JPH0848721A - Preparation of water-absorptive resin - Google Patents

Abstract

PURPOSE:To obtain a water-absorptive resin having a very large average primary particle size without causing abnormal polymn. and with little deposition thereof on a polymerizer by effecting water-in-oil type reverse phase suspension polymn. of a partially neutralized acrylic acid monomer in the presence of a specific surfactant. CONSTITUTION:An aq. soln. of acrylic acid monomers having a monomer concn. of at least 20wt.% after neutralization, wherein pref. at least 50mol% COOH groups are neutralized with an alkali metal and the acrylic acid monomers are selected from among (meth)acrylic acid and alkali metal salts thereof, is subjected to water-in-oil type reverse phase suspension polymn. in an inert solvent in the presence of a cross-linking agent, a surfactant and a free radical polymn. initiator, wherein a mixture of a sorbitan fatty ester of 2 to 12 in HLB (e.g. sorbitan monostearate), a 20-50C alkane and/or alkene and a copolymer (deriv.) of a 20-50C alkene with a polybasic alpha,beta-unsatd. carboxylic acid anhydride (e.g. maleic acid) is obtd. as the surfactant.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a water absorbent resin. More specifically, the present invention relates to a method for producing a water absorbent resin by subjecting a partially neutralized salt of an acrylic acid-based monomer to a water-in-oil type reverse phase suspension polymerization using a specific surfactant.

The water-absorbent resin obtained by the production method of the present invention is excellent in water absorbability and absorbs a large amount of water to swell, but is insoluble in water, the particle size of the polymer is large, and the resin itself is easy. Since it can be pulverized into various types, it can be advantageously used for the production of various absorbent materials or various materials that are used in a swollen state by absorbing water.

[0003]

2. Description of the Related Art Conventionally, paper, pulp, nonwoven fabric, sponge-like urethane resin and the like have been used as water retention agents in sanitary napkins, paper diapers, various sanitary materials and various agricultural materials. However, since the water absorption amount of these materials is only about 10 to 50 times their own weight, a large amount of material is required to absorb or retain a large amount of water, and not only becomes extremely bulky, There is a defect that water is easily separated when pressure is applied to the absorbed material. In order to improve the above-mentioned drawbacks of this type of water absorbing material, various high water absorbing polymer materials have been proposed in recent years. For example, starch graft polymers (Japanese Patent Publication No. 53-46199, etc.), cellulose modifiers (Japanese Patent Laid-Open No. 50-80376, etc.),
A cross-linked product of a water-soluble polymer (Japanese Patent Publication No. 43-23462, etc.), a self-crosslinking alkali metal acrylate polymer (Japanese Patent Publication No. 54-30710, etc.) and the like have been proposed.
However, these water-absorbing polymer materials are still insufficient in the amount of water absorption, and in some of the above-mentioned publications, the resin obtained by drying is extremely hard, and it is difficult to pulverize easily, and a large mechanical pulverizing force is required. There were many problems in practical or industrial scale production such as need.

The present inventors have already proposed a method for producing a water-absorbing material that has improved the above-mentioned drawbacks of the conventional water-absorbing materials (Japanese Patent Laid-Open Nos. 61-157513 and 62-62807).
No. However, the water absorbing material produced by the method disclosed in the above publication also has various drawbacks. That is, in JP-A-61-157513, a polymerization reaction is extremely stable because a nonionic surfactant having an HLB of 3 to 6 is used as a surfactant in the water-in-oil type reverse phase suspension polymerization method. The obtained polymer has an average particle size of 100 μm or less and becomes extremely fine powder. Therefore, when handling powder, it is necessary to take measures against dust. On the other hand, the present inventors, in JP-A-62-62807, use a copolymer of an α-olefin and an α, β-unsaturated polycarboxylic acid anhydride or a derivative thereof as a surfactant, By polymerizing in the presence of a small amount of a cross-linking agent, a method for producing a beaded superabsorbent polymer which has an extremely high water absorption capacity, a high gel strength, a large particle size of the polymer, and can be easily pulverized was proposed. However, in this method, as a result of various detailed studies, suspended particles tend to adhere during the polymerization reaction,
As a result, it becomes a massive state and causes abnormal polymerization,
It was found that not only is it extremely dangerous in operation, but it is difficult to obtain the desired bead-shaped water-absorbing polymer. on the other hand,
As a method for solving the above problems, Japanese Patent Laid-Open No. 62-95308
In the publication, a method of polymerizing in the presence of hydroxyethyl cellulose was further proposed.

[0005]

However, the above-mentioned method using hydroxyethyl cellulose allows stable polymerization without causing abnormal polymerization.
And, although it is possible to obtain a water-absorbent resin having an extremely large average primary particle diameter of the polymer, the polymer is strongly adhered to heating equipment such as a polymerization container, and it is extremely difficult to continuously and stably produce the polymer. There is a problem.

[0006]

Under the circumstances, the present invention has the following background.
As a result of diligent studies on a method of simultaneously satisfying all of the three conditions in which the obtained polymer has a large primary average particle size, can be stably polymerized without causing abnormal polymerization, and has very little polymer adhesion to the polymerization vessel, etc. As a surfactant in turbid polymerization, (1) sorbitan fatty acid ester having HLB = 2 to 12 (2) alkane and / or alkene having 20 to 50 carbon atoms (3) alkene having 20 to 50 carbon atoms, and α, β- It has been found that the above object can be achieved by using a mixture of a copolymer with an unsaturated polycarboxylic acid anhydride or a derivative thereof, and the present invention has been completed. That is, the present invention provides an aqueous solution containing an acrylic acid-based monomer selected from the group consisting of acrylic acid and / or methacrylic acid and alkali metal salts thereof in an amount of 20% by weight or more after neutralization as a cross-linking agent and an interface. In the method of reverse-phase suspension polymerization of water-in-oil type in an inert solvent using a radical polymerization initiator in the presence of an activator, (1) a sorbitan fatty acid ester having HLB = 2 to 12 ( 2) Alkane and / or alkene having 20 to 50 carbon atoms (3) Use of a mixture of a copolymer of an alkene having 20 to 50 carbon atoms and an α, β-unsaturated polycarboxylic acid anhydride or a derivative thereof. And a method for producing a water-absorbent resin. Hereinafter, each requirement of the method of the present invention will be specifically described.

(Monomer) The monomer used in the present invention is acrylic acid and / or methacrylic acid, and 50 mol% or more, preferably 65 mol% or more of all the carboxyl groups thereof are neutralized with an alkali metal salt. Is an acrylic acid-based monomer. If the degree of neutralization is too low, the amount of absorbed water will be significantly small, and the strength of the absorbed gel will be small. To neutralize the acid monomer into an alkali metal salt, an alkali metal hydroxide or bicarbonate can be used, but an alkali metal hydroxide is preferable,
Specific examples thereof include sodium hydroxide, potassium hydroxide and lithium hydroxide. Sodium hydroxide is most preferable from the viewpoint of industrial availability, price, and safety. The larger the amount of the acrylic acid-based monomer used in the present invention, the better. Specifically, the monomer concentration after neutralization with respect to water is 20% by weight or more, preferably 30% by weight or more. The higher the monomer concentration is, the more advantageous it is at the completion of a unit batch, and the easier the dehydration operation after polymerization is, which is economically advantageous.

(Crosslinking Agent) The crosslinking agent used in the production method of the present invention has two or more double bonds in the molecule, has good copolymerizability with the acrylic acid-based monomer, and has an efficient crosslinking structure. Should be given. Examples of such a cross-linking agent include ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, polypropylene glycol di (meth) acrylate, glycerin tri ( (Meth) acrylate, N, N′-methylenebis (meth) acrylamide, diallyl phthalate, diallyl malate, diallyl terephthalate, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate and the like, among which polyethylene glycol di (Meth) acrylate and N, N'-methylenebisacrylamide are preferred. The amount of these crosslinking agents used is 0.001 to 10% by weight, preferably 0.01 to 2% by weight, based on the acrylic acid-based monomer. 0.00
If it is less than 1% by weight, the water absorption capacity will be extremely high, but the gel strength upon absorption of water will be extremely weak, and if it exceeds 10% by weight, the water absorption gel strength will be remarkably improved, but the water absorption capacity will be considerably small. It becomes a problem in practical use.

(Radical Polymerization Initiator) The polymerization initiator used in the production method of the present invention is basically soluble in an aqueous solution of an acrylic acid type monomer. Specific examples of such an initiator include hydrogen peroxide, persulfates such as ammonium persulfate and potassium persulfate, hydroperoxides such as t-butyl hydroperoxide and cumene hydroperoxide, and azoisobutyronitrile. , 2,2-
An azo-based initiator such as azobis (2-amidinopropane) dihydrochloride is used. Further, these radical initiators may be used as a redox-type initiator in combination with a reducing substance such as sodium bisulfite and L-ascorbic acid, or an amine. The amount of these radical initiators used is 0.01 to 0.01 with respect to the acrylic acid-based monomer.
It is 10% by weight, preferably 0.1 to 2% by weight.

(Inert Solvent) As the inert solvent used in the present invention, in principle, all liquid hydrocarbons can be used as long as they do not participate in the polymerization and are not mixed with water. For example, aromatic hydrocarbons such as benzene, ethylbenzene, toluene and xylene, alicyclic hydrocarbons such as cyclohexane, methylcyclohexane, cyclooctane and decalin, aliphatic hydrocarbons such as hexane, pentane, heptane and octane, chlorobenzene and bromine. Examples thereof include halogenated hydrocarbons such as benzene and dichlorobenzene. Among these, alicyclic hydrocarbons such as cyclohexane and methylcyclohexane, and aliphatic hydrocarbons such as hexane and heptane are particularly preferable. In addition, these inert solvents can be used alone or in combination of two or more kinds. The amount of these inert solvents used is 0.5 to 10 times the volume of the aqueous monomer solution in order to make the polymerization reaction system a water-in-oil type and to remove heat of the polymerization reaction.
The amount is preferably 0.8 to 5 times.

(Surfactant) The surfactant used in the present invention is (1) sorbitan fatty acid ester having HLB = 2 to 12 (2) alkane and / or alkene having 20 to 50 carbon atoms (3) carbon number It is characterized by using a mixture of a copolymer of an alkene of 20 to 50 and an α, β-unsaturated polycarboxylic acid anhydride or a derivative thereof.
Examples of the sorbitan fatty acid ester having HLB = 2 to 12 of (1) include sorbitan tristearate, sorbitan distearate, sorbitan monostearate, sorbitan monooleate, sorbitan monopalmitate, and sorbitan monolaurylate. Used as one kind or as a mixture of two or more kinds. Among these, sorbitan monostearate can be mentioned as a particularly preferable example in the present invention. 20 to 50 carbon atoms of (2)
The alkane and / or alkene has 20 to 20 carbon atoms.
It is an alkane and / or alkene within the range of 50, and regardless of linear, branched, 1-olefin, internal olefin, etc., any one can be used, or a mixture thereof may be used. Among these alkanes and / or alkenes, those having 25 to 35 carbon atoms are particularly preferable in the present invention. (3) has 20 to 20 carbon atoms
The 50 alkene is an alkene having 25 to 35 carbon atoms, preferably a 1-olefin having 25 to 35 carbon atoms. These alkenes may be the same as the alkenes shown in (2). Further, examples of the α, β-unsaturated polycarboxylic acid anhydride include maleic anhydride, citraconic anhydride, itaconic anhydride, and the like, of which maleic anhydride is preferable. The derivative of these copolymers is a partially esterified product, partially amidated product or partially neutralized product of the copolymer. Examples of the partially esterified product include copolymer monomethyl ester, monoethyl ester, and monobutyl ester. Examples of the partially amidated product of the copolymer include monoethylamide, monopropylamide and monobutylamide of the copolymer. Examples of the partially neutralized product include neutralized products with alkali metals, alkaline earth metals, and ammonium. Among these, alkali metals such as sodium, potassium and lithium are preferable in the present invention. In addition, in the present invention, the α, β-unsaturated polycarboxylic acid of the above-mentioned copolymer may be in the state of an acid anhydride or in the state of being partially or wholly opened when used. The composition of the alkene and the α, β-unsaturated polycarboxylic acid anhydride in the copolymer is 99 to 50: 1 to 50 by weight, preferably 99 to 30: 1.
~ 70. Further, the molecular weight of the copolymer or its derivative is 1,000 to 100,000, preferably 3,000.
~ 50,000. The composition of the mixture of (1) to (3) above is a weight ratio of (1) :( 2) + (3) = 20 to 80:80 to 20, (2) :( 3) = 2 to 70:98. -30 It is (1) :( 2) + (3) = 30-70: 70-30 (2) :( 3) = 5-60: 95-40 preferably. The amount of these surfactants used as a mixture is 0.01 to 10% by weight, preferably 0.
It is from 05 to 5% by weight.

(Polymerization conditions etc.) Typical embodiments of the polymerization reaction of the present invention are as follows. That is, a crosslinking agent and a radical polymerization initiator are added and dissolved in a previously neutralized acrylic acid-based monomer aqueous solution, and an inert gas such as nitrogen is introduced to perform deaeration. At this time, water-soluble chain transfer agents such as thiols, thiol acids, secondary alcohols, amines and hypophosphites may coexist in the aqueous monomer solution. On the other hand, a surfactant is put in an inert solvent and slightly heated to dissolve it if necessary, and an inert gas such as nitrogen is introduced to degas. The above monomer aqueous solution is poured into this and heated to a predetermined temperature. During this period, the aqueous solution of the reaction system becomes fine droplets and is dispersed and suspended in the inert solvent. In addition, the injection of the monomer aqueous solution at this time may be a batch method in which the entire amount is injected at a time or a dropping method. After the initiation of polymerization, cooling or heating is appropriately performed depending on the state of heat generation. The polymerization reaction temperature is 60 to 100 ° C, preferably 60 to 80 ° C.
The polymerization reaction of the present invention is carried out using a water-in-oil type reverse phase suspension polymerization method. For that purpose, as described above, as a surfactant, (1) sorbitan fatty acid ester having HLB = 2 to 12 (2) carbon Alkane and / or alkene having a number of 20 to 50 (3) A mixture of a copolymer of an alkene having a number of carbon atoms of 20 to 50 and an α, β-unsaturated polycarboxylic acid anhydride or a derivative thereof is used. The important point at this time is to achieve the object by appropriately adjusting the ratio of the inert solvent and the monomer aqueous solution and further combining appropriate means such as stirring. That is, the weight ratio of the inert solvent to the aqueous monomer solution is usually 10: 1 to 1: 1.
It is 0.5: 1, preferably 5: 1 to 0.8: 1. If the ratio of the inert solvent to the aqueous monomer solution is too large, the polymerization stability increases, but the productivity per batch, for example, deteriorates. On the other hand, if it is too small, it becomes difficult to remove the heat of polymerization, which causes a problem in polymerization stability. The absolute value of the stirring speed cannot be uniquely shown because it depends on the type of stirring blade used and the scale of the polymerization reaction tank, but it cannot be shown uniquely, but if the surfactant mixture of the present invention is used, a suitable rotation speed can be obtained. Number of stable emulsion particles can be formed, and 2
An extremely large average particle size of 00 μ or more can be obtained, and the polymerization can be stably carried out. At this time, excessive stirring not only reduces the average particle size of the obtained polymer particles,
It is not preferable because the water absorption performance may be significantly impaired.

The polymer obtained by the production method of the present invention is composed of swollen bead-like particles and can be separated from the inert solvent by decantation or evaporation. Further, if necessary, further azeotropic dehydration in the presence of an inert solvent or direct dehydration by heating to obtain a dried powdery polymer. Further, in the present invention, surface crosslinking may be carried out by setting an appropriate moisture content during the drying. The polymer thus obtained is usually a powder having a large spherical primary particle having an average particle diameter of 200 μ or more or a small amount of secondary particles obtained by partially aggregating them.
The secondary particles can also be easily crushed with a slight mechanical force. This has great advantages in terms of polymer production and use.

[0014]

EXAMPLES The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples. The physiological saline absorption capacity and average particle size described in these examples are the results measured by the following test methods. A. Saline water absorption capacity 1 g of water-absorbent resin in 400 mesh nylon bag (10c
m × 10 cm) and soak in 1 liter of 0.9% saline for 1 hour. After 1 hour, the nylon bag was pulled up, drained for 15 minutes, weighed, blank-corrected, and the weight of 0.9% physiological saline in which 1 g of the water-absorbent resin was absorbed was taken as the water absorption amount. B. Average particle size Among JIS standard sieves, 12, 24, 35, 4
The particle size distribution on the basis of mass was determined using a sieve of 2, 60, 80, 100, 150, 200, 325 mesh, and the 50% particle size on the basis of mass was taken as the average particle size.

Example 1 Into a 1-liter four-necked round-bottomed flask equipped with a stirrer, a reflux condenser, a thermometer, and a nitrogen gas inlet tube, 319.1 g of cyclohexane was placed, and HLB = 4.7. Sorbitan monostearate 1.117 g, carbon number 28-3
Alkene of 8 (R ₁ CH = CH ₂ R ₂ : 16%, R ₁ R
_{2 C = CH 2: 29%} , CH 2 = CH-R: 55%)
0.537 g, a copolymer of an alkene having 28 to 38 carbon atoms and maleic anhydride (molecular weight 3000-25000)
0.581 g of a surfactant mixture (sorbitan monostearate: alkene + copolymer = 50: 50, alkene: copolymer = 48: 52) was added and dissolved, and the internal temperature was adjusted to 20 ° C. under a nitrogen gas atmosphere. did. Separate capacity 500c
To a conical flask of c, 56.0 g of water was added while cooling 116.9 g of acrylic acid from the outside, and 181.6 g of 25% caustic soda was added to 70% of the carboxyl groups.
Neutralized. The monomer concentration with respect to water in this case corresponds to 40% by weight as the monomer concentration after neutralization. Then, add N, N'-methylenebisacrylamide 0.8 to this.
18 g and 0.1215 g of potassium persulfate, and 0.111 g of sodium hypophosphite hydrate as a water-soluble chain transfer agent.
Was added and dissolved. The contents of the 500 cc flask were added to the contents of the above-described four-necked round bottom flask, and the mixture was stirred and suspended. It should be noted that stirring was performed using a pitch paddle blade, 1
It was performed at 70 rpm. Next, when the temperature was raised at the same number of revolutions and the temperature was raised to about 50 ° C., the polymerization started and reached a peak at about 73 ° C. Thereafter, it was kept at 70 ° C. for 1 hour. After reacting for 1 hour, the rotation speed was set to 330 rpm, heating was further performed, and dehydration was performed by azeotropic distillation with cyclohexane. After dehydration,
When the stirring was stopped, the wet polymer particles settled to the bottom of the flask and could be easily separated from the cyclohexane phase by decantation. The separated polymer was heated at 90 ° C. to remove attached cyclohexane and trace water. The resulting dry polymer was a free-flowing, easily millable powder.

Example 2 The alkene having 28 to 38 carbon atoms in Example 1 was replaced with 0.0
56g, 1.06 of alkene and maleic anhydride copolymer
A dried polymer was obtained by the same operation and method as in Example 1 except that the amount was 2 g (alkene: copolymer = 5: 95).

Example 3 The alkene having 28 to 38 carbon atoms in Example 1 was replaced with 0.7.
83 g, 0.33 of alkene and maleic anhydride copolymer
A dry polymer was obtained by the same operation and method as in Example 1, except that the amount was 5 g (alkene: copolymer = 70: 30).

Example 4 The sorbitan monostearate in Example 1 was adjusted to 0.6
71 g, 0.751 g of alkene having 28 to 38 carbon atoms, 0.814 g of alkene and maleic anhydride copolymer (sorbitan monostearate: alkene + copolymer = 30: 7)
0, alkene: copolymer = 48: 52) except that the same procedure and method as in Example 1 were used to obtain a dry polymer.

Example 5 The sorbitan monostearate in Example 1 was set to 1.5.
65 g, 0.322 g of alkene having 28 to 38 carbon atoms, 0.349 g of alkene and maleic anhydride copolymer (sorbitan monostearate: alkene + copolymer = 70: 3)
0, alkene: copolymer = 48: 52) except that the same procedure and method as in Example 1 were used to obtain a dry polymer.

Example 6 Instead of the sorbitan monostearate in Example 1, sorbitan monolaurate 1.11 with HLB = 8.6.
A dried polymer was obtained by the same operation and method as in Example 1 except that the amount was 7 g.

Example 7 Instead of the sorbitan monostearate in Example 1, sorbitan tristearate 1.11 with HLB = 2.1
A dried polymer was obtained by the same operation and method as in Example 1 except that the amount was 7 g.

Example 8 Example 1 was repeated except that the alkene and maleic anhydride copolymer used in Example 1 were treated with methanol under reflux for about 8 hours and excess methanol was removed under reduced pressure.
A dried polymer was obtained by the same operation and method.

Example 9 The alkene and maleic anhydride copolymer used in Example 1 were treated with a large excess of butylamine under reflux at 70 ° C. for about 8 hours, and the excess butylamine was removed under reduced pressure. A dried polymer was obtained by the same operation and method as in Example 1.

Comparative Example 1 The same operation as in Example 1 except that only 2.235 g of sorbitan monostearate was used as the surfactant in Example 1.
A dry polymer was obtained by the same method.

Comparative Example 2 The same operation as in Example 1 except that 1.073 g of an alkene having 28 to 38 carbon atoms and 1.162 g of an alkene / maleic anhydride copolymer were used as the surfactant in Example 1.
Polymerization was carried out by the same method, but the polymerization was extremely unstable and was often agglomerated.

Comparative Example 3 Polymerization was carried out by the same operation and method as in Example 1, except that only 2.235 g of the alkene and maleic anhydride copolymer was used as the surfactant in Example 1, but the polymerization was extremely unstable. Yes, and often agglomerated.

Comparative Example 4 The same operation as in Example 1 except that a mixture of sorbitan monostearate (1.118 g), alkene and maleic anhydride copolymer (1.118 g) was used as the surfactant in Example 1.
Polymerization was carried out by the same method, but the polymerization was extremely unstable and was often agglomerated.

Comparative Example 5 The alkene having 28 to 38 carbon atoms in Example 1 was changed to 0.8.
94g, 0.22 of alkene and maleic anhydride copolymer
A dried polymer was obtained by the same operation and method as in Example 1 except that the amount was 4 g. Table 1 summarizes the results shown in the above Examples and Comparative Examples. As is clear from the present results, by using the surfactant of the present invention, abnormal polymerization is not caused at all, and the adhesion of the polymer to the polymerization container or the like is significantly improved, and the continuous and stable high-performance water-absorbing property is obtained. It can be seen that the resin can be produced and that the average particle size of the primary particles is 200 μm or more, which is extremely large. Therefore, in the present invention, as described above, the obtained polymer has a large primary average particle diameter,
It is intended to provide a method capable of stably polymerizing without causing abnormal polymerization, and simultaneously satisfying all three conditions such as adhesion of a polymer to a polymerization container and the like which are extremely small.

[0029]

[Table 1]

[0030]

According to the method of the present invention, a water-absorbing resin having basic water-absorbing properties such as water-absorbing ability and water-absorbing speed and having an extremely large average primary particle size can be polymerized without causing abnormal polymerization at all. There is very little polymer adhesion to containers, etc., and it is possible to manufacture safely, continuously and stably.
Its industrial value is great.

Claims (3)

[Claims] 1. A cross-linking agent and a surface active agent are prepared as an aqueous solution containing an acrylic acid-based monomer selected from the group consisting of acrylic acid and / or methacrylic acid and alkali metal salts thereof in an amount of 20% by weight or more after neutralization. In the method of carrying out a water-in-oil type reverse phase suspension polymerization in an inert solvent using a radical polymerization initiator in the presence of an agent, (1) a sorbitan fatty acid ester of HLB = 2 to 12 (2) ) Alkane and / or alkene having 20 to 50 carbon atoms (3) Use of a mixture comprising a copolymer of an alkene having 20 to 50 carbon atoms and an α, β-unsaturated polycarboxylic acid anhydride or a derivative thereof. A method for producing a water-absorbent resin, which is characterized. 2. The method according to claim 1, wherein 50 mol% or more of all the carboxyl groups of the acrylic acid-based monomer are neutralized with an alkali metal salt. 3. The composition of surfactants (1), (2), and (3) in a weight ratio of (1) :( 2) + (3) = 20 to 80:80 to 20, (2): (3) = 2-70: 98-30, The manufacturing method of Claim 1.