CA1295481C

CA1295481C - Heat-sensitive recording material

Info

Publication number: CA1295481C
Application number: CA000571259A
Authority: CA
Inventors: Toshimi Satake; Toshiaki Minami; Tomoaki Nagai; Fumio Fujimura
Original assignee: Jujo Paper Co Ltd
Current assignee: Nippon Paper Industries Co Ltd
Priority date: 1988-07-06
Filing date: 1988-07-06
Publication date: 1992-02-11
Anticipated expiration: 2009-02-11

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is a heat-sensitive recording material comprising a substrate and a color-developing layer which comprises a near infrared-absorbing fluoran-type leuco dye, an electron acceptor and an electron donor. The electron acceptor and the electron donor react with each other and form a chelate when heated. A typical electron acceptor is a polyvalent hydroxyaromatic compound and a typical electron donor is a double metal salt of a higher fatty acid. This heat-sensitive recording material provides a superior optical readability in near infrared region.

Description

12~54fii 71~23-11 T 1 TT ,E OF THE INVENTION
Heat-sensitive recording material BACKGROUND OE' THE INVENTION
1. Field of the Invention This invention relates to a heat-sensitive recording material which has a superior optical readability in near infrared region. The subject matter of this application is closely related to Canadian Application Serial Nos. 513,278, 521,898 and 538,428.
10 2. Prior Art A heat-sensitive recording utilizes a heat-color-forming reaction which takes place between a colorless or pale-colored chromogenic dyestuff and an acidic color-developing substance such as a phenol material or an organic acid. A heat-sensitive recording sheet is disclosed, for example, in Japanese Patent Publication Nos. 4160/1968 and 14039/1970 and in Japanese Laid-Open Patent Publication No. 27736/1973, and is now widely applied for practical use.
In general, a heat-sensitive recording sheet is produced 20 by applying on a support sheet, such as paper, film etc., a coating composition which is prepared by individually grinding and dispersing a colorless chromogenic dyestuff and a color-developing material into fine particles, mixing the resultant dispersion with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. The coating, when heated, undergoes instantaneously a chemical reaction which forms a color.

These heat-sensitive recording sheets have now been finding a wide range of applications, including medical or industrial measurement recording instruments, terminal printers of computer and information communication systems, facsimile equipments, printers of electronic calculators, automatic ticket vending machines, and so on.
Further, these heat-sensitive recording sheets comprising a combination of a leuco-dyestuff and a color-developing agent are utilized as thermosensitive labels in POS-system. However, since the color formation is in the visible region, these recording sheets cannot be adapted for reading by a semi-conductor laser in the near infrared region which is used as a bar code scanner.
Besides the heat-sensitive color-developing system in which the above colorless leuco dyestuff is used, a chelate type color-developing system with the use of metal compounds is known.
For examples, in Japanese Patent Publication No.
8787jl957 there is described a combined use of iron stearate (electron acceptor) with tannic acid or gallic acid, and in Japanese Patent Publication No. 6485tl959 there is described a combined use of an electron acceptor such as silver stearate, iron stearate, gold stearate, copper stearate or mercury behenate with an electron donor such as methyl gallate, ethyl gallate, propyl gallate, butyl gallate or dodecyl gallate.
Although these heat-sensitive recording sheets using a chelate type color-developing system are utilized as thermo-71023-ll sensitive label in POS-system, etc, they have a drawback that the readability in using semiconductor of near infrared region as a bar-code-scanner is insufficient.
SUM~RY OF THE INVENTION
It is attempted according to the present invention to provide a heat-sensitive recording material having a superior optical readability in near infrared region.
Thus, the present invention provides a heat-sensitive recording material having a substrate sheet and thereon a heat-sensitive color-developing layer containing an electxon acceptor and an electron donor which react with each other forming a chelate, wherein the color-developing layer further comprises a near infrared-absorbing fluoran-type leuco dye of the following general formula (I):

R ~ (I) C = O

(Rg)~

[wherein at least one of Rl, R2, R3, R4, Rs, R6, R7, R8 and Rg 129~481 represents a p-tp-aminophenylamino)anilino group of the formula (V):

N _ / ~ N ~ N \ T3 (V) the remainders of Rl, R2, R3, R4, R5, R6, R7~ R8 and Rg~
which may be the same or different, represent a hydrogen atom, an alkyl group, an alkoxy group, a cycloalkyl group, a halogen atom, a nitro group, a hydroxy group, an amino group, a substituted amino group, an aralkyl group a substituted aralkyl group, an aryl group or a substituted aryl group;
Tl, T2 and T3, which may be the same or different, represent a hydrogen atom, a Cl-Cg alkyl group, a C3-Cg alkenyl group or a C3-Cg alkynyl group;
T4 represents a hydrogen atom, a Cl-Cg alkyl group, a C3-Cg alkenyl group, a C3-Cg alkynyl group or a phenyl group; in addition, T3 and T4 taken together with the nitrogen to which they are attached, may re~resent a morpholino group, a pyrrolidino group, a piperidino group or a hexamethylenimino group; and n represents an integer from 0 to 4].
The alkyl group in formula (I) is preferably a lower alkyl having 1 to 4 carbon atoms, such as, methyl, ethyl, etc.
:20 the alkoxy group in formula (I) is preferably a lower alkoxy having 1 to 4 carbon atoms, such as, methoxy, ethoxy, etc.
The cycloalkyl group in formula (I) is usually a cyclo-:~ alkyl having 3 to 6 carbon atoms, such as cyclopropyl, cyclohexyl, etc.

.. . .

129~

The substituted amino group in formula (I) is preferably a mono- or di-(lower alkyl)substituted amino, such as dimethyl-amino, diethylamino, etc.
The aralkyl group in formula (I) is preferably phenyl-Cl_4 alkyl group (such as benzyl) substituted with a usual sub-stituent.
The aryl group in formula (I) is preferably phenyl or naphthyl and the substituted aryl group is preferably phenyl sub-stituted with a usual substituent.
Preferred compounds among the leuco dyes of the general formula (I) are those in which:
one of R2, R3, R6 and R7 is the p-(p-aminophenylamino)-anilino group of the formula (V):
one or two of the remainders of Rl, R2, R3, R4, Rs, R6, R7, R8 and Rg ars other than hydrogen;
all of the still remaining groups are hydrogen;
Tl and T2 are each hydrogen; and T3 is hydrogen and T4 is phenyl or T3 and T4 are each Cl 8 alkyl.
An aspect of the present invention provides a method of developing color on sheet, the color being both in the visible region and in the near infrared region. This process comprises applying heat to a selected portion of the heat-sensitive record-ing material, thereby reacting the electron donor with the elec-tron acceptor to form a chelate and reacting the electron donor with the leuco dye to form a reacted dye. The formed chelate absorbs light in the visible and the near infrared region but only .

129~48~

weakly in the near infrared region. The reacted dye absorbs strongly light in the near infrared region.
DETAILED DESCRIPTION OF THE INVENTION
The kind of electron donor is not particularly limited.
However, a preferred electron donor is a metal double salt of higher fatty acid. The metal double salt of higher fatty acid used in this invention means a metal double salt having at least two metal atoms as higher fatty acid-metal in the molecule. Owing to the double salt, the metal double salt of higher fatty acid is clearly different in physical-chemical properties from a higher fatty acid metal salt (metal single salt) containing only one metal atom. The metal double salt of higher fatty acid is synthesized by causing the reaction between alkali metal salt or ammonium salt of higher fatty acid and an inorganic metal salt with the use of at least two inorganic metals. Hence, the kind and the mixing ratio of two metal atoms in double salt are unrestrictedly controlled in this synthesis. For example, iron-zinc double salt of behenic acid containing iron and zinc of a mixed ratio 2:1 is obtained by causing a reaction between an aqueous solution of sodium behenate and an aqueous solution of ferric chloride and zinc chloride having a mixed ratio of 2:1.
Suitable metals in the metal double salt of higher fatty acid are polyvalent metals, for example, iron, zinc, calcium, magnesium, aluminum, barium, lead, manganese, tin, nickel, cobalt, ~: ~ copper, silver, mercury, etc. Preferable metals are zinc, calcium, aluminum, magnesium and silver. In this invention, there are used metal double salt of higher fatty acid having saturated :~
: ' 129~4~1 71023-11 or unsaturated aliphatic group with 16 - 35 carbon atoms.
Representative metal double salts of higher fatty acids include the following substances, but they are not limited to these substances.
1) iron-zinc double salt of stearic acid, 2) iron-zinc double salt of montanic acid, 3) iron-zinc double salt of acid wax, 4) iron-zinc double salt of behenic acid, 5) iron-calcium double salt of behenic acid, 6) iron-aluminum double salt of behenic acid, 7) iron-magnesium double salt of behenic acid, 8) silver-calcium double salt of behenic acid, 9) silver-aluminum double salt of behenic acid, 10) silver-magnesium double salt of behenic acid, and 11) calcium-aluminum double salt of behenic acid.
These metal double salts of higher fatty acids may be used alone as an electron acceptor of heat- sensitive recording sheet. It is possible to use two or more metal double salts of higher fatty acids, simultaneously.
The electron donors of this invention used with the above metal double salt of higher fatty acid include polyvalent hydroxyaromatic compounds, diphenylcarbazide, diphenylcarbazone, hexamethylenetetramine, spirobenzopyran, l-formyl-4-phenylsemicar-bazide, etc.
Particularly preferred are polyvalent hydroxyaromatic compounds of the following general formula (VI):

12954~

(OH)n ~ X - R (VI) twhere R represents alkyl group having 18 - 35 carbon atoms, ~ , CH2 ~ 1 ~ Rl ~
~ 1 or ~ Rl (where Rl is an alkyl group having 18 - 35 carbon atoms); n represents an integer of 2 or 3, and -X- represents -CH2-, -C02-, -CO-, -O-, -CONH- or -CON-R' (R' is an alkyl group having 5 - 30 carbon atoms)~.
In preparing the coating by dispersing the above poly-valent phenolic derivative in aqueous-system or solvent-system binder, care should be taken so that these phenol derivatives do not react with the electron-acceptor, and that the solvent-resistance and the dispersing stability of the phenolic deriva-tive~s are improved. For this reason, the polyvalent hydroxyaro-~; matic compounds having a substituent group with a carbon number of 18 to 35 other than the color forming group are particularly desirable. Further, it is preferable that the hydroxyl groups are adjacent to one another.
These polyvalent phenol derivatives can be used ~: :

~ - 8 -; '~
:~ :
~ :

12~5481 independently. It is possible to use two or more polyvalent phenols, if necessary.
Among the fluoran-type leuco dyes of the formula (I), dyes of the following general formula (II) is preferable.

R4 R5 Tl T2 ~ ~ N - ~ N ~ N / (II) (Rg)n ~ f wherein Rl, R2, R3, R4, Rs, R6, R7, Rg, Rg, Tl, T2, T3~ T4 and n are a~ defined above.

~2~5~

Taking the productivity, costs and performances into consideration, 2-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran (m.p: 197 -203C) of the following formula (III) and 2-chloro-3-methyl-6-p-(p-phenylaminophenyl)aminoanilinofluoran (m.p: 191.5 - 196C) of the following formula (IV) are most preferable.

~ -N ~ ~H ~ N(CH~)2 ~\C=~

~ ~ NH ~ NH ~

~\C=O

The fluoran type leuco dyes of this invention are not particularly limited and include, for example, .~ ~
2-methyl-6-p-tp-dimethylaminophenyl)aminoamilinofluoran, 2-methoxy-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-chloro-6-p-(p-dimethylaminophenyl)aminoanilinofluoran, p-nitro-6-p-(p-diethylaminophenyl)aminoanilinofluoran, ~ 2-amino-6-p-(p-diethylaminophenyl)aminoanil;nofluoran, :: :

.... .

2-diethylamino-6-p-(p-diethylaminophenyl)aminoanilino-fluoran, 2-phenyl-6-p-(p-phenylaminophenyl)amino-anilinofluoran, 2-benzyl-6-p-(p-penylaminophenyl)amino-anilinofluoran, 2-hydroxy-6-p-(p-phenylaminophenyl) aminoanilinofluoran, 3-methyl-6-p-(p-dimethylamino-phenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-dibuthylaminophenyl)aminoanilinofluoran, 3-methyl-7-p-(p-dimethylaminophenyl)aminoanilino-fluoran, 3-methoxy-7-p-(p-dimethylaminophenyl)amino-anilinofluoran, 3-chloro-7-p-(p-dimethylaminophenyl) aminoanilinofluoran, 3-nitro-7-p-(p-diethylaminophenyl) aminoan-linofluoran, 3-amino-7-p-(p-diethylaminophenyl) aminoanilinofluoran, 3-diethylamino-7-p-(p-diethylamino-phenyl)aminoanilinofluoran, 3-phenyl-7-p-(p-phenylamino-phenyl)aminoanilinofluoran, 3-benzyl-7-p-(p-phenylamino-phenyl)aminoanilinofluoran, 3-hydroxy-7-p-(p-phenyl-aminophenyl)aminoanilinofluoran, 2-methyl-7-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-diethylamino-7-p-(p-diethylamino)-7-p-(p-diethylaminophenyl)amino-anilinofluoran, 2-diethylamino-7-p-(p-dibutylamino-phenyl)aminoanilinofluoran, 2-p-(p-dimethylaminophenyl) aminoanilino-6-methylfluoran, 2-p-(p-dimethylamino-phenyl)aminoanilino-6-methoxyfluoran, 2-p-(p-dimethyl-129~4~1 71023-11 aminophenyl)aminoanilino-6-chlorofluoran, 2-p-(p-diethylaminophenyl)aminoanilino-6-nitrofluoran, 2-p-(p-diethylaminophenyl)aminoanilino-6-aminofluoran, 2-p-(p-diethylaminophenyl)aminoanilino-6-diethylaminofluoran, 2-p-(p-phenylaminophenyl)aminoanilino-6-phenyl-fluoran, 2-p-~p-phenylaminophenyl)aminoanilino-6-benzyl-fluoran, 2-p-(p-phenylaminophenyl)aminoanilino-6-hydroxyfluoran, 2-p-(p-dimethylaminophenyl)aminoanilino-6-methylfluoran, 2-p-(p-diethylaminophenyl)aminoanilino-6-diethylaminofluoran, 2-p-(p-phenylaminophenyl)amino-anilino-6-diethylaminofluoran, 3-p-(p-dimethylamino-phenyl)aminoanilino-7-methylfluoran, 3-p-(p-dimethyl-aminophenyl)aminoanilino-7-methoxyfluoran, 3-p-(p-dimethylaminophenyl)aminoanilino-7-chlorofluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-nitrofluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-aminofluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-diethylaminofluoran, 3-p-(p-phenylaminophenyl)aminoanilino-7-phenylfluoran, 3-p-(p-phenylaminophenyl)aminoanilino-7-benzylfluoran, 3-p-(p-phenylaminophenyl)aminoanilino-7-hydroxyfluoran, 3-p-(p-dimethylaminophenyl)aminoanilino-7-methylfluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-diethylamino-fluoran, and 3-p-(p-phenylaminophenyl)aminoanilino-7-diethylaminofluoran.

As the binders of this invention, there can be mentioned, for example, a fully saponified polyvinyl alcohol having a polymerization degree of 200 - 1900, a partially saponified polyvinyl alcohol, carboxylated polyvinyl alcohol, amide-modified polyvinyl alcohol, sulfonic acid-modified polyvinyl alcohol, butyral-modified polyvinyl alcohol, other modified polyvinyl alcohol, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, styrene/malic acid anhydride copolymers, styrene/butadiene copolymers, cellulose delivatives such as ethyl cellulose, acetyl cellulose, etc.; polyvinyl chloride, polyvinyl acetate, polyacryl amide, polyacrylic acid ester, polyvinyl butyrol, polystyrol and copolymers thereof; polyamide resin, silicone resin, petroleum resin, terpene resin, ketone resin and cumaron resin.
These polymeric materials may be used after they were dissolved in a solvent such as water, alcohol, ketone, ester hydrocarbon, etc., or after they were emusified or dispersed in water or a solvent other than water.
The species and the amount of electron ;~ acceptor such as metal double salt of higher fatty acid, electron doner such as phenolic derivatives, near . :

3 Z9~481 infrared-absorbing leuco dye, binder and other ingredients, which are used in this invention, are determined depending upon the performance and recording aptitude required for the recording material, and are not otherwise limited. However, in ordinary cases, the color-developing layer contains:
(a) 5 to 30% by weight of the binder, (b) 15 to 70% by weight of the filler, (c) 5 to 40% by weight of the electron acceptor, (d) 5 to 30% by weight of the electron donor, and (e) 0.5 to 10~ by weight of the colorless leuco dye, all based on the total solid content of the color-developing layer.
It is also suitable to use 1 - 6 parts by weight of the electron donor, 0.5 - 5 parts by weight of the near infrared-absorbing leuco dye, 0.5 - 4 parts by weight of the binder and 5 - 20 parts by weight of the filler, based on 1 - 9 parts by weight of the electron acceptor.
The aimed heat-sensitive recording material may be obtained by a well known method, such as, by coating the above coating composition on a substrate such as paper, synthetic paper, film, etc.
The above electron acceptor, the above electron donor and the above basic colorless dye are ground to a particle size of several microns or smaller by means of a grinder or emulsifier such as a ball mill, attritor, sand grinder, etc., and binders and various additives in accordance with the purpose, are added thereto to prepare coating color. Such additives are as follows:
filler; releasing agent for prevention of sticking, such as fatty ~2g 5 ~1 71023-11 acid metal salt; anti-fogging agent such as fatty acid amide, ethylenebisamide, montan wax, polyethylene wax etc.; dispersant such as sodium dioctylsulfosuccinate, sodium dodecylbenzene sulfonate, sodium lauryl alcohol sulfate, sodium alginate;
UV-absorber such as benzophenone type or triazole type;
antifoamer; fluorescent brightening agent; water resistance agent;
and so on.
As filler, there may be used any organic or inorganic filler usually used in the paper-manufacturing field. Examples for fillers of this invention include clay, talc, silica, magnesium carbonate, alumina, aluminum hydroxide, magnesium hydroxide, barium sulfate, kaolin, titanium dioxide, zinc oxide, calcium carbonate, aluminum oxide, urea-formalin resin, poly-styrene resin, phenol resin, etc.
(Function) The reason why the heat-sensitive recording sheet of this invention provides a superior optical readability in the near infrared region is thought as follows. As a result of a color-developing process in which a chelate is formed by the heat-melting reaction between the electron acceptor such as a metaldouble salt of higher fatty acid and an electron donor such as a polyvalent phenol derivative, a colored image in the visible and near infrared region is formed. In other words, the formed chelate absorbs light in the visible and near infrared region.
However, the chelate formed from the combination of the electron acceptor and the electron donor has only a low absorption of near infrared rays, and consequently can not be used practically.

1295i481 710~3-11 Hence, there is added a near infrared-absorbing-fluoran-type leuco dye of the formula (I) which undergoes a heat-melt reaction with the electron donor such as the polyvalent phenol derivative to form a color which can absorb light in the near infrared region efficiently. In this manner, there is obtained a heat-sensitive recording material which provides a superior optical readability in the near infrared region.
(Examples) This invention will be described by way of example hereunder. Throughout the specification the parts are units by weight.
~Example 1 (Test Nos. 1-4]
Liquid A (dispersion of electron acceptor) metal double salt of higher fatty acid (see Table 1) 9.0 parts 10~ aqueous solution of polyvinyl alcohol 10.0 parts water 6.0 parts 129548i 71023-11 Liquid B (dispersion of electron donor) polyvalent phenol;c derivative (see Table 1) 6.0 parts zinc stearate 1.5 parts 10% aqueous solution of polyvinyl alcohol 13.75 parts water 8.25 parts Liquid C (dispersion of near infrared-absorbing dye) near infrared-absorbing dye (see Table 1) 0.5 parts 10~ aqueous solution of polyvinyl alcohol 1.25 parts water 0.75 parts The liquids A, B and C of the above-mentioed composition were individually ground to a particle size of 3 microns by attritor. Then, the dispersions were :~: mixed in the following portion to prepare the coating ` color.
Liquid A 25.0 parts 20 ~ Liquid B 29.5 parts Liquid C 2.5 parts ~:: : Kaolin clay 12.0 parts , ~;
(50% aqueous dispersion) :

, .

12954~

The coating color was applied on one side of a base paper weighing 50 g/m2 at a coating weight of 6.0 g/m2 and was then dried. The resultant paper was treated to a smoothness of 200 - 600 seconds by a supercalender. In this manner, a heat-sensitive recording sheet was obtained.

[Comparative Example (Test Nos. 13-16)]
Liquid D (disPersion of electron acceptor) metal double salt of higher fatty acid (see Table 1) 9.0 parts 10% aqueous solution of polyvinyl alcohol 10.0 parts water 6.0 parts Liquid F (dispersion of electron donor) polyvalent phenolic derivative (see Table 1) 6.0 parts zinc stearate 1.5 parts 10% aqueous solution of polyvinyl ;alcohol 13.75 parts 2~0 ~ water 8.25 parts . ~ ~

' 1295~1 The liquids D and F of the above-mentioned composition were individually ground to a particle size of 3 microns by attritor. Then, the dispersions were mixed in the same portion as in Example to prepare the coating color Liquid D 25.0 parts Liquid F 29.5 parts Kaolin clay 12.0 parts (50% aqueous dispersion) Using the above coating color, a heat-sensitive recording sheet was obtained in the same manner as in Example 1.
The heat-sensitive recording sheets obtained in Example and Comparative Example were tested for the following articles. The test results were shown in Table 1.

.

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~295481 71023-11 Note (1) Image density: A heat-sensitive recording sheet was recorded in a pulse width of 3.2 milliseconds and an impressed voltage of 18.03 volts by using the thermal facsimile KB-4800 manufactured by TOSHIBA CORPORATION and the optical density of the recorded image was measured by a Macbeth densitometer. (using RD-514 and amber filter, the same in the following) (2) Infrared reflectance (~): The infrared reflectance of the image portion recorded by the method of Note (1) was measured by a spectrophotometer (waelength: 800 nm) (Effects of this invention) The effects of this invention are as follows:
(1) Superior optical readability in near infrared region.

.

Claims

1. A heat-sensitive recording material comprising a substrate sheet having thereon a color-developing layer which contains an electron acceptor and an electron donor which react with each other to form a chelate when heated, wherein the color-developing recording layer further contains a near infrared-absorbing fluoran-type leuco dye of the following general formula (I):
(I) (wherein at least one of R1, R2, R3, R4, R5, R6, R7, R8 and R9 represents a p-(p-aminophenylamino)anilino group of the formula (V):

(V) the remainders of R1, R2, R3, R4, Rs, R6, R7, R8 and R9 which may be the same or different, represent a hydrogen atom, an alkyl group, an alkoxy group, a cycloalkyl group, a halogen atom, a nitro group, a hydroxy group, an amino group, a substituted amino group, an aralkyl group, a substituted aralkyl group, an aryl group, or a substituted aryl group;
T1, T2 and T3 which may be the same or different, represent a hydrogen atom, a C1 - C8 alkyl group, a C3 - C9 alkenyl group, or a C3 - C9 alkynyl group;
T4 represents a hydrogen atom, a C1 - C8 alkyl group, a C3 - C9 alkenyl group, a C3 - C9 alkynyl group or a phenyl group;
in addition, T3 and T4 taken together with the nitrogen to which they are attached, may represent a morpholino group, a pyrrolidino group, a piperidino group, or a hexamethyleneimino group; and n represents an integer from 0 to 4).

2. The heat-sensitive recording material according to claim 1, wherein the electron acceptor comprises a metal double salt of a higher fatty acid having 16 - 35 carbon atoms.

3. The heat-sensitive recording material according to claim 2, wherein the metal double salt of higher fatty acid comprises at least two metals selected from the group consisting of iron, zinc, calcium, magnesium, aluminum, barium, lead, manganese, tin, nickel, cobalt, copper, silver and mercury.

4. The heat-sensitive recording material according to claim 2, wherein the electron donor is at least one member selected from a group consisting of polyvalent hydroxyaromatic compound, diphenylcarbazide, diphenylcarbazone, hexamethylenetetramine, spirobenzopyran, and 1-formyl-4-phenylsemicarbazide.

5. The heat-sensitive recording material according to claim 4, wherein the electron donor is a polyvalent hydroxyaromatic compound represented by the following general formula (VI):

(VI) (wherein R represents alkyl group having 18 - 35 carbon atoms, or (R1 is an alkyl group having 18 - 35 carbon atoms); n represents an integer of 2 or 3, and -X- represents -CH2-, -CO2, -CO-, -O-, -CONH- or (R' is an alkyl group having 5 - 30 carbon atoms)).

6. The heat-sensitive recording material according to claim 2, 3 or 5, wherein the fluoran-type leuco dye of the general formula (I) is at least one dye selected from a group consisting of 2-chloro-3-methyl-6-p-(p-phenylamino-phenyl)aminoanilinofluoran and 2-methyl-6-p-(p-dimethylaminophenyl)aminoanilinofluoran.

7. The heat-sensitive recording material according to claim 2, 3 or 5, wherein said color-developing layer comprises 1 - 6 parts by weight of the electron donor, 0.5 -6 parts by weight of the near infrared-absorbing fluoran-type leuco dye, 0.5 - 4 parts by weight of a binder and 5 - 20 parts by weight of a filler, based on 1 - 9 parts by weight of the electron acceptor.

8. The heat-sensitive recording material according to claim 2, 3 or 5, wherein the substrate is at least one member selected from a group consisting of paper, synthetic paper, film and plastic.

9. A heat-sensitive recording material comprising a substrate sheet having thereon a color-developing layer which contains a binder, a filler, an electron acceptor, an electron donor and a colorless chromogenic dyestuff, wherein:
(1) the electron acceptor comprises a metal double salt of a higher fatty acid having 16 to 35 carbon atoms in which the double salt comprises two metals selected from the group consisting of iron, zinc, calcium, magnesium, aluminum, barium, lead, manganese, tin, nickel, cobalt, copper, silver and mercury;
(2) the electron donor is a polyvalent hydroxyaromatic compound represented by the formula:

(VI) (wherein R represents an alkyl group having 18 to 35 carbon atoms, or (R1 is an alkyl group having 18 to 35 carbon atoms);
n represents an integer of 2 or 3;
X represents -CH2-, -CO2-, -CO-, -O-, -CONH- or (R' is an alkyl group having 5 to 30 carbon atoms); and the OH groups are adjacent to each other):
(3) the colorless chromogenic dyestuff is a near infrared-absorbing fluoran-type leuco dye of the formula:

(I) (wherein one of R1, R2, R3, R4, R5, R6, R7 and R8 represents a p-(p-aminophenylamino)anilino group (V);

the remainders of R1, R2, R3, R4, R5, R6, R7, R8 and R9, which may be the same or different, represent hydrogen, c1-4 alkyl, C1-4 alkoxy, C3-6 cycloalkyl, halogen, nitro, hydroxy, amino mono- or di(C1-4alkyl)amino, benzyl or phenyl;
T1, T2 and T3, which may be the same or different, represent hydrogen, C1-8 alkyl, C3-9 alkenyl or C3-9 alkynyl;
T4 represents hydrogen, C1-8 alkyl, C3-9 alkenyl, C3-9 alkynyl or phenyl; or T3 and T4 together with the nitrogen atom to which they are attached represent morpholino, pyrrolidino, piperidino or hexamethyleneimino; and n represents an integer of 0 to 4); and (4) the electron acceptor and the electron donor are capable of reacting with each other to form a chelate which absorbs light in the visible and near infrared region and the electron donor is capable of forming a near infrared-absorbing color with the colorless leuco dye when the heat-sensitive recording material is heated.

10. The heat-sensitive recording material according to claim 9, wherein in the formula (I):
one of R2, R3, R6 and R7 is the p-(p-aminophenylamino)-anilino group of the formula (V);
one or two of the remainders of R1, R2, R3, R4, R5, R6, R7, R8 and R9 are other than hydrogen all of the still remaining groups are hydrogen:
T1 and T2 are each hydrogen; and T3 is hydrogen and T4 is phenyl or T3 and T4 are each C1-8 alkyl.

11. The heat-sensitive recording material according to claim 9, wherein the colorless chromogenic dyestuff is a member selected from the group consisting of 2-methyl-6-p-(p-dimethylaminophenyl)-aminoanilinofluroan, 2-methoxy-6-p-(p-dimethylaminophenyl)amino-anilinofluroan, 2-chloro-6-p-(p-dimethylaminophenyl)aminoanilino-fluroan, p-nitro-6-p-(p-diethylaminophenyl)aminoanilinofluroan, 2-amino-6-p-(p-diethylaminophenyl)aminoanilinofluroan, 2-diethylamino-6-p-(p-diethylaminophenyl)aminoanilino-fluoran, 2-phenyl-6-p-(p-phenylaminophenyl)amino-anilinofluoran, 2-benzyl-6-p-(p-penylaminophenyl)amino-anilinofluoran, 2-hydroxy-6-p-(p-phenylaminophenyl) aminoanilinofluoran, 3-methyl-6-p-(p-dimethylamino-phenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-diethylaminophenyl)aminoanilinofluoran, 3-diethylamino-6-p-(p-dibuthylaminophenyl)aminoanilinofluoran, 3-methyl-7-p-(p-dimethylaminophenyl)aminoanilino-fluoran, 3-methoxy-7-p-(p-dimethylaminophenyl)amino-anilinofluoran, 3-chloro-7-p-(p-dimethylaminophenyl) aminoanilinofluoran, 3-nitro-7-p-(p-diethylaminophenyl) aminoanilinofluoran, 3-amino-7-p-(p-diethylaminophenyl) aminoanilinofluoran, 3-diethylamino-7-p-(p-diethylamino-phenyl)aminoanilinofluoran, 3-phenyl-7-p-(p-phenylamino-phenyl)aminoanilinofluoran, 3-benzyl-7-p-(p-phenylamino-phenyl)aminoanilinofluoran, 3-hydroxy-7-p-(p-phenyl-aminophenyl)aminoanilinofluoran, 2-methyl-7-p-(p-dimethylaminophenyl)aminoanilinofluoran, 2-diethylamino-7-p-(p-diethylamino)-7-p-(p-diethylaminophenyl)amino-anilinofluoran, 2-diethylamino-7-p-(p-dibutylamino-phenyl)aminoanilinofIuoran, 2-p-(p-dimethylaminophenyl) aminoanilino-6-methylfluoran, 2-p-(p-dimethylamino-phenyl)aminoanilino-6-methoxyfluoran, 2-p-(p-dimethyl-aminophenyl)aminoanilino-6-chlorofluoran, 2-p-(p-diethylaminophenyl)aminoanilino-6-nitrofluoran, 2-p-(p-diethylaminophenyl)aminoanilino-6-aminofluaran, 2-p-(p-diethylaminophenyl)aminoanilino-6-diethylaminofluoran, 2-p-(p-phenylaminophenyl)aminoanilino-6-phenyl-fluoran, 2-p-(p-phenylaminophenyl)aminoanilino-6-benzyl-fluoran, 2-p-(p-phenylaminophenyl)aminoanilino-6-hydroxyfluoran, 2-p-(p-dimethylaminophenyl)aminoanilino-6-methylfluoran, 2-p-(p-diethylaminophenyl)aminoanilino-6-diethylaminofluoran, 2-p-(p-phenylaminophenyl)amino-anilino-6-diethylaminofluoran, 3-p-(p-dimethylamino-phenyl)aminoanilino-7-methylfluoran, 3-p-(p-dimethyl-aminophenyl)aminoanilino-7-methoxyfluoran, 3-p-(p-dimethylaminophenyl)aminoanilino-7-chlorofluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-nitrofluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-aminofluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-diethylaminofluoran, 3-p-(p-phenylaminophenyl)aminoanilino-7-phenylfluoran, 3-p-(p-phenylaminophenyl)aminoanilino-7-benzylfluoran, 3-p-(p-phenylaminophenyl)aminoanilino-7-hydroxyfluoran, 3-p-(p-dimethylaminophenyl)aminoanilino-7-methylfluoran, 3-p-(p-diethylaminophenyl)aminoanilino-7-diethylamino-fluoran, and 3-p-(p-phenylaminophenyl)aminoanilino-7-diethylaminofluoran.

12. The heat-sensitive recording material according to claim 9, wherein the color-developing layer contains:
(a) 5 to 30% by weight of the binder, (b) 15 to 70% by weight of the filler, (c) 5 to 40% by weight of the electron acceptor, (d) 5 to 30% by weight of the electron donor, and (e) 0.5 to 10% by weight of the colorless leuco dye, all based on the total solid content of the color-developing layer;
the electron donor is a polyvalent hydroxyaromatic compound of the formula:

(VI) (wherein R represents an alkyl group having 18 to 35 carbon atoms, X represents -CO2- , -CONH- or -CO-, n represents 3, and the three hydroxyl groups are adjacent to one another).

13. The heat-sensitive recording material according to claim 10 or 11, wherein the color-developing layer contains:
(a) 5 to 30% by weight of the binder, (b) 15 to 70% by weight of the filler, (c) 5 to 40% by weight of the electron acceptor, (d) 5 to 30% by weight of the electron donor, and (e) 0.5 to 10% by weight of the colorless leuco dye, all based on the total solid content of the color-developing layer;
the electron donor is a polyvalent hydroxyaromatic compound of the formula:

(VI) (wherein R represents an alkyl group having 18 to 35 carbon atoms, X represents -CO2- , -CONH- or -CO-, n represents 3, and the three hydroxyl groups are adjacent to one another).

14. A method of developing color on sheet, the said color being both in the visible region and in the near infrared region, which method comprises:
applying heat to a selected portion of the heat-sensitive recording material as defined in claim 1, thereby reacting the said electron donor with the said electron acceptor to form a chelate and reacting the said electron donor with the fluoran-type leuco dye to form a reacted dye, wherein the said chelate absorbs light in the visible region but absorbs only weakly light in the near infrared region and the reacted dye absorbs light in the near infrared region.

15. The method according to claim 14, wherein the heat-sensitive recording material is as defined in claim 9 or 10.

16. The method according to claim 14, wherein the heat-sensitive recording material is as defined in claim 11 or 12.