WO2021187323A1

WO2021187323A1 - Method for producing water-absorbing resin particles

Info

Publication number: WO2021187323A1
Application number: PCT/JP2021/009867
Authority: WO
Inventors: 萌西田
Original assignee: 住友精化株式会社
Priority date: 2020-03-18
Filing date: 2021-03-11
Publication date: 2021-09-23
Also published as: JPWO2021187323A1

Abstract

A method for producing water-absorbing resin particles is disclosed. This method comprises mixing a powder of polymer particles with a crosslinking-agent solution to form a premix and heating the premix to form particles containing the crosslinking agent introduced. If the sum of the water content in the polymer particles and the water content in the crosslinking-agent solution at the time when the powder is mixed with the crosslinking-agent solution is expressed by Y1, the water content in the premix is expressed by Y2, and the mass of the premix from which the water has been removed is expressed by Z2, then the premix is heated until the premix comes to have a water retention, as calculated with the equation Water retention (mass%) = (Y2/Y1)×100, of 85 mass% or less and a water content, as calculated with the equation Water content (mass%) = (Y2/Z2)×100, of 10 mass% or less.

Description

Method for producing water-absorbent resin particles

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

Patent Document 1 discloses a method for producing surface-crosslinked water-absorbent resin particles that can be used for absorbent articles such as sanitary products.

Japanese Unexamined Patent Publication No. 7-242709

Cross-linking treatment such as surface cross-linking tends to improve the absorption performance of water-absorbent resin particles under pressure. However, the cross-linking method has a limitation in its application because it tends to reduce the centrifuge holding capacity (CRC), which is the basic water absorption performance of the absorbent resin particles.

One aspect of the present invention provides a method for producing water-absorbent resin particles, which can improve the absorption performance of the water-absorbent resin particles under pressure while suppressing the decrease in CRC by cross-linking the polymer particles.

One aspect of the present invention relates to a method for producing water-absorbent resin particles containing polymer particles. The method comprises mixing a polymer and a powder of polymer particles containing water with a cross-linking agent and a cross-linking agent solution containing water to form a premix, and premixing the premix with the premix. By heating so that a part of the water contained in the mixture is removed, thereby forming the cross-linking agent-introduced particles containing the polymer and the cross-linking agent, and further heating the cross-linking agent-introduced particles. It comprises forming polymer particles containing the polymer crosslinked by the cross-linking agent. The total of the water content of the polymer particles at the time when the powder is mixed with the cross-linking agent solution and the water content of the cross-linking agent solution is Y1 (g), and the water content of the premix is Y2 (g). When the mass of the portion obtained by removing water from the premix is Z2 (g), the premix is used to form the cross-linking agent-introduced particles.
Formula: Moisture residual rate (mass%) = (Y2 / Y1) × 100
The water residual ratio of the premix calculated in is 85% by mass or less, and
Formula: Moisture content (mass%) = (Y2 / Z2) x 100
It is heated until the moisture content of the premix calculated in 1 is 10% by mass or less.

By the above method, it is possible to produce water-absorbent resin particles having greatly improved absorption performance under pressure while suppressing a decrease in CRC due to cross-linking. When the premix containing the polymer particles and the cross-linking agent solution is heated so that water is removed, the cross-linking is performed because the absorption of the cross-linking agent solution and the evaporation of water from the inside of the polymer particles compete with each other near the particle surface. It is considered that the agent does not easily penetrate deep inside the polymer particles. As a result, the cross-linking agent-introduced particles in which the cross-linking agent stays near the surface of the polymer particles are formed, and when the cross-linking proceeds in that state, the cross-linking density is increased in a relatively thin region near the surface of the polymer particles. As a result, polymer particles in which a region having a relatively low crosslink density is widely maintained can be obtained, which is presumed to contribute to the suppression of CRC decrease.

According to one aspect of the present invention, there is provided a method for producing water-absorbent resin particles, which can improve the absorption performance of the water-absorbent resin particles under pressure while suppressing the decrease in CRC by cross-linking the polymer particles.

It is sectional drawing which shows one Embodiment of an absorbent article. It is a schematic diagram which shows the measuring method of the absorption ratio under pressure (AAP).

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

In the present specification, "(meth) acrylic" means both acrylic and methacryl. Similarly, "acrylate" and "methacrylate" are also referred to as "(meth) acrylate". This is also true for other similar terms. "(Poly)" means both with and without the "poly" prefix. In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value of the numerical range of one step can be arbitrarily combined with the upper limit value or the lower limit value of the numerical range of another step. In the numerical range described in the present specification, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples. "Water-soluble" means that it exhibits a solubility in water of 5% by mass or more at 25 ° C. The materials exemplified in the present specification may be used alone or in combination of two or more. "Saline" refers to a 0.9% by mass sodium chloride aqueous solution. "Room temperature" means 25 ° C.

One embodiment of the method for producing water-absorbent resin particles forms a premix by mixing the polymer and the powder of the polymer particles containing water with a cross-linking agent and a cross-linking agent solution containing water. That is, the premix is heated so that a part of the water contained in the premix is removed, thereby forming the crosslinker-introduced particles containing the polymer and the crosslinker, and the crosslinker-introduced particles are further added. It includes forming polymer particles containing a polymer crosslinked by a cross-linking agent by heating.

The polymer forming the polymer particles may be any as long as it can impart water absorption to the polymer particles. The monomer constituting the polymer may be an ethylenically unsaturated monomer. The polymer may be a crosslinked polymer.

The ethylenically unsaturated monomer constituting the polymer is, for example, (meth) acrylic acid and a salt thereof, 2- (meth) acrylamide-2-methylpropanesulfonic acid and a salt thereof, (meth) acrylamide, N, N. -Dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, N-methylol (meth) acrylamide, polyethylene glycol mono (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-diethylaminopropyl ( It may contain at least one compound selected from the group consisting of meta) acrylate and diethylaminopropyl (meth) acrylamide. When the ethylenically unsaturated monomer contains an amino group, the amino group may be quaternized. The ethylenically unsaturated monomer constituting the polymer contains at least one compound selected from the group consisting of acrylic acid and its salt, methacrylic acid and its salt, acrylamide, methacrylicamide and N, N-dimethylacrylamide. It may contain acrylic acid and its salt, methacrylic acid and its salt, and at least one compound selected from the group consisting of acrylamide, from acrylic acid and its salt, and methacrylic acid and its salt. It may contain at least one compound selected from the group.

The polymer forming the polymer particles may have a functional group that reacts with the cross-linking agent. This functional group may be, for example, a carboxyl group, an amino group, or a combination thereof. The carboxyl group can be, for example, a group derived from acrylic acid, methacrylic acid or salts thereof.

The polymer may contain a monomer unit derived from a monomer other than the ethylenically unsaturated monomer. The proportion of the monomer unit derived from the ethylenically unsaturated monomer (particularly (meth) acrylic acid and a salt thereof) in the polymer may be 70 to 100 mol% with respect to the total amount of the monomer.

The polymer particles can be obtained by, for example, a polymerization method selected from a reverse phase suspension polymerization method, an aqueous solution polymerization method, a bulk polymerization method, and a precipitation polymerization method. Polymer particles containing a crosslinked polymer can be obtained by self-crosslinking during polymerization, reaction with an internal crosslinking agent, or a combination thereof.

An example of a method for obtaining polymer particles by an aqueous solution polymerization method is to polymerize an ethylenically unsaturated monomer in a monomer aqueous solution containing an ethylenically unsaturated monomer and water to contain the polymer and water. Forming a lumpy hydrogel polymer, crushing the hydrogel polymer to form a crushed product, drying the crushed product to obtain a dried product, and further crushing the dried product. To obtain polymer particles. The powder of the polymer particles after pulverization may be classified by a sieve or the like.

When the monomer has an acid group such as (meth) acrylic acid, 2- (meth) acrylamide-2-methylpropanesulfonic acid, etc., the acid group is neutralized with an alkaline neutralizer if necessary. You may. Examples of alkaline neutralizers include alkali metal salts such as sodium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium hydroxide, potassium carbonate; and ammonia. The degree of neutralization of the monomer by the alkaline neutralizing agent may be 10 to 100 mol%, 30 to 90 mol%, 40 to 85 mol%, or 50 to 80 mol%. The degree of neutralization means the ratio of neutralized acid groups based on the amount of all acid groups contained in the monomer.

The monomer aqueous solution may contain a polymerization initiator. The polymerization initiator may be a photoradical polymerization initiator, a thermal radical polymerization initiator, or a combination thereof. The polymerization initiator may be water-soluble. The thermal radical polymerization initiator may be, for example, an azo compound, a peroxide, or a combination thereof. The amount of the polymerization initiator is, for example, 0.001 to 1 mol, 0.005 to 0.5 mol, 0.008 to 0.3 mol, or 0.01 to 0.2 mol with respect to 100 mol of the monomer. It may be. The monomer aqueous solution may further contain a chain transfer agent or the like, if necessary.

The monomer aqueous solution may contain an internal cross-linking agent, in which case polymer particles containing a cross-linked polymer cross-linked by the internal cross-linking agent can be obtained. The internal cross-linking agent may be a compound having two or more reactive functional groups (for example, polymerizable unsaturated groups). Examples of internal cross-linking agents are di or tri (meth) acrylics of polyols such as (poly) ethylene glycol, (poly) propylene glycol, trimethylolpropane, glycerin polyoxyethylene glycol, polyoxypropylene glycol, (poly) glycerin. Acid esters; Unsaturated polyesters obtained by reacting the above polyol with unsaturated acids (maleic acid, fumaric acid, etc.); (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, (poly) ) Glycidyl group-containing compounds such as glycerin diglycidyl ether and glycidyl (meth) acrylate; bisacrylamides such as N, N'-methylenebis (meth) acrylamide; Tri (meth) acrylic acid esters; di (meth) acrylic acid carbamil esters obtained by reacting polyisocyanates (trilylene diisocyanate, hexamethylene diisocyanate, etc.) with hydroxyethyl (meth) acrylic acid; allylated starch Alylated cellulose; diallyl phthalate; N, N', N "-triallyl isocyanurate; divinylbenzene; pentaerythritol; ethylenediamine; polyethyleneimine. From the viewpoint of easily increasing CRC and excellent reactivity at low temperature. From the viewpoint, the internal cross-linking agent is composed of polyethylene glycol diacrylate, trimethylpropantriacrylate, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, and (poly) glycerin diglycidyl ether. It may contain at least one selected.

The coarsely crushed product obtained by coarsely crushing the hydrogel polymer may be in the form of particles, or may have an elongated shape in which a plurality of particles are connected. The minimum width of the coarse crushed product may be, for example, about 0.1 to 15 mm or 1.0 to 10 mm. The maximum width of the coarse crushed product may be about 0.1 to 200 mm or 1.0 to 150 mm. Examples of devices for coarse crushing include centrifugal crushers, kneaders (eg, pressurized kneaders, double-armed kneaders, etc.), meat choppers, cutter mills, and pharma mills.

Most of the water in the crushed product is removed by drying the crushed product. The moisture content of the dried product obtained by drying may be, for example, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The water content can be measured by the same method as the “moisture content of polymer particles” described later. The drying method may be a general method such as natural drying, heat drying, spray drying, vacuum drying or a combination thereof. The coarsely crushed product may be dried under normal pressure or reduced pressure. The heating temperature for drying under normal pressure may be 70 to 250 ° C. or 80 to 200 ° C.

The method of crushing the dried product is not particularly limited. For example, the dried product can be crushed by using a crusher such as a centrifugal crusher, a roller mill, a stamp mill, a jet mill, a high-speed rotary crusher, and a container-driven mill.

The powder of the polymer particles obtained by pulverization may be classified. Classification means an operation of dividing a particle group (powder) into two or more particle groups having different particle size distributions. A part of the powder of the polymer particles after the classification may be pulverized and classified again.

The classification method is not particularly limited, but may be, for example, screen classification or wind power classification. Screen classification is a method of classifying particles on a screen into particles that pass through the mesh of the screen and particles that do not pass through the screen by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary shifter, a cylindrical stirring sieve, a blower shifter, or a low-tap shaker. Wind power classification is a method of classifying particles using the flow of air.

The medium particle size of the polymer particles before being mixed with the cross-linking agent solution, which is obtained through pulverization and, if necessary, particle size adjustment such as classification, may be, for example, 200 to 500 μm.

The CRC of the polymer particles before being mixed with the cross-linking agent solution, which is used in the step for surface cross-linking described later, may be, for example, 30 to 50 g / g. The water content of the polymer particles before being mixed with the cross-linking agent solution may be 0 to 20% by mass, 0 to 10% by mass, or more than 0% by mass and 5% by mass or less. When the polymer particles are preheated as described later, the water content of the polymer particles after the preheating may be within the above range. In the present specification, the "moisture content of the polymer particles" means the ratio of the water content in the polymer particles based on the total mass of the polymer particles containing water. Usually, when the polymer particles containing water are heated at 200 ° C. for 2 hours, the difference in mass of the polymer particles before and after heating can be regarded as the amount of water in the polymer particles. Details of the method for measuring the water content will be described in Examples described later.

The temperature of the polymer particles may be raised by preheating the powder of the polymer particles before mixing with the cross-linking agent solution. In this case, the powder of the polymer particles whose temperature has been raised by the preheating is mixed with the cross-linking agent solution. By preheating, the effect of suppressing the decrease in CRC can be exerted even more remarkably. Preheating may be performed while stirring the powder. The temperature reached by the polymer particles by preheating may be in the same range as the heating temperature of the premix. The preheating time may be in the range where the temperature of the polymer particles is equal to or higher than a predetermined temperature, and may be, for example, 2 to 60 minutes. The heating means for preheating is not particularly limited and can be selected from ordinary methods.

If necessary, the preheated polymer particle powder is mixed with the cross-linking agent solution. By heating the formed premix, some of the water in the premix is removed, thereby forming crosslinker-introduced particles containing the crosslinker and the polymer. The amount of the cross-linking agent solution mixed with the powder of the polymer particles may be 0.1 to 20 parts by mass with respect to 100 parts by mass of the polymer particles. Additional materials other than the polymer particle powder and the cross-linking agent solution may be further added to the premix, but the amount of the non-water component of the additional material is the polymer particle powder and cross-linking. It may be, for example, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, or 1% by mass or less with respect to the total mass of the agent solution.

The temperature at which the premix is heated (the temperature of the premix) is adjusted within a range in which the water in the premix is gradually removed and the progress of the reaction by the cross-linking agent is suppressed. Therefore, the temperature of the premix to be heated may be lower than the heating temperature for cross-linking the polymer in the polymer particles with a cross-linking agent. Specifically, the temperature of the premix to be heated is 50 ° C. or higher, 55 ° C. or higher, 60 ° C. or higher, 65 ° C. or higher, 70 ° C. or higher, 75 ° C. or higher, 80 ° C. or higher, 85 ° C. or higher, 90 ° C. or higher. It may be 95 ° C. or higher or 100 ° C. or higher, and may be 180 ° C. or lower or 170 ° C. or lower. The temperature of the premix to be heated is 50 ° C or higher, 55 ° C or higher, 60 ° C or higher, 65 ° C or higher, 70 ° C or higher, 75 ° C or higher, 80 ° C or higher, 85 ° C or higher, 90 ° C or higher, 95 ° C or higher or 100. ° C or higher and may be 180 ° C or lower, 50 ° C or higher, 55 ° C or higher, 60 ° C or higher, 65 ° C or higher, 70 ° C or higher, 75 ° C or higher, 80 ° C or higher, 85 ° C or higher, 90 ° C or higher, 95 ° C. It may be higher than or equal to 100 ° C. or higher and 170 ° C. or lower. When the heating temperature is high, the decrease in CRC can be suppressed more effectively, and the heating time of the cross-linking agent-introduced particles tends to be shortened. For example, these temperature ranges may be applied when the cross-linking agent contains an alkylene carbonate compound. The heating temperature of the premix does not have to be kept constant and may vary.

The heating time of the premix may be, for example, 2 minutes or more, 5 minutes or more, 60 minutes or less, 50 minutes or less, or 40 minutes or less. The heating time of the premix may be 2 minutes or more and 60 minutes or less, 50 minutes or less, or 40 minutes or less, and 5 minutes or more and 60 minutes or less, 50 minutes or less, or 40 minutes or less. .. When the heating temperature of the premix varies, the time for the premix to be heated to the above temperature range may be 2 minutes or more, or 5 minutes or more, 60 minutes or less, 50 minutes or less, or 40 minutes or less. You may. When the heating temperature of the premix varies, the time for the premix to be heated to the above temperature range may be 60 minutes or less, 50 minutes or less or 40 minutes or less in 2 minutes or more, and 60 minutes in 5 minutes or more. Hereinafter, it may be 50 minutes or less or 40 minutes or less. The heating time here can be the time from the time when the powder of the polymer particles is mixed with the total amount of the cross-linking agent solution. The time (mixing time) from the start of mixing the polymer particle powder with the cross-linking agent solution to the time when the polymer particle powder is mixed with the total amount of the cross-linking agent solution is, for example, 1 second or more. It may be present, or it may be within 120 seconds.

The heating conditions of the premix (for example, heating temperature and heating time) are controlled so that the residual water content and the water content of the premix are within a predetermined range, which are calculated by the following formulas. As a result, the decrease in CRC associated with cross-linking can be effectively suppressed. Controlling the residual water content is also advantageous for suppressing the formation of coarse particles due to the aggregation of particles.
Moisture residual rate (mass%) = (Y2 / Y1) x 100
Moisture content (mass%) = (Y2 / Z2) x 100
Here, Y1 is the sum of the water content of the polymer particles at the time when the powder of the polymer particles is mixed with the cross-linking agent solution and the water content of the cross-linking agent solution mixed with the polymer particles, and is usually used. , Corresponds to the initial water content in the premix. Y2 is the amount of water in the premixture, and Z2 is the mass of the portion of the premixture excluding water. Y1, Y2 and Z2 can be determined from, for example, the amount of the polymer particles and the cross-linking agent solution charged, and the measured values of the water content of the polymer particles and the premixture.

The water residual ratio in the premixture (crosslinking agent-introduced particles) after heating is a reserve relative to the total amount of water contained in the polymer particles before forming the premixture and water contained in the crosslinking agent solution. It corresponds to the percentage of the amount of water remaining in the mixture. If any material containing water is added to the premix, the amount of water contained in that material is also added to Y1. That is, Y1 is determined by regarding water derived from some material added other than the polymer particles and the cross-linking agent solution as a part of the water introduced by the cross-linking agent solution. The water residual ratio of the premix after heating is usually 85% by mass or less. The water residual ratio may be 80% by mass or less, 70% by mass or less, 65% by mass or less, 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, or 40% by mass or more. The residual water content of the premix after heating is 85% by mass or less, 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, or 40% by mass. It may be 80% by mass or less, 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, or 40% by mass or more. It may be 70% by mass or less and 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, or 40% by mass or more, 65. In mass% or less, it 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, 35% by mass or more, or 40% by mass or more.

The water content of the premix after heating (crosslinking agent-introduced particles) corresponds to the ratio of the amount of water remaining in the premix to the mass of the portion of the premix after heating excluding water. This water content can be calculated from, for example, the total mass of the premix containing water and the water content of the premix. The water content of the premix can be measured in the same manner as the water content of the polymer particles described above. The moisture content of the premix after heating is 10% by mass or less. This corresponds to a small initial water content of the premix. Even if the water content of the premix after heating is 9% by mass or less, 8% by mass or less, 7% by mass or less, 6% by mass or less, 5% by mass or less, 4% by mass or less or 3.5% by mass or less. It may be 0.1% by mass or more.

The heating means for heating the premix is not particularly limited and can be selected from ordinary methods. The premix may be heated with stirring. When preheating the polymer particles, the powder of the polymer particles is preheated in the mixing container, the cross-linking agent solution is added to the powder in the preheated state, and the powder is preheated in the same mixing container as the preheating. The mixture may continue to be heated.

The cross-linking agent solution can be water or a solution containing a cross-linking agent dissolved in water. The solvent contained in the cross-linking agent solution may be substantially only water. The proportion of the solvent other than water may be 10% by mass or less, 5% by mass or less, or 1% by mass or less based on the mass of the cross-linking agent solution. The solvent other than water may be a hydrophilic organic solvent, and examples thereof include ethyl alcohol, isopropyl alcohol, and acetone.

The cross-linking agent contained in the cross-linking agent solution is mainly used to increase the cross-linking density near the surface of the polymer particles. Therefore, the cross-linking agent may be referred to as a "surface cross-linking agent". Examples of surface cross-linking agents include polyol compounds such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin; (poly) ethylene glycol. Polyglycidyl compounds such as diglycidyl ether, (poly) glycerin diglycidyl ether, (poly) glycerin triglycidyl ether, trimethylpropan triglycidyl ether (poly) propylene glycol polyglycidyl ether, and (poly) glycerol polyglycidyl ether; epichlorohydrin , Epibromhydrin, and haloepoxy compounds such as α-methylepicrolhydrin; isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol. , 3-Butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and oxetane compounds such as 3-butyl-3-oxetaneethanol; 1,2-ethylenebisoxazoline and the like. Oxazoline compounds; alkylene carbonate compounds such as ethylene carbonate; hydroxyalkylamide compounds such as bis [N, N-di (β-hydroxyethyl)] adipamide. These surface cross-linking agents may be used alone or in combination of two or more. The surface cross-linking agent may contain an alkylene carbonate compound, a polyol compound, or a combination thereof, or may contain an alkylene carbonate compound. The ratio of the alkylene carbonate compound in the surface cross-linking agent is 25 to 100 mol%, 30 to 100 mol%, 30 to 80 mol%, 35 to 70 mol%, or 35 based on the total amount of substance (mol) of the surface cross-linking agent. It may be up to 55 mol%.

From the viewpoint of water absorption performance under pressure of the water-absorbent resin particles, the amount of the cross-linking agent (surface cross-linking agent) is 0.00001 to 0. Per mol of the monomer unit constituting the polymer in the polymer particles. It may be 03 mol, 0.00005 to 0.02 mol, or 0.0001 to 0.01 mol.

The concentration of the cross-linking agent (surface cross-linking agent) in the cross-linking agent solution may be, for example, 0.1 to 50.0% by mass based on the mass of the cross-linking agent solution. The concentration of the cross-linking agent here means the total concentration of the cross-linking agent solutions when the cross-linking agent solution contains two or more kinds of cross-linking agents.

By further heating the cross-linking agent-introduced particles formed by heating the premix until the water residual ratio and the water content are equal to or less than the predetermined values, the polymer crosslinked mainly by the surface cross-linking agent in the vicinity of the particle surface is obtained. The containing polymer particles, that is, the surface-crosslinked polymer particles are formed.

The heating temperature and heating time for surface cross-linking are adjusted so that the cross-linking reaction proceeds appropriately in consideration of the type of surface cross-linking agent and the like. For example, the heating temperature for surface cross-linking may exceed 180 ° C. or 190 ° C. or higher. The heating temperature for surface cross-linking may be 250 ° C. or lower. For example, these temperature ranges may be applied when the surface cross-linking agent contains an alkylene carbonate compound. The heating time for surface cross-linking may be, for example, 5 to 90 minutes.

According to the method according to the present embodiment, polymer particles (water-absorbent resin particles) in which AAP is increased can be obtained while suppressing a decrease in CRC due to surface cross-linking. That is, the CRC and AAP of the polymer particles before being mixed with the cross-linking agent solution (or before preheating) were surface-crosslinked with "CRC (before surface cross-linking)" and "AAP (before surface cross-linking)", respectively. When the CRC and AAP of the polymer particles are "CRC (after surface cross-linking)" and "AAP (after surface cross-linking)", respectively, the sum of ΔCRC and ΔAAP (ΔCRC + ΔAAP) calculated by the following formula is large.
ΔCRC = CRC (after surface cross-linking) -CRC (before surface cross-linking)
ΔAAP = AAP (after surface cross-linking) -AAP (before surface cross-linking)
Surface cross-linking tends to reduce CRC and increase AAP. Therefore, ΔCRC has a negative value, and ΔAAP has a positive value in many cases. A large ΔCRC + ΔAAP means that the AAP increased while suppressing the decrease in CRC associated with surface cross-linking. According to the method according to the present embodiment, for example, the polymer particles so that ΔCRC + ΔAAP is 12.0 g / g or more, 13.0 g / g or more, 14.0 g / g or more, or 15.0 g / g or more. Can be surface crosslinked. The upper limit of ΔCRC + ΔAAP is not particularly limited, but is usually about 60 g / g.

The surface-crosslinked polymer particles may be further dried or classified if necessary. Inorganic particles may be attached to the surface of the polymer particles. Examples of inorganic particles include silica particles such as amorphous silica.

The produced water-absorbent resin particles are used to form an absorbent body that constitutes an absorbent article such as a diaper, for example. FIG. 1 is a cross-sectional view showing an example of an absorbent article. The absorbent article 100 shown in FIG. 1 includes a sheet-shaped absorber 10, core wraps 20a and 20b, a liquid permeable sheet 30, and a liquid permeable sheet 40. In the absorbent article 100, the liquid permeable sheet 40, the core wrap 20b, the absorbent body 10, the core wrap 20a, and the liquid permeable sheet 30 are laminated in this order. The absorber 10 has water-absorbent resin particles 10a produced by the method according to the above-described embodiment, and a fiber layer 10b containing a fibrous material. The water-absorbent resin particles 10a are dispersed in the fiber layer 10b.

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

1. 1. Measurement method The water content, centrifuge holding capacity (CRC), absorption ratio under pressure (AAP), water absorption rate by the Vortex method, and medium particle size were measured by the following procedures.

1-1. Moisture content (wetness standard)
1.0 g of a sample (polymer particle or premixture) is placed in an aluminum foil case (No. 8) having a constant amount (W1 (g)) in advance, and the mouth of the aluminum foil case is lightly closed. The total mass W2 (g) of this sample-containing aluminum foil case is precisely weighed. The aluminum foil case containing the sample is dried for 2 hours in a hot air dryer (manufactured by ADVANTEC, model: FV-320) whose internal temperature is set to 200 ° C. Allow the dried aluminum foil case containing the sample to cool to room temperature in a desiccator. The total mass W3 (g) of the aluminum foil case containing the sample after allowing to cool is measured. The water content of the sample is calculated from the following formula.
Moisture content (mass%) = [{(W2-W1)-(W3-W1)} / (W2-W1)] x 100

1-2. Centrifuge retention capacity (CRC)
The CRC is measured in an environment of a temperature of 23 ° C. ± 2 ° C. and a humidity of 50 ± 10% according to the following procedure according to the EDANA method (NWSP 241.0.R2 (15), page.769-778).

Adjust the size to 60 mm x 85 mm by folding a non-woven fabric with a size of 60 mm x 170 mm (product name: Heat Pack MWA-18, manufactured by Nippon Paper Papylia Co., Ltd.) in half in the longitudinal direction. Nonwoven fabrics are heat-sealed against each other on both sides extending in the longitudinal direction. This prepares a non-woven fabric bag having a size of 60 mm in width and 85 mm in length and having an opening at one end in the longitudinal direction. On both sides along the longitudinal direction of the non-woven fabric bag, a crimping portion having a width of 5 mm is formed in which a heat seal is interposed between the non-woven fabrics. 0.2 g of particles to be measured is housed inside the non-woven fabric bag. Next, the non-woven fabric bag is closed by crimping the opening of the non-woven fabric bag with a heat seal.

Float a plurality of non-woven fabric bags containing particles on 1000 g of physiological saline contained in a stainless steel vat (240 mm × 320 mm × 45 mm) so as not to overlap each other. One minute after the non-woven fabric bag is put into the saline solution, the non-woven fabric bag is completely immersed in the saline solution with a spatula.

After the non-woven fabric bag was put into the saline solution, when a total of 30 minutes had passed, 1 minute for the non-woven fabric bag to float in the saline solution and 29 minutes for the non-woven fabric bag to be immersed in the saline solution, from the saline solution, Remove the non-woven bag containing the gel formed by water absorption. Using a centrifuge (manufactured by Kokusan Co., Ltd., model number: H-122), the gel in the non-woven fabric bag is dehydrated for 3 minutes under the condition that the centrifugal force is 250 G. After dehydration, the mass Ma of the non-woven fabric bag containing the mass of the gel is weighed. The mass Mb of the non-woven fabric bag, which has been subjected to the same operation as the above operation without accommodating the particles to be measured, is measured. The CRC is calculated by the following formula. Mc is a scaled value of 0.2 g of the mass of the particles used in the measurement.
CRC [g / g] = {(Ma-Mb) -Mc} / Mc

1-3. Absorption rate under pressure (AAP)
Using the measuring device 110 shown in FIG. 2, the absorption ratio (AAP) under pressure at a pressure of 2.07 kPa (0.3 psi) is measured. The measuring device 110 is composed of a weight 112, a plastic cylinder 114 having an inner diameter of 60 mm, and a wire mesh 116 having a 400 mesh (opening 38 μm). The wire mesh 116 closes one opening of the cylinder 114, and the cylinder 114 and the wire mesh 116 are arranged so that the wire mesh 116 is horizontal. The weight 112 has a disc portion 112a, a rod-shaped portion 112b extending from the center of the disc portion 112a in a direction perpendicular to the disc portion 112a, and a cylindrical portion 112c having a through hole in the center. The rod-shaped portion 112b is inserted into the through hole. The disk portion 112a has a diameter substantially equal to the inner diameter of the cylinder 114 so that it can move in the vertical direction of the cylinder 114 inside the cylinder 114. The diameter of the cylindrical portion 112c is smaller than the diameter of the disc portion 112a. The weight of the weight 112 is adjusted so that a pressure of 2.07 kPa is applied to the particles to be measured. Inside the cylinder 114, 0.90 g of particles 120 to be measured are uniformly scattered on the wire mesh 116. The weight 112 inserted in the cylinder 114 is placed on the sprayed particles. In this state, the total mass Wa [g] of the measuring device 110 and the particles 120 before absorbing the liquid is measured.

A glass filter 140 (ISO4793 P-250) having a diameter of 90 mm and a thickness of 7 mm is placed in the center of the bottom surface (diameter 150 mm) in the recess of the stainless steel petri dish 130. Next, 0.90 mass% sodium chloride aqueous solution (25 ° C. ± 2 ° C.) is added to the stainless steel petri dish until the water surface is level with the upper surface of the glass filter 140. A sheet of filter paper 150 with a diameter of 90 mm (ADVANTEC Toyo Co., Ltd., product name: (No. 3), thickness 0.23 mm, reserved particle diameter 5 μm) is placed on the glass filter 140, and the entire surface of the filter paper 150 is sodium chloride. Wet with an aqueous solution to remove excess sodium chloride aqueous solution. Subsequently, the measuring device 110 in which the particles 120 before liquid absorption are charged is placed on the filter paper 150, and the particles 120 are made to absorb the sodium chloride aqueous solution while pressurizing the particles 120 at a pressure of 2.07 kPa. One hour after the start of absorption, the measuring device 110 is lifted and the total mass Wb [g] of the measuring device 110 and the particles 120 after absorbing the liquid is measured.

From Wa and Wb, the absorption ratio under pressure (AAP) [g / g] is calculated by the following formula.
AAP [g / g] = (Wb [g] -Wa [g]) /0.90 [g]

1-4. A rotor (8 mm × 30 mm, without ring) and 50 g of physiological saline are placed in a beaker having a water absorption rate of 100 mL by the Vortex method, and the beaker containing the physiological saline is held at 25 ° C. in a constant temperature bath. Next, 2.0 g of the particles for measurement are put into a vortex of physiological saline stirred at 600 rpm, and at the same time, measurement with a stopwatch is started. The time point at which the vortex on the liquid surface converges is set as the end point, and the time (seconds) up to that point is taken as the water absorption rate.

1-5. Medium particle size 10 g of powder of polymer particles or water-absorbent resin particles is mixed with a continuous fully automatic sonic vibration type sieving measuring instrument (Robot Shifter RPS-205, manufactured by Seishin Enterprise Co., Ltd.) and JIS standard opening 850 μm, 500 μm. , 425 μm, 300 μm, 250 μm, 180 μm and 106 μm sieves and a saucer are used for sieving. The mass of the particles remaining on each sieve is calculated as a mass percentage with respect to the total amount. The mass percentages of the particles remaining on each sieve are integrated in order from the one with the largest particle size, and the relationship between the mesh size of the sieve and the integrated value of the mass percentages of the particles remaining on the sieve is plotted on logarithmic probability paper. .. By connecting the plots on the probability paper with a straight line, the particle size corresponding to the cumulative mass percentage of 50% by mass is obtained, and this is defined as the medium particle size.

2. Preparation of Polymer Particles Before Surface Crosslinking 520 g (7.22 mol) of 100% acrylic acid was placed in a round-bottomed cylindrical separable flask having an inner diameter of 11 cm and an internal volume of 2 L equipped with a stirrer. While stirring the acrylic acid, 445.36 g of ion-exchanged water was added into the separable flask. Then, 454.04 g of 48% by mass sodium hydroxide was added dropwise to the separable flask under an ice bath to partially neutralize the acrylic acid (monomer concentration 45.08% by mass, neutralization rate 75.44 mol%). ) 1419.4 g was obtained. This operation was repeated twice to obtain a total of about 2800 g of a partially neutralized acrylic acid solution.

To 2781.72 g of the acrylic acid partial neutralizing solution, 406.25 g of ion-exchanged water and 3.55 g of polyethylene glycol diacrylate were added. The obtained reaction solution was degassed in a nitrogen gas atmosphere for 30 minutes. Next, the reaction solution was supplied to a stainless steel double-armed kneader having two sigma-type blades and a jacket, and the inside of the kneader was replaced with nitrogen gas while keeping the reaction solution at 30 ° C. Subsequently, 92.63 g of a 2.0 mass% sodium persulfate aqueous solution and 15.85 g of a 0.5 mass% L-ascorbic acid aqueous solution were added to the reaction solution with stirring. Approximately 1 minute later, the temperature of the reaction solution began to rise with the start of polymerization, and 3 minutes later, the maximum temperature reached 93 ° C. Then, the internal temperature was kept at 60 ° C., and 60 minutes after the start of the polymerization, the produced hydrogel-like polymer was taken out from the kneader.

The hydrogel polymer was sequentially put into a meat chopper 12VR-750SDX manufactured by Kiren Royal Co., Ltd. and coarsely crushed. The diameter of the hole in the plate located at the outlet of the meat chopper was 6.4 mm. The coarsely crushed product of the hydrogel polymer was spread over a wire mesh having a mesh size of 0.8 cm × 0.8 cm and placed, and dried with hot air at 160 ° C. for 60 minutes to obtain a dried product.

The dried product was crushed using a centrifugal crusher (Resch, ZM200, screen diameter 1 mm, 6000 rpm). The obtained pulverized material was passed through a sieve having a mesh size of 850 μm, and amorphous crushed particles (medium particle size: 331 μm) that did not pass through a sieve having a mesh size of 180 μm were obtained as polymer particles before surface cross-linking. rice field. The obtained polymer particles exhibited a centrifuge holding capacity (CRC) of 39.9 g / g, an absorption ratio under pressure (AAP) of 8.1 g / g, and a water content of 5.4% by mass. ..

3. 3. Preparation of surface-crosslinked polymer particles (water-absorbent resin particles) (Example 1)
<Preheating>
A round-bottomed cylindrical separable flask equipped with a stirrer and having an inner diameter of 11 cm and an internal volume of 2 L was prepared. A fluororesin anchor blade having an outer diameter of 9 cm was attached to the stirrer. 25 g of polymer particles were placed in a separable flask. The polymer particles were preheated for 35 minutes by immersing the separable flask in an oil bath at 200 ° C. while stirring the polymer particles at a rotation speed of 300 rpm. Preheating raised the temperature of the polymer particles to 170 ° C. When a part of the polymer particles at this time was taken out and the water content α1 was measured, it was 1.65% by mass.

<Heating of premixture>
0.70 g of a cross-linking agent solution containing ethylene carbonate and propylene glycol as a surface cross-linking agent was added dropwise to the polymer particles heated to 170 ° C. The cross-linking agent solution contained 11.13% by mass of ethylene carbonate, 17.77% by mass of propylene glycol, and 71.10% by mass of deionized water based on the total mass thereof. The premix containing the polymer particles and the cross-linking agent solution was stirred for 3 minutes after the completion of dropping the cross-linking agent solution. During that time, the temperature of the polymer particles was maintained at about 170 ° C. by heating with an oil bath at 200 ° C. When the premix was stirred with heating for 3 minutes, the premix (crosslinking agent-introduced particles) was removed from the separable flask. The water content α2 of the premix (crosslinking agent-introduced particles) after the heat treatment was measured and found to be 1.68% by mass.

<Surface cross-linking>
The cross-linking agent-introduced particles taken out from the separable flask are immediately surface-crosslinked with the polymer particles by heating them in a hot air dryer (manufactured by ADVANTEC, model: FV-320) set to an internal temperature of 200 ° C. for 90 minutes. Was advanced. By classifying the powder of the surface-crosslinked polymer particles with a wire mesh having an opening of 850 μm, water-absorbent resin particles which were fractions passing through the wire mesh of 850 μm were obtained.

<Moisture residual rate and moisture content in premixture (crosslinking agent-introduced particles)>
From the charged amount of the polymer particles before preheating and the water content of 5.4% by mass, the mass (dry mass) Z0 of the portion of the polymer particles before preheating excluding water was calculated by the following formula.
Z0 (g) = 25 x (1-5.4 / 100) = 23.65
From Z0 and the water content α1 (1.65% by mass) of the polymer particles after preheating, the water content X1 (g) in the polymer particles at the time of mixing with the cross-linking agent solution after preheating is the following relational expression. It was calculated based on. The calculated X1 was 0.3968 g.
X1 / (Z0 + X1) = α1 / 100
The total Y1 of the water content X1 in the polymer particles and the water content in the cross-linking agent solution was calculated by the following formula.
Y1 (g) = X1 + 0.70 × (71.10 / 100) = 0.8945
The mass of the portion of the premix after heating (crosslinking agent-introduced particles) excluding water, that is, the total of the dry mass Z0 of the polymer particles before being mixed with the crosslinking agent solution and the crosslinking agent in the crosslinking agent solution. The mass Z2 of was calculated by the following formula.
Z2 (g) = Z0 + 0.70 × (11.13 / 100) + 0.70 × (17.77 / 100) = 23.8523
The water content Y2 in the premix (crosslinking agent-introduced particles) after heating was calculated based on the following relational expression regarding the water content α2 (1.68% by mass) of the premix and Z2 and Y2. The calculated Y2 was 0.4076 g.
Y2 / (Z2 + Y2) = α2 / 100
From Y1, Y2 and Z2, the residual water content and the water content in the premix (crosslinking agent-introduced particles) after heating were calculated by the following formulas.
Moisture residual rate = (Y2 / Y1) x 100 = 45.6% by mass
Moisture content = (Y2 / Z2) x 100 = 1.71% by mass

(Example 2)
Water-absorbent resin particles, which are surface-crosslinked polymer particles, were obtained in the same manner as in Example 1 except that the time for stirring the premix while heating was changed to 5 minutes. The water residual ratio and the water content in the premix (or the cross-linking agent-introduced particles) were determined by the same method as in Example 1.

(Example 3)
Water-absorbent resin particles, which are surface-crosslinked polymer particles, were obtained in the same manner as in Example 1 except that the time for stirring the premix while heating was changed to 35 minutes. The water residual ratio and the water content in the premix (or the cross-linking agent-introduced particles) were determined by the same method as in Example 1.

(Example 4)
The surface was the same as in Example 1 except that the temperature of the polymer particles was adjusted to 130 ° C. by changing the temperature of the oil bath for preheating the polymer particles and heating the premixture to 150 ° C. Water-absorbent resin particles, which are crosslinked polymer particles, were obtained. The water residual ratio and the water content in the premix (or the cross-linking agent-introduced particles) were determined by the same method as in Example 1. The time for stirring the premix while heating was also set to 3 minutes as in Example 1.

(Example 5)
Water-absorbent resin particles, which are surface-crosslinked polymer particles, were obtained in the same manner as in Example 4 except that the time for stirring the premix while heating was changed to 35 minutes. The water residual ratio and the water content in the premix (or the cross-linking agent-introduced particles) were determined by the same method as in Example 1.

(Example 6)
The surface was the same as in Example 1 except that the temperature of the polymer particles was adjusted to 90 ° C. by changing the temperature of the oil bath for preheating the polymer particles and heating the premixture to 110 ° C. Water-absorbent resin particles, which are crosslinked polymer particles, were obtained. The water residual ratio and the water content in the premix (or the cross-linking agent-introduced particles) were determined by the same method as in Example 1. However, the time for stirring the premix while heating was set to 35 minutes.

(Comparative Example 1)
The surface was the same as in Example 1 except that the temperature of the polymer particles was adjusted to 40 ° C. by changing the temperature of the oil bath for preheating the polymer particles and heating the premixture to 45 ° C. Water-absorbent resin particles, which are crosslinked polymer particles, were obtained. The water residual ratio and the water content in the premix (or the cross-linking agent-introduced particles) were determined by the same method as in Example 1. However, the time for stirring the premix while heating was set to 5 minutes.

(Comparative Example 2)
25 g of the polymer particles were placed in the same separable flask as in Example 1. The polymer particles were stirred for 35 minutes at a rotation speed of 300 rpm in a room temperature environment. The temperature of the polymer particles was 25 ° C. Next, 0.7 g of the same cross-linking agent solution as in Example 1 was placed in a separable flask, and the formed premix was stirred for 1 minute. The premix was heated in a hot air dryer (manufactured by ADVANTEC, model: FV-320) set to an internal temperature of 200 ° C. for 90 minutes. The powder of the polymer particles after heating was classified with a wire mesh having an opening of 850 μm to obtain water-absorbent resin particles which were fractions passing through the wire mesh of 850 μm. The residual water content and the water content in the premix were determined by the same method as in Example 1.

(Comparative Example 3)
Water-absorbent resin particles were obtained in the same manner as in Comparative Example 2 except that the time for stirring the premix in the separable flask was changed to 35 minutes. The residual water content and the water content in the premix were determined by the same method as in Example 1.

4. Evaluation of Water-absorbent Resin Particles For the water-absorbent resin particles, which are surface-crosslinked polymer particles, the water absorption rate and the medium particle size were measured by the CRC, AAP, and Vortex methods. The amount of change in CRC from CRC of the polymer particles before surface cross-linking ΔCRC and the amount of change in AAP from AAP of the polymer particles before surface cross-linking ΔAAP were obtained, and the sum of the two (ΔCRC + ΔAAP) was also calculated.

5. Results Tables 1 and 2 show the evaluation results. In each example, heating of the premix causes formation of cross-linking agent-introduced particles having a water content of 85% by mass or less and a water content of 10% by mass or less, and subsequent surface cross-linking reduces CRC. It was confirmed that AAP could be improved while suppressing it.

10 ... Absorbent, 10a ... Water-absorbent resin particles, 10b ... Fiber layer, 20a, 20b ... Core wrap, 30 ... Liquid permeable sheet, 40 ... Liquid permeable sheet, 100 ... Absorbent article, 110 ... Measuring device, 112 ... Weight, 112a ... Disc, 112b ... Rod, 112c ... Cylindrical, 114 ... Cylindrical, 116 ... Wire mesh, 120 ... Particles to be measured, 130 ... Stainless petri dish, 140 ... Glass filter, 150 ... Filter paper.

Claims

A method for producing water-absorbent resin particles containing polymer particles.
To form a premixture by mixing the polymer and water-containing polymer particle powder with a cross-linking agent and a water-containing cross-linking agent solution.
The premix is heated so that a part of the water contained in the premix is removed, thereby forming crosslinker-introduced particles containing the polymer and the crosslinker.
By further heating the cross-linking agent-introduced particles, polymer particles containing the polymer cross-linked by the cross-linking agent can be formed.
With
The total of the water content of the polymer particles at the time when the powder is mixed with the cross-linking agent solution and the water content of the cross-linking agent solution is Y1 (g), and the water content of the premix is Y2 (g). In order to form the cross-linking agent-introduced particles, when the mass of the portion obtained by removing water from the premix is Z2 (g), the premix is of the formula:
Moisture residual ratio (mass%) = (Y2 / Y1) × 100, the water residual ratio of the premix is 85% by mass or less, and the formula:
Moisture content (mass%) = (Y2 / Z2) x 100
The method of heating until the moisture content of the premix calculated in 1 is 10% by mass or less.
The method according to claim 1, wherein the method further comprises preheating the powder, and the powder in a preheated state is mixed with the cross-linking agent solution.
The method according to claim 1 or 2, wherein in order to form the cross-linking agent-introduced particles, the premix is heated to a temperature lower than the heating temperature for cross-linking the polymer with the cross-linking agent.
The method according to any one of claims 1 to 3, wherein the premix is heated to 100 to 180 ° C. to form the crosslinker-introduced particles.
The method according to any one of claims 1 to 4, wherein the premix is heated for 2 minutes or more in order to form the cross-linking agent-introduced particles.