JP2006169267A - Method for producing modified water absorbing resin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a modified water absorbing resin having excellent water absorption characteristics. <P>SOLUTION: The method for producing the modified water absorbing resin comprises (a) mixing a water absorbing resin with a water-soluble radical polymerization initiator without adding an ethylenically unsaturated monomer and (b) irradiating the resultant mixture with active energy rays. The AAP (absorption capacity under load) and SFC (saline flow conductivity) are especially improved. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a modified water-absorbent resin, and more specifically, an active energy ray is added to a water-absorbent resin mixed with a water-soluble radical polymerization initiator without adding an ethylenically unsaturated monomer. It is related with the method which modifies water-absorbing resin by irradiating.

  Conventionally, a water-absorbing resin has been used as a constituent material of sanitary materials that absorb sanitary cotton, disposable diapers, or other body fluids. Examples of such a water-absorbing resin include a hydrolyzate of starch-acrylonitrile graft polymer, a neutralized product of starch-acrylic acid graft polymer, a saponified product of vinyl acetate-acrylic ester copolymer, and an acrylonitrile copolymer. Examples thereof include hydrolysates of polymers or acrylamide copolymers, cross-linked products thereof, and cross-linked products of partially neutralized polyacrylic acid. All of these have an internal crosslinking structure and are insoluble in water.

  Properties desired for such a water-absorbent resin include a high absorption ratio, an excellent absorption rate, a high gel strength, and an excellent suction force for sucking water from the substrate. However, since the water absorption property is affected by the crosslink density, the relationship between the properties does not necessarily show a positive correlation, for example, the gel strength increases but the water absorption amount decreases as the crosslink density increases. In particular, the absorption magnification and the absorption rate, gel strength, suction force, and the like are in a contradictory relationship. For this reason, in the water absorbent resin having improved absorption capacity, when it comes into contact with the liquid, water absorption is not performed uniformly, and a part of the water absorbent resin is formed or water does not diffuse throughout the water absorbent resin particles. For this reason, the absorption rate may be extremely reduced.

  A method of coating the surface of water-absorbent resin particles with a surfactant or a non-volatile hydrocarbon in order to alleviate such a phenomenon and obtain a water-absorbent resin having a high absorption rate and a relatively good absorption rate. There is. This method improves the dispersibility of water absorbed in the initial stage, but is not sufficiently effective in improving the absorption speed and suction force of individual particles.

  Further, as a method for producing a polyacrylic acid-based superabsorbent polymer with improved water absorption characteristics, an aqueous composition of a polymer having a partial alkali metal salt of polyacrylic acid as a main component and a low crosslinking density is used as a water-soluble polymer. There is a method of heating in the presence of an oxide radical initiator and introducing crosslinking by radical crosslinking (Patent Document 1). It is difficult to uniformly distribute the internal crosslinks in the polymer, and it is not easy to adjust the crosslink density. For this reason, after obtaining a polymer having a low crosslinking density and a water-soluble polyacrylic acid gel, a persulfate as a polymerization initiator is added and heated. In this Patent Document 1, by adjusting the initiator addition amount, it is possible to precisely control the crosslinking density, and since the crosslinking is uniformly present in the polymer, an excellent water absorption specification is obtained, and the adhesiveness is high. No water-absorbing resin was obtained.

  The persulfate used in Patent Document 1 is decomposed by heat, but is also decomposed by ultraviolet rays to generate radicals (Non-Patent Document 1). Since persulfate has an action as a polymerization initiator, radical crosslinking occurs at the same time as polymerization by irradiating an aqueous solution of a water-soluble vinyl monomer with light energy, and a hydrogel can be produced (Patent Document 2). In addition to the hydrophilic polymer component and the photopolymerization initiator, there is also a reaction system in which a crosslinking agent is further added to form internal crosslinking by ultraviolet irradiation (Patent Document 3).

  On the other hand, there is a method in which the surface of the water absorbent resin is treated with a crosslinking agent to increase the crosslink density of the surface of the water absorbent resin (for example, Patent Document 4 and Patent Document 5). Reactive functional groups exist on the surface of the water-absorbent resin as shown in the above example. If a surface cross-linking agent capable of reacting with such a functional group is added to introduce cross-linking between functional groups, the surface cross-linking density of the water-absorbing resin increases, and a water-absorbing resin having excellent water-absorbing properties even under pressure can do.

In addition, when the surface cross-linking agent is used, the cross-linking reaction requires a high temperature or a long period of time, and there is a problem such as remaining unreacted cross-linking agent. There is also a method in which the resin is heated to introduce crosslinks to polymer molecular chains in the vicinity of the surface of the resin through decomposition of the radical initiator (Patent Document 6). In the examples, heating with 130 ° C. superheated steam was performed for 6 minutes to obtain a water-absorbing resin with improved water absorption ratio.
U.S. Pat. No. 4,783,510 JP 2004-99789 A WO2004 / 031253 U.S. Pat. No. 4,666,983 US Pat. No. 5,422,405 JP 63-99211 A J. et al. Phys. Chem. , 1975, 79, 2693, J. Am. Photochem. Photobiol. , A, 1988, 44, 243

  The purpose of introducing surface cross-linking into the water-absorbent resin is a method for producing a water-absorbent resin having an excellent balance between absorption capacity and absorption rate. In general, a cross-linking agent having at least two functional groups that can react with a functional group on the surface of the water absorbent resin needs to act on the water absorbent resin. Examples of such cross-linking agents include polyhydric alcohols, polyhydric glycidyl ethers, haloepoxy compounds, polyhydric aldehydes, polyhydric amines, polyvalent metal salts, etc., but generally have low reactivity. It is necessary to carry out the reaction at a high temperature, and in some cases, it may be left under heating for a long time. For this reason, much energy and time are required.

  In the surface treatment method of Patent Document 6 using a peroxide radical initiator as a cross-linking agent, in order to advance the reaction efficiently, humidification for maintaining a high reaction temperature and moisture necessary for the progress of the reaction is required. It is necessary and further improvement in production efficiency is required.

  Under such circumstances, the present invention provides a method for producing a modified water-absorbing resin that is excellent in production efficiency and excellent in absorption capacity under pressure, absorption speed, gel strength, liquid permeability, and the like. Objective.

  A detailed study of a method for producing a water-absorbent resin with a surface modification of the water-absorbent resin revealed that a persulfate that has been used as a polymerization initiator in the past was used. It has been found that radicals are generated and a crosslinked structure can be easily formed on the surface of the water absorbent resin. In addition, according to this method, it was found that surface crosslinking can be introduced without using a surface crosslinking agent that was an essential component in the conventional method, and that the water-absorbing resin obtained has an excellent balance of water absorption properties. Completed the invention.

  Conventionally, during surface crosslinking, high-temperature treatment at 100 to 300 ° C. was required depending on the type of surface crosslinking agent to be blended, but in the present invention, irradiation with active energy rays was performed without using a surface crosslinking agent. Surface cross-linking can be introduced only by itself. For this reason, it is possible to modify the water absorbent resin without exposing it to a high temperature, and it is possible to prevent thermal deterioration of the water absorbent resin during the modification.

  Moreover, since the persulfate is water-soluble, it can be dissolved in an aqueous solution and mixed with the water-absorbent resin, and uniform surface crosslinking can be generated. As a result, the modified water-absorbent resin has extremely high properties desired for the water-absorbent resin, such as absorption capacity, absorption rate, gel strength, and suction force.

  Since the method for producing a modified water-absorbing resin of the present invention performs surface cross-linking by irradiation with active energy rays, the water-absorbing resin can be modified in a shorter time compared to the conventional production method.

The first of the present invention is a method for producing a modified water absorbent resin,
a) Without adding an ethylenically unsaturated monomer, mixing the water-absorbent resin and the water-soluble radical polymerization initiator,
b) irradiating the resulting mixture with active energy rays;
Is a method for producing a modified water-absorbent resin. Hereinafter, the modified water-absorbing resin production method according to the present invention will be described in detail, but the scope of the present invention is not limited to these explanations, and the gist of the present invention is not limited to the following examples. The present invention can be carried out with appropriate modifications within a range not impairing.

(A) Water-absorbing resin The water-absorbing resin that can be used in the present invention is a water-swellable, water-insoluble crosslinked polymer capable of forming a hydrogel. In the present invention, “water swellability” means that the free swelling ratio is essentially 2 g / g or more, preferably 5 to 100 g / g, more preferably 10 in a 0.9 mass% sodium chloride aqueous solution (saline). Those that absorb -60 g / g of physiological saline. The “water-insoluble” is preferably 0 to 50% by mass, more preferably 25% by weight or less, and still more preferably 15% by weight or less of an uncrosslinked water-soluble component (water-soluble polymer) in the water-absorbent resin. Particularly preferably, it means 10% by weight or less. In addition, the free swell ratio and the numerical values of the water-soluble component are determined by the measurement method defined in the examples described later. “Modification” refers to all physical or chemical effects on the water-absorbent resin, and includes surface cross-linking, pore formation, hydrophilization, and hydrophobicity of the water-absorbent resin.

  The water-absorbing resin that can be used in the present invention is not particularly limited as long as it is obtained by polymerization using a conventionally known method using a monomer component that essentially contains an ethylenically unsaturated monomer. .

  The ethylenically unsaturated monomer is not particularly limited and is preferably a monomer having an unsaturated double bond at the terminal, such as (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2- Anionic monomers such as (meth) acryloylpropane sulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid and salts thereof; (meth) acrylamide, N-substituted (meta ) Nonionic hydrophilic group-containing monomers such as acrylamide, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate; N, N-dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (Meth) acrylate, N, N-diethylaminopropyl (meth) acrylate, N, N- Methylamino propyl (meth) acrylamide, amino group-containing unsaturated monomers and their quaternized products and the like; or the like can be cited, and may be used alone or two or more selected from these. Preferably, (meth) acrylic acid, 2- (meth) acryloylethanesulfonic acid, 2- (meth) acrylamido-2-methylpropanesulfonic acid, salts thereof, N, N-dimethylaminoethyl (meth) acrylate, N , N-dimethylaminoethyl (meth) acrylate quaternized product, (meth) acrylamide, particularly preferably acrylic acid and / or a salt thereof.

  When acrylate is used as the monomer, a monovalent salt of acrylic acid selected from alkali metal salts, ammonium salts, and amine salts of acrylic acid is preferable from the viewpoint of water absorption performance of the water absorbent resin. More preferred are alkali metal acrylates, and particularly preferred are acrylates selected from sodium salts, lithium salts, and potassium salts.

  When producing the water-absorbent resin, other monomer components other than the above-mentioned monomers can be used within a range not impairing the effects of the present invention. For example, an aromatic ethylenically unsaturated monomer having 8 to 30 carbon atoms, an aliphatic ethylenically unsaturated monomer having 2 to 20 carbon atoms, and an alicyclic ethylenically unsaturated monomer having 5 to 15 carbon atoms And hydrophobic monomers such as alkyl group (meth) acrylic acid alkyl ester having 4 to 50 carbon atoms. Generally the ratio of these hydrophobic monomers is the range of 0-20 mass parts with respect to 100 mass parts of said ethylenically unsaturated monomers. If the hydrophobic monomer exceeds 20 parts by mass, the water absorption performance of the resulting water absorbent resin may be reduced.

  The water absorbent resin used in the present invention becomes insoluble due to the formation of internal crosslinks. Such internal crosslinking may be a self-crosslinking type that does not use a crosslinking agent, but uses an internal crosslinking agent having two or more polymerizable unsaturated groups and / or two or more reactive functional groups in one molecule. Can be formed.

  Such an internal cross-linking agent is not particularly limited, and preferably, for example, N, N′-methylenebis (meth) acrylamide, N-methylol (meth) acrylamide, glycidyl (meth) acrylate, (poly) ethylene glycol diester (Meth) acrylate, (poly) propylene glycol di (meth) acrylate, glycerol tri (meth) acrylate, glycerol acrylate methacrylate, (meth) acrylic acid polyvalent metal salt, trimethylolpropane tri (meth) acrylate, triallylamine, tri List allyl cyanurate, triallyl isocyanurate, triallyl phosphate, ethylene glycol diglycidyl ether, (poly) glycerol glycidyl ether, polyethylene glycol diglycidyl ether, etc. It can be. Two or more of these internal cross-linking agents may be used in combination.

  The amount of the internal crosslinking agent used is preferably 0.0001 to 1 mol%, more preferably 0.001 to 0.5 mol%, based on the total amount of monomer components used in producing the water absorbent resin. More preferably, it is 0.005-0.2 mol%. When the amount is less than 0.0001 mol%, internal crosslinking is not introduced into the resin. On the other hand, when the amount exceeds 1 mol%, the gel strength of the water-absorbent resin becomes too high, and the water absorption ratio may be lowered. When a crosslinked structure is introduced into the polymer using the internal cross-linking agent, the internal cross-linking agent is added to the reaction system before, during or after the polymerization of the monomer, or after the polymerization or neutralization. What should I do?

  In order to obtain a water-absorbing resin, the monomer component including the monomer and the internal crosslinking agent may be polymerized in an aqueous solution. In this case, usable polymerization initiators include persulfates such as potassium persulfate, ammonium persulfate and sodium persulfate; potassium peracetate, sodium peracetate, potassium percarbonate, sodium percarbonate, t-butyl hydroperoxide; Hydrogen peroxide; azo compounds such as 2,2′-azobis (2-amidinopropane) dihydrochloride, water-soluble radical polymerization initiators such as 2-hydroxy-2-methyl-1-phenyl-propane-1- There are photopolymerization initiators such as ON. Further, for example, a reducing agent such as sulfite, L-ascorbic acid or ferric salt may be combined with the water-soluble radical polymerization initiator and used as a redox initiator.

  Although there is no restriction | limiting in particular in the density | concentration of the monomer in the said monomer aqueous solution, Preferably it is 15-90 mass%, More preferably, it is 35-80 mass%. If it is less than 15% by mass, the obtained hydrogel has a large amount of water, which requires a heat amount and time for drying, which is disadvantageous.

  The polymerization method is not particularly limited, and well-known methods such as aqueous solution polymerization, reverse phase suspension polymerization, precipitation polymerization, bulk polymerization and the like can be employed. Among these methods, aqueous solution polymerization in which a monomer is dissolved in an aqueous solution for polymerization and reverse phase suspension polymerization are preferable from the viewpoint of ease of control of the polymerization reaction and performance of the obtained water-absorbent resin.

  When starting the above polymerization, the above polymerization initiator is used. In addition to the aforementioned polymerization initiator, active energy rays such as ultraviolet rays, electron beams, and γ rays may be used alone or in combination with the polymerization initiator. The temperature at the start of polymerization depends on the type of polymerization initiator used, but is preferably in the range of 15 to 130 ° C, and more preferably in the range of 20 to 120 ° C. If the temperature at the start of polymerization is out of the above range, it is preferable because the residual monomer of the obtained water-absorbent resin may increase or excessive self-crosslinking reaction may occur to lower the water-absorbing performance of the water-absorbent resin. Absent.

  The reverse phase suspension polymerization is a polymerization method in which an aqueous monomer solution is suspended in a hydrophobic organic solvent. For example, U.S. Pat. Nos. 4,093,764, 4,367,323, 4,446,261, 4,683,274, and 5,244,735. Are described in US patents. Aqueous solution polymerization is a method of polymerizing an aqueous monomer solution without using a dispersion solvent. For example, US Pat. Nos. 4,462,001, 4,873,299, 4,286,082, 4,973,632, 4,985,518, 5,124,416, 5,250,640 are used. No. 5,264,495, US Pat. No. 5,145,906 and US Pat. No. 5,380,808, and European patents such as European Patents 081636, 09555086, and 0922717. Monomers and initiators exemplified in these polymerization methods can also be applied in the present invention.

  When aqueous solution polymerization is performed, a partially neutralized product such as acrylic acid is polymerized, or an acid group-containing monomer such as acrylic acid is polymerized and then polymerized with an alkali compound such as sodium hydroxide or sodium carbonate. Can also be neutralized. In addition, the neutralization rate (mol% of the neutralized acid group in all the acid groups) of the water absorbent resin obtained is in the range of 25 to 100 mol%, preferably in the range of 50 to 90 mol%. .

  After polymerization, it is usually a hydrogel crosslinked polymer. In the present invention, the water-containing gel-like crosslinked polymer can be used as it is as a water-absorbing resin, but is preferably dried to have a water content (100-solid content) described later.

  In the present invention, as will be described later, the water-absorbent resin is modified by irradiation with a water-soluble radical polymerization initiator and an active energy ray. Such modification is performed by the active radical polymerization initiator with respect to the polymer main chain. This is due to action. Therefore, not only the water-absorbing resin obtained by polymerizing the water-soluble ethylenically unsaturated monomer described above, but also water-absorbing resins such as polyvinyl alcohol crosslinked body, polyethylene oxide crosslinked body, polyaspartic acid crosslinked body, carboxymethylcellulose crosslinked body. It can also be modified.

  On the other hand, the water-absorbent resin used in the present invention is preferably a powdery water-absorbent resin obtained by polymerizing a monomer mainly composed of acrylic acid (salt). The hydrogel polymer obtained by polymerization is preferably pulverized after drying to obtain a water-absorbing resin. The drying may be performed, for example, using a dryer such as a hot air dryer, preferably at 100 to 220 ° C, more preferably at 120 to 200 ° C.

  As a pulverizer that can be used for such pulverization, for example, among the pulverization model names classified in Table 1.10 of the Powder Engineering Handbook (Edition of the Powder Engineering Society, first edition), a shear pulverizer, It is classified into an impact crusher and a high-speed rotary crusher, and those having one or more crushing mechanisms such as cutting, shearing, impact, and friction can be preferably used. Among crushers corresponding to these models, cutting and shearing A crusher whose mechanism is the main mechanism can be used particularly preferably. For example, a roll mill (roll rotary type) pulverizer is preferable.

  The water-absorbent resin used in the present invention is preferably in the form of powder. More preferably, it is a powdery water-absorbing resin containing 90 to 100% by mass, particularly preferably 95 to 100% by mass of particles having a particle size in the range of 150 to 850 μm. When the water-absorbent resin larger than 850 μm is used for, for example, the obtained modified water-absorbent resin in a diaper or the like, the touch may be poor and the top sheet of the diaper may be broken. On the other hand, if the particles smaller than 150 μm exceed 10% by mass, fine powder may be scattered or clogged at the time of use, and the water absorption performance of the modified water absorbent resin may be lowered. The average particle size is in the range of 10 to 1,000 μm, preferably 200 to 600 μm. If the average particle size is less than 10 μm, it may be unfavorable for safety and health. On the other hand, if it exceeds 1,000 μm, it may not be used for diapers. In addition, the said particle size is the value measured with the measuring method of the particle size distribution of the Example mentioned later.

  The water content of the water absorbent resin used in the method for producing a modified water absorbent resin of the present invention is not particularly limited as long as the water absorbent resin has fluidity. Preferably, the moisture content after drying at 105 ° C. for 3 hours is in the range of 0 to 20% by mass, preferably 0 to 10% by mass, more preferably 0 to 5% by mass.

  The water-absorbent resin used in the present invention is not limited to the one produced by the above method, and may be one prepared by another method. The water-absorbing resin obtained by the above method is a water-absorbing resin that is not surface-crosslinked. However, as the water-absorbing resin used in the method for producing a modified water-absorbing resin of the present invention, a polyvalent A water-absorbing resin having an alcohol, a polyvalent epoxy compound, an alkylene carbonate, an oxazolidone compound and the like and surface-crosslinked can also be used.

(B) Water-soluble radical polymerization initiator In the method for producing a modified water-absorbing resin of the present invention, a water-soluble radical polymerization initiator and the water-absorbing resin are added without adding an ethylenically unsaturated monomer. Mix. Conventionally, surface cross-linking of a water-absorbing resin has been generally performed by blending a surface cross-linking agent. If a surface cross-linking agent is blended, the functional group on the resin surface and the surface cross-linking agent are chemically strongly bonded, and thereby a stable surface cross-linking structure can be introduced on the resin surface. Further, the surface crosslinking distance can be easily adjusted by appropriately selecting the chain length of the surface crosslinking agent, and the crosslinking density can be controlled by adjusting the blending amount. However, in the present invention, the water-absorbing resin is modified only by using a water-soluble radical polymerization initiator without blending such a surface cross-linking agent. Specifically, a cross-linked structure is formed on the surface of the water-absorbing resin. It was found that can be introduced. In the present invention, “without adding an ethylenically unsaturated monomer” means that the surface crosslinking agent exists in addition to the ethylenically unsaturated monomer, but is irradiated with active energy. This is because surface crosslinking can be introduced only by ethylenically unsaturated monomers, and is excluded only in this case.

  In the present invention, the reason why surface crosslinking is possible with a water-soluble radical polymerization initiator and active energy rays is not clear, but a cross-linked structure is formed even in the absence of a crosslinkable compound. It is considered that a water-soluble radical polymerization initiator activated by irradiation with active energy rays acts at a plurality of positions on the main chain or side chain to bind them together by some action. For example, it is inferred that the hydrogen atom is extracted from the main chain of the water-absorbent resin to activate carbon atoms, such carbon atoms are bonded in the vicinity, and as a result, a crosslinked structure is randomly formed.

  In the present invention, the reason why the “water-soluble radical polymerization initiator” is particularly used is that it can be easily and uniformly dispersed on the surface of a water-absorbing resin excellent in hydrophilicity and water absorption. As a result, a water-absorbing resin having excellent water absorption characteristics can be produced.

  The water-soluble radical polymerization initiator used in the present invention is one that dissolves in water (25 ° C.) in an amount of 1% by mass or more, preferably 5% by mass or more, more preferably 10% by mass or more. Persulfates such as sodium and potassium persulfate; hydrogen peroxide; 2,2′-azobis-2-amidinopropane dihydrochloride, 2-2′-azobis [2-2 (-imidazolin-2-yl) propane] 2 Examples thereof include water-soluble azo compounds such as hydrochloride. Among these, the use of persulfate is particularly preferable because the modified water-absorbent resin has excellent absorption capacity under pressure, saline flow conductivity, and free swelling ratio.

  The amount of the water-soluble radical polymerization initiator used is preferably in the range of 0.01 to 20 parts by mass, more preferably in the range of 0.1 to 15 parts by mass, particularly preferably 1 to 1 part by mass with respect to 100 parts by mass of the water absorbent resin. The range is 10 parts by mass. If the mixing amount of the water-soluble radical polymerization initiator is less than 0.01 parts by mass, the water-absorbent resin may not be modified even when irradiated with active energy rays. On the other hand, when the mixing amount of the water-soluble radical polymerization initiator is more than 20 parts by mass, the water absorption performance of the modified water absorbent resin may be lowered.

  In the present invention, when a water-soluble radical polymerization initiator is essentially used and no water-soluble radical polymerization initiator is used, for example, only an oil-soluble radical polymerization initiator, particularly an oil-soluble photopolymerization initiator is used. Excellent physical properties can be achieved compared to cases. The oil-soluble photopolymerization initiator referred to here is, for example, a compound having a solubility in water of less than 1% by mass.

  In the present invention, a water-soluble radical polymerization initiator selected from persulfate, hydrogen peroxide, and a water-soluble azo compound is essential, but an initiator other than the water-soluble radical polymerization initiator may be used in combination. Good. As other polymerization initiators that can be used in combination, photopolymerization initiators such as oil-soluble benzoin derivatives, benzyl derivatives, and acetophenone derivatives, oil-soluble ketone peroxides, peroxyketals, hydroperoxides, dialkyl peroxides, Examples thereof include oil-soluble organic peroxides such as peroxyesters and peroxycarbonates. Such a photopolymerization initiator may be a commercially available product, trade names of Ciba Specialty Chemicals Irgacure 184 (hydroxycyclohexyl-phenyl ketone), Irgacure 2959 (1- [4- (2-hydroxyethoxy) -phenyl] -2-hydroxy -2-methyl-1-propan-1-one) and the like.

  When other initiators are used in combination according to the present invention, the amount used is 0 to 20 parts by mass, preferably 0 to 15 parts by mass, particularly 0 to 10 parts by mass with respect to 100 parts by mass of the water absorbent resin. The use ratio is less than that of the water-soluble radical polymerization initiator, for example, 1/2 or less, more preferably 1/10 or less, particularly 1/50 or less of the mass ratio of the water-soluble radical polymerization initiator.

(C) Mixing of water-absorbing resin and water-soluble radical polymerization initiator To mix the water-soluble radical polymerization initiator and the water-absorbing resin, the water-soluble radical polymerization initiator may be mixed with the water-absorbing resin as it is. Preferably, this is dissolved in an aqueous solution, and the resulting aqueous solution is mixed with a water absorbent resin. Since the water-absorbent resin has water-absorbing properties, the water-soluble radical polymerization initiator is uniformly dispersed on the surface of the water-absorbent resin by dissolving and supplying the water-soluble radical polymerization initiator in the aqueous solution. Can be mixed. In addition, as aqueous solution, the other solvent may be included in the range which does not impair the solubility of water-soluble radical polymerization initiator other than water.

  The amount of the aqueous solution to be used is in the range of 1 to 20 parts by mass with respect to 100 parts by mass of the water absorbent resin (converted to a solid content of 100% by mass). When the amount of the aqueous solution is less than 1 part by mass, surface crosslinking may not be performed sufficiently even when the water-soluble radical polymerization initiator is irradiated with active energy. On the other hand, if the amount of the aqueous solution is more than 20 parts by mass, a large amount of energy is required in the drying process after irradiation with active energy rays, which is disadvantageous. In addition, the water absorbent resin may be decomposed. The aqueous solution is not necessarily used as an aqueous solution for dissolving the water-soluble radical polymerization initiator. You may mix with the aqueous solution of the said range, after mixing a water-soluble radical polymerization initiator and a water absorbing resin. Therefore, what dried the hydrogel crosslinked body obtained by superposing | polymerizing a monomer component in the water content of 0-20 mass% can mix a water-soluble radical polymerization initiator directly with this.

  On the other hand, a surfactant, a water-soluble polymer, a hydrophilic organic solvent, and a water-soluble inorganic compound may be contained in the aqueous solution in order to improve the mixing property between the aqueous solution and the water-absorbent resin. As such a surfactant, at least one surfactant selected from the group consisting of a nonionic surfactant having an HLB of 7 or more or an anionic surfactant can be used. For example, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyglycerin fatty acid ester, polyoxyethylene alkyl ether, polyoxyethylene alkylphenol ether, polyoxyethylene acyl ester, sucrose fatty acid ester, higher alcohol sulfate ester salt, alkylnaphthalene sulfone Examples thereof include acid salts, alkyl polyoxyethylene sulfate salts, and dialkyl sulfosuccinates.

  Examples of the water-soluble polymer include polyvinyl alcohol, polyethylene oxide, polyethylene glycol, polypropylene glycol, polyacrylamide, polyacrylic acid, sodium polyacrylate, polyethyleneimine, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, dextrin, Examples include sodium alginate and starch.

  Examples of hydrophilic organic solvents include alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol and t-butyl alcohol; ketones such as acetone and methyl ethyl ketone; dioxane and alkoxy (poly) ethylene glycol , Ethers such as tetrahydrofuran; amides such as ε-caprolactam and N, N-dimethylformamide; sulfoxides such as dimethyl sulfoxide; ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, 1,3 -Propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, glycerin, 2-butene-1,4-dioe 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, And polyhydric alcohols such as triethanolamine, polyoxypropylene, pentaerythritol, and sorbitol; and one or more of these can be used.

  Examples of water-soluble inorganic compounds include alkali metal salts such as sodium and potassium and hydroxides, polyvalent metal salts such as calcium, magnesium, aluminum and zirconium, hydrogen carbonate, dihydrogen phosphate, hydrogen phosphate, etc. Non-reducing alkali metal salt pH buffering agents are exemplified. These surfactant, water-soluble polymer, hydrophilic organic solvent, and inorganic compound are preferably 0 to 40% by mass, more preferably 0.01 to 30% by mass, and still more preferably 0. 0% by mass based on the total amount of the aqueous solution. It is good to use at 1-10 mass%.

  In addition, as a method of mixing the water-absorbing resin and the water-soluble radical polymerization initiator, an ordinary mixer such as a V-type mixer, a ribbon-type mixer, a screw-type mixer, a rotating disk-type mixer, an airflow type, or the like is used. Examples of the mixing method include a mixer, a batch kneader, a continuous kneader, a paddle type mixer, and a vertical mixer.

(D) Active energy rays In the production of a water-absorbent resin, it is known to irradiate active energy rays to improve the polymerization rate. For example, an insoluble water-absorbing resin having an internal crosslink can be prepared by blending an internal crosslinking agent and a photopolymerization initiator in a polymerizable monomer component and irradiating active energy rays such as ultraviolet rays, electron beams, and γ rays. it can. It is also known that a surface cross-linking agent is used as a surface cross-linking method of a water-absorbent resin, and a surface cross-linking agent is used to promote reaction under heating conditions to form surface cross-linking. As such surface cross-linking of the water-absorbent resin, a compound having a plurality of functional groups in one molecule, such as polyhydric alcohol, polyhydric glycidyl ether, haloepoxy compound, polyhydric aldehyde, is used. Generally, when heated to 100 to 300 ° C., these functional groups react with carboxyl groups and the like on the surface of the water absorbent resin, and a crosslinked structure is formed on the surface of the water absorbent resin. However, in the present invention, a crosslinked structure can be formed on the surface of the water-absorbent resin by irradiation with a water-soluble radical polymerization initiator and active energy rays even in the absence of such a surface crosslinking agent or polymerizable monomer. There is a feature in the point. In addition, this makes it possible to improve the absorption capacity under load (AAP) and saline flow conductivity (SFC) of the modified water absorbent resin.

  In the present invention, the irradiation with the active energy ray may be performed during the mixing of the water-absorbent resin and the water-soluble radical polymerization initiator, or may be performed after mixing both. However, a method of preparing a mixture of a water-absorbing resin and an aqueous solution containing a water-soluble radical polymerization initiator, and irradiating the resulting mixture with active energy rays, because it can form uniform surface cross-linking It is.

  Examples of active energy rays include one or more of ultraviolet rays, electron beams, and gamma rays. Of these, ultraviolet rays and electron beams are preferable. Considering the influence of the active energy rays on the human body, ultraviolet rays are more preferable, ultraviolet rays having a wavelength of 300 nm or less, particularly preferably 180 to 290 nm are more preferable.

Irradiation conditions, when ultraviolet rays are used, preferably, irradiation intensity 3~1000mW / cm ^2, irradiation amount is 100~10000mJ / cm ^2. Examples of the apparatus for irradiating ultraviolet rays include a high pressure mercury lamp, a low pressure mercury lamp, a metal halide lamp, a xenon lamp, and a halogen lamp. As long as ultraviolet rays are irradiated, preferably as long as ultraviolet rays of 300 nm or less are irradiated, other radiations and wavelengths may be included, and the method is not particularly limited. In the case of using an electron beam, preferably the acceleration voltage is 50 to 800 kV and the absorbed dose is 0.1 to 100 Mrad.

  In general, the irradiation time of active energy rays may be preferably 0.1 minutes or more and less than 60 minutes, more preferably 0.2 minutes or more and less than 30 minutes, and further preferably 1 minute or more and less than 15 minutes. When a conventional surface cross-linking agent is used, the time for the surface cross-linking treatment can be shortened as compared with the same cross-linking density, which may exceed 60 minutes.

  When surface treatment is performed by irradiating active energy rays, it is not necessary to heat the surface. However, when irradiated with active energy rays, radiant heat may be generated. In general, the water-absorbent resin is preferably treated at a temperature of less than 150 ° C, more preferably less than 120 ° C, still more preferably from room temperature to 100 ° C, particularly preferably from 50 to 100 ° C. Therefore, the treatment temperature can be set lower than the conventional surface treatment temperature.

  It is preferable to stir the water-absorbent resin during irradiation with active energy rays. The active energy ray can be uniformly irradiated to the mixture of the water-soluble radical polymerization initiator and the water-absorbent resin by stirring. The devices that can stir the water-absorbent resin during irradiation with active energy rays include vibration mixers, vibration feeders, ribbon mixers, conical ribbon mixers, screw mixers, air flow mixers, and batches. Examples thereof include a kneader, a continuous kneader, a paddle type mixer, a high-speed flow type mixer, and a floating flow type mixer.

  In general, it is known that a reaction using a radical as an active species is inhibited by oxygen. However, in the production method of the present invention, the physical properties of the surface-treated water-absorbing resin were not lowered even when oxygen was present in the system. For this reason, it is not necessary to make the atmosphere inert when irradiating the active energy rays.

(E) Other treatments After irradiation with active energy rays, the water-absorbent resin may be heat-treated at a temperature of 50 to 250 ° C. as necessary for drying and the like.

  Further, after the active energy ray irradiation, surface cross-linking may be formed using a conventionally known surface cross-linking agent such as polyhydric alcohol, polyhydric epoxy compound, alkylene carbonate or the like.

  In the method for producing a modified water-absorbent resin of the present invention, a liquid permeability improver may be added to the water-absorbent resin before or after irradiation with active energy rays. Examples of such liquid permeability improvers include talc, kaolin, fullerite, bentonite, activated clay, barite, natural asphalt, strontium ore, ilmenite, perlite, and other mineral products; aluminum sulfate 14-18 water Salts (or anhydrides), potassium aluminum sulfate 12-hydrate, sodium aluminum sulfate 12-hydrate, aluminum compounds such as aluminum chloride, polyaluminum chloride, aluminum oxide, and aqueous solutions thereof; other polyvalent metal salts; hydrophilic Amorphous silica (eg, dry method: Reolosil QS-20, Tokuyama Corporation, precipitation method: DEGUSSA Sipernat22S, Sipernat 2200); 0), silicon oxide-aluminum oxide complex, and the like oxide composite such as silicon oxide, magnesium oxide complex. Such a liquid permeability improver is 0 to 20 parts by mass, more preferably 0.01 to 10 parts by mass, and particularly preferably 0.1 to 5 parts by mass with respect to 100 parts by mass of the modified water absorbent resin. Mix the parts. In the liquid permeability improving agent, those that are soluble in water can be added as an aqueous solution, and those that are not soluble can be added as a powder or slurry. Further, these liquid permeability improvers may be mixed with a radical polymerization initiator and added. In addition, as an additive, an antibacterial agent, a deodorant, a chelating agent, and the like may be appropriately added within the above range.

(F) Modified Water Absorbent Resin When the method for producing a modified water absorbent resin of the present invention is performed, the absorption capacity under pressure of the obtained water absorbent resin is improved. Conventionally, it is known that when surface cross-linking is formed, the free swelling ratio is slightly reduced, but the ability to maintain liquid absorption even under pressure, that is, the absorption capacity under pressure, is improved. According to the method of the present invention, the absorption capacity under pressure of 4.83 kPa of the water absorbent resin is increased by 1 g / g or more without using a surface crosslinking agent or an ethylenically unsaturated monomer. This is considered to indicate that a crosslinked structure was introduced on the surface of the water absorbent resin by the method of the present invention. The physical properties after modification are preferably 8 g / g or more, more preferably 12 g / g or more, still more preferably 15 g / g or more, and particularly preferably 20 g / g or more. The modified water-absorbent resin of the present invention has an absorption capacity under pressure of 4.83 kPa of 8 to 40 g / g. In addition, although an upper limit is not ask | required in particular, about 40 g / g may be enough from the cost increase by manufacture difficulty.

  The free swelling ratio (GV) is preferably 8 g / g or more, more preferably 15 g / g or more, still more preferably 20 g / g or more, and particularly preferably 25 g / g or more. Although an upper limit is not specifically limited, Preferably it is 50 g / g or less, More preferably, it is 40 g / g or less, More preferably, it is 35 g / g or less. When the free swell ratio (GV) is less than 8 g / g, the amount of absorption is too small and it is not suitable for the use of sanitary materials such as diapers. On the other hand, when the free swelling ratio (GV) is larger than 50 g / g, the gel strength is weak and there is a possibility that a water absorbent resin having excellent liquid permeability cannot be obtained.

The modified water-absorbent resin obtained by the present invention has a saline flow conductivity (SFC) of preferably 10 (× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, more preferably It is 30 (× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more, more preferably 50 (× 10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more. In addition, let these numerical values be the values measured by the method specified by the Example mentioned later.

  The shape of the surface-treated water-absorbent resin obtained by the present invention can be adjusted as appropriate depending on the treatment conditions such as the shape of the water-absorbent resin before treatment, granulation / molding after treatment, etc. . Such powder has a weight average particle size (specified by sieve classification) of 10 to 1,000 μm, preferably 200 to 600 μm, preferably 150 to 850 μm in content of 90 to 100% by mass, more preferably It is 95-100 mass%.

  The production method of the present invention has the effect of granulating fine powder generated during the production of the water-absorbent resin during surface crosslinking of the water-absorbent resin. For this reason, even if fine powder is contained in the water absorbent resin before modification, the fine powder contained is granulated when the method for producing a modified water absorbent resin of the present invention is performed. It is possible to reduce the amount of fine powder contained in the water absorbent resin. The particle size distribution of the modified water-absorbing resin obtained shifts to the higher particle size side compared to the water-absorbing resin before the modification. However, the rate of shift is the type and amount of water-soluble radical polymerization initiator mixed with the water-absorbent resin, and when this is used as an aqueous solution, the ratio of water, irradiation conditions of active energy rays, and how to flow during irradiation It changes by etc.

  The modified water-absorbing resin obtained by the method of the present invention is formed with uniform and high cross-linking density across the entire surface of the water-absorbent resin, and the properties desired for the water-absorbing resin, such as absorption capacity, absorption rate, The gel strength and suction force can be made extremely high. In addition, when surface-crosslinking the acrylic acid water-absorbing resin using a surface cross-linking agent such as polyhydric alcohol, polyvalent epoxy compound, alkylene carbonate, the speed and degree of surface cross-linking depends on the neutralization rate of the water-absorbing resin. It was dependent. Specifically, when the neutralization rate is low, surface crosslinking proceeds quickly, and when the neutralization rate is high, surface crosslinking is difficult. Moreover, in order to surface-crosslink the water-absorbing resin obtained by post-neutralization of polyacrylic acid, it has been necessary to uniformly post-neutralize after the surface cross-linking treatment. However, in the present invention, it is possible to produce a water-absorbing resin excellent in water-absorbing characteristics by modifying the water-absorbing resin without depending on the neutralization rate of the water-absorbing resin and the uniformity of post-neutralization. Since surface cross-linking depends on the action of the water-soluble radical polymerization initiator on the main chain of the water-absorbent resin, it proceeds without any problem whether the carboxyl group is an acid or a salt. It is thought that.

  If the present invention is carried out in the presence of an ethylenically unsaturated monomer, the water-soluble radical polymerization initiator is consumed for the polymerization of the ethylenically unsaturated monomer, which is not intended by the present invention. .

  According to the present invention, when the water-absorbing resin is surface-treated, it can be sufficiently surface-treated even at a reaction temperature near room temperature, and the obtained surface-treated water-absorbing resin has an absorption capacity, an absorption rate, a gel strength. The properties desired for the water-absorbing resin such as suction force are at a very high level. Therefore, the water-absorbent resin obtained by the present invention is optimal as a sanitary material that absorbs sanitary cotton, disposable diapers, or other bodily fluids, or an agricultural water-absorbent resin.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these. Hereinafter, for convenience, “mass part” may be simply referred to as “part”, and “liter” may be simply referred to as “L”. Measurement methods and evaluation methods in Examples and Comparative Examples are shown below.

(1) Particle size distribution 10 g of the water-absorbent resin before and after the surface treatment is sieved with a test sieve (made by IIDA Seisakusho) having a diameter of 75 mm and openings of 850 μm, 600 μm and 300 μm, and the respective masses are measured. % By mass was determined. The sieving was performed by shaking for 5 minutes using a SIEVE SHAKER ES-65 type manufactured by IIDA Seisakusho. The water-absorbent resin was measured after drying at 60 ± 5 ° C. under reduced pressure (less than 1 mmHg (133.3 pa)) for 24 hours.

(2) Measurement of solid content 1 g of water-absorbing resin was uniformly spread on the bottom of the aluminum bag cup in an aluminum cup having a bottom diameter of 4 cm and a height of 2 cm. This was left in a hot air dryer adjusted to 180 ° C. for 3 hours, and the solid content (%) of the water absorbent resin was calculated from the weight loss before and after.

(3) Free swelling ratio (GV)
Put 0.2 g of water-absorbent resin uniformly in a non-woven bag (Nangoku Pulp Industries Co., Ltd., trade name: Heaton paper, model: GSP-22, size: 60 mm × 60 mm), and a large excess (usually (500 mL) of 0.9 mass% sodium chloride aqueous solution (saline) at room temperature (25 ± 2 ° C.). After 30 minutes, the bag was pulled up and drained with a centrifugal force of 250 G for 3 minutes using a centrifuge, and then the mass W ₁ (g) of the bag was measured. Moreover, the same operation was performed without using a water absorbent resin, and the mass W ₂ (g) of the bag at that time was measured. _Then, the W 1, _{W 2,} was calculated free swelling capacity (GV) (g / g) by the following equation.

(4) Absorption capacity under pressure (AAP)
A stainless steel 400 mesh wire mesh (aperture size 38 μm) is fused to the bottom of a plastic support cylinder with an inner diameter of 60 mm, and water is absorbed on the wire mesh at room temperature (25 ± 2 ° C.) and humidity 50 RH%. 0.900 g of the water-soluble resin was uniformly sprayed, and the outer diameter was adjusted to be able to uniformly apply a load of 4.83 kPa to the water-absorbent resin. A piston and a load in which no gap was formed between the wall surface and the vertical movement was not hindered were placed in this order, and the mass Wa (g) of this measuring device set was measured.

  A glass filter with a diameter of 90 mm (manufactured by Mutual Riken Glass Co., Ltd., pore diameter: 100 to 120 μm) is placed inside a petri dish with a diameter of 150 mm, and a 0.9 mass% sodium chloride aqueous solution (saline) (20 to 25 ° C.) was added to the same level as the upper surface of the glass filter. On top of that, a sheet of filter paper having a diameter of 90 mm (manufactured by ADVANTEC Toyo Co., Ltd., product name: (JIS P 3801, No. 2), thickness 0.26 mm, retained particle diameter 5 μm) was placed so that the entire surface was wetted. Excess liquid was removed.

A set of measuring devices was placed on the wet filter paper, and the liquid was absorbed under load for a predetermined time. This absorption time was calculated from the start of measurement and was 1 hour later. Specifically, after 1 hour, the measuring device set was lifted and its mass W _b (g) was measured. This mass measurement must be performed as quickly as possible and without vibrations. Then, the absorption capacity under pressure (AAP) (g / g) was calculated from W _a and W _b by the following formula.

(5) Saline flow conductivity (SFC)
The saline flow conductivity (SFC) is a value indicating the liquid permeability when the water absorbent resin particles swell. It shows that it has high liquid permeability, so that the value of SFC is large.

  This was performed according to the saline flow conductivity (SFC) test described in JP-T 9-509591.

  Using the apparatus shown in FIG. 1, the water-absorbent resin particles (0.900 g) uniformly placed in the container 40 are swollen in an artificial urine under a pressure of 0.3 psi (2.07 kPa) for 60 minutes to obtain a gel of gel 44 The height of the layer was recorded, and then 0.69 mass% saline solution 33 was passed through the swollen gel layer from the tank 31 with a constant hydrostatic pressure under a pressure of 0.3 psi (2.07 kPa). The SFC test was performed at room temperature (20-25 ° C.). Using a computer and a balance, the amount of liquid passing through the gel layer at 20 second intervals as a function of time was recorded for 10 minutes. The flow rate Fs (T) passing through the swollen gel 44 (mainly between the particles) was determined in units of g / s by dividing the increased mass (g) by the increased time (s). Let Ts be the time at which a constant hydrostatic pressure and a stable flow rate were obtained, use only the data obtained between Ts and 10 minutes for the flow rate calculation, and use the flow rate obtained between Ts and 10 minutes. The value of Fs (T = 0), ie the initial flow rate through the gel layer, was calculated. Fs (T = 0) was calculated by extrapolating the result of the least squares method of Fs (T) versus time to T = 0.

here,
Fs (t = 0): flow rate expressed in g / s,
L0: height of the gel layer expressed in cm,
ρ: density of NaCl solution (1.003 g / cm ³ ),
A: Area above the gel layer in the cell 41 (28.27 cm ² ),
ΔP: Hydrostatic pressure applied to the gel layer (4920 dyne / cm ² ), and the unit of SFC value is (10 ⁻⁷ · cm ³ · s · g ⁻¹ ).

  In the apparatus shown in FIG. 1, a glass tube 32 is inserted into the tank 31, and the lower end of the glass tube 32 is 5 cm from the bottom of the swollen gel 44 in the cell 41 with 0.69 mass% saline 33. It was arranged so that it could be maintained at the upper height. The 0.69 mass% saline solution 33 in the tank 31 was supplied to the cell 41 through the L-shaped tube 34 with a cock. Under the cell 41, a container 48 for collecting the passed liquid is disposed, and the collection container 48 is installed on an upper pan balance 49. The inner diameter of the cell 41 is 6 cm. A 400 stainless steel wire mesh (aperture 38 μm) 42 was installed. There is a hole 47 sufficient for the liquid to pass through the lower part of the piston 46, and a glass filter 45 with good permeability is attached to the bottom so that the water absorbent resin particles or the swollen gel thereof does not enter the hole 47. It was. The cell 41 was placed on a table on which the cell was placed, and the surface of the table in contact with the cell was placed on a stainless steel wire mesh 43 that did not prevent liquid permeation.

  The artificial urine is composed of 0.25 g of calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, phosphoric acid Hydrogen diammonium 0.15g and pure water 994.25g are added.

(6) Soluble amount
Weigh out 184.3 g of 0.900 wt% sodium chloride aqueous solution in a plastic container with a lid of 250 ml capacity (diameter 6 cm x height 9 cm), add 1.00 g of particulate water-absorbing resin to the aqueous solution, and continue for 16 hours. By using a magnetic stirrer having a diameter of 8 mm and a length of 25 mm and stirring at a rotation speed of 500 rpm, a soluble component in the resin was extracted. 50.0 g of the filtrate obtained by filtering this extract using one filter paper (ADVANTEC Toyo Co., Ltd., product name: JIS P 3801 No. 2, thickness: 0.26 mm, reserved particle diameter 5 μm) A measurement solution was obtained.

  First, only 0.900 wt% sodium chloride aqueous solution was titrated to pH 10 with 0.1N NaOH aqueous solution, and then titrated to pH 2.7 with 0.1N HCl aqueous solution ([bNaOH]). ml, [bHCl] ml).

  The titration ([NaOH] ml, [HCl] ml) was determined by performing the same titration operation on the measurement solution.

  For example, in the case of a water-absorbing resin composed of a known amount of acrylic acid and its sodium salt, based on the average molecular weight of the monomer and the titration amount obtained by the above operation, the soluble content in the water-absorbing resin is expressed by the following equation: Can be calculated. In the case of an unknown amount, the average molecular weight of the monomer was calculated using the neutralization rate obtained by titration.

(Production Example 1)
To a kneader equipped with two sigma blades, an aqueous acrylate monomer solution (monomer concentration: 38 mass%, neutralization rate: 75 mol%) consisting of sodium acrylate, acrylic acid and water. It was prepared, and polyethylene glycol diacrylate (average number of ethylene oxide units: n = 8) as an internal cross-linking agent was dissolved to 0.05 mol% with respect to the monomer.

  Next, nitrogen gas was blown into the aqueous solution to reduce the oxygen concentration in the aqueous solution and to replace the entire reaction vessel with nitrogen. Subsequently, 0.05 mol% (based on monomer) sodium persulfate and 0.0006 mol% (based on monomer) L-ascorbine were used as polymerization initiators while rotating two sigma blades. An acid was added and stirring polymerization was performed in a kneader. After 40 minutes, a hydrogel polymer having an average particle diameter of 2 mm was obtained.

  The obtained hydrogel polymer was dried in a hot air dryer set at 170 ° C. for 45 minutes, and then the dried product was pulverized with a roll mill pulverizer and classified with a sieve having an opening of 850 μm, so that the particle size was 850 μm. By removing larger particles, a powdery water absorbent resin (A) as a base polymer was obtained.

  Various evaluation results of the water-absorbent resin (A) as the obtained base polymer are shown in Table 1.

  In addition, Table 2 shows the particle size distribution of the water-absorbent resin (A) as the obtained base polymer.

Example 1
10 g of the water-absorbing resin (A) as a base polymer was added to a quartz separable flask, and 1.05 g of a 23.8 wt% ammonium persulfate aqueous solution was added while stirring with a stirring blade. After stirring for 10 minutes, using an ultraviolet irradiation device (UV-152 / 1MNSC3-AA06) equipped with a metal halide lamp (USHIO, UVL-1500M2-N1), the irradiation intensity was 60 mW / cm ² . The surface-treated water-absorbing resin (1) was obtained by irradiating with ultraviolet rays at room temperature for a minute. Table 3 shows surface treatment conditions and water absorption characteristics.

(Example 2)
A surface-treated water-absorbing resin (2) was obtained in the same manner as in Example 1 except that 1.30 g of 38.5 wt% ammonium persulfate aqueous solution was used.

(Example 3)
A surface-treated water-absorbing resin (3) was obtained in the same manner as in Example 2 except that the irradiation time of ultraviolet rays was changed to 5 minutes.

Example 4
A surface-treated water-absorbent resin (4) was obtained in the same manner as in Example 1 except that 1.30 g of a 38.5% by weight sodium persulfate aqueous solution was used.

(Comparative Example 1)
A surface-treated comparative water-absorbing resin (1) was obtained in the same manner as in Example 2 except that heating was performed in a hot water bath at 80 ° C. for 10 minutes instead of irradiation with ultraviolet rays.

(Production Example 2)
A hydrogel polymer was obtained by performing polymerization in the same manner as in Production Example 1 except that the amount of the internal crosslinking agent in Production Example 1 was changed to 0.065 mol% with respect to the monomer. The obtained hydrogel polymer was dried in a hot air dryer set at 175 ° C. for 50 minutes, and then the dried product was pulverized with a roll mill pulverizer and classified with a sieve having an opening of 500 μm and a sieve having an opening of 300 μm. Then, by removing particles having a particle size larger than 500 μm and particles having a particle size smaller than 300 μm, a water-absorbing resin (B) as a base polymer was obtained.

  Various evaluation results of the water-absorbent resin (B) as the obtained base polymer are shown in Table 1.

  In addition, Table 2 shows the particle size distribution of the water-absorbent resin (B) as the obtained base polymer.

(Example 5)
A surface-treated water-absorbing resin (5) was obtained in the same manner as in Example 1 except that 10 g of the water-absorbing resin (B) was used as a base polymer and 1.3 g of a 38.5% by weight aqueous sodium persulfate solution was used. It was.

(Comparative Example 2)
A surface-treated comparative water-absorbing resin (2) was obtained in the same manner as in Example 5 except that 0.8 g of ion-exchanged water was used without using a water-soluble radical polymerization initiator.

(Comparative Example 3)
A comparative water-absorbent resin (3) was obtained in the same manner as in Example 5 except that, instead of irradiating with ultraviolet rays, heating was performed in a hot air dryer adjusted to 180 ° C. for 1 hour.

(Example 6)
A surface-treated water-absorbing resin (6) was obtained in the same manner as in Example 5 except that a mixed solution of 1.3 g of a 38.5 wt% aqueous sodium persulfate solution and 0.2 g of a 50 wt% aqueous aluminum sulfate solution was used. Obtained.

(Comparative Example 4)
A surface-treated comparative water-absorbing resin (4) was obtained in the same manner as in Example 5 except that 0.2 g of 50% by weight aqueous aluminum sulfate solution was used.

(Comparative Example 5)
A comparative water-absorbent resin (5) was obtained in the same manner as in Example 6 except that, instead of irradiating with ultraviolet rays, heating was performed in a hot air dryer adjusted to 180 ° C. for 1 hour.

(Production Example 3)
A hydrogel polymer was obtained by performing polymerization in the same manner as in Production Example 1 except that the amount of the internal crosslinking agent in Production Example 1 was 0.09 mol% based on the monomer. The obtained hydrogel polymer was dried in a hot air dryer set at 175 ° C. for 50 minutes, and then the dried product was pulverized with a roll mill pulverizer and classified with a sieve having an opening of 600 μm, so that the particle size was 600 μm. By removing larger particles, a particulate water-absorbing resin (C) as a base polymer was obtained.

  Various evaluation results of the particulate water-absorbing resin (C) as the obtained base polymer are shown in Table 1.

  Table 2 shows the particle size distribution of the obtained particulate water-absorbing resin (C) as the base polymer.

(Example 7)
A surface-treated water-absorbing resin was obtained in the same manner as in Example 5 except that 10 g of the water-absorbing resin (C) was used as the base polymer. The obtained water-absorbing resin was allowed to stand for 12 hours under reduced pressure in a vacuum dryer adjusted to 60 ° C. to obtain a water-absorbing resin (7). The obtained water-absorbent resin (7) had a solid content (defined by loss on drying at 180 ° C. for 3 hours) of 94.0%.

(Example 8)
A water-absorbent resin (8) was obtained in the same manner as in Example 7, except that a mixed solution of 1.3 g of a 38.5 wt% aqueous sodium persulfate solution and 0.2 g of a 50 wt% aqueous aluminum sulfate solution was used. The obtained water-absorbent resin (8) had a solid content (specified by loss on drying at 180 ° C. for 3 hours) of 93.3%.

Example 9
Example 7 except that 1.3 g of a 38.5% by weight aqueous sodium persulfate solution and 0.2 g of a 5: 1 mixed solution of a 50% by weight aqueous aluminum sulfate solution and a 50% by weight aqueous sodium lactate solution were used. In the same manner as described above, a water absorbent resin (9) was obtained. The obtained water-absorbent resin (9) had a solid content (specified by loss on drying at 180 ° C. for 3 hours) of 93.7%.

  Various evaluation results of the surface-treated water-absorbing resin thus obtained are shown in Table 1.

  According to the present invention, when modifying a water-absorbent resin, it can be sufficiently surface-treated even at a reaction temperature near room temperature, and the resulting modified water-absorbent resin is excellent in water-absorbing properties, so that it can be used as a paper diaper or the like. Can be industrially useful.

It is the schematic of the measuring apparatus used for the measurement of a saline flow conductivity (SFC).

Explanation of symbols

31 ... Tank,
32 ... Glass tube,
33 ... 0.69 mass% sodium chloride aqueous solution,
34 ... L-shaped tube with cock,
35 ... cook,
40 ... container,
41 ... cell,
42 ... stainless steel wire mesh,
43 ... stainless steel wire mesh,
44 ... swelling gel,
45 ... Glass filter,
46 ... piston,
47 ... hole in piston,
48 ... collection container,
49...

Claims (11)

A method for producing a modified water absorbent resin,
a) Without adding an ethylenically unsaturated monomer, mixing the water-absorbent resin and the water-soluble radical polymerization initiator,
b) irradiating the resulting mixture with active energy rays;
A process for producing a modified water-absorbent resin, comprising:   The method for producing a modified water-absorbent resin according to claim 1, wherein the water-soluble radical polymerization initiator is a persulfate.   The method for producing a modified water-absorbing resin according to claim 1 or 2, wherein an amount of the water-soluble radical polymerization initiator added to 100 parts by mass of the water-absorbing resin is 0.01 to 20 parts by mass.   The method for producing a modified water-absorbing resin according to claim 1, wherein the water-soluble radical polymerization initiator is mixed in an aqueous solution.   In mixing the water absorbent resin and the water-soluble radical polymerization initiator, 1 to 20 parts by mass of water is further mixed with 100 parts by mass of the water absorbent resin. A method for producing a modified water absorbent resin.   The method for producing a modified water-absorbent resin according to claim 1, wherein the active energy ray is ultraviolet rays.   The modified water-absorbent resin according to any one of claims 1 to 6, wherein the water-absorbent resin is a powdered water-absorbent resin obtained by polymerizing a monomer mainly composed of acrylic acid (salt). Method.   The method for producing a modified water-absorbent resin according to any one of claims 1 to 7, wherein the water-absorbent resin contains 90 to 100% by mass of particles having a particle size in the range of 150 to 850 µm.   The modified water-absorbent resin according to any one of claims 1 to 8, wherein the absorption capacity under pressure of 4.83 kPa of the water-absorbent resin is 1 g / g higher than the absorption capacity under pressure before modification. Production method.   The method for producing a modified water absorbent resin according to any one of claims 1 to 9, wherein the water absorbent resin after modification has an absorption capacity under pressure of 4.83 kPa of 8 to 40 g / g. The method for producing a water absorbent resin according to claim 1, wherein the modified water absorbent resin has a saline flow conductivity of 10 (10 ⁻⁷ · cm ³ · s · g ⁻¹ ) or more.

Images