JP6555832B2 - Method for producing aqueous liquid absorbent resin - Google Patents

Description

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

  Conventionally, hydrophilic cross-linked polymers called water-absorbing resins have been used as powdered and granular absorbents that have the ability to absorb aqueous liquids. Hygiene products such as paper diapers and sanitary products, anti-condensation agents, and water retention for agriculture and horticulture. The range of application for various industrial fields such as chemicals is expanding. The aqueous liquid absorbent resin used for these applications is required to have a high water retention amount.

  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 chains to water) / polymer crosslinking density”. That is, the crosslink density is involved in the performance of the aqueous liquid absorbent resin. As a method for increasing the water retention amount, reducing the amount of the crosslinking agent used is a commonly practiced means. However, in the aqueous solution polymerization of an aqueous monomer solution containing a radical polymerizable monomer and a crosslinking agent, a chain transfer agent is used. There has been proposed a method of coexisting (see, for example, Patent Document 1).

JP-A-3-179008

  However, the conventional method using a chain transfer agent has a certain effect on the improvement of the water retention amount, but further improvement in performance is desired. In addition, the conventional method using a chain transfer agent has a problem that the resulting aqueous liquid absorbent resin particles are likely to generate odor and coloring. Although the cause of this odor and coloring is not clear, it is thought that it originates from the residue of the chain transfer agent incorporated in the polymer chain and the low molecular component that increases with the use of the chain transfer agent. Particularly in sanitary products such as paper diapers, in addition to improved absorption performance, the appearance of an aqueous liquid absorbent resin that is less likely to cause odor and coloring from the viewpoint of safety and cleanliness has been desired.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have carried out polymerization under specific conditions in the presence of a specific chain transfer agent, so that the amount of water retention is high, and further, an aqueous liquid with less odor and coloring. It has been found that an absorbent resin can be obtained. That is, the present invention is a method for producing an aqueous liquid absorbent resin comprising a step of radical polymerization of a radically polymerizable monomer (a) containing acrylic acid as a main component in the presence of an internal crosslinking agent (b) and water. , Radical polymerization is carried out in the presence of hypophosphorous acid (salt) (c), the upper limit of the concentration of the monomer (a) is less than 30% by weight based on the weight of the polymerization solution, and polymerization during polymerization A method for producing an aqueous liquid absorbent resin, characterized in that the highest temperature of the liquid is less than its boiling point.

  The aqueous liquid absorbent resin obtained by the production method of the present invention has a high water retention amount, and further has less odor and coloring.

  The production method of the present invention is a method for producing an aqueous liquid absorbent resin comprising a step of radical polymerization of a radically polymerizable monomer (a) mainly composed of acrylic acid in the presence of an internal crosslinking agent (b) and water. Radical polymerization is carried out in the presence of hypophosphorous acid (salt) (c), the upper limit of the concentration of the monomer (a) is less than 30% by weight based on the weight of the polymerization solution, and The maximum temperature of the polymerization liquid is less than its boiling point.

  The radical polymerizable monomer (a) (hereinafter also simply referred to as the 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%, particularly preferably in order to improve the absorption characteristics of the aqueous liquid absorbent resin. Is 90 to 100 mol%.

  The monomer (a) may contain a monomer other than acrylic acid as necessary. Although it does not specifically limit as monomers other than acrylic acid, Specifically, for example, acrylic acid salts, such as sodium acrylate, potassium acrylate, and ammonium acrylate salt; Methacrylic acid, (anhydrous) maleic acid , Itaconic acid, cinnamic acid, allyl toluene sulfonic acid, vinyl toluene sulfonic acid, 2- (meth) acryloylethane sulfonic acid, 2-hydroxyethyl (meth) acryloyl phosphate, vinyl sulfonic acid, styrene sulfonic acid, 2- ( Acid group-containing monomers such as (meth) acrylamido-2-methylpropanesulfonic acid, 2- (meth) acryloylethanesulfonic acid and 2- (meth) acryloylpropanesulfonic acid, and salts thereof; mercaptan group-containing unsaturated monomers; phenolic hydroxyl groups Containing unsaturated monomers; ) Acrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, 2- Nonionic properties such as hydroxypropyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloylpyrrolidine and N-vinylacetamide Hydrophilic group-containing unsaturated monomer; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate DOO and N, N- dimethylaminopropyl (meth) cationic unsaturated monomers such as cationic group forming unsaturated monomers and salts of these acrylamide. These monomers may be used independently and may use 2 or more types together.

  In the present invention, when a monomer other than acrylic acid is used, the amount thereof is preferably 30 mol% or less, more preferably 20 mol% or less, particularly preferably 10 mol, based on the total number of moles of the monomer (a). % Or less. By using a monomer other than the acrylic acid in the above proportion, 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.

  As the internal cross-linking agent (b) used in the production method of the present invention, at least one functional group capable of reacting with the functional group of the monomer (a), the compound (b1) having two or more radically polymerizable double bonds, is used. And a compound (b3) having at least one radical polymerizable double bond and a compound (b3) having two or more functional groups capable of reacting with the functional group of the monomer (a).

  Examples of the compound (b1) having two or more radically polymerizable double bonds include bis (meth) acrylamides such as N, N′-methylenebisacrylamide; (poly) alkylene glycol, trimethylolpropane, glycerin, Poly (meth) acrylates of polyhydric alcohols such as pentaerythritol and sorbitol; divinyl compounds such as divinylbenzene; poly (meth) allyl of polyhydric alcohols such as (poly) alkylene glycol, trimethylolpropane, glycerin, pentaerythritol and sorbitol Examples thereof include polyvalent (meth) allyl compounds such as ether, tetraallyloxyethane and triallyl isocyanurate.

  The compound (b2) having at least one functional group capable of reacting with the functional group of the monomer (a) and having at least one radical polymerizable double bond includes a carboxylic acid (salt) group, a hydroxyl group. Or a radical polymerizable compound having a functional group capable of reacting with an amino group or the like. For example, a radical polymerizable monomer having a hydroxyl group such as hydroxyethyl (meth) acrylate and N-methylol (meth) acrylamide and glycidyl (meta ) Radical polymerizable monomers having an epoxy group such as acrylate.

  The compound (b3) having two or more functional groups capable of reacting with the functional group of the monomer (a) has two or more functional groups capable of reacting with a carboxylic acid (salt) group, a hydroxyl group or an amino group. Polyfunctional compounds such as glyoxal; polycarboxylic acids such as phthalic acid and adipic acid; polyhydric alcohols such as (poly) alkylene glycol, glycerin and sorbitol; (poly) alkylene polyamines such as ethylenediamine; ethylene glycol diglycidyl ether And polyglycidyl ethers of polyhydric alcohols such as glycerin triglycidyl ether and sorbitol polyglycidyl ether.

  Among these, the compound (b1) having two or more radically polymerizable double bonds is preferable, and more preferable is a polyvalent (from the viewpoint of increasing the water retention amount of the obtained aqueous liquid absorbent resin. A meta) allyl compound, particularly preferred is a polyvalent allyl compound, and most preferred is a polyvalent allyl compound having 8 to 20 carbon atoms. These internal crosslinking agents (b) may be used alone or in combination of two or more.

  The amount of the internal crosslinking agent (b) 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 obtained resin has a small gel strength upon water absorption and becomes a sol, which may deteriorate the productivity and increase the amount of water-soluble components. On the other hand, if it exceeds 5% by weight, the gel strength is excessively increased, and the water retention amount may decrease.

  Hypophosphorous acid (salt) (c) used in the production method of the present invention includes hypophosphorous acid, alkali metal salts of hypophosphorous acid (such as sodium hypophosphite and potassium hypophosphite), hypochlorous acid, and the like. Examples include alkaline earth metal salts of phosphoric acid (such as calcium hypophosphite and barium hypophosphite), ammonium hypophosphite and hydrates thereof. These hypophosphorous acid (salt) (c) may be used independently and may use 2 or more types together.

  The amount of hypophosphorous acid (salt) (c) 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 amount of (c) used is less than 0.001% by weight, the effect of using (c), that is, the effect of improving the water retention amount of the aqueous liquid absorbent resin may be insufficient. On the other hand, if it exceeds 1% by weight, the polymerization rate of the monomer (a) may not be sufficiently increased, or the polymerization rate may be slow, and it may be necessary to increase the polymerization time or the aging time. , Productivity may be worse. Moreover, an odor and coloring may become easy to generate | occur | produce in the aqueous liquid absorptive resin obtained.

In the present invention, the monomer (a) is radically polymerized in the presence of water. The upper limit of the polymerization concentration, that is, the charged concentration of the monomer (a) in the polymerization solution is the weight of the polymerization solution, that is, usually the monomer (a), water, the internal crosslinking agent (b) and hypophosphorous acid ( Based on the total weight of the salt). The charged concentration range of the monomer (a) is preferably 10 to 29.5% by weight, more preferably 20 to 29% by weight, from the viewpoint of dry removal of water after polymerization.
If the polymerization concentration exceeds 30% by weight, the molecular weight of the resulting polymer will be low, and side reactions such as self-crosslinking will occur, resulting in a decrease in the water retention amount of the resulting aqueous liquid absorbent resin.

In the present invention, the maximum temperature reached by the polymerization solution during polymerization is less than its boiling point (that is, the polymerization solution), and is preferably 100 ° C. or less. When the maximum temperature reaches the boiling point of the polymerization solution, the molecular weight of the resulting polymer becomes low and side reactions such as self-crosslinking occur, resulting in a decrease in the water retention amount of the resulting aqueous liquid absorbent resin, Moreover, odor and coloring generate | occur | produce in the aqueous liquid absorptive resin obtained.
The boiling point of the polymerization liquid can be measured as a 10% distillation temperature of a non-reactive mixture obtained by removing the polymerization initiator from the polymerization liquid. Specifically, the boiling point of the polymerization liquid is a known heating distillation determined by JISK0066 “Method for Distillation Test of Chemical Products” It can be measured by a method using a tester.
In addition to the above method, when the maximum temperature reached during polymerization exceeds the boiling point of the polymerization solution, water vapor violently blows out from the interface of the polymerization solution or foaming of the polymerization gel occurs, so that the determination can be made visually.

  In the present invention, the polymerization start temperature is appropriately selected according to various conditions such as the polymerization concentration, the polymerization catalyst to be used, the maximum temperature reached during the polymerization, etc., but is preferably 15 ° C. or less, more preferably 10 ° C. or less. is there.

  The polymerization method in the present invention may be any conventionally known method carried out in the presence of water, for example, an aqueous solution polymerization method, a suspension polymerization method and a reverse phase suspension weight, which usually use a radical polymerization catalyst. Legal etc. are mentioned. Moreover, the method of starting superposition | polymerization by irradiating a radiation, an electron beam, an ultraviolet-ray, etc. can also be taken. Among these, the aqueous solution polymerization method is preferable because it is not necessary to use an organic solvent or the like and is advantageous in terms of production cost. In particular, an aqueous solution absorptive polymerization method is preferable because an aqueous liquid absorbent resin having a large amount of water retention and a small amount of water-soluble components can be obtained, and temperature control during polymerization is unnecessary.

  As a polymerization catalyst used for polymerization using a radical polymerization catalyst, a conventionally known catalyst can be used. For example, azo compounds [azobisisobutyronitrile, azobiscyanovaleric acid and 2,2′-azobis ( 2-amidinopropane) hydrochloride, etc.], inorganic peroxides [hydrogen peroxide, ammonium persulfate, potassium persulfate, sodium persulfate, etc.], organic peroxides [benzoyl peroxide, di-t-butyl peroxide, cumene] Hydroperoxide, succinic acid peroxide and di (2-ethoxyethyl) peroxydicarbonate etc.] and redox catalyst [alkali metal sulfite or bisulfite, ammonium sulfite, ammonium bisulfite and ascorbic acid and the like, and , Alkali metal persulfate, ammonium persulfate, peracid Those consisting of a combination of an oxidizing agent such as hydrogen and organic peroxide], and the like. These catalysts may be used alone or in combination of two or more thereof.

  The amount of the radical polymerization catalyst 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 neutralization degree of the acid group in the polymer is preferably 50 to 80 mol%. When the degree of neutralization is less than 50 mol%, the resulting water-containing gel polymer has high tackiness, and the workability during production and use may deteriorate. Further, the water retention amount of the obtained aqueous liquid absorbent resin may be reduced. On the other hand, when the degree of neutralization exceeds 80%, the pH of the obtained resin becomes high, and there is a concern that the safety of human skin may be concerned.
Neutralization may be performed at any stage after polymerization in the production of an aqueous liquid absorbent resin. For example, a method such as neutralization in the state of a hydrogel polymer that is a polymerization product is exemplified as a preferred example. The

  In the present invention, if necessary, a compound (for example, polyglycidyl such as ethylene glycol diglycidyl ether) having at least two groups capable of reacting with the carboxylic acid (salt) group in the water-containing gel state of the obtained polymer neutralized product. Compounds, polyhydric alcohols such as ethylene glycol, diethylene glycol and glycerin, (poly) alkylene polyamines such as ethylenediamine, and polyvalent metal compounds capable of forming ionic crosslinks). By this crosslinking, an aqueous liquid absorbent resin having high gel strength and a small amount of water-soluble component can be produced.

In the present invention, the obtained hydrogel polymer is dried and pulverized into particles as necessary.
The drying method is a method of drying with hot air at a temperature of 80 to 230 ° C., a thin film drying method using a drum dryer or the like heated to 100 to 230 ° C., a (heating) vacuum drying method, a freeze drying method, and an infrared ray. Ordinary methods such as a drying method may be used.

The particle shape of the aqueous liquid absorbent resin to be pulverized is not particularly limited, and examples thereof include an irregular crushed shape, a flake shape, a pearl shape, and a granulated shape. The amorphous crushed form obtained by aqueous solution polymerization is preferable in that it is easily entangled with the fibrous material for use as a paper diaper and there is no fear of dropping off from the fibrous material.
There is no particular limitation on the pulverization method, and ordinary devices such as a hammer pulverizer, an impact pulverizer, a roll pulverizer, and a shet airflow pulverizer can be used. The obtained pulverized product is sieved if necessary.
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. Usually, the content of particles of 100 μm or less is 3% or less, and preferably the content of particles of 150 μm or less is 3% or less.

  The water retention amount 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, more preferably 55 g / g or more. The water retention amount is measured by the method described later.

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

  As a method for surface cross-linking the aqueous liquid absorbent resin, a conventionally known method, for example, after making the aqueous liquid absorbent resin particles, a mixed solution of a surface cross-linking agent (d), water and a solvent is mixed, The method of heating reaction is mentioned.

  Examples of the surface cross-linking agent (d) include polyglycidyl compounds such as ethylene glycol diglycidyl ether, glycerol diglycidyl ether and polyglycerol polyglycidyl ether, polyhydric alcohols such as glycerin and ethylene glycol, alkylene carbonates such as ethylene carbonate, Examples include polyamines and polyvalent metal compounds such as aluminum sulfate, sodium alum, potassium alum, aluminum chloride, polyaluminum chloride, aluminum lactate, zirconium acetate, ammonium zirconium carbonate, zirconium oxychloride, zirconium nitrate and zirconium sulfate. Among these, a polyglycidyl compound is preferable in that a crosslinking reaction can be performed 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) 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 cross-linking agent (d) used is less than 0.001% by weight, the degree of surface cross-linking is insufficient, and the effect of improving the amount of absorption under load may be insufficient. On the other hand, when the amount of use of (d) exceeds 5% by weight, the degree of cross-linking on the surface becomes excessive, and the water retention amount may decrease.

  The amount of water used during 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 cross-linking agent (d) into the aqueous liquid absorbent resin particles may be insufficient, and the effect of improving the amount of absorption under load may be poor. On the other hand, if the amount of water used exceeds 10% by weight, penetration of the surface cross-linking agent (d) into the inside becomes excessive, and although an improvement in the amount of absorption under load is observed, the amount of water retained may decrease.

As the solvent used in combination with water at the time of surface crosslinking, a conventionally known solvent can be used. The degree of penetration of the surface crosslinking agent (d) into the aqueous liquid absorbent resin particles, the surface crosslinking agent (d) In consideration of reactivity, etc., it can be appropriately selected and used. Preferably, lower alcohols (methanol, propylene glycol, 1,3-propanediol, ethylene glycol, diethylene glycol, etc.), ether alcohols (diethylene glycol, etc.), etc. A hydrophilic organic solvent which can be dissolved in water, more preferably a lower alcohol. A solvent may be used independently and may use 2 or more types together.
Although the usage-amount of a solvent can be suitably adjusted with the kind of solvent, Preferably it is 1-10 weight% based on the weight of the aqueous liquid absorptive resin before surface crosslinking. Moreover, although the ratio of the solvent with respect to water can also be adjusted arbitrarily, Preferably it is 20 to 80 weight% on a weight basis, More preferably, it is 30 to 70 weight%.

  In order to perform the 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. Although reaction time can be suitably adjusted with reaction temperature, Preferably it is 3 to 60 minutes, More preferably, it is 10 to 40 minutes. The particulate aqueous liquid absorbent resin obtained by surface cross-linking can be further subjected to surface cross-linking using the same or different type of surface cross-linking agent as the first used surface cross-linking agent.

  The particulate aqueous liquid absorbent resin obtained by surface crosslinking is sieved as necessary to adjust the particle size. The average particle size of the obtained particles is preferably 100 to 600 μm, more preferably 200 to 500 μm. The content of fine particles is preferably small, 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, if necessary, at any stage of the production method of the present invention, an antiseptic, a fungicide, an antibacterial agent, an antioxidant, an ultraviolet absorber, an antioxidant, a colorant, an aromatic, a deodorant, Inorganic powder, organic fibrous material, and the like can be added, and the amount thereof is usually 5% by weight or less based on the weight of the obtained aqueous liquid absorbent resin. Further, if necessary, a treatment for forming a foamed structure may be performed at any stage in the method of the present invention, and granulation and molding may be performed.

EXAMPLES Hereinafter, although an Example demonstrates this invention further, this invention is not limited to these. Unless otherwise specified, “parts” means “parts by weight” and “%” means “% by weight”. The water retention amount of the aqueous liquid absorbent resin, the absorption amount under a load of 40 g / cm ² or 60 g / cm ² , whiteness, and odor are measured by the following methods.

[Measurement method of water retention amount]
A tea bag (20 cm long, 10 cm wide) made of 250 mesh nylon net was charged with 1.000 g of an aqueous liquid absorbent resin and immersed in physiological saline (ion exchange aqueous solution with a NaCl concentration of 0.90%) for 60 minutes. Then, pull up, hang for 15 minutes, drain the water, put the tea bag together in a centrifugal dehydrator, perform centrifugal dehydration at 150G for 90 seconds, remove excess water, and measure the weight (h1) including the tea bag did. 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 amount was determined from the following equation. In addition, the temperature of the used physiological saline and measurement atmosphere was 25 degreeC +/- 2 degreeC.
Water retention amount (g / g) = (h1)-(h2)

[Measurement method of absorption under load]
Place 0.160 g of aqueous liquid absorbent resin in a plastic cylinder (25 mm inner edge, 30 mm height) with a 250 mesh nylon net on the bottom, and evenly level the outer liquid 25 mm on this aqueous liquid absorbent resin. A 200 g weight that smoothly moves up and down in the cylinder was placed. The load at this time corresponds to about 40 g / cm ² .
A plastic cylinder containing an aqueous liquid absorbent resin and a weight was immersed in a petri dish (diameter: 12 cm) containing 60 ml of physiological saline with the nylon mesh side facing down and left standing. The weight increased by the aqueous liquid absorbent resin after absorbing physiological saline was measured after 60 minutes, and the value was converted to a value per gram of the aqueous liquid absorbent resin and the amount absorbed under a load of 40 g / cm ^2. did. The amount of absorption under a load of 60 g / cm ² is determined by performing the same measurement using a 300 g weight having the same outer diameter.

[Measurement method of whiteness (WB value)]
For the initial coloration of the aqueous liquid absorbent resin (coloring immediately after production) and the ease of colorization in long-term storage or applied products, use a digital colorimetric color difference meter (ND-1001DP type manufactured by Nippon Denshoku Industries Co., Ltd.). It was evaluated by measuring the whiteness (WB value) before and after the acceleration test. The whiteness (WB) indicates that the larger the value, the more the coloring is suppressed. In addition, the procedure of a coloring promotion test is as follows.
A glass petri dish having an inner diameter of 90 mm was charged with 10 g of an aqueous liquid absorbent resin, and uniformly leveled so that the surface became flat. This was 60 ± 2 ° C., 80 ± 2% R.I. H. For 14 days. Thereafter, the petri dish was taken out from the thermo-hygrostat and returned to room temperature, and then the whiteness (WB value) after the acceleration 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 in a 100 ml beaker, 20 g of 0.9 wt% sodium chloride aqueous solution was added, the beaker was sealed with a film, and left at 37 ° C. for 1 hour. Thereafter, an odor sensory test was conducted by 10 adult subjects, and an average score was calculated based on the following scores.
0: no unpleasant odor 1: slightly unpleasant odor 2: unpleasant odor 3: particularly unpleasant odor

<Example 1>
A monomer aqueous solution is prepared by mixing 270 parts of acrylic acid, 0.88 part of pentaerythritol triallyl ether (manufactured by Daiso) as a crosslinking agent and 712 parts of ion-exchanged water, and the mixture is put into a polymerization tank capable of adiabatic polymerization. did. By introducing nitrogen gas into the solution, the amount of dissolved oxygen in the solution was 0.2 ppm or less, and the solution temperature was 5 ° C. To this polymerization solution, 0.14 parts of sodium hypophosphite monohydrate, 1.1 parts of 1% aqueous hydrogen peroxide solution, 2.0 parts of 2% aqueous ascorbic acid solution and 2% 2,2′-azobisbis 13.5 parts of an amidinopropane dihydrochloride aqueous solution was added and mixed (monomer concentration 27%). About 2 hours after the rise in temperature indicating the start of polymerization was confirmed, an equilibrium was reached at 80 ° C., and aging was further performed for 5 hours to obtain a hydrogel polymer. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 102 degreeC.
While this hydrogel polymer was crushed into small pieces using a meat chopper, 220 parts of a 49% NaOH aqueous solution was added, and about 72 mol% of the carboxyl groups in the polymer were converted to sodium salts. This neutralized water-containing gel was air-dried using a ventilated 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 / sec until the water content became 4%. The dried product was pulverized with a juicer mixer (OSTERIZER BLENDER manufactured by Oster), and then sieved to adjust the particle size to a particle size range of 710 to 150 μm to obtain an aqueous liquid absorbent resin (A1-1).

<Example 2>
In Example 1, except that the amount of sodium hypophosphite monohydrate was changed from 0.14 part to 0.20 part, the same operation as in Example 1 was performed, and the aqueous liquid absorbent resin (A1- 2) was obtained. The monomer concentration during polymerization was 27%, and the temperature reached at equilibrium was 80 ° C. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 102 degreeC.

<Example 3>
In Example 1, except that the amount of pentaerythritol triallyl ether was changed from 0.88 parts to 1.2 parts, the same operation as in Example 1 was performed to obtain an aqueous liquid absorbent resin (A1-3). It was. The monomer concentration during polymerization was 27%, and the temperature reached at equilibrium was 80 ° C. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 102 degreeC.

<Example 4>
In Example 1, except changing the solution temperature at the time of superposition | polymerization from 5 degreeC to 15 degreeC, operation similar to Example 1 was performed and aqueous liquid absorptive resin (A1-4) was obtained. The monomer concentration during polymerization was 27%, and the temperature reached at equilibrium was 89 ° C. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 102 degreeC.

<Example 5>
In Example 1, except having changed the quantity of ion-exchange water from 712 parts to 643 parts, operation similar to Example 1 was performed and the aqueous liquid absorptive resin (A1-5) was obtained. The monomer concentration during polymerization was 29%, and the temperature reached at equilibrium was 90 ° C. In addition, when the boiling point of this polymerization liquid was measured using the 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 5 Except changing from 15 degreeC to 15 degreeC, operation similar to Example 1 was performed and aqueous liquid absorptive resin (A1-6) was obtained. The monomer concentration during polymerization was 29%, and the temperature reached at equilibrium was 99 ° C. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 103 degreeC.

<Comparative Example 1>
A mixture of 222 g of acrylic acid and 582 g of water was added and 181 g of a 49% NaOH aqueous solution was gradually added to neutralize about 72 mol% of acrylic acid while keeping the solution temperature not exceeding 35 ° C. while performing external cooling. . Next, 0.88 part of pentaerythritol triallyl ether (manufactured by Daiso) was mixed as a crosslinking agent to prepare an aqueous monomer solution, and this mixed solution was put into a polymerization tank capable of adiabatic polymerization. By introducing nitrogen gas into the solution, the amount of dissolved oxygen in the solution was 0.2 ppm or less, and the solution temperature was 5 ° C. To this polymerization solution, 0.14 parts of sodium hypophosphite monohydrate, 1.1 parts of 1% aqueous hydrogen peroxide solution, 2.0 parts of 2% aqueous ascorbic acid solution and 2% 2,2′-azobisbis 13.5 parts of an amidinopropane dihydrochloride aqueous solution was added and mixed (monomer concentration 27%). About 2 hours after the rise in temperature indicating the start of polymerization was confirmed, an equilibrium was reached at 80 ° C., and aging was further performed for 5 hours to obtain a hydrogel polymer. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 103 degreeC.
The water-containing gel-like polymer was crushed into small pieces using a meat chopper, and using a ventilated hot air dryer (manufactured by Inoue Metal), water content under conditions of a supply air temperature of 150 ° C. and a wind speed of 1.5 m / sec. Was dried by aeration until it reached 4%. The dried product was pulverized with a juicer mixer (Osterizer BLENDER manufactured by Oster Co., Ltd.), sieved, and adjusted to a particle diameter range of 710 to 150 μm, and a comparative aqueous liquid absorbent resin (R1-1) was prepared. Obtained.

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

<Comparative Example 3>
In Comparative Example 1, the same operation as in Comparative Example 1 was carried out except that 0.88 part of N, N′-methylenebisacrylamide was used instead of 0.88 part of pentaerythritol triallyl ether, and an aqueous liquid for comparison was used. Absorbent resin (R1-3) was obtained. The monomer concentration during polymerization was 27%, and the temperature reached at equilibrium was 80 ° C. In addition, when the boiling point of this polymerization liquid was measured using the 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 a comparative aqueous liquid absorbent resin (R1-4). The monomer concentration during polymerization was 27%, and the temperature reached at equilibrium was 80 ° C. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 102 degreeC.

<Comparative Example 5>
In Example 1, the same operation as in Example 1 was carried out except that 0.14 part of triethylene glycol dimercaptan was used instead of 0.14 part of sodium hypophosphite monohydrate. A liquid absorbent resin (R1-5) was obtained. The monomer concentration during polymerization was 27%, and the temperature reached at equilibrium was 80 ° C. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 102 degreeC.

<Comparative Example 6>
In Example 1, except that the solution temperature at the start of polymerization was changed from 5 ° C. to 30 ° C., the same operation as in Example 1 was performed to obtain a comparative aqueous liquid absorbent resin (R1-6). The monomer concentration during the polymerization was 27%. During the polymerization, the temperature of the mixture reached the boiling point and foaming occurred.

<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 5 Except for changing from ° C to 25 ° C, the same operation as in Example 1 was performed to obtain a comparative aqueous liquid absorbent resin (R1-7). The monomer concentration during the polymerization was 29%. During the polymerization, the temperature of the mixture reached the boiling point and foaming occurred.

<Comparative Example 8>
In Example 1, except having changed the quantity of ion-exchange water from 712 parts to 583 parts, operation similar to Example 1 was performed and the comparative aqueous liquid absorbent resin (R1-8) was obtained. The monomer concentration at the time of polymerization was 31%, and the temperature reached at the time of equilibrium was 97 ° C. In addition, when the boiling point of this polymerization liquid was measured using the heating type distillation tester, it was 104 degreeC.

<Example 7>
While stirring 100 g of aqueous liquid absorbent resin (A1-1) (high-speed stirring turbulizer manufactured by Hosokawa Micron: rotation speed 2000 rpm), a solution consisting of 0.14 g of ethylene glycol diglycidyl ether, 4 g of water and 6 g of methanol was added. The mixture was mixed and heated at 140 ° C. for 40 minutes for surface crosslinking to obtain an aqueous liquid absorbent resin (A2-1).

<Examples 8 to 12>
In place of the aqueous liquid absorbent resin (A1-1), the aqueous liquid absorbent resin (A2) is used in the same manner as in Example 7 except that the aqueous liquid absorbent resins (A1-2) to (A1-6) are used. -2) to (A2-6) were obtained.

<Example 13>
While stirring 100 g of the aqueous liquid absorbent resin (A1-1) (high speed stirring turbulizer manufactured by Hosokawa Micron: rotation speed 2000 rpm), 0.14 g of ethylene glycol diglycidyl ether, 3.6 g of water and 2.8 g of propylene glycol are used. The solution was added and mixed, and heated at 140 ° C. for 40 minutes for surface crosslinking to obtain an aqueous liquid absorbent resin (A2-1P).

<Example 14>
An aqueous liquid absorbent resin (A2-2P) was obtained in the same manner as in Example 13 except that the aqueous liquid absorbent resin (A1-2) was used instead of the aqueous liquid absorbent resin (A1-1). .

<Comparative Examples 9-16>
Comparative aqueous liquid in the same manner as in Example 7 except that comparative aqueous liquid absorbent resins (R1-1) to (R1-8) are used instead of the aqueous liquid absorbent resin (A1-1). Absorbent resins (R2-1) to (R2-8) were obtained.

<Comparative Example 17>
A comparative aqueous liquid absorbent resin (R2-) was used in the same manner as in Example 13 except that a comparative aqueous liquid absorbent resin (R1-4) was used instead of the aqueous liquid absorbent resin (A1-1). 4P) was obtained.

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

  From the results of Table 1 and Table 2, the aqueous liquid absorbent resin obtained by the production method of the present invention has a higher water retention amount and less coloring and odor than the aqueous liquid absorbent resin of the comparative example. I understand that. In particular, from the results in Table 2, the water retention amount is improved while having an absorption amount under a load equal to or greater than that of the aqueous liquid absorbent resin of the comparative example, and the absorption performance is dramatically improved. I understand.

By the production method of the present invention, it is possible to obtain an aqueous liquid absorbent resin that has a large amount of water retention and little coloration or odor. In addition, the particulate aqueous liquid absorbent resin obtained by surface cross-linking exhibits a high water retention amount and also exhibits a high absorption amount under load, so that it has an excellent balance between the water retention amount and the absorption amount under load. There is.
Because of the above 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 goods such as paper diapers.

Claims (8)

A method for producing an aqueous liquid absorbent resin comprising a step of radical polymerization of a radically polymerizable monomer (a) having acrylic acid as a main component in the presence of an internal crosslinking agent (b) and water, the monomer (a ) Contains 90 to 100 mol% of acrylic acid, performs radical polymerization in the presence of hypophosphorous acid (salt) (c), and the upper limit of the concentration of the monomer (a) is based on the weight of the polymerization solution. A method for producing an aqueous liquid absorbent resin, characterized in that it is less than 30% by weight and the maximum temperature of the polymerization solution during polymerization is less than its boiling point.
  The production method according to claim 1, wherein the amount of the hypophosphorous acid (salt) (c) is 0.001 to 1% by weight based on the weight of the monomer (a).   The production method according to claim 1 or 2, wherein the maximum temperature of the polymerization solution is 100 ° C or lower.   The production method according to any one of claims 1 to 3, wherein the polymerization start temperature is 15 ° C or lower.   The production method according to claim 1, wherein the internal crosslinking agent (b) is a polyvalent allyl compound.   The production method according to any one of claims 1 to 5, further comprising a step of neutralizing the hydrogel polymer obtained by radical polymerization.   The production method according to any one of claims 1 to 6, further comprising a step of making the aqueous liquid absorbent resin particles, and a step of surface cross-linking the aqueous liquid absorbent resin particles with a surface cross-linking agent (d).   The manufacturing method according to any one of claims 1 to 7, wherein a water retention amount of the obtained aqueous liquid absorbent resin with respect to physiological saline is 50 g / g or more.