Abstract:
A gel manufactured by irradiating ionizing radiation in the range of 5-1000 kGy to an obtained paste compound of a density of 10% by weight or more after adding water to carboxymethyl carrageenan and wherein each gram of dry gel obtained is capable of absorbing 20 grams or more of water. The ionizing radiation may be gamma rays, electron beams or X-rays.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a biodegradable and water-absorbing polymer gel, which uses carboxymethyl carrageenan as a raw material. In particular, the present invention relates to a method of manufacturing a water-absorbing polymer gel obtained by irradiating ionizing radiation after mixing carboxymethyl carrageenan with water. 
     2. Prior Art 
     As for the hydrogel, it is known that it can be obtained by causing radiation crosslinking by irradiating ionizing radiation to an aqueous solution of polyethylene oxide, polyvinyl alcohol, polyacrylamide, and polyvinylpyrrolidone etc. The hydrogel thus obtained is used as a moisture retention material and with hygienic articles such as a disposable diaper, etc. in the field of medical treatment and cosmetics because a significant amount of water can be absorbed and contained. The material used for these articles is chiefly sodium polyacrylate. 
     A method of preparing a hydrogel by irradiating ionizing radiation to an aqueous solution of carboxymethylated polymers has been disclosed, for instance, in JP 2001-2703 A1. 
     BRIEF SUMMARY OF THE INVENTION 
     As described above, the water absorbing gel obtained by radiation crosslinking of water-soluble polymers such as sodium polyacrylate, etc. is widely used for hygiene articles such as a disposable diaper, etc. The disposable diapers used in a home or a hospital are disposed of by incineration processing. However, the combustion temperature decreases when the wet diapers are put in the incinerator, resulting in the generation of dioxin and an increase in carbon dioxide. They are not decomposed in the underground disposal, and stay in the soil for a long period. As a result, the environment load may be increased. Therefore, in order to decrease the environment load the application of hydrogels such as sodium polyaspartate and sodium polyglutamate decomposed in the soil is attempted. 
     There is a method of synthesizing the water absorbing material by chemical crosslinking natural materials such as starch and cellulose, in which a reagent such as formalin, glutaraldehyde, and epichlorohydrin is used. However, because these chemical substances are toxic, there are problems of environmental pollution in a work site and residual contamination in water absorbing materials. A safe cross-linking technique is required, because aldehyde pollutes the work environment and residual aldehyde may occasionally irritate the skin in this method. 
     Therefore, water absorbing materials which reduce the environment load are required. Especially, biodegradable polymers which are decomposed and digested by the microorganisms in the soil and are easy to process after use are considered to be a low environmental load material. If a collection system after use can be constructed, biodegradable and water-absorbing materials become recyclable material which can be used as a resource because they can be disposed of as fertilizer by composting. 
     An object of the present invention is to provide a method of manufacturing a gel in which the gel can be manufactured cheaply using high water-absorbing polymers as raw materials, which are decomposed and digested by the microorganisms in the soil. 
     The inventors carried out hard research from the above-mentioned viewpoint. As a result, when radiation was irradiated to a highly concentrated paste of synthesized carboxymethyl carrageenan, it was found out that the crosslinking of carboxymethyl carrageenan occurred. Carrageenans are well-known material. For example, San-Ei Gen F.F.I. Inc. whose head office is in Osaka City is supplying it to the market as a gelatinizer for food. 
     It is difficult to form the gel by radiation-induced crosslinking with carboxymethyl carrageenan in a state of a solid or a low concentration solution (10% or less), because decomposition occurs first in the polymer chain. In the present invention, it became possible to crosslink by adding water to carboxymethyl carrageenan, kneading well to make a paste of constant concentration (10% or more), and irradiating the ionizing radiation. 
     The hydrogel of carboxymethyl carrageenan manufactured based on the present invention is basically synthesized as follows. After adding water to the carboxymethyl carrageenan and mixing it, ionizing radiation of more than a fixed dose is irradiated to the obtained paste sample of a certain concentration. As a result, a hydrogel excellent in heat resistance which does not dissolve at 50° C. or more can be obtained. 
     More concretely, carboxymethyl carrageenan is mixed with water well to obtain a high concentration sample in a paste-state. This is put in a bag made of a laminated product of polyethylene and nylon. The heat seal is provided after vacuum deaeration, and gamma-rays are irradiated. Although this paste is soft before irradiation, the paste becomes rubbery by the gamma-irradiation and thus an elastic gel is obtained. The concentration of the paste should be 10% or more to form the radiation crosslinking, preferably from 20% to 40%. The decomposition occurs first in the state of the solid or the density of 10% or less, and thus the formation of a gel by crosslinking is not observed. 
     Gamma-rays, electron beams, or X-rays can be used as the ionizing radiation and the dose of the crosslinking may be in the range of 0.1-1000 kGy. Although the desirable dose of crosslinking is 5 kGy or more, 20 kGy or more is the best. As for the ionizing radiation, the gamma-rays obtained from cobalt  60  or the electron beam caused by an accelerator is desirable because of the industrial production. As for an electron accelerator, a high-energy electron accelerator of middle energy to high energy of acceleration voltage 1 MeV or more which can irradiate thick materials is the most desirable. If the sample before irradiation is worked like a film by pressure, the gel can be obtained by the radiation crosslinking even with a low energy electron accelerator of 1 MeV or less because the electron beam penetrates. Although the influence on crosslinking of oxygen under irradiation is little, it is preferable to cover the upper surface of the paste with a plastic film, etc. such as polyester and to irradiate it in order to control the evaporation prevention of the moisture under irradiation and the decrease in crosslinking density. 
     The gel manufactured according to the present invention can be used for many kinds of products. Because this gel has a biodegradation characteristic, it is possible to dispose of it by composting. The environmental load does not increase because it is possible to use it as fertilizer afterwards. It is, therefore, particularly effective for hygiene articles when using it for face masks, disposable diapers or make-up articles. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows the relationship between gel fraction and water absorption when gamma-rays of 50 kGy are irradiated to carboxymethyl carrageenan solutions of various concentrations on which one cycle process is performed. 
         FIG. 2  shows the relationship between gel fraction and water absorption when gamma-rays of 50 kGy are irradiated to carboxymethyl carrageenan solutions of various concentrations on which three cycle processes are performed. 
         FIG. 3  shows gel fraction when gamma-rays of 0-100 kGy are irradiated to carboxymethyl carrageenan of different concentrations on which one cycle process is performed. 
         FIG. 4  shows gel fraction when gamma-rays of 0-100 kGy are irradiated to carboxymethyl carrageenan of various concentrations on which three cycle processes are performed. 
         FIG. 5  shows water absorption when gamma-rays of 0-100 kGy are irradiated to carboxymethyl carrageenan of various concentrations on which one cycle process is performed. 
         FIG. 6  shows gel fraction when gamma-rays of 0-100 kGy are irradiated to carboxymethyl carrageenan of various concentrations on which three cycle processes are performed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method of manufacturing a gel according to the present invention is explained more concretely hereinafter with reference to some embodiments and comparative examples. Each of these embodiments was basically performed at room temperature under atmospheric pressure. Further, the present invention is not limited only to these embodiments. 
     Comparative Example 1 
     The carboxymethyl carrageenan used in comparative example 1 is the carrageenan carboxymethylated by using a conventional method. Namely, as one step, carrageenan was distributed to a mixed solution of 40% sodium hydroxide solution/isopropyl alcohol, reacted at 40° C. for 3 hours by adding monochloroacetic acid, filtered after neutralization, and dried. Gamma-rays of 0 to 100 kGy were irradiated to the less than 10% of carboxymethyl carrageenan aqueous solution and the carboxymethyl carrageenan of a solid-state at room temperature. Although molecular weight decreased as a result, the carboxymethyl carrageenan came to dissolve in water easily. However, water insoluble gel components were not formed. Accordingly, it can be estimated that crosslinking has not been formed in such a condition. 
     Embodiment 1 
     Embodiment 1 is explained referring to  FIG. 1 .  FIG. 1  shows the relationship between gel fraction and water absorption when gamma-rays of 50 kGy are irradiated to carboxymethyl carrageenan solutions of various concentrations on which one cycle process was performed. The 30% and 40% samples of paste-state were made by kneading well the carboxymethyl carrageenan used in comparative example 1 with water, and gamma-rays of 50 kGy were irradiated. As understood clearly from  FIG. 1 , the crosslinking occurred to the carboxymethyl carrageenan by gamma-irradiation, and water insoluble gels were formed. Then, the water-absorbing hydrogel was obtained by soaking the gel in a large amount of water. 
     For the irradiation with gamma-rays of 50 kGy, when the concentration of the carboxymethyl carrageenan was 30%, the gel fraction was 30.5% and the water absorption of the obtained gel was 71 (g water/1 g dry gel). When the concentration of the carboxymethyl carrageenan was 40%, the gel fraction was 35.5%, and the water absorption of the obtained gel was 69 (g water/1 g dry gel). 
     Embodiment 2 
     Embodiment 2 is explained referring to  FIG. 2 .  FIG. 2  shows the relationship between gel fraction and water absorption when gamma-rays of 50 kGy are irradiated to carboxymethyl carrageenan solutions of various concentrations on which three cycle processes are performed. As for the carboxymethyl carrageenan used in embodiment 2, the reaction with monochloroacetic acid was carried out at 40 for 3 hours in an alkali environment (1 cycle). The carrageenan was carboxylated by repeating this cycle three times (3 cycles). The degree of substitution of the carboxymethyl group in the carrageenan was increased more than that in embodiment 1. The 20%, 30% and 40% samples of paste-state were made by kneading well the carboxymethyl carrageenan used in comparative example 1 with water, and gamma-rays of 50 kGy were irradiated. As understood clearly from  FIG. 2 , the crosslinking occurred to the polymer of the carboxymethyl carrageenan by gamma-irradiation, and water insoluble gels were formed. Then, the water-absorbing absorbing hydrogel was obtained by soaking the gel in a large amount of water. 
     Comparative Example 2 
     The carboxymethyl carrageenan used in embodiment 2 of solid-state and less than 10% of aqueous solution were irradiated with gamma-rays of 0 to 100 kGy at room temperature. Although molecular weight decreased as a result, the carboxymethyl carrageenan came to dissolve in water easily. However, water insoluble gel components were not formed. Accordingly, it can be estimated that crosslinking has not been formed in such a condition. 
     For the irradiation with gamma-rays of 50 kGy when the concentration of the carboxymethyl carrageenan was 20%, the gel fraction was 59.1% and the water absorption of the obtained gel was 97 (g water/1 g dry gel). When the concentration of the carboxymethyl carrageenan was 30%, the gel fraction was 67.8% and the water absorption of the obtained gel was 42 (g water/1 g dry gel). And, when the concentration of the carboxymethyl carrageenan was 40%, the gel fraction was 73.7%, and the water absorption of the obtained gel was 22 (g water/1 g dry gel). 
     Embodiment 3 
     To investigate the properties of the carboxymethyl carrageenan gels more in detail, the inventors irradiated gamma-rays of 0-100 kGy to paste samples of the concentration of 10%, 20%, 30% and 40% of carboxymethyl carrageenan, and examined gel fraction (%) and water absorption (g water/1 g dry gel) of each of the carboxymethyl carrageenan gels. The results are graphed in  FIGS. 3 to 6 .  FIG. 3  shows gel fraction when gamma-rays from 0 to 100 kGy are irradiated to paste samples of various concentrations on which one cycle process is performed.  FIG. 4  shows gel fraction when gamma-rays from 0 to 100 kGy are irradiated to paste samples of various concentrations on which three cycle processes are performed.  FIG. 5  shows water absorption when gamma-rays from 0 to 100 kGy are irradiated to paste samples of various concentrations on which one cycle process is performed. And,  FIG. 6  shows gel fraction when gamma-rays from 0 to 100 kGy are irradiated to paste samples of various concentrations on which three cycle processes are performed. 
     As understood from  FIG. 3  and  FIG. 4 , the gel fraction increases rapidly between 5 kGy and 20 kGy, and it increases gradually after passing a critical point of 20 kGy. Further, as understood from  FIG. 5  and  FIG. 6 , the water absorption decreases rapidly to the dose of about 30 kGy and it decreases gradually after passing a critical point of about 30 kGy. 
     Because the experiments carried out in embodiment 3 have been performed also in embodiments 1 and 2 and comparative examples 1 and 2 under the same conditions, the results of each embodiment and each comparative example are included in the results of embodiment 3. Moreover, in  FIG. 4 , only data measured with increase in the concentration of 10% is shown. Although it is, therefore, shown that for carboxymethyl carrageenan on which three cycle processes are performed, the gel is not formed in the concentration of 10%, and it is formed for the first time in the density of 20%, it has been understood through sampling experiments that gels with the gel fraction and water absorption for practical use can be obtained in the case of concentrations between 15% and 60%. 
     Moreover, as for the evaluation of the gel manufactured according to the present invention, the gel fraction and water absorption are calculated as follows. 
     (1) Gel Fraction 
     The gel fraction is obtained as follows. The obtained gel after irradiation is freeze-dried, and dried in a 50° C. vacuum oven to constant weight. The dry sample is soaked in a large amount of water for 48 hours. A soluble component called a sol where the crosslinking is not formed is dissolved in water, and only a gel which is an insoluble component is collected. After that, the gel is dried at 50° C. for 24 hours. The gel fraction is calculated by the following equation.
 
Gel fraction (%)=(gel weight except soluble components/initial dry weight)×100
 
(2) Water Absorption
 
     The water absorption is expressed by the amount of water absorbed by the dry gel of 1 g when the gel formed by irradiating the radiation to the paste sample is dried and then the gel dried is soaked in a large amount of water at 25° C. 
     The gel manufactured as mentioned above can be used in various fields such as industry, agriculture, medical treatment, and food. Although the present invention has been illustrated and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the present invention. Therefore, the present invention should not be understood as limited to the specific embodiments set out above but to include all possible embodiments, which can be embodied within a scope encompassed and equivalent thereof with respect to the features set out in the appended claims.