JP2716485B2 - Method for producing superabsorbent polymer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a superabsorbent polymer.

The superabsorbent polymer obtained in the present invention has high safety, excellent water absorption performance, and has a large average particle size. Furthermore, since the fluidity and transportability of the product are good, it can be advantageously used for various water absorbing materials.

2. Related Art In recent years, superabsorbent polymers are used not only in the field of sanitary materials such as sanitary products and disposable diapers, but also in industrial applications such as waterproofing materials, anti-condensation materials, freshness-maintaining materials, solvent dehydrating materials, etc., greening, agricultural and horticultural applications, etc. It is a synthetic polymer that is being put to practical use, and its application range is expected to be further expanded in the future.

Examples of this type of superabsorbent polymer include a hydrolyzate of a starch-acrylonitrile graft copolymer, a cross-linked carboxymethyl cellulose, a cross-linked polyacrylic acid (salt),
Acrylic acid (salt) -vinyl alcohol copolymers, crosslinked polyethylene oxides, and the like are known.

However, although all of these superabsorbent polymers almost satisfy the water absorbing ability, it is hard to say that they simultaneously satisfy various water absorbing properties such as a water absorbing speed and a water retaining ability. That is, a polymer having a high water absorption capacity generally has a low water absorption rate and poor water retention capacity.

As a method for improving the water absorption rate of the superabsorbent polymer, a method of mixing a so-called lubricant, such as a lipophilic surfactant, a non-volatile hydrocarbon or potassium stearate, and increasing the crosslink density of the polymer itself. Methods for reducing the hydrophilicity of polymers have been proposed. However, although the former method is only physical mixing and the lubricant is not chemically fixed to the polymer, although the improvement is seen at the very beginning, the effect is not permanent. .

In addition, the latter method has a problem that the water absorbing ability itself of the polymer is reduced.

Next, regarding the average particle diameter, particularly, the superabsorbent polymer synthesized by the inverse suspension polymerization method has many fine particles,
Many are difficult to handle.

As a method for increasing the average particle size of the superabsorbent polymer,
A method of adding a thickener to the monomer phase and polymerizing the same, a method of granulating the generated fine particles using a binder, and the like are generally used. However, the former method is not a good method because the polymer particles adhere to each other to increase the viscosity of the monomer phase and the whole or a part thereof becomes a mass. The latter is disadvantageous in terms of safety and cost because a binder is used.

In addition, it has been proposed to use various surfactants for the purpose of increasing various water absorbing properties or increasing the average particle diameter.

As such, for example, a method using a surfactant of HLB8-12 described in JP-A-56-131608 JP-A-57-158209, cellulose-based protection described in JP-A-57-158210 A method using a colloidal agent A method using a sucrose fatty acid ester described in JP-A-61-43606, a method using a polyglycerin fatty acid ester of HLB2-16 described in JP-A-62-172006, and the like.

However, although the above-mentioned method improves the average particle size of the obtained polymer and reduces the number of fine particles, it is not always satisfactory in that the polymer adheres strongly to the wall of the polymerization vessel at the time of polymerization and stable operation is performed. Not a thing.

In addition, in the method, if a small amount of the cellulose-based protective colloid agent is left on the polymer surface, it melts during drying, and the polymer portion may become a lump or may cause problems in fluidity and transportability. Because of this, it is hard to say that it is a good method.

Although the method (1) has high safety and can obtain a polymer having a relatively large particle size, it tends to be inferior in water absorption performance, particularly in water absorption rate and water retention capacity, as compared with the case where another surfactant is used.

Is very narrow in the allowable range of production conditions for obtaining a polymer having a large particle size, and if the conditions are deviated, bulk polymerization occurs.In consideration of industrial stable production, this method is not advantageous. Difficult to say.

Accordingly, there is still no superabsorbent polymer having a large average particle size and sufficiently satisfying the various properties required for the superabsorbent polymer. Taking into account the diameter and also the production stability, there is still room for improvement.

[Summary of the Invention]

The present invention seeks to provide a method for easily producing a highly water-absorbing polymer having high safety, excellent water absorption properties, a large average particle size, fluidity as a product, and good transportability. Is what you do.

The present inventors have conducted various studies for the purpose of solving the above-mentioned problems, and as a result, in the above-mentioned reversed-phase suspension polymerization method, (a) as a surfactant, a solid-state appearance at least at room temperature and HLB By using a mixed surfactant of a sorbitan fatty acid ester having a value in the range of 1 to 8 and a sucrose fatty acid ester having a (LB) HLB value in the range of 1 to 6, high safety and various water absorption properties are obtained. It has been found that a highly water-absorbing polymer which is excellent in fluidity, has a large average particle size, and has good fluidity and transportability as a product can be obtained, thereby completing the present invention.

That is, the method for producing a superabsorbent polymer according to the present invention comprises an acrylic acid-based monomer selected from the group consisting of acrylic acid and / or methacrylic acid and an alkali metal salt thereof.
In a method in which a reverse polymerization is carried out in a hydrocarbon solvent using a radical polymerization initiator in the presence of a cross-linking agent, the surfactant used in the reverse polymerization is (A) a solid at least at room temperature. It is a mixed surfactant of a sorbitan fatty acid ester having an appearance of a state and having an HLB value in the range of 1 to 8 and a sucrose fatty acid ester having a (ii) HLB value in the range of 1 to 6. It is assumed that.

Effect The superabsorbent polymer produced by the present invention is excellent in various water absorbing properties such as water absorbing ability, water absorbing speed, and water retaining ability. Moreover, it has a large average particle size, and has good fluidity and transportability as a product.

[Specific description of the invention]

Monomer As the monomer to be polymerized in the present invention, an acrylic acid monomer selected from the group consisting of acrylic acid and / or methacrylic acid and acrylic metal salts thereof is used. The alkali metal salt referred to herein means a carboxyl group of acrylic acid and / or methacrylic acid, for example,
It is a salt obtained by neutralization with an alkali metal hydroxide such as sodium hydroxide, potassium hydroxide, lithium hydroxide and the like. In terms of the performance, price, etc. of the produced polymer, a salt obtained by neutralizing acrylic acid and / or methacrylic acid with sodium hydroxide is particularly preferably used. The degree of neutralization of acrylic acid and / or methacrylic acid is one of the very important factors when considering the performance of the superabsorbent polymer in the present invention. When considering the balance, it is desirable that 50 to 95 mol% of the total carboxyl groups of the acrylic acid monomer be neutralized. When the degree of neutralization is less than 50 mol%, it tends to be poor in water absorption ability, and it is difficult to maintain a well-balanced performance. Conversely, if the neutralization exceeds 95 mol%, the polymerization rate becomes extremely slow, so that the merit of the neutralization is small.

The concentration of these acrylic acid monomers in solution is
It is preferably at least 20%, especially 30% to 60%.

In the present invention, in addition to the acrylic acid monomers described above, monomers copolymerizable therewith, such as maleic acid (salt), itaconic acid (salt), acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, (Meth) acryloylethanesulfonic acid, 2-hydroxyethyl (meth) acrylate, and the like can also be used in combination within a range that does not substantially impair the water absorbing performance of the water-absorbing polymer.

Crosslinking agent The above acrylic acid-based monomer, even if no crosslinking agent is used, causes some degree of self-crosslinking to become a highly water-absorbing polymer. It is necessary to add an agent component. As the crosslinking agent component, a water-soluble compound having two or more polymerizable unsaturated groups in the molecule and showing copolymerizability with the acrylic acid monomer, for example, N, N'-methylenebisacrylamide, N, Bisacrylamides such as N'-methylenebismethacrylamide are common and preferred.

The cross-linking agent component is usually
About 0.005 to 0.5% by weight, preferably about 0.01 to 0.3% by weight,
It is appropriate to use within the range.

Radical polymerization initiator In the reversed phase suspension polymerization method of the present invention, it is usual to dissolve a water-soluble radical polymerization initiator in an aqueous monomer solution in advance. Typical examples of the water-soluble radical polymerization initiator used here include persulfates such as potassium persulfate and ammonium persulfate, and azo-based initiators such as azobis- (2-amidinopropane) hydrochloride. These radical polymerization initiators are used in an amount of usually about 0.001 to 5.0% by weight, preferably about 0.01 to 1.0% by weight, based on the aqueous monomer solution.

Solvent As the solvent for the inverse suspension polymerization used in the present invention, an aliphatic hydrocarbon or an alicyclic hydrocarbon is preferable. Specific examples include normal pentane, normal hexane, normal heptane, cyclohexane, ligroin, and the like. Among them, cyclohexane and normal hexane are particularly preferable from the viewpoint of removing the heat of polymerization and drying the obtained polymer.

Surfactants The surfactants used in the present invention, that is, sorbitan fatty acid esters and sucrose fatty acid esters, as known as food emulsifiers, are safe ones that meet the standard items of the official food additive standard, It is a very preferable surfactant from the viewpoint of safety.

When these surfactants are mixed and used, it is essential that the sorbitan fatty acid ester (a) has a solid-state appearance at least at room temperature and has an HLB value in the range of 1 to 8. .

Sorbitan fatty acid ester that does not satisfy the conditions of
If even a small amount remains on the polymer surface, the fluidity and transportability of the product polymer deteriorate, and on the other hand, sorbitan fatty acid esters that do not satisfy the above conditions are not preferred because they tend to reduce the average particle size and performance.

The other sucrose fatty acid ester (b) preferably has an HLB value in the range of 1 to 6. When the sucrose fatty acid ester has an HLB value exceeding 6, the particle size of the produced polymer tends to be small.

The amount of these surfactants to be added is usually 0.1 to 10% by weight, preferably 0.1 to 10% by weight, based on the amount of the monomers mixed.
0.5 to 5% by weight. 0.1% by weight
If it is less than 10%, the desired dispersion effect cannot be obtained.
If the amount is more than the weight percentage, the polymer tends to aggregate when the polymer is dried, which is not preferable.

The mixing ratio of these surfactants is a very important factor in establishing various properties of water absorption and particle size, and is usually 80:20 to 20:80 by weight ratio of sorbitan fatty acid ester to sucrose fatty acid ester. , Preferably in the range of 60:40 to 40:60.

Examples of typical sorbitan fatty acid esters include sorbitan monopalmitate (HLB: 6.7), sorbitan monostearate (HLB = 4.7), sorbitan sesquistearate, (HLB = 4.2), and sorbitan tristearate (HLB).
B = 2.1).

Polymerization As described above, the present invention provides an acrylic acid monomer selected from the group consisting of acrylic acid and / or methacrylic acid and an alkali metal salt thereof by using a crosslinking agent, a radical polymerization initiator and two surfactants. Below, polymerization is carried out by a reversed-phase suspension polymerization method. The order of addition, the mode of addition and the polymerization operation of the acrylic acid-based monomer, crosslinking agent, radical polymerization initiator and surfactant are not limited as long as the object of the present invention is not violated. For example, (a) an acrylic acid-based monomer aqueous solution in which a crosslinking agent and a radical polymerization initiator are dissolved is added to a hydrocarbon solvent to which a surfactant has been added, suspended, and then heated to perform polymerization. Method (b): adding a surfactant to an aqueous solution of an acrylic acid monomer in which a crosslinking agent and a radical polymerization initiator are dissolved,
A method in which this is added to a hydrocarbon solvent and suspended, and polymerization is performed by heating. (C) An acrylic acid-based monomer aqueous solution in which a crosslinking agent and a radical polymerization initiator are dissolved is added to a hydrocarbon solvent and suspended. After that, a method of adding a surfactant and heating to carry out polymerization is exemplified.

The polymerization temperature is from 40 to 120 ° C, preferably from 60 to 90 ° C. The polymerization time varies depending on the polymerization temperature and the like, but is generally about 30 minutes to 6 hours, preferably 1 hour to 4 hours. After completion of the polymerization, the polymer is separated from the hydrocarbon solvent and dried to obtain the desired powdery superabsorbent polymer.

(Experimental example)

Hereinafter, the present invention will be described in detail with reference to experimental examples. The various water-absorbing properties of the superabsorbent polymer represent water-absorbing ability, water-absorbing speed (diffusion speed), and water-holding power, and were measured as follows. The method for measuring the average particle size is also described.

Water absorption capacity 1g of superabsorbent polymer 400g nylon bag (10c
mx 10 cm) and immersed in 1 liter of 0.9% saline for 30 minutes. After 30 minutes, pull up the nylon bag,
After draining for 15 minutes, the weight was measured, the blank was corrected, and the weight of the 0.9% physiological saline in which 1 g of the superabsorbent polymer had absorbed water was determined to have a water absorbing ability.

Water absorption rate (diffusion rate) It was measured using the apparatus shown in FIG. 1.0 g of superabsorbent polymer is placed on the nonwoven fabric on a support plate with small holes. When the superabsorbent polymer was brought into contact with 0.9% physiological saline from the lower surface, the amount of 0.9% physiological saline absorbed was measured. After the start, the water absorption rate (diffusion rate) was defined as the amount of 0.9% physiological saline absorbed in 10 minutes.

Water retention capacity Using the device shown in Fig. 1, 1 g of superabsorbent polymer
0.9%-25 ml of physiological saline is uniformly absorbed. This water-absorbing gel is placed in a 400 mesh nylon bag (10 cm × 10 cm), and the entrance is sealed. Place the top and bottom of this sample on paper (10
(cm × 10cm) sandwiched between 10 pieces, and apply a load of 45 g / cm ² for 3 minutes. The 0.9% physiological saline solution transferred to the paper was measured, and the water retention capacity was determined from the water absorption before and after pressurization.

Average particle size 42, 60, 80, 100, 15 of JIS standard screens
The particle size distribution based on the mass standard was determined using a sieve of 0, 200, or 325 mesh, and the 50% particle size based on the mass standard was defined as the average particle size.

Example 1 Cyclohexane 282 g was placed in a 1-liter four-necked round-bottom flask equipped with a stirrer, a reflux condenser, a thermometer, and a nitrogen gas inlet tube, and sorbitan monostearate 1.
After adding and dissolving 05 g and 1.05 g of sucrose fatty acid ester (trade name "DK-ester F-10" manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), nitrogen gas was blown in to expel dissolved oxygen.

Separately, in a 500 ml conical beaker, 28.6 g of 95% pure sodium hydroxide dissolved in 185.85 g of water was added to 70 g of acrylic acid while cooling with 70 g of acrylic acid from outside using 70 mol% of the total carboxyl groups. Was neutralized. The monomer concentration in water in this case corresponds to 30% by weight. Next, 0.098 g of N, N'-methylenebisacrylamide and 0.243 g of potassium persulfate were added and dissolved, and dissolved oxygen was expelled by blowing nitrogen gas.

To the contents of the four-necked round bottom flask,
Add the contents of the conical beaker of l, stir and disperse, place in an oil bath while bubbling nitrogen gas, and raise the temperature inside the flask. After several tens of minutes, the temperature reached 75 ° C. Next, the internal temperature is maintained at 60 to 65 ° C.
Allowed to react for hours. The stirring was performed at 250 rpm. After the reaction for 3 hours, when the stirring was stopped, the wet polymer particles settled at the bottom of the round-bottom flask, so that they could be easily separated from the cyclohexane phase by decantation. The separated wet polymer was transferred to a reduced-pressure drier and heated to 80 to 90 ° C. to remove attached cyclohexane and water, thereby obtaining about 90 g of a smooth powder polymer.

Example 2 In Example 1, 1.75 g of sorbitan monostearate and sucrose fatty acid ester (DK-
About 90 g of a powdered polymer was obtained in the same manner except that 1.75 g of ester F-10) was used.

Example 3 In Example 1, 1.40 g of sorbitan monostearate and sucrose fatty acid ester (DK-
About 90 g of powdered polymer was obtained in the same manner except that 0.7 g of ester F-10) was used.

Example 4 In Example 1, as a surfactant, sorbitan monostearate (1.05 g) and sucrose fatty acid ester (trade name “DK-Ester F-20” manufactured by Daiichi Kogyo Seiyaku Co., Ltd.)
About 90 g of a powdered polymer was obtained in the same manner except that 05 g was used.

Example 5 In Example 1, 1.40 g of sorbitan monostearate and sucrose fatty acid ester (DK-
About 90 g of powdered polymer was obtained in the same manner except that 0.7 g of ester F-20) was used.

Example 6 In Example 1, 1.05 g of sorbitan tristearate and sucrose fatty acid ester (DK-
About 90 g of a powdered polymer was obtained in the same manner except that 1.05 g of the ester F-10) was used.

Example 7 In Example 1, 1.05 g of sorbitan tristearate and sucrose fatty acid ester (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., trade name "DK-ester F-50") were used as surfactants.
About 90 g of a powdered polymer was obtained in the same manner except that 05 g was used.

Comparative Example 1 About 90 g of a powdered polymer was obtained in the same manner as in Example 1, except that 2.1 g of sucrose fatty acid ester (DK-ester F-10) was used as the surfactant.

Comparative Example 2 About 90 g of a powdered polymer was obtained in the same manner as in Example 1, except that 2.1 g of sucrose fatty acid ester (DK-ester F-20) was used as a surfactant.

Comparative Example 3 About 90 g of a powdered polymer was obtained in the same manner as in Example 1, except that 2.1 g of sorbitan monostearate was used as the surfactant.

Comparative Example 4 In Example 1, sorbitan monostearate 1.89 g and sucrose fatty acid ester (DK-
About 90 g of a powdered polymer was obtained in the same manner except that 0.21 g of the ester F-10) was used.

Comparative Example 5 In Example 1, 0.21 g of sorbitan monostearate and sucrose fatty acid ester (DK-
About 90 g of a powdered polymer was obtained in the same manner except that 1.89 g of ester F-10) was used.

Comparative Example 6 In Example 1, hydroxyethyl cellulose (manufactured by Fuji Chemical Co., Fuji Hec A-5000) was used in the monomer phase.
About 90 g of a powdered polymer was obtained in the same manner except that 2.3 g was added.

The results of various properties of water absorption, average particle size, and fluidity of the polymer are shown in Table 1 below.

As is clear from the results shown in Table 1, the superabsorbent polymer of the present invention is excellent in various water absorbing properties such as water absorbing ability, water absorbing speed, and water holding power, and has a large average particle diameter and fluidity. It turns out that it is favorable.

[Brief description of the drawings]

FIG. 1 is a drawing schematically showing an apparatus used for measuring the water absorption rate and water retention capacity of a superabsorbent polymer. 1 ... superabsorbent polymer (1 g), 2 ... support plate with small holes, 3 ... nonwoven fabric, 4 ... 0.9% saline, 5 ... burette, 6 ... rubber stopper, 7, 8 ... valve, 9 ... air inlet.

Claims (1)

(57) [Claims] An acrylic acid monomer selected from the group consisting of acrylic acid and / or methacrylic acid and an alkali metal salt thereof is reversely reacted in a hydrocarbon solvent using a radical polymerization initiator in the presence of a crosslinking agent. In the method of phase suspension polymerization, the surfactant used for the reverse phase suspension polymerization is (a) a solid-state appearance at least at room temperature and an HLB value of 1
Sorbitan fatty acid esters in the range of ~ 8 and (b) HL
A method for producing a superabsorbent polymer, wherein the surfactant is a mixed surfactant with a sucrose fatty acid ester having a B value in the range of 1 to 6.