JPH0776244B2 - Method for producing water-absorbent resin having low water-soluble content - Google Patents

Description

TECHNICAL FIELD The present invention relates to a novel method for producing a water absorbent resin having a low water-soluble content.

(Prior art) Water-absorbent resins are used as sanitary items, diapers, disposable sanitary items, and other agricultural and horticultural items such as water retention agents, as well as coagulating sludge, preventing dew condensation on building materials, dehydrating oils, etc. It is also used for.

Examples of this type of water absorbent resin include crosslinked carboxymethyl cellulose, crosslinked polyoxyethylene, hydrolyzate of starch-acrylonitrile graft copolymer, starch- (meth) acrylic acid graft copolymer, and (meth) acrylic acid salt. Polymer crosslinked products, (meth) acrylate-based copolymer crosslinked products, etc. are known. Of these, carboxymethylcellulose crosslinked products and polyoxyethylene crosslinked products have not yet been obtained that have satisfactory water absorption and water retention capabilities.

Thus, (meth) acrylate polymer cross-linked product, (meth)
The crosslinked acrylate copolymer can satisfy the water absorption capacity, water retention capacity and quality stability.

Conventionally, an aqueous solution polymerization method, a reverse phase emulsion polymerization method, a reverse phase suspension polymerization method, and the like are known as methods for producing a crosslinked (meth) acrylate polymer. Among these methods, particularly in the case of aqueous solution polymerization, conventionally, the obtained water-absorbent resin contains a considerable amount of a water-soluble resin which is a water-soluble component, and therefore, when used in the above-mentioned hygiene article. Had a disadvantage in that it was associated with discomfort such as stickiness during use due to the water-soluble content.
As a method of reducing the water-soluble content in the water-absorbent resin, there is also a method of increasing the amount of the crosslinkable monomer used during the polymerization, but in such a method the water-absorbing ability of the water-absorbent resin obtained is remarkably high. Tends to decline.

(Problems to be Solved by the Invention) Based on the above circumstances, the water-soluble component in the (meth) acrylate polymer crosslinked product does not deteriorate the various performances as the original water-absorbing resin. The development of a water-absorbent resin production method capable of reducing water consumption has been earnestly desired in the industry.

Here, the object of the present invention is to obtain a water-absorbent resin having excellent properties such as water-absorbing ability of a crosslinked product of a (meth) acrylate-based polymer and having a very small water-soluble content of the resin by an aqueous solution polymerization method. It is to provide the technology to produce.

(Means for Solving Problems) In order to achieve the above object, the present inventor considers the above circumstances.
In particular, the inventors have earnestly studied various polymerization conditions in aqueous solution polymerization of a (meth) acrylate polymer cross-linked product. As a result, it was surprisingly found that this object can be achieved by allowing the water-absorbent resin powder to be present during the polymerization. The present invention has been completed based on such findings.

That is, the present invention provides a method for producing a water-absorbent resin by aqueous solution polymerization of an aqueous monomer solution containing an alkali metal acrylate and / or an alkali metal methacrylate as a main monomer. The present invention relates to a method for producing a water-absorbent resin, which is carried out in the presence of a water-absorbent resin powder of a crosslinked polymer having an alkali metal salt of an acid and / or an alkali metal salt of methacrylic acid as a main monomer.

In the present invention, the main monomer unit constituting the objective water-absorbent resin is an alkali metal (meth) acrylic acid salt. Here, the alkali metal salt means a sodium salt or a potassium salt. These alkali metal salts are obtained by neutralizing (meth) acrylic acid with sodium hydroxide, potassium hydroxide, etc. These are not necessarily completely neutralized products, but partially neutralized products However, the neutralization degree is usually in the range of 50 to 100%, more preferably 6
It is preferable to set it in the range of 0 to 85%. If it is less than 50%, the water absorption capacity of the resulting water absorbent resin tends to decrease, which is not preferable.

In the present invention, the main monomer is as described above (meta)
Although it is an alkali metal acrylate, if necessary, a divinyl compound can be used in combination as a crosslinkable compound. Preferred divinyl compounds include, for example, divinylbenzene, N,
Examples thereof include N'-methylenebisacrylamide, N, N'-methylenebismethacrylamide, polyethylene glycol diacrylate, polypropylene glycol diacrylate, etc. These may be used alone or in combination. These divinyl compounds may be used alone or in combination.

The amount of the divinyl compound used is appropriately determined in consideration of the water absorption capacity of the resulting water absorbent resin, the gel strength, the stability of the gel strength over time, etc.
It is about 0% by weight or less, preferably 1.0% by weight or less. By the way, when adopting the conventional aqueous solution polymerization method, it is necessary to use at least about 0.001% by weight of the crosslinkable monomer, and at such an amount, water-soluble resin obtained is water-soluble. It contains a considerable amount of minutes. On the other hand, the water absorbent resin obtained by the method of the present invention does not need to contain the crosslinkable monomer as an essential component, and the amount of the crosslinkable monomer used is about 0.001% by weight or less. Even if it exists, there is an advantage that it is possible to obtain a water absorbent resin having a very small amount of water-soluble components. If it exceeds 5.0% by weight, the water absorption capacity tends to decrease, which is not preferable.

Furthermore, in addition to the (meth) acrylic acid alkali metal salt which is the essential monomer, other monomers can be used in combination, if necessary. The other monomer is preferably an acrylic or methacrylic monomer, and specific examples thereof include acrylamide, acrylamido-2-methylpropanesulfonate, lower acrylic acid ester, and lower methacrylic acid ester. The amount of the monomer used is preferably about 20% by weight or less based on all the monomers in consideration of the water absorption capacity, water retention capacity, gel strength and the like of the water absorbent resin to be obtained. Further, a surfactant can be used if necessary. Specific examples thereof include polyoxyethylene alkyl ether, polyoxyethylene alkylphenyl ether, polyoxyethylene sorbitan fatty acid ester, higher alcohol sulfuric acid ester salt, alkylbenzene sulfonic acid salt, and polyoxyethylene sulfuric acid ester salt. The amount used is 0.01-5 with respect to all monomers.
It is set to about% by weight.

The polymerization initiator used in the present invention is not particularly limited, and various known ones such as ammonium persulfate, persulfate such as potassium persulfate, redox system of the persulfate and sulfite, various water-soluble azo compounds, etc. Can be used. Specific examples of the water-soluble azo compound include azobisisobutyronitrile, 4-t-butylazo-4-cyanovaleric acid, 4,4′-azobis (4-cyanovaleric acid) and 2,2′-azobis ( 2-amidinopropane) dihydrochloride, 2,2'-azobis (N, N'-dimethyleneisobutylamidine) dihydrochloride, 2,2'-azobis (N, N'-dimethyleneisobutylamidine) and the like. be able to.

In the present invention, the initiation of the aqueous solution polymerization may be performed by heating or UV irradiation. In the case of ultraviolet polymerization, water-soluble azo compounds are preferable among the above-mentioned initiators, and among them, 2,2'-azobis (2-amidinopropane) dihydrochloride and 2,2'-azobis (N, N'-dimethylene) are preferred. Isobutylamidine) dihydrochloride, 2,2'-azobis (N, N '
-Dimethylene isobutylamidine) and the like are preferable. In this case, diacetyl, benzoin, benzyl, anthraquinone as a photopolymerization initiator generally used for photopolymerization,
Acetophenone, diphenyl disulfide, benzophenone and various derivatives thereof are not suitable. This is because when such an initiator is used, the polymerization is not completed and a considerable amount of unreacted monomer remains, so that the water-absorbent resin obtained has a sticky feeling during water absorption.

In the method of the present invention, it is an essential requirement that a water-absorbent resin powder of a cross-linked polymer having an alkali metal acrylate and / or an alkali metal methacrylate as a main monomer be present at the time of polymerization. For the first time, the above-mentioned object of the present invention can be achieved. The method for allowing the water-absorbent resin powder to exist is not particularly limited, and it is usually sufficient to uniformly disperse the resin powder in the reaction system to some extent.

The water-absorbent resin powder used at the time of polymerization is not particularly limited, and various known powders can be arbitrarily used, but from the viewpoint of the effect of reducing the water-soluble content in the water-absorbent resin obtained in general, (meth) acrylic An acid polymer crosslinked product is more preferable.
As the (meth) acrylic acid-based polymer crosslinked product, polyacrylic acid salt crosslinked product, polymethacrylic acid salt crosslinked product, polyacrylic acid-methacrylic acid salt crosslinked product, hydrolyzate of starch-acrylonitrile graft copolymer, starch Examples thereof include- (meth) acrylic acid graft copolymers.
The particle size of the water-absorbent resin powder is also not particularly limited, but is usually about 1000 μm or less, preferably
It is assumed to be 300 μm or less. The amount of the water-absorbent resin powder used is not particularly limited, but it is usually about 0.5 to 20% by weight based on all monomers used in aqueous solution polymerization. If it is less than 0.5% by weight, the effect of its addition is insufficient, and if it is used in excess of 20% by weight, it is meaningless to use it in excess.

The production method of the present invention is substantially similar to the conventionally known method except that a water-absorbent resin powder is also used during polymerization.
As such a conventionally known method, for example, JP-A-541598
23, JP-A-55-84304, JP-A-55-108407
JP-A-55-133413, JP-A-56-36504, JP-A-56-91837, JP-A-58-71907,
Examples thereof include JP-A-63-43912 and JP-A-63-317519.

More specifically, the production method of the present invention is carried out as follows. First of all, a predetermined amount of an alkali metal acrylate and / or an alkali metal acrylate and other monomer, a crosslinkable compound, a surfactant, etc., which can be used if necessary, are added to water in predetermined amounts and dissolved. Then, an aqueous monomer solution is prepared. The concentration of all the monomers in the aqueous monomer solution is usually 30 to 80% by weight, preferably 40 to 75% by weight when polymerized by heating. When it is irradiated with ultraviolet rays, the amount is usually 25 to 65% by weight, preferably 30 to 60% by weight. Here, if the monomer concentration is less than the lower limit, the degree of polymerization of the resulting water-absorbent resin tends to decrease, while if it exceeds the upper limit, the reaction temperature during the reaction becomes too high and the water-absorbing property obtained. The resin tends to be porous, which tends to reduce water retention.

Then, the polymerization initiator is mixed with stirring in the aqueous monomer solution to dissolve the polymerization initiator. The amount of the polymerization initiator used is
Although not particularly limited, usually 0.001 relative to all monomers
It is suitable to be about 5.0% by weight, preferably 0.01 to 1.0% by weight. Next, after charging this mixed solution in an appropriate reaction vessel, the above reaction system is heated or the reaction system is irradiated with ultraviolet rays to start a polymerization reaction. However, when the monomer concentration during the polymerization is about 60% by weight or more, no heating operation is required during the polymerization, and the reaction can be completed only by the heat of polymerization. In the polymerization reaction of the present invention, the form of the reaction vessel is not limited, but it is preferable to use an endless belt or an open vessel having a large surface area from the viewpoint of convenience such as workability in drying and pulverizing the water-absorbent resin to be obtained. Also, the thickness of the reaction solution at the time of supply is not particularly limited, but it is usually preferably about 5 cm or less from the viewpoint of sufficiently transmitting ultraviolet rays, particularly when irradiated with ultraviolet rays. The amount of UV light is not particularly limited, but is usually 5 to 2000 mJou.
L / cm ² is good. If it is less than this range, the polymerization and crosslinking tend to be insufficient. The preferred amount of light is
It is about 50 to 1000 mJoul / cm ² . Further, as the light source used for ultraviolet irradiation, conventionally known light sources, for example, incandescent light bulbs, halogen lamps, fluorescent lamps, xenon lamps, sodium lamps, mercury lamps, metal halide lamps, etc. can be used as they are, and the liquid thickness etc. It is appropriately selected and used in consideration of reaction conditions. Thus, when the reaction system is heated or irradiated with ultraviolet light, the reaction starts immediately,
The polymerization reaction proceeds at about 70 to 100 ° C. The ultraviolet irradiation time at the time of ultraviolet polymerization is appropriately determined so as to have the above-mentioned light amount, but in the case of using an endless belt, for example, if the irradiation spot under the above conditions is passed for a short time of usually several seconds to several minutes, the reaction Is completed. Incidentally, the preparation temperature of the mixture of the monomer aqueous solution and the photopolymerization initiator to be subjected to the irradiation reaction is 0 to 40.
It is good to set the temperature to about ℃, preferably 10 to 25 ℃. If the temperature is less than 0 ° C, the mixture may coagulate, and if it exceeds 40 ° C, the reaction system temperature becomes too high in the subsequent polymerization reaction, so that the water-absorbing resin tends to have a low water retention rate. is there.

The water absorbent resin thus obtained has a water content of usually about 35 to 75% by weight, and its appearance is a transparent rubber-like elastic body. Therefore, if necessary depending on the application, the polymer can be formed into a powdery material or a granular material through a step of subsequently drying and pulverizing the polymer. For these steps, a known method may be applied as it is, and it is not necessary to use a special operation or device. For example, a hot air dryer, an infrared dryer, a fluidized bed dryer or the like can be used as the drying device, and the drying temperature may be usually about 70 to 200 ° C. The dried water-absorbent resin thus obtained can be crushed to a desired particle size using, for example, a vibration crusher or an impact crusher. Incidentally, in the above polymerization reaction, when heat-polymerized at a high monomer concentration, a water content of about 10% by weight or less can be obtained immediately upon completion of the reaction,
It is possible to directly grind without passing through the subsequent drying step.

The water-absorbent resin obtained by the method of the present invention, which is essential to use together with the water-absorbent resin powder at the time of polymerization, is water-soluble as compared with the water-absorbent resin obtained by adopting the above various conventional aqueous solution polymerization methods. Despite the fact that the content of the minute component is extremely low, various properties such as the water absorbing ability inherent to the water absorbent resin are hardly deteriorated. Incidentally, among the conventional techniques, the water-absorbent resin obtained by the method described in JP-A-63-43912 or JP-A-63-317519 has a water content higher than that obtained by other known technologies. Although the content of the soluble component is considerably low, it goes without saying that the water-soluble component can be further reduced by applying the method of the present invention to the method.

The size, shape, etc. of the water absorbent resin obtained by the method of the present invention are
It is not particularly limited and can be appropriately selected depending on the application. For example, when it is used as a sanitary material, it is usually in the form of particles, and those having a particle size of about 100 to 1000 μm are preferable.

The water-absorbent resin obtained by the method of the present invention can be used for the above-mentioned various uses, and can be particularly suitable for sanitary products such as sanitary products and disposable diapers. The water-absorbent resin is used alone or in combination with an inorganic powder such as silicon dioxide powder or titanium oxide powder or an organic filler such as rubber depending on the application. Examples of the silicon dioxide powder include colloidal silica, white carbon and ultrafine particle silica.

Furthermore, the water-absorbent resin pulverized product obtained by the above method can be post-modified if necessary. For example, a water-absorbent resin pulverized product obtained by reacting a carboxylate contained in the water-absorbent resin with a known crosslinking agent such as a polyepoxy compound represented by a water-soluble diglycidyl ether compound, an aldehyde compound, and a polyvalent metal salt. It is also possible to modify the surface of the above, and the modified product can be used for the same purpose as described above.

(Examples) Hereinafter, the present invention will be described more specifically with reference to Reference Examples, Examples, and Comparative Examples, but it goes without saying that the present invention is not limited to the Examples.

Reference Example 1 As water-absorbent resin powder, polyacrylate cross-linked product (trade name "Arasorb S-100", manufactured by Arakawa Chemical Industry Co., Ltd.)
Was screened with an 80-mesh wire net (hereinafter referred to as water-absorbent resin powder a), and its water absorption capacity was 55 g / g.

Reference Example 2 As a water-absorbent resin powder, a polyacrylate cross-linked product (trade name "Aquaric CA", manufactured by Nippon Shokubai Chemical Co., Ltd.)
Was screened with a wire mesh of 80 mesh (hereinafter referred to as water-absorbent resin powder b), and its water absorption capacity was 44 g / g.

Reference Example 3 As a water-absorbent resin powder, a starch-acrylic acid type graft copolymer (trade name "Sunwet IM-1500", manufactured by Sanyo Kasei Co., Ltd.) was sieved with an 80-mesh wire mesh (hereinafter This is referred to as water-absorbent resin powder c), and its water absorption capacity is 47 g / g.

Example 1 Under ice-cooling, 328 g of acrylic acid and 543.2 g of water were neutralized by adding 136.55 g of sodium hydroxide (corresponding to 75 mol% to acrylic acid), and then N, N'-methylenebisacrylamide.
0.032g (0.008% by weight based on total monomer) and 2,2 '
-Azobis (N, N'-dimethyleneisobutylamidine)
After 0.403 g of dihydrochloride (0.1% by weight based on all monomers) was added and dissolved, nitrogen gas was blown to remove dissolved oxygen and the liquid temperature was adjusted to 20 ° C. to obtain a preparation liquid having a total monomer concentration of 40% by weight.

20.16 g of the water-absorbent resin a (5% by weight based on all monomers) was added to the prepared liquid and stirred to disperse the water-absorbent resin powder.
Take 6.7g and add it to a glass petri dish with an inner diameter of 150mm and a depth of 25mm. Conveyor type UV device (manufactured by Eye Graphic Co., Ltd., high pressure mercury lamp 2Kw x 2 lamps, 80W / cm, emission length 250)
mm), the belt speed is 10m / so that the light intensity is 900mJoul / cm ^2.
UV irradiation was performed for 1 minute and an irradiation time of 10.8 seconds to obtain a rubber-containing hydrogel cross-linked polymer having a thickness of 9 mm. Then, this gel-like crosslinked polymer was cut into 2 to 5 mm with a cutter, dried in a hot air dryer at 140 ° C for 2 hours, then pulverized with a pulverizer, and further dried at 130 ° C for 30 minutes to absorb water. Resin powder A was obtained.

Example 2 In Example 1, the addition amount of the water absorbent resin powder a was 40.33 g.
Polymerization, drying and pulverization were carried out in the same manner except that the content was changed to (10% by weight based on all monomers) to obtain a water absorbent resin powder B.

Example 3 Polymerization was carried out in the same manner as in Example 1, except that the water absorbent resin powder b was used in place of the water absorbent resin powder a, and the addition amount was changed to 40.33 g (10% by weight based on all monomers). Dry,
Pulverization was performed to obtain a water absorbent resin powder C.

Example 4 Polymerization was carried out in the same manner as in Example 1, except that the water absorbent resin powder c was used instead of the water absorbent resin powder a, and the addition amount was changed to 40.33 g (10% by weight based on all monomers). Dry,
Crushing was performed to obtain a water absorbent resin powder D.

Example 5 30 g of acrylic acid was added to 9.24 g of water, and 20.6 g of potassium hydroxide having a purity of 85% (corresponding to 75 mol% with respect to acrylic acid) was added to neutralize the mixture, followed by N, N'-methylene. Bisacrylamide (0.009 g, 0.021% by weight based on all monomers) was added to prepare an aqueous potassium acrylate solution having a monomer concentration of 70% by weight. Water-absorbent resin powder a4.19 g (10% by weight based on all monomers) was added to the prepared liquid, and the mixture was stirred to disperse the resin powder and then kept at 70 ° C. To this was added a solution in which 1 g of water and 0.280 g of 2,2'-azobis (2-amidinopropane) dihydrochloride were dissolved, and immediately spread on the bottom of a cylindrical reactor having an inner diameter of about 10 cm. Polymerization started after several seconds, the reaction was completed within 1 minute, and a crosslinked polymer foamed by the heat of polymerization and dried was obtained. This was pulverized to obtain a crosslinked polymer powder. Then, the crosslinked polymer powder was heated at 130 ° C in a hot air dryer.
And dried for 30 minutes to obtain a water absorbent resin powder E.

Example 6 Under ice-cooling, 350 g of acrylic acid and 504.62 g of water were neutralized by adding 145.71 g of sodium hydroxide (corresponding to 75 mol% to acrylic acid), and then 0.086 g of N, N′-methylenebisacrylamide ( (0.02% by weight based on all monomers) and 2.15 g of sorbitan fatty acid ester (Kao Atlas Co., Ltd., trade name "Span 20") were added and dissolved, and then nitrogen gas was blown in to expel dissolved oxygen and raise the liquid temperature to 40 ° C. did.

Water-absorbent resin powder a43.01g (1 to all monomers)
(0% by weight) and stirred to disperse the water-absorbent resin powder, then 0.15 g of ammonium persulfate and sodium bisulfite
0.05 g was added and dissolved. This solution was placed in a 150 mm square container lined with tetrafluoroethylene. The polymerization proceeds gently and the temperature of the reaction system is 55 ~ 2 hours after the initiation of the polymerization.
It reached 60 ° C. Seven hours after the initiation of the polymerization, the gel-like crosslinked polymer as the content was taken out. Then, the polymer was cut into about 2 to 5 mm with a cutter, dried in a hot air dryer at 140 ° C. for 2 hours, and then ground with a grinder to obtain a water-absorbent resin powder F.

Example 7 In Example 1, N, N′-methylenebisacrylamide was not added, and the amount of the water absorbent resin powder a added was 40.33 g.
Polymerization, drying, powdery polymerization, drying, and pulverization were carried out in the same manner except that the amount was changed to (10% by weight based on all monomers) to obtain a water absorbent resin powder G.

Comparative Example 1 Polymerization, drying and pulverization were performed in the same manner as in Example 1 except that the water absorbent resin powder a was not added to obtain a water absorbent resin powder H.

Comparative Example 2 In Example 1, the addition amount of N, N'-methylenebisacrylamide was 0.121 g (0.03% by weight based on all monomers).
Polymerization, drying and pulverization were carried out in the same manner except that the water absorbent resin powder a was not added and the water absorbent resin powder I was obtained.

Comparative Example 3 Water absorbing resin powder J was obtained by polymerizing, drying and pulverizing in the same manner as in Example 5, except that water absorbing resin powder a was not added.

Comparative Example 4 Water absorbing resin powder K was obtained by carrying out polymerization, drying and crushing in the same manner as in Example 6 except that water absorbing resin powder a was not added.

Comparative Example 5 Polymerization, drying and pulverization were performed in the same manner as in Example 7 except that the water absorbent resin powder a was not added to obtain a water absorbent resin powder L.

Table 1 shows the measurement results of the water-absorbing ability and the water-soluble content of the water-absorbent resin powders (A to L) obtained in the above various Examples and Comparative Examples.

The water absorption capacity means the amount of physiological saline absorbed per 1 g of the water absorbent resin powder.

(Measurement of water-soluble content) Add 300 ml of distilled water to a 500 ml Erlenmeyer flask, add 0.2 ± 0.02 g of water-absorbent resin powder to this, add it with stirring, and leave it in the dark for 24 hours. . Then, vacuum filtration is performed using a glass fiber filter made by Millipore. The filtrate is placed in a dryer for 10
Heat at 5 ° C for 18 hours, measure the heating residue, and calculate the water-soluble content by the following formula.

Water-soluble content = (heating residue x 100) / sample weight (Effects of the Invention) According to the method of the present invention, a water-absorbing resin which has an extremely small amount of water-soluble components and also has excellent original performances of the water-absorbing resin despite adopting an aqueous solution polymerization method similar to the conventional method A remarkable effect is that the resin can be easily provided.

Claims (3)

[Claims] 1. A method for producing a water absorbent resin by aqueous solution polymerization of an aqueous monomer solution containing an alkali metal acrylate and / or an methacrylic acid alkali metal salt as a main monomer, wherein the aqueous solution polymerization is acrylic acid. A method for producing a water-absorbent resin, which is carried out in the presence of a water-absorbent resin powder of a crosslinked polymer containing an alkali metal salt and / or an alkali metal methacrylic acid as a main monomer. 2. The production method according to claim 1, wherein a divinyl compound is contained as a crosslinkable compound during the aqueous solution polymerization. 3. The water-absorbent resin powder is 0.5 to 2 with respect to all monomers.
The production method according to claim 1 or 2, which comprises 0% by weight.