JP2848882B2 - Manufacturing method of super absorbent resin - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a superabsorbent resin. More specifically, the present invention relates to a method for producing a superabsorbent resin having excellent swelling gel strength and a small wet-back amount, and which can be suitably used as, for example, a disposable diaper, a sanitary napkin, a soil water retention agent, a cement admixture, and the like.

[Related Art] In recent years, a resin having a high water-absorbing ability with respect to an aqueous liquid and having high water swelling properties, that is, a water-insoluble resin, has been recently developed and used for various applications. As such a highly water-absorbing resin, starch-sodium acrylate graft polymer, hydrolysis product of starch-acrylonitrile graft polymer, and the like are used. The monomer having a hydrophilic group is used.

However, the superabsorbent resin made of such a monomer having a hydrophilic group as a raw material, when it comes into contact with a liquid, becomes muddy, and has insufficient gel strength at the time of swelling, and has a low water absorption rate. It has the disadvantage of being small.

As a method for producing a superabsorbent resin having a desired water absorbing ability, which has solved the disadvantages of the above superabsorbent resin, and has both high gel strength and a high water absorption rate after absorbing water, it has been obtained by suspension polymerization. After adjusting the water in the polymer particles to a predetermined amount by azeotropic dehydration, a post-crosslinking agent is added,
A method of performing a crosslinking reaction on the surface of a polymer is known (JP-A-60-1204).

In the above method, although the obtained highly water-absorbent resin has improved gel strength and water absorption strength, the unreacted post-crosslinking agent may remain in the highly water-absorbent resin, The amount of water in the polymer particles must be increased as much as possible, and the amount of the post-crosslinking agent added must be reduced.Therefore, at the start of the post-crosslinking reaction, the particles adhere to each other due to the water in the polymer particles, and between the particles. There is a problem that the crosslink density varies, and further, the crosslink proceeds to the inside of the polymer particles, and the effect exerted by the crosslink on the surface becomes insufficient. When a water-absorbent resin having such a problem is used, for example, as a water-absorbent resin for disposable diapers, urine returns frequently, causing problems such as causing stuffiness on the skin and giving discomfort during use. Therefore, the superabsorbent resin obtained by the above method has very low practical value.

[Problems to be Solved by the Invention] In view of the above prior art, the present inventors have intensively studied to obtain a highly water-absorbent resin having excellent gel strength and a small wet-back amount while maintaining a desired water-absorbing ability. As a result, the inventors finally found a method for producing such a superabsorbent resin and completed the present invention.

[Means for Solving the Problems] That is, the method for producing a superabsorbent resin according to the present invention is characterized in that the water content of the polymer containing a carboxyl group is adjusted to 10 to 30% by weight, and then the surface of the polymer is impregnated with a crosslinking agent. When the amount of the remaining cross-linking agent becomes 25 to 80% by weight of the compounding amount of the cross-linking agent at the start of the reaction, the cross-linking reaction is started at 5 to 20% with respect to the water-containing polymer at the start of the reaction. It is characterized in that the crosslinking reaction is continued by further adding water by weight.

[Action and Examples] According to the method for producing a superabsorbent resin of the present invention, the water content of the carboxyl group-containing polymer is adjusted to 10 to 30% by weight, and then the crosslinking reaction of the polymer surface containing water with a crosslinking agent is performed. At the time when the amount of the remaining cross-linking agent becomes 25 to 80% by weight of the compounding amount of the cross-linking agent at the start of the reaction, 5 to 20% by weight with respect to the water-containing polymer at the start of the reaction
By further adding water to continue the crosslinking reaction, the desired superabsorbent resin can be obtained.

Examples of the carboxyl group-containing polymer used in the present invention include crosslinked acrylic acid-acrylate polymer, crosslinked methacrylic acid-methacrylate polymer, acrylic acid (ester) -vinyl acetate copolymer Powders such as cross-linked saponified denbun-acrylic acid (salt) graft copolymer, cross-linked maleic anhydride-modified polyvinyl alcohol cross-linking products, and among them, when used as products such as diapers, for example, such as water absorption rate Powders of a crosslinked product of an acrylate polymer such as a crosslinked product of an acrylic acid-sodium acrylate copolymer are preferred because of improved physical properties.

These carboxyl-containing polymers are generally produced by synthesizing a polymer by a method such as reverse phase suspension polymerization, reverse phase emulsion polymerization, aqueous solution polymerization or reaction in an organic solvent, and drying the polymer as it is. , Or after drying and then through a pulverizing step, the polymer used in the present invention is not limited to these methods, and may be manufactured by any method.

The average particle size of the carboxyl group-containing polymer cannot be determined unequivocally because it varies depending on the type of the superabsorbent resin powder and the like, but is 100 to 500 μm, preferably 150 to 400 μm in consideration of the use of diapers and the like. It is desirable. If the average particle size exceeds the above range,
When the water absorption rate tends to decrease, and when the water absorption rate is smaller than the above range, dust tends to be generated or mamako tends to be generated when the dust comes into contact with body fluid.

In the present invention, the water content of the carboxyl group-containing polymer is adjusted so as to be 10 to 30% (the same applies to weight% or less). When the water content is less than 10%, the initial surface cross-linking reaction rate becomes extremely low, and
If it exceeds 30%, the cross-linking proceeds to the inside of the polymer, and the surface cross-linking effect is weakened, and powder agglomeration and uneven cross-linking density tend to occur. In addition, such a water content is preferably 15 to 25%.

As described above, in the present invention, since the water content of the polymer containing a carboxyl group is adjusted to be 10 to 30%, the crosslinking is prevented from progressing to the inside of the polymer during the crosslinking reaction, In addition, the polymer is selectively crosslinked only on the surface of the polymer, and as a result, a highly water-absorbent resin having excellent gel strength while maintaining high water-absorbing ability can be obtained.

After the carboxyl group-containing polymer is hydrated, its surface is then crosslinked with a crosslinking agent.

The crosslinking agent can be used without particular limitation as long as it is a compound having two or more functional groups capable of reacting with a carboxyl group. Representative examples of the crosslinking agent include a polyglycidyl compound and a cyclic epoxy compound. Specific examples of the polyglycidyl compound include ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, and glycerin triglycidyl ether. Specific examples of the cyclic epoxy compound include, for example, a compound represented by the formula: Celloxide 2021 (trade name, manufactured by Daicel Chemical Industries, Ltd.). Among these crosslinking agents, ethylene glycol diglycidyl ether (hereinafter, referred to as EGDGE) and celloxide 2021 can be suitably used because they provide a highly water-absorbent resin having a small wet back amount.

The amount of the cross-linking agent varies depending on the type of the cross-linking agent and the like and cannot be unconditionally determined.
It is adjusted so that it becomes 05-3.0%, especially 0.001-1.0%. When the amount of the crosslinking agent is less than 0.0005%, the gel strength and the like tend to decrease, and
If it exceeds 3.0%, the water absorption performance tends to be significantly reduced.

The reaction between the carboxyl group-containing polymer and the cross-linking agent is performed by dissolving and dispersing the carboxyl-containing polymer and a solvent such as water, benzene, toluene, gixylene, and cyclohexane as needed. Start by mixing. The celloxide 2021 is dissolved in cyclohexane, and the EGDGE is dissolved in water. Usually 40-90 ° C to accelerate the reaction
Temperature. The time required for the reaction is the blending amount of the carboxyl group-containing polymer and the crosslinking agent,
Although it depends on the heating temperature, it is usually about 30 to 180 minutes.

Next, when the residual amount of the crosslinking agent becomes 25 to 80% of the blending amount, 5 to 20% of water is added to the water-containing carboxyl group-containing polymer at a time or several times. Add in portions. In this way, while the cross-linking reaction is proceeding, the reaction may be performed while checking the remaining amount of the cross-linking agent one by one to confirm that the remaining amount of the cross-linking agent has reached 25 to 80%.

The addition amount of the water is set to 5 to 20% with respect to the water-containing polymer having a carboxyl group, when the addition amount of the water is less than 5%, the effect of promoting the crosslinking effect is small. If the amount exceeds 20%, powder agglomeration occurs. The amount of the water is preferably 5 to 18%.

After the completion of the reaction, the obtained superabsorbent resin is dried. There is no particular limitation on the drying method. In addition, as a specific example of such a drying method, for example, in the atmosphere at 50 to 80 ° C. for 1 to 2 hours,
For example, a method at 90 ° C. for about 1 to 2 hours can be used.

The superabsorbent resin obtained by the method for producing a superabsorbent resin of the present invention has no local block, has little solvent remaining, is sanitary and safe, and has excellent gel strength and wet back amount. And the water absorption capacity is much better than conventional products.

Next, the method for producing the superabsorbent resin of the present invention will be described in more detail based on Examples and Comparative Examples, but the present invention is not limited to only these Examples.

Production Example 1 [Production of finely divided sodium polyacrylate crosslinked product by reversed-phase suspension polymerization] 100 g of acrylic acid was placed in a 500 ml beaker, and 157 g of a 25.9% aqueous sodium hydroxide solution was added thereto under cooling at 35 ° C or lower. Thus, acrylic acid was neutralized to obtain an acrylic acid monomer in which 73 mol% was partially neutralized (hereinafter, referred to as a partially neutralized acrylic acid aqueous solution). Transfer this to a 300 ml dropping funnel,
It was carried out for 10 minutes N ₂ bubbling. Then 7% APS
3 ml of an aqueous solution of (ammonium persulfate) and 1 ml of a 1% aqueous solution of N, N'-methylenebisacrylamide were added and mixed well to prepare an adjusted solution.

760 ml of cyclohexane is placed in a separable flask having a capacity of 2 l, 4 g of sorbitan monostearate (HLB 4.7) is dissolved, and then N ₂ bubbling is performed at 25 ° C. for 30 minutes (total N ₂ volume: 10 l) to dissolve dissolved oxygen and After removing the air in the gas phase, the temperature inside the system was raised to 72 ° C. The adjusted solution was dropped into the flask over 1 hour, polymerization was carried out under stirring, and further maintained at 72 ° C. for 3 hours to complete the polymerization.

Next, the obtained crosslinked acrylic acid-sodium acrylate copolymer was subjected to azeotropic dehydration at a water content of 22.5%. After cooling to room temperature, the generated particles were separated using a 325 mesh wire mesh, and then air-dried in a fume hood for about 1 hour to obtain a crosslinked acrylic acid-sodium acrylate copolymer having a water content of 22.5%. I got it. The crosslinked product was pearl-like particles having an average particle size of 110 μm. This was used as a polymer used in the following examples.

Examples 1 to 5 193.5 g of the polymer obtained in Production Example 1 was charged into a one-volume kneader, and the crosslinking agent shown in Table 1 was added to 4.8 g of cyclohexane when the crosslinking agent was celloxide 2021,
When the cross-linking agent was EGDGE, a solution dissolved in 4.8 g of water was added, and then the temperature of the kneader jacket was increased to 70 ° C. while kneading, and this temperature was maintained.

After the elapse of 40 minutes and 80 minutes, the amount of the cross-linking agent shown in Table 1 remained, so the amount of water shown in Table 1 was added.

After completion of the reaction, the resulting superabsorbent resin was taken out by drying at 70 ° C. for 1 hour in a kneader, and further dried at 90 ° C. for 1.5 hours in a vacuum drier to obtain a water content of about 5%. About 160 g of superabsorbent resin was obtained.

Comparative Examples 1 and 2 219 g of the polymer obtained in Production Example 1 was charged into a one-volume kneader, and the crosslinking agent shown in Table 1 was added to water (Comparative Example 1).
Alternatively, after adding solutions each dissolved in 3.5 g of cyclohexane (Comparative Example 2), the temperature was raised to 70 ° C. while kneading,
This temperature was maintained.

Thereafter, the reaction was carried out for 2 hours without adding water, followed by drying in the same manner as in Examples 1 to 5, to obtain about 160 g of a superabsorbent resin.

As the physical properties of the obtained superabsorbent resin, the water absorbing ability, swelling gel strength and wet back value were examined according to the following methods. Table 1 shows the results.

Comparative Examples 3 to 4 When the residual ratio of the crosslinking agent after 40 minutes was the ratio shown in Table 1,
The procedure was performed in the same manner as in Example 4 except that the amount of water shown in Table 1 was added.

 Table 1 shows the results.

Comparative Examples 5 to 6 When the residual ratio of the crosslinking agent is the ratio shown in Table 1 (Comparative Example 5
100 minutes later, Comparative Example 6 was 5 minutes later), except that the amount of water shown in Table 1 was added.

 Table 1 shows the results.

(B) Water absorption capacity 0.2g of superabsorbent resin (in dry state) in a 500ml beaker
And 60 g of saline (0.9% aqueous sodium chloride solution)
Alternatively, add 200 g of pure water and stir gently with a glass rod.
The mixture was allowed to stand at room temperature for a period of time, filtered through a 325-mesh wire net, and the weight of the gel after filtration was measured.

(Water absorption capacity) (g / g) = (gel weight after filtration (g)-0.2 (g)) / 0.2 (g) (b) Swelling gel strength 1 g of superabsorbent resin and saline in a 100 ml beaker 25 g of (0.9% aqueous sodium chloride solution) was gently mixed and left to stand to obtain a swollen gel swollen 25 times.

After leaving the obtained swelling gel at room temperature for 5 hours, the swelling gel strength was measured with a neo card meter (M-302 type, manufactured by Iio Electric Co., Ltd.).

(C) Wet back value 5.0 g of the granules obtained in the central part (100 × 240 mm) of a sheet-shaped cotton-like valve (120 × 280 × 5 mm) are evenly dispersed, and then the cotton-like valve and A test diaper was prepared by adhering a cotton-like valve sheet of the same shape from the upper surface.

About 75 g of artificial urine was injected into the center of the upper surface of the diaper through a dropping funnel. After 20 minutes, inject about 75 g of artificial urine and after 20 minutes, about 60 g of paper (100 x 100 mm)
We put 3.5 kg of weights in order and left for 3 minutes.

Thereafter, the weight and the paper were sequentially removed, the weight of the paper was measured, and the amount of increase in the weight of the paper was determined to determine the amount of urine returned (wet back value).

As is clear from the results shown in Table 1, the superabsorbent resin obtained by the production method of the present invention has excellent water absorbing ability, high swelling gel strength, and extremely low wet back value. You can see that.

[Effect of the Invention] Since the superabsorbent resin obtained by the production method of the present invention has better water absorption ability than the conventional one, and has a large swelling gel strength and a small wet back amount, For example, it can be suitably used for a wide range of uses as a disposable diaper, sanitary napkin, soil water retention agent, cement admixture, and the like.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C08F 8/00 C08F 8/14

Claims (2)

(57) [Claims] 1. After the water content of a carboxyl group-containing polymer is adjusted to 10 to 30% by weight, a cross-linking reaction of the surface of the polymer impregnated with a cross-linking agent is started, and the amount of the remaining cross-linking agent is determined at the start of the reaction. 25 to 80% by weight of the compounding amount of the crosslinking agent in
A method for producing a super water-absorbent resin, characterized by further adding 5 to 20% by weight of water to the water-containing polymer at the start of the reaction at the time of the reaction to continue the crosslinking reaction. 2. The method according to claim 1, wherein the polymer is a crosslinked product of an acrylic acid-sodium acrylate copolymer.