Stabilizers for dye-donor element used in thermal dye transfer

This invention 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 about 1000 and about 10,000.

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 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. Pat. No. 4,621,271, the disclosure of which is 
hereby incorporated by reference. 
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. These changes generally result in 
sensitometric variations, nonuniform printing due to light or dark spots 
and dye smearing from a high density to a low density area of the print. 
U.S. Pat. No. 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 on 
donor storage or keeping. However, the use of compounds according to this 
invention for such a purpose is not disclosed. 
U.S. Pat. No. 5,266,551 discloses the use of polycarbonate polyols which 
can react with multifunctional isocyanates to generate a crosslinked 
polymer network for a dye-receiver 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 upon storage and keeping are 
minimized and dye efficiency during printing is maintained. 
These 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 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 about 
1000 and about 10,000, preferably from about 1000 to about 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 carbonate groups and having two terminal hydroxy 
groups. 
In another preferred embodiment of the invention, the stabilizer can be 
represented by the following formula: 
##STR1## 
wherein R and R' each independently represents 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 
dye-donor elements upon keeping can be minimized. Also, the stabilizers 
used in the present invention exhibit improved solubility characteristics 
in preferred coating solvents. 
The polycarbonate polyols used in the invention may 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 in excess to give a chloroformate-terminated 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: 
##STR2## 
Examples of bis(chloroformates) which can be used in the above reaction 
include: 
##STR3## 
Examples of diols which can be used in addition to diethylene glycol, 
butanediol, pentanediol, nonanediol include the following: 
##STR4## 
The above monomers and other aliphatic and aromatic diols may be combined 
to form a variety of compositions, and compounds of different chain 
lengths and end groups. The polyol could have terminal aliphatic hydroxyl 
end groups (e.g., diethylene glycol end groups) or phenolic terminal 
groups (e.g., bisphenol A end groups). In addition, appropriate amounts of 
multifunctional (&gt;2) alcoholic groups of either phenolic or aliphatic 
nature, such as glycerol, could be added to yield branched oligomeric 
polyols. 
Following are examples of stablizers which may be used in the invention: 
__________________________________________________________________________ 
##STR5## 
moles of monomer ratio* 
Stabilizer M2 M4 M6 alip./arom. 
MW** 
__________________________________________________________________________ 
G-1 (Tg = 53.degree. C.) 
0.50 0.325 0.325 10/13 1950 
G-2 (Tg = 51.degree. C.) 
0.50 0.30 0.30 10/12 2850 
__________________________________________________________________________ 
*represents aliphatic/aromatic monomer ratio, yielding aliphatic end 
groups 
**refers to polystyrene equivalent molecular weight - 
##STR6## 
mole of monomer ratio* 
STABILIZER R M2 M4 M5 alip./arom. 
MW** 
__________________________________________________________________________ 
G-3 (Tg = 29.degree. C.) 
H 0.26 0.20 -- 13/10 2780 
G-4 (Tg = 32.degree. C.) 
H 0.24 0.20 -- 12/10 3740 
G-5 (Tg = 77.degree. C.) 
Cl 0.12 -- 0.10 12/10 3600 
G-6 (Tg = 64.degree. C.) 
Cl 0.13 -- 0.10 13/10 3100 
G-7(Tg = 58.degree. C.) 
Cl 0.15 -- 0.10 15/10 2600 
__________________________________________________________________________ 
*represents aliphatic/aromatic monomer ratio, yielding aliphatic end 
groups 
**refers to polystyrene equivalent molecular weight 
In a preferred embodiment of the invention, the stablizer is present at a 
concentration of from about 5 to about 25% by weight of the dye layer. 
Any 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. 
Especially good results have been obtained with sublimable dyes such as 
anthraquinone dyes, e.g., Sumikalon Violet RS.RTM. (product of Sumitomo 
Chemical Co., Ltd.), Dianix Fast Violet 3R-FS.RTM. (product of Mitsubishi 
Chemical Industries, Ltd.), and Kayalon Polyol Brilliant Blue N-BGM.RTM. 
and KST Black 146.RTM. (products of Nippon Kayaku Co., Ltd.); azo dyes 
such as Kayalon Polyol Brilliant Blue BM.RTM., Kayalon Polyol Dark Blue 
2BM.RTM., and KST Black KR.RTM. (products of Nippon Kayaku Co., Ltd.), 
Sumickaron Diazo Black 5G.RTM. (product of Sumitomo Chemical Co., Ltd.), 
and Miktazol Black 5GH.RTM. (product of Mitsui Toatsu Chemicals, Inc.); 
direct dyes such as Direct Dark Green B.RTM. (product of Mitsubishi 
Chemical Industries, Ltd.) and Direct Brown M.RTM. and Direct Fast Black 
D.RTM. (products of Nippon Kayaku Co. Ltd.); acid dyes such as Kayanol 
Milling Cyanine 5R.RTM. (product of Nippon Kayaku Co. Ltd.); basic dyes 
such as Sumicacryl Blue 6G.RTM. (product of Sumitomo Chemical Co., Ltd.), 
and Aizen Malachite Green.RTM. (product of Hodogaya Chemical Co., Ltd.); 
##STR7## 
or any of the dyes disclosed in U.S. Pat. Nos. 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 
disclosures of which are hereby incorporated by reference. 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/m.sup.2 and are preferably 
hydrophobic. 
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. Pat. No. 4,716,144. 
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 head. Such materials include polyesters such as 
poly(ethylene terephthalate); polyamides; polycarbonates; cellulose esters 
such as cellulose acetate; fluorine polymers such as polyvinylidene 
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 polyether-imides. The support generally has a 
thickness of from about 5 to about 200 mm. It may also be coated with a 
subbing layer, if desired, such as those materials described in U.S. Pat. 
Nos. 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. Pat. No. 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/m.sup.2. 
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.degree. 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. Pat. 
Nos. 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. 
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 0.001 to about 2 g/m.sup.2. 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. 
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 
baryta-coated paper, polyethylene-coated paper, an ivory paper, a 
condenser paper or a synthetic paper such as DuPont Tyvek.RTM.. 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-co-butyral), poly(vinyl alcohol-co-benzal), 
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/m.sup.2. 
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 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. 
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. 
Thermal printing heads which can be used to transfer dye from the dye-donor 
elements of the invention are available commercially. There can be 
employed, for example, a Fujitsu Thermal Head (FTP-040 MCSOO1), a TDK 
Thermal Head F415 HH7-1089 or a Rohm Thermal Head KE 2008-F3. 
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. 
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 
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.

The following examples are provided to illustrate the invention. 
EXAMPLE 1 
Preparation of polycarbonate polyol from excess diethylene glycol 
bis(chloroformate) and bisphenol A-terminal aliphatic hydroxyl groups 
(G-4). 
A 1-liter three-necked, round-bottomed 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 mole), bisphenol A (45.7 g, 0.2 
mole), ethyl acetate (325 ml) and cooled to 5.degree. -10.degree. C. with 
an ice bath. A solution of triethylamine (40.48 g, 0.4 mole) in ethyl 
acetate (75 ml) was slowly added over a 45 min period while stirring under 
an argon flow. The mixture was filtered from the white precipitate, rinsed 
with ethyl acetate, the combined ethyl acetate solutions were treated with 
20 ml water and 50 ml 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 3 times with a 
600 ml 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 into 3 liters of a 50/50 methanol/ice 
water mixture. The soft taffy was ground in a blender with water to a 
hardened solid, filtered and air dried. 
EXAMPLE 2 
Control cyan dye donor 1 was prepared by coating 0.12 g/m.sup.2 Tyzor 
TBT.RTM. titanium tetrabutoxide (DuPont Corp.) in a propyl acetate/butanol 
solvent mixture on both sides of a 6 .mu.m poly(ethylene terephthalate) 
support. 
On one side of the support was coated a slipping layer of 0.48 g/m.sup.2 
poly(vinyl acetal) (Sekisui Co.) binder, 0.007 g/m.sup.2 BYKS732 copolymer 
of polypropylene oxide and polymethyloctyl siloxane, 98% Stoddard solvent 
(BYK Chemie), 0.010 g/m.sup.2 PS513 aminopropyl-terminated 
polydimethylsiloxane (Huels Co.) and 0.0003 g/m.sup.2 p-toluenesulfonic 
acid coated from a diethyl ketone/methanol solvent mixture. 
On the reverse side of the support was coated cyan dye C-1 illustrated 
above, (0.37 g/m2), cyan dye C-2 illustrated above (0.11 g/m2), cellulose 
acetate propionate (CAP) binder (2.5% acetyl, 45% propionyl) (0.35 g/m2), 
S363NI micronized blend of polyethylene, polypropylene, and oxidized 
polyethylene particles (Shamrock Technologies, Inc.), (0.02 g/m2), and 
Flourad.RTM. FC 430 surfactant (3M Corp.) (0.002 g/m.sup.2) dissolved in 
and coated from a toluene/methanol/cyclopentanone mixture. 
Comparison dye-donor 2 was prepared similar to Control dye-donor 1 except 
that 0.11 g/m.sup.2 of a Comparison Stabilizer (shown below) was added to 
the dye layer and the CAP binder was adjusted to 0.24 g/m.sup.2 in order 
to achieve equivalent sensitometric response. 
Dye-donors according to the invention were prepared similar to control 
dye-donor 2 except that they contained the stabilizers as listed in Table 
1 instead of the Comparison Stabilizer. 
##STR8## 
Comparison Stabilizer 
2,4,6,-trimethylaniline-substituted phyenylindane 
A dye-receiver element was prepared by 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) subbing layer of Dow Z-6020 (an aminoalkylene aminotrimethoxysilane) 
(Dow Corning Co.) (0.11 g/m.sup.2) coated from ethanol; 
2) dye-receiving layer composed of Makrolon.RTM. KL3-1013 bisphenol A 
polycarbonate (Myles Laboratories) (1.776 g/m2), Lexan.RTM. 141-112 
polycarbonate (General Electric Co.) (1.453 g/m2), dibutyl phthalate 
(0.323 g/m2), diphenyl phthalate (0.323 g/m.sup.2) and FC431.RTM. 
fluorosurfactant (3M Corp.) (0.011 g/m.sup.2) coated from dichloromethane; 
3) overcoat layer of a linear condensation copolycarbonate of bisphenol A 
(50 mole-%), diethylene glycol (49 mole-%), and 2,500 MW 
polydimethylsiloxane block units (1 mole-%) (11 g/m2), Fluorad FC431.RTM. 
(0.02 g/m.sup.2) and Dow-Corning 510 Silicone Fluid (0.01 g/m.sup.2) 
coated from dichloromethane. 
Accelerated keeping tests were performed by hand-winding samples of the 
donors on plastic cores under constant tension against the poly(vinyl 
acetal) slipping layer described above and placing them (sealed at 40% RH 
in a foil-lined bag) into accelerated keeping ovens at 60.degree. C./70% 
RH for 3 days. The results are listed in Table 1. 
Eleven step sensitometric images were printed using incubated and room-kept 
donor samples. The dye side of a dye-donor element strip approximately 10 
cm.times.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 30.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 between the printing head and roller at 11.1 mm/s. 
Coincidentally, the resistive elements in the thermal print head were 
pulsed for 64 .mu.s/pulse at 129 .mu.s intervals during the 16.9 .mu.s/dot 
printing time. A stepped density image was generated by incrementally 
increasing the number of pulses/dot from 0 to 127. The voltage supplied to 
the print head was approximately 14.5 volts, resulting in an instantaneous 
peak power of 0.429 watts/dot and a maximum total energy of 3.49 mJ/dot. 
The Dmax values (step 11) and changes with incubation of Status A red 
densities at O.D.=0.50 are listed in Table 1 as follows: 
TABLE 1 
______________________________________ 
Red Density (Steps 4 or 5) 
Before After 
Stabilizer D-max Incub. Incub. 
Diff. 
______________________________________ 
None 2.45 0.55 0.96 0.41 
Comparison 2.46 0.57 0.66 0.09 
G-1 2.45 0.47 0.45 -0.02 
G-2 2.44 0.46 0.47 0.01 
G-3 2.68 0.44 0.43 -0.01 
G-4 2.64 0.42 0.51 0.09 
______________________________________ 
The above data show that the stabilizers of this invention (G-1 through 
G-4) decrease the sensitometric change of the dye-donors after incubation 
relative to the no stabilizer control. Stabilizers G-1 through G-3 also 
showed a decrease in sensitometric change after incubation relative to the 
Comparison Stabilizer. In addition, a 7%-9% increase in Dmax was achieved 
when using stabilizers G-3 and G-4 relative to the stabilizer-free control 
and the Comparison Stabilizer. 
EXAMPLE 3 
Example 2 was repeated but using the materials identified in Table 2 as 
follows: 
TABLE 2 
______________________________________ 
Red Density (Steps 4 or 5) 
Before After 
Stabilizer D-max Incub. Incub. 
Diff. 
______________________________________ 
None 2.36 0.49 0.85 0.36 
Comparison 2.45 0.55 0.58 0.03 
G-5 2.48 0.58 0.64 0.06 
G-6 2.39 0.60 0.66 0.06 
G-7 2.63 0.60 0.67 0.07 
______________________________________ 
The above data show that stabilizers G-5 through G-7 all decrease the 
sensitometric change of the donors after incubation relative to the 
stabilizer-free control. This demonstrates that the reduction in 
sensitometric changes achievable with the stabilizers, of this invention 
are independent of the molecular weight of the stabilizer used. 
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.