JPH03195713A - Production of polymer having high water absorption - Google Patents

Abstract

PURPOSE:To obtain the title polymer having high water absorption properties, high water absorption rate and gel strength in polymerizing a monomer consisting essentially of (meth)acrylic acid and a salt thereof by water-in-oil type reversed phase suspension polymerization, by carrying out the reaction in the presence of a crosslinking agent in a specific dispersion medium. CONSTITUTION:An acrylic acid-based monomer consisting essentially of (meth) acrylic acid and au alkaline metallic salt or ammonium salt thereof is polymerized in cyclohexanone solvent in the presence of a water-soluble radical polymerization initiator, hydroxyethyl cellulose and a crosslinking agent [e.g. polyethylene glycol di(meth)acrylate] by using a sorbitan fatty acid ester (e.g. sorbitan monostearate) having 3-12 HLB as a dispersant. A monomer consisting essentially of a monomer containing >=50% total carboxyl group neutralized with an alkali metal salt or ammonium salt is preferable as the raw material monomer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Background of the Invention] (Industrial Application Field) The present invention relates to a method for producing a superabsorbent polymer.

The polymer obtained by the production method of the present invention has high water absorption capacity and water absorption gel strength, has a large polymer particle size, and has a significantly small amount of residual organic solvent, so it can be used in sanitary materials, industrial materials, agriculture and horticulture, etc. can be used advantageously for a variety of materials.

(Prior art) In recent years, superabsorbent polymers have been used not only for sanitary materials such as disposable diapers and sanitary products, but also for water-stopping agents, anti-condensation materials, freshness-preserving materials,
It has come to be used for industrial purposes such as solvent dehydration materials, greening, agricultural and horticultural purposes, and various products have been proposed so far.

Known examples of this type of superabsorbent polymer include hydrolysates of starch-acrylonitrile graft copolymers, carboxymethyl cellulose, polyacrylic acid (salts), acrylic acid (salts)-vinyl alcohol copolymers, and polyethylene oxide. ing.

However, it was difficult to say that any of these superabsorbent polymers simultaneously satisfied all required performances such as water absorption capacity, water absorption rate, gel strength, cohesiveness, and safety. For example, in the water-in-oil reverse phase suspension polymerization of aqueous solutions of alkali metal acrylates, etc., it is used as a dispersant.
Sorbitan fatty acid ester with HLB 3 to 6 described in Japanese Patent Publication No. 30710, nonionic surfactant with HLB 6 to 9 described in JP-A-57-167302, Japanese Patent Publication No. 60-2
When using the surfactants with HLB 8 to 12 described in Publication No. 5045, fine superabsorbent polymers with particle sizes of about 10 to 100 μm are obtained, so dust countermeasures are required when handling, and so-called so-called There was a problem in that it was easy to cause a sticky phenomenon, resulting in a slow water absorption rate.

On the other hand, Japanese Patent Publication No. 63-36321, Japanese Patent Publication No. 63-3
When a lipophilic carboxyl group-containing polymer is used as a dispersant as described in Publication No. 6322, a polymer with a particle size of several hundred μm can be obtained, but the affinity between the dispersant and the monomer is poor. Due to its high content, there was a problem in that it easily formed into lumps during the polymerization reaction.

As a method for increasing the particle size of a super absorbent polymer, Komei No. 1-17482 and JP-A-57-158210
The publication describes a method of using an oil-soluble cellulose ester or cellulose ether as a dispersant, but such a method involves the risk of the remaining dispersant melting when the polymer is dried, causing the polymer to aggregate. There was a problem that it easily adhered to the walls of the vessel.

Further, when a copolymer of α-olefin and α,β-unsaturated carboxylic acid anhydride or a derivative thereof is used as a dispersant described in JP-A-62-95308, and polymerized in the presence of hydroxyethyl cellulose. has a particle size of 10
A polymer with a diameter of 0 to 2000 μm can be obtained, and the polymer does not adhere to equipment, etc. However, the water absorption capacity of such a super absorbent polymer is at most 600 to 800 times its own weight, which cannot be said to be sufficient. Ta.

On the other hand, in JP-A-56-76419, in the absence of a crosslinking agent, a highly concentrated aqueous alkali metal acrylic salt aqueous solution of 35% by weight or more is used as a dispersant, and a sorbitan fatty acid ester with an HLB of 3 to 6 is used. A water-absorbing polymer with a particle size of 10 to 300 μm has been obtained by polymerization in the presence of hydroxyethyl cellulose, but although a water-absorbing polymer with a water absorption capacity as high as 700 to 900 times can be obtained, the problem is that the gel strength is weak. Furthermore, since a large amount of n-hexane used as a dispersion medium remains in this polymer, its use poses a major safety problem.

[Summary of the invention]

(Summary) The present invention improves the above problems and provides a method for producing a superabsorbent polymer that simultaneously satisfies all required performances such as water absorption capacity, water absorption rate, gel strength, cohesiveness, and safety. This is what we are trying to provide.

As a result of intensive research aimed at improving the above-mentioned problems, the inventors of the present invention discovered that, in the method described in JP-A-56-76419, polymerization of acrylic acid, etc., in the presence of a cross-linking agent was carried out. The present invention was achieved by discovering that high water absorption capacity and gel strength can be obtained at the same time, and that the residual solvent in the polymer can be significantly reduced by using cyclohexane instead of n-hexane as a dispersion medium. be.

That is, in the method for producing a superabsorbent polymer of the present invention, an acrylic acid monomer containing acrylic acid and/or methacrylic acid and an alkali metal salt or ammonium salt thereof as a main component is combined with a water-soluble radical polymerization initiator and a water-soluble radical polymerization initiator. In the presence of hydroxyethylcellulose, H as a dispersant
When polymerizing by water-in-oil reverse phase suspension polymerization using sorbitan fatty acid esters of LB3 to 6, the method is characterized in that the polymerization of acrylic acid monomers is carried out in a cyclohexane solvent in the presence of a crosslinking agent. It is something.

(Effects) According to the present invention, it is possible to produce a superabsorbent polymer that simultaneously satisfies all required performances such as water absorption capacity, water absorption rate, gel strength, cohesiveness, and safety.

[Detailed description of the invention]

(Acrylic acid monomer) The acrylic acid monomer used in the polymerization reaction of the present invention has acrylic acid and/or methacrylic acid and an alkali metal salt or ammonium salt thereof as main components. A preferred specific example of such an acrylic acid monomer is one in which 20% or more, preferably 50% or more of the total carboxyl groups in the specimen are neutralized with an alkali metal salt or ammonium salt. It is something. In this case, if the degree of neutralization is less than 20%, the water absorption capacity will be low and the strength of the obtained water absorption gel will be extremely low. The upper limit of the degree of neutralization is about 90%.

As the alkali agent for neutralizing the acid monomer to the alkali metal salt, alkali metal hydroxides, bicarbonates, and the like can be used, but alkali metal hydroxides are preferable. Specific examples of such alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide.

Sodium hydroxide is most preferred in terms of industrial availability, price, and safety.

Such an acrylic acid monomer is dissolved in an aqueous medium to form a solution. As the aqueous medium, water is often used, but one consisting of water and a small amount of a water-soluble organic solvent can also be used.

The larger the amount of the acrylic acid monomer in the solution, the better. Specifically, the concentration of the acrylic acid monomer is preferably 20% by weight or more, particularly 30% by weight or more as the monomer concentration after neutralization in the aqueous medium. The higher the monomer concentration, the better the yield per unit batch, and also the easier the dehydration operation after polymerization, which is economically advantageous.

In the present invention, monomers other than those mentioned above, such as (1) itaconic acid, maleic acid, fumaric acid, 2-acrylamide-2
- Methylpropanesulfonic acid, 2-acryloylethanesulfonic acid, 2-acryloylpropanesulfonic acid and its salts, ■ Alkyl or alkoxyalkyl esters of dicarboxylic acids such as itaconic acid, maleic acid, fumaric acid, ■ (meth)acrylamide, ■Vinyl sulfonic acid, ■Methyl acrylate, ethyl acrylate, etc., ■(meth)
Hydroxyethyl acrylate, hydroxypropyl (meth)acrylate, ■Polyethylene glycol mono(meth)acrylate,
N-methylol (meth)acrylamide, glycidyl (
A monomer having a copolymerizable double bond in the molecule, such as meth)acrylate, can also be used in combination.

(Water-soluble radical polymerization initiator) As the water-soluble radical polymerization initiator used in the present invention, those well known in the field of polymer chemistry can be used. Specific examples of such water-soluble radical polymerization initiators include inorganic or organic peroxides, such as persulfates (ammonium salts, alkali metal salts (especially potassium salts), etc.), hydrogen peroxide, ditertiary butyl peroxide, etc. , acetyl peroxide, and the like. In addition to these peroxides, if a predetermined aqueous solution is obtained, an azo compound or other radical polymerization initiator, such as 2゜2'-azobis(2-amidinopropane) dihydrochloride, 2,2'-
Azobis(N,N'-dimethyleneisobutyramidine) dihydrochloride, 4,4'-azobis(4-cyanovaleric acid), etc. can also be used.

Polymerization is initiated by the decomposition of these water-soluble radical polymerization initiators, and in the present invention, the decomposition of the water-soluble radical polymerization initiators is accomplished by not only heating, which is a conventional means, but also decomposition of the water-soluble radical polymerization initiators by chemical substances. It can also be carried out by well-known methods such as promoting the decomposition of . When the polymerization initiator is a peroxide, the decomposition promoting substance is a reducing compound (water-soluble in the present invention) such as an acidic sulfite, ascorbic acid, an amine, etc. for persulfates, and Polymerization initiators consisting of a combination of a peroxide and a reducing compound are well known in the field of polymer chemistry as "redox initiators." Therefore, in the present invention, the term "polymerization initiator" refers to combinations with such decomposition promoting substances, especially redox initiators,
This includes:

The amount of water-soluble radical polymerization initiator used as described above is 0.001 to 1 in the membrane, based on the acrylic acid monomer.
0% by weight, preferably 0.01 to 1% by weight.

(Hydroxyethylcellulose) Any hydroxyethylcellulose used in the present invention can be used as long as it dissolves in the aqueous acrylic acid monomer solution. Specific examples of such hydroxycellulose include those with a degree of substitution (i.e., a value indicating how many of the three OH groups in the glucose unit constituting cellulose are etherified) of 0.4 to 2, EO The number of moles added (that is, the number of moles of ethylene oxide added per glucose unit) is preferably 1 to 5.

If the degree of substitution or the number of moles of EO added is smaller than the above values, hydroxyethyl cellulose will not dissolve in the acrylic acid monomer and cannot be used in the present invention.

The amount of hydroxyethylcellulose used is 0.1 to 20% by weight, preferably 0.1 to 20% by weight, based on the acrylic acid monomer.
5 to 10% by weight. As the amount of hydroxyethylcellulose used increases, the particle size of the resulting polymer increases and the water absorption capacity improves, but if it exceeds 20% by weight, it is difficult to dissolve hydroxyethylcellulose in the monomer, and it is not economically advantageous. I can not say.

(Dispersant) In the present invention, a sorbitan fatty acid ester with an HLB of 3 to 6 is used as a dispersant, and is solid at room temperature.
Sorbitan monostearate is particularly preferred.

The amount of dispersant to be used is 0.00% relative to the acrylic acid monomer.
It is 5 to 20% by weight, preferably 1 to 10% by weight.

(Crosslinking agent) The crosslinking agent used in the production method of the present invention has two or more double bonds in the molecule and is copolymerizable with acrylic acid monomers, such as polyethylene glycol di(meth).
Acrylate, polyethylene glycol di(meth)acrylate, glycerin tri(meth)acrylate, N,
N'-methylenebis(meth)acrylamide, etc., or a compound having two or more functional groups that can react with a functional group such as a carboxyl group in an acrylic acid monomer during polymerization or during drying after polymerization, such as , ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, di- or polyglycidyl ether of aliphatic polyhydric alcohol, and the like.

The amount of these crosslinking agents used is 1 to 10% by weight, preferably 0.01 to 2% by weight, based on the acrylic acid monomer. If it is less than 0.001% by weight, the water absorption capacity will increase, but the gel strength of the polymer during water absorption will be extremely weak, and if it is more than 10% by weight, the water absorption gel strength will be particularly improved, but the water absorption capacity will be small. This results in a practical problem.

(Dispersion medium) In the present invention, cyclohexane is used as a dispersion medium. By the way, it is not clear why when n-hexane is used instead of cyclohexane, the amount remaining in the resulting polymer is extremely large, but the affinity between sorbitan fatty acid ester and n-hexane is high, and hydroxyethyl cellulose and sorbitan fatty acid ester are This is thought to be because it is easily incorporated into the surface layer of the polymer particles formed.

In the present invention, the amount of cyclohexane to be used is 0.5 to 5 %, based on the aqueous solution containing the acrylic acid monomer, in order to make the polymerization reaction system a water-in-oil type and to remove the heat of the polymerization reaction. It is desirable to use a weight ratio.

(Specific manufacturing method) An example of a specific embodiment of the method for manufacturing a superabsorbent polymer according to the present invention is as follows.

Neutralize acrylic acid and (or) methacrylic acid in advance to obtain an aqueous alkali metal salt solution or an aqueous ammonium salt solution,
A crosslinking agent, a water-soluble radical polymerization initiator, and hydroxyethyl cellulose are added and dissolved therein, and an inert gas such as nitrogen is introduced to perform deaeration.

Separately, sorbitan fatty acid ester with HLB 3 to 6 is added to dichlorohexane and suspended by slightly heating if necessary.
Deaeration is performed by introducing an inert gas such as nitrogen. Adding an aqueous solution containing the above acrylic acid monomer to this,
Polymerize by heating to a predetermined temperature.

The polymer after polymerization consists of wet bead-like particles, which can be easily separated from the dispersion medium by decantation or evaporation, either as is or after azeotropic dehydration. Then, the wet polymer is
For example, by drying at a temperature of 120° C. or lower, a powdery polymer is obtained. The polymer obtained in this way usually has a particle size of 100 μm or more, and is in the form of granules containing a small amount of true spherical primary particles or secondary particles that are partially agglomerated. It is something.

These secondary particles can be easily crushed into primary particles by a small amount of mechanical force.

[Experiment example]

The following experimental examples are for more specifically explaining the present invention. The water absorption capacity and gel strength of the superabsorbent polymer were measured according to the following method.

Water Absorption Capacity ■Pure Water Water Absorption Capacity Approximately 0.2 g of super absorbent polymer is accurately weighed in 1000 cc of pure water, and the water is allowed to absorb water for 1 hour while stirring with a magnetic stirrer. After water absorption, the swollen gel was drained for 15 minutes using a 100-mesh sieve, and then the weight of the swollen gel was measured, and the pure water absorption capacity was calculated according to the following formula.

■069% physiological saline water absorption capacity Precisely weigh approximately 0.5 g of the superabsorbent polymer, put it in a 400 mesh nylon bag (10 cm x 10 cm), and soak it in 500 cc of 0.9% physiological saline for 1 hour. Soak. After that, the nylon bag was pulled up and drained for 15 minutes, then its weight was measured, the blank was corrected, and 0 was calculated using the same formula as above.
．． 9% physiological saline water absorption capacity was calculated.

Absorb 100 g of pure water in 0.5 g of gel strength super absorbent polymer (
The gel after water absorption was measured using a rheometer (Fudo Kogyo Model NMR-2002J), and the force at which the cell entered the gel was defined as the gel strength.

Example 1 Cyclohexane 1371r and 0.509 g of sorbitan monostearate (HLB 4.7) were added and dissolved in a 1500 cc four-day round bottom flask equipped with a stirrer, reflux condenser, thermometer and nitrogen gas inlet tube. After that, nitrogen gas was introduced to drive out dissolved oxygen.

Separately, in a conical flask with a capacity of 200 cc, 33.9 g of acrylic acid was cooled from the outside, and 1 liter of water was added to it in advance.
An alkaline aqueous solution prepared from 6.3 g and 52.7 g of 25% sodium hydroxide was added to remove 7 of the carboxyl group.
Neutralized 0%. The monomer concentration relative to water in this case corresponds to 40% by weight as the monomer concentration after neutralization.

Next, 0.0119 g of N,N'-methylenebisacrylamide as a crosslinking agent, hydroxyethyl cellulose (manufactured by Fuji Chemical Co., Ltd., part name: rAX15),
Degree of substitution -1, number of moles of EO added -1.6 to 1.8) 0.6
After adding and dissolving 78 g and 0.118 g of potassium persulfate as a water-soluble radical polymerization initiator, nitrogen gas was blown in to drive out dissolved oxygen.

Add this 200cc to the contents of the four-day round bottom flask mentioned above.
The contents of the conical flask were added at 15°C, dispersed with stirring, and heated at a rate of 2.4°C/min.
Polymerization was carried out at 70°C for about 1 hour. Subsequently, azeotropic dehydration was performed for 4 hours under cyclohexane reflux. In addition, stirring is 2
It was performed at 5 orpm.

When stirring was stopped, the wet beaded polymer particles settled to the bottom of the flask and could be easily separated from the cyclohexane layer by decantation. The separated polymer was transferred to a vacuum dryer and heated to 80 to 90°C to remove adhering cyclohexane and water, yielding a smooth powdery polymer. The average particle size of the polymer is 35
The polymer had a diameter of 5 μm, and analysis of residual solvent by headspace gas chromatography revealed that the polymer contained 35 ppm of cyclohexane.

Example 2 A polymer having an average particle size of 271 μm and 3 ppm of residual cyclohexane was obtained by carrying out the same procedure as in Example 1, except that the amount of N,N'methylenebisacrylamide used as a crosslinking agent was changed to 0.0475 g.

Example 3 A polymer with an average particle size of 202 μm and a residual cyclohexane of 18 ppm was prepared in the same manner as in Example 1, except that the amount of sorbitan monostearate used was changed to 1.02K and the amount of hydroxyethyl cellulose used was changed to 0.509 g. I got it.

Example 4 In Example 1, the amount of sorbitan monostearate used was changed to 1.02 g, and the amount of hydroxyethyl cellulose used was changed to FAX-15J.
The average particle size was 203 μm using the same procedure except that 8 g was used.
A polymer containing 33 ppm of cyclohexane remaining was obtained.

Example 5 121 g of cyclohexane and 0.0 g of sorbitan monostearate were placed in a 500 cc four-day round bottom flask equipped with a stirrer, reflux condenser, thermometer and nitrogen gas inlet tube.
After adding and dissolving 0.09 g, nitrogen gas was introduced to drive out dissolved oxygen.

Separately, in a conical flask with a capacity of 200 cc, 30. While cooling the mixture from the outside, an aqueous alkaline solution prepared in advance from 44.9 g of water and 46.6 g of 25% sodium hydroxide was added to neutralize 70% of the carboxyl groups. The monomer concentration relative to water in this case corresponds to 30% by weight as the monomer concentration after neutralization. Next, to this was added 0.042 g of N,N'-methylenebisacrylamide as a crosslinking agent, 75 g of FAX-15J O as hydroxyethyl cellulose, and 0.101 g of potassium persulfate as a water-soluble radical polymerization initiator.
After adding r and dissolving it, nitrogen gas was blown in to drive out the dissolved oxygen.

Add this 200cc to the contents of the four-day round bottom flask mentioned above.
The contents of the conical flask were added at 15°C, dispersed under stirring, and allowed to warm at a rate of 2.4°C/min.
Polymerization was carried out at 70°C for about 1 hour. Subsequently, azeotropic dehydration was performed for 4 hours and 30 minutes under cyclohexane reflux. Note that stirring was performed at 250 rpm.

When stirring was stopped, the wet beaded polymer particles settled to the bottom of the flask and could be separated into the cyclohexane layer and the container by decantation. The separated polymer was transferred to a vacuum dryer and heated to 80 to 90° C. to remove adhering cyclohexane and water, yielding a smooth powdered polymer with an average particle size of 320 μm. The residual cyclohexane was 11 ppm.

Example 6 121 g of cyclohexane and 0.0 g of sorbitan monostearate were placed in a 500 cc four-day round bottom flask equipped with a stirrer, reflux condenser, thermometer and nitrogen gas inlet tube.
After adding and dissolving 531sr, nitrogen gas was introduced to drive out dissolved oxygen.

Separately, in a conical flask with a capacity of 200 cc, 35.4 sr of acrylic acid was added in advance to 5.08 g of water and 55 sr of 25% sodium hydroxide while being cooled from the outside. 70% of the carboxyl groups were neutralized by adding an alkaline aqueous solution prepared from step (o). The monomer concentration with respect to water in this case corresponds to 45% by weight as a heptamer concentration after neutralization. Next, 0.0124 g of N,N'-methylenebisacrylamide was added as a crosslinking agent, and 0.637 g of FAX-15J was added as hydroxyethyl cellulose.
and 0 potassium persulfate as a water-soluble radical polymerization initiator.
．． After adding and dissolving 123 g, nitrogen gas was blown in to drive out dissolved oxygen.

Add this 200ec to the contents of the four-day round bottom flask.
The contents of the conical flask were added at 15°C, dispersed under stirring, and heated at a rate of 2.4°C/min.
Polymerization was carried out at 70°C for about 1 hour. Subsequently, evaporative dehydration was performed under cyclohexane reflux for 3 and a half hours. Note that stirring was performed at 25 Or pm.

When stirring was stopped, the wet beaded polymer particles settled to the bottom of the flask and could be easily separated from the cyclohexane layer by decantation. The separated polymer was transferred to a vacuum dryer and heated to 80 to 90° C. to remove adhering cyclohexane and water, yielding a smooth powdered polymer with an average particle size of 320 μm. The residual cyclohexane was 11 ppm.

Comparative Example 1 The same procedure as in Example 1 was repeated except that the amount of sorbitan monostearate used was changed to 1.02 g, and hydroxyethyl cellulose was not used.
A finely powdered polymer was obtained.

The residual cyclohexane was 4 ppm.

Comparative Example 2 A polymer with an average particle size of 283 μm was obtained in the same manner as in Example 1, except that n-hexane was used as the dispersion medium instead of cyclohexane, and the amount of sorbitan monostearate was changed to 1.02 g. Ta. The residual normal hexanes were extremely high at 3,750 ppm.

Comparative Example 3 In Example 1, the amount of sorbitan monostearate used was changed to 1.02. Instead, the same procedure was performed except that no crosslinking agent was used to obtain a polymer having an average particle size of 310 μm and a residual cyclohexane of 35 ppm.

Table 1 summarizes the water absorption capacity, gel strength, average particle diameter, and amount of residual solvent of the superabsorbent polymers obtained in Examples and Comparative Examples.

As is clear from Table 1, the superabsorbent polymer produced according to the present invention has excellent water absorption capacity and gel strength, and has a significantly low amount of residual solvent.

Table 1

Claims (1)

[Claims] An acrylic acid monomer mainly composed of acrylic acid and/or methacrylic acid and an alkali metal salt or ammonium salt thereof is mixed with HLB 3 to 6 as a dispersant in the presence of a water-soluble radical polymerization initiator and hydroxyethyl cellulose. When polymerizing sorbitan fatty acid ester by water-in-oil reverse phase suspension polymerization, the polymerization of acrylic acid monomer is carried out in the presence of a crosslinking agent.
A method for producing a superabsorbent polymer, characterized in that it is carried out in a cyclohexane solvent.