Stabilised dye-donor element for use in thermal dye transfer

Description

[0001] This invention relates to the use of certain stabilizers in dye-donor elements for thermal dye transfer systems, and more particularly to stabilizers for naphthol-p-phenylenediamine cyan dyes in such elements.

[0002] In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta or yellow signal. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. patent 4,621,271.

[0003] An important requirement for any thermal dye-donor element is to maintain performance over its useful lifetime without degradation in the quality of the image. The dye layer of a dye-donor element for resistive head thermal dye transfer generally comprises a polymeric binder and diffusible dyes. The percentage of dye in the layer is typically quite high, in the range of 20 to 80 %. The dye is usually dissolved in the binder or phase-separated into small domains. During keeping of the donor, the temperature and humidity may be elevated and the dye layer is in contact with a slipping layer coated on the back side of the donor element when it is wound up in spool form. The slipping layer may contain mobile lubricating oils or materials which can act as plasticizers or solvents for the dye layer. This enables the dye to become mobile, allowing changes to occur in the layer including further phase separation, migration of the dye to the surface, and even crystallization of the dye. Dye may also transfer to the slipping layer.

[0004] U.S. Patent 4,695,287 relates to dye-donor elements for thermal dye transfer containing a p-phenylenediamine cyan dye. There is a problem with these elements is that the p-phenylenediamine cyan dye may be subject to degradation during storage, when the dye layer comes into contact with humidity from the atmosphere or when the elements are stored at elevated temperatures.

[0005] U.S. Patent 3,868,252 relates to the use of nitroxyl-containing polymers as oxidants in a silver halide-based color diffusion transfer process. However, there is no disclosure that such polymers are useful in a dye-donor element to improve raw stock keeping performance.

[0006] It is an object of this invention to provide addenda for the dye layer of a thermal dye-donor element to assure stabilization of a naphthol-p-phenylenediamine cyan dye present.

[0007] This and other objects are achieved in accordance with this invention which relates to a dye-donor element for thermal dye transfer comprising a support having thereon a dye layer comprising a thermally-transferable naphthol-p-phenylenediamine cyan dye in a polymeric binder, the dye layer also containing a stabilizer, the stabilizer comprising a compound containing a nitroxyl free radical and having a molecular weight of at least 400 or a hydroxylamine moiety and having a molecular weight of at least 330, the stabilizer being present in the amount of 5-10 mole% based on the weight of the dye.

[0008] By use of the stabilizers of the invention, dye degradation of a dye-donor element during raw stock keeping is minimized.

[0009] Nitroxyl-free radical compounds or hydroxylamines useful in the invention can comprise, for example,

wherein R₁=O· or OH and n is from 1 to 15.

[0010] In a preferred embodiment of the invention, n in the above compound is 8 and the compound has the following structure:

Compound 1

[0011] This compound is made using Tinuvin 770® from Ciba-Geigy as a starting material and modifying it by the procedure described in J.Phys.Chem., 97, 1138 (1993), to create the free radical.

[0012] Compounds containing a hydroxylamine moiety useful in the invention can comprise, for example:

wherein R² and R³ each independently represents an alkyl group of from 1 to 15 carbon atoms.

[0013] In another preferred embodiment of the invention, in the above formula, both R² and R³ are t-C₅H₁₁ which would result in the following structure:

Compound 2

[0014] Any naphthol-p-phenylenediamine cyan dye can be used in the dye-donor employed in the invention provided it is transferable to the dye-receiving layer by the action of heat. Examples of such dyes include



The above dyes may be employed singly or in combination. The dyes may be used at a coverage of from about 0.05 to about 1 g/m² and are preferably hydrophobic.

[0015] A dye-barrier layer may be employed in the dye-donor elements of the invention to improve the density of the transferred dye. Such dye-barrier layer materials include hydrophilic materials such as those described and claimed in U. S. Patent 4,716,144.

[0016] The dye layer of the dye-donor element may be coated on the support or printed thereon by a printing technique such as a gravure process.

[0017] Any material can be used as the support for the dye-donor element of the invention provided it is dimensionally stable and can withstand the heat of the thermal head. Such materials include polyesters such as poly(ethylene terephthalate); polyamides; polycarbonates; cellulose esters; fluorine polymers; polyethers; polyacetals; polyolefins; and polyimides. The support generally has a thickness of from about 5 to about 200 µm. It may also be coated with a subbing layer, if desired, such as those materials described in U. S. Patents 4,695,288 or 4,737,486.

[0018] The dye in the dye-donor element of the invention is dispersed in a polymeric binder such as a cellulose derivative, e.g., cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate or any of the materials described in U. S. Patent 4,700,207; a polycarbonate; polyvinyl acetate, poly(styrene-co-acrylonitrile), a poly(sulfone) or a poly(phenylene oxide). The binder may be used at a coverage of from about 0.1 to about 5 g/m².

[0019] The reverse side of the dye-donor element may be coated with a slipping layer to prevent the printing head from sticking to the dye-donor element. Such a slipping layer would comprise either a solid or liquid lubricating material or mixtures thereof, with or without a polymeric binder or a surface active agent. Preferred lubricating materials include oils or semi-crystalline organic solids that melt below 100°C such as poly(vinyl stearate), beeswax, perfluorinated alkyl ester polyethers, poly(caprolactone), silicone oil, poly(tetrafluoroethylene), carbowax, poly(ethylene glycols), or any of those materials disclosed in U. S. Patents 4,717,711; 4,717,712; 4,737,485; and 4,738,950. Suitable polymeric binders for the slipping layer include poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-acetal), poly(styrene), poly(vinyl acetate), cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate or ethyl cellulose.

[0020] The amount of the lubricating material to be used in the slipping layer depends largely on the type of lubricating material, but is generally in the range of about .001 to about 2 g/m². If a polymeric binder is employed, the lubricating material is present in the range of 0.05 to 50 weight %, preferably 0.5 to 40, of the polymeric binder employed.

[0021] The dye-receiving element that is used with the dye-donor element of the invention usually comprises a support having thereon a dye image-receiving layer. The support may be a transparent film such as a poly(ether sulfone), a polyimide, a cellulose ester such as cellulose acetate, a poly(vinyl alcohol-co-acetal) or a poly(ethylene terephthalate). The support for the dye-receiving element may also be reflective such as barytacoated paper, polyethylene-coated paper, an ivory paper, a condenser paper or a synthetic paper such as DuPont Tyvek®. Pigmented supports such as white polyester (transparent polyester with white pigment incorporated therein) may also be used.

[0022] The dye image-receiving layer may comprise, for example, a polycarbonate, a polyurethane, a polyester, polyvinyl chloride, poly(styrene-co-acrylonitrile), poly(caprolactone), a poly(vinyl acetal) such as poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-cobenzal), poly(vinyl alcohol-co-acetal) or mixtures thereof. The dye image-receiving layer may be present in any amount which is effective for the intended purpose. In general, good results have been obtained at a concentration of from about 1 to about 5 g/m².

[0023] As noted above, the dye-donor elements of the invention are used to form a dye transfer image. Such a process is defined by claim 7 and comprises imagewise heating a dye-donor element as described above and transferring a dye image to a dye-receiving element to form the dye transfer image.

[0024] The dye-donor element of the invention may be used in sheet form or in a continuous roll or ribbon. If a continuous roll or ribbon is employed, it may have alternating areas of dyes such as sublimable cyan and/or magenta and/or yellow and/or black or other dyes. Thus, one-, two-, three- or four-color elements (or higher numbers also) are included within the scope of the invention.

[0025] In a preferred embodiment of the invention, the dye-donor element comprises a poly(ethylene terephthalate) support coated with sequential repeating areas of cyan, yellow and magenta, and the above process steps are sequentially performed for each color to obtain a three-color dye transfer image. Of course, when the process is only performed for a single color, then a monochrome dye transfer image is obtained.

[0026] Thermal printing heads which can be used to transfer dye from the dye-donor elements of the invention are available commercially.

[0027] A thermal dye transfer assemblage of the invention comprises

a) a dye-donor element as described above, and

b) a dye-receiving element as described above, the dye-receiving element being in a superposed relationship with the dye-donor element so that the dye layer of the donor element is in contact with the dye image-receiving layer of the receiving element.

[0028] The above assemblage comprising these two elements may be preassembled as an integral unit when a monochrome image is to be obtained. This may be done by temporarily adhering the two elements together at their margins. After transfer, the dye-receiving element is then peeled apart to reveal the dye transfer image.

[0029] When a three-color image is to be obtained, the above assemblage is formed three times using different dye-donor elements. After the first dye is transferred, the elements are peeled apart. A second dye-donor element (or another area of the donor element with a different dye area) is then brought in register with the dye-receiving element and the process repeated. The third color is obtained in the same manner.

[0030] The following examples are provided to illustrate the invention.

Example 1

[0031] The following control stabilizer materials were used in the experimental work:

[0032] Cyan dye donor samples for incubation testing were prepared by coating onto an unsubbed 100 µm poly(ethylene terephthalate) support a dye layer containing 0.27 g/m² of cyan dye C-1 above and various addenda to be tested in the amounts indicated in Table 1, in a cellulose acetate propionate binder (2.5% acetyl, 45% propionyl) from a toluene, methanol, cyclopentanone (70:25:5 wt-ratio) solvent mixture.

[0033] These samples were incubated against bare Estar® (Eastman Chemical Co.) for six weeks at 60°C/50% RH. Samples of each were also stored at 0°C as a check condition. After incubation, each sample was analyzed by high performance liquid chromatography (HPLC) to determine the change in cyan dye coverage. The cyan dye concentration was determined from external standards and converted to dye coverage. The amount of dye remaining for each sample was normalized against the result for Compound 1 in each case at 5 mole %. (Compound 1 was thus 1.0, meaning that there was no dye degradation). A relative ranking for dye stability as compared to Compound 1 for each sample was thus obtained as follows:

TABLE 1

Stabilizer	Coverage of Addendum (mole % of Dye)	Molecular Weight of Addendum	Relative Ranking for Dye Stability
None	0	-	0.42
Compound 1	1	510	0.56
Compound 1	5	510	1.00
Compound 1	10	510	1.01
Control 3	10	156	0.57
Control 11	10	480	0.32

[0034] The above results show that Compound 1 is effective at levels from about 5 to 10 mole % with respect to the dye content. A lower molecular weight nitroxyl analog, Control 3, as well as the free amine Control 11 were found to be ineffective, even when used at levels of 10 mole %.

Example 2

[0035] This example was similar to Example 1 to show compounds of the invention compared to various other types of free radical compounds, including nitroxyl, hydrazyl, and galvinoxyl free radical-carrying compounds. The stabilizers were all used at 5 mole % of dye. The following results were obtained:

TABLE 2

Stabilizer	Molecular Weight (g/mole)	Type of Compound	Relative Ranking for Dye Stability
None	-	-	0.34
Compound 1	510	nitroxyl	1.00
Compound 2	446	hydroxylamine	0.74
Control 5	186	nitroxyl	0.38
Control 4	141	nitroxyl	0.46
Control 2	185	nitroxyl	0.44
Control 1	183	nitroxyl	0.49
Control 6	422	carboxyl	0.41
Control 7	394	hydrazyl	0.40

[0036] The above results show that, when compared against other types of free radical carriers, the compounds of the invention are superior in their dye stabilization effectiveness.

Example 3

[0037] This example was similar to Example 2 to show compounds of the invention compared to various other commonly used antioxidants or stabilizers. The following results were obtained:

TABLE 3

Stabilizer	Relative Ranking for Dye Stability
None	0.54
Compound 1	1.00
Control 3	0.47
Control 13	0.40
Control 11	0.46
Control 12	0.44
Control 10	0.49
Control 8	0.47
Control 9	0.44
Control 14	0.42

[0038] The above results show that, when compared against other types of free radical carriers, the compounds of the invention are superior in their dye stabilization effectiveness.

Claims

1. A dye-donor element for thermal dye transfer comprising a support having thereon a dye layer comprising a thermally-transferable naphthol-p-phenylenediamine cyan dye in a polymeric binder, said dye layer also containing a stabilizer, said stabilizer comprising a compound containing a nitroxyl free radical and having a molecular weight of at least 400 or a hydroxylamine moiety and having a molecular weight of at least 330, said stabilizer being present in the amount of 5-10 mole% based on the weight of the dye.

2. The element of Claim 1 wherein said stabilizer is

wherein R₁=O· or OH and n is from 1 to 15.

3. The element of Claim 2 wherein n is 8.

4. The element of Claim 1 wherein said stabilizer is

wherein R² and R³ each independently represents an alkyl group of from 1 to 15 carbon atoms.

5. The element of Claim 4 wherein both R² and R³ are t-C₅H₁₁.

6. The element of Claim 1 wherein the side of the support opposite the side containing said dye layer has a slipping layer thereon.

7. A process of forming a thermal dye transfer image comprising:

a) contacting at least one dye-donor element as defined in any of claims 1 to 6, with a dye-receiving element comprising a support having, thereon a polymeric dye image-receiving layer the dye receiving element being in a superposed relationship with the dye-donor element so that the dye layer of the dye-donor element is in contact with the dye-receiving layer of the dye receiving element;

b) imagewise-heating said dye-donor element; and thereby

c) transferring a dye image to said dye-receiving element to form said thermal dye transfer image.

8. The process of Claim 7 wherein said dye-donor element is as defined in claims 2 or 4.

9. A thermal dye transfer assemblage comprising:

(a) a dye donor element as defined in any of the claims 1 to 6, and

(b) a dye-receiving element comprising a support having thereon a dye image-receiving layer,
said dye-receiving element being in superposed relationship with said dye-donor element so that said dye layer is in contact with said dye image-receiving layer.

10. The assemblage of Claim 9 wherein said dye-donor element is as defined in claims 2 or 4.

Ansprüche

1. Farbstoff-Donorelement für eine thermische Farbstoff-Übertragung, umfassend einen Träger, der darauf eine Farbstoffschicht aufweist, welche einen thermisch übertragbaren Naphthol-p-phenylendiamin-Cyanfarbstoff in einem polymeren Bindemittel umfaßt, wobei die Farbstoffschicht auch einen Stabilisator enthält, wobei der Stabilisator eine Verbindung umfaßt, die ein freies Nitroryl-Radikal enthält und ein Molekulargewicht von mindestens 400 aufweist oder eine Hydroxyl-Einheit enthält und ein Molekulargewicht von mindestens 330 aufweist, wobei der Stabilisator in einer Menge von 5 - 10 Mol-%, bezogen auf das Gewicht des Farbstoffes, anwesend ist.

2. Element nach Anspruch 1, in welchem der Stabilisator

ist, worin R₁ = O· oder OH und n 1 bis 15 beträgt.

3. Element nach Anspruch 1, in welchem n 8 beträgt.

4. Element nach Anspruch 1, in welchem der Stabilisator

ist, worin R² und R³ unabhängig eine Alkylgruppe mit 1 bis 15 Kohlenstoffatomen darstellen.

5. Element nach Anspruch 4, in welchem sowohl R² als auch R³ t-C₅H₁₁ ist.

6. Element nach Anspruch 1, in welchem die Seite des Trägers, welche entgegengesetzt zu der Seite liegt, die die Farbstoffschicht enthält, darauf eine Gleitschicht aufweist.

7. Verfahren zur Erzeugung eines Farbstoff-Thermotransfer-Bildes, umfassend:

a) Inkontaktbringen mindestens eines Farbstoff-Donorelementes, wie in irgendeinem der Ansprüche 1 bis 6 definiert, mit einem Farbstoff-Empfangselement, welches einen Träger umfaßt, der darauf eine polymere Farbstoffbild-Empfangsschicht aufweist, wobei das Farbstoff-Empfangselement in überlagerter Beziehung mit dem Farbstoff-Donorelement steht, so daß die Farbstoffschicht des Farbstoff-Donorelementes in Kontakt mit der Farbstoff-Empfangsschicht des Farbstoff-Empfangselementes steht;

b) bildweises Erwärmen des Farbstoff-Donorelementes; und dadurch

c) Übertragen eines Farbstoffbildes auf das Farbstoff-Empfangselement, um das Farbstoff-Thermotransfer-Bild zu erzeugen.

8. Verfahren nach Anspruch 7, in welchem das Farbstoff-Donorelement wie in den Ansprüchen 2 oder 4 definiert ist.

9. Zusammenstellung zur thermischen Farbstoff-Übertragung, umfassend:

(a) ein Farbstoff-Donoerelement, wie in irgendeinem der Ansprüche 1 bis 6 definiert, und

(b) ein Farbstoff-Empfangselement, welches einen Träger umfaßt, der darauf eine Farbstoffbild-Empfangsschicht aufweist, wobei das Farbstoff-Empfangselement in überlagerter Beziehung mit dem Farbstoff-Donorelement steht, so daß die Farbstoffschicht in Kontakt mit der Farbstoffbild-Empfangsschicht steht.

10. Zusammenstellung nach Anspruch 9, in welcher das Farbstoff-Donorelement wie in den Ansprüchen 2 oder 4 definiert ist.

Revendications

1. Elément donneur de colorant utilisé dans un procédé de transfert de colorant par la chaleur comprenant un support revêtu d'une couche de colorant comprenant un colorant cyan naphtol-p-phénylènediamine capable d'être transféré par la chaleur dans un liant polymère, ladite couche de colorant contenant également un agent stabilisant, ledit agent stabilisant comprenant un composé contenant un radical libre nitroxyle et ayant un poids moléculaire d'au moins 400 ou un radical hydroxylamine et ayant un poids moléculaire d'au moins 300, ledit agent stabilisant étant utilisé selon une quantité de 5 à 10% en moles par rapport au poids du colorant.

2. Elément selon la revendication 1, dans lequel ledit agent stabilisant est :

où :

R₁ = O· ou OH et n est de 1 à 15.

3. Elément selon la revendication 2, dans lequel n est 8.

4. Elément selon la revendication 1, dans lequel ledit agent stabilisant est :

où R² et R³ représentent chacun indépendamment un groupe alkyle de 1 à 15 atomes de carbone.

5. Elément selon la revendication 4, dans lequel R² et R³ sont t-C₅H₁₁.

6. Elément selon la revendication 1, dans lequel le côté du support opposé au côté contenant ladite couche de colorant est revêtu d'une couche favorisant le glissement.

7. Procédé de formation d'une image par transfert de colorant par la chaleur, comprenant :

a) la mise en contact d'au moins un élément donneur de colorant tel que défini dans l'une quelconque des revendications 1 à 6, avec un élément récepteur de colorant comprenant un support revêtu d'une couche réceptrice d'image de colorant polymère, ledit élément récepteur de colorant étant superposé à l'élément donneur de colorant, de manière que la couche de colorant de l'élément donneur de colorant soit en contact avec la couche réceptrice de colorant de l'élément récepteur de colorant ;

b) le chauffage, en conformité avec l'image, dudit élément donneur de colorant ; et

c) le transfert d'une image de colorant sur ledit élément récepteur de colorant, afin de former ladite image par transfert de colorant par la chaleur.

8. Procédé selon la revendication 7, dans lequel ledit élément donneur de colorant est tel que défini dans les revendications 2 ou 4.

9. Assemblage pour le transfert de colorant par la chaleur, comprenant :

(a) un élément donneur de colorant tel que défini dans l'une quelconque des revendications 1 à 6, et

(b) un élément récepteur de colorant comprenant un support revêtu d'une couche réceptrice d'image de colorant, ledit élément récepteur de colorant étant superposé audit élément donneur de colorant, de manière que ladite couche de colorant soit en contact avec ladite couche réceptrice d'image de colorant.

10. Assemblage selon la revendication 9, dans lequel ledit élément donneur de colorant est tel que défini dans les revendications 2 ou 4.