JPH09241391A - Production of highly water-absorbent polymer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a highly water-absorbent polymer safely at a high productivity without using an org. solvent by adding a specific cross-linker to an aq. soln. of a specific water-sol. polymer to cause cross-linking and to simultaneously carry out water removal and granulation. SOLUTION: This polymer is produced by kneading 0.005-5wt.% cross-linker having at least two carboxyl-reactive groups with a 30-95wt.% aq. soln. of a carboxylated polymer having a wt. average mol-wt. of 20,000-3.000,000 on a kneader at 50-200 deg.C under a pressure of 100-500Torr for 5min to 5hr to cause cross-linking and to simultaneously carry out water removal and granulation. Finer granules can be obtd. by kneading in the presence of a surfactant or an inert inorg. powder. If necessary the cross-linking is caused in the presence of a polymerizable monomer and a polymn. initiator to simultaneously cause polymn., thus giving a polymer with a high gel strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a super absorbent polymer which constitutes an absorbent body of absorbent articles such as disposable diapers, sanitary napkins, adult sheets, tampons and sanitary cotton.

[0002]

2. Description of the Related Art A method for producing a superabsorbent polymer is a method of adding a cross-linking agent or a polymerization initiator accompanied by self-crosslinking during polymerization to a polymerizable monomer having a carboxyl group. , A method of producing a highly water-absorbing polymer by simultaneously advancing polymerization and crosslinking by heating, and synthesizing a linear polymer by polymerizing a polymerizable monomer, and then adding a suitable crosslinking agent, etc. It is roughly divided into two methods, that is, a method of producing a superabsorbent polymer by using it and crosslinking.

In the above method, since the crosslinking progresses as the polymerization progresses, the gelation of the polymer rapidly progresses and the heat generated by the polymerization (polymerization heat) is large. For this reason, the reaction liquid is in a bumping state during the polymerization and crosslinking reactions, which is not only difficult to control the reaction but also dangerous. On the other hand, in the above method, the heat of polymerization in the polymerization is as large as in the above method, but since the viscosity of the reaction solution is relatively low, it will not be in the bumping state, and the polymer during crosslinking after the polymerization is When it turns into a gel, there is almost no heat generated by the reaction. Therefore, although the superabsorbent polymer can be safely produced, there are still the following problems in terms of production.

JP-A-58-79006 and JP-B-59-45695 disclose a method for producing a superabsorbent polymer by crosslinking a water-soluble polymer. However, the methods disclosed in these publications are methods in which a cross-linking agent is added and mixed in an aqueous polymer solution, and then the mixture is heated in a sheet form or a block form to perform cross-linking. Generally, a super absorbent polymer used as a hygiene material is usually used in a powder form in order to improve the water absorption rate,
According to this method, a subsequent step of removing water from the sheet-like or block-like polymer and crushing is required. Moreover, a great amount of energy is required to mechanically grind the dried polymer by removing water.

Further, in JP-A-62-230813, a water-soluble or water-swellable polymer particle formed from a first monomer in a non-aqueous solvent is added with a crosslinking agent and a water-soluble ethylenically unsaturated second monomer. In addition, a method for producing water-soluble or water-swellable polymer particles having a controlled particle size by performing crosslinking and polymerization is disclosed. According to this production method, a particulate superabsorbent polymer can be obtained without crushing, but since an organic solvent is used, there are still problems in danger and working environment. Further, since the above-mentioned organic solvent is used in a large amount, the cost is increased and a facility for collecting and reusing the organic solvent is required.

Therefore, an object of the present invention is to provide a method for producing a highly water-absorbing polymer safely and with good productivity without using an organic solvent.

[0007]

Means for Solving the Problems As a result of intensive studies, the present inventors have added a specific cross-linking agent to an aqueous solution of a specific water-soluble polymer to cause a cross-linking reaction, and at the same time remove water (dry) and produce it. It was found that the above-mentioned object can be achieved by carrying out graining.

The present invention has been made on the basis of the above findings, and a crosslinking agent having two or more functional groups capable of reacting with a carboxyl group is added to an aqueous solution of a polymer having a carboxyl group to carry out a crosslinking reaction to enhance the reaction. It is intended to provide a method for producing a superabsorbent polymer, characterized in that, when producing the superabsorbent polymer, the above crosslinking reaction is carried out, and at the same time, moisture removal and granulation are performed.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing the superabsorbent polymer of the present invention will be described in detail below. As an aqueous solution of a polymer having a carboxyl group used in the present invention,
An unsaturated carboxylic acid such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, and a monomer composition containing at least one member selected from the group consisting of neutralized products thereof is used. An aqueous solution of a polymer obtained by polymerizing a monomer can be used. Further, as the aqueous solution of the above-mentioned polymer having a carboxyl group, an aqueous solution of a copolymer of the above monomer and another monomer copolymerizable with the above monomer may be used. In this case, examples of the other monomer include vinyl sulfonic acid, 2-hydroxyethyl acrylate, dimethylaminoethyl acrylate, etc., and the content of the other monomer in the copolymer is 25. The other monomers can be used up to a weight percentage.

When obtaining the above-mentioned polymer having a carboxyl group, a polymerization initiator can be used. As the polymerization initiator, a known water-soluble radical initiator can be used, and examples thereof include persulfates such as potassium persulfate, sodium persulfate, ammonium persulfate, hydrogen peroxide, and t.
Examples thereof include hydroperoxides such as -butyl hydroperoxide and cumene hydroperoxide, and azo compounds such as 2,2'-azobis-2'-amidinopropane dihydrochloride. These polymerization initiators may be used alone or in combination of two or more. Furthermore,
A redox initiator obtained by combining the above-mentioned polymerization initiator with a reducing agent such as a sulfite salt such as sodium sulfite, sodium bisulfite or potassium sulfite, a ferric salt or L-ascorbic acid can also be used. Among these, it is preferable to use a water-soluble azo compound that is unlikely to cause self-crosslinking.

The molecular weight (weight average molecular weight) of the above-mentioned polymer having a carboxyl group is preferably from 20,000 to.
3,000,000, more preferably 50,000-
1,000,000. If the molecular weight is less than 20,000, it may not be insolubilized in water, or even if it is insolubilized, it has a high water-soluble content and may have a sticky gel upon water absorption. Further, the molecular weight is 3,000
If it exceeds 50,000, the viscosity becomes high, and the viscosity becomes too high when the crosslinking reaction is carried out, and the crosslinking tends to become nonuniform. In this case, the viscosity can be lowered by diluting to reduce the concentration of the polymer, but the amount of water to be evaporated per unit polymer is increased and the efficiency of the apparatus is lowered.

The concentration of the aqueous solution of the above-mentioned polymer having a carboxyl group is preferably 30 to 95% by weight,
More preferably, it is 40 to 70% by weight. The concentration is 30
If it is less than wt%, the actual throughput will be small,
It may be economically disadvantageous, and if it exceeds 95% by weight, the cross-linking agent may not easily penetrate into the interior, and the cross-linking may be non-uniform.

As the cross-linking agent having two or more functional groups capable of reacting with the carboxyl group used in the present invention, polyhydric glycidyl ether, polyhydric alcohol, polyhydric isocyanate, polyhydric aziridine, haloepoxy compound, polyhydric aldehyde, polyhydric aldehyde, Examples include divalent amines and polyvalent metal salts.

Examples of the polyvalent glycidyl ethers include ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, sorbitol polyglycidyl ether, Examples thereof include pentaerythritol polyglycidyl ether, propylene glycol glycidyl ether, polypropylene glycol diglycidyl ether and the like.

Examples of the polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, glycerin, polyglycerin, propylene glycol, diethanolamine, triethanolamine, polyoxypropyl, oxye. Examples thereof include thienoxypropylene block copolymer, pentaerythritol and sobitol.

Further, as the above-mentioned polyisocyanate,
Examples thereof include 2,4-toluylene diisocyanate and hexamethylene diisocyanate. Examples of the polyhydric aziridine include 2,2-bishydroxymethylbutanol-tris [3- (1-acylidinyl) propionate], 1,6-hexamethylenediethyleneurea, diphenylmethane-bis-4,4'-. N,
N'-diethylene urea etc. are mentioned. Examples of the haloepoxy compound include epichlorohydrin and α-methylchlorohydrin.

Examples of the polyhydric aldehyde include glutaraldehyde and glyoxal. Examples of the polyvalent amine include ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, polyethyleneimine and the like. The polyvalent metal salts include, for example, aluminum chloride,
Examples thereof include magnesium chloride, carboxyl chloride, aluminum sulfate, magnesium sulfate and carboxyl sulfate.

The amount of the cross-linking agent used is preferably 0.005 to the polymer having a carboxyl group.
It is 5% by weight, and more preferably 0.01 to 0.5% by weight. If the amount of the crosslinking agent used is less than 0.005 parts by weight, the polymer may not be insolubilized in water, and if it exceeds 5% by weight, the water absorption capacity may decrease.

The cross-linking agent is added after the polymerization for obtaining the polymer having a carboxyl group is completed. If the cross-linking agent is added simultaneously with the polymerization, cross-linking will be formed during the polymerization.

When the above crosslinking reaction is carried out, a polymerizable monomer and a polymerization initiator are added, and at the same time, the polymerizable monomer is polymerized, so that the saturated water absorption is equivalent and the stickiness at the time of swelling is increased. It is possible to obtain a polymer having less gel strength, that is, higher gel strength. The reason for this is that the crosslinking reaction and the polymerization of the polymerizable monomer occur at the same time, and while the polymerizable monomer is polymerized, the polymer having the carboxyl group formed by the crosslinking reaction (linear polymer). It is thought that this is due to the formation of a molecular interpenetrating network structure by intertwining with the network of.

The above-mentioned polymerizable monomer used here is the same as that used in obtaining an aqueous solution of the above-mentioned polymer having a carboxyl group, that is, an unsaturated carboxylic acid such as acrylic acid, Methacrylic acid,
A monomer containing at least one selected from the group consisting of crotonic acid, itaconic acid, maleic acid, and neutralized products thereof can be used, and the monomer and the monomer can be used together. It may be a mixture with another monomer such as a polymerizable vinyl sulfonic acid, 2-hydroxyethyl acrylate, dimethylaminoethyl acrylate, and the like.
It may be a mixture with a monomer containing two or more polymerizable unsaturated double bonds such as N'-methylenebis (meth) acrylamide. The amount of the polymerizable monomer used is less than 50% by weight with respect to the polymer having a carboxyl group. When the amount of the polymerizable monomer used exceeds 50% by weight, the heat generated by the polymerization reaction that occurs at the same time as the cross-linking reaction becomes large, and bumping occurs or temperature control becomes difficult.

As the above-mentioned polymerization initiator, known ones such as radical polymerization initiator can be used.

In the present invention, when a superabsorbent polymer is produced by adding the above-mentioned crosslinking agent to an aqueous solution of the above-mentioned polymer having a carboxyl group and carrying out a crosslinking reaction, the above-mentioned crosslinking reaction is carried out and water is removed at the same time. By performing (drying) and granulating, a powdery superabsorbent polymer can be obtained more efficiently than in the conventional method. The reason for this is that the cross-linking of the polymer progresses halfway, and the fragmentation is performed in a relatively low-strength state. This is because it becomes possible to make a powder form with less energy as compared with the case of pulverizing from.

The apparatus for carrying out the above-mentioned crosslinking reaction is not particularly limited as long as it has high power, and for example, Chemical Engineering Handbook, pages 917 to 919 (Maruzen Co., Ltd. 1992).
The kneading machine described in April 15, 2015) can be preferably used. Among these kneaders, a double-arm kneader, a self-cleaning kneader, and a continuous muller are more preferable because they have good granulation properties.

As the above-mentioned kneader, a container having a stirring blade and having a single or plural stirring shafts can be used. The shape of the stirring blade is sigma type, S
Type, Banbury type, etc. can be used.

The heating temperature for carrying out the above crosslinking reaction is
Preferably 50 to 200 ° C, more preferably 70 to 15
0 ° C. When the heating temperature is lower than 50 ° C, not only the crosslinking rate is slow but also it takes a long time to dry the water, and when the heating temperature is higher than 200 ° C, the crosslinking is dried before the crosslinking proceeds sufficiently and a sufficient crosslinking reaction occurs. The effect of does not tend to be obtained.

The time for carrying out the crosslinking reaction is preferably 5 minutes to 5 hours, more preferably 20 minutes to
2 hours.

Moisture removal, which is carried out at the same time as the above-mentioned crosslinking reaction, is carried out at the above-mentioned heating temperature and for a suitable pressure and operating method. Here, the pressure may be normal pressure or reduced pressure, but is preferably 100 to 500 torr.
The operation method may be a batch method or a continuous method.

The granulation carried out at the same time as the above crosslinking reaction is carried out by anionic surfactants such as polyoxyethylene alkyl ether sulfates, cationic surfactants such as quaternary alkyl ammonium salts, and sorbitan fatty acid esters. It is carried out by adding a surface active agent such as a nonionic surface active agent, etc., and an inorganic powder inert to the reaction such as a silicon dioxide powder, an aluminum oxide powder, a titanium oxide powder. By the above-mentioned granulation, reaggregation of the polymer can be reduced and a superabsorbent polymer having a small particle size can be obtained.

According to the manufacturing method of the present invention, the average particle size is 1
It is possible to easily obtain a super absorbent polymer having a particle size of 00 to 400 μm and a maximum particle size of 800 μm or less.

The super absorbent polymer obtained by the production method of the present invention is used as an absorbent polymer constituting an absorbent body in absorbent articles such as paper diapers, sanitary napkins, adult sheets, tampons and sanitary cotton. , And other uses similar to conventional absorption polymers.

[0032]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Example 1 102 parts by weight of an 80% by weight aqueous solution of acrylic acid was neutralized with 108.8 parts by weight of a 30% by weight aqueous sodium hydroxide solution, and then 3% by weight of 2.
10 parts by weight of a 2'-azobis- (2-amidinopropane) dihydrochloride aqueous solution was added, and the mixture was stirred so as to be uniform.
Then, the temperature was raised to 75 ° C. with stirring under a nitrogen stream to polymerize, and a linear polyacrylic acid partially neutralized product was obtained after 2 hours. The concentration of the partially neutralized polyacrylic acid in the aqueous solution was 45% by weight. The weight average molecular weight of this polymer was 650,000 (measured by GPC method, the same applies hereinafter). This polymer was manufactured by Irie Seisakusho 1L Bench Kneader PNV-
After transferring to 1H, 2 parts by weight of 30% by weight ethylene glycol diglycidyl ether aqueous solution was added thereto, and the mixture was stirred uniformly for 5 minutes. And dehydration.
As a result, a powdery superabsorbent polymer was obtained after 1 hour. The average particle size of the obtained super absorbent polymer is 280 μm.
The saturated water absorption was 56.3 g / g, and the gel strength when swollen was 38.1 g / cm ² .

Here, the saturated water absorption amount was obtained by dispersing about 1 g of the polymer in a large excess of physiological saline, allowing it to swell sufficiently, and then filtering with a wire mesh of 80 mesh to obtain the weight (W) of the obtained swollen polymer. Measure this value first (before swelling)
It was expressed as a value obtained by dividing the weight of the polymer (W ₀ ) and subtracting 1 from the value. That is, saturated water absorption (g / g) = W / W
_{It was set to 0} -1. The gel strength when swollen is 2
g to absorb 60 g of physiological saline to make a swollen gel,
A neocard meter (manufactured by Iio Electric Co., Ltd.) was used to measure the elastic force until the gel broke, and the value was taken as the gel strength (g / cm ² ) when swollen.

Example 2 A 3% by weight aqueous solution of 2,2'-azobis- (2-amidinopropane) dihydrochloride was added to 4.5%.
Example 1 except that the aqueous solution of potassium persulfate was changed to wt%
The same operation as above was performed to obtain a super absorbent polymer. In this case, the weight average molecular weight of the polymer before crosslinking (partially neutralized polyacrylic acid) was 470,000. The average particle size of the crosslinked polymer (obtained superabsorbent polymer) is 24.
It was 0 μm, the saturated water absorption was 58.3 g / g, and the gel strength upon swelling was 36.2 g / cm ² .

Example 3 102 parts by weight of an 80% by weight aqueous solution of acrylic acid was neutralized with 108.8 parts by weight of a 30% by weight aqueous solution of sodium hydroxide, and then 3% by weight of 2,
10 parts by weight of a 2'-azobis- (2-amidinopropane) dihydrochloride aqueous solution was added, and the mixture was stirred so as to be uniform.
Then, the temperature was raised to 75 ° C. with stirring under a nitrogen stream to polymerize, and a linear polyacrylic acid partially neutralized product was obtained after 2 hours. The weight average molecular weight of this polymer was 650,000. 3 parts by weight of an aqueous solution of ethylene glycol diglycidyl ether of 30% by weight, 20.4 parts by weight of an aqueous solution of acrylic acid of 80% by weight and 30% by weight of this aqueous polymer solution were added.
21.8% by weight of aqueous sodium hydroxide solution was added to neutralize the acrylic acid. Then, 2.0 parts by weight of ammonium persulfate was added. This was transferred to 1L Bench Kneader PNV-1H manufactured by Irie Seisakusho, and after uniformly stirring for 5 minutes,
The jacket is heated to 120 ° C and the inside of the tank is set to 200 torr.
The pressure was reduced to 1, and crosslinking, dehydration and polymerization were performed. as a result,
After 1 hour, a powdery super absorbent polymer was obtained. The superabsorbent polymer thus obtained had an average particle size of 370 μm, a saturated water absorption of 55.8 g / g, and a gel strength of 4 when swollen.
It was 3.8 g / cm ² . The saturated water absorption amount and the gel strength at the time of swelling are values obtained by measurement in the same manner as in Example 1.

Example 4 A highly water-absorbent polymer was obtained in the same manner as in Example 3 except that triethylenetetramine was used instead of ethylene glycol diglycidyl ether. In this case, the weight average molecular weight of the polymer before crosslinking (partially neutralized polyacrylic acid) was 650,000. Also,
The average particle size of the polymer after cross-linking (obtained super absorbent polymer) was 380 μm, and the saturated water absorption was 52.9 g / g.
And the gel strength when swollen was 46.3 g / cm ² .

[0038]

According to the method for producing a superabsorbent polymer of the present invention, the superabsorbent polymer can be produced safely and with good productivity without using an organic solvent.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Shida 1334 Minato Minato, Wakayama, Wakayama Prefecture Kao Research Institute

Claims (3)

[Claims] 1. A method of producing a superabsorbent polymer by adding a crosslinking agent having two or more functional groups capable of reacting with a carboxyl group to an aqueous solution of a polymer having a carboxyl group to produce a superabsorbent polymer. A method for producing a superabsorbent polymer, characterized in that water removal and granulation are carried out at the same time as the above step. 2. The method for producing a superabsorbent polymer according to claim 1, wherein the cross-linking reaction, the removal of water and the granulation are performed using a kneader. 3. The high water absorption according to claim 1, wherein a polymerizable monomer and a polymerization initiator are added at the time of carrying out the crosslinking reaction, and the polymerizable monomer is simultaneously polymerized. Of producing water-soluble polymer.