JPH0257389A - Heat transfer sheet - Google Patents

Abstract

PURPOSE:To improve the sensitivity of a heat transfer sheet and to reduce the wear of a thermal head with conductive carbon by forming a back layer of specific substance and binder as main agents, and incorporating the conductive carbon therein. CONSTITUTION:A back layer is formed of polyisocyanate and binder as main agents with the rich polyisocyanate thereby to form the layer of very reduced thickness. Accordingly, a heat transfer sheet having high sensitivity is provided. If the binder to be reacted with the polyisocyanate is made of two or more types of resins having different softening points, even if the carbon to be contained is relatively small in amount, sufficient charge preventing property is imparted. Therefore, a problem of the wear of a thermal head with the carbon can be solved. The polyisocyanate compound uses total amount of 50wt.% or more of the polyisocyanate and the binder to generate a coating film remarkably adapted for the back layer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a thermal transfer sheet, and more particularly to a novel thermal transfer sheet having an excellent antistatic and heat-resistant slip layer (back layer) made of a specific material.

(Prior Art) Conventionally, when printing output prints from computers or word processors by a thermal transfer method, a thermal transfer sheet having a heat-melting ink layer provided on one side of a base film has been used.

This conventional thermal transfer sheet has a thickness of 1 mm as a base film.
Using paper such as capacitor paper or paraffin paper with a thickness of 0 to 20 μm, or a plastic film such as polyester or cellophane with a thickness of 3 to 20 μm, a layer of heat-melting ink made by mixing coloring agents such as pigments and dyes with wax is applied. It is manufactured by coating.

Furthermore, if a heat-resistant material such as a plastic film is used as the base film, the thermal head may stick during printing, impairing the peeling and slipping properties of the thermal head, or tearing the base film. To avoid such problems, we form a heat-resistant layer made of silicone resin, etc.
In addition, a lubricant or the like is included in these layers for the purpose of improving the slip properties. Further, in order to prevent charging during printing or other handling, conductive carbon is included to provide antistatic properties.

(Problem that the invention seeks to solve) G. The back layer in conventional thermal transfer sheets needs to be formed relatively thick to prevent tearing of the base film, which reduces the sensitivity of the thermal transfer sheet. It has the disadvantage of

In addition, in the manufacturing process, when silicone oil or silicone resin is used, there is a lot of repellency of the coating agent for forming the back layer on the base film, making it difficult to form an even coating film.

Furthermore, conductive carbon is added to these back layers for the purpose of preventing static electricity, but in order to provide sufficient conductivity, a combination of the binder forming the heat-resistant layer and the conductive carbon is required. The problem is that more than 1% of conductive carbon is required, and as a result, the conductive carbon causes significant wear on the thermal head.

Therefore, an object of the present invention is to provide a thermal transfer sheet that solves the following drawbacks.

(Means for solving problems) The above objects are achieved by the present invention as described below.

That is, the present invention provides a thermal transfer sheet comprising a transfer ink layer that is melted by heating on one side of a base film and a back layer on the other side of the base film that is in contact with a thermal head. The thermal transfer sheet is formed from polyisocyanate and a binder as main ingredients, and contains a conductive carphone.

(Function) By forming the back layer using polyisocyanate and a binder as main ingredients and enriching the polyisocyanate, a very thin back layer is formed, so a highly sensitive thermal transfer sheet can be provided.

In particular, in the present invention, when the binder that reacts with the polyisocyanate is composed of two types of resins with different softening points, sufficient antistatic properties can be obtained even with a relatively small amount of conductive carbon. Therefore, the problem of wear of the thermal head due to conductive carbon could not be solved satisfactorily.

(Preferred Embodiments) Next, the present invention will be explained in more detail with reference to preferred embodiments.

As the base film used in the present invention, the same base film used in conventional thermal transfer sheets can be used as is, and other films can also be used, and there are no particular limitations.

Specific examples of preferred base films include polyester, polypropylene, cellophane, polycarbonate, cellulose acetate, polyethylene, polyvinyl chloride,
Polystyrene, nylon, polyimide, polyvinylidene chloride, polyvinyl alcohol, fluororesin, chlorinated rubber,
Examples include plastics such as ionomers, papers such as condenser paper and paraffin paper, nonwoven fabrics, and base films composited with stiff straw.

The thickness of this base film can be changed as appropriate depending on the material so that its strength and thermal conductivity are appropriate, but the thickness is preferably, for example, 2 to 25 μm.
It is.

The back layer, which mainly characterizes the present invention, is provided on one side of the above-mentioned base film, and is effective when a material with low heat resistance, such as a plastic film, is used as the base film. .

In the present invention, the above-mentioned back layer is formed from polyisocyanate and a binder as main ingredients, and is characterized by containing 11 conductive carbons.

Conventionally, it has been known to use polyurethane made of polyol and polyisocyanate to form various paints, coating agents, or back layers, or to use polyisocyanate as a crosslinking agent in other resin liquids to form crosslinked resin layers. However, in these known methods, the polyisocyanate is used in a very small amount relative to the resin to be crosslinked, so-called simply as a crosslinking agent.

In the present invention, unlike these conventional examples, a polyisocyanate compound is used as the main ingredient of the back layer, that is, in an amount accounting for 50% or more of the total of the polyisocyanate and the binder. It was found that a coating film was formed.

As the polyisocyanate used in the present invention, general polyisocyanates can be used, but preferred are polyurethane polyisocyanates having a relatively high molecular weight and reacted with polyisocyanate dimers, trimers, or polyol compounds. Specifically, for example, a cyclic trimer of 2,4-tolylene diisocyanate, a cyclic trimer of 2,6-tolylene diisocyanate, a trimer of diphenylmethane-4,4'-diisocyanate, 3 mol reaction product of diphenylmethane-4,4'-diisocyanate and 1 mol of trimethylolpropane, reaction product of 3 mol of 2,4-tolylene diisocyanate and 1 mol of trimethylolpropane, 3 mol of 2.
6-1 Reaction product of lylene diisocyanate and 1 mole of trimethylolpropane, reaction product of 3 moles of 2,4-tolylene diisocyanate and 1 mole of trimethylolethane 53 moles of 2,6-tolylene diisocyanate and 1 mole of trimethylolethane, the reaction product of 3 moles of mixed 2.4- and 2.6-tolylene diisocyanate and 1 mole of trimethylolpropane, 2.4- and 2. 6-) Stabilized polyisocyanates obtained by blocking a mixed cyclic trimer of lylene diisocyanate with phenol or cresol, etc. can be mentioned.

These polyisocyanates are, for example, bamboonate (
Product names such as Takeda Pharmaceutical Co., Ltd.), Parnock (Dainippon Ink Chemical Co., Ltd.), Coronate (Nippon Polyurethane Co., Ltd.), Duranate (Asahi Kasei Co., Ltd.), Dismodur (Bayer Co., Ltd.), Polyisocyanate It can be obtained and used in the present invention.

In the present invention, the polyisocyanate described above is preferably used in combination with a binder.

Examples of these binders include ethyl cellulose, droxyethyl cellulose, ethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methyl cellulose, cellulose acetate, cellulose acetate, cellulose resins such as nitrified surfaces, polyvinyl alcohol, polyvinyl acetate, and polyvinyl butyral. , polyvinyl acetal, polyvinylpyrrolidone, vinyl resins such as polyacrylamide, polyester resins, polyurethane resins, silicone-modified or fluorine-modified urethanes, etc. Among these, cellulose-based, acetal-based, butyral-based, polyester-based, Those having some reactive groups, for example, hydroxyl groups therein, such as polyurethane, are preferred.

The weight ratio of the binder and polyisocyanate is such that the polyisocyanate is at least 50% by weight, preferably 50 to 95% by weight of the total weight of the binder and polyisocyanate. If the amount of polyisocyanate used is less than 50% by weight, the heat resistance of the back layer will be insufficient, while if no binder is used, the film will become brittle.

In addition, in the present invention, when forming the back layer from the above-mentioned materials, in order to improve the slip properties of the back layer, thermal mold release agents such as wax, high grade acid amide, ester, surfactant, etc., lubricants, or fluorine are added. Organic powders such as resins, electrical particles such as silica, clay, talc, calcium carbonate, etc. can be included. In addition, in order to improve the heat resistance of the back layer, Hydrotalcy 1-DHT-4A was added as a heat resistant agent.
(manufactured by Kyowa Kagaku 1 Gyo), Talc Micro Ace L-1 (E1
(manufactured by genuine talc), Teflon Rubron [, -2 (manufactured by Daikin Industries), graphite fluoride 5CP-10 (manufactured by Sanbo Chemical Nigyo), graphite AT40S (manufactured by Oriental Sangyo), or silica, calcium carbonate, precipitated barium, urea crosslinking powder,
Fine particles such as acrylic crosslinked powder, silicone powder, wood powder, 5-molybdenum sulfide, and boron nitride can be included.

Another feature of the present invention is that when forming the back layer from the material described in 1 above, conductive carbon is incorporated therein.
It is to add anti-static properties to the back layer.

According to each study of the present invention, satisfactory static protection i1=+1
In order to impart this to the back layer, it is required to include more than 10% by weight of conductive carbon in the back layer. This significantly solves the problem of not being able to obtain antistatic IF properties of -1 unless the amount of conductive carbon is reduced.

As the conductive carbon used in the present invention, any conductive carbon conventionally used for conductive plastics and antistatic IF for plastics can be used.

Preferred specific examples include those which can be obtained manually from the market under names such as Ketchen Black EC and Ketchen Black EC600JD.

In the present invention, the above-mentioned conductive carbon is used as the back layer.
It is used in an amount of less than 1% by weight, preferably 1 to 10% by weight, more preferably 3 to 10% by weight.

If the amount of conductive carbon used exceeds the above range, the antistatic properties will be sufficient, but the problem of wear of the thermal head will occur, while if the amount of conductive carbon is too small, satisfactory antistatic properties will not be obtained.

In particular, in the present invention, when using the above-mentioned conductive carbon, by using a combination of a binder with a high softening point and a binder with a low softening point as the example binder, it is possible to reduce the amount of conductive carbon used with a small Li1. It has been found that a satisfactory charging property can be obtained even in -[-minutes].

As a binder with a high softening point, C has a softening point of 200°C or higher, while as a binder with a low softening point, it is preferable to use a binder with a softening point of about 100 to 120"C, and the ratio of the two is preferably the former. /Latter-50notake80150
~2o nails! A ratio of 4 is preferable, and if the ratio is too high, the antistatic properties will be poor, while if it is too low, the heat resistance of the back layer itself will deteriorate, which is not preferred.

To form the back layer, prepare a coating solution by dissolving or dispersing the above materials in a suitable solvent such as acetone, methyl ethyl ketone, toluene, xylene, etc., and apply this coating L solution to a gravure coater or roll coater. It is formed by coating and drying using a conventional coating method such as a wire bar or the like.

The coating amount-4-, that is, the thickness of the back layer is also important,
In the present invention, a back layer having sufficient performance can be formed with a thickness of 0.5 g/rn" or less, preferably 0.1 to 0.5 g/m" based on solid content.

In addition, in the present invention, after forming the back layer, unreacted isocyanate groups often remain in the layer.
It is preferable to perform an aging treatment for 1 minute after forming the layer. Through such treatment, the back layer has a pencil hardness of about H to 2H.

It is also effective to form a brimer layer made of polyurethane resin or the like prior to forming the above-mentioned back layer.

In the present invention, a heat-melting ink layer is further formed on the other surface of the base film using an ink containing necessary materials.

The ink for forming the hot-melt ink layer used in the present invention comprises a colorant and a vehicle, and may further contain various additives as necessary.

Among organic or inorganic pigments or dyes, it is preferable that the colorant has 4T characteristics that are good as a recording material, for example, one that has sufficient color density and does not discolor due to light, heat, temperature, etc. .

Further, it may be a substance that is colorless when not heated but develops color when heated, or a substance that develops color when it comes into contact with something coated on the transfer target. In addition to the colorants forming cyan, magenta, yellow, and black, colorants of various other colors can also be used.

The vehicle used includes wax as a main component, and mixtures of wax with drying oil, resin, mineral oil, cellulose, rubber derivatives, and the like.

Representative examples of wax include microcrystalline wax, carnauba wax, paraffin wax, and the like. In addition, we have a wide range of waxes including Fischer-Tropsch wax, various low molecular weight polyethylenes, wood wax, beeswax, spermaceti wax, privet wax, wool wax, shellac wax, candelilla wax, betrobutam, partially modified waxes, fatty acid esters, fatty acid amides, etc. used.

Furthermore, a thermally conductive substance can be blended into the hot-melt ink in order to provide the hot-melt ink layer with good thermal conductivity and melt transferability. Examples of this substance include carbonaceous substances such as carbon black, aluminum, copper, tin oxide, and molybdenum disulfide.

In addition to hot melt coating, methods for forming a hot melt ink layer directly or indirectly on a base film include:
Examples include hot lacquer coating, gravure coating, gravure reverse coating, roll coating, and many other methods of applying the ink. The thickness of the ink layer formed should be determined to balance the required concentration and thermal sensitivity and is in the range of 0.1 to 30 μm, preferably in the range of 1 to 20 μm.

In the present invention, a surface layer can be further formed on the ink layer. The surface layer forms part of the transfer film, forms the surface in contact with the transfer paper, seals the printed area of the transfer paper during transfer, prevents background smearing, and protects the ink layer. It has the function of improving adhesion to the transfer paper.

The wax used to form the surface layer is the same substance as the wax used in the hot-melt ink layer described above.

The surface layer made of the wax is formed by coating and cooling a wax melt, coating and drying an organic solvent solution containing the wax, coating and drying an aqueous dispersion containing particles of the wax, and the like.

The coating of the surface layer, as well as the formation of the ink layer, can also be done by various techniques. In the present invention, the thickness of this layer is preferably 0.5 mm in order to avoid insufficient sensitivity even when the printing energy is low, such as in a high-speed Type 9 printer.
It is 1 μm or more and less than 5 μm.

It is recommended that an appropriate amount of extender pigment be added to the above surface layer. This is because bleeding and trailing of printed characters can be better prevented.

Thermal transfer images generally have glossy and beautiful prints, but the documents may become difficult to read, so matte prints are sometimes desirable. In such a case, for example, as proposed by the applicant (Japanese Patent Application No. 58-208306),
A thermal transfer sheet is formed by coating a base film with an inorganic pigment such as silica, calcium carbonate, etc. dispersed in an appropriate solvent to provide a matte layer, and then coating a heat-melting ink layer. good. Alternatively, the base film itself may be matted and used (this is also the technique of Japanese Patent Application No. 1983-208307 proposed by the applicant).

Since it goes without saying that the present invention can be applied to thermal transfer sheets for color printing, multicolor thermal transfer sheets are also included within the scope of the present invention.

(Effects) According to the present invention as described above, by forming the back layer using polyisocyanate and a binder as main ingredients and enriching the polyisocyanate, a very thin back layer is formed, which improves sensitivity. A thermal transfer sheet with a high temperature is provided.

In particular, in the present invention, when the binder to be reacted with polyisocyanate is composed of two or more resins with different softening points, sufficient antistatic properties are imparted even with a relatively small amount of conductive carbon, so that the conductive carbon is The problem of thermal head wear due to carbon has been fully resolved.

(Example) Hereinafter, the present invention will be explained in more detail by giving Examples and Comparative Examples. In the text, parts or percentages are based on weight unless otherwise specified.

Example 1 17.0 parts of polyvinyl butyral resin (Eslec BX-1, softening point 240°C, manufactured by Block Chemical Co., Ltd.) Polyvinyl butyral resin (Eslec B[, -5, Softening point 105℃,
5.6 parts conductive carbon (Ketchen Black E (: DJ-600, manufactured by Lion Akzo)
5. 3-part heat release agent (mark Fl, -11
3. Manufactured by Adeka Argus ■) 5.0 parts Teflon powder (Fluon L-170J, Asahi Glass ■
! Polyisocyanate
22.6 parts Polyisocyanate TD [(III (-430 curing agent, Moroboshi Ink ■)) 22.6 parts Polyisocyanate TD [(III (-430 curing agent, Moroboshi Ink ■) 22.6 parts After thoroughly kneading the above composition, the solid content was reduced to 10 with a toluene/methyl ethyl ketone equivalent mixture. An ink composition for the back layer was prepared by diluting the ink composition to
Apply the ink composition in an amount of 0.5g using Myabah #4.
After coating to give a solid content of /ba (solid content), it was dried with warm air.

The obtained film was further cured by heating 5 parts in an oven at 60° C. for 1 hour to form a back layer. The ratio of the polyisocyanate to the binder NC01011 in the ink composition for the back layer is 1.8, and the proportion of conductive carbon in the back layer is 5.3%.

Separately, transfer the transfer ink composition with the following composition into a blade kneader.
The mixture was kneaded for 6 hours while heating to 90°C.

Nbarafin wax 10 parts Carnauba wax 10 parts Polybutene V11
-100, manufactured by Nippon Oil ■) 1 part carbon black (Geest 3, manufactured by Tokai Electrode ■)
2 parts The above ink composition was heated at a temperature of 100°C and applied to the other surface of the above base film using a hot melt roll coating method so that the coating amount was about 5.0 g/rn''. A thermal transfer ink layer was formed thereon to obtain a thermal transfer sheet of the present invention.

Example 2 Except for replacing the ink composition for the back layer with the following composition,
A thermal transfer sheet of the present invention was formed in the same manner as in Example 1.

5.27 parts of polyvinyl butyral resin (ESLETSUKU BX1, softening point 240℃, made by Tsukiki Kagaku@), 0 .88 parts silicone acrylic resin (XS810,
Tg-95℃, manufactured by Toagosei Kagaku T Gyo ■)
3.12 parts conductive carbon (Ketchen Black+;
c I) J-600, Lion @! ! ＞
0.98 parts fluorocarbon 0.18
Partial heat mold release agent (Mark F (Knee 113, Adeka Argus ■)
) 0.09 parts methyl ethyl ketone 69.23 parts toluene
20.15 parts] - 00 parts of the agglomerated acid and polyisocyanate TDI (Ito430 hardening agent, manufactured by Moroboshi Ink ■) ' 3.375 parts polyisocyanate MDI (XEL hardening agent (D), various W inks ■ 1.5 parts of toluene/methyl ethyl ketone (1:1) (147.0 parts) were mixed.

In the above composition, the ratio of polyisocyanate-1. to binder N(:010H) was 1.1, and the amount of conductive carbon was 6.5%.

Example 3 Except for replacing the ink composition for the back layer with the following composition,
A thermal transfer sheet of the present invention was formed in the same manner as in Example 1.

1 Polyvinyl butyral resin (S-LEC BX-1, softening point 240°C, manufactured by Miki Kagaku ■) 4.18 parts Polyvinyl butyral resin (S-LEC BL-5, softening point 105°C, manufactured by Mikki Chemical ■) 3.22 parts Conductive Carbon (Ketchen Black E (: DJ-600, manufactured by Aion Akzo)
1.93 parts Teflon powder (Fluon L-1
70J, made by Asahi Glass) 2
．． 67 parts Methyl ethyl ketone 63.59 parts Toluene 24.41 parts 100 parts of the above composition and 3.53 parts of polyisocyanate TDI (curing agent for IIR-430, manufactured by Soya Ink ■) Catalyst (desmoravit p, p, Sumitomo Bayer) Made of urethane) 0.018 parts toluene/methyl ethyl ketone/isopropyl alcohol (4/4/
2) 160. , 0 parts were mixed.

In the above composition, the ratio of polyisocyanate to binder N(:01011) was 2.1, and the amount of conductive carbon was 6.5%.

When measuring the IR spectrum of the back layer of the thermal transfer sheet of the present invention obtained as described above, absorption based on the NGO group was observed near 2,272c "1", and the amount of residual isocyanate was quite large based on the magnitude of this absorption. When stored under high temperature and high humidity, moisture in the air and residual isocyanate in addition to the hydroxyl groups and acetyl groups of the binder react as shown in the following formula, and the density increases in 3 to 5 days at 60°C. It was found that a crosslinked product was formed.

III Example 4 An ink was prepared in the same manner as in Example 3, except that desmoravid p and p were not used in Example 3, and a back layer was formed in the same manner. A 2% solution of DesmoRapid p,p in methyl ethyl ketone/toluene (]:I) was then prepared and further lightly coated with Myaber #2 on the back layer. The obtained film was lightly dried with hot air using a dryer to obtain a thermal transfer sheet of the present invention.

Example 5 The procedure of Example 1 was repeated, except that prior to forming the back layer on the polyester film, a plier layer was formed using an ink composition for a brimer layer having the following composition, and then a back layer was formed on the brimer layer. A thermal transfer sheet of the present invention was produced in the same manner.

Boo Maan Acrylic Polyol (Dianal LR-23745%
Solution, manufactured by Mitsubishi Rayon■) 41.2 parts toluene
26.3 parts methyl ethyl ketone
26.3 parts polyisocyanate (Parnock D-
750, manufactured by Dainippon Ink Chemical ■) 6.2 parts of the above composition was coated on a polyester film using Myabah #8 in an amount of 1.2 parts. Og/rn" (solid content) and then dried with hot air. A back layer was provided on the brimer layer in the same manner as in Example 1, and a thermal transfer ink layer was formed on the opposite surface of the polyester film. Ink was applied.

Examples 6 to 10 Thermal transfer sheets of the present invention were obtained in the same manner as in Example 1, except that the amount of conductive carbon was adjusted to the following proportions.

Shakao 1 1% Husband example 3% Disaster example 5% Example 1 10%
A thermal transfer sheet of the present invention was obtained in the same manner as in Example 1 except that L-5 was not used.

Husband example ↓Unislec anal-5 is made into 22 and 6 parts, and Eslec BX-
A thermal transfer sheet L of the present invention was obtained in the same manner as in Example 1 except that Example 1 was not used.

Comparative Example 1 A thermal transfer sheet of a comparative example was obtained in the same manner as in Example 1 except that no back layer was formed.

Comparative Example 2 A thermal transfer sheet of a comparative example was obtained in the same manner as in Example 1 except that conductive carbon was not used.

Comparative Example 3 Instead of polyisocyanate, the same Esletsukuro ×-1
A thermal transfer sheet of a comparative example was obtained in the same manner as in Example 1 except that .

The surface distance per sheet is 4XlO-'ern'') Transcript: L: Quality paper (manufactured by Yamakaikokusaku Bulb, YP 46 size 1:15 kg) Comparison of test results of the following Examples and Comparative Examples The thermal rolling γJ receipt surface resistance value was measured, and the head abrasion, head running performance, and post-printing condition were observed when printing was performed under the conditions listed in F, and the results shown in Table 1 below were obtained.

Printing equipment used: Thin film thermal head Printing energy: 1.70 mJ/dot (1 dora Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Implementation Example 11 Example 12 Comparative Example 1 Comparative Example 2 Comparative Example 3 <106 <106 0a 10+4 Ni copy Y-V-1 (- spatula 0.1 0.1 0.1 0.1 0.1 0°0 0. 0 0.0 0, 2 0.2 0.1 0.1 0, 1 0.1 1.0 +if The evaluation in Table 1 was performed as follows.

Surface resistance value (Ω/mouth) Two-surface resistance measuring device (Tl1860
I, manufactured by Atopan Test Co., Ltd.).

Head wear (μm) - 30,000 m of thermal transfer sheet is printed in a standard pattern, and the wear of the thermal head after printing is on the surface? It was measured with a shape measuring device (Surf Kneader-5E3C1 manufactured by Kosaka Institute).

Staking: Judgment was made by measuring the sound during printing, the appearance of the transferred object and thermal transfer sheet after printing, and the deviation of the printed pattern when the two were stacked on a table.

O: Can print well without sticking X: The thermal transfer sheet has holes and breaks, making it impossible to print. Judgment was made by second visual inspection of the condition after printing.

O: Good ×: Unable to print Furthermore, the heat-resistant sheet according to the present invention obtained as described above has the necessary properties to enable smooth conveyance without wrinkles when printing under the above-mentioned printing conditions. As a result of examining the coating amount of the back layer ink composition, it was found that 0.2 g/rn' was sufficient. The coating amount of the ink composition is 5.0
g1d, a printing energy of 1.0 mJ/dot is required to obtain a sufficient printing density with the transfer ink layer, but if the coating amount of the back layer is 0.2 g/rn'', then the printing energy is 0.0 mJ/dot. 7 mJ/dot is sufficient and printing energy can be reduced by about 30%.

Patent applicant: Dai Nippon Printing Co., Ltd.

Claims (5)

[Claims] (1) A thermal transfer sheet comprising a transfer ink layer that melts when heated on one side of a base film and a back layer on the other side of the base film that contacts the thermal head, where the back layer is made of polyisocyanate. 1. A thermal transfer sheet comprising a binder and a binder as main ingredients, and containing conductive carbon. (2) The thermal transfer sheet according to claim 1, wherein the polyisocyanate forming the back layer accounts for 50% or more by weight of the total of the polyisocyanate and the binder. (3) The amount of conductive carbon is 1 to 10% by weight of the back layer
The thermal transfer sheet according to claim 1, wherein the thermal transfer sheet occupies . (4) The thermal transfer sheet according to claim 1, wherein the thickness of the back layer is 0.5 μm or less. (5) The thermal transfer sheet according to claim 1, wherein the binder comprises two or more resins having different softening points.