WO2009001954A1

WO2009001954A1 - Method of producing water-absorbent resin

Info

Publication number: WO2009001954A1
Application number: PCT/JP2008/061801
Authority: WO
Inventors: Yorimichi Dairoku; Kenji Tada; Shinichi Fujino; Yoshio Irie
Original assignee: Nippon Shokubai Co., Ltd.
Priority date: 2007-06-27
Filing date: 2008-06-24
Publication date: 2008-12-31
Also published as: JP2010531366A

Abstract

A method of the present invention for producing a water-absorbent resin is a method which uses a continuous polymerizing device (20) including an endless belt (8a) and a net belt (8b). A monomer liquid (4) is continuously supplied to the endless belt (8a), and is gelatinized, thereby obtaining a hydrous gel (7). Thereafter, the hydrous gel (7) is transferred to the net belt (8b), and is dried. Thus, a method for producing a water-absorbent resin which prevents deterioration of a surface of a conveyor belt is provided.

Description

DESCRIPTION

METHOD OF PRODUCING WATER-ABSORBENT RESIN

TECHNICAL FIELD

The present invention relates to a method for producing a water-absorbent resin.

BACKGROUND ART

Water-absorbent resins have been widely used for various purposes such as sanitary supplies including paper diapers, sanitary napkins and adult incontinence products, as well as water retaining agents for soil. This is because the water-absorbent resin has an aqueous liquid absorbing property, which is capable of absorbing large amounts of aqueous liquids, from several times more to hundreds times more of its own weight. Consequently, the water-absorbent resin has been produced and consumed in large amounts. The water-absorbent resin (also referred as a superabsorbent resin, or a superabsorbent polymer) is already well-known. For example, the water-absorbent resin is disclosed in the Japanese Industrial Standard (JIS) K7223- 1996, and has been introduced in many commercial reference books.

Conventionally, one method is known as a method for producing the water-absorbent resin, in which a hydrophilic polymer containing an acid group such as a carboxyl group or a sulfonic acid group, particularly a hydrophilic polymer whose main component is acrylic acid and/ or a salt thereof, is polymerized in an aqueous solution form. For example, the following is a method for obtaining the water-absorbent resin. A hydrophilic monomer in an aqueous solution form is polymerized. This obtains a hydrous gel. The hydrous gel thus obtained is crushed while the hydrous gel is stirred. Thereafter, the crushed hydrous gel is dried and classified to obtain the water-absorbent resin. Another example of obtaining the water-absorbent resin is as follows. An aqueous solution containing the hydrophilic monomer is polymerized without stirring the aqueous solution, thereby obtaining a hydrous gel. The obtained hydrous gel is crushed, and the crushed hydrous gel is further dried and classified to obtain the water-absorbent resin.

The latter method of producing the water-absorbent resin which polymerization of the aqueous solution containing the hydrophilic monomer is performed without stirring may be, for example, performed by using a reaction device which includes a continuously conveyable endless belt. This method is useful in a view that the water-absorbent resin is continuously obtained with ease . Examples of the polymerization in the aqueous solution form are disclosed in Patent Documents 1 to 3. A production process of the water-absorbent resin has a problem that a hydrous gel polymerized from a monomer whose main component is acrylic acid and/ or a salt thereof is strongly adhesive . This causes the hydrous gel to readily adhere to the polymerizing device. More specifically, if the hydrous gel adheres at the end of the endless belt and does not peel off, problems occur such as the hydrous gel rolling into the bottom side of the endless belt.

In order to solve the problems, each of Patent Documents 4 and 5 discloses a method for producing water-absorbent resin which uses a polymerizing device having an endless belt coated with a polymer. The polymer coated on the endless belt has a high release property with the hydrous gel. Patent Document 6 discloses a method for producing water-absorbent resin which uses a device having an endless belt whose surface in contact with the monomer solution has a predetermined melting viscosity. According to the production method disclosed in Patent Document 6, the water-absorbent resin is produced while maintaining the releasability of the hydrous gel, as well as securing high productivity. [Patent Document 1 ]

Japanese Unexamined Patent Publication, Tokukai, No. 2000- 1507 (published January 7, 2000) [Patent Document 2] Japanese Unexamined Patent Publication, Tokukai, No .

2004-51967 (published February 19 , 2004)

[Patent Document 3]

Japanese Unexamined Patent Publication, Tokukai, No. 2000-2 122 15 (published August 2 , 2000)

[Patent Document 4]

Japanese Unexamined Patent Publication, Tokukaihei, No.

7-228605 (published August 29 , 1995)

[Patent Document 5] Japanese Unexamined Patent Publication, Tokukai, No.

2002-3509 (published January 9 , 2002)

[Patent Document 6]

Japanese Unexamined Patent Publication, Tokukai, No.

2006- 199862 (published August 3 , 2006)

DISCLOSURE OF INVENTION

Adoption of the conventional polymerizing device or the conventional method for producing the water-absorbent resin however still cannot attain sufficiently satisfactory production of the water-absorbent resin (or the hydrous gel) . That is to say, in a conventional polymerizing device , the monomer is polymerized on the endless belt. This causes the monomer aqueous solution which has not been subjected to polymerization and the hydrous gel to be produced to be placed on the same endless belt. In view of this, the inventors of the present invention found out that a surface of the belt deteriorates in course of operation time .

The present invention is made in view of the conventional problems, and an object thereof is to provide a method for producing a water-absorbent resin capable of suppressing deterioration in a surface of a continuously conveyable conveyor belt such as an endless belt, for example.

Another object of the present invention is to provide a method for producing a water-absorbent resin in order to obtain a hydrous gel having high solid content and a low amount of residual monomer.

In order to attain the objects, a method of the present invention for producing water-absorbent resin is a method including polymerizing a monomer aqueous solution whose main component is acrylic acid and/ or a salt thereof by using a polymerizing device which includes a conveyor, so as to obtain a hydrous gel; and drying the hydrous gel, the step of polymerizing being such that the polymerizing device is a polymerizing device including a plurality of conveyors which operate in combination with each other.

In addition, it is preferable for the monomer aqueous solution to be gelatinized in the polymerizing device by continuously supplying and polymerizing the monomer aqueous solution on a first conveyor that is one of the plurality of conveyors.

According to the present invention, a polymerizing device includes a plurality of conveyors which operate in combination with each other. A monomer aqueous solution gelatinizes in a state where a monomer only has contact with a first conveyor belt. This reduces an amount of the conveyor belt which readily deteriorate, with respect to the whole polymerizing device . Even if the first conveyor belt deteriorates, only the first conveyor belt requires replacement. Therefore, as compared to a polymerizing device having a single conveyor, the polymerizing device of the present invention is advantageous in cost and time.

The method for producing the water-absorbent resin is preferably arranged such that solid content concentration of the hydrous gel is increased by drying the hydrous gel while the hydrous gel is traveled on a second and latter conveyor(s) , each of which is one of the plurality of conveyors.

This thus allows an increase in solid content of the hydrous gel with no remarkable deterioration occurring in the conveyor belt. This thus allows ease in latter steps of gel crushing and reducing residual monomer amount.

The method for producing the water-absorbent resin preferably includes: washing a first conveyor belt of the first conveyor after the hydrous gel is transferred from the first conveyor to the second conveyor, each of which is one of the plurality of conveyors .

Hydrous gel remaining on the first conveyor belt is removed by washing the first conveyor belt. This thus suppresses the deterioration of the first conveyor belt. The method for producing the water-absorbent resin is preferably arranged such that the plurality of conveyors which operate in combination with each other are conveyors having at least two types of different conveyor belts.

This enables to select a most appropriate belt material for each stage . The production conditions can be selected more freely.

The method for producing the water-absorbent resin is preferably arranged such that a ratio of an effective length of the first conveyor to a total length of an effective length of each of the second and latter conveyors, each of which are one of the plurality of conveyors, is not less than 1 : 0.5 and not more than 1 : 5. In addition, it is preferable that a time period required for an arbitrary point on the first conveyor to reach a position where the first conveyor is first washed is not less than 30 seconds and less than 5 minutes, the arbitrary point being a position where the monomer aqueous solution is supplied on the first conveyor.

The above arrangement enables to wash the first conveyor belt relatively soon after the first conveyor belt gets contact with the monomer liquid and the hydrous gel. Thus, the deterioration on the surface of the first conveyor belt is reduced even more.

A polymerization temperature of the monomer aqueous solution is preferably in a range of not lower than 50⁰C to not higher than 140⁰C. Concentration of a monomer component in the monomer aqueous solution is preferably at least 40 wt % .

With the method of the present invention for producing the water-absorbent resin, even if the temperature for the polymerization or the solid content concentration of the hydrous gel is in the above range, it is possible to suppress the deterioration of the first conveyor belt. Specifically, even in an extreme polymerization condition, suppression of the deterioration in the first conveyor belt is realized.

In the method of the present invention for producing water-absorbent resin, the solid content concentration of the hydrous gel discharged by the polymerizing device is to be not less than 50 wt % to not more than 80 wt %, and a particle-shaped water-absorbent resin is preferably obtained by continuously crushing the hydrous gel discharged from the polymerizing device with a crusher. The solid content concentration of the hydrous gel is preferably in a range of not less than 55 wt % to not more than 80 wt %, further preferably in a range of not less than 60 wt % to not more than 80 wt %, and particularly preferable in a range of not less than 65 wt % to not more than 80 wt %. By thus increasing the solid content concentration as in the above range , an amount of residual monomer contained in the hydrous gel is relatively reduced. Furthermore, it is possible to provide an easily crushable hydrous gel, in a case where the hydrous gel is to be crushed. By crushing the hydrous gel, it is possible to provide particle- shaped water-absorbent resin suitably used for sanitary supplies, water retaining agents for soil, and the like.

Additional objects, features, and strengths of the present invention will be made clear by the description below.

Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a cross sectional view illustrating one embodiment of a continuous polymerizing device according to the present embodiment.

Fig. 2 is a cross sectional view illustrating a trough and its vicinity in a continuous polymerizing device according to the present embodiment.

REFERENCE NUMERALS

1 Hydrous gel production section (conveyor, first conveyor) 2 Ageing section (conveyor, second conveyor)

3 Monomer liquid supplying nozzle

4 Monomer liquid

5 UV lamp 6 Trough

7 Hydrous gel

8a Endless belt (first conveyor belt)

8b Net belt (second conveyor belt)

9 Washing nozzle 10 Fluororesin layer

20 Continuous polymerizing device

BEST MODE FOR CARRYING OUT THE INVENTION

One embodiment of the present invention is described below with reference to Figs. 1 and 2. A method according to the present invention for producing water-absorbent resin may be performed, for example, by using a continuous polymerizing device 20 as illustrated in Fig. 1.

Fig. 1 is a cross sectional view illustrating a continuous polymerizing device 20 used in the method according to the present invention for producing water-absorbent resin. The continuous polymerizing device 20 includes a plurality of conveyors operating in combination with each other. The continuous polymerizing device 20 includes conveyors which operate as a hydrous gel production section 1 or an ageing section 2 , respectively. On an upper part of the hydrous gel production section 1 , a monomer liquid supplying nozzle 3 is provided for supplying a monomer liquid 4. Furthermore, on the upper part of the hydrous gel production section 1 , a UV lamp 5 is provided for irradiating ultraviolet rays in order to cure the monomer liquid 4. In the hydrous gel production section 1 , a trough 6 is provided. On the trough 6, a sheet-shaped hydrous gel 7 is to be formed. The hydrous gel production section 1 and the ageing section 2 have an endless belt 8a (first conveyor belt) and a net belt 8b (second conveyor belt) , respectively.

A state where a plurality of conveyors are operating in combination with each other is a state where the sheet-shaped hydrous gel 7 is conveyed by a plurality of conveyors, not in a so-called hauled state or a cut state.

Although the belt speed of each of the conveyors depend on the change in form of the hydrous gel and may be same or different to each other, each belt speed is not greatly different from each other. The plurality of conveyors may be operated with one driving motor, or one driving motor may be provided for each conveyor. The conveyor may be a belt conveyor and/ or a chain conveyor. The conveyors are an endless-type, and an endless belt is attached.

In the method according to the present embodiment for producing water-absorbent resin, the monomer liquid 4 is continuously supplied onto the endless belt 8a. The monomer liquid 4 is polymerized, thereby the hydrous gel 7 is obtained.

The continuous polymerizing device 20 supplies the monomer liquid 4 onto the endless belt 8a provided in the hydrous gel production section 1 , and gelatinizes the monomer liquid 4.

The endless belt 8a also operates so as to convey the hydrous gel 7.

A roller is provided on each end of the continuous polymerizing device 20. The rollers rotate in the direction of the arrow as shown in Fig. 1 . This allows the endless belt 8a to travel in a loop-shape. The endless belt 8a, in order to improve the releasability of the hydrous gel, is processed as follows. The belt is provided with a releasing film on its surface . The releasing film is formed by soaking a belt base material such as fiber or rubber in polymer resin, and then thermally sticking the polymer resin on the surface of the belt material.

The releasing film is sufficient as long as the releasing film has releasability with the hydrous gel 7, and examples encompass, as a polymer film, a polymer film made of fluororesin, polyether-ketone, aromatic polyimidazole, and silicon resin .

A resin belt made of silicon resin or fluororesin may also be used. Examples of the fluororesin which may be used encompass: tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA) , tetrafluoroethylene-hexafluoropropylene copolymer (FEP) , tetrafluoroethylene-ethylene copolymer (ETFE) , polychlorotrifluoro-ethylene (PCTFE) , chlorotrifluoroethylene-ethylene copolymer (ETCFE) , polyvinylidene fluoride (PVDF) , and polyvinyl fluoride (PVF) .

Other than the above, as long as the endless belt 8a shows releasability, the endless belt 8a may be for example: a steel belt made of stainless steel which has the release film; a steel belt further mounted with the resin belt or a single resin film; or a chain conveyor formed by a punching plate mounted with the resin belt or the single resin film.

The monomer liquid supplying nozzle 3 is provided on an upper side of the traveling loop of the hydrous gel production section 1. The monomer liquid 4 is continuously supplied from the monomer liquid supplying nozzle 3 onto the surface of the hydrous gel production section 1 which travels in a clock-wise direction as shown in Fig. 1 .

The monomer liquid 4 is a material of the hydrous gel 7, and is an aqueous solution containing a monomer component. A supplying amount of the monomer liquid 4 is altered as appropriate depending on a desired thickness and amount produced of the hydrous gel 7. The monomer liquid 4 is supplied on the trough 6. The structure of the trough 6 is later described with reference to Fig. 2. The monomer liquid 4 is not particularly limited as long as the monomer liquid 4 becomes a water-absorbent resin by polymerization. For example, the following may be used as the monomer liquid 4 : an anionic unsaturated monomer and/ or a salt thereof such as (meth)acrylic acid, (anhydrous) maleic acid, itaconic acid, cinnamic acid, vinyl sulfonic acid, allyltoluene sulfonic acid, vinyltoluene sulfonic acid, styrene sulfonic acid, 2-(meth) acrylamide-2-methylpropane sulfonic acid, 2- (meth)acryloylethane sulfonic acid,

2-(meth)acryloylpropane sulfonic acid, and 2-hydroxyethyl(meth)acryloyl phosphate; an unsaturated monomer containing a mercaptan group; an unsaturated monomer containing a phenolic hydroxyl group; an unsaturated monomer containing an amide group such as (meth)acrylamide, N-ethyl(meth)acrylamide, and N,N-dimethyl(meth)acrylamide; and an unsaturated monomer containing an amino group, such as

N,N-dimethylaminoethyl(meth)acrylate,

N,N-dimethylaminopropyl(meth)acrylate, and

N,N-dimethylaminopropyl(meth)acyrlamide. The monomers may be used solely, or two or more of the monomers may be used in combination as appropriate. However, from the view of performance and cost of the obtained water-absorbent resin, it is preferable to use acrylic acid and/ or a salt thereof (for example, salts of sodium, lithium, potassium, ammonium, amine and the like, preferably sodium salt from the view of cost) as its main component.

An amount of acrylic acid and/ or the salt thereof contained in the monomer liquid 4 is preferably 70 to 100 mol % with respect to a whole monomer component, and is more preferably not less than 80 mol %, further preferably not less than 90 mol %, and particularly preferable as not less than 95 mol % . The maximum limit of the contained amount of acrylic acid and / or the salt thereof is 100 mol %. Although there are particularly no limits in a neutralization rate for the monomer containing an acid group, for purposes in which the monomer may have contact with the human body such as sanitary materials, the amount of acrylic acid and/ or the salt thereof is preferably contained, together in view of the not having a necessity of neutralization, in a range of not less than 40 mol % to not more than 90 mol %, and is more preferable in a range of not less than 50 mol % to not more than 80 mol %.

Concentration of the monomer liquid 4 (monomer concentration) at the time of polymerization is not particularly limited, however is preferably not less than 40 wt %, and is more preferably not less than 45 wt % . Generally, low concentration of the monomer liquid 4 decreases the productivity, and high concentration of the monomer liquid 4 increases the viscosity of the hydrous gel 7 generated by polymerization. The high viscosity of the hydrous gel 7 causes the hydrous gel 7 to readily adhere . An advantage of the present invention appears clearly in a high concentration of at least 45 wt %. The concentration of the monomer liquid 4 is preferably in a range of 40 wt % to 70 wt %, more preferably in a range of 45 wt % to 70 wt %, further preferably in a range of 48 wt % to 70 wt %, even more preferably in a range of 50 wt % to 70 wt %, and even further preferably in a range of 52 wt % to 70 wt %. If the monomer concentration is lower than 40 wt %, the productivity becomes low, and if the monomer concentration exceeds 70 wt %, the absorbing capacity decreases.

A polymerization temperature at which the monomer liquid 4 is polymerized is not particularly limited. The polymerization temperature, specifically, is the temperature range of the monomer liquid 4 in which the monomer aqueous solution starts to polymerize, then gelatinizes, and finally reaches a maximum temperature . However, the polymerization temperature is preferably in a range of not less than 50⁰C to not more than 140°C, and is further preferably in a range of not less than 80⁰C to not more than 120°C . A high polymerization temperature allows active evaporation of water, which increases the solid content concentration of the hydrous gel 7 to be obtained. This additionally improves the productivity of the hydrous gel 7. However, on the other hand, the hydrous gel 7 with high solid content concentration readily adheres on the endless belt 8a.

However, with the present invention, deterioration of the surface of the endless belt 8a is suppressed. Therefore, the advantage of the present invention is further demonstrated under a condition in which the hydrous gel 7 is readily adhered. By increasing the solid content concentration to the above range, it is possible to relatively reduce the amount of residual monomer contained in the hydrous gel 7. Furthermore, it is possible to provide an easily crushable hydrous gel 7, in a case where the hydrous gel 7 is to be crushed.

The polymerization initiator added to the monomer liquid 4 is not particularly limited, and may be selected and used from one type or two or more types of the polymerization initiators used in normal water-absorbent resin production, depending on the type of monomer to be polymerized and the polymerization conditions.

For example, the polymerization initiator may be a thermal decomposition type initiator, a photodecomposition type initiator, or the like. Examples of the thermal decomposition initiator encompass: persulfates such as sodium persulfate, potassium persulfate, and ammonium persulfate; peroxides such as hydrogen peroxide, t-butyl peroxide, methyl ethyl ketone peroxide; azo compounds such as an azonitrile compound, an azoamidine compound, a cyclic azoamidine compound, an azoamide compound, an alkyl-azo compound, 2,2 '-azobis(2-amidinopropane) dihydrochloride and 2 ,2 '-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride. Examples of the photodecomposition initiator encompass: benzoin derivatives; benzyl derivatives; acetophenone derivatives; benzophenone derivatives; and azo compounds.

From the view of cost and reduction ability of residual monomers, the persulfate is preferred of the above polymerization initiators. A reducing agent which accelerates decomposition of the polymerization initiator may be j ointly used, together serving as a redox initiator. Examples of the reducing agent encompass: sodium sulfite, (bi) sulfite and/ or a salt thereof such as sodium bisulfite, L-ascorbic acid and/ or a salt thereof, a reductive metal and / or a salt thereof such as ferrous salt, and amines. However, the reducing agent is not particularly limited to these examples. It is more preferable to jointly use the photodecomposition type initiator and the thermal decomposition type initiator. An amount of the polymerization initiators used is not particularly limited, however is normally, with respect to the monomer in the monomer liquid 4 , in a range of 0.001 wt % to 2 wt %, and is preferably in a range of 0.01 wt % to 0.5 wt %.

The polymerization initiator is normally mixed with the monomer liquid 4 prior to supplying the monomer liquid 4 onto the endless belt 8a. When mixing the polymerization initiator and the monomer liquid 4 , if the mixing or the supplying of the initiator and the monomer liquid 4 takes time or a part of the mixed liquid resides in a pipe, the monomer could start polymerizing before the monomer is supplied on the endless belt 8a, effected by the polymerization initiator. This causes the polymerized component to adhere and grow in size in the pipe such as the monomer liquid supplying nozzle 3 , which may cause the pipe to be blocked. Therefore, the monomer liquid 4 and the polymerization initiator are preferably quickly mixed and supplied to the reaction device. For example, it is desirable to apply a technique suggested in Japanese Unexamined Patent Publication, Tokukai, No. 2004- 155963. Although the polymerization initiator is mixed with the monomer liquid 4 in a solution or a fluid dispersion state, it is also possible to mix the polymerization initiator with the monomer liquid as it is as long as the polymerization initiator is in a liquid state.

An internal crosslinking agent may be used in the polymerization if necessary. A conventional well-known internal crosslinking agent may be used for the internal crosslinking agent. Detailed examples of the internal crosslinking agent encompass: N,N'-methylene bis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth) acrylate, trimethylolpropane tri(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylate methacrylate, ethylene oxide denatured trimethylolpropane tri(meth)acrylate, pentaerythritol hexa(meth)acrylate , triallylcyanurate, triallylisocyanurate, triallylphosphate, triallylamine, poly(meth) allyoxyalkane , (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, 1 ,4-butanediol, pentaerythritol, ethylenediamine, ethylene carbonate, propylene carbonate, polyethyleneimine and glycidyl (meth)acrylate. One type or two or more types of the internal crosslinking agent may be used, in view of reactivity.

Particularly, it is preferable to essentially use a compound having at least two polymerizable unsaturated groups as the internal crosslinking agent. An amount used of the internal crosslinking agent may be determined as appropriate from a desired physical property of the water-absorbent resin. However, normally, the amount used is preferably in a range of 0.0001 to 10 mol %, and is more preferably in a range of 0.001 to 1 .0 mol %, each with respect to the monomer component. An insufficient amount of the internal crosslinking agent causes the decrease in gel strength. Accordingly, the soluble content of the gel increases. On the other hand, an excessive amount of the internal crosslinking agent causes decrease in the absorbency of the gel. As similar to the aforementioned polymerization initiator, the internal crosslinking agent is suitably mixed in the monomer liquid 4 when used .

In the polymerization, a hydrophilic polymer, a chain transfer agent, and a chelating agent may be added in a reaction system. Examples of the hydrophilic polymer encompass: starch, a starch derivative, cellulose, a cellulose derivative, polyvinyl alcohol, polyacrylic acid and/ or a salt thereof, and a crosslinking agent of polyacrylic acid and/ or a salt thereof. An example of the chain transfer agent is hypophosphorous acid and/ or a salt thereof. An amount of these contained when added to the reaction system is preferably in a range of 0 to 30 wt % with respect to the monomer component. Like the aforementioned polymerization initiator, the hydrophilic polymer, the chain transfer agent, and/ or the chelating agent may be added to the reaction system by being mixed in the monomer solution. The polymerization is normally performed under a regular pressure for ease of the device and the operation. However, it is also preferable to perform the polymerization in a decompressed atmosphere so as to lower the boiling temperature of the polymerizing system.

The UV lamp 5 irradiates light (i. e. ultraviolet rays) to the monomer liquid 4 supplied on the endless belt 8a. This light is required for the polymerization reaction . The UV lamp 5 is positioned on a downstream side of the monomer liquid supplying nozzle 3 in a horizontal traveling path of the hydrous gel production section 1 . A well-known UV lamp may be used for the UV lamp 5. Although the UV lamp 5 is provided in the continuous polymerizing device 20, the UV lamp 5 is not required when a redox initiator or a thermal decomposition initiator is used in accordance with the type of the monomer liquid 4. In this case, a heater may be provided instead of the UV lamp 5.

The hydrous gel 7 is the monomer liquid 4 polymerized in a sheet-form on the trough 6 by the light irradiated from the UV lamp 5. A process in obtaining the hydrous gel 7 is later described with reference to Fig. 2. The hydrous gel 7 thus obtained is conveyed from the hydrous gel production section 1 to the ageing section 2 by the endless belt 8a and the net belt 8b (second conveyor belt) .

Solid content concentration of the hydrous gel 7 discharged from the continuous polymerizing device 20 is not particularly limited, however its minimum limit is preferably not less than 30 wt % and is further preferably not less than 50 wt %. It is more preferable to be not less than 55 wt %, is further preferable to be not less than 60 wt %, and is most preferred to be not less than 65 wt % . The maximum value is preferably not more than 80 wt %.

Generally, when a solid content concentration of the hydrous gel is low, burden on a dry hydrous gel increases. This causes the productivity to decrease. On the other hand, when a solid content concentration of the hydrous gel is high, although viscosity of the gel increases thereby causing the hydrous gel to readily adhere, the productivity of the hydrous gel increases in crushing the hydrous gel. Therefore, the advantage of the present invention is demonstrated with a high solid content concentration of the hydrous gel of at least 50 wt % and not more than 80 wt %. Furthermore, the solid content concentration is preferably in a range of 60 wt % to 78 wt %, more preferably in a range of 60 wt % to 75 wt %, further preferably in a range of 60 wt % to 73 wt %, and is even further preferably in a range of 66 wt % to 73 wt %.

The solid content concentration of the hydrous gel 7 to be obtained in the hydrous gel production section 1 may be measured by, for example, sampling the hydrous gel 7 which travels between the endless belt 8a and the net belt 8b. The amount of water of the hydrous gel 7 dried in the ageing section 2 is later described.

A polymerization rate of the hydrous gel 7 to be obtained from the hydrous gel production section 1 is at least 90 wt %, preferably not less than 95 wt %, and particularly preferable as not less than 98 wt %. The polymerization rate is calculated by the following procedures: (i) the hydrous gel 7 is sampled at the aforementioned position; (ii) reaction is stopped by rapidly cooling the hydrous gel 7 with solid carbon dioxide or liquid nitrogen; (iii) residual monomer of the hydrous gel 7 is analyzed; and (iv) the ratio of the residual monomer amount in the solid content of the hydrous gel 7 is found and is subtracted from 100%. The endless belt 8a passes the vicinity of the UV lamp 5 by traveling in a horizontal direction, then curves from a horizontal direction to a downward direction. Thereafter, the endless belt 8a travels in a horizontal direction, however opposite to the direction of which the endless belt 8a traveled horizontally immediately before. A band-shaped hydrous gel 7 which is traveled on the endless belt 8a separates from the endless belt 8a of the hydrous gel production section 1 at a position B l . The position B l is an end of the traveling path of the endless belt 8a. The hydrous gel 7 then extends in a directly downward direction, and is placed on a position A2 of the belt of the ageing section 2 , which is the latter stage. The part of the endless belt 8a on the lower side of the hydrous gel production section 1 travels horizontally in the opposite direction of the upper side of the endless belt 8a, having a surface of the endless belt 8a, that is, the fluororesin layer, facing downwards.

In the method of the present invention for producing the water-absorbent resin, the endless belt 8a is preferably washed after the hydrous gel 7 is transferred from the endless belt 8a to the net belt 8b. It is possible to wash the endless belt 8a by using a washing nozzle 9 as described as follows.

The washing nozzle 9 is provided on the lower side of the hydrous gel production section 1 . The washing nozzle 9 washes the non-polymerized monomer liquid 4 and remaining hydrous gel 7 on the endless belt 8a, by spraying heated water, a washing solution, or the like to the surface of the endless belt 8a. The washing nozzle 9 is sufficient as long as the washing nozzle 9 can wash the endless belt 8a. A well-known washing device may be used, including ones which also use a washing brush .

In the hydrous gel production section 1 , a heater is provided to heat the endless belt 8a (however illustrations are omitted) . The heater enables to keep warm (a) the endless belt 8a in a part which passes a position between the monomer liquid supplying nozzle 3 and the UV lamp 5, and (b) the roller on the exit side of the hydrous gel production section 1 . An exhaust pipe is connected to a condenser and a blower on the upper side of the hydrous gel production section 1 , so as to collect water vapor that has generated due to polymerization heat during the polymerization.

In the method according to the present invention for producing the water-absorbent resin, the hydrous gel 7 is transferred to the net belt 8b, and is dried. The drying of the hydrous gel 7 is performed in the ageing section 2. The ageing section 2 heats the hydrous gel 7 obtained in the hydrous gel production section 1 , and promotes the polymerization reaction to progress. In the ageing section 2 , the net belt 8b travels in a loop-shape due to rollers positioned on each end of the net belt 8b. The net belt 8b is driven by the rollers. The net belt 8b is made of metal, and is constructed with wires whose surface is coated with Teflon (registered trademark) resin. A structure of the net belt 8b is not limited to this, and any structure may be used as the net belt 8b as long as the belt has releasability with the hydrous gel 7. The net belt 8b may be, for example, a steel belt conveyor which has a punching plate made of stainless steel whose surface is provided with the release film, or a chain conveyor constructed combined with the punching plate . Furthermore, the conveyor may be solely used as the net belt 8b even without the release film, as long as the conveyor demonstrates releasability. Other than the metal net belt whose surface is coated with resin, a net belt made of resin or fiber material, or a so-called wire belt may be used as the net belt 8b. A belt which has no spaces therein may be used as similar to the endless belt 8a, however it is preferable to use a net belt which easily vaporizes water from the surface of the hydrous gel 7. As such, different types of belts may be used as appropriate. This enables to select a most suitable belt material for the endless belt 8a in the hydrous gel production section 1 and the net belt 8b in the ageing section 2 , as an arrangement of the continuous polymerizing device 20. The production conditions can be selected more freely.

The ageing section 2 is covered with a box (not illustrated) which functions as a partition. In the box of the ageing section 2 , hot air is introduced. This thus allows application of heat to the hydrous gel 7 , as similar to the hydrous gel production section 1 . Fig. 1 illustrates an ageing section 2 having only one tier. However, the ageing section 2 may be arranged so that the ageing section 2 has a plurality of tiers, for a continuous polymerizing device which performs extended drying (ageing) . The ageing section may be transported in an opposite direction to that of Fig. 1 , so that the ageing section 2 has a different transporting direction to the hydrous gel production section 1 . Furthermore, in the case the ageing section 2 has a plurality of tiers, the ageing section 2 may have the tiers arranged vertically to each other. In such an arrangement, the hydrous gel 7 will be conveyed alternately in opposite directions.

The hydrous gel 7 is dried in the ageing section 2. An amount of water contained in the hydrous gel 7 that is dried in the ageing section 2 is preferably more than 0 wt % and less than 5 wt % with respect to the weight of the hydrous gel 7 at the time of transfer to the ageing section 2. The amount of water dried is more preferably less than 3 wt %, and is further preferable to be less than 2 wt %. Drying of the amount of water not less than 5 wt % of the hydrous gel 7 decreases the productivity of the water-absorbent resin. This is because the hydrous gel 7 which has not been subjected to crushing has a smaller surface area as compared to the hydrous gel 7 which has been subjected to crushing. Drying efficiency is decreased when not less than 5 wt % of water amount of the hydrous gel 7 is dried. Therefore, this is not preferred.

It is preferable for the hydrous gel 7 to be dried so that only the surface of the hydrous gel 7 and its vicinity is reduced in the amount of water, and the amount of water included in the hydrous gel 7 as a whole is hardly reduced. The crushability of the hydrous gel is thus improved, and the amount of the residual monomer is also reduced. The continuous polymerizing device 20 is of a multi-stage type having the hydrous gel production section 1 and the ageing section 2. Therefore, as compared to a polymerizing device of a single stage, the continuous polymerizing device 20 of the multi-stage type can transfer the hydrous gel 7 to the ageing section 2 in a heated state, heated to a high temperature at the hydrous gel production section 1 . Although the endless belt 8a placing the hydrous gel 7 which just has been polymerized readily deteriorates, it is possible to suppress the deterioration of the surface of the endless belt 8a. This is because the continuous polymerizing device 20 includes the hydrous gel production section 1 and the ageing section 2. This enables to wash the surface of the endless belt 8a with the washing nozzle 9 in a short period of time. It is desirable for the endless belt 8a to have an effective length in a shortest length possible, that is, the shortest length in which the hydrous gel 7 is obtainable by the gelatinization due to the polymerization reaction. The effective length indicates a length between a position (place) A l where the monomer liquid 4 is supplied and a position B l where the hydrous gel 7 is released from the endless belt 8a. This effective length is appropriately determined based on the type of the monomer liquid 4 , the desired thickness of the hydrous gel 7, the polymerization condition of the monomer liquid 4, and other factors . As such, the effective length of the endless belt 8a is difficult to set as a specific value . However, a ratio of the effective length of the endless belt 8a to the net belt 8b is to be in a range of 1 : 0.5 to 1 : 5, in a direction of which the hydrous gel 7 is to be conveyed. The ratio is preferably in a range of 1 : 0.6 to 1 : 4, is further preferably in a range of

1 : 0.7 to 1 : 3 , and is particularly in a range of 1 : 0.8 to 1 : 2. The effective length in the ratio of the above range enables to wash the endless belt 8a with the washing nozzle 9 in a shorter time . The effective length of the net belt 8b indicates a length from a position A2 to a releasing point B2 of the net belt 8b. In the case of the net belt 8b which is constructed of the plurality of net belt tiers, a total length of each of the effective lengths of the belts is calculated as the effective length of the net belt 8b. Furthermore, a sum total length of the endless belt 8a and the total length of the net belt 8b is preferably not less than 9 m however not more than 100 m. The advantage of the present invention appears clearly with a longer conveyor belt.

It is preferable for the monomer liquid 4 and the hydrous gel 7 to have a dwell time period on the endless belt

8a (namely, the dwell time period on the effective length of the belt) of less than 3 minutes, is further preferably less than 2 minutes, and is particularly preferable to be less than 1.5 minutes. The continuous polymerizing device 20 of the present invention is, in other words, a multi-stage type polymerizing device including the hydrous gel production section 1 and the ageing section 2. It is possible to say that the time period required to perform the series of processes from supplying the monomer liquid 4 to washing the endless belt 8a is short in this device . A traveling speed of the endless belt 8a and the net belt 8b is appropriately adjusted by a driving device incorporated in the hydrous gel production section 1 and the ageing section 2. The time period required to perform the series of processes from supplying the monomer liquid 4 onto the first conveyor belt until washing the part of the first conveyor belt on which the monomer liquid was supplied, is appropriately changed based on the type of the monomer liquid 4, the desired thickness of the hydrous gel 7 , the polymerization condition of the monomer liquid 4 , and other factors. The time period required is, in other words, the time period required to travel from the position A l to the position C l in Fig. 1 . The position A l is where the monomer liquid 4 is supplied on the endless belt 8a. The position C l is where the part of the endless belt 8a on which the monomer liquid 4 is supplied is washed. More specifically, the time period may be set as not less than 30 seconds to less than 5 minutes, is preferably set as not less than 30 seconds to less than 3 minutes, is further preferably set as not less than 30 seconds to less than 2 minutes, and is particularly preferable to be set as not less than 30 seconds to less than 1 .5 minutes. The above preferable time period shortens the time of the hydrous gel 7 dwelled on the endless belt 8a, and enables to wash the surface of the endless belt 8a with the washing nozzle 9 in a short time period.

The ageing section 2 is positioned in a lower position to the hydrous gel production section 1 . As such, it is preferable for the ageing section 2 and the hydrous gel production section 1 to be positioned at different heights . The "different height" here denotes a distance between upper surfaces of the two endless belts. It is preferable to position the two endless belts to have the different height in a range of 0.2 m to 10 m, more preferably in a range of 0.2 m to 5 m, and further preferably in a range of 0.2 m to 2 m. By having the ageing section 2 positioned lower than the hydrous gel production section 1 , passing of the hydrous gel between the belts is carried out smoothly, and makes it difficult for problems to occur caused by bending of the hydrous gel. The hydrous gel 7 aged at the ageing section 2 , next may be crushed by a crusher (not illustrated) , and made into a particle-shaped hydrous gel. The crusher is not particularly limited, and a well-known crusher may be used. For example, a crusher of a shearing/ cutting method such as a cutter mill may be used. Thereafter, the crushed hydrous gel 7 is dried, thereby obtaining the water-absorbent resin.

Examples of physical properties of an obtained water-absorbent resin include soluble content, absorption capacity without pressure, and residual monomer amount. The soluble content is a value indicating the soluble amount of the water-absorbent resin in a predetermined solvent.

The absorption capacity without pressure (GV) is a value indicating an absorbable amount of liquid with respect to a unit weight of the water-absorbent resin. The absorption capacity without pressure is preferably in a range of not less than 20 fold to not more than 60 fold of the weight of the water-absorbent resin. The use of the water-absorbent resin having the absorption capacity without pressure in the above range in products such as paper diapers allows the attainment of a high quality product in which leaking or returning of liquid hardly occurs.

It is necessary to contain many crosslinking agents in order to produce water-absorbent resin particles if the range of the absorption capacity without pressure is below the above range . This makes the amount of a water-soluble component extremely small, which causes decrease in granular strength. Not only this, stickiness occurs such as an increase in the amount of liquid returning in the absorbent of the paper diaper or the like, due to insufficient absorbency of the water-absorbent resin. Therefore, the absorption capacity without pressure in a range lesser than the above range is not preferable . On the other hand, if the range of the absorption capacity without pressure is exceeding the above range, the water-soluble amount increases, which causes the water-absorbent resin to readily aggregate when granulized. In addition, the water-absorbent resin gel particles which have swelled by absorbing liquid decreases in gel strength. This causes change in shape of the gel particles under pressure such as body pressure. As a result, the volume of the gel layer of the water-absorbent resin decreases, thereby causing possibility of leakage from the absorbent.

The residual monomer amount is a value indicating an amount of a monomer remaining in the water-absorbent resin, and is preferably a low value also as from the view of safety.

The following description explains a structure of the trough 6 with reference to Fig. 2. Fig. 2 is a cross sectional view illustrating the trough 6 from a direction along the flowing direction of the hydrous gel 7. The trough 6 guides the endless belt 8a. The trough 6 also is a position where the monomer liquid 4 is supplied from the monomer liquid supplying nozzle 3, and is polymerized. More specifically, the trough 6 is a reactor. As illustrated in Fig. 2 , the cross section of the trough 6 shows an upper shape of the trough 6, arranged to have a flat central section and an inclined section on both sides of the central section, the inclined section inclined to a higher position. A flexible endless belt 8a curves inwards so as to form a groove in the center and inclines upwards in both sides, as the flexible endless belt 8a travels along the trough

6, in accordance with the upper surface shape of the trough 6. A fluororesin layer 10 is formed on a surface of the endless belt 8a. The endless belt 8a returns to a flat state after passing the position of the trough 6. The following description explains an operation of the continuous polymerizing device 20. First, the monomer liquid 4 is supplied onto the endless belt 8a from the monomer liquid supplying nozzle 3. A supplied amount of the monomer liquid 4 is adjusted depending on the desired thickness of the hydrous gel 7.

The monomer liquid 4 is continuously supplied from the monomer liquid supplying nozzle 3 onto the endless belt 8a. The supplied monomer liquid 4 is collected in a groove section of the endless belt 8a in which the center of the trough 6 is recessed. Therefore, the monomer liquid 4 does not leak or flow out to the outside of the endless belt 8a. The monomer liquid 4 collected in a recessed section of the endless belt 8a on the trough 6 is heated with a heater (not illustrated), and is polymerized due to light irradiation from the UV lamp 5. This forms the band-shaped hydrous gel 7. Other than the trough form, a technique suggested in Japanese Patent Unexamined Publication, Tokukai, No . 2000-017004 , where the shape of the belt is one which has a side dam, may also be applied as the shape of the belt. The endless belt 8a travels in order to convey the hydrous gel 7 to the ageing section 2. A part of the endless belt 8a in which the hydrous gel 7 is adhered to moves in a clock-wise direction of the hydrous gel production section 1 , and travels so as to come close to the washing nozzle 9 provided on the lower side of the hydrous gel production section 1. By thus independently providing the hydrous gel production section 1 and the ageing section 2 , the surface of the endless belt 8a is washed at a quicker stage .

Comparing with a polymerizing device in which the hydrous gel production section 1 and the ageing section 2 are provided as one and the washing nozzle 9 is provided on a lower side of the ageing section 2 , the continuous polymerizing device 20 can shorten the length from the endless belt 8a to the washing nozzle 9 by around one third. Namely, it is possible to wash the endless belt 8a with the washing nozzle 9 in a more quicker stage . Therefore, it is possible to suppress the deterioration of the surface of the endless belt 8a.

In an arrangement as in the present embodiment where the hydrous gel production section 1 and the ageing section 2 are separately arranged, it is necessary to provide a driving device and a heater independently for each of the hydrous gel production section 1 and the ageing section 2. This causes the arrangement of the device to be relatively complex. Consequently, such arrangement has conventionally not been adopted. However, in the conventional arrangement, the deterioration of the belt is remarkable, and the belt requires frequent replacement. This causes a disadvantage that cost and time required for the replacement greatens. The hydrous gel 7 dried at the ageing section 2 may be used as it is or may be used in a further crushed state, depending on the purpose of use . The hydrous gel 7 is dried on the net belt 8b with a heater (not illustrated) provided in the ageing section 2. The amount of time the hydrous gel 7 is dried is appropriately determined, depending on the thickness of the hydrous gel 7 and the type of the monomer liquid 4 which is the original material of the hydrous gel 7. The hydrous gel 7 is continuously discharged from the ageing section 2 , after completing the ageing process. It is preferable for the hydrous gel 7 thus discharged to be continuously crushed by a crusher (not illustrated) , thereby obtaining the water-absorbent resin.

Normally, the hydrous gel 7 thus crushed is further dried and crushed, so as to be used as the water-absorbent resin. The water-absorbent resin is further processed as appropriate. The water-absorbent resin may undergo the following processes: a classification process, a surface crosslinking process, a granulation process, or other processes. Following these processes, the water-absorbent resin is used as a water-absorbent resin product. Well-known techniques (for example, a technique disclosed in Japanese Unexamined Patent Publication, Tokukai, No . 2002-2 12204) may be adopted for the crushing, drying and surface crosslinking processes of the hydrous gel 7. The invention being thus described, it will be obvious that the same way may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

EXAMPLES

The following description describes the present invention in detail by presenting examples. However, the present invention is not limited to these. In the following description, unless otherwise specified, "parts by weight" may simply be worded as "parts", and "% by weight" may simply be worded as "%" .

Physical properties of the water-absorbent resin obtained in Example and Comparative Example were measured as follows. Each of the measurements were carried out at a temperature range of 23 ± 2°C.

Approximately 0.2 g of water-absorbent resin was accurately measured (this weight will be the "weight of water-absorbent resin" in the following formula) , and was evenly put into a bag made of nonwoven fabric (having a size of 60 mm * 60 mm) . The bag made of nonwoven fabric which includes the water-absorbent resin was immersed in 0.9% sodium chloride aqueous solution (physiological saline) . The bag was taken out after 30 minutes, and was drained for 3 minutes by using a centrifugal separator at a speed of 250 * 9.81 m/ s² (250 G) . After draining the bag, the bag was measured to attain a weight W l (g) . The same procedures were carried out by using a bag made of nonwoven fabric however without including the water-absorbent resin. This bag also was measured to attain a weight WO (g) . Absorption capacity without pressure was consequently calculated in accordance with the following formula, from the weight W l and WO :

GV (g/ g)

= [(W l - WO) / weight of water-absorbent resin] - 1

A soluble content in a resin was extracted by the following procedures: (i) 184.3 g of 0.9% sodium chloride aqueous solution (physiological saline) was measured into a plastic container which had a volume of 250 ml and a lid attached thereto, (ii) 1.00 g of water-absorbent resin was added to the aqueous solution, and (iii) the aqueous solution containing the water-absorbent resin was stirred for 16 hours. The aqueous solution in which the soluble content of the resin was extracted was filtered with a filter paper. This obtained a filtrate . Next, 50.0 g of the filtrate thus obtained was measured. This served as the measuring solution.

First, just the physiological saline was titrated with NaOH aqueous solution of 0. 1 N until pH 10. Next, the physiological saline was titrated with HCl aqueous solution of 0. 1 N until pH 2.7. Thus, a blank titer was found ([bNaOH] ml,

[bHCl] ml) . The same titration operation was performed with the measuring solution, thus finding a titer ([NaOH] ml, [HCl] ml) . Thereafter, based on, for example, in a case of a water-absorbent resin made of acrylic acid and a sodium salt thereof, a molecular amount of the monomer and the titer thus obtained by the above operation, the soluble content was calculated by the following calculation:

Soluble content (%) = 0. 1 x Mw x 184.3 * 100 *

([HCl] - [bHCl]) / 1000 / 1 .0 / 50.0,

where Mw = 72.06 * ( 1 - neutralization rate / 100) + 94.04 x neutralization rate / 100 ; and Neutralization rate (mol %)

= [ 1 - ([NaOH] - [bNaOH]) / ([HCl] - [bHCl]) ] x 100

A residual monomer amount in the water-absorbent resin can be measured by a conventional well-known method. For example, the following method may be used: (i) 1 .0 g of water-absorbent resin is added in 184.3 g of 0.90 mass % sodium chloride aqueous solution (physiological saline) , (ii) the aqueous solution is extracted over two hours while the aqueous solution is stirred, (iii) the water-absorbent resin swelled in a form of a gel is filtered with a filter paper, and (iv) a residual monomer amount in the filtrate is analyzed by a liquid chromatography. In such case, a calibration curve obtained by analyzing a monomer standard solution of a known concentration served as an external standard. Thus, it was possible to find the residual monomer amount in the water-absorbent resin in view of a dilute strength of the filtrate.

[Example 1 ] A water-absorbent resin was produced by using a continuous polymerizing device. A continuous polymerizing device which had an endless belt made of glass fiber material was used in producing the water-absorbent resin. The glass fiber material had a fluororesin layer thereon, by soaking and thermal sealing. The fluororesin layer was a fluororesin whose main component was tetrafluoroethylene polymer (PTFE) . The fluororesin layer was formed on the surface of the belt with a thickness of approximately 50 μm. The endless belt in a part between the position of the monomer liquid supplying nozzle and the UV lamp, and the roller on an exit side of the continuous polymerizing device were kept at a temperature of approximately 100⁰C by a heater which heated the endless belt.

The ageing section traveled in a loop-shape by having an endless net belt be driven by a sprocket positioned on both ends of the ageing section. The endless net belt was made of metal, and had an endless chain. The net belt was arranged so as to form three tiers in a box of the ageing section, different to the continuous polymerizing device as illustrated in Fig. 1 . The ageing section introduced hot air of a temperature of 100⁰C in its box, and heated the hydrous gel as similar to the hydrous gel production section. A distance between an upper surface of the endless belt of the hydrous gel production section and an upper surface of the ageing section was 40 cm.

Firstly, a monomer liquid was prepared. The following were continuously supplied and mixed in a mixer: 48.5% sodium hydroxide aqueous solution; acrylic acid; an aqueous solution in which 1 % polyethylene glycol diacrylate (average molecular amount of 523) and pentasodium diethylenetriaminepantaacetate aqueous solution (I) were dissolved; and water, each with a flow speed set as 35 kg/ h; 43.7 kg/ h; 9.8 kg/ h; and 9.8 kg/ h, respectively. Thus, the monomer liquid 4 was prepared. A temperature of the monomer liquid 4 was 98⁰C. Next, 1 % V50 (produced by Wako Pure Chemical Industries, Ltd.) aqueous solution was added to the monomer liquid 4 in a flow speed of 1 .8 kg/ h, at a pipe connecting the mixer and the monomer liquid supplying nozzle. The 1 % V50 aqueous solution served as an azo-type polymerization initiator. Thereafter, the mixture was continuously supplied on the belt. The mixture supplied on the belt was then polymerized, thereby produced a band-shaped hydrous gel.

After the hydrous gel was transferred from the endless belt to the net belt, the part of the endless belt which had contact with the hydrous gel was washed by a shower nozzle . A time period required to perform the series of processes from supplying the monomer liquid on the endless belt to washing the endless belt was 1 minute and 32 seconds. The band-shaped hydrous gel discharged from the ageing section

2 was crushed with a cutter mill having a screen of a diameter of 10 mm. Thus, a crushed particle-shaped hydrous gel subject to drying was obtained.

The endless belt had an effective length of 3.2 m. The effective length indicates a length between a position where the monomer liquid was supplied onto the endless belt (namely, a position directly under the monomer liquid supplying nozzle) and a roller provided on the ageing section side. The endless belt was set so as to travel in a speed of 2.3 m/ minute . Accordingly, a dwell time of the monomer liquid and the hydrous gel on the endless belt was approximately 1 .4 minutes. A length of the net belt in the ageing section where the hydrous gel was mounted was approximately 6.7 m. A traveling speed of the net belt was adjusted so as to match the speed of the band-shaped hydrous gel continuously discharged from the hydrous gel production section. A dwell time of the hydrous gel in the ageing section was measured, which resulted as 2.9 minutes. The dwell time in the ageing section denotes the time required for the hydrous gel to be traveled from the point where the hydrous gel was conveyed to the net belt to the point where the hydrous gel was discharged from the ageing section. That is to say, a total amount of polymerization time calculated approximately 4.3 minutes. A small time required for the hydrous gel to travel from the hydrous gel production section and the ageing section was not included in the calculation.

The continuous production was operated for a total of approximately 70 hours. Even after 70 hours of operation, adhering of the hydrous gel was hardly recognized on the belts in both of the hydrous gel production section and the ageing section. The operation of the continuous production was thus possible without any problems occurring caused by adhering of gel on both belts.

A small amount of the hydrous gel on the endless belt immediately after light irradiation by the UV lamp, and a small amount of the hydrous gel immediately after the hydrous gel was discharged from the ageing section were cut off and immediately cooled. This cooled hydrous gel was quickly fragmented with scissors to a 3 * 3 mm square, and 5 g of this fragmented hydrous gel was measured on a laboratory dish. Thereafter, the hydrous gel was dried for 24 hours in a drier of 180⁰C . Solid content concentration of the hydrous gel was calculated, whereby the concentration result was 69 wt % and 71 wt %, respectively. The crushed particle-shaped hydrous gel thus obtained was dried by hot air for 30 minutes at a temperature of 180⁰C, then further crushed with a roller mill (mill-type crusher) . Thereafter, particles of the hydrous gel which was in a range of 300 μm to 600 μm was classified by using a sieve, thereby obtained a water-absorbent resin particle ( 1 ) . Physical properties of the obtained water-absorbent resin particle were measured, whereby the physical properties resulted in having 32 g/ g of absorption capacity without pressure (GV) , 7.2% of soluble content, and 500 ppm of residual monomer. [Comparative Example 1 ]

Polymerization was performed by using a device as similar to the continuous polymerizing device in Example 1 as a polymerizing device . Specifically, the polymerizing device is a belt polymerizing device having a Teflon (registered trademark) resin-soaked glass fiber material belt formed in a trough-shape, whose bottom surface of the endless belt and an exit side roller were kept at approximately 10O⁰C . However, the ageing section was not provided, and the endless belt effective length was extended to 6 m. That is to say, the polymerization was performed by having no latter ageing section.

Hydrous gel discharged in a band-shape from the polymerizing device was crushed by a cutter mill having a screen of a diameter 10 mm. Thus, a crushed particle-shaped hydrous gel subject to drying was obtained. A traveling speed of the belt was set as 1 .4 m/ minute, so as to have a polymerization time of 4.3 minutes, as the same as Example 1.

A small amount of the hydrous gel peeled off of from the endless belt was cut off and cooled. The hydrous gel thus cooled was quickly fragmented by using a pair of scissors to a 3 x 3 mm square . Thereafter, 5 g of this fragmented hydrous gel was measured on a laboratory dish. The hydrous gel was then dried for 24 hours at a temperature of 180⁰C . Solid content concentration of the hydrous gel in the laboratory dish was calculated, whereby the result showed 70 wt % .

After the crushed particle-shaped hydrous gel was dried with hot air for 30 minutes at a temperature of 180⁰C, the hydrous gel was crushed by a roller mill (mill-type crusher) . Thereafter, the hydrous gel was classified, separating particles in a range of 300 μm to 600 μm by using a sieve. Thus, a comparative water-absorbent resin particle ( 1 ) was obtained. Physical properties were measured of the obtained comparative water-absorbent resin particle ( 1 ) , which gave a result of having 32 g/ g of absorption capacity without pressure (GV) , 8.5% of soluble content, and 630 ppm of residual monomer.

Adhering of the hydrous gel to the endless belt gradually started to show after a total of approximately 30 hours of continuous production. After approximately 50 hours, the adhering of the hydrous gel on the endless belt became remarkable . Thereafter, the hydrous gel could not peel off from the endless belt, thereby caused a problem that the hydrous gel was drawn into the lower section of the endless belt. This made the continuation of the production impossible.

The belt at this point was examined, which showed that the smoothness in texture of the surface was lost. The belt was examined by using a microscope, which showed that multiple holes were generated on the Teflon (registered trademark) resin layer on the surface layer of the belt. It was thus found that the adhering occurred by progression of deterioration of the belt.

The method of the present invention for producing water-absorbent resin is a method including polymerizing a monomer aqueous solution whose main component is acrylic acid and/ or a salt thereof by using a polymerizing device which includes a conveyor, so as to obtain a hydrous gel; and drying the hydrous gel, the step of polymerizing being such that the polymerizing device is a polymerizing device including a plurality of conveyors which operate in combination with each other.

By thus having a plurality of conveyors in a polymerizing device, a monomer aqueous solution supplied on a first conveyor belt is polymerized and gelatinized to a hydrous gel on the first conveyor belt, thereafter conveyed to a second conveyor belt. Therefore, a length of time in which the hydrous gel is placed on the first conveyor belt is accordingly- shortened as compared to a polymerizing device which has a single conveyor. An effect is thus attained such that deterioration in a part of the first conveyor belt which has contact with the hydrous gel is suppressed.

The embodiments and concrete examples of implementation discussed in the foregoing detailed explanation serve solely to illustrate the technical details of the present invention, which should not be narrowly interpreted within the limits of such embodiments and concrete examples, but rather may be applied in many variations within the spirit of the present invention, provided such variations do not exceed the scope of the patent claims set forth below. INDUSTRIAL APPLICABILITY

With a continuous polymerizing device of the present invention, deterioration of the surface of the conveyor belt such as an endless belt in the continuous polymerizing device can be prevented in polymerizing to obtain a hydrous gel.

Consequently, durability of the continuous polymerizing device is improved. Thus, the present invention is widely applicable in industries which produce hydrous gel or water-absorbent resins.

Claims

1. A method for producing water-absorbent resin comprising: polymerizing a monomer aqueous solution whose main component is acrylic acid and/ or a salt thereof by using a polymerizing device which includes a conveyor, so as to obtain a hydrous gel; and drying the hydrous gel, the step of polymerizing being such that the polymerizing device is a polymerizing device including a plurality of conveyors which operate in combination with each other.

2. The method for producing water-absorbent resin as set forth in claim 1 , wherein the monomer aqueous solution is gelatinized in the polymerizing device by continuously providing and polymerizing the monomer aqueous solution on a first conveyor that is one of the plurality of conveyors.

3. The method for producing water-absorbent resin as set forth in any one of claims 1 and 2 , wherein solid content concentration of the hydrous gel is increased by drying the hydrous gel while the hydrous gel is traveled on a second and latter conveyor(s) included in the plurality of conveyors.

4. The method for producing water-absorbent resin as set forth in any one of claims 1 to 3, comprising: washing a first conveyor belt of a/ the first conveyor after the hydrous gel is transferred from the first conveyor to a/ the second conveyor, each of which is one of the plurality of conveyors.

5. The method for producing water-absorbent resin as set forth in any one of claims 1 to 4 , wherein the plurality of conveyors which operate in combination with each other are conveyors having at least two types of different conveyor belts.

6. The method for producing water-absorbent resin as set forth in any one of claims 1 to 5, wherein a ratio of an effective length of a/ the first conveyor to a total length of an effective length of each of a/ the second and latter conveyors, each of which are included in the plurality of conveyors, is not less than 1 : 0.5 and not more than 1 : 5.

7. A method for producing water-absorbent resin as set forth in any one of claims 4 to 6, wherein a time period required for an arbitrary point on the first conveyor to reach a position where the first conveyor is first washed is not less than 30 seconds and less than 5 minutes, the arbitrary point being a position where the monomer aqueous solution is supplied on the first conveyor.

8. A method for producing water-absorbent resin as set forth in any one of claims 1 to 7 , wherein a polymerization temperature of the monomer aqueous solution is in a range of not lower than 50⁰C to not higher than 140⁰C .

9. The method for producing water-absorbent resin as set forth in any one of claims 1 to 8, wherein a concentration of a monomer component in the monomer aqueous solution is at least 40 wt %.

10. The method for producing water-absorbent resin as set forth in any one of claims 1 to 9 , wherein solid content concentration of the hydrous gel discharged by the polymerizing device is not less than 50 wt % to not more than 80 wt %, and a particle-shaped water-absorbent resin is obtained by continuously crushing the hydrous gel discharged from the polymerizing device with a crusher.