EP0260347B1

EP0260347B1 - Ink sheet repeatedly usable in thermal recording

Info

Publication number: EP0260347B1
Application number: EP86117680A
Authority: EP
Inventors: Hiroo Fujitsu Limited Ueda; Hiroshi Fujitsu Limited Sasao; Kohei Fujitsu Limited Kiyota; Koji Fujitsu Limited Uchiyama
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1985-12-18
Filing date: 1986-12-18
Publication date: 1992-06-17
Anticipated expiration: 2006-12-18
Also published as: KR920001485B1; DE3685754T2; EP0260347A2; KR880003753A; DE3685754D1; EP0260347A3

Description

The present invention relates to thermal recording (heat transfer recording), and more particularly to an improved thermal ink sheet or ribbon which is repeatedly usable for the thermal recording. It further relates to an improved composition of thermal ink material.
As is well known, in a thermal recording process, an ink sheet having a solid ink composition layer coated on a substrate such as a polyester film is used. A thermal printing head contacts the substrate with some pressure, and transfers heat to the solid ink composition layer. The heat is distributed selectively on the contact surface of the printing head corresponding to an image pattern to be reproduced. The heated solid ink material contained in the layer melts and adheres to the surface of a receiving sheet, i.e. a printing paper, which is closely in contact with the ink sheet. Consequently, the image pattern is transferred onto the printing paper.
The thermal recording process is used in various recording means for information apparatus, such as facsimiles, word processors, personal computers, automatic ticket issuing machines, etc. The thermal recording process has substantial advantages, such as low noise printing, compact size, low cost, and low running cost by the use of plain paper as the receiving sheet. However, there are certain disadvantages.
In a prior art thermal process, a printed image is transferred onto a receiving sheet like a decalcomania. As the receiving sheet, therefore, a smooth plain paper having Bekk smoothness higher than 200 seconds is required in order to obtain clear printing quality. For a plain paper having a rougher surface to be used, the thermal solid ink material (hereinafter, simply 'ink material') must contain more resinous components to strengthen the adhesion of the printed image to the plain paper, or have improved film forming ability and bridging property. As a result, the melting point of the thermal ink material rises, causing the printing ink sheet to have lower printing sensitivity. Consequently, the printing energy must be increased, requiring a heavy load and shortening the life of the associated thermal printing head. Furthermore, various counter measures have been taken: increase in the printing pressure to the thermal ink sheet through the thermal printing head, improvement in the timing and the peeling angle when the thermal ink sheet is peeled off the printing paper after the transfer of the printing image, and adoption of an improved structure of the printing head to achieve sharper point contact between the printing head and the thermal ink sheet.
However, the most remarkable disadvantage of the prior art thermal recording is the vast consumption of ink sheets, because in a single thermal recording step, all the ink material existing on the areas of the substrate of the ink sheet corresponding to the record pattern is transferred, making it impossible to use the ink sheet again in a succeeding thermal recording step. An ink sheet of this type is referred to as a 'one-time' ink sheet, and is used up each time the printing image is transferred to the printing paper. This increases the running cost of the thermal recording process.
Recently, various types of reusable, or 'multi-time' ink sheets which withstand repeated use for thermal recording have been developed to solve the problems described above. They are disclosed, for example, in US-A-3 392 042 issued on July 9, 1968, to Hugh T. Findlay, et al., Japanese laid open patent application publication No. 57-160691 of Uchiyama et al., published on October 4, 1982, and Japanese laid open patent application publication No. 58-183297 of Ohnishi et al., published on October 26, 1983. In these ink sheets, a thermal ink composition layer is disposed on a substrate, for example a polyester film. The ink composition layer includes a porous transfer layer, and ink material is contained in the pores of the porous transfer layer. When heat and pressure are applied to the ink sheet in the area corresponding to a recording pattern through contact with the associated printing head, the applied heat is transmitted through the substrate to raise the temperature of the ink material to cause it to melt, decreasing its viscosity. The heated ink material flows easily through the porous structure of the transfer layer, is expressed under the pressure toward the printing paper, usually a plain paper, and penetrates thereinto. The porous structure of the transfer layer acts as a flow resistance, limiting the quantity of the ink material which is expressed for one printing to an adequate quantity. Thus, repeated use of the thermal ink sheet becomes possible. Thereafter, the thermal printing ink sheet is peeled off the printing paper and the thermal recording process is completed.
The reusable thermal printing ink sheet of the prior art must be three to five times thicker than one-time thermal printing ink sheet in order to withstand the repeated thermal printing operation, causing low printing sensitivity and requiring more printing energy. Particularly when a plain paper having a rough surface is used, a more resinous ink material is needed, having a higher melting point and a higher melted viscosity as described above. In a cold environment, therefore, such prior art reusable thermal printing ink sheets cannot be used for a conventional thermal printing apparatus.
Furthermore, the prior art multi-time thermal printing ink sheet has other problems, namely, background noise of the printed paper and ghost images which are transferred to the printing paper. The background noise is caused by the character of the surface of the associated ink sheet, a powdery and adhesive surface of the ink sheet having an adverse effect. A printing paper is contaminated only by the friction between the surfaces of the ink sheet and printing paper during their storage or printing operation. Accordingly, the background noise is found in both the one time ink sheet and the multi-time use ink sheet of the prior art.
The ghost image is caused by the brittleness of the solid ink material. Fig. 1 shows schematic, enlarged cross-sectional and plan views of an ink sheet and images transferred onto a printing paper, illustrating the states of the surface of the printing paper (upper row), the ink composition layer (middle row) and the cross-section of the ink composition layer (lower row), at the periods respectively before printing, after printing, and during succeeding printing stages where no printing signal but only printing pressure is applied (blank printing). As shown in the cross-sectional views, a substrate 1 is coated with an ink composition layer 3 which has a porous layer therein formed of coagulated carbon powder 4. Ink material 5 stored in the pores of the porous structure contains low temperature melting compounds (waxes) and coloring agents soluble therein. After an image is transferred onto the paper, the originally smooth surface of the ink layer 3 is roughened when the ink sheet is peeled off the printing paper 10. A lot of micro peaks 6 are left which are formed by half-melted viscous ink material pulled in a direction normal to the surface of the ink sheet. These peaks 6 are distributed in the form of a reverse image 8. If the ink material 5 is brittle in its solid state, namely, at room temperature, these peaks 6 are easily collapsed and tend to be transferred to the printing paper 10 when only a printing pressure of the printing head (represented by a pressing roller 11) is applied to the ink sheet in the next printing step. As a result, a faint image 7 is left on the paper 10. The faint image 7 is referred to as a ghost image. If a new pattern is printed in the succeeding step, the new pattern image and the ghost image 7 are transferred on the same portion of the printing paper 10. Thus, the printing quality is lowered. The background noise and the ghost images are frequently found in thermal recording using a prior art ink sheet comprising low temperature melting compounds such as fatty acid compounds, fatty acid amide compounds, or ester compounds, because these materials are brittle or have powdery surfaces in their solid state.
An ink sheet containing a low temperature melting urethane compound having (NHCO) atomic bonding is proposed in Japanese laid open patent application publication No. 58-199195, issued on November 19, 1983, by Kuroda et al. The urethane compound features a narrow and sharply distinct melting temperature zone, leading to a clear printed image on a printing paper. On the other hand, thermal ink material containing the urethane compound has a strong adhesion force to the paper and is quite viscous near its melting temperature. In a multi-time use of the ink sheet including the urethane compound alone, some background noise is found which is believed to be due to the high viscosity of the urethane compound. When the ink sheet is peeled off the printing paper just after the completion of the transfer of the printing image, the ink material still in a half-melted state is pulled leaving sharp, elongated peaks of the solid ink material. Such elongated peaks collapse very easily, even though the urethane compound is not so brittle, causing a ghost image on the printing paper. In addition, the resulting transferred image has a rather low optical density, caused by a non-uniform printed image which has many white spots where no ink material is transferred locally. In general, the observed optical density of a printed image is regarded as an average optical density. Consequently, a non-uniform printed image has low optical density. The non-uniform printed image is also believed to be due to the high viscosity of the ink material of this type. Furthermore, the melting point is also rather high, requiring large printing energy and the use of relatively smooth plain paper as the receiving sheet.
Accordingly, an ink sheet containing only the urethane compound is not suitable as a reusable multi-time ink sheet. There is thus a need for a further improved thermal recording ink sheet for practical use.
Embodiments of the present invention can provide an improved reusable thermal recording ink sheet which gives little or no ghost image on the printing paper during the thermal recording process.
Embodiments of the present invention can also provide an improved reusable thermal recording ink sheet which is capable of transferring a reproduced printing image onto a plain paper having a relatively rough surface.
They can also provide an improved reusable thermal recording ink sheet which allows thermal recording even in a cold environment.
The present invention comprises a thermal recording ink sheet wherein a layer of an ink composition is formed over a substrate, which may be of polyester or the like. The ink composition comprises a filler, e.g. fine carbon powder, and ink material which is solid at room temperature but melts on heating to a fairly low temperature. The ink material is sometimes referred to as solid ink material. The ink material contains a low temperature melting compound and coloring agent which are soluble in one other. The low temperature melting compound comprises urethane compound as base material and at least one additive material selected from esters, fatty acid amides and fatty acids. These compounds are generally wax-like materials. The additive materials may also include paraffin wax to reduce the viscosity of the ink material. The particles of the filler, e.g. fine carbon powder referred to as carbon black, tend to coagulate to form a porous layer and accommodate the ink material in the pores of the porous layer as described above. When the ink sheet is heated and the solid ink material is melted, the fluid ink material is expressed from the pores under pressure provided by the relevant printing head and transferred onto a printing paper. The porous layer acts as a transfer control layer.
The ink material may also contain plasticizer.
In the ink material according to the present invention, the merit of urethane compounds that they are not brittle in the solid state, thus substantially preventing background noise, is still maintained, but their demerit of having a rather high melting point and high viscosity in the molten state, causing low optical density of the transferred printing image, is compensated by the additive materials. The additive materials such as esters, fatty acids, and fatty acid amides act as viscosity modulators of the associated ink material in the molten state, lowering the viscosity of the urethane compound. The viscosity of the new ink material at the printing temperature is fairly low, providing the melted ink material with flowability sufficient to make the ink material flow and reach the surface of the printing paper even if the surface of the paper is rather rough and uneven. The flowing ink material penetrates into the paper to some degree or forms a film adhered to the surface of the paper.
As a result, the new ink sheet can provide a clear and uniform thermal printing image with a satisfactory optical density and accompanied by no ghost images. Furthermore, rough plain papers become available as printing paper. It has been confirmed that a new printing ink sheet according to the present invention can provide better images of satisfactory printing quality even after more than ten thermal printing processes. These features and advantages will be apparent from the following more detailed description of the invention with reference to the accompanying drawings, in which:

Fig. 1 shows schematic, enlarged cross-sectional views and plan views of an ink sheet and images transferred onto a printing paper, illustrating the occurrence of a ghost image;
Fig. 2 is a schematic cross-sectional view of a reusable ink sheet according to the present invention;
Fig. 3 is a graph illustrating the result of repeated printing using a thermal solid ink sheet of a first embodiment of the invention, in which an ester is added to the urethane compound, wherein the optical density O.D. is plotted on the ordinate and the number of repetitions on the abscissa;
Fig. 4 is a graph showing the variation of optical density of a printed image with the weight ratio of ester to urethane compound by weight;
Fig. 5 is a graph illustrating the result of a repeated printing test employing an ink sheet of a second embodiment of the invention, in which a fatty acid amide is added to the urethane compound;
Fig. 6 shows the relationship between the weight ratio of fatty acid amide to urethane compound and the optical density of a printed image transferred onto a printing paper using an ink sheet of the second embodiment;
Fig. 7 is a graph illustrating the result of a repeated printing test employing an ink sheet of a third embodiment of the invention, in which a fatty acid is added to the urethane compound;
Fig. 8 is a graph illustrating the result of repeated printing tests employing an ink sheet of a fourth embodiment in which both a fatty acid amide and an ester are added to the urethane compound;
Fig. 9 is a graph illustrating the relationship between the weight ratio of fatty acid amide to urethane compound, or of fatty acid amide to urethane compound plus ester, and the optical density of a printed image transferred to a printing paper using an ink sheet of the fourth embodiment;
Fig. 10 illustrates the relationship between the weight ratio of fatty acid to urethane compound, or of fatty acid to urethane compound plus fatty acid amide and the optical density of a printed image transferred on a printing paper using the ink material of a sixth embodiment;
Fig. 11 is a viscosity characteristics diagram of a solid ink material of the sixth embodiment, illustrating the relationship between temperature and viscosity;
Fig. 12 is a characteristics diagram of a solid ink material of a seventh embodiment incorporating a plasticizer, illustrating the relationship between temperature and the optical density of a printed image; and
Fig. 13 is a characteristics diagram of a solid ink material of the seventh embodiment, illustrating the relationship between the proportion of plasticizer and the optical density of the printed image.

Fig. 2 is a schematic cross-sectional view of an example of a reusable ink sheet according to the present invention. The ink sheet comprises a substrate 21 which is coated by an intermediate layer 22. A solid ink composition layer 23 is further formed over the intermediate layer 22, which serves to maintain good adhesion between the substrate 21 and the solid ink composition layer 23. As the material of the substrate 21, any material may be used that can withstand the heat of the thermal printing head, i.e. any conventional material which does not soften, melt, or deform upon heating of the head. The materials conventionally used include polyester film, polyimide film, polycarbonate film, and other polymeric films, condenser papers, and other thin papers. The intermediate layer 22 prevents the solid ink composition layer 23 from coming off the surface of the substrate 21 when the ink sheet is heated. The material of the intermediate layer 22, therefore, must be a material that can be coated very thinly and adhere to both the substrate 21 and solid ink composition layer 23. In view of this point, polyester resin or polyamide resin is most suitable.
Thus the substrate 21 may be a polyester film 6 µm in thickness, coated by a 3 µm thick intermediate layer and formed of polyester resin and polyamide resin, and the solid ink layer may be 10 µm in dry thickness.
Seven embodiments of the invention will now be described in each of which the structure of the layers of the ink sheet is as described above, but the composition of the ink material contained in the ink composition layer 23 is varied, as summarised in Table 1.
In each of the examples the urethane compound was a reaction product of hexamethylene diisocyanate and ethanol supplied by Nippon Oil and Fats Ltd. as "Alflow DH-20", and having the chemical formula

C₂H₅0-CO-NH-C₆H₁₂-NH-CO-OC₂H₅,

the coloring agent was Kayaset Black (product of Nippon Kayaku Ltd.) and the filler was carbon black (product of Tokai Carbon Ltd. in the first example and of Nippon Kayaku Ltd. in the other examples).

Embodiment 1

In an example of this embodiment the solid ink composition was prepared by thoroughly blending the following components using acetone as solvent;
The carnauba wax, having cerotic acid as its main component, was a product of Nikko Fine Products Ltd., and the paraffin wax was a product of Nippon Seiro Ltd.
The reusable ink sheet of this example of the first embodiment was cut into a ribbon ink sheet and tested with a conventional serial thermal printing machine employing a printing paper, such as a Xerographic paper of PA4 type, a product of Kisyu Paper Ltd., having a rather rougher surface (Bekk smoothness of 60 seconds) than ordinary high quality thermal printing paper (Bekk smoothness of 250 seconds). The printing energy employed was 30 mJ/mm², and the printing pulse width was 1 ms. The results of a repeated printing test are shown in Fig. 3, wherein the optical density O.D. of the printed images is plotted as the ordinate and the number of the repetition as the abscissa. The printing image pattern is a solid area which is repeatedly printed on the Xerographic paper using the same portion of the solid ink sheet under testing. The O.D. value starts from approximately 1.0 and decreases to around 0.6 after printing five times. This implies that the ink material including urethane compound mixed with ester compound has an improved film formation ability on a printing paper, resulting in a high optical density of the printed images and the elimination of the ghost image.
Fig. 4 illustrates the relationship between the ratio of ester to urethane compound by weight arid the optical density of the printed image transferred to a printing paper using the relevant ink material. The maximum optical density of the printed image is obtained at the ratio of 0.2 by weight and uniform, clear printing images are achieved. Without the addition of ester, namely with the ink material containing only urethane compound as the low temperature melting compound, the optical density is low. With ratios higher than 0.5 or lower than 0.1, ghost image formation occurs. A low temperature melting compound having a ratio of ester to urethane compound by weight ranging from 0.1 to 0.5 is therefore preferable. The decrease in the optical density O.D. at weight ratios over 0.3 is due to low mutual solubility between the coloring agents and the ester compounds.

Embodiment 2

In this embodiment a fatty acid amide is used as an additive material to the low temperature melting compound instead of the ester of the first embodiment. The blending composition of the material of the ink composition layer 23 in an example of this second embodiment is as follows, using acetone as solvent:
After cutting the solid ink sheet to a ribbon sheet, the repeated printing test of a solid ink sheet of the second embodiment was conducted in just the same manner as with the first embodiment. The pattern of the printing image was also a solid area.
The result, shown in Fig. 5, is very similar to that of Fig. 3, of the first embodiment. Fig. 6 shows the relationship between the weight ratio of fatty acid amide to urethane compound and the optical density of a printed image transferred onto a printing paper using the relevant ink material. The maximum optical density of the printed image is obtained at the ratio of 0.2 by weight, and uniform, clear printing images are achieved. Without the addition of fatty acid amide, namely with an ink material containing only urethane compound as the low temperature melting compound, the optical density is low. For ratios lower than approximately 0.1, or higher than 0.5, ghost images background noise arise and the optical density also falls below 0.9. The ratio by weight range from 0.1 to 0.5 is therefore preferable.

Embodiment 3

In this embodiment a fatty acid is used as additive material to the low temperature melting compound, instead of the ester of the first embodiment. The composition of the material of an ink composition layer 23 in an example is as follows, acetone being used as solvent.
A repeated printing test of the solid ink sheet of the third embodiment is performed in just the same manner as with the first embodiment. The pattern of the printing image is also a solid area. The result, illustrated in Fig. 7, is very similar to that of the first embodiment shown in Fig. 3. The fall in the optical density of the printed image is rather sharper, down to 0.5 with five printing times. However, a remarkable feature of the third embodiment is the low viscosity of the ink material in the melted state (cf. Table 1) and its low melting temperature. This implies an essential effectiveness of the further application of the fatty acids to the solid ink material.
In the solid ink sheets of the preceding embodiments, a single additive material is added to the urethane compound of the low temperature melting compound. In the following embodiments, solid ink material containing combinations of two additive materials will be disclosed.

Embodiment 4

In this embodiment a combination of fatty acid amide and ester is used as combined additive material. The composition of the material of the ink composition layer 23 in an example is as follows. Acetone was used as solvent.
Forming the solid ink sheet into a ribbon sheet, the repeated printing test of the solid ink sheet of the fourth embodiment was conducted in just the same manner as the first embodiment. The patterns of the printing image were solid area and ordinary Japanese characters printed at random. The results are illustrated in Fig. 8. With a solid area pattern, the resulting optical density of the printed image decreases from 1.0 at the first printing to 0.5 at the fifth printing. However, with ordinary Japanese character images, the optical density of the relevant image only decreased slightly and can be maintained at approximately 0.9 on average, which is satisfactory in practice even after ten repeated printings. The ghost image is completely eliminated, achieving high quality printing with uniform optical density of characters printed on the paper and being free from 'white spot' (partially non-transferred portion of printed images).
In the fourth embodiment, several other low temperature melting compound compositions can be used. As fatty acid amides, instead of Diamid 0-200 the following components are applicable with good printing image: erucic acid amide, commercially available under trade name Alflow P-10 (product of Nippon Oil and Fats Ltd.), N-stearyl oleic acid amide, trade name Nikkaamaido SO (product of Nippon Kasei Chemical Ltd.), ricinoleic acid amide, trade name Diamid H (product of Nippon Kasei Chemical Ltd.). As esters, instead of carnauba wax, the following components can be used with good results: beeswax, stearyl behenate (product of Nippon Oil and Fats Ltd.), cane sugar fatty acid amide, trade name Sugar Wax FA-10E (product of Dai Ichi Kogyo Jeiyaku Ltd.).
Fig. 9 illustrates the relationship between the weight ratio of fatty acid amide to urethane compound or of fatty acid amide to urethane compound plus ester and the optical density of a printed image transferred to a printing paper using the relevant ink material. In this example the weight ratio of ester to urethane compound is maintained at 0.2. The optical density of the printed image is over 1.0 for the range from zero to 0.6 of the ratio of fatty acid amide to the urethane compound higher than 0.5 (upper scale of the Figure), implying that excess addition of fatty acid amide makes the solidified ink material brittle. Weight ratios not higher than 0.5 are therefore preferable. The viscosity of the ink material in its molten state, for example at 75°C, is fairly low resulting in a uniform printing image.

Embodiment 5

The structure of a solid ink sheet of the fifth embodiment is the same as that of the first embodiment with the addition of fatty acids such as myristic acid, palmitic acid, stearic acid and behenic acid as additive agents to the low temperature melting compound. The additive agent thus comprises esters and fatty acid. The same evaluating tests as that of the first embodiment are performed with satisfactory results. Optical density above 1.0 is achieved, and a uniform printing image without spots was attained even when Xerographic paper with rough surface was used as the receiving paper. In addition, no ghost image is found in the printed papers tested.

Embodiment 6

The structure of a solid ink sheet of the sixth embodiment is the same as that of the second embodiment, with the addition of fatty acids such as myristic acid, palmitic acid, stearic acid and behenic acid. That is, the additive agent to the low temperature melting compound of the solid ink material comprises fatty acids and fatty acid amides.
An example of a composition of the solid ink material of the sixth embodiment is as follows, using acetone as solvent:
In the above description, C_n designates an alkyl group having n atoms of carbon, and C₁₃ corresponds to myristic acid, C₁₅ corresponds to palmitic acid, C₁₆+C₁₈ corresponds to stearic acid, and C₂₁ corresponds to behenic acid. These fatty acids may be added to the solid ink material singly or with two or more fatty acids in combination. The same evaluating tests as that of the first embodiment are performed with a solid ink sheet of the sixth embodiment with satisfactory results. An optical density above 1.0 is achieved, and uniform printing image is attained on a Xerographic paper with a rough surface. In addition, no ghost image is found in the printed papers tested. The viscosity of the ink material at 75°C is also fairly low as the result of the addition of fatty acids (cf. Table 1).
However, if the number of carbon atoms in the alkyl group contained in the fatty acid is below twelve (C₁₁ corresponds to lauric acid), the melting point is low, below 50°C, and background noise is caused only by the friction between the printing paper and the associated printing ink sheet. On the contrary, with a number of carbon atoms of alkyl group contained in the fatty acid is higher than twenty-four (C₂₃ corresponds to lignoceric acid), the melting point of the ink material becomes too high, requiring large printing energy during thermal printing operation, and background noise arises. Therefore, the fatty acids containing an alkyl group having a number of carbon atoms ranging from fourteen to twenty-two are preferred.
Fig. 10 illustrates the relationship between the ratio of fatty acid (stearic acid) to urethane compound by weight (upper scale) or the ratio of fatty acid to urethane compound plus fatty acid amide by weight (lower scale), and the optical density of a printed image transferred on a printing paper using the relevant ink material of the sixth embodiment. The weight ratio of fatty acid amide to urethane compound is maintained at 0.25. The optical density of the printed image is maintained over 1.0 for the range from zero to 0.5 of the ratio of fatty acid to urethane compound by weight. No ghost image is found for the ratio range 0 to 0.5 when the weight ratio of fatty acid amide to urethane compound is 0.25.
Fig. 11 is a viscosity characteristics diagram of solid ink materials, illustrating the relationship between temperature and viscosity. Curve A illustrates the characteristic of an ink material without addition of fatty acid and curve B with the addition of a fatty acid (stearic acid). From the curves, the effect of the fatty acid for reducing the viscosity of the ink material is clearly seen, particularly at lower temperature.

Embodiment 7

The structure of a solid ink sheet of a seventh embodiment is the same as that of the second embodiment, with the addition of plasticizer. For example, composition of the solid ink material of the seventh embodiment is illustrated as follows:
Plasticizers that may be used include, for example, phthalic acid esters such as dioctyl phthalate or diisodecyl phthalate, fatty acid esters such as dioctyl azelate or dibutyl sebacate, maleic acid and fumaric acid esters such as dibutyl maleate and dioctyl fumarate, respectively and orthophosphoric acid esters such as tributyl phosphate and trioctyl phosphate.
The same evaluating tests as that of the first embodiment were performed with a solid ink sheet of the seventh embodiment under environmental temperature ranging from 5°C to 40°C with satisfactory results. An optical density above 1.0 was achieved, and a uniform printing image was attained on Xerographic paper with a rough surface. The presence of plasticizer in the thermal ink sheet of the seventh embodiment can overcome the problem that at a low temperature, the portion of the ink composition layer 23 which is heated for the thermal printing is apt to come off from the substrate 21 locally when the ink sheet is peeled off the associated printing paper after the printing operation. Further, a clear thermal image is achieved with no ghost image and no background noise at high temperature.
Fig. 12 is a characteristics diagram of a solid ink material of an example of the seventh embodiment, illustrating the relationship between temperature and the optical density of a printed image. Curve A illustrates the characteristic of the ink material with addition of plasticizer, here tributyl phosphate, and curve B without the addition of plasticiser. From the curves, the effect of the plasticizer in improving the optical density of the printed image at lower temperatures can be clearly seen. In fact, the addition of the plasticizer below the weight ratio of 3% is no more effective for a cold environmental temperature.
Fig. 13 is a characteristics diagram of the solid ink material above described, illustrating the relationship between the content of the plasticizer and the optical density of the printed image. The thermal printing is performed at a room temperature of 22°C. As can be seen from the curve, the ink material containing the plasticizer having a ratio of the plasticizer to the total solid ink material lower than 3% by weight still has some effect, because the thermal printing is performed at fairly high temperature. As the additive ratio of the tributyl phosphate increases, the optical density tends to decrease, but the uniformity of the printed images is somewhat improved. However, with ink material having the ratio of the plasticizer higher than 15% there can be some storage problems at high temperature. Consequently, ink material containing a plasticizer in a ratio of plasticizer to ink material from 3% to 15% is preferable.

Claims

A reusable ink sheet for thermal printing comprising a substrate (21) and a thermal ink composition layer (23) formed over said substrate (21) and comprising a thermal ink composition containing a coloring agent, a filler material, and a low temperature melting compound, said low temperature melting compound being solid at normal temperature and containing a urethane compound,
characterised in that said low temperature melting compound further contains at least one additional compound selected from the group of esters, fatty acid amides and fatty acids.
A reusable ink sheet according to claim 1, wherein the ratio of one of said additional compounds to said urethane compound ranges from 10% to 50% by weight.
A reusable ink sheet according to claim 1 or claim 2 wherein said additional compound is an ester.
A reusable ink sheet according to claim 1 or claim 2, wherein said additional compound is a fatty acid amide.
A reusable ink sheet according to claim 1 or claim 2, wherein said additional compound is a fatty acid.
A reusable ink sheet according to claim 1 or claim 2, containing as additional compounds a fatty acid amide and an ester.
A reusable ink sheet according to claim 1 or claim 2, containing as additional compounds a fatty acid amide and a fatty acid.
A reusable ink sheet according to claim 1 or claim 2, containing as additional compounds a fatty acid and an ester.
A reusable ink sheet according to any one of claims 1, 2, 5, 7 and 8, wherein the number of carbon atoms of the alkyl group contained in said fatty acid is from fourteen to twenty-two.
A reusable ink sheet according to any of claims 6 to 8, wherein the ratio of each additional compound to said urethane compound ranges from 10% to 50% by weight.
A reusable ink sheet according to any preceding claim which also contains paraffin wax.
A reusable ink sheet for thermal printing according to any preceding claim, wherein said low temperature melting compound further contains a plasticizer.
A reusable ink sheet according to claim 12, wherein said plasticizer is composed of one or more of: phthalic acid ester, fatty acid ester, maleic acid ester, fumaric acid ester, and orthophosphoric acid ester.
A reusable ink sheet according to claim 12 or claim 13, wherein the ratio of said plasticizer to said thermal ink material ranges from 3% to 15% by weight.