This invention relates to the use of a certain polymeric binder for
a thermal transfer donor element. The donor element is used to produce binary text
on a thermal receiver element for optical character recognition (OCR) and bar
codes which can be read by scanners.
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. The process is then
repeated for the other two colors. A color hard copy is thus obtained which corresponds
to the original picture viewed on a screen. Further details of this process and
an apparatus for carrying it out are contained in U.S. patent 4,621,271.
Dye diffusion thermal printing can be used to produce bar codes for
use in a diversity of areas including packaging, sales, passports and ID cards.
Bar codes require only a binary image composed of a very high density, machine-readable
black and a low minimum density. The black density in the bar code can be produced
by printing dyes sequentially from yellow, magenta and cyan donor elements to the
same area of the thermal receiver or by printing from a single dye-donor element
which contains the dye mixture necessary to produce black. The same technique can
be used to produce alphanumeric characters which can be optically read. In either
case it is necessary to incorporate near infrared dyes or optically recognizable
alphanumerics into the bar code to accommodate the various scanning devices used.
The spectral response range for scanners is considered to be from 600 to 1000 nm
which includes the red and near infrared portions of the electromagnetic spectrum.
The near infrared dyes and visible dyes used in dye diffusion thermal
printing must be stable to thermal degradation in the dye-donor element, easily
transferred to the thermal receiver at low printing energies, and stable to degradation
by heat and light after transfer to the receiver.
The dye-donor of a diffusion thermal transfer system usually contains
the dyes and a non-transferable polymeric binder which adheres the dyes to the
donor substrate. The polymeric binder is chosen such that sticking of donor to
receiver during printing at high densities is minimal, preferably non-existent.
As the time for printing (line time) is decreased, additional energy
is applied to the dye-donor element to maintain high dye density in the thermal
receiver. As the power increases, the propensity of donor/receiver sticking increases
because of the higher temperatures attained, not only because of increased energy
but also because of lower heat loss to the surroundings.
U.S. Patent 5,514,637 relates to a typical dye diffusion donor wherein
a continuous tone image can be printed rendering the appropriate gray scales. In
this system, the binder of the dye-donor element usually does not transfer to the
receiving element. There is a problem with using this system to print bar codes,
however, in that high levels of dye are required to produce a binary image composed
of a very high density, machine-readable black.
It is an object of this invention to provide a thermal transfer donor
element wherein higher densities can be obtained than using a dye diffusion transfer
element. It is another object of this invention to provide a binder for a thermal
transfer donor element which has good adhesion to a receiver element.
These and other objects are achieved in accordance with this invention
which relates to a thermal transfer donor element comprising a support having thereon
a dye layer comprising a dye dispersed in a polymeric binder, the dye layer being
capable of being thermally transferred to a receiver element, wherein the polymeric
binder is a phenoxy resin.
Another embodiment of the invention relates to a process of forming
a dye transfer image comprising:
a) imagewise-heating the thermal transfer donor element described above, and
b) transferring portions of the dye layer to a dye-receiving element to form
the dye transfer image.
By using the thermal transfer donor element of the invention, 100%
of the dye is transferred (together with the binder) to the receiver during the
printing step. Since less dye is used in the thermal transfer donor element, it
also has better shelf stability to crystallization since the dye concentration
in the polymer is lower.
The binder may be used at any concentration effective for the intended
purpose. In general, good results are obtained when the binder is used at a coverage
of from about 0.1 to about 5 g/m2. The binder may be present at a concentration
of from about 15 to about 35 % by weight of the dye layer.
Any phenoxy resin known to those skilled in the art may be used in
the invention. For example, there may be employed the following: Paphen® resins
such as Phenoxy Resins PKHC®, PKHH® and PKHJ® from Phenoxy Associates,
Rock Hill, S.C.; and 045A and 045B resins from Scientific Polymer Products, Inc.
Ontario, N.Y. which have a mean number molecular weight of greater than about
10,000. In a preferred embodiment of the invention, the phenoxy resin is a Phenoxy
Resin PKHC®, PKHH® or PKHJ® having the following formula:
In another embodiment of the invention, various crosslinking agents
may be employed with the binder such as titanium alkoxides, polyisocyanates, melamine-formaldehyde,
phenol-formaldehyde, urea-formaldehyde, vinyl sulfones and silane coupling agents
such as tetraethylorthosilicate. In still another embodiment of the invention,
the crosslinking agent is a titanium alkoxide such as titanium tetra-isopropoxide
or titanium butoxide. In general, good results have been obtained when the crosslinking
agent is present in an amount of from about 0.01 g/m2 to 0.045 g/m2.
Any image dye can be used in the thermal transfer donor element 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 any of the dyes
used in the examples hereafter or those 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 about 0.05 to about 1 g/m2 and
are preferably hydrophobic. In a preferred embodiment of the invention, a mixture
of cyan, magenta and yellow image dyes and an infrared-absorbing dye is employed.
Infrared-absorbing dyes which may be used in the invention include
cyanine infrared-absorbing dyes as described in U.S. Patent 4,973,572, or other
dyes as described in the following U.S. Patents: 4,948,777; 4,950,640; 4,950,639;
4,948,776; 4,948,778; 4,942,141; 4,952,552; 5,036,040; and 4,912,083.
The dye-receiving element that is used in the invention 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, white polyester (polyester with white pigment
incorporated therein), an ivory paper, a condenser paper, a synthetic paper such
as DuPont Tyvek®, or a laminated, microvoided, composite packaging film support
as described in U.S. Patent 5,244,861.
The dye image-receiving layer may comprise, for example, a polycarbonate,
a polyurethane, a polyester, poly(vinyl chloride), poly(styrene-co-acrylonitrile),
polycaprolactone or mixtures thereof. The dye image-receiving layer may be present
in any amount which is effective for the intended purpose. In general, good results
have been obtained at a concentration of from about 1 to about 5 g/m2.
Any material can be used as the support for the thermal transfer
donor element of the invention provided it is dimensionally stable and can withstand
the heat of the thermal head. Such materials include polyesters such as poly(ethylene
terephthalate); polyamides; polycarbonates; cellulose esters; fluorine polymers;
polyethers; polyacetals; polyolefins; and polyimides. The support generally has
a thickness of from about 5 to about 200 µm. It may also be coated with a subbing
layer, if desired, such as those materials described in U. S. Patents 4,695,288
The reverse side of the thermal transfer donor element may be coated
with a slipping layer to prevent the printing head from sticking to the thermal
transfer donor element. Such a slipping layer would comprise either a solid or
liquid lubricating material or mixtures thereof, with or without a polymeric binder
or a surface-active agent. Preferred lubricating materials include oils or semi-crystalline
organic solids that melt below 100°C such as poly(vinyl stearate), beeswax, perfluorinated
alkyl ester polyethers, polycaprolactone, silicone oil, polytetrafluoroethylene,
carbowax, poly(ethylene glycols), or any of those materials disclosed in U.S. Patents
4,717,711; 4,717,712; 4,737,485; and 4,738,950. Suitable polymeric binders for
the slipping layer include poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-acetal),
polystyrene, poly(vinyl acetate), cellulose acetate butyrate, cellulose acetate
propionate, cellulose acetate or ethyl cellulose.
A thermal dye transfer assemblage of the invention comprises
a) a thermal transfer 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 thermal transfer
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 an 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.
The following example is provided to illustrate the invention:
The following dyes were used in the experimental work:
(Cyasorb 165® available from Glendale Protective Technologies)
A. Donor Elements
A thermal transfer donor element was prepared by coating on a 6.4
µm poly(ethylene terephthalate) substrate (DuPont) which had been coated with
Tyzor TBT® titanium tetrabutoxide (DuPont). On that side of this donor substrate
was coated a slipping layer composed of poly(vinyl acetal) (Sekisui) (0.383 g/m2),
candelilla wax (Strahl & Pitsch) (0.022 g/m2), p-toluenesulfonic
acid (0.0003 g/m2), and PS-513, (an aminopropyl dimethyl terminated
polydimethyl siloxane), (United Chemical Technologies) (0.010 g/m2).
On the opposite side of the so-prepared donor support was coated one of the dye
layers as outlined below, from a toluene/n-propanol/cyclopentanone (60:35:5 wt-%)
solvent mixture, using a slot head for delivery. Drying was performed at 38-43°C.
This was the same as Thermal Transfer Donor 3 except that IR-Dyes
1 and 2 were replaced by IR-Dye 5 and IR-Dye 3.
Thermal Transfer Donor 5
This was the same as Thermal Transfer Donor 3 except that the level
of phenoxy resin was reduced to 0.538 g/m2.
Thermal Transfer Donor 6
This was the same as Thermal Transfer Donor 3 except that the level
of phenoxy resin was reduced to 0.269 g/m2.
Thermal Transfer Donor 7 (Comparison)
This was the same as Thermal Transfer Donor 2 except that the KS-1
(polyvinylacetal, Sekisui) was used in place of the PKHJ phenoxy resin.
Thermal Transfer Donor 8
This was the same as Thermal Transfer Donor 4 except that IR-Dye
4 was substituted for IR-Dye 5.
The formulation was designed to function as a dye diffusion thermal
transfer donor with cellulose acetate propionate (CAP) as the binder which did
not stick to the receiver. The materials and coating weights were as follows:
MATERIAL COATING WEIGHT (g/m2) Dye 10.150 Dye 20.226 Dye 30.040 Dye 40.226 Dye 50.323 IR-Dye 10.430 IR-Dye 20.108 2 µm divinylbenzene beads0.027 CAP 482-20 (20 sec viscosity) (Eastman Chemical Co.)0.074 CAP 482-0.5 (0.5 sec viscosity) (Eastman Chemical Co.)0.602 Fluorad® FC-430 (fluorosurfactant) (3M Corp.)0.011
B. Receiver Element
The receiver element consisted of four layers coated on 175 µm Estar®
(Eastman Kodak Co.) support.
The first layer, which was coated directly onto the support, consisted
of a copolymer of butyl acrylate and acrylic acid (50/50 wt. %) at 8.07 g/m2,
1,4-butanediol diglycidyl ether (Eastman Kodak) at 0.565 g/m2, tributylamine
at 0.323 g/m2, Fluorad® FC-431 (3M Corp.) at 0.016 g/m2.
The second layer consisted of a copolymer of 14 mole-% acrylonitrile,
79 mole-% vinylidine chloride and 7 mole-% acrylic acid at 0.538 g/m2,
and DC-1248 silicone fluid (Dow Corning) at 0.016 g/m2.
The third layer consisted of Makrolon® KL3-1013 polycarbonate
(Bayer AG) at 1.77 g/m2, Lexan 141-112 polycarbonate (General Electric
Co.) at 1.45 g/m2, Fluorad® FC-431 at 0.011 g/m2, dibutyl
phthalate at 0.323 g/m2, and diphenylphthalate at 0.323 g/m2.
The fourth, topmost layer of the receiver element, consisted of a
copolymer of 50 mole-% bisphenol A, 49 mole-% diethylene glycol and 1 mole-% of
a polydimethylsiloxane block at a laydown of 0.646 g/m2, Fluorad®
FC-431 at 0.054 g/m2, and DC-510 (Dow Corning) at 0.054 g/m2.
C. Printing Conditions
The dye side of a donor element as described above was placed in
contact with the topmost layer of the receiver element. The assemblage was placed
between a motor driven platen (35 mm in diameter) and a Kyocera KBE-57-12MGL2
thermal print head which was pressed against the slip layer side of the thermal
transfer donor element with a force of 31.2 Newtons.
The Kyocera print head has 672 independently addressable heaters
with a resolution of 11.81 dots/mm of 1968 Ω average resistance. The imaging
electronics were activated and the assemblage was drawn between the printing head
and the roller at 26.67 mm/sec. Coincidentally, the resistance elements in the
thermal print head were pulsed on for 87.5 microseconds every 91 microseconds.
Printing maximum density required 32 pulses "on" time per printed line of 3.175
milliseconds. The maximum voltage supplied was 12.0 volts resulting in an energy
of 3.26 J/cm2 to print a maximum Status A density of 2.2 to 2.3. The
image was printed with a 1:1 aspect ratio.
The results in Table I represent the Status A densities measured
with an X-Rite densitometer(X-Rite Corp.) in the visible region and the infrared
densities obtained at 820 and 915 nm using a Lambda 12 Spectrophotometer with
an integrating sphere from Perkin-Elmer Corporation.
Thermal Transfer Donor Element Status A Red Status A Green Status A Blue Density Region 820 nm 915 nm 12.982.992.811.101.11 22.702.702.631.161.16 32.552.418.104.22.168 42.992.792.541.160.77 52.592.642.321.191.18 62.602.522.291.121.09 7 (Comparison)2.592.562.531.201.17 82.462.222.214.171.124 Control0.640.590.570.170.22
The above results show that the values for the Thermal Transfer Donors
1 through 8 indicate substantial density increases in the printed receiver over
that for the dye diffusion control for both the visible and infrared regions of
the spectrum. This was found even when the dye level of the visible dyes had been
decreased by 60% (Thermal Transfer Donor 3) from that of the dye diffusion control.
Whereas Thermal Transfer Dye-Donor 7 gave high density values, it exhibited lower
adhesion to the receiver surface (see below) than did the Thermal Transfer Donors
of the invention.
Adhesion was measured by a Scotch® tape pull test of the receiver
having the following test materials transferred thereto: Elvacite® 1010 and
1020 acrylic resins (ICI Acrylics), Matrimid® 5218 polyamide (Ciba-Geigy),
polyvinylacetal (Sekisui) and PKHJ® phenoxy resin (Phenoxy Associates). The
Scotch® tape was applied with finger pressure and rapidly pulled off. The
following results were obtained:
The above results show that the acrylic resins (Elvacite®) and
polyamide (Matrimid®) both have poor adhesion to the topmost layer of thermal
receiver elements containing polysiloxanes. Poly(vinyl acetal) gave moderate adhesion,
whereas the phenoxy resin adhered very well to the receiver element.
Bar Code Printing Test
Scans were performed on a scanner from Kronos Inc.. The bar codes
for this test were printed at a line time of 3.175 milliseconds at an applied power
of 3.26 J/cm2. The bar code was scanned 10 times. The following results
Sample Performance*Dye Diffusion Dye-Donor (control)0/10 Thermal Transfer Donor 110/10 Thermal Transfer Donor 210/10 Thermal Transfer Donor 310/10 Thermal Transfer Donor 410/10 Thermal Transfer Donor 510/10 Thermal Transfer Donor 610/10 Thermal Transfer Donor 7 (comparison)0/10 Thermal Transfer Donor 810/10
* Performance is the number of correct scans per number
The above results show that when a bar code printed from Thermal
Transfer Donors 1 through 6 and Thermal Transfer Donor 8 is compared to a bar code
from the dye diffusion control, the readability is better (10 correct scans per
10 attempts) than that of the dye diffusion control (0 correct scans per 10 attempts).
Thermal Transfer Donor 7 gave poor readability because of the poorer adhesion of
the poly(vinyl acetal) binder to the receiver surface (see Table II).
Daylight Exposure Test
The printed samples were exposed to a Xenon lamp at an intensity
of 50 Klux for 7 days. The spectral output of the lamp was adjusted to a daylight
exposure with appropriate filters. The absorbance at 820 nm and 915 nm was measured
using a Perkin Elmer Lambda 12 spectrophotometer (Perkin Elmer Corp.) before and
after exposure to the lamp and the % absorbance change was calculated. The following
results were obtained:
Sample % Absorbance Change of Infrared Dyes 820 nm 915 nm Dye Diffusion Dye-Donor (Control)-30-26 Thermal Transfer Donor 124
The above results show that IR-Dye 1 and IR-Dye 2 (Dye-Donor 1) show
excellent stability to fading by exposure to daylight compared to the control produced
by dye diffusion.
A thermal transfer donor element comprising a support having thereon a dye
layer comprising a dye dispersed in a polymeric binder, said dye layer being capable
of being thermally transferred to a receiver element, wherein said polymeric binder
is a phenoxy resin.
The element of Claim 1 wherein said binder is present at a concentration of
from 15 to 35 % by weight of said dye layer.
The element of Claim 1 wherein said phenoxy resin comprises
A process of forming a dye transfer image comprising:
a) imagewise-heating a thermal transfer donor element comprising a support
having thereon a dye layer comprising a dye dispersed in a polymeric binder, and
b) transferring portions of said dye layer to a dye-receiving element to form
said dye transfer image,
wherein said binder is a phenoxy resin.
The process of Claim 4 wherein said binder is present at a concentration of
from 15 to 35 % by weight of said dye layer.
The process of Claim 4 wherein said phenoxy resin comprises
A thermal dye transfer assemblage comprising:
a) a thermal transfer donor element comprising a support having thereon a dye
layer comprising a dye dispersed in a polymeric binder, said dye layer being capable
of being thermally transferred to a receiver element, and
b) a receiver element comprising a support having thereon an image-receiving
layer, said receiver element being in superposed relationship with said thermal
transfer donor element so that said dye layer is in contact with said image-receiving
wherein said polymeric binder is a phenoxy resin.
The assemblage of Claim 7 wherein said binder is present at a concentration
of from 15 to 35 % by weight of said dye layer.
The assemblage of Claim 7 wherein said phenoxy resin comprises