JPS61258791A - Black color thermal transfer recording sheet - Google Patents

Abstract

PURPOSE:To facilitate controlling an ink for forming a thermal transfer layer and enhance uniformity of thermal transfer, a coated layer and a black color, by using a graphite powder as an ink coloring material and an ink carrier material. CONSTITUTION:The thermal transfer layer 40 comprising a hot-melt binder material 20 and a graphite powder 30 is provided on a surface 11 of a heat-resistant base 10. The weight percentage of the amount of the graphite powder 32, 32a penetrating through layer 40 in the thickness direction and protruding from the surface 41 of the layer 40 to form a rugged surface based on the amount of the powder 30, 30a may be so set that at least one, preferably at least four, particles 32, 32a are present per unit recording picture element. A mixed suspended liquid for forming the thermal transfer layer is dissolved in a solvent while heating it to a temperature not lower than the melting or boiling point of the hot-melt binder material, followed by agitating and grinding while precipitating fine particles in a cold condition. The precipitated and suspended liquid is applied to the surface 41 of the base by solvent coating, and the coated layer is heated to a temperature not lower than the melting or softening point of the precipitated hot-melt particles to completely evaporate off the solvent and melt the particles, whereby a uniform thermal transfer layer 40 is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention uses a thermal recording head or the like to control temperature increase and record continuously 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 that thermally transfers and records black ink material in gradation density.

Conventional technology A coloring material and a hot-melt material are contained on a sheet-like heat-resistant substrate surface such as a polyethylene terephthalate (PET) film or capacitor paper, and the viscosity of the material is reduced and controlled by heating recording control, and the recording medium The ink material in a relationship that imparts transferability to
Room-temperature solid particles having at least one of the softening point and decomposition temperature higher than the hot-melt binder material are mixed and dispersed in the ink material as an ink carrier, and a thermal transfer layer is formed by forming an uneven surface with the room-temperature solid particles. According to the thermal transfer recording sheet, the thermal transfer recording density is continuously controlled in response to temperature rising recording control, and halftone images can be recorded (
(Refer to the specification of Japanese Patent Application No. 59-227155).

Problems to be Solved by the Invention In the above-mentioned thermal transfer recording sheet, the thermal conductivity of the room-temperature solid particles as the ink carrier is usually selected to be higher than that of the hot melt binder material, for example, the average particle size is 2.6 to 10.
Alumina powder of about μm size is used. In addition, since it is necessary for at least a portion of the solid particles at room temperature to penetrate the ink material layer and protrude onto the surface of the thermal transfer layer to form an uneven surface, a material that is soluble in a solvent at room temperature is selected as the hot melt binder material and contains a large amount of solvent. The thermal transfer layer is applied by a solvent coating method, and the evaporation of the solvent makes the thickness of the ink material layer smaller than at least some of the room-temperature solid particles. In the construction of a black thermal transfer recording sheet, fine carbon black powder with a secondary particle size of 1 μm or less is used as a pigment coloring material, but it contains alumina particles with a larger particle size than these, and is used together with a hot melt binder. When an ink for forming a thermal transfer layer is manufactured with a large amount of solvent contained, alumina particles with a large specific gravity tend to settle, making it difficult to manage the ink material, and also hindering uniform dispersion during layering in wet coating. easy to be

In addition, since alumina particles are white, this non-uniform dispersion tends to impede the original black uniformity of thermal transfer recording.

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

The present invention was made in view of the above problems, and an object of the present invention is to provide a useful black thermal transfer recording sheet.

Means for Solving the Problems In the present invention, a binder material whose viscosity is reduced and controlled by temperature increase recording control and imparts transferability to a recording medium is placed on a sheet-like heat-resistant substrate surface; A thermal transfer layer containing graphite powder particles is laminated, and this graphite powder has a particle size distribution, and some of the powder particles penetrate through the thickness direction of the thermal transfer layer and protrude onto the surface of the thermal transfer layer. A black thermal transfer recording sheet is constructed by forming an uneven surface.

In this case, when artificial graphite is used as the graphite powder, its particle size is more spherical than that of flaky graphite or earthy graphite, and a good black thermal transfer image can be obtained.

Function: In the black thermal transfer recording sheet according to the present invention, both the small-sized black material forming the coloring material and the large-sized room-temperature solid particles serving as the ink carrier are composed of graphite powder. Therefore, the conventional sedimentation phenomenon due to the difference in specific gravity between the coloring material and the solid particles at room temperature is improved, and since the color is the same, the management of the ink for forming the thermal transfer layer, the uniformity of coating layer formation, the uniform black thermal transferability, etc. are improved. That will happen.

Generally, in thermal transfer recording, a thermal recording head is placed on the back side of a heat-resistant substrate of a recording sheet, a recording platen is placed on the back side of a recording medium, the surface of the recording medium is pressed against the surface of the thermal transfer layer, and the thermal transfer layer is transferred through the heat-resistant substrate using the thermal head. Control temperature rise recording.

Now, the graphite powder mixed in the hot melt binder material constituting the thermal transfer layer is composed of fine powder whose particle size is sufficiently small compared to the thickness of the thermal transfer layer, and the thermal transfer layer has a uniform thickness. , assume that it is a simple black ink layer with a flat surface.

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

However, since the surface of the ink layer is unmelted and in a solid state at a temperature below the melting point of the hot melt binder material, the ink material cannot adhere to or be transferred to the recording medium. The adhesion and transfer of the ink material to the recording medium occurs only when the thermal head supplies the amount of heat necessary for the progress of melting of the ink layer in the thickness direction to reach the surface portion of the ink layer. In this case, the ink that has melted and become less viscous in the thickness direction of the ink layer is attached to and transferred to the recording medium all at once, so the recording density is binary, and the recording density is controlled in response to temperature increase recording control. So-called continuous tone thermal transfer recording is difficult.

However, the black thermal transfer recording sheet according to the present invention has graphite powder with a dog particle size, which is dispersed in the above-mentioned ink layer, penetrates the above-mentioned ink layer in the thickness direction, and is coated on the surface of the ink layer. Let's take a look at the cases that stand out.

The thermal conductivity of commonly used hot melt materials is low < 6 to 9
X10 cal17cm, Sec, °C, while graphite is 1.4 X 1O-2cal/crA, sec
It has a high thermal conductivity of , °C, and its melting point is 3500'
Melting point of hot melt binder material (usually 6o~ao'
c) has a much higher value.

Therefore, for temperature increase recording control, large particle diameter graphite powder has the role of a heat conductive medium that penetrates the ink layer in the thickness direction, and melts the ink material attached to or adjacent to the surface of the ink layer.
Reduces viscosity.

Therefore, even if the amount of heating due to temperature increase recording is small and the ink layer melts only on the back side, the graphite powder part with a large particle size will penetrate in the thickness direction of the ink layer along the surface. to melt the ink material and reduce its viscosity.

The ink material, which has been melted and reduced in viscosity in response to temperature-raising recording control, is affected by the thermal expansion of the hot-melt binder material during melting, the pressure applied through the surface of the recording medium, and the interaction between the surface of the recording medium and large-grained graphite. Due to capillary action in the contact gap, the particles penetrate and adhere to the surface of the recording medium through the surface of the large graphite particles. When the thermal transfer recording sheet is peeled off from the recording medium after the temperature increase recording control has been completed and before the molten ink has cooled and solidified, the amount of heating exceeds a certain level and the large graphite particles are surrounded by sufficient melting. If ink is present, the large graphite particles themselves are also attached and transferred to the surface of the recording medium with the unpenetrated molten ink material attached to the surface.

Therefore, large-sized graphite particles are penetrated by molten ink.

It plays the role of a so-called ink carrier for transfer, and the transfer recording density is continuous in accordance with the amount of melted ink, that is, the amount of heating for recording at elevated temperature. The maximum transfer recording density is achieved when the entire ink layer in the thickness direction is melted and has a low viscosity.

In this way, it is possible to change the particle size distribution from the small particle size fine powder that forms the usual coloring material for ink as graphite powder to include the large particle size particles that exhibit the effect as an ink carrier as described above. The above-mentioned infiltration and adhesion transfer continuously play the role of the black ink material and ink carrier according to the particle size of the graphite powder, that is, the heat capacity, and a thermal transfer recording sheet with good black continuous gradation can be realized. .

Example Hereinafter, the present invention will be described in detail with reference to Examples.

FIG. 1 is a cross-sectional structural diagram of an embodiment of a black thermal transfer recording sheet according to the present invention. For convenience of explanation, all graphite powder particles are shown as spherical.

1o is a heat-resistant substrate such as a film of polyethylene terephthalate (PET), polyimide, etc. or capacitor paper having a thickness of 3 to 15 μm. Heat resistant substrate 1
A thermal transfer layer 4o made of a hot melt binder material 2o and graphite powder 3o is laminated on the surface 11 of the black thermal transfer recording sheet 1Q.

is formed. Temperature increase recording control by the recording head is performed from the back surface 12 side of the substrate. In this example, graphite powder 30 divided into two types of particle size distributions, large and small, is mixed and used.

The graphite powder 31 has a small particle size and is embedded in the thermal transfer layer surface 41 to be used as a black ink coloring material to form the above-mentioned so-called ink layer. On the other hand, the graphite powder 32 has a large particle size, penetrates the thermal transfer layer 40, protrudes from its surface 41, and forms an uneven surface on the layer 40, and acts as the aforementioned ink carrier effect particles and also itself for transfer recording. It also serves as both black and white wood.

Thickness t of the ink layer (layer 4 in the gap between particles 32
The average thickness of the powder 31 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 becomes too small, the recording density will be low due to insufficient ink amount, while t
If it becomes too large, recording sensitivity and continuous gradation properties tend to decrease due to the increase in heat capacity. The particle size of the powder 32 is selected to be larger than the above-mentioned value of t since it is necessary to form an uneven surface on the layer 4o. For example, the maximum particle size is preferably 15 μm or less; if it is more than 15 μm, the ink penetration path becomes too long, making it difficult for the molten ink to penetrate as described above, resulting in a decrease in recording sensitivity, and the transfer of the powder 32 itself may sometimes result in poor recording. It reduces the continuous gradation property of the density range and deteriorates the recorded image quality into dots. The particle size of the powder 32 is preferably selected to be greater than the thickness t and less than 10 μm.

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

As shown in Fig. 2, the graphite powder 30a in this example has a single average particle size without mixing two types of particle size sorted graphite, and the particle size distribution is continuous, such as a normal distribution. Graphite powder consisting of particles with varying particle sizes is used.

The particles having a certain size are used as the black coloring material 31a buried in the layer surface 41, and the particles having a certain size or more protruding from the layer surface 41 serve as the ink carriers 32&.

The maximum value of the distribution particle size that defines the maximum particle size of the ink carrier 322L in the graphite 30a is 16 μm, preferably within 10 μm, as described in FIG. 1. The particle size distribution of the graphite powder 30a is normally within the range of 0 to this maximum value. The weight percentage of the graphite powders 30 (FIG. 1) and 301L (FIG. 2) in the thermal transfer layer 40 is selected, for example, in the range of 10 to 80%; if it is less than 10%, the blackness will decrease, and if it exceeds 80%, the blackness will decrease. Due to the shortage of the hot melt binder material 20, the thermal transferability deteriorates, and the strength of transfer recording onto the recording medium also decreases. Graphite powder 30.30&
It is particularly recommended that the weight % of is selected within the range of 20 to 70%. The coating amount of the thermal transfer layer 40 is preferably in the range of 162 to 10 g/door, and is 1.29/n? Below, the transfer recording density is low, and when it exceeds 10y/77/, the thermal transfer sensitivity decreases due to the increase in heat capacity, and the continuous gradation property particularly in the low density region deteriorates. A particularly recommended range is 1.5 to egyrrl.

On the other hand, in FIGS. 1 and 2, the graphite powder 30° 301 penetrates the thermal transfer layer 40 in the thickness direction and protrudes onto the layer surface 41, forming an uneven surface. When the powders 32, 32a have a single particle size and are perfectly spherical, the percentage by weight is extremely small, preferably at least one particle per unit recording pixel, and four or more particles per unit recording pixel. However, since the actual powder is not in such an ideal state, it is practically selected within the range of 5 to 7%, for example. If the weight % is less than 5%, the continuous gradation property in the low recording density region will be impaired due to the lack of ink carrier particles 30.3C1O, and the recorded image will still look rough, especially in the low recording density region. If the weight % exceeds 70%, the continuous gradation property in the low density region will deteriorate due to thermal transfer to the recording medium, resulting in a decrease in the quality of the recorded image. A particularly recommended range is 15-60%.

Examples of the hot melt binder material 2o include carnauba wax, candelilla wax, monothane wax, acid wax, and ester wax.

Partially oxidized ester wax, wax such as solid baraffy y, ethylene vinyl acetate copolymer (87 people).

The constituent materials include one or more resins such as petroleum resins, acrylic resins, polyamide resins, low-molecular polyethylene, and polybutene, and auxiliary agents such as plasticizers, mineral oil 2 surfactants, and antioxidants are added to these as necessary. Composed of mixtures.

The formation of an uneven surface by the protrusion of the surface 41 of the graphite terminal 32.32a means that the mixed material of the thermal transfer layer 4o containing the hot melt binder material 20 and the graphite powder 30. This can be easily done by coating the substrate surface 11 to a limited thickness using a coating method and then evaporating the solvent from the coating layer.

In this case, hot melt waxes and resins U generally do not dissolve in the solvent at room temperature in many cases. When using hot melt binder material 2Q containing such components, a solvent having a boiling point higher than the melting point or softening point of the room temperature insoluble hot melt material is used in the solvent coating method, and a mixture of these materials for the thermal transfer layer is used. The liquid is first heated to near or above the melting point or boiling point of the room temperature insoluble hot melt material to dissolve it in a solvent, and then cooled while stirring and pulverizing to form fine particles of the room temperature insoluble hot melt material. is precipitated by so-called cold precipitation. This precipitated suspension is coated with a solvent on the substrate surface 41, and after pre-drying the solvent in this coating layer, or immediately heating this coating layer to a temperature higher than the melting point or softening point of the precipitated hot melt particles, the solvent is completely removed. While evaporating, the precipitated hot melt particles are melted.

According to the above manufacturing method, the precipitated hot melt particles lose their particulate nature and are compatible with other hot melt materials, so there is an excellent advantage that a uniform thermal transfer layer 40 can be formed. In addition, the first
2, the exposed top surfaces of graphite powders 32, 32a are thinly coated with hot melt binder material 20' containing fine graphite powders 31', 31m', but even if these are not present, Make it good.

〔Example〕

Graphite powder 3oIL (31a, 32&) whose surface was treated with stearic acid, average particle size 1.0 μm (6 parts% weight value)
2 Artificial graphite with a particle size distribution of 0 to 10 μm (Hmu G-150-st manufactured by Nippon Graphite Co., Ltd.) 53.2 parts (hereinafter all parts by weight), an alicyclic saturated hydrocarbon resin (Kyokaku) as the hot melt binder material 20 point 70°C) 21.6 parts, paraffin (
melting point 50-62°C) 14.4 parts, carnauba wax (
melting point 80-83°C) 3.6 parts, ETA (MA: 2 parts%,
MI: 400° Softening point 86°C) 7.2 parts total 100
250 parts of xylene (boiling point: 138-144 DEG C.) as a solvent was added to 1 part to prepare a suspension for a thermal transfer layer, and heated to 100 DEG C. with stirring in a ball mill.

ETA and carnauba wax do not completely dissolve in xylene at room temperature. By heating at 100'C, the hot-melt binder materials including these proceed to melt and become mutually compatible. When the hot melt binder material is completely melted, the ball milling is continued while the mixture is naturally cooled to room temperature, and the ball milling is further continued for 2 hours.

During this cooling, stirring and dispersion process, the incompatible ETA and carnauba wax precipitate into fine particles and are uniformly dispersed together with the artificial graphite powder.

This precipitation mixed suspension was applied at room temperature to the surface 11 of a heat-resistant substrate 10 made of a 9 μm thick PET film using a commercially available #5 percoater, and after preliminary evaporation of the xylene solvent in the coating layer, the precipitation The entire coating layer was passed through a heating drying oven at 120'C, which is higher than the softening point and melting point of both EV (softening point: 86°C) and carnauba wax (melting point: 80-83°C) particles. Aiya 7, 7 Eo yeah. 5 o,
1. Melt the precipitated particles and reduce their viscosity. By this heating and melting, the precipitated particles lose their granularity, and their compatibility with other hot melt binder material components also progresses, so that a uniform hot melt binder material 20 is formed.

Further, as the xylene solvent evaporates, the layer surface of the coating layer is lowered, and as illustrated in FIG. A thermal transfer layer 4o with an uneven surface is formed by protruding from the layer surface 41 with its top portion thinly coated with melt binder material 20'. The coating weight of layer 40 in this example was 3.61y&.

FIG. 3 shows an example of transfer recording characteristics using the black thermal transfer recording sheet exemplified in FIG. 2 manufactured in this manner.

A known linear type thermal recording head of 4 dots (pixels)/MH was used for the temperature increase recording control, and a polypropylene resin-based recording paper having a thickness of 150 μm was used as the recording medium (image receptor). The main scanning line recording speed is 16.7 ms/line, the sub-scanning line density is 4 lines//ff, and the recording electrical signal is 6
The pulse width is modulated by bits, and the maximum modulated pulse width (Pw) is 4 mS.

The applied power was 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 on the recording paper surface rises smoothly from the paper surface density DO in response to the modulation pulse width Pw. Extremely excellent continuous tone transfer recording characteristics can be obtained from low to medium to high recording density areas, and when an image signal is used as the recording electrical signal, clear black halftones with excellent image quality can be obtained from the line head to the top. The image could be recorded by thermal transfer. The presence of large-diameter ink carrier particles 32a, which particularly effectively contribute to continuous gradation in the low transfer recording density region, can be detected by observing relatively large black transfer recording dots with a diameter of about 10 μm using a microscope. When the modulation pulse width Pw=1 ms, the density of the recording dots is approximately 4oO/M11I, and one recording electrode of the thermal head (that is, one recording pixel is 0.25
xo, 2 parts mA) It was about 25 pieces per surface.

[Comparative example]

Instead of the graphite powder in the above example, particle size distribution of 0 to 1.
A thermal transfer recording sheet was manufactured in the same manner using 2 μm artificial graphite fine powder. The coating weight of the thermal transfer layer was about 3.51/d as in the examples, and the average thickness of the thermal transfer layer was 2.6 μm. This fact is reflected in the above example.
This means that artificial graphite having a particle size distribution of about 2.6 μm or more can form an ink carrier 32a (FIG. 2) that forms irregularities on the surface of the thermal transfer layer.

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

Similarly, thermal transfer recording experiments were conducted on the sheet of this comparative example, and the recording density from the low transfer recording density area to the medium transfer recording density area was unstable depending on the location, and the sheet had a binary density and lacked continuous gradation. Unfortunately, thermal transfer recording of a good halftone image could not be performed.

Although artificial graphite was used in the above embodiment, earthy graphite or scaly graphite may be used, or two or more of these three types of graphite may be used in combination. In particular, in the configuration shown in FIG. 1, the small particle size graphite powder 31 can be used as scaly graphite, and the large particle size graphite powder 32 can be used as artificial or earthy graphite. In this case, it is recommended to use artificial graphite as the powder 32 because it provides good image quality and continuous gradation.

However, when these three types of graphite powder are used alone,
Artificial graphite has the best continuous tone characteristics and image quality, and its use is recommended. Next, it tends to decrease in the order of earthy graphite and scaly graphite. Particularly in low recording density areas, the image quality tends to become dot-like and rough.

The particle shape of artificial graphite powder is closest to a spherical shape, followed by clay-like graphite and scale-like graphite, which are far from spherical in order, making smooth penetration transfer through the surface of the ink carrier 32. This is assumed to be due to the fact that .32& itself is easily transferred and tends to reduce the continuity of transferred recording density.

As described above, by using graphite powder as an ink coloring material and an ink carrier material, the ink for forming a thermal transfer layer coating can be easily managed and thermal transfer can be performed.

A black thermal transfer recording sheet with improved coating layer uniformity and black uniformity can be realized.

Further, the black thermal transfer recording sheet according to the present invention has excellent continuous tone properties and can thermally transfer record halftone images of good quality, and its industrial effects are extremely impressive.

In the above description, an insulating material is used as the heat-resistant base 10, but the base 1o may be made resistive by mixing carbon or graphite powder.

In this case, since the thermal transfer layer 4o in the present invention is also resistive, a linear recording electrode head is brought into contact with the back surface 12 of the substrate as a common electrode on one side of the thermal transfer layer 40'1jH, and amplitude modulation, pulse width modulation, and amplitude control are performed. (Continuous tone thermal transfer recording can be performed by selectively applying a pulse width modulated recording electric signal and controlling the temperature of the thermal transfer layer 40 by Joule heat of the current flowing through the substrate 10° thermal transfer layer 40. Alternatively, in the above method, a vapor-deposited conductive film of aluminum or the like is installed between the substrate surface 11 and the thermal transfer layer 40, and this conductive film is used as a common electrode to use Joule heat of the resistive substrate 1Q to control the heating and recording of the thermal transfer layer. You can also.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a black thermal transfer recording sheet according to an embodiment of the present invention, FIG. 2 is a cross-sectional view of a black thermal transfer recording sheet according to another embodiment of the present invention, and FIG. 3 is a cross-sectional view of a black thermal transfer recording sheet according to another embodiment of the present invention. FIG. 2 is a characteristic diagram showing thermal transfer recording characteristics of a thermal transfer recording sheet. 1o...Heat-resistant substrate, 2o, 20'...
・Hot melt binder material, 30, 31. 31', 31
a. 31&', 32, 322L-old... graphite powder, 40
...Thermal transfer layer, 100... Mountain black thermal transfer recording sheet. Name of agent: Patent attorney Toshio Nakao (1 person)
...Asexuality resistance

Claims (1)

[Claims] A thermal transfer layer containing graphite powder particles and a hot melt binder material whose viscosity is controlled to be reduced by temperature increase recording control and impart transferability to a recording medium is layered on a sheet-like heat-resistant substrate surface, Further, the graphite powder has a particle size distribution, and some of the powder particles penetrate through the thickness direction of the thermal transfer layer and protrude onto the surface of the thermal transfer layer, thereby forming an uneven surface on the thermal transfer layer. Record sheet.