JP2015178608A - Method for producing polyacrylic acid (salt)-based water-absorbing resin - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a water-absorbing resin that even if produced using an acrylic acid having a high content of acrylic acid dimer, can reduce a residual monomer without deteriorating the productivity and without using a treatment agent.SOLUTION: The method for producing a polyacrylic acid (salt)-based water-absorbing resin includes a step of preparing a monomer aqueous solution from an acrylic acid having an acrylic acid dimer content of 150-2,000 ppm and a step of polymerizing the monomer aqueous solution. The production method further includes a step of neutralizing the acrylic acid with a basic compound in the step of preparing the monomer aqueous solution in which the maximum temperature in the neutralization step is 70°C or higher.

Description

  The present invention relates to a method for producing a polyacrylic acid (salt) water-absorbing resin. In detail, it is related with the manufacturing method of the water absorbing resin used for the absorber of sanitary goods, such as a paper diaper and a sanitary napkin. More specifically, a polyacrylic acid (salt) -based water-absorbing resin that can efficiently obtain a water-absorbing resin with a small amount of residual monomer even if acrylic acid with a large content of acrylic acid dimer is used as a raw material. It relates to a manufacturing method.

  Water-absorbing resins have been developed in recent years as highly water-absorbing compounds, and are mainly used as absorbents for sanitary products such as paper diapers and sanitary napkins, water retention agents for agriculture and horticulture, and industrial water-stopping agents. Widely used in disposable applications. There are a wide variety of water-absorbent resins in the water-absorbent resin, and many monomers and hydrophilic polymers are present as raw materials. Among them, polyacrylic acid (salt) -based water-absorbing resins using acrylic acid and / or a salt thereof as a monomer are industrially most produced due to their high water absorption performance (Patent Documents 5 to 8). .

  In the production of the polyacrylic acid (salt) -based water-absorbent resin, acrylic acid used includes a dimer of acrylic acid represented by the following formula (hereinafter referred to as “acrylic acid dimer”).

  Although the acrylic acid dimer can be removed by purification operations such as distillation and crystallization in the production process of acrylic acid, the acrylic acid dimer becomes 1 while the obtained acrylic acid is stored in a product tank or the like. About several tens to several hundreds of ppm per day is produced, and the content in acrylic acid may be several weight% depending on the storage conditions (temperature, time, moisture, etc.) (Non-Patent Documents 1 and 2).

  Further, since the acrylic acid dimer has a polymerizable double bond, it is copolymerized with a monomer such as acrylic acid and incorporated into the main chain polymer. Therefore, it is decomposed from the main chain to produce acrylic acid in a process such as a drying process of the water absorbent resin. As a result, there is a problem that the residual monomer in the water absorbent resin increases.

  Accordingly, in order to reduce the amount of the residual monomer, acrylic acid is excessively neutralized with a basic compound and kept alkaline (temperature is 20 to 50 ° C.), and then acrylic acid is added to obtain a partially neutralized product. Technology (Patent Document 1) and technology for adding sodium bisulfite to reduce the residual monomer in the water-absorbent resin when producing the water-absorbent resin using acrylic acid having an acrylic acid dimer content of 200 ppm or more (Patent Document 2) has been proposed.

  In addition, since acrylic acid dimer is removed from acrylic acid and the treatment after polymerization is expensive, acrylic acid dimer in acrylic acid leads to an increase in residual monomers as described above. Technology using acrylic acid with low dimer content (Patent Documents 3 and 10), technology using acrylic acid with low acrylic acid oligomer content including acrylic acid dimer (Patent Document 4), purity less than 99.8% by weight A technique (Patent Document 9) using acrylic acid has been proposed. In addition, in these techniques, like patent documents 5-8, after performing the refining process (especially distillation) of acrylic acid, the storage time until it is used for manufacture of a water absorbing resin is made as short as possible, The residual monomer in the water absorbent resin is reduced.

  Moreover, although water absorbing resin is partially neutralized polyacrylic acid, patent documents 11-15 are proposed as a method of neutralizing acrylic acid for water absorbing resin. Patent Document 11 discloses that the neutralization method of acrylic acid affects the coloring of the water-absorbent resin, and Patent Document 12 discloses continuous neutralization in a state where the neutralization temperature is maintained below 70 ° C. Furthermore, regarding the neutralization temperature, from the viewpoint of danger during polymerization, a technique has been proposed in which the neutralization temperature is less than 40 ° C., and further less than 35 ° C. (Patent Document 16).

JP-A-2-209906 International Publication No. 2012/163930 Pamphlet JP-A-6-21934 Special table 2006-509085 gazette JP 2002-212204 A JP 2004-155963 A JP 2005-36100 A International Publication No. 2013/122246 Pamphlet International Publication No. 2012/164081 Pamphlet JP 2006-219661 A International Publication No. 2009/123197 Pamphlet International Publication No. 2007/028751 Pamphlet International Publication No. 2007/028747 Pamphlet International Publication No. 2007/028746 Pamphlet International Publication No. 2003/095410 Pamphlet International Publication No. 98/49221 Pamphlet

“Acrylic acid and its polymer (1)” by Eizo Omori, Shosodo Co., Ltd. (1973) p. 34 "Modern Superabsorbent Polymer Technology" (1998) p. 24-28

  The above-described conventional technology has various problems listed below. That is, it is difficult or expensive to obtain high-purity acrylic acid with a low content of acrylic acid dimer (Patent Documents 3, 4, 10, etc.), and neutralization of acrylic acid at a low temperature is performed when cooling. Is disadvantageous in terms of energy (Patent Documents 1, 12, 16, etc.), the operation of reducing the residual monomer is complicated (Patent Documents 1, 2, 9, etc.), and the number of processes is increased. And a new problem occurs due to the addition of an extra treating agent.

  In addition, the liquid level in the storage tank is kept as low as possible in order to shorten the storage time (residence time) until it is used for the production of the water-absorbent resin after the purification step (especially distillation) of acrylic acid. Measures such as suppression have been taken (Patent Document 11).

  However, when continuous production is temporarily stopped by production adjustment, or when there is a suspension due to troubles in the water absorbent resin manufacturing plant, acrylic acid is stored in the storage tank, but the suspension or suspension period is prolonged. Even with high-purity acrylic acid, the acrylic acid dimer content in the tank increases, and it is necessary to return the acrylic acid to the purification process again.

  Therefore, the problem to be solved by the present invention is not limited to the use of high-purity acrylic acid, which is disadvantageous in terms of cost and procurement, and lowers productivity even with acrylic acid having a high acrylic acid dimer content. Furthermore, it is to provide a method for producing a water-absorbent resin in which residual monomers are reduced without using a treating agent.

  As a result of intensive studies by the present inventors in order to solve the above-mentioned problems, an operation for removing the acrylic acid dimer is not required by setting the maximum temperature to 70 ° C. or higher for the temperature condition during neutralization. The present inventors have found that a water-absorbing resin comparable to that obtained when acrylic acid having a low acrylic acid dimer content is used can be obtained. The “polymerization composition” means a monomer, a polymer, water, a polymerization initiator, and the like.

  That is, the method for producing a polyacrylic acid (salt) water-absorbing resin according to the present invention includes a step of preparing a monomer aqueous solution with acrylic acid having an acrylic acid dimer content of 150 to 2000 ppm, and the monomer aqueous solution. A method for producing a polyacrylic acid (salt) -based water-absorbing resin, comprising a step of polymerizing a monomer, and further comprising a step of neutralizing the acrylic acid with a basic compound in the monomer aqueous solution preparation step. A method for producing a polyacrylic acid (salt) water-absorbing resin, wherein the maximum temperature in the neutralization step is 70 ° C. or higher.

  According to the present invention, even when acrylic acid having a high content of acrylic acid dimer is used as a raw acid for the water-absorbing resin, a water-absorbing resin with reduced residual monomers due to the acrylic acid dimer can be obtained. Furthermore, since an operation for removing the acrylic acid dimer is not required, productivity can be improved and costs can be reduced.

  Hereinafter, although the manufacturing method of the polyacrylic acid (salt) water-absorbing resin according to the present invention will be described in detail, the scope of the present invention is not limited to these descriptions, and the present invention is not limited to the following examples. Modifications and implementations can be made as appropriate without departing from the spirit of the invention. Specifically, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the claims, and technical means disclosed in different embodiments are appropriately combined. Embodiments obtained in this manner are also included in the technical scope of the present invention.

[1] Definition of terms (1-1) "Water absorbent resin"
The “water-absorbent resin” in the present invention refers to a water-swellable, water-insoluble polymer gelling agent and satisfies the following physical properties. That is, as “water swellability”, CRC specified by ERT441.2-02 is 5 g / g or more, and as “water-insoluble”, Ext specified by ERT470.2-02 is 50% by weight or less. It refers to a polymer gelling agent that satisfies the above.

  The water-absorbent resin can be appropriately designed according to its use, and is not particularly limited, but is preferably a hydrophilic cross-linked polymer obtained by cross-linking an unsaturated monomer having a carboxyl group. Moreover, the whole amount (100 weight%) is not limited to the form which is a polymer, The water absorbing resin composition containing the additive etc. may be sufficient in the range which satisfies the said physical property (CRC, Ext).

  Furthermore, the water-absorbent resin in the present invention is not limited to the final product, but is an intermediate in the manufacturing process of the water-absorbent resin (for example, a water-containing gel-like crosslinked polymer after polymerization, a dried polymer after drying, a water absorption before surface crosslinking). In combination with the above water-absorbent resin composition, all of which are collectively referred to as “water-absorbent resin”. Examples of the shape of the water absorbent resin include a sheet shape, a fiber shape, a film shape, a particle shape, and a gel shape. In the present invention, a particulate water absorbent resin is preferable.

(1-2) "Polyacrylic acid (salt)"
The “polyacrylic acid (salt)” in the present invention refers to polyacrylic acid and / or a salt thereof, and acrylic acid and / or a salt thereof (hereinafter referred to as “acrylic acid (salt)”) as a main component. A polymer containing a repeating component and a graft component as an optional component is meant.

  The “main component” means that the used amount (content) of acrylic acid (salt) is usually 50 to 100 mol% based on the entire monomer (excluding the internal crosslinking agent) used for polymerization, Preferably it is 70-100 mol%, More preferably, it is 90-100 mol%, More preferably, it means substantially 100 mol%.

  The polyacrylic acid salt as a polymer contains a water-soluble salt, preferably a monovalent salt, more preferably an alkali metal salt or an ammonium salt, still more preferably an alkali metal salt, particularly preferably a sodium salt. .

(1-3) “EDANA” and “ERT”
“EDANA” is an abbreviation for European Disposables and Nonwovens Associations, and “ERT” is an abbreviation for a method of measuring water-absorbent resin of the European standard (almost the world standard) (EDANA Recommended Test Methods). . In the present invention, unless otherwise specified, the physical properties of the water-absorbent resin are measured according to the original ERT (revised in 2002 / known literature).

(1-3-1) “CRC” (ERT441.2-02)
“CRC” is an abbreviation for Centrifugation Retention Capacity (centrifuge retention capacity), and means the water absorption capacity of the water absorbent resin under no pressure (sometimes referred to as “water absorption capacity”). Specifically, 0.2 g of the water-absorbing resin was put in a non-woven bag, and then immersed in a large excess of 0.9 wt% sodium chloride aqueous solution for 30 minutes to freely swell, and then centrifuged (250 G ) Is the water absorption rate (unit: g / g) after draining.

(1-3-2) “AAP” (ERT442.2-02)
“AAP” is an abbreviation for Absorption Against Pressure, which means the water absorption capacity of a water absorbent resin under pressure. Specifically, after swelling 0.9 g of water-absorbing resin with a large excess of 0.9 wt% sodium chloride aqueous solution for 1 hour under a load of 2.06 kPa (21 g / cm ² , 0.3 psi) Of water absorption (unit: g / g). In some cases, the load condition is changed to 4.83 kPa (49 g / cm ² , 0.7 psi). In addition, ERT442.2-02 describes “Absorption Under Pressure”, which has substantially the same content.

(1-3-3) "PSD" (ERT420.2-02)
“PSD” is an abbreviation for Particle Size Distribution, and means a particle size distribution of a water-absorbent resin measured by sieving classification. The weight average particle diameter (D50) and the logarithmic standard deviation (σζ) of the particle size distribution are described in “(3) Mass-Average Particle Diameter (D50) and Logarithmic Standard Deviation (σζ) of US Pat. It measures by the method similar to "Particle Diameter Distribution."

(1-3-4) “Ext” (ERT470.2-02)
“Ext” is an abbreviation for Extractables, which means the water-soluble component (water-soluble component amount) of the water-absorbent resin. Specifically, it refers to the amount of dissolved polymer (unit:% by weight) after adding 1.0 g of water-absorbing resin to 200 ml of 0.9 wt% aqueous sodium chloride solution and stirring at 500 rpm for 16 hours. The amount of dissolved polymer is measured using pH titration.

(1-3-5) “Moisture Content” (ERT430.2-02)
“Moisture Content” means the water content of the water-absorbent resin. Specifically, it refers to a value (unit:% by weight) calculated from loss on drying when 4.0 g of water-absorbent resin is dried at 105 ° C. for 3 hours. In some cases, the water-absorbent resin is measured at 1.0 g and the drying temperature is changed to 180 ° C.

(1-3-6) “Residual Monomers” (ERT410.2-02)
“Residual Monomers” means the amount of monomer (monomer) remaining in the water-absorbent resin (hereinafter referred to as “residual monomer”). Specifically, it refers to the amount of residual monomer (unit: ppm) after adding 1.0 g of water-absorbing resin to 200 ml of 0.9 wt% sodium chloride aqueous solution and stirring at 500 rpm for 1 hour. The amount of residual monomer is measured using high performance liquid chromatography (HPLC).

(1-3-7) Other properties of water-absorbing resin specified by EDANA “pH” (ERT400.2-02): means the pH of the water-absorbing resin.

  “Flow Rate” (ERT450.2-02): The flow rate of the water-absorbing resin.

  “Density” (ERT460.2-02): means the bulk specific gravity of the water-absorbent resin.

  “Respirable Particles” (ERT480.2-02): Respirable area dust of water absorbent resin.

  “Dust” (ERT490.2-02): means dust contained in the water absorbent resin.

(1-4) “Liquid permeability”
“Liquid permeability” of the water-absorbent resin in the present invention refers to the fluidity of liquid passing between particles of the swollen gel under load or no load. As a typical measurement method, SFC (Saline) is used. Flow Conductivity / Saline Flow Inductivity) and GBP (Gel Bed Permeability / Gel Bed Permeability).

  “SFC” refers to the liquid permeability of a 0.69 wt% sodium chloride aqueous solution to a water absorbent resin under a 2.07 kPa load, and is measured according to the SFC test method disclosed in US Pat. No. 5,669,894. .

  “GBP” refers to the liquid permeability of a 0.9 wt% aqueous sodium chloride solution to a water-absorbent resin under load or free swelling, and conforms to the GBP test method disclosed in International Publication No. 2005/016393. Measured.

(1-5) Others In this specification, “X to Y” indicating a range means “X or more and Y or less”. Further, unless otherwise noted, “t (ton)” as a unit of weight means “Metric ton”, and “ppm” means “weight ppm” or “mass ppm”. Furthermore, “weight” and “mass”, “part by weight” and “part by mass”, “% by weight” and “% by mass” are treated as synonyms. Further, “˜acid (salt)” means “˜acid and / or salt thereof”, and “(meth) acryl” means “acryl and / or methacryl”.

[2] Method for Producing Polyacrylic Acid (Salt) Water Absorbent Resin Hereinafter, a method for producing a polyacrylic acid (salt) water absorbent resin according to the present invention will be described.

(2-1) Preparation Step of Monomer Aqueous Solution This step is a step of preparing an aqueous solution containing acrylic acid (salt) as a main component (hereinafter referred to as “monomer aqueous solution”). In addition, although the slurry liquid of a monomer can also be used in the range by which the water absorption performance of the water-absorbing resin obtained does not fall, in this section, monomer aqueous solution is demonstrated for convenience.

  Further, the above “main component” means that the content (usage amount) of acrylic acid (salt) is usually based on the whole monomer (excluding the internal cross-linking agent) subjected to the polymerization reaction of the water absorbent resin. It means 50 mol% or more, preferably 70 mol% or more, more preferably 90 mol% or more (the upper limit is 100 mol%).

(Acrylic acid (salt))
In the present invention, acrylic acid and / or a salt thereof (hereinafter referred to as “acrylic acid (salt)”) is used as a monomer from the viewpoint of physical properties and productivity of the obtained water-absorbent resin. As the “acrylic acid”, acrylic acid containing a trace inhibitor such as a polymerization inhibitor or an organic impurity can be used.

  As the polymerization inhibitor, preferably methoxyphenols, more preferably p-methoxyphenol, from the viewpoint of the polymerization of acrylic acid and the color tone of the water-absorbent resin, preferably 10 to 300 ppm, more preferably 30 to 200 ppm, further Preferably 50 to 150 ppm, particularly preferably 60 to 100 ppm. In addition, when content of the said methoxyphenol is less than 10 ppm, the danger that the polymerization reaction of acrylic acid will start by the time of manufacturing a water absorbing resin increases. Moreover, when this content exceeds 300 ppm, since the polymerization reaction for manufacturing a water absorbing resin is delayed or the color tone of the obtained water absorbing resin deteriorates, it is not preferable.

  As one of the organic impurities, in the present invention, attention is paid to “acrylic acid dimer obtained by addition reaction of two molecules of acrylic acid”. In addition, acrylic acid dimer content in acrylic acid of this invention is 150-2000 ppm, Preferably it is 200-1500 ppm, More preferably, it is 250-1000 ppm, More preferably, it is 300-600 ppm. When the acrylic acid dimer content is less than 150 ppm, it is necessary to increase the cost of acrylic acid or to use it immediately after purification of acrylic acid. On the other hand, when the content exceeds 2000 ppm, the residual monomer increases remarkably when the water-absorbing resin in the manufacturing process is heated to a high temperature in the water-absorbing resin manufacturing process (particularly the drying process or the heat treatment process). Is not preferable.

  The acrylic acid of the present invention may contain organic impurities other than the above-mentioned acrylic acid dimer and water. The content of the organic impurities is preferably 2% by weight or less, more preferably 1% by weight or less, still more preferably 0.1% by weight or less, and the water content is preferably 2% by weight or less. More preferably, it is 1 weight% or less, More preferably, it is 0.1 weight% or less, Most preferably, it is 0.01 weight% or less.

  For trace components such as polymerization inhibitors and impurities in acrylic acid other than those described above, the compounds described in Patent Document 10 and US Patent Application Publication No. 2008/0161512 are applied to the present invention.

  The “acrylate” is obtained by neutralizing the above acrylic acid with the following basic composition. As the acrylate, a commercially available acrylate (for example, an aqueous solution or powder of sodium acrylate) is used. )) Or may be obtained by neutralization (hereinafter referred to as “neutralization step”) in the water-absorbent resin production plant.

(Other monomers)
In the present invention, the “other monomer” refers to a monomer other than the acrylic acid (salt), and can be used in combination with acrylic acid (salt) to produce a water-absorbing resin.

  Examples of the other monomers include water-soluble or hydrophobic unsaturated monomers. Specifically, compounds described in U.S. Pat. Nos. 4,893,999, 6,241,928, 6,987,151, 6,671,141, U.S. Patent Application Publication No. 2005/0215734, etc. (excluding acrylic acid) are used. The invention also applies. In addition, what uses the said water-soluble or hydrophobic unsaturated monomer as a copolymerization component is contained in the water absorbing resin of this invention.

(Basic composition)
In the present invention, the “basic composition” refers to a composition containing a basic compound, such as a commercially available sodium hydroxide aqueous solution.

  Specific examples of the basic compound include alkali metal carbonates and hydrogen carbonates, alkali metal hydroxides, ammonia, and organic amines. Among these, strong basicity is desired from the viewpoint of the physical properties of the obtained water-absorbent resin. That is, hydroxides of alkali metals such as sodium hydroxide, potassium hydroxide and lithium hydroxide are preferable, and sodium hydroxide is more preferable.

(Neutralization process)
This step is a step of neutralizing acrylic acid with a basic composition in order to obtain an acrylate. The neutralization step is neutralization with respect to acrylic acid (before polymerization) or neutralization with respect to a hydrogel crosslinked polymer obtained by crosslinking polymerization of acrylic acid (after polymerization) (hereinafter referred to as “post-neutralization”). ) May be used, or may be used in combination.

  These neutralization steps may be continuous or batch and are not particularly limited, but are preferably continuous from the viewpoint of production efficiency and the like.

  The neutralization rate in the present invention is preferably 60 to 90 mol%, more preferably 60 to 85 mol%, still more preferably 60 to 80 mol%, particularly preferably 65 to 75 mol, based on the acid group of the monomer. Mol%. When the neutralization rate is less than 60 mol%, the water absorption ratio may decrease. On the other hand, when the neutralization rate exceeds 90 mol%, a water absorbent resin having a high water absorption capacity under pressure may not be obtained.

  The neutralization rate is the same even in the case of post-neutralization. Moreover, the said neutralization rate is applied also about the neutralization rate of the water absorbing resin as a final product. The neutralization rate of 75 mol% means a mixture of 25 mol% acrylic acid and 75 mol% acrylate. Moreover, this mixture may be called an acrylic acid partial neutralized material.

  In the present invention, the maximum temperature of the system in the neutralization step is 70 ° C. or higher, preferably 80 ° C. or higher, more preferably 90 ° C. or higher. In addition, as an upper limit, it should just be below the boiling point of monomer aqueous solution, Preferably it should just be 100 degrees C or less. The “boiling point” means the boiling point at normal pressure of the neutralization system.

  In the case of neutralization with respect to acrylic acid, that is, when the neutralized product is a monomer aqueous solution (hereinafter sometimes referred to as “neutralization solution”), the neutralization is performed after reaching the maximum temperature. The liquid is preferably cooled to 70 ° C. or lower, more preferably 50 ° C. or lower, still more preferably 30 ° C. or lower, and then polymerization may be performed. In addition, as a minimum temperature of cooling, it is about 0 degreeC from a viewpoint of the solubility of neutralization salt.

  In the present invention, it is preferable to perform the following operations after reaching the maximum temperature. That is, after reaching the maximum temperature, it is preferably 20 to 1200 seconds, more preferably 30 to 800 seconds, still more preferably 60 to 600 seconds, and particularly preferably 90 to 500 seconds (hereinafter, this time is referred to as “warming time”). It is preferable to keep the temperature at 70 ° C. or higher.

  In addition, the above-mentioned heat retention time includes the time for the monomer aqueous solution and the time for performing the polymerization reaction. Specifically, starting from the time when the temperature of the neutralization system reaches the maximum temperature (for example, 87 ° C.), the polymerization reaction starts at 80 ° C. after 40 seconds, and the temperature of the polymerization composition reaches the maximum after 70 seconds. Examples include a case where the temperature (boiling point of the polymerization composition) is reached and the temperature of the polymerization composition drops to 70 ° C. after 200 seconds. The temperature pattern is one of the most preferred embodiments of the present invention.

  In addition, the acrylic acid dimer can also be neutralized in this neutralization step to form an acrylic acid dimer salt. Therefore, the ratio (number of moles) of the acrylic acid dimer (including salt) in the aqueous monomer solution obtained in this step to the acrylic acid dimer in the acrylic acid before neutralization is preferably 80% or less, more Preferably it is 70% or less, More preferably, it is 60% or less, Most preferably, it is 50% or less.

  In addition, about other neutralization conditions, such as the apparatus which performs neutralization, and residence time, the conditions described in patent document 11 (International Publication No. 2009/123197) are applied to this invention.

(Internal crosslinking agent)
As the internal crosslinking agent used in the present invention, the compounds described in US Pat. No. 6,241,928 are also applied to the present invention. From these, one or more compounds are selected in consideration of reactivity.

  Further, from the viewpoint of water absorption performance and the like of the obtained water absorbent resin, preferably a compound having two or more polymerizable unsaturated groups, more preferably a compound having thermal decomposability at the following drying temperature, more preferably (poly) alkylene. A compound having two or more polymerizable unsaturated groups having a glycol structural unit is used as an internal cross-linking agent.

  The polymerizable unsaturated group is preferably an allyl group, a (meth) acrylate group, more preferably a (meth) acrylate group. Moreover, polyethylene glycol is preferable as the (poly) alkylene glycol structural unit, and the n number is preferably 1 to 100, more preferably 6 to 50.

  Therefore, in the present invention, (poly) alkylene glycol di (meth) acrylate or (poly) alkylene glycol tri (meth) acrylate is preferably used, and more preferably (poly) ethylene glycol di (meth) acrylate is used.

  The amount of the internal crosslinking agent used is preferably 0.005 to 2 mol%, more preferably 0.01 to 1 mol%, still more preferably 0.05 to 0.5 mol%, based on the entire monomer. It is. A desired water-absorbing resin can be obtained by setting the amount used within the above range. In addition, when there is too little this usage-amount, it exists in the tendency for gel strength to fall and a water soluble content to increase, and since there exists a tendency for a water absorption factor to fall when this usage-amount is too much, it is unpreferable.

  In the present invention, a method in which a predetermined amount of an internal cross-linking agent is previously added to the monomer aqueous solution and a cross-linking reaction is performed simultaneously with the polymerization is preferably applied. On the other hand, in addition to this method, a method of adding an internal cross-linking agent during or after polymerization and post-crosslinking, a method of radical cross-linking using a radical polymerization initiator, radiation using active energy rays such as electron beams and ultraviolet rays A method of crosslinking and the like can also be employed. Moreover, these methods can also be used together.

(Other substances added to the monomer aqueous solution)
In the present invention, from the viewpoint of improving the physical properties of the resulting water-absorbent resin, the following substances may be added during preparation of the aqueous monomer solution.

  Specifically, hydrophilic polymer such as starch, starch derivative, cellulose, cellulose derivative, polyvinyl alcohol, polyacrylic acid (salt), polyacrylic acid (salt) cross-linked product, preferably 50% by weight or less, more preferably Is added in an amount of 20% by weight or less, more preferably 10% by weight or less, particularly preferably 5% by weight or less (the lower limit is 0% by weight), foaming agents such as carbonates, azo compounds and bubbles, surfactants, chelating agents. An agent, a chain transfer agent and the like are preferably added at 5% by weight or less, more preferably 1% by weight or less, still more preferably 0.5% by weight or less (the lower limit is 0% by weight).

  Moreover, the said substance may be added not only in the form added to monomer aqueous solution but in the middle of superposition | polymerization, and these forms can also be used together.

  When a water-soluble resin or a water-absorbing resin is used as the hydrophilic polymer, a graft polymer or a water-absorbing resin composition (for example, starch-acrylic acid polymer, PVA-acrylic acid polymer, etc.) is obtained. It is done. These polymers and water-absorbing resin compositions are also within the scope of the present invention.

(2-2) Polymerization step In this step, the acrylic acid (salt) monomer aqueous solution obtained in the monomer aqueous solution preparation step is polymerized to form a hydrogel crosslinked polymer (hereinafter referred to as "hydrogel"). It is a process of obtaining.

(Polymerization initiator)
The polymerization initiator used in the present invention is not particularly limited because it is appropriately selected depending on the polymerization form and the like. For example, the thermal decomposition type polymerization initiator, the photodecomposition type polymerization initiator, or the decomposition of these polymerization initiators is used. And a redox polymerization initiator in combination with a reducing agent that promotes the reaction. Specifically, one or two polymerization initiators disclosed in U.S. Pat. Nos. 4,893,999, 6,241,928, 6,987,151, 6,671,141, U.S. Patent Application Publication No. 2005/0215734, and the like. More than seeds are used. From the viewpoint of handling of the polymerization initiator and physical properties of the water-absorbent resin, a peroxide or an azo compound is preferably used, more preferably a peroxide, and still more preferably a persulfate.

  The amount of the polymerization initiator used is preferably 0.001 to 1 mol%, more preferably 0.001 to 0.5 mol%, based on the monomer. The amount of the reducing agent used is preferably 0.0001 to 0.02 mol% with respect to the monomer.

  In addition, it may replace with the said polymerization initiator and may irradiate active energy rays, such as a radiation, an electron beam, and an ultraviolet-ray, and may implement a polymerization reaction, and these active energy rays and a polymerization initiator may be used together.

(Polymerization form)
The polymerization form applied to the present invention is not particularly limited, but from the viewpoint of water absorption characteristics and ease of polymerization control, preferably spray droplet polymerization, aqueous solution polymerization, reverse phase suspension polymerization, more preferably aqueous solution polymerization. , Reverse phase suspension polymerization, more preferably aqueous solution polymerization. Among them, continuous aqueous solution polymerization is particularly preferable, and either continuous belt polymerization or continuous kneader polymerization is applied, but continuous belt polymerization is most preferable.

  As specific polymerization forms, continuous belt polymerization is disclosed in U.S. Pat. Nos. 4,893,999, 6,241,928, 6,906,159, and 7,091,253, U.S. Patent Application Publication No. 2005/0215734, etc. Nos. 6987151 and 6710141, respectively. By adopting such continuous aqueous solution polymerization, the production efficiency of the water-absorbent resin is improved.

  Moreover, as a preferable form of the continuous aqueous solution polymerization, “high temperature initiation polymerization” and “high concentration polymerization” can be mentioned. Although these polymerization forms are described in detail in Patent Documents 5 to 8, they can be applied as they are as the polymerization forms of the present invention.

  Specifically, “high temperature initiation polymerization” means that the temperature of the monomer aqueous solution is preferably 60 ° C. or higher, more preferably 70 ° C. or higher, further preferably 80 ° C. or higher, particularly preferably 90 ° C. or higher (the upper limit is the boiling point). The polymerization starts at a temperature of), and “high concentration polymerization” means that the monomer concentration is preferably 30% by weight or more, more preferably 35% by weight or more, still more preferably 40% by weight or more, and particularly preferably 45%. It refers to a form in which polymerization is initiated at a weight percent or more (the upper limit is a saturated concentration). These polymerization forms can also be used in combination.

  In the above high temperature initiation polymerization, the temperature difference (ΔT) between the temperature at the start of polymerization and the highest temperature reached at the time of polymerization is preferably 60 ° C. or less, more preferably 50 ° C. or less, still more preferably 40 ° C. or less, particularly It is preferably controlled to be 30 ° C. or lower, and most preferably 25 ° C. or lower. When the ΔT exceeds 60 ° C., the physical properties of the resulting water-absorbent resin are lowered, which is not preferable.

  Moreover, in this invention, you may raise solid content of monomer aqueous solution during a superposition | polymerization period with said superposition | polymerization form. The solid content of the monomer aqueous solution is a value obtained from the following formula (1). In addition, also about the water-containing gel-like crosslinked polymer obtained after superposition | polymerization, let the value calculated | required from the following Formula (1) be solid content.

  In the above formula (1), the “component in the polymerization system” refers to a monomer aqueous solution, a graft component, a water-absorbing resin, and other solid components (for example, water-insoluble fine particles). Hydrophobic solvents in phase suspension polymerization are not included.

  In the present invention, the solid content of the hydrogel crosslinked polymer after polymerization is not particularly limited, but is preferably 30 to 80% by weight, more preferably 40 to 75% by weight, still more preferably 50 to 70% by weight, particularly Preferably it is 55 to 65% by weight. When the solid content exceeds 80% by weight, problems such as a reduction in water absorption and an increase in soluble content occur. On the other hand, when the solid content is less than 30% by weight, the load in the drying process is large, which is not preferable.

  Further, the solid content can be defined by the following formula (2), with the rate of change during the polymerization period (indicator of solid content increase) as the “concentration ratio”.

  Although the said concentration ratio in this invention is not specifically limited, Preferably it is 1.10 or more, More preferably, it is 1.15 or more, More preferably, it is 1.20 or more, Most preferably, it is 1.25 or more.

  In the high temperature initiation / high concentration polymerization preferably applied in the present invention, the maximum temperature reached during the polymerization period reaches the boiling point of the polymerization composition, and water vapor is generated. Therefore, the solid content increases during the polymerization period. The water vapor contains a raw material for a water-absorbing resin such as acrylic acid. Therefore, it is desired to recover the water vapor and reuse it as a monomer aqueous solution. In this case, the recovery rate of acrylic acid is preferably 0.5% by weight or more, more preferably 1% by weight or more, still more preferably 2% by weight, based on the weight of the total acrylic acid used (before neutralization). That's it.

  Moreover, in this invention, although superposition | polymerization can also be performed in an air atmosphere, it is preferable to superpose | polymerize in inert gas atmosphere, such as nitrogen and argon, from a viewpoint of the color tone of the water absorbent resin obtained. In addition, the above high temperature initiation polymerization is preferable because it can be carried out in an air atmosphere and it is not necessary to consider an accident (oxygen deficiency) of a worker.

(2-3) Gel pulverization step In this step, the hydrogel obtained in the polymerization step is subjected to gel pulverization with a screw pulverizer such as a kneader or meat chopper, or a gel pulverizer such as a cutter mill. This is a step of obtaining a hydrous gel (hereinafter referred to as “particulate hydrous gel”). In addition, when the said superposition | polymerization process is kneader polymerization, the superposition | polymerization process and the gel grinding | pulverization process are implemented simultaneously.

  Regarding gel grinding conditions and forms other than those described above, the contents disclosed in International Publication No. 2011/126079 are preferably applied to the present invention.

(2-4) Drying step This step is a step of obtaining a dry polymer by drying the particulate hydrogel obtained in the polymerization step and / or the gel grinding step to a desired resin solid content. The resin solid content is determined from loss on drying (weight change when 1 g of water-absorbent resin is heated at 180 ° C. for 3 hours), preferably 80% by weight or more, more preferably 85 to 99% by weight, and still more preferably 90%. It is -98 weight%, Most preferably, it is 92-97 weight%.

  The method for drying the particulate hydrous gel is not particularly limited. For example, heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drum dryer drying, azeotropy with a hydrophobic organic solvent. Examples include drying by dehydration and high-humidity drying using high-temperature steam. Among these, hot air drying is preferable from the viewpoint of drying efficiency, and band drying in which hot air drying is performed on a ventilation belt is more preferable.

  The drying temperature (hot air temperature) in the hot air drying is preferably 120 to 250 ° C, more preferably 150 to 200 ° C, from the viewpoint of the color tone of the water absorbent resin and the drying efficiency. The dew point of the hot air is not particularly limited, but is preferably 50 to 90 ° C, more preferably 60 to 80 ° C, and still more preferably 65 to 75 ° C from the viewpoint of reducing the residual monomer. If the dew point of the hot air is less than 50 ° C., the residual monomer reducing effect is small, and if the dew point of the hot air exceeds 90 ° C., condensation is likely to occur in the device and promotes deterioration of the device, which is not preferable. .

  Note that drying conditions other than the above drying temperature and dew point, such as the wind speed and drying time of hot air, can be appropriately set according to the moisture content and total weight of the particulate hydrous gel to be dried and the desired resin solid content. When performing band drying, various conditions described in International Publication Nos. 2006/100300, 2011/025012, 2011/025013, 2011/111657, etc. are appropriately applied. .

  By setting the above drying temperature, drying time, and dew point within the above ranges, the desired water-absorbent resin CRC (water absorption ratio), Ext (water-soluble), and color tone (see [3] below) It can be.

(2-5) Grinding step, classification step In this step, the dried polymer obtained in the drying step is pulverized (pulverization step), adjusted to a predetermined particle size (classification step), and water-absorbing resin powder ( This is a step of obtaining a powdery water-absorbing resin before surface cross-linking for the sake of convenience as “water-absorbing resin powder”.

  As an apparatus used in the pulverization process of the present invention, a roll mill and / or a roll granulator are essential, but as other combined apparatuses, for example, high-speed rotary pulverization such as a hammer mill, a screw mill, a pin mill, etc. Machine, vibration mill, knuckle type pulverizer, cylindrical mixer and the like.

  In addition, the particle size adjustment method in the classification step of the present invention is not particularly limited, and examples thereof include sieve classification using JIS standard sieve (JIS Z8801-1 (2000)), airflow classification, and the like. The particle size adjustment of the water-absorbing resin is not limited to the above pulverization step and classification step, but a polymerization step (especially reverse phase suspension polymerization or spray droplet polymerization), other steps (for example, granulation step, fine powder collection step) ).

  The water-absorbent resin powder obtained in the present invention has a weight average particle diameter (D50) of preferably 200 to 600 μm, more preferably 200 to 550 μm, still more preferably 250 to 500 μm, and particularly preferably 350 to 450 μm. The ratio of particles having a particle diameter of less than 150 μm is preferably 10% by weight or less, more preferably 5% by weight or less, and further preferably 1% by weight or less. The ratio of particles having a particle diameter of 850 μm or more is preferably 5% or less. % By weight or less, more preferably 3% by weight or less, still more preferably 1% by weight or less. The lower limit of the ratio of these particles is preferably as small as possible in any case, and is preferably 0% by weight, but may be about 0.1% by weight. Furthermore, the logarithmic standard deviation (σζ) of the particle size distribution is preferably 0.20 to 0.50, more preferably 0.25 to 0.40, and still more preferably 0.27 to 0.35. These particle sizes are measured using a standard sieve according to the measurement methods disclosed in US Pat. No. 7,638,570 and EDANA ERT420.2-02.

  The particle size described above is applied not only to the water-absorbent resin after surface crosslinking (hereinafter sometimes referred to as “water-absorbent resin particles” for convenience) but also to the water-absorbent resin as the final product. Therefore, in the water-absorbent resin particles, it is preferable that the surface cross-linking treatment (surface cross-linking step) is performed so as to maintain the particle size in the above range, and the particle size is adjusted by providing a sizing step after the surface cross-linking step. preferable.

(2-6) Surface cross-linking step This step is a step of providing a portion having a higher cross-linking density in the surface layer of the water-absorbent resin powder obtained through the above-described steps (portion of several tens of micrometers from the surface of the water-absorbent resin powder). It is comprised from a mixing process, a heat treatment process, and a cooling process (optional).

  In the surface cross-linking step, a water-absorbing resin (water-absorbing resin particles) surface-crosslinked by radical cross-linking or surface polymerization on the surface of the water-absorbent resin powder, a cross-linking reaction with a surface cross-linking agent, or the like is obtained.

(Surface cross-linking agent)
Although it does not specifically limit as a surface crosslinking agent used by this invention, An organic or inorganic surface crosslinking agent is mentioned. Among these, an organic surface cross-linking agent that reacts with a carboxyl group is preferable from the viewpoint of the physical properties of the water-absorbing resin and the handleability of the surface cross-linking agent. Examples thereof include one or more surface cross-linking agents (excluding polyvalent metal compounds) disclosed in US Pat. No. 6,071,976. Further, the disclosed contents of the US Patent Document are applied to the present invention regarding the amount of the surface cross-linking agent used, water as a solvent, and the type and amount of a hydrophilic organic solvent used as necessary.

(Mixing process)
This step is a step of mixing the water absorbent resin powder and the surface cross-linking agent. The method for mixing the surface cross-linking agent is not particularly limited, but a surface cross-linking agent solution is prepared in advance, and the liquid is preferably sprayed or dropped on the water-absorbent resin powder, and more preferably sprayed. And mixing them.

  An apparatus for performing the mixing is not particularly limited, but a high-speed stirring type mixer is preferable, and a high-speed stirring type continuous mixer is more preferable.

(Heat treatment process)
In this step, heat is applied to the mixture discharged from the mixing step to cause a crosslinking reaction on the surface of the water absorbent resin powder.

  As for the heat treatment conditions in the heat treatment step, the contents disclosed in US Pat. No. 6,071,976 are applied to the present invention.

(Cooling process)
This step is an optional step that is installed as necessary after the heat treatment step.

  The apparatus for performing the cooling is not particularly limited, but is preferably an apparatus having the same specifications as the apparatus used in the heat treatment step, and more preferably a paddle dryer. It is because it can be used as a cooling device by changing the heat medium to a refrigerant. In the cooling step, the water-absorbent resin particles obtained in the heat treatment step are forcibly cooled to 40 to 80 ° C., more preferably 50 to 70 ° C. as necessary.

(2-7) Rehumidification step This step is performed by adding the following polyvalent metal salt compound, polycationic polymer, chelating agent, inorganic reducing agent, hydroxycarboxylic acid compound to the water absorbent resin particles obtained in the surface cross-linking step. A step of adding at least one additive selected from the group consisting of:

  In addition, since the said additive is added with aqueous solution or slurry liquid, a water-absorbent resin particle is swollen with water again. For this reason, this process is referred to as a “rehumidification process”. As described above, the additive can be mixed with the water-absorbent resin powder simultaneously with the surface cross-linking agent (aqueous solution).

(Polyvalent metal salt and / or cationic polymer)
In the present invention, it is preferable to add a polyvalent metal salt and / or a cationic polymer from the viewpoint of the water absorption rate, liquid permeability, hygroscopic fluidity and the like of the water absorbent resin obtained.

  As the polyvalent metal salt and / or cationic polymer, specifically, compounds disclosed in “[7] Polyvalent metal salt and / or cationic polymer” of International Publication No. 2011/040530 and the amount of use thereof Applies to the present invention.

(Chelating agent)
In the present invention, it is preferable to add a chelating agent from the viewpoint of the color tone (coloring prevention) and deterioration prevention of the water-absorbent resin obtained.

  As the chelating agent, specifically, the compounds disclosed in “[2] chelating agent” of WO 2011/040530 and the amount used thereof are applied to the present invention.

(Inorganic reducing agent)
In the present invention, it is preferable to add an inorganic reducing agent from the viewpoints of color tone (anti-coloring), deterioration prevention, residual monomer reduction and the like of the water-absorbing resin obtained.

  As the inorganic reducing agent, specifically, compounds disclosed in “[3] Inorganic reducing agent” of WO 2011/040530 and the amount used thereof are applied to the present invention.

(Α-hydroxycarboxylic acid compound)
In this invention, it is preferable to add (alpha) -hydroxycarboxylic acid from viewpoints, such as color tone (coloring prevention) of the water absorbing resin obtained. The “α-hydroxycarboxylic acid compound” is a carboxylic acid having a hydroxyl group in the molecule or a salt thereof, and is a hydroxycarboxylic acid having a hydroxyl group at the α-position.

  As the α-hydroxycarboxylic acid compound, specifically, the compounds disclosed in “[6] α-hydroxycarboxylic acid compound” of International Publication No. 2011/040530 and the amount used thereof are applied to the present invention. .

(2-8) Other additive addition process In this invention, in order to add various functions to water-absorbent resin, additives other than the additive mentioned above can also be added. Specific examples of such additives include surfactants, compounds having phosphorus atoms, oxidizing agents, organic reducing agents, water-insoluble inorganic fine particles, organic powders such as metal soaps, deodorants, antibacterial agents, pulp and thermoplastics. Examples thereof include fibers. The surfactant is a compound disclosed in International Publication No. 2005/077500, and the water-insoluble inorganic fine particles are disclosed in International Publication No. 2011/040530 “[5] Water-insoluble inorganic fine particles”. Each of these compounds is applied to the present invention.

  The amount of the additive used (addition amount) is appropriately determined depending on the intended use of the water absorbent resin to be obtained, and is not particularly limited, but is preferably 3 parts by weight or less with respect to 100 parts by weight of the water absorbent resin powder. More preferably, it is 1 part by weight or less. Moreover, this additive can be added in any manufacturing process of polyacrylic acid (salt) type water absorbing resin.

(2-9) Other Steps In the present invention, in addition to the steps described above, a granulation step, a sizing step, a fine powder removal step, a fine powder reuse step, and the like can be provided as necessary. Moreover, you may further include 1 type, or 2 or more types of processes, such as a transport process, a storage process, a packing process, and a storage process. In addition, the “granulating step” includes a step of performing classification and pulverization when the fine powder removing step after the surface cross-linking step and the water absorbent resin are aggregated and exceed a desired size. In addition, the “fine powder recycling step” includes a step of adding the fine powder as it is as in the present invention to a large water-containing gel and adding it to any step of the production process of the water absorbent resin.

[3] Physical properties of polyacrylic acid (salt) water-absorbing resin The polyacrylic acid (salt) water-absorbing resin obtained by the production method according to the present invention uses the water-absorbing resin in sanitary goods, particularly paper diapers. In the case, among the physical properties listed in the following (3-1) to (3-10), at least one or more, preferably two or more including AAP, more preferably three or more including AAP, Most preferably, it is desired to control all physical properties within a desired range. When these physical properties do not satisfy the following ranges, the effects of the present invention cannot be obtained sufficiently, and there is a possibility that sufficient performance will not be exhibited in high-concentration paper diapers.

  In addition, the polyacrylic acid (salt) water-absorbing resin obtained by the production method according to the present invention is not particularly limited with respect to its shape, but is preferably in the form of particles. In this section, the physical properties of the particulate water-absorbing resin which is a preferred embodiment will be described. The following physical properties were measured according to the EADANA method unless otherwise specified.

(3-1) CRC (absorption capacity under no pressure)
The CRC (water absorption capacity under no pressure) of the water absorbent resin of the present invention is usually 5 g / g or more, preferably 15 g / g or more, more preferably 25 g / g or more. The upper limit is not particularly limited and is preferably as high as possible, but from the viewpoint of balance with other physical properties, it is preferably 70 g / g or less, more preferably 50 g / g or less, and still more preferably 40 g / g or less.

  When the CRC is less than 5 g / g, the amount of absorption is small and it is not suitable as an absorbent material for sanitary goods such as paper diapers. In addition, when the CRC exceeds 70 g / g, the rate of absorbing body fluids such as urine and blood is reduced, so that it is not suitable for use in high water absorption type paper diapers. CRC can be controlled by an internal crosslinking agent, a surface crosslinking agent, or the like.

(3-2) AAP (absorption capacity under pressure)
The AAP (water absorption capacity under pressure) of the water absorbent resin of the present invention is preferably 20 g / g or more, more preferably 22 g / g or more, still more preferably 23 g / g or more, particularly preferably 24 g / g or more, most preferably It is 25 g / g or more. The upper limit is not particularly limited, but is preferably 30 g / g or less.

  When the AAP is less than 20 g / g, the amount of liquid returned when pressure is applied to the absorbent body (usually referred to as “Re-Wet”) increases, and the absorbent article for sanitary goods such as paper diapers. Not suitable as. AAP can be controlled by particle size, surface cross-linking agent, and the like.

(3-3) Particle size (particle size distribution, weight average particle size (D50), logarithmic standard deviation (σζ) of particle size distribution)
The particle size (particle size distribution, weight average particle size (D50), logarithmic standard deviation (σζ)) of the water absorbent resin of the present invention is the same as the particle size of the water absorbent resin powder before surface crosslinking. Controlled.

(3-4) Ext (water-soluble component)
The Ext (water-soluble content) of the water-absorbent resin of the present invention is usually 50% by weight or less, preferably 35% by weight or less, more preferably 25% by weight or less, and further preferably 15% by weight or less. Although it does not specifically limit about a lower limit, Preferably it is 0 weight%, More preferably, it is about 0.1 weight%.

  When this Ext exceeds 50 weight%, there exists a possibility that it may become a water absorbing resin with weak gel strength and inferior liquid permeability. Furthermore, since rewetting increases, it is not suitable as an absorbent material for sanitary goods such as paper diapers. Ext can be controlled with an internal cross-linking agent or the like.

(3-5) Water content The water content of the water-absorbent resin of the present invention is preferably more than 0% by weight and 15% by weight or less, more preferably 1 to 13% by weight, still more preferably 2 to 10% by weight, particularly Preferably, it is 2 to 9% by weight.

  By setting the moisture content within the above range, a water-absorbing resin excellent in powder characteristics (for example, fluidity, transportability, damage resistance, etc.) can be obtained.

(3-6) Residual monomer From the viewpoint of safety, the residual monomer contained in the water-absorbent resin of the present invention is preferably 500 ppm or less, more preferably 400 ppm or less, and even more preferably 300 ppm or less. Although it does not specifically limit about a lower limit, Preferably it is 0 ppm, More preferably, it is about 10 ppm.

  By setting the content of the residual monomer within the above range, a water-absorbing resin that can reduce irritation to human skin and the like can be obtained.

(3-7) SFC (saline flow conductivity)
The SFC (saline flow conductivity) of the water-absorbent resin of the present invention is preferably 50 × 10 ⁻⁷ · cm ³ · s · g ⁻¹ or more, more preferably 60 × 10 ⁻⁷ · cm ³ · s ·. g ⁻¹ or more, more preferably 70 × 10 ⁻⁷ · cm ³ · s · g ⁻¹ or more, and particularly preferably 80 × 10 ⁻⁷ · cm ³ · s · g ⁻¹ or more. Although it does not specifically limit about an upper limit, Preferably it is 3000 * 10 <^-7> * cm < ^{3 >} * s <^-1> or less, More preferably, it is 2000 * 10 <^-7> * cm < ^{3 >} * s * g <^-1> or less.

When the SFC is less than 50 × 10 ⁻⁷ · cm ³ · s · g ⁻¹ , the liquid permeability of body fluids such as urine and blood is low, so it is not suitable as an absorbent material for sanitary goods such as paper diapers. In addition, when the SFC exceeds 3000 × 10 ⁻⁷ · cm ³ · s · g ⁻¹ , body fluids such as urine and blood may not be sufficiently absorbed and liquid leakage may occur. Not suitable as a sanitary product absorber. SFC can be controlled by particle size, surface cross-linking agent, polyvalent metal salt, cationic polymer, and the like.

(3-8) FSR (Water absorption rate)
The FSR (water absorption rate) of the water absorbent resin of the present invention is preferably 0.10 g / g / s or more, more preferably 0.15 g / g / s or more, still more preferably 0.20 g / g / s or more, Preferably it is 0.25 g / g / s or more. Although it does not specifically limit about an upper limit, Preferably it is 5.0 g / g / s or less, More preferably, it is 3.0 g / g / s or less.

  If the FSR is less than 0.10 g / g / s, body fluids such as urine and blood may not be sufficiently absorbed and liquid leakage may occur, which is not suitable as an absorbent material for sanitary articles such as paper diapers. . The FSR can be controlled by foam polymerization, particle size, and the like.

(3-9) Initial color tone The initial color tone of the water-absorbent resin of the present invention is preferably 88 or more, more preferably 89 or more, and still more preferably 90 or more in the Hunter Lab color system. The upper limit is 100, but if it shows at least 88, there will be no problem with color tone. The a value is preferably −3 to 3, more preferably −2 to 2, and still more preferably −1 to 1. Furthermore, b value becomes like this. Preferably it is 0-12, More preferably, it is 0-10, More preferably, it is 0-9. In addition, the whiteness increases as the L value approaches 100, and the a value and the b value become substantially white with low coloration as the value approaches 0.

(3-10) Color tone with time The color tone with time of the water-absorbent resin of the present invention is preferably 80 or more, more preferably 81 or more, still more preferably 82 or more, and particularly preferably 83 or more in the Hunter Lab color system. It is. The upper limit is 100, but if it shows at least 80, there will be no problem with color tone. The a value is preferably −3 to 3, more preferably −2 to 2, and still more preferably −1 to 1. Furthermore, b value becomes like this. Preferably it is 0-15, More preferably, it is 0-12, More preferably, it is 0-10. In addition, the whiteness increases as the L value approaches 100, and the a value and the b value become substantially white with low coloration as the value approaches 0.

[4] Use of polyacrylic acid (salt) -based water-absorbing resin The use of the water-absorbing resin of the present invention is not particularly limited, but is preferably used as an absorbent material for sanitary products such as paper diapers, sanitary napkins, and incontinence pads. Can be mentioned. In particular, it can be used as an absorber for high-concentration paper diapers (a large amount of water-absorbing resin used per paper diaper), which has been a problem with odor, coloring and the like derived from raw materials. Furthermore, a remarkable effect can be expected when used in the upper layer of the absorber.

  In addition to the water-absorbent resin, an absorbent material such as pulp fiber can be used as the absorbent body. In this case, the content (core concentration) of the water absorbent resin in the absorbent body is preferably 30 to 100% by weight, more preferably 40 to 100% by weight, still more preferably 50 to 100% by weight, and still more preferably. 60 to 100% by weight, particularly preferably 70 to 100% by weight, and most preferably 75 to 95% by weight.

  By making the said core density | concentration into the said range, when this absorber is used for the upper layer part of an absorbent article, an absorbent article can maintain the white state with a clean feeling. Furthermore, since the diffusibility of body fluids such as urine and blood is excellent, the amount of absorption can be improved by efficient liquid distribution.

  The present invention will be described more specifically with reference to the following examples and comparative examples. However, the present invention is not construed as being limited thereto, and examples obtained by appropriately combining technical means disclosed in each example. Are also included in the scope of the present invention.

  In addition, as long as there is no notice, the electrical device (including the physical property measurement of a water absorbing resin) used by a manufacture example, an Example, and a comparative example used the power supply of 200V or 100V. Further, various physical properties of the water-absorbent resin of the present invention were measured under conditions of room temperature (20 to 25 ° C.) and relative humidity of 50% RH ± 10% unless otherwise noted.

  For convenience, “liter” may be expressed as “l” or “L”, and “wt%” may be expressed as “wt%”. Further, in the measurement of trace components, the value below the detection limit is expressed as “ND” (Non Detected).

[Measurement of physical properties of water-absorbing resin]
(A) CRC (absorption capacity under no pressure)
The CRC (water absorption capacity under no pressure) of the water absorbent resin of the present invention was measured according to the EDANA method (ERT441.2-02).

(B) AAP (water absorption magnification under pressure)
The AAP (water absorption capacity under pressure) of the water-absorbent resin of the present invention was measured according to the EDANA method (ERT442.2-02). The load condition was changed to 4.83 kPa (0.7 psi).

(C) Particle size (particle size distribution, weight average particle size (D50), logarithmic standard deviation of particle size distribution (σζ))
The particle size (particle size distribution, weight average particle size (D50), logarithmic standard deviation (σζ)) of the water-absorbent resin of the present invention is described in columns 27 and 28 of US Pat. No. 7,638,570 “(3) Mass-Average Particle Diameter (D50) and Logical Standard Deviation (σζ) of Particle Diameter Distribution ”.

(D) Ext (water soluble component)
The Ext (water-soluble content) of the water-absorbent resin of the present invention was measured according to the EDANA method (ERT470.2-02).

(E) Water content and resin solid content The water content of the water-absorbent resin of the present invention was measured in accordance with the EDANA method (ERT430.2-02). In the present invention, the measurement was performed by changing the sample amount to 1.0 g and the drying temperature to 180 ° C.

  In addition, resin solid content (weight%) was prescribed | regulated by (100-water content) (weight%).

(F) Residual monomer and trace components (acetic acid, acrylic acid dimer, etc.)
The residual monomer contained in the water absorbent resin of the present invention was measured according to the EDANA method (ERT410.2-02). Moreover, it measured based on EDANA method (ERT410.2-02) also about trace components (for example, an acetic acid, acrylic acid dimer, etc.) other than a residual monomer.

  Further, the trace amount component (for example, acetic acid and the like) in the hydrogel crosslinked polymer was measured by changing the sample amount to 2 g and the stirring time to 24 hours.

(G) SFC (saline flow conductivity)
The SFC (saline flow conductivity) of the water-absorbent resin of the present invention was measured according to the measurement method disclosed in US Pat. No. 5,669,894.

(H) FSR (Water absorption speed)
The FSR (water absorption rate) of the water-absorbent resin of the present invention was measured based on the measurement method disclosed in International Publication No. 2011/078298.

(I) Initial color tone and temporal color tone The initial color tone and temporal color tone of the water-absorbent resin of the present invention were measured in accordance with the measurement method disclosed in International Publication No. 2011/040530.

[Production Example 1]
When commercially available acrylic acid was obtained and the contents of acrylic acid dimer and p-methoxyphenol were measured, the acrylic acid dimer was 5000 ppm and p-methoxyphenol was 70 ppm. The acrylic acid was referred to as acrylic acid (A) for convenience.

  Next, by re-distilling the acrylic acid (A), repurified acrylic acid having an acrylic acid dimer content of 80 ppm and a p-methoxyphenol content of 0.1 ppm was obtained. Thereafter, p-methoxyphenol was added to prepare acrylic acid (B) having a content of 70 ppm.

  Next, the acrylic acid (A) and acrylic acid (B) were mixed at the following weight ratios to produce three types of acrylic acid having different acrylic acid dimer contents.

  That is, acrylic acid (1) is acrylic acid (A): acrylic acid (B) = 1: 53.9 (weight ratio), and acrylic acid (2) is acrylic acid (A): acrylic acid (B). = 1: 8.65 (weight ratio), and acrylic acid (3) was obtained by mixing acrylic acid (A): acrylic acid (B) = 1: 3.97 (weight ratio). The acrylic acid dimer content was 170 ppm for acrylic acid (1), 590 ppm for acrylic acid (2), and 1070 ppm for acrylic acid (3). Further, the acrylic acid content of p-methoxyphenol was 70 ppm.

[Example 1]
About acrylic acid (1) obtained by the said manufacture example 1, the following operation was performed.

  That is, 48.5 wt% sodium hydroxide aqueous solution 297 g / min, acrylic acid (1) 367 g / min, 30 wt% polyethylene glycol diacrylate (number average molecular weight 523) aqueous solution 4.0 g / min, 20 wt% 4.5 g / min of a solution prepared by dissolving 0.989 parts by weight of 2-hydroxymethyl-2-methylpropiophenone, 1.08 parts by weight of trisodium diethylenetriaminepentaacetate in 97.9 parts by weight of an aqueous acrylic acid solution, and ion-exchanged water 314 g / min was continuously supplied to the mixer to prepare a mixed solution (1). At this time, the mixed liquid (1) rose to a maximum temperature of 95 ° C. due to heat of dissolution and heat of neutralization.

  In Example 1, the neutralization rate was 70 mol% and the monomer concentration was 45 wt%. The neutralized acrylic acid dimer was 90 ppm in terms of the content in acrylic acid.

  Next, when the temperature of the mixed liquid (1) is lowered to 90 ° C., 20.1 g / min of a 3 wt% aqueous sodium persulfate solution is added to the mixed liquid (1) to obtain an aqueous monomer solution ( After 1), it was continuously fed to the polymerization apparatus. Starting from the time when the temperature of the mixed solution (1) reached the maximum temperature, the time until the monomer aqueous solution (1) was supplied to the polymerization apparatus was 300 seconds.

  The polymerization apparatus is a continuous belt polymerization machine having an effective length of 3.2 m, which is defined by the length of time until the hydrogel crosslinked polymer is discharged from the supply port of the aqueous monomer solution. The belt was an endless belt in which the surface of a glass fiber substrate was coated with a fluororesin, and the running speed was set to 1.7 m / min. The surface temperature of the belt was kept at about 100 ° C.

  The monomer aqueous solution (1) was continuously supplied on the belt of the continuous belt polymerization machine so as to have a thickness of 4.9 cm. The temperature of the monomer aqueous solution (1) at this time was 88 ° C., and this temperature was set as the polymerization start temperature.

  The aqueous monomer solution (1) supplied on the belt quickly started polymerization and expanded while generating water vapor. In addition, 109 ° C. (the boiling point of the polymerization composition (1)) was recorded as the highest temperature reached. In the expansion process, the water-containing gel-like crosslinked polymer was irregularly peeled from the belt, but contracted about 60 seconds after the start of polymerization.

Subsequently, the contracted water-containing gel-like crosslinked polymer was irradiated with ultraviolet rays having a light amount of 210 mJ / cm ² using an ultraviolet irradiation device (VB15201BY / USHIO). In addition, the light quantity of the ultraviolet rays was measured using an ultraviolet ray integrating light meter (UIT-150 / Ushio Electric) equipped with a light receiver (UVD-S365 / Ushio Electric). By the above operation, a sheet-like hydrogel crosslinked polymer (1) was obtained.

  The obtained sheet-like hydrogel crosslinked polymer (1) was subjected to gel pulverization with a meat chopper (VR-400K / Iizuka Kogyo; die hole diameter 10 mm), and then heated with a hot air dryer at a temperature of 180 ° C. and a dew point of 70 ° C. Drying for a minute gave a dry polymer (1).

  After the dried polymer (1) is pulverized with a roll mill, it is classified using a JIS standard sieve having openings of 850 μm, 710 μm, 600 μm, 500 μm, 300 μm, and 150 μm, and mixed at the following weight ratios to absorb water: Resin powder (1) was obtained.

  That is, 15% by weight of particles having a particle size of 150 μm or more and less than 300 μm, 45% by weight of particles of 300 μm or more and less than 500 μm, 25% by weight of particles of 500 μm or more and less than 600 μm, and 10% by weight of particles of 600 μm or more and less than 710 μm. %, Particles of 710 μm or more and less than 850 μm were mixed so as to be 5% by weight.

  Table 1 shows the physical properties (CRC, residual monomer, soluble content) of the water-absorbent resin powder (1) obtained by the above operation.

  Subsequently, surface cross-linking consisting of 0.34 parts by weight of 1,4-butanediol, 0.56 parts by weight of propylene glycol and 3.0 parts by weight of ion-exchanged water with respect to 100 parts by weight of the water absorbent resin powder (1). The agent solution was uniformly mixed, heat-treated at 208 ° C. for 40 minutes, and then forcedly cooled to 60 ° C. (surface crosslinking step).

  Thereafter, the mixture was pulverized until passing through a 850 μm JIS standard sieve (granulation step) to obtain a surface-crosslinked water-absorbing resin (1). Table 1 shows the physical properties (CRC, AAP, residual monomer) of the water absorbent resin (1) obtained.

[Example 2]
Except having changed into acrylic acid (2) in the said Example 1, operation similar to Example 1 was performed and the water absorbent resin powder (2) was obtained. The physical properties (CRC, residual monomer, soluble content) of the water-absorbent resin powder (2) obtained are shown in Table 1. In addition, the acrylic acid dimer after neutralization was 260 ppm when converted to the content in acrylic acid.

  Then, the surface-crosslinked water-absorbing resin (2) was obtained by performing the same surface-crosslinking step and sizing step as in Example 1. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the water absorbent resin (2) obtained.

[Example 3]
Except having changed into acrylic acid (3) in the said Example 1, operation similar to Example 1 was performed and the water absorbing resin powder (3) was obtained. The physical properties (CRC, residual monomer, soluble content) of the water-absorbent resin powder (3) obtained are shown in Table 1. In addition, the acrylic acid dimer after neutralization was 540 ppm when converted to the content in acrylic acid.

  Then, the surface-crosslinked water-absorbing resin (3) was obtained by performing the same surface cross-linking step and granulating step as in Example 1. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the water absorbent resin (3) obtained.

[Example 4]
After putting 300 g of the mixed liquid (1) obtained by continuous neutralization of Example 1 into a separable flask having a volume of 500 ml, the mixture was immediately immersed in an ice water bath and rapidly cooled to 20 ° C. When the temperature of the mixed solution (1) reached around 21 ° C., the ice water bath was removed and an air bath was used.

  Subsequently, when the temperature of the mixed liquid (1) reached 20 ° C. in a nitrogen atmosphere, 2.4 g of a 5 wt% aqueous sodium persulfate solution and 0.5 g of a 1 wt% L-ascorbic acid aqueous solution were added, It added under stirring and was set as monomer aqueous solution (4).

  After 10 minutes, the monomer aqueous solution (4) gradually became cloudy and polymerization started. The temperature of the monomer aqueous solution (4) at this time was 30 ° C., and this temperature was used as the polymerization start temperature.

  About 30 minutes after the start of polymerization, 85 ° C. was recorded as the maximum temperature reached. From this point, the separable flask was immersed in a 85 ° C. hot water bath and heating was continued for 1 hour. Thereafter, the hot water bath was removed and the mixture was allowed to cool, and the hydrogel crosslinked polymer (4) was taken out.

  The obtained hydrogel crosslinked polymer (4) was subjected to gel pulverization with a meat chopper (VR-400K / Iizuka Kogyo; die pore diameter 10 mm) and then dried for 30 minutes in a hot air dryer with a temperature of 180 ° C. and a dew point of 70 ° C. A dry polymer (4) was obtained.

  After the dried polymer (4) is pulverized with a roll mill, it is classified using a JIS standard sieve having openings of 850 μm, 710 μm, 600 μm, 500 μm, 300 μm, and 150 μm, and mixed at the following weight ratios to absorb water: Resin powder (4) was obtained.

  That is, 15% by weight of particles having a particle size of 150 μm or more and less than 300 μm, 45% by weight of particles of 300 μm or more and less than 500 μm, 25% by weight of particles of 500 μm or more and less than 600 μm, and 10% by weight of particles of 600 μm or more and less than 710 μm. %, Particles of 710 μm or more and less than 850 μm were mixed so as to be 5% by weight.

  Table 1 shows the physical properties (CRC, residual monomer, soluble content) of the water-absorbent resin powder (4) obtained by the above operation.

  Subsequently, a surface-crosslinked water-absorbing resin (4) was obtained by carrying out the same surface-crosslinking step and sizing step as in Example 1. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the water absorbent resin (4) obtained.

[Example 5]
Except having changed into acrylic acid (2) in the said Example 4, operation similar to Example 4 was performed and the water absorbing resin powder (5) was obtained. Table 1 shows the physical properties (CRC, residual monomer, soluble content) of the water-absorbent resin powder (5) obtained. In addition, the acrylic acid dimer after neutralization was 260 ppm when converted to the content in acrylic acid.

  Subsequently, a surface-crosslinked water-absorbing resin (5) was obtained by performing the same surface-crosslinking step and sizing step as in Example 4. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the water absorbent resin (5) obtained.

[Example 6]
Except having changed into acrylic acid (3) in the said Example 4, operation similar to Example 4 was performed and the water absorbent resin powder (6) was obtained. The physical properties (CRC, residual monomer, soluble content) of the water absorbent resin powder (6) obtained are shown in Table 1. In addition, the acrylic acid dimer after neutralization was 540 ppm when converted to the content in acrylic acid.

  Subsequently, a surface-crosslinked water-absorbing resin (6) was obtained by carrying out the same surface-crosslinking step and sizing step as in Example 4. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the water absorbent resin (6) obtained.

[Comparative Example 1]
After charging 79.2 g of acrylic acid (1) obtained in Production Example 1, 0.52 g of polyethylene glycol diacrylate (number average molecular weight 523), and 57.3 g of ion-exchanged water into a separable flask having a volume of 500 ml, The mixture was stirred with a magnetic stirrer under a nitrogen atmosphere. The separable flask was immersed in an ice water bath.

  Subsequently, a solution obtained by adding 100 g of ion-exchanged water to 63.5 g of a 48.5 wt% aqueous sodium hydroxide solution was gradually added. At this time, neutralization was performed while removing heat so that the liquid temperature in the separable flask did not exceed 30 ° C.

  In Comparative Example 1, the neutralization rate was 70 mol% and the monomer concentration was 30 wt%. The neutralized acrylic acid dimer was 200 ppm in terms of the content in acrylic acid.

  Subsequently, when the liquid temperature in the separable flask reached 20 ° C. in a nitrogen atmosphere, 2.4 g of a 5 wt% aqueous sodium persulfate solution and 0.5 g of a 1 wt% aqueous L-ascorbic acid solution were added, It added under stirring and it was set as comparative monomer aqueous solution (1).

  After 10 minutes, the comparative monomer aqueous solution (1) gradually became cloudy and polymerization started. The temperature of the comparative monomer aqueous solution (1) at this time was 30 ° C., and this temperature was used as the polymerization start temperature.

  About 30 minutes after the start of polymerization, 82 ° C. was recorded as the maximum temperature reached. From this point, the separable flask was immersed in a 85 ° C. hot water bath and heating was continued for 1 hour. Thereafter, the hot water bath was removed and the mixture was allowed to cool, and the comparative hydrogel crosslinked polymer (1) was taken out.

  The obtained comparative hydrous gel-like cross-linked polymer (1) was subjected to gel pulverization with a meat chopper (VR-400K / Iizuka Kogyo; dice pore diameter 10 mm), and then dried in a hot air drier at a temperature of 180 ° C. and a dew point of 70 ° C. for 30 minutes. As a result, a comparative dry polymer (1) was obtained.

  After the comparative dry polymer (1) was pulverized with a roll mill, it was classified using a JIS standard sieve having openings of 850 μm, 710 μm, 600 μm, 500 μm, 300 μm, and 150 μm, and mixed at the following weight ratios for comparison. Water-absorbent resin powder (1) was obtained.

  That is, 15% by weight of particles having a particle size of 150 μm or more and less than 300 μm, 45% by weight of particles of 300 μm or more and less than 500 μm, 25% by weight of particles of 500 μm or more and less than 600 μm, and 10% by weight of particles of 600 μm or more and less than 710 μm. %, Particles of 710 μm or more and less than 850 μm were mixed so as to be 5% by weight.

  Table 1 shows the physical properties (CRC, residual monomer, soluble content) of the comparative water absorbent resin powder (1) obtained by the above operation.

  Subsequently, a surface-crosslinked comparative water-absorbing resin (1) was obtained by performing the same surface-crosslinking step and sizing step as in Example 1. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the comparative water absorbent resin (1) obtained.

[Comparative Example 2]
A comparative water absorbent resin powder (2) was obtained in the same manner as in Comparative Example 1 except that the acrylic acid (2) was changed. Table 1 shows the physical properties (CRC, residual monomer, soluble content) of the comparative water absorbent resin powder (2) obtained. In addition, the acrylic acid dimer after neutralization was 600 ppm when converted to the content in acrylic acid.

  Subsequently, a surface-crosslinked comparative water-absorbing resin (2) was obtained by performing the same surface cross-linking step and sizing step as in Comparative Example 1. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the comparative water absorbent resin (2) obtained.

[Comparative Example 3]
A comparative water absorbent resin powder (3) was obtained in the same manner as in Comparative Example 1 except that the acrylic acid (3) was changed. Table 1 shows the physical properties (CRC, residual monomer, soluble content) of the comparative water absorbent resin powder (3) obtained. In addition, the acrylic acid dimer after neutralization was 1100 ppm when converted to the content in acrylic acid.

  Subsequently, a surface-crosslinked comparative water-absorbing resin (3) was obtained by performing the same surface cross-linking step and granulation step as in Comparative Example 1. Table 1 shows the physical properties (CRC, AAP, residual monomer) of the comparative water absorbent resin (3) obtained.

(Summary)
About the influence of the temperature at the time of neutralization, in Examples 1-6 and Comparative Examples 1-3, although the neutralization rate is the same as 70 mol%, when the maximum temperature at the time of neutralization is 95 degreeC (Examples 1- 3) and the case where it does not exceed 30 ° C. (Comparative Examples 1 to 3), the acrylic acid dimer content after neutralization is greatly different. It can be seen that the acrylic acid dimer is reduced by increasing the temperature during neutralization.

  Moreover, about the influence of a polymerization method, it is judged from the comparison with Examples 1-3 and Examples 4-6 that the highest reached temperature at the time of superposition | polymerization has affected the amount of residual monomers in a water absorbent resin powder. . That is, it can be seen that in Examples 1 to 3, in which the maximum temperature reached during polymerization was 109 ° C, the residual monomer was reduced compared to Examples 4 to 6 in which the maximum temperature reached during polymerization was 85 ° C.

  That is, since the decomposition of the acrylic acid dimer is promoted by setting it to a high temperature state in the neutralization step or the polymerization step, the acrylic acid dimer incorporated in the main chain portion of the water absorbent resin is reduced. Therefore, since the acrylic acid dimer to be decomposed is relatively decreased by the heating operation in the drying step or the surface cross-linking step, it is estimated that the residual monomer is reduced as a result.

  The method for producing a polyacrylic acid (salt) water-absorbing resin according to the present invention can be applied to production of water-absorbing resin using acrylic acid containing a large amount of acrylic acid dimer, particularly mass production. Moreover, the water-absorbent resin obtained by the present invention is suitable for use as an absorbent body for sanitary goods such as paper diapers.

Claims (11)

A method for producing a polyacrylic acid (salt) water-absorbing resin, comprising a step of preparing an aqueous monomer solution with acrylic acid having an acrylic acid dimer content of 150 to 2000 ppm, and a step of polymerizing the aqueous monomer solution. There,
The step of preparing the aqueous monomer solution further includes a step of neutralizing the acrylic acid with a basic compound, and the maximum temperature in the neutralization step is 70 ° C. or higher. Manufacturing method of resin.   The manufacturing method according to claim 1, wherein p-methoxyphenol is further contained in the acrylic acid, and the content thereof is 50 to 150 ppm.   3. The production method according to claim 1, wherein in the neutralization step, after reaching the maximum temperature, the time for keeping the temperature at 70 ° C. or more is 20 to 1200 seconds.   The said neutralization process WHEREIN: The manufacturing method of any one of Claims 1-3 which cools the neutralization liquid which made the maximum temperature 70 degreeC or more to 70 degrees C or less.   The ratio (number of moles) of acrylic acid dimer (including salt) in the aqueous monomer solution obtained in the neutralization step to acrylic acid dimer in acrylic acid before neutralization is 80% or less. 3. The production method according to 3 or 4.   The manufacturing method of any one of Claims 3-5 whose neutralization rate of acrylic acid (salt) obtained at the said neutralization process is 60-90 mol%.   The production method according to any one of claims 1 to 6, wherein polymerization is started at a monomer concentration of the monomer aqueous solution of 30% by weight or more.   The polymerization process according to any one of claims 1 to 7, wherein the polymerization step is a high temperature initiation polymerization that starts polymerization at a temperature of the monomer aqueous solution of 60 ° C or higher, and the highest temperature reached in the polymerization step is the boiling point of the monomer aqueous solution. The production method according to claim 1.   The manufacturing method of any one of Claims 1-8 which performs the said superposition | polymerization process in air atmosphere.   The method according to any one of claims 1 to 9, further comprising a step of drying the hydrogel crosslinked polymer obtained in the polymerization step, wherein the drying step is performed using hot air having a dew point of 50 to 90 ° C. The manufacturing method as described.   The manufacturing method of any one of Claims 1-10 whose residual monomer in the said polyacrylic acid (salt) type water absorbing resin is 300 ppm or less.