JP2722695B2 - Method for producing water-absorbing polymer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Object of the Invention "Industrial application field" The present invention relates to a novel method for producing a water-absorbing polymer, and the water-absorbing polymer is used for sanitary articles, diapers, disposable rags and the like. In addition to being used as agricultural and horticultural products such as sanitary products and water retention agents, it is also used for applications such as solidification of sludge, prevention of dew condensation of building materials, dehydration of oils, etc. It is widely used in the industry and in the chemical industry for producing water-absorbing polymers.

[Prior art] Conventionally, water-absorbing polymers include carboxymethylcellulose crosslinked products, polyoxyethylene crosslinked products, starch-acrylonitrile graft copolymer hydrolysates, starch-acrylic acid graft copolymers, and acrylate polymer crosslinked products. Products, acrylate-based copolymer crosslinked products, and the like are known.

Among these, crosslinked acrylate polymer and crosslinked acrylate copolymer can satisfy water absorption capacity, water retention capacity, quality stability, etc., but the polymerization method has various problems. There is.

That is, as a method for producing a crosslinked acrylate polymer or a crosslinked acrylate copolymer, various polymerization methods such as aqueous solution polymerization, reversed-phase emulsion polymerization, and reversed-phase suspension polymerization have been adopted. Each of these methods has the following problems.

For example, in the case of reverse-phase emulsion polymerization, reverse-phase suspension polymerization, and the like, it is essential to use an organic solvent in the polymerization step. In addition, there is a problem that the temperature and pressure of the reaction system abnormally rise, which may cause explosion and fire, or deteriorate the working environment.

On the other hand, in the case of aqueous solution polymerization, the method of performing thermal polymerization in a batch system is the mainstream from the viewpoint of easy reaction control.However, in order to improve the yield, when polymerizing a high-concentration aqueous monomer solution, The reaction proceeds intensely, and the heat of the reaction rapidly raises the temperature of the system to a boiling state, thereby preventing the release of water vapor, so that the reaction runs away and a popcorn phenomenon occurs in the gel. In addition, a gel effect phenomenon, in which the polymerization rate is remarkably increased due to the increase in the viscosity of the solution, is added, so that it is more difficult to control the temperature, and it is difficult to obtain a product of favorable quality. In addition, workability such as taking out of the product is remarkably deteriorated.

To solve this problem, that is, a method of carrying out a polymerization reaction at a relatively low temperature in order to facilitate the control of the reaction temperature has been considered, but this method has a disadvantage that the reaction time is long and the production efficiency is low. .

On the other hand, a relatively high-concentration aqueous monomer solution, which should solve such productivity problems, is heated in advance, and a polymerization reaction initiator is added to the aqueous solution without external heating. A high production efficiency production method that does not require a drying step, in which continuous polymerization is performed and moisture is vaporized, has also been proposed.In this method, the production heat is high, but the polymerization heat based on severe polymerization conditions is high. The resulting resin tends to be porous due to the evaporation of water, and the obtained resin has a low water retention rate and the water once absorbed upon pressurization is released, which is a problem that a so-called return phenomenon occurs. In addition, there is also a problem in that a large amount of low molecular weight substances are generated, so that a sticky feeling is generated at the time of water absorption.

Also, as a method of adding a high-concentration aqueous monomer solution and a polymerization reaction initiator, there has been proposed a method of mixing with a jetting force of a fluid, but there is a possibility that the monomer is polymerized near the injection hole and the injection hole is blocked. Yes, it is not impossible to continue the polymerization operation stably for a long period of time, but this requires careful attention and effort.

In any of these production methods, it is necessary to perform post-crosslinking, surface treatment, and the like in order to improve the water absorption rate of the obtained resin.

"Problem to be Solved by the Invention" The present invention relates to an α, β-
The above problems when polymerizing a monomer mainly composed of an unsaturated carboxylic acid or a salt thereof into a water-absorbing polymer are solved, and a water-absorbing polymer having excellent productivity, workability, and excellent physical properties is obtained. To provide a manufacturing method.

(B) Configuration of the Invention "Means for Solving the Problems" In view of the above-mentioned circumstances, the present inventors have conducted various studies in order to achieve the above-mentioned object. According to a method in which polymerization is carried out while preventing boiling in a reaction system by pressure polymerization which has never been described, a simple reaction comprising α, β-unsaturated carboxylic acid such as acrylic acid or acrylate or a salt thereof as a main component. From aqueous solution
The inventors have found that a water-absorbing polymer excellent in productivity, workability and physical properties can be obtained by solving the above-mentioned problems, and filed an application for the invention (Japanese Patent Application No. 63-281353).
issue).

Further, the present inventors have continued the study, and in the polymerization, when the monomer aqueous solution to be supplied to the reactor is dropped, the oxygen in the monomer aqueous solution, which is always necessarily required, is replaced with nitrogen. The inventors have found that the step (nitrogen substitution step) can be omitted and that the productivity is further improved, and the present invention has been completed.

That is, the present invention supplies a monomer aqueous solution mainly containing an α, β-unsaturated carboxylic acid or a salt thereof dropwise to a polymerization reactor pressurized with an inert gas, The present invention relates to a method for producing a water-absorbing polymer, which comprises polymerizing a medium while preventing boiling of a medium to obtain a gel polymer.

The α, β-unsaturated carboxylic acid or a salt thereof in the present invention refers to an unsaturated carboxylic acid represented by acrylic acid, methacrylic acid, itaconic acid, maleic acid or the like, or a metal salt of the carboxylic acid such as sodium or potassium. The monomer aqueous solution containing these as a main component is composed of one or more of those monomers, or contains them and other hydrophilic monomers such as acrylamide, 2-hydroxyethyl A vinyl-based hydrophilic monomer such as (meth) acrylate, 2- (meth) acryloylethanesulfonic acid, 2-acrylamide sodium 2-ethylpropanesulfonate, a quaternary salt of dimethylaminoethyl acrylate, and a crosslinked structure can be introduced.
It is an aqueous solution mixture with a hydrophilic polyfunctional monomer such as N, N-methylenebisacrylamide and ethylene glycol diacrylate. Of course, as the monomer aqueous solution,
It may be added with starch, cellulose or the like conventionally used in the production of water-absorbing polymers.

A preferred monomer aqueous solution for the present invention is a monomer aqueous solution containing at least 20% by weight of acrylic acid and an alkali metal acrylate, and the ratio (molar ratio) of acrylic acid to the alkali metal acrylate is 0 to 80: 20-100. In addition, a mixture of acrylic acid and an alkali metal acrylate is very easily prepared by partially neutralizing acrylic acid with an alkali metal salt and used in the present invention.

In the present invention, the supply of the monomer aqueous solution to the polymerization reactor is carried out in a dropping state, but there is no particular method as a specific method, and the monomer solution is dropped in a water drop state directly from the supply port. And a method of spraying by providing a spray nozzle at the supply pipe opening.

The droplet diameter of the monomer aqueous solution to be dropped is 5 μ to 2000.
is preferable, and it is more preferable for the present invention to drop as a droplet having a diameter of 10 μm to 1,000 μm. If the droplet is dropped as a droplet having a diameter of less than 5 μm, the mist entrainment rate to the exhaust gas system increases, and the pipe may be clogged.
With a droplet exceeding μ, even under pressure, there is no effect of nitrogen substitution, and it is difficult to perform uniform and stable polymerization.

In the present invention, the pressure applied when employing a means for preventing the aqueous medium from boiling during polymerization by pressure includes a monomer aqueous solution or a polymer formed by polymerization of a monomer aqueous solution and a monomer aqueous solution. The pressure may be any pressure as long as it can prevent the boiling of the reaction system (aqueous solution), particularly the boiling of the aqueous medium, and the pressurization produces a uniform gel without boiling the gel. obtain.

Since the boiling pressure of the reaction system (aqueous solution) fluctuates variously depending on the concentration of the aqueous monomer solution and the polymerization initiation temperature, the polymerization may be carried out by appropriately setting a pressure sufficient to prevent boiling according to the pressure. But generally 0.5K above boiling pressure
g / cm ^{2 It} is preferable to polymerize under pressure of ² G or more, 2 kg / cm
^It is more preferable to polymerize under a pressure of ² G or more. The upper limit of the pressurization is not limited by the properties of the obtained water-absorbing polymer, but is determined mainly from the viewpoint of economical efficiency and difficulty in operation in manufacturing equipment.

The pressurization is performed to prevent the polymerization temperature from becoming high and the aqueous medium from boiling, so that the reaction system containing the monomer aqueous solution or the polymer formed by the polymerization of the monomer aqueous solution with the monomer aqueous solution is used. It is sufficient that the boiling of the (aqueous solution) is suppressed, but from the viewpoint of easiness of operation, it is desirable to constantly apply a constant pressure higher than the maximum boiling pressure during the polymerization period.

The polymerization of the monomer aqueous solution is preferably performed continuously,
At this time, the monomer concentration can be arbitrarily adjusted within a range in which the monomer does not precipitate from the aqueous solution due to the solubility, which is also a feature of the present invention. Naturally, it also allows polymerization near the precipitation concentration, which can maximize production efficiency.

For example, the solubility of a partially neutralized salt of acrylic acid (degree of neutralization: 70% of a mixture of acrylic acid and acrylate) in water is 48% at room temperature, and according to the present invention, at such a concentration, It also enables a polymerization reaction.

The polymerization initiation concentration is not particularly limited and may be set according to the catalyst system to be used, and there is no problem if the temperature is set at a temperature at which the reaction rate does not significantly decrease.

As the initiator, one or two or more peroxides such as persulfate, hydrogen peroxide, succinic peroxide, t-butylperoxymaleic acid, or these peroxides and sodium sulfite, ascorbic acid, Redox initiators and azo compounds combined with a reducing agent such as sodium erythorbate are used.
It is 0.05 to 0.5% by weight.

As an example of the pressurized continuous apparatus, there is a rigid cylindrical pressurized reaction tank, each raw material is continuously supplied in a fixed amount from the upper part of the reactor, and the gel generated by the polymerization reaction is also continuously continuously supplied from the lower part. It is extracted. The pressure in the reactor is controlled by the pressure of an inert gas such as nitrogen gas and a pressure regulating valve.

The raw material supplied to the reactor polymerizes rapidly and reaches a reaction temperature peak in about 10 minutes. The generated gel is pushed out of the reactor by a piston flow due to the vapor pressure generated by the reaction and the pressure, and is discharged into the atmosphere outside the system after a predetermined aging time. The removed gel is shredded, dried and pulverized according to the purpose.

"Action" According to the present invention, a polymerization reaction is performed while a monomer aqueous solution is supplied dropwise to a polymerization reactor pressurized by an inert gas, and the pressure is prevented from boiling the aqueous medium. If, for example, a nitrogen replacement step is not required, the boiling of the gel is suppressed, and the boiling point is not considered, the polymerization reaction can be advanced even in a batch type or continuously, even in a high-concentration aqueous monomer solution without concern for the boiling point. A uniform gel of the water-absorbing polymer can be produced. Furthermore, since the reaction is performed under pressure, it is possible to easily and continuously take out the gel after the completion of the reaction using the self-pressure, and to produce a water-absorbing polymer with good productivity and workability. I can do it. Although it is presumed to be especially for pressurization, the gel of the water-absorbing polymer obtained by pressure polymerization contains a myriad of fine bubbles, greatly improving the water absorption rate of the water-absorbing polymer, and becoming a powder. A water-absorbing polymer having an unexpectedly excellent performance that is difficult to obtain is obtained. And this becomes more remarkable by high concentration polymerization.

Further, by performing continuous polymerization, it is possible to produce a gel with little change in physical properties, and it is also possible to obtain a gel having excellent water absorption ability.

In general, the finer the particle size of the water-absorbing polymer powder, the larger the surface area, and the higher the water absorption rate.However, when a certain particle size is reached, the particles are stuck together during the water absorption, and the sprinkling occurs. , Which causes a decrease in the water absorption rate. For this reason, the inorganic fine powder is coated on the surface, or by post-treatment such as surface cross-linking, this is intended to be solved, but according to the pressure continuous polymerization,
Such a step can be eliminated.

"Examples" Example 1 Aqueous solution obtained by adding 22.8 parts of water to 35.3 parts of acrylic acid
The mixture was neutralized by adding 42 parts of a 32% aqueous solution of caustic soda with stirring. 38.45 g of this aqueous solution cooled to 20 ° C. was continuously pressurized to 4 kg / cm ² G with nitrogen gas at a rate of 4 kg / cm ² G per minute. The mixture was spray-added to the pressure polymerization reactor at a droplet diameter of 45 μm using a spray nozzle to carry out polymerization.

The monomer aqueous solution was 1.06 g / min of a 3% aqueous solution of methylenebisacrylamide (hereinafter referred to as MBAM), 0.5% / min of a 6.56% aqueous solution of ammonium persulfate (hereinafter referred to as APS), and sodium erythorbate (Elbit N). : 0.328% aqueous solution of Fujisawa Pharmaceutical Co., Ltd.) per minute
It was supplied at a rate of 0.5 g.

The gel was continuously discharged so that the liquid level in the polymerization reactor was maintained at a position of 300 mm from the lower outlet. The discharge was easily performed only by pressurizing the system.

This mixture has a neutralization degree of 70% and an apparent monomer concentration.
48%.

The temperature of the system rose to 135 ° C., and the polymerization reaction was completed in about 10 minutes.

 The resulting gel was a milky white elastic gel.

The product was shredded and dried in a hot air dryer at 120 ° C., and the dried product was pulverized to obtain a resin powder. The powdered resin was sieved to select particles having a particle size of 60 to 100 mesh.

Measurement of water absorption rate (1) Connect a glass filter (11G2) and a 50 ml burette with a rubber tube, pour 0.9% NaCl aqueous solution, and sufficiently remove the air under the filter. Raise and lower the bullet. 60-100mes
0.1 g of a powder sample having a particle size of h is precisely weighed and distributed uniformly on a filter, and the amount of water absorbed after 1 minute is measured (hereinafter, this method is referred to as a CAP method).

Measurement of water absorption rate (2) Place 50 ml of 0.9% NaCl aqueous solution in a 100 ml beaker and rotate with a magnetic stirrer at 600 rpm. In this, 60-100mesh
Add 2 g of powder sample with a concentration of and read the time until the solution surface is flat. Check the state of the flour after the measurement (white lump) (hereinafter, this method is referred to as a spiral method).

Measurement of saturation magnification 0.1 g of a powder sample having a particle size of 60 to 100 mesh is precisely weighed and put into a 300 ml beaker. Put 200 ml of 0.9% NaCl aqueous solution into this,
Stir for 3 hours with a magnetic stirrer. Three hours later,
Filter with a 100 mesh wire mesh (70 × 70 × 70 mm), leave for 5 minutes, wipe the water of the wire mesh with a paper towel, and weigh.

Comparative Example 1 Example 1 was repeated except that the spray nozzle of Example 1 was changed to a pipe having an opening having an inner diameter of 4φ, and was allowed to continuously flow down.
Polymerization was carried out in the same manner as described above.

 The polymerization was non-uniform and mostly unpolymerized.

Example 2 Polymerization was carried out in the same manner as in Example 1, except that the spray nozzle was changed to a nozzle having 13 holes of 0.2φ. The polymerization reaction took 12 minutes.

The product was shredded and dried in a hot air dryer at 120 ° C., and the dried product was pulverized to obtain a resin powder. The powdered resin was sieved to select particles having a particle size of 60 to 100 mesh.

Example 3 Aqueous solution obtained by adding 33.7 parts of water to 22.1 parts of acrylic acid
The mixture was neutralized by adding 26.2 parts of a 32% aqueous solution of caustic soda with stirring. 28.24 g of this aqueous solution cooled to 20 ° C per minute
Continuously pressurized to 3 Kg / cm ² G with nitrogen gas, a cylindrical pressure polymerization reactor with an inner diameter of 100 mm and a height of 700 mm, the surface of which has been treated with a fluororesin, has an opening with an inner diameter of 1φ It was dropped at a droplet diameter of 2000 μ from the end of the pipe. In the monomer aqueous solution, 0.689 g of a 3% aqueous solution of MBAM per minute and 5.05% of APS were used.
An aqueous solution was supplied at a rate of 0.356 g / min, and a 0.126% aqueous solution of Elbit N was supplied at a rate of 0.356 g / min for polymerization.

The gel was continuously discharged so that the liquid level of the polymerization reactor was maintained at a position of 340 mm from the lower discharge port. The discharge was easily performed only by pressurizing the system.

This mixture has a neutralization degree of 70% and an apparent monomer concentration.
30%.

The temperature of the system rose to 90 ° C., and the polymerization reaction was completed in about 21 minutes.

 The resulting gel was a milky white elastic gel.

The product was shredded and dried in a hot air dryer at 120 ° C., and the dried product was pulverized to obtain a resin powder. The powdered resin was sieved to select particles having a particle size of 60 to 100 mesh.

Measurement of water absorption performance The water absorption performance of the resin powder obtained as described above was measured and summarized in Table 1.

As is evident from Table 1, in the case of continuous polymerization under pressure, the water absorption rate is improved, and there is no sprinkling of the gel after any water absorption. In particular, those obtained by pressure polymerization at a high concentration have a remarkably improved water absorption rate without post-treatment.

(C) Effects of the Invention The present invention exhibits the following excellent effects.

1. Continuous polymerization can be performed with good productivity and workability, and a water-absorbing polymer can be obtained efficiently.

2. Uniform water-absorbing polymer capable of high-concentration aqueous solution reaction can be obtained.

3. No heat removal device for reaction heat is required.

4. A water-absorbing polymer with countless fine bubbles is obtained,
A water-absorbing polymer having a high water-absorbing rate can be obtained without performing post-treatment or the like.

5. Easy removal of gel.

6. Since polymerization can be performed at a high concentration, the drying process can be greatly reduced, and the equipment scale and energy cost can be reduced.

7. The nitrogen replacement step can be omitted, and the productivity can be further improved.

8. The water-absorbing polymer obtained by the present invention is excellent in the above-mentioned properties, and is used for sanitary products such as sanitary products, diapers, disposable rags, agricultural and horticultural products such as water retention agents, sludge solidification, and condensation of building materials. It is used for prevention, dehydration of oils, and the like, and can exert more excellent effects than conventional ones.

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Claims (1)

(57) [Claims] 1. A monomer aqueous solution mainly comprising an α, β-unsaturated carboxylic acid or a salt thereof is supplied dropwise to a polymerization reactor pressurized with an inert gas, and the pressurized water is supplied to the polymerization reactor. A method for producing a water-absorbing polymer, wherein a gel-like polymer is obtained by polymerization while preventing boiling of an aqueous medium.