DE69602499T2 - Stabilizers for dye-donor element used in thermal dye transfer - Google Patents

Description

This invention relates to the use of certain polycarbonate polyol stabilizers in dye-donor elements for thermal dye transfer systems.

In recent years, thermal transfer systems have been developed to obtain prints from images generated electronically by a color video camera. According to one way of obtaining such prints, an electronic image is first subjected to color separation using color filters. The respective color-separated images are then converted into electrical signals. These signals are then processed to generate cyan, magenta and yellow electrical signals. These signals are then transferred 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 print head and a roller. A line-type thermal print head is used to apply heat from the back of the dye-donor sheet. The thermal print head has many heating elements and is heated sequentially in response to the cyan, magenta or yellow signal. The process is then repeated for the other two colors. A color print is thus obtained that corresponds to the original image viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in US Patent 4,621,271.

An important requirement for any thermal dye-donor element is to maintain its performance throughout its useful life without deterioration in the quality of the image. The dye layer of a dye-donor A resistive head thermal dye transfer element 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 storage of the donor, the temperature and humidity may be elevated and the dye layer is in contact with a slip layer coated on the back of the donor element when it is wound in spool form. The slip layer may contain mobile lubricating oils or materials that can act as plasticizers or solvents for the dye layer. This allows the dye to become mobile, allowing changes in the layer to occur, including further phase separation, migration of the dye to the surface, and even crystallization of the dye. Dye may also be transferred to the slip layer. These changes generally result in sensitometric variations, uneven printing due to light and dark spots, and dye smearing from a high density to a low density area of the print.

US Patent 5,288,691 relates to the use of mixtures of monomeric or oligomeric glass materials or compounds containing a phenylindane moiety as stabilizers for dye-donor elements to minimize sensitometric changes during storage and preservation of the donor. However, the use of compounds according to this invention for such a purpose is not disclosed.

U.S. Patent 5,266,551 discloses the use of polycarbonate polyols capable of reacting with multifunctional isocyanates to form a crosslinked polymer network for a dye-receiving element for thermal dye transfer printing. However, there is no disclosure in this patent that such materials would be useful as a stabilizer in a dye-donor element.

It is an object of this invention to provide a stabilizer for a dye-donor element in which sensitometric changes during storage and preservation are minimized and dye performance during printing is maintained.

These and other objects are achieved according to this invention which relates to a dye-donor element for thermal dye transfer comprising a support having thereon a dye layer comprising an image dye in a polymeric binder, and wherein the dye layer also contains a stabilizer comprising an oligomeric polycarbonate polyol having a molecular weight between 1000 and 10,000, preferably 1000 to 5000.

In a preferred embodiment of the invention, the stabilizer comprises an oligomeric polycarbonate polyol comprising a mixture of aromatic and/or aliphatic diols linked by a carbonate group and having two terminal hydroxy groups.

In a further preferred embodiment of the invention, the stabilizer can be represented by the following formula:

wherein R and R' each independently represent a divalent aliphatic group having from about 2 to about 16 carbon atoms or an aromatic radical having from about 6 to about 30 carbon atoms and n is between 2 and 10.

By using the stabilizers of the present invention, sensitometric changes of the dye-donor elements during storage can be minimized. The stabilizers used in the invention exhibit improved solubility properties in preferred coating solvents.

The polycarbonate polyols used in the invention can be formed by the reaction of a bis(chloroformate) with a diol. One of the monomers is used in excess and becomes the end group. Alternatively, the bis(chloroformate) could be present in excess to give a chloroformate end-blocked oligomer which is then hydrolyzed to form a hydroxyl group. Polyols can be prepared from these monomers with either R or R' in excess according to the following equation:

Examples of bis(chloroformates) that can be used in the above reaction include: M1: Bisphenol A bis(chloroformate) M2: Diethylene glycol bis(chloroformate) or M3: Butanediolbis(chloroformate).

Examples of diols that can be used in addition to diethylene glycol, butanediol, pentanediol, nonanediol include the following: M4: Bisphenol A M5: 2,2',6,6'-tetrachlorobisphenol A M6: 4,4'-(Octahydro-4,7-methano-5H-inden-5-ylidene)bisphenol M7: 4,4'-Bicyclo[2.2.2]hept-2-ylidenebisphenol

The above monomers and other aliphatic and aromatic diols can be combined to form a variety of compositions and compounds with different chain lengths and end groups. The polyol could end permanent aliphatic hydroxyl end groups (e.g. diethylene glycol end groups) or phenolic end groups (e.g. bisphenol A end groups). In addition, appropriate amounts of multifunctional (> 2) alcoholic groups of either phenolic or aliphatic nature, such as glycerin, could be added to provide branched oligomeric polyols.

The following are examples of stabilizers that can be used in the invention:

* represents the aliphatic/aromatic monomer ratio which provides aliphatic end groups

** refers to polystyrene equivalent molecular weight

* represents the aliphatic/aromatic monomer ratio which provides aliphatic end groups

** refers to polystyrene equivalent molecular weight

In a preferred embodiment of the invention, the stabilizer is present at a concentration of about 5 to about 25% by weight of the dye layer.

Any dye can be used in the dye-donor used in the invention, provided that it is transferable to the dye-receiving layer by the action of heat. Particularly good results have been obtained with sublimable dyes such as

or any of the dyes disclosed in U.S. Patents 4,541,830, 4,698,651, 4,695,287, 4,701,439, 4,757,046, 4,743,582, 4,769,360 and 4,753,922. The above dyes may be used singly or in combination.

The dyes can be used at a coverage of about 0.05 to about 1 g/m2 and are preferably hydrophobic.

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

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

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; fluoropolymers; polyethers; polyacetals; polyolefins; and polyimides. The support generally has a thickness of from about 5 to about 200 mm. It can also be coated, if desired, with an adhesive layer such as those materials described in U.S. Patents 4,695,288 or 4,737,486.

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 can be used at a coverage of from about 0.1 to about 5 g/m².

The back side of the dye-donor element may be coated with a slipping layer to prevent the printhead to the dye-donor element. Such a slip layer would comprise either a solid or liquid lubricating material or mixtures thereof with or without a polymeric binder or surfactant. 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 sliding layer include poly(vinyl alcohol cobutyral), poly(vinyl alcohol coacetal), poly(styrene), poly(vinyl acetate), cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate or ethyl cellulose.

The amount of lubricating materials to be used in the sliding layer depends largely on the type of lubricating material, but is generally in the range of about 0.001 to about 2 g/m². If a polymeric binder is used, the lubricating material is in the range of 0.05 to 50% by weight, preferably 0.5 to 40%, of the polymeric binder used.

The dye-receiving element used with the dye-donor element of the invention usually comprises a support having a dye image-receiving layer thereon. The support may be a transparent film such as a poly(ethersulfone), a polyimide, a cellulose ester such as cellulose acetate, a poly(vinyl alcohol coacetal) or a poly(ethylene terephthalate). The support for the dye-receiving element may also be reflective such as baryta-coated 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.

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-cobutyral), poly(vinyl alcohol-co-benzal), poly(vinyl alcohol-coacetal), or mixtures thereof. The dye image-receiving layer may be present in any amount effective for the intended purpose. Generally, good results have been obtained at a concentration of from about 1 to about 5 g/m².

As noted above, the dye-donor elements of the invention are used to form a dye transfer image. One such process 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.

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

In a preferred embodiment of the invention, the dye-donor element comprises a poly(ethylene terephthalate) support coated with successive repeating areas of cyan, yellow and magenta, and the above process steps are carried out for each color in turn to give a three-color dye transfer image. Of course, if the process is carried out for only a single color, then a single-color dye transfer image is obtained.

A dye thermal transfer assembly of the invention comprises

a) a dye-donor element as described above and

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

The above assembly comprising these two elements may be preassembled into an integral unit if a single color image is to be obtained. This may be accomplished by temporarily adhering the two elements together at their edges. After transfer, the dye-receiving element is then peeled away to reveal the dye transfer image.

If 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 into precise registration with the dye-receiving element and the process is repeated. The third color is obtained in the same way.

The following examples are provided to illustrate the invention.

EXAMPLE 1 - Preparation of polycarbonate polyol from excess diethylene glycol bis(chloroformate) and aliphatic hydroxyl-terminated bisphenol A (G-4).

A 1 liter three-necked round bottom flask equipped with an argon inlet, a mechanical stirrer, and an addition funnel was charged with diethylene glycol bis(chloroformate) (55.4 g, 0.24 mol), bisphenol A (45.7 g, 0.2 mol), ethyl acetate (325 mL) and cooled to 5-10°C with an ice bath. A solution of triethylamine (40.48 g, 0.4 mol) in ethyl acetate (75 mL) was added slowly over a period of 45 minutes while stirring under a stream of argon. The mixture was filtered from the white precipitate, rinsed with ethyl acetate, the combined ethyl acetate solutions were treated with 20 mL of water and 50 mL of acetone followed by 12 g of pyridine to hydrolyze the chloroformate end groups. The solution was washed with 600 mL of water containing 6 mL of concentrated hydrochloric acid, washed three times with 600 mL of sodium chloride solution and dried over anhydrous potassium carbonate. The solution was filtered, condensed on a rotary evaporator to 50 to 60% solids, and precipitated in 3 liters of a 50/50 methanol/ice water mixture. The soft caramel-like material was ground in a blender with water to a hardened solid, filtered, and air dried.

EXAMPLES 2

The control cyan dye donor 1 was prepared by coating 0.12 g/m² of titanium tetrabutoxide Tyzor TBT® (DuPont Corp.) in a propyl acetate/butanol solvent mixture on both sides of a 6 µm poly(ethylene terephthalate) support.

On one side of the support was coated a slip layer of 0.48 g/m² poly(vinyl acetal) binder (Sekisui Co.), 0.007 g/m² copolymer of polypropylene oxide and polymethyloctylsiloxane BYKS732, 98% Stoddard solvent (BYK Chemie), 0.010 g/m² aminopropyl endblocked polydimethylsiloxane PS513 (Huels Co.), and 0.0003 g/m² p-toluenesulfonic acid applied from a diethyl ketone/methanol solvent mixture.

On the other side of the support were coated the cyan dye C-1 illustrated above (0.37 g/m²), the cyan dye C-2 illustrated above (0.11 g/m²), cellulose acetate propionate (CAP) binder (2.5% acetyl, 45% propionyl) (0.35 g/m²), a micronized mixture of Polyethylene, polypropylene and oxidized polyethylene particles S363NI (Shamrock Technologies, Inc.) (0.02 g/m²) and surfactant Fluorad® FC 430 (3M Corp.) (0.002 g/m²) dissolved in and applied from a toluene/methanol/cyclopentanone mixture.

Comparative Dye Donor 2 was prepared similarly to Control Dye Donor 1, except that 0.11 g/m² of a comparative stabilizer (shown below) was added to the dye layer and the CAP binder was adjusted to 0.24 g/m² to achieve an equivalent sensitometric response.

Dye donors according to the invention were prepared similarly to Control Dye Donor 2 except that they contained the stabilizers listed in Table 1 in place of the comparative stabilizer. Comparative Stabilizer 2,4,6-trimethylaniline substituted phenylindane.

A dye-receiving element was prepared by extrusion laminating a paper inner layer with a 38 µm thick microvoided composite film (OPPalyte® 350TW, Mobil Chemical Co.) as disclosed in U.S. Patent 5,244,861. The composite film side of the resulting laminate was then coated with the following layers in the order listed:

1) Dow Z-6020 (an aminoalkyleneaminotrimethoxysilane) (Dow Corning Co.) (0.11 g/m2) adhesive layer applied from ethanol;

2) Dye-receiving layer composed of bisphenol A polycarbonate Makrolon® KL3-1013 (Myles Laboratories) (1.776 g/m²), polycarbonate Lexan® 141-112 (General Electric Co.) (1.453 g/m²), dibutyl phthalate (0.323 g/m²), diphenyl phthalate (0.323 g/m²) and fluorosurfactant FC431® (3M Corp.) (0.011 g/m²), coated from dichloromethane;

3) Topcoat of a linear condensation copolycarbonate of bisphenol A (50 mol%), diethylene glycol (49 mol%) and polydimethylsiloxane block units of MW 2500 (1 mol%) (11 g/m²), Fluorad FC431® (0.02 g/m²) and Dow Corning 510 silicone fluid (0.01 g/m²) applied from dichloromethane.

Accelerated storage tests were performed by hand-winding donor samples under constant tension onto plastic cores against the poly(vinyl acetal) slip layer described above and placing them (tightly enclosed in a foil-lined bag at 40% RH) in accelerated storage ovens at 60ºC/70% RH for 3 days. The results are shown in Table 1.

Sensitometric images of 11 levels were printed using the incubated and room-held donor samples. The dye side of a dye-donor element strip of approximately 10 cm x 13 cm area was placed in contact with the dye image-receiving layer of the dye-receiving element of the same area. The assembly was clamped to a 60 mm diameter rubber roller driven by a stepper motor, and a TDK thermal head (No. L-231) (thermostatically controlled at 30ºC) was pressed against the dye-donor element side of the assembly with a force of 24.4 newtons (2.5 kg), pressing it against the rubber roller.

The image recording electronics were switched on, causing the donor/receiver assembly to be pulled between the print head and the roller at 11.1 mm/s. At the same time, the resistance elements in the Thermal print head was pulsed for 64 us/pulse at 129 us intervals during the 16.9 us/dot print time. A graded density image was produced by incrementally increasing the number of pulses/dot from 0 to 127. The voltage applied to the print head was approximately 14.5 volts, resulting in an instantaneous maximum power of 0.429 watts/dot and a maximum total energy of 3.49 mJ/dot. The Dmax values (level 11) and changes in Status A red densities at OD = 0.50 with incubation are shown in Table 1 below: TABLE 1

The above data demonstrate that the stabilizers of this invention (G-1 through G-4) reduce the sensitometric change of the dye donors after incubation relative to the control without stabilizer. Stabilizers G-1 through G-3 also showed a decrease in sensitometric changes after incubation relative to the control stabilizer. In addition, a 7-9% increase in Dmax relative to the control without stabilizer and the control stabilizer was achieved when using stabilizers G-3 and G-4.

EXAMPLE 3

Example 2 was repeated but using the materials identified in Table 2 as follows: TABLE 2

The above data show that stabilizers G-5 through G-7 all reduce the sensitometric change of the donor after incubation relative to the stabilizer-free control. This demonstrates that the reduction in sensitometric changes achievable with the stabilizers of this invention is independent of the molecular weight of the stabilizer used.

Claims (10)

1. A dye-donor element for thermal dye transfer comprising a support having thereon a dye layer comprising an image dye in a polymeric binder, wherein the dye layer further comprises a stabilizer comprising an oligomeric polycarbonate polyol having a molecular weight of between 1000 and 10,000. 2. The element of claim 1 wherein the stabilizer is present in a concentration of 5 to 25 percent by weight of the dye layer. 3. The element of claim 1 wherein the stabilizer comprises an oligomeric polycarbonate polyol having a mixture of aromatic and/or aliphatic diols linked by carbonate groups and having two terminal hydroxy groups. 4. The element of claim 1, wherein the stabilizer is represented by the following formula: wherein R and R' each independently represent a divalent aliphatic group having 2 to 16 carbon atoms or an aromatic radical having 6 to 30 carbon atoms and wherein n is a number between 2 and 10. 5. Element according to claim 1, in which the stabilizer consists of: 6. Element according to claim 1, in which the stabilizer consists of: 7. A process for producing a thermal dye transfer image comprising: I) contacting at least one dye-donor element comprising a support having thereon a dye layer comprising an image dye in a polymeric binder with a dye-receiving element comprising a support having thereon a polymeric dye image-receiving layer; in which II) heating the dye-donor element imagewise; and wherein III) transferring a dye image to the dye-receiving element to form the thermal transfer image, wherein the dye layer further contains a stabilizer comprising an oligomeric polycarbonate polyol having a molecular weight between 1000 and 10,000. 8. The process of claim 7, wherein the stabilizer comprises an oligomeric polycarbonate polyol containing a mixture of aromatic and/or aliphatic diols linked together by carbonate groups and having two terminal hydroxy groups. 9. Thermal dye transfer kit with: (I) a dye-donor element comprising a support having thereon a dye layer comprising an image dye dispersed in a polymeric binder, and (II) a dye-receiving element comprising a support having thereon a dye image-receiving layer, the dye-receiving layer being in superior relationship to the dye-donor element such that the dye layer comes into contact with the dye image-receiving layer, in which the dye layer further contains a stabilizer comprising an oligomeric polycarbonate polyol having a molecular weight between 1000 and 10,000. 10. A composition according to claim 9, wherein the stabilizer comprises an oligomeric polycarbonate polyol of a mixture of aromatic and/or aliphatic diols linked together by carbonate groups and having two terminal hydroxy groups.