JPH07258302A - Manufacture of water-absorptive resin - Google Patents

Abstract

PURPOSE:To provide a process for easily manufacturing a water-absorptive resin at a low cost which has a high water absorption capacity and a low water soluble content. CONSTITUTION:A water-soluble unsatd. monomer and a crosslinking agent in a water solution are polymerized under adiabatic conditions without applying any shearing force. Before the temp. in a polymerizer which has been resin by heat of polymerization reaches the max. temp., the resultant hydrous gel is taken out of the polymerizer, chopped, and dried to provide a water- absorptive resin. This process is advantageous in that in chopping the water-soluble hydrous gel polymer, hydrous gels do not stick to one another, obviating the need to use an additive such as a release agent.

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 water absorbent resin having a high water absorption capacity and a low water-soluble content.

[0002]

2. Description of the Related Art As water-absorbent resins, crosslinked polyacrylic acid salts, saponified acrylic acid ester-vinyl acetate copolymers, crosslinked polyvinyl alcohol modified products, crosslinked isobutylene-maleic anhydride copolymers, starch-acrylic acid graft polymers, etc. It is known to be used in a wide range of applications such as sanitary products, sanitary materials such as paper diapers, drip absorbents, soil water retention agents, and dew condensation prevention for building materials. As a method for producing these water-absorbent resins, reverse phase suspension polymerization can be carried out, for example, in JP-A-56-1.
The methods described in 61408, 57-158209, and 57-198714 are known, and as an aqueous solution polymerization method, for example, JP-A-2-
170808 and those described in JP-A-55-108407 are known. In addition, as a method of polymerizing the polymer gel while breaking and cooling by stirring in a double-arm kneader, those described in, for example, JP-A-57-34101 and JP-A-5-112654 are known. Further, a method of polymerizing a high-concentration aqueous solution on a belt and performing the polymerization and the drying at the same time is described in, for example, JP-A-58-71907.
And so on. However, the reversed-phase suspension polymerization method uses an organic solvent and thus has poor workability, and there is a risk of ignition and explosion.Measures must be taken for that purpose, and the cost of the organic solvent and its removal are high. Becomes Further, it is more costly to completely remove the organic solvent from the product. On the other hand, in the aqueous solution polymerization, there is no problem as described above, and since the aqueous solution polymerization is carried out under pressure in the method described in JP-A-2-170808, boiling of the aqueous solution is suppressed and polymerization at a high concentration is possible, and high water absorption. Although it has the advantage of being able to produce a water-absorbing resin with high productivity with high productivity, the temperature of the aqueous monomer solution in the polymerization vessel, that is, the aqueous solution during the reaction, or the aqueous gel produced will be extremely high. It has a problem that the amount of dissolved components increases. The method disclosed in JP-A-57-34101, JP-A-5-112654, etc. is a method of performing aqueous solution polymerization while subdividing the gel formed with the progress of polymerization by the shearing force of a stirring blade. Although it is a method capable of producing a subdivided hydrogel polymer having a cross-linked structure, it has a problem that it requires a special polymerization apparatus and that it is difficult to produce a water absorbent resin having a high water absorption capacity. is doing. The method described in JP-A-58-71907 is a method in which a high-concentration monomer aqueous solution is preliminarily heated, and a polymerization initiator is added to continuously polymerize on an endless bell without external heating. Since this method does not require a drying step because the water content is vaporized, and the method has good productivity, it has a drawback of having a large amount of water-soluble components.

The water-soluble component in the water-absorbent resin is leached from the water-absorbent resin when it contacts a liquid to be absorbed such as water, urine or body fluid to form a hydrogel structure, and is extracted by the liquid to be absorbed. The water-soluble components thus generated accelerate the deterioration of the water absorbent resin. In addition, it creates an unfavorable situation such as giving a discomfort due to the sliminess and contaminating the liquid to be absorbed. Therefore, a method for producing a water absorbent resin having a high water absorption capacity and a low water-soluble content is strongly desired.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a water-absorbent resin having a simple process, good productivity, high water absorption capacity and low water-soluble content.

[0005]

Means for Solving the Problems The inventors of the present invention have solved the above problems, and have earnestly studied about a method for producing a water absorbent resin having excellent productivity, workability, and excellent quality. The invention was completed. That is, the present invention is
The water-soluble unsaturated monomer and the cross-linking agent in the aqueous solution are polymerized under adiabatic conditions without applying shearing force, and the temperature rises due to the heat of polymerization. The present invention relates to a method for producing a water-absorbent resin, which is characterized in that it is further taken out and shredded and dried.

The present invention will be described in detail below. The method of the present invention may be carried out under normal pressure or under pressure, and the polymerization under pressure can be carried out according to the conditions disclosed in JP-A-2-170808. As the water-soluble unsaturated monomer used in the present invention, specifically,
(Meth) acrylic acid, (anhydrous) maleic acid, fumaric acid, crotonic acid, itaconic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acryloylpropanesulfonic acid, 2-
Anionic monomers such as (meth) acrylamide-2-methylpropanesulfonic acid and salts thereof; (meth) acrylamide,
N-substituted (meth) acrylate, 2-hydroxyethyl (meth)
Nonionic hydrophilic group-containing monomers such as acrylate, 2-hydroxypropyl (meth) acrylate, methoxy polyethylene glycol (meth) acrylate, polyethylene glycol (meth) acrylate; N, N-dimethylaminoethyl (meth) acrylate, N , N-dimethylaminopropyl
Specific examples thereof include amino group-containing unsaturated monomers such as (meth) acrylamide and quaternized products thereof. As the water-soluble unsaturated monomer component, one kind or two or more kinds can be selected and used from these, but considering the water absorption characteristics of the finally obtained water-absorbent resin (meth)
Acrylic acid (salt), 2- (meth) acryloylethanesulfonic acid (salt), 2- (meth) acrylamido-2-methylpropanesulfonic acid (salt), (meth) acrylamide, methoxypolyethylene glycol (meth) acrylate, N , N-dimethylaminoethyl (meth) acrylate or one or more selected from the group consisting of quaternary compounds thereof are preferable, and
Those containing (meth) acrylic acid (salt) as an essential component are preferable. In this case, it is most preferable that 20 to 100 mol% of (meth) acrylic acid is neutralized with a basic substance.
Any (meth) acrylic acid salt can be extremely easily prepared by partially neutralizing (meth) acrylic acid with an alkali metal salt and used in the present invention. The aqueous solution concentration of the water-soluble unsaturated monomer used for polymerization is not particularly limited, but is usually 20 to 50% by weight. In the present invention, starch, cellulose or the like can be added to the above-mentioned water-soluble unsaturated monomer for polymerization as in the conventional case. Further, in an amount that does not extremely hinder the hydrophilicity of the resulting hydrogel polymer, for example, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylates and vinyl acetate and vinyl acetate. Alternatively, a hydrophobic monomer such as vinyl propionate may be used in combination.

The cross-linking agent used in the present invention is a component which makes a polymer obtained from a water-soluble unsaturated monomer into a cross-linking structure, and has a polymerizable unsaturated group or a reactive functional group in the molecule or at the terminal of the molecule. I have. Usually, the amount of the cross-linking agent used is 0.0001 to 5% by weight, preferably 0.005 to 3% by weight, based on the monomer components. Examples of these cross-linking agents include, for example, N, N'-methylenebis (meth) acrylamide, (poly) ethylene glycol (meth) acrylate, glycerin tri (meth) acrylate, trimethylolpropane tri (meth) acrylate, triallylamine. , Triallyl cyanurate, glycidyl (meth) acrylate, (poly) ethylene glycol, diethylene glycol, (poly) glycerin, propylene glycol, diethanolamine, trimethylolpropane, pentaerythritol, (poly) ethylene glycol diglycidyl ether, (poly) glycerol Polyglycidyl ether,
Epichlorohydrin, ethylenediamine, polyethyleneimine, (poly) aluminum chloride, aluminum sulfate,
Specific examples thereof include calcium chloride and magnesium sulfate, and among these, one or two or more can be used in consideration of reactivity.

In the present invention, as the polymerization initiator for polymerizing the water-soluble unsaturated monomer and the crosslinking agent, those conventionally used can be applied as they are and added to the aqueous monomer solution. Examples of preferred polymerization initiators include ammonium persulfate, potassium persulfate, sodium persulfate, hydrogen peroxide, t-butyl hydroperoxide,
One or more peroxides such as succinic acid peroxide and t-butylperoxymaleic acid, or a redox initiator in which these peroxides are combined with a reducing agent such as sodium sulfite and ascorbic acid. And water-soluble radical initiators such as azo compounds such as 2,2′-azobis-amidinopropane dihydrochloride. The addition amount of the polymerization initiator is preferably 0.0001 to 5% by weight, more preferably 0.005 to 1% by weight, based on the weight of the monomer. 0.
If it is less than 0001% by weight, the polymerization may not proceed, and if it is more than 5% by weight, a resin having a desired water absorption capacity may not be obtained.

The polymerization initiation temperature is not particularly limited,
It may be set according to the catalyst system used, and there is no problem if it is set to a temperature at which the reaction rate does not significantly decrease. Usually 0-3
The polymerization may be initiated at a temperature of 0 ° C. The polymerization reaction container used in the present invention is not particularly limited, but a pressurizable container is preferable, and by using such a reaction container, the hydrogel polymer can be continuously and easily taken out. The process required to produce the water absorbent resin of the present invention is an adiabatic polymerization reaction. The adiabatic polymerization reaction referred to here is to use a polymerization reaction container having a capacity such that the polymerized hydrogel polymer is not extremely cooled. The production method of the present invention does not utilize shearing force, for example, a double-arm kneader, and the most general adiabatic polymerization reaction without stirring is applied. The capacity of the polymerization reaction container is not particularly limited, but a container having an inner diameter of 30 cm or more is preferable, and such a polymerization reaction container can be used with good productivity and adiabatic polymerization reaction.

Since the polymerization reaction of the water-soluble unsaturated monomer and the cross-linking agent is an exothermic reaction, the method of adiabatic polymerization in the polymerization reaction vessel is such that after the polymerization is started at a predetermined temperature,
Although it depends on the concentration of the aqueous solution of the water-soluble unsaturated monomer, the temperature inside the polymerization reaction vessel rises until it reaches a temperature of about 80 to 140 ° C. In the production of a general water-absorbent resin, in order to completely complete the polymerization reaction even after reaching the maximum temperature and reduce the unreacted monomers remaining in the produced hydrogel polymer, A means of leaving (aging) at temperature is adopted. However, the present inventors believe that the resulting hydrogel polymer is exposed to such a temperature to lower the molecular weight, and also the cross-linking point is cleaved to increase the water-soluble content. Was reached. That is, the present invention, in order to avoid such a phenomenon,
Before the temperature in the polymerization reaction vessel reached the maximum temperature, the water-soluble gelled polymer produced was taken out from the polymerization reaction vessel and shredded and dried to reduce the water-soluble content. The lower the temperature in the polymerization reaction vessel is, the better the removal of the produced gel is because the water-soluble content decreases, but it is economically problematic to remove it when the polymerization is not progressing so much. The degree of the polymerization reaction may be determined in consideration of the soluble content.

As a method of shredding the hydrogel polymer taken out from the polymerization reaction vessel, there is a method of using a device capable of cutting and extruding a rubber-like elastic body, for example, a cutter-type shredder and a chopper. This can be easily achieved by using a known device such as a die shredder or a kneader type shredder. As a method for drying the shredded hydrogel polymer, an ordinary dryer or a heating furnace can be used, for example, a hot air dryer, a fluidized bed dryer, a gas stream dryer, an infrared dryer, a dielectric heating dryer. Machines and the like. A conventionally known pulverization method can be adopted for pulverizing the dried gel. For example, a high speed rotary crusher, a screw mill, a roll mill and the like can be mentioned.

[0012]

According to the present invention, that is, in the adiabatic polymerization, the gel produced is taken out before the temperature in the polymerization reaction vessel reaches the maximum temperature. The action of reducing the water-soluble content of the generated water-absorbent resin is exhibited.
In addition, due to the small amount of water-soluble components, when the water-absorbing hydrous gel-like polymer is shredded, the hydrous gels do not stick to each other and it is not necessary to add additives such as a release agent. The effect is also produced.

[0013]

The present invention will be described in detail below with reference to examples, but the scope of the present invention is not limited to these examples. Further, the water absorption capacity as a water-absorbent resin of a dry pulverized product of the particulate hydrogel polymer described in these examples,
The water-soluble content and residual monomer are the values measured by the following test methods. A. Water absorption capacity: 0.5 g of water-absorbent resin powder was precisely weighed in a beaker of 300 ml, and a 0.9% sodium chloride aqueous solution 20
After adding 0 ml and stirring for 60 minutes, the content is 120 mmHg
The mixture was transferred onto a glass filter connected to a suction filtration device whose pressure was reduced in step 1, sucked for 10 minutes, then weighed, and calculated by the following formula 1.

[0014]

[Formula 1] Water absorption capacity (g / g) = hydrogel weight after suction (g) /0.5 (g) B.I. Water-soluble content: 0.5 g of water-absorbent resin powder was precisely weighed and 1
The mixture was dispersed in 000 ml of deionized water, stirred for 12 hours, filtered with filter paper, and the filtrate was concentrated and dried at 130 ° C.
The water-soluble content was determined according to the following mathematical formula 2.

[0015]

[Formula 2] Water-soluble content (% by weight) = (dry solid (g) x 1000
/0.5 x filtrate (g)) x 100

C. Residual monomer: 0.4 g of water-absorbent resin powder was precisely weighed in a 300 ml beaker, 200 ml of 0.9% sodium chloride aqueous solution was added, stirred for 3 hours, filtered through a membrane filter, and the filtrate was subjected to high performance liquid chromatography. Was analyzed. On the other hand, a calibration curve obtained by similarly analyzing a monomer standard solution having a known concentration was used as an external standard, and the amount of residual monomer in the water absorbent resin was determined in consideration of the dilution ratio of the filtrate.

Example 1 Sodium acrylate 75 was placed in a suitable heat-resistant pressure-resistant reaction vessel.
10 kg aqueous solution of monomer component consisting of mol% and acrylic acid 25 mol% (40 wt% of monomer component) and N as a cross-linking agent,
N'-methylenebisacrylamide 2.0 g (0.05% by weight)
(To the monomer component), nitrogen gas was blown in to expel dissolved oxygen, and the temperature of the aqueous solution was adjusted to 10 ° C. 2.0 g (0.0) of ammonium persulfate as a polymerization initiator was added to this aqueous solution.
5% by weight of monomer component), 0.2 g of L-ascorbic acid (0.1%)
(005 wt% relative to the monomer component) was added, and at the same time, the pressure in the container was increased to 5 kg / cm ² . Polymerization with heat generation started 3 minutes after the addition of the polymerization initiator, and about 20 minutes after the internal temperature reached 90 ° C (the maximum temperature expected to reach 125 ° C), the hydrogel polymer was taken out from the reaction vessel, and then chopper type. It was cut into about 2 mm by a shredder. The shredded hydrogel polymer was cooled to about 70 ° C. The shredded hydrogel polymer was dried with a hot air dryer at 130 ° C. for 60 minutes, and then pulverized with a roll mill pulverizer to select a water-absorbent resin powder having a particle size in the range of 300 μm to 500 μm. did. The water absorption capacity, water-soluble content and residual monomer of the obtained water-absorbent resin were measured by the above test method, and the results are shown in Table 1.

Example 2 A water-absorbent resin powder was obtained in the same manner as in Example 1 except that the hydrogel polymer was taken out of the reaction vessel when the internal temperature reached 70 ° C. It was Evaluation was performed by the same test method as in Example 1, and the results are shown in Table 1.

Comparative Example 1 The same procedure as in Example 1 was carried out except that the hydrous gel polymer was taken out of the reactor at the time when the internal temperature reached the maximum temperature (125 ° C.). Gelled polymer particles were obtained. The obtained hydrogel polymer particles were obtained in Example 1
In the same manner as in (1), hot air drying, pulverization, and evaluation by the same test method, the results are shown in Table 1.

Example 3 In Example 1, the monomer component was 30% by weight, and trimethylolpropane triacrylate as a cross-linking agent was 1.
5 g (0.05 wt% vs. monomer component), ammonium persulfate as a polymerization initiator 1.5 g (0.05 wt% vs. monomer component), L-ascorbic acid 0.15 g (0.005 wt% vs. (Monomer component), polymerization is carried out without pressurization, and the internal temperature is 80 ° C.
A water-absorbent resin powder was obtained in the same manner as in Example 1 except that the hydrous gel polymer was taken out of the reactor when the temperature reached (the expected maximum temperature of 90 ° C.). Evaluation was performed by the same test method as in Example 1, and the results are shown in Table 1.

Example 4 The procedure of Example 3 was repeated, except that the hydrogel polymer was taken out of the reactor when the internal temperature reached 65 ° C. Obtained. Evaluation was carried out by the same test method as in Example 1, and the results are shown in Table 1.

Comparative Example 2 The same operation as in Example 3 was carried out except that the hydrogel polymer was taken out of the reactor at the time when the internal temperature reached the maximum reached temperature (about 90 ° C.). A water absorbent resin powder was obtained. Evaluation was carried out in the same manner as in Example 1, and the results are shown in Table 1.

Example 5 In Example 1, the monomer component was 25% by weight, the starting temperature was 20 ° C., and 1.25 g of N, N′-methylenebisacrylamide (0.5% by weight per unit amount) was used as a crosslinking agent. 1.25 g (0.05% ammonium persulfate) as a polymerization initiator
% By weight of monomer component), L-ascorbic acid 0.125 g
(0.005% by weight relative to the monomer component), polymerization was carried out without pressurization, and when the internal temperature reached 70 ° C (expected maximum temperature reached 80 ° C), the hydrogel polymer was removed from the reaction vessel. The same operation as in Example 1 was carried out except that the water-absorbent resin powder was obtained. Evaluation was carried out by the same test method as in Example 1, and the results are shown in Table 1.

Comparative Example 3 In Example 5, the same operation as in Example 5 was performed except that the hydrogel polymer was taken out from the reactor when the internal temperature reached the maximum reached temperature (about 80 ° C.). . The water-absorbent resin powder obtained was evaluated by the same test method as in Example 1, and the results are shown in Table 1.

Example 6 In Example 1, 75 mol% sodium acrylate,
A monomer component consisting of 20 mol% of acrylic acid and 5 mol% of sodium 2-acrylamido-2-methylpropanesulfonate was 30% by weight, and 1.5 g of trimethylolpropane triacrylate (0.05% by weight per unit amount) was used as a crosslinking agent. Monomer component), ammonium persulfate 1.5 as a polymerization initiator
g (0.05% by weight relative to monomer component), L-ascorbic acid
15g (0.005% by weight of monomer component), polymerization was carried out without pressurization, and the internal temperature was 80 ° C (expected maximum temperature 90 ° C)
At this point, the same procedure as in Example 1 was carried out except that the hydrogel polymer was taken out of the reactor to obtain a water absorbent resin powder. Evaluation was performed by the same test method as in Example 1, and the results are shown in Table 1.

Comparative Example 4 The same operation as in Example 3 was carried out except that the hydrous gel polymer was taken out of the reactor at the time when the internal temperature reached the maximum reached temperature (about 90 ° C.). A water absorbent resin powder was obtained. Evaluation was performed by the same test method as in Example 1, and the results are shown in Table 1.

[0027]

[Table 1]

In the results of Examples 1 to 6 and Comparative Examples 1 to 4, the concentration of the residual monomer remaining in the water absorbent resin was in the range of 200 to 300 ppm in all cases, which was almost unchanged. As can be seen from Table 1 above, the water-absorbent resin obtained by the method of the present invention absorbs water as compared with the case where the polymerization is carried out until the temperature in the polymerization reaction vessel reaches the maximum temperature (Comparative Examples 1 to 4). Although the magnification is almost the same, the water-soluble content is extremely small.

[0029]

EFFECTS OF THE INVENTION The present invention is based on the fact that the water-soluble hydrogel polymer does not stick to each other when the water-absorbent hydrogel polymer is shredded due to its low water-soluble content, and thus an additive such as a release agent is added. It is possible to provide an excellent water-absorbent resin manufacturing method that does not require
Further, it has an excellent effect that a water absorbent resin having a low water-soluble content can be provided.

Claims (1)

[Claims] 1. A water-soluble unsaturated monomer and a cross-linking agent in an aqueous solution are polymerized under adiabatic conditions without applying a shearing force, and are produced by the heat of polymerization before the temperature in the polymerization vessel reaches the maximum temperature. A method for producing a water-absorbent resin, which comprises removing a hydrogel from a polymerization container and chopping and drying.