Thermally sensitive recording medium

Abstract: A thermally sensitive recording medium comprising a thermally sensitive color developing layer which comprises (a) a colorless or pale colored dye precursor, (b) a color developer which can react with the dye precursor to develop a color when heated and (c) a polyurea compound. Said polyurea compound comprises units of formula (1):

Description: This invention relates to the thermally sensitive recording medium comprising a poly urea compound in a color developing layer. The preservative stability of a recorded image is superior to that of conventional thermally sensitive recording media, and is suited to uses for which long term preservative stability is required. In general, a thermally sensitive recording medium is prepared by the following procedure. A colorless or pale colored dye precursor which is ordinarily an electron donating compound and a color developer which is an electron accepting compound are separately ground to fine particles and dispersed, then mixed together. A binder, a filler, a sensitizer, a lubricant and other stabilizers are added, and the obtained coating fluid is coated on a substrate such as paper, synthetic paper, film or plastics, which develops a color by an instantaneous chemical reaction caused by heating with a thermally sensitive head, a hot stamp or a laser beam. These thermally sensitive recording media are widely used in a measuring recorder, a thermal printer of a computer, a facsimile, an automatic ticket vender or a bar code label. However, recently, along with a diversification of recording apparatuses for thermally sensitive recording medium and a remarkable progress toward high quality, the required quality of the thermally sensitive recording medium is becoming higher. Further, since recording methods on normal paper such as an electrophotographic method or an ink jet method are becoming more popular, the thermally sensitive recording method is often compared with normal paper recording methods. Therefore, for the thermally sensitive recording method, it is strongly required to improve the stability of the recorded part (image) and the stability of the not recorded part before and after recording (ground part or blank part) to the similar quality level of that of normal paper recording method. Especially, from the view point of image preservative stability of a recorded part, a thermally sensitive recording medium which is superior in light resistance, oil resistance, water resistance and plasticizer resistance is required. To solve the above mentioned problems, methods to contain various kind of stabilizers in a color developing layer are provided. For instance, metallic salts disclosed in Japanese patent laid open publication 63-22683, metallic salts of phospholic ester disclosed in Japanese patent laid open publication 4-303682, metallic salts of benzoic acid derivatives disclosed in Japanese Patent publication 2-26874 or Japanese Patent Publication 2-39994 can be mentioned. In these prior art documents, the image preserving effect is achieved by containing the above mentioned chemicals in a color developing layer. Further, an epoxy compound disclosed in Japanese Patent laid open Publication 4-97887 and an aziridine compound disclosed in Japanese Patent laid open Publication 4-113888 display good effects in the improvement of oil resistance and water resistance, and an aliphatic dicarboxylic acid compound disclosed in Japanese Patent laid open Publication 6-32054 is effective for the improvement of oil resistance. An acylacetanilide compound disclosed in Japanese Patent laid open Publication 8-72406 and p-hydroxybenzoic acid anilide disclosed in Japanese Patent laid open Publication 8-258430 also have good effects on oil resistance. US 5 380 693 discloses a transparent thermosensitive recording medium comprising a transparent support, a thermosensitive recording layer containing an electron-donating coloring compound, an electron-accepting compound and a binder resin, and a protective layer formed on the thermosensitive recording layer. The transparent support, the thermosensitive recording layers and the protective layer have substantially the same refractive index. It is preferred that the binder resin includes a hydroxyl group in the molecule. US 4 243 716 discloses a thermal sensitive paper having a heat sensitive layer containing a color-forming lactone compound and a color-developing phenol compound, and having incorporated therein, a finally divided urea-formaldehyde resin. EP-0 748 698 discloses a thermal recording sheet comprises a substrate, a thermal color developing layer comprising (a) a colorless or pale colored basic aromatic dye and an organic color developer, and (b) aluminium hydroxide and urea-formaldehyde resin. GB-A-2 007 858 discloses a thermally responsive record material comprising a substrate coated with a composition of a substantially colorless color former, a phenolic co-reactant and a thermographically acceptable binder, wherein a substantially water-insoluble, cross-linked urea-formaldehyde agglomerated resin pigment is dispersed in particulate form within the composition. Among the above mentioned stabilizers, a stabilizer which uses metallic salt has a good effect on the preservative stability of the image. However, since it has a problem that the heat resistance of the ground color is not good, such stabilizer is difficult to use practically. In the case of a non-metallic salt compound, there are not so many stabilizers which are good not only in respect of oil resistance and water resistance but also in respect of plasticizer resistance, therefore it is necessary to use plural kinds of stabilizers simultaneously. The method to add plural kinds of stabilizers together with and to improve preservative stability of the image for all items has many problems from the view point of productivity and economics and is also practically difficult to put to industrial use. For practical industrial use, one stabilizer compound which is superior in image preservative stability in respect of all of oil resistance, plasticizer resistance and water resistance is needed. The object of this invention is to provide a thermally sensitive recording medium providing improved image preservative stability of the recording portion especially in plasticizer resistance, oil resistance and water resistance at a cheap price. The invention provides a thermally sensitive recording medium comprising a substrate having thereon a thermally sensitive color developing layer which comprises (a) a colorless or pale colored dye precursor, (b) a color developer which can react with the dye precursor to develop a color when heated and (d) a poly urea compound which comprises units of formula (1): wherein A 1 represents a divalent group of one of the following formulae: The recording medium of the invention displays excellent functions concerning the image preservative stability such as plasticizer resistance, oil resistance and water resistance. The invention uses a poly urea compound comprising units of formula (1) as a component of the stabilizer. The poly urea compound of formula (1) preferably comprises a repeating unit chosen from formulae (2) to (7) below: wherein R 1 and R 2 represent an alkyl group, an alkoxy group or an electron accepting group, o and p represent an integer from 0 to 4, and A 2 represents a divalent group as defined for A 1 above, wherein R 3 represents an alkyl group, an alkoxy group or an electron accepting group, q is an integer from 0 to 4 and A 3 represents a divalent group as defined for A 1 above, wherein r is an integer from 2 to 12 and A 4 represents a divalent group as defined for A 1 above, wherein A 5 represents a divalent group, wherein A 6 represents a divalent group as defined for A 1 above, wherein R 4 and R 5 represent an alkyl group, alkoxy group and electron accepting group, s and t are an integer from 0 to 8 and A 7 represents a divalent group as defined for A 1 above. Substituents R 1 to R 5 above do not obstruct the color development and image preservative stability. Suitable examples of substituents R 1 to R 5 are C 1 - 4 alkyl and C 1 - 4 alkoxy. As electron accepting groups R 1 to R 5 , halogen atoms (such as chlorine, bromine and fluorine) or a nitro group are preferred. Referring to a poly urea compound having a structure of a -NHCONH-b, there are three specific ways of combining the aromatic or aliphatic hydrocarbon compound with a or b as follows: i) When both a and b are an aliphatic hydrocarbon, the electron density on the nitrogen atom of the urea increases because of the electron donating feature of the aliphatic compound, and it is more difficult to cationate the hydrogen atom. Therefore, the color developing ability deteriorates and image preservative stability decreases compared to iii) below. ii) When both a and b are an aromatic hydrocarbon, since the structure of the aromatic compound is generally flat and the structural feature of it is stiff, the poly urea compound easily forms fibrous or film-like material. Consequently, at the fabrication of the thermally sensitive recording medium, the poly urea compound is mixed with water containing polyvinylalcohol, ground by a pulverizer or an emulsifier such as a ball mill, an attriter or a sand grinder, then a poly urea dispersion is prepared. However, in this case, it is very difficult to obtain fine granulated particles and a homogeneously distributed dispersion. Therefore, the improvement in image preservative stability is not as great as might be expected, compared to iii) below. iii) When one of a or b is an aliphatic compound and the other is an aromatic compound, the color developing ability and the image preservative stability are improved sufficiently and also the dispersion becomes good and the most balanced poly urea can be obtained. Preferably then, the divalent groups A 2 , A 3 and A 5 of poly urea compounds represented by formulae (2), (3) and (5) which are bonded at one end with an aromatic hydrocarbon are aliphatic hydrocarbon and the divalent groups A 4 , A 6 and A 7 of the poly urea compounds represented by formulae (4), (6) and (7) which are bonded at one end with an aliphatic hydrocarbon are aromatic hydrocarbon. Especially, as A 2 , A 3 and A 5 , a normal chain or a partially branched chain hydrocarbon are desirable. And, as A 4 , A 6 and A 7 , an aromatic hydrocarbon in which hetero atom is not included is suited. The poly urea compound useful in the material of this invention has a color developing ability which is reactable with a dye precursor. And the application to use this compound as a color developer is already disclosed in Japanese Patent laid open No. 10-181217 (application No. 8-349482) not pre-published. Since, the poly urea compound is insoluble in oil, plasticizer or various kind of solvents because it is a compound of high molecular weight, it is not dissolved by them even if it is exposed to them, and as a result, the phenomenon of image vanishing caused by dissociation with dye is not observed and an excellent image preservative stability can be obtained. The image preservative stability of the poly urea used in the material of this invention is remarkably superior to that of conventional color developers such as phenols, low molecular weight urea or urethane, therefore it is especially useful for applications in which long term image preservative stability of the recorded part is required. Meanwhile, recently, in addition to image preservative stability, the requirement to improve a color developing property so as to obtain sufficient color density by lower impressive energy is becoming more serious. The present inventors have found that to add the defined poly urea compound to the thermally sensitive recording media which uses conventional well known color developer is effective. When they are used together, an excellent thermal sensitive recording medium can be obtained which is endowed with both the good color developing ability of a conventional well known color developer and with excellent image preservative stability. Further, the thermal sensitive recording medium of this invention has a strong point that the developed image does not vanish when it is contacted with plasticizer, still further since it does not have problems such as line fading, hazing or blotting, it is superior at a fine line image such as a numeral figure or a character. The amount of poly urea compound used in the material of this invention in a color developing layer is changeable according to the required quality. However, in general when the amount is smaller than 0.01 part to 1 part of a color developer the effect on the image preservative stability is not sufficient, and when the amount is bigger than 2 parts to 1 part of a color developer the initial color developing density is not sufficient. Therefore, the amount of poly urea compound to be contained is suitably from 0.01 to 2 parts and desirably smaller than 1 part to 1 part of color developer. As examples of compounds of formulae (1) to (7) used in this invention, the following compounds are mentioned, however it is not intended to be limited to them. And, these mentioned poly urea compounds can be used alone or by mixing together. The poly urea compounds used in the material of this invention can be synthesized by a conventional well known method. The following methods can be mentioned as typical conventional well known methods. (a) The method to dissolve diisocyanate and diamine in an inert solvent such as dimethylacetoamide, acetone, dimethylformamide, chlorobenzene or dimethylsulfoxide, mix them in the inert gas atmosphere for several minutes to several hours by constant stirring at room temperature and react them. [E. L. Lawton et al., Appl. Polym. Sci., 25 , 187(1980) or C. S. Marvel, J.H.Johnson, J. Am. Chem. Soc., 72 , 1674(1950)] (b) The synthetic method by mixing diamine with urea and heating, then de-ammonia. [Mitsui Toatsu, U.S.Patent., 2973342(1961)] (c) The synthetic method by reaction of diamine and phosgene by way of carbamic acid chloride. [P. Bomer et al., Makromol. Chem., 101, 1(1967) or L. Alexandru, L. Dascalu, J. Polym. Sci., 52 , 331(1961)] (d) The synthetic method by heating diamine and carbamate [Brit. Pat., 528437(1940) or U.S. Pat., 2181663 (1940)] (e) The synthetic method by heating diamine and carbon dioxide under high pressure. [N. Yamazaki et al., J. Polym. Sci. PartC., 12 , 517(1974)] (f) The synthetic method by heating diamine and carbon oxysulfide under lower pressure. [G. J. M. Van d. Kerk, Recueil. Trav. Chim., 74 , 1301 (1955)] (g) The synthetic method by reacting diamine and diphenyl carbonate or di(p-nitrophenyl) carbonate. [R.D. Katsarava et al., Makromol. Chem., 194 , 3209 (1993)] (h) The synthetic method from diisocyanate and benzoic acid in dimethylsulfoxide. [W. R. Sorensen, J. Org. Chem., 24 , 978 (1959)] In the case of synthetic method using diisocyanate as a starting material, since diphenylmethane-4,4'-diisocyanate <commodity name : MDI>, tolylene-2,4-diisocyanate <2,4-TDI>, tolylene-2,6-diisocyanate <2,6-TDI>, 1,6-hexamethylenediisocyanate <HDI>, 1,5-naphthylenediisocyanate <NDI>, isophorone-diisocyanate and dicyclohexylmethane-4,4'-diisocyanate which can be a starting material, are produced commercially in the market, they can be easily bought by lower price from the market. And for the production of poly urea, they can be synthesized by high productivity without special equipment. Therefore, when the polyurea compound useful in the material of this invention is fabricated using the above mentioned compound as a starting material, the production cost becomes very low. The poly urea compound of formula (1) to (7) can be synthesized by any methods mentioned above, and among them (a) method which synthesize it using diisocyanate is most convenient. Since the poly urea compound useful in the material of this invention is insoluble or very difficult to dissolve in any kind of solvents, the measurement of molecular weight of the compound is impossible. Therefore, it is very difficult to confirm that these compounds are apparently high molecular compounds. However, from the view point that they do not have a constant and sharp melting point and they have a good spinnability which is observed by sticking and pulling up the molten fluid of these compounds with a glass bar, further they indicate very high viscosity when they are dissolved in conc sulfuric acid, it is possible to presume that these compounds are high molecular compounds. For the fabrication of the thermally sensitive recording medium of this invention, various conventional well known producing methods can be used. Specifically, it can be fabricated by the following method. That is, poly urea compound, dye precursor, color developer and sensitizer are ground and granulated by a pulverizer or an emulsifier such as a ball mill, an attriter or a sand grinder, fillers and additives are added, the moulting material is then dispersed in an aqueous solution of water soluble binder, and the coating is obtained. The thermally sensitive recording medium can also be obtained by coating the obtained coating on a surface of a substrate by means of an air knife coater, a blade coater or a roll coater. As the dye precursor to be used in the thermally sensitive recording medium, the conventional well known chemical compounds can be used. The examples of dye precursors used in the thermally sensitive recording medium are listed below, however it is not intended to be limited to them. These dye precursor can be used alone or used by mixing together. 3,3-bis(4-dimethylaminophenyl)-6-dimethylaminophtalide <commodity name : CVL>, 3-diethylamino-6-methyl-7-anilinofluoran <OBD>, 3-(N-isoamyl-N-ethylamino)-6-methyl-7-anilinofluoran <S-205>, 3-diethylamino-7-m-trifluoromethylanilinofluoran <Black-100>, 3-dibutylamino-7-o-chloroanilinofluoran <TH-107>, 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran <PSD-150)>, 3-diethylamino-7-anilinofluoran <Green-2>, 3,3-bis(4-dimethylaminophenyl)phthalide <MGL>, tris[4-(dimethylamino)phenyl]methane <LCV>, 3,3-bis(1-ethyl-2-methylindole-3-yl)phthalide <Indolyl red>, 3-cyclohexylamino-6-chlorofluoran <OR-55>, 3,3-bis[2-(p-dimethylamtnophenyl)-2-(p-methoxyphenyl)ethenyl]-4,5,6,7-tetrachlorophthalide <NIR-Black>, 1,1,5,5-tetrakis(p-dimethylaminophenyl)-3-methoxy-1,4-pentadiene, and 1,1,5,5-tetrakis(p-dimethylaminophenyl)-3-(p-dimethylamino phenyl)-1,4-pentadiene As the color developer to be used in the thermally sensitive recording medium of this invention, the conventional well known chemical compounds can be used. Suitable examples of color developer are listed below, however it is not intended to be limited to them. Bisphenols such as 2,2-bis(4-hydroxyphenyl)propane, 1,7-di(4-hydroxyphenylthio)-3,5-dioxaheptane and 4,4'-cyclohexilidendiphenol, 4-hydroxy benzoic esters such as 4-hydroxy benzyl benzoate, 4-hydroxy ethyl benzoate, 4-hydroxy normalpropyl benzoate, 4-hydroxy isopropyl benzoate and 4-hydroxy butyl benzoate, 4-hydroxy phthalic diesters such as 4-hydroxy dimethyl phthalate, 4-hydroxy diisopropyl phthalate and 4-hydroxy dihexyl phthalate, Phthalic monoester such as monobenzyl phthalate, monocyclohexyl phthalate, monophenyl phthalate and monomethylphenyl phthalate, Bishydroxyphenylsulfides such as bis(4-hydroxy-3-tert-buthyl-6-methylphenyl)sulfide, bis(4-hydroxy-2,5-dimethylphenyl)sulfide and bis(4-hydroxy-2-methyl-5-ethylphenyl)sulfide, 4-hydroxyphenylarylsulfones such as 4-hydroxy-4'-isopropoxydiphenylsulfone, 4-hydroxy-4'-methyldiphenylsulfone and 4-hydroxy-4'-normalpropoxydiphenylsulfone, 4-hydroxyphenylarylsulfonates such as 4-hydroxyphenylbenzenesulfonate, 4-hydroxyphenyl-p-tolylsulfonate and 4-hydroxyphenyl-p-chlorobenzenesulfonate, 1,3-di[2-(hydroxyphenyl)-2-propyl]benzenes such as 1,3-di[2-(4-hydroxyphenyl)-2-propyl]benzene and 1,3-di[2-(4-hydroxy-3-methylphenyl)-2-propyl]benzene, 4-hydroxybenzoyloxibenzoic esters such as benzyl 4-hydroxybenzoyloxybenzoate, methyl 4-hydroxybenzoyloxybenzoate, ethyl 4-hydroxybenzoyloxybenzoate, normalpropyl 4-hydroxybenzoyloxybenzoate, isopropyl 4-hydroxybenzoyloxybenzoate and buthyl 4-hydroxybenzoyloxy benzoate, Bishydroxyphenylsulfones such as bis(3-tert-butyl-4-hydroxy-6-methylphenyl)sulfone, bis(3-ethyl-4-hydroxyphenyl)sulfone, bis(3-propyl-4-hydroxyphenyl)sulfone, bis(3-isopropyl-4-hydroxyphenyl)sulfone, bis(3-ethyl-4-hydroxyphenyl)sulfone bis(4-hydroxyphenyl)sulfone 2-hydroxyphenyl-4'-hydroxyphenyl)sulfone bis(3-chloro-4-hydroxyphenyl)sulfone and bis(3-bromo-4-hydroxyphenyl)sulfone, Phenols such as p-tert-butylphenol, p-phenylphenol, p-benzylphenol, 1-naphthol and 2-naphthol, Metallic salts of aromatic hydrocarbon such as benzoic acid, p-tert-butyl benzoic acid, trichloro benzoic acid, 3-sec-butyl-4-hydroxybenzoic acid, 3-cyclohexyl-4-hydroxybenzoic acid, 3,5-dimethyl-4-hydroxybenzoic acid, terephthalic acid, salicylic acid, 3-isopropylsalicylic acid and 3-tert-butylsalicylic acid N-phenyl-N'-sulfamoylphenylureas such as N-phenyl-N'-(p-sulfamoyl)phenylurea and N-phenyl-N'-(m-sulfamoyl)phenylurea, N-phenyl-N'-sulfamoylphenylthioureas such as N-phenyl-N'-(p-sulfamoyl)phenylthiourea and N-phenyl-N'-(m-sulfamoyl)phenylthiourea, N-benzenesulfoneyl-phenylureylenebenzamides such as N-benzenesulfoneyl-p-(phenylureylene)benzamide, N-(4-toluenesulfoneyl)-p-(phenylureylene)benzamide and N-(4-ethylphenylsulfonyl)-p-(phenylureylene)benzamide, and N-benzenesulfoneyl-phenylthioureylenebenzamides such as N-benzenesulfoneyl-p-(phenylthioureylene)benzamide, N-(4-toluenesulfoneyl)-p-(phenylthioureylene)benzamide and N-(4-ethylphenylsulfoneyl)-p-(phenylthioureylene)benzamide. Among these compounds, bisphenols, 4-hydroxyphenylaryl-sulfones and bishydroxyphenylsulfones are preferably used from the view point of color developing. Especially, since 2,2-bis(4-hydroxyphenyl)propane, 4-hydroxy-4'-isopropoxydiphenylsulfone and bis(4-hydroxyphenyl)sulfone are comparatively cheap and the expected effects can be obtained in good balance, they are good for industrial use. Generally, in the thermally sensitive recording medium which uses a dye precursor and a color developer as the color developing components, a sensitizer is usually used to improve the color developing sensitivity. Suitable examples of sensitizer are listed below, however it is not intended to be limited to them. These sensitizers can be used alone or used by mixing together. Stearic acid, stearamide, palmitic acid amide, oleic acid amide, behenic acid, ethylenebisstearamide, coconut fatty acid amide, montan wax, polyethylene wax, phenyl- α-naphthylcarbonate, di-p-tolylcabonate, diphenylcarbonate, 4-biphenyl-p-tolylether, p-benzylbiphenyl, m-terphenyl, triphenylmethane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,2-bis(3-methylphenoxy)ethane, 1,2-bisphenoxyethane, 1,2-bis(4-methylphenoxy)ethane, 1,4-bisphenoxybutane, 1,4-bisphenoxybutene, 2-naphthylbenzyl ether, 1,4-diethoxynaphthalene, 1,4-dimethoxynaphthalene, phenyl 1-hydroxy-2-naphthoate, methyl 1-hydroxy-2-naphthoate, methyl 1-hydroxy-2-naphthoate, phenyl 2-naphthoate, benzyl p-benzyloxybenzoate, dibenzyl terephthalate, dimethyl terephthalate, 1,1-diphenylethanol, 1,1-diphenyl-2-propanol, 1,3-diphenoxy-2-propanol, p-(benzyloxy)benzylalcohol, normal octadecylcarbamoyl-p-methoxycarbonylbenzene, normal octadecylcarbamoylbenzene. In this invention, various stabilizers can be added to inprove the stability of the recorded image. Suitable examples of stabilizers are listed below, however it is not intended to be limited to them. Zinc stearate, aluminum stearate, calcium stearate, zinc palmitate, zinc behenate; metallic salt of p-chlorobenzoic acid (Zn, Ca), metallic salt of monobenzyl phthalate (Zn, Ca) and 4,4'-isopropylidene bis(3-methyl-6-tert-butyl)phenol. As a binder used in the thermal sensitive recording medium of this invention, well known compounds can be used. Suitable examples of binders are listed below, however it is not intended to be limited to them. Full saponificated polyvinylalcohol whose degree of polymerization is smaller than 2000, partially saponificated polyvinylalcohol, carboxy modified polyvinylalcohol, amide modified polyvinylalcohol, sulfonic acid modified polyvinylalcohol, other kind of modified polyvinylalcohol, cellulose derivatives such as hydroxyethylcellulose, methyl cellulose, carboxymethyl cellulose and acetyl cellulose, polymer or co-polymer such as casein, gelatin, styrene/maleic anhydride copolymer, styrene/butadiene copolymer, styrene, vinyl acetate, acrylamide and acrylic acid ester, polyamide resin, silicon resin, petroleum resin, terpene resin, ketone resin, coumarone resin and others. Above mentioned natural and synthetic high molecular compounds are used by dissolving in water or organic solvents such as alcohol, or emulsified or dispersed in an emulsion or a paste-like state. They can be used alone or in combination. As a filler to be used in this invention, clay, calcined clay, diatomaceous earth, talc, kaolin, calcium carbonate, basic magnesium carbonate, barium sulfate, barium carbonate, aluminum hydroxide, zinc oxide, silica, magnesium hydroxide, titanium oxide, urea-formaldehyde resin, polystyrene resin, phenol resin and other natural or synthetic, inorganic or organic fillers can be mentioned, however it is not intended to be limited to them. These fillers can be used alone or used in combination. In addition to the above, it is further possible to use an ultraviolet ray absorber, a defoaming agent, a fluorescence paint, a water resistance agent and a slip agent as an additive, however it is not intended to be limited to them. The amount of dye precursor and color developer, and amount and type of other main components used in the thermally sensitive recording medium of this invention are determined in acoordance with the required quality and the recording adaptability and are not specially limited, however it is usually preferable to use from 1 to 8 parts of color developer, from 1 to 20 parts of fillers to 1 part of dye precursor, and from 10 to 25 % of binders in an amount of total solid is preferably used. As a suitable substrate for the thermal sensitive recording medium of this invention, a high quality paper, a middle quality paper, a coated paper, a synthetic paper or a plastic film can be mentioned, however, the present invention is not limited to them. Further, for the purpose of improving the preservative stability, an overcoat layer comprising high molecular compound can be prepared on the thermally sensitive color developing layer. Furthermore, for the purpose of improving both preservation and sensitivity, an undercoat layer containing an organic or an inorganic filler can be prepared between the color developing layer and the substrate. EXAMPLES Examples for synthesis of the poly urea compound used in this invention and Examples for preparation of the thermal sensitive recording medium are illustrated below, however it is not intended to be limited to the Examples. -Synthesis of polyurea compounds [Synthetic Example 1] Synthesis of poly urea compound (A-01) by MDI and 4,4'-diaminodiphenylmethane 3.0 g of 4,4'-diaminodiphenylmethane is dissolved in 20 ml of acetone anhydride. The solution prepared by dissolving 3.75 g of MDI in 20 ml of acetone anhydride is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by a vacuum desiccator and 6.22g of white solid (A-01) is obtained (yield 92%). The obtained solid is heated and molten at the temperature higher than a decomposition point or a melting point. The confirmation test whether the molten compound indicates a property of spinnability is carried out by sticking a glass bar to the molten compound, by pulling up the bar and by observing the formation of fine filaments. Further, the 0.2 g/dl solution of this compound in 95% concentrated sulfuric acid is prepared and the viscosity of this solution is measured by Canon-Fenske viscometer (Shibata Kagaku Kiki Industries, based on JIS K2283 method) at 25°C. In continued synthetic Examples, the spinnability and viscosity of obtained compound are measured by same procedure. And the spinnability is estimated as follows. That is when the white solid becomes viscous liquid by heating and fine fibers are observed the spinnability is estimated as "good" and when the white solid changes to yellow, brown or black color by heating and smoke is observed, then ash or charcoal remains the spinnability is estimated as "poor" . <Decomposition point> Higher than 300°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3306, 3019, 1649, 1595, 1540, 1508, 1407, 1304, 1229, 1199, 1178, 810, 501 <Spinnability> poor <Viscosity> 19.9 mPa.s [Synthetic Example 2] Synthesis of poly urea compound (A-02) by MDI and 1,2-ethylenediamine 1.92g of 1,2-ethylenediamine is dissolved in 52 ml of dimethylformamide. The solution prepared by dissolving 8.0 g of MDI in 100 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by methanol. Then dried up by vacuum desiccator and 9.70g of white solid(A-02) is obtained (yield 98%). The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 290 ∼ 292°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3307, 3111, 3028, 2925, 1639, 1592, 1557, 1542, 1510, 1408, 1305, 1228, 1108, 1017, 864, 817, 771, 666, 619, 508 <Spinnability> poor <Viscosity> 20.6 mPa.s [Synthetic Example 3] Synthesis of poly urea compound (A-03) by MDI and 1,6-hexamethylenediamine 1.86 g of 1,6-hexamethylenediamine is dissolved in 40 ml of dimethylacetamide. The solution prepared by dissolving 4.00 g of MDI in 40 ml of dimethylacetamide is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 4.65 g (yield 79%) of white solid (A-03) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 260 ∼ 270°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3314, 2929, 2851, 1639, 1596, 1541, 1510, 1411, 1307, 1236 <Spinnability> good <Viscosity> 20.3 mPa.s [Synthetic Example 4] Synthesis of poly urea compound (A-04) by MDI and 1,12-dodecanediamine 4.48 g of 1,12-dodecanediamine is dissolved in 120 ml of chloroform. The solution prepared by dissolving 5.6 g of MDI in 70 ml of chloroform is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by methanol. Then dried up by vacuum desiccator and 9.18 g (yield 91%) of white solid (A-04) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 254 ∼ 256°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3322, 3113, 3031, 2923, 2851, 1650, 1597, 1557, 1511, 1408, 1309, 1231, 1109, 1068, 1018, 814, 773, 720, 652, 508 <Spinnability> good <Viscosity> 20.9 mPa.s [Synthetic Example 5] Synthesis of poly urea compound (A-05) by MDI and 1,2-propanediamine 2.37 g of 1,2-propanediamine is dissolved in 64 ml of dimethylformamide. The solution prepared by dissolving 8.0 g of MDI in 100 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by methanol. Then dried up by vacuum desiccator and 10.2 g (yield 99%) of white solid (A-05) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 274 ∼ 276°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3316, 3115, 3030, 2970, 2925, 1651, 1597, 1544, 1511, 1409, 1312, 1229, 1107, 815, 762, 664, 509 <Spinnability> poor <Viscosity> 20.3 mPa.s [Synthetic Example 6] Synthesis of poly urea compound (A-06) by MDI and 2-methyl-1,5-diaminopentane 2.97 g of 2-methyl-1,5-diaminopentane is dissolved in 80 ml of dimethylformamide. The solution prepared by dissolving 8.0 g of MDI in 100 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by methanol. Then dried up by vacuum desiccator and 8.41 g (yield 90%) of white solid (A-06) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 250 ∼ 270°C. <IR spectrum> (by KBr pellet method, cm-1) 3378, 3116, 3030, 2925, 2867, 1652, 1598, 1558, 1541, 1508, 1408, 1308, 1229, 1107, 1018, 814, 771, 667, 508 <Spinnability> good <Viscosity> 20.5 mPa.s [Synthetic Example 7] Synthesis of poly urea compound (A-07) by MDI and 1,2-diaminocyclohexane 2.92 g of 1,2-diaminocyclohexane is dissolved in 79 ml of dimethylformamide. The solution prepared by dissolving 6.4 g of MDI in 80 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of small amount of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by methanol. Then dried up by vacuum desiccator and 9.03 g (yield 97%) of white solid (A-07) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 272 ∼ 280°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3320, 3119, 3029, 2930, 2856, 1654, 1599, 1545, 1511, 1409, 1313, 1228, 1109, 814, 761, 662, 509 <Spinnability> poor <Viscosity> 20.0 mPa.s [Synthetic Example 8] Synthesis of poly urea compound (A-08) by MDI and 4,4'-diaminodicyclohexylmethane 4.71 g of 4,4'-diaminodicyclohexylmethane is dissolved in 130 ml of dimethylformamide. The solution prepared by dissolving 5.6 g of MDI in 70 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by methanol. Then dried up by vacuum desiccator and 10.0 g (yield 97%) of white solid (A-08) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 285 ∼ 292°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3421, 3030, 2924, 2852, 1654, 1558, 1541, 1520, 1455, 1409, 1316, 1226, 1124, 1036, 818, 762, 659, 507 <Spinnability> good <Viscosity> 19.6 mPa.s [Synthetic Example 9] Synthesis of poly urea compound (A-09) by MDI and ethyleneglycolbis(3-aminopropylether) 3.95 g of ethyleneglycolbis(3-aminopropylether) is dissolved in 100 ml of dimethylformamide. The solution prepared by dissolving 5.60 g of MDI in 70 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 9.40 g (yield 98%) of white solid (A-09) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 245°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3310, 3114, 3046, 3032, 2861, 1650, 1636, 1597, 1558, 1541, 1508, 1407, 1302, 1233, 1104, 1018, 809, 773, 621, 505 <Spinnability> good <Viscosity> 21.7 mPa.s [Synthetic Example 10] Synthesis of poly urea compound (A-10) by MDI and 3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5,5]undecane 5.27 g of 3,9-bis(3-aminopropyl)-2,4,8,10-tetraoxaspiro[5,5]undecane is dissolved in 140 ml of dimethylformamide. The solution prepared by dissolving 4.80 g of MDI in 60 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by methanol. Then dried up by vacuum desiccator and 9.80 g (yield 97%) of white solid (A-10) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 240°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3387, 2922, 2853, 1653, 1601, 1558, 1541, 1508, 1457, 1408, 1310, 1233, 1167, 1149, 941, 667,511 <Spinnability> good <Viscosity> 19.3 mPa.s [Synthetic Example 11] Synthesis of poly urea compound (A-11) by MDI and p-xylylenediamine 3.49 g of p-xylylenediamine is dissolved in 90 ml of dimethylformamide. The solution prepared by dissolving 6.40 g of MDI in 80 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. Stirred for 2 hours at the room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 9.39g (yield 99%) of white solid(A-11) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 280°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3294, 3121, 3027, 2919, 2875, 1653, 1558, 1541, 1507, 1405, 1302, 1221, 1095, 1052, 1016, 806, 760, 657, 614, 544, 502 <Spinnability> good <Viscosity> 19.4 mPa.s [Synthetic Example 12] Synthesis of poly urea compound (A-12) by MDI and m-phenylenediamine 2.42 g of m-phenylenediamine is dissolved in 65 ml of chloroform. The solution prepared by dissolving 5.61 g of MDI in 70 ml of chloroform is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at the room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 7.42 g (yield 92%) of white solid (A-12) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> higher than 300°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3300, 3030, 1646, 1598, 1542, 1512, 1490, 1407, 1302, 1215, 1203, 1107, 1017, 855, 774, 750, 687, 666 <Spinnability> good <Viscosity> 21.1 mPa.s [Synthetic Example 13] Synthesis of poly urea compound (A-13) by MDI and 4,4'-thiodianiline 4.85 g of 4,4'-thiodianiline is dissolved in 130 ml of dimethylformamide. The solution prepared by dissolving 5.60g of MDI in 70 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 7.29 g (yield 70%) of white solid (A-13) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> Higher than 300°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3301, 3029, 1646, 1592, 1538, 1510, 1491, 1409, 1396, 1306, 1233, 1177, 1107, 1083, 1014, 816, 769, 638, 508 <Spinnability> poor <Viscosity> 20.6 mPa.s [Synthetic Example 14] Synthesis of poly urea compound (A-17) by MDI and 3,3'-diethyl-4,4'-diaminodiphenylmethane 4.07 g of 3,3'-diethyl-4,4'-diaminodiphenylmethane is dissolved in 110 ml of chloroform. The solution prepared by dissolving 4.00 g of MDI in 50 ml of chloroform is dropped into said solution in nitrogen gas atmosphere. During the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 8.01 g (yield 99%) of white solid (A-17) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 270°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3286, 3124, 3027, 2962, 2927, 2871, 1653, 1593, 1539, 1507, 1408, 1296, 1238, 1197, 1097, 1056, 1017, 810, 753, 660 <Spinnability> good <Viscosity> 22.0 mPa.s [Synthetic Example 15] Synthesis of poly urea compound (A-18) by MDI and 4,4'-diaminodiphenylthiourea 4.96 g of 4,4'-diaminodiphenylthiourea is dissolved in 130 ml of dimethylacetoamide. The solution prepared by dissolving 4.8 g of MDI in 60 ml of dimethylacetamide is dropped into said solution in nitrogen gas atmosphere. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 9.70 g (yield 99%) of white solid (A-18) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 260°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3282, 3031, 2927, 1663, 1602, 1507, 1408, 1305, 1227, 1195, 1112, 1015, 829, 745, 718, 508 <Spinnability> poor <Viscosity> 22.0 mPa.s [Synthetic Example 16] Synthesis of poly urea compound (A-21) by 2,4-TDI and 1,6-hexamethylenediamine 2.67 g of 1,6-hexamethylenediamine is dissolved in 40 ml of dimethylformamide. The solution prepared by dissolving 3.29 ml of 2,4-TDI in 40 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. Immediately after the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 5.41 g (yield 81%) of white solid (A-21) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 230 ∼ 245°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3326, 2930, 2856, 1633, 1546, 1446, 1413, 1215, 1011, 649, 591 <Spinnability> good <Viscosity> 20.7 mPa.s [Synthetic Example 17] Synthesis of poly urea compound (A-23) by 2,4-TDI and 4,4'-diaminodiphenylmethane 3.42 g of 4,4'-diaminodiphenylmethane is dissolved in 20 ml of acetone anhydride. The solution prepared by dissolving 2.47 ml of 2,4-TDI in 20 ml of acetone anhydride is dropped into said solution in nitrogen gas atmosphere. Immediately after the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 6.14 g (yield 96%) of white solid (A-23) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> Higher than 300°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3293, 2272, 1645, 1596, 1540, 1510, 1409, 1304, 1218, 1203, 810, 662, 507 <Spinnability> good <Viscosity> 20.1 mPa.s [Synthetic Example 18] Synthesis of poly urea compound (A-24) by 2,4-TDI and 4,4'-diaminodiphenylthiourea 4.00 g of 4,4'-diaminodiphenylthiourea is dissolved in 40 ml of dimethylformamide. 2.22 ml of 2,4-TDI is dropped into said solution in nitrogen gas atmosphere. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 6.65 g (yield 99%) of white solid (A-24) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 250°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3400, 1653, 1607, 1539, 1508, 1407, 1307, 1214, 1125, 1016, 832, 668 <Spinnability> poor <Viscosity> 23.0 mPa.s [Synthetic Example 19] Synthesis of poly urea compound (A-27) by 2,6-TDI and 1,6-hexamethylenediamine 2.67 g of 1,6-hexamethylenediamine is dissolved in 40 ml of dimethylformamide. The solution prepared by dissolving 4.00 g of 2,6-TDI in 40 ml of dimethylformamide is dropped into said solution in nitrogen gas atmosphere. Immediately after the dropping the generation of white precipitation is observed. Stirred for 2 hours at room temperature. After the reaction, the obtained fluid is thrown into 500 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 6.34 g (yield 95%) of white solid (A-27) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> Higher than 250°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3320, 2930, 2857, 1636, 1558, 1472, 1438, 1294, 1241, 1066, 783, 668 <Spinnability> good <Viscosity> 20.8 mPa.s [Synthetic Example 20] Synthesis of poly urea compound (A-31) by HDI and 1,6-hexamethylenediamine 3.45 g of 1,6-hexamethylenediamine is dissolved in 93 ml of methylethylketone. The solution prepared by dissolving 5.00 g of HDI in 63 ml of methylethylketone is dropped into said solution in nitrogen gas atmosphere. Immediately after the dropping the generation of white precipitation is observed. Stirred for 1 hours at room temperature. After the reaction, the obtained fluid is thrown into 400 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 5.32 g (yield 63%) of white solid (A-31) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 274 ∼ 276°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3358, 3136, 2933, 2856, 1628, 1571, 1477, 1461, 1251, 1214, 1074, 625, 603 <Spinnability> good <Viscosity> 20.2 mPa.s [Synthetic Example 21] Synthesis of poly urea compound (A-39) by HDI and 4,4'-diaminobenzanilide 4.05 g of 4,4'-diaminobenzanilide is dissolved in 110 ml of methylethylketone. The solution prepared by dissolving 3.00 g of HDI in 40 ml of methylethylketone is dropped into said solution in nitrogen gas atmosphere. Immediately after the dropping the generation of white precipitation is observed. Stirred for 1 hours at room temperature. After the reaction, the obtained fluid is thrown into 400 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 4.73 g (yield 67%) of white solid (A-39) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> Higher than 300°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3310, 2930, 2856, 1641, 1607, 1556, 1512, 1403, 1309, 1231, 1181, 1109, 835, 761, 666, 636, 523 <Spinnability> good <Viscosity> 20.0 mPa.s [Synthetic Example 22] Synthesis of poly urea compound (A-44) by NDI and 1,6-hexamethylenediamine 2.64 g 1,6-hexamethylenediamine of is dissolved in 71 ml of methylethylketone. The solution prepared by dissolving 5.04 g of NDI in 63 ml of methylethylketone is dropped into said solution in nitrogen gas atmosphere. Immediately after the dropping the generation of white precipitation is observed. Stirred for 1 hours at room temperature. After the reaction, the obtained fluid is thrown into 400 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 5.99 g (yield 77%) of white solid (A-44) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> Higher than 300°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3315, 3114, 3069, 2929, 2856, 1634, 1558, 1543, 1418, 1329, 1239, 779, 668 <Spinnability> poor <Viscosity> 20.2 mPa.s [Synthetic Example 23] Synthesis of poly urea compound (A-57) by isopholonediisocyanate and m-phenylenediamine 2.43 g of m-phenylenediamine is dissolved in 66 ml of methylethylketone. The solution prepared by dissolving 5.00 g of isopholonediisocyanate in 63 ml of methylethylketone is dropped into said solution in nitrogen gas atmosphere. Stirred for 1 hours at room temperature. After the reaction, the obtained fluid is thrown into 400 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 2.87 g (yield 39%) of white solid (A-57) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 287 ∼ 290°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3376, 2951, 2916, 1656, 1606, 1543, 1490, 1304, 1228, 866, 777, 690 <Spinnability> good <Viscosity> 20.2 mPa.s [Synthetic Example 24] Synthesis of poly urea compound (A-71) by dicyclohexylmethane-4,4'-diisocyanate and 2,4-diaminotoluene 4.01 g of 2,4-diaminotoluene is dissolved in 108 ml of methylethylketone. The solution prepared by dissolving 6.03 g of dicyclohexylmethane-4,4'-diisocyanate in 75 ml of methylethylketone is dropped into said solution in nitrogen gas atmosphere. Immediately after the dropping the generation of white precipitation is observed. Stirred for 1 hour at room temperature. After the reaction, the obtained fluid is thrown into 400 ml of methanol and the generated precipitation is separated by filtration and rinsed by acetone. Then dried up by vacuum desiccator and 5.50 g (yield 62%) of white solid (A-71) is obtained. The confirmation test of spinnability and the measurement of viscosity are carried out same as to the Synthetic Example 1. <Decomposition point> 283 ∼ 290°C. <IR spectrum> (by KBr pellet method, cm -1 ) 3344, 2923, 2851, 1647, 1596, 1538, 1448, 1413, 1377, 1308, 1275, 1222, 1129, 894, 812, 663 <Spinnability> good <Viscosity> 19.4 mPa.s Test results of viscosity and spinnability of Synthetic Examples Synthetic Example viscosity spinnability 1 19.9 ×(poor) 2 20. 6 × 3 20.3 O(good) 4 20.9 O 5 20.3 × 6 20.5 O 7 20. 0 × 8 19.6 O 9 21.7 O 10 19.3 O 11 19.4 O 12 21.1 O 13 20.6 × 14 22.0 O 15 22. 0 × 16 20.7 O 17 20.1 O 18 23. 0 × 19 20.8 O 20 20.2 O 21 20.0 O 22 20.2 × 23 20.2 O 24 19.4 O -Fabrication of thermal sensitive recording medium- [Examples 1 ∼ 48] The thermal sensitive recording medium composed by following components are fabricated. As the first step, a dye dispersion (liquid A), a color developer dispersion (liquid B) and a poly urea dispersion (liquid C) are separately ground to average particles diameter of 1µ m by a sand grinder. (liquid A : dispersion of dye) 3-N,N-diethylamino-6-methyl-7-anilinofluoran 2.0 parts 10% aqueous solution of polyvinyl alcohol 4.6 parts water 2.6 parts (liquid B : dispersion of color developer) color developer (refer to Table 1) 6.0 parts 10% aqueous solution of polyvinyl alcohol 18.8 parts water 11.2 parts (liquid C : dispersion of poly urea) compound used in this invention (refer to Table 1) 4.0 parts 10% aqueous solution of polyvinyl alcohol 12.5 parts water 7.5 parts Then, a thermal sensitive coating is prepared by mixing liquid A, liquid B, liquid C and a dispersion of kaolin clay by following combination ratio. Liquid A : dispersion of dye 9.2 parts Liquid B : dispersion of color developer 36.0 parts Liquid C : dispersion of poly urea 24.0 parts Kaolin clay (50% aqueous dispersion) 12.0 parts The prepared thermal sensitive coating is coated over the one side surface of 50 g/m 2 base paper, dried and super calendered to a flatness of 500 to 600 seconds to obtain a thermal sensitive recording medium with a coating amount of 6.0 to 6.5 g/m2. In above explanations, parts and % respectively indicate parts by weight and weight %. [Comparative Examples 1 ∼ 2] A thermal sensitive coating without (liquid C : dispersion of poly urea) is prepared, and thermal sensitive recording media are prepared by the same procedure as in Examples 1 ∼ 48. - Evaluation methods ofthe thermal sensitive recording media - [Method for color developing] Thermal recording is carried out on the prepared thermal sensitive recording media using an UBI Printer 201 (UBI) at an application energy of 450 mJ/mm 2 . Then the recording density of recording part and blank part are measured by a Macbeth densitometer (RD-914, amber filter used). Following tests are carried out on the specimen obtained as above. [Plasticizer resistance test] : Specimen for test is contacted to a polyvinylchloride film (DIAWRAP 300G, product of Mitsubishi Resin), allowed to leave alone for 4 hours at 40°C and the density of recorded part is measured by a Macbeth densitometer. [Oil resistance test] : Specimen for test is dipped into salad oil for 1 hour, then wiped off, allowed to leave alone for 24 hours in room temperature and the density of recorded part is measured by a Macbeth densitometer. [Water resistance test] : Specimen for test is dipped into city water for 24 hours, dried at 30°C for 2 hours then the density of recorded part is measured by a Macbeth densitometer. The combination ratio of image preservative stability tests are summarized in Table 2 and the obtained results are shown in Table 3. In Table 3, the bigger value of Macbeth densitometer indicates good image preservative stability. combination of image preservative stability test No. color developer compound used in this invention Ex. 1 4-hydroxy-4'-isopropoxydiphenyl sulfone A-01 Ex. 2 2,2-bis(4-hydroxyphenyl)propane A-01 Ex. 3 4-hydroxy-4'-isopropoxydiphenyl sulfone A-02 Ex. 4 2,2-bis(4-hydroxyphenyl)propane A-02 Ex. 5 4-hydroxy-4'-isopropoxydiphenyl sulfone A-03 Ex. 6 2,2-bis (4-hydroxyphenyl) propane A-03 Ex. 7 4-hydroxy-4'-isopropoxydiphenyl sulfone A-04 Ex. 8 2,2-bis(4-hydroxyphenyl)propane A-04 Ex. 9 4-hydroxy-4'-isopropoxydiphenyl sulfone A-05 Ex. 10 2,2-bis(4-hydroxyphenyl)propane A-05 Ex. 11 4-hydroxy-4'-isopropoxydiphenyl sulfone A-06 Ex. 12 2,2-bis(4-hydroxyphenyl)propane A-06 combination of image preservative stability test No. color developer compound used in this invention Ex. 13 4-hydroxy-4'-isopropoxydiphenyl sulfone A-07 Ex. 14 2,2-bis(4-hydroxyphenyl)propane A-07 Ex. 15 4-hydroxy-4'-isopropoxydiphenyl sulfone A-08 Ex. 16 2,2-bis(4-hydroxyphenyl)propane A-08 Ex. 17 4-hydroxy-4'-isopropoxydiphenyl sulfone A-09 Ex. 18 2,2-bis(4-hydroxyphenyl)propane A-09 Ex. 19 4-hydroxy-4'-isopropoxydiphenyl sulfone A-10 Ex. 20 2,2-bis(4-hydroxyphenyl)propane A-10 Ex. 21 4-hydroxy-4'-isopropoxydiphenyl sulfone A-11 Ex. 22 2,2-bis(4-hydroxyphenyl)propane A-11 Ex. 23 4-hydroxy-4'-isopropoxydiphenyl sulfone A-12 Ex. 24 2,2-bis(4-hydroxyphenyl)propane A-12 combination of image preservative stability test No. color developer compound used in this invention Ex. 25 4-hydroxy-4'-isopropoxydiphenyl sulfone A-13 Ex. 26 2,2-bis(4-hydroxyphenyl)propane A-13 Ex. 27 4-hydroxy-4'-isopropoxydiphenyl sulfone A-17 Ex. 28 2,2-bis(4-hydroxyphenyl)propane A-17 Ex. 29 4-hydroxy-4'-isopropoxydiphenyl sulfone A-18 Ex. 30 2,2-bis(4-hydroxyphenyl)propane A-18 Ex. 31 4-hydroxy-4'-isopropoxydiphenyl sulfone A-21 Ex. 32 2,2-bis(4-hydroxyphenyl)propane A-21 Ex. 33 4-hydroxy-4'-isopropoxydiphenyl sulfone A-23 Ex. 34 2,2-bis(4-hydroxyphenyl) propane A-23 Ex. 35 4-hydroxy-4'-isopropoxydiphenyl sulfone A-24 Ex. 36 2,2-bis(4-hydroxyphenyl)propane A-24 combination of image preservative stability test No. color developer compound used in this invention Ex. 37 4-hydroxy-4'-isopropoxydiphenyl sulfone A-27 Ex. 38 2,2-bis(4-hydroxyphenyl)propane A-27 Ex. 39 4-hydroxy-4'-isopropoxydiphenyl sulfone A-31 Ex. 40 2,2-bis(4-hydroxyphenyl)propane A-31 Ex. 41 4-hydroxy-4'-isopropoxydiphenyl sulfone A-39 Ex. 42 2,2-bis(4-hydroxyphenyl)propane A-39 Ex. 43 4-hydroxy-4'-isopropoxydiphenyl sulfone A-44 Ex. 44 2,2-bis(4-hydroxyphenyl)propane A-44 Ex. 45 4-hydroxy-4'-isopropoxydiphenyl sulfone A-57 Ex. 46 2,2-bis(4-hydroxyphenyl)propane A-57 Ex. 47 4-hydroxy-4'-isopropoxydiphenyl sulfone A-71 Ex. 48 2,2-bis(4-hydroxyphenyl)propane A-71 Compar. Ex. 1 4-hydroxy-4'-isopropoxydiphenyl sulfone none Compar. Ex. 2 2,2-bis(4-hydroxyphenyl)propane none Test results of image preservation stability test No. color density plasticiser resistance oil resistance water resistance Example 1 1. 39 1. 25 1. 10 1. 29 Example 2 1. 36 1. 23 1. 09 1. 29 Example 3 1. 35 1. 20 1.10 1. 25 Example 4 1. 32 1.18 1.10 1.22 Example 5 1. 34 1. 29 1.14 1.22 Example 6 1. 33 1. 29 1.15 1.21 Example 7 1.32 1.25 1.10 1.15 Example 8 1.30 1.25 1.11 1.10 Example 9 1.30 1.20 1.11 1.16 Example 10 1.24 1.13 1.05 1.09 Example 11 1.29 1.18 1.12 1.12 Example 12 1.28 1.11 1.09 1.13 Example 13 1.25 1.15 1.04 1.10 Example 14 1.25 1.15 1.03 1.10 Example 15 1.19 1.02 1.01 1.11 Example 16 1.18 1. 00 0. 98 1. 08 Example 17 1.26 1.20 1.12 1.15 Example 18 1.27 1.21 1.10 1.13 Example 19 1.18 1.01 1.08 1.10 Example 20 1.18 0.98 1.05 1.10 Example 21 1.10 1.00 1.00 1.10 Example 22 1.09 1.01 0.99 1.03 Example 23 1.16 1.05 1.09 1.12 Example 24 1.10 0.99 1.01 1.03 Test results of image preservation stability test No. color density plasticiser resistance oil resistance water resistance Example 25 1.33 1.28 1.10 1.21 Example 26 1.33 1.27 1.09 1.22 Example 27 1.38 1.22 1.08 1.20 Example 28 1.31 1.20 1.09 1.21 Example 29 1.33 1.15 1.01 1.18 Example 30 1.33 1.14 1.06 1.20 Example 31 1.12 1.12 1.01 1.10 Example 32 1.08 1.00 1.01 1.04 Example 33 1.27 1.10 1.09 1.16 Example 34 1.20 1.05 1.03 1.10 Example 35 1.33 1.14 1.05 1.15 Example 36 1.33 1.12 1.05 1.13 Example 37 1.12 1.08 1.01 1.10 Example 38 1.09 1.01 1.00 1.01 Example 39 0. 91 0. 81 0. 85 0.88 Example 40 0. 87 0. 80 0. 82 0. 83 Example 41 1. 31 1. 20 1. 09 1. 22 Example 42 1.29 1.17 1.10 1.18 Example 43 1.17 1.03 1.01 1.10 Example 44 1.12 1.01 0.98 1.04 Example 45 1.16 1.05 1.01 1.10 Example 46 1.11 1.04 1.00 1.08 Example 47 0. 99 0. 83 0. 86 0. 91 Example 48 0.97 0.81 0.81 0.90 Compar. Example 1 1.46 0.36 0.23 1.24 Compar. Example 2 1.41 0.38 0.33 1.02 As clearly shown from these results, Examples 1 to 48 which contain a poly urea compound as defined herein in a color developing layer, are superior to Comparative Examples 1 and 2 which do not contain poly urea compound, in respect of image preservative stability of the recording part. Since the thermally sensitive recording medium which contains the poly urea compound as defined herein in the thermally sensitive color developing layer is superior in the image preservative stability of the recording part and can be produced at a low price, it provides a very useful and convenient recording medium.

Claims: A thermally sensitive recording medium comprising a substrate having thereon a thermally sensitive color developing layer which comprises (a) a colorless or pale colored dye precursor, (b) a color developer which can react with the dye precursor to develop a color when heated and (c) a polyurea compound which comprises units of formula (1): wherein A 1 represents a divalent group of one of the following formulae: A medium according to claim 1 wherein the polyurea compound comprises units of formula (2): wherein R 1 and R 2 respectively represent an alkyl group, an alkoxy group or an electron accepting group, o and p are an integer from 0 to 4 and A 2 represents a divalent group as defined for A 1 in claim 1. A medium according to claim 1 wherein the polyurea compound comprises units of formula (3): wherein R 3 represents an alkyl group, an alkoxy group or an electron accepting group, q is an integer from 0 to 4 and A 3 represents a divalent group as defined for A 1 in claim 1. A medium according to claim 1 wherein the polyurea compound comprises units of formula (4): wherein r is an integer from 2 to 12 and A 4 represents a divalent group as defined for A 1 in claim 1. A medium according to claim 1 wherein the polyurea compound comprises units of formula (5): wherein A 5 represents a divalent group as defined for A 1 in claim 1. A medium according to claim 1 wherein the polyurea compound comprises units of formula (6): wherein A 6 represents a divalent group as defined for A 1 in claim 1.. A medium according to claim 1 wherein the polyurea compound comprises units of formula (7): wherein R 4 and R 5 respectively represent an alkyl group, an alkoxy group or an electron accepting group, s and t are an integer from 0 to 8 and A 7 represents a divalent group as defined for A 1 in claim 1. A medium according to any one of the preceding claims, wherein the content of polyurea compound is from 0.01 to 2 parts per 1 part color developer.

Inventors: NAKANO TOMOYUKI, OHASHI REIJI, SEKI JUNKO, YANAI KOICHI, YOSHIOKA HIDETOSHI, NAKANO, TOMOYUKI, OHASHI, REIJI, SEKI, JUNKO, YANAI, KOICHI, YOSHIOKA, HIDETOSHI
Applicants: JUJO PAPER CO LTD, NIPPON PAPER INDUSTRIES CO., LTD.
Application year: 1998
Main classification: B41M5
IPC codes: B41M5