Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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B CKGROUND OF THE INVENTION
The present invention relates to a heat
sensitive transfer medium Eor use in a multi-gradation
printing process in which the density ox a print image
can be changed. More particularly, it relates to a
multi-gradation heat sensitive transfer medium for use in
a multi-gradation thermal transfer process for adjusting
the density of a transfer image by superimposing ink
layers on a recording medium a plurality of times by
thermal transfer
As a conventional multi-gradation thermal
transfer method, there is known a technique commonly
called 3L method (see Japan Denshi Tsushin Gakkai's
technical report IE 81-63, p 45 to 52, Sept. 25, 1981).
This 3L method is intended to obtain a
multiplicity of gradations through combinations of dot
number variations in a picture element with reflection
density variations in thermally transferred ink layers.
With this method, however, it has been very difficult to
obtain a multiplicity of image gradations close to
natural tones and yet having a high degree of resolution,
becuase it has a limitation in the adjustment of
reflection density of the ink layers and because the
number of dots is subject to limitation relative to
resolution.
In order to overcome the drawback of the 3L
method, there may be considered a method in which
printing is carried out by superimposing ink layers of
the same color which have the same density or different
densities, thereby adjusting the density of the print
image. According to this method, it is possible to use
the number of times of superimposing as a means for
providing multi-gradation and to improve resolution by
decreasing the number of dots per picture element but yet
obtain a some number of gradations.
Ho~7ever, such multi-gradation method utilizing
superimposing printing technique has a disadvantage that
the density of a subsequently transferred ink layer
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produces a stronger effect than that of a previously
transferred ink layer so that it is difficult to obtain a
progressive increase in density, thus the print image
produced being likely to be of a foggy and light tone
even at a maximum reElection density.
SUMMARY OF THE INVENTION
The object of this invention is to overcome
such difficulty and make it possible to obtain clear
print images of a multiplicity of gradations by thermal
transfer printing.
This and other objects of the invention will
become apparent from the description hereinafter.
To this end, the invention provides a heat
sensitive transfer medium having improved thermal transfer
ink layers each of which contains a color pigment having
a high transparency and a high tinting strength in an
amount smaller than sufficient to provide a maximal
degree of reflection density, and has a light
transmittance of not less than 65 % in a region where the
color pigment does not have any absorption band.
BRIEF DESCRIPTION OF TOE DRAWINGS
Fig. 1 is a schematic illustration showing an
embodiment of a multi-gradation heat sensitive transfer
medium according to the present invention wherein ink
layers are arranged in color dandara pattern.
Figs. 2, 3 and 4 are graphical representations
showing the relation between reflection density GOD) and
amount of color medium with respect to print images as
obtained in Examples 1 and 2 and Comparative Example,
respectively.
DETAILED DESCRIPTION
. . .
By printing the above-mentioned specific ink
layers so that they are superimposed with each other on a
recording medium, it is possible to obtain a print image
having high reflection density, much higher in the upper
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limit thereof than any conventionally produced print
image, and which is very clear and visually free of foggy
and fuzzy tone.
In this invention, the amoun-t of color medium
of an ink layer defined below must be less than
sufficient to give a maximal degree of reflection density
to a print image, since if it exceeds the upper limit of
reflection density for the image in the particular color,
multi-gradation is not attainable.
The amount of color medium is defined as
follows:
Amount of color medium (g/m ) =
color pigment contentl x amount of coating
l (% by weight) J l (g/m ) J
100
The maximal degree of reflection density of a
print image is intended to mean the upper limit that
reflection density never exceeds even if the amount of
color medium is increased by any amount over the limit.
The visible light transmittance of each ink
layer in a region where a color pigment used does not
have any absorption band (hereinafter this region is
referred to as "non-absorption band") should be not less
than about 65 %, preferably not less than 70 %. If it is
lower than 65 %, the upper level of reflection density of
the image produced can never be sufficient, and no clear
print image can be obtained either. The ligh-t
transmittance is preferably as high as feasible.
Printing using the transfer medium of the
pesent invention is carried out by melt-transferring the
ink layer of the transfer medium in the form of dots on a
recordirlg medium hy means of a thermal head having a
plurality of heating elements. In the invention, the
superimposing of ink layers is intended to mean that the
dots of an ink layer subsequently transferred are
substantially superimposed over the dots of an ink layer
transferred previously on a recording medium.
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The present invention is more particularly
described and explained by means of the followirlg
Examples. These Examples are intended to illustrate the
invention and not be construed to limit the scope of the
invention. It is to be understood that various changes
and modifications may be made in the invention without
departing from the spirit and scope thereof.
Example l
On a film base there were coated three partial
ink layers Yl, Y2, Y3 in yellow Y; three partial ink
layers Cl, C2, ~3 in cyan C; and three partial ink layers
Ml, M2, M3 in magenta M in a "Dandara" pattern as shown
in Fig. l (Dandara: trademark of Fuji Kagakushi Kogyo
Co., Ltd.). The three partial ink layers for each color
were different from each other in the amount of color
medium.
For the film base was used a polyester film
having a thickness of 9 I. As a vehicle for the ink
layers was used one having a high degree of transparency,
the composition of which was as follows:
Ingredient % by weight
25 Carnauba wax No. 1 20
Paraffin wax (melting point 65C) 35
Ester wax 25
Petroleum resin 10
Spindle oil 10
As color mediums for the ink layers were used
the following color pigments each having a high degree of
transperency: cyanine blue for cyan, rhodamine lake Y for
magenta, and benzidine yellow for yellow.
Table 1 shows the content of color pigment for
each respective partial ink layer, and the reflection
density ~pD) of a print image as thermally transferred
one time by a thermal printer directly on a plain paper,
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and the light transmittance of each ink layer in the
non~absorption band and the wavelength of t:he non-
absorption band.
The reflection density GOD) oE a print image is
defined as follows:
Reflection density GOD) - [Reflection density of
a print image] - [Reflection density of unprinted
portion]
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Then, superimposing printing of the partial ink
layers in each color was carried out two or more times by
using a thermal printer. With respect to the print
images thus obtained, the relationship between the
reflection density GOD) of the print image and the
amount of color medium is graphically presented in Fig.
2. In Fig. 2 (as well as in Figs. 3 and 4), the print
images obtained from Yl, Cl or Ml are signified by markO ,
the print images obtained from Y2, C2 or M2 by mark, and
the print images obtained from Y3, C3 or M3 by mark.
As is clear from Fig. 2, with each color, once
the amount of color medium exceeded a certain limit, the
reflection density GOD) of the print image produced
never did exceed the certain value even if the amount of
color medium was increased by any amount over the limit.
About 10 gradations of print images were obtained for
e-ach color, each print image being found as having an
exceptionally good definition.
Example 2
The same procedures as in Example 1 except that
0.5 part by weight of titanium oxide was employed together
with 1 part by weight of each color pigment as used in
Example 1 were repeated to produce a multi-gradation
heat sensitive transfer medium. Printing was carried out
by using the obtained transfer medium in the same manner
as described in Example 1. The relationship between the
reflection density GOD) of the print image produced and
the amount of color medium is graphically shown in Fig. 3
The light transmittance in non-absortion band
was about 70 % with a layer corresponding to the
aforesaid ink layer Yl, about 76 with a layer
corresponding to aforesaid ink layer Cl, and about 73
with a layer coresponding to the aforesaid ink layer Ml.
As is apparent from Fig. 3, a maximal value of
reflection density GOD) substantially as high as that in
Example 1 was obtained for each respective color.
Further, about 8 gradations of print iamges were obtained
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for each color, each print image being of an
exceptionally good definition.
comparative Example
The same procedures as 1n Exarnple 1 except that
4 parts by weight of titanium oxide was employed together
with 1 part by weight of each color pigment as used in
Example 1 were repeated to produce a multi-gradation heat
sensitive transfer medium. The light transmittance in
non-absorption band was about 45 with a layer
corresponding to the aforesaid ink layer Yl, about 49
with a layer corresponding to the aforesaid ink layer Cl,
and about 47 with a layer corresponding to the
aforesaid ink layer Ml. Printing was carried out by
using the obtained transfer medium in the same manner as
described in Example 1. The relationship between the
reflection density GOD) of the print image obtained and
the amount of color medium is graphically shown in Fig. 4.
As can be clearly seen fom Fig. 4, the print
image obtained in Comparative Example had a much lower
reflection density in each color as compared with those
in Examples 1 and 2, the image being of foggy tone.
In the above experiments, titanium oxide was
used to lower the light transmittance in order to prove
the effect of change in light transmittance, in view of
the fact that the use of a different color pigment would
be reflected in a difference in hue which would be
inconvenient from the standpoint of comparison.
In Figs. 2 to 4, it is noted, mark O indicates
the reflection density GOD) of a print image produced by
using an ink layer which was formed by applying an ink
having a color pigment content equal to that of one used
in an ink layer marked, in amount of coating ox 2 g/m~.
As can be clearly seen from Figs. 2 to 4,
reflection density GOD) varies depending upon the amount
of color medium.
In many cases, the thickness of an ink layer is
preferably selected so that the total thickness of
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superimposed prints is about 35 lam or less.
In cases where, not in aforesaid Examples alone,
a vehicle or color pigment having good transperency, Eor
example, any of those men-tioned below, was used, good
results similar to those observed in the above Examples
were obtained. Maximal reflection density, light
transmittance in non-absorption band, and amount of
color medium at which the maximal reflection density was
reached varied depending upon the kind of the material
used. In every case, however, a maximal or higher level
of density was visually observed.
As a color pigment in yellow was used one kind
or a mixture of two or more kinds of pigments such as
naphthol yellow S, Hansa yellow 5G, permanent yellow NCG,
and quinoline yellow lake. Good results were obtained as
in Example l.
- As a color pigment in magenta was used one kind
or a mixture of two or more kinds of pigments such as
brilliant fast scarlet, brilliant carmine BS, permanent
carmine FB, lithol red, permanent red F5R, brilliant
carmine 6B, pigment scarlet 3B, rhodamine lake s, and
alizarin lake. Again, good results were obtained as in
Example l.
As a color pigment in cyan was used one kind or
a mixture of two or more kinds of pigments such as
Victoria blue lake, metal-free phthalocyanine blue,
phthalocyanine blue, and fast sky blue. Again, good
results were obtained as in Example 1.
Carbon black or the like was used as a color
pigment in black, and in this case, the results were also
satisfactory as in Example l.
With respect to the composition of a vehicle,
it is desirable to use solid wax having a penetration of
lO to 30 (at 25 C) as a binder in order to obtain an
improved melt-transferability of ink layers. For
example, waxes such as carnauba wax, microcrystalline
wax, Japan wax, beeswax, ceresin wax and spermaceti are
used. Further, any readily hot-meltable material such as
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low molecular weight polyethylene, oxidized wax or ester
wax may be used in combination.
As a softening agent may be advantageously used
any readily hot-meltable material such as petroleum resin,
polyvinyl acetate, polystyrene, styrene-butadiene
-opolymer, cellulose esters, cellulose ethers or acrylic
resins, or lubricating oils.
Furthermore, for the purpose of the present
invention, it is possible to use a heat-conductive
powdery material and/or an extender pigment in order to
give good heat-conductivity and melt-transferability to
such heat sensitive ink layer.
As such heat-conductive powdery material may be
advantageously used aluminum, copper, or zinc, for
example, which has a heat-conductivity of 6.0 x 10 4 to
25.0 x 10 4 cal/sec.cm.C.
As extender pigments may be used colloidal
silica, magnesium carbonate, calcium carbonate, clay,
kaolin, calcium silicate, highly dispersive silicic acid
anhydride (commercially available under the name
"Aerosil" made by Nippon Aerosil Kabushiki Kaisha), and
white carbon, for example, which all have relatively high
transparency.
Such heat-conductive material and extender
pigment may be used in an amount of 0 to 30 parts by
weight and 0 to lO parts by weight per 100 parts by
weight of the total dry weight of the ink composition for
each color, respectively.
It is noted that the combination and amounts of
the above ingredients of which the vehicle consists
should be selected so that the transparency of the
vehicle itself may not be affected adversely.
As a foundation may be used thin papers such as
thin condenser paper, insulating condenser paper,
one-time carbon base paper, parchment paper, glassine
paper, India paper and wax paper; plastic films such as
polyester film, polyimide film and polyvinyl chloride
film; and cellophane.
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The foundation may have a highly heat-resistant
resin layer coated thereon in order to prevent sticking
or a highly heat-conduct.ive layer coated thereon in order
to improve transferability.
The arrangement of ink layers relative to the
foundation may not be limited to one such as shown in
Fig. 1, but such layers may be arranged in any
conventional pattern.
Again, not only is it possible to apply ink
layers in different colors on one foundation, but it is
possible as well to apply ink layers in different colors
to separate foundations on a color by color basis or to
change the foundation according to the difference in
density.
