JPH02111484A - Production of highly water absorptive sheet - Google Patents

Abstract

PURPOSE:To form the highly water absorptive film by applying and fixing an aq. compsn. essentially consisting of a specific compounding ratio of acrylic acid and its salt to a flexible sheet, then executing moisture absorption in a specific humidity atmosphere. CONSTITUTION:The aq. compsn. of the monomers essentially consisting of the acrylic acid and its salt having 50:50 to 0:100 molar ratio is applied to the flexible sheet and thereafter, the solvent of the above-mentioned compsn. is substantially removed and solidified. After 3 to 50 pts.wt. moisture per 100 pts.wt. solid content is absorbed in the compsn. in the atmosphere of 40 to 98% relative humidity, the compsn. is irradiated with electron beams of 0.007 to 5.5Mrad.sec<2> of (dose)<3>/(Dose rate)<2> in a moisture absorption state to produce the highly water absorptive sheet. The more specific examples of the above- mentioned flexible sheet include a fiber assembly, rubber or porous plastic sheet, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a super absorbent sheet, and more particularly to a method for producing a super absorbent sheet with excellent initial water absorption capacity.

Conventional technology So-called super absorbent substances that absorb a large amount of water and swell are used in disposable diapers, sanitary products, disposable miscellaneous items, water retention agents for agricultural and horticultural purposes, etc., and are also used as sludge coagulants and dew condensation agents. Its use as a preventive agent is also expanding to civil engineering and construction-related fields.

This superabsorbent material can be used in a variety of forms, but is most often used as a sheet.

Conventionally, several methods have been proposed for forming a water-absorbing substance into a sheet. When these methods are divided into sheets, there are two methods: a method in which a water-absorbing substance is produced and then it is made into a sheet, and a method in which the production of a water-absorbing substance and the production into a sheet are carried out simultaneously. In addition, when divided by the manufacturing method of water-absorbing substances,
There are methods using chemical reactions and methods using reactions caused by energy ray irradiation. Therefore, methods have been proposed in which various combinations of the sheet-forming time and the manufacturing method of the water-absorbing substance are used.

First, there are two methods for producing a water-absorbing material and then forming it into a sheet by some means. In the first method, it is combined with a base material to form a sheet. That is, the polymeric water-absorbing substance is made into powder, particles, etc., and is applied to a sheet-like base material to perform a fixing treatment. In the second method, a fibrous water-absorbing substance is formed into a sheet.

The first method is more common and requires the steps of manufacturing the water-absorbing material and fixing it onto the sheet. The fixing method is to sprinkle the powdered water-absorbing material onto the base sheet, layer it with another sheet, and then emboss it, or add a small amount of moisture to slightly gel the powdered water-absorbing material. The so-called wet method (JP-A-54125293, JP-A-51-40497, JP-A-54-141099, and JP-A-583645)
(see No. 2), or a coating method in which a resin compound consisting of a highly water-absorbent substance, a resin binder, and a solvent is applied to the roof material, and the solvent is evaporated to form a film on the base material (
(See Japanese Patent Application Laid-Open No. 5111-1ot047). However, these methods for obtaining sheet-like water-absorbing materials have some problems. For example, in the first method, the process becomes complicated because it requires separate processes from the manufacturing process of powdered or particulate water-absorbing material and the manufacturing process of the sheet, or the processing in the sheeting process is difficult. In addition, in the second method, the cost is high and the water absorbing capacity of the resulting water absorbent sheet is low.

Next, unlike the above-mentioned methods, several methods have been proposed in which the production of a water-absorbing substance and the formation of a sheet are simultaneously performed by directly polymerizing monomers on a sheet-like base material. For example, there is a method in which polymerization is carried out by impregnating a base material with an acrylate monomer solution and adding a Lanocal polymerization initiator or a mixture of it and a reducing agent in the form of a mist (Japanese Patent Laid-Open No. 60149609). reference). However, in this method, it is difficult to uniformly disperse the polymerization initiator, and a uniform polymer cannot be obtained. As a result, a large amount of monomer remains, resulting in poor quality. In addition, a method in which monomers, a polymerization initiator, a reducing agent, etc. are uniformly mixed and the mixture is applied to a base material in the form of a mist to perform polymerization (see JP-A No. 62-53309), and a similar method. Method using a crosslinking agent for small bonds (Japanese Patent Application Laid-Open No. 1983
2-62829), and a similar method in which only the reducing agent is added afterwards (see Japanese Patent Laid-Open No. 62-97979). However, all of these methods have the following drawbacks.

That is, since a polymerization initiator (catalyst) is used, some of the polymerization initiator remains after polymerization, and there is a possibility that a large amount of monomer also remains. In addition, the chemical reactions utilized in these methods are generally difficult to control. Therefore, it is not possible to precisely control the reaction as in the present invention, and it is difficult to adjust the water absorption capacity of the resulting water absorbent sheet.

Furthermore, a method has been proposed in which monomers are chemically polymerized on a sheet-like substrate and then irradiated with radiation to reduce the monomers remaining on the water-absorbing sheet (
(See Japanese Patent Application Laid-open No. 10638/1983). However, since the reaction used in this method is chemical polymerization, it has drawbacks such as residual catalyst, and even if monomers can be reduced by radiation irradiation, crosslinking reactions etc. occur due to radiation irradiation. It is difficult to control the quality of the final water absorbent sheet due to changes in water absorption capacity after chemical polymerization.

As a method other than chemical polymerization, a method has been proposed in which a photosensitizer is added to an acrylate and a divinyl compound, and the mixture is polymerized by irradiation with ultraviolet light (Japanese Patent Laid-Open No. 63-43912). reference).

However, this method has the disadvantage that the photosensitizer remains, and if there is a concern about the toxicity of the photosensitizer, it cannot be applied to products that are used in direct contact with the body, such as sanitary napkins and disposable diapers. It becomes a big problem.

Finally, there is a method in which a reaction by radiation irradiation is used to polymerize the monomer on a sheet-like substrate.

Japanese Patent Publication No. 57-500546 discloses a method of applying an acrylic acid type m-form aqueous solution onto a fiber base material and converting it into a superabsorbent polymer by sufficiently irradiating it with radiation. This method is more advanced than the Moeki methods in that it does not use catalysts that may cause harm if they remain, and it uses radiation. However, when ``sufficient irradiation'' is performed, the crosslinking reaction generally progresses, which hinders the swelling of the resulting water-absorbing substance, resulting in a significantly inferior water-absorbing ability, resulting in what is called ``high water absorption''.
You don't get the sort. Furthermore, this method does not allow the water absorption capacity to be adjusted to any desired level as in the present invention. It is also considered that in this method, a large amount of monomer remains.

As described above, the conventional methods for producing superabsorbent sheets have various problems.

Against this background, the present inventors applied an aqueous composition containing acrylic acid and acrylate monomers in a specific molar ratio range in a specific concentration range to a specific line 1 on a sheet-like base material. We have completed the invention of an excellent manufacturing method for super absorbent sheets using electron beam irradiation, which involves polymerization and curing by electron beam irradiation within a range, and have already filed a patent application (Japanese Patent Laid-Open No. 63-1
(See No. 05044). In this method, high water absorption capacity can only be obtained by combining the selected specific range of monomer molar ratio, monomer concentration in the composition, and electron beam irradiation conditions. This method is equivalent to or better than the sheet-forming method of powdered water-absorbing material. Furthermore, since this method involves almost a single step, it is also superior in terms of process to the method of forming a powdery water-absorbing material into a sheet.

Furthermore, since catalysts and photosensitizers are not used, there is no problem of their remaining.

The manufacturing method of JP-A No. 63-105044 is thus excellent, but although it achieves high water absorption capacity as a result,
It was not possible to precisely adjust the water absorption capacity to the desired level. Therefore, as a result of continuing research to solve this problem, the inventors of the present invention found that the purpose could be achieved by defining the irradiation conditions using the control factor of (line ff1)'/(dose rate). They discovered this and have already filed a patent application for this (Japanese Patent Application No. 1983-202236).

Invention or Problem to be Solved As mentioned above, the present inventors have made a series of developments regarding a method for producing a super absorbent sheet based on irradiating acrylic acid and acrylate monomers with electron beams on a substrate. 1986-105044 and Japanese Patent Application No. 612
The manufacturing method of No. 02236 had a common unresolved problem. In other words, in both manufacturing methods, an aqueous composition mainly consisting of an aqueous solution is applied to the substrate and the electron beam is irradiated in that state, so that the water content evaporates during the electron beam irradiation because it contains a considerable amount of water. , there is a risk that it may condense in the cooling section of the irradiation equipment and cause water droplets to adhere to the irradiated object.■
There are process problems such as the need for extra irradiation voltage and dose because the electron beam is absorbed by water, or the electron beam is irradiated with the aqueous composition filling the gap formed by the base material. As a result, the hardened and produced resin also fills the gaps formed by the base material, impairing the base material's original water absorption ability due to capillary action, and when used as a water absorbent sheet, the initial water absorption This is a quality problem such as poor water absorption ability.

Means for Solving the Problems Under these circumstances, the inventors of the present invention have conducted further intensive research and have found that after applying an aqueous composition to a substrate, the solvent can be removed by drying to reduce the gaps in the substrate to a certain extent. When the composition is solidified in the regenerated form, then rehydrated by controlling the relative humidity within a specific range, and then irradiated with an electron beam, the curing reaction occurs without loss of voids, and the specific surface area is reduced. It has been discovered that a resin with a large amount of resin is produced by curing, and the present invention has been completed.

That is, the present invention provides (a) a molar ratio of 50:50 to 0:1;
00 acrylic acid and acrylate as main components to a flexible sheet, (b) substantially removing the solvent of the composition and solidifying it, (c) The solidified product 1 in an atmosphere with a humidity of 40 to 98%
00 parts by weight of moisture, and then (d) in this hygroscopic state, (dose) 3/(dose rate) 2 is 0.0.
The present invention provides a method for producing a highly water-absorbent sheet, characterized in that the sheet is cured by irradiation with an electron beam of 07-5, 5 M rad-see2.

The flexible sheet serving as the base material in the present invention is flexible and inherently has gaps so that a certain amount of gaps can be regenerated when the monomolecular aqueous composition is applied and then dried and solidified. Any one having L) can be used. Examples of such flexible sheets include fiber aggregates, porous sheets of rubber or plastic, rubber-like plastic nets, metal foils, metal nets, etc.
In particular, fiber aggregates are preferable because of their good porosity, flexibility, strength, etc. Fibers include hydrophilic fibers and hydrophobic fibers,
The method using hydrophilic fibers is better from the viewpoint of water absorption speed, but the method using hydrophobic fibers is superior from the viewpoint of the strength of the sheet when water is absorbed.
It can be selected as appropriate depending on the purpose.

The super absorbent sheet obtained by the manufacturing method of the present invention is an inseparable integral body of the above-described flexible sheet and water absorbent resin, but in manufacturing such a super absorbent sheet, First, a monomer aqueous composition is prepared. Here, the "monomer aqueous composition" as used herein refers to an aqueous solution of a specific monomer or to which other components are added as desired. Therefore, although water is the main solvent in the aqueous composition, a low-boiling non-aqueous solvent that can be removed together with water in the drying/solidification process described later can also be used in combination.

Use acrylic acid and acrylate as monomers. Examples of the acrylate include alkali metal salts such as sodium salts, potassium salts, and lithium salts, alkaline earth metal salts such as calcium salts and magnesium salts, and preferably alkali metal salts are used. Aqueous compositions of the monomers can be prepared by mixing amounts of acrylic acid, acrylate, and water. Alternatively, it can also be prepared by mixing predetermined amounts of acrylic acid, a basic metal hydroxide, and water, and neutralizing a portion of the acrylic acid to convert it into an acrylate. It is also possible to add a small amount of another type of monomer to this aqueous composition to adjust the curing reaction during subsequent electron beam irradiation. As such an In body for adjustment,
Any water-soluble bifunctional monomer may be used as long as it has miscibility with the aqueous composition, and water-soluble difunctional monomers such as polyethylene glycol noacrylate, N,N''-methylenebisacrylamide, etc. are particularly preferably used. If desired, colorants, inorganic or organic fillers, etc. may be added to the aqueous composition, and polyvinyl alcohol may be added to change the liquid properties of the aqueous composition to make it easier to apply to the substrate. , thickeners such as polyethylene oxide, or surfactants may be added.

In preparing the aqueous composition, the molar ratio of acrylic acid and acrylate is in the range of 50:50 to 0:100, mainly due to the reactivity during electron beam irradiation described later. If the proportion of acrylic acid is larger than this range, the amount of evaporation of acrylic acid will increase in the subsequent drying process, which is inappropriate. In addition, since the reactivity is relatively reduced when the molar ratio is 98 to 100, the molar ratio is preferably in the range of 70 to 98.

The monomer aqueous composition thus prepared is then applied to the base taro flexible sheet. Examples of the application method include a coating method, an impregnation method, a spray method, and a printing method, but the impregnation method is particularly preferred because of the porous base material.

After such application, drying substantially removes the solvent. Drying is usually done by heating. Assimilation occurs due to solvent removal;
Upon solidification, the volume of the composition decreases and some of the original porosity of the substrate is restored. When observed, for example, when the base material is fiber, it is observed that the solidified material has a granular shape along each fiber, and some portions are crystalline.

Following solidification, the humidity is controlled in an atmosphere with a relative humidity of 40 to 98% to rehydrate the solidified product. In this way, moisture is added in the gas phase, and immersion in water is not appropriate as it may damage the form of the solidified product. The amount of water m to be contained in the solidified composition is in the range of 3 to 50 parts by weight, preferably 5 to 40 parts by weight, based on 100 parts by weight of the solidified composition. If the amount of triplex is less than 1 part, the reactivity during electron beam irradiation will decrease, making it impossible to achieve the purpose.On the other hand, if it exceeds 501 parts, the solidified material will lose its void-retaining form due to deliquescence and become liquid. This is inappropriate because the situation is close to .

Note that with regard to the humidity conditioning time, care must be taken as the moisture absorption capacity after drying varies considerably depending on the type of acrylate used. In other words, potassium acrylate has a high hygroscopic ability, so it can be used in a normal atmosphere (20-30°C, relative humidity 5.
0 to 70%) for several minutes to more than ten minutes in a high humidity atmosphere (30 to 4%).
At 0° C. and relative humidity of 80 to 95%), it is sufficient to leave it for a few minutes or less.

On the other hand, in the case of sodium acrylate, the moisture absorption ability is low, and it is preferable to add a moisture absorption promoter to the monomer composition. As such a moisture absorption accelerator, a nonionic one is preferable since an ionic one affects the water absorption ability of the resulting water absorbent sheet.

Examples of nonionic moisture absorption enhancers include glycerin, ethylene glycol, triethylene glycol, polyethylene glycol, and the like, and inorganic powders such as silica gel and activated alumina.

, After 1llll wetness is completed, electron beam is irradiated with moisture retained.The irradiation conditions are selected within the range of (dose) 3/(dose rate)' from 0.007 to 5.5 Mrad-sec. As described in detail in Japanese Patent Application No. 63-202236, the water absorbing capacity level of the water absorbent sheet obtained after curing is arbitrarily changed by the control factor of (dose) 3/(linear m ratio) 2. In this case, even if the dose is the same, polymerization and crosslinking reactions occur more when the dose rate is low than when the dose rate is high, resulting in lower water absorption.Here, the dose rate is (In the case of continuous irradiation, it is expressed as the value obtained by dividing the ray 1 by the time it takes for the irradiated object to pass through the electron beam band, that is, the irradiation time.) If seconds are used as the unit of V time, linear rate - dose / Time [M
rad/sec], so (line ff1)3/(dose rate)1[Mrad3/ (M rad/ 5ec)'l
-= (line f!ri'/(dose 7 hours (dose)·(time)'[Mrad-sec'l). Therefore, it may be considered that the dose and irradiation time are controlled.

The relationship with specific values is (line ff1)3/(dose rate)! If the value is less than 0.007, curing will be insufficient, while if it exceeds 5.5, crosslinking will proceed too much, which will not only reduce the water absorption capacity but also cause damage to the base material. Appropriate. However, if irradiation speed is important, set the dose a little higher and increase the dose accordingly.
In other words, the irradiation speed may be increased. From the above, in electron beam irradiation, the following factors are considered: l) type of acrylate, 2) ratio of acrylic acid to acrylate, 3) moisture absorption rate, 4)
Conditions should be set in consideration of factors related to water absorption capacity, such as the type and amount of the crosslinking promoting additive. Note that the irradiation voltage is not particularly limited, but is generally 150 to 3
000KV range. Depending on the thickness of the object to be irradiated including the base material, a voltage that sufficiently transmits the object may be selected from the above range.

The irradiation atmosphere may be in the air, but is preferably carried out in an inert gas, for example, nitrogen atmosphere.

Polymerization and crosslinking of single polymers occur due to electron beam irradiation.

Acrylic acid and acrylate monomers are monofunctional, and conventional polymerization reactions yield only water-soluble linear polymers. Therefore, in order to obtain a highly water-absorbent polymer that swells with water, crosslinking is required in addition to polymerization, and an appropriate balance between such polymerization and crosslinking is essential to achieve high water absorbency. In the present invention, a highly water-absorbent sheet can be obtained because the polymerization and crosslinking occur in a well-balanced manner by electron beam irradiation under the above conditions in a relatively low nematode population, and the reaction does not generate heat. This is probably because the reaction can be carried out with good controllability.

Thus, the superabsorbent sheet obtained after electron beam irradiation is
The resin is cured with a certain amount of voids remaining and is integrated with the base material, resulting in a large specific surface area and an excellent initial water absorption capacity during use.

The superabsorbent sheet produced by the manufacturing method of the present invention can be made into sheets of various shapes by appropriately selecting the type of porous base material, and can be used for sanitary products, diapers, disposable rags, etc.
It can be used in a wide range of fields, including agricultural and horticultural materials, construction-related materials, civil engineering-related materials, and cold and heat storage materials.

The present invention will be explained in more detail with reference to Examples below.

Examples Examples! An aqueous monomer solution was prepared by mixing 72.0 g of acrylic acid and 144.2 g of an aqueous potassium hydroxide solution containing 53.2 g of potassium hydroxide. In this monomer aqueous solution, the molar ratio of acrylic acid and acrylate is 5:9.
5. The total monomer concentration is 50% by weight.

Next, a nonwoven fabric base material made of partially fused polyester fibers with a fiber length of 56 m and a 1.3 denier and a basis weight of 40 g/m'' was impregnated with the prepared monomer aqueous solution and attached 11
41 g/m''.

After impregnation, the monomer composition was dried for 8 minutes in a dryer at 105°C to solidify it on the fiber substrate, and then left in an atmosphere of 20°C and 65% relative humidity for 10 minutes to absorb moisture. Immediately, using an area beam type electron beam accelerator manufactured by Nissin High Voltage Co., Ltd., in a nitrogen atmosphere, the acceleration voltage was 200 KV, the dose was 2 Mrad, and the dose rate was 12.8 M.
rad/sec, (dose)3/(dose rate)”=0
, 049 Mrad-see'' conditions to obtain a super absorbent sheet.The water content of the solidified product immediately before irradiation was 41% of the weight of the dried monomer composition.
Met.

The superabsorbent sheet produced by the method of the present invention thus obtained had a water absorption capacity of 350 times that of the entire sheet and 680 times that of the superabsorbent material, and was of high quality with excellent initial water absorption capacity ( The magnification used in water absorption capacity indicates how many times the weight of absorbed water is relative to the weight of the reference material.) Further, no dew condensation was observed during electron beam irradiation.

Example 2 72.0 g of acrylic acid and 113111.3 g of an aqueous sodium hydroxide solution containing 38° Og of sodium hydroxide.

A monomer aqueous composition was prepared by mixing 02 g of polyethylene glycol diacrylate having a molecular weight of 600 and 10.5 g of glycerin. In this monomer aqueous composition, the molar ratio of acrylic acid to sodium acrylate is 5:95, and the total monomer concentration is 35% by weight.

This composition was deposited on the same substrate as in Example 1 in an amount of 62 g.
/11' and dried in a dryer at 105° C. for 10 minutes to solidify the composition on the fiber base material,
Next, moisture was absorbed for 30 minutes in an atmosphere of 20°C and relative humidity of 65%, and in that state, the area beam type electron beam accelerator manufactured by Nissin High Voltage was immediately used in a nitrogen atmosphere at an acceleration voltage of 200 KV and a dose 5 Mrac! , dose rate 3
2, 1 Mrad/sec, (line m)3/(dose rate)
A super absorbent sheet was obtained by electron beam irradiation under the conditions of "= 0, I 22 Mrad sec'.
The moisture content of the solidified product immediately before irradiation was 17.6% based on the weight of the dried monomer composition.

Thus, the superabsorbent sheet obtained by the manufacturing method of the present invention had a water absorption capacity of 125 times that of the entire sheet and 350 times that of the superabsorbent material, but was of high quality with excellent initial water absorption capacity. Further, no dew condensation was observed during electron beam irradiation.

Comparative example 1 (when irradiated in solution form without drying) Example 2
Using the same nonwoven fabric as above, it was impregnated with an aqueous monomer solution having the following composition so that the amount of adhesion was 60 g/m''.

Ingredients Quantity Acrylic acid 72g Sodium hydroxide 32g Polyethylene glycol diac 0.2g Lylate (molecular weight 600) Water 1 1
Immediately after the og impregnation, using the same electron beam irradiation device as in Example 1, in a nitrogen atmosphere, an acceleration voltage of 200 KV, a beam ffi 2
Mrad, dose rate 5, 38 Mrad/see,
(Dose) 3/(line ff141)” = 0, 27 M
irradiation with electron beam under rad-see' conditions, then 1
It was dried at 05°C for 3 minutes to obtain a super absorbent sheet. This material had a water absorption capacity 120 times that of the entire sheet.

The sheet of Example 2 produced by the manufacturing method of the present invention and the sheet of Comparative Example 1 have approximately the same water absorption capacity. However, as is clear from FIG. 1 of the accompanying drawings, the sheet of Example 2 produced by the manufacturing method of the present invention had a higher initial water absorption rate than the sheet of Comparative Example 1, and was found to be superior in initial water absorption capacity.

Effects of the Invention By the manufacturing method of the present invention, the superabsorbent substance adheres to the base material in granular form, increasing the number of gaps, so that it is possible to obtain a superabsorbent sheet that has excellent initial water absorption ability and has the desired water absorption ability level. Furthermore, process improvements have been made in that there is no dew condensation within the equipment during electron irradiation, eliminating the need for extra water evaporation energy, and improving irradiation efficiency.

[Brief explanation of the drawing]

FIG. 1 is a graph comparing the change in water absorption amount over time for the water absorbent sheet of Example 2 produced by the manufacturing method of the present invention and the control sheet of Comparative Example 1.

Claims (4)

[Claims] (1) (a) Applying a single aqueous composition containing acrylic acid and acrylate as main components in a molar ratio of 50:50 to 0:100 to a flexible sheet, and (b) providing a solvent for the composition. (c) a relative humidity of 40 to 98%;
3 to 50 parts by weight of moisture is absorbed per 100 parts by weight of the solidified material in an atmosphere of (d) in the hygroscopic state (
Dose)^3/(dose rate)^2 is 0.007 to 5.5Mr
A method for producing a highly water-absorbent sheet, characterized by curing it by irradiating it with an electron beam of ad.sec^2. (2) The manufacturing method according to claim (1), wherein the flexible sheet is a fiber aggregate. (3) Claim No. (1) characterized in that the flexible sheet is a porous sheet of rubber or plastic.
Manufacturing method described in section. (4) The manufacturing method according to claim (1), wherein the flexible sheet is a rubber or plastic net.