Reversible thermosensitive recording material

A reversible thermosensitive recording material is composed of a reversible thermosensitive recording layer capable of reversibly assuming a transparent state and a white opaque state depending on the temperature thereof, in which recording layer an organic low-molecular-weight material is dispersed in a matrix resin, the organic low-molecular-weight material being composed of a eutectic mixture containing as the main component a saturated higher fatty acid with 23 to 32 carbon atoms and a higher fatty acid.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a reversible thermosensitive recording 
material which is capable of forming images therein and erasing the same 
therefrom by utilizing the performance that a reversible thermosensitive 
recording layer of the recording material reversibly assumes a transparent 
state and a white opaque state depending upon the temperature thereof. 
2. Discussion of the Background 
Reversible thermosensitive recording materials are conventionally known, as 
disclosed in Japanese Laid-Open Patent Applications 54-119377 and 
55-154198. These conventional reversible thermosensitive recording 
materials have the shortcoming that a temperature range where the 
reversible thermosensitive recording material assumes a transparent state 
is as narrow as 2.degree. to 4.degree. C. Because of such a narrow 
temperature range, temperature control is difficult in forming images in 
the recording material by utilizing the reversible change between the 
transparent state and the white opaque state of the reversible 
thermosensitive recording material. 
To eliminate the above-mentioned shortcoming, a particular higher fatty 
acid ester is used as a eutectic agent in the reversible thermosensitive 
recording material, as disclosed in Japanese Laid-Open Applications 
63-39378 and 63-130380. In the above-mentioned applications, however, the 
temperature range is increased toward the low temperature side. There 
remains a problem of the obtained images disappearing at a temperature of 
50.degree. to 60.degree. C. 
In Japanese Laid-Open Patent Application 3-2089, the inventors of the 
present invention have proposed to use an aliphatic dicarboxylic acid 
having a high melting point in the reversible thermosensitive recording 
layer in order to increase the temperature range where the reversible 
thermosensitive recording material can assume a transparent state toward 
the high temperature side. As a result, the preservability of the images 
can be ensured at temperatures of 50.degree. to 60.degree. C. 
In line with diversified usage and with the development of the market for 
reversible thermosensitive recording materials, there is an increasing 
demand for the capability of preserving the image at temperatures as high 
as 70.degree. to 80.degree. C. More specifically, the above-mentioned 
demand is directed to the case where the reversible thermosensitive 
recording material is used or stored, for example, exposed to direct 
sunlight, or left in a closed car in the summer. Accordingly, it is 
necessary to further increase the temperature range where the reversible 
thermosensitive recording material assumes a transparent state toward the 
high temperature side. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a reversible 
thermosensitive recording material, free from the above-mentioned 
conventional shortcomings, with improved durability and temperature 
controllability capable of yielding high image contrast between a 
transparent portion and a white opaque portion. 
The above-mentioned object of the present invention can be achieved by a 
reversible thermosensitive recording material comprising a reversible 
thermosensitive recording layer capable of reversibly assuming a 
transparent state and a white opaque state depending on the temperature 
thereof, which recording layer comprises a matrix resin and an organic 
low-molecular-weight material dispersed in the matrix resin, the organic 
low-molecular-weight material comprising a eutectic mixture comprising as 
the main component a saturated higher fatty acid having 23 to 32 carbon 
atoms and a higher fatty acid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A reversible thermosensitive recording layer of the recording material 
according to the present invention comprises a matrix resin and an organic 
low-molecular-weight material dispersed in the matrix resin. This organic 
low-molecular-weight material comprises a eutectic mixture comprising a 
saturated higher fatty acid having 23 to 32 carbon atoms (hereinafter 
referred to as a saturated higher fatty acid A) and a higher fatty acid 
(hereinafter referred to as a higher fatty acid B). 
It is preferable that the amount of the saturated higher fatty acid A be 60 
to 90 wt. % of the total weight of the eutectic mixture. In addition, the 
higher fatty acid B capable of forming the eutectic mixture together with 
the above-mentioned saturated higher fatty acid A is preferably a 
saturated dibasic acid represented by the following formula (I) or (II): 
##STR1## 
(Wherein n is an integer of 13 to 28.) 
Specific examples of the saturated higher fatty acid A for use in the 
present invention are as follows: tricosanoic acid (C.sub.23), 
octacosanoic acid (C.sub.28), tetracosanoic acid (C.sub.24), nonacosanoic 
acid (C.sub.29), pentacosanoic acid (C.sub.25), triacontanoic acid 
(C.sub.30), hexacosanoic acid (C.sub.26), dotriacontanoic acid (C.sub.32), 
and heptacosanoic acid (C.sub.27). 
Specific examples of the structure of the saturated dibasic acid 
represented by the above formula (I) or (II) used as the higher fatty acid 
B are as follows: 
Formula (I): 
CH.sub.3 (CH.sub.2).sub.13 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.14 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.15 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.16 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.17 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.18 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.19 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.20 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.21 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.22 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.23 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.24 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.25 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.26 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.27 CH(COOH).sub.2, 
CH.sub.3 (CH.sub.2).sub.28 CH(COOH).sub.2, 
##STR2## 
Further, the aforementioned dibasic acids may have a branched chain in 
their structure. 
The reversible thermosensitive recording material of the present invention 
can be switched from a transparent state to a white opaque state, and vice 
versa, depending upon the temperature thereof. It is presumed that the 
difference between the transparent state and the white opaque state of the 
reversible thermosensitive recording material is based on the following 
principles: 
(i) In the transparent state, the organic low-molecular-weight material 
dispersed in the matrix resin consists of large crystals, or is compatible 
with the matrix resin, so that the light which enters the recording layer 
from one side passes therethrough to the opposite side, without being 
scattered, thus the reversible thermosensitive recording material appears 
transparent. 
(ii) In the white opaque state, the organic low-molecular-weight material 
is composed of polycrystals consisting of numerous small crystals, or the 
crystallographic axes of crystals are pointed to various directions 
because of phase-separation, so that the light which enters the recording 
layer is scattered a number of times on the interface of crystals of the 
low-molecular-weight material. As a result, the thermosensitive recording 
layer becomes opaque in a white color. 
The transition of the state of the reversible thermosensitive recording 
layer depending upon the temperature thereof will now be explained by 
referring to FIG. 1. 
In FIG. 1, it is supposed that a reversible thermosensitive recording 
material comprising a matrix resin and a low-molecular-weight material 
dispersed in the matrix resin is initially in a white opaque state at room 
temperature T.sub.0 or below. When the recording material is heated to any 
temperature between temperature T.sub.1 and temperature T.sub.2, the 
recording material becomes transparent. Even if the recording material 
which is already in the maximum transparent state is cooled to room 
temperature T.sub.0 or below, the maximum transparent state is maintained. 
It is considered that this is because the organic low-molecular-weight 
material changes its state from a polycrystalline state to a single 
crystalline state via a semi-melted state during the above-mentioned 
heating and cooling steps. 
When the recording material in the maximum transparent state is further 
heated to temperature T.sub.3 or more, it assumes a medium state which is 
between the maximum transparent state and the maximum white opaque state. 
When the recording material in the medium state at temperature T.sub.3 is 
cooled to room temperature T.sub.0 or below, the recording material 
returns to the original maximum white opaque state, without passing 
through any transparent state. It is considered that this is because the 
organic low-molecular-weight material is melted when heated to temperature 
T.sub.3 or above, and the polycrystals of the organic low-molecular-weight 
material separate out when it is cooled. If the recording material in the 
white opaque state is heated to any temperature between temperature 
T.sub.1 and temperature T.sub.2, and then cooled to a temperature below 
the room temperature T.sub.0, the recording material assumes an 
intermediate state between the transparent state and the white opaque 
state. 
When the recording material in the transparent state at room temperature 
T.sub.0 is again heated to temperature T.sub.3 or above, and then cooled 
to room temperature T.sub.0, the recording material returns to the white 
opaque state. Thus, the reversible thermosensitive recording material 
according to the present invention can assume a maximum white opaque 
state, a maximum transparent state and an intermediate state between the 
aforementioned two states at room temperature. 
Therefore, a white opaque image can be obtained on a transparent 
background, or a transparent image can also be obtained on a white opaque 
background by selectively applying the thermal energy to the reversible 
thermosensitive recording material according to the present invention. 
Further, such image formation and erasure can be repeated many times. 
When a colored sheet is placed behind the reversible thermosensitive 
recording material, the colored image can be obtained on the white opaque 
background or the white opaque image can be obtained on the colored 
background. 
In the case where the reversible thermosensitive recording material of the 
present invention is projected using an OHP (Over Head Projector), a white 
opaque portion in the recording material appears dark and a transparent 
portion in the recording material, through which the light passes becomes 
a bright portion on the screen. 
In the recording material according to the present invention capable of 
reversibly forming an image therein and erasing the same therefrom, the 
saturated higher fatty acid A having 23 to 32 carbon atoms which is the 
main component in the eutectic mixture has a melting point of 79.degree. 
to 96.degree. C., which is higher than the melting point of the saturated 
higher fatty acids conventionally used as the low-molecular-weight 
materials in the reversible thermosensitive recording layer of this type. 
Furthermore, when the saturated dibasic acid represented by the 
above-mentioned formula (I) or (II) is used as the higher fatty acid B in 
the recording material of the present invention, the melting point of such 
a higher fatty acid B is as high as 120.degree. C. or more. 
As a result, the temperature at which the reversible thermosensitive 
recording layer assumes the transparent state can be elevated and the 
temperature range where the reversible thermosensitive recording layer 
maintains the transparent state can be increased toward the high 
temperature side. Also, even if a protective layer is formed on the 
reversible thermosensitive recording layer, the transparency of the 
reversible thermosensitive recording layer in the transparent state is not 
decreased. 
To produce the reversible thermosensitive recording material of the present 
invention, (1) a solution in which both the matrix resin and the eutectic 
mixture for use in the present invention are dissolved, or (2) a 
dispersion prepared by dispersing finely-divided particles of the eutectic 
mixture for use in the present invention in a matrix resin solution may be 
coated on a support such as a plastic film, a sheet of synthetic paper, a 
glass plate, or a metallic plate, then dried, so that a reversible 
thermosensitive recording layer can be formed on the support. 
A solvent used for the formation of the reversible thermosensitive 
recording layer can be selected depending on the kind of matrix resin and 
the type of organic low-molecular-weight material. For example, 
tetrahydrofuran, tetrahydropyran, dioxane, methyl ethyl ketone, methyl 
isobutyl ketone, chloroform, carbon tetrachloride, ethanol, toluene and 
benzene can be employed. 
Not only when the above-mentioned dispersion (2) is used, but also when the 
solution (1) is used in forming the reversible thermosensitive recording 
layer, the organic low-molecular-weight material separates out in the form 
of finely-divided particles and is dispersed in the reversible 
thermosensitive recording layer. 
The matrix resin for use in the reversible thermosensitive recording layer 
can form the recording layer in which finely-divided particles of the 
organic low-molecular-weight material are uniformly dispersed and impart 
high transparency to the recording layer when the recording layer is in a 
maximum transparent state. Therefore, it is preferable that the matrix 
resin for use in the reversible thermosensitive recording layer have high 
transparency, high mechanical stability and high film-forming properties. 
Examples of the resin used as the matrix resin include polyvinyl chloride; 
polystyrene; vinyl chloride copolymers such as vinyl chloride - vinyl 
acetate copolymer, vinyl chloride - vinyl acetate - vinyl alcohol 
copolymer, vinyl chloride - vinyl acetate - maleic acid copolymer, and 
vinyl chloride - acrylate copolymer; polyvinylidene chloride; vinylidene 
chloride copolymers such as vinylidene chloride - vinyl chloride 
copolymer, and vinylidene chloride - acrylonitrile copolymer; polyester; 
polyamide; polyacrylate, polymethacrylate and acrylate - methacrylate 
copolymer; and silicone resin. These resins can be used alone or in 
combination. 
It is preferable that the ratio by weight of the eutectic mixture in the 
organic low-molecular-weight material to the matrix resin be in the range 
of about (2:1) to (1:16), more preferably in the range of (1:1) to (1:3) 
in the reversible thermosensitive recording layer. When the mixing ratio 
of the eutectic mixture to the matrix resin is within the above-mentioned 
range, there is no problem of the film-forming properties of the 
reversible thermosensitive recording layer, and at the same time, the 
contrast between the transparent portion and white opaque portion of the 
recording layer is sufficient for practical use. 
It is preferable that the amount of the saturated higher fatty acid A be 60 
to 90 wt. % of the total weight of the eutectic mixture. More preferably, 
the mixing ratio of the saturated higher fatty acid A to the higher fatty 
acid B may be determined in such a fashion that the freezing point of the 
obtained eutectic mixture is within the range of 75.degree. to 115.degree. 
C. 
The thickness of the reversible thermosensitive recording layer is 
preferably in the range of 1 to 30 .mu.m. Within the above range, the 
thermal sensitivity does not decrease, and the image contrast is not 
degraded. 
The reversible thermosensitive recording layer for use in the present 
invention may further comprise other additives to maintain the temperature 
range where the reversible thermosensitive recording layer can assume the 
transparent state even when the reversible thermosensitive recording layer 
is repeatedly heated. For example, the following plasticizers can be 
employed: tributyl phosphate, tri-2-ethylhexyl phosphate, triphenyl 
phosphate, tricresyl phosphate, butyl oleate, dimethyl phthalate, diethyl 
phthalate, dibutyl phthalate, diheptyl phthalate, di-n-octyl phthalate, 
di-2-ethylhexyl phthalate, diisononyl phthalate, dioctyldecyl phthalate, 
diisodecyl phthalate, butylbenzyl phthalate, dibutyl adipate, di-n-hexyl 
adipate, di-2-ethylhexyl adipate, di-2-ethylhexyl azelate, dibutyl 
sebacate, di-2-ethylhexyl sebacate, diethylene glycol dibenzoate, 
triethylene glycol-2-ethyl butyrate, methyl acetylricinoleate, butyl 
acetylricinoleate, butylphthalyl butyl glycolate and tributyl 
acetylcitrate. It is preferable that the ratio of weight of the organic 
low-molecular-weight material to the aforementioned additives in the 
recording layer be in the range of about (1:0.01) to (1:0.8). 
A protective layer can be formed on the reversible thermosensitive 
recording layer when necessary. As the material for the protective layer, 
a silicone rubber and a silicone resin as disclosed in Japanese Laid-Open 
Patent Application 63-221087, a mixture of finely-divided particles of a 
poylsiloxane graft polymer and a resin as disclosed in Japanese Laid-Open 
Patent Application 63-317385, and a polyamide resin can be employed. In 
any case, the above-mentioned material is dissolved in a solvent to 
prepare a coating liquid for the protective layer, and the thus prepared 
coating liquid is coated on the reversible thermosensitive recording 
layer. It is not desirable that the matrix resin and the organic 
low-molecular-weight material for use in the reversible thermosensitive 
recording layer be easily dissolved in such a solvent used for the 
protective layer coating liquid. 
Preferable examples of the above-mentioned solvent for use in a coating 
liquid for the protective layer include n-hexane, methyl alcohol, ethyl 
alcohol, and isopropyl alcohol. In particular, alcohol-based solvents are 
preferred from the viewpoint of cost. 
Other features of this invention will become apparent in the course of the 
following description of exemplary embodiments which are given for 
illustration of the invention and are not intended to be limited thereof. 
Example 1 
[Formation of Reversible Thermosensitive Recording Layer] 
The following components were mixed to prepare a coating liquid for a 
reversible thermosensitive recording layer: 
______________________________________ 
Parts by Weight 
______________________________________ 
n-pentacosanedioic acid 
0.8 
(m.p.: 81-83.degree. C.) 
Saturated dibasic acid of the 
0.2 
following formula: 
CH.sub.3 (CH.sub.2).sub.17 CH(COOH).sub.2 
(m.p.: 121-123.degree. C.) 
(freezing point of the obtained 
eutectic mixture: 81.0.degree. C.) 
vinyl chloride - vinyl acetate 
2.5 
copolymer (Trademark "VYHH" made 
by Union Carbide Japan K.K.) 
Di-2-ethylhexyl phthalate 
0.3 
tetrahydrofuran 20 
______________________________________ 
The above prepared coating liquid was coated by a wire bar on an 
aluminum-deposited surface of a polyester film (Trademark "Metallumy" made 
by Toray Industries, Inc.) with a thickness of about 50 .mu.m serving as a 
support, and dried at 110.degree. to 120.degree. C., so that a reversible 
thermosensitive recording layer with a thickness of about 5 .mu.m was 
formed on the support. 
[Formation of Intermediate Layer] 
The following components were mixed to prepare a coating liquid for an 
intermediate layer: 
______________________________________ 
Parts by weight 
______________________________________ 
Polyamide resin (Trademark 
1.0 
"CM8000" made by Toray 
Industries, Inc.) 
Methanol 9.0 
______________________________________ 
The above prepared coating liquid was coated on the reversible 
thermosensitive recording layer by a wire bar, and dried under the 
application of heat thereto, so that an intermediate layer with a 
thickness of about 1 .mu.m was formed on the reversible thermosensitive 
recording layer. 
[Formation of Protective Layer] 
A commercially available butyl acetate solution of ultraviolet-curing 
urethane acrylate resin (Trademark "Unidic 17-824-9" made by Dainippon Ink 
& Chemicals, Incorporated) was coated on the above formed intermediate 
layer by a wire bar, dried under the application of heat thereto, and 
cured by the irradiation of an ultraviolet lamp of 80 W/cm for 5 sec., so 
that a protective layer with a thickness of about 2 .mu.m was formed on 
the intermediate layer. 
Thus, a reversible thermosensitive recording material No. 1 according to 
the present invention was obtained. 
Example 2 
The procedure for preparation of the reversible thermosensitive recording 
material No. 1 in Example 1 was repeated except that n-pentacosanedioic 
acid and the saturated dibasic acid in the formulation of the coating 
liquid for the reversible thermosensitive recording layer employed in 
Example 1 were respectively replaced by n-tricontanoic acid (m.p.: 
91.5.degree.-93.5.degree. C.) and a saturated dibasic acid having the 
formula of 
##STR3## 
(m.p. 108.5.degree.-110.degree. C.) to prepare a coating liquid for a 
reversible thermosensitive recording layer. 
The freezing point of the obtained eutectic mixture of the above components 
was 83.5.degree. C. 
Thus, a reversible thermosensitive recording material No. 2 according to 
the present invention was obtained. 
Example 3 
The procedure for preparation of the reversible thermosensitive recording 
material No. 1 in Example 1 was repeated except that 0.8 parts by weight 
of n-pentacosanedioic acid and 0.2 parts by weight of the saturated 
dibasic acid in the formulation of the coating liquid for the reversible 
thermosensitive recording layer employed in Example 1 were respectively 
replaced by 0.90 parts by weight of n-tetracosanoic acid (m.p.: 
82.degree.-85.degree. C.) and 0.1 parts by weight of a saturated dibasic 
acid having the formula of CH.sub.3 (CH.sub.2).sub.21 CH(COOH).sub.2 
(m.p.: 125.degree.-126.degree. C.) to prepare a coating liquid for a 
reversible thermosensitive recording layer. 
The freezing point of the obtained eutectic mixture of the above components 
was 86.degree. C. 
Thus, a reversible thermosensitive recording material No. 3 according to 
the present invention was obtained. 
Example 4 
The procedure for preparation of the reversible thermosensitive recording 
material No. 1 in Example 1 was repeated except that 0.8 parts by weight 
of n-pentacosanedioic acid and 0.2 parts by weight of the saturated 
dibasic acid in the formulation of the coating liquid for the reversible 
thermosensitive recording layer employed in Example 1 were respectively 
replaced by 0.75 parts by weight of n-triacontanoic acid and 0.25 parts by 
weight of a saturated dibasic acid having the formula of CH.sub.3 
(CH.sub.2).sub.17 CH(COOH).sub.2 to prepare a coating liquid for a 
reversible thermosensitive recording layer. 
The freezing point of the obtained eutectic mixture of the above components 
was 88.degree. C. Thus, a reversible thermosensitive recording material 
No. 4 according to the present invention was obtained. 
Example 5 
The procedure for preparation of the reversible thermosensitive recording 
material No. 1 in Example 1 was repeated except that 0.8 parts by weight 
of n-pentacosanedioic acid and 0.2 parts by weight of the saturated 
dibasic acid in the formulation of the coating liquid for the reversible 
thermosensitive recording layer employed in Example 1 were respectively 
replaced by 0.6 parts by weight of n-hexacosanoic acid (m.p.: 
86.degree.-88.degree. C.) and 0.4 parts by weight of a saturated dibasic 
acid having the formula of CH.sub.3 (CH.sub.2).sub.18 CH(COOH).sub.2 
(m.p.: 125.degree.-128.degree. C.) to prepare a coating liquid for a 
reversible thermosensitive recording layer. 
The freezing point of the obtained eutectic mixture of the above components 
was 84.degree. C. thus, a reversible thermosensitive recording material 
No. 5 according to the preset invention was obtained. 
Comparative Example 1 
The procedure for preparation of the reversible thermosensitive recording 
material No. 1 in Example 1 was repeated except that 0.8 parts by weight 
of n-pentacosanedioic acid and 0.2 parts by weight of the saturated 
dibasic acid in the formulation of the coating liquid for the reversible 
thermosensitive recording layer employed in Example 1 were respectively 
replaced by 80 parts by weight of behenic acid with a purity of 95% and 20 
parts by weight of a dibasic acid having the formula of 
HOOC(CH.sub.2).sub.18 COOH with a purity of 99% to prepare a coating 
liquid for a reversible thermosensitive recording layer. 
Thus, a comparative reversible thermosensitive recording material No. 1 was 
obtained. 
Comparative Example 2 
The procedure for preparation of the reversible thermosensitive recording 
material No. 1 in Example 1 was repeated except that 0.8 parts by weight 
of n-pentacosandedioic acid and 0.2 parts by weight of the saturated 
dibasic acid in the formulation of the coating liquid for the reversible 
thermosensitive recording layer employed in Example 1 were respectively 
replaced by 80 parts by weight of behenic acid with a purity of 80% and 20 
parts by weight of a dibasic acid having the formula of 
HOOC(CH.sub.2).sub.18 COOH with a purity of 86% to prepare a coating 
liquid for a reversible thermosensitive recording layer. 
Thus, a comparative reversible thermosensitive recording material No. 2 was 
obtained. 
The above prepared reversible thermosensitive recording materials No. 1 to 
No. 5 according to the present invention obtained in Examples 1 to 5 and 
the comparative reversible thermosensitive recording materials No. 1 and 
No. 2 obtained in Comparative Examples 1 and 2 assumed a white opaque 
state at the initial stage. Each of the reversible thermosensitive 
recording materials was gradually heated from 65.degree. C. to a high 
temperature range of 120.degree. to 130.degree. C. by 0.5.degree. C. Every 
time the recording material was heated, the reversible thermosensitive 
recording material was cooled to room temperature and a reflection density 
of each reversible thermosensitive recording material was measured by a 
Macbeth reflection-type densitometer RD-514, with a black paper with a 
density of 1.92 placed behind the recording material. When the reflection 
density of the recording material exceeded 1.0, the recording material was 
considered to assume a transparent state. The temperature range in which 
the recording material assumed a transparent state was thus obtained, and 
the temperature width was calculated. The results are shown in Table 1. 
The temperature at which the recording material initiated to assume a 
transparent state (hereinafter referred to as a 
transparent-state-initiation-temperature) was expressed by the temperature 
at which the reflection density of the recording material exceeded 0.7 
while the reversible thermosensitive recording material in the maximum 
white opaque state was heated from 65.degree. C. 
The transparent-state-initiation temperature, and the densities of the 
recording material in the maximum white opaque state and the maximum 
transparent state are also shown in Table 1. 
In addition, image samples were prepared by using the reversible 
thermosensitive recording materials obtained in Examples 1 to 5 and 
Comparative Examples 1 and 2, and a heat-resistant preservation test was 
carried out in such a manner that each image sample was stored in a 
temperature-controlled bath of 80.degree. C. The reflection density of 
each image sample was measured before the storage, and after one hour, 6 
hours, and 24 hours. The results are shown in Table 2. 
TABLE 1 
__________________________________________________________________________ 
Transparent- 
Temperature 
Temperature 
Density in 
Density in 
State-Initiation 
Range of 
Width during 
Maximum 
Maximum 
Temperature 
Transparent 
Transparent 
Transparent 
White Opaque 
(.degree.C.) 
State (.degree.C.) 
State (.degree.C.) 
State State 
__________________________________________________________________________ 
Example 1 
81.0 82.5-96.0 
13.5 1.83 0.41 
Example 2 
82.5 83.5-96.0 
12.5 1.82 0.39 
Example 3 
84.5 87.0-100.0 
13.0 1.70 0.37 
Example 4 
89.0 91.0-100.5 
9.5 1.91 0.40 
Example 5 
81.0 83.0-100.5 
17.5 1.69 0.42 
Comp. Ex. 1 
75.0 78.0-96.0 
18.0 1.75 0.36 
Comp. Ex. 2 
68.0 70.0-83.0 
14.0 1.78 0.34 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Reflection Density of Image Sample of Reversible 
Thermosensitive Recording Material 
Before After After After 
Storage 
1 hour 6 hours 24 hours 
______________________________________ 
Example 1 0.41 0.53 0.55 0.60 
Example 2 0.39 0.50 0.54 0.58 
Example 3 0.37 0.47 0.49 0.50 
Example 4 0.40 0.45 0.46 0.49 
Example 5 0.42 0.54 0.57 0.63 
Comp. Ex. 1 0.36 1.73 1.75 1.75 
Comp. Ex. 2 0.34 1.77 1.78 1.78 
______________________________________ 
As is apparent from Table 1, the transparent-state-initiation temperature 
of the reversible thermosensitive recording materials obtained in Example 
1 to Example 5 is 81.degree. C. or more, and the lower limit of the 
temperature range of the transparent state shown in Table 1 is as high as 
82.5.degree. C. in the case of the recording materials of the present 
invention. 
In addition, as can be seen from the results in Table 2, deterioration of 
the image samples obtained in the reversible thermosensitive recording 
materials according to the present invention is very little when the 
recording materials are stored at hot temperatures. 
In particular, the reversible thermosensitive recording material No. 3 of 
the present invention has the advantage that the temperature range where 
it can assume a transparent state is wide. Further, the decrease in the 
whiteness degree of the image sample can be effectively prevented after 
the heat-resistant preservation test. 
As previously explained, according to the present invention, the 
temperature range where the reversible thermosensitive recording material 
can assume the transparent state can be increased toward the high 
temperature side and the reversible thermosensitive recording material is 
capable of yielding images with high contrast between a transparent 
portion and a white opaque portion. Furthermore, the images formed in the 
recording material have excellent environmental stability.