Thermal dye transfer assemblage with low Tg polymeric receiver mixture

Abstract

A thermal dye transfer assemblage comprising:(I) a dye-donor element comprising a support having thereon a dye layer comprising a dye dispersed in a polymeric binder, the dye being a deprotonated cationic dye which is capable of being reprotonated to a cationic dye having a N-H group which is part of a conjugated system, and(II) 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 is in contact with the polymeric dye image-receiving layer, the polymeric dye image-receiving layer comprising a mixture ofa) a polymer having a Tg of less than about 19.degree. C. and having no or only slight acidity; andb) sub-micron, non film-forming, organic, acidic particles which are capable of reprotonating the cationic dye transferred to the dye image-receiving layer from the dye layer.

Description

FIELD OF THE INVENTION

This invention relates to a thermal dye transfer receiver element of a thermal dye transfer assemblage and, more particularly, to a polymeric dye image-receiving layer containing a mixture of materials capable of reprotonating a deprotonated cationic dye transferred to the receiver from a suitable donor.

BACKGROUND OF THE INVENTION

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 one of the cyan, magenta or yellow signals, and 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. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.

Dyes for thermal dye transfer imaging should have bright hue, good solubility in coating solvents, good transfer efficiency and good light stability. A dye receiver polymer should have good affinity for the dye and provide a stable (to heat and light) environment for the dye after transfer. In particular, the transferred dye image should be resistant to damage caused by handling, or contact with chemicals or other surfaces such as the back of other thermal prints, adhesive tape, and plastic folders such as poly(vinyl chloride), generally referred to as "retransfer".

Commonly-used dyes are nonionic in character because of the easy thermal transfer achievable with this type of compound. The dye-receiver layer usually comprises an organic polymer with polar groups to act as a mordant for the dyes transferred to it. A disadvantage of such a system is that since the dyes are designed to be mobile within the receiver polymer matrix, the prints generated can suffer from dye migration over time.

A number of attempts have been made to overcome the dye migration problem which usually involves creating some kind of bond between the transferred dye and the polymer of the dye image-receiving layer. One such approach involves the transfer of a cationic dye to an anionic dye-receiving layer, thereby forming an electrostatic bond between the two. However, this technique involves the transfer of a cationic species which, in general, is less efficient than the transfer of a nonionic species.

In one type of thermal dye transfer printing, deprotonated nonionic dyes may be transferred to an acid-containing receiver where a reprotonation process may take place to convert the dyes to their protonated form by interaction with the acid moiety in the dye-receiving layer. The dyes are thus rendered cationic. As a consequence, the transferred dyes are anchored in the receiving layer and form a strong electrostatic bond. The reprotonation reaction also causes a hue shift of the transferred dyes from their deprotonated form to their protonated form. In a practical sense, it is always desirable to complete this protonation process as fast as possible at a rate known as the dye conversion rate.

DESCRIPTION OF RELATED ART

U.S. Pat. Nos. 5,523,274 and 5,534,479 relate to the transfer of a deprotonated cationic dye to a dye image-receiving layer containing an organic acid moiety as part of a polymer chain having a Tg of less than 25.degree. C. which is capable of reprotonating the deprotonated cationic dye. However, there is a problem when using a receiver layer containing only an acid-containing polymer in that the dye will bind strongly to these acid sites as soon as it is transferred to the receiver and will tend to stratify at the surface of the receiver. This results in poor or low density images and poor image keeping.

U.S. Pat. No. 5,627,128 relates to the transfer of a deprotonated cationic dye to a polymeric dye image-receiving layer comprising a mixture of an organic polymeric or oligomeric acid which is capable of reprotonating the deprotonated cationic dye and a polymer having a Tg of less than about 19.degree. C. and having no or only slight acidity. There is a problem with this polymer mixture in that the rate of reprotonation of the deprotonated cationic dyes is not as fast as one would like it to be.

It is an object of this invention to provide a thermal dye transfer assemblage which will reprotonate a deprotonated cationic dye transferred to the receiver of the assemblage. It is another object of the invention to provide a thermal dye transfer assemblage which has a receiver with an improved dye conversion rate and which has good physical properties, water resistance, and reduced sensitivity in densitometric and dye conversion rate variability to the humidity of receiver raw stock storage and printing.

SUMMARY OF THE INVENTION

These and other objects are achieved in accordance with this invention which relates to a thermal dye transfer assemblage comprising:

(I) a dye-donor element comprising a support having thereon a dye layer comprising a dye dispersed in a polymeric binder, the dye being a deprotonated cationic dye which is capable of being reprotonated to a cationic dye having a N-H group which is part of a conjugated system, and (II) 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 is in contact with the polymeric dye image-receiving layer, the polymeric dye image-receiving layer comprising a mixture of a) a polymer having a Tg of less than about 19.degree. C. and having no or only slight acidity; and b) sub-micron, non film-forming, organic, acidic particles which are capable of reprotonating the cationic dye transferred to the dye image-receiving layer from the dye layer.

The receiver mixture of the present invention which contains sub- micron, non film-forming, organic, acidic particles ensures that the acidic moiety will be the dispersed phase within the low-Tg polymer binder, thus rendering the receiver mixture less sensitive to changes in environmental humidity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sub-micron, non film-forming, organic, acidic particles employed in the invention have enough strong acid sites to effect protonation of the dye and are non film-forming at the temperature of coating and drying of the element. These particles have an acid group, such as sulfonic acid, carboxylic acid, or phosphoric acid, which is capable of protonating the dye. The particles may be used in an amount of from about 1 to about 90 wt. % of the receiver layer, preferably from about 5 to 50 wt. % of the receiver layer, and most preferably from about 10 to 40 wt. % of the receiver layer.

The sub-micron, non film-forming, organic, acidic particles useful in the invention include addition type polymers or copolymers prepared from ethylenically unsaturated monomers such as acrylates, including acrylic acid, methacrylates, including methacrylic acid, acrylamides, methaclylamides, itaconic acid, styrenes, including substituted styrenes, styrene-sulfonic acid, acrylonitrile, methacrylonitrile, vinyl acetates, vinylidene halides, and olefins and substituted olefins. Other polymers include water-dispersible polyesters, polyurethanes, polyamides, and epoxies. For these non-film-forming polymers, the Tg of the polymer or copolymer is preferably above the coating and drying temperatures used during the coating procedure.

Typical coating and drying conditions for the receiver can be varied but need to be high enough to ensure evaporation of the solvent or water employed. Reasonable lower limits are about 50-60.degree. C. The upper limit will depend on the capabilities of the machine. Thus, the non film-forming acidic particles in general have a Tg greater than about 60.degree. C., preferably greater than about 80.degree. C., to ensure that the particles do not flow and coalesce during the drying. The Tg of the polymer is defined herein as the midpoint in the change in heat capacity measured by differential scanning calorimetry at 20.degree. C./min.

In addition, lower Tg polymer and copolymer acidic particles can be rendered non film-forming and useful in the invention by incorporating crosslinking and graft-linking monomers into them such as divinyl benzene, 1,4-butyleneglycol methacrylate, trimethylpropane triacrylate, ethyleneglycol dimethacrylate, bisphenol A dimethacrylate, 1,3-butanediol diacrylates and dimethacrylates, N-N-diallylmethacrylamide, N,N-divinylaniline, N,N-methylenebisacrylamide, etc.

In order to minimize light scattering effects that may lead to haziness of the coatings, the acidic particles preferably have an average particle size less than about 0.5 .mu.m, preferably less than about 0.1 .mu.m.

Examples of sub-micron, non film-forming, organic, acidic particles useful in the invention include:

A-1 Random copolymer of methyl methacrylate (92 wt. %) and acrylic acid (8 wt. %), Tg=117.degree. C., size: 53 nm A-2 Random copolymer of methyl methacrylate (91 wt. %) and �1-propanesulfonic acid, 2-methyl-2-( (1-oxo-2-propenyl)amino)! (9 wt. %),Tg=106.degree. C., size: 43 nm A-3 Random copolymer of methyl methacrylate (83 wt. %) and �1-propanesulfonic acid, 2-methyl-2-( (1-oxo-2-propenyl)amino)! (17 wt. %),Tg=106.degree. C., size:49 nm A-4 Random copolymer of methyl methacrylate (84 wt. %) and �1-propanesulfonic acid, 2-methyl-2-((1-oxo-2-propenyl)amino)! (15 wt. %), crosslinked with 1 wt. % ethyleneglycol dimethacrylate, Tg=109.degree. C., size:69 nm A-5 Random copolymer of methyl methacrylate (66 wt. %) and �1 -propanesulfonic acid, 2-methyl-2-((1-oxo-2-propenyl)amino)! (33 wt. %), crosslinked with 1 wt. % ethyleneglycol dimethacrylate, Tg=101.degree. C., size: 62 nm A-6 Random copolymer of styrene (66 wt. %) and p-styrene sulfonic acid (33 wt. %), crosslinked with 1 wt. % divinyl benzene, Tg=106.degree. C., size: 58 nm A-7 Random copolymer of styrene (49.5 wt. %) and p-styrene sulfonic acid (49.5 wt. %), crosslinked with 1 wt % divinyl benzene, size: 80 nm A-8 Random copolymer of butyl acrylate (68 wt. %) and �1-propanesulfonic acid, 2-methyl-2-((1-oxo-2-propenyl)amino)!(30 wt. %), crosslinked with 2 wt. % ethyleneglycol dimethacrylate, Tg=-43.degree. C., size: 53 nm A-9 Random copolymer of butyl acrylate (65 wt %) and �1-propanesulfonic acid, 2-methyl-2-((1-oxo-2-propenyl)amino)!(33 wt. %), crosslinked with 2 wt. % ethyleneglycol dimethacrylate, Tg=-44.degree. C., size: 49 nm

The particle size of the above acidic particles is the mean measured by photocorrelation spectroscopy or microtrac Ultrafine Particle Analyzer (UPA).

Deprotonated cationic dyes useful in the invention which are capable of being reprotonated to a cationic dye having a N-H group which is part of a conjugated system are described in U.S. Pat. No. 5,523,274, the disclosure of which is hereby incorporated by reference.

In a preferred embodiment of the invention, the deprotonated cationic dye employed in the invention and the corresponding cationic dye having a N-H group which is part of a conjugated system have the following structures: ##STR1## wherein:

X, Y and Z form a conjugated link between nitrogen atoms selected from CH, C-alkyl, N, or a combination thereof, the conjugated link optionally forming part of an aromatic or heterocyclic ring;

R represents a substituted or unsubstituted alkyl group from about 1 to about 10 carbon atoms;

R.sup.1 and R.sup.2 each individually represents a substituted or unsubstituted phenyl or naphthyl group or a substituted or unsubstituted alkyl group from about 1 to about 10 carbon atoms; and

n is an integer of from 0 to 11.

The deprotonated cationic dyes according to the above formula are disclosed in U.S. Pat. Nos. 4,880,769, 4,137,042 and 5,559,076, and in K. Venkataraman ed., The Chemistry of Synthetic Dyes, Vol. IV, p. 161, Academic Press, 1971, the disclosures of which are hereby incorporated by reference. Specific examples of such dyes include the following (the X max values and color descriptions in parentheses refer to the dye in its protonated form): ##STR2##

The dyes described above may be employed in any amount effective for the intended purpose. In general, good results have been obtained when the dye is present in an amount of from about 0.05 to about 1.0 g/m.sup.2, preferably from about 0.1 to about 0.5 g/m.sup.2. Dye mixtures may also be used.

Any type of polymer may be employed in the receiver of the invention, e.g., condensation polymers such as polyesters, polyurethanes, polycarbonates, etc.; addition polymers such as polystyrenes, vinyl polymers, acrylic polymers, etc.; block copolymers containing large segments of more than one type of polymer covalently linked together; or blends thereof, provided such polymeric material has the low Tg as described above. In a preferred embodiment of the invention, the dye image-receiving layer comprises an acrylic polymer, a styrene polymer, a polyester or a vinyl polymer or mixtures thereof. These polymers having a Tg of less than about 19.degree. C. employed in the invention may contain groups which are slightly acidic to improve water dispersibility. However, these acid groups are generally insufficient to protonate the dye.

Following are examples of low Tg polymers that may be used in the invention:

Polymer P-1: poly(butyl acrylate-co-allyl methacrylate) 98:2 wt core/poly(glycidyl methacrylate) 10 wt shell, (Tg=-40.degree. C.) Polymer P-2: poly(butyl acrylate-co-allyl methacrylate) 98:2 wt core/poly(ethyl methacrylate) 30 wt shell, (Tg=-41.degree. C.) Polymer P-3: poly(butyl acrylate-co-allyl methacrylate) 98:2 wt core/poly(2-hydroxypropyl methacrylate) 10 wt shell, (Tg=-40.degree. C.) Polymer P-4: poly(butyl acrylate-co-ethylene glycol dimethacrylate) 98:2 wt core/poly(glycidyl methacrylate 10 wt shell, Tg=-42.degree. C.) Polymer P-5: poly(butyl acrylate-co-allyl methacrylate-co-glycidyl methacrylate) 89:2:9 wt, (Tg=-34.degree. C.) Polymer P-6: poly(butyl acrylate-co-ethylene glycol dimethacrylate-co-glycidyl methacrylate) 89:2:9 wt (Tg=-28.degree. C.) Polymer P-7: poly(butyl methacrylate-co-butyl acrylate-co-allyl methacrylate) 49:49:2 wt core/poly(glycidyl methacrylate) 10 wt shell, (Tg=-18.degree. C.) Polymer P-8: poly(methyl methacrylate-co-butyl acrylate-co-2-hydroxyethyl methacrylate-co-2-sulfoethyl methacrylate sodium salt) 30:50:10:10 wt, (Tg==3.degree. C.) Polymer P-9: poly(methyl methacrylate-co-butyl acrylate-co-2-hydroxyethyl methacrylate-co-styrenesulfonic acid sodium salt) 40:40:10:10 wt, (Tg=0.degree. C.) Polymer P-10: poly(methyl methacrylate-co-butyl acrylate-co-2-sulfoethyl methacrylate sodium salt-co-ethylene glycol dimethacrylate) 44:44:10:2 wt, (Tg=14.degree. C.) Polymer P-11: poly(butyl acrylate-co-Zonyl TM.RTM.-co-2-acrylamido-2-methyl-propanesulfonic acid sodium salt) 50:45:5 wt (Tg=-39.degree. C.) (Zonyl TM.RTM. is a monomer from the DuPont Company) Polymer P-12: XU31066.50 (experimental polymer based on a styrene butadiene copolymer from Dow Chemical Company) (Tg=-31.degree. C.) Polymer P-13: AC540.RTM. nonionic emulsion (Allied Signal Co.) (Tg=-55.degree. C.) Polymer P-14: a polyester having the formula: ##STR3## wherein R is polyethylene glycol having a molecular weight of 200, the polyester having a Tg of -22.degree. C.

The polymer in the dye image-receiving layer may be present in any amount which is effective for its intended purpose. In general, good results have been obtained at a concentration of from about 0.5 to about 20 g/m.sup.2. The polymers may be coated from organic solvents or water, if desired.

The support for the dye-receiving element employed in the invention may be transparent or reflective, and may comprise a polymeric, synthetic or cellulosic paper support, or laminates thereof. Examples of transparent supports include films of poly(ether sulfone)s, poly(ethylene naphthalate), polyimides, cellulose esters such as cellulose acetate, poly(vinyl alcohol-co-acetal)s, and poly(ethylene terephthalate). The support may be employed at any desired thickness, usually from about 10 .mu.m to 1000 .mu.m. Additional polymeric layers may be present between the support and the dye image-receiving layer. For example, there may be employed a polyolefm such as polyethylene or polypropylene. White pigments such as titanium dioxide, zinc oxide, etc., may be added to the polymeric layer to provide reflectivity. In addition, a subbing layer may be used over this polymeric layer in order to improve adhesion to the dye image-receiving layer. Such subbing layers are disclosed in U.S. Pat. Nos. 4,748,150, 4,965,238, 4,965,239, and 4,965241, the disclosures of which are incorporated by reference. The receiver element may also include a backing layer such as those disclosed in U.S. Pat. Nos. 5,011,814 and 5,096,875, the disclosures of which are incorporated by reference. In a preferred embodiment of the invention, the support comprises a microvoided thermoplastic core layer coated with thermoplastic surface layers as described in U.S. Pat. No. 5,244,861, the disclosure of which is hereby incorporated by reference.

Resistance to sticking during thermal printing may be enhanced by the addition of release agents to the dye-receiving layer or to an overcoat layer, such as silicone-based compounds, as is conventional in the art.

Any material can be used as the support for the dye-donor element employed in the invention, provided it is dimensionally stable and can withstand the heat of the thermal print heads. Such materials include polyesters such as poly(ethylene terephthalate); polyamides; polycarbonates; glassine paper; condenser paper; cellulose esters such as cellulose acetate; fluorine polymers such as poly(vinylidene fluoride) or poly(tetrafluoroethylene-co-hexafluoropropylene); polyethers such as polyoxymethylene; polyacetals; polyolefins such as polystyrene, polyethylene, polypropylene or methylpentene polymers; and polyimides such as polyimide amides and polyetherimides. The support generally has a thickness of from about 2 to about 30 .mu.m.

Dye-donor elements that are used with the dye-receiving element of the invention conventionally comprise a support having thereon a dye layer containing the dyes as described above 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. Pat. No. 4,700,207; or a poly(vinyl acetal) such as poly(vinyl alcohol-co-butyral). The binder may be used at a coverage of from about 0.1 to about 5 g/m.sup.2.

As noted above, dye-donor elements are used to form a dye transfer image. Such a process comprises imagewise-heating a dye-donor element and transferring a dye image to a dye-receiving element as described above to form the dye transfer image.

In a preferred embodiment of the invention, a dye-donor element is employed which comprises a poly(ethylene terephthalate) support coated with sequential repeating areas of deprotonated dyes, as described above, capable of generating a cyan, magenta and yellow dye and the dye transfer 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.

Thermal print heads which can be used to transfer dye from dye-donor elements to the receiving elements of the invention are available commercially. There can be employed, for example, a Fujitsu Thermal Head (FTP-040 MCS001), a TDK Thermal Head F415 HH7-1089 or a Rohm Thermal Head KE 2008-F3. Alternatively, other known sources of energy for thermal dye transfer may be used, such as lasers as described in, for example, GB No. 2,083,726A.

When a three-color image is to be obtained, the assemblage described above is formed on three occasions during the time when heat is applied by the thermal print head. 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 into register with the dye-receiving element and the process repeated. The third color is obtained in the same manner. After thermal dye transfer, the dye image-receiving layer contains a thermally-transferred dye image.

The following examples are provided to further illustrate the invention.

EXAMPLES

Example 1--Preparation of Acidic Particles A-5

In a 1.0 liter flask equipped with a Teflon paddle stirrer was placed 800 ml of nitrogen deaerated, 0.2 .mu.m filtered, distilled water. To this was added 12 g of sodium dodecylsulfate, 1 g ethyleneglycol dimethacrylate, 100 g methyl methacrylate and 50 g 2-acrylamido-2-methyl-1-propane sulfonic acid sodium salt. The mixture was heated with stirring to 65.degree. C. and a solution of 0.64 g potassium persulfate and 0.60 g sodium metabisulfite in 20 ml of water was added. Heating was continued overnight at 65.degree. C. The mixture was cooled and dialyzed against distilled water for 48 hours. The latex was passed through an Amberlite.RTM. IR 120 column to convert the sodium sulfonic acid groups to the acid form. From this was obtained a latex having a 0.0622 .mu.m size (by UPA) containing 36.0% 2-acrylamido-2-methyl-1-propane sulfonic acid.

Acidic particles A-1 through A-4 and A-6 through A-9 can be prepared in a similar manner.

Dye-Donor Elements

Individual dye-donor elements were prepared by coating the following compositions in the order listed on a 6 .mu.m poly(ethylene terephthalate) support:

1) a subbing layer of Tyzor TBT.RTM., a titanium tetrabutoxide, (DuPont Company) (0.16 g/m.sup.2) coated from 1-butanol/propyl acetate (15/85 wt. %); and 2) an imaging dye layer coated from a tetrahydrofuran/cylopentanone (95/5) solvent mixture, whereby two different binder polymer mixtures with the selected dye as shown in Table 1 were used: DB-1 propionate ester of bisphenol A copolymer with epichlorohydrin (prepared by techniques similar to those described in U.S. Pat. No. 5,244,862); DB-2 poly(butyl methacrylate-co-Zonyl TM.RTM.) (75/25) where Zonyl TM.RTM. is a perfluoro monomer available from DuPont.

Details of dye and binder laydowns are summarized in the following Table 1:

TABLE 1______________________________________ DB-1 DB-2Dye-Donor Deprotonated Dye Laydown, Laydown, Laydown,Element Dye (g/m.sup.2) (g/m.sup.2) (g/m.sup.2)______________________________________Yellow Dye 5 0.28 0.27 0.07Cyan Dye 1 0.15 0.18 0.05______________________________________

On the back side of the dye-donor element were coated the following compositions in the order listed:

1) a subbing layer of Tyzor TBT.RTM., a titanium tetrabutoxide, (DuPont Company) (0.13 g/m.sup.2) coated from 1-butanol/propyl acetate (15/85 wt. %); and 2) a slipping layer of 0.38 g/m.sup.2 poly(vinyl acetal) (Sekisui), 0.022 g/m.sup.2 Candelilla wax dispersion (7% in methanol), 0.011 g/m.sup.2 PS513 amino-terminated polydimethylsiloxane (Huels) and 0.0003 g/m.sup.2 p-toluenesulfonic acid coated from a 3-pentanone/distilled water (98/2) solvent mixture.

Dye-Receivers Elements

Receiver Element E-1:

This element was prepared by first extrusion laminating a paper core with a 38 .mu.m thick microvoided composite film (OPPalyte.RTM. 350TW, Mobil Chemical Co.) as disclosed in U.S. Pat. No. 5,244,861. The composite film side of the resulting laminate was then coated with the following layers in the order recited:

1) a subbing layer of Polymine P, polyethyleneimine (0.02 g/m.sup.2) (BASF Corp.) coated from water; and 2) a dye-receiving layer of a mixture 2.58 g/m.sup.2 of acidic particles A-1, and 3.88 g/m.sup.2 of the low Tg polymer P-1, coated from distilled water.

Receiver Elements E-2 through E-14.

These were prepared as described for Element E-1, with the various ratios of acidic particles to low Tg polymer listed in Table 2.

Control Elements C-1 and C-2.

These were prepared as described for Receiver Element E-1, except that they used film-forming acidic polymers (control acid sources CA-1 and CA-2) and low Tg polymer mixtures as listed in Table 2.

Control Acid Sources:

CA-1: poly�isophthalic acid-co-5-sulfoisophthalic acid (90:10 molar ratio)-diethylene glycol (100 molar ratio)!, Mw=20,000 (sulfonic acid of AQ29D, Eastman Chemical Co., acidic substance A-1 of U.S. Pat. No. 5,627,128) CA-2: Ammonium salt of AQ29D which has the following structure: ##STR4## wherein M.sup.+ is NH.sub.4.sup.+ TABLE 2______________________________________ Ratio AcidReceiver Acid Source Low Tg Polymer Source/Low TgElement (g/m.sup.2) P-1 (g/m.sup.2) Polymer______________________________________E-1 A-1 (2.58) 3.88 40/60E-2 A-2 (2.58) 3.88 40/60E-3 A-3 (2.58) 3.88 40/60E-4 A-4 (1.29) 5.17 20/80E-5 A-4 (2.58) 3.88 40/60E-6 A-5 (1.29) 5.17 20/80E-7 A-5 (1.94) 4.52 30/70E-8 A-6 (1.29) 5.17 20/80E-9 A-7 (0.65) 5.81 10/90E-10 A-7 (1.29) 5.17 20/80E-11 A-8 (1.29) 5.17 20/80E-12 A-8 (2.58) 3.88 40/60E-13 A-9 (1.29) 5.17 20/80E-14 A-9 (2.58) 3.88 40/60C-1 CA-1 (2.58) 3.88 40/60C-2 CA-2 (2.58) 3.88 40/60______________________________________

Receiver Element 15.

The receiver element can be comprised of two or more 5 layers, coated separately or simultaneously, and having varying combinations of the acidic particles and the low Tg polymer. In the example illustrated by Table 3 below, the lower layer is coated closest to the subbing layer and the upper layer is coated over the lower layer, that is. furthest from the subbing layer.

TABLE 3______________________________________Receiver Element Upper Layer (g/m.sup.2) Lower Layer (g/m.sup.2)______________________________________E-15 A-7 (0.11)/P-1 (1.91) A-7 (0.43)/P-1 (3.58)______________________________________

This multilayer also contained 7 wt % ML-160, a carnauba wax aqueous dispersion, added as a coating aid.

Receiver Elements 16-17.

In this experiment, two low Tg polymers were combined as shown in Table 4:

TABLE 4______________________________________ Low Tg Low Tg Ratio of AcidReceiver Acidic Particle Polymer Polymer Particle/Low TgElement (g/m.sup.2) P-1 (g/m.sup.2) P-14 (g/m.sup.2) Polymers______________________________________E-16 A-5 (1.29) 1.93 3.22 20/80E-17 A-5 (1.29) 3.22 1.93 20/80______________________________________

Preparation and Evaluation of Thermal Dye Transfer Images

Eleven-step sensitometric cyan and green (yellow+cyan) thermal dye transfer images were prepared from the above dye-donor elements and dye-receiver elements. The dye side of the dye-donor element approxiately 10 cm.times.15 cm in area was placed in contact with a receiving-layer side of a dye-receiving element of the same area. This assemblage was clamped to a stepper motor-driven, 60 mm diameter rubber roller. A thermal head TDK model no. L-231, resolution of 5.4 dots/mm, thermostated at 25.degree. C. was pressed with a force of 24.4 Newton (2.5 kg) against the dye-donor element side of the assemblage, pushing it against the rubber roller.

The imaging electronics were activated causing the donor-receiver assemblage to be drawn through the print head/roller nip at 40.3 mm/sec. Coincidentally, the resistive elements in the thermal print head were pulsed for 127.75 .mu.s/pulse at 130.75 .mu.s intervals during a 4.575 msec/dot printing cycle (including a 0.391 msec/dot cool-down interval). A stepped image density was generated by incrementally increasing the number of pulses/dot from a minimum of 0 to a maximum of 32 pulses/dot. The voltage supplied to the thermal head was approximately 13.0 volts resulting in an instantaneous peak power of 0.318 watts/dot and a maximum total energy of 1.30 mJ/dot. This procedure was done using the yellow dye-donor element and then repeated on a portion of the yellow image with the cyan dye-donor element to produce a green stepped image. Print room humidity: 61%RH.

For images containing a cyan dye (cyan or green images), the rate of protonation is proportional to the rate of color change from the deprotonated dye form (magenta) to the protonated dye form (cyan). This color change can be monitored by measuring status A red (cyan) and green (magenta) densities at various time intervals and calculating the red/green ratio for each time interval. Complete protonation (conversion) of the cyan dye was equivalent to the red/green ratio after incubating prints at 50.degree. C./50% RH for 3 Hours and a % dye conversion can be calculated.

After printing, the dye-donor element was separated from the imaged receiving element and the Status A reflection red and green densities at step 10 in the stepped-image were measured for the green image using a X-Rite 820.RTM. reflection densitometer after 1.0 minutes and 120 minutes at room temperature. The prints were then placed in a 50.degree. C./50% relative humidity oven for 3.0 hours and the red and green densities were reread �red/green (R/G) incubated endpoint!. A R/G ratio (minus the baseline) was calculated for the cyan dye in the cyan and green images in each receiver at the above mentioned time intervals. The % dye conversion at 1.0 minutes was calculated from the ratio of the R/G ratio at 1.0 minutes to that at 120 minutes. The results are summarized in Table 5 below.

TABLE 5______________________________________ Ratio of AcidRe- Particle/ R/G % Conversion at 1 minceiver Acid Low Tg Incubated relative to endpoint @ 120Element Source Polymers Endpoints min green cyan green cyan______________________________________E-1 A-1 40/60 3.04 4.16 49 65E-2 A-2 40/60 3.83 4.38 45 69E-3 A-3 40/60 4.03 3.27 71 79E-4 A-4 20/80 4.27 4.74 75 80B-5 A-4 40/60 3.93 4.15 52 73E-6 A-5 20/80 4.33 4.07 75 87B-7 A-5 30/70 3.51 3.24 83 89B-8 A-6 20/80 3.54 3.73 62 80E-9 A-7 10/90 4.51 4.23 68 88E-10 A-7 20/80 3.54 3.09 85 91E-11 A-8 20/80 4.64 4.56 42 74B-12 A-8 40/60 3.92 3.74 83 92B-13 A-9 20/80 4.83 4.70 44 82B-14 A-9 40/60 4.42 4.20 87 97E-15 A-7 9/91 4.55 4.17 43.2 97.5B-16 A-5 20/80 4.27 4.25 51.2 75.7B-17 A-5 20/80 5.02 4.96 61.3 77.5C-1 CA-1 40/60 5.42 4.69 35 70C-2 CA-2 40/60 5.62 5.78 23 51______________________________________

The above results in Table 5 show that adding the acidic particles (A-1 through A-9) to the receiver element in accordance with the invention improves the % dye conversion of deprotonated cationic dyes after printing at 1 minute in one or both the green and cyan channels over that which is obtained with the control acid-contaning polymers CA-1 and CA-2. Thus, using the receiver of the invention improves the dye conversion of deprotonated cationic dyes after printing at short times and no change in color balance occurs over time.

Also, the above results show that the ultimate dye conversion (RIG endpoint of incubated samples) is not sacrificed, staying over a value of 3.0.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

1. A thermal dye transfer assemblage comprising:

(I) a dye-donor element comprising a support having thereon a dye layer comprising a dye dispersed in a polymeric binder, said dye being a deprotonated cationic dye which is capable of being reprotonated to a cationic dye having a N-H group which is part of a conjugated system, and

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

a) a polymer having a Tg of less than about 19.degree. C. and having no or only slight acidity; and

b) sub-micron, non film-forming, organic, acidic particles which are capable of reprotonating the cationic dye transferred to said dye image-receiving layer from said dye layer.

2. The assemblage of claim 1 wherein said polymer having a Tg of less than about 19.degree. C. is an acrylic polymer, a styrene polymer, a polyester or a vinyl polymer.

3. The assemblage of claim 1 wherein said deprotonated cationic dye has the following formula: ##STR5## wherein: X, Y and Z form a conjugated link between nitrogen atoms selected from CH, C-alkyl, N, or a combination thereof, the conjugated link optionally forming part of an aromatic or heterocyclic ring;

R represents a substituted or unsubstituted alkyl group from about 1 to about 10 carbon atoms;

R.sup.1 and R.sup.2 each individually represents substituted or unsubstituted phenyl or naphthyl or a substituted or unsubstituted alkyl group from about 1 to about 10 carbon atoms; and

n is 0 to 11.

4. The assemblage of claim 1 wherein said acidic particles have a Tg greater than about 60.degree. C.

5. The assemblage of claim 1 wherein said acidic particles are cross-linked.

6. The assemblage of claim 5 wherein said acidic particles are present in an amount of from about 1 to about 90% of said polymeric dye image- receiving layer.

7. A process of forming a dye transfer image comprising imagewise-heating a dye-donor element comprising a support having thereon a dye layer comprising a dye dispersed in a polymeric binder, said dye being a deprotonated cationic dye which is capable of being reprotonated to a cationic dye having a N-H group which is part of a conjugated system, and imagewise transferring said dye to a dye-receiving element to form said dye transfer image, said dye-receiving element comprising a support having thereon a polymeric dye image-receiving layer, said polymeric dye image-receiving layer comprising a mixture of

a) a polymer having a Tg of less than about 19.degree. C. and having no or only slight acidity; and

b) sub-micron, non film-forming, organic, acidic particles which are capable of reprotonating the cationic dye transferred to said dye image-receiving layer from said dye layer.

8. The process of claim 7 wherein said polymer having a Tg of less than about 19.degree. C. is an acrylic polymer, a styrene polymer, a polyester or a vinyl polymer.

9. The process of claim 7 wherein said deprotonated cationic dye has the following formula: ##STR6## wherein: X, Y and Z form a conjugated link between nitrogen atoms selected from CH, C-alkyl, N, or a combination thereof, the conjugated link optionally forming part of an aromatic or heterocyclic ring;

R represents a substituted or unsubstituted alkyl group from about 1 to about 10 carbon atoms;

R.sup.1 and R.sup.2 each individually represents substituted or unsubstituted phenyl or naphthyl or a substituted or unsubstituted alkyl group from about 1 to about 10 carbon atoms; and

n is 0 to 11.

10. The process of claim 7 wherein said acidic particles have a Tg greater than about 60.degree. C.

11. The process of claim 7 wherein said acidic particles are cross-linked.

12. The process of claim 7 wherein said acidic particles are present in an amount of from about 1 to about 90 % of said polymeric dye image-receiving layer.