WO1994004370A1

WO1994004370A1 - Thermal transfer printing

Info

Publication number: WO1994004370A1
Application number: PCT/EP1993/002129
Authority: WO
Inventors: Richard Anthony Hann; David Wood Jarvis
Original assignee: Imperial Chemicals Industries Plc
Priority date: 1992-08-17
Filing date: 1993-08-11
Publication date: 1994-03-03
Also published as: GB9217476D0

Abstract

A thermal transfer printing sheet comprising a substrate having a coating comprising a mixture of dyes of formula (1) wherein rings A and B are optionally substituted in the free positions by non-ionic groups; formula (2) wherein: R1 is H or alkyl; R2 is alkyl; X is -H or a group of formula (a) in which R?3 and R4¿ each independently is -H or alkyl; Y is -OH or -NH¿2?; and ring A is as hereinbefore defined; and formula (3) wherein: R?5 and R6¿ each independently is alkyl, substituted alkyl, phenyl, substituted phenyl, cycloalkyl and substituted cycloalkyl; R7 is H, alkyl, alkoxy, alkylcarbonylamino or halogen; R8 is H, alkyl, phenyl, alkylphenyl or -CN; and R?9 and R10¿ each independently is H, alkyl, alkoxy, alkylthio or halogen and a process for selectively transferring the dye from the transfer sheet to a receiver sheet by the application of heat.

Description

THERMAL TRANSFER PRINTING Introduction

This specification describes an invention relating to dye diffusion thermal transfer printing (DDTTP), especially to a transfer sheet carrying a dye mixture which has an improved print stability and better resistance to visible surface crystallisation (snail tracking) and to a transfer printing process in which the dye mixture is transferred from the transfer sheet to a receiver sheet by the application of heat. It is known to print woven or knitted textile material by a thermal transfer printing (TTP) process. In such a process a sublimable dye is applied to a paper substrate (usually as an ink also containing a resinous or polymeric binder to bind the dye to the substrate until it is required for printing) in the form of a pattern, to produce a transfer sheet comprising a paper substrate printed with a pattern which it is desired to transfer to the textile. Substantially all the dye is then transferred from the transfer sheet to the textile material, to form an identical pattern on the textile material, by placing the patterned side of the transfer sheet in contact with the textile material and heating the sandwich, under light pressure from a heated plate, to a temperature from 180-220°C for a period of 30-120 seconds.

As the surface of the textile substrate is fibrous and uneven it will not be in contact with the printed pattern on the transfer sheet over the whole of the pattern area. It is therefore necessary for the dye to be sublimable and vaporise during passage from the transfer sheet to the textile substrate in order for dye to be transferred from the transfer sheet to the textile substrate over the whole of the pattern area. As heat is applied evenly over the whole area of the sandwich over a sufficiently long period for equilibrium to be established, conditions are substantially isothermal, the process is non-selective and the dye penetrates deeply into the fibres of the textile material. In DDTTP, a transfer sheet is formed by applying a heat- transferable dye (usually in the form of a solution or dispersion in a liquid also containing a polymeric or resinous binder to bind the dye to the substrate) to a thin (usually <20 micron) substrate having a smooth plain surface in the form of a continuous even film over the entire printing area of the transfer sheet. Dye is then selectively transferred from the transfer sheet by placing it in contact with a material having a smooth surface with an affinity for the dye, hereinafter called the receiver sheet, and selectively heating discrete areas of the reverse side of the transfer sheet for periods from about 1 to 20 milliseconds (msec) and temperatures up to 300°C, in accordance with a pattern information signal, whereby dye from the selectively heated regions of the transfer sheet melts and diffuses from the transfer sheet to the receiver sheet and forms a pattern thereon in accordance with the pattern in which heat is applied to the transfer sheet. The shape of the pattern is determined by the number and location of the discrete areas which are subjected to heating and the depth of shade in any discrete area is determined by the period of time for which it is heated and the temperature reached.

Heating is generally, though not necessarily, effected by a line of heating elements, over which the receiver and transfer sheets are passed together. Each element is approximately square in overall shape, although the element may optionally be split down the centre, and may be resistively heated by an electrical current passed through it from adjacent circuitry. Each element normally corresponds to an element of image information and can be separately heated to 300°C to 400°C, in less than 20 msec and preferably less than 10 msec, usually by an electric pulse in response to a pattern information signal. During the heating period the temperature of an element will rise from about 70°C to 300-400°C over about 5-8 msec. With increase in temperature and time more dye will diffuse from the transfer sheet to the receiver sheet and thus the amount of dye transferred onto, and the depth of shade at, any discrete area on the receiver sheet will depend on the period for which an element is heated while it is in contact with the reverse side of the transfer sheet.

As heat is applied through individually energised elements for very short periods of time the process is selective in terms of location and quantity of dye transferred and the transferred dye remains close to the surface of the receiver sheet. As an alternative heating may be effected using a light source in a light-induced thermal transfer (LITT) printer where the light source can be focused, in response to an electronic pattern information signal, on each area of the transfer sheet to be heated. The heat for effecting transfer of the dye from the transfer sheet is generated in the dyesheet which has an absorber for the inducing light. The absorber is selected according to the light source used and converts the light to thermal energy, at a point at which the light is incident, sufficient to transfer the dye at that point to the corresponding position on the receiver sheet. The inducing light usually has a narrow waveband and may be in the visible, infra-red or ultra violet regions although infra- red emitting lasers are particularly suitable.

It is clear that there are significant distinctions between TTP onto synthetic textile materials and DDTTP onto smooth polymeric surfaces and thus dyes which are suitable for the former process are not necessarily suitable for the latter.

In DDTTP it is important that the surfaces of the transfer sheet and receiver sheet are even so that good contact can be achieved between the printed surface of the transfer sheet and the receiving surface of the receiver sheet over the entire printing area because it is believed that the dye is transferred substantially by diffusion in the molten state in condensed phases. Thus, any defect or speck of dust which prevents good contact over any part of the printing area will inhibit transfer and lead to an unprinted portion on the receiver sheet on the area where good contact is prevented, which can be considerably larger than the area of the speck or defect. The surfaces of the substrate of the transfer and receiver sheets are usually a smooth polymeric film, especially of a polyester, which has some affinity for the dye.

Important criteria in the selection of a dye for DDTTP are its thermal properties, fastness properties, such as light fastness, and facility for transfer by diffusion into the substrate in the DDTTP process. For suitable performance the dye or dye mixture should transfer evenly and rapidly, in proportion to the heat applied to the transfer sheet so that the amount transferred to the receiver sheet is proportional to the heat applied. After transfer the dye should preferably not migrate or crystallise and should have excellent fastness to light, heat, rubbing, especially rubbing with a oily or greasy object, e.g. a human finger, such as would be encountered in normal handling of the printed receiver sheet. As the dye should be sufficiently mobile to migrate from the transfer sheet to the receiver sheet at the temperatures employed, 100-400°C, in the short time-scale, generally <20 msec, it is preferably free from ionic and/or water-solubilising groups, and is thus not readily soluble in aqueous or water-miscible media, such as water and ethanol. Many potentially suitable dyes are also not readily soluble in the solvents which are commonly used in, and thus acceptable to, the printing industry; for example, alcohols such as i.-propanol, ketones such as methyl ethyl ketone (MEK) , methyl i.-butyl ketone (MIBK) and cyclohexanone, ethers such as tetrahydrofuran and aromatic hydrocarbons such as toluene. The dye can be applied as a dispersion in a suitable medium or as a solution in a suitable solvent to the substrate from a solution. In order to achieve the potential for a high optical density (OD) on the receiver sheet it is desirable that the dye should be readily soluble or readily dispersable in the ink medium. It is also important that a dye which has been applied to a transfer sheet from a solution should be resistant to crystallisation so that it remains as an amorphous layer on the transfer sheet for a considerable time. Crystallisation not only produces defects which prevent good contact between the transfer receiver sheet but gives rise to uneven prints.

The following combination of properties is highly desirable for a dye which is to be used in DDTTP:

Ideal spectral characteristics (narrow absorption curve) Correct thermochemical properties (high thermal stability and efficient transferability with heat). High optical densities on printing.

Good solubility in solvents acceptable to printing industry: this is desirable to produce solution coated dyesheets alternatively good dispersion in acceptable media is desirable to produce dispersion coated dyesheets.

Stable dyesheets (resistant to dye migration or crystallisation). Stable printed images on the receiver sheet (resistant to heat, migration, crystallisation, grease, rubbing and light). The achievement of good light fastness in DDTTP is extremely difficult because of the unfavourable environment of the dye, close to the surface of the polyester receiver sheet. Many known dyes for polyester fibre have high light fastness (>6 on the International Scale of 1-8) on polyester fibre when applied by TTP because dye penetration into the fibres is good, but the same dyes exhibit very poor light fastness on a polyester receiver sheet when applied by DDTTP because of poor penetration into the substrate.

DDTTP is generally used for printing images on a suitable substrate. The Invention

According to the present invention there is provided a thermal transfer printing sheet comprising a substrate having a coating comprising a mixture of dyes of Formula (1):

wherein:

Rings A and B independently are optionally substituted in the free positions by non-ionic groups; Formula (2):

wherein: R¹ is H or alkyl; R² is alkyl; X is -H or a group of Formula

in which R³ and R⁴ each independently is -H or alkyl; Y is -OH or -NH₂; and Ring A is as hereinbefore defined; and Formula (3) :

Formu la C -0

wherein: R⁵ and R⁶ each independently is H, alkyl, substituted alkyl, phenyl, substituted phenyl, cycloalkyl and substituted cycloalkyl; R⁷ is H, alkyl, alkoxy, alkylcarbonylamino or halogen; R⁸ is H, alkyl, phenyl, alkylphenyl or -CN; and R⁹ and R¹⁰ each independently is H, alkyl, alkoxy, alkylthio or halogen.

Where the groups represented by R¹, R², R³ and R* are each independently alkyl these are preferably C₁_₁₂-alkyl and more preferably C-_₉-alkyl.

Where the groups represented by R⁵, R⁶, R⁷, R⁸, R⁹ and R¹⁰ each independently are alkyl these are preferably Cι_₆-alkyl, more preferably C^-alkyl. The alkyl groups represented by R¹ to R¹⁰ may be straight or branched chain alkyl groups. Where R⁵ and R⁶ represent cycloalkyl this is preferably C_,_₈-cycloalkyl, more preferably cyclohexyl. Where R⁷, R⁹ or R¹⁰ represents an alkoxy group this may be straight or branched chain and is preferably a C^g- alkoxy, more preferably a Cι_₄-alkoxy group. Where R⁷ represents an alkylcarbonylamino group this is preferably a

carbonylamino group, more preferably -NHCOCH₃. Where R⁷, R⁹ or R¹⁰ represents halogen this is preferably -F, -Cl or -Br, more preferably -F or -Cl.

Where R⁸ represents alkylphenyl this is preferably C^- alkyl phenyl, more preferably benzyl. Where R⁹ and R¹⁰ represents alkylthio these are preferably

Ci..-alkylthio, more preferably methylthio, ethylthio and butylthio.

Suitable non-ionic group substituents for Rings A and B include C_j.g-alkyl, Cj.g-alkylphenyl (especially -CH₂phenyl), Cι_₆- alkoxy, -OCOOCi.g-alkyl, -COCi.g-alkyl, -COOCi.g-alkyl, -OC_j.g-alkyl, -CN, -N0₂,

-F, -Cl, -Br, NH₂, -OH, -NHtCi.g-alkyl) (especially -NH(CH₃), -

NH(C₂H₅),

-NH(iso-C₃H₇) and -NHCH₂phenyl) ,

and -NHaryl (especially

-NHphenyl and -NHnaphthyl) , phenyl or C_,_₈-cycloalkyl.

Where Ring A carries substituents these are preferably in the 5- and in the 5- and 8-positions.

In dyes of Formula (2) R¹ is preferably H and R² is preferably in the p-position or R¹ and R² are preferably in the m- and p-positions or R¹ and R² are preferably in both m-positions relative to the -0-.

Where R⁵ and R⁶ are substituted alkyl, phenyl or cycloalkyl groups, suitable substituents may be selected from any of the substituents described above for Rings A and B.

The mixture of dyes preferably comprises from 52 to 552, more preferably 252 to 502 especially from 352 to 452 of the dye of

Formula (1), from 802 to 402, more preferably from 602 to 302, especially from 452 to 352 of the dye of Formula (2) and from 52 to 502, more preferably from 102 to 402, especially from 102 to 302 of the dye of Formula (3) .

A preferred mixture of dyes comprises from 352 to 452 of a dye of Formula (1) wherein Rings A and B carry no further substituents; from 452 to 352 of a dye of Formula (2) wherein R¹ is -H or methyl;

R² is C_j._g-alkyl preferably methyl, isopropyl, tertiary butyl, tertiary octyl or n-nonyl; and Rings A and B carry no further substituents; and from 102 to 302 of a dye of Formula (3) wherein R⁵ is C^g-alkyl preferably methyl, ethyl, tertiary butyl, sec butyl or benzyl; R⁶ is Cj.g-alkyl preferably methyl, ethyl or sec butyl; or C_j.g-alkyl substituted by carbonyl-C^-alkoxy such as acetoxyethyl and acetoxybutyl; R⁷ is C^-alkyl or C^-alkylcarbonylamino preferably methyl, ethyl or acetylamino; R⁸ is

such as methyl or ethyl; and R⁹ and R¹⁰ are both H. An especially preferred mixture of dyes comprises from 382 to 422 of a dye of Formula (1) wherein Rings A and B carry no further substituents; from 422 to 382 of a dye of Formula (2) wherein R¹ is -H; and R² is p-methyl, p-isopropyl or p-tertiary octyl; and from 162 to 242 of a dye of Formula (3) wherein R⁵ is methyl, ethyl, secbutyl or benzyl; R⁶ is ethyl, acetoxyethyl or acetoxybutyl; R⁷ is methyl or acetylamino; R⁸ is methyl and R⁹ and R¹⁰ are both H.

Especially preferred dyes of Formula (1) are l-amino-4- hydroxy-2-phenoxyanthraquinone. Especially preferred dyes of Formula (2) are l-amino-4- hydroxy-2-(4-methylphenoxy)anthraquinone, l-amino-4-hydroxy-2-(4- isopropylphenoxy) anthraquinone, l-amino-4-hydroxy-2-(4- tertiaryoctylphenoxy)anthraquinone.

Especially preferred dyes of Formula (3) are N-(2- acetoxyethyl)-N-ethyl-3-methyl-4-[ (4-cyano-3-methylisothiazol-5- yl)azo]aniline, N-(2-acetoxyethyl)-N-(sec butyl)-3-acetylamino-4-[ ( - cyano-3-methylisothiazol-5-yl)azo]aniline, N-ethyl-N-benzyl-3- acetylamino-4-[(4-cyano-3-methylisothiazol-5-yl)azo]aniline. The Coating The coating suitably comprises a binder together with a mixture of dyes of Formulae (1), (2) and (3). The ratio of binder to dye is preferably at least 0.7:1 and more preferably from 1:1 to 4:1 and especially preferably 1:1 to 2:1 in order to provide good adhesion between the dye and the substrate and inhibit migration of the dye during storage. The coating may also contain other additives, such as curing agents, preservatives, etc., these and other ingredients being described more fully in EP 133011A, EP 133012A and EP 111004A. The Binder The binder may be any resinous or polymeric material suitable for binding the dye to the substrate which has acceptable solubility in the ink medium, i.e. the medium in which the dye and binder are applied to the transfer sheet. It is preferred however, that the dye is soluble in the binder so that it can exist as a solid solution in the binder on the transfer sheet. In this form it is generally more resistant to migration and crystallisation during storage. Examples of binders include cellulose derivatives, such as ethylhydroxyethylcellulose (EHEC), hydroxypropyl cellulose (HPC), ethylcellulose, methylcellulose, cellulose acetate and cellulose acetate butyrate; carbohydrate derivatives, such as starch; alginic acid derivatives; alkyd resins; vinyl resins and derivatives, such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinyl acetoacetal and polyvinyl pyrrolidone; polycarbonates such as AL-71 from Mitsubishi Gas Chemicals and MAKRO ON 2040 (from Bayer, MAKROLON is a trademark) polymers and co-polymers derived from acrylates and acrylate derivatives, such as polyacrylic acid, polymethyl methacrylate and styrene-acrylate copolymers, styrene derivatives such as polystyrene, polyester resins, polyamide resins, such as melamines; polyurea and polyurethane resins; organosilicones, such as polysiloxanes, epoxy resins and natural resins, such as gum tragacanth and gum arabic. Mixtures of two or more of the above resins may also be used, mixtures preferably comprise a vinyl resin or derivative and a cellulose derivative, more preferably the mixture comprises polyvinyl butyral and ethylcellulose. It is also preferred to use a binder or mixture of binders which is soluble in one of the above-mentioned commercially acceptable organic solvents.

The mixture of dyes of Formulae (1), (2) and (3) have good thermal properties giving rise to even prints on the receiver sheet, whose depth of shade is accurately proportional to the quantity of applied heat so that a true grey scale of coloration can be attained. The mixture of dyes of Formulae (1), (2) and (3) also have strong absorbance properties and are soluble in a wide range of solvents, especially those solvents which are widely used and accepted in the printing industry, for example, alkanols, such as i.- propanol and butanol; aromatic hydrocarbons, such as toluene, ethers, such as tetrahydrofuran and ketones such as MEK, MIBK and cyclohexanone. Alternatively the mixture of dyes may be dispersed by high shear mixing in suitable media such as water, in the presence of dispersing agents. This produces inks (solvent plus mixture of dyes and binder) which are stable and allow production of solution or dispersion coated dyesheets. The latter are stable, being resistant to dye crystallisation or migration during prolonged storage.

The combination of strong absorbance properties and good solubility in the preferred solvents allows the achievement of good OD of the mixture of dyes of Formulae (1), (2) and (3) on the receiver sheet. The transfer sheets of the present invention have good stability and produce receiver sheets with good OD and which are fast to both light and heat. Furthermore, the printed receiver sheets should be able to withstand handling without damage occurring to the coloured image. The ability to withstand such damage is simulated in a 'snail tracking' test where the printed side of one receiver sheet is rubbed against the back of another receiver sheet, following incubation faded areas or 'snail tracks' on the printed side of the receiver sheet are quantified. The smaller the area which has faded or 'snail tracked' the better is the resistance to damage during handling. The present transfer sheets comprising mixtures of dyes of Formulae (1), (2) and (3) produce receiver sheets which have good resistance to snail tracking. The Substrate

The substrate may be any sheet material preferably having at least one smooth even surface and capable of withstanding the temperatures involved in DDTTP, i.e. up to 400°C for periods up to

20 msec, yet thin enough to transmit heat applied on one side through to the dyes on the other side to effect transfer of the dye onto a receiver sheet within such short periods. Examples of suitable materials are polymers, especially polyester, polyacrylate, polyamide, cellulosic and polyalkylene films, metallised forms thereof, including co-polymer and laminated films, especially laminates incorporating a smooth even polyester receptor layer on which the dye is deposited. Thin (<20 micron) high quality paper of even thickness and having a smooth coated surface, such as capacitor paper, is also suitable. A laminated substrate preferably comprises a backcoat, on the opposite side of the laminate from the receptor layer, which, in the printing process, holds the molten mass together, such as a thermosetting resin, e.g a silicone, acrylate or polyurethane resin, to separate the heat source from the polyester and prevent melting of the latter during the DDTTP operation. The thickness of the substrate depends to some extent upon its thermal conductivity but it is preferably less than 20μm and more preferably less than lOμ-m. The DDTTP Process

According to a further feature of the present invention there is provided a dye diffusion thermal transfer printing process which comprises contacting a transfer sheet comprising a coating comprising a mixture of dyes of Formulae (1), (2) and (3) with a receiver sheet, so that the coating is in contact with the receiver sheet and selectively heating an area on the reverse side of the transfer sheet whereby the dye on the opposite side of the sheet to the heated area is selectively transferred to the receiver sheet. Heating in the selected areas can be effected by contact with heating elements, which can be heated to 200-450°C, preferably 200-400°C, over periods of 1 to 20 msec preferably, 2 to 10 msec, whereby the dye mixture may be heated to 150-300°C, depending on the time of exposure, and thereby caused to transfer, substantially by diffusion, from the transfer to the receiver sheet. Good contact between coating and receiver sheet at the point of application is essential to effect transfer. The density of the printed image is related to the time period for which the transfer sheet is heated. The Receiver Sheet

The receiver sheet conveniently comprises a polyester sheet material, especially a white polyester film, preferably of polyethylene terephthalate (PET). Although some dyes of Formulae (1), (2) and (3) are known for the coloration of textile materials made from PET, the coloration of textile materials, by dyeing or printing is carried out under such conditions of time and temperature that the dye can penetrate into the PET and become fixed therein. In thermal transfer printing, the time period is so short that penetration of the PET is much less effective and the substrate is preferably provided with a receptive layer, on the side to which the dye is applied, into which the dye mixture more readily diffuses to form a stable image. Such a receptive layer, which may be applied by co-extrusion or solution coating techniques, may comprise a thin layer of a modified polyester or a different polymeric material which is more permeable to the dye than the PET substrate. While the nature of the receptive layer will affect to some extent the depth of shade and quality of the print obtained it has been found that the mixture of dyes of Formulae (1), (2) and (3) give particularly strong and good quality prints (e.g. fast to light, heat and storage) on any specific transfer or receiver sheet, compared with other dyes of similar structure which have been proposed for thermal transfer printing processes. The design of receiver and transfer sheets is discussed further in EP 133,011 and EP 133012.

The invention is further illustrated by the following examples in which all parts and percentages are by weight. Ink 1 This was prepared by dissolving a mixture of 0.215 parts of l-amino-4-hydroxy-2-phenoxyanthraquinone (Dye A), 0.215 parts of 1- amino-4-hydroxy-2-(4-isopropylphenoxy)anthraquinone (Dye B), 0.108 parts of N-(2-acetoxyethyl)-N-ethyl-3-methyl-4-[ (4-cyano-3-methyliso thiazol-5-yl)azo]aniline (Dye C), 0.473 parts of polyvinylbutyral and 0.118 parts of ethyl cellulose in 8.871 parts of tetrahydrofuran (THF). Ink 2

As for Ink 1 except that l-amino-4-hydroxy-2-(4-t- octylphenoxy) anthraquinone was used in place of the l-amino-4- hydroxy-2-(4-isopropylphenoxy)anthraquinone. Ink 3

As for Ink 1 except that l-amino-4-hydroxy-2-(4- methylphenoxy) anthraquinone was used in place of the l-amino-4- hydroxy-2-(4-isopropyl phenoxy)anthraquinone. Comparative Ink A

This was prepared by dissolving a mixture of 0.43 parts of Dye A, 0.108 parts of Dye C, 0.473 parts of polyvinylbutyral and 0.118 parts of ethyl cellulose in 8.871 parts of THF. Transfer Sheet TS1

This was prepared by applying Ink 1 to a 6μm polyester film (substrate) using a wire-wound metal Meyer-bar (K-bar No 2) to produce a wet film of ink on the surface of the sheet. The ink was then dried with hot air to give a 0.54μm dry film on the surface of the substrate.

Transfer Sheets TS2. TS3 and TSA

These were prepared as described for TS1 above except that Inks 2, 3 and A respectively were used in place of Ink 1. Printed Receiver Sheet RSI A sample of TS1 was contacted with a receiver sheet, comprising a composite structure based in a white polyester base having a receptive coating layer on the side in contact with the printed surface of TS1. The receiver and transfer sheets were placed together on the drum of a transfer printing machine and passed over a matrix of closely-spaced elements which were selectively heated in accordance with a pattern information signal to a temperature of >300°C for periods from 2 to 10 msec, whereby a quantity of the dye, in proportion to the heating period, at the position on the transfer sheet in contact with an element while it was hot was transferred from the transfer sheet to the receiver sheet. After passage over the array of elements the transfer sheet was separated from the receiver sheet. Printed Receiver Sheets RS2 and RSA

This was prepared as described for RSI above except that TS2 and TSA respectively were used in place of TS1.

Evaluation of Inks. Transfer Sheets and Printed Receiver Sheets

The stability of the ink was assessed by visual inspection. An ink was considered to be stable if there was no precipitation over a period of two weeks at ambient. The quality of the transfer sheet was assessed in respect of its stability by placing a first transfer sheet, dyecoat side down, on a piece of A4 paper and placing a second transfer sheet, dyecoat side down, on the first transfer sheet such that the dyecoat of the second transfer sheet is in contact with the back of the first transfer sheet to simulate the relative positions of the transfer sheets during storage on a spool. A second piece of A4 paper was placed on top of the second transfer sheet followed by an A4 sized metal plate through which a weight of 5kg was evenly applied. The paper, transfer sheets and weight were placed in a humidity oven at 55°C and 602 relative humidity. The dyecoat of the second transfer sheet was inspected at daily intervals under a microscope for the presence of crystals >3μm in length. Transfer sheet stability is quoted in Table 1 as the number of days elapsing before proliferation of such crystals and the longer it takes for crystals to form the more stable the transfer sheet.

Table 1

The quality of the printed impression on the receiver sheet was assessed in respect of: i) Reflected optical density (OD) of the receiver sheet was measured by means of a densitometer (Sakura Digital Transmission densitometer) . The results of the assessments are shown in Table 2:

Table 2

ii) Visible Surface Crystallisation (snail tracking)

Receiver sheets were printed at an OD of 1.0 and the printed sides were rubbed up and down once against the back of a blank receiver sheet to simulate damage in handling. The receiver sheets were incubated at 45°C and 852 relative humidity for 15 days before quantifying the areas of faded 'snail tracks' by placing the receiver sheets under a transparent 2mm grid and counting the affected squares. The smaller the area where fading has occurred the more resistant the receiver sheet is to snail tracking.

The results are expressed as a percentage area where fading has occurred. The results of the assessments is shown in Table 3:

Table 3

Claims

0437016CLAIMS

1. A thermal transfer printing sheet comprising a substrate having a coating comprising a mixture of dyes of Formula (1):

wherein:

Rings A and B are optionally substituted in the free positions by non-ionic groups; Formula (2):

wherein: R¹ is H or alkyl;

R² is alkyl;

X is -H or a group of formula

in which R³ and R⁴ each independently is -H or alkyl; Y is -OH or -NH₂5 and

Ring A is as hereinbefore defined; and Formula (3) :

Formula Q 3^~)

wherein:

R⁵ and R⁶ each independently is H, alkyl, substituted alkyl, phenyl, substituted phenyl, cycloalkyl and substituted cycloalkyl;

R⁷ is H, alkyl, alkoxy, alkylcarbonylamino or halogen; R⁸ is H, alkyl, phenyl, alkylphenyl or -CN; and R⁹ and R¹⁰ each independently is H, alkyl, alkoxy, alkylthio or halogen.

2. A thermal transfer printing sheet according to Claim 1 in which R¹, R³ and R^A each independently is H or C₁.₁₂-alkyl, R² is C,_₁₂- alkyl, R⁵ and R⁶ each independently is H or Ci.g-alkyl, phenyl, C_,_₈- cycloalkyl each of which may be substituted by a group selected from Cj.g-alkyl, Ci.g-alkylphenyl, C^g-alkoxy, -OCOOCj.g-alkyl, -COCi. - alkyl, -COOC^g-alkyl,

-OCi.g-alkyl, -CN, -N0₂, -F, -Cl, -Br, -NH₂, -OH, -NHJC^g-alkyl) ,

-NH(C₂H₅) , -NH(iso-C₃H₇) and -NHCH₂phenyl , -N(C_:_₆-alkyl )₂, and -

NHaryl , phenyl or Cή_₈-cycloalkyl , R⁷ is H , Cj.g-alkyl , Ci.g-alkoxy , C₁_ _l-alkylcarbonylamino , -F, -Cl or -

Br ,

R⁸ is H, Ci.g-alkyl, phenyl, C^-alkylphenyl or -CN,

R⁹ and R¹⁰ each independently is H, Ci.g-alkyl, Ci. -alkoxy, Cj..,- alkylthio, -F, -Cl or -Br.

3. A thermal transfer printing sheet according to Claim 1 wherein the mixture of dyes comprises from 52 to 552 of the dye of Formula (1), from 802 to 402 of the dye of Formula (2) and from 5 to 502 of the dye of Formula (3).

4. A thermal transfer printing sheet according to Claim 1 wherein the mixture of dyes comprises from 352 to 452 of the dye of Formula (1) wherein Rings A and B carry no further substituents, from 452 to 352 of the dye of Formula (2) in which R¹ is -H or methyl, R² is methyl, isopropyl, tertiary octyl or n-nonyl and Rings A and B carry no further substituents and from 102 to 302 of the dye of Formula (3) in which R⁵ is methyl, ethyl, tertiary butyl, sec butyl or benzyl, R⁶ is acetoxyethyl or acetoxybutyl, R⁷ is methyl, ethyl or acetylamino, R⁸ is methyl, ethyl, benzyl or -CN and R⁹ and R¹⁰ are both -H.

5. A thermal transfer printing sheet according to Claim 1 wherein the mixture of dyes comprises from 382 to 422 of a dye of Formula (1) wherein Rings A and B carry no further substituents, from 422 to 382 of a dye of Formula (2) wherein R¹ is -H, R² is p-methyl, p-isopropyl or p-tertiaryoctyl; and from 162 to 242 of a dye of Formula (3) wherein R⁵ is methyl, ethyl, sec butyl or benzyl, R⁶ is ethyl acetoxyethyl or acetoxybutyl; R⁷ is methyl or acetylamino; R⁸ is methyl and R⁹ and R¹⁰ are both H.

6. A thermal transfer printing sheet according to any one of Claims 1 to 5 wherein the mixture of dyes comprises a dye of Formula

(1) is l-amino-4-hydroxy-2-phenoxyanthraquinone, the dye of Formula

(2) is l-amino-4-hydroxy-2-(4-methylphenoxy)anthraquinone, l-amino-4- hydroxy-2-(4-isopropylphenoxy) anthraquinone, l-amino-4-hydroxy-2-(4- tertiary octylphenoxy)anthraquinone, and the dye of Formula (3) is N- (2-acetoxyethyl)-N-ethyl-3-methyl-4-[(4-cyano-3-methyl isothiazol-5- yl)azo]aniline, N-(2-acetoxyethyl)-N-(sec butyl)-3-acetylamino-4-[ (4- cyano-3-methylisothiazol-5-yl)azo]aniline, N-ethyl-N-benzyl-3- acetyla ino-4-[(4-cyano-3-methylisothiazol-5-yl)azo]aniline.