Thermal transfer sheet

There is provided a thermal transfer sheet comprising: a substrate film; and a hot-melt ink layer provided on one side of the substrate film, the hot-melt ink layer containing a bismuth oxide/borosilicate glass frit having a softening point of 400.degree. to 500.degree. C.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a thermal transfer sheet having a hot-melt ink 
layer. More particularly, this invention relates to a thermal transfer 
sheet which is suitable for use in printing of an image having stability 
such as chemical resistance and heat resistance. 
2. Background Art 
A thermal transfer sheet comprising a substrate and a hot-melt ink layer 
provided on one side of the substrate has hitherto been used as a thermal 
transfer recording medium for use with a thermal printer, a facsimile 
machine, a bar code printer and the like. 
In the conventional thermal transfer sheet, about 10 to 20 .mu.m-thick 
paper, such as a capacitor paper or a paraffin paper, or about 3 to 20 
.mu.m-thick plastic film, such as a polyester film or cellophane, is 
provided as a substrate film, and a hot-melt ink of wax with a colorant, 
such as a pigment or a dye, incorporated therein is coated on the 
substrate film to form a hot-melt ink layer. 
In use, heat and pressure are applied through the back side of the 
substrate film by means of a thermal head to melt the hot-melt ink layer 
in its areas corresponding to printing areas and to transfer the melt onto 
a printing paper, thereby conducting printing. 
In recent years, in the production of members like assemblies of precision 
components, such as cathode-ray tubes and IC circuits, the above thermal 
transfer sheet is, in some cases, used in such a manner that bar codes are 
printed using the thermal transfer sheet on a label and the printed label 
is applied to members for purposes of process control. These members, in 
the course of production process, are treated under severe conditions, 
that is, undergo sintering at a high temperature of about 500.degree. C. 
or surface treatment with a chemical, such as an acid. In this case, the 
bar code label is also exposed to the same severe conditions. In some 
thermal transfer sheets for such an application, a heat-resistant glass 
frit is dispersed in a binder for the hot-melt ink layer of the thermal 
transfer sheet (Japanese Patent Laid-Open No. 198195/1984). 
In the case of the thermal transfer sheet containing a glass frit in the 
binder of the hot-melt ink layer, lead glass has been used as the glass 
frit, posing a problem that lead becomes a toxic water-soluble compound. 
Therefore, this thermal transfer sheet is unfavorable from the viewpoint 
of biological safety. 
Further, when the conventional thermal transfer sheet is used for the above 
process control, the binder of the hot-melt ink layer is evaporated from 
the printed label upon heat treatment, leaving only a pigment. Therefore, 
the printed image is not fixed on the label, posing a problem that the 
image has no storage stability. 
An object of the present invention is to solve the above problems and to 
provide a thermal transfer sheet which can provide an image having 
excellent storage stability and biological safety, that is, creating no 
toxicity, in applications where sintering at a high temperature and 
surface treatment with a chemical are conducted. 
SUMMARY OF THE INVENTION 
According to the present invention, the above object can be attained by a 
thermal transfer sheet comprising: a substrate film; and a hot-melt ink 
layer provided on one side of the substrate film, the hot-melt ink layer 
containing a bismuth oxide/borosilicate glass frit having a softening 
point of 400.degree. to 500.degree. C. 
According to a preferred embodiment of the present invention, the bismuth 
oxide/borosilicate glass frit contains an alkali metal. 
The construction of the thermal transfer sheet of the present invention is 
such that a hot-melt ink layer is provided on one side of a substrate 
film, the hot-melt ink layer containing a bismuth oxide/borosilicate glass 
frit having a softening point of 400.degree. to 500.degree. C. The use of 
a glass frit having a softening point falling within the above range and 
below the sintering temperature used (generally 400.degree. to 500.degree. 
C.) permits the glass frit to be melted so as to cover the color pigment, 
ensuring the fixation of a printed image onto a label. The softening point 
of the glass frit used is preferably about 5.degree. C. lower than the 
sintering temperature. 
Incorporation of an alkali metal in the glass frit can provide a printed 
image having improved resistance to surface treatment with an acidic 
chemical. 
Further, since a bismuth oxide/borosilicate glass frit is used instead of 
the lead glass frit which creates toxicity, the printed image creates no 
toxicity and is biologically safe. 
DETAILED DESCRIPTION OF THE INVENTION 
Embodiments of the thermal transfer sheet of the present invention will be 
described in detail. 
The thermal transfer sheet of the present invention comprises a substrate 
film and a hot-melt ink layer provided on one side of the substrate film. 
In the thermal transfer sheet of the present invention, a matte layer may 
be optionally provided between the substrate film and the hot-melt ink 
layer in order to provide a matte print. Further, a backside layer may be 
optionally provided on the surface of the substrate film remote from the 
hot-melt ink layer. 
Substrate film 
The substrate film used in the thermal transfer sheet of the present 
invention is not particularly limited. Specifically, substrate films used 
in the conventional thermal transfer sheet as such may be used in the 
present invention, and, further, other substrate films may also be used. 
Specific preferred examples of the substrate film include: films of 
plastics, such as polyester, polypropylene, cellophane, polycarbonate, 
cellulose acetate, polyethylene, polyvinyl chloride, polystyrene, nylon, 
polyimide, polyvinylidene chloride, polyvinyl alcohol, fluororesin, 
chlorineted rubber, and ionomar; papers such as condenser paper and 
paraffin paper; nonwoven fabrics; and laminates of these materials. 
The thickness of the substrate film may be varied depending upon the 
material so as to have suitable strength and thermal conductivity. For 
example, it is preferably 2 to 25 .mu.m. 
Backside layer 
A backside layer may be provided on the surface of the substrate film 
remote from the hot-melt ink layer from the viewpoint of preventing 
blocking between the thermal transfer sheet and a thermal head and, at the 
same time, of improving the slip property of the thermal transfer sheet. 
The backside layer may be formed of preferably a binder resin with 
additives, such as a slip agent, a surfactant, inorganic particles, 
organic particles, and a pigment, incorporated therein. 
Binder resins usable in the backside layer include, for example, cellulosic 
resins, such as ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl 
cellulose, methyl cellulose, cellulose acetate, cellulose acetate 
butyrate, and nitrocellulose, vinyl resins, such as polyvinyl alcohol, 
polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, polyvinyl 
pyrrolidone, acrylic resin, polyacrylamide, and acrylonitrile/styrene 
copolymer, polyester resin, and silicone-modified or fluorine-modified 
urethane resin. 
The use of a crosslinked resin among them is preferred. In this case, a 
resin having several reactive groups, for example, hydroxyl groups, is 
used in combination with a crosslinking agent, such as a polyisocyanate. 
The backside layer may be formed by dissolving or dispersing the above 
binder resin, containing additives, such as a slip agent, a surfactant, 
inorganic particles, organic particles, and pigments, in a suitable 
solvent to prepare a coating liquid, coating the coating liquid by means 
of conventional coating means, such as a gravure coater, a roll coater, or 
a wire bar, and drying the coating. 
Matte layer 
A matte layer may be provided between the substrate film and the hot-melt 
ink layer for the following purposes. 
In the case of a thermally transferred image, the surface of the print is 
generally glossy and beautiful. In some cases, however, letters in the 
print are illegible. Therefore, matte prints are often desired. The matte 
layer may be provided, for example, as proposed by the present inventors 
(Japanese Patent Application No. 208306/1983), that is, between the 
substrate and the hot-melt ink layer by coating a dispersion of a suitable 
binder resin and an inorganic pigment, such as silica or calcium 
carbonate, in a suitable solvent and drying the coating. 
The thickness of the matte layer is preferably about 0.1 to 1 .mu.m. When 
the thickness is less than 0.1 .mu.m, the function of the matte layer is 
unsatisfactory, while when it exceeds 1 .mu.m, much printing energy is 
unfavorable necessary. 
Hot-melt ink layer 
The hot-melt ink layer contains a bismuth oxide/borosilicate glass frit 
having a softening point of 400.degree. to 500.degree. C., and the 
conventional colorant and binder. If necessary, various additives, such as 
mineral oils, vegetable oils, higher fatty acids, such as steatic acid, 
plasticizers, thermoplastic resins, and fillers, may be incorporated in 
the hot-melt ink layer. 
Waxes usable as the binder include, for example, microcrystalline wax, 
carnauba wax, and paraffin wax. Further, other various waxes, such as 
Fischer-Tropsh wax, various polyethylenes having low molecular weights, 
Japan wax, beeswax, spermaceti, insect wax, wool wax, shellac wax, 
candelilla wax, petrolatum, polyester wax, partially modified wax, fatty 
acid esters, and fatty acid amides. Among them, those having a melting 
point of 50.degree. to 85.degree. C. are preferred. When the melting point 
is below 50.degree. C., a problem of storage stability occurs, while when 
the melting point exceeds 85.degree. C., the sensitivity is 
unsatisfactory. 
Resins usable as the binder include, for example, ethylene/vinyl acetate 
copolymer, ethylene/acrylic ester copolymer, polyethylene, polystyrene, 
polypropylene, polybutene, petroleum resin, vinyl chloride resin, vinyl 
chloride/vinyl acetate copolymer, polyvinyl alcohol, vinylidene chloride 
resin, methacrylic resin, polyamide, polycarbonate, fluororesin, 
polyvinylformal, polyvinyl butyral, acetyl cellulose, nitrocellulose, 
polyvinyl acetate, polyisobutylene, ethyl cellulose, and polyacetal. 
Resins, having a relatively low softening point, for example, a softening 
point of 50.degree. to 80.degree. C., which have hitherto been used as a 
heat-sensitive adhesive, are particularly preferred. 
The bismuth oxide/borosilicate glass frit used in the present invention 
comprises Bi.sub.2 O.sub.3 and B.sub.2 O.sub.3 as essential components. In 
the bismuth oxide/borosilicate glass frit, the contents of Bi.sub.2 O3, 
B.sub.2 O.sub.3, SiO.sub.2 and the like are regulated to bring the 
softening point to 400.degree. to 500.degree. C. and to form a network 
structure. For example, a glass frit comprising, by weight, 75 to 85% of 
Bi.sub.2 O.sub.3, 12 to 18% of B.sub.2 O.sub.3, 0.1 to 5.0% of SiO.sub.2, 
0.05 to 3.0% of CeO.sub.2, 0.1 to 3.0% of Li.sub.2 O, and 0.05 to 3.0% of 
Na.sub.2 O has a softening point of 400.degree. to 500.degree. C. 
Further, the average particle diameter of the bismuth oxide/borosilicate 
glass frit is preferably 0.1 to 10 .mu.m from the viewpoint of good 
dispersibility in the formulation of an ink. 
A bismuth oxide/borosilicate glass frit with an oxide of an alkali metal, 
such as sodium, potassium or lithium, added thereto is preferred from the 
viewpoint of improving the resistance of the printed image to surface 
treatment with an acidic chemical. 
The colorant may be selected from the conventional organic or inorganic 
pigments, and colorants having satisfactory color density and causing 
neither color change nor fading upon exposure to light, heat and the like. 
For example, inorganic colorants, such as carbon black, graphite, a 
sulfide and oxide of iron, titanium, nickel, chromium, manganese, cobalt, 
cadmium and like, and compound oxides of these metals, are particularly 
preferred. 
The color of the colorant is not limited to cyan, magenta, yellow, and 
black, and colorants of various colors may be used. 
Further, a thermally conductive material may be incorporated as a filler 
for the binder into the hot-melt ink layer from the viewpoint of imparting 
good heat conductivity and hot-melt transferability. Fillers usable herein 
include, for example, carbonaceous materials, such as carbon black, and 
metals and metallic compounds, such as aluminum, copper, tin oxide, and 
molybdenum disulfide. 
The content of the colorant is preferably 5 to 80% based on the solid 
content of the whole ink layer, and the content of the bismuth 
oxide/borosilicate glass frit is preferably 1 to 60% based on the solid 
content of the whole ink layer. The proportion of the weight of the 
bismuth oxide/borosilicate glass frit to the weight of the colorant is 
preferably 50 to 200% from the viewpoint of the storage stability of an 
image in the print. 
The hot-melt ink layer may be formed by formulating the above colorant 
component, binder component, bismuth oxide/borosilicate glass frit, and 
optionally a solvent component, such as water or an organic solvent, to 
prepare a coating liquid for a hot-melt ink layer and coating the coating 
liquid by any conventional coating method, such as hot-melt coating, 
hot-lacquer coating, gravure coating, gravure reverse coating, or roll 
coating. Alternatively, it may be formed by using a coating liquid of an 
aqueous or nonaqueous emulsion. 
The thickness of the hot-melt ink layer should be determined so as to offer 
a good balance between the necessary print density and the heat 
sensitivity. It is generally about 0.to 30 .mu.m, preferably about 1 to 20 
.mu.m. 
The thermal transfer sheet of the present invention can, of course, cope 
with color printing, and thermal transfer sheets for multi-color printing 
also fall within the scope of the present invention. 
The following examples further illustrate the present invention but are not 
intended to limit it. In the following examples and comparative examples, 
all "parts" or "%" are by weight unless otherwise specified.

EXAMPLE 1 
A 4.5 .mu.m-thick polyethylene terephthalate film was provided as a 
substrate film, and a coating liquid, for a matte layer, having the 
following composition was coated on one side of the substrate film by 
means of a gravure coater, and the coating was dried to form a 0.5 
.mu.m-thick matte layer. A coating liquid, for a hot-melt ink layer, 
having the following composition was coated on the matte layer by means of 
a hot-melt coater, and the coating was dried to form a 10 .mu.m-thick 
hot-melt ink layer. 
A coating liquid, for a backside layer, having the following composition 
was coated on the other side of the substrate film by means of a roll 
coater, and the coating was dried to form a 0.1 .mu.m-thick backside 
layer. Thus, a thermal transfer sheet of the present invention was 
prepared. 
______________________________________ 
Coating liquid for matte layer 
Polyester resin (Vylon 200, 
60 parts 
manufactured by Toyobo 
Co., Ltd.) 
Carbon black (Diablack, 
40 parts 
manufactured by Mitsubishi 
Chemical Corporation) 
Methyl ethyl ketone 100 parts 
Toluene 200 parts 
Coating liquid for hot-melt ink layer 
Fired pigment: Fe.sub.2 O.sub.3.CoO.Cr.sub.2 O.sub.3 
45 parts 
Bismuth oxide/borosilicate glass 
30 parts 
frit (softening point: 440-445.degree. C.) 
.alpha.-Olefin/maleic anhydride copolymer 
20 parts 
Ethylene/vinyl acetate copolymer 
6 parts 
(Evaflex 200W, manufactured 
by DuPont-Mitsui Polychemicals 
Co., Ltd.) 
Polyester wax 7 parts 
Carnauba wax 7 parts 
Paraffin wax 10 parts 
(SP-0160, manufactured by 
Nippon Seiro Co., Ltd.) 
Rosin ester 5 parts 
Coating liquid for backside layer 
Polyvinyl butyral resin 
20 parts 
(S-lec BX-1, manufactured by 
Sekisui Chemical Co., Ltd.) 
Talc (Microace L-1, manufactured by 
30 parts 
Nippon Talc Co., Ltd.) 
Fine particles of melamine resin 
30 parts 
(Epostar-S, manufactured by Nippon 
Shokubai Kagaku Kogyo Co., Ltd.) 
Polyisocyanate (Takenate A-3, 
40 parts 
Takeda Chemical Industries, Ltd.) 
Toluene/methyl ethyl ketone (1/1) 
900 parts 
______________________________________ 
EXAMPLE 2 
A thermal transfer sheet of the present invention was prepared in the same 
manner as in Example 1, except that the coating liquid for a hot-melt ink 
layer had the following composition. 
______________________________________ 
Coating liquid for hot-melt ink layer 
______________________________________ 
Fired pigment: Fe.sub.2 O.sub.3.CoO.Cr.sub.2 O.sub.3 
45 parts 
Bismuth oxide/borosilicate glass 
40 parts 
frit (softening point: 440-445.degree. C.) 
.alpha.-Olefin/maleic anhydride copolymer 
20 parts 
Ethylene/vinyl acetate copolymer 
6 parts 
(Evaflex 200W, manufactured 
by DuPont-Mitsui Polychemicals 
Co., Ltd.) 
Polyester wax 7 parts 
Carnauba wax 7 parts 
Paraffin wax 10 parts 
(SP-0160, manufactured by 
Nippon Seiro Co., Ltd.) 
Rosin ester 5 parts 
______________________________________ 
EXAMPLE 3 
A thermal transfer sheet of the present invention was prepared in the same 
manner as in Example 1, except that the coating liquid for a hot-melt ink 
layer had the following composition. 
______________________________________ 
Coating liquid for hot-melt ink layer 
______________________________________ 
Fired pigment: Fe.sub.2 O.sub.3.CoO.Cr.sub.2 O.sub.3 
45 parts 
Bismuth oxide/borosilicate glass 
50 parts 
frit (softening point: 440-445.degree. C.) 
.alpha.-Olefin/maleic anhydride copolymer 
20 parts 
Ethylene/vinyl acetate copolymer 
6 parts 
(Evaflex 200W, manufactured 
by DuPont-Mitsui Polychemicals 
Co., Ltd.) 
Polyester wax 7 parts 
Carnauba wax 7 parts 
Paraffin wax 10 parts 
(SP-0160, manufactured by 
Nippon Seiro Co., Ltd.) 
Rosin ester 5 parts 
______________________________________ 
EXAMPLE 4 
A thermal transfer sheet of the present invention was prepared in the same 
manner as in Example 2, except that no matte layer was provided. 
Comparative Example 1 
A thermal transfer sheet of the present invention was prepared in the same 
manner as in Example 4, except that the coating liquid for a hot-melt ink 
layer had the following composition and the thickness of the hot-melt ink 
layer was 4 .mu.m. 
______________________________________ 
Coating liquid for hot-melt ink layer 
______________________________________ 
Carbon black (Diablack, 
10 parts 
manufactured by Mitsubishi 
Chemical Corporation) 
Ethylene/vinyl acetate copolymer 
10 parts 
(Evaflex 200W, manufactured 
by DuPont-Mitsui Polychemicals 
Co., Ltd.) 
Carnauba wax 15 parts 
Paraffin wax 60 parts 
(SP-0160, manufactured by 
Nippon Seiro Co., Ltd.) 
______________________________________ 
Comparative Example 2 
A thermal transfer sheet of the present invention was prepared in the same 
manner as in Example 4, except that the coating liquid for a hot-melt ink 
layer had the following composition the thickness of the hot-melt ink 
layer was 8 .mu.m. 
______________________________________ 
Coating liquid for hot-melt ink layer 
______________________________________ 
Fired pigment: Fe.sub.2 O.sub.3.CoO.Cr.sub.2 O.sub.3 
45 parts 
.alpha.-Olefin/maleic anhydride copolymer 
20 parts 
Ethylene/vinyl acetate copolymer 
6 parts 
(Evaflex 200W, manufactured 
by DuPont-Mitsui Polychemicals 
Co., Ltd.) 
Polyester wax 7 parts 
Carnauba wax 7 parts 
Paraffin wax 10 parts 
(SP-0160, manufactured by 
Nippon Seiro Co., Ltd.) 
Rosin ester 5 parts 
______________________________________ 
Printing was performed using the thermal transfer sheets prepared in the 
above examples and comparative examples under the following printing 
conditions, and the prints thus obtained were fired at 450.degree. C. for 
two periods of time, 30 min and 180 min. The prints were evaluated for 
sensitivity in printing, heat resistance, and acid resistance according to 
the following criteria. 
Printing conditions 
Printer: BC8MKII, manufactured by Auto Nics Co., Ltd. 
Printing energy: 0.3 mj/dot (different conditions were used for the 
evaluation of the sensitivity in printing) 
Printing speed: 67 mm/sec 
Label: Label for firing (layer with ceramic bonded thereto/inorganic fiber 
cloth) 
Sensitivity in printing 
Printing of bar codes was carried out with the printing energy varied in 16 
stages from 0.23 mj/dot to 0.39 mj/dot, and the prints were read with a 
bar code scanner. In this case, the minimum printing energy value was 
determined which could provide bar codes readable with the bar code 
scanner without creating any error. The sensitivity in printing was 
evaluated in terms of the energy level. 
.largecircle.: Bar codes were readable when the printing energy was between 
0.23 mj/dot and 0.28 mj/dot. 
.DELTA.: Bar codes were readable when the printing energy was between 0.29 
mj/dot and 0.34 mj/dot. 
Heat resistance 
Bar codes were printed on a label under the above printing conditions, and 
firing was carried out at 450.degree. C. for two periods of time, 30 min 
and 180 min. The reflectance of black of the bar codes was measured with a 
bar code scanner (AUTOSCAN 7000, manufactured by RJS ENTERPRISES INC.) 
before and after the firing, and the heat resistance of the print was 
evaluated from the measured values. 
.largecircle.: Bar codes after firing were readable with a bar code 
scanner, and the reflectance of black area was 2 to 3% before firing and 
not more than 12% after firing. 
.DELTA.: Bar codes after firing were readable with a bar code scanner, and 
the reflectance of black area was 2 to 3% before firing and not less than 
12% after firing. 
x: Bar codes after firing were unreadable with a bar code scanner. 
Acid resistance 
Solid printing was carried out on a label under the above conditions, and 
the print was fired in an oven of 450.degree. C. for 30 min. A strong 
acid, such as amidosulfonic acid, was dropped on the print, and, after 
standing in this state for a while, the print was washed with water. After 
the residual water was wiped off, the print was rubbed with a finger to 
evaluate the fixation of the transferred ink to the label. 
.circleincircle.: Completely fixed. 
.largecircle.: Fairly fixed, and rubbing resulted in about 10% lowered ink 
density. 
x: Not fixed, and rubbing resulted in separation of ink. 
Evaluation results 
The results of evaluation on the prints prepared using the thermal transfer 
sheets of the examples and the comparative examples are summarized in 
Table 1. 
TABLE 1 
______________________________________ 
Heat resistance 
Heat resistance 
Sensitivity (fired for (fired for Acid 
in printing 30 min) 180 min) resistance 
______________________________________ 
Example 1 
.DELTA. .largecircle. 
.largecircle. 
.largecircle. 
Example 2 
.DELTA. .largecircle. 
.largecircle. 
.largecircle. 
Example 3 
.DELTA. .largecircle. 
.largecircle. 
.circleincircle. 
Example 4 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
Comparative 
.largecircle. 
X X X 
Example 1 
Comparative 
.largecircle. 
.largecircle. 
.DELTA. X 
Example 2 
______________________________________