JPH0714664B2 - Black thermal transfer recording sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Field of the Invention The present invention controls temperature rising recording using a thermal recording head or the like and continuously prints on a recording medium (image receptor) such as uncoated paper, coated paper, or plastic film. The present invention relates to a black thermal transfer recording sheet for thermal transfer recording of a black ink material with gradation density.

Conventional technology A sheet of heat-resistant substrate such as polyethylene terephthalate (PET) film or condenser paper contains a coloring material and a hot-melt material, and its viscosity is controlled to decrease by the temperature-increasing recording control. The ink material has a transferability-imparting relationship and at least a particle diameter larger than the thickness of the layer made of the ink material, and at least one of its melting point, softening point, and decomposition temperature is hot melt. According to the thermal transfer recording sheet in which the normal temperature solid particles higher than the binder material are mixed and dispersed in the ink material as the ink carrier, and the thermal transfer layer having the uneven surface formed by the normal temperature solid particles is formed, The thermal transfer recording density can be continuously controlled to record a halftone image (see Japanese Patent Application No. 59-227155).

Problems to be Solved by the Invention In the above thermal transfer recording sheet, the thermal conductivity of the solid particles at room temperature as an ink carrier is usually selected to be larger than that of the hot melt binder material, for example, an alumina powder having an average diameter of about 2.5 to 10 μm. Is used. Further, since at least a part of the room temperature solid particles penetrates the ink material layer and protrudes to the surface of the thermal transfer layer to form an uneven surface, the hot melt binder material is selected as a solvent soluble material at room temperature and contains a large amount of solvent. The thermal transfer layer is applied by the solvent coating method, and the thickness of the ink material layer is configured to be smaller than at least some of the solid particles at room temperature by evaporation of the solvent. In the constitution of the black thermal transfer recording sheet, fine carbon black powder having a secondary particle diameter of 1 μm or less is used as a pigment coloring material, but it contains alumina particles having a large particle diameter as compared with these, and together with a hot melt binder. If the ink for forming the thermal transfer layer is manufactured with a large amount of solvent contained, alumina particles having a large specific gravity tend to be deposited and it is difficult to manage the ink material, and the uniform dispersion arrangement is likely to be disturbed during the layer formation in the solvent coating. . In addition, since the alumina particles are white, this uneven distribution
It tends to hinder the original black uniformity of thermal transfer recording.

Thus, in the conventional black thermal transfer memory sheet, there is room for improvement in management of the ink for forming the thermal transfer layer, uniformity of coating layer formation, and black uniform thermal transfer recording property.

The present invention has been made in view of such problems, and an object thereof is to provide a useful black thermal transfer recording sheet.

Means for Solving the Problems In the present invention, a hot melt binder material is provided on a sheet-shaped heat-resistant substrate surface, the viscosity of which is controlled to be reduced by temperature rising recording control, and transferability to a recording medium is imparted. , A thermal transfer layer containing at least graphite powder particles as a coloring material is formed, and this graphite powder has a particle size distribution, and some of the powder particles penetrate the thickness direction of the thermal transfer layer and project to the surface of the thermal transfer layer. A black thermal transfer recording sheet is formed by forming an uneven surface on the thermal transfer layer.

In this case, when artificial graphite is used as the graphite powder, the particle size thereof is closer to a sphere than that of flake graphite or earth graphite, and a good black thermal transfer image can be obtained.

Operation In the black thermal transfer recording sheet according to the present invention, the black-colored material having a small particle size to form the color material and the large-sized ink carrier having room temperature solid particles are both composed of graphite powder. Therefore, since the deposition phenomenon due to the difference in specific gravity between the color material and the solid particles at room temperature is improved as in the conventional case and the same color is obtained, the management of the ink for forming the thermal transfer layer, the uniformity of coating layer formation, the uniform thermal transfer of black color, etc. are improved. Will be.

Generally, in thermal transfer recording, a thermal recording head is arranged on the back side of the heat-resistant substrate of the recording sheet, a recording platen is arranged on the back side of the recording medium, and the surface of the recording medium is pressed against the surface of the thermal transfer layer. Temperature rising recording control.

Now, the particle size of the graphite powder mixed in the hot-melt binder material that constitutes the thermal transfer layer is composed of fine powder that is sufficiently smaller than the thickness of the thermal transfer layer, and the thermal transfer layer has a uniform thickness. Assuming that the ink layer is a so-called black ink layer having a flat surface.

By the temperature rise recording control, the hot melt binder material, that is, the ink layer, is melted and reduced in viscosity while absorbing the heat of fusion from the back surface side (that is, the heat resistant substrate front surface side) according to the amount of heating.

However, since the temperature of the ink layer surface is in the unmelted and solid state below the melting point of the hot melt binder material, the ink material cannot adhere to or be transferred to the recording medium. Adhesion and transfer of the ink material to the recording medium occurs only after the heating head supplies a sufficient amount of heat necessary for the progress of the melting of the ink layer in the thickness direction to reach the surface portion of the ink layer. In this case, the ink whose thickness is melted and reduced in viscosity in the thickness direction of the ink layer adheres to and is transferred to the recording medium at one time, so that the recording density is binary, and the recording density is controlled according to the temperature increase recording control. Continuous tone thermal transfer recording is difficult.

However, like the black thermal transfer recording sheet according to the present invention, it has graphite powder having a large particle size, which is dispersed and arranged in the above-mentioned ink layer, penetrates in the thickness direction of the above-mentioned ink layer, and is formed on the surface of the ink layer. Consider the case where it is protruding.

The thermal conductivity of commonly used hot melt materials is low 6
~ 9 × 10 ^-4 cal / cm, sec, ℃, while graphite has 1.4
It has a high thermal conductivity of × 10 ^-2 cal / cm, sec, ℃, and its melting point is 3500 ℃.
It has a much higher value for (80 ℃).

Therefore, the graphite powder having a large particle size has a role as a heat conduction medium penetrating in the thickness direction of the ink layer for the temperature increase recording control, and melts the ink material adhering to or adjoining the surface thereof,
Reduces viscosity.

Therefore, even if the heating amount due to the temperature rise recording is small and the ink layer is only melted on the back side, the graphite powder portion with a large particle size penetrates along the surface in the thickness direction of the ink layer. To melt the ink material and reduce its viscosity.

In this way, the ink material that has been melted and reduced in viscosity in response to the temperature rising recording control has a thermal expansion during the melting of the hot melt vine material,
Due to pressing through the surface of the recording medium and capillary action in the contact gap between the surface of the recording medium and the large-sized graphite, the particles penetrate the surface of the large-sized graphite particles and permeate and adhere to the surface side of the recording medium. Then, when the thermal transfer recording sheet is peeled from the recording medium in the state before the temperature rising recording control is finished and the molten ink is cooled and solidified, the heating amount is not less than a certain value and the large-sized graphite particles surround and sufficiently melt around it. When the ink is present, the large-diameter graphite particles themselves are also adhered and transferred to the surface of the recording medium in a state where the impermeable molten ink material is adhered to the surface.

Therefore, the graphite particles having a large particle size have a role of a so-called ink carrier for permeating and transferring the molten ink, and the transfer recording density is continuous corresponding to the amount of the molten ink, that is, the heating amount for heating and recording. The maximum transfer recording density is given in a state in which the entire thickness direction of the ink layer is melted and the viscosity is reduced.

Thus, from the small particle size fine powder that forms the usual coloring material for ink as the graphite powder, it is possible to provide the particle size distribution in the form of including the large particle size which exhibits the effect as the ink carrier as described above. The above-mentioned permeation / adhesion transfer continuously plays a role as the above-mentioned black ink material and ink carrier according to the particle size of the graphite powder, that is, the heat capacity, and it is possible to realize a thermal transfer recording sheet with good continuous gradation of black. .

Examples Hereinafter, embodiments of the present invention will be described with reference to Examples. FIG. 1 is a sectional structural view of an embodiment of such a black thermal transfer recording sheet of the present invention. Note that, for convenience of explanation, all the particle shapes of the graphite powder are shown as spherical.

Reference numeral 10 is a heat-resistant substrate such as a film of polyethylene terephthalate (PET) or polyimide having a thickness of 3 to 15 μm or a condenser paper. Surface 11 of heat resistant substrate 10
A thermal transfer layer 40 composed of a hot melt binder material 20 and graphite powder 30 is layered on the black thermal transfer recording sheet 100. Temperature rise recording control by the recording head
Do from the side. In this example, the graphite powder 30 is used as a mixture of two types of particle size distribution, large and small.

The graphite powder 31 has a small particle size and is embedded in the surface 41 of the thermal transfer layer to be used as a black ink coloring material forming the so-called ink layer. On the other hand, the graphite powder 32 has a large particle size and is a thermal transfer layer.
It penetrates through 40 and projects on the surface 41 to form an uneven surface on the layer 40, which acts as the above-mentioned ink carrier effect particles and also serves as both a black-colored material to be transferred and recorded.

The thickness t of the ink layer (the average thickness of the layer 40 in the gap between the particles 32) is selected to be, for example, about 1 μm to 5 μm, and the particle size of the powder 31 is selected to be smaller than t. If t is too small, the recording density is low due to insufficient ink amount, while if t is too large, the recording capacity or continuous gradation tends to be deteriorated due to an increase in the heat capacity. The particle size of the powder 32 is selected to be equal to or larger than the above-mentioned value t because it is necessary to form an uneven surface on the layer 40. The maximum particle size is, for example, 15 μm or less, and if it is 15 μm or more, the ink permeation path becomes too long and it becomes difficult to permeate the melted ink as described above, and the recording sensitivity is lowered, and the powder is
The transfer of 32 itself sometimes deteriorates the continuous gradation property in the low recording density region, and deteriorates the recorded image quality in a dot shape.
The particle size of the powder 32 is preferably chosen to be above the thickness t and below 10 μm.

FIG. 2 is a sectional structural view of another embodiment of the black thermal transfer recording sheet according to the present invention.

In the graphite powder 30a in this example, the particle size is displayed as a single average particle size without mixing two kinds of particle size-selective graphite as shown in FIG. 2, and the particle size distribution is continuous such as normal distribution. A graphite powder composed of particles of which the particle size changes is used.

A black color material whose grain size is below a certain level is buried in the layer surface 41.
It is used as 31a, and a certain particle size or more protruding from the layer surface 41 functions as the above-mentioned ink carrier 32a.

In the graphite 30a, the maximum value of the distribution particle size that defines the maximum particle size of the ink carrier 32a is 15 μm as described in FIG.
It is preferably selected within 10 μm. Regarding the particle size distribution of the graphite powder 30a, the one having a distribution particle size within the range of 0 to this maximum value is usually used. Graphite powder 30 occupying the thermal transfer layer 40 (first
%), 30a (FIG. 2) is selected in the range of, for example, 10 to 80%, and when the content is less than 10%, the blackness decreases, and when it exceeds 80%, the thermal transfer property is deteriorated due to the shortage of the hot melt binder material 20. Along with the decrease, the strength of transfer recording on the recording medium decreases. It is recommended that the weight% of the graphite powder 30,30a be selected within the range of 20 to 70%. The coating amount of the thermal transfer layer 40 may have a range of 1.2~10g / m ^2, thin transfer recording density is 1.2 g / m ² or less, 10 g
When it exceeds / m ² , the thermal transfer sensitivity decreases due to the increase in the heat capacity, and the continuous gradation especially in the low density region deteriorates. 1.
5-6 g / m ² is a particularly recommended range.

On the other hand, in FIGS. 1 and 2, the graphite powders 30 and 30a are graphite powders 32 and 32a which are penetrating in the thickness direction of the thermal transfer layer 40 and project on the layer surface 41 to form an uneven surface. If the powder 32, 32a is a perfect sphere with a single particle size, the weight percentage occupies at least one, preferably four or more per unit recording pixel, which is an extremely small value. However, since the actual powder is not in such an ideal state, it is practically selected within the range of, for example, 5 to 70%. When the weight% is less than 5%, the lack of the ink carrier particles 30, 30a impairs the continuous gradation in the low recording density region, and the recorded image looks rough especially in the low recording density region. When the weight% exceeds 70%, the continuous gradation property in the low density region is deteriorated due to the thermal transfer to the recording medium and the recorded image quality is deteriorated. 15-50% is a particularly recommended range.

Examples of the hot melt binder material 20 include carnauba wax, candelilla wax, montan wax,
Consists of one or more of acid wax, ester wax, partially oxidized ester wax, wax such as solid paraffin, ethylene vinyl acetate copolymer (EVA), petroleum resin, acrylic resin, polyamide resin, low molecular weight polyethylene, polybutene and other resins As a material, these are mixed with an auxiliary agent such as a plasticizer, a mineral oil, a surface-active agent, and an antioxidant, if necessary.

In order to form the uneven surface due to the projection of the surface 41 of the graphite terminals 32, 32a, a mixed material of the hot melt binder material 20 and the thermal transfer layer 40 containing the graphite powders 30, 30a, respectively, is used for gravure printing, for example.
Substrate surface by solvent coating method such as flexographic printing
It can be easily performed by applying the coating with a limited thickness to 11 and evaporating the solvent from the coating layer.

In this case, the hot melt wax and resins are generally insoluble in the solvent at room temperature in many cases. In the use of such components in the hot-melt binder material 20, a solvent having a boiling point equal to or higher than the melting point or softening point of the room-temperature insoluble hot-melt material is used for the solvent coating method, and a mixed suspension of these thermal transfer layers is used. Is once heated to a temperature around or above the melting point and boiling point of the room temperature insoluble hot melt material to dissolve it in a solvent, and then stirred and ground while being cooled to cool fine particles of the room temperature insoluble hot melt material. So-called cold precipitation is performed. This precipitation suspension is solvent-coated on the surface 41 of the substrate, and the solvent of this coating layer is pre-dried, or immediately this coating layer is heated to the melting point or softening point of the precipitation hot melt particles or higher to completely remove the solvent. Evaporate and melt the precipitated hot melt particles.

According to the above-mentioned manufacturing method, the deposited hot melt particles lose the particle property and are compatible with other hot melt materials, and have an excellent advantage that a uniform thermal transfer layer 40 can be formed. The first
In FIG. 2 and FIG. 2, the exposed top surface of the graphite powder 32, 32a is thinly coated with the hot-melt binder material 20 'containing the fine graphite powder 31', 31a ', but even if they are not present, Let's be good.

〔Example〕

Graphite powder 30a (31a, 32a) surface treated with stearic acid Average particle size 1.0μm (50% weight value), particle size distribution 0-10
53.2 parts of artificial graphite (HAG-150-st manufactured by Nippon Graphite Co., Ltd.) (all by weight below), 21.6 parts of alicyclic saturated hydrocarbon resin (softening point 70 ° C.) as hot melt binder material 20, parfin (melting point 50 ~ 52 ℃) 14.4 parts, Carnauba wax (melting point 80-83 ℃) 3.6 parts, EVA (VA: 28%, MI: 400, softening point 85
X) as a solvent (boiling point 138
To 144 ° C.) is added to prepare a suspension for the thermal transfer layer, and the mixture is heated to 100 ° C. while being stirred by a ball mill.

EVA and carnauba wax are not completely soluble in xylene at room temperature. By heating at 100 ° C, the hot-melt binder materials including these materials are dissolved and become compatible with each other. When the hot-melt binder material is completely melted, the ball mill is continuously cooled to room temperature while continuing to be ball-milled, and the ball-mill is further applied for 2 hours. During this cooling, stirring, and dispersion process, incompatible EVA and carnauba wax are precipitated in the form of fine particles and are evenly dispersed together with the artificial graphite powder.

The precipitate-mixed suspension was applied to a surface 11 of a heat-resistant substrate 10 made of a PET film having a thickness of 9 μm using a commercially available # 5 bar coater.
Applied at room temperature and pre-evaporated the xylene solvent in the coating layer, then the softening point of the precipitated EVA (softening point 85 ° C) and carnauba wax (melting point 80-83 ° C) particles, higher than the melting point It is passed through a heating and drying oven at 120 ° C. to heat the coating layer to completely evaporate the xylene solvent, melt the precipitated particles and reduce the viscosity. Due to this heating and melting, the precipitated particles lose their granularity, and the compatibility with other hot melt binder material components also progresses, resulting in a more uniform hot melt binder material.
20 are composed.

Further, as the xylene solvent evaporates, the layer surface of the coating layer lowers, and as shown in FIG. 2, the large-sized graphite particles 32a forming the ink carrier are hot particles containing graphite fine particles 32a '. The thermal transfer layer on the uneven surface is projected from the layer surface 41 in a state where the nip portion is thinly covered with the melt binder material 20 '.
40 are composed. The coating amount of the layer 40 in this example is 3.5 g / m ²
Met.

FIG. 3 shows an example of transfer recording characteristics using the black thermal transfer recording sheet illustrated in FIG. 2 thus manufactured.

A known linear thermal recording head of 4 dots (pixels) / mm was used for temperature increase recording control, and polypropylene resin-based recording paper having a thickness of 150 μm was used as a recording medium (image receiving body). The main scanning line recording speed is 16.7 ms / line, the sub-scanning line density is 4 lines / mm, the recording electric signal is pulse width modulated with 6 bits, and the maximum modulation pulse width (Pw) is 4 ms. The applied power is 0.6 W / dot.

As is clear from FIG. 3, according to the black thermal transfer recording sheet of this embodiment, the transfer recording density D on the recording paper surface rises smoothly from the paper surface density Do in accordance with the modulation pulse width Pw. Then, it is possible to obtain extremely excellent continuous gradation transfer recording characteristics from the low recording density area to the medium and high recording density areas. When an image signal is used as a recording electric signal, a clear black halftone image with excellent image quality in a line-sequential manner is obtained. Was able to record thermal transfer. The presence of the large-diameter ink carrier particles 32a that particularly effectively contributes to continuous gradation in the low transfer recording density region can be detected by observing a relatively large black transfer recording dot having a diameter of about 10 μm with a microscope. When the modulation pulse width Pw = 1 ms, the density of the above-mentioned recording dots is about 400 / mm ² , and about 25 per recording electrode (ie, one recording pixel 0.25 × 0.25 mm ² ) surface of the thermal head.
It was an individual.

[Comparative example]

Particle size distribution 0 to 1.2 instead of graphite powder in the above example
A thermal transfer recording sheet was manufactured in the same manner as described below using the artificial graphite fine powder of μm. The coating amount of the thermal transfer layer was about 3.5 g / m ² as in the example, and the average thickness of the thermal transfer layer was 2.5 μm. This fact means that in the above-mentioned embodiment, the artificial graphite having a distribution particle size of about 2.5 μm or more can form the ink carrier 32a (FIG. 2) which forms the unevenness on the surface of the thermal transfer layer.

However, the particle size of the artificial graphite fine powder in this reference is all smaller than the thickness of the thermal transfer layer, indicating that the ink carrier 32a cannot be formed.

Similarly, when a thermal transfer recording experiment is carried out, in the sheet of this comparative example, the recording density D from the low transfer recording density area to the medium transfer recording density area is unstable depending on the location, and the density is binary and lacks continuous gradation. Therefore, good thermal transfer recording of a halftone image could not be performed.

Although artificial graphite was used in the above-mentioned examples, earth graphite and scaly graphite may be used, or two or more of these three types of graphite may be mixed and used. In particular, in the configuration shown in FIG. 1, the graphite powder 31 having a small particle size can be used as scale graphite and the graphite powder 32 having a large particle size can be used as artificial or earth graphite. In this case, it is recommended that the powder 32 be artificial graphite because the image quality and continuous gradation are good.

However, when each of these three types of graphite powder is used alone,
Artificial graphite is the best for continuous tone characteristics and image quality, and its use is recommended. Next, it becomes easier to decrease in the order of soil graphite and scaly graphite. Especially in the low recording density range, the image quality tends to be rough with dots.

In these, the particle shape of the artificial graphite powder is the closest to the sphere shape, then the soil shape and the scaly graphite are far from the sphere shape in that order, and smooth permeation transfer through the surface of the ink carrier 32, 32a becomes difficult, and therefore, the lump shape 32 , 32a itself is easily transcribed,
It is assumed that this is because the continuity of the transfer recording density is easily lowered.

EFFECTS OF THE INVENTION As described in detail above, by using graphite powder as the ink coloring material and the ink carrier material, it is easy to manage the ink for forming the thermal transfer layer coating, and the thermal transfer, the uniformity of the coating layer, and the black uniformity can be achieved. An improved black thermal transfer recording sheet can be realized.

Further, the black thermal transfer recording sheet according to the present invention is excellent in continuous gradation and can perform thermal transfer recording of a good quality halftone image, and its industrial effect is extremely great.

Although an insulating material is used as the heat resistant substrate 10 in the above description, the substrate 10 can be made resistive by mixing carbon or graphite powder. In this case, since the thermal transfer layer 40 in the present invention is also resistive, a linear type recording electrode head is brought into contact with the rear surface 12 of the substrate using the thermal transfer layer 40 as one common electrode to perform amplitude modulation, pulse width modulation and amplitude pulse width modulation. It is possible to perform continuous gradation thermal transfer recording by selectively applying the recording electric signal of No. 2 and controlling the temperature rising recording of the thermal transfer layer 40 by Joule heat of the current flowing through the substrate 10 and the thermal transfer layer 40.
Alternatively, in the above, a vapor-deposited conductive film such as aluminum is provided between the substrate surface 11 and the thermal transfer layer 40, and the Joule heat of the resistive substrate 10 is used as the common electrode with the conductive film as a common electrode to control the heating and recording of the thermal transfer layer. You can also do it.

[Brief description of drawings]

FIG. 1 is a sectional view of a black thermal transfer recording sheet according to an embodiment of the present invention, FIG. 2 is a sectional view of a black transfer recording sheet according to another embodiment of the present invention, and FIG. 3 is a black portion according to the present invention. FIG. 4 is a characteristic diagram showing thermal transfer recording characteristics for explaining a thermal transfer recording sheet. 10 ... Heat resistant substrate, 20, 20 '... Hot melt binder material, 30, 31, 31', 31a, 31a ', 32, 32a ... Graphite powder, 40 ...
… Thermal transfer layer, 100… Black thermal transfer recording sheet.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 59-79788 (JP, A) JP 59-131496 (JP, A) JP 59-101399 (JP, A)

Claims (1)

[Claims] 1. A hot-melt binder material whose viscosity is controlled to be reduced by temperature rising recording control and transferability to a recording medium is provided on a sheet-shaped heat-resistant substrate surface, and at least graphite powder particles as a coloring material. And a graphite powder having a particle size distribution, and some of the powder particles penetrate the thickness of the heat transfer layer and project to the surface of the heat transfer layer, forming an uneven surface on the heat transfer layer. A graphite thermal transfer recording sheet characterized in that