JPS648006B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a water-absorbing resin that can highly absorb aqueous liquid and stably retain it. Traditionally, paper, pulp, sponge, etc. have been used as water-absorbing or water-retaining materials in sanitary materials such as sanitary napkins and paper diapers, or in the agricultural field. However, if pressure is applied, a large portion of it will be squeezed out. As alternatives to these materials, several water-absorbing materials have been proposed in recent years, such as hydrolysates of starch-polyacrylonitrile graft polymers, modified cellulose ethers, and modified polyethylene oxides. However, these materials do not have sufficient water absorption capacity, or even if they do have sufficient water absorption capacity, the manufacturing process is complicated, or they are made from natural materials, so there is a possibility of them spoiling, so they cannot be said to be sufficient water absorption materials. It's difficult. The present inventor conducted research on water-absorbing agents to meet these objectives, and previously proposed a highly safe water-absorbing material (Japanese Patent Publication No. 30710/1983). This special public service 1977-
The polymer disclosed in Publication No. 30710 is an acrylic acid alkali metal salt polymer produced by optimizing polymerization conditions and employing a reversed-phase suspension polymerization method, and has a large water absorption capacity. are doing. However, in general, water-absorbing polymers have significantly improved water-absorbing performance compared to conventional absorbents (pulp, paper, etc.), but on the other hand, they are considerably more expensive. Therefore, from an economical point of view, there is a desire for the emergence of a polymer with higher water absorption, which can make the polymer more absorbent and reduce the amount of polymer used. Therefore, as a result of extensive research in order to achieve this objective, the present inventors found that by combining two types of polymerization initiators when producing the self-crosslinking type acrylic acid alkali metal salt polymer, the desired powder form was obtained. The present inventors discovered that a water-absorbing agent having a very large water-absorbing capacity can be obtained by using the above methods, and completed the present invention. That is, the present invention provides a 30% by weight solution containing two types of water-soluble radical polymerization initiators, persulfate and hydroperoxide, in a petroleum-based aliphatic hydrocarbon solvent.
The present invention provides a method for producing a water-absorbing resin, which is characterized in that an aqueous solution of an alkali metal acrylate salt up to a saturated concentration is polymerized in a state in which it is dispersed and suspended in the presence of a sorbitan fatty acid ester of HLB 2 to 9. be. The technology of the reverse phase suspension polymerization method used in the present invention is disclosed in Japanese Patent Publication No. 34-10644, Japanese Patent Application Publication No. 48-1082, and Japanese Patent Application Publication No. 51-41090.
and JP-A-51-47930 and other publications mainly disclose the production of acrylamide polymers and acrylamide copolymers, and the techniques can be applied here as well. In the production process of the alkali metal acrylic salt polymer according to the present invention, neutralization of acrylic acid must be carried out before the start of polymerization, when the acrylic acid becomes a polymer, since it is difficult to powderize the polymer after it has been formed. It must be neutralized with an alkali metal hydroxide to give a Its degree of neutralization is 50-95 mol%, preferably 65-80 mol%. Examples of the alkali metal hydroxide used for neutralization include sodium hydroxide, potassium hydroxide, and lithium hydroxide. Among these, sodium hydroxide is said to be the most preferable from the viewpoint of safety to living organisms and economical efficiency. The organic solvent used in the present invention must be a petroleum-based aliphatic hydrocarbon with a boiling point of 30 to 200°C.
It is included in the scope of. Examples include n-hexane and ligroin. Most preferred is n-hexane. In order to carry out reverse phase suspension polymerization in this way, the organic solvent used and the surfactant for dispersing the acrylate monomer aqueous solution in the oil are important, and aromatic solvents such as benzene, toluene or xylene are important. as an auxiliary agent to form a W/O emulsion using
Even if sorbitan fatty acid esters of HLB 2 to 9, such as sorbitan monostearate, are used as a surfactant, the polymer obtained after polymerization will gel.
It does not turn into the desired powder form. Next, the surfactant used for dispersing/suspending the aqueous solution of alkali metal acrylate in an organic solvent is preferably a nonionic surfactant with HLB 2 to 9 that is solid at room temperature. is necessary for the surface state of the polymer, that is, the property that the obtained polymer particles do not stick to each other and do not form agglomerates, and sorbitan fatty acid esters, which are solid at room temperature, are particularly preferable, sorbitan monostearate. . If a surfactant with an HLB greater than 9 is used, a W/O emulsion cannot be stably formed, and if an HLB is less than 2, the surfactant will be oily even if a W/O emulsion is formed. Both of these are undesirable because they adhere to the surface of the polymer particles after polymerization, making the particles sticky and making them difficult to handle. The appropriate amount of surfactant to be added is usually 1 to 100% of the monomer.
The amount is in the range of 10wt%, but the particle size produced depends on the amount. The present inventor has found that the concentration of the aqueous acrylic acid salt aqueous solution used for polymerization has a very deep correlation with the degree of crosslinking or swelling of the resulting polymer; however, in order to increase the degree of swelling of the polymer, The higher the concentration of the aqueous solution of alkali metal acrylate, the better; it is usually 30 wt% or more up to saturation concentration. Preferably it is a saturation concentration. Incidentally, the saturation concentration of sodium acrylate at room temperature at a neutralization degree of 75% is approximately 45 wt%. Polymerization at such high concentrations provides characteristically self-crosslinked polymers without the use of crosslinking agents normally required to produce crosslinked polymers. In the present invention, a crosslinking agent may be used in a small amount. When a crosslinking agent is used, the water absorption capacity of the polymer naturally decreases as the amount used increases, but the water absorption rate increases. Therefore, the use of a crosslinking agent of about 0.2% or less is permissible, and the resulting polymer has a superior water absorption rate, although its water absorption ability is inferior to that obtained when no crosslinking agent is used. Crosslinking agents that can be used include N, N'-
Examples include methylenebis(meth)acrylamide, N-methylol(meth)acrylamide, and glycidyl(meth)acrylate. When a crosslinking agent is used, the resulting polymer can be obtained in powder form even if the concentration of the aqueous alkali metal acrylate solution is somewhat low. The concentration of this aqueous solution can be lowered depending on the amount of crosslinking agent used, and about 30% by weight is sufficient. In the practice of this invention, the ratio of the amounts of aqueous monomer solution to organic solvent can be varied over a wide range. Generally, when the amount of organic solvent is small, it is advantageous in that the amount of monomer charged per batch in the reactor can be increased, but it becomes difficult to eliminate heat generation. Therefore, this ratio is determined from the heat removal capacity of the reactor. A capacity ratio between 1:1 and 1:4 is usually suitable. In the present invention, by using a combination of two types of water-soluble radical polymerization initiators, persulfate and hydroperoxide, it is possible to obtain a polymer with extremely high water absorption capacity. Examples of the persulfate include sodium persulfate, potassium persulfate, and ammonium persulfate, and examples of the hydroperoxide include hydrogen peroxide, t-butyl hydroperoxide, and cumene hydroperoxide. Any combination of initiators may be used as long as the amount of persulfate is 0.01 to 1 wt% and the amount of hydroperoxide is 0.01 to 1 wt% based on the monomer. However, when the initiator is only a persulfate, the maximum water absorption capacity is 500 g/g or less. In addition, in the case of only hydroperoxide, the water absorption capacity cannot be measured because it aggregates into pieces of 3 to 5 mm and the polymer is solubilized. Polymerization is carried out in the same manner as described in Japanese Patent Publication No. 54-30710. That is, a solution in which an initiator and, if necessary, a crosslinking agent are added and dissolved in a monomer aqueous solution that has been neutralized in advance is mixed with an organic solvent in which a surfactant is dissolved, and nitrogen gas is introduced while stirring to eliminate oxygen in the system. kick out During this time, the aqueous solution portion of the reaction system becomes minute droplets and is dispersed and suspended in the organic solvent. Next, it is heated to a predetermined temperature to initiate polymerization, but depending on the state of heat generation, cooling or heating is performed as appropriate. The polymerization reaction proceeds in dispersed minute droplets, and when the stirring is stopped after the reaction is complete, the polymer particles swollen with water settle out, but these are separated from the organic solvent by means such as tilting, filtration, or centrifugation. After that, it may be dried, or it may be dried by directly distilling off the organic solvent and water. The polymer thus obtained usually has a diameter of about 0.01 to 0.1 mm, depending on the amount of surfactant used, and is composed of true spherical primary particles whose surface is covered with surfactant, or secondary aggregation of some of them. It is a granule containing a small amount of secondary particles. These secondary particles can also be easily pulverized by slight mechanical force.
This has great advantages in the production and use of polymers. In other words, when a polymer gel is made by ordinary aqueous polymerization and then dried, the polymer is not powdered, so a process of crushing the dried material with extremely large mechanical force is required.
Fine grinding is an extremely difficult process. The most effective embodiment of the present invention is as follows. That is, 40 to 45 wt of sodium acrylate containing 0.01 to 1 wt% of persulfate and 0.01 to 1 wt% of hydroperoxide based on monomer in n-hexane and neutralized to 65 to 80 mol%.
% aqueous solution in the presence of sorbitan monostearate and polymerize it, and then distill off n-hexane and water to form a powder with a very high water absorption capacity (maximum water absorption capacity of about 1200g/g). ) polymer can be obtained. In this way, the present invention has made it possible to have an extremely large water absorption capacity, but in the previous application,
When compared to the invention disclosed in Publication No. 30710, its greatest feature lies in the use of a combination of persulfate and hydroperoxide as a polymerization initiator. Moreover, by using the ultra-high water-absorbing polymer obtained by combining two types of initiators in this way, the amount added can be half of the amount of conventional water-absorbing agents, making it extremely economical. This makes it possible to further expand the range of uses. The water absorption capacity of the polymer thus obtained is very high, so even when a small amount of a crosslinking agent is used during polymerization and crosslinking is performed, the resulting polymer has a substantially satisfactory water absorption capacity. In this way, a polymer that is slightly cross-linked using a small amount of cross-linking agent has a lower water absorption capacity than a polymer that is not cross-linked, but the water absorption rate increases significantly, so that it can be used, for example, when added to paper diapers. It has the effect of absorbing a large amount of liquid in a short period of time and reducing the discomfort caused by getting wet. In the present invention, water absorption capacity is a value determined by the following operation. That is, about 1 g of polymer was dispersed in a large excess of ion-exchanged water, left to stand at room temperature for 30 minutes to sufficiently swell, and then passed through an 80-mesh wire mesh. The value obtained by measuring the weight (W) of the obtained swollen polymer and dividing this value by the initial polymer weight (Wo), that is, water absorption capacity (g/g) = W/
It was Wo. In addition, the water absorption rate is 0.5 at room temperature and normal pressure.
It is expressed as the time required to absorb 5 ml of 0.9% saline solution into 5 ml of polymer. The present invention will be specifically explained below using Examples and Comparative Examples. Example 1 Hexane 230 was placed in a 500 ml four-necked flask equipped with a stirrer, reflux condenser, dropping funnel, and nitrogen gas inlet tube.
After adding and dissolving 1.8 g of sorbitan monostearate, nitrogen gas was blown into the solution to drive out dissolved oxygen. Separately, in a beaker, 13.4 g of acrylic acid dissolved in 40 g of water was cooled with ice from the outside.
78% of the carboxyl groups were neutralized with 98% caustic soda. The monomer concentration in this aqueous solution was 45 wt%. Next, add 0.1g of potassium persulfate to this aqueous solution.
After adding and dissolving 0.04 g of 30% hydrogen peroxide, the pressure was reduced to remove oxygen present in the solution. The contents of the beaker were added to the four-necked flask and dispersed, and stirring was continued for 3 hours while the internal temperature of the flask was maintained at 60 to 65° C. with a water bath while introducing a slight amount of nitrogen gas. The hexane was distilled off under reduced pressure, and the remaining swollen polymer portion was dried under reduced pressure at 30 to 80°C to obtain a powdery polymer. Example 2 In the charging recipe in Example 1, 0.3 g of potassium persulfate and 30% hydrogen peroxide were added in the polymerization initiator part.
A powdered polymer was obtained in the same manner as in Example 1 except that the amount was 0.12 g. Example 3 A powdered polymer was obtained in the same manner as in Example 1, except that in the charging recipe in Example 1, 50 g of water, that is, the monomer concentration was 40%, was used in the part related to the aqueous phase. Example 4 A powdered polymer was obtained in the same manner as in Example 1, except that 0.1 g of potassium persulfate and 0.04 g of 80% t-butyl hydroperoxide were used as the polymerization initiator in the charging recipe of Example 1. Ta. Example 5 A powdered polymer was obtained in the same manner as in Example 1, except that 0.1 g of potassium persulfate and 0.06 g of cumene hydroperoxide were used as the polymerization initiator. Example 6 In the charging recipe of Example 1, 0.09 g of ammonium persulfate and 30% hydrogen peroxide were added in the polymerization initiator part.
A powdered polymer was obtained in the same manner as in Example 1 except that the amount was 0.04 g. Comparative Example 1 Synthesized in the same manner as in Japanese Patent Publication No. 54-30710. That is, a powdered polymer was obtained in the same manner as in Example 1, except that only 0.1 g of potassium persulfate was used as the polymerization initiator in the charging recipe of Example 1. Comparative Example 2 80% of the polymerization initiator in the formulation of Example 1
A powdered polymer was obtained in the same manner as in Example 1, except that 0.04 g of t-butyl hydroperoxide was used. This polymer was almost soluble in water and it was impossible to measure its water absorption capacity. Comparative Example 3 In the charging recipe in Example 1, the polymerization initiators were 0.1 g of potassium persulfate and ammonium persulfate.
A powdered polymer was obtained in the same manner as in Example 1 except that the amount was 0.09 g. The water absorption capacity of the polymers obtained in Examples 1 to 6 and Comparative Examples 1 to 3 was measured, and the results are shown in Table 1.

[Table] It is clear from Table 1 that the performance of the water-absorbing polymer produced by the production method of the present invention is extremely high. Example 7 A powdered polymer was obtained in the same manner as in Example 1, except that 6 mg of N,N'-methylenebisacrylamide was further dissolved as a crosslinking agent in the monomer aqueous solution portion of the charging recipe of Example 1. Example 8 A powdered polymer was obtained in the same manner as in Example 1, except that 15 mg of N,N'-methylenebisacrylamide was further dissolved as a crosslinking agent in the monomer aqueous solution portion of the charging recipe of Example 1. The water absorption rates of the polymers obtained in Examples 7 and 8 and Comparative Example 1 were measured, and the results are shown in Table 2.

【table】

Claims (1)

[Claims] 1. An alkali metal acrylate containing two types of water-soluble radical polymerization initiators, persulfate and hydroperoxide, in a petroleum-based aliphatic hydrocarbon solvent at a saturation concentration of 30% by weight or more. salt solution, HLB2
A method for producing a water-absorbing resin, characterized in that polymerization is carried out in a dispersed and suspended state in the presence of the sorbitan fatty acid ester of ~9. 2 Acrylic acid alkali metal salt is 50% of acrylic acid
The method for producing a water-absorbing resin according to claim 1, wherein ~95 mol% of the water-absorbing resin is neutralized with an alkali metal hydroxide. 3. The method for producing a water-absorbing resin according to claim 1, wherein the sorbitan fatty acid ester is sorbitan monostearate. 4. The method for producing a water-absorbing resin according to claim 1, wherein the petroleum-based aliphatic hydrocarbon is normal hexane.