a kneader, the polymerization step and the coarse crushing step may be carried out substantially simultaneously.

The crosslinked polymer (crushed gel, crushed polymer) after crushing may be in the form of particles, or may have an elongated shape in which particles are linked together. The size of the smallest side of the crushed gel may be, for example, about 0.1 to 15 mm, and preferably about 1.0 to 10 mm. The size of the largest side of the crushed gel may be, for example, about 0.1 to 200 mm, and preferably about 1.0 to 150 mm.

[Drying process]
In the production method according to the present embodiment, the drying step is carried out so that the initial rate of change of the dry basis moisture content, as shown in the following formula, is 6.0%/min or less. The dry basis moisture content before the drying step is 100% by mass or more.
Initial rate of change of dry basis moisture content (%/min)=[{(dry basis moisture content before drying process−dry basis moisture content after 15 minutes of drying process)/dry basis moisture content before drying process}×100]/15

In this specification, drying refers to removing at least a portion of the moisture contained in the crosslinked polymer by placing the crosslinked polymer in an environment of 80°C or higher. In this specification, the dry moisture content is the ratio (mass%) of the moisture content in the crosslinked polymer to the solid content (dry weight) in the crosslinked polymer. Therefore, when the dry moisture content is 100 mass%, the moisture content and solid content contained in the crosslinked polymer are the same. In addition, the initial rate of change in the dry moisture content shown in the above formula means the average rate of change in the dry moisture content per minute for 15 minutes from the start of the drying process.

According to the manufacturing method of this embodiment, by controlling the degree of drying of the cross-linked polymer in the early stage of the drying process (15 minutes after the start of the drying process), the cross-linked polymer can be prevented from adhering to the metal surface of the drying device after the drying process, and the peelability can be improved. According to the manufacturing method of this embodiment, after the drying process is completed, the cross-linked polymer can be peeled off from the metal surface by natural falling without applying a force to the cross-linked polymer from the outside, for example.

The cross-linked polymer is preferably dried by placing it on a metal surface. The metal surface may be, for example, a wire mesh or a metal plate with holes. Metals that have been resin-treated, such as fluororesin-treated, tend to have a lower heat transfer rate than the original untreated metal, so in order to perform efficient drying, it is preferable that the metal surface of the drying device is not resin-treated. According to the manufacturing method of this embodiment, even if a metal that has not been resin-treated is used for at least a part of the metal surface of the drying device, the cross-linked polymer is excellent in peelability from the drying device.

In this specification, the dry weight moisture content before the drying process refers to the dry weight moisture content immediately before the crosslinked polymer is subjected to the drying process. For example, when drying is performed using a drying device, immediately before the crosslinked polymer is placed at a predetermined location in the drying device. When the production method includes a process of coarsely crushing the crosslinked polymer, the dry weight moisture content of the crosslinked polymer immediately after coarse crushing may be used as the dry weight moisture content before the drying process.

The dry weight moisture content before the drying process is 100% by mass or more, and may be 110% by mass or more, 120% by mass or more, 130% by mass or more, or 135% by mass or more. The dry weight moisture content before the drying process is preferably 233% by mass or less from the viewpoint of suppressing the elution of monomers constituting the crosslinked polymer during the drying process and improving the peelability. The dry weight moisture content before the drying process may be 220% by mass or less, 210% by mass or less, 200% by mass or less, 190% by mass or less, 180% by mass or less, 170% by mass or less, or 160% by mass or less.

The moisture content on a dry basis before the drying step can be adjusted, for example, by adjusting the moisture content of the aqueous monomer solution used in the polymerization of the crosslinked polymer. This is because the moisture content of the aqueous monomer solution affects the moisture content of the lumpy hydrogel obtained by polymerization and the moisture content on a dry basis of the crosslinked polymer after crushing. The moisture content on a dry basis before the drying step may be reduced by carrying out the polymerization step under a nitrogen stream, by distilling off the steam generated when the gel becomes hot due to the reaction heat during the polymerization step, and/or by subjecting the lumpy hydrogel obtained by polymerization to a crushing step under a nitrogen stream.

The drying temperature for 15 minutes from the start of the drying process and after 15 minutes has elapsed is 80°C or higher, and may be 90°C or higher, 100°C or higher, 120°C or higher, 140°C or higher, 160°C or higher, 170°C or higher, 180°C or higher, 190°C or higher, or 195°C or higher. The drying temperature may be a temperature equal to or higher than the boiling point of water. The drying temperature may be 220°C or lower, 210°C or lower, or 200°C or lower. In this specification, the drying temperature may be the set temperature of the drying device or the exposure atmosphere temperature of the hydrogel in the drying process. The drying process may be performed at normal pressure or under reduced pressure. During the drying process, the environmental temperature may temporarily become lower than the predetermined drying temperature or lower than 80°C.

It is preferable that the drying apparatus for drying the cross-linked polymer is set in advance to a temperature of 80°C or higher, preferably a predetermined drying temperature, and that drying is started as soon as the cross-linked polymer is placed in the drying apparatus.

The cumulative time of the drying process may be set so that the dry weight moisture content after drying is within an appropriate range depending on conditions such as the drying temperature. The cumulative time of the drying process may be, for example, 15 minutes or more, 20 minutes or more, 25 minutes or more, or 30 minutes or more, and may be 120 minutes or less, 90 minutes or less, or 60 minutes or less.

The crosslinked polymer can be dried using a drying device such as a hot air dryer, a vacuum dryer, a ventilated belt dryer, a ventilated band dryer, a rotary ventilated dryer, an agitator dryer, a fluidized bed dryer, a vibrating fluidized dryer, or a vacuum dryer.

The initial rate of change of the dry basis moisture content is 6.0%/min or less, and may be 5.8%/min or less, 5.6%/min or less, 5.3%/min or less, 5.0%/min or less, 4.5%/min or less, 4.2%/min or less, 4.0%/min or less, 3.8%/min or less, or 3.5%/min or less.

The initial rate of change of the dry basis moisture content may be 3.0%/min or more, and preferably 3.5%/min or more. When the initial rate of change is 3.5%/min or more, the peelability of the crosslinked polymer after drying can be further improved. The initial rate of change may be 4.5%/min or more, or 5.0%/min or more.

The initial rate of change of the moisture content on a dry basis can be adjusted, for example, by adjusting the drying temperature, the amount of moisture volatilized from the crosslinked polymer and released outside the system, etc., during the first 15 minutes of the drying process.

The dry weight moisture content after 15 minutes of the drying process means the dry weight moisture content after 15 minutes from the start of the drying process. The dry weight moisture content after 15 minutes of the drying process may be, for example, 10% by mass or more, 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, 35% by mass or more, 40% by mass or more, 45% by mass or more, 50% by mass or more, 55% by mass or more, or 60% by mass or more. The dry weight moisture content after 15 minutes of the drying process may be, for example, 80% by mass or less, 75% by mass or less, 70% by mass or less, 65% by mass or less, 60% by mass or less, 55% by mass or less, 50% by mass or less, 45% by mass or less, 40% by mass or less, 35% by mass or less, or 30% by mass or less.

In the manufacturing method according to the present embodiment, the manner of drying after 15 minutes from the start of the drying process is not particularly important. For example, in order to perform more efficient drying, the drying process may be performed under stronger drying conditions than in the early stages of the drying process. The drying temperature after 15 minutes from the start of the drying process may be higher or lower than in the early stages of the drying process.

In this specification, the final dry weight moisture content is the dry weight moisture content immediately after the drying process of the crosslinked polymer is completed. For example, when drying is performed using a drying device, immediately after the crosslinked polymer is removed from the drying device. For example, when multiple drying devices are used, immediately after the crosslinked polymer is removed from the drying device used last. When the crosslinked polymer is pulverized after the drying process, for example, the moisture content of the crosslinked polymer immediately before pulverization may be the final dry weight moisture content. However, when moisture is added to the crosslinked polymer by adding an aqueous additive solution to the crosslinked polymer after the drying process and before the pulverization process, the moisture content of the crosslinked polymer before the addition of moisture is the final dry weight moisture content. The final dry weight moisture content can be reduced, for example, by increasing the drying temperature or extending the drying time.

The final dry weight moisture content may be 15% by mass or less, 10% by mass or less, 8% by mass or less, 6% by mass or less, or 5% by mass or less. When a grinding process of the crosslinked polymer is performed after the drying process, the final dry weight moisture content is preferably 15% by mass or less from the viewpoint of reducing adhesion of the crosslinked polymer to the grinder, reducing the load on the grinder, and suppressing mechanical failure. The final dry weight moisture content may be, for example, 1% by mass or more, 2% by mass or more, 3% by mass or more, 3.5% by mass or more, or 4% by mass or more.

[Peeling]
After the drying step, the dried cross-linked polymer can be removed from the drying device. If the cross-linked polymer is placed on a metal surface during the drying step, the cross-linked polymer is peeled off from the metal surface. According to the manufacturing method of the present embodiment, the cross-linked polymer can be peeled off from the metal surface of the drying device by, for example, natural drop without applying an external force.

[Crushing]
The crosslinked polymer (crushed and dried polymer) after the crushing and drying steps is preferably further crushed. That is, the production method according to the present embodiment preferably includes a crushing step of the crosslinked polymer after the drying step. By crushing, a particulate crosslinked polymer (polymer particles) having a smaller particle size can be obtained.

For the grinding, for example, a grinding machine such as a roller mill (roll mill), stamp mill, jet mill, high-speed rotary grinder (hammer mill, pin mill, rotor beater mill, etc.), or container-driven mill (rotary mill, vibration mill, planetary mill, etc.) can be used. Preferably, a high-speed rotary grinder is used. The grinder may have an opening on the outlet side, such as a perforated plate, screen, or grid, that controls the maximum particle size of the ground particles. The shape of the opening may be polygonal, circular, etc., and the maximum diameter of the opening may be 0.1 to 5 mm, 0.3 to 3.0 mm, or 0.5 to 1.5 mm.

[Classification, particle size adjustment]
The particulate crosslinked polymer obtained by pulverization may be further classified. The manufacturing method according to the present embodiment may include a step of classifying the crosslinked polymer after pulverization. Classification refers to an operation of dividing a certain particle group into two or more particle groups having different particle size distributions according to the particle size. In addition, the classified particles may be pulverized again, and a plurality of classification steps may be performed, such as repeating the pulverization step and the classification step, or the classification step may be performed after the surface crosslinking step described below. The classification of the particles may be performed, for example, by a method such as screen classification or air classification. The particles may be granulated as necessary. The crosslinked polymers of each particle size obtained by classification may be mixed again as necessary to adjust the particle size so as to have a desired particle size distribution.

[Surface crosslinking]
The method for producing water-absorbent resin particles according to the present embodiment may include a step of performing surface cross-linking of polymer particles. The surface cross-linking can be performed, for example, by adding a cross-linking agent (surface cross-linking agent) for performing surface cross-linking to the polymer particles and allowing them to react.

The surface cross-linking agent may contain, for example, two or more functional groups (reactive functional groups) that are reactive with functional groups derived from ethylenically unsaturated monomers. Examples of the surface cross-linking agent include polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; polyglycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether; haloepoxy compounds such as epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin; 2,4 -tolylene diisocyanate, hexamethylene diisocyanate, and other isocyanate compounds having two or more reactive functional groups; oxetane compounds such as 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, 3-butyl-3-oxetaneethanol, and other oxetane compounds; oxazoline compounds such as 1,2-ethylenebisoxazoline; carbonate compounds such as ethylene carbonate; and hydroxyalkylamide compounds such as bis[N,N-di(β-hydroxyethyl)]adipamide.

[Water-absorbing resin particles]
The water-absorbent resin particles obtained by the manufacturing method according to the present embodiment include the particulate crosslinked polymer (polymer particles) described above. The water-absorbent resin particles may consist of only polymer particles, or may further include additional components such as a gel stabilizer, a metal chelating agent (ethylenediaminetetraacetic acid and its salts, diethylenetriaminepentaacetic acid and its salts, such as pentasodium diethylenetriaminepentaacetate, etc.), a flowability improver (lubricant), etc. The additional components may be disposed inside or on the surface of the polymer particles, or both.

The water-absorbent resin particles may include a plurality of inorganic particles arranged on the surfaces of the polymer particles. The manufacturing method according to the present embodiment may further include a step of attaching the inorganic particles to the surfaces of the polymer particles.

The shape of the water-absorbent resin particles obtained by the manufacturing method according to this embodiment may be, for example, a crushed shape or a shape formed by agglomeration of crushed particles. The median particle diameter of the water-absorbent resin particles may be 130 to 800 μm, 200 to 850 μm, 250 to 700 μm, 300 to 600 μm, or 300 to 450 μm.

The water-absorbent resin particles obtained by the manufacturing method according to this embodiment have excellent water absorption properties and can be used in fields such as sanitary materials such as paper diapers and sanitary products, agricultural and horticultural materials such as water retention agents and soil conditioners, and industrial materials such as waterproofing agents and dew condensation prevention agents.

The present invention will be explained in more detail below with reference to examples, however, the present invention is not limited to these examples.

<Production Example 1>
[Preparation of Aqueous Monomer Solution]
339.39 g (4.71 mol) of acrylic acid was placed in a 2 L separable flask. 292.30 g of ion-exchanged water was added to the acrylic acid in the separable flask while stirring. Next, 297.45 g of a 48% by mass aqueous sodium hydroxide solution was added dropwise in an ice water bath at about 3° C. to prepare a partially neutralized solution of acrylic acid with a monomer concentration of 45.0% by mass (neutralization rate 75.8 mol%).

[Preparation of Crushed Gel]
(Polymerization process)
889.28 g of the above partially neutralized acrylic acid solution, 143.40 g of ion-exchanged water, 0.412 g of polyethylene glycol diacrylate (n≒9) as an internal crosslinking agent (NOF Corporation, Blenmar ADE-400A), and 16.14 g of a 2% by mass aqueous potassium persulfate solution were placed in a fluororesin-coated 18-8 stainless steel tray (external dimensions: 297 mm x 232 mm x height 50 mm) and stirred with two stirrers (diameter 8 mm, length 45 mm) to form a uniform mixture in the stainless steel tray. Then, the top of the stainless steel tray was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel tray to 25°C, a nitrogen introduction tube (flow rate 200 ml/min) made of fluororesin with an inner diameter of 3 mm was inserted, and the mixture in the tray was replaced with nitrogen to adjust the dissolved oxygen content to 0.1 ppm or less. Next, while the mixture was stirred at 300 rpm, 3.39 g of a 0.5% by mass aqueous solution of L-ascorbic acid was added dropwise using a syringe (10 mL disposable syringe manufactured by Terumo Corporation, injection needle manufactured by Terumo Corporation).

The polymerization reaction started 3 minutes after the L-ascorbic acid solution was added dropwise. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the reaction solution gelled. 14 minutes after the L-ascorbic acid solution was added dropwise, the thermometer indicated 86.0°C, and the temperature then began to drop.

The stainless steel tray containing the hydrous gel (a hydrous gel-like cross-linked polymer) formed by the gelation of the reaction liquid was immersed in a water bath at 75°C, and the hydrous gel was heated in that state for 20 minutes to fully complete the polymerization reaction.

(Crushing process)
The entire amount of the hydrogel after the polymerization step was taken out of the container, and cut by making cuts at 5 cm intervals on the long side. The cut hydrogel was sequentially put into a meat chopper 12VR-750SDX manufactured by Kiryu Royal Co., Ltd. and coarsely crushed (fragmented) to obtain a coarsely crushed gel. The diameter of the hole in the plate located at the outlet of the meat chopper was 6.4 mm. The moisture content of the coarsely crushed gel at this time (moisture content A based on wet weight before drying step) was 58.2% by mass.

<Production Example 2>
[Preparation of Aqueous Monomer Solution]
340.60 g (4.73 mol) of acrylic acid was placed in a 2 L separable flask. 292.87 g of ion-exchanged water was added to the acrylic acid in the separable flask while stirring. Next, 295.98 g of a 48% by mass aqueous sodium hydroxide solution was added dropwise in an ice water bath at about 3° C. to prepare a partially neutralized solution of acrylic acid with a monomer concentration of 45.0% by mass (neutralization rate 75.1 mol%).

[Preparation of Crushed Gel]
(Polymerization process)
888.10 g of the above partially neutralized acrylic acid solution, 144.55 g of ion-exchanged water, 0.413 g of polyethylene glycol diacrylate (n≒9) as an internal crosslinking agent (NOF Corporation, Blenmar ADE-400A), and 16.17 g of a 2% by mass aqueous potassium persulfate solution were placed in a fluororesin-coated 18-8 stainless steel tray (external dimensions: 297 mm x 232 mm x height 50 mm) and stirred with two stirrers (diameter 8 mm, length 45 mm) to form a uniform mixture in the stainless steel tray. Then, the top of the stainless steel tray was covered with a polyethylene film. After adjusting the temperature of the mixture in the stainless steel tray to 25°C, a nitrogen introduction tube (flow rate 200 ml/min) made of fluororesin with an inner diameter of 3 mm was inserted, and the mixture in the tray was replaced with nitrogen to adjust the dissolved oxygen content to 0.1 ppm or less. Next, while the mixture was stirred at 300 rpm, 3.40 g of a 0.5% by mass aqueous solution of L-ascorbic acid was added dropwise using a syringe (10 mL disposable syringe manufactured by Terumo Corporation, injection needle manufactured by Terumo Corporation).

The polymerization reaction started 1 minute after the L-ascorbic acid aqueous solution was added dropwise. As the polymerization reaction progressed, the viscosity of the reaction solution increased, and then the reaction solution gelled. 10 minutes after the end of the addition of the L-ascorbic acid aqueous solution, the installed thermometer indicated 94.6°C, after which the temperature began to drop. The subsequent steps were carried out in the same manner as in Production Example 1. The wet-based moisture content of the crushed gel obtained after the crushing step (wet-based moisture content A before the drying step) was 58.1% by mass.

<Production Example 3>
The polymerization step was carried out in the same manner as in Production Example 2. One minute after the L-ascorbic acid aqueous solution was dropped, the polymerization reaction started. As the polymerization reaction proceeded, the viscosity of the reaction solution increased, and then the reaction solution gelled. 11 minutes after the end of the dropwise addition of the L-ascorbic acid aqueous solution, the installed thermometer indicated 72.6°C, and the temperature then began to decrease. The subsequent steps were also carried out in the same manner as in Production Example 2. The wet-based moisture content of the crushed gel obtained after the crushing step (wet-based moisture content A before the drying step) was 58.6% by mass.

Example 1
(drying process)
60 g of the crushed gel obtained in Production Example 1 was placed uniformly within a range of 15 cmΦ from the center of a JIS sieve (diameter 20 cm) with a mesh size of 1.7 mm. Then, a metal petri dish (167 g) with a diameter of 15 cm was placed on the crushed gel, and a weight of 250 g was placed on the petri dish, and a uniform load was applied to the entire surface of the crushed gel, so that the crushed gel was pressed against the JIS sieve for 10 seconds. Then, the metal petri dish and the weight were removed, and the upper opening of the JIS sieve on which the crushed gel was placed was covered with aluminum foil (cooking foil, manufactured by Toyo Aluminum Echo Products Co., Ltd.). The JIS sieve covered with aluminum foil was placed in a hot air dryer (manufactured by ADVANTEC Co., Ltd., FV-320) previously set at 200 ° C., and a drying process was performed for 30 minutes to obtain a crushed and dried polymer. After the drying process, the JIS sieve on which the crushed and dried polymer was placed was removed from the dryer, and a peeling rate test was immediately carried out. Thereafter, the wet-based moisture content of the crushed and dried polymer (final wet-based moisture content C) was measured, and the dry-based moisture content (final dry-based moisture content c) was calculated. The final wet-based moisture content C was 5.1% by mass.

Separately, the top opening of the JIS sieve was covered with aluminum foil in the same manner as above, and the coarsely crushed gel was removed 15 minutes after the start of the drying process at 200°C. The wet-based moisture content of the removed coarsely crushed gel (wet-based moisture content B after 15 minutes of drying process) was measured, and the dry-based moisture content (dry-based moisture content b after 15 minutes of drying process) was calculated. The wet-based moisture content B after 15 minutes of drying process was 41.0% by mass.

Example 2
(drying process)
The upper opening of the JIS sieve was covered with aluminum foil, and then 24 vent holes having a diameter of 3 mm were opened at intervals of 2 to 3 cm in the aluminum foil, and the drying step was carried out in the same manner as in Example 1. The wet basis moisture content of the coarsely pulverized and dried polymer obtained after the drying step (final wet basis moisture content C) was 4.7 mass%.

Separately, in the same manner as above, the top opening of the JIS sieve was covered with aluminum foil, 24 3 mm diameter ventilation holes were made in the aluminum foil at intervals of 2 to 3 cm, and the coarsely crushed gel was removed 15 minutes after the start of the drying process at 200°C. The wet basis moisture content of the removed coarsely crushed gel (wet basis moisture content B after 15 minutes of drying process)) was measured and found to be 35.6% by mass.

Example 3
(drying process)
Using the crushed gel obtained in Production Example 2, the crushed gel was pressed against a JIS sieve for 10 seconds using a metal petri dish and a weight in the same manner as in Example 1. The metal petri dish and weight were removed, and the JIS sieve on which the crushed gel was placed was placed in a hot air dryer (manufactured by ADVANTEC, FV-320) previously set at 180 ° C. After 15 minutes, the JIS sieve on which the crushed gel was placed was removed from the hot air dryer and immediately transferred to another hot air dryer (manufactured by ADVANTEC, FV-320) previously set at 200 ° C., and a drying process was performed for another 15 minutes to obtain a crushed and dried polymer. After a total of 30 minutes of drying, a peeling rate test was immediately performed. Thereafter, the wet basis moisture content of the crushed and dried polymer (final wet basis moisture content C) was measured and found to be 3.7% by mass.

Separately, the wet moisture content of the crushed gel taken out 15 minutes after the start of the drying process at 180°C as described above (wet moisture content B after 15 minutes of drying process) was measured and found to be 19.7% by mass.

Example 4
The drying step was carried out in the same manner as in Example 1, except that the temperature of the 30-minute drying step was changed to 180°C, and the upper opening of the JIS sieve was not covered with aluminum foil. The wet-based moisture content of the resulting crushed and dried polymer (final wet-based moisture content C) was 5.1% by mass. Separately, the wet-based moisture content of the crushed gel taken out 15 minutes after the start of the drying step at 180°C as described above (wet-based moisture content B after 15 minutes of drying step) was measured and found to be 19.5% by mass.

Example 5
The drying step was carried out in the same manner as in Example 3, except that the drying step temperature for 15 minutes from the start of the drying step was changed to 160° C. using the crushed gel obtained after the crushing step in Production Example 3. The wet-based water content of the resulting crushed and dried polymer (final wet-based water content C) was 3.6% by mass. Separately, the wet-based water content of the crushed gel taken out 15 minutes after the start of the drying step at 160° C. as above (wet-based water content B after 15 minutes of drying step) was measured and found to be 23.0% by mass.

<Comparative Example 1>
The drying step was carried out in the same manner as in Example 1, except that the JIS sieve was not covered with aluminum foil. The wet-based moisture content of the resulting crushed and dried polymer (final wet-based moisture content C) was 2.7% by mass. Separately, the wet-based moisture content of the crushed gel taken out 15 minutes after the start of the drying step at 200° C. as described above (wet-based moisture content B after 15 minutes of drying step) was measured and found to be 8.3% by mass.

<Comparative Example 2>
The drying step was carried out in the same manner as in Example 3, except that the order of the 180°C hot air dryer and the 200°C hot air dryer was reversed, and the polymer was dried in the 200°C hot air dryer for 15 minutes, and then quickly transferred to a 180°C dryer. The wet-based water content of the resulting crushed and dried polymer (final wet-based water content C) was 3.7% by mass. Separately, the wet-based water content of the crushed gel taken out 15 minutes after the start of the drying step at 200°C as described above (wet-based water content B after 15 minutes of drying step) was measured and found to be 8.4% by mass.

[Wet standard moisture content measurement]
The wet basis moisture content A before the drying process, the wet basis moisture content B after 15 minutes of the drying process, and the final wet basis moisture content C of the measurement sample were measured by the following method. For the measurement of the wet basis moisture content A before the drying process, 20.0 g of the crushed gel obtained in the crushing process was sampled and used as the measurement sample. For the measurement of the wet basis moisture content B after 15 minutes of the drying process, 20.0 g of the crushed gel taken out 15 minutes after the start of the drying process was sampled and used as the measurement sample. For the measurement of the final wet basis moisture content C, 20.0 g of the crushed and dried polymer obtained immediately after the entire drying process (160 to 200 ° C, 30 minutes) was sampled and used as the measurement sample.

The above measurement sample was placed in a fluororesin-coated stainless steel tray (external dimensions: 185 mm x 140 mm x height 30 mm) previously adjusted to a constant weight (W1 (g)), and the total mass W2 (g) of the stainless steel tray and the measurement sample was precisely weighed. The precisely weighed measurement sample was dried for 2 hours in a hot air dryer (manufactured by ADVANTEC, model: FV-320) with the internal temperature set to 200°C. After the dried measurement sample was allowed to cool in a desiccator, the total mass W3 (g) of the stainless steel tray and the measurement sample was precisely weighed. The wet basis moisture content of the measurement sample was calculated from the following formula.
Wet standard moisture content (mass%) = [{(W2-W1)-(W3-W1)}/(W2-W1)] x 100

[Calculation of moisture content on dry basis]
Using the values of the wet basis moisture content A before the drying process, the wet basis moisture content B after 15 minutes of the drying process, and the final wet basis moisture content C obtained by the above-mentioned method, the dry basis moisture content a before the drying process, the dry basis moisture content b after 15 minutes of the drying process, and the final dry basis moisture content c were calculated according to the following formulas.
Dry basis moisture content (mass%) = [wet basis moisture content / (100 - wet basis moisture content)] × 100

[Initial rate of change of moisture content on dry basis]
The initial rate of change in the dry basis moisture content of the hydrogel was calculated from the following formula using the calculated dry basis moisture content a before the drying process and the dry basis moisture content b 15 minutes after the drying process.
Initial rate of change of dry basis moisture content (%/min) = [{(dry basis moisture content a before drying process - dry basis moisture content b after 15 minutes of drying process) / dry basis moisture content a before drying process} x 100] / 15 (min)

[Peeling rate test]
After the drying process, the JIS sieve on which the crushed and dried polymer was placed was removed from the hot air dryer while keeping the mesh surface of the JIS sieve parallel to the ground and without vibration. Within 5 seconds after removal (during which the JIS sieve was not vibrated and the mesh surface of the JIS sieve was kept parallel to the ground), the JIS sieve was turned upside down by 180° over 3 seconds, and the crushed and dried polymer that had fallen from the JIS sieve at that time was collected in a tub. The weight W4 (g) of the crushed and dried polymer that had fallen and the weight W5 (g) of the crushed and dried polymer that remained attached to the JIS sieve were measured. The peeling rate was calculated by the following formula. The results are shown in Table 1.
Peeling rate (mass%) = [W4/(W4+W5)] x 100

In the manufacturing method of the example in which the initial rate of change in the dry basis moisture content was 6.0% or less, the crosslinked polymer after the drying step exhibited excellent releasability. In particular, in Examples 3 and 5, the crosslinked polymer after the drying step exhibited high releasability even when the drying conditions were made stronger after 15 minutes had elapsed from the start of the drying step. On the other hand, in the comparative example in which the initial rate of change in the dry basis moisture content was more than 6.0%/min, the peel rate was low even when the drying conditions were made milder after 15 minutes had elapsed from the start of the drying step.

Claims (5)

A method for producing water-absorbent resin particles containing a crosslinked polymer, comprising the steps of:
A drying step of drying the crosslinked polymer,
The crosslinked polymer has a dry weight basis moisture content of 100% by mass or more before the drying step;
The method according to claim 1, wherein the drying step is carried out so that an initial rate of change of the dry basis moisture content, as shown in the following formula, is 6.0%/min or less.
Initial rate of change of dry basis moisture content (%/min)=[{(dry basis moisture content before drying process−dry basis moisture content after 15 minutes of drying process)/dry basis moisture content before drying process}×100]/15 The method of claim 1, wherein the drying step is carried out so that the initial rate of change of the dry basis moisture content is 5.6%/min or less. The method according to claim 1 or 2, wherein the drying step is carried out so that the initial rate of change of the dry basis moisture content is 3.0%/min or more. The method according to any one of claims 1 to 3, further comprising crushing the crosslinked polymer prior to the drying step. The method of any one of claims 1 to 4, further comprising grinding the crosslinked polymer after the drying step.