WO2007119730A1 - Method for production of modified water absorbent resin - Google Patents

Method for production of modified water absorbent resin Download PDF

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Publication number
WO2007119730A1
WO2007119730A1 PCT/JP2007/057936 JP2007057936W WO2007119730A1 WO 2007119730 A1 WO2007119730 A1 WO 2007119730A1 JP 2007057936 W JP2007057936 W JP 2007057936W WO 2007119730 A1 WO2007119730 A1 WO 2007119730A1
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Prior art keywords
absorbent resin
water absorbent
water
weight
production
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PCT/JP2007/057936
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French (fr)
Inventor
Yoshiro Mitsukami
Makoto Matsumoto
Taku Iwamura
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Nippon Shokubai Co., Ltd.
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Priority to JP2008526310A priority Critical patent/JP2009533485A/en
Publication of WO2007119730A1 publication Critical patent/WO2007119730A1/en

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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J3/00—Processes of treating or compounding macromolecular substances
    • C08J3/28—Treatment by wave energy or particle radiation
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2333/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
    • C08J2333/04—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters
    • C08J2333/06—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters of esters containing only carbon, hydrogen, and oxygen, the oxygen atom being present only as part of the carboxyl radical
    • C08J2333/08—Homopolymers or copolymers of acrylic acid esters

Abstract

This invention is to provide a method for producing a modified water absorbent resin having excellent production efficiency, and manifesting excellent absorbency against pressure, absorption speed, gel strength, a liquid permeability, and the like. This invention relates to a method for the production of a modified water absorbent resin, which comprises a) a mixing step comprising mixing a water absorbent resin, water, and a water-soluble radical polymerization initiator without addition of an ethylenically unsaturated monomer, to obtain a water absorbent resin composition, and b) an irradiating step comprising irradiating said water absorbent resin composition obtained in the mixing step with active energy rays, wherein a surface water content of said water absorbent resin in said water absorbent resin composition at least at any point of time in the irradiating step is controlled to a level of not lower than 3.0% by weight based on 100% by weight of the water absorbent resin.

Description

DESCRPTION
METHOD FOR PRODUCTION OF MODIFIED WATER ABSORBENT RESIN
Technical Field:
This invention relates to a method for the production of a modified water absorbent resin. More particularly, it relates to a method for the production of a modified water absorbent resin by irradiating active energy rays to a water absorbent resin.
Background Art:
A water absorbent resin has been hitherto used as a component for hygienic materials such as sanitary cotton, disposable diaper, and absorbents for other kinds of body fluid. As typical examples of the water absorbent resin, hydrolyzate of starch-acrylonitrile graft polymer, neutralized starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic ester copolymer, hydrolyzate of acrylonitrile copolymer or acrylamide copolymer, and cross-linked product thereof, and partially neutralized cross-linked acrylic acidmaybe cited. These water absorbent resins have an internal cross-linked structure and are in-soluble in water. The characteristics demanded with such a water absorbent resin include high absorption capacity, high absorption speed, high gel strength, and fully satisfactory suction power necessary for sucking water from a medium, for example. Since the water absorbing properties are affected by cross-link density, however, they do not necessarily manifest positive correlations with one another as evinced by the fact that an increase in the cross-link density leads to an increase in the gel strength but a decrease in the amount of water absorbed. Particularly, the absorption capacity is in a contradictory relation with the absorption speed, the gel strength, and the suction power, for example. The water absorbent resin having improved absorption capacity, therefore, would possibly induce inhomogeneous absorption of water and form aggregations when the water absorbent resin particles contact with water, and also induce dramatic decrease in absorption speed because the water is not diffused throughout the entire volumes of water absorbent resin particles .
For the purpose of relaxing this phenomenon and obtaining a water absorbent resin having high absorption capacity and a comparatively satisfactory absorption speed, a method for coating a surface of water absorbent resin particles with a surfactant or a nonvolatile hydrocarbon has been available. This method indeed can exalt the dispersibility of initially absorbed water but bring no sufficient effects on enhancing absorption speed and suction power of individual resin particles.
As a means to produce a polyacrylic acid based water absorbent polymer having improvedwater absorbingproperties , a method which comprises heating an aqueous composition of a polymer having a partial alkali metal salt of polyacrylic acid as a main component and having a low cross-link density in the presence of a water-soluble peroxide radical initiating agent thereby introducing a cross-link therein by radical cross-linking has been proposed (U.S. Patent No. 4,910,250). It is difficult to distribute uniformly internal cross-links in the polymer and uneasy to adjust the cross-link density. Accordingly, a polymer which contains water-soluble polyacrylic acid gel having low cross-link density is obtained and then the polymer is heated together for example with a persulfate added thereto as a polymerization initiator. U.S. Patent No. 4,910,250 states that excellent water absorbing properties can be attained and a water absorbent resin having no stickiness can be obtained because the adjustment of the amount of the initiator to be added can allow precise control of cross-link density and uniform presence of cross-link in the polymer.
While the persulfate which is used in the U.S. Patent No. 4,910,250 is decomposed by heat, it is also decomposed by ultraviolet rays to generate radicals (J. Phys . Chem. , 1975, 79, 2693, J. Photochem. Photobiol., A. 1988, 44, 243). Since the persulfate acts as a polymerization initiator, the aqueous solution of a water-soluble vinyl monomer, when exposed to radiation, undergoes polymerization and radical cross-linkage simultaneously to produce a hydrogel (JP-A 2004-99,789) . A reaction system has been also known, which comprises adding a hydrophilic polymer component and a photo-polymerization initiator, further adding a cross-linking agent thereto, and irradiating them with ultraviolet rays to form an internal cross-link (WO 2004/031253) .
Meanwhile, a method which comprises treating a surface of a water absorbent resin to increase cross-link density of the surface of water absorbent resin has been also known
(U.S. Patent Nos .4, 666, 983 and 5, 422, 405, for example) . Such water absorbent resins as cited in the preceding publications entail the presence of a reactive functional group on their surfaces. By adding a surface cross-linking agent capable of reacting with the functional groups to a water absorbent resin to introduce a cross-link between the functional groups , cross-link density on the surface of water absorbent resin can be increased and a water absorbent resin having excellent water absorbing properties even under pressure can be obtained.
Further, since the use of the surface cross-linking agent requires a high temperature and a long time for the reaction of forming cross-link and entails the problem of suffering persistence of unaltered cross-linking agent, a method which comprises contacting an aqueous solution containing a peroxide radical initiating agent with a resin, heating the resin to decompose the radical initiating agent and introduce cross-links into polymer molecular chains in the neighborhood of the surface of the resin has been proposed (U.S. Patent No. 4,783,510) . In the working example, a water absorbent resin exhibiting exalted absorption capacity was obtained by heating with superheated steam at 1300C for β minutes.
Here, a technique wherein on surface cross-linking of water absorbent resin particles by adding a surface crosslinking agent, a water absorbent property of the water absorbent resin particles is improved by controlling a water content or a water amount of the whole of said water absorbent resin particles, is known (for example, U.S. Patent Nos. 4,541,871 and 4,507,438).
Disclosure of Invention: The object of introducing surface cross-links into a water absorbent resin is directedtoward a method for producing a water absorbent resin having a perfect balance between absorption capacity and absorption speed. Generally, this object requires a cross-linking agent having at least two functional groups capable of reacting with the functional group present on the surface of the water absorbent resin to act on the water absorbent resin. As typical examples of the cross-linking agent, polyhydric alcohols, polyvalent glycidyl ethers, haloepoxy compounds, polyvalent aldehydes, polyvalent amines, and polyvalent metal salts may be cited. Since the cross-linking agent has low reactivity, the relevant reaction is required to be carried out at an elevated temperature and occasionally to be retained in a heated state for a long time. The reaction, therefore, demands copious amounts of energy and time.
Even in a method as described in the U.S. Patent Nos. 4,541,871 and 4,507,438, wherein on introducing a cross-linking structure on surface of water absorbent resin particles by adding a surface cross-linking agent, a water content or a water amount of the whole of the water absorbent resin particles is controlled, it is demanded to introduce surface cross-links more effectively and to produce a water absorbent resin having more excellent water absorbent properties. A satisfactory production method, however, has not been provided, which is an existing state.
In such an existing state, this invention is aimed at providing a method for the production of a water absorbent resin which is so modified as to excel in the efficiency of production and in such properties as absorbency against pressure, absorption speed, gel strength, and permeability of liquid.
The present inventors have studied in detail production methods of a water absorbent resin of which a surface is modified. In the process, they have intensively searched the causes why in conventional methods wherein a water content or a water amount of the whole of water absorbent resin particles is controlled, a water absorbent resin having sufficiently excellent water absorbent properties cannot effectively be produced. As a result, they have found that surfaces of the particles are required to retain water to some extent in order to effectively introduce a cross-linking structure on surfaces of water absorbent resin particles. Namely, they have found that in conventional methods, even if a water content or a water amount of the whole of water absorbent resin particles is controlled, when surfaces of the particles are not humid to some extent, introduction of a cross-linking structure on surfaces of particles cannot effectively be attained, and therefore a water absorbent resin having excellent water absorbent properties cannot be produced. Based on such knowledge, the present inventors have attempted to conduct surface treatment on a surface of water absorbent resin particles in a state of being humid to some extent. In addition, the present inventors have also found that cross-linking efficiency and water absorbent properties of the resultant water absorbent resin can be improved, by adopting as a means of introducing surface cross-links irradiation with active energy rays without using conventional addition of a surface cross-linking agent.
Consequently, the present invention has been accomplished.
Namely, the present invention is to provide a method for the production of a modified water absorbent resin, which comprises a) a mixing step comprising mixing a water absorbent resin, water, and a water-soluble radical polymerization initiator without addition of an ethylenically unsaturated monomer, to obtain a water absorbent resin composition, and b) an irradiating step comprising irradiating said water absorbent resin composition obtained in the mixing step with active energy rays, wherein a surface water content of said water absorbent resin in said water absorbent resin composition at least at any point of time in the irradiating step is controlled to a level of not lower than 3.0% by weight based on 100% by weight of the water absorbent resin.
According to the method of this invention, a cross-linked structure can be uniformly introduced on a surface of water absorbent particles. As a result, the water absorbent resin which has been modified by the method remarkably excels in such characteristics as absorption capacity, absorption speed, gel strength, and suction power which the water absorbent resin is expected to have.
Since the method of this invention for the production of a modified water absorbent resin comprises the surface cross-linkage by irradiation with the active energy rays, the water absorbent resin can be modified in a shorter period as compared with the conventional method.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.
Brief Description of Drawings:
Fig. 1 is a schematic diagram of a measuring device to be used in determining the saline flow conductivity (SFC) .
Best Mode for Carrying Out the Invention:
This invention is directed toward a method for the production of a modified water absorbent resin, which comprises a) a mixing step comprising mixing a water absorbent resin, water, and a water-soluble radical polymerization initiator without addition of an ethylenically unsaturated monomer, to obtain a water absorbent resin composition, and b) an irradiating step comprising irradiating said water absorbent resin composition obtained in the mixing step with active energy rays, wherein a surface water content of said water absorbent resin in said water absorbent resin composition at least at any point of time in the irradiating step is controlled to a level of not lower than 3.0% by weight based on 100% by weight of the water absorbent resin.
Now, the method for the production of a modified water absorbent resin according to this invention will be described in detail below. The scope of this invention does not need to be restricted by this description but may be executed in other than the following illustrations as properly altered without departure from the purport of the invention, (a) Water absorbent resin
The water absorbent resin which can be used in this invention is a water-swellable, water-insoluble, cross-linked polymer which can form a hydrogel . As mentioned in detail below, the present invention is characterized in controlling a surface water content of the water absorbent resin in the water absorbent resin composition at least at any point of time in the irradiating step to a level of not lower than 3.0% by weight based on 100% by weight of the water absorbent resin. Accordingly, when the water absorbent resin has been already in a hydrated state prior to mixing step and manifests a surface water content of not less than 3.0% by weight without any addition of water at the start of the next irradiating step, additional water need not to be added in the mixing step. The term "ability to swell in water" as used in this invention refers to the free swelling capacity of a given sample in an aqueous 0.9% by weight sodium chloride solution (physiological saline), i.e. the ability of the sample to absorb the physiological saline essentially not lower than 2 g/g and preferably in the range of 5 - 100 g/g and more preferably in the range of 10 - 60 g/g. The term "insoluble in water" as used herein means that an uncross-linked water-soluble component (a water-soluble polymer; hereinafter also called as "an elutable and soluble portion") in the water absorbent resin, which is preferably in the range of 0 - 50% by weight, more preferably not more than 25% by weight, still more preferably not more than 15% by weight, and particularly preferably not more than 10% by weight. In this connection, as a value of centrifuge retention capacity, a value measured by the method specified in the working example cited below is adopted. And as a value of an elutable and soluble portion, a value measured by a method described below is adopted. [Method for measuring an elutable and soluble portion] In a covered plastic container (a diameter of 6 cm and a height of 9 cm) having a volume of 250 mL, 184.3 g of physiological saline is placed, 1.00 g of a water absorbent resin is added therein. An elutable and soluble portion in a resin is extracted by stirring the mixture for 16 hours with a magnetic stirrer having a diameter of 8 mm and a length of 25 mm at a rotation speed of 500 rpm. This extract is filtrated with one sheet of a filter paper (0.26 mm in thickness and 5 μm in retained particle diameter; made by Advantec Toyo K. K. and sold under the product name of "JIS P 3801 No. 2") . Then, 50.0 g of the resultant filtrate is taken by measuring, make a solution for measuring.
First, only physiological saline is titrated with 0.1 N of an aqueous solution of sodium hydroxide to pH of 10. Then, it is titrated with 1 N of an aqueous solution of hydrochloric acid to pH of 2.7, to obtain blank titration amounts (called as [bNaOH] and [bHCl] , respectively) .
By conducting the same operation of titration concerning solutions for measuring, titration amounts (called as [NaOH] and [HCl] , respectively) were obtained.
For instance, in the case of a water absorbent resin consisting of known amounts of acrylic acid and sodium salt thereof, an elutable and soluble portion in the water absorbent resin can be calculated on the basis of an average molecular weight of the monomers and the titration amounts obtained by the above described operation in accordance to the equation described below. In the case of an unknown amount, an average molecular weight of a monomer is calculated, by using a neutralization ratio obtained by titration according to the equation described below.
Elutable and soluble portion (% by weight) = 0.1 x (Average molecular weight of monomer) x 184.3 x 100 x ([HCl] - [bHCl]) / 1000 / 1.0 / 50.0
Neutralization ratio (% by mol)
= [1 - ([NaOH] - [bNaOH] ) / ([HCl] - [bHCl])] x 100
As used herein, the term "modification" refers to all physical or chemical actions performed on a water absorbent resin , including surface cross-linkage of a water absorbent resin, formation of pores therein, and impartation of hydrophilic property or hydrophobic property thereto, for example .
The water absorbent resin which can be used in this invention does not need to be particularly restricted but is only requiredto be capable ofbeing obtainedbypolymerizing a monomer component essentially containing an ethylenically unsaturated monomer by means of any of the known methods.
The ethylenically unsaturated monomer is not particularly restrictedbut is preferred to be amonomer having an unsaturateddoublebond at the terminal thereof. As typical examples thereof, anionic monomers such as (meth) acrylic acid, 2- (meth) acryloyl ethane sulfonic acid, 2- (meth) acryloyl propane sulfonic acid, 2- (meth) acrylamide-2-methyl propane sulfonic acid, vinyl sulfonic acid, and styrene sulfonic acid, and salts thereof; nonionic hydrophilic group-containing monomers such as (meth) acrylamide, N-substituted (meth) acrylamide, 2-hydroxyethyl (meth) acrylate, and 2-hydroxypropyl (meth) acrylate; and amino group-containing unsaturated monomers such as N,N~dimethylaminoethyl (meth) acrylate, N, N-diethylaminoethyl (meth) acrylate, N,N-diethylaminopropyl (meth) acrylate, and N,N-dimethylaminopropyl (meth) acrylamide, and quaternized products thereof may be cited. These monomers may be used either singly or in the form of a mixture of two or more members . Among monomers cited above, (meth) acrylic acid, 2- (meth) acryloyl ethane sulfonic acid, 2- (meth) acrylamide-2-methylpropane sulfonic acid, and salts thereof, N, N-dimethylaminoethyl (meth) acrylate and quaternized N, N-dimethylaminoethyl (meth) acrylate, and (meth) acrylamide prove preferable, and acrylic acid and/or the salt thereof are particularly preferable.
When an acrylic acid salt is used as the monomer, the monovalent salt of acrylic acid selected among alkali metal salt, ammonium salt, and amine salt of acrylic acid may be preferably used from the viewpoint of ability of a water absorbent resin to absorb water. More preferably, alkali metal salts of acrylic acid may be used, and acrylic acid salts selected among sodium salt, lithium salt, and potassium salt thereof may be particularly preferably used.
In the production of a water absorbent resin, another monomer component other than the monomers cited above may be used in such an amount as to impair effects of this invention . As typical examples of such another monomer component, hydrophobic monomers such as aromatic ethylenically unsaturated monomers having 8 - 30 carbon atoms, aliphatic ethylenically unsaturatedmonomers having 2 - 20 carbon atoms, alicyclic ethylenically unsaturated monomers having 5 - 15 carbon atoms, and alkyl esters of (meth) acrylic acid containing alkyl groups having 4 - 50 carbon atoms may be cited. The proportion of such a hydrophobic monomer is generally in the range of 0 - 20 parts by weight, based on 100 parts by weight of the ethylenically unsaturated monomer . If the proportion of the hydrophobic monomer exceeds 20 parts by weight, water absorbing properties of the produced water absorbent resin would be degraded.
The water absorbent resin which can be used in this invention is insolubilized by the formation of an internal cross-link. This internal cross-link may be of self-cross-linkage type using no cross-linking agent, or alternatively can be formed by using an internal cross-linking agent having not less than two polymerizable unsaturated groups and/or not less than two reactive functional groups in one molecular unit.
The internal cross-linking agent does not need to be particularly restricted. As typical examples of the inner cross-linking agent, N, N' -methylenebis (meth) acrylamide,
N-methylol (meth) acrylamide, glycidyl (meth) acrylate,
(poly) ethylene glycol di (meth) acrylate, (poly) propylene glycol di (meth) acrylate, glycerin tri (meth) acrylate, glycerin acrylate methacrylate, polyvalent metal salts of (meth) acrylic acid, trimethylol propane tri (meth) acrylate, triallyl amine, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, ethylene glycol diglycidyl ether, (poly) glycerol glycidyl ether, and polyethylene glycol diglycidyl ether may be cited. These internal cross-linking agents may be used singly or in the form of a mixture of two or more members .
The amount of the internal cross-linking agent to be used is preferably in the range of 0.0001 - 1 mol%, more preferably 0.001 - 0.5 mol%, and still more preferably 0.005 - 0.2 mol%, based on the total amount of monomer components used in the production of a water absorbent resin. If this amount is less than 0.0001 mol%, the internal cross-linking agent would not be introduced into the resin. Conversely, if the amount exceeds 1 mol%, gel strength of the water absorbent resin wouldbe too heightened to lower the absorption capacity thereof. For the introduction of a cross-linked structure into an interior of the polymer by using the internal cross-linking agent, the internal cross-linking agent can be added into the reaction system prior to, during, or after the polymerization of monomers, or after the neutralization of the produced polymer.
For the purpose of producing a water absorbent resin, monomer components including the monomers and the internal cross-linking agent as mentioned above are polymerized in an aqueous solution form. A polymerization initiator which can be used in this case includes water-soluble radical polymerization initiators such as persulfates such as potassium persulfate, ammonium persulfate, and sodium persulfate; potassium peracetate, sodium peracetate, potassium percarbonate, sodium percarbonate, and t-butyl hydroperoxide; hydrogen peroxide; azo compounds such as 2,2' -azobis (2-amidinopropane) -dihydrochloride, and photopolymerization initiators such as 2-hydroxy-2-methyl-l-phenyl-propan-l-on, for example. The water-soluble radical polymerization initiators may be combined with a reducing agent such as a sulfite, L-ascorbic acid, or a ferric salt, soas tobeusedas a redox type initiator . The concentration of the monomer in the aqueous monomer solution is not particularly restricted but falls preferably within the range of 15 - 90% by weight, and more preferably 35 - 80% by weight. If this concentration is less than 15% by weight, the shortage would be at a disadvantage in necessitating consumption of heat and time for drying because the resultant hydrogel has an unduly large content of water.
A method for the polymerization is not particularly restricted but may be selected among well-known methods such as solution polymerization, reversed-phase suspension polymerization, precipitation polymerization, and bulk polymerization. Among these methods, the aqueous solution polymerization which comprises dissolving a monomer in an aqueous solution and polymerizing it in the aqueous solution, and the reversed phase suspension polymerization may be particularly advantageous on account of the ease of control of polymerization reaction and the performance of a produced water absorbent resin.
In initiating the polymerization, the polymerization initiator is used to start the polymerization. Besides the polymerization initiator, such active energy rays as ultraviolet rays, electron radiation, and Y rays may be used either singly or in combination with a polymerization initiator. Though the temperature in initiating the polymerization depends on the kind of polymerization initiator to be used, it falls preferably in the range of 15 - 130°C, andmore preferably 20 - 120°C. If the temperature in initiating the polymerization deviates from the range mentioned above, the deviation would be at a disadvantage in increasing the residual monomer in the produced water absorbent resin and suffering the self cross-linking reaction to proceed excessively and consequently degrading water absorbing properties of the produced water absorbent resin.
The "reversed phase suspension polymerization" is a method of polymerization in which an aqueous monomer solution is suspended in a hydrophobic organic solvent . It is disclosed in U.S. Patent Nos . 4,093,776, 4,367,323, 4,446,261, 4, 683,274, and5, 244, 735, forexample. The "aqueous solution polymerization" is a method for polymerizing an aqueous monomer solution without using a dispersing solvent. It is disclosed inU. S. Patent Nos.4,625,001, 4,873,299, 4,286,082, 4,973,632, 4,985,518, 5,124,416, 5,250,640, 5,264,495, 5, 145, 906, and 5, 380, 808, and European Patent Nos. 0811636,
0 955 086, and 0 922 717, for example. The monomers and the initiators which are cited by way of illustration in these methods of polymerization can be applied to this invention.
The aqueous solution polymerization may be performed by polymerizing partially neutralized acrylic acid or by polymerizing an acid group-containing monomer such as acrylic acid and the like and subsequently neutralizing the resultant polymer with such an alkali compound as sodium hydroxide or sodium carbonate. Accordingly, the water absorbent resin to be used in this invention preferably has an acid group and a specific neutralization ratio (mol% of the neutralized acids group in the whole of acid groups) . In this case, the neutralization ratio of the produced water absorbent resin (mol% of the neutralized acids group in the whole of acid groups) is in the range of 25 - 100 mol%, and preferably 50 - 90 mol%, more preferably of 50 - 75 mol%, and most preferably 60 - 70 mol%. Accordingly, the preferable embodiment of this invention is to provide a method for the production of a modified water absorbent resin, which comprises a) mixing a water absorbent resin, water, and persulfate as a radical polymerization initiator without addition of an ethylenically unsaturated monomer and b) irradiating the resultant mixture with active energy rays, wherein the water absorbent resin contains an acid group and has a neutralization ratio (mol% of the neutralized acids group in the whole of acid groups) in the range of 50 to 75 mol%. After the completion of the polymerization, a hydrogel-like cross-linked polymer is generally obtained. While this invention permits this hydrogel-like cross-linked polymer in its unaltered form as a water absorbent, resin, the polymer is preferably dried so as to give a water content (% by weight) [100- (Solid content) (% by weight) ] which will be specifically described herein below.
Incidentally, in this invention, a water absorbent resin composition is obtained by mixing a water absorbent resin, a water-soluble radical polymerization initiator and/or a heat-degradable radical polymerization initiator (in the present specification, referred collectively to as "radical polymerization initiator"), and water, which will be described specifically herein below. Then, the resultant composition is irradiated with active energy rays to modify the water absorbent resin. This modification results from the action of active radicals generated from the polymerization initiator on the main chain of the polymer. This modification, therefore, does not need to be limited to a water absorbent resin which is obtained by polymerizing a water-soluble ethylenically unsaturated monomer as described above but may be effected on such a water absorbent resin as cross-linked polyvinyl alcohol, cross-linked polyethylene oxide, cross-linked polyaspartic acid, and cross-linked carboxymethyl cellulose, for example.
The water absorbent resin which can be used in this invention is preferably a powdery water absorbent resin which is obtained by polymerizing a monomer having acrylic acid
(salt) particularly as its main component . The hydrogel-like polymer which is obtained by polymerization is preferably dried and subsequently pulverized to a water absorbent resin. The drying may be effected by using a drier such as a hot air drier at a temperature in the range of 100 - 220°C, and more preferably 120 - 200°C.
For use in the pulverization, among shear primary crushers, impact shredders, and high speed rotary grinders included in the names of the powdering machines classified in Table 1.10 of Particle Technology Handbook (first edition, compiled by Particle Technology Association) , the powdering machines having at least one of the powdering mechanisms such as cutting, shearing, striking, and rubbing can be adopted particularly favorably. Among the powdering machines, the powdering machines which have cutting and shearing as main mechanisms can be used particularly advantageously. A roll mill (roll rotary type) powdering machine may be cited as a preferred example. The water absorbent resin which can be used in this invention is preferably in a powdery form. More preferably, it is a powdery water absorbent resin which contains particles of a diameter in the range of 150 - 850 μm (as defined by sieve classification) in a proportion in the range of 90 - 100 % by weight, and particularly preferably 95 - 100% by weight. When the modified water absorbent resin having a particle diameter exceeding 850 μm is used in disposable diapers, for example, it would impart a disagreeable feel to the user's skin and possibly inflict a rupture on the top sheet of a diaper . If the proportion of particles of a diameter smaller than 150 μm exceeds 10 % by weight based on weight of the water absorbent resin, the fine particles would scatter and clog the texture while in use and would degrade water absorbing properties of the modified water absorbent resin. The weight average particle diameter of the water absorbent resin may be in the range of 10 - 1,000 μm, and preferably 200 - 600 μm. If the weight average particle diameter is less than 10 μm, the shortage would possibly prove unfavorable in terms of safety and health. Conversely, if it exceeds 1, 000 μm, the water absorbent resin could not be used in disposable diapers, for example. In this connection, as a value of the particle diameter, a value measured by a measuring method of a particle size distribution specified in the working example cited below is adopted.
In addition or alternatively, the water absorbent resin to be used in this invention is preferably obtainedbyproducing a water absorbent resin precursor having a low neutralization ratio, and mixing the water absorbent resin precursor with a base. A multifunctional surface-treatment agent has been conventionally used for the surface-treatment (surface cross-linkage) . The multifunctional surface-treatment agent serves to react with a carboxyl group (-COOH) in a water absorbent resin but do not react with the salt thereof (for example, -COONa) . Accordingly, uniform cross-linkage can be attained by preparing an ethylenically unsaturated monomer mixture (for example, a mixture of acrylic acid with sodium acrylate) in which -COOH/-COONa ratio has been adjusted within a suitable range in advance, polymerizing the resultant mixture to produce a water absorbent resin having the -COOH and -COONa groups uniformly distributed therein, and subjecting the resultant water absorbent resin to the surface cross-linkage with a multifunctional surface-treatment agent . On the other hand, when a water absorbent resin is obtained by polymerizing a monomer mixture including an acid type ethylenically unsaturated monomer like acrylic acid as a main component, and then neutralizing the resultant polymer with an alkali compound such as sodium hydroxide and sodium carbonate, the resultant water absorbent resin has a small elutable portion and high gel strength. It, however, when subj ected to the surface cross-linkage with a multifunctional surface-treatment agent, has degraded water absorbency, because the -COOH and -COONa groups are not uniformly distributed in the water absorbent resin. Accordingly, the water absorbent resin to be produced by the latter method is not desirably subjected to such a conventional surface cross-linkage with amultifunctional surface-treatment agent . Conversely, according to the method of this invention, since a water-soluble radical polymerization initiator or a heat-degradable radical polymerization initiator induces cross-linkage by extracting a hydrogen in a main chain to form a radical and using the radical for coupling, but not by reacting with -COOH, the cross-linking reaction is not affected by whether or not -COOH groups are uniformly distributed in the water absorbent resin. As a result, according to the method of this invention, a water absorbent resin which is obtained by polymerizing a monomer or a monomer mixture including as a main component an acid type ethylenically unsaturatedmonomer like acrylic acid to obtain a water absorbent resin precursor having a low neutralization ratio, and then neutralizing the water absorbent resin precursor with an alkali compound such as sodium hydroxide and sodium carbonate can be modified, and the resultant modified water absorbent resin to be obtained by this method can manifest high gel strength and excellent water absorbency.
In the case when a water absorbent resin is obtained by polymerizing a monomer or a monomer mixture including as a main component an acid type ethylenically unsaturated monomer to obtain a water absorbent resin precursor having a low neutralization ratio, and then adding a base to the water absorbent resin precursor, the base may be added either in a solid form or in a liquid form. Preferably, the base is added in a liquid form, particularly in an aqueous solution form. When the base is added in an aqueous solution form, a step for adding a base to a water absorbent resin precursor and a step for producing a water absorbent resin composition can be simultaneously carried out . The base which can be used in this embodiment is not particularly limited so long as it permits the neutralization of the water absorbent resin precursor having a low neutralization ratio to a desired neutralization ratio . Well-known inorganic and organic salt and acid can be used. Typically, sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, sodium carbonate, potassium carbonate, ammonium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium bicarbonate, sodium phosphate, potassium phosphate, ammonium phosphate, sodiumborate, potassiumborate, ammoniumpentaborate, sodium acetate, potassiumacetate, ammoniumacetate, sodiumlactate, potassium lactate, ammonium lactate, sodium propionate, potassium propionate, ammonium propionate, and the like can be cited. These bases can be used singly or in a mixed form of two or more members . Among the bases described above, when a water absorbent resin precursor having a low neutralization ratio is obtainedbypolymerizing amonomer or amonomermixture including as a main component an acid type ethylenically unsaturated monomer such as acrylic acid, hydroxide of monovalent cation such as sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium hydroxide; and carbonate of monovalent cation such as sodium carbonate, potassium carbonate, ammonium carbonate, sodiumbicarbonate, potassium bicarbonate, and ammonium bicarbonate may be preferably used in terms that they can be commercially available easily, have excellent physical properties, and permits efficient adjustment of neutralization ratio to a desired level. In this embodiment, the amount of base added is not particularly limited and can be suitably selected so that the water absorbent resin used in the mixing step a) has a desired neutralization ratio adjusted within the preferable range as mentioned above.
In this invention, the expression "water absorbent resin precursor having a low neutralization ratio" is referred to as a water absorbent resin precursor having a low neutralization ratio (mol% of the neutralized acids group in the whole of acid groups) or having no neutralized acid groups (i.e., the neutralization ratio is zero) , and typically referred to as a water absorbent resin precursor having a neutralization ratio (mol% of the neutralized acids group in the whole of acid groups) in the approximate range of 0 to 50mol%, more preferably Oto 20mol%. Such awater absorbent resin precursor having a low neutralization ratio can be obtained by the same method as mentioned above by using a monomer mixture including as a main component an acid group-containing monomer like acrylic acid wherein a neutralization ratio is preferably adjusted within the above range. Thus the detailed explanation of the precursor will be omitted. The water content of the water absorbent resin prior to the modification to be used in the method for production of a modified water absorbent resin contemplated by this invention has no particular restriction so long as the water absorbent resinhas fluidity. The water absorbent resin after being dried at 180°C for three hours has a water content falling in the preferable range of 0 - 50% by weight, 0 - 40% by weight, 0 - 30% by weight, 0 - 20% by weight, 0 - 10% by weight, and 0 - 5% by weight in increasing order of preference. The water absorbent resin to be used in this invention is not limited to the product of the method as described above but may be any product obtained by some other method. While the water absorbent resin which is obtained by the method described above is a water absorbent resin having undergone no surface cross-linkage, for use in the method for producing a modified water absorbent resin of this invention, the water absorbent resin which has undergone surface cross-linkage in advance with a polyhydric alcohol, a polyvalent epoxy compound, an alkylene carbonate, or an oxazolidone compound can be adopted.
(b) Water absorbent resin composition
In a method for the production of a modified water absorbent resin according to the present invention, in the step a) , a water absorbent resin composition is obtained by mixing water and a radical polymerization initiator (a water-soluble radical polymerization initiator and/or a heat-degradable radical polymerization initiator) with the water absorbent resin, without addition of an ethylenically unsaturated monomer. Hitherto, the surface cross-linkage of a water absorbent resin has been generally effected by using a surface cross-linking agent. The incorporation of the surface cross-linking agent results in strong, chemical binding between the functional groups present on the surface of resin and the surface cross-linking agent, thereby introducing a stable surface cross-link structure into the resin surface. Then, by properly selecting a chain length of the surface cross-linking agent, the distance between cross-links can be adjusted easily. By adjusting an amount of the surface cross-linking agent to be incorporated, the cross-link density can be controlled. This invention, however, permits the modification of a water absorbent resin, specifically the introduction of a cross-link structure to the surface of the water absorbent resin, by merely using a radical polymerization initiator without requiring the incorporation of the surface cross-linking agent . Further, by additionally adding water to obtain a water absorbent composition and irradiating the water absorbent resin composition with active energy rays, a cross-linked structure can be effectively introduced to the surface of the water absorbent resin particles and at the same time, the produced modified water absorbent resin has improved water absorption properties. In addition to the merits as described above, the addition of water in a relatively large amount to the water absorbent resin in the step a) permits the efficient introduction of a cross-linking structure on a surface of the water absorbent resin in the step b) described in detail below, and thus has also a merits as of shortening an irradiation time required for improving absorbency against pressure (AAP) and the saline flow conductivity (SFC) of the modified water absorbent resin to a desired level. This invention uses the expression "without addition of an ethylenically unsaturated monomer" with the object of preventing a radical polymerization initiator from reacting with an ethylenically unsaturated monomer to avoid the consumption of the radical polymerization initiator that is activated by the irradiation with active energy rays prior to the action on the surface of the water absorbent resin in the step b) .
In the step a) , water is mixed with a water absorbent resin. In this case, mixing of a water absorbent resin and water may be conducted by adding water alone, or by adding water in a form of an aqueous solution containing another component. As the aqueous solution, for example, an aqueous solution containing a radical polymerization initiator stated later, an aqueous solution containing a mixing aid also stated later, and the like may be included.
In the step a) , an amount of water mixed with a water absorbent resin is not especially limited, however, as a value in a water absorbent resin composition obtained, is in the preferable range of 1 to 100 parts by weight, 1 to 70 parts by weight, 1 to 50 parts by weight, 2 to 40 parts by weight, 3 to 30 parts by weight, 4 to 20 parts by weight, and 5 to 10 parts by weight, in increasing order of preference, based on 100 parts by weight of a water absorbent resin (as reduced to as 100 parts by weight of a solid content) . If the amount of water is less than 1 part by weight, uniform cross-linking would not be attained in some cases even when the water absorbent resin composition is irradiated with active energy rays in the step b) described in detail below, because the surface water content of the water absorbent resin can not reach 3.0% by weight. Conversely, if the amount of water exceeds 100 parts by weight, unduly large energy would be necessary during a drying step after the irradiation with active energy rays. In addition, the water absorbent resin would be possibly decomposed. Particularly, the addition of water in a relatively large amount to the water absorbent resin in the step a) permits the efficient introduction of a cross-linking structure on a surface of the water absorbent resin in the step b) described in detail below, and thus an irradiation time required for improving absorbency against pressure (AAP) and the saline flow conductivity (SFC) of the modified water absorbent resin to a desired level can be shortened.
Further, in the step a) , in addition to the water, a water-soluble radical polymerization initiator and/or a heat-degradable radical polymerization initiator are mixed as a radical polymerization initiator with the water absorbent resin composition. Incidentally, hereinafter, ΛΛa water-soluble radical polymerization initiator and/or a heat-degradable radical polymerization initiator" are sometimes called collectively as a radical polymerization initiator.
In this step, when "a water-soluble radical polymerization initiator" is mixed with a water absorbent resin, the initiator can be easily dispersed uniformly on the surface of the water absorbent resin which excels in hydrophilic property and water absorbing property. Thus, a water absorbent resin excelling in water absorbing properties can be produced. The water-soluble radical polymerization initiator to be used in this invention has solubility in water (25°C) of not less than 1% by weight, preferably not less than 5% by weight, and more preferably not less than 10% by weight. As typical examples thereof, persulfates such as ammonium persulfate, sodium persulfate, and potassium persulfate; hydrogen peroxide; and water-soluble azo compounds such as 2, 2 ' -azobis-2-amidinopropane dihydrochloride and 2, 2 ' -azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride may be cited. The use of a persulfate particularly preferable among them proves in respect that the modified water absorbent resin excels in water absorption properties including absorbency of physiological saline against pressure (in this specification, referred simply to as "absorbency against pressure"), and saline flow conductivity.
The term heat-degradable radical polymerization initiator to be used in this invention is a compound which generates a radical by heating. A heat-degradable radical polymerization initiator having 10 hour half-life decomposition temperature in the range of 0 to 120 °C, more preferably 20 to 100 °C, may be preferably used in this invention. In consideration of temperature during the irradiation with active energy rays, a heat-degradable radical polymerization initiator having 10 hour half-life decomposition temperature in the range of 40 to 80 0C can be particularly preferably used in this invention. If the lower limit of 10 hour half-life decomposition temperature is less than 0 °C (lower limit) , the heat-degradable radical polymerization initiator would be too unstable during the storage. Conversely, if the upper limit thereof exceeds 120 °C (upper limit) , the chemical stability of the heat-degradable radical polymerization initiator would be too high to lower reactivity.
In the step, when λλa heat-degradable radical polymerization initiator" is mixed with a water absorbent resin, the surface modification can be carried out at a low temperature for a short period of time, and the resultant modified water absorbent resin can manifest high gel strength and excellent water-absorbing properties. The heat-degradable radical polymerization initiator to be used in this invention may be either oil-soluble or water-soluble . The decomposition rate of an oil-soluble heat-degradable radical polymerization initiator is less sensitive to a pH value and ion strength as compared to that of a water-soluble heat-degradable radical polymerization initiator . However, a water-soluble heat-degradable radical polymerization initiator may be more preferably used in respect of its permeability to a water absorbent resin because the water absorbent resin is hydrophilic. The heat-degradable radical polymerization initiator has advantages in respect that it is relatively inexpensive and the process and devices for the production thereof can be simplifiedbecause the strict light-shielding is not always required, as compared with a compound which has been commercially available as a photo-degradable radical polymerization initiator. As representative examples of the heat-degradable radical polymerization initiator, persulfates such as sodium persulfate, ammonium persulfate, and potassium persulfate; percarbonates such as sodium percarbonate; peracetates such as peracetic acid, and sodium peracetate; hydrogen peroxide; and azo compounds such as 2,2' -azobis (2-amidinopropane) dihydrochloride,
2,2 '-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, and 2, 2 ' -azobis (2-methylpropionitrile) may be cited. Among the heat-degradable radical polymerization initiators cited above, persulfates including sodium persulfate, ammonium persulfate, and potassium persulfate, and azo compounds including 2, 2 ' -azobis (2-amidinopropane) dihydrochloride, 2, 2 ' -azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride, and 2, 2 ' -azobis (2-methylpropionitrile) which have 10 hour half-life decomposition temperature in the range of 40 to 80 °C can be used preferably. Particularly, persulfates may be preferably used in respect of excellent absorbency of physiological saline against pressure, saline flow conductivity, and free swelling capacity.
The amount of the radical polymerization initiator is preferably in the range of 0.01 - 20 parts by weight, more preferably 0.1 -15 parts by weight, and particularly preferably 1-10 parts by weight, based on 100 parts by weight of the water absorbent resin. If the amount of the radical polymerization initiator to be mixed is less than 0.01 part by weight, the water absorbent resin would not be sufficiently modified even by the exposure to the active energy rays. Conversely, if the amount of the radical polymerization initiator to be mixed exceeds 20 parts by weight, water absorbing properties of the modified water absorbent resin would be degraded.
In this invention, by essentially using the water-soluble radical polymerization initiator and/or a heat-degradable radical polymerization initiator, excellent properties can be accomplished as compared with the case of omitting the use of such a radical polymerization initiator, for example, the case of using solely an oil-soluble photopolymerization initiator. Incidentally, the term "oil-soluble photopolymerization initiator" as used herein means a compound having water-solubility of less than 1% by weight.
While this invention essentially uses a water-soluble radical polymerization initiator and/or a heat-degradable radical polymerization initiator, another initiator other than the radical polymerization initiator can be additionally used. As typical examples of the another polymerization initiator which can be additionallyused, photopolymerization initiators such as oil-soluble benzoin derivatives, benzyl derivatives, and acetophenone derivatives, and oil-soluble organic peroxides such as oil-soluble ketone peroxide, peroxyketal, hydroperoxide, dialkyl peroxide, peroxy esters, and peroxycarbonate may be cited. These photopolymerization initiators may be commercially available products such as, for example, products fromCiba Specialty Chemicals sold under the trademark designations of Irgacure 184
(hydroxycyclohexyl-phenyl ketone) and Irgacure 2959
(1- [4- (2-hydroxyethoxy) -phenyl] -2-hydroxy-2-methyl-l-pro pan-l-on) .
When another initiator is to be used in combination in this invention, the amount of the another initiator to.be used is in the range of 0 - 20 parts by weight, preferably 0 - 15 parts by weight, and particularly preferably 0 - 10 parts by weight, based on 100 parts by weight of the water absorbent resin. This rate corresponds to a smaller amount than the radical polymerization initiator such as, for example, not more than 1/2, further not more than 1/10, and particularly not more than 1/50 of the weight ratio of the water-soluble radical polymerization initiator. When a water-soluble radical polymerization initiator and/or a heat-degradable radical polymerization initiator are to be used in combination, the amount of the radical polymerization initiator is referred to a total amount thereof. While the mixing of the radical polymerization initiator and the water absorbent resin mentioned above may be accomplished by mixing the radical polymerization initiator to be mixed in its unmodified form with the water absorbent resin, it is preferably performed by dissolving the initiator in an aqueous solution and then mixing the resultant aqueous solution with the water absorbent resin. Since the water absorbent resin is capable of absorbing water, the radical polymerization initiator can be uniformly dispersed on the surface of the water absorbent resin and uniformly mixed with the water absorbent resin bymixing the radical polymerization initiator in an aqueous solution form. The aqueous solution may contain, besides water, some other solvent in an amount son as not to impair solubility of the radical polymerization initiator.
Further, when a radical polymerization initiator is added in a form of an aqueous solution, an amount of water in an aqueous solution used is not limited, and it may be enough to ensure that an amount of water in a water absorbent resin composition is fallen within the preferable range as described above. In this connection, a form of mixing water into a water absorbent resin is not limited to a case where mixing is conducted in a form of an aqueous solution containing a radical polymerization initiator. After mixing a radical polymerization initiator and a water absorbent resin, water or an aqueous solution may be mixed therewith. Therefore, a hydrogel-like cross-linked product to be obtained by polymerizing a monomer component may be dried so as to give a water content of 0 to 50% by weight, and then directly mixed with a radical polymerization initiator, to obtain a water absorbent resin composition.
For the purpose of exalting the mixing property of the aqueous solution with a water absorbent resin composition, a mixing aid is preferably added to the water absorbent resin composition. Although the time of adding a mixing aid is not particularly limited, the mixing aid is preferably added at the same time as or prior to the mixing step a) . The mixing aid is not particularly limited, as long as it is a water-soluble or water-dispersible compound except an ethylenically unsaturated monomer or a radical polymerization initiator, and it can repress the agglomeration of the water absorbent resin with water and improve the mixing of the aqueous solution with the water absorbent resin . The mixing aid is preferably a water-soluble or water-dispersible compound. As such a water-soluble or water-dispersible compound, surfactants, water-soluble polymers, hydrophilic organic solvents, water-soluble inorganic compounds, inorganic acids, inorganic acid salts, organic acids, and organic acid salts can be typically used. In this specification, the term "water-soluble compound" is referred to as a compound having solubility in 100 g of water at room temperature of not less than 1 g, preferably not less than 10 g. Since the addition of the mixing aid can repress the agglomeration of the water absorbent resin with water and induce the uniform mixing of the aqueous solution with the water absorbent resin, the active energy rays, when irradiated in the subsequent step, can be irradiated equally and evenly to the water absorbent resin and thus the uniform surface cross-linkage of the entire water absorbent resin can be attained.
When a mixing aid is to be used, the form of the mixing aid to be used is not particularly limited, and it may be used in a powdery form, or may be dissolved, dispersed, or suspended in a solution. Preferably, it is used in the form of an aqueous solution.
Further, in the case of using a mixing aid, the order of the addition of the mixing aid is not also particularly limited. Any method such as a method which comprises adding a mixing aid to a water absorbent resin, and then adding and mixing water and a radical polymerization initiator (in some cases, an aqueous solution containing them) to the mixture, and a method which comprises dissolving a mixing aid in an aqueous solution, and simultaneously mixing the resultant solution with a water absorbent resin can be used.
As the surfactant to be used herein, at least one kind of surfactant which is selected from the group consisting of nonionic surfactants and anionic surfactants having an HLB of not less than 7 may be adopted. As typical examples of such surfactants, sorbitan aliphatic esters, polyoxyethylene sorbitan aliphatic esters, polyglycerin aliphatic esters, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, polyoxyethylene acyl esters, sucrose aliphaatic esters, higher alcohol sulfuric esters, alkyl naphthalene sulfonates, alkylpolyoxyethylene sulfate, and dialkyl sulfosuccinates may be cited. Among these surfactants, polyoxyethylene alkyl ethers can be preferably used. The number average molecular weight of the polyoxyethylene alkyl ether is preferably in the range of 200 to 100,000, more preferably 500 to 10,000. If the number average molecular weight is too large, the solubility in water would decrease and thus the mixing with the water absorbent resin would become inefficient because the concentration of the surfactant in the solution can not be increased and the viscosity of the solution is also increased. Conversely, if the number average molecular weight is too small, the surfactant would become less effective as a mixing aid. As typical examples of the water-soluble polymer, polyvinyl alcohol, polyethylne oxide, polyethylene glycol, polypropylene glycol, polyacrylamide, polyacrylic acid, sodium polyacrylate, polyethylene imine, methyl cellulose, carboxyrαethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, dextrin, sodiumalginate, and starch may be cited. Among these polymers, polyethylene glycol can be preferably used. The number average molecular weight of the polyethylene glycol, like polyoxyethylene alkyl ether, is preferably in the range of 200 to 100,000, more preferably 500 to 10,000.
As typical examples of the hydrophilic organic solvent, alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, isobutyl alcohol, and t-butyl alcohol; ketones such as acetone and methylethyl ketone; ethers such as dioxane, alkoxy (poly) ethylene glycol, and tetrahydrofuran; amides such as e-caprolactam and N,N-dimethyl formamide; sulfxides such as dimethyl sulfoxide; and polyhydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, 1,3-propane diol, dipropylene glycol, 2, 2, 4-trimethyl-l, 3-pentane diol, glycerin, 2-butene-l, 4-diol, 1,3-butane diol, 1,4-butane diol, 1,5-pentane diol, 1,6-hexane diol, 1, 2-cyclohexane dimethanol,. 1, 2-cyclohexanol, trimethylol propane, diethanol amine, triethanol amine, polyoxypropylene, pentaerythritol, and sorbitol may be cited. These hydrophilic organic solvents may be used either singly or in the form of a mixture of two or more members.
As typical examples of the water-soluble inorganic compound, alkali metal salts such as sodium chloride, sodium hydrogen sulfate, and sodium sulfate, ammonium salts such as ammonium chloride, ammonium hydrogen sulfate, and ammonium sulfate, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, polyvalent metal salts such as aluminium chloride, polyaluminium chloride, aluminium sulfate, potassium alum, calcium chloride, alkoxy titanium, zirconium ammonium carbonate, zirconium acetate, and non-reducible alkali metal salt pH buffer agents such as hydrogencarbonate, dihydrogen phosphate, and monohydrogen phosphate may be cited.
Further, as typical examples of the inorganic acid (salt), hydrochloric acid, sulfuric acid, phosphoric acid, carbonic acid, and boric acid, and the salts thereof, for example, alkali metal salts thereof, and alkali earth metal salts thereof may be cited. As typical examples of the organic acid
(salt) , acetic acid, propionic acid, lactic acid, citric acid, succinic acid, malic acid, and tartaric acid, and the salts thereof, for example, alkali metal salts thereof, and alkali earth metal salts thereof may be typically cited.
Among the compounds cited above, at least one water-soluble or water-dispersible compound selected from the group consisting of polyoxyethylene alkyl ethers, polyethylene glycol, water-soluble polyvalent metals, sodium chloride, ammonium hydrogen sulfate, ammonium sulfate, sulfuric acid, and hydrochloric acid may be preferably used as the mixing aid.
These mixing aids can be used singly or in the mixed form of two or more members. The amount of the mixing aid to be added is not particularly limited as long as it can repress the aggregation of the water absorbent resin with water, and improves the mixing of the aqueous solution with the water absorbent resin, as mentioned above. Typically, the mixing aid is preferably added in an amount in the range of 0.01 to 40 parts by weight, more preferably 0.1 to 5 parts by weight, to 100 parts by weight of the water absorbent resin.
In the step a) according to this invention, the conditions for mixing a water absorbent resin, water, and a radical polymerization initiator, and optionally a mixing aid are not particularly limited. For example, the mixing temperature in the step a) is preferably in the range of 0 to 150 °C, 10 to 120 °C, 20 to 100 °C, 30 to 90 °C, 40 to 70 0C, in increasing order of preference. If the mixing temperature exceeds 150 °C, the water absorbent resin would be degraded with heat, and the surface water content of the water absorbent resin in the step b) would not be maintained at a level of not less than 3.0%by weight because of evaporation of water and the like. Conversely, if the mixing temperature is less than 0 °C, water would be condensed to inhibit the stable operation. The mixing step at an elevated temperature is preferable in view that a radical polymerization initiator can act upon less irradiation due to the heat. Accordingly, in such a case, a mixing/irradiation system may be preferably closed so as to repress excessive leakage of steam and to increase a surface water content of a water absorbent resin in the step b) to a level of not less than 3.0% by weight. The temperatures of water absorbent resin and water prior to the step a) are not also particularly limited. For example, the temperatures of water absorbent resin prior to the step a) is preferably in the range of 0 to 150 0C, 10 to 120 °C, 20 to 100 °C, 50 to 100 °C, in increasing order of preference. If the temperatures of water absorbent resin prior to the step a) exceeds 150 °C, the water absorbent resin would be degraded with heat. Conversely, if the mixing temperature is less than 0 0C, water would be condensed to inhibit the stable operation. The temperature of water prior to the step a) is preferably in the range of 5 to 80 0C, more preferably 10 to 60 0C, particularly preferably 20 to 50 °C. If the temperatures of water prior to the step a) exceeds 80 °C, water would evaporate excessively prior to the mixing step a) and thus a sufficient amount of water can not be mixed with a water absorbent resin to prevent a surface water content of the water absorbent resin from reaching 3.0% by weight. Conversely, if it is less than 5 °C, water would be condensed to inhibit the stable operation. Further, the mixing time in the step a) is not also particularly limited so as that the above components can be mixed uniformly. Typically, the mixing time is preferably in the range of 0.1 second to 60 minutes, more preferably 1 second to 30 minutes, further more preferably 2 seconds to 20 minutes, most preferably 5 seconds to 10 minutes. If the mixing time is less than the lower limit, a water absorbent resin, water, and a radical polymerization initiator, and optionally a mixing aid would not be mixed uniformly. Conversely, if the mixing time exceeds the upper limit and becomes unduly long, an excess amount of water would penetrate into an inner part of the water absorbent resin, to induce the decrease of surface water content to a level less than 3.0% by weight and to repress the surface treatment by irradiation with active energy rays. Further, the excessive decrease in an amount of water to be mixed with a water absorbent resin is not also preferable in terms of inducing the decrease of surface water content to a level less than 3.0% by weight. As regards a method for mixing a water absorbent resin, water, and a radical polymerization initiator to obtain a water absorbent resin composition, a method which effects the mixture by the use of an ordinary mixing device such as, for example, V-shape mixer, ribbon type mixer, screw type mixer, rotary circular plate type mixer, air-current type mixer, batch kneader, continuous kneader, paddle type mixer, or space type mixer may be cited as an example, (c) Active energy rays
A fact that in the production of a water absorbent resin, the rate of polymerization is exaltedby the exposure to active energy rays has been well-known in the art. For example, by formulating a polymerizable monomer component and an internal cross-linking agent and a photopolymerization initiator together and irradiating the resultant mixture with active energy rays such as ultraviolet rays, electron radiation, or Y rays, a water-insoluble absorbent resin having internal cross-links can be prepared. Then, as a method for cross-linking the surface of the water absorbent resin, the formation of a surface cross-linkage attained by using a surface cross-linking agent and promoting the relevant reaction by application of heat has also well-known in the art. For the surface cross-linkage of the water absorbent resin, compounds such as polyhydric alcohols, polyvalent glycidyl ethers, haloepoxy compounds, and polyvalent aldehydes which contain a plurality of functional groups in one molecular unit may be used. Generally, by heating at 100 - 300 °C, these functional groups can be reacted with a carboxyl group present on the surface of the water absorbent resin to give rise to a cross-linked structure on the surface of the water absorbent resin. This invention, however, is characterized in that a cross-linked structure can be formed on a surface of a water absorbent resin by combining a radical polymerization initiator and exposure with active energy rays without requiring the presence of a surface cross-linking agent and a polymerizable monomer. By the method of this invention, absorbency against pressure (AAP) and the saline flow conductivity (SFC) of the modified water absorbent resin can be improved.
The method of this invention has a feature in that in an irradiating step for irradiating a water absorbent resin composition with active energy rays, a surface water content of a water absorbent resin in the water absorbent resin composition is controlled to a level of not lower than a predetermined value. Specifically, in the irradiating step, a surface water content of the water absorbent resin in the water absorbent resin composition is controlled to a level of not lower than 3.0% by weight . In this connection, the surface water content may be controlled to a level of not lower than 3.0% by weight at any point of time in the irradiating step, and it is not necessary to control to a level of not lower than 3.0% by weight throughout the course from the beginning to the end of the irradiating step. When a surface water content of the water absorbent resin is controlled to a level of not lower than 3.0% by weight throughout the course from the beginning to the end of the irradiating step, the modification (for example, introduction of a cross-linking structure) of a surface of the water absorbent resin would not efficiently conducted in some cases. And as a result, a modified water absorbent resin to be obtained would not show sufficiently excellent water absorbent properties.
As described above, the surface water content may be controlled to a level of not lower than 3.0% by weight at least at any point of time in the irradiating step. It is preferably controlled to a level of not lower than 3.5% by weight, and more preferably not lower than 4.0% by weight. From the viewpoint of attaining functions and effects by the present invention, the upper limit of the surface water content is not particularly limited, and it may be appropriately selected in accordance with purposes. However, the surface water content is usually not higher than 60.0% by weight, preferably not higher than 50.0% by weight, more preferably not higher than 40.0% by weight, and further preferably not higher than 30.0% by weight. When the surface water content is too high, water absorbent resin particles would adhere or agglomerate to each other, and irradiation with active energy rays would not effectively carried out onto a surface of the water absorbent resin.
In the preferable embodiment of the present invention, a surface water content at the beginning of the irradiating step is controlled so as to be fallen within the above-described range. By this embodiment, functions and effects by the present invention can be surely attained. It should be noted that during the irradiating step, a value of the surface water content may be varied. Namely, a surface water content may be increasedor decreased as comparedwith that at the beginning of the irradiating step. Meanwhile, it is sufficient that at least at any point of time in the irradiating step, a surface water content is controlled so as to be fallen within the above-described range. The surface water content is controlled to a level within the above-described range, in preferably not lower than 30%, more preferably not lower than 60%, further preferably not lower than 90%, and particularly preferably 100% (that is, the whole period) , of the whole period of the irradiating step. In this invention, the term "a surface water content" is referred to a weight percentage of a water amount existing in the vicinity of a surface of a particle based on a weight of a water absorbent resin particle. It is essentially different from a concept of a water amount or a water content in the whole particle. The surface water content is measured by the method specified in the working example cited below. The measuring method is briefIy explained, as follows. Water is extracted by adding a hydrophilic organic solvent to a water absorbent resin composition obtained in the step a) , a water amount in the extract is quantitatively determined by Karl Fischer method, and thus a value of a surface water content can be calculated. In the irradiating step, a method of controlling a surface water content of a water absorbent resin a level within the above-described range is not especially limited. For an example thereof, to attain a preferable surface water content, a method which comprises adding a sufficient amount of water into a water absorbent resin composition obtained in the step a) , or a method which comprises promoting the permeation of water into an inner part of a water absorbent resin particle or suppressing 'water from evaporating into an atmosphere in the mixing step a) and in the irradiating step b) may be used. Since an extent of permeation of water into an inner part of a water absorbent resin particle is influenced by time and temperature, it is preferable to appropriately control a temperature in a system during the mixing step a) . Further, to suppress evaporation of water, it is preferable to make the system a state near a closed one as far as possible, as well as controlling of a mixing time and a temperature in a system. In the irradiating step b) , when a stirring apparatus having a structure of a box or a cylinder is used, for instance, a state near a closed one can be attained by covering an opening part for irradiation with a material capable of transmitting active energy rays, such as quartz glass. As a method for promoting permeation of water to an inner part of a water absorbent resin, such methods are exemplified, as that a mixing time in the mixing step a) is extended, that a water absorbent resin composition is put in a closed system, and that heat treatment is conducted on a water absorbent resin composition at a temperature of not higher than a boiling point of water. On the other hand, as a method for promoting diffusion of water from a surface, such methods are exemplified, as that an air stream is introduced to a water absorbent resin composition, that a water absorbent resin composition is put in an open system, and that heat treatment is conducted on a water absorbent resin composition at a temperature of not lower than a boiling point of water. In order to control a surface water content in a water absorbent resin within the above-described range, it is preferable to conduct irradiation with active energy rays while appropriately monitoring a surface water content. In this case, a water absorbent resin composition may be dried to a certain range, or a predetermined amount of water may be added to a water absorbent resin composition, depending on the monitored value. Moreover, when water is added, penetration and diffusion of water from a surface to an inner part of a water absorbent resinmay appropriatelybe controlled, or volatilization of water from a surface of a water absorbent resin may appropriately be controlled.
In this invention, the irradiation with active energy rays may be conducted while a water absorbent resin, water and a radical polymerization initiator are mixed, or the irradiation may be conducted after mixing at least two of these. However, from a viewpoint of being able to form a uniform surface cross linkage, preferably after obtaining a water absorbent resin composition containing a water absorbent resin, water and a water-soluble radical polymerization initiator, the resultant water absorbent resin composition is irradiated with active energy rays.
As typical examples of the active energy rays, ultraviolet rays, electron radiation, and γ rays may be cited. These active energy rays may be used either singly or in the form of a combination of two or more members. Among these active energy rays, ultraviolet rays and electron radiation prove advantageous. In consideration of influence of active energy rays on human body, ultraviolet rays are more preferable and ultraviolet rays having a wavelength not exceeding 300 nm and particularly preferably in the range of 180 - 290 nm are particularly preferable. As regards irradiating conditions, when the ultraviolet rays are used, intensity of irradiation is preferably in the range of 3 - 1, 000 mW/cm2, and dose of irradiation is preferably in the range of 100 - 10,000 mJ/cm2. As typical examples of the device for irradiating ultraviolet rays, high-pressure mercury-vapor lamp, low-pressure mercury-vapor lamp, metal halide lamps, xenon lamp, and halogen lamps may be cited. So long as ultraviolet rays, preferably ultraviolet rays of a wavelength of not more than 300 nm, is used, it may contain another radiation or wavelength and the procedure is not particularly restricted. When the electron radiation is used, voltage of acceleration is preferably in the range of 50 - 800 kV and absorbed dose is preferably in the range of 0.1 - 100 Mrad.
Generally, the duration of irradiating with active energy rays is preferably not less than 0.1 minute and less than 60 minutes, more preferably not less than 0.2 minute and less than 30 minutes, and more preferably not less than 1 minute and less than 20 minutes, although it can be suitably determined depending on mixing device, amount of water absorbent resin composition, irradiation intensity of lamp, and the like. The duration possibly exceeds 60 minutes in the case of a conventional surface cross-linking agent. For the fixed cross-link density, this invention can decrease a duration of surface cross-linking treatment. When the surface treatment is effected by irradiation of active energy rays, no application of heat is required. The irradiation of active energy rays, however, possibly results in inducing generation of radiant heat. Generally, a water absorbent resin can be treated at a temperature preferably less than 1500C, more preferably less than 1200C, still more preferably in the range of room temperature to 1000C, and particularly preferably in the range of 50 - 1000C. Thus, this invention allows a treating temperature to be set at a lower level than a conventional surface treating temperature.
During the irradiation of active energy rays, a water absorbent resin is preferably kept stirred. By this stirring, the mixture of a radical polymerization initiator and a water absorbent resin can be uniformly irradiated with the active energy rays. As typical examples of a device for stirring a water absorbent resin during the irradiation of active energy rays, shaking mixer, shaking feeder, ribbon type mixer, conical ribbon type mixer, screw type mixing extruder, air current type mixer, batch kneader, continuous kneader, paddle type mixer, high-speed fluidifying mixer, and buoyant fluidifying mixer may be cited. Further, active energy rays may be irradiated from the surrounding of an apparatus, while a water absorbent resin composition is made flow in the apparatus having a form of a box or a cylinder. In this case, to make a mixture flow, a pressure of gas such as air or the like may be utilized, as is used in flowing a powder with air. When air is used, it is preferable to humidify the air for the purpose of preventing a water absorbent resin composition from drying. When irradiation with active energy rays is conducted from many directions, uniform surface treatment can be conducted in a short period. In this connection, a material composing the above-described apparatus is not particularly limited so long as it does not obstruct irradiation with active energy rays onto a water absorbent resin composition, and for example, quartz glass is included, (d) Other treatment
After the irradiation of active energy rays, a water absorbent resin may be optionally subjected to heat treatment at a temperature in the range of 50 - 250°C as for the purpose of drying.
Further, after the irradiation of active energy rays, a water absorbent resin may be surface cross-linked by using a conventional surface cross-linking agent such as polyhydric alcohols, polyvalent epoxy compounds, and alkylene carbonates .
In the method for producing a modified water absorbent resin of the present invention, a water absorbent resin may be added with an agent for enhancing liquid-permeability before or after or during the irradiation of active energy rays. As typical examples of such an agent, minerals such as talc, kaolin, fuller's earth, bentonite, activated clay, barite, natural asphaltum, strontium ore, ilmenite, and pearlite; aluminum compounds such as aluminum sulfates 14 - 18 hydrates (or anhydrides) , potassium aluminum sulfates 12 hydrate, sodium aluminum sulfate 12 hydrate, aluminum chloride, aluminum polychloride, and aluminum oxide, and aqueous solutions thereof; other polyvalent metal salts; hydrophilic amorphous silica (such as, for example, a product by drymethodmade by Tokuyama K. K. and sold under the trademark designation of "Reolosil QS-20" and products by precipitation method made by DEGUSSA Corp. and sold under the trademark designation of "Sipernat 22S" and "Sipernat 2200") ; and oxide composites such as silicon oxide-aluminum oxide-magnesium oxide composite (such as, for example, a product made by ENGELHARD Corp. and sold under the trademark designation of "Attagel #50") , silicon oxide-aluminum oxide composite, and silicon oxide-magnesium oxide composite may be cited. Such a liquid-permeability enhancing agent is in an amount preferably in the range of 0 - 20 parts by weight, more preferably 0.01 - 10 parts by weight, and particularly preferably 0.1 - 5 parts by weight with 100 parts by weight of a water absorbent resin which has been modified. The liquid-permeability enhancing agent can be added in the form of aqueous solution when it is water-soluble or in the form of powder or slurry when it is water-insoluble. The liquid-permeability enhancing agent may be also added in a mixed form with a radical polymerization initiator. Other additives such as antibacterial agent, deodorant, and chelating agent may be properly used additionally in an amount in the range as mentioned above.
(e) Modified water absorbent resin
According to the method for producing a modified water absorbent resin of this invention, the produced water absorbent resin can manifest improved absorbency against pressure. It has been hitherto known that the formation of surface cross-linkage results in slightly lowering free swelling capacity but exalting ability to retain absorbed liquid even under pressed state, namely absorbency against pressure. By the method of this invention, the absorbency against pressure of 4.83 kPa of the water absorbent resin can be improved by not less than 1 g/g comparing with the absorption against pressure of the resin prior to the modification. This fact may be thought to indicate that a cross-linked structure is introduced to the surface of the water absorbent resin by the method of this invention. The increase in the absorbency against pressure after the modification is preferablynot less than 8 g/g, more preferably not less than 12 g/g, still more preferably not less than 15 g/g, and particularly preferably not less than 20 g/g, most preferably not less than 22 g/g. The modified water absorbent resin of this invention may exhibit absorbency against pressure of 4.83 kPa in the range of 8 - 40 g/g.
The centrifuge retention capacity (CRC) of the modified water absorbent resin is preferably not more than 50 g/g, more preferably not more than 40 g/g, still more preferably not more than 35 g/g. Although the lower limit thereof is not particularly limited, it is preferably not less than 10 g/g, morepreferablynot less than 20 g/g, stillmore preferably not less than 25 g/g. If the centrifuge retention capacity
(CRC) exceeds 50 g/g, gel strength would be lowered, to induce decrease in absorbency against pressure. On the hand, if the centrifuge retention capacity (CRC) is less than 10 g/g, sufficient water absorption capacity could not be attained, to induce leaking of urine when used in a disposable diaper.
The modified water absorbent resin which is obtained by this invention has saline flow conductivity (SFC) preferably of not less than 10 (x 10~7 ■ cm3 • s • g"1) , more preferably not less than 30 (x 10~7"cm3*s#g~1) , and still more preferably not less than 50 (x 10"7#cm3>s-g~1) , particularly preferably not less than 70 (* lO^'cir^'S'g"1) , most preferably not less than 100 (x 10~7 • cm3 • s • g"1) . The value is to be determined by the method specified in the working example cited herein below.
Further, the modified water absorbent resin which is obtained by this invention has a feature in having an extremely low residualmonomer content. This is consideredtobebecause the initiator radicals to be formed by the irradiation on a radical polymerization initiator with active energy rays react with remaining monomers in a water absorbent resin. Since the water absorbent resin is used in hygienic materials such as disposable diaper, the residual monomer content is preferably as small as possible in terms of odor and safety. While a residual monomer content of a water absorbent resin as a base polymer is generally in the range of 200 to 500 ppm, the residual monomer content of the water absorbent resin surface-treated by this invention is mostly not more than 200 ppm (the lower limit is 0 ppm) . The residual monomer content of the modified water absorbent resin is preferably not more than 200 ppm, more preferably not more than 150 ppm, particularly not more than 100 ppm (the lower limit is 0 ppm) .
Further, the modified water absorbent resin which is obtained by this invention has a smaller solid content as compared with a modified water absorbent resin to be obtained by a conventional modifying method which comprises adding a surface-treatment agent to a water absorbent resin as a base polymer andheating the mixture at an elevated temperature . This is because according to the method of this invention, the reaction does not require an elevated temperature and thus most of water contained in the aqueous solution which is added to a water absorbent resin as a base polymer remains even after the reaction. The large water content of the water absorbent resin has such effects that an amount of fine powder having a particle size of not more than 150 μm which is not desirable in terms of health can be decreased, the generation of static electricity on particle surface which causes blocking during the pneumatic conveying can be prevented, and the degradation of physical properties by physical damage during the pneumatic conveying can be also repressed. The solid content of the modified water absorbent resin is preferably not more than 95%, more preferably not more than 93% , particularly not more than 91% . Although the lower limit is not critical, a solid content of not more than 70% has a possibility of not being desirable in some uses, because in such a case, absorbency per weight of a water absorbent resin decreases. The form of the surface-treated water absorbent resin which is obtained by this invention can be properly adjusted by treatment conditions such as a form of a water absorbent resin before the treatment, and agglomeration and molding processes of a water absorbent resin after the treatment. Generally, the modified water absorbent resin is in a powdery form. This powder has a weight average particle diameter (specified by classification with sieves) in the range of 10 - 1,000 μm, and preferably 200 - 600 μm. In this powder, the content of particles having diameters of 150 - 850 μm is preferably in the range of 90 - 100 % by weight, and more preferably 95 - 100 % by weight, based on the weight of the water absorbent resin.
The method of this invention manifests an effect of agglomerating a fine powder generated in the production of a water absorbent resin during the course of surface cross-linking of the water absorbent resin. Accordingly, even when a water absorbent resin prior to the modification happens to contain a fine powder, the method for producing a modified water absorbent resin of this invention permits the agglomeration of the contained fine powder, which can lead decrease of the amount of fine powder to be contained in the resuitantmodifiedwater absorbent resin. The particle size distribution of the produced modified water absorbent resin is shifted toward a larger particle size as compared with the water absorbent resin prior to the modification. The degree of the shift, however, may vary with the kind and amount of a radical polymerization initiator to be mixed with a water absorbent resin and, particularly when it is added as an aqueous solution, also with a water content, conditions of irradiation with active energy rays, and a flowing process during the irradiation. The modified water absorbent resin which' is obtained by the method of this invention has a surface cross-linkage formed uniformly at a high cross-link density throughout on the entire surface of the water absorbent resin and can exalt to extremely high levels such characteristics as absorption capacity, absorption speed, gel strength, and suction power which a water absorbent resin is expected to possess . A speed and extent of the surface cross-linkage have been found to depend on a ratio of neutralization, when an acrylic acid type water absorbent resin is subjected to surface cross-linking by using such a surface cross-linking agent as polyhydric alcohol, polyvalent epoxy compound, or alkylene carbonate. To be specific, the surface cross-linking proceeds fast when the ratio of neutralization is low, while the surface cross-linkage proceeds with difficulties when the ratio of neutralization is high. For the purpose of surface cross-linking a water absorbent resin to be obtained by the post-neutralization of polyacrylic acid, the post-neutralization was required to be performed uniformly after the surface cross-linking treatment . On the other hand, according to this invention, a water absorbent resin can be modified to produce a water absorbent resin excelling in water absorbing properties without dependence on a ratio of neutralization of a water absorbent resin or on uniformity ofpost-neutralization. It is inferred that since the surface cross-linkage depends on the action of a radical polymerization initiator on a main chain of a water absorbent resin, the surface cross-linkage can proceed regardless of whether a carboxyl group is present in the form of an acid or a salt.
The case when this invention is executed in the presence of an ethylenically unsaturated monomer does not conform to the object of this invention, because the radical polymerization initiator is consumed by the polymerization of the ethylenically unsaturated monomer.
In accordance with this invention, the surface treatment of a water absorbent resin can be carried out fully satisfactorily even at a reaction temperature in the neighborhood of room temperature, and the surface-treated water absorbent resin consequently obtained can manifest at extremely high levels such characteristics as absorption capacity, absorption speed, gel strength, and suction power which the water absorbent resin is expected to possess. Accordingly, the water absorbent resin which is obtained by this invention is optimally usable for sanitary cotton, disposable diapers, and other sanitary materials for absorbing body fluid and for agricultural activities.
Examples :
Now, this invention will be described more specifically below with reference to working examples and comparative examples. This invention is not limited thereto. Hereinafter, the "parts by weight" may be expressed simply as "parts" and the "liters" simply as "L" for the sake of convenience. The method of determination and the method of evaluation indicated in the working examples and the comparative example will be shown below.
(1) Particle size distribution: weight average particle diameter (D50) and logarithmic standard deviation (σζ) A water absorbent resin or particulate absorbent of 10 g is passed through JIS standard sieves having mesh sizes of 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm,
150 μm, 106 μm, and 45 μm (THE IIDA TESTING SIEVE, made by Iida Seisakusho K. K. , 8 cm in diameter) , at room temperature
(20 to 25 0C) and at humidity of 50 RH%, and then classified by using a sieve shaker (IIDA SIEVE SHAKER, TYPE: ES-65type,
SER. No.0501, made by Iida Seisakusho K. K.) for 5 minutes.
As for a weight average diameter, residual percentage R is plotted on a logarithmic probability paper, and from this plotting, a particle diameter corresponding to R = 50 wt% reads as a weight average diameter (D50) .
Further, the particle diameters when R is 84.1% by weight and 15.9% by weight are referred to as Xl and X2, respectively. The logarithmic standard deviation (σζ) is represented by the following formula. Specifically, it means that the smaller the value σζ is, the narrower the particle size distribution is. σζ = 0.5 x ln(X2/Xl)
(2) Surface water content
500 g of a water absorbent resin as a base polymer was added to 5 liters of Loedige mixer (made by Loedige Co . , Ltd. , Type: M5R) , and a treating solution obtained by preliminarily mixing 5.0 g of ammonium persulfate, 2.5 g of a monomethyl ether of a polyethylene glycol (a number average molecular weight of about 2, 000) and 40 g of water, was sprayed thereto under stirring at 300 rpm. After being mixed by stirring for 3 min at room temperature, the stirring was terminated. The resultant mixture of 1 g was added to a screw tube (made by Maruemu Corporation, Screw Tube No.5), and 4 g of methanol anhydride was added. Then, the mixture was shaken for 30 seconds with a mini-shaker MSl made by IKA K. K. , and thereafter it was absorbed with a syringe, then was filtrated with a filter (made by Zeal Science Co., Ltd.; Water type 25 A (a pore diameter of 0.45 μm) ) . An amount of water contained in a filtrate was measured by a method below with a Karl Fischer moisture meter (made by Kyoto Electronics Manufacturing Co., Ltd.; Type: MKS-IS).
[Measurement of amount of water with a Karl Fischer moisture meter] 1. Principle for measurement
This is a method of measuring an amount of water using a volumetric analysis, wherein a Karl Fischer reagent in which water reacts quantitatively with iodine and sulfurous acid gas in the presence of methyl alcohol and pyridine, is used as a titrant.
Polarization is conducted by making slight constant electric current between two platinum electrodes immersed in a solution, and an end point of titration is determined by a Dead Stop method wherein a potential change caused by excessive iodine at an end point is detected.
To measure an amount of water by a Karl Fischer method, a sample is put in a flask for titration, titrated with a Karl Fischer reagent, and an amount of water in the sample is determined as a product of a titration amount of a Karl Fischer reagent and a titer.
W = K x F wherein W is an amount of water (mg) in a sample; K is a titration amount of a Karl Fischer reagent (mL) ; and F is a titer of a Karl Fischer reagent (mg/mL) . 2. Measuring method
50 mL of a solvent for measurement (a mixed product of 5OmL of an acetic acid (special grade) , 5OmL of Buffer solution (HYDRNAL-Buffer) , and 900 mL ofmethanol anhydride) is charged until electrodes in a Karl Fischer moisture meter are immersed therein. Then, titration is conducted with a Karl Fischer reagent by pushing a "START" key, to make an inner part of a flask for titration in an anhydrous state.
A sample is put into a flask for titration, titration is conducted with a Karl Fischer reagent by pushing a "START" key. A weight of the sample (a) [mg] and an amount of Karl Fischer titration (b) [mL] are recorded. Measuring was conducted by three times in all, and an average value is calculated.
By assigning a weight of the sample (a) and an amount of Karl Fischer titration (b) into the equation (1) below, a water content (c) [wt%] in methanol anhydride, which is used in extraction of water from a mixture containing a water absorbent resin, is calculated.
As for F (titer of a Karl Fischer reagent) , measuring is conducted by using HYDRNAL-Composite 5K (about 5mgH2θ/mL) , and it is preliminarily calculated by assigning the value into the equation (2) below.
(C) = ((b) x F / (a)) x 100 (1) F (mg/mL)
= [HYDRNAL-Composite 5K (about 5 mg H2O/mL) ] x [a solution amount of HYDRNAL-Composite 5K [mL] ] / [a titration amount of a Karl Fischer reagent [mL] ] (2)
A total (d) [wt %] of a water concentration contained in methanol anhydride itself preliminarily measured and a water concentration derived from water contained in a water absorbent resin before addition of a treating agent is subtracted from a water concentration (c) in methanol anhydride which is used in extraction of water from a mixture containing a water absorbent resin mixture calculated in the above described equation (1) , to obtain a concentration (e) , namely (c) - (d) = (e) . An amount of water (g) [mg] which is extracted from a water absorbent resinmixture is calculated by using the concentration (e) and an amount of methanol anhydride (f) [mg] to be used in extraction of water from the water absorbent resin mixture, in accordance with the following equation (3) .
(g) = ((C) - (d)) x (f) = (e) x (f) (3)
Further, an amount of water (h) [mg] derived from a treating solution, which is on calculation contained in a water absorbent resin mixture (a), can be calculated by using the following equation (4), from a weight (i) [mg] of the treating solution added per 1,000 mg of a water absorbent resin and a weight of water (j) [mg] contained in the treating solution.
(h) = (a) x ((J) / (1000 + (i)) (4)
A content of water (g) extracted from a water absorbent resin to an amount of water (h) derived from a treating solution, which is on calculation contained in a water absorbent resin mixture (a) , is calculated from the following equation (5) , which is made as an extraction ratio (k) [wt %] . (k) = ((g) / (h)) x 100 (5)
A product of a weight ratio (1) [wt %] of an amount of water contained in a treating solution added to a water absorbent resin relative to an amount of a water absorbent resin and an extraction ratio (k) is multiplied together in accordance with the following equation (6), to obtain a surface water content (m) [wt %] . (m) = (1) x ((k) / 100) (6)
(3) Centrifuge Retention Capacity (CRC)
CRC indicates absorbency in an aqueous 0.90 wt . % sodium chloride solution (hereinafter also called simply as "physiological saline") without load for 30 min.
0.200 g of a water absorbent resin is uniformly put in a pouch (85 mm * 60 mm) made of an non-woven fabric (made by Nangoku Pulp Kogyo K. K. , Product Name; Heatlon Paper, Model GSP-22) , and heat-sealed. Then, the pouch is immersed at room temperature in large excess (about 500 mL) of physiological saline. After 30 min, the pouch is pulled out, and water is removed with a centrifuge (made by Kokusan Co.,Ltd., Type:
H-122) by centrifugal force (250G) as described in "edena ABSORBENCY II 441.1-99" for 3 min. Then, a weight of the pouch is measured, which is regarded as Wl (g) . Further, the same operation is conductedwithout using the water absorbent resin, to measure a weight, which is regarded as WO (g) . From these values, Wl and WO, CRC (g/g) is calculated according to the equation described below.
CRC (g/g) = [ (Wl - WO) / Weight of water absorbent resin] - 1
(4) Absorbency against pressure (AAP) A 400-mesh wire gauze of stainless steel (38 μm in mesh size) is welded to a bottom of a plastic supporting cylinder 60mmin inside diameter . Under conditions of roomtemperature (25 ± 2°C) and 50 RH% of humidity, 0.900 g of a given water absorbent resin is uniformly scattered on the wire gauze. A piston and a load, each of which is adjusted to exert a load of 4.83 kPa uniformly on the water absorbent resin, has an outside diameter slightly smaller than 60 mm but produces no gap relative to the inner wall surface of the supporting cylinder, and does not have its unobstructed vertical motion prevented, were mounted thereon sequentially in the order mentioned, and the whole weight W3 (g) of the resultant measuring device is determined.
A glass filter 90 mm in diameter (pore diameter: 100 - 120 μm: made by Sogo Rikagaku Glass Manufactory K. K.) is placed inside a petri dish 150 mm in diameter. An aqueous 0.9 wt .% sodium chloride solution (physiological saline) (20 - 25°C) is added to the petri dish so as to give the same level as the upper surface of the glass filter. One filter paper 90 mm in diameter (0.26 mm in thickness and 5 μm in retained particle diameter; made by Advantec Toyo K. K. and sold under the product name of "JIS P 3801, No. 2") is mounted on the surface of physiological saline so as to have the surface thereof thoroughly wetted and the excess solution is removed.
The resultant measuring device is wholly mounted on the wetted filter paper and the water absorbent resin is allowed to absorb the solution under load for a prescribed time. This absorption time is set at one hour from the start of the measurement. To be specific, the whole measuring device is lifted after the one hour' s standing, and the weight thereof Wb (g) is determined. This determination of the weight must be performed as quickly as possible without exposing the device to anyvibration. Theabsorbency against pressure (AAP) (g/g) is calculated in accordance with the following formula using Wa and Wb.
AAP (g/g) = [Wb (g) - Wa (g) ] /Weight of water absorbent resin (g)
(5) Total water content In an aluminum cup having a bottom with a diameter of
4 cm and a height of 2 cm, 1.00 g of a water absorbent resin is spreaduniformly on the bottom. The aluminumcup containing the water absorbent resin is weighed [W4 (g) ] . The cup is left in a hot air drier kept at 180 °C for 3 hours. Immediately
(within at least 1 minute) after the cup is taken out of the hot air drier, the aluminum cup containing the water absorbent resin is weighed [W5 (g) ] . The total water content is calculated from the values W4 and W5 by the following formula. Total water content (% by weight)
= [ (W4 (g) -W5 (g) ) / (Weight of water absorbent resin (g) ) ] x 100
(6) Saline flow conductivity (SFC)
SFC is a value which indicates a degree of liquid permeability exhibited by water absorbent resin particles in a swollen state. The larger SFC means higher liquid permeability.
The SFC is determined in accordance with the test for the saline flow conductivity (SFC) described in JP-T-9 (1997) -509591 with necessary modification.
Specifically, by the use of a device illustrated in Fig. 1, SFC is determined. In the device illustrated in Fig. 1, a tank 31 has a glass tube 32 inserted therein and the lower end of the glass tube 32 is disposed so that an aqueous 0.69 wt. % sodium chloride solution 33 can be maintained to a height of 5 cm from the bottom of the swelled gel 44 held in a cell 41. The aqueous 0.69 wt. % sodium chloride solution in the tank 31 is supplied to the cell 41 via an L-letter tube 34 fitted with a cock. Below the cell 41, a container 48 for collecting the passed liquid is disposed and the collecting container 48 is set on a pan scale 49. The cell 41 has an inside diameter of 6 cm. A wire gauze (opening of sieve: 38 μm) 42 of stainless steel of No. 400 is disposed on the bottom surface in the lower part of the cell . A piston 46 is provided in the lower part thereof with holes 47 sufficient for passing a liquid, and fitted in the bottom part thereof with a glass filter 45 having good permeability so as to prevent the water absorbent resin or the swelled gel thereof from entering the hole 47. The cell 41 is laid on a stand for mounting the cell . The surface of the stand contacting the cell is placed on a wire gauze 43 of stainless steel incapable of obstructing the passage of liquid.
An artificial urine is preparedbymixing 0.25 gof calcium chloride dihydrate, 2.0 g of potassium chloride, 0.50 g of magnesium chloride hexahydrate, 2.0 g of sodium sulfate, 0.85 g of ammonium dihydrogen phosphate, 0.15 g of diammonium hydrogen phosphate, and 994.25 g of purified water together .
As a measuring operation, a water absorbent resin (0.900 g) is uniformly placed in a container 40 and left swelling with an artificial urine under a pressure of 0.3 psi (2.07 kPa) for 60 minutes, and a height of a gel layer of gel 44 is recorded. Subsequently, under a pressure of 0.3 psi (2.07 kPa) , an aqueous 0.69 wt. % sodium chloride solution 33 from a tank 31 is passed under a stated hydrostatic pressure through the swelled gel layer. By means of a computer and a balance, the amounts of liquidpassing through the gel layer at intervals of 20 seconds are recorded as a function of time over 10 minutes . A flow speed Fs (t) through the swelled gel 44 (mainly between adjacent particles) is determined in unit of [g/s] by dividing an increased weight (g) by an increased time (second) . The time in which the constant hydrostatic pressure and the stable flow speed are attained is denoted by Ts. The data obtained for 10 minutes and Ts are exclusively used for the calculation of a flow speed. The value Fs (t = 0) , namely an initial flow speed through the gel layer, is calculated by using the flow speed obtained over 10 minutes and Ts. Specifically, the Fs
(t = 0) is calculated by extrapolating the result of the least-squares method performed on the Fs (t) against time into t = 0.
SFC = [Fs (t = 0) x LO] / (p x A xΔP) = [Fs (t = 0) x LO] / 139506 wherein Fs (t) stands for a flow speed expressed in units of [g/s], LO stands for a height of a gel layer expressed in units of cm, p stands for a density of an aqueous 0.69 wt . % sodium chloride solution (1.003 g/cm3) ,
A stands for an upper side area of a gel layer in a cell 41 (28.27 cm2) ,
ΔP stands for a hydrostatic pressure exerted on a gel layer
(4920 dynes/cm2), and a unit of SFC is (lO^-cm^s-g"1) .
(Production Example 1)
In a reaction vessel which is formed from a jacketed, double-armtype kneader of stainless steel with an inner volume of 10 L and provided with two sigma-type blades, and a lid further attached thereto, 5,433 g of an aqueous solution of sodium acrylate (a monomer concentration: 39 wt. %) having a neutralization ratio of 70 mol% was charged. Then, 12.83 g of a polyethylene glycol diacrylate (a number of average ethylene oxide units: n = 9) as an internal cross-linking agent was dissolved into the aqueous solution, to prepare a reaction solution. Further, the reaction solution was deaerated under a nitrogen atmosphere. Subsequently, 29.43 g of an aqueous 10 wt. % sodium persulfate solution as a polymerization initiator and 24.53 g of an aqueous 0.1 wt . % L-ascorbic acid solution were added to the reaction solution while stirring. As a result, polymerizationbegan after about one minute. While pulverizing a gel formed, polymerization was conducted at 20 to 95°C, and a hydrogel-like cross-linked polymer was taken out after 30 minutes from the beginning of the polymerization. The particle diameter of the hydrogel-like cross-linked polymer obtained was not larger than 5 mm. The pulverized hydrogel-like cross-linked polymers were scattered on a wire mesh of 50 mesh (opening of sieve : 300 μm) , and were dried in a hot air at 175°C for 50 minutes. Thus, easily pulverizable powdery agglomerates having an amorphous form were obtained.
The resultant powdery agglomerates were pulverized with a roll mill, and further were classified with a JIS standard sieve having an opening of 710 μm. Next, particles which passed through the sieve having an opening of 710 μm in the above-described operation, were classified with a JIS standard sieve having an opening of 150 μm, to remove particles which passed through the sieve having an opening of 150 μm. Thus, a water absorbent resin (A) was obtained.
The particle size distribution of the resultant water absorbent resin (A) is shown in Table 1 below, and various properties thereof are shown in Table 2 below. (Production Example 2)
In a reaction vessel which is formed from a jacketed, double-armtype kneader of stainless steel with an inner volume of 10 L and provided with two sigma-type blades, and a lid further attached thereto, 5,437 g of an aqueous solution of sodium acrylate (a monomer concentration: 39 wt . %) having a neutralization ratio of 60 mol% was charged. Then, 7.90 g of a polyethylene glycol diacrylate (a number of average ethylene oxide units: n = 9) as an internal cross-linking agent was dissolved into the aqueous solution, to prepare a reaction solution. Further, the reaction solution was deaerated under a nitrogen atmosphere. Subsequently, 30.19 g of an aqueous 10 wt. % sodium persulfate solution as a polymerization initiator and 25.16 g of an aqueous 0.1 wt. % L-ascorbic acid solution were added to the reaction solution while stirring. As a result, polymerization began after about one minute. While pulverizing a gel formed, polymerization was conducted at 20 to 95°C, and a hydrogel-like cross-linked polymer was taken out after 30 minutes from the beginning of the polymerization. A particle diameter of the hydrogel-like cross-linked polymer obtained was not larger than 5 mm. The pulverized hydrogel-like cross-linked polymers were scattered on a wire mesh of 50 mesh (opening of sieve : 300 μm) , and were dried in a hot air at 175°C for 50 minutes. Thus, easily pulverizable powdery agglomerates having an amorphous form were obtained.
The resultant powdery agglomerates were pulverized with a roll mill, and further were classified with a JIS standard sieve having an opening of sieve of 710 μm. Next, particles which passed through a sieve having an opening of sieve of 710 μm in the above-described operation, were classified with a JIS standard sieve having a opening of sieve of 150 μm, to remove particles which passed through a sieve having a opening of sieve of 150 μm. Thus, a water absorbent resin (B) was obtained.
The various properties of the resultant water absorbent resin (B) are shown in Table 2 below. Inthiscase, theparticle size distribution of the resultant water absorbent resin (B) was the same as that of the water absorbent resin (A) . (Production Example 3) In a reaction vessel which is formed from a jacketed, double-armtype kneader of stainless steel with an inner volume of 10 L and provided with two sigma-type blades, and a lid further attached thereto, 5,443 g of an aqueous solution of sodium acrylate (a monomer concentration: 39 wt. %) having a neutralization ratio of 90 mol% was charged. Then, 6.11 g of a polyethylene glycol diacrylate (a number of average ethylene oxide units: n = 9) as an internal cross-linking agent was dissolved into the aqueous solution, to prepare a reaction solution. Further, the reaction solution was deaerated under a nitrogen atmosphere. Subsequently, 28.02 g of an aqueous 10 wt. % sodium persulfate solution as a polymerization initiator and 23.35 g of an aqueous 0.1 wt . % L-ascorbic acid solution were added to the reaction solution while stirring. As a result, polymerizationbegan after about one minute. While pulverizing a gel formed, polymerization was conducted at 20 to 95°C, and a hydrogel-like cross-linked polymer was taken out after 30 minutes from the beginning of the polymerization. A particle diameter of the hydrogel-like cross-linked polymer obtained was not larger than 5 mm. The pulverized hydrogel-like cross-linked polymers were scattered on a wire mesh of 50 mesh (opening of sieve : 300 μm) , and were dried in a hot air at 175°C for 50 minutes. Thus, easily pulverizable powdery agglomerates having an amorphous form were obtained.
The resultant powdery agglomerates were pulverized with a roll mill, and further were classified with a JIS standard sieve having an opening of sieve of 710 μm. Next, particles which passed through a sieve having an opening of sieve of 710 μm in the above-described operation, were classified with a JIS standard sieve having a opening of sieve of 150 μm, to remove particles which passed through a sieve having a opening of sieve of 150 μm. Thus, a water absorbent resin (C) was obtained.
The various properties of the resultant water absorbent resin (C) are shown in Table 2 below. In this case, theparticle size distribution of the resultant water absorbent resin (C) was the same as that of the water absorbent resin (A) .
In the Table 1, "not less than 850 μm" is referred to as a ratio (% by weight) of a water absorbent resin which has remained on a sieve having a mesh size of 850 μm following the classification process. Also, "not more than 45 μm" is referred to as a ratio (% by weight) of a water absorbent resin which has passed through a sieve having a mesh size of 45 μm following the classification process. Then, "x to y is referred to as a ratio (% by weight) of a water absorbent resin which has passed through a sieve having a mesh size of x μm and also remained on a sieve having a mesh size of y μm following the classification process.
Table 1
Figure imgf000066_0001
(Example 1)
500 g of the water absorbent resin (A) as a base polymer was added to 5L of Loedige mixer (made by Loedige Co., Ltd., Type: M5R) . A treating solution which had been preliminarily- prepared by mixing 5.0 g of ammonium persulfate, 2.5 g of polyethylene glycol monomethyl ether (a number average molecular weight of about 2, 000) and 40 g of water, was sprayed under stirring at 300 rpm. After mixing was continued under stirring for additional 3 minutes at room temperature in order to make the added water permeate and diffuse into an inner part of particles, stirring was terminated once, and a sample charging port of the Loedige mixer was taken off. The surface water content of the water absorbent resin composition (1) thus obtained was found to be 4.6% by weight.
After placing a glass plate made of quartz and having a thickness of 3 mm at the opening of the mixer, stirring of the water absorbent resin composition (1) was restarted (the time required for restart was 30 seconds) . An ultraviolet rays radiating device (made by Ushio Denki K. K., UV-152/IMNSC3-AA06) furnished with a metal halide lamp of
1 kW (made by the same company, UVL-1500M2-N1) was set so as to make the distance between the center of the lamp and the plate of quartz 8 cm. Then, the water absorbent resin composition (1) was irradiated with ultraviolet rays at room temperature for 15 minutes, to obtain a modified water absorbent resin (1) .
The modified water absorbent resin (1) obtained was rated for various properties, and the results are shown in Table
2 below. In the Table 2, "CRC after correction with total water content" and "AAP after correction with total water content" are calculated by the following formulae. In the following formulae, "CRC before correction with total water content" is referred to as a centrifuge retention capacity
(CRC) of water absorbent resin prior to determination of total water content by the above (5), and "AAP before correction with total water content" is referred to as an absorbency against pressure (AAP) of water absorbent resin prior to determination of total water content by the above (5) .
CRC after correction with total water content (g/g)
= [ (CRC before correction with total water content) (g/g) + 1] / (100 - Total water content of water absorbent resin) ] x 100 - 1
AAP after correction with total water content (g/g)
= AAP before correction with total water content (g/g) / (100 - Total water content of water absorbent resin) ] x 100 (Example 2)
By repeating the same procedure as described in Example
1 except that the mixing time of the treating solution was changed to lOminutes tomake water furtherpermeate and diffuse into an inner part of particles, a water absorbent resin composition (2) having a surface water content of 4.4 % by weight was obtained. Further, by the same procedure as described in Example 1, the water absorbent resin composition (2) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (2) .
The modified water absorbent resin (2) obtained was rated for various properties, and the results are shown in Table
2 below. (Example 3)
By repeating the same procedure as described in Example
1 except that the mixing time of the treating solution was changed to 30 minutes tomake water furtherpermeate anddiffuse into an inner part of particles, a water absorbent resin composition (3) having a surface water content of 3.4 % by weight was obtained. Further, by the same procedure as described in Example 1, the water absorbent resin composition
(3) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (3) . Themodified water absorbent resin (3) obtained was rated for various properties, and the results are shown in Table
2 below. (Example 4)
The same procedure as described in Example 2 was repeated except that the water absorbent resin (B) was used as a base polymer instead, and the amount of ammonium persulfate in the treating solution was changed to 12.5 g, to obtain a water absorbent resin composition (4) having a surface water content of 4.5 %byweight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (4) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (4) .
The modified water absorbent resin (4) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 5) The same procedure as described in Example 4 was repeated except that the water amount in the treating solution was changed to 80 g, to obtain a water absorbent resin composition
(5) having a surface water content of 7.9 % byweight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (5) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (5) .
The modified water absorbent resin (2) obtained was rated for various properties, and the results are shown in Table 2 below.
(Example 6)
The same procedure as described in Example 4 was repeated except that the water amount in the treating solution was changed to 120 g, to obtain a water absorbent resin composition (6) having a surface water content of 10.4 % byweight. Further, by the same procedure as described in Example 1, the water absorbent resin composition (6) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (6). The modified water absorbent resin (6) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 7)
The same procedure as described in Example 4 was repeated except that the water amount in the treating solution was changed to 160 g, to obtain a water absorbent resin composition (7) havinga surface water content of 12.5 % by weight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (7) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (7) . The modified water absorbent resin (7) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 8)
The same procedure as described in Example 4 was repeated except that the water amount in the treating solution was changed to 200 g, to obtain a water absorbent resin composition
(8) having a surface water content of 15.5 % byweight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (8) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (8) .
The modified water absorbent resin (8) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 9)
A water absorbent resin composition (9) having a surface water content of 12.5 % by weight was obtained by repeating the same procedure as described in Example 7. Further, by the same procedure as described in Example 1, the water absorbent resin composition (9) was irradiated with ultraviolet rays for 1 minute, to obtain a modified water absorbent resin (9). Themodifled water absorbent resin (9) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 10) The same procedure as described in Example 5 was repeated except that the water absorbent resin (C) was used as a base polymer instead, to obtain a water absorbent resin composition
(10) having a surface water content of 5.2 % byweight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (10) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (10) .
The modified water absorbent resin (10) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 11)
The same procedure as described in Example 10 was repeated except that the water amount in the treating solution was changed to 120 g, to obtain a water absorbent resin composition (11) havinga surface water content of 6.5 %byweight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (11) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (11). The modified water absorbent resin (11) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 12)
The sameprocedure as described in Example 10 was repeated except that the water amount in the treating solution was changed to 160 g, to obtain a water absorbent resin composition (12) havinga surface water content of 6.7 % byweight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (12) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (12) . The modified water absorbent resin (12) obtained was rated for various properties, and the results are shown in Table 2 below. (Example 13)
The same procedure as described in Example 10 was repeated except that the water amount in the treating solution was changed to 200 g, to obtain a water absorbent resin composition (13) having a surface water content of 9.6 % byweight . Further, by the same procedure as described in Example 1, the water absorbent resin composition (13) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin (13) .
The modified water absorbent resin (13) obtained was rated for various properties, and the results are shown in Table 2 below. (Comparative Example 1)
The same procedure as described in Example 1 was repeated except that themixing time of the treating solutionwas changed to 60 minutes, to obtain a water absorbent resin composition for comparison having a surface water content of 2.8 % by weight. Further, bythe same procedure as described inExample 1,'the water absorbent resin composition for comparison was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin for comparison (1) .
The modified water absorbent resin for comparison (1) obtained was rated for various properties, and the results are shown in Table 2 below. (Comparative Example 2) The same procedure as described in Example 4 was repeated except that the water amount in the treating solution was changed to 20 g, to obtain a water absorbent resin composition for comparison (2) having a surface water content of 1.9 % by weight. Further, by the same procedure as described in Example 1, the water absorbent resin composition for comparison (2) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin for comparison (2) . The modified water absorbent resin for comparison (2) obtained was rated for various properties, and the results are shown in Table 2 below. (Comparative Example 3)
The same procedure as described in Example 9 was repeated except that the water amount in the treating solution was changed to 20 g, to obtain a water absorbent resin composition for comparison (3) having a surface water content of 1.1 % by weight. Further, by the same procedure as described in Example 1, the water absorbent resin composition for comparison (3) was irradiated with ultraviolet rays for 15 minutes, to obtain a modified water absorbent resin for comparison (3) .
The modified water absorbent resin for comparison (3) obtained was rated for various properties, and the results are shown in Table 2 below.
[Table 2]
Figure imgf000074_0001
PEG-OMe: Polyethylene glycol monomethyl ether (a number average molecular weight of about 2,000), W: Pure water
It is noted from the results shown in Table 2 that according to the method for the production of this invention, by irradiating with active energy rays a water absorbent resin composition containing a water absorbent resin, water and a water-soluble radical polymerization initiator with a surface water content of the water absorbent resin being controlled to a level of not lower than a predetermined value, the modification of the surface of the water absorbent resin particle can be effectively conducted, to produce a water absorbent resin having excellent water absorbent properties . Further, it is also noted from the results of Example 9 that by irradiating water absorbent resin with active energy rays while controlling a surface water content thereof a water absorbent resin to a level of not less than a prescribed value by adding relatively large amount of water thereto, the modification of surface of water absorbent resin particles can be efficiently carried out even in a short time and a water absorbent resin having excellent water absorbent properties can be produced.
Industrial Applicability:
This invention is industrially applicable, because the surface treatment can be satisfactorily carried out even at a reaction temperature approximating normal roomtemperature . Further, since the modified water absorbent resin consequently obtained excels in water absorbing properties, it can be used for disposable diapers, for example.
The entire disclosure of Japanese Patent Application Nos. 2006-108081 and 2006-157577 filed on April 10, 2006 and
June 6, 2006, respectively, including specification, claims, drawings and summary are incorporated herein by reference in its entirety.

Claims

1. A method for the production of a modified water absorbent resin, which comprises: a) a mixing step comprising mixing a water absorbent resin, water, and a water-soluble radical polymerization initiator without addition of an ethylenically unsaturated monomer, to obtain a water absorbent resin composition, and b) an irradiating step comprising irradiating said water absorbent resin composition obtained in the mixing step with active energy rays, wherein a surface water content of said water absorbent resin in said water absorbent resin composition at least at any point of time in the irradiating step is controlled to a level of not lower than 3.0% by weight based on 100% by weight of the water absorbent resin.
2. The method for the production of a modified water absorbent resin according to claim 1, wherein said water-soluble radical polymerization initiator is at least one member selected from the group consisting of persulfates , hydrogen peroxide and water-soluble azo compounds.
3. A method for the production of a modified water absorbent resin, which comprises: a) a mixing step comprising mixing a water absorbent resin, water, and a heat-degradable radical polymerization initiator without addition of an ethylenically unsaturated monomer, to obtain a water absorbent resin composition, and b) an irradiating step comprising irradiating said water absorbent resin composition obtained in the mixing step with active energy rays, and wherein an amount of water mixed in said step a) exceeds 20 parts by weight and is not more than 100 parts by weight based on 100 parts by weight of the water absorbent resin, and a surface water content of said water absorbent resin in said water absorbent resin composition at least at any point of time in the irradiating step b) is controlled to a level of not lower than 3.0% by weight based on 100% by weight of the water absorbent resin.
4. The method for the production of a modified water absorbent resin according to claim 3, wherein said water-soluble radical polymerization initiator is at least one member selected fromthe group consisting of persulfates, hydrogen peroxide and water-soluble azo compounds.
5. The method for the production of a modified water absorbent resin according to any one of claims 1 to 4, wherein the surface water content at the time of beginning the said irradiating step is controlled to a level of not lower than 3.0% by weight.
6. The method for the production of a modified water absorbent resin according to any one of claims 1 to 5, wherein a mixing amount of said radical polymerization initiator in the mixing step is in the range of 0.01 to 20 parts by weight, based on 100 parts by weight of the water absorbent resin.
7. The method for the production of a modified water absorbent resin according to any one of claims 1 to β, wherein in the mixing step, said radical polymerization initiator is mixed in a form of an aqueous solution.
8. The method for the production of a modified water absorbent resin according to any one of claims 1 to 7, wherein a mixing amount of said water in the mixing step is in the range of 1 to 100 parts by weight, based on 100 parts by weight of the water absorbent resin.
9. The method for the production of a modified water absorbent resin according to any one of claims 1 to 8, wherein a mixing aid is added to the water absorbent resin at the same time as or prior to the mixing step a) .
10. The method for the production of a modified water absorbent resin according to claim 9, wherein said mixing aid is at least one water-soluble or water-dispersible compound selected from the group consisting of surfactants, water-soluble polymers, hydrophilic organic solvents, water-soluble inorganic compounds, inorganic acids, inorganic acid salts, organic acids, and organic acid salts .
11. The method for the production of a modified water absorbent resin according to claim 9 or 10, wherein said mixing aid is at least one water-soluble or water-dispersible compound selected from the group consisting of polyoxyethylene alkyl ethers, polyethylene glycols, water-soluble polyvalent metal salts, sodium chloride, ammonium hydrogen sulfate, ammonium sulfate, sulfuric acid, and hydrochloric acid.
12. The method for the production of a modified water absorbent resin according to any one of claims 9 to 11, wherein said mixing aid is added in an amount in the range of 0.01 to 40 parts by weight based on 100 parts by weight of the water absorbent resin.
13. The method for the production of a modified water absorbent resin according to any one of claims 1 to 12, wherein said water absorbent resin contains an acid group and has a neutralization ratio (mol% of the neutralized acids group in the whole of acid groups) in the range of 50 to 75 mol%.
14. The method for the production of a modified water absorbent resin according to any one of claims 1 to 13, wherein said active energy rays are ultraviolet rays.
15. The method for the production of a modified water absorbent resin according to any one of claims 1 to 14 , wherein said water absorbent resin is a powdery water absorbent resin obtained by polymerizing a monomer containing an acrylic acid (salt) as a main component.
16. The method for the production of a modified water absorbent resin according to any one of claims 1 to 15, wherein said water absorbent resin is obtained by producing a water absorbent resin precursor having a low neutralization ratio, and mixing said water absorbent resin precursor with a base .
17. The method for the production of a modified water absorbent resin according to any one of claims 1 to 16, wherein said water absorbent resin contains particles having a particle diameter in the range of 150 to 850 μm in a ratio in the range of 90 to 100% by weight.
18. The method for the production of a modified water absorbent resin according to any one of claims 1 to 17 , wherein the absorbency of physiological saline against pressure of 4.83 kPa of the water absorbent resin after the modification is higher by not less than 1 g/g/ as compared with the absorbency against pressure of the resin prior to the modification.
19. The method for the production of a modified water absorbent resin according to any one of claims 1 to 18 , wherein the absorbency of physiological saline against pressure of 4.83 kPa of the water absorbent resin after the modification is in the range of 8 to 40 g/g.
20. The method for the production of a modified water absorbent resin according to any one of claims 1 to 19, wherein the saline flow conductivity of the water absorbent resin after the modification is not less than 10 (* 10~7 • cm3 • s • g"1) .
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JPS6399211A (en) * 1986-06-04 1988-04-30 Hayashikane Zosen Kk Production of modified water-absorbing resin
JPS63260907A (en) * 1987-04-17 1988-10-27 Mitsubishi Petrochem Co Ltd Manufacture of highly water-absorptive polymer
JP2005097585A (en) * 2003-08-27 2005-04-14 Nippon Shokubai Co Ltd Process for producing surface-treated particulate water-absorbing resin
JP2007056071A (en) * 2005-08-22 2007-03-08 Procter & Gamble Co Method for preparing modified water absorbent resin
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JP2005097585A (en) * 2003-08-27 2005-04-14 Nippon Shokubai Co Ltd Process for producing surface-treated particulate water-absorbing resin
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