EP0372585A2

EP0372585A2 - Thermal transfer recording medium

Info

Publication number: EP0372585A2
Application number: EP89122716A
Authority: EP
Inventors: Makoto C/O Seiko Epson Corporation Taniguchi; Michinari C/O Seiko Epson Corporation Tsukahara
Original assignee: Seiko Epson Corp
Current assignee: Seiko Epson Corp
Priority date: 1988-12-09
Filing date: 1989-12-08
Publication date: 1990-06-13
Also published as: EP0372585A3

Abstract

A thermal transfer recording medium (2) for use with a thermal transfer printer (7, 8, 9) comprises a substrate (5), and a thermal transfer ink layer (1) coated on the substrate (2). The substrate includes a plurality of first minute regions randomly coated with the thermal transfer ink layer (1), and a plurality of second minute regions uncoated therewith. The thermal transfer ink layer (1) may comprise a plurality of minute concave and convex portions randomly provided on the first minute regions.

Description

The present invention relates to a thermal transfer recording medium for use with a thermal transfer printer.
Prior art transfer recording media have a uniform thickness of ink layer coated on the entire surface of the transfer area except for pinholes and other defects that might occur accidentally. This prior art technique, however, has experienced several problems during thermal transfer, such as low resolution of tone reproduction and jumps from medium to high density.
In the prior art, with a view to performing sharp and faithful ink transfer onto receiving sheet during thermal transfer with thermal transfer printers, a binder that melts in the same temperature range is selected and colorants such as a pigment and a dye are dispersed in the binder to formulate a heat-fusible ink, which is coated in uniform thickness over a substrate. Alternatively, the heat-fusible ink is coated as interspersed dots to prevent ink spreading or chaining as described in Japanese Patent Application No. 59-224389.
However, when a full-color image is formed by successive transfer of three colored surfaces with such conventional thermal transfer recording media using yellow, magenta and cyan inks, not only jumps from medium to high density occur but also inks run in the high density region, thus deteriorating the tone reproduction characteristics and producing a blurred image having a limited range of reproduced colors.
An object, therefore, of the present invention is to provide a thermal transfer recording medium that is capable of achieving faithful tone reproduction upon thermal transfer.
This object is solved by the thermal transfer recording medium of the independent claims. Further advantageous features of the thermal transfer recording medium of the invention are evident from the dependent claims.
The present invention provides a thermal transfer recording medium that has sufficiently improved tone reproduction characteristics of ink to enable the formation of a sharp color image with a broad range of reproduced colors when multi-colored surfaces are successively transferred.
According to one aspect of the present invention, a thermal transfer recording medium is provided that has a thermal transfer ink layer coated on a substrate in such a way that the coated area is composed of randomly spaced small coated and uncoated regions.
According to another aspect of the present invention a thermal transfer recording medium is provided that has a thermal transfer ink layer coated on a substrate in such a way that said ink layer has randomly spaced small asperities across its thickness, with the thickness of any area having a recess being preferably no more than one half the thickness of any area having a projection. More preferably, any area having a recess is not thicker than 5 µm.
According to still another aspect of the present invention a thermal transfer recording medium is provided that has a thermal transfer ink layer coated on a substrate in such a way that the coated area is composed of randomly spaced small coated and uncoated regions, and that said ink layer has randomly spaced small asperities across its thickness, with the thickness of any area having a recess being preferably no more than one half the thickness of any area having a projection, and with any area having a recess being not thicker than 5 µm in a more preferred embodiment.
The thermal transfer recording medium of the present invention is characterized by a thermal transfer ink layer which is coated in the form of isolation interspersed on a substrate. Desirably, the coated ink layer assumes 40 - 95% of the total area of the substrate.
The thermal transfer recording medium of the present invention which has the construction described above has the advantage of achieving faithful tone reproduction upon thermal transfer. A particular advantage of this recording medium achieves faithful tone reproduction in the medium to high density regions upon thermal transfer. With the recording medium of the present invention, the ink is not transferred all at once at a certain temperature but is transferred gradually over a broad temperature range. Further, the ink layer is coated as interspersed islands on a substrate. Because of these two major features, ink chaining, running and spreading are effectively prevented to insure good tone reproduction characteristics in the low to high density regions, thereby producing a sharp image with a broad range of reproduced colors.

Fig. 1 illustrates a first embodiment of the present invention, with Fig. 1A showing a heat-fusible ink layer coated on a substrate and Fig. 1B being an enlarged view of the essential part of Fig. 1A;
Fig. 2 is a graph showing tone reproduction characteristic;
Fig. 3 illustrates a second embodiment of the present invention, with Fig. 3A showing a heat-fusible ink layer coated on a substrate and Fig. 3B being an enlarged view of the essential part of Fig. 3A;
Fig. 4 is a graph showing the tone reproduction characteristic of a thermal transfer recording medium prepared according to the present invention as compared to that of a prior art product;
Fig. 5 illustrates a third embodiment of the present invention, with Fig. 5A showing a heat-fusible ink layer coated on a substrate and Fig. 5B being an enlarged view of the essential part of Fig. 5A;
Fig. 6 is a graph showing the tone reproduction characteristic of another thermal transfer recording medium prepared according to the present invention as compared to that of a prior art product;
Fig. 7 illustrates a comparative thermal transfer recording medium, with Fig. 7A showing a heat-fusible ink layer coated on a substrate and Fig. 7B being an enlarged view of the essential part of Fig. 7A;
Figs. 8A and 8B show schematically the process of transfer with a prior art thermal transfer recording medium;
Figs. 9A, 9B and 9C show schematically the process of transfer with the thermal transfer recording medium of the present invention;
Fig. 10 shows schematically the constitution of the thermal transfer recording medium of the present invention;
Fig. 11 is a cross section showing the constitution of the thermal transfer recording medium samples prepared in Examples 1 - 4 of the present invention;
Fig. 12 is a cross section showing the constitution of the thermal transfer recording medium prepared in Comparative Example 1;
Fig. 13 is a cross section showing the constitution of the thermal transfer recording medium prepared in Comparative Example 2;
Fig. 14 shows schematically the process of printing with a thermal head using the thermal recording transfer medium samples prepared in Examples 1 - 4 and Comparative Examples 1 - 3;
Fig. 15 is a graph correlating printing energy with transfer density to show the tone reproduction characteristic of magenta ink which was the first color of the inks used in Examples 1 - 4;
Fig. 16 is a graph correlating printing energy with transfer density to show the tone reproduction characteristic of cyan ink which was the second color of the inks used in Examples 1 - 4;
Fig. 17 is a graph correlating printing energy with transfer density to show the tone reproduction characteristic of yellow ink which was the third color of the inks used in Examples 1 - 4;
Fig. 18 is a graph correlating printing energy with transfer density to show the tone reproduction characteristic of magenta ink which was the first color of the inks used in Comparative Examples 1 - 2;
Fig. 19 is a graph correlating printing energy with transfer density to show the tone reproduction characteristic of cyan ink which was the second color of the inks used in Comparative Examples 1 - 2; and
Fig. 20 is a graph correlating printing energy with transfer density to show the tone reproduction characteristic of yellow ink which was the third color of the inks used in Comparative Examples 1 - 2.

The present invention is described in greater detail with reference to preferred embodiments and comparative examples.
The first embodiment of the present invention is illustrated in Figs. 1A and 1B. Fig. 1A shows the surface state of a heat-fusible ink layer 101 coated on a substrate 102. The encircled portion of Fig. 1A is shown enlarged in Fig. 1B, in which numeral 111 denotes the area where the heat-fusible ink layer is deposited and numeral 113 denotes the area where it is not deposited. Numeral 112 denotes the enlarged portion of the substrate 102, which may be made of any substrate material for thermal transfer recording media such as polyethylene terephthalate, polyamide, polyimide, polyether sulfone, capacitor paper, etc.

The composition of the heat-fusible ink used in the first embodiment is shown in the following Table 1.

Table 1

Component	Parts by weight
Phthalocyanine Blue
	20
Carnauba wax No. 1	20
Paraffin wax	15
Oxidized polyethylene wax	20
Polystyrene resin	25
Toluene	450
Methyl ethyl ketone	450

Fig. 2 is a graph showing the tone reproduction characteristic of the recording medium of the present invention (a) and that of a prior art product (b). Comparing curves (a) and (b), one can see that the recording medium of the present invention shows a very gradual change in density. In other words, transfer density can be easily controlled by changing transfer energy, thus leading to the production of high-quality prints with improved tone reproduction. The vertical axis of the graph in Fig. 2 plots reflection optical density (OD) with maximum value normalized for 1.0, and the horizontal axis plots transfer energy.
The second embodiment of the present invention is illustrated in Fig. 3A and 3B. Fig. 3A shows schematically a heat-fusible ink layer 101 coated on a substrate 102. The encircled portion of Fig. 3A is shown enlarged in Fig. 3B. The hatched area 111 is a projection of the heat-fusible ink, and 113′ denotes a recess in the heat-fusible ink layer. Numeral 112 denotes the enlarged portion of the substrate 102, which may be made of any substrate material for thermal transfer recording media such as polyethylene terephthalate, polyamide, polyimide, polyether sulfone, capacitor paper, etc. The thickness of the ink coating is not limited to any particular value but preferably, the thickness of the recessed ink layer 113 is no more than one half the thickness of the projecting portion 111 or is no greater than 5 µm.

The composition of the heat-fusible ink used in the second embodiment is shown in the following Table 2. Needless to say, the present invention which relates to the structure of a thermal transfer recording medium is by no means limited to the ink composition shown in Table 2.

Table 2

Component	Amount(wt%)
Carmine 6B	20
Carnauba wax No. 1	15
α-Olefin/maleic anhydride copolymer	30
CN-Paraffin wax	10
polyethylene wax	20
EEA	5
Solids/toluene = 1/9
EEA: ethylene-ethyl acrylate copolymer

Fig. 4 is a graph showing the tone reproduction characteristic of the recording medium of the present invention (301) and that of a prior art product (302). The vertical axis of the graph in Fig. 4 plots reflection OD with maximum value normalized for 1.0 and the horizontal axis plots transfer energy. Comparing curves (301) and (302), one can see that the recording medium of the present invention shows a very gradual change in density in the medium to high density regions where transfer energy varies from 24 to 28 mJ/mm².
The third embodiment of the present invention is illustrated in Figs. 5A and 5B. Fig. 5A shows schematically a heat-fusible ink layer 101 coated on a substrate 102. The encircled portion of Fig. 5A is shown enlarged in Fig. 5B. The hatched area 111 is a projection of the heat-fusible ink, and 113 denotes a recess in the heat-fusible ink layer. Numeral 114 denotes the area where the heat-fusible ink layer is not deposited. Numeral 112 denotes the enlarged portion of the substrate 102, which may be made of any substrate material for thermal transfer recording media such as polyethylene terephthalate, aromatic polyamide, aliphatic polyamide, polyimide, polyether sulfone, capacitor paper, etc. The thickness of the ink coating is not limited to any particular value but preferably, the thickness of the recessed ink layer is no more than one half the thickness of the projecting portion or is no greater than 5 µm.

The composition of the heat-fusible ink used in the third embodiment is shown in the following Table 3. Needless to say, the present invention which relates to the structure of a thermal recording medium is by no means limited to the ink composition shown in Table 3.

Table 3

Component	Amount(wt%)
Carmine 6B	20
Carnauba wax No. 1	20
α-Olefin/maleic anhydride copolymer	20
Microcrystalline wax	10
polyethylene wax	25
EEA	5
Solids/toluene = 1/9
EEA: ethylene-ethyl acrylate copolymer

Fig. 6 is a graph showing the tone reproduction characteristic of the recording medium of the present invention (301) and that of a prior art product (302). The vertical axis of the graph in Fig. 6 plots reflection OD with maximum value normalized for 1.0 and the horizontal axis plots transfer energy. Comparing curves (301) and (302), one can see that the recording medium of the present invention shows a very gradual change in density in the medium to high density regions where transfer energy varies from 25 to 29 mJ/mm².
The surface stage of a heat-fusible ink layer coated by the prior art method is illustrated as a comparison in Figs. 7A and 7B. As shown in Fig. 7A, the heat-fusible ink layer indicated by 201 is coated on a substrate 202. The essential part of Fig. 7A is shown enlarged in Fig. 7B, in which 211 denotes the enlarged portion of 201 and 212 denotes the enlarged portion of 202. The difference in the coating thickness of heat-fusible ink layer as deliberately created between 111 and 113 according to the present invention is not observed in Figs. 7A and 7B. Nor is observed the small region as indicated by 114 where the heat-fusible ink is not deposited in the ink coated area.
Figs. 8A and 8B show a model case where a prior art heat-fusible ink layer 404 melts by heat generation from heating elements 407 in a thermal head 406. The areas where the ink melts are delineated by ABCD and EFGH. The substrate is shown by 403 and the ink which is transferred onto receiving sheet as a result of heat generation is shown by 405.
As transfer energy is increased, the distance between A and B and that between E and F will increase whereas the distance between B and E will decrease. Thus, the force of adhesion 401 between the unmelted ink portion BEDG and the substrate will decrease with increasing transfer energy. On the other hand, the force 402 which resists the withdrawal of ABCD and EFGH from the thermal transfer recording medium is little affected by thermal energy. Hence, if thermal energy increases to a certain level, the force 402 will become stronger than 401, whereupon the ink BEDG which should remain on the substrate will be transferred simultaneously with ABCD and EFGH, thus providing a transfer density much bigger than what should be yielded.
Figs. 9A, 9B and 9C show the process of forming asperities in an ink layer according to the present invention. They show a model case where a heat-fusible ink layer 504 melts by heat generation from heating elements 507 in a thermal head 506. The areas where the ink melts are delineated by A′B′C′D′ and E′F′G′H′. The substrate is shown by 503 and the ink which is transferred onto receiving sheet as a result of heat generating is shown by 505.
If a region where the ink layer is deposited in small thickness is present in the area B′E′D′F′ as in the present invention, force 502 becomes smaller than force 402. On the other hand, force 501 is equal to force 401, so if 501<502, the distance between E′ and B′ will become shorter than that between E and B. Hence, a satisfactory transfer density will have been achieved by the time the area B′E′D′G′ is transferred unintentionally. In other words, the thermal transfer recording medium obtained achieves faithful tone reproduction, with reduced jumps from medium to high density.
With reference Figs. 9A to 9C, the region where no ink layer is deposited offers the advantage of reducing force 502 which resists ink withdrawal to zero. Hence, the relation 502<501 always holds to provide a thermal transfer recording medium that also achieves faithful tone reproduction, with reduced jumps from medium to high density.
A fourth embodiment of the present invention is hereunder described with reference to specific data.
Tables 4 - 7 show the thermal transfer ink compositions for three colors, yellow, magenta and cyan, used in Examples 1 - 4 of the present invention. The figures in these tables show the amounts of respective ingredients in parts by weight. The ink compositions shown in Tables 4 - 7 contained both high-molecular weight polymers that would not exhibit a distinct melting phenomenon and a plurality of waxes having different melting points (hereinafter abbreviated as m.p.). So, taken as a whole, these ink compositions did not show a distinct melting phenomenon at certain specified temperatures.
The thermal transfer inks were prepared by the following procedure. First, polymers and waxes were added to solvent capable of dissolution or uniform dispersion of these components and then mixed with stirring. Subsequently, colorants were added and dispersed in a ball mill for 18 h until uniform dispersion of the binders and colorants was confirmed.

Thermal transfer inks suitable for use in the present invention can be prepared not only by the solvent process described above but also by other methods such as hot melt and emulsion processes.

Table 5

(Example 2)
Component	Y	M	C
Hansa Yellow G	10
Brilliant Carmine 6B		10
Phthalocyanine Blue			10
Polyester
Polystyrene
	20	20	20
EVA
EEA
	5	5	5
Carnauba wax	30	30	30
Paraffin wax	20	20	20
Polyethylene wax	15	15	15
Total	100	100	100

Table 6

(Example 3)
Component	Y	M	C
Hansa Yellow G	10
Brilliant Carmine 6B		10
Phthalocyanine Blue			10
Polyester	20	20	20
Polystyrene
EVA
	15	15	15
EEA
Candelilla wax
	20	20	20
Paraffin wax	20	20	20
Saxol wax	15	15	15
Total	100	100	100

Table 7

(Example 4)
Component	Y	M	C
Hansa Yellow G	10
Brilliant Carmine 6B		10
Phthalocyanine Blue			10
Polyester	15	15	15
Polystyrene	10	10	10
EVA	5	5	5
Microcrystalline wax	15	15	15
Carnauba wax	15	15	15
Paraffin wax	15	15	15
Polyethylene wax	15	15	15
Total	100	100	100

Using the ink compositions shown in Tables 4 - 7, thermal transfer recording media were prepared by the following procedure. The solvent-dispersed inks prepared by the solvent method described above were coated onto a 6-µm thick substrate 5 as shown in Fig. 10. The substrate was made of PET (polyethylene terephthalate). The solvent in the inks was removed by thermal or vacuum drying so as to form a thermal transfer recording medium having a heat-fusible ink layer 1 (0.5 - 5 µm) on the substrate 5.
In the experiments described above, toluene was used as a solvent but this is not the sole solvent that can be used in the present invention and depending upon the components of ink, organic solvents such as methyl ethyl ketone, tetrahydrofuran, acetone, methyl isobutyl ketone, cyclohexanone, butyl acetate, ethyl acetate, ethanol, methanol and carbon tetrachloride may be used either independently or as admixtures with themselves or water.
Besides the polymers shown in Tables 4 -7, acrylic resins, polyurethane, polyvinyl acetal, polyamides, nylon, rosin resins, polyethylene, polycarbonates, vinylidene chloride resins, polyvinyl alcohol, cellulosic resins epoxy resins, vinyl acetate resin, vinyl chloride resin, etc. may be used in thermal transfer inks.
Besides the waxes shown in Tables 4 - 7, montan wax, synthetic oxidized waxes, α-olefin/maleic anhydride copolymers, animal and vegetable waxes, lanolin, etc. may be used in the present invention.
Besides the organic pigments shown in Tables 4 - 7, inorganic pigments, dyes, carbon black, etc. may be used as colorants in the present invention.
Fig. 11 shows schematically the constitution of the thermal transfer recording media prepared using the ink compositions of Examples 1 - 4 of the present invention, together with the ink coverages of the substrate surface. One recognizes that thermal transfer medium 2 contains ink layer 1 and substrate 5.
Fig. 12 shows schematically the constitution of a thermal transfer recording medium prepared according to Comparative Example 1 using the ink compositions of Example. As shown, the ink compositions were applied to the substrate to give an area coverage of about 100%.
Fig. 13 shows schematically the constitution of a thermal transfer recording medium prepared according to Comparative Example 2 using the ink compositions of Example 1. As shown, the ink compositions were applied to the substrate to give an area coverage of about 30%.
Using the thermal transfer inks described above, a printing test was conducted by three-color overprinting with a thermal head. The mechanism of printing with a thermal head using these thermal transfer inks is shown schematically in Fig. 15. The thermal transfer recording medium prepared by the method described above makes contact with a heating element 8 in the thermal head 7 at the surface which is not coated with the ink layer 1. The surface of the medium which is coated with the ink layer makes contact with a receiving sheet 4 on the drum 9. When the heating element 8 generates heat according to print information, its thermal energy is transmitted to the ink layer 1 through the substrate 5, whereupon the heated portions of the ink layer soften and are transferred as patterned dots 6 onto the receiving sheet 4 to form an image.
The patterned image to be printed on the receiving sheet (smooth-surfaced paper) consisted of horizontal stripes having a density graduation of 16 scales. The printing energy was adjusted to such a constant value that a full density would be attained at scale 14 when ink of the first color was transferred, with excess energy being applied at scales 15 and 16. In order to evaluate transfer characteristics, the transfer density at each scale was measured in terms of reflection OD values using a Macbeth densitometer Model TR-9-27 (Kollmorgan Corp.) equipped with filters for complementary colors.
The tone reproduction on the transfer recording media of Examples 1 - 4 and Comparative Examples 1 - 2 was evaluated and the results are shown graphically in Figs. 15 - 20, in which the vertical axis plots reflection OD with maximum value normalized for 1.0 and the horizontal axis plots transfer energy. Fig. 15 shows the tone reproduction of magenta which was the first color of the inks prepared in Examples 1 - 4. Fig. 16 shows the tone reproduction of cyan which was the second color of the inks prepared in Examples 1 - 4. Fig. 17 shows the tone reproduction of yellow which was the third color of the inks prepared in Examples 1 - 4. Fig. 18 shows the tone reproduction of magenta which was the first color of the inks prepared in Comparative Examples 1 - 2. Fig. 19 shows the tone reproduction of cyan which was the second color of the inks prepared in Comparative Examples 1 - 2. Fig. 20 shows the tone reproduction of yellow which was the third color of the inks prepared in Comparative Examples 1 - 2.
As Figs. 15 - 17 show, the thermal transfer inks of Examples 1 - 4 exhibited good tone reproduction over a broad range of transfer energies without the occurrence of ink running even in the high density region where high printing energy was applied. On the other hand, as shown in Figs. 18 - 20, the inks of Comparative Examples 1 was poorly transferred in the low density region. In addition, they experienced a sudden increase from medium to high density. Further, the ink of Comparative Example 1 not only caused scumming over the entire printing surface but also experienced running in the high density region. The ink of Comparative Example 2 was transferred in such a small amount that satisfactory density could not be achieved.
Using the seven samples of thermal transfer ink, full-color image was formed by three-color overprinting, with the tone of each color being varied at 64 scales. The images obtained with the thermal transfer recording media of Examples 1 - 4 were smooth and sharp with a wide range of reproduced colors. On the other hand, the images obtained with the recording media of Comparative Examples 1 - 2 had a narrow range of reproduced colors and their quality deteriorated because of such phenomena as granular appearance due to density jumps, ink running, scumming and insufficient density.
The above results show that in order to obtain a full-color image and other prints of high quality that are sharp, that have good tone reproduction over a wide range of colors, and that are free from such defects as granular appearance, ink running, scumming and insufficient density, it is effective to use a thermal transfer recording medium in which a layer of heat-fusible ink that does not show a distinct hot melting phenomenon within a specified temperature range is coated in the form of islands interspersed on the substrate. Desirably, the coated ink layer assumes 40 - 95% of the total area of the substrate.
The foregoing description of the preferred embodiments of the present invention assumes that image is formed by three-color overprinting with a thermal head. It should, however, be noted that this is not the sole mode of recording using the thermal transfer recording medium of the present invention and that it is applicable to electro-thermorecording and other methods thermal transfer.
The foregoing description of the preferred embodiments of the present invention also assumes that color image consisting of three colors, yellow, magenta and cyan, is formed. It should, however, be noted that the thermal transfer recording medium of the present invention is also useful with four-color (including black) printers, as well as monochromatic and two-color printers.
As described on the foregoing pages, according to one aspect of the present invention, a thermal transfer recording medium is provided that has a heat-fusible ink layer coated on a substrate in such a way that the coated area is composed of randomly spaced small coated and uncoated regions. This recording medium has the advantage of providing good tone reproduction.
According to another aspect of the present invention, there is provided a thermal transfer recording medium that has a heat-fusible ink layer coated on a substrate in such a way that said ink layer has randomly spaced small asperities across its thickness, with the thickness of any area having a recess being preferably no more than one half the thickness of any area having a projection. More preferably, any area having a recess is not thicker than 5 µm. This thermal transfer recording medium has the particular advantage of providing good tone reproduction in the medium to high density regions upon thermal transfer.
According to still another aspect of the present invention, there is provided a thermal transfer recording medium that has a heat-fusible ink layer coated on a substrate in such a way that the coated area is composed of randomly spaced small coated and uncoated regions, and that said ink layer has randomly spaced small asperities across its thickness, with the thickness of any area having a recess being preferably no more than one half the thickness of any area having a projection, with any area having a recess being not thicker than 5 µm in a more preferred embodiment. This thermal transfer recording medium also has the particular advantage of providing good tone reproduction in the medium to high density regions upon thermal transfer.
As will be understood from the foregoing description, the thermal transfer recording medium of the present invention offers the advantage of forming a full-color image and other prints that are sharp and which achieve good reproduction of a wide range of colors in all density regions.

Claims

1. A thermal transfer recording medium for use with a thermal transfer printer comprising:
A substrate (102); and
a thermal transfer ink layer (101) coated on said substrate (102),
wherein said substrate (102) includes a plurality of first
minute regions (111) randomly coated with said thermal transfer ink layer (101), and a plurality of second minute regions (113) uncoated therewith.

2. A thermal transfer recording medium as claimed in Claim 1, wherein said thermal transfer ink layer (101) comprises a plurality of minute concave and convex portions randomly provided on said first minute regions (111).

3. A thermal transfer recording medium as claimed in Claim 2, wherein the thickness of said concave portions of said thermal transfer ink layer (101) is no more than one half the thickness of said convex portions.

4. A thermal transfer recording medium as claimed in Claim 2 or 3, wherein the thickness of said concave portions of said thermal transfer ink layer (101) is not thicker than 5 µm.

5. A thermal transfer recording medium for use with a thermal transfer printer, comprising:
a substrate (102); and
a thermal transfer ink layer (101) coated on said substrate (102), said thermal transfer ink layer (101) comprising minute concave and convex portions provided at random.

6. A thermal transfer recording medium as claimed in Claim 5, wherein the thickness of said concave portions of said thermal transfer ink layer is no more than one half the thickness of said convex portions.

7. A thermal transfer recording medium as claimed in Claim 5, wherein the thickness of said concave portions of said thermal transfer ink layer (101) is not thicker than 5 µm.

8. A thermal transfer recording medium for use with a thermal transfer printer, comprising:
a substrate (102); and
a heat-softening ink layer (101) coated on said substrate so as to be interspersed in the form of isolation.

9. A thermal transfer recording medium as claimed in one of the preceding claims, wherein the coated ink layer assumes 40-95% of the total area of the substrate (102).