JPH05139034A - Production of thermal recording material - Google Patents

Abstract

PURPOSE:To produce a thermal recording material having high sensitivity and excellent color forming uniformity by laminating a thermal layer to a support having a heat insulating under layer using plastic non-foamed hollow bodies by a blade coating method. CONSTITUTION:In a thermal recording material formed by laminating a thermal layer to a base material having an under layer by a blade coating layer, the absorption factor Ha to water of the under layer due to a Bristol method (J. TAPPI paper pulp testing method No. 51-87) is set to 0.25ml/m<2>.msec<1/2> or more and the contact angle with water thereof is set to 60 deg. or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-sensitive recording material utilizing a color-forming reaction between a leuco dye and a color developer on a support, and more particularly to a blade of the heat-sensitive recording material having an under layer excellent in color uniformity. The present invention relates to a manufacturing method by a coating method.

[0002]

2. Description of the Related Art In various recording devices using thermal recording paper, there is a strong demand for high sensitivity of the thermal recording paper due to downsizing, cost reduction and speeding up. Further, it is desired to improve productivity in order to reduce the cost of thermal recording paper, and blade coating is said to be suitable for high-concentration liquid even during high-concentration liquid coating, which does not burden the drying capacity due to high-speed coating. Engineering is mainly used. Various proposals have been made for higher sensitivity, but it has been proposed that a plastic non-foamed hollow body is effective in order to give the under layer heat insulation. Above all, in order to exert the effect particularly, it is important to use Plastec latex, which has similar thermal conductivity as the binder.

However, JP-A-62-5886, 64-14075,
When a thermal layer is formed by a blade coating method using a hollow body such as JP-A 1-285383 or 2-214688 and a latex resin-based underlayer, a unique color formation of scaly spots is formed, resulting in uniform coloring. However, there is a defect that the color density is partially lowered and the recording paper does not have a function.

Further, Japanese Patent Laid-Open No. 2-204086 proposes improvement of streaks and scratches by a blade coating method, but is characterized by lowering the absorption coefficient Ka for water, and in the underlayer system, It tends to reduce the color formation. Even if Ka is between 0.40 and 0.25,
When the contact angle with water exceeds 60 degrees, a new defect occurs in which the thermal layer forming liquid does not adhere in the form of pinholes.

Further, in Japanese Patent Laid-Open No. 61-154991, it has been proposed to use a water resistant resin having good wettability with water for the under layer. However, since an organic solvent is used, the under layer remains, so that background fog is caused. It has drawbacks. In addition, since the water-resistant resin layer is formed, the absorption coefficient Ka as the under layer is lowered, and when the thermal liquid is applied by blade coating, the thermal liquid does not reach the immobilization point and stagnation of the liquid There is a drawback that unevenness occurs and the uniformity of color development is extremely poor.

[0006]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and its purpose is to provide a support having a heat-insulating underlayer using a plastic non-foamed hollow body by a blade coating method. It is an object of the present invention to provide a method capable of producing a heat-sensitive recording material having high sensitivity and excellent color uniformity in a heat-sensitive recording material having a thermal layer laminated thereon.

[0007]

The present invention relates to a thermal recording material obtained by laminating a thermal layer on a substrate having an under layer by a blade coating method, and the under layer is made by the Bristow method (J. TAPPI paper pulp test). Method No. 51-8
Absorption coefficient Ka to water according to 7) is 0.25 ml / m
² ． It is characterized by having a msec ^1/2 or more and a contact angle with water of 60 degrees or less.

The under layer according to the present invention can be formed by dry coating on a support together with a plastic non-foamed hollow body and a resin. In this case, the particle diameter of the plastic non-foamed hollow body is preferably 10 μm or less. Further, this plastic non-foamed hollow body can be used together with various fillers used for conventional general paper. Examples of such fillers include calcium carbonate, kaolin, talc, zinc oxide, aluminum hydroxide, silica, magnesium silicate, poly-sterene resin, and urea-formaldehyde resin. When used in combination with such a filler, the mixing ratio is not particularly limited, but in general, the above filler is used in a ratio of 0.1 to 1 part by weight relative to 1 part by weight of the plastic non-foamed hollow body. To be done.

The resin used for the under layer is S
It is desirable to add 5% or more of a water-soluble resin to a latex resin such as BR latex to the total resin solid content,
Particularly, 5% to 30% is preferable. The total solid content of the resin used in the under layer is 5 with respect to the total solid content of the under layer.
% To 40% is preferable. As the water-soluble resin added to the under layer resin, polyvinyl alcohol, modified polyvinyl alcohol, methyl cellulose and hydroxyethyl cellulose are desirable. If the addition amount of such a water-soluble resin is less than 5% with respect to the total resin, the effect of changing the absorption coefficient Ka and the contact angle is insufficient, and if it exceeds 30%, the effect is saturated. And economically disadvantageous.

On the other hand, as the resin used for the under layer, S
When only a latex resin such as BR latex is used, it is desirable to use a polycarboxylic acid alkali salt as an additive (surfactant) in or on the under layer. The amount added is not particularly limited, but is preferably 0.05% to 5% with respect to the total solid content of the underlayer.
If the addition amount is less than 0.05%, the effect of changing the absorption coefficient Ka and the contact angle is insufficient, and if it exceeds 5%, the effect is saturated and it is economically disadvantageous. The polycarboxylic acid alkali salt is not particularly limited, but includes 2-propenoic acid homopolymer sodium salt, a sodium salt of a copolymer of 2.5-furandion and 2,4,4-trimethylpentene, and butane dicarboxylic acid. Acid sulfo 1,4-bis (2
-Ethylhexyl) ester sodium salt is preferred.

If necessary, an organic filler may be added to the under layer according to the present invention. Examples of the organic fillers include phenol derivatives having color forming ability, paraffin waxes, amide waxes, and the like.

The blade coating in the present invention is not limited to a coating method using a bevel type or a vent type blade, but includes a coating method using a roll blade or a bill blade coating method. Also, in dwell time, you are not restricted to long and short. In addition, the blade coating is not limited to off-machine coating, and may be performed by an on-machine coater installed on the paper machine.

Next, the thermal layer forming material used in the present invention will be described. The leuco dyes used in the present invention may be applied alone or as a mixture of two or more kinds, and as such leuco dyes, those applied to the recording material of this kind are arbitrarily applied, for example, triphenyl Methanephthalide type, triallylmethane type, fluorane type, phenothiazine type, thiofluorane type, xanthene type, indophthalyl type, spiropyran type, azaphthalide type, chromenopyrazole type, methine type, rhodamine anilinolactam type, rhodamine lactam type, quinazoline And leuco compounds such as diazaxanthenes and bislactones are preferably used. Examples of such compounds include those shown below.

3,3-bis (p-dimethylaminophenyl) -phthalide, 3,3-bis (p-dimethylaminophenyl) -6-dimethylaminophthalide 3,3-bis (p-dimethylaminophenyl)- 6-Diethylaminophthalide 3,3-bis (p-dimethylaminophenyl) -6-chlorophthalide 3,3-bis (p-dibutylaminophenyl) phthalide 3-cyclohexylamino-6-chlorofluorane 3-dimethylamino-5 , 7-Dimethylfluorane 3- (N-methyl-N-isoamylamino) -6-methyl-7-anilinofluorane 3- (N-methyl-N-isobutylamino) -6-methyl-7-anilino Fluoran 3- (Np-tolyl-N-ethylamino) -6-methyl-7-anilinofluorane 3- (N-methyl-N- Amylamino) -6-methyl-
7-anilinofluorane 3- (N-methyl-N-cyclohexylamino) 6-methyl-7-anilinofluorane 3- (N-methyl-N-iso-propylamino) -6
-Methyl-7-anilinofluorane 3- (N-ethyl-N-tetrahydrofurfurylamino) -6-methyl-7-anilinofluorane 3-diethylamino-7,8-benzfluorane 3-diethylamino-7 -Chlorofluorane 3-diethylamino-7-methylfluorane 3-diethylamino-6-methyl-7-chlorofluorane 3-pyrrolidino-6-methyl-7-anilinofluorane 3-diethylamino-6-methyl-7- (M-Trichloroanilino) fluorane 3-diethylamino-7- (o-chloroanilino) fluorane 3-dibutylamino-7- (o-chloroanilino) fluorane 3-diethylamino-6-methyl-7-anilinofluorane 3-dibutyl Amino-6-methyl-7-anilinofluorane 3-diethylamino- - methyl-7- (2 ', 4'-
Dimethylanilino) fluorane 3- (N, N-diethylamino) -5-methyl-7-
(N, N-Dibenzylamino) fluorane benzoyl leuco methylene blue 6'-chloro-8'-methoxybenzoindolinospiropyran 6'-bromo-8'-methoxybenzoindolinospiropyran 3- (2'-hydroxy-4'- Dimethylaminophenyl) -3- (2'-methoxy-5'-chlorophenyl)
Phthalide 3- (2'-hydroxy-4'-dimethylaminophenyl) -3- (2'-methoxy-5'-nitrophenyl)
Phthalide 3- (2′-hydroxy-4′-diethylaminophenyl) -3- (2′-methoxy-5′-methylphenyl)
Phthalide 3- (2'-methoxy-4'-dimethylaminophenyl) -3- (2'-hydroxy-4'-chloro-5'-
Methylphenyl) phthalide 3-morpholino-7- (N-propyltrifluoromethylanilino) fluorane 3-pyrrolidino-7-trifluoromethylanilinofluorane 3-diethylamino-5-chloro-7- (N-benzyl-tri Fluoromethylanilino) fluorane 3-pyrrolidino-7- (di-p-chlorophenyl) methylaminofluorane 3-diethylamino-5-chloro-7- (α-phenylethylamino) fluorane 3- (N-ethyl-N- p-toluidino) -7- (α-
Phenylethylamino) fluorane 3-diethylamino-7- (o-methoxycarbonylphenylethyl) fluorane 3-diethylamino-5-methyl-7- (α-phenylethylamino) fluorane 3-diethylamino-7-piperidinoaminofluorane 2-chloro-3- (N-methyltoluidino) -7- (p
-N-butylanilino) fluorane 3,6-bis (dimethylamino) fluorene spiro (9,3 ')-6'-dimethylaminophthalide 3- (N-ethyl-N-cyclohexylamino) -5,
6-benzo-7-α-naphthylamino-4′-bromofluorane 3-diethylamino-6-chloro-7-anilinofluorane 3- (N-ethyl-N-2-ethoxypropylamino)
-6-Methyl-7-anilinofluorane 3- (N-ethyl-tetrafurfurylamino) -6-methyl-7-anilinofluorane 3-diethylamino-6-methyl-7-mesitidino-
4 ', 5'-benzofluorane 3- (p-dimethylaminophenyl) -3- {1,1-
Bis (p-dimethylaminophenyl) ethylene-2-yl} phthalide 3- (p-dimethylaminophenyl) -3- {1,1-
Bis (p-dimethylaminophenyl) ethylene-2-yl} -6-dimethylaminophthalide 3- (p-dimethylaminophenyl) -3- (1-p-
Dimethylaminophenyl-1-phenylethylene-2-
Yl) phthalide 3- (p-dimethylaminophenyl) -3- (1-p-
Dimethylaminophenyl-1-p-chlorophenylethylene-2-yl) -6-dimethylaminophthalide 3- (4'-dimethylamino-2'-methoxy) -3-
(1 "-p-dimethylaminophenyl-1" -p-chlorophenyl-1 ", 3" -butadiene-4 "-yl) benzophthalide 3- (4'-dimethylamino-2'-benzyloxy)
-3- (1 "-p-dimethylaminophenyl-1" -phenyl-1 ", 3" -butadiene-4 "-yl) benzophthalide 3-dimethylamino-6-dimethylamino-fluorene-9-spiro-3 '(6'-Dimethylamino) phthalide 3,3-bis {2- (p-dimethylaminophenyl)-
2- (p-methoxyphenyl) ethenyl} -4,5,
6,7-Tetrachlorophthalide 3-bis {1,1-bis (4-pyrrolidinophenyl) ethylene-2-yl} -5,6-dichloro-4,7-dibromophthalide bis (p-dimethylamino) Styryl) -1-naphthalenesulfonylmethane bis (p-dimethylaminostyryl) -1-p-tolylsulfonylmethane and the like.

In the present invention, various electron-accepting compounds, such as phenolic compounds, are used as color developers.
Thiophenolic compounds, thiourea derivatives, organic acids and metal salts thereof can be used, and specific examples thereof include, but are not limited to, those shown below.

4,4'-isopropylidenediphenol, 4,4'-isopropylidenebis-o-methylphenol, 4,4'-sec-butylidenebisphenol, 4,4'-isopropylidenebis (2-tert) −
Butylphenol, zinc p-nitrobenzoate, 1,3
5-tris (4-tert-butyl-3-hydroxy-
2,6-dimethylbenzyl isocyanuric acid, 2,2-
(3,4′-dihydroxyphenyl) propane, bis (4-hydroxy-3-methylphenyl) sulfide,
4- (β (p-methoxyphenoxy) ethoxy) salicylic acid, 1,7-bis (4-hydroxyphenylthio)
3,5-dioxaheptane, 1,5-bis (4-hydroxyphenylthio) -3-oxapentane, phthalic acid monobenzyl ester monocalcium salt, 4,4′-cyclohexylidene diphenol, 4,4′- Isopropylidene bis (2-chlorophenol), 2,2'-methylene bis (4-methyl-6-tert-butylphenol), 4,4'-butylidene bis (6-tert-butyl-2-methylphenol), 1,1 , 3-Tris (2
-Methyl-4-hydroxy-5-cyclohexylphenyl) butane, 4,4'-thiobis (6-tert-butyl-2-methylphenol), 4,4'-diphenol sulfone, 4-isopropoxy-4'- Hydroxydiphenyl sulfone, 4-benzyloxy-4'-hydroxydiphenyl sulfone, 4,4'-diphenol sulfoxide, isopropyl p-hydroxybenzoate, benzyl p-hydroxybenzoate, benzyl protocatechuate,
Stearyl gallate, lauryl gallate, octyl gallate, 1,3-bis (4-hydroxyphenylthio) propane, N, N'-diphenylthiourea, N, N'-di (m-chlorophenyl) thiourea, salicyl Anilid,
Methyl bis- (4-hydroxyphenyl) acetate, benzyl bis (4-hydroxyphenyl) acetate, 1,3-bis (4-hydroxyphenyl) benzene, 1,4-bis (4-hydroxyphenyl) benzene, 2,4 '-Diphenol sulfone, 2,2'-diallyl-4,4'-hydroxyphenyl sulfone, 3,4-dihydroxy-
4′-methyldiphenyl sulfone, 1-acetyloxy-2-naphthoate zinc, 2-acetyloxy-3-naphthoate zinc, 2-acetyloxy-1-naphthoate zinc, α, α-bis (4-hydroxyphenyl) ) Α-Methyltoluene, antipyrine complex of zinc thiocyanate, tetrabromobisphenol A, tetrabromobisphenol S, 4,4′-thiobis (2-methylphenol-
4,4'-thiobis (2-chlorophenol) and the like.

To produce the heat-sensitive recording material of the present invention, a leuco dye, a color developer, and other auxiliary agents may be supported and bound on a support. As the binder in this case, various conventional binders can be used in appropriate amounts, and specific examples thereof include the following.

Polyvinyl alcohol, starch and its derivatives, methoxy cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, methyl cellulose,
Cellulose derivatives such as ethyl cellulose, sodium polyacrylate, polyvinylpyrrolidone, acrylamide / acrylic acid ester copolymer, acrylamide / acrylic acid ester / methacrylic acid terpolymer, styrene /
Water-soluble polymers such as alkali salt of maleic anhydride copolymer, isobutylene / maleic anhydride copolymer alkali salt, polyacrylamide, sodium alginate, gelatin, casein, polyvinyl acetate, polyurethane, polyacrylic ester, poly Methacrylic acid ester, vinyl chloride / vinyl acetate copolymer, ethylene / vinyl acetate copolymer emulsion, and styrene / butadiene / acrylic copolymer latex.

Further, in the present invention, various heat-fusible substances can be used in combination as the sensitivity improving agent, and specific examples thereof include the followings, but the present invention is not limited thereto. ..

Fatty acids such as stearic acid and behenic acid, fatty acid amides such as stearic acid amide and palmitic acid amide, zinc stearate, aluminum stearate,
Fatty acid metal salts such as calcium stearate, zinc palmitate and zinc behenate, p-benzylbiphenyl, terphenyl, triphenylmethane, benzyl p-benzyloxybenzoate, β-benzyloxynaphthalene, β
-Phenyl naphthoate, phenyl 1-hydroxy-2-naphthoate, methyl 1-hydroxy-2-naphthoate,
Diphenyl carbonate, glare coal carbonate, dibencil terephthalate, dimethyl terephthalate,
1,4-dimethoxynaphthalene, 1,4-diethoxynaphthalene, 1,4-dibenzyloxynaphthalene, 1,2-diphenoxyethane, 1,2-bis (3-methylphenoxy) ethane, 1,2-bis (4-methylphenoxy) ethane, 1,4
-Diphenoxybutane, 1,4-diphenoxy-2-butene, 1,2-bis (4-methoxyphenylthio) ethane,
Dibenzoylmethane, 1,4-diphenylthiobutane, 1,
4-diphenylthio-2-butene, 1,3-bis (2-vinyloxyethoxy) benzene, 1,4-bis (2-vinyloxyethoxy) benzene, p- (2-vinyloxyethoxy) biphenyl, p- Aryloxybiphenyl, p-propargyloxybiphenyl, dibenzoyloxymethane, dibenzoyloxypropane, dibenzyl disulfide, 1,1-diphenylethanol, 1,1-diphenylpropanol, p-benzyloxybenzyl alcohol, 1,3- Phenoxy-2-propanol, N-
Octadecylcarbamoyl-p-methoxycarbonylbenzene, N-octadecylcarbamoylbenzene, 1,2
-Bis (4-methoxyphenoxy) propane, 1,5-bis (4-methoxyphenoxy) -3-oxapentane,
Dibenzyl oxalate, bis (4-methylbenzyl) oxalate, bis (4-chlorobenzyl oxalate) and the like.

In the present invention, various conventional fillers can be used in appropriate amounts, and specific examples thereof include calcium carbonate, magnesium carbonate, titanium oxide, zinc oxide, silica, aluminum hydroxide, zinc hydroxide. , Barium sulfate,
In addition to clay, talc, inorganic fine powder of surface-treated calcium carbonate and silica, urea-formalin resin can be used. As other additive substances, lubricants such as zinc stearate and calcium stearate, various surfactants and antifoaming agents are added as necessary.

Examples of the support include paper, synthetic paper, film and the like. A protective layer can be provided if necessary, and the surface can be treated with a super calendar or the like.

[0023]

EXAMPLES Examples are shown below for illustrating the present invention more specifically, but the present invention is not limited thereto. Further, parts and% in the examples are parts by weight and% by weight, unless otherwise specified.

Example 1 A mixture having the following composition was stirred and dispersed with a disper to prepare an under layer forming liquid [Liquid A]. [Liquid A] Hollow body of styrene / acrylic copolymer (solid content 37%) 60 parts (hollow ratio 30 to 35%, particle outer diameter 0.4 to 0.5 μm) Sterene / butadiene copolymer (solid content 50 %) 10 parts Itaconic acid-modified polyvinyl alcohol aqueous solution (solid content 5%) 15 parts Water 15 parts Further, a mixture consisting of the following ingredients is crushed and dispersed by a sand grinder so that the average particle diameter becomes 1 to 2 μm. B
Liquid] [C liquid]. [Solution B] 3-dibutylamino-6methyl-7anilinofluorane 30 parts Silicon oxide pigment 10 parts 10% aqueous polyvinyl alcohol solution 20 parts Water 40 parts [Solution C] Bisphenol A 10 parts Dibenzyl oxalate 10 parts Hydroxylation Aluminum 20 parts Sulfon-modified polyvinyl alcohol aqueous solution (solid content 10%) 20 parts Water 40 parts Subsequently, [B liquid]: [C liquid] = 1: 6 by mixing and stirring to prepare a thermal layer forming liquid. .. Next, [A liquid] is applied and dried on a commercially available high-quality paper having a basis weight of 50 g / m ^{2 by} a blade coating method so that the dry adhesion amount becomes 5 g / m ^2, and the thermal layer forming liquid is dried and adhered on this. An amount of 4 g / m ² was applied and dried by a blade coating method, and surface treatment was further performed by a super calender to obtain a heat-sensitive recording material by the production method of the present invention.

Example 2 In Example 1 [Solution A], an itaconic acid-modified polyvinyl aqueous solution (solid content: 5%) was added to a methyl cellulose aqueous solution (solid content: 5%).
%) Was obtained in the same manner as in Example 1 to obtain a thermosensitive recording medium according to the production method of the present invention.

Example 3 In Example 1 [solution A], an aqueous solution of itaconic acid-modified polyvinyl (solid content 5%) was mixed with hydroxyethyl cellulose (solid content 5).
%) Was obtained in the same manner as in Example 1 to obtain a thermosensitive recording medium according to the production method of the present invention.

Example 4 [Liquid D] Hollow body of styrene / acrylic copolymer (solid content 37%) 60% (hollowness 30 to 35%, particle outer diameter 0.4 to 0.5 μm) methylene / butadiene copolymer Combined (solid content 50%) 10% 2-propenoic acid homopolymer sodium salt 0.1 part Water 29.9 parts Example 1 The same as Example 1 except that [Solution A] was changed to [Solution D]. A thermosensitive recording medium was obtained by the manufacturing method of the present invention.

Example 5 The sodium salt of 2-propenoic acid homopolymer in Example 4 [Solution D] was changed to the sodium salt of the copolymer of 2,5-furandione and 2,4,4-trimethylpentene. A thermosensitive recording medium according to the production method of the present invention was obtained in the same manner as in Example 1 except for the above.

Example 6 Example 1 was repeated except that the sodium salt of 2-propenoic acid homopolymer in [D liquid] was changed to sodium salt of butanedioic acid sulfo-1,4-bis (2-ethylhexyl) ester. Similarly, a heat-sensitive recording material was obtained by the manufacturing method of the present invention.

Comparative Example 1 A thermosensitive recording medium was obtained in the same manner as in Example 1 except that the aqueous solution of itaconic acid-modified polyvinyl alcohol in Example 1 [solution A] was removed.

Comparative Example 2 A thermosensitive recording medium was prepared in the same manner as in Example 1 except that 15 parts of an itaconic acid-modified polyvinyl alcohol aqueous solution in [solution A] was replaced with 0.75 parts of ethyl cellulose and 14.25 parts of toluene. Got

Comparative Example 3 A thermosensitive recording medium was obtained in the same manner as in Example 1 except that the itaconic acid-modified polyvinyl alcohol in Example 1 [Liquid A] was changed to a styrene / maleic sizing agent.

Comparative Example 4 A thermosensitive recording medium was obtained in the same manner as in Example 1 except that 15 parts of ethylene glycol (5% aqueous solution) was added to Example 1 [solution A].

<Evaluation> (1) Regarding the support provided with the undercoat layer, the Bristow method (J. TAPPI paper pulp test method No. 51) was used.
According to -87), the absorption coefficient for water was measured. (2) The contact angle of the support provided with the undercoat layer was measured with an automatic contact angle meter CA-Z manufactured by Kyowa Interface Science Co., Ltd. with 4 μl of a dropping solution. (3) Printing characteristics: Thermal head printing power of 0.68 w / dot / line in a thermal printing device manufactured by Matsushita Electronic Parts Co., Ltd. Recording time 5 msec / l, scanning line density 8 × 7.7 dots / mm Width of 0.25 msec and 0.
Printing was performed for 3 msec, and the image density and background density at that time were measured using a Macbeth density system RD-914, and in addition, the good color development of the printed image was evaluated as good or bad.

[0035]

[Table 1]

[0036]

As is apparent from the above description, according to the present invention, when a thermal layer is laminated on a support having a heat-insulating under layer using a plastic non-foamed hollow body by a blade coating method, It was confirmed that this is a manufacturing method capable of providing a thermosensitive recording medium having high sensitivity and excellent color uniformity.

Claims (3)

[Claims] 1. A thermal recording material comprising a substrate having an under layer and a thermal layer laminated thereon by a blade coating method, wherein the under layer is formed by the Bristow method (J. TAPPI paper pulp test method No. 51-87). Absorption coefficient Ka for water is 0.25 ml / m ² . A method for producing a thermosensitive recording medium, characterized in that it has a msec ^1/2 or more and a contact angle with water of 60 degrees or less. 2. The method for producing a thermosensitive recording medium according to claim 1, wherein the amount of the water-soluble resin used as the under layer forming resin is 5% or more based on the total solid content of the resin. 3. The method for producing a thermosensitive recording medium according to claim 1, wherein an alkali salt of polycarboxylic acid is used as an additive (surfactant) in or on the under layer.