EP0313355A2

EP0313355A2 - Thermal transfer material

Info

Publication number: EP0313355A2
Application number: EP88309866A
Authority: EP
Inventors: Tetsuo Hasegawa; Naoki Kushida; Yasuyuki Tamura; Koichi Tohma; Hisao Yaegashi; Takayuki Suzuki
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1987-10-21
Filing date: 1988-10-20
Publication date: 1989-04-26
Also published as: EP0313355A3; JPH01108089A

Abstract

A thermal transfer material comprising a support and at least a first ink layer and a second ink layer disposed in the order named on the support, wherein the adhesion strength F₁ between the support and the first ink layer and the adhesion strength F₂ between the first and second ink layers satisfy the relation of F₁ > F₂ at 90 °C. The thermal transfer material also provides a peeling strength of 1 - 5 g/cm at 90 °C. Two color recording is effected by superposing the thermal transfer material on plain paper, applying a pattern of heat and separating the thermal transfer material from the paper while changing the time from heating until the separation, i.e., temperature at the time of separation. Sharp edge-cutting of the heated portion and the selective transferability of the second ink layer are ensured by the definition of the peeling strength.

Description

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to a thermal transfer material for use in a recording method of transferring two-color images onto a recording medium such as plain paper.
The thermal or heat-sensitive transfer recording method has recently been widely used because it has general advantages of the thermal recording method such that the apparatus employed is light in weight, compact, free of noise, excellent in operability and adapted to easy maintenance, and also has other advantages such that it does not require a color-formation type converted paper but provides recorded images with excellent durability.
Further, there is also a commercial demand for a method of obtaining two-color images while retaining the advantages of the thermal transfer recording method as described above. Accordingly, there have been proposed several techniques for obtaining two-color images.
In order to obtain two-color images on plain paper by the thermal transfer recording method, Japanese Laid-Open Patent Application No. 148591/1981 discloses a two-color type thermal transfer recording element (transfer material) comprising a substrate and two heat-fusible ink layers including a high-melting point ink layer A and a low-melting point ink layer B containing mutually different colorants disposed in this order on the substrate. When a low thermal input energy is applied to the element, only the low-melting point layer B is transferred onto plain paper. On the other hand, when a high thermal input energy is applied to the element, both the heat-fusible ink layers A and B are transferred onto the plain paper. As a result, two-color images can be obtained.
Further, Japanese Laid-Open Patent Application No. 64389/1984 discloses a two-color thermal transfer ink sheet which comprises, on a substrate, an ink layer comprising an ink which melt-exudes at a lower temperature and another ink which is melt-peeled at a higher temperature than the melt-exudation temperature.
In the methods using the above-mentioned thermal transfer materials, two-color recording is effected by changing the energy applied to a thermal head at two levels so as to change the temperature of the ink layers. However, when a high energy is supplied to the ink layers to provide a high temperature, a lower temperature portion is formed at the periphery of a higher temperature portion due to heat diffusion, so that a bordering of a lower temperature color is formed around the higher temperature printed image. Further, when a high energy is supplied to a thermal head, it requires a relatively long time until the thermal head is cooled so that a higher-temperature printed image is liable to be accompanied with a trailing of a lower-temperature color. Further, in any of the above methods, there is a constraint that a relatively low melting material is required for providing an ink to be transferred at a lower temperature, whereby they give rise to problems such as ground soiling and low storability of the thermal transfer material.
As a technique for dissolving above-mentioned problems, our research group has proposed a recording method as disclosed in Japanese Laid-Open Patent Application (JP-A, KOKAI) No. 137789/1986 (U.S. Patent Application Serial No. 819,497). In this recording method, there is employed a thermal transfer material comprising a support and at least a first ink layer and a second ink layer disposed in this order on the support, and after heat is applied to the thermal transfer material, a length of time from the heat application until the separation between the transfer material and recording medium is so controlled that the second ink layer is selectively, or both the first and second ink layers are, transferred to the recording medium.
Our research group has further proposed, as a thermal transfer material for use in such recording method, one as disclosed in Japanese Laid-Open Patent Application No. 295075/1986 and one as disclosed in Japanese Laid-Open Patent Application No. 295079/1986. Japanese Laid-Open Patent Application No. 295075/1986 discloses a thermal transfer material wherein at least one of a first ink layer and a second ink layer contains a silicone oil or a fluorine-containing surfactant so as to promote separation between the first and second ink layers. Japanese Laid-Open Patent Application No. 295079/1986 discloses a thermal transfer material wherein a fine powder layer not meltable under application of a heat energy for recording is disposed between a first ink layer and a second ink layer so as to easily cause separation therebetween.
The above-mentioned recording method disclosed in Japanese Laid-Open Patent Application No. 137789/1986 (U.S. Patent Application Serial No. 819,497), has solved the problems of bordering, trailing, etc., in the prior art. In this new two-color recording method, however, a further improvement in transferred image quality is still desired.
In order to obviate a problem that the first ink is mixed into an image of the second ink when the second ink layer is selectively transferred in the above recording method, our research group has also proposed to separate the thermal transfer material from a recording medium under the action of a peeling force of not less than 20 g-f (gram-force) and less than 200 g-f in a direction perpendicular to and leaving from the surface of the recording medium toward the thermal transfer material (U.S. Patent Application Serial No. 58,852).
The quality of a recorded image is also improved by promoting sharp edge-cutting of a transferred image. It is, however, not always easy to effect such sharp edge-cutting in a transfer operation because it is influenced by various actual recording conditions such as a heat-application condition and a peeling condition among others.
Incidentally, as a technique for improving the edge-cutting of a transferred image obtained in the above-mentioned recording method disclosed in Japanese Laid-Open Patent Application No. 137789/1986 (U.S. Patent Application Serial No. 819,497), our research group has proposed a thermal transfer material wherein the total ink layers on a support have a tensile strength in the range of 8 - 20 kg/cm², as disclosed in Japanese Patent Application No. 226823/1986 (U.S. Patent Application Serial No. 98,320).
In order to effect sharp edge-cutting and to prevent a first ink from mixing into the image of a second ink when the second ink layer is selectively transferred in the above-mentioned recording method, our research group has further proposed a thermal transfer material wherein the first ink layer comprises a binder having a glass transition temperature (Tg) of 0 °C or below and 25 to 85 wt. % of a pigment (U.S. Patent No. 106,819).
However, there has recently been a commercial demand for a transferred image having a further high definition or resolution, and in order to meet such demand, a thermal head of high definition having a large number of heating element dots has also been developed. In a case where such thermal head of high definition is used, it is preferred to use a thermal transfer material adapted to a high definition recording. If a conventional thermal transfer material is subjected to a recording using the above-mentioned thermal head of high definition, an irregular transfer occurs at the periphery of a heat-applied portion and the edge portion of a printed image becomes uneven or irregular in some cases. Therefore, there has strongly been desired a thermal transfer material capable of providing a clear image even in combination with a thermal head of high definition.
As a thermal transfer material for meeting the above commercial demand, our research group has proposed one wherein the adhesion (F₁) between a support and a first ink layer is in the range of 1.0 g/cm - 10 g/cm (U.S. Patent Application Serial No. 162,574).

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide a thermal transfer material by which clear recorded images having a good color separation characteristic can be formed on plain paper by a simple method.
As a result of further study of ours, it has been found critical for ink layers including first and second layers not only to satisfy a specific magnitude relationship between the adhesion (strength) F₁ between a support and the first ink layer and the adhesion (strength) F₂ between the first and second ink layers at a specific temperature, but also to provide a specific range of peeling strength as an index to the adhesion F₂, under specific conditions, in order to enhance the selective transferability of the first and second ink layers in the above-described recording methods of Japanese Laid-Open Patent Application No. 137789/1986.
The thermal transfer material according to the present invention is based on the above knowledge and comprises: a support and at least a first ink layer and a second ink layer disposed in the order named on the support, wherein the adhesion strength F₁ between the support and the first ink layer and the adhesion strength F₂ between the first and second ink layers satisfy the relation of F₁ > F₂ at 90 °C; the thermal transfer material providing a peeling strength of 1 - 5 g/cm at 90 °C.
These and other objects features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, wherein like parts are denoted by like reference numerals. In the description appearing hereinafter, "part(s)" and "%" used for describing quantities are by weight unless otherwise noted specifically.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic sectional view across the thickness of an embodiment of the thermal transfer material according to the present invention;
Figure 2 is a schematic sectional view showing another laminar structure of the thermal transfer material according to the present invention; and
Figure 3 is a view illustrating a thermal transfer recording apparatus for two-color recording and a mode of operation thereof using a thermal transfer material according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1, a thermal transfer material 1 according to the present invention comprises a support 2, and a first ink layer 3 and a second ink layer 4 disposed in this order on the support.
In the thermal transfer material of the present invention, it is essential that the adhesion (strength) F₂ between the first ink layer 3 and the second ink layer 4 and the adhesion (strength) F₁ between the first ink layer 3 and the support 2 satisfy a relation of F₁ > F₂ at 90 °C, and that the peeling strength at this temperature is 1 - 5 g/cm.
More specifically, the above-mentioned relation between the adhesions F₁ and F₂ and the peeling strength value at the above-mentioned temperature may be confirmed in the following manner, for example.
A thermal transfer material 1 in a sheet form having a width (d) of about 1 - 2 cm and a length of about 3 cm as shown in Figure 1 is superposed on a recording medium such as plain paper, e.g., one having a Bekk smoothness of 200 sec so that the second ink layer 4 of the transfer material 1 contacts the recording medium, and heat is applied in a pattern (e.g., solid print pattern) to the transfer material 1 from its support 2 side by means of a thermal head as in the ordinary thermal transfer recording method. After the heat application, the transfer material 1 and the plain paper are, as they are without peeling, loaded on a tensile strength tester (Tensilon RTM-100, Toyo Boldwin K.K.) provided with a thermostat chamber capable of temperature control in the range of -60 °C to +270 °C so as to allow peeling at a peeling angle of 180 degrees. By using the above tester, the transfer material 1 and the plain paper are peeled from each other at a peeling speed of 300 mm/min at an environmental temperature (temperature in the thermostat chamber) of 90 °C, and a load (or force) required for such peeling is measured from the beginning to the end of the peeling. Then, the average value of the thus measured load represented by 1/2 x (maximum value + minimum value) is determined, and the peeling strength used in the present invention may be obtained by dividing the average value by the width (d) of the sample thermal transfer material.
On the other hand, the thermal transfer material 1 is superposed on plain paper and the thermal transfer material is bonded to the paper by heat in the same manner as described above. When the transfer material 1 and the plain paper are peeled from each other at a peeling speed of 300 mm/min, at an environmental temperature of 90 °C, and at a peeling angle of 180 degrees in the same manner as described above, the second ink layer 4 is selectively transferred to the plain paper as a recording medium (confirmation of F₁ > F₂). In this instance, when the first and second ink layers are caused to contain colorants of different color tones, an image with the color of the second ink is obtained at the environmental temperature of 90 °C.
In this way, the above-mentioned relation between the adhesions F₁ and F₂ can be easily confirmed.
In the thermal transfer material according to the present invention having the above-mentioned characteristics, it is preferred that both the first ink layer 3 and the second ink layer 4 are transferred to the recording medium at an environmental temperature (temperature in the thermostat chamber) of 40 °C (i.e., the relationship of F₁ < F₂ is satisfied at 40 °C), in order to miniaturize the thermal transfer recording apparatus actually used for recording. In such case, the above-mentioned relation of F₁ < F₂ may easily be confirmed by the fact that an image with the color of the first ink is obtained on the recording medium at the environmental temperature of 40 °C.
Incidentally, the above-mentioned adhesion (F₂) between the second and first ink layers and the adhesion (F₁) between the first ink layer and the support are evaluated according to relative easiness between the separation between the second and first ink layers, and the separation between the first ink layer and the support, when transfer recording is effected on a recording medium. Such evaluation of the adhesions is not affected by the form of separation between ink layers (e.g., whether or not the separation between the second and first ink layers has occurred strictly at the boundary between these layers, or whether or not some separation layer described hereinafter, if any, remains on the thermal transfer material).
In the thermal transfer material according to the present invention, the above-mentioned peeling strength is required to be 1 - 5 g/cm and may preferably be 2 - 4 g/cm. If the peeling strength is smaller than 1 g/cm, the separation of the second ink layer 4 can be easily caused therein to decrease the reproducibility in the transfer thereof. Alternatively, edge-cutting at the boundary between a heat-applied portion and a non-heat-applied portion is decreased with respect to the separation between the first and second ink layers thereby to decrease the selectivity in the separation therebetween.
On the other hand, if the peeling strength is larger than 5 g/cm, the adhesion strength between the first ink layer and the second ink layer bonded to a recording medium by heat application becomes too large, and the difference with the adhesion (F₁) between the support and the first ink layer and/or the adhesion (F₃) between the second ink layer and the recording medium is difficult to be made sufficiently large. As a result, the selective transferability of the second ink layer becomes insufficient, whereby clear color separation cannot be obtained and color reproducibility becomes poor.
On the contrary, when the above-mentioned peeling strength is in the range of 1 - 5 g/cm, good color separation is obtained without breaking a recording medium such as plain paper, whereby clear two-color recorded images are obtained.
As described above, the thermal transfer material according to the present invention satisfies a specific relationship with respect to the adhesions F₁ and F₂ and also provides a specific peeling strength at the above-mentioned specific temperature (90 °C). Incidentally, the above-mentioned temperature 90 °C (or 40 °C) should be used only in order to define the characteristic of the thermal transfer material 1 per se, but these temperatures do not define the using (or thermal transfer recording) conditions for the thermal transfer material.
In the thermal transfer material 1 of the present invention having the above-mentioned characteristics, it is further preferred that the respective ink layers satisfy the following conditions.
Between the first and second ink layers, it is preferred that the cohesion of the first ink layer is larger than that of the second ink layer at 90 °C. As the difference in cohesion between the first and second ink layers becomes larger, a recorded image of clearer color can be obtained by selective transfer of the second ink layer with less mixing of the first ink. Further, in addition to a large difference in cohesion, it is preferred that the first and second ink layers are composed of different materials which are mutually insoluble. In order to satisfy the large cohesion difference and the mutual insolubility, it is for example desired that the binder constituting the first ink layer contains 50 parts or more of a resin as described hereinbelow, and the binder constituting the second ink layer contains 50 parts or more of a wax as described below, per 100 parts of the respective binders. Particularly, the second ink layer may preferably contain 70 parts or more of a wax per 100 parts of the binder, in order to more suitably attain a peeling strength of 1 - 5 g/cm.
In an embodiment wherein a separation layer is not disposed, the melt viscosity of the second ink layer may preferably be 5,000 - 9,000 mPa·S, more preferably 6,000 - 8,000 mPa·S at 140 °C, in order to suitably obtain a peeling strength of 1 - 5 g/cm.
In the present invention, as shown in Fig. 2, it is possible to form a separation layer (or adhesive layer) 5 between the first ink layer 3 and the second ink layer 4. The separation layer 5 is a layer for controlling the adhesion (F₂) between the first and second ink layers. If the separation layer 5 is composed of a material having properties similar to those required of the second ink layer explained with reference to Figure 1, the second ink layer 4 is not necessarily required to satisfy the properties explained with reference to Figure 1. As a result, it becomes possible to compose the second ink layer of a material which can show a large adhesion to a recording medium, e.g., a material similar to one for constituting the first ink layer in the embodiment shown in Figure 1, thus being further advantageous in improving the recorded image quality. In order to retain the adhesion to a recording medium, it is preferred that the melt viscosity of the second ink layer 4 is selected so that it provides 200 cps or more at a temperature 30 °C higher than the softening temperature of the second ink layer 4.
In an embodiment wherein a separation layer is disposed, the melt viscosity of the separation layer may preferably be 5,000 - 9,000 mPa·S, more preferably 6,000 - 8,000 mPa·S at 140 °C, in order to suitably obtain a peeling strength of 1 - 5 g/cm.
Further, the structure shown in Figure 2 is advantageous as compared with a structure not having a separation layer 5, in that substantially no separation is caused within the second ink layer 4 and therefore no fluctuation in density of recorded images results.
Incidentally, even in such embodiment as shown in Fig. 2, if the above-mentioned peeling strength is smaller than 1 g/cm, the color separation between the first and second ink layers becomes poor. The reason for this may for example be considered that in such case, the melt viscosity of the separation layer 5 becomes too low at the above-mentioned temperature, and the separation layer 5 may partially be crushed easily due to the pressure of a thermal head actually used (ordinarily, about 100 - 300 g as a load).
Hereinbelow, the structure or composition of the respective parts in the thermal transfer material 1 of the present invention is supplemented.
As the support 2 of the thermal transfer material, it is possible to use films of, e.g., polyester, aramide resin, nylon, polycarbonate, or paper such as capacitor paper, preferably having a thickness of about 3 to 12 microns. Too thick a support is not desirable because the heat conductivity becomes inferior. If a sufficient heat resistance and a strength are attained, a support can be thinner than 3 microns. It is sometimes advantageous to coat the back surface (opposite to the face on which the ink layers are disposed) with a layer for supplementing the heat resistance.
In the thermal transfer material 1 of the present invention, the ink layers and the separation layer, if any, on the support 2 may preferably have a thickness of not exceeding 20 microns in total. Further, it is preferred that each of the first ink layer 3, second ink layer 4, and separations layer 5, has a thickness in the range of 0.5 - 10 microns.
As a heating means for the thermal transfer recording method using the thermal transfer material of the present invention, ordinary heat sources such as infrared rays and laser beam may also be used in place of a thermal head. Further, in order to provide a conduction heating system, i.e., a system wherein a thermal transfer material itself generates a heat due to a current passing therethrough, a thin layer of a conductive material such as aluminum may be disposed as a return electrode between the support 2 and the first ink layer 3, or the ink layer per se may be made conductive, as desired.
The first ink layer 3 constituting a thermal transfer layer on the support 2 may be formed by dispersing a colorant of a first color tone in a binder (not intended to exclude a case where the colorant is dissolved in the binder), and the second ink layer 4 may be formed by dispersing a colorant of a second color tone in a binder.
In the thermal transfer material of the present invention, when the color of the first ink layer 3 and the second ink layer 4 are desired to be obtained substantially as they are, it is preferred to dispose a first ink layer 3 of a dark color such as black and a second ink layer 4 of a brighter color than that of the first ink layer such as yellow. Further, when the color of the second ink layer 4 and the mixed color due to mixing of the first and second ink layers are desired to be obtained, there may for example be disposed a first ink layer 3 of yellow and a second ink layer 4 of magenta, whereby two-color images with magenta and red portions can be obtained in the same manner as described above. Various two-color combinations can be obtained by using different kinds and concentrations of colorants and/or different proportions in thickness of ink layers.
The colorant may be selected from all of the known dyes and pigments including: carbon black, Nigrosin dyes, lamp black, Sudan Black SM, Alkali Blue, Fast Yellow G, Benzidine Yellow, Pigment Yellow, Indo Fast Orange, Irgadine Red, Paranitroaniline Red, Toluidine Red, Carmine FB, Permanent Bordeaux FRR, Pigment Orange R, Lake Red C, Rhodamine FB, Rhodamine B Lake, Methyl Violet B Lake, Phthalocyanine Blue, Pigment Blue, Brilliant Green B, Phthalocyanine Green, Oil Yellow GG, Zapon Fast Yellow CGG, Kayaset Y963, Kayaset TG, Smiplast Yellow GG, Zapon Fast Orange RR, Oil Scarlet, Smiplast Orange G, Orasol Brosn B, Zapon Fast Scarlet CG, Aizen Spiron Red BEH, Oil Pink OP, Victoria Blue F4R, Fastgen Blue 5007, Sudan Blue, and Oil Peacock Blue. Two or more of these colorant may be used in a combination of two or more species as desired. Further, metal powder such as copper powder and aluminum powder or powder of mineral such as mica may also be used as a colorant. Further, other additives such as surfactants, plasticizers, mineral oils, vegetable oils, fillers, etc., may also be added to an ink layer or separation layer.
The binders for constituting the first and second ink layers and the materials for constituting the separation layer may be selected, as a single species or a combination of two or more species as desired, from the following materials: waxes including: natural waxes such as whale wax, beeswax, lanolin, carnauba wax, candelilla wax, montan wax and ceresin wax; petroleum waxes such as paraffin wax and micro-crystalline wax; synthetic waxes such as oxidized wax, ester wax, low-molecular weight polyethylene, Fischer-Tropsch wax and the like; higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, and behenic acid; higher alcohols such as stearyl alcohol and behenyl alcohol; esters such as fatty acid esters of sucrose and fatty acid esters of sorbitane; amides such as oleic amide; or resins including: polyolefin resins, polyamide resins, polyester resin, epoxy resins, polyurethane resins, acrylic resins, polyvinyl chloride resins, cellulose resins, polyvinyl alcohol resins, petroleum resins, phenolic resins, polystyrene resins, polyvinyl acetate resins; elastomers such as natural rubber, styrene-butadiene rubber, isoprene rubber, chloroprene rubber; polyisobutylene, polybutene, etc.
The content of the colorant in each of the first and second ink layers may preferably be in the range of 1 - 90 %, particularly 2 - 80 %.
The ink layers and separation layer having the desired properties as described above may be obtained by appropriately controlling the properties such as molecular weights, crystallinities, etc., of the above mentioned materials or appropriately mixing a plurality of the above-mentioned materials.
In a preferred embodiment of the present invention, the second ink layer 4 may preferably comprise an ethylenevinyl acetate resin, paraffin wax and a colorant; or an ethylenevinyl acetate resin, oxidized polyethylene and a colorant. Further, the separation layer 5, if any, may preferably comprise paraffin wax or carnanba wax.
The thermal transfer material according to the invention may be obtained by forming the respective layers by mixing the materials constituting the respective layers and an organic solvent such as methyl ethyl ketone, xylene and tetrahydrofuran capable of dissolving the binders and applying the thus formed coating liquids successively on the support. Alternatively, the so-called hot-melt coating method may be adopted, including the steps of blending, hot-melting and applying the materials in a molten state for the respective layers. The materials for the respective layers may be formed into aqueous emulsions by the addition of a dispersant such as a surfactant, and the aqueous emulsion may be applied to form the respective layers. Further, the respective layers of the transfer material may also be formed by using the above mentioned coating methods in combination, i.e., by using different methods for the respective layers.
As described above, according to the present invention, there is provided a thermal transfer material which comprises at least a first and a second ink layer on a support and shows a specific relationship between the adhesion between the support and the first ink layer (F₁) and the adhesion between the first and second ink layers (F₂) at a specific temperature, and which also shows a specific peeling strength at this temperature.
By using the thermal transfer material of the present invention, it is possible to provide beautiful two-color images on a recording medium such as plain paper only by supplying a pattern of energy and separating the thermal transfer material from the recording medium while changing the length of time from the heat application until the separation.
Hereinbelow, the present invention will be explained more specifically while referring to specific examples of practice. Incidentally, the melt viscosity of a sample was measured by means of a rotational viscometer (E-type).

Example 1

<Ink 1>

Polyamide resin aqueous dispersion (melt viscosity = 4x10³ mPa·S at 140 °C, weight-average molecular weight = 8,000, acid value = 4 amine value = 2, softening point = 108 °C)
80 parts
Carbon black aqueous dispersion	20 parts
(The amounts, physical properties such as melt viscosities and molecular weights of aqueous dispersions for providing an ink formulation, in this example and the other examples are all expressed based on their solid contents.)

The above components were sufficiently mixed to prepare an ink 1. The ink 1 was applied onto a 6 micron-thick PET (polyethylene terephthalate) film and dried at 70 °C to form a first ink layer at a coating rate of 2 g/m².

<Ink 2>

Ethylene-vinyl acetate resin aqueous dispersion (melt viscosity = 1x10⁵ mPa·S at 150 °C)	30 parts

Paraffin wax aqueous dispersion (melt viscosity = 5x10² mPa·S at 100 °C)	50 parts
Blue pigment (Cyanine Blue) aqueous dispersion	20 parts
Fluorine-containing surfactant	0.1 part

The above components were sufficiently mixed to prepare an ink 2, which was applied onto the above prepared first ink layer and dried at 70 °C to form a second ink layer at a coating rate of 3 g/m², whereby a thermal transfer material (I) having a structure as shown in Figure 1 was obtained.

Example 2

<Ink 3>

Paraffin wax aqueous dispersion (melt viscosity = 5x10² mPa·S at 100 °C)	100 parts
Fluorine-containing surfactant	0.1 part

<Ink 4>

Oxidized polyethylene aqueous dispersion (number-average molecular weight = 2000, melt viscosity = 2x10² mPa·S at 140 °C)
60 parts

Ethylene-vinyl acetate resin aqueous dispersion (melt viscosity = 1x10⁵ mPa·S at 150 °C)	40 parts
Blue pigment aqueous dispersion	25 parts
Fluorine-containing surfactant	0.1 part

The above components were respectively sufficiently mixed to prepare inks 3 and 4.
A first ink layer of the ink 1 was formed on a PET support in the same manner as in Example 1. Then, a separation layer was formed on the first ink layer by the ink 3 at a coating rate of 1.0 g/m², and further a second ink layer was formed on the separation layer by the ink 4 at a coating rate of 2.5 g/m², whereby a thermal transfer material (II) having a laminar structure as shown in Figure 2 was prepared.

Example 3

<Ink 5>

Alkyl acrylates copolymer aqueous dispersion (weight ratio of methyl methacrylate: butyl acrylate = 95:5, melt viscosity = 5x10⁴ mPa·S at 150 °C)
80 parts
Carbon black aqueous dispersion	20 parts
Fluorine-containing surfactant	0.1 part

<Ink 6>

Carnauba wax aqueous dispersion (melt viscosity = 1x10¹ mPa·S at 130 °C)	100 parts
Fluorine-containing surfactant	0.1 part

The above components were respectively sufficiently mixed to prepare inks 5 and 6.
A first ink layer in a coating amount of 2.0 g/m² was formed on a 4.5 micron-thick PET support by the above ink 5, and a separation layer in a coating amount of 1.0 g/cm² was formed on the first ink layer by the ink 6, and then a second ink layer in a coating amount of 2.5 g/m² was formed on the separation layer by the ink 4, whereby a thermal transfer material (III) was prepared.
The thermal transfer material (I), (II) and (III) were respectively cut into a sheet form having a width of 2 cm and a length of 10 cm and then were superposed on wood-free paper having a surface smoothness of 200 sec so that their second ink layers contacted the paper. Then, heat corresponding to a solid print pattern was applied to the transfer material under the conditions of a pulse cycle of 1.4 msec., a pulse duration of 0.7 msec, and an applied energy of 13 mJ/mm², whereby the thermal transfer materials and the wood-free paper were heat-bonded. Each of the superpositions of the thermal transfer material and the paper was loaded on a tensile tester (Tensilon RTM-100, mfd. by Toyo Boldwin K.K.). Then, the transfer material and the plain paper were peeled from each other at a peeling speed of 300 mm/min, at a peeling angle of 180 degrees and at an environmental temperature of 90 °C, and a load required for such peeling was measured from the beginning to the end of the peeling. Then, the average value of the thus measured load represented by 1/2 x (maximum value + minimum value) was determined, and the peeling strength was obtained by dividing the average value by the above-mentioned width of the thermal transfer material. The thus obtained results are shown in a table appearing hereinafter.
In the above-mentioned peeling test, when any of thermal transfer materials (I), (II) and (III) was used, clear blue recorded images were obtained through selective transfer of the second ink layers, and the first ink layers remained on the PET supports (F₁ > F₂).
Then, the above thermal transfer materials (I), (II) and (III) were respectively used for recording by means of a thermal transfer recording apparatus for an English typewriter (Typestar 6, mfd. by Canon K.K.).
More specifically, a system as schematically shown in Figure 3 was used. Referring to Figure 3, the thermal transfer material 1 wound off from a supplying core 9a was moved to a heat-applying position, where it was pressed against a recording medium 7 supported by a platen 10 by means of a thermal head 6 so that the second ink layer thereof contacted the recording medium 7 and simultaneously a pattern of heat was applied to the thermal transfer material 1 from the thermal head 6. When the thermal transfer material 1 was peeled from the recording medium 7 at the rear end 6a of the thermal head 6 immediately after the heat application, only the second ink layer was transferred to the recording medium 7. On the other hand, when a peeling-control member 8 was moved in the direction of an arrow A to a position 8a indicated by dashed lines, the thermal transfer material 1 and the recording medium 7 were pressed against each other, a pattern of heat was supplied from the thermal head, and the thermal transfer material 1 was peeled from the recording medium 7 at the position 8a of the control member 8, both the first and second ink layers were transferred to the recording medium 7. Incidentally, as shown in Figure 3, the thermal transfer material 1 peeled from the recording medium 7 was wound up about a winding core 9b.
Referring again to Figure 3, as the thermal head 6, one prepared by Rohm K.K., having a length from the center of the heat generating part to the trailing end 6a of 350 microns was used. A carriage 11 loading the thermal head 6 and the ribbon of the thermal transfer material 1 was moved in the direction of an arrow B, at a moving velocity of 50 mm/sec. As a result, the time from heating until the peeling-off of the thermal transfer material 1 from the recording medium was about 7 msec when the thermal transfer material was peeled from the recording medium immediately after the heat application. In order to delay the time of the peeling, a control member 8 for controlling the peeling was disposed at about 5 mm (i.e., ℓ = 5 mm as shown in Figure 3) after the trailing end 6a of the thermal head (i.e., downstream side of the trailing end 6a with respect to the moving direction of the thermal transfer material 1).
As a result, when the control member 8 was moved toward the recording medium 7, the delayed time of peeling-off was about 100 msec after the heating. Incidentally, it was confirmed that the result of the recording was not substantially different from the case of ℓ = 5 mm, even if the position of the control member 8 was changed in the range of from 2 mm to 20 mm (i.e., ℓ = 2 - 20 mm) after the trailing end 6a of the thermal head 6.
When the transfer recording was conducted on plain paper by the use of the thermal transfer material (I), (II) and (III), blue images were obtained if the transfer material was peeled immediately after the heat application, and black images were obtained if the transfer material was peeled at the delayed time. The results are also shown in the table appearing hereinafter.

Comparative Example 1

<Ink 7>

Carboxylated NBR latex (melt viscosity = 1x10⁴ mPa·S at 150 °C)	40 parts

Oxidized polyethylene aqueous dispersion (number-average molecular weight = 5000, melt viscosity = 5x10³ mPa·S at 140 °C)
60 parts
Blue pigment aqueous dispersion	25 parts
Fluorine-containing surfactant	0.1 part

The above components were sufficiently mixed to prepare an ink 7.
A thermal transfer material (IV) was prepared in the same manner as in Example 3 except that the second ink layer was formed by using the above-mentioned ink 7.

Comparative Example 2

<Ink 8>

Terpene-phenol resin aqueous dispersion (melt viscosity = 3x10² mPa·S at 150 °C)	100 parts
Fluorine-containing surfactant	0.1 part

The above components were sufficiently mixed to prepare an ink 8.
A thermal transfer material (V) was prepared in the same manner as in Example 3 except that the separation layer was formed by using the above-mentioned ink 8.
The thus obtained thermal transfer materials (IV) and (V) were respectively heat-bonded to wood-free paper and the peeling strengths were measured by using the the tensile tester in the same manner as in the case of the thermal transfer materials (I), (II) and (III) according to the present invention.
Then, the thermal transfer materials (IV) and (V) were used for recording by means of the transfer recording apparatus (Typestar 6) in the same manner as in the case of the transfer material (I). As a result, when the thermal transfer material (IV) was peeled from a recording medium immediately after the heat application, it caused lacking of image and could not provide images reproducibly. On the other hand, when the thermal transfer material (V) was peeled from a recording medium immediately after the heat application, it only provided bluish black images wherein numerous black spots were mixed in a blue recorded image. As a result, both of the above-mentioned images provided by the thermal transfer materials (IV) and (V) were not usable in practice.
The thus obtained results of the peeling strength measurements and two-color image recordings with respect to the thermal transfer materials (I) to (V) are shown in the following table.

Transfer material Peeling strength (g/cm) Image quality of two-color image

Example 1 I 3 Good

2 II 2 Good

3 III 4 Good

Comparative Example 1 IV 0.3 *1

2 V 10 *2

*1 Blue recorded images could not be provided reproducibly.

*2 Blue recorded images were unclear.

Claims

1. A thermal transfer material, comprising: a support and at least a first ink layer and a second ink layer disposed in the order named on the support, wherein the adhesion strength F₁ between the support and the first ink layer and the adhesion strength F₂ between the first and second ink layers satisfy the relation of F₁ > F₂ at 90 °C; said thermal transfer material providing a peeling strength of 1 - 5 g/cm at 90 °C.

2. A thermal transfer material according to Claim 1, which provides a peeling strength of 2 - 4 g/cm.

3. A thermal transfer material according to Claim 1, wherein the adhesions F₁ and F₂ satisfy the relation of F₁ < F₂ at 40 °C.

4. A thermal transfer material according to Claim 1, which further comprises a separation layer between the first and second ink layers.

5. A thermal transfer material according to Claim 1, wherein the second ink layer comprises an ethylene-vinyl acetate resin, paraffin wax and a colorant.

6. A thermal transfer material according to Claim 1, wherein the second ink layer comprises an ethylene-vinyl acetate resin, oxidized polyethylene and a colorant.

7. A thermal transfer material according to Claim 6, which further comprises a separation layer comprising paraffin wax between the first and second ink layers.

8. A thermal transfer material according to Claim 6, which further comprises a separation layer comprising carnauba wax between the first and second ink layers.