EP0460463B1 - Use of azo dyes in thermal transfer printing - Google Patents

Description

für den Thermotransferdruck, in der die Substituenten folgende Bedeutung haben:

• X einen Rest der Formellla oder Ilb
  
  wobei
• R¹ Wasserstoff, eine C_i-C₆-Alkylgruppe oder eine Phenylgruppe, die C1-C4-Alkyl, C1-C2-Alkoxy, Chlor, Brom oder Cyano als Substituenten tragen kann, bedeutet,
• n für 1 oder 2 steht und
• R² Wasserstoff, C1-C4-Alkyl, C, -C4-Alkoxy, Chlor oder Brom bezeichnet;
• K den Rest einer Kupplungskomponente III
  
  aus der Anilin-, Aminonaphthalin-, Pyrazol-, Hydroxypyridon- oder Tetrahydrochinolinreihe sowie ein Verfahren zur Übertragung dieser Azofarbstoffe durch Diffusion von einem Träger auf ein mit Kunststoff beschichtetes Substrat mit Hilfe eines Thermokopfes.

The present invention relates to the use of azo dyes of the general formula I.

for thermal transfer printing, in which the substituents have the following meaning:

• X is a radical of the formula la or Ilb
  
  in which
• R ^{1 is} hydrogen, a C _i -C ₆ alkyl group or a phenyl group which can carry C1-C4-alkyl, C1-C2-alkoxy, chlorine, bromine or cyano as substituents,
• n stands for 1 or 2 and
• R ² denotes hydrogen, C1-C4-alkyl, C, -C4-alkoxy, chlorine or bromine;
• K the rest of a coupling component III
  
  from the aniline, aminonaphthalene, pyrazole, hydroxypyridone or tetrahydroquinoline series and a process for transferring these azo dyes by diffusion from a support onto a plastic-coated substrate with the aid of a thermal head.

The technique of thermal transfer printing is well known; In addition to the laser and IR lamp, the main heat source used is a thermal head with which short heating pulses lasting a fraction of a second can be emitted.

In this preferred embodiment of thermal transfer printing, a transfer sheet which contains the dye to be transferred together with one or more binders, a carrier material and possibly other auxiliaries such as release agents or crystallization-inhibiting substances is heated from the rear by the thermal head. The dye diffuses from the transfer sheet into the surface coating of the substrate, e.g. into the plastic layer of a coated paper.

The main advantage of this method is that the amount of ink transferred and thus the color gradation can be controlled in a targeted manner via the energy to be delivered to the thermal head.

Thermal transfer printing generally uses the three subtractive primary colors yellow, magenta and cyan, possibly also black, whereby the dyes used must have the following properties for optimal color recording: easy thermal transferability, low tendency to migrate within or from the surface coating of the recording medium at room temperature , high thermal and photochemical stability as well as resistance to moisture and chemicals, no tendency to crystallize when storing the transfer sheet, a suitable color for the subtractive color mixture, a high molar absorption coefficient and easy technical accessibility.

At the same time, these requirements are very difficult to meet. Therefore, most of the dyes proposed for thermal transfer printing do not meet the required property profile. This also applies, for example, to the azo dyes described in US Pat. No. 4,764,178 and recommended for thermal transfer printing, which have coupling components from the aniline, tetrahydroquinoline, aminoquinoline or julolidine series, and to EP-A-258 856 and US-A-4,698,651 for the same use, known azo dyes with coupling components based on aniline, which are known inter alia all differ from the azo dyes I by the nature of the substituent ortho to the nitrogen atom in the thiazole ring.

Furthermore, EP-A-346 729 describes azo dyes with coupling components from the diaminopyridine series for thermal transfer printing.

The azo dyes I themselves are known from the older German patent applications P 38 10 643.4 and P 38 16 698.4 or can be obtained by the methods mentioned there.

The invention was therefore based on the object of finding suitable red and yellow dyes for thermal transfer printing which come closer to the required property profile than the dyes known hitherto.

Accordingly, the use of the azo dyes I defined at the outset was found for thermal transfer printing.

In addition, a method for transferring azo dyes by diffusion from a support onto a plastic-coated substrate using a thermal head was found, which is characterized in that a support is used for this purpose, on which one or more of the azo dyes I defined at the outset are located.

Suitable alkyl radicals R ¹ or R ² are especially methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl and tert-butyl. Alkyl radicals R ¹ can also be pentyl, isopentyl, neopentyl, tert-pentyl, hexyl and 2-methylpentyl.

Suitable alkoxy radicals R ² are, for example, methoxy, ethoxy, propoxy, isopropoxy, butoxy and isobutoxy.

If the radical R ¹ is a phenyl group bearing substituents, the following may be mentioned, for example: methyl, ethyl, methoxy, ethoxy, chlorine, bromine and cyanophenyl, the substituents of which are each in the 2, 3 or 4 position to sit.

• Methoxy-, Ethoxy-, Propoxy-, Butoxymethyl,
• 2-Methoxy-, 2-Ethoxy-, 2-Propoxy-, 2-Butoxy-, 2-Pentyloxy- und
• 2-Hexyloxyethyl;
• 2-, 3- und 4-Pyridyl.

Preferred radicals X of the formula Ila or Ilb are, for example:

• Methoxy, ethoxy, propoxy, butoxymethyl,
• 2-methoxy, 2-ethoxy, 2-propoxy, 2-butoxy, 2-pentyloxy and
• 2-hexyloxyethyl;
• 2-, 3- and 4-pyridyl.

• - Anilinderivate der allgemeinen Formel Illa
  
• - Aminonaphthalinderivate der allgemeinen Formel Illb
  
• - Pyrazolderivate der allgemeinen Formel IIIc
  
• - Hydroxypyridonderivate der allgemeinen Forml Illd
  
• - Tetrahydrochinolinderivate der allgemeinen Formel IIIe
  

Preferred coupling components 111 are:

• - Aniline derivatives of the general formula Illa
  
• - Aminonaphthalene derivatives of the general formula Illb
  
• - Pyrazole derivatives of the general formula IIIc
  
• - Hydroxypyridone derivatives of the general formula III
  
• - Tetrahydroquinoline derivatives of the general formula IIIe
  

• _R3, _R ⁴ Wasserstoff; C₁-C₁₀-Alkyl, deren C-Kette durch ein bis drei Sauerstoffatome in Etherfunktion unterbrochen sein kann und die folgende Substituenten tragen können: Cyano, Hydroxy, Phenyl, Phenoxy, Phenylaminocarbonyloxy, Benzyloxy, Benzoyloxy, das C₁-C₄-Alkyl, C1-C4-Alkoxy, Fluor, Chlor oder Brom als Substituenten tragen kann, C1-C4-Alkanoyloxy, C₁-C₆-Alkoxycarbonyloxy, Ci-C₈-Alkoxycarbonyl, Mono- oder Di-C₁-C₈-alkylaminocarbonyloxy, wobei die C-Kette der drei letztgenannten Substituenten jeweils durch ein oder zwei Sauerstoffatome in Etherfunktion unterbrochen sein kann; C₃-C₅-Alkenyl oder C₅-C₇-Cycloalkyl; Phenyl, das C₁-C₄-Alkyl, C₁-C₄-Alkoxy, C₁-C₄-Dialkylamino, Acetylamino, Fluor, Chlor oder Brom als Substituenten tragen kann;
• _R ⁵ Wasserstoff, Chlor; C₁-C₄-Alkyl, C₁-C₄-Alkoxy, C1-C4-Alkanoylamino, das C₁-C₄-Alkoxy, Phenoxy oder Chlor als Substituenten tragen kann, C₂-C₃-Alkenoylamino, Benzoylamino, Ureido, Mono- oder Di-C₁-C₄-alkylureido oder C1-C4-Alkylsulfonylamino;
• R⁶ Wasserstoff, Chlor, C₁-C₄-Alkyl oder C₁-C₄-Alkoxy;
• R⁷ Wasserstoff, C₁-C₈-Alkyl oder Phenyl;
• R⁸ Wasserstoff, C₁-C₈-Alkyl, das Phenyl, Furyl oder Thienyl als Substituenten tragen kann, C_s-C₇-Cycloalkyl oder Phenyl.

The substituents have the following meaning:

• _R 3, _R ^{4 are} hydrogen; C ₁ -C ₁₀ alkyl, the C chain of which can be interrupted by one to three oxygen atoms in ether function and which can carry the following substituents: cyano, hydroxy, phenyl, phenoxy, phenylaminocarbonyloxy, benzyloxy, benzoyloxy, the C ₁ -C ₄ - Can carry alkyl, C1-C4-alkoxy, fluorine, chlorine or bromine as substituents, C1-C4-alkanoyloxy, C ₁ -C ₆ -alkoxycarbonyloxy, Ci-C ₈ -alkoxycarbonyl, mono- or di-C ₁ -C ₈ - alkylaminocarbonyloxy, where the C chain of the latter three substituents can each be interrupted by one or two oxygen atoms in ether function; C ₃ -C ₅ alkenyl or C ₅ -C ₇ cycloalkyl; Phenyl which can carry C ₁ -C ₄ alkyl, C ₁ -C ₄ alkoxy, C ₁ -C ₄ dialkylamino, acetylamino, fluorine, chlorine or bromine as substituents;
• _R ^{5 is} hydrogen, chlorine; C ₁ -C ₄ alkyl, C ₁ -C ₄ alkoxy, C1-C4 alkanoylamino, which can carry C ₁ -C ₄ alkoxy, phenoxy or chlorine as substituents, C ₂ -C ₃ alkenoylamino, benzoylamino, ureido , Mono- or di-C ₁ -C ₄ -alkylureido or C1-C4-alkylsulfonylamino;
• R ^{6 is} hydrogen, chlorine, C ₁ -C ₄ alkyl or C ₁ -C ₄ alkoxy;
• R ^{7 is} hydrogen, C ₁ -C ₈ alkyl or phenyl;
• R ^{8 is} hydrogen, C ₁ -C ₈ alkyl, which can carry phenyl, furyl or thienyl as substituents, C _s -C ₇ cycloalkyl or phenyl.

Suitable alkyl radicals R ³ , R ⁴ , R ^s , R ⁶ , R ⁷ or R ⁸ are especially methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl and tert-butyl.

Alkyl radicals R ³ , R ⁴ , R ⁷ or R ⁸ are furthermore, for example, pentyl, isopentyl, neopentyl, tert-pentyl, hexyl, 2-methylpentyl, heptyl, octyl and 2-ethylhexyl, R ³ , R ³ ', R ⁴ or R ⁴ 'additionally, for example, nonyl and decyl.

If the C chain of the alkyl radicals R ³ or R ^{4 is} interrupted by one to three oxygen atoms, the following groups may be mentioned, for example: 2-methoxy, 2-ethoxy, 2-propoxy, 2-butoxyethyl, 2- and 3 -Methoxypropyl, 1-methoxyprop-2-yl, 2-ethoxypropyl, 2-propoxypropyl, 4,7-dioxaoctyl, 4,7-dioxanonyl, 4,8-dioxadecyl, 4,7,10-trioxaundecyl and 4,7,10 Trioxadodecyl.

• Cyanomethyl, 2-Cyanoethyl und 3-Cyanopropyl;
  • 2-Hydroxyethyl, 2-Hydroxypropyl, 1-Hydroxyprop-2-yl, 2-Hydroxybutyl, 1-Hydroxybut-2-yl, 4-Hydroxybutyl und 8-Hydroxy-4-oxaoctyl. Weiterhin geeignete Alkylreste R³ oder R⁴ weisen Phenyl, Phenoxy, Phenylaminocarbonyloxy sowie Benzyloxy oder Benzoyloxy als Substituenten auf, z.B.:
  • Benzyl, 1-Phenylethyl und 2-Phenylethyl, 2-Phenoxy-ethyl, 6-Phenoxy-4-oxahexyl und 2-(Phenylaminocarbonyloxy)ethyl;
  • 3-Benzyloxypropyl und 2-Benzoyloxyethyl, 2-(2-Methylbenzoyloxy)-ethyl, 2-(4-Methylbenzoyloxy)ethyl, 2-(4-Chlorbenzoyloxy)ethyl, 2-(4-Methoxybenzoyloxy)ethyl, 2-Benzoyloxypropyl und 2-Benzyloxybutyl.
• Tragen die Alkylreste R³ oder R⁴ Alkanoyloxy, Alkoxycarbonyloxy, Alkoxycarbonyl oder Alkylaminocarbonyloxy als Substituenten, so kommen beispielsweise folgende Gruppen in Betracht:
  • 2-Acetyloxy-, 2-Propionyloxy-, 2-Pentanoyloxyethyl,
  • 2-Acetyloxy-, 3-Acetyloxy-, 2-Propionyloxypropyl,
  • 2-Acetyloxy-, 4-Acetyloxy-, 2-Propionyloxybutyl und
  • 8-Acetyloxy-4-oxaoctyl;
  • 2-(Ethoxycarbonyloxy)ethyl, 2-(Butoxycarbonyloxy)ethyl und
  • 4-(Ethoxycarbonyloxy)butyl;
  • Methoxycarbonyl-, Ethoxycarbonyl-, Propoxycarbonyl-, Butoxycarbonylmethyl, 1-(Methoxycarbonyl)-, 2-(Methoxycarbonyl)-, 2-(Ethoxycarbonyl)-, 2-(Propoxycarbonyl)-, 2-(Butoxycarbonyl)-, 2-(lsobutoxycarbonyl)-, 2-(2-Ethylhexyloxycarbonyl)ethyl,
  • 2-(3-Oxabutyloxycarbonyl)-, 2-(3-Oxapentyloxycarbonyl)- und
  • 2-(3-Oxaheptyloxycarbonyl)ethyl;
  • 2-(Diethylaminocarbonyloxy)ethyl.
• Handelt es sich bei den Resten R³ oder R⁴ um Alkenyl-, Cycloalkyl- oder Substituenten tragende Phenylgruppen, so sind z.B. zu nennen:
  • Allyl und Methallyl;
  • Cyclopentyl, Cyclohexyl, Methylcyclohexyl und Cycloheptyl;
  • 2-, 3- und 4-Methylphenyl, 2- und 4-Methoxyphenyl, 2- und 4-Ethoxyphenyl, 4-Dimethylaminophenyl, 4-Acetylaminophenyl, 5-Chlorphenyl und 2,4-Dichlorphenyl.

Alkyl radicals R ³ or R ⁴ can additionally carry cyano and hydroxy as substituents; examples are:

• Cyanomethyl, 2-cyanoethyl and 3-cyanopropyl;
  • 2-hydroxyethyl, 2-hydroxypropyl, 1-hydroxyprop-2-yl, 2-hydroxybutyl, 1-hydroxybut-2-yl, 4-hydroxybutyl and 8-hydroxy-4-oxaoctyl. Suitable alkyl radicals R ³ or R ⁴ have phenyl, phenoxy, phenylaminocarbonyloxy and benzyloxy or benzoyloxy as substituents, for example:
  • Benzyl, 1-phenylethyl and 2-phenylethyl, 2-phenoxy-ethyl, 6-phenoxy-4-oxahexyl and 2- (phenylaminocarbonyloxy) ethyl;
  • 3-benzyloxypropyl and 2-benzoyloxyethyl, 2- (2-methylbenzoyloxy) ethyl, 2- (4-methylbenzoyloxy) ethyl, 2- (4-chlorobenzoyloxy) ethyl, 2- (4-methoxybenzoyloxy) ethyl, 2-benzoyloxypropyl and 2 -Benzyloxybutyl.
• If the alkyl radicals R ³ or R ⁴ carry alkanoyloxy, alkoxycarbonyloxy, alkoxycarbonyl or alkylaminocarbonyloxy as substituents, the following groups are suitable, for example:
  • 2-acetyloxy, 2-propionyloxy, 2-pentanoyloxyethyl,
  • 2-acetyloxy, 3-acetyloxy, 2-propionyloxypropyl,
  • 2-acetyloxy, 4-acetyloxy, 2-propionyloxybutyl and
  • 8-acetyloxy-4-oxaoctyl;
  • 2- (ethoxycarbonyloxy) ethyl, 2- (butoxycarbonyloxy) ethyl and
  • 4- (ethoxycarbonyloxy) butyl;
  • Methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonylmethyl, 1- (methoxycarbonyl) -, 2- (methoxycarbonyl) -, 2- (ethoxycarbonyl) -, 2- (propoxycarbonyl) -, 2- (butoxycarbonyl) -, 2- (isobutoxycarbonyl ) -, 2- (2-ethylhexyloxycarbonyl) ethyl,
  • 2- (3-oxabutyloxycarbonyl) -, 2- (3-oxapentyloxycarbonyl) - and
  • 2- (3-oxaheptyloxycarbonyl) ethyl;
  • 2- (diethylaminocarbonyloxy) ethyl.
• If the R ³ or R ⁴ radicals are phenyl groups bearing alkenyl, cycloalkyl or substituents, the following may be mentioned, for example:
  • Allyl and methallyl;
  • Cyclopentyl, cyclohexyl, methylcyclohexyl and cycloheptyl;
  • 2-, 3- and 4-methylphenyl, 2- and 4-methoxyphenyl, 2- and 4-ethoxyphenyl, 4-dimethylaminophenyl, 4-acetylaminophenyl, 5-chlorophenyl and 2,4-dichlorophenyl.

Suitable alkoxy radicals R ⁵ or R ⁶ are, for example, methoxy, ethoxy, propoxy, isopropoxy, butoxy and isobutoxy.

• Acetylamino, Propionylamino, Methoxy-, Ethoxy-, Chlor- und Phenoxy-acetylamino;
• Acryloylamino und Methacryloylamino;
• N-Methylureido, N-Butylureido und N,N-Dimethylureido;
• Methylsulfonylamino, Ethylsulfonylamino, Propylsulfonylamino und Butylsulfonylamino.

As R ⁵ continues alkanoylamino, alkenoylamino, benzoylamino, alkylureido or alkylsulfonylamino suitable as:

• Acetylamino, propionylamino, methoxy, ethoxy, chloro and phenoxy acetylamino;
• Acryloylamino and methacryloylamino;
• N-methylureido, N-butylureido and N, N-dimethylureido;
• Methylsulfonylamino, ethylsulfonylamino, propylsulfonylamino and butylsulfonylamino.

The radical R ⁸ can, for example, also be substituted alkyl as follows: benzyl, 1- and 2-phenylethyl, fur-2-ylmethyl, 2- (fur-2-yl) ethyl, 2- (thien-2-yl) ethyl and 2 - (Pyrid-2-yl) ethyl.

Of the coupling components H-K listed above, those of the formulas Illa, IIIc, Illd and Ille are particularly preferred.

• R³, R⁴ Wasserstoff; C₁-C₈-Alkyl, deren C-Kette durch ein Sauerstoffatom unterbrochen sein kann und die Cyano, Hydroxy, C1-C4-Alkanoyloxy oder C_i-C₈-A!koxycarbony! als Substituenten tragen können; C₅-C₇-Cycloalkyl;
• R⁵ Wasserstoff, Methyl, Methoxy oder Acetylamino;
• _R ⁶ Wasserstoff;
• R⁷ Methyl.

Very particularly preferred coupling components are aniline derivatives Illa and tetrahydroquinoline derivatives Ille with the following meaning of the substituents:

• R ³ , R ^{4 are} hydrogen; C ₁ -C ₈ alkyl, the C chain of which may be interrupted by an oxygen atom and the cyano, hydroxy, C1-C4-alkanoyloxy or C _i -C ₈ -A! Koxycarbony! can wear as substituents; C ₅ -C ₇ cycloalkyl;
• R ^{5 is} hydrogen, methyl, methoxy or acetylamino;
• _R ^{6 is} hydrogen;
• R ⁷ methyl.

Preferred azo dyes 1 can be found in the examples.

The dyes I to be used according to the invention are distinguished from the red and yellow dyes previously used for thermal transfer printing by the following properties: easier thermal transferability despite the relatively high molecular weight, improved migration properties in the recording medium at room temperature, significantly higher light fastness, better resistance to moisture and chemicals , better solubility in the production of the printing ink, higher color strength and easier technical accessibility.

In addition, the azo dyes 1 show a significantly better color purity, in particular in dye mixtures, and result in improved black prints.

The transfer sheets required as dye donors for the thermal transfer printing process according to the invention are prepared as follows: The azo dyes I are dissolved in an organic solvent, e.g. Isobutanol, methyl ethyl ketone, methylene chloride, chlorobenzene, toluene, tetrahydrofuran or mixtures thereof, processed with one or more binders and possibly other auxiliaries such as release agents or substances which hinder crystallization to give a printing ink which preferably contains the dyes in molecularly dispersed solution. The printing ink is then applied to an inert support and dried.

Suitable binders for the use of the azo dyes I according to the invention are all materials which are soluble in organic solvents and which are known to serve for thermal transfer printing, i.e. Cellulose derivatives such as methyl cellulose, hydroxypropyl cellulose, cellulose acetate or cellulose acetobutyrate, especially ethyl cellulose and ethyl hydroxyethyl cellulose, starch, alginates, alkyd resins such as polyvinyl alcohol or polyvinyl pyrrolidone, and especially polyvinyl acetate and polyvinyl butyrate. In addition, there are polymers and copolymers of acrylates or their derivatives such as polyacrylic acid, polymethyl methacrylate or styrene acrylate copolymers, polyester resins, polyamide resins, polyurethane resins or natural resins such as e.g. Gum arabic into consideration.

Mixtures of these binders are often recommended, e.g. those made of ethyl cellulose and polyvinyl butyrate in a weight ratio of 2: 1.

The weight ratio of binder to dye is usually 8: 1 to 1: 1, preferably 5: 1 to 2: 1.

As aids e.g. Release agents based on perfluorinated alkylsulfonamidoalkyl esters or silicones, as described in EP-A-227 092 or EP-A-192 435, and especially organic additives which prevent the transfer dyes from crystallizing out when the ink ribbon is stored and heated, for example cholesterol or vanillin used.

Inert carrier materials are, for example, tissue paper, blotting paper or glassine paper as well as films made of heat-resistant plastics such as polyesters, polyamides or polyimides, these films also being able to be metal-coated.

The inert carrier can additionally be coated with a lubricant on the side facing the thermal head in order to prevent the thermal head from sticking to the carrier material. Suitable lubricants are, for example, silicones or polyurethanes, as described in EP-A-216 483.

The thickness of the dye carrier is generally 3 to 30 µm, preferably 5 to 10 µm.

The substrate to be printed, for example paper, must in turn be coated with a binder which absorbs the dye during the printing process. Polymeric materials whose glass transition temperature Tg is between 50 and 100 _° C., for example polycarbonates and polyester, are preferably used for this purpose. Further details can be found in EP-A-227 094, EP-A-133 012, EP-A-133 011, JP-A-199 997/1986 or JP-A 283 595/1986.

For the process according to the invention, a thermal head is used which can be heated to temperatures up to over 300 ° C., so that the dye transfer takes place within a maximum time of 15 msec.

Examples

Initially, transfer sheets (donors) were produced in a conventional manner from 8 μm thick polyester film, which was provided with an approximately 5 μm thick transfer layer made of a binder B, which each contained 0.25 g of azo dye I. The weight ratio of binder to dye was 4: 1 in each case.

The substrate (slave) to be printed consisted of paper of approximately 120 μm thick, which was coated with an 8 μm thick plastic layer (Hitachi Color Video Print Paper).

The coated side of the encoder and slave was placed on top of one another, wrapped with aluminum foil and heated to a temperature between 70 and 80 ° C. for two minutes between two heating plates. With similar samples, this process was repeated three times at a higher temperature between 80 and 120 _° C.

The amount of dye diffused into the plastic layer of the receiver is proportional to the optical density, which was determined as absorbance A photometrically after the respective heating to the temperatures given above.

R: allg. GaskonstantePlotting the logarithm of the measured extinction values A against the associated reciprocal absolute temperature gives straight lines, from the slope of which the activation energy AE _T for the transfer experiment can be calculated:

R: general gas constant

The temperature T ^* at which the extinction reaches the value 2 can also be taken from the application, that is to say the transmitted light intensity is one hundredth of the incident light intensity. The smaller the temperature T ^* , the better the thermal transferability of the investigated dye.

Tables 1 to 9a list the azo dyes I tested for their thermal transfer behavior and their color.

The associated Tables 1 to 9b are the binder B used for the respective experiment (where: EC = ethyl cellulose, PVB = polyvinyl butyrate, MS = EC: PVB = 2: 1, EHEC = ethyl hydroxyethyl cellulose, CA = cellulose acetate) and the parameters already mentioned T ^* [° C] and AE _T - [kcal / mol].

Claims (2)

1. The use in thermal transfer printing of azo dyes of the general formula I

in which the substituents have the following meanings:

X is a radical of the formula Ila or IIb

where

R¹ is hydrogen, Ci-C6-alkyl, or phenyl which may be substituted by Ci-C4-alkyl, C₁-C₂-alkoxy, chlorine, bromine or cyano,

n is 1 or 2, and

R² is hydrogen, C₁-C₄-alkyl, C₁-C₄-alkoxy, chlorine or bromine, and

K is the radical of a coupling component III

of the aniline, aminonaphthalene, pyrazole, hydroxypyridone or tetrahydroquinoline series.

2. A process for transferring azo dyes by diffusion from a transfer to a plastic-coated substrate with the aid of a thermal printing head, which comprises using a transfer on which there is or are situated one or more azo dyes of the formula I as set forth in claim 1.