Thermosensitive recording material

Abstract: An undercoat composition is prepared by mixing in water 80 - 95 parts by wt. of an oil-absorbable pigment, preferably of oil absorption (JIS-K5101) of 70 ml / 100 g or more (e.g. kaolin), 5 - 20 parts of a binder, e.g. a water-soluble polymer (e.g. starch), and 1 - 5% of the wt. of the pigment of carboxymethyl cellulose of etherification degree 0.6 to 0.8 and mean mol. wt. 20,000 to 200,000, and optionally 0.5 to 10% by wt. of the pigment of a wax (e.g. paraffin wax); the mixture is blade coated preferably by an on-machine coater, in an amount of 1 - 20 g/m² on a paper support (e.g. of base weight 30 - 70 g/m²) which has a Stöckigt sizing degree (JIS P-8122) of 5 to 10 seconds; and the undercoated layer has excellent surface smoothness and high proportion of voids. On the undercoat is coated a known thermosensitive layer, e.g. containing an electron-donating dye precursor and an electron-accepting color developer in a binder, e.g. in an amount of 2 - 7 g/m². The resulting thermosensitive recording material has high sensitivity, high image quality and low adhesion to a thermal head when used for thermosensitive printing.

Description: This invention is directed to thermosensitive recording materials for use in facsimile machines, thermal printers and hot-pen recording systems. More particularly, it relates to a low cost thermosensitive recording material which has a high sensitivity and high image quality. Recording materials containing an electron-donating dye precursor and an electron-accepting compound are well known as materials for use in such applications as pressure-sensitive recording paper, thermosensitive recording paper, photo- and thermosensitive recording paper and electro-thermosensitive recording paper. Such applications are disclosed in detail for example in GB-A.2,140,449, US-A-4,480,052, US-A-4,436,920, JP-B-60-23922 ("JP-B" means an unexamined Japanese patent publication), JP-A-57-179836 ("JP-A" means an "unexamined published Japanese patent application"), JP-A-123556 and JP-A-60-123557. Because of the advance in the development of high speed, low energy recording instruments triggered by the recently expanding use of thermosensitive recording materials in various fields, there is a great demand for a thermosensitive recording material which possesses a high sensitivity and a high image quality, but has a low tendency to cause stains on a thermal head. To satisfy these demands, various techniques have been proposed such as a process in which an undercoat layer is inserted between a support and a thermosensitive recording layer and, as disclosed in JP-A-2-1369, a process in which components of a coating solution and their contents and the like are specified. These prior art processes, however, do not show satisfactory results. The production cost of a thermosensitive recording material greatly depends on its support material. A material having a low Stöckigt sizing degree of 10 seconds or below may be useful as a low cost support. Such a support, however, is apt to cause deterioration of the surface conditions of an undercoat layer, especially when a blade coater is used, and therefore to cause the problem of decreased sensitivity and image quality of the resulting thermosensitive recording material. In view of the above, it is an object of the present invention to provide a thermosensitive recording material, containing a low cost support, which possesses a high sensitivity, a high image quality and a low capacity to adhere stains to a thermal head and can thus be applied to a high speed recording system. EP-A-361501, EP-A-341715, EP-A-329384, EP-A-186375, GB-A-2198856 and GB-A-2183354 each describe thermosensitive recording materials comprising a support, an undercoat layer comprising an oil-absorbing pigment and a thermosensitive recording layer coated on said undercoat layer, and that carboxymethyl cellulose (CMC) can be a component of the intermediate layer. However, none of these publications specify the etherification degree or the mean molecular mass of the carboxymethyl cellulose used. GB 2183354 describes that the paper support is made from pulp containing a sizing agent, but the sizing degree is not mentioned. We have now found that the thermal sensitivity and image quality of a thermosensitive recording material are greatly dependent upon the capacity of the support; and that sufficient thermal sensitivity and image quality can be obtained by employing an undercoat layer containing a specific CMC even when an inexpensive support having a Stöckigt sizing degree of 10 seconds or less is used. According to the present invention a thermosensitive recording material comprises a paper support and coated thereon in order (1) an undercoat layer comprising (a) a pigment capable of absorbing an oil, (b) a binder and (c) a carboxymethyl cellulose in an amount of from 1 to 5% by weight based on the pigment and (2) a thermosensitive recording layer, characterised in that said carboxymethyl cellulose has an etherification degree of from 0.6 to 0.8 and a mean molecular weight of from 20,000 to 200,000, and wherein the paper support has a Stöckigt sizing degree of from 5 to 10 seconds. The Stöckigt sizing degree of the support is measured in accordance with the Japanese Industrial Standard (JIS) P-8122. This test measures the water-resistance of the support, in terms of the time taken for a red color to develop in the test. The base weight of the support is preferably from 30 to 70 g/m² more preferably 35 to 50 g/m². Directly coated on the said support is the undercoat layer, containing an oil-absorbable pigment and a binder as its main components, as well as the specific amount of a specific carboxymethyl cellulose. The undercoat layer preferably contains at least 80 % of oil absorbable pigment and 5 % binder, by weight as solid contents. Illustrative examples of the oil-absorbable pigment include baked kaolin, aluminum oxide, magnesium carbonate, calcium carbonate, amorphous silica, baked diatomaceous earth, aluminum silicate, magnesium aluminosilicate and aluminium hydroxide. The most preferred pigments are those having an oil absorption value (based on JIS-K5101) of 70 ml/100 g or more. A binder for use in the undercoat may be selected from (a) water-soluble polymers such as starch (including modified starch), casein, polyvinyl alcohol, methyl cellulose, hydroxyethyl cellulose, polyacrylic acid and (b) latexes such as a styrene-butadiene copolymer or a methylmethacrylate-butadiene copolymer . The binder may be used generally in an amount of from 7 to 20 parts based on 100 parts of the oil-absorbable pigment, depending on the intended film strength of a coating layer, and the thermal sensitivity of a thermosensitive recording layer . Too much binder is harmful because desired thermal sensitivity cannot be obtained due to insufficient thermal insulation caused by a reduction in the percentage of void in the undercoat layer, though one of the purposes of employing an oil-absorbable pigment is to improve thermal insulation. Also, too much binder may result in adhesion of stains to a thermal head. If the amount of binder is too small, it will have no significant results but rather reduce film strength and adhesion capacity of a coating layer. In addition to said pigment and binder, the coating solution for the undercoat layer contains said carboxymethyl cellulose so as to give fluidity to the coating composition at the time of coating, and consequently excellent surface smoothness and other surface characteristics to the undercoat formed. The aforesaid carboxymethyl cellulose is added to the coating solution in an amount of from 1 to 5% by weight, preferably from 1 to 3% by weight, based on the weight of the oil-absorbable pigment. The presence of the carboxymethyl cellulose in the coating solution permits the use of a support having a low Stöckigt sizing degree of 5 to 10 seconds. If the amount of the carboxymethyl cellulose were less than 1% by weight, the effect of the oil-absorbable pigment in giving fluidity to the coating solution under a high shearing stress will deteriorate markedly, which will cause various defects on the surface of the layer such as streaking and stalactites. Such troubles have occurred frequently in the prior art, especially when a support having a low Stöckigt sizing degree is used, as in the present invention. Addition of more than 5% by weight of the carboxymethyl cellulose would increase static viscosity, reducing workability of the composition. Further, electrolytic corrosion of a thermal head is exacerbated by the increased sodium ion content. To prepare the coating solution for the undercoat layer, the oil-absorbable pigment may be added while stirred to water with a dispersing agent so as to form a dispersion, and then the binder may be added. The resultant undercoat layer has an excellent surface smoothness and other surface characteristics, and a high percentage of void (due to the effect of the oil-absorbable pigment). Because of these excellent properties, unlike an undercoat layer without such a carboxymethyl cellulose, the undercoat layer in the present invention can contribute greatly to the production of a thermosensitive recording material which has a high thermal sensitivity and a high image quality and is almost free from adhesion of stains to a thermal head. A further improved effect can be obtained by adding a wax to the undercoat layer in the material of the invention. The addition of a wax increases the water-retentivity of a coating solution for making a thermosensitive recording layer coated on the undercoat layer. An increase in the water-retentivity renders possible the formation of a homogeneous thermosensitive recording layer. Such an embodiment, therefore, is especially preferable in the present invention. As such a wax, paraffin wax is most preferred, in an amount from 0.5 to 10% by weight based on the oil-absorbable pigment, more preferably from 1.5 to 5% by weight. If necessary, other pigments, dispersants or various auxiliaries may be added to the coating solution for the undercoat layer, provided that these additives are used in amounts which do not impair the properties of the resulting thermosensitive recording material. Coverage of the undercoat layer, though not especially limited, may preferably be adjusted in the range of about 1 to 20 g/m², more preferably 5 to 15 g/m², depending on the desired properties of the thermosensitive recording material. Blade coating techniques for use in coating the undercoat layer in the present invention include not only a method in which a bevel type blade or a vent type blade is employed but also a rod blade coating method and a billblade coating method . Blade coating may be effected by the use of not only an off machine coater but also an on-machine coater attached to a paper machine. Such an on-machine coater is especially preferable for use in the present invention, because the paper support is subjected to the coating step before the support exhibits its sizing effect. In the present invention, there is no special limitation with regard to the combination of coloring components in a thermosensitive recording layer which is coated on the undercoat layer. For example, any combination of coloring components can be used, provided that these compounds can contact each other and show a coloring reaction when they are exposed to heat. Illustrative examples of such combinations include a combination of an electron-donating dye precursor with an electron-accepting compound, a combination of a higher fatty acid metal salt such as ferric stearate with a phenol such as gallic acid and a combination of a diazonium compound with a coupler and a base compound Among these combinations, combinations of an electron-donating dye precursor (a color former) and an electron-accepting compound (a color developer) may be most preferable for the present invention in view of the thermal sensitivity and image quality they afford. A color former for use in the present invention may be selected from various known compounds such as triarylmethane compounds, diphenylmethane compounds, xanthene compounds, thiazine compounds and spiropyran compounds. Illustrative examples of these compounds are disclosed for instance in JP-A-55-227253 which include: triarylmethane compounds such as 3,3- bis ( p -dimethylaminophenyl)-6-dimethylamino phthalide, 3,3- bis ( p -dimethylaminophenyl) phthalide, 3-( p -dimethylaminophenyl)-3-(1,3 dimethylindole-3-yl) phthalide and 3-( p -dimethylaminophenyl)-3-(2-methylindole3-yl)phthalide, diphenylmethane compounds such as 4,4- bis -dimethylaminobenzhydrin benzyl ether, N-halophenyl leucoauramine and N-2,4,5-trichlorophenyl leucoauramine; xanthene compounds such as Rhodamine B anilinolactam, Rhodamine-( p -nitrino) lactam, 2-(dibenzylamino) fluoran, 2-anilino-3-methyl-6-diethylaminofluoran, 2-anilino-3-methyl-6-dibutylaminofluoran, 2-anilino-3-methyl-6-N-ethyl-N-isoamylaminofluran, 2-anilino-3-methyl-6-N-methyl-N-cyclohexylaminofluran, 2-anilino-3-chloro-6-diethylaminofluoran, 2-anilino-3-methyl-6-N-ethyl-N-isobutylfluoran; 2-anilino-6-dibutylaminofluoran, 2-anilino-3-methyl-6-N-methyl-N-tetrahydrofurfurylaminofluoran, 2-anilino-3-methyl-6-piperidinoaminofluoran, 2-( o -chloroanilino)-6-diethylaminofluoran and 2-(3,4-dichloroanilino)-6-diethylaminofluoran; thiazine compounds such as benzoyl Leucomethylene Blue, p -nitrobenzyl Leucomethylene Blue ; spiropyran compounds such as 3-methyl- spiro -dinaphthopyran, 3-ethyl- spiro -dinaphthopyran, 3-3'-dichloro- spiro -dinaphthopyran, 3-benzyl- spiro -dinaphthopyran, 3-methyl-naphtho-(3-methoxybenzo)-spiropyran ant 3-propyl- spiro -dibenzopyran; and many other related compounds. A color developer for use in the present invention may be selected from various compounds, but preferably from phenolic compounds, salicylic acid derivatives and polyvalent metal salts thereof, to prevent surface fogging. Illustrative examples of such phenolic compounds include 2,2'-bis(4-hydroxyphenyl) propane (namely, bisphenol A), 4-t-butylphenol, 4-phenylphenol, 4-hydroxydiphenoxide, 1,1'bis(4-hydroxyphenyl) cyclohexane, 1,1'- bis (3-chloro-4-hydroxyphenyl) cyclohexane, 1,1'- bis (3-chloro-4-hydroxyphenyl)2-ethyl butane, 4,4'- sec -isooctylidene diphenol, 4,4'-sec-butylidene diphenol, 4- tert -octyl phenol, 4-p-methylphenyl phenol, 4,4'-methylcyclohexylidene phenol, 4,4'-isopentylidene phenol and benzyl p -hydroxybenzoate. Illustrative examples of the salicylic acid derivatives include 4-pentadecyl salicylic acid, 3,5-di(α-methylbenzyl) salicylic acid, 3,5-di( tert -octyl) salicylic acid, 5-octadecyl salicylic acid, 5-α-( p -α-methylbenzylphenyl) ethyl salicylic acid, 3-α-methylbenzyl-5-tert-octyl salicylic acid, 5-tetradecyl salicylic acid, 4-hexyloxy salicylic acid, 4-cyclophexyloxy salicylic acid, 4-decyloxy salicylic acid, 4-dodecyloxy salicylic acid, 4-pentadecyloxy salicylic acid and 4-octadecyloxy salicylic acid and zinc, aluminum, calcium, copper and lead salts of these salicylic compounds. These color developers may preferably be used in an amount of from 50 to 800% by weight based on the color former, more preferably from 100 to 500% by weight. An amount of less than 50% would cause insufficient color development and the addition of more than 800% would provide no proportionally greater effect. For the purpose of improving the thermal response of the thermosensitive recording material of the present invention, a heat-fusible compound may be included in the thermosensitive recording layer. Illustrative examples of the heat fusible compound appropriate for the present invention include benzyl p -benzyloxybenzoate, β-naphthylbenzyl ether, stearic acid amide, stearylurea, p -benzylbiphenyl, di(2-methylphenoxy) ethane, di(2-methoxyphenoxy) ethane, β-naphthol-( p -methylbenzyl) ether, α-naphthylbenzyl ether, 1,4-butanediol- p -methylphenyl ether, 1,4-butanediol-p-isopropylphenyl ether, 1,4-butanediol- p - tert -octylphenyl ether, 1-phenoxy-2-(4-ethylphenoxy) ethane, 1-phenoxy-2-(chlorophenoxy) ethane, 1,4-butanediolphenyl ether and diethyleneglycol- bis (4-methoxyphenyl) ether. These heat-fusible compounds may be used alone or as a mixture thereof. For the purpose of obtaining sufficient thermal response, the heat-fusible compound is preferably used in an amount of from 10 to 400% by weight on the basis of the color developer, more preferably from 50 to 250%. To make such a thermosensitive recording layer, the said components are preferably dispersed in a water-soluble binder, which preferably has a solubility of 5% by weight or more in water at 25°C. Illustrative examples of such water-soluble binders include a polyvinyl alcohol, a methyl cellulose, a carboxymethyl cellulose, starch materials (including a modified starch), gelatin, gum arabic, casein and a saponified product of a styrene-maleic anhydride copolymer. These water-soluble binders may be used not only at the time of the dispersion step but also for the purpose of improving the film strength of the thermosensitive layer. For this purpose, the water-soluble binder may be used jointly with a synthetic polymer latex binder such as a styrene-butadiene copolymer, a vinyl acetate copolymer, an acrylonitrile-butadiene copolymer, a methylacrylate-butadiene copolymer or a polyvinylidene chloride. These recording layer components are made into a coating solution by dispersing them separately or simultaneously, using a mixer or a grinder such as a ball mill, an attritor or a sand mill. If desired, the coating solution may be further mixed with other additives such as a pigment, a metallic soap, a wax article, a surfactant, an antistatic agent, an ultraviolet ray absorption agent, an antifoaming agent or a fluorescent dyestuff. Calcium carbonate, barium sulfate, lithopone, agalmatolite, kaolin, baked kaolin, amorphous silica or aluminum hydroxide may be used as a pigment additive. As a metallic soap, metal salts of higher fatty acids may be useful, for instance zinc stearate, calcium stearate and aluminum stearate. As a wax, a paraffin wax, a microcrystalline wax, a carnauba wax, methylol stearoamide, a polyethylene wax, a polystyrene wax or a fatty acid amide wax may be used alone or as a mixture. An alkali metal salt of sulfosuccinic acid or a fluorine-containing surfactant may be useful as the surfactant. The addition of an anti-achromation agent to the thermosensitive recording layer is desirable for the purpose of preventing achromation of image printing parts and thereby solidifying the formed image. As such an agent, phenolic compounds, especially hindered phenol compounds, may be effective, such as 1,1,3- tris (2-methyl-4-hydroxy- tert -butylphenyl) butane, 1,1,3- tris (2-ethyl-4-hydroxy-5- tert -butylphenyl) butane,1,1,3- tris (3,5-di- tert -butyl-4-hydroxyphenyl)butane, 1,1,3- tris (2-methyl-4-hydroxy-5- tert -butylphenyl) propane, 2,2'-methylene- bis (6- tert -butyl-4-methylphenol), 2,2'-methylene- bis -(6- tert -butyl-4-ethylphenol), 4,4'-butylidene- bis (6- tert -butyl-3-methylphenol) and 4,4'-thio- bis (3-methyl-6- tert -butylphenol). Such phenolic compounds are preferably used in an amount of from 1 to 200% by weight on the basis of the color developer, more preferably from 5 to 50%. The foregoing materials are respectively mixed and coating composition then applied to the undercoated support. The coating step can be effected, e.g. by an air knife coater, a roll coater, a blade coater or a curtain flow coater. The coating is then dried and may be subjected to smoothing treatments such as calendering, prior to use of the material. A coating solution for a thermosensitive recording layer, is preferably coated on a support with a glue spread of about 2 to 7 g/m² as dry weight. The invention is now illustrated by the following Examples, wherein the parts and percentages are by weight unless otherwise noted. EXAMPLE 1 Preparation of undercoat solution (1) : Undercoat solution (1) for use in the coating of an undercoat layer was prepared by mixing and dispersing the following composition: 100 parts of a baked kaolin (Ansilex 90, manufactured by Engelhard Corp.; oil absorption, 75 ml/100 g), 1 part of sodium hexametaphosphate, 20 parts of 30% aqueous solution of an oxidized starch, 15 parts of a styrene-butadiene copolymer latex (48%), 40 parts of 5% aqueous solution of a carboxymethyl cellulose (etherification degree, 0.65; mean molecular weight, 100,000), and 66 parts of water. Formation of undercoat layer (1): A predetermined amount of the thus prepared undercoat solution (1) was coated with a blade coater on a support having a Stöckigt sizing degree of 7 seconds and a basis weight of 45 g/m². Thereafter, the coated solution was dried to obtain undercoat layer (1) having a dry coated weight of 8 g/m². The support was made of paper made of Laubholz bleached kraft pulp. Preparation of thermosensitive recording layer solution: Solution A A dispersant having a mean particle size of 1.0 µm was prepared by mixing the following composition using a sand mill: 10 parts of 3-dibutylamino-6-methyl-7-anilinofluoran and 50 parts of 5% solution of a polyvinyl alcohol (PVA-105). Solution B A dispersant having a mean particle size of 1.0 µm was prepared by mixing the following composition using a sand mill: 20 parts of bisphenol A. 20 parts of naphthylbenzyl ether, and 200 parts of 5% solution of a polyvinyl alcohol. Solution C A pigment dispersant having a mean particle size of 2.0 µm was prepared by mixing the following composition using a sand mill: 40 parts of precipitated calcium carbonate, 1 part of 40% solution of sodium polyacrylate, and 60 parts of water. A coating solution for use in a thermosensitive recording layer was obtained by mixing 60 parts of the Solution A with 240 parts of the Solution B, 101 parts of the Solution C and 25 parts of a 21% water-dispersion of zinc stearate. Preparation of thermosensitive recording material A predetermined amount of the thus prepared thermosensitive recording layer solution was coated on the previously prepared undercoat layer using an air knife coater. Thereafter, the coated solution was dried and subjected to calendering to obtain a thermosensitive recording material with its thermosensitive recording layer having a glue spread of 5.5 g/m². EXAMPLE 2 Preparation of undercoat solution (2) Undercoat solution (2) for use in the coating of an undercoat layer was prepared by mixing and dispersing the following composition: 100 parts of a baked kaolin (Ansilex 90, manufactured by Engelhard Corp.; oil absorption, 75 ml/100 g), 1 part of sodium hexametaphosphate, 20 parts of 30% aqueous solution of an oxidized starch, 15 parts of a styrene-butadiene copolymer latex (48%), 40 parts of 5% aqueous solution of a carboxymethyl cellulose (etherification degree, 0.80; mean molecular weight, 100,000), and 66 parts of water. Formation of undercoat layer (2) A predetermined amount of the thus prepared undercoat solution (2) was coated on a support having a Stökigt sizing degree of 7 seconds and a basis weight of 45 g/m² using a blade coater. Thereafter, the coated solution was dried to obtain undercoat layer (2) having a glue spread of 8 g/m². Preparation of thermosensitive recording material A thermosensitive recording material was obtained by repeating the process of Example 1 except that the undercoat layer (1) used in Example 1 was replaced by the undercoat layer (2). EXAMPLE 3 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the support used in Example 1 was replaced by another support having a Stöckigt sizing degree of 5 seconds and a basis weight of 45 g/m². EXAMPLE 4 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 100,000 used in Example 1 as the undercoat layer solution (1) was replaced by a carboxymethyl cellulose having an etherification degree of 0.65 and the mean molecular weight of 180,000. EXAMPLE 5 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 100,000 used in Example 1 as the undercoat layer solution (1) was replaced by a carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 30,000. EXAMPLE 6 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the undercoat layer solution (1) used in Example 1 was replaced by an undercoat layer solution prepared by mixing and dispersing the following composition: 100 parts of baked kaolin (Ansilex 90, manufactured by Engelhard Corp.; oil absorption, 75 ml/100 g). 1 part of sodium hexametaphosphate, 20 parts of 30% aqueous solution of an oxidized starch, 15 parts of a styrene-butadiene copolymer latex (48%), 40 parts of 5% aqueous solution of a carboxymethyl cellulose (etherification degree, 0.80; mean molecular weight, 100,000), 10 parts of 30% water-dispersion of paraffin wax (135°F), and 66 parts of water. COMPARATIVE EXAMPLE 1 An undercoat layer solution was prepared by repeating the process of Example 1 except that the carboxymethyl cellulose used in Example 1 was replaced by another carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 300,000. The thus prepared undercoat solution was coated on a support having a Stöckigt sizing degree of 7 seconds and a basis weight of 45 g/m² using a blade coater. The undercoat solution was used in such an amount that a glue spread would become 8 g/m² when the coated solution was dried. In this attempt, however, an undercoat layer did not form because of a high static viscosity (2,300 mPa.s) which lowered workability of the process to almost impossible level. COMPARATIVE EXAMPLE 2 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 100,000 used in Example 1 as the undercoat layer solution (1) was replaced by another carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 15,000. COMPARATIVE EXAMPLE 3 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the support used in Example 1 was replaced by a support having a Stöckigt sizing degree of 4 seconds and a basis weight of 45 g/m² . COMPARATIVE EXAMPLE 4 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 100,000 used in Example 1 as the undercoat layer solution (1) was replaced by a carboxymethyl cellulose having an etherification degree of 0.50 and a mean molecular weight of 100,000. COMPARATIVE EXAMPLE 5 A thermosensitive recording material was obtained by repeating the process of Example 1 except that the carboxymethyl cellulose having an etherification degree of 0.65 and a mean molecular weight of 100,000 used in Example 1 as the undercoat layer solution (1) was replaced by a carboxymethyl cellulose having an etherification degree of 1.0 and a mean molecular weight of 100,000. The thermosensitive recording materials obtained in Examples 1 to 6 and Comparative Examples 1 to 5 were tested for their dynamic sensitivities, image qualities, stain adhesion frequencies to a thermal head and surface characteristics in accordance with the following evaluation methods. The results are shown in Table 1. Dynamic sensitivity Printing on the thus obtained thermosensitive recording materials was carried out by using an experimental apparatus for thermosensitive printing, which was equipped with a thermal head (Trade name, KTL-2168-MPD1; manufactured by Kyocera Corp.) and a pressure roll (100 kg/cm²) attached directly to the thermal head, under the pressure rolling conditions of 24 V head voltage and 10 ms pulse cycle, with a pulse width of 0.8 ms. Print density on the thus printed material was measured using a Macbeth-type reflection density meter, RD-918. Image quality The printed samples thus obtained were evaluated macroscopically using the following evaluation criteria: A Good B Bad C Extremely bad Stain adhesion to thermal head A two-meter long test pattern with 100% blackness was printed on each thermosensitive recording material using the aforementioned experimental apparatus for thermosensitive printing, and the amount of stains adhered to the thermal head was observed macroscopically with the following evaluation criteria: A Good B Bad C Extremely bad Surface conditions The degree of track mark defects such as streaking and stalactites and of coating workability were evaluated as the surface characteristics using the following evaluation criteria: A Markedly good B Problematic C Extremely problematic / Not coatable Thus, it is apparent that the thermosensitive recording material of the invention has a high sensitivity, high image quality and low stain adhesion to a thermal head.

Claims: A thermosensitive recording material, comprising a paper support and coated thereon in order (1) an undercoat layer comprising (a) a pigment capable of absorbing an oil, (b) a binder and (c) a carboxymethyl cellulose in an amount of from 1 to 5% by weight based on the pigment and (2) a thermosensitive recording layer characterised in that said carboxymethyl cellulose has an etherification degree of from 0.6 to 0.8 and a mean molecular weight of from 20,000 to 200,000, and wherein the paper support has a Stockigt sizing degree of from 5 to 10 seconds. A recording material according to Claim 1, wherein the amount of the binder in the undercoat is 7 to 20 parts by weight per 100 parts of said pigment. A recording material according to Claim 1 or 2, wherein the undercoat layer contains at least 80 weight % of the pigment and at least 5 wt % of binder. A recording material according to Claim 1, 2 or 3, wherein the amount of the carboxymethyl cellulose is 1 to 3% by weight. A recording material according to any preceding claim, wherein said undercoat layer further contains 0.5 to 10% by weight of paraffin wax based on said oil-absorbing pigment. A recording material according to any preceding claim, wherein the undercoat layer is present in a dry weight of 1 - 20 g/m². A recording material according to any preceding claim, wherein the thermosensitive layer contains an electron-donating dye precursor and an electron-accepting developer compound. A method of making a recording material as defined in any preceding claim, wherein the undercoat layer is blade coated on the support by use of a bevel type blade, vent type blade, rod blade or bill blade and the thermosensitive layer is coated on the undercoat layer. A method according to Claim 8, wherein the blade coating is carried out by an on-machine coater.

Inventors: HOSHI YOSHIYUKI C O FUJI PHOTO, MITSUO HIROFUMI C O FUJI PHOTO, HOSHI, YOSHIYUKI C/O FUJI PHOTO FILM CO., LTD., MITSUO, HIROFUMI, MITSUO, HIROFUMI, C/O FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1991
Main classification: B41M5
IPC codes: B41M5