Method for producing superabsorbent resin

Abstract

To provide a method for producing a super absorbent polymer which enables super absorbent characteristics to be stabilized for a longer period of time by a crosslinking reaction performed at a lower temperature and / or in a shorter period of time.SOLUTION: A method for producing a super absorbent polymer, comprises: preparing a salt (I) of a copolymer of isobutylene and maleic anhydride and an alkaline substance, a catalyst (II) for a crosslinking reaction, and a crosslinking agent (III) which is a polyfunctional compound having two or more functional groups; and performing a crosslinking reaction of making the salt (I) crosslink by the crosslinking agent (III) in the presence of the catalyst (II). Preferably, the preparation includes preparing an aqueous solution comprising the salt (I), the catalyst (II), and the crosslinking agent (III) under heating, and the crosslinking reaction includes producing a solid matter by drying the aqueous solution, and performing the crosslinking reaction by subjecting the solid matter produced to a heat treatment.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing a superabsorbent resin.

Background Art

[0002] Superabsorbent resins have the property of instantaneously absorbing a large amount of water when they come into contact with water and not releasing the absorbed water even when a certain pressure is applied. Therefore, they are widely used in applications such as sanitary materials for physiological products and diapers, soil conditioners, civil engineering chemicals, sealing materials, optical fiber water transmission prevention tapes, chemical warmers, roofing materials, cat litter, casting cement, deodorants, fragrances, detergents, abrasives, heat storage materials, and cold storage materials (see paragraph 0002 of the specification of Patent Document 1). In particular, superabsorbent resins improved to satisfy practical salt water resistance, wet heat resistance, and durability, and methods for producing the same are disclosed in Patent Documents 1, 2, 3, and 4.

[0003] However, in the conventional method for producing a superabsorbent resin, there has been a problem that the crosslinking reaction for forming the superabsorbent resin requires a long time. Moreover, even after desirable water absorption characteristics are obtained, the crosslinking reaction proceeds slowly, resulting in unstable water absorption characteristics of the superabsorbent resin. When the crosslinking reaction proceeds excessively, problems such as resinification and a decrease in the water absorption amount, or when pressure is applied to the superabsorbent resin that has absorbed water, seepage of water occurs, leading to partial deterioration problems.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention has been made in view of the above problems, and provides a method for producing a superabsorbent resin, which can obtain a superabsorbent resin having high water absorption characteristics that are stable over a longer period by a crosslinking reaction at a lower temperature and / or for a shorter time.

Means for Solving the Problems

[0006] In order to achieve the above object, a first aspect of the present invention is a method for producing a superabsorbent resin, which includes preparing a salt (I) of a copolymer of isobutylene and maleic anhydride and an alkaline substance, a catalyst (II) for a crosslinking reaction, and a crosslinking agent (III) which is a polyfunctional compound having two or more functional groups, and performing a crosslinking reaction of crosslinking the salt (I) with the crosslinking agent (III) in the presence of the catalyst (II).

[0007] According to this configuration, since the crosslinking reaction is carried out in the presence of a catalyst, a superabsorbent resin having desirable high water absorption can be obtained by a crosslinking reaction at a lower temperature and / or for a shorter time. Moreover, the high water absorption characteristics of the obtained superabsorbent resin are stable over a longer period.

[0008] A second aspect of the present invention is the method for producing a superabsorbent resin according to the first aspect, wherein the preparation includes preparing an aqueous solution containing the salt (I), the catalyst (II), and the crosslinking agent (III) while heating. And performing the crosslinking reaction includes generating a solid by drying the aqueous solution and performing a heat treatment on the generated solid to cause the crosslinking reaction.

[0009] According to this configuration, a powdery superabsorbent resin can be obtained. Note that the crosslinking reaction proceeds to some extent even in the process of preparing an aqueous solution containing salt (I), catalyst (II), and crosslinking agent (III) while heating, but mainly proceeds by subjecting the solid matter to heat treatment.

[0010] The third aspect of the present invention is a method for producing a superabsorbent resin according to the first aspect, wherein the preparing includes preparing an aqueous solution containing the salt (I), the catalyst (II), and the crosslinking agent (III) while heating. And causing the crosslinking reaction to occur includes mixing the aqueous solution with heated paraffin and subjecting the mixed aqueous solution to heat treatment to cause the crosslinking reaction to occur. Further, the method for producing the superabsorbent resin further includes removing the paraffin at room temperature after the crosslinking reaction.

[0011] According to this configuration, a spherical superabsorbent resin can be obtained. Note that the crosslinking reaction proceeds to some extent even in the process of preparing an aqueous solution containing salt (I), catalyst (II), and crosslinking agent (III) while heating, but mainly proceeds by subjecting the aqueous solution mixed with paraffin to heat treatment.

[0012] The fourth aspect of the present invention is a method for producing a superabsorbent resin according to the first aspect, wherein the preparing includes preparing an aqueous solution containing the salt (I), the catalyst (II), and the crosslinking agent (III) while heating. And causing the crosslinking reaction to occur includes coating the aqueous solution on a coating substrate and subjecting the coated aqueous solution to heat treatment to cause the crosslinking reaction to occur.

[0013] According to this configuration, a sheet-like or film-like superabsorbent resin can be obtained. Note that the crosslinking reaction proceeds to some extent even in the process of preparing an aqueous solution containing salt (I), catalyst (II), and crosslinking agent (III) while heating, but mainly proceeds by subjecting the coated aqueous solution to heat treatment.

[0014] A fifth aspect of the present invention is a method for producing a superabsorbent resin according to any one of the first to fourth aspects, wherein the catalyst (II) is an acidic catalyst, formic acid, methanesulfonic acid, P-toluenesulfonic acid, phosphoric acid, and at least one selected from dodecylbenzenesulfonic acid, or a basic catalyst, 1H-imidazole, 2-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-undec Sil Imidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-phenylimidazole, N,N'methylbenzylamine, picoline, and at least one selected from pyridine.

[0015] According to this configuration, since a catalyst having excellent characteristics as a catalyst is used in the crosslinking reaction, a superabsorbent resin having high water absorption characteristics that are stable over a longer period can be obtained by a crosslinking reaction at a lower temperature and / or for a shorter time.

[0016] A sixth aspect of the present invention is a method for producing a superabsorbent resin according to any one of the first to fifth aspects, wherein the functional group is at least one selected from a hydroxyl group, an epoxy group, and an amino group.

[0017] According to this configuration, since a polyfunctional compound having excellent characteristics is used as a crosslinking agent, the crosslinking reaction proceeds smoothly.

Effects of the Invention

[0018] As described above, according to the present invention, a method for producing a superabsorbent resin is realized in which a superabsorbent resin having high water absorption characteristics that are stable over a longer period can be obtained by a crosslinking reaction at a lower temperature and / or for a shorter time.

Modes for Carrying Out the Invention

[0019] The method for producing a superabsorbent resin according to the present invention is, as described above, (a) preparing a salt (I) of a copolymer of isobutylene and maleic anhydride and an alkaline substance, a catalyst (II) for a crosslinking reaction, and a crosslinking agent (III) which is a polyfunctional compound having two or more functional groups; and (b) performing a crosslinking reaction of crosslinking the salt (I) with the crosslinking agent (III) in the presence of the catalyst (II).

[0020] Hereinafter, preferred forms of each material substance and preferred forms of the manufacturing process will be exemplified. Regarding desirable examples of the salt (I) and the crosslinking agent (III), as exemplified below, the findings described in Patent Document 1 can be used as they are. First, the molecular weight of the copolymer (especially an alternating copolymer) of isobutylene and maleic anhydride preferably has a value of the limiting viscosity [η] measured at 30 ° C. in a dimethylformamide solution in the range of 0.1 to 8 (dl / g), and particularly preferably in the range of 0.2 to 5 (dl / g). When a copolymer having a limiting viscosity lower than 0.1 is used, it is difficult to obtain a resin having a large water absorption ratio, and in addition, the durability of the superabsorbent resin may decrease. On the other hand, when a copolymer having a limiting viscosity higher than 8 is used, the solution viscosity when made into an aqueous solution becomes high, and thus manufacturing problems may occur.

[0021] Examples of the alkaline substance include sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, ammonium hydroxide, ammonia gas, methylamine, ethylamine, propylamine, dimethylamine, diethylamine, triethylamine, monoethanolamine, diethanolamine, sodium carbonate, potassium carbonate, sodium acetate, potassium acetate, ammonium acetate, disodium phosphate, trisodium phosphate and the like. Among these, sodium hydroxide, potassium hydroxide and ammonia are preferable, and these alkaline substances may be used in combination of two or more kinds.

[0022] The reaction ratio of an alkaline substance to an alternating copolymer of isobutylene and maleic anhydride, that is, the ratio of neutralization (hereinafter referred to as "degree of neutralization"), is important. Assuming that the degree of neutralization (α) is 100% when 2 moles of an alkaline substance react with 1 mole of maleic anhydride, it is desirable that the degree of neutralization (α) be in the range of 10 to 90%. If the degree of neutralization (α) is lower than 10%, sufficient water absorption performance may not be obtained. Also, if the degree of neutralization (α) is higher than 90%, the gel strength in the swollen state may decrease. A particularly preferred range is 30 to 70%. The pH assumed in advance is almost determined by the degree of neutralization, but depending on the selection and amount of the desired catalyst, that is, an acid catalyst and a basic catalyst, some adjustment is possible. When the degree of neutralization is almost neutral, either of them may be used. When the degree of neutralization is small, an acid catalyst can be selected, and when it is large, a basic catalyst can be selected.

[0023] The crosslinking agent (III) is preferably used in a proportion of 0.01 to 5% by weight, more preferably 0.1 to 2% by weight, based on the salt of (I).

[0024] The crosslinking agent (III) that can be used is a polyfunctional compound having two or more functional groups such as a hydroxyl group, an epoxy group, and an amino group. Examples thereof include polyhydric epoxy compounds such as ethylene glycol, propylene glycol, glycerin, diethylene glycol, polyethylene glycol, polypropylene glycol, polyglycerin, ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, glycerin diglycidyl ether, glycerin triglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polyvalent amines such as polyethyleneimine, ethanolamine, ethylenediamine, propylenediamine, trimethylol melamine, pentaerythritol, urea, triethylenediamine, triethylenetetramine, hexamethylenetetramine, diethylenetriamine, triethyltetramine, tetraethylenepentamine, pentaethylenehexamine, etc. These crosslinking agents preferably have a molecular weight of 60 to 100,000, and more preferably 60 to 5,000. Two or more kinds may be used in combination.

[0025] The catalyst (II) for the crosslinking reaction is preferably, as an acidic catalyst, formic acid, toluenesulfonic acid, and as a basic catalyst, an imidazole group, pyridine, picoline, N,N'-methylbenzylamine, etc. The amount of the catalyst may be adjusted as appropriate, but basically, it is used in a proportion of 0.01 to 5% by weight, more preferably 0.1 to 3% by weight, according to the amount of the crosslinking agent.

[0026] In the conventional manufacturing method without using a catalyst, it takes a long time to reach the target crosslinking level, and moreover, the reaction still proceeds even after reaching the target level. For this reason, the high water absorption property is not stable forever. On the other hand, in the manufacturing method of the embodiment of the present invention, since the crosslinking reaction is carried out in the presence of a catalyst, when comparing at the same heat treatment temperature, the crosslinking reaction reaches the target level in a shorter time. Moreover, after reaching the target level, the progress of the reaction is almost non-existent.

[0027] Regarding the processing temperature during the manufacturing process, in the initial solution state, maintain 80°C to 95°C at the time of neutralization, maintain 40°C to 60°C when adding the crosslinking agent and the catalyst, maintain drying conditions around 110°C in the powder forming method, maintain at about 95°C in paraffin in the spheroidization method, and coat at about 50°C in the sheet coating method. In the final heat treatment, when no catalyst is added, the target is about 160°C to about 190°C, and when a catalyst is added, about 130°C to about 150°C is used as a guide.

[0028] Thus, in the manufacturing method of the embodiment of the present invention, when compared at the same heat treatment temperature, a superabsorbent resin having desirable high water absorbency can be obtained by a crosslinking reaction in a shorter time than the conventional manufacturing method. Moreover, the high water absorption characteristics of the obtained superabsorbent resin are stable over a longer period. Also, when compared at the same heat treatment time, in the manufacturing method according to the embodiment of the present invention, a superabsorbent resin having desirable high water absorbency can be obtained by a crosslinking reaction at a lower temperature than the conventional manufacturing method. Moreover, the high water absorption characteristics of the obtained superabsorbent resin are stable over a longer period. The excellent effects of the manufacturing method according to the embodiment of the present invention are supported by the examples described below. In addition, as an interesting discovery through the following examples, when a crosslinking agent was added to an aqueous solution of a salt to which an alkaline substance was added and further a catalyst was added and heated at 60°C to 95°C, it was found that the crosslinking reaction also started in the aqueous solution.

Examples

[0029] As an example of the manufacturing process, methods known as the "powdering method" and the "spheroidization method" can be adopted. Also, a method that should be called the "sheet coating method" can be adopted. Hereinafter, examples of trial-producing a superabsorbent resin by embodying the manufacturing method of the present invention by these methods will be described.

[0030] [Example 1. (Powdering method)] As Example 1, a superabsorbent resin was trial-produced using the powdering method as follows. 100 parts by weight of an isobutylene-maleic anhydride copolymer ("Isoban 10" manufactured by Kuraray Co., Ltd., having an intrinsic viscosity (η) of 1.01 at 30°C in a dimethylformamide solution, and a molar ratio in the copolymer of isobutylene:maleic anhydride = 1:1), 31 parts by weight of sodium hydroxide, and 305 parts by weight of water were mixed and heated from room temperature to 95°C with stirring to prepare a uniform aqueous solution of the sodium neutralized product of the isobutylene maleic anhydride copolymer. The degree of neutralization α was 0.6.

[0031] Next, 0.5 parts by weight of polyethyleneimine with a molecular weight of 300 and 0.5 g of 1H-imidazole as a catalyst were added to 436 parts by weight of the above aqueous solution, and after thorough mixing, the mixture was poured into a square vat with a side of 70 cm coated with fluororesin to prepare a film. Next, after drying at 95°C for 1 hour, heating was performed at 120°C for 1 hour, and further heat treatment was carried out at 150°C for 2 hours. Drying was performed at 100°C or lower in order to suppress the boiling of water and remove air. The plate-like product obtained by the heat treatment was pulverized to obtain a water-absorbing resin powder with a 20-mesh pass.

[0032] The product thus obtained was filtered through a 20-mesh cloth, wrung by hand, and the water absorption ratio was measured, and a durability test was conducted. The test results are shown in Table 1 together with the comparative examples to be compared.

[0033]

Table 1

[0034] In the comparative examples, in the process of Example 1, an aqueous solution was prepared without adding a catalyst, and then, by passing through the same process, a water-absorbing resin powder was obtained. However, the temperature and time of the final heat treatment were set to three cases: 150°C × 2 hours, 165°C × 2 hours, and 180°C × 2 hours. These are designated as Comparative Examples 1-1, 1-2, and 1-3, respectively.

[0035] The test method is as follows. For the water absorption ratio test, before conducting the durability test, 1 gram of the specimen was weighed on a balance, put into a beaker containing 350 ml (milliliters) of purified water, left at room temperature for 2 hours, then filtered through a 200-mesh nylon cloth, and after that, gently wrung to remove free water, and then the swollen specimen was weighed to measure how many times it swelled. In Durability Test A, 350 g of water was added to 1 g of the water-absorbing resin, sealed so that the water would not evaporate, left at 25°C for 1 year, and then the state of the water-containing gel was observed. Also, the water absorption ratio was measured. In Durability Test B, 350 g of water was added to 1 g of the water-absorbing resin, and the mixture was sealed to prevent water evaporation. After heating at 70 °C for 30 days, the state of the water-containing gel was observed. Also, the water absorption ratio was measured. In Durability Test C, 350 g of water was added to 1 g of the water-absorbing resin, and the mixture was sealed to prevent water evaporation. After heating at 100 °C for 48 hours, the state of the water-containing gel was observed. Also, the water absorption ratio was measured. In Heat Resistance Test D, the water-absorbing resin was heated at 150 °C for 8 hours, and then the state of the water-containing gel was observed. Also, the water absorption ratio was measured. In Heat Resistance Test E, the water-absorbing resin was heated at 190 °C for 4 hours, and then the state of the water-containing gel was observed. Also, the water absorption ratio was measured. In Durability Tests A, B, C, D, and E, for the test of the absorption ratio after the durability test by standing or heating, the sample placed in the water-containing state was gently squeezed with a 200-mesh nylon cloth to remove free water, and then the swollen specimen was weighed to measure how many times it had swollen.

[0036] In the examples with a catalyst added, the initial water absorption ratio remained stable until the end, and a heat treatment at 150 °C for 2 hours was sufficient to obtain sufficient performance. In contrast, in Comparative Examples 1-1, 1-2, and 1-3, the crosslinking was not sufficient, and for this reason, the water absorption ratio was not stable unless sufficient heat treatment was performed. Although countermeasures were taken by raising the heat treatment temperature above 190 °C or further lengthening the heat treatment time, when this was done, the sample turned yellow and resinified, resulting in partial deterioration. By comparing the examples and the comparative examples, it is understood that a stable water-absorbing resin can be obtained by adding a catalyst.

[0037] [Example 2. (Spheroidization method)] Patent Document 3 describes the following manufacturing method as "Example 1". To 233 parts of a 14% aqueous sodium hydroxide solution, 100 parts of an isobutylene maleic anhydride copolymer (“Isoban 10” manufactured by Kuraray Co., Ltd.) were added, and the mixture was heated and stirred to prepare a uniform aqueous solution with a neutralization degree of 0.1. Next, 0.3 parts of polyethyleneimine with a molecular weight of 300 (“Epomin SP-003” manufactured by Nippon Shokubai Kagaku Kogyo Co., Ltd.) as a cross-linking agent was added to the prepared aqueous solution, and the mixture was thoroughly mixed. The resulting mixture was then added to 333 parts of liquid paraffin in which 1 part of an oil-soluble surfactant with an HLB of 1.8 (“Leodol SP-030” manufactured by Kao Corporation) was dissolved. While stirring, the temperature was maintained at 95°C for 25 hours to allow sufficient evaporation of water. Then, after solid-liquid separation, the product was washed with hexane and dried to obtain a spherical superabsorbent resin with a particle size of 32 to 100 mesh and a water absorption capacity of 350 times.

[0038] As Example 2 of the present invention, in the above method of Patent Document 3, when adding 0.3 parts of polyethyleneimine SP-003, 0.3 g of 1H-imidazole was added as a catalyst, and while controlling the stirring speed, the temperature was maintained at 95°C for 5 hours to allow sufficient evaporation of water. Then, after solid-liquid separation, the product was washed with hexane and dried to obtain spherical objects with a diameter of 0.5 mm to 2 mm. Further, by performing a heat treatment at 150°C for 2 hours, a spherical superabsorbent resin was obtained. It was found that this superabsorbent resin was a spherical gel with a water absorption capacity of about 220 times in distilled water and also had a sustained release effect, and thus could be expected to be applied as a fragrance or deodorant. In addition, it had excellent long-term stability. The product hydrated with distilled water withstood a durability test at 70°C for 30 days and retained a water absorption capacity of 200 times even after a durability test at room temperature for 1 year.

[0039] On the other hand, as Comparative Example 2, in the reaction system without adding a catalyst, no water absorption was observed after 1 year. The manufacturing method of Comparative Example 2 was the same as that of Example 2 except that no catalyst was added. The comparison results between Example 2 and Comparative Example 2 are shown in Table 2.

[0040]

Table 2

[0041] Also, in each of the methods of Example 2 and Comparative Example 2, the rate of heating the sample introduced into the paraffin while stirring was slowed down to confirm whether superabsorbent resin particles of large size could be obtained. As a result, as shown in Table 2, in the method of Example 2, products with diameters of 2 mm to 15 mm were obtained.

[0042] [Example 3. (Sheet Coating Method)] As Example 3, a superabsorbent resin was prototyped by the following method. 100 parts by weight of an isobutylene-maleic anhydride copolymer (a total of 100 parts by weight of 60 parts by weight of Isoban 10 (weight average molecular weight of about 165,000) and 40 parts by weight of Isoban 06 (weight average molecular weight of about 85,000)), 31 parts by weight of sodium hydroxide, and 305 parts by weight of water were mixed, heated from room temperature to 95°C, and stirred to prepare a uniform aqueous solution of the sodium neutralized product of the isobutylene-maleic anhydride copolymer. Next, this aqueous solution was brought to 50°C, and 0.5 part by weight of polyethylene having a molecular weight of 600 and 0.5 g of 2-methylimidazole as a catalyst were added to 436 parts by weight of the aqueous solution, and after mixing, a coating solution was obtained. The coating solution has a solid content of 30% and a slightly higher viscosity at 30°C, so it is in a state easy to coat.

[0043] A PE / PP nonwoven fabric was laid on the prepared release paper, and a stainless steel punching plate (a perforated plate with holes having a diameter of 5 mm) was placed thereon, and coating was performed using a bar coater. This was used as a surface-coated sheet, and with the nonwoven fabric gel-coated on the release paper still placed thereon, the sample was placed in an oven at 30°C, the temperature was raised, and finally heat treatment was performed at 145°C for 30 minutes to obtain a sheet (Example 3-1).

[0044] Furthermore, in order to prevent the gel from falling off after water absorption, the nonwoven fabric was changed to rayon / polyester, and after coating, a sandwich structure was formed with the nonwoven fabric. With the nonwoven fabric gel-coated on the release paper still placed thereon, the sample was placed in an oven at 30°C, the temperature was raised, and finally heat treatment was performed at 145°C for 30 minutes to obtain a composite sheet (Example 3-2).

[0045] As a comparative example, a surface-coated sheet was obtained through the same steps as in Example 3-1 without adding 2-methylimidazole as the catalyst (Comparative Example 3-1). Due to the heat treatment, not only the gel but also the adherend undergoes deterioration, tearing, brittleness, and yellowing. The test results for these prototypes are shown in Table 3.

[0046]

Table 3

[0047] By using the catalyst, the following advantages could be confirmed. (1) The width of the molecular weight distribution can be expanded. Thereby, the range of coating conditions can be broadened. (2) The concentration of the coating solution can be increased. Thereby, the range of coating conditions can be broadened. (3) The range of selection of the types of materials for the sheet to be coated can be significantly broadened, and particularly with regard to heat resistance, deterioration can be prevented. (4) A decrease in the temperature and time of the heat treatment can be expected. Thereby, an energy-saving effect can be obtained. (5) Information on the viscosity and pH of the coating solution is easy to understand. Therefore, it is easy to respond to on-site situations.

[0048] [Example 4.] It was experimentally investigated how various factors related to the production method of the superabsorbent resin affect the properties of the produced superabsorbent resin. The evaluation method was based on the formulations of Example 1 and Comparative Example 1, with light weight, heating, and stirring performed. The final heat treatment was 180°C × 4 hours when no catalyst was used and 150°C × 2 hours when a catalyst was used. The results are shown in Table 4. What is characteristic in Table 4 is that the mixed solution, that is, powdered isobutylene maleic anhydride, is stirred in water while raising the temperature. First, an alkaline substance is added, neutralized and dissolved in water, and a crosslinking agent is added to cause crosslinking. During this process, the solution viscosity was measured 1 hour after reaching 50°C. Furthermore, when the solution viscosity was measured after adding the catalyst, as can be seen from the solution viscosities of Comparative Example 4 and Example 4, it was found that the viscosity increased due to heating in the solution. That is, it was discovered that crosslinking starts in the aqueous solution state. As shown in Table 4, it was found that the presence of the catalyst broadens the molecular weight range of isobutylene maleic anhydride and slightly lowers the pH. Also, the heat treatment conditions become lower, and it was found that productivity is improved. In addition, various processing methods become possible, various forms can be considered, and it is expected that the applications will expand.

[0049]

Table 4

[0050] In the experiment, the following points were noted. (1) Use the copolymer of isobutylene - maleic anhydride. Furthermore, in order to obtain an optimal water - absorbent resin, there is an optimal range for the molecular weight. (2) To start the crosslinking reaction in water, use a neutralizing agent that is soluble in water and determine the degree of neutralization α according to the amount of the alkaline substance. There is an appropriate amount for the amount of the alkaline substance. (3) Select a crosslinking agent that is soluble in water, add an appropriate amount to the alkaline neutralized solution of isobutylene - maleic anhydride, and stir and mix well. (4) Select a liquid catalyst rather than a solid catalyst for the catalyst, and appropriately select an acidic catalyst or a basic catalyst according to the degree of neutralization. In the neutral region, either is acceptable. (5) The criterion for causing the crosslinking reaction is confirmed by the solution viscosity, and the end point of the reaction is selected according to the shape and performance of the water - absorbent resin finally obtained.

[0051] Regarding the performance of a roughly defined water-absorbing resin, when measuring the water absorption ratio of a water-containing gel in a water-absorbed state, it is filtered and wrung out with a 200-mesh cloth. At that time, a rough estimate can be made based on the presence or absence and hardness of the slime of the water (free water) coming out of the cloth. Expressions such as "firm", "with slime", and "presence or absence of a slippery feeling" were considered to be easy-to-understand expressions. The quality of productivity was indicated by symbols such as "double circle", "circle", and "triangle", but those with a low heat treatment temperature, a short heat treatment time, and a large water absorption ratio (those that are "firm") were judged as "double circle". It is understood that the performance of the obtained product is more stable when a catalyst is used.

Claims

1. preparing a salt (I) of a copolymer of isobutylene and maleic anhydride and an alkaline substance, a catalyst (II) for a crosslinking reaction, and a crosslinking agent (III) which is a polyfunctional compound having two or more functional groups; causing a crosslinking reaction in which the salt (I) is crosslinked with the crosslinking agent (III) in the presence of the catalyst (II); A method for producing a superabsorbent resin, comprising:

2. The preparing includes preparing an aqueous solution containing the salt (I), the catalyst (II), and the crosslinking agent (III) while heating, The causing the crosslinking reaction includes: generating a solid by drying the aqueous solution; causing the crosslinking reaction by subjecting the generated solid to a heat treatment; The method for producing a superabsorbent resin according to Claim 1, comprising:

3. The preparing includes preparing an aqueous solution containing the salt (I), the catalyst (II), and the crosslinking agent (III) while heating, The causing the crosslinking reaction includes: mixing the aqueous solution with heated paraffin; causing the crosslinking reaction by subjecting the mixed aqueous solution to a heat treatment; The method for producing a superabsorbent resin further includes removing the paraffin at room temperature after the crosslinking reaction. The method for producing a superabsorbent resin according to Claim 1.

4. The preparing includes preparing an aqueous solution containing the salt (I), the catalyst (II), and the crosslinking agent (III) while heating, The causing the crosslinking reaction includes: coating the aqueous solution on a coating substrate; causing the crosslinking reaction by subjecting the coated aqueous solution to a heat treatment; The method for producing a superabsorbent resin according to Claim 1, comprising:

5. The catalyst (II) is at least one selected from formic acid, methanesulfonic acid, P-toluenesulfonic acid, phosphoric acid, and dodecylbenzenesulfonic acid, which are acidic catalysts, or At least one selected from 1H-imidazole, 2-methylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 1,2-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, 1-cyanoethyl-2-methylimidazole, 1-cyanoethyl-2-undecylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 1-cyanoethyl-2-phenylimidazole, N,N'-methylbenzylamine, picoline, and pyridine, which are basic catalysts The method for producing a superabsorbent resin according to any one of claims 1 to 4, which is any of the above

6. The method for producing a superabsorbent resin according to any one of claims 1 to 5, wherein the functional group is at least one selected from a hydroxyl group, an epoxy group, and an amino group

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