Image-receiving sheet for thermal dye-transfer recording

A thermal dye-transfer recording, image-receiving sheet, comprising a support having thereon an image-receiving layer containing a butyral resin for receiving a transferred image from a coloring material-transferring sheet, is disclosed. In this image-receiving sheet, sharp recorded images can be obtained, and the images exhibit excellent storage stability and abrasion resistance.

FIELD OF THE INVENTION 
The present invention relates to an image-receiving sheet for thermal 
dye-transfer recording, and more particularly to an improvement in an 
image-receiving sheet for thermal dye-transfer recording utilizing 
heat-sublimable dyes. According to the present invention, thermal 
dye-transfer recorded images having greatly improved storage stability and 
abrasion resistance can be obtained. 
BACKGROUND OF THE INVENTION 
A thermal recording system for obtaining recorded images simultaneously 
with the application of input signals is widely used in facsimile 
machines, computer terminal printers, and printers for measuring equipment 
because the apparatus used in a thermal recording system is relatively 
simple and inexpensive, and it is of low noise. 
The recording medium most commonly used in such a thermal recording system 
is a so-called color formation-type heat-sensitive recording paper, which 
is provided with a recording layer which undergoes physical and chemical 
changes on heating to cause color formation. This recording medium, 
however, has several disadvantages. One of the disadvantages is that the 
recording medium is liable to cause unnecessary color formation during the 
production or storage thereof. Another disadvantage is that the storage 
stability of images recorded on this medium is poor. For example, images 
so produced exhibit a fading phenomenon when brought into contact with 
organic solvents or chemicals. 
In order to overcome the above problems, a recording system has been 
proposed in which a recording medium utilizing a coloring material which 
is colored itself is used in place of the above color formation-type 
heat-sensitive recording paper. For example, Japanese Patent Application 
(OPI) No. 15446/76 (the term "OPI" as used herein refers to a "published 
unexamined Japanese patent application") discloses a recording system in 
which a substrate, such as paper and a polymer film, coated with a 
coloring material which is solid or semi-solid at room temperature is 
superposed on a recording paper (image-receiving paper) in such a manner 
that the coloring material coated on the substrate comes into contact with 
the recording paper. Then the coloring material is selectively transferred 
to the recording paper by heating the substrate with a thermal recording 
head, thereby recording a desired image. 
In this recording system, the coloring material on the substrate is melted, 
evaporated, and sublimated by the application of heat. Then it is 
transferred to the recording paper and fixed thereon by the action of 
sticking, adsorption, and dye-fixing, to thereby form a recorded image. 
One of the characteristic features of this recording system is that plain 
paper (non-coated paper) can be used as the recording paper. However, when 
plain paper is used as the recording paper, the dye-fixing, in particular, 
is difficult to accomplish. As a result, not only is the resulting 
recorded image low in color density, but a serious fading phenomenon also 
occurs over a lapse of time. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved 
image-receiving sheet for use in the thermal dye-transfer recording in 
which a coloring material, particularly a heat-sublimable dye, is 
thermally-transferred. 
Another object of the present invention is to provide an image-receiving 
sheet capable of providing recorded images which are very sharp, high in 
color density, and greatly improved in storage stability and abrasion 
resistance. 
To achieve the objects and in accordance with the purposes of the 
invention, as embodied and broadly described herein, the thermal 
dye-tansfer recording image receiving sheet of this invention comprises a 
support having thereon an image-receiving layer containing a butyral resin 
for receiving a transferred image from a coloring material-transferring 
sheet. 
DETAILED DESCRIPTION OF THE INVENTION 
The butyral resin which is contained in the image-receiving layer of the 
image-receiving sheet of the present invention can be generally prepared 
by reacting polyvinyl alcohol and butyl aldehyde and can be obtained in 
the form of a vinyl butyral/vinyl alcohol copolymer by appropriately 
controlling the degree of substitution. It is preferred that the degree of 
butyralization be at least 50 mol%. In particular, a butyral resin having 
a degree of butyralization of from 55 to 75 mol% is most preferred because 
it has excellent dye-fixing properties and thus, provides an 
image-receiving layer capable of forming a recorded image having excellent 
storage stability. 
An unsaponified vinyl acetate group resulting from the starting polyvinyl 
alcohol may adversely affect the heat resistance of the image-receiving 
layer, and therefore, it is desirable that the unsaponified vinyl acetate 
group content in the butyral resin be controlled to 20 mol% or less, with 
the range of 10 mol% or less being more preferred. 
The butyral resin is generally dissolved in a suitable organic solvent such 
as benzene, toluene, xylene, ethyl acetate, acetone, and methyl ethyl 
ketone, to be adjusted to a suitable concentration and viscosity depending 
on the type of a coating head, and then coated on a support by means of 
coating equipment such as, for example, a blade coater, an air knife 
coater, a bar coater, a roll coater, a gravure coater, or a curtain 
coater, and then dried. 
In preparing the coating composition, various additives can be added, if 
desired. For example, vinyl polymers such as polystyrenes and 
polyacrylates and condensation polymers such as polyesters, 
polycarbonates, and polysulfones can be added for the purpose of improving 
the physical properties of the coating composition and the recording 
characteristics. In addition, for the purpose of improving the physical 
properties of the surface of the image-receiving layer, such as improving 
the writing properties of the image-receiving sheet, inorganic or organic 
pigments such as natural ground calcium carbonate, precipitated calcium 
carbonate, talc, clay, natural or synthetic silicic acids, titanium oxide, 
aluminum hydroxide, zinc oxide, or a powdered urea/formaldehyde resin, and 
various auxiliary agents can be added. 
Since the butyral resin used in the present invention contains hydroxyl 
groups in the molecule, the physical properties of the image-receiving 
layer can be improved upon heating. Combining the butyral resin with a 
cross-linking agent gives rise to a marked improvement in the heat 
resistance of the image-receiving layer. Suitable examples of 
cross-linking agents which can be used include polyfunctional 
cross-linking agents such as polyisocyanates, epoxy compounds, and 
polymethylols; and polyfunctional monomers having two or more unsaturated 
groups in the molecule, such as polyfunctional polyesters, polyfunctional 
epoxy acrylates, polyfunctional ether acrylates, and polyfunctional 
polyester acrylates. The amount of the cross-linking agent added is 
generally 50% by weight or less based on the butyral resin. If desired, 
catalysts can be used in combination with the cross-linking agent, or the 
cross-linking can be attained by heating or irradiating with actinic 
radiation such as ultraviolet light, electron beams, or X-rays. 
The amount of the butyral resin coated on the support can be varied widely, 
depending on the purpose for which the image-receiving sheet is to be 
used. In general, the butyral resin is coated in an amount (on a dry 
weight basis) of from 2 to 15 g per square meter of the support. 
As the support, plain paper, synthetic paper, synthetic resin films, and so 
on can be used. In general, the preferred support is plain paper because 
it exhibits excellent thermal properties. Examples of useful types of 
plain paper include paper produced by adding to the cellulose pulp, which 
is the main component, certain additives, such as paper strengthening 
agents, sizing agents, fixing agents, and inorgaic or organic fillers 
followed by the usual paper-making procedures; and paper produced by size 
pressing with oxidized starch or providing a pre-coat layer made mainly of 
a pigment, such as clay to thereby improve the physical properties of the 
surface thereof. 
The image-receiving sheet for thermal dye-transfer recording to the present 
invention exhibits excellent performance, particularly when used in 
combination with a coloring material-transferring sheet containing a 
heat-sublimable dye. That is, in this case, the image-receiving sheet 
produces recorded images which are sharp, high in color density, and 
greatly improved in storage stability and abrasion resistance. 
Although exact reasons why the above excellent effects can be obtained are 
not always clear, it is considered that, in view of the fact that the 
image-receiving sheet of the present invention is excellent in 
dye-receiving ability and gives sharp recorded images particularly 
resistant to light, during the thermal-transfer, the dye is probably 
absorbed in the butyral resin present in the image-receiving layer to 
exhibit its dissolved color. At the same time, it is presumably diffused 
in molecular form and thus stabilized in the butyral resin matrix. This 
theory is supported by the fact that when the dye is not sufficiently 
diffused because of insufficient heat absorption during the recording so 
that the dissolved color is not satisfactorily exhibited, a recorded image 
having greatly improved storage stability can still be obtained by 
subjecting the image-receiving sheet to a post-heat treatment, for 
example, by pressing it against a hot plate or irradiating it with a flash 
lamp. 
The term "heat-sublimable dye" as referred to herein means a dye which does 
not transfer a coloring material when contacted with the image-receiving 
sheet under the usual handling conditions but which, when heated to 
60.degree. C. or more, tranfers the coloring material as the result of 
melting, evaporation, and sublimation. The heat-sublimable dye is selected 
appropriately from various dyes such as disperse dyes represented by azo-, 
nitro-, anthraquinone-, and quinoline-based dyes, basic dyes represented 
by triphenylmethane- and fluoran-based dyes, and oil-soluble dyes. 
The image-receiving sheet of the present invention is useful in the thermal 
recording system including not only the contact heating type, in which the 
sheet is contact heated by the use of a hot plate or the thermal head of a 
thermal printing unit, but also the non-contact heating type, in which the 
sheet is irradiated with rays such as infrared light, YAG laser, carbon 
dioxide laser, and the like.

The present invention is described in greater detail with reference to the 
following examples although it is not limited thereto. All parts and 
percents (%) are by weight unless otherwise indicated. 
EXAMPLE 1 
Ten parts of a butyral resin (degree of butyralization: 67 mol%, residual 
acetyl group: 5 mol%, average degree of polymerization: 500) was dissolved 
in a mixed solvent of 40 parts of toluene and 40 parts of methyl ethyl 
ketone, and 10 parts of surface-treated calcium carbonate (trade name: 
Lyton A, produced by Bihoku Funka K.K.) was dispersed in the 
above-prepared solution to prepare a coating composition. This coating 
composition was at a coating weight (dry basis) of 10 g/m.sup.2 coated on 
a plain paper having a basis weight of 60 g/m.sup.2 and then dried. The 
coated paper was subjected to a super calender treatment at a pressure of 
150 kg/cm to prepare an image-receiving sheet for thermal dye-transfer 
recording. 
COMATIVE EXAMPLE 1 
An image-receiving sheet was prepared in the same manner as in Example 1 
except that the butyral resin was replaced by a saturated polyester resin 
(trade name: Vylon 200, produced by Toyobo Co., Ltd.). 
COMATIVE EXAMPLE 2 
The same plain paper (basis weight: 60 g/m.sup.2) as used in Example 1 was 
used as an image-receiving sheet without application of any coating. 
EXAMPLES 2 TO 7 
Image-receiving sheets were prepared in the same manner as in Example 1 
except that butyral resins and pigments as shown in Table 1 were used. 
TABLE 1 
__________________________________________________________________________ 
Butyral Resin 
Type 
Residual 
Average 
Degree of Acetyl 
Degree of Pigment 
Example 
Butyralization 
Group 
Polymeriza- 
Amount Amount 
No. (mol %) (mol %) 
tion (parts) 
Type (parts) 
__________________________________________________________________________ 
2 60 3 250 5 special clacined 
15 
kaolin clay.sup.(1) 
3 63 6 500 12 calcined kaolin 
8 
clay.sup.(2) 
4 65 2 500 10 super-finely 
10 
divided amorphous 
silica.sup.(3) 
5 65 3 2,000 10 super-finely 
10 
divided silicic 
acid.sup.(4) 
6 68 3 1,000 10 titanium oxide 
10 
7 70 1 1,000 15 super-finely 
5 
divided hydrated 
silicic acid.sup.(5) 
__________________________________________________________________________ 
Note: 
.sup.(1) Trade name: Tysin; produced by Burgess Pigment Co. 
.sup.(2) Trade name: Burgess KE; produced by Burgess Pigment Co. 
.sup.(3) Trade name: Finesil X37; produced by Tokuyama Soda Co., Ltd. 
.sup.(4) Trade name: Silton R2; produced by Mizusawa Industrial Chemicals 
Ltd. 
.sup.(5) Trade name: Nipsil E220A; produced by Nippon Silica Industrial 
Corporation 
The thus prepared nine image-receiving sheets were subjected to the quality 
test described below. 
First, three types of coloring material-transferring sheets were prepared 
as follows: One part of each of three heat-sublimable dyes (Disperse 
Yellow 3, Disperse Red 60, and Solvent Blue 36), 1.5 parts of 
hydroxypropyl cellulose, and 15 parts of isopropyl alcohol were mixed, 
pulverized, and dispersed in a ball mill, to prepare three kinds of dye 
inks. Each dye ink was gravure printed at a coating weight (dry basis) of 
1.8 g/m.sup.2 on a 12 .mu.m thick condensor paper to prepare a coloring 
material-transferring sheet. 
The coloring material-transferring sheet was superposed on the 
image-receiving sheet in such a manner that the coated sides of the sheets 
came in contact with each other. Thereafter, heat was applied from the 
back side of the coloring material-transferring sheet by the use of a 
thermal head (16 V, 4 ms) to obtain a thermal dye-transferred recorded 
image on the image-receiving layer of the image-receiving sheet. Then, the 
density of each color of yellow, red, and blue was measured with a Macbeth 
color densitometer. The results are shown in Table 2. 
Further, the heat resistance of the recorded image was evaluated by heating 
the recorded image at 50.degree. C. for 5 hours. The light resistance was 
evaluated by exposing the recorded image to a xenon lamp (150 W) for 3 
hours. The changes in the density and resolution properties of the 
recorded images were evaluated based on the criterion shown below. 
A: No change. 
B: Slight change, but suitable for practical use. 
C: Serious change and unsuitable for practical use. 
The results are shown in Table 2. 
TABLE 2 
______________________________________ 
Heat Resistance 
Light 
Image Density Den- Resolution 
Resistance 
Yellow Red Blue sity Properties 
Density 
______________________________________ 
Example 1 
1.21 1.05 1.11 A B A 
Com- 1.11 1.07 1.10 A B B 
parative 
Example 1 
Com- 0.56 0.54 0.65 C C C 
parative 
Example 2 
Example 2 
1.21 1.07 1.14 A B A 
Example 3 
1.27 1.05 1.1 A B A 
Example 4 
1.30 1.10 1.21 A A A 
Example 5 
1.31 1.10 1.20 A A A 
Example 6 
1.29 1.12 1.20 A B A 
Example 7 
1.20 1.09 1.15 A B A 
______________________________________ 
It can be seen from Table 2 that in all the image-receiving sheets of the 
present invention, sharp recorded images can be obtained and, further, the 
recorded images exhibit excellent storage stability. 
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. Thus, it is intended that the 
present invention cover the modifications and variations of this invention 
provided they come within the scope of the appended claims and their 
equivalents.