in the thermosensitive recording material of the present invention , a thermosensitive colored image - forming layer is arranged on a surface of a sheet substrate , and comprises a substantially colorless dye precursor , a specific color developing agent reactive with the dye precursor upon heating to thereby develop a color , and a binder . the specific color developing agent comprises at least one member selected from the group consisting of the specific methacryloylimido compounds of the formulae ( i ) to ( v ) and the aromatic imido compounds of the formulae ( vi ) to ( viii ). the compounds of the formulae ( i ) to ( viii ) have an imido group of the formula : in the specific compounds of the formulae ( i ) to ( viii ), the alkyl groups represented by r 1 to r 6 , r 8 , r 10 , r 12 and r 13 preferably have 1 to 18 carbon atoms and are selected from methyl , ethyl , propyl , butyl , pentyl and octadecyl , the aryl groups represented by r 1 to r 6 , r 8 , r 10 , r 12 and r 13 are preferably selected from phenyl , naphthyl and anthryl groups , the aralkyl groups represented by r 1 to r 6 , r 8 , r 10 , r 12 and r 13 are preferably selected from benzyl , phenethyl and naphthylmethyl , the alkenyl groups represented by r 1 to r 6 , r 8 , r 10 , r 12 and r 13 are preferably selected from propenyl , butenyl and pentenyl , the cycloalkyl groups represented by r 1 , r 5 , and r 6 are preferably selected from cyclohexyl and cyclopropyl , and the aromatic cyclic groups represented by r 7 , r 9 and r 11 are preferably selected from benzene , naphthalene , phenanthrene and anthracene ring groups . also , in the formulae ( vi ) to ( viii ), the alkyl groups contained , as substituents , in r 7 to r 13 preferably have 1 to 4 carbon atoms and are selected from methyl , ethyl , propyl and butyl , the alkoxyl groups contained , as substituents , in r 7 to 7 13 preferably have 1 to 3 carbon atoms and are selected from methoxy , ethoxy and propoxy groups , the acyl groups contained , as substituents , in r 7 to r 13 are preferably selected from acetyl and benzoyl groups , the aryl groups contained , as substituents , in r 7 , r 9 and r 11 are preferably selected from phenyl and naphthyl , and the halogen atoms contained , as substituents , in r 7 to r 13 are preferably selected from chlorine , bromine and iodine atoms . the aromatic imido compounds of the formulae ( vi ) to ( viii ) are preferably selected from the group consisting of the compounds of the formulae 101 ) to 167 ): ## str8 ## the methacryloylimido compounds of the formulae ( i ) to ( v ) can be prepared in accordance with the following reactions : also , the aromatic imido compounds of the formulae ( vi ) to ( viii ) can be prepared in accordance with the following reactions : ## str9 ## in the above formulae , r 1 , r 2 , r 3 , r 4 , r 5 and r 6 are as defined above . ## str10 ## in the above - mentioned formulae , r 7 , r 8 , r 9 , r 10 , r 11 , r 12 and r 13 are as defined above . the specific color - developing agent of the present invention comprises one or two or more of the above - mentioned compounds of the formulae ( i ) to ( viii ). the thermosensitive colored image - forming layer optionally contains at least one conventional color - developing compound in addition to the specific color developing agent of the present invention , to further enhance the color - forming performance of the colored image - forming layer . the conventional color developing compounds include color developing phenolic and organic acid compounds , and are preferably selected from the group consisting of 2 , 2 - bis ( 4 - hydroxyphenyl ) propane ( namely bisphenol a ), 1 , 1 - bis ( 4 - hydroxyphenyl )- 1 - phenylethane , 1 , 4 - bis [ 1 - methyl - 1 -( 4 &# 39 ;- hydroxyphenyl ) ethyl ] benzene , 1 , 3 - bis [ 1 - methyl - 1 -( 4 &# 39 ;- hydroxyphenyl ) ethyl ] benzene , dihydroxydiphenylether ( disclosed in jp - a - 1 - 180 , 382 ), benzyl p - hydroxy - benzoate ( disclosed in jp - a - 52 - 140 , 483 ), bisphenol s , 4 - hydroxy - 4 &# 39 ;- isopropoxydiphenylsulfone ( disclosed in jp - a - 60 - 13 , 852 ), 1 , 1 - di -( 4 - hydroxyphenyl ) cyclohexane , 1 , 7 - di ( 4 - hydroxyphenylthio )- 3 , 5 - dioxaheptane ( disclosed in jp - a - 59 - 52 , 694 ), and 3 , 3 &# 39 ;- diallyl - 4 , 4 &# 39 ;- dihydroxydiphenylsulfone ( disclosed in jp - a60 - 208 , 286 ). the above - mentioned conventional color developing compounds can be employed alone or as a mixture of two or more thereof . preferably , the conventional color developing compounds are contained in an amount of 5 to 30 % based on the total weight of the colored image - forming layer . in the thermosensitive recording material of the present invention , the thermosensitive colored image - forming layer preferably further contains a colored image - stabilizing agent comprising at least one aromatic aziridine compound having at least one aziridinyl group . the aromatic aziridinyl compound usable for the colored image - stabilizing agent is preferably selected from the group consisting of 2 , 4 - bis ( 1 - aziridinylcarbonylamino ) toluene , bis [ 4 -( 1 - aziridinylcarbonylamino ) phenyl ] methane , bis [ 3 - chloro - 4 -( 1 - aziridinylcarbonylamino ) phenyl ] methane , 2 , 2 - bis [ 4 -( 1 - aziridinylcarbonyloxy ) phenyl ] propane , 1 , 4 - bis ( 1 - aziridinylcarbonyloxy ) benzene and 1 , 4 - bis ( 1 - aziridinylcarbonyl ) benzene . the aziridinine compounds are used alone or as a mixture of two or more thereof . the aziridinine compounds effectively enhance the resistance of the resultant colored images to oily and fatty substances , plasticizers , heat , and moisture , even immediately after the formation of the colored images . the dye precursor usable for the colored image - forming layer of the present invention comprises at least one member selected from conventional triphenylmethane , fluoran and diphenylmethane leuco dyes , for example , 3 -( 4 - diethylamino - 2 - ethoxyphenyl )- 3 -( 1 - ethyl - 2 - methylindole - 3 - il )- 4 - azaphthalide , crystal violet lactone , 3 -( n - ethyl - n - isopentylamino )- 6 - methyl - 7 - anilinofluoran , 3 - diethylamino - 6 - methyl - 7 - anilinofluoran , 3 - diethylamino - 6 - methyl - 7 -( 2 &# 39 ;, 4 &# 39 ;- dimethyl - anilino ) fluoran , 3 -( n - ethyl - n - p - toluidino )- 6 - methyl - 7 - anilinofluoran , 3 - pyrrolidino - 6 - methyl - 7 - anilinofluoran , 3 - dibutylamino - 6 - methyl - 7 - anilinofluoran , 3 -( n - cyclohexyl - n - methylamino )- 6 - methyl - 7 - anilinofluoran , 3 - diethylamino - 7 -( o - chloroanilino ) fluoran , 3 - diethylamino - 7 -( m - trifluoromethylanilino ) fluoran , 3 - diethylamino - 6 - methyl - 7 - chlorofluoran , 3 - diethylamino - 6 - methylfluoran , 3 - cyclohexylamino - 6 - chlorofluoran and 3 -( n - ethyl - n - hexylamino )- 6 - methyl - 7 -( p - chloroanilino ) fluoran . the binder usable for the present invention preferably comprises at least one member selected from water - soluble polymeric materials , for example , various types of polyvinyl alcohol resins which have a different molecular weight from each other , starch and starch derivatives , cellulose derivatives , for example , methoxy cellulose , carboxymethyl cellulose , methyl cellulose and ethyl cellulose , sodium polyacrylate , polyvinyl pyrrolidine , acrylic acid amide - acrylic acid ester copolymers , acrylic acid amide - acrylic acid ester - methacrylic acid terpolymers , alkali salts of styrene - maleic anhydride copolymers , polyacrylic acid amide , sodium alginate , gelatine and casein , and water - insoluble polymeric materials , for example , polyvinyl acetate resins , polyurethane resins , styrene - butadiene copolymer resins , polyacrylic acid resins , polyacrylic acid ester resins , vinyl chloride - vinyl acetate copolymer resins , polybutyl acrylate , ethylene - vinyl acetate copolymer resins and styrene - butadiene - acrylic compound - terpolymer resins , used in the form of a latex . in the thermosensitive colored image - forming layer of the present invention , the dye precursor is present in an amount of 5 to 20 % by weight , the color developing agent is present in an amount of 10 to 40 % by weight , and the binder is present in an amount of 5 to 20 % by weight , based on the total dry weight of the colored image - forming layer . when the content of the specific color developing agent of the present invention is less than 10 %, the resultant colored image forming layer exhibits an unsatisfactory color - developing activity , and even if the content of the specific color developing agent is increased to more than 40 %, the color developing activity of the resultant colored image - forming layer is saturated , and accordingly , is not further enhanced , and thus causes an economical disadvantage . also , in the thermosensitive colored image - forming layer of the present invention , the colored image - stabilizing agent is preferable present in an amount of 1 to 30 % based on the total dry weight of the colored image - forming layer . if the content of the colored image - stabilizing agent is less than 1 %, the resultant colored image - stabilizing effect is sometimes unsatisfactory . also , even if the colored image - stabilizing agent is used in an amount of more than 30 %, no further enhancement of the colored image - stabilizing effect is obtained . the thermosensitive colored image - forming layer of the present invention optionally further comprises a heat - fusible organic substance , usually referred to as a sensitizing agent , inorganic and organic pigments , antioxidants , for example , hindered phenol compounds , ultraviolet ray - absorbers , and waxes . the sensitizing agent comprises at least one organic compound having a melting point of from 50 ° c . to 150 ° c ., for example , phenyl 1 - hydroxy - 2 - naphthoate ( jp - a - 57 - 191 , 089 ), p - benzyl - biphenyl ( jp - a - 60 - 82 , 382 ), benzylnaphthylether ( jp - a - 58 - 87 , 094 ), dibenzyl terephthalate ( jp - a - 58 - 98 , 285 ), benzyl p - benzyloxybenzoate ( jp - a - 57 - 201 , 691 ), diphenyl carbonate , ditolyl carbonate ( jp - a - 58 - 136 , 489 ), m - therphenyl ( jp - a - 57 - 89 , 994 ), 1 , 2 - bis ( m - tolyloxy ) ethane ( jp - a - 60 - 56 , 588 ), 1 , 5 - bis ( p - methoxyphenoxy )- 3 - oxapentane ( jp - a - 62 - 181 , 183 ), oxalic acid diesters ( jp - a - 64 - 1 , 583 ) and 1 , 4 - bis ( p - tolyloxy ) benzene ( jp - a - 2 - 153 , 783 ). the inorganic and organic pigments usable for the present invention are preferably selected from inorganic fine particles of , for example , calcium carbonate , silica , zinc oxide , titanium dioxide , aluminum hydroxide , zinc hydroxide , barium sulfate , clay , anhydrous clay , talc , and surface - treated calcium carbonate and silica and organic fine particles of , for example , urea - formaldehyde resins , styrene - methacrylate copolymer resins and polystyrene resins . the waxes usable for the present invention preferably comprise at least one member selected from , for example , paraffin waxes , carnauba wax , microcrystalline waxes , polyethylene waxes , amide type waxes , bisimide type waxes , higher fatty acid amide waxes , for example , stearic acid amide , ethylene - bis - stearoamide wax , higher fatty acid esters and metal salts , for example , zinc stearate , aluminum stearate calcium stearate and zinc oleate . in the colored image - forming layer of the present invention , the sensitizing agent is preferably contained in an amount of 5 to 40 % by weight , and the wax and organic or inorganic pigment are optionally contained in amounts of 2 to 20 % by weight and 5 to 50 % by weight , respectively , based on the total dry weight of the colored image - forming layer . the sheet substrate usable for the present invention is not limited to a specific group of materials , and usually the sheet substrate comprises a member selected from fine paper sheets , coated paper sheet having a clay or latex - coated layer , cast - coated paper sheets , paper boards , plastic resin films , synthetic paper sheets comprising a plastic resin such as a polyolefin resin and a multi - layer structure , and laminated composite sheets . preferably , the sheet substrate has a basis weight of 40 to 170 g / m 2 . the colored image - forming layer can be formed on a surface of a sheet substrate , by applying a coating liquid containing the above - mentioned components , and by drying and solidifying the coating liquid layer on the sheet substrate . the colored image - forming layer is preferably present in a dry weight of from 1 to 15 g / m 2 , more preferably 2 to 10 g / m 2 . in the present thermosensitive recording material , a protective layer and / or printed layer may be formed on the colored image - forming layer . in the thermosensitive colored image - forming layer of the present invention , the colored image - forming layer containing , as a specific color developing agent , the specific compound of the formulae ( i ) to ( viii ) exhibits a satisfactory color developing effect for practical use and has a high whiteness . also , when the azirinine compound is contained in the colored image - forming layer in addition to the specific color developing compound of the present invention , the resultant colored images exhibit an excellent resistance to oily or fatty substances and to a plasticizer , even immediately after the formation of the colored images , and thus have a superior persistency for a long time . the present invention will be further explained by the following specific examples , which are merely representative and do not in any way restrict the scope of the present invention . a thermosensitive recording paper sheet was prepared by the following procedures . ______________________________________ ( 1 ) preparation of an aqueous dye precursordispersion a in the followinq compositioncomponent part by weight______________________________________3 -( n - isopentyl - n - ethylamino )- 6 - methyl - 207 - anilinofluoran10 % aqueous solution of polyvinyl alcohol 10water 70______________________________________ the composition was dispersed in a sand grinder to an extent such that the resultant dispersed solid particles had an average size of 1 μm or less . ______________________________________ ( 2 ) preparation of an aqueous color - developingagent dispersion b in the following compositioncomponent part by weight______________________________________methacryloylimido compound of the formula 32 2010 % aqueous solution of polyvinyl alcohol 10water 70______________________________________ the composition was dispersed in a sand grinder to an extent such that resultant dispersed solid particles had an average size of 1 μm or less . a coating liquid was prepared by mixing an aqueous dispersion , prepared by dispersing 85 parts by weight of anhydrous clay available under the trademark of ansilex , from engelhard corporation , in 320 parts by weight of water , with 40 parts by weight of an aqueous emulsion of a styrene - butadiene copolymer in a solid concentration of 50 % by weight and 50 parts by weight of a 10 % aqueous oxidized starch solution . the coating liquid was coated on a surface of a fine paper sheet having a basis weight of 48 g / m 2 , to form a coating layer having a dry weight of 7 . 0 g / m 2 , whereby a coated paper sheet was obtained . a coating liquid was prepared by evenly mixing 80 parts by weight of the aqueous dye precursor dispersion a , and 160 parts by weight of the aqueous color - developing agent dispersion b , with 30 parts by weight of a calcium carbonate pigment , 20 parts by weight of an aqueous 25 % zinc stearate dispersion , 15 parts by weight of an aqueous 30 % paraffin dispersion , and 120 parts by weight of an aqueous 10 % polyvinyl alcohol solution , by agitating the mixture . a surface of the pigment coated paper sheet was coated with the resultant coating liquid and dried . a thermosensitive colored image - forming layer was formed in a weight of 5 . 0 g / m 2 , to provide a thermosensitive recording paper sheet . the recording sheet was treated by a super calender , and the calendered surface of the recording sheet had a bekk smoothness of 600 to 1000 seconds . a specimen of the resultant thermosensitive recording sheet was subjected to a colored image - developing test in 64 lines by using a dynamic color - developing tester provided by modifying a thermosensitive facsimile printer , at a one line recording time of 10 m sec ., at a scanning line density of 8 × 8 dot / mm , and with an applied energy of 0 . 54 mj / dot . the resultant black colored images were clear and had a high color density of 1 . 2 or more determined by macbeth reflection color density tester rd - 914 ( trademarks ). the results of the above - mentioned tests are shown in table 1 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 1 except that , in the preparation of the aqueous dispersion b , the compound of the formula 32 was replaced by the methacryloylimido compound of the formula 17 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 1 except that , in the preparation of the aqueous color developing agent dispersion b , the compound of the formula 32 was replaced by 2 , 2 - bis ( 4 - hydroxyphenyl ) propane ( i . e ., bisphenol a ). the test results are shown in table 1 . table 1______________________________________ item whiteness (%) of color density of colored image - example no . colored images ( d ) forming layer______________________________________example 1 1 . 35 85 . 6 2 1 . 34 83 . 4comparative example 1 1 . 35 70 . 4______________________________________ a thermosensitive recording paper sheet was prepared by the same procedures as in example 1 , with the following exceptions . ______________________________________ ( 1 ) additionally , an aqueous aziridinine compounddispersion c was prepared in the following composition . component part by weight______________________________________bis [ 4 -( 1 - aziridinylcarbonylamino ) 20phenyl ] methane10 % polyvinyl alcohol aqueous solution 10water 70 the composition was dispersed in a sand grinder to an extent such that the resultant dispersed solid particles had an average size of 1 μm or less . a coating liquid was prepared by mixing 50 parts of the aqueous dispersion a and 100 parts of the aqueous dispersion b as mentioned in example 1 and 100 parts by weight of the above - mentioned aqueous dispersion c , with 30 parts by weight of a calcium carbonate pigment dispersion , 20 parts by weight of an aqueous 25 % aqueous zinc stearate dispersion , 15 parts by weight of an aqueous 30 % paraffin dispersion and 120 parts by weight of an aqueous 10 % polyvinyl alcohol solution , by agitating the mixture . a surface of the pigment coated paper sheet was coated with the resultant coating liquid and the resultant coated layer was dried . a thermosensitive colored image - forming layer was formed in a dry weight of 5 . 0 g / m 2 , to provide a thermosensitive recording paper sheet . the recording sheet was treated by a super calender , and the resultant calendered surface of the recording sheet had a bekk smoothness of 600 to 1000 seconds . a specimen of the resultant thermosensitive recording sheet was subjected to a colored image - developing test in 64 lines by using a dynamic color - developing tester provided by modifying a thermosensitive facsimile printer , at a one line recording time of 10 m sec ., at a scanning line density of 8 × 8 dot / mm , and with an applied energy of 0 . 54 mj / dot . the resultant black colored images were clear and had a high color density of 1 . 0 or more determined by macbeth reflection color density tester rd - 914 ( trademarks ). the color image - developed specimen was subjected within 30 minutes from the completion of the color image - developing procedure , to a salad oil resistance test in such a manner that a salad oil was applied to the colored image - developed surface of the specimen by using a cotton applicator , the salad oil - applied specimen was left to stand at room temperature for 30 minutes , and thereafter , the remaining colored images were evaluated by a naked eye observation . the same test procedures as mentioned above were carried out , except that the salad oil was replaced by dioctyl phthalate to evaluate the resistance of the colored images to the plasticizer . the results of the above - mentioned tests are shown in table 2 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 3 except that , in the preparation of the aqueous color developing agent dispersion b , the compound of the formula 32 was replaced by the compound of the formula 17 . the obtained black colored images had a color density of 1 . 32 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 3 except that , in the preparation of the aqueous color developing agent dispersion b , the compound of the formula 32 was replaced by the compound of the formula 28 . the resultant black colored images had a color density of 1 . 30 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 3 except that , in the preparation of the aqueous aziridinine compound dispersion c , the bis [ 4 -( 1 - aziridinylcarbonylamino ) phenyl ] methane was replaced by 2 , 4 - bis ( 1 - aziridinylcarbonylamino ) toluene of the formula : ## str13 ## the resultant black colored images had a color density of 1 . 32 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 3 except that , in the preparation of the aqueous color developing agent * dispersion b , the compound of the formula 32 was replaced by 2 , 2 - bis ( 4 - hydroxyphenyl ) propane , namely bisphenol a . the resultant black colored images had a color density of 1 . 31 . the same procedures as in comparative example 1 were carried out , except that the resultant thermosensitive recording paper sheet specimen was subjected to the tests for the salad oil and plasticizer resistances of the colored images . table 2______________________________________ item resistance of colored resistance of colored images to salad oil images to dioctylexample no . (*). sub . 1 phosphate (*). sub . 1______________________________________example 3 3 3 4 3 3 5 3 3 6 3 3comparative 2 2 1example 3 1 1______________________________________ note : 3 . . . remaining colored images were clear 2 . . . remaining colored images were dim 1 . . . remaining colored images disappeared table 1 clearly shows that the specific color developing agent of the present invention consisting of a methacryloylimido compound of the formulae ( i ) to ( v ) exhibited a satisfactory color developing performance comparable to a conventional typical color developing agent consisting of bisphenol a , and advantageously caused the resultant colored image - forming layer to exhibit a significantly high whiteness in comparison with the conventional colored image - forming layer . also , table 2 clearly shows that the colored images formed on the colored image - forming layer of the present invention containing the aziridinine compound in addition to the specific color - developing agent exhibited an excellent resistance to the oily and fatty substances , and to the plasticizers , even immediately after the formation of the colored images . a thermosensitive recording paper sheet was prepared by the following procedures . ______________________________________ ( 1 ) preparation of an aqueous dye precursordispersion a in the following compositioncomponent part by weight______________________________________3 -( n - isopentyl - n - ethylamino )- 6 - methyl - 207 - anilinofluoran10 % aqueous solution of polyvinyl alcohol 10water 70______________________________________ the composition was dispersed in a sand grinder to an extent such that the resultant dispersed solid particles had an average size of 1 μm or less . ______________________________________ ( 2 ) preparation of an aqueous color - developingagent dispersion b in the following compositioncomponent part by weight______________________________________aromatic imido compound of the formula 101 2010 % aqueous solution of polyvinyl alcohol 10water 70______________________________________ the composition was dispersed in a sand grinder to an extent such that the resultant dispersed solid particles had an average size of 1 μm or less . a coating liquid was prepared by mixing an aqueous dispersion , prepared by dispersing 85 parts by weight of anhydrous clay available under the trademark of ansilex , from engelhard corporation , in 320 parts by weight of water , with 40 parts by weight of an aqueous emulsion of a styrene - butadiene copolymer in a solid concentration of 50 % by weight and 50 parts by weight of a 10 % aqueous oxidized starch solution . the coating liquid was coated on a surface of a paper sheet having a basis weight of 48 g / m 2 , to a coating layer having a dry weight of 7 . 0 g / m 2 , whereby a coated paper sheet was obtained . a coating liquid was prepared by evenly mixing 80 parts by weight of the aqueous dye precursor dispersion a , and 160 parts by weight of the aqueous color - developing agent dispersion b , with 30 parts by weight of a calcium carbonate pigment , 20 parts by weight of an aqueous 25 % zinc stearate dispersion , 15 parts by weight of an aqueous 30 % paraffin dispersion , and 120 parts by weight of an aqueous 10 % polyvinyl alcohol solution , by agitating the mixture . a surface of the pigment coated paper sheet was coated with the resultant coating liquid and dried . a thermosensitive colored image - forming layer was formed in a weight of 5 . 0 g / m 2 , to provide a thermosensitive recording paper sheet . the recording sheet was treated by a super calender , and the calendered surface of the recording sheet had a bekk smoothness of 600 to 1000 seconds . a specimen of the resultant thermosensitive recording sheet was subjected to the same colored image - developing test in 64 lines as in example 1 . the resultant black colored images were clear and had a high color density of 1 . 2 or more determined by macbeth reflection color density tester rd - 914 ( trademark ). also , the reflectance of the non - color - developed portions of the specimen was measured by using a hunter whiteness tester ( trademark ) with a blue filter . the whiteness of the specimen was represented by the measured reflectance value . the results of the above - mentioned tests are shown in table 3 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 7 except that , in the preparation of the aqueous color developing agent dispersion b , the compound of the formula 101 was replaced by the compound of the formula 114 . the obtained black colored images had a color density of 1 . 34 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 7 except that , in the preparation of the aqueous dispersion b , the compound of the formula 101 was replaced by an aromatic imido compound of the formula 122 . the resultant black colored images had a color density of 1 . 34 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 7 except that , in the preparation of the aqueous dispersion b , the compound of the formula 101 was replaced by 2 , 2 - bis ( 4 - hydroxyphenyl ) propane , namely bisphenol a . the resultant black colored images had a color density of 1 . 35 . table 3______________________________________ item color density ( d ) whiteness of coloredexample of colored images image - forming layer (%) ______________________________________example 7 1 . 35 85 . 6 8 1 . 34 83 . 4 9 1 . 34 85 . 6comparative 1 . 35 70 . 0example 4______________________________________ a thermosensitive recording paper sheet was prepared by the same procedures as in example 7 with the following exceptions . ______________________________________ ( 1 ) additionally an aqueous aziridinine compounddispersion was prepared in the following composition . component part by weight______________________________________bis [ 4 -( 1 - aziridinylcarbonylamino ) 20phenyl ] methane10 % aqueous polyvinyl alcohol solution 10water 70______________________________________ the composition was dispersed in a sand grinder to an extent such that the resultant dispersed solid particles had an average size of 1 μm or less . a coating liquid was produced by mixing 50 parts by weight of the aqueous dye precursor dispersion and 100 parts of the aqueous color developing agent dispersion ( b ) of example 7 and 100 parts by weight of the above - mentioned aqueous aziridinine compound dispersion c , with 30 parts by weight of a calcium carbonate pigment , 20 parts by weight of a 25 % zinc stearate dispersion , 15 parts by weight of a 30 % paraffin dispersion and 120 parts by weight of a 10 % polyvinyl alcohol aqueous solution , by agitating the mixture . the resultant coating liquid was applied to a surface of the pigment - coated paper sheet and dried , to provide a thermosensitive colored image - forming layer having a dry weight of 5 . 0 g / m 2 . a thermosensitive recording paper sheet was obtained . the resultant recording sheet was treated by a super calender to provide a smoothed surface thereof having a bekk smoothness of 600 to 1000 seconds . the resultant recording sheet was subjected to the same tests as mentioned in example 3 . the obtained black colored images were clear and had a high color density of 1 . 0 or more , measured by a macbeth reflection color density tester rd - 914 ( trademark ). the results of the salad oil and dioclyl phosphate resistance tests are shown in table 4 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 10 except that , in the preparation of the color developing agent dispersion b , the compound of the formula 101 was replaced by an aromatic imido compound of the formula 114 . the resultant black colored images had a color density of 1 . 29 . a thermosensitive recording sheet was prepared by the same procedures as in example 10 except that , in the preparation of the color developing agent dispersion b , the compound of the formula 101 was replaced by an aromatic imido compound of the formula 122 . the resultant black colored images had a color density of 1 . 30 . a thermosensitive recording paper sheet was obtained by the sam procedures as in example 10 except that , in the preparation of the color developing agent dispersion , the compound of the formula 101 was replaced by an aromatic imido compound of the formula 126 . the resultant black colored images had a color density of 1 . 28 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 10 except that , in the preparation of the aziridinine compound dispersion c , the bis [ 4 -( 1 - aziridinylcarbonylamino ) phenyl ] methane was replaced by 2 , 4 - bis ( 1 - aziridinylcarbonylamino ) toluene . the resultant black colored images had a color density of 1 . 30 . a thermosensitive recording paper sheet was prepared by the same procedures as in example 10 except that , in the preparation of the color developing agent dispersion b , the compound of the formula 101 was replaced by 2 , 2 - bis ( 4 - hydroxyphenyl ) propane , i . e ., bisphenol a . the same thermosensitive recording paper sheet as in comparative example 4 was subjected to the same salad oil and dioctyl phosphate resistance tests as in example 3 . table 4______________________________________ item resistance of resistance of colored images to colored images toexample no . salad oil (*). sub . 1 dioctyl phosphate (*). sub . 1______________________________________example 10 3 3 11 3 3 12 3 3 13 3 3 14 3 3comparative 5 2 1example 6 1 1______________________________________ note : 3 -- remaining colored images were clear . 2 -- remaining colored images were dim . 1 -- remaining colored images disappeared . table 3 clearly shows that the specific color developing agent of the present invention consisting of an aromatic imido compound of the formulae ( vi ) to ( viii ) exhibited a satisfactory color developing performance comparable to a conventional typical color developing agent consisting of bisphenol a , and advantageously caused the resultant colored image - forming layer to exhibit a significantly high whiteness in comparison with the conventional colored image - forming layer . also , table 4 clearly shows that the colored images formed on the colored image - forming layer of the present invention containing the aziridinine compound in addition to the specific color - developing agent exhibited an excellent resistance to the oily and fatty substances , and to the plasticizers , even immediately after the formation of the colored images .