MATERIAL FOR INCREASING DYE TRANSFER EFFICIENCY IN DYE-DONOR ELEMENTS USED IN THERMAL DYE TRANSFER

Abstract: A dye-donor element for thermal dye transfer comprising a support having on one side thereof a dye dispersed in a polymeric binder, characterized in that the polymeric binder comprises a mixed cellulose ester and the dye-donor element contains a colorless, nonpolymeric material for increasing dye transfer efficiency having the following formula: wherein both R groups represent -CO₂J or -O₂CJ and are located either ortho or meta to each other; and each J independently represents a substituted or unsubstituted phenyl group, or a substituted or unsubstituted carbomonocyclic or carbobicyclic ring having from about 5 to about 12 carbon atoms, with the proviso that when either or both of J is substituted, then the total number of substituent carbon atoms in each J group is about 8 or less.

Description: This invention relates to materials which can be added to a dye-donor element in order to improve the dye transfer efficiency. 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 and yellow signals. 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 No. 4,621,271 by Brownstein entitled "Apparatus and Method For Controlling A Thermal Printer Apparatus, " issued November 4, 1986. It is always desirable to transfer as much dye as possible with the lowest thermal energy in dye transfer systems using a thermal head. The amount of dye which can be transferred from a dye-donor to a receiving element by thermal dye transfer depends upon the dye transfer efficiency. In JP 61/286,199, there is a disclosure of a dye-donor element containing a "heat-fusible" compound of a certain formula. In Example 1 of that reference, benzoic acid phenyl ester is employed in a binder of ethyl cellulose and in Example 3, similar compounds are employed in a binder of cellulose acetate. As will be shown by comparative tests hereinafter, the use of a cellulose acetate binder with the compounds employed in this invention gives poor transfer density and the use of an ethyl cellulose binder gives poor density after incubation. In addition, as will be shown by comparative tests hereinafter, not all the compounds shown in the JP 61/286,199 reference provide a significant increase in density upon transfer. It is an object of this invention to provide a material for increasing dye transfer efficiency which would not promote crystallization of the dye in the dye-donor and which would retain its beneficial effects upon storage. These and other objects are achieved in accordance with this invention which comprises a dye-donor element for thermal dye transfer comprising a support having on one side thereof a dye dispersed in a polymeric binder, characterized in that the polymeric binder comprises a mixed cellulose ester and the dye-donor element contains a colorless, nonpolymeric material for increasing dye transfer efficiency having the following formula: wherein both R groups represent -CO₂J or -O₂CJ and are located either ortho or meta to each other; and each J independently represents a substituted (e.g., alkyl, alkoxy, acyl, etc.) or unsubstituted phenyl group, or a substituted (e.g., alkyl alkoxy, acyl, etc.) or unsubstituted carbomonocyclic or carbobicyclic ring having from 5 to 12 carbon atoms, with the proviso that when either or both of J is substituted, then the total number of substituent carbon atoms in each J group is 8 or less. These compounds provide an increase in dye transfer efficiency by providing equivalent density with less energy than a dye-donor which does not contain the compounds. The phenyl group in the above formula may be substituted with groups such as CH₃, C₂H₅, t-C₄H₉, OC₂H₅, CH₂OCH₃, COCH₃, NO₂, etc. As noted above, the polymeric binder employed in the invention comprises a mixed cellulose ester. Such esters include cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate hydrogen phthalate, etc. In a preferred embodiment of the invention, the binder is cellulose acetate propionate or cellulose acetate butyrate. The binder may be used at a coverage of from 0.1 to 5 g/m². In another preferred embodiment of the invention, both R groups represent -O₂CJ and J is phenyl. In still another preferred embodiment of the invention, both R groups represent -CO₂J, wherein J represents -C₆H₅, cyclo-C₆H₁₁, -C₆H₄(4-OCCH₃), -C₆H₄(3-OCH₃), -C₆H₄(3-CH₃), -C₆H₄(2,6-t-C₄H₉), -C₆H₄(2,6-i-C₃H₇), or fenchyl. In yet still another preferred embodiment of the invention, the material for increasing the dye transfer efficiency has the formula: The materials described above may be incorporated directly into the dye layer of the dye-donor or in an adjacent layer where it will be in effective contact with the dye. The material may be employed in any amount which is effective for the intended use. In general, good results have been obtained at a concentration of from 0.05 to 0.3 g/m² or 30% to 300% by weight of coated dye. Compounds included within the scope of the invention include the following: Any dye can be used in the dye layer of the dye-donor element of the invention provided it is transferable to the dye-receiving layer by the action of heat. Especially good results have been obtained with sublimable dyes such as or any of the dyes disclosed in U.S. Patent 4,541,830. The above dyes may be employed singly or in combination to obtain a monochrome. The dyes may be used at a coverage of from 0.05 to 1 g/m² and are preferably hydrophobic. 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. 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 printing heads. Such materials include polyesters such as poly(ethylene terephthalate); polyamides; polycarbonates; glassine paper; condenser paper; cellulose esters; fluorine polymers; polyethers; polyacetals; polyolefins; and polyimides. The support generally has a thickness of from 2 to 30 µm. It may also be coated with a subbing layer, if desired. 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 a lubricating material such as a surface active agent, a liquid lubricant, a solid lubricant or mixtures thereof, with or without a polymeric binder. 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 poly(ethylene terephthalate) or reflective such as baryta-coated paper, polyethylene-coated paper, white polyester (polyester with white pigment incorporated therein), etc. The dye image-receiving layer may comprise, for example, a polycarbonate, a polyurethane, a polyester, polyvinyl chloride, poly(styrene- co -acrylonitrile), poly(caprolactone) 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 1 to 5 g/m². As noted above, the dye-donor elements of the invention are used to form a dye transfer image. Such a 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 may be used in sheet form or in a continuous roll or ribbon. If a continuous roll or ribbon is employed, it may have only one dye or may have alternating areas of other different dyes, such as sublimable cyan and/or magenta and/or yellow and/or black or other dyes. Such dyes are disclosed in U. S. Patents 4,541,830; 4,698,651; 4,695,287; and 4,701,439. Thus, one-, two-, three- or four-color elements (or higher numbers also) 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 sequential repeating areas of cyan, magenta and yellow dye, 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. A thermal dye transfer assemblage using 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. 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. When a three-color image is to be obtained, the above assemblage is formed on three occasions during the time when heat is applied by the thermal printing 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 in register with the dye-receiving element and the process repeated. The third color is obtained in the same manner. The following examples are provided to illustrate the invention. Example 1 - Comparative Tests A dye-receiver was prepared by obtaining a commercially produced paper stock 6.5 mil (165 µm) thick 40 lb/1000 ft²) (195 g/m²) mixture of hard woodkraft and soft wood-sulfite bleached pulp. The paper stock was then extrusion overcoated with an approximately 1:4 ratio of medium density:high density polyethylene (2.5 lb/1000 ft²) (12 g/m²) with approximately 6 wt. percent anatase titanium dioxide and 1.5 wt. percent zinc oxide (layer thickness 12 µm). The support was then coated with the following layers: (a) Subbing layer of poly(acrylonitrile)- co -vinylidene chloride- co -acrylic acid (14:79:7 wt. ratio) (0.54 g/m²) coated from a butanone and cyclopentanone solvent mixture; (b) Dye-receiving layer of Makrolon 5705® polycarbonate (Bayer AG) (2.9 g/m²), 1,4-didecoxy-2,5-dimethoxybenzene (0.38 g/m²), and polycaprolactone (0.38 g/m²) coated from methylene chloride; and (c) Overcoat of polycaprolactone (0.11 g/m²), DC 510® (Dow Corning Corp.) surfactant (0.011 g/m²) and FC-431® surfactant (3M Co.) (0.011 g/m²) coated from methylene chloride. The back side of the receiver was coated with a polyethylene layer and an overcoat layer. Control cyan, magenta and yellow dye-donor elements were prepared as follows. On one side of a 6 µm poly(ethylene terephthalate) support, a subbing layer of titanium n-butoxide (duPont Tyzor TBT®) (0.12 g/m²) was coated from a n-propyl acetate and 1-butanol solvent mixture. On top of this layer were coated repeating color patches of cyan, magenta and yellow dyes. The cyan coating contained the cyan dye illustrated above (0.28 g/m²) and cellulose acetate propionate (2.5% acetyl, 45% propionyl) binder (0.44 g/m²) from a toluene, methanol and cyclopentanone solvent mixture. The magenta coating contained the magenta dye illustrated above (0.15 g/m²) in the same binder as the cyan dye (0.32 g/m²). The yellow coating contained the yellow dye illustrated above (0.14 g/m²) in the same binder as the cyan dye (0.25 g/m²). Dye-donor elements in accordance with the invention were prepared similar to the control elements except that they also contained either 0.054 g/m² or 0.16 g/m² of diphenyl phthalate. Comparison dye-donor elements were made similar to the control elements except that they contained plasticizer compounds as listed in Table 1. On the reverse side of each dye-donor was coated a subbing layer of Tyzor TBT® titanium n-butoxide (duPont Corp.) (0.12 g/m²) coated from n-propyl acetate and 1-butanol solvent mixture and a slipping layer of Emralon 329® (Acheson Colloids Co.) lubricant (0.54 g/m²) of poly(tetrafluoroethylene)particles in a cellulose nitrate binder and a propyl acetate, toluene, isopropyl alcohol and 2-butanone solvent mixture. The dye-side of the dye-donor element strip 4 inches (10. cm) wide was placed in contact with the dye image-receiving layer of a dye-receiver element strip of the same width. The assemblage was fastened in a clamp on a rubber-roller of 2.25 in (5.65 cm) diameter driven by a stepper motor. A TDK L-231 Thermal Head was pressed at a force of 8 pounds (3.6 kg) against the dye-donor element side of the assemblage pushing it against the rubber roller. The imaging electronics were activated causing the device to draw the assemblage between the printing head and roller at 0.25 inches/sec (6.4 mm/sec). Coincidentally the resistive elements in the thermal print were heated using a supplied voltage of approximately 25.5 v, representing approximately 0.36 watts/pixel (8.5 mjoules/pixel group) at Dmax. Graduated density test images were generated using a "pulse-imaging" technique as described in the Brownstein patent referred to above. The energy required to produce a 1.0 density image was calculated. In this manner, the relative efficiency of thermal transfer (energy for 1.0 density) can be efficiently compared. Each dye-donor was then incubated for 2 weeks at 49°C, 50% RH. The imaging test was then repeated to re-evaluate the thermal transfer efficiency (energy for 1.0 density). The following results were obtained: Material Added to Dye-Donor @ 0.054 g/m² Energy (mjoules/pixel)/1.0 density Blue Green Red Init. Inc. Init. Inc. Init. Inc. None (Control) 5.3 5.4 5.6 5.8 5.6 5.8 A (Comparison) 4.9 5.4 5.1 5.8 5.1 5.7 B (Comparison) 5.1 5.5 5.4 5.8 5.4 5.8 C (Comparison) 5.1 5.5 5.3 5.7 5.2 5.7 D (Comparison) 5.2 5.5 5.5 5.8 5.4 5.8 Diphenyl phthalate 4.9 5.1 5.1 5.3 5.2 5.4 Material Added to Dye-Donor @ 0.16 g/m² Energy (mjoules/pixel)/1.0 density Blue Green Red Init. Inc. Init. Inc. Init. Inc. None (Control) 5.3 5.4 5.6 5.8 5.6 5.8 A (Comparison) 4.2 5.4 4.6 5.5 4.6 5.7 B (Comparison) 4.8 5.3 5.2 5.6 4.9 5.6 C (Comparison) 4.3 5.4 4.7 5.7 4.6 5.2 E (Comparison) 5.1 5.5 5.4 5.9 5.5 5.8 Diphenyl phthalate 4.0 4.1 4.3 4.3 4.8 4.9 Compound A is 1,3-diphenyl-2-propanone. Compound B is diphenyl carbonate. Compound C is benzyl benzoate. Compound D is diphenyl methane. Compound E is bibenzyl. The above results indicate that although the comparison dye-donor elements initially provided equivalent density with less energy than the control, this benefit was lost when the coatings were incubated. The dye-donor element of the invention, however, remained relatively unchanged (less than 0.2 mjoules) in producing a 1.0 density image before and after incubation of the dye-donor. Example 2 - Cellulosic Binder Comparisons A magenta dye-donor element was prepared by coating on a 6 µm poly(ethylene terephthalate) support: 1) a subbing layer of a titanium alkoxide (duPont Tyzor TBT®) (0.12 g/m²) coated from a n-propyl acetate and n-butyl alcohol solvent mixture, and 2) a dye layer containing the magenta dye illustrated above (0.17 g/m²) in the cellulosic binder indicated below at either 0.26 or 0.34 g/m² coated from a toluene, methanol and cyclopentanone solvent mixture, and diphenyl phthalate (0.09 or 1.7 g/m²). On the back side of the dye-donor was coated: 1) a subbing layer of a titanium alkoxide (duPont Tyzor TBT® ) (0.12 g/m²) coated from a n-propyl acetate and n-butyl alcohol solvent mixture, and 2) a slipping layer of Petrarch Systems PS513® amino-terminated polysiloxane (0.004 g/m²); p-toluenesulfonic acid (2.5% of the wt¸ of the polysiloxane); Emralon 329® (Acheson Colloids Corp.) dry film lubricant of poly(tetrafluoroethylene) particles in a cellulose nitrate resin binder (0.54 g/m²); and BYK-320® (BYK Chemie, USA) copolymer of a polyalkylene oxide and a methyl alkylsiloxane (0.0075 g/m²), coated from a n-propyl acetate, toluene, isopropyl alcohol and n-butyl alcohol solvent mixture. A dye-receiver was prepared by obtaining a commercially produced paper stock 6.5 mil (165 µm) thick 40 lb/1000 ft² (195 g/m²) mixture of hard woodkraft and soft wood-sulfite bleached pulp. The paper stock was then extrusion overcoated with an approximately 1:4 ratio of medium density:high density polyethylene (2.5 lb/1000 ft²) (12 g/m²) with approximately 6 wt. percent anatase titanium dioxide and 1.5 wt. percent zinc oxide (layer thickness 12 µm). The support was then coated with the following layers: (a) Subbing layer of poly(acrylonitrile)- co -vinylidene chloride- co -acrylic acid (14:79:7 wt. ratio) (0.54 g/m²) coated from a butanone and cyclopentanone solvent mixture; and (b) Dye-receiving layer of Makrolon 5705® polycarbonate (Bayer AG) (2.9 g/m²), 1,4-didecoxy-2,5-dimethoxybenzene (0.38 g/m²), and FC-431® (3M Corp.) surfactant (0.016 g/m²) coated from methylene chloride. The dye side of the dye-donor element strip approximately 10 cm x 13 cm in area was placed in contact with the dye image-receiving layer of the dye-receiver element of the same area. The assemblage was clamped to a stepper-motor driven 60 mm diameter rubber roller and a TDK Thermal Head (No. L-231) (thermostatted at 26°C) was pressed with a force of 8.0 pounds (3.6 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 between the printing head and roller at 6.9 mm/sec. Coincidentally, the resistive elements in the thermal print head were pulsed for 29 µsec/pulse at 128 µsec intervals during the 33 msec/dot printing time. A stepped density image was generated by incrementally increasing the number of pulses/dot from 0 to 255. The voltage supplied to the print head was approximately 23.5 volts, resulting in an instantaneous peak power of 1.3 watts/dot and a maximum total energy of 9.6 mjoules/dot. The dye-receiver was separated from the dye-donor and the Status A green densities of each transferred image consisting of a series of eight graduated density steps one cm square were read, and the maximum density, D-max, was tabulated. Another portion of each dye-donor was incubated at 49°C, approximately 50% RH for 7 days. The same imaging procedure described above was used for these incubated donors and D-max values were compared. The percent density loss from D-max was calculated. The following results were obtained. The above results show that only dye-donors that employ diphenylphthalate and use binders of mixed cellulose esters in accordance with the invention gave an initial increase in transferred dye density that was maintained upon dye-donor incubation. The comparison binder of cellulose acetate gave low transferred densities regardless of whether the dye-donor was fresh or incubated. The comparison binder of ethyl cellulose showed increased dye transfer when fresh, but this was not maintained upon incubation of the dye-donor. Example 3 A magenta dye-donor element was prepared by coating on a 6 µm poly(ethylene terephthalate) support: 1) a subbing layer of a titanium alkoxide (duPont Tyzor TBT®) (0.12 g/m²) coated from a n-propyl acetate and n-butyl alcohol solvent mixture, and 2) a dye layer containing the magenta dye illustrated above (0.17 g/m²) in a cellulose acetate propionate binder (2.5% acetyl, 45% propionyl) (0.31 g/m²) coated from a toluene, methanol and cyclopentanone solvent mixture, and the aromatic ester indicated in Tables 3 and 4 (0.17 g/m²). On the back side of the dye-donor was coated: 1) a subbing layer of a titanium alkoxide (duPont Tyzor TBT®) (0.12 g/m²) coated from a n-propyl acetate and n-butyl alcohol solvent mixture, and 2) a slipping layer containing the following material for Sets A, B, and D: Emralon 329® (Acheson Colloids Corp.) dry film lubricant of poly(tetrafluoroethylene) particles in a cellulose nitrate resin binder (0.54 g/m²), coated from a n-propyl acetate, toluene, isopropyl alcohol and n-butyl alcohol solvent mixture; For Set C, the same as Sets A, B, and D but also containing Petrarch Systems PS513® amino-terminated polysiloxane (0.004 g/m²); p-toluenesulfonic acid (2.5% of the wt. of the polysiloxane); and BYK-320® (BYK Chemie, USA) copolymer of a polyalkylene oxide and a methyl alkylsiloxane (0.008 g/m²); For Set E, the same as Sets A, B and D, but also containing S-232® wax (Shamrock Technologies) (micronized blend of polyethylene and carnauba wax particles (0.016 g/m²). The dye-donors were then incubated for 7 days at 49°C, 50% RH. The dye-receiver of Example 2 was employed in this Example. The dye side of the dye-donor element strip approximately 10 cm x 13 cm in area was placed in contact with the dye image-receiving layer of the dye-receiver element of the same area. The assemblage was clamped to a stepper-motor driven 60 mm diameter rubber roller and a TDK Thermal Head (No. L-231) (thermostatted at 26°C) was pressed with a force of 8.0 pounds (3.6 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 between the printing head and roller at 6.9 mm/sec. Coincidentally, the resistive elements in the thermal print head were pulsed for 29 µsec/pulse at 128 µsec intervals during the 33 msec/dot printing time. Graduated density test images were generated using a "pulsed-imaging" technique as described in U.S. Patent 4,621,271 of Brownstein referred to above. Pulses/dot were incrementally increased from 0 to 255. The voltage supplied to the print head was approximately 23.5 volts, resulting in an instantaneous peak power of 1.3 watts/dot and a maximum total energy of 9.6 mjoules/dot. The dye-receiver was separated from the dye-donor and the Status A green densities of each transferred image consisting of a series of eleven graduated density steps one cm square were read, and the maximum density, D-max, was tabulated. The energy (number of pulses) required to produce a density of 2.0 was also calculated. In this manner, the relative efficiency of thermal dye transfer (pulses for 2.0 density) can be effectively compared. In general, materials suitable for the practice of the invention had to produce a 2.0 density after donor incubation with at least 5% less energy (approximately 12-15 pulses less) and not show a maximum density loss greater than 0.5 as compared to a control with no material added. The following results were obtained. The above results indicate that materials used in accordance with the invention produced at least a 2.0 density after donor incubation, with at least 5% less energy (approximately 12-15 pulses less), without showing a maximum density loss greater than 0.5 as compared to a control with no material added.

Claims: A dye-donor element for thermal dye transfer comprising a support having on one side thereof a dye dispersed in a polymeric binder, characterized in that said polymeric binder comprises a mixed cellulose ester and said dye-donor element contains a colorless, non polymeric material for increasing dye transfer efficiency having the following formula: wherein both R groups represent -CO₂J or -O₂CJ and are located either ortho or meta to each other; and each J independently represents a substituted or unsubstituted phenyl group, or a substituted or unsubstituted carbomonocyclic or carbobicyclic ring having from 5 to 12 carbon atoms, with the proviso that when either or both of J is substituted, then the total number of substituent carbon atoms in each J group is 8 or less. The element of Claim 1 characterized in that both R groups represent -O₂CJ and J is phenyl. The element of Claim 1 characterized in that said polymeric binder comprises cellulose acetate propionate or cellulose acetate butyrate. The element of Claim 1 characterized in that both R groups represent -CO₂J. The element of Claim 4 characterized in that said J represents -C₆H₅, cyclo-C₆H₁₁, -C₆H₄(4-OCCH₃), -C₆H₄(3-OCH₃), -C₆H₄(3-CH₃), -C₆H₄(2,6-t-C₄H₉), -C₆H₄(2,6-i-C₃H₇), or fenchyl. The element of Claim 1 characterized in that said material has the formula: The element of Claim 1 characterized in that said support comprises poly(ethylene terephthalate) and the dye layer comprises sequential repeating areas of cyan, magenta and yellow dye.

Inventors: LUM, KIN KWONG C/O EASTMAN KODAK COMPANY, VANIER, NOEL RAWLE C/O EASTMAN KODAK COMPANY
Applicants: EASTMAN KODAK COMPANY (A NEW JERSEY CORPORATION)
Application year: 1988
Main classification: B41M5
IPC codes: B41M5