This invention relates to a thermal transfer printing (TTP) dye sheet, in particular to a dye sheet having an improved dye binder.

Thermal transfer printing is a printing process in which a dye is caused, by thermal stimuli, to transfer from a dye sheet to a receiver sheet thereby to form an image on the receiver sheet. In such a process, the dye sheet and receiver sheet are placed in intimate contact, the thermal stimuli are applied to the dye sheet to cause dye transfer and the dye sheet and receiver sheet are then separated. By applying the thermal stimuli to pre-determined areas in the dye sheet, the dye is selectively transferred to the receiver to form the desired image. The thermal stimuli may be provided by a programmable print head which is in contact with the dye sheet or by for example, a laser in a light-induced thermal transfer process ( ITT). Dye-sheets conventionally comprise a substrate having on one surface thereof a dye coat which typically comprises a thermally transferable dye dispersed or dissolved in a binder. Dye-sheets may also comprise a back coat to impart desirable properties for example, good handling and thermal characteristics to the dye sheet. Further, a primer or subbing layer may be employed between the substrate and the dye coat and/or the substrate and the back coat for example to improve the adhesion of the coat to the substrate.

Many materials have been suggested for use as the binder including, cellulosic polymers as disclosed in J0326 393, polyvinylbutyral as disclosed in EP-A-141678, epoxy resins and phenolic resins. J59199295 discloses an ink composition for sublimation dye transfer consisting of cellulose acetate, polystyrene, a disperse dye, silica and methylethyl ketone.

During the TTP process, application of a thermal stimulus to an area of the dye coat of the dye sheet heats that area of the sheet to a temperature typically in excess of 100°C as a result of which the dye in that area of the dye sheet is transferred to the receiver sheet. On removal of the thermal stimulus the temperature of the heated area then decreases to the ambient operating temperature in the process. However, once the thermal stimulus is removed and the temperature of the heated area of the the dye sheet is decreasing, there may be

still be sufficient heat to cause unwanted, uncontrolled residual transfer of dye to the receiver sheet which may cause a reduction in image quality. This effect is known as low temperature thermal transfer. We have now found that by employing a dye sheet binder comprising a polymer having a monomeric unit of an optionally substituted styrene provided that where unsubstituted styrene is present in the polymer then the polymer is a copolymer problems due to low temperature thermal transfer may be reduced or avoided and other significant advantages may also be secured.

Accordingly, a first aspect of the invention provides a thermal transfer printing dye sheet which comprises a substrate having on one side, a dye coat comprising a binder and a thermally transferable dye wherein the binder comprises a mixture of a polymer having, as at least 10Z of the monomeric units of the polymer, a monomeric vmit of formula (I);

-[H ₂ C-CXY]- (I)

wherein X is H or methyl and Y is an optionally substituted phenyl group, a cyano group or an ester group of formula -CO∑Z wherein Z is an optionally substituted Ci to CQ hydrocarbyl group with the proviso that where Y is an unsubstituted phenyl group the second- polymer also has a substituted styrene and/or a non-styrene monomeric unit. Dye-sheets according to the present invention have been found to exhibit a particularly advantageous combination of characteristics. In particular, good resistance to low temperature thermal transfer may be achieved which provides for improved image quality and images may be produced which have excellent optical density.

Also, we have found that when the dye sheet is to be used to produce multi-colour images by using a dye sheet comprising a plurality of panels of different uniform colours, usually magenta, yellow and cyan, improved colour balance between the dyes by matching the three colours may be achieved. A practical benefit of this is that an excess of one or more of the colours leading to a colour imbalance and hence a tinge of the image may be reduced or avoided, such imbalance being

particularly undesirable and visually prominent in neutral colours for example grey.

The polymer may be a homopolymer, the monomeric unit of which is derived from parahydroxystyrene, the phenyl group of which may or may not be further substituted.

Alternatively, the polymer may be a copolymer (block, random, alternating or graft) and formed of monomeric units according to formula 1 where Y is substituted phenyl and monomeric units according to formula 1 where Y is unsubstituted phenyl or an ester group of formula -CO2 where Z is an optionally substituted Ci to C5 hydrocarbyl group, and/or monomeric units derived from maleic anhydride.

According to a further alternative, the polymer may be a copolymer formed of monomeric units according to formula 1 where Y is unsubstituted phenyl and monomeric units derived from maleic anhydride. In another alternative, the polymer may be a copolymer formed of monomeric units according to formula 1 where Y is unsubstituted phenyl and monomeric units according to formula 1 where Y is an ester group of formula

-CO2 where Z is an optionally substituted C ^** L to Cs hydocarbyl group. Preferably, the further substituent on the phenyl group is halogen (most preferably bromine), glycidyl neodecanoate, hydroxymethyl, dimethylaminomethyl, 2-hydroxyethylmethylaminomethyl, t-butyl, sulphonate, styrene or benzoate.

According to a further preferred feature, the hydrocarbyl group is hydroxyethyl, hydroxypropyl or hydroxybutyl.

A most preferred thermal transfer printing dye sheet comprises a substrate having on one side, a dye coat comprising a binder and a thermally transferable dye wherein the binder comprises a polymer selected from the group consisting of : a parahydroxystyrene homopolymer, a bromo-parahydroxystyrene homopolymer, a coplymer of parahydroxystyrene and parahydroxystyrene further substituted with halogen( eg bromine), glycidylneodecanoate, hydroxymethyl, dimethylaminomethyl, 2-hydroxyethylmethylaminomethyl, t-butyl, sulphonate, styrene or benzoate,

a coplymer of parahydroxystyrene with or without a further substituentituent on the phenyl group and any one or a mixture of methyl(meth)acrylate, 2-hydroxy(meth) crylate, butyl (meth)acrylate, fluoro(meth)acrylate and acrylic acid

Where the polymer is a copolymer, suitably at least 102 and preferably at least 252 of the monomeric units of the polymer, based on the total number of monomeric units in the polymer, are of formula (I). The two monomeric units are suitably present in a ratio in the range 15:85 to 85:15 and preferably 25:75 to 75:25.

The glass transition temperature (Tg) of the polymer is suitably in the range 50 to 200°C and preferably 60 to 185°C.

The optionally substituted styrene polymer is suitably present in the dye-coat in an amount of at least 52 by weight of the total dye coat weight.

A further aspect of the invention provides a thermal transfer printing method which comprises placing a dye-sheet according to any aspect of the present invention in contact with a receiver sheet which comprises a substrate having on one side a dye-receptive surface, applying thermal stimuli to the dye-sheet at pre-determined locations to effect thermal transfer of the dye from the said locations to the dye-receptive surface thereby to form a pre-determined dye image and separating the dye-sheet and receiver sheet. The dyecoat is formed by coating the substrate, or a layer on the substrate, with an ink prepared by dissolving or dispersing one or more thermal transfer dyes and the binder in a liquid vehicle to form a coating composition; then removing any volatile liquids. Any dye capable of being thermally transferred in the manner described above, may be selected as required. Dyes known to thermally transfer, come from a variety of dye classes, e.g. from such nonionic dyes as azo dyes, anthraquinone dyes, azomethine dyes, methine dyes, indoaniline dyes, naphthoquinone dyes, quinophthalone dyes and nitro dyes.

The ink may also include dispersing agents, antistatic agents, antifoaming agents, and oxidation inhibitors, and can be coated onto the

substrate as described for the formation of the latter. The thickness of the dyecoat is suitably 0.1-5 μ_m, preferably 0.5-3 μm.

The dye and binder are suitably present in a weight ratio of 0.1 to 3:1 of dye:binder. The precise ratio of dye to binder will be

5 determined by a consideration of the particular dye and binder to be employed and the application for which the dye sheet is intended to be used.

Preferably, the dye sheet comprises a backcoat disposed on the opposite side of the substrate to the dye-coat to provide suitable heat

^" "-- ^• resistance and slip and handling properties. Suitable backcoats having a desirable balance of properties include those described in EP-A-314348 and especially those described in EP-A-458522. Particularly preferred backcoats include those in which the backcoat comprises the reaction product of radically co-polymerising in a layer of coating composition,

¹ 5 the following constituents: a) at least one organic compound having a plurality of radically polymerisable saturated groups per molecule and b) at least one organic compound having a single radically polymerisable unsaturated group

2 ⁽ - ⁾ the backcoat also containing an effective amount, as slip agent, of c) a metallic salt of a phosphate ester.

In cases, where the dye sheet is to be used in a LITT process, a light absorbing material may be included in the dye-coat or, if desired, a separate absorber layer comprising a light absorbing material disposed - between the dye-coat and the substrate may be employed. The light-absorbing material suitably comprises a material which is an absorber for the inducing light to convert it into the required thermal energy to effect thermal transfer of the dye.

If present, the absorber is preferably carbon black, as this

-in provides good absorption and conversion to heat, of a broad spectrum of wavelengths, and hence is not critical to the inducing light source employed for the printing, further, it is also relatively cheap.

However, any suitable absorber materials known in the art may be employed as desired. For lasers operating in the near infrared, there C

- are also a number of organic materials known to absorb at the laser wavelengths. Examples of such materials include the substituted

phthalocyanines described in EP-B-157,568, which can readily be selected to match laser diode radiation at 750-900 nm, for example.

A variety of materials can be used for the substrate, including transparent polymer films of polyesters, polyamides, polyimides, polycarbonates, polysulphones, polypropylene and cellophane, for example. Biaxially orientated polyester film is the most preferred, in view of its mechanical strength, dimensional stability and heat resistance,. The thickness of the substrate is suitably 1-50 μm, and preferably 2-30 μ-m. Various coating methods may be employed to coat the dye-coat and if present, other coats for example a backcoat, onto the substrate, including, for example, roll coating, gravure coating, screen coating and fountain coating. After removal of any solvent, the coating can be cured by heating or by irradiating with electromagnetic radiation, such as ultraviolet light, electron beams and gamma rays, as appropriate. .

Typical curing conditions are heating at 50-150°C for 0.5-10 minutes (in the case of thermal curing), or exposure to radiation for 1-60 s from an ultraviolet lamp of 80 W/cm power output, positioned about 15 cm from the coating surface (in case of ultraviolet light curing). In-line UV curing may utilise a higher powered lamp, eg up to 120 W/cm power output, focused on the coating as it passes the lamp in about 0.1-10 ms. The coating is preferably applied with a thickness such that after drying and curing the thickness is 0.1-5 μm, preferably 0.2-3 μ , and will depend on the concentration of the coating composition. The dye sheet may be elongated in the form of a ribbon and housed in a cassette for convenience, enabling it to be wound on to expose fresh areas of the dyecoat after each print has been made.

Dyesheets designed for producing multicolour prints have a plurality of panels of different uniform colours, usually three: yellow, magenta and cyan, although the provision of a fourth panel containing a black dye, has also previously been suggested. When supported on a substrate elongated in the form of a ribbon, these different panels are suitably in the form of transverse panels, each the size of the desired print, and arranged in a repeated sequence of the colours employed. During printing, panels of each colour in turn are held against a dye-receptive surface of the receiver sheet, as the two sheets are

imagewise selectively irradiated to transfer the dye selectively where required, the first colour being overprinted by each subsequent colour in turn to make up the full colour image.

The invention is illustrated by the following non-limiting examples.

Example 1

A selection of dye sheets were produced by coating a dye coat of composition listed in Table 1 onto a 6μm thick polyethylene terephthalate substrate having a subcoat (onto which the dye coat was coated) on one side and a back coat on the opposite side using a wire bar. For all the dye sheets the substrate, sub coat and back coat were the same. Once coated, the dye coat was dried in an oven for 20 seconds at 110°C to produce a dye sheet having a dry dye coat thickness of lμ . Examples IB, ID and IF illustrate the present invention and Examples 1A, ιc and IE are comparative Examples according to the prior art.

* comparative Example ci was 3-acetylamino-4-(3-cyano-5-phenylazothiophenyl-2-ylazo)-N,N- diet- hyl aniline; C2 was CI solvent blue 63; M0 was CI disperse red

60; M3 was 3-methyl-4(3-methyl-4-cyanoisothiazol-5-ylazo)-N-ethyl-N-ace toxy- ethyl aniline;

Ethyl cellulose was grade ECT-10 available from Hercules; Polyvinyl butyral had a molecular weight of 100000, a Tg of 85°C and was grade SLEC BX1 available from Sekisui;

S/PHS copolymer was a styrene/parahydroxystyrene copolymer and

PPHS was polyparahydroxystyrene, both available from Hoechst

Celanese. Example 2

A receiver sheet was produced by coating onto a polyethylene terephthalate substrate having a backcoat and a subcoat, a dye-receptive layer of the following composition:

Vylon 200 11.602 Tegomer HSi 2210 0.082

( (bis-hydroxyalkyl polydimethyl- siloxane from Goldschmidt)

Cymel 303 0.162

Di-n-butyl amine blocked toluene 0.052 sulphonic acid catalyst

Tinuvin 234 0.122

(UV stabiliser)

Toluene/Methyl ethyl ketone to 1002

47.5/52.5 solvent mixture

The dye-receptive coat was dried for 3 minutes at 140°C to provide a dry coat thickness of 4μm.

The dye sheets produced in Example 1 were each brought into contact with a sample of the receiver sheet and thermal transfer printing was effected by means of a programmable print head supplying heat pulses of 2 to 14 millisecond duration to the backcoat of the dye sheet to provide a gradation in the optical density of the print image. Printing in the cases of Examples IE and IF was effected using a different printer. The dye sheet and receiver sheet were separated following printing and the reflection optical densities on the receiver sheet were measured using a Sakura densitometer and are shown in Table 2. Example 3

In order to simulate the conditions under which unwanted low temperature thermal transfer under normal print conditions may occur, samples of the dye sheets produced in Example 1 and the receiver sheet produced in Example 2 were fed, in register, through a 2-roll laminator (OZATEC HRL350 hot roll laminator available from Hoechst) at 0.2 ms ^_1 . The rolls of the laminator were maintained at a temperature of 60°C and the pressure between them was 5 bar. The reflection optical densities on the receiver sheet were measured using a Sakura densitometer and are shown in Table 2, the column headings denoting which dye sheet was used in the test.

Low Temperature 0.36 0.18 0.24 0.14 0.28 0.14 Thermal Transfer (at 60°C) * comparative Example

The above results illustrate that significant reductions in dye transferred as a result of low temperature thermal transfer may be secured, by using dye sheets according to the present invention whilst maintaining acceptable optical densities in normal printing as compared with dye sheets of the prior art.