Heat-sensitive recording material

A heat-sensitive recording material comprising a substantially transparent support and a heat-sensitive recording layer formed on one side thereof, wherein the thermal recording energy required for increasing the transmission density of recorded areas as measured with a Macbeth transmission densitometer TD904 (visual filter) to a value higher by 0.1 than the transmission density of non-recorded areas is from 10 mJ/mm.sup.2 to 40 mJ/mm.sup.2, and the energy required for obtaining a transmission density of 2.5 in recorded areas as measured with a Macbeth transmission densitometer TD904 (visual filter) is from 80 mJ/mm.sup.2 to 130 mJ/mm.sup.2. The recording material provides a highly precise gradated image using a conventional thermal head.

FIELD OF THE INVENTION 
The present invention relates to a heat-sensitive recording material. More 
particularly, the present invention relates to a heat-sensitive recording 
material which provides high image quality in a stable manner, especially 
when recording is conducted with a thermal head and the recorded images 
are observed with transmitted light. 
BACKGROUND OF THE INVENTION 
The use of thermal recording is expanding in the fields of facsimile 
telegraphs and printers, labels including POS, etc. This is because 
thermal recording has the following advantages: (1) development is 
unnecessary, (2) the quality of paper supports for thermal recording is 
similar to that of general paper, (3) handling is easy, (4) 
developed-color densities are high, (5) the recording apparatus is simple 
and inexpensive, and (6) the recording operation is quiet. Because of the 
increasing number of applications, the performance requirements of 
heat-sensitive recording materials have become diverse. A heat-sensitive 
recording material capable of multicolor recording and a transparent 
heat-sensitive recording material for overhead projectors have been 
developed as described, for example, in JP-A-63-265682. (The term "JP-A" 
as used herein means an "unexamined published Japanese patent 
application.") 
With the recent trend toward the use of electronic medical equipment such 
as ultrasonic scanners, CT scanners, and X-ray apparatus, a transparent 
heat-sensitive recording material has also been developed for directly 
recording images from digital signals that are received from this type of 
equipment. 
However, conventional heat-sensitive recording materials comprising a 
transparent support and a heat-sensitive recording layer formed thereon 
are disadvantageous in that image density unevenness in thermal-head 
recording caused by, e.g., fluctuation in thermal-head resistivity, 
surface roughness of a glazing layer, and fluctuation in the moving speed 
of the thermal head or recording material, tends to be more noticeable. 
This is because the recorded images are observed with transmitted light. A 
second disadvantage is that conventional heat-sensitive recording 
materials are subject to density fluctuations with changes in temperature 
and humidity. Density fluctuations are troublesome in those applications 
where high image precision is required such as in medical images. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a 
heat-sensitive recording material which is capable of providing highly 
precise gradated images, such as medical images, in a stable manner using 
a conventional thermal head. 
The above object of the present invention is achieved by providing a 
heat-sensitive recording material comprising a substantially transparent 
support and a heat-sensitive recording layer formed on one side of the 
support, said recording material being adapted for thermal recording 
comprising imagewise applying thermal energy to the recording layer to 
obtain recorded and non-recorded areas of the recording material, wherein 
the thermal recording energy required for increasing the transmission 
density of recorded areas as measured with a Macbeth transmission 
densitometer TD904 (visual filter) to a value higher by 0.1 than the 
transmission density of non-recorded areas is from 10 mJ/mm.sup.2 to 40 
mJ/mm.sup.2, and the thermal recording energy required for obtaining a 
transmission density of 2.5 in recorded areas as measured with a Macbeth 
transmission densitometer TD904 (visual filter) is from 80 mJ/mm.sup.2 to 
130 mJ/mm.sup.2. 
DETAILED DESCRIPTION OF THE INVENTION 
In the recording material of the present invention, if the energy required 
for increasing the transmission density as measured with a Macbeth 
transmission densitometer TD904 (visual filter) to a value higher by 0.1 
than the transmission density of non-recorded areas is below 10 
mJ/mm.sup.2, the background becomes considerably colored during storage. 
On the other hand, if that energy exceeds 40 mJ/mm.sup.2, not only do 
density fluctuations with a change in environmental conditions become 
larger, but also severe unevenness results. The energy required for 
increasing the transmission density to a value higher by 0.1 than the 
transmission density of non-recorded areas is preferably from 20 
mJ/mm.sup.2 to 30 mJ/mm.sup.2. Furthermore, if the energy required for 
obtaining a transmission density of 2.5 as measured with a Macbeth 
transmission densitometer TD904 (visual filter) is below 80 mJ/mm.sup.2, 
not only do density fluctuations with a change in environmental conditions 
become larger, but severe unevenness results. If the energy required for a 
transmission density of 2.5 exceeds 130 mJ/mm.sup.2, the support becomes 
deformed. The energy required for obtaining a transmission density of 2.5 
is preferably from 90 mJ/mm.sup.2 to 110 mJ/mm.sup.2. 
The substantially transparent support of the present invention is a polymer 
resin film having a haze value represented by the equation 
##EQU1## 
of 40% or lower. The support is preferably a polymer resin film having a 
haze of 20% or lower, more preferably 10% or lower. 
In the present invention, the support is not particularly limited as long 
as it is substantially transparent. The support preferably comprises a 
transparent polymer resin which has been colored so as to have a color 
within the quadrilateral region defined by the four points of A (x=0.2805, 
y=0.3005), B (x=0.2820, y=0.2970), C (x=0.2885, y=0.3015), and D 
(x=0.2870, y=0.3040) in the chromaticity coordinate system described in 
JIS-Z8701. Examples of methods for coloring a transparent polymer resin 
include a method in which a material for forming a transparent sheet, made 
of, e.g., poly(ethylene terephthalate), poly(butylene terephthalate), 
cellulose triacetate, polypropylene, polystyrene, polyethylene, 
poly(vinylidene chloride), polyacrylic, or polycarbonate, is kneaded with 
a blue dye before the material is formed into a resin film; and a method 
in which a blue dye is dissolved in an appropriate solvent to prepare a 
coating liquid, which coating liquid is applied to a transparent and 
colorless resin film by a known coating technique, e.g., gravure coating, 
roller coating, or wire coating, and then dried. Preferred of such 
supports is a film produced by incorporating a blue dye into a polyester 
resin such as poly(ethylene terephthalate) or poly(ethylene naphthalate) 
by kneading, forming the dyed resin into a film, and subjecting this film 
to thermal treatment, stretching, and antistatic treatment. 
Dyes for use in coloring the support are not particularly limited. However, 
dyes for use in blue or violaceous film bases for X-ray photographs are 
generally preferred. Examples of such dyes include the dyes described in 
JP-B-47-8734, JP-B-47-30294, and JP-B-51-25335. (The term "JP-B" as used 
herein means an "examined Japanese patent publication.") Dyes may be used 
alone or in admixture. Specific examples of such blue or violaceous dyes 
include the compounds represented by the following structural formulae. 
##STR1## 
The support for use in the present invention is preferably dyed in a color 
within the quadrilateral region defined by the four points of A (x=0.2805, 
y=0.3005), B (x=0.2820, y=0.2970), C (x=0.2885, y=0.3015), and D 
(x=0.2870, y=0.3040) in the chromaticity coordinate system described in 
JIS-Z8701. This is accomplished by changing the kinds and amounts of dyes 
for use in the coloring. If the color of the dyed support is above the 
straight line A-D in the chromaticity coordinate system with x as abscissa 
and y as ordinate, the color undesirably is greenish blue. If the color of 
the dyed support is below the straight line B-C, red is undesirably 
enhanced. Furthermore, if the color of the dyed support is on the right of 
the straight line D-C, yellow is enhanced, which is undesirable especially 
when observing images formed in highlight parts. 
The thickness of the support is not particularly limited, but the preferred 
range thereof is generally from 25 to 200 .mu.m. 
In the present invention, the thermal energy applied to the heat-sensitive 
recording material with a thermal head is defined by the following 
equation. 
EQU Energy=[energy (V) applied per dot of the thermal head/resistivity 
(.OMEGA.) per dot].sup.2 .times.pulse width (msec).div.(recording area 
(mm.sup.2) per dot per pulse) 
An anti-reflection layer may be formed on the recording material of the 
present invention to reduce the gloss of the side opposite the 
heat-sensitive recording layer to 50% or lower, especially preferably 30% 
or lower, measured at an incident light angle of 20.degree.. The 
glossiness of the support on the side of the anti-reflection layer is 
measured using a specular glossmeter as glossiness to a glass surface at 
an incident angle of 20.degree., according to "Method 5" (specular gloss 
at an incident angle of 20.degree., Gs(20.degree.)) as described in 
JIS-Z8741, 349-353 (1983). The anti-reflection layer contains a polymeric 
binder and a fine particulate substance. Examples of the polymeric binder 
include methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, 
starches, gelatin, modified gelatins, poly(vinyl alcohol), carboxylated 
poly(vinyl alcohol), polyacrylamide, polystyrene and styrene copolymers, 
polyesters and ester copolymers, polyethylene and ethylene copolymers, 
epoxy resins, acrylate or methacrylate resins and acrylate or methacrylate 
copolymers, polyurethane resins, and polyamide resins. 
In order to prevent reflection from the support of light having wavelengths 
which make images blurry, to thereby obtain distinct images having reduced 
gloss, a suitable fine particulate substance is selected so as to provide 
a gloss of 50% or lower, especially preferably 30% or lower, as measured 
at an incident light angle of 20.degree.. Fine particulate substances 
having a particle diameter of from 1 to 20 .mu.m, in particular from 1 to 
10 .mu.m, are especially preferred because such particulate substances can 
be used for regulating gloss without deteriorating image quality. 
Examples of the fine particulate substance include fine particles of starch 
obtained from barley, wheat, corn, rice, and beans; fine particles of 
synthetic polymers such as cellulose fibers, polystyrene resins, epoxy 
resins, polyurethane resins, urea-formalin resins, poly(meth)acrylate 
resins, poly(methyl (meth)acrylate) resins, vinyl chloride/vinyl acetate 
copolymer resins, and polyolefins; and fine particles of inorganic 
substances such as calcium carbonate, titanium oxide, kaolin, smectite 
clay, aluminum hydroxide, silica, and zinc oxide. These fine particulate 
substances may be used in combination of two or more thereof. To obtain a 
heat-sensitive recording material having good transparency, fine 
particulate substances having a refractive index of from 1.45 to 1.75 are 
preferred. 
The finely particulate substance is used in an amount of from 0.5 to 10% by 
weight, preferably from 1 to 5% by weight, based on the amount of the 
polymeric binder contained in the anti-reflection layer. Amounts of the 
particulate substance smaller than 0.5% by weight are undesirable in that 
the anti-reflective effect is insufficient, while amounts thereof 
exceeding 10% by weight are undesirable in that unclear images result, 
although gloss is reduced. 
The heat-sensitive recording material of the present invention may 
optionally have a ultraviolet filter layer for preventing from color 
fading due to light. The ultraviolet filter layer contains an ultraviolet 
absorber in the form of dispersions, emulsions, or microcapsules in an 
amount of generally from 0.1 to 5 g/m.sup.2, preferably from 0.5 to 2.0 
g/m.sup.2. The ultraviolet absorber for use in the present invention is 
not particularly limited. However, preferred examples of the ultraviolet 
absorber include benzotriazole type absorbers and benzophenone type 
absorbers. Details of the ultraviolet filter layer are described, for 
example, in JP-B-53-23205. The ultraviolet filter layer may be provided on 
the side of the support opposite the heat-sensitive recording layer, 
between the support and the heat-sensitive recording layer, or on the 
heat-sensitive recording layer. 
In the present invention, an image is recorded by the reaction of a color 
former with a developer, which components are kept apart from each other 
at ordinary storage temperatures (generally from 0.degree. to 35.degree. 
C.). Upon heating, the color former and the developer come into contact 
with each other to develop a color. 
The color former and the developer each is substantially colorless before 
color development. Upon contact with each other, the two ingredients 
undergo a color developing reaction. Examples of combinations of the two 
ingredients include the following (a) to (m). 
(a) Combinations of a photo-decomposable diazo compound and a coupler. 
(b) Combinations of an electron-donating dye precursor and an 
electron-accepting compound. 
(c) Combinations of an organic metal salt such as silver behenate or silver 
stearate and a reducing agent such as protocatechuic acid, spiroindane, or 
hydroquinone. 
(d) Combinations of a long-chain fatty acid salt such as ferric stearate or 
ferric myristate and a phenol derivative such as gallic acid or ammonium 
salicylate. 
(e) Combinations of a salt of an organic acid such as acetic acid, stearic 
acid, or palmitic acid with a heavy metal such as nickel, cobalt, lead, 
copper, iron, mercury, or silver and a sulfide of an alkaline earth metal 
such as calcium sulfide, strontium sulfide, or potassium sulfide; or 
combinations of such a heavy metal salt of an organic acid and an organic 
chelating agent such as s-diphenylcarbazide or diphenylcarbazone. 
(f) Combinations of a (heavy) metal sulfate such as silver sulfide, lead 
sulfide, mercury sulfide, or sodium sulfide and a sulfur compound such as 
Na-tetrathionate, sodium thiosulfate, or thiourea. 
(g) Combinations of a ferric salt of a fatty acid such as ferric stearate 
and an aromatic polyhydroxy compound such as 
3,4-dihydroxytetraphenylmethane. 
(h) Combinations of a noble metal salt of an organic acid such as silver 
oxalate or mercury oxalate and an organic polyhydroxy compound such as a 
polyhydroxy alcohol, glycerol, or a glycol. 
(i) Combinations of a ferric salt of a fatty acid such as ferric 
pelargonate or ferric laurate and either thiocetylcarbamide or an 
isothiocetylcarbamide derivative. 
(j) Combinations of a lead salt of an organic acid such as lead caproate, 
lead pelargonate, or lead behenate and a thiourea derivative such as 
ethylenethiourea or N-dodecylthiourea. 
(k) Combinations of a heavy metal salt of a higher fatty acid such as 
ferric stearate or copper stearate and a zinc dialkyldithiocarbamate. 
(l) Combinations which form an oxazine dye, such as combinations of 
resorcinol and a nitroso compound. 
(m) Combinations of a formazan compound and a reducing agent and/or a metal 
salt. 
Preferred of these in the present invention are combinations (a), i.e., 
combinations of a photo-decomposable diazo compound and a coupler, 
combinations (b), i.e., combinations of an electron-donating dye precursor 
and an electron-accepting compound, and combinations (c), i.e., 
combinations of an organic metal salt and a reducing agent, with 
combinations (a) and combinations (b) being especially preferred. Namely, 
the use a combination of an electron-donating dye precursor (color former) 
and an acid substance (developer) or a combination of a diazo compound 
(color former) and a coupling compound (developer) is preferred. 
Especially from the standpoint of image clearness, the former combination 
is preferably employed. 
The electron-donating colorless dye for use in the present invention is not 
particularly limited, as long as it is a substantially colorless 
substance. However, the electron-donating dye is preferably a 
substantially colorless compound which has the property of developing a 
color upon donation of an electron or upon reception of a proton from an 
acid, etc., and which has a lactone, lactam, sultone, spiropiran, ester, 
or amide skeleton or a similar skeleton and undergoes ring opening or 
cleavage at this skeleton upon contact with a developer. Examples of the 
color former include various compounds such as triphenylmethanephthalide 
compounds, fluoran compounds, phenothiazine compounds, indolylphthalide 
compounds, leuco auramine compounds, rhodamine lactam compounds, 
triphenylmethane compounds, triazene compounds, spiropyran compounds, and 
fluorene compounds. Specific examples of the phthalide compounds are 
described in U.S. Reissued Patent U.S. Pat. No. 23,024 and U.S. Pat. Nos. 
3,491,111, 3,491,112, 3,491,116, and 3,509,174. Specific examples of the 
fluoran compounds are described in U.S. Pat. Nos. 3,624,107, 3,627,787, 
3,641,011, 3,462,828, 3,681,390, 3,920,510, and 3,959,571. Specific 
examples of spirodipyran compounds are described in U.S. Pat. No. 
3,971,808. Specific examples of pyridine compounds and pyrazine compounds 
are described in U.S. Pat. Nos. 3,775,424, 3,853,869, and 4,246,318. 
Specific examples of the fluorene compounds are described in 
JP-A-63-94878. 
Particularly effective of these color formers are black-developing 
2-arylamino-3-(H, halogen, alkyl, or alkoxy)-6-(substituted 
amino)fluorans. Specific examples thereof include 
2-anilino-3-methyl-6-diethylaminofluoran, 
2-anilino-3-methyl-6-N-cyclohexyl-N-methylaminofluoran, 
2-p-chloroanilino-3-methyl-6-dibutylaminofluoran, 
2-anilino-3-methyl-6-dioctylaminofluoran, 
2-anilino-3-chloro-6-diethylaminofluoran, 
2-anilino-3-methyl-6-N-ethyl-N-isoamylaminofluoran, 
2-anilino-3-methyl-6-N-ethyl-N-dodecylaminofluoran, 
2-anilino-3-methoxy-6-dibutylaminofluoran, 
2-o-chloroanilino-6-dibutylaminofluoran, 
2-p-chloroanilino-3-ethyl-6-N-ethyl-N-isoamylaminofluoran, 
2-o-chloroanilino-6-p-butylanilinofluoran, 
2-anilino-3-pentadecyl-6-diethylaminofluoran, 
2-anilino-3-ethyl-6-dibutylaminofluoran, 
2-o-toluidino-3-methyl-6-diisopropylaminofluoran, 
2-anilino-3-methyl-6-N-isobutyl-N-ethylaminofluoran, 
2-anilino-3-methyl-6-N-ethyl-N-tetrahydrofurfurylaminofluoran, 
2-anilino-3-chloro-6-N-ethyl-N-isoamylaminofluoran, 
2-anilino-3-methyl-6-N-methyl-N-.gamma.-ethoxypropylaminofluoran, 
2-anilino-3-methyl-6-N-ethyl-N-.gamma.-ethoxypropylaminofluoran, and 
2-anilino-3-methyl-6-N-ethyl-N-.gamma.-propoxypropylaminofluoran. 
The developer for use in combination with these color formers may be a 
phenol compound or an acid substance such as an organic acid or a metal 
salt thereof or a hydroxybenzoic acid ester. Examples of such compounds 
are described, e.g., in JP-A-61-291183. Specific examples of the developer 
include bisphenols and analogues thereof such as 
2,2-bis(4'-hydroxyphenyl)propane (generally known as bisphenol A), 
2,2-bis(4'-hydroxyphenyl)pentane, 
2,2-bis(4'-hydroxy-3',5'-dichlorophenyl)propane, 
1,1-bis(4'-hydroxyphenyl)cyclohexane, 2,2-bis(4'-hydroxyphenyl)hexane, 
1,1-bis(4'-hydroxyphenyl)propane, 1,1-bis(4'-hydroxyphenyl)butane, 
1,1-bis(4'-hydroxyphenyl)pentane, 1,1-bis(4'-hydroxyphenyl)hexane, 
1,1-bis(4'-hydroxyphenyl)heptane, 1,1-bis(4'-hydroxyphenyl)octane, 
1,1-bis(4'-hydroxyphenyl)-methylpentane, 
1,1-bis(4'-hydroxyphenyl)-2-ethylhexane, 
1,1-bis(4'-hydroxyphenyl)dodecane, 1,4-bis(p-hydroxyphenylcumyl)benzene, 
1,3-bis(p-hydroxyphenylcumyl)benzene, bis(p-hydroxyphenyl) sulfone, 
bis(3-allyl-4-hydroxyphenyl) sulfone, and benzyl 
bis(p-hydroxyphenyl)acetate; salicylic acid derivatives such as 
3,5-di-.alpha.-methylbenzylsalicylic acid, 3,5-di-t-butylsalicylic acid, 
3-.alpha.,.alpha.-dimethylbenzylsalicylic acid, and 
4-(.beta.-p-methoxyphenoxyethoxy)salicylic acid and salts of these acids 
with a polyvalent metal (especially preferably zinc or aluminum); 
hydroxybenzoic acid esters and analogues thereof such as benzyl 
p-hydroxybenzoate, 2-ethylhexyl p-hydroxybenzoate, and 2-phenoxyethyl 
.beta.-resorcylate; and phenols and analogues thereof such as 
p-phenylphenol, 3,5-diphenylphenol, cumylphenol, 
4-hydroxy-4'-isopropoxydiphenyl sulfone, and 4-hydroxy-4'-phenoxydiphenyl 
sulfone. Of these compounds, the bisphenols and analogues thereof are 
preferred for improving color-developing properties. The addition amount 
of the developer is preferably from 50 to 800% by weight, more preferably 
from 100 to 500% by weight, based on the amount of the color former. The 
electron-accepting compounds described above may be used in a combination 
of two or more thereof. 
The diazo compound for use in the present invention is a photo-decomposable 
diazo compound which produces a desired hue upon reaction with a developer 
called a coupling ingredient (described below). Upon exposure to light 
having a specific wavelength prior to contact with the coupling 
ingredient, the diazo compound decomposes so that it does not produce a 
color even upon contact with the coupling ingredient. The hue obtained 
with this color-developing system is determined by the diazo dye formed by 
the reaction of the diazo compound with the coupling ingredient. It is 
therefore possible to easily obtain various developed colors by changing 
the chemical structure of the diazo compound or of the coupling 
ingredient, as is well known. Most colors can be developed depending on 
the combination of the two ingredients. 
The photo-decomposable diazo compound for use in the present invention 
typically is an aromatic diazo compound. More particularly, the 
photo-decomposable diazo compound includes aromatic diazonium salt, a 
diazo sulfonate compound, or a diazo amino compound. The diazonium salt is 
a compound represented by the general formula ArN.sub.2.sup.+ X.sup.- 
(wherein Ar represents a substituted or unsubstituted aromatic moiety, 
N.sub.2.sup.+ represents a diazonium group, and X.sup.- represents an acid 
anion). A large number of diazo sulfonate compounds are known, which are 
obtained by treating the corresponding diazonium salts with a sulfite. The 
diazo amino compound is obtained by coupling a diazo group with 
dicyandiamide, sarcosine, methyltaurine, N-ethylanthranilic 
acid-5-sulfonic acid, monoethanolamine, diethanolamine, guanidine, or the 
like. Details of these diazo compounds are described, e.g., in 
JP-A-2-136286. 
Examples of the coupling ingredient for coupling with the diazo compound in 
this invention include 2-hydroxy-3-naphthoic acid anilide and the coupling 
compounds described in JP-A-62-146678 including resorcinol. 
In the case where a combination of a diazo compound and a coupling 
ingredient is used in this invention, a basic substance may be added in 
order to accelerate the coupling reaction. This basic substance may be a 
water-insoluble or slightly water-soluble basic substance or a substance 
which generates an alkali upon heating. Examples of these substances 
include nitrogen-containing compounds such as inorganic and organic 
ammonium salts, organic amines, amides, urea and thiourea and derivatives 
thereof, thiazole, pyrrole, pyrimidine, piperazine, guanidine, indole, 
imidazole, imidazoline, triazole, morpholine, piperidine, amidines, 
formamidine, pyridine, and the like and derivatives of such 
nitrogen-containing compounds. Specific examples of these compounds are 
described, e.g., in JP-A-61-291183. These basic compounds may be used in a 
combination of two or more thereof. 
The color former or developer for use in the present invention may be 
incorporated into the heat-sensitive recording layer in the form of solid 
dispersions by a known method. However, it is preferable to encapsulate 
the ingredient to improve the transparency of the heat-sensitive recording 
layer and to attain good shelf stability (antifogging) by preventing the 
color former and the developer from coming into contact with each other at 
ordinary temperatures, and also to control the sensitivity for color 
development, i.e., developing a color with a desired heat energy. 
When the color former or developer is used in the form of solid 
dispersions, the smaller the particle diameter of the solid dispersions 
is, the lower energy required for developing a color is. Therefore, in the 
present invention, the use of the combination of rather small-size 
dispersions having a 50% volume average particle diameter of from 0.1 to 
0.6 .mu.m (measured with a laser diffraction particle size distribution 
analyzer, LA-500; manufactured by Horiba Ltd.) and rather large-size 
dispersions having a 50% volume average particle diameter of from 0.6 to 
2.0 .mu.m in admixture is preferable for developing a color with a desired 
heat energy. The mixing ratio of the small dispersions to the large 
dispersions is generally from 1:10 to 10:1. 
When the color former or developer is used in the form of microcapsules, a 
color develops with low energy in the case of that the particle diameter 
of microcapsules is small, the glass transition temperature of a material 
of a microcapsule wall is low, or the thickness of a microcapsule wall is 
thin. On the other hand, a color develops with high energy in the case of 
that the particle diameter of microcapsules is large, the glass transition 
temperature of a material of a microcapsule wall is high, or the thickness 
of a microcapsule wall is thick. Therefore, microcapsules which develop a 
color with low energy and microcapsules which develop a color with high 
energy are preferably used in combination for developing a color with a 
desired heat energy. The microcapsules which develop with low energy 
preferably have a particle diameter of from 0.1 to 2 .mu.m, and the wall 
of the microcapsules preferably has a thickness of from 0.005 to 0.1 .mu.m 
and comprises a material having a glass transition temperature of from 
60.degree. to 110.degree. C. The microcapsules which develop with high 
energy preferably have a particle diameter of from 0.1 to 2 .mu.m, and the 
wall of the microcapsules preferably has a thickness of from 0.001 to 0.2 
.mu.m and comprises a material having a glass transition temperrature of 
from 110.degree. to 170.degree. C. The mixing ratio of the former to the 
latter is generally from 1:10 to 10:1. 
For producing microcapsules for use in the present invention, any of an 
interfacial polymerization method, internal polymerization method, and 
external polymerization method can be employed. Especially preferred is an 
interfacial polymerization technique in which a core-forming substance 
containing an electron-donating colorless dye, a diazonium salt, etc. is 
emulsified into an aqueous solution containing a water-soluble compound 
dissolved therein, and a wall of a polymeric substance is then formed on 
the entire surface of each resulting oil droplet. 
The reactants for forming the polymer are added to the inside and/or the 
outside of the oil droplets. Examples of the polymeric substance include 
polyurethanes, polyureas, polyamides, polyesters, polycarbonates, 
urea-formaldehyde resins, melamine resins, polystyrene, 
styrene-methacrylate copolymers, and styrene-acrylate copolymers. 
Preferred polymeric substances are polyurethanes, polyureas, polyamides, 
polyesters, and polycarbonates, with polyurethanes and polyureas being 
especially preferred. These polymeric substances may be used in 
combination of two or more thereof. Examples of the water-soluble polymer 
include gelatin, polyvinylpyrrolidone, and poly(vinyl alcohol). 
For example, in the case of using a polyurea as a capsule wall material, 
microcapsule walls can be easily formed by reacting a polyisocyanate, 
e.g., a diisocyanate, a triisocyanate, a tetraisocyanate, or a 
polyisocyanate prepolymer, with a polyamine, e.g., a diamine, a triamine, 
or a tetramine, a prepolymer having two or more amino groups, piperazine 
or a derivative thereof, a polyol, or the like in an aqueous medium by 
interfacial polymerization. 
Furthermore, capsule walls made of, for example, a polyurea/polyamide 
composite material or a polyurethane/polyamide composite material can be 
produced, for example, from a polyisocyanate and an acid chloride or from 
a polyamine and a polyol by regulating the pH of the dispersion medium as 
a reaction medium and then heating the system. Details of a process for 
producing capsule walls made of a polyurea/polyamide composite material 
are described in JP-A-58-66948. 
For controlling the energy required for color development, a mixture of 
microcapsules of two or more kinds having capsule walls having different 
glass transition temperatures may be employed in the present invention if 
desired. 
For forming microcapsule walls for use in the present invention which 
become substance-permeable at lower temperatures, a plasticizer which is 
solid at ordinary temperatures and preferably has a melting point of 
50.degree. C. or higher, but preferably not higher than 120.degree. C., 
may be selected from plasticizers suitable for polymers for use as a 
microcapsule wall material. For example, in the case of a polyurea or 
polyurethane wall material, preferred plasticizers include hydroxy 
compounds, carbamic esters, aromatic alkoxy compounds, organic sulfonamide 
compounds, aliphatic amide compounds, and arylamide compounds. 
For imparting satisfactory storage stability to the recording material, it 
is preferred to microencapsulate the color former and to incorporate the 
developer in the form of an emulsion. 
In the present invention, the developer may also be used in the form of 
dispersed solid particles. However, for improving the transparency of the 
heat-sensitive recording layer to impart satisfactory light transmission 
to the heat-sensitive recording material, it is especially preferred to 
use the developer in the form of an emulsified dispersion. The dispersion 
may be obtained by dissolving the developer into a slightly water-soluble 
or water-insoluble organic solvent, and mixing the solution with an 
aqueous phase containing a water-soluble polymer having a surfactant as a 
protective colloid to emulsify the solution. 
The organic solvent for use in the emulsification may be suitably selected 
from high-boiling oils. Preferred oils include esters, 
dimethylnaphthalene, diethylnaphthalene, diisopropylnaphthalene, 
dimethylbiphenyl, diisopropylbiphenyl, diisobutylbiphenyl, 
1-methyl-1-dimethyphenyl-2-phenylmethane, 
1-ethyl-1-dimethylphenyl-1-phenylmethane, 
1-propyl-1-dimethylphenyl-1-phenylmethane, triallylmethanes (e.g., 
tritolylmethane and tolyldiphenylmethane), terphenyl compounds (e.g., 
terphenyl), alkyl compounds, alkylated diphenyl ethers (e.g., 
propyldiphenyl ether), hydrogenated terphenyls (e.g., hexahydroterphenyl), 
and diphenyl ether. Of these organic solvents, esters are especially 
preferably used to impart stability to the emulsion state. 
Examples of the esters include phosphoric esters (e.g., triphenyl 
phosphate, tricresyl phosphate, butyl phosphate, octyl phosphate, and 
cresyl phenyl phosphate), phthalic esters (e.g., dibutyl phthalate, 
2-ethylhexyl phthalate, ethyl phthalate, octyl phthalate, and butyl benzyl 
phthalate), dioctyl tetrahydrophthalate, benzoic esters (e.g., ethyl 
benzoate, propyl benzoate, butyl benzoate, isopentyl benzoate, and benzyl 
benzoate), abietic esters (e.g., ethyl abietate and benzyl abietate), 
dioctyl adipate, isodecyl succinate, dioctyl azelate, oxalic esters (e.g., 
dibutyl oxalate and dipentyl oxalate), diethyl malonate, maleic esters 
(e.g., dimethyl maleate, diethyl maleate, and dibutyl maleate), tributyl 
citrate, sorbic esters (e.g., methyl sorbate, ethyl sorbate, and butyl 
sorbate), sebacic esters (e.g., dibutyl sebacate and dioctyl sebacate), 
ethylene glycol esters (e.g., mono- and diesters with formic acid, mono- 
and diesters with butyric acid, mono- and diesters with lauric acid, mono- 
and diesters with palmitic acid, mono- and diesters with stearic acid, and 
mono- and diesters with oleic acid), triacetin, diethyl carbonate, 
diphenyl carbonate, ethylene carbonate, propylene carbonate, and boric 
esters (e.g., tributyl borate and tripentyl borate). Of these esters, 
tricresyl phosphate alone or in combination with other esters is 
especially preferred for best emulsion stability. The above described oils 
may be used in combination with each other or with other oil(s). 
An auxiliary solvent as a low-boiling dissolution aid may be added to the 
above-described organic solvent in the present invention. Especially 
preferred examples of the auxiliary solvent include ethyl acetate, 
isopropyl acetate, butyl acetate, and methylene chloride. 
The water-soluble polymer incorporated as a protective colloid into the 
aqueous phase, which aqueous phase is mixed with an oil phase comprising 
the above described ingredients, suitably selected from known anionic, 
nonionic, and ampholytic polymers. However, poly(vinyl alcohol), gelatin, 
and cellulose derivatives are especially preferred. 
The surfactant incorporated into the aqueous phase is a surfactant which 
does not cause the protective colloid to precipitate or flocculate, and is 
suitably selected from anionic and nonionic surfactants. Preferred 
examples of the surfactant include sodium alkylbenzenesulfonates, sodium 
alkylsulfates, dioctyl sulfosuccinate sodium salt, and poly(alkylene 
glycol)s (e.g., polyoxyethylene nonyl phenyl ether). 
The emulsified dispersion in the present invention is easily obtained by 
mixing an oil phase comprising the above described ingredients with an 
aqueous phase containing a protective colloid and a surfactant using known 
means for microemulsification, e.g., high-speed agitation or ultrasonic 
dispersion, to disperse the oil phase. 
The proportion of the oil phase to the aqueous phase (weight of the oil 
phase/weight of the aqueous phase) is preferably from 0.02 to 0.6, more 
preferably from 0.1 to 0.4. If the proportion of the oil phase is below 
0.02, the dispersion is too dilute to obtain sufficient color development. 
If the oil-phase proportion is above 0.6, the viscosity of the dispersion 
is too high. This results in handling difficulties and reduced stability 
of the coating liquid. 
For coating a support with the thus-prepared liquid for forming a 
heat-sensitive layer, a known coating means for applying a water-based or 
organic-solvent-based coating liquid may be used. In this case, methyl 
cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, a starch, 
gelatin, poly(vinyl alcohol), carboxylated poly(vinyl alcohol), 
polyacrylamide, polystyrene or a styrene copolymer, a polyester or an 
ester copolymer, polyethylene or an ethylene copolymer, an epoxy resin, an 
acrylate or methacrylate resin or an acrylate or methacrylate copolymer, a 
polyurethane resin, a polyamide resin, or the like may be used in this 
invention along with the microcapsules in order to safely and evenly apply 
the liquid for forming a heat-sensitive layer and to maintain the strength 
of the coating film. 
A pigment, a wax, a hardener, and the like may be added to the 
heat-sensitive recording layer as needed. The heat-sensitive recording 
layer is preferably applied such that the total amount of the color former 
and the developer is from 0.1 to 10 g/m.sup.2 and the thickness of the 
layer is from 1 to 20 .mu.m. 
The heat-sensitive recording material of the present invention may comprise 
a multilayer coating comprising two or more heat-sensitive recording 
layers having different recording sensitivities to obtain a desired 
heat-sensitive recording material (e.g., sensitive to different levels of 
thermal energy applied by a thermal head). 
When a combination of a photo-decomposable diazo compound and a coupler is 
used, such recording layers described above each having a different 
color-developing property can be prepared, for example, by using bases 
each having a different basicity, or by using couplers each having a 
different coupling ability. In case of a combination of an 
electron-donating dye precursor and an electron-accepting compound, such 
recording layers can be prepared, for example, by using electron-donating 
dye precursors each having a different basicity, by using 
electron-accepting compounds each having a different acidity, or by using 
sensitizing agents each having a different melting point. In case of a 
combination of an organic metal salt and a reducing agent is used, such 
recording layers can be prepared, for example, by using reducing agents 
each having a different reducing ability, or by using auxiliary melting 
agents each having a different menting point. 
In the present invention, a subbing layer is preferably formed on the 
support prior to forming the heat-sensitive recording layer containing 
microcapsules etc. and an anti-reflection layer. The subbing layer 
increases adhesion to thereby prevent the heat-sensitive layer and the 
anti-reflection layer from peeling off the support. For forming the 
subbing layer, an acrylic ester copolymer, poly(vinylidene chloride), SBR, 
a water-soluble polyester, or the like may be used. The thickness of the 
subbing layer is preferably from 0.1 to 0.5 .mu.m. Two or more of the 
subbing layers may be formed on the support for enhancing adhesion. 
When forming the heat-sensitive recording layer and anti-reflection layer 
on the subbing layer by coating with a coating liquid, the subbing layer 
may swell due to water contained in the coating liquid. This swelling may 
deteriorate images recorded on the heat-sensitive recording layer. It is 
therefore preferable to harden the subbing layer with a hardener. Examples 
of the hardener include dialdehydes such as glutaraldehyde, 
2,3-dihydroxy-1,4-dioxane, etc. and boric acid, etc. These hardeners may 
be added in an amount suitable for the desired hardness, generally in the 
range of from 0.20 to 3.0% by weight based on the total amount of the 
subbing layer. 
A protective layer is preferably formed on the heat-sensitive recording 
layer in the present invention in order to avoid light scattering on the 
heat-sensitive layer surface, which scattering may result, e.g., in a 
decrease in apparent transparency. This protective layer may be formed by 
a known method. Such protective layers are described in detail, e.g., in 
"Kami Pulp Gijutsu Times (Paper Pulp Technology Times)" (September 1985), 
pp. 2-4 and JP-A-63-318546. 
To form a protective layer having satisfactory transparency, it is 
especially preferred to use wholly saponified poly(vinyl alcohol), 
carboxylated poly(vinyl alcohol), silica-modified poly(vinyl alcohol), or 
the like. The protective layer may contain a known hardener, a wax, a 
pigment, etc. Along with or in place of the conventional protective layer 
described above, a protective layer containing a silicone resin as a main 
component may be formed in the present invention. The silicone-based 
protective layer imparts good water resistance without impairing the 
transparency of the heat-sensitive recording layer. 
The heat-sensitive recording layer, protective layer, anti-reflection 
layer, and subbing layer are formed by a known coating method, e.g., blade 
coating, air-knife coating, gravure coating, roll coating, spray coating, 
dip coating, or bar coating. 
As described above, the heat-sensitive recording material of the present 
invention comprises a substantially transparent support and a 
heat-sensitive recording layer formed on one side of the support. The 
recording material is characterized in that the thermal recording energy 
required for increasing the transmission density of recorded areas as 
measured with a Macbeth transmission densitometer TD904 (visual filter) to 
a value higher by 0.1 than the transmission density of non-recorded areas 
is from 10 mJ/mm.sup.2 to 40 mJ/mm.sup.2, and that the energy required for 
obtaining a transmission density of 2.5 as measured with a Macbeth 
transmission densitometer TD904 (visual filter) is from 80 mJ/mm.sup.2 to 
130 mJ/mm.sup.2. Consequently, the heat-sensitive recording material not 
only exhibits little unevenness in thermal-head recording caused by 
fluctuations in thermal-head resistivity, surface roughness of a glazing 
layer, etc., but also is stable to changes in environmental conditions 
including temperature and humidity. Thus, highly precise images such as 
medical images are stably obtained using the heat-sensitive recording 
material of the present invention and a conventional thermal head. 
The present invention will be explained below in detail by reference to the 
following Examples, but the invention should not be construed as being 
limited to these Examples. Hereinafter, unless otherwise noted, all 
concentration values are given in terms of % by weight.

EXAMPLE 1 
(Preparation of Microcapsule Dispersion A) 
Into a mixed solvent containing 0.08 g of butanol and 3.6 g of ethyl 
acetate were dissolved a color former ingredient containing of 1.9 g of 
2-anilino-3-methyl-6-N-ethyl-N-butylaminofluoran, 0.25 g of color former 
GN-2 (manufactured by Yamamoto Chemicals, Inc.), 0.36 g of color former 
GN-169 (manufactured by Yamamoto Chemicals, Inc.), 0.59 g of 
1-methyl-3-methoxy-6-diethylaminofluoran, 0.2 g of Pergascript RED I-6B 
(manufactured by Ciba-Geigy Ltd.), and 0.08 g of Tinuvin P (manufactured 
by Ciba-Geigy Ltd.) and a capsule wall-forming material containing 0.4 g 
of a trimethylolpropane/xylylene diisocyanate adduct (isocyanate content, 
11.6%; Takenate D-110N, trade name of Takeda Chemical Industries, Ltd.), 
1.1 g of 1:2 by mol adduct of bisphenol A with xylylene diisocyanate, and 
1.0 g of Sumidur N3200 (trade name for a capsule wall-forming agent 
manufactured by Sumitomo Chemical Co., Ltd.). The solution thus obtained 
was mixed with an aqueous phase prepared by mixing 7.5 g of an 8 wt % 
aqueous solution of poly(vinyl alcohol) (PVA 217C, manufactured by Kuraray 
Co., Ltd.) with 2.6 g of ion-exchanged water. This mixture was then 
emulsified with an Ace Homogenizer (manufactured by Nippon Seiki Co., 
Ltd.) at 10,000 rpm for 5 minutes. To the thus-obtained emulsion was added 
1.55 g of a 4% aqueous tetraethylenepentamine solution. An encapsulation 
reaction was then conducted at 40.degree. C. for 3 hours to prepare a 
capsule dispersion having an average particle diameter of 0.7 .mu.m. All 
average particle diameters are given as a 50% volume-average particle 
diameter measured with a laser diffraction particle size distribution 
analyzer (manufactured by Horiba Ltd.). 
(Preparation of Microcapsule Dispersion B) 
Into a mixed solvent containing 0.08 g of butanol and 3.6 g of ethyl 
acetate were dissolved a color former ingredient containing 1.3 g of 
2-anilino-3-methyl-6-N-ethyl-N-butylaminofluoran, 0.17 g of color former 
GN-2 (manufactured by Yamamoto Chemicals, Inc.), 0.25 g of color former 
GN-169 (manufactured by Yamamoto Chemicals, Inc.), 0.40 g of 
1-methyl-3-methoxy-6-diethylaminofluoran, 0.14 g of Pergascript RED I-6B 
(manufactured by Ciba-Geigy Ltd.), and 0.05 g of Tinuvin P (manufactured 
by Ciba-Geigy Ltd.) and a capsule wall-forming material containing 4.00 g 
of a trimethylolpropane/xylylene diisocyanate adduct (isocyanate content, 
12.5%; Takenate D-110NH, trade name of Takeda Chemical Industries, Ltd.). 
The solution thus obtained was mixed with an aqueous phase prepared by 
mixing 7.5 g of an 8 wt % aqueous solution of poly(vinyl alcohol) (PVA 
217C, manufactured by Kuraray Co., Ltd.) with 2.6 g of ion-exchanged 
water. This mixture was then emulsified with an Ace Homogenizer 
(manufactured by Nippon Seiki Co., Ltd.) at 10,000 rpm for 5 minutes. To 
the thus-obtained emulsion was added 1.55 g of 4% aqueous 
tetraethylenepentamine solution. An encapsulation reaction was then 
conducted at 40.degree. C. for 3 hours to prepare a capsule dispersion 
having an average particle diameter of 0.7 .mu.m. All average particle 
diameters are given as a 50% volume-average particle diameter measured 
with a laser diffraction particle size distribution analyzer (manufactured 
by Horiba Ltd.). 
(Preparation of Developer Emulsion A) 
Into a mixed solvent containing 0.08 g of tricresyl phosphate, 0.04 g of 
diethyl maleate, and 1.5 g of ethyl acetate were dissolved 0.34 g of 
n-butyl bis(4-hydroxyphenyl)acetate, 0.83 g of the developer represented 
by structural formula (1): 
##STR2## 
0.83 g of the developer represented by structural formula (2): 
##STR3## 
0.58 g of the developer represented by structural formula (3): 
##STR4## 
0.39 g of Sumilizer MDP-S (trade name of Sumitomo Chemical Co., Ltd.), and 
0.35 g of Tinuvin 328 (trade name of Ciba-Geigy Ltd.). The solution thus 
obtained was mixed with an aqueous phase prepared by mixing 4.1 g of an 8 
wt % aqueous solution of poly(vinyl alcohol) (PVA 217C, manufactured by 
Kuraray Co., Ltd.) with 8.2 g of ion-exchanged water, 0.9 g of a 2% 
aqueous solution of sodium (p-nonylbenzene butyl ether)sulfonate, and 0.9 
g of a 2% aqueous solution of sodium dodecylsulfonate. This mixture was 
emulsified with an Ace Homogenizer (manufactured by Nippon Seiki Co., 
Ltd.) at 10,000 rpm to obtain an emulsion having an average particle 
diameter of 0.5 .mu.m. 
(Preparation of Developer Emulsion B) 
Into a mixed solvent containing 0.08 g of tricresyl phosphate, 0.04 g of 
diethyl maleate, and 1.5 g of ethyl acetate were dissolved 0.93 g of the 
developer represented by structural formula (4): 
##STR5## 
0.93 g of the developer represented by structural formula (5): 
##STR6## 
0.37 g of Sumilizer MDP-S (trade name of Sumitomo Chemical Co., Ltd.), 
0.37 g of Sumilizer BBM-S (trade name of Sumitomo Chemical Co., Ltd.), 
0.37 g of 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, and 0.35 g 
of Tinuvin 328 (trade name of Ciba-Geigy Ltd.). The solution thus obtained 
was mixed with an aqueous phase prepared by mixing 4.1 g of an 8 wt % 
aqueous solution of poly(vinyl alcohol) (PVA 217C, manufactured by Kuraray 
Co., Ltd.) with 8.2 g of ion-exchanged water, 0.9 g of a 2% aqueous 
solution of sodium (p-nonylbenzene butyl ether)sulfonate, and 0.9 g of a 
2% aqueous solution of sodium dodecylsulfonate. This mixture was 
emulsified with an Ace Homogenizer (manufactured by Nippon Seiki Co., 
Ltd.) at 10,000 rpm to obtain an emulsion having an average particle 
diameter of 0.5 .mu.m. 
(Preparation of Protective-Layer Liquid) 
(1) Preparation of Pigment Dispersion 
To 99.0 g of water was added 24.8 g of kaolin (Kaogloss, manufactured by 
Mizusawa Industrial Chemicals, Ltd.). This mixture was stirred for 3 
hours. To 99.3 g of this dispersion were added 4.6 g of a 40% dispersant 
(POIZ 532A, manufactured by Kao Corporation), 10 g of a 10% aqueous 
solution of poly(vinyl alcohol) (PVA 105, manufactured by Kuraray Co., 
Ltd.), 10.4 g of a 30 wt % zinc stearate dispersion (Z-7-30, manufactured 
by Chukyo Yushi Co., Ltd.), 0.6 g of a 10% aqueous solution of sodium 
dodecylbenzenesulfonate, and 6.8 g of water. This mixture was treated with 
a ball mill to disperse the kaolin to an average particle diameter of 0.8 
.mu.m. 
(2) Preparation of Protective-Layer Liquid 
The kaolin dispersion described above was added in an amount of 28.2 g to a 
mixture of 51.3 g of water, 407.7 g of a 6 wt % aqueous solution of 
poly(vinyl alcohol) (PVA 124C, manufactured by Kuraray Co., Ltd.), 0.42 g 
of a 20.5 wt % zinc stearate dispersion (F155, manufactured by Chukyo 
Yushi Co., Ltd.), and 21.4 g of 1.14% aqueous boric acid solution. This 
mixture was mixed with 3.11 g of a 30 wt % dispersion of a wax (paraffin 
wax having a melting point of 68.degree. C.; average particle diameter, 
0.25 .mu.m), 4.5 g of a 4% aqueous fluoropolymer dispersion (ME413 
manufactured by Daikin Industries, Ltd.), 10.5 g of a 10% aqueous solution 
of sodium dodecylbenzenesulfonate, 16.7 g of a 2 wt % aqueous solution of 
the compound represented by structural formula (6): 
EQU C.sub.8 F.sub.17 SO.sub.2 N(C.sub.3 H.sub.7)CH.sub.2 COOK (6) 
and 0.9 g of 40% aqueous glyoxal solution. Thus, a protective-layer liquid 
was obtained. 
(Preparation of Capsules for Ultraviolet Filter Layer) 
To 8.18 g of ethyl acetate were added 1.58 g of Tinuvin PS (trade name of 
Ciba-Geigy Ltd.), 6.28 g of Tinuvin 328 (trade name of Ciba-Geigy Ltd.), 
5.22 g of Tinuvin 343 (trade name of Ciba-Geigy Ltd.), 1.44 g of the 
compound represented by structural formula (7): 
##STR7## 
and 7.28 g of the compound represented by the following structural formula 
(8) 
EQU O.dbd.P--(OCH.sub.2 CH.sub.2 CH(CH.sub.3)CH.sub.2 C(CH.sub.3).sub.2 
CH.sub.3).sub.3 (8) 
This mixture was heated to 70.degree. C. to obtain a solution, which was 
then cooled to 35.degree. C. Thereto were added 0.9 g of Takenate D110N 
(trade name for a capsule wall-forming agent manufactured by Takeda 
Chemical Industries, Ltd.) and 0.3 g of Burhock D750 (trade name of a 
capsule wall-forming agent manufactured by Dainippon Ink & Chemicals, 
Inc.) as capsule wall-forming agents. This mixture was warmed at 
35.degree. C. for 40 minutes. The solution thus obtained was mixed with an 
aqueous phase obtained by mixing 116.3 g of a 15 wt % aqueous solution of 
poly(vinyl alcohol) (PVA 205, manufactured by Kuraray Co., Ltd.) with 8.0 
g of a 10 wt % aqueous solution of sodium dodecylbenzenesulfonate. This 
mixture was emulsified with an Ace Homogenizer (manufactured by Nippon 
Seiki Co., Ltd.) at 15,000 rpm for 15 minutes to obtain an emulsion having 
a 50% volume-average particle diameter of 0.25 .mu.m. To the thus-obtained 
emulsion were added 58 g of water and 0.15 g of tetraethylenepentamine. 
An encapsulation reaction was then conducted at 40.degree. C. for 3 hours 
to prepare a capsule dispersion having an average particle diameter of 
0.25 .mu.m. 
(Preparation of Coating Liquid for Ultraviolet Filter Layer) 
To a mixture of 42.31 g of water and 42.0 g of a 9.54 wt % aqueous solution 
of silanol-modified poly(vinyl alcohol) (R2105, manufactured by Kuraray 
Co., Ltd.) was added 13.36 g of the above-described capsule dispersion for 
an ultraviolet filter layer (solid content, 24.15%). This mixture was 
mixed with 16.87 g of a 50 wt % aqueous solution of the compound 
represented by structural formula (9): 
##STR8## 
and 65.3 g of 20% colloidal silica (Snowtex O, manufactured by Nissan 
Chemical Industries, Ltd.). Thus, a coating liquid for ultraviolet filter 
layer was prepared. 
(Preparation of Anti-reflection Layer Liquid) 
To 47.82 g of water was added 0.105 g of rice starch having an average 
particle diameter of 5 .mu.m (manufactured by Matsutani Kagaku Kogyo Co., 
Ltd.). After the starch was sufficiently dispersed, this mixture was mixed 
with 2.5 g of a 2 wt % aqueous solution of di(2-ethyl)hexyl 
sulfosuccinate, 1.66 g of a 2 wt % aqueous solution of the compound 
represented by structural formula (10): 
EQU O.dbd.P--(OCH.sub.2 CH.sub.2 CH(CH.sub.3)CH.sub.2 C(CH.sub.3).sub.2 
CH.sub.3).sub.3 (10) 
and 16.67 g of 20% colloidal silica (Snowtex O, manufactured by Nissan 
Chemical Industries, Ltd.). Thus, a coating liquid for anti-reflection 
layer was prepared. 
(Production of Transparent Support) 
A 175 .mu.m-thick poly(ethylene terephthalate) (PET) film, which support 
had been colored blue as defined by x=0.2850 and y=0.2995 in the 
chromaticity coordinate system described in JIS-Z8701, was coated on one 
side thereof with a SBR latex in an amount of 0.32 g/m.sup.2 on a dry 
basis. A coating liquid prepared by mixing 20 g of a 5 wt % aqueous 
solution of gelatin (Nitta Gelatin #810) with 0.048 g of 
polymethylmethacrylamide particles having a particle diameter of 2.2 
.mu.m, 0.12 g of 3 wt % aqueous 1,2-benzothiazolin-3-one solution, and 
1.01 g of a 2 wt % aqueous solution of di(2-ethyl)hexyl sulfosuccinate was 
applied to the SBR-coated surface in an amount of 0.09 g/m.sup.2 on a dry 
basis. The other side of the film was then coated in the same manner to 
prepare a transparent support having a subbing layer on both side thereof. 
(Production of Heat-Sensitive Recording Material) 
The coating liquid for an ultraviolet filter layer described above was 
applied onto one of the subbing layers of the above transparent support in 
an amount of 1.8 g/m.sup.2 on a dry basis, and dried. Subsequently, the 
coating liquid for a anti-reflection layer described above was applied to 
the thus-formed ultraviolet filter layer in an amount of 2.2 g/m.sup.2 on 
a dry basis, and dried. A coating liquid prepared by mixing 16.3 g of 
microcapsule dispersion A described above (solid content, 20.9%) with 25.2 
g of microcapsule dispersion B described above (solid content, 30.7%), 
114.4 g of developer emulsion B described above (solid content, 21.0%), 
1.8 g of a 50 wt % aqueous solution of the compound represented by 
structural formula (11): 
##STR9## 
240 g of a 15 w % aqueous solution of poly(vinyl alcohol) (PVA 205C, 
manufactured by Kuraray Co., Ltd.), and 24 g of Snowtex O (colloidal 
silica manufactured by Nissan Chemical Industries, Ltd.) was then applied 
to the support on the side opposite the ultraviolet filter layer and the 
anti-reflection layer in an amount of 8.96 g/m.sup.2 on a dry basis, and 
dried. On the other hand, 8.92 g of a coating liquid prepared by mixing 
8.8 g of capsule dispersion A described above (solid content, 20.9%) with 
0.3 g of a 50 wt % aqueous solution of the compound represented by 
structural formula (12): 
##STR10## 
was mixed with 18.6 g of a coating liquid prepared by mixing 28.2 g of 
developer emulsion A described above (solid content, 21.3%) with 0.18 g of 
40 wt % aqueous glyoxal solution and 0.18 g of water, and further mixed 
with 1.8 g of Snowtex O (colloidal silica manufactured by Nissan Chemical 
Industries, Ltd.). This mixture was immediately applied onto the 
microcapsule-containing layer in an amount of 5.97 g/m.sup.2 on a dry 
basis, and dried. Subsequently, the coating liquid for a protective layer 
described above was applied to the thus-formed coating film in an amount 
of 2.5 g/m.sup.2 on a dry basis, and dried. Thus, a heat-sensitive 
recording material according to the present invention was obtained. 
EXAMPLE 2 
(Preparation of Microcapsule Dispersion C) 
To 5 g of the diazonium compound represented by structural formula (13): 
##STR11## 
were added 15 g of methylene chloride, 5 g of tricresyl phosphate, 15 g of 
trimethylolpropane trimethacrylate, and 20 g of a 75 wt % ethyl acetate 
solution of a 3:1 adduct of m-xylylene diisocyanate with 
trimethylolpropane (Takenate D 110N, manufactured by Takeda Chemical 
Industries, Ltd.). This mixture was stirred to produce a homogeneous oil 
phase solution. 
The oil phase solution thus obtained was mixed with an aqueous phase 
containing 60 g of a 7 wt % aqueous solution of poly(vinyl alcohol) (PVA 
217E, manufactured by Kuraray Co., Ltd.), and this mixture was emulsified 
with an Ace Homogenizer (manufactured by Nippon Seiki Co., Ltd.) at 8,000 
rpm for 5 minutes. To the thus-obtained emulsion was added 50 g of water. 
An encapsulation reaction was then conducted at 40.degree. C. for 3 hours 
to prepare a capsule dispersion having an average particle diameter of 1.5 
.mu.m. After the encapsulation reaction, 10 ml of an ion-exchange resin 
(MB-3, manufactured by Japan Organo Co., Ltd.) was added to this 
dispersion. This mixture was stirred for 30 minutes and then filtered to 
obtain microcapsule dispersion C. 
(Preparation of Microcapsule Dispersion D) 
To 5 g of the diazonium compound represented by structural formula (14): 
##STR12## 
were added 15 g of methylene chloride, 5 g of tricresyl phosphate, 15 g of 
trimethylolpropane trimethacrylate, and 20 g of a 75 wt % ethyl acetate 
solution of a 3:1 adduct of tolylene diisocyanate with trimethylolpropane 
(Burnock D750, manufactured by Dainippon Ink & Chemicals, Inc.). This 
mixture was stirred to produce a homogeneous oil phase solution. 
The oil phase solution thus obtained was mixed with an aqueous phase 
containing 60 g of a 7 wt % aqueous solution of poly(vinyl alcohol) (PVA 
217E, manufactured by Kuraray Co., Ltd.), and this mixture was emulsified 
with an Ace Homogenizer (manufactured by Nippon Seiki Co., Ltd.) at 8,000 
rpm for 5 minutes. To the thus-obtained emulsion was added 50 g of water. 
An encapsulation reaction was then conducted at 40.degree. C. for 3 hours 
to prepare a capsule dispersion having an average particle diameter of 1.5 
.mu.m. To this dispersion was added 10 ml of an ion-exchange resin (MB-3, 
manufactured by Japan Organo Co., Ltd.). This mixture was stirred for 30 
minutes and then filtered to obtain microcapsule dispersion D. 
Into 25 g of ethyl acetate were dissolved 4.3 g of the coupler compound 
represented by structural formula (15): 
##STR13## 
0.7 g of the coupler compound represented by structural formula (16): 
##STR14## 
5 g of 1,2,3-triphenylguanidine, 0.8 g of tricresyl phosphate, and 0.2 g 
of diethyl maleate. The solution thus obtained was mixed with an aqueous 
phase prepared by mixing 40 g of an 8 wt % aqueous poly(vinyl alcohol) 
solution with 15 g of water and 0.5 g of sodium dodecylbenzenesulfonate. 
This mixture was emulsified with an Ace Homogenizer (manufactured by 
Nippon Seiki Co., Ltd.) at 10,000 rpm to obtain an emulsion having an 
average particle diameter of 0.5 .mu.m. 
(Production of Heat-Sensitive Recording Material) 
Ten grams of the coupler emulsion was mixed with 2.5 g each of microcapsule 
dispersions C and D described above each containing a diazonium compound. 
This mixture was applied to the same poly(ethylene terephthalate) (PET) 
support having subbing layers on each side thereof as in Example 1, in an 
amount of 15 g/m.sup.2 on a dry basis. The coating was dried to form a 
heat-sensitive recording layer. The same protective-layer liquid as in 
Example 1 was then applied to the thus-formed heat-sensitive recording 
layer at a thickness of 2 .mu.m on a dry basis, and dried to produce a 
transparent heat-sensitive recording material. Thereafter, the same 
anti-reflection layer liquid as in Example 1 was applied to the 
heat-sensitive recording material on the side opposite to the 
heat-sensitive layer in an amount of 1.0 g/m.sup.2 on a dry basis, and 
dried to form an anti-reflection layer. Thus, a transparent heat-sensitive 
recording material according to the present invention was produced. 
COMATIVE EXAMPLE 1 
A heat-sensitive recording material was obtained in the same manner as in 
Example 1, except that a heat-sensitive recording layer was formed as 
follows. 
Three separate dispersions were, respectively, prepared by adding to 150 g 
of a 5% aqueous solution of poly(vinyl alcohol) (Kuraray PVA-105) each of 
30 g 2-anilino-3-methyl-6-N-dibutylaminofluoran as a color former, 30 g of 
bisphenol A as a developer, and 30 g of .beta.-naphthyl benzyl ether as a 
sensitizer. Each of these mixtures were treated with a Dynomill (Type: 
KDL; manufactured by Shinmaru Enterprises Corp.) along with 230 cc of 
glass beads having a particle diameter of 0.8 mm, until each of the color 
former, developer, and sensitizer was dispersed to an average particle 
diameter of 0.5 .mu.m. The particle diameter of each dispersion is given 
as a 50% volume-average particle diameter measured with a laser 
diffraction particle size distribution analyzer (LA-500, manufactured by 
Horiba Ltd.). The color former dispersion, developer dispersion, and 
sensitizer dispersion thus prepared were mixed together in a proportion of 
5:10:10 by weight. To this mixture was added a 10% aqueous solution of 
poly(vinyl alcohol) (Kuraray PVA-105) in an amount of 40% by weight on a 
dry basis based on the total amount of all solid ingredients. Thus, a 
coating liquid was obtained. 
The same transparent support as in Example 1, on which the ultraviolet 
filter layer and anti-reflection layer had been formed, was coated with 
the above-described coating liquid with a wire-wound bar in an amount of 
15 g/m.sup.2 on a dry basis, and the coating was dried to form a 
heat-sensitive recording layer. The protective-layer liquid obtained in 
Example 1 was then applied to the thus-formed heat-sensitive recording 
layer at a thickness of 2 .mu.m on a dry basis, and dried to produce a 
heat-sensitive recording material. 
COMATIVE EXAMPLE 2 
A heat-sensitive recording material was obtained in the same manner as in 
Example 1, except that a heat-sensitive recording layer was formed as 
follows. 
A coating liquid prepared by mixing 45.3 g of microcapsule dispersion A 
(solid content, 20.9%) with 114.4 g of developer emulsion A described 
above (solid content, 21.3%), 1.8 g of a 50 wt % aqueous solution of the 
compound represented by structural formula (17): 
##STR15## 
80 g of a 15 wt % aqueous solution of poly(vinyl alcohol) (PVA 205C, 
manufactured by Kuraray Co., Ltd.), and 24 g of Snowtex O (colloidal 
silica manufactured by Nissan Chemical Industries, Ltd.) was applied in an 
amount of 15.0 g/m.sup.2 on a dry basis, and dried. The coating liquid for 
a protective layer obtained in Example 1 was then applied to the 
thus-formed coating film in an amount of 2.5 g/m.sup.2 on a dry basis, and 
dried to obtain a heat-sensitive recording material. 
COMATIVE EXAMPLE 3 
A heat-sensitive recording material was obtained in the same manner as in 
Example 1, except that a heat-sensitive recording layer was formed as 
follows. 
A coating liquid prepared by mixing 45.3 g of microcapsule dispersion A 
(solid content, 20.9%) with 114.4 g of developer emulsion A described 
above (solid content, 21.3%), 1.8 g of a 50 wt % aqueous solution of the 
compound represented by structural formula (18): 
##STR16## 
480 g of a 15 wt % aqueous solution of poly(vinyl alcohol) (PVA 205C, 
manufactured by Kuraray Co., Ltd.), and 24 g of Snowtex O (colloidal 
silica manufactured by Nissan Chemical Industries, Ltd.) was applied in an 
amount of 15.0 g/m.sup.2 on a dry basis, and dried. The coating liquid for 
a protective layer obtained in Example 1 was then applied to the 
thus-formed coating film in an amount of 2.5 g/m.sup.2 on a dry basis, and 
dried to obtain a heat-sensitive recording material. 
The heat-sensitive recording materials obtained above were used for 
recording with a thermal printer (FTI-1000, manufactured by Fuji Photo 
Film Co., Ltd.) under conditions of 30.degree. C., 90% RH and 10.degree. 
C., 20% RH while changing the recording energy from 1 mJ/mm.sup.2 to 140 
mJ/mm.sup.2 to compare the gradation properties of the recording 
materials. A CT image was further printed on the heat-sensitive recording 
materials to compare the image qualities of these materials. 
The results obtained are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Environmental Comparative 
Comparative 
Comparative 
conditions 
Example 1 
Example 2 
Example 1 
Example 2 
Example 3 
__________________________________________________________________________ 
Energy A.sup.1) 
10.degree. C., 20% RH 
35 35 18 31 51 
(mJ/mm.sup.2) 
30.degree. C., 90% RH 
28 27 11 21 42 
Energy B.sup.2) 
10.degree. C., 20% RH 
127 101 52 62 91 
(mJ/mm.sup.2) 
30.degree. C., 90% RH 
114 87 45 52 83 
Density 
-- 0.29 0.34 0.89 --.sup.4) 
0.68 
difference.sup.3) 
Unevenness 
-- G G U U N 
of CT image.sup.5) 
__________________________________________________________________________ 
Remarks: 
.sup.1) Energy A is the energy required for increasing the transmission 
density of recorded areas to a value higher by 0.1 than the transmission 
density of nonrecorded areas. 
.sup.2) Energy B is the energy required for obtaining a transmission 
density of 2.5 in recorded areas. 
.sup.3) Density difference means the difference in density at an energy o 
60 mJ/mm.sup.2 between 10.degree. C., 20% RH and 30.degree. C., 90% RH. 
.sup.4) No density difference was observed because the saturation density 
had been reached. 
.sup.5) Unevenness of CT image was evaluated in accordance with the 
following criteria: 
G: good 
N: slightly noticeable 
U: unsuitable for practical use 
The above results show that the heat-sensitive recording material of the 
present invention provides highly precise images, e.g., medical images, in 
a stable manner using a conventional thermal head under various 
environmental conditions. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.