Source: https://patents.google.com/patent/TWI406883B/en
Timestamp: 2020-01-24 06:19:56
Document Index: 227509714

Matched Legal Cases: ['Application No. 2005', 'Application No. 2005', 'Application No. 2005', 'Application No. 2005', 'Application No. 2005', 'Application No. 2005', 'Application No. 2005', 'Application No. 2005', 'Application No. 2005', 'Application No. 2005']

TWI406883B - Water absorbing agent, water absorbing article and method for production of water absorbing agent - Google Patents
Water absorbing agent, water absorbing article and method for production of water absorbing agent Download PDF
TWI406883B
TWI406883B TW95104551A TW95104551A TWI406883B TW I406883 B TWI406883 B TW I406883B TW 95104551 A TW95104551 A TW 95104551A TW 95104551 A TW95104551 A TW 95104551A TW I406883 B TWI406883 B TW I406883B
TW95104551A
TW200706566A (en
Fujino Shin-Ichi
2006-02-10 Application filed by Nippon Catalytic Chem Ind filed Critical Nippon Catalytic Chem Ind
2007-02-16 Publication of TW200706566A publication Critical patent/TW200706566A/en
2013-07-19 First worldwide family litigation filed litigation Critical https://patents.darts-ip.com/?family=36407948&utm_source=google_patent&utm_medium=platform_link&utm_campaign=public_patent_search&patent=TWI406883(B) "Global patent litigation dataset” by Darts-ip is licensed under a Creative Commons Attribution 4.0 International License.
2013-09-01 Publication of TWI406883B publication Critical patent/TWI406883B/en
239000006096 absorbing agents Substances 0 title 2
Water absorbing agent, water absorbing material and method for producing water absorbing agent
The present invention relates to a water absorbing agent, a water absorbing material, and a method of producing a water absorbing agent.
In recent years, water absorbing agents for absorbing a large amount of water have been widely used in sanitary articles such as disposable diapers, sanitary napkins, and adult diapers. The polyacrylic acid crosslinked polymer which is usually neutralized in such a water absorbing agent is used in various forms such as a sheet form, a fiber form, and a film form.
The characteristics of such water absorbing agents include water absorbing properties such as high absorbency when unstressed and stressed. In addition, when applied to sanitary articles such as disposable diapers, hygiene and taste problems must also be considered. Hygiene and taste problems are related to the water absorbing material itself. Among them, unreacted residual trace monomers in the water absorbing agent are the most important factors. Therefore, the amount of residual monomers in the water absorbing agent must be as low as possible.
The method for reducing residual monomers in the water-absorbing resin comprises: (1) increasing the ratio of the radical polymerization initiator added to the monomer solution by more than two times or more (JP-B No. S63-7203), ( 2) After polymerization at the time of polymerization, a radical polymerization initiator is added to the water gel polymer (JP-A No. 2004-517179), (3) a reducing agent is added to the water gel polymer before drying or drying. Medium (JP-B No.H7-98847). And (4) adding a reducing agent when the water-absorbing resin is mixed with the aqueous surface crosslinking agent solution (JP-A No. H04-106108), and (5) a water-absorbing resin and a mixed activator when the surface crosslinking treatment is completed, for example When a surfactant and a small particle insoluble in water are mixed, a reducing agent (International Publication No. 91/03497) or the like is added.
Although the amount of residual monomers can be reduced by the methods (1) and (2) above, the obtained water-absorbing resin has a color problem. Therefore, the water-absorbing resin can be seen from the upper layer of the absorbent, so that the value of the product becomes low. In addition, due to the severe process conditions at the time of reducing the amount of residual monomers, the polymer is deteriorated and the physical properties are deteriorated. Therefore, it is difficult to balance the reduction of the residual monomer while maintaining or improving the absorption, such as the absorption force without force and the absorption force at the time of stress. In the methods (3) and (4) above, there is a problem that odor is released upon subsequent heat treatment, which may be caused by the reaction of the added reducing agent with the impurity component in the raw material to produce a new product. Further research is needed. Therefore, these processes are not suitable for producing absorbent articles such as disposable diapers. Further, in the method of the above (5), after the water absorbing agent is removed from the absorbent such as a disposable diaper, the amount of residual monomers in the absorbent is measured, and it is found that although the step of reducing the amount of residual monomers has been passed, the absorbent The amount of residual monomer in the water absorbing agent will still vary, and the residual monomer does not fall to the required standard or exceeds the specification. This causes problems in taste and hygiene, or products that are out of specification. In some applications, it is required to reduce the thickness of the absorbent, which causes a decrease in the amount of hydrophilic fibers such as wood pulp, which constitutes the absorbent, and causes the water absorbent to The possibility of skin contact is greatly increased.
SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a water absorbent having a low residual monomer content, a small variation in residual monomer amount in a different particle size distribution range, a preferred absorbent property, and which can be used as a sanitary article, and a water absorbing agent and a water absorbing material.
In order to solve the above problems, the present inventors have found that variations in the amount of residual monomers of water absorbing agents in absorbent articles such as disposable diapers are derived from variations in the particle size distribution of the water absorbing agent because the amount of residual monomers in the water absorbing agent having different particle size distribution ranges is different. Therefore, the amount of residual monomer is related to the variation of the particle size distribution. Further, in the particles of the water absorbing agent, particles having a small particle diameter have a large amount of residual monomers.
More specifically, when the powdery water absorbing agent is supplied to the water absorbing material from the apparatus for producing an absorbent such as a disposable diaper, the water absorbing agent produces a remarkable particle size separation. Therefore, the amount of monomers remaining in the water absorbing agent in different absorbent materials such as disposable diapers may be different, and the particle size distribution may also be different. Further, if the particle diameter is used to measure the amount of residual monomers in each water absorbing agent, it is found that the amount of residual monomers is different in different particle size distribution ranges. More specifically, although the amount of residual monomers is not much different among particles having the same particle size range; for example, in particles having a particle diameter of not less than 300 μm, residues of particles having a particle diameter of not less than 300 μm in the same disposable diaper The amount of monomer and residual monomer having a particle diameter of less than 300 μm is different, and it is shown that there are differences in different particle size distribution ranges. In other words, the amount of residual monomer in the water absorbing agent varies in different particle size distribution ranges, and the variation of the residual monomer of the water absorbing agent in the water absorbing agent shows that the particle size separation results in different particle size distribution ranges, and there are different residues. The amount of monomer.
The reasons for the difference in the amount of residual monomers in the range of different particle size distributions are as follows. In the case of water-absorbing resin obtained by aqueous polymerization, the amount of residual monomers in the water-gel polymer after completion of polymerization is usually from 50,000 to 100,000 ppm, but the amount of residual monomers after the polymer is dried becomes several hundred ppm. This is because the polymerization initiator remaining in the water-gel polymer particles is polymerized with the residual monomers at a high temperature during drying. At this time, the water vapor content on the surface of the polymer gel and the small gel particles is rapidly lowered due to rapid drying, so that the residual initiator does not function, or the monomer and the residual initiator are reduced due to the decrease in the moisture content. The probability of action. As a result, the polymerization of the residual monomers is terminated. Therefore, a part of the monomer which is easily dried may leave a large amount of monomers, particularly such as a surface of a dried product and small polymer gel particles. At the same time, the monomer gas evaporated during drying is absorbed by the surface of other dried materials, resulting in excessive monomer content on the surface of the dried object. Moreover, upon drying, the polymer which completes the polymerization and is in contact with oxygen terminates the polymerization of the residual monomers, so that the initiator radicals on the surface of the gel and the small gel particles cannot react due to the action of oxygen in the air.
As described above, in the step of drying the water-gel polymer, the amount of residual monomers in the gel particles is also different, so that even in a single particle, the content of the residual monomer is high on the surface, thereby causing a difference in content. .
In addition, when the dried material of the water-gel polymer needs to be ground, the small particles may contain a large amount of residual monomers, and although it is related to the grinding equipment used, the grinding usually destroys a portion of the surface containing a large amount of residual monomers.
In the case of a water-absorbing resin obtained by inverse suspension polymerization, it is usually not necessary to carry out a grinding step, however, the amount of residual monomers may vary depending on the particle size distribution. Since water and a suspension solvent (cyclohexane, n-hexane, etc.) are removed by distillation after completion of the polymerization, part of the residual monomers are removed together with the solvent. This will result in differences in residual monomers in different particle size distribution ranges. Even if the amount of residual monomer is independent of the state of the gel, the surface area per unit volume between the small particles and the large particles will be different. Since the amount released from the inside of the gel is related to the surface area, the large particles are more likely to contain residual monomers.
According to the above description, the present invention introduces the "residual monomer index (RMI: residual monomer index) of different particle size distribution ranges" calculated according to the formula (1) as a range of control of different particle size distribution, residual monomers in the water absorbing agent. The index of the amount variation is controlled by the residual monomer index within a desired range to achieve a low residual monomer content required for sanitary products and a water absorbing agent having a small variation in residual monomer amount in a range of different particle size distributions.
RMI=|RM 1 -RM 2 |/RM A (1)
Wherein RM 1 represents the amount of residual monomers in the water absorbing agent having a particle diameter of less than 300 μm; RM 2 represents the amount of residual monomers in the water absorbing agent having a particle diameter of 300 to 850 μm; and RM A represents the amount of residual monomers in the water absorbing agent; . When RM A is 0, the RMI should also be zero. |RM 1 -RM 2 | indicates the absolute value of (RM 1 -RM 2 ).
The water absorbing agent of the present invention is a water absorbing agent containing a particulate water-absorbing resin as a main component, which has a crosslinked structure extended by constituent units of unsaturated carboxylic acid and/or a salt thereof, which is a surface cross-linking agent pair The surface is subjected to surface crosslinking treatment, and the particles constituting the water absorbing agent have a particle diameter of 300 to 850 μm and less than 300 μm, wherein the amount of residual monomers is not less than 0 but not higher than 500 ppm, and is calculated from the above formula (1). The residual monomer index (RMI) is not more than 0.30.
The water absorbing material according to the present invention has the above water absorbing agent.
The method for producing a water absorbing agent according to the present invention comprises: (1) a step of polymerizing an aqueous monomer solution containing an unsaturated carboxylic acid and/or a salt thereof and a crosslinking agent to prepare a water gel polymer; and (2) drying the water gel The polymer is then ground and classified to adjust the preparation step of the granular water-absorbing resin precursor having a particle diameter of 300 to 850 μm and a particle diameter of less than 300 μm as a main component, and (3) heating the mixture of the water-absorbing resin precursor and the surface crosslinking agent. a step of forming an ester bond on the surface of the water-absorbing resin precursor to prepare a water-absorbing resin, (4) a step of adding an aqueous solution containing a sulfur reducing agent to the water-absorbing resin by spraying, and (5) not lower than A step of heat-treating a mixture of water-absorbing resin and the aqueous solution in hot air at 40 ° C but not higher than 120 ° C.
The water absorbing agent of the present invention contains a low amount of residual monomers and a residual monomer amount having a small variation in a range of particle size distribution, and when applied to a water absorbing material, variation in residual monomer amount in the water absorbing material is reduced due to particle size separation. . Therefore, it is possible to apply the absorbent of the thin sanitary article widely used in recent years.
As described above, the inventors have found that residual monomers in the water absorbing agent are not uniformly present in the water absorbing agent particles, but are particularly present in the water absorbing agent obtained by water polymerization, in the surface portion with respect to the water absorbing agent particles. There will be more residual monomers in the small particles. Conventional methods for reducing residual monomers, assuming that residual monomers are uniformly distributed throughout the particles and particles, in order to reduce internal residual monomers, excessive treatment is performed to reduce residual monomers of the entire water absorbing agent (particle interior, large particles) . This will destroy various physical properties of the water absorbing agent. It is therefore difficult to achieve balance in reducing residual monomers and preferred physical properties such as high force absorption and suppression of discoloration. However, according to the production method of the present invention, residual monomers in the surface and small particle water absorbing agent can be selectively treated, and thus can be optimized in terms of residual monomers and preferable physical properties.
The production method of the water absorbing agent of the present invention is not particularly limited. In the method of the present invention which can selectively treat residual monomers in the surface and small particles of the water absorbing agent, for example, the production method of the present invention can be used (after the surface crosslinking treatment, a small amount of an aqueous solution containing a sulfur reducing agent is added and The heat treatment is carried out under the conditions required, and the method disclosed in Japanese Patent Application No. 2005-03812 can also be used. According to the production method of the present invention, the purpose of uniformly adding the surface cross-linking can be achieved by spraying a small amount of the above aqueous solution. Further, according to the production method using only a small amount of aqueous solution according to the present invention, the aqueous solution tends to act on the surface and small particles, and therefore can be selectively heat-treated (dried) on the surface and the small particles of the water absorbing agent by the required hot air. Residual monomer in.
As described above, according to the production method of the water absorbing agent of the present invention, the aqueous solution containing the sulfur reducing agent is uniformly mixed with the water-absorbing resin by spraying and heat-treated. Therefore, in the water-absorbing resin in which the residual monomer amount varies depending on the particle size distribution range, the sulfur-containing reducing agent can effectively react with the residual monomer, so that a residual monomer having a low residual amount and a residual particle size range can be obtained. A small amount of water absorbing agent. Therefore, the water absorbing agent produced does not discolor, does not generate an odor, and has preferable physical properties without affecting the absorption property particularly under stress due to deterioration of the polymer.
Since the water absorbing material of the present invention contains a low amount of residual monomers and a water absorbing agent having a small amount of residual monomer variation in a different particle size range, it can be used in sanitary articles because it does not discolor, and there is no fear of residual monomers caused by particle size separation. Amount of variation and specifications outside the product.
Next, a method for producing a water absorbing agent, a water absorbing material and a water absorbing agent of the present invention will be described based on preferred embodiments, however, the scope of the present invention is not limited to the description. Other additional examples, or variations that do not depart from the scope of the invention, are included in the scope of the invention.
The first step of the method for producing a water absorbing agent according to the present invention is a step of polymerizing a monomer containing an unsaturated carboxylic acid and/or a salt thereof to produce a water gel polymer (hereinafter referred to as a polymer gel). The unsaturated carboxylic acid to be used is not particularly limited as long as it contains one or more carboxyl groups in one molecule, and examples thereof include acrylic acid, methacrylic acid, maleic acid, itaconic acid, cinnamic acid, crotonic acid and the like. From the viewpoint of the characteristics and cost of the water absorbing agent, it is preferred to use acrylic acid and/or a salt thereof as a main component. Acrylic acid and/or its salt as a main component, that is, a polyacrylic acid (partially neutralized) cross-linked polymer as a main component, and in addition to the above-mentioned unsaturated carboxylic acid, it can be used together with other monomers. . Such monomers include: anionic unsaturated monomers such as maleic anhydride, vinyl sulfonic acid, propylene toluene sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, 2-methyl acrylamide phthalamide-2-methylpropane Sulfonic acid, 2-methylpropaneethanesulfonic acid, 2-methylpropanepropanesulfonic acid and 2-hydroxyethylmethylacrylic acid phosphate, and salts thereof; sulfhydryl groups containing unsaturated monomers; a phenolic hydroxyl group; a guanamine group containing an unsaturated monomer such as methacrylamide, N-ethyl methacrylamide and N,N-dimethyl methacrylamide; Amino groups of saturated monomers such as N,N-dimethylaminoethyl methacrylic acid, N,N-dimethylaminopropyl methacrylic acid and N,N-dimethylaminopropyl group Acrylamide; and the like. These monomers may be used alone in combination with the above unsaturated carboxylic acid and/or a salt thereof, or two or more of these monomers may be used in combination.
Salts of unsaturated carboxylic acids such as acrylic acid include sodium salts, lithium salts, potassium salts, ammonium salts and amine salts, and the like. Among them, from the viewpoint of cost, it is preferred to use a sodium salt. The amount of acrylic acid and/or its salt is desirably not less than 70% molar ratio, preferably not less than 80% molar ratio, even preferably not less than 90% molar ratio per monomer component, and Not less than 95% molar ratio (except for the internal cross-linking agent described later). The upper limit is 100% molar ratio. The neutralization ratio is not particularly limited if the monomer is an acid group-containing monomer including acrylic acid, and may be neutralized after polymerization if necessary. In applications where contact with the human body, such as hygiene products, the non-necessity of neutralization after polymerization must be considered, the neutralization ratio is preferably not less than 40% molar ratio but not more than 90% molar ratio, and the lower limit is 50% molar ratio and upper limit is 80% molar ratio.
The polymerization may be carried out by aqueous polymerization or reversed phase suspension polymerization. Although the concentration of the aqueous monomer solution is not particularly limited, it is preferably not less than 10% by weight but not more than 70% by weight, preferably not less than 20% by weight but not more than 60% by weight. When performing aqueous polymerization or reverse phase suspension polymerization, a solvent other than water can be used. The type of solvent is not particularly limited. The concentration of the aqueous monomer solution is usually expressed by the solid content of the water-based gel polymer. The solid content of the water-gel polymer can be determined from the residual product obtained by drying at 180 ° C for 6 hours.
A radical polymerization initiator can be used for the polymerization. The radical polymerization initiator is not particularly limited, but one or more initiators commonly used to produce a water absorbing agent may be selected depending on the type of the polymerization monomer and the polymerization conditions. For example, persulfates in hot starters such as sodium persulfate, potassium persulfate and ammonium persulfate; peroxides such as hydrogen peroxide, t-butyl peroxide and methyl ethyl ketone peroxide; azo a compound such as an azonitrile compound, an azomethine compound, a cycloazolyl compound, an azoamine bond compound, an alkylazo compound, 2,2'-azobis(2-amidinopropane) Hydrogen chloride (2,2'-azobis(2-amidinopropane)dihydrochloride) and 2,2'azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride) (2,2'-azobis[ 2-(2-imidazoline-2-yl)propane]dihydrochloride); and photoinitiators such as benzoin extensions, benzene extensions, acetophenone extensions, benzophenone extensions, azo compounds, and the like. From the standpoint of cost and benefit of reducing residual monomers, it is recommended to use a hot starter, preferably a persulphate.
It is also possible to use a redox initiator together with a reducing agent to accelerate the decomposition of the above radical polymerization initiator. Such reducing agents include sulfites and bisulfites such as sodium sulfite and sodium hydrogen sulfite, L-ascorbic acid (ascorbate), reducing metals (salts) such as ferrous salts, amine salts and the like. Preferably, the photoinitiator is used with a thermal initiator. In this polymerization step, the amount of the radical polymerization initiator per 100 parts by weight of the monomer is desirably not less than 0.001 part by weight but not more than 2 parts by weight, and the lower limit is 0.01 part by weight, and the upper limit is 0.05 part by weight. The amount of the radical polymerization initiator to be used may not be less than 0.001 part by weight because the amount of the residual monomer in the water absorbent increases as the amount of the unreacted monomer is too large. In contrast, it is not desirable to use more than 2 parts by weight because of the water absorption characteristics, particularly when the water absorbing agent is stressed, the absorption force is lowered. In addition, it may cause discoloration of the water absorbing agent. In the present invention, the residual monomer can be removed using mild processing conditions, so that the conventional physical property degradation due to severe polymerization conditions and drying conditions can be avoided. In the polymerization step, in addition to the use of a radical polymerization initiator, the polymerization reaction can be caused by irradiation of high-energy light such as radiation, electron beam, ultraviolet light or the like.
Since the water absorbing agent of the present invention is a crosslinked structure produced by unsaturated carboxylic acid and/or a salt thereof, a self-crosslinking type crosslinked polymer is formed in the polymerization step or a crosslinked polymer is formed with an added internal crosslinking agent. . The condition to be an internal crosslinking agent is that it is necessary to have two or more polymerizable unsaturated groups or reactive groups in one molecule. A material that can be used as an internal crosslinking agent such as N,N'-methylenebis(meth)acrylamide, polyethylene glycol dimethacrylate, polypropylene ethylene Alcohol dimethacrylic acid, trimethylolpropane trimethacrylic acid, glyceryl trimethacrylate, propylene glyceryl methacrylate, ethylene oxide modified trimethylolpropane trimethacrylate, pentaerythritol Pentaerythritol hexamethyacrylate, triacryl cyanurate, tripropyl methacrylate, triacrylophosphoric acid, cyanuric acid, polyallyloxyalkane, Polyethylene glycol diepoxypropyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1,4-butanediol, Isopentanol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethylenimine, glycidyl methacrylate, and the like. One or two or more of them can be used. However, a crosslinkable monomer such as an internal crosslinking agent may be added after one or several times of polymerization to form a crosslinked structure. In addition, it can also be added to the water-gel polymer after polymerization to cause cross-linking.
The water-absorbing resin of the present invention is not limited to the production method disclosed above, and a cross-linked polymer having the above monomer as a constituent unit can be used. For example, the above-mentioned monomer is used as a constituent unit, and a polymerizable double bond structure in each monomer is opened to carry out a polymerization reaction (C=C double bond becomes a -C-C-single bond).
The amount of the internal crosslinking agent may be arbitrarily determined depending on the desired characteristics of the water absorbing agent, but is generally not desirably less than 0.001 mole per 100 moles of monomer, but not more than 5 moles. When the amount of internal cross-linking agent is less than 0.001 mol, the gel strength will decrease and the extractables will increase. In contrast, the absorbency decreases when the amount is above 5 moles. The internal crosslinking agent can be added to the reaction system in one or several portions. In the polymerization step, various forming agents such as hydrogencarbonate, carbon dioxide, azo compounds and inert organic solvents (for example, about 0 to 30 parts by weight per 100 parts by weight of monomers); hydrophilic polymers such as starch and cellulose, starch And a cellulose derivative, a polyvinyl alcohol, a polyacrylic acid (acrylic ester) or a polyacrylic acid (acrylate) crosslinked polymer (for example, about 0 to 30 parts by weight per 100 parts by weight of the monomer); various surfactants; A transfer agent such as hypophosphorous acid (sub-phosphoric acid) (for example, about 0 to 1 part by weight per 100 parts by weight of the monomer) can be added to the reaction system as necessary.
Although the polymerization temperature is not particularly limited, it is generally not desirable to be lower than 10 ° C but not higher than 140 ° C. When the polymerization temperature is lower than 10 ° C, the polymerization time becomes too long to cause a decrease in the yield, and the physical properties of the water-absorbing resin are also deteriorated. In contrast, when the polymerization temperature exceeds 140 ° C, the physical properties of the water-absorbing resin may also be deteriorated. The polymerization time is not particularly limited, but it must be determined depending on the type of the monomer, the polymerization initiator, the polymerization temperature, and the like. Moreover, the polymerization can be carried out under normal pressure equipment for ease of operation or at low pressure to lower the boiling temperature of the polymerization system.
The water gel polymer solid content obtained by concentration polymerization of the above aqueous monomer solution is not less than 10% by weight but not more than 70% by weight, and after drying, grinding and classification, a water-absorbing resin precursor can be obtained (No. Two steps). The water gel polymer preferably performs the second step with a particle size of 0.1 to 5 mm. The drying method is not particularly limited, and a usual method such as a general drying oven or a heating oven can be employed. The drying temperature is preferably not lower than 100 ° C but not higher than 250 ° C. The lower limit is 120 ° C, preferably 150 ° C, and the upper limit is desirably 220 ° C, preferably 200 ° C. The drying time is not particularly limited, but the time is determined according to the solid content required for the dried product, for example, not less than 80% by weight, or 85 to 100% by weight, and preferably 90 to 100% by weight. . The weight of the residue obtained by heating and drying at 180 ° C for 3 hours is divided by the weight before drying to obtain a solid content after drying, preferably not less than 90% by weight to facilitate grinding. Although it must be determined according to the particle size of the polymer gel, the drying temperature, and the gas flow rate used, the drying time is generally increased within two hours to increase the production efficiency.
The dried polymer needs to be ground. Although it is generally a gel polymer dried product obtained in the polishing and drying step, it may be polished before the polymer is dried. It is preferred to grind to obtain particles of the desired particle size and to select suitable milling conditions to produce more particles having a particle size of 150 to 850 μm. The grinding method is not particularly limited and any known method can be used.
If it is granular, the particle size distribution can be controlled by dispersion polymerization and dispersion drying, such as reversed phase suspension polymerization. However, in the case of aqueous polymerization, it can be ground and classified after drying to obtain a desired particle size distribution. The product obtained by the grinding step must be classified to obtain particles having a mass median diameter (D50) of not less than 200 μm but not more than 850 μm, thereby obtaining a desired particle size distribution. Grading can be done in any conventional manner.
In order to obtain the water absorbing agent of the present invention, the mass median diameter (D50) of the particle size of the ground product must fall within a small range of 200 to 850 μm or 200 to 710 μm, or even 250 to 600 μm, preferably 300 to 500 μm. Further, the content of particles smaller than 150 μm is desirably controlled at 0 to 5% by weight, or 0 to 3% by weight, preferably 0 to 2% by weight, or even 0 to 1% by weight. Further, in the above-mentioned abrasive product, the weight ratio of the particles having a particle diameter of 300 to 850 μm to the particle diameter of less than 300 μm is not less than 5/95 but not more than 95/5. Further, it is desirable to control the particle diameter to not less than 150 μm but not more than 600 μm, and the weight ratio is not less than 90% by weight of the abrasive product, or not less than 95% by weight, preferably not less than 98% by weight.
For example, in order to adjust the particle size distribution to reduce the amount of particles smaller than 150 μm and control the mass median diameter (D50) to fall within a small range of 250 to 600 μm, it is necessary to remove large particles and small particles by ordinary equipment such as a sieve after grinding. Grading. This step removes large particles with a particle size between 600 and 5000 μm or particles with a particle size between 850 and 2000 μm. Further, the particle size of the removed small particles is preferably less than 150 μm or less than 200 μm. Therefore, the large particles that are screened out can be directly discarded, but the above grinding step is generally repeated.
The selected small particles may be directly discarded, or may be formed into larger particles by a granulation step or agglomerated into a particulate state for use in the water-absorbing resin of the present invention. The method can be used in any of the steps disclosed in U.S. Patent Nos. 6,229,930, 5,264,495, 4,506, 062, 5, 478, 879 and European Patent 844 270, and Japanese Patent Application No. 2005-38123.
According to the procedure disclosed in Japanese Patent Application No. 2005-38123, when small particles containing a large amount of residual monomers are recovered, small particles are recovered after reducing the amount of residual monomers. This method can be used as one of the methods for obtaining the water absorbing agent of the present invention.
In the method disclosed in Japanese Patent Application No. 2005-38123, instead of adding a radical polymerization initiator to a polymer gel, a thermal initiator (thermal decomposition radical polymerization initiator) is added to produce water absorption. The agglomerated particles obtained from the small particles in the resin are dried to allow the agglomerated particles to coexist with the polymer gel. The residual monomer can be more effectively reduced by adding the same amount (weight per unit of water-absorbing resin obtained per unit) as compared with the case of directly adding a thermal initiator to the polymer gel. The same improvement can be achieved when an oxidizing agent or a reducing agent is used in place of the hot initiator.
In particular, the method for producing a water-absorbing resin disclosed in Japanese Patent Application No. 2005-38123 includes a step of preparing a water-absorbent polymer gel; and preparing an agglomerated gel by adding an aqueous solution of a hot starter, an oxidizing agent, and a reduction a step of adding at least one additive to the small particles produced by the water-absorbing resin and a mass median diameter falling within a range of 10 to 150 μm; and a step of allowing the agglomerated gel and the polymer gel to coexist when dried.
According to the method disclosed in Japanese Patent Application No. 2005-38123, an equilibrium can be achieved between reducing the small particles obtained in the production step and lowering the amount of residual monomers in the water-absorbing resin. More particularly, according to the invention, the water-absorbing resin containing a small amount of residual monomers has better physical properties and effectively utilizes small particles generated in the water-absorbing resin process without reducing residual monomers (heating The amount of the additive is increased by one of the initiator, the oxidizing agent and the reducing agent. In other words, this method of producing a water-absorbing resin can effectively reduce residual monomers, and it is also possible to recover small particles using a conventional agglomeration method in terms of production cost. Therefore, the production method disclosed in Japanese Patent Application No. 2005-38123 can be one of the examples of the production method of the water absorbing agent of the present invention.
Next, the production method disclosed in Japanese Patent Application No. 2005-38123 will be described as an example of a production method of the water absorbing agent of the present invention. However, the production method is not limited to the following description, and the following examples can be changed and changed without departing from the scope of the invention.
The production method includes a step of preparing a water-absorbent polymer gel (hereinafter also referred to as "polymerization step"). Typical subsequent steps include, but are not limited to, a step of drying a polymer gel (hereinafter also referred to as a "drying step"), a step of grinding a polymer gel dried product (hereinafter also referred to as "grinding step"), and a stepwise ground product. The step (hereinafter also referred to as "classification step"). The production method may adopt a method suitable for continuously producing water-absorbing resin, wherein water having a small amount of residual monomers can be obtained by reusing small particles obtained in the production of water-absorbing resin. A resin (for example, small particles removed in the above-mentioned classification step waste, etc.).
More importantly, in the production method, it is necessary to add an aqueous solution (hereinafter also referred to as "essential additive") containing at least one of a hot initiator, an oxidizing agent and a reducing agent to the small particles obtained by producing the water-absorbing resin. The mass median particle size of the small particles falls within the range of 10 to 150 μm, thereby obtaining an agglomerated gel (hereinafter also referred to as "agglomerate gel" or "agglomerated particles"), and drying the agglomerated gel and the polymer gel. They coexist. In other words, the agglomerated particles and the aqueous solution obtained by mixing the small particles must be dried together with the polymer gel obtained in the polymerization step.
In the above method for producing agglomerated particles, the particles may have a plurality of small particles, and the median diameter of the agglomerated particles is not more than 20 mm, and is desirably 0.3 to 10 mm, preferably 0.35 to 5 mm. Therefore, when a large number of group gels are obtained, the mixed small particles and aqueous solution must be dried and ground.
The water content of the agglomerated particles is desirably not more than 75% by weight, preferably not more than 70% by weight, or even not more than 65% by weight to shorten the drying time (the lower limit is greater than 0% by weight, preferably not Less than 5% by weight). When the water vapor content of the agglomerated particles is higher than the water vapor content of the polymer gel, the problem of incomplete drying may occur when dried together with the polymer gel. According to the production method, the inventors have found that when the agglomerated particles having a low moisture content and the polymer gel are dried together by a known technique, the amount of residual monomers of the water-absorbing resin is remarkably increased, and therefore, it is necessary to maintain the high concentration of the agglomerated particles. The moisture content, although this heats the residual monomer in a subsequent drying step. However, according to the production method, even if the water vapor content is relatively low, the residual monomer can be reduced. When the water vapor content is within the above range, the production method can be used to obtain significant benefits.
The present production method can obtain small particles having a particle diameter smaller than that of the water-absorbing resin, and these small particles are generally regarded as waste as described above. It is generally desired that the mass median particle size (D50) of the water-absorbing resin (classified according to JIS standard sieve) is in the range of 200 to 800 μm, and the mass median particle diameter (D50) of the water-absorbing resin obtained by the production method is 200. ~450μm. These small particles are the remaining residues, so that the mass median diameter (D50) of the obtained water-absorbing resin falls within the above-mentioned required range, particularly in the range of 10 to 150 μm as described above. It is generally desired that the particle size is much smaller than 150 μm (classified according to JIS standard screen) and the content is 70 to 100% by weight or even 90 to 100% by weight. Further, it is desirable that the shape of the small particles obtained by the polymerization of the aqueous system is irregular rather than the spherical shape obtained by inverse suspension polymerization to increase the agglomeration strength. Moreover, the small particles can be selected for the surface cross-linking treatment which is conventionally used in the production of water-absorbing resins or mixtures thereof.
All of the small particles obtained by the production of the water-absorbing resin can be agglomerated as described above. The small particles obtained in the usual classification step can be used first, and are not limited to the small particles obtained in the classification step, and the small particles removed by the sieve or other production steps can also be used in the agglomeration step. In addition, small particles obtained in different steps, or small particles obtained in separate processes (other production equipment) can also be used in combination. In addition, the composition of the small particles may be the same as or different from the polymer gel that is dried together. However, the small particles dried together are preferably the same as the polymer gel component.
In terms of aqueous solution mixing and drying efficiency, the temperature of the small particles is desirably not lower than 35 ° C, or 40 to 100 ° C, preferably 45 to 80 ° C. The temperature of the small particles can be arbitrarily adjusted by constant temperature, heating, cooling, or the like in each step of producing the water-absorbing resin.
The aqueous solution used to agglomerate the small particles is to dissolve the above-mentioned necessary additives in a solvent. The solvent is not particularly limited, but may be, for example, water, an aqueous solution including a hydrophilic organic solvent (for example, a lower alcohol such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol and tert-butanol; ketones such as Acetone; ethers such as dioxane and tetrahydrofuran; amines such as N,N-dimethylformamide; hydrazines such as dimethyl hydrazine, etc. The range of water content in the solvent in terms of physical properties and agglomeration strength In the weight ratio of 90 to 100%, preferably 99 to 100% by weight, even more preferably only water. Moreover, the aqueous solution may contain a small amount of other additives such as a crosslinking agent, a chelating agent, without impairing the production method. Surfactant. Crosslinking agents which can be used are various surface crosslinking agents as described later. By adding a crosslinking agent to an aqueous solution, the water-soluble component is lowered and the strength of agglomeration is improved.
Although the hot initiator can be used as an essential additive in the aqueous solution, it is not particularly limited, and can only react with the monomer when the agglomerated particles and the polymer gel are coexistently dried, for example, potassium persulfate or ammonium persulfate. Sodium sulfate, t-butylhydroperoxide, hydrogen peroxide, 2,2'-azobis(2-amidinopropane) dihydrogen 2,2'-azobis (2-amidinopropane) Dihydrochloride and so on. Among them, it is preferred to use a peroxide and a persulfate such as sodium persulfate. These hot starters may be used singly or in combination of two or more.
Although an oxidizing agent can be used as an essential additive in an aqueous solution, as long as agglomerated particles and a polymer gel coexist and can be reacted with a monomer when dried, for example, inorganic oxidizing agents: chlorate, bromate, chlorite, and inorganic Peroxides or organic peroxides such as the above-mentioned thermal initiators such as hypochlorite, persulfate, hydrogen peroxide, t-butyl peroxide and benzoic acid peroxide . Among them, persulfate and hydrogen peroxide, especially persulfate, are preferably used. These oxidizing agents may be used singly or in combination of two or more.
Although a reducing agent can be used as an essential additive in an aqueous solution, as long as the agglomerated particles and the polymer gel coexist, an additive which can react with the monomer, whether an organic reducing agent or an inorganic reducing agent, can be used, but it is preferably used, for example. A sulfur, phosphorus or nitrogen based reducing agent. Specific examples are sulfites (such as sodium sulfite, potassium sulfite, ammonium sulfite, etc.), bisulfites (such as sodium bisulfite, potassium bisulfite, ammonium bisulfite, etc.), bisulfite, even Disulfite, trisulfonate, tetrasulfonate, thiosulfate, nitrite, dimethylhydrazine, sulfur dioxide, phosphorous acid, nitrogen-containing organic compounds such as amino acids and ethanolamine, and the like. Among these sulfur-based reducing agents, sulfite, disulfite, bisulfite and dithionite are preferably used. The sodium salt, potassium salt and ammonium salt are preferably used among these salts. Among them, sodium sulfite and sodium hydrogen sulfite are preferred. These reducing agents may be used singly or in combination of two or more.
It is recommended to use the above-mentioned hot starter in the necessary additives, in particular, a persulfate as an essential component to achieve an excellent effect of reducing residual monomers.
The content of the necessary additives in the aqueous solution is not particularly limited, but it is desirable that each small particle has a weight ratio of 0.0001 to 1%. When the content is less than 0.0001% by weight, the residual monomer cannot be lowered to a sufficient extent, and when it exceeds 1% by weight, the dried water-absorbing resin may cause discoloration.
Further, in the polymerization step, necessary additives (especially hot starters) may also be used as necessary. In this case, with respect to the content of the necessary additives in the aqueous solution, the content of the necessary additives of the small particles before agglomeration is generally desired to be from 1 to 500% by weight, preferably from 5 to 400% by weight, or even from 10 to 300% by weight. Ratio (content per unit weight of monomer component). When the proportion of the necessary additives in the aqueous solution exceeds the above range in each polymerization step, the advantages of the production method cannot be attained. As described above (when the necessary additives are used in the polymerization step), the necessary additives added in the polymerization step and in the aqueous solution may be the same or different kinds.
The method of production is characterized by the addition of the necessary additives to the aqueous solution. The necessary additives (especially the hot starter) may be separately added to the aqueous solution at any step of the production process, for example, added to the polymer gel before the drying step, and added to the dried product before the grinding step (dry Polymer gel) and the like. However, from the viewpoint of the amount of addition, the benefit may be lower than the above.
The amount of the aqueous solution used is not particularly limited, but it is not desirable to be less than 25 parts by weight but not more than 280 parts by weight per 100 parts by weight of the small particles. It is best not to exceed 200 cents, or even more than 150 cents. When the amount of the aqueous solution exceeds 280 parts by weight, a large amount of group gel is generated, so that it is necessary to further dry and grind the gel state material to produce agglomerated particles, and to use strict conditions for drying. In contrast, when the amount of the aqueous solution is less than 25 parts by weight, the agglomeration strength may be insufficient, and a final product having excellent characteristics cannot be obtained, and it is not easy to agglomerate due to uneven mixing.
Although it is only necessary to mix small particles and an aqueous solution when agglomerating small particles, it is preferred to heat the aqueous solution before agglomeration. According to a preferred embodiment, the heated aqueous solution and small particles can be rapidly mixed to achieve agglomeration. Therefore, a large amount of group gel is not produced but a small amount of agglomerated particles can be directly controlled. Therefore, it is not necessary to further dry and grind the gel state material to obtain agglomerated particles, and in addition, the problem of generating a large amount of group gel can be avoided, and the main chain of the water-absorbing resin itself is inferior to the solution, and the like, the mixing becomes more difficult and the property is deteriorated. And to prevent the gel from becoming a block.
In the preferred embodiment of the agglomeration, the temperature at which the aqueous solution is heated is usually not lower than 40 ° C, desirably not lower than 50 ° C, or not lower than 60 ° C, preferably not lower than 70 ° C. Further, the upper limit of the temperature is lower than the boiling point of the aqueous solution, and the boiling point can be adjusted by adding a salt, other solvent or changing the pressure (pressure reduction and pressurization) and the like. However, even if the heating temperature exceeds 100 ° C, no significant change occurs, and therefore, a temperature not higher than 100 ° C can be used. When the aqueous solution is preheated, the necessary additives are preferably formulated separately so that the aqueous solution can be brought to a higher concentration at room temperature or upon cooling, and then mixed with other heated large amounts of aqueous solution before being mixed with the small particles.
In the preferred embodiment of the agglomeration, in addition to preheating the aqueous solution, it is desirable to heat the small particles themselves. The heating temperature of the small particles is also not lower than 40 ° C, and it is generally not desirable to be not lower than 50 ° C. Because even if the temperature is higher than 100 ° C, no significant change will occur, and a temperature not higher than 100 ° C can be used. The procedure for preheating the small particles themselves is not particularly limited, for example, heating in a constant temperature manner after drying and heating, or heating by external separation.
In the preferred embodiment of the agglomeration, when the heated aqueous solution and the small particles are rapidly mixed, the rapid mixing means that the completion of the mixing of the aqueous solution and the small particles to produce agglomerated particles is short. In particular, the time from the start of contact of the aqueous solution with the small particles to the generation of agglomerated particles, that is, the mixing time is short. The mixing time is desirably no more than 3 points, or no more than 1 minute, and preferably 1 second to 60 seconds. When the mixing time is too long, the aqueous solution and the small particles become less likely to be uniformly mixed, and a large amount of group gel is formed. Further, when the mixing time is too long, it is possible that the necessary additives in the aqueous solution are decomposed before the step of producing the agglomerated particles to be dried together with the polymer gel, so that the necessary additives cannot be obtained in a sufficient amount in the drying step. Further, after the completion of the mixing, if the mixing is continued for a long time, the characteristics of the obtained water-absorbing resin are lowered, for example, the amount of water outflow is increased, and the absorption force at the time of stress is lowered.
The method of rapid mixing includes mixing and stirring the heated aqueous solution with small particles at a time. In other words, if a method of gradually adding an aqueous solution to mixing, such as a spraying method, is employed, since the small particles may agglomerate during handling, the characteristics of the water-absorbing resin are lowered. The time for heating the aqueous solution is desirably not more than 60 seconds, or not more than 30 seconds, and preferably not more than 10 seconds. Further, small particles may be mixed into the heated aqueous solution and stirred with respect to the above process. In this example, the time for mixing the small particles is desirably no more than 60 seconds, preferably no more than 30 seconds, and most preferably no more than 10 seconds. In addition, a method of simultaneously mixing small particles with a heated aqueous solution at the same time is also included in the method of rapid mixing. In this example, the mixing time of the two is desirably no more than 60 seconds, or no more than 30 seconds, and preferably no more than 10 seconds. In addition, it is also possible to continuously mix the two at the same time to continuously obtain agglomerated particles. It is considered that the necessary additives may be decomposed, so that the dry agglomerated particles and the polymer gel allow them to coexist for as long as possible. In the above preferred embodiment, the polymer gel continuously obtained in the polymerization step is quickly mixed with the agglomerated particles before the drying step.
Whether or not the agglomerates are agglomerated particles can be confirmed by using an optical microscope to determine whether or not the particles are agglomerated by using a plurality of discrete particles which are moisturized at the time of water absorption or maintaining their shape.
When the dry agglomerated particles and the polymer gel are allowed to coexist, the difference in the solid content (%) of the polymer gel and the agglomerated particles (or the difference in the moisture content of the polymer gel and the agglomerated particles) is desirably as small as possible. In particular, the ratio of the solid content A (%) of the agglomerated particles (agglomerate gel) to the solid content B (%) of the polymer gel (A/B) is generally not less than 1/3 but not more than 3, or not Less than 1/2 but not more than 2, preferably not less than 2/3 but not more than 3/2, preferably not less than 4.5/5.5 but not more than 5.5/4.5, preferably 1 (that is, solid The same content). When the ratio (A/B) of the solid content A of the agglomerated particles to the solid content B of the polymer gel is out of range, the drying of the two mixed components is liable to cause unevenness. Therefore, it is possible to cause one of the problems of excessive drying or insufficient drying, which causes problems in production and quality.
"Solid content" means the removal of volatile components (mainly water) from the gel state water-absorbing resin (polymer gel or agglomerate gel), that is, the resin component of the water-absorbing resin. The weight of the solid as described above is referred to as "solid amount", and the ratio of the solid amount in the gel state water-absorbing resin containing a volatile component per unit weight is referred to as "solid content (%)". The "water vapor content" is the proportion (%) of water in the gel state water-absorbing resin, which is equivalent to 100% minus the above solid content (%).
When the agglomerated particles and the polymer gel coexist in the drying, the ratio of the agglomerated particles to the polymer gel can be obtained (in other words, the amount of small particles recovered per unit solid content in the polymer gel at the polymerization step) ), and the solid content of the agglomerated particles (agglomerate gel), that is, the solid content of the small particles before agglomeration, is desirably not more than 40% by weight, or not more than 35% by weight, not more than 30% by weight, not more than 25 % by weight, not more than 20% by weight, preferably not more than 15% by weight. Any lower limit of more than 0% by weight may be tolerated. Generally, depending on the production efficiency in the polymerization step, the recovery amount of small particles per unit solid amount of the polymer gel is difficult to exceed 40% by weight.
Assuming that the recovery amount of the small particles is at most 40% by weight, the necessary additive added to the aqueous solution is small in proportion to the amount of the water-absorbing resin obtained by drying the agglomerated particles together with the polymer gel, and is usually less than 0.3% by weight. ratio. When the amount of small particles recovered is about 10 or several parts by weight, the amount of the necessary additives may be less than 0.1% by weight. In other words, the production method can reduce the residual monomer to a desired degree by adding a small amount of necessary additives to the aqueous solution, and it is necessary to reduce the residual monomer to a desired level as compared with other methods. Add all necessary additives to the gel.
In the production method, the agglomerated particles and the polymer gel are allowed to coexist. It is desirable that at least a portion of the agglomerated particles are in contact with at least a portion of the polymeric gel upon drying. Further, it is preferred to mix the agglomerated particles with a polymer gel and dry. The drying can be carried out in a state where the agglomerated particles are uniformly mixed with the polymer gel, or the agglomerated particles are slightly mixed or heavily mixed with the polymer gel. In other words, according to the production method, the drying can be carried out simply by mild or heavy mixing, and the residual monomers can be sufficiently reduced as described above without being specially mixed into a uniform state. In particular, when a polymer gel is transferred to a drying device using a pipe or a conveyor belt, the agglomerated particles must be introduced into the pipe or supplied to the conveyor belt for drying. In the conventional method of reducing residual monomers by adding persulfate or the like to a polymer gel, it is necessary to uniformly disperse an additive such as a persulfate or the like throughout the polymer gel. However, sometimes it is necessary to mix separately, so the manner of mixing also affects the mechanism of reducing residual monomers. In other words, according to the production method, the necessary additives in the agglomerated particles cannot be reduced by uniformly mixing with the polymer gel forming the agglomerated particles, but it is possible to produce some effects upon drying. . Of course, it can uniformly mix the agglomerated particles with the polymer gel.
The small particle agglomerated gel obtained in the above step (the procedure disclosed in Japanese Patent Application No. 2005-38123) has a low amount of residual monomers in the particles. In the grinding performed in order to obtain the desired particle size distribution after the drying step (when the drying step allows the agglomerated gel and the polymer gel to coexist), small particles generated by the agglomerated gel in the grinding step (regeneration small) The granules contain a smaller amount of residual monomers than the small particles (primary small particles) produced by drying and grinding directly from the polymer gel. As a result, since the total amount of residual monomers in the small particles having a particle diameter of less than, for example, 300 μm is decreased, the difference in residual monomers between particles having a particle diameter of, for example, 300 to 850 μm and particles having a particle diameter of less than 300 μm is reduced, so the present invention can A small RMI water absorbing agent is obtained.
In the above step of the present invention, the content of the regenerated water-absorbing resin contained in the water-absorbing resin particles is preferably from 0 to 50% by weight, or from 5 to 40% by weight, or even from 10 to 30% by weight. When the water-absorbing resin regenerated in the above step is used as the water-absorbing resin particles of the present invention, since there is a larger surface area than that which is not regenerated, there is a faster water absorption speed.
The bulk density of the above-mentioned water-absorbing resin precursor can be adjusted in the range of 0.40 to 0.90 g/ml (according to JIS K-3362, 1998) or 0.50 to 0.80 g/ml to obtain the water absorbing agent of the present invention. Moreover, it will be adjusted to 150~600μm particles to total particles of 90~100% by weight, or 95~100% by weight, or even 98~100% by weight. The logarithmic standard deviation (σζ) of the particle size distribution is 0.20 to 0.50, or 0.20 to 0.45, preferably 0.20 to 0.40.
The water-absorbing resin precursor obtained by the present invention can be adjusted to have the above-mentioned specific particle size distribution as described above, and the absorption force of the physiological saline solution before the surface cross-linking is not less than 32 g/g, or 35 to 70 g/g. Even 40~65g/g, preferably 45~60g/g. The control of the absorption force can be controlled by the above polymerization conditions, drying conditions such as an internal crosslinking agent, and the like.
The above-mentioned water-absorbing resin precursor is subjected to surface crosslinking treatment to allow the surface of the particles to be crosslinked (third step). According to the surface treatment, the water absorbing agent of the present invention can reduce the absorption force of the force before the surface cross-linking [CRC: centrifuge retention capacity] is preferably 95 to 50%, or even 90 to 60%.
Preferably, the surface crosslinking agent is selected from surface crosslinking agents which form an ester bond with the carboxyl group. a surface crosslinking agent which forms an ester bond (anhydroester ester bond) with a polycarboxylic acid-based functional group (carboxyl group) in the water-absorbing resin, and includes a surface crosslinking agent having a hydroxyl group such as a polyvalent alcohol or an amino alcohol; The surface generates a hydroxyl group-containing surface crosslinking agent such as an alkylene carbonate, an oxazolidinone, a propylene oxide ring, and an epoxy resin compound.
Surface crosslinking agents include polyvalent alcohol compounds such as ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, 1,3-propanediol, 1-methyl-1 , 3-propanediol, 2-methyl-1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentanediol, 2,3,4-trimethyl- 1,3-pentanediol, polypropylene glycol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,4-butanediol, 1,3-butane Glycol, 1,5-pentanediol, 1,6-hexanediol, 1,2-hydroxymethylcyclohexane, 1,2-dimethylolcyclohexane, 1,2-cyclohexane Glycol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, ethylene oxide-propylene oxide segment copolymer, isovalerol, meso-erythritol, D-sorbitol and Sorbitol; epoxy resin compounds such as ethylene glycol diepoxypropyl ether, polyethylene glycol diepoxypropyl ether, glycerol polyglycidyl ether, diglycerol polyglycidol ether (diglycerol polyglycidyl ether), polyglycerol polycondensation Polyglycerol polyglycidyl ether, propylene glycol diepoxypropyl ether, polypropylene glycol diepoxypropyl ether and glycidol; polyamine salt compounds such as ethylenediamine, secondary ethyltriamine, triethylenetetra Amine, tetravinylpentylamine, pentaethyleneamine, polyethylenimine, polyamidepolyamine, and inorganic or organic salts thereof (aziridinium salts, etc.); isocyanate compounds such as 2,4-toluene diisocyanate and hexa Methyl diisocyanate; halogenated epoxy resin compounds such as epichlorohydrin, epibromohydrin and α-methylepichlorohydrin; polyoxazoline compounds such as 1,2 ethylenebisoxazoline (1,2) -ethylenebisoxazoline); oxazolidinone compounds such as N-acyl oxazolidinone and 2-oxazolidinone; akylene carbonate compounds such as 1,3-dioxan-2-one ( 1,3-dioxolane-2-one), 4-methy-1,3-dioxolane-2-one, 4,5-methyl-1 ,3-Dioxy-2-one (4,5-dimethyl-1,3-dioxolane-2-one), 4,4-methyl-1,3-dioxide-2 -ketone (4,4-dimethyl-1,3-dioxolane-2-one), 4-ethyl-1,3-dioxol-2-one (4-ethyl-1,3-dioxolane-2-one) , 4-hydroxymehtyl-1,3-dioxolane-2-one, 1,3--2-one (1,3-dioxane-2- One), 4-methyl-1,3-di 2-keto-1,4-dioxane-2-one, 4,6-dimethyl-1,3-di -2 ketone (4,6-dimethyl-1,3-dioxane-2-one) and 1,3-di-2 ketone (1,3-dioxopane-2-one); cyclic urea compound; epoxidized ring compound For example, propylene oxide ring, 2-methylepoxypropyl ring, 3-methyl-3-hydroxymthyloxetane and 3-ethyl-3-hydroxymethyl epoxy 3-ethyl-3-hydroxymthyloxetane; a polyvalent metal compound such as a hydroxide or a chloride such as zinc, calcium, magnesium, aluminum, iron, zirconium or the like. Among these surface crosslinking agents, at least one compound of one of a polyvalent alcohol compound, an epoxy resin compound, a polyamine salt compound and a salt thereof, a glycidyl compound and an alkylene carbonate compound is selected. The surface crosslinking agent may be selected from one or more of the following compounds, such as a polyvalent alcohol having 3 to 6 carbon atoms and 2 to 3 hydroxyl groups in the molecule, and an epoxy resin compound having 6 to 12 carbon atoms, having 3 to 5 An alkylene carbonate of carbon atoms, and a propylene oxide compound having 3 to 10 carbon atoms. One, two or more of these surface crosslinking agents may be selected depending on the reactivity. The surface crosslinking step can be performed more than once according to the desired characteristics. In this example, the second and subsequent steps may use the same surface cross-linking agent as the first, or a different surface cross-linking agent may be used.
In the amino alcohol as a surface crosslinking agent, for example, ethanolamine can be used in addition to the above.
The amount of the surface crosslinking agent is desirably less than 0.001 part by weight of the water-absorbing resin precursor particles but not more than 10 parts by weight, or the lower limit is 0.01 parts by weight and the upper limit is 5 parts by weight per 100 parts by weight. However, it can be adjusted depending on the compound and combination used. The surface cross-linking agent in this range can make the cross-linking density of the surface of the water-absorbing resin higher than the internal cross-linking density, but it is not desirable that the amount of the surface cross-linking agent exceeds 10 parts by weight because the efficiency is deteriorated, and too much cross-linking agent will be in the absorbent. The best cross-linking structure is formed. The amount of the surface crosslinking agent is not desirably less than 0.001 part by weight because the purpose of improving the water absorption characteristics such as the absorption force of the force or the like may not be achieved. When surface crosslinking is carried out, it is desirable to use water as a solvent when mixing the water-absorbing resin and the surface crosslinking agent. The water content of the solid content water-absorbing resin per 100 parts by weight may exceed 0 parts by weight, but it is not desirable to be more than 20 parts by weight, preferably not less than 0.5 parts by weight but not more than 10 parts by weight, but according to the water absorption tree The type, particle size, water vapor content, and the like of the ester precursor are adjusted.
When surface crosslinking is carried out, it may be used together with a hydrophilic organic solvent when mixing the water-absorbing resin precursor and the surface crosslinking agent. Hydrophilic organic solvents which can be used in this step include lower alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol and tert-butanol; ketones such as acetone; ethers such as dioxane Anthracene and tetrahydrofuran; amines such as N,N-dimethylformamide; and anthraquinones such as dimethylammonium. The hydrophilic organic solvent may be used in an amount according to the type, particle diameter, water vapor content, etc. of the water-absorbing resin precursor, but preferably not less than 0 part by weight but not more than 20 parts by weight, and preferably not less than 0 part by weight. However, it is not higher than 10 parts by weight per 100 parts by weight of the water-absorbing resin precursor particles. It is best not to use a hydrophilic organic solvent (0 weight) to reduce the source of impurities that can cause odor.
After surface water crosslinking is carried out after mixing water and/or a water-repellent organic solvent and the above surface crosslinking agent, the aqueous solution may be mixed by spraying or dropping into the water-absorbing resin precursor. Ideally, it is sprayed. The size of the spray droplets is preferably in the range of 0.1 to 300 μm, and the average particle diameter is preferably in the range of 0.1 to 200 μm.
A mixing device of a mixed water-absorbing resin precursor, a surface crosslinking agent, water, and a water-reducing organic solvent preferably produces a large mixing force to achieve uniform mixing. Mixing equipment including cylindrical mixer, conical double mixer, fast mixing mixer, V-type mixer, ribbon mixer, screw mixer, double arm coupler, grinding coupler, rotor mixer, airflow mixing , agitator, batch mixer, continuous mixer, etc.
After the surface-crosslinking is carried out after mixing the water-absorbing resin precursor and the surface crosslinking agent, heat treatment must be performed to cause cross-linking of the surface of the water-absorbing resin precursor to obtain the desired water-absorbing resin. In particular, in order to allow the crosslinking agent to react with the surface of the water-absorbing resin precursor, it is preferable to carry out heat treatment in consideration of the reactivity of the crosslinking agent, simplification of equipment, and improvement of production efficiency. The treatment temperature for the heat treatment is desirably not lower than 80 ° C but not higher than 250 ° C, although the temperature is related to the crosslinking agent used. However, it is not desirable to treat the temperature below 80 ° C, so as not to affect the yield due to excessive heat treatment time, and also to achieve uniform surface cross-linking, which deteriorates the absorbency under stress and causes surface cross-linking agent to remain. The treatment temperature is also not expected to be higher than 250 ° C because the same water-absorbing resin precursor may produce a heat inferior solution.
The heat treatment equipment can be a common drying oven or a heating oven. For example, heat conduction type, heat radiation type, hot air flow type, dielectric heating drying oven or heating oven. For example, belt type, groove agitating type, screw type, rotor type, disc type, twist type, fluidized bed type, air flow type, infrared type, electron beam type drying oven or heating oven.
Although it is possible to produce small particles due to subsequent heat treatment, the generated small particles may be removed in the final classification step or may be subjected to a particle size increasing step together with the small particles obtained in the classification step before the abrasive particles are heated ( Agglomeration step; the above-described regenerating step to obtain larger particles or agglomeration into granules). The small particles having an increased particle size are subjected to a weight ratio of the small particles to the small particles before the heat treatment after heat treatment of not more than 5%, or not more than 3% by weight. The amount of the small particles after the heat treatment is not desirably more than 5% by weight, so as not to cause a decrease in the strength and the absorption force of the particles having a large particle diameter.
In order to obtain the above-mentioned water-absorbing resin, an aqueous solution having a sulfur-containing reducing agent may be added by spraying if necessary (fourth step). More specifically, this method can be used in the above-mentioned step of recovering small particles containing a large amount of residual monomers to obtain a water absorbing agent having a low residual monomer content and a small variation in particle size distribution range, without using small particles. Recycling method of residual monomers (Japanese Patent Application No. 2005-38123).
Surface crosslinking can achieve the goal of selectively adding a small amount of aqueous solution to the surface of the particles and small particles. A chelating agent, a plant component, a bactericide, a water-soluble polymer, an inorganic salt, etc. may be contained in an aqueous solution. In the water containing the sulfur reducing agent, the amount of water is preferably not less than 0.5 part by weight but not more than 15 parts by weight, preferably not less than 1 part by weight but not higher than 100 parts by weight of the water-absorbing resin. 12 parts by weight, preferably not less than 1 part by weight but not more than 10 parts by weight, even not less than 1 part by weight but not more than 5 parts by weight. When the water content is less than the above lower limit of 0.5 part by weight, the aqueous solution having the sulfur-containing reducing agent is difficult to uniformly mix with the water-absorbing resin. The water content is not recommended to exceed the upper limit of 15 cents, so as to avoid drying, it takes too much energy to reach the required water vapor content, which is not in line with the benefits. The treatment of the mixture after the aqueous solution is mixed becomes difficult, and thus it is not desirable from the viewpoint of stable production. Further, when the content exceeds the upper limit of 15 parts by weight, in order to shorten the drying time, if drying is performed at a high temperature, odor may be generated. Further, when water having a weight of not less than 15 parts by weight is added, the water-absorbing resin can produce strong agglomeration, and therefore must be ground after drying. In the grinding step, the crosslinked layer on which the surface has been formed is destroyed, so that the absorption force at the time of stress is lowered.
Further, the amount of the sulfur-containing reducing agent is desirably less than 0.05 parts by weight but not more than 10 parts by weight, preferably not less than 0.1 part by weight but not more than 8 parts by weight per 100 parts by weight of the water-absorbing resin. When the amount of the sulfur-containing reducing agent is less than 0.05% by weight of the above lower limit, the purpose of lowering the residual monomer may not be achieved. In contrast, when the amount exceeds the upper limit of 10 parts by weight, the amount of addition required to lower the residual monomer is also exceeded, resulting in a decrease in physical properties and an increase in cost. Therefore, it is desirable that the additive containing a sulfur-containing reducing agent is not less than 0.001% by weight in the aqueous solution but not more than 50% by weight. In the present invention, it is selectively on the surface of the particles and the surface of the small particles or enters the inside thereof, so that it is sufficient as long as it is added in a small amount without deteriorating the physical properties.
Sulfur-containing reducing agents include sodium sulfite, potassium sulfite, calcium sulfite, ammonium sulfite, sulfurous acid, sodium hydrogen sulfite, potassium hydrogen sulfite, ammonium hydrogen sulfite, sodium thiosulfate, potassium thiosulfate, thiosulfuric acid Ammonium, thiosulfate, cysteine, cystine, and the like. One or more of the reducing agents may be used. From the standpoint of reducing residual monomers and cost, it is preferred to use sulfites and bisulfites.
The resulting aqueous solution having a sulfur-containing reducing agent can be added to the water-absorbing resin by droplets to be mixed. It is best to choose a spray method that can control the diameter of the tiny droplets. The average particle diameter of the spray droplets is desirably less than 0.1 μm but not more than 300 μm, and the upper limit is preferably 200 μm. Although the average particle diameter of the droplets is less than 0.1 μm and does not cause a particular problem, it is not easily practical to be limited by the cost of the nozzle and equipment. Further, when the average particle diameter of the droplets exceeds 300 μm, the number of droplets per unit volume of the aqueous solution may decrease. Therefore, the probability of mixing with the water-absorbing resin is also lowered and it is not possible to mix well. Therefore, even if the purpose of reducing the amount of residual monomers can be achieved, the particles may be mutated in the amount of residual monomers and the desired residual monomer index may not be achieved.
The mixing device used to add the aqueous solution is preferably capable of providing a large mixing force. Such mixing equipment includes, for example, cylindrical mixers, conical double-layer mixers, fast mixing mixers, V-type mixers, ribbon mixers, screw mixers, double-arm couplers, grinding couplers, rotor mixers , gas mixers, mixers, batch mixers, continuous mixers, etc.
When the aqueous solution is added by spraying, the aqueous solution may contain other additives such as a chelating agent, a plant component, a bactericide, a water-soluble polymer, an inorganic salt or the like in addition to the sulfur-containing reducing agent. The amount of the additive in this example can be freely selected, but it is preferably not less than 0.001% by weight of the aqueous solution but not more than 50% by weight. As for the chelating agent, it is preferred to have a high degree of chelation separation for Fe and Cu. For example, a chelating agent having a stability constant of Fe ions of not less than 10, or a chelating agent having a stability constant of not less than 20, preferably a polyvalent aminocarboxylic acid (carboxylic acid) and a salt thereof, or having three or more carboxyl groups Aminocarboxylic acids and salts thereof. The polyvalent carboxylic acid includes diethylenetriamine pentaacetic acid, triethylenetetraamine hexaacetic acid, cyclohexane-1,2-ethanediaminetetraacetic acid, N-hydroxyethyl Ethylenediamine triacetic acid, ethylenediamine tetrapropionacetic acid, ethylenediamine tetrapropionacetic acid, N-alkyl-N'-carbonylmethyl-aspartate N-alkyl-N'-carboxymethyl aspartic acid, N-alkenyl-N'-carboxymethyl aspartic acid and alkali metal salts, alkaline earth Metal salts, ammonium salts or amino acid salts. The salts used may be partially neutralized or fully neutralized, or a mixture thereof. Among them, diethylenetriamine pentaacetic acid, triethylenetetraamine hexaacetic acid, N-hydroxyethylethylenediamine triacetic acid and salts thereof are preferred. The amount of the water-absorbing resin per 100 parts by weight is usually from 0.00001 to 10 parts by weight, or from 0.0001 to 1 part by weight.
The above plant ingredients have a mixing range of 0 to 10 parts by weight, 0.001 to 5 parts by weight, preferably 0.002 to 3 parts by weight per 100 parts by weight of the water-absorbing resin to achieve the deodorizing effect. The plant component selectable in the present invention may be a compound containing at least one of polyphenols, flavonoids and other analogs and caffeine, or tannin, tannic acid, Chinese gallnut, gallic acid and gallic acid. At least one of the ingredients. Further, the above-mentioned bactericide may be any bactericidal agent having a bactericidal action without particular limitation, for example, the bactericide disclosed in JP-A No. H11-267500.
The water-absorbing resin to which the aqueous solution having the sulfur-containing reducing agent is added must be subjected to heat drying treatment to prepare a water absorbing agent in an agglomerated state (fifth step). From the viewpoint of agglomeration strength, the weight loss after heat treatment at 180 ° C for 3 hours is divided by the weight before drying to obtain a water gas content of not less than 1% by weight but not more than 15% by weight. The lower limit of the moisture content is 2% by weight, preferably 2.5% by weight, and the upper limit is 13% by weight, and preferably 12% by weight.
Heating is heated using hot air. The heating temperature (heating medium temperature or material temperature) is desirably not lower than 40 ° C but not higher than 120 ° C, lower limit is 50 ° C and upper limit is 100 ° C. The heating time is not expected to be shorter than 1 minute but not more than 2 hours. The ideal heating temperature and time are: heating at 60 ° C for 0.1 to 1.5 hours, or heating at 100 ° C for 0.1 to 1 hour.
When the heating condition exceeds this range, for example, when the temperature is too low or the time is too short, since the surface state of the obtained water absorbing agent may be too wet, the bonding force is too strong, so that the particles are not easily handled. In contrast, when the temperature is too high and the time is too long, energy and waste of the cross-linking reaction of the water-absorbing agent surface cross-linking agent are wasted.
The addition and heating of the aqueous solution can be carried out in the same equipment or in different equipment. Heating can be carried out with stirring or simply heating, as long as the temperature and moisture content can be controlled to fall within the desired range, but it is preferable to simply heat to allow the ripening to form a loose mass. The heating can be carried out in such a manner that the water-absorbing resin-coated aqueous solution has a height of about 1 to 100 cm. The lower limit of the height is 5 cm, or 10 cm, and the upper limit is 80 cm, or 70 cm. The aged water absorbing agent may be further subjected to grinding or classification to obtain the water absorbing agent of the present invention, if necessary. The above-mentioned definition of aging refers to a step of obtaining a granule by also performing a wettability treatment on the surface of the water absorbing agent particles in the above heat treatment step.
In the present invention: (A) a plant component, (B) a polyvalent metal salt of an organic acid, (C) inorganic small particles (including a composite hydrated oxide of (D)), etc. may be used as the above-mentioned chelating agent, botanical component and A secondary constituent of the biocide. Various functions of the water absorbing agent of the present invention can be increased.
The amount of (A)~(D) and (E) other additives may be adjusted according to the purpose and desired function. However, depending on the amount of the compound added, the amount of the water-absorbing resin per 100 parts by weight is generally from 0 to 10 parts by weight. Within the range, or 0.001 to 5 points weight, or even 0.002 to 3 points weight. If the amount is less than 0.001 part by weight, significant benefits and functions may not be produced, and when the amount is not less than 10 parts by weight, undue effect or even decrease in absorbency may occur.
(A) Botanical component The water absorbing agent according to the present invention may be mixed with the plant component of the above ratio to achieve a deodorizing function. In addition to the above-mentioned botanical ingredients, it includes: Japanese camellia (Camellia), Japanese elm and Japanese thick-skinned tea plants, rice (rice), alfalfa, bamboo plants, maize (cereals), wheat and other gramineous plants. , and coffee plants in the forage plants. The plant component used in the present invention may be a plant extract (composition oil) produced by a plant or food processing industry, a plant itself, a plant residue, and a residue after plant extraction, etc., but is not limited thereto.
(B) Polyvalent metal salt The water absorbing agent of the present invention may be mixed with a polyvalent metal salt of the above ratio, particularly a polyvalent metal salt of an organic acid to improve particle flowability and avoid agglomeration due to water absorption. The organic acid polyvalent metal salt and the mixing method which can be used can be referred to, for example, a metal such as a fatty acid other than an alkali metal salt, a petroleum acid, a polymer acid or the like as disclosed in International Patent Application No. PCT/2004/JP1355. Salts, organic acid polyvalent metal salts having seven or more carbon atoms in the molecule are suitable for use in the present invention. Organic acids having a polyvalent metal salt include long-chain or branched-chain fatty acids such as caproic acid, caprylic acid, octicoyl acid, capric acid, lauric acid, myristic acid, palmitic acid, oleic acid and stearic acid; petroleum acids such as benzene Formic acid, myristic acid, naphthenic acid, naphthoic acid and naphthyloxyacetic acid; polymeric acids such as polymethacrylic acid, and polydecanoic acid. However, it is preferably an organic acid having a carboxyl group in the molecule, and a fatty acid such as hexanoic acid, octanoic acid, octicoyl acid, citric acid, lauric acid, myristic acid, palmitic acid, oleic acid, stearic acid, beef fatty acid and hydrogenated hydrazine. sesame oil. The organic acid is preferably a fatty acid having no unsaturated bond in the molecule such as caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid and stearic acid. The organic acid is preferably a long-chain fatty acid having 12 or more carbon atoms in the molecule and having no unsaturated bond, such as lauric acid, myristic acid, palmitic acid or stearic acid.
(C) Inorganic Small Particles The water absorbing agent of the present invention may be mixed with inorganic small particles, particularly inorganic small particles which are insoluble in water to avoid agglomeration caused by water absorption. Inorganic particles suitable for use in the present invention include metal oxides such as cerium oxide and titanium oxide, cerium salts such as natural zeolites and artificial zeolites, kaolin, talc, clay, bentonite and the like. Among them, cerium oxide and ceric acid salt are preferably used, and the average particle diameter of cerium oxide and cerium salt using a particle size analyzer method is preferably from 0.001 to 200 μm.
(D) Composite hydrated oxide The water absorbing agent of the present invention may be mixed with a composite hydrated oxide including zinc, bismuth, zinc and aluminum to improve the fluidity after water absorption (the fluidity of the water absorbing resin or the water absorbing agent after absorbing water) Excellent deodorizing function.
For example, in the water absorbing agent obtained by the production method of the present invention, quantitative analysis by HPLC (High Performance Liquid Chromatography) is used, and the amount of residual monomers is not less than 0 but not more than 500 ppm. The amount of residual monomer is desirably not lower than 0 but not higher than 400 ppm, preferably not lower than 0 but not higher than 300 ppm, preferably not lower than 0 but not higher than 250 ppm, preferably not lower than 0 but not higher than 200 ppm, not lower than 0 but not higher than 150 ppm, and not lower than 0 but not higher than 100 ppm. If the main component of the monomer used in the above polymerization is acrylic acid and/or a salt thereof, the amount of unreacted acrylic acid and/or its salt is not more than 500 ppm. When the amount of the residual monomer in the water absorbing agent of the present invention exceeds 500 ppm, when it is used as a water absorbing material such as a disposable diaper, the odor is absorbed after the urine is absorbed. In addition, when considering the possibility of contact with human skin, hygiene issues must also be considered. In addition, based on the health of the manufacturing site staff, scattered particles can have adverse health effects, so it is not desirable to exceed this level.
For example, in the example of the water absorbing agent obtained by the above production method of the present invention, the residual monomer index (RMI) obtained by the formula (1) is not more than 0.30. RMI is desirably not less than 0 but not more than 0.26, preferably not less than 0 but not more than 0.20, or not less than 0 but not more than 0.15, and not less than 0 but not more than 0.10.
RMI=|RM 1 -RM 2 |/RMA (1)
Wherein RM 1 represents the amount of residual monomers of particles having a particle diameter of less than 300 μm in the water absorbing agent; RM 2 represents the amount of residual monomers in the particles having a particle diameter of 300 to 850 μm in the water absorbing agent; and RM A represents the amount of residual monomers in the water absorbing agent; . When RM A is 0, the RMI should also be zero.
The residual monomer index can be determined as follows: the water absorbing agent is filtered and classified using a sieve and a tray of JIS standard 850 μm and 300 μm mesh; the amount of residual monomers having a particle diameter of less than 300 μm and a particle diameter of 300 to 850 μm are respectively determined; The residual monomer value of the water absorbing agent is substituted into the above formula (1). The residual monomer index is not expected to exceed 0.30. Since the water absorbing agent of different particle size distribution ranges may have variations in the amount of residual monomers, when the water absorbing agent is applied to the water absorbing material, the particle size separation may cause different particle diameters of the water absorbing agent. The deviation of the distribution produces a variation in the amount of residual monomer.
The water absorbing agent of the present invention is preferably not less than 30 g/g in physiological saline, and the test method described later is preferably not less than 30 g/g. When the unabsorbed force is less than 30 g/g, application to a water absorbing material such as a disposable diaper may cause insufficient water absorption. Although there is no limit to the upper limit, it will cause production and cost problems when it exceeds 60g/g. The CRC is preferably not less than 31 g/g but not more than 55 g/g, or not less than 32 g/g but not more than 50 g/g. The absorption capacity (CRC) of the water absorbing agent of the present invention when it is not subjected to force is desirably controlled to not less than 30 g/g, and must be controlled to not less than 30 g/g because the aqueous solution containing the sulfur reducing agent is added before the fourth step. It is possible to reduce the water absorption of the water-absorbing resin, that is, the third step of obtaining the surface-crosslinked water-absorbing resin according to the amount of the aqueous solution added.
The absorption of physiological saline by the water-absorbing resin of the present invention at a high force of 4.8 kPa (AAP 4.8 kPa) must be not less than 20 g/g as measured by the measurement method described later. When the absorption force at a high force is less than 20 g/g, the desired high absorbency cannot be obtained when applied to a water absorbing material such as a disposable diaper. Although the upper limit is not limited, when it exceeds 35g/g, it may cause problems in production and cost. The absorption force at the time of high force is preferably not less than 21 g/g, and not less than 22 g/g.
The absorption force (AAP 4.8 kPa) of the water absorbing agent of the present invention at a high stress is desirably controlled to be not less than 20 g/g, and must be controlled to not less than 20 g/g because the aqueous solution containing the sulfur reducing agent is added in the fourth step. It is possible to lower the water absorption of the water-absorbing resin, that is, the third step of obtaining the surface-crosslinked water-absorbing resin according to the amount of the aqueous solution added.
The water absorbing agent of the present invention has an absorption force of not less than 30 g/g when not subjected to force and a suction force of not less than 20 g/g at a high force, and has a preferable gel strength. Therefore, when a water absorbing agent composed mainly of a water absorbing agent and a hydrophilic fiber is used, the water absorbing agent and the hydrophilic fiber per unit weight may contain a large amount of water absorbing agent (core concentration). In this example, the core concentration is 30 to 100% by weight, or 40 to 90% by weight, or 50 to 80% by weight.
In the water absorbing agent of the present invention, the absorption force at the time of no force is not less than 30 g/g, and the absorption force at the time of high force is not less than 20 g/g, which is a reference parameter, and the other parameter is physiological saline. Total absorption (TAC), total absorption can be obtained from the force of unstressed force (CRC) and the absorption force of 1.9 kPa (SAAP) single layer force according to formula (2), the total absorption force is expected to be not less than 65g /g, or not less than 67g/g, or not less than 70g/g. The total absorption is preferably not less than 65 g/g because the desired high absorbency cannot be achieved when applied to absorbent articles such as disposable diapers. Although the upper limit is not limited, there will be problems in terms of production and cost in excess of 100 g/g.
TAC(g/g)=CRC+SAAP (2)
Among them, CRC represents the absorption force without force (g/g), and SAAP (single layer AAP) represents the absorption force when the single layer is subjected to a force of 1.9 kPa (g/g).
The total absorption capacity (TAC) of the water absorbing agent of the present invention must be not less than 65 g/g, and must be controlled to not less than 65 g/g because it is possible to lower the water-absorbing resin before adding the aqueous solution containing the sulfur reducing agent in the fourth step. The water absorption, that is, the third step, the surface-crosslinked water-absorbing resin obtained according to the amount of the aqueous solution added.
The physiological saline solution of the present invention has a total absorbent force of not less than 65 g/g, and therefore has an absorption force of not less than 30 g/g and a high absorption force of not less than 20 g/g. The agent will have a higher unabsorbed force. Therefore, the core concentration of the absorbent can be lower than the above, and the absorbent of the desired absorbent can be achieved with a smaller amount of the water absorbing agent. In this example, the core concentration is preferably from 10 to 70% by weight, or from 20 to 60% by weight, or from 30 to 50% by weight.
The water absorbing agent can simultaneously or individually satisfy the unabsorbable absorption force of not less than 30 g/g, and the absorption force at the time of high force is not less than 20 g/g, or the total absorption force of the water absorbing agent is not less than 65 g/g. Parameter condition.
The present inventors have found that the amount of residual monomers in the water absorbing agent is related to different particle size distribution ranges. This is not a conventional knowledge, and the amount of residual monomers in different particle size distribution ranges is compared, and the amount of residual monomers is found to be 300 μm. The change is greatest, and a particle size of 150 μm produces a more pronounced variation in the amount of residual monomer. Therefore, in order to control the residual monomer index of the present invention to fall within a specific range, it is necessary to strictly control the particle size distribution, that is, the weight ratio of particles having a particle diameter of 300 to 850 μm and particles having a particle diameter of less than 300 μm, and the amount of small particles having a particle diameter of less than 150 μm. . If these values exceed the above range, it is difficult to control the residual monomer index.
Further, when a conventional water-absorbent-containing absorbent absorbent such as a disposable diaper has a large particle diameter, for example, when a large amount of particles having a particle diameter of more than 850 μm is contained, a large-particle water absorbing agent may irritate the user's skin to cause a user. Feeling unwell. Therefore, a water absorbing agent containing too many large particle diameter particles is not suitable as an absorbent.
Further, the particle size distribution and the mass median diameter of the water absorbing agent are related to the value of the surface area of the water absorbing agent. The surface area of the water absorbing agent is related to the contact area between the water absorbing agent and the aqueous solution such as body fluid, and thus has an influence on the absorption behavior and absorption force of the above water absorbing agent. When the number of small particles is too large, gel agglomeration may cause a decrease in the absorption of the absorbent. In addition, the working environment may be poor due to the dust generated by the absorbent during the production process. In contrast, when the number of large particles is too large, the absorption force of the water absorbing material is lowered due to a decrease in the surface area of the water absorbing agent, so that the water absorbing speed becomes slow.
Therefore, the control of the particle size distribution of the water absorbing agent is important for the absorption capacity of the water absorbing agent as well as the water absorbing material and the working environment. The water absorbing agent of the present invention based on the above considerations is strictly limited to be within this range.
In the entire water absorbing agent. The water absorbing agent of the present invention must control particles having a particle diameter of not more than 850 μm to be not less than 95% by weight, and the content of particles smaller than 150 μm is not more than 5% by weight, not more than 2% by weight, and not more than 1%. weight ratio. The weight ratio of the particles having a particle diameter of 300 to 850 μm to the particles having a particle diameter of less than 300 μm must be not less than 5/95 but not more than 95/5, preferably not less than 20/80 but not more than 80/20, and not less than 30/ 70 but not more than 70/30. Further, it is preferable to control the particle content of not less than 150 μm but not more than 600 μm to be not less than 90% by weight, not less than 95% by weight, and preferably not less than 98% by weight of the entire water absorbing agent.
The mass median diameter (D50) of the water absorbing agent must be controlled within a small range of 200 to 710 μm, preferably 250 to 600 μm, and 300 to 500 μm, and the ratio of particles smaller than 150 μm must be controlled at 0 to 5% by weight, 0 ~3% by weight, and preferably 0 to 2% by weight, and 0 to 1% by weight. In addition, the median particle size of the water absorbing agent after agglomeration in the fourth step and the fifth step may increase by more than 5 to 30% than the mass median particle size of the water-absorbing resin before agglomeration. In this example, the lower ratio is expected to increase by 7%, and 9%, while the upper limit is 25%, and 20%. Since the production method of the present invention has an agglomeration step, the water absorbing agent may have a small amount such as small particles having a particle diameter of less than 150 μm although the mass median particle diameter is small.
The logarithmic standard deviation (σζ) of the particle size distribution of the water absorbing agent must be controlled to be not less than 0.20 but not more than 0.50. I hope the upper limit can reach 0.45 or even 0.40.
The bulk density of the water absorbing agent of the present invention (according to JIS K-3362, 1998) must be controlled to not less than 0.40 g/ml but not more than 0.90 g/ml. It is desirable that the lower limit of the density of the block must be controlled at 0.50 g/ml, and the upper limit must be controlled at 0.80 g/ml.
The shape of the water absorbing agent is preferably an irregular abrasive shape to fix a hydrophilic fiber such as wood pulp. The irregularly ground shape indicates that the ground product is preferably an abrasive product in aqueous polymerization and has the above particle size distribution.
Moreover, according to the measurement method described later, the sulfur-based volatile component in the water absorbing agent is preferably 0 to 2.5 ppm, or 0 to 2.0 ppm, or 0 to 1.5 ppm, or 0 to 1.0 ppm, or even 0 ppm (undetectable). . When the detected sulfur-based volatile component exceeds the above upper limit of 2.5 ppm, the sulfur-containing reducing agent added in the above step is released from the moistened gel. If it is used in absorbent, it will cause discomfort to the user and is not suitable for use in hygiene products.
When an aqueous solution having a sulfur-containing reducing agent is added to the water-absorbing resin after the surface crosslinking treatment to obtain a water-absorbing agent for reducing residual monomers, the absorption of the water-absorbing agent must be sufficiently considered, and the amount of the sulfur-based volatile component in the water-absorbing agent must be controlled. Fall within the above range. This aspect is described in detail below.
In order to control the sulfur-based volatile component in the water absorbing agent to fall within the above range, the aqueous solution and the water-absorbing resin must be uniformly mixed using a device having a large mixing force after the surface cross-linking treatment. However, in a mixing apparatus having a large mixing force, the interconnecting layer formed by the front surface may be destroyed in the apparatus. In particular, when a large droplet of a sulfur-containing reducing agent aqueous solution is added to the water-absorbing resin, the probability of contact with the water-absorbing resin becomes low, and the water-absorbing resin cannot contact the liquid droplets to cause the "ineffective mixing" in the general mixing equipment. ". Therefore, the cross-linked layer of the surface will become dry after the heat treatment, and the scraping by the wall of the mixing device or the stirring member is seriously affected by the uniformity of the surface cross-linking, so that the absorbed water absorbing agent absorbs under high force. (AAP 4.8 kPa) dropped significantly. Therefore, although the water absorbing agent has an unabsorbable force (CRC), it is difficult to maintain a certain absorption force at a high force (AAP 4.8 kPa). More specifically, in the surface-crosslinked water-absorbing resin and the sulfur-containing reducing agent water absorbing agent, the water absorbing agent can be achieved conventionally, and has an unabsorbable absorption force (CRC) and a high force absorption force (AAP). 4.8 kPa), and the characteristics of the sulfur-based volatile components falling within a specific range.
According to the production method of the present invention, by mixing the droplets of the above preferred diameter range with the aqueous solution by spraying, an aqueous solution having a sulfur-containing reducing agent may be added to the water-absorbing resin after the surface is crosslinked to obtain a water absorbing agent. In particular, it is possible to obtain an unabsorbed force (CRC) of not less than 32 g/g and a high force (AAP of 4.8 kPa) of not less than 20 g/g, while strictly limiting sulfur-based volatilization. A water absorbing agent with a composition in the range of 0 to 3 ppm. Further, by adding an aqueous solution, a water absorbing agent having an amount of residual monomers in a desired range can be obtained. The reason why the water absorbing agent can achieve high absorbency in the production method of the present invention is unknown. However, it can be confirmed that since the surface of the water-absorbing resin becomes wet or moist, the gel elasticity is increased so that the surface cross-linking layer can be prevented from being broken by the stirring member of the wall of the mixing device, and thus the water-absorbing resin and the aqueous solution droplet can be increased in the above method. The probability of contact.
Further, the water absorbing agent of the present invention has a Hunter brightness of not less than 70, or not less than 71, and preferably not less than 72. The water vapor content per total weight is 1 to 15% by weight.
The water absorbing agent of the present invention can be used for applications such as absorbent articles such as disposable diapers, sanitary napkins and pads, and is not particularly limited. It is used in paper urine with a high concentration of water absorbing agent (a large amount of water-absorbing resin in each piece of paper diaper) can achieve good results, avoiding the problems of odor, discoloration and the like caused by the conventional water absorbing material.
The water absorbing material of the present invention has a sheet-like absorbent core formed of a water absorbing agent and hydrophilic fibers; the liquid permeable layer is provided on the front side; and the liquid permeable layer is provided on the back side. If hydrophilic fibers are not used, the absorbent core may be in the form of fixing the water absorbing agent to paper and/or non-woven fabric. The absorbent of the present invention, in particular, a children's disposable diaper, an adult diaper and a sanitary napkin, may be mixed or laminated with, for example, a fibrous substrate and a water absorbing agent of the present invention to produce an absorbent core, for example, sandwiching an absorbent core with a liquid permeation layer (front side) The substrate of the layer and the substrate having the liquid impermeable layer (back surface) may be provided with an elastomer, a diffusion layer, a pressure sensitive tape or the like if necessary.
The content of the water absorbing agent (core concentration) absorbing the core in the water absorbent is preferably not less than 10% by weight, or not less than 20% by weight, or not less than 30% by weight, preferably not less than 70% by weight. . The above absorbent core is preferably compressed so as to have a density of not less than 0.06 g/cc but not more than 0.50 g/cc, and a basis weight of not less than 0.01 g/cm 2 but not more than 0.20 g/cm 2 . Fiber substrates which can be used are, for example, basic wood pulp, cotton linters and hydrophilic fibers such as crosslinked cellulosic fibers, crepe fibers, cotton fibers, wool fibers, acetate fibers, virgin fibers, and the like.
The water absorbing material of the present invention has a water absorbing agent having a low residual monomer content and a variation in residual monomer amount which is small in a wide range of particle size distribution. Since the amount of residual monomers is very low, it can be applied to absorbents of thin sanitary products.
Next, the advantages of the present invention will be explained with reference to the following examples, however, the scope of the present invention is not limited to these examples. Various characteristics of the water absorbing agent can be measured according to the method described later, and various properties of the water absorbing resin and the water absorbing agent in the disposable diaper can also be measured by the same method. In addition, comparative tests of commercially available water-absorbing resin and water absorbing agent and water absorbing agent in paper diapers were also carried out. The test is to let it absorb water vapor in a dispersed manner and dry at a low pressure (for example, 60 to 80 ° C for about 16 hours) to achieve a balanced water vapor content of the dried water absorbent (about 5% by weight, 2 to 8% by weight). Than) and then compare.
[Preparation of water-absorbing resin (1)] 5500 g of 4.0 g of polyethylene glycol diacrylic acid (average molar number of ethylene oxide added: 9) was dissolved in an aqueous sodium acrylate solution containing a ratio of 75% molar ratio (Monomer concentration: 38% by weight) A reaction mixture was obtained. The reaction mixture was then placed in a two-blade stainless steel dual-arm coupler reaction tank having a volumetric dimension of 10 L and a lid. Nitrogen gas was introduced into the reaction mixture and the reaction mixture was maintained at 25 ° C, and nitrogen was allowed to replace the oxygen dissolved in the reaction mixture. Then, 28.3 g of a 10% by weight aqueous sodium persulfate solution and 1.4 g of a 1% by weight aqueous solution of L-ascorbic acid were added to the reaction mixture and stirred. After one minute, the polymerization started. After 17 minutes from the start of the polymerization, the maximum polymerization temperature of 86 ° C was reached, and a water-gel polymer was obtained 35 minutes after the start of the polymerization. Therefore, the produced hydrogel polymer was divided into particles of 1 to 4 mm, and these divided polymers were spread on a 50 mesh metal mesh (mesh size: 300 μm), and then dried by hot air at 160 ° C for 60 minutes. Then, the dried product was ground by a barrel method, and classified by a mesh having a mesh size of 850 μm and 150 μm. Therefore, the crosslinked polymer particles (a) having an irregularly ground shape can be obtained as the water-absorbing resin precursor.
100 parts by weight of crosslinked polymer particles (a) 3.53 parts by weight of a 0.5 part by weight aqueous solution of propylene glycol aqueous surface crosslinking agent, 0.03 parts by weight of ethylene glycol diglycidyl ether, 0.3 parts by weight of 1,4-butyl The alkanediol and 2.7 parts by weight of water were mixed by spraying. The mixture was heat-treated at a temperature of 210 ° C in a mixer for 45 minutes to obtain a water-absorbing resin (1).
[Preparation of water-absorbing resin (2)] 2.5 g of polyethylene glycol diacrylic acid (average molar number of ethylene oxide added: 9) was dissolved in 5500 g of a neutralizing sodium acrylate aqueous solution having a ratio of 75% molar ratio ( Monomer concentration: 38% by weight) to obtain a reaction mixture. The reaction mixture was then placed in the same reaction tank as the water-absorbing resin (1). Nitrogen gas was introduced into the reaction mixture and the reaction mixture was maintained at 25 ° C to allow nitrogen to displace the oxygen dissolved in the reaction mixture. Then, 28.3 g of a 10% by weight aqueous sodium persulfate solution and 1.4 g of a 1% by weight aqueous solution of L-ascorbic acid were added to the reaction mixture and stirred. After about one minute, the polymerization started. 17 minutes after the start of the polymerization, the maximum polymerization temperature of 86 ° C was reached, and a water gel polymer was obtained 35 minutes after the start of the polymerization. The water gel polymer is divided into particles of 1 to 6 mm. The water-gel polymer was dried and ground in the same manner as above, and classified by a mesh having a mesh size of 850 μm and 150 μm. Therefore, the crosslinked polymer particles (b) having an irregular abrasive shape can be obtained as the water-absorbing resin precursor.
100 parts by weight of the crosslinked polymer particles (b) were sprayed with a 3.53 part by weight aqueous surface crosslinking agent solution, the composition of which was the same as above. The mixture was heat-treated at 195 ° C for 40 minutes in a mixer to obtain a water-absorbing resin (2).
[Preparation of water-absorbing resin (3)] 1.7 g of polyethylene glycol diacrylic acid (average molar number of ethylene oxide added: 9) was dissolved in 2000 g of a sodium acrylate aqueous solution having a ratio of 75% molar ratio ( Monomer concentration: 35% by weight) to obtain a reaction mixture. The reaction mixture was injected into a stainless steel carrier having a length, a width, and a height of 320 mm, 220 mm, and 50 mm, respectively. The reaction mixture was added at a height of 23 mm. The upper surface of the stainless steel carrier was sealed with a polyethylene film and filled with a portion of nitrogen, a vent and a feed port of a polymerization initiator, and placed in a water bath at 30 °C. The reaction mixture was purged with nitrogen and the reaction mixture was maintained at a temperature of 30 ° C to remove oxygen dissolved in the solution. Thereafter, nitrogen gas was continuously injected into the space above the reaction tank and discharged from the other side. 1.6 g of 10% by weight aqueous solution of 2,2'-azobis(2-amidinopropane) dihydrochloride was added thereto, and 1.6 g of 10% by weight was added. An aqueous solution of sodium sulfate, 2.9 g of a 1% by weight aqueous solution of L-ascorbic acid and 0.3 g of a 3.5% by weight aqueous hydrogen peroxide solution were thoroughly mixed with a word agitator. The polymerization was started one minute after the addition of the polymerization initiator, and the stainless steel was bubbled in a water bath at 12 ° C, the liquid was soaked to a height of 10 mm from the bottom of the tray, and the polymerization temperature was controlled. After 12 minutes from the start of the polymerization, the maximum polymerization temperature of 74 ° C was reached. During the gel aging, the carrier was bubbled in a water bath at 60 ° C with a liquid height of 10 mm from the bottom of the carrier and maintained for 20 minutes. The obtained water-gel polymer was pulverized by a meat grinder having a pore diameter of 9.5 mm, and spread on a metal mesh of 50 mesh (mesh size: 300 μm), and dried by hot air at 160 ° C for 60 minutes. Then, the dried product was ground by a barrel method and classified by a mesh having a mesh size of 850 μm and 150 μm. A crosslinked polymer (c) having an irregular abrasive shape can be obtained as a water-absorbing resin precursor.
100 parts by weight of the crosslinked polymer particles (c) sprayed with 5.03 parts by weight of a 1 part by weight aqueous surface crosslinking agent solution of propylene glycol, 0.03 parts by weight of ethylene glycol diglycidyl ether, 1 part by weight The propanol is mixed with 3 parts by weight of water. The mixture was heat-treated in a mixer at a temperature of 195 ° C for 40 minutes to obtain a water-absorbing resin (3).
[Preparation of water-absorbing resin (4)] 4.0 g of polyethylene glycol diacrylic acid (average molar number of ethylene oxide added: 9) was dissolved in 5500 g of a neutralizing sodium acrylate aqueous solution having a ratio of 75% molar ratio ( Monomer concentration: 38% by weight) to obtain a reaction mixture. The reaction mixture was then placed in a stainless steel reaction vessel with a lid having a two-bladed double-arm coupler and having a volume of 10 L. The reaction mixture was maintained at 25 ° C and nitrogen was introduced to displace the oxygen dissolved in the reaction mixture. Then, while stirring, 47.2 g of a 10% by weight aqueous sodium persulfate solution and 1.4 g of a 1% by weight aqueous solution of L-ascorbic acid were added to the reaction mixture. The aggregation will start immediately. 15 minutes after the start of the polymerization, the highest temperature of polymerization was 92 ° C, and a water-gel polymer was obtained 35 minutes after the start of the polymerization. Therefore, the produced water gel polymer is divided into particles of 1 to 4 mm in size. These water-repellent polymers were dispersed in a 50-mesh mesh (mesh size: 300 μm), and then dried by hot air at 160 ° C for 60 minutes. Then, the dried product was ground by a barrel mill and classified by a mesh having a mesh size of 850 μm and 150 μm. Therefore, the crosslinked polymer particles (d) which are precursors of the water-absorbing resin exhibit an irregular abrasive shape.
100 parts by weight of the crosslinked polymer particles (d) were sprayed with a 3.53 part by weight aqueous surface crosslinking agent solution containing 0.5 parts by weight of propylene glycol and 0.03 parts by weight of ethylene glycol diglycidyl ether. 0.3 parts by weight of 1,4-butanediol and 2.7 parts by weight of water. The mixture was heat-treated at 210 ° C for 45 minutes in a mixer to obtain a water-absorbing resin (4).
[Example 1] A chemical specific grade produced by Wako Pure Chemical Industries, Ltd. was added by spraying in a 100-weight by weight water-absorbing resin (1) containing 5 parts by weight of water and 1 part by weight of sodium hydrogen sulfite (Wako Pure Chemical Industries, Ltd.). The aqueous solution is used as a sulfur-containing reducing agent. The spray addition method was carried out using a hollow conical nozzle 1/4M-K010 (manufactured by Ikeuchi Co., Ltd., droplet diameter: 70 μm to 150 μm). The obtained mixture was covered to a thickness of 5 cm, and aged for 30 minutes in a vent type hot air dryer at a temperature of 80 °C. The aged mixture was sieved with a metal mesh having a mesh size of 850 μm to obtain a water absorbing agent (1).
[Example 2] The above aqueous solution containing 10 parts by weight of water and 1 part by weight of sodium hydrogen sulfite was added by way of spraying to 100 parts by weight of the water-absorbing resin (1). The hollow cone nozzle 1/4M-K010 (droplet diameter: 70 μm to 150 μm) described above was used for the spraying. The obtained mixture was dispersed to a thickness of 5 cm, and aged in a vent type hot air dryer at a temperature of 80 ° C for 30 minutes. The aged mixture was sieved through a mesh of a mesh size of 850 μm to obtain a water absorbing agent (2).
[Comparative Example 1] The water-absorbing agent (1) which was directly treated with the untreated water-absorbing resin (1) as a control.
[Comparative Example 2] One-part by weight of the above-mentioned sodium hydrogen sulfite particles were added to 100 parts by weight of the water-absorbing resin (1), and sufficiently mixed to obtain a comparative water-absorbing agent (2).
[Example 3] The above-obtained 100 parts by weight of the water-absorbing resin (2) was spray-added to the above-mentioned water containing 3 parts by weight and 0.5 parts by weight of an aqueous solution of sodium hydrogen sulfite. The hollow cone nozzle 1/4M-K010 (droplet diameter: 70 μm to 150 μm) described above was used for spraying. The obtained mixture was covered to a thickness of 5 cm, and aged in a vent type hot air dryer at a temperature of 80 ° C for 30 minutes. The aged mixture was sieved through a metal mesh having a mesh size of 850 μm to obtain a water absorbing agent (3).
[Example 4] The above aqueous solution containing 10 parts by weight of water and 1 part by weight of sodium hydrogen sulfite was sprayed in a 100 part by weight of the water-absorbing resin (2). The hollow cone nozzle 1/4M-K010 (droplet diameter: 70 μm to 150 μm) described above was used for spraying. The obtained mixture was covered to a thickness of 5 cm, and aged in a vent type hot air dryer at a temperature of 80 ° C for 30 minutes. The aged mixture was sieved through a metal mesh having a mesh size of 850 μm to obtain a water absorbing agent (4).
[Comparative Example 3] A water absorbing agent (3) as a control was directly used as the untreated water-absorbing resin (2).
[Example 5] The above aqueous solution containing 5 parts by weight of water and 0.5 parts by weight of sodium hydrogen sulfite was added by spraying in 100 parts by weight of the water-absorbing resin (3). The hollow cone nozzle 1/4M-K010 (droplet diameter: 70 μm to 150 μm) described above was used for spraying. The obtained mixture was covered to a thickness of 5 cm, and aged in a vent type hot air dryer at a temperature of 80 ° C for 30 minutes. The aged mixture was sieved through a metal mesh having a mesh size of 850 μm to obtain a water absorbing agent (5).
[Comparative Example 4] A water absorbing agent (4) was used as a control directly with the untreated water-absorbing resin (3).
[Example 6] In Example 5, an aqueous solution containing 5 parts by weight of water and 0.5 parts by weight of sodium hydrogen sulfite was mixed, and a hollow cone nozzle was replaced by a straight tube having an inner diameter of 1 mm, and the aqueous solution was a droplet having a diameter of 2 mm. The way to add and mix. Then, the same treatment as in Example 5 was carried out to obtain a water absorbing agent (6).
[Comparative Example 5] The above-mentioned untreated water-absorbing resin (4) was used as a water absorbing agent (5) as a control.
[Preparation of water absorbing agent (7) according to Example 7] 2.3 g of polyethylene glycol acrylate (average molar number of ethylene oxide added: 9) was dissolved in 2000 g with a neutralization ratio of 75% molar ratio (single Body concentration: 39% by weight of an aqueous solution of sodium acrylate to obtain a reaction mixture. The above reaction mixture was injected into a stainless steel carrier having a length of 320 mm, a width of 220 mm and a height of 50 mm. The height of the reaction mixture was 23 mm. The upper portion of the stainless steel carrier was sealed with a polyethylene film having a nitrogen gas injection port, an exhaust port, and a polymerization initiator injection port, and placed in a water bath at 25 °C. The reaction mixture was maintained at a temperature of 25 ° C and nitrogen was injected to remove oxygen dissolved in the solution. Nitrogen gas is then continuously injected into the space above the reaction tank and vented from the other side. 8.0 g of a 10% by weight aqueous solution of sodium persulfate was added as a polymerization initiator and 1.4 g of a 1% by weight aqueous solution of L-ascorbic acid, followed by thorough mixing with a magnetic stir bar. The polymerization was started two minutes after the addition of the polymerization initiator, and the stainless steel was bubbled in a water bath at a liquid temperature of 12 ° C, and the liquid surface was soaked 10 mm from the bottom of the carrier to control the polymerization temperature. After 15 minutes from the start of the polymerization, the polymerization reached a maximum temperature of 85 °C. The carrier was bubbled at a temperature of 60 ° C and the liquid surface was aged 10 mm from the bottom of the carrier to maintain the gel for 20 minutes. The obtained hydrocolloid polymer was pulverized by a chopper having a pore size of 9.5 mm (No. 32 meat grinder manufactured by Hiraga Seisakusho Co., Ltd.) to obtain a granulated hydrocolloid polymer (polymer gel). The water gel polymer (polymer gel) was spread on a mesh of 50 mesh (mesh size: 300 μm), and then dried by hot air at 180 ° C for 30 minutes using a hot air circulating batch drying oven (Satake Chemical Equipment Mfg., Ltd. "type 71-S6" manufactured). Then, the dried product is subjected to a gel grinding step in a barrel manner, and the ground product is classified by a mesh having a mesh size of 850 μm and 150 μm to obtain an interconnected polymer particle (e) having an irregularly ground shape having a particle diameter of 150 μm to 850 μm. And crosslinked polymer particles (f) having a particle diameter of less than 150 μm.
The above operation was repeated to prepare 300 g of the crosslinked polymer particles (f). The obtained 300 g of the crosslinked polymer particles (f) was placed in a 5 L 臼 research mixer (manufactured by NISHI NIPPON SHIKENKI: KK) and placed in a water bath at 80 ° C, and the aqueous solution (s1) was placed once for small particle agglomeration. And maintained at 80 ° C, the mixing blade of the 臼 research mixer was subjected to high-speed stirring at 60 Hz / 100V. The aqueous solution (s1) to be placed was an aqueous solution prepared by dissolving 0.08 g of sodium persulfate in 450 g of water at 80 ° C, and was used immediately after dissolution. After the addition for 10 seconds, the particulate crosslinked polymer particles (f) were mixed with the aqueous solution to start agglomeration. After 10 minutes from the addition of the aqueous solution, an agglomerated gel having a particle size of 3 to 10 mm was produced. Then, 200 g of the agglomerated gel obtained by the above polymerization step and the above gel grinding step and 1800 g of a granulated hydrogel polymer (polymer gel) were slightly stirred, and the mixture was spread on a mesh of 50 mesh ( Mesh size: 300 μm), and dried by hot air drying at 180 ° C for 30 minutes using a hot air circulation type batch drying oven ("type 71-S6" manufactured by Satake Chemical Equipment Mfg., Ltd.) to obtain a dried product. The dried product was ground by a barrel method, and the ground product was classified by a sieve of 850 μm and 150 μm mesh to obtain crosslinked polymer particles (g) having an irregularly ground shape of a particle diameter of 150 μm to 850 μm.
100 parts by weight of the crosslinked polymer particles (g) were sprayed with 3.5 parts by weight of an aqueous surface crosslinking agent solution containing 0.3 parts by weight of 1,4-butanediol and 0.2 parts by weight of 3-ethyl group. 3-hydroxymethylepoxypropyl ring and 3 parts by weight of water. These mixtures were subjected to heat treatment at 210 ° C for 40 minutes in a mixer to obtain a water-absorbing resin (7) of Example 7.
[Comparative Example 6] In the small particle agglomeration step, the same crosslinked polymer particles (g), crosslinked polymer particles (h) having a particle diameter of 150 μm to 850 μm and having an irregularly ground shape, and 450 g of water at 80 ° C were used. Replace the aqueous solution (s1). The crosslinked polymer particles (h) produced by 100 parts by weight are sprayed and mixed with a 3.5 weight by weight aqueous surface crosslinking agent solution, and the aqueous surface crosslinking agent solution contains 0.3 parts by weight of 1,4-butanediol and 0.2. Part by weight of 3-ethyl-3-hydroxymethyloxetane and 3 parts by weight of water. The mixture was subjected to heat treatment at 210 ° C for 40 minutes in a mixer to obtain a comparative water absorbing agent (6).
[Comparative Example 7] 0.3 parts by weight of sodium hydrogen sulfite (a chemical special grade manufactured by Wako Pure Chemical Industries, Ltd.) and 5 parts by weight of water were contained in 100 parts by weight of the crosslinked polymer particles (h). The aqueous solution (s2) was mixed. Further, a 3.5-part weight aqueous surface crosslinking agent solution was spray-mixed in a mixture of 100 parts by weight, the solution containing 0.3 parts by weight of 1,4-butanediol and 0.2 parts by weight of 3-ethyl- 3-hydroxy-3-hydroxymethyloxetane and 3 parts by weight of water. The mixture was subjected to heat treatment at 210 ° C for 40 minutes in a mixer to obtain a control water absorbing agent (7) according to Comparative Example 7.
[Comparative Example 8] A 0.05 part by weight of a water absorbing agent (6) obtained in Comparative Example 6 was added with 0.05 parts by weight of sodium hydrogen sulfite (a chemical special grade manufactured by Wako Pure Chemical Industries, Ltd.). And 30 parts by weight of an aqueous solution of water (s3) and mixed. The obtained mixture was dried at 70 ° C for 2 hours using a hot air circulation type batch drying oven ("type 71-S6" manufactured by Satake Chemical Equipment Mfg., Ltd.). Since the dried product after drying became a hard block, it could not be sieved with a mesh having a mesh size of 850 μm as in the case of Example 1. Therefore, it was necessary to grind the dried product again by a barrel method, and then sieved with a metal mesh having a mesh size of 850 μm to obtain a comparative water absorbing agent (8).
The amount of the residual monomer of the water absorbing agent, the residual monomer index, the absorption force without force, the absorption force at a force of 4.8 kPa, the single layer absorption force at a force of 1.9 kPa, and the particle diameter of less than 150 μm were measured according to the following method. Weight ratio, Hunter brightness, and moisture content. The measured temperature and relative humidity conditions were 25 ° C ± 2 ° C, 50% RH, respectively. The physiological saline used was a 0.90% by weight aqueous solution of sodium chloride.
[Residual monomer (ppm) amount] The water absorbing agent in 0.500 g was dispersed in 1000 ml of deionized water and stirred with a magnetic stirrer of 50 mm length for 2 hours to extract residual monomers. Then, the gel of the sputum was filtered with a filter paper (Toyo Roshi Kaisha Ltd., manufactured by No. 2, filter particle size according to JIS P 3801: 5 μm). The mixture was filtered through a filter paper and filtered through a sieve chromatography disk 25A to carry out a pretreatment for measuring residual monomers by HPLC (manufactured by Kurabo Industries Ltd. for an aqueous solution system, pore size: 0.45 μm). A sample of the residual monomer was measured and analyzed by a high performance liquid chromatography analyzer (HPLC). A calibration curve is obtained using a monomer concentration solution of known concentration. The amount of residual monomer in the water absorbing agent is analyzed quantitatively in consideration of the dilution ratio of the water absorbing agent in deionized water. The measurement conditions of HPLC were as follows.
Carrier solution: diluted with 3 ml of phosphoric acid (85% by weight, a chemical special grade manufactured by Wako Pure Chemical Industries, Ltd.) in 1000 ml of ultrapure water (resistance: not less than 15 MΩ.cm) The obtained aqueous phosphoric acid solution.
Carrier solution flow rate: 0.7 ml/min.
Column: SHODEX RSpak DM-614 (manufactured by Showa Denko K.K.) Column temperature: 23±2°C Wavelength: UV 205nm
[Absorption of physiological saline (CRC) without force] 0.20 g of the water absorbing agent was uniformly placed in a bag made of non-woven fabric (60 mm X 85 mm) and bubbled in physiological saline at 25 ± 2 ° C water. After 30 minutes, the bag was taken out of the solution, and dehydrated by a centrifuge (Model Centrifuge Model H-122 manufactured by Kokusan Corporations) at 250 G (250 x 9.81 m/sec 2 ) for 3 minutes, and then the bag was weighed to obtain W2 (g). . The weight W1 (g) of the bag without the water absorbing agent in the bag was measured in the same manner. The absorption force (g/g) was determined from the following formula by the weight of W1 and W2.
CRC(g/g)=(W2-W1)/W0-1
Wherein W0 represents the weight of the water absorbing agent.
[4.8 kPa absorption of physiological saline at high stress (AAP 4.8 kPa: absorption force under force)] 0.900 g of water absorbing agent was uniformly dispersed in a 400 mesh stainless steel metal mesh welded to an inner diameter of 60 mm plastic cylinder (Mesh size: 38 μm). Using a piston with an outer diameter of slightly less than 60 mm, there is no gap between the inner surface of the piston and the cylinder and it can slide up and down and press against the water absorbing agent. Measure the weight of the plastic cylinder, water absorbing agent and piston W3 (g). The weight of the water absorbing agent can be uniformly squeezed at 4.8 kPa, and the calculated weight includes the weight of the piston, thus completing the erection of the measuring device. A glass sieve having a diameter of 90 mm and a thickness of 5 mm was placed on a petri dish having a diameter of 150 mm, and poured into a physiological saline solution controlled at a temperature of 25 ± 2 ° C to bring the liquid surface to the upper surface of the glass filter. A filter paper of diameter 9 (Toyo Roshi Kaisha Ltd. No. 2) was placed on the surface of the glass filter to be wetted, and excess solution was removed.
Place the measuring device on the filter paper to allow the liquid to be absorbed by the stressed water absorbing agent. When the liquid level is lower than the upper surface of the glass filter, the liquid is added to maintain the liquid level constant. After one hour, the measuring device was taken out and weighed W4 (g) (cylinder, moisturizing agent and weight of the piston). The absorption force (g/g) at a high force of 4.8 kPa was obtained from the weights W3 and W4 according to the following formula.
AAP 4.8kPa (g / g) = (W4-W3) / W0
[1.9 kPa force to single saline absorption of physiological saline (SAAP; single layer AAP 1.9 kPa: absorption force when single layer is stressed)] The measurement method is the same as the above 4.8 kPa high force absorption method. The weight of the water absorbing agent was changed from 0.900 g to 0.200 g, and the weight of the weight was adjusted so that 1.9 kPa was uniformly applied to the water absorbing agent, including the weight of the piston. The single layer absorption force (g/g) at a force of 1.9 kPa can be obtained by the following formula.
SAAP(g/g)=(W6-W5)/W0 (wherein W5 represents a cylinder, the weight of the water absorbing agent before water absorption and the weight of the piston; W6 represents a cylinder, the water absorbing agent absorbs water and the weight of the piston; and W0 represents the weight of the water absorbing agent) .
[Particle weight ratio of particle diameter less than 150 μm] The water absorbing agent was sieved and classified by a JIS standard sieve of meshes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, 106 μm and 45 μm. A weight ratio of particles smaller than 150 μm. Further, the particle size distribution ratio R is plotted on a log-square paper. From the figure, the mass median diameter (D50) was determined from the particle diameter of R = 50% by weight. Further, the logarithmic standard deviation (σζ) is expressed as follows, wherein the smaller σζ indicates that the particle size distribution is more concentrated.
Ζζ=0.5 x ln(X2/X1)
Here, X1 and X2 represent a particle diameter of R = 84.1% by weight and a particle diameter of R = 15.9% by weight, respectively.
When the sieve is classified, 10.00 g of the water absorbing agent is placed in a JIS standard sieve (IIDA TESTING SIEVE: 80 mm inner diameter) having a different mesh size and a sieve shaker (Model ES-65 shake of Iida Seisakusho Co., Ltd.) is used. The machine was shaken for 5 minutes.
[Hunter brightness (WB)] The Hanter brightness was measured using a Korean characteristic meter. The larger the value of Hunter brightness, the higher the brightness.
[Water vapor content (H) of water absorbing agent] An accurate 2.000 g of water absorbing agent was weighed and placed in an aluminum cup having a bottom surface diameter of about 50 mm, and then the total weight W7 (g) of the water absorbing agent and the aluminum cup was weighed. It was then dried in an oven at 180 ° C for 3 hours. After three hours, the aluminum cup and the water absorbing agent were taken out from the oven and cooled to room temperature in a dry box. The total weight W8 (g) of the water absorbing agent and the aluminum cup after drying was measured, and the water vapor content was determined according to the following formula.
H (% by weight) = (W7-W8) / W0
[Sulfur-based volatile component (ppm) of water absorbing agent] 6.00 g of the water absorbing agent obtained in each of the examples and the comparative examples was uniformly dispersed in a glass petri dish (SOGO LABORATORY GLASS WORKS CO., LTD. Code: 305-07 in the A-1000 in the catalog, outer cymbal: 120mm x height: 25mm). The water absorbing agent was then covered with a circular heat permeable heatlon paper (Nangoku Pulp Technical Corporation, type: GSP-22) cut into a diameter of 116 mm. Non-woven fabrics can also be used instead of heatlon paper. The outer ring of the heatlon paper or non-woven fabric was fixed to the inner surface of the glass petri dish with three 10 mm x 10 mm tapes. One side of the 3 L suction bag was opened (manufactured by OMI ODORAIR SERVICE Corporation), and the glass petri dish having the dispersed water-absorbing resin was placed in an air bag, and the side where the air bag was opened was closed with a tape. After the air in the inhalation bag is discharged from the glass tube on the inhalation bag, it is filled with 1.2 L of odorless air, and 30 ml of a 0.90% by weight aqueous solution of sodium chloride (physiological saline solution) is adjusted to 25 ± 2 After the temperature of °C, pour into the plate in the suction bag. To avoid contamination by air, use a glass tube connected to a Teflon tube (registered trade name). Then let the water absorbing agent evenly moisturize, and seal the tape with silicone rubber to achieve watertightness. The bag was placed in a 37 ° C incubator to make it moist. After 60 minutes, the bag was taken out and placed at room temperature. After returning to room temperature for 10 minutes, the silicone was removed, and the atmosphere concentration was measured using a gas sampler (GV-100S manufactured by Gastec Corporation) and a gas detecting tube (No. 5Lb manufactured by Gastec Corporation for measuring sulfur dioxide). Avoid being polluted by the surrounding air. The measured atmospheric concentration is defined as the sulfur-based volatile component (ppm) volatilized by the water absorbing agent.
At the time of measurement, even if there is only physiological saline and no water absorbing agent is used, the gas detecting tube may cause discoloration. Therefore, the blank control value obtained from physiological saline alone must be subtracted as a correction (detection limit: 0.05 ppm).
The measurement results of the above physical properties are as shown in Tables 1 and 2.
[Odor Test] Sensory evaluation was performed on the water absorbing agents (1) to (7) and the comparative water absorbing agents (1) to (8), respectively. About 1 part by weight of the water absorbing agent was moistened with 20 parts by weight of a 0.9% by weight physiological saline solution, and maintained at 37 ° C and water-tight for 1 hour. Then ten adults were selected for the odor test. As for the evaluation method, if there is no odor, it is 0; the odor is recorded as 5 points; and it is evaluated according to the degree of dissatisfaction of the tester. The odor score was based on the average score of 10 testers. The lower the odor score, the less odor is produced. The obtained odor scores are as shown in Tables 1 and 2. The water-absorbing resin (1) was used as a standard for comparison. Therefore, the odor score evaluated by the water-absorbing resin (1) was set to 3.0.
From the results of the odor test, it can be seen that the control water absorbing agent (2) prepared directly from the sodium bisulphite particle state as a sulfur-containing reducing agent produces a special odor of sodium hydrogen sulfite, which causes the tester's discomfort. . Even if sodium hydrogen sulfite in an aqueous solution state is added, if the subsequent step is subjected to a high temperature heat treatment (for example, at 210 ° C in Comparative Example 7), the unpleasant feeling of the tester increases. In contrast, when the residual monomer in the small particle agglomeration step is lowered as in Example 7, or after the surface crosslinking is completed after the high temperature treatment, the sulfur-containing reducing agent is added to the water-absorbing resin, the tester's unpleasant feeling is lowered.
[Evaluation of absorption core (evaluation of variation of residual monomer amount)] Using water absorbing agent (3) and short fiber wood pulp of Example 3 as hydrophilic fibers, a water absorbing core containing 10 g of water absorbing agent and 10 g of short fiber wood pulp was prepared. . The detailed preparation method of the water absorbing core is explained below. 100 parts by weight of the water absorbing agent (3) was placed in the feed port, and the water absorbing agent was intermittently dropped onto the metal mesh according to the vibration feeder and the switch of the conveyor belt to adjust the amount of the feed. On the other hand, the amount required to drip the short-fiber wood pulp into the metal mesh is set to the point of time before the start of the dripping of the water absorbing agent (3) to the end point required for the amount of the water absorbing agent (3) to be dropped. The time between them can be unwrapped. The water absorbing core was prepared by dropping the water absorbing agent (3) and the short fiber wood pulp on a metal wire having a length of 12 cm, a width of 12 cm, and a mesh of 150 μm, and sucked from the bottom of the metal mesh. First, supply the short-fiber wood pulp, and then open the vibrating feeder of the water absorbing agent (3). After the formation of the short-fiber wood pulp layer on the metal mesh, start to drip the required amount of water absorbing agent within a predetermined time (3) )time. Repeat this step to prepare 10 pieces of water-absorbing cores (1) to (10). Each of the water absorbing cores (1) to (10) was placed in a polyethylene bag (length: 20 cm, width: 24 cm). After closing the opening of the bag, shake the bag. The separated water absorbing agent (3)-1~(3)-10 can be obtained by separating the water absorbing agent (3) from the short fiber wood pulp. The residual monomers of the respective water absorbing agents are as shown in Table 3.
The above water-repellent agent (5)-1 to (5)-10 was obtained by repeating the above procedure with reference to the control water absorbing agent (5) obtained in Example 5. The residual monomer of each water absorbing agent is as shown in Table 3.
In Tables 1 and 2, Comparative Examples 1, 2 and Comparative Example 1, the results of Comparative Examples 3 and 4 and Comparative Example 3 show that the water absorbents of Examples 1 to 4 contain low amounts of residual monomers, and A large range of particle size distributions with small residual monomer variability and compliance with hygiene. Moreover, Comparative Example 2 and Comparative Example 2, and Comparative Examples 5 and 6 show that the sulfur-containing reducing agent and the aqueous sulfur-containing reducing agent solution are uniformly mixed with the water-absorbing resin to obtain a water-absorbing agent having the above physical properties. Further, although the addition of the amount of the polymerization initiator in Comparative Example 5 can reduce the amount of residual monomers, in other examples, the residual monomer index cannot be lowered to the desired range, and in addition, the Hunter brightness is lowered. And the absorption force at 4.8 kPa with high force.
Comparative Example 7 and Comparative Example 6 show that when the amount of small particle residual monomers in the small particle agglomerating gel step is lowered, the residual monomer and residual monomer index in the obtained water absorbing agent are also lowered. Moreover, according to Comparative Example 5, the method selected in Example 7 can more effectively reduce the residual monomer and residual monomer index compared to the amount of polymerization initiator in the polymerization process.
From the comparison of Example 7 with Comparative Example 8, it is understood that although the residual monomer can be lowered according to the method of Comparative Example 8, the residual monomer index cannot fall within the desired range. Further, since the water vapor content is too high, the proportion (solid content) of the water-absorbing resin component in the water absorbing agent in Comparative Example 8 is lowered, so that the absorption force without force and the absorption force at high stress are lowered. Moreover, in Comparative Example 8, the grinding step was performed again after the surface cross-linking treatment to grind the hard agglomerate, but this step destroyed the cross-linked layer of the surface, and the absorption force at the time of the force-reducing was lower than the decrease in the solid content. Big.
As in Tables 1 and 2, Example 3 and Comparative Example 5 had the same residual monomers. However, as can be seen from the third table, the variation of each of the water-absorbing cores in Example 3 was small and the residual monomer index could be lowered, although the amount of the residual monomers was substantially the same as in Comparative Example 5.
As described above, according to the method for producing a water absorbing agent of the present invention, a water absorbing agent of a low residual monomer and a residual monomer amount which is small in variation in a particle size distribution range can be obtained. The water absorbing agent does not discolor, does not emit an odor, and has a balance of preferable physical properties without causing a problem that physical properties are deteriorated due to deterioration of the polymer.
The water absorbing agent of the present invention has better physical properties, a low amount of residual monomers, and a variation in residual monomer amount in a small particle size distribution range, and therefore, when applied to a water absorbing material such as a disposable diaper, the product can be reduced. Variation and use as a thin absorbent for hygiene products.
The water absorbing agent of the present invention can be applied to water absorbing agents requiring water absorption and water retention such as sanitary articles such as disposable diapers and sanitary napkins, water retaining agents for medical use, water retaining agents for agricultural and horticultural use, and other industrial dehydrating agents.
The above-described examples are for illustrative purposes only, and various modifications may be made without departing from the scope of the invention.
A particulate water absorbing agent having a water-absorbing resin as a main component, wherein the absorption resin has a crosslinked structure composed of unsaturated carboxylic acid and/or a salt thereof as a constituent unit and is surface-treated with a crosslinking agent In the cross-linking treatment, the water absorbing agent has particles having a particle diameter of 300 to 850 μm and particles having a particle diameter of less than 300 μm, wherein the amount of residual monomers is not less than 0 but not higher than 500 ppm, and the residual monomer index calculated by the formula (1) RMI) is not more than 0.30: RMI = | RM 1 - RM 2 | / RM A (1) wherein RM 1 represents the residual monomer amount of the water absorbing agent particles having a particle diameter of less than 300 μm; RM 2 represents a water absorbing agent having a particle diameter of 300 to 850 μm. The residual monomer amount of the particles; and RM A represents the residual monomer amount of the water absorbing agent.
The water absorbing agent of claim 1, wherein the amount of the residual monomer is not less than 0 but not more than 300 ppm.
A water absorbing agent according to claim 1, wherein the main component of the unsaturated carboxylic acid is acrylic acid, and the residual monomer is acrylic acid and/or a salt thereof.
The water absorbing agent according to claim 1, wherein the weight ratio of the particles having a particle diameter of 300 to 850 μm to the particles having a particle diameter of less than 300 μm is not less than 20/80 but not more than 80/20.
For example, in the water absorbing agent of claim 1, wherein the physiological saline solution has a force absorption force (CRC) of not less than 30 g/g, and absorption of a physiological saline solution of 4.8 kPa (AAP 4.8 kPa) at a high force. The force is not less than 20g/g.
For example, in the water absorbing agent of claim 1, wherein the total absorption of physiological saline (TAC) is calculated by the formula (2) as follows, and is not less than 65 g/g: TAC (g/g) = CRC + SAAP (2) Wherein CRC represents the unabsorbed force (g/g), while SAAP (single layer AAP) represents a single layer of absorbance at 1.9 kPa (g/g).
The water absorbing agent according to claim 1, wherein the content of the particles having a particle diameter of less than 150 μm is not more than 2% by weight.
The water absorbing agent of claim 1, wherein the particles have an irregular abrasive shape.
A water absorbing agent according to claim 1, which has an oxidizing agent, a reducing agent or a persulfate salt and a water-absorbing resin.
A water absorbing agent according to claim 1, which has a sulfur-containing reducing agent and a water-absorbing resin.
The water absorbing agent according to claim 1, wherein the water-absorbing resin has a crosslinked structure composed of unsaturated carboxylic acid and/or a salt thereof and is surface-crosslinked by a surface crosslinking agent on the surface thereof; Sulfur reducing agent; The absorption capacity (CRC) of the physiological saline solution is not less than 32g/g, and the absorption force of the physiological saline solution at a high force of 4.8kPa (AAP 4.8kPa) is not less than 20g/g, by gas detection. The test tube measures the concentration of the sulfur-based volatile component of not less than 0 but not more than 2.5 ppm.
A water absorbing material having any one of the water absorbing agents of the first to eleventh aspects of the patent application.
A method for producing a water absorbing agent, comprising: a first step of preparing a water gel polymer by polymerizing an aqueous monomer solution containing an unsaturated carboxylic acid and/or a salt thereof and a crosslinking agent; drying the water gel polymer a second step of grinding and classifying the particle size distribution to prepare particles having a particle size of 300 to 850 μm and particles having a particle diameter of less than 300 μm as a main component of the particulate water-absorbing resin precursor; by heating the water-absorbing resin precursor and surface a third step of preparing a mixture of crosslinking agents to form an ester bond on the surface of the water-absorbing resin precursor to prepare a water-absorbing resin; and a fourth step of adding an aqueous solution containing a sulfur-containing reducing agent to the water-absorbing resin by spraying; And a fifth step of heat-treating the mixture of the water-absorbing resin and the aqueous solution under hot air of not lower than 40 ° C but not higher than 120 ° C.
The method for producing a water absorbing agent according to claim 13 , wherein the water absorption resin obtained by the third step has an unabsorbable force (CRC) of not less than 30 g/g on physiological saline, and is physiological saline. The absorption force at a high force of 4.8 kPa (AAP 4.8 kPa) is not less than 20 g/g.
The method for producing a water absorbing agent according to claim 13 , wherein the total absorption (TAC) of the water-absorbing resin obtained by the third step to physiological saline is not less than 65 g/g, wherein the TAC can be determined by the formula (3) Calculated: TAC(g/g)=CRC+SAAP (3) where CRC represents unabsorbed force (g/g) and SAAP (single layer AAP) represents a single layer of 1.9 kPa (g/g) ) The absorption of force.
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