JPH06329716A - Post-treatment method for water absorbent resin - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a method for post-treating a water absorbent resin without significantly reducing the water absorbing ability. A method for post-treatment of a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer, comprising one or more of the following post-treatment steps (1) to (4) A method characterized in that it is carried out at a temperature not exceeding a temperature higher than the glass transition temperature of the resin by 50 ° C. when the water content of the resin is 25% or more. (1) Dehydration step of excess water after polymerization (2) Surface modification step of resin (3) Drying step (4) Granulation step

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a post-treatment method for a water absorbent resin. More specifically, the present invention relates to a method for post-treating a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer without significantly reducing the water-absorbing ability of the resin.

[0002]

2. Description of the Related Art In recent years, water-absorbent resins are used for sanitary products, disposable paper diapers, sanitary products such as disposable rags, agricultural and horticultural products such as water retention agents, soil conditioners, sludge coagulants, anti-condensation agents and oils. It is used in various applications such as dehydrating agents. In particular, the demand for sanitary products and sanitary products such as disposable diapers has been increasing year by year, and as anti-condensation agents, they are used in a wide range of fields such as building materials, container transportation, and marine transportation. As described above, the water absorbent resin has greatly contributed to our social life.

The water-absorbent resin is generally produced by polymerizing a water-soluble ethylenically unsaturated monomer. As such a water-absorbent resin, an acrylic acid polymer crosslinked product, an acrylic acid ester-vinyl acetate copolymer saponified product, a starch-acrylic acid salt graft copolymer crosslinked product, etc. are known. . These water-absorbent resins are generally reverse phase suspension polymerization, reverse phase emulsion polymerization, after synthesizing the polymer by reaction in an aqueous solution or reaction in an organic solvent, and then dehydrating excess water contained in the polymer, Alternatively, after dehydrating a part of the water, it is subjected to a surface modification treatment and then dried.
It is made into a product through various post-treatment steps after polymerization.

As a method for polymerizing the water-absorbent resin, for example, US Pat. No. 4,698,404 and JP-A-2-25580 are available.
No. 4, JP-A-2-300210, etc.
Various polymerization methods for improving the water absorption capacity have been reported.
However, according to these methods, although the water absorption capacity as a water-absorbent resin after the polymerization step is improved, a post-treatment such as a dehydration step after the polymerization step, a surface modification step, and a drying step for completing an industrial product. By carrying out the process, the very high water absorption capacity obtained after the polymerization is greatly reduced, and the final product form shows only a small water absorption capacity.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances of the prior art, and an object thereof is to provide a method for post-treating a water-absorbent resin without significantly reducing the water-absorbing capacity.

[0006]

[Means for Solving the Problems]

[Summary of the Invention] The present inventors have conducted extensive research on various post-treatment steps after polymerization carried out in an industrial process, and as a result, temperature conditions in the step and the glass transition temperature of the water-absorbing water-absorbent resin in the step, and The present invention has been completed by obtaining a specific correlation with respect to the performance of the water absorbent resin.
That is, the post-treatment method of the water-absorbent resin of the present invention is a post-treatment method of the water-absorbent resin obtained by polymerization of a water-soluble ethylenically unsaturated monomer, the post-treatment step after polymerization (excess water dehydration step, One or more of the surface modification step, the drying step and the granulation step) and does not exceed a temperature higher than the glass transition temperature Tg of the resin by 50 ° C. when the water content of the resin in the step is 25% or more. It is characterized in that it is carried out at a temperature.

Detailed Description of the Invention <Water Absorbent Resin> The water absorbent resin to which the method of the present invention is applied is a resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer. Examples of such a water-absorbent resin include (meth) acrylate polymer cross-linked products, (meth)
Acrylic ester-saponified vinyl acetate copolymer crosslinked product, starch-acrylic acid salt graft copolymer crosslinked product,
Examples include crosslinked products of saponified starch-acrylonitrile graft copolymer, and crosslinked products of saponified starch- (meth) acrylic acid graft copolymer. Further, a copolymer obtained by copolymerizing these with the water-absorbent resin may be used as long as the monomer gives good performance to the water-absorbent resin. As the monomer that gives such performance, a water-soluble ethylenic group having a group derived from a carboxylic acid or (and) salt thereof, phosphoric acid or (and) salt thereof, sulfonic acid or (and) salt as a functional group is used. Unsaturated monomers are mentioned. Specifically, maleic acid or a salt thereof, itaconic acid or a salt thereof, vinylsulfonic acid or a salt thereof, 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof, 2-acryloylethanesulfonic acid or a salt thereof, 2- Acryloyl propane sulfonic acid or its salt, 2-itacroyl ethane sulfonic acid or its salt, vinylphosphonic acid or its salt, etc. can be illustrated, and these 1 type (s) or 2 or more types can be used.

Further, in addition to a monomer having a group derived from a carboxylic acid or / and a salt thereof, a phosphoric acid or / and a salt thereof, a sulfonic acid or / and a salt thereof as the functional group, Polymerizable monomer such as (meth) acrylamide, 2-hydroxyethyl (meth) acrylamide, (poly) ethylene glycol mono (meth) acrylate, 2-hydroxyethyl (meth)
The water-absorbent resin that produces acrylate or the like may be copolymerized in an amount within a range that does not deteriorate the performance of the water-absorbent resin. When the water-absorbent resin is a carboxylate type, that is, a salt type, sodium, potassium, and other alkali metal salt types, magnesium, calcium, and other alkaline earth metal salt types, and the like are particularly preferable. Is an alkali metal salt type.

In the production of the water absorbent resin used in the present invention, it is possible to add a crosslinking agent or an additive for improving the performance of the resin. Examples of the cross-linking agent include divinyl compounds copolymerizable with the above-mentioned monomers, such as N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol (meth) acrylates, ethylene glycol diglycidyl ether, and polyethylene glycol diglycidyl. Polyglycidyl ether, such as ether,
Water-soluble compounds having two or more functional groups capable of reacting with functional groups such as carboxylic acid, phosphoric acid and sulfonic acid, such as polyols such as glycerin and pentaerythritol, polyamines such as ethylenediamine, haloepoxy compounds and polyaldehydes Etc. can be preferably used. Examples of the additives include inactive inorganic powders such as fine particle silica, titanium dioxide powder, and alumina powder, and surfactants, which are added at appropriate times and in appropriate amounts according to the desired purpose.

The polymerization initiator used for producing the water-absorbent resin may be any one that is water-soluble and can be dissolved in an aqueous solution of a water-soluble ethylenically unsaturated monomer. Specific examples include (a) persulfates such as hydrogen peroxide, potassium persulfate, sodium persulfate, and ammonium persulfate; (b) peroxides such as t-butyl hydroperoxide and cumene hydroperoxide; ) Azo initiators such as azoisobutyronitrile and 2,2'-azobis (2-amidinopropane) dihydrochloride are used. Among these polymerization initiators, initiators exhibiting oxidative properties such as persulfates and hydroperoxides can be used as reducing initiators such as sodium bisulfite, L-ascorbic acid and ferrous salts. It can be used as a redox initiator in combination with a substance or amines. The amount of these initiators used is generally 0.01 to 10 parts by weight, preferably 0.1 to 2 parts by weight, based on the water-soluble ethylenically unsaturated monomer.

Among the water-absorbent resins to which the present invention is applied, those produced by reverse phase suspension polymerization, solution polymerization, or reverse phase emulsion polymerization, the solvent and surfactant used during the polymerization are Any aqueous solution of the ethylenically unsaturated monomer can be used as long as it is stable in forming a water-in-oil type dispersion having an aqueous phase as an aqueous phase and a solvent as an oil phase and is inert to polymerization. Such solvents include aliphatic hydrocarbons, alicyclic hydrocarbons, or aromatic hydrocarbons, and the aliphatic hydrocarbons include normal pentane and
Suitable are normal hexane, normal heptane, etc., suitable cycloaliphatic hydrocarbons are cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, etc., and aromatic hydrocarbons are benzene, toluene, xylene, etc. In particular, normal hexane, normal heptane, and cyclohexane are preferable because they have industrially constant quality, are easily available, and are inexpensive.

As the surfactant, polyoxyethylene fatty acid ester, sorbitan fatty acid ester, sorbitol fatty acid ester, sorbitan fatty acid ester ether, sorbitol fatty acid ester ether, glycerin fatty acid ester, sucrose fatty acid ester and the like can be used. In particular, sorbitol monostearate, sorbitol monolaurate, sorbitan monostearate, sorbitan monolaurate, sucrose distearate, sucrose mono-distearate and the like are preferable because they have a certain quality in industrial use and are easily available.

Regarding the method for producing a water-absorbent resin by polymerizing a water-soluble ethylenically unsaturated monomer, for example, JP-B-60-25045, JP-B-61-36663, JP-A-57-158210, and JP-A-57-158210. -21405,
The details are described in JP-A-58-71907 and JP-A-57-98513. The following are typical examples of polymerization. Example-1 Initiating radical polymerization by suspending an α, β-unsaturated carboxylic acid and its alkali metal salt aqueous solution in a petroleum hydrocarbon solvent containing a sucrose fatty acid ester in the presence or absence of a crosslinking agent. A method of polymerizing in the presence of an agent. Example-2 A method in which an aqueous solution of an α, β-unsaturated carboxylic acid and an alkali metal salt thereof is polymerized in the presence or absence of a crosslinking agent in the presence of a radical polymerization initiator. Example-3 A method of suspending acrylic acid and an aqueous solution of an alkali salt of acrylic acid in an alicyclic or aliphatic hydrocarbon solvent sharing a surfactant of HLB 8 to 12 and polymerizing them in the presence of a water-soluble radical polymerization initiator. . Example 4 A reaction product obtained by grafting 1 to 20% of α, β-unsaturated carboxylic acid or its anhydride onto a monoolefin polymer having a molecular weight of 750 to 10,000, or a monoolefin polymer to finally obtain an acid value. The product obtained by oxidation so that the water content is 10 to 100 is used as a protective colloid, and the aqueous monomer solution is suspended in a polymerization-inert and hydrophobic liquid to prevent the presence of a water-soluble radical polymerization initiator. How to polymerize into. Example-5 At least one kind of starch and cellulose (A) and at least one kind of water-soluble monomer having an addition-polymerizable double bond, or water-soluble by hydrolysis (C) and a cross-linking agent (C) Is polymerized as an essential component, and is hydrolyzed if necessary to obtain a polymer. Example 6 A polymerization reaction initiator was added to a warm aqueous solution containing acrylic acid and an acrylic acid alkali salt and a water-miscible or water-soluble divinyl compound, and the concentration of these monomers being in the range of 55 to 80% by weight. Or by irradiating with an electron beam to cause polymerization without external heating.

The water-absorbent resin which is the object of the present invention is not limited to these methods and may be polymerized by any method. In addition, in order to improve various functions (durability, water absorption ability, etc.) of the water absorbent resin, US Pat.
98404, JP-A-2-300210, JP-A-2-255804, and JP-A-3-179008.
Polymerization may be carried out by adding various additives described in Japanese Patent Publication No. The effect of the present invention is more remarkably observed as the water-absorbent resin having higher water-absorbing ability after polymerization as described in these documents.

<Post-Processing Step> The water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer has a property that after polymerization, the excess water contained in the polymer is dehydrated or, if necessary, the surface of the polymer. It is obtained as an industrial product after carrying out various post-treatment steps such as modification. Typical post-treatment processes described in the above-mentioned publications are shown below. (A) In the suspension polymerization system, "the liquid after polymerization after suspension polymerization is heated with stirring while the polymer particles are suspended, the excess water is evaporated together with the hydrocarbon solvent, the water is removed, and the evaporated carbonization is carried out. The hydrogen solvent returns a part or the whole amount to the suspension. "A dehydration process including operations such as dehydration operation and operations such as receiving and discharging slurry incidental to this. (B) Dehydration operation in the aqueous solution polymerization system, in which the high water content polymer after polymerization is heated and dehydrated by a dryer such as a belt dryer, an airflow dryer, a rotary dryer, a paddle dryer, etc. Dehydration process including operations such as receiving and discharging. (C) The water content of the resin is adjusted to an appropriate degree by the dehydration operation of the above (A) and (A), and then the slurry concentration of the resin is adjusted by an operation such as solid-liquid separation, if necessary, and then the surface modification A surface modification process that includes the operation of adding a chemical agent to carry out the surface modification reaction and the operations of receiving, discharging, and heating the slurry or powder incidental thereto. (D) After the solid-liquid separation operation is performed on the water-containing resin containing excess water after surface modification as necessary, the drying operation to adjust the water content to the required level of the product and the reception of the slurry or powder incidental thereto Drying process including operations such as discharge, heating, etc. (E) Dehydration by heating or drying operation for adjusting the water content of the water-containing resin containing excess water after aqueous solution polymerization to a predetermined water content. (F) As a granulation for adjusting the particle size of the resin to the required quality, a granulation step of humidifying as necessary, mixing a granulation aid (binder), heating and drying, crushing, and classifying . It goes without saying that the method of the present invention is applicable without being limited to these representative operations.

<Water Content of Resin and Glass Transition Temperature Tg> Regarding the relationship between the water content of a general polymer and the glass transition temperature Tg, “Polymer and Moisture” (edited by The Polymer Society of Japan, published by Koshobo), “ Polymer Physical Properties Engineering "(Kanamaru Race: published by Chijin Shokan)
As described in the above-mentioned book, when the water content of the polymer increases, Tg shifts to the low temperature side, and the motion of the polymer segment becomes active at Tg or higher. Thus, the theoretical elucidation that the Tg of the polymer changes depending on the water content has not been made yet. As a result of examining the above-mentioned water-absorbent resin, the present inventors have found that although the absolute values are different in each water-absorbent resin, the same tendency as that of a general polymer material shows that the water content and the Tg are the same.
Got about. Then, as a result of comparing and examining this Tg value and the temperature condition at the time of the above-mentioned post-treatment step, the temperature at which each treatment step does not exceed 50 ° C. higher than the Tg of the water-containing resin in that step, preferably 40 ° C. It was found that the decrease in the water absorption capacity of the resin due to the post-treatment is significantly suppressed when the temperature is not exceeded. The reason for this is not clear, but by operating with the motion of the polymer segment suppressed to a certain level or less, suppression of self-crosslinking reaction due to heat, reaction suppression of residual radicals, etc. are expressed, and water absorption obtained by polymerization It is presumed that this is because the performance of the functional resin is maintained as much as possible. It is interesting that it was confirmed that the polymer reaction due to heat first occurred under the temperature condition considerably higher than Tg (the reaction is presumed to be caused by the fact that the water absorption capacity of the resin is lowered).

<Post-treatment by the method of the present invention> In the method of the present invention, at least one of the above-mentioned post-treatment steps is carried out when the water content of the resin in the step is 25% or more. It is carried out at a temperature not exceeding 50 ° C. higher. Specific operations for carrying out the method of the present invention are described below. In the dehydration step (a) described above, the step temperature is the boiling point under the set pressure of the hydrocarbon solvent and the water system, so that the step temperature is performed under reduced pressure so that the step temperature falls within the above temperature range. At the initial stage of dehydration, the water content of the resin is high, so Tg is low, and therefore the required set temperature is considerably low. However, as dehydration progresses, the required set temperature rises as Tg rises, which facilitates industrial operation. . In the above-mentioned dehydration step (a), since the step temperature is substantially the same as the boiling point of water, in order to keep the step temperature within the above temperature range,
It may be carried out under reduced pressure as in the step (a). However, even if the process temperature at the initial stage of dehydration in these processes is equal to or higher than the required set temperature, that is, 50 ° C. or higher than the Tg of the resin, the heating time under such temperature is the process (a) or 30% or less of the total required time in (a),
The effect of the present invention can be obtained if it is preferably 10% or less.

In the above surface modification step (c), in order to carry out the surface modification reaction according to the method of the present invention at a low temperature, (1) the surface modification is carried out to increase the concentration of the surface modifier with respect to the resin. After or before mixing the agents, the hydrocarbon solvent is removed, (2) the catalyst for the surface modification reaction is added, and (3) the surface modifier that reacts at a low temperature in a short time is changed. In step (c), (d), (e), or (f), the step (a) is carried out under reduced pressure in the same manner as step (a), if necessary.

In the granulating step (f), the water content of the resin at the time of mixing the binder for granulation, water, and the resin is described in JP-A-2-284927 (water content adjusted to 35 to 60%). JP-A-2-308820 (water content 10 to 60%
Adjustment), JP-A-3-137129 (water content 10 to 8).
Adjusted to 0%), JP-A-61-97333 (water content 1 to
(Adjusted to 30%) As shown in each publication, it is generally adjusted to a high water content, but it is preferable to carry out the mixing / granulation operation and the subsequent drying operation according to the method of the present invention unlike the conventional method. Among the above-mentioned post-treatment steps, in particular, the water absorption capacity of the resin having a high water content in the dehydration step is significantly decreased by heat (the difference between Tg and the treatment temperature is generally large), and therefore the method of the present invention is applied at least in the dehydration step. However, in addition to this, it is desirable to add application to other processes as necessary.

[0020]

The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited thereto. Incidentally, for the water-absorbent resin obtained in the example,
The following measurements were performed. <Artificial urine water absorption capacity> 1 g of water-absorbent resin sample was placed in a 400-mesh nylon bag (size of 10 cm x 10 cm),
Immerse in 1 liter of artificial urine for 30 minutes (at the same time immerse a blank nylon bag). After 30 minutes, the nylon bag was pulled up, drained for 15 minutes, weighed, blank-corrected, and the weight of the artificial urine absorbed by 1 g of the water-absorbent resin sample was taken as the artificial urine water-absorption capacity. The influence of the water content of the resin sample was corrected by the following equation, and the water absorption capacity of the water absorbent resin having a different water content was compared as the corrected water absorption capacity of the water absorbent resin having a water content of 5%.

[0021]

[Equation 1] The composition of the artificial urine used here is as follows. Artificial urine composition Urine 1.94% Sodium chloride 0.80% Calcium chloride 0.06% Magnesium sulfate 0.11% Pure water 97.09% <Glass transition temperature Tg> Tg is measured by differential scanning calorimetry (D
SC) device (temperature increase rate 10 ° C./min), and point B shown in FIG. 1 was taken as Tg. Here, FIG. 1 is a schematic diagram of the DSC curve of the glass transition of a polymer, “Polymer Measurement Method (Structure and Physical Properties)”, Volume 1, edited by The Society of Polymer Science, published by Baifukan Co., Ltd., January 30, 1973, FIG. 1 cited on page 181 is cited.

Production Example 1 SUS3 equipped with a stirrer, jacket, and nitrogen gas inlet pipe
Charge 04 parts of cyclohexane to 100 parts by weight of
0.8 part by weight of sorbitan monostearate of LB3 was added and dispersed. Nitrogen gas was blown in to expel the dissolved oxygen, the temperature was raised to 60 ° C., sorbitan monostearate was dissolved under stirring, and then the cooled liquid was used as the (A) liquid. Next, 100 parts by weight of an 80% by weight acrylic acid aqueous solution was placed in a jacketed SUS304 stirring tank, and while cooling from the outside, 131.47 parts by weight of a 25.4% by weight caustic soda aqueous solution was added dropwise to 75 mol. %, N, N'-methylenebisacrylamide 0.03
A solution obtained by adding 1 part by weight, 0.8 part by weight of sodium hypophosphite, and 0.12 part by weight of potassium persulfate to form a solution (a) was prepared.
100 parts by weight of solution (a) and 80 parts by weight of solution (a) were added to a polymerization reactor made of SUS304 equipped with a stirrer, a jacket, a vulcanization cooler, and a nitrogen gas inlet tube to disperse the system again. After sufficiently purging with nitrogen, raise the temperature and cover the jacket with 70
The polymerization reaction was carried out by holding at ˜75 ° C. for 15 minutes and then at 65 ° C. for 1 hour to obtain a polymerization liquid of a water absorbent resin (c) having a water content of 57%.

Comparative Example 1 Polymerization liquid (c) of Production Example 1 was heated to evaporate cyclohexane and water contained in the polymer, and after condensation, cyclohexane was continuously refluxed into the system in a total amount of 72 to 7.
This was carried out at 6 ° C. to obtain a water absorbent resin (D) having a water content of 25%. Dehydration was continued for this resin by the same operation to make the water content 17%, followed by settling and separation, and 60 parts by weight of the supernatant was removed, and γ-glycidoxypropyltrimethoxysilane was added as a surface modifier to 0%. After adding 0.034 part by weight and stirring for 30 minutes, the temperature was raised to 105 ° C. to evaporate cyclohexane, and then the mixture was held for 30 minutes to perform surface modification.
The resin was heated to 120 ° C. and dried for 3 hours to give a water content of 5
% Water absorbent resin (e) was obtained.

Example 1 Except for operating at 50 to 54 ° C. under reduced pressure, the same dehydration operation as in Comparative Example 1 was carried out to obtain a water absorbent resin (f) having a water content of 25%. This resin was subjected to additional dehydration, surface modification and drying in exactly the same manner as in Comparative Example 1 to obtain a water absorbent resin (K) having a water content of 5%.

Comparative Example 2 0.074 parts by weight of ethylene glycol diglycidyl ether was added as a surface modifier, and the mixture was stirred for 30 minutes, heated to 110 ° C. to evaporate and remove cyclohexane, and then maintained for 60 minutes to improve the surface. The same operation as in Comparative Example 1 was carried out except that the quality was changed to obtain a water absorbent resin (K) having a water content of 5%.

Example 2 Example 1 except that the surface modification was carried out by the method of Comparative Example 2.
Exactly the same operation as above, water-absorbent resin (ke) with a water content of 5%
Got

Production Example 2 80% by weight in a jacketed SUS304 stirred tank
While taking 100 parts by weight of the acrylic acid aqueous solution of 31.4% and cooling from the outside, 25.4% by weight of a caustic soda aqueous solution 131.4
After 7 parts by weight was added dropwise to neutralize 75 mol%, N,
0.0984 parts by weight of N'-methylenebisacrylamide, Span 20 as a surfactant (Kao Atlas Co., Ltd.)
0.492 parts by weight, and the liquid temperature is 4 under nitrogen atmosphere.
After the temperature was set to 0 ° C., 0.0343 parts by weight of ammonium persulfate and 0.0115 parts by weight of sodium hydrogen sulfite were added and uniformly dissolved. Polymerization proceeds gently, 55-80 ° C
By holding for about 7 hours, a water absorbent resin (K) having a water content of 57% was obtained.

Comparative Example 3 The gel water-absorbing resin (Co) obtained in Production Example 2 was shredded and then dried at 120 ° C. for 2 hours with a hot air dryer to obtain a water content of 2
5% of water absorbent resin (SA) was obtained. The resin was further dried under the same conditions to obtain a water absorbent resin (shi) having a water content of 5%.

Example 3 The gel-like water absorbent resin (Co) obtained in Production Example 2 was shredded and then dried at 50 ° C. for 3 hours in a vacuum dryer to obtain a water content of 25.
% Water absorbent resin (s) was obtained. This resin was additionally dried at 120 ° C. using the same hot air dryer as in Comparative Example 2 to obtain a water absorbent resin (C) having a water content of 5%.

Production Example 3 414 parts by weight of acrylic acid, 4380 parts by weight of a 37% by weight aqueous solution of sodium acrylate, and polyethylene glycol diacrylate (n = 8) 5 as a crosslinking agent.
4.97 parts by weight, sodium hypophosphite monohydrate 9.76
40 parts by weight, using 380 parts by weight of deionized water, a concentration of 40
%, And a neutralization degree of 75% were obtained, and then nitrogen gas was blown in to expel dissolved oxygen. A stainless steel double-armed beading machine (kneader) with a jacket having a sigma type blade with an internal capacity of 10 liters is attached with a lid, and the above monomer aqueous solution (so) is fed into this reactor, and nitrogen gas is blown into it. The inside of the reaction system was replaced with nitrogen. Then, while rotating the two sigma type blades and heating the jacket with warm water of 35 ° C., 2.62 parts by weight of ammonium persulfate and 0.12 part of sodium bisulfite were used as a polymerization initiator.
Polymerization was initiated by adding parts by weight. After 60 minutes from the start of polymerization, the lid was removed and the hydrogel polymer (Ta) was taken out.

Comparative Example 4 The polymer (Ta) obtained in Production Example 3 was spread on a 50-mesh wire net and dried with hot air at a temperature of 120 ° C. to obtain a water content of 25%.
To obtain a dried product (h). The dried product was further dried under the same conditions to have a water content of 7%, and then crushed by a hammer crusher,
A 20-mesh wire mesh was used for sieving to obtain a 20-mesh water-absorbent resin (tsu).

Example 4 The polymer (ta) obtained in Production Example 3 was spread on a 50-mesh wire net and dried at 50 ° C. in a vacuum dryer to give a dried product (te) having a water content of 25%. Got This dried product was used as Comparative Example 4
After drying under the same conditions as above to obtain a water content of 7%, it was crushed with a hammer-type crusher and sieved with a 20-mesh wire mesh to 20
A water-absorbent resin (g) that passed through the mesh was obtained. Table 1 shows the results of Tg and water absorption capacity of artificial urine measured by the above-mentioned methods for the water-absorbent resins obtained in the above Examples and Comparative Examples.

[0033]

[Table 1]

[0034]

According to the method of the present invention, it is possible to post-treat a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer without significantly reducing the water-absorbing ability of the resin. Becomes That is, according to the present invention, it is possible to enjoy the high water-absorbing ability of the water-absorbent resin at the end of the polymerization even in the final product. Therefore, the industrial contribution of the present invention is extremely high in the field of products requiring high water absorption such as various sanitary products.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a DSC curve of glass transition of a polymer.

Claims (1)

[Claims] 1. A post-treatment method for a water-absorbent resin obtained by polymerizing a water-soluble ethylenically unsaturated monomer, which comprises one or more of the following post-treatment steps (1) to (4): The method is characterized in that it is carried out at a temperature not exceeding a temperature higher than the glass transition temperature of the resin by 50 ° C. when the water content of the resin is 25% or more. (1) Dehydration step of excess water after polymerization (2) Surface modification step of resin (3) Drying step (4) Granulation step