JP4408691B2 - Manufacturing method of water absorbent resin - Google Patents

Description

  The present invention relates to a method for producing a water-absorbent resin mainly composed of unsaturated ammonium carboxylate. More specifically, the present invention relates to a method for producing a water-absorbent resin having a high water absorption performance under pressure and a small amount of residual monomer.

  In recent years, a water-absorbing resin, which is a synthetic polymer that gels by absorbing a large amount of water, has been developed and used as a sanitary material such as disposable diapers and absorbent bands for sanitary products, or as a water retention agent and dehydrating agent in the agricultural and civil engineering fields. Widely used.

  Examples of such a water-absorbing resin include a hydrolyzate of starch-acrylonitrile graft polymer (Patent Document 1), a neutralized product of starch-acrylic acid graft polymer (Patent Document 2), and vinyl acetate-acrylic acid ester. Saponified copolymer (Patent Document 3), hydrolyzate of acrylonitrile copolymer or acrylamide copolymer (Patent Document 4), or self-crosslinked sodium polyacrylate obtained by reverse phase suspension polymerization (Patent Document 3) Many are known, such as a literature 5) and a polyacrylic acid partial neutralized product cross-linked product (Patent Literature 6). In recent years, high water absorption performance under high pressure such as high water absorption under high pressure has been demanded for these water-absorbing resins due to the demands from sanitary materials. In order to satisfy such a requirement, in the usual method for producing a water-absorbent resin, cross-linking in the vicinity of the surface is performed after polymerization and drying.

  Many conventional methods have been proposed as methods for treating the vicinity of the surface of a water-absorbent resin. For example, a method using a polyhydric alcohol (Patent Documents 7 and 8), a method using an alkylene carbonate (Patent Document 9), a method using glyoxal (Patent Document 10), a method using a polyvalent metal (Patent Documents 10 to 13) , A method using a silane coupling agent (Patent Documents 14 to 16), a method in which a water-absorbing resin is dispersed in a mixed solvent of water and a hydrophilic organic solvent (Patent Document 17), and a specific amount of water is allowed to coexist. There are known methods for crosslinking a water-absorbent resin (Patent Documents 18 and 19), a method for crosslinking by coexisting inorganic powder and water (Patent Document 20), a method for irradiating electromagnetic radiation (Patent Document 21), and the like.

  However, these water-absorbing resins have the disadvantage that the residual monomer remains in the range of about 500 to 3000 ppm.

  On the other hand, many methods for reducing residual monomers in water-absorbing resins and water-soluble resins have been proposed. For example, a) a method of changing polymerization conditions or increasing the polymerization rate by radiation (Patent Documents 22 to 25), b) a method of adding an additive such as amines or sulfites after the polymerization (Patent Documents 26 to 29) C) a method of extracting the residual monomer with an organic solvent or the like (Patent Document 30), d) a method of decomposing the residual monomer with a microorganism (Patent Document 31), e) a method of volatilizing the residual monomer at a high temperature (Patent Document 32) F) Technology using a monomer obtained by a specific neutralization method or a monomer with less heavy metal (Patent Documents 33 and 34), g) To the ammonium salt and alkali metal salt of the acid group-containing monomer used There is known a method (Patent Document 35) that defines the neutralization amount.

  However, the method a) has an insufficient reduction effect, and various physical properties of the water-absorbent resin are deteriorated due to severe polymerization and post-treatment conditions. In the methods b) and c), the additives and organic solvents used remain in the water-absorbent resin, which has a problem in safety. The method d) has a safety problem because it uses microorganisms. The method e) had little effect because the unsaturated carboxylic acid did not volatilize. The method f) has no particular problem for safety, but the effect is insufficient. Although the method of g) did indeed have a sufficient effect for reducing the residual monomer, there was no description of the water absorption performance of the water absorbent resin under pressure, and it was insufficient as a practical method for producing a water absorbent resin. Met.

That is, in the conventional technology, in order to give the water absorbent resin high water absorption performance under pressure, in the method for producing the water absorbent resin, a step of crosslinking the surface vicinity after polymerization and drying is required. However, there was no cross-linking method in the vicinity of the surface that could reduce the residual monomer at the same time while maintaining a high water absorption capacity under high pressure.
Japanese Patent Publication No.49-43395 JP 51-125468 JP 52-14689 A Japanese Patent Publication No. 53-15959 JP-A-53-46389 Japanese Patent Laid-Open No. 55-84304 JP 58-180233 A JP 61-16903 A DE-4020780C publication JP 52-17393 A JP-A-51-136588 JP-A-61-257235 JP 62-7745 JP-A-61-211305 JP-A-61-252212 JP-A-61-264006 JP 57-44627 A JP 58-117222 A JP 59-62665 A USP-4587308 Publication JP 63-43930 A Japanese Patent Laid-Open No. 50-96689 JP 56-72005 JP 63-43930 A Japanese Unexamined Patent Publication No. 63-260906 JP 50-40689 A Japanese Unexamined Patent Publication No. 55-135110 JP-A 64-62317 DE-3724707 JP-A-1-292003 Japanese Patent Publication No. 60-29523 Japanese Patent Laid-Open No. 54-119588 JP-A-2-209906 JP-A-3-31306 JP-A-6-122708

  Accordingly, an object of the present invention is to provide a method for producing a water-absorbent resin that exhibits excellent physical properties such as a sufficiently high water absorption capacity under pressure, is sufficiently practical, has little residual monomer, and is highly safe. That is.

  As a result of intensive studies to achieve the above object, the present inventor dried a gel-like polymer obtained by polymerizing an aqueous monomer solution mainly containing an unsaturated carboxylic acid and / or a salt thereof. Later, when mixing with a crosslinking agent and performing surface crosslinking by heat treatment, it was found that the above problems can be solved by surface crosslinking under conditions represented by certain parameters, and the present invention was completed.

That is, this invention is following (1)-(5).
(1) (Meth) acrylic acid ammonium salt exceeds 50, 100 mol% or less, (meth) acrylic acid alkali metal salt is 0 or more and less than 50 mol%, (meth) acrylic acid is 0 or more and less than 50 mol%, others After drying a gel polymer obtained by polymerizing an aqueous monomer solution mainly composed of an unsaturated carboxylate consisting of 0 to less than 50 mol% of the monomer, the mixture is mixed with a crosslinking agent, and the following formula A method for producing a water-absorbent resin, characterized in that surface crosslinking is carried out under heat treatment under conditions satisfying 1.
420 ≦ (T−100) ² × t ≦ 1500 (1)
(In Formula 1, T (° C.) is the surface crosslinking temperature, and t (time) is the surface crosslinking time.)
( 2 ) Production of water-absorbent resin according to (1), wherein surface crosslinking is performed using one or two or more kinds of crosslinking agents comprising a polyhydric alcohol compound, a polyvalent glycidyl compound, a polyvalent amine compound, and an alkylene carbonate as a crosslinking agent. Method.
( 3 ) Mixing with the cross-linking agent and heat-treating one or two or more cross-linking agents comprising a polyhydric alcohol compound, a polyvalent glycidyl compound, a polyvalent amine compound, and an alkylene carbonate in the vicinity of the water-absorbent resin surface The method for producing a water-absorbent resin according to (1) or (2) , which is a heat treatment in addition to.
( 4 ) The method for producing a water-absorbent resin according to any one of (1) to (3) , wherein surface crosslinking is performed while performing heat treatment under conditions satisfying the following mathematical formula 2 instead of the mathematical formula (1).
700 ≦ (T−100) ² × t ≦ 1000 (2)
(In Formula 2, T (° C.) is the surface crosslinking temperature, and t (time) is the surface crosslinking time.)
( 5 ) The method for producing a water-absorbent resin according to any one of (1) to (4) , wherein the crosslinking temperature is higher than 100 ° C and lower than 150 ° C.

  According to the production method of the present invention, in order to obtain a high water absorption capacity under pressure, in the production method of the water absorbent resin, in the step of cross-linking the vicinity of the surface after polymerization and drying, the residual monomer can be reduced at the same time. The obtained water-absorbing resin has a high water absorption performance under high pressure, is sufficiently practical and has excellent safety, and can be used in a wide range of fields such as sanitary materials, foodstuffs, civil engineering, and agriculture. In particular, it is most suitable for hygiene materials because of its high water absorption performance under pressure.

    Hereinafter, the present invention will be described in more detail.

  In the present invention, first, a gel polymer can be obtained by radical polymerization of an aqueous monomer (hereinafter abbreviated as monomer) aqueous solution containing an unsaturated carboxylic acid and / or a salt thereof as a main component by an ordinary method. It is essential. As the monomer used in the present invention, an ammonium salt is particularly preferable from the viewpoint that the amount of residual monomer can be reduced. Therefore, it is preferable to include at least an ammonium salt as the monomer. The amount of the unsaturated carboxylic acid ammonium salt is preferably more than 50 and 100 mol% or less, more preferably 70 to 100 mol%, still more preferably 90 to 100 mol%.

As the monomer mainly composed of unsaturated carboxylic acid and / or a salt thereof used in the present invention, a monomer that can be converted into a water-absorbing resin by polymerization is used without particular limitation. Examples of such unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid and the like. In these, acrylic acid and methacrylic acid are preferable and acrylic acid is more preferable.

In the present invention, a monomer having an unsaturated carboxylic acid and / or a salt thereof as a main component is used as the main component of the monomer, but other hydrophilic monomers and / or hydrophobic monomers are used. May be used in combination.

  Examples of hydrophilic monomers that can be used in combination include acrylamide, methacrylamide, N-ethyl (meth) acrylamide, Nn-propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, N, N-dimethyl (meth) ) Acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate, polyethylene glycol mono (meth) acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, Nonionic hydrophilic monomers such as N-acryloylpyrrolidine; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N, N-dimethylaminopropyl (meth) acrylate, N, N- It can be mentioned cationic hydrophilic monomers such as methyl amino propyl (meth) acrylamide and their quaternary salts and the like, may be used or at least one or two kinds selected from these groups. Among these hydrophilic monomers that can be used in combination, methoxypolyethylene glycol (meth) acrylate, N, N, -dimethylaminoethyl (meth) acrylate and its quaternary salt, and acrylamide are preferred.

  Examples of hydrophobic monomers that can be used in combination include styrene, vinyl chloride, butadiene, isobutene, ethylene, propylene, stearyl (meth) acrylate, lauryl (meth) acrylate, and the like. When these monomers having no acid group are used, they are used in the range of 0 to less than 50 mol%, preferably 0 to 30 mol%, more preferably 0 to 10 mol% in all monomers. Good. In particular, when a hydrophobic monomer is used, it is desirable to use it to such an extent that the water absorption performance of the water absorbent resin does not fall from the practical range.

  In addition, from the viewpoint of water absorption rate and water absorption speed of the water-absorbing resin obtained, a part or all of the unsaturated carboxylic acid in the monomer is neutralized to form a salt, or an unsaturated carboxylate is used from the beginning. It is desirable to use it. At that time, the neutralization rate is 50 to 100 mol%, preferably 70 to 100 mol%, more preferably 90 to 100 mol%.

  Examples of the basic substance used when neutralizing a part or the whole of the unsaturated carboxylic acid include alkali metal carbonate (hydrogen) salt, hydroxide, ammonia, various amino acids, organic amines, etc. From the viewpoint of various physical properties, sodium hydroxide and / or potassium hydroxide and / or ammonia are preferable, and ammonia is particularly preferable. Further, urea may be added to the monomer as an ammonia precursor for polymerization.

  Moreover, when using unsaturated carboxylate for a monomer, the alkali metal salt and / or ammonium salt of unsaturated carboxylic acid are preferable, and ammonium salt is more preferable.

  In the present invention, there is no restriction on further post-neutralization during and / or after the polymerization. However, when a strong base such as sodium hydroxide is used for the post-neutralization, attention must be paid to hydrolysis at the crosslinking point, It is desirable to use the water-absorbing resin so that the water-absorbing performance of the water-absorbing resin does not fall from the practical range.

  The unsaturated carboxylic acid and / or salt thereof used in the present invention has an ammonium salt ratio exceeding 50 and not more than 100 mol%, preferably 70 to 100 mol%, more preferably 70 to 100 mol%, and still more preferably 90. ~ 100 mol%. The proportion of the alkali metal salt is 0 to less than 50 mol%, more preferably 0 to 20 mol%. Within such a range, it is possible to obtain a water-absorbent resin that is excellent in various physical properties and has few residual monomers. The neutralization with a very small amount of polyvalent metal salt does not change the gist of the present invention.

  In the present invention, the above monomer is polymerized and crosslinked to obtain a water-absorbing resin. The crosslinking method used is not particularly limited. For example, a method in which a water-soluble resin is obtained by polymerizing the monomer of the present invention, and then a crosslinking agent is added during or after polymerization, followed by crosslinking. Examples include radical crosslinking with a polymerization initiator, radiation crosslinking with an electron beam, photocrosslinking with light, etc. In order to produce a water-absorbing resin with excellent performance with high productivity, a predetermined amount of a crosslinking agent is used in advance. It is desirable to carry out the polymerization by adding to the body and to carry out a crosslinking reaction simultaneously with and / or after the polymerization.

  As a crosslinking agent used in a method in which a predetermined amount of a crosslinking agent is added to a monomer in advance to perform polymerization, and a crosslinking reaction is performed simultaneously with and / or after polymerization, N, N ′,-methylenebisacrylamide, (poly) Ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, trimethylolpropane di (meth) acrylate, (poly) ethylene glycol (β-acryloyloxypropionate) ), Poly (meth) allyloxyalkane, (poly) ethylene glycol diglycidyl ether, (poly) ethylene glycol, glycerin, pentaerythritol, ethylenediamine, polyethyleneimine, and the like. Moreover, the amount of its use is 0.005-5 mol% normally with respect to a monomer, More preferably, it is 0.01-1 mol%. Among these crosslinking agents, two or more polymerizable unsaturated groups are contained in the molecule because of the durability of the water-absorbing resin obtained, the water absorption performance, and the ease of handling of the hydrogel during production. It is desirable to use a polymerizable crosslinking agent.

In the present invention, when polymerizing using the above-mentioned monomers, bulk polymerization or precipitation polymerization can also be performed, but from the viewpoint of the performance of the water-absorbing resin obtained and ease of control of the polymerization reaction, etc. It is desirable to polymerize the body as a solution. The polymerization solvent is not particularly limited as long as it is a liquid in which the monomer is dissolved, and examples thereof include water, methanol, ethanol, acetone, dimethylformamide, dimethyl sulfoxide, etc., but water or a mixed liquid of a water-soluble organic solvent and water. Is desirable. The concentration of the monomer in the solution is not particularly limited and may exceed the saturation concentration and may be in a slurry state, but is usually in the range of 20% by weight to the saturation concentration, more preferably 25 to 50% by weight. is there.
If the monomer concentration is too low, productivity will be unfavorable, and if the monomer concentration is too high, various physical properties of the resulting water-absorbent resin will be affected due to the influence of partial low-molecular polymerization before the polymerization reaction, etc. It is not preferable because it causes a decrease.

  In the polymerization, water-soluble chain transfer agents such as hypophosphites, thiols, and thiolic acids, and hydrophilic polymers such as starch, cellulose, polyvinyl alcohol, polyacrylic acid, and cross-linked polyacrylate are used. Polymerization may be performed in addition to the monomer. The amount used is usually 5 parts by weight or less for the former and 50 parts by weight or less for the latter.

  The method for polymerizing the monomer in the present invention is not particularly limited, and examples thereof include polymerization with a radical polymerization initiator, radiation polymerization, electron beam polymerization, and ultraviolet polymerization with a photosensitizer. In order to improve various properties of the water-absorbing resin obtained, polymerization with a radical polymerization initiator is desirable. The radical polymerization method is not particularly limited. For example, cast polymerization performed in a mold, polymerization on a belt conveyor, polymerization performed while subdividing a hydrogel polymer, various aqueous solution polymerizations, reverse polymerization Although known polymerization methods such as phase suspension polymerization and reverse phase emulsion polymerization can be mentioned, reverse phase suspension polymerization and aqueous solution polymerization are particularly desirable. These polymerization methods may be either continuous or batch.

  The polymerization conditions for the monomer are not particularly limited, and are performed under normal conditions. The pressure may be any of reduced pressure, normal pressure, and increased pressure. Moreover, about temperature, initial temperature is 0-40 degreeC normally, Preferably 10-30 degreeC is good. The temperature during the polymerization is usually 50 to 100 ° C., preferably 70 to 90 ° C., excluding the peak temperature due to the heat of polymerization. The aging temperature after completion of the polymerization is usually 50 to 120 ° C, preferably 70 to 100 ° C.

  Examples of radical polymerization initiators used for polymerization include persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; organic peroxides such as t-butyl hydroperoxide and cumene hydroperoxide; hydrogen peroxide Azo compounds such as 2,2′-azobis (2-amidinopropane) dihydrochloride; and other known initiators such as chlorite, hypochlorite, permanganate, etc. Among these, at least one or two or more selected from the group consisting of persulfate, hydrogen peroxide, and azo compounds is preferable. Moreover, when using an oxidizing radical polymerization initiator, reducing agents, such as a sulfite and L-ascorbic acid, can be used together, and when using an azo polymerization initiator, you may use an ultraviolet-ray together. These radical polymerization initiators may be added to the polymerization system all at once or sequentially, but the amount used is usually 0.001 to 2 mol%, preferably 0.01 to 1 mol%.

  In the present invention, the gel polymer obtained according to the above procedure is heat-treated.

  The heat treatment temperature is usually in the range of 100 to 300 ° C, preferably 100 to 260 ° C, more preferably 100 to 200 ° C. If the heat treatment temperature is too high, the water-absorbent resin may have adverse effects such as deterioration of various physical properties. The heating time is usually 1 minute to 10 hours, preferably 10 minutes to 5 hours. In order to further achieve the object of the present invention, the polymer used in the heat treatment preferably has a large surface area, for example, a film shape, a string shape, a particle shape, or the like.

  In the heat treatment, the state of the polymer is not particularly limited, and examples thereof include a gel polymer after polymerization, a dispersion in an organic solvent, and a polymer in a dry state. Further, during the heat treatment, the solid content of the crosslinked body may be kept constant, or the solid content may be increased by volatilizing the solvent.

  In the present invention, the time for performing the heat treatment is not particularly limited, and examples thereof include a drying step, a surface crosslinking step, a reheating step after drying, a granulation step, and a step of adding an additive. It is desirable to perform heat treatment in the drying process. The drying method is not particularly limited, and examples thereof include known drying methods such as hot air drying, infrared drying, azeotropic dehydration, and the like. In particular, the heat treatment in the surface crosslinking step will be described later.

  Further, an additive such as a surfactant, inorganic powder, or bisulfite may be added to the water-absorbent resin after polymerization or after drying, and the particle size may be adjusted by pulverization or granulation. . For example, in the case of a powdery water absorbent resin, the average particle size is adjusted to 10 to 2000 μm, more preferably 100 to 1000 μm, and still more preferably about 150 to 850 μm.

  In the present invention, in order to improve various physical properties of the obtained water-absorbent resin and to reduce the residual monomer, it is essential to subject the dried gel polymer obtained by the above method to surface crosslinking treatment. In the present invention, the crosslinking in the vicinity of the surface may be performed by crosslinking with radiation or the like, but is usually performed by adding a crosslinking agent in the vicinity of the surface. The crosslinking agent used is not particularly limited, and known crosslinking agents are used. For example, (poly) ethylene glycol, (poly) propylene glycol, (poly) glycerin, 1,6-hexanediol, trimethylolpropane, diethanolamine, triethanolamine, Various polyhydric alcohols such as ethanolamine, polyoxypropylene, pentaerythritol and sorbitol; Various polyhydric epoxy compounds such as (poly) ethylene glycol diglycidyl ether; Various polyvalent amine compounds such as ethylenediamine and polyethyleneimine; -Various polyaziridine compounds such as bishydroxymethylbutanol-tris (3- (1-aziridinyl) propionate); Various alkylene carbonate compounds such as 1,3-dioxolan-2-one; Various types such as glyoxal and glutaraldehyde Polyvalent aldehyde compounds; various polyvalent isocyanate compounds such as 2,4-tolylene diisocyanate; various polyvalent oxazoline compounds such as 1,2-ethylenebisoxazoline; various haloepoxy compounds such as epichlorohydrin; aluminum, iron, zirconium, etc. Various polyvalent metal salts such as hydroxides and chlorides of the above; other compounds containing a plurality of functional groups capable of causing a crosslinking reaction with carboxylic acid groups or salts thereof present in the dried product of the gel polymer Can do.

  In the present invention, among these crosslinking agents, at least one or two or more selected from the group consisting of polyhydric alcohols, polyvalent glycidyl compounds, polyvalent amines, and alkylene carbonates are used as the crosslinking agent. However, polyhydric alcohol suitable for heat treatment at high temperature is most preferable.

  In the present invention, the amount of the crosslinking agent used is not particularly limited, but is usually 0.01 to 20 parts by weight, preferably 0.05 to 10 parts by weight, based on 100 parts by weight of the solid content of the water absorbent resin. Range.

  As a method of adding a crosslinking agent near the surface of the water absorbent resin, a known method is used, a method of adding a crosslinking agent directly to the water absorbent resin, a method of adding the crosslinking agent to the water absorbent resin as a solution or dispersion, Examples thereof include a method of adding a crosslinking agent to the water-absorbent resin dispersed in a solvent. As the solvent for the solution or dispersion, it is preferable to use a hydrophilic organic solvent such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, acetone, tetrahydrofuran, and water. In addition, the usage-amount is 0-30 weight part normally with respect to 100 weight part of solid content of a water absorbing resin, Preferably it is the range of 0-10 weight part.

  In the present invention, in order to improve various physical properties of the obtained water-absorbent resin and reduce the residual monomer, it is preferable that water is present when the water-absorbent resin and the crosslinking agent are mixed and then heat-treated. Therefore, when using a dried water absorbent resin, it is desirable to add water. At that time, the addition of water may be performed simultaneously with the addition of the crosslinking agent or may be performed separately, but the amount used is usually 20 parts by weight or less with respect to 100 parts by weight of the solid content of the water absorbent resin. Preferably it is the range of 0.5-10 weight part.

  In the present invention, the heat treatment for surface cross-linking is essential after mixing the water-absorbent resin and the cross-linking agent according to the above-described method.

  The heat treatment temperature is in the range of more than 100 ° C. and less than 150 ° C., preferably 110 to 145 ° C., more preferably 120 to 140 ° C. If it is less than 100 ° C., uniform and strong surface cross-linking is difficult to achieve, and the residual monomer reducing effect at the time of surface cross-linking is small, and the residual amount of the added cross-linking agent is also large. On the other hand, at 150 ° C. or higher, heat treatment does not require a great deal of energy, and the water-absorbent resin is prone to thermal degradation, which causes an increase in soluble content and a decrease in physical properties such as water absorption capacity under pressure. Absent. As for the heating time, an appropriate crosslinking time is determined in accordance with the set heating temperature so as to satisfy the parameter range in Equation 1. As long as the surface cross-linking treatment is performed within this parameter range, the residual monomer can be sufficiently reduced, and a water-absorbing resin having a high water absorption capacity under pressure can be produced.

  As a method for performing the heat treatment, known means are used. (1) A method in which a crosslinking agent is directly added to the water-absorbent resin and then heat-treated as it is, and (2) a crosslinking agent is added to the water-absorbent resin dispersed in a solvent. Examples of the method include a method of heat-treating while being dispersed after the addition, and a method of (3) filtering and heat-treating from the dispersion medium. Among these methods, the method of (1) is easy because of heat treatment. Is preferred. In addition, there is no restriction | limiting in particular about heat processing apparatus, Well-known apparatuses, such as a hot air dryer, a fluidized bed dryer, and a Nauter type dryer, are used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, the scope of the present invention is not necessarily limited by these Examples. In addition, the various physical properties described in the examples indicate values measured by the following test methods.

(1) Tea bag method water absorption capacity 0.5 g of water-absorbing resin was uniformly placed in a non-woven tea bag bag (95 × 70 mm) and immersed in 500 g of a 0.9 wt% sodium chloride aqueous solution. After 1 hour, the tea bag-type bag was pulled up, drained for 10 minutes, the weight of the tea bag-type bag was measured, and the water absorption capacity was calculated by the following formula.

Tea bag method water absorption (g / g)
= ((Weight of tea bag after water absorption)-(weight of tea bag before water absorption)) / (weight of water absorbent resin)

(2) Water absorption capacity under pressure 0.16g of water-absorbing resin is placed in a special container made of acrylic resin with an inner diameter of 25mm, an outer diameter of 35mm, and a height of 30mm. The bottom of the cylinder is sealed with nylon 400 mesh. By loading a cylindrical weight with a predetermined weight (278.33 g) with an outer diameter of 24.5 mm evenly onto the bottom mesh, and slowly inserting from the top, 0.8 psi vertical downward pressure is applied. The special container was gently immersed in 60 g of a 0.9 wt% sodium chloride aqueous solution in a SUS petri dish having an inner diameter of 120 mm and a height of 28 mm. After 1 hour, pull up the dedicated container, wipe off the moisture on the side of the dedicated container, drain the water by leaving the moisture on the bottom of the dedicated container on the filter paper for 3 seconds, and then measure the weight of the dedicated container together with the weight. The water absorption capacity under pressure was calculated by the following formula.

0.8 psi water absorption capacity under pressure (g / g) = ((dedicated container after water absorption + weight of weight)-
(Special container before water absorption + weight of weight) / (weight of water-absorbing resin)

(3) Residual monomer
After dispersing 0.16 g of water-absorbing resin in 40 g of 0.9% by weight sodium chloride aqueous solution,
Stirred for a while. Thereafter, the residual liquid in the treatment liquid obtained by filtering the dispersion liquid is treated.
The amount of the monomer was measured using high performance liquid chromatography, and was measured per treated water absorbent resin.
The amount of residual monomer (weight ppm / SAP) was converted into a value. (SAP: super absorbent polymer)

Example of production of water-absorbing resin before surface crosslinking Using acrylic acid as a special grade reagent and 28% by weight ammonia water and distilled water as special grade reagent.
A 40 wt% aqueous solution of ammonium acrylate (pH = 7.0) was prepared under ice cooling. In order to remove the polymerization inhibitor present in the 40 wt% ammonium acrylate aqueous solution, the 40 wt% acrylic
Ammonium acid aqueous solution is treated with activated carbon, polymerization inhibitor-free 40% by weight acrylic acid ammonium
An aqueous nium solution was obtained.

100 g of the 40 wt% ammonium acrylate aqueous solution and N, N′-methylenebisacryla
0.09 mol% of imide was charged into a 500 ml separable flask, degassed by bubbling with nitrogen gas for 30 minutes, and then 0.001 mol% of L-ascorbic acid and 0.07 mol% of ammonium persulfate were added at 35 ° C. with stirring with a stirrer. After about 1 to 2 minutes, an increase in the internal temperature was observed, and polymerization was started. Immediately before the progress of the polymerization, the viscosity of the liquid increased and stirring with the stirrer became impossible. Stirring was stopped, and the bubbling nitrogen gas was switched to gas phase substitution to continue the polymerization. Eventually, when the polymerization peak temperature reached 92 ° C. and the internal temperature began to drop, the separable flask was immersed in a water bath, and aging at 70 ° C. was performed for 2 hours.

  The water-containing gel polymer thus obtained was taken out and subdivided to about 5 mm, then vacuum-dried at 100 ° C. for 2 hours, and the resulting dried product was crushed with a commercially available crusher under cooling with dry ice. Further, after vacuum drying at 100 ° C. for 2 hours, the obtained dried product was classified to 850 μm or less to obtain a water absorbent resin. The residual monomer of the obtained water absorbent resin was 187 ppm by weight / SAP.

Example 1
After adding 1 part by weight of glycerin, 30 parts by weight of isopropanol and 3 parts by weight of distilled water as a cross-linking agent to 100 parts by weight of the water-absorbing resin before surface cross-linking obtained in the above production example, and thoroughly mixing the inert oven Then, heat treatment (surface crosslinking treatment) was performed at 120 ° C. for 2 hours. The results are shown in Table 1.

Example 2
The heat treatment (surface cross-linking treatment) was carried out in the same manner as in Example 1 except that the heat treatment (surface cross-linking treatment conditions) was 150 ° C. and 0.33 hours. The results are shown in Table 1.

Comparative Example 1
The heat treatment (surface cross-linking treatment) was carried out in the same manner as in Example 1 except that the heat treatment (surface cross-linking treatment conditions) was 110 ° C. for 3 hours. The results are shown in Table 1.
Comparative Example 2
The heat treatment (surface cross-linking treatment) was performed in the same manner as in Example 1 except that the heat treatment (surface cross-linking treatment conditions) was 120 ° C. for 1 hour. The results are shown in Table 1.

Comparative Example 3
A heat treatment (surface cross-linking treatment) was performed in the same manner as in Example 1 except that the heat treatment (surface cross-linking treatment conditions) was 150 ° C. for 1 hour. The results are shown in Table 1.

  As is apparent from the results in Table 1, in the method for producing a water-absorbing resin of the present invention, a water-absorbing resin having a small amount of residual monomer and having a high water absorption capacity under pressure can be easily obtained.

  The method for producing a water-absorbent resin of the present invention can be suitably used in fields where high water absorption performance under pressure and safety such as sanitary materials are required.

Claims (5)

(Meth) acrylic acid ammonium salt exceeds 50, 100 mol% or less, (meth) acrylic acid alkali metal salt is 0 or more and less than 50 mol%, (meth) acrylic acid is 0 or more and less than 50 mol%, other single amount A gel polymer obtained by polymerizing an aqueous monomer solution mainly composed of an unsaturated carboxylate having a body of 0 or more and less than 50 mol% is dried and then mixed with a crosslinking agent to satisfy the following formula 1. A method for producing a water-absorbent resin, characterized in that surface crosslinking is carried out while heat-treating under conditions.
420 ≦ (T−100) ² × t ≦ 1500 (1)
(In Formula 1, T (° C.) is the surface crosslinking temperature, and t (time) is the surface crosslinking time.) The method for producing a water-absorbent resin according to claim 1, wherein the surface crosslinking is performed using one or two or more kinds of crosslinking agents comprising a polyhydric alcohol compound, a polyvalent glycidyl compound, a polyvalent amine compound, and an alkylene carbonate as a crosslinking agent. Mixing with the cross-linking agent and heat treatment can be performed by adding one or more cross-linking agents composed of a polyhydric alcohol compound, a polyvalent glycidyl compound, a polyvalent amine compound, and an alkylene carbonate to the vicinity of the water-absorbent resin surface. method for producing a water-absorbent resin according to claim 1 or 2 is to process. The method for producing a water-absorbent resin according to any one of claims 1 to 3 , wherein surface crosslinking is performed while heat treatment is performed under conditions satisfying the following mathematical formula 2 instead of the mathematical formula (1).
700 ≦ (T−100) ² × t ≦ 1000 (2)
(In Formula 2, T (° C.) is the surface crosslinking temperature, and t (time) is the surface crosslinking time.) The method for producing a water-absorbent resin according to any one of claims 1 to 4 , wherein the crosslinking temperature is higher than 100 ° C and lower than 150 ° C.