Thermosensitive recording medium

A thermosensitive recording medium including a support, and at least one layer provided on the support, wherein one of the at least one layer is a thermosensitive layer capable of forming a color image when heated imagewise, and one of the at least one layer contains a zinc dithiocarbamate of the formula: ##STR1## wherein R.sup.1 and R.sup.2 stand independently from each other for an alkyl group having 1-4 carbon atoms or a phenyl group. The zinc dithiocarbamate is contained in the thermosensitive layer or an intermediate layer interposed between the thermosensitive layer and the support.

BACKGROUND OF THE INVENTION 
This invention relates to a thermosensitive recording medium having a 
support and a thermosensitive color-developing layer formed on the support 
and containing a leuco dye and a color developer capable of reacting with 
the leuco dye at an elevated temperature to develop a color. 
Thermosensitive recording media capable of thermally recording an image 
through coloring reaction of a colorless or light-color leuco dye with a 
developer such as a phenol compound are now increasingly utilized in 
various fields such as recording papers for printers of computers, medical 
measuring instruments and facsimile machines, automatic ticket vending 
machines, thermosensitive copying machines and POS (point of sales) 
labels. 
One problem of the known thermosensitive recording media is that the images 
and background are apt to be discolored when exposed to light such as 
sunlight or room lamps. In particular, the image density is gradually 
lowered or the background is colored when irradiated with light. To cope 
with this problem, there is a proposal to incorporate a benzotriazole 
compound as a UV absorbing agent into the thermosensitive layer 
(JP-A-63-307981, etc.). The use of such a benzotriazole compound is, 
however, still unsatisfactory because the lightfastness is gradually lost 
with time. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a 
thermosensitive recording medium having excellent lightfastness. 
Another object of the present invention is to provide a thermosensitive 
recording medium free of discoloration and deterioration of recorded 
images and free of coloration of the background even when exposed to light 
irradiation for a long time. 
In accomplishing the foregoing object, there is provided in accordance with 
the present invention a thermosensitive recording medium comprising a 
support, and a thermosensitive layer provided on said support and capable 
of forming a color image when heated imagewise, said thermosensitive layer 
containing a zinc dithiocarbamate of the formula: 
##STR2## 
wherein R.sup.1 and R.sup.2 are independently selected from the group 
consisting of alkyl groups having 1-4 carbon atoms and a phenyl group. 
The present invention also provides a thermosensitive recording medium 
comprising a support, a thermosensitive layer provided on said support and 
capable of forming a color image when heated imagewise, and an 
intermediate layer interposed between said support and said 
thermosensitive layer and containing a zinc dithiocarbamate of the 
formula: 
##STR3## 
wherein R.sup.1 and R.sup.2 are independently selected from the group 
consisting of alkyl groups having 1-4 carbon atoms and a phenyl group. 
Other objects, features and advantages of the present invention will become 
apparent from the detailed description of the preferred embodiments of the 
invention to follow. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The thermosensitive recording medium of the present invention has a support 
on which a thermosensitive color-developing layer containing a leuco dye 
and a color developer is supported. Any conventionally used support, such 
as paper, a plastic film or a synthetic paper, may be suitably used for 
the purpose of the present invention. If desired, an intermediate layer 
may be interposed between the support and the thermosensitive 
color-developing layer. 
It is important that either the thermosensitive color-developing layer or 
the intermediate layer should contain a zinc dithiocarbamate of the 
formula: 
##STR4## 
wherein R.sup.1 and R.sup.2 are independently selected from alkyl groups 
having 1-4 carbon atoms and a phenyl group. The phenyl group may have one 
or more substituents such as an alkyl group or a halogen atom. From the 
standpoint of sensitivity and lightfastness, the use of a zinc 
dithiocarbamate having a melting point of 190.degree.-250.degree. C. is 
preferred. 
The zinc dithiocarbamate is suitably used in an amount of 0.1-3 parts by 
weight per part by weight of the leuco dye. 
Any leuco dye customarily employed in the field of thermosensitive 
recording material may be suitably used for the purpose of the present 
invention. For example, triphenylmethane leuco compounds, triarylmethane 
leuco compounds, fluoran leuco compounds, phenothiazine leuco compounds, 
thiofluorane leuco compounds, xanthene leuco compounds, indolylphthalide 
leuco compounds, spiropyran leuco compounds, azaphthalide leuco compounds, 
chromenopyrazol leuco compounds, methyne leuco compounds, rhodamine 
anilinolactam leuco compounds, rhodamine lactam leuco compounds, 
quinazoline leuco compounds, diazaxanthene leuco compounds and bislactone 
leuco compounds. 
Illustrative of suitable leuco compounds are 
3,3-bis(p-dimethylaminophenyl)phthalide, 
3,3-bis(p-dimethylaminophenyl)-6-dimethylaminophthalide (or Crystal Violet 
Lactone), 3,3-bis(p-dimethylaminophenyl)-6-diethylaminophthalide, 
3,3-bis(p-dimethylaminophenyl)-6-chlorophthalide, 
3,3-bis(p-dibutylaminophenyl)phthalide, 3-cyclohexylamino-6-chlorofluoran, 
dimethylamino-5,7-dimethylfluoran, 3-diethylamino-7-chlorofluoran, 
3-diethylamino-7-methylfluoran, 3-diethylamino-7,8-benzfluoran, 
3-diethylamino-6-methyl-7-chlorofluoran, 
3-dibutylamino-6-methyl-7-anilinofluoran, 
3-dipentylamino-6-methyl-7-anilinofluoran, 
3-(N-p-tolyl-N-ethylamino)-6-methyl-7-anilinofluoran, 
3-pyrrolidono-6-methyl-7-anilinofluoran, 
2-N-m-trifluoromethylphenylamino-6-diethylaminofluoran, 
2-3,6-bis(diethylamino)-9-(o-chloroanilino)-xanthylbenzoic acid lactam!, 
3-diethylamino-6-methyl-7-(m-trichloromethylanilino)fluoran, 
3-diethylamino-7-(o-chloroanilino)fluoran, 
3-dibutylamino-7-(o-chloroanilino)-fluoran, 
3-N-methyl-N-amylamino-6-methyl-7-anilinofluoran, 
3-N-methyl-N-cyclohexylamino-6-methyl-7-anilinofluoran, 
3-N-methyl-N-isoamylamino-6-methyl-7-anilinofluoran, 
3-N-methyl-N-isobutylamino-6-methyl-7-anilinofluoran, 
3-diethylamino-6-chloro-7-anilinofluoran, 
3-(N-ethyl-N-2-ethoxypropylamino)-6-methyl-7-anilinofluoran, 
3-N-ethyl-N-tetrafurfurylamino-6-methyl-7-anilinofluoran, 
3-diethylamino-6-methyl-7-anilinofluoran, 
3-diethylamino-5-methyl-7-dibenzylaminofluoran, benzoleuco methylene blue, 
6-chloro-8-methoxy-2,3-dihydrobenzoindole-2-spiro-2'-(2',3'-dihydro-benzof 
uran), 
6-bromo-8-methoxy-2,3-dihydrobenzoindole-2-spiro-2'-(2',3'-dihydrobenzofur 
an), 
3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'-chlorophenyl)phthal 
ide, 
3-(2'-hydroxy-4'-dimethylaminophenyl)-3-(2'-methoxy-5'-nitrophenyl)phthali 
de, 
3-(2'-hydroxy-4'-diethylaminophenyl)-3(2'-methoxy-5'-methylphenyl)phthalid 
e, 3-diethylamino-6-methyl-7-(2',4-dimethylanilino)fluoran, 
3-(2'-methoxy-4'-dimethylaminophenyl)-3-(2'-hydroxy-4'-chloro-5'-methylphe 
nyl) 
phthalide,3-morpholino-7-(N-propyl-N-o-trifluoromethylphenylamino)fluoran, 
3-morpholino-7-(N-propyl-N-m-trifluoromethylphenylamino) fluoran, 
3-morpholino-7-(N-propyl-N-p-trifluoromethylphenylamino) fluoran, 
3-pyrrolidino-7-o-trifluoromethylanilinofluoran, 
3-pyrrolidino-7-m-trifluoromethylanilinofluoran, 
3-pyrrolidino-7-p-trifluoromethylanilinofluoran, 
3-diethylamino-5-chloro-7-(N-benzyl-N-trifluoromethylphenylamino)fluoran, 
3-pyrrolidino-7-(di-p-chlorophenyl)methylaminofluoran, 
3-diethylamino-5-chloro-7-(.alpha.-phenylethylamino)fluoran, 
3-N-ethyl-N-p-methylphenylamino-7-(.alpha.-phenylethylamino)fluoran, 
3-diethylamino-7-(o-methoxycarbonylphenylamino) fluoran, 
3-diethylamino-5-methyl-7-(.alpha.-phenylethylamino)fluoran, 
3-diethylamino-7-piperidinofluoran, 
2-chloro-3-N-methyl-N-o-methylphenylamino-7-(p-n-butylanilino)fluoran, 
2-chloro-3-N-methyl-N-m-methylphenylamino-7-(p-n-butylanilino)fluoran, 
2-chloro-3-N-methyl-N-p-methylphenylamino-7-(p-n-butylanilino)-fluoran, 
3-N-benzyl-N-cyclohexylamino-5,6-benzo-7-.alpha.-naphthylamino-4'-bromoflu 
oran, 3-diethylamino-6-methyl-7-mesidino-4',5'-benzofluoran, 
3-(p-dimethylaminophenyl)-3-2,2-bis(p-dimethylaminophenyl)ethenyl!phthali 
de, 
3-(p-dimethylaminophenyl)-3-2,-bis(p-dimethylaminophenyl)-ethenyl!-6-dime 
thylaminophthalide, 
3-(p-dimethylaminophenyl)3-(2-p-dimethylaminophenyl-2-phenyletheyl)phthali 
de, 
3-(p-dimethylaminophenyl)-3-(2-p-dimethylaminophenyl-2-p-chlorophenylethen 
yl)-6-dimethylaminophthalide, 
3-(4'-dimethylamino-2'-methoxyphenyl)-3-(4"-p-dimethylaminophenyl-4"-p-chl 
orophenyl-1",3"-butadienyl)-5,6-benzophthalide, 
3-(4'-dimethylamino-2'-benzyloxyphenyl)-3-(4"-p-dimethylaminophenyl-4"-phe 
nyl-1",3"-butadienyl)-5,6-benzophthalide, 
3-dimethylamino-6-dimethylaminofluorene-9-spiro-3'-(6'-dimethylamino)phtha 
lide, 
3,3-bis2-(p-dimethylaminophenyl)-2-(p-methoxyphenyl)ethenyl!-4,5,6,7-tetr 
achlorophthalide, 
3-bis2,2-bis(4-pyrrolidinophenyl)-ethenyl!-5,6-dichloro-4, 
7-dibromophthalide, 
bis(p-dimethyl-aminostyryl)-1-naphthalenesulfonylmethane and 
bis(p-dimethyl-aminostyryl)-1-p-tolylsulfonylmethane. These leuco dyes may 
be used singly or in combination of two or more thereof. 
The leuco dye is generally used in an amount of 10-50% by weight, 
preferably 20-40% by weight based on the weight of the thermosensitive 
color-developing layer for reasons of color density and storage stability. 
Any developer customarily used in the field of thermosensitive recording 
material may be suitably used for the purpose of the present invention. 
Illustrative of suitable developers are 4,4'-isopropylidenediphenol, 
4,4'-isopropylidenebis(o-methylphenol), 4,4'-5-butylidenediphenol, 
4,4'-isopropylidenebis(2-t-butylphenol), zinc p-nitrobenzoate, 
1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl) isocyanuric acid, 
2,2-(3',4'-dihydroxyphenyl)propane, bis(4 
-hydroxy-3-methylphenyl)sulphide, 4-.beta.-(p-methoxyphenoxy)ethoxy!salic 
ylic acid, 1,7-bis(4-hydroxyphenylthio)-3,5-dioxaheptane, 
1,5-bis(4-hydroxyphenylthio)-5-oxapentane, monocalcium 
monobenzylphthalate, 4,4'-cyclohexylidenediphenol, 
4,4'-isopropylidenebis(2-chlorophenol), 
2,2-methylenebis(4-methyl-6-tert-butylphenol), 
2,2'-methylenebis(4-ethyl-6-tert-butylphenol), 
4,4'butylidenebis(6-tert-butyl-2-methylphenol), 
1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 
1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 
4,4'-thiobis(6-tert-butyl-2-methylphenol), 4,4'-diphenolsulfone, 
4-isopropoxy-4'-hydroxydiphenylsulfone, 
4-benzyloxy-4'-hydroxydiphenylsulfone, 4,4'-diphenolsulfoxide, isopropyl 
p-hydroxybenzoate, benzyl p-hydroxybenzoate, benzyl protocatechuate, 
stearyl gallate, lauryl gallate, octyl gallate, 
1,3-bis(4-hydroxyphenylthio)propane, N,N'-diphenylthiourea, 
N,N'-di(m-chlorophenyl)thiourea, salicylanilide, methyl 
bis(4-hydroxyphenyl)acetate, benzyl bis(4-hydroxyphenyl)acetate, 
2,4'-diphenolsulfone, 2,2'-diallyl-4,4'-diphenolsulfone, 
3,4-dihydroxyphenyl-4'-methyldiphenylsulfone, zinc 1-acetyloxy-2-nathoate, 
zinc 2-acetyloxy-1-naphthoate, zinc 2-acetyloxy-3-naphthoate, 
.alpha.,.alpha.-bis(4-hydroxyphenyl)-.alpha.-methyltoluene, antipyrine 
complex of zinc thiocyanate, tetrabromobisphenol A, tetrabromobisphenol s, 
4,4'-thiobis(2-methylphenol) and 4,4'-thiobis(2-chlorophenol). The above 
color developers may be used singly or in combination of two or more 
thereof. 
The color developer is generally used in an amount of 1-5 times, preferably 
2-4 times, the weight of the leuco dye for reasons of color density and 
storage stability. 
The thermosensitive layer may contain a binder to firmly bond the layer to 
the support. Any binder conventionally used in the field of the 
thermosensitive recording medium may be employed for the purpose of the 
present invention. Illustrative of suitable binders are water-soluble 
polymers such as polyvinyl alcohol, carboxyl group-modified polyvinyl 
alcohol, starch, starch derivatives, hydroxyethylcellulose, 
hydroxymethylcellulose, ethylcellulose, methylcellulose, 
carboxymethylcellulose, sodium polyacrylate, polyvinyl pyrrolidone, an 
acrylamide-acrylate copolymer, an acrylamide-acrylate-methacrylic acid 
terpolymer, a salt of a styrene-maleic anhydride copolymer, a salt of a 
styrene-acrylic acid copolymer, a salt of isobutylene-maleic anhydride 
copolymer, polyacrylamide, sodium alginate, gelatin and casein; and 
aqueous emulsions such as a styrene-butadiene copolymer, a 
styrene-butadiene-acrylic acid (or acrylate) copolymer latex, polyvinyl 
acetate, a vinyl acetate-acrylic acid copolymer, a styrene-acrylate 
copolymer, polyurethane, polyacrylate, polymethacrylate, a 
vinylchloride-vinyl acetate copolymer and an ethylene-vinyl acetate 
copolymer. 
Various additives may be further incorporated into the thermosensitive 
layer. The additives include an inorganic or organic filler such as 
silica, zinc oxide, aluminum hydroxide, zinc hydroxide, barium sulfate, 
kaolin, clay, talc, calcium carbonate, magnesium carbonate, calcined clay, 
titanium oxide, diatomaceous earth, anhydrous silica, activated clay, 
surface treated calcium, styrene-methacrylic acid copolymer powder, nylon 
powder, polyethylene powder, polystyrene powder or urea-formaldehyde resin 
powder; a fusible substance (or lubricant) having a melting point of 
50.degree.-200.degree. C., such as a fatty acid, a fatty acid ester, a 
fatty amide, a fatty acid salt (e.g. zinc stearate or calcium stearate), a 
wax (e.g. a stearate wax, polyethylene wax, carnauba wax, microcrystalline 
wax or carboxyl-modified paraffin wax), an aromatic carboxylic acid-amine 
condensation product, phenyl benzoate, higher straight-chain glycol, 
dialkyl 3,4-epoxy-hexahydrophthalate, higher ketone, p-benzylbiphenyl, 
dibenzyl oxalate, di-p-methylbenzyl oxalate or a metaterphenyl; and a 
surfactant. 
If desired, various other additional layers may be provided in the 
thermosensitive recording medium. For example, a protecting layer may be 
provided over the surface of the thermosensitive color-developing layer 
for protecting same and, in particular, for improving matching property of 
the thermosensitive recording medium with a thermal head. The overcoat 
layer is preferably formed of a water-soluble organic polymeric material 
such as polyvinyl alcohol, carboxyl group-modified polyvinyl alcohol, 
amino group-modified polyvinyl alcohol, starch, starch derivatives, 
methoxycellulose, hydroxyethylcellulose, carboxymethylcellulose, 
hydroxymethylcellulose, ethylcellulose, methylcellulose, polyacrylic acid, 
polyacrylic derivatives, a styrene-acrylic acid copolymer, styrene-acrylic 
acid copolymer derivatives, a styrene-acrylic acid-acrylamide terpolymer, 
polyethylene imine, a water-soluble polyester, water soluble polyurethane, 
an isobutylene-maleic anhydride copolymer or isobutylene-maleic anhydride 
copolymer derivatives. If desired, water insoluble polyester, 
polyurethane, an acrylate copolymer, styrene-acrylate copolymer, an epoxy 
resin, polyvinyl acetate, polyvinylidene chloride or polyvinyl chloride 
may also be used for the formation of the protecting layer. 
The protecting layer may further contain a water-proof agent such as 
formalin, chrome alum, glyoxal, melamine, a melamin-formalin resin, a 
polyamide resin or a polyamide-epichlorhydrin resin. Various additives 
such as a filler, a fusible substance, a surfactant and an agent for 
preventing the color development by pressure may be incorporated into the 
protecting layer, if desired. The filler and the fusible substance may be 
as exemplified above with reference to the thermosensitive layer. 
Preferably, an intermediate layer is provided between the support and the 
thermosensitive color-developing layer. The intermediate layer preferably 
contains a filler in the form of hollow particles (empty beads) having an 
average particle size of 2-10 .mu.m, an outside diameter D.sub.o and an 
inside diameter D.sub.i which is at least 90% of the outside diameter 
D.sub.o. The hollow filler is generally used in an amount of at least 30%, 
preferably 50-80%, of the volume of the intermediate layer. Illustrative 
of suitable hollow fillers are those formed of a polymer having a high 
elasticity such as a vinylidene chloride-acrylonitrile copolymer. A binder 
such as described above may be used for supporting the beads in the 
intermediate layer and for bonding the layer to the adjacent layer and the 
support. 
The thermosensitive color-developing layer may be formed on the support by 
applying a coating of a composition containing the above ingredients. For 
example, the above ingredients are homogeneously dispersed in a suitable 
dispersing medium such as water and the resulting dispersion is coated by 
a wire bar or any other suitably coating means to a predetermined 
thickness over the support, followed by drying of the coat. The 
color-developing layer generally has a thickness of 2-10 .mu.m, preferably 
4-8 .mu.m. 
The thermosensitive recording medium thus constructed may be heated 
imagewise with a thermal head to give a color-developed image.

The following examples will further illustrate the present invention. Parts 
and percentages are by weight. 
EXAMPLE 1 
Preparation of Dispersion A: 
The following components were ground with a sand mill into an average 
particle diameter of 2 .mu.m to obtain Dispersion A. 
______________________________________ 
3-dipentylamino-6-methyl-7-anilinofluoran 
20 parts 
10% aqueous polyvinyl alcohol solution 
20 parts 
Water 60 parts 
______________________________________ 
Preparation of Dispersion B: 
The following components were ground with a sand mill into an average 
particle diameter of 2 .mu.m to obtain Dispersion B. 
______________________________________ 
di-p-methylbenzyl oxalate 
10 parts 
10% aqueous polyvinyl alcohol solution 
12.5 parts 
calcium carbonate 15 parts 
water 97.5 parts 
______________________________________ 
Preparation of Dispersion C: 
The following components were ground with a sand mill into an average 
particle diameter of 2 .mu.m to obtain Dispersion C. 
______________________________________ 
zinc dibutyldithiocarbamate 
20 parts 
10% aqueous polyvinyl alcohol solution 
20 parts 
Water 60 parts 
______________________________________ 
Preparation of Dispersion D: 
The following components were blended to obtain Dispersion D. 
______________________________________ 
hollow filler of styrene-acrylate copolymer 
45 parts 
(solid content: 27.5%, D.sub.i /D.sub.o : 0.5) 
styrene-butadiene copolymer latex 
15 parts 
(solid content: 47.5%) 
water 50 parts 
______________________________________ 
Preparation of Dispersion F: 
The following components were blended to obtain Dispersion F for forming a 
protecting layer. 
______________________________________ 
10% aqueous polyvinyl alcohol solution 
63 parts 
epichlorohydrin solution of polyamide 
10 parts 
(solid content: 25%) 
silica 3 parts 
zinc stearate 1 part 
water 23 parts 
______________________________________ 
Preparation of Thermosensitive Recording Medium: 
Dispersion D obtained above was coated over the surface of high quality 
paper and dried to form an intermediate layer having a dried weight of 5 
g/m.sup.2. Dispersions A, B and C were blended with a weight ratio of 
A:B:C of 2:18:1 and the blend was applied over the intermediate layer and 
dried to form a thermosensitive color-developing layer having a dried 
weight of 6.5 g/m.sup.2. Dispersion F was then applied over the surface of 
the thermosensitive color-developing layer and dried to form a protecting 
layer having a dried weight of 5 g/m.sup.2. This was calendared at a 
pressure of 30 kgf/cm.sup.2, thereby obtaining a thermosensitive recording 
medium according to the present invention. 
EXAMPLE 2 
Example 1 was repeated in the same manner as described except that zinc 
diethyldithiocarbamate was substituted for zinc dibutyldithiocarbamate. 
EXAMPLE 3 
Example 1 was repeated in the same manner as described except that zinc 
dimethyldithiocarbamate was substituted for zinc dibutyldithiocarbamate. 
EXAMPLE 4 
Example 1 was repeated in the same manner as described except that zinc 
ethylphenyldithiocarbamate was substituted for zinc 
dibutyldithiocarbamate. 
EXAMPLE 5 
The above Dispersion C was blended with the above Dispersion D with a 
weight ratio of D:C of 1:0.5. This blend was then coated over the surface 
of high quality paper and dried to form an intermediate layer having a 
dried weight of 5.5 g/m.sup.2. Dispersions A and B were blended with a 
weight ratio of A:B of 1:9 and the blend was applied over the intermediate 
layer and dried to form a thermosensitive color-developing layer having a 
dried weight of 6 g/m.sup.2. Dispersion F was then applied over the 
surface of the thermosensitive color-developing layer and dried to form a 
protecting layer having a dried weight of 5 g/m.sup.2. This was calendared 
at a pressure of 30 kgf/cm.sup.2, thereby obtaining a thermosensitive 
recording medium according to the present invention. 
EXAMPLE 6 
Example 5 was repeated in the same manner as described except that zinc 
dimethyldithiocarbamate was substituted for zinc dibutyldithiocarbamate. 
EXAMPLE 7 
Example 5 was repeated in the same manner as described except that zinc 
ethylphenyldithiocarbamate was substituted for zinc 
dibutyldithiocarbamate. 
EXAMPLE 8 
Preparation of Dispersion E: 
The following components were blended to obtain Dispersion E. 
______________________________________ 
hollow filler of vinylidene chloride- 
35 parts 
acrylonitrile copolymer 
(solid content: 40%, D.sub.i /D.sub.o : 0.9) 
styrene-butadiene copolymer latex 
15 parts 
(solid content: 47.5%) 
water 50 parts 
______________________________________ 
Example 6 was repeated in the same manner as described except that 
Dispersion E was substituted for Dispersion D. 
EXAMPLE 9 
Example 7 was repeated in the same manner as described except that 
Dispersion E was substituted for Dispersion D. 
Comparative Example 1 
Example 8 was repeated in the same manner as described except that 
Dispersion C containing zinc dimethyldithiocarbamate was not used. 
Comparative Example 2 
Preparation of Dispersion G: 
The following components were ground with a sand mill into an average 
particle diameter of 2 .mu.m to obtain Dispersion G. 
______________________________________ 
4,4'-butylidene-bis(3-methyl-6- 
20 parts 
t-butylphenol) 
10% aqueous polyvinyl alcohol solution 
20 parts 
Water 60 parts 
______________________________________ 
Dispersion E obtained above was coated over the surface of high quality 
paper and dried to form an intermediate layer having a dried weight of 5 
g/m.sup.2. Dispersions A, B and G were blended with a weight ratio of 
A:B:G of 1:9:1 and the blend was applied over the intermediate layer and 
dried to form a thermosensitive color-developing layer having a dried 
weight of 7 g/m.sup.2. Dispersion F was then applied over the surface of 
the thermosensitive color-developing layer and dried to form a protecting 
layer having a dried weight of 5 g/m.sup.2. This was calendared at a 
pressure of 30 kgf/cm.sup.2, thereby obtaining a thermosensitive recording 
medium for comparative purposes. 
Comparative Example 3 
Comparative Example 2 was repeated in the same manner as described except 
that 2-(2-hydroxy-5-methylphenyl)benzotriazole was substituted for 
4,4'-butylidene-bis(3-methyl-6-t-butylphenol), thereby obtaining a 
thermosensitive recording medium for comparative purposes. 
The thermosensitive recording media thus obtained were tested for their 
thermal sensitivity and lightfastness and backside heat resistance to 
obtain the results shown in Table 1 below. The test methods are as 
follows. 
Lightfastness: 
A sample recording medium is recorded with a heating block at a temperature 
of 130.degree. C., a contact pressure of 2 kg/cm.sup.2 and a contact time 
of 1 second. The sample is then irradiated with light of 0.35W/m.sup.2 
(340 .mu.m) from a xenon lamp (weatherometer ATLAS Ci35A manufactured by 
Toyo Seiki Inc.) for 24 hours. Thereafter, the color densities of the 
background (BG) and the image (IM) are measured by McBeth Densitometer 
RD-914 using amber and blue filters, respectively. 
Thermal Sensitivity: 
Using a simulator having (manufactured by Matsushita Electronic Components 
Co., Ltd.), a sample recording medium is recorded at an electric power 
(applied to the thermal head) of 0.45 W/dot, a recording speed of 4 
msec/line, a scanning line density of 8.times.3.85 dot/mm and a pulse 
width of 0.45 and 0.50 msec. The density of the image thus developed is 
measured by McBeth Densitometer RD-914. 
TABLE 1 
______________________________________ 
Before Light- After Light- Sensitivity 
Example 
fastness Test fastness Test 
at 0.45 
at 0.50 
No. IM BG IM BG msec* msec* 
______________________________________ 
1 1.37 0.10 1.32 0.45 0.80 1.05 
2 1.37 0.09 1.28 0.36 0.80 1.03 
3 1.36 0.08 1.29 0.24 0.76 1.01 
4 1.36 0.07 1.30 0.22 0.75 0.99 
5 1.35 0.09 1.30 0.40 0.80 1.04 
6 1.36 0.08 1.29 0.16 0.78 1.02 
7 1.36 0.08 1.27 0.15 0.74 1.01 
8 1.37 0.08 1.30 0.17 1.05 1.20 
9 1.36 0.08 1.28 0.15 1.00 1.19 
Comp.1 1.35 0.07 0.50 0.18 1.02 1.21 
Comp.2 1.38 0.08 0.60 0.17 0.95 1.18 
Comp.3 1.37 0.07 0.16 0.16 0.98 1.20 
______________________________________ 
*pulse width 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all the 
changes which come within the meaning and range of equivalency of the 
claims are therefore intended to be embraced therein.