JPH0356513A - Production of water-absorptive polymer - Google Patents

Abstract

PURPOSE:To obtain the title polymer to be used for sanitary articles, sludge solidification, etc. by putting an aqueous solution of alpha, beta - unsaturated carboxylic acid ( salt ) monomer to polymerization in the presence of a polyfunctional monomer while preventing the aqueous medium from boiling through pressurization to enable high-concentration aqueous solution reaction. CONSTITUTION:An aqueous solution of monomer predominant in alpha, beta - unsaturated carboxylic acid ( salt ) ( pref. containing >= 20 wt.% of acrylic acid plus its alkali metal salt ) is put to polymerization in the presence of a polyfunctional monomer ( pref. ethylene glycol diacrylate ) while preventing the aqueous medium from boiling through pressurization ( pref. >= 2 kg/cm<2>G ), thus obtaining the objective polymer.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Object of the invention "Field of industrial application" The present invention relates to a new method for producing a water-absorbing polymer. In addition to being used as sanitary products and agricultural and horticultural supplies such as water retention agents, it is also used for purposes such as coagulating sludge, preventing condensation on building materials, and dehydrating oils.
It is widely used in these various industries and in the chemical industry that manufactures water-absorbing polymers.

"Prior art" Conventional water-absorbing polymers include carboxymethylcellulose crosslinked products, polyoxyethylene crosslinked products, starch-acrylonitrile graft copolymer hydrolysates, starch-acrylic acid graft copolymers, and acrylate polymer crosslinks. thing,
Crosslinked acrylate copolymers are known.

Among these, carboxymethylcellulose crosslinked products and polyoxyethylene crosslinked products have not yet been found to have satisfactory water absorption and water retention abilities.

In addition, the hydrolyzate of starch-acrylonitrile graft copolymer and the starch-acrylic acid graft copolymer have relatively high water absorption and water retention capacity, but because they use starch, which is a natural polymer, they have low heat resistance, There are drawbacks to rotting and decomposition, etc.
Another problem is that the manufacturing method thereof is complicated.

Furthermore, although crosslinked acrylate polymers and crosslinked acrylate copolymers can satisfy water absorption capacity, water retention capacity, quality stability, etc., there are various problems with their polymerization methods. be.

That is, various polymerization methods such as aqueous solution polymerization, reverse-phase emulsion polymerization, and reverse-phase suspension polymerization have been adopted as methods for producing crosslinked acrylate polymers or crosslinked acrylate copolymers. All of these methods have the following problems.

For example, in the case of reverse-phase emulsion polymerization, reverse-phase suspension polymerization, etc., it is essential to use an organic solvent in the polymerization process. , the temperature and pressure of the reaction system may rise abnormally, leading to problems such as the risk of explosion or fire, or deterioration of the working environment.

On the other hand, in the case of aqueous solution polymerization, batch thermal polymerization is the mainstream method because it is easy to control the reaction, but when trying to polymerize a highly concentrated monomer aqueous solution with the aim of improving the yield, The reaction progresses rapidly, and the temperature of the system rapidly rises due to the reaction heat, reaching a boiling state, which prevents the release of water vapor, causing the reaction to run out of control and causing a popcorn phenomenon in the gel. Furthermore, due to the increase in the viscosity of the solution, there is also a gel effect phenomenon in which the polymerization rate increases significantly, making temperature control more difficult and making it difficult to obtain a product of favorable quality. In addition, the workability of taking out products, etc. becomes significantly inferior.

In order to solve these problems, that is, to make it easier to control the temperature of the reaction, a method of conducting the polymerization reaction at a relatively low temperature has been considered, but this method requires a long reaction time and has low production efficiency. Defects arise.

On the other hand, in order to solve these productivity problems, a relatively high concentration monomer aqueous solution is heated in advance, a polymerization reaction initiator is added to it, and an endless belt can be used without external heating. A production method with high production efficiency that does not require a drying process has also been proposed, in which continuous polymerization is carried out at the same time as moisture is vaporized.However, although this method has high production efficiency, it does not require high production heat due to the harsh polymerization conditions. The resulting resin tends to become porous due to water evaporation due to water evaporation, and the resulting resin has a low water retention rate, causing the so-called rebound phenomenon in which water that has been absorbed is released when pressurized. Furthermore, there is the problem that a sticky feeling occurs when water is absorbed due to the formation of a large amount of low molecular weight substances. In addition, regardless of the production method, post-crosslinking, surface treatment, etc. must be performed in order to improve the water absorption rate of the resulting resin.

``The secret problem to be solved by the invention'' The present invention is directed to polymerization of a monomer mixture containing a polyfunctional monomer mainly composed of α,β monounsaturated carboxylic acid or its salt such as acrylic acid or acrylic acid salt. It is an object of the present invention to provide a manufacturing method that solves the above-mentioned problems when producing a water-absorbing polymer by producing a water-absorbing polymer with excellent productivity, workability, and excellent physical properties.

(Example) Structure of the Invention or "Means for Solving the Problem" In view of the above-mentioned circumstances, the present inventor has devised a method that has been practically performed in the production of water-absorbing polymers in the past in order to achieve the above-mentioned object. By polymerizing under pressure while preventing boiling in the reaction system, it is possible to polymerize from an aqueous solution of a monomer mixture mainly consisting of α, β monounsaturated carboxylic acids such as acrylic acid or acrylates, or salts thereof. discovered that it was possible to solve the above problems and obtain a water-absorbing polymer with excellent productivity, workability, and physical properties, and filed an application for the invention (Japanese Patent Application No. 63-281353). Furthermore, the present inventors continued their studies and found that an even more excellent water-absorbing polymer could be obtained by carrying out the polymerization in the presence of a polyfunctional monomer, particularly a hydrophilic polyfunctional monomer. It was completely destroyed.

That is, in the present invention, an aqueous monomer solution mainly composed of α,β monounsaturated carboxylic acid or a salt thereof in the coexistence of a polyfunctional monomer,
The present invention relates to a method for producing a water-absorbing polymer, characterized in that polymerization is carried out while preventing boiling of an aqueous medium by applying pressure.

In the present invention, α,β monounsaturated carboxylic acids or salts thereof, etc., and the monomer aqueous solution mainly composed of them,
Consisting of one or more of these monomers, or together with other hydrophilic monomers, such as acrylamide, 2-hydroxyethyl (meth)acrylate, 2-(meth)acryloylethanesulfonic acid, It is a mixture of 2-acrylamide and vinyl hydrophilic monomers such as sodium 2-ethylpropanesulfonate and quaternary salt of dimethylaminoethyl acrylate. In addition, the polyfunctional monomer refers to a monomer such as N,N-methylenebisacrylamide and ethylene glycol diacrylate that can introduce a crosslinked structure into a water-absorbing polymer, and is a preferred polyfunctional monomer for the present invention. is a hydrophilic polyfunctional monomer such as ethylene glycol diacrylate, and other hydrophilic polyfunctional monomers include ethylene glycol dimethacrylate, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, and polyethylene glycol diglycidyl. Ether (commercially available products with main chain n (degree of condensation) of 2 to
8, such as SR-2EC, SR manufactured by Sakamoto Pharmaceutical Co., Ltd.
-8EG, EX-8 2 1 manufactured by Nagase Kakai Kogyo), polypropylene glycol diglycidyl ether, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, polyether polyacrylate (commercially available products manufactured by Toa Chemical Industry) Aronix M-240, M-245, M-260, M-305
A more preferred hydrophilic polyfunctional monomer for the present invention is polyethylene glycol diglycidyl ether (n in the main chain is 2 to 4). Polyfunctional monomers improve the water absorption rate and improve the feel of the resin after water absorption, and polyfunctional monomers with hydrophilic properties are particularly effective in this regard.

However, if the resin is too hydrophilic, it tends to increase the adhesiveness of the resin, easily generate joint powder, and reduce the water absorption rate.

Furthermore, as the monomer aqueous solution of the present invention, those to which starch, cellulose, etc., which have been conventionally used in the production of water-absorbing polymers, may be used.

A preferred aqueous monomer solution for the present invention is a monomer aqueous solution containing 20% by weight or more of acrylic acid and an alkali metal salt of acrylate, and the ratio (molar ratio) of acrylic acid to an alkali metal salt of acrylate is 0 to 80: 20 to 100. Any mixture of acrylic acid and an alkali metal salt of acrylic acid can be prepared very easily by partially neutralizing acrylic acid with an alkali metal salt, and the use of the polyfunctional monomer used in the present invention is very easy. The amount is preferably 0.02 to 0.5% by weight, more preferably 0.05 to 0.2% by weight based on the pure monomer content.

Polymerization of an aqueous monomer solution is carried out batchwise or continuously in an aqueous solution, but the monomer concentration at this time can be adjusted arbitrarily to the extent that the monomer does not precipitate from the aqueous solution due to solubility. This is also a feature of the present invention. Naturally, it also allows polymerization near the precipitate concentration, which can maximize production efficiency.

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

In the present invention, in order to prevent the aqueous medium from boiling during polymerization, it is necessary to pressurize the aqueous medium. It must be able to prevent boiling of the reaction system (aqueous solution) containing the polymer, especially boiling of the aqueous medium.

Applying pressure can produce a uniform gel without boiling, but the degree of pressurization should be adjusted accordingly, as the pressure at boiling varies depending on the monomer mixture concentration and polymerization initiation temperature. , may be set appropriately to prevent boiling, but generally it is 0. It is preferable to polymerize under pressure of 5 Kg/cm2G or more, more preferably 2K
This is polymerization under pressure of g/cm2G or more. The upper limit of pressurization is not limited by the characteristics of the water-absorbing polymer to be obtained, but is determined mainly by the economical efficiency of production equipment and the difficulty of operation.

Pressurization is performed to prevent the polymerization temperature from increasing and the aqueous medium from boiling. The pressure may be slightly higher than the vapor pressure at the boiling temperature of the (aqueous solution), but from the viewpoint of ease of operation, it is desirable to apply the pressure set as described above during the polymerization period.

The polymerization initiation temperature is not particularly limited and may be set depending on the catalyst system used, and there will be no problem as long as it is set at a temperature that does not significantly reduce the reaction rate.

As an initiator, one or more peroxides such as persulfate, hydrogen peroxide, succinic peroxide, t-butylperoxymaleic acid, or these peroxides and sodium sulfite, ascorbic acid, A redox initiator combined with a reducing agent such as sodium erythorbate and an azo compound are used, and the amount added is usually 0.05 to 0.5% by weight based on the monomer.

"Effect" When attempting to produce a water-absorbing polymer by the above-mentioned aqueous solution polymerization, unless the temperature of the reaction system is controlled so as not to exceed the boiling point of the aqueous solution, the reaction will not only run out of control, but will also produce a uniform gel. It was considered difficult to do. The methods adopted for this purpose were to suppress the reaction heat by performing belt polymerization, lowering the concentration of the heptopomer, or lowering the reaction start temperature as much as possible so as not to exceed the boiling point of the aqueous solution. . For this reason, there were problems in that the reaction time was long, the amount obtained per reactor volume was limited, and it was difficult to obtain a water-absorbing polymer with excellent physical properties.

However, according to the present invention in which the polymerization reaction is carried out under pressure, that is, by pressurizing the polymerization reaction solution to a level higher than the vapor pressure of the polymerization reaction aqueous solution at the reaction temperature during the polymerization reaction, boiling of the gel is suppressed, and boiling points are not a concern. There is no need for this, and the polymerization reaction can proceed with good control even in a highly concentrated monomer aqueous solution.
A uniform gel of water-absorbing polymer can be produced.

Furthermore, since the reaction is carried out under pressure, the gel can be easily taken out after the reaction is completed using its own pressure.

In particular, according to the present invention, although it is presumed that this is due to pressurization, the resulting water-absorbing polymer gel contains countless fine bubbles, which greatly improves the water absorption rate of the water-absorbing polymer, and
A water-absorbing polymer is obtained that has unexpectedly excellent performance in that it is resistant to joint powder. This becomes more noticeable when polymerization is carried out at a high concentration.

In addition, since the present invention uses a polyfunctional monomer that can introduce a crosslinked structure into the water-absorbing polymer, the water absorption rate of the water-absorbing polymer is further improved, and the gel after water absorption has a smooth feel. It is possible to impart excellent properties such as the ability to become a product.

In general, the finer the particle size of water-absorbing polymer powder, the larger the surface area, which improves the water absorption rate.However, when the particle size reaches a certain point, the particles stick together while absorbing water, causing joint powder. This causes the water absorption rate to decrease. Therefore, attempts have been made to solve this problem by post-processing such as coating the surface with inorganic fine powder or crosslinking the surface, but according to the present invention, such steps can be eliminated.

"Examples" Example 1 To an aqueous solution obtained by adding 22.8 parts of water to 35.3 parts of acrylic acid, 42 parts of a 32% aqueous solution of caustic soda was added with stirring to neutralize the solution. After cooling to 20'C, 0.11 parts of hydrophilic polyfunctional monomer diethylene glycol diglycidyl ether was added to this aqueous solution, and after nitrogen bubbling in a pressure polymerization reactor, ammonium persulfate (hereinafter referred to as AP) was added.
S) 0.09 parts, sodium erythorbate (Elvit N: trade name manufactured by Fujisawa Pharmaceutical Co., Ltd.) 0.00
45 parts of each were added as lO% aqueous solutions and polymerized under a pressure of 4 Kg/cm''G.

Note that this mixture has a degree of neutralization of 70% and an apparent monomer concentration of 48%.

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

The product was shredded and dried in a hot air oven at 120'C, and the dried material was ground to obtain a resin powder. This powdered resin was sieved to select particles with a particle size of 60 to 1 Q Qmesh.

Connect a 50ml burette with a 50ml burette using a rubber tube, add a 0.9% NaCl aqueous solution, and remove enough air from the bottom of the filter until the filter surface is smeared with liquid. Adjust the liquid level by moving the burette up and down. A powder sample with a particle size of 60 to 100 mesh 0. 1
Accurately weigh the amount of water, spread it evenly on the filter, and measure the amount of water absorbed after 1 minute (hereinafter this method will be referred to as CA
(referred to as P method).

Detailed description 1D and column (shave 100D beaker with 0.9% NaCl aqueous solution 50!II
Rotate with a magnetic stirrer at 600 rpm. Add 2 g of a powder sample with a particle size of 60 to 100 mesh to this, and read the time until the surface of the solution becomes flat.

Check the status of the powder (white mass) after measurement (hereinafter this method is referred to as the swirl method).

Put 0.5g of powder sample with a particle size of 60-100mesh into a 90φ watch glass, spread it flat, and evenly add 5ml of pure water. Check the whitening state of the gel after 1 minute. do.

Judgment: A - Complete whitening B - Half whitening C - Slight whitening D - No whitening Furthermore, this measurement method correlates well with the smooth feeling of the gel after water absorption.

Example 2 To an aqueous solution obtained by adding 51.2 parts of water to 22.1 parts of acrylic acid, 26.2 parts of a 32% aqueous solution of caustic soda was added with stirring to neutralize the solution. After cooling to 20'C, 0.075 part of hydrophilic polyfunctional monomer diethylene glycol diglycidyl ether was added to this aqueous solution, and after bubbling with nitrogen in a pressure polymerization reactor, 0.06 part of APS and Elvit NO. O 2 O a parts were each added as a 10% aqueous solution, and polymerized under a pressure of 3 Kg/cm 2 G. This mixture has a degree of neutralization of 70% and an apparent monomer concentration of 30%.
%.

The temperature of the system rose to 90°C, and the polymerization reaction was completed in about 20 minutes.

The product was shredded and dried in a hot air oven at 120°C;
The dried product was pulverized to obtain resin powder. This powdered resin was sieved to select particles with a particle size of 60 to 1 Q Qmesh.

Comparative Example 1 A quasi-metallic material similar to Example 1 was prepared and polymerized under normal pressure. When the temperature exceeded 106°C during the reaction, the gel popped out.

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

The water absorption performance of the resin powder obtained in the above manner after one suction was measured and summarized in Table 1.

= (Margin below) 111 Table 1 As is clear from Table 1, the water absorption rate of the polymers subjected to pressure polymerization with the addition of hydrophilic polyfunctional monomers is improved compared to those without the addition of hydrophilic polyfunctional monomers. However, no joint powder is generated in the gel after water absorption. In particular, products polymerized under pressure at high concentrations can be treated without post-treatment.
Water absorption speed is dramatically improved.

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

1. A highly concentrated aqueous solution reaction is possible and a uniform water-absorbing polymer can be obtained.

2. A heat removal device for reaction heat is not required.

3. A water-absorbing polymer having countless microscopic bubbles can be obtained, and a water-absorbing polymer with a high water absorption rate can be obtained without any post-treatment.

4. Gel can be easily removed.

5. Since it can be polymerized at a high concentration, the drying process can be significantly shortened, and equipment scale and energy costs can be reduced.

6. By using a polyfunctional monomer, particularly a hydrophilic monomer having appropriate hydrophilicity, the water absorption rate can be further improved and a smooth feel can be imparted to the gel after water absorption.

7. Because of the above-mentioned excellent properties, the water-absorbing polymer obtained in the present invention can be used for sanitary products such as sanitary products, diapers, and disposable rags, for agricultural and gardening products such as water retention agents, and for coagulation of sludge.
It is used to prevent condensation on building materials, dehydrate oil, etc., and can produce even better effects than conventional ones.

Claims (1)

[Claims] 1. It is characterized by polymerizing an aqueous monomer solution mainly composed of α,β-unsaturated carboxylic acid or its salt in the coexistence of a polyfunctional monomer while preventing boiling of the aqueous medium by applying pressure. A method for producing a water-absorbing polymer.