CN107406542B - Method for producing aqueous liquid absorbent resin - Google Patents

Abstract

The present invention provides a method for producing an aqueous liquid-absorbent resin having a large water retention capacity and little coloration and odor. The present invention is a method for producing an aqueous liquid-absorbent resin, which comprises radically polymerizing a radically polymerizable monomer (a) containing acrylic acid as a main component in the presence of an internal crosslinking agent (b) and water. step, the production method is characterized in that radical polymerization is carried out in the presence of hypophosphorous acid (salt) (c), and the upper limit of the charging concentration of the monomer (a) is less than 30% by weight based on the weight of the polymerization solution, And the maximum attained temperature of the polymerization liquid at the time of superposition|polymerization is lower than the boiling point.

Description

Manufacturing method of water-based liquid-absorbent resin

technical field

The present invention relates to a method for producing a resin excellent in the absorption performance of an aqueous liquid. Specifically, it relates to a method for producing an aqueous liquid-absorbent resin having high water retention and less odor and coloration.

Background technique

In the past, as a powdery and granular absorbent having the ability to absorb aqueous liquids, hydrophilic cross-linked polymers called water-absorbent resins have been used. Various industrial fields such as agricultural/horticultural water-retaining agents. The water-holding capacity of the aqueous liquid-absorbent resin used for these applications is expected to be high.

In general, the water absorption capacity (water retention amount) of an aqueous liquid-absorbent resin under normal pressure is conceptually proportional to "(ion osmotic pressure + affinity of polymer chain for water)/crosslinking density of polymer". That is, the performance of the aqueous liquid-absorbent resin is related to the crosslinking density. As a method for increasing the water retention, a method is generally carried out by reducing the amount of the crosslinking agent used, and a method has been further proposed: when an aqueous monomer solution containing a radical polymerizable monomer and a crosslinking agent is subjected to aqueous solution polymerization, A chain transfer agent coexists (for example, see Patent Document 1).

prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Laid-Open No. 3-179008

SUMMARY OF THE INVENTION

The problem to be solved by the invention

However, although the conventional method using a chain transfer agent has confirmed the effect of improving water retention to a certain extent, further improvement in performance is desired. In addition, in the conventional method using a chain transfer agent, there is a problem that the obtained aqueous liquid-absorbent resin particles tend to generate odor and color. The cause of the odor and coloring is not certain, but it is considered to be derived from the residue of the chain transfer agent incorporated in the polymer chain and the low-molecular component which increases with the use of the chain transfer agent. In particular, for use in sanitary products such as disposable diapers, an aqueous liquid-absorbent resin that is less likely to generate odor and coloration from the viewpoints of safety and cleanliness, in addition to further improved absorption performance, is desired.

means of solving problems

The present inventors have conducted intensive studies to solve the above-mentioned problems, and found that an aqueous liquid absorbent resin with high water retention and less odor and coloration can be obtained by carrying out polymerization under specific conditions in the presence of a specific chain transfer agent. That is, the present invention is a method for producing an aqueous liquid-absorbent resin comprising radically subjecting a radically polymerizable monomer (a) containing acrylic acid as a main component to the presence of an internal crosslinking agent (b) and water. The process of polymerization, the production method is characterized in that radical polymerization is performed in the presence of hypophosphorous acid (salt) (c), and the upper limit of the concentration of the monomer (a) is less than 30 weight based on the weight of the polymerization solution. %, and the highest temperature of the polymerization solution during polymerization is lower than its boiling point.

Invention effect

The aqueous liquid-absorbent resin obtained by the production method of the present invention has a high water retention capacity and less odor and coloration.

Detailed ways

The production method of the present invention is production of an aqueous liquid-absorbent resin including a step of radically polymerizing a radically polymerizable monomer (a) containing acrylic acid as a main component in the presence of an internal crosslinking agent (b) and water The method is to carry out free radical polymerization in the presence of hypophosphorous acid (salt) (c), based on the weight of the polymerization solution, the upper limit of the feed concentration of the above-mentioned monomer (a) is less than 30% by weight, and the polymerization solution during polymerization. The highest attainable temperature is below its boiling point.

The radically polymerizable monomer (a) (hereinafter also simply referred to as the above-mentioned monomer (a)) used in the production method of the present invention contains acrylic acid as a main component. The content of acrylic acid in the monomer (a) is preferably 70 to 100 mol %, more preferably 80 to 100 mol %, and particularly preferably 90 to 100 mol % in order to improve the absorption characteristics of the aqueous liquid-absorbent resin.

The said monomer (a) may contain monomers other than acrylic acid as needed. The monomers other than acrylic acid are not particularly limited, and specific examples thereof include acrylates such as sodium acrylate, potassium acrylate, and ammonium acrylate; methacrylic acid, maleic acid (anhydride), itaconic acid, cinnamic acid, Allyltoluenesulfonic acid, vinyltoluenesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-hydroxyethyl(meth)acryloyl phosphate, vinylsulfonic acid, styrenesulfonic acid, 2 -(meth)acrylamido-2-methylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid and 2-(meth)acryloylpropanesulfonic acid and other acid group-containing monomers and their salts ; Unsaturated monomers containing thiol groups; Unsaturated monomers containing phenolic hydroxyl groups; (meth)acrylamide, N-ethyl (meth)acrylamide, N-n-propyl (meth)acrylamide, N-isopropyl (meth)acrylamide, N,N-dimethyl (meth)acrylamide, (meth)acrylate-2-hydroxyethyl, (meth)acrylate-2-hydroxypropyl, Methoxy polyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine and N-vinylacetamide and other non-ionic unsaturated monomers containing hydrophilic groups; (meth)acrylic acid-N,N-dimethylaminoethyl ester, (meth)acrylic acid-N,N-diethylaminoethyl ester , Cationic group-forming unsaturated monomers such as (meth)acrylic acid-N,N-dimethylaminopropyl ester and N,N-dimethylaminopropyl (meth)acrylamide, and cationic groups such as their salts Unsaturated monomers, etc. These monomers may be used alone or in combination of two or more.

In the present invention, when a monomer other than acrylic acid is used, the amount is preferably 30 mol % or less, more preferably 20 mol % or less, and particularly preferably 10 mol % or less, based on the total number of moles of the above-mentioned monomer (a). mol% or less. By using monomers other than the above-mentioned acrylic acid in the above-mentioned ratio, the absorption characteristics of the finally obtained aqueous liquid-absorbent resin can be further improved, and the aqueous liquid-absorbent resin can be obtained at a lower cost.

Examples of the internal crosslinking agent (b) used in the production method of the present invention include a compound (b1) having two or more radically polymerizable double bonds; A functional group reactive with a functional group and a compound (b2) having at least one radically polymerizable double bond; a compound (b3) having two or more functional groups reactive with the functional group of the monomer (a), and the like.

Examples of compounds (b1) having two or more radically polymerizable double bonds include bis(meth)acrylamides such as N,N'-methylenebisacrylamide; (poly)alkylene diacylamides. Poly(meth)acrylates of polyols such as alcohol, trimethylolpropane, glycerol, pentaerythritol and sorbitol; divinyl compounds such as divinylbenzene; (poly)alkylene glycol, trimethylol Poly(meth)allyl compounds such as poly(meth)allyl ethers of polyols such as propane, glycerol, pentaerythritol and sorbitol, tetraallyloxyethane, and triallyl isocyanurate Wait.

Examples of the compound (b2) having at least one functional group reactive with the functional group of the monomer (a) and having at least one radically polymerizable double bond include those having a carboxylic acid (salt) group, a hydroxyl group, and Radical polymerizable compounds having functional groups such as amino groups and the like can be exemplified by radical polymerizable monomers having hydroxyl groups such as hydroxyethyl (meth)acrylate and N-methylol (meth)acrylamide, and (methyl) Radical polymerizable monomers etc. which have an epoxy group, such as glycidyl acrylate.

Examples of the compound (b3) having two or more functional groups reactive with the functional group of the monomer (a) include compounds having two or more functional groups reactive with a carboxylic acid (salt) group, a hydroxyl group, an amino group, or the like. Examples of polyfunctional compounds include glyoxal; polycarboxylic acids such as phthalic acid and adipic acid; polyols such as (poly)alkylene glycol, glycerol, and sorbitol; and (poly)alkylenes such as ethylenediamine. polyglycidyl ethers of polyalcohols such as ethylene glycol diglycidyl ether, glycerol triglycidyl ether and sorbitol polyglycidyl ether, etc.

Among these, compounds (b1) having two or more radically polymerizable double bonds are preferable, and polyvalent (meth)allyl compounds are more preferable from the viewpoint of being able to increase the water retention of the obtained aqueous liquid-absorbent resin. , polyallyl compounds are particularly preferred, and polyallyl compounds having 8 to 20 carbon atoms are most preferred. These internal crosslinking agents (b) may be used alone or in combination of two or more.

The amount of the internal crosslinking agent (b) to be used is preferably 0.001 to 5% by weight, more preferably 0.01 to 2% by weight, based on the weight of the monomer (a). When the amount of (b) is less than 0.001% by weight, the resulting resin may have a low gel strength at the time of water absorption and become a sol, resulting in poor productivity and an increase in the amount of water-soluble components. On the other hand, when it exceeds 5 weight%, on the contrary, the gel strength becomes too large, and the water retention amount may decrease.

Examples of hypophosphorous acid (salt) (c) used in the production method of the present invention include hypophosphorous acid, alkali metal salts of hypophosphorous acid (sodium hypophosphite, potassium hypophosphite, etc.), and alkaline earth metal salts of hypophosphorous acid (calcium hypophosphite). and barium hypophosphite, etc.), ammonium hypophosphite and their hydrates. These hypophosphorous acid (salt) (c) may be used alone or in combination of two or more.

The amount of hypophosphorous acid (salt) (c) to be used is preferably 0.001 to 1% by weight, more preferably 0.005 to 0.3% by weight, based on the weight of the monomer (a). When the usage-amount of (c) is less than 0.001 weight%, the effect of using (c), ie, the effect of raising the water retention of an aqueous liquid-absorbent resin, may be insufficient. On the other hand, if it exceeds 1 wt%, the polymerization rate of the monomer (a) may be insufficiently improved, or the polymerization rate may become slow, and the polymerization time or aging time may need to be prolonged, resulting in poor productivity. In addition, the obtained aqueous liquid-absorbent resin tends to generate odor and color in some cases.

In the present invention, the above-mentioned monomer (a) is radically polymerized in the presence of water. Based on the weight of the polymerization solution, that is, usually the total weight of the above-mentioned monomer (a), water, internal crosslinking agent (b) and hypophosphorous acid (salt), the polymerization concentration, that is, the amount of the above-mentioned monomer (a) in the polymerization solution The upper limit of the feed concentration is less than 30% by weight. From the viewpoint of drying and removal of water after polymerization, the range of the charging concentration of the monomer (a) is preferably 10 to 29.5% by weight, and more preferably 20 to 29% by weight.

When the polymerization concentration exceeds 30% by weight, the resulting polymer has a low molecular weight and a side reaction such as self-crosslinking occurs, so that the water holding capacity of the resulting aqueous liquid-absorbent resin decreases.

In the present invention, the maximum attained temperature of the polymerization liquid at the time of polymerization is lower than the (ie, the boiling point of the polymerization liquid), preferably 100° C. or lower. When the maximum temperature exceeds the boiling point of the polymerization liquid, the resulting polymer has a low molecular weight and side reactions such as self-crosslinking occur, resulting in a decrease in the water retention of the resulting aqueous liquid-absorbent resin, and the resulting aqueous liquid-absorbent resin. Resin produces odor and coloration.

The boiling point of the polymerization solution can be measured as the 10% distillation temperature of the non-reactive mixture obtained by removing the polymerization initiator from the polymerization solution. Specifically, it can be determined by using JIS K 0066 "Chemicals, Distillation Test Method" ” was measured by the method of a known heating distillation tester.

It should be noted that, in addition to the above-mentioned methods, when the maximum attained temperature during the polymerization exceeds the boiling point of the polymerization liquid, water vapor is violently ejected from the interface of the polymerization liquid, or foaming of the polymerization gel occurs, so it is also possible to use Judgment by sight.

In the present invention, the polymerization initiation temperature can be appropriately selected in accordance with various conditions such as the polymerization concentration, the polymerization catalyst used, and the maximum attaining temperature during polymerization, but is preferably 15°C or lower, more preferably 10°C or lower.

The polymerization method in the present invention may be any of conventionally known methods carried out in the presence of water, and examples thereof include aqueous solution polymerization, suspension polymerization, and reversed-phase suspension polymerization generally using a radical polymerization catalyst. In addition, a method of starting polymerization by irradiation with radiation, electron beams, ultraviolet rays, or the like may also be employed. Among these, an aqueous solution polymerization method is preferable because it is advantageous in terms of production cost without using an organic solvent or the like. In particular, an aqueous solution adiabatic polymerization method is preferable because an aqueous liquid-absorbent resin having a large water retention amount and a small amount of water-soluble components can be obtained, and temperature control during polymerization is not required.

As the polymerization catalyst used in the polymerization using a radical polymerization catalyst, conventionally known catalysts can be used, and examples thereof include azo compounds [azobisisobutyronitrile, azobiscyanovaleric acid, and 2,2' -Azobis(2-amidinopropane) hydrochloride, etc.], inorganic peroxides [hydrogen peroxide, ammonium persulfate, potassium persulfate and sodium persulfate, etc.], organic peroxides [benzoyl peroxide , di-tert-butyl peroxide, cumene hydroperoxide, succinic acid peroxide, and bis(2-ethoxyethyl) peroxydicarbonate, etc.] and redox catalysts [by alkali metal sulfurous acid Salts or bisulfites, ammonium sulfite, ammonium bisulfite and ascorbic acid and other reducing agents and alkali metal persulfate, ammonium persulfate, hydrogen peroxide and organic peroxides etc. Combination of oxidizing agents] etc. These catalysts may be used alone, or two or more of them may be used in combination.

The amount of the radical polymerization catalyst to be used is preferably 0.0005 to 5% by weight, more preferably 0.001 to 2% by weight, based on the weight of the monomer (a).

In the present invention, the hydrogel polymer obtained after polymerization is usually neutralized. The degree of neutralization of acid groups in the polymer is preferably 50 to 80 mol %. When the degree of neutralization is less than 50 mol %, the adhesiveness of the obtained hydrogel polymer may increase, and the workability at the time of production and use may be deteriorated. Moreover, the water retention amount of the obtained aqueous liquid-absorbent resin may fall. On the other hand, when the degree of neutralization exceeds 80%, the pH of the obtained resin may increase, and there is concern about the safety to human skin.

Neutralization can be performed at any stage after polymerization in the production of the aqueous liquid-absorbent resin, and as a preferable example, a method such as neutralization in the state of a hydrogel polymer as a polymerization product can be exemplified.

In the present invention, a compound having at least two groups capable of reacting with a carboxylic acid (salt) group (for example, a polyglycidyl compound such as ethylene glycol diglycidyl ether, ethylene glycol, Polyols such as glycols and glycerin, (poly)alkylene polyamines such as ethylenediamine, polyvalent metal compounds capable of forming ion crosslinks, etc.) make the obtained polymer neutralized product more stable in the state of a water-containing gel. Uniformly cross-linked. By this crosslinking, an aqueous liquid-absorbent resin having high gel strength and a small amount of water-soluble components can be produced.

In the present invention, the obtained hydrogel polymer is dried and pulverized into pellets as necessary.

The drying method may be a method of drying with hot air at a temperature of 80 to 230°C, a thin film drying method using a drum dryer heated to 100 to 230°C, a drying method under reduced pressure (heating), a freeze-drying method, and the like. A usual method such as a drying method using infrared rays.

The particle shape after grinding of the aqueous liquid-absorbent resin is not particularly limited, and examples thereof include an amorphous crushed shape, a scaly shape, a pearl shape, a granulated shape, and the like. From the viewpoint of good entanglement with the fibrous material and no fear of falling off the fibrous material in paper diaper applications and the like, the amorphous crushed form obtained by aqueous solution polymerization is preferred.

The pulverization method is also not particularly limited, and ordinary devices such as a hammer mill, an impact pulverizer, a roll pulverizer, and a jet pulverizer can be used. The obtained pulverized product can be screened as needed.

The average particle diameter of the pulverized aqueous liquid-absorbent resin particles is usually 100 to 600 μm, preferably 200 to 500 μm. The content of fine particles is preferably small, and the content of particles of 100 μm or less is usually 3% or less, and the content of particles of 150 μm or less is preferably 3% or less.

The water retention capacity of the aqueous liquid-absorbent resin obtained by the production method of the present invention with respect to physiological saline is preferably 50 g/g or more, and more preferably 55 g/g or more. In addition, the water retention amount was measured by the method mentioned later.

In the present invention, by surface-crosslinking the obtained aqueous liquid-absorbent resin, the gel strength can be improved, and the desired water retention and absorption under load can be satisfied in actual use.

As a method for surface-crosslinking the aqueous liquid-absorbent resin, there are known methods. For example, after the aqueous liquid-absorbent resin is made into a granular form, a mixed solution of a surface crosslinking agent (d), water and a solvent is used. A method of mixing and performing a heating reaction.

As the surface crosslinking agent (d), for example, polyglycidyl compounds such as ethylene glycol diglycidyl ether, glycerol diglycidyl ether, and polyglycerol polyglycidyl ether, polyglycidyl compounds such as glycerin and ethylene glycol, and the like can be mentioned. Alkylene carbonates such as alcohols, ethylene carbonate, polyamines, aluminum sulfate, sodium aluminum sulfate, potassium aluminum sulfate, aluminum chloride, polyaluminum chloride, aluminum lactate, zirconium acetate, ammonium zirconium carbonate, zirconium oxychloride , polyvalent metal compounds such as zirconium nitrate and zirconium sulfate. Among these, a polyglycidyl compound is preferable because the crosslinking reaction can proceed at a relatively low temperature. These surface crosslinking agents may be used alone or in combination of two or more.

The amount of the surface crosslinking agent (d) to be used is preferably 0.001 to 5% by weight, more preferably 0.005 to 2% by weight, based on the weight of the aqueous liquid-absorbent resin before crosslinking. When the amount of the surface crosslinking agent (d) used is less than 0.001% by weight, the degree of surface crosslinking is insufficient, and the effect of improving the absorption under load may be insufficient. On the other hand, when the usage-amount of (d) exceeds 5 weight%, the crosslinking degree of a surface becomes excessive, and water retention may fall.

The amount of water to be used in surface crosslinking is preferably 1 to 10% by weight, more preferably 2 to 7% by weight, based on the weight of the aqueous liquid-absorbent resin before crosslinking. When the amount of water used is less than 1% by weight, the penetration of the surface crosslinking agent (d) into the aqueous liquid-absorbent resin particles is insufficient, and the effect of improving the amount of absorption under load may be poor. On the other hand, when the amount of water used exceeds 10% by weight, the penetration of the surface crosslinking agent (d) into the interior is excessive, and an increase in the absorption under load is observed, but the water retention may decrease.

As the solvent used in combination with water at the time of surface crosslinking, a conventionally known solvent can be used, and the degree of penetration of the surface crosslinking agent (d) into the aqueous liquid-absorbent resin particles and the reaction of the surface crosslinking agent (d) can be considered. Properties, etc., are appropriately selected and used, preferably lower alcohols (methanol, propylene glycol, 1,3-propanediol, ethylene glycol, diethylene glycol, etc.) and ether alcohols (diethylene glycol, etc.) that can dissolve in water. organic solvents, more preferably lower alcohols. The solvent may be used alone or in combination of two or more.

The amount of the solvent to be used can be appropriately adjusted depending on the type of the solvent, but is preferably 1 to 10% by weight based on the weight of the aqueous liquid-absorbent resin before surface crosslinking. Moreover, although the ratio of a solvent with respect to water can be adjusted arbitrarily, it is preferable that it is 20 to 80 weight% on a weight basis, and it is more preferable that it is 30 to 70 weight%.

In order to perform surface crosslinking, a mixed solution of the surface crosslinking agent (d), water, and a solvent is mixed with the aqueous liquid-absorbent resin particles by a conventionally known method, and a heating reaction is performed. The reaction temperature is preferably 100 to 230°C, more preferably 120 to 160°C. The reaction time can be appropriately adjusted according to the reaction temperature, but is preferably 3 to 60 minutes, and more preferably 10 to 40 minutes. The particulate aqueous liquid-absorbent resin obtained by surface crosslinking may be further surface-crosslinked using a surface crosslinking agent of the same or different type as the surface crosslinking agent used initially.

The particulate aqueous liquid-absorbent resin obtained by surface-crosslinking is screened as necessary to adjust the particle size. The average particle diameter of the obtained particles is preferably 100 to 600 μm, more preferably 200 to 500 μm. The content of fine particles is preferably small, and the content of particles of 100 μm or less is preferably 3% by weight or less, and the content of particles of 150 μm or less is more preferably 3% by weight or less.

In the present invention, preservatives, antifungal agents, antibacterial agents, antioxidants, ultraviolet absorbers, antioxidants, colorants, fragrances, deodorants, and inorganic powders may be added as necessary at any stage of the production method of the present invention. and organic fibrous materials, etc., the amount thereof is usually 5% by weight or less based on the weight of the obtained aqueous liquid-absorbent resin. In addition, a treatment such as forming a foamed structure may be performed at any stage in the method of the present invention as necessary, and granulation or molding may also be performed.

Example

The present invention will be further described below through examples, but the present invention is not limited to these examples. Hereinafter, unless otherwise specified, parts represent parts by weight and % represent % by weight. In addition, the water retention capacity of the aqueous liquid-absorbent resin, the absorption amount under a load of 40 g/cm ² or 60 g/cm ² , the whiteness, and the odor were measured by the following methods.

[Measurement method of water retention amount]

A tea bag (length 20 cm, width 10 cm) made of 250-mesh nylon mesh was added with 1.000 g of an aqueous liquid-absorbent resin, immersed in physiological saline (ion-exchange aqueous solution with a NaCl concentration of 0.90%) for 60 minutes, lifted up, and hung for 15 After dehydration for 10 minutes, the tea bags were put into a centrifugal dehydrator, centrifuged at 150G for 90 seconds, the remaining water was removed, and the weight (h1) including the tea bags was measured. The weight (h2) of the tea bag after centrifugal dehydration was measured in the same manner as above except that the measurement sample was not used, and the water retention was obtained from the following formula. In addition, the temperature of the physiological saline and measurement atmosphere used was set to 25 degreeC±2 degreeC.

Water retention (g/g)=(h1)-(h2)

[Method for measuring absorption under load]

Into a plastic cylinder (25mm inner edge, 30mm height) with a 250-mesh nylon mesh attached to the bottom surface, 0.160 g of a water-based liquid absorbent resin was added to uniformly flatten it, and an outer edge of 25 mm was placed on the water-based liquid absorbent resin. The weight of 200g is smoothly moved up and down in the cylinder. The load at this time corresponds to about 40 g/cm ² .

The plastic cylinder to which the aqueous liquid-absorbent resin and the weight were added was immersed in a petri dish (diameter: 12 cm) containing 60 ml of physiological saline with the nylon mesh side as the bottom, and placed. After 60 minutes, the weight increased by the absorption of physiological saline by the aqueous liquid-absorbent resin was measured, and the value was converted to a value per 1 g of the aqueous liquid-absorbent resin to obtain the amount of absorption under a load of 40 g/cm ² . The amount of absorption under a load of 60 g/cm ² was obtained by performing the same measurement using a weight of 300 g having the same outer diameter.

[Measurement method of whiteness (WB value)]

The easiness of initial coloring (coloring immediately after production) and coloring in long-term storage or applied products of the aqueous liquid-absorbent resin was measured by using a digital colorimeter (ND-1001DP type manufactured by Nippon Denshoku Industries Co., Ltd.) Evaluation was performed by promoting the whiteness (WB value) before and after the test. The larger the value of the whiteness (WB), the more suppressed the coloring. In addition, the process of a coloring promotion test is as follows.

10 g of an aqueous liquid-absorbent resin was added to a glass plate having an inner diameter of 90 mm, and the surface was uniformly flattened to make the surface flat. It was stored in a constant temperature and humidity machine at 60±2°C and 80±2% R.H. for 14 days. Then, the plate was taken out from the constant temperature and humidity machine, and after returning to room temperature, the whiteness (WB value) after the accelerated test was measured.

[Odor test method]

The odor of the aqueous liquid-absorbent resin was evaluated by the following test method.

1 g of the aqueous liquid absorbent resin was put into a 100-ml beaker, and 20 g of a 0.9 wt % sodium chloride aqueous solution was added, and then the beaker was sealed with a membrane and left at 37° C. for 1 hour. Then, the odor sensory test was performed by 10 adult subjects, and the average score was calculated based on the following scores.

0: No unpleasant odor

1: Slightly unpleasant odor

2: Unpleasant odor

3: Unpleasant odor is particularly strong

<Example 1>

270 parts of acrylic acid, 0.88 parts of pentaerythritol triallyl ether (manufactured by Daiso) as a crosslinking agent, and 712 parts of ion-exchanged water were mixed to prepare an aqueous monomer solution, and the mixed solution was put into a polymerization tank capable of adiabatic polymerization. By introducing nitrogen gas into the solution, the dissolved oxygen amount in the solution was set to 0.2 ppm or less, and the solution temperature was set to 5°C. To this polymerization solution were added 0.14 parts of sodium hypophosphite monohydrate, 1.1 parts of a 1% aqueous hydrogen peroxide solution, 2.0 parts of a 2% aqueous solution of ascorbic acid, and 13.5 parts of a 2% aqueous solution of 2,2'-azodiamidinopropane dihydrochloride. portion and mixed (monomer concentration 27%). After confirming that the temperature indicating the initiation of polymerization was increased, approximately 2 hours later, the temperature was approximately equilibrated at 80° C., and aging was further performed for 5 hours to obtain a hydrogel-like polymer. In addition, it was 102 degreeC as a result of actual measurement of the boiling point of this polymerization liquid using a heating type distillation tester.

This hydrogel-like polymer was minced into small pieces using a meat grinder, and 220 parts of a 49% NaOH aqueous solution was added to make about 72 mol% of the carboxyl groups in the polymer as a sodium salt. The neutralized water-containing gel was air-dried until the water content reached 4% under the conditions of a supply air temperature of 150° C. and a wind speed of 1.5 m/sec using a vented hot air dryer (manufactured by Inoue Metal). The dried body was pulverized with a juicer (OSTERIZER BLENDER manufactured by Oster), and then sieved, and adjusted to a particle size range of 710 to 150 μm in mesh to obtain an aqueous liquid-absorbent resin (A1-1).

<Example 2>

In Example 1, except having changed the quantity of sodium hypophosphite monohydrate from 0.14 part to 0.20 part, the same operation as Example 1 was performed, and the aqueous liquid absorbent resin (A1-2) was obtained. The monomer concentration at the time of polymerization was 27%, and the reaching temperature at the time of equilibrium was 80°C. In addition, it was 102 degreeC as a result of actual measurement of the boiling point of this polymerization liquid using a heating type distillation tester.

<Example 3>

In Example 1, except having changed the amount of pentaerythritol triallyl ether from 0.88 part to 1.2 part, the same operation as Example 1 was performed, and the aqueous liquid absorbent resin (A1-3) was obtained. The monomer concentration at the time of polymerization was 27%, and the reaching temperature at the time of equilibrium was 80°C. In addition, it was 102 degreeC as a result of actual measurement of the boiling point of this polymerization liquid using a heating type distillation tester.

<Example 4>

In Example 1, except having changed the solution temperature at the time of polymerization start from 5 degreeC to 15 degreeC, the same operation as Example 1 was performed, and the aqueous liquid absorbent resin (A1-4) was obtained. The monomer concentration at the time of polymerization was 27%, and the reaching temperature at the time of equilibrium was 89°C. In addition, it was 102 degreeC as a result of actual measurement of the boiling point of this polymerization liquid using a heating type distillation tester.

<Example 5>

In Example 1, except having changed the quantity of ion-exchanged water from 712 parts to 643 parts, the same operation as Example 1 was performed, and the aqueous liquid absorbent resin (A1-5) was obtained. The monomer concentration at the time of polymerization was 29%, and the reaching temperature at the time of equilibrium was 90°C. In addition, when the boiling point of this polymerization liquid was measured using a heating type distillation tester, it was 103 degreeC.

<Example 6>

In Example 1, the amount of ion-exchanged water was changed from 712 parts to 643 parts, the amount of sodium hypophosphite monohydrate was changed from 0.14 parts to 0.20 parts, and the solution temperature at the start of polymerization was changed from 5°C to 15 degreeC, except having performed the same operation as Example 1, the aqueous liquid-absorbent resin (A1-6) was obtained. The monomer concentration at the time of polymerization was 29%, and the reaching temperature at the time of equilibrium was 99°C. In addition, when the boiling point of this polymerization liquid was measured using a heating type distillation tester, it was 103 degreeC.

<Comparative Example 1>

222 g of acrylic acid and 582 g of water were mixed, and 181 g of a 49% NaOH aqueous solution was slowly added so that the solution temperature did not exceed 35° C. while external cooling was performed to neutralize about 72 mol % of acrylic acid. Next, 0.88 part of pentaerythritol triallyl ether (manufactured by Daiso) was mixed as a crosslinking agent to prepare an aqueous monomer solution, and the mixed solution was put into a polymerization tank capable of adiabatic polymerization. By introducing nitrogen gas into the solution, the dissolved oxygen amount in the solution was set to 0.2 ppm or less, and the solution temperature was set to 5°C. To this polymerization solution were added 0.14 parts of sodium hypophosphite monohydrate, 1.1 parts of a 1% aqueous hydrogen peroxide solution, 2.0 parts of a 2% aqueous solution of ascorbic acid, and 13.5 parts of a 2% aqueous solution of 2,2'-azodiamidinopropane dihydrochloride. portion and mixed (monomer concentration 27%). After confirming that the temperature indicating the initiation of polymerization was increased, approximately 2 hours later, the temperature was approximately equilibrated at 80° C., and aging was further performed for 5 hours to obtain a hydrogel-like polymer. In addition, when the boiling point of this polymerization liquid was measured using a heating type distillation tester, it was 103 degreeC.

The water-containing gel-like polymer was chopped into small pieces using a meat grinder, and then air-dried using a ventilation hot air dryer (manufactured by Inoue Metal) under the conditions of a supply air temperature of 150°C and a wind speed of 1.5 m/s until the water contained rate reaches 4%. The dried body was pulverized with a juicer (OSTERIZER BLENDER, manufactured by Oster), and then sieved, and adjusted to a particle size range of 710 to 150 μm in mesh to obtain an aqueous liquid absorbent resin (R1-1) for comparison.

<Comparative Example 2>

In Comparative Example 1, except that 0.88 part of ethylene glycol diglycidyl ether was used in place of 0.88 part of pentaerythritol triallyl ether, the same operations as in Comparative Example 1 were performed to obtain an aqueous liquid-absorbent resin for comparison ( R1-2). The monomer concentration at the time of polymerization was 27%, and the reaching temperature at the time of equilibrium was 78°C. In addition, when the boiling point of this polymerization liquid was measured using a heating type distillation tester, it was 103 degreeC.

<Comparative Example 3>

In Comparative Example 1, except that 0.88 part of N,N'-methylenebisacrylamide was used instead of 0.88 part of pentaerythritol triallyl ether, the same operation as in Comparative Example 1 was carried out to obtain an aqueous liquid absorption for comparison. Sexual resin (R1-3). The monomer concentration at the time of polymerization was 27%, and the reaching temperature at the time of equilibrium was 80°C. In addition, when the boiling point of this polymerization liquid was measured using a heating type distillation tester, it was 103 degreeC.

<Comparative Example 4>

In Example 1, except that sodium hypophosphite monohydrate was not added, the same operation as in Example 1 was performed to obtain an aqueous liquid-absorbent resin (R1-4) for comparison. The monomer concentration at the time of polymerization was 27%, and the reaching temperature at the time of equilibrium was 80°C. In addition, it was 102 degreeC as a result of actual measurement of the boiling point of this polymerization liquid using a heating type distillation tester.

<Comparative Example 5>

In Example 1, except having used 0.14 part of triethylene glycol dithiol in place of 0.14 part of sodium hypophosphite monohydrate, the same operation as in Example 1 was carried out to obtain an aqueous liquid-absorbent resin (R1) for comparison. -5). The monomer concentration at the time of polymerization was 27%, and the reaching temperature at the time of equilibrium was 80°C. In addition, it was 102 degreeC as a result of actual measurement of the boiling point of this polymerization liquid using a heating type distillation tester.

<Comparative Example 6>

In Example 1, except having changed the solution temperature at the time of polymerization start from 5 degreeC to 30 degreeC, the same operation as Example 1 was performed, and the aqueous liquid absorbent resin (R1-6) for comparison was obtained. The monomer concentration during the polymerization was 27%, and during the polymerization, the temperature of the mixture reached the boiling point, causing foaming.

<Comparative Example 7>

In Example 1, the amount of ion-exchanged water was changed from 712 parts to 643 parts, the amount of sodium hypophosphite monohydrate was changed from 0.14 parts to 0.20 parts, and the solution temperature at the start of polymerization was changed from 5°C to 25 degreeC, except having performed the same operation as Example 1, the aqueous liquid-absorbent resin (R1-7) for comparison was obtained. The monomer concentration during polymerization was 29%, and during the polymerization, the temperature of the mixture reached the boiling point, causing foaming.

<Comparative Example 8>

In Example 1, except having changed the quantity of ion-exchanged water from 712 parts to 583 parts, the same operation as Example 1 was performed, and the aqueous liquid absorbent resin (R1-8) for comparison was obtained. The monomer concentration at the time of polymerization was 31%, and the reaching temperature at the time of equilibrium was 97°C. In addition, the boiling point of this polymerization liquid was measured using a heating type distillation tester, and it was 104 degreeC as a result.

<Example 7>

0.14 g of ethylene glycol diglycidyl ether, 4 g of water, and 6 g of methanol were added while stirring 100 g of the aqueous liquid absorbent resin (A1-1) (high-speed stirring paddle mixer manufactured by Hosokawamicron: 2000 rpm rotation speed). The solution was mixed, heated at 140° C. for 40 minutes, and surface-crosslinked to obtain an aqueous liquid-absorbent resin (A2-1).

<Examples 8 to 12>

Aqueous liquid-absorbent resins (A2- 2) to (A2-6).

<Example 13>

0.14 g of ethylene glycol diglycidyl ether, 3.6 g of water, and 2.8 g of propylene glycol were added while stirring 100 g of the aqueous liquid-absorbent resin (A1-1) (a high-speed stirring paddle mixer manufactured by Hosokawamicron: 2000 rpm rotation speed). The solution composed of g was mixed, and heated at 140° C. for 40 minutes to perform surface crosslinking to obtain an aqueous liquid-absorbent resin (A2-1P).

<Example 14>

Except having used the aqueous liquid absorbent resin (A1-2) instead of the aqueous liquid absorbent resin (A1-1), it carried out similarly to Example 13, and obtained the aqueous liquid absorbent resin (A2-2P).

<Comparative Examples 9 to 16>

An aqueous liquid for comparison was obtained in the same manner as in Example 7, except that the aqueous liquid-absorbent resins (R1-1) to (R1-8) for comparison were used in place of the aqueous liquid-absorbent resin (A1-1). Absorbent resins (R2-1) to (R2-8).

<Comparative Example 17>

A comparative aqueous liquid-absorbent resin (R2- 4P).

Evaluation of water retention, whiteness, and odor of the obtained aqueous liquid-absorbent resins (A1-1) to (A1-6) and comparative aqueous liquid-absorbent resins (R1-1) to (R1-8) The results are shown in Table 1.

In addition, the obtained water-based liquid-absorbent resins (A2-1) to (A2-6), (A2-1P), (A2-2P) and comparative water-based liquid-absorbent resins (R2-1) to ( Table 2 shows the evaluation results of water retention, absorption under load, whiteness, and odor of R2-8) and (R2-4P).

【Table 1】

【Table 2】

From the results in Tables 1 and 2, the aqueous liquid-absorbent resin obtained by the production method of the present invention has higher water retention and lower coloring and odor than the aqueous liquid-absorbent resin of the comparative example. In particular, as can be seen from the results in Table 2, compared with the aqueous liquid-absorbent resin of the comparative example, it has an absorption amount under load equal to or higher than that, and the water retention amount is improved, and the absorption performance is significantly improved.

Industrial Applicability

According to the production method of the present invention, an aqueous liquid-absorbent resin having a large water retention capacity and little coloration and odor can be obtained. In addition, the particulate aqueous liquid-absorbent resin obtained by surface crosslinking exhibits a high water retention capacity and also exhibits a high absorption capacity under load, so it has the characteristic of being excellent in the balance between the water retention capacity and the absorption capacity under load .

Because of the above-mentioned effects, the aqueous liquid-absorbent resin obtained by the method of the present invention can be suitably used for aqueous liquid-absorbent articles, particularly sanitary products such as disposable diapers.

Claims (8)

1. A method for producing an aqueous liquid-absorbent resin, comprising a step of radically polymerizing a radically polymerizable monomer (a) containing acrylic acid as a main component in the presence of an internal crosslinking agent (b) and water , the content of acrylic acid in the radically polymerizable monomer (a) is 90 mol % to 100 mol %, and the production method is characterized in that: Radical polymerization is carried out in the presence of hypophosphorous acid (salt) (c), based on the weight of the polymerization solution, the upper limit of the concentration of the monomer (a) is less than 30% by weight, and the maximum concentration of the polymerization solution during polymerization is The temperature is reached below the boiling point of the polymerization liquid. 2 . The production method according to claim 1 , wherein the amount of the hypophosphorous acid (salt) (c) is 0.001% by weight to 1% by weight based on the weight of the monomer (a). 3 . The manufacturing method of Claim 1 or 2 whose maximum attained temperature of the said polymerization liquid is 100 degrees C or less. 4 . The production method according to claim 1 , wherein the polymerization initiation temperature is 15° C. or lower. 5 . The manufacturing method of Claim 1 or 2 whose said internal crosslinking agent (b) is a polyvalent allyl compound. 6. The production method according to claim 1 or 2, further comprising a step of neutralizing the hydrogel polymer obtained by radical polymerization. 7. The production method according to claim 1 or 2, further comprising: a step of making the aqueous liquid-absorbent resin into a granular form; and surface-forming the aqueous liquid-absorbent resin particles with a surface crosslinking agent (d). cross-linking process. The production method according to claim 1 or 2, wherein the water retention capacity of the obtained aqueous liquid-absorbent resin with respect to physiological saline is 50 g/g or more.