JPS63210108A - Manufacture of highly water-absorptive polymer - Google Patents

Abstract

PURPOSE:To obtain the title polymer having a high strength and a uniform particle diameter, by subjecting an acrylic acid monomer to water-in-oil type reversed phase suspension polymerization, adding a dried powder of a highly water-absorptive polymer and water to the polymerized liquid, and effecting azeotropic dehydration of the water in the polymer. CONSTITUTION:An acrylic acid monomer which comprises (meth)acrylic acid, its alkali metal salt and/or ammonium salt as main components, and a water-soluble radical initiator are dissolved in water. Then, the aqueous solution is dispersed in an organic solvent, and subjected to water-in-oil type reversed phase suspension polymerization. To the resulting polymerized liquid are added a dried powder of a highly water-absorptive polymer and water. Then, under reflux of the organic solvent, the azeotropic dehydration is continued until the water content of the polymer reduces to 20wt.% or below to obtain the aimed highly water-absorptive polymer. This polymer is superior in flow properties, miscibility, dispersibility, etc., and can be suitably used for the production of sanitary materials.

Description

DETAILED DESCRIPTION OF THE INVENTION [Background of the Invention] Industrial Field of Application The present invention relates to a method for producing superabsorbent polymers. The superabsorbent polymer obtained by the present invention is strong, has a narrow particle size distribution, and has uniform particle size, so it has good fluidity, good mixing with other substances, and dispersibility, and has a high water absorption rate. Due to its large size, it can be advantageously used in the production of various materials such as sanitary materials, agricultural and horticultural materials, and industrial materials.

Conventional technology In recent years, superabsorbent polymers have come to be used in sanitary materials such as disposable diapers and sanitary products, agriculture and horticulture, and industrial materials, and various products have been proposed so far. . For example, starch graft polymer (Japanese Patent Publication No. 53-461
No. 99, etc.), cellulose-modified materials (Japanese Patent Application Laid-open No. 1983-1980),
376, etc.), crosslinked products of water-soluble polymers (Japanese Patent Publication No. 43)
-23462, etc.), and self-crosslinking acrylic acid/alkali metal salt polymers (Japanese Patent Publication No. 54-30710, etc.).

However, most of these superabsorbent polymers are
Contains a lot of fine powder below mesh size, and when using it,
There are the following problems.

(1) Environmental measures are required because dust is generated.

(2) Since the fluidity of the polymer is poor, when a hopper is used, bridges are formed and the hopper becomes clogged, causing problems in quantitative performance.

(3) Poor mixability or dispersibility when mixed with other substances.

(4) If it comes into contact with liquid, it is likely to become sticky.

Possible solutions to these problems include removal of fine powder and granulation using an organic binder. However, removing the fine powder requires a new separation device, and the separated fine powder must be disposed of, which is economically disadvantageous in any case. On the other hand, when an organic binder is used, there remain problems with safety when used in hygiene products, and problems such as a decline in the performance of the superabsorbent resin itself remain.

In addition to the above-mentioned methods, there is also a method of granulating a superabsorbent resin powder by spraying an aqueous liquid onto the surface thereof (Japanese Patent Application Laid-open No. 1983-97333), a method of mixing a dry resin with a liquid containing a superabsorbent resin, A method of heating and drying in a dryer while stirring (Japanese Patent Application Laid-open No. 57
-117551) etc. have been proposed. However, the method described in JP-A No. 61-97333 has the disadvantage that due to slight variations in droplet size, spraying method, etc., it becomes a granular polymer with a wide particle size distribution. Japanese Unexamined Patent Publication 1983
In the method described in Japanese Patent Publication No. 117551, when drying is carried out while stirring, fine powder is generated again due to friction between the particles, and as a result, it is not possible to produce a uniform granulated product.

In addition, the reverse phase suspension polymerization method is known as one of the methods for producing water absorbent resins, but in the case of spherical polymers produced by this method, the contact area between the polymers is small, so the strength of the granules is weak. , and it is difficult to form uniform granules.

[Summary of the invention]

Problems to be Solved by the Present Invention The present invention aims to improve the above-mentioned drawbacks and to provide a superabsorbent polymer obtained by reverse-phase suspension polymerization that has strength, that is, can be easily used in the drying process and post-treatment process. The object of the present invention is to provide a method for obtaining a polymer having a narrow particle size distribution, large particle size, and uniform size without pulverization.

Means for Solving the Problems In view of the prior art, the present inventors have conducted various studies on methods for granulating superabsorbent polymers obtained by reverse-phase suspension polymerization. , by adding dry powder polymer and water and performing azeotropic dehydration under refluxing organic solvent until the water concentration in the polymer becomes 20% by weight or less, the strength and particle size distribution are narrow, the particle size is large, and The present inventors have discovered that a uniform polymer can be produced and that performance such as water absorption rate is improved, leading to the completion of the present invention.

That is, the method for producing a superabsorbent polymer according to the present invention is as follows:
An aqueous solution containing an acrylic acid monomer mainly composed of acrylic acid and/or methacrylic acid and their alkali metal salts and/or ammonium salts together with a water-soluble radical initiator is dispersed in an organic solvent and polymerized. Dry powder of a highly water-absorbent polymer and water are added to the polymerization solution obtained by subjecting it to water-in-oil reverse phase suspension polymerization, and the water concentration in the polymer is reduced to 20% by weight or less under reflux of the organic solvent. It is characterized by carrying out azeotropic dehydration until .

Effects of the Invention According to the method of the present invention, a super absorbent polymer as a reverse phase suspension polymerization product, which has not been achieved by conventional granulation methods, can be produced with strength, a narrow particle size distribution, and a large particle size. Moreover, it can be obtained as a uniform granular polymer.

These granular polymers are strong and contain less fine powder, which eliminates the deterioration of the working environment due to dust generation.
The fluidity or mixability is improved, and the occurrence of lumps is reduced. Furthermore, various physical properties such as water absorption rate are improved, so it can be used in a wide range of fields such as sanitary materials such as disposable diapers and sanitary products, agriculture and horticulture, and industrial materials.

[Detailed description of the invention]

Monomer 1 When the monomer used in the present invention is acrylic acid and/or methacrylic acid and their alkali metal salt and/or ammonium salt, the acrylic acid monomer accounts for 20% or more of the total carboxyl groups, preferably It is preferable that at least 50% of the alkali metal salt and/or ammonium salt be neutralized. If the degree of neutralization is too low, a polymer with excellent water absorption cannot be obtained. The alkalis used for neutralization include lithium,
Hydroxides of alkali metals such as sodium and potassium are suitable, but sodium hydroxide is often used industrially.

The acrylic acid monomer used in the present invention has the above-mentioned acrylic acid compound as a main component. Therefore, the term "acrylic acid monomer" as used in the present invention includes not only the case where the monomer is composed only of the acrylic acid compound as described above, but also the case where the monomer contains a small proportion of a comonomer. It is desirable that such comonomers are also water-soluble, but water-soluble monomers other than the above-mentioned acrylic acid compounds are, for example, (
Meth)acrylamides are well known to those skilled in the art, examples of which are disclosed, for example, in the prior art cited above and below. One group of such comonomers are crosslinker monomers having multiple ethylenically unsaturated bonds. Such crosslinking monomers may also include, for example, N, N
- methylene bisacrylamide, ethylene glycol di(meth)acrylate, as well as well known to those skilled in the art.

Polymerization The polymerization method used in the present invention is a water-in-oil type reverse phase suspension polymerization method.

In water-in-oil type reverse-phase suspension polymerization, an aqueous monomer solution is dispersed as droplets in an organic solvent, and the monomers are polymerized within the droplets by the action of a water-soluble radical polymerization solvent added thereto. It consists of

Any protective colloid or inert solvent that can provide a stable water-in-oil phase structure during the polymerization reaction can be used. To give a specific example, (1) using a copolymer of α-olefin and α,β-unsaturated polycarboxylic acid anhydride or its derivative 1 as a protective colloid, hydrocarbon Method using a system solvent (Japanese Patent Application No. 60-234878, Japanese Patent Application No. 60-2
34877), (2) H as a protective colloid.
A method using sorbitan fatty acid esters of LB3 to 6 and a petroleum-based aliphatic hydrocarbon solvent (Japanese Patent Publication No. 54-307
10), (3) A method using an oil-soluble cellulose ester or cellulose ether as a protective colloid and a hydrocarbon solvent (JP-A-58-32607), (4) A nonion with an HLB of 6 to 9 as a protective colloid. method using a hydrocarbon solvent using a surfactant (JP-A-57-167302), (5) using a nonionic surfactant with an HLB of 8 to 12 as a protective colloid, and alicyclic or aliphatic carbonization. Method using hydrogen solvent, (
6) Using a basic nitrogen-containing polymer as a protective colloid,
Method using hydrocarbon solvent (Japanese Patent Application Laid-Open No. 57-98513
(7) A method using a carboxyl group-containing polymer as a protective colloid and a hydrocarbon solvent (Japanese Unexamined Patent Publication No. 1983-94011), (8) A method using a sucrose fatty acid ester as a protective colloid to create a hydrophobic Method using organic solvent (JP-A-61-43606, JP-A-61
-87702), (9) As a protective colloid (A
) Styrene and/or its alkyl substituted derivatives 50-9
7 mol%, (B) 3 to 50 mol% of dialkylaminoalkyl-acrylate or monomethacrylate and/or dialkylaminoalkyl-acrylamide or monomethacrylamide, and (C) unsaturation copolymerizable with (A) and (B). A method using a copolymer consisting of 0 to 30 mol% of monomers and a hydrophobic organic solvent (Japanese Patent Application Laid-Open No. 1986-
40309), (10) sorbitan fatty acid ester and/or sucrose fatty acid ester as a protective colloid, and (A) 50 to 97 mol% of styrene and/or its alkyl-substituted derivative, (B) dialkylaminoalkyl acrylate or - 3 to 50 moles of methacrylate and/or dialkylaminoalkyl-acrylamide or monomethacrylamide 96 and (C) 0 to 30 unsaturated monomers copolymerizable with (A) and (B)
A method using a hydrophobic organic solvent using a copolymer consisting of mol% (Japanese Patent Application Laid-open No. 61-53308), (11)
Examples include a method using a mixture of a sorbitan fatty acid ester with an HLB of 3 to 9 and a polyoxyalkylene monoether type nonionic surfactant as a protective colloid, and a hydrophobic solvent (Japanese Patent Application Laid-Open No. 61-97301).

Water-soluble radical initiator The water-soluble radical initiator used in the present invention includes (a)
Hydrogen peroxide, (b) persulfates such as ammonium persulfate and potassium persulfate, (c) hydroperoxides such as tert-butyl hydroperoxide and cumene hydroperoxide, (d) azoisobutyronitrile, 2 .2'
- Azo initiators such as azobis(2-amidinopropane) dihydrochloride, and the like. These water-soluble radical polymerization initiators may also be used as a redox type initiator in combination with a reducing substance such as sodium hydrogen sulfite or an amine compound. The amount of these water-soluble radical initiators used is about 0.01 to 10% by weight, preferably about 0.1 to 2% by weight, based on the acrylic acid monomer.

Drying of the polymerization reaction solution In the method of the present invention, a dry powder of a highly water-absorbing polymer and water are added to the polymerization reaction solution obtained by the above operation, and the organic solvent in the polymerization reaction solution is co-dried under reflux. A granular superabsorbent polymer is obtained by boiling and dehydrating.

The composition and polymerization method of the dry powder polymer to be added do not matter as long as it is a superabsorbent polymer, but it is preferable that the monomer composition is substantially the same as that of the polymer in the target polymerization reaction solution, and that the polymer is in an opposite phase. It is desirable that the polymer be produced by suspension polymerization. These polymers may be subjected to so-called post-treatment such as surface crosslinking.

Regarding the particle size of the dry powder polymer to be added, it is preferable that 50% or more of the particle size be fine powder of 100 mesh or less in order to narrow the particle size distribution width and obtain granules with a uniform shape.

Generally, super absorbent polymers are made into products by cutting fine powder after the drying process in order to maintain constant fluidity, water absorption properties, etc., so this fine powder must be added to the polymerization reaction solution. It is preferable to use it as a dry polymer powder from both economical and process standpoints.

The amount of dry powder polymer added to the polymerization reaction solution is 10 to 100% by market weight based on the polymer present after the polymerization reaction.
is preferred. If the amount of the polymer added is less than 10% by weight, no significant granulation effect will be obtained, while if it exceeds 100% by weight, there is a high probability that the secondary polymer particles will combine with each other to form a lumpy polymer.

On the other hand, the amount of water added to the polymerization solution is preferably 100 to 300% by weight based on the polymer present in the system. If the amount of water in the system is less than 100% by weight, the resulting polymer may become lumpy, while if it exceeds 300% by weight, azeotropic dehydration will take a long time, and economically, the polymer will This is also disadvantageous in terms of performance. As a method for adding water, there is a method of newly adding water, but a process of recycling and adding water during dehydration is economically preferable.

EXPERIMENTAL EXAMPLE Super absorbent polymer production example 1 A super absorbent polymer was produced based on Example 1 of JP-A-61-157513.

That is, 375 g of cyclohexane was placed in a 1 liter four-day round bottom flask equipped with a stirrer, reflux condenser, thermometer, and nitrogen gas inlet tube, and 4.5 g of sorbitan monostearate was added and dissolved. Dissolved oxygen was expelled with gas.

Separately, put 75 liters of acrylic acid in a flask with a capacity of 1,500 cc.
Add 124.g of 25% aqueous sodium hydroxide solution while cooling with ice water. Add gg and 107.2g of water,
75.0% of carboxyl groups were neutralized. The monomer concentration relative to water in this case corresponds to 30% by weight. Next, 0.05 g of N,N'-methylenebisacrylamide and 0.26 g of potassium persulfate were added and dissolved, and then nitrogen gas was blown in to drive out dissolved oxygen.

The contents of this 500ml flask were added to the contents of the four-bottle flask, stirred to disperse, and the temperature inside the flask was raised in an oil bath without bubbling nitrogen gas. After reaching that point, the internal temperature suddenly increased.
The temperature reached 72°C several tens of minutes later. Its internal temperature is 60~
The reaction was carried out for 3 hours while maintaining the temperature at 65°C and stirring. Note that stirring was set at 250 revolutions per minute.

Super absorbent polymer production example 2 A super absorbent polymer was produced based on Example 1 of Japanese Patent Publication No. 54-30710.

That is, 228 ml of n-hexane was placed in a 500 ml four-day round bottom flask equipped with a stirrer, reflux condenser, dropping funnel, and nitrogen gas inlet tube, and 1.8 g of sorbitan monostearate was added and dissolved. Afterwards, nitrogen gas was blown in to drive out the dissolved oxygen.

Separately in an Erlenmeyer flask 30. external −18
- 13.1 g of 39 g of water dissolved in this while cooling with water
An aqueous solution of 95% pure caustic soda was added to neutralize 75% of the carboxyl groups. The monomer concentration in the aqueous phase is 45
weight%. Next, 0.1 g of potassium persulfate was added and dissolved, and then nitrogen gas was blown in to drive out dissolved oxygen.

Add the contents of this Erlenmeyer flask to the fourth flask described in - and disperse it, while introducing nitrogen gas little by little.
The reaction was carried out for 6 hours while maintaining the internal temperature of the flask at 60 to 65°C using an oil bath.

Super absorbent polymer production example 3 A super absorbent polymer was produced based on Example 1 of JP-A-56131608.

That is, 30 g of acrylic acid was placed in a flask with a capacity of 100 m1 (7), and while cooling and stirring, 22.6 g of 96
58.7 f of an aqueous solution of caustic soda was added dropwise to neutralize 80% of the acrylic acid. This was then added with potassium persulfate
, 1 g was added and stirred to dissolve at room temperature.

Separately, a
00ml flask, 163.4 g of cyclohexane,
and sorbitan monostearate 1 with HLB of 8,69
．． After 9 g of the solution was charged and stirred at room temperature to dissolve the surfactant, the above-mentioned partially neutralized solution of acrylic acid to which potassium persulfate had been added was added dropwise and suspended. After sufficiently purging the system with nitrogen gas again, the temperature is raised to bring the oil bath temperature to 55-60℃.
The reaction was carried out for 3 hours while maintaining the temperature at °C.

Example 1 10
45 g of a dry powder polymer (monomer composition is the same as in Production Example 1) produced by reverse phase suspension polymerization with 68% by weight of 0 mesh or less and 55.2 g of water were added, and the mixture was heated under cyclohexane reflux for 6 hours. It was azeotropically dehydrated. The water concentration of the polymer at this point was 15%. This polymer was dried in a vacuum dryer for 3 hours and dried at 142.
5 g of granular polymer was obtained.

Example 2 A polymerization solution obtained in the same manner as in Production Example 1 was used, and 100 mesh or less was added to 68 FRfi.
Azeotropic dehydration was carried out in the same manner as in Example 1, except that 28 g of dry powder polymer and 25.2 g of water were used, and after drying,
126 g of granular polymer was obtained.

Example 3 Using a polymerization solution obtained in the same manner as in Production Example 1 above, 74g of a dry powder polymer was produced by reverse phase suspension polymerization in which the amount of 100 meshes or less added was 68% by weight. Azeotropic dehydration was carried out in the same manner as in Example 1, except that the amount of water was changed to 117 g, and after drying, 177 g of granular polymer was obtained.

Example 4 Using a polymerization solution obtained in the same manner as in Production Example 1 above, 45 g of a dry powder polymer and water were produced by reverse phase suspension polymerization such that 100 meshes or less added thereto weighed 68 wt. Azeotropic dehydration was carried out in the same manner as in Example 1, except that the amount was changed to 25 g, and 141.8 g of granular polymer was obtained.

Example 5 100% of the polymerization solution obtained in the same manner as in Production Example 2
19 g of a dry powder polymer (monomer composition is the same as in Production Example 2) produced by reverse-phase suspension polymerization with a mesh content of 68% by weight and 67.4 f of water were added, and the mixture was heated under refluxing n-hexane for 5 hours. to perform azeotropic dehydration. The water concentration in the polymer at this point was 13%.

This polymer was dried in a vacuum dryer for 3 hours, and 59
．． 9 g of granular polymer was obtained.

Comparative Example 1 A polymerization solution obtained in the same manner as in Production Example 1 was allowed to stand, and the cyclohexane phase was separated by decantation.

The separated wet polymer was dried in a vacuum dryer for 3 hours to obtain 96.6 g of dry polymer.

The water absorption rate and particle size distribution of the polymers obtained in each of the above Examples and Comparative Examples were evaluated by the following methods. The results are shown in Table 1.

A. Weigh out about 200g of saline solution with a concentration of 0.9% by weight into a beaker with a water absorption rate of 300ml, add about 0.5g of polymer to it, disperse it, and leave it for each predetermined time (1 minute, 3 minutes,
5 minutes) to swell by standing still.

After each predetermined time, drain the water using a 100-mesh sieve, weigh the amount of filtrate, calculate the water absorption capacity for each time using the formula below, and compare.

Amount of saline solution (g) B6 particle size distribution 24 mesh, 42 mesh, 100 mesh and 2
00 mesh standard stainless steel sieves are stacked, 40 g of polymer is placed on top and shaken for 5 minutes, and the amount remaining on each sieve is expressed in weight %.

Claims (1)

[Claims] 1. An aqueous solution containing an acrylic acid monomer mainly composed of acrylic acid and/or methacrylic acid and their alkali metal salts and/or ammonium salts together with a water-soluble radical initiator in an organic solvent. Dry powder of a highly water-absorbing polymer and water are added to the polymerization solution obtained by subjecting it to water-in-oil reverse-phase suspension polymerization, which consists of dispersing and polymerizing the polymer. A method for producing a superabsorbent polymer, which comprises carrying out azeotropic dehydration until the water concentration therein becomes 20% by weight or less. 2. 50% or more of the super absorbent polymer dry powder added to the polymerization reaction solution has a mesh size of 100 mesh or less,
The amount added is 1 for the polymer present after the polymerization reaction.
0 to 100% by weight, and the amount of water added is 1% to the polymer present in the system after addition of the dry powder polymer.
The method for producing a superabsorbent polymer according to claim 1, wherein the amount is such that the amount is 00 to 300% by weight. 3. Claims 1 to 2, wherein the superabsorbent polymer added to the polymerization reaction liquid has substantially the same monomer composition as the polymer in the polymerization reaction liquid, and is optionally surface-treated. A method for producing a superabsorbent polymer according to any one of paragraphs.