JPH0476362B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Field of Application> The present invention relates to a method for producing a water-absorbing resin.
More specifically, the present invention relates to a method for producing a water-absorbing resin having excellent manufacturing safety and high water-absorbing capacity.

<Prior art and problems to be solved by the invention> In recent years, water-absorbing resins with high water-absorbing and water-retaining abilities have been used for disposable diapers, sanitary products, culture media for microorganisms and plants, carriers for liquid chromatography, etc. ing. As a type of such water-absorbing resin, a crosslinked polyacrylic acid metal salt type is widely used.

This metal acrylate-based water-absorbing resin is produced by adding a polymerization initiator to an aqueous solution of an alkali metal acrylate such as sodium acrylate or potassium acrylate, polymerizing the mixture, and then removing water. At this time, examples of the polymerization initiator include persulfates such as potassium persulfate and sodium persulfate;
A redox catalyst consisting of a combination of an oxidizing agent and a reducing agent (for example, the persulfate and monovalent copper ion, hydrogen persulfate and divalent iron ion, etc.) is used, but these persulfates and redox catalysts When used as an initiator, the polymerization reaction is extremely rapid; for example, even if the polymerization initiation temperature is room temperature, heat is generated rapidly as the polymerization progresses. In particular, when it is carried out in large quantities and at a high monomer concentration, such as in industrial production, it is difficult to control unless special consideration is taken, and runaway reactions sometimes occur, which is extremely dangerous. From this point of view, in order to prevent runaway reactions, the acrylate salt is polymerized by lowering the monomer concentration and using a temperature control system.
Lowering the monomer concentration requires a large amount of heat to evaporate water, which is uneconomical, and there is also the problem that equipment equipped with a temperature control system is expensive.

<Purpose> This invention was made in view of the above problems, and by using a special polymerization initiator,
It is an object of the present invention to provide a method for producing a water-absorbing resin in which polymerization of an acrylic acid metal salt proceeds under extremely mild conditions, which is highly safe and has a high water-absorbing capacity.

<Means for solving the problem> The method for producing a water absorbent resin of the present invention, which was made to solve the above problems, uses acrylic acid whose degree of neutralization with monovalent cations is 30% or more and less than 100%. This method is characterized in that after polymerization with a sulfur-containing reducing agent, an oxidizing agent is added for treatment.

In this specification, the degree of neutralization refers to the ratio of acrylate to the total of acrylic acid and acrylate in the charged monomers, and is defined by the following formula.

Degree of neutralization (%) = acrylate (mol) x 100/acrylic acid + acrylate (mol) In the above configuration, the neutralizing agent used to neutralize acrylic acid is It is selected as appropriate depending on the purpose of use of the resin, but for example,
Alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, lithium hydroxide, alkali metal carbonates or hydrogen carbonates such as sodium carbonate, potassium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, ammonium hydroxide, triethylamine,
Bases that can act as monovalent cations, such as ethanolamines, pyridine, picoline, and other organic amines, can be selected arbitrarily, but depending on economic efficiency, safety, and water absorption performance of the resulting water absorbent resin, etc. Considering this, sodium hydroxide is appropriate.

In addition, the degree of neutralization of acrylic acid is 30% or more, preferably 50% or more, and more preferably 70%.
A ratio of 95% or less is preferable in view of the water absorption performance of the resulting water absorbent resin. In addition, 1 of acrylic acid
The acrylate aqueous solution may be prepared by mixing a valence cation salt and acrylic acid in an appropriate ratio. Considering the feature of this invention that the concentration of the polymerization solution is low in heat generation, the higher the monomer concentration in the solution, the more remarkable the technical, equipment, and performance effects of this invention are. When performing neutralization, sodium acrylate will precipitate if the water concentration falls below 45% to 50%, so if the monomer concentration is to be increased, it is necessary to homogenize the solution by mixing and stirring.

In addition, in order to modify the water absorption capacity, gel strength, etc. of the water absorbent resin obtained by this invention, acrylamide, methylenebisacrylamide, N-vinylpyrrolidone, etc. may be added to the acrylic acid to form a copolymer. good.

The sulfur-containing reducing agent used in this invention is not particularly limited as long as it is a sulfur-containing reducing agent that can initiate polymerization of acrylate, but sulfites such as sodium sulfite and potassium sulfite, sodium hydrogen sulfite, Examples include hydrogen sulfites such as potassium hydrogen sulfite, thiosulfates such as sodium thiosulfate and potassium thiosulfate, dithionites such as sodium dithionite, and mixtures thereof. Although these sulfur-containing reducing agents can be used in powder form, they are usually used as an aqueous solution.

Examples of the oxidizing agent include conventional oxidizing agents, such as peroxodisulfates such as sodium peroxodisulfate, potassium peroxodisulfate, and ammonium peroxodisulfate, which may be used alone or as a mixture. Although it can be used in powder form, it is usually used as an aqueous solution.

The amount of sulfur-containing reducing agent used in polymerization depends on the type of reducing agent, amount of oxidizing agent, polymerization temperature, concentration of monomer aqueous solution, etc., but is usually 0.001 parts by weight per 100 parts by weight of monomer. ~0.5 parts by weight, preferably 0.01 to 0.1 parts by weight are added. If the amount of the reducing agent added is less than 0.001 parts by weight, the reaction will be slow and the polymerization time will be long, and then when an oxidizing agent is added, there is a tendency for a large exothermic reaction like that seen in the polymerization of acrylates due to ordinary peroxides, etc. occurs, and the water absorption capacity also decreases. Further, if more than 0.5 parts by weight is added, the polymerization solution becomes highly viscous, and stirring and mixing becomes difficult unless the monomer concentration is lowered.

The amount of the oxidizing agent added varies depending on the type of oxidizing agent, the reaction temperature, the amount of the reducing agent used, the characteristics required of the water-absorbing resin, etc., but it is usually 0.001 parts by weight per 100 parts by weight of the monomer. ~0.5 parts by weight, preferably 0.01 to 0.1 parts by weight are added. If the amount of the oxidizing agent added is less than 0.001 part by weight, the resulting water absorbent resin will have a large soluble portion, and if it is added in excess of 0.5 part by weight, the resulting water absorbent resin will become hard and its water absorption capacity will decrease.

As described above, the water-absorbing resin obtained by the production method of the present invention can have its water-absorbing ability, gel hardness after water absorption, etc. changed as appropriate depending on the amounts of the reducing agent and oxidizing agent used. In the general formula, for example,
When the amount of reducing agent used is changed while the amount of oxidizing agent added is constant, the tendency to solubilize increases beyond a certain range in which the water absorption capacity increases as the amount of reducing agent used increases. Furthermore, when the amount of the oxidizing agent used is increased while the amount of the reducing agent used is constant, the amount of water absorbed decreases and the gel state after water absorption becomes hard, whereas when the amount is decreased, the gel state tends to become soft. However, when each is used in a proportionately increased or decreased amount within the above-mentioned addition range, no significant difference is observed in the performance of the resulting polymers.

Furthermore, in the production method of the present invention, by adding aluminum compounds such as aluminum salts such as aluminum nitrate, aluminum acetate, aluminum chloride, and aluminum sulfate, and aluminates such as aluminum alcoholate and sodium aluminate, the polymer can be crosslinked. You can also adjust the degree. In particular, water-absorbent resins produced under conditions with low catalyst concentration tend to contain soluble portions or become soft after water absorption, so adding the above aluminum compound in such cases will reduce the amount of water absorbed. It is possible to increase the gel strength and improve the gel strength. In addition, in the above aluminum compound,
Aluminum salts, etc. can be used by adding them to either the monomer aqueous solution or the catalyst aqueous solution, but in the case of aluminates, adding them to the catalyst aqueous solution may cause the alkali to decompose the catalyst, so they should be added to the monomer aqueous solution. It is better. These aluminum compounds are usually added in an amount of 5 parts by weight or less, preferably about 0.1 to 0.5 parts by weight, per 100 parts by weight of the monomer.

Next, an example of the manufacturing method of the present invention will be explained in detail.

First, acrylic acid is neutralized with the neutralizing agent aqueous solution to a desired degree of neutralization and a concentration suitable for polymerization. The aforementioned reducing agent aqueous solution is added to this solution to initiate polymerization. As the polymerization progresses, the viscosity of the solution increases, and when the polymerization progresses to a certain extent and the viscosity becomes high, the oxidizing agent aqueous solution is added to the reaction system,
After heating and processing as necessary, the reaction solution is dehydrated and dried to obtain the water absorbent resin of the present invention.

In the above steps, polymerization using a reducing agent usually proceeds sufficiently at room temperature, but heating or heating may be performed as necessary. When heated or heated, the polymerization reaction proceeds rapidly and the viscosity rapidly increases, so the timing of addition of the oxidizing agent is important and greatly affects the water absorption ability of the resulting resin. As a general tendency, if the oxidizing agent is added late, the high viscosity will prevent the added oxidizing agent from homogeneous dispersion, resulting in a non-homogeneous polymer, which may result in soluble portions. . Therefore, when warming or heating, it is preferable to use a polymerization retarder such as allyl alcohol or propylene glycol in combination.

The treatment temperature after adding the oxidizing agent may be at or above the temperature at which the oxidizing agent used is decomposed or activated. Considering the fact that moisture in the reaction system must be removed and dried after treatment with an oxidizing agent, and that no change in performance or other properties was observed even after treatment with an oxidizing agent at around 100°C, moisture coexisting in the system can be reduced. It is convenient to process at around the boiling point of .

<Function> The present invention has the above-mentioned structure, and unlike the conventional polymerization using persulfates or redox initiators, the polymerization of acrylic acids using a sulfur-containing reducing agent has extremely low heat generation and can be carried out quickly under mild conditions. After the polymerization has progressed to a certain extent, it is treated with an oxidizing agent and further polymerization or crosslinking reaction is carried out, which makes it easy to control the temperature during polymerization and prevent runaway reactions.

Furthermore, since the polymerization proceeds under mild conditions, there are few side reactions, etc., and a water-absorbing resin with high water-absorbing capacity can be obtained.

<Examples> The present invention will be described in more detail below with reference to Examples.

Example 1 When 10 g of acrylic acid is neutralized with 20 g of a 25% sodium hydroxide aqueous solution and 1 g of a 1.0% potassium pyrosulfite aqueous solution is added at room temperature, it polymerizes in about 5 minutes to obtain a stirrable viscous material. It will be done. The temperature within the system during this polymerization is 35°C to 45°C, and this temperature does not change much even if the amount of monomer solution is increased. The obtained viscous substance tends to separate into solid and liquid over time after several hours, but before separation it has thixotropic properties and becomes a water-like low viscosity liquid when stirred, so it cannot be used in this state. When 1 g of a 1% potassium peroxodisulfate aqueous solution is added, a homogeneous solution is obtained, but when this solution is heated above 60° C., it becomes a rubbery polymer with no tackiness after a few minutes. In this case, if the heating temperature is high, the treatment time will be shortened, but no heat generation or bumping phenomenon was observed, and no difference in water absorption performance depending on the treatment temperature was observed. The resulting polymer, which was dehydrated and dried, exhibited water absorption performance that was approximately 1000 times greater in tap water and approximately 100 times greater in 0.9% saline.

Example 2 10 g of acrylic acid was neutralized by adding 20 g of a 25% sodium hydroxide aqueous solution, and the mixture was heated and stirred with 3 g of a 1% potassium pyrosulfite aqueous solution in a mixer that can be heated to 60°C or higher. Before the polymerization was sufficiently completed, 1 g of a 1% potassium peroxodisulfate aqueous solution was added.
Continue heating, mixing and stirring. Thereafter, it is dehydrated and dried to obtain a polymer. The water absorption performance of the obtained polymer was similar to that of Example 1.

Example 3 10 g of acrylic acid was neutralized by adding 20 g of a 25% aqueous sodium hydroxide solution, 0.3 g of acrylamide was dissolved, and 1.5 g of a 2% aqueous sodium dithionite solution was added at room temperature (approximately 30°C). Upon addition, polymerization begins immediately and the viscosity increases with slight heat generation. After 1 minute and 30 seconds, add 2% each of potassium peroxodisulfate and aluminum nitrate nonahydrate.
When 2 g of the dissolved aqueous solution is added and mixed, the viscosity increases further and becomes rubbery within a few minutes, but the product obtained by heating and drying this at 60°C or higher has a viscosity of 0.9
% saline solution.

In the above example, the concentration of the sodium dithionite aqueous solution and the mixed aqueous solution of potassium peroxodisulfate and aluminum nitrate nonahydrate was set to 1%, and the amounts added were set to 3 g and 2 g, respectively, and water absorbent resins were produced in the same manner. did. The obtained water absorbent resin is
It exhibited 150 to 170 times the water absorption performance of 0.9% saline, and the gel after water supply was slightly soft, but there was no soluble portion.

<Effects> As described above, according to the method for producing a water-absorbent resin of the present invention, polymerization proceeds under mild conditions by using a special polymerization initiator, and temperature control is facilitated, resulting in safety. Moreover, polymerization conditions can be set within a wide range. In addition, it is technically and equipment-wise easy and simple, and has excellent safety and economical efficiency. The resulting water-absorbing resin has excellent water-absorbing performance, and its properties and performance can be changed by appropriately changing the amounts of the reducing agent and oxidizing agent used, making it possible to obtain the desired product and expand its application range. Can be expanded. Furthermore, in its manufacturing method,
A wide variety of methods can be used, including those known in the art. It can be made into a powder by spraying into an inert gas or air. In addition, there are various methods such as heating after coating and impregnating shaped objects and non-woven fabrics, kneading and heating in rolls, screws, or mixers, and inverse emulsion polymerization to dehydrate powder using the boiling point of the solvent. It has the advantage that the manufacturing method can be used.

Claims (1)

[Claims] 1. Degree of neutralization by monovalent cations is 30% or more and 100%.
1. A method for producing a water-absorbing resin, which comprises polymerizing acrylic acid with a sulfur-containing reducing agent and then adding an oxidizing agent. 2. The method for producing a water-absorbing resin according to claim 1, wherein the sulfur-containing reducing agent is a sulfite, a hydrogen sulfite, a thiosulfate, or a dithionite. 3. The method for producing a water-absorbing resin according to claim 1, wherein the oxidizing agent is peroxodisulfate.