JP2006219661A - Method for producing water absorbing resin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a water absorbing resin, capable of producing the resin having low contents of a residual monomer and water-soluble matter, without being affected by a polymerization procedure. <P>SOLUTION: This method for producing the water absorbing resin includes a process using an acrylic monomer (A) which contains an acrylic acid salt in an amount of at least 50 mol% based on a number of moles of the acrylic monomer (A) and has a content of a dimer acid (salt) of not more than 0.5 wt% based on a weight of the monomer (A) and polymerizing the monomer (A) in such a state that a content of iron atom in a polymerization liquid is not more than 5×10<SP>-6</SP>wt% based on a weight of the monomer (A). The water absorbing resin produced by the method is useful for sanitary goods, such as a paper diaper, a sanitary napkin, and a medical blood holding agent. Further, the resin is used for a pet urine absorbing agent, a urine gelling agent for a portable toilet, a freshness keeping agent for a vegetable, a fruit, etc., a drip absorbing agent for meet and fishery products, a cold reserving agent, a disposable pocket warmer, a gelling agent for a battery, a water holding agent for a plant, soil, etc., a dew condensation preventing agent, a water stopping agent, a packing agent, artificial snow, etc. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

As a method for producing a water-absorbing resin having a low content of residual monomers and water-soluble components, a polymer obtained by polymerizing acrylic acid having a dimer acid content of 0.5% by weight or less in acrylic acid is used. A manufacturing method including a step of summing is known (Patent Document 1).
Japanese Patent No. 2995276

However, in the conventional production method, even if the content of the dimer acid of acrylic acid is 0.5% by weight or less, when polymerization is performed using an acrylic monomer containing an acrylate, the residual monomer or soluble content is contained. It is difficult to produce a water-absorbing resin with a small amount. That is, a polymerization method using an acrylic monomer containing at least 50 mol% of acrylate instead of 100 mol% of acrylic acid {reverse phase suspension polymerization method or an acrylic monomer containing at least 50 mol% of acrylate There is a problem in the production using the aqueous solution polymerization method} using.
The objective of this invention is providing the manufacturing method which can manufacture a water-absorbing resin with little content of a residual monomer and a water soluble matter, without being influenced by the polymerization method.

The inventor has intensively studied to achieve the above object and has reached the present invention. That is, the production method of the water-absorbent resin of the present invention is characterized in that it contains at least 50 mol% of acrylate based on the number of moles of the acrylic monomer (A) and is dimer acid based on the weight of (A) ( A polymerization step of polymerizing the acrylic monomer (A) having a salt content of 0.5% by weight or less,
The gist is that it includes a polymerization step in which the content of iron atoms in the polymerization liquid is polymerized at 5 × 10 ⁻⁶ wt% or less based on the weight of (A).

  The production method of the present invention can produce a water-absorbing resin with little residual monomer and water-soluble content, regardless of the polymerization method.

  Examples of the acrylic monomer (A) include the radical polymerizable monomer (a) and the radical polymerizable internal cross-linking agent (b) described in JP 2001-200006 A or JP 2001-220415 A. It is.

Among the acrylic monomers described in these publications, from the viewpoint of water absorption performance, a radical polymerizable monomer having a carboxylic acid (salt) group, a sulfonic acid (salt) group or a hydroxyl group is preferable, and more preferably a carboxylic acid. A radically polymerizable monomer having a (salt) group, most preferably acrylic acid and acrylate.
The salt is preferably an alkali metal salt or an alkaline earth metal salt, more preferably an alkali metal salt, and particularly preferably a sodium salt.

  The radical polymerizable internal cross-linking agent (b) is preferably a radical polymerizable internal cross-linking agent having two or more ethylenically unsaturated groups, more preferably bis (meth) acrylamide having 8 to 12 carbon atoms, 2 to 2 carbon atoms. 10 polyol poly (meth) acrylate, 2 to 10 carbon polyallylamine and 2 to 10 polyol poly (meth) allyl ether, most preferably N, N'-methylenebis (meth) acrylamide and pentaerythritol Tetraallyl ether.

The content (% by weight) of the radical polymerizable monomer (a) in the acrylic monomer (A) is preferably 60 to 100, more preferably 70 to 100, particularly preferably based on the weight of (A). 80-100, then preferably 90-100, most preferably 95-100. Within this range, the water absorption performance is further improved.
The content (% by weight) of the radically polymerizable internal crosslinking agent (b) in the acrylic monomer (A) is preferably 0 to 40, more preferably 0 to 30, particularly preferably based on the weight of (A). Preferably 0-20, then preferably 0-10, most preferably 0-5. Within this range, the water absorption performance is further improved. When the radically polymerizable internal crosslinking agent (b) is included, the lower limit of the content (% by weight) of (b) is preferably 0.001, more preferably 0.005, particularly preferably 0.01, and then preferably Is 0.03, most preferably 0.06. Within this range, the water absorption performance (especially the amount absorbed under load) is further improved.

The content (mol%) of the acrylate in the acrylic monomer (A) is preferably at least 50, more preferably 50 to 100, particularly preferably based on the number of moles of the acrylic monomer (A). 60-90, then preferably 70-80, most preferably 70-75.
The content (mol%) of the acrylic monomer other than the acrylate is preferably 50 or less, more preferably 0 to 50, and particularly preferably 10 to 40, based on the number of moles of the acrylic monomer (A). Then, preferably 20-30, most preferably 25-30.

  The content (% by weight) of the dimer acid (salt) in the acrylic monomer (A) is preferably 0.5 or less, more preferably 0.35 or less, particularly preferably based on the weight of (A). 0.2 or less, then preferably 0.15 or less, most preferably 0.1 or less. Within this range, the water-soluble content and the residual monomer content of the resulting water-absorbent resin can be further reduced. It is presumed that monomer and water-soluble components increase due to thermal decomposition of dimer acid (salt) and a decrease in polymerization rate.

Dimer acid (salt) is a dimer produced by an addition reaction of two molecules of acrylic acid (salt).
In addition, content of a dimer acid (salt) is measured based on JISK0124: 2002. For example, it is measured under the following conditions.
Pretreatment: Hydrogen chloride is added to convert dimer acid (salt) to dimer acid.
Apparatus: Hi-SepLC, manufactured by Shimadzu Corporation Column: SCR-101H, 0.3 m × 7.9 mmφ
Developing solvent: 0.015 wt% phosphoric acid aqueous solution Flow rate: 0.5 ml / min
Sample: 0.1 wt% aqueous solution sample injection amount: 100 μl
Detector: UV detector, wavelength 195 nm

  Dimer acid is produced during the production of acrylic acid or the like, but most of it is usually removed by distillation operation or the like. However, dimer acid is gradually formed during storage of acrylic acid and can reach several weight percent order depending on humidity, residual moisture content, pH and / or temperature (especially in summer). In order to reduce the content of dimer acid (salt), the storage period is shortened (preferably 3 months or less, more preferably 1 month or less, particularly preferably 15 days or less), and low temperature (30 ° C. or less is preferable). More preferably, it should be stored at 20 ° C. or less, particularly preferably 15 ° C. or less.

The content (% by weight) of iron atoms in the polymerization solution is preferably 5 × 10 ⁻⁶ or less, more preferably 4 × 10 ⁻⁶ or less, particularly preferably based on the weight of the acrylic monomer (A). It is 3 × 10 ⁻⁶ or less, preferably 2 × 10 ⁻⁶ or less, and most preferably 1 × 10 ⁻⁶ or less. Within this range, the residual monomer and water-soluble components are further reduced. This is presumed to be because when an excessive amount of iron atoms is present in the polymerization solution, the initiator decomposes abnormally and the iron atoms themselves act as a chain transfer agent.

The form of the iron atom may be any of metal, ion or compound.
As an ion containing an iron atom, a 1-3 valent iron ion, an iron complex ion, etc. are included.
Compounds containing iron atoms include iron fluoride, iron bromide, iron iodide, iron hydroxide, iron oxide, ammonium sulfate iron, potassium iron sulfate, iron nitrate, iron phosphate, iron acetate, iron oxalate, magnesium oxide Although ferric, manganese ferric oxide, nickel ferric oxide, zinc ferric oxide and the like, and hydrates thereof can be considered, it is not limited thereto.

Possible routes for iron atoms to enter the polymerization process include mixing from raw materials and mixing from equipment.
As mixing from the raw materials, acrylic monomer (a), alkali used when neutralizing acrylic acid to make acrylate, initiator (d), graft substrate (e), chain transfer agent ( f), an internal crosslinking agent (g), a reaction solvent (c) and the like are conceivable. Of these, water and alkali are likely to be mixed with iron atoms.
As mixing from the equipment, although it can not be said unconditionally depending on the equipment, melting out from the tank or the line can be considered. For example, contamination of iron atoms from an alkali stock tank or an acrylic acid stock tank (corrosion of tank or line by alkali or acid) is considered. In addition to these, contamination of iron atoms due to mechanical wear due to opening and closing of line mixers and valves, stirring, and the like can be considered.
Therefore, as a method of keeping the content of iron atoms in the polymerization solution low, a countermeasure for preventing mixing from raw materials and equipment can be considered.
As measures for preventing contamination from raw materials, it is effective to use water as ion-exchanged water or distilled water, and to use alkali with a grade having a low iron atom content.
As a measure to prevent contamination from the equipment, polymer coating or pre-cleaning the line or tank with phosphoric acid in advance to prevent the dissolution of iron atoms due to corrosion of the line or tank, so that the iron atoms can be dissolved. It is effective to prevent this.

In addition, content of an iron atom is measured as follows.
After accurately weighing 2.07 g of nitric acid for ultra high purity precision analysis (for example, TAMAPURE AA-100, manufactured by Tama Chemical Co., Ltd.) in a 50 mL polypropylene (PP) container, ultra pure water having an electric conductivity of 3 MΩ · cm or more To prepare a 4.14 wt% aqueous nitric acid solution. After precisely weighing 0.15 g of the polymerization solution sample in a 15 mL PP container, it is diluted with a 3% by weight nitric acid aqueous solution so that the total amount becomes 15 g to obtain a measurement sample solution, and this measurement sample solution is subjected to an atomic absorption analysis furnace method (for example, The following conditions are used): {Use a calibration curve prepared using a standard solution for atomic absorption analysis of known concentration (manufactured by Nacalai Tesque)} Determine the content of iron atoms.
Measurement condition example Measurement device: GFA-6500 (manufactured by Shimadzu Corporation)
Measurement wavelength: 248.3nm
Measurement signal: BKG correction signal Calculation: Peak height Slit width: 0.2 nm
Time constant: 0.1 seconds Lamp current: 12.5 mA
Photomultiplier voltage: 380V

A polymerization liquid means the liquid containing the acrylic monomer (A) used for a superposition | polymerization process. If necessary, the polymerization solution may contain a reaction solvent (c), an initiator (d), a graft substrate (e), a chain transfer agent (f) and / or an internal crosslinking agent (g).
Examples of the reaction solvent (c) include water and a hydrophobic solvent.
Examples of the hydrophobic solvent include cyclohexane, normal hexane, normal heptane, toluene and xylene.
Of these, water, cyclohexane and normal hexane are preferred, water and cyclohexane are more preferred, and water is particularly preferred.
When using the reaction solvent (c), the use amount (% by weight) of the reaction solvent (c) is preferably 100 to 600, more preferably 150 to 500, based on the weight of the acrylic monomer (A). Especially preferably, it is 200-450.

  As the initiator (d) and the graft substrate (e), those described in JP-A No. 2001-200006 or JP-A No. 2001-220415 can be used. When the initiator (d) is used, the amount (% by weight) of the initiator (d) is preferably 0.005 to 0.5, more preferably based on the weight of the acrylic monomer (A). It is 0.007 to 0.4, particularly preferably 0.009 to 0.3. Moreover, when using a graft base material (e), 0.1-3 are preferable based on the weight of an acrylic monomer (A), as for content (weight%) of a graft base material (e), Furthermore, Preferably it is 0.3-2.7, Most preferably, it is 0.5-2.

  Examples of the chain transfer agent (f) include pyrogallol, p-methoxyphenol, gallic acid, gallic acid ester, tannic acid, flavonoids, thiourea and hypophosphorous acid (salt). These may be used alone or in combination of two or more. When the chain transfer agent (f) is used, the amount (% by weight) of the chain transfer agent (f) used is preferably 0.001 to 30, more preferably based on the weight of the acrylic monomer (A). 0.005 to 20, particularly preferably 0.01 to 10.

The internal cross-linking agent (g) is an internal cross-linking agent other than the radical polymerizable internal cross-linking agent (b), and includes polyhydric alcohol, polyhydric glycidyl, polyhydric amine, polyhydric aziridine, polyhydric isocyanate and the like. .
Examples of such an internal crosslinking agent (g) include polyhydric alcohols, polyhydric glycidyl, polyhydric amines described in JP-A No. 58-180233 (corresponding to US Pat. No. 4,666,983) and JP-A No. 59-189103, Polyvalent aziridines and polyvalent isocyanates can be used.
Examples of the polyvalent glycidyl compound include ethylene glycol diglycidyl ether and glycerin diglycidyl ether.
Examples of the polyvalent amine compound include ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, and polyethyleneimine.
As a polyvalent aziridine compound, trade name: Chemitite PZ-33 {2,2-bishydroxymethylbutanol-tris (3- (1-aziridinyl) propinate)}, trade name: Chemitite HZ-22 {1,6-hexa Methylenediethylene urea} and trade name: Chemite DZ-22 {diphenylmethane-bis-4, 4′-N, N′-diethylene urea} (these are trade names manufactured by Nippon Shokubai Chemical Industry Co., Ltd.).
Examples of the polyvalent isocyanate compound include 2,4-tolylene diisocyanate and hexamethylene diisocyanate.
These internal crosslinking agents may be used alone or in combination of two or more.
When the internal crosslinking agent (g) is used, the amount used (% by weight) is preferably 0.001 to 5, more preferably 0.002 to 2, based on the weight of the acrylic monomer (A). Most preferably, it is 0.003-1.6. Within this range, the water absorption performance (especially the amount of water absorption and the water absorption speed) is further improved. In addition, when using an internal crosslinking agent (g) and a radically polymerizable internal crosslinking agent (b), these use weight ratios (g / b) are preferably 20-1, more preferably 10-2, and particularly preferably. Is 5-3.

As a polymerization method in the polymerization step, for example, a conventionally known method can be applied, and any of a solution polymerization method, an emulsion polymerization method, a suspension polymerization method, a reverse phase suspension polymerization method, and the like may be used. Further, the shape of the polymerization solution at the time of polymerization may be a thin film shape, a spray shape, or the like. As a polymerization control method, an adiabatic polymerization method, a temperature control polymerization method, an isothermal polymerization method, or the like can be applied.
When suspension polymerization or reverse phase suspension polymerization is applied as the polymerization method, conventionally known dispersants (such as sucrose ester, phosphate ester and sorbitan ester) and protective colloid (Poval, α) -Olefin-maleic anhydride copolymer, polyethylene oxide, etc.) can be used.

  In this invention, superposition | polymerization conditions are not specifically limited, For example, although a polymerization start temperature can be suitably determined with the kind etc. of initiator to be used, 0-100 degreeC is preferable, More preferably, it is 5-80 degreeC. The polymerization time can be appropriately determined depending on the type of initiator used, the polymerization temperature, and the like, but is preferably 0.5 to 20 hours, more preferably 1 to 10 hours, and particularly preferably 2 to 5 hours.

The form of the hydrogel obtained at the end of the polymerization process varies depending on the polymerization method. In the solution polymerization method, a bulk hydrogel, an emulsion polymerization method, a suspension polymerization method, and a reverse phase suspension polymerization method yield a particulate hydrogel. It is done.
The hydrogel can be crushed as necessary. The size (longest diameter) of the hydrogel after crushing is preferably 50 μm to 10 cm, more preferably 100 μm to 2 cm, and particularly preferably 1 mm to 1 cm. Within this range, the drying property in the drying process is further improved.
The crushing can be performed by a known method, and for example, crushing can be performed using a normal apparatus such as a Bex mill, a rubber chopper, a pharma mill, a mincing machine, an impact crusher, and a roll crusher.

A water-absorbent resin can be obtained by distilling off the solvent (including water) from the obtained hydrogel.
When the solvent contains an organic solvent, the content (% by weight) of the organic solvent after distillation is preferably 10 to 0.01, more preferably 5 to 0.05, particularly based on the weight of the water absorbent resin. Preferably it is 3-0.1, Most preferably, it is 1-0.5. Within this range, the water absorption performance (particularly the amount of water retained) of the water absorbent resin is further improved.
Moreover, 0-20 are preferable based on the weight of the water-absorbent resin, more preferably 0-10, particularly preferably 0-5, and most preferably 0-2, after removal of water (% by weight). . Within this range, the water absorption performance (particularly the water retention amount) and the handling properties after drying (powder fluidity of particles made of a water absorbent resin, etc.) are further improved.
In addition, the content and moisture of the organic solvent are an infrared moisture meter (JE400 manufactured by KETT Co., Ltd.): 120 ± 5 ° C., 30 minutes, atmospheric humidity before heating 50 ± 10% RH, lamp specification 100V, 40W ) Is obtained from the weight loss of the crosslinked polymer (A) before and after heating.
The method of distilling off the solvent is a method of distilling (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. Ordinary methods such as drying, infrared drying, decantation and filtration may be used.

Further, the water absorbent resin can be pulverized after drying, if necessary.
When pulverizing, the weight average particle diameter (μm) of the water-absorbent resin after pulverization is preferably 100 to 800, more preferably 200 to 500, and particularly preferably 300 to 400. Within this range, handling properties after pulverization (powder fluidity of particles made of water-absorbent resin, etc.) are further improved.

  The weight average particle diameter is measured in accordance with JIS Z8815-1994, and the sieves with an aperture of 710 μm, 500 μm, 300 μm, 150 μm, and 106 μm with an inner diameter of 150 mm and a depth of 45 mm are arranged with a narrow aperture screen. In order, put 50 g of the measurement sample on the top 710 μm sieve with the widest opening, sieve for 10 minutes with a sieve vibrator, and measure the weight of the measurement sample remaining on each sieve. Measured by determining the weight percent of the measurement sample remaining on each sieve based on the weight of the first measurement sample (each particle size distribution of the sample is the particle size on the horizontal axis and the content based on weight on the vertical axis) Plot on logarithmic probability paper to determine particle size occupying 50% by weight of total weight).

The smaller the content of fine particles, the better the water absorption performance, and the content of fine particles of 106 μm or less in all particles is preferably 3% by weight or less, and more preferably the content of fine particles of 150 μm or less in all particles is 3% by weight. It is as follows.
The content of the fine particles can be determined using a plot created when determining the above weight average particle diameter.

The pulverization method is not particularly limited, and usual 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 as necessary to adjust the particle size.
The shape of the particles made of the water-absorbent resin is not particularly limited, and examples thereof include an irregular crushed shape, a flake shape, a pearl shape, and a rice grain shape. Among these, from the viewpoint of good entanglement with the fibrous material for use in paper diapers and the like and no fear of dropping off from the fibrous material, an irregular crushed shape is preferable.

  The water-absorbent resin can be subjected to surface crosslinking as necessary. The surface cross-linking agent for performing the surface cross-linking is not particularly limited, but from the viewpoint of the water absorption performance of the water-absorbent resin, etc. Surface cross-linking agent having at least two groups {polyvalent glycidyl described in JP-A-59-189103, etc., polyvalent glycidyl described in JP-A-58-180233, JP-A-61-16903, etc. Alcohols, polyvalent amines, polyvalent aziridines and polyvalent isocyanates, silane coupling agents described in JP-A-61-211305 or JP-A-61-225212, and JP-A-51-136588 or Multivalent metals described in JP-A-61-257235, etc.} are preferred, more preferred are polyvalent glycidyls, and most preferred are ethyl acetates. Glycol diglycidyl ether and glycerin diglycidyl ether, and most preferably ethylene glycol diglycidyl ether.

In the case of surface crosslinking, the amount (% by weight) of the surface crosslinking agent used is preferably 0.001 to 7, more preferably 0.002 to 5, based on the weight of the water absorbent resin (including the surface crosslinking agent). Most preferably, it is 0.003-4. Within this range, the water absorption performance is further improved.
Surface cross-linking can be achieved by a method of spraying or impregnating a water-absorbing resin with an aqueous solution containing a surface cross-linking agent, followed by heat treatment (100 to 200 ° C.).

Water-absorbent resin contains additives (preservatives, fungicides, antibacterial agents, antioxidants, UV absorbers, colorants, fragrances, deodorants, organic fibrous materials, etc.) as necessary. Can be made.
When the additive is contained, the content (% by weight) of the additive is preferably 0.001 to 10, more preferably 0.01 to 5, particularly preferably 0.05 based on the weight of the water absorbent resin. ~ 1, most preferably 0.1-0.5.

  When the method for producing a water-absorbent resin of the present invention is applied, a water-absorbent resin having a low water-soluble content and a small content of residual monomer can be produced regardless of the polymerization method. In addition, when the water-absorbent resin produced by the production method of the present invention is applied to sanitary goods such as disposable diapers and sanitary napkins, it is difficult to cause the undesired phenomenon and the water absorption performance is remarkably improved. Furthermore, since it is difficult to cause clogging troubles in machines that produce sanitary goods such as disposable diapers and sanitary napkins, the feed can be stably and evenly dispersed.

  In addition, the water-absorbent resin produced by the production method of the present invention is not only used for the sanitary products described above, but also for pet urine absorbent, urine gelling agent for portable toilets, freshness-keeping agents such as fruits and vegetables, meat and fish and shellfish It is also useful for various applications such as drip absorbents, cryogens, disposable warmers, gelling agents for batteries, water retention agents for plants and soil, anti-condensation agents, water-stopping agents and packing agents, and artificial snow.

  Hereinafter, although an example and a comparative example explain the present invention further, the present invention is not limited to these. Residual monomer content, water-soluble content, water retention, absorption under load, and moisture absorption blocking rate were measured by the following methods. Unless otherwise specified, “part” means “part by weight” and “%” means “% by weight”.

<Measurement method of residual monomer content>
In a 300 ml beaker, 1.0 g of a measurement sample and 249.0 g of 0.9% saline were added and stirred at 20 to 30 ° C. for 3 hours, and then the insoluble matter was filtered off to obtain a filtrate. The filtrate was subjected to high-performance liquid chromatography (under the following conditions) to determine the content of residual monomer {using a calibration curve prepared using a water-soluble vinyl monomer at a known concentration}.
Measurement condition column: SCR-101H (length 0.3 m × inner diameter 7.9 mm Shimadzu Corporation) developing solution: 0.015% phosphoric acid aqueous solution flow rate: 0.5 ml / min
Sample injection volume: 100 μl
Detector: UV detector, wavelength 195 nm
Temperature: 40 ° C

<Method for measuring water-soluble content>
In the same manner as described in JP-A-62-54751, the water-soluble content after extraction for 3 hours was measured.

<Measurement method of water retention amount>
1.00 g of a measurement sample is placed in a tea bag (20 cm long, 10 cm wide) made of a nylon net having a mesh size of 63 μm (conforming to JIS Z8801-1: 2001), and physiological saline (saline concentration 0.9%) 1, After immersing in 000 cc for 1 hour without stirring, suspend for 15 minutes to drain water, put the tea bag in a centrifuge, centrifuge at 150G for 90 seconds, and measure the weight (a) after centrifugal dehydration. . Furthermore, the same operation was performed only with a tea bag without a measurement sample, the weight (b) was measured, and the water retention amount was determined by the following equation. In addition, the used physiological saline temperature and the measurement temperature were 25 degreeC.
Water retention amount = ab

<Measurement method of absorption under load>
Weigh 0.1 g of measurement sample in a cylindrical plastic tube (inner diameter 30 mm, height 60 mm) with a nylon mesh of 63 μm (conforming to JIS Z8801-1: 2001) pasted on the bottom, and make the plastic tube vertical. A measurement sample was arranged on a nylon net so as to have a substantially uniform thickness, and a weight of 29.5 mm × 22 mm in outer diameter was placed on the measurement sample so as to have a load of 20 g / cm ² . In a petri dish (diameter: 12 cm) containing 60 ml of physiological saline (salt concentration 0.9%), place a plastic tube containing a measurement sample and a weight vertically, immerse the nylon mesh side on the bottom, and let it stand, After 60 minutes, the plastic tube containing the sample and the weight is weighed, and the weight of the measurement sample that has absorbed physiological saline is calculated. The value obtained by multiplying this increased weight by 10 times the weight of physiological saline (g / G).

<Hygroscopic blocking rate (Hygroscopic BL rate)>
10 g of a measurement sample that has passed through a wire mesh having an opening of 850 μm (conforming to JIS Z8801-1: 2001) is uniformly placed in an aluminum cylindrical dish having a diameter of 5 cm, and constant temperature and humidity of 40 ± 1 ° C. and relative humidity of 80 ± 5%. It left still in the tank for 3 hours. After measuring the total weight (a) of the measurement sample after standing for 3 hours, this was tapped five times with a 1400 μm wire mesh (conforming to JIS Z8801-1: 2001), sieved, and blocked by moisture absorption to 1400 μm. The weight (b) of the measurement sample remaining on the wire mesh was measured, and the moisture absorption blocking rate was determined by the following equation.
Moisture absorption blocking rate = (b / a) × 100

<Example 1>
In a glass reaction vessel, 83.5 parts of acrylic acid was diluted with 178.3 parts of deionized water and neutralized by adding 131.1 parts of a 25% aqueous sodium hydroxide solution while cooling to 30-20 ° C. . To this solution, 0.15 part of N, N′-methylenebisacrylamide was charged, and the temperature of the polymerization solution (monomer aqueous solution) was kept at 3 ° C. while stirring and mixing. The content of acrylate in the polymerization solution (monomer aqueous solution) was 72 mol%, the content of dimer acid (salt) was 0.48%, and the content of iron atoms was 4.9 × 10 ⁻⁶ %. .
In addition, the content of dimer acid (salt) in the polymerization solution is the same as that of acrylic acid having increased dimer acid by storing fresh acrylic acid having a dimer acid content of 0.10% by weight at 20 ° C. for 2 months. And using fresh acrylic acid (hereinafter the same). Moreover, the content of iron atoms in the polymerization solution was adjusted by purchasing sodium hydroxide having different iron atom contents (the same applies hereinafter).
After flowing nitrogen into the polymerization solution to make the dissolved oxygen amount 1 ppm or less, 0.3 part of 1% aqueous solution of hydrogen peroxide, 0.8 part of 0.2% aqueous solution of ascorbic acid and 2,2′-azobis Addition and mixing of 0.8 parts of a 2% aqueous solution of amidinopropane dihydrochloride to initiate polymerization, and after the polymerization solution reaches 80 ° C., it is polymerized at a polymerization temperature of 80 ± 2 ° C. for about 5 hours, thereby containing water. A gel polymer was obtained. The content of iron atoms in the obtained hydrous gel was 4.9 × 10 ⁻⁶ %. The water content (120 ± 5 ° C. × 30 minutes) was 74.6%.
Next, 400 parts of the hydrogel polymer was kneaded for 5 minutes at 25 ° C. with a mincing machine (diameter hole diameter: 6 mm, 12VR-400K, Iizuka Kogyo Co., Ltd.), then 135 ° C. and wind speed 2.0 m / sec. It dried with the ventilation type band dryer of the conditions of the above, and the polymer dried material was obtained.
This polymer dried product was pulverized with a commercially available juicer mixer, adjusted to a particle size of 250 to 600 μm using a sieve having an opening of 600 and 250 μm, and then 100 parts of this polymer was stirred at high speed (high speed stirring turbulizer manufactured by Hosokawa Micron). 2 parts of a 15% water / methanol mixed solution of ethylene glycol diglycidyl ether (water / methanol weight ratio = 70/30) while being sprayed and mixed while mixing at 140 ° C. for 30 minutes. The water absorbent resin (1) was obtained by standing still and heat-crosslinking.

<Example 2>
Except for using a polymerization liquid (monomer aqueous solution) in which the content of dimer acid (salt) in the polymerization liquid (monomer aqueous solution) is 0.10% and the content of iron atoms is 0.8 × 10 ⁻⁶ % In the same manner as in Example 1, a water absorbent resin (2) was obtained. The content of iron atoms in the hydrogel obtained after polymerization was 0.8 × 10 ⁻⁶ %.

<Example 3>
145.4 parts of acrylic acid was diluted with 9.4 parts of water and neutralized by adding 242.3 parts of 25% aqueous sodium hydroxide while cooling to 30-20 ° C. 0.09 part of ethylene glycol diglycidyl ether (manufactured by Nagase Kasei Kogyo Co., Ltd., Denacol EX-810), 0.0146 part of sodium hypophosphite monohydrate and 0.0727 part of potassium persulfate are dissolved in this solution. To give a polymerization solution (monomer aqueous solution). The content of acrylate in the polymerization solution (monomer aqueous solution) is 75 mol%, the content of dimer acid (salt) is 0.47%, and the content of iron atoms is 3.0 × 10 ⁻⁶ %. Met.
Subsequently, 624 parts of cyclohexane was put into a reaction vessel equipped with a stirrer, a reflux condenser, a thermometer, and a nitrogen gas introduction tube, and polyoxyethylene octylphenyl ether phosphate ester (Daiichi Kogyo Seiyaku Co., Ltd., trade name: (Blysurf A210G) 1.56 parts was added and dissolved, and then purged with nitrogen while stirring, and then heated to 70 ° C. And while maintaining at 70 degreeC, the prepared superposition | polymerization liquid (monomer aqueous solution) is dripped at 6.6 parts / min for 6 minutes, and after hold | maintaining at 75 degreeC for 15 minutes, the remaining superposition | polymerization liquid (monomer aqueous solution) is 6. It was added dropwise over 6 minutes at 6 parts / minute. Thereafter, aging was performed at 75 ° C. for 30 minutes. The content of iron atoms in the hydrogel obtained after aging was 3.0 × 10 ⁻⁶ %. Thereafter, water was removed by azeotropy with cyclohexane until the water content of the resin became about 20% (infrared moisture meter (FD-100 type, manufactured by Kett, 180 ° C., measured in 20 minutes)). When the stirring was stopped, the resin particles settled, and the resin particles and cyclohexane were separated by decantation, and 80 parts of the resin particles and 140 parts of cyclohexane were placed in a reaction vessel, and glycerin polyglycidyl ether was added thereto. (Nagase Kasei Kogyo Co., Ltd., trade name: Denacol EX-314) After adding 3.4 parts of cyclohexane solution containing 0.25%, heating at 60 ° C. and holding for 30 minutes, further heating to reflux cyclohexane Then, the resin particles were obtained by filtration and dried under reduced pressure at 80 ° C. to obtain water-absorbing resin particles (3).

<Example 4>
Except for using a polymerization liquid (monomer aqueous solution) in which the content of dimer acid (salt) in the polymerization liquid (monomer aqueous solution) is 0.10% and the content of iron atoms is 0.8 × 10 ⁻⁶ % In the same manner as in Example 3, a water absorbent resin (4) was obtained. The content of iron atoms in the hydrogel obtained after aging was 0.8 × 10 ⁻⁶ %.

<Comparative Example 1>
Except for using a polymerization liquid (monomer aqueous solution) in which the dimer acid (salt) content in the polymerization liquid (monomer aqueous solution) is 0.80% and the iron atom content is 7.0 × 10 ⁻⁶ %. A comparative water absorbent resin (H1) was obtained in the same manner as in Example 1. The content of iron atoms in the hydrogel obtained after polymerization was 7.0 × 10 ⁻⁶ %.

<Comparative example 2>
Except for using a polymerization liquid (monomer aqueous solution) in which the content of dimer acid (salt) in the polymerization liquid (monomer aqueous solution) is 0.30% and the content of iron atoms is 7.0 × 10 ⁻⁶ %. A comparative water absorbent resin (H2) was obtained in the same manner as in Example 1. The content of iron atoms in the hydrogel obtained after polymerization was 7.0 × 10 ⁻⁶ %.

<Comparative Example 3>
Except for using a polymerization liquid (monomer aqueous solution) in which the content of dimer acid (salt) in the polymerization liquid (monomer aqueous solution) is 0.70% and the content of iron atoms is 4.3 × 10 ⁻⁶ %. A comparative water absorbent resin (H3) was obtained in the same manner as in Example 1. The content of iron atoms in the hydrogel obtained after polymerization was 4.3 × 10 ⁻⁶ %.

<Comparative example 4>
Except for using a polymerization liquid (monomer aqueous solution) in which the content of dimer acid (salt) in the polymerization liquid (monomer aqueous solution) is 0.10% and the content of iron atoms is 6.8 × 10 ⁻⁶ %. A comparative water absorbent resin (H4) was obtained in the same manner as in Example 3. The content of iron atoms in the hydrogel obtained after aging was 6.8 × 10 ⁻⁶ %.

As can be seen from Table 1, the water-absorbent resin (Examples 1 to 4) obtained by the production method of the present invention has a residual monomer content and a water-soluble content as compared with the water-absorbent resins of Comparative Examples 1 to 4. Very few. Furthermore, the water retention amount, absorption amount under load, and moisture absorption BL ratio of the water absorbent resin obtained by the production method of the present invention showed extremely excellent values.
In addition, when the water-absorbent resin produced by the production method of the present invention is applied to sanitary goods such as disposable diapers and sanitary napkins, the moisture absorption BL rate is low, so that there is little mechanical trouble and it can be dispersed uniformly.

The water-absorbent resin produced by the production method of the present invention is useful for sanitary products such as disposable diapers, sanitary napkins and medical blood retention agents. Also, pet urine absorbent, mobile toilet urine gelling agent and freshness maintaining agent such as fruits and vegetables, meat and seafood drip absorbent, cold insulation, disposable warmer, battery gelling agent, water retention agent for plants and soil, etc. It can also be used in various applications such as anti-condensation agents, water-stopping agents and packing agents, and artificial snow.

Claims (1)

An acrylic containing at least 50 mol% acrylate based on the number of moles of acrylic monomer (A) and having a dimer acid (salt) content of 0.5 wt% or less based on the weight of (A) A polymerization step of polymerizing the monomer (A),
A method for producing a water-absorbent resin, comprising a polymerization step of polymerizing a content of iron atoms in a polymerization solution at 5 × 10 ⁻⁶ wt% or less based on the weight of (A).