in the present invention , the alkyl cellulose derivative serving as a starting material is carboxyalkyl cellulose ( a ), hydroxyalkyl cellulose ( b ), alkyl cellulose ( c ), a mixture of these celluloses , or a mixture of at least one of ( a ), ( b ) and ( c ) as a main component with an unmodified cellulose . each of these celluloses has at least one hydroxyl group or carboxyl group on average per glucose unit . carboxyalkyl cellulose ( a ) serving as a starting material in the present invention is produced by substituting the hydrogen atom of a hydroxyl group in a cellulose molecule by a carboxymethyl group , a carboxyethyl group , or a carboxypropyl group . carboxyalkyl cellulose ( a ) is preferably carboxymethyl cellulose or carboxyethyl cellulose . in the aforementioned carboxyalkyl cellulose , 20 % or more , preferably 40 % or more , of the entirety of carboxyl groups form an alkali metal salt , an ammonium salt , or an amine salt . examples of alkali metal salts include a sodium salt , a potassium salt , and a lithium salt . of these , a sodium salt is preferable . when the percentage of carboxyl groups which constitute the salt is less than 20 %, the cellulose is difficult to be uniformly mixed with water or solved in water . the upper limit for the percentage of carboxyl groups which form the salt is not particularly limited , and may be as high as 100 %. hydroxyalkyl cellulose ( b ) serving as a starting material in the present invention is produced by reacting the hydrogen atom of a hydroxyl group in a cellulose molecule with , for example , ethylene oxide or propylene oxide . therefore , the hydrogen atom is substituted by a hydroxyethyl group (— c 2 h 4 oh ), a hydroxyisopropyl group (— c 3 h 6 oh ), or a hydroxy - n - propyl group (— c 3 h 6 oh ); or by a polyoxyalkylene ether - substituted group which is formed by reacting thus obtained terminal hydroxyl group with 1 to 10 molecules of ethylene oxide or propylene oxide . hydroxyalkyl cellulose ( b ) is preferably hydroxyethyl cellulose ( hec ) or hydroxypropyl cellulose ( hpc ). taking hec as an example , hec is produced through the following process : cellulose is reacted with ethylene oxide ( eo ) in the presence of a hydrophilic organic solvent and sodium hydroxide ; and subsequently the resultant reaction mixture is subjected to neutralization , purification , drying , and pulverizing , to thereby produce hec . during the course of hydroxyethylation of cellulose , formed hydroxyethyl groups are further reacted with eo . when the amount of alkali and water during hydroxyethylation is appropriately selected hec which contains uniformly distributed substituents and exhibits resistance to enzymatic hydrolysis is produced . the amount by mole of eo added to cellulose is represented by “ molar substitution ” ( ms ). when ms is 1 , the degree of substitution ( ds ) of cellulose is 0 . 7 . in the case of a commercially available hec , ms is 1 . 5 – 3 . 0 , ds is 0 . 9 – 1 . 4 . hec having an ms of 1 or more ( ds : 0 . 7 or more ) is water - soluble . alkyl cellulose ( c ) serving as a starting material in the present invention is produced by partially substituting hydrogen atoms of hydroxyl groups in a cellulose molecule with a methyl group , an ethyl group , or a propyl group . alkyl cellulose ( c ) is preferably methyl cellulose . the degree of alkyletherification of the aforementioned alkyl cellulose is 66 % or less , preferably 50 % or less , more preferably 33 % or less . the average polymerization degree of the aforementioned alkyl cellulose derivative is not particularly limited , but in practice , the degree is about 10 – 2 , 000 , preferably about 50 – 1 , 000 , more preferably about 200 – 800 . the average etherification degree ( i . e ., the degree to which the hydrogen atom of a hydroxyl group on cellulose is substituted by a carboxyalkyl group , a hydroxyalkyl group , or an alkyl group ) of the alkyl cellulose derivative is 0 . 5 or more , preferably 0 . 8 or more , more preferably 1 . 1 or more . the upper limit is 3 . when the average etherification degree is less than 0 . 5 , satisfactory cross - linking does not result . the alkyl cellulose derivative serving as a starting material in the present invention may be a product produced through a known process , and particularly , commercially available products may be used . carboxyalkyl cellulose can be produced through a variety of processes , such as a slurry process ( high concentration solution process ) or a kneader process ( low concentration solution process ), which are conventionally employed . for example , carboxyalkyl cellulose can be produced through a process including a mercerization step and a carboxyalkylation step . in the mercerization step , cellulose is reacted with an alkali , to thereby form alkali cellulose . in the carboxyalkylation step , the alkali cellulose resulting from the mercerization step is reacted with monochloroacetic acid , to thereby produce carboxymethyl cellulose ; or the alkali cellulose is reacted with an acrylate , and then the resultant ester is subjected to hydrolysis , to thereby produce carboxyethyl cellulose . hydroxyalkyl cellulose is produced by reacting a hydroxyl group of cellulose with alkylene oxide . for example , hydroxyethyl cellulose and hydroxypropyl cellulose are produced by reacting hydroxyl groups of cellulose with ethylene oxide and propylene oxide , respectively . such hydroxyalkyl cellulose may further be reacted with alkylene oxide . for example , ethylhydroxyethyl cellulose is produced by reacting hydroxyethyl cellulose with ethylene oxide . alkyl cellulose can be produced through the reaction between alkali cellulose and alkyl chloride or dialkyl sulfate . for example , methyl cellulose is produced through the reaction between alkali cellulose and methyl chloride or dimethyl sulfate ; and ethyl cellulose is produced through the reaction between alkali cellulose and ethyl chloride or diethyl sulfate . cellulose obtained from a variety of materials such as wood pulp and linter pulp may be employed . the alkali may be an alkali metal such as lithium , potassium , or sodium ; ammonia ; or amine . of these , sodium is usually employed in the form of hydroxide or aqueous solution . the amount of alkali ( e . g ., sodium hydroxide ) which is employed in the mercerization step is usually about 30 – 80 parts by weight , preferably about 40 – 75 parts by weight , on the basis of 100 parts by weight of cellulose . the amount of alkali ( e . g ., sodium hydroxide ) which is employed in the slurry process is usually 35 – 70 parts by weight , preferably about 45 – 65 parts by weight , on the basis of 100 parts by weight of cellulose . in order to carry out mercerization , cellulose is employed usually in an amount of about 1 – 7 wt . % and about 10 – 25 wt . %, in the slurry process and the kneader process , respectively . the amount of alkali employed in the mercerization step in the slurry process differs from the amount of alkali employed in the mercerization step in the kneader process . in the slurry process , mercerization may be carried out in an aqueous medium containing an alkali in an amount of about 1 – 10 wt . %. in the kneader process , mercerization may be carried out in an aqueous medium containing an alkali in an amount of about 2 – 15 wt . %. mercerization may be carried out in the presence of an appropriate solvent . examples of such solvents include water ; alcohols such as ethanol and isopropanol ; ketones such as acetone ; and cellosolves such as methyl cellosolve and ethyl cellosolve . the thus - produced carboxyalkyl cellulose can be purified through moisture - evaporation , washing , and then drying . if necessary , after the reaction is complete , the carboxyalkyl cellulose may be treated with a peroxide such as hydrogen peroxide or peracetic acid to control the viscosity . in the present invention , a mixture of a starting alkyl cellulose derivative and water , in which the amount of water is 5 to 2 , 000 parts by weight on the basis of 100 parts by weight of the alkyl cellulose derivative , is irradiated with radioactive rays . as described above , when a starting alkyl cellulose derivative such as cmc is irradiated with radioactive rays , decomposition of the derivative precedes cross - linking thereof . however , in the presence of water , hydroxy radicals are generated through the irradiation , and cross - linking of the derivative proceeds through the intervention of the hydroxy radicals . a mixture of the alkyl cellulose derivative and water may assume a form such that the derivative contains water absorbed in its structure , giving rise to a paste or an aqueous solution . preferably , the alkyl cellulose derivative and water are uniformly mixed . when the amount of water falls below the above range , the degree of decomposition of the starting alkyl cellulose derivative increases , whereas when the amount of water is in excess of the above range , it is difficult for the derivative to undergo cross - linking . examples of water employed in the present invention include city water , industrial water , degassed water , deionized water , gel - filtered water , and distilled water . preferably , water not containing oxygen or ions is employed . examples of the types of radioactive rays employed for irradiation in the present invention include α - rays , β - rays , γ - rays , x - rays , electron beams , and uv - rays . of these , x - rays , electron beams , or γ - rays from cobalt 60 are preferable . particularly , irradiation with γ - rays or irradiation with electron beams by use of an electron accelerator can be effectively employed for the formation of a cross - linking structure . in the present invention , the dose of radioactive rays varies between whether enhancement of water absorbability is intended or enhancement of gel strength is intended . also , the dose varies in relation to the mixing ratio of the starting alkyl cellulose derivative and water . in order to enhance water absorbability , the dose of radioactive rays is 0 . 1 – 50 kgy , preferably 0 . 3 – 20 kgy , more preferably 0 . 5 – 10 kgy , as reduced to γ - rays . when the dose falls below the above range , cross - linking of the starting alkyl cellulose derivative does not proceed , and thus the water - absorbability of the resultant resin is unsatisfactory . in contrast , when the dose is in excess of the above range , cross - linking of the derivative is excessive , and thus the water - absorbability of the resultant resin is unsatisfactory . in order to obtain a high strength of the gel , the dose of radioactive rays is 20 – 300 kgy , preferably 30 – 200 kgy , and more preferably 50 – 100 kgy , as reduced to γ - rays . when the dose falls below the above range , the strength of the resultant gel , particularly compressive strength decreases . in contrast , when the dose is in excess of the above range , production costs increase . when the alkyl cellulose derivative is irradiated with radioactive rays in the absence of oxygen , the derivative can be cross - linked effectively ( i . e ., at a low irradiation dose ). this is because , when irradiation is carried out in the presence of oxygen , the extent of oxidative decomposition of the derivative increases . in order to enhance water absorbability , the gel fraction of the above - produced self - cross - linking alkyl cellulose derivative is 0 . 1 – 50 %, preferably 0 . 5 – 40 %, more preferably 1 – 30 %. when the gel fraction of the self - cross - linking alkyl cellulose derivative falls below the above range , cross - linking of the derivative is unsatisfactory . in contrast , when the gel fraction is in excess of the above range , cross - linking of the derivative is excessive , and thus the water - absorbability of the resultant resin is unsatisfactory . in order to obtain a high strength of the gel , the gel fraction of the self - cross - linking alkyl cellulose derivative is 30 % or more , preferably 50 % or more , more preferably 60 % or more . the upper limit of the gel fraction is 100 %. when the gel fraction of the derivative falls below the above range , the strength of the resultant gel is unsatisfactory . the gel fraction is obtained through the following procedure : the self - cross - linking alkyl cellulose derivative is soaked in a large amount of distilled water ( e . g ., the amount is 10 – 100 times that of the derivative ) for 48 hours ; the resultant derivative is subjected to filtration through use of a 20 - mesh stainless steel sieve ; and the ratio of the insoluble derivative remaining on the sieve is obtained as the gel fraction of the derivative . the gel fraction is obtained from the following formula : ( wherein w 1 represents the weight of the dried starting alkyl cellulose derivative employed , and w 2 represents the weight of the dried insoluble derivative after filtration of the cross - linked derivative ). the biodegradability of the above - produced self - cross - linking alkyl cellulose derivative is measured through the following procedure . the self - cross - linking alkyl cellulose derivative produced through irradiation with radioactive rays is dried , and the dried derivative ( 0 . 2 g ) is added to an aqueous acetic acid solution ( buffer having a ph of 4 . 5 ) ( 10 ml ) containing 0 . 5 wt . % of cellulase , which is an enzyme employed for testing biodegradability . the resultant solution is allowed to stand at 40 ° c . for 0 to 8 hours , to thereby permit biodegradation of the derivative . the elapsed time and the percentage of the remaining self - cross - linking alkyl cellulose derivative are measured . the biodegradation (%) is obtained by subtracting the percentage of the remaining derivative from 100 %. in order to enhance water absorbability , the percent biodegradation of the self - cross - linking alkyl cellulose derivative during the above - mentioned period of time is 50 % or more , preferably 70 % or more , more preferably 80 % or more , much more preferably 90 % or more . in terms of the time versus percent biodegradation , eight - hour biodegradation desirably attains a percent biodegradation of 50 % or more , preferably 70 % or more , more preferably 80 % or more , much more preferably 90 % or more ; preferably , five - hour biodegradation is required to attain a percent biodegradation of 50 % or more , preferably 70 % or more , more preferably 80 % or more , much more preferably 90 % or more ; more preferably , four - hour biodegradation desirably attains a percent biodegradation of 50 % or more , preferably 70 % or more , more preferably 80 % or more , much more preferably 90 % or more ; and more preferably , three - hour biodegradation desirably attains a percent biodegradation of 50 % or more , preferably 70 % or more , more preferably 80 % or more , much more preferably 90 % or more . the time necessary to biodegrade the derivative to 100 % is controlled by determining the degrees of etherification or cross - linking . in order to obtain a high strength of the gel , the percent biodegradation of the self - cross - linking alkyl cellulose derivative at eight hours is 40 % or more , preferably 50 % or more , more preferably 60 % or more . the time necessary to biodegrade the derivative to 100 % is controlled by determining the degree of etherification or cross - linking . in the present invention , the self - cross - linking alkyl cellulose derivative irradiated with radioactive rays may be dried , and employed in the form of a solid or powder . the method for drying the self - cross - linking alkyl cellulose derivative is not particularly limited , and the derivative may be dried by means of a known method , for example , through heating or vacuuming , to thereby control the moisture in the derivative to a desired level . the water absorption ratio of the self - cross - linking alkyl cellulose derivative of the present invention is represented by the amount of distilled water which 1 g of the dried self - cross - linking alkyl cellulose derivative ( gel ) absorbs . the water absorption ratio varies with the degree of gelation of the derivative , and the ratio is 30 g – 1000 g / g - dried gel , preferably 100 g / g - dried gel or more , more preferably 200 g / g - dried gel or more . the self - cross - linking alkyl cellulose derivative of the present invention exhibits water - absorbability , high gel strength , and / or biodegradability , in addition to the characteristics of the starting alkyl cellulose derivative . therefore , the derivative can be employed for a variety of uses described in general publications or publications described in the “ background art ” of the present specification . in addition , the aforementioned characteristics allow the derivative to be used in products that must demonstrate a higher level of performance . the self - cross - linking alkyl cellulose derivative which absorbs water 30 times or more its own weight can be suitably employed in medical products , cosmetic products , sanitary products , or agricultural water retention agents . the derivative is particularly suitable for diapers or sanitary napkins . the self - cross - linking alkyl cellulose derivative having a gel compressive strength of 100 g / cm 2 or more is suitably employed in chromatography carriers , industrial materials , ground - improving agents , or soil - improving agents . the present invention will next be described in more detail by way of examples , which should not be construed as limiting the invention thereto . starting carboxyalkyl celluloses ( product of daicel chem . ind ., ltd .) employed in the examples are as follows . a : cmc , viscosity of 10 wt . % aqueous solution : 73 ( mpa · s ) at 20 ° c ., average etherification degree : 1 . 27 , percentage of carboxyl groups forming sodium chloride salts : 100 % b : cmc , viscosity of 10 wt . % aqueous solution : 161 ( mpa · s ) at 20 ° c ., average etherification degree : 2 . 21 , percentage of carboxyl groups forming sodium chloride salts : 100 % c : cmc , viscosity of 10 wt . % aqueous solution : 168 ( mpa · s ) at 20 ° c ., average etherification degree : 0 . 86 , percentage of carboxyl groups forming sodium chloride salts : 100 % d : cmc , viscosity of 10 wt . % aqueous solution : 250 ( mpa · s ) at 20 ° c ., average etherification degree : 1 . 29 , percentage of carboxyl groups forming sodium chloride salts : 100 % e : cmc , viscosity of 10 wt . % aqueous solution : 3670 ( mpa · s ) at 20 ° c ., average etherification degree : 1 . 22 , percentage of carboxyl groups forming sodium chloride salts : 100 % f : cmc , viscosity of 10 wt . % aqueous solution : 244 ( mpa · s ) at 20 ° c ., average etherification degree : 1 . 32 , percentage of carboxyl groups forming sodium chloride salts : 100 % g : carboxyethyl cellulose , viscosity of 10 wt . % aqueous solution : 200 ( mpa · s ) at 20 ° c ., average etherification degree : 1 . 32 , percentage of carboxyl groups forming sodium chloride salts : 100 % hpc1 : hydroxypropyl cellulose , viscosity of 2 wt . % aqueous solution : 280 ( mpa · s ) at 20 ° c . hpc2 : hydroxypropyl cellulose , viscosity of 2 wt . % aqueous solution : 2 , 500 ( mpa · s ) at 20 ° c . mc5 : methyl cellulose , viscosity of 2 wt . % aqueous solution : 25 ( mpa · s ) at 20 ° c . mc6 : methyl cellulose , viscosity of 2 wt . % aqueous solution : 7 , 500 ( mpa · s ) at 20 ° c . aqueous solutions each containing starting material f in an amount of 5 , 10 , 20 , or 30 wt . % were irradiated with γ - rays . the results are shown in fig1 . the x - axis of the graph shown in fig1 represents the dose of γ - rays ( unit : kgy ), and the y - axis represents the gel fraction ( wt . %) of cmc after irradiation . aqueous solutions each containing one of starting materials a through e in an amount of 20 wt . % were irradiated with γ - rays . the results are shown in fig2 . the x - axis of the graph shown in fig2 represents the dose of γ - rays ( unit : kgy ), and the y - axis represents the gel fraction ( wt . %) of cmc after irradiation . aqueous solutions each containing starting material f in an amount of 5 , 10 , 20 , or 30 wt . % were irradiated with γ - rays . after completion of irradiation , the resultant cmc was dried , and the water absorption of the dried cmc was measured . the results are shown in fig3 . the x - axis of the graph shown in fig3 represents the dose of γ - rays ( unit : kgy ), and the y - axis represents the amount of water ( g ) absorbed in 1 g of the dried gel of the irradiated cmc . aqueous solutions each containing starting material f in an amount of 20 or 30 wt . % were irradiated with γ - rays of 20 kgy . after completion of irradiation , the respective cmc solutions were subjected to biodegradation by use of cellulase . the percent biodegradation of each of the cmc solutions is shown in fig4 . the x - axis of the graph shown in fig4 represents the biodegradation time ( hours ), and the y - axis represents the remaining cmc (%). the percent biodegradation is obtained by subtracting the remaining cmc (%) from 100 %. in the case in which an enzyme cellulase is employed , the percent biodegradation of cmc after irradiation is higher than that of the cmc before irradiation . also , in the case in which a compost is employed , the percent biodegradation of the cmc after irradiation is higher than that of the cmc before irradiation . an aqueous solution of starting material g in an amount of 20 wt . % was irradiated with γ - rays . similar to the case of example 1 , the gel fraction of the resultant product increases as the dose of γ - rays increases . aqueous solutions each containing hpc1 , hpc2 , mc5 , or mc6 in an amount of 30 wt . % were irradiated with γ - rays . the results are shown in fig5 . the x - axis of the graph shown in fig5 represents the dose of γ - rays ( unit : kgy ), and the y - axis represents the gel fraction ( wt . %) of hpc1 , hpc2 , mc5 , and mc6 after irradiation . as is apparent from fig5 , each of hpc1 and hpc2 exhibits a peak of the gel fraction when the dose of γ - rays is 10 – 40 kgy , demonstrating achievement of satisfactory cross - linking . in the meantime , mc6 is cross - linked when the dose of γ - rays is high , and mc5 , which has a low polymerization degree , is cross - linked when the dose is higher . aqueous solutions each containing hpc1 or hpc2 in an amount of 40 wt . % were irradiated with γ - rays . the results are shown in fig6 . the x - axis of the graph shown in fig6 represents the dose of γ - rays ( unit : kgy ), and the y - axis represents the gel fraction ( wt . %) of hpc1 and hpc2 after irradiation . as is apparent from fig6 , the degree of cross - linking of the aqueous solution containing hpc in an amount of 40 wt . % is higher than that of the aqueous solution containing each of hpc in an amount of 30 wt . %. according to the present invention , a self - cross - linking alkyl cellulose derivative is produced . a water - absorbable resin or a gel of high strength can be produced depending on the type of starting alkyl cellulose derivative , the percentage of water during irradiation , or the dose of radioactive rays . in addition , by modifying such conditions , a biodegradable self - cross - linking alkyl cellulose derivative is produced .
a process for producing a self - cross - linking alkyl cellulose derivative , which includes irradiating , with radioactive rays , a mixture of a starting alkyl cellulose derivative and water , and thus obtained self - cross - linking alkyl cellulose derivative has an improved biodegradability and excellent water - absorbability .