WO1994005505A1

WO1994005505A1 - Thermal transfer printing receiver sheet

Info

Publication number: WO1994005505A1
Application number: PCT/GB1993/001846
Authority: WO
Inventors: Michael Robert Dayman; Paul Andrew Edwards
Original assignee: Imperial Chemical Industries Plc
Priority date: 1992-09-01
Filing date: 1993-09-01
Publication date: 1994-03-17

Abstract

Thermal transfer printing receiver sheets comprising a temperature sensitive substrate having a receiver layer comprising a polyester, a cross-linking agent and a release agent wherein the receiver layer is curable at a temperature of not more than 115 °C are disclosed. The cross-linking agent is preferably an alkoxylated alkylamine resin wherein the alkoxy moiety of the said resin has at least three carbon atoms. The receiver sheet is particularly suitable, when the receiver layer has a thickness of less than 2.5 νm for use as an image donor sheet in an image re-transfer process.

Description

Thermal Transfer Printing Receiver Sheet

This invention relates to a thermal transfer printing receiver sheet and in particular to a receiver sheet having a receiver layer which is capable of being cured at a relatively low temperature. 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. In such processes, the dye sheet and receiver sheet are placed in intimate contact, the thermal stimuli are applied to the dye sheet 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 dye sheet and receiver sheet are then separated.

Receiver sheets conventionally comprise a substrate with a dye-receiving surface on one side, into which a dye is thermally transferable and retainable. The dye-receiving surface may be provided by one side of the substrate itself. However, receiver sheets typically comprise a substrate supporting a receiver layer, the receiver layer having a dye-receptive polymer, a cross-linking agent and a release system. Typically the receiver layer is applied to the substrate by a coating process in which a composition comprising the components of the receiver layer in a liquid medium are coated onto the substrate and then dried to remove the medium and conventionally are cured at a temperature of at least 130°C to provide the desired receiver layer. Receiver sheets having a receiver layer comprising a methoxylated alkyl amine cross-linking agent and a silicone release system are known although an acceptable degree of curing is attainable only where conventional curing temperatures are employed.

Conventionally employed substrates include thermoplastic films for example polyethylene terephthalate and polyolefins. Significant markets also exist for receiver sheets having a substrate which is somewhat more sensitive to temperature than a thermoplastic film substrate, for example cellulose fibre paper. A cellulose fibre substrate allows the receiver sheet to be used in applications in which drafting marks are to e applied to the sheet for example as greetings cards and post cards. A drawback with temperature sensitive substrates, for example cellulose paper fibre is that they may be prone to discoloration if the receiver layer is cured by heating to a conventional curing temperature. Further, the use of substrates comprising thermoplastic materials which melt or soften at relatively low temperatures may be limited by the requirement of curing the receiver layer at a relatively high temperature. Further, to reduce the problem of the dye sheet and the receiver sheet sticking together during or following printing, it is desirable that the receiver layer comprises a release system. We have found that by employing a receiver layer which is curable at low temperatures and which comprises a compatible release system in receiver sheets having temperature sensitive substrates, problems due to discoloration and/or inadequate release following printing may be ameliorated. Accordingly, the invention provides a thermal transfer printing receiver sheet comprising a temperature .sensitive substrate having on one side a receiver layer capable of receiving a thermally transferable dye comprising a polyester, a cross-linking agent and a release agent wherein the receiver layer is curable at a temperature of not more than 115°C and preferably not more than 110°C.

As the receiver layer of a receiver sheet according to the invention is curable at a low temperature, temperature sensitive substrates may be used without a significant risk of impairment, for example by discoloration, of the substrate, and hence the receiver sheet as a whole, during manufacture. Moreover, receiver sheets according to the invention have good print stability and light fastness together with good print performance for example, images of high optical density may be printed onto the receiver sheet without there being any significant unwanted dye transfer in addition to the desired dye transfer. Preferably the cross-linking agent is an amine resin, and we have found that alkoxylated alkyl amines having a higher alkoxy moiety provide excellent low temperature curable receiver layers.

According to a further aspect of the invention there is provided a thermal transfer printing receiver sheet comprising a temperature sensitive substrate having on one side a receiver layer capable of receiving a thermally transferable dye comprising a polyester, a hydroxylated silicone as a release agent and an alkoxylated alkylamine resin as a cross-linking agent, wherein the alkoxyl moiety of the said resin has at least three carbon atoms. Suitably, the alkoxy alkylamine resin-containing receiver layer is curable at a temperature of not more than 115°C and preferably not more than 110°C.

Alkoxy alkyl amine resins wherein the alkoxy moiety has at least 3 carbon atoms are advantageous over alkoxyalkyl amine resins having a terminal methoxy moiety as they are curable to an acceptable degree at a lower temperature and, for a given temperature, provide a greater degree of curing than methoxy alkyl amine resins. Furthermore, excellent print performance is achievable with receiver sheets according to the invention.

The cross-linking agent may be any suitable amine resin but is preferably a urea, melamine or guanamine resin for example, melamine formaldehyde, benzoguanamine and imino melamine resins. Especially preferred amine resins include alkoxylated alkyl amine resins wherein the terminal alkoxy moiety has at least 4 carbon atoms for example, n-butoxy and iso-butoxy moieties. It is preferred that the alkoxy alkyl moiety of the alkoxyalkyl amine resin does not comprise methoxyl or ethoxyl even if, in the case of di-substituted amine groups, the other alkoxy alkyl substituent comprises an alkoxyl moiety having at least 3 carbon atoms.

Examples of highly preferred amine cross-linking agents include those available under the trade name BEETLE resins from British Industrial Plastics, (grades BE692 and BE 659 being especially preferred) and CYMEL 1158 available from American Cyanamid. Other cross-linking materials may be employed if desired for example urea formaldehyde, provided that the receiver layer is curable at or below 115°C.

The cross-linking agent is suitably present in an amount of 0.3 to 10 and preferably 0.5 to 5 parts by weight based on 100 parts of the polyester.

Suitably the release agent comprises a hydroxylated silicone. Preferred hydroxylated silicones comprise 2 hydroxyl groups, for example, a copolymer of a silicone and a polyoxyalkylene which provides - A -

a molecule having terminal hydroxyl groups in which case the cross-linking agent preferably has a functionality of at least 3 to provide a multiple cross-linking structure. Examples of preferred silicone materials include TEGOMER HSi 2311, HSi 2211 and HSi 2111 available from Goldschmidt.

The silicone is suitably present in an amount of 0.05 to 10, preferably 0.1 to 5 desirably 0.2 to 3.0 parts by weight based on 100 parts of the polyester.

Cross-linking is suitably effected in the presence of an acid catalyst for example p-toluene sulphonic acid (PTSA) which is desirably present in an amount of 0.1 to 10 parts by weight based on 100 parts of the polyester. The catalyst is preferably blocked and suitable examples include amine-blocked PTSA and ammonium tosylate.

Polyesters which are suitable for use in the receiver layer include VYLON polyesters (available from Toyobo), including grade 200 and especially grades 103 and 290 or mixtures thereof.

Other components which may be present in the dye-image donor sheet include an ultra violet absorber for example TINϋVIN 900 available from Ciba Geigy. Suitably, receiver sheets according to the present invention have a receiver layer having a thickness of up to 6μ_m, preferably, 0.5 to 4μ_m and especially 0.5 to 2.5μm. The improved release characteristics of the receiver layer enable a thin receiver layer to be employed to achieve a desired release performance which provides further printing performance benefits for example improved optical density.

The combination of the cross-linking agent and the release agent in the present invention provide a receiver sheet having excellent release properties, that is the separation of the receiver sheet from an adjacent surface following printing is improved and problems due to melt adhesion of the receiver with the said surface may be ameliorated. These properties allow the receiver sheets to be used in thermal transfer printing processes in which the receiver layer is in contact with a surface of, for example, a dye sheet which does not itself possess good release characteristics. Thus, receiver sheets according to the invention find application with a wide range of dye sheets which need not themselves provide a satisfactory release function. An example of another type of surface which may be used with a receiver of the present invention due to the excellent release properties of the receiver sheet is the dye-receptive surface of an article for example a ceramic or textile article, to which it is desired to transfer an image from the receiver sheet by means of an image retransfer process.

Image retransfer processes typically comprise placing a receiver sheet having a dye image in its receiver layer in contact with the dye-receptive surface of the article, the receiver sheet is then heated to effect thermal transfer of the dye image to the dye-receptive surface and the receiver sheet and the article are then separated to provide an imaged article. In this context, the receiver sheet acts as an image donor sheet.

Although the image donor sheet is suitably employed to produce an image on a second surface, it is desirable that the image on the sheet is of sufficient quality and optical density to provide a commercially acceptable image and thus allow the sheet, as an alternative to being used as a dye-image donor sheet, to be marketed as a print for example as a post card. A receiver sheet used as an image donor sheet typically comprises a substrate having an image layer on one side of the substrate which is conventionally at least 3.5 μm thick, for example 4 to 6 μm thick.

It has been surprisingly found that by employing as an image donor sheet the receiver sheet of the invention having a thin image layer, a dye image of excellent optical density may be provided on the dye-receptive surface and prior to transfer of the dye image, also on the image layer.

Accordingly a further aspect of the invention provides a method of producing a dye image on a dye-receptive surface which comprises placing _a dye-image donor sheet comprising an image layer having a thermally transferable dye defining an image to be transferred, in contact with the dye-receptive surface, heating the said sheet to effect thermal transfer of the dye image to the dye-receptive surface wherein the said image layer comprises a polyester, a cross-linking agent and a release agent, has a cure temperature of not more than 115°C and a thickness of not more than 2.5 μ_m. Desirably, the image layer has a maximum thickness of not more than 2μm, and preferably not more than 1.5 μm for example lμm. An acceptable image on the second surface may be obtained with an image layer of thickness less than 0.5 μm. However, it is generally desirable that the image layer has a thickness of at least 0.5μm and preferably 0.7 μm as this reduces the possibility of the image layer being saturated by dye when the image is applied to it. Such saturation is generally undesirable as during printing, the dye-image donor sheet may bond to either the dye-receptive surface or, if the image is applied to the dye-image donor sheet by a TTP process, to the dye sheet employed to transfer ink to the said sheet to form the image. Additionally production difficulties associated with providing a very thin image layer may be avoided.

By employing an image layer of reduced thickness in the dye-image donor sheet, it has been found that the proportion of the dye in the donor sheet transferable to the dye-receptive surface provides an image having excellent optical density.

A further aspect of the invention provides a method of producing a dye-image on a dye-receptive surface which comprises placing a dye-image donor sheet comprising an image layer having a thermally transferable dye defining an image to be transferred, in contact with the dye-receptive surface, heating the said sheet to effect thermal transfer of the dye-image to the said surface wherein at least 20Z, and preferably at least 40Z of the dye in the image layer is transferred to the said surface.

The invention also provides a method of producing a dye-image on a dye-receptive surface which comprises placing a dye-image donor sheet comprising an image layer having a thermally transferable dye defining an image to be transferred, in contact with the dye-receptive surface, heating the said sheet to effect thermal transfer of the dye image to the second surface wherein the optical density of the dye image on the dye-receptive surface is at least 202 and preferably at least 402 of the optical density of the dye image in the image layer prior to the thermal transfer. A further aspect of the invention provides for the use of a receiver sheet according to the invention having in its receiver layer, a thermally transferable dye which defines an image as a dye image donor sheet. Yet another aspect of the invention provides a method of producing a dye image on a dye receptive surface which comprises placing a receiver sheet according to the invention having in its receiver layer, a thermally transferable dye which defines an image, in contact with the said surface, heating the receiver sheet to effect thermal transfer of the dye image to the said surface and separating the receiver sheet and the dye-receptive surface.

In effecting thermal transfer of the dye image to the dye-receptive surface, part of the dye, when heated, may move in a direction towards the substrate of the image donor sheet rather than transfer to the dye-receptive surface; this effect is herein referred to as back-diffusion.

Suitably, the dye-image donor sheet comprises means for reducing back-diffusion of the dye image. Such means desirably comprises a physical barrier for example the substrate of the image donor sheet or a separate sub-layer interposed between the substrate and the receiver layer which substrate or sub-layer is less permeable to the dye than the receiver layer and which is preferably substantially impermeable to the said dye.

The receiver sheet/image donor sheet may also comprise a backcoat on the opposite side to the receiver layer to improver handling properties. Such a backcoat may comprise a cross-linked polymer matrix and suitably has a surface texture. Examples of suitable backcoats are disclosed in EP-A-409526 and our co-pending European Patent Application 92300627.4. The invention also provides a method of producing a receiver sheet which comprises providing a temperature sensitive substrate, coating onto one side of the substrate, a curable composition comprising a polyester, a cross-linking agent and a release agent in a liquid vehicle, removing substantially all of the liquid vehicle to provide an uncured layer if the said composition and curing the said composition by heating to a temperature of not more than 115°C and preferably not more than 110°C.

Application of the image layer onto the substrate suitably comprises applying a composition comprising the components of the image layer in a liquid vehicle, for example a conventional coating solvent, by means of a conventional coating technique for example by roll, gravure, reverse-gravure, bead and screen coating.

The receiver layer may be cured rapidly and still provide an acceptable degree of curing. It is known to cure a receiver layer using a line speed of for example 6 metres per minute (mpm) . However the composition which provides the receiver layer in receiver sheets of the present invention may be cured at higher line speeds due to the rapid curing of the components of the composition. If desired, the receiver layer is cured at a line speed of at least lOmpm, preferably at least 15 mpm, for example 25mpm.

The rapid low temperature cure simplifies the manufacture procedure, renders it more efficient and moreover avoids the exposure of the substrate to elevated temperatures for prolonged periods.

Dyes typically employed in thermal transfer printing are cyan, yellow or magenta in colour and the proportion of dye transferred may vary depending on the particular dye. In the case of re-transfer, suitably at least 202 of the cyan dye, at least 402 of the yellow dye and at least 402 of the magenta dye in the image to be transferred is transferred to the dye receptive surface. Most preferably, similar proprtions of cyan, yellow and magenta are transferred as this allows the colour balance of the image to be maintained on the dye-receptive surface.

The significant proportion of dye transferred provides the advantageous practical benefit of requiring less dye to secure an image of comparable optical density to that achievable with a donor sheet having an image layer of greater thickness but in which a lower proportion of the dye is trans erable.

A further benefit of requiring less dye in the image donor sheet is that the image layer is not saturated with dye when the dye image is formed in it. This permits the image donor sheet to be marketed as a print itself if desired thus allowing increased user flexibility. Transfer of the dye image from the image donor sheet to the dye-receptive surface is suitably effected in a printing press by heating the donor sheet and dye-receptive surface to a temperature of at least 120°C, and preferably 150 to 250°C. Suitably transfer of the dye image is effected under elevated pressure and preferably at least 1 bar gauge, particularly 2 to 3 bar gauge to provide improved intimate contact between the dye-image donor sheet and the dye-receptive surface thus reducing the risk of uneven transfer of the dye image. Transfer of the dye image to the dye-receptive surface is believed to be a dynamic process and thus the transfer is suitably effected for sufficient time for the transfer of the dye between the image donor sheet and the said surface to approach equilibrium. Desirably, the transfer is effected for at least 1 minute and preferably 1.5 to 3 minutes. The time required to effect suitable dye transfer will depend on several factors including the nature of the image layer, the dye-receptive surface, the dye(s) and the temperature and pressure of transfer and be selected accordingly to provide an image of the desired optical density. The substrate may comprise material conventionally used as a substrate including thermoplastic films for example polyethylene terephthalate and polyolefins which may be filled or voided as desired for example pearl film. A particularly preferred substrate is cellulose fibre paper, especially a cellulose fibre paper having a layer of polyolefin, for example polyethylene and polypropylene, on both sides of the cellulose core, as this allows the user the option of using the image donor sheet as a print for greetings cards, post cards and the like.

The dye-receptive surface may be any material suitable for receiving thermally transferable dye, examples of which include polyester fibres and cross linked epoxy resin which may be applied as a coating on an article to be imaged.

The dye-receptive surface to which the image from the image donor sheet is to be transferred may be a surface of a dye-receptive sheet, but is suitably the surface of an article for example a ceramic or textile article. A further aspect of the invention provides an imaged article which comprises an article comprising a dye-receptive surface in which is located a thermally transferable dye defining an image wherein the dye has been applied to said article by placing an image donor sheet comprising an image layer containing said dye in contact with the article, heating the said sheet and said article to effect thermal transfer of the dye to the article and separating the sheet and article wherein the image layer has a thickness of no more than 2 μm.

The shape of an article to which the image is to be transferred is not critical but the article should be constructed of a material that is capable of withstanding the conditions under which it is to receive the image.

Ceramic articles for example plates, cups and pots and textile articles, for example shirts and hats, are particularly suitable for receiving an image in a method according to the present invention.

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

Example 1

A series of receiver sheets were produced by coating onto a cellulose fibre paper substrate coated on both sides with polyethylene, a composition comprising the following components (amounts are in parts by weight) :

VYLON 200 60 (polyester having Tg 47°C-Toyobo)

VYLON 103 40

(polyester having Tg 67°C-Toyobo)

TINϋVIN 900 1

(uv absorber-Ciba Geigy)

TEGOMER HSi2211 1.1

(hydroxylated siloxane-Goldschmidt)

BEETLE BE 692 1.25

(butylated melamine resin-BIP)

BEETLE BE 659 1.4

(butylated benzoguana ine resin-BIP)

Catalyst 0.4 (di-N-butylamine salt of p-toluene sul honic acid)

Toluene/ ethylethyl ketone (50/50) to 52 solids

The sheets were cured at 110°C for 3 minutes at a line speed of lOmpm and differed only in the thickness of the coating applied to the substrate. Thus sheets having a receiver layer of 6μm (comparative example), 4.3μjn (comparative example), and l.Aμm were produced by using Meier K bars 5, 4 and 2 respectively to apply the coating.

An image comprising a line of yellow squares and a line of cyan squares was applied to the dye-image donor sheet by passing the sheet through a Hitachi VY200 thermal printer. Each square for the two dyes was produced using a different print level on the printer (temperature of the thermal stimuli to effect dye transfer was increased for each square) . The imaged receiver sheets were then used as dye-image donor sheets to transfer the dye image to a dye-receptive surface on a ceramic article. The dye-receptive surface was an epoxy amine resin applied to the article by CACTUS (USA). A 75x75mm piece of each image donor sheet was attached to the article which was then placed in a press (made by RPL (USA)) which comprised two preheated metal blocks having a complementary shape to that of the dye-receptive surface. The article and sheet were then heated in the press for 3 minutes at a temperature of about 218°C. The article was then cooled in air and the image donor sheet removed to reveal the transferred image on the article.

The optical density of the transferred image was determined indirectly using a Sakura densitometer in the reflection mode by measuring the optical density of the image on the image donor sheet prior to and after the image transfer process and recording the difference in optical density as a percentage based on the pre-transfer optical density. The results are shown in Table 1.

Table 1 Image layer thickness ( m)

Print level; 6

Optical density before transfer: Optical density after transfer:

Reduction in optical density (2):

Print level: 7

Optical density before transfer:

Optical density after transfer: Reduction in optical density (2):

Print level: 8

Optical density before transfer:

Optical density after transfer:

Reduction in optical density (2):

comparative example

The results illustrate that a significant improvement in optical densi of the transferred image may be obtained with receiver sheets according to the present invention.

The dye sheet and the dye-receptive surface of the article to be image both readily separated from the receiver sheet and there was no evidence of melt bonding of the receiver sheet to the dye sheet or the dye-receptive surface. Example 2

Receiver sheets according to the present invention were produced by coating onto a cellulose fibre paper substrate coated on both sides with polyethylene, a composition comprising the following components (amounts ar in parts by weight): Composition 2A Composition 2B Composition

VYLON 290 (polyester-Toyobo) 100

VYLON 200 (polyester-Toyobo) 60

VYLON 103 (polyester-Toyobo) 100 40

TINUVIN 900 1 1 1

(uv absorber-Ciba Geigy)

TEGOMER HSi2211 2.5 2.0 0.6

(hydroxylated siloxane-Goldschmidt)

TEGOMER HSi2111 0.8 0.2

(hydroxylated siloxane-Goldschmidt)

BEETLE BE 692 2.5

(butylated melamine resin-BIP)

BEETLE BE 659 - 2.8 3.5 1.5

(butylated benzoguanamine resin-BIP)

Catalyst 0.8 1.2 0.4

(di-N-butylamine salt of p-toluene sulphonic acid)

Toluene/MEK (50/50) to 52 solids to 52 solids

Toluene/MEK (15/85) to 212 soli

Compositions 2A and 2B were cured at 110°C for 3 minutes at a line sp of lOmpm and composition 2C was cured at 110°C for 1 minute at a line spee of 25mpm.

A receiver sheet having an image layer of composition 2C was tested t assess its performance in various respects as follows:

Release Ageing Performance

An unprinted receiver sheets was placed in a humidity controlled oven, and subjected to accelerated ageing conditions of 45°C at 852 relative hiin.-iri-.t-y (RH) , for a period of 15 days. After this time an image was prin onto the sample using full power black on a range of commercial printers. The print was assessed for total transfer of dye to the print.

Result: No total transfer was observed after 15 days. - 1A -

Print Crystallisation performance

A series of yellow, magenta, cyan and black images, with various pri times were produced on the receiver sheet. The printed sheet was placed a humidity controlled oven, at A5°C/85Z RH for a period of 15 days. Afte this time the print was evaluated for dye crystallisation, otherwise know

"snail tracking" .

Results: The print crystallisation performance was found to be very good

Low Temperature Thermal Transfer Performance

This evaluation was carried out with the use of magenta dye panels a hot roll laminator. The dye panel was placed on the receiver sheet, and sandwich was passed through the laminator, at a specified speed and press

The temperature of the laminator rolls was varied. The optical density of any dye transferred to the receiver was measured at a variety of temperatures.

Results : The performance was found to be acceptable.

Optical Density Build-up

A receiver sheet was printed with yellow, magenta and cyan panels at range of print times. The optical density of the resultant printed recei was measured using on a Sakura densitometer.

Results : Excellent optical density build ups were achieved.

Light Fastness

A receiver sheet was printed with yellow, magenta cyan and black and colour co-ordinates (L*a*b*) were measured. The printed receivers were t placed in a Cl 35 fadometer for 100 hours. The colour co-ordinates were t remeasured. the change in these co-ordinates (Delta E) was then be calculated.

Results: The lightfastness results were very good.

Claims

1. A thermal transfer printing receiver sheet comprising a temperature sensitive substrate having on one side a receiver layer capable of receivi a thermally transferable dye, comprising a polyester, a cross-linking agen and a release agent wherein the receiver layer is curable at a temperature not more than 115°C.

2. A receiver sheet according to claim 1, in which the cross-linking age is an amine resin.

3. A receiver sheet according to claim 2 in which the amine resin is a urea, melamine or guanamine resin.

4. A receiver sheet according to any preceding claim, in which the cross-linking agent is an alkoxylated alkylamine resin wherein the alkoxy moiety has at least three carbon atoms.

5. A receiver sheet according to any preceding claim, in which the cross-linking agent is present in an amount of 0.3 to 10 parts by weight based on 100 parts of the polyester.

6. A receiver sheet according to any preceding claim in which the releas agent is a hydroxylated silicone.

7 A receiver sheet according to claim 6, in which the silicone is a copolymer of a silicone and a polyoxyalkylene.

8 A receiver sheet according to claim 6 or 7, in which the silicone is present in an amount of 0.1 to 10 parts by weight based on 100 parts of th polyester.

9. A method of producing a receiver sheet comprising providing a temperature sensitive substrate, coating, on to one side of the substrate, a curable composition comprising a polyester, a cross-linking agent and a release agent in a liquid vehicle, removing substantially all of the liqui vehicle to provide an uncured layer of the composition and curing said composition by heating to a temperature of not more than 115°C.

10. A method according to claim 9, in which the curing temperature is not more than 110°C.

11. A method of producing a dye image on a dye-receptive surface which comprises placing a dye-image donor sheet comprising an image layer having thermally transferable dye defining an image to be transferred, in contact with the dye-receptive surface, heating the said sheet to effect thermal transfer of the dye image to the dye-receptive surface wherein the said image layer comprises a polyester, a cross-linking agent and a release agent, has a cure temperature of not more than 115°C and a thickness of not more than 2.5 μm.

12. A method according to claim 11 in which the image layer has a thickness of not more than 1.5μm.

13. A method according to claim 11 or 12, in which at least 202 of the dye in the dye image is transferred to the said surface.

14. A receiver sheet according to any of claims 1 to 8, having in its receiver layer, a thermally transferable dye which defines an image, and capable of being used as a dye image donor sheet.